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Versions: (draft-vasseur-ccamp-inter-domain-pd-path-comp) 00 01 02 03 04 05 06 RFC 5152

Networking Working Group                                JP. Vasseur, Ed.
Internet-Draft                                        Cisco Systems, Inc
Intended status: Standards Track                        A. Ayyangar, Ed.
Expires: October 2007                                      Nuova Systems
                                                                R. Zhang
                                                              BT Infonet
                                                              April 2007

   A Per-domain path computation method for establishing Inter-domain
          Traffic Engineering (TE) Label Switched Paths (LSPs)

             draft-ietf-ccamp-inter-domain-pd-path-comp-05

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Abstract

   This document specifies a per-domain path computation technique for
   establishing inter-domain Traffic Engineering (TE) Multiprotocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) Label Switched
   Paths (LSPs).  In this document a domain refers to a collection of
   network elements within a common sphere of address management or path
   computational responsibility such as IGP areas and Autonomous
   Systems.

   Per-domain computation applies where the full path of an inter-domain
   TE LSP cannot be or is not determined at the ingress node of the TE
   LSP, and is not signaled across domain boundaries.  This is most
   likely to arise owing to TE visibility limitations.  The signaling
   message indicates the destination and nodes up to the next domain
   boundary.  It may also indicate further domain boundaries or domain

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   identifiers.  The path through each domain, possibly including the
   choice of exit point from the domain, must be determined within
   the domain.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].









































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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  General assumptions  . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Common assumptions . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Example of topology for the inter-area TE case . . . . . .  7
     3.3.  Example of topology for the inter-AS TE case . . . . . . .  8
   4.  Per-domain path computation procedures . . . . . . . . . . . .  9
     4.1.  Example with an inter-area TE LSP  . . . . . . . . . . . . 12
       4.1.1.  Case 1: T0 is a contiguous TE LSP  . . . . . . . . . . 12
       4.1.2.  Case 2: T0 is a stitched or nested TE LSP  . . . . . . 13
     4.2.  Example with an inter-AS TE LSP  . . . . . . . . . . . . . 14
       4.2.1.  Case 1: T1 is a contiguous TE LSP  . . . . . . . . . . 14
       4.2.2.  Case 2: T1 is a stitched or nested TE LSP  . . . . . . 15
   5.  Path optimality/diversity  . . . . . . . . . . . . . . . . . . 15
   6.  Reoptimization of an inter-domain TE LSP . . . . . . . . . . . 15
     6.1.  Contiguous TE LSPs . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Stitched or nested (non-contiguous) TE LSPs  . . . . . . . 16
     6.3.  Path characteristics after reoptimization  . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Copyright Statements . . . . . . . . . . 21






















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1.  Terminology

   Terminology used in this document

   AS: Autonomous System.

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

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

   Boundary LSR: a boundary LSR is either an ABR in the context of
   inter-area TE or an ASBR in the context of inter-AS TE.

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

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

   LSR: Label Switching Router.

   LSP: Label Switched Path.

   TE LSP: Traffic Engineering Label Switched Path.

   PCE: Path Computation Element: an entity (component, application or
   network node) that is capable of computing a network path or route
   based on a network graph and applying computational constraints.

   TED: Traffic Engineering Database.

   The notion of contiguous, stitched and nested TE LSPs is defined in
   [RFC4726] and will not be repeated here.


2.  Introduction

   The requirements for inter-domain Traffic Engineering (inter-area and
   inter-AS TE) have been developed by the Traffic Engineering Working
   Group and have been stated in [RFC4105] and [RFC4216].  The framework
   for inter-domain MPLS Traffic Engineering has been provided in
   [RFC4726].

   Some of the mechanisms used to establish and maintain inter-domain TE
   LSPs are specified in [I-D.ietf-ccamp-inter-domain-rsvp-te] and
   [I-D.ietf-ccamp-lsp-stitching].

   This document exclusively focuses on the path computation aspects and


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   defines a method for establishing inter-domain TE LSP where each node
   in charge of computing a section of an inter-domain TE LSP path is
   always along the path of such TE LSP.

   When the visibility of an end to end complete path spanning multiple
   domains is not available at the Head-end LSR, one approach described
   in this document consists of using a per-domain path computation
   technique during LSP setup to determine the inter-domain TE LSP as it
   traverses multiple domains.

   The mechanisms proposed in this document are also applicable to MPLS
   TE domains other than IGP areas and ASs.

   The solution described in this document does not attempt to address
   all the requirements specified in [RFC4105] and [RFC4216].  This is
   acceptable according to [RFC4216] which indicates that a solution may
   be developed to address a particular deployment scenario and might,
   therefore, not meet all requirements for other deployment scenarios.

