Segment Routing Path MTU in BGP
draft-ietf-idr-sr-policy-path-mtu-13

Interdomain Routing Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Standards Track Y. Zhu
Expires: 3 June 2026 China Telecom
 A. Sawaf
 Saudi Telecom Company
 Z. Li
 G. Zeng
 Huawei Technologies
 November 2025
 Segment Routing Path MTU in BGP
 draft-ietf-idr-sr-policy-path-mtu-13
Abstract
 Segment Routing is a source routing paradigm that explicitly
 indicates the forwarding path for packets at the ingress node. An SR
 policy is a set of SR Policy candidate paths consisting of one or
 more segments with the appropriate SR path attributes. BGP
 distributes each SR Policy candidate path as combination of an prefix
 plus a the BGP Tunnel Encapsulation(Tunnel-Encaps) attribute
 containing an SR Policy Tunnel TLV with information on the SR Policy
 candidate path as a tunnel. However, the path maximum transmission
 unit (MTU) information for a segment list for SR path is not
 currently passed in the BGP Tunnel-Encaps attribute. . This document
 defines extensions to BGP to distribute path MTU information within
 SR policies.
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 5 May 2026.
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Copyright Notice
 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.
 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 . . . . . . . . . . . . . . . . . . 5
 3. SR Policy for Path MTU . . . . . . . . . . . . . . . . . . . 5
 3.1. Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . . . 6
 4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 7
 5. Implementation Status . . . . . . . . . . . . . . . . . . . . 7
 5.1. Huawei's Commercial Delivery . . . . . . . . . . . . . . 7
 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
 7. Manageability Considerations . . . . . . . . . . . . . . . . 8
 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
 11.1. Normative References . . . . . . . . . . . . . . . . . . 9
 11.2. Informative References . . . . . . . . . . . . . . . . . 10
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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 Segment Routing Policy ( SR Policy) as defined in [RFC9256].
 In order to distribute SR policies to the headend, [RFC9830]
 specifies a BGP mechanism to pass SR Policies and Candidate SR
 Policies in BGP UPDATE message. Each SR Candidate Path is passed as
 combination of a specific type of NLRI and BGP Tunnel Encapsulation
 Attribute (Tunnel-Encaps) with SR Policy Tunnel type tunnel. The
 NLRI must contain either be the IPv4 Unicast AFI with SR Policy SAFI
 (AFI=1/SAFI=73), the IPv6 Unicast AFI with the SR Policy SAFI (AFI=2/
 SAFI=73).
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 The maximum transmission unit (MTU) is the largest size packet or
 frame, in bytes, that can be sent in a network. An MTU that is too
 large might cause retransmissions. Too small an MTU might cause the
 router to send and handle relatively more header overhead and
 acknowledgments.
 When an LSP is created across a set of links with different MTU
 sizes, the ingress router needs to know what the smallest MTU is on
 the LSP path. If this MTU is larger than the MTU of one of the
 intermediate links, traffic might be dropped, because MPLS packets
 cannot be fragmented. Also, the ingress router may not be aware of
 this type of traffic loss, because the control plane for the LSP
 would still function normally. [RFC3209] specifies the mechanism of
 MTU signaling in RSVP. Similarly, the SRv6 packets will be dropped
 if the packet size is larger than the path MTU, since IPv6 packet
 cannot be fragmented on transmission [RFC8200].
 The host may discover the PMTU by Path MTU Discovery (PMTUD)
 [RFC8201] or other mechanisms. But the ingress router still needs to
 examine the packet size for dropping too large packets to avoid
 malicious traffic or error traffic. Also, the packet size may
 exceeds the PMTU because of the new encapsulation of SR-MPLS or SRv6
 packet at the ingress router.
 In order to check whether the Packet size exceeds the PMTU or not,
 the ingress node needs to know the Path MTU associated to the
 forwarding path. However, the path maximum transmission unit (MTU)
 information for SR path is not currently distributed in the BGP
 Tunnel-Encaps attribute TLV for the SR Policy Tunnel.
 This document defines a new sub-TLV for the BGP Tunnel-Encaps
 attribute for the SR Policy Tunnel type to specify Maximum Path MTU
 for a Segment list (Sub-TLV). The Maximum Path MTU can be calculated
 as the maximum of individual Link MTU information. The Link MTU
 information can be obtained via BGP-LS [I-D.ietf-idr-bgp-ls-link-mtu]
 or some other means. based on all Link MTUs, the controller can
 compute the PMTU and convey the information via the BGP SR policy.
