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Network Working Group                            Adrian Farrel (editor)
Internet Draft                                       Old Dog Consulting
Category: Standards Track
Expires: April 2004                                  Arun Satyanarayana
                                                   Movaz Networks, Inc.

                                                          Atsushi Iwata
                                                        Norihito Fujita
                                                        NEC Corporation

                                                          Gerald R. Ash
                                                                   AT&T

                                                   Simon Marshall-Unitt
                                                   Data Connection Ltd.

                                                           October 2003

      Crankback Signaling Extensions for MPLS Signaling
             <draft-iwata-mpls-crankback-07.txt>

Status of this Memo

   This document is an Internet-Draft and is in full
   conformance with all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be
   accessed at http://www.ietf.org/shadow.html.

Abstract

   In a distributed, constraint-based routing environment, the
   information used to compute a path may be out of date. This means
   that Multiprotocol Label Switching (MPLS) label switched path (LSP)
   setup requests may be blocked by links or nodes without sufficient
   resources. Crankback is a scheme whereby setup failure information is
   returned from the point of failure to allow new setup attempts to be
   made avoiding the blocked resources. Crankback can also be applied to
   LSP restoration to indicate the location of the failed link or node.

   This document specifies crankback signaling extensions for use in
   MPLS signaling using RSVP-TE as defined in "RSVP-TE: Extensions to
   RSVP for LSP Tunnels", RFC3209, so that the LSP setup request can be
   retried on an alternate path that detours around blocked links or
   nodes. This offers significant improvements in the successful setup
   and recovery ratios for LSPs, especially in situations where a large
   number of setup requests are triggered at the same time.

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Table of Contents

   Section A : Problem Statement

   1. Summary for Sub-IP Area..........................................3
   1.1. Summary........................................................3
   1.2. Related documents..............................................3
   1.3. Where does it fit in the Picture of the Sub-IP Work............3
   1.4. Why is it Targeted at this WG..................................3
   1.5. Justification..................................................3
   2. Introduction and Framework.......................................4
   2.1. Background.....................................................4
   2.2. Repair and Restoration.........................................4
   3. Discussion: Explicit Versus Implicit Re-routing Indications......5
   4. Required Operation...............................................7
   4.1. Resource Failure or Unavailability.............................8
   4.2. Computation of an Alternate Path...............................8
   4.2.1 Information Required for Re-routing...........................8
   4.2.2 Signaling a New Route.........................................9
   4.3. Persistence of Error Information...............................9
   4.4. Handling Re-route Failure......................................9
   4.5. Limiting Re-routing Attempts..................................10
   5. Existing Protocol Support for Crankback Re-routing..............10
   5.1. RSVP-TE [RFC 3209]............................................11
   5.2. GMPLS-RSVP-TE [RFC 3473]......................................11

   Section B : Solution

   6. Control of Crankback Operation..................................12
   6.1. Requesting Crankback and Controlling In-Network Re-routing....12
   6.2. Action on Detecting a Failure.................................12
   6.3. Limiting Re-routing Attempts..................................13
   6.3.1 New Status Codes for Re-routing..............................13
   6.4. Protocol Control of Re-routing Behavior.......................13
   7. Reporting Crankback Information.................................14
   7.1. Required Information..........................................14
   7.2. Protocol Extensions...........................................14
   7.2.1 Guidance for Use of IF_ID Error Spec TLVs....................18
   7.2.2 Alternate Path identification................................20
   7.3. Action on Receiving Crankback Information.....................20
   7.3.1 Re-route Attempts............................................20
   7.3.2 Location Identifiers of Blocked Links or Nodes...............21
   7.3.3 Locating Errors within Loose or Abstract Nodes...............21
   7.3.4 When Re-routing Fails........................................21
   7.3.5 Aggregation of Crankback Information.........................22
   7.4. Notification of Errors........................................22
   7.4.1 ResvErr Processing...........................................22
   7.4.2 Notify Message Processing....................................23
   7.5. Error Values..................................................23
   7.6. Backward Compatibility........................................23
   8. Routing Protocol Interactions...................................23
   9. LSP Restoration Considerations..................................24
   9.1. Upstream of the Fault.........................................24
   9.2. Downstream of the Fault.......................................25
   10. IANA Considerations............................................25
   10.1. Error Codes..................................................25
   10.2. IF_ID_ERROR_SPEC TLVs........................................25

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   10.3. LSP_ATTRIBUTES Object........................................25
   11. Security Considerations........................................26
   12. Acknowledgments................................................26
   13. Intellectual Property Considerations...........................26
   14. Normative References...........................................26
   15. Informational References.......................................27
   16. Authors' Addresses.............................................27
   17. Full Copyright Statement.......................................28


Section A : Problem Statement


1. Summary for Sub-IP Area

1.1. Summary

   This document describes requirements, procedures and
   protocol extensions for Crankback Routing in MPLS and
   GMPLS networks. These extensions address some of the
   requirements laid out by the ITU-T for the Automatically
   Switched Optical Network (ASON). This is recognized in
   [ASON-REQ].

1.2. Related documents

   See the References Sections.

1.3. Where does it fit in the Picture of the Sub-IP Work

   This work is applicable to MPLS and GMPLS signaling protocols.

1.4. Why is it Targeted at this WG

   MPLS is a product of the MPLS WG, GMPLS is worked on by
   the CCAMP WG. This document provides common extensions
   for use in MPLS and GMPLS and so is appropriate for
   consideration by the CCAMP WG.

   The CCAMP charter now contains the work item:

    - Define signaling and routing mechanisms to make possible the
      creation of paths that span multiple IGP areas, multiple ASes,
      and multiple providers, including techniques for crankback.

1.5. Justification

   Crankback Signaling is a requirement in large and multi-
   area networks, in networks with rapidly changing
   topologies or resource usage, or in networks where setup
   latency may be high.

   The requirement for Crankback Routing in the Automatically
   Switched Optical Network (ASON) has been identified by the
   ITU-T [G8080] and recognized by the IETF in [ASON-REQ].



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2. Introduction and Framework

2.1. Background

   RSVP-TE (RSVP Extensions for LSP Tunnel) [RFC3209] can be
   used for establishing explicitly routed LSPs in an MPLS
   network. Using RSVP-TE, resources can also be reserved
   along a path to guarantee or control QoS for traffic
   carried on the LSP. To designate an explicit path that
   satisfies QoS constraints, it is necessary to discern the
   resources available to each link or node in the network.
   For the collection of such resource information, routing
   protocols, such as OSPF and IS-IS , can be extended to
   distribute additional state information [RFC2702].

   Explicit paths can be computed based on the distributed
   information at the LSR initiating a LSP and signaled as
   Explicit Routes during LSP establishment. Explicit Routes
   may contain 'loose hops' and 'abstract nodes' that convey
   routing through any of a collection of nodes. This
   mechanism may be used to devolve parts of the path
   computation to intermediate nodes such as area border LSRs.

