Networking Working Group                                JP. Vasseur, Ed.
Internet-Draft                                        Cisco Systems, Inc
Intended status: Informational                                  R. Zhang
Expires: March 4, 2007                                        BT Infonet
                                                                N. Bitar
                                                             JL. Le Roux
                                                          France Telecom
                                                         August 31, 2006

 A Backward Recursive PCE-based Computation (BRPC) procedure to compute
     shortest inter-domain Traffic Engineering Label Switched Paths

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   The ability to compute constrained shortest Traffic Engineering (TE)
   Label Switched Paths (LSPs) in Multiprotocol Label Switching (MPLS)

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   and Generalized MPLS (GMPLS) networks across multiple domains (where
   a domain is referred to as a collection of network elements within a
   common sphere of address management or path computational
   responsibility such as IGP areas and Autonomous Systems) has been
   identified as a key requirement .  This document specifies a
   procedure relying on the use of multiple Path Computation Elements
   (PCEs) in order to compute such inter-domain shortest constraint
   paths along a determined sequence of domains, using a backward
   recursive path computation technique while preserving confidentiality
   across domains, which is sometimes required when domains are managed
   by different Service Providers.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Table of Contents

   1.  History  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  General assumptions  . . . . . . . . . . . . . . . . . . . . .  5
   5.  BRPC Procedure . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Domain path selection  . . . . . . . . . . . . . . . . . .  6
     5.2.  Mode of Operation  . . . . . . . . . . . . . . . . . . . .  7
   6.  PCEP Protocol Extensions . . . . . . . . . . . . . . . . . . .  9
   7.  Inter-AS TE Links  . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Usage in conjunction with per-domain path computation  . . . .  9
   9.  BRPC procedure completion failure  . . . . . . . . . . . . . . 10
   10. Applicability  . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Diverse end-to-end path computation  . . . . . . . . . . . 10
     10.2. Path optimality  . . . . . . . . . . . . . . . . . . . . . 11
   11. Reoptimization of an inter-domain TE LSP . . . . . . . . . . . 11
   12. Metric normalization . . . . . . . . . . . . . . . . . . . . . 11
   13. Manageability Considerations . . . . . . . . . . . . . . . . . 12
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     17.2. Informative References . . . . . . . . . . . . . . . . . . 13
     17.3. Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Proposed Status and Discussion [To Be Removed
                Upon Publication] . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  History

   The aim of this document is to specify a Backward Recursive PCE-based
   Computation (BRPC) procedure to compute shortest constrained inter-
   domain (G)MPLS TE LSP.  Such procedure had been initially documented
   in draft-vasseur-ccamp-inter-domain-path-comp (Scenario 2) and is now
   moved to a separated document in the light of the progress made by
   the PCE Working Group.

2.  Terminology

   ABR: routers used to connect two IGP areas (areas in OSPF or levels
   in IS-IS).

   ASBR: routers used to connect together ASs of a different or the same
   Service Provider via one or more Inter-AS links.

   Boundary Node (BN): a boundary node is either an ABR in the context
   of inter- area TE or an ASBR in the context of inter-AS TE.

   Entry BN of domain(n): a BN connecting domain(n-1) to domain(n).

   Exit BN of domain(n): a BN connecting domain(n) to domain(n+1).

   Inter-AS TE LSP: A TE LSP that crosses an AS boundary.

   Inter-area TE LSP: A TE LSP that crosses an IGP area boundary.

   LSR: Label Switching Router.

   LSP: Label Switched Path.

   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.

   PCE(i) is a PCE with the scope of domain(i).

   TED: Traffic Engineering Database.

   VSPT: Virtual Shortest Path Tree.

   The notion of contiguous, stitched and nested TE LSPs is defined in
   [I-D.ietf-ccamp-inter-domain-framework] and will not be repeated

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3.  Introduction

   The requirements for inter-area and inter-AS MPLS Traffic Engineering
   have been developed by the Traffic Engineering Working Group (TE WG)
   and have been stated in [RFC4105] and [RFC4216], respectively.

   The framework for inter-domain MPLS Traffic Engineering has been
   provided in [I-D.ietf-ccamp-inter-domain-framework].

