    Network Working Group                                 J.-L. Le Roux, Ed.
    Request for Comments: 4105                                France Telecom
    Category: Informational                               J.-P. Vasseur, Ed.
                                                         Cisco Systems, Inc.
                                                               J. Boyle, Ed.
                                                                      PDNETs
                                                                   June 2005

             Requirements for Inter-Area MPLS 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 Internet Society (2005).

    Abstract

       This document lists a detailed set of functional requirements for the
       support of inter-area MPLS Traffic Engineering (inter-area MPLS TE).
       It is intended that solutions that specify procedures and protocol
       extensions for inter-area MPLS TE satisfy these requirements.

    Table of Contents

       1. Introduction ....................................................2
       2. Conventions Used in This Document ...............................3
       3. Terminology .....................................................3
       4. Current Intra-Area Uses of MPLS Traffic Engineering .............4
          4.1. Intra-Area MPLS Traffic Engineering Architecture ...........4
          4.2. Intra-Area MPLS Traffic Engineering Applications ...........4
               4.2.1. Intra-Area Resource Optimization ....................4
               4.2.2. Intra-Area QoS Guarantees ...........................5
               4.2.3. Fast Recovery within an IGP Area ....................5
          4.3. Intra-Area MPLS TE and Routing .............................6
       5. Problem Statement, Requirements, and Objectives of Inter-Area ...6
          5.1. Inter-Area Traffic Engineering Problem Statement ...........6
          5.2. Overview of Requirements for Inter-Area MPLS TE ............7
          5.3. Key Objectives for an Inter-Area MPLS-TE Solution ..........8
               5.3.1. Preserving the IGP Hierarchy Concept ................8
               5.3.2. Preserving Scalability ..............................8
       6. Application Scenario.............................................9

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       7. Detailed Requirements for Inter-Area MPLS TE ...................10
          7.1. Inter-Area MPLS TE Operations and Interoperability ........10
          7.2. Inter-Area TE-LSP Signaling ...............................10
          7.3. Path Optimality ...........................................11
          7.4. Inter-Area MPLS-TE Routing ................................11
          7.5. Inter-Area MPLS-TE Path Computation .......................12
          7.6. Inter-Area Crankback Routing ..............................12
          7.7. Support of Diversely-Routed Inter-Area TE LSPs ............13
          7.8. Intra/Inter-Area Path Selection Policy ....................13
          7.9. Reoptimization of Inter-Area TE LSP .......................13
          7.10. Inter-Area LSP Recovery ..................................14
                7.10.1. Rerouting of Inter-Area TE LSPs ..................14
                7.10.2. Fast Recovery of Inter-Area TE LSP ...............14
          7.11. DS-TE support ............................................15
          7.12. Hierarchical LSP Support .................................15
          7.13. Hard/Soft Preemption .....................................15
          7.14. Auto-Discovery of TE Meshes ..............................16
          7.15. Inter-Area MPLS TE Fault Management Requirements .........16
          7.16. Inter-Area MPLS TE and Routing ...........................16
       8. Evaluation criteria ............................................17
          8.1. Performances ..............................................17
          8.2. Complexity and Risks ......................................17
          8.3. Backward Compatibility ....................................17
       9. Security Considerations ........................................17
       10. Acknowledgements ..............................................17
       11. Contributing Authors ..........................................18
       12. Normative References ..........................................19
       13. Informative References ........................................19

    1.  Introduction

       The set of MPLS Traffic Engineering components, defined in [RSVP-TE],
       [OSPF-TE], and [ISIS-TE], which supports the requirements defined in
       [TE-REQ], is used today by many network operators to achieve major
       Traffic Engineering objectives defined in [TE-OVW].  These objectives
       include:

          - Aggregated Traffic measurement
          - Optimization of network resources utilization
          - Support for services requiring end-to-end QoS guarantees
          - Fast recovery against link/node/Shared Risk Link Group (SRLG)
            failures

       Furthermore, the applicability of MPLS to traffic engineering in IP
       networks is discussed in [TE-APP].

       The set of MPLS Traffic Engineering mechanisms, to date, has been
       limited to use within a single Interior Gateway Protocol (IGP) area.

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       This document discusses the requirements for an inter-area MPLS
       Traffic Engineering mechanism that may be used to achieve the same
       set of objectives across multiple IGP areas.

       Basically, it would be useful to extend MPLS TE capabilities across
       IGP areas to support inter-area resources optimization, to provide
       strict QoS guarantees between two edge routers located within
       distinct areas, and to protect inter-area traffic against Area Border
       Router (ABR) failures.

       First, this document addresses current uses of MPLS Traffic
       Engineering within a single IGP area.  Then, it discusses a set of
       functional requirements that a solution must or should satisfy in
       order to support inter-area MPLS Traffic Engineering.  Because the
       scope of requirements will vary between operators, some requirements
       will be mandatory (MUST), whereas others will be optional (SHOULD).
       Finally, a set of evaluation criteria for any solution meeting these
       requirements is given.