   It must be pointed out that the inter-domain path computation
   technique proposed in this document is one among many others and the
   choice of the appropriate technique must be driven by the set of
   requirements for the paths attributes and the applicability to a
   particular technique with respect to the deployment scenario.  For
   example, if the requirement is to get an end-to-end constraint-based
   shortest path across multiple domains, then a mechanism using one or
   more distributed PCEs could be used to compute the shortest path
   across different domains (see [RFC4655]).  Other offline mechanisms
   for path computation are not precluded either.  Note also that a
   Service Provider may elect to use different inter-domain path
   computation techniques for different TE LSP types.


3.  General assumptions

3.1.  Common assumptions

   - Each domain in all the examples below is assumed to be capable of
     doing Traffic Engineering (i.e. running OSPF-TE or ISIS-TE and
     RSVP-TE).  A domain may itself comprise multiple other domains.
     E.g., An AS may itself be composed of several other sub-AS(es) (BGP
     confederations) or areas/levels.  In this case, the path
     computation technique described for inter-area and inter-AS MPLS
     Traffic Engineering just recursively applies.

   - The inter-domain TE LSPs are signaled using RSVP-TE ([RFC3209] and
     [RFC3473]).



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   - The path (ERO) for an inter-domain TE LSP may be signaled as a set
     of (loose and/or strict) hops.

   - The hops may identify:

     * The complete strict path end-to-end across different domains

     * The complete strict path in the source domain followed by
       boundary LSRs (or domain identifiers, e.g.  AS numbers)

     * The complete list of boundary LSRs along the path

     * The current boundary LSR and the LSP destination.

     The set of (loose or strict) hops can either be statically
     configured on the Head-end LSR or dynamically computed.  A
     per-domain path computation method is defined in this document with
     an optional Auto-discovery mechanism based on IGP, BGP, policy
     routing information yielding the next-hop boundary node (domain
     exit point, such as ABR/ASBR) along the path as the TE LSP is being
     signaled, along with potential crankback mechanisms.  Alternatively
     the domain exit points may be statically configured on the Head-end
     LSR in which case next-hop boundary node auto-discovery would not
     be required.

   - Boundary LSRs are assumed to be capable of performing local path
     computation for expansion of a loose next-hop in the signaled ERO
     if the path is not signaled by the Head-end LSR as a set of strict
     hops or if the strict hop is an abstract node (e.g. an AS).  In any
     case, no topology or resource information needs to be distributed
     between domains (as mandated per [RFC4105] and [RFC4216]), which is
     critical to preserve IGP/BGP scalability and confidentiality in the
     case of TE LSPs spanning multiple routing domains.

   - The paths for the intra-domain Hierarchical LSPs (H-LSP) or
     Stitched LSPs (S-LSP) or for a contiguous TE LSP within the domain
     may be pre-configured or computed dynamically based on the arriving
     inter-domain LSP setup request (depending on the requirements of
     the transit domain).  Note that this capability is explicitly
     specified as a requirement in [RFC4216].  When the paths for the
     H-LSPs/S-LSP are pre-configured, the constraints as well as other
     parameters like local protection scheme for the intra-domain
     H-LSP/S-LSP are also pre-configured.

   - While certain constraints like bandwidth can be used across
     different domains, certain other TE constraints like resource
     affinity, color, metric, etc. as listed in [RFC2702] may need to be
     translated at domain boundaries.  If required, it is assumed that,
     at the domain boundary LSRs, there will exist some sort of local

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     mapping based on policy agreement in order to translate such
     constraints across domain boundaries.  It is expected that such an
     assumption particularly applies to inter-AS TE: for example, the
     local mapping would be similar to the Inter-AS TE Agreement
     Enforcement Polices stated in [RFC4216].

   - The procedures defined in this document are applicable to any node
     (not just boundary node) that receives a Path message with an ERO
     that constains a loose hop or an abstract node that is not a simple
     abstract node (that is, an abstract node that identifies more than
     one LSR).

3.2.  Example of topology for the inter-area TE case

   The following example will be used for the inter-area TE case in this
   document.

    <-area 1-><-- area 0 --><--- area 2 --->
    ------ABR1------------ABR3-------
    |    /   |              |  \     |
   R0--X1    |              |   X2---X3--R1
    |        |              |  /     |
    ------ABR2-----------ABR4--------
   <=========== Inter-area TE LSP =======>

      Figure 1 - Example of topology for the inter-area TE case

   Description of Figure 1:

   - ABR1, ABR2, ABR3 and ABR4 are ABRs,
   - X1: an LSR in area 1,
   - X2, X3: LSRs in area 2,
   - An inter-area TE LSP T0 originated at R0 in area 1 and terminating
     at R1 in area 2.

