



SPRING Working Group                                             S. Peng
Internet-Draft                                                  D. Dhody
Intended status: Standards Track                                  Huawei
Expires: 17 September 2026                                 K. Talaulikar
                                                           Cisco Systems
                                                               G. Mishra
                                                            Verizon Inc.
                                                           16 March 2026


            Path MTU (PMTU) for Segment Routing (SR) Policy
                  draft-ietf-spring-pmtu-sr-policy-04

Abstract

   This document defines the Path MTU (PMTU) for Segment Routing (SR)
   Policy (called SR-PMTU).  It applies to both Segment Routing over
   IPv6 (SRv6) and SR-MPLS.  This document specifies the framework of
   SR-PMTU for SR Policy including the link MTU collection, the SR-PMTU
   computation, the SR-PMTU enforcement, and the handling behaviours on
   the headend.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 17 September 2026.

Copyright Notice

   Copyright (c) 2026 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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   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
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  SR-PMTU Definition for SR Policy  . . . . . . . . . . . . . .   4
     4.1.  SR-PMTU of a Segment List . . . . . . . . . . . . . . . .   4
     4.2.  SR-PMTU of a Candidate Path . . . . . . . . . . . . . . .   4
     4.3.  SR-PMTU of an SR Policy . . . . . . . . . . . . . . . . .   5
   5.  The Framework of SR-PMTU for SR Policy  . . . . . . . . . . .   5
     5.1.  Link MTU Collection . . . . . . . . . . . . . . . . . . .   5
     5.2.  SR-PMTU Computation . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  Loose Path  . . . . . . . . . . . . . . . . . . . . .   6
       5.2.2.  Strict TE Path  . . . . . . . . . . . . . . . . . . .   6
       5.2.3.  Mixed Path  . . . . . . . . . . . . . . . . . . . . .   6
       5.2.4.  Binding Path  . . . . . . . . . . . . . . . . . . . .   6
       5.2.5.  TI-LFA  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  SR-PMTU Enforcement . . . . . . . . . . . . . . . . . . .   7
     5.4.  Handling behaviors on the headend . . . . . . . . . . . .   8
       5.4.1.  SR-PMTU Constraints and Optimization  . . . . . . . .   8
       5.4.2.  Fragmentation processing  . . . . . . . . . . . . . .   8
     5.5.  Link MTU Change . . . . . . . . . . . . . . . . . . . . .   9
   6.  SRv6-Specific Handling  . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
   along any given path.  The headend is a node where the instructions
   for source routing (i.e., segments) are encoded in the packet and
   hence becomes the starting node for a specific segment routing path.
   Intermediate per-path states are eliminated thanks to source routing.

   A Segment Routing Policy (SR Policy) [RFC9256] is an ordered list of
   segments (i.e., instructions) that represent a source-routed policy.
   The headend node is said to steer a flow into a SR Policy.  The



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   packets steered into an SR Policy have an ordered list of segments
   associated with that SR Policy written into them.  [RFC8660]
   describes the representation and processing of this ordered list of
   segments as an MPLS label stack for SR-MPLS, while [RFC8754] and
   [RFC8986] describe the same for Segment Routing over IPv6 (SRv6) with
   the use of the Segment Routing Header (SRH).

   [RFC8402] introduces the SR Policy construct and provides an overview
   of how it is leveraged for Segment Routing use-cases.  [RFC9256]
   updates [RFC8402] to specify detailed concepts of SR Policy and
   steering packets into an SR Policy.

   This document extends the SR Policy to also include the Path MTU
   information to SR Policy and applies to both SRv6 and SR-MPLS.  The
   SRv6-specific handling is specified in Section 6.

1.1.  Motivation

   The motivation for handling SR-PMTU for the SR paths includes (but is
   not limited to):

   *  Being able to avoid fragmentation by being aware of the SR-PMTU
      associated with the SR paths and policies at the headend.

   *  Being able to generate ICMP messages at the headend.

   *  When fragmentation is unavoidable, the ability to do it correctly
      at the headend.

   *  Ability to use SR-PMTU as path computation constraint and
      optimization criteria at the headend or controller/PCE.

