Network File System (NFS) Version 4 Minor Version 1 Protocol

Internet-Draft	Working Toward rfc5661bis	June 2022
Noveck	Expires 17 December 2022	[Page]

Abstract

This document describes the Network File System (NFS) version 4 minor version 1, including features retained from the base protocol (NFS version 4 minor version 0, which is specified in RFC 7530) and protocol extensions made subsequently. The later minor version has no dependencies on NFS version 4 minor version 0, and was, until recently, documented as a completely separate protocol.¶

This document will give rise to a suite of documents which collectively obsolete RFC 8881. In addition to many corrections and clarifications, it will rely on NFSv4-wide documents to substantially revise the treatment of protocol extension, internationalization, and security, superseding the descriptions of those aspects of the protocol appearing in RFCs 5661 and 8881.¶

1. Introduction to this Update

This document is intended to be the basis for a revised and updated specification of NFSv4.1. Unlike [35], which provided a limited-function update to [65], this document will revise any text known to be wrong, rather than retaining it because it is outside a pre-specified limited change scope.¶

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as specified in BCP 14 [1] [31] when, and only when, they appear in all capitals, as shown here.¶

The above differs from the corresponding statement in earlier specification versions (RFC5661 [65] and RFC8881 [35]) which only referred to RFC2119. For further discussion of this change, see Section 1.3.3 ¶

In some cases, such keywords will appear in all capitals within quotations, direct or indirect, from earlier documents. In such cases, these terms are to be interpreted just as above, except where it is explicitly noted that such an interpretation is not to be inferred. In some cases, it might be that this document's approach to the matter would not use those key words for reasons explained in the text. Such a shift might cause compatibility issues, if the previous keyword were actually relied upon but it also possible that it was not relied upon.¶

The reader should be aware that, as discussed in Appendix A.1, there are uses of the keywords listed above in RFCs 5661 and 8881 might not have been appropriate, even though the interpretation specified above was intended when the text was written and submitted for publication. In some cases the text in this document has been updated to correct the issue but it should be understood that not all such questionable uses have been addressed and that this state of affairs might continue to exist until the descendant documents are submitted for publication.¶

1.2. Organizational Changes for This Update

It is important to note that there are a number of important organizational changes discussed below that will be made in this updated specification. As work on this document progresses, the status of those changes, together with other necessary changes, will be summarized in Section 1.3.¶

The updated specification will depend on a number of NFSv4-wide documents, as described in Section 1.2.1, rather than try to deal with every aspect of the protocol description itself.¶

In the case of security, there will have to be decisions on how v4.1-specific security-related features will be addressed. See Section 1.2.2 for details.¶
The updated specification will not consist of a single document as previously done. Instead, it will consist of a suite of documents, as described in Section 1.2.3 ¶

1.2.1. Reliance on NFSv4-wide Documents

In many cases, matters previously described within the NFSv4.1 specification, will be addressed in separate NFSv4-wide documents.¶

The process of protocol extension and creation of new minor versions is described in a separate NFSv4-wide document, RFC8178 [32], dealing with the issue for the NFSv4 protocols as a whole.¶
Internationalization will be described in a separate document describing internationalization for all of the NFSv4 protocols, currently [37]. The only v4.1-specific feature is the fs_charset_cap attribute, which is described in the current document rather than the internationalization document.¶
Security will also be described in a separate document applying to all minor versions. The handling is different because there is a wider range of security-relevant features within v4.1, despite the fact that the fundamental approach is the same for all minor versions. As a result, for some features, the security document will have the lead role while, for others, the v4.1 specification will be the main source of information about the feature, although the basic security functionality will be as defined by the NFSv4-wide security document.¶

1.2.2. Adaptation of NFSv4-wide the NFSv4-wide Security Document to v4.1-specific Features

The v4.1-specific security-related features are dealt with as described below:¶

Security issues regarding pNFS will be the responsibility of the v4.1 specification document. In doing this, it will rely, where it can, on the security facilities described in the NFSv4-wide security document.¶

This is necessary because some layout types will access data without using the RPC-based facilities that discussed in the security document. In addition the requirements for security-related co-ordination between data server and metadata server are best dealt with in the security document, including cases in which RPC is used by both the data server and the metadata server, in which the necessary co-ordination requirements are defined by the layout type specification document¶
The description of the SECINFO_NO_NAME operation, will remain in the v4.1 specification, even though the description of SECINFO will be consolidated in the security document.¶

This shift is necessary because the description of SECINFO needs to be revised to allow negotiation of security-related transport characteristics in addition to authentication flavors, mechanisms and RPCSEC_GSS services.¶

Since the results of SECINFO an SECINFO_NONAME are identical,¶

The circumstances of use of SECINFO will be different for different minor version, because of the limited availability among other reasons. To deal with this situation, the security document will discuss these issue in separate sections, directed to v4.0 and v4.1+.¶
The description of the MACH_CRED and SSV features will remain in the v4.1 specification document and will only be mentioned in passing in the security document.¶

Instead, the focus with regard to state protection will be on client-peer authentication which applies to all minor versions.¶
sessions¶

1.2.3. Conversion of the NFSv4.1 specification into a Document Suite

The main specification for this minor version will be divided into multiple parts. There will certainly be a division of the specification into two part with discussion/explanation of the protocol appearing in one document, while reference material, defining each error, request, operation, and callback appearing separately.¶

This transformation is being done to make the revision and publication progress more manageable since the size of the specification as a single document makes it hard to deal with. Given that need, it is likely that the discussion/explanation will be further subdivided as discussed below.¶

With regard to the proposed documents discussed below, it needs to be understood that each document will have additional material in addition to the cited core material but that we intend also to take steps to avoid unhelpful duplication. Specifically,¶

Each document will have it's own abstract highlighting its role as part of the nfsv4.1 specification.¶
Each document will have a short introductory section describing its relationships with other documents in the specification. Part of this will deal with the possible absence of some routine sections because matters are dealt with in other documents in the specification.¶
We will try to avoid having a full Requirements Language section in each document, typically using a reference to the extended treatment of the subject in the introductory document.¶
It doesn't seem that each document needs an IANA Considerations section, given that most of them will empty.¶

However, if omitting them causes confusion in the RFC editing process, many documents will receive empty pro forma sections.¶
Having a large number of Security Considerations sections would add confusion and needs to be avoided especially since the primary Security Considerations section will be in the NFSv4-wide security document rather than in the v4.1 specification per se.¶

There will probably need to be a Security Considerations section in the pNFS document but no others.¶

An explanation of how security is addressed for NFSv4.1 as whole will appear in the introductory document but it would not be a Security Considerations section.¶
Each document will have it's own reference section. It is likely that (normative) intra-specification references will have their own sub-section.¶
There will only be a single acknowledgements section, applying to the specification as a whole. It is expected that appear in the introductory document¶

It is likely that the discussion/explanation section could be further divided as listed below. Although this listing identifies the specific material to be assigned to individual documents, it should not be assumed that the set of top-level sections will remain the same. There will be the opportunity to sub-divide top-level sections to make tables of contents more useful, particularly in the case of documents derived from a single current top-level section.¶

An Introductory document would be derived from sections 1 through 7, together with Appendix A and Acknowledgments.¶
The main discussion document would be derived from sections 8 through 14 together with sections 25 and 26 ¶
The document regarding multi-server namespace would be derived from Section 15.¶
The document regarding pNFS would be derived from Section 16.¶
The document regarding the file layout type would be derived from Section 17.¶

There is no similar opportunity to sub-divide the reference document since doing so would undercut its usefulness. That document will be derived from sections 19 through 19.3. It is likely that a replacement for [10] be treated as a second reference document within the NFSv4,1 specification document suite.¶

1.3. Introduction to The Changes Being Made in this Update

Like all internet drafts, this document is a work in progress. In this particular case, that designation is particularly appropriate since there are specific changes that need to be made and either have not made or have been started but not completed. Information regarding changes made or to be made in this update is to be found in Sections 1.3.1 through 1.3.4.¶

The current form in which the material is presented is designed for internal use within the working group, in order to help track the document's progress towards its goals.¶

Ultimately, the material regarding these changes will be re-organized in an eventual RFC.¶

1.3.1. Changes Completed So Far in this Update

Work on the necessary changes discussed below has already been completed, although necessary review might not yet have occurred.¶

The discussion of minor versioning has been updated to refer to RFC8178 [32], instead of the former approach which allowed each minor version to make its own versioning rules.¶

The document has been updated to eliminate the current (erroneous) treatment of internationalization, derived from earlier NFSv4.1 specifications [65], [35]. The section dealing with internationalization has been deleted, since it was never implemented. In its place, the specification has been modified to reference an external document which is to define the appropriate handling for internationalization for the NFSv4 protocols as a whole. Currently, that document is the I-D draft-ietf-nfsv4-internationalization [37]. In addition, the treatment of the fs_charset_cap attribute has been revised to reflect the analysis presented in the internationalization document.¶

Despite the completion of the internationalization work within this document, there remains work to do, most of which involves the completion of review for the NFSv4-wide internationalization document¶

The new document was based on the treatment of internationalization within RFC7530 [66], which served as a useful starting point, since implementation of all NFSv4 minor version followed the same approach to internationalization issues. However the following issues still need to be addressed:¶
There are is a need to update the treatment within RFC7530 to reflect IDNA changes made soon after the document was published.¶
There is a need to deal better with client name-caching issues, especially in the context of case-insensitive file systems. Text has been written and submitted but review is needed.¶

However, these issues will be addressed in the context of the internationalization document, rather than the NFSv4.1 specification. That document's transition to be a working group document is a good time to focus in order to get them addressed.¶

1.3.2. Changes Being Made in this Update to Address NFSv4.1 Errata Reports

Work is ongoing to deal with errata reports associated previous NFSv4.1 specifications. These include a large set of errata reports associated with RFC5661 and a few associated with RFC8881. This work can be categorized as follows:¶

The following errata reports associated with RFC5661 were dealt with in RFC8881, either because their substance related to issues to be dealt with in RFC8881 or because the simplicity of the needed change and its non-controversial nature made it simple to address the report as part of the RFC editing process for RFC8881: 2062, 2280, 2324, 2330, 2548, 3558.¶
Work needs to be done to address many errata reports relevant to RFC 5661 that were not addressed in RFC8881, because the change was too large or too controversial to address in the context of RFC editing for RFC8661.¶
There are a number of errata reports associated with RFC8881, that also need to be addressed.¶
A few of the existing errata reports appear to have implications for RFC5662 and would need to be addressed by an eventual rfc5662bis RFC.¶

The errata reports that remain and that will need to be addressed in this document include reports currently assigned a range of statuses in the errata reporting system, including reports marked Verified, Hold for Document Update, and Rejected. These statuses are relevant to the processing of the associated errata but not in a way as direct as might be anticipated since errata reports marked Rejected might be addressed, as a result of a justified change in working group consensus.¶

The following errata reports associated with RFC5661, have already been incorporated into this document draft: 2005, 2006, 2249, 2299, 2327, 2328, 2722, 3653, 3714, 4215, 5040, 5467, 5476, 6015.¶
The following errata reports associated with RFC5661, have not been incorporated and are being ignored because they deal with internationalization-related material which will be deleted in the handling of internationalization applying to rfc5661bis: 4914.¶
The following errata reports associated with RFC88811 already been incorporated into this document draft: 6308, 6337.¶
The following errata reports associated with RFC5661 have not yet been incorporated in this document draft but are expected to be resolved before initial submission of the descendant working group documents: 2291, 3208, 3379, 2556, 4118, 4119, 4492, 4712, 5212.¶
The remaining errata reports, all associated with RFC56661, need to be resolved considerably before WGLC for the descendent working group documents.¶

1.3.3. Changes Being Made Now in this Update

Work on the necessary changes discussed below has started but is not yet complete. This includes cases in which work to be completed is not within this document, but in a document referred to by this document. In such cases, matters formerly dealt within the NFSv4.1 specification, in the form of a single document, need to be addressed in a number of documents, each dealing with all NFSv4 minor versions together.¶

As noted previously, there are significant problems with the treatment of security within previous NFSv4.1 specifications [39], [65], and within other current NFSv4 specifications (e.g. RFC7530 [66], RFC8881 [35]). These are listed in Section 25.1. Work has started on these issues, although it is not as advanced as that for internationalization, since many important decisions need to be made. There is a preliminary security I-D [69] which serves as a guide to the decisions that will need to be made to guide the writing of of a working group standards-track document discussing security for all the NFSv4 protocols, which this document will eventually refer to normatively.¶

Work has begun in Sections 2.7 and 5.3 to make the presentation more suitable to an environment in which RPC makes transport-level encryption and client-host authentication available. However, there is a need for some working group decisions to be made before completion of the transition to a security framework that fully embraces these new elements. In addition, the writing of a new Security Considerations section will require substantial progress on a standards-track security document for NFSv4 as a whole. Once that work is done, there will need to be a re-organization of those sections and their role will primarily be to refer to the standards-track security document.¶

In addition, there is work being done to address security issues for NFSv4.1-specific features:¶

Significant work has been done to clarify security implications of pNFS.¶

This work has primarily consisted of a major revision of Section 16.9 although there are significant updates to Sections 2.7 and 2.8.2.¶

It has been made clear that the only cases in which there are essentially no security consequences from the use of pNFS, are those in which RPC is used by the storage protocol, correcting text in previous specifications which gives a contrary impression.¶

The text has been revised to take account of the existence of services provided by rpc-tls including encryption and client host authentication.¶

There has been a re-organization of Section 16.9, including separate subsections dealing with non-RPC-based storage protocols and RPC-based storage protocols with either loose or tight coupling between storage server and metadata server.¶
Significant work has been done to provide rpc-tls-based state protection which can be taken advantage even by clients who have not implemented SP4_MACH_CRED or SP4_SSV or who are using AUTH_SYS.¶

The Section 5.5.3 has been revised to allow, when SP4_NONE is used, to allow client host authentication to be used for state protection.¶

It is made clear in Section 7.8.3 that the use of SP4_NONE, when host-client authentication is active provides state protection against other clients rather than waiving state protection.¶

For many of the changes mentioned above, the definitive treatment will appear in the NFSv4-wide security document and there might also be a temporary references to the preliminary security I-D.¶

Further changes along these lines will most likely be necessary wherever in the document the SP4_* values are referred to.¶

There are a number of issues relating to the use of the key words defined in [1]. While the issues below could be treated individually and distributed among Sections 1.3.1 through 1.3.4, for now, we will treat them together.¶

A shift has been made from only citing RFC2119 [1] to citing RFC8174 [31]. While it is sometimes said that, in the absence of RFC8174, "must" and "MUST" are to be considered synonymous, the working group has never interpreted RFC2119 in that way, although the clarification provided by RFC8174 was helpful. In light of this, it might be considered that all the necessary work has been done, apart from necessary review. However, given the working group discussion about this issue in connection with RFC8881, it appears that the working group will need to further discuss this issue soon after this document becomes a working group document. That would enable us to consider this aspect of the work complete.¶
The use of the term "RECOMMENDED" to describe NFSv4 attributes which are not MANDATORY as been addressed by switching to the term "Recommended", since "RECOMMENDED" is not in accord with RFC2119 [1]. This work is considered complete.¶
For many of the existing uses of the terms "SHOULD" and "SHOULD NOT", it is not clear that the meaning is compatible with RFC2119. The difficulty is compounded by uncertainty left as to the proper use of these terms, about which there may be disagreement within the working group. See Appendix A.1.1 for a detailed discussion of issues that need to be resolved. Some work has started on this area by adjusting text in certain sections but the work cannot be completed until there is agreement about the proper use of these terms in this document.¶

These issues have been addressed by changes in Sections 5.7.2, 5.7.3, and 7.6.1). In addition, similar changes will be made in [69].¶
There are some cases in which the terms "MUST" and "MUST NOT" have essentially been ignored by implementations or other reasons to believe that these terms may have been used inappropriately. See Appendix A.1.2 for a detailed discussion. Some work has been done toward addressing these issues but it is not complete, because further discussion is needed regarding changes to be made in Section 7.6.2.¶

These issues have been addressed by changes in Sections 5.7.1 and 5.7.2.¶

1.3.4. Changes That Will Need to be Made Later in this Update

Work on necessary changes discussed below has not started yet, although some discussion and planning may have occurred, possibly together with preliminary specification text within Appendix A proposing likely changes to be made late in the specification proper.¶

Work also has to be done to divide the specification into two parts, dealing with protocol discussion/explanation and reference material respectively. That work is expected to be done as part of the transition of this effort into a set of working group documents.¶

There is a significant amount of pNFS-related work which it may make sense to address together.¶

Work needs to incorporate the material within RFC 8434 [68], which establishes the requirements for parallel NFS (pNFS) layout types, which are not clearly specified in RFC 5661, leading to confusion.¶
There are a significant number of pNFS-related errata that need to be addressed: 2505, 2751, 3064-3068, 3208, 3901, 4118, 4119.¶
¶

There are a number of issues that need to be addressed regarding the treatment of Directory Delegations. As initial discussion of these issues has resulted in no clear consensus on how these issues should be addressed in this minor version, further working group discussion will be needed. See Appendix A.3. for details.¶

There are issues that need to be addressed regarding how retried requests are to be terminated, including the fact the most common client handling of this situation violates a "MUST" in the existing specification. This expected to take the form of a revised Section 7.6.2, as discussed in Appendix A.2.¶

5. Core Infrastructure

5.1. Introduction

NFSv4.1 relies on core infrastructure common to nearly every operation. This core infrastructure is described in the remainder of this section.¶

5.2. RPC and XDR

The NFSv4.1 protocol is a Remote Procedure Call (RPC) application that uses RPC version 2 and the corresponding eXternal Data Representation (XDR) as defined in [3] and [2]. The transport-level encryption and client-host authentication facilities described in [36] can also be used.¶

5.3. RPC-Based Security

In addition to the above, as discussed in Section 5.3.1, some security flavors provide additional security services.¶

NFSv4.1 clients and servers MUST implement RPCSEC_GSS. (This requirement to implement is not a requirement to use.) Other flavors, such as AUTH_NONE and AUTH_SYS, can be implemented as well, although the security implications of doing so need to be carefully considered, particularly when the client host is not itself authenticated. In particular, it is RECOMMENDED by rpc-tls [36] that AUTH_SYS not be used when client host authentication is not in effect.¶

5.3.1. RPCSEC_GSS and Security Services

RPCSEC_GSS [4] uses the functionality of GSS-API [7]. This allows for the use of various security mechanisms by the RPC layer without the additional implementation overhead of adding RPC security flavors.¶

5.3.1.1. GSS Server Principal

Regardless of what security mechanism under RPCSEC_GSS is being used, the NFS server MUST identify itself in GSS-API via a GSS_C_NT_HOSTBASED_SERVICE name type. GSS_C_NT_HOSTBASED_SERVICE names are of the form:¶

     service@hostname

For NFS, the "service" element is¶

nfs

Implementations of security mechanisms will convert nfs@hostname to various different forms. For Kerberos V5, the following form is RECOMMENDED:¶

     nfs/hostname

5.4. COMPOUND and CB_COMPOUND

A significant departure from the versions of the NFS protocol before NFSv4 is the introduction of the COMPOUND procedure. For the NFSv4 protocol, in all minor versions, there are exactly two RPC procedures, NULL and COMPOUND. The COMPOUND procedure is defined as a series of individual operations and these operations perform the sorts of functions performed by traditional NFS procedures.¶

The operations combined within a COMPOUND request are evaluated in order by the server, without any atomicity guarantees. A limited set of facilities exist to pass results from one operation to another. Once an operation returns a failing result, the evaluation ends and the results of all evaluated operations are returned to the client.¶

With the use of the COMPOUND procedure, the client is able to build simple or complex requests. These COMPOUND requests allow for a reduction in the number of RPCs needed for logical file system operations. For example, multi-component look up requests can be constructed by combining multiple LOOKUP operations. Those can be further combined with operations such as GETATTR, READDIR, or OPEN plus READ to do more complicated sets of operation without incurring additional latency.¶

NFSv4.1 also contains a considerable set of callback operations in which the server makes an RPC directed at the client. Callback RPCs have a similar structure to that of the normal server requests. In all minor versions of the NFSv4 protocol, there are two callback RPC procedures: CB_NULL and CB_COMPOUND. The CB_COMPOUND procedure is defined in an analogous fashion to that of COMPOUND with its own set of callback operations.¶

The addition of new server and callback operations within the COMPOUND and CB_COMPOUND request framework provides a means of extending the protocol in subsequent minor versions.¶

Except for a small number of operations needed for session creation, server requests and callback requests are performed within the context of a session. Sessions provide a client context for every request and support robust replay protection for non-idempotent requests.¶

5.5. Client Identifiers and Client Owners

For each operation that obtains or depends on locking state, the specific client needs to be identifiable by the server.¶

Each distinct client instance is represented by a client ID. A client ID is a 64-bit identifier representing a specific client at a given time. The client ID is changed whenever the client re-initializes, and may change when the server re-initializes. Client IDs are used to support lock identification and crash recovery.¶

During steady state operation, the client ID associated with each operation is derived from the session (see Section 7) on which the operation is sent. A session is associated with a client ID when the session is created.¶

Unlike NFSv4.0, the only NFSv4.1 operations possible before a client ID is established are those needed to establish the client ID.¶

A sequence of an EXCHANGE_ID operation followed by a CREATE_SESSION operation using that client ID (eir_clientid as returned from EXCHANGE_ID) is required to establish and confirm the client ID on the server. Establishment of identification by a new incarnation of the client also has the effect of immediately releasing any locking state that a previous incarnation of that same client might have had on the server. Such released state would include all byte-range lock, share reservation, layout state, and -- where the server supports neither the CLAIM_DELEGATE_PREV nor CLAIM_DELEG_CUR_FH claim types -- all delegation state associated with the same client with the same identity. For discussion of delegation state recovery, see Section 14.2.1. For discussion of layout state recovery, see Section 16.7.1.¶

Releasing such state requires that the server be able to determine that one client instance is the successor of another. Where this cannot be done, for any of a number of reasons, the locking state will remain for a time subject to lease expiration (see Section 12.3) and the new client will need to wait for such state to be removed, if it makes conflicting lock requests.¶

Client identification is encapsulated in the following client owner data type:¶

struct client_owner4 {
        verifier4       co_verifier;
        opaque          co_ownerid<NFS4_OPAQUE_LIMIT>;
};

The first field, co_verifier, is a client incarnation verifier, allowing the server to distinguish successive incarnations (e.g., reboots) of the same client. The server will start the process of canceling the client's leased state if co_verifier is different than what the server has previously recorded for the identified client (as specified in the co_ownerid field).¶

The second field, co_ownerid, is a variable length string that uniquely defines the client so that subsequent instances of the same client bear the same co_ownerid with a different verifier.¶

There are several considerations for how the client generates the co_ownerid string:¶

The string should be unique so that multiple clients do not present the same string. The consequences of two clients presenting the same string range from one client getting an error to one client having its leased state abruptly and unexpectedly cancelled.¶
The string should be selected so that subsequent incarnations (e.g., restarts) of the same client cause the client to present the same string. The implementor is cautioned from an approach that requires the string to be recorded in a local file because this precludes the use of the implementation in an environment where there is no local disk and all file access is from an NFSv4.1 server.¶
The string should be the same for each server network address that the client accesses. This way, if a server has multiple interfaces, the client can trunk traffic over multiple network paths as described in Section 7.5. (Note: the precise opposite was advised in the NFSv4.0 specification [39].)¶
The algorithm for generating the string should not assume that the client's network address will not change, unless the client implementation knows it is using statically assigned network addresses. This includes changes between client incarnations and even changes while the client is still running in its current incarnation. Thus, with dynamic address assignment, if the client includes just the client's network address in the co_ownerid string, there is a real risk that after the client gives up the network address, another client, using a similar algorithm for generating the co_ownerid string, would generate a conflicting co_ownerid string.¶

Given the above considerations, an example of a well-generated co_ownerid string is one that includes:¶

If applicable, the client's statically assigned network address.¶
Additional information that tends to be unique, such as one or more of:¶
- The client machine's serial number (for privacy reasons, it is best to perform some one-way function on the serial number).¶
- A Media Access Control (MAC) address (again, a one-way function should be performed).¶
- The timestamp of when the NFSv4.1 software was first installed on the client (though this is subject to the previously mentioned caution about using information that is stored in a file, because the file might only be accessible over NFSv4.1).¶
- A true random number. However, since this number ought to be the same between client incarnations, this shares the same problem as that of using the timestamp of the software installation.¶
For a user-level NFSv4.1 client, it should contain additional information to distinguish the client from other user-level clients running on the same host, such as a process identifier or other unique sequence.¶

The client ID is assigned by the server (the eir_clientid result from EXCHANGE_ID) and should be chosen so that it will not conflict with a client ID previously assigned by the server. This applies across server restarts.¶

In the event of a server restart, a client may find out that its current client ID is no longer valid when it receives an NFS4ERR_STALE_CLIENTID error. The precise circumstances depend on the characteristics of the sessions involved, specifically whether the session is persistent (see Section 7.6.5), but in each case the client will receive this error when it attempts to establish a new session with the existing client ID and receives the error NFS4ERR_STALE_CLIENTID, indicating that a new client ID needs to be obtained via EXCHANGE_ID and the new session established with that client ID.¶

When a session is not persistent, the client will find out that it needs to create a new session as a result of getting an NFS4ERR_BADSESSION, since the session in question was lost as part of a server restart. When the existing client ID is presented to a server as part of creating a session and that client ID is not recognized, as would happen after a server restart, the server will reject the request with the error NFS4ERR_STALE_CLIENTID.¶

In the case of the session being persistent, the client will re-establish communication using the existing session after the restart. This session will be associated with the existing client ID but may only be used to retransmit operations that the client previously transmitted and did not see replies to. Replies to operations that the server previously performed will come from the reply cache; otherwise, NFS4ERR_DEADSESSION will be returned. Hence, such a session is referred to as "dead". In this situation, in order to perform new operations, the client needs to establish a new session. If an attempt is made to establish this new session with the existing client ID, the server will reject the request with NFS4ERR_STALE_CLIENTID.¶

When NFS4ERR_STALE_CLIENTID is received in either of these situations, the client needs to obtain a new client ID by use of the EXCHANGE_ID operation, then use that client ID as the basis of a new session, and then proceed to any other necessary recovery for the server restart case (see Section 12.4.2).¶

See the descriptions of EXCHANGE_ID (Section 22.35) and CREATE_SESSION (Section 22.36) for a complete specification of these operations.¶

5.5.1. Upgrade from NFSv4.0 to NFSv4.1

To facilitate upgrade from NFSv4.0 to NFSv4.1, a server may compare a value of data type client_owner4 in an EXCHANGE_ID with a value of data type nfs_client_id4 that was established using the SETCLIENTID operation of NFSv4.0. A server that does so will allow an upgraded client to avoid waiting until the lease (i.e., the lease established by the NFSv4.0 instance client) expires. This requires that the value of data type client_owner4 be constructed the same way as the value of data type nfs_client_id4. If the latter's contents included the server's network address (per the recommendations of the NFSv4.0 specification [39]), and the NFSv4.1 client does not wish to use a client ID that prevents trunking, it should send two EXCHANGE_ID operations. The first EXCHANGE_ID will have a client_owner4 equal to the nfs_client_id4. This will clear the state created by the NFSv4.0 client. The second EXCHANGE_ID will not have the server's network address. The state created for the second EXCHANGE_ID will not have to wait for lease expiration, because there will be no state to expire.¶

5.5.2. Server Release of Client ID

NFSv4.1 introduces a new operation called DESTROY_CLIENTID (Section 22.50), which the client uses to destroy a client ID it no longer needs. This permits graceful, bilateral release of a client ID. The operation cannot be used if there are sessions associated with the client ID, or state with an unexpired lease.¶

If the server determines that the client holds no associated state for its client ID (associated state includes unrevoked sessions, opens, locks, delegations, layouts, and wants), the server MAY choose to unilaterally release the client ID in order to conserve resources. If the client contacts the server after this release, the server MUST ensure that the client receives the appropriate error so that it will use the EXCHANGE_ID/CREATE_SESSION sequence to establish a new client ID. The server ought to be very hesitant to release a client ID since the resulting work on the client to recover from such an event will be the same burden as if the server had failed and restarted. Typically, a server would not release a client ID unless there had been no activity from that client for many minutes. As long as there are sessions, opens, locks, delegations, layouts, or wants, the server MUST NOT release the client ID. See Section 7.13.1.4 for discussion on releasing inactive sessions.¶

5.5.3. Resolving Client Owner Conflicts

When the server gets an EXCHANGE_ID for a client owner that currently has no state, or that has state but the lease has expired, the server MUST allow the EXCHANGE_ID and confirm the new client ID if followed by the appropriate CREATE_SESSION.¶

When the server gets an EXCHANGE_ID for a new incarnation of a client owner that currently has an old incarnation with state and an unexpired lease, the server is allowed to dispose of the state of the previous incarnation of the client owner if one of the following is true:¶

The client ID was created without explicit state protection (i.e. SP4_NONE was used) and without client host authentication while the current EXCHANGE_ID shares those characteristics and the principals used for client Id creation and the current EXCHANGE_ID match as well.¶
The client ID was created without explicit state protection (i.e. SP4_NONE was used) and with client host authentication while the current EXCHANGE_ID shares those characteristics with the EXCHANGE_ID used to create the client ID while the authenticated client hosts match as well.¶
The principal that created the client ID for the client owner is the same as the principal that is sending the EXCHANGE_ID operation. Note that if the client ID was created with SP4_MACH_CRED state protection (Section 22.35), the principal MUST be based on RPCSEC_GSS authentication, the RPCSEC_GSS service used MUST be integrity or privacy, and the same GSS mechanism and principal MUST be used as that used when the client ID was created.¶
The client ID was established with SP4_SSV protection (Section 22.35, Section 7.8.3) and the client sends the EXCHANGE_ID with the security flavor set to RPCSEC_GSS using the GSS SSV mechanism (Section 7.9).¶
The client ID was established with SP4_SSV protection, and under the conditions described herein, the EXCHANGE_ID was sent with SP4_MACH_CRED state protection. Because the SSV might not persist across client and server restart, and because the first time a client sends EXCHANGE_ID to a server it does not have an SSV, the client MAY send the subsequent EXCHANGE_ID without an SSV RPCSEC_GSS handle. Instead, as with SP4_MACH_CRED protection, the principal MUST be based on RPCSEC_GSS authentication, the RPCSEC_GSS service used MUST be integrity or privacy, and the same GSS mechanism and principal MUST be used as that used when the client ID was created.¶

If none of the above situations apply, the server MUST return NFS4ERR_CLID_INUSE.¶

If the server accepts the principal and co_ownerid as matching that which created the client ID, and the co_verifier in the EXCHANGE_ID differs from the co_verifier used when the client ID was created, then after the server receives a CREATE_SESSION that confirms the client ID, the server deletes state. If the co_verifier values are the same (e.g., the client either is updating properties of the client ID (Section 22.35) or is attempting trunking (Section 7.5), the server MUST NOT delete state.¶

5.6. Server Owners

The server owner is similar to a client owner (Section 5.5), but unlike the client owner, there is no shorthand server ID. The server owner is defined in the following data type:¶

struct server_owner4 {
 uint64_t       so_minor_id;
 opaque         so_major_id<NFS4_OPAQUE_LIMIT>;
};

The server owner is returned from EXCHANGE_ID. When the so_major_id fields are the same in two EXCHANGE_ID results, the connections that each EXCHANGE_ID were sent over can be assumed to address the same server (as defined in Section 2.5). If the so_minor_id fields are also the same, then not only do both connections connect to the same server, but the session can be shared across both connections. The reader is cautioned that multiple servers may deliberately or accidentally claim to have the same so_major_id or so_major_id/so_minor_id; the reader should examine Sections 7.5 and 22.35 in order to avoid acting on falsely matching server owner values.¶

The considerations for generating an so_major_id are similar to that for generating a co_ownerid string (see Section 5.5). The consequences of two servers generating conflicting so_major_id values are less dire than they are for co_ownerid conflicts because the client can use RPCSEC_GSS to compare the authenticity of each server (see Section 7.5).¶

5.7. Transport Layers

5.7.1. REQUIRED and RECOMMENDED Properties of Transports

NFSv4.1 works over Remote Direct Memory Access (RDMA) and non-RDMA-based transports with the following attributes:¶

The transport supports reliable delivery of data, which NFSv4.1 requires. However the possibility of connections breaking is addressed in NFSv4.1 by a session-based replay cache to prevent the spurious re-execution of non-idempotent requests or modifying idempotent requests.¶
The transport delivers data in the order it was sent. Ordered delivery simplifies detection of transmit errors, and simplifies the sending of arbitrary sized requests and responses via the record marking protocol [3].¶

Because efficient handling is required when sending large amounts of data, congestion control facilities are a significant concern.¶

When NFSv4.1 is used over an IP-based network protocol, it is REQUIRED that the transport provide congestion control.¶
When NFSv4.1 is used over a non-IP network protocol, it is RECOMMENDED that the transport provide congestion control.¶

To enhance the possibilities for interoperability, it is strongly recommended that NFSv4.1 client and server implementations support operation over the TCP transport protocol.¶

It is permissible for a connectionless transport to be used under NFSv4.1; however, reliable and in-order delivery of data combined with congestion control by the connectionless transport is REQUIRED. As a consequence, UDP by itself MUST NOT be used as an NFSv4.1 transport, although transports to be used for NFSv4.1 may be layered on UDP. NFSv4.1 assumes that a client transport address and server transport address used to send data over a transport together constitute a connection, even if the underlying transport eschews the concept of a connection.¶

5.7.2. Client and Server Transport Behavior

When a connection-oriented transport (e.g., TCP) is used, the client and server are normally expected to maintain the use of connections already established for a considerable length of time. This is for a number reasons:¶

This will prevent the weakening of the transport's congestion control mechanisms by the confusion resulting from dispersing a single burst of network traffic into multiple connections.¶
This will improve performance for the WAN environment by eliminating the need for connection setup handshakes. This is particularly so given that each new connection requires the re-establishment of a connection id, and setting up new connections.¶
The NFSv4.1 callback model requires the client and server to maintain a client-created backchannel (see Section 7.3.1) for the server to use so that any dropping of the connection will interfere wit the use of the backchannel.¶

Although it is not fatal for a requester to retry without a disconnect between the request and retry, there are good reasons to avoid this practice. The retry does consume resources, especially with RDMA, where each request, retry or not, consumes a credit. Retries for no reason, especially retries sent shortly after the previous attempt, are a poor use of network bandwidth and defeat the purpose of a transport's inherent congestion control system.¶

There is no situation in which a replier is allowed to silently drop a request, whether the request is a retry or not. (The silent drop behavior of RPCSEC_GSS [4] is not within scope here since this behavior happens at the RPCSEC_GSS layer, which is at a lower layer in the request processing.) While the replier MAY disconnect the connection, if it does not do so, it is obligated to execute the request or return an appropriate error based on the contents of the reply cache (see Section 7.6.1).¶

When sending a reply, the replier MUST send the reply to the same full network address (e.g., if using an IP-based transport, the source port of the requester is part of the full network address) from which the requester sent the request. If using a connection-oriented transport, replies MUST be sent on the connection from which the request was received.¶

If a connection is dropped after the replier receives the request but before the replier sends the reply, the replier might have a pending reply. If a connection is established with the same source and destination full network address as the dropped connection, then the replier MUST NOT send the reply until the requester retries the request. The reason for this prohibition is that the requester MAY retry a request over a different connection (provided that connection is associated with the original request's session).¶

When using RDMA transports, there are other reasons for avoiding retries over the same connection:¶

RDMA transports use "credits" to enforce flow control, where a credit is a right to a peer to transmit a message. If one peer were to retransmit a request (or reply), it would consume an additional credit. If the replier retransmitted a reply, it would certainly result in an RDMA connection loss, since the requester would typically only post a single receive buffer for each request. If the requester retransmitted a request, the additional credit consumed on the server might lead to RDMA connection failure unless the client accounted for it and decreased its available credit, leading to wasted resources.¶
RDMA credits present a new issue to the reply cache in NFSv4.1. The reply cache may be used when a connection within a session is lost, such as after the client reconnects. Credit information is a dynamic property of the RDMA connection, and stale values must not be replayed from the cache. This implies that the reply cache contents must not be blindly used when replies are sent from it, and credit information appropriate to the channel must be refreshed by the RPC layer.¶

In addition, as described in Section 7.6.2, while a session is active, an NFSv4.1 requester that ceases to wait for an outstanding reply MUST take appropriate care to avoid that situation vitiating guarantees needed to maintain the exactly-once semantics needed for the successful operation of the session-based reply cache.¶

5.7.3. Ports

Historically, NFSv3 servers have listened over TCP port 2049. The registered port 2049 [44] for the NFS protocol should be the default configuration for NFSv4.1, although the port 20049 is used for NFSv4.1 layered on RPC-over-RDMA.¶

The use of a reserved port has been common for NFS implementations and it is expected that this will apply to NFSv4.1 as well. While the use of RPC binding protocols as described in RFC1833 [45] is a possibility, there is no requirement that servers provide support for such use.¶

In light of this, a client should avoid this sort of use unless it has good reason to expect such support to be present on the server, while accessing NFS services at the appropriate well-known port depending on the transport to be used.¶

7. Session

NFSv4.1 clients and servers MUST support and MUST use the session feature as described in this section.¶

7.1. Motivation and Overview

Previous versions and minor versions of NFS have suffered from the following:¶

Lack of support for Exactly Once Semantics (EOS). This includes lack of support for EOS through server failure and recovery.¶
Limited callback support, including no support for sending callbacks through firewalls, and races between replies to normal requests and callbacks.¶
Limited trunking over multiple network paths.¶
Requiring machine credentials for fully secure operation.¶

Through the introduction of a session, NFSv4.1 addresses the above shortfalls with practical solutions:¶

EOS is enabled by a reply cache with a bounded size, making it feasible to keep the cache in persistent storage and enable EOS through server failure and recovery. One reason that previous revisions of NFS did not support EOS was because some EOS approaches often limited parallelism. As will be explained in Section 7.6, NFSv4.1 supports both EOS and unlimited parallelism.¶
The NFSv4.1 client (defined in Section 2.5) creates transport connections and provides them to the server to use for sending callback requests, thus solving the firewall issue (Section 22.34). Races between responses from client requests and callbacks caused by the requests are detected via the session's sequencing properties that are a consequence of EOS (Section 7.6.3).¶
The NFSv4.1 client can associate an arbitrary number of connections with the session, and thus provide trunking (Section 7.5).¶
The NFSv4.1 client and server produce a session key independent of client and server machine credentials which can be used to compute a digest for protecting critical session management operations (Section 7.8.3).¶
The NFSv4.1 client can also create secure RPCSEC_GSS contexts for use by the session's backchannel that do not require the server to authenticate to a client machine principal (Section 7.8.2).¶

A session is a dynamically created, long-lived server object created by a client and used over time from one or more transport connections. Its function is to maintain the server's state relative to the connection(s) belonging to a client instance. This state is entirely independent of the connection itself, and indeed the state exists whether or not the connection exists. A client may have one or more sessions associated with it so that client-associated state may be accessed using any of the sessions associated with that client's client ID, when connections are associated with those sessions. When no connections are associated with any of a client ID's sessions for an extended time, such objects as locks, opens, delegations, layouts, etc. are subject to expiration. The session serves as an object representing a means of access by a client to the associated client state on the server, independent of the physical means of access to that state.¶

A single client may create multiple sessions. A single session MUST NOT serve multiple clients.¶

7.2. NFSv4 Integration

Sessions are part of NFSv4.1 and not NFSv4.0. Normally, a major infrastructure change such as sessions would require a new major version number to an Open Network Computing (ONC) RPC program like NFS. However, because NFSv4 encapsulates its functionality in a single procedure, COMPOUND, and because COMPOUND can support an arbitrary number of operations, sessions have been added to NFSv4.1 with little difficulty. COMPOUND includes a minor version number field, and for NFSv4.1 this minor version is set to 1. When the NFSv4 server processes a COMPOUND with the minor version set to 1, it expects a different set of operations than it does for NFSv4.0. NFSv4.1 defines the SEQUENCE operation, which is required for every COMPOUND that operates over an established session, with the exception of some session administration operations, such as DESTROY_SESSION (Section 22.37).¶

7.2.1. SEQUENCE and CB_SEQUENCE

In NFSv4.1, when the SEQUENCE operation is present, it MUST be the first operation in the COMPOUND procedure. The primary purpose of SEQUENCE is to carry the session identifier. The session identifier associates all other operations in the COMPOUND procedure with a particular session. SEQUENCE also contains required information for maintaining EOS (see Section 7.6). Session-enabled NFSv4.1 COMPOUND requests thus have the form:¶

    +-----+--------------+-----------+------------+-----------+----
    | tag | minorversion | numops    |SEQUENCE op | op + args | ...
    |     |   (== 1)     | (limited) |  + args    |           |
    +-----+--------------+-----------+------------+-----------+----

and the replies have the form:¶

    +------------+-----+--------+-------------------------------+--//
    |last status | tag | numres |status + SEQUENCE op + results |  //
    +------------+-----+--------+-------------------------------+--//
            //-----------------------+----
            // status + op + results | ...
            //-----------------------+----

A CB_COMPOUND procedure request and reply has a similar form to COMPOUND, but instead of a SEQUENCE operation, there is a CB_SEQUENCE operation. CB_COMPOUND also has an additional field called "callback_ident", which is superfluous in NFSv4.1 and MUST be ignored by the client. CB_SEQUENCE has the same information as SEQUENCE, and also includes other information needed to resolve callback races (Section 7.6.3).¶

7.2.2. Client ID and Session Association

Each client ID (Section 5.5) can have zero or more active sessions. A client ID and associated session are required to perform file access in NFSv4.1. Each time a session is used (whether by a client sending a request to the server or the client replying to a callback request from the server), the state leased to its associated client ID is automatically renewed.¶

State (which can consist of share reservations, locks, delegations, and layouts (Section 3)) is tied to the client ID. Client state is not tied to any individual session. Successive state changing operations from a given state owner MAY go over different sessions, provided the session is associated with the same client ID. A callback MAY arrive over a different session than that of the request that originally acquired the state pertaining to the callback. For example, if session A is used to acquire a delegation, a request to recall the delegation MAY arrive over session B if both sessions are associated with the same client ID. Sections 7.8.1 and 7.8.2 discuss the security considerations around callbacks.¶

7.3. Channels

A channel is not a connection. A channel represents the direction ONC RPC requests are sent.¶

Each session has one or two channels: the fore channel and the backchannel. Because there are at most two channels per session, and because each channel has a distinct purpose, channels are not assigned identifiers.¶

The fore channel is used for ordinary requests from the client to the server, and carries COMPOUND requests and responses. A session always has a fore channel.¶

The backchannel is used for callback requests from server to client, and carries CB_COMPOUND requests and responses. Whether or not there is a backchannel is decided by the client; however, many features of NFSv4.1 require a backchannel. NFSv4.1 servers MUST support backchannels.¶

Each session has resources for each channel, including separate reply caches (see Section 7.6.1). Note that even the backchannel requires a reply cache (or, at least, a slot table in order to detect retries) because some callback operations are non-idempotent.¶

7.3.1. Association of Connections, Channels, and Sessions

Each channel is associated with zero or more transport connections (whether of the same transport protocol or different transport protocols). A connection can be associated with one channel or both channels of a session; the client and server negotiate whether a connection will carry traffic for one channel or both channels via the CREATE_SESSION (Section 22.36) and the BIND_CONN_TO_SESSION (Section 22.34) operations. When a session is created via CREATE_SESSION, the connection that transported the CREATE_SESSION request is automatically associated with the fore channel, and optionally the backchannel. If the client specifies no state protection (Section 22.35) when the session is created, then when SEQUENCE is transmitted on a different connection, the connection is automatically associated with the fore channel of the session specified in the SEQUENCE operation.¶

A connection's association with a session is not exclusive. A connection associated with the channel(s) of one session may be simultaneously associated with the channel(s) of other sessions including sessions associated with other client IDs.¶

It is permissible for connections of multiple transport types to be associated with the same channel. For example, both TCP and RDMA connections can be associated with the fore channel. In the event an RDMA and non-RDMA connection are associated with the same channel, it is desirable for the maximum number slots to be at least one more than the total number of RDMA credits (Section 7.6.1). This way, if all RDMA credits are used, the non-RDMA connection can have at least one outstanding request. If a server supports multiple transport types, it MUST allow a client to associate connections from each transport to a channel.¶

It is permissible for a connection of one type of transport to be associated with the fore channel, and a connection of a different type to be associated with the backchannel.¶

7.4. Server Scope

Servers each specify a server scope value in the form of an opaque string eir_server_scope returned as part of the results of an EXCHANGE_ID operation. The purpose of the server scope is to allow a group of servers to indicate to clients that a set of servers sharing the same server scope value has arranged to use distinct values of opaque identifiers so that the two servers never assign the same value to two distinct objects. Thus, the identifiers generated by two servers within that set can be assumed compatible so that, in certain important cases, identifiers generated by one server in that set may be presented to another server of the same scope.¶

The use of such compatible values does not imply that a value generated by one server will always be accepted by another. In most cases, it will not. However, a server will not inadvertently accept a value generated by another server. When it does accept it, it will be because it is recognized as valid and carrying the same meaning as on another server of the same scope.¶

When servers are of the same server scope, this compatibility of values applies to the following identifiers:¶

Filehandle values. A filehandle value accepted by two servers of the same server scope denotes the same object. A WRITE operation sent to one server is reflected immediately in a READ sent to the other.¶
Server owner values. When the server scope values are the same, server owner value may be validly compared. In cases where the server scope values are different, server owner values are treated as different even if they contain identical strings of bytes.¶

The coordination among servers required to provide such compatibility can be quite minimal, and limited to a simple partition of the ID space. The recognition of common values requires additional implementation, but this can be tailored to the specific situations in which that recognition is desired.¶

Clients will have occasion to compare the server scope values of multiple servers under a number of circumstances, each of which will be discussed under the appropriate functional section:¶

When server owner values received in response to EXCHANGE_ID operations sent to multiple network addresses are compared for the purpose of determining the validity of various forms of trunking, as described in Section 15.5.2.¶
When network or server reconfiguration causes the same network address to possibly be directed to different servers, with the necessity for the client to determine when lock reclaim should be attempted, as described in Section 12.4.2.1.¶

When two replies from EXCHANGE_ID, each from two different server network addresses, have the same server scope, there are a number of ways a client can validate that the common server scope is due to two servers cooperating in a group.¶

If both EXCHANGE_ID requests were sent with RPCSEC_GSS ([4], [9], [22]) authentication and the server principal is the same for both targets, the equality of server scope is validated. It is RECOMMENDED that two servers intending to share the same server scope and server_owner major_id also share the same principal name. In some cases, this simplifies the client's task of validating server scope.¶
The client may accept the appearance of the second server in the fs_locations or fs_locations_info attribute for a relevant file system. For example, if there is a migration event for a particular file system or there are locks to be reclaimed on a particular file system, the attributes for that particular file system may be used. The client sends the GETATTR request to the first server for the fs_locations or fs_locations_info attribute with RPCSEC_GSS authentication. It may need to do this in advance of the need to verify the common server scope. If the client successfully authenticates the reply to GETATTR, and the GETATTR request and reply containing the fs_locations or fs_locations_info attribute refers to the second server, then the equality of server scope is supported. A client may choose to limit the use of this form of support to information relevant to the specific file system involved (e.g. a file system being migrated).¶

7.5. Trunking

Trunking is the use of multiple connections between a client and server in order to increase the speed of data transfer. NFSv4.1 supports two types of trunking: session trunking and client ID trunking.¶

In the context of a single server network address, it can be assumed that all connections are accessing the same server, and NFSv4.1 servers MUST support both forms of trunking. When multiple connections use a set of network addresses to access the same server, the server MUST support both forms of trunking. NFSv4.1 servers in a clustered configuration MAY allow network addresses for different servers to use client ID trunking.¶

Clients may use either form of trunking as long as they do not, when trunking between different server network addresses, violate the servers' mandates as to the kinds of trunking to be allowed (see below). With regard to callback channels, the client MUST allow the server to choose among all callback channels valid for a given client ID and MUST support trunking when the connections supporting the backchannel allow session or client ID trunking to be used for callbacks.¶

Session trunking is essentially the association of multiple connections, each with potentially different target and/or source network addresses, to the same session. When the target network addresses (server addresses) of the two connections are the same, the server MUST support such session trunking. When the target network addresses are different, the server MAY indicate such support using the data returned by the EXCHANGE_ID operation (see below).¶

Client ID trunking is the association of multiple sessions to the same client ID. Servers MUST support client ID trunking for two target network addresses whenever they allow session trunking for those same two network addresses. In addition, a server MAY, by presenting the same major server owner ID (Section 5.6) and server scope (Section 7.4), allow an additional case of client ID trunking. When two servers return the same major server owner and server scope, it means that the two servers are cooperating on locking state management, which is a prerequisite for client ID trunking.¶

Distinguishing when the client is allowed to use session and client ID trunking requires understanding how the results of the EXCHANGE_ID (Section 22.35) operation identify a server. Suppose a client sends EXCHANGE_IDs over two different connections, each with a possibly different target network address, but each EXCHANGE_ID operation has the same value in the eia_clientowner field. If the same NFSv4.1 server is listening over each connection, then each EXCHANGE_ID result MUST return the same values of eir_clientid, eir_server_owner.so_major_id, and eir_server_scope. The client can then treat each connection as referring to the same server (subject to verification; see Section 7.5.1 below), and it can use each connection to trunk requests and replies. The client's choice is whether session trunking or client ID trunking applies.¶

Session Trunking.

