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RFC 4726 - A Framework for Inter-Domain Multiprotocol Label Swit


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Network Working Group                                          A. Farrel
Request for Comments: 4726                            Old Dog Consulting
Category: Informational                                    J.-P. Vasseur
                                                     Cisco Systems, Inc.
                                                             A. Ayyangar
                                                           Nuova Systems
                                                           November 2006

       A Framework for Inter-Domain Multiprotocol Label Switching
                          Traffic Engineering

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2006).

Abstract

   This document provides a framework for establishing and controlling
   Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
   Traffic Engineered (TE) Label Switched Paths (LSPs) in multi-domain
   networks.

   For the purposes of this document, a domain is considered to be any
   collection of network elements within a common sphere of address
   management or path computational responsibility.  Examples of such
   domains include Interior Gateway Protocol (IGP) areas and Autonomous
   Systems (ASes).

Table of Contents

   1. Introduction ....................................................3
      1.1. Nested Domains .............................................3
   2. Signaling Options ...............................................4
      2.1. LSP Nesting ................................................4
      2.2. Contiguous LSP .............................................5
      2.3. LSP Stitching ..............................................5
      2.4. Hybrid Methods .............................................6
      2.5. Control of Downstream Choice of Signaling Method ...........6
   3. Path Computation Techniques .....................................6
      3.1. Management Configuration ...................................7
      3.2. Head-End Computation .......................................7
           3.2.1. Multi-Domain Visibility Computation .................7
           3.2.2. Partial Visibility Computation ......................7
           3.2.3. Local Domain Visibility Computation .................8
      3.3. Domain Boundary Computation ................................8
      3.4. Path Computation Element ...................................9
           3.4.1. Multi-Domain Visibility Computation ................10
           3.4.2. Path Computation Use of PCE When Preserving
                  Confidentiality ....................................10
           3.4.3. Per-Domain Computation Elements ....................10
      3.5. Optimal Path Computation ..................................11
   4. Distributing Reachability and TE Information ...................11
   5. Comments on Advanced Functions .................................12
      5.1. LSP Re-Optimization .......................................12
      5.2. LSP Setup Failure .........................................13
      5.3. LSP Repair ................................................14
      5.4. Fast Reroute ..............................................14
      5.5. Comments on Path Diversity ................................15
      5.6. Domain-Specific Constraints ...............................16
      5.7. Policy Control ............................................17
      5.8. Inter-Domain Operations and Management (OAM) ..............17
      5.9. Point-to-Multipoint .......................................17
      5.10. Applicability to Non-Packet Technologies .................17
   6. Security Considerations ........................................18
   7. Acknowledgements ...............................................19
   8. Normative References ...........................................19
   9. Informative References .........................................20

1.  Introduction

   The Traffic Engineering Working Group has developed requirements for
   inter-area and inter-AS Multiprotocol Label Switching (MPLS) Traffic
   Engineering in [RFC4105] and [RFC4216].

   Various proposals have subsequently been made to address some or all
   of these requirements through extensions to the Resource Reservation
   Protocol Traffic Engineering extensions (RSVP-TE) and to the Interior
   Gateway Protocols (IGPs) (i.e., Intermediate System to Intermediate
   System (IS-IS) and OSPF).

   This document introduces the techniques for establishing Traffic
   Engineered (TE) Label Switched Paths (LSPs) across multiple domains.
   In this context and within the remainder of this document, we
   consider all source-based and constraint-based routed LSPs and refer
   to them interchangeably as "TE LSPs" or "LSPs".

   The functional components of these techniques are separated into the
   mechanisms for discovering reachability and TE information, for
   computing the paths of LSPs, and for signaling the LSPs.  Note that
   the aim of this document is not to detail each of those techniques,
   which are covered in separate documents referenced from the sections
   of this document that introduce the techniques, but rather to propose
   a framework for inter-domain MPLS Traffic Engineering.

   Note that in the remainder of this document, the term "MPLS Traffic
   Engineering" is used equally to apply to MPLS and Generalized MPLS
   (GMPLS) traffic.  Specific issues pertaining to the use of GMPLS in
   inter-domain environments (for example, policy implications of the
   use of the Link Management Protocol [RFC4204] on inter-domain links)
    are covered in separate documents such as [GMPLS-AS].

   For the purposes of this document, a domain is considered to be any
   collection of network elements within a common sphere of address
   management or path computational responsibility.  Examples of such
   domains include IGP areas and Autonomous Systems.  Wholly or
   partially overlapping domains (e.g., path computation sub-domains of
   areas or ASes) are not within the scope of this document.