2.  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.  Terminology

      Link MTU: As per [RFC8899], this could more properly be called the
      IP MTU.  It is the size in bytes of the largest IP packet,
      including the IP header and payload, that can be transmitted over
      a link.  In case of MPLS, it also includes the label stack, and in
      case of IPv6, it includes IPv6 extension headers (including SRH).




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      Path MTU (PMTU): See [RFC8899].  In the scope of this document,
      this is also called SR-PMTU for the SR paths and policies.

      SR overhead: The SR-MPLS label stack or SRH.  The link MTU takes
      the SR overhead into consideration.

4.  SR-PMTU Definition for SR Policy

   The Segment Routing policy architecture is specified in [RFC9256].
   An SR Policy is associated with one or more candidate paths.  A
   candidate path is either dynamic, explicit, or composite.  The
   related concepts with the SR-PMTU definition in this document are
   listed as follows.

   An explicit/dynamic candidate path is expressed as a Segment-List or
   a set of Segment-Lists directly or by computation.  If a candidate
   path is associated with a set of Segment-Lists, each Segment-List is
   associated with weight for weighted load balancing.  The default
   weight is 1.

   A composite candidate path is defined in [RFC9256].

4.1.  SR-PMTU of a Segment List

   A Segment-List represents a specific source-routed path to send
   traffic from the headend to the endpoint of the corresponding SR
   policy [RFC9256].  The SR-PMTU of a segment list is defined as the
   minimum Link MTU of all the links in a path between a source node and
   a destination node.  Refer to Section 5.2 for specific handling for
   Node, Adjacency and Binding SID (as well as their combinations).

4.2.  SR-PMTU of a Candidate Path

   In the case of an explicit/dynamic candidate path, if it is expressed
   as a single Segment-List, then the SR-PMTU of the candidate path is
   the same as that of the SR-PMTU of the segment list as described in
   Section 4.1.

   In the case of an explicit/dynamic candidate path, if it is expressed
   as a set of Segment-Lists (for load-balancing), then the SR-PMTU of
   the candidate path is defined as the minimum SR-PMTU of all the
   Segment-Lists in the set.

   In the case of a composite candidate path, the SR-PMTU is defined as
   the minimum SR-PMTU of all the constituent SR Policies.






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4.3.  SR-PMTU of an SR Policy

   According to [RFC9256], an SR Policy is associated with one or more
   candidate paths.  A candidate path is selected when it is valid and
   it is determined to be the best path of the SR Policy.  The selected
   path is referred to as the "active path" of the SR policy.  Then the
   SR-PMTU for an SR Policy is defined as the SR-PMTU of the selected/
   active candidate path of this SR policy.

   In the case of an explicit/dynamic candidate path, the SR-PMTU
   definition can be referred to in Section 4.2.

   In the case of a composite candidate path, the SR-PMTU is defined as
   the minimum SR-PMTU of all the constituent SR policies.  Since the
   constituent SR Policies of a composite candidate path can only be
   explicit/dynamic candidate paths, then the SR-PMTU definition of
   explicit/dynamic candidate path is as per Section 4.2.

5.  The Framework of SR-PMTU for SR Policy

   The framework of SR-PMTU for SR Policy includes link MTU collection,
   SR-PMTU computation, SR-PMTU enforcement, and handling behaviors on
   the headend.

                          +------------------+
                 +--------|Network Controller| SR-PMTU computation
                 |        +--------/|\-------+
                 |                  |
       SR-PMTU enforcement   Link MTU Collection
                 |                  |
              +-\|/-+   +-----------|-----------+   +-----+
    Handling  |Head |---|    Segment Routing    |---|End  |
    behaviors |end  |   |    Network Domain     |   |Point|
              +-----+   +-----------------------+   +-----+
                 <---------Link MTU collection---------|



            Figure 1. The Framework of SR-PMTU for SR Policy

5.1.  Link MTU Collection

   The SR-PMTU of a segment list is defined as the minimum Link MTU of
   all the links in a path, see Section 4.1.  The Link MTU can be
   collected in network through various mechanisms such as the ones
   defined in [I-D.hu-lsr-igp-path-mtu] and
   [I-D.ietf-idr-bgp-ls-link-mtu] without the knowledge of the services.




