BGP SR Policy Extensions for Segment List Identifier
draft-ietf-idr-sr-policy-seglist-id-13

IDR                                                               C. Lin
Internet-Draft                                      New H3C Technologies
Intended status: Standards Track                                W. Cheng
Expires: 21 February 2027                                   China Mobile
                                                                  Y. Liu
                                                                     ZTE
                                                           K. Talaulikar
                                                           Cisco Systems
                                                                 M. Chen
                                                    New H3C Technologies
                                                          20 August 2026

          BGP SR Policy Extensions for Segment List Identifier
                 draft-ietf-idr-sr-policy-seglist-id-13

Abstract

   Segment Routing (SR) is a source routing paradigm that explicitly
   indicates the forwarding path for packets at the ingress node.  An SR
   Policy is a set of candidate paths, each consisting of one or more
   segment lists.  This document defines extensions to BGP SR Policy to
   specify the identifier of a segment list.

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
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   This Internet-Draft will expire on 21 February 2027.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Segment List Identifier in SR Policy  . . . . . . . . . . . .   3
     2.1.  Segment List ID Sub-TLV . . . . . . . . . . . . . . . . .   5
   3.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .   7
     6.1.  New H3C Technologies  . . . . . . . . . . . . . . . . . .   8
     6.2.  ZTE Corp  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Cross WG Information . . . . . . . . . . . . . . . .  11
     A.1.  Spring WG . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  PCE WG  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     A.3.  SRv6OPS . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at an 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 Policy Candidate Paths to the headend
   (i.e., the node where the instructions for source routing are written
   into the packet), [RFC9830] specifies a mechanism by using BGP.

   However, there is no identifier for segment lists in BGP SR Policy,
   which may cause inconvenience for other mechanisms to designate
   segment lists distributed by BGP.

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   Consider a scenario where a network controller distributes SR Policy
   Candidate Paths to the headend nodes, which need to collect traffic
   forwarding statistics per segment list.  When a headend node reports
   each statistic to the controller, it needs to specify the segment
   list to which the statistic belongs.  Due to the lack of an
   identifier, the headend node usually reports all SIDs in the
   associated segment list along with the statistic, and then the
   controller needs to compare the SIDs one by one to recognize which
   segment list it is.  The advertisement of all SIDs in the segment
   list consumes a lot of octets, and the comparison of SIDs can be
   complicated.

   Consider a second example where a network controller distributes SR
   Policy Candidate Paths using BGP, and then uses YANG [RFC7950]
   /NETCONF [RFC6241] to set some configurations of the segment lists
   that are not suitable to be carried in BGP.  The controller needs to
   specify which segment list these configurations belong to when it
   issues them.  In this case, an identifier of the segment list can
   also be helpful.

   An identifier of a segment list may also serve as a user-friendly
   attribute for debugging and troubleshooting purposes, such as
   displaying an invalid segment list when its associated BFD session is
   down.

   Section 5.7.4 of [RFC9857] defines an SR Segment List Identifier sub-
   TLV for reporting the identifier associated with a specific SID list.
   This document defines extensions to BGP SR Policy to specify the
   identifier of an SR Segment List.  Note that the Segment List ID is
   scoped to a Candidate Path.  It does not uniquely identify a segment
   list across an SR Policy or headend node, nor does it imply that the
   SID sequence is immutable.  A controller may keep the same Segment
   List ID while updating the SIDs within the segment list.  Any
   external reference that uses this identifier MUST also specify the
   associated Candidate Path.

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.  Segment List Identifier in SR Policy

   As defined in Section 2.2 of [RFC9830], the SR Policy Encoding
   structure is as follows:

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   SR Policy SAFI NLRI: <Distinguisher, Color, Endpoint>
   Attributes:
      Tunnel Encapsulation Attribute (23)
         Tunnel Type: SR Policy (15)
             Binding SID
             SRv6 Binding SID
             Preference
             Priority
             SR Policy Name
             SR Policy Candidate Path Name
             Explicit NULL Label Policy (ENLP)
             Segment List
                 Weight
                 Segment
                 Segment
                 ...
             ...