If the eia_clientowner argument is the same in two different EXCHANGE_ID requests, and the eir_clientid, eir_server_owner.so_major_id, eir_server_owner.so_minor_id, and eir_server_scope results match in both EXCHANGE_ID results, then the client is permitted to perform session trunking. If the client has no session mapping to the tuple of eir_clientid, eir_server_owner.so_major_id, eir_server_scope, and eir_server_owner.so_minor_id, then it creates the session via a CREATE_SESSION operation over one of the connections, which associates the connection to the session. If there is a session for the tuple, the client can send BIND_CONN_TO_SESSION to associate the connection to the session.¶

Of course, if the client does not desire to use session trunking, it is not required to do so. It can invoke CREATE_SESSION on the connection. This will result in client ID trunking as described below. It can also decide to drop the connection if it does not choose to use trunking.¶

Client ID Trunking.

If the eia_clientowner argument is the same in two different EXCHANGE_ID requests, and the eir_clientid, eir_server_owner.so_major_id, and eir_server_scope results match in both EXCHANGE_ID results, then the client is permitted to perform client ID trunking (regardless of whether the eir_server_owner.so_minor_id results match). The client can associate each connection with different sessions, where each session is associated with the same server.¶

The client completes the act of client ID trunking by invoking CREATE_SESSION on each connection, using the same client ID that was returned in eir_clientid. These invocations create two sessions and also associate each connection with its respective session. The client is free to decline to use client ID trunking by simply dropping the connection at this point.¶

When doing client ID trunking, locking state is shared across sessions associated with that same client ID. This requires the server to coordinate state across sessions and the client to be able to associate the same locking state with multiple sessions.¶

It is always possible that, as a result of various sorts of reconfiguration events, eir_server_scope and eir_server_owner values may be different on subsequent EXCHANGE_ID requests made to the same network address.¶

In most cases, such reconfiguration events will be disruptive and indicate that an IP address formerly connected to one server is now connected to an entirely different one.¶

Some guidelines on client handling of such situations follow:¶

When eir_server_scope changes, the client has no assurance that any IDs that it obtained previously (e.g., filehandles) can be validly used on the new server, and, even if the new server accepts them, there is no assurance that this is not due to accident. Thus, it is best to treat all such state as lost or stale, although a client may assume that the probability of inadvertent acceptance is low and treat this situation as within the next case.¶
When eir_server_scope remains the same and eir_server_owner.so_major_id changes, the client can use the filehandles it has, consider its locking state lost, and attempt to reclaim or otherwise re-obtain its locks. It might find that its filehandle is now stale. However, if NFS4ERR_STALE is not returned, it can proceed to reclaim or otherwise re-obtain its open locking state.¶
When eir_server_scope and eir_server_owner.so_major_id remain the same, the client has to use the now-current values of eir_server_owner.so_minor_id in deciding on appropriate forms of trunking. This may result in connections being dropped or new sessions being created.¶

7.5.1. Verifying Claims of Matching Server Identity

When the server responds using two different connections that claim matching or partially matching eir_server_owner, eir_server_scope, and eir_clientid values, the client does not have to trust the servers' claims. The client may verify these claims before trunking traffic in the following ways:¶

For session trunking, clients SHOULD reliably verify if connections between different network paths are in fact associated with the same NFSv4.1 server and usable on the same session, and servers MUST allow clients to perform reliable verification. When a client ID is created, the client SHOULD, unless client host authentication is in effect, specify that BIND_CONN_TO_SESSION is to be verified according to the SP4_SSV or SP4_MACH_CRED (Section 22.35) state protection options. For SP4_SSV, reliable verification depends on a shared secret (the SSV) that is established via the SET_SSV (see Section 22.47) operation.¶

When a new connection is associated with the session (via the BIND_CONN_TO_SESSION operation, see Section 22.34), if the client specified SP4_SSV state protection for the BIND_CONN_TO_SESSION operation, the client MUST send the BIND_CONN_TO_SESSION with RPCSEC_GSS protection, using integrity or privacy, and an RPCSEC_GSS handle created with the GSS SSV mechanism (see Section 7.9).¶

If the client mistakenly tries to associate a connection to a session of a wrong server, the server will either reject the attempt because it is not aware of the session identifier of the BIND_CONN_TO_SESSION arguments, or it will reject the attempt because the RPCSEC_GSS authentication fails. Even if the server mistakenly or maliciously accepts the connection association attempt, the RPCSEC_GSS verifier it computes in the response will not be verified by the client, so the client will know it cannot use the connection for trunking the specified session.¶

If the client specified SP4_MACH_CRED state protection, the BIND_CONN_TO_SESSION operation will use RPCSEC_GSS integrity or privacy, using the same credential that was used when the client ID was created. Mutual authentication via RPCSEC_GSS assures the client that the connection is associated with the correct session of the correct server.¶
For client ID trunking, the client has at least two options for verifying that the same client ID obtained from two different EXCHANGE_ID operations came from the same server. The first option is to use RPCSEC_GSS authentication when sending each EXCHANGE_ID operation. Each time an EXCHANGE_ID is sent with RPCSEC_GSS authentication, the client notes the principal name of the GSS target. If the EXCHANGE_ID results indicate that client ID trunking is possible, and the GSS targets' principal names are the same, the servers are the same and client ID trunking is allowed.¶

The second option for verification is to use SP4_SSV protection. When the client sends EXCHANGE_ID, it specifies SP4_SSV protection. The first EXCHANGE_ID the client sends always has to be confirmed by a CREATE_SESSION call. The client then sends SET_SSV. Later, the client sends EXCHANGE_ID to a second destination network address different from the one the first EXCHANGE_ID was sent to. The client checks that each EXCHANGE_ID reply has the same eir_clientid, eir_server_owner.so_major_id, and eir_server_scope. If so, the client verifies the claim by sending a CREATE_SESSION operation to the second destination address, protected with RPCSEC_GSS integrity using an RPCSEC_GSS handle returned by the second EXCHANGE_ID. If the server accepts the CREATE_SESSION request, and if the client verifies the RPCSEC_GSS verifier and integrity codes, then the client has proof the second server knows the SSV, and thus the two servers are cooperating for the purposes of specifying server scope and client ID trunking.¶

7.6. Exactly Once Semantics

Via the session, NFSv4.1 offers exactly once semantics (EOS) for requests sent over a channel. EOS is supported on both the fore channel and backchannel.¶

Each COMPOUND or CB_COMPOUND request that is sent with a leading SEQUENCE or CB_SEQUENCE operation MUST be executed by the receiver exactly once. This requirement holds regardless of whether the request is sent with reply caching specified (see Section 7.6.1.3). The requirement holds even if the requester is sending the request over a session created between a pNFS data client and pNFS data server. To understand the rationale for this requirement, divide the requests into three classifications:¶

Non-idempotent requests.¶
Idempotent modifying requests.¶
Idempotent non-modifying requests.¶

An example of a non-idempotent request is RENAME. Obviously, if a replier executes the same RENAME request twice, and the first execution succeeds, the re-execution will fail. If the replier returns the result from the re-execution, this result is incorrect. Therefore, EOS is required for non-idempotent requests.¶

An example of an idempotent modifying request is a COMPOUND request containing a WRITE operation. Repeated execution of the same WRITE has the same effect as execution of that WRITE a single time. Nevertheless, enforcing EOS for WRITEs and other idempotent modifying requests is necessary to avoid data corruption.¶

Suppose a client sends WRITE A to a noncompliant server that does not enforce EOS, and receives no response, perhaps due to a network partition. The client reconnects to the server and re-sends WRITE A. Now, the server has outstanding two instances of A. The server can be in a situation in which it executes and replies to the retry of A, while the first A is still waiting in the server's internal I/O system for some resource. Upon receiving the reply to the second attempt of WRITE A, the client believes its WRITE is done so it is free to send WRITE B, which overlaps the byte-range of A. When the original A is dispatched from the server's I/O system and executed (thus the second time A will have been written), then what has been written by B can be overwritten and thus corrupted.¶

An example of an idempotent non-modifying request is a COMPOUND containing SEQUENCE, PUTFH, READLINK, and nothing else. The re-execution of such a request will not cause data corruption or produce an incorrect result. Nonetheless, to keep the implementation simple, the replier MUST enforce EOS for all requests, whether or not idempotent and non-modifying.¶

Note that true and complete EOS is not possible unless the server persists the reply cache in stable storage, and unless the server is somehow implemented to never require a restart (indeed, if such a server exists, the distinction between a reply cache kept in stable storage versus one that is not is one without meaning). See Section 7.6.5 for a discussion of persistence in the reply cache. Regardless, even if the server does not persist the reply cache, EOS improves robustness and correctness over previous versions of NFS because the legacy duplicate request/reply caches were based on the ONC RPC transaction identifier (XID). Section 7.6.1 explains the shortcomings of the XID as a basis for a reply cache and describes how NFSv4.1 sessions improve upon the XID.¶

7.6.1. Slot Identifiers and Reply Cache

The RPC layer provides a transaction ID (XID), which, while required to be unique, is not convenient for tracking requests for two reasons. First, the XID is only meaningful to the requester; it cannot be interpreted by the replier except to test for equality with previously sent requests. When consulting an RPC-based duplicate request cache, the opaqueness of the XID requires a computationally expensive look up (often via a hash that includes XID and source address). NFSv4.1 requests use a non-opaque slot ID, which is an index into a slot table, which is far more efficient. Second, because RPC requests can be executed by the replier in any order, there is no bound on the number of requests that may be outstanding at any time. To achieve perfect EOS, using ONC RPC would require storing all replies in the reply cache. XIDs are 32 bits; storing over four billion (2³²) replies in the reply cache is not practical. In practice, previous versions of NFS have chosen to store a fixed number of replies in the cache, and to use a least recently used (LRU) approach to replacing cache entries with new entries when the cache is full. In NFSv4.1, the number of outstanding requests is bounded by the size of the slot table, and a sequence ID per slot is used to tell the replier when it is safe to delete a cached reply.¶

In the NFSv4.1 reply cache, when the requester sends a new request, it selects a slot ID in the range 0..N, where N is the replier's current maximum slot ID granted to the requester on the session over which the request is to be sent. The value of N starts out as equal to ca_maxrequests - 1 (Section 22.36), but can be adjusted by the response to SEQUENCE or CB_SEQUENCE as described later in this section. The slot ID must be unused by any of the requests that the requester has already active on the session. "Unused" here means the requester has no outstanding request for that slot ID.¶

A slot contains a sequence ID and the cached reply corresponding to the request sent with that sequence ID. The sequence ID is a 32-bit unsigned value, and is therefore in the range 0..0xFFFFFFFF (2³² - 1). The first time a slot is used, the requester MUST specify a sequence ID of one (Section 22.36). Each time a slot is reused, the request MUST specify a sequence ID that is one greater than that of the previous request on the slot. If the previous sequence ID was 0xFFFFFFFF, then the next request for the slot MUST have the sequence ID set to zero (i.e., (2³² - 1) + 1 mod 2³²).¶

The sequence ID accompanies the slot ID in each request. It is for the critical check at the replier: it used to efficiently determine whether a request using a certain slot ID is a retransmit or a new, never-before-seen request. It is not feasible for the requester to assert that it is retransmitting to implement this, because for any given request the requester cannot know whether the replier has seen it unless the replier actually replies. Of course, if the requester has seen the reply, the requester would not retransmit.¶

The replier compares each received request's sequence ID with the last one previously received for that slot ID, to see if the new request is:¶

A new request, in which the sequence ID is one greater than that previously seen in the slot (accounting for sequence wraparound). The replier proceeds to execute the new request, and the replier MUST increase the slot's sequence ID by one.¶
A retransmitted request, in which the sequence ID is equal to that currently recorded in the slot. If the original request has executed to completion, the replier returns the cached reply. See Section 7.6.2 for direction on how the replier deals with retries of requests that are still in progress.¶
A misordered retry, in which the sequence ID is less than (accounting for sequence wraparound) that previously seen in the slot. The replier MUST return NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or CB_SEQUENCE).¶
A misordered new request, in which the sequence ID is two or more than (accounting for sequence wraparound) that previously seen in the slot. Note that because the sequence ID MUST wrap around to zero once it reaches 0xFFFFFFFF, a misordered new request and a misordered retry cannot be distinguished. Thus, the replier MUST return NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or CB_SEQUENCE).¶

Unlike the XID, the slot ID is always within a specific range; this has two implications. The first implication is that for a given session, the replier need only cache the results of a limited number of COMPOUND requests. The second implication derives from the first, which is that unlike XID-indexed reply caches (also known as duplicate request caches - DRCs), the slot ID-based reply cache cannot be overflowed. Through use of the sequence ID to identify retransmitted requests, the replier does not need to actually cache the request itself, reducing the storage requirements of the reply cache further. These facilities make it practical to maintain all the required entries for an effective reply cache.¶

The slot ID, sequence ID, and session ID therefore take over the traditional role of the XID and source network address in the replier's reply cache implementation. This approach is considerably more portable and completely robust -- it is not subject to the reassignment of ports as clients reconnect over IP networks. In addition, the RPC XID is not used in the reply cache, enhancing robustness of the cache in the face of any rapid reuse of XIDs by the requester. While the replier does not care about the XID for the purposes of reply cache management (but the replier MUST return the same XID that was in the request), nonetheless there are considerations for the XID in NFSv4.1 that are the same as all other previous versions of NFS. The RPC XID remains in each message and needs to be formulated in NFSv4.1 requests as in any other ONC RPC request. The reasons include:¶

The RPC layer retains its existing semantics and implementation.¶
The requester and replier must be able to interoperate at the RPC layer, prior to the NFSv4.1 decoding of the SEQUENCE or CB_SEQUENCE operation.¶
If an operation is being used that does not start with SEQUENCE or CB_SEQUENCE (e.g., BIND_CONN_TO_SESSION), then the RPC XID is needed for correct operation to match the reply to the request.¶
The SEQUENCE or CB_SEQUENCE operation may generate an error. If so, the embedded slot ID, sequence ID, and session ID (if present) in the request will not be in the reply, and the requester has only the XID to match the reply to the request.¶

Given that well-formulated XIDs continue to be required, this raises the question: why do SEQUENCE and CB_SEQUENCE replies have a session ID, slot ID, and sequence ID? Having the session ID in the reply means that the requester does not have to use the XID to look up the session ID, which would be necessary if the connection were associated with multiple sessions. Having the slot ID and sequence ID in the reply means that the requester does not have to use the XID to look up the slot ID and sequence ID. Furthermore, since the XID is only 32 bits, it is too small to guarantee the re-association of a reply with its request [46]; having session ID, slot ID, and sequence ID in the reply allows the client to validate that the reply in fact belongs to the matched request.¶

The SEQUENCE (and CB_SEQUENCE) operation also carries a "highest_slotid" value, which carries additional requester slot usage information. The requester MUST always indicate the slot ID representing the outstanding request with the highest-numbered slot value. The requester should in all cases provide the most conservative value possible, although it can be increased somewhat above the actual instantaneous usage to maintain some minimum or optimal level. This provides a way for the requester to yield unused request slots back to the replier, which in turn can use the information to reallocate resources.¶

The replier responds with both a new target highest_slotid and an enforced highest_slotid, described as follows:¶

The target highest_slotid is an indication to the requester of the highest_slotid the replier wishes the requester to be using. This permits the replier to withdraw (or add) resources from a requester that has been found to not be using them, in order to more fairly share resources among a varying level of demand from other requesters. The requester must always comply with the replier's value updates, since they indicate newly established hard limits on the requester's access to session resources. However, because of request pipelining, the requester may have active requests in flight reflecting prior values; therefore, the replier must not immediately require the requester to comply.¶
The enforced highest_slotid indicates the highest slot ID the requester is permitted to use on a subsequent SEQUENCE or CB_SEQUENCE operation. The replier's enforced highest_slotid SHOULD be no less than the highest_slotid the requester indicated in the SEQUENCE or CB_SEQUENCE arguments.¶

A requester can be intransigent with respect to lowering its highest_slotid argument to a Sequence operation, i.e. the requester continues to ignore the target highest_slotid in the response to a Sequence operation, and continues to set its highest_slotid argument to be higher than the target highest_slotid. This can be considered particularly egregious behavior when the replier knows there are no outstanding requests with slot IDs higher than its target highest_slotid. When faced with such intransigence, the replier is free to take more forceful action, and MAY reply with a new enforced highest_slotid that is less than its previous enforced highest_slotid. Thereafter, if the requester continues to send requests with a highest_slotid that is greater than the replier's new enforced highest_slotid, the server MAY return NFS4ERR_BAD_HIGH_SLOT, unless the slot ID in the request is greater than the new enforced highest_slotid and the request is a retry.¶

The replier SHOULD retain the slots it wants to retire until the requester sends a request with a highest_slotid less than or equal to the replier's new enforced highest_slotid.¶

The requester can also be intransigent with respect to sending non-retry requests that have a slot ID that exceeds the replier's highest_slotid. Once the replier has forcibly lowered the enforced highest_slotid, the requester is only allowed to send retries on slots that exceed the replier's highest_slotid. If a request is received with a slot ID that is higher than the new enforced highest_slotid, and the sequence ID is one higher than what is in the slot's reply cache, then the server can both retire the slot and return NFS4ERR_BADSLOT (however, the server MUST NOT do one and not the other). The reason it is safe to retire the slot is because by using the next sequence ID, the requester is indicating it has received the previous reply for the slot.¶
The requester SHOULD use the lowest available slot when sending a new request. This way, the replier may be able to retire slot entries faster. However, where the replier is actively adjusting its granted highest_slotid, it will not be able to use only the receipt of the slot ID and highest_slotid in the request. Neither the slot ID nor the highest_slotid used in a request may reflect the replier's current idea of the requester's session limit, because the request may have been sent from the requester before the update was received. Therefore, in the downward adjustment case, the replier may have to retain a number of reply cache entries at least as large as the old value of maximum requests outstanding, until it can infer that the requester has seen a reply containing the new granted highest_slotid. The replier can infer that the requester has seen such a reply when it receives a new request with the same slot ID as the request replied to and the next higher sequence ID.¶

7.6.1.1. Caching of SEQUENCE and CB_SEQUENCE Replies

When a SEQUENCE or CB_SEQUENCE operation is successfully executed, its reply MUST always be cached. Specifically, session ID, sequence ID, and slot ID MUST be cached in the reply cache. The reply from SEQUENCE also includes the highest slot ID, target highest slot ID, and status flags. Instead of caching these values, the server MAY re-compute the values from the current state of the fore channel, session, and/or client ID as appropriate. Similarly, the reply from CB_SEQUENCE includes a highest slot ID and target highest slot ID. The client MAY re-compute the values from the current state of the session as appropriate.¶

Regardless of whether or not a replier is re-computing highest slot ID, target slot ID, and status on replies to retries, the requester MUST NOT assume that the values are being re-computed whenever it receives a reply after a retry is sent, since it has no way of knowing whether the reply it has received was sent by the replier in response to the retry or is a delayed response to the original request. Therefore, it may be the case that highest slot ID, target slot ID, or status bits may reflect the state of affairs when the request was first executed. Although acting based on such delayed information is valid, it may cause the receiver of the reply to do unneeded work. Requesters MAY choose to send additional requests to get the current state of affairs or use the state of affairs reported by subsequent requests, in preference to acting immediately on data that might be out of date.¶

7.6.1.2. Errors from SEQUENCE and CB_SEQUENCE

Any time SEQUENCE or CB_SEQUENCE returns an error, the sequence ID of the slot MUST NOT change. The replier MUST NOT modify the reply cache entry for the slot whenever an error is returned from SEQUENCE or CB_SEQUENCE.¶

7.6.1.3. Optional Reply Caching

On a per-request basis, the requester can choose to direct the replier to cache the reply to all operations after the first operation (SEQUENCE or CB_SEQUENCE) via the sa_cachethis or csa_cachethis fields of the arguments to SEQUENCE or CB_SEQUENCE. The reason it would not direct the replier to cache the entire reply is that the request is composed of all idempotent operations [43]. Caching the reply may offer little benefit. If the reply is too large (see Section 7.6.4), it may not be cacheable anyway. Even if the reply to idempotent request is small enough to cache, unnecessarily caching the reply slows down the server and increases RPC latency.¶

Whether or not the requester requests the reply to be cached has no effect on the slot processing. If the result of SEQUENCE or CB_SEQUENCE is NFS4_OK, then the slot's sequence ID MUST be incremented by one. If a requester does not direct the replier to cache the reply, the replier MUST do one of following:¶

The replier can cache the entire original reply. Even though sa_cachethis or csa_cachethis is FALSE, the replier is always free to cache. It may choose this approach in order to simplify implementation.¶
The replier enters into its reply cache a reply consisting of the original results to the SEQUENCE or CB_SEQUENCE operation, and with the next operation in COMPOUND or CB_COMPOUND having the error NFS4ERR_RETRY_UNCACHED_REP. Thus, if the requester later retries the request, it will get NFS4ERR_RETRY_UNCACHED_REP. If a replier receives a retried Sequence operation where the reply to the COMPOUND or CB_COMPOUND was not cached, then the replier,¶
- MAY return NFS4ERR_RETRY_UNCACHED_REP in reply to a Sequence operation if the Sequence operation is not the first operation (granted, a requester that does so is in violation of the NFSv4.1 protocol).¶
- MUST NOT return NFS4ERR_RETRY_UNCACHED_REP in reply to a Sequence operation if the Sequence operation is the first operation.¶
If the second operation is an illegal operation, or an operation that was legal in a previous minor version of NFSv4 and MUST NOT be supported in the current minor version (e.g., SETCLIENTID), the replier MUST NOT ever return NFS4ERR_RETRY_UNCACHED_REP. Instead the replier MUST return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or NFS4ERR_NOTSUPP as appropriate.¶
If the second operation can result in another error status, the replier MAY return a status other than NFS4ERR_RETRY_UNCACHED_REP, provided the operation is not executed in such a way that the state of the replier is changed. Examples of such error statuses include NFS4ERR_SEQUENCE_POS,NFS4ERR_REQ_TOO_BIG, and NFS4ERR_NOTSUPP returned for an operation that is legal but not REQUIRED in the current minor version and but is not supported by the replier or its file system.¶

The discussion above assumes that the retried request matches the original one. Section 7.6.1.3.1 discusses what the replier might do, and MUST do when original and retried requests do not match. Since the replier may only cache a small amount of the information that would be required to determine whether this is a case of a false retry, the replier may send to the client any of the following responses:¶

The cached reply to the original request (if the replier has cached it in its entirety and the users of the original request and retry match).¶
A reply that consists only of the Sequence operation with the error NFS4ERR_SEQ_FALSE_RETRY.¶
A reply consisting of the response to Sequence with the status NFS4_OK, together with the second operation as it appeared in the retried request with an error of NFS4ERR_RETRY_UNCACHED_REP or other error as described above.¶
A reply that consists of the response to Sequence with the status NFS4_OK, together with the second operation as it appeared in the original request with an error of NFS4ERR_RETRY_UNCACHED_REP or other error as described above.¶

7.6.1.3.1. False Retry

If a requester sent a Sequence operation with a slot ID and sequence ID that are in the reply cache but the replier detected that the retried request is not the same as the original request, including a retry that has different operations or different arguments in the operations from the original and a retry that uses a different principal in the RPC request's credential field that translates to a different user, then this is a false retry. When the replier detects a false retry, it is permitted (but not always obligated) to return NFS4ERR_SEQ_FALSE_RETRY in response to the Sequence operation when it detects a false retry.¶

Translations of particularly privileged user values to other users due to the lack of appropriately secure credentials, as configured on the replier, should be applied before determining whether the users are the same or different. If the replier determines the users are different between the original request and a retry, then the replier MUST return NFS4ERR_SEQ_FALSE_RETRY.¶

If an operation of the retry is an illegal operation, or an operation that was legal in a previous minor version of NFSv4 and MUST NOT be supported in the current minor version (e.g., SETCLIENTID), the replier MAY return NFS4ERR_SEQ_FALSE_RETRY (and MUST do so if the users of the original request and retry differ). Otherwise, the replier MAY return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or NFS4ERR_NOTSUPP as appropriate. Note that the handling is in contrast for how the replier deals with retries requests with no cached reply. The difference is due to NFS4ERR_SEQ_FALSE_RETRY being a valid error for only Sequence operations, whereas NFS4ERR_RETRY_UNCACHED_REP is a valid error for all operations except illegal operations and operations that MUST NOT be supported in the current minor version of NFSv4.¶

7.6.2. Retry and Replay of Reply

[DN: Need to clean up this section, once the working group has had time to discuss the issues involved, particularly the viability of modifying an existing MUST to say something else, even though the MUST in question has not been strictly adhered to.¶

A requester MUST NOT retry a request, unless the connection it used to send the request disconnects. The requester can then reconnect and re-send the request, or it can re-send the request over a different connection that is associated with the same session.¶

If the requester is a server wanting to re-send a callback operation over the backchannel of a session, the requester of course cannot reconnect because only the client can associate connections with the backchannel. The server can re-send the request over another connection that is bound to the same session's backchannel. If there is no such connection, the server MUST indicate that the session has no backchannel by setting the SEQ4_STATUS_CB_PATH_DOWN_SESSION flag bit in the response to the next SEQUENCE operation from the client. The client MUST then associate a connection with the session (or destroy the session).¶

Note that it is not fatal for a requester to retry without a disconnect between the request and retry. However, the retry does consume resources, especially with RDMA, where each request, retry or not, consumes a credit. Retries for no reason, especially retries sent shortly after the previous attempt, are a poor use of network bandwidth and defeat the purpose of a transport's inherent congestion control system.¶

A requester MUST wait for a reply to a request before using the slot for another request. If it does not wait for a reply, then the requester does not know what sequence ID to use for the slot on its next request. For example, suppose a requester sends a request with sequence ID 1, and does not wait for the response. The next time it uses the slot, it sends the new request with sequence ID 2. If the replier has not seen the request with sequence ID 1, then the replier is not expecting sequence ID 2, and rejects the requester's new request with NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or CB_SEQUENCE).¶

RDMA fabrics do not guarantee that the memory handles (Steering Tags) within each RPC/RDMA "chunk" [27] are valid on a scope outside that of a single connection. Therefore, handles used by the direct operations become invalid after connection loss. The server must ensure that any RDMA operations that must be replayed from the reply cache use the newly provided handle(s) from the most recent request.¶

A retry might be sent while the original request is still in progress on the replier. The replier SHOULD deal with the issue by returning NFS4ERR_DELAY as the reply to SEQUENCE or CB_SEQUENCE operation, but implementations MAY return NFS4ERR_MISORDERED. Since errors from SEQUENCE and CB_SEQUENCE are never recorded in the reply cache, this approach allows the results of the execution of the original request to be properly recorded in the reply cache (assuming that the requester specified the reply to be cached).¶

7.6.3. Resolving Server Callback Races

It is possible for server callbacks to arrive at the client before the reply from related fore channel operations. For example, a client may have been granted a delegation to a file it has opened, but the reply to the OPEN (informing the client of the granting of the delegation) may be delayed in the network. If a conflicting operation arrives at the server, it will recall the delegation using the backchannel, which may be on a different transport connection, perhaps even a different network, or even a different session associated with the same client ID.¶

The presence of a session between the client and server alleviates this issue. When a session is in place, each client request is uniquely identified by its { session ID, slot ID, sequence ID } triple. By the rules under which slot entries (reply cache entries) are retired, the server has knowledge whether the client has "seen" each of the server's replies. The server can therefore provide sufficient information to the client to allow it to disambiguate between an erroneous or conflicting callback race condition.¶

For each client operation that might result in some sort of server callback, the server SHOULD "remember" the { session ID, slot ID, sequence ID } triple of the client request until the slot ID retirement rules allow the server to determine that the client has, in fact, seen the server's reply. Until the time the { session ID, slot ID, sequence ID } request triple can be retired, any recalls of the associated object MUST carry an array of these referring identifiers (in the CB_SEQUENCE operation's arguments), for the benefit of the client. After this time, it is not necessary for the server to provide this information in related callbacks, since it is certain that a race condition can no longer occur.¶

The CB_SEQUENCE operation that begins each server callback carries a list of "referring" { session ID, slot ID, sequence ID } triples. If the client finds the request corresponding to the referring session ID, slot ID, and sequence ID to be currently outstanding (i.e., the server's reply has not been seen by the client), it can determine that the callback has raced the reply, and act accordingly. If the client does not find the request corresponding to the referring triple to be outstanding (including the case of a session ID referring to a destroyed session), then there is no race with respect to this triple. The server SHOULD limit the referring triples to requests that refer to just those that apply to the objects referred to in the CB_COMPOUND procedure.¶

The client must not simply wait forever for the expected server reply to arrive before responding to the CB_COMPOUND that won the race, because it is possible that it will be delayed indefinitely. The client should assume the likely case that the reply will arrive within the average round-trip time for COMPOUND requests to the server, and wait that period of time. If that period of time expires, it can respond to the CB_COMPOUND with NFS4ERR_DELAY. There are other scenarios under which callbacks may race replies. Among them are pNFS layout recalls as described in Section 16.5.5.2.¶

7.6.4. COMPOUND and CB_COMPOUND Construction Issues

Very large requests and replies may pose both buffer management issues (especially with RDMA) and reply cache issues. When the session is created (Section 22.36), for each channel (fore and back), the client and server negotiate the maximum-sized request they will send or process (ca_maxrequestsize), the maximum-sized reply they will return or process (ca_maxresponsesize), and the maximum-sized reply they will store in the reply cache (ca_maxresponsesize_cached).¶

If a request exceeds ca_maxrequestsize, the reply will have the status NFS4ERR_REQ_TOO_BIG. A replier MAY return NFS4ERR_REQ_TOO_BIG as the status for the first operation (SEQUENCE or CB_SEQUENCE) in the request (which means that no operations in the request executed and that the state of the slot in the reply cache is unchanged), or it MAY opt to return it on a subsequent operation in the same COMPOUND or CB_COMPOUND request (which means that at least one operation did execute and that the state of the slot in the reply cache does change). The replier SHOULD set NFS4ERR_REQ_TOO_BIG on the operation that exceeds ca_maxrequestsize.¶

If a reply exceeds ca_maxresponsesize, the reply will have the status NFS4ERR_REP_TOO_BIG. A replier MAY return NFS4ERR_REP_TOO_BIG as the status for the first operation (SEQUENCE or CB_SEQUENCE) in the request, or it MAY opt to return it on a subsequent operation (in the same COMPOUND or CB_COMPOUND reply). A replier MAY return NFS4ERR_REP_TOO_BIG in the reply to SEQUENCE or CB_SEQUENCE, even if the response would still exceed ca_maxresponsesize.¶

If sa_cachethis or csa_cachethis is TRUE, then the replier MUST cache a reply except if an error is returned by the SEQUENCE or CB_SEQUENCE operation (see Section 7.6.1.2). If the reply exceeds ca_maxresponsesize_cached (and sa_cachethis or csa_cachethis is TRUE), then the server MUST return NFS4ERR_REP_TOO_BIG_TO_CACHE. Even if NFS4ERR_REP_TOO_BIG_TO_CACHE (or any other error for that matter) is returned on an operation other than the first operation (SEQUENCE or CB_SEQUENCE), then the reply MUST be cached if sa_cachethis or csa_cachethis is TRUE. For example, if a COMPOUND has eleven operations, including SEQUENCE, the fifth operation is a RENAME, and the tenth operation is a READ for one million bytes, the server may return NFS4ERR_REP_TOO_BIG_TO_CACHE on the tenth operation. Since the server executed several operations, especially the non-idempotent RENAME, the client's request to cache the reply needs to be honored in order for the correct operation of exactly once semantics. If the client retries the request, the server will have cached a reply that contains results for ten of the eleven requested operations, with the tenth operation having a status of NFS4ERR_REP_TOO_BIG_TO_CACHE.¶

A client needs to take care that, when sending operations that change the current filehandle (except for PUTFH, PUTPUBFH, PUTROOTFH, and RESTOREFH), it does not exceed the maximum reply buffer before the GETFH operation. Otherwise, the client will have to retry the operation that changed the current filehandle, in order to obtain the desired filehandle. For the OPEN operation (see Section 22.16), retry is not always available as an option. The following guidelines for the handling of filehandle-changing operations are advised:¶

Within the same COMPOUND procedure, a client SHOULD send GETFH immediately after a current filehandle-changing operation. A client MUST send GETFH after a current filehandle-changing operation that is also non-idempotent (e.g., the OPEN operation), unless the operation is RESTOREFH. RESTOREFH is an exception, because even though it is non-idempotent, the filehandle RESTOREFH produced originated from an operation that is either idempotent (e.g., PUTFH, LOOKUP), or non-idempotent (e.g., OPEN, CREATE). If the origin is non-idempotent, then because the client MUST send GETFH after the origin operation, the client can recover if RESTOREFH returns an error.¶
A server MAY return NFS4ERR_REP_TOO_BIG or NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a filehandle-changing operation if the reply would be too large on the next operation.¶
A server SHOULD return NFS4ERR_REP_TOO_BIG or NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a filehandle-changing, non-idempotent operation if the reply would be too large on the next operation, especially if the operation is OPEN.¶
A server MAY return NFS4ERR_UNSAFE_COMPOUND to a non-idempotent current filehandle-changing operation, if it looks at the next operation (in the same COMPOUND procedure) and finds it is not GETFH. The server SHOULD do this if it is unable to determine in advance whether the total response size would exceed ca_maxresponsesize_cached or ca_maxresponsesize.¶

7.6.5. Persistence

Since the reply cache is bounded, it is practical for the reply cache to persist across server restarts. The replier MUST persist the following information if it agreed to persist the session (when the session was created; see Section 22.36):¶

The session ID.¶
The slot table including the sequence ID and cached reply for each slot.¶

The above are sufficient for a replier to provide EOS semantics for any requests that were sent and executed before the server restarted. If the replier is a client, then there is no need for it to persist any more information, unless the client will be persisting all other state across client restart, in which case, the server will never see any NFSv4.1-level protocol manifestation of a client restart. If the replier is a server, with just the slot table and session ID persisting, any requests the client retries after the server restart will return the results that are cached in the reply cache, and any new requests (i.e., the sequence ID is one greater than the slot's sequence ID) MUST be rejected with NFS4ERR_DEADSESSION (returned by SEQUENCE). Such a session is considered dead. A server MAY re-animate a session after a server restart so that the session will accept new requests as well as retries. To re-animate a session, the server needs to persist additional information through server restart:¶

The client ID. This is a prerequisite to let the client create more sessions associated with the same client ID as the re-animated session.¶
The client ID's sequence ID that is used for creating sessions (see Sections 22.35 and 22.36). This is a prerequisite to let the client create more sessions.¶
The principal that created the client ID. This allows the server to authenticate the client when it sends EXCHANGE_ID.¶
The SSV, if SP4_SSV state protection was specified when the client ID was created (see Section 22.35). This lets the client create new sessions, and associate connections with the new and existing sessions.¶
The properties of the client ID as defined in Section 22.35.¶

A persistent reply cache places certain demands on the server. The transition between the execution of the sequence of operations (starting with SEQUENCE) and the placement of its results in the persistent cache MUST be atomic. If a client retries a sequence of operations that was issued to the server, the only acceptable outcomes are an indication that the request is still being processed, the cached reply reflecting the completion of the request, or an indication that the client ID or session has been lost (indicating a catastrophic loss of the reply cache or a session that has been deleted because the client failed to use the session for an extended period of time).¶

The possibility exists of situations in which a server can fail and restart in the middle of a COMPOUND procedure that contains one or more non-idempotent or idempotent-but-modifying operations. If the server allows COMPOUND procedures to be continued after failure, it creates a even greater challenge for the execution of such requests and the atomic placement of results in the reply cache.¶

When a server providing a persistent reply cache does not continue a COMPOUND procedure that was interrupted by a server failure, the error NFS4ERR_DEADSESSION is returned.¶

7.7. RDMA Considerations

A complete discussion of the operation of RPC-based protocols over RDMA transports is in [27]. A discussion of the operation of NFSv4, including NFSv4.1, over RDMA is in [28]. Where RDMA is considered, this specification assumes the use of such a layering; it addresses only the upper-layer issues relevant to making best use of RPC/RDMA.¶

7.7.1. RDMA Connection Resources

RDMA requires its consumers to register memory and post buffers of a specific size and number for receive operations.¶

Registration of memory can be a relatively high-overhead operation, since it requires pinning of buffers, assignment of attributes (e.g., readable/writable), and initialization of hardware translation. Preregistration is desirable to reduce overhead. These registrations are specific to hardware interfaces and even to RDMA connection endpoints; therefore, negotiation of their limits is desirable to manage resources effectively.¶

Following basic registration, these buffers must be posted by the RPC layer to handle receives. These buffers remain in use by the RPC/NFSv4.1 implementation; the size and number of them must be known to the remote peer in order to avoid RDMA errors that would cause a fatal error on the RDMA connection.¶

NFSv4.1 manages slots as resources on a per-session basis (see Section 7), while RDMA connections manage credits on a per-connection basis. This means that in order for a peer to send data over RDMA to a remote buffer, it has to have both an NFSv4.1 slot and an RDMA credit. If multiple RDMA connections are associated with a session, then if the total number of credits across all RDMA connections associated with the session is X, and the number of slots in the session is Y, then the maximum number of outstanding requests is the lesser of X and Y.¶

7.7.2. Flow Control

Previous versions of NFS do not provide flow control; instead, they rely on the windowing provided by transports like TCP to throttle requests. This does not work with RDMA, which provides no operation flow control and will terminate a connection in error when limits are exceeded. Limits such as maximum number of requests outstanding are therefore negotiated when a session is created (see the ca_maxrequests field in Section 22.36). These limits then provide the maxima within which each connection associated with the session's channel(s) must remain. RDMA connections are managed within these limits as described in Section 3.3 of [27]; if there are multiple RDMA connections, then the maximum number of requests for a channel will be divided among the RDMA connections. Put a different way, the onus is on the replier to ensure that the total number of RDMA credits across all connections associated with the replier's channel does exceed the channel's maximum number of outstanding requests.¶

The limits may also be modified dynamically at the replier's choosing by manipulating certain parameters present in each NFSv4.1 reply. In addition, the CB_RECALL_SLOT callback operation (see Section 24.8) can be sent by a server to a client to return RDMA credits to the server, thereby lowering the maximum number of requests a client can have outstanding to the server.¶

7.7.3. Padding

Header padding is requested by each peer at session initiation (see the ca_headerpadsize argument to CREATE_SESSION in Section 22.36), and subsequently used by the RPC RDMA layer, as described in [27]. Zero padding is permitted.¶

Padding leverages the useful property that RDMA preserve alignment of data, even when they are placed into anonymous (untagged) buffers. If requested, client inline writes will insert appropriate pad bytes within the request header to align the data payload on the specified boundary. The client is encouraged to add sufficient padding (up to the negotiated size) so that the "data" field of the WRITE operation is aligned. Most servers can make good use of such padding, which allows them to chain receive buffers in such a way that any data carried by client requests will be placed into appropriate buffers at the server, ready for file system processing. The receiver's RPC layer encounters no overhead from skipping over pad bytes, and the RDMA layer's high performance makes the insertion and transmission of padding on the sender a significant optimization. In this way, the need for servers to perform RDMA Read to satisfy all but the largest client writes is obviated. An added benefit is the reduction of message round trips on the network -- a potentially good trade, where latency is present.¶

The value to choose for padding is subject to a number of criteria. A primary source of variable-length data in the RPC header is the authentication information, the form of which is client-determined, possibly in response to server specification. The contents of COMPOUNDs, sizes of strings such as those passed to RENAME, etc. all go into the determination of a maximal NFSv4.1 request size and therefore minimal buffer size. The client must select its offered value carefully, so as to avoid overburdening the server, and vice versa. The benefit of an appropriate padding value is higher performance.¶

                 Sender gather:
     |RPC Request|Pad  bytes|Length| -> |User data...|
     \------+----------------------/      \
             \                             \
              \    Receiver scatter:        \-----------+- ...
         /-----+----------------\            \           \
         |RPC Request|Pad|Length|   ->  |FS buffer|->|FS buffer|->...

