RFC 0000 BGP LS extensions for Segment Routing July 2019
Previdi, et al. Standards Track [Page]
Stream:
Internet Engineering Task Force (IETF)
RFC:
0000
Category:
Standards Track
Published:
ISSN:
2070-1721
Authors:
S. Previdi
Huawei Technologies
K. Talaulikar, Ed.
Cisco Systems, Inc.
C. Filsfils
Cisco Systems, Inc.
H. Gredler
RtBrick Inc.
M. Chen
Huawei Technologies

BGP Link-State extensions for Segment Routing

Abstract

Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by routing protocols e.g. by the link state routing protocols (IS-IS, OSPFv2 and OSPFv3) within IGP topologies.

This document defines extensions to the BGP Link-state address-family in order to carry segment routing information via BGP.

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc0000.

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Table of Contents

1. Introduction

Segment Routing (SR) allows for a flexible definition of end-to-end paths by combining sub-paths called "segments". A segment can represent any instruction: topological or service-based. A segment can have a local semantic to an SR node or global semantic within a domain. Within IGP topologies, an SR path is encoded as a sequence of topological sub-paths, called "IGP segments". These segments are advertised by the link-state routing protocols (IS-IS, OSPFv2 and OSPFv3).

[RFC8402] defines the Link-State IGP segments - Prefix, Node, Anycast and Adjacency segments. Prefix segments, by default, represent an ECMP-aware shortest-path to a prefix, as per the state of the IGP topology. Adjacency segments represent a hop over a specific adjacency between two nodes in the IGP. A prefix segment is typically a multi-hop path while an adjacency segment, in most of the cases, is a one-hop path. Node and anycast segments are variations of the prefix segment with their specific characteristics.

When Segment Routing is enabled in an IGP domain, segments are advertised in the form of Segment Identifiers (SIDs). The IGP link-state routing protocols have been extended to advertise SIDs and other SR-related information. IGP extensions are described for: IS-IS [ISIS-SR-EXT], OSPFv2 [OSPF-SR-EXT] and OSPFv3 [OSPFv3-SR-EXT]. Using these extensions, Segment Routing can be enabled within an IGP domain.

Segment Routing (SR) allows advertisement of single or multi-hop paths. The flooding scope for the IGP extensions for Segment routing is IGP area-wide. Consequently, the contents of a Link State Database (LSDB) or a Traffic Engineering Database (TED) has the scope of an IGP area and therefore, by using the IGP alone it is not enough to construct segments across multiple IGP Area or AS boundaries.

In order to address the need for applications that require topological visibility across IGP areas, or even across Autonomous Systems (AS), the BGP-LS address-family/sub-address-family have been defined to allow BGP to carry Link-State information. The BGP Network Layer Reachability Information (NLRI) encoding format for BGP-LS and a new BGP Path Attribute called the BGP-LS attribute are defined in [RFC7752]. The identifying key of each Link-State object, namely a node, link, or prefix, is encoded in the NLRI and the properties of the object are encoded in the BGP-LS attribute.

                        +------------+
                        |  Consumer  |
                        +------------+
                              ^
                              |
                              v
                    +-------------------+
                    |    BGP Speaker    |         +-----------+
                    | (Route-Reflector) |         | Consumer  |
                    +-------------------+         +-----------+
                          ^   ^   ^                       ^
                          |   |   |                       |
          +---------------+   |   +-------------------+   |
          |                   |                       |   |
          v                   v                       v   v
    +-----------+       +-----------+             +-----------+
    |    BGP    |       |    BGP    |             |    BGP    |
    |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
    +-----------+       +-----------+             +-----------+
          ^                   ^                         ^
          |                   |                         |
         IGP                 IGP                       IGP
Figure 1: Link State Info Collection

Figure 1 denotes a typical deployment scenario. In each IGP area, one or more nodes are configured with BGP-LS. These BGP speakers form an IBGP mesh by connecting to one or more route-reflectors. This way, all BGP speakers (specifically the route-reflectors) obtain Link-State information from all IGP areas (and from other ASes from EBGP peers). An external component connects to the route-reflector to obtain this information (perhaps moderated by a policy regarding what information is or isn't advertised to the external component) as described in [RFC7752].

