Network Working Group                                        S. Yasukawa
Internet Draft                                                       NTT
Category: Informational                                        A. Farrel
Created: May 12, 2008                                 Old Dog Consulting
Expires: November 12, 2008

       PCC-PCE Communication Requirements for Point to Multipoint
       Multiprotocol Label Switching Traffic Engineering (MPLS-TE)


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   The Path Computation Element (PCE) provides path computation
   functions in support of traffic engineering in Multi-Protocol 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 "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.

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Conventions used in this document

   Although this document is not a protocol specification, the key words
   "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" are to be
   interpreted as described in RFC 2119 [RFC2119] for clarity of
   description of requirements.

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 VPNs [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 Swiching Routers (LSRs).

   The applicability of the PCE-based path computation architecture to
   P2MP MPLS TE is described in a companion document [PCE-P2MP-APP]. 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].

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

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

2.1. 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.

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2.1.1. Indication of P2MP Path Computation Request

   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

2.1.2. Indication of P2MP Objective Functions

   [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 [PCE-OF] 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.

2.1.3. Non-Support of P2MP Path Computation

   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

2.1.4. 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 2.1.1. Such
   legacy implementations will not be able to make use of the new
   reason code described in Section 2.1.3.

   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 is the
   parameter described in Section 2.1.1.

2.1.5. Specification of Destinations

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

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2.1.6. Indication of P2MP Paths

   The Path Computation Response MUST be able to carry the path of a
   P2MP LSP.

   P2MP paths can be expressed as a compacted series of routes as
   described in [RFC4875]. The Path Computation Response MUST be able to
   carry the P2MP path as either a compacted path (but not necessarily
   using the identical encoding as described in [RFC4875]), or as a
   non-compacted path comprising a series of source-to-leaf point-to-
   point (P2P) paths (known as S2L sub-paths).

   By default, the path returned by the PCE SHOULD use the compacted

   The request from the PCC MAY allow the PCC to express a preference
   for receiving a compacted or non-compacted P2MP path in the response.

2.1.7. Multi-Message Requests and Responses

   A single P2MP LSP may have very 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.

2.1.8. 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.

   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.

2.1.9. Path Modification and Path Diversity

   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 on 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 compacted paths or as a non-compacted paths (see Section
   2.1.6) according to the preference of the PCC.

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2.1.10. Reoptimization of P2MP TE LSPs

   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 2.1.9.

   Because P2MP LSPs are more complex it is often the case that small
   optimization improvements can be made after changes in network
   resource availability. But 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 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 [PCE-POLICY].

   Path reoptimization responses SHOULD indicate which of the routes (as
   supplied according to Section 2.1.6) have been modified from the
   paths supplied on the request.

2.1.11. Addition and Removal of Destinations from Existing Paths

   A variation of path modification described in Section 2.1.9 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.

   To support this function for leaf additions it MUST be possible to
   make the following indications on a path computation request:

   - The path of an existing P2MP LSP (as described in Section 2.1.9).

   - Which destinations are new additions to the tree.

   - Which destinations of the existing tree must not have their paths

   It MAY also be possible to indicate on a path computation request a
   cost-benefit reoptimization threshold such that the tree and/or a new
   path to any individual destination is not supplied unless a certain
   improvement is made. Compare with Section 2.1.10.

   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

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   tree. The recomputation function in this case is essentially the same
   as that described in Section 2.1.10, 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) on the Path Computation

   For both addition and deletion of destinations, the Path Computation
   Response SHOULD indicate which of the routes (as supplied according
   to Section 2.1.6) have been modified from the paths supplied on the
   request as described in Section 2.1.10.

   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

2.1.12. 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. That is, 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.

   The PCC MUST be able to specify on a Path Computation Request a list
   of nodes that constitues a limiting superset of the branch nodes for
   a P2MP path computation.

   A PCC MUST be able to specify on a Path Computation Request a list of
   nodes that must not be used as branch nodes for a P2MP path

2.1.13. Capabilities Exchange

   PCE capabilities exchange forms part of PCE discovery [RFC4674], but
   may also be included in the PCECP message exchanges [RFC4657].

   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 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 2.1.2.

   These requirements do not represent a change to [RFC4657] except to
   add more capabilities and objective functions.

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3. Manageability Considerations

3.1. 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 2.1.13.

   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.

3.2. 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.

3.3. Liveness Detection and Monitoring

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

3.4. 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.

3.5. 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

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   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].

3.6. Impact on Network Operation

   The use of a PCE to compute P2MP paths is not expected to have
   significant impact on network operations. But 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.

4. 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

5. IANA Considerations

   This document makes no requests for IANA action.

6. Acknowledgments

   Thanks to Dean Cheng, Young Lee, Quintin Zhao, and Daniel King for
   their comments and suggestions on this document.

7. References

7.1. Normative Reference

   [RFC2119]      Bradner, S., "Key words for use in RFCs to indicate
                  requirements levels", RFC 2119, March 1997.

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   [RFC4657]      Ash, J., and Le Roux, J.L., "Path Computation Element
                  (PCE) Communication Protocol Generic Requirements",
                  RFC 4657, September 2006.

   [PCE-P2MP-APP] S. Yasukawa et al., "Applicability of the Path
                  Computation Element to Point-to-Multipoint Traffic
                  Engineering", draft-yasukawa-pce-p2mp-app, work in

   [PCE-POLICY]   Bryskin, I., Papadimitriou, D., and Berger, L.,
                  "Policy-Enabled Path Computation Framework",
                  draft-ietf-pce-policy-enabled-path-comp, work in

7.2. Informative Reference

   [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]      S. Yasukawa, Editor "Signaling Requirements for
                  Point-to-Multipoint Traffic Engineered MPLS LSPs",
                  RFC4461, April 2006.

   [RFC4655]      Farrel, A., Vasseur, J.P., and Ash, G., "A Path
                  Computation Element (PCE)-Based Architecture",
                  RFC 4655, August 2006.

   [RFC4674]      J.L. Le Roux, Editor, "Requirements for Path
                  Computation Element (PCE) Discovery", RFC 4674,
                  October 2006.

   [RFC4834]      Morin, T., "Requirements for Multicast in Layer 3
                  Provider-Provisioned Virtual Private Networks
                  (PPVPNs)", RFC 4834, April 2007.

   [RFC4875]      Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
                  "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.P, and Le Roux, J.L., Editors, "IGP Routing
                  Protocol Extensions for Discovery of Traffic
                  Engineering Node Capabilities", RFC 5073, December

   [PCE-OF]       Le Roux, J.L., Vasseur, J.P., and Lee, Y., "Encoding
                  of Objective Functions in Path Computation Element
                  communication Protocol (PCEP)", draft-ietf-pce-of,
                  work in progress.

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8. Authors' Addresses

   Seisho Yasukawa
   NTT Corporation (R&D Strategy Department)
   3-1, Otemachi 2-Chome Chiyodaku, Tokyo 100-8116 Japan

   Adrian Farrel
   Old Dog Consulting

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   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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   This document and the information contained herein are provided on an

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