In the above case, the server may recycle unused buffers to the next posted receive if unused by the actual received request, or may pass the now-complete buffers by reference for normal write processing. For a server that can make use of it, this removes any need for data copies of incoming data, without resorting to complicated end-to-end buffer advertisement and management. This includes most kernel-based and integrated server designs, among many others. The client may perform similar optimizations, if desired.¶

7.7.4. Dual RDMA and Non-RDMA Transports

Some RDMA transports (e.g., RFC 5040 [8]) permit a "streaming" (non-RDMA) phase, where ordinary traffic might flow before "stepping up" to RDMA mode, commencing RDMA traffic. Some RDMA transports start connections always in RDMA mode. NFSv4.1 allows, but does not assume, a streaming phase before RDMA mode. When a connection is associated with a session, the client and server negotiate whether the connection is used in RDMA or non-RDMA mode (see Sections 22.36 and 22.34).¶

7.8. Session Security

7.8.1. Session Callback Security

Via session/connection association, NFSv4.1 improves security over that provided by NFSv4.0 for the backchannel. The connection is client-initiated (see Section 22.34) and subject to the same firewall and routing checks as the fore channel. At the client's option (see Section 22.35), connection association is fully authenticated before being activated (see Section 22.34). Traffic from the server over the backchannel is authenticated exactly as the client specifies (see Section 7.8.2).¶

7.8.2. Backchannel RPC Security

When the NFSv4.1 client establishes the backchannel, it informs the server of the security flavors and principals to use when sending requests. If the security flavor is RPCSEC_GSS, the client expresses the principal in the form of an established RPCSEC_GSS context. The server is free to use any of the flavor/principal combinations the client offers, but it MUST NOT use combinations not offered. This way, the client need not provide a target GSS principal for the backchannel as it did with NFSv4.0, nor does the server have to implement an RPCSEC_GSS initiator as it did with NFSv4.0 [39].¶

The CREATE_SESSION (Section 22.36) and BACKCHANNEL_CTL (Section 22.33) operations allow the client to specify flavor/principal combinations.¶

Also note that the SP4_SSV state protection mode (see Sections 22.35 and 7.8.3) has the side benefit of providing SSV-derived RPCSEC_GSS contexts (Section 7.9).¶

7.8.3. Protection from Unauthorized State Changes

As described to this point in the specification, the state model of NFSv4.1 is vulnerable to an attacker that sends a SEQUENCE operation with a forged session ID and with a slot ID that it expects the legitimate client to use next. When the legitimate client uses the slot ID with the same sequence number, the server returns the attacker's result from the reply cache, which disrupts the legitimate client and thus denies service to it. Similarly, an attacker could send a CREATE_SESSION with a forged client ID to create a new session associated with the client ID. The attacker could send requests using the new session that change locking state, such as LOCKU operations to release locks the legitimate client has acquired. Setting a security policy on the file that requires RPCSEC_GSS credentials when manipulating the file's state is one potential work around, but has the disadvantage of preventing a legitimate client from releasing state when RPCSEC_GSS is required to do so, but a GSS context cannot be obtained (possibly because the user has logged off the client).¶

NFSv4.1 provides three options to a client for state protection, which are specified when a client creates a client ID via EXCHANGE_ID (Section 22.35).¶

The first (SP4_NONE) is to simply waive state protection, except for that provided by client host authentication.¶

The other two options (SP4_MACH_CRED and SP4_SSV) share several traits:¶

An RPCSEC_GSS-based credential is used to authenticate client ID and session maintenance operations, including creating and destroying a session, associating a connection with the session, and destroying the client ID.¶
Because RPCSEC_GSS is used to authenticate client ID and session maintenance, the attacker cannot associate a rogue connection with a legitimate session, or associate a rogue session with a legitimate client ID in order to maliciously alter the client ID's lock state via CLOSE, LOCKU, DELEGRETURN, LAYOUTRETURN, etc.¶
In cases where the server's security policies on a portion of its namespace require RPCSEC_GSS authentication, a client may have to use an RPCSEC_GSS credential to remove per-file state (e.g., LOCKU, CLOSE, etc.). The server may require that the principal that removes the state match certain criteria (e.g., the principal might have to be the same as the one that acquired the state). However, the client might not have an RPCSEC_GSS context for such a principal, and might not be able to create such a context (perhaps because the user has logged off). When the client establishes SP4_MACH_CRED or SP4_SSV protection, it can specify a list of operations that the server MUST allow using the machine credential (if SP4_MACH_CRED is used) or the SSV credential (if SP4_SSV is used).¶

The SP4_MACH_CRED state protection option uses a machine credential where the principal that creates the client ID MUST also be the principal that performs client ID and session maintenance operations. The security of the machine credential state protection approach depends entirely on safeguarding the per-machine credential. Assuming a proper safeguard using the per-machine credential for operations like CREATE_SESSION, BIND_CONN_TO_SESSION, DESTROY_SESSION, and DESTROY_CLIENTID will prevent an attacker from associating a rogue connection with a session, or associating a rogue session with a client ID.¶

There are at least three scenarios for the SP4_MACH_CRED option:¶

The system administrator configures a unique, permanent per-machine credential for one of the mandated GSS mechanisms (e.g., if Kerberos V5 is used, a "keytab" containing a principal derived from a client host name could be used).¶
The client is used by a single user, and so the client ID and its sessions are used by just that user. If the user's credential expires, then session and client ID maintenance cannot occur, but since the client has a single user, only that user is inconvenienced.¶
The physical client has multiple users, but the client implementation has a unique client ID for each user. This is effectively the same as the second scenario, but a disadvantage is that each user needs to be allocated at least one session each, so the approach suffers from lack of economy.¶

The SP4_SSV protection option uses the SSV (Section 2.5), via RPCSEC_GSS and the SSV GSS mechanism (Section 7.9), to protect state from attack. The SP4_SSV protection option is intended for the situation comprised of a client that has multiple active users and a system administrator who wants to avoid the burden of installing a permanent machine credential on each client. The SSV is established and updated on the server via SET_SSV (see Section 22.47). To prevent eavesdropping, a client SHOULD send SET_SSV via RPCSEC_GSS with the privacy service or use tls encryption on the connection making the request. Several aspects of the SSV make it intractable for an attacker to guess the SSV, and thus associate rogue connections with a session, and rogue sessions with a client ID:¶

The arguments to and results of SET_SSV include digests of the old and new SSV, respectively.¶
Because the initial value of the SSV is zero, therefore known, the client that opts for SP4_SSV protection and opts to apply SP4_SSV protection to BIND_CONN_TO_SESSION and CREATE_SESSION MUST send at least one SET_SSV operation before the first BIND_CONN_TO_SESSION operation or before the second CREATE_SESSION operation on a client ID. If it does not, the SSV mechanism will not generate tokens (Section 7.9). A client SHOULD send SET_SSV as soon as a session is created.¶
A SET_SSV request does not replace the SSV with the argument to SET_SSV. Instead, the current SSV on the server is logically exclusive ORed (XORed) with the argument to SET_SSV. Each time a new principal uses a client ID for the first time, the client SHOULD send a SET_SSV with that principal's RPCSEC_GSS credentials, with RPCSEC_GSS service set to RPC_GSS_SVC_PRIVACY.¶

Here are the types of attacks that can be attempted by an attacker named Eve on a victim named Bob, and how SP4_SSV protection foils each attack:¶

Suppose Eve is the first user to log into a legitimate client. Eve's use of an NFSv4.1 file system will cause the legitimate client to create a client ID with SP4_SSV protection, specifying that the BIND_CONN_TO_SESSION operation MUST use the SSV credential. Eve's use of the file system also causes an SSV to be created. The SET_SSV operation that creates the SSV will be protected by the RPCSEC_GSS context created by the legitimate client, which uses Eve's GSS principal and credentials. Eve can eavesdrop on the network while her RPCSEC_GSS context is created and the SET_SSV using her context is sent. Even if the legitimate client sends the SET_SSV with RPC_GSS_SVC_PRIVACY, because Eve knows her own credentials, she can decrypt the SSV. Eve can compute an RPCSEC_GSS credential that BIND_CONN_TO_SESSION will accept, and so associate a new connection with the legitimate session. Eve can change the slot ID and sequence state of a legitimate session, and/or the SSV state, in such a way that when Bob accesses the server via the same legitimate client, the legitimate client will be unable to use the session.¶

The client's only recourse is to create a new client ID for Bob to use, and establish a new SSV for the client ID. The client will be unable to delete the old client ID, and will let the lease on the old client ID expire.¶

Once the legitimate client establishes an SSV over the new session using Bob's RPCSEC_GSS context, Eve can use the new session via the legitimate client, but she cannot disrupt Bob. Moreover, because the client SHOULD have modified the SSV due to Eve using the new session, Bob cannot get revenge on Eve by associating a rogue connection with the session.¶

The question is how did the legitimate client detect that Eve has hijacked the old session? When the client detects that a new principal, Bob, wants to use the session, it SHOULD have sent a SET_SSV, which leads to the following sub-scenarios:¶
- Let us suppose that from the rogue connection, Eve sent a SET_SSV with the same slot ID and sequence ID that the legitimate client later uses. The server will assume the SET_SSV sent with Bob's credentials is a retry, and return to the legitimate client the reply it sent Eve. However, unless Eve can correctly guess the SSV the legitimate client will use, the digest verification checks in the SET_SSV response will fail. That is an indication to the client that the session has apparently been hijacked.¶
- Alternatively, Eve sent a SET_SSV with a different slot ID than the legitimate client uses for its SET_SSV. Then the digest verification of the SET_SSV sent with Bob's credentials fails on the server, and the error returned to the client makes it apparent that the session has been hijacked.¶
- Alternatively, Eve sent an operation other than SET_SSV, but with the same slot ID and sequence that the legitimate client uses for its SET_SSV. The server returns to the legitimate client the response it sent Eve. The client sees that the response is not at all what it expects. The client assumes either session hijacking or a server bug, and either way destroys the old session.¶
Eve associates a rogue connection with the session as above, and then destroys the session. Again, Bob goes to use the server from the legitimate client, which sends a SET_SSV using Bob's credentials. The client receives an error that indicates that the session does not exist. When the client tries to create a new session, this will fail because the SSV it has does not match that which the server has, and now the client knows the session was hijacked. The legitimate client establishes a new client ID.¶
If Eve creates a connection before the legitimate client establishes an SSV, because the initial value of the SSV is zero and therefore known, Eve can send a SET_SSV that will pass the digest verification check. However, because the new connection has not been associated with the session, the SET_SSV is rejected for that reason.¶

In summary, an attacker's disruption of state when SP4_SSV protection is in use is limited to the formative period of a client ID, its first session, and the establishment of the SSV. Once a non-malicious user uses the client ID, the client quickly detects any hijack and rectifies the situation. Once a non-malicious user successfully modifies the SSV, the attacker cannot use NFSv4.1 operations to disrupt the non-malicious user.¶

Note that neither the SP4_MACH_CRED nor SP4_SSV protection approaches prevent hijacking of a transport connection that has previously been associated with a session. If the goal of a counter-threat strategy is to prevent connection hijacking, the use of IPsec or TLS is RECOMMENDED.¶

If a connection hijack occurs, the hijacker could in theory change locking state and negatively impact the service to legitimate clients. However, if the server is configured to require the use of RPCSEC_GSS with integrity or privacy on the affected file objects, and if EXCHGID4_FLAG_BIND_PRINC_STATEID capability (Section 22.35) is in force, this will thwart unauthorized attempts to change locking state.¶

7.9. The Secret State Verifier (SSV) GSS Mechanism

The SSV provides the secret key for a GSS mechanism internal to NFSv4.1 that NFSv4.1 uses for state protection. Contexts for this mechanism are not established via the RPCSEC_GSS protocol. Instead, the contexts are automatically created when EXCHANGE_ID specifies SP4_SSV protection. The only tokens defined are the PerMsgToken (emitted by GSS_GetMIC) and the SealedMessage token (emitted by GSS_Wrap).¶

The mechanism OID for the SSV mechanism is iso.org.dod.internet.private.enterprise.Michael Eisler.nfs.ssv_mech (1.3.6.1.4.1.28882.1.1). While the SSV mechanism does not define any initial context tokens, the OID can be used to let servers indicate that the SSV mechanism is acceptable whenever the client sends a SECINFO or SECINFO_NO_NAME operation (see Section 6.3).¶

The SSV mechanism defines four subkeys derived from the SSV value. Each time SET_SSV is invoked, the subkeys are recalculated by the client and server. The calculation of each of the four subkeys depends on each of the four respective ssv_subkey4 enumerated values. The calculation uses the HMAC [52] algorithm, using the current SSV as the key, the one-way hash algorithm as negotiated by EXCHANGE_ID, and the input text as represented by the XDR encoded enumeration value for that subkey of data type ssv_subkey4. If the length of the output of the HMAC algorithm exceeds the length of key of the encryption algorithm (which is also negotiated by EXCHANGE_ID), then the subkey MUST be truncated from the HMAC output, i.e., if the subkey is of N bytes long, then the first N bytes of the HMAC output MUST be used for the subkey. The specification of EXCHANGE_ID states that the length of the output of the HMAC algorithm MUST NOT be less than the length of subkey needed for the encryption algorithm (see Section 22.35).¶

/* Input for computing subkeys */
enum ssv_subkey4 {
        SSV4_SUBKEY_MIC_I2T     = 1,
        SSV4_SUBKEY_MIC_T2I     = 2,
        SSV4_SUBKEY_SEAL_I2T    = 3,
        SSV4_SUBKEY_SEAL_T2I    = 4
};

The subkey derived from SSV4_SUBKEY_MIC_I2T is used for calculating message integrity codes (MICs) that originate from the NFSv4.1 client, whether as part of a request over the fore channel or a response over the backchannel. The subkey derived from SSV4_SUBKEY_MIC_T2I is used for MICs originating from the NFSv4.1 server. The subkey derived from SSV4_SUBKEY_SEAL_I2T is used for encryption text originating from the NFSv4.1 client, and the subkey derived from SSV4_SUBKEY_SEAL_T2I is used for encryption text originating from the NFSv4.1 server.¶

The PerMsgToken description is based on an XDR definition:¶

/* Input for computing smt_hmac */
struct ssv_mic_plain_tkn4 {
  uint32_t        smpt_ssv_seq;
  opaque          smpt_orig_plain<>;
};

/* SSV GSS PerMsgToken token */
struct ssv_mic_tkn4 {
  uint32_t        smt_ssv_seq;
  opaque          smt_hmac<>;
};

The field smt_hmac is an HMAC calculated by using the subkey derived from SSV4_SUBKEY_MIC_I2T or SSV4_SUBKEY_MIC_T2I as the key, the one-way hash algorithm as negotiated by EXCHANGE_ID, and the input text as represented by data of type ssv_mic_plain_tkn4. The field smpt_ssv_seq is the same as smt_ssv_seq. The field smpt_orig_plain is the "message" input passed to GSS_GetMIC() (see Section 2.3.1 of [7]). The caller of GSS_GetMIC() provides a pointer to a buffer containing the plain text. The SSV mechanism's entry point for GSS_GetMIC() encodes this into an opaque array, and the encoding will include an initial four-byte length, plus any necessary padding. Prepended to this will be the XDR encoded value of smpt_ssv_seq, thus making up an XDR encoding of a value of data type ssv_mic_plain_tkn4, which in turn is the input into the HMAC.¶

The token emitted by GSS_GetMIC() is XDR encoded and of XDR data type ssv_mic_tkn4. The field smt_ssv_seq comes from the SSV sequence number, which is equal to one after SET_SSV (Section 22.47) is called the first time on a client ID. Thereafter, the SSV sequence number is incremented on each SET_SSV. Thus, smt_ssv_seq represents the version of the SSV at the time GSS_GetMIC() was called. As noted in Section 22.35, the client and server can maintain multiple concurrent versions of the SSV. This allows the SSV to be changed without serializing all RPC calls that use the SSV mechanism with SET_SSV operations. Once the HMAC is calculated, it is XDR encoded into smt_hmac, which will include an initial four-byte length, and any necessary padding. Prepended to this will be the XDR encoded value of smt_ssv_seq.¶

The SealedMessage description is based on an XDR definition:¶

/* Input for computing ssct_encr_data and ssct_hmac */
struct ssv_seal_plain_tkn4 {
  opaque          sspt_confounder<>;
  uint32_t        sspt_ssv_seq;
  opaque          sspt_orig_plain<>;
  opaque          sspt_pad<>;
};

/* SSV GSS SealedMessage token */
struct ssv_seal_cipher_tkn4 {
  uint32_t      ssct_ssv_seq;
  opaque        ssct_iv<>;
  opaque        ssct_encr_data<>;
  opaque        ssct_hmac<>;
};

The token emitted by GSS_Wrap() is XDR encoded and of XDR data type ssv_seal_cipher_tkn4.¶

The ssct_ssv_seq field has the same meaning as smt_ssv_seq.¶

The ssct_encr_data field is the result of encrypting a value of the XDR encoded data type ssv_seal_plain_tkn4. The encryption key is the subkey derived from SSV4_SUBKEY_SEAL_I2T or SSV4_SUBKEY_SEAL_T2I, and the encryption algorithm is that negotiated by EXCHANGE_ID.¶

The ssct_iv field is the initialization vector (IV) for the encryption algorithm (if applicable) and is sent in clear text. The content and size of the IV MUST comply with the specification of the encryption algorithm. For example, the id-aes256-CBC algorithm MUST use a 16-byte initialization vector (IV), which MUST be unpredictable for each instance of a value of data type ssv_seal_plain_tkn4 that is encrypted with a particular SSV key.¶

The ssct_hmac field is the result of computing an HMAC using the value of the XDR encoded data type ssv_seal_plain_tkn4 as the input text. The key is the subkey derived from SSV4_SUBKEY_MIC_I2T or SSV4_SUBKEY_MIC_T2I, and the one-way hash algorithm is that negotiated by EXCHANGE_ID.¶

The sspt_confounder field is a random value.¶

The sspt_ssv_seq field is the same as ssvt_ssv_seq.¶

The field sspt_orig_plain field is the original plaintext and is the "input_message" input passed to GSS_Wrap() (see Section 2.3.3 of [7]). As with the handling of the plaintext by the SSV mechanism's GSS_GetMIC() entry point, the entry point for GSS_Wrap() expects a pointer to the plaintext, and will XDR encode an opaque array into sspt_orig_plain representing the plain text, along with the other fields of an instance of data type ssv_seal_plain_tkn4.¶

The sspt_pad field is present to support encryption algorithms that require inputs to be in fixed-sized blocks. The content of sspt_pad is zero filled except for the length. Beware that the XDR encoding of ssv_seal_plain_tkn4 contains three variable-length arrays, and so each array consumes four bytes for an array length, and each array that follows the length is always padded to a multiple of four bytes per the XDR standard.¶

For example, suppose the encryption algorithm uses 16-byte blocks, and the sspt_confounder is three bytes long, and the sspt_orig_plain field is 15 bytes long. The XDR encoding of sspt_confounder uses eight bytes (4 + 3 + 1-byte pad), the XDR encoding of sspt_ssv_seq uses four bytes, the XDR encoding of sspt_orig_plain uses 20 bytes (4 + 15 + 1-byte pad), and the smallest XDR encoding of the sspt_pad field is four bytes. This totals 36 bytes. The next multiple of 16 is 48; thus, the length field of sspt_pad needs to be set to 12 bytes, or a total encoding of 16 bytes. The total number of XDR encoded bytes is thus 8 + 4 + 20 + 16 = 48.¶

GSS_Wrap() emits a token that is an XDR encoding of a value of data type ssv_seal_cipher_tkn4. Note that regardless of whether or not the caller of GSS_Wrap() requests confidentiality, the token always has confidentiality. This is because the SSV mechanism is for RPCSEC_GSS, and RPCSEC_GSS never produces GSS_wrap() tokens without confidentiality.¶

There is one SSV per client ID. There is a single GSS context for a client ID / SSV pair. All SSV mechanism RPCSEC_GSS handles of a client ID / SSV pair share the same GSS context. SSV GSS contexts do not expire except when the SSV is destroyed (causes would include the client ID being destroyed or a server restart). Since one purpose of context expiration is to replace keys that have been in use for "too long", hence vulnerable to compromise by brute force or accident, the client can replace the SSV key by sending periodic SET_SSV operations, which is done by cycling through different users' RPCSEC_GSS credentials. This way, the SSV is replaced without destroying the SSV's GSS contexts.¶

SSV RPCSEC_GSS handles can be expired or deleted by the server at any time, and the EXCHANGE_ID operation can be used to create more SSV RPCSEC_GSS handles. Expiration of SSV RPCSEC_GSS handles does not imply that the SSV or its GSS context has expired.¶

The client MUST establish an SSV via SET_SSV before the SSV GSS context can be used to emit tokens from GSS_Wrap() and GSS_GetMIC(). If SET_SSV has not been successfully called, attempts to emit tokens MUST fail.¶

The SSV mechanism does not support replay detection and sequencing in its tokens because RPCSEC_GSS does not use those features (see "Context Creation Requests", Section 5.2.2 of [4]). However, Section 7.10 discusses special considerations for the SSV mechanism when used with RPCSEC_GSS.¶

7.10. Security Considerations for RPCSEC_GSS When Using the SSV Mechanism

When a client ID is created with SP4_SSV state protection (see Section 22.35), the client is permitted to associate multiple RPCSEC_GSS handles with the single SSV GSS context (see Section 7.9). Because of the way RPCSEC_GSS (both version 1 and version 2, see [4] and [9]) calculate the verifier of the reply, special care must be taken by the implementation of the NFSv4.1 client to prevent attacks by a man-in-the-middle. The verifier of an RPCSEC_GSS reply is the output of GSS_GetMIC() applied to the input value of the seq_num field of the RPCSEC_GSS credential (data type rpc_gss_cred_ver_1_t) (see Section 5.3.3.2 of [4]). If multiple RPCSEC_GSS handles share the same GSS context, then if one handle is used to send a request with the same seq_num value as another handle, an attacker could block the reply, and replace it with the verifier used for the other handle.¶

There are multiple ways to prevent the attack on the SSV RPCSEC_GSS verifier in the reply. The simplest is believed to be as follows.¶

Each time one or more new SSV RPCSEC_GSS handles are created via EXCHANGE_ID, the client SHOULD send a SET_SSV operation to modify the SSV. By changing the SSV, the new handles will not result in the re-use of an SSV RPCSEC_GSS verifier in a reply.¶
When a requester decides to use N SSV RPCSEC_GSS handles, it SHOULD assign a unique and non-overlapping range of seq_nums to each SSV RPCSEC_GSS handle. The size of each range SHOULD be equal to MAXSEQ / N (see Section 5 of [4] for the definition of MAXSEQ). When an SSV RPCSEC_GSS handle reaches its maximum, it SHOULD force the replier to destroy the handle by sending a NULL RPC request with seq_num set to MAXSEQ + 1 (see Section 5.3.3.3 of [4]).¶
When the requester wants to increase or decrease N, it SHOULD force the replier to destroy all N handles by sending a NULL RPC request on each handle with seq_num set to MAXSEQ + 1. If the requester is the client, it SHOULD send a SET_SSV operation before using new handles. If the requester is the server, then the client SHOULD send a SET_SSV operation when it detects that the server has forced it to destroy a backchannel's SSV RPCSEC_GSS handle. By sending a SET_SSV operation, the SSV will change, and so the attacker will be unavailable to successfully replay a previous verifier in a reply to the requester.¶

Note that if the replier carefully creates the SSV RPCSEC_GSS handles, the related risk of a man-in-the-middle splicing a forged SSV RPCSEC_GSS credential with a verifier for another handle does not exist. This is because the verifier in an RPCSEC_GSS request is computed from input that includes both the RPCSEC_GSS handle and seq_num (see Section 5.3.1 of [4]). Provided the replier takes care to avoid re-using the value of an RPCSEC_GSS handle that it creates, such as by including a generation number in the handle, the man-in-the-middle will not be able to successfully replay a previous verifier in the request to a replier.¶

7.11. Session Mechanics - Steady State

7.11.1. Obligations of the Server

The server has the primary obligation to monitor the state of backchannel resources that the client has created for the server (RPCSEC_GSS contexts and backchannel connections). If these resources vanish, the server takes action as specified in Section 7.13.2.¶

7.11.2. Obligations of the Client

The client SHOULD honor the following obligations in order to utilize the session:¶

Keep a necessary session from going idle on the server. A client that requires a session but nonetheless is not sending operations risks having the session be destroyed by the server. This is because sessions consume resources, and resource limitations may force the server to cull an inactive session. A server MAY consider a session to be inactive if the client has not used the session before the session inactivity timer (Section 7.12) has expired.¶
Destroy the session when not needed. If a client has multiple sessions, one of which has no requests waiting for replies, and has been idle for some period of time, it SHOULD destroy the session.¶
Maintain GSS contexts and RPCSEC_GSS handles for the backchannel. If the client requires the server to use the RPCSEC_GSS security flavor for callbacks, then it needs to be sure the RPCSEC_GSS handles and/or their GSS contexts that are handed to the server via BACKCHANNEL_CTL or CREATE_SESSION are unexpired.¶
Preserve a connection for a backchannel. The server requires a backchannel in order to gracefully recall recallable state or notify the client of certain events. Note that if the connection is not being used for the fore channel, there is no way for the client to tell if the connection is still alive (e.g., the server restarted without sending a disconnect). The onus is on the server, not the client, to determine if the backchannel's connection is alive, and to indicate in the response to a SEQUENCE operation when the last connection associated with a session's backchannel has disconnected.¶

7.11.3. Steps the Client Takes to Establish a Session

If the client does not have a client ID, the client sends EXCHANGE_ID to establish a client ID. If it opts for SP4_MACH_CRED or SP4_SSV protection, in the spo_must_enforce list of operations, it SHOULD at minimum specify CREATE_SESSION, DESTROY_SESSION, BIND_CONN_TO_SESSION, BACKCHANNEL_CTL, and DESTROY_CLIENTID. If it opts for SP4_SSV protection, the client needs to ask for SSV-based RPCSEC_GSS handles.¶

The client uses the client ID to send a CREATE_SESSION on a connection to the server. The results of CREATE_SESSION indicate whether or not the server will persist the session reply cache through a server that has restarted, and the client notes this for future reference.¶

If the client specified SP4_SSV state protection when the client ID was created, then it SHOULD send SET_SSV in the first COMPOUND after the session is created. Each time a new principal goes to use the client ID, it SHOULD send a SET_SSV again.¶

If the client wants to use delegations, layouts, directory notifications, or any other state that requires a backchannel, then it needs to add a connection to the backchannel if CREATE_SESSION did not already do so. The client creates a connection, and calls BIND_CONN_TO_SESSION to associate the connection with the session and the session's backchannel. If CREATE_SESSION did not already do so, the client MUST tell the server what security is required in order for the client to accept callbacks. The client does this via BACKCHANNEL_CTL. If the client selected SP4_MACH_CRED or SP4_SSV protection when it called EXCHANGE_ID, then the client SHOULD specify that the backchannel use RPCSEC_GSS contexts for security.¶

If the client wants to use additional connections for the backchannel, then it needs to call BIND_CONN_TO_SESSION on each connection it wants to use with the session. If the client wants to use additional connections for the fore channel, then it needs to call BIND_CONN_TO_SESSION if it specified SP4_SSV or SP4_MACH_CRED state protection when the client ID was created.¶

At this point, the session has reached steady state.¶

7.12. Session Inactivity Timer

The server MAY maintain a session inactivity timer for each session. If the session inactivity timer expires, then the server MAY destroy the session. To avoid losing a session due to inactivity, the client MUST renew the session inactivity timer. The length of session inactivity timer MUST NOT be less than the lease_time attribute (Section 10.8.1.11). As with lease renewal (Section 12.3), when the server receives a SEQUENCE operation, it resets the session inactivity timer, and MUST NOT allow the timer to expire while the rest of the operations in the COMPOUND procedure's request are still executing. Once the last operation has finished, the server MUST set the session inactivity timer to expire no sooner than the sum of the current time and the value of the lease_time attribute.¶

7.13. Session Mechanics - Recovery

7.13.1. Events Requiring Client Action

The following events require client action to recover.¶

7.13.1.1. RPCSEC_GSS Context Loss by Callback Path

If all RPCSEC_GSS handles granted by the client to the server for callback use have expired, the client MUST establish a new handle via BACKCHANNEL_CTL. The sr_status_flags field of the SEQUENCE results indicates when callback handles are nearly expired, or fully expired (see Section 22.46.3).¶

7.13.1.2. Connection Loss

If the client loses the last connection of the session and wants to retain the session, then it needs to create a new connection, and if, when the client ID was created, BIND_CONN_TO_SESSION was specified in the spo_must_enforce list, the client MUST use BIND_CONN_TO_SESSION to associate the connection with the session.¶

If there was a request outstanding at the time of connection loss, then if the client wants to continue to use the session, it MUST retry the request, as described in Section 7.6.2. Note that it is not necessary to retry requests over a connection with the same source network address or the same destination network address as the lost connection. As long as the session ID, slot ID, and sequence ID in the retry match that of the original request, the server will recognize the request as a retry if it executed the request prior to disconnect.¶

If the connection that was lost was the last one associated with the backchannel, and the client wants to retain the backchannel and/or prevent revocation of recallable state, the client needs to reconnect, and if it does, it MUST associate the connection to the session and backchannel via BIND_CONN_TO_SESSION. The server SHOULD indicate when it has no callback connection via the sr_status_flags result from SEQUENCE.¶

7.13.1.3. Backchannel GSS Context Loss

Via the sr_status_flags result of the SEQUENCE operation or other means, the client will learn if some or all of the RPCSEC_GSS contexts it assigned to the backchannel have been lost. If the client wants to retain the backchannel and/or not put recallable state subject to revocation, the client needs to use BACKCHANNEL_CTL to assign new contexts.¶

7.13.1.4. Loss of Session

The replier might lose a record of the session. Causes include:¶

Replier failure and restart.¶
A catastrophe that causes the reply cache to be corrupted or lost on the media on which it was stored. This applies even if the replier indicated in the CREATE_SESSION results that it would persist the cache.¶
The server purges the session of a client that has been inactive for a very extended period of time.¶
As a result of configuration changes among a set of clustered servers, a network address previously connected to one server becomes connected to a different server that has no knowledge of the session in question. Such a configuration change will generally only happen when the original server ceases to function for a time.¶

Loss of reply cache is equivalent to loss of session. The replier indicates loss of session to the requester by returning NFS4ERR_BADSESSION on the next operation that uses the session ID that refers to the lost session.¶

After an event like a server restart, the client may have lost its connections. The client assumes for the moment that the session has not been lost. It reconnects, and if it specified connection association enforcement when the session was created, it invokes BIND_CONN_TO_SESSION using the session ID. Otherwise, it invokes SEQUENCE. If BIND_CONN_TO_SESSION or SEQUENCE returns NFS4ERR_BADSESSION, the client knows the session is not available to it when communicating with that network address. If the connection survives session loss, then the next SEQUENCE operation the client sends over the connection will get back NFS4ERR_BADSESSION. The client again knows the session was lost.¶

Here is one suggested algorithm for the client when it gets NFS4ERR_BADSESSION. It is not obligatory in that, if a client does not want to take advantage of such features as trunking, it may omit parts of it. However, it is a useful example that draws attention to various possible recovery issues:¶

If the client has other connections to other server network addresses associated with the same session, attempt a COMPOUND with a single operation, SEQUENCE, on each of the other connections.¶
If the attempts succeed, the session is still alive, and this is a strong indicator that the server's network address has moved. The client might send an EXCHANGE_ID on the connection that returned NFS4ERR_BADSESSION to see if there are opportunities for client ID trunking (i.e., the same client ID and so_major_id value are returned). The client might use DNS to see if the moved network address was replaced with another, so that the performance and availability benefits of session trunking can continue.¶
If the SEQUENCE requests fail with NFS4ERR_BADSESSION, then the session no longer exists on any of the server network addresses for which the client has connections associated with that session ID. It is possible the session is still alive and available on other network addresses. The client sends an EXCHANGE_ID on all the connections to see if the server owner is still listening on those network addresses. If the same server owner is returned but a new client ID is returned, this is a strong indicator of a server restart. If both the same server owner and same client ID are returned, then this is a strong indication that the server did delete the session, and the client will need to send a CREATE_SESSION if it has no other sessions for that client ID. If a different server owner is returned, the client can use DNS to find other network addresses. If it does not, or if DNS does not find any other addresses for the server, then the client will be unable to provide NFSv4.1 service, and fatal errors should be returned to processes that were using the server. If the client is using a "mount" paradigm, unmounting the server is advised.¶
If the client knows of no other connections associated with the session ID and server network addresses that are, or have been, associated with the session ID, then the client can use DNS to find other network addresses. If it does not, or if DNS does not find any other addresses for the server, then the client will be unable to provide NFSv4.1 service, and fatal errors should be returned to processes that were using the server. If the client is using a "mount" paradigm, unmounting the server is advised.¶

If there is a reconfiguration event that results in the same network address being assigned to servers where the eir_server_scope value is different, it cannot be guaranteed that a session ID generated by the first will be recognized as invalid by the first. Therefore, in managing server reconfigurations among servers with different server scope values, it is necessary to make sure that all clients have disconnected from the first server before effecting the reconfiguration. Nonetheless, clients should not assume that servers will always adhere to this requirement; clients MUST be prepared to deal with unexpected effects of server reconfigurations. Even where a session ID is inappropriately recognized as valid, it is likely either that the connection will not be recognized as valid or that a sequence value for a slot will not be correct. Therefore, when a client receives results indicating such unexpected errors, the use of EXCHANGE_ID to determine the current server configuration is RECOMMENDED.¶

A variation on the above is that after a server's network address moves, there is no NFSv4.1 server listening, e.g., no listener on port 2049. In this example, one of the following occur: the NFSv4 server returns NFS4ERR_MINOR_VERS_MISMATCH, the NFS server returns a PROG_MISMATCH error, the RPC listener on 2049 returns PROG_UNVAIL, or attempts to reconnect to the network address timeout. These SHOULD be treated as equivalent to SEQUENCE returning NFS4ERR_BADSESSION for these purposes.¶

When the client detects session loss, it needs to call CREATE_SESSION to recover. Any non-idempotent operations that were in progress might have been performed on the server at the time of session loss. The client has no general way to recover from this.¶

Note that loss of session does not imply loss of byte-range lock, open, delegation, or layout state because locks, opens, delegations, and layouts are tied to the client ID and depend on the client ID, not the session. Nor does loss of byte-range lock, open, delegation, or layout state imply loss of session state, because the session depends on the client ID; loss of client ID however does imply loss of session, byte-range lock, open, delegation, and layout state. See Section 12.4.2. A session can survive a server restart, but lock recovery may still be needed.¶

It is possible that CREATE_SESSION will fail with NFS4ERR_STALE_CLIENTID (e.g., the server restarts and does not preserve client ID state). If so, the client needs to call EXCHANGE_ID, followed by CREATE_SESSION.¶

7.13.2. Events Requiring Server Action

The following events require server action to recover.¶

7.13.2.1. Client Crash and Restart

As described in Section 22.35, a restarted client sends EXCHANGE_ID in such a way that it causes the server to delete any sessions it had.¶

7.13.2.2. Client Crash with No Restart

If a client crashes and never comes back, it will never send EXCHANGE_ID with its old client owner. Thus, the server has session state that will never be used again. After an extended period of time, and if the server has resource constraints, it MAY destroy the old session as well as locking state.¶

7.13.2.3. Extended Network Partition

To the server, the extended network partition may be no different from a client crash with no restart (see Section 7.13.2.2). Unless the server can discern that there is a network partition, it is free to treat the situation as if the client has crashed permanently.¶

7.13.2.4. Backchannel Connection Loss

If there were callback requests outstanding at the time of a connection loss, then the server MUST retry the requests, as described in Section 7.6.2. Note that it is not necessary to retry requests over a connection with the same source network address or the same destination network address as the lost connection. As long as the session ID, slot ID, and sequence ID in the retry match that of the original request, the callback target will recognize the request as a retry even if it did see the request prior to disconnect.¶

If the connection lost is the last one associated with the backchannel, then the server MUST indicate that in the sr_status_flags field of every SEQUENCE reply until the backchannel is re-established. There are two situations, each of which uses different status flags: no connectivity for the session's backchannel and no connectivity for any session backchannel of the client. See Section 22.46 for a description of the appropriate flags in sr_status_flags.¶

7.13.2.5. GSS Context Loss

The server SHOULD monitor when the number of RPCSEC_GSS handles assigned to the backchannel reaches one, and when that one handle is near expiry (i.e., between one and two periods of lease time), and indicate so in the sr_status_flags field of all SEQUENCE replies. The server MUST indicate when all of the backchannel's assigned RPCSEC_GSS handles have expired via the sr_status_flags field of all SEQUENCE replies.¶

7.14. Parallel NFS and Sessions

A client and server can potentially be a non-pNFS implementation, a metadata server implementation, a data server implementation, or two or three types of implementations. The EXCHGID4_FLAG_USE_NON_PNFS, EXCHGID4_FLAG_USE_PNFS_MDS, and EXCHGID4_FLAG_USE_PNFS_DS flags (not mutually exclusive) are passed in the EXCHANGE_ID arguments and results to allow the client to indicate how it wants to use sessions created under the client ID, and to allow the server to indicate how it will allow the sessions to be used. See Section 17.1 for pNFS sessions considerations.¶

Table 1
Data Type	Definition
int32_t	typedef int int32_t;
uint32_t	typedef unsigned int uint32_t;
int64_t	typedef hyper int64_t;
uint64_t	typedef unsigned hyper uint64_t;
attrlist4	typedef opaque attrlist4<>;¶ Used for file/directory attributes.¶
bitmap4	typedef uint32_t bitmap4<>;¶ Used in attribute array encoding.¶
changeid4	typedef uint64_t changeid4;¶ Used in the definition of change_info4.¶
clientid4	typedef uint64_t clientid4;¶ Shorthand reference to client identification.¶
count4	typedef uint32_t count4;¶ Various count parameters (READ, WRITE, COMMIT).¶
length4	typedef uint64_t length4;¶ The length of a byte-range within a file.¶
mode4	typedef uint32_t mode4;¶ Mode attribute data type.¶
nfs_cookie4	typedef uint64_t nfs_cookie4;¶ Opaque cookie value for READDIR.¶
nfs_fh4	typedef opaque nfs_fh4<NFS4_FHSIZE>;¶ Filehandle definition.¶
nfs_ftype4	enum nfs_ftype4;¶ Various defined file types.¶
nfsstat4	enum nfsstat4;¶ Return value for operations.¶
offset4	typedef uint64_t offset4;¶ Various offset designations (READ, WRITE, LOCK, COMMIT).¶
qop4	typedef uint32_t qop4;¶ Quality of protection designation in SECINFO.¶
sec_oid4	typedef opaque sec_oid4<>;¶ Security Object Identifier. The sec_oid4 data type is not really opaque. Instead, it contains an ASN.1 OBJECT IDENTIFIER as used by GSS-API in the mech_type argument to GSS_Init_sec_context. See [7] for details.¶
sequenceid4	typedef uint32_t sequenceid4;¶ Sequence number used for various session operations (EXCHANGE_ID, CREATE_SESSION, SEQUENCE, CB_SEQUENCE).¶
seqid4	typedef uint32_t seqid4;¶ Sequence identifier used for locking.¶
sessionid4	typedef opaque sessionid4[NFS4_SESSIONID_SIZE];¶ Session identifier.¶
slotid4	typedef uint32_t slotid4;¶ Sequencing artifact for various session operations (SEQUENCE, CB_SEQUENCE).¶
utf8string	typedef opaque utf8string<>;¶ UTF-8 encoding for strings.¶
utf8str_cis	typedef utf8string utf8str_cis;¶ Case-insensitive UTF-8 string.¶
utf8str_cs	typedef utf8string utf8str_cs;¶ Case-sensitive UTF-8 string.¶
utf8str_mixed	typedef utf8string utf8str_mixed;¶ UTF-8 strings with a case-sensitive prefix and a case-insensitive suffix.¶
component4	typedef utf8str_cs component4;¶ Represents pathname components.¶
linktext4	typedef utf8str_cs linktext4;¶ Symbolic link contents ("symbolic link" is defined in an Open Group [11] standard).¶
pathname4	typedef component4 pathname4<>;¶ Represents pathname for fs_locations.¶
verifier4	typedef opaque verifier4[NFS4_VERIFIER_SIZE];¶ Verifier used for various operations (COMMIT, CREATE, EXCHANGE_ID, OPEN, READDIR, WRITE) NFS4_VERIFIER_SIZE is defined as 8.¶

Table 2
name	#	Data Type	Description
threshold4_read_size	0	length4	If a file's length is less than the value of threshold4_read_size, then it is RECOMMENDED that the client read from the file via the MDS and not a storage device.
threshold4_write_size	1	length4	If a file's length is less than the value of threshold4_write_size, then it is RECOMMENDED that the client write to the file via the MDS and not a storage device.
threshold4_read_iosize	2	length4	For read I/O sizes below this threshold, it is RECOMMENDED that the client read data using the MDS.
threshold4_write_iosize	3	length4	For write I/O sizes below this threshold, it is RECOMMENDED that the client write data using the MDS.