This document describes extensions to BGP-LS to advertise the SR information. An external component (e.g., a controller) can collect SR information from across an SR domain (as described in [RFC8402]) and construct the end-to-end path (with its associated SIDs) that need to be applied to an incoming packet to achieve the desired end-to-end forwarding. SR operates within a trusted domain consisting of a single or multiple ASes managed by the same administrative entity e.g. within a single provider network.

2. BGP-LS Extensions for Segment Routing

This document defines SR extensions to BGP-LS and specifies the TLVs and sub-TLVs for advertising SR information within the BGP-LS Attribute. Section 2.4 and Section 2.5 lists the equivalent TLVs and sub-TLVs in IS-IS, OSPFv2 and OSPFv3 protocols.

BGP-LS [RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link NLRI or a Prefix NLRI. BGP-LS [RFC7752] defines the TLVs that map link-state information to BGP-LS NLRI within the BGP-LS Attribute. This document adds additional BGP-LS Attribute TLVs in order to encode SR information. It does not introduce any changes to the encoding of the BGP-LS NLRIs.

2.1. Node Attributes TLVs

The following Node Attribute TLVs are defined:

Table 1: Node Attribute TLVs
Type Description Section
1161 SID/Label Section 2.1.1
1034 SR Capabilities Section 2.1.2
1035 SR Algorithm Section 2.1.3
1036 SR Local Block Section 2.1.4
1037 SRMS Preference Section 2.1.5

These TLVs should only be added to the BGP-LS Attribute associated with the Node NLRI describing the IGP node that is originating the corresponding IGP TLV/sub-TLV described below.

2.1.1. SID/Label TLV

The SID/Label TLV is used as a sub-TLV by the SR Capabilities (Section 2.1.2) and Segment Routing Local Block (SRLB) (Section 2.1.4) TLVs. This information is derived from the protocol specific advertisements.

The TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Type            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      SID/Label (variable)                    //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SID/Label TLV Format

Where:

  • Type:
    1161
    Length:
    Variable. Either 3 or 4 depending whether the value is encoded as a label or as an index/SID.
    SID/Label:
    If length is set to 3, then the 20 rightmost bits represent a label (the total TLV size is 7) and the 4 leftmost bits are set to 0. If length is set to 4, then the value represents a 32 bit SID (the total TLV size is 8).

2.1.2. SR Capabilities TLV

The SR Capabilities TLV is used in order to advertise the node's SR Capabilities including its Segment Routing Global Base (SRGB) range(s). In the case of IS-IS, the capabilities also include the IPv4 and IPv6 support for the SR-MPLS forwarding plane. This information is derived from the protocol specific advertisements.

The SR Capabilities TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Type            |          Length               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Flags    |   Reserved    | 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Range Size 1                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                SID/Label sub-TLV 1                           //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Range Size N                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                SID/Label sub-TLV N                           //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SR Capabilities TLV Format
Where:
Type:
1034
Length:
Variable. Minimum length is 12.
Flags:
1 octet of flags as defined in Section 3.1 of [ISIS-SR-EXT]MUST be set to 0 and ignored on receipt.
Reserved:
1 octet that MUST be set to 0 and ignored on receipt.

One or more entries, each of which have the following format:

  • Range Size: 3 octet with a non-zero value indicating the number of labels in the range.
  • SID/Label TLV (as defined in Section 2.1.1) used as sub-TLV which encodes the first label in the range. Since the SID/Label TLV is used to indicate the first label of the SRGB range, only label encoding is valid under the SR Capabilities TLV.

2.1.3. SR Algorithm TLV

The SR Algorithm TLV is used in order to advertise the SR Algorithms supported by the node. This information is derived from the protocol specific advertisements.

The SR Algorithm TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type               |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Algorithm 1  |  Algorithm... |  Algorithm N  |                
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SR Algorithm TLV Format

Where:

Type:
1035
Length:
Variable. Minimum length is 1 and maximum can be 256.
Algorithm:
One or more fields of 1 octet each identifying the algorithm.

