Network
Networking Working Group                               JP Vasseur (Editor)
IETF Internet draft                                JP. Vasseur, Ed.
Internet-Draft                                        Cisco Systems, Inc. Inc
Proposed Status: Standard                       Arthi Ayyangar (Editor)                                A. Ayyangar, Ed.
Expires: August 11, 2006                                Juniper Networks
                                                          Raymond
                                                                R. Zhang
                                                              BT Infonet Service Corporation

Expires: April
                                                        February 7, 2006
                                                           October 2005

           draft-ietf-ccamp-inter-domain-pd-path-comp-01.txt

   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-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

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

 Vasseur, Ayyangar and Zhang                                         1 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 identifiers.  The path through each domain, possibly including
   the choice of exit point from the domain, must be determined within
   the domain.

Conventions used in this document

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].

Table of content Contents

   1. Terminology..................................................3  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2. Introduction.................................................3  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  General assumptions..........................................4
3.1 assumptions  . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Common assumptions..........................................4
3.2 assumptions . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Example of topology for the inter-area TE case .............5
3.3 . . . . . .  7
     3.3.  Example of topology for the inter-AS TE case ...............6 . . . . . . .  7
   4.  Per-domain path computation procedures.......................7
4.1 procedures . . . . . . . . . . . .  9
     4.1.  Example with an inter-area TE LSP...........................10
4.1.1 LSP  . . . . . . . . . . . . 12
       4.1.1.  Case 1: T1 T0 is a contiguous TE LSP.........................10
4.1.2 LSP  . . . . . . . . . . 12
       4.1.2.  Case 2: T1 T0 is a stitched or nested TE LSP.................11
4.2 LSP  . . . . . . 13
     4.2.  Example with an inter-AS TE LSP.............................11
4.2.1 LSP  . . . . . . . . . . . . . 13
       4.2.1.  Case 1: T1 is a contiguous TE LSP.........................12
4.2.2 LSP  . . . . . . . . . . 14
       4.2.2.  Case 2: T1 is a stitched or a nested TE LSP...............12 LSP  . . . . . . 14
   5.  Path optimality/diversity....................................13 optimality/diversity  . . . . . . . . . . . . . . . . . . 15
   6.  Reoptimization of an inter-domain TE LSP.....................13
6.1 LSP . . . . . . . . . . . 15
     6.1.  Contiguous TE LSPs..........................................13 LSPs . . . . . . . . . . . . . . . . . . . . 15
     6.2.  Stitched or nested (non-contiguous) TE LSPs................13
6.3 LSPs  . . . . . . . 16
     6.3.  Path characteristics after reoptimization...................15 reoptimization  . . . . . . . . 17
   7. Security  IANA Considerations .....................................15  . . . . . . . . . . . . . . . . . . . . . 17
   8. Intellectual Property  Security Considerations ........................15  . . . . . . . . . . . . . . . . . . . 17
   9. Acknowledgments..............................................15  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   10. References..................................................15
10.1 Normatives References .....................................16
10.2 . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     10.2. Informative References ....................................17

 Vasseur, Ayyangar . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
   Intellectual Property and Zhang                                         2 Copyright Statements . . . . . . . . . . 21

1.  Terminology

   Terminology used in this document

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

   ASBR Routers: 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
   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 Router.

   LSP: Label Switched Path Path.

   TE LSP: Traffic Engineering Label Switched Path 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 Database.

   The notion of contiguous, stitched and nested TE LSPs is defined in
[INT-DOMAIN-FRWK]
   [I-D.ietf-ccamp-inter-domain-framework] 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 [INT-AREA-REQS] [RFC4105] and [INT-AS-REQS]. [RFC4216].  The framework
   for inter-domain MPLS Traffic Engineering has been provided in [INT-DOMAIN-FRWK].
   [I-D.ietf-ccamp-inter-domain-framework].

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

   This document exclusively focuses on the path computation aspects and
   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

 Vasseur, Ayyangar and Zhang                                         3 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 ASes. ASs.

