IDR Working Group                                                Y. Liu
Internet-Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: October 30, 2026                          New H3C Technologies
                                                                R. Chen
                                                                    ZTE
                                                               T. Zhang
                                                                   BUPT
                                                                G. Zeng
                                                                 Huawei
                                                         April 30, 2026



          BGP Extension for SRv6 Policy Segment List optimization


                 draft-liu-idr-sr-segment-list-optimize-06


Abstract

   In some use cases, an SRv6 policy's segment list ends with the
   policy endpoint's node SID, and the traffic steered (over policy)
   already ensures that it is taken to the policy endpoint.  In such
   cases, the SID list can be optimized by excluding the endpoint Node
   SID when installing the policy.

   This document specifies a BGP extension to indicate whether the
   endpoint's node SID needs to be included or excluded when installing
   the SRv6 Policy. This optimization can improve the forwarding
   efficiency of data packets when End SID and Service SID are present.

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), its areas, and its working groups.  Note that
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   Drafts.

   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."




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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on October 30 2026.

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   document authors. All rights reserved.

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Table of Contents


   1. Introduction...................................................3
   2. Terminology....................................................3
   3. Extension......................................................4
   4. Operation......................................................4
   5. Use Case.......................................................5
      5.1. Data packet Processing to VPN.............................6
      5.2. OAM Packet Processing to the Egress Node..................7
   6. IANA Considerations............................................7
   7. Security Considerations........................................8
   8. References.....................................................8
      8.1. Normative References......................................8
      8.2. Informative References....................................9
   9. Acknowledgments................................................9
   Appendix A. Cross WG Information.................................10
      A.1. Link to Spring WG........................................10
      A.2. Interaction with PCE WG..................................10
      A.3. Interaction with SRV6ops Recommendations.................10
   Authors' Addresses...............................................12





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   1. Introduction

   Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
   along any path.  A Segment Routing Policy (SR Policy) [RFC8402] is
   an ordered list of segments that represent a source-routed policy.
   The headend node is said to steer a flow into an SR Policy.  The
   packets steered into an SR Policy have an ordered list of segments
   associated with that SR Policy written into them. Segment Routing
   Policy Architecture [RFC9256] updates [RFC8402] as it details the
   concepts of SR Policy and steering into an SR Policy.  [RFC8986]
   describes the representation and processing of this ordered list of
   segments for Segment Routing over IPv6 (SRv6).  [RFC9830] document
   specifies how BGP may distribute SR Policy candidate paths.

   In some use cases, an SRv6 Policy's SID list ends with the policy
   endpoint's node SID, and the traffic steered (over policy) already
   ensures that it is taken to the policy endpoint.  In such cases, the
   SID list can be optimized by excluding the endpoint Node SID when
   installing the policy.  [I-D. draft-ietf-spring-srv6-policy-sid-
   list-opt] specifies procedures to indicate whether the endpoint's
   node SID needs to be included or excluded when installing the SRv6
   Policy.

   This document specifies a BGP extension to indicate whether the
   endpoint's node SID needs to be included or excluded when installing
   the SRv6 Policy. This optimization can improve the forwarding
   efficiency of data packets when End SID and Service SID are present.

   2. Terminology

   The following terminologies are used in this document.

   SR: Segment Routing

   SRv6: SR for IPv6

   SRH: Segment Routing Header

   SID: Segment Identifier

   CE: Customer Edge

   PE: Provider Edge

   VPN: Virtual Private Network

   PSP: Penultimate Segment Pop


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   3. Extension

   IFN-flag (Install Final Node-sid flag) is proposed in the Candidate
   Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
   sr-policy-admin-flags]. The bit position for the flag is to be
   defined by IANA.

   A flag is introduced in the Candidate Path Administrative Flags Sub-
   TLV [I.D draft-lin-idr-sr-policy-admin-flags] to advertise the IFN-
   flag property:

     Value: TBD

     Description: IFN-flag (Install Final Node-sid flag)

     where:

       -  IFN-flag (Bit TBD): indicate the endpoint node SID is included
          in installing SID list(s) of the Candidate Path (CP) when set.

          *  If set to 1, the endpoint node SID MUST be included when
             installing the SR Policy SID list(s) used to carry the
             data traffic.

