Support for Path MTU (PMTU) in the Path Computation Element (PCE) Communication Protocol (PCEP)
draft-ietf-pce-pcep-pmtu-09

PCE Working Group                                                S. Peng
Internet-Draft                                                     C. Li
Updates: 5440 (if approved)                          Huawei Technologies
Intended status: Standards Track                                  L. Han
Expires: 24 August 2026                                     China Mobile
                                                               L. Ndifor
                                                            MTN Cameroon
                                                                S. Sidor
                                                                   Cisco
                                                        20 February 2026

   Support for Path MTU (PMTU) in the Path Computation Element (PCE)
                     Communication Protocol (PCEP)
                      draft-ietf-pce-pcep-pmtu-09

Abstract

   The Path Computation Element (PCE) provides path computation
   functions in support of traffic engineering in Multiprotocol Label
   Switching (MPLS) and Generalized MPLS (GMPLS) networks.

   The Source Packet Routing in Networking (SPRING) architecture
   describes how Segment Routing (SR) can be used to steer packets
   through an IPv6 or MPLS network using the source routing paradigm.  A
   Segment Routed Path can be derived from a variety of mechanisms,
   including an IGP Shortest Path Tree (SPT), explicit configuration, or
   a Path Computation Element (PCE).

   Since the SR does not require signaling, the path maximum
   transmission unit (MTU) information for the SR path is unavailable at
   the headend.  This document specifies the extension to PCE
   Communication Protocol (PCEP) to carry path MTU as a new metric type
   in the PCEP messages for SR, but not limited to it.

   This document also updates RFC 5440 to allow metric bounds to be
   minimum as needed in the case of path MTU.

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 24 August 2026.

Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  PCEP Extention  . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Extensions to METRIC Object . . . . . . . . . . . . . . .   4
       4.1.1.  Update to RFC 5440  . . . . . . . . . . . . . . . . .   5
     4.2.  Stateful PCE and PCE Initiated LSPs . . . . . . . . . . .   6
     4.3.  Segment Routing . . . . . . . . . . . . . . . . . . . . .   6
       4.3.1.  Multi-Path Handling . . . . . . . . . . . . . . . . .   7
       4.3.2.  Path MTU Adjustment . . . . . . . . . . . . . . . . .   7
       4.3.3.  MSD . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Other Path Setup Types  . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  METRIC Type . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  PCEP Error Type and Value . . . . . . . . . . . . . . . .   8
     6.3.  Security Considerations . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   [RFC5440] describes the Path Computation Element (PCE) Communication
   Protocol (PCEP).  PCEP enables communication between a Path
   Computation Client (PCC) and a PCE, or between PCE and PCE, for the
   purpose of computation of Multiprotocol Label Switching (MPLS) as
   well as Generalized MPLS (GMPLS) Traffic Engineering Label Switched
   Path (TE LSP) characteristics.

   [RFC8231] specifies a set of extensions to PCEP to enable stateful
   control of TE LSPs within and across PCEP sessions in compliance with
   [RFC4657].  It includes mechanisms to effect LSP State
   Synchronization between PCCs and PCEs, delegation of control over
   LSPs to PCEs, and PCE control of timing and sequence of path
   computations within and across PCEP sessions.  The model of operation
   where LSPs are initiated from the PCE is described in [RFC8281].

   As per [RFC8402], with Segment Routing (SR), a node steers a packet
   through an ordered list of instructions, called segments.  A segment
   can represent any instruction, topological or service-based.  A
   segment can have a semantic local to an SR node or global within an
   SR domain.  SR enables the enforcement of a flow through a specific
   path and service chain while ensuring that the per-flow state is
   maintained only at the ingress node of the SR domain.  Segments can
   be derived from different components: IGP, BGP, Services, Contexts,
   Locators, etc.  The SR architecture can be applied to the MPLS
   forwarding plane without any change, in which case an SR path
   corresponds to an MPLS Label Switching Path (LSP).  The SR is applied
   to the IPv6 forwarding plane using SRH.  An SR path can be derived
   from an IGP Shortest Path Tree (SPT), but SR-TE paths may not follow
   IGP SPT.  Such paths may be chosen by a suitable network planning
   tool or a PCE and provisioned on the ingress node.