   The solution described in this document does not attempt to address
   all the requirements specified in [INT-AREA-REQS] [RFC4105] and [INT-AS-REQS]. [RFC4216].  This is
   acceptable according to [INT-AS-REQS] [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 [PCE-ARCH]). [I-D.ietf-pce-architecture]).  Other
   offline mechanisms for path computation are not precluded either.
   Note also that a Service Provider may elect to use different inter-domain inter-
   domain path computation techniques for different TE LSP types.

3.  General assumptions

In the rest of this document, we make the following set of 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).
   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 ([RSVP-TE]). ([RFC3209]).

   - 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

 Vasseur, Ayyangar and Zhang                                         4 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 and/or BGP 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 [INT-AREA-REQS] [RFC4105] and [INT-AS-REQS]), [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 S-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 [INT-AS-REQS]. [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.
   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 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 [INT-AS-REQS]. [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 =======>

 Vasseur, Ayyangar and Zhang                                         5

      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.

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 [INT-AS-REQS]: [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
         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 [INT-AS-REQS]. [RFC4216].

   Description of Figure 2:

 Vasseur, Ayyangar and Zhang                                         6

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

   - The ASBRs in different ASes 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 [OSPF-TE], [ISIS-TE], [G-OSPF] [RFC3630], [RFC3784], [RFC4203] and [G-ISIS]). [RFC4205]).  In
   other words, the ASes ASs 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 ASes 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 ASes ASs where
   some of those
ASes 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 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 area, and exert a strict control on
   the path in other areas (defining strict hops).

   When a boundary an LSR (e.g. 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 [INTER-DOMAIN-
SIG]. [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 boundary LSR is not present in the TED.

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

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

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

 Vasseur, Ayyangar and Zhang                                         7

   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 a PathErr PErr message upstream with error code=24
   ("Routing Problem") and error subcode as described in section 4.3.4
   of
[RSVP-TE]. [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 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 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) If 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. TED

   a) 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 a PathErr PErr message SHOULD be sent
   for the inter-domain TE LSP based on section 4.3.4 of [RSVP-TE]. [RFC3209].

   b) Case of stitched or nested LSP

   i) If the boundary LSR is a candidate LSR for intra-
                area intra-area H-LSP/S-LSP
   setup (the LSR has local policy for nesting or stitching), 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 H-LSP/S-LSP to the
   next-hop boundary LSR.  If pre-configured H-
                LSP(s) H-LSP(s) or S-LSP(s)
   already exist, then it will try to select from among those intra-domain intra-
   domain LSPs.  Depending on local policy, it MAY signal a new H-LSP/S-LSP 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 [INTER-DOMAIN-SIG]. [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

 Vasseur, Ayyangar and Zhang                                         8 computation and signaling failure and a PathErr PErr 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 path computation and
   signaling failure and a PathErr PErr SHOULD be sent upstream for the
                inter-domain inter-
   domain TE LSP.  In both these cases procedures described in section
   4.3.4 of [RSVP-TE] [RFC3209] SHOULD be followed to handle the failure.

   ii) 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), 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.
   ASs.  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. 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

   The links interconnecting ASBRs are usually not TE-enabled and no IGP
   is running at the AS boundaries.  An implementation supporting
   inter-AS MPLS TE MUST allow the set up of inter-AS TE LSP over the
   region interconnecting multiple ASBRs.  In other words, an ASBR
   compliant with this document MUST support the set up of TE LSP over
   inter-ASBR links and MUST be able to perform all the usual operations
   related to MPLS Traffic Engineering (call admission control, ...).

   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.

 Vasseur, Ayyangar and Zhang                                         9

   Note that no summarized TE information is leaked between ASes ASs which is
   compliant with the requirements listed in [INT-AREA-REQS] [RFC4105] and [INT-AS-
REQS]. [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
   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

4.1.1.  Case 1: T1 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-ASR1-ABR3(loose)-X2-
X3-R1 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 inter-area TE LSP source and destination.  In case of set
   up failure or when an RSVP PathErr PErr is received indicating the TE LSP has
   suffered a failure, an implementation might support the possibility
   to retry a

 Vasseur, Ayyangar and Zhang                                        10 particular path option configurable amount of times
   (optionally with dynamic intervals between each trial) before trying
   a lower priority path option.

