Signaling Maximum SID Depth (MSD) Using OSPFApstra, Inc.jefftant.ietf@gmail.comHuawei Technologiesuma.chunduri@huawei.comGoogle, Inc.aldrin.ietf@gmail.comCisco Systemsppsenak@cisco.comBGP-LSSIDMSDOSPFThis document defines a way for an Open Shortest Path First (OSPF)
router to advertise multiple
types of supported Maximum SID Depths (MSDs) at node and/or link
granularity. Such advertisements allow entities (e.g., centralized
controllers) to determine whether a particular Segment
Identifier (SID) stack can be supported
in a given network. This document only refers to the Signaling MSD as
defined in RFC 8491, but it defines an encoding that can support
other MSD types. Here, the term "OSPF" means both OSPFv2 and OSPFv3.
When Segment Routing (SR) paths are computed by a centralized
controller, it is critical that the controller learn the Maximum SID
Depth (MSD) that can be imposed at each node/link on a given SR path.
This ensures that the Segment Identifier (SID) stack depth of a computed
path doesn't exceed the number of SIDs the node is capable of
imposing. defines how to signal MSD in the
Path Computation Element Communication Protocol (PCEP). However, if PCEP
is not supported/configured on the head-end of an SR tunnel or a
Binding-SID anchor node, and the controller does not participate in IGP
routing, it has no way of learning the MSD of nodes and
links. BGP&nbhy;LS (Distribution of Link-State and TE Information Using
BGP) defines a way to
expose topology and associated attributes and capabilities of the nodes
in that topology to a centralized controller. MSD signaling by BGP-LS
has been defined in . Typically, BGP-LS is
configured on a small number of nodes that do not necessarily act as
head-ends. In order for BGP-LS to signal MSD for all the nodes and links
in the network for which MSD is relevant, MSD capabilities SHOULD be
advertised by every OSPF router in the network.Other types of MSDs are known to be useful. For example,
defines Entropy Readable Label Depth (ERLD),
which is used by a head&nbhy;end to insert an Entropy Label (EL) at a
depth where it can be read by transit nodes.This document defines an extension to OSPF used to advertise one or
more types of MSDs at node and/or link granularity.
In the future, it is expected that new MSD-Types will be defined to
signal additional capabilities, e.g., ELs, SIDs that can be
imposed through recirculation, or SIDs associated with another data
plane such as IPv6. MSD advertisements MAY be useful even if SR itself is
not enabled. For example, in a non-SR MPLS network, MSD defines the
maximum label depth.This memo makes use of the terms defined in .Distribution of Link-State and TE Information Using
BGPOpen Shortest Path FirstMaximum SID Depth - the number of SIDs supported by a
node or a link on a nodeSegment Identifier as defined in Imposition is the act of modifying and/or
adding labels to the outgoing label stack associated with a packet. This
includes:
replacing the label at the top of the label stack with a new
labelpushing one or more new labels onto the label stackThe number of labels imposed is then the sum of the number of
labels that are replaced and the number of labels that are
pushed. See for further details.Path Computation Element Communication ProtocolSegment RoutingLink State AdvertisementRouter InformationThe 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
when,
and only when, they appear in all capitals, as shown here.The Node MSD TLV within the body of the OSPF RI Opaque LSA is defined to carry the provisioned SID depth of the
router originating the RI LSA. Node MSD is the smallest MSD supported
by the node on the set of interfaces configured for use by the
advertising IGP instance. MSD values may be learned via a hardware API
or may be provisioned.
Type: 12Length: variable (multiple of 2 octets); represents the total
length of the value field in octets.Value: consists of one or more pairs of a 1&nbhy;octet MSD-Type and
1&nbhy;octet MSD-Value.MSD-Type: one of the values defined in the "IGP MSD-Types" registry
defined in .MSD-Value: a number in the range of 0-255. For all MSD-Types, 0
represents the lack of ability to impose an MSD stack of any depth; any
other value represents that of the node. This value MUST represent the
lowest value supported by any link configured for use by the advertising
OSPF instance.This TLV is optional and is applicable to both OSPFv2 and OSPFv3.
The scope of the advertisement is specific to the deployment.When multiple Node MSD TLVs are received from a given router, the
receiver MUST use the first occurrence of the TLV in the Router
Information (RI) LSA. If the Node MSD TLV appears in multiple RI
LSAs that have different flooding scopes, the Node
MSD TLV in the RI LSA with the area-scoped
flooding scope MUST be used. If the Node MSD TLV appears in
multiple RI LSAs that have the same flooding scope,
the Node MSD TLV in the RI LSA with the
numerically smallest Instance ID MUST be used and other
instances of the Node MSD TLV MUST be ignored.