   Notes:

   - The terminology used in the example above corresponds to OSPF but
     the path computation technique proposed in this document equally
     applies to the case of an IS-IS multi-level network.

   - Just a few routers in each area are depicted in the diagram above
     for the sake of simplicity.

   - The example depicted in Figure 1 shows the case where the Head-end
     and Tail-end areas are connected by means of area 0.  The case of
     an inter-area TE LSP between two IGP areas that does not transit
     through area 0 is not precluded.


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3.3.  Example of topology for the inter-AS TE case

   We consider the following general case, built on a superset of the
   various scenarios defined in [RFC4216]:


         <-- AS 1 ---> <------- AS 2 -----><--- AS 3 ---->

                  <---BGP--->            <---BGP-->
   CE1---R0---X1-ASBR1-----ASBR4--R3---ASBR7----ASBR9----R6
         |\     \ |       / |   / |   / |          |      |
         | \     ASBR2---/ ASBR5  | --  |          |      |
         |  \     |         |     |/    |          |      |
         R1-R2---ASBR3-----ASBR6--R4---ASBR8----ASBR10---R7---CE2

         <======= Inter-AS TE LSP(LSR to LSR)===========>

   or

   <======== Inter-AS TE LSP (CE to ASBR =>

   or

   <================= Inter-AS TE LSP (CE to CE)===============>

          Figure 2 - Example of topology for the inter-AS TE case

   The diagram depicted in Figure 2 covers all the inter-AS TE
   deployment cases described in [RFC4216].

   Description of Figure 2:

   - Three interconnected ASs, respectively AS1, AS2, and AS3.  Note
     that in some scenarios described in [RFC4216] AS1=AS3.

   - The ASBRs in different ASs are BGP peers.  There is usually no IGP
     running on the single hop links interconnecting the ASBRs and also
     referred to as inter-ASBR links.

   - Each AS runs an IGP (IS-IS or OSPF) with the required IGP TE
     extensions (see [RFC3630], [RFC3784], [RFC4203] and [RFC4205]).  In
     other words, the ASes are TE enabled,

   - Each AS can be made of several IGP areas.  The path computation
     technique described in this document applies to the case of a
     single AS made of multiple IGP areas, multiples ASs made of a
     single IGP areas or any combination of the above.  For the sake of
     simplicity, each routing domain will be considered as single area
     in this document.  The case of an Inter-AS TE LSP spanning multiple

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     ASs where some of those ASs are themselves made of multiple IGP
     areas can be easily derived from the examples above: the per-domain
     path computation technique described in this document is applied
     recursively in this case.

   - An inter-AS TE LSP T1 originated at R0 in AS1 and terminating at R6
     in AS3.


4.  Per-domain path computation procedures

   The mechanisms for inter-domain TE LSP computation as described in
   this document can be used regardless of the nature of the inter-
   domain TE LSP (contiguous, stitched or nested).

   Note that any path can be defined as a set of loose and strict hops.
   In other words, in some cases, it might be desirable to rely on the
   dynamic path computation in some domain, and exert a strict control
   on the path in other domains (defining strict hops).

   When an LSR that is a boundary node such as an ABR/ASBR receives a
   Path message with an ERO that contains a loose hop or an abstract
   node that is not a simple abstract node (that is, an abstract node
   that identifies more than one LSR), then it MUST follow the
   procedures as described in [I-D.ietf-ccamp-inter-domain-rsvp-te].  In
   addition, the following procedures describe the path computation
   procedures that SHOULD be carried out on the LSR:

   1) If the next hop is not present in the TED.

   If the loose next-hop is not present in the TED, the following
   conditions MUST be checked:

   o  If the IP address of the next hop boundary LSR is outside of the
      current domain,

   o  If the domain is PSC (Packet Switch Capable) and uses in-band
      control channel

   If the two conditions above are satisfied then the boundary LSR
   SHOULD check if the next-hop has IP reachability (via IGP or BGP).
   If the next-hop is not reachable, then a signaling failure occurs and
   the LSR SHOULD send back an RSVP PathErr message upstream with error
   code=24 ("Routing Problem") and error subcode as described in section
   4.3.4 of [RFC3209].  If the next-hop is reachable, then it SHOULD
   find a domain boundary LSR (domain boundary point) to get to the
   next-hop.  The determination of domain boundary point based on
   routing information is what we term as "auto-discovery" in this


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   document.  In the absence of such an auto-discovery mechanism, a) the
   ABR in the case of inter-area TE or the ASBR in the next-hop AS in
   the case of inter-AS TE should be the signaled loose next-hop in the
   ERO and hence should be accessible via the TED or b) there needs to
   be an alternate scheme that provides the domain exit points.
   Otherwise the path computation for the inter-domain TE LSP will fail.

