IDR                                                            W. Jiang
Internet Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: October 21, 2026                          New H3C Technologies
                                                                R. Chen
                                                        ZTE Corporation
                                                         April 21, 2026


          BGP Extensions of SR Policy for Composite Candidate Path
                draft-jiang-idr-sr-policy-composite-path-05


Abstract

   SR Policy Architecture [RFC9256] defines the concept of a Composite
   Candidate Path. A regular SR Policy Candidate Path outputs traffic
   to a set of Segment Lists, while an SR Policy Composite Candidate
   Path outputs traffic recursively to a set of SR Policies on the same
   headend. This document defines extensions to BGP to distribute SR
   policies carrying composite candidate path information.  So that
   composite candidate paths can be installed when the SR policy is
   applied.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 21, 2026.

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   Copyright (c) 2026 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



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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
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Table of Contents


   1. Introduction...................................................2
      1.1. Requirements Language.....................................3
   2. Constituent SR Policy Attributes in SR Policy..................3
      2.1. Constituent SR Policy Sub-TLV.............................4
      2.2. Per-Flow Forwarding Class Sub-TLV.........................5
   3. Procedures.....................................................6
   4. Error Handling.................................................7
   5. Security Considerations........................................8
   6. IANA Considerations............................................8
   7. References.....................................................8
      7.1. Normative References......................................8
      7.2. Informative References....................................9
   Authors' Addresses...............................................10



1. Introduction

   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]. In order to distribute SR policies to the headend,
   [RFC9830] specifies a mechanism by using BGP.

   SR Policy Architecture [RFC9256] defines the concept of a Composite
   Candidate Path. A regular SR Policy Candidate Path outputs traffic
   to a set of Segment Lists, while an SR Policy Composite Candidate
   Path outputs traffic recursively to a set of SR Policies on the same
   headend.

   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, for a
   load-balanced steering of packet flows over its constituent SR
   Policies.


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   Composite Candidate Paths can be distributed via the Path
   Computation Element Communication Protocol (PCEP), as described in
   section 3.6 of [I-D. draft-ietf-pce-multipath].

   This document defines extensions to Border Gateway Protocol (BGP) to
   distribute SR policies carrying composite candidate path
   information. The SR policy in the composite path can be distributed
   by BGP as described in [RFC9830], by PCEP as described in [RFC
   9862], or through static configuration. This document does not alter
   the existing deployment methods for SR policies; it only extends BGP
   to support the distribution of composite candidate paths based on
   [RFC9830].

1.1. Requirements Language

   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.

2. Constituent SR Policy Attributes in SR Policy

   As defined in section 2.2 of [RFC9830], the SR policy encoding
   structure is as follows:

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
          Attributes:
              Tunnel Encaps Attribute (23)
                  Tunnel Type: SR Policy
                      Binding SID
                      SRv6 Binding SID
                      Preference
                      Priority
                      Policy Name
                      Policy Candidate Path Name
                      Explicit NULL Label Policy (ENLP)
                      Segment List
                          Weight
                          Segment
                          Segment
                          ...
                      ...
               Figure 1: SR Policy Encoding

   As described in section 2.2 in [RFC9256], 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.  Therefore a constituent SR Policy is

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   referenced only by color in the composite candidate path since its
   headend and endpoint are identical to the parent SR policy.

   SR policy with composite candidate path information is expressed as
   below:



      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
          Attributes:
              Tunnel Encaps Attribute (23)
                  Tunnel Type: SR Policy
                      Binding SID
                      SRv6 Binding SID
                      Preference
                      Priority
                      Policy Name
                      Policy Candidate Path Name
                      Explicit NULL Label Policy (ENLP)
                      Segment List
                          Weight
                          Segment
                          Segment
                          ...
                      Constituent SR Policy
                          Color
                          Weight
                          Fowarding Class
                      ...
   Figure 2: SR policy with composite candidate path Encoding

   SR Policy Architecture [RFC9256] defines the concept of a Composite
   Candidate Path.  A regular SR Policy Candidate Path outputs traffic
   to a set of Segment Lists, while an SR Policy Composite Candidate
   Path outputs traffic recursively to a set of SR Policies on the same
   headend.