2.1.4. SR Local Block TLV

The SR Local Block (SRLB) TLV contains the range(s) of labels the node has reserved for local SIDs. Local SIDs are used, e.g., in IGP (IS-IS, OSPF) for Adjacency-SIDs, and may also be allocated by components other than IGP protocols. As an example, an application or a controller may instruct a node to allocate a specific local SID. Therefore, in order for such applications or controllers to know the range of local SIDs available, it is required that the node advertises its SRLB.

This information is derived from the protocol specific advertisements.

The SRLB TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Type            |               Length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Flags    |   Reserved    | 
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Sub-Range Size 1                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                SID/Label sub-TLV 1                           //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Sub-Range Size N                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                SID/Label sub-TLV N                           //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: SRLB TLV Format
Where:
Type:
1036
Length:
Variable. Minimum length is 12.
Flags:
1 octet of flags. The flags are as defined in Section 3.3 of [ISIS-SR-EXT]MUST be set to 0 and ignored on receipt.
Reserved:
1 octet that MUST be set to 0 and ignored on receipt.

One or more entries corresponding to sub-range(s), each of which have the following format:

  • Range Size: 3 octet value indicating the number of labels in the range.
  • SID/Label TLV (as defined in Section 2.1.1) used as sub-TLV which encodes the first label in the sub-range. Since the SID/Label TLV is used to indicate the first label of the SRLB sub-range, only label encoding is valid under the SR Local Block TLV.

2.1.5. SRMS Preference TLV

The Segment Routing Mapping Server (SRMS) Preference TLV is used in order to associate a preference with SRMS advertisements from a particular source. [LDP-INTEROP] specifies the SRMS functionality along with SRMS preference of the node advertising the SRMS Prefix-to-SID Mapping ranges.

This information is derived from the protocol specific advertisements.

The SRMS Preference TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type               |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference    |
+-+-+-+-+-+-+-+-+
Figure 6: SRMS Preference TLV Format

Where:

TType:
1037
Length:
1.
Preference:
1 octet carrying an unsigned 8 bit SRMS preference.

2.3. Prefix Attribute TLVs

The following Prefix Attribute TLVs are defined:

Table 4: Prefix Attribute TLVs
Type Description Section
1158 Prefix SID Section 2.3.1
1159 Range Section 2.3.4
1170 Prefix Attribute Flags Section 2.3.2
1171 Source Router-ID Section 2.3.3

These TLVs should only be added to the BGP-LS Attribute associated with the Prefix NLRI describing the prefix of the IGP node that is originating the corresponding IGP TLV/sub-TLV described below.

2.3.1. Prefix SID TLV

The Prefix SID TLV is used in order to advertise information related to a Prefix SID. This information is derived from Prefix-SID sub-TLV of IS-IS (Section 2.1 of [ISIS-SR-EXT]Section 5 of [OSPF-SR-EXT]Section 6 of [OSPFv3-SR-EXT]

The Prefix SID TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Type            |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Flags     |   Algorithm   |           Reserved            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       SID/Index/Label (variable)             //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Prefix SID TLV Format

Where:

Type: 1158
Length: Variable. 7 or 8 depending on Label or Index encoding of the SID

Flags: 1 octet value which should be set as:

Algorithm: 1 octet value identify the algorithm. The semantics of algorithm are described in Section 3.1.1 of [RFC8402]
Reserved: 2 octets that MUST be set to 0 and ignored on receipt.

SID/Index/Label:

The Flags and, as an extension, the SID/Index/Label fields of this TLV are interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing these fields.

2.3.2. Prefix Attribute Flags TLV

The Prefix Attribute Flags TLV carries IPv4/IPv6 prefix attribute flags information. These flags are defined for OSPFv2 in Section 2.1 of [RFC7684]Appendix A.4.1.1 of [RFC5340]Section 2.1 of [RFC7794]

The Prefix Attribute Flags TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type               |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Flags (variable)                      //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Prefix Attribute Flags TLV Format

Where:

Type: 1170
Length: Variable.