   An implementation MAY support the ability to disable such IP
   reachability fall-back option should the next hop boundary LSR not be
   present in the TED.  In other words, an implementation MAY support
   the possibility to trigger a signaling failure whenever the next-hop
   is not present in the TED.

   2) Once the next-hop boundary LSR has been determined (according to
   the procedure described in 1)) or if the next-hop boundary is present
   in the TED

   o  Case of a contiguous TE LSP.  The boundary LSR that processes the
      ERO SHOULD perform an ERO expansion (unless not allowed by policy)
      after having computed the path to the next loose hop (ABR/ASBR)
      that obeys the set of required constraints.  If no path satisfying
      the set of constraints can be found, then this SHOULD be treated
      as a path computation and signaling failure and an RSVP PathErr
      message SHOULD be sent for the inter-domain TE LSP based on
      section 4.3.4 of [RFC3209].

   o  Case of stitched or nested LSP

      *  If the boundary LSR is a candidate LSR for intra-area H-LSP/
         S-LSP setup (the LSR has local policy for nesting or
         stitching), the TE LSP is a candidate for hierarchy/nesting
         (the "Contiguous LSP" bit defined in
         [I-D.ietf-ccamp-inter-domain-rsvp-te] is not set) and if there
         is no H-LSP/S-LSP from this LSR to the next-hop boundary LSR
         that satisfies the constraints, it SHOULD signal a H-LSP/S-LSP
         to the next-hop boundary LSR.  If pre-configured H-LSP(s) or
         S-LSP(s) already exist, then it will try to select from among
         those intra-domain LSPs.  Depending on local policy, it MAY
         signal a new H-LSP/S-LSP if this selection fails.  If the
         H-LSP/S-LSP is successfully signaled or selected, it propagates
         the inter-domain Path message to the next-hop following the
         procedures described in [I-D.ietf-ccamp-inter-domain-rsvp-te].
         If, for some reason the dynamic H-LSP/S-LSP setup to the next-
         hop boundary LSR fails, then this SHOULD be treated as a path
         computation and signaling failure and an RSVP PathErr message
         SHOULD be sent upstream for the inter-domain LSP.  Similarly,
         if selection of a preconfigured H-LSP/S-LSP fails and local
         policy prevents dynamic H-LSP/S this SHOULD be treated as a


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         path computation and signaling failure and an RSVP PathErr
         SHOULD be sent upstream for the inter-domain TE LSP.  In both
         these cases procedures described in section 4.3.4 of [RFC3209]
         SHOULD be followed to handle the failure.

      *  If, however, the boundary LSR is not a candidate for intra-
         domain H-LSP/S-LSP (the LSR does not have local policy for
         nesting or stitching) or the TE LSP is a not candidate for
         hierarchy/nesting (the "Contiguous LSP" bit defined in
         [I-D.ietf-ccamp-inter-domain-rsvp-te] is set), then it SHOULD
         apply the same procedure as for the contiguous case.

   Note that in both cases, path computation and signaling process may
   be stopped due to policy violation.

   The ERO of an inter-domain TE LSP may comprise abstract nodes such as
   ASes.  In such a case, upon receiving the ERO whose next hop is an
   AS, the boundary LSR has to determine the next-hop boundary LSR which
   may be determined based on the "auto-discovery" process mentioned
   above.  If multiple ASBRs candidates exist the boundary LSR may apply
   some policies based on peering contracts that may have been pre-
   negotiated.  Once the next-hop boundary LSR has been determined a
   similar procedure as the one described above is followed.

   Note related to the inter-AS TE case:

     In terms of computation of an inter-AS TE LSP path, an interesting
     optimization technique consists of allowing the ASBRs to flood the
     TE information related to the inter-ASBR link(s) although no IGP TE
     is enabled over those links (and so there is no IGP adjacency over
     the inter-ASBR links).  This of course implies for the inter-ASBR
     links to be TE-enabled although no IGP is running on those links.
     This allows an LSR (could be entry ASBR) in the previous AS to make
     a more appropriate route selection up to the entry ASBR in the
     immediately downstream AS taking into account the constraints
     associated with the inter-ASBR links.  This reduces the risk of
     call set up failure due to inter-ASBR links not satisfying the
     inter-AS TE LSP set of constraints.  Note that the TE information
     is only related to the inter-ASBR links: the TE LSA/LSP flooded by
     the ASBR includes not only the TE-enabled links contained in the AS
     but also the inter-ASBR links.

