| Internet-Draft | SR Policy Composite Path in BGP-LS | October 2025 |
| Lin, et al. | Expires 23 April 2026 | [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 [RFC9552], 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.jiang-idr-sr-policy-composite-path] defines extensions for BGP to distribute SR policies carrying composite candidate path information. While as defined in Section 3.6 of [I-D.ietf-pce-multipath], PCEP signals the 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.¶
[RFC9552] 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 [RFC9552].¶
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 Composite Candidate Path TLV is used to report the constituent SR policy(s) of a composite candidate path. Only a single instance of this TLV is advertised for a given candidate path. If multiple instances are present, then the first valid (i.e., not determined to be malformed as per section 8.2.2 of [RFC9552]) one is used and the rest are ignored.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:¶
Type: to be assigned by IANA.¶
Length: the total length of the value field not including Type and Length fields.¶
Reserved: 32 bits reserved and MUST be set to 0 on transmission and MUST be ignored on receipt.¶
Color: 4 octets that indicates the color of the constituent SR Policy.¶
Weight: 4 octet field that indicates the weight associated with the SID-List for weighted load-balancing. Refer Section 2.2 and 2.11 of [RFC9256].¶
Sub-TLVs: variable and contains any other optional attributes associated with the Composite Candidate Path. Currently, the sub-TLV only defines the Per-Flow Forwarding Class TLV.¶
Per-Flow Candidate Path builds on top of the concept of the Composite Candidate Path. Each Path in a Per-Flow Candidate Path is assigned a 3-bit forward class value, which allows Quality of Service (QoS) classified traffic to be steered depending on the forward class. The Per-Flow Forwarding Class TLV is an optional sub-TLV of the Composite Candidate Path TLV.Only a single instance of this sub-TLV is advertised for a given candidate path. If multiple instances are present, then the first valid (i.e., not determined to be malformed as per section 8.2.2 of [RFC9552]) one is used and the rest are ignored.¶
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 | FC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
Type (16 bits): TBD1 for "PER-FLOW-FORWARD-CLASS" TLV.¶
Length (16 bits): 4.¶
Reserved: This field MUST be set to zero on transmission and MUST be ignored on receipt.¶
FC (3 bits): Forward class value that is given by the QoS classifier to traffic entering the given Candidate Path. Different classes of traffic that enter the given Candidate Path can be differentially steered into different Colors.¶
The document does not bring new operation beyond the description of operations defined in [RFC9552] and [I-D.draft-ietf-idr-bgp-ls-sr-policy]. The existing operations defined in [RFC9552] 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 carrying 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 [RFC9552] and [I-D.draft-ietf-idr-bgp-ls-sr-policy], as well as the reception.¶
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 | Composite Candidate Path TLV | This document |
| TBA | Per-Flow Forwarding Class TLV | This document |