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   Network Working Group                              Eiji Oki (Editor)
   Internet Draft                                                   NTT
   Category: Informational
   Expires: May 2006
                                                          November 2005

        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].

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.

   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 technology (e.g., PSC, TDM VC4, TDM VC12, LSC) or a
   distinction between client and server networking roles. 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

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   compute end-to-end paths across layers, as known as inter-layer path
   computation, and mechanisms for control and management of VNT by
   setting up and releasing LSPs in 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 use of PCE for the control of the VNT is for further study.

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

4. Inter-Layer Traffic Engineering

   This section describes key topics of inter-layer traffic engineering
   in MPLS and GMPLS networks.

4.1.  Inter-Layer Path Computation

   [LSP-HIER] defines a way to signal an upper-layer LSP, whose explicit
   route includes lower-layer(s) LSP paths. The computation of end-to-
   end paths across layers is called Inter-Layer Path Computation.

   An LSR in the higher-layer may not have information on the lower-
   layer topology, particularly in an overlay or augmented model, and
   hence may 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 single PCE path computation where a single PCE have topology
   information on multiple layers, and can compute an end-to-end path
   considering all layers' topology, or on multiple PCEs computation
   where a set of PCEs have information of a single layer topology and
   collaborate together to build an end-to-end path.

   A two-layer network is considered. The higher-layer network can be
   considered as a packet-based IP/MPLS network or GMPLS network. The
   lower-layer network is considered as a GMPLS optical network. The
   bandwidth granularity of the lower layer is coarse. For example, the
   bandwidth is equal to 2.5 Gbit/s or 10 Gbit/s. On the other hand, the
   granularity of the higher layer is flexible and well engineered.

   Consider the case where higher-layer LSPs are to be established end-
   to-end across a lower-layer network. For example, packet LSPs carried
   across an optical core. Connectivity across the lower-layer is
   achieved by tunneling the higher-layer LSPs within lower-layer LSPs.
   However, when the bandwidths of the higher-layer LSPs are much
   smaller than the capacity of the lower-layer LSPs, the resources in
   the lower layer are not fully utilized unless a mechanism is provided
   to aggregate multiple higher-layer LSPs into a single lower-layer LSP.

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   There are two main options for routing a higher-layer LSP over a
   lower-layer network. A single hop route uses a single edge-to-edge
   lower-layer LSP that is managed as a single hop within the higher
   layer. A multiple hop route uses a series of lower-layer LSPs each of
   which appears to the higher-layer LSP as a single hop.

   Lower-layer LSPs form a Virtual Network Topology, which can be used
   for routing higher-layer LSPs or to carry IP traffic. Inter-layer
   path computation for end-to-end LSPs in the higher-layer network that
   span the lower-layer network may utilize the VNT, and PCE is a
   candidate for computing the paths of such higher-layer LSPs within
   the higher-layer network. PCEs could:
   - perform a single computation on behalf of the ingress LSR using
   information gathered from more than one layer. This mode is referred
   as to Single PCE Computation in [PCE-ARCH].
   - perform a set of cooperated path computations on behalf of the
   ingress LSR through cooperation between PCEs responsible for each
   layer. This mode is referred as to Multiple PCE Computation with
   inter-PCE communication in [PCE-ARCH].
   - perform separate path computations on behalf of the TE LSP head-end
   and each transit LSR that is the entry point to a new layer. This
   mode is referred as to Multiple PCE Computation (without inter-PCE
   communication) in [PCE-ARCH]. This option utilizes per-layer path
   computation performed independently by successive PCEs. Since there
   is no PCE-PCE communication, and since each PCE is responsible for a
   single layer only, there are no requirements placed on the PCC-PCE
   communications protocol above those already defined for single domain
   operation described in [PCE-COM-REQ]. Therefore this option is not
   discussed further in this document.

   When PCE returns to PCC a computed explicit path that would be
   acceptable for use for MPLS and GMPLS LSPs once converted to an
   Explicit Route Object (ERO) for use in RSVP-TE signaling, two options
   could be considered as:
   -Option 1: Mono-layer path. There are two cases. The first case is
   that the PCE computes a path that includes already established lower
   layer-LSPs: that is the ERO includes sub-object(s) corresponding to
   lower layer hierarchical LSPs. This does not trigger new lower layer-
   LSP setup but the utilization of existing lower-layer LSPs. The other
   is that the PCE computes a path that includes loose hop(s). The
   higher layer would select which lower layers to use and would select
   the entry and exit points from those layers, but would not select the
   path across the layers. A transit LSR corresponding to the entry
   point is expected to expand the loose hop. Path expansion process on
   border LSR may result either in the selection of an existing lower
   layer LSP, or in the computation and setup of a new lower-layer LSP.
   -Option 2: Multi-layer path. The PCE computes a "multi-layer" path
   that can include the complete path of one or more lower-layer LSPs
   not yet established. In that case the ERO contains paths of lower-
   layer LSPs to be established. The signaling of the higher-layer LSP
   will trigger the establishment of the lower-layer LSPs (nested

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4.2.  VNT Management

   As a result of inter-layer path computation, PCE may determine that
   there are insufficient lower-layer LSPs to support this or future
   higher-layer LSPs. New lower-layer LSPs are needed in order to
   satisfy the high-layer LSP requests or to efficiently manage the
   utilization of lower-layer network resources. In other words, the VNT
   needs to be controlled or managed in cooperation with inter-layer
   path computation.