   Note that no summarized TE information is leaked between ASes which
   is compliant with the requirements listed in [RFC4105] and [RFC4216].

   For example, consider the diagram depicted in Figure 2: when ASBR1
   floods its IGP TE LSA ((opaque LSA for OSPF)/LSP (TLV 22 for IS-IS))
   in its routing domain, it reflects the reservation states and TE


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   properties of the following links: X1-ASBR1, ASBR1-ASBR2 and ASBR1-
   ASBR4.

   Thanks to such an optimization, the inter-ASBRs TE link information
   corresponding to the links originated by the ASBR is made available
   in the TED of other LSRs in the same domain that the ASBR belongs to.
   Consequently, the path computation for an inter-AS TE LSP path can
   also take into account the inter-ASBR link(s).  This will improve the
   chance of successful signaling along the next AS in case of resource
   shortage or unsatisfied constraints on inter-ASBR links and it
   potentially reduces one level of crankback.  Note that no topology
   information is flooded and these links are not used in IGP SPF
   computations.  Only the TE information for the outgoing links
   directly connected to the ASBR is advertised.

   Note that an Operator may decide to operate a stitched segment or
   1-hop hierarchical LSP for the inter-ASBR link.

4.1.  Example with an inter-area TE LSP

4.1.1.  Case 1: T0 is a contiguous TE LSP

   The Head-end LSR (R0) first determines the next hop ABR (which could
   be manually configured by the user or dynamically determined by using
   auto-discovery mechanism).  R0 then computes the path to reach the
   selected next hop ABR (ABR1) and signals the Path message.  When the
   Path message reaches ABR1, it first determines the next hop ABR from
   its area 0 along the LSP path (say ABR3), either directly from the
   ERO (if for example the next hop ABR is specified as a loose hop in
   the ERO) or by using the auto-discovery mechanism specified above.

   - Example 1 (set of loose hops): R0-ABR1(loose)-ABR3(loose)-R1(loose)

   - Example 2 (mix of strict and loose hops): R0-X1-ABR1-ABR3(loose)-
     X2-X3-R1

   Note that a set of paths can be configured on the Head-end LSR,
   ordered by priority.  Each priority path can be associated with a
   different set of constraints.  It may be desirable to systematically
   have a last resort option with no constraint to ensure that the
   inter-area TE LSP could always be set up if at least a TE path exists
   between the inter-area TE LSP source and destination.  In case of set
   up failure or when an RSVP PathErr is received indicating the TE LSP
   has suffered a failure, an implementation might support the
   possibility to retry a particular path option configurable amount of
   times (optionally with dynamic intervals between each trial) before
   trying a lower priority path option.



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   Once it has computed the path up to the next hop ABR (ABR3), ABR1
   sends the Path message along the computed path.  Upon receiving the
   Path message, ABR3 then repeats a similar procedure.  If ABR3 cannot
   find a path obeying the set of constraints for the inter-area TE LSP,
   the signaling process stops and ABR3 sends a PathErr message to ABR1.
   Then ABR1 can in turn trigger a new path computation by selecting
   another egress boundary LSR (ABR4 in the example above) if crankback
   is allowed for this inter-area TE LSP (see
   [I-D.ietf-ccamp-crankback]).  If crankback is not allowed for that
   inter-area TE LSP or if ABR1 has been configured not to perform
   crankback, then ABR1 MUST stop the signaling process and MUST forward
   a PathErr up to the Head-end LSR (R0) without trying to select
   another ABR.

4.1.2.  Case 2: T0 is a stitched or nested TE LSP

   The Head-end LSR (R0) first determines the next hop ABR (which could
   be manually configured by the user or dynamically determined by using
   auto-discovery mechanism).  R0 then computes the path to reach the
   selected next hop ABR and signals the Path message.  When the Path
   message reaches ABR1, it first determines the next hop ABR from its
   area 0 along the LSP path (say ABR3), either directly from the ERO
   (if for example the next hop ABR is specified as a loose hop in the
   ERO) or by using an auto-discovery mechanism, specified above.

   ABR1 then checks if it has a H-LSP or S-LSP to ABR3 matching the
   constraints carried in the inter-area TE LSP Path message.  If not,
   ABR1 computes the path for a H-LSP or S-LSP from ABR1 to ABR3
   satisfying the constraint and sets it up accordingly.  Note that the
   H-LSP or S-LSP could have also been pre-configured.