   [I-D.ietf-ccamp-inter-domain-pd-path-comp] defines a technique for
   establishing inter-domain (G)MPLS TE LSP whereby the path is computed
   during the signalling process on a per-domain basis by the entry
   boundary node of each domain (each node in charge of computing a
   section of an inter-domain TE LSP path is always along the path of
   such TE LSP).  Such path computation technique fulfills some of the
   requirements stated in [RFC4105] and [RFC4216] but not all of them.
   In particular, it cannot guarantee to find an optimal (shortest)
   inter-domain constrained path.  Furthermore, it cannot be efficiently
   used to compute a set of inter-domain diversely routed TE LSPs.

   The aim of this document is to describe a PCE-based TE LSP
   computation procedure to compute optimal inter-domain constrained

   Qualifying a path as optimal requires some clarification.  Indeed, a
   globally optimal TE LSP placement usually refers to a set of TE LSPs
   whose placements optimize the network resources with regards to a
   specified objective function (e.g. a placement that reduces the
   maximum or average network load while satisfying the TE LSP
   constraints).  In this document, an optimal inter-domain constrained
   TE LSP is defined as the shortest path satisfying the set of required
   constraints that would be obtained in the absence of multiple domains
   (in other words, in a totally flat IGP network between the source and
   destination of the TE LSP).

4.  General assumptions

   In the rest of this document, we make the following set of
   assumptions common to inter-area and inter-AS MPLS TE:

   - Each IGP area or AS is assumed to be Traffic Engineering enabled
   (i.e. running OSPF-TE or ISIS-TE and RSVP-TE).

   - No topology or resource information is distributed between domains
   (as mandated per [RFC4105] and [RFC4216]), which is critical to
   preserve IGP/BGP scalability and confidentiality.

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   - While certain constraints like bandwidth can be used across
   different domains, other TE constraints like resource affinity,
   color, metric, etc. as listed in [RFC2702] could be translated at
   domain boundaries.  If required, it is assumed that, at the domain
   boundary nodes, there will exist some sort of local mapping based on
   policy agreement, in order to translate such constraints across
   domain boundaries during the inter-PCE communication process.

   - The various ASBRs are BGP peers, without any IGP running on the
   inter-ASBR links.

   - Each AS can be made of several IGP areas.  The path computation
   procedure described in this document applies to the case of a single
   AS made of multiple IGP areas, multiples ASs made of a single IGP
   area or any combination of the above.  For the sake of simplicity,
   each AS will be considered to be comprised of a single area in this
   document.  The case of an Inter-AS TE LSP spanning multiple ASs where
   some of those ASs are themselves made of multiple IGP areas can be
   easily derived from this case by applying the BRPC procedure
   described in this document, recursively.

   - The domain path (set of domains traversed to reach the destination
   domain) is either administratively pre-determined or discovered by
   some means (outside of the scope of this document).

5.  BRPC Procedure

   The BRPC procedure is a Multiple-PCE path computation technique as
   described in [I-D.ietf-pce-architecture].  A possible model consists
   of hosting the PCE function on boundary nodes (e.g., ABR or ASBR) but
   this is not mandated by the BRPC procedure.

   The BRPC procedure does not make any assumptions with regards to the
   nature of the inter-domain TE LSP that could be contiguous, nested or

   Furthermore, no assumption is made on the actual path computation
   algorithm in use by a PCE (it can be any variant of CSPF, algorithm
   based on linear-programming to solve multi-constraints optimization
   problems and so on).

5.1.  Domain path selection

   The PCE-based BRPC procedure applies to the computation of an optimal
   constrained inter-domain TE LSP.  The sequence of domains to be
   traversed can either be determined a priori or during the path
   computation procedure.  The BRPC procedure guarantees to compute the

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   optimal path across a specific set of traversed domains (which
   constitutes an additional constraint).  In the case of an arbitrary
   set of meshed domains, the BRPC procedure can be used to compute the
   optimal path across each domain set in order to get to optimal
   constrained path between the source and the destination of the TE

5.2.  Mode of Operation

   Definition of VSPT(i)

   In each domain i:

   * There is a set of X-en(i) entry BNs noted BN-en(k,i) where BN-
   en(k,i) is the kth entry BN of domain(i).