10. File Attributes

To meet the requirements of extensibility and increased interoperability with non-UNIX platforms, attributes need to be handled in a flexible manner. The NFSv3 fattr3 structure contains a fixed list of attributes that not all clients and servers are able to support or care about. The fattr3 structure cannot be extended as new needs arise and it provides no way to indicate non-support. With the NFSv4.1 protocol, the client is able to query what attributes the server supports and construct requests with only those supported attributes (or a subset thereof).¶

To this end, attributes are divided into three groups: REQUIRED, Recommended and named. Both REQUIRED and Recommended attributes are supported in the NFSv4.1 protocol by a specific and well-defined encoding and are identified by number. They are requested by setting a bit in the bit vector sent in the GETATTR request; the server response includes a bit vector to list what attributes were returned in the response. New REQUIRED or Recommended attributes may be added to the NFSv4 protocol as part of a new minor version by publishing a Standards Track RFC that allocates a new attribute number value and defines the encoding for the attribute. Similarly, Recommended attributes may be added to an existing extensible version by publishing a Standards Track RFC that allocates a new attribute number value and defines the encoding for the attribute. Such extensions are allowed wince Recommended attributes are considered OPTIONAL with regard to the NFSv4 extension framework. See [32] for further details¶

Named attributes are accessed by the new OPENATTR operation, which accesses a hidden directory of attributes associated with a file system object. OPENATTR takes a filehandle for the object and returns the filehandle for the attribute hierarchy. The filehandle for the named attributes is a directory object accessible by LOOKUP or READDIR and contains files whose names represent the named attributes and whose data bytes are the value of the attribute. For example:¶

Table 3
LOOKUP	"foo"	; look up file
GETATTR	attrbits
OPENATTR		; access foo's named attributes
LOOKUP	"x11icon"	; look up specific attribute
READ	0,4096	; read stream of bytes

Named attributes are intended for data needed by applications rather than by an NFS client implementation. NFS implementors are strongly encouraged to define their new attributes as Recommended attributes by bringing them to the IETF Standards Track process.¶

The set of attributes that are classified as REQUIRED is deliberately small since servers need to do whatever it takes to support them. A server should support as many of the Recommended attributes as possible but, by their definition, the server is not required to support all of them. Attributes are defined as REQUIRED if the data is both needed by a large number of clients and is not otherwise reasonably computable by the client when support is not provided on the server.¶

Note that the hidden directory returned by OPENATTR is a convenience for protocol processing. The client should not make any assumptions about the server's implementation of named attributes and whether or not the underlying file system at the server has a named attribute directory. Therefore, operations such as SETATTR and GETATTR on the named attribute directory are undefined.¶

10.1. REQUIRED Attributes

These MUST be supported by every NFSv4.1 client and server in order to ensure a minimum level of interoperability. The server MUST store and return these attributes when requested. A client may ask for the value of any of these attributes to be returned by setting a bit in the GETATTR request, and the server MUST return their value.¶

The client is expected to be able to function with an attribute set limited to these attributes. With just the REQUIRED attributes some client functionality may be impaired or limited in some ways. In the case of servers not supporting the owner, mode, or acl-related attributes, there would be no ability to provide substantial security-related functionality.¶

10.2. Recommended Attributes

These attributes are understood well enough to warrant support in the NFSv4.1 protocol. However, they might not be supported on all clients and servers. A client may ask for any of these attributes to be returned by setting a bit in the GETATTR request but must handle the case where the server does not return them. A client MAY ask for the set of attributes the server supports and has no reason to request attributes the server does not support. A server is REQUIRED to be tolerant of requests for unsupported attributes simply not returning them rather than considering the request an error.¶

It is a reasonable expectation that servers will support all attributes they comfortably can and only fail to support attributes that are difficult to support in their operating environments. A server can provide attributes whenever it doesn't have to "tell lies" to the client. For example, a file modification time should be either an accurate time or should not be supported by the server. At times this will pose difficulties for clients, but a client is better positioned to decide whether and how to fabricate or construct an attribute or whether to do without the attribute, in cases in which the server is unable to provide an accurate value.¶

Previous versions of the NFSv4.1 specification [65] [35] have described these attributes as "RECOMMENDED" even though, as discussed in the first paragraph of this section, that description is not accord with RFC2119 [1]. The NFSv4.0 specification [66] still uses "RECOMMENDED" although explicitly disclaiming the assumption that the RFC2119 definition applies in this case. For the purposes of protocol extension and minor versioning, these attributes are to be treated as OPTIONAL.¶

10.3. Named Attributes

These attributes are not supported by direct encoding in the NFSv4 protocol but are accessed by string names rather than numbers and correspond to an uninterpreted stream of bytes that are stored with the file system object. The namespace for these attributes may be accessed by using the OPENATTR operation. The OPENATTR operation returns a filehandle for a virtual "named attribute directory", and further perusal and modification of the namespace may be done using operations that work on more typical directories. In particular, READDIR may be used to get a list of such named attributes, and LOOKUP and OPEN may select a particular attribute. Creation of a new named attribute may be the result of an OPEN specifying file creation.¶

Once an OPEN is done, named attributes may be examined and changed by normal READ and WRITE operations using the filehandles and stateids returned by OPEN.¶

Named attributes and the named attribute directory may have their own (non-named) attributes. Each of these objects MUST have all of the REQUIRED attributes and may have additional RECOMMENDED attributes. However, the set of attributes for named attributes and the named attribute directory need not be, and typically will not be, as large as that for other objects in that file system.¶

Named attributes and the named attribute directories might be the target of delegations (in the case of the named attribute directory, these will be directory delegations). However, since granting of delegations is at the server's discretion, a server need not support delegations on named attributes or on named attribute directories.¶

It is RECOMMENDED that servers support arbitrary named attributes. A client should not depend on the ability to store any named attributes in the server's file system. If a server does support named attributes, a client that is also able to handle them should be able to copy a file's data and metadata with complete transparency from one location to another; this would imply that names allowed for regular directory entries are valid for named attribute names as well.¶

In NFSv4.1, the structure of named attribute directories is restricted in a number of ways, in order to prevent the development of non-interoperable implementations in which some servers support a fully general hierarchical directory structure for named attributes while others support a limited but adequate structure for named attributes. In such an environment, clients or applications might come to depend on non-portable extensions. The restrictions are:¶

CREATE is not allowed in a named attribute directory. Thus, such objects as symbolic links and special files are not allowed to be named attributes. Further, directories may not be created in a named attribute directory, so a hierarchical structure of named attributes for a single object is not allowed.¶
If OPENATTR is done on a named attribute directory or on a named attribute, the server MUST return NFS4ERR_WRONG_TYPE.¶
Doing a RENAME of a named attribute to a different named attribute directory or to an ordinary (i.e., non-named-attribute) directory is not allowed.¶
Creating hard links between named attribute directories or between named attribute directories and ordinary directories is not allowed.¶

Names of attributes will not be controlled by this document or other IETF Standards Track documents. See Section 26.2 for further discussion.¶

10.4. Classification of Attributes

Each of the REQUIRED and Recommended attributes can be classified in one of three categories: per server (i.e., the value of the attribute will be the same for all file objects that share the same server owner; see Section 5.6 for a definition of server owner), per file system (i.e., the value of the attribute will be the same for some or all file objects that share the same fsid attribute (Section 10.8.1.9) and server owner), or per file system object. Note that it is possible that some per file system attributes may vary within the file system, depending on the value of the "homogeneous" (Section 10.8.2.16) attribute. Note that the attributes time_access_set and time_modify_set are not listed in this section because they are write-only attributes corresponding to time_access and time_modify, and are used in a special instance of SETATTR.¶

The per-server attribute is:¶
- lease_time¶
The per-file system attributes are:¶
- supported_attrs, suppattr_exclcreat, fh_expire_type, link_support, symlink_support, unique_handles, aclsupport, cansettime, case_insensitive, case_preserving, chown_restricted, files_avail, files_free, files_total, fs_locations, homogeneous, maxfilesize, maxname, maxread, maxwrite, no_trunc, space_avail, space_free, space_total, time_delta, change_policy, fs_status, fs_layout_type, fs_locations_info, fs_charset_cap¶
The per-file system object attributes are:¶
- type, change, size, named_attr, fsid, rdattr_error, filehandle, acl, archive, fileid, hidden, maxlink, mimetype, mode, numlinks, owner, owner_group, rawdev, space_used, system, time_access, time_backup, time_create, time_metadata, time_modify, mounted_on_fileid, dir_notif_delay, dirent_notif_delay, dacl, sacl, layout_type, layout_hint, layout_blksize, layout_alignment, mdsthreshold, retention_get, retention_set, retentevt_get, retentevt_set, retention_hold, mode_set_masked¶

For quota_avail_hard, quota_avail_soft, and quota_used, see their definitions below for the appropriate classification.¶

10.5. Set-Only and Get-Only Attributes

Some REQUIRED and Recommended attributes are set-only; i.e., they can be set via SETATTR but not retrieved via GETATTR. Similarly, some REQUIRED and Recommended attributes are get-only; i.e., they can be retrieved via GETATTR but not set via SETATTR. If a client attempts to set a get-only attribute or get a set-only attributes, the server MUST return NFS4ERR_INVAL.¶

10.6. REQUIRED Attributes - List and Definition References

The list of REQUIRED attributes appears in Table 4. The meaning of the columns of the table are:¶

Name:: The name of the attribute.¶
Id:: The number assigned to the attribute. In the event of conflicts between the assigned number and [10], the latter is likely authoritative, but should be resolved with Errata to this document and/or [10]. See [51] for the Errata process.¶
Data Type:: The XDR data type of the attribute.¶
Acc:: Access allowed to the attribute. R means read-only (GETATTR may retrieve, SETATTR may not set). W means write-only (SETATTR may set, GETATTR may not retrieve). R W means read/write (GETATTR may retrieve, SETATTR may set).¶
Defined in:: The section of this specification that describes the attribute.¶

Table 4
Name	Id	Data Type	Acc	Defined in:
supported_attrs	0	bitmap4	R	Section 10.8.1.1
type	1	nfs_ftype4	R	Section 10.8.1.2
fh_expire_type	2	uint32_t	R	Section 10.8.1.3
change	3	uint64_t	R	Section 10.8.1.4
size	4	uint64_t	R W	Section 10.8.1.5
link_support	5	bool	R	Section 10.8.1.6
symlink_support	6	bool	R	Section 10.8.1.7
named_attr	7	bool	R	Section 10.8.1.8
fsid	8	fsid4	R	Section 10.8.1.9
unique_handles	9	bool	R	Section 10.8.1.10
lease_time	10	nfs_lease4	R	Section 10.8.1.11
rdattr_error	11	enum	R	Section 10.8.1.12
filehandle	19	nfs_fh4	R	Section 10.8.1.13
suppattr_exclcreat	75	bitmap4	R	Section 10.8.1.14

10.7. Recommended Attributes - List and Definition References

The Recommended attributes are defined in Table 5. The meanings of the column headers are the same as Table 4; see Section 10.6 for the meanings.¶

Table 5
Name	Id	Data Type	Acc	Defined in:
acl	12	nfsace4<>	R W	Section 10.14
aclsupport	13	uint32_t	R	Section 10.14
archive	14	bool	R W	Section 10.8.2.1
cansettime	15	bool	R	Section 10.8.2.2
case_insensitive	16	bool	R	Section 10.8.2.3
case_preserving	17	bool	R	Section 10.8.2.4
change_policy	60	chg_policy4	R	Section 10.8.2.5
chown_restricted	18	bool	R	Section 10.8.2.6
dacl	58	nfsacl41	R W	Section 10.14
dir_notif_delay	56	nfstime4	R	Section 10.11.1
dirent_notif_delay	57	nfstime4	R	Section 10.11.2
fileid	20	uint64_t	R	Section 10.8.2.7
files_avail	21	uint64_t	R	Section 10.8.2.8
files_free	22	uint64_t	R	Section 10.8.2.9
files_total	23	uint64_t	R	Section 10.8.2.10
fs_charset_cap	76	uint32_t	R	Section 10.8.2.11
fs_layout_type	62	layouttype4<>	R	Section 10.12.1
fs_locations	24	fs_locations	R	Section 10.8.2.12
fs_locations_info	67	fs_locations_info4	R	Section 10.8.2.13
fs_status	61	fs4_status	R	Section 10.8.2.14
hidden	25	bool	R W	Section 10.8.2.15
homogeneous	26	bool	R	Section 10.8.2.16
layout_alignment	66	uint32_t	R	Section 10.12.2
layout_blksize	65	uint32_t	R	Section 10.12.3
layout_hint	63	layouthint4	W	Section 10.12.4
layout_type	64	layouttype4<>	R	Section 10.12.5
maxfilesize	27	uint64_t	R	Section 10.8.2.17
maxlink	28	uint32_t	R	Section 10.8.2.18
maxname	29	uint32_t	R	Section 10.8.2.19
maxread	30	uint64_t	R	Section 10.8.2.20
maxwrite	31	uint64_t	R	Section 10.8.2.21
mdsthreshold	68	mdsthreshold4	R	Section 10.12.6
mimetype	32	utf8str_cs	R W	Section 10.8.2.22
mode	33	mode4	R W	Section 10.14
mode_set_masked	74	mode_masked4	W	Section 10.14
mounted_on_fileid	55	uint64_t	R	Section 10.8.2.23
no_trunc	34	bool	R	Section 10.8.2.24
numlinks	35	uint32_t	R	Section 10.8.2.25
owner	36	utf8str_mixed	R W	Section 10.8.2.26
owner_group	37	utf8str_mixed	R W	Section 10.8.2.27
quota_avail_hard	38	uint64_t	R	Section 10.8.2.28
quota_avail_soft	39	uint64_t	R	Section 10.8.2.29
quota_used	40	uint64_t	R	Section 10.8.2.30
rawdev	41	specdata4	R	Section 10.8.2.31
retentevt_get	71	retention_get4	R	Section 10.13.3
retentevt_set	72	retention_set4	W	Section 10.13.4
retention_get	69	retention_get4	R	Section 10.13.1
retention_hold	73	uint64_t	R W	Section 10.13.5
retention_set	70	retention_set4	W	Section 10.13.2
sacl	59	nfsacl41	R W	Section 10.14
space_avail	42	uint64_t	R	Section 10.8.2.32
space_free	43	uint64_t	R	Section 10.8.2.33
space_total	44	uint64_t	R	Section 10.8.2.34
space_used	45	uint64_t	R	Section 10.8.2.35
system	46	bool	R W	Section 10.8.2.36
time_access	47	nfstime4	R	Section 10.8.2.37
time_access_set	48	settime4	W	Section 10.8.2.38
time_backup	49	nfstime4	R W	Section 10.8.2.39
time_create	50	nfstime4	R W	Section 10.8.2.40
time_delta	51	nfstime4	R	Section 10.8.2.41
time_metadata	52	nfstime4	R	Section 10.8.2.42
time_modify	53	nfstime4	R	Section 10.8.2.43
time_modify_set	54	settime4	W	Section 10.8.2.44

10.8. Attribute Definitions

10.8.1. Definitions of REQUIRED Attributes

10.8.1.1. Attribute 0: supported_attrs

The bit vector that would retrieve all REQUIRED and Recommended attributes that are supported for this object. The scope of this attribute applies to all objects with a matching fsid.¶

10.8.1.2. Attribute 1: type

Designates the type of an object in terms of one of a number of special constants:¶

NF4REG designates a regular file.¶
NF4DIR designates a directory.¶
NF4BLK designates a block device special file.¶
NF4CHR designates a character device special file.¶
NF4LNK designates a symbolic link.¶
NF4SOCK designates a named socket special file.¶
NF4FIFO designates a fifo special file.¶
NF4ATTRDIR designates a named attribute directory.¶
NF4NAMEDATTR designates a named attribute.¶

Within the explanatory text and operation descriptions, the following phrases will be used with the meanings given below:¶

The phrase "is a directory" means that the object's type attribute is NF4DIR or NF4ATTRDIR.¶
The phrase "is a special file" means that the object's type attribute is NF4BLK, NF4CHR, NF4SOCK, or NF4FIFO.¶
The phrases "is an ordinary file" and "is a regular file" mean that the object's type attribute is NF4REG or NF4NAMEDATTR.¶

10.8.1.3. Attribute 2: fh_expire_type

Server uses this to specify filehandle expiration behavior to the client. See Section 9 for additional description.¶

10.8.1.4. Attribute 3: change

A value created by the server that the client can use to determine if file data, directory contents, or attributes of the object have been modified. The server may return the object's time_metadata attribute for this attribute's value, but only if the file system object cannot be updated more frequently than the resolution of time_metadata.¶

10.8.1.5. Attribute 4: size

The size of the object in bytes.¶

10.8.1.6. Attribute 5: link_support

TRUE, if the object's file system supports hard links.¶

10.8.1.7. Attribute 6: symlink_support

TRUE, if the object's file system supports symbolic links.¶

10.8.1.8. Attribute 7: named_attr

TRUE, if this object has named attributes. In other words, object has a non-empty named attribute directory.¶

10.8.1.9. Attribute 8: fsid

Unique file system identifier for the file system holding this object. The fsid attribute has major and minor components, each of which are of data type uint64_t.¶

10.8.1.10. Attribute 9: unique_handles

TRUE, if two distinct filehandles are guaranteed to refer to two different file system objects.¶

10.8.1.11. Attribute 10: lease_time

Duration of the lease at server in seconds.¶

10.8.1.12. Attribute 11: rdattr_error

Error returned from an attempt to retrieve attributes during a READDIR operation.¶

10.8.1.13. Attribute 19: filehandle

The filehandle of this object (primarily for READDIR requests).¶

10.8.1.14. Attribute 75: suppattr_exclcreat

The bit vector that would set all REQUIRED and Recommended attributes that are supported by the EXCLUSIVE4_1 method of file creation via the OPEN operation. The scope of this attribute applies to all objects with a matching fsid.¶

10.8.2. Definitions of Uncategorized Recommended Attributes

The definitions of most of the Recommended attributes follow. Collections that share a common category are defined in other sections.¶

10.8.2.1. Attribute 14: archive

TRUE, if this file has been archived since the time of last modification (deprecated in favor of time_backup).¶

10.8.2.2. Attribute 15: cansettime

TRUE, if the server is able to change the times for a file system object as specified in a SETATTR operation.¶

10.8.2.3. Attribute 16: case_insensitive

TRUE, if file name comparisons on this file system are case insensitive.¶

10.8.2.4. Attribute 17: case_preserving

TRUE, if file name case on this file system is preserved.¶

10.8.2.5. Attribute 60: change_policy

A value created by the server that the client can use to determine if some server policy related to the current file system has been subject to change. If the value remains the same, then the client can be sure that the values of the attributes related to fs location and the fss_type field of the fs_status attribute have not changed. On the other hand, a change in this value does necessarily imply a change in policy. It is up to the client to interrogate the server to determine if some policy relevant to it has changed. See Section 8.3.6 for details.¶

This attribute MUST change when the value returned by the fs_locations or fs_locations_info attribute changes, when a file system goes from read-only to writable or vice versa, or when the allowable set of security flavors for the file system or any part thereof is changed.¶

10.8.2.6. Attribute 18: chown_restricted

If TRUE, the server will reject any request to change either the owner or the group associated with a file if the caller is not a privileged user (for example, "root" in UNIX operating environments or, in Windows 2000, the "Take Ownership" privilege).¶

10.8.2.7. Attribute 20: fileid

A number uniquely identifying the file within the file system.¶

10.8.2.8. Attribute 21: files_avail

File slots available to this user on the file system containing this object -- this should be the smallest relevant limit.¶

10.8.2.9. Attribute 22: files_free

Free file slots on the file system containing this object -- this should be the smallest relevant limit.¶

10.8.2.10. Attribute 23: files_total

Total file slots on the file system containing this object.¶

10.8.2.11. Attribute 76: fs_charset_cap

Character set capabilities for this file system. See Section 18.1.¶

10.8.2.12. Attribute 24: fs_locations

Locations where this file system may be found. If the server returns NFS4ERR_MOVED as an error, this attribute MUST be supported. See Section 15.16 for more details.¶

10.8.2.13. Attribute 67: fs_locations_info

Full function file system location. See Section 15.17.2 for more details.¶

10.8.2.14. Attribute 61: fs_status

Generic file system type information. See Section 15.18 for more details.¶

10.8.2.15. Attribute 25: hidden

TRUE, if the file is considered hidden with respect to the Windows API.¶

10.8.2.16. Attribute 26: homogeneous

TRUE, if this object's file system is homogeneous; i.e., all objects in the file system (all objects on the server with the same fsid) have common values for all per-file-system attributes.¶

10.8.2.17. Attribute 27: maxfilesize

Maximum supported file size for the file system of this object.¶

10.8.2.18. Attribute 28: maxlink

Maximum number of links for this object.¶

10.8.2.19. Attribute 29: maxname

Maximum file name size supported for this object.¶

10.8.2.20. Attribute 30: maxread

Maximum amount of data the READ operation will return for this object.¶

10.8.2.21. Attribute 31: maxwrite

Maximum amount of data the WRITE operation will accept for this object. This attribute SHOULD be supported if the file is writable. Lack of this attribute can lead to the client either wasting bandwidth or not receiving the best performance.¶

10.8.2.22. Attribute 32: mimetype

MIME body type/subtype of this object.¶

10.8.2.23. Attribute 55: mounted_on_fileid

Like fileid, but if the target filehandle is the root of a file system, this attribute represents the fileid of the underlying directory.¶

UNIX-based operating environments connect a file system into the namespace by connecting (mounting) the file system onto the existing file object (the mount point, usually a directory) of an existing file system. When the mount point's parent directory is read via an API like readdir(), the return results are directory entries, each with a component name and a fileid. The fileid of the mount point's directory entry will be different from the fileid that the stat() system call returns. The stat() system call is returning the fileid of the root of the mounted file system, whereas readdir() is returning the fileid that stat() would have returned before any file systems were mounted on the mount point.¶

Unlike NFSv3, NFSv4.1 allows a client's LOOKUP request to cross other file systems. The client detects the file system crossing whenever the filehandle argument of LOOKUP has an fsid attribute different from that of the filehandle returned by LOOKUP. A UNIX-based client will consider this a "mount point crossing". UNIX has a legacy scheme for allowing a process to determine its current working directory. This relies on readdir() of a mount point's parent and stat() of the mount point returning fileids as previously described. The mounted_on_fileid attribute corresponds to the fileid that readdir() would have returned as described previously.¶

While the NFSv4.1 client could simply fabricate a fileid corresponding to what mounted_on_fileid provides (and if the server does not support mounted_on_fileid, the client has no choice), there is a risk that the client will generate a fileid that conflicts with one that is already assigned to another object in the file system. Instead, if the server can provide the mounted_on_fileid, the potential for client operational problems in this area is eliminated.¶

If the server detects that there is no mounted point at the target file object, then the value for mounted_on_fileid that it returns is the same as that of the fileid attribute.¶

The mounted_on_fileid attribute is Recommended, and the server SHOULD provide it if possible. For a UNIX-based server, this is straightforward. Usually, mounted_on_fileid will be requested during a READDIR operation, in which case it is trivial (at least for UNIX-based servers) to return mounted_on_fileid since it is equal to the fileid of a directory entry returned by readdir(). If mounted_on_fileid is requested in a GETATTR operation, the server should obey an invariant that has it returning a value that is equal to the file object's entry in the object's parent directory, i.e., what readdir() would have returned. Some operating environments allow a series of two or more file systems to be mounted onto a single mount point. In this case, for the server to obey the aforementioned invariant, it will need to find the base mount point, and not the intermediate mount points.¶

10.8.2.24. Attribute 34: no_trunc

If this attribute is TRUE, then if the client uses a file name longer than name_max, an error will be returned instead of the name being truncated.¶

10.8.2.25. Attribute 35: numlinks

Number of hard links to this object.¶

10.8.2.26. Attribute 36: owner

The string name of the owner of this object.¶

10.8.2.27. Attribute 37: owner_group

The string name of the group ownership of this object.¶

10.8.2.28. Attribute 38: quota_avail_hard

The value in bytes that represents the amount of additional disk space beyond the current allocation that can be allocated to this file or directory before further allocations will be refused. It is understood that this space may be consumed by allocations to other files or directories.¶

10.8.2.29. Attribute 39: quota_avail_soft

The value in bytes that represents the amount of additional disk space that can be allocated to this file or directory before the user may reasonably be warned. It is understood that this space may be consumed by allocations to other files or directories though there is a rule as to which other files or directories.¶

10.8.2.30. Attribute 40: quota_used

The value in bytes that represents the amount of disk space used by this file or directory and possibly a number of other similar files or directories, where the set of "similar" meets at least the criterion that allocating space to any file or directory in the set will reduce the "quota_avail_hard" of every other file or directory in the set.¶

Note that there may be a number of distinct but overlapping sets of files or directories for which a quota_used value is maintained, e.g., "all files with a given owner", "all files with a given group owner", etc. The server is at liberty to choose any of those sets when providing the content of the quota_used attribute, but should do so in a repeatable way. The rule may be configured per file system or may be "choose the set with the smallest quota".¶

10.8.2.31. Attribute 41: rawdev

Raw device number of file of type NF4BLK or NF4CHR. The device number is split into major and minor numbers. If the file's type attribute is not NF4BLK or NF4CHR, the value returned SHOULD NOT be considered useful.¶

10.8.2.32. Attribute 42: space_avail

Disk space in bytes available to this user on the file system containing this object -- this should be the smallest relevant limit.¶

10.8.2.33. Attribute 43: space_free

Free disk space in bytes on the file system containing this object -- this should be the smallest relevant limit.¶

10.8.2.34. Attribute 44: space_total

Total disk space in bytes on the file system containing this object.¶

10.8.2.35. Attribute 45: space_used

Number of file system bytes allocated to this object.¶

10.8.2.36. Attribute 46: system

This attribute is TRUE if this file is a "system" file with respect to the Windows operating environment.¶

10.8.2.37. Attribute 47: time_access

The time_access attribute represents the time of last access to the object by a READ operation sent to the server. The notion of what is an "access" depends on the server's operating environment and/or the server's file system semantics. For example, for servers obeying Portable Operating System Interface (POSIX) semantics, time_access would be updated only by the READ and READDIR operations and not any of the operations that modify the content of the object [13], [14], [15]. Of course, setting the corresponding time_access_set attribute is another way to modify the time_access attribute.¶

Whenever the file object resides on a writable file system, the server should make its best efforts to record time_access into stable storage. However, to mitigate the performance effects of doing so, and most especially whenever the server is satisfying the read of the object's content from its cache, the server MAY cache access time updates and lazily write them to stable storage. It is also acceptable to give administrators of the server the option to disable time_access updates.¶

10.8.2.38. Attribute 48: time_access_set

Sets the time of last access to the object. SETATTR use only.¶

10.8.2.39. Attribute 49: time_backup

The time of last backup of the object.¶

10.8.2.40. Attribute 50: time_create

The time of creation of the object. This attribute does not have any relation to the traditional UNIX file attribute "ctime" or "change time".¶

10.8.2.41. Attribute 51: time_delta

Smallest useful server time granularity.¶

10.8.2.42. Attribute 52: time_metadata

The time of last metadata modification of the object.¶

10.8.2.43. Attribute 53: time_modify

The time of last modification to the object.¶

10.8.2.44. Attribute 54: time_modify_set

Sets the time of last modification to the object. SETATTR use only.¶

10.9. Interpreting owner and owner_group

The Recommended attributes "owner" and "owner_group" (and also users and groups within the "acl" attribute) are represented in terms of a UTF-8 string. To avoid a representation that is tied to a particular underlying implementation at the client or server, the use of the UTF-8 string has been chosen. Note that Section 6.1 of RFC 2624 [53] provides additional rationale. It is expected that the client and server will have their own local representation of owner and owner_group that is used for local storage or presentation to the end user. Therefore, it is expected that when these attributes are transferred between the client and server, the local representation is translated to a syntax of the form "user@dns_domain". This will allow for a client and server that do not use the same local representation the ability to translate to a common format that can be interpreted by both.¶

Similarly, security principals may be represented in different ways by different security mechanisms. Servers normally translate these representations into a common format, generally that used by local storage, to serve as a means of identifying the users corresponding to these security principals. When these local identifiers are translated to the form of the owner attribute, associated with files created by such principals, they identify, in a common format, the users associated with each corresponding set of security principals.¶

The translation used to interpret owner and group strings is not specified as part of the protocol. This allows various solutions to be employed. For example, a local translation table may be consulted that maps a numeric identifier to the user@dns_domain syntax. A name service may also be used to accomplish the translation. A server may provide a more general service, not limited by any particular translation (which would only translate a limited set of possible strings) by storing the owner and owner_group attributes in local storage without any translation or it may augment a translation method by storing the entire string for attributes for which no translation is available while using the local representation for those cases in which a translation is available.¶

Servers that do not provide support for all possible values of the owner and owner_group attributes SHOULD return an error (NFS4ERR_BADOWNER) when a string is presented that has no translation, as the value to be set for a SETATTR of the owner, owner_group, or acl attributes. When a server does accept an owner or owner_group value as valid on a SETATTR (and similarly for the owner and group strings in an acl), it is promising to return that same string when a corresponding GETATTR is done. Configuration changes (including changes from the mapping of the string to the local representation) and ill-constructed name translations (those that contain aliasing) may make that promise impossible to honor. Servers should make appropriate efforts to avoid a situation in which these attributes have their values changed when no real change to ownership has occurred.¶

The "dns_domain" portion of the owner string is meant to be a DNS domain name, for example, user@example.org. Servers should accept as valid a set of users for at least one domain. A server may treat other domains as having no valid translations. A more general service is provided when a server is capable of accepting users for multiple domains, or for all domains, subject to security constraints.¶

In the case where there is no translation available to the client or server, the attribute value will be constructed without the "@". Therefore, the absence of the @ from the owner or owner_group attribute signifies that no translation was available at the sender and that the receiver of the attribute should not use that string as a basis for translation into its own internal format. Even though the attribute value cannot be translated, it may still be useful. In the case of a client, the attribute string may be used for local display of ownership.¶

To provide a greater degree of compatibility with NFSv3, which identified users and groups by 32-bit unsigned user identifiers and group identifiers, owner and group strings that consist of decimal numeric values with no leading zeros can be given a special interpretation by clients and servers that choose to provide such support. The receiver may treat such a user or group string as representing the same user as would be represented by an NFSv3 uid or gid having the corresponding numeric value. A server is not obligated to accept such a string, but may return an NFS4ERR_BADOWNER instead. To avoid this mechanism being used to subvert user and group translation, so that a client might pass all of the owners and groups in numeric form, a server SHOULD return an NFS4ERR_BADOWNER error when there is a valid translation for the user or owner designated in this way. In that case, the client must use the appropriate name@domain string and not the special form for compatibility.¶

The owner string "nobody" may be used to designate an anonymous user, which will be associated with a file created by a security principal that cannot be mapped through normal means to the owner attribute. Users and implementations of NFSv4.1 SHOULD NOT use "nobody" to designate a real user whose access is not anonymous.¶

10.10. Character Case Attributes

With respect to the case_insensitive and case_preserving attributes, each UCS-4 character (which UTF-8 encodes) can be mapped to an equivalent character of different case or compared in a case-insensitive manner. The details vary based on the Unicode version implemented by the server for the current file system. Details of the process and how the client can best deal with uncertainty about the process will be discussed in the NFSv4-wide internationalization document (See [37] for the latest version)¶

10.11. Directory Notification Attributes

As described in Section 22.39, the client can request a minimum delay for notifications of changes to attributes, but the server is free to ignore what the client requests. The client can determine in advance what notification delays the server will accept by sending a GETATTR operation for either or both of two directory notification attributes. When the client calls the GET_DIR_DELEGATION operation and asks for attribute change notifications, it should request notification delays that are no less than the values in the server-provided attributes.¶

10.11.1. Attribute 56: dir_notif_delay

The dir_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to the directory's attributes.¶

10.11.2. Attribute 57: dirent_notif_delay

The dirent_notif_delay attribute is the minimum number of seconds the server will delay before notifying the client of a change to a file object that has an entry in the directory.¶

10.12. pNFS Attribute Definitions

10.12.1. Attribute 62: fs_layout_type

The fs_layout_type attribute (see Section 8.3.13) applies to a file system and indicates what layout types are supported by the file system. When the client encounters a new fsid, the client SHOULD obtain the value for the fs_layout_type attribute associated with the new file system. This attribute is used by the client to determine if the layout types supported by the server match any of the client's supported layout types.¶

10.12.2. Attribute 66: layout_alignment

When a client holds layouts on files of a file system, the layout_alignment attribute indicates the preferred alignment for I/O to files on that file system. Where possible, the client should send READ and WRITE operations with offsets that are whole multiples of the layout_alignment attribute.¶

10.12.3. Attribute 65: layout_blksize

When a client holds layouts on files of a file system, the layout_blksize attribute indicates the preferred block size for I/O to files on that file system. Where possible, the client should send READ operations with a count argument that is a whole multiple of layout_blksize, and WRITE operations with a data argument of size that is a whole multiple of layout_blksize.¶

10.12.4. Attribute 63: layout_hint

The layout_hint attribute (see Section 8.3.19) may be set on newly created files to influence the metadata server's choice for the file's layout. If possible, this attribute is one of those set in the initial attributes within the OPEN operation. The metadata server may choose to ignore this attribute. The layout_hint attribute is a subset of the layout structure returned by LAYOUTGET. For example, instead of specifying particular devices, this would be used to suggest the stripe width of a file. The server implementation determines which fields within the layout will be used.¶

10.12.5. Attribute 64: layout_type

This attribute lists the layout type(s) available for a file. The value returned by the server is for informational purposes only. The client will use the LAYOUTGET operation to obtain the information needed in order to perform I/O, for example, the specific device information for the file and its layout.¶

10.12.6. Attribute 68: mdsthreshold

This attribute is a server-provided hint used to communicate to the client when it is more efficient to send READ and WRITE operations to the metadata server or the data server. The two types of thresholds described are file size thresholds and I/O size thresholds. If a file's size is smaller than the file size threshold, data accesses SHOULD be sent to the metadata server. If an I/O request has a length that is below the I/O size threshold, the I/O SHOULD be sent to the metadata server. Each threshold type is specified separately for read and write.¶

The server MAY provide both types of thresholds for a file. If both file size and I/O size are provided, the client SHOULD reach or exceed both thresholds before sending its read or write requests to the data server. Alternatively, if only one of the specified thresholds is reached or exceeded, the I/O requests are sent to the metadata server.¶

For each threshold type, a value of zero indicates no READ or WRITE should be sent to the metadata server, while a value of all ones indicates that all READs or WRITEs should be sent to the metadata server.¶

The attribute is available on a per-filehandle basis. If the current filehandle refers to a non-pNFS file or directory, the metadata server should return an attribute that is representative of the filehandle's file system. It is suggested that this attribute is queried as part of the OPEN operation. Due to dynamic system changes, the client should not assume that the attribute will remain constant for any specific time period; thus, it should be periodically refreshed.¶

10.13. Retention Attributes

Retention is a concept whereby a file object can be placed in an immutable, undeletable, unrenamable state for a fixed or infinite duration of time. Once in this "retained" state, the file cannot be moved out of the state until the duration of retention has been reached.¶

When retention is enabled, retention MUST extend to the data of the file, and the name of file. The server MAY extend retention to any other property of the file, including any subset of REQUIRED, Recommended, and named attributes, with the exceptions noted in this section.¶

Servers MAY support or not support retention on any file object type.¶

The five retention attributes are explained in the next subsections.¶

10.13.1. Attribute 69: retention_get

If retention is enabled for the associated file, this attribute's value represents the retention begin time of the file object. This attribute's value is only readable with the GETATTR operation and MUST NOT be modified by the SETATTR operation (Section 10.5). The value of the attribute consists of:¶

const RET4_DURATION_INFINITE    = 0xffffffffffffffff;
struct retention_get4 {
        uint64_t        rg_duration;
        nfstime4        rg_begin_time<1>;
};

The field rg_duration is the duration in seconds indicating how long the file will be retained once retention is enabled. The field rg_begin_time is an array of up to one absolute time value. If the array is zero length, no beginning retention time has been established, and retention is not enabled. If rg_duration is equal to RET4_DURATION_INFINITE, the file, once retention is enabled, will be retained for an infinite duration.¶

If (as soon as) rg_duration is zero, then rg_begin_time will be of zero length, and again, retention is not (no longer) enabled.¶

10.13.2. Attribute 70: retention_set

This attribute is used to set the retention duration and optionally enable retention for the associated file object. This attribute is only modifiable via the SETATTR operation and MUST NOT be retrieved by the GETATTR operation (Section 10.5). This attribute corresponds to retention_get. The value of the attribute consists of:¶

struct retention_set4 {
        bool            rs_enable;
        uint64_t        rs_duration<1>;
};

If the client sets rs_enable to TRUE, then it is enabling retention on the file object with the begin time of retention starting from the server's current time and date. The duration of the retention can also be provided if the rs_duration array is of length one. The duration is the time in seconds from the begin time of retention, and if set to RET4_DURATION_INFINITE, the file is to be retained forever. If retention is enabled, with no duration specified in either this SETATTR or a previous SETATTR, the duration defaults to zero seconds. The server MAY restrict the enabling of retention or the duration of retention on the basis of the ACE4_WRITE_RETENTION ACL permission. The enabling of retention MUST NOT prevent the enabling of event-based retention or the modification of the retention_hold attribute.¶

The following rules apply to both the retention_set and retentevt_set attributes.¶

As long as retention is not enabled, the client is permitted to decrease the duration.¶
The duration can always be set to an equal or higher value, even if retention is enabled. Note that once retention is enabled, the actual duration (as returned by the retention_get or retentevt_get attributes; see Section 10.13.1 or Section 10.13.3) is constantly counting down to zero (one unit per second), unless the duration was set to RET4_DURATION_INFINITE. Thus, it will not be possible for the client to precisely extend the duration on a file that has retention enabled.¶
While retention is enabled, attempts to disable retention or decrease the retention's duration MUST fail with the error NFS4ERR_INVAL.¶
If the principal attempting to change retention_set or retentevt_set does not have ACE4_WRITE_RETENTION permissions, the attempt MUST fail with NFS4ERR_ACCESS.¶

10.13.3. Attribute 71: retentevt_get

Gets the event-based retention duration, and if enabled, the event-based retention begin time of the file object. This attribute is like retention_get, but refers to event-based retention. The event that triggers event-based retention is not defined by the NFSv4.1 specification.¶

10.13.4. Attribute 72: retentevt_set

Sets the event-based retention duration, and optionally enables event-based retention on the file object. This attribute corresponds to retentevt_get and is like retention_set, but refers to event-based retention. When event-based retention is set, the file MUST be retained even if non-event-based retention has been set, and the duration of non-event-based retention has been reached. Conversely, when non-event-based retention has been set, the file MUST be retained even if event-based retention has been set, and the duration of event-based retention has been reached. The server MAY restrict the enabling of event-based retention or the duration of event-based retention on the basis of the ACE4_WRITE_RETENTION ACL permission. The enabling of event-based retention MUST NOT prevent the enabling of non-event-based retention or the modification of the retention_hold attribute.¶

10.13.5. Attribute 73: retention_hold

Gets or sets administrative retention holds, one hold per bit position.¶

This attribute allows one to 64 administrative holds, one hold per bit on the attribute. If retention_hold is not zero, then the file MUST NOT be deleted, renamed, or modified, even if the duration on enabled event or non-event-based retention has been reached. The server MAY restrict the modification of retention_hold on the basis of the ACE4_WRITE_RETENTION_HOLD ACL permission. The enabling of administration retention holds does not prevent the enabling of event-based or non-event-based retention.¶

If the principal attempting to change retention_hold does not have ACE4_WRITE_RETENTION_HOLD permissions, the attempt MUST fail with NFS4ERR_ACCESS.¶

10.14. Access Control Attributes

The use of the following access control attributes is described in Section 5 of the NFSv4-wide security document, currently [69].¶

The owner and owner_group attributes, described in Sections 10.8.2.26 and 10.8.2.27 ¶
The mode attribute described in Section 5.3.1 of the NFSv4-wide security document.¶
The acl, sacl, dacl and aclsupport attributes, described in Section 5.5 through 5.8 [will need to be updated] of the NFSv4-wide security document.¶

12. State Management

Integrating locking into the NFS protocol necessarily causes it to be stateful. With the inclusion of such features as share reservations, file and directory delegations, recallable layouts, and support for mandatory byte-range locking, the protocol becomes substantially more dependent on proper management of state than the traditional combination of NFS and NLM (Network Lock Manager) [54]. These features include expanded locking facilities, which provide some measure of inter-client exclusion, but the state also offers features not readily providable using a stateless model. There are three components to making this state manageable:¶

clear division between client and server¶
ability to reliably detect inconsistency in state between client and server¶
simple and robust recovery mechanisms¶

In this model, the server owns the state information. The client requests changes in locks and the server responds with the changes made. Non-client-initiated changes in locking state are infrequent. The client receives prompt notification of such changes and can adjust its view of the locking state to reflect the server's changes.¶

Individual pieces of state created by the server and passed to the client at its request are represented by 128-bit stateids. These stateids may represent a particular open file, a set of byte-range locks held by a particular owner, or a recallable delegation of privileges to access a file in particular ways or at a particular location.¶

In all cases, there is a transition from the most general information that represents a client as a whole to the eventual lightweight stateid used for most client and server locking interactions. The details of this transition will vary with the type of object but it always starts with a client ID.¶

12.1. Client and Session ID

A client must establish a client ID (see Section 5.5) and then one or more sessionids (see Section 7) before performing any operations to open, byte-range lock, delegate, or obtain a layout for a file object. Each session ID is associated with a specific client ID, and thus serves as a shorthand reference to an NFSv4.1 client.¶

For some types of locking interactions, the client will represent some number of internal locking entities called "owners", which normally correspond to processes internal to the client. For other types of locking-related objects, such as delegations and layouts, no such intermediate entities are provided for, and the locking-related objects are considered to be transferred directly between the server and a unitary client.¶

12.2. Stateid Definition

When the server grants a lock of any type (including opens, byte-range locks, delegations, and layouts), it responds with a unique stateid that represents a set of locks (often a single lock) for the same file, of the same type, and sharing the same ownership characteristics. Thus, opens of the same file by different open-owners each have an identifying stateid. Similarly, each set of byte-range locks on a file owned by a specific lock-owner has its own identifying stateid. Delegations and layouts also have associated stateids by which they may be referenced. The stateid is used as a shorthand reference to a lock or set of locks, and given a stateid, the server can determine the associated state-owner or state-owners (in the case of an open-owner/lock-owner pair) and the associated filehandle. When stateids are used, the current filehandle must be the one associated with that stateid.¶

All stateids associated with a given client ID are associated with a common lease that represents the claim of those stateids and the objects they represent to be maintained by the server. See Section 12.3 for a discussion of the lease.¶

The server may assign stateids independently for different clients. A stateid with the same bit pattern for one client may designate an entirely different set of locks for a different client. The stateid is always interpreted with respect to the client ID associated with the current session. Stateids apply to all sessions associated with the given client ID, and the client may use a stateid obtained from one session on another session associated with the same client ID.¶

12.2.1. Stateid Types

With the exception of special stateids (see Section 12.2.3), each stateid represents locking objects of one of a set of types defined by the NFSv4.1 protocol. Note that in all these cases, where we speak of guarantee, it is understood there are situations such as a client restart, or lock revocation, that allow the guarantee to be voided.¶

Stateids may represent opens of files.¶

Each stateid in this case represents the OPEN state for a given client ID/open-owner/filehandle triple. Such stateids are subject to change (with consequent incrementing of the stateid's seqid) in response to OPENs that result in upgrade and OPEN_DOWNGRADE operations.¶
Stateids may represent sets of byte-range locks.¶

All locks held on a particular file by a particular owner and gotten under the aegis of a particular open file are associated with a single stateid with the seqid being incremented whenever LOCK and LOCKU operations affect that set of locks.¶
Stateids may represent file delegations, which are recallable guarantees by the server to the client that other clients will not reference or modify a particular file, until the delegation is returned. In NFSv4.1, file delegations may be obtained on both regular and non-regular files.¶

A stateid represents a single delegation held by a client for a particular filehandle.¶
Stateids may represent directory delegations, which are recallable guarantees by the server to the client that other clients will not modify the directory, until the delegation is returned.¶

A stateid represents a single delegation held by a client for a particular directory filehandle.¶
Stateids may represent layouts, which are recallable guarantees by the server to the client that particular files may be accessed via an alternate data access protocol at specific locations. Such access is limited to particular sets of byte-ranges and may proceed until those byte-ranges are reduced or the layout is returned.¶

A stateid represents the set of all layouts held by a particular client for a particular filehandle with a given layout type. The seqid is updated as the layouts of that set of byte-ranges change, via layout stateid changing operations such as LAYOUTGET and LAYOUTRETURN.¶

12.2.2. Stateid Structure

Stateids are divided into two fields, a 96-bit "other" field identifying the specific set of locks and a 32-bit "seqid" sequence value. Except in the case of special stateids (see Section 12.2.3), a particular value of the "other" field denotes a set of locks of the same type (for example, byte-range locks, opens, delegations, or layouts), for a specific file or directory, and sharing the same ownership characteristics. The seqid designates a specific instance of such a set of locks, and is incremented to indicate changes in such a set of locks, either by the addition or deletion of locks from the set, a change in the byte-range they apply to, or an upgrade or downgrade in the type of one or more locks.¶

When such a set of locks is first created, the server returns a stateid with seqid value of one. On subsequent operations that modify the set of locks, the server is required to increment the "seqid" field by one whenever it returns a stateid for the same state-owner/file/type combination and there is some change in the set of locks actually designated. In this case, the server will return a stateid with an "other" field the same as previously used for that state-owner/file/type combination, with an incremented "seqid" field. This pattern continues until the seqid is incremented past NFS4_UINT32_MAX, and one (not zero) is the next seqid value.¶

The purpose of the incrementing of the seqid is to allow the server to communicate to the client the order in which operations that modified locking state associated with a stateid have been processed and to make it possible for the client to send requests that are conditional on the set of locks not having changed since the stateid in question was returned.¶

Except for layout stateids (Section 16.5.3), when a client sends a stateid to the server, it has two choices with regard to the seqid sent. It may set the seqid to zero to indicate to the server that it wishes the most up-to-date seqid for that stateid's "other" field to be used. This would be the common choice in the case of a stateid sent with a READ or WRITE operation. It also may set a non-zero value, in which case the server checks if that seqid is the correct one. In that case, the server is required to return NFS4ERR_OLD_STATEID if the seqid is lower than the most current value and NFS4ERR_BAD_STATEID if the seqid is greater than the most current value. This would be the common choice in the case of stateids sent with a CLOSE or OPEN_DOWNGRADE. Because OPENs may be sent in parallel for the same owner, a client might close a file without knowing that an OPEN upgrade had been done by the server, changing the lock in question. If CLOSE were sent with a zero seqid, the OPEN upgrade would be cancelled before the client even received an indication that an upgrade had happened.¶

When a stateid is sent by the server to the client as part of a callback operation, it is not subject to checking for a current seqid and returning NFS4ERR_OLD_STATEID. This is because the client is not in a position to know the most up-to-date seqid and thus cannot verify it. Unless specially noted, the seqid value for a stateid sent by the server to the client as part of a callback is required to be zero with NFS4ERR_BAD_STATEID returned if it is not.¶

In making comparisons between seqids, both by the client in determining the order of operations and by the server in determining whether the NFS4ERR_OLD_STATEID is to be returned, the possibility of the seqid being swapped around past the NFS4_UINT32_MAX value needs to be taken into account. When two seqid values are being compared, the total count of slots for all sessions associated with the current client is used to do this. When one seqid value is less than this total slot count and another seqid value is greater than NFS4_UINT32_MAX minus the total slot count, the former is to be treated as lower than the latter, despite the fact that it is numerically greater.¶

12.2.3. Special Stateids

Stateid values whose "other" field is either all zeros or all ones are reserved. They may not be assigned by the server but have special meanings defined by the protocol. The particular meaning depends on whether the "other" field is all zeros or all ones and the specific value of the "seqid" field.¶

