SR Policy Extensions for Path Segment and Bidirectional Path
draft-ietf-idr-sr-policy-path-segment-15

Interdomain Routing Working Group                                  C. Li
Internet-Draft                                                     Z. Li
Intended status: Standards Track                     Huawei Technologies
Expires: 23 November 2026                                         Y. Yin
                                                           China Telecom
                                                                W. Cheng
                                                            China Mobile
                                                           K. Talaulikar
                                                           Cisco Systems
                                                             22 May 2026

      SR Policy Extensions for Path Segment and Bidirectional Path
                draft-ietf-idr-sr-policy-path-segment-15

Abstract

   BGP SR Policy address-family is used for signaling of individual
   candidate paths of a Segment Routing Policy.  This document specifies
   extensions for the signaling of a Path Segment Identifier associated
   with the Segment List(s) of a candidate path.  It also specifies
   extensions for the signaling of the Segment List(s) in the reverse
   direction when Bidirectional SR Policies are used.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 23 November 2026.

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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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  BGP Extensions  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . .   5
     4.2.  Reverse Segment List Sub-TLV  . . . . . . . . . . . . . .   7
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Error Handling and Fault Management . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Existing Registry: BGP Tunnel Encapsulation Attribute
           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 SR Policy as defined in [RFC9256].  BGP SR Policy SAFI
   [RFC9830] is used for the signaling of SR Policy candidate paths to
   headend nodes.

   In many use cases such as performance measurement, the path to which
   the packets belong is required to be identified.  In some scenarios,
   (e.g., Mobile backhaul transport networks), there are Requirements to
   support bidirectional path.  This document defines the extensions to
   BGP SR Policy address-family [RFC9830] to signal Path Segment for
   individual Segment List and the Reverse Segment List to support
   instantiation of bidirectional SR Policies.

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   The Path Segment can be a Path Segment in SR-MPLS [RFC9545] or SRv6
   [I-D.ietf-spring-srv6-path-segment].

2.  Terminology

   This document makes use of the terms defined in [RFC8402], [RFC9256],
   [RFC9545], and [RFC9830].  Some of terms are listed below for
   reference.

   *  SR: Segment Routing.

   *  SR-MPLS: Segment Routing over MPLS data plane.

   *  SRv6: Segment Routing over IPv6 data plane.

   *  PSID: Path Segment Identifier.

   *  SRPM: SR Policy Module.

2.1.  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.

3.  Overview

   As defined in [RFC9830], the SR Policy Candidate Path encoding
   structure is as follows:

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

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           Figure 1: SR Policy Candidate Path encoding structure

   As defined in [RFC9256], a candidate path includes multiple segment
   list specified by SID list.  A Path Segment [RFC9545]
   [I-D.ietf-spring-srv6-path-segment] can be used for identifying a
   segment list, candidate path, or SR Policy (depending on its context)
   at the endpoint (i.e., tail-end) of a SR Policy.

   A Segment List Sub-TLV that contains a set of segment Sub-TLVs and
   other Sub-TLVs as shown in Figure 2.  This document defines a new
   Path Segment Sub-TLV within Segment List Sub-TLV as described in
   section 3.1.

   The new SR Policy encoding structure with Path Segment Sub-TLV is
   expressed as below:

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

     Figure 2: SR Policy encoding structure with Path Segment Sub-TLVs

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   In some scenariose, for example, mobile backhaul transport network,
   there are requirements to support bidirectional path.  In SR, a
   bidirectional path can be represented as a binding of two
   unidirectional SR paths.  This document also defines a Reverse
   Segment List Sub-TLV to describe the reverse path.  *When a SR policy
   includes a bidirectional path, both the forward and reverse segment
   lists MUST be encoded in the BGP UPDATE message as adjacent Sub-TLVs
   under the Tunnel Encapsulation attribute.* An SR policy carrying SR
   bidirectional 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
               Preference
               Priority
               Policy Name
               Explicit NULL Label Policy (ENLP)
               Segment List
                   Weight
                   Path Segment
                   Segment
                   Segment
                   ...
               Reverse Segment List
                   Path Segment
                   Segment
                   Segment
                   ...

       Figure 3: SR Policy carrying SR bidirectional path information

4.  BGP Extensions

4.1.  SR Path Segment Sub-TLV

   An SR Path Segment Sub-TLV is included in the segment list Sub-TLV to
   identify an SID list.  It 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     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Path Segment ID (Variable)                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //     SRv6 Endpoint Behavior and SID Structure (optional)     //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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                       Figure 4: Path Segment Sub-TLV

   Where:

   *  Type (TBA1): SR Path Segment Sub-TLV (to be assigned by IANA).

   *  Length: the total length of the value field not including Type and
      Length fields.