   In a distributed routing environment, however, the
   resource information used to compute a constraint-based
   path may be out of date. This means that a setup request
   may be blocked, for example, because a link or node along
   the selected path has insufficient resources.

   In RSVP-TE, a blocked LSP setup may result in a PathErr
   message sent to the initiator or a ResvErr sent to the
   terminator (egress LSR). These messages may result in the
   LSP setup being abandoned. In Generalized MPLS [RC3473]
   the Notify message may additionally be used to expedite
   notification of LSP failures to ingress and egress LSRs,
   or to a specific "repair point".

   These existing mechanisms provide a certain amount of
   information about the path of the failed LSP.

2.2. Repair and Restoration

   If the ingress LSR or intermediate area border LSR knows
   the location of the blocked link or node, the LSR can
   designate an alternate path and then reissue the setup
   request. Determination of the identity of the blocked
   link or node can be achieved by the mechanism known as
   crankback routing [PNNI, ASH1]. In RSVP-TE, crankback
   signaling requires notifying an upstream LSR of the
   location of the blocked link or node. In some cases this
   requires more information than is currently available in
   the signaling protocols.

   On the other hand, various restoration schemes for link
   or node failures have been proposed in [RFC3469] and
   others including fast restoration. These schemes rely on
   the existence of a backup LSP to protect the primary, but

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   if both the primary and backup paths fail it is necessary
   to reestablish the LSP on an end-to-end basis avoiding
   the known failures. Similarly, fast restoration by
   establishing a restoration path on demand after failure
   requires computation of a new LSP that avoids the known
   failures. End-to-end restoration for alternate routing
   requires the location of the failed link or node.
   Crankback routing schemes could also be used to notify
   upstream LSRs of the location of the failure.

   Furthermore, in situations where many link or node
   failures occur at the same time, the difference between
   the distributed routing information and the real-time
   network state becomes much greater than in normal LSP
   setups. LSP restoration might, therefore, be performed
   with inaccurate information, which is likely to cause
   setup blocking. Crankback routing could improve failure
   recovery in these situations.

   Generalized MPLS [RFC3471] extends MPLS into networks
   that manage Layer2, TDM and lambda resources. In a
   network without wavelength converters, setup requests are
   likely to be blocked more often than in a conventional
   MPLS environment because the same wavelength must be
   allocated at each Optical Cross-Connect on an end-to-end
   explicit path. Furthermore, end-to-end restoration is the
   only way to recover LSP failures. This implies that
   crankback routing would also be useful in a GMPLS
   network, in particular in dynamic LSP re-routing cases
   (no backup LSP pre-establishment).

3. Discussion: Explicit Versus Implicit Re-routing Indications

   There have been problems in service provider networks
   when "inferring" from indirect information that re-
   routing is allowed. This document proposes the use of an
   explicit re-routing indication that explicitly authorizes
   re-routing.

   Various existing protocol options and exchanges including
   the error values of PathErr message [RFC2205, RFC3209]
   and the Notify message [RFC3473] allow an implementation
   to infer a situation where re-routing can be done. This
   allows for recovery from network errors or resource
   contention.

   However, such inference of recovery signaling is not
   always desirable since it may be doomed to failure.
   Experience of using release messages in TDM-based
   networks for analogous purposes provides some guidance.
   One can use the receipt of a release message with a cause
   value (CV) indicating "link congestion" to trigger a re-
   routing attempt at the originating node. However, this
   sometimes leads to problems.




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       *--------------------*  *-----------------*
       |                    |  |                 |
       |  N2 ----------- N3-|--|----- AT--- EO2  |
       |  |              | \|  |    / |          |
       |  |              |  |--|-  /  |          |
       |  |              |  |  | \/   |          |
       |  |              |  |  | /\   |          |
       |  |              |  |--|-  \  |          |
       |  |              | /|  |    \ |          |
       |  N1 ----------- N4-|--|----- EO1        |
       |                    |  |                 |
       *--------------------*  *-----------------*
                A-1                  A-2

           Figure 1. Example of network topology

   Figure 1 illustrates four examples based on service-
   provider experiences with respect to crankback (i.e.,
   explicit indication) versus implicit indication through a
   release with CV. In this example, N1, N2,N3, and N4 are
   located in one area (A-1), and AT, EO1, and EO2 are in
   another area (A-2).

   Note that two distinct areas are used in this example to
   expose the issues clearly. In fact, the issues are not
   limited to multi-area networks, but arise whenever path
   computation is distributed throughout the network. For
   example where loose routes, AS routes or path computation
   domains are used.

   1. A connection request from node N1 to EO1 may route to N4
      and then find "all circuits busy". N4 returns a release
      message to N1 with CV34 indicating all circuits busy.
      Normally, a node such as N1 is programmed to block a
      connection request when receiving CV34, although there is
      good reason to try to alternate route the connection request
      via N2 and N3.

      Some service providers have implemented a technique called
      route advance (RA), where if a node that is RA capable
      receives a release message with CV34, it will use this as an
      implicit re-route indication and try to find an alternate
      route for the connection request if possible. In this
      example, alternate route N1-N2-N3-EO1 can be tried and may
      well succeed.

   2. Suppose a connection request goes from N2 to N3 to AT
      trying to reach EO2 and is blocked at link AT-EO2. Node AT
      returns a CV34 and with RA, N2 may try to re-route N2-N1-N4-
      AT-EO2, but of course this fails again. The problem is that
      N2 does not realize where this blocking occurred based on
      the CV34, and in this case there is no point in further
      alternate routing.





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   3. However, in another case of a connection request from N2
      to E02, suppose that link N3-AT is blocked. In this case N3
      should return crankback information (and not CV34) so that
      N2 can alternate route to N1-N4-AT-EO2, which may well be
      successful.

   4. In a final example, for a connection request from EO1 to
      N2, EO1 first tries to route the connection request directly
      to N3. However, node N3 may reject the connection request
      even if there is bandwidth available on link N3-EO1 (perhaps
      for priority routing considerations, e.g., reserving
      bandwidth for high priority connection requests). However,
      when N3 returns CV34 in the release message, EO1 blocks the
      connection request (a normal response to CV34 especially if
      E01-N4 is already known blocked) rather than trying to
      alternate route through AT-N3-N2, which might be successful.
      If N3 returns crankback information, EO1 could respond by
      trying the alternate route.

   It is certainly the case that with topology exchange,
   such as OSPF, the ingress LSR could infer the re-routing
   condition. However, convergence of routing information is
   typically slower than the expected LSP setup times. One
   of the reasons for crankback is to avoid the overhead of
   available-link-bandwidth flooding, and to more efficiently
   use local state information to direct alternate routing at
   the ingress-LSR.

   [ASH1] shows how event-dependent-routing can just use crankback,
   and not available-link-bandwidth flooding, to decide on the re-
   route path in the network through "learning models". Reducing
   this flooding reduces overhead and can lead to the ability to
   support much larger AS sizes.