          *  If set to 0, the endpoint node SID MUST NOT be included
             when installing the SR Policy SID list(s) used to carry
             the data traffic.

       -  The unassigned bits in the Flags field MUST be set to zero
          upon transmission and MUST be ignored upon receipt.

   4. Operation

   When the controller distributes the SRv6 Policy configuration to the
   head node through BGP, set the IFN-flag is proposed in the Candidate
   Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
   sr-policy-admin-flags].

   After receiving the SRv6 Policy configuration with the IFN-Flag (set
   to zero) of the Candidate Path Administrative Flags, the ingress
   node will not simultaneously arrange the End SID and Service SID of
   the egress node into the SRH.SegmentList of packet.

   For data packets forwarded to VPN through this SRv6 Policy, the
   SRH.SegmentList will not encapsulate the End SID corresponding to
   the egress node in the SID list of SRv6 Policy.



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   If the forwarding path does not include the service SID of the
   egress node, then the End SID of the egress node should be
   encapsulated in SRH.SegmentList.

   For OAM detection packets of the SR policy, the SRH.SegmentList is
   encapsulated according to the SID list of the SR policy, only
   encapsulating node SIDs.

   5. Use Case

   Taking Figure 1 as an example, describe how SRv6 data packets and
   OAM packets are forwarded in the SRv6 network based on the optimized
   Segment List arrangement mechanism.

                           +------------+
                           | Controller |
                           +------------+
                         /               \
                       /                   \
                     /                       \
       +---+    +---+     +---+     +---+     +---+   +---+
       |CE1|----|PE1|-----| P1|-----| P2|-----|PE2|---|CE2|
       +---+    +---+     +---+     +---+     +---+   +---+
                2::2       3::3     4::4      5::5
                                              End.DT4 SID: 5::100
                                 Figure 1

   CE1 and CE2 are VPN access devices that connect to the IPv6 backbone
   network through PE. PE1 has a locator 2::/64. P1 has a locator
   3::/64. P2 has an End SID 4::4 with PSP Flavor. PE2 has a locator
   5::/64 and a VPN SID 5::100. The traffic from CE1 to CE2 is
   forwarded along the path PE1->P1->P2->PE2.

   P2 needs to perform the PSP behavior to remove the SRH extension
   header.

   The controller calculates the SRv6 forwarding path from PE1 to PE2
   based on the collected topology and configuration information, and
   distributes the SRv6 Policy to PE1 through BGP. The Endpoint address
   is 5::5 of PE2. There is only one candidate path. The candidate path
   contains a Segment list <3::3, 4::4, 5::5> with IFN-Flag set zero in
   in the Candidate Path Administrative Flags Sub-TLV.

   PE2 advertises a BGP VPN route to PE1, and the next hop of the BGP
   route is the endpoint address 5::5. After receiving the BGP route,
   PE1 iterates to the SRv6 Policy using the color and the next hop of
   the route.


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   There are two types of packets sent from PE1 to PE2: data packets
   and OAM packets.

   5.1. Data packet Processing to VPN

   After PE1 receives the data packet from CE1 to CE2, it looks up the
   VPN instance routing table and iterates to SRv6 Policy.

   PE1 adds the SRH extension header to the packet and encapsulates the
   Segment List of the SRv6 Policy. The Segment List in the SRH
   extension header is encapsulated as <3::3, 4::4, 5::100>, and the SL
   is set to 2.

   The Segment List in SRH is shown in Figure 2.

                             +--------+
             Segment List[0] | 5::100 | ==> PE2's End.DT4 SID
                             +--------+
             Segment List[1] | 4::4   |
                             +--------+
             Segment List[2] | 3::3   |
                             +--------+
                        Figure 2

   The segment list optimization method proposed in this document is
   suitable for both SRv6 SID compressed [RFC9800] and non-compressed
   scenarios. If the END SID and VPN SID of the egress node share a
   common Locator-Block with a sequence of consecutive nodes, the SIDs
   of the egress node can also be arranged in a compressed Segment
   List.

   In order to improve compression efficiency and reduce the overhead
   of SRv6 packet header, the compressed Segment List can only contain
   the compressed VPN SID.