   * There is a set of X-ex(i) exit BN noted BN-ex(k,i) where BN-ex(k,i)
   is the kth exit BN of domain(i).

   VSPT(i): MP2P (MultiPoint To Point) tree returned by PCE(i) to

               Root (TE LSP destination)
               /         I            \
         BN-en(1,i)   BN-en(2,i) ... BN-en((j), i).

   Where j<= [X-en(i)]

   Each link of tree VSPT(i) represents the shortest path between BN-
   en(j,i) (identified by its TE Router-ID) and the destination that
   satisfies the set of required constraints for the TE LSP (bandwidth,
   affinities, ...).  These are path segments to reach the destination
   from BN-en(j,i).

   Note that PCE(i) only considers the entry BNs that provide
   connectivity from domain(i-1).  That is, the set BN-en(k,i-1) is only
   made of those BNs that provide connectivity from domain (i-1) to
   domain(i).  Furthermore, some BNs may be excluded according to policy
   constraints (either due to local policy or policies signaled in the
   path computation request).

   Step 1: the PCC needs to first determine the PCE capable of serving
   its path computation request.  The path computation request is then
   relayed until reaching a PCE(n) such that the TE LSP destination
   resides in the domain(n).  At each step of the process, the next PCE

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   can either be statically configured or dynamically discovered via
   IGP/BGP extensions.  If no next PCE can be found or the next hop PCE
   of choice is unavailable, the procedure stops and a path computation
   error is returned (see section Section 9).  If multiple PCEs are
   discovered, the PCE may select a subset of these PCEs based on some
   local policies or heuristics.  Note also that a sequence of PCEs
   might be enforced by policy on the PCC and this constraint can be
   either carried in the PCEP path computation request (defined in

   Step 2: PCE(n) computes VSPT(n) made of the list of shortest
   constrained path(s) between every BN-en(j,n) and the TE LSP
   destination using a suitable path computation algorithm (e.g.  CSPF)
   and returns the computed VSPT(n) to PCE(n-1).

   Step i:

   - For i=n-1 to 2: PCE(i) concatenates the topology of domain (i)
   (using its TED) with the received VSPT(i+1).  In the case of Inter-AS
   TE LSP computation, this requires to also add the inter-AS TE links
   connecting the domain (i) to the domain (i+1).  Then PCE(i) computes
   VSPT(i) (P2MP tree made of the shortest constrained paths between
   each BN-en(j,i) and the TE LSP destination).


   Finally PCE(1) computes the end-to-end shortest constrained path from
   the source to the destination and returns the corresponding path to
   the requesting PCC.

   Each branch of the VSPT tree (path) may be returned in the form of an
   explicit path (in which case all the hops along the path segment are
   listed) or a loose path (in which case only the BR is specified) so
   as to preserve confidentiality along with the respective cost.  In
   the later case, various techniques can used in order to retrieve the
   computed explicit paths on a per domain basis during the signaling
   process thanks to the use of path keys as described in

   BRPC guarantees to find the optimal (shortest) constrained inter-
   domain TE LSP according to a set of defined domains to be traversed.
   Note that other variants of the BRPC procedure relying on the same
   principles are also possible.

   Note also that in case of ECMP paths, more than one path could be
   returned to the requesting LSR.

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6.  PCEP Protocol Extensions

   The BRPC procedure requires the specification of a new flag of the RP
   object carried within the PCReq message (defined in
   [I-D.ietf-pce-pcep]), the aim of which is to specify that the
   shortest path(s) satisfying the constraints from the destination to
   the set of entry boundary nodes are requested (such set of path(s)
   forms the downstream VSPT as specified in Section 5.2).

   The following new flag of the RP object is defined: VSPT (V) flag:
   0x20.  When set, this indicates that the PCC requests the computation
   of an inter-domain TE LSP using the BRPC procedure.

   Because path segment(s) computed by a downstream PCE in the context
   of the BRPC procedure must be provided along with their respective
   path cost(s), the C flag of the RP object carried within the PCReq
   message MUST be set.  It is the choice of the requester to
   appropriately set the O bit of the RP object.

