Support for Path MTU (PMTU) in the Path Computation Element (PCE) communication Protocol (PCEP).
draft-li-pce-pcep-pmtu-02

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Replaces draft-dhody-pce-pcep-pmtu
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PCE Working Group                                                S. Peng
Internet-Draft                                                     C. Li
Intended status: Standards Track                     Huawei Technologies
Expires: February 24, 2021                                        L. Han
                                                            China Mobile
                                                         August 23, 2020

   Support for Path MTU (PMTU) in the Path Computation Element (PCE)
                     communication Protocol (PCEP).
                       draft-li-pce-pcep-pmtu-02

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 SR path is not available.
   This document specify the extension to PCE communication protocol
   (PCEP) to carry path (MTU) in the PCEP messages.

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.

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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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on February 24, 2021.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  PCEP Extention  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Extensions to METRIC Object . . . . . . . . . . . . . . .   5
     3.2.  Stateful PCE and PCE Initiated LSPs . . . . . . . . . . .   6
     3.3.  Segment Routing . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Path MTU Adjustment . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  METRIC Type . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   [RFC5440] describes the Path Computation Element (PCE) Communication
   Protocol (PCEP).  PCEP enables the 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 Generalzied MPLS (GMPLS) Traffic Engineering Label Switched
   Path (TE LSP) characteristics.

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   [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 allows to enforce a flow through any path and service
   chain while maintaining per-flow state 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 IPV6 forwarding plane using SRH.  A 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 a SR-
   TE LSP for MPLS data plane.  [I-D.ietf-pce-segment-routing-ipv6]
   extend PCEP to support SR for IPv6 data plane.

   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 need 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] specify the
   mechanism of MTU signaling in RSVP.

   Since the SR does not require signaling, the path MTU information for
   SR path is not available.  This document specify the extension to
   PCEP to carry path MTU in the PCEP messages.  It is assumed that the

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   PCE is aware of the link MTU as part of the Traffic Engineering
   Database (TED) population.  This could be done via IGP, BGP-LS 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 independent of SR.

2.  Terminology

   This draft refers to the terms defined in [RFC8201], [RFC4821] and
   [RFC3988].

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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.

3.  PCEP Extention

3.1.  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 bit and C bit).  This document defines a new
   type for the METRIC object for Path MTU.

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   o  T = TBD: Path MTU.

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

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

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

   o  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 ask PCE to optimize the path MTU
   during path computation.  In this case, the B bit MUST be cleared.

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

3.2.  Stateful PCE and PCE Initiated LSPs

   [RFC8231] specifies a set of extensions to PCEP to enable stateful
   control of MPLS-TE and GMPLS 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 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

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   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.  A PCC MAY
   include the path MTU metric as a bound constraint or to indicate
   optimization criteria (similar to PCReq).

3.3.  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.

   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,
   [I-D.ietf-pce-segment-routing-ipv6] adds the support for IPv6 data
   plane in SR.

   The new metric type for path MTU is applicable for the SR-TE path and
   require no additional extensions.

3.4.  Path MTU Adjustment

   The path MTU metric can be used for both primary and protection path.

   The minimal value of the link MTU along the path is collected, based
   on which minor adjustment is made to cater for overhead introduced by
   the protection mechanisms such as TI-LFA.  The path MTU is the value
   of the minimum link MTU minus the overhead.  In this way, the ingress
   node can use the path MTU directly.

4.  Security Considerations

   This document defines a new METRIC type that do 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

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   with adverse effect.  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 based on TLS
   is considered a security enhancement and thus is much better suited
   for the sensitive information.

5.  IANA Considerations

   This document makes following requests to IANA for action.

5.1.  METRIC Type

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

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

6.  Acknowledgement

   We would like to thank Dhruv Dhody for his contributions for this
   document.

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>.

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   [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>.

7.2.  Informative References

   [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>.

   [RFC4657]  Ash, J., Ed. and J. 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>.

   [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>.

   [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>.

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   [I-D.ietf-pce-segment-routing-ipv6]
              Li, C., Negi, M., Koldychev, M., Kaladharan, P., and Y.
              Zhu, "PCEP Extensions for Segment Routing leveraging the
              IPv6 data plane", draft-ietf-pce-segment-routing-ipv6-06
              (work in progress), July 2020.

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

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