1.1.  Nested Domains

   Nested domains are outside the scope of this document.  It may be
   that some domains that are nested administratively or for the
   purposes of address space management can be considered as adjacent
   domains for the purposes of this document; however, the fact that the
   domains are nested is then immaterial.  In the context of MPLS TE,
   domain A is considered to be nested within domain B if domain A is

   wholly contained in domain B, and domain B is fully or partially
   aware of the TE characteristics and topology of domain A.

2.  Signaling Options

   Three distinct options for signaling TE LSPs across multiple domains
   are identified.  The choice of which options to use may be influenced
   by the path computation technique used (see section 3), although some
   path computation techniques may apply to multiple signaling options.
   The choice may further depend on the application to which the TE LSPs
   are put and the nature, topology, and switching capabilities of the
   network.

   A comparison of the usages of the different signaling options is
   beyond the scope of this document and should be the subject of a
   separate applicability statement.

2.1.  LSP Nesting

   Hierarchical LSPs form a fundamental part of MPLS [RFC3031] and are
   discussed in further detail in [RFC4206].  Hierarchical LSPs may
   optionally be advertised as TE links.  Note that a hierarchical LSP
   that spans multiple domains cannot be advertised in this way because
   there is no concept of TE information that spans domains.

   Hierarchical LSPs can be used in support of inter-domain TE LSPs.  In
   particular, a hierarchical LSP may be used to achieve connectivity
   between any pair of Label Switching Routers (LSRs) within a domain.
   The ingress and egress of the hierarchical LSP could be the edge
   nodes of the domain in which case connectivity is achieved across the
   entire domain, or they could be any other pair of LSRs in the domain.

   The technique of carrying one TE LSP within another is termed LSP
   nesting.  A hierarchical LSP may provide a TE LSP tunnel to transport
   (i.e., nest) multiple TE LSPs along a common part of their paths.
   Alternatively, a TE LSP may carry (i.e., nest) a single LSP in a
   one-to-one mapping.

   The signaling trigger for the establishment of a hierarchical LSP may
   be the receipt of a signaling request for the TE LSP that it will
   carry, or may be a management action to "pre-engineer" a domain to be
   crossed by TE LSPs that would be used as hierarchical LSPs by the
   traffic that has to traverse the domain.  Furthermore, the mapping
   (inheritance rules) between attributes of the nested and the
   hierarchical LSPs (including bandwidth) may be statically pre-
   configured or, for on-demand hierarchical LSPs, may be dynamic

   according to the properties of the nested LSPs.  Even in the dynamic
   case, inheritance from the properties of the nested LSP(s) can be
   complemented by local or domain-wide policy rules.

   Note that a hierarchical LSP may be constructed to span multiple
   domains or parts of domains.  However, such an LSP cannot be
   advertised as a TE link that spans domains.  The end points of a
   hierarchical LSP are not necessarily on domain boundaries, so nesting
   is not limited to domain boundaries.

   Note also that the Interior/Exterior Gateway Protocol (IGP/EGP)
   routing topology is maintained unaffected by the LSP connectivity and
   TE links introduced by hierarchical LSPs even if they are advertised
   as TE links.  That is, the routing protocols do not exchange messages
   over the hierarchical LSPs, and LSPs are not used to create routing
   adjacencies between routers.

   During the operation of establishing a nested LSP that uses a
   hierarchical LSP, the SENDER_TEMPLATE and SESSION objects remain
   unchanged along the entire length of the nested LSP, as do all other
   objects that have end-to-end significance.

2.2.  Contiguous LSP

   A single contiguous LSP is established from ingress to egress in a
   single signaling exchange.  No further LSPs are required to be
   established to support this LSP so that hierarchical or stitched LSPs
   are not needed.

   A contiguous LSP uses the same Session/LSP ID along the whole of its
   path (that is, at each LSR).  The notions of "splicing" together
   different LSPs or of "shuffling" Session or LSP identifiers are not
   considered.

2.3.  LSP Stitching

   LSP Stitching is described in [STITCH].  In the LSP stitching model,
   separate LSPs (referred to as a TE LSP segments) are established and
   are "stitched" together in the data plane so that a single end-to-end
   Label Switched Path is achieved.  The distinction is that the
   component LSP segments are signaled as distinct TE LSPs in the
   control plane.  Each signaled TE LSP segment has a different source
   and destination.

   LSP stitching can be used in support of inter-domain TE LSPs.  In
   particular, an LSP segment may be used to achieve connectivity
   between any pair of LSRs within a domain.  The ingress and egress of
   the LSP segment could be the edge nodes of the domain in which case

   connectivity is achieved across the entire domain, or they could be
   any other pair of LSRs in the domain.

   The signaling trigger for the establishment of a TE LSP segment may
   be the establishment of the previous TE LSP segment, the receipt of a
   setup request for TE LSP that it plans to stitch to a local TE LSP
   segment, or a management action.

   LSP segments may be managed and advertised as TE links.

