





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

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.

   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
   http://www.rfc-editor.org/info/rfc5862.







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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
   (http://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.

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].








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2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
        computation.

        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
        document.






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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
        destinations.

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.




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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



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        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:

        - The path of an existing P2MP LSP (as described in
          Section 3.1.9).

        - Which destinations are new additions to the tree.

        - 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.





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        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.





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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.






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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
   results.

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
   failure.

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
   requests.



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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.





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   [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
   JAPAN
   EMail: yasukawa.seisho@lab.ntt.co.jp


   Adrian Farrel
   Old Dog Consulting
   EMail: adrian@olddog.co.uk
























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