2.1. Constituent SR Policy Sub-TLV

   The Constituent SR Policy sub-TLV is an optional sub-TLV of BGP
   Tunnel Encapsulation Attribute, and MAY appear multiple times in the
   SR Policy encoding.  The ordering of Constituent SR Policy sub-TLVs
   does not matter.  The Constituent SR Policy sub-TLV MAY contain a
   Weight sub-TLV.

   The Constituent SR Policy sub-TLV and the Segment List sub-TLV MUST
   NOT appear in the same candidate path.

   The Constituent SR Policy sub-TLV has the 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                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 3:  Constituent SR Policy sub-TLV format

      where:

      *  Type: to be assigned by IANA.

       *  Length: The total length (not including the Type and Length
          fields) of the sub-TLVs encoded within the Constituent SR
          Policy sub-TLV in terms of octets.

       *  RESERVED: 1 octet of reserved bits. This field MUST be set to
         Zero on transmission and MUST be ignored on receipt.

      *  Color: 4 octets that carry an unsigned non-zero integer value
         indicating the Color of the Constituent SR Policy. As
         described in section 2.2 in [RFC9256], the endpoints of the
         constituent SR Policies and the parent SR Policy MUST be
         identical, thus different constituent SR Policies can be
         distinguished by Color.

       *  Weight: 4 octets carrying an unsigned integer value indicating
          the weight associated with a segment list as described in
          Section 2.11 of [RFC9256].  A weight value of zero is
          invalid.

      *  sub-TLVs currently defined: An optional single Per-Flow
         Forwarding Class sub-TLV which is defined in section 2.2 on
         this document. The other Sub-TLVs in Constituent SR Policy Sub-
         TLV are out of scope of this document.


2.2. Per-Flow Forwarding Class Sub-TLV

   Per-Flow Forwarding Path builds on top of the concept of the
   Composite Candidate Path.  Each Path in a Per-Flow Forwarding Path
   is assigned a 3-bit Forward Class (FC) value, which allows QoS

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   classified traffic to be steered depending on the FC. The Per-Flow
   FC sub-TLV is an optional sub-TLV of the Constituent SR Policy TLV.

   The Per-flow FC sub-TLV is OPTIONAL and it MUST NOT appear more than
   once inside the Constituent SR Policy sub-TLV.

   The Per-flow FC sub-TLV has the 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      |     Flags     | RESERVED-1    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        RESERVED-2                       | FC  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4 Per-Flow FC Sub-TLV


      where:

      *  Type: to be assigned by IANA.

      *  Length: Specifies the length of the value field (not including
         Type and Length fields) in terms of octets.  The value MUST be
         6.

      *  Flags: 1 octet of flags.  No flags are defined in this
         document. The Flags field MUST be set to zero on transmission
         and MUST be ignored on receipt.

      *  RESERVED-1: 1 octet of reserved bits.  This field MUST be set
         to zero on transmission and MUST be ignored on receipt.

      *  Reserved-2(29 bits): 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.


3. Procedures

   The document does not bring new operation beyond the description of
   operations defined in [RFC9830].  The existing operations defined in
   [RFC9830] can apply to this document directly.

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   Typically but not limit to, the SR policies carrying composite
   candidate path information are configured by a controller.

   After configuration, the SR policies carrying path composite
   candidate path information will be advertised by BGP update
   messages.

   The operation of advertisement is the same as defined in [RFC9830],
   as well as the reception.

   Note the differences among several color TLVs. The Tunnel Egress
   Endpoint and Color sub-TLVs of the Tunnel Encapsulation Attribute,
   as defined in [RFC9012], are not utilized for SR Policy encodings;
   see more details in Section 2.3 of [RFC9830].

   The Color Extended Community (as defined in [RFC9012]) is used to
   steer traffic into an SR Policy, as described in Section 8.8 of
   [RFC9256] and Section 3 of [RFC9830].

   The color of the Constituent SR Policy is identified by its color,
   as described in Section 2.1.

4. Error Handling

   The error handling of the BGP Update messages for BGP SR Policy SAFI
   with the extensions defined in this document follows the procedures
   in section 5 of [RFC9830].