2.4.  Hybrid Methods

   There is nothing to prevent the mixture of signaling methods
   described above when establishing a single, end-to-end, inter-domain
   TE LSP.  It may be desirable in this case for the choice of the
   various methods to be reported along the path, perhaps through the
   Record Route Object (RRO).

   If there is a desire to restrict which methods are used, this must be
   signaled as described in the next section.

2.5.  Control of Downstream Choice of Signaling Method

   Notwithstanding the previous section, an ingress LSR may wish to
   restrict the signaling methods applied to a particular LSP at domain
   boundaries across the network.  Such control, where it is required,
   may be achieved by the definition of appropriate new flags in the
   SESSION-ATTRIBUTE object or the Attributes Flags TLV of the
   LSP_ATTRIBUTES object [RFC4420].  Before defining a mechanism to
   provide this level of control, the functional requirement to control
   the way in which the network delivers a service must be established.
   Also, due consideration must be given to the impact on
   interoperability since new mechanisms must be backwards compatible,
   and care must be taken to avoid allowing standards-conformant
   implementations that each supports a different functional subset in
   such a way that they are not capable of establishing LSPs.

3.  Path Computation Techniques

   The discussion of path computation techniques within this document is
   limited significantly to the determination of where computation may
   take place and what components of the full path may be determined.

   The techniques used are closely tied to the signaling methodologies
   described in the previous section in that certain computation
   techniques may require the use of particular signaling approaches and
   vice versa.

   Any discussion of the appropriateness of a particular path
   computation technique in any given circumstance is beyond the scope
   of this document and should be described in a separate applicability
   statement.

   Path computation algorithms are firmly out of the scope of this
   document.

3.1.  Management Configuration

   Path computation may be performed by offline tools or by a network
   planner.  The resultant path may be supplied to the ingress LSR as
   part of the TE LSP or service request, and encoded by the ingress LSR
   as an Explicit Route Object (ERO) on the Path message that is sent
   out.

   There is no reason why the path provided by the operator should not
   span multiple domains if the relevant information is available to the
   planner or the offline tool.  The definition of what information is
   needed to perform this operation and how that information is
   gathered, is outside the scope of this document.

3.2.  Head-End Computation

   The head-end, or ingress, LSR may assume responsibility for path
   computation when the operator supplies part or none of the explicit
   path.  The operator must, in any case, supply at least the
   destination address (egress) of the LSP.

3.2.1.  Multi-Domain Visibility Computation

   If the ingress has sufficient visibility of the topology and TE
   information for all of the domains across which it will route the LSP
   to its destination, then it may compute and provide the entire path.
   The quality of this path (that is, its optimality as discussed in
   section 3.5) can be better if the ingress has full visibility into
   all relevant domains rather than just sufficient visibility to
   provide some path to the destination.

   Extreme caution must be exercised in consideration of the
   distribution of the requisite TE information.  See section 4.

3.2.2.  Partial Visibility Computation

   It may be that the ingress does not have full visibility of the
   topology of all domains, but does have information about the
   connectedness of the domains and the TE resource availability across
   the domains.  In this case, the ingress is not able to provide a

   fully specified strict explicit path from ingress to egress.
   However, for example, the ingress might supply an explicit path that
   comprises:

      - explicit hops from ingress to the local domain boundary
      - loose hops representing the domain entry points across the
        network
      - a loose hop identifying the egress.

   Alternatively, the explicit path might be expressed as:

      - explicit hops from ingress to the local domain boundary
      - strict hops giving abstract nodes representing each domain in
        turn
      - a loose hop identifying the egress.

   These two explicit path formats could be mixed according to the
   information available resulting in different combinations of loose
   hops and abstract nodes.

   This form of explicit path relies on some further computation
   technique being applied at the domain boundaries.  See section 3.3.

   As with the multi-domain visibility option, extreme caution must be
   exercised in consideration of the distribution of the requisite TE
   information.  See section 4.

3.2.3.  Local Domain Visibility Computation

   A final possibility for ingress-based computation is that the ingress
   LSR has visibility only within its own domain, and connectivity
   information only as far as determining one or more domain exit points
   that may be suitable for carrying the LSP to its egress.

   In this case, the ingress builds an explicit path that comprises
   just:

      - explicit hops from ingress to the local domain boundary
      - a loose hop identifying the egress.

3.3.  Domain Boundary Computation

   If the partial explicit path methods described in sections 3.2.2 or
   3.2.3 are applied, then the LSR at each domain boundary is
   responsible for ensuring that there is sufficient path information
   added to the Path message to carry it at least to the next domain
   boundary (that is, out of the new domain).

