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Networking Working Group                                JP. Vasseur, Ed.Request for Comments: 5152                           Cisco Systems, Inc.Category: Standards Track                               A. Ayyangar, Ed.                                                        Juniper Networks                                                                R. Zhang                                                                      BT                                                           February 2008A Per-Domain Path Computation Method for Establishing Inter-DomainTraffic Engineering (TE) Label Switched Paths (LSPs)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.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 Interior Gateway Protocol (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   identifiers.  The path through each domain, possibly including the   choice of exit point from the domain, must be determined within the   domain.Vasseur, et al.             Standards Track                     [Page 1]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008Table of Contents1. Introduction ....................................................22. Terminology .....................................................32.1. Requirements Language ......................................43. General Assumptions .............................................43.1. Common Assumptions .........................................43.2. Example of Topology for the Inter-Area TE Case .............63.3. Example of Topology for the Inter-AS TE Case ...............74. Per-Domain Path Computation Procedures ..........................84.1. Example with an Inter-Area TE LSP .........................114.1.1. Case 1: T0 Is a Contiguous TE LSP ..................114.1.2. Case 2: T0 Is a Stitched or Nested TE LSP ..........124.2. Example with an Inter-AS TE LSP ...........................134.2.1. Case 1: T1 Is a Contiguous TE LSP ..................134.2.2. Case 2: T1 Is a Stitched or Nested TE LSP ..........145. Path Optimality/Diversity ......................................146. Reoptimization of an Inter-Domain TE LSP .......................156.1. Contiguous TE LSPs ........................................156.2. Stitched or Nested (non-contiguous) TE LSPs ...............166.3. Path Characteristics after Reoptimization .................177. Security Considerations ........................................178. Acknowledgements ...............................................189. References .....................................................189.1. Normative References ......................................189.2. Informative References ....................................181.  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 [RFC5151] and [RFC5150].   This document exclusively focuses on the path computation aspects and   defines a method for establishing inter-domain TE LSPs where each   node in charge of computing a section of an inter-domain TE LSP path   is always along the path of such a TE LSP.   When the visibility of an end-to-end complete path spanning multiple   domains is not available at the Head-end LSR (the LSR that initiated   the TE LSP), 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.Vasseur, et al.             Standards Track                     [Page 2]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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.  The   choice of the appropriate technique must be driven by the set of   requirements for the path 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 off-line 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.2.  Terminology   Terminology used in this document:   AS: Autonomous System.   ABR: Area Border Router, a router used to connect two IGP areas   (areas in OSPF or levels in Intermediate System to Intermediate   System (IS-IS)).   ASBR: Autonomous System Border Router, a router used to connect   together ASs 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.   ERO: Explicit Route Object.   IGP: Interior Gateway Protocol.   Inter-AS TE LSP: A TE LSP that crosses an AS boundary.   Inter-area TE LSP: A TE LSP that crosses an IGP area.Vasseur, et al.             Standards Track                     [Page 3]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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.2.1.  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 inRFC 2119 [RFC2119].3.  General Assumptions3.1.  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 (Resource Reservation Protocol Traffic Engineering)).  A     domain may itself comprise multiple other domains, e.g., an AS may     itself be composed of several other sub-ASs (BGP confederations) or     areas/levels.  In this case, the path computation technique     described for inter-area and inter-AS MPLS Traffic Engineering     applies recursively.   - The inter-domain TE LSPs are signaled using RSVP-TE ([RFC3209] and     [RFC3473]).   - The path (specified by an ERO (Explicit Route Object) in an RSVP-TE     Path message) 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)Vasseur, et al.             Standards Track                     [Page 4]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008      * 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 be either 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 (e.g., based on IGP, BGP, policy routing   information) yielding the next-hop boundary node (domain exit point,   such as an Area Border Router (ABR) or an Autonomous System Border   Router (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 LSP (H-LSP) or Stitched     LSP (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-LSP/S-LSP are pre-configured, the constraints as well as other     parameters like a 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     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].Vasseur, et al.             Standards Track                     [Page 5]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   - The procedures defined in this document are applicable to any node     (not just a boundary node) that receives a Path message with an ERO     that constrains 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 is an LSR in area 1.   - X2 and X3 are LSRs in area 2.   - An inter-area TE LSP T0 originated at R0 in area 1 and terminated     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.Vasseur, et al.             Standards Track                     [Page 6]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 20083.3.  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]:            <-- AS1 ----> <------- AS2 ------><--- AS3 ----->                     <---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 ASs are TE enabled.   - CE: Customer Edge router.   - 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, multiple ASs made of a single     IGP area, or any combination of the above.  For the sake of     simplicity, each routing domain will be considered as a single areaVasseur, et al.             