                        Figure 1: SR Policy Encoding

   SR policy with segment list identifier is expressed as below:

   SR Policy SAFI NLRI: <Distinguisher, Color, Endpoint>
   Attributes:
      Tunnel Encapsulation Attribute (23)
         Tunnel Type: SR Policy (15)
             Binding SID
             SRv6 Binding SID
             Preference
             Priority
             SR Policy Name
             SR Policy Candidate Path Name
             Explicit NULL Label Policy (ENLP)
             Segment List
                 Weight
                 Segment List ID
                 Segment
                 Segment
                 ...
             ...

         Figure 2: SR policy with segment list identifier Encoding

   The segment list identifier can be advertised using the Segment List
   ID sub-TLV, as defined in Section 2.1.

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   The notation used in Figure 1 and Figure 2 is a logical
   representation of the BGP attribute hierarchy.  It is not a new
   encoding specification but a common convention to illustrate the
   nesting of TLVs and sub-TLVs.  The top-level line represents the
   NLRI, and each indented line represents a TLV or sub-TLV contained
   within the parent.  Angle brackets (<...>) enclose a tuple of fields
   that together form an NLRI.  Ellipses (...) indicate that additional
   instances of the preceding item type or additional sub-TLVs may be
   present.  This notation is consistent with the SR Policy encoding
   conventions used in [RFC9830] Section 2.2.  The actual TLV encoding
   follows the TLV rules defined in the BGP Tunnel Encapsulation
   Attribute specification [RFC9012].

2.1.  Segment List ID Sub-TLV

   This document defines the Segment List ID sub-TLV.  When present, it
   MUST be carried within a Segment List sub-TLV (type 128) of an SR
   Policy Tunnel Type TLV (type 15) in the BGP Tunnel Encapsulation
   Attribute [RFC9012].  It MUST NOT be carried directly as a sub-TLV of
   the SR Policy Tunnel Type TLV.

   The Segment List ID sub-TLV specifies the identifier of the segment
   list by a 4-octet number.  The Segment List ID is unique within the
   context of a Candidate Path.  An originator that assigns non-zero
   Segment List IDs MUST assign a different value to each Segment List
   within the same Candidate Path.  Multiple Segment Lists MAY carry the
   value zero because zero indicates that no identifier has been
   assigned.

   The Segment List ID sub-TLV is an optional sub-TLV.  It MUST NOT
   appear more than once in a given segment list.  If multiple instances
   are present, only the first instance is used and the rest are
   ignored.  The presence of multiple instances is not considered
   malformed, as per Section 2.4 of [RFC9830].

   The Segment List ID 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    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Segment List ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: Segment List ID sub-TLV

   where:

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   *  Type: 19.

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

   *  Segment List ID: 4 octets which carry a 32-bit unsigned number.
      The values are interpreted as follows:

      -  Values 1 through 0xffffffff: Segment List ID.

      -  Value 0: indicates that no Segment List ID has been assigned.
         Multiple Segment Lists within the same Candidate Path MAY carry
         the value zero.

      The scope of the Segment List ID is the SR Policy Candidate Path.

3.  Error Handling

   The validation and error handling of an SR Policy NLRI with the
   Segment List ID sub-TLV in the BGP tunnel encapsulation attribute
   [RFC9012] follow the procedures in Sections 4.2 and 5 of [RFC9830].

   A Segment List ID sub-TLV with a length other than 6 octets is
   considered malformed.  If malformed, the associated BGP SR Policy
   NLRI MUST also be considered malformed, and the "treat-as-withdraw"
   strategy of [RFC7606] MUST be applied.