The following combinations of "other" and "seqid" are defined in NFSv4.1:¶

When "other" and "seqid" are both zero, the stateid is treated as a special anonymous stateid, which can be used in READ, WRITE, and SETATTR requests to indicate the absence of any OPEN state associated with the request. When an anonymous stateid value is used and an existing open denies the form of access requested, then access will be denied to the request. This stateid MUST NOT be used on operations to data servers (Section 17.6).¶
When "other" and "seqid" are both all ones, the stateid is a special READ bypass stateid. When this value is used in WRITE or SETATTR, it is treated like the anonymous value. When used in READ, the server MAY grant access, even if access would normally be denied to READ operations. This stateid MUST NOT be used on operations to data servers.¶
When "other" is zero and "seqid" is one, the stateid represents the current stateid, which is whatever value is the last stateid returned by an operation within the COMPOUND. In the case of an OPEN, the stateid returned for the open file and not the delegation is used. The stateid passed to the operation in place of the special value has its "seqid" value set to zero, except when the current stateid is used by the operation CLOSE or OPEN_DOWNGRADE. If there is no operation in the COMPOUND that has returned a stateid value, the server MUST return the error NFS4ERR_BAD_STATEID. As illustrated in Figure 6, if the value of a current stateid is a special stateid and the stateid of an operation's arguments has "other" set to zero and "seqid" set to one, then the server MUST return the error NFS4ERR_BAD_STATEID.¶
When "other" is zero and "seqid" is NFS4_UINT32_MAX, the stateid represents a reserved stateid value defined to be invalid. When this stateid is used, the server MUST return the error NFS4ERR_BAD_STATEID.¶

If a stateid value is used that has all zeros or all ones in the "other" field but does not match one of the cases above, the server MUST return the error NFS4ERR_BAD_STATEID.¶

Special stateids, unlike other stateids, are not associated with individual client IDs or filehandles and can be used with all valid client IDs and filehandles. In the case of a special stateid designating the current stateid, the current stateid value substituted for the special stateid is associated with a particular client ID and filehandle, and so, if it is used where the current filehandle does not match that associated with the current stateid, the operation to which the stateid is passed will return NFS4ERR_BAD_STATEID.¶

12.2.4. Stateid Lifetime and Validation

Stateids must remain valid until either a client restart or a server restart or until the client returns all of the locks associated with the stateid by means of an operation such as CLOSE or DELEGRETURN. If the locks are lost due to revocation, as long as the client ID is valid, the stateid remains a valid designation of that revoked state until the client frees it by using FREE_STATEID. Stateids associated with byte-range locks are an exception. They remain valid even if a LOCKU frees all remaining locks, so long as the open file with which they are associated remains open, unless the client frees the stateids via the FREE_STATEID operation.¶

It should be noted that there are situations in which the client's locks become invalid, without the client requesting they be returned. These include lease expiration and a number of forms of lock revocation within the lease period. It is important to note that in these situations, the stateid remains valid and the client can use it to determine the disposition of the associated lost locks.¶

An "other" value must never be reused for a different purpose (i.e., different filehandle, owner, or type of locks) within the context of a single client ID. A server may retain the "other" value for the same purpose beyond the point where it may otherwise be freed, but if it does so, it must maintain "seqid" continuity with previous values.¶

One mechanism that may be used to satisfy the requirement that the server recognize invalid and out-of-date stateids is for the server to divide the "other" field of the stateid into two fields.¶

an index into a table of locking-state structures.¶
a generation number that is incremented on each allocation of a table entry for a particular use.¶

And then store in each table entry,¶

the client ID with which the stateid is associated.¶
the current generation number for the (at most one) valid stateid sharing this index value.¶
the filehandle of the file on which the locks are taken.¶
an indication of the type of stateid (open, byte-range lock, file delegation, directory delegation, layout).¶
the last "seqid" value returned corresponding to the current "other" value.¶
an indication of the current status of the locks associated with this stateid, in particular, whether these have been revoked and if so, for what reason.¶

With this information, an incoming stateid can be validated and the appropriate error returned when necessary. Special and non-special stateids are handled separately. (See Section 12.2.3 for a discussion of special stateids.)¶

Note that stateids are implicitly qualified by the current client ID, as derived from the client ID associated with the current session. Note, however, that the semantics of the session will prevent stateids associated with a previous client or server instance from being analyzed by this procedure.¶

If server restart has resulted in an invalid client ID or a session ID that is invalid, SEQUENCE will return an error and the operation that takes a stateid as an argument will never be processed.¶

If there has been a server restart where there is a persistent session and all leased state has been lost, then the session in question will, although valid, be marked as dead, and any operation not satisfied by means of the reply cache will receive the error NFS4ERR_DEADSESSION, and thus not be processed as indicated below.¶

When a stateid is being tested and the "other" field is all zeros or all ones, a check that the "other" and "seqid" fields match a defined combination for a special stateid is done and the results determined as follows:¶

If the "other" and "seqid" fields do not match a defined combination associated with a special stateid, the error NFS4ERR_BAD_STATEID is returned.¶
If the special stateid is one designating the current stateid and there is a current stateid, then the current stateid is substituted for the special stateid and the checks appropriate to non-special stateids are performed.¶
If the combination is valid in general but is not appropriate to the context in which the stateid is used (e.g., an all-zero stateid is used when an OPEN stateid is required in a LOCK operation), the error NFS4ERR_BAD_STATEID is also returned.¶
Otherwise, the check is completed and the special stateid is accepted as valid.¶

When a stateid is being tested, and the "other" field is neither all zeros nor all ones, the following procedure could be used to validate an incoming stateid and return an appropriate error, when necessary, assuming that the "other" field would be divided into a table index and an entry generation.¶

If the table index field is outside the range of the associated table, return NFS4ERR_BAD_STATEID.¶
If the selected table entry is of a different generation than that specified in the incoming stateid, return NFS4ERR_BAD_STATEID.¶
If the selected table entry does not match the current filehandle, return NFS4ERR_BAD_STATEID.¶
If the client ID in the table entry does not match the client ID associated with the current session, return NFS4ERR_BAD_STATEID.¶
If the stateid represents revoked state, then return NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or NFS4ERR_DELEG_REVOKED, as appropriate.¶
If the stateid type is not valid for the context in which the stateid appears, return NFS4ERR_BAD_STATEID. Note that a stateid may be valid in general, as would be reported by the TEST_STATEID operation, but be invalid for a particular operation, as, for example, when a stateid that doesn't represent byte-range locks is passed to the non-from_open case of LOCK or to LOCKU, or when a stateid that does not represent an open is passed to CLOSE or OPEN_DOWNGRADE. In such cases, the server MUST return NFS4ERR_BAD_STATEID.¶
If the "seqid" field is not zero and it is greater than the current sequence value corresponding to the current "other" field, return NFS4ERR_BAD_STATEID.¶
If the "seqid" field is not zero and it is less than the current sequence value corresponding to the current "other" field, return NFS4ERR_OLD_STATEID.¶
Otherwise, the stateid is valid and the table entry should contain any additional information about the type of stateid and information associated with that particular type of stateid, such as the associated set of locks, e.g., open-owner and lock-owner information, as well as information on the specific locks, e.g., open modes and byte-ranges.¶

12.2.5. Stateid Use for I/O Operations

Clients performing I/O operations need to select an appropriate stateid based on the locks (including opens and delegations) held by the client and the various types of state-owners sending the I/O requests. SETATTR operations that change the file size are treated like I/O operations in this regard.¶

The following rules, applied in order of decreasing priority, govern the selection of the appropriate stateid. In following these rules, the client will only consider locks of which it has actually received notification by an appropriate operation response or callback. Note that the rules are slightly different in the case of I/O to data servers when file layouts are being used (see Section 17.9.1).¶

If the client holds a delegation for the file in question, the delegation stateid SHOULD be used.¶
Otherwise, if the entity corresponding to the lock-owner (e.g., a process) sending the I/O has a byte-range lock stateid for the associated open file, then the byte-range lock stateid for that lock-owner and open file SHOULD be used.¶
If there is no byte-range lock stateid, then the OPEN stateid for the open file in question SHOULD be used.¶
Finally, if none of the above apply, then a special stateid SHOULD be used.¶

Ignoring these rules may result in situations in which the server does not have information necessary to properly process the request. For example, when mandatory byte-range locks are in effect, if the stateid does not indicate the proper lock-owner, via a lock stateid, a request might be avoidably rejected.¶

The server however should not try to enforce these ordering rules and should use whatever information is available to properly process I/O requests. In particular, when a client has a delegation for a given file, it SHOULD take note of this fact in processing a request, even if it is sent with a special stateid.¶

12.2.6. Stateid Use for SETATTR Operations

Because each operation is associated with a session ID and from that the clientid can be determined, operations do not need to include a stateid for the server to be able to determine whether they should cause a delegation to be recalled or are to be treated as done within the scope of the delegation.¶

In the case of SETATTR operations, a stateid is present. In cases other than those that set the file size, the client may send either a special stateid or, when a delegation is held for the file in question, a delegation stateid. While the server SHOULD validate the stateid and may use the stateid to optimize the determination as to whether a delegation is held, it SHOULD note the presence of a delegation even when a special stateid is sent, and MUST accept a valid delegation stateid when sent.¶

12.3. Lease Renewal

Each client/server pair, as represented by a client ID, has a single lease. The purpose of the lease is to allow the client to indicate to the server, in a low-overhead way, that it is active, and thus that the server is to retain the client's locks. This arrangement allows the server to remove stale locking-related objects that are held by a client that has crashed or is otherwise unreachable, once the relevant lease expires. This in turn allows other clients to obtain conflicting locks without being delayed indefinitely by inactive or unreachable clients. It is not a mechanism for cache consistency and lease renewals may not be denied if the lease interval has not expired.¶

Since each session is associated with a specific client (identified by the client's client ID), any operation sent on that session is an indication that the associated client is reachable. When a request is sent for a given session, successful execution of a SEQUENCE operation (or successful retrieval of the result of SEQUENCE from the reply cache) on an unexpired lease will result in the lease being implicitly renewed, for the standard renewal period (equal to the lease_time attribute).¶

If the client ID's lease has not expired when the server receives a SEQUENCE operation, then the server MUST renew the lease. If the client ID's lease has expired when the server receives a SEQUENCE operation, the server MAY renew the lease; this depends on whether any state was revoked as a result of the client's failure to renew the lease before expiration.¶

Absent other activity that would renew the lease, a COMPOUND consisting of a single SEQUENCE operation will suffice. The client should also take communication-related delays into account and take steps to ensure that the renewal messages actually reach the server in good time. For example:¶

When trunking is in effect, the client should consider sending multiple requests on different connections, in order to ensure that renewal occurs, even in the event of blockage in the path used for one of those connections.¶
Transport retransmission delays might become so large as to approach or exceed the length of the lease period. This may be particularly likely when the server is unresponsive due to a restart; see Section 12.4.2.1. If the client implementation is not careful, transport retransmission delays can result in the client failing to detect a server restart before the grace period ends. The scenario is that the client is using a transport with exponential backoff, such that the maximum retransmission timeout exceeds both the grace period and the lease_time attribute. A network partition causes the client's connection's retransmission interval to back off, and even after the partition heals, the next transport-level retransmission is sent after the server has restarted and its grace period ends.¶

The client MUST either recover from the ensuing NFS4ERR_NO_GRACE errors or it MUST ensure that, despite transport-level retransmission intervals that exceed the lease_time, a SEQUENCE operation is sent that renews the lease before expiration. The client can achieve this by associating a new connection with the session, and sending a SEQUENCE operation on it. However, if the attempt to establish a new connection is delayed for some reason (e.g., exponential backoff of the connection establishment packets), the client will have to abort the connection establishment attempt before the lease expires, and attempt to reconnect.¶

If the server renews the lease upon receiving a SEQUENCE operation, the server MUST NOT allow the lease to expire while the rest of the operations in the COMPOUND procedure's request are still executing. Once the last operation has finished, and the response to COMPOUND has been sent, the server MUST set the lease to expire no sooner than the sum of current time and the value of the lease_time attribute.¶

A client ID's lease can expire when it has been at least the lease interval (lease_time) since the last lease-renewing SEQUENCE operation was sent on any of the client ID's sessions and there are no active COMPOUND operations on any such sessions.¶

Because the SEQUENCE operation is the basic mechanism to renew a lease, and because it must be done at least once for each lease period, it is the natural mechanism whereby the server will inform the client of changes in the lease status that the client needs to be informed of. The client should inspect the status flags (sr_status_flags) returned by sequence and take the appropriate action (see Section 22.46.3 for details).¶

The status bits SEQ4_STATUS_CB_PATH_DOWN and SEQ4_STATUS_CB_PATH_DOWN_SESSION indicate problems with the backchannel that the client may need to address in order to receive callback requests.¶
The status bits SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRING and SEQ4_STATUS_CB_GSS_CONTEXTS_EXPIRED indicate problems with GSS contexts or RPCSEC_GSS handles for the backchannel that the client might have to address in order to allow callback requests to be sent.¶
The status bits SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED, SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED, SEQ4_STATUS_ADMIN_STATE_REVOKED, and SEQ4_STATUS_RECALLABLE_STATE_REVOKED notify the client of lock revocation events. When these bits are set, the client should use TEST_STATEID to find what stateids have been revoked and use FREE_STATEID to acknowledge loss of the associated state.¶
The status bit SEQ4_STATUS_LEASE_MOVE indicates that responsibility for lease renewal has been transferred to one or more new servers.¶
The status bit SEQ4_STATUS_RESTART_RECLAIM_NEEDED indicates that due to server restart the client must reclaim locking state.¶
The status bit SEQ4_STATUS_BACKCHANNEL_FAULT indicates that the server has encountered an unrecoverable fault with the backchannel (e.g., it has lost track of a sequence ID for a slot in the backchannel).¶

12.4. Crash Recovery

A critical requirement in crash recovery is that both the client and the server know when the other has failed. Additionally, it is required that a client sees a consistent view of data across server restarts. All READ and WRITE operations that may have been queued within the client or network buffers must wait until the client has successfully recovered the locks protecting the READ and WRITE operations. Any that reach the server before the server can safely determine that the client has recovered enough locking state to be sure that such operations can be safely processed must be rejected. This will happen because either:¶

The state presented is no longer valid since it is associated with a now invalid client ID. In this case, the client will receive either an NFS4ERR_BADSESSION or NFS4ERR_DEADSESSION error, and any attempt to attach a new session to that invalid client ID will result in an NFS4ERR_STALE_CLIENTID error.¶
Subsequent recovery of locks may make execution of the operation inappropriate (NFS4ERR_GRACE).¶

12.4.1. Client Failure and Recovery

In the event that a client fails, the server may release the client's locks when the associated lease has expired. Conflicting locks from another client may only be granted after this lease expiration. As discussed in Section 12.3, when a client has not failed and re-establishes its lease before expiration occurs, requests for conflicting locks will not be granted.¶

To minimize client delay upon restart, lock requests are associated with an instance of the client by a client-supplied verifier. This verifier is part of the client_owner4 sent in the initial EXCHANGE_ID call made by the client. The server returns a client ID as a result of the EXCHANGE_ID operation. The client then confirms the use of the client ID by establishing a session associated with that client ID (see Section 22.36.3 for a description of how this is done). All locks, including opens, byte-range locks, delegations, and layouts obtained by sessions using that client ID, are associated with that client ID.¶

Since the verifier will be changed by the client upon each initialization, the server can compare a new verifier to the verifier associated with currently held locks and determine that they do not match. This signifies the client's new instantiation and subsequent loss (upon confirmation of the new client ID) of locking state. As a result, the server is free to release all locks held that are associated with the old client ID that was derived from the old verifier. At this point, conflicting locks from other clients, kept waiting while the lease had not yet expired, can be granted. In addition, all stateids associated with the old client ID can also be freed, as they are no longer reference-able.¶

Note that the verifier must have the same uniqueness properties as the verifier for the COMMIT operation.¶

12.4.2. Server Failure and Recovery

If the server loses locking state (usually as a result of a restart), it must allow clients time to discover this fact and re-establish the lost locking state. The client must be able to re-establish the locking state without having the server deny valid requests because the server has granted conflicting access to another client. Likewise, if there is a possibility that clients have not yet re-established their locking state for a file and that such locking state might make it invalid to perform READ or WRITE operations. For example, if mandatory locks are a possibility, the server must disallow READ and WRITE operations for that file.¶

A client can determine that loss of locking state has occurred via several methods.¶

When a SEQUENCE (most common) or other operation returns NFS4ERR_BADSESSION, this may mean that the session has been destroyed but the client ID is still valid. The client sends a CREATE_SESSION request with the client ID to re-establish the session. If CREATE_SESSION fails with NFS4ERR_STALE_CLIENTID, the client must establish a new client ID (see Section 12.1) and re-establish its lock state with the new client ID, after the CREATE_SESSION operation succeeds (see Section 12.4.2.1).¶
When a SEQUENCE (most common) or other operation on a persistent session returns NFS4ERR_DEADSESSION, this indicates that a session is no longer usable for new, i.e., not satisfied from the reply cache, operations. Once all pending operations are determined to be either performed before the retry or not performed, the client sends a CREATE_SESSION request with the client ID to re-establish the session. If CREATE_SESSION fails with NFS4ERR_STALE_CLIENTID, the client must establish a new client ID (see Section 12.1) and re-establish its lock state after the CREATE_SESSION, with the new client ID, succeeds (Section 12.4.2.1).¶
When an operation, neither SEQUENCE nor preceded by SEQUENCE (for example, CREATE_SESSION, DESTROY_SESSION), returns NFS4ERR_STALE_CLIENTID, the client MUST establish a new client ID (Section 12.1) and re-establish its lock state (Section 12.4.2.1).¶

12.4.2.1. State Reclaim

When state information and the associated locks are lost as a result of a server restart, the protocol must provide a way to cause that state to be re-established. The approach used is to define, for most types of locking state (layouts are an exception), a request whose function is to allow the client to re-establish on the server a lock first obtained from a previous instance. Generally, these requests are variants of the requests normally used to create locks of that type and are referred to as "reclaim-type" requests, and the process of re-establishing such locks is referred to as "reclaiming" them.¶

Because each client must have an opportunity to reclaim all of the locks that it has without the possibility that some other client will be granted a conflicting lock, a "grace period" is devoted to the reclaim process. During this period, requests creating client IDs and sessions are handled normally, but locking requests are subject to special restrictions. Only reclaim-type locking requests are allowed, unless the server can reliably determine (through state persistently maintained across restart instances) that granting any such lock cannot possibly conflict with a subsequent reclaim. When a request is made to obtain a new lock (i.e., not a reclaim-type request) during the grace period and such a determination cannot be made, the server must return the error NFS4ERR_GRACE.¶

Once a session is established using the new client ID, the client will use reclaim-type locking requests (e.g., LOCK operations with reclaim set to TRUE and OPEN operations with a claim type of CLAIM_PREVIOUS; see Section 13.11) to re-establish its locking state. Once this is done, or if there is no such locking state to reclaim, the client sends a global RECLAIM_COMPLETE operation, i.e., one with the rca_one_fs argument set to FALSE, to indicate that it has reclaimed all of the locking state that it will reclaim. Once a client sends such a RECLAIM_COMPLETE operation, it may attempt non-reclaim locking operations, although it might get an NFS4ERR_GRACE status result from each such operation until the period of special handling is over. See Section 15.11.9 for a discussion of the analogous handling lock reclamation in the case of file systems transitioning from server to server.¶

During the grace period, the server must reject READ and WRITE operations and non-reclaim locking requests (i.e., other LOCK and OPEN operations) with an error of NFS4ERR_GRACE, unless it can guarantee that these may be done safely, as described below.¶

The grace period may last until all clients that are known to possibly have had locks have done a global RECLAIM_COMPLETE operation, indicating that they have finished reclaiming the locks they held before the server restart. This means that a client that has done a RECLAIM_COMPLETE must be prepared to receive an NFS4ERR_GRACE when attempting to acquire new locks. In order for the server to know that all clients with possible prior lock state have done a RECLAIM_COMPLETE, the server must maintain in stable storage a list clients that may have such locks. The server may also terminate the grace period before all clients have done a global RECLAIM_COMPLETE. The server SHOULD NOT terminate the grace period before a time equal to the lease period in order to give clients an opportunity to find out about the server restart, as a result of sending requests on associated sessions with a frequency governed by the lease time. Note that when a client does not send such requests (or they are sent by the client but not received by the server), it is possible for the grace period to expire before the client finds out that the server restart has occurred.¶

Some additional time in order to allow a client to establish a new client ID and session and to effect lock reclaims may be added to the lease time. Note that analogous rules apply to file system-specific grace periods discussed in Section 15.11.9.¶

If the server can reliably determine that granting a non-reclaim request will not conflict with reclamation of locks by other clients, the NFS4ERR_GRACE error does not have to be returned even within the grace period, although NFS4ERR_GRACE must always be returned to clients attempting a non-reclaim lock request before doing their own global RECLAIM_COMPLETE. For the server to be able to service READ and WRITE operations during the grace period, it must again be able to guarantee that no possible conflict could arise between a potential reclaim locking request and the READ or WRITE operation. If the server is unable to offer that guarantee, the NFS4ERR_GRACE error must be returned to the client.¶

For a server to provide simple, valid handling during the grace period, the easiest method is to simply reject all non-reclaim locking requests and READ and WRITE operations by returning the NFS4ERR_GRACE error. However, a server may keep information about granted locks in stable storage. With this information, the server could determine if a locking, READ or WRITE operation can be safely processed.¶

For example, if the server maintained on stable storage summary information on whether mandatory locks exist, either mandatory byte-range locks, or share reservations specifying deny modes, many requests could be allowed during the grace period. If it is known that no such share reservations exist, OPEN request that do not specify deny modes may be safely granted. If, in addition, it is known that no mandatory byte-range locks exist, either through information stored on stable storage or simply because the server does not support such locks, READ and WRITE operations may be safely processed during the grace period. Another important case is where it is known that no mandatory byte-range locks exist, either because the server does not provide support for them or because their absence is known from persistently recorded data. In this case, READ and WRITE operations specifying stateids derived from reclaim-type operations may be validly processed during the grace period because of the fact that the valid reclaim ensures that no lock subsequently granted can prevent the I/O.¶

To reiterate, for a server that allows non-reclaim lock and I/O requests to be processed during the grace period, it MUST determine that no lock subsequently reclaimed will be rejected and that no lock subsequently reclaimed would have prevented any I/O operation processed during the grace period.¶

Clients should be prepared for the return of NFS4ERR_GRACE errors for non-reclaim lock and I/O requests. In this case, the client should employ a retry mechanism for the request. A delay (on the order of several seconds) between retries should be used to avoid overwhelming the server. Further discussion of the general issue is included in [55]. The client must account for the server that can perform I/O and non-reclaim locking requests within the grace period as well as those that cannot do so.¶

A reclaim-type locking request outside the server's grace period can only succeed if the server can guarantee that no conflicting lock or I/O request has been granted since restart.¶

A server may, upon restart, establish a new value for the lease period. Therefore, clients should, once a new client ID is established, refetch the lease_time attribute and use it as the basis for lease renewal for the lease associated with that server. However, the server must establish, for this restart event, a grace period at least as long as the lease period for the previous server instantiation. This allows the client state obtained during the previous server instance to be reliably re-established.¶

The possibility exists that, because of server configuration events, the client will be communicating with a server different than the one on which the locks were obtained, as shown by the combination of eir_server_scope and eir_server_owner. This leads to the issue of if and when the client should attempt to reclaim locks previously obtained on what is being reported as a different server. The rules to resolve this question are as follows:¶

If the server scope is different, the client should not attempt to reclaim locks. In this situation, no lock reclaim is possible. Any attempt to re-obtain the locks with non-reclaim operations is problematic since there is no guarantee that the existing filehandles will be recognized by the new server, or that if recognized, they denote the same objects. It is best to treat the locks as having been revoked by the reconfiguration event.¶
If the server scope is the same, the client should attempt to reclaim locks, even if the eir_server_owner value is different. In this situation, it is the responsibility of the server to return NFS4ERR_NO_GRACE if it cannot provide correct support for lock reclaim operations, including the prevention of edge conditions.¶

The eir_server_owner field is not used in making this determination. Its function is to specify trunking possibilities for the client (see Section 7.5) and not to control lock reclaim.¶

12.4.2.1.1. Security Considerations for State Reclaim

During the grace period, a client can reclaim state that it believes or asserts it had before the server restarted. Unless the server maintained a complete record of all the state the client had, the server has little choice but to trust the client. (Of course, if the server maintained a complete record, then it would not have to force the client to reclaim state after server restart.) While the server has to trust the client to tell the truth, the negative consequences for security are limited to enabling denial-of-service attacks in situations in which AUTH_SYS is supported. The fundamental rule for the server when processing reclaim requests is that it MUST NOT grant the reclaim if an equivalent non-reclaim request would not be granted during steady state due to access control or access conflict issues. For example, an OPEN request during a reclaim will be refused with NFS4ERR_ACCESS if the principal making the request does not have access to open the file according to the acl, dacl, or mode attributes on the file.¶

Nonetheless, it is possible that a client operating in error or maliciously could, during reclaim, prevent another client from reclaiming access to state. For example, an attacker could send an OPEN reclaim operation with a deny mode that prevents another client from reclaiming the OPEN state it had before the server restarted. The attacker could perform the same denial of service during steady state prior to server restart, as long as the attacker had permissions. Given that the attack vectors are equivalent, the grace period does not offer any additional opportunity for denial of service, and any concerns about this attack vector, whether during grace or steady state, are addressed the same way: use RPCSEC_GSS for authentication and limit access to the file only to principals that the owner of the file trusts.¶

Note that if prior to restart the server had client IDs with the EXCHGID4_FLAG_BIND_PRINC_STATEID (Section 22.35) capability set, then the server SHOULD record in stable storage the client owner and the principal that established the client ID via EXCHANGE_ID. If the server does not, then there is a risk a client will be unable to reclaim state if it does not have a credential for a principal that was originally authorized to establish the state.¶

12.4.3. Network Partitions and Recovery

If the duration of a network partition is greater than the lease period provided by the server, the server will not have received a lease renewal from the client. If this occurs, the server may free all locks held for the client or it may allow the lock state to remain for a considerable period, subject to the constraint that if a request for a conflicting lock is made, locks associated with an expired lease do not prevent such a conflicting lock from being granted but MUST be revoked as necessary so as to avoid interfering with such conflicting requests.¶

If the server chooses to delay freeing of lock state until there is a conflict, it may either free all of the client's locks once there is a conflict or it may only revoke the minimum set of locks necessary to allow conflicting requests. When it adopts the finer-grained approach, it must revoke all locks associated with a given stateid, even if the conflict is with only a subset of locks.¶

When the server chooses to free all of a client's lock state, either immediately upon lease expiration or as a result of the first attempt to obtain a conflicting a lock, the server may report the loss of lock state in a number of ways.¶

The server may choose to invalidate the session and the associated client ID. In this case, once the client can communicate with the server, it will receive an NFS4ERR_BADSESSION error. Upon attempting to create a new session, it would get an NFS4ERR_STALE_CLIENTID. Upon creating the new client ID and new session, the client will attempt to reclaim locks. Normally, the server will not allow the client to reclaim locks, because the server will not be in its recovery grace period.¶

Another possibility is for the server to maintain the session and client ID but for all stateids held by the client to become invalid or stale. Once the client can reach the server after such a network partition, the status returned by the SEQUENCE operation will indicate a loss of locking state; i.e., the flag SEQ4_STATUS_EXPIRED_ALL_STATE_REVOKED will be set in sr_status_flags. In addition, all I/O submitted by the client with the now invalid stateids will fail with the server returning the error NFS4ERR_EXPIRED. Once the client learns of the loss of locking state, it will suitably notify the applications that held the invalidated locks. The client should then take action to free invalidated stateids, either by establishing a new client ID using a new verifier or by doing a FREE_STATEID operation to release each of the invalidated stateids.¶

When the server adopts a finer-grained approach to revocation of locks when a client's lease has expired, only a subset of stateids will normally become invalid during a network partition. When the client can communicate with the server after such a network partition heals, the status returned by the SEQUENCE operation will indicate a partial loss of locking state (SEQ4_STATUS_EXPIRED_SOME_STATE_REVOKED). In addition, operations, including I/O submitted by the client, with the now invalid stateids will fail with the server returning the error NFS4ERR_EXPIRED. Once the client learns of the loss of locking state, it will use the TEST_STATEID operation on all of its stateids to determine which locks have been lost and then suitably notify the applications that held the invalidated locks. The client can then release the invalidated locking state and acknowledge the revocation of the associated locks by doing a FREE_STATEID operation on each of the invalidated stateids.¶

When a network partition is combined with a server restart, there are edge conditions that place requirements on the server in order to avoid silent data corruption following the server restart. Two of these edge conditions are known, and are discussed below.¶

The first edge condition arises as a result of the scenarios such as the following:¶

Client A acquires a lock.¶
Client A and server experience mutual network partition, such that client A is unable to renew its lease.¶
Client A's lease expires, and the server releases the lock.¶
Client B acquires a lock that would have conflicted with that of client A.¶
Client B releases its lock.¶
Server restarts.¶
Network partition between client A and server heals.¶
Client A connects to a new server instance and finds out about server restart.¶
Client A reclaims its lock within the server's grace period.¶

Thus, at the final step, the server has erroneously granted client A's lock reclaim. If client B modified the object the lock was protecting, client A will experience object corruption.¶

The second known edge condition arises in situations such as the following:¶

Client A acquires one or more locks.¶
Server restarts.¶
Client A and server experience mutual network partition, such that client A is unable to reclaim all of its locks within the grace period.¶
Server's reclaim grace period ends. Client A has either no locks or an incomplete set of locks known to the server.¶
Client B acquires a lock that would have conflicted with a lock of client A that was not reclaimed.¶
Client B releases the lock.¶
Server restarts a second time.¶
Network partition between client A and server heals.¶
Client A connects to new server instance and finds out about server restart.¶
Client A reclaims its lock within the server's grace period.¶

As with the first edge condition, the final step of the scenario of the second edge condition has the server erroneously granting client A's lock reclaim.¶

Solving the first and second edge conditions requires either that the server always assumes after it restarts that some edge condition occurs, and thus returns NFS4ERR_NO_GRACE for all reclaim attempts, or that the server record some information in stable storage. The amount of information the server records in stable storage is in inverse proportion to how harsh the server intends to be whenever edge conditions arise. The server that is completely tolerant of all edge conditions will record in stable storage every lock that is acquired, removing the lock record from stable storage only when the lock is released. For the two edge conditions discussed above, the harshest a server can be, and still support a grace period for reclaims, requires that the server record in stable storage some minimal information. For example, a server implementation could, for each client, save in stable storage a record containing:¶

the co_ownerid field from the client_owner4 presented in the EXCHANGE_ID operation.¶
a boolean that indicates if the client's lease expired or if there was administrative intervention (see Section 12.5) to revoke a byte-range lock, share reservation, or delegation and there has been no acknowledgment, via FREE_STATEID, of such revocation.¶
a boolean that indicates whether the client may have locks that it believes to be reclaimable in situations in which the grace period was terminated, making the server's view of lock reclaimability suspect. The server will set this for any client record in stable storage where the client has not done a suitable RECLAIM_COMPLETE (global or file system-specific depending on the target of the lock request) before it grants any new (i.e., not reclaimed) lock to any client.¶

Assuming the above record keeping, for the first edge condition, after the server restarts, the record that client A's lease expired means that another client could have acquired a conflicting byte-range lock, share reservation, or delegation. Hence, the server must reject a reclaim from client A with the error NFS4ERR_NO_GRACE.¶

For the second edge condition, after the server restarts for a second time, the indication that the client had not completed its reclaims at the time at which the grace period ended means that the server must reject a reclaim from client A with the error NFS4ERR_NO_GRACE.¶

When either edge condition occurs, the client's attempt to reclaim locks will result in the error NFS4ERR_NO_GRACE. When this is received, or after the client restarts with no lock state, the client will send a global RECLAIM_COMPLETE. When the RECLAIM_COMPLETE is received, the server and client are again in agreement regarding reclaimable locks and both booleans in persistent storage can be reset, to be set again only when there is a subsequent event that causes lock reclaim operations to be questionable.¶

Regardless of the level and approach to record keeping, the server MUST implement one of the following strategies (which apply to reclaims of share reservations, byte-range locks, and delegations):¶

Reject all reclaims with NFS4ERR_NO_GRACE. This is extremely unforgiving, but necessary if the server does not record lock state in stable storage.¶
Record sufficient state in stable storage such that all known edge conditions involving server restart, including the two noted in this section, are detected. It is acceptable to erroneously recognize an edge condition and not allow a reclaim, when, with sufficient knowledge, it would be allowed. The error the server would return in this case is NFS4ERR_NO_GRACE. Note that it is not known if there are other edge conditions.¶

In the event that, after a server restart, the server determines there is unrecoverable damage or corruption to the information in stable storage, then for all clients and/or locks that may be affected, the server MUST return NFS4ERR_NO_GRACE.¶

A mandate for the client's handling of the NFS4ERR_NO_GRACE error is outside the scope of this specification, since the strategies for such handling are very dependent on the client's operating environment. However, one potential approach is described below.¶

When the client receives NFS4ERR_NO_GRACE, it could examine the change attribute of the objects for which the client is trying to reclaim state, and use that to determine whether to re-establish the state via normal OPEN or LOCK operations. This is acceptable provided that the client's operating environment allows it. In other words, the client implementor is advised to document for his users the behavior. The client could also inform the application that its byte-range lock or share reservations (whether or not they were delegated) have been lost, such as via a UNIX signal, a Graphical User Interface (GUI) pop-up window, etc. See Section 14.5 for a discussion of what the client should do for dealing with unreclaimed delegations on client state.¶

For further discussion of revocation of locks, see Section 12.5.¶

12.5. Server Revocation of Locks

At any point, the server can revoke locks held by a client, and the client must be prepared for this event. When the client detects that its locks have been or may have been revoked, the client is responsible for validating the state information between itself and the server. Validating locking state for the client means that it must verify or reclaim state for each lock currently held.¶

The first occasion of lock revocation is upon server restart. Note that this includes situations in which sessions are persistent and locking state is lost. In this class of instances, the client will receive an error (NFS4ERR_STALE_CLIENTID) on an operation that takes client ID, usually as part of recovery in response to a problem with the current session), and the client will proceed with normal crash recovery as described in the Section 12.4.2.1.¶

The second occasion of lock revocation is the inability to renew the lease before expiration, as discussed in Section 12.4.3. While this is considered a rare or unusual event, the client must be prepared to recover. The server is responsible for determining the precise consequences of the lease expiration, informing the client of the scope of the lock revocation decided upon. The client then uses the status information provided by the server in the SEQUENCE results (field sr_status_flags, see Section 22.46.3) to synchronize its locking state with that of the server, in order to recover.¶

The third occasion of lock revocation can occur as a result of revocation of locks within the lease period, either because of administrative intervention or because a recallable lock (a delegation or layout) was not returned within the lease period after having been recalled. While these are considered rare events, they are possible, and the client must be prepared to deal with them. When either of these events occurs, the client finds out about the situation through the status returned by the SEQUENCE operation. Any use of stateids associated with locks revoked during the lease period will receive the error NFS4ERR_ADMIN_REVOKED or NFS4ERR_DELEG_REVOKED, as appropriate.¶

In all situations in which a subset of locking state may have been revoked, which include all cases in which locking state is revoked within the lease period, it is up to the client to determine which locks have been revoked and which have not. It does this by using the TEST_STATEID operation on the appropriate set of stateids. Once the set of revoked locks has been determined, the applications can be notified, and the invalidated stateids can be freed and lock revocation acknowledged by using FREE_STATEID.¶

12.6. Short and Long Leases

When determining the time period for the server lease, the usual lease trade-offs apply. A short lease is good for fast server recovery at a cost of increased operations to effect lease renewal (when there are no other operations during the period to effect lease renewal as a side effect). A long lease is certainly kinder and gentler to servers trying to handle very large numbers of clients. The number of extra requests to effect lock renewal drops in inverse proportion to the lease time. The disadvantages of a long lease include the possibility of slower recovery after certain failures. After server failure, a longer grace period may be required when some clients do not promptly reclaim their locks and do a global RECLAIM_COMPLETE. In the event of client failure, the longer period for a lease to expire will force conflicting requests to wait longer.¶

A long lease is practical if the server can store lease state in stable storage. Upon recovery, the server can reconstruct the lease state from its stable storage and continue operation with its clients.¶

12.7. Clocks, Propagation Delay, and Calculating Lease Expiration

To avoid the need for synchronized clocks, lease times are granted by the server as a time delta. However, there is a requirement that the client and server clocks do not drift excessively over the duration of the lease. There is also the issue of propagation delay across the network, which could easily be several hundred milliseconds, as well as the possibility that requests will be lost and need to be retransmitted.¶

To take propagation delay into account, the client should subtract it from lease times (e.g., if the client estimates the one-way propagation delay as 200 milliseconds, then it can assume that the lease is already 200 milliseconds old when it gets it). In addition, it will take another 200 milliseconds to get a response back to the server. So the client must send a lease renewal or write data back to the server at least 400 milliseconds before the lease would expire. If the propagation delay varies over the life of the lease (e.g., the client is on a mobile host), the client will need to continuously subtract the increase in propagation delay from the lease times.¶

The server's lease period configuration should take into account the network distance of the clients that will be accessing the server's resources. It is expected that the lease period will take into account the network propagation delays and other network delay factors for the client population. Since the protocol does not allow for an automatic method to determine an appropriate lease period, the server's administrator may have to tune the lease period.¶

12.8. Obsolete Locking Infrastructure from NFSv4.0

There are a number of operations and fields within existing operations that no longer have a function in NFSv4.1. In one way or another, these changes are all due to the implementation of sessions that provide client context and exactly once semantics as a base feature of the protocol, separate from locking itself.¶

The following NFSv4.0 operations MUST NOT be implemented in NFSv4.1. The server MUST return NFS4ERR_NOTSUPP if these operations are found in an NFSv4.1 COMPOUND.¶

SETCLIENTID since its function has been replaced by EXCHANGE_ID.¶
SETCLIENTID_CONFIRM since client ID confirmation now happens by means of CREATE_SESSION.¶
OPEN_CONFIRM because state-owner-based seqids have been replaced by the sequence ID in the SEQUENCE operation.¶
RELEASE_LOCKOWNER because lock-owners with no associated locks do not have any sequence-related state and so can be deleted by the server at will.¶
RENEW because every SEQUENCE operation for a session causes lease renewal, making a separate operation superfluous.¶

Also, there are a number of fields, present in existing operations, related to locking that have no use in minor version 1. They were used in minor version 0 to perform functions now provided in a different fashion.¶

Sequence ids used to sequence requests for a given state-owner and to provide retry protection, now provided via sessions.¶
Client IDs used to identify the client associated with a given request. Client identification is now available using the client ID associated with the current session, without needing an explicit client ID field.¶

Such vestigial fields in existing operations have no function in NFSv4.1 and are ignored by the server. Note that client IDs in operations new to NFSv4.1 (such as CREATE_SESSION and DESTROY_CLIENTID) are not ignored.¶

To support Win32 share reservations, it is necessary to provide operations that atomically open or create files. Having a separate share/unshare operation would not allow correct implementation of the Win32 OpenFile API. In order to correctly implement share semantics, the previous NFS protocol mechanisms used when a file is opened or created (LOOKUP, CREATE, ACCESS) need to be replaced. The NFSv4.1 protocol defines an OPEN operation that is capable of atomically looking up, creating, and locking a file on the server.¶

13.1. Opens and Byte-Range Locks

It is assumed that manipulating a byte-range lock is rare when compared to READ and WRITE operations. It is also assumed that server restarts and network partitions are relatively rare. Therefore, it is important that the READ and WRITE operations have a lightweight mechanism to indicate if they possess a held lock. A LOCK operation contains the heavyweight information required to establish a byte-range lock and uniquely define the owner of the lock.¶

13.1.1. State-Owner Definition

When opening a file or requesting a byte-range lock, the client must specify an identifier that represents the owner of the requested lock. This identifier is in the form of a state-owner, represented in the protocol by a state_owner4, a variable-length opaque array that, when concatenated with the current client ID, uniquely defines the owner of a lock managed by the client. This may be a thread ID, process ID, or other unique value.¶

Owners of opens and owners of byte-range locks are separate entities and remain separate even if the same opaque arrays are used to designate owners of each. The protocol distinguishes between open-owners (represented by open_owner4 structures) and lock-owners (represented by lock_owner4 structures).¶

Each open is associated with a specific open-owner while each byte-range lock is associated with a lock-owner and an open-owner, the latter being the open-owner associated with the open file under which the LOCK operation was done. Delegations and layouts, on the other hand, are not associated with a specific owner but are associated with the client as a whole (identified by a client ID).¶

13.1.2. Use of the Stateid and Locking

All READ, WRITE, and SETATTR operations contain a stateid. For the purposes of this section, SETATTR operations that change the size attribute of a file are treated as if they are writing the area between the old and new sizes (i.e., the byte-range truncated or added to the file by means of the SETATTR), even where SETATTR is not explicitly mentioned in the text. The stateid passed to one of these operations must be one that represents an open, a set of byte-range locks, or a delegation, or it may be a special stateid representing anonymous access or the special bypass stateid.¶

If the state-owner performs a READ or WRITE operation in a situation in which it has established a byte-range lock or share reservation on the server (any OPEN constitutes a share reservation), the stateid (previously returned by the server) must be used to indicate what locks, including both byte-range locks and share reservations, are held by the state-owner. If no state is established by the client, either a byte-range lock or a share reservation, a special stateid for anonymous state (zero as the value for "other" and "seqid") is used. (See Section 12.2.3 for a description of 'special' stateids in general.) Regardless of whether a stateid for anonymous state or a stateid returned by the server is used, if there is a conflicting share reservation or mandatory byte-range lock held on the file, the server MUST refuse to service the READ or WRITE operation.¶

Share reservations are established by OPEN operations and by their nature are mandatory in that when the OPEN denies READ or WRITE operations, that denial results in such operations being rejected with error NFS4ERR_LOCKED. Byte-range locks may be implemented by the server as either mandatory or advisory, or the choice of mandatory or advisory behavior may be determined by the server on the basis of the file being accessed (for example, some UNIX-based servers support a "mandatory lock bit" on the mode attribute such that if set, byte-range locks are required on the file before I/O is possible). When byte-range locks are advisory, they only prevent the granting of conflicting lock requests and have no effect on READs or WRITEs. Mandatory byte-range locks, however, prevent conflicting I/O operations. When they are attempted, they are rejected with NFS4ERR_LOCKED. When the client gets NFS4ERR_LOCKED on a file for which it knows it has the proper share reservation, it will need to send a LOCK operation on the byte-range of the file that includes the byte-range the I/O was to be performed on, with an appropriate locktype field of the LOCK operation's arguments (i.e., READ*_LT for a READ operation, WRITE*_LT for a WRITE operation).¶

Note that for UNIX environments that support mandatory byte-range locking, the distinction between advisory and mandatory locking is subtle. In fact, advisory and mandatory byte-range locks are exactly the same as far as the APIs and requirements on implementation. If the mandatory lock attribute is set on the file, the server checks to see if the lock-owner has an appropriate shared (READ_LT) or exclusive (WRITE_LT) byte-range lock on the byte-range it wishes to READ from or WRITE to. If there is no appropriate lock, the server checks if there is a conflicting lock (which can be done by attempting to acquire the conflicting lock on behalf of the lock-owner, and if successful, release the lock after the READ or WRITE operation is done), and if there is, the server returns NFS4ERR_LOCKED.¶

For Windows environments, byte-range locks are always mandatory, so the server always checks for byte-range locks during I/O requests.¶

Thus, the LOCK operation does not need to distinguish between advisory and mandatory byte-range locks. It is the server's processing of the READ and WRITE operations that introduces the distinction.¶

Every stateid that is validly passed to READ, WRITE, or SETATTR, with the exception of special stateid values, defines an access mode for the file (i.e., OPEN4_SHARE_ACCESS_READ, OPEN4_SHARE_ACCESS_WRITE, or OPEN4_SHARE_ACCESS_BOTH).¶

For stateids associated with opens, this is the mode defined by the original OPEN that caused the allocation of the OPEN stateid and as modified by subsequent OPENs and OPEN_DOWNGRADEs for the same open-owner/file pair.¶
For stateids returned by byte-range LOCK operations, the appropriate mode is the access mode for the OPEN stateid associated with the lock set represented by the stateid.¶
For delegation stateids, the access mode is based on the type of delegation.¶

When a READ, WRITE, or SETATTR (that specifies the size attribute) operation is done, the operation is subject to checking against the access mode to verify that the operation is appropriate given the stateid with which the operation is associated.¶

In the case of WRITE-type operations (i.e., WRITEs and SETATTRs that set size), the server MUST verify that the access mode allows writing and MUST return an NFS4ERR_OPENMODE error if it does not. In the case of READ, the server may perform the corresponding check on the access mode, or it may choose to allow READ on OPENs for OPEN4_SHARE_ACCESS_WRITE, to accommodate clients whose WRITE implementation may unavoidably do reads (e.g., due to buffer cache constraints). However, even if READs are allowed in these circumstances, the server MUST still check for locks that conflict with the READ (e.g., another OPEN specified OPEN4_SHARE_DENY_READ or OPEN4_SHARE_DENY_BOTH). Note that a server that does enforce the access mode check on READs need not explicitly check for conflicting share reservations since the existence of OPEN for OPEN4_SHARE_ACCESS_READ guarantees that no conflicting share reservation can exist.¶

