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rfc:rfc4105

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

Le Roux, et al. Informational [Page 1] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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:
  1. Aggregated Traffic measurement
  2. Optimization of network resources utilization
  3. Support for services requiring end-to-end QoS guarantees
  4. 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.

Le Roux, et al. Informational [Page 2] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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.

Le Roux, et al. Informational [Page 3] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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:
  1. 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.

Le Roux, et al. Informational [Page 4] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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

Le Roux, et al. Informational [Page 5] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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-

Le Roux, et al. Informational [Page 6] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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:
  1. Perform inter-area resource optimization.
  2. Provide inter-area QoS guarantees for traffic between edge nodes

located in different areas.

  1. 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:
  1. 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.

Le Roux, et al. Informational [Page 7] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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:
  1. Inter-area resources optimization.
  2. Strict inter-area QoS guarantees.
  3. 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.

Le Roux, et al. Informational [Page 8] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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

  1. –area1——–area0——area2–
  2. —–R1-ABR1-R2——-ABR3——-

| \ | / | |

    | R0     \  | /         |      R4 |
    | R5      \ |/          |         |
     ---------ABR2----------ABR4-------
  1. ABR1, ABR2: Area0-Area1 ABRs
  2. ABR3, ABR4: Area0-Area2 ABRs
  1. R0, R1, R5: LSRs in area 1
  2. R2: an LSR in area 0
  3. 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.

Le Roux, et al. Informational [Page 9] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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.

Le Roux, et al. Informational [Page 10] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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).

Le Roux, et al. Informational [Page 11] RFC 4105 Inter-Area MPLS TE Reqs June 2005

7.5. Inter-Area MPLS-TE Path Computation

 Several methods may be used for path computation, including the
 following:
  1. 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.
  1. 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.

Le Roux, et al. Informational [Page 12] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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.

Le Roux, et al. Informational [Page 13] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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

Le Roux, et al. Informational [Page 14] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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.

Le Roux, et al. Informational [Page 15] RFC 4105 Inter-Area MPLS TE Reqs June 2005

 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.

Le Roux, et al. Informational [Page 16] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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.

Le Roux, et al. Informational [Page 17] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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

Le Roux, et al. Informational [Page 18] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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.

Le Roux, et al. Informational [Page 20] RFC 4105 Inter-Area MPLS TE Reqs June 2005

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

Le Roux, et al. Informational [Page 21] RFC 4105 Inter-Area MPLS TE Reqs June 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Le Roux, et al. Informational [Page 22]

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