   *  Flags: 8 bits of flags.  Following flags are defined:

     0  1  2  3  4  5  6  7
    +--+--+--+--+--+--+--+--+
    |    Reserved     |B |L |
    +--+--+--+--+--+--+--+--+

   *  -  L-Flag: Local flag.  Set when the Path Segment has local
         significance on an SR node.

      -  B-Flag: This flag, when set, indicates the presence of the SRv6
         Endpoint Behavior and SID Structure encoding specified in
         Section 2.4.4.2.4. of [RFC9830].  It MUST be ignored when the
         value of length field is smaller than 18.

      -  The rest bits of Flag are reserved and MUST be set to 0 on
         transmission and MUST be ignored on receipt.

   *  Path Segment ID: if the length is 2, then no Path Segment ID is
      present.  If the length is 6 then the Path Segment ID is encoded
      in 4 octets [RFC9545] using the format below.  TC, S, TTL (Total
      of 12 bits) are RESERVED and SHOULD be set to zero and MUST be
      ignored.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Path Segment Label            | TC  |S|       TTL     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: SR-MPLS Path Segment Sub-TLV

   If the length is 18 then the Path Segment ID contains a 16-octet SRv6
   Path Segment ID [I-D.ietf-spring-srv6-path-segment].

   If the length is larger than 18 and B-flag is set, then SRv6 Endpoint
   Behavior and SID Structure TLVs is included as per Section 2.4.4.2.4.
   of [RFC9830].

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   The Path Segment is used to identified an SR path, and it can be used
   in OAM or IOAM use cases.  When all the SID Lists within a candidate
   path share the same Path Segment ID, the Path Segment can be used to
   collect the aggregated information of the candidate path.  Multiple
   Path Segment MAY be included in a Segment List for different use
   cases.  In SR-MPLS, one, or some or all of them MAY be inserted into
   the SID List as the requirement of the use case.  However, in SRv6,
   only one Path Segment ID can be encoded in a SRH.  Therefore, an
   implementation MUST decide how to choose a Path Segment ID from the
   multiple Path Segment IDs.  In order to simplify the implementation,
   this document suggests to encode only one Path Segment Sub-TLV for a
   segment list, while the rest Path Segment SHOULD be ignored.

4.2.  Reverse Segment List Sub-TLV

   A Reverse Segment List Sub-TLV is defined to specify an SR reverse
   path associated with the path specified by the Segment List, and it
   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            |   RESERVED    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sub-TLVs (Variable)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: SR Reverse Segment List Sub-TLV

   where:

   Type (TBA2): Reverse Segment List Sub-TLV (to be assigned by IANA).

   Length: the total length of the Sub-TLVs encoded within the Reverse
   Path Segment List Sub-TLV not including the Type and Length fields.

   RESERVED: 1 octet of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   Sub-TLVs, reuse the Sub-TLVs in Segment List defined in [RFC9830] and
   [RFC9831].

   *  One or more mandatory SR Path Segment Sub-TLVs that contains the
      Path Segments of the reverse SR path.

   *  One or more Segment Sub-TLVs to specify the reverse SR path.

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   The Segment sub-TLVs in the Reverse Segment List sub-TLV provides the
   information of the reverse SR path.  This Reverse Segment list can be
   used for directing egress BFD peer to use specific path for the
   reverse direction of the BFD session [RFC9612] or other applications.

   A Reverse Segment List TLV MUST immediately follow its corresponding
   Segment List TLV in the attribute as this forms the one-to-one
   correlation of the forward and reverse segment lists.  A Reverse
   Segment List TLV not encoded in the attribute in this manner MUST be
   considered as malformed.  However, a Segment List TLV that is not
   immediately followed by a Reverse Segment List TLV simply indicates
   that the forward segment list does not have its corresponding reverse
   segment list and this condition MUST NOT be considered as an error.

5.  Operations

   This document defines new Sub-TLVs under the extensions for SR policy
   defined in [RFC9830], therefore, the description of operations
   defined in [RFC9830], can apply to this document directly, including
   advertisement of SR policies and reception of SR policy NLRI.

   Typically but not limit to, the unidirectional or bidirectional SR
   policies carrying path identification infomation are configured by a
   controller.

   After configuration, the unidirectional or bidirectional SR policies
   carrying path identification infomation will be advertised by BGP
   update messages.  The operation of advertising this SR policy is the
   same as defined in [RFC9830], as well as the reception.

   The consumer of the unidirectional or bidirectional SR policies is
   not the BGP process, it can be any applications, such as performance
   measurement [I-D.ietf-spring-stamp-srpm-srv6].  The operation of
   sending information to consumers is out of scope of this document.

6.  Error Handling and Fault Management

   This document extends the error handling defined in [RFC9830] for the
   new TLVs and sub-TLVs introduced herein.  In the event of any of the
   TLVs and sub-TLVs introduced in this document being found to be
   malformed, the "Treat-as-withdraw" error handling [RFC7606] MUST be
   performed.