Standards Track                     [Page 7]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008     in this document.  The case of an inter-AS TE LSP spanning multiple     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 terminated 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 domains, 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 strict node, the procedures   specified in [RFC3209] apply and no further action is needed.   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 [RFC5151].   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, the two following      conditions MUST be checked:      o  Whether the IP address of the next-hop boundary LSR is outside         of the current domain      o  Whether the next-hop 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 insection4.3.4 of [RFC3209].  If the available routing information indicatesVasseur, et al.             Standards Track                     [Page 8]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   that next hop is reachable, the selected route will be expected to   pass through a domain boundary via a domain boundary LSR.  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 TE LSP will fail.   An implementation MAY support the ability to disable such an 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.  Unless not allowed by policy, the         boundary LSR that processes the ERO SHOULD perform an ERO         expansion (a process consisting of computing the constrained         path up to the next loose hop and adding the list of hops as         strict nodes in the ERO).  If no path satisfying the set of         constraints can be found, then this is treated as a path         computation and signaling failure and an RSVP PathErr message         SHOULD be sent for the inter-domain TE LSP based onsection4.3.4 of [RFC3209].      o  Case of a stitched or nested TE LSP         *  If the boundary LSR is a candidate LSR for intra-area H-LSP/            S-LSP setup (the boundary has local policy for nesting or            stitching), the TE LSP is a candidate for hierarchy/nesting            (the "Contiguous LSP" bit defined in [RFC5151] 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 an H-LSP/S-LSP to the next-hop boundary LSR.  If a            pre-configured H-LSP(s) or S-LSP(s) already exists, 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 [RFC5151].  If for some reason the dynamic H-LSP/S-LSP            setup to the next-hop boundary LSR fails, then this SHOULDVasseur, et al.             Standards Track                     [Page 9]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008            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 pre-            configured 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 an RSVP PathErr            message SHOULD be sent upstream for the inter-domain TE LSP.            In both of these cases, procedures described insection4.3.4 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 boundary LSR does not have            local policy for nesting or stitching) or the TE LSP is not            a candidate for hierarchy/nesting (the "Contiguous LSP" bit            defined in [RFC5151] is set), then it SHOULD apply the same            procedure as for the contiguous case.   The ERO of an inter-domain TE LSP may comprise abstract nodes such as   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 ASBR 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 the following 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 that the inter-ASBR links   be TE-enabled although no IGP is running on those links.            <-- AS1 ----> <------- AS2 ------><--- AS3 ----->                     <---BGP--->            <---BGP-->      CE1---R0---X1-ASBR1-----ASBR4--R3---ASBR7----ASBR9----R6            |\     \ |       / |   / |   / |          |      |            | \     ASBR2---/ ASBR5  | --  |          |      |            |  \     |         |     |/    |          |      |            R1-R2---ASBR3-----ASBR6--R4---ASBR8----ASBR10---R7---CE2      Figure 3 - Flooding of the TE-related information for                 the inter-ASBR linksVasseur, et al.             Standards Track                    [Page 10]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   Referring to Figure 3, ASBR1 for example would advertise in its OSPF   Link State Advertisement (LSA)/IS-IS LSP the Traffic Engineering TLVs   related to the link ASBR1-ASBR4.   This allows an LSR (could be the 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   setup 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 ASs, 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   properties of the following links: X1-ASBR1, ASBR1-ASBR2, and   ASBR1-ASBR4.   Thanks to such an optimization, the inter-ASBR TE link information   corresponding to the links originated by the ASBR is made available   in the TED of other LSRs in the same domain to which the ASBR   belongs.  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   The following example uses Figure 1 as a reference.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   the auto-discovery mechanism).  R0 then computes the path to reach   the selected next-hop ABR (ABR1) and signals the Path message.  WhenVasseur, et al.             Standards Track                    [Page 11]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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   setup failure or when an RSVP PathErr is received indicating that the   TE LSP has suffered a failure, an implementation might support the   possibility of retrying a particular path option a 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 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 [RFC4920]).  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   the 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.Vasseur, et al.             Standards Track                    [Page 12]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   ABR1 then checks whether it has an 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 an 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 [RFC5151], ABR1 sends the Path   message for the 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 [RFC4920]).  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.4.2.  Example with an Inter-AS TE LSP   The following example uses Figure 2 as a reference.   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-ASBR 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 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 either   must belong to the same AS or must be the ASBR directly connected to   the next hop AS.  In this latter case, the ASBR reachability is   announced in the IGP TE LSA/LSP originated by its neighboring ASBR.   In example 1 above, the TE LSP path is defined as: ASBR4(loose)-   ASBR9(loose)-R6(loose).  This implies that R0 must compute the pathVasseur, et al.             Standards Track                    [Page 13]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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 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 [RFC4920]), or 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 setup   failure due to an outdated IGP TE database in some downstream AS.  