   As shown in Figure 3, PE1, P1, P2, and PE3 share the common Locator-
   block A:0:0:0/64 (represented by LB in Figure3).

       +---+    +---+     +---+     +---+     +---+   +---+
       |CE1|----|PE1|-----| P1|-----| P2|-----|PE2|---|CE2|
       +---+    +---+     +---+     +---+     +---+   +---+
              LB:2:1::   LB:2:2::  LB:2:3::   LB:2:4::
                                             End.DT4 SID: LB:2:100::
                              Figure 3

   The compressed Segment List optimized in SRH is shown in Figure 4.



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           +-----------+-------+-------+-------+------+
           |  A:0:0:0  |  2:2  |  2:3  |  2:100|   0  |
           +-----------+-------+-------+-------+------+
              64bits    16bits   16bits  16bits
                              Figure 4

   5.2. OAM Packet Processing to the Egress Node

   If the head node enables OAM function and detects a fault in the
   SRv6 Policy forwarding path, PE1 will send OAM detection messages to
   PE2, such as BFD packets.

   The OAM detection message sends by PE1 encapsulate the segment list
   corresponding to the SRv6 Policy. Since the message does not need to
   be sent to VPN, the Segment List of the SRH extension header is
   encapsulated as <3::3, 4::4, 5::5>.

   The Segment List in SRH is shown in Figure 5.

                             +--------+
             Segment List[0] | 5::5   | ==> PE2's End SID
                             +--------+
             Segment List[1] | 4::4   |
                             +--------+
             Segment List[2] | 3::3   |
                             +--------+
                              Figure 5

   6. IANA Considerations

   IFN-flag (Install Final Node-sid flag) is proposed in the Candidate
   Path Administrative Flags Sub-TLV specified in [I.D draft-lin-idr-
   sr-policy-admin-flags].

    - IFN-Flag (Bit TBD): indicate the endpoint node SID is included in
      installing SID list(s) of the Candidate Path (CP) when set.

      * If set to 1, the endpoint node SID MUST be included when
        installing the SR Policy SID list(s) used to carry the data
        traffic.

      * If set to 0, the endpoint node SID MUST NOT be included when
        installing the SR Policy SID list(s) used to carry the data
        traffic.





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   7. Security Considerations

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

   The Candidate Path Administrative Flags Sub-TLV, as defined in the
   relevant draft, introduces a flag to advertise the IFN-flag
   property. This flag indicates whether the endpoint's Node SID should
   be included or excluded during the installation of the SRv6 Policy,
   which ultimately influences packet forwarding behavior.
   Consequently, when configuring, querying, or reporting the IFN-flag
   via BGP, it is essential to implement safeguards to protect this
   mission-critical or commercially sensitive information.

   8. References

   8.1. Normative References

   [I-D. draft-lin-idr-sr-policy-admin-flags] C, Lin, "BGP SR Policy
             Extensions for Administrative Flags ", Work in Progress,
             Internet-Draft, draft-lin-idr-sr-policy-admin-flags-04, 28
             February 2026,<https://datatracker.ietf.org/doc/html/
             draft-lin-idr-sr-policy-admin-flags-04>.

   [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>.

   [RFC8400] Chen, H., Liu, A., Saad, T., Xu, F., and L. Huang,
             "Extensions to RSVP-TE for Label Switched Path (LSP)
             Egress Protection", RFC 8400, DOI 10.17487/RFC8400, June
             2018, <https://www.rfc-editor.org/info/rfc8400>.

   [RFC8679] Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
             Michel, C., and H. Chen, "MPLS Egress Protection
             Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
             <https://www.rfc-editor.org/info/rfc8679>.

   [RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
             Matsushima, S., and D. Voyer, "IPv6 Segment Routing
             Header(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
             <https://www.rfc-editor.org/info/rfc8754>.






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   [RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
             D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
             (SRv6) Network Programming", RFC 8986, DOI
             10.17487/RFC8986, February 2021, <https://www.rfc-
             editor.org/info/rfc8986>.

   [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>.

