Network Working Group                             J.-L. Le Roux (Editor)
Internet Draft                                            France Telecom






Category: Informational
Expires: June 2007
                                                           December 2006


 PCE Communication Protocol (PCECP) Specific Requirements for Inter-Area
    Multi Protocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
                           Traffic Engineering


             draft-ietf-pce-pcecp-interarea-reqs-05.txt


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Abstract

   For scalability purposes a network may comprise multiple Interior
   Gateway Protocol (IGP) areas. An inter-area Traffic Engineered-Label
   Switched Path (TE-LSP) is an LSP that transits through at least two
   IGP areas. In a multi-area network, topology visibility remains local
   to a given area, and a head-end Label Switching Router (LSR) cannot
   compute an inter-area shortest constrained path. One key application
   of the Path Computation Element (PCE) based architecture is the
   computation of inter-area TE-LSP paths. The PCE Communication
   Protocol (PCECP) is used to communicate computation requests from
   Path Computation Clients (PCC) to PCEs, and to return computed paths
   in responses. This document lists a detailed set of PCECP specific
   requirements for support of inter-area TE-LSP path computation. It
   complements the generic requirements for a PCE Communication
   Protocol.

Conventions used in this document

   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.

Table of Contents

   1.      Terminology.................................................3
   2.      Introduction................................................3
   3.      Motivations for PCE-based Inter-Area Path Computation.......4
   4.      Detailed Inter-Area Specific Requirements on PCECP..........5
   4.1.    Control and Recording of Area Crossing......................5
   4.2.    Area Recording..............................................5
   4.3.    Strict Explicit Path and Loose Path.........................6
   4.4.    PCE-list Enforcement and Recording in Multiple PCE
             Computation...............................................6
   4.5.    Inclusion of Area IDs in Request............................6
   4.6.    Area Inclusion/Exclusion....................................7
   4.7.    Inter-area Diverse Path computation.........................7
   4.8.    Inter-area Policies.........................................8
   4.9.    Loop Avoidance..............................................8
   5.      Manageability Considerations................................8
   6.      Security Considerations.....................................8
   7.      Acknowledgments.............................................9
   8.      IANA Considerations.........................................9
   9.      References..................................................9
   9.1.    Normative References........................................9
   9.2.    Informative References......................................9
   10.     Editor Address:.............................................9
   11.     Contributors' Addresses....................................10
   12.     Intellectual Property Statement............................11



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. Terminology

      LSR: Label Switching Router.

      LSP: MPLS Label Switched Path.

      TE-LSP: Traffic Engineering Label Switched Path.

      IGP area: OSPF Area or IS-IS level.

      ABR: IGP Area Border Router, a router that is attached to more
      than one IGP areas (ABR in OSPF or L1/L2 router in IS-IS).

      Inter-Area TE LSP: TE LSP that traverses more than one IGP area.

      CSPF: Constrained Shortest Path First.

      SRLG: Shared Risk Link Group.

      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.

      PCC: Path Computation Client, any application that request path
      computation to be performed by a PCE.

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

      ERO: RSVP-TE Explicit Route Object. It encodes the explicit path
      followed by a TE-LSP.


. Introduction

   [RFC4105] lists a set of motivations and requirements for setting up
   TE-LSPs across IGP area boundaries. These LSPs are called inter-area
   TE-LSPs. These requirements include the computation of inter-
   area shortest constrained paths with key guideline being to respect
   the IGP hierarchy concept, and particularly the containment of
   topology information. The main challenge with inter-area MPLS-TE lies
   in path computation. Indeed the head-end LSR cannot compute an
   explicit path across areas, as its topology visibility is limited to
   its own area.

   Inter-area path computation is one of the key applications of the PCE
   based architecture [RFC4655]. The computation of optimal inter-area
   paths may be achieved using the services of one or more PCEs.


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   Such PCE-based inter-area path computation could rely for instance on
   a single multi-area PCE that has the TE database of all the areas in
   the IGP domain and can directly compute an end-to-end constrained
   shortest path.  Alternatively, this could rely on the cooperation
   between PCEs whereby each PCE covers one or more IGP areas and the
   full set of PCEs covers all areas.