    2.  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 [RFC2119].

    3.  Terminology

       LSR:               Label Switching Router

       LSP:               Label Switched Path

       TE LSP:            Traffic Engineering Label Switched Path

       Inter-area TE LSP: TE LSP whose head-end LSR and tail-end LSR do not
                          reside within the same IGP area or whose head-end
                          LSR and tail-end LSR are both in the same IGP area
                          although the TE-LSP transiting path is across
                          different IGP areas.

       IGP area:          OSPF area or IS-IS level.

       ABR:               Area Border Router, a router used to connect two
                          IGP areas (ABR in OSPF, or L1/L2 router in IS-IS).

       CSPF:              Constraint-based Shortest Path First.

       SRLG:              Shared Risk Link Group.

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    4.  Current Intra-Area Uses of MPLS Traffic Engineering

       This section addresses architecture, capabilities, and uses of MPLS
       TE within a single IGP area.  It first summarizes the current MPLS-TE
       architecture, then addresses various MPLS-TE capabilities, and
       finally lists various approaches to integrate MPLS TE into routing.
       This section is intended to help define the requirements for MPLS-TE
       extensions across multiple IGP areas.

    4.1.  Intra-Area MPLS Traffic Engineering Architecture

       The MPLS-TE control plane allows establishing explicitly routed MPLS
       LSPs whose paths follow a set of TE constraints.  It is used to
       achieve major TE objectives such as resource usage optimization, QoS
       guarantee and fast failure recovery.  It consists of three main
       components:

       - The routing component, responsible for the discovery of the TE
         topology.  This is ensured thanks to extensions of link state IGP:
         [ISIS-TE], [OSPF-TE].
       - The path computation component, responsible for the placement of
         the LSP.  It is performed on the head-end LSR thanks to a CSPF
         algorithm, which takes TE topology and LSP constraints as input.
       - The signaling component, responsible for the establishment of the
         LSP (explicit routing, label distribution, and resources
         reservation) along the computed path.  This is ensured thanks to
         RSVP-TE [RSVP-TE].

    4.2.  Intra-Area MPLS Traffic Engineering Applications

    4.2.1.  Intra-Area Resource Optimization

       MPLS TE can be used within an area to redirect paths of aggregated
       flows away from over-utilized resources within a network.  In a small
       scale, this may be done by explicitly configuring a path to be used
       between two routers.  On a grander scale, a mesh of LSPs can be
       established between central points in a network.  LSPs paths can be
       defined statically in configuration or arrived at by an algorithm
       that determines the shortest path given administrative constraints
       such as bandwidth.  In this way, MPLS TE allows for greater control
       over how traffic demands are routed over a network topology and
       utilize a network's resources.

       Note also that TE LSPs allow measuring traffic matrix in a simple and
       scalable manner.  The aggregated traffic rate between two LSRs is
       easily measured by accounting of traffic sent onto a TE LSP
       provisioned between the two LSRs in question.

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    4.2.2.  Intra-Area QoS Guarantees

       The DiffServ IETF working group has defined a set of mechanisms
       described in [DIFF-ARCH], [DIFF-AF], and [DIFF-EF] or [MPLS-DIFF],
       that can be activated at the edge of or over a DiffServ domain to
       contribute to the enforcement of a QoS policy (or set of policies),
       which can be expressed in terms of maximum one-way transit delay,
       inter-packet delay variation, loss rate, etc.  Many Operators have
       some or full deployment of DiffServ implementations in their networks
       today, either across the entire network or at least at its edge.

       In situations where strict QoS bounds are required, admission control
       inside the backbone of a network is in some cases required in
       addition to current DiffServ mechanisms.  When the propagation delay
       can be bounded, the performance targets, such as maximum one-way
       transit delay, may be guaranteed by providing bandwidth guarantees
       along the DiffServ-enabled path.

       MPLS TE can be simply used with DiffServ: in that case, it only
       ensures aggregate QoS guarantees for the whole traffic.  It can also
       be more intimately combined with DiffServ to perform per-class of
       service admission control and resource reservation.  This requires
       extensions to MPLS TE called DiffServ-Aware TE, which are defined in
       [DSTE-PROTO].  DS-TE allows ensuring strict end-to-end QoS
       guarantees.  For instance, an EF DS-TE LSP may be provisioned between
       voice gateways within the same area to ensure strict QoS to VoIP
       traffic.

       MPLS TE allows computing intra-area shortest paths, which satisfy
       various constraints, including bandwidth.  For the sake of
       illustration, if the IGP metrics reflects the propagation delay, it
       allows finding a minimum propagation delay path, which satisfies
       various constraints, such as bandwidth.

    4.2.3.  Fast Recovery within an IGP Area

       As quality-sensitive applications are deployed, one of the key
       requirements is to provide fast recovery mechanisms, allowing traffic
       recovery to be guaranteed on the order of tens of msecs, in case of
       network element failure.  Note that this cannot be achieved by
       relying only on classical IGP rerouting.