   If the LSR at the domain boundary has full visibility to the egress
   then it can supply the entire explicit path.  Note, however, that the
   ERO processing rules of [RFC3209] state that it should only update
   the ERO as far as the next specified hop (that is, the next domain
   boundary if one was supplied in the original ERO) and, of course,
   must not insert ERO subobjects immediately before a strict hop.

   If the LSR at the domain boundary has only partial visibility (using
   the definitions of section 3.2.2), it will fill in the path as far as
   the next domain boundary, and will supply further domain/domain
   boundary information if not already present in the ERO.

   If the LSR at the domain boundary has only local visibility into the
   immediate domain, it will simply add information to the ERO to carry
   the Path message as far as the next domain boundary.

   Domain boundary path computations are performed independently from
   each other.  Domain boundary LSRs may have different computation
   capabilities, run different path computation algorithms, apply
   different sets of constraints and optimization criteria, and so
   forth, which might result in path segment quality that is
   unpredictable to and out of the control of the ingress LSR.  A
   solution to this issue lies in enhancing the information signaled
   during LSP setup to include a larger set of constraints and to
   include the paths of related LSPs (such as diverse protected LSPs) as
   described in [GMPLS-E2E].

   It is also the case that paths generated on domain boundaries may
   produce loops.  Specifically, the paths computed may loop back into a
   domain that has already been crossed by the LSP.  This may or may not
   be a problem, and might even be desirable, but could also give rise
   to real loops.  This can be avoided by using the recorded route (RRO)
   to provide exclusions within the path computation algorithm, but in
   the case of lack of trust between domains it may be necessary for the
   RRO to indicate the previously visited domains.  Even this solution
   is not available where the RRO is not available on a Path message.
   Note that when an RRO is used to provide exclusions, and a loop-free
   path is found to be not available by the computation at a downstream
   border node, crankback [CRANKBACK] may enable an upstream border node
   to select an alternate path.

3.4.  Path Computation Element

   The computation techniques in sections 3.2 and 3.3 rely on topology
   and TE information being distributed to the ingress LSR and those
   LSRs at domain boundaries.  These LSRs are responsible for computing
   paths.  Note that there may be scaling concerns with distributing the
   required information; see section 4.

   An alternative technique places the responsibility for path
   computation with a Path Computation Element (PCE) [RFC4655].  There
   may be either a centralized PCE, or multiple PCEs (each having local
   visibility and collaborating in a distributed fashion to compute an
   end-to-end path) across the entire network and even within any one
   domain.  The PCE may collect topology and TE information from the
   same sources as would be used by LSRs in the previous paragraph, or
   though other means.

   Each LSR called upon to perform path computation (and even the
   offline management tools described in section 3.1) may abdicate the
   task to a PCE of its choice.  The selection of PCE(s) may be driven
   by static configuration or the dynamic discovery.

3.4.1.  Multi-Domain Visibility Computation

   A PCE may have full visibility, perhaps through connectivity to
   multiple domains.  In this case, it is able to supply a full explicit
   path as in section 3.2.1.

3.4.2.  Path Computation Use of PCE When Preserving Confidentiality

   Note that although a centralized PCE or multiple collaborative PCEs
   may have full visibility into one or more domains, it may be
   desirable (e.g., to preserve topology confidentiality) that the full
   path not be provided to the ingress LSR.  Instead, a partial path is
   supplied (as in section 3.2.2 or 3.2.3), and the LSRs at each domain
   boundary are required to make further requests for each successive
   segment of the path.

   In this way, an end-to-end path may be computed using the full
   network capabilities, but confidentiality between domains may be
   preserved.  Optionally, the PCE(s) may compute the entire path at the
   first request and hold it in storage for subsequent requests, or it
   may recompute each leg of the path on each request or at regular
   intervals until requested by the LSRs establishing the LSP.

   It may be the case that the centralized PCE or the collaboration
   between PCEs may define a trust relationship greater than that
   normally operational between domains.

3.4.3.  Per-Domain Computation Elements

   A third way that PCEs may be used is simply to have one (or more) per
   domain.  Each LSR within a domain that wishes to derive a path across
   the domain may consult its local PCE.

   This mechanism could be used for all path computations within the
   domain, or specifically limited to computations for LSPs that will
   leave the domain where external connectivity information can then be
   restricted to just the PCE.

3.5.  Optimal Path Computation

   There are many definitions of an optimal path depending on the
   constraints applied to the path computation.  In a multi-domain
   environment, the definitions are multiplied so that an optimal route
   might be defined as the route that would be computed in the absence
   of domain boundaries.  Alternatively, another constraint might be
   applied to the path computation to reduce or limit the number of
   domains crossed by the LSP.

   It is easy to construct examples that show that partitioning a
   network into domains, and the resulting loss or aggregation of
   routing information may lead to the computation of routes that are
   other than optimal.  It is impossible to guarantee optimal routing in
   the presence of aggregation / abstraction / summarization of routing
   information.