   The generic requirements for a PCE Communication Protocol (PCECP),
   which allows a PCC to send path computation requests to a PCE and the
   PCE to send path computation responses to a PCC, are set forth in
   [RFC4657]. The use of a PCE-based approach for inter-area path
   computation implies specific requirements on a PCE Communication
   Protocol, in addition to the generic requirements already listed in
   [RFC4657]. This document complements these generic requirements by
   listing a detailed set of PCECP requirements specific to inter-area
   path computation.

   It is expected that PCECP procedures be defined to satisfy these
   requirements.

   Note that PCE-based inter-area path computation may require a
   mechanism for automatic PCE discovery across areas, which is out of
   the scope of this document. Detailed requirements for such a
   mechanism are discussed in [RFC4674].

. Motivations for PCE-based Inter-Area Path Computation

   IGP hierarchy enables improved IGP scalability, by dividing the IGP
   domain into areas and limiting the flooding scope of topology
   information to within area boundaries. A router in an area has full
   topology information for its own area but only information about
   reachability to destinations in other areas._ Thus, a head-end LSR
   cannot compute an end-to-end path that crosses the boundary of its
   IGP area(s).

   A current solution for computing inter-area TE-LSP path relies on a
   per domain path computation ([PD-COMP]). It is based on loose hop
   routing with an ERO expansion on each ABR. This allows an LSP to be
   set up following a constrained path, but faces two major limitations:
   - This does guarantee the use of an optimal constrained path;
   - This may lead to several crankback signaling messages and hence
     delay the LSP setup, and may also invoke possible alternate routing
     activities.

   Note that, here, by optimal constrained path we mean the shortest
   constrained path across multiple areas, taking into account either
   the IGP or TE metric [METRIC]. In other words, such a path is the
   path that would have been computed by making use of some CSPF
   algorithm in the absence of multiple IGP areas.

   The PCE based architecture [RFC4655] is well suited to inter-area
   path computation, as it allows the path computation limitations

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   resulting from the limited topology visibility to be overcome by
   introducing path computation entities with more topology visibility,
   or by allowing cooperation between path computation entities in each
   area.

   There are two main approaches for the computation of an inter-area
   optimal path:
   - Single PCE computation: The path is computed by a single PCE that
     has topology visibility in all areas and can compute an end-
     to-end optimal constrained path on its own.
   - Multiple PCE computation with inter-PCE communication: The path
     computation is distributed on multiple PCEs, which have partial
     topology visibility. They compute path segments in their domains of
     visibility and collaborate with each other so as to arrive at an
     end-to-end optimal constrained path. Such collaboration is ensured
     thanks to inter-PCE communication.

   Note that the use of a PCE-based approach, to perform inter-area path
   computation implies specific functional requirements in a PCECP, in
   addition to the generic requirements listed in [RFC4657]. These
   specific requirements are discussed in next section.

. Detailed Inter-Area Specific Requirements on PCECP

   This section lists a set of additional requirements for the PCECP
   that complement requirements listed in [RFC4657] and are specific to
   inter-area (G)MPLS TE path computation.

4.1. Control and Recording of Area Crossing

   In addition to the path constraints specified in [RFC4657], the
   request message MUST allow indicating whether area crossing is
   allowed or not. Indeed, when the source and destination reside in the
   same IGP area, there may be intra-area and inter-area feasible paths.
   As set forth in [RFC4105], if the shortest path is an inter-area
   path, an operator either may want to avoid, as far as possible,
   crossing areas and thus may prefer selecting a sub-optimal intra-area
   path or, conversely, may prefer to use a shortest path, even if it
   crosses areas.

   Also, when the source and destination reside in the same area it may
   be useful to know whether the returned path is an inter-area path.
   Hence the response message MUST allow indicating whether the computed
   path is crossing areas.

4.2. Area Recording

   It may be useful for the PCC to know the set of areas crossed by an
   inter-area path and the corresponding path segments. Hence the
   response message MUST allow identifying the crossed areas. Also the
   response message MUST allow segmenting the returned path and marking


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   each segment so that it is possible to tell which piece of the path
   lie within which area.