       Various recovery mechanisms can be used to protect traffic carried
       onto TE LSPs.  They are defined in [MPLS-RECOV].  Protection
       mechanisms are based on the provisioning of backup LSPs that are used
       to recover traffic in case of failure of protected LSPs.  Among those
       protection mechanisms, local protection (also called Fast Reroute) is
       intended to achieve sub-50ms recovery in case of link/node/SRLG

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       failure along the LSP path [FAST-REROUTE].  Fast Reroute is currently
       used by many operators to protect sensitive traffic inside an IGP
       area.

       [FAST-REROUTE] defines two modes for backup LSPs.  The first, called
       one-to-one backup, consists of setting up one detour LSP per
       protected LSP and per element to protect.  The second, called
       facility backup, consists of setting up one or several bypass LSPs to
       protect a given facility (link or node).  In case of failure, all
       protected LSPs are nested into the bypass LSPs (benefiting from the
       MPLS label stacking property).

    4.3.  Intra-Area MPLS TE and Routing

       There are several possibilities for directing traffic into intra-area
       TE LSPs:

       1) Static routing to the LSP destination address or any other
          addresses.
       2) IGP routes beyond the LSP destination, from an IGP SPF perspective
          (IGP shortcuts).
       3) BGP routes announced by a BGP peer (or an MP-BGP peer) that is
          reachable through the TE LSP by means of a single static route to
          the corresponding BGP next-hop address (option 1) or by means of
          IGP shortcuts (option 2).  This is often called BGP recursive
          routing.
       4) The LSP can be advertised as a link into the IGP to become part of
          IGP database for all nodes, and thus can be taken into account
          during SPF for all nodes.  Note that, even if similar in concept,
          this is different from the notion of Forwarding-Adjacency, as
          defined in [LSP-HIER].  Forwarding-Adjacency is when the LSP is
          advertised as a TE-link into the IGP-TE to become part of the TE
          database and taken into account in CSPF.

    5.  Problem Statement, Requirements, and Objectives of Inter-Area
        MPLS TE

    5.1.  Inter-Area Traffic Engineering Problem Statement

       As described in Section 4, MPLS TE is deployed today by many
       operators to optimize network bandwidth usage, to provide strict QoS
       guarantees, and to ensure sub-50ms recovery in case of link/node/SRLG
       failure.

       However, MPLS-TE mechanisms are currently limited to a single IGP
       area.  The limitation comes more from the Routing and Path
       computation components than from the signaling component.  This is
       basically because the hierarchy limits topology visibility of head-

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       end LSRs to their IGP area, and consequently head-end LSRs can no
       longer run a CSPF algorithm to compute the shortest constrained path
       to the tail-end, as CSPF requires the whole topology to compute an
       end-to-end shortest constrained path.

       Several operators have multi-area networks, and many operators that
       are still using a single IGP area may have to migrate to a multi-area
       environment, as their network grows and single area scalability
       limits are approached.

       Thus, those operators may require inter-area traffic engineering to:

       - Perform inter-area resource optimization.
       - Provide inter-area QoS guarantees for traffic between edge nodes
         located in different areas.
       - Provide fast recovery across areas, to protect inter-area traffic
         in case of link or node failure, including ABR node failures.

       For instance, an operator running a multi-area IGP may have voice
       gateways located in different areas.  Such VoIP transport requires
       inter-area QoS guarantees and inter-area fast protection.

       One possible approach for inter-area traffic engineering could
       consist of deploying MPLS TE on a per-area basis, but such an
       approach has several limitations:

       - Traffic aggregation at the ABR levels implies some constraints that
         do not lead to efficient traffic engineering.  Actually, this per-
         area TE approach might lead to sub-optimal resource utilization, by
         optimizing resources independently in each area.  What many
         operators want is to optimize their resources as a whole; in other
         words, as if there was only one area (flat network).
       - This does not allow computing an inter-area constrained shortest
         path and thus does not ensure end-to-end QoS guarantees across
         areas.
       - Inter-area traffic cannot be protected with local protection
         mechanisms such as [FAST-REROUTE] in case of ABR failure.

       Therefore, existing MPLS TE mechanisms have to be enhanced to support
       inter-area TE LSPs.

    5.2.  Overview of Requirements for Inter-Area MPLS TE

       For the reasons mentioned above, it is highly desired to extend the
       current set of MPLS-TE mechanisms across multiple IGP areas in order
       to support the intra-area applications described in Section 4 across
       areas.

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       The solution MUST allow setting up inter-area TE LSPs; i.e., LSPs
       whose path crosses at least two IGP areas.

       Inter-area MPLS-TE extensions are highly desired in order to provide:

       - Inter-area resources optimization.
       - Strict inter-area QoS guarantees.
       - Fast recovery across areas, particularly to protect inter-area
         traffic against ABR failures.

