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Internet Engineering Task Force (IETF) S. Yasukawa Request for Comments: 5862 NTT Corporation Category: Informational A. Farrel ISSN: 2070-1721 Old Dog Consulting

                                                             June 2010
  Path Computation Clients (PCC) - Path Computation Element (PCE)
            Requirements for Point-to-Multipoint MPLS-TE


 The Path Computation Element (PCE) provides path computation
 functions in support of traffic engineering in Multiprotocol Label
 Switching (MPLS) and Generalized MPLS (GMPLS) networks.
 Extensions to the MPLS and GMPLS signaling and routing protocols have
 been made in support of point-to-multipoint (P2MP) Traffic Engineered
 (TE) Label Switched Paths (LSPs).  The use of PCE in MPLS networks is
 already established, and since P2MP TE LSP routes are sometimes
 complex to compute, it is likely that PCE will be used for P2MP LSPs.
 Generic requirements for a communication protocol between Path
 Computation Clients (PCCs) and PCEs are presented in RFC 4657, "Path
 Computation Element (PCE) Communication Protocol Generic
 Requirements".  This document complements the generic requirements
 and presents a detailed set of PCC-PCE communication protocol
 requirements for point-to-multipoint MPLS/GMPLS traffic engineering.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Yasukawa & Farrel Informational [Page 1] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

Copyright Notice

 Copyright (c) 2010 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
 ( 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.

1. Introduction

 The Path Computation Element (PCE) defined in [RFC4655] is an entity
 that is capable of computing a network path or route based on a
 network graph, and applying computational constraints.  The intention
 is that the PCE is used to compute the path of Traffic Engineered
 Label Switched Paths (TE LSPs) within Multiprotocol Label Switching
 (MPLS) and Generalized MPLS (GMPLS) networks.
 Requirements for point-to-multipoint (P2MP) MPLS TE LSPs are
 documented in [RFC4461], and signaling protocol extensions for
 setting up P2MP MPLS TE LSPs are defined in [RFC4875].  P2MP MPLS TE
 networks are considered in support of various features, including
 layer 3 multicast virtual private networks [RFC4834].
 Path computation for P2MP TE LSPs presents a significant challenge,
 and network optimization of multiple P2MP TE LSPs requires
 considerable computational resources.  PCE offers a way to offload
 such path computations from Label Switching Routers (LSRs).
 The applicability of the PCE-based path computation architecture to
 P2MP MPLS TE is described in a companion document [RFC5671].  No
 further attempt is made to justify the use of PCE for P2MP MPLS TE
 within this document.
 This document presents a set of PCC-PCE communication protocol
 (PCECP) requirements for P2MP MPLS traffic engineering.  It
 supplements the generic requirements documented in [RFC4657].

Yasukawa & Farrel Informational [Page 2] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].
 Although this document is not a protocol specification, this
 convention is adopted for clarity of description of requirements.

3. PCC-PCE Communication Requirements for P2MP MPLS Traffic Engineering

 This section sets out additional requirements specific to P2MP MPLS
 TE that are not covered in [RFC4657].

3.1. PCC-PCE Communication

 The PCC-PCE communication protocol MUST allow requests and replies
 for the computation of paths for P2MP LSPs.
 This requires no additional messages, but requires the addition of
 the parameters described in the following sections to the existing
 PCC-PCE communication protocol messages.

3.1.1. Indication of P2MP Path Computation Request

 R1:  Although the presence of certain parameters (such as a list of
      more than one destination) MAY be used by a protocol
      specification to allow an implementation to infer that a Path
      Computation Request is for a P2MP LSP, an explicit parameter
      SHOULD be placed in a conspicuous place within a Path
      Computation Request message to allow a receiving PCE to easily
      identify that the request is for a P2MP path.

3.1.2. Indication of P2MP Objective Functions

 R2:  [RFC4657] includes the requirement to be able to specify the
      objective functions to be applied by a PCE during path
      This document makes no change to that requirement, but it should
      be noted that new and different objective functions will be used
      for P2MP computation.  Definitions for core objective functions
      can be found in [RFC5541] together with usage procedures.  New
      objective functions for use with P2MP path computations will
      need to be defined and allocated codepoints in a separate

Yasukawa & Farrel Informational [Page 3] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

3.1.3. Non-Support of P2MP Path Computation

 R3:  PCEs are not required to support P2MP path computation.
      Therefore, it MUST be possible for a PCE to reject a P2MP Path
      Computation Request message with a reason code that indicates no
      support for P2MP path computation.

