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Internet Engineering Task Force (IETF) T. Takeda, Ed. Request for Comments: 6457 NTT Category: Informational A. Farrel ISSN: 2070-1721 Old Dog Consulting

                                                         December 2011

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

Abstract

 The Path Computation Element (PCE) provides functions of path
 computation in support of traffic engineering in networks controlled
 by Multi-Protocol Label Switching (MPLS) and Generalized MPLS
 (GMPLS).

 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 RFC 4657, "Path
 Computation Element (PCE) Communication Protocol Generic
 Requirements".  Generic requirements for a PCE discovery protocol are
 presented in RFC 4674, "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.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6457.

Takeda, et al. Informational [Page 1] RFC 6457 PCE Inter-Layer Requirements December 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology ................................................3
 2. Motivation for PCE-Based Inter-Layer Path Computation ...........4
 3. PCC-PCE Communication and Discovery Requirements for
    Inter-Layer .....................................................4
    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
         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
    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
    4.3. Liveness Detection and Monitoring ..........................8
    4.4. Verifying Correct Operation ................................9
    4.5. Requirements on Other Protocols and Functional Components ..9
    4.6. Impact on Network Operation ................................9
 5. Security Considerations ........................................10
 6. Acknowledgments ................................................10
 7. References .....................................................10
    7.1. Normative References ......................................10
    7.2. Informative References ....................................10

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
 the separation of technologies (e.g., Packet Switch Capable (PSC),
 Time Division Multiplex (TDM), lambda switch capable (LSC)) into
 GMPLS regions [RFC3945], the separation of data plane switching

Takeda, et al. Informational [Page 2] RFC 6457 PCE Inter-Layer Requirements December 2011

 granularity levels (e.g., PSC-1 and PSC-2 or Virtual Circuit 4 (VC4)
 and VC12) into GMPLS layers [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 [RFC5623].

 This document presents sets of requirements for communication between
 Path Computation Clients (PCCs) and PCEs using the PCE Communication
 Protocol (PCEP) and for PCE discovery for inter-layer traffic
 engineering.  It supplements the generic requirements documented in
 [RFC4657], [RFC4674], and the framework provided in [RFC5623].

1.1. Terminology

 LSP:  Label Switched Path.

 LSR:  Label Switching Router.

 PCC:  A Path Computation Client is any client entity (component,
       application or network node) requesting a path computation to
       be performed by a Path Computation Element.

 PCE:  A Path Computation Element is an entity that is capable of
       computing a network path or route based on a network graph and
       applying computational constraints.

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

Takeda, et al. Informational [Page 3] RFC 6457 PCE Inter-Layer Requirements December 2011

 Although this requirements document is informational and not a
 protocol specification, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
 and "OPTIONAL" 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;
 hence, it 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 they 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-controlled 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 it 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 set up 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 [RFC5623].

3. PCC-PCE Communication and Discovery Requirements for Inter-Layer

  Traffic Engineering

 This section provides additional requirements specific to the
 problems of multi-layer TE that are not covered in [RFC4657] or
 [RFC4674].

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3.1. PCC-PCE Communication

 PCEP MUST allow requests and replies for inter-layer path
 computation.

 This requires no additional messages, but it implies the following
 additional constraints to be added to PCEP.

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 - Traffic Engineering
 (RSVP-TE) signaling.  There are two options [RFC5623].

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, 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.

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 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),
 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 or not triggered signaling is required.

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

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

 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, 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 of a higher-layer or lower-layer LSP
 may be present in multiple layers.  Thus, even 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 in the sense used
 in [RFC5212] 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 underlying TDM network might be optimized for fewest hops.

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3.1.4. Adaptation Capability

 The concept of adaptation is used here as introduced in [RFC5212].

 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 in scope of a different 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].

 PCEP MUST allow requests and replies for multiple PCE inter-layer
 path computation.

3.1.6. Inter-Layer Diverse Paths

 PCEP 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, Shared Risk Link Group (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 RFC 4674) be able to advise the following PCE
 capabilities related to inter-layer path computation:

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

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3.3. Supported Network Models

 PCEP SHOULD allow several architectural alternatives for interworking
 between MPLS- and GMPLS-controlled 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.

 A further discussion of policy-enabled path computation can be found
 in [RFC5394].

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
 (Autonomous System) 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 PCEP request
 to the originating PCC.

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 In particular, where a request is forwarded between multiple PCEs,
 neither the PCC nor the first PCE can monitor the liveness of all
 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 they
 are expected already to be covered by general, and by inter-AS and
 inter-area, implementations.  Such a procedure is specified in
 [RFC5441].  In addition, [RFC5886] 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 are used to collect the information to build the
 topology graph, the 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

 This section examines the impact on network operations of the use of
 a PCE for inter-layer traffic engineering.  It does not present any
 further requirements on the PCE or PCC, for the PCEP or for
 deployment.

 The use of a PCE to compute inter-layer paths is not expected to have
 significant impact on network operations if the upper-layer traffic
 engineering practices are aware of the frequent changes that might
 occur in the VNT.  It should also 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.

Takeda, et al. Informational [Page 9] RFC 6457 PCE Inter-Layer Requirements December 2011

 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 used 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
 [RFC5623], 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. Acknowledgments

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

7. References

7.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

 [RFC3945]   Mannie, E., Ed., "Generalized Multi-Protocol Label
             Switching (GMPLS) Architecture", RFC 3945, October 2004.

 [RFC4206]   Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
             Hierarchy with Generalized Multi-Protocol Label Switching
             (GMPLS) Traffic Engineering (TE)", RFC 4206, October
             2005.

7.2. Informative References

 [RFC4655]   Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
             Computation Element (PCE)-Based Architecture", RFC 4655,
             August 2006.

 [RFC4657]   Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
             Element (PCE) Communication Protocol Generic
             Requirements", RFC 4657, September 2006.

 [RFC4674]   Le Roux, J., Ed., "Requirements for Path Computation
             Element (PCE) Discovery", RFC 4674, October 2006.

Takeda, et al. Informational [Page 10] RFC 6457 PCE Inter-Layer Requirements December 2011

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

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

 [RFC5212]   Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
             M., and D. Brungard, "Requirements for GMPLS-Based Multi-
             Region and Multi-Layer Networks (MRN/MLN)", RFC 5212,
             July 2008.

 [RFC5394]   Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
             "Policy-Enabled Path Computation Framework", RFC 5394,
             December 2008.

 [RFC5623]   Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
             "Framework for PCE-Based Inter-Layer MPLS and GMPLS
             Traffic Engineering", RFC 5623, September 2009.

 [PCEP-MIB]  A. Koushik, and E. Stephan, "PCE communication protocol
             (PCEP) Management Information Base", Work in Progress,
             July 2010.

 [RFC5441]   Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
             "A Backward-Recursive PCE-Based Computation (BRPC)
             Procedure to Compute Shortest Constrained Inter-Domain
             Traffic Engineering Label Switched Paths", RFC 5441,
             April 2009.

 [RFC5886]   Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set
             of Monitoring Tools for Path Computation Element
             (PCE)-Based Architecture", RFC 5886, June 2010.

Takeda, et al. Informational [Page 11] RFC 6457 PCE Inter-Layer Requirements December 2011

Contributing Authors

 Eiji Oki
 University of Electro-Communications
 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
 EMail: ke-kumaki@kddi.com

Authors' Addresses

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

 Adrian Farrel
 Old Dog Consulting
 EMail: adrian@olddog.co.uk

Takeda, et al. Informational [Page 12]