   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 PErr 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 [CRANKBACK]). [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 PErr up to the
   Head-end LSR (R0) without trying to select another ABR.

4.1.2

4.1.2.  Case 2: T1 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 [INTER-DOMAIN-SIG], [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 PErr 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 [CRANKBACK]). [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 PErr
   up to the inter-area Head-end LSR (R0) without trying to select
   another egress LSR.

4.2.  Example with an inter-AS TE LSP

 Vasseur, Ayyangar and Zhang                                        11

   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 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). 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 PErr 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
[CRANKBACK]). [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
   PErr 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).

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.

 Vasseur, Ayyangar and Zhang                                        12

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" '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 no 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 [PCE-
ARCH]). [I-D.ietf-pce-architecture]).

6.  Reoptimization of an inter-domain TE LSP

   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.

[LOOSE-REOPT]

   [I-D.ietf-ccamp-loose-path-reopt] 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 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

 Vasseur, Ayyangar and Zhang                                        13 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 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 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" "Make-Before-Break" techniques discussed in [RSVP-TE].
   [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 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
   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
   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
   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.

   Note that stitching or nesting rely on local optimization: the
   reoptimization process allows to locally reoptimize each TE S-LSP or
   H-LSP: hence, the reoptimization is not global and consequently the
end to end
   end-to-end path may no longer be optimal should it be optimal when
   being set up.

   Procedures described in [LOOSE-REOPT] [I-D.ietf-ccamp-loose-path-reopt] 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.

 Vasseur, Ayyangar and Zhang                                        14

6.3.  Path characteristics after reoptimization

   Note that in the case of loose hop reoptimization of contiguous inter-
domain
   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 that have
   been described in section 7 of [INTER-DOMAIN-SIG]; [I-D.ietf-ccamp-inter-domain-rsvp-te];
   however the path computation aspects specified in this document do
   not raise additional security concerns.

8. Intellectual Property Considerations

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain

9.  Acknowledgements

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

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights.  Information
on the procedures with respect RFCs to rights in RFC documents can be
found in Indicate
              Requirement Levels", BCP 78 14, RFC 2119, March 1997.

   [RFC2205]  Braden, B., Zhang, L., Berson, S., Herzog, S., and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat S.
              Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
              Functional Specification", RFC 2205, September 1997.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and any
assurances of licenses to be made available, or the result of an
attempt made J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, September 1999.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard.  Please address the information to the IETF at ietf-
ipr@ietf.org.

9. Acknowledgments

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

10. References

 Vasseur, Ayyangar and Zhang                                        15
   10.1. Normative References

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

[RFC3979] Bradner, S., Ed., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3979, March 2005.

[RSVP] Braden, et al, "Resource ReSerVation Protocol (RSVP)" Version
1, Functional Specification", RFC 2205, September 1997.

[RSVP-TE] Awduche, et al, "Extensions to RSVP RSVP for LSP
              Tunnels", RFC 3209, December 2001.

[OSPF-TE]

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

[ISIS-TE] Li, T.,

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

[G-OSPF]Rekhter Y., Kompella K, et al.,

   [RFC4203]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
              of Generalized Multi-Protocol Label Switching", draft-ietf-ccamp-ospf-
gmpls-extensions, work in progress.

[G-ISIS] Rekhter Y., Kompella K, et al., "IS-IS 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", draft-ietf-isis-gmpls-
extensions, work in progress. Switching (GMPLS)",
              RFC 4205, October 2005.

10.2.  Informative references

[CRANKBACK] Farrel References

   [I-D.ietf-ccamp-crankback]
              Farrel, A., et al., "Crankback Signaling Extensions for MPLS and
              GMPLS RSVP-TE", draft-ietf-ccamp-crankback, work draft-ietf-ccamp-crankback-05 (work in progress.

[INT-AREA-REQ] Le Roux, J.L., Vasseur, J.P., Boyle, J., "Requirements
for inter-area MPLS Traffic Engineering", RFC 4105, June
              progress), May 2005.