3.1.4. Non-Support by Back-Level PCE Implementations

 It is possible that initial PCE implementations will be developed
 without support for P2MP path computation and without the ability to
 recognize the explicit parameter described in Section 3.1.1.  Such
 legacy implementations will not be able to make use of the new reason
 code described in Section 3.1.3.
 R4:  Therefore, at least one parameter required for inclusion in a
      P2MP Path Computation Request message MUST be defined in such a
      way as to cause automatic rejection as unprocessable or
      unrecognized by a back-level PCE implementation without
      requiring any changes to that PCE.  It is RECOMMENDED that the
      parameter that causes this result be the parameter described in
      Section 3.1.1.

3.1.5. Specification of Destinations

 R5:  Since P2MP LSPs have more than one destination, it MUST be
      possible for a single Path Computation Request to list multiple

3.1.6. Indication of P2MP Paths

 R6:  The Path Computation Response MUST be able to carry the path of
      a P2MP LSP.
 P2MP paths can be expressed as a compressed series of routes as
 described in [RFC4875].  The Path Computation Response MUST be able
 to carry the P2MP path as either a compressed path (but not
 necessarily using the identical encoding as described in [RFC4875]),
 or as a non-compressed path comprising a series of source-to-leaf
 point-to-point (P2P) paths (known as S2L sub-paths).
 R7:  By default, the path returned by the PCE SHOULD use the
      compressed format.
      The request from the PCC MAY allow the PCC to express a
      preference for receiving a compressed or non-compressed P2MP
      path in the response.

Yasukawa & Farrel Informational [Page 4] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

3.1.7. Multi-Message Requests and Responses

 R8:  A single P2MP LSP may have many destinations, and the computed
      path (tree) may be very extensive.  In these cases, it is
      possible that the entire Path Computation Request or Response
      cannot fit within one PCE message.  Therefore, it MUST be
      possible for a single request or response to be conveyed by a
      sequence of PCE messages.
 Note that there is a requirement in [RFC4657] for reliable and
 in-order message delivery, so it is assumed that components of the
 sequence will be delivered in order and without missing components.

3.1.8. Non-Specification of Per-Destination Constraints and Parameters

 [RFC4875] requires that all branches of a single P2MP LSP have the
 same characteristics, and achieves this by not allowing the signaling
 parameters to be varied for different branches of the same P2MP LSP.
 R9:  It MUST NOT be possible to set different constraints, traffic
      parameters, or quality-of-service requirements for different
      destinations of a P2MP LSP within a single computation request.

3.1.9. Path Modification and Path Diversity

 R10: No changes are made to the requirement to support path
      modification and path diversity as described in [RFC4657].
      Note, however, that a consequence of this requirement is that it
      MUST be possible to supply an existing path in a Path
      Computation Request.  This requirement is unchanged from
      [RFC4657], but it is a new requirement that such paths MUST be
      able to be P2MP paths.  The PCC MUST be able to supply these
      paths as compressed paths or as non-compressed paths (see
      Section 3.1.6) according to the preference of the PCC.

3.1.10. Reoptimization of P2MP TE LSPs

 R11: Reoptimization MUST be supported for P2MP TE LSPs as described
      for P2P LSPs in [RFC4657].  To support this, the existing path
      MUST be supplied as described in Section 3.1.9.
      Because P2MP LSPs are more complex, it is often the case that
      small optimization improvements can be made after changes in
      network resource availability.  However, re-signaling any LSP
      introduces risks to the stability of the service provided to the
      customer and the stability of the network, even when techniques
      like make-before-break [RFC3209] are used.  Therefore, a P2MP
      Path Computation Request SHOULD contain a parameter that allows

Yasukawa & Farrel Informational [Page 5] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

      the PCC to express a cost-benefit reoptimization threshold for
      the whole LSP, as well as per destination.  The setting of this
      parameter is subject to local policy at the PCC and SHOULD be
      subject to policy at the PCE [RFC5394].
      Path reoptimization responses SHOULD indicate which of the
      routes (as supplied according to Section 3.1.6) have been
      modified from the paths supplied in the request.

3.1.11. Addition and Removal of Destinations from Existing Paths

 A variation of path modification described in Section 3.1.9 is that
 destinations may be added to, or removed from, existing P2MP TE LSPs.
 In the case of the addition of one or more destinations, it is
 necessary to compute a path for a new branch of the P2MP LSP.  It may
 be desirable to recompute the whole P2MP tree, to add the new branch
 as a simple spur from the existing tree, or to recompute part of the
 P2MP tree.
 R12: To support this function for leaf additions, it MUST be possible
      to make the following indications in a Path Computation Request:
  1. The path of an existing P2MP LSP (as described in

Section 3.1.9).