       It may be desired to compute inter-area shortest paths that satisfy
       some bandwidth constraints or any other constraints, as is currently
       possible within a single IGP area.  For the sake of illustration, if
       the IGP metrics reflects the propagation delay, it may be necessary
       to be able to find the optimal (shortest) path satisfying some
       constraints (e.g., bandwidth) across multiple IGP areas.  Such a path
       would be the inter-area path offering the minimal propagation delay.

       Thus, the solution SHOULD provide the ability to compute inter-area
       shortest paths satisfying a set of constraints (i.e., bandwidth).

    5.3.  Key Objectives for an Inter-Area MPLS-TE Solution

       Any solution for inter-area MPLS TE should be designed with
       preserving IGP hierarchy concept, and preserving routing and
       signaling scalability as key objectives.

    5.3.1.  Preserving the IGP Hierarchy Concept

       The absence of a full link-state topology database makes the
       computation of an end-to-end optimal path by the head-end LSR not
       possible without further signaling and routing extensions.  There are
       several reasons that network operators choose to break up their
       network into different areas.  These often include scalability and
       containment of routing information.  The latter can help isolate most
       of a network from receiving and processing updates that are of no
       consequence to its routing decisions.  Containment of routing
       information MUST not be compromised to allow inter-area traffic
       engineering.  Information propagation for path-selection MUST
       continue to be localized.  In other words, the solution MUST entirely
       preserve the concept of IGP hierarchy.

    5.3.2.  Preserving Scalability

       Achieving the requirements listed in this document MUST be performed
       while preserving the IGP scalability, which is of the utmost
       importance.  The hierarchy preservation objective addressed in the
       above section is actually an element to preserve IGP scalability.

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       The solution also MUST not increase IGP load unreasonably, which
       could compromise IGP scalability.  In particular, a solution
       satisfying those requirements MUST not require the IGP to carry some
       unreasonable amount of extra information and MUST not unreasonably
       increase the IGP flooding frequency.

       Likewise, the solution MUST also preserve scalability of RSVP-TE
       ([RSVP-TE]).

       Additionally, the base specification of MPLS TE is architecturally
       structured and relatively devoid of excessive state propagation in
       terms of routing or signaling.  Its strength in extensibility can
       also be seen as an Achilles heel, as there is no real limit to what
       is possible with extensions.  It is paramount to maintain
       architectural vision and discretion when adapting it for use for
       inter-area MPLS TE.  Additional information carried within an area or
       propagated outside of an area (via routing or signaling) should be
       neither excessive, patchwork, nor non-relevant.

       Particularly, as mentioned in Section 5.2, it may be desired for some
       inter-area TE LSP carrying highly sensitive traffic to compute a
       shortest inter-area path, satisfying a set of constraints such as
       bandwidth.  This may require an additional routing mechanism, as base
       CSPF at head-end can no longer be used due to the lack of topology
       and resource information.  Such a routing mechanism MUST not
       compromise the scalability of the overall system.

    6.  Application Scenario

          ---area1--------area0------area2--
           ------R1-ABR1-R2-------ABR3-------
          |       \   |  /        |         |
          | R0     \  | /         |      R4 |
          | R5      \ |/          |         |
           ---------ABR2----------ABR4-------

          - ABR1, ABR2: Area0-Area1 ABRs
          - ABR3, ABR4: Area0-Area2 ABRs

          - R0, R1, R5: LSRs in area 1
          - R2: an LSR in area 0
          - R4: an LSR in area 2

       Although the terminology and examples provided in this document make
       use of the OSPF terminology, this document equally applies to IS-IS.

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       Typically, an inter-area TE LSP will be set up between R0 and R4,
       where both LSRs belong to different IGP areas.  Note that the
       solution MUST support the capability to protect such an inter-area TE
       LSP from the failure on any Link/SRLG/Node within any area and the
       failure of any traversed ABR.  For instance, if the TE LSP R0->R4
       goes through R1->ABR1->R2, then it can be protected against ABR1
       failure, thanks to a backup LSP (detour or bypass) that may follow
       the alternate path R1->ABR2->R2.

       For instance, R0 and R4 may be two voice gateways located in distinct
       areas.  An inter-area DS-TE LSP with class-type EF is set up from R1
       to R4 to route VoIP traffic classified as EF.  Per-class inter-area
       constraint-based routing allows the DS-TE LSP to be routed over a
       path that will ensure strict QoS guarantees for VoIP traffic.

       In another application, R0 and R4 may be two pseudo wire gateways
       residing in different areas.  An inter-area LSP may be set up to
       carry pseudo wires.

       In some cases, it might also be possible to have an inter-area TE LSP
       from R0 to R5 transiting via the backbone area (or any other levels
       with IS-IS).  There may be cases where there are no longer enough
       resources on any intra area path R0-to-R5, and where there is a
       feasible inter-area path through the backbone area.

    7.  Detailed Requirements for Inter-Area MPLS TE

    7.1.  Inter-Area MPLS TE Operations and Interoperability

       The inter-area MPLS TE solution MUST be consistent with requirements
       discussed in [TE-REQ], and the derived solution MUST interoperate
       seamlessly with current intra-area MPLS TE mechanisms and inherit its
       capability sets from [RSVP-TE].