   It is beyond the scope of this document to define what is an optimum
   path for an inter-domain TE LSP.  This debate is abdicated in favor
   of requirements documents and applicability statements for specific
   deployment scenarios.  Note, however, that the meaning of certain
   computation metrics may differ between domains (see section 5.6).

4.  Distributing Reachability and TE Information

   Traffic Engineering information is collected into a TE Database (TED)
   on which path computation algorithms operate either directly or by
   first constructing a network graph.

   The path computation techniques described in the previous section
   make certain demands upon the distribution of reachability
   information and the TE capabilities of nodes and links within domains
   as well as the TE connectivity across domains.

   Currently, TE information is distributed within domains by additions
   to IGPs [RFC3630], [RFC3784].

   In cases where two domains are interconnected by one or more links
   (that is, the domain boundary falls on a link rather than on a node),
   there should be a mechanism to distribute the TE information
   associated with the inter-domain links to the corresponding domains.
   This would facilitate better path computation and reduce TE-related
   crankbacks on these links.

   Where a domain is a subset of an IGP area, filtering of TE
   information may be applied at the domain boundary.  This filtering
   may be one way or two way.

   Where information needs to reach a PCE that spans multiple domains,
   the PCE may snoop on the IGP traffic in each domain, or play an
   active part as an IGP-capable node in each domain.  The PCE might
   also receive TED updates from a proxy within the domain.

   It is possible that an LSR that performs path computation (for
   example, an ingress LSR) obtains the topology and TE information of
   not just its own domain, but other domains as well.  This information
   may be subject to filtering applied by the advertising domain (for
   example, the information may be limited to Forwarding Adjacencies
   (FAs) across other domains, or the information may be aggregated or
   abstracted).

   Before starting work on any protocols or protocol extensions to
   enable cross-domain reachability and TE advertisement in support of
   inter-domain TE, the requirements and benefits must be clearly
   established.  This has not been done to date.  Where any cross-domain
   reachability and TE information needs to be advertised, consideration
   must be given to TE extensions to existing protocols such as BGP, and
   how the information advertised may be fed to the IGPs.  It must be
   noted that any extensions that cause a significant increase in the
   amount of processing (such as aggregation computation) at domain
   boundaries, or a significant increase in the amount of information
   flooded (such as detailed TE information) need to be treated with
   extreme caution and compared carefully with the scaling requirements
   expressed in [RFC4105] and [RFC4216].

5.  Comments on Advanced Functions

   This section provides some non-definitive comments on the constraints
   placed on advanced MPLS TE functions by inter-domain MPLS.  It does
   not attempt to state the implications of using one inter-domain
   technique or another.  Such material is deferred to appropriate
   applicability statements where statements about the capabilities of
   existing or future signaling, routing, and computation techniques to
   deliver the functions listed should be made.

5.1.  LSP Re-Optimization

   Re-optimization is the process of moving a TE LSP from one path to
   another, more preferable path (where no attempt is made in this
   document to define "preferable" as no attempt was made to define
   "optimal").  Make-before-break techniques are usually applied to
   ensure that traffic is disrupted as little as possible.  The Shared

   Explicit style is usually used to avoid double booking of network
   resources.

   Re-optimization may be available within a single domain.
   Alternatively, re-optimization may involve a change in route across
   several domains or might involve a choice of different transit
   domains.

   Re-optimization requires that all or part of the path of the LSP be
   re-computed.  The techniques used may be selected as described in
   section 3, and this will influence whether the whole or part of the
   path is re-optimized.

   The trigger for path computation and re-optimization may be an
   operator request, a timer, information about a change in availability
   of network resources, or a change in operational parameters (for
   example, bandwidth) of an LSP.  This trigger must be applied to the
   point in the network that requests re-computation and controls re-
   optimization and may require additional signaling.

   Note also that where multiple mutually-diverse paths are applied
   end-to-end (i.e., not simply within protection domains; see section
   5.5) the point of calculation for re-optimization (whether it is PCE,
   ingress, or domain entry point) needs to know all such paths before
   attempting re-optimization of any one path.  Mutual diversity here
   means that a set of computed paths has no commonality.  Such
   diversity might be link, node, Shared Risk Link Group (SRLG), or even
   domain disjointedness according to circumstances and the service
   being delivered.

   It may be the case that re-optimization is best achieved by
   recomputing the paths of multiple LSPs at once.  Indeed, this can be
   shown to be most efficient when the paths of all LSPs are known, not
   simply those LSPs that originate at a particular ingress.  While this
   problem is inherited from single domain re-optimization and is out of
   scope within this document, it should be noted that the problem grows
   in complexity when LSPs wholly within one domain affect the re-
   optimization path calculations performed in another domain.