   Therefore, the alternate routing should be indicated based on
   an explicit indication (as in examples 3 and 4), and it is best
   to know the following information separately:

     a) where blockage/congestion occurred (as in examples 1-2),

        and

     b) whether alternate routing "should" be attempted even if
        there is no "blockage" (as in example 4).

4. Required Operation

   Section 2 identifies some of the circumstances under which
   crankback may be useful. Crankback routing is performed as
   described in the following procedures, when an LSP setup
   request is blocked along the path or when an existing LSP fails.







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4.1. Resource Failure or Unavailability

   When an LSP setup request is blocked due to unavailable
   resources, an error message response with the location
   identifier of the blockage should be returned to the LSR
   initiating the LSP setup (ingress LSR), the area border
   LSR, the AS border LSR, or to some other repair point.

   This error message carries an error specification
   according to [RFC3209] - this indicates the cause of the
   error and the node/link on which the error occurred.
   Crankback operation may require further information as
   detailed in section 6.

4.2. Computation of an Alternate Path

   In a flat network without partitioning, when the ingress
   LSR receives the error message it computes an alternate
   path around the blocked link or node to satisfy QoS
   constraints using link state information about the area.
   If an alternate path is found, a new LSP setup request is
   sent over this path.

   On the other hand, in a network partitioned into areas
   such as with hierarchical OSPF, an area border LSR may
   intercept and terminate the error response, and perform
   alternate (re-)routing within the downstream area.

   In a third scenario, any node within an area may act as a
   repair point. In this case, the LSR behaves much as an
   area border LSR as described above. It can intercept and
   terminate the error response, and perform alternate
   routing. This may be particularly useful where domains of
   computation are applied within the network, however if
   all nodes in the network perform re-routing it is
   possible to spend excessive network and CPU resources on
   re-routing attempts that would be better made only at
   designated re-routing nodes. This scenario is somewhat
   like 'MPLS fast re-route' [FASTRR], in which any node in
   the MPLS domain can establish 'local repair' LSPs after
   failure notification.

4.2.1 Information Required for Re-routing

   In order to correctly compute a route that avoids the
   blocking problem, a repair point LSR must gather as much
   crankback information as possible. Ideally, the repair
   node will be given the node, link and reason for the
   failure.

   However, this information may not be enough to help with
   re-computation. Consider for instance an explicit route
   that contains a non-explicit abstract node or a loose
   hop. In this case, the failed node and link is not
   necessarily enough to tell the repair point which hop in
   the explicit route has failed. The crankback information
   needs to provide the context into the explicit route.

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4.2.2 Signaling a New Route

   If the crankback information can be used to compute a new
   route avoiding the blocking problem, the route can be
   signaled as an Explicit Route.

   However, it may be that the repair point does not have
   sufficient topology information to compute an Explicit
   Route that is guaranteed to avoid the failed link or
   node. In this case, Route Exclusions [EXCLUDE] may be
   particularly helpful. To achieve this, [EXCLUDE] allows
   the crankback information to be presented as route exclusions
   to force avoidance of the failed node, link or resource.

4.3. Persistence of Error Information

   The repair point LSR that computes the alternate path
   should store the location identifiers of the blockages
   indicated in the error message until the LSP is
   successfully established or until the LSR abandons re-
   routing attempts. Since crankback routing may happen more
   than once while establishing a specific LSP, a history
   table of all experienced blockages for this LSP SHOULD be
   maintained (at least until the routing protocol updates
   the state of this information) to perform an accurate
   path computation to detour all blockages.

   If a second error response is received by a repair point
   (while it is performing crankback re-routing) it should
   update the history table that lists all experienced
   blockages, and use the entire gathered information when
   making a further re-routing attempt.

4.4. Handling Re-route Failure

   Multiple blockages (for the same LSP) may occur, and
   successive setup retry attempts may fail. Retaining error
   information from previous attempts ensures that there is
   no thrashing of setup attempts, and knowledge of the
   blockages increases with each attempt.

   It may be that after several retries, a given repair
   point is unable to compute a path to the destination
   (that is, the egress of the LSP) that avoids all of the
   blockages. In this case, it must pass the error
   indication upstream. It is most useful to the upstream
   nodes (and in particular the ingress LSR) that may,
   themselves, attempt new routes for the LSP setup if the
   error indication in this case identifies all of the
   downstream blockages and also the node that has been
   unable to compute an alternate path.







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4.5. Limiting Re-routing Attempts

   It is important to prevent an endless repetition of LSP
   setup attempts using crankback routing information after
   error conditions are signaled, or during periods of high
   congestion. It may also be useful to reduce the number of
   retries, since failed retries will increase setup latency
   and degrade performance.

   The maximum number of crankback re-routing attempts
   allowed may be limited in a variety of ways. The number
   may be limited by LSP, by node, by area or by AS. Control
   of the limit may be applied as a configuration item per
   LSP, per node, per area or per AS.

   When the number of retries at a particular node, area or
   AS is exceeded, the LSR handling the current failure
   reports the failure upstream to the next node, area or AS
   where further re-routing attempts may be attempted. It is
   important that the crankback information provided
   indicates that routing back through this node, area or AS
   will not succeed - this situation is similar to that in
   section 4.4. Note that in some circumstances, such a
   report will also mean that no further re-routing attempts
   can possibly succeed - for example, when the egress node
   is within the failed area.

   When the maximum number of retries for a specific LSP has
   been exceeded, the LSR handling the current failure
   should send an error message upstream indicating "Maximum
   number of re-routings exceeded". This error will be
   passed back to the ingress LSR with no further re-routing
   attempts. The ingress LSR may choose to retry the LSP
   setup according to local policy and might choose to re-
   use its original path or seek to compute a path that
   avoids the blocked resources. In the latter case, it may
   be useful to indicate the blocked resource in this error
   message.

5. Existing Protocol Support for Crankback Re-routing

   Crankback re-routing is appropriate for use with RSVP-TE.

   1) Path establishment may fail because of an inability to
      route, perhaps because links are down. In this case a
      PathErr message is returned to the initiator.

   2) Path establishment may fail because resources are
      unavailable. This is particularly relevant in GMPLS where
      explicit label control may be in use. Again, a PathErr
      message is returned to the initiator.

   3) Resource reservation may fail in the upstream direction,
      as the Resv is processed, and resources are reserved. If
      resources are not available on the required link or at a
      specific node, a ResvErr message is returned to the egress
      node indicating "Admission Control failure" [RFC2205]. The

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      egress is allowed to change the FLOWSPEC and try again, but
      in the event that this is not practical or not supported
      (particularly in the GMPLS context), the egress LSR may
      choose to take any one of the following actions.

      - Ignore the situation and allow recovery to happen through
        Path refresh message and refresh timeout [RFC2205].
      - Send a PathErr message towards the initiator indicating
        "Admission Control failure".
      - Send a ResvTear message towards the initiator to abort
        the LSP setup.