   8.2. Informative References

   [I-D. draft-ietf-spring-srv6-policy-sid-list-opt] Z. Ali, C. Lin, Y.
             Liu, R. Chen and C. Li, "SRv6 Policy SID List
             Optimization", Work in Progress, Internet-Draft, draft-
             ietf-spring-srv6-policy-sid-list-opt, 25 November
             2025,<https://datatracker.ietf.org/doc/html/draft-ietf-
             spring-srv6-policy-sid-list-opt-00>.

   [I-D. draft-ietf-srv6ops-srv6-deployment] McBride, M., Liu, Y., Li,
             Z., Durmus, M., Erdogan, E., Mishra, G., and Horn, J.,
             "SRv6 Deployment Options", Work in Progress, Internet-
             Draft, draft-ietf-srv6ops-srv6-deployment, 25 February
             2026,<https://datatracker.ietf.org/doc/html/draft-ietf-
             srv6ops-srv6-deployment-02>.

   [I-D. draft-all-pce-srv6-policy-sid-list-optimization] Z. Ali, C.
             Lin, Y. Liu, R. Chen and C. Li, "Path Computation Element
             Communication Protocol (PCEP) extensions for SRv6 Policy
             SID List Optimization", Work in Progress, Internet-Draft,
             draft-all-pce-srv6-policy-sid-list-optimization, 02 March
             2026, <https://datatracker.ietf.org/doc/html/draft-all-
             pce-srv6-policy-sid-list-optimization-04>.

   [RFC9800] Cheng, W., Ed., Filsfils, C., Li, Z., Decraene, B., and F.
             Clad, Ed., "Compressed SRv6 Segment List Encoding",RFC
             9800, DOI 10.17487/RFC9800, June 2025, <https://www.rfc-
             editor.org/info/rfc9800>.

   9. Acknowledgments

   TBD






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Appendix A. Cross WG Information

   This section describes cross-working group information for use
   during the IETF review process. This section will be removed by the
   RFC editor prior to publication.

A.1. Link to Spring WG

   This document implements the procedures in Section 3.2 of [I-D.
   draft-ietf-spring-srv6-policy-sid-list-opt] via BGP. This spring
   feature requires both this document and [I-D. draft-lin-idr-sr-
   policy-admin-flags] to be implemented.

   The work is split into two drafts due to their distinct technical
   scopes and design hierarchy:

    * [I-D. draft-lin-idr-sr-policy-admin-flags] defines a foundational
      extension for managing the administrative state (e.g., Up/Down) of
      SR Policies, applicable universally.

    * This document specifies a specialized extension for the path
      optimization features described in [I-D. draft-ietf-spring-srv6-
      policy-sid-list-opt].

   This work separation ensures deployment flexibility, allowing
   independent implementation of basic operational control and advanced
   optimization logic. It also reflects a clean layered architecture,
   reducing complexity and promoting modular protocol design.

   If only one of the two IDR drafts is implemented, the spring
   requirements for a complete and operable SRv6 Policy control
   plane cannot be fully satisfied.

A.2. Interaction with PCE WG

   The optimization feature implemented in BGP by this document also
   could be implemented in PCEP according to Section 3.1 of [I-D.
   draft-ietf-spring-srv6-policy-sid-list-opt].

   [I-D. draft-all-pce-srv6-policy-sid-list-optimization] specifies a
   PCEP extension to indicate whether the endpoint's node SID needs to
   be included or excluded when installing the SRv6 Policy.

A.3. Interaction with SRV6ops Recommendations

   For SRv6 deployment recommendations of the optimization feature
   implemented in BGP by this document, please refer to Section 5.2 of
   [I-D. draft-ietf-srv6ops-srv6-deployment].

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   This optimization feature could be distributed to external
   components using BGP-LS extension defined in [I-D. draft-ali-idr-
   srv6-policy-sl-opt-distribution].












































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Authors' Addresses


   Yisong Liu
   China Mobile

   Email: liuyisong@chinamobile.com

   Changwang Lin
   New H3C Technologies

   Email: linchangwang.04414@h3c.com

   Ran Chen
   ZTE Corporation

   Email: chen.ran@zte.com.cn

   Tiankui Zhang
   BUPT

   Email: zhangtiankui@bupt.edu.cn

   Guanming Zeng
   Huawei

   Email: zengguanming@huawei.com





















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