4.3. Strict Explicit Path and Loose Path

   A Strict Explicit Path is defined as a set of strict hops, while a
   Loose Path is defined as a set of at least one loose hop and zero,
   one or more strict hops. An inter-area path may be strictly explicit
   or loose (e.g. a list of ABRs as loose hops). It may be useful to
   indicate to the PCE if a Strict Explicit path is required or not.
   Hence the PCECP request message MUST allow indicating whether a
   Strict Explicit Path is required/desired.

4.4. PCE-list Enforcement and Recording in Multiple PCE Computation

   In case of multiple-PCE inter-area path computation, a PCC may want
   to indicate a preferred list of PCEs to be used, one per area.
   In each area the preferred PCE should be tried before another PCE be
   selected. Note that if there is no preferred PCE indicated for an
   area, any PCE in that area may be used.

   Hence the PCECP request message MUST support the inclusion of a list
   of preferred PCEs per area. Note that this requires that a PCC in one
   area have knowledge of PCEs in other areas. This could rely on
   configuration or on a PCE discovery mechanism, allowing discovery
   across area boundaries (see [RFC4674]).

   Also it would be useful to know the list of PCEs which effectively
   participated in the computation. Hence the request message MUST
   support a request for PCE recording and the response message MUST
   support the recording of the set of one or more PCEs that took part
   in the computation.

   It may also be useful to know the path segments computed by each PCE.
   Hence the request message SHOULD allow a request for the
   identification of path segments computed by a PCE, and the response
   message SHOULD allow identifying the path segments computed by each
   PCE.

4.5. Inclusion of Area IDs in Request

   The knowledge of the areas in which the source and destination lie
   would allow a PCE to select an appropriate downstream PCE. This would
   be useful when the area ID(s) of a PCE (i.e. the area(s) where it has
   visibility) is/are known, which can be achieved by the PCE Discovery
   Protocol (see [RFC4674]) or any other mean.

   A PCE may not have any visibility of the source/destination area and
   hence may not be able to determine the area of the
   source/destination. In such a situation it would be useful that a PCC
   indicates the source and destination area IDs in its request message.


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   For that purpose the request message MUST support the inclusion of
   the source and destination area IDs. Note that this information could
   be learned by the PCC through configuration.

4.6. Area Inclusion/Exclusion

   In some situations, it may be useful that the request message
   indicate one or more area(s) that must be followed by the path to be
   computed. It may also be useful that the request message indicate one
   or more area(s) that must be avoided by the path to be computed (e.g.
   request for a path between LSRs in two stub areas connected to the
   same ABR(s), which must not cross the backbone area). Hence the
   request message MUST allow indicating a set of one or more area(s)
   that must be explicitly included in the path, and a set of one or
   more area(s) that must be explicitly excluded from the path.

4.7. Inter-area Diverse Path computation

   For various reasons, including protection and load balancing, the
   computation of diverse inter-area paths may be required.
   There are various levels of diversity in an inter-area context:
        -Per-area diversity (intra-area path segments are link, node or
         SRLG disjoint)
        -Inter-area diversity (end-to-end inter-area paths are link,
         node or SRLG disjoint)

   Note that two paths may be disjoint in the backbone area but non-
   disjoint in peripheral areas. Also two paths may be node disjoint
   within areas but may share ABRs, in which case path segments within
   an area are node disjoint but end-to-end paths are not node-disjoint.

   The request message MUST allow requesting the computation of a set of
   inter-area diverse paths between the same node pair or between
   distinct node pairs. It MUST allow indicating the required level of
   diversity of a set of inter-area paths (link, node, SRLG diversity),
   as well as the required level of diversity of a set of intra-area
   segments of inter-area paths (link, node, SRLG diversity) on a per-
   area basis.

   The response message MUST allow indicating the level of diversity of
   a set of computed inter-area loose paths (link, node, SRLG
   diversity), globally, and on a per-area basis (link, node, SRLG
   diversity of intra-area path segments).

   Note that, in order to ensure SRLG consistency, SRLG identifiers
   within the IGP domain should be assigned and allocated by the same
   entity.


   Note that specific objective functions may be requested for diverse
   path computation, such as minimizing the cumulated cost of a set of
   diverse paths as set forth in [RFC4657].