      Note that in multi-area networks, the ResvErr might be
      intercepted and acted on at an area border router.

   4) It is also possible to make resource reservations on the
      forward path as the Path message is processed. This choice
      is compatible with LSP setup in GMPLS networks [RFC3471]. In
      this case if resources are not available, a PathErr message
      is returned to initiator indicating "Admission Control
      failure".

   Crankback information would be useful to an upstream node
   (such as the ingress) if it is supplied on a PathErr or a
   Notify message that is sent upstream.

5.1. RSVP-TE [RFC 3209]

   In RSVP-TE a failed LSP setup attempt results in a PathErr
   message returned upstream. The PathErr message carries an
   ERROR_SPEC object, which indicates the node or interface
   reporting the error and the reason for the failure.

   Crankback re-routing can be performed explicitly avoiding
   the node or interface reported.

5.2. GMPLS-RSVP-TE [RFC 3473]

   GMPLS extends the error reporting described above by
   allowing LSRs to report the interface that is in error in
   addition to the identity of the node reporting the error.
   This further enhances the ability of a re-computing node
   to route around the error.

   GMPLS introduces a targeted Notify message that may be
   used to report LSP failures direct to a selected node.
   This message carries the same error reporting facilities
   as described above. The Notify message may be used to
   expedite the propagation of error notifications, but in a
   network that offers crankback routing at multiple nodes
   there would need to be some agreement between LSRs as to
   whether PathErr or Notify provides the stimulus for
   crankback operation. Otherwise, multiple nodes might
   attempt to repair the LSP at the same time, in particular
   because 1) these messages can flow through different
   paths before reaching the ingress LSR and 2) the destination
   of the Notify message might not be the ingress LSR.

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Section B : Solution

6. Control of Crankback Operation

6.1. Requesting Crankback and Controlling In-Network Re-routing

   When a request is made to set up an LSP tunnel, the ingress
   LSR should specify whether it wants crankback information to
   be collected in the event of a failure and whether it requests
   re-routing attempts by any or specific intermediate nodes. For
   this purpose, a Re-routing Flag field is added to the protocol
   setup request messages. The corresponding values are mutually
   exclusive.

   No Re-routing          Intermediate nodes SHOULD NOT attempt
                          re-routing after failure. Nodes detecting
                          failures MUST report an error and MAY supply
                          crankback information. This is the default
                          and backwards compatible option.

   End-to-end Re-routing  Intermediate nodes SHOULD NOT attempt
                          re-routing after failure. Nodes detecting
                          failures MUST report an error and SHOULD
                          supply crankback information.

   Boundary Re-routing    Intermediate nodes MAY attempt re-routing
                          after failure only if they are Area Border
                          Routers or AS Border Routers. The boundary
                          ABR/ASBR can either decide to forward the
                          Path Error message upstream to the Head-end
                          LSR or try to select another egress boundary
                          LSR. Other nodes SHOULD NOT attempt re-
                          routing. Nodes detecting failures MUST
                          report an error and SHOULD supply crankback
                          information.

   Segment-based Re-routing
                          All intermediate nodes MAY attempt re-
                          routing after failure. Nodes detecting
                          failures MUST report an error and SHOULD
                          supply full crankback information.

6.2. Action on Detecting a Failure

   A node that detects the failure to setup an LSP or the
   failure of an established LSP SHOULD act according to the
   Re-routing Flag passed on the LSP setup request.

   If Segment-based Re-routing is allowed or if Boundary Re-
   routing is allowed and the detecting node is an ABR or ASBR,
   the detecting node MAY immediately attempt to re-route.

   If End-to-end Re-routing is indicated, or if Segment-based or
   Boundary Re-routing is allowed and the detecting node chooses
   not to make re-routing attempts (or has exhausted all possible
   re-routing attempts), the detecting node returns a protocol
   error indication and SHOULD include full crankback information.

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6.3. Limiting Re-routing Attempts

   Each repair point should apply a locally configurable
   limit to the number of attempts it makes to re-route an
   LSP. This helps to prevent excessive network usage in the
   event of significant faults and allows back-off to other
   repair points which may have a better chance of routing
   around the problem.

6.3.1 New Status Codes for Re-routing

   An error code/value of "Routing Problem"/"Re-routing
   limit exceeded" (24/TBD) is used to identify that a node
   has abandoned crankback re-routing because it has reached
   a threshold for retry attempts.

   A node receiving an error response with this status code
   MAY also attempt crankback re-routing, but it is RECOMMENDED
   that such attempts be limited to the ingress LSR.

6.4. Protocol Control of Re-routing Behavior

   The Session Attributes Object in RSVP-TE is used on Path
   messages to indicate the capabilities and attributes of the
   session. This object contains an 8-bit flag field which is
   used to signal individual Boolean capabilities or attributes.
   The Re-Routing Flag described in section 5.1 would fit
   naturally into this field, but there is a scarcity of bits, so
   use is made of the new LSP_ATTRIBUTES object defined in
   [LSP-ATTRIB]. Three bits are defined for inclusion in the LSP
   Attributes TLV as follows. The values below are suggested and
   actual values are TBD by IETF consensus.

      0x01 End-to-end re-routing desired
           This flag indicates the end-to-end re-
           routing behavior for an LSP under
           establishment. This MAY also be used
           for specifying the behavior of end-to-
           end LSP restoration for established LSPs.

      0x02 Boundary re-routing desired.
           This flag indicates the boundary re-routing
           behavior for an LSP under establishment.
           This MAY also be used for specifying the
           segment-based (hierarchical) LSP restoration
           for established LSPs. The boundary ABR/ASBR
           can either decide to forward the PathErr
           message upstream to the Head-end LSR or try
           to select another egress boundary LSR.

      0x04 Segment-based re-routing desired.
           This flag indicates the segment-based
           re-routing behavior for an LSP under
           establishment. This MAY also be used
           for specifying the segment-based LSP
           restoration for established LSPs.


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7. Reporting Crankback Information

7.1. Required Information

   As described above, full crankback information should
   indicate the node, link and other resources, which have
   been attempted but have failed because of allocation
   issues or network failure.

   The default crankback information SHOULD include the
   interface and the node address.

7.2. Protocol Extensions

   [RFC3473] defines an IF_ID ERROR_SPEC Object that can be
   used on PathErr, ResvErr and Notify messages to convey
   the information carried in the Error Spec Object defined
   in [RFC 3209]. Additionally, it has scope for carrying
   TLVs that help identify the identity of the link
   associated with the error.

   The TLVs for use with this object are defined in
   [RFC3471], and are as follows. They are used to identify
   links in the IF_ID PHOP Object and in the IF_ID
   ERROR_SPEC Object to identify the failed resource which
   is usually the downstream resource from the reporting
   node.