  1. Which destinations are new additions to the tree.
  1. Which destinations of the existing tree must not have their

paths modified.

      It MAY also be possible to indicate in a Path Computation
      Request a cost-benefit reoptimization threshold, such that the
      addition of new leaves will not cause reoptimization of the
      existing P2MP tree unless a certain improvement is made over
      simply grafting the new leaves to the existing tree.  (Compare
      with Section 3.1.10.)
      In the case of the deletion of one or more destinations, it is
      not necessary to compute a new path for the P2MP TE LSP, but
      such a computation may yield optimizations over a simple pruning
      of the tree.  The recomputation function in this case is
      essentially the same as that described in Section 3.1.10, but
      note that it MAY be possible to supply the full previous path of
      the entire P2MP TE LSP (that is, before the deletion of the
      destinations) in the Path Computation Request.

Yasukawa & Farrel Informational [Page 6] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

      For both addition and deletion of destinations, the Path
      Computation Response SHOULD indicate which of the routes (as
      supplied according to Section 3.1.6) have been modified from the
      paths supplied in the Path Computation Request, as described in
      Section 3.1.10.
      Note that the selection of all of these options is subject to
      local policy at the PCC and SHOULD be subject to policy at the
      PCE [RFC5394].

3.1.12. Specification of Applicable Branch Nodes

 For administrative or security reasons, or for other policy reasons,
 it may be desirable to limit the set of nodes within the network that
 may be used as branch points for a given LSP, i.e., to provide to the
 path computation a limiting set of nodes that can be used as branches
 for a P2MP path computation, or to provide a list of nodes that must
 not be used as branch points.
 R13: The PCC MUST be able to specify in a Path Computation Request a
      list of nodes that constitutes a limiting superset of the branch
      nodes for a P2MP path computation.
      A PCC MUST be able to specify in a Path Computation Request a
      list of nodes that must not be used as branch nodes for a P2MP
      path computation.

3.1.13. Capabilities Exchange

 PCE capabilities exchange forms part of PCE discovery [RFC4674], but
 may also be included in the PCECP message exchanges [RFC4657].
 R14: The ability to perform P2MP path computation and the objective
      functions supported by a PCE SHOULD be advertised as part of PCE
      discovery.  In the event that the PCE's ability to perform P2MP
      computation is not advertised as part of PCE discovery, the
      PCECP MUST allow a PCC to discover which PCEs with which it
      communicates support P2MP path computation, and which objective
      functions specific to P2MP path computation are supported by
      each PCE.
 The list of objective functions is assumed to be coordinated with
 those that can be requested as described in Section 3.1.2.
 These requirements do not represent a change to [RFC4657], except to
 add more capabilities and objective functions.

Yasukawa & Farrel Informational [Page 7] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

3.1.14. Path-Tree Diversity

 Section 3.1.9 sets out the requirement to be able to request multiple
 diverse paths.  Additionally, with P2MP trees, it may be that only
 parts of the tree can be, or need to be, diverse.
 R15: The PCC SHOULD be able to request a PCE to compute a secondary
      P2MP path tree with partial path diversity for specific leaves
      or a specific S2L sub-path.

4. Manageability Considerations

4.1. Control of Function and Policy

 PCE implementations MAY provide a configuration switch to allow
 support of P2MP MPLS TE computations to be enabled or disabled.  When
 the level of support is changed, this SHOULD be re-advertised as
 described in Section 3.1.13.
 Support for, and advertisement of support for, P2MP MPLS TE path
 computation MAY be subject to policy, and a PCE MAY hide its P2MP
 capabilities from certain PCCs by not advertising them through the
 discovery protocol and not reporting them to the specific PCCs in any
 PCECP capabilities exchange.  Further, a PCE MAY be directed by
 policy to refuse a P2MP path computation for any reason including,
 but not limited to, the identity of the PCC that makes the request.

4.2. Information and Data Models

 PCECP protocol extensions to support P2MP MPLS TE SHOULD be
 accompanied by MIB objects for the control and monitoring of the
 protocol and the PCE that performs the computations.  The MIB objects
 MAY be provided in the same MIB module as used for general PCECP
 control and monitoring or MAY be provided in a new MIB module.
 The MIB objects SHOULD provide the ability to control and monitor all
 aspects of PCECP relevant to P2MP MPLS TE path computation.

