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Path MTU (PMTU) for Segment Routing Policy
draft-peng-spring-pmtu-sr-policy-01

Document Type Active Internet-Draft (individual)
Authors Shuping Peng , Dhruv Dhody , Ketan Talaulikar , Gyan Mishra
Last updated 2022-10-23
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draft-peng-spring-pmtu-sr-policy-01
SPRING Working Group                                             S. Peng
Internet-Draft                                                  D. Dhody
Intended status: Standards Track                     Huawei Technologies
Expires: 26 April 2023                                     K. Talaulikar
                                                              Arrcus Inc
                                                               G. Mishra
                                                            Verizon Inc.
                                                         23 October 2022

               Path MTU (PMTU) for Segment Routing Policy
                  draft-peng-spring-pmtu-sr-policy-01

Abstract

   This document defines the Path MTU (PMTU) for SR Policy (called SR-
   PMTU) and it applies to both SRv6 and SR-MPLS.  The framework of SR-
   PMTU for SR Policy is specified, including link MTU collection, SR-
   PMTU Computation, SR-PMTU Enforcement, and Handling behaviors 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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 April 2023.

Copyright Notice

   Copyright (c) 2022 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
   and restrictions with respect to this document.  Code Components

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   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 . . . . . . . . . . . . . . .   5
     4.3.  SR-PMTU of an SR Policy . . . . . . . . . . . . . . . . .   5
   5.  The Framework of SR-PMTU for SR Policy  . . . . . . . . . . .   5
     5.1.  Link MTU Collection . . . . . . . . . . . . . . . . . . .   6
     5.2.  SR-PMTU Computation . . . . . . . . . . . . . . . . . . .   6
       5.2.1.  Loose TE Path . . . . . . . . . . . . . . . . . . . .   6
       5.2.2.  Strict TE Path  . . . . . . . . . . . . . . . . . . .   7
       5.2.3.  Mixed Path  . . . . . . . . . . . . . . . . . . . . .   7
       5.2.4.  Binding Path  . . . . . . . . . . . . . . . . . . . .   7
       5.2.5.  TI-LFA  . . . . . . . . . . . . . . . . . . . . . . .   7
       5.2.6.  Others  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  SR-PMTU Enforcement . . . . . . . . . . . . . . . . . . .   8
     5.4.  Handling behaviors on the headend . . . . . . . . . . . .   9
       5.4.1.  SR-PMTU Constraints and Optimization  . . . . . . . .   9
       5.4.2.  Fragmentation processing  . . . . . . . . . . . . . .   9
   6.  SRv6-Specific Handling  . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Segment Routing (SR) [RFC8402] allows a node to steer a packet flow
   along any path.  The headend is a node where the instructions for
   source routing (i.e., segments) are written into 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)
   [I-D.ietf-spring-segment-routing-policy] 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.
   [I-D.ietf-spring-segment-routing-policy] 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 a separate section.

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 the 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 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

3.  Terminology

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      Link MTU: As per [RFC4821], the Maximum Transmission Unit, i.e.,
      maximum IP packet size in bytes, that can be conveyed in one piece
      over a link.  This includes the IP header, but excludes link layer
      headers and other framing that is not part of IP or the IP
      payload.  In case of MPLS, it also includes the label stack and in
      case of IPv6, it includes IPv6 extension headers (including SRH).

      Path MTU, or PMTU: The minimum link MTU of all the links in a path
      between a source node and a destination node.  In the scope of
      this document, this is also called SR-PMTU for the SR paths and
      policies.  Note that the link MTU takes the SR overhead (label
      stack or SRH) into consideration.

4.  SR-PMTU Definition for SR Policy

   Segment Routing policy architecture is specified in
   [I-D.ietf-spring-segment-routing-policy].  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.  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 acts as a container for grouping SR
   Policies.  The composite candidate path construct enables the
   combination of SR Policies, each with explicit candidate paths and/or
   dynamic candidate paths with potentially different optimization
   objectives and constraints, for load-balanced steering of packet
   flows over its constituent SR Policies
   [I-D.ietf-spring-segment-routing-policy].

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 [I-D.ietf-spring-segment-routing-policy].  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 Section 5.2
   for specific handling for Node, Adjacency and Binding SID (as well as
   their combinations).

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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
   the above section.

   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, then the SR-PMTU of the
   composite candidate path is defined as the minimum SR-PMTU of all the
   constituent SR Policies of this composite candidate path.  The SR-
   PMTU of each SR Policy is defined in the following subsection.

4.3.  SR-PMTU of an SR Policy

   According to [I-D.ietf-spring-segment-routing-policy], 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 the above subsection.

   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 do not use
   composite candidate paths, only explicit/dynamic candidate paths will
   be used, then the SR-PMTU definition of explicit/dynamic candidate
   path can be referred to in the above subsection.

