| Internet-Draft | SR Policy Composite Path in BGP-LS | August 2023 |
| Li, et al. | Expires 1 March 2024 | [Page] |
Segment Routing is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. An SR Policy is associated with one or more candidate paths, and each candidate path is either dynamic, explicit or composite. This document specifies the extensions to BGP Link State (BGP-LS) to carry composite candidate path information in the advertisement of an SR policy.¶
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As described in [RFC7752], BGP Link State (BGP-LS) provides a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol.¶
Segment routing (SR) [RFC8402] is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. The ingress node steers packets into a specific path according to the Segment Routing Policy (SR Policy) as defined in [RFC9256].¶
An SR Policy is associated with one or more candidate paths. A composite candidate path acts as a container for grouping of SR Policies. As described in section 2.2 in [RFC9256], the composite candidate path construct enables combination of SR Policies, each with explicit candidate paths and/or dynamic candidate paths with potentially different optimization objectives and constraints, for a load-balanced steering of packet flows over its constituent SR Policies.¶
[I-D.draft-jiang-spring-parent-sr-policy-use-cases] describes some use cases for SR policy group composite candidate path.¶
[I-D.draft-ietf-idr-bgp-ls-sr-policy] describes a mechanism to collect the SR policy information that is locally available in a node and advertise it into BGP-LS updates. This document extends it to provide some extra information to carry composite candidate path information in the BGP-LS advertisement.¶
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.¶
[RFC7752] defines the BGP-LS NLRI that can be a Node NLRI, a Link NLRI or a Prefix NLRI. The corresponding BGP-LS attribute is a Node Attribute, a Link Attribute or a Prefix Attribute. [I-D.draft-ietf-idr-bgp-ls-sr-policy] describes a mechanism to collect the SR Policy information that is locally available in a node and advertise it into BGP Link State (BGP-LS) updates. This section defines a new sub-TLV which is carried in the optional non-transitive BGP Attribute "LINK_STATE Attribute" defined in [RFC7752].¶
Segment Routing Policy (SR Policy) architecture is specified in [RFC9256]. A SR Policy can comprise of one or more candidate paths, and each candidate path is either dynamic, explicit or composite. A composite candidate path can comprise of one or more constituent SR policies. The endpoints of the constituent SR Policies and the parent SR Policy MUST be identical, and the colors of each of the constituent SR Policies and the parent SR Policy MUST be different.¶
The Constituent SR Policy TLV is used to report the constituent SR policy(s) of a composite candidate path. The TLV has 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Color | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-TLVs (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
The document does not bring new operation beyond the description of operations defined in [RFC7752] and [I-D.draft-ietf-idr-bgp-ls-sr-policy]. The existing operations defined in [RFC7752] and [I-D.draft-ietf-idr-bgp-ls-sr-policy] can apply to this document directly.¶
Typically but not limit to, the BGP-LS messages carring composite candidate path information along with the SR policy are distributed to a controller.¶
After configuration, the composite candidate path information will be advertised by BGP update messages. The operation of advertisement is the same as defined in [RFC7752] and [I-D.draft-ietf-idr-bgp-ls-sr-policy], as well as the receiption.¶
Procedures and protocol extensions defined in this document do not affect the security considerations discussed in [I-D.draft-ietf-idr-bgp-ls-sr-policy].¶
This document defines a new TLV in the BGP-LS Link Descriptor and Attribute TLVs:¶
| Value | Description | Reference |
|---|---|---|
| TBA | Constituent SR Policy TLV | This document |