   Type Length Format     Description
   --------------------------------------------------------------------
    1      8   IPv4 Addr. IPv4                    (Interface address)
    2     20   IPv6 Addr. IPv6                    (Interface address)
    3     12   Compound   IF_INDEX                (Interface index)
    4     12   Compound   COMPONENT_IF_DOWNSTREAM (Component interface)
    5     12   Compound   COMPONENT_IF_UPSTREAM   (Component interface)

   Two new TLVs are defined for use in the IF_ID PHOP Object
   and in the IF_ID Error Spec Object. Note that the Type
   values shown here are only suggested values - final
   values are TBD and to be determined by IETF consensus.

   Type Length Format     Description
   --------------------------------------------------------------------
    6     16   See below  UNUM_COMPONENT_IF_DOWN  (Component interface)
    7     16   See below  UNUM_COMPONENT_IF_UP    (Component interface)

   In order to facilitate reporting of crankback
   information, the following additional TLVs are defined.
   Note that the Type values shown here are only suggested
   values - final values are TBD and to be determined by
   IETF consensus.







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   Type Length Format     Description
   --------------------------------------------------------------------
    8    var   See below  DOWNSTREAM_LABEL        (GMPLS label)
    9    var   See below  UPSTREAM_LABEL          (GMPLS label)
   10      8   See below  NODE_ID                 (Router Id)
   11      x   See below  OSPF_AREA               (Area Id)
   12      x   See below  ISIS_AREA               (Area Id)
   13      8   See below  AUTONOMOUS_SYSTEM       (Autonomous system)
   14    var   See below  ERO_CONTEXT             (ERO subobject)
   15    var   See below  ERO_NEXT_CONTEXT        (ERO subobjects)
   16      8   IPv4 Addr. PREVIOUS_HOP_IPv4       (Node address)
   17     20   IPv6 Addr. PREVIOUS_HOP_IPv6       (Node address)
   18      8   IPv4 Addr. INCOMING_IPv4           (Interface address)
   19     20   IPv6 Addr. INCOMING_IPv6           (Interface address)
   20     12   Compound   INCOMING_IF_INDEX       (Interface index)
   21     12   Compound   INCOMING_COMP_IF_DOWN   (Component interface)
   22     12   Compound   INCOMING_COMP_IF_UP     (Component interface)
   23     16   See below  INCOMING_UNUM_COMP_DOWN (Component interface)
   24     16   See below  INCOMING_UNUM_COMP_UP   (Component interface)
   25    var   See below  INCOMING_DOWN_LABEL     (GMPLS label)
   26    var   See below  INCOMING_UP_LABEL       (GMPLS label)
   27      8   See below  REPORTING_NODE_ID       (Router Id)
   28      x   See below  REPORTING_OSPF_AREA     (Area Id)
   29      x   See below  REPORTING_ISIS_AREA     (Area Id)
   30      8   See below  REPORTING_AS            (Autonomous system)
   31    var   See below  PROPOSED_ERO            (ERO subobjects)
   32    var   See below  NODE_EXCLUSIONS         (List of nodes)
   33    var   See below  LINK_EXCLUSIONS         (List of interfaces)

   For types 1, 2, 3, 4 and 5, the format of the Value field
   is already defined in [RFC3471].

   For types 16 and 18, they format of the Value field is
   the same as for type 1.

   For types 17 and 19, the format of the Value field is the
   same as for type 2.

   For types 20, 21 and 22, the formats of the Value fields
   are the same as for types 3, 4 and 5 respectively.

   For types 6, 7, 23 and 24 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            IP Address                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Interface ID                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Component ID                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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       IP Address: 32 bits

          The IP address field may carry either an IP
          address associated with the router, where
          associated address is the value carried in
          a router address TLV of routing.

       Interface ID: 32 bits

          The Interface ID identifier of the
          unnumbered link.

       Component ID: 32 bits

          A bundled component link. The special value
          0xFFFFFFFF can be used to indicate the same
          label is to be valid across all component
          links.

   For types 8, 9, 25 and 26 the length field is variable
   and the Value field is a label as defined in [RFC3471].
   As with all uses of labels, it is assumed that any node
   that can process the label information knows the syntax
   and semantics of the label from the context. Note that
   all TLVs are zero-padded to a multiple four octets so
   that if a label is not itself a multiple of four octets
   it must be disambiguated from the trailing zero pads by
   knowledge derived from the context.

   For types 10 and 27 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Router Id                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Router Id: 32 bits

          The Router Id used to identify the node within the IGP.

   For types 11 and 28 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     OSPF Area Identifier                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       OSPF Area Identifier

          The 4-octet area identifier the node is part of. In the case of
          ABRs, this identifies the area where the failure has occurred.





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   For types 12 and 29 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Length      |     ISIS Area Identifier                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                     ISIS Area Identifier (continued)          ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Length

          Length of the actual (non-padded) ISIS Area Identifier
          in octets. Valid values are from 2 to 11 inclusive.

       ISIS Area Identifier

          The variable-length ISIS area identifier. Padded with
          trailing zeroes to a four-octet boundary.

   For types 13 and 30 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Autonomous System Number                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Autonomous System Number: 32 bits

          The AS Number of the associated Autonomous System. Note
          that if 16-bit AS numbers are in use, the low order bits
          (16 through 31) should be used and the high order bits
          (0 through 15) should be set to zero.

   For types 14, 15 and 31 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                       ERO Subobjects                          ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       ERO Subobjects:

          A sequence of ERO subobjects. Any ERO subobjects are
          allowed whether defined in [RFC3209], [RFC3473] or other
          documents. Note that ERO subobjects contain their own
          type and length fields.







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   For type 32 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                       Node Identifiers                        ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Node Identifiers:

          A sequence of TLVs as defined here of types 1, 2 or 10
          that indicates downstream nodes that have already
          participated in crankback attempts and have been declared
          unusable for the current LSP setup attempt.

   For type 33 the Value field has the format:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                       Link Identifiers                        ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Link Identifiers:

          A sequence of TLVs as defined here of types 3, 4, 5, 6 or 7
          that indicates incoming interfaces at downstream nodes that
          have already participated in crankback attempts and have
          been declared unusable for the current LSP setup attempt.

7.2.1 Guidance for Use of IF_ID Error Spec TLVs

   If Crankback is not being used but an IF-ID Error_Spec
   Object is included in a PathErr, ResvErr or Notify
   message, the sender SHOULD include one of the TLVs of
   type 1 through 5 as described in [RFC3473]. A sender that
   wishes to report an error with a component link of an
   unnumbered bundle SHOULD use the new TLVs of type 6 or 7
   as defined in this document. A sender MAY include
   additional TLVs from the range 8 through 33 to report
   crankback information, although this information will at
   most only be used for logging.

   If Cranback is being used, the sender of a PathErr,
   ResvErr or Notify message MUST use the IF_ID Error_Spec
   Object and MUST include at least one of the TLVs in the
   range 1 through 7 as described in [RFC3473] and the
   previous paragraph. Additional TLVs SHOULD also be
   included to report further information. Note that all
   such TLVs are optional and MAY be omitted. Inclusion of
   the optional TLVs SHOULD be performed where doing so
   helps to facilitate error reporting and crankback. The
   TLVs fall into three categories: those that are essential

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   to report the error, those that provide additional
   information that is or may be fundamental to the utility
   of cranback, and those that provide additional
   information that may be useful for crankback in some
   circumstances.