The READ bypass special stateid (all bits of "other" and "seqid" set to one) indicates a desire to bypass locking checks. The server MAY allow READ operations to bypass locking checks at the server, when this special stateid is used. However, WRITE operations with this special stateid value MUST NOT bypass locking checks and are treated exactly the same as if a special stateid for anonymous state were used.¶

A lock may not be granted while a READ or WRITE operation using one of the special stateids is being performed and the scope of the lock to be granted would conflict with the READ or WRITE operation. This can occur when:¶

A mandatory byte-range lock is requested with a byte-range that conflicts with the byte-range of the READ or WRITE operation. For the purposes of this paragraph, a conflict occurs when a shared lock is requested and a WRITE operation is being performed, or an exclusive lock is requested and either a READ or a WRITE operation is being performed.¶
A share reservation is requested that denies reading and/or writing and the corresponding operation is being performed.¶
A delegation is to be granted and the delegation type would prevent the I/O operation, i.e., READ and WRITE conflict with an OPEN_DELEGATE_WRITE delegation and WRITE conflicts with an OPEN_DELEGATE_READ delegation.¶

When a client holds a delegation, it needs to ensure that the stateid sent conveys the association of operation with the delegation, to avoid the delegation from being avoidably recalled. When the delegation stateid, a stateid open associated with that delegation, or a stateid representing byte-range locks derived from such an open is used, the server knows that the READ, WRITE, or SETATTR does not conflict with the delegation but is sent under the aegis of the delegation. Even though it is possible for the server to determine from the client ID (via the session ID) that the client does in fact have a delegation, the server is not obliged to check this, so using a special stateid can result in avoidable recall of the delegation.¶

13.2. Lock Ranges

The protocol allows a lock-owner to request a lock with a byte-range and then either upgrade, downgrade, or unlock a sub-range of the initial lock, or a byte-range that overlaps -- fully or partially -- either with that initial lock or a combination of a set of existing locks for the same lock-owner. It is expected that this will be an uncommon type of request. In any case, servers or server file systems may not be able to support sub-range lock semantics. In the event that a server receives a locking request that represents a sub-range of current locking state for the lock-owner, the server is allowed to return the error NFS4ERR_LOCK_RANGE to signify that it does not support sub-range lock operations. Therefore, the client should be prepared to receive this error and, if appropriate, report the error to the requesting application.¶

The client is discouraged from combining multiple independent locking ranges that happen to be adjacent into a single request since the server may not support sub-range requests for reasons related to the recovery of byte-range locking state in the event of server failure. As discussed in Section 12.4.2, the server may employ certain optimizations during recovery that work effectively only when the client's behavior during lock recovery is similar to the client's locking behavior prior to server failure.¶

13.3. Upgrading and Downgrading Locks

If a client has a WRITE_LT lock on a byte-range, it can request an atomic downgrade of the lock to a READ_LT lock via the LOCK operation, by setting the type to READ_LT. If the server supports atomic downgrade, the request will succeed. If not, it will return NFS4ERR_LOCK_NOTSUPP. The client should be prepared to receive this error and, if appropriate, report the error to the requesting application.¶

If a client has a READ_LT lock on a byte-range, it can request an atomic upgrade of the lock to a WRITE_LT lock via the LOCK operation by setting the type to WRITE_LT or WRITEW_LT. If the server does not support atomic upgrade, it will return NFS4ERR_LOCK_NOTSUPP. If the upgrade can be achieved without an existing conflict, the request will succeed. Otherwise, the server will return either NFS4ERR_DENIED or NFS4ERR_DEADLOCK. The error NFS4ERR_DEADLOCK is returned if the client sent the LOCK operation with the type set to WRITEW_LT and the server has detected a deadlock. The client should be prepared to receive such errors and, if appropriate, report the error to the requesting application.¶

13.4. Stateid Seqid Values and Byte-Range Locks

When a LOCK or LOCKU operation is performed, the stateid returned has the same "other" value as the argument's stateid, and a "seqid" value that is incremented (relative to the argument's stateid) to reflect the occurrence of the LOCK or LOCKU operation. The server MUST increment the value of the "seqid" field whenever there is any change to the locking status of any byte offset as described by any of the locks covered by the stateid. A change in locking status includes a change from locked to unlocked or the reverse or a change from being locked for READ_LT to being locked for WRITE_LT or the reverse.¶

When there is no such change, as, for example, when a range already locked for WRITE_LT is locked again for WRITE_LT, the server MAY increment the "seqid" value.¶

13.5. Issues with Multiple Open-Owners

When the same file is opened by multiple open-owners, a client will have multiple OPEN stateids for that file, each associated with a different open-owner. In that case, there can be multiple LOCK and LOCKU requests for the same lock-owner sent using the different OPEN stateids, and so a situation may arise in which there are multiple stateids, each representing byte-range locks on the same file and held by the same lock-owner but each associated with a different open-owner.¶

In such a situation, the locking status of each byte (i.e., whether it is locked, the READ_LT or WRITE_LT type of the lock, and the lock-owner holding the lock) MUST reflect the last LOCK or LOCKU operation done for the lock-owner in question, independent of the stateid through which the request was sent.¶

When a byte is locked by the lock-owner in question, the open-owner to which that byte-range lock is assigned SHOULD be that of the open-owner associated with the stateid through which the last LOCK of that byte was done. When there is a change in the open-owner associated with locks for the stateid through which a LOCK or LOCKU was done, the "seqid" field of the stateid MUST be incremented, even if the locking, in terms of lock-owners has not changed. When there is a change to the set of locked bytes associated with a different stateid for the same lock-owner, i.e., associated with a different open-owner, the "seqid" value for that stateid MUST NOT be incremented.¶

13.6. Blocking Locks

Some clients require the support of blocking locks. While NFSv4.1 provides a callback when a previously unavailable lock becomes available, this is an OPTIONAL feature and clients cannot depend on its presence. Clients need to be prepared to continually poll for the lock. This presents a fairness problem. Two of the lock types, READW_LT and WRITEW_LT, are used to indicate to the server that the client is requesting a blocking lock. When the callback is not used, the server should maintain an ordered list of pending blocking locks. When the conflicting lock is released, the server may wait for the period of time equal to lease_time for the first waiting client to re-request the lock. After the lease period expires, the next waiting client request is allowed the lock. Clients are required to poll at an interval sufficiently small that it is likely to acquire the lock in a timely manner. The server is not required to maintain a list of pending blocked locks as it is used to increase fairness and not correct operation. Because of the unordered nature of crash recovery, storing of lock state to stable storage would be required to guarantee ordered granting of blocking locks.¶

Servers may also note the lock types and delay returning denial of the request to allow extra time for a conflicting lock to be released, allowing a successful return. In this way, clients can avoid the burden of needless frequent polling for blocking locks. The server should take care in the length of delay in the event the client retransmits the request.¶

If a server receives a blocking LOCK operation, denies it, and then later receives a nonblocking request for the same lock, which is also denied, then it should remove the lock in question from its list of pending blocking locks. Clients should use such a nonblocking request to indicate to the server that this is the last time they intend to poll for the lock, as may happen when the process requesting the lock is interrupted. This is a courtesy to the server, to prevent it from unnecessarily waiting a lease period before granting other LOCK operations. However, clients are not required to perform this courtesy, and servers must not depend on them doing so. Also, clients must be prepared for the possibility that this final locking request will be accepted.¶

When a server indicates, via the flag OPEN4_RESULT_MAY_NOTIFY_LOCK, that CB_NOTIFY_LOCK callbacks might be done for the current open file, the client should take notice of this, but, since this is a hint, cannot rely on a CB_NOTIFY_LOCK always being done. A client may reasonably reduce the frequency with which it polls for a denied lock, since the greater latency that might occur is likely to be eliminated given a prompt callback, but it still needs to poll. When it receives a CB_NOTIFY_LOCK, it should promptly try to obtain the lock, but it should be aware that other clients may be polling and that the server is under no obligation to reserve the lock for that particular client.¶

A share reservation is a mechanism to control access to a file. It is a separate and independent mechanism from byte-range locking. When a client opens a file, it sends an OPEN operation to the server specifying the type of access required (READ, WRITE, or BOTH) and the type of access to deny others (OPEN4_SHARE_DENY_NONE, OPEN4_SHARE_DENY_READ, OPEN4_SHARE_DENY_WRITE, or OPEN4_SHARE_DENY_BOTH). If the OPEN fails, the client will fail the application's open request.¶

Pseudo-code definition of the semantics:¶

        if (request.access == 0) {
          return (NFS4ERR_INVAL)
        } else {
          if ((request.access & file_state.deny)) ||
             (request.deny & file_state.access)) {
            return (NFS4ERR_SHARE_DENIED)
        }
        return (NFS4ERR_OK);

When doing this checking of share reservations on OPEN, the current file_state used in the algorithm includes bits that reflect all current opens, including those for the open-owner making the new OPEN request.¶

The constants used for the OPEN and OPEN_DOWNGRADE operations for the access and deny fields are as follows:¶

const OPEN4_SHARE_ACCESS_READ   = 0x00000001;
const OPEN4_SHARE_ACCESS_WRITE  = 0x00000002;
const OPEN4_SHARE_ACCESS_BOTH   = 0x00000003;

const OPEN4_SHARE_DENY_NONE     = 0x00000000;
const OPEN4_SHARE_DENY_READ     = 0x00000001;
const OPEN4_SHARE_DENY_WRITE    = 0x00000002;
const OPEN4_SHARE_DENY_BOTH     = 0x00000003;

13.8. OPEN/CLOSE Operations

To provide correct share semantics, a client MUST use the OPEN operation to obtain the initial filehandle and indicate the desired access and what access, if any, to deny. Even if the client intends to use a special stateid for anonymous state or READ bypass, it must still obtain the filehandle for the regular file with the OPEN operation so the appropriate share semantics can be applied. Clients that do not have a deny mode built into their programming interfaces for opening a file should request a deny mode of OPEN4_SHARE_DENY_NONE.¶

The OPEN operation with the CREATE flag also subsumes the CREATE operation for regular files as used in previous versions of the NFS protocol. This allows a create with a share to be done atomically.¶

The CLOSE operation removes all share reservations held by the open-owner on that file. If byte-range locks are held, the client SHOULD release all locks before sending a CLOSE operation. The server MAY free all outstanding locks on CLOSE, but some servers may not support the CLOSE of a file that still has byte-range locks held. The server MUST return failure, NFS4ERR_LOCKS_HELD, if any locks would exist after the CLOSE.¶

The LOOKUP operation will return a filehandle without establishing any lock state on the server. Without a valid stateid, the server will assume that the client has the least access. For example, if one client opened a file with OPEN4_SHARE_DENY_BOTH and another client accesses the file via a filehandle obtained through LOOKUP, the second client could only read the file using the special read bypass stateid. The second client could not WRITE the file at all because it would not have a valid stateid from OPEN and the special anonymous stateid would not be allowed access.¶

13.9. Open Upgrade and Downgrade

When an OPEN is done for a file and the open-owner for which the OPEN is being done already has the file open, the result is to upgrade the open file status maintained on the server to include the access and deny bits specified by the new OPEN as well as those for the existing OPEN. The result is that there is one open file, as far as the protocol is concerned, and it includes the union of the access and deny bits for all of the OPEN requests completed. The OPEN is represented by a single stateid whose "other" value matches that of the original open, and whose "seqid" value is incremented to reflect the occurrence of the upgrade. The increment is required in cases in which the "upgrade" results in no change to the open mode (e.g., an OPEN is done for read when the existing open file is opened for OPEN4_SHARE_ACCESS_BOTH). Only a single CLOSE will be done to reset the effects of both OPENs. The client may use the stateid returned by the OPEN effecting the upgrade or with a stateid sharing the same "other" field and a seqid of zero, although care needs to be taken as far as upgrades that happen while the CLOSE is pending. Note that the client, when sending the OPEN, may not know that the same file is in fact being opened. The above only applies if both OPENs result in the OPENed object being designated by the same filehandle.¶

When the server chooses to export multiple filehandles corresponding to the same file object and returns different filehandles on two different OPENs of the same file object, the server MUST NOT "OR" together the access and deny bits and coalesce the two open files. Instead, the server must maintain separate OPENs with separate stateids and will require separate CLOSEs to free them.¶

When multiple open files on the client are merged into a single OPEN file object on the server, the close of one of the open files (on the client) may necessitate change of the access and deny status of the open file on the server. This is because the union of the access and deny bits for the remaining opens may be smaller (i.e., a proper subset) than previously. The OPEN_DOWNGRADE operation is used to make the necessary change and the client should use it to update the server so that share reservation requests by other clients are handled properly. The stateid returned has the same "other" field as that passed to the server. The "seqid" value in the returned stateid MUST be incremented, even in situations in which there is no change to the access and deny bits for the file.¶

13.10. Parallel OPENs

Unlike the case of NFSv4.0, in which OPEN operations for the same open-owner are inherently serialized because of the owner-based seqid, multiple OPENs for the same open-owner may be done in parallel. When clients do this, they may encounter situations in which, because of the existence of hard links, two OPEN operations may turn out to open the same file, with a later OPEN performed being an upgrade of the first, with this fact only visible to the client once the operations complete.¶

In this situation, clients may determine the order in which the OPENs were performed by examining the stateids returned by the OPENs. Stateids that share a common value of the "other" field can be recognized as having opened the same file, with the order of the operations determinable from the order of the "seqid" fields, mod any possible wraparound of the 32-bit field.¶

When the possibility exists that the client will send multiple OPENs for the same open-owner in parallel, it may be the case that an open upgrade may happen without the client knowing beforehand that this could happen. Because of this possibility, CLOSEs and OPEN_DOWNGRADEs should generally be sent with a non-zero seqid in the stateid, to avoid the possibility that the status change associated with an open upgrade is not inadvertently lost.¶

13.11. Reclaim of Open and Byte-Range Locks

Special forms of the LOCK and OPEN operations are provided when it is necessary to re-establish byte-range locks or opens after a server failure.¶

To reclaim existing opens, an OPEN operation is performed using a CLAIM_PREVIOUS. Because the client, in this type of situation, will have already opened the file and have the filehandle of the target file, this operation requires that the current filehandle be the target file, rather than a directory, and no file name is specified.¶
To reclaim byte-range locks, a LOCK operation with the reclaim parameter set to true is used.¶

Reclaims of opens associated with delegations are discussed in Section 14.2.1.¶

14. Client-Side Caching

Client-side caching of data, of file attributes, and of file names is essential to providing good performance with the NFS protocol. Providing distributed cache coherence is a difficult problem, and previous versions of the NFS protocol have not attempted it. Instead, several NFS client implementation techniques have been used to reduce the problems that a lack of coherence poses for users. These techniques have not been clearly defined by earlier protocol specifications, and it is often unclear what is valid or invalid client behavior.¶

The NFSv4.1 protocol uses many techniques similar to those that have been used in previous protocol versions. The NFSv4.1 protocol does not provide distributed cache coherence. However, it defines a more limited set of caching guarantees to allow locks and share reservations to be used without destructive interference from client-side caching.¶

In addition, the NFSv4.1 protocol introduces a delegation mechanism, which allows many decisions normally made by the server to be made locally by clients. This mechanism provides efficient support of the common cases where sharing is infrequent or where sharing is read-only.¶

14.1. Performance Challenges for Client-Side Caching

Caching techniques used in previous versions of the NFS protocol have been successful in providing good performance. However, several scalability challenges can arise when those techniques are used with very large numbers of clients. This is particularly true when clients are geographically distributed, which classically increases the latency for cache revalidation requests.¶

The previous versions of the NFS protocol repeat their file data cache validation requests at the time the file is opened. This behavior can have serious performance drawbacks. A common case is one in which a file is only accessed by a single client. Therefore, sharing is infrequent.¶

In this case, repeated references to the server to find that no conflicts exist are expensive. A better option with regards to performance is to allow a client that repeatedly opens a file to do so without reference to the server. This is done until potentially conflicting operations from another client actually occur.¶

A similar situation arises in connection with byte-range locking. Sending LOCK and LOCKU operations as well as the READ and WRITE operations necessary to make data caching consistent with the locking semantics (see Section 14.3.2) can severely limit performance. When locking is used to provide protection against infrequent conflicts, a large penalty is incurred. This penalty may discourage the use of byte-range locking by applications.¶

The NFSv4.1 protocol provides more aggressive caching strategies with the following design goals:¶

Compatibility with a large range of server semantics.¶
Providing the same caching benefits as previous versions of the NFS protocol when unable to support the more aggressive model.¶
Requirements for aggressive caching are organized so that a large portion of the benefit can be obtained even when not all of the requirements can be met.¶

The appropriate requirements for the server are discussed in later sections in which specific forms of caching are covered (see Section 14.4).¶

14.2. Delegation and Callbacks

Recallable delegation of server responsibilities for a file to a client improves performance by avoiding repeated requests to the server in the absence of inter-client conflict. With the use of a "callback" RPC from server to client, a server recalls delegated responsibilities when another client engages in sharing of a delegated file.¶

A delegation is passed from the server to the client, specifying the object of the delegation and the type of delegation. There are different types of delegations, but each type contains a stateid to be used to represent the delegation when performing operations that depend on the delegation. This stateid is similar to those associated with locks and share reservations but differs in that the stateid for a delegation is associated with a client ID and may be used on behalf of all the open-owners for the given client. A delegation is made to the client as a whole and not to any specific process or thread of control within it.¶

The backchannel is established by CREATE_SESSION and BIND_CONN_TO_SESSION, and the client is required to maintain it. Because the backchannel may be down, even temporarily, correct protocol operation does not depend on them. Preliminary testing of backchannel functionality by means of a CB_COMPOUND procedure with a single operation, CB_SEQUENCE, can be used to check the continuity of the backchannel. A server avoids delegating responsibilities until it has determined that the backchannel exists. Because the granting of a delegation is always conditional upon the absence of conflicting access, clients MUST NOT assume that a delegation will be granted and they MUST always be prepared for OPENs, WANT_DELEGATIONs, and GET_DIR_DELEGATIONs to be processed without any delegations being granted.¶

Unlike locks, an operation by a second client to a delegated file will cause the server to recall a delegation through a callback. For individual operations, we will describe, under IMPLEMENTATION, when such operations are required to effect a recall. A number of points should be noted, however.¶

The server is free to recall a delegation whenever it feels it is desirable and may do so even if no operations requiring recall are being done.¶
Operations done outside the NFSv4.1 protocol, due to, for example, access by other protocols, or by local access, also need to result in delegation recall when they make analogous changes to file system data. What is crucial is if the change would invalidate the guarantees provided by the delegation. When this is possible, the delegation needs to be recalled and MUST be returned or revoked before allowing the operation to proceed.¶
The semantics of the file system are crucial in defining when delegation recall is required. If a particular change within a specific implementation causes change to a file attribute, then delegation recall is required, whether that operation has been specifically listed as requiring delegation recall. Again, what is critical is whether the guarantees provided by the delegation are being invalidated.¶

Despite those caveats, the implementation sections for a number of operations describe situations in which delegation recall would be required under some common circumstances:¶

For GETATTR, see Section 22.7.4.¶
For OPEN, see Section 22.16.4.¶
For READ, see Section 22.22.4.¶
For REMOVE, see Section 22.25.4.¶
For RENAME, see Section 22.26.4.¶
For SETATTR, see Section 22.30.4.¶
For WRITE, see Section 22.32.4.¶

On recall, the client holding the delegation needs to flush modified state (such as modified data) to the server and return the delegation. The conflicting request will not be acted on until the recall is complete. The recall is considered complete when the client returns the delegation or the server times its wait for the delegation to be returned and revokes the delegation as a result of the timeout. In the interim, the server will either delay responding to conflicting requests or respond to them with NFS4ERR_DELAY. Following the resolution of the recall, the server has the information necessary to grant or deny the second client's request.¶

At the time the client receives a delegation recall, it may have substantial state that needs to be flushed to the server. Therefore, the server should allow sufficient time for the delegation to be returned since it may involve numerous RPCs to the server. If the server is able to determine that the client is diligently flushing state to the server as a result of the recall, the server may extend the usual time allowed for a recall. However, the time allowed for recall completion should not be unbounded.¶

An example of this is when responsibility to mediate opens on a given file is delegated to a client (see Section 14.4). The server will not know what opens are in effect on the client. Without this knowledge, the server will be unable to determine if the access and deny states for the file allow any particular open until the delegation for the file has been returned.¶

A client failure or a network partition can result in failure to respond to a recall callback. In this case, the server will revoke the delegation, which in turn will render useless any modified state still on the client.¶

14.2.1. Delegation Recovery

There are three situations that delegation recovery needs to deal with:¶

client restart¶
server restart¶
network partition (full or backchannel-only)¶

In the event the client restarts, the failure to renew the lease will result in the revocation of byte-range locks and share reservations. Delegations, however, may be treated a bit differently.¶

There will be situations in which delegations will need to be re-established after a client restarts. The reason for this is that the client may have file data stored locally and this data was associated with the previously held delegations. The client will need to re-establish the appropriate file state on the server.¶

To allow for this type of client recovery, the server MAY extend the period for delegation recovery beyond the typical lease expiration period. This implies that requests from other clients that conflict with these delegations will need to wait. Because the normal recall process may require significant time for the client to flush changed state to the server, other clients need be prepared for delays that occur because of a conflicting delegation. This longer interval would increase the window for clients to restart and consult stable storage so that the delegations can be reclaimed. For OPEN delegations, such delegations are reclaimed using OPEN with a claim type of CLAIM_DELEGATE_PREV or CLAIM_DELEG_PREV_FH (see Sections 14.5 and 22.16 for discussion of OPEN delegation and the details of OPEN, respectively).¶

A server MAY support claim types of CLAIM_DELEGATE_PREV and CLAIM_DELEG_PREV_FH, and if it does, it MUST NOT remove delegations upon a CREATE_SESSION that confirm a client ID created by EXCHANGE_ID. Instead, the server MUST, for a period of time no less than that of the value of the lease_time attribute, maintain the client's delegations to allow time for the client to send CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH requests. The server that supports CLAIM_DELEGATE_PREV and/or CLAIM_DELEG_PREV_FH MUST support the DELEGPURGE operation.¶

When the server restarts, delegations are reclaimed (using the OPEN operation with CLAIM_PREVIOUS) in a similar fashion to byte-range locks and share reservations. However, there is a slight semantic difference. In the normal case, if the server decides that a delegation should not be granted, it performs the requested action (e.g., OPEN) without granting any delegation. For reclaim, the server grants the delegation but a special designation is applied so that the client treats the delegation as having been granted but recalled by the server. Because of this, the client has the duty to write all modified state to the server and then return the delegation. This process of handling delegation reclaim reconciles three principles of the NFSv4.1 protocol:¶

Upon reclaim, a client reporting resources assigned to it by an earlier server instance must be granted those resources.¶
The server has unquestionable authority to determine whether delegations are to be granted and, once granted, whether they are to be continued.¶
The use of callbacks should not be depended upon until the client has proven its ability to receive them.¶

When a client needs to reclaim a delegation and there is no associated open, the client may use the CLAIM_PREVIOUS variant of the WANT_DELEGATION operation. However, since the server is not required to support this operation, an alternative is to reclaim via a dummy OPEN together with the delegation using an OPEN of type CLAIM_PREVIOUS. The dummy open file can be released using a CLOSE to re-establish the original state to be reclaimed, a delegation without an associated open.¶

When a client has more than a single open associated with a delegation, state for those additional opens can be established using OPEN operations of type CLAIM_DELEGATE_CUR. When these are used to establish opens associated with reclaimed delegations, the server MUST allow them when made within the grace period.¶

When a network partition occurs, delegations are subject to freeing by the server when the lease renewal period expires. This is similar to the behavior for locks and share reservations. For delegations, however, the server may extend the period in which conflicting requests are held off. Eventually, the occurrence of a conflicting request from another client will cause revocation of the delegation. A loss of the backchannel (e.g., by later network configuration change) will have the same effect. A recall request will fail and revocation of the delegation will result.¶

A client normally finds out about revocation of a delegation when it uses a stateid associated with a delegation and receives one of the errors NFS4ERR_EXPIRED, NFS4ERR_ADMIN_REVOKED, or NFS4ERR_DELEG_REVOKED. It also may find out about delegation revocation after a client restart when it attempts to reclaim a delegation and receives that same error. Note that in the case of a revoked OPEN_DELEGATE_WRITE delegation, there are issues because data may have been modified by the client whose delegation is revoked and separately by other clients. See Section 14.5.1 for a discussion of such issues. Note also that when delegations are revoked, information about the revoked delegation will be written by the server to stable storage (as described in Section 12.4.3). This is done to deal with the case in which a server restarts after revoking a delegation but before the client holding the revoked delegation is notified about the revocation.¶

14.3. Data Caching

When applications share access to a set of files, they need to be implemented so as to take account of the possibility of conflicting access by another application. This is true whether the applications in question execute on different clients or reside on the same client.¶

Share reservations and byte-range locks are the facilities the NFSv4.1 protocol provides to allow applications to coordinate access by using mutual exclusion facilities. The NFSv4.1 protocol's data caching must be implemented such that it does not invalidate the assumptions on which those using these facilities depend.¶

14.3.1. Data Caching and OPENs

In order to avoid invalidating the sharing assumptions on which applications rely, NFSv4.1 clients should not provide cached data to applications or modify it on behalf of an application when it would not be valid to obtain or modify that same data via a READ or WRITE operation.¶

Furthermore, in the absence of an OPEN delegation (see Section 14.4), two additional rules apply. Note that these rules are obeyed in practice by many NFSv3 clients.¶

First, cached data present on a client must be revalidated after doing an OPEN. Revalidating means that the client fetches the change attribute from the server, compares it with the cached change attribute, and if different, declares the cached data (as well as the cached attributes) as invalid. This is to ensure that the data for the OPENed file is still correctly reflected in the client's cache. This validation must be done at least when the client's OPEN operation includes a deny of OPEN4_SHARE_DENY_WRITE or OPEN4_SHARE_DENY_BOTH, thus terminating a period in which other clients may have had the opportunity to open the file with OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH access. Clients may choose to do the revalidation more often (i.e., at OPENs specifying a deny mode of OPEN4_SHARE_DENY_NONE) to parallel the NFSv3 protocol's practice for the benefit of users assuming this degree of cache revalidation.¶

Since the change attribute is updated for data and metadata modifications, some client implementors may be tempted to use the time_modify attribute and not the change attribute to validate cached data, so that metadata changes do not spuriously invalidate clean data. The implementor is cautioned in this approach. The change attribute is guaranteed to change for each update to the file, whereas time_modify is guaranteed to change only at the granularity of the time_delta attribute. Use by the client's data cache validation logic of time_modify and not change runs the risk of the client incorrectly marking stale data as valid. Thus, any cache validation approach by the client MUST include the use of the change attribute.¶
Second, modified data must be flushed to the server before closing a file OPENed for OPEN4_SHARE_ACCESS_WRITE. This is complementary to the first rule. If the data is not flushed at CLOSE, the revalidation done after the client OPENs a file is unable to achieve its purpose. The other aspect to flushing the data before close is that the data must be committed to stable storage, at the server, before the CLOSE operation is requested by the client. In the case of a server restart and a CLOSEd file, it may not be possible to retransmit the data to be written to the file, hence, this requirement.¶

14.3.2. Data Caching and File Locking

For those applications that choose to use byte-range locking instead of share reservations to exclude inconsistent file access, there is an analogous set of constraints that apply to client-side data caching. These rules are effective only if the byte-range locking is used in a way that matches in an equivalent way the actual READ and WRITE operations executed. This is as opposed to byte-range locking that is based on pure convention. For example, it is possible to manipulate a two-megabyte file by dividing the file into two one-megabyte ranges and protecting access to the two byte-ranges by byte-range locks on bytes zero and one. A WRITE_LT lock on byte zero of the file would represent the right to perform READ and WRITE operations on the first byte-range. A WRITE_LT lock on byte one of the file would represent the right to perform READ and WRITE operations on the second byte-range. As long as all applications manipulating the file obey this convention, they will work on a local file system. However, they may not work with the NFSv4.1 protocol unless clients refrain from data caching.¶

The rules for data caching in the byte-range locking environment are:¶

First, when a client obtains a byte-range lock for a particular byte-range, the data cache corresponding to that byte-range (if any cache data exists) must be revalidated. If the change attribute indicates that the file may have been updated since the cached data was obtained, the client must flush or invalidate the cached data for the newly locked byte-range. A client might choose to invalidate all of the non-modified cached data that it has for the file, but the only requirement for correct operation is to invalidate all of the data in the newly locked byte-range.¶
Second, before releasing a WRITE_LT lock for a byte-range, all modified data for that byte-range must be flushed to the server. The modified data must also be written to stable storage.¶

Note that flushing data to the server and the invalidation of cached data must reflect the actual byte-ranges locked or unlocked. Rounding these up or down to reflect client cache block boundaries will cause problems if not carefully done. For example, writing a modified block when only half of that block is within an area being unlocked may cause invalid modification to the byte-range outside the unlocked area. This, in turn, may be part of a byte-range locked by another client. Clients can avoid this situation by synchronously performing portions of WRITE operations that overlap that portion (initial or final) that is not a full block. Similarly, invalidating a locked area that is not an integral number of full buffer blocks would require the client to read one or two partial blocks from the server if the revalidation procedure shows that the data that the client possesses may not be valid.¶

The data that is written to the server as a prerequisite to the unlocking of a byte-range must be written, at the server, to stable storage. The client may accomplish this either with synchronous writes or by following asynchronous writes with a COMMIT operation. This is required because retransmission of the modified data after a server restart might conflict with a lock held by another client.¶

A client implementation may choose to accommodate applications that use byte-range locking in non-standard ways (e.g., using a byte-range lock as a global semaphore) by flushing to the server more data upon a LOCKU than is covered by the locked range. This may include modified data within files other than the one for which the unlocks are being done. In such cases, the client must not interfere with applications whose READs and WRITEs are being done only within the bounds of byte-range locks that the application holds. For example, an application locks a single byte of a file and proceeds to write that single byte. A client that chose to handle a LOCKU by flushing all modified data to the server could validly write that single byte in response to an unrelated LOCKU operation. However, it would not be valid to write the entire block in which that single written byte was located since it includes an area that is not locked and might be locked by another client. Client implementations can avoid this problem by dividing files with modified data into those for which all modifications are done to areas covered by an appropriate byte-range lock and those for which there are modifications not covered by a byte-range lock. Any writes done for the former class of files must not include areas not locked and thus not modified on the client.¶

14.3.3. Data Caching and Mandatory File Locking

Client-side data caching needs to respect mandatory byte-range locking when it is in effect. The presence of mandatory byte-range locking for a given file is indicated when the client gets back NFS4ERR_LOCKED from a READ or WRITE operation on a file for which it has an appropriate share reservation. When mandatory locking is in effect for a file, the client must check for an appropriate byte-range lock for data being read or written. If a byte-range lock exists for the range being read or written, the client may satisfy the request using the client's validated cache. If an appropriate byte-range lock is not held for the range of the read or write, the read or write request must not be satisfied by the client's cache and the request must be sent to the server for processing. When a read or write request partially overlaps a locked byte-range, the request should be subdivided into multiple pieces with each byte-range (locked or not) treated appropriately.¶

14.3.4. Data Caching and File Identity

When clients cache data, the file data needs to be organized according to the file system object to which the data belongs. For NFSv3 clients, the typical practice has been to assume for the purpose of caching that distinct filehandles represent distinct file system objects. The client then has the choice to organize and maintain the data cache on this basis.¶

In the NFSv4.1 protocol, there is now the possibility to have significant deviations from a "one filehandle per object" model because a filehandle may be constructed on the basis of the object's pathname. Therefore, clients need a reliable method to determine if two filehandles designate the same file system object. If clients were simply to assume that all distinct filehandles denote distinct objects and proceed to do data caching on this basis, caching inconsistencies would arise between the distinct client-side objects that mapped to the same server-side object.¶

By providing a method to differentiate filehandles, the NFSv4.1 protocol alleviates a potential functional regression in comparison with the NFSv3 protocol. Without this method, caching inconsistencies within the same client could occur, and this has not been present in previous versions of the NFS protocol. Note that it is possible to have such inconsistencies with applications executing on multiple clients, but that is not the issue being addressed here.¶

For the purposes of data caching, the following steps allow an NFSv4.1 client to determine whether two distinct filehandles denote the same server-side object:¶

If GETATTR directed to two filehandles returns different values of the fsid attribute, then the filehandles represent distinct objects.¶
If GETATTR for any file with an fsid that matches the fsid of the two filehandles in question returns a unique_handles attribute with a value of TRUE, then the two objects are distinct.¶
If GETATTR directed to the two filehandles does not return the fileid attribute for both of the handles, then it cannot be determined whether the two objects are the same. Therefore, operations that depend on that knowledge (e.g., client-side data caching) cannot be done reliably. Note that if GETATTR does not return the fileid attribute for both filehandles, it will return it for neither of the filehandles, since the fsid for both filehandles is the same.¶
If GETATTR directed to the two filehandles returns different values for the fileid attribute, then they are distinct objects.¶
Otherwise, they are the same object.¶

14.4. Open Delegation

When a file is being OPENed, the server may delegate further handling of opens and closes for that file to the opening client. Any such delegation is recallable since the circumstances that allowed for the delegation are subject to change. In particular, if the server receives a conflicting OPEN from another client, the server must recall the delegation before deciding whether the OPEN from the other client may be granted. Making a delegation is up to the server, and clients should not assume that any particular OPEN either will or will not result in an OPEN delegation. The following is a typical set of conditions that servers might use in deciding whether an OPEN should be delegated:¶

The client must be able to respond to the server's callback requests. If a backchannel has been established, the server will send a CB_COMPOUND request, containing a single operation, CB_SEQUENCE, for a test of backchannel availability.¶
The client must have responded properly to previous recalls.¶
There must be no current OPEN conflicting with the requested delegation.¶
There should be no current delegation that conflicts with the delegation being requested.¶
The probability of future conflicting open requests should be low based on the recent history of the file.¶
The existence of any server-specific semantics of OPEN/CLOSE that would make the required handling incompatible with the prescribed handling that the delegated client would apply (see below).¶

There are two types of OPEN delegations: OPEN_DELEGATE_READ and OPEN_DELEGATE_WRITE. An OPEN_DELEGATE_READ delegation allows a client to handle, on its own, requests to open a file for reading that do not deny OPEN4_SHARE_ACCESS_READ access to others. Multiple OPEN_DELEGATE_READ delegations may be outstanding simultaneously and do not conflict. An OPEN_DELEGATE_WRITE delegation allows the client to handle, on its own, all opens. Only one OPEN_DELEGATE_WRITE delegation may exist for a given file at a given time, and it is inconsistent with any OPEN_DELEGATE_READ delegations.¶

When a client has an OPEN_DELEGATE_READ delegation, it is assured that neither the contents, the attributes (with the exception of time_access), nor the names of any links to the file will change without its knowledge, so long as the delegation is held. When a client has an OPEN_DELEGATE_WRITE delegation, it may modify the file data locally since no other client will be accessing the file's data. The client holding an OPEN_DELEGATE_WRITE delegation may only locally affect file attributes that are intimately connected with the file data: size, change, time_access, time_metadata, and time_modify. All other attributes must be reflected on the server.¶

When a client has an OPEN delegation, it does not need to send OPENs or CLOSEs to the server. Instead, the client may update the appropriate status internally. For an OPEN_DELEGATE_READ delegation, opens that cannot be handled locally (opens that are for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH or that deny OPEN4_SHARE_ACCESS_READ access) must be sent to the server.¶

When an OPEN delegation is made, the reply to the OPEN contains an OPEN delegation structure that specifies the following:¶

the type of delegation (OPEN_DELEGATE_READ or OPEN_DELEGATE_WRITE).¶
space limitation information to control flushing of data on close (OPEN_DELEGATE_WRITE delegation only; see Section 14.4.1)¶
an nfsace4 specifying read and write permissions¶
a stateid to represent the delegation¶

The delegation stateid is separate and distinct from the stateid for the OPEN proper. The standard stateid, unlike the delegation stateid, is associated with a particular lock-owner and will continue to be valid after the delegation is recalled and the file remains open.¶

When a request internal to the client is made to open a file and an OPEN delegation is in effect, it will be accepted or rejected solely on the basis of the following conditions. Any requirement for other checks to be made by the delegate should result in the OPEN delegation being denied so that the checks can be made by the server itself.¶

The access and deny bits for the request and the file as described in Section 13.7.¶
The read and write permissions as determined below.¶

The nfsace4 passed with delegation can be used to avoid frequent ACCESS calls. The permission check should be as follows:¶

If the nfsace4 indicates that the open may be done, then it should be granted without reference to the server.¶
If the nfsace4 indicates that the open may not be done, then an ACCESS request must be sent to the server to obtain the definitive answer.¶

The server may return an nfsace4 that is more restrictive than the actual ACL of the file. This includes an nfsace4 that specifies denial of all access. Note that some common practices such as mapping the traditional user "root" to the user "nobody" (see Section 10.9) may make it incorrect to return the actual ACL of the file in the delegation response.¶

The use of a delegation together with various other forms of caching creates the possibility that no server authentication and authorization will ever be performed for a given user since all of the user's requests might be satisfied locally. Where the client is depending on the server for authentication and authorization, the client should be sure authentication and authorization occurs for each user by use of the ACCESS operation. This should be the case even if an ACCESS operation would not be required otherwise. As mentioned before, the server may enforce frequent authentication by returning an nfsace4 denying all access with every OPEN delegation.¶

14.4.1. Open Delegation and Data Caching

An OPEN delegation allows much of the message overhead associated with the opening and closing files to be eliminated. An open when an OPEN delegation is in effect does not require that a validation message be sent to the server. The continued endurance of the "OPEN_DELEGATE_READ delegation" provides a guarantee that no OPEN for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH, and thus no write, has occurred. Similarly, when closing a file opened for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH and if an OPEN_DELEGATE_WRITE delegation is in effect, the data written does not have to be written to the server until the OPEN delegation is recalled. The continued endurance of the OPEN delegation provides a guarantee that no open, and thus no READ or WRITE, has been done by another client.¶

For the purposes of OPEN delegation, READs and WRITEs done without an OPEN are treated as the functional equivalents of a corresponding type of OPEN. Although a client SHOULD NOT use special stateids when an open exists, delegation handling on the server can use the client ID associated with the current session to determine if the operation has been done by the holder of the delegation (in which case, no recall is necessary) or by another client (in which case, the delegation must be recalled and I/O not proceed until the delegation is returned or revoked).¶

With delegations, a client is able to avoid writing data to the server when the CLOSE of a file is serviced. The file close system call is the usual point at which the client is notified of a lack of stable storage for the modified file data generated by the application. At the close, file data is written to the server and, through normal accounting, the server is able to determine if the available file system space for the data has been exceeded (i.e., the server returns NFS4ERR_NOSPC or NFS4ERR_DQUOT). This accounting includes quotas. The introduction of delegations requires that an alternative method be in place for the same type of communication to occur between client and server.¶

In the delegation response, the server provides either the limit of the size of the file or the number of modified blocks and associated block size. The server must ensure that the client will be able to write modified data to the server of a size equal to that provided in the original delegation. The server must make this assurance for all outstanding delegations. Therefore, the server must be careful in its management of available space for new or modified data, taking into account available file system space and any applicable quotas. The server can recall delegations as a result of managing the available file system space. The client should abide by the server's state space limits for delegations. If the client exceeds the stated limits for the delegation, the server's behavior is undefined.¶

Based on server conditions, quotas, or available file system space, the server may grant OPEN_DELEGATE_WRITE delegations with very restrictive space limitations. The limitations may be defined in a way that will always force modified data to be flushed to the server on close.¶

With respect to authentication, flushing modified data to the server after a CLOSE has occurred may be problematic. For example, the user of the application may have logged off the client, and unexpired authentication credentials may not be present. In this case, the client may need to take special care to ensure that local unexpired credentials will in fact be available. This may be accomplished by tracking the expiration time of credentials and flushing data well in advance of their expiration or by making private copies of credentials to assure their availability when needed.¶

14.4.2. Open Delegation and File Locks

When a client holds an OPEN_DELEGATE_WRITE delegation, lock operations are performed locally. This includes those required for mandatory byte-range locking. This can be done since the delegation implies that there can be no conflicting locks. Similarly, all of the revalidations that would normally be associated with obtaining locks and the flushing of data associated with the releasing of locks need not be done.¶

When a client holds an OPEN_DELEGATE_READ delegation, lock operations are not performed locally. All lock operations, including those requesting non-exclusive locks, are sent to the server for resolution.¶

14.4.3. Handling of CB_GETATTR

The server needs to employ special handling for a GETATTR where the target is a file that has an OPEN_DELEGATE_WRITE delegation in effect. The reason for this is that the client holding the OPEN_DELEGATE_WRITE delegation may have modified the data, and the server needs to reflect this change to the second client that submitted the GETATTR. Therefore, the client holding the OPEN_DELEGATE_WRITE delegation needs to be interrogated. The server will use the CB_GETATTR operation. The only attributes that the server can reliably query via CB_GETATTR are size and change.¶

Since CB_GETATTR is being used to satisfy another client's GETATTR request, the server only needs to know if the client holding the delegation has a modified version of the file. If the client's copy of the delegated file is not modified (data or size), the server can satisfy the second client's GETATTR request from the attributes stored locally at the server. If the file is modified, the server only needs to know about this modified state. If the server determines that the file is currently modified, it will respond to the second client's GETATTR as if the file had been modified locally at the server.¶

Since the form of the change attribute is determined by the server and is opaque to the client, the client and server need to agree on a method of communicating the modified state of the file. For the size attribute, the client will report its current view of the file size. For the change attribute, the handling is more involved.¶

For the client, the following steps will be taken when receiving an OPEN_DELEGATE_WRITE delegation:¶

The value of the change attribute will be obtained from the server and cached. Let this value be represented by c.¶
The client will create a value greater than c that will be used for communicating that modified data is held at the client. Let this value be represented by d.¶
When the client is queried via CB_GETATTR for the change attribute, it checks to see if it holds modified data. If the file is modified, the value d is returned for the change attribute value. If this file is not currently modified, the client returns the value c for the change attribute.¶

For simplicity of implementation, the client MAY for each CB_GETATTR return the same value d. This is true even if, between successive CB_GETATTR operations, the client again modifies the file's data or metadata in its cache. The client can return the same value because the only requirement is that the client be able to indicate to the server that the client holds modified data. Therefore, the value of d may always be c + 1.¶

While the change attribute is opaque to the client in the sense that it has no idea what units of time, if any, the server is counting change with, it is not opaque in that the client has to treat it as an unsigned integer, and the server has to be able to see the results of the client's changes to that integer. Therefore, the server MUST encode the change attribute in network order when sending it to the client. The client MUST decode it from network order to its native order when receiving it, and the client MUST encode it in network order when sending it to the server. For this reason, change is defined as an unsigned integer rather than an opaque array of bytes.¶

For the server, the following steps will be taken when providing an OPEN_DELEGATE_WRITE delegation:¶

Upon providing an OPEN_DELEGATE_WRITE delegation, the server will cache a copy of the change attribute in the data structure it uses to record the delegation. Let this value be represented by sc.¶
When a second client sends a GETATTR operation on the same file to the server, the server obtains the change attribute from the first client. Let this value be cc.¶
If the value cc is equal to sc, the file is not modified and the server returns the current values for change, time_metadata, and time_modify (for example) to the second client.¶
If the value cc is NOT equal to sc, the file is currently modified at the first client and most likely will be modified at the server at a future time. The server then uses its current time to construct attribute values for time_metadata and time_modify. A new value of sc, which we will call nsc, is computed by the server, such that nsc >= sc + 1. The server then returns the constructed time_metadata, time_modify, and nsc values to the requester. The server replaces sc in the delegation record with nsc. To prevent the possibility of time_modify, time_metadata, and change from appearing to go backward (which would happen if the client holding the delegation fails to write its modified data to the server before the delegation is revoked or returned), the server SHOULD update the file's metadata record with the constructed attribute values. For reasons of reasonable performance, committing the constructed attribute values to stable storage is OPTIONAL.¶

As discussed earlier in this section, the client MAY return the same cc value on subsequent CB_GETATTR calls, even if the file was modified in the client's cache yet again between successive CB_GETATTR calls. Therefore, the server must assume that the file has been modified yet again, and MUST take care to ensure that the new nsc it constructs and returns is greater than the previous nsc it returned. An example implementation's delegation record would satisfy this mandate by including a boolean field (let us call it "modified") that is set to FALSE when the delegation is granted, and an sc value set at the time of grant to the change attribute value. The modified field would be set to TRUE the first time cc != sc, and would stay TRUE until the delegation is returned or revoked. The processing for constructing nsc, time_modify, and time_metadata would use this pseudo code:¶

    if (!modified) {
        do CB_GETATTR for change and size;

        if (cc != sc)
            modified = TRUE;
    } else {
        do CB_GETATTR for size;
    }

    if (modified) {
        sc = sc + 1;
        time_modify = time_metadata = current_time;
        update sc, time_modify, time_metadata into file's metadata;
    }