   As per [RFC8664], it is possible to use a stateful PCE for computing
   one or more SR-TE paths taking into account various constraints and
   objective functions.  Once a path is chosen, the stateful PCE can
   initiate an SR-TE path on a PCC using PCEP extensions specified in
   [RFC8281] using the SR specific PCEP extensions specified in
   [RFC8664].  [RFC8664] specifies PCEP extensions for supporting an SR-
   TE LSP for MPLS data plane.  [RFC9603] extend PCEP to support SR for
   IPv6 data plane.

   [I-D.ietf-spring-pmtu-sr-policy] specifies the link maximum
   transmission unit (MTU) and SR Path MTU (SR-PMTU) in the context of
   SR paths and policies.  It also states the motivation, link MTU
   collection, SR-PMTU Computation, SR-PMTU Enforcement, and handling
   behaviors on the headend.

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   Since the SR does not require signaling, the path MTU information for
   SR path is not available.  This document specifies the extension to
   PCEP to carry path MTU in the PCEP messages.  It is assumed that the
   PCE is aware of the link MTU as part of the Traffic Engineering
   Database (TED) population.  This could be done via IGP, BGP-LS
   [I-D.ietf-idr-bgp-ls-link-mtu] or some other means.  Thus, the PCE
   can find the path MTU at the time of path computation and include
   this information as part of the PCEP messages.

   Though the key use case for path MTU is SR, the PCEP extension (as
   specified in this document) creates a new metric type for path MTU,
   making this a generic extension that can be used for any path setup
   type (PST) (see Section 4.4).

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 [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 the link
      layer headers and other framing that is not part of IP or the IP
      payload.  In the 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 SR,
      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.  PCEP Extention

4.1.  Extensions to METRIC Object

   The METRIC object is defined in Section 7.8 of [RFC5440], comprising
   metric-value and metric-type (T field), and a flags field, comprising
   a number of bit flags (B "Bound" bit and C "Computed Metric" bit).
   This document defines a new type for the METRIC object for Path MTU.

   *  T = TBD: Path MTU.

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   *  A network comprises of a set of N links {Li, (i=1...N)}.

   *  A path P of an LSP is a list of K links {Lpi,(i=1...K)}.

   *  A Link MTU of link L is denoted M(L).

   *  A Path MTU metric for the path P = Min {M(Lpi), (i=1...K)}.

   The Path MTU metric type of the METRIC object in PCEP represents the
   minimum of the Link MTU of all links along the path.

   When PCE computes the path, it can also find the Path MTU (based on
   the above criteria) and include this information in the METRIC object
   with the above metric type in the PCEP message when replying to the
   PCC.  In a Path Computation Reply (PCRep) message, the PCE MAY insert
   the METRIC object with an Explicit Route Object (ERO) so as to
   provide the METRIC (path MTU) for the computed path.  The PCE MAY
   also insert the METRIC object with a NO-PATH object to indicate that
   the metric constraint could not be satisfied.

   Further, a PCC MAY use the Path MTU metric in a Path Computation
   Request (PCReq) message to request a path meeting the MTU requirement
   of the path.  In this case, the B bit MUST be set to suggest a bound
   (a maximum) for the Path MTU metric that MUST NOT be exceeded for the
   PCC to consider the computed path as acceptable.  The Path MTU metric
   must be less than or equal to the value specified in the metric-value
   field.

   A PCC can also use this metric to request that the PCE optimize the
   path MTU during path computation.  In this case, the B bit MUST be
   cleared.

