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   Network Working Group                              Eiji Oki (Editor)
   Internet Draft                                                   NTT
   Category: Informational
   Expires: April 2007
                                                           October 2006

        PCC-PCE Communication Requirements for Inter-Layer Traffic


   Status of this Memo

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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

   Conventions used in this document

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   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...................6
   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.................................................7
   9. References......................................................7
   9.1.  Normative Reference..........................................7
   9.2.  Informative Reference........................................7
   10.  Authors' Addresses............................................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 [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 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 [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-FRWK].

   This document presents a set of PCC-PCE communication protocol
   (PCECP) requirements for inter-layer traffic engineering. It
   supplements the generic requirements documented in [RFC4657].

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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-FRWK].

5. PCC-PCE Communication Requirements for Inter-Layer Traffic

   This section sets out additional requirements not covered in
   [RFC4657] 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-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 [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 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 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 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.

   A response from a PCE to a PCC MUST allow indicating whether the
   computed path triggers nested signaling or not.

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 5.1.3. Communication of Inter-Layer Constraints

   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

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8. Acknowledgments

  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

   [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.

   [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-FRWK] E. Oki et al., "Framework for PCE-Based Inter-
   Layer MPLS and GMPLS Traffic Engineering", draft-oki-pce-inter-layer-
   frwk (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

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   Email: jeanlouis.leroux@francetelecom.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

11.     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
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   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at ietf-

   Disclaimer of Validity

   This document and the information contained herein are provided on an

   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 rights.

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