7.  Inter-AS TE Links

   In the case of Inter-AS TE LSP path computation, the BRPC procedure
   requires the knowledge of the traffic engineering attributes of the
   Inter-AS TE links: the process by which the PCE acquires this
   information is out of the scope of the BRPC procedure (which is
   compliant with the PCE architecture defined in

   That said, a straitghforward solution consists of allowing the ASBRs
   to flood the TE information related to the inter-ASBR link(s)
   although no IGP TE is enabled over those links (there is no IGP
   adjacency over the inter-ASBR links).  This allows the PCE of a
   domain to get entire TE visibility up to the set of entry ASBRs in
   the downstream domain.

8.  Usage in conjunction with per-domain path computation

   The BRPC procedure may be used to compute path segments and could be
   used in conjunction with other path computation techniques (such as
   the per-domain path computation technique defined in
   [I-D.ietf-ccamp-inter-domain-pd-path-comp]) to compute the end-to-end
   path.  In this case end-to-end path optimality can no longer be

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9.  BRPC procedure completion failure

   If the BRPC procedure cannot be completed because a PCE along the
   domain path does not support the procedure, a PCErr message is
   returned to the upstream PCE with a Error-Type "BRPC procedure
   completion failure".  The PCErr message MUST be relayed to the
   requesting PCC.

   PCEP-ERROR objects are used to report a PCEP protocol error and are
   characterized by an Error-Type which specifies the type of error and
   an Error-value that provides additional information about the error
   type.  Both the Error-Type and the Error-Value are managed by IANA.
   A new Error-Type is defined that relates to the BRPC procedure.

 Error-type          Meaning
     10              BRPC procedure completion failure
                          1: BRPC procedure not supported by one or more PCEs
                             along the domain path

10.  Applicability

   As discussed in section 3, the requirements for inter-area and
   inter-AS MPLS Traffic Engineering have been developed by the Traffic
   Engineering Working Group (TE WG) and have been stated in [RFC4105]
   and [RFC4216], respectively.  Among the set of requirements, both
   documents indicate the need for some solution providing the ability
   to compute an optimal (shortest) constrained inter-domain TE LSP and
   to compute a set of diverse inter-domain TE LSPs.

10.1.  Diverse end-to-end path computation

   PCEP allows a PCC to request the computation of a set of diverse TE
   LSPs thanks to the SVEC object by setting the flags L, N or S to
   request link, node or SRLG diversity respectively.  Such request MUST
   be taken into account by each PCE along the path computation chain
   during the VSPT computation.  In the context of the BRPC procedure, a
   set of diversely routed TE LSP between two LSRs can be computed since
   the paths segment(s) of the VSPT are simultaneously computed by a
   given PCE.  The BRPC path procedure allows for the computation of
   diverse paths under various objective functions (such as minimizing
   the sum of the costs of the N diverse paths, etc) thus avoiding the
   well-known "trapping" problem.  Indeed, with a 2-step approach
   consisting of computing the first path followed by the computation of
   the second path after having removed the set of network elements
   traversed by the first path (if that does not violate confidentiality
   preservation), one cannot guarantee that a solution will be found

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   even if such solution exists.  Furthermore, even if a solution is
   found, it may not be the most optimal one with respect to objective
   function such as minimizing the sum of the paths costs, bounding the
   path delays of both paths and so on.  Finally, it must be noted that
   such a 2-step path computation approach is usually less efficient in
   term of signalling delays since it requires two serialized TE LSP set

10.2.  Path optimality

   BRPC guarantees that the optimal (shortest) constrained inter-domain
   path will always be found subject to policy constraints.  When
   combined with other local path computation techniques (e.g. in the
   case of stitched/nested TE LSP) and in the case where a domain has
   more than one BR-en or more than one BR-ex, optimality after some
   network change within the domain can only be guaranteed by re-
   executing the BRPC procedure.

11.  Reoptimization of an inter-domain TE LSP

   The ability to reoptimize an existing inter-domain TE LSP path has
   been explicitly listed as a requirement in [RFC4105] and [RFC4216].
   In the case of a TE LSP reoptimization request, regular procedures
   apply as defined in PCEP where the path in use (if available on the
   head-end) is provided within the path computation request in order
   for the PCEs involved in the reoptimization request to avoid double
   bandwidth accounting.

