Segment Routing Path MTU in BGP
draft-ietf-idr-sr-policy-path-mtu-12

Interdomain Routing Working Group                                  C. Li
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                  Y. Zhu
Expires: 16 April 2026                                     China Telecom
                                                                A. Sawaf
                                                   Saudi Telecom Company
                                                                   Z. Li
                                                     Huawei Technologies
                                                         13 October 2025

                    Segment Routing Path MTU in BGP
                  draft-ietf-idr-sr-policy-path-mtu-12

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 16 April 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/
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   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.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 List
   Sub- TLVs" [RFC9830]:

   Value    Description                                  Reference
   ---------------------------------------------------------------------
    TBA     Path MTU sub-TLV                            This document

7.  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.

   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.

8.  Contributors

   Jun Qiu

   Huawei Technologies

   China

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   Email: qiujun8@huawei.com

9.  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.

10.  References

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>.

   [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>.

   [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>.

10.2.  Informative References

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   [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-12, 3 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-path-segment-12>.

   [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>.

   [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

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   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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