[INT-AS-REQ] Zhang, R., Vasseur, J.P., "MPLS Inter-AS Traffic
Engineering Requirements", draft-ietf-tewg-interas-mpls-te-req, work in
progress.

[INT-DOMAIN-FRWK]

   [I-D.ietf-ccamp-inter-domain-framework]
              Farrel, A., Vasseur, J.P., Ayyangar, A., "A Framework for Inter-Domain MPLS Traffic
              Engineering", draft-ietf-ccamp-inter-
domain-framework, work draft-ietf-ccamp-inter-domain-framework-04
              (work in progress. progress), July 2005.

   [I-D.ietf-ccamp-inter-domain-pd-path-comp]
              Vasseur, Ayyangar and Zhang                                        16

[INTER-DOMAIN-SIG] J., "A Per-domain path computation method for
              establishing Inter-domain Traffic  Engineering (TE) Label
              Switched Paths (LSPs)",
              draft-ietf-ccamp-inter-domain-pd-path-comp-01 (work in
              progress), October 2005.

   [I-D.ietf-ccamp-inter-domain-rsvp-te]
              Ayyangar, A., A. and J. Vasseur, JP. "Inter-domain MPLS "Inter domain GMPLS Traffic
              Engineering - RSVP RSVP-TE extensions", draft-ietf-ccamp-inter-domain-
rsvp-te, work
              draft-ietf-ccamp-inter-domain-rsvp-te-02 (work in progress.

[LSP-STITCHING]
              progress), October 2005.

   [I-D.ietf-ccamp-loose-path-reopt]
              Vasseur, J., "Reoptimization of Multiprotocol Label
              Switching (MPLS) Traffic Engineering  (TE) loosely routed
              Label Switch Path (LSP)",
              draft-ietf-ccamp-loose-path-reopt-02 (work in progress),
              February 2006.

   [I-D.ietf-ccamp-lsp-stitching]
              Ayyangar, A., A. and J. Vasseur, JP. "Label Switched Path
              Stitching with Generalized MPLS Traffic Engineering", draft-ietf-ccamp-
lsp-stitching-00, work under progress.

[LOOSE-PATH-REOPT] Vasseur, Ikejiri and Zhang "Reoptimization of an
explicit loosely routed MPLS TE paths", draft-ietf-ccamp-loose-path-
reopt, work in Progress.

[LSP-HIER] Kompella K., Rekhter Y., "LSP Hierarchy with Generalized
MPLS TE", draft-ietf-mpls-lsp-hierarchy-08.txt, March 2002.

[PCE-ARCH] Farrel
              draft-ietf-ccamp-lsp-stitching-02 (work in progress),
              September 2005.

   [I-D.ietf-pce-architecture]
              Farrel, A., Vasseur J.P, Ash J, "Path "A Path Computation Element (PCE) Based
              Architecture", draft-ietf-pce-architecture, work draft-ietf-pce-architecture-04 (work in progress.
              progress), January 2006.

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

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

Authors' Address:

Jean-Philippe Addresses

   JP Vasseur (Editor) (editor)
   Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , Inc
   1414 Massachusetts Avenue
   Boxborough, MA -  01719
   USA

   Email: jpv@cisco.com

   Arthi Ayyangar (Editor) (editor)
   Juniper Networks, Inc Networks
   1194 N.Mathilda Ave Avenue
   Sunnyvale, CA  94089
   USA
e-mail:

   Email: arthi@juniper.net

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

   Email: raymond_zhang@infonet.com

Full Copyright raymond_zhang@bt.infonet.com

Intellectual Property Statement

Copyright (C)

   The Internet Society (2005).  This IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document is subject or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the rights, licenses and restrictions contained procedures with respect to rights in RFC documents can be
   found in BCP 78, 78 and except
as set forth therein, BCP 79.

   Copies of IPR disclosures made to the authors retain all their rights. IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET

 Vasseur, Ayyangar and Zhang                                        17
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 Vasseur, Ayyangar

Copyright Statement

   Copyright (C) The Internet Society (2006).  This document is subject
   to the rights, licenses and Zhang                                        18 restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Acknowledgment

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