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5.2.  SR-PMTU Computation

   The collected link MTU of all the related links are sent to the
   network controller or the headend where the SR-PMTU is computed.
   Depending upon the path type, the computation methods are different,
   which are described in the following subsections.

5.2.1.  Loose Path

   In a loose path [RFC7855], only Node SIDs are used along the path.
   Between two adjacent Node-SIDs, generally, there are equal-cost
   multipaths (ECMP).  The SR-PMTU of the loose path is computed by
   finding the minimum SR-PMTU of all the ECMPs between two adjacent
   Node SIDs along the loose path.

5.2.2.  Strict TE Path

   In a strict TE path [RFC7855], only Adj-SIDs are used along the path.
   Since the link MTU of all the links being indicated by the Adj-SIDs
   of the strict TE path are known to the network controller, the SR-
   PMTU of the strict SR-TE path is computed by finding out the minimum
   link MTU of all the links in the strict SR-TE path between its source
   node and destination node.

5.2.3.  Mixed Path

   In a mixed path, both Node SIDs-and Adj-SIDs are used along the path.
   The PMTU of the mixed TE path is computed by finding the minimum SR-
   PMTU of all the ECMPs between two adjacent Node SIDs and the link MTU
   of all the links indicated by the Adj SIDs.

5.2.4.  Binding Path

   The Binding SID (BSID) [RFC8402] is bound to an SR Policy,
   instantiation of which may involve a list of SIDs.  The SR-PMTU of
   the binding path is the same as that of an SR Policy as specified in
   the above section modulo that it also includes the encapsulation
   overhead associated with it (i.e. the additional label stack pushed
   in case of SR-MPLS and the outer IPv6 header with its own SRH in case
   of SRv6).  This is done to make sure the headend of the SR path that
   includes a BSID is able to compute the SR-PMTU correctly by taking
   the correct SR-PMTU of the binding path into consideration along with
   other SIDs in the SR path.








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5.2.5.  TI-LFA

   Topology Independent Loop-free Alternate Fast Re-route (TI-LFA)
   [I-D.ietf-rtgwg-segment-routing-ti-lfa], aims to provide protection
   for node and adjacency segments within the SR framework.  The PMTU of
   the repair path might be different from the original path's, which
   could lead to fragmentation while the repair path is in use.

   To avoid fragmentation, it is possible for the headend (or
   controller) to consider the FRR overhead when computing the SR-PMTU
   of the original path.

5.3.  SR-PMTU Enforcement

   SR Policy as per [RFC9256] does not include SR-PMTU in the SR Policy
   encoding structure.  As specified in
   [I-D.ietf-idr-sr-policy-path-mtu], the SR-PMTU is encoded in the SR
   policy structure as shown in Figure 2.  After the SR-PMTU
   computation, the SR-PMTU is enforced along with the SR Policy to the
   headend of the corresponding path.

         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 (SR-PMTU)
                       Segment
                       Segment
                       ...
                   ...

            Figure 2. The SR Policy encoding structure with SR-PMTU

   When there are multiple paths that can be selected, the one with the
   highest SR-PMTU will be used in order to avoid fragmentation on the
   headend.

   The PCEP extension to handle PMTU is specified in
   [I-D.ietf-pce-pcep-pmtu].





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5.4.  Handling behaviors on the headend

   After the SR-PMTU is computed, the headend performs the handling
   behaviors such as encapsulation and fragmentation, if needed.  Note
   that this behavior is similar to the existing behaviors of MPLS and
   IPv6 dataplane.

5.4.1.  SR-PMTU Constraints and Optimization

   Generally, considering the services being carried, the operators set
   an SR-PMTU constraint aiming for a proper path selection that
   fulfills packet size requirements hence avoiding fragmentation.
   Furthermore, the encapsulation on the headend will introduce the
   overhead on top of the packet to be encapsulated.  Generally, the
   encapsulation overhead has to be estimated according to the possible
   path hops and sometimes the repair paths.  Therefore, the SR-PMTU
   constraint is set considering both the carried services and the
   encapsulation overhead.