   If a receiver encounters multiple segment lists with the same non-
   zero Segment List ID within a Candidate Path, the semantic error MUST
   be handled by the SR Policy Module (SRPM) component as described in
   Section 5 of [RFC9830].  The receiver MAY either treat all segment
   lists as having no Segment List ID associated.

   An implementation SHOULD log any errors found during the above
   validation for further analysis.

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4.  Operational Considerations

   As specified in Section 4.2.2 of [RFC9830], an SR Policy Candidate
   Path carrying an unrecognized or unsupported sub-TLV is, by default,
   not considered usable.  This means that a headend node that does not
   implement this extension may treat a Candidate Path carrying the
   Segment List ID sub-TLV as invalid.  Operators should be aware of
   this deployment consideration when introducing this extension into a
   network.

5.  Security Considerations

   The protocol extensions defined in this document do not affect the
   base BGP security model.  The security requirements and mechanisms
   described in [RFC9830] also apply to this document.  SR operates
   within a trusted SR domain [RFC8402] and its security considerations
   also apply to BGP sessions when carrying SR Policy information.

   The Segment List ID sub-TLV is an optional sub-TLV that specifies an
   identifier associated with a segment list.  The scope of this
   identifier is the SR Policy Candidate Path.  The Segment List ID
   uniquely identifies a segment list within an SR Policy Candidate
   Path.

   The Segment List ID is assigned by the originator (often a
   controller), distributed via BGP, and used as an identifier for the
   segment list.  Since this identifier may expose mission-critical or
   commercially sensitive network information, it introduces a
   confidentiality risk.

   It is the responsibility of the network operator to ensure that only
   trusted nodes (that include both routers and controller applications)
   within the SR domain are configured to receive such information.

   The management of the Segment List ID itself (e.g., via YANG/NETCONF)
   is out of scope for this document.  As mentioned in Section 1, the
   identifier may be used to cross-reference configurations delivered
   via other mechanisms such as NETCONF, but the definition of such
   management interfaces is left to future documents.

6.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942].]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].

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   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

6.1.  New H3C Technologies

   *  Organization: New H3C Technologies.

   *  Implementation: H3C CR16000, CR19000 series routers
      implementation.

   *  Description: All sections including all the "MUST" and "SHOULD"
      clauses have been implemented in above-mentioned New H3C Products
      (running Version 7.1.099 and above).

   *  Maturity Level: Product

   *  Coverage: All sections.

   *  Version: Draft-03

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: linchangwang.04414@h3c.com

   *  Last updated: February 10, 2025

6.2.  ZTE Corp

   *  Organization: ZTE Corporation

   *  Implementation: ZTE's ZXR10 core router

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   *  Description: The implementation in lab has been completed.  The
      commercial implementation is under development.

   *  Maturity Level: Product

   *  Coverage: All

   *  Version: Draft-03

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: feng.jun99@zte.com.cn

   *  Last updated: February 6, 2025

7.  IANA Considerations

   This document defines a new Sub-TLV in the registry "SR Policy
   Segment List Sub-TLVs" [RFC9830] [IANA-SR-POLICY-SEGMENT-LIST].

   Value     Description                   Reference
   -------------------------------------------------------
   19        Segment List ID sub-TLV       This document

8.  Acknowledgments

   The authors would like to acknowledge Hao Li, Haiyang Zhang, Jie
   Dong, Yongqing Zhu, Nat Kao, Allan Michael, Yisong Liu, Ran Chen,
   Libin Liu, Lancheng Qin, Xinxin Yi, Wei Wang, Jiangbo Wang,
   Wisdomtan, Zheng Zhang, Yujia Gao, Alex Lee, David Wright, Hongwei
   Li, Quan Xiong, Xiao Min and Xiaolan Wang for their review and
   discussion of this document.

   The authors would also like to thank Susan Hares for her shepherd
   review and helpful comments to improve this document.

   Thanks to Henning Rogge for the RTGDIR, and Alexey Melnikov for the
   SECDIR Early review.