This would return to the client (that sent GETATTR) the attributes it requested, but make sure size comes from what CB_GETATTR returned. The server would not update the file's metadata with the client's modified size.¶

In the case that the file attribute size is different than the server's current value, the server treats this as a modification regardless of the value of the change attribute retrieved via CB_GETATTR and responds to the second client as in the last step.¶

This methodology resolves issues of clock differences between client and server and other scenarios where the use of CB_GETATTR break down.¶

It should be noted that the server is under no obligation to use CB_GETATTR, and therefore the server MAY simply recall the delegation to avoid its use.¶

14.4.4. Recall of Open Delegation

The following events necessitate recall of an OPEN delegation:¶

potentially conflicting OPEN request (or a READ or WRITE operation done with a special stateid)¶
SETATTR sent by another client¶
REMOVE request for the file¶
RENAME request for the file as either the source or target of the RENAME¶

Whether a RENAME of a directory in the path leading to the file results in recall of an OPEN delegation depends on the semantics of the server's file system. If that file system denies such RENAMEs when a file is open, the recall must be performed to determine whether the file in question is, in fact, open.¶

In addition to the situations above, the server may choose to recall OPEN delegations at any time if resource constraints make it advisable to do so. Clients should always be prepared for the possibility of recall.¶

When a client receives a recall for an OPEN delegation, it needs to update state on the server before returning the delegation. These same updates must be done whenever a client chooses to return a delegation voluntarily. The following items of state need to be dealt with:¶

If the file associated with the delegation is no longer open and no previous CLOSE operation has been sent to the server, a CLOSE operation must be sent to the server.¶
If a file has other open references at the client, then OPEN operations must be sent to the server. The appropriate stateids will be provided by the server for subsequent use by the client since the delegation stateid will no longer be valid. These OPEN requests are done with the claim type of CLAIM_DELEGATE_CUR. This will allow the presentation of the delegation stateid so that the client can establish the appropriate rights to perform the OPEN. (See Section 22.16, which describes the OPEN operation, for details.)¶
If there are granted byte-range locks, the corresponding LOCK operations need to be performed. This applies to the OPEN_DELEGATE_WRITE delegation case only.¶
For an OPEN_DELEGATE_WRITE delegation, if at the time of recall the file is not open for OPEN4_SHARE_ACCESS_WRITE/OPEN4_SHARE_ACCESS_BOTH, all modified data for the file must be flushed to the server. If the delegation had not existed, the client would have done this data flush before the CLOSE operation.¶
For an OPEN_DELEGATE_WRITE delegation when a file is still open at the time of recall, any modified data for the file needs to be flushed to the server.¶
With the OPEN_DELEGATE_WRITE delegation in place, it is possible that the file was truncated during the duration of the delegation. For example, the truncation could have occurred as a result of an OPEN UNCHECKED with a size attribute value of zero. Therefore, if a truncation of the file has occurred and this operation has not been propagated to the server, the truncation must occur before any modified data is written to the server.¶

In the case of OPEN_DELEGATE_WRITE delegation, byte-range locking imposes some additional requirements. To precisely maintain the associated invariant, it is required to flush any modified data in any byte-range for which a WRITE_LT lock was released while the OPEN_DELEGATE_WRITE delegation was in effect. However, because the OPEN_DELEGATE_WRITE delegation implies no other locking by other clients, a simpler implementation is to flush all modified data for the file (as described just above) if any WRITE_LT lock has been released while the OPEN_DELEGATE_WRITE delegation was in effect.¶

An implementation need not wait until delegation recall (or the decision to voluntarily return a delegation) to perform any of the above actions, if implementation considerations (e.g., resource availability constraints) make that desirable. Generally, however, the fact that the actual OPEN state of the file may continue to change makes it not worthwhile to send information about opens and closes to the server, except as part of delegation return. An exception is when the client has no more internal opens of the file. In this case, sending a CLOSE is useful because it reduces resource utilization on the client and server. Regardless of the client's choices on scheduling these actions, all must be performed before the delegation is returned, including (when applicable) the close that corresponds to the OPEN that resulted in the delegation. These actions can be performed either in previous requests or in previous operations in the same COMPOUND request.¶

14.4.5. Clients That Fail to Honor Delegation Recalls

A client may fail to respond to a recall for various reasons, such as a failure of the backchannel from server to the client. The client may be unaware of a failure in the backchannel. This lack of awareness could result in the client finding out long after the failure that its delegation has been revoked, and another client has modified the data for which the client had a delegation. This is especially a problem for the client that held an OPEN_DELEGATE_WRITE delegation.¶

Status bits returned by SEQUENCE operations help to provide an alternate way of informing the client of issues regarding the status of the backchannel and of recalled delegations. When the backchannel is not available, the server returns the status bit SEQ4_STATUS_CB_PATH_DOWN on SEQUENCE operations. The client can react by attempting to re-establish the backchannel and by returning recallable objects if a backchannel cannot be successfully re-established.¶

Whether the backchannel is functioning or not, it may be that the recalled delegation is not returned. Note that the client's lease might still be renewed, even though the recalled delegation is not returned. In this situation, servers SHOULD revoke delegations that are not returned in a period of time equal to the lease period. This period of time should allow the client time to note the backchannel-down status and re-establish the backchannel.¶

When delegations are revoked, the server will return with the SEQ4_STATUS_RECALLABLE_STATE_REVOKED status bit set on subsequent SEQUENCE operations. The client should note this and then use TEST_STATEID to find which delegations have been revoked.¶

14.4.6. Delegation Revocation

At the point a delegation is revoked, if there are associated opens on the client, these opens may or may not be revoked. If no byte-range lock or open is granted that is inconsistent with the existing open, the stateid for the open may remain valid and be disconnected from the revoked delegation, just as would be the case if the delegation were returned.¶

For example, if an OPEN for OPEN4_SHARE_ACCESS_BOTH with a deny of OPEN4_SHARE_DENY_NONE is associated with the delegation, granting of another such OPEN to a different client will revoke the delegation but need not revoke the OPEN, since the two OPENs are consistent with each other. On the other hand, if an OPEN denying write access is granted, then the existing OPEN must be revoked.¶

When opens and/or locks are revoked, the applications holding these opens or locks need to be notified. This notification usually occurs by returning errors for READ/WRITE operations or when a close is attempted for the open file.¶

If no opens exist for the file at the point the delegation is revoked, then notification of the revocation is unnecessary. However, if there is modified data present at the client for the file, the user of the application should be notified. Unfortunately, it may not be possible to notify the user since active applications may not be present at the client. See Section 14.5.1 for additional details.¶

14.4.7. Delegations via WANT_DELEGATION

In addition to providing delegations as part of the reply to OPEN operations, servers MAY provide delegations separate from open, via the OPTIONAL WANT_DELEGATION operation. This allows delegations to be obtained in advance of an OPEN that might benefit from them, for objects that are not a valid target of OPEN, or to deal with cases in which a delegation has been recalled and the client wants to make an attempt to re-establish it if the absence of use by other clients allows that.¶

The WANT_DELEGATION operation may be performed on any type of file object other than a directory.¶

When a delegation is obtained using WANT_DELEGATION, any open files for the same filehandle held by that client are to be treated as subordinate to the delegation, just as if they had been created using an OPEN of type CLAIM_DELEGATE_CUR. They are otherwise unchanged as to seqid, access and deny modes, and the relationship with byte-range locks. Similarly, because existing byte-range locks are subordinate to an open, those byte-range locks also become indirectly subordinate to that new delegation.¶

The WANT_DELEGATION operation provides for delivery of delegations via callbacks, when the delegations are not immediately available. When a requested delegation is available, it is delivered to the client via a CB_PUSH_DELEG operation. When this happens, open files for the same filehandle become subordinate to the new delegation at the point at which the delegation is delivered, just as if they had been created using an OPEN of type CLAIM_DELEGATE_CUR. Similarly, this occurs for existing byte-range locks subordinate to an open.¶

14.5. Data Caching and Revocation

When locks and delegations are revoked, the assumptions upon which successful caching depends are no longer guaranteed. For any locks or share reservations that have been revoked, the corresponding state-owner needs to be notified. This notification includes applications with a file open that has a corresponding delegation that has been revoked. Cached data associated with the revocation must be removed from the client. In the case of modified data existing in the client's cache, that data must be removed from the client without being written to the server. As mentioned, the assumptions made by the client are no longer valid at the point when a lock or delegation has been revoked. For example, another client may have been granted a conflicting byte-range lock after the revocation of the byte-range lock at the first client. Therefore, the data within the lock range may have been modified by the other client. Obviously, the first client is unable to guarantee to the application what has occurred to the file in the case of revocation.¶

Notification to a state-owner will in many cases consist of simply returning an error on the next and all subsequent READs/WRITEs to the open file or on the close. Where the methods available to a client make such notification impossible because errors for certain operations may not be returned, more drastic action such as signals or process termination may be appropriate. The justification here is that an invariant on which an application depends may be violated. Depending on how errors are typically treated for the client-operating environment, further levels of notification including logging, console messages, and GUI pop-ups may be appropriate.¶

14.5.1. Revocation Recovery for Write Open Delegation

Revocation recovery for an OPEN_DELEGATE_WRITE delegation poses the special issue of modified data in the client cache while the file is not open. In this situation, any client that does not flush modified data to the server on each close must ensure that the user receives appropriate notification of the failure as a result of the revocation. Since such situations may require human action to correct problems, notification schemes in which the appropriate user or administrator is notified may be necessary. Logging and console messages are typical examples.¶

If there is modified data on the client, it must not be flushed normally to the server. A client may attempt to provide a copy of the file data as modified during the delegation under a different name in the file system namespace to ease recovery. Note that when the client can determine that the file has not been modified by any other client, or when the client has a complete cached copy of the file in question, such a saved copy of the client's view of the file may be of particular value for recovery. In another case, recovery using a copy of the file based partially on the client's cached data and partially on the server's copy as modified by other clients will be anything but straightforward, so clients may avoid saving file contents in these situations or specially mark the results to warn users of possible problems.¶

Saving of such modified data in delegation revocation situations may be limited to files of a certain size or might be used only when sufficient disk space is available within the target file system. Such saving may also be restricted to situations when the client has sufficient buffering resources to keep the cached copy available until it is properly stored to the target file system.¶

14.6. Attribute Caching

This section pertains to the caching of a file's attributes on a client when that client does not hold a delegation on the file.¶

The attributes discussed in this section do not include named attributes. Individual named attributes are analogous to files, and caching of the data for these needs to be handled just as data caching is for ordinary files. Similarly, LOOKUP results from an OPENATTR directory (as well as the directory's contents) are to be cached on the same basis as any other pathnames.¶

Clients may cache file attributes obtained from the server and use them to avoid subsequent GETATTR requests. Such caching is write through in that modification to file attributes is always done by means of requests to the server and should not be done locally and should not be cached. The exception to this are modifications to attributes that are intimately connected with data caching. Therefore, extending a file by writing data to the local data cache is reflected immediately in the size as seen on the client without this change being immediately reflected on the server. Normally, such changes are not propagated directly to the server, but when the modified data is flushed to the server, analogous attribute changes are made on the server. When OPEN delegation is in effect, the modified attributes may be returned to the server in reaction to a CB_RECALL call.¶

The result of local caching of attributes is that the attribute caches maintained on individual clients will not be coherent. Changes made in one order on the server may be seen in a different order on one client and in a third order on another client.¶

The typical file system application programming interfaces do not provide means to atomically modify or interrogate attributes for multiple files at the same time. The following rules provide an environment where the potential incoherencies mentioned above can be reasonably managed. These rules are derived from the practice of previous NFS protocols.¶

All attributes for a given file (per-fsid attributes excepted) are cached as a unit at the client so that no non-serializability can arise within the context of a single file.¶
An upper time boundary is maintained on how long a client cache entry can be kept without being refreshed from the server.¶
When operations are performed that change attributes at the server, the updated attribute set is requested as part of the containing RPC. This includes directory operations that update attributes indirectly. This is accomplished by following the modifying operation with a GETATTR operation and then using the results of the GETATTR to update the client's cached attributes.¶

Note that if the full set of attributes to be cached is requested by READDIR, the results can be cached by the client on the same basis as attributes obtained via GETATTR.¶

A client may validate its cached version of attributes for a file by fetching both the change and time_access attributes and assuming that if the change attribute has the same value as it did when the attributes were cached, then no attributes other than time_access have changed. The reason why time_access is also fetched is because many servers operate in environments where the operation that updates change does not update time_access. For example, POSIX file semantics do not update access time when a file is modified by the write system call [15]. Therefore, the client that wants a current time_access value should fetch it with change during the attribute cache validation processing and update its cached time_access.¶

The client may maintain a cache of modified attributes for those attributes intimately connected with data of modified regular files (size, time_modify, and change). Other than those three attributes, the client MUST NOT maintain a cache of modified attributes. Instead, attribute changes are immediately sent to the server.¶

In some operating environments, the equivalent to time_access is expected to be implicitly updated by each read of the content of the file object. If an NFS client is caching the content of a file object, whether it is a regular file, directory, or symbolic link, the client SHOULD NOT update the time_access attribute (via SETATTR or a small READ or READDIR request) on the server with each read that is satisfied from cache. The reason is that this can defeat the performance benefits of caching content, especially since an explicit SETATTR of time_access may alter the change attribute on the server. If the change attribute changes, clients that are caching the content will think the content has changed, and will re-read unmodified data from the server. Nor is the client encouraged to maintain a modified version of time_access in its cache, since the client either would eventually have to write the access time to the server with bad performance effects or never update the server's time_access, thereby resulting in a situation where an application that caches access time between a close and open of the same file observes the access time oscillating between the past and present. The time_access attribute always means the time of last access to a file by a read that was satisfied by the server. This way clients will tend to see only time_access changes that go forward in time.¶

14.7. Data and Metadata Caching and Memory Mapped Files

Some operating environments include the capability for an application to map a file's content into the application's address space. Each time the application accesses a memory location that corresponds to a block that has not been loaded into the address space, a page fault occurs and the file is read (or if the block does not exist in the file, the block is allocated and then instantiated in the application's address space).¶

As long as each memory-mapped access to the file requires a page fault, the relevant attributes of the file that are used to detect access and modification (time_access, time_metadata, time_modify, and change) will be updated. However, in many operating environments, when page faults are not required, these attributes will not be updated on reads or updates to the file via memory access (regardless of whether the file is local or is accessed remotely). A client or server MAY fail to update attributes of a file that is being accessed via memory-mapped I/O. This has several implications:¶

If there is an application on the server that has memory mapped a file that a client is also accessing, the client may not be able to get a consistent value of the change attribute to determine whether or not its cache is stale. A server that knows that the file is memory-mapped could always pessimistically return updated values for change so as to force the application to always get the most up-to-date data and metadata for the file. However, due to the negative performance implications of this, such behavior is OPTIONAL.¶
If the memory-mapped file is not being modified on the server, and instead is just being read by an application via the memory-mapped interface, the client will not see an updated time_access attribute. However, in many operating environments, neither will any process running on the server. Thus, NFS clients are at no disadvantage with respect to local processes.¶
If there is another client that is memory mapping the file, and if that client is holding an OPEN_DELEGATE_WRITE delegation, the same set of issues as discussed in the previous two bullet points apply. So, when a server does a CB_GETATTR to a file that the client has modified in its cache, the reply from CB_GETATTR will not necessarily be accurate. As discussed earlier, the client's obligation is to report that the file has been modified since the delegation was granted, not whether it has been modified again between successive CB_GETATTR calls, and the server MUST assume that any file the client has modified in cache has been modified again between successive CB_GETATTR calls. Depending on the nature of the client's memory management system, this weak obligation may not be possible. A client MAY return stale information in CB_GETATTR whenever the file is memory-mapped.¶
The mixture of memory mapping and byte-range locking on the same file is problematic. Consider the following scenario, where a page size on each client is 8192 bytes.¶
- Client A memory maps the first page (8192 bytes) of file X.¶
- Client B memory maps the first page (8192 bytes) of file X.¶
- Client A WRITE_LT locks the first 4096 bytes.¶
- Client B WRITE_LT locks the second 4096 bytes.¶
- Client A, via a STORE instruction, modifies part of its locked byte-range.¶
- Simultaneous to client A, client B executes a STORE on part of its locked byte-range.¶

Here the challenge is for each client to resynchronize to get a correct view of the first page. In many operating environments, the virtual memory management systems on each client only know a page is modified, not that a subset of the page corresponding to the respective lock byte-ranges has been modified. So it is not possible for each client to do the right thing, which is to write to the server only that portion of the page that is locked. For example, if client A simply writes out the page, and then client B writes out the page, client A's data is lost.¶

Moreover, if mandatory locking is enabled on the file, then we have a different problem. When clients A and B execute the STORE instructions, the resulting page faults require a byte-range lock on the entire page. Each client then tries to extend their locked range to the entire page, which results in a deadlock. Communicating the NFS4ERR_DEADLOCK error to a STORE instruction is difficult at best.¶

If a client is locking the entire memory-mapped file, there is no problem with advisory or mandatory byte-range locking, at least until the client unlocks a byte-range in the middle of the file.¶

Given the above issues, the following are permitted:¶

Clients and servers MAY deny memory mapping a file for which they know there are byte-range locks.¶
Clients and servers MAY deny a byte-range lock on a file they know is memory-mapped.¶
A client MAY deny memory mapping a file that it knows requires mandatory locking for I/O. If mandatory locking is enabled after the file is opened and mapped, the client MAY deny the application further access to its mapped file.¶

14.8. Name and Directory Caching without Directory Delegations

The NFSv4.1 directory delegation facility (described in Section 14.9 below) is OPTIONAL for servers to implement. Even where it is implemented, it may not always be functional because of resource availability issues or other constraints. Thus, it is important to understand how name and directory caching are done in the absence of directory delegations. These topics are discussed in the next two subsections.¶

14.8.1. Name Caching

The results of LOOKUP and READDIR operations may be cached to avoid the cost of subsequent LOOKUP operations. Just as in the case of attribute caching, inconsistencies may arise among the various client caches. To mitigate the effects of these inconsistencies and given the context of typical file system APIs, an upper time boundary is maintained for how long a client name cache entry can be kept without verifying that the entry has not been made invalid by a directory change operation performed by another client.¶

When a client is not making changes to a directory for which there exist name cache entries, the client needs to periodically fetch attributes for that directory to ensure that it is not being modified. After determining that no modification has occurred, the expiration time for the associated name cache entries may be updated to be the current time plus the name cache staleness bound.¶

When a client is making changes to a given directory, it needs to determine whether there have been changes made to the directory by other clients. It does this by using the change attribute as reported before and after the directory operation in the associated change_info4 value returned for the operation. The server is able to communicate to the client whether the change_info4 data is provided atomically with respect to the directory operation. If the change values are provided atomically, the client has a basis for determining, given proper care, whether other clients are modifying the directory in question.¶

The simplest way to enable the client to make this determination is for the client to serialize all changes made to a specific directory. When this is done, and the server provides before and after values of the change attribute atomically, the client can simply compare the after value of the change attribute from one operation on a directory with the before value on the subsequent operation modifying that directory. When these are equal, the client is assured that no other client is modifying the directory in question.¶

When such serialization is not used, and there may be multiple simultaneous outstanding operations modifying a single directory sent from a single client, making this sort of determination can be more complicated. If two such operations complete in a different order than they were actually performed, that might give an appearance consistent with modification being made by another client. Where this appears to happen, the client needs to await the completion of all such modifications that were started previously, to see if the outstanding before and after change numbers can be sorted into a chain such that the before value of one change number matches the after value of a previous one, in a chain consistent with this client being the only one modifying the directory.¶

In either of these cases, the client is able to determine whether the directory is being modified by another client. If the comparison indicates that the directory was updated by another client, the name cache associated with the modified directory is purged from the client. If the comparison indicates no modification, the name cache can be updated on the client to reflect the directory operation and the associated timeout can be extended. The post-operation change value needs to be saved as the basis for future change_info4 comparisons.¶

As demonstrated by the scenario above, name caching requires that the client revalidate name cache data by inspecting the change attribute of a directory at the point when the name cache item was cached. This requires that the server update the change attribute for directories when the contents of the corresponding directory is modified. For a client to use the change_info4 information appropriately and correctly, the server must report the pre- and post-operation change attribute values atomically. When the server is unable to report the before and after values atomically with respect to the directory operation, the server must indicate that fact in the change_info4 return value. When the information is not atomically reported, the client should not assume that other clients have not changed the directory.¶

14.8.2. Directory Caching

The results of READDIR operations may be used to avoid subsequent READDIR operations. Just as in the cases of attribute and name caching, inconsistencies may arise among the various client caches. To mitigate the effects of these inconsistencies, and given the context of typical file system APIs, the following rules should be followed:¶

Cached READDIR information for a directory that is not obtained in a single READDIR operation must always be a consistent snapshot of directory contents. This is determined by using a GETATTR before the first READDIR and after the last READDIR that contributes to the cache.¶
An upper time boundary is maintained to indicate the length of time a directory cache entry is considered valid before the client must revalidate the cached information.¶

The revalidation technique parallels that discussed in the case of name caching. When the client is not changing the directory in question, checking the change attribute of the directory with GETATTR is adequate. The lifetime of the cache entry can be extended at these checkpoints. When a client is modifying the directory, the client needs to use the change_info4 data to determine whether there are other clients modifying the directory. If it is determined that no other client modifications are occurring, the client may update its directory cache to reflect its own changes.¶

As demonstrated previously, directory caching requires that the client revalidate directory cache data by inspecting the change attribute of a directory at the point when the directory was cached. This requires that the server update the change attribute for directories when the contents of the corresponding directory is modified. For a client to use the change_info4 information appropriately and correctly, the server must report the pre- and post-operation change attribute values atomically. When the server is unable to report the before and after values atomically with respect to the directory operation, the server must indicate that fact in the change_info4 return value. When the information is not atomically reported, the client should not assume that other clients have not changed the directory.¶

14.9. Directory Delegations

14.9.1. Introduction to Directory Delegations

Directory caching for the NFSv4.1 protocol, as previously described, is similar to file caching in previous versions. Clients typically cache directory information for a duration determined by the client. At the end of a predefined timeout, the client will query the server to see if the directory has been updated. By caching attributes, clients reduce the number of GETATTR calls made to the server to validate attributes. Furthermore, frequently accessed files and directories, such as the current working directory, have their attributes cached on the client so that some NFS operations can be performed without having to make an RPC call. By caching name and inode information about most recently looked up entries in a Directory Name Lookup Cache (DNLC), clients do not need to send LOOKUP calls to the server every time these files are accessed.¶

This caching approach works reasonably well at reducing network traffic in many environments. However, it does not address environments where there are numerous queries for files that do not exist. In these cases of "misses", the client sends requests to the server in order to provide reasonable application semantics and promptly detect the creation of new directory entries. Examples of high miss activity are compilation in software development environments. The current behavior of NFS limits its potential scalability and wide-area sharing effectiveness in these types of environments. Other distributed stateful file system architectures such as AFS and DFS have proven that adding state around directory contents can greatly reduce network traffic in high-miss environments.¶

Delegation of directory contents is an OPTIONAL feature of NFSv4.1. Directory delegations provide similar traffic reduction benefits as with file delegations. By allowing clients to cache directory contents (in a read-only fashion) while being notified of changes, the client can avoid making frequent requests to interrogate the contents of slowly-changing directories, reducing network traffic and improving client performance. It can also simplify the task of determining whether other clients are making changes to the directory when the client itself is making many changes to the directory and changes are not serialized.¶

Directory delegations allow improved namespace cache consistency to be achieved through delegations and synchronous recalls, in the absence of notifications. In addition, if time-based consistency is sufficient, asynchronous notifications can provide performance benefits for the client, and possibly the server, under some common operating conditions such as slowly-changing and/or very large directories.¶

14.9.2. Directory Delegation Design

NFSv4.1 introduces the GET_DIR_DELEGATION (Section 22.39) operation to allow the client to ask for a directory delegation. The delegation covers directory attributes and all entries in the directory. If either of these change, the delegation will be recalled synchronously. The operation causing the recall will have to wait before the recall is complete. Any changes to directory entry attributes will not cause the delegation to be recalled.¶

In addition to asking for delegations, a client can also ask for notifications for certain events. These events include changes to the directory's attributes and/or its contents. If a client asks for notification for a certain event, the server will notify the client when that event occurs. This will not result in the delegation being recalled for that client. The notifications are asynchronous and provide a way of avoiding recalls in situations where a directory is changing enough that the pure recall model may not be effective while trying to allow the client to get substantial benefit. In the absence of notifications, once the delegation is recalled the client has to refresh its directory cache; this might not be very efficient for very large directories.¶

The delegation is read-only and the client may not make changes to the directory other than by performing NFSv4.1 operations that modify the directory or the associated file attributes so that the server has knowledge of these changes. In order to keep the client's namespace synchronized with that of the server, the server will notify the delegation-holding client (assuming it has requested notifications) of the changes made as a result of that client's directory-modifying operations. This is to avoid any need for that client to send subsequent GETATTR or READDIR operations to the server. If a single client is holding the delegation and that client makes any changes to the directory (i.e., the changes are made via operations sent on a session associated with the client ID holding the delegation), the delegation will not be recalled. Multiple clients may hold a delegation on the same directory, but if any such client modifies the directory, the server MUST recall the delegation from the other clients, unless those clients have made provisions to be notified of that sort of modification.¶

Delegations can be recalled by the server at any time. Normally, the server will recall the delegation when the directory changes in a way that is not covered by the notification, or when the directory changes and notifications have not been requested. If another client removes the directory for which a delegation has been granted, the server will recall the delegation.¶

14.9.3. Attributes in Support of Directory Notifications

See Section 10.11 for a description of the attributes associated with directory notifications.¶

14.9.4. Directory Delegation Recall

The server will recall the directory delegation by sending a callback to the client. It will use the same callback procedure as used for recalling file delegations. The server will recall the delegation when the directory changes in a way that is not covered by the notification. However, the server need not recall the delegation if attributes of an entry within the directory change.¶

If the server notices that handing out a delegation for a directory is causing too many notifications to be sent out, it may decide to not hand out delegations for that directory and/or recall those already granted. If a client tries to remove the directory for which a delegation has been granted, the server will recall all associated delegations.¶

The implementation sections for a number of operations describe situations in which notification or delegation recall would be required under some common circumstances. In this regard, a similar set of caveats to those listed in Section 14.2 apply.¶

For CREATE, see Section 22.4.4.¶
For LINK, see Section 22.9.4.¶
For OPEN, see Section 22.16.4.¶
For REMOVE, see Section 22.25.4.¶
For RENAME, see Section 22.26.4.¶
For SETATTR, see Section 22.30.4.¶

14.9.5. Directory Delegation Recovery

Recovery from client or server restart for state on regular files has two main goals: avoiding the necessity of breaking application guarantees with respect to locked files and delivery of updates cached at the client. Neither of these goals applies to directories protected by OPEN_DELEGATE_READ delegations and notifications. Thus, no provision is made for reclaiming directory delegations in the event of client or server restart. The client can simply establish a directory delegation in the same fashion as was done initially.¶

15. Multi-Server Namespace

NFSv4.1 supports attributes that allow a namespace to extend beyond the boundaries of a single server. It is desirable that clients and servers support construction of such multi-server namespaces. Use of such multi-server namespaces is OPTIONAL; however, and for many purposes, single-server namespaces are perfectly acceptable. The use of multi-server namespaces can provide many advantages by separating a file system's logical position in a namespace from the (possibly changing) logistical and administrative considerations that cause a particular file system to be located on a particular server via a single network access path that has to be known in advance or determined using DNS.¶

15.1. Terminology

In this section as a whole (i.e., within all of Section 15), the phrase "client ID" always refers to the 64-bit shorthand identifier assigned by the server (a clientid4) and never to the structure that the client uses to identify itself to the server (called an nfs_client_id4 or client_owner in NFSv4.0 and NFSv4.1, respectively). The opaque identifier within those structures is referred to as a "client id string".¶

It is particularly important to clarify the distinction between trunking detection and trunking discovery. The definitions we present are applicable to all minor versions of NFSv4, but we will focus on how these terms apply to NFS version 4.1.¶

Trunking detection refers to ways of deciding whether two specific network addresses are connected to the same NFSv4 server. The means available to make this determination depends on the protocol version, and, in some cases, on the client implementation.¶

In the case of NFS version 4.1 and later minor versions, the means of trunking detection are as described in this document and are available to every client. Two network addresses connected to the same server can always be used together to access a particular server but cannot necessarily be used together to access a single session. See below for definitions of the terms "server-trunkable" and "session-trunkable".¶
Trunking discovery is a process by which a client using one network address can obtain other addresses that are connected to the same server. Typically, it builds on a trunking detection facility by providing one or more methods by which candidate addresses are made available to the client, who can then use trunking detection to appropriately filter them.¶

Despite the support for trunking detection, there was no description of trunking discovery provided in RFC 5661 [65], making it necessary to provide those means in this document.¶

The combination of a server network address and a particular connection type to be used by a connection is referred to as a "server endpoint". Although using different connection types may result in different ports being used, the use of different ports by multiple connections to the same network address in such cases is not the essence of the distinction between the two endpoints used. This is in contrast to the case of port-specific endpoints, in which the explicit specification of port numbers within network addresses is used to allow a single server node to support multiple NFS servers.¶

Two network addresses connected to the same server are said to be server-trunkable. Two such addresses support the use of client ID trunking, as described in Section 7.5.¶

Two network addresses connected to the same server such that those addresses can be used to support a single common session are referred to as session-trunkable. Note that two addresses may be server-trunkable without being session-trunkable, and that, when two connections of different connection types are made to the same network address and are based on a single file system location entry, they are always session-trunkable, independent of the connection type, as specified by Section 7.5, since their derivation from the same file system location entry, together with the identity of their network addresses, assures that both connections are to the same server and will return server-owner information, allowing session trunking to be used.¶

Regarding the terminology that relates to the construction of multi-server namespaces out of a set of local per-server namespaces:¶

Each server has a set of exported file systems that may be accessed by NFSv4 clients. Typically, this is done by assigning each file system a name within the pseudo-fs associated with the server, although the pseudo-fs may be dispensed with if there is only a single exported file system. Each such file system is part of the server's local namespace, and can be considered as a file system instance within a larger multi-server namespace.¶
The set of all exported file systems for a given server constitutes that server's local namespace.¶
In some cases, a server will have a namespace more extensive than its local namespace by using features associated with attributes that provide file system location information. These features, which allow construction of a multi-server namespace, are all described in individual sections below and include referrals (Section 15.5.6), migration (Section 15.5.5), and replication (Section 15.5.4).¶
A file system present in a server's pseudo-fs may have multiple file system instances on different servers associated with it. All such instances are considered replicas of one another. Whether such replicas can be used simultaneously is discussed in Section 15.11.1, while the level of coordination between them (important when switching between them) is discussed in Sections 15.11.2 through 15.11.8 below.¶
When a file system is present in a server's pseudo-fs, but there is no corresponding local file system, it is said to be "absent". In such cases, all associated instances will be accessed on other servers.¶

Regarding the terminology that relates to attributes used in trunking discovery and other multi-server namespace features:¶

File system location attributes include the fs_locations and fs_locations_info attributes.¶
File system location entries provide the individual file system locations within the file system location attributes. Each such entry specifies a server, in the form of a hostname or an address, and an fs name, which designates the location of the file system within the server's local namespace. A file system location entry designates a set of server endpoints to which the client may establish connections. There may be multiple endpoints because a hostname may map to multiple network addresses and because multiple connection types may be used to communicate with a single network address. However, except where explicit port numbers are used to designate a set of servers within a single server node, all such endpoints MUST designate a way of connecting to a single server. The exact form of the location entry varies with the particular file system location attribute used, as described in Section 15.2.¶

The network addresses used in file system location entries typically appear without port number indications and are used to designate a server at one of the standard ports for NFS access, e.g., 2049 for TCP or 20049 for use with RPC-over-RDMA. Port numbers may be used in file system location entries to designate servers (typically user-level ones) accessed using other port numbers. In the case where network addresses indicate trunking relationships, the use of an explicit port number is inappropriate since trunking is a relationship between network addresses. See Section 15.5.2 for details.¶
File system location elements are derived from location entries, and each describes a particular network access path consisting of a network address and a location within the server's local namespace. Such location elements need not appear within a file system location attribute, but the existence of each location element derives from a corresponding location entry. When a location entry specifies an IP address, there is only a single corresponding location element. File system location entries that contain a hostname are resolved using DNS, and may result in one or more location elements. All location elements consist of a location address that includes the IP address of an interface to a server and an fs name, which is the location of the file system within the server's local namespace. The fs name can be empty if the server has no pseudo-fs and only a single exported file system at the root filehandle.¶
Two file system location elements are said to be server-trunkable if they specify the same fs name and the location addresses are such that the location addresses are server-trunkable. When the corresponding network paths are used, the client will always be able to use client ID trunking, but will only be able to use session trunking if the paths are also session-trunkable.¶
Two file system location elements are said to be session-trunkable if they specify the same fs name and the location addresses are such that the location addresses are session-trunkable. When the corresponding network paths are used, the client will be able to able to use either client ID trunking or session trunking.¶

Discussion of the term "replica" is complicated by the fact that the term was used in RFC 5661 [65] with a meaning different from that used in this document. In short, in [65] each replica is identified by a single network access path, while in the current document, a set of network access paths that have server-trunkable network addresses and the same root-relative file system pathname is considered to be a single replica with multiple network access paths.¶

Each set of server-trunkable location elements defines a set of available network access paths to a particular file system. When there are multiple such file systems, each of which containing the same data, these file systems are considered replicas of one another. Logically, such replication is symmetric, since the fs currently in use and an alternate fs are replicas of each other. Often, in other documents, the term "replica" is not applied to the fs currently in use, despite the fact that the replication relation is inherently symmetric.¶

15.2. File System Location Attributes

NFSv4.1 contains attributes that provide information about how a given file system may be accessed (i.e., at what network address and namespace position). As a result, file systems in the namespace of one server can be associated with one or more instances of that file system on other servers. These attributes contain file system location entries specifying a server address target (either as a DNS name representing one or more IP addresses or as a specific IP address) together with the pathname of that file system within the associated single-server namespace.¶

The fs_locations_info Recommended attribute allows specification of one or more file system instance locations where the data corresponding to a given file system may be found. In addition to the specification of file system instance locations, this attribute provides helpful information to do the following:¶

Guide choices among the various file system instances provided (e.g., priority for use, writability, currency, etc.).¶
Help the client efficiently effect as seamless a transition as possible among multiple file system instances, when and if that should be necessary.¶
Guide the selection of the appropriate connection type to be used when establishing a connection.¶

Within the fs_locations_info attribute, each fs_locations_server4 entry corresponds to a file system location entry: the fls_server field designates the server, and the fl_rootpath field of the encompassing fs_locations_item4 gives the location pathname within the server's pseudo-fs.¶

The fs_locations attribute defined in NFSv4.0 is also a part of NFSv4.1. This attribute only allows specification of the file system locations where the data corresponding to a given file system may be found. Servers SHOULD make this attribute available whenever fs_locations_info is supported, but client use of fs_locations_info is preferable because it provides more information.¶

Within the fs_locations attribute, each fs_location4 contains a file system location entry with the server field designating the server and the rootpath field giving the location pathname within the server's pseudo-fs.¶

15.3. File System Presence or Absence

A given location in an NFSv4.1 namespace (typically but not necessarily a multi-server namespace) can have a number of file system instance locations associated with it (via the fs_locations or fs_locations_info attribute). There may also be an actual current file system at that location, accessible via normal namespace operations (e.g., LOOKUP). In this case, the file system is said to be "present" at that position in the namespace, and clients will typically use it, reserving use of additional locations specified via the location-related attributes to situations in which the principal location is no longer available.¶

When there is no actual file system at the namespace location in question, the file system is said to be "absent". An absent file system contains no files or directories other than the root. Any reference to it, except to access a small set of attributes useful in determining alternate locations, will result in an error, NFS4ERR_MOVED. Note that if the server ever returns the error NFS4ERR_MOVED, it MUST support the fs_locations attribute and SHOULD support the fs_locations_info and fs_status attributes.¶

While the error name suggests that we have a case of a file system that once was present, and has only become absent later, this is only one possibility. A position in the namespace may be permanently absent with the set of file system(s) designated by the location attributes being the only realization. The name NFS4ERR_MOVED reflects an earlier, more limited conception of its function, but this error will be returned whenever the referenced file system is absent, whether it has moved or not.¶

Except in the case of GETATTR-type operations (to be discussed later), when the current filehandle at the start of an operation is within an absent file system, that operation is not performed and the error NFS4ERR_MOVED is returned, to indicate that the file system is absent on the current server.¶

Because a GETFH cannot succeed if the current filehandle is within an absent file system, filehandles within an absent file system cannot be transferred to the client. When a client does have filehandles within an absent file system, it is the result of obtaining them when the file system was present, and having the file system become absent subsequently.¶

It should be noted that because the check for the current filehandle being within an absent file system happens at the start of every operation, operations that change the current filehandle so that it is within an absent file system will not result in an error. This allows such combinations as PUTFH-GETATTR and LOOKUP-GETATTR to be used to get attribute information, particularly location attribute information, as discussed below.¶

The Recommended file system attribute fs_status can be used to interrogate the present/absent status of a given file system.¶

15.4. Getting Attributes for an Absent File System

When a file system is absent, most attributes are not available, but it is necessary to allow the client access to the small set of attributes that are available, and most particularly those that give information about the correct current locations for this file system: fs_locations and fs_locations_info.¶

15.4.1. GETATTR within an Absent File System

As mentioned above, an exception is made for GETATTR in that attributes may be obtained for a filehandle within an absent file system. This exception only applies if the attribute mask contains at least one attribute bit that indicates the client is interested in a result regarding an absent file system: fs_locations, fs_locations_info, or fs_status. If none of these attributes is requested, GETATTR will result in an NFS4ERR_MOVED error.¶

When a GETATTR is done on an absent file system, the set of supported attributes is very limited. Many attributes, including those that are normally REQUIRED, will not be available on an absent file system. In addition to the attributes mentioned above (fs_locations, fs_locations_info, fs_status), the following attributes SHOULD be available on absent file systems. In the case of Recommended attributes, they should be available at least to the same degree that they are available on present file systems.¶

change_policy:: This attribute is useful for absent file systems and can be helpful in summarizing to the client when any of the location-related attributes change.¶
fsid:: This attribute should be provided so that the client can determine file system boundaries, including, in particular, the boundary between present and absent file systems. This value must be different from any other fsid on the current server and need have no particular relationship to fsids on any particular destination to which the client might be directed.¶
mounted_on_fileid:: For objects at the top of an absent file system, this attribute needs to be available. Since the fileid is within the present parent file system, there should be no need to reference the absent file system to provide this information.¶

Other attributes SHOULD NOT be made available for absent file systems, even when it is possible to provide them. The server should not assume that more information is always better and should avoid gratuitously providing additional information.¶

When a GETATTR operation includes a bit mask for one of the attributes fs_locations, fs_locations_info, or fs_status, but where the bit mask includes attributes that are not supported, GETATTR will not return an error, but will return the mask of the actual attributes supported with the results.¶

Handling of VERIFY/NVERIFY is similar to GETATTR in that if the attribute mask does not include fs_locations, fs_locations_info, or fs_status, the error NFS4ERR_MOVED will result. It differs in that any appearance in the attribute mask of an attribute not supported for an absent file system (and note that this will include some normally REQUIRED attributes) will also cause an NFS4ERR_MOVED result.¶

15.4.2. READDIR and Absent File Systems

A READDIR performed when the current filehandle is within an absent file system will result in an NFS4ERR_MOVED error, since, unlike the case of GETATTR, no such exception is made for READDIR.¶

Attributes for an absent file system may be fetched via a READDIR for a directory in a present file system, when that directory contains the root directories of one or more absent file systems. In this case, the handling is as follows:¶

If the attribute set requested includes one of the attributes fs_locations, fs_locations_info, or fs_status, then fetching of attributes proceeds normally and no NFS4ERR_MOVED indication is returned, even when the rdattr_error attribute is requested.¶
If the attribute set requested does not include one of the attributes fs_locations, fs_locations_info, or fs_status, then if the rdattr_error attribute is requested, each directory entry for the root of an absent file system will report NFS4ERR_MOVED as the value of the rdattr_error attribute.¶
If the attribute set requested does not include any of the attributes fs_locations, fs_locations_info, fs_status, or rdattr_error, then the occurrence of the root of an absent file system within the directory will result in the READDIR failing with an NFS4ERR_MOVED error.¶
The unavailability of an attribute because of a file system's absence, even one that is ordinarily REQUIRED, does not result in any error indication. The set of attributes returned for the root directory of the absent file system in that case is simply restricted to those actually available.¶

15.5. Uses of File System Location Information

The file system location attributes (i.e., fs_locations and fs_locations_info), together with the possibility of absent file systems, provide a number of important facilities for reliable, manageable, and scalable data access.¶

When a file system is present, these attributes can provide the following:¶

The locations of alternative replicas to be used to access the same data in the event of server failures, communications problems, or other difficulties that make continued access to the current replica impossible or otherwise impractical. Provisioning and use of such alternate replicas is referred to as "replication" and is discussed in Section 15.5.4 below.¶
The network address(es) to be used to access the current file system instance or replicas of it. Client use of this information is discussed in Section 15.5.2 below.¶

Under some circumstances, multiple replicas may be used simultaneously to provide higher-performance access to the file system in question, although the lack of state sharing between servers may be an impediment to such use.¶

When a file system is present but becomes absent, clients can be given the opportunity to have continued access to their data using a different replica. In this case, a continued attempt to use the data in the now-absent file system will result in an NFS4ERR_MOVED error, and then the successor replica or set of possible replica choices can be fetched and used to continue access. Transfer of access to the new replica location is referred to as "migration" and is discussed in Section 15.5.4 below.¶

When a file system is currently absent, specification of file system location provides a means by which file systems located on one server can be associated with a namespace defined by another server, thus allowing a general multi-server namespace facility. A designation of such a remote instance, in place of a file system not previously present, is called a "pure referral" and is discussed in Section 15.5.6 below.¶

Because client support for attributes related to file system location is OPTIONAL, a server may choose to take action to hide migration and referral events from such clients, by acting as a proxy, for example. The server can determine the presence of client support from the arguments of the EXCHANGE_ID operation (see Section 22.35.3).¶

15.5.1. Combining Multiple Uses in a Single Attribute

A file system location attribute will sometimes contain information relating to the location of multiple replicas, which may be used in different ways:¶

File system location entries that relate to the file system instance currently in use provide trunking information, allowing the client to find additional network addresses by which the instance may be accessed.¶
File system location entries that provide information about replicas to which access is to be transferred.¶
Other file system location entries that relate to replicas that are available to use in the event that access to the current replica becomes unsatisfactory.¶

In order to simplify client handling and to allow the best choice of replicas to access, the server should adhere to the following guidelines:¶

All file system location entries that relate to a single file system instance should be adjacent.¶
File system location entries that relate to the instance currently in use should appear first.¶
File system location entries that relate to replica(s) to which migration is occurring should appear before replicas that are available for later use if the current replica should become inaccessible.¶

15.5.2. File System Location Attributes and Trunking

Trunking is the use of multiple connections between a client and server in order to increase the speed of data transfer. A client may determine the set of network addresses to use to access a given file system in a number of ways:¶

When the name of the server is known to the client, it may use DNS to obtain a set of network addresses to use in accessing the server.¶
The client may fetch the file system location attribute for the file system. This will provide either the name of the server (which can be turned into a set of network addresses using DNS) or a set of server-trunkable location entries. Using the latter alternative, the server can provide addresses it regards as desirable to use to access the file system in question. Although these entries can contain port numbers, these port numbers are not used in determining trunking relationships. Once the candidate addresses have been determined and EXCHANGE_ID done to the proper server, only the value of the so_major_id field returned by the servers in question determines whether a trunking relationship actually exists.¶

When the client fetches a location attribute for a file system, it should be noted that the client may encounter multiple entries for a number of reasons, such that when it determines trunking information, it may need to bypass addresses not trunkable with one already known.¶

The server can provide location entries that include either names or network addresses. It might use the latter form because of DNS-related security concerns or because the set of addresses to be used might require active management by the server.¶

Location entries used to discover candidate addresses for use in trunking are subject to change, as discussed in Section 15.5.7 below. The client may respond to such changes by using additional addresses once they are verified or by ceasing to use existing ones. The server can force the client to cease using an address by returning NFS4ERR_MOVED when that address is used to access a file system. This allows a transfer of client access that is similar to migration, although the same file system instance is accessed throughout.¶