   Many of the TLVs report the specific resource that has
   failed. For example, TLV type 1 can be used to report that
   the setup attempt was blocked by some form of resource
   failure on a specific interface identified by the IP
   address supplied. TLVs in this category are 1 through 13.
   These TLVs SHOULD be supplied whenever the node detecting
   and reporting the failure with crankback information has
   the information available. The use of TLVs of type 10, 11,
   12 and 13, MAY, however, be omitted according to local
   policy and relevance of the information.

   Reporting nodes SHOULD also supply TLVs from the range 14
   through 26 as appropriate for reporting the error. The
   reporting nodes MAY also supply TLVs from the range 27
   through 33.

   Note that in deciding whether a TLV in the range 14
   through 26 "is appropriate", the reporting node should
   consider amongst other things, whether the information is
   pertinent to the cause of the failure. For example, when
   a cross-connection fails it may be that the outgoing
   interface is faulted, in which case only the interface
   (for example, TLV type 1) needs to be reported, but if
   the problem is that the incoming interface cannot be
   connected to the outgoing interface because of temporary
   or permanent cross-connect limitations, the node should
   also include reference to the incoming interface (for
   example, TLV type 18).

   Some TLVs help to locate the fault within the context of
   the path of the LSP that was being set up. TLVs of types
   14, 15, 16 and 17 help to set the context of the error
   within the scope of an explicit path that has loose hops
   or non-precise abstract nodes. The ERO context
   information is not always a requirement, but a node may
   notice that it is a member of the next hop in the ERO
   (such as a loose or non-specific abstract node) and
   deduce that its upstream neighbor may have selected the
   path using next hop routing. In this case, providing the
   ERO context will be useful to the node further that
   performs re-routing.

   Four TLVs (27, 28, 29 and 30) allow the location of the
   reporting node to be expanded upon. These TLVs would not
   be included if the information is not of use within the
   local system, but might be added by ABRs relaying the
   error. Note that the Reporting Node Id (TLV 27) need not
   be included if the IP address of the reporting node as
   indicated in the Error Spec itself, is sufficient to
   fully identify the node.


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   The last three TLVs (31, 32, and 33) provide additional
   information for recomputation points. The reporting node
   (or some node forwarding the error) may supply
   suggestions about the ERO that could have been used to
   avoid the error. As the error propagates back upstream
   and as crankback routing is attempted and fails, it is
   beneficial to collect lists of failed nodes and links so
   that they will not be included in further computations
   performed at upstream nodes. Theses lists may also be
   factored into route exclusions [EXCLUDE].

   Note that there is no ordering requirement on any of the
   TLVs within the IF_ID Error Spec, and no implication
   should be drawn from the ordering of the TLVs in a
   received IF_ID Error Spec.

   It is left as an implementation detail precisely when to
   include each of the TLVs according to the capabilities of
   the system reporting the error.

7.2.2 Alternate Path identification

   No new object is used to distinguish between Path/Resv
   messages for an alternate LSP. Thus, the alternate LSP
   uses the same SESSION and SENDER_TEMPLATE/FILTER_SPEC
   objects as the ones used for the initial LSP under re-
   routing.

7.3. Action on Receiving Crankback Information

7.3.1 Re-route Attempts

   As described in section 3, a node receiving crankback
   information in a PathErr must first check to see whether
   it is allowed to perform re-routing. This is indicated by
   the Re-routing Flags in the SESSION_ATTRIBUTE object
   during LSP setup request.

   If a node is not allowed to perform re-routing it should
   forward the PathErr message, or if it is the ingress
   report the LSP as having failed.

   If re-routing is allowed, the node should attempt to
   compute a path to the destination using the original
   (received) explicit path and excluding the failed/blocked
   node/link. The new path should be added to an LSP setup
   request as an explicit route and signaled.

   LSRs performing crankback re-routing should store all received
   crankback information for an LSP until the LSP is successfully
   established or until the node abandons its attempts to re-route
   the LSP. This allows the combination of crankback information
   from multiple failures when computing an alternate path.

   It is an implementation decision whether the crankback
   information is discarded immediately upon successful LSP
   establishment or retained for a period in case the LSP fails.

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7.3.2 Location Identifiers of Blocked Links or Nodes

   In order to compute an alternate path by crankback re-
   routing, it is necessary to identify the blocked links or
   nodes and their locations. The common identifier of each
   link or node in an MPLS network should be specified. Both
   protocol-independent and protocol- dependent identifiers
   may be specified. Although a general identifier that is
   independent of other protocols is preferable, there are a
   couple of restrictions on its use as described in the
   following subsection.

   In link state protocols such as OSPF and IS-IS , each
   link and node in a network can be uniquely identified.
   For example, by the context of a Router ID and the Link
   ID. If the topology and resource information obtained by
   OSPF advertisements is used to compute a constraint-based
   path, the location of a blockage can be represented by
   such identifiers.

   Note that, when the routing-protocol-specific link
   identifiers are used, the Re-routing Flag on the LSP
   setup request must have been set to show support for
   boundary or segment-based re-routing.

   In this document, we specify routing protocol specific
   link and node identifiers for OSPFv2 for IPv4, IS-IS for
   IPv4, OSPF for IPv6, and IS-IS for IPv6. These
   identifiers may only be used if segment-based re-routing
   is supported, as indicated by the Routing Behavior flag
   on the LSP setup request.

7.3.3 Locating Errors within Loose or Abstract Nodes

   The explicit route on the original LSP setup request may
   contain a loose or an Abstract Node. In these cases, the
   crankback information may refer to links or nodes that
   were not in the original explicit route.

   In order to compute a new path, the repair point may need
   to identify the pair of hops (or nodes) in the explicit
   route between which the error/blockage occurred.

   To assist this, the crankback information reports the top
   two hops of the explicit route as received at the
   reporting node. The first hop will likely identify the
   node or the link, the second hop will identify a 'next'
   hop from the original explicit route.

7.3.4 When Re-routing Fails

   When a node cannot or chooses not to perform crankback re-
   routing it must forward the PathErr message further upstream.

   However, when a node was responsible for expanding or
   replacing the explicit route as the LSP setup was
   processed it MUST update the crankback information with

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   regard to the explicit route that it received. Only if
   this is done will the upstream nodes stand a chance of
   successfully routing around the problem.

7.3.5 Aggregation of Crankback Information

   When a setup blocking error or an error in an established
   LSP occurs and cranback information is sent in an error
   notification message, some node upstream may choose to
   attempt crankback re-routing. If that node's attempts at
   re-routing fail the node will accumulate a set of failure
   information. When the node gives up it must propagate the
   failure message further upstream and include crankback
   information when it does so.

