Network Working Group                                    T. Takeda (Ed.)
Internet Draft                                                       NTT
Category: Informational                                        A. Farrel (Ed.)
Created: December 2008 August 21, 2009                              Old Dog Consulting
Expires: February 21, 2010

         PCC-PCE Communication and PCE Discovery Requirements for
                      Inter-Layer Traffic Engineering

                   draft-ietf-pce-inter-layer-req-09.txt

                   draft-ietf-pce-inter-layer-req-10.txt

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Abstract

   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 requirements.

   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".

   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 engineering.

Table of Contents

   1. Introduction...................................................3
   1.1. Terminology..................................................3
   2. Motivation for PCE-Based Inter-Layer Path Computation..........4
   3. PCC-PCE Communication and Discovery Requirements for
      Inter-Layer Traffic Engineering...............................................5 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.................5
   3.1.3. Communication of Inter-Layer Constraints...................6 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................7 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..................................8 Models..................................9
   4.3. Liveness Detection and Monitoring............................9
   4.4. Verifying Correct Operation..................................9
   4.5. Requirements on Other Protocols and Functional Components....9
   4.6. Impact on Network Operation.................................10
   5. Security Considerations.......................................10
   6. IANA Considerations...........................................10
   7. Acknowledgments...............................................10
   8. References....................................................10 References....................................................11
   8.1. Normative References........................................10 References........................................11
   8.2. Informative References......................................11
   9. Authors' Addresses............................................12

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) [RFC5212], 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, Label Switched Paths (LSPs) 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) [RFC5212].

   In layered networks under the operation of Multiprotocol Label
   Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS)
   protocols, it is important to provide mechanisms to allow global
   optimization of network resources. That is, to take into account all
   layers, rather than optimizing resource utilization at each layer
   independently. This allows better network efficiency to be achieved.
   This is what we call Inter-layer traffic engineering. This includes
   mechanisms allowing computation of end-to-end paths across layers
   (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 [RFC5212].

   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 sets of requirements for communication between
   path computation clients (PCCs) and PCEs using the PCE protocol
   (PCEP), and for PCE discovery for inter-layer traffic engineering. It
   supplements the generic requirements documented in [RFC4657] and
   [RFC4674].

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 constraints.

   PCEP: PCE Communication Protocol, a protocol for communication
   between PCCs and PCEs.

   Although this requirements document is an informational document not
   a protocol specification, the key words "MUST", "MUST NOT",
   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD 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 an MPLS or a 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 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
   as TE 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
   [RFC4674].

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
   protocol.

3.1.1. Control of Inter-Layer Path Computation

   A request from a PCC to a PCE MUST support the inclusion of an
   optional indication of 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.

3.1.2. 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].

   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 PCEP MUST allow the PCC to select the path types to be computed,
   and 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, PCEP
   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 (triggered signaling). The
   triggered signaling may increase the higher-layer connection setup
   latency. An ingress LSR for the higher-layer LSP, or a PCC, needs to
   know whether triggered signaling is required or not.

   A request from a PCC to a PCE MUST allow indicating whether triggered
   signaling is acceptable or not.

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

   Note that a PCE may not be able to distinguish virtual TE links from
   regular TE links. In such cases, even if a request from a PCC to a
   PCE indicates that triggered signaling is not acceptable, a PCE may
   choose virtual TE links in path computation. Therefore, when a
   network uses virtual TE links and a PCE is not able to distinguish
   virtual TE links from regular TE links, it MUST be understood that a
   PCE may choose virtual TE links even if a request from a PCC to a PCE
   indicates triggered signaling is not acceptable.

   Also note that an ingress LSR may be present in multiple layers.
   Thus, when a mono-layer path is requested or supplied, PCEP MUST be
   able to indicate the required/provided path layer.

3.1.3. 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.

   Furthermore, it may be desirable to constrain the number of layer
   boundaries crossed (i.e., the number of adaptations performed on the
   end-to-end path), so PCEP SHOULD include a constraint or objective
   function to minimize or cap the number of adaptations on a path, and
   a mechanism to report that number when a path is supplied.

   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.

3.1.4. 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 adaptations.

3.1.5. 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
   [RFC4655].

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

3.1.6. 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 requests, with the desired level of
   diversity at each layer (link, node, SRLG).

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 inter-layer constraints
   - 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 [RFC3945], [RFC5145],
   [RFC5146].

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-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 PCEP 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.

4.2. Information and Data Models

   PCEP 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
   PCEP 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 PCEP 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 PCEP 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 nor 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 PCEP request relies on each PCE
   operating correct monitoring procedures and correlating any failures
   back to the PCEP 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. Such a procedure is specified in [BRPC]. [RFC5441].
   In addition, [PCEP-MON] specifies mechanisms to gather various state
   metrics along the path computation chain.

4.4. Verifying Correct Operation

   There are no additional requirements beyond those expressed in
   [RFC4657] for verifying the correct operation of the PCEP. 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 results.

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 [RFC5212].

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 supported.

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.

6. IANA Considerations

   This Informational document makes no requests for IANA action.

7. Acknowledgments

   We would like to thank Kohei Shiomoto, Ichiro Inoue, and Dean Cheng
   for their useful comments. Thanks to members of ITU-T Study Group 15
   Question 14 for their constructive comments during the liaison
   process.

8. References

8.1. Normative References

   [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.

8.2. Informative References

   [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.

   [RFC5145] K. Shiomoto, "Framework for MPLS-TE to GMPLS Migration",
             RFC 5145, March 2008.

   [RFC5146] K. Kumaki et al., "Interworking Requirements to Support
             Operation of MPLS-TE over GMPLS Networks", RFC 5146, March
             2008.

   [RFC5212] K. Shiomoto et al., "Requirements for GMPLS-Based Multi-
             Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
             2008.

   [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 draft-ietf-pce-pcep-
             mib (work in progress).

   [RFC5145] K. Shiomoto, "Framework for MPLS-TE to GMPLS Migration",
             RFC 5145, March 2008.

   [RFC5146] K. Kumaki et al., "Interworking Requirements to Support
             Operation of MPLS-TE over GMPLS Networks", RFC 5146, March
             2008.

   [BRPC]

   [RFC5441] JP. Vasseur (Ed.), "A Backward Recursive PCE-based Backward-Recursive PCE-Based
             Computation (BRPC) Procedure To to Compute Shortest
             Constrained Inter-domain Inter-Domain Traffic Engineering Label Switched
             Paths", draft-ietf-pce-brpc (work in progress). RFC 5441, April 2009.

   [PCEP-MON] JP. Vasseur (Ed.), "A set of monitoring tools for Path
             Computation Element based Architecture", draft-ietf-pce-
             Monitoring (work in progress).

9. Authors' Addresses

   Eiji Oki
   University of Electro-Communications
   Tokyo
   Tokyo, Japan
   Email: oki@ice.uec.ac.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

   Tomonori Takeda
   NTT
   3-9-11 Midori-cho,
   Musashino-shi, Tokyo 180-8585, Japan
   Email: takeda.tomonori@lab.ntt.co.jp

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