15.5.3. File System Location Attributes and Connection Type Selection

Because of the need to support multiple types of connections, clients face the issue of determining the proper connection type to use when establishing a connection to a given server network address. In some cases, this issue can be addressed through the use of the connection "step-up" facility described in Section 22.36. However, because there are cases in which that facility is not available, the client may have to choose a connection type with no possibility of changing it within the scope of a single connection.¶

The two file system location attributes differ as to the information made available in this regard. The fs_locations attribute provides no information to support connection type selection. As a result, clients supporting multiple connection types would need to attempt to establish connections using multiple connection types until the one preferred by the client is successfully established.¶

The fs_locations_info attribute includes the FSLI4TF_RDMA flag, which is convenient for a client wishing to use RDMA. When this flag is set, it indicates that RPC-over-RDMA support is available using the specified location entry. A client can establish a TCP connection and then convert that connection to use RDMA by using the step-up facility.¶

Irrespective of the particular attribute used, when there is no indication that a step-up operation can be performed, a client supporting RDMA operation can establish a new RDMA connection, and it can be bound to the session already established by the TCP connection, allowing the TCP connection to be dropped and the session converted to further use in RDMA mode, if the server supports that.¶

15.5.4. File System Replication

The fs_locations and fs_locations_info attributes provide alternative file system locations, to be used to access data in place of or in addition to the current file system instance. On first access to a file system, the client should obtain the set of alternate locations by interrogating the fs_locations or fs_locations_info attribute, with the latter being preferred.¶

In the event that the occurrence of server failures, communications problems, or other difficulties make continued access to the current file system impossible or otherwise impractical, the client can use the alternate locations as a way to get continued access to its data.¶

The alternate locations may be physical replicas of the (typically read-only) file system data supplemented by possible asynchronous propagation of updates. Alternatively, they may provide for the use of various forms of server clustering in which multiple servers provide alternate ways of accessing the same physical file system. How the difference between replicas affects file system transitions can be represented within the fs_locations and fs_locations_info attributes, and how the client deals with file system transition issues will be discussed in detail in later sections.¶

Although the location attributes provide some information about the nature of the inter-replica transition, many aspects of the semantics of possible asynchronous updates are not currently described by the protocol, which makes it necessary for clients using replication to switch among replicas undergoing change to familiarize themselves with the semantics of the update approach used. Due to this lack of specificity, many applications may find the use of migration more appropriate because a server can propagate all updates made before an established point in time to the new replica as part of the migration event.¶

15.5.4.1. File System Trunking Presented as Replication

In some situations, a file system location entry may indicate a file system access path to be used as an alternate location, where trunking, rather than replication, is to be used. The situations in which this is appropriate are limited to those in which both of the following are true:¶

The two file system locations (i.e., the one on which the location attribute is obtained and the one specified in the file system location entry) designate the same locations within their respective single-server namespaces.¶
The two server network addresses (i.e., the one being used to obtain the location attribute and the one specified in the file system location entry) designate the same server (as indicated by the same value of the so_major_id field of the eir_server_owner field returned in response to EXCHANGE_ID).¶

When these conditions hold, operations using both access paths are generally trunked, although trunking may be disallowed when the attribute fs_locations_info is used:¶

When the fs_locations_info attribute shows the two entries as not having the same simultaneous-use class, trunking is inhibited, and the two access paths cannot be used together.¶

In this case, the two paths can be used serially with no transition activity required on the part of the client, and any transition between access paths is transparent. In transferring access from one to the other, the client acts as if communication were interrupted, establishing a new connection and possibly a new session to continue access to the same file system.¶
Note that for two such location entries, any information within the fs_locations_info attribute that indicates the need for special transition activity, i.e., the appearance of the two file system location entries with different handle, fileid, write-verifier, change, and readdir classes, indicates a serious problem. The client, if it allows transition to the file system instance at all, must not treat any transition as a transparent one. The server SHOULD NOT indicate that these two entries (for the same file system on the same server) belong to different handle, fileid, write-verifier, change, and readdir classes, whether or not the two entries are shown belonging to the same simultaneous-use class.¶

These situations were recognized by [65], even though that document made no explicit mention of trunking:¶

It treated the situation that we describe as trunking as one of simultaneous use of two distinct file system instances, even though, in the explanatory framework now used to describe the situation, the case is one in which a single file system is accessed by two different trunked addresses.¶
It treated the situation in which two paths are to be used serially as a special sort of "transparent transition". However, in the descriptive framework now used to categorize transition situations, this is considered a case of a "network endpoint transition" (see Section 15.9).¶

15.5.5. File System Migration

When a file system is present and becomes inaccessible using the current access path, the NFSv4.1 protocol provides a means by which clients can be given the opportunity to have continued access to their data. This may involve using a different access path to the existing replica or providing a path to a different replica. The new access path or the location of the new replica is specified by a file system location attribute. The ensuing migration of access includes the ability to retain locks across the transition. Depending on circumstances, this can involve:¶

The continued use of the existing clientid when accessing the current replica using a new access path.¶
Use of lock reclaim, taking advantage of a per-fs grace period.¶
Use of Transparent State Migration.¶

Typically, a client will be accessing the file system in question, get an NFS4ERR_MOVED error, and then use a file system location attribute to determine the new access path for the data. When fs_locations_info is used, additional information will be available that will define the nature of the client's handling of the transition to a new server.¶

In most instances, servers will choose to migrate all clients using a particular file system to a successor replica at the same time to avoid cases in which different clients are updating different replicas. However, migration of an individual client can be helpful in providing load balancing, as long as the replicas in question are such that they represent the same data as described in Section 15.11.8.¶

In the case in which there is no transition between replicas (i.e., only a change in access path), there are no special difficulties in using of this mechanism to effect load balancing.¶
In the case in which the two replicas are sufficiently coordinated as to allow a single client coherent, simultaneous access to both, there is, in general, no obstacle to the use of migration of particular clients to effect load balancing. Generally, such simultaneous use involves cooperation between servers to ensure that locks granted on two coordinated replicas cannot conflict and can remain effective when transferred to a common replica.¶
In the case in which a large set of clients is accessing a file system in a read-only fashion, it can be helpful to migrate all clients with writable access simultaneously, while using load balancing on the set of read-only copies, as long as the rules in Section 15.11.8, which are designed to prevent data reversion, are followed.¶

In other cases, the client might not have sufficient guarantees of data similarity or coherence to function properly (e.g., the data in the two replicas is similar but not identical), and the possibility that different clients are updating different replicas can exacerbate the difficulties, making the use of load balancing in such situations a perilous enterprise.¶

The protocol does not specify how the file system will be moved between servers or how updates to multiple replicas will be coordinated. It is anticipated that a number of different server-to-server coordination mechanisms might be used, with the choice left to the server implementer. The NFSv4.1 protocol specifies the method used to communicate the migration event between client and server.¶

In the case of various forms of server clustering, the new location may be another server providing access to the same physical file system. The client's responsibilities in dealing with this transition will depend on whether a switch between replicas has occurred and the means the server has chosen to provide continuity of locking state. These issues will be discussed in detail below.¶

Although a single successor location is typical, multiple locations may be provided. When multiple locations are provided, the client will typically use the first one provided. If that is inaccessible for some reason, later ones can be used. In such cases, the client might consider the transition to the new replica to be a migration event, even though some of the servers involved might not be aware of the use of the server that was inaccessible. In such a case, a client might lose access to locking state as a result of the access transfer.¶

When an alternate location is designated as the target for migration, it must designate the same data (with metadata being the same to the degree indicated by the fs_locations_info attribute). Where file systems are writable, a change made on the original file system must be visible on all migration targets. Where a file system is not writable but represents a read-only copy (possibly periodically updated) of a writable file system, similar requirements apply to the propagation of updates. Any change visible in the original file system must already be effected on all migration targets, to avoid any possibility that a client, in effecting a transition to the migration target, will see any reversion in file system state.¶

15.5.6. Referrals

Referrals allow the server to associate a file system namespace entry located on one server with a file system located on another server. When this includes the use of pure referrals, servers are provided a way of placing a file system in a location within the namespace essentially without respect to its physical location on a particular server. This allows a single server or a set of servers to present a multi-server namespace that encompasses file systems located on a wider range of servers. Some likely uses of this facility include establishment of site-wide or organization-wide namespaces, with the eventual possibility of combining such together into a truly global namespace, such as the one provided by AFS (the Andrew File System) [64].¶

Referrals occur when a client determines, upon first referencing a position in the current namespace, that it is part of a new file system and that the file system is absent. When this occurs, typically upon receiving the error NFS4ERR_MOVED, the actual location or locations of the file system can be determined by fetching a locations attribute.¶

The file system location attribute may designate a single file system location or multiple file system locations, to be selected based on the needs of the client. The server, in the fs_locations_info attribute, may specify priorities to be associated with various file system location choices. The server may assign different priorities to different locations as reported to individual clients, in order to adapt to client physical location or to effect load balancing. When both read-only and read-write file systems are present, some of the read-only locations might not be absolutely up-to-date (as they would have to be in the case of replication and migration). Servers may also specify file system locations that include client-substituted variables so that different clients are referred to different file systems (with different data contents) based on client attributes such as CPU architecture.¶

If the fs_locations_info attribute lists multiple possible targets, the relationships among them may be important to the client in selecting which one to use. The same rules specified in Section 15.5.5 below regarding multiple migration targets apply to these multiple replicas as well. For example, the client might prefer a writable target on a server that has additional writable replicas to which it subsequently might switch. Note that, as distinguished from the case of replication, there is no need to deal with the case of propagation of updates made by the current client, since the current client has not accessed the file system in question.¶

Use of multi-server namespaces is enabled by NFSv4.1 but is not required. The use of multi-server namespaces and their scope will depend on the applications used and system administration preferences.¶

Multi-server namespaces can be established by a single server providing a large set of pure referrals to all of the included file systems. Alternatively, a single multi-server namespace may be administratively segmented with separate referral file systems (on separate servers) for each separately administered portion of the namespace. The top-level referral file system or any segment may use replicated referral file systems for higher availability.¶

Generally, multi-server namespaces are for the most part uniform, in that the same data made available to one client at a given location in the namespace is made available to all clients at that namespace location. However, there are facilities provided that allow different clients to be directed to different sets of data, for reasons such as enabling adaptation to such client characteristics as CPU architecture. These facilities are described in Section 15.17.3.¶

Note that it is possible, when providing a uniform namespace, to provide different location entries to different clients in order to provide each client with a copy of the data physically closest to it or otherwise optimize access (e.g., provide load balancing).¶

15.5.7. Changes in a File System Location Attribute

Although clients will typically fetch a file system location attribute when first accessing a file system and when NFS4ERR_MOVED is returned, a client can choose to fetch the attribute periodically, in which case, the value fetched may change over time.¶

For clients not prepared to access multiple replicas simultaneously (see Section 15.11.1), the handling of the various cases of location change are as follows:¶

Changes in the list of replicas or in the network addresses associated with replicas do not require immediate action. The client will typically update its list of replicas to reflect the new information.¶
Additions to the list of network addresses for the current file system instance need not be acted on promptly. However, to prepare for a subsequent migration event, the client can choose to take note of the new address and then use it whenever it needs to switch access to a new replica.¶
Deletions from the list of network addresses for the current file system instance do not require the client to immediately cease use of existing access paths, although new connections are not to be established on addresses that have been deleted. However, clients can choose to act on such deletions by preparing for an eventual shift in access, which becomes unavoidable as soon as the server returns NFS4ERR_MOVED to indicate that a particular network access path is not usable to access the current file system.¶

For clients that are prepared to access several replicas simultaneously, the following additional cases need to be addressed. As in the cases discussed above, changes in the set of replicas need not be acted upon promptly, although the client has the option of adjusting its access even in the absence of difficulties that would lead to the selection of a new replica.¶

When a new replica is added, which may be accessed simultaneously with one currently in use, the client is free to use the new replica immediately.¶
When a replica currently in use is deleted from the list, the client need not cease using it immediately. However, since the server may subsequently force such use to cease (by returning NFS4ERR_MOVED), clients might decide to limit the need for later state transfer. For example, new opens might be done on other replicas, rather than on one not present in the list.¶

15.6. Trunking without File System Location Information

In situations in which a file system is accessed using two server-trunkable addresses (as indicated by the same value of the so_major_id field of the eir_server_owner field returned in response to EXCHANGE_ID), trunked access is allowed even though there might not be any location entries specifically indicating the use of trunking for that file system.¶

This situation was recognized by [65], although that document made no explicit mention of trunking and treated the situation as one of simultaneous use of two distinct file system instances. In the explanatory framework now used to describe the situation, the case is one in which a single file system is accessed by two different trunked addresses.¶

15.7. Users and Groups in a Multi-Server Namespace

As in the case of a single-server environment (see Section 10.9), when an owner or group name of the form "id@domain" is assigned to a file, there is an implicit promise to return that same string when the corresponding attribute is interrogated subsequently. In the case of a multi-server namespace, that same promise applies even if server boundaries have been crossed. Similarly, when the owner attribute of a file is derived from the security principal that created the file, that attribute should have the same value even if the interrogation occurs on a different server from the file creation.¶

Similarly, the set of security principals recognized by all the participating servers needs to be the same, with each such principal having the same credentials, regardless of the particular server being accessed.¶

In order to meet these requirements, those setting up multi-server namespaces will need to limit the servers included so that:¶

In all cases in which more than a single domain is supported, the requirements stated in RFC 8000 [26] are to be respected.¶
All servers support a common set of domains that includes all of the domains clients use and expect to see returned as the domain portion of an owner or group in the form "id@domain". Note that, although this set most often consists of a single domain, it is possible for multiple domains to be supported.¶
All servers, for each domain that they support, accept the same set of user and group ids as valid.¶
All servers recognize the same set of security principals. For each principal, the same credential is required, independent of the server being accessed. In addition, the group membership for each such principal is to be the same, independent of the server accessed.¶

Note that there is no requirement in general that the users corresponding to particular security principals have the same local representation on each server, even though it is most often the case that this is so.¶

When AUTH_SYS is used, the following additional requirements must be met:¶

Only a single NFSv4 domain can be supported through the use of AUTH_SYS.¶
The "local" representation of all owners and groups must be the same on all servers. The word "local" is used here since that is the way that numeric user and group ids are described in Section 10.9. However, when AUTH_SYS or stringified numeric owners or groups are used, these identifiers are not truly local, since they are known to the clients as well as to the server.¶

Similarly, when stringified numeric user and group ids are used, the "local" representation of all owners and groups must be the same on all servers, even when AUTH_SYS is not used.¶

15.8. Additional Client-Side Considerations

When clients make use of servers that implement referrals, replication, and migration, care should be taken that a user who mounts a given file system that includes a referral or a relocated file system continues to see a coherent picture of that user-side file system despite the fact that it contains a number of server-side file systems that may be on different servers.¶

One important issue is upward navigation from the root of a server-side file system to its parent (specified as ".." in UNIX), in the case in which it transitions to that file system as a result of referral, migration, or a transition as a result of replication. When the client is at such a point, and it needs to ascend to the parent, it must go back to the parent as seen within the multi-server namespace rather than sending a LOOKUPP operation to the server, which would result in the parent within that server's single-server namespace. In order to do this, the client needs to remember the filehandles that represent such file system roots and use these instead of sending a LOOKUPP operation to the current server. This will allow the client to present to applications a consistent namespace, where upward navigation and downward navigation are consistent.¶

Another issue concerns refresh of referral locations. When referrals are used extensively, they may change as server configurations change. It is expected that clients will cache information related to traversing referrals so that future client-side requests are resolved locally without server communication. This is usually rooted in client-side name look up caching. Clients should periodically purge this data for referral points in order to detect changes in location information. When the change_policy attribute changes for directories that hold referral entries or for the referral entries themselves, clients should consider any associated cached referral information to be out of date.¶

15.9. Overview of File Access Transitions

File access transitions are of two types:¶

Those that involve a transition from accessing the current replica to another one in connection with either replication or migration. How these are dealt with is discussed in Section 15.11.¶
Those in which access to the current file system instance is retained, while the network path used to access that instance is changed. This case is discussed in Section 15.10.¶

15.10. Effecting Network Endpoint Transitions

The endpoints used to access a particular file system instance may change in a number of ways, as listed below. In each of these cases, the same fsid, client IDs, filehandles, and stateids are used to continue access, with a continuity of lock state. In many cases, the same sessions can also be used.¶

The appropriate action depends on the set of replacement addresses that are available for use (i.e., server endpoints that are server-trunkable with one previously being used).¶

When use of a particular address is to cease, and there is also another address currently in use that is server-trunkable with it, requests that would have been issued on the address whose use is to be discontinued can be issued on the remaining address(es). When an address is server-trunkable but not session-trunkable with the address whose use is to be discontinued, the request might need to be modified to reflect the fact that a different session will be used.¶
When use of a particular connection is to cease, as indicated by receiving NFS4ERR_MOVED when using that connection, but that address is still indicated as accessible according to the appropriate file system location entries, it is likely that requests can be issued on a new connection of a different connection type once that connection is established. Since any two non-port-specific server endpoints that share a network address are inherently session-trunkable, the client can use BIND_CONN_TO_SESSION to access the existing session with the new connection.¶
When there are no potential replacement addresses in use, but there are valid addresses session-trunkable with the one whose use is to be discontinued, the client can use BIND_CONN_TO_SESSION to access the existing session using the new address. Although the target session will generally be accessible, there may be rare situations in which that session is no longer accessible when an attempt is made to bind the new connection to it. In this case, the client can create a new session to enable continued access to the existing instance using the new connection, providing for the use of existing filehandles, stateids, and client ids while supplying continuity of locking state.¶
When there is no potential replacement address in use, and there are no valid addresses session-trunkable with the one whose use is to be discontinued, other server-trunkable addresses may be used to provide continued access. Although the use of CREATE_SESSION is available to provide continued access to the existing instance, servers have the option of providing continued access to the existing session through the new network access path in a fashion similar to that provided by session migration (see Section 15.12). To take advantage of this possibility, clients can perform an initial BIND_CONN_TO_SESSION, as in the previous case, and use CREATE_SESSION only if that fails.¶

15.11. Effecting File System Transitions

There are a range of situations in which there is a change to be effected in the set of replicas used to access a particular file system. Some of these may involve an expansion or contraction of the set of replicas used as discussed in Section 15.11.1 below.¶

For reasons explained in that section, most transitions will involve a transition from a single replica to a corresponding replacement replica. When effecting replica transition, some types of sharing between the replicas may affect handling of the transition as described in Sections 15.11.2 through 15.11.8 below. The attribute fs_locations_info provides helpful information to allow the client to determine the degree of inter-replica sharing.¶

With regard to some types of state, the degree of continuity across the transition depends on the occasion prompting the transition, with transitions initiated by the servers (i.e., migration) offering much more scope for a nondisruptive transition than cases in which the client on its own shifts its access to another replica (i.e., replication). This issue potentially applies to locking state and to session state, which are dealt with below as follows:¶

An introduction to the possible means of providing continuity in these areas appears in Section 15.11.9 below.¶
Transparent State Migration is introduced in Section 15.12. The possible transfer of session state is addressed there as well.¶
The client handling of transitions, including determining how to deal with the various means that the server might take to supply effective continuity of locking state, is discussed in Section 15.13.¶
The source and destination servers' responsibilities in effecting Transparent State Migration of locking and session state are discussed in Section 15.14.¶

15.11.1. File System Transitions and Simultaneous Access

The fs_locations_info attribute (described in Section 15.17) may indicate that two replicas may be used simultaneously, although some situations in which such simultaneous access is permitted are more appropriately described as instances of trunking (see Section 15.5.4.1). Although situations in which multiple replicas may be accessed simultaneously are somewhat similar to those in which a single replica is accessed by multiple network addresses, there are important differences since locking state is not shared among multiple replicas.¶

Because of this difference in state handling, many clients will not have the ability to take advantage of the fact that such replicas represent the same data. Such clients will not be prepared to use multiple replicas simultaneously but will access each file system using only a single replica, although the replica selected might make multiple server-trunkable addresses available.¶

Clients who are prepared to use multiple replicas simultaneously can divide opens among replicas however they choose. Once that choice is made, any subsequent transitions will treat the set of locking state associated with each replica as a single entity.¶

For example, if one of the replicas become unavailable, access will be transferred to a different replica, which is also capable of simultaneous access with the one still in use.¶

When there is no such replica, the transition may be to the replica already in use. At this point, the client has a choice between merging the locking state for the two replicas under the aegis of the sole replica in use or treating these separately until another replica capable of simultaneous access presents itself.¶

15.11.2. Filehandles and File System Transitions

There are a number of ways in which filehandles can be handled across a file system transition. These can be divided into two broad classes depending upon whether the two file systems across which the transition happens share sufficient state to effect some sort of continuity of file system handling.¶

When there is no such cooperation in filehandle assignment, the two file systems are reported as being in different handle classes. In this case, all filehandles are assumed to expire as part of the file system transition. Note that this behavior does not depend on the fh_expire_type attribute and supersedes the specification of the FH4_VOL_MIGRATION bit, which only affects behavior when fs_locations_info is not available.¶

When there is cooperation in filehandle assignment, the two file systems are reported as being in the same handle classes. In this case, persistent filehandles remain valid after the file system transition, while volatile filehandles (excluding those that are only volatile due to the FH4_VOL_MIGRATION bit) are subject to expiration on the target server.¶

15.11.3. Fileids and File System Transitions

In NFSv4.0, the issue of continuity of fileids in the event of a file system transition was not addressed. The general expectation had been that in situations in which the two file system instances are created by a single vendor using some sort of file system image copy, fileids would be consistent across the transition, while in the analogous multi-vendor transitions they would not. This poses difficulties, especially for the client without special knowledge of the transition mechanisms adopted by the server. Note that although fileid is not a REQUIRED attribute, many servers support fileids and many clients provide APIs that depend on fileids.¶

It is important to note that while clients themselves may have no trouble with a fileid changing as a result of a file system transition event, applications do typically have access to the fileid (e.g., via stat). The result is that an application may work perfectly well if there is no file system instance transition or if any such transition is among instances created by a single vendor, yet be unable to deal with the situation in which a multi-vendor transition occurs at the wrong time.¶

Providing the same fileids in a multi-vendor (multiple server vendors) environment has generally been held to be quite difficult. While there is work to be done, it needs to be pointed out that this difficulty is partly self-imposed. Servers have typically identified fileid with inode number, i.e. with a quantity used to find the file in question. This identification poses special difficulties for migration of a file system between vendors where assigning the same index to a given file may not be possible. Note here that a fileid is not required to be useful to find the file in question, only that it is unique within the given file system. Servers prepared to accept a fileid as a single piece of metadata and store it apart from the value used to index the file information can relatively easily maintain a fileid value across a migration event, allowing a truly transparent migration event.¶

In any case, where servers can provide continuity of fileids, they should, and the client should be able to find out that such continuity is available and take appropriate action. Information about the continuity (or lack thereof) of fileids across a file system transition is represented by specifying whether the file systems in question are of the same fileid class.¶

Note that when consistent fileids do not exist across a transition (either because there is no continuity of fileids or because fileid is not a supported attribute on one of instances involved), and there are no reliable filehandles across a transition event (either because there is no filehandle continuity or because the filehandles are volatile), the client is in a position where it cannot verify that files it was accessing before the transition are the same objects. It is forced to assume that no object has been renamed, and, unless there are guarantees that provide this (e.g., the file system is read-only), problems for applications may occur. Therefore, use of such configurations should be limited to situations where the problems that this may cause can be tolerated.¶

15.11.4. Fsids and File System Transitions

Since fsids are generally only unique on a per-server basis, it is likely that they will change during a file system transition. Clients should not make the fsids received from the server visible to applications since they may not be globally unique, and because they may change during a file system transition event. Applications are best served if they are isolated from such transitions to the extent possible.¶

Although normally a single source file system will transition to a single target file system, there is a provision for splitting a single source file system into multiple target file systems, by specifying the FSLI4F_MULTI_FS flag.¶

15.11.4.1. File System Splitting

When a file system transition is made and the fs_locations_info indicates that the file system in question might be split into multiple file systems (via the FSLI4F_MULTI_FS flag), the client SHOULD do GETATTRs to determine the fsid attribute on all known objects within the file system undergoing transition to determine the new file system boundaries.¶

Clients might choose to maintain the fsids passed to existing applications by mapping all of the fsids for the descendant file systems to the common fsid used for the original file system.¶

Splitting a file system can be done on a transition between file systems of the same fileid class, since the fact that fileids are unique within the source file system ensure they will be unique in each of the target file systems.¶

15.11.5. The Change Attribute and File System Transitions

Since the change attribute is defined as a server-specific one, change attributes fetched from one server are normally presumed to be invalid on another server. Such a presumption is troublesome since it would invalidate all cached change attributes, requiring refetching. Even more disruptive, the absence of any assured continuity for the change attribute means that even if the same value is retrieved on refetch, no conclusions can be drawn as to whether the object in question has changed. The identical change attribute could be merely an artifact of a modified file with a different change attribute construction algorithm, with that new algorithm just happening to result in an identical change value.¶

When the two file systems have consistent change attribute formats, and this fact is communicated to the client by reporting in the same change class, the client may assume a continuity of change attribute construction and handle this situation just as it would be handled without any file system transition.¶

15.11.6. Write Verifiers and File System Transitions

In a file system transition, the two file systems might be cooperating in the handling of unstably written data. Clients can determine if this is the case by seeing if the two file systems belong to the same write-verifier class. When this is the case, write verifiers returned from one system may be compared to those returned by the other and superfluous writes can be avoided.¶

When two file systems belong to different write-verifier classes, any verifier generated by one must not be compared to one provided by the other. Instead, the two verifiers should be treated as not equal even when the values are identical.¶

15.11.7. READDIR Cookies and Verifiers and File System Transitions

In a file system transition, the two file systems might be consistent in their handling of READDIR cookies and verifiers. Clients can determine if this is the case by seeing if the two file systems belong to the same readdir class. When this is the case, readdir class, READDIR cookies, and verifiers from one system will be recognized by the other, and READDIR operations started on one server can be validly continued on the other simply by presenting the cookie and verifier returned by a READDIR operation done on the first file system to the second.¶

When two file systems belong to different readdir classes, any READDIR cookie and verifier generated by one is not valid on the second and must not be presented to that server by the client. The client should act as if the verifier were rejected.¶

15.11.8. File System Data and File System Transitions

When multiple replicas exist and are used simultaneously or in succession by a client, applications using them will normally expect that they contain either the same data or data that is consistent with the normal sorts of changes that are made by other clients updating the data of the file system (with metadata being the same to the degree indicated by the fs_locations_info attribute). However, when multiple file systems are presented as replicas of one another, the precise relationship between the data of one and the data of another is not, as a general matter, specified by the NFSv4.1 protocol. It is quite possible to present as replicas file systems where the data of those file systems is sufficiently different that some applications have problems dealing with the transition between replicas. The namespace will typically be constructed so that applications can choose an appropriate level of support, so that in one position in the namespace, a varied set of replicas might be listed, while in another, only those that are up-to-date would be considered replicas. The protocol does define three special cases of the relationship among replicas to be specified by the server and relied upon by clients:¶

When multiple replicas exist and are used simultaneously by a client (see the FSLIB4_CLSIMUL definition within fs_locations_info), they must designate the same data. Where file systems are writable, a change made on one instance must be visible on all instances at the same time, regardless of whether the interrogated instance is the one on which the modification was done. This allows a client to use these replicas simultaneously without any special adaptation to the fact that there are multiple replicas, beyond adapting to the fact that locks obtained on one replica are maintained separately (i.e., under a different client ID). In this case, locks (whether share reservations or byte-range locks) and delegations obtained on one replica are immediately reflected on all replicas, in the sense that access from all other servers is prevented regardless of the replica used. However, because the servers are not required to treat two associated client IDs as representing the same client, it is best to access each file using only a single client ID.¶
When one replica is designated as the successor instance to another existing instance after the return of NFS4ERR_MOVED (i.e., the case of migration), the client may depend on the fact that all changes written to stable storage on the original instance are written to stable storage of the successor (uncommitted writes are dealt with in Section 15.11.6 above).¶
Where a file system is not writable but represents a read-only copy (possibly periodically updated) of a writable file system, clients have similar requirements with regard to the propagation of updates. They may need a guarantee that any change visible on the original file system instance must be immediately visible on any replica before the client transitions access to that replica, in order to avoid any possibility that a client, in effecting a transition to a replica, will see any reversion in file system state. The specific means of this guarantee varies based on the value of the fss_type field that is reported as part of the fs_status attribute (see Section 15.18). Since these file systems are presumed to be unsuitable for simultaneous use, there is no specification of how locking is handled; in general, locks obtained on one file system will be separate from those on others. Since these are expected to be read-only file systems, this is not likely to pose an issue for clients or applications.¶

When none of these special situations applies, there is no basis within the protocol for the client to make assumptions about the contents of a replica file system or its relationship to previous file system instances. Thus, switching between nominally identical read-write file systems would not be possible because either the client does not use the fs_locations_info attribute, or the server does not support it.¶

15.11.9. Lock State and File System Transitions

While accessing a file system, clients obtain locks enforced by the server, which may prevent actions by other clients that are inconsistent with those locks.¶

When access is transferred between replicas, clients need to be assured that the actions disallowed by holding these locks cannot have occurred during the transition. This can be ensured by the methods below. Unless at least one of these is implemented, clients will not be assured of continuity of lock possession across a migration event:¶

Providing the client an opportunity to re-obtain his locks via a per-fs grace period on the destination server, denying all clients using the destination file system the opportunity to obtain new locks that conflict with those held by the transferred client as long as that client has not completed its per-fs grace period. Because the lock reclaim mechanism was originally defined to support server reboot, it implicitly assumes that filehandles will, upon reclaim, be the same as those at open. In the case of migration, this requires that source and destination servers use the same filehandles, as evidenced by using the same server scope (see Section 7.4) or by showing this agreement using fs_locations_info (see Section 15.11.2 above).¶

Note that such a grace period can be implemented without interfering with the ability of non-transferred clients to obtain new locks while it is going on. As long as the destination server is aware of the transferred locks, it can distinguish requests to obtain new locks that contrast with existing locks from those that do not, allowing it to treat such client requests without reference to the ongoing grace period.¶
Locking state can be transferred as part of the transition by providing Transparent State Migration as described in Section 15.12.¶

Of these, Transparent State Migration provides the smoother experience for clients in that there is no need to go through a reclaim process before new locks can be obtained; however, it requires a greater degree of inter-server coordination. In general, the servers taking part in migration are free to provide either facility. However, when the filehandles can differ across the migration event, Transparent State Migration is the only available means of providing the needed functionality.¶

It should be noted that these two methods are not mutually exclusive and that a server might well provide both. In particular, if there is some circumstance preventing a specific lock from being transferred transparently, the destination server can allow it to be reclaimed by implementing a per-fs grace period for the migrated file system.¶

15.11.9.1. Security Consideration Related to Reclaiming Lock State after File System Transitions

Although it is possible for a client reclaiming state to misrepresent its state in the same fashion as described in Section 12.4.2.1.1, most implementations providing for such reclamation in the case of file system transitions will have the ability to detect such misrepresentations. This limits the ability of unauthenticated clients to execute denial-of-service attacks in these circumstances. Nevertheless, the rules stated in Section 12.4.2.1.1 regarding principal verification for reclaim requests apply in this situation as well.¶

Typically, implementations that support file system transitions will have extensive information about the locks to be transferred. This is because of the following:¶

Since failure is not involved, there is no need to store locking information in persistent storage.¶
There is no need, as there is in the failure case, to update multiple repositories containing locking state to keep them in sync. Instead, there is a one-time communication of locking state from the source to the destination server.¶
Providing this information avoids potential interference with existing clients using the destination file system by denying them the ability to obtain new locks during the grace period.¶

When such detailed locking information, not necessarily including the associated stateids, is available:¶

It is possible to detect reclaim requests that attempt to reclaim locks that did not exist before the transfer, rejecting them with NFS4ERR_RECLAIM_BAD (Section 19.1.9.4). SN¶
It is possible when dealing with non-reclaim requests, to determine whether they conflict with existing locks, eliminating the need to return NFS4ERR_GRACE (Section 19.1.9.2) on non-reclaim requests.¶

It is possible for implementations of grace periods in connection with file system transitions not to have detailed locking information available at the destination server, in which case, the security situation is exactly as described in Section 12.4.2.1.1.¶

15.11.9.2. Leases and File System Transitions

In the case of lease renewal, the client may not be submitting requests for a file system that has been transferred to another server. This can occur because of the lease renewal mechanism. The client renews the lease associated with all file systems when submitting a request on an associated session, regardless of the specific file system being referenced.¶

In order for the client to schedule renewal of its lease where there is locking state that may have been relocated to the new server, the client must find out about lease relocation before that lease expire. To accomplish this, the SEQUENCE operation will return the status bit SEQ4_STATUS_LEASE_MOVED if responsibility for any of the renewed locking state has been transferred to a new server. This will continue until the client receives an NFS4ERR_MOVED error for each of the file systems for which there has been locking state relocation.¶

When a client receives an SEQ4_STATUS_LEASE_MOVED indication from a server, for each file system of the server for which the client has locking state, the client should perform an operation. For simplicity, the client may choose to reference all file systems, but what is important is that it must reference all file systems for which there was locking state where that state has moved. Once the client receives an NFS4ERR_MOVED error for each such file system, the server will clear the SEQ4_STATUS_LEASE_MOVED indication. The client can terminate the process of checking file systems once this indication is cleared (but only if the client has received a reply for all outstanding SEQUENCE requests on all sessions it has with the server), since there are no others for which locking state has moved.¶

A client may use GETATTR of the fs_status (or fs_locations_info) attribute on all of the file systems to get absence indications in a single (or a few) request(s), since absent file systems will not cause an error in this context. However, it still must do an operation that receives NFS4ERR_MOVED on each file system, in order to clear the SEQ4_STATUS_LEASE_MOVED indication.¶

Once the set of file systems with transferred locking state has been determined, the client can follow the normal process to obtain the new server information (through the fs_locations and fs_locations_info attributes) and perform renewal of that lease on the new server, unless information in the fs_locations_info attribute shows that no state could have been transferred. If the server has not had state transferred to it transparently, the client will receive NFS4ERR_STALE_CLIENTID from the new server, as described above, and the client can then reclaim locks as is done in the event of server failure.¶

15.11.9.3. Transitions and the Lease_time Attribute

In order that the client may appropriately manage its lease in the case of a file system transition, the destination server must establish proper values for the lease_time attribute.¶

When state is transferred transparently, that state should include the correct value of the lease_time attribute. The lease_time attribute on the destination server must never be less than that on the source, since this would result in premature expiration of a lease granted by the source server. Upon transitions in which state is transferred transparently, the client is under no obligation to refetch the lease_time attribute and may continue to use the value previously fetched (on the source server).¶

If state has not been transferred transparently, either because the associated servers are shown as having different eir_server_scope strings or because the client ID is rejected when presented to the new server, the client should fetch the value of lease_time on the new (i.e., destination) server, and use it for subsequent locking requests. However, the server must respect a grace period of at least as long as the lease_time on the source server, in order to ensure that clients have ample time to reclaim their lock before potentially conflicting non-reclaimed locks are granted.¶

15.12. Transferring State upon Migration

When the transition is a result of a server-initiated decision to transition access, and the source and destination servers have implemented appropriate cooperation, it is possible to do the following:¶

Transfer locking state from the source to the destination server in a fashion similar to that provided by Transparent State Migration in NFSv4.0, as described in [67]. Server responsibilities are described in Section 15.14.2.¶
Transfer session state from the source to the destination server. Server responsibilities in effecting such a transfer are described in Section 15.14.3.¶

The means by which the client determines which of these transfer events has occurred are described in Section 15.13.¶

15.12.1. Transparent State Migration and pNFS

When pNFS is involved, the protocol is capable of supporting:¶

Migration of the Metadata Server (MDS), leaving the Data Servers (DSs) in place.¶
Migration of the file system as a whole, including the MDS and associated DSs.¶
Replacement of one DS by another.¶
Migration of a pNFS file system to one in which pNFS is not used.¶
Migration of a file system not using pNFS to one in which layouts are available.¶

Note that migration, per se, is only involved in the transfer of the MDS function. Although the servicing of a layout may be transferred from one data server to another, this not done using the file system location attributes. The MDS can effect such transfers by recalling or revoking existing layouts and granting new ones on a different data server.¶

Migration of the MDS function is directly supported by Transparent State Migration. Layout state will normally be transparently transferred, just as other state is. As a result, Transparent State Migration provides a framework in which, given appropriate inter-MDS data transfer, one MDS can be substituted for another.¶

Migration of the file system function as a whole can be accomplished by recalling all layouts as part of the initial phase of the migration process. As a result, I/O will be done through the MDS during the migration process, and new layouts can be granted once the client is interacting with the new MDS. An MDS can also effect this sort of transition by revoking all layouts as part of Transparent State Migration, as long as the client is notified about the loss of locking state.¶

In order to allow migration to a file system on which pNFS is not supported, clients need to be prepared for a situation in which layouts are not available or supported on the destination file system and so direct I/O requests to the destination server, rather than depending on layouts being available.¶

Replacement of one DS by another is not addressed by migration as such but can be effected by an MDS recalling layouts for the DS to be replaced and issuing new ones to be served by the successor DS.¶

Migration may transfer a file system from a server that does not support pNFS to one that does. In order to properly adapt to this situation, clients that support pNFS, but function adequately in its absence, should check for pNFS support when a file system is migrated and be prepared to use pNFS when support is available on the destination.¶

15.13. Client Responsibilities When Access Is Transitioned

For a client to respond to an access transition, it must become aware of it. The ways in which this can happen are discussed in Section 15.13.1, which discusses indications that a specific file system access path has transitioned as well as situations in which additional activity is necessary to determine the set of file systems that have been migrated. Section 15.13.2 goes on to complete the discussion of how the set of migrated file systems might be determined. Sections 15.13.3 through 15.13.5 discuss how the client should deal with each transition it becomes aware of, either directly or as a result of migration discovery.¶

The following terms are used to describe client activities:¶

"Transition recovery" refers to the process of restoring access to a file system on which NFS4ERR_MOVED was received.¶
"Migration recovery" refers to that subset of transition recovery that applies when the file system has migrated to a different replica.¶
"Migration discovery" refers to the process of determining which file system(s) have been migrated. It is necessary to avoid a situation in which leases could expire when a file system is not accessed for a long period of time, since a client unaware of the migration might be referencing an unmigrated file system and not renewing the lease associated with the migrated file system.¶

15.13.1. Client Transition Notifications

When there is a change in the network access path that a client is to use to access a file system, there are a number of related status indications with which clients need to deal:¶

If an attempt is made to use or return a filehandle within a file system that is no longer accessible at the address previously used to access it, the error NFS4ERR_MOVED is returned.¶

Exceptions are made to allow such filehandles to be used when interrogating a file system location attribute. This enables a client to determine a new replica's location or a new network access path.¶

This condition continues on subsequent attempts to access the file system in question. The only way the client can avoid the error is to cease accessing the file system in question at its old server location and access it instead using a different address at which it is now available.¶
Whenever a client sends a SEQUENCE operation to a server that generated state held on that client and associated with a file system no longer accessible on that server, the response will contain the status bit SEQ4_STATUS_LEASE_MOVED, indicating that there has been a lease migration.¶

This condition continues until the client acknowledges the notification by fetching a file system location attribute for the file system whose network access path is being changed. When there are multiple such file systems, a location attribute for each such file system needs to be fetched. The location attribute for all migrated file systems needs to be fetched in order to clear the condition. Even after the condition is cleared, the client needs to respond by using the location information to access the file system at its new location to ensure that leases are not needlessly expired.¶

Unlike NFSv4.0, in which the corresponding conditions are both errors and thus mutually exclusive, in NFSv4.1 the client can, and often will, receive both indications on the same request. As a result, implementations need to address the question of how to coordinate the necessary recovery actions when both indications arrive in the response to the same request. It should be noted that when processing an NFSv4 COMPOUND, the server will normally decide whether SEQ4_STATUS_LEASE_MOVED is to be set before it determines which file system will be referenced or whether NFS4ERR_MOVED is to be returned.¶

Since these indications are not mutually exclusive in NFSv4.1, the following combinations are possible results when a COMPOUND is issued:¶

The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is asserted.¶

In this case, transition recovery is required. While it is possible that migration discovery is needed in addition, it is likely that only the accessed file system has transitioned. In any case, because addressing NFS4ERR_MOVED is necessary to allow the rejected requests to be processed on the target, dealing with it will typically have priority over migration discovery.¶
The COMPOUND status is NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is clear.¶

In this case, transition recovery is also required. It is clear that migration discovery is not needed to find file systems that have been migrated other than the one returning NFS4ERR_MOVED. Cases in which this result can arise include a referral or a migration for which there is no associated locking state. This can also arise in cases in which an access path transition other than migration occurs within the same server. In such a case, there is no need to set SEQ4_STATUS_LEASE_MOVED, since the lease remains associated with the current server even though the access path has changed.¶
The COMPOUND status is not NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is asserted.¶

In this case, no transition recovery activity is required on the file system(s) accessed by the request. However, to prevent avoidable lease expiration, migration discovery needs to be done.¶
The COMPOUND status is not NFS4ERR_MOVED, and SEQ4_STATUS_LEASE_MOVED is clear.¶

In this case, neither transition-related activity nor migration discovery is required.¶

Note that the specified actions only need to be taken if they are not already going on. For example, when NFS4ERR_MOVED is received while accessing a file system for which transition recovery is already occurring, the client merely waits for that recovery to be completed, while the receipt of the SEQ4_STATUS_LEASE_MOVED indication only needs to initiate migration discovery for a server if such discovery is not already underway for that server.¶

The fact that a lease-migrated condition does not result in an error in NFSv4.1 has a number of important consequences. In addition to the fact that the two indications are not mutually exclusive, as discussed above, there are number of issues that are important in considering implementation of migration discovery, as discussed in Section 15.13.2.¶

Because SEQ4_STATUS_LEASE_MOVED is not an error condition, it is possible for file systems whose access paths have not changed to be successfully accessed on a given server even though recovery is necessary for other file systems on the same server. As a result, access can take place while:¶

The migration discovery process is happening for that server.¶
The transition recovery process is happening for other file systems connected to that server.¶

15.13.2. Performing Migration Discovery

Migration discovery can be performed in the same context as transition recovery, allowing recovery for each migrated file system to be invoked as it is discovered. Alternatively, it may be done in a separate migration discovery thread, allowing migration discovery to be done in parallel with one or more instances of transition recovery.¶

In either case, because the lease-migrated indication does not result in an error, other access to file systems on the server can proceed normally, with the possibility that further such indications will be received, raising the issue of how such indications are to be dealt with. In general:¶

No action needs to be taken for such indications received by any threads performing migration discovery, since continuation of that work will address the issue.¶
In other cases in which migration discovery is currently being performed, nothing further needs to be done to respond to such lease migration indications, as long as one can be certain that the migration discovery process would deal with those indications. See below for details.¶
For such indications received in all other contexts, the appropriate response is to initiate or otherwise provide for the execution of migration discovery for file systems associated with the server IP address returning the indication.¶

This leaves a potential difficulty in situations in which the migration discovery process is near to completion but is still operating. One should not ignore a SEQ4_STATUS_LEASE_MOVED indication if the migration discovery process is not able to respond to the discovery of additional migrating file systems without additional aid. A further complexity relevant in addressing such situations is that a lease-migrated indication may reflect the server's state at the time the SEQUENCE operation was processed, which may be different from that in effect at the time the response is received. Because new migration events may occur at any time, and because a SEQ4_STATUS_LEASE_MOVED indication may reflect the situation in effect a considerable time before the indication is received, special care needs to be taken to ensure that SEQ4_STATUS_LEASE_MOVED indications are not inappropriately ignored.¶

A useful approach to this issue involves the use of separate externally-visible migration discovery states for each server. Separate values could represent the various possible states for the migration discovery process for a server:¶

Non-operation, in which migration discovery is not being performed.¶
Normal operation, in which there is an ongoing scan for migrated file systems.¶
Completion/verification of migration discovery processing, in which the possible completion of migration discovery processing needs to be verified.¶

Given that framework, migration discovery processing would proceed as follows:¶

While in the normal-operation state, the thread performing discovery would fetch, for successive file systems known to the client on the server being worked on, a file system location attribute plus the fs_status attribute.¶
If the fs_status attribute indicates that the file system is a migrated one (i.e., fss_absent is true, and fss_type != STATUS4_REFERRAL), then a migrated file system has been found. In this situation, it is likely that the fetch of the file system location attribute has cleared one of the file systems contributing to the lease-migrated indication.¶
In cases in which that happened, the thread cannot know whether the lease-migrated indication has been cleared, and so it enters the completion/verification state and proceeds to issue a COMPOUND to see if the SEQ4_STATUS_LEASE_MOVED indication has been cleared.¶
When the discovery process is in the completion/verification state, if other requests get a lease-migrated indication, they note that it was received. Later, the existence of such