   When SR-PMTU-based path optimization is done, PCE will select the
   path with the highest SR-PMTU among all the possible paths.

   Even if the SR-PMTU is not considered by the PCE at the time of path
   computation, the computed SR-PMTU is useful at the headend for the
   reasons already stated in Section 1.1.

   Once the SR-PMTU constraint is set on the headend, it is supposed to
   be the lowest bound of the SR-PMTUs of all the paths being computed
   locally or enforced by the controller in order to avoid
   fragmentation.

5.4.2.  Fragmentation processing

   If the SR-PMTU of all the paths being computed locally or enforced by
   the controller is smaller than the SR-PMTU constraint set on the
   headend, the fragmentation will have to be handled.  If fragmentation
   is not possible, the headend could generate the ICMP messages
   [RFC4443] to notify the traffic source.

   Over this selected path, on the headend, the packets are fragmented
   in order to guarantee the size of the encapsulated packets is smaller
   than the PMTU of the selected path.









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5.5.  Link MTU Change

   The Link MTU collected as described in Section 5.1 may change over
   time due to factors such as device configuration updates or
   topological modifications, such as the addition of a new link with a
   lower MTU.  These changes can impact the SR-PMTU of the data path,
   and the computed SR-PMTU value may remain outdated until the control
   plane converges.  This behavior is similar to changes in other link
   metrics.

6.  SRv6-Specific Handling

   In the case of SRv6, the SRH is included in the calculation of the
   Link MTU and thus in the SR-PMTU.  Note that the PMTU considerations
   for IPv6 [RFC8201] apply for the SRv6.  [RFC8754] also specify the
   MTU considerations related to encapsulation with an outer IPv6 header
   with SRH.

7.  Security Considerations

   [RFC9256] specifies in detail the SR Policy construct (introduced in
   [RFC8402]) and its security considerations.  The additional SR-MTU
   attribute information can be sensitive in some deployments and could
   be used to influence SR path setup and selection with adverse effect.
   The protocol extensions that include SR-PMTU need to take this into
   consideration.  This document does not define any new protocol
   extensions and thus does not introduce any further security
   considerations.

8.  IANA Considerations

   This document does not include any IANA requests.

9.  Acknowledgement

   Thanks to xx for useful discussions and comments.

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>.






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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>.

   [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>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [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>.

   [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>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.











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   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              21, 12 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
              segment-routing-ti-lfa-21>.

10.2.  Informative References

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <https://www.rfc-editor.org/info/rfc7855>.

   [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.hu-lsr-igp-path-mtu]
              Hu, Z., Peng, S., and X. Xi, "IGP Extensions for Path
              MTU", Work in Progress, Internet-Draft, draft-hu-lsr-igp-
              path-mtu-00, 19 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-hu-lsr-igp-
              path-mtu-00>.

   [I-D.ietf-idr-sr-policy-path-mtu]
              Li, C., Zhu, Y., El Sawaf, A., Li, Z., and G. Zeng,
              "Segment Routing Path MTU in BGP", Work in Progress,
              Internet-Draft, draft-ietf-idr-sr-policy-path-mtu-14, 5
              January 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-idr-sr-policy-path-mtu-14>.

   [I-D.ietf-pce-pcep-pmtu]
              Peng, S., Li, C., Han, L., Ndifor, L., and S. Sidor,
              "Support for Path MTU (PMTU) in the Path Computation



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              Element (PCE) Communication Protocol (PCEP)", Work in
              Progress, Internet-Draft, draft-ietf-pce-pcep-pmtu-09, 20
              February 2026, <https://datatracker.ietf.org/doc/html/
              draft-ietf-pce-pcep-pmtu-09>.

Authors' Addresses

   Shuping Peng
   Huawei
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: pengshuping@huawei.com


   Dhruv Dhody
   Huawei
   India
   Email: dhruv.ietf@gmail.com


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


   Gyan Mishra
   Verizon Inc.
   United States of America
   Email: gyan.s.mishra@verizon.com



















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