       The proposed solution MUST allow provisioning at the head-end with
       end-to-end RSVP signaling (potentially with loose paths) traversing
       across the interconnected ABRs, without further provisioning required
       along the transit path.

    7.2.  Inter-Area TE-LSP Signaling

       The solution MUST allow for the signaling of inter-area TE LSPs,
       using RSVP-TE.

       In addition to the signaling of classical TE constraints (bandwidth,
       admin-groups), the proposed solution MUST allow the head-end LSR to
       specify a set of LSRs explicitly, including ABRs, by means of strict
       or loose hops for the inter-area TE LSP.

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       In addition, the proposed solution SHOULD also provide the ability to
       specify and signal certain resources to be explicitly excluded in the
       inter-area TE-LSP path establishment.

    7.3.  Path Optimality

       In the context of this requirement document, an optimal path is
       defined as the shortest 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.

       As mentioned in Section 5.2, the solution SHOULD provide the
       capability to compute an optimal path dynamically, satisfying a set
       of specified constraints (defined in [TE-REQ]) across multiple IGP
       areas.  Note that this requirement document does not mandate that all
       inter-area TE LSPs require the computation of an optimal (shortest)
       inter-area path.  Some inter-area TE-LSP paths may be computed via
       some mechanisms that do not guarantee an optimal end-to-end path,
       whereas some other inter-area TE-LSP paths carrying sensitive traffic
       could be computed by making use of mechanisms allowing an optimal
       end-to-end path to be computed dynamically.  Note that regular
       constraints such as bandwidth, affinities, IGP/TE metric
       optimization, path diversity, etc., MUST be taken into account in the
       computation of an optimal end-to-end path.

    7.4.  Inter-Area MPLS-TE Routing

       As mentioned in Section 5.3, IGP hierarchy does not allow the head-
       end LSR to compute an end-to-end optimal path.  Additional mechanisms
       are required to compute an optimal path.  These mechanisms MUST not
       alter the IGP hierarchy principles.  Particularly, in order to
       maintain containment of routing information and to preserve the
       overall IGP scalability, the solution SHOULD avoid any dynamic-TE-
       topology-related information from leaking across areas, even in a
       summarized form.

       Conversely, this does not preclude the leaking of non-topology-
       related information that is not taken into account during path
       selection, such as static TE Node information (TE router ids or TE
       node capabilities).

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    7.5.  Inter-Area MPLS-TE Path Computation

       Several methods may be used for path computation, including the
       following:

       - Per-area path computation based on ERO expansion on the head-end
         LSR and on ABRs, with two options for ABR selection:

             1) Static configuration of ABRs as loose hops at the head-end
                LSR.
             2) Dynamic ABR selection.

       - Inter-area end-to-end path computation, which may be based on (for
         instance) a recursive constraint-based searching thanks to
         collaboration between ABRs.

       Note that any path computation method may be used provided that it
       respect key objectives pointed out in Section 5.3.

       If a solution supports more than one method, it should allow the
       operator to select by configuration, and on a per-LSP basis, the
       desired option.

    7.6.  Inter-Area Crankback Routing

       Crankback routing, as defined in [CRANKBACK], may be used for inter-
       area TE LSPs.  For paths computed thanks to ERO expansions with a
       dynamic selection of downstream ABRs, crankback routing can be used
       when there is no feasible path from a selected downstream ABR to the
       destination.  The upstream ABR or head-end LSR selects another
       downstream ABR and performs ERO expansion.

       Note that this method does not allow computing an optimal path but
       just a feasible path.  Note also that there can be 0(N^2) LSP setup
       failures before finding a feasible path, where N is the average
       number of ABR between two areas.  This may have a non-negligible
       impact on the LSP setup delay.

       Crankback may also be used for inter-area LSP recovery.  If a
       link/node/SRLG failure occurs in the backbone or tail-end area, the
       ABR upstream to the failure computes an alternate path and reroutes
       the LSP locally.

       An inter-area MPLS-TE solution MAY support [CRANKBACK].  A solution
       that does, MUST allow [CRANKBACK] to be activated/deactivated via
       signaling, on a per-LSP basis.

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    7.7.  Support of Diversely-Routed Inter-Area TE LSPs

       There are several cases where the ability to compute diversely-routed
       TE-LSP paths may be desirable.  For instance, in the case of LSP
       protection, primary and backup LSPs should be diversely routed.
       Another example is the requirement to set up multiple diversely-
       routed TE LSPs between a pair of LSRs residing in different IGP
       areas.  For instance, when a single TE LSP satisfying the bandwidth
       constraint cannot be found between two end-points, a solution would
       consist of setting up multiple TE LSPs so that the sum of their
       bandwidth satisfy the bandwidth requirement.  In this case, it may be
       desirable to have these TE LSPs diversely routed in order to minimize
       the impact of a failure, on the traffic between the two end-points.

