Network Working Group                                    Seisho Yasukawa
Internet Draft                                                       NTT
Category: Informational                                    Adrian Farrel
Expires: August 2007                                  Old Dog Consulting
                                                           February 2007

       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). Since P2MP TE LSP routes are
   sometimes complex to compute, and given the use of PCE in MPLS
   networks it is likely that PCE will be used in P2MP MPLS-TE networks.

   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 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 [P2MP-RSVP]. P2MP MPLS TE
   networks are considered in support of various features including
   layer 3 multicast VPNs.

   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, requires the addition of the
   parameters described in the following sections to the 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 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.

2.1.2. Non-Support of P2MP Path Computation

   Not all PCEs are 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.

2.1.3. 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.
   Therefore, at least one parameter required for a P2MP path
   computation request 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.

2.1.4. 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.

2.1.5. Indication of P2MP Paths

   The Path Computation Response MUST be able to carry the path of a
   P2MP LSP. This SHOULD be expressed as a compacted series of routes as
   described in [P2MP-RSVP] although not necessarily using an identical
   encoding. This path MAY be expressed as a non-compacted series of
   source-to-destination routes.

2.1.6. 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 messages.

   Note that there is a requirement in [RFC4657] for reliable and

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   in-order message delivery, so it is assumed that components of the
   sequence will be delivered in order and without missing components.

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

   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.

2.1.8. 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.

2.1.9. 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 2.1.8.

   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 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 2.1.5) have been modified from the
   paths supplied on the request.

2.1.10. Addition and Removal of Destinations from Existing Paths

   A variation of path modification described in Section 2.1.8 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.

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   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 2.1.8).

   - 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 2.1.9.

   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 2.1.9, 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 2.1.5) have been modified from the paths supplied on the
   request as described in Section 2.1.9.

   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

2.1.11. Capabilities Exchange

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

   The ability to perform P2MP path computation 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

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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 2.1.11.

   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 MUST 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 MUST 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 MUST include the requisite information. If the TE
   extensions to the routing protocol are used, these SHOULD be as
   described in [TE-NODE-CAP].

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.

   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 for his comments 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.

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

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   [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.

   [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

   [P2MP-RSVP]    Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
                  "Extensions to RSVP-TE for Point to Multipoint TE
                  LSPs", draft-ietf-mpls-rsvp-te-p2mp, work in progress.

   [TE-NODE-CAP]  Vasseur, J.P, and Le Roux, J.L., Editors, "IGP Routing
                  Protocol Extensions for Discovery of Traffic
                  Engineering Node Capabilities", draft-ietf-ccamp-te-
                  node-cap, work in progress.

8. Authors' Addresses

   Seisho Yasukawa
   NTT Corporation
   (R&D Strategy Department)
   3-1, Otemachi 2-Chome Chiyodaku, Tokyo 100-8116 Japan
   Phone: +81 3 5205 5341

   Adrian Farrel
   Old Dog Consulting

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10. Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

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