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     Network Working Group                              Eiji Oki (Editor)
     Internet Draft                                                   NTT
     Category: Informational
     Expires: May 2008
                                                            November 2007
      PCC-PCE Communication and PCE Discovery Requirements for Inter-
                         Layer Traffic Engineering
     Status of this Memo
     By submitting this Internet-Draft, each author represents that any
     applicable patent or other IPR claims of which he or she is aware
     have been or will be disclosed, and any of which he or she becomes
     aware will be disclosed, in accordance with Section 6 of BCP 79.
     Internet-Drafts are working documents of the Internet Engineering
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     The Path Computation Element (PCE) provides functions of path
     computation in support of traffic engineering in Multi-Protocol
     Label Switching (MPLS) and Generalized MPLS (GMPLS) networks.
     MPLS and GMPLS networks may be constructed from layered
     client/server networks. It is advantageous for overall network
     efficiency to provide end-to-end traffic engineering across
     multiple network layers. PCE is a candidate solution for such
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     Generic requirements for a communication protocol between Path
     Computation Clients (PCCs) and PCEs are presented in “PCE
     Communication Protocol Generic Requirements". Generic requirements
     for PCE discovery protocol are presented in “Requirements for Path
     Computation Element (PCE) DiscoveryE
     This document complements the generic requirements and presents
     detailed sets of PCC-PCE communication protocol requirements and
     PCE discovery protocol requirements for inter-layer traffic
     Table of Contents
     1. Introduction.................................................3
     1.1.  Terminology...............................................4
     2. Motivation for PCE-Based Inter-Layer Path Computation........4
     3. PCC-PCE Communication and Discovery Requirements for Inter-
     Layer Traffic Engineering........................................5
     3.1.  PCC-PCE Communication.....................................5
     3.1.1.  Control of Inter-Layer Path Computation.................5
     3.1.2.  Control of The Type of Path to be Computed..............6
     3.1.3.  Communication of Inter-Layer Constraints................7
     3.1.4.  Adaptation Capability...................................7
     3.1.5.  Cooperation Between PCEs................................7
     3.1.6.  Inter-Layer Diverse paths...............................7
     3.2.  Capabilities Advertisements for PCE Discovery.............8
     3.3.  Supported Network Models..................................8
     4. Manageability considerations.................................8
     4.1.  Control of Function and Policy............................8
     4.2.  Information and Data Models...............................9
     4.3.  Liveness Detection and Monitoring.........................9
     4.4.  Verifying Correct Operation...............................9
     4.5.  Requirements on Other Protocols and Functional Components 10
     4.6.  Impact on Network Operation..............................10
     5. Security Considerations.....................................10
     6. Acknowledgments.............................................11
     7. References..................................................11
     7.1.  Normative Reference......................................11
     7.2.  Informative Reference....................................11
     8. AuthorsEAddresses..........................................12
     9. Intellectual Property Statement.............................12
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  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.
     A network may comprise multiple layers. These layers may represent
     separations of technologies (e.g., packet switch capable (PSC),
     time division multiplex (TDM), lambda switch capable (LSC))
     [RFC3945], separation of data plane switching granularity levels
     (e.g., PSC-1 and  PSC-2, or VC4 and VC12) [MLN-REQ], or a
     distinction between client and server networking roles (e.g.,
     commercial or administrative separation of client and server
     networks). In this multi-layer network, LSP in lower layers are
     used to carry upper-layer LSPs. The network topology formed by
     lower-layer LSPs and advertised to the higher layer is called a
     Virtual Network Topology (VNT) [MLN-REQ].
     It is important to optimize network resource utilization globally,
     i.e. taking into account all layers, rather than optimizing
     resource utilization at each layer independently. This allows
     achieving better network efficiency. This is what we call Inter-
     layer traffic engineering. This includes mechanisms allowing to
     compute end-to-end paths across layers, as known as inter-layer
     path computation, and mechanisms for control and management of the
     VNT by setting up and releasing LSPs in the lower layers [MLN-REQ].