       Thus, the solution MUST be able to establish diversely-routed inter-
       area TE LSPs when diverse paths exist.  It MUST support all kinds of
       diversity (link, node, SRLG).

       The solution SHOULD allow computing an optimal placement of
       diversely-routed LSPs.  There may be various criteria to determine an
       optimal placement.  For instance, the placement of two diversely
       routed LSPs for load-balancing purposes may consist of minimizing
       their cumulative cost.  The placement of two diversely-routed LSPs
       for protection purposes may consist of minimizing the cost of the
       primary LSP while bounding the cost or hop count of the backup LSP.

    7.8.  Intra/Inter-Area Path Selection Policy

       For inter-area TE LSPs whose head-end and tail-end LSRs reside in the
       same IGP area, there may be intra-area and inter-area feasible paths.
       If the shortest path is an inter-area path, an operator either may
       want to avoid, as far as possible, crossing area 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.  Thus, the solution
       should allow IGP area crossing to be enabled/disabled, on a per-LSP
       basis, for TE LSPs whose head-end and tail-end reside in the same IGP
       area.

    7.9.  Reoptimization of Inter-Area TE LSP

       The solution MUST provide the ability to reoptimize in a minimally
       disruptive manner (make before break) an inter-area TE LSP, should a
       more optimal path appear in any traversed IGP area.  The operator
       should be able to parameterize such a reoptimization according to a
       timer or event-driven basis.  It should also be possible to trigger
       such a reoptimization manually.

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       The solution SHOULD provide the ability to reoptimize an inter-area
       TE LSP locally within an area; i.e., while retaining the same set of
       transit ABRs.  The reoptimization process in that case MAY be
       controlled by the head-end LSR of the inter-area LSP, or by an ABR.
       The ABR should check for local optimality of the inter-area TE LSPs
       established through it on a timer or event driven basis.  The option
       of a manual trigger to check for optimality should also be provided.

       In some cases it is important to restrict the control of
       reoptimization to the Head-End LSR only.  Thus, the solution MUST
       allow for activating/deactivating ABR control of reoptimization, via
       signaling on a per LSP-basis.

       The solution SHOULD also provide the ability to perform an end-to-end
       reoptimization, potentially resulting in a change on the set of
       transit ABRs.  Such reoptimization can only be controlled by the
       Head-End LSR.

       In the case of head-end control of reoptimization, the solution
       SHOULD provide the ability for the inter-area head-end LSR to be
       informed of the existence of a more optimal path in a downstream area
       and keep a strict control over the reoptimization process.  Thus, the
       inter-area head-end LSR, once informed of a more optimal path in some
       downstream IGP areas, could decide to perform a make-before-break
       reoptimization gracefully (or not to), according to the inter-area
       TE-LSP characteristics.

    7.10.  Inter-Area LSP Recovery

    7.10.1.  Rerouting of Inter-Area TE LSPs

       The solution MUST support rerouting of an inter-area TE LSP in case
       of SRLG/link/node failure or preemption.  Such rerouting may be
       controlled by the Head-End LSR or by an ABR (see Section 7.6, on
       crankback).

    7.10.2.  Fast Recovery of Inter-Area TE LSP

       The solution MUST provide the ability to benefit from fast recovery,
       making use of the local protection techniques specified in
       [FAST-REROUTE] both in the case of an intra-area network element
       failure (link/SRLG/node) and in that of an ABR node failure.  Note
       that different protection techniques SHOULD be usable in different
       parts of the network to protect an inter-area TE LSP.  This is of the
       utmost importance, particularly in the case of an ABR node failure,
       as this node typically carries a great deal of inter-area traffic.
       Moreover, the solution SHOULD allow computing and setting up a backup
       tunnel following an optimal path that offers bandwidth guarantees

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       during failure, along with other potential constraints (such as
       bounded propagation delay increase along the backup path).

       The solution SHOULD allow ABRs to be protected, while providing the
       same level of performances (recovery delay, bandwidth consumption) as
       provided today within an area.

       Note that some signaling approaches may have an impact on FRR
       performances (recovery delay, bandwidth consumption).  Typically,
       when some intra-area LSPs (LSP-Segment, FA-LSPs) are used to support
       the inter-area TE LSP, the protection of ABR using [FAST-REROUTE] may
       lead to higher bandwidth consumption and higher recovery delays.  The
       use of [FAST-REROUTE] to protect ABRs, although ensuring the same
       level of performances, currently requires a single end-to-end RSVP
       session (contiguous LSP) to be used, without any intra-area LSP.
       Thus, the solution MUST provide the ability, via signalling on a
       per-LSP basis, to allow or preclude the use of intra-area LSPs to
       support the inter-area LSPs.

    7.11.  DS-TE support

       The proposed inter-area MPLS TE solution SHOULD also satisfy core
       requirements documented in [DSTE-REQ] and interoperate seamlessly
       with current intra-area MPLS DS-TE mechanism [DSTE-PROTO].