   There is not scope in the protocol extensions described
   in this document to supply a full list of all of the
   failures that have occurred. Such a list would be
   indefinitely long and would include more detail than is
   required. However, TLVs 32 and 33 allow lists of unusable
   links and nodes to be accumulated as the failure is
   passed back upstream.

   Aggregation may involve reporting all links from a node
   as unusable by flagging the node as unusable, or flagging
   an ABR as unusable when there is no downstream path
   available, and so on. The precise details of how
   aggregation of crankback information is performed are
   beyond the scope of this document.

7.4. Notification of Errors

7.4.1 ResvErr Processing

   As described above, the resource allocation failure for
   RSVP-TE may occur on the reverse path when the Resv
   message is being processed. In this case, it is still
   useful to return the received crankback information to
   the ingress LSR. However, when the egress LSR receives
   the ResvErr message, per RFC 2205 it still has the option
   of re-issuing the Resv with different resource
   requirements (although not on an alternate path).

   When a ResvErr carrying crankback information is received at
   an egress LSR, the egress LSR MAY ignore this object and
   perform the same actions as for any other ResvErr. However,
   if the egress LSR supports the crankback extensions defined
   in this document, and after all local recovery procedures
   have failed, it SHOULD generate a PathErr message carrying
   the crankback information and send it to the ingress LSR.

   If a ResvErr reports on more than one FILTER_SPEC
   (because the Resv carried more than one FILTER_SPEC) then
   only one set of crankback information should be present
   in the ResvErr and it should apply to all FILTER_SPEC
   carried. In this case, it may be necessary per [RFC 2205]
   to generate more than one PathErr.

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7.4.2 Notify Message Processing

   [RFC3473] defines the Notify message to enhance error
   reporting in RSVP-TE networks. This message is not
   intended to replace the PathErr and ResvErr messages. The
   Notify message is sent to addresses requested on the Path
   and Resv messages. These addresses could (but need not)
   identify the ingress and egress LSRs respectively.

   When a network error occurs, such as the failure of link
   hardware, the LSRs that detect the error MAY send Notify
   messages to the requested addresses. The type of error
   that causes a Notify message to be sent is an
   implementation detail.

   In the event of a failure, an LSR that supports [RFC3473]
   and the crankback extensions defined in this document MAY
   choose to send a Notify message carrying crankback
   information. This would ensure a speedier report of the
   error to the ingress/egress LSRs.

7.5. Error Values

   Error values for the Error Code "Admission Control
   Failure" are defined in [RFC2205]. Error values for the
   error code "Routing Problem" are defined in [RFC 3209]
   and [RFC 3473].

   A new error value is defined for the error code "Routing
   Problem". "Re-routing limit exceeded" indicates that re-
   routing has failed because the number of crankback re-
   routing attempts has gone beyond the predetermined
   threshold at an individual LSR.

7.6. Backward Compatibility

   It is recognized that not all nodes in an RSVP-TE network
   will support the extensions defined in this document. It
   is important that an LSR that does not support these
   extensions can continue to process a PathErr, ResvErr or
   Notify message even if it carries the newly defined IF_ID
   ERROR_SPEC information (TLVs).

8. Routing Protocol Interactions

   If the routing-protocol-specific link or node identifiers
   are used in the Link and Node IF_ID ERROR_SPEC TLVs
   defined above, the signaling has to interact with the
   OSPF/IS-IS routing protocol.

   For example, when an intermediate LSR issues a PathErr
   message, the signaling module of the intermediate LSR
   should interact with the routing logic to determine the
   routing-protocol-specific link or node ID where the
   blockage or fault occurred and carry this information
   onto the Link TLV and Node TLV inside the IF_ID
   ERROR_SPEC object. The ingress LSR, upon receiving the

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   error message, should interact with the routing logic to
   compute an alternate path by pruning the specified link
   ID or node ID in the routing database.

   Procedures concerning these protocol interactions are out
   of scope of this document.

9. LSP Restoration Considerations

   LSP restoration is performed to recover an established
   LSP when a failure occurs along the path. In the case of
   LSP restoration, the extensions for crankback re-routing
   explained above can be applied for improving performance.
   This section gives an example of applying the above
   extensions to LSP restoration. The goal of this example
   is to give a general overview of how this might work, and
   not to give a detailed procedure for LSP restoration.

   Although there are several techniques for LSP
   restoration, this section explains the case of on-demand
   LSP restoration, which attempts to set up a new LSP on
   demand after detecting an LSP failure.

9.1. Upstream of the Fault

   When an LSR detects a fault on an adjacent downstream
   link or node, a PathErr message is sent upstream. In
   GMPLS, the ERROR_SPEC object may carry a
   Path_State_Remove_Flag indication. Each LSR receiving the
   message then releases the corresponding LSP. (Note that
   if the state removal indication is not present on the
   PathErr message, the ingress node must issue a PathTear
   message to cause the resources to be released.) If the
   failed LSP has to be restored at an upstream LSR, the
   IF_ID ERROR SPEC that includes the location information
   of the failed link or node is included in the PathErr
   message. The ingress, intermediate area border LSR, or
   indeed any repair point permitted by the Re-routing
   Flags, that receives the PathErr message can terminate
   the message and then perform alternate routing.

   In a flat network, when the ingress LSR receives the
   PathErr message with the IF_ID ERROR_SPEC TLVs, it
   computes an alternate path around the blocked link or
   node satisfying the QoS constraints. If an alternate path
   is found, a new Path message is sent over this path
   toward the egress LSR.

   In a network segmented into areas, the following
   procedures can be used. As explained in Section 8.2, the
   LSP restoration behavior is indicated in the Flags field
   of the SESSION_ATTRIBUTE object of the Path message. If
   the Flags indicate "End-to-end re-routing", the PathErr
   message is returned all the way back to the ingress LSR,
   which may then issue a new Path message along another
   path, which is the same procedure as in the flat network
   case above.

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   If the Flags field indicates Boundary re-routing, the
   ingress area border LSR MAY terminate the PathErr message
   and then perform alternate routing within the area for
   which the area border LSR is the ingress LSR.

   If the Flags field indicates segment-based re-routing,
   any node MAY apply the procedures described above for
   Boundary re-routing.

9.2. Downstream of the Fault

   This section only applies to errors that occur after an
   LSP has been established. Note that an LSR that generates
   a PathErr with Path_State_Remove Flag SHOULD also send a
   PathTear downstream to clean up the LSP.

   A node that detects a fault and is downstream of the
   fault MAY send a PathErr or Notify message containing an
   IF_ID ERROR SPEC that includes the location information
   of the failed link or node, and MAY send a PathTear to
   clean up the LSP at all other downstream nodes. However,
   if the reservation style for the LSP is Shared Explicit (SE)
   the detecting LSR MAY choose not to send a PathTear - this
   leaves the downstream LSP state in place and facilitates
   make-before-break repair of the LSP re-utilizing downstream
   resources. Note that if the detecting node does not send a
   PathTear immediately then unused sate will timeout according
   to the normal rules of [RFC2205].