   Once ABR1 has selected the H-LSP/S-LSP for the inter-area LSP, using
   the signaling procedures described in
   [I-D.ietf-ccamp-inter-domain-rsvp-te], ABR1 sends the Path message
   for inter-area TE LSP to ABR3.  Note that irrespective of whether
   ABR1 does nesting or stitching, the Path message for the inter-area
   TE LSP is always forwarded to ABR3.  ABR3 then repeats the exact same
   procedures.  If ABR3 cannot find a path obeying the set of
   constraints for the inter-area TE LSP, ABR3 sends a PathErr message
   to ABR1.  Then ABR1 can in turn either select another H-LSP/S-LSP to
   ABR3 if such an LSP exists or select another egress boundary LSR
   (ABR4 in the example above) if crankback is allowed for this inter-
   area TE LSP (see [I-D.ietf-ccamp-crankback]).  If crankback is not
   allowed for that inter-area TE LSP or if ABR1 has been configured not
   to perform crankback, then ABR1 forwards the PathErr up to the inter-
   area Head-end LSR (R0) without trying to select another egress LSR.




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4.2.  Example with an inter-AS TE LSP

   The path computation procedures for establishing an inter-AS TE LSP
   are very similar to those of an inter-area TE LSP described above.
   The main difference is related to the presence of inter-ASBRs
   link(s).

4.2.1.  Case 1: T1 is a contiguous TE LSP

   The inter-AS TE path may be configured on the Head-end LSR as a set
   of strict hops, loose hops or a combination of both.

   - Example 1 (set of loose hops): ASBR4(loose)-ASBR9(loose)-R6(loose)

   - Example 2 (mix of strict and loose hops): R2-ASBR3-ASBR2-ASBR1-
     ASBR4-ASBR10(loose)-ASBR9-R6

   In the example 1 above, a per-AS path computation is performed,
   respectively on R0 for AS1, ASBR4 for AS2 and ASBR9 for AS3.  Note
   that when an LSR has to perform an ERO expansion, the next hop must
   either belong to the same AS, or must be the ASBR directly connected
   to the next hops AS.  In this later case, the ASBR reachability is
   announced in the IGP TE LSA/LSP originated by its neighboring ASBR.
   In the example 1 above, the TE LSP path is defined as: ASBR4(loose)-
   ASBR9(loose)-R6(loose).  This implies that R0 must compute the path
   from R0 to ASBR4, hence the need for R0 to get the TE reservation
   state related to the ASBR1-ASBR4 link (flooded in AS1 by ASBR1).  In
   addition, ASBR1 must also announce the IP address of ASBR4 specified
   in the T1's path configuration.

   Once it has computed the path up to the next hop ASBR, ASBR1 sends
   the Path message for the inter-area TE LSP to ASBR4 (supposing that
   ASBR4 is the selected next hop ASBR).  ASBR4 then repeats the exact
   same procedures.  If ASBR4 cannot find a path obeying the set of
   constraints for the inter-AS TE LSP, then ASBR4 sends a PathErr
   message to ASBR1.  Then ASBR1 can in turn either select another ASBR
   (ASBR5 in the example above) if crankback is allowed for this
   inter-AS TE LSP (see [I-D.ietf-ccamp-crankback]).  If crankback is
   not allowed for that inter-AS TE LSP or if ASBR1 has been configured
   not to perform crankback, ABR1 stops the signaling process and
   forwards a PathErr up to the Head-end LSR (R0) without trying to
   select another egress LSR.  In this case, the Head-end LSR can in
   turn select another sequence of loose hops, if configured.
   Alternatively, the Head-end LSR may decide to retry the same path;
   this can be useful in case of set up failure due an outdated IGP TE
   database in some downstream AS.  An alternative could also be for the
   Head-end LSR to retry to same sequence of loose hops after having
   relaxed some constraint(s).


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4.2.2.  Case 2: T1 is a stitched or nested TE LSP

   The path computation procedures are very similar to the inter-area
   LSP setup case described earlier.  In this case, the H-LSPs or S-LSPs
   are originated by the ASBRs at the entry to the AS.


5.  Path optimality/diversity

   Since the inter-domain TE LSP is computed on a per domain (area, AS)
   basis, one cannot guarantee that the optimal inter-domain path can be
   found.

   Moreover, computing two diverse paths using a per-domain path
   computation approach may not be possible in some topologies (due to
   the well-known 'trapping' problem).

   As already pointed out, the required path computation method can be
   selected by the Service Provider on a per LSP basis.