    7.12.  Hierarchical LSP Support

       In the case of a large inter-area MPLS deployment, potentially
       involving a large number of LSRs, it may be desirable/necessary to
       introduce some level of hierarchy in order to reduce the number of
       states on LSRs (such a solution implies other challenges).  Thus, the
       proposed solution SHOULD allow inter-area TE-LSP aggregation (also
       referred to as LSP nesting) so that individual TE LSPs can be carried
       onto one or more aggregating LSPs.  One such mechanism, for example,
       is described in [LSP-HIER].

    7.13.  Hard/Soft Preemption

       As defined in [MPLS-PREEMPT], two preemption models are applicable to
       MPLS: Soft and Hard Preemption.

       An inter-area MPLS-TE solution SHOULD support the two models.

       In the case of hard preemption, the preempted inter-area TE LSP
       should be rerouted, following requirements defined in Section 7.10.1.

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       In the case of soft preemption, the preempted inter-area TE LSP
       should be re-optimized, following requirements defined in Section
       7.9.

    7.14.  Auto-Discovery of TE Meshes

       A TE mesh is a set of LSRs that are fully interconnected by a full
       mesh of TE LSPs.  Because the number of LSRs participating in some TE
       mesh might be quite large, it might be desirable to provide some
       discovery mechanisms allowing an LSR to discover automatically the
       LSRs members of the TE mesh(es) that it belongs to.  The discovery
       mechanism SHOULD be applicable across multiple IGP areas, and SHOULD
       not impact the IGP scalability, provided that IGP extensions are used
       for such a discovery mechanism.

    7.15.  Inter-Area MPLS TE Fault Management Requirements

       The proposed solution SHOULD be able to interoperate with fault
       detection mechanisms of intra-area MPLS TE.

       The solution SHOULD support [LSP-PING] and [MPLS-TTL].

       The solution SHOULD also support fault detection on backup LSPs, in
       case [FAST-REROUTE] is deployed.

    7.16.  Inter-Area MPLS TE and Routing

       In the case of intra-area MPLS TE, there are currently several
       possibilities for routing traffic into an intra-area TE LSP.  They
       are listed in Section 4.2.

       In the case of inter-area MPLS TE, the solution MUST support static
       routing into the LSP, and also BGP recursive routing with a static
       route to the BGP next-hop address.

       ABRs propagate IP reachability information (summary LSA in OSPF and
       IP reachability TLV in ISIS), that MAY be used by the head-end LSR to
       route traffic to a destination beyond the TE-LSP tail-head LSR (e.g.,
       to an ASBR).

       The use of IGP shortcuts MUST be precluded when TE-LSP head-end and
       tail-end LSRs do not reside in the same IGP area.  It MAY be used
       when they reside in the same area.

       The advertisement of an inter-area TE LSP as a link into the IGP, in
       order to attract traffic to an LSP source, MUST be precluded when
       TE-LSP head-end and tail-end LSRs do not reside in the same IGP area.
       It MAY be used when they reside in the same area.

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    8.  Evaluation criteria

    8.1.  Performances

       The solution will be evaluated with respect to the following
       criteria:

       (1) Optimality of the computed inter-area TE-LSP primary and backup
           paths, in terms of path cost.
       (2) Capability to share bandwidth among inter-area backup LSPs
           protecting independent facilities.
       (3) Inter-area TE-LSP setup time (in msec).
       (4) RSVP-TE and IGP scalability (state impact, number of messages,
           message size).

    8.2.  Complexity and Risks

       The proposed solution SHOULD not introduce complexity to the current
       operating network to such a degree that it would affect the stability
       and diminish the benefits of deploying such a solution over SP
       networks.

    8.3.  Backward Compatibility

       In order to allow for a smooth migration or co-existence, the
       deployment of inter-area MPLS TE SHOULD not affect existing MPLS TE
       mechanisms.  In particular, the solution SHOULD allow the setup of an
       inter-area TE LSP among transit LSRs that do not support inter-area
       extensions, provided that these LSRs do not participate in the
       inter-area TE procedure.  For illustration purposes, the solution MAY
       require inter-area extensions only on end-point LSRs, on ABRs, and,
       potentially, on Points of Local Repair (PLR) protecting an ABR.

    9.  Security Considerations

       This document does not introduce new security issues beyond those
       inherent in MPLS TE [RSVP-TE] and an inter-area MPLS-TE solution may
       use the same mechanisms proposed for that technology.  It is,
       however, specifically important that manipulation of administratively
       configurable parameters be executed in a secure manner by authorized
       entities.

    10.  Acknowledgements

       We would like to thank Dimitri Papadimitriou, Adrian Farrel, Vishal
       Sharma, and Arthi Ayyangar for their useful comments and suggestions.