5.2.  LSP Setup Failure

   When an inter-domain LSP setup fails in some domain other than the
   first, various options are available for reporting and retrying the
   LSP.

   In the first instance, a retry may be attempted within the domain
   that contains the failure.  That retry may be attempted by nodes
   wholly within the domain, or the failure may be referred back to the
   LSR at the domain boundary.

   If the failure cannot be bypassed within the domain where the failure
   occurred (perhaps there is no suitable alternate route, perhaps
   rerouting is not allowed by domain policy, or perhaps the Path
   message specifically bans such action), the error must be reported
   back to the previous or head-end domain.

   Subsequent repair attempts may be made by domains further upstream,
   but will only be properly effective if sufficient information about
   the failure and other failed repair attempts is also passed back
   upstream [CRANKBACK].  Note that there is a tension between this
   requirement and that of topology confidentiality although crankback
   aggregation may be applicable at domain boundaries.

   Further attempts to signal the failed LSP may apply the information
   about the failures as constraints to path computation, or may signal
   them as specific path exclusions [EXCLUDE].

   When requested by signaling, the failure may also be systematically
   reported to the head-end LSR.

5.3.  LSP Repair

   An LSP that fails after it has been established may be repaired
   dynamically by re-routing.  The behavior in this case is either like
   that for re-optimization, or for handling setup failures (see
   previous two sections).  Fast Reroute may also be used (see below).

5.4.  Fast Reroute

   MPLS Traffic Engineering Fast Reroute ([RFC4090]) defines local
   protection schemes intended to provide fast recovery (in 10s of
   msecs) of fast-reroutable packet-based TE LSPs upon link/SRLG/Node
   failure.  A backup TE LSP is configured and signaled at each hop, and
   activated upon detecting or being informed of a network element
   failure.  The node immediately upstream of the failure (called the
   PLR, or Point of Local Repair) reroutes the set of protected TE LSPs
   onto the appropriate backup tunnel(s) and around the failed resource.

   In the context of inter-domain TE, there are several different
   failure scenarios that must be analyzed.  Provision of suitable
   solutions may be further complicated by the fact that [RFC4090]
   specifies two distinct modes of operation referred to as the "one to
   one mode" and the "facility back-up mode".

   The failure scenarios specific to inter-domain TE are as follows:

   - Failure of a domain edge node that is present in both domains.
     There are two sub-cases:

     - The Point of Local Repair (PLR) and the Merge Point (MP) are in
       the same domain.

     - The PLR and the MP are in different domains.

   - Failure of a domain edge node that is only present in one of the
     domains.

   - Failure of an inter-domain link.

   Although it may be possible to apply the same techniques for Fast
   Reroute (FRR) to the different methods of signaling inter-domain LSPs
   described in section 2, the results of protection may be different
   when it is the boundary nodes that need to be protected, and when
   they are the ingress and egress of a hierarchical LSP or stitched LSP
   segment.  In particular, the choice of PLR and MP may be different,
   and the length of the protection path may be greater.  These uses of
   FRR techniques should be explained further in applicability
   statements or, in the case of a change in base behavior, in
   implementation guidelines specific to the signaling techniques.

   Note that after local repair has been performed, it may be desirable
   to re-optimize the LSP (see section 5.1).  If the point of re-
   optimization (for example, the ingress LSR) lies in a different
   domain to the failure, it may rely on the delivery of a PathErr or
   Notify message to inform it of the local repair event.

   It is important to note that Fast Reroute techniques are only
   applicable to packet switching networks because other network
   technologies cannot apply label stacking within the same switching
   type.  Segment protection [GMPLS-SEG] provides a suitable alternative
   that is applicable to packet and non-packet networks.

5.5.  Comments on Path Diversity

   Diverse paths may be required in support of load sharing and/or
   protection.  Such diverse paths may be required to be node diverse,
   link diverse, fully path diverse (that is, link and node diverse), or
   SRLG diverse.

   Diverse path computation is a classic problem familiar to all graph
   theory majors.  The problem is compounded when there are areas of
   "private knowledge" such as when domains do not share topology

   information.  The problem can be resolved more efficiently (e.g.,
   avoiding the "trap problem") when mutually resource disjoint paths
   can be computed "simultaneously" on the fullest set of information.

   That being said, various techniques (out of the scope of this
   document) exist to ensure end-to-end path diversity across multiple
   domains.

   Many network technologies utilize "protection domains" because they
   fit well with the capabilities of the technology.  As a result, many
   domains are operated as protection domains.  In this model,
   protection paths converge at domain boundaries.

   Note that the question of SRLG identification is not yet fully
   answered.  There are two classes of SRLG:

   - those that indicate resources that are all contained within one
     domain

   - those that span domains.