12.  Metric normalization

   In the case of inter-area TE, the same IGP/TE metric scheme is
   usually adopted for all the IGP areas (e.g. based on the link-speed,
   propagation delay or some other combination of link attributes).
   Hence, the proposed set of mechanisms always computes the shortest
   path across multiple areas obeying the required set of constraints
   with respect to a well-specified objective function.  Conversely, in
   the case of Inter-AS TE, in order for this path computation to be
   meaningful, a metric normalization between ASs may be required.  One
   solution to avoid IGP metric modification would be for the SPs to
   agree on a TE metric normalization scheme and use the TE metric for
   TE LSP path computation (in that case, this must be requested in the
   PCEP Path computation request) thanks to the COST object.

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13.  Manageability Considerations

   To be added in a further revision of this document.

14.  IANA Considerations

   A new flag of the RP object (specified in [I-D.ietf-pce-pcep]) is
   defined in this document.

   Name: VSPT (V)

   Value: 0x20.

   When set, this indicates that the PCC requests the computation of an
   inter-domain TE LSP using the BRPC procedure.

   A new Error-Type is defined in this document (Error-Type and Error-
   value to be assigned by IANA).

 Error-type          Meaning
     10              BRPC procedure completion failure
                          1: BRPC procedure not supported by one or PCEs
                             along the domain path

15.  Security Considerations

   The BRPC procedure does not introduce any additional security issues
   beyond the ones related to inter-PCE communication.

16.  Acknowledgements

   The authors would like to thank Arthi Ayyangar and Dimitri
   Papadimitriou for their useful comments.  A special thank to Adrian
   Farrel for his useful comments and suggestions.

17.  References

17.1.  Normative References

              Farrel, A., "A Path Computation Element (PCE) Based
              Architecture", draft-ietf-pce-architecture-05 (work in
              progress), April 2006.

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              Vasseur, J., "Path Computation Element (PCE) communication
              Protocol (PCEP) - Version 1", draft-ietf-pce-pcep-02 (work
              in progress), June 2006.

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

17.2.  Informative References

17.3.  Informative References

              Bradford, R., "Preserving Topology Confidentiality in
              Inter-Domain Path Computation and  Signaling",
              draft-bradford-pce-path-key-00 (work in progress),
              June 2006.

              Farrel, A., "A Framework for Inter-Domain Multiprotocol
              Label Switching Traffic  Engineering",
              draft-ietf-ccamp-inter-domain-framework-05 (work in
              progress), July 2006.

              Vasseur, J., "A Per-domain path computation method for
              establishing Inter-domain Traffic  Engineering (TE) Label
              Switched Paths (LSPs)",
              draft-ietf-ccamp-inter-domain-pd-path-comp-03 (work in
              progress), August 2006.

              Ayyangar, A. and J. Vasseur, "Inter domain GMPLS Traffic
              Engineering - RSVP-TE extensions",
              draft-ietf-ccamp-inter-domain-rsvp-te-03 (work in
              progress), March 2006.

              Roux, J., "IGP protocol extensions for Path Computation
              Element (PCE) Discovery",
              draft-ietf-pce-disco-proto-igp-02 (work in progress),
              June 2006.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, September 1999.

   [RFC4105]  Le Roux, J., Vasseur, J., and J. Boyle, "Requirements for

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              Inter-Area MPLS Traffic Engineering", RFC 4105, June 2005.

   [RFC4216]  Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
              (AS) Traffic Engineering (TE) Requirements", RFC 4216,
              November 2005.

Appendix A.  Proposed Status and Discussion [To Be Removed Upon

   This Internet-Draft is being submitted for eventual publication as an
   RFC with a proposed status of Informational.  Discussion of this
   proposal should take place on the following mailing list:

Authors' Addresses

   JP Vasseur (editor)
   Cisco Systems, Inc
   1414 Massachusetts Avenue
   Boxborough, MA  01719


   Raymond Zhang
   BT Infonet
   2160 E. Grand Ave.
   El Segundo, CA  90025


   Nabil Bitar
   40 Sylvan Road
   Waltham, MA  02145


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   JL Le Roux
   France Telecom
   2, Avenue Pierre-Marzin
   Lannion,   22307


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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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   retain all their rights.

   This document and the information contained herein are provided on an

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   The IETF takes no position regarding the validity or scope of any
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