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     Network Working Group                              Eiji Oki (Editor)
     Internet Draft                                                   NTT
     Category: Informational
     Expires: August 2006
                                                            February 2006
         PCC-PCE Communication Requirements for Inter-Layer Traffic
     Status of this Memo
     By submitting this Internet-Draft, each author represents that any
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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 service
     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 requirements.
     Generic requirements for a communication protocol between Path
     Computation Clients (PCCs) and PCEs are presented in "PCE
     Communication Protocol Generic Requirements". This document
     complements the generic requirements and presents a detailed set of
     PCC-PCE communication protocol requirements for inter-layer traffic
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     Conventions used in this document
     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     this document are to be interpreted as described in RFC 2119
     Table of Contents
     1. Contributors..................................................2
     2. Terminology...................................................2
     3. Introduction..................................................3
     4. Motivation for PCE-Based Inter-Layer Path Computation.........4
     5. PCC-PCE Communication Requirements for Inter-Layer Traffic
     5.1.  PCC-PCE Communication......................................4
     5.1.1.  Control of Inter-Layer Path Computation..................5
     5.1.2.  Control of The Type of Path to be Computed...............5
     5.1.3.  Communication of Inter-Layer Constraints.................5
     5.1.4.  Cooperation Between PCEs.................................6
     5.1.5.  Inter-Layer Diverse paths................................6
     5.2.  Supportive Network Models..................................6
     6. Manageability considerations..................................6
     7. Security Considerations.......................................6
     8. Acknowledgments...............................................6
     9. References....................................................7
     9.1.  Normative Reference........................................7
     9.2.  Informative Reference......................................7
     10.  AuthorsEAddresses..........................................7
     11.  Intellectual Property Statement.............................8
  1. Contributors
     The following are the authors that contributed to the present
     Eiji Oki (NTT)
     Jean-Louis Le Roux (France Telecom)
     Kenji Kumaki (KDDI)
     Adrian Farrel (Old Dog Consulting)
  2. 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.
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     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 constraints.
     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.
  3. Introduction
     The Path Computation Element (PCE) defined in [PCE-ARCH] 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 of 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) [MRN-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 [MRN-REQ].
    Inter-layer traffic engineering is included in the scope of the PCE
    architecture [PCE-ARCH], 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 a set of PCC-PCE communication protocol
     (PCECP) requirements for inter-layer traffic engineering. It
     supplements the generic requirements documented in [PCE-COM-REQ].
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  4. Motivation for PCE-Based Inter-Layer Path Computation
     [RFC4206] defines a way to signal a higher-layer LSP, whose
     explicit route includes hops traversed by LSPs in lower layers. 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 lower-
     layer topology, 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
     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' TE topologies.
     Further discussion of the application of PCE to inter-layer path
     computation can be found in [PCE-INTER-LAYER-APP].
  5. PCC-PCE Communication Requirements for Inter-Layer Traffic
     This section sets out additional requirements not covered in [PCE-
     COM-REQ] specific to the problems of multi-layer TE.
  5.1.  PCC-PCE Communication
     The PCC-PCE communication protocol MUST allow requests and replies
     for inter-layer path computation.
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     This requires no additional messages, but implies the following
     additional constraints to be added to the PCC-PCE communication
   5.1.1. 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.
     Therefore, a request from a PCC to a PCE MUST support the inclusion
     of such an indication.
   5.1.2. 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-APP].
     - 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 [RFC4206].
     A request from a PCC to a PCE MUST allow control of the type of the
     path to be computed by selection from the following list:
     - A mono-layer path that includes already established lower-layer
     - a mono-layer path that includes loose hop(s)
     - a multi-layer path that can include the complete path of one or
     more lower-layer LSPs not yet established.
     When multi-layer path computation is requested, a response from a
     PCE to a PCC 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 includes loose hop(s), or 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.
     A response from a PCE to a PCC MUST allow indicating whether the
     computed path triggers nested signaling or not.
   5.1.3. Communication of Inter-Layer Constraints
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     A request from a PCC to a PCE MUST support the inclusion of
     constraints for multiple layers. This includes the switching
     type(s) and encoding type(s) that can, must, or must not be used in
     the computed path.
   5.1.4. 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.
   5.1.5. 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 request, with the desired
     level of diversity at each layer (link, node, SRLG).
  5.2.  Supportive Network Models
     The PCC-PCE communication protocol SHOULD allow several
     architectural alternatives for interworking between MPLS and GMPLS
     networks: overlay, integrated and augmented models [RFC3945].
  6. Manageability considerations
     Manageability of inter-layer traffic engineering with PCE must
     address the following consideration for section 5.1.
     - need for a MIB module for control and monitoring
     - need for built-in diagnostic tools
     - configuration implication for the protocol
  7. 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.
     It is expected that solutions for inter-layer protocol extensions
     will address these issues in detail using security techniques such
     as authentication.
  8. Acknowledgments
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    We would like to thank Kohei Shiomoto, Ichiro Inoue, and Dean Cheng
    for their useful comments.
  9. References
  9.1.  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.
  9.2.  Informative Reference
     [PCE-ARCH] A. Farrel, JP. Vasseur and J. Ash, "Path Computation
     Element (PCE) Architecture", draft-ietf-pce-architecture (work in
     [PCE-COM-REQ] J. Ash, J.L Le Roux et al., "PCE Communication
     Protocol Generic Requirements", draft-ietf-pce-comm-protocol-gen-
     reqs (work in progress).
     [PCE-DISC-REQ] JL Le Roux et al., "Requirements for Path
     Computation Element (PCE) Discovery", draft-ietf-pce-discovery-reqs
     (work in progress).
     [MRN-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-APP] E. Oki et al., "PCE Applicability for Inter-
     Layer MPLS and GMPLS Traffic Engineering", draft-oki-pce-inter-
     layer-app (work in progress)
  10.     Authors' Addresses
     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@francetelecom.com
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     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
  11.     Intellectual Property Statement
     The IETF takes no position regarding the validity or scope of any
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     Copies of IPR disclosures made to the IETF Secretariat and any
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     specification can be obtained from the IETF on-line IPR repository
     at http://www.ietf.org/ipr.
     The IETF invites any interested party to bring to its attention any
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     Disclaimer of Validity
     This document and the information contained herein are provided on
     Copyright Statement
     Copyright (C) The Internet Society (2006).  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
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