4.3. Liveness Detection and Monitoring

 No changes are necessary to the liveness detection and monitoring
 requirements as already embodied in [RFC4657].  However, it should be
 noted that, in general, P2MP computations are likely to take longer
 than P2P computations.  The liveness detection and monitoring
 features of the PCECP SHOULD take this into account.

Yasukawa & Farrel Informational [Page 8] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

4.4. Verifying Correct Operation

 There are no additional requirements beyond those expressed in
 [RFC4657] for verifying the correct operation of the PCECP.  Note
 that verification of the correct operation of the PCE and its
 algorithms is out of scope for the protocol requirements, but a PCC
 MAY send the same request to more than one PCE and compare the

4.5. Requirements on Other Protocols and Functional Components

 A PCE operates on a topology graph that may be built using
 information distributed by TE extensions to the routing protocol
 operating within the network.  In order that the PCE can select a
 suitable path for the signaling protocol to use to install the P2MP
 LSP, the topology graph must include information about the P2MP
 signaling and branching capabilities of each LSR in the network.
 Whatever means is used to collect the information to build the
 topology graph, the graph MUST include the requisite information.  If
 the TE extensions to the routing protocol are used, these SHOULD be
 as described in [RFC5073].

4.6. Impact on Network Operation

 The use of a PCE to compute P2MP paths is not expected to have
 significant impact on network operations.  However, it should be
 noted that the introduction of P2MP support to a PCE that already
 provides P2P path computation might change the loading of the PCE
 significantly, and that might have an impact on the network behavior,
 especially during recovery periods immediately after a network

5. Security Considerations

 P2MP computation requests do not raise any additional security issues
 for the PCECP, as there are no new messages and no new PCC-PCE
 relationships or transactions introduced.
 Note, however, that P2MP computation requests are more CPU-intensive
 and also use more link bandwidth.  Therefore, if the PCECP was
 susceptible to denial of service attacks based on the injection of
 spurious Path Computation Requests, the support of P2MP path
 computation would exacerbate the effect.
 It would be possible to consider applying different authorization
 policies for P2MP Path Computation Requests compared to other

Yasukawa & Farrel Informational [Page 9] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

6. Acknowledgments

 Thanks to Dean Cheng, Young Lee, Quintin Zhao, Daniel King,
 Fabien Verhaeghe, and Francis Dupont for their comments and
 suggestions on 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, March 1997.
 [RFC4657]   Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
             Element (PCE) Communication Protocol Generic
             Requirements", RFC 4657, September 2006.
 [RFC5394]   Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
             "Policy-Enabled Path Computation Framework", RFC 5394,
             December 2008.
 [RFC5671]   Yasukawa, S. and A. Farrel, Ed., "Applicability of the
             Path Computation Element (PCE) to Point-to-Multipoint
             (P2MP) MPLS and GMPLS Traffic Engineering (TE)",
             RFC 5671, October 2009.

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, December 2001.
 [RFC4461]   Yasukawa, S., Ed., "Signaling Requirements for Point-to-
             Multipoint Traffic-Engineered MPLS Label Switched Paths
             (LSPs)", RFC 4461, April 2006.
 [RFC4655]   Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
             Computation Element (PCE)-Based Architecture", RFC 4655,
             August 2006.
 [RFC4674]   Le Roux, J., Ed., "Requirements for Path Computation
             Element (PCE) Discovery", RFC 4674, October 2006.
 [RFC4834]   Morin, T., Ed., "Requirements for Multicast in Layer 3
             Provider-Provisioned Virtual Private Networks (PPVPNs)",
             RFC 4834, April 2007.

Yasukawa & Farrel Informational [Page 10] RFC 5862 PCC-PCE and P2MP MPLS-TE June 2010

 [RFC4875]   Aggarwal, R., Ed., Papadimitriou, D., Ed., and
             S. Yasukawa, Ed., "Extensions to Resource Reservation
             Protocol - Traffic Engineering (RSVP-TE) for Point-to-
             Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
             May 2007.
 [RFC5073]   Vasseur, J., Ed., and J. Le Roux, Ed., "IGP Routing
             Protocol Extensions for Discovery of Traffic Engineering
             Node Capabilities", RFC 5073, December 2007.
 [RFC5541]   Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
             Objective Functions in the Path Computation Element
             Communication Protocol (PCEP)", RFC 5541, June 2009.

Authors' Addresses

 Seisho Yasukawa
 NTT Corporation
 9-11, Midori-Cho 3-Chome
 Musashino-Shi, Tokyo 180-8585
 Adrian Farrel
 Old Dog Consulting

Yasukawa & Farrel Informational [Page 11]

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