   The validation of the TLVs/sub-TLVs introduced in this document and
   defined in their respective subsections of Section 2 MUST be
   performed to determine if they are malformed or invalid.

   The Constituent SR Policy sub-TLV and the Segment List sub-TLV MUST
   NOT appear in the same candidate path. If Constituent SR Policy sub-
   TLV does not match the above description, or its format is
   considered malformed, the associated BGP SR Policy NLRI is
   considered malformed and the "treat-as-withdraw" strategy of
   [RFC7606] MUST be applied.

   The Per-flow FC sub-TLV is optional and MUST NOT appear more than
   once for one Constituent SR Policy sub-TLV.  The Per-flow FC sub-TLV
   is considered malformed if its format does not match the above
   description.  If the Per-flow FC sub-TLV appears more than once, or
   its format is considered malformed, the associated BGP SR Policy
   NLRI is considered malformed and the "treat-as-withdraw" strategy of
   [RFC7606] MUST be applied.




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5. Security Considerations

   The security considerations of BGP [RFC4271] and BGP SR policy
   [RFC9830] apply to this document.

   This document defines BGP extensions for distributing SR policies
   that carry composite candidate path information. These functions
   extend the risks associated with SR Policy into the dynamic realm.
   Misconfiguration or errors in configuring an SR Policy Composite
   Candidate Path may lead to packets being forwarded along unintended
   paths for the affected routes.

6. IANA Considerations

   This document defines a sub-TLV in the "BGP Tunnel Encapsulation
   Attribute Sub-TLVs" registry under the "Border Gateway Protocol
   (BGP) Tunnel Encapsulation" registry group.

   +=======+===============================+===============+
   | Value | Description                   | Reference     |
   +=======+===============================+===============+
   | TBA   | Constituent SR Policy Sub-TLV | This document |
   +-------+-------------------------------+---------------+


   This document creates a new registry called "Constituent SR Policy
   sub-TLV" under the "Border Gateway Protocol (BGP) Tunnel
   Encapsulation" registry group.

   +=======+===============================+===============+
   | Value | Description                   | Reference     |
   +=======+===============================+===============+
   | TBA   | Per-Flow FC Sub-TLV           | This document |
   +-------+-------------------------------+---------------+


7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
             Border Gateway Protocol 4 (BGP-4)", RFC 4271, DOI
             10.17487/RFC4271, January 2006, <https://www.rfc-
             editor.org/info/rfc4271>.



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   [RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and
             K.Patel, "Revised Error Handling for BGP UPDATE Messages",
             RFC 7606, DOI 10.17487/RFC7606, August 2015,
             <https://www.rfc-editor.org/info/rfc7606>.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, 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, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC9830] Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
             D. Jain, "Advertising Segment Routing Policies in BGP",
             RFC 9830, DOI 10.17487/RFC9830, September 2025,
             <https://www.rfc-editor.org/info/rfc9830>.

7.2. Informative References

   [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>.

   [RFC9862] Koldychev, M., Sivabalan, S., Sidor, S., Barth, C., Peng,
             S., and H. Bidgoli, "Path Computation Element
             Communication Protocol (PCEP) Extensions for Segment
             Routing (SR) Policy Candidate Paths", RFC 9862, DOI
             10.17487/RFC9862, October 2025, <https://www.rfc-
             editor.org/rfc/rfc9862>.

   [I-D.ietf-pce-multipath] Koldychev, M., Sivabalan, S., Saad, T.,
             Beeram, V. P., Bidgoli, H., Peng, S., and S. Sidor, "Path
             Computation Element Communication Protocol (PCEP)
             Extensions for Signaling Multipath Information", Work in
             Progress, Internet-Draft, draft-ietf-pce-multipath-23, 13
             April 2026, <https://datatracker.ietf.org/doc/html/draft-
             ietf-pce-multipath-23>.










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Authors' Addresses


   Wenying Jiang
   China Mobile
   Beijing
   China
   Email: jiangwenying@chinamobile.com

   Changwang Lin
   New H3C Technologies
   Beijing
   China
   Email: linchangwang.04414@h3c.com

   Ran Chen
   ZTE Corporation
   Email: chen.ran@zte.com.cn































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