   If the per-domain path computation technique does not meet the set of
   requirements for a particular TE LSP (e.g. path optimality,
   requirements for a set of diversely routed TE LSPs, ...) other
   techniques such as PCE-based path computation techniques may be used
   (see [RFC4655]).


6.  Reoptimization of an inter-domain TE LSP

   As stated in [RFC4216]and in [RFC4105], the ability to reoptimize an
   already established inter-domain TE LSP constitutes a requirement.
   The reoptimization process significantly differs based upon the
   nature of the TE LSP and the mechanism in use for the TE LSP
   computation.

   The following mechanisms can be used for reoptimization and are
   dependent on the nature of the inter-domain TE LSP.

6.1.  Contiguous TE LSPs

   After an inter-domain TE LSP has been set up, a more optimal route
   might appear within any traversed domain.  Then in this case, it is
   desirable to get the ability to reroute an inter-domain TE LSP in a
   non-disruptive fashion (making use of the so-called Make-Before-Break
   procedure) to follow such more optimal path.  This is a known as a TE
   LSP reoptimization procedure.




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   [RFC4736] proposes a mechanism that allows the Head-end LSR to be
   notified of the existence of a more optimal path in a downstream
   domain.  The Head-end LSR may then decide to gracefully reroute the
   TE LSP using the so-called Make-Before-Break procedure.  In case of a
   contiguous LSP, the reoptimization process is strictly controlled by
   the Head-end LSR which triggers the make-before-break procedure,
   regardless of the location of the more optimal path.

6.2.  Stitched or nested (non-contiguous) TE LSPs

   In the case of a stitched or nested inter-domain TE LSP, the
   reoptimization process is treated as a local matter to any domain.
   The main reason is that the inter-domain TE LSP is a different LSP
   (and therefore different RSVP session) from the intra-domain S-LSP or
   H-LSP in an area or an AS.  Therefore, reoptimization in a domain is
   done by locally reoptimizing the intra-domain H-LSP or S-LSP.  Since
   the inter-domain TE LSPs are transported using S-LSP or H-LSP across
   each domain, optimality of the inter-domain TE LSP in a domain is
   dependent on the optimality of the corresponding S-LSP or H-LSPs.
   If, after an inter-domain LSP is setup, a more optimal path is
   available within an domain, the corresponding S-LSP or H-LSP will be
   reoptimized using "Make-Before-Break" techniques discussed in
   [RFC3209].  Reoptimization of the H-LSP or S-LSP automatically
   reoptimizes the inter-domain TE LSPs that the H-LSP or the S-LSP
   transports.  Reoptimization parameters like frequency of
   reoptimization, criteria for reoptimization like metric or bandwidth
   availability, etc can vary from one domain to another and can be
   configured as required, per intra-domain TE S-LSP or H-LSP if it is
   preconfigured or based on some global policy within the domain.

   Hence, in this scheme, since each domain takes care of reoptimizing
   its own S-LSPs or H-LSPs, and therefore the corresponding inter-
   domain TE LSPs, the Make-Before-Break can happen locally and is not
   triggered by the Head-end LSR for the inter-domain LSP.  So, no
   additional RSVP signaling is required for LSP reoptimization and
   reoptimization is transparent to the Head-end LSR of the inter-domain
   TE LSP.

   If, however, an operator desires to manually trigger reoptimization
   at the Head-end LSR for the inter-domain TE LSP, then this solution
   does not prevent that.  A manual trigger for reoptimization at the
   Head-end LSR SHOULD force a reoptimization thereby signaling a "new"
   path for the same LSP (along the more optimal path) making use of the
   Make-Before-Break procedure.  In response to this new setup request,
   the boundary LSR may either initiate new S-LSP setup, in case the
   inter-domain TE LSP is being stitched to the intra-domain S-LSP or it
   may select an existing or new H-LSP in case of nesting.  When the LSP
   setup along the current path is complete, the Head-end LSR should


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   switchover the traffic onto that path and the old path is eventually
   torn down.  Note that the Head-end LSR does not know a priori whether
   a more optimal path exists.  Such a manual trigger from the Head-end
   LSR of the inter-domain TE LSP is, however, not considered to be a
   frequent occurrence.

   Procedures described in [RFC4736] MUST be used if the operator does
   not desire local reoptimization of certain inter-domain LSPs.  In
   this case, any reoptimization event within the domain MUST be
   reported to the Head-end node.  This SHOULD be a configurable policy.