Flags: a variable length flag field (according to the length field). Flags are routing protocol specific and are to be set as below:

The Flags field of this TLV is interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing this field.

2.3.3. Source Router Identifier (Source Router-ID) TLV

The Source Router-ID TLV contains the IPv4 or IPv6 Router-ID of the originator of the Prefix. For the IS-IS protocol this is derived from the IPv4/IPv6 Source Router ID sub-TLV as defined in Section 2.2 of [RFC7794]Section 4 of [OSPF-PREFIX-ORIG]

The Source Router-ID TLV has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Type               |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   4 or 16 octet Router-ID                    //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Source Router-ID TLV Format

Where:

Type: 1171
Length: Variable. 4 or 16 in case of IS-IS and 4 in case of OSPF.
Router-ID: the IPv4 or IPv6 Router-ID in case of IS-IS and the OSPF Router-ID in the case of OSPF.

2.3.4. Range TLV

The Range TLV is used in order to advertise a range of prefix-to-SID mappings as part of the Segment Routing Mapping Server (SRMS) functionality [LDP-INTEROP], as defined in the respective underlying IGP SR extensions [OSPF-SR-EXT], Section 4[OSPFv3-SR-EXT], Section 5[ISIS-SR-EXT], Section 2.4

A Prefix NLRI, that been advertised with a Range TLV, is considered a normal routing prefix (i.e. prefix reachability) only when there is also an IGP metric TLV (TLV 1095) associated it. Otherwise, it is considered only as the first prefix in the range for prefix-to-SID mapping advertisement.

The format of the Range TLV is as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Type              |             Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     Flags     | Reserved      |             Range Size        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           sub-TLVs                           //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: Range TLV Format

Where:

  • Type: 1159
  • Length: Variable. 11 or 12 depending on Label or Index encoding of the SID

  • Flags: 1 octet value which should be set as:

  • Reserved: 1 octet that MUST be set to 0 and ignored on receipt.
  • Range Size: 2 octets that carry the number of prefixes that are covered by the advertisement..

The Flags field of this TLV is interpreted according to the respective underlying IS-IS, OSPFv2 or OSPFv3 protocol. The Protocol-ID of the BGP-LS Prefix NLRI is used to determine the underlying protocol specification for parsing this field.

The prefix-to-SID mappings are advertised using sub-TLVs as below:

IS-IS:
    SID/Label Range TLV
        Prefix-SID sub-TLV

OSPFv2/OSPFv3:
    OSPFv2/OSPFv3 Extended Prefix Range TLV
        Prefix SID sub-TLV

BGP-LS:
    Range TLV
        Prefix-SID TLV (used as a sub-TLV in this context)

The prefix-to-SID mapping information for the BGP-LS Prefix-SID TLV (used as sub-TLV in this context) is encoded as described in Section 2.3.1.

2.4. Equivalent IS-IS Segment Routing TLVs/Sub-TLVs

This section illustrate the IS-IS Segment Routing Extensions TLVs and sub-TLVs mapped to the ones defined in this document.

The following table, illustrates for each BGP-LS TLV, its equivalence in IS-IS.

Table 5: IS-IS Segment Routing Extensions TLVs/Sub-TLVs
Description IS-IS TLV/sub-TLV Reference
SR Capabilities SR-Capabilities sub-TLV (2) [ISIS-SR-EXT]
SR Algorithm SR-Algorithm sub-TLV (19) [ISIS-SR-EXT]
SR Local Block SR Local Block sub-TLV (22) [ISIS-SR-EXT]
SRMS Preference SRMS Preference sub-TLV (19) [ISIS-SR-EXT]
Adjacency SID Adj-SID sub-TLV (31) [ISIS-SR-EXT]
LAN Adjacency SID LAN-Adj-SID sub-TLV (32) [ISIS-SR-EXT]
Prefix SID Prefix-SID sub-TLV (3) [ISIS-SR-EXT]
Range SID/Label Binding TLV (149) [ISIS-SR-EXT]
SID/Label SID/Label sub-TLV (1) [ISIS-SR-EXT]
Prefix Attribute Flags Prefix Attributes Flags sub-TLV (4) [RFC7794]
Source Router-ID IPv4/IPv6 Source Router ID sub-TLV (11/12) [RFC7794]
L2 Bundle Member Attributes L2 Bundle Member Attributes TLV (25) [L2-BUNDLE-ISIS]