   At a well-known merge point, an ABR or an ASBR, a similar
   decision might also be made so as to better facilitate
   make-before-break repair. In this case a received
   PathTear might be 'absorbed' and not propagated further
   downstream for an LSP that has SE reservation style.
   Note, however, that this is a divergence from the protocol
   and might severely impact normal tear-down of LSPs.

10. IANA Considerations

10.1 Error Codes

   A new error value is defined for the RSVP-TE "Routing
   Problem" error code that is defined in [RFC3209].

   TBD     Re-routing limit exceeded.

10.2 IF_ID_ERROR_SPEC TLVs

   Note that the IF_ID_ERROR_SPEC TLV type values are not
   currently tracked by IANA. This might be a good
   opportunity to move them under IANA control.

10.3 LSP_ATTRIBUTES Object

   Three bits are defined for inclusion in the LSP
   Attributes TLV of the LSP_ATTRIBUTES object. IANA is
   requested to assign those bits.

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11. Security Considerations

   It should be noted that while the extensions in this
   document introduce no new security holes in the
   protocols, should a malicious user gain protocol access
   to the network, the crankback information might be used
   to prevent establishment of valid LSPs.

   The implementation of re-routing attempt thresholds are
   particularly important in this context.

   The crankback routing extensions and procedures for LSP
   restoration as applied to RSVP-TE introduce no further
   new security considerations. Refer to [RFC2205],
   [RFC3209] and [RFC3473] for a description of applicable
   security considerations.

12. Acknowledgments

   We would like to thank Juha Heinanen and Srinivas Makam
   for their review and comments, and Zhi-Wei Lin for his
   considered opinions. Thanks, too, to John Drake for
   encouraging us to resurrect this document and consider
   the use of the IF-ID ERROR SPEC object. Thanks for a
   welcome and very thorough review by Dimitri Papadimitriou.

13. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication 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 implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

14. Normative References

   [RFC2205]    R. Braden, et al., "Resource ReSerVation Protocol (RSVP)
                Version 1 Functional Specification", RFC2205, September
                1997.

   [RFC3209]    D. Awduche, et al., "RSVP-TE: Extensions to RSVP for LSP
                Tunnels", RFC3209, December 2001.


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   [RFC3471]    P. Ashwood-Smith and L. Berger, et al., "Generalized
                MPLS - Signaling Functional Description", RFC 3471,
                January 2003.

   [RFC3473]    L. Berger, et al., "Generalized MPLS Signaling - RSVP-TE
                Extensions", RFC 3473, January 2003.

   [LSP-ATTRIB] A. Farrel, D. Papadimitriou, JP. Vasseur, "Encoding of
                Attributes for Multiprotocol Label Switching (MPLS)
                Label Switched Path (LSP) Establishment Using RSVP-TE",
                draft-farrel-mpls-rsvpte-attributes-00.txt, October
                2003, work in progress.

   [ASON-REQ]   D. Papadimitriou, J. Drake, J. Ash, A. Farrel, L. Ong,
                "Requirements for Generalized MPLS (GMPLS) Signaling
                Usage and Extensions for Automatically Switched Optical
                Network (ASON)", daft-ietf-ccamp-gmpls-ason-reqts-03.txt
                October 2003, work in progress.

15. Informational References

   [ASH1]       G. Ash, ITU-T Recommendations E.360.1 --> E.360.7, "QoS
                Routing & Related Traffic Engineering Methods for IP-,
                ATM-, & TDM-Based Multiservice Networks", May, 2002.

   [FASTRR]     Ping Pan, et al., "Fast Reroute Extensions to RSVP-TE
                for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-fastreroute-
                03.txt, July 2003 (work in progress).

   [G8080]      ITU-T Recommendation G.808/Y.1304, Architecture for the
                Automatically Switched Optical Network (ASON), November
                2001.

   [EXCLUDE]    C-Y. Lee, A. Farrel and S De Cnodder, "Exclude Routes -
                Extension to RSVP-TE", draft-ietf-ccamp-rsvp-te-exclude-
                route-00.txt, June 2003 (work in progress).

   [PNNI]       ATM Forum, "Private Network-Network Interface
                Specification Version 1.0 (PNNI 1.0)", <af-pnni-
                0055.000>, May 1996.

   [RFC2702]    D. Awduche, et al., "Requirements for Traffic
                Engineering Over MPLS", RFC2702, September 1999.

   [RFC3469]    V. Sharma, et al., "Framework for MPLS-based Recovery",
                RFC 3469, February 2003.

   [INTER-AS]   JP. Vasseur, and R. Zhang, "Inter-AS MPLS Traffic
                Engineering", draft-vasseur-inter-as-te-01.txt, June
                2003, work in progress.

16. Authors' Addresses

   Adrian Farrel (editor)
   Old Dog Consulting
   Phone:  +44 (0) 1978 860944
   EMail:  adrian@olddog.co.uk

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   Arun Satyanarayana
   Movaz Networks, Inc.
   7926 Jones Branch Drive, Suite 615
   McLean, VA 22102
   Phone:  (+1) 703-847-1785
   EMail:  aruns@movaz.com

   Atsushi Iwata
   NEC Corporation
   Networking Research Laboratories
   1-1, Miyazaki, 4-Chome, Miyamae-ku,
   Kawasaki, Kanagawa, 216-8555, JAPAN
   Phone: +81-(44)-856-2123
   Fax:   +81-(44)-856-2230
   EMail: a-iwata@ah.jp.nec.com

   Norihito Fujita
   NEC Corporation
   Networking Research Laboratories
   1-1, Miyazaki, 4-Chome, Miyamae-ku,
   Kawasaki, Kanagawa, 216-8555, JAPAN
   Phone: +81-(44)-856-2123
   Fax:   +81-(44)-856-2230
   EMail: n-fujita@bk.jp.nec.com

   Gerald R. Ash
   AT&T
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748, USA
   Phone: (+1) 732-420-4578
   Fax:   (+1) 732-368-8659
   EMail: gash@att.com

   Simon Marshall-Unitt
   Data Connection Ltd.
   100 Church Street
   Enfield, Middlesex
   EN2 6BQ, UK
   Phone: (+44) (0)-208-366-1177
   EMail: smu@dataconnection.com

17. Full Copyright Statement

   Copyright (c) The Internet Society (2003). All Rights
   Reserved. This document and translations of it may be
   copied and furnished to others, and derivative works that
   comment on or otherwise explain it or assist in its
   implementation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of
   any kind, provided that the above copyright notice and
   this paragraph are included on all such copies and
   derivative works. However, this document itself may not
   be modified in any way, such as by removing the copyright
   notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose
   of developing Internet standards in which case the

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   procedures for copyrights defined in the Internet
   Standards process must be followed, or as required to
   translate it into languages other than English.

   The limited permissions granted above are perpetual and
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