



Spring Working Group                                              Z. Ali
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                                  C. Lin
Expires: 23 April 2026                              New H3C Technologies
                                                                  Y. Liu
                                                            China Mobile
                                                                 R. Chen
                                                         ZTE Corporation
                                                                   C. Li
                                                     Huawei Technologies
                                                         20 October 2025


                   SRv6 Policy SID List Optimization
         draft-ali-spring-srv6-policy-sid-list-optimization-01

Abstract

   Segment Routing (SR) allows a node to steer a packet flow along any
   path.  SR Policy is an ordered list of segments (i.e., instructions)
   that represent a source-routed policy.  The packets steered into an
   SR Policy carry an ordered list of segments associated with that SR
   Policy.  An SR Policy can be instantiated SR-MPLS and SRv6 data
   planes.

   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.  This draft specifies procedures to indicate
   whether the endpoint's node SID needs to be included or excluded when
   installing the SRv6 Policy.

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.

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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Table of Contents

   1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  SID List Optimization During Policy Creation  . . . . . . . .   4
     3.1.  Policy Creation when using PCEP . . . . . . . . . . . . .   4
     3.2.  Policy Creation when using BGP  . . . . . . . . . . . . .   4
   4.  SID List Optimization Advertisement . . . . . . . . . . . . .   5
   5.  OAM Considerations  . . . . . . . . . . . . . . . . . . . . .   5
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Terminology

   This document uses the following terms defined in [RFC5440]: PCC,
   PCE, PCEP.



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   SR: Segment Routing.

   SID: Segment Identifier.

   SRv6: Segment Routing over IPv6 data plane.

2.  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.  An SR Policy can be
   instantiated SR-MPLS and SRv6 data planes.  [RFC8986] describes the
   representation and processing of this ordered list of segments for
   Segment Routing over IPv6 (SRv6).  [RFC9603] specifies PCEP
   extensions to support SR for the IPv6 data plane.

   The SRv6 Policy SID list may end with the policy endpoint's Node SID
   or the penultimate hop adjacency SID.  If the computed SID list ends
   with the policy endpoint's node SID and the overlay SID in the
   steered traffic (over policy) already ensures that the traffic is
   taken to the policy endpoint with the same intent, the SRv6 policy
   endpoint device needs to process back-to-back local node SIDs.
   Examples of overlay SID containing the local node SID are a service
   SID, a binding SID for transit policies, an EPE SID, etc.  From a
   compression efficiency viewpoint, carrying back-to-back end-point
   node SID is inefficient.  The SID list in the packet can be optimized
   by excluding the end-point node SID when installing the policy.  End-
   point node SID exclusion improves the compression efficiency and
   makes packet processing more efficient for the policy endpoint.

   Excluding the policy endpoint's node SID is possible in most use
   cases, but not all.  For example, if the SRv6 Policy is used to carry
   MPLS traffic, as described in [I-D.ietf-spring-srv6-mpls-
   interworking], it is not possible to exclude the policy endpoint's
   node SID.  Specifically, the endpoint's node SID inclusion or
   exclusion is a policy attribute.

   This draft specifies procedures needed to include or exclude the node
   SID when installing the SRv6 Policy.







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3.  SID List Optimization During Policy Creation

3.1.  Policy Creation when using PCEP

   The following procedure is applicable to both PCC-initiated Mode and
   PCE-initiated Mode.

   A PCE always computes the SRv6 TE Policy SID list from the headend to
   the endpoint (node SID).

   PCC and PCE exchange capabilities during the PCEP initialization
   phase to indicate the support for the SID list optimization.

   If the PCEP peers are capable of supporting the SID list
   optimization, the PCE indicates the inclusion or exclusion of the
   last SID in the ERO as follows:

   *  If the computed SID list ends with the policy endpoint's Node SID
      and the traffic being steered over policy does not take the
      traffic to the policy endpoint, the PCE MUST signal the
      installation of the last node SID to the PCC.  In this case, the
      PCC MUST install the last SID (the endpoint node SID) when
      installing the SRv6 Policy sid list(s) used to carry data traffic.
      In this case, the PCE considers the the policy endpoint's Node SID
      in the MSD consideration procedure.  However, in the case when the
      installation of the last SID is not required, the PCE does not
      consider the the policy endpoint's Node SID in the MSD
      consideration procedure.  Specifically, suppose the size of the
      SRv6 TE Policy SID list computed by PCE is L.  In the case when
      the the last SID is not installed by PCC, the PCE uses the sid-
      list length L-1 in the headend MSD consideration procedure.  This
      is because the endpoint node SID is suppressed.

   *  If the computed SID list does not ends with the policy endpoint's
      Node SID, e.g., penultimate hop Adjacency SID, the PCC MUST
      install the last SID when installing the SRv6 Policy sid list(s)
      used to carry data traffic.

   Protocol extension details can be found in [I-D.all-pce-srv6-policy-
   sid-list-optimization].

3.2.  Policy Creation when using BGP

   As defined in [RFC9256], an SR Policy is associated with one or more
   candidate paths.  An SR Policy Controller [RFC9256] defines the set
   of policies and advertises them to policy headend routers (typically
   ingress routers).  These policy advertisements use the BGP SR Policy
   SAFI [I-D.ietf-idr-sr-policy-safi].



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   When the SR Policy Controller completes computation of the SID list
   path:

   *  If the computed SID list ends with the policy endpoint's Node SID
      and the traffic being steered over policy does not take the
      traffic to the policy endpoint, the SR Policy Controller
      installation of the last node SID to SRv6 Policy headend routers.
      In this case, the headend router MUST install the last SID (the
      endpoint node SID) when installing the SRv6 Policy sid list(s)
      used to carry data traffic.