   The authors would like to thank Gunter Van de Velde, Mohamed
   Boucadair, Eric Vyncke, Mike Bishop, Mahesh Jethanandani, Roman
   Danyliw, Deb Cooley, and Jim Guichard for the IESG review.

9.  References

9.1.  Normative References

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   [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>.

   [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, 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>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/info/rfc9012>.

   [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>.

   [RFC9830]  Previdi, S., Filsfils, C., Talaulikar, K., Ed., 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>.

9.2.  Informative References

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

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   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC9857]  Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H.,
              and J. Tantsura, "Advertisement of Segment Routing
              Policies Using BGP - Link State", RFC 9857,
              DOI 10.17487/RFC9857, October 2025,
              <https://www.rfc-editor.org/info/rfc9857>.

   [IANA-SR-POLICY-SEGMENT-LIST]
              IANA, "SR Policy Segment List Sub-TLVs",
              <https://www.iana.org/assignments/bgp-tunnel-
              encapsulation#sr-policy-segment-list-sub-tlvs>.

   [I-D.ietf-pce-multipath]
              Koldychev, M. and S. Sidor, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Signaling
              Multipath Information", Work in Progress, Internet-Draft,
              draft-ietf-pce-multipath-29, 23 June 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              multipath-29>.

Appendix A.  Cross WG Information

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [I-D.ietf-pce-multipath].]

   This section describes cross-working group information for the IETF
   review process.

A.1.  Spring WG

   [RFC9256] details SR Policy architecture, but does not define any
   identifier for a Segment List.  The Segment List ID was introduced in
   BGP-LS (Section 5.7.4 of [RFC9857]) and PCEP (Section 4.2 of
   [I-D.ietf-pce-multipath]).  Note that the names are different due to
   the objects/TLVs in respective protocols but the semantics are
   identical.

   A Segment List belongs to a Candidate Path.  Note that the Segment
   List is meant to realize the objectives of a specific Candidate Path.
   Therefore, the segment list ID is scoped under the candidate path
   hierarchy.  The segment list ID is a 32-bit non-zero number that
   serves as the identifier associated with a segment list.  Currently,
   in BGP-LS and PCEP extensions, the segment list ID is scoped under
   the candidate path.

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   This document defines extensions to BGP SR Policy to specify the
   identifier of an SR Segment List.  The definition of the segment list
   ID in this document is consistent with the specification of segment
   list ID in BGP-LS and PCEP extensions.

A.2.  PCE WG

   The BGP extension in this document for the identifier of segment list
   also could be implemented via PCEP according to Section 4.2 of
   [I-D.ietf-pce-multipath].  The Path ID uniquely identifies a Path
   within the context of an LSP.  Path IDs MUST be unique across all
   these path types within the same LSP.  In this document, Path IDs
   correspond to the identifier of a segment list under CP.

   Sections 4.3 and 4.4 of [I-D.ietf-pce-multipath] describe the usage
   of Path IDs in load-balancing and multiple paths for protection
   scenarios, respectively.  Additionally, Appendix A of
   [I-D.ietf-pce-multipath] specifies how Path IDs are encapsulated
   within the PCEP protocol.

A.3.  SRv6OPS

   This document is unrelated to SRv6OPS and only relates to BGP-TE
   extensions.

Authors' Addresses

   Changwang Lin
   New H3C Technologies
   8 Yongjia North Road
   Beijing
   Haidian District, 100094
   China
   Email: linchangwang.04414@h3c.com

   Weiqiang Cheng
   China Mobile
   32 Xuanwumen West Street
   Beijing
   Xicheng District, 100053
   China
   Email: chengweiqiang@chinamobile.com

   Yao Liu
   ZTE
   China

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   Email: liu.yao71@zte.com.cn

   Ketan Talaulikar
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com

   Mengxiao Chen
   New H3C Technologies
   China
   Email: chen.mengxiao@h3c.com

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