2.5. Equivalent OSPFv2/OSPFv3 Segment Routing TLVs/Sub-TLVs

This section illustrate the OSPFv2 and OSPFv3 Segment Routing Extensions TLVs and sub-TLVs mapped to the ones defined in this document.

The following table, illustrates for each BGP-LS TLV, its equivalence in OSPFv2 and OSPFv3.

Table 6: OSPFv2 Segment Routing Extensions TLVs/Sub-TLVs
Description OSPFv2 TLV/sub-TLV Reference
SR Capabilities SID/Label Range TLV (9) [OSPF-SR-EXT]
SR Algorithm SR-Algorithm TLV (8) [OSPF-SR-EXT]
SR Local Block SR Local Block TLV (14) [OSPF-SR-EXT]
SRMS Preference SRMS Preference TLV (15) [OSPF-SR-EXT]
Adjacency SID Adj-SID sub-TLV (2) [OSPF-SR-EXT]
LAN Adjacency SID LAN Adj-SID sub-TLV (3) [OSPF-SR-EXT]
Prefix SID Prefix SID sub-TLV (2) [OSPF-SR-EXT]
Range OSPF Extended Prefix Range TLV (2) [OSPF-SR-EXT]
SID/Label SID/Label sub-TLV (1) [OSPF-SR-EXT]
Prefix Attribute Flags Flags of OSPFv2 Extended Prefix TLV (1) [RFC7684]
Source Router-ID Prefix Source Router-ID sub-TLV (TBD) [OSPF-PREFIX-ORIG]
Table 7: OSPFv3 Segment Routing Extensions TLVs/Sub-TLVs
Description OSPFv3 TLV/sub-TLV Reference
SR Capabilities SID/Label Range TLV (9) [OSPF-SR-EXT]
SR Algorithm SR-Algorithm TLV (8) [OSPF-SR-EXT]
SR Local Block SR Local Block TLV (14) [OSPF-SR-EXT]
SRMS Preference SRMS Preference TLV (15) [OSPF-SR-EXT]
Adjacency SID Adj-SID sub-TLV (5) [OSPFv3-SR-EXT]
LAN Adjacency SID LAN Adj-SID sub-TLV (6) [OSPFv3-SR-EXT]
Prefix SID Prefix SID sub-TLV (4) [OSPFv3-SR-EXT]
Range OSPFv3 Extended Prefix Range TLV (9) [OSPFv3-SR-EXT]
SID/Label SID/Label sub-TLV (7) [OSPFv3-SR-EXT]
Prefix Attribute Flags Prefix Option Fields of Prefix TLV types 3,5,6 [RFC8362]
Source Router-ID Prefix Source Router-ID sub-TLV (TBD) [OSPF-PREFIX-ORIG]

3. IANA Considerations

Early allocation of codepoints has been done by IANA for this document from the registry "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" under the "BGP-LS Parameters" registry based on Table 8. The column "IS-IS TLV/Sub-TLV" defined in the registry does not require any value and should be left empty.

3.1. TLV/Sub-TLV Code Points Summary

This section contains the global table of all TLVs/sub-TLVs defined in this document.

Table 8: Summary Table of TLV/Sub-TLV Codepoints
TLV Code Point Description Reference
1034 SR Capabilities Section 2.1.2
1035 SR Algorithm Section 2.1.3
1036 SR Local Block Section 2.1.4
1037 SRMS Preference Section 2.1.5
1099 Adjacency SID Section 2.2.1
1100 LAN Adjacency SID Section 2.2.2
1158 Prefix SID Section 2.3.1
1159 Range Section 2.3.4
1161 SID/Label Section 2.1.1
1170 Prefix Attribute Flags Section 2.3.2
1171 Source Router-ID Section 2.3.3
1172 L2 Bundle Member Attributes Section 2.2.3

4. Manageability Considerations

This section is structured as recommended in [RFC5706].