   *  If the computed SID list does not ends with the policy endpoint's
      Node SID, e.g., penultimate hop Adjacency SID, the SR Policy
      headend router MUST install this SID when installing the SRv6
      Policy SID list(s) for data traffic.

   Protocol extension details can be found in [I-D.liu-idr-sr-segment-
   list-optimize] and [I-D.lin-idr-sr-policy-admin-flags].

4.  SID List Optimization Advertisement

   [I-D.ietf-idr-bgp-ls-sr-policy] describes a mechanism to distribute
   SR policy information to external components using BGP-LS.  SR policy
   information can be used by external components for path computation,
   re-optimization, service placement, network visualization, etc.

   BGP-LS needs to be extended to indicate whether the endpoint's node
   SID is included or excluded in installing SID list(s) of the
   Candidate Path (CP) of an SRv6 Policy.

   Protocol extension details can be found in [I-D.ali-idr-srv6-policy-
   sl-opt-distribution].

5.  OAM Considerations

   The SID list optimization outlined in this draft applies to the case
   where service traffic, like L3VPN, L2VPN, etc. is steered over
   policy.  It does not apply to the probe packets used for OAM
   (Operational and Management) of the SRv6 policy in the transport
   network.  When performing OAM operation over the policy, e.g., ping,
   traceroute, BFD, STAMP, etc. over policy, the initiator always
   includes the last SID of the policy segment list.

   Please note that both data traffic and OAM packets are routed by the
   locator of the policy endpoint node.  Therefore, the data traffic and
   OAM packets always follow the same route in the network.





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6.  Operational Considerations

   This document specifies the detailed construction of the SRv6 Policy
   with inclusion or exclusion of end-point node SID and its operational
   mechanisms.  Therefore, the manageability considerations of [RFC9256]
   apply.

   Support of the inclusion or exclusion of end-point node SID
   capability defined in this document SHOULD be configurable.  The
   control of this capability for the candidate path of an SRv6 Policy
   can be achieved via BGP, PCEP, YANG, or CLI.  This capability itself
   does not alter packet forwarding behavior but rather optimizes the
   segment list orchestration for service packets, thereby enhancing
   forwarding performance.

   A YANG model for the configuration and operation of SR Policy has
   been defined in [SR-POLICY-YANG].  In later versions, the capability
   to control the inclusion or exclusion of end-point node SID will be
   added under the candidate-path node of SRv6 Policy.

7.  Security Considerations

   [RFC8754] defines the notion of an SR domain and use of SRH within
   the SR domain.  Procedures for securing an SR domain are defined the
   section 5.1 and section 7 of [RFC8754].  This document does not
   impose any additional security challenges to be considered beyond
   security threats described in [RFC8754], [RFC8679] and [RFC8986].

8.  IANA Considerations

   None

9.  Contributors

   The following people have contributed to this document:

      Rajesh M Venkateswaran
      Cisco Systems, Inc.
      Email: rmelarco@cisco.com


      Yuanxiang Qiu
      New H3C Technologies
      Email: qiuyuanxiang@h3c.com


10.  References




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10.1.  Normative References

   [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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC9603]  Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
              and Y. Zhu, "Path Computation Element Communication
              Protocol (PCEP) Extensions for IPv6 Segment Routing",
              RFC 9603, DOI 10.17487/RFC9603, July 2024,
              <https://www.rfc-editor.org/info/rfc9603>.




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10.2.  Informative References

   [I-D.ali-idr-srv6-policy-sl-opt-distribution]
              Ali, Z., Lin, C., Liu, Y., and R. Chen, "SRv6 Policy SID
              List Optimization Advertisement", Work in Progress,
              Internet-Draft, draft-ali-idr-srv6-policy-sl-opt-
              distribution-01, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ali-idr-srv6-
              policy-sl-opt-distribution-01>.

   [I-D.all-pce-srv6-policy-sid-list-optimization]
              Ali, Z., Lin, C., Liu, Y., Chen, R., 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, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-all-pce-srv6-
              policy-sid-list-optimization-02>.

   [I-D.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, 6 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
              ls-sr-policy-17>.

   [I-D.ietf-idr-sr-policy-safi]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
              D. Jain, "Advertising Segment Routing Policies in BGP",
              Work in Progress, Internet-Draft, draft-ietf-idr-sr-
              policy-safi-13, 6 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
              policy-safi-13>.

   [I-D.ietf-spring-srv6-mpls-interworking]
              Agrawal, S., Filsfils, C., Voyer, D., Dawra, G., Li, Z.,
              and S. Hegde, "SRv6 and MPLS interworking", Work in
              Progress, Internet-Draft, draft-ietf-spring-srv6-mpls-
              interworking-01, 7 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-mpls-interworking-01>.









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   [I-D.lin-idr-sr-policy-admin-flags]
              Lin, C., Liu, Y., and R. Chen, "BGP SR Policy Extensions
              for Administrative Flags", Work in Progress, Internet-
              Draft, draft-lin-idr-sr-policy-admin-flags-02, 13
              September 2025, <https://datatracker.ietf.org/doc/html/
              draft-lin-idr-sr-policy-admin-flags-02>.

   [I-D.liu-idr-sr-segment-list-optimize]
              Liu, Y., Lin, C., and R. Chen, "BGP Extension for SRv6
              Policy Segment List optimization", Work in Progress,
              Internet-Draft, draft-liu-idr-sr-segment-list-optimize-01,
              20 February 2025, <https://datatracker.ietf.org/doc/html/
              draft-liu-idr-sr-segment-list-optimize-01>.

Authors' Addresses

   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com


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


   Yisong Liu
   China Mobile
   Email: liuyisong@chinamobile.com


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


   Cheng Li
   Huawei Technologies
   Email: c.l@huawei.com












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