    Inter-layer traffic engineering is included in the scope of the
    PCE architecture [RFC4655], and PCE can provide a suitable
    mechanism for resolving inter-layer path computation issues. The
    applicability of the PCE-based path computation architecture to
    inter-layer traffic engineering is described in [PCE-INTER-LAYER-
     This document presents sets of PCC-PCE communication protocol
     (PCECP) and PCE Discovery protocol requirements for inter-layer
     traffic engineering. It supplements the generic requirements
     documented in [RFC4657] and [RFC4674].
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  1.1.  Terminology
     LSP: Label Switched Path.
     LSR: Label Switching Router.
     PCC: Path Computation Client: any client application requesting a
     path computation to be performed by a Path Computation Element.
     PCE: Path Computation Element: an entity (component, application
     or network node) that is capable of computing a network path or
     route based on a network graph and applying computational
     PCECP: PCE Communication Protocol, a protocol for communication
     between PCCs and PCEs.
     TED: Traffic Engineering Database which contains the topology and
     resource information of the domain. The TED may be fed by IGP
     extensions or potentially by other means.
     TE LSP: Traffic Engineering Label Switched Path.
     TE LSP head-end: head/source/ingress of the TE LSP.
     TE LSP tail-end: tail/destination/egress of the TE LSP.
     Although this requirements document is an informational document
     not a protocol specification, the key words "MUST", "MUST NOT",
     "RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to be
     interpreted as described in RFC 2119 [RFC2119] for clarity of
     requirement specification.
  2. Motivation for PCE-Based Inter-Layer Path Computation
     [RFC4206] defines a way to signal a Multiprotocol Label Switching
     (MPLS) or Generalized MPLS (GMPLS) LSP with an explicit route in a
     higher layer of a network that includes hops traversed by LSPs in
     lower layers of the network. The computation of end-to-end paths
     across layers is called Inter-Layer Path Computation.
     An LSR in the higher-layer might not have information on the
     topology of lower-layers, particularly in an overlay or augmented
     model, and hence might not be able to compute an end-to-end path
     across layers.
     PCE-based inter-layer path computation, consists of relying on one
     or more PCEs to compute an end-to-end path across layers. This
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     could rely on a single PCE path computation where the PCE has
     topology information about multiple layers and can directly
     compute an end-to-end path across layers considering the topology
     of all of the layers. Alternatively, the inter-layer path
     computation could be performed as a multiple PCE computation where
     each member of a set of PCEs has information about the topology of
     one or more layers, but not all layers, and collaborate to compute
     an end-to-end path.
     Consider a two-layer network where the higher-layer network is a
     packet-based IP/MPLS or GMPLS network and the lower-layer network
     is a GMPLS optical network. An ingress LSR in the higher-layer
     network tries to set up an LSP to an egress LSR also in the
     higher-layer network across the lower-layer network, and needs a
     path in the higher-layer network. However, suppose that there is
     no TE link between border LSRs, which are located on the boundary
     between the higher-layer and lower-layer networks, and that the
     ingress LSR does not have topology visibility in the lower layer.
     If a single-layer path computation is applied for the higher-layer,
     the path computation fails. On the other hand, inter-layer path
     computation is able to provide a route in the higher-layer and a
     suggestion that a lower-layer LSP be setup between border LSRs,
     considering both layersETE topologies.
     Further discussion of the application of PCE to inter-layer path
     computation can be found in [PCE-INTER-LAYER-FRWK].
  3. PCC-PCE Communication and Discovery Requirements for Inter-Layer
    Traffic Engineering
     This section sets out additional requirements specific to the
     problems of multi-layer TE that are not covered in [RFC4657] or
  3.1.  PCC-PCE Communication
     The PCC-PCE communication protocol MUST allow requests and replies
     for inter-layer path computation.