An   alternative could also be for the Head-end LSR to retry the 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.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).   For example, consider the following simple topology:                            +-------+                           /         \                          A----B-----C------D                               \           /                                +---------+                Figure 4 - Example of the "trapping" problemVasseur, et al.             Standards Track                    [Page 14]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   In the simple topology depicted in Figure 4, with a serialized   approach using the per-domain path computation technique specified in   this document, a first TE LSP may be computed following the path   A-B-C-D, in which case no diverse path could be found although two   diverse paths actually exist: A-C-D and A-B-D.  The aim of that   simple example that can easily be extended to the inter-domain case   is to illustrate the potential issue of not being able to find   diverse paths using the per-domain path computation approach when   diverse paths exist.   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 [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 better 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 a better path.  This is a known as a TE LSP   reoptimization procedure.   [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 Make-Before-Break procedure.  In case of a   contiguous LSP, the reoptimization process is strictly controlled by   the Head-end LSR that triggers the Make-Before-Break procedure as   defined in [RFC3209], regardless of the location of the better path.Vasseur, et al.             Standards Track                    [Page 15]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 20086.2.  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-LSP.  If   after an inter-domain LSP is set up a more optimal path is available   within a 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 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 pre-configured 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 either may 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   switch over 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.Vasseur, et al.             Standards Track                    [Page 16]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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 the 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.  Security Considerations   Signaling of inter-domain TE LSPs raises security issues (discussed   insection 7 of [RFC5151]).   [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 Fast Reroute ([RFC4090], since the Merge Point (MP)   and Point of Local Repair (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]):   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.Vasseur, et al.             Standards Track                    [Page 17]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   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 ASs 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.8.  Acknowledgements   We would like to acknowledge input and helpful comments from Adrian   Farrel, Jean-Louis Le Roux, Dimitri Papadimitriou, and Faisal Aslam.9.  References9.1.  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., Ed., "Generalized Multi-Protocol Label               Switching (GMPLS) Signaling Resource ReserVation               Protocol-Traffic Engineering (RSVP-TE) Extensions",RFC3473, January 2003.9.2.  Informative References   [RFC4920]   Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,               N., and G. Ash, "Crankback Signaling Extensions for MPLS               and GMPLS RSVP-TE",RFC 4920, July 2007.   [RFC5151]   Farrel, A., Ed., Ayyangar, A., and JP. Vasseur, "Inter-               Domain MPLS and GMPLS Traffic Engineering -- Resource               Reservation Protocol-Traffic Engineering (RSVP-TE)               Extensions",RFC 5151, February 2008.   [RFC5150]   Ayyangar, A., Kompella, K., Vasseur, JP., and A. Farrel,               "Label Switched Path Stitching with Generalized               Multiprotocol Label Switching Traffic Engineering (GMPLS               TE)",RFC 5150, February 2008.Vasseur, et al.             Standards Track                    [Page 18]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   [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.   [RFC4090]   Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast               Reroute Extensions to RSVP-TE for LSP Tunnels",RFC 4090,               May 2005.   [RFC4105]   Le Roux, J.-L., Ed., Vasseur, J.-P., Ed., and J. Boyle,               Ed., "Requirements for Inter-Area MPLS Traffic               Engineering",RFC 4105, June 2005.   [RFC4203]   Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF Extensions               in Support of Generalized Multi-Protocol Label Switching               (GMPLS)",RFC 4203, October 2005.   [RFC4205]   Kompella, K., Ed., and Y. Rekhter, Ed., "Intermediate               System to Intermediate System (IS-IS) Extensions in               Support of Generalized Multi-Protocol Label Switching               (GMPLS)",RFC 4205, October 2005.   [RFC4216]   Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS Inter-               Autonomous System (AS) Traffic Engineering (TE)               Requirements",RFC 4216, November 2005.   [RFC4655]   Farrel, A., Vasseur, J.-P., and J. Ash, "A Path               Computation Element (PCE)-Based Architecture",RFC 4655,               August 2006.   [RFC4726]   Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A Framework               for Inter-Domain Multiprotocol Label Switching Traffic               Engineering",RFC 4726, November 2006.Vasseur, et al.             Standards Track                    [Page 19]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008   [RFC4736]   Vasseur, JP., Ed., 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)   Juniper Networks   1194 N. Mathilda Ave   Sunnyvale, CA  94089   USA   EMail: arthi@juniper.net   Raymond Zhang   BT   2160 E. Grand Ave.   El Segundo, CA  90025   USA   EMail: raymond.zhang@bt.comVasseur, et al.             Standards Track                    [Page 20]

RFC 5152          Path Comp. for Inter-Domain TE LSPs      February 2008Full Copyright Statement   Copyright (C) The IETF Trust (2008).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST AND   THE INTERNET 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.Intellectual Property   The 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 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 to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the 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 athttp://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.Vasseur, et al.             Standards Track                    [Page 21]