The new protocol extensions introduced in this document augment the existing IGP topology information that is distributed via [RFC7752]. Procedures and protocol extensions defined in this document do not affect the BGP protocol operations and management other than as discussed in the Manageability Considerations section of [RFC7752]. Specifically, the malformed attribute tests for syntactic checks in the Fault Management section of [RFC7752] now encompass the new BGP-LS Attribute TLVs defined in this document. The semantic or content checking for the TLVs specified in this document and their association with the BGP-LS NLRI types or their BGP-LS Attribute is left to the consumer of the BGP-LS information (e.g. an application or a controller) and not the BGP protocol.

A consumer of the BGP-LS information retrieves this information over a BGP-LS session (refer to Section 1 and 2 of [RFC7752]). The handling of semantic or content errors by the consumer would be dictated by the nature of its application usage and hence is beyond the scope of this document.

This document only introduces new Attribute TLVs and any syntactic error in them would result in only that specific attribute being discarded with an error log. The SR information introduced in BGP-LS by this specification, may be used by BGP-LS consumer applications like a SR path computation engine (PCE) to learn the SR capabilities of the nodes in the topology and the mapping of SR segments to those nodes. This can enable the SR PCE to perform path computations based on SR for traffic engineering use-cases and to steer traffic on paths different from the underlying IGP based distributed best path computation. Errors in the encoding or decoding of the SR information may result in the unavailability of such information to the SR PCE or incorrect information being made available to it. This may result in the SR PCE not being able to perform the desired SR based optimization functionality or to perform it in an unexpected or inconsistent manner. The handling of such errors by applications like SR PCE may be implementation specific and out of scope of this document.

The extensions, specified in this document, do not introduce any new configuration or monitoring aspects in BGP or BGP-LS other than as discussed in [RFC7752]. The manageability aspects of the underlying SR features are covered by [SR-YANG], [IS-IS-SR-YANG] and [OSPF-SR-YANG].

5. Security Considerations

The new protocol extensions introduced in this document augment the existing IGP topology information that is distributed via [RFC7752]. The advertisement of the SR link attribute information defined in this document presents similar risk as associated with the existing set of link attribute information as described in [RFC7752]. The Security Considerations section of [RFC7752] also applies to these extensions. The procedures and new TLVs defined in this document, by themselves, do not affect the BGP-LS security model discussed in [RFC7752].

The TLVs introduced in this document are used to propagate IGP defined information ([ISIS-SR-EXT], [OSPF-SR-EXT] and [OSPFv3-SR-EXT]). These TLVs represent the SR information associated with the IGP node, link and prefix. The IGP instances originating these TLVs are assumed to support all the required security and authentication mechanisms (as described in [ISIS-SR-EXT], [OSPF-SR-EXT] and [OSPFv3-SR-EXT]) in order to prevent any security issue when propagating the TLVs into BGP-LS.

BGP-LS SR extensions enable traffic engineering use-cases within the Segment Routing domain. SR operates within a trusted domain [RFC8402] and its security considerations also apply to BGP-LS sessions when carrying SR information. The SR traffic engineering policies using the SIDs advertised via BGP-LS are expected to be used entirely within this trusted SR domain (e.g. between multiple AS/domains within a single provider network). Therefore, precaution is necessary to ensure that the link-state information (including SR information) advertised via BGP-LS sessions is limited to consumers in a secure manner within this trusted SR domain. BGP peering sessions for address-families other than Link-State may be setup to routers outside the SR domain. The isolation of BGP-LS peering sessions is recommended to ensure that BGP-LS topology information (including the newly added SR information) is not advertised to an external BGP peering session outside the SR domain.