   The former might be identified using a combination of a globally
   scoped domain ID, and an SRLG ID that is administered by the domain.
   The latter requires a global scope to the SRLG ID.  Both schemes,
   therefore, require external administration.  The former is able to
   leverage existing domain ID administration (for example, area and AS
   numbers), but the latter would require a new administrative policy.

5.6.  Domain-Specific Constraints

   While the meaning of certain constraints, like bandwidth, can be
   assumed to be constant across different domains, other TE constraints
   (such as resource affinity, color, metric, priority, etc.) may have
   different meanings in different domains and this may impact the
   ability to support Diffserv-aware MPLS, or to manage preemption.

   In order to achieve consistent meaning and LSP establishment, this
   fact must be considered when performing constraint-based path
   computation or when signaling across domain boundaries.

   A mapping function can be derived for most constraints based on
   policy agreements between the domain administrators.  The details of
   such a mapping function are outside the scope of this document, but
   it is important to note that the default behavior must either be that
   a constant mapping is applied or that any requirement to apply these
   constraints across a domain boundary must fail in the absence of
   explicit mapping rules.

5.7.  Policy Control

   Domain boundaries are natural points for policy control.  There is
   little to add on this subject except to note that a TE LSP that
   cannot be established on a path through one domain because of a
   policy applied at the domain boundary may be satisfactorily
   established using a path that avoids the demurring domain.  In any
   case, when a TE LSP signaling attempt is rejected due to non-
   compliance with some policy constraint, this should be reflected to
   the ingress LSR.

5.8.  Inter-Domain Operations and Management (OAM)

   Some elements of OAM may be intentionally confined within a domain.
   Others (such as end-to-end liveness and connectivity testing) clearly
   need to span the entire multi-domain TE LSP.  Where issues of
   topology confidentiality are strong, collaboration between PCEs or
   domain boundary nodes might be required in order to provide end-to-
   end OAM, and a significant issue to be resolved is to ensure that the
   end-points support the various OAM capabilities.

   The different signaling mechanisms described above may need
   refinements to [RFC4379], [BFD-MPLS], etc., to gain full end-to-end
   visibility.  These protocols should, however, be considered in the
   light of topology confidentiality requirements.

   Route recording is a commonly used feature of signaling that provides
   OAM information about the path of an established LSP.  When an LSP
   traverses a domain boundary, the border node may remove or aggregate
   some of the recorded information for topology confidentiality or
   other policy reasons.

5.9.  Point-to-Multipoint

   Inter-domain point-to-multipoint (P2MP) requirements are explicitly
   out of the scope of this document.  They may be covered by other
   documents dependent on the details of MPLS TE P2MP solutions.

5.10.  Applicability to Non-Packet Technologies

   Non-packet switching technologies may present particular issues for
   inter-domain LSPs.  While packet switching networks may utilize
   control planes built on MPLS or GMPLS technology, non-packet networks
   are limited to GMPLS.

   On the other hand, some problems such as Fast Reroute on domain
   boundaries (see section 5.4) may be handled by the GMPLS technique of
   segment protection [GMPLS-SEG] that is applicable to both packet and
   non-packet switching technologies.

   The specific architectural considerations and requirements for
   inter-domain LSP setup in non-packet networks are covered in a
   separate document [GMPLS-AS].

6.  Security Considerations

   Requirements for security within domains are unchanged from [RFC3209]
   and [RFC3473], and from [RFC3630] and [RFC3784].  That is, all
   security procedures for existing protocols in the MPLS context
   continue to apply for the intra-domain cases.

   Inter-domain security may be considered as a more important and more
   sensitive issue than intra-domain security since in inter-domain
   traffic engineering control and information may be passed across
   administrative boundaries.  The most obvious and most sensitive case
   is inter-AS TE.

   All of the intra-domain security measures for the signaling and
   routing protocols are equally applicable in the inter-domain case.
   There is, however, a greater likelihood of them being applied in the
   inter-domain case.

   Security for inter-domain MPLS TE is the subject of a separate
   document that analyzes the security deployment models and risks.
   This separate document must be completed before inter-domain MPLS TE
   solution documents can be advanced.

   Similarly, the PCE procedures [RFC4655] are subject to security
   measures for the exchange computation information between PCEs and
   for LSRs that request path computations from a PCE.  The requirements
   for this security (set out in [RFC4657]) apply whether the LSR and
   PCE (or the cooperating PCEs) are in the same domain or lie across
   domain boundaries.

   It should be noted, however, that techniques used for (for example)
   authentication require coordination of secrets, keys, or passwords
   between sender and receiver.  Where sender and receiver lie within a
   single administrative domain, this process may be simple.  But where
   sender and receiver lie in different administrative domains, cross-
   domain coordination between network administrators will be required
   in order to provide adequate security.  At this stage, it is not
   proposed that this coordination be provided through an automatic
   process or through the use of a protocol.  Human-to-human

   coordination is more likely to provide the required level of
   confidence in the inter-domain security.