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4.8. Inter-area Policies

   In addition to the policy requirements discussed in [RFC4657], the
   application of inter-area path computation policies requires some
   additional information to be carried in the PCECP request messages.
   The request message MUST allow for the inclusion of the address of
   the originating PCC. This may be useful in a multiple PCE
   computation, so as to apply policies not only based on the PCECP peer
   but also based on the originating PCC.

   Note that work on supported policy models and the corresponding
   requirements/implications is being undertaken as a separate work item
   in the PCE working group ([PCE-POL-FMWK]).

4.9. Loop Avoidance

   In case of multiple-PCE inter-area path computation, there may be
   risks of PCECP request loops. A mechanism MUST be defined to detect
   and correct PCECP request message loops. This may rely, for instance,
   on the recording, in the request message, of the set of traversed
   PCEs.

   Also the returned path in a response message MUST be loop free.


. Manageability Considerations

   The inter-area application implies some new manageability
   requirements in addition to those already listed in [RFC4657]. The
   PCECP PCC and PCE MIB modules MUST allow recording the proportion of
   inter-area requests and the success rate of inter-area requests. The
   PCEP PCC MIB module MUST also allow recording the performances of a
   PCE chain (minimum, maximum and average response time), in case of
   multiple-PCE inter-area path computation.

   A built in diagnostic tool MUST be defined to monitor the
   performances of a PCE chain, in case of multiple-PCE inter-area path
   computation. It MUST allow determining the minimum maximum and
   average response time globally for the chain, and on a per PCE basis.

. Security Considerations

   IGP areas are administrated by the same entity. Hence the inter-area
   application does not imply a new trust model, or new security issues
   beyond those already defined in [RFC4657].





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. Acknowledgments

   We would also like to thank Adrian Farrel, Jean-Philippe Vasseur,
   Bruno Decraene, Yannick Le Louedec, Dimitri Papadimitriou and Lou
   Berger for their useful comments and suggestions.

. IANA Considerations

      This document makes no requests for IANA action.

. References

9.1. Normative References

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

   [RFC4105] Le Roux J.L., Vasseur J.P., Boyle, J., et al. "Requirements
   for inter-area MPLS-TE" RFC 4105, June 2005.

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

   [RFC4657] J. Ash, J.L Le Roux et. al., "PCE Communication Protocol
   Generic Requirements", RFC4657, September 2006.

9.2. Informative References

   [RFC4674] J.L. Le Roux et. al., "Requirements for Path Computation
   Element (PCE) Discovery", RFC4674, October 2006.

   [PD-COMP] Vasseur, J.P., Ayyangar, A., Zhang, R., "A Per-domain path
   computation method for computing Inter-domain Traffic Engineering
   (TE) Label Switched Path (LSP)", work in progress.

   [PCE-POL-FMWK] I. Bryskin, D. Papadimitriou, L. Berger "Policy-
   Enabled Path Computation Framework", draft-ietf-pce-policy-enabled-
   path-comp, work in progress.

   [METRIC] Le Faucheur et al., "Use of Interior Gateway Protocol (IGP)
   Metric as a second MPLS Traffic Engineering (TE) Metric", RFC3785,
   May 2004.

. Editor Address:

   Jean-Louis Le Roux
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   FRANCE
   Email: jeanlouis.leroux@orange-ftgroup.com


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. Contributors' Addresses

   Jerry Ash
   AT&T
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748, USA
   Phone: +1-(732)-420-4578
   Email: gash@att.com

   Nabil Bitar
   Verizon
   40 Sylvan Road
   Waltham, MA 02145
   Email: nabil.bitar@verizon.com

   Dean Cheng
   Cisco Systems Inc.
   3700 Cisco Way
   San Jose CA 95134 USA
   Phone: +1 408 527 0677
   Email: dcheng@cisco.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

   Eiji Oki
   NTT
   Midori-cho 3-9-11
   Musashino-shi, Tokyo 180-8585, JAPAN
   Email: oki.eiji@lab.ntt.co.jp

   Raymond Zhang
   BT
   2160 E. Grand Ave.
   El Segundo, CA 90245
   USA
   raymond.zhang@bt.com

   Renhai Zhang
   Huawei Technologies
   No. 3 Xinxi Road, Shangdi,
   Haidian District,
   Beijing City,
   P. R. China
   Email: zhangrenhai@huawei.com


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