



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

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   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
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   license-info) in effect on the date of publication of this document.



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   Please review these documents carefully, as they describe your rights
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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.





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   [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:










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   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.




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   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].







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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,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              ls-sr-policy-17>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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, <https://www.rfc-editor.org/info/rfc8402>.

   [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,
              <https://www.rfc-editor.org/info/rfc9256>.

7.2.  Informative References




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   [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,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              multipath-14>.

   [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,
              <https://datatracker.ietf.org/doc/html/draft-jiang-idr-sr-
              policy-composite-path-02>.

   [RFC9552]  Talaulikar, K., "Distribution of Link-State and Traffic
              Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, January 2024,
              <https://www.rfc-editor.org/info/rfc9552>.

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








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