   One new security concept is introduced by inter-domain MPLS TE.  This
   is the preservation of confidentiality of topology information.  That
   is, one domain may wish to keep secret the way that its network is
   constructed and the availability (or otherwise) of end-to-end network
   resources.  This issue is discussed in sections 3.4.2, 5.2, and 5.8
   of this document.  When there is a requirement to preserve inter-
   domain topology confidentiality, policy filters must be applied at
   the domain boundaries to avoid distributing such information.  This
   is the responsibility of the domain that distributes information, and
   it may be adequately addressed by aggregation of information as
   described in the referenced sections.

   Applicability statements for particular combinations of signaling,
   routing, and path computation techniques to provide inter-domain MPLS
   TE solutions are expected to contain detailed security sections.

7.  Acknowledgements

   The authors would like to extend their warmest thanks to Kireeti
   Kompella for convincing them to expend effort on this document.

   Grateful thanks to Dimitri Papadimitriou, Tomohiro Otani, and Igor
   Bryskin for their review and suggestions on the text.

   Thanks to Jari Arkko, Gonzalo Camarillo, Brian Carpenter, Lisa
   Dusseault, Sam Hartman, Russ Housley, and Dan Romascanu for final
   review of the text.

8.  Normative References

   [RFC3031]     Rosen, E., Viswanathan, A., and R. Callon,
                 "Multiprotocol Label Switching Architecture", RFC 3031,
                 January 2001.

   [RFC3209]     Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                 V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                 LSP Tunnels", RFC 3209, December 2001.

   [RFC3473]     Berger, L., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Signaling Resource ReserVation
                 Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
                 3473, January 2003.

   [RFC3630]     Katz, D., Kompella, K., and D. Yeung, "Traffic
                 Engineering (TE) Extensions to OSPF Version 2", RFC
                 3630, September 2003.

   [RFC3784]     Smit, H. and T. Li, "Intermediate System to
                 Intermediate System (IS-IS) Extensions for Traffic
                 Engineering (TE)", RFC 3784, June 2004.

9.  Informative References

   [BFD-MPLS]    Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
                 "BFD For MPLS LSPs", Work in Progress, June 2006.

   [CRANKBACK]   Farrel, A., et al., "Crankback Signaling Extensions for
                 MPLS Signaling", Work in Progress, May 2005.

   [EXCLUDE]     Lee, CY., Farrel, A., and DeCnodder, "Exclude Routes -
                 Extension to RSVP-TE", Work in Progress, August 2005.

   [RFC4090]     Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
                 Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
                 2005.

   [GMPLS-AS]    Otani, T., Kumaki, K., Okamoto, S., and W. Imajuku,
                 "GMPLS Inter-domain Traffic Engineering Requirements",
                 Work in Progress, August 2006.

   [GMPLS-E2E]   Lang, J.P., Rekhter, Y., and D. Papadimitriou, Editors,
                 "RSVP-TE Extensions in support of End-to-End
                 Generalized Multi-Protocol Label Switching (GMPLS)-
                 based Recovery", Work in Progress, April 2005.

   [GMPLS-SEG]   Berger, L., Bryskin, I., Papadimitriou, D., and A.
                 Farrel, "GMPLS Based Segment Recovery", Work in
                 Progress, May 2005.

   [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.

   [RFC4105]     Le Roux, J.-L., Vasseur, J.-P., and J. Boyle,
                 "Requirements for Inter-Area MPLS Traffic Engineering",
                 RFC 4105, June 2005.

   [RFC4204]     Lang, J., "Link Management Protocol (LMP)", RFC 4204,
                 October 2005.

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

   [RFC4379]     Kompella, K. and G. Swallow, "Detecting Multi-Protocol
                 Label Switched (MPLS) Data Plane Failures", RFC 4379,
                 February 2006.

   [RFC4420]     Farrel, A., Papadimitriou, D., Vasseur, J.-P., and A.
                 Ayyangar, "Encoding of Attributes for Multiprotocol
                 Label Switching (MPLS) Label Switched Path (LSP)
                 Establishment Using Resource ReserVation Protocol-
                 Traffic Engineering (RSVP-TE)", RFC 4420, February
                 2006.

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

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

   [STITCH]      Ayyangar, A. and J.-P. Vasseur, "LSP Stitching with
                 Generalized MPLS TE", Work in Progress, September 2005.

Authors' Addresses

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

   Jean-Philippe Vasseur
   Cisco Systems, Inc
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA
   EMail: jpv@cisco.com

   Arthi Ayyangar
   Nuova Systems
   EMail: arthi@nuovasystems.com

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