   Note that [RFC5440] allow two metric object instances for
   optimization (B flag cleared) and thus the Path MTU metric object
   might be used alongside other metric types as well.

   The error handling and processing of the METRIC object is as
   specified in [RFC5440].

4.1.1.  Update to RFC 5440

   For the handling of B bit in METRIC Object, [RFC5440] states: "When
   set in a PCReq message, the metric-value indicates a bound (a
   maximum) for the path metric that must not be exceeded for the PCC to
   consider the computed path as acceptable.  The path metric must be
   less than or equal to the value specified in the metric-value field."

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   The new metric type path MTU defined in this document is different.
   The bound for the path MTU indicates a minimum value instead of
   maximum.  That is when the metric type is set to TBD for path MTU,
   the metric-value indicates a bound (a minimum path MTU) for the path
   metric that must not be subceeded for the PCC to consider the
   computed path as acceptable.  The path metric for path MTU must be
   greater than or equal to the value specified in the metric-value
   field.

   Further, a PCC MAY request that PCE optimizes an individual path
   computation request to maximize the path MTU of the computed path by
   clearing the B bit in the METRIC object with metric-type=TBD for path
   MTU.

4.2.  Stateful PCE and PCE Initiated LSPs

   [RFC8231] specifies a set of extensions to PCEP to enable stateful
   control of MPLS-TE LSPs via PCEP and the maintaining of these LSPs at
   the stateful PCE.  It further distinguishes between an active and a
   passive stateful PCE.  A passive stateful PCE uses LSP state
   information learned from PCCs to optimize path computations but does
   not actively update the LSP state.  In contrast, an active stateful
   PCE utilizes the LSP delegation mechanism to update LSP parameters in
   those PCCs that delegated control over their LSPs to the PCE.
   [RFC8281] describes the setup, maintenance, and teardown of PCE-
   initiated LSPs under the stateful PCE model.  The document defines
   the PCInitiate message that is used by a PCE to request a PCC to set
   up a new LSP.

   The new metric type defined in this document can also be used with
   the stateful PCE extensions.  The format of PCEP messages described
   in [RFC8231] and [RFC8281] uses <intended-attribute-list> and
   <attribute-list>, respectively, (where the <intended-attribute-list>
   is the attribute-list defined in Section 6.5 of [RFC5440]).

   A PCE MAY include the path MTU metric in PCInitiate or PCUpd message
   to inform the PCC of the path MTU calculated for the path or as a
   bound constraint.  A PCC MAY include the path MTU metric as a bound
   constraint or to indicate optimization criteria (similar to PCReq).

4.3.  Segment Routing

   A Segment Routed path (SR path) can be derived from an IGP Shortest
   Path Tree (SPT).  Segment Routed Traffic Engineering paths (SR-TE
   paths) may not follow IGP SPT.  Such paths may be chosen by a
   suitable network planning tool and provisioned on the source node of
   the SR-TE path.

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   It is possible to use a PCE for computing one or more SR-TE paths
   taking into account various constraints and objective functions.
   Once a path is chosen, the PCE can inform an SR-TE path on a PCC
   using PCEP extensions specified in [RFC8664].  Further, [RFC9603]
   adds the support for IPv6 data plane in SR.

   Refer [I-D.ietf-spring-pmtu-sr-policy] for SR-PMTU considerations.

4.3.1.  Multi-Path Handling

   [I-D.ietf-spring-pmtu-sr-policy] specify the handling of SR-PMTU at
   the SR Policy, Candidate paths, and at each segment list level.  In
   PCEP, support for multiple segment list is added in
   [I-D.ietf-pce-multipath].  The METRIC object is currently encoded at
   the candidate path level.  A future update of the document could
   investigate and include mechanisms to support SR-PMTU at each segment
   list level.

4.3.2.  Path MTU Adjustment

   As per [I-D.ietf-spring-pmtu-sr-policy], 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 where the overhead is allowed to be configured by
   an operator.

