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 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). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." 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. Li, et al. Expires 23 November 2026 [Page 1] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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 and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are 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. Li, et al. Expires 23 November 2026 [Page 2] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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: 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 ... ... Li, et al. Expires 23 November 2026 [Page 3] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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: 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 Li, et al. Expires 23 November 2026 [Page 4] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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: 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) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Li, et al. Expires 23 November 2026 [Page 5] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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]. Li, et al. Expires 23 November 2026 [Page 6] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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. Li, et al. Expires 23 November 2026 [Page 7] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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: Li, et al. Expires 23 November 2026 [Page 8] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 * *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 Li, et al. Expires 23 November 2026 [Page 9] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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. Li, et al. Expires 23 November 2026 [Page 10] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, 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, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 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, . [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, . [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, . Li, et al. Expires 23 November 2026 [Page 11] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 [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, . [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, . [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, . 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, . [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, . Authors' Addresses Cheng Li Huawei Technologies Huawei Campus, No. 156 Beiqing Rd. Beijing 100095 China Email: c.l@huawei.com Li, et al. Expires 23 November 2026 [Page 12] Internet-Draft Path ID and Bi-directional Path in BGP May 2026 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 Li, et al. Expires 23 November 2026 [Page 13]