6.3.  Path characteristics after reoptimization

   Note that in the case of loose hop reoptimization of contiguous
   inter-domain TE LSP or local reoptimization of stitched/nested S-LSP
   where boundary LSRs are specified as loose hops, the TE LSP may
   follow a preferable path within one or more domain(s) but would still
   traverse the same set of boundary LSRs.  In contrast, in the case of
   PCE-based path computation techniques, because end to end optimal
   path is computed, the reoptimization process may lead to following a
   completely different inter-domain path (including a different set of
   boundary LSRs).


7.  IANA Considerations

   This document makes no request for any IANA action.


8.  Security Considerations

   Signaling of inter-domain TE LSPs raises security issues (discussed
   in section 7 of [I-D.ietf-ccamp-inter-domain-rsvp-te]).

   [RFC4726] provides an overview of the requirements for security in an
   MPLS-TE or GMPLS multi-domain environment.  In particular, when
   signaling an inter-domain RSVP-TE LSP, an operator may make use of
   the security features already defined for RSVP-TE ([RFC3209]).  This
   may require some coordination between the domains to share the keys
   (see [RFC2747] and [RFC3097]), and care is required to ensure that
   the keys are changed sufficiently frequently.  Note that this may
   involve additional synchronization, should the domain border nodes be
   protected with FRR, since the MP and PLR should also share the key.
   For an inter-domain TE LSP, especially when it traverses different
   administrative or trust domains, the following mechanisms SHOULD be
   provided to an operator (also see [RFC4216]):




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   1) A way to enforce policies and filters at the domain borders to
   process the incoming inter-domain TE LSP setup requests (Path
   messages) based on certain agreed trust and service levels/contracts
   between domains.  Various LSP attributes such as bandwidth, priority,
   etc. could be part of such a contract. 2) A way for the operator to
   rate-limit LSP setup requests or error notifications from a
   particular domain. 3) A mechanism to allow policy-based outbound RSVP
   message processing at the domain border node, which may involve
   filtering or modification of certain addresses in RSVP objects and
   messages.

   This document relates to inter-domain path computation.  It must be
   noted that the process for establishing paths described in this
   document does not increase the information exchanged between ASes and
   preserves topology confidentiality, in compliance with [RFC4105] and
   [RFC4216].  That being said, the signaling of inter-domain TE LSP
   according to the procedure defined in this document requires path
   computation on boundary nodes that may be exposed to various attacks.
   Thus it is RECOMMENDED to support policy decisions to reject the ERO
   expansion of an inter-domain TE LSP if not allowed.


9.  Acknowledgements

   We would like to acknowledge input and helpful comments from Adrian
   Farrel, Jean-Louis Le Roux, Dimitri Papadimitriou and Faisal Aslam.

   Adrian Farrel prepared the final verison of this document for IESG
   review.

10.  References

10.1.  Normative References

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

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

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






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10.2.  Informative References

   [I-D.ietf-ccamp-crankback]
              Farrel, A., "Crankback Signaling Extensions for MPLS and
              GMPLS RSVP-TE", draft-ietf-ccamp-crankback (work in
              progress).

   [I-D.ietf-ccamp-inter-domain-rsvp-te]
              Ayyangar, A. and J. Vasseur, "Inter domain MPLS and GMPLS
              Traffic Engineering - RSVP-TE extensions",
              draft-ietf-ccamp-inter-domain-rsvp-te (work in
              progress.

   [I-D.ietf-ccamp-lsp-stitching]
              Ayyangar, A., "Label Switched Path Stitching with
              Generalized MPLS Traffic Engineering",
              draft-ietf-ccamp-lsp-stitching (work in progress).

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

   [RFC2747]  Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
              Authentication", RFC 2747, January 2000.

   [RFC3097]  Braden, R. and L. Zhang, "RSVP Cryptographic
              Authentication -- Updated Message Type Value", RFC 3097,
              April 2001.

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

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

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

   [RFC4203]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4203, October 2005.

   [RFC4205]  Kompella, K. and Y. Rekhter, "Intermediate System to
              Intermediate System (IS-IS) Extensions in Support of
              Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4205, October 2005.


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

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

   [RFC4726]  Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework for
              Inter-Domain Multiprotocol Label Switching Traffic
              Engineering", RFC 4726, November 2006.

   [RFC4736]  Vasseur, J., Ikejiri, Y., and R. Zhang, "Reoptimization of
              Multiprotocol Label Switching (MPLS) Traffic Engineering
              (TE) Loosely Routed Label Switched Path (LSP)", RFC 4736,
              November 2006.


Authors' Addresses

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

   Email: jpv@cisco.com


   Arthi Ayyangar (editor)
   Nuova Systems
   2600 San Tomas Expressway
   Santa Clara, CA  95051
   USA

   Email: arthi@nuovasystems.com


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

   Email: raymond_zhang@bt.infonet.com







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