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    11.  Contributing Authors

       This document was the collective work of several authors.  The text
       and content of this document was contributed by the editors and the
       co-authors listed below (the contact information for the editors
       appears in Section 14 and is not repeated below):

       Ting-Wo Chung                         Yuichi Ikejiri
       Bell Canada                           NTT Communications Corporation
       181 Bay Street, Suite 350,            1-1-6, Uchisaiwai-cho,
       Toronto,                              Chiyoda-ku, Tokyo 100-8019
       Ontario, Canada, M5J 2T3              JAPAN

       EMail: ting_wo.chung@bell.ca          EMail: y.ikejiri@ntt.com

       Raymond Zhang                         Parantap Lahiri
       Infonet Services Corporation          MCI
       2160 E. Grand Ave.                    22001 Loudoun Cty Pky
       El Segundo, CA 90025                  Ashburn, VA 20147
       USA                                   USA

       EMail: raymond_zhang@infonet.com      EMail: parantap.lahiri@mci.com

       Kenji Kumaki
       KDDI Corporation
       Garden Air Tower
       Iidabashi, Chiyoda-ku,
       Tokyo 102-8460,
       JAPAN

       EMail: ke-kumaki@kddi.com

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    12.  Normative References

       [RFC2119]      Bradner, S., "Key words for use in RFCs to indicate
                      requirements levels", RFC 2119, March 1997.

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

       [DSTE-REQ]     Le Faucheur, F. and W. Lai, "Requirements for Support
                      of Differentiated Services-aware MPLS Traffic
                      Engineering", RFC 3564, July 2003.

    13.  Informative References

       [TE-OVW]       Awduche, D., Chiu, A., Elwalid, A., Widjaja, I., and
                      X. Xiao, "Overview and Principles of Internet Traffic
                      Engineering", RFC 3272, May 2002.

       [RSVP-TE]      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.

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

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

       [TE-APP]       Boyle, J., Gill, V., Hannan, A., Cooper, D., Awduche,
                      D., Christian, B., and W. Lai, "Applicability
                      Statement for Traffic Engineering with MPLS", RFC
                      3346, August 2002.

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

       [LSP-PING]     Kompella, K., Pan, P., Sheth, N., Cooper, D., Swallow,
                      G., Wadhwa, S., Bonica, R., "Detecting Data Plane
                      Liveliness in MPLS", Work in Progress.

       [MPLS-TTL]     Agarwal, P. and B. Akyol, "Time To Live (TTL)
                      Processing in Multi-Protocol Label Switching (MPLS)
                      Networks", RFC 3443, January 2003.

    Le Roux, et al.              Informational                     [Page 19]

    RFC 4105                Inter-Area MPLS TE Reqs                June 2005

       [LSP-HIER]     Kompella, K., and Y. Rekhter, "LSP Hierarchy with
                      Generalized MPLS TE", Work in Progress.

       [MPLS-RECOV]   Sharma, V. and F. Hellstrand, "Framework for Multi-
                      Protocol Label Switching (MPLS)-based Recovery", RFC
                      3469, February 2003.

       [CRANKBACK]    Farrel, A., Ed., "Crankback Signaling Extensions for
                      MPLS Signaling", Work in Progress.

       [MPLS-DIFF]    Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
                      Vaananen, P., Krishnan, R., Cheval, P., and J.
                      Heinanen, "Multi-Protocol Label Switching (MPLS)
                      Support of Differentiated Services", RFC 3270, May
                      2002.

       [DSTE-PROTO]   Le Faucheur, F., et al., "Protocol Extensions for
                      Support of Differentiated-Service-aware MPLS Traffic
                      Engineering",  Work in Progress.

       [DIFF-ARCH]    Blake, S., Black, D., Carlson, M., Davies, E., Wang,
                      Z., and W. Weiss, "An Architecture for Differentiated
                      Service", RFC 2475, December 1998.

       [DIFF-AF]      Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
                      "Assured Forwarding PHB Group", RFC 2597, June 1999.

       [DIFF-EF]      Davie, B., Charny, A., Bennet, J.C., Benson, K., Le
                      Boudec, J., Courtney, W., Davari, S., Firoiu, V., and
                      D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop
                      Behavior)", RFC 3246, March 2002.

       [MPLS-PREEMPT] Farrel, A., "Interim Report on MPLS Pre-emption", Work
                      in Progress.

       [METRIC]       Le Faucheur, F., Uppili, R., Vedrenne, A., Merckx, P.,
                      and T. Telkamp, "Use of Interior Gateway Protocol
                      (IGP) Metric as a second MPLS Traffic Engineering (TE)
                      Metric", BCP 87, RFC 3785, May 2004.

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    14.  Editors' Addresses

       Jean-Louis Le Roux
       France Telecom
       2, avenue Pierre-Marzin
       22307 Lannion Cedex
       France

       EMail: jeanlouis.leroux@francetelecom.com

       Jean-Philippe Vasseur
       Cisco Systems, Inc.
       300 Beaver Brook Road
       Boxborough, MA - 01719
       USA

       EMail: jpv@cisco.com

       Jim Boyle

       EMail: jboyle@pdnets.com

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    Full Copyright Statement

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    Acknowledgement

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