6. Contributors

The following people have substantially contributed to the editing of this document:

Peter Psenak 
Cisco Systems
Email: ppsenak@cisco.com
Les Ginsberg 
Cisco Systems
Email: ginsberg@cisco.com
Acee Lindem
Cisco Systems
Email: acee@cisco.com
Saikat Ray
Individual
Email: raysaikat@gmail.com
Jeff Tantsura
Apstra Inc.
Email: jefftant.ietf@gmail.com

7. References

7.1. Normative References

[ISIS-SR-EXT]
Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC YYYY, , <https://www.rfc-editor.org/rfc/rfcYYYY>.
[L2-BUNDLE-ISIS]
Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and E. Aries, "Advertising L2 Bundle Member Link Attributes in IS-IS", RFC ZZZZ, , <https://www.rfc-editor.org/rfc/rfcZZZZ>.
[OSPF-PREFIX-ORIG]
Wang, A., Lindem, A., Dong, J., Talaulikar, K., and P. Psenak, "OSPF Extension for Prefix Originator", RFC AAAA, , <https://www.rfc-editor.org/rfc/rfcAAAA>.
[OSPF-SR-EXT]
Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF Extensions for Segment Routing", RFC BBBB, , <https://www.rfc-editor.org/rfc/rfcBBBB>.
[OSPFv3-SR-EXT]
Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions for Segment Routing", RFC CCCC, , <https://www.rfc-editor.org/rfc/rfcCCCC>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC4202]
Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4202, DOI 10.17487/RFC4202, , <https://www.rfc-editor.org/info/rfc4202>.
[RFC5340]
Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, , <https://www.rfc-editor.org/info/rfc5340>.
[RFC7684]
Psenak, P., Gredler, H., Shakir, R., Henderickx, W., Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute Advertisement", RFC 7684, DOI 10.17487/RFC7684, , <https://www.rfc-editor.org/info/rfc7684>.
[RFC7752]
Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, , <https://www.rfc-editor.org/info/rfc7752>.
[RFC7794]
Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, , <https://www.rfc-editor.org/info/rfc7794>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8362]
Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and F. Baker, "OSPFv3 Link State Advertisement (LSA) Extensibility", RFC 8362, DOI 10.17487/RFC8362, , <https://www.rfc-editor.org/info/rfc8362>.
[RFC8402]
Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, , <https://www.rfc-editor.org/info/rfc8402>.
[RFC8571]
Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of IGP Traffic Engineering Performance Metric Extensions", RFC 8571, DOI 10.17487/RFC8571, , <https://www.rfc-editor.org/info/rfc8571>.

7.2. Informative References

[IS-IS-SR-YANG]
Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J. Tantsura, "YANG Data Model for IS-IS Segment Routing", Work in Progress, draft-ietf-isis-sr-yang-06, .
[LDP-INTEROP]
Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., and S. Litkowski, "Segment Routing interworking with LDP", Work in Progress, draft-ietf-spring-segment-routing-ldp-interop-15, .
[OSPF-SR-YANG]
Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, "YANG Data Model for OSPF SR (Segment Routing) Protocol", Work in Progress, draft-ietf-ospf-sr-yang-08, .
[RFC5706]
Harrington, D., "Guidelines for Considering Operations and Management of New Protocols and Protocol Extensions", RFC 5706, DOI 10.17487/RFC5706, , <https://www.rfc-editor.org/info/rfc5706>.
[SR-YANG]
Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J. Tantsura, "YANG Data Model for Segment Routing", Work in Progress, draft-ietf-spring-sr-yang-13, .

Acknowledgements

The authors would like to thank Jeffrey Haas, Aijun Wang, Robert Raszuk and Susan Hares for their review of this document and their comments. The authors would also like to thank Alvaro Retana for his extensive review and comments which helped correct issues and improve the document.

Authors' Addresses

Stefano Previdi
Huawei Technologies
Rome
Italy
Ketan Talaulikar (editor)
Cisco Systems, Inc.
India
Cisco Systems, Inc.
Clarence Filsfils
Brussels
Belgium
Hannes Gredler
RtBrick Inc.
China
100095
Beijing
Huawei Building, No. 156 Beiqing Rd.
Huawei Technologies
Mach(Guoyi) Chen