2. Terminology
 This memo makes use of the terms defined in [RFC8402] and [RFC3209].
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 MTU: Maximum Transmission Unit, the size in bytes of the largest IP
 packet, including the IP header and payload, that can be
 transmitted on a link or path. Note that this could more properly
 be called the IP MTU, to be consistent with how other standards
 organizations use the acronym MTU.
 Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet
 size in bytes, that can be conveyed in one piece over a link. Be
 aware that this definition is different from the definition used
 by other standards organizations.
 For IETF documents, link MTU is uniformly defined as the IP MTU
 over the link. This includes the IP header, but excludes link
 layer headers and other framing that is not part of IP or the IP
 payload.
 Be aware that other standards organizations generally define link
 MTU to include the link layer headers.
 For the MPLS data plane, this size includes the IP header and data (or
 other payload) and the label stack but does not include any lower-layer
 headers. A link may be an interface (such as Ethernet or Packet-over-
 SONET), a tunnel (such as GRE or IPsec), or an LSP.
 Path: The set of links traversed by a packet between a source node
 and a destination node.
 Path MTU, or PMTU: The minimum link MTU of all the links in a path
 between a source node and a destination node.
 For the MPLS data plane, it is the MTU of an LSP from a given LSR to
 the egress(es), over each valid (forwarding) path. This size includes
 the IP header and data (or other payload) and any part of the label
 stack that was received by the ingress LSR before it placed the packet
 into the LSP (this part of the label stack is considered part of the
 payload for this LSP). The size does not include any lower-level
 headers.
 Note that: The PMTU value may be modified by subtracting some overhead
 introduced by protection mechanism, like TI-LFA. Therefore, the value
 of PMTU dilivered to the ingress node MAY be smaller than the minimum
 link MTU of all the links in a path between a source node and a
 destination node.
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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. SR Policy for Path MTU
 As defined in [RFC9830] , the SR policy 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
 ...
 ...
 As introduced in Section 1, each SR path has it's path MTU. SR
 policy with SR path MTU 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 MTU
 Segment
 Segment
 ...
 ...
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3.1. Path MTU Sub-TLV
 A Path MTU sub-TLV is an Optional sub-TLV. When it appears, it must
 appear only once at most within a Segment List sub-TLV. If multiple
 Path MTU sub-TLVs appear within a Segment List sub-TLV, the NLRI MUST
 be treated as a malformed NLRI.
 As per [RFC9830], when the error determined allows for the router to
 skip the malformed NLRI(s) and continue processing of the rest of the
 update message, then it MUST handle such malformed NLRIs as 'Treat-
 as-withdraw'. This document does not define new error handling rules
 for Path MTU sub-TLV, and the error handling rules defined in
 [RFC9830] apply to this document.
 A Path MTU sub-TLV is associated with an SR path specified by a
 segment list sub-TLV or a path segment [RFC9545]
 [I-D.ietf-spring-srv6-path-segment]. The Path MTU 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 | RESERVED |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Path MTU |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Figure 1. Path MTU sub-TLV
 Where:
 Type: to be assigned by IANA.
 Length: the total length in octets the value field not including Type
 and Length fields. The value must be 6.
 Reserved: 16 bits reserved and MUST be set to 0 on transmission and
 MUST be ignored on receipt.
 Path MTU: 4 bytes value of path MTU in octets. The value can be
 calculated by a central controller or other devices based on the
 information that learned via IGP of BGP-LS or other means.
 Whenever the path MTU of a physical or logical interface is changed,
 a new SR policy with new path MTU information should be updated
 accordingly by BGP.
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4. Operations
 The document does not bring new operation beyond the description of
 operations defined in [RFC9830]. The existing operations defined in
 [RFC9830] can apply to this document directly.
 Typically but not limit to, the SR policies carrying path MTU
 infomation are configured by a controller.
 After configuration, the SR policies carrying path MTU infomation
 will be advertised by BGP update messages. The operation of
 advertisement is the same as defined in [RFC9830], as well as the
 receiption.
 The consumer of the SR policies is not the BGP process. The
 operation of sending information to consumers is out of scope of this
 document.
5. 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].
 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".
5.1. Huawei's Commercial Delivery
 The feature has been implemented on Huawei VRP8.
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 * Organization: Huawei
 * Implementation: Huawei's Commercial Delivery implementation based
 on VRP8.
 * Description: The implementation has been done.