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.

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                          +------------------+
                 +--------|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

   SR-PMTU is defined as the minimum link MTU of all the links in a path
   between a source node and a destination node.  The link MTU needs to
   be first collected.  The link MTU can be collected through various
   protocols such as IGP [I-D.hu-lsr-igp-path-mtu] and BGP-LS
   [I-D.ietf-idr-bgp-ls-link-mtu], etc.

5.2.  SR-PMTU Computation

   The collected link MTU of all the related links are sent to the
   network controller 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 TE Path

   In a loose TE 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 TE path is computed by
   finding out the minimum SR-PMTU of all the ECMPs between two adjacent
   Node SIDs along the loose TE path.

   Note that an implementation could maintain the SR-PMTU value
   associated with Node SIDs at the time of best path computation.  The
   details of which are out of the scope of this document.

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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 sent 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 out the minimum
   value 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. in case of SR-MPLS, 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.

5.2.5.  TI-LFA

   Topology Independent Loop-free Alternate Fast Re-route (TI-LFA)
   [I-D.ietf-rtgwg-segment-routing-ti-lfa], aimed at providing
   protection of node and adjacency segments within the SR framework.
   The repair path is to pre-compute SPT_new(R,X) for each destination,
   that is, the Shortest Path Tree rooted at node R in the state of the
   network after the resource X has failed.  An implementation is free
   to use any local optimization to provide smaller SID lists by
   combining Node SIDs and Adjacency SIDs.  In addition, the usage of
   Node-SIDs allows to maximize ECMPs over the repair path.  Note that
   while the PMTU of repair path might be different from the original
   path, which could lead to fragmentation while the repair path is in
   use.  When the controller has computed the new path, its new PMTU
   would be updated to the headend.

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   Note that it is possible for the headend implementation to take an
   FRR overhead into consideration when determining if fragmentation
   would be needed for the SR Path with TI-LFA enabled.  If this is
   used, an implementation SHOULD allow the value to be configured by an
   operator.

5.2.6.  Others

   All other types of path can be considered here in future updates.

5.3.  SR-PMTU Enforcement

   Currently, there is still no SR-PMTU in the SR Policy encoding
   structure [I-D.ietf-spring-segment-routing-policy].  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 enforced 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 and enforced on the headend, the
   headend is going to perform the handling behaviors such as
   encapsulation, fragmentation, etc.  Note that this behavior is
   similar to the existing behavior of MPLS and IPv6 dataplane.

5.4.1.  SR-PMTU Constraints and Optimization

   Generally, considering its services being carried, the operators set
   an SR-PMTU limit 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 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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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

   [I-D.ietf-spring-segment-routing-policy] specifies in detail the SR
   Policy construct (introduced [RFC8402]) and its security
   consideration.  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 an IANA request.

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>.

   [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>.

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   [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>.

   [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-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-22, 22 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-22.txt>.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., 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-
              08, 21 January 2022, <https://www.ietf.org/archive/id/
              draft-ietf-rtgwg-segment-routing-ti-lfa-08.txt>.

10.2.  Informative References

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

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   [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>.

   [I-D.ietf-idr-bgp-ls-link-mtu]
              Zhu, Y., Hu, Z., Peng, S., and R. Mwehaire, "Signaling
              Maximum Transmission Unit (MTU) using BGP-LS", Work in
              Progress, Internet-Draft, draft-ietf-idr-bgp-ls-link-mtu-
              03, 30 May 2022, <https://www.ietf.org/archive/id/draft-
              ietf-idr-bgp-ls-link-mtu-03.txt>.

   [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://www.ietf.org/archive/id/draft-hu-lsr-igp-path-
              mtu-00.txt>.

   [I-D.ietf-idr-sr-policy-path-mtu]
              Li, C., Zhu, Y., Sawaf, A. E., and Z. Li, "Segment Routing
              Path MTU in BGP", Work in Progress, Internet-Draft, draft-
              ietf-idr-sr-policy-path-mtu-05, 21 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-idr-sr-policy-
              path-mtu-05.txt>.

   [I-D.ietf-pce-pcep-pmtu]
              Peng, S., Li, C., Han, L., and L. Ndifor, "Support for
              Path MTU (PMTU) in the Path Computation Element (PCE)
              communication Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-pmtu-02, 6 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-pcep-pmtu-
              02.txt>.

Authors' Addresses

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

Peng, et al.              Expires 26 April 2023                [Page 12]
Internet-Draft                   SR-PMTU                    October 2022

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   India
   Email: dhruv.ietf@gmail.com

   Ketan Talaulikar
   Arrcus Inc
   India
   Email: ketant.ietf@gmail.com

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

Peng, et al.              Expires 26 April 2023                [Page 13]