     This requires no additional messages, but implies the following
     additional constraints to be added to the PCC-PCE communication
           Control of Inter-Layer Path Computation
     A request from a PCC to a PCE SHOULD indicate whether inter-layer
     path computation is allowed. In the absence of such an indication,
     the default is that inter-layer path computation is not allowed.
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     Therefore, a request from a PCC to a PCE MUST support the
     inclusion of such an indication.
           Control of The Type of Path to be Computed
     The PCE computes and returns a path to the PCC that the PCC can
     use to build a higher-layer or lower-layer LSP once converted to
     an Explicit Route Object (ERO) for use in RSVP-TE signaling. There
     are two options [PCE-INTER-LAYER-FRWK].
     - Option 1: Mono-layer path. The PCE computes a "mono layer" path,
     i.e. a path that includes only TE-links from the same layer.
     - Option 2: Multi-layer path. The PCE computes a "multi-layer"
     path, i.e. a path that includes TE links from distinct layers
     It may be necessary or desirable for a PCC to control the type of
     path that is produced by a PCE. For example, a PCC may know that
     it is not possible for technological or policy reasons to signal a
     multi-layer path and that a mono-layer path is required, or the
     PCC may know that it does not wish the layer border node to have
     control of path computation. In order to make this level of
     control possible, the PCECP MUST allow the PCC to select the path
     types that may be returned by choosing one or more from the
     following list:
     - A mono-layer path that is specified by strict hop(s). The path
     may include virtual TE link(s).
     - A mono-layer path that includes loose hop(s).
     - A multi-layer path that can include the path (as strict or loose
     hops) of one or more lower-layer LSPs not yet established.
     The path computation response from a PCE to a PCC MUST report the
     type of path computed, and where a multi-layer path is returned,
     PCECP MUST support the inclusion, as part of end-to-end path, of
     the path of the lower-layer LSPs to be established.
     If a response message from a PCE to PCC carries a mono-layer path
     that is specified by strict hops but includes virtual TE link(s),
     or includes loose hop(s), or carries a multi-layer path that can
     include the complete path of one or more lower-layer LSPs not yet
     established, the signaling of the higher-layer LSP may trigger the
     establishment of the lower-layer LSPs (nested signaling). The
     nested signaling may increase the higher-layer connection setup
     latency. An ingress LSR for the higher-layer LSP, or a PCC, needs
     to know whether nested signaling is required or not.
     A request from a PCC to a PCE MUST allow indicating whether nested
     signaling is acceptable or not.
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     A response from a PCE to a PCC MUST allow indicating whether the
     computed path triggers nested signaling or not.
          Communication of Inter-Layer Constraints
     A request from a PCC to a PCE MUST support the inclusion of
     constraints for a multi-layer path. This includes control over
     which network layers may, must, or must not be included in the
     computed path. Such control may be expressed in terms of the
     switching types of the layer networks.
     The path computation request MUST also allow for different
     objective functions to be applied within different network layers.
     For example, the path in a packet-network may need to be optimized
     for least delay using the IGP metric as a measure of delay, while
     the path in an under-lying TDM network might be optimized for
     fewest hops.
           Adaptation Capability
     It MUST be possible for the path computation request to indicate
     the desired adaptation function at the end points of the lower-
     layer LSP that is being computed. This will be particularly
     important where the ingress and egress LSR participate in more
     than one layer network but may not be capable of all associated
           Cooperation Between PCEs
     When each layer is controlled by a PCE, which only has access to
     the topology information of its layer, the PCEs of each layer need
     to cooperate to perform inter-layer path computation. In this case,
     communication between PCEs is required for inter-layer path
     computation. A PCE that behaves as a client is defined as a PCC
     The PCC-PCE communication protocol MUST allow requests and replies
     for multiple PCE inter-layer path computation.