4.3.3.  MSD

   In SR, the term Maximum SID Depth (MSD) [RFC8491] refers to the
   maximum number of SIDs that an ingress is capable of imposing on a
   packet.  In contrast, the PMTU determines whether IP fragmentation
   can be avoided, making it an unrelated factor.

4.4.  Other Path Setup Types

   While SR is the primary use case for the extension defined in this
   document, the extension in itself is not limited to use within SR.

   The PMTU metric type can be used for any path setup type.  An
   implementation MAY choose not to support the use of this metric type
   for a particular PST as a local policy, in which case it MUST respond
   to its peer with a PCErr message with the Error-Type = 5 ("Policy
   Violation") and Error-value = TBD2 ("Metric Type not supported with
   this PST").

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5.  Security Considerations

   This document defines a new METRIC type that does not add any new
   security concerns beyond those discussed in [RFC5440] in itself.
   Some deployments may find the path MTU information to be extra
   sensitive and could be used to influence path computation and setup
   with adverse effects.  Additionally, snooping of PCEP messages with
   such data or using PCEP messages for network reconnaissance may give
   an attacker sensitive information about the operations of the
   network.  Thus, such deployment should employ suitable PCEP security
   mechanisms like TCP Authentication Option (TCP-AO) [RFC5925] or
   Transport Layer Security (TLS) [RFC8253].  The procedure is based on
   TLS is considered a security enhancement and thus is much better
   suited for sensitive information.

6.  IANA Considerations

   This document makes the following requests to IANA for action.

6.1.  METRIC Type

   IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
   registry group.  Within this registry group, IANA maintains a
   registry for "METRIC Object T Field".  IANA is requested to make the
   following allocation:

   Value                  Description                  Reference
   ---------------------- ---------------------------- --------------
   TBD1                   Path MTU                     This document

6.2.  PCEP Error Type and Value

   IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
   registry group.  Within this registry group, IANA maintains a
   registry for "PCEP-ERROR Object Error Types and Values".  IANA is
   requested to make the following allocation:

   Error-Type   Meaning                  Error-Value   Reference
   ------------ ------------------------ ------------- --------------
   5            Policy violation                       [RFC5440]
                                         TBD2: Metric  This document
                                         Type not
                                         supported
                                         with this
                                         PST

6.3.  Security Considerations

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7.  References

7.1.  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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [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>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [I-D.ietf-spring-pmtu-sr-policy]
              Peng, S., Dhody, D., Talaulikar, K., and G. S. Mishra,
              "Path MTU (PMTU) for Segment Routing (SR) Policy", Work in
              Progress, Internet-Draft, draft-ietf-spring-pmtu-sr-
              policy-03, 5 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              pmtu-sr-policy-03>.

7.2.  Informative References

   [RFC4657]  Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

   [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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   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [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>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [I-D.ietf-pce-multipath]
              Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
              Bidgoli, H., Peng, S., and S. Sidor, "Path Computation
              Element Communication Protocol (PCEP) Extensions for
              Signaling Multipath Information", Work in Progress,
              Internet-Draft, draft-ietf-pce-multipath-19, 2 February
              2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pce-multipath-19>.

   [RFC9603]  Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
              and Y. Zhu, "Path Computation Element Communication
              Protocol (PCEP) Extensions for IPv6 Segment Routing",
              RFC 9603, DOI 10.17487/RFC9603, July 2024,
              <https://www.rfc-editor.org/info/rfc9603>.

   [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>.

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Authors' Addresses

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

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: c.l@huawei.com

   Liuyan Han
   China Mobile
   Beijing
   100053
   China
   Email: hanliuyan@chinamobile.com

   Luc-Fabrice Ndifor
   MTN Cameroon
   Cameroon
   Email: Luc-Fabrice.Ndifor@mtn.com

   Samuel Sidor
   Cisco
   Slovakia
   Email: ssidor@cisco.com

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