   While PCE is responsible for inter-layer path computation, VNT
   management may be performed by other network elements.  The
   relationship between VNT management and PCE for inter-layer
   computation is for further study.

5. Inter-layer Path Computation Models

   The generic PCC-PCE communication protocol requirements [PCE-COM-REQ]
   are limited to basic path computation scenarios and generic concerns.
   They do not necessarily cover all the requirements for inter-layer
   traffic engineering and further requirements are stated in section 6
   of this document to address the specific problem statements set out
   in this section.

   As stated in Section 4.1, two PCE modes defined in the PCE
   architecture can be used to perform inter-layer path computation.
   They are discussed below.

5.1.  Single PCE Inter-Layer Path Computation

   In Figure 1, higher-layer LSRs (H1, H2, H3 and H4) are connected by
   an end-to-end higher-layer LSP. This is supported by a lower-layer
   LSP (H2-L1-L2-H3) that traverses the lower-layer LSRs (L1 and L2) but
   is presented as a single hop (H2-H3) in the higher-layer. A single
   PCE manages the entire network and has visibility into both layers.

                          | PCE |

       -----    -----                  -----    -----
      | LSR |--| LSR |................| LSR |--| LSR |
      | H1  |  | H2  |                | H3  |  | H4  |
       -----    -----\                /-----    -----
                      \-----    -----/
                      | LSR |--| LSR |
                      | L1  |  | L2  |
                       -----    -----

             Figure 1 : Single PCE with Multi-Layer Visibility

5.2.  Multiple PCE Inter-Layer Path Computation

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   In Figure 2, there is one PCE in each layer. PCEs of each layer
   collaborate together to compute an end-to-end path across layers. PCE
   Hi is responsible for computations in the higher layer and may
   “consultEwith PCE Lo to compute paths across the lower layer. PCE Lo
   is responsible for path computation in lower layer. A simple
   cooperation could be: PCE Hi requests a path H2-H3 to PCE Lo. Of
   course more complex cooperation may be required if an end-to-end
   optimal path is desired.

                               | PCE |
                               | Hi  |
       -----    -----             |            -----    -----
      | LSR |--| LSR |............|...........| LSR |--| LSR |
      | H1  |  | H2  |            |           | H3  |  | H4  |
       -----    -----\          --+--         /-----    -----
                      \        | PCE |       /
                       \       | Lo  |      /
                        \       -----      /
                         \                /
                          \-----    -----/
                          | LSR |--| LSR |
                          | L1  |  | L2  |
                           -----    -----

             Figure 2 : Cooperating Single-Layer PCEs

6. 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.

6.1.  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

6.1.1 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.

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6.1.2 Control of Type of Path to be Computed

   A request from a PCC to a PCE MUST allow controlling the type path to
   be computed: A mono-layer path that includes already established
   lower layer-LSP, 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.

   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.

6.1.3 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.

6.1.4 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 [PCE-

   The PCC-PCE communication protocol MUST allow requests and replies
   for cooperative inter-layer path computation.

6.1.5 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).

6.2.  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].

7. Manageability considerations

   Manageability of inter-layer traffic engineering with PCE must
   address the following consideration for section 6.1.

   - need for a MIB module for control and monitoring
   - need for built-in diagnostic tools
   - configuration implication for the protocol

8. Security Considerations

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   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

9. Acknowledgment

  We would like to thank Kohei Shiomoto and Ichiro Inoue for their
  useful comments.

10.     References

10.1 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.

10.2 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

   [PCE-DISC-REQ] JL Le Roux et al., "Requirements for Path Computation
   Element (PCE) Discovery", draft-ietf-pce-discovery-reqs (work in

   [MRN-REQ] K. Shiomoto et al., "Requirements for GMPLS-based multi-
   region networks (MRN) ", draft-shiomoto-ccamp-gmpls-mrn-reqs (work in

11.     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

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   France Telecom R&D,
   Av Pierre Marzin,
   22300 Lannion, France
   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

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   Copyright Statement

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   Copyright (C) The Internet Society (2005).  This document is subject
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   except as set forth therein, the authors retain all their rights.

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