Network Working Group C. Lin Internet-Draft New H3C Technologies Intended status: Standards Track W. Cheng Expires: 23 April 2026 China Mobile Z. Ali A. MahendraBabu Cisco Systems, Inc 20 October 2025 Signaling Composite Candidate Path of SR Policy using BGP-LS draft-li-idr-bgpls-sr-policy-composite-path-08 Abstract 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. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 23 April 2026. Copyright Notice Copyright (c) 2025 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Lin, et al. Expires 23 April 2026 [Page 1] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. BGP-LS Extensions for Composite Candidate Path . . . . . . . 3 3.1. Composite Candidate Path TLV . . . . . . . . . . . . . . 3 3.2. Per-Flow Forwarding Class TLV . . . . . . . . . . . . . . 4 4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction 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. Lin, et al. Expires 23 April 2026 [Page 2] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 [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. 2. Terminology 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. 3. BGP-LS Extensions for Composite Candidate Path [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]. 3.1. Composite Candidate Path TLV 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: Lin, et al. Expires 23 April 2026 [Page 3] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 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. 3.2. 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. Lin, et al. Expires 23 April 2026 [Page 4] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 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. 4. Operations 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. 5. Security Considerations 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]. Lin, et al. Expires 23 April 2026 [Page 5] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 6. IANA Considerations 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 | +-------+-------------------------------+---------------+ Table 1 7. References 7.1. Normative References [I-D.draft-ietf-idr-bgp-ls-sr-policy] Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and J. Tantsura, "Advertisement of Segment Routing Policies using BGP Link-State", Work in Progress, Internet-Draft, draft- ietf-idr-bgp-ls-sr-policy-17, 10 September 2025, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, July 2018, . [RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, . 7.2. Informative References Lin, et al. Expires 23 April 2026 [Page 6] Internet-Draft SR Policy Composite Path in BGP-LS October 2025 [I-D.ietf-pce-multipath] Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P., Bidgoli, H., Yadav, B., Peng, S., Mishra, G. S., and S. Sidor, "Path Computation Element Communication Protocol (PCEP) Extensions for Signaling Multipath Information", Work in Progress, Internet-Draft, draft-ietf-pce- multipath-14, 5 September 2025, . [I-D.jiang-idr-sr-policy-composite-path] Wenying, J., Lin, C., and R. Chen, "BGP Extensions of SR Policy for Composite Candidate Path", Work in Progress, Internet-Draft, draft-jiang-idr-sr-policy-composite-path- 02, 13 September 2025, . [RFC9552] Talaulikar, K., "Distribution of Link-State and Traffic Engineering Information Using BGP", RFC 9552, DOI 10.17487/RFC9552, January 2024, . Authors' Addresses Changwang Lin New H3C Technologies Email: linchangwang.04414@h3c.com Weiqiang Cheng China Mobile Email: chengweiqiang@chinamobile.com Zafar Ali Cisco Systems, Inc Email: zali@cisco.com Aravind Babu MahendraBabu Cisco Systems, Inc Email: aramahen@cisco.com Lin, et al. Expires 23 April 2026 [Page 7]