The RI LSA can be advertised at any of the defined opaque flooding
scopes (link, area, or Autonomous System (AS)). For the purpose of
Node MSD TLV advertisement, area-scoped flooding is RECOMMENDED. The Link MSD sub-TLV is defined to carry the MSD of the interface
associated with the link. MSD values may be learned via a hardware API
or may be provisioned.Type:For OSPFv2, the link-level MSD-Value is advertised as an
optional sub-TLV of the OSPFv2 Extended Link TLV as defined in
and has a type of 6.For OSPFv3, the link-level MSD-Value is advertised as an
optional sub-TLV of the E-Router-LSA TLV as defined in and has a type of 9.
Length: variable; same as defined in .Value: consists of one or more pairs of a 1&nbhy;octet MSD-Type and
1&nbhy;octet MSD-Value. MSD-Type: one of the values defined in the "IGP MSD-Types" registry
defined in .The MSD-Value field contains the Link MSD of the router originating
the corresponding LSA as specified for OSPFv2 and OSPFv3. The Link MSD
is a number in the range of 0-255. For all MSD-Types, 0 represents the
lack of ability to impose an MSD stack of any depth; any other value
represents that of the particular link when used as an outgoing
interface.
If this sub-TLV is advertised multiple times for the same link in
different OSPF Extended Link Opaque LSAs / E-Router-LSAs originated
by the same OSPF router, the sub-TLV in the OSPFv2 Extended Link
Opaque LSA with the smallest Opaque ID or in the OSPFv3
E-Router-LSA with the smallest Link State ID MUST be used by
receiving OSPF routers. This situation SHOULD be logged as an error.
When Link MSD is present for a given MSD-Type, the value of the Link
MSD MUST take precedence over the Node MSD. When a Link MSD&nbhy;Type
is not signaled but the Node MSD-Type is, then the Node MSD&nbhy;Type
value MUST be considered as the MSD value for that link.In order to increase flooding efficiency, it is RECOMMENDED that
routers with homogenous Link MSD values advertise just the Node MSD
value.The meaning of the absence of both Node and Link MSD advertisements
for a given MSD-Type is specific to the MSD-Type. Generally, it can only
be inferred that the advertising node does not support advertisement of
that MSD-Type. However, in some cases the lack of advertisement might
imply that the functionality associated with the MSD-Type is not
supported. Per , the correct interpretation MUST
be specified when an MSD-Type is defined.This specification updates several existing OSPF registries.IANA has allocated TLV type 12 from the "OSPF Router
Information (RI) TLVs" registry as defined by .
IANA has allocated sub-TLV type 6 from
the "OSPFv2 Extended Link TLV Sub-TLVs" registry.
IANA has allocated sub-TLV type 9 from the "OSPFv3 Extended-LSA
Sub&nbhy;TLVs" registry.
Security concerns for OSPF are addressed in ,
, and .
Further security analysis for the OSPF protocol is done in . Security considerations as specified by , , and
are applicable to this document.Implementations MUST ensure that malformed TLVs and sub-TLVs defined in
this document are detected and do not provide a vulnerability for
attackers to crash the OSPF router or routing process. Reception
of malformed TLVs or sub-TLVs SHOULD be counted and/or logged for
further analysis. Logging of malformed TLVs and sub-TLVs SHOULD be
rate&nbhy;limited to prevent a Denial-of-Service (DoS) attack (distributed
or otherwise) from overloading the OSPF control plane.Advertisement of an incorrect MSD value may have negative
consequences. If the value is smaller than supported, path computation
may fail to compute a viable path. If the value is larger than
supported, an attempt to instantiate a path that can't be supported by
the head-end (the node performing the SID imposition) may occur.The presence of this information may also inform an attacker of how
to induce any of the aforementioned conditions.There's no DoS risk specific to this extension, and it is
not vulnerable to replay attacks.PCEP Extensions for Segment RoutingSignaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-StateSignaling Entropy Label Capability and Entropy Readable Label-stack Depth Using OSPFThe authors would like to thank Acee Lindem, Ketan Talaulikar,
Tal Mizrahi, Stephane Litkowski, and Bruno Decraene for their reviews
and valuable comments.The following person contributed to this document:Les GinsbergEmail: ginsberg@cisco.com