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PCE Working Group                                               D. Dhody
Internet-Draft                                                     C. Li
Intended status: Standards Track                     Huawei Technologies
Expires: January 3, 2019                                   July 02, 2018


      Support for Path MTU (PMTU) in the Path Computation Element
                     Communication Protocol (PCEP).
                      draft-dhody-pce-pcep-pmtu-00

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any



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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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 and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  PCEP Extention  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Extensions to METRIC Object . . . . . . . . . . . . . . .   4
     2.2.  Stateful PCE and PCE Initiated LSPs . . . . . . . . . . .   5
     2.3.  Segement Routing  . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  METRIC Types  . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.

   [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



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   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 [I-D.ietf-spring-segment-routing], 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 [I-D.ietf-pce-segment-routing], 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 [I-D.ietf-pce-segment-routing].
   [I-D.ietf-pce-segment-routing] specifies PCEP extensions for
   supporting a SR-TE LSP for MPLS data plane.
   [I-D.negi-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.  PCEP Extention

2.1.  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.

   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.



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   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].

2.2.  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
   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).

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





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

3.  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
   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.

4.  IANA Considerations

   This document makes following requests to IANA for action.

4.1.  METRIC Types

   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

5.  Acknowledgments

6.  References

6.1.  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>.



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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>.

   [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>.

6.2.  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>.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-12 (work in progress), June
              2018.








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   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

   [I-D.negi-pce-segment-routing-ipv6]
              Negi, M., Kaladharan, P., Dhody, D., and S. Sivabalan,
              "PCEP Extensions for Segment Routing leveraging the IPv6
              data plane", draft-negi-pce-segment-routing-ipv6-02 (work
              in progress), June 2018.

Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   Email: dhruv.ietf@gmail.com


   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: chengli13@huawei.com





















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