   The following conditions MUST be considered as making an UPDATE
   message malformed:

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   *  *Path Segment Sub-TLV:* The length of the sub-TLV is not 2/6/18/
      larger than 18 octets, or the value fields are outside their
      defined ranges.

   *  *Reverse Segment List Sub-TLV:* A Reverse Segment List Sub-TLV is
      present in the Tunnel Encapsulation Attribute but does not
      immediately follow a Segment List Sub-TLV.

7.  IANA Considerations

   This document defines new Sub-TLVs in following registries:

7.1.  Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs

   *This document defines a new Sub-TLV in the registry "SR Policy
   Segment List Sub-TLVs" [RFC9830] to be assigned by IANA:*

        Codepoint   Description                           Reference
        -------------------------------------------------------------
        TBA(17)     Path Segment Sub-TLV                  This document

   *This document also defines a new Sub-TLV in the registry "BGP Tunnel
   Encapsulation Attribute sub-TLVs" [RFC9830] to be assigned by IANA:*

        Codepoint   Description                           Reference
        -------------------------------------------------------------
        TBA2     Reverse Segment List Sub-TLV          This document

8.  Security Considerations

   The security considerations of RFC 9830 apply to this document.

   Additionally, specific to the Path Segment ID and Reverse Path
   Segment, the Path Segment information is critical to the path, and an
   incorrect Path Segment ID may cause unexpected forwarding actions and
   results.  Implementations must ensure the correctness of the Path
   Segment ID value, especially in SR-MPLS networks.  Furthermore, the
   distribution of Path Segment information from a controller to an
   ingress router must be protected.  The security considerations
   outlined in the Path Segment related documents, such as "draft-ietf-
   spring-srv6-path-segment" and "RFC 9545", apply to this distribution
   procedure.

9.  Contributors

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      Guanming Zeng
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: zengguanming@huawei.com

      Mach(Guoyi) Chen
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: Mach.chen@huawei.com

      Jie Dong
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: jie.dong@huawei.com

      James N Guichard
      Futurewei Technologies
      2330 Central Express Way
      Santa Clara
      USA

      Email: james.n.guichard@futurewei.com

      Huanan Chen
      China Telecom
      109 West Zhongshan Ave
      Guangzhou
      China

      Email: chenhuan6@chinatelecom.cn

10.  Acknowledgements

   Many thanks to Shraddha Hedge, Susan Hares for their detailed reviews
   and comments.

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11.  References

11.1.  Normative References

   [I-D.ietf-spring-srv6-path-segment]
              Li, C., Cheng, W., Zeng, G., Dhody, D., and Y. Zhu, "Path
              Segment Identifier (PSID) in SRv6 (Segment Routing in
              IPv6)", Work in Progress, Internet-Draft, draft-ietf-
              spring-srv6-path-segment-15, 15 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-path-segment-15>.

   [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>.

   [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>.

   [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>.

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   [RFC9545]  Cheng, W., Ed., Li, H., Li, C., Ed., Gandhi, R., and R.
              Zigler, "Path Segment Identifier in MPLS-Based Segment
              Routing Networks", RFC 9545, DOI 10.17487/RFC9545,
              February 2024, <https://www.rfc-editor.org/info/rfc9545>.

   [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>.

   [RFC9831]  Talaulikar, K., Ed., Filsfils, C., Previdi, S., Mattes,
              P., and D. Jain, "Segment Type Extensions for BGP Segment
              Routing (SR) Policy", RFC 9831, DOI 10.17487/RFC9831,
              September 2025, <https://www.rfc-editor.org/info/rfc9831>.

11.2.  Informative References

   [I-D.ietf-spring-stamp-srpm-srv6]
              Gandhi, R., Filsfils, C., Janssens, B., Chen, M., and R.
              F. Foote, "Performance Measurement Using Simple Two-Way
              Active Measurement Protocol (STAMP) for Segment Routing
              over the IPv6 (SRv6) Data Plane", Work in Progress,
              Internet-Draft, draft-ietf-spring-stamp-srpm-srv6-01, 2
              April 2026, <https://datatracker.ietf.org/doc/html/draft-
              ietf-spring-stamp-srpm-srv6-01>.

   [RFC9612]  Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen,
              "Bidirectional Forwarding Detection (BFD) Reverse Path for
              MPLS Label Switched Paths (LSPs)", RFC 9612,
              DOI 10.17487/RFC9612, July 2024,
              <https://www.rfc-editor.org/info/rfc9612>.

Authors' Addresses

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: c.l@huawei.com

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   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: lizhenbin@huawei.com

   Yuanyang Yin
   China Telecom
   Guangzhou
   China
   Email: yinyuany@chinatelecom.cn

   Weiqiang Cheng
   China Mobile
   Beijing
   China
   Email: chengweiqiang@chinamobile.com

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

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