 * Maturity Level: Product
 * Contact: guokeqiang@huawei.com
6. IANA Considerations
 This document defines a new Sub-TLV in registries "SR Policy Segment
 List Sub-TLVs" [RFC9830]:
 Value Description Reference
 ---------------------------------------------------------------------
 TBA Path MTU sub-TLV This document
7. Manageability Considerations
 Currently, there is no standardized YANG data model that explicitly
 exposes the Path MTU value carried in the BGP SR Policy Path MTU Sub-
 TLV to network management systems or operators. Similarly, while
 BGP-LS can distribute link-level MTU information (as defined in
 [I-D.ietf-idr-bgp-ls-link-mtu]), it does not natively convey end-to-
 end Path MTU values associated with specific SR Policies.
 Standardized manageability support would significantly improve
 operational visibility and automation. Therefore, the authors
 recommend a YANG Module Extension: Extend the YANG data model for SR
 Policies (e.g., ietf-sr-policy or vendor-specific models) to include
 a read-only leaf for path-mtu, reflecting the value received via BGP
 or configured locally. This would enable NETCONF/RESTCONF-based
 retrieval and integration with network assurance systems.
8. Security Considerations
 This document defines the extension to BGP to distribute path MTU
 information within SR policies. Therefore, the security mechanisms
 of the base BGP security model [RFC4271] and the security
 considereations in [RFC9830] apply to this document. The path MTU
 extension is included in the SR Policy extension [RFC9830], so it
 does not introduce extra security problems comparing the existing SR
 policy entension.
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 The path MTU information is critical to the path, and a wrong path
 MTU may cause packet dropping in the forwarding. An implementation
 needs to make sure that the value of the link MTU is correctly
 collected from some means, such as BGP-LS. It also must ensure the
 processing and calculation of path MTU is correct to avoid packet
 dropping in forwarding. In addition, the path MTU distribution from
 a controller to an ingress router has to be protected. The security
 considereations in [RFC9830] apply to this distribution procedure.
9. Contributors
 Jun Qiu
 Huawei Technologies
 China
 Email: qiujun8@huawei.com
10. Acknowledgements
 Authors would like to thank Ketan Talaulikar, Aijun Wang, Weiqiang
 Cheng, Huanan Chen, Chongfeng Xie, Stefano Previdi, Taishan Tang,
 Keqiang Guo, Chen Zhang, Susan Hares, Weiguo Hao, Gong Xia, Bing
 Yang, Linda Dunbar, Shunwan Zhuang, Huaimo Chen, Mach Chen, Jingring
 Xie, Zhibo Hu, Jimmy Dong and Jianwei Mao for their proprefessional
 comments and help.
11. References
11.1. 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>.
 [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>.
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 [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>.
 [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>.
11.2. Informative References
 [I-D.ietf-idr-bgp-ls-link-mtu]
 Zhu, Y., Hu, Z., Peng, S., and R. Mwehair, "Signaling
 Maximum Transmission Unit (MTU) using BGP-LS", Work in
 Progress, Internet-Draft, draft-ietf-idr-bgp-ls-link-mtu-
 10, 18 September 2025,
 <https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
 ls-link-mtu-10>.
 [I-D.ietf-spring-srv6-path-segment]
 Li, C., Cheng, W., Chen, M., 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-13, 13 October 2025,
 <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
 srv6-path-segment-13>.
 [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
 and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
 Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
 <https://www.rfc-editor.org/info/rfc3209>.
 [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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 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
 (IPv6) Specification", STD 86, RFC 8200,
 DOI 10.17487/RFC8200, July 2017,
 <https://www.rfc-editor.org/info/rfc8200>.
 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
 "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
 DOI 10.17487/RFC8201, July 2017,
 <https://www.rfc-editor.org/info/rfc8201>.
 [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>.
Authors' Addresses
 Cheng Li
 Huawei Technologies
 Huawei Campus, No. 156 Beiqing Rd.
 Beijing
 100095
 China
 Email: c.l@huawei.com
 YongQing Zhu
 China Telecom
 109, West Zhongshan Road, Tianhe District.
 Guangzhou
 China
 Email: zhuyq8@chinatelecom.cn
 Ahmed El Sawaf
 Saudi Telecom Company
 Riyadh
 Saudi Arabia
 Email: aelsawaf.c@stc.com.sa
 Zhenbin Li
 Huawei Technologies
 Huawei Campus, No. 156 Beiqing Rd.
 Beijing
 100095
 China
 Email: lizhenbin@huawei.com
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 Guanming Zeng
 Huawei Technologies
 Huawei Campus, No. 156 Beiqing Rd.
 Beijing
 100095
 China
 Email: zengguanming@huawei.com
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