           Inter-Layer Diverse paths
     The PCE communication protocol MUST allow for the computation of
     diverse inter-Layer paths. A request from a PCC to a PCE MUST
     support the inclusion of multiple path requests, with the desired
     level of diversity at each layer (link, node, SRLG).
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  3.2.  Capabilities Advertisements for PCE Discovery
     In the case where there are several PCEs with distinct
     capabilities available, a PCC has to select one or more
     appropriate PCEs.
     For that purpose, the PCE discovery mechanism MAY support the
     disclosure of some detailed PCE capabilities.
     A PCE MAY (to be consistent with the above text and RFC4674) be
     able to advise the following inter-layer-path-computation-related
     PCE capabilities:
          - Support for inter-layer path computation
          - Support for mono-layer/multi-layer paths
          - Support for Adaptation Capability
          - Support for Inter-PCE communication
          - Support for inter-layer diverse path computation
  3.3.  Supported Network Models
     The PCC-PCE communication protocol SHOULD allow several
     architectural alternatives for interworking between MPLS and GMPLS
     networks: overlay, integrated and augmented models
  4. Manageability considerations
  4.1.  Control of Function and Policy
     An individual PCE MAY elect to support inter-layer computations
     and advertise its capabilities as described in the previous
     sections. PCE implementations MAY provide a configuration switch
     to allow support of inter-layer path computations to be enabled or
     disabled. When the level of support is changed, this SHOULD be re-
     However, a PCE MAY also elect to support inter-layer computations,
     but not to advertise the fact, so that only those PCCs configured
     to know of the PCE and its capabilities can use it.
    Support for, and advertisement of support for, inter-layer path
    computation MAY be subject to policy and a PCE MAY hide its inter-
    layer 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 an inter-layer path
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    computation request 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 inter-layer computations MUST
     be accompanied by MIB objects for the control and monitoring of
     the protocol and of 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  [PCEP-MIB] 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 inter-layer path computation.
  4.3. 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 inter-layer path computations might require extended
     cooperation between PCEs (as is also the case for inter-AS and
     inter-area computations) and so the liveness detection and
     monitoring SHOULD be applied to each PCECP communication and
     aggregated to report the behavior of an individual PCECP request
     to the originating PCC.
     In particular, where a request is forwarded between multiple PCEs
     neither the PCC not the first PCE can monitor the liveness of all
     inter-PCE-PCE connections or of the PCEs themselves. In this case,
     suitable performance of the original PCECP request relies on each
     PCE operating correct monitoring procedures and correlating any
     failures back to the PCECP requests that are outstanding. These
     requirements are no different from those for any cooperative PCE
     usage, and are expected to be already covered by general, and by
     inter-AS and inter-area implementations.
  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
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  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
     inter-layer LSP, the topology graph must include information about
     the inter-layer signaling and forwarding (i.e. adaptation)
     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 satisfy
     the requirements as described in [MLN-REQ].
  4.6. Impact on Network Operation
     The use of a PCE to compute inter-layer paths is not expected to
     have significant impact on network operations. But it should be
     noted that the introduction of inter-layer support to a PCE that
     already provides mono-layer path computation might change the
     loading of the PCE and that might have an impact on the network
     behavior especially during recovery periods immediately after a
     network failure.
     On the other hand, it is envisioned that the use of inter-layer
     path computation will have significant benefits to the operation
     of a multi-layer network including improving the network resource
     usage and enabling a greater number of higher-layer LSPs to be
  5. Security Considerations
     Inter-layer traffic engineering with PCE may raise new security
     issues when PCE-PCE communication is done between different layer
     networks for inter-layer path computation. Security issues may
     also exist when a single PCE is granted full visibility of TE
     information that applies to multiple layers.
     The formal introduction of a VNT Manager component as described in
     [PCE-INTER-LAYER-FRWK] provides the basis for the application of
     inter-layer security and policy.
     It is expected that solutions for inter-layer protocol extensions
     will address these issues in detail.
