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   Network Working Group                              Eiji Oki (Editor)
   Internet Draft                                                   NTT
   Category: Informational
   Expires: September 2007
                                                             March 2007

        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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC 2119

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   Table of Contents

   1. Contributors...................................................2
   2. Terminology....................................................2
   3. Introduction...................................................3
   4. Motivation for PCE-Based Inter-Layer Path Computation..........3
   5. PCC-PCE Communication Requirements for Inter-Layer Traffic
   5.1.  PCC-PCE Communication.......................................4
   5.1.1.  Control of Inter-Layer Path Computation...................4
   5.1.2.  Control of The Type of Path to be Computed................4
   5.1.3.  Communication of Inter-Layer Constraints..................5
   5.1.4.  Adaptation Capability.....................................5
   5.1.5.  Cooperation Between PCEs..................................5
   5.1.6.  Inter-Layer Diverse paths.................................6
   5.2.  Supportive Network Models...................................6
   6. Manageability considerations...................................6
   6.1. Control of Function and Policy...............................6
   6.2. Information and Data Models..................................6
   6.3. Liveness Detection and Monitoring............................6
   6.4. Verifying Correct Operation..................................7
   6.5. Requirements on Other Protocols and Functional Components....7
   6.6. Impact on Network Operation..................................7
   7. Security Considerations........................................7
   8. Acknowledgments................................................8
   9. References.....................................................8
   9.1.  Normative Reference.........................................8
   9.2.  Informative Reference.......................................8
   10.  Authors' Addresses...........................................8
   11.  Intellectual Property Statement..............................9

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.

   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.

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   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) [MRN-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].

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

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   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.

   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:

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   - 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.

 5.1.3. Communication of Inter-Layer Constraints

   A request from a PCC to a PCE MUST support the inclusion of
   constraints for 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.

 5.1.4. Adaptation Capability

   It MUST be possible for the path computation request to indicate the
   desired adaptation function at the egress of the LSP that is being
   computed. This will be particularly important where the egress LSR
   participates in more than one layer network but may not be capable
   of all associated adaptations.

 5.1.5. 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.

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 5.1.6. 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

6.1.  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-advertised.

   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 computation request for any
  reason including, but not limited to, the identity of the PCC that
  makes the request.

6.2. 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 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.

6.3. 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.

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   In particular, where a request is forwarded between PCEs multiple
   times neither the PCC not the first PCE can monitor the liveness of
   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.

6.4. 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

6.5. 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 [MRN-REQ].

6.6. 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 supported.

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.

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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
   Email: jeanlouis.leroux@orange-ftgroup.com

   Kenji Kumaki
   KDDI Corporation
   Garden Air Tower
   Iidabashi, Chiyoda-ku,

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   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
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   Information on the procedures with respect to rights in RFC
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   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 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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