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  6. Acknowledgments
    We would like to thank Kohei Shiomoto, Ichiro Inoue, and Dean
    Cheng for their useful comments.
  7. References
  7.1.  Normative Reference
     [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
     requirements levels", RFC 2119, March 1997.
     [RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching
     Architecture", RFC 3945, October 2004.
     [RFC4206] Kompella, K., and Rekhter, Y., "Label Switched Paths
     (LSP) Hierarchy with Generalized Multi-Protocol Label Switching
     (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.
  7.2.  Informative Reference
     [RFC4655] A. Farrel, JP. Vasseur and J. Ash, "A Path Computation
     Element (PCE)-Based Architecture", RFC 4655, September 2006.
     [RFC4657] J. Ash, J.L Le Roux et al., " Path Computation Element
     (PCE) Communication Protocol Generic Requirements", RFC 4657,
     September 2006.
     [RFC4674] JL Le Roux et al., "Requirements for Path Computation
     Element (PCE) Discovery", RFC 4674, September 2006.
     [MLN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi-
     region and multi-layer networks (MRN/MLN)", draft-ietf-ccamp-
     gmpls-mln-reqs (work in progress).
     [PCE-INTER-LAYER-FRWK] E. Oki et al., "Framework for PCE-Based
     Inter-Layer MPLS and GMPLS Traffic Engineering", draft-ietf-pce-
     inter-layer-frwk (work in progress).
     [PCEP-MIB] A. Koushik, and E. Stephan, "PCE communication
     protocol(PCEP) Management Information Base", draft-kkoushik-pce-
     pcep-mib (work in progress).
     [INTWORK-FRWK] K. Shiomoto, “Framework for MPLS-TE to GMPLS
     migration,Edraft-ietf-ccamp-mpls-gmpls-interwork-fmwk (work in
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     [INTWORK-REQ] K. Kumaki et al., “Interworking Requirements to
     Support operation of MPLS-TE over GMPLS Networks,Edraft-ietf-
     ccamp-mpls-gmpls-interwork-reqts (work in progress).
  8. AuthorsEAddresses
     Eiji Oki
     3-9-11 Midori-cho,
     Musashino-shi, Tokyo 180-8585, Japan
     Email: oki.eiji@lab.ntt.co.jp
     Jean-Louis Le Roux
     France Telecom R&D,
     Av Pierre Marzin,
     22300 Lannion, France
     Email: jeanlouis.leroux@orange-ftgroup.com
     Kenji Kumaki
     KDDI Corporation
     Garden Air Tower
     Iidabashi, Chiyoda-ku,
     Tokyo 102-8460, JAPAN
     Phone: +81-3-6678-3103
     Email: ke-kumaki@kddi.com
     Adrian Farrel
     Old Dog Consulting
     Email: adrian@olddog.co.uk
  9. Intellectual Property Statement
     The IETF takes no position regarding the validity or scope of any
     Intellectual Property Rights or other rights that might be claimed
     to pertain to the implementation or use of the technology
     described in this document or the extent to which any license
     under such rights might or might not be available; nor does it
     represent that it has made any independent effort to identify any
     such rights.  Information on the procedures with respect to rights
     in RFC documents can be found in BCP 78 and BCP 79.
     Copies of IPR disclosures made to the IETF Secretariat and any
     assurances of licenses to be made available, or the result of an
     attempt made to obtain a general license or permission for the use
     of such proprietary rights by implementers or users of this
     specification can be obtained from the IETF on-line IPR repository
     at http://www.ietf.org/ipr.
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     The IETF invites any interested party to bring to its attention
     any copyrights, patents or patent applications, or other
     proprietary rights that may cover technology that may be required
     to implement this standard. Please address the information to the
     IETF at ietf-ipr@ietf.org.
     Disclaimer of Validity
     This document and the information contained herein are provided on
     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.
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