Network Working Group                             Adrian Farrel (editor)
Internet Draft                                        Old Dog Consulting
Category: Standards Track
Expires: April August 2005                                  Arun Satyanarayana
                                                    Movaz Networks, Inc.

                                                           Atsushi Iwata
                                                         Norihito Fujita
                                                         NEC Corporation

                                                           Gerald R. Ash (AT&T)

                                                            October 2004
                                                                    AT&T

                                                           February 2005

        Crankback Signaling Extensions for MPLS and GMPLS Signaling
                 <draft-ietf-ccamp-crankback-03.txt>
                 <draft-ietf-ccamp-crankback-04.txt>

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, I certify each
   author represents that any applicable patent or other IPR claims of
   which I am he or she is aware have been disclosed, or will be disclosed, and any of
   which I he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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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   information used to compute a path may be out of date.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 and Generalized MPLS
   (GMPLS) Traffic Engineered (TE) 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 and GMPLS signaling as defined in
   "Generalized Multi-Protocol Label Switching (GMPLS) Signaling
   Functional Description", RFC3473. These extensions mean 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.

Table of Contents

   Section A : Problem Statement

   1. Terminology......................................................3 Terminology......................................................4
   1.1. Control Plane and Data Plane Separation........................4
   2. Introduction and Framework.......................................3 Framework.......................................5
   2.1. Background.....................................................3 Background.....................................................5
   2.2. Repair and Restoration.........................................4 Restoration.........................................6
   2.3. Interaction with TE Flooding Mechanisms .......................6
   3. Discussion: Explicit Versus Implicit Re-routing Indications......5 Indications......7
   4. Required Operation...............................................6 Operation...............................................8
   4.1. Resource Failure or Unavailability.............................6 Unavailability.............................8
   4.2. Computation of an Alternate Path...............................6 Path...............................8
   4.2.1 Information Required for Re-routing...........................7 Re-routing...........................9
   4.2.2 Signaling a New Route.........................................7 Route.........................................9
   4.3. Persistence of Error Information...............................7 Information...............................9
   4.4. Handling Re-route Failure......................................7 Failure.....................................10
   4.5. Limiting Re-routing Attempts...................................8 Attempts..................................10
   5. Existing Protocol Support for Crankback Re-routing...............8 Re-routing..............11
   5.1. RSVP-TE [RFC 3209].............................................9 ......................................................12
   5.2. GMPLS-RSVP-TE [RFC 3473].......................................9 ................................................12

   Section B : Solution

   6. Control of Crankback Operation..................................10 Operation..................................12
   6.1. Requesting Crankback and Controlling In-Network Re-routing....10 Re-routing....12
   6.2. Action on Detecting a Failure.................................11 Failure.................................13
   6.3. Limiting Re-routing Attempts..................................11 Attempts..................................14
   6.3.1 New Status Codes for Re-routing..............................11 Re-routing..............................14
   6.4. Protocol Control of Re-routing Behavior.......................11 Behavior.......................14
   7. Reporting Crankback Information.................................12 Information.................................15
   7.1. Required Information..........................................12 Information..........................................15
   7.2. Protocol Extensions...........................................12 Extensions...........................................15
   7.3 Guidance for Use of IF_ID Error Spec TLVs......................16 TLVs......................19
   7.3.1 General Principles...........................................16 Principles...........................................19
   7.3.2 Error Report TLVs............................................17 TLVs............................................20
   7.3.3 Fundamental Crankback TLVs...................................17 TLVs...................................20
   7.3.4 Additional Crankback TLVs....................................18 TLVs....................................20
   7.3.5 Grouping TLVs by Failure Location............................19 Location............................22
   7.3.6 Alternate Path identification................................20 identification................................23
   7.4. Action on Receiving Crankback Information.....................20 Information.....................23
   7.4.1 Re-route Attempts............................................20 Attempts............................................23
   7.4.2 Location Identifiers of Blocked Links or Nodes...............20 Nodes...............23
   7.4.3 Locating Errors within Loose or Abstract Nodes...............21 Nodes...............24
   7.4.4 When Re-routing Fails........................................21 Fails........................................24
   7.4.5 Aggregation of Crankback Information.........................21 Information.........................24
   7.5. Notification of Errors........................................22 Errors........................................25
   7.5.1 ResvErr Processing...........................................22 Processing...........................................25
   7.5.2 Notify Message Processing....................................22 Processing....................................25
   7.6. Error Values..................................................23 Values..................................................26
   7.7. Backward Compatibility........................................23 Compatibility........................................26
   8. Routing Protocol Interactions...................................23 Interactions...................................26
   9. LSP Restoration Considerations..................................24 Considerations..................................26
   9.1. Upstream of the Fault.........................................24 Fault.........................................27
   9.2. Downstream of the Fault.......................................25 Fault.......................................27
   10. IANA Considerations............................................25 Considerations............................................28
   10.1. Error Codes..................................................25 Codes..................................................28
   10.2. IF_ID_ERROR_SPEC TLVs........................................25 TLVs........................................28
   10.3. LSP_ATTRIBUTES Object........................................25 Object........................................28
   11. Security Considerations........................................26 Considerations........................................28
   12. Acknowledgments................................................26 Acknowledgments................................................29
   13. Intellectual Property Considerations...........................26 Considerations...........................29
   14. Normative References...........................................26 References...........................................29
   15. Informational References.......................................27 References.......................................30
   16. Authors' Addresses.............................................28 Addresses.............................................31
   17. Disclaimer of Validity.........................................29 Validity.........................................32
   18. Full Copyright Statement.......................................29 Statement.......................................32
   A.  Experience of Crankback in TDM-based Networks..................30 Networks..................33

Section A : Problem Statement

0. Changes

(This section to be removed before publication as an RFC.)

0.1 Changes from 03 to 04 Version

   - Content of NODE_ID TLV changes from Router ID to TE Router ID.
   - Clarification that the MPLS LSPs referenced are TE LSPs.
   - More open inclusion of GMPLS alongside MPLS.
   - Note that bundling draft changes obsoletes the use of component ID
     TLVs. Remove unnumbered component interface id TLVs and renumber
     other TLVs.
   - New section explaining control plane and data plane separation.
   - New section on the interaction with TE flooding mechanisms.
   - Clarify the use of the history table.
   - Clarify the way that the retry counting is used.
   - Typos.

0.2 Changes from 01 to 02, and 02 to 03 Versions

   - Update IPR and copyright
   - Update references

0.2

0.3 Changes from 00 to 01 Versions

   - Removal of background descriptive material pertaining to TDM
     network experience from section 3 to an Appendix.
   - Removal of definition of Error Spec TLVs for unnumbered bundled
     links from section 7.2 to a separate document.
   - More detailed guidance on which Error Spec TLVs to use when.
   - Change LSP_ATTRIBUTE flags from hex values to bit numbers.
   - Typographic errors fixed.
   - Update references.

1. Terminology

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

2. Introduction

1.1. Control Plane and Framework

2.1. Background

   RSVP-TE (RSVP Extensions for LSP Tunnels) [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 Data Plane Separation

   Throughout this document the LSP. To designate an explicit path processes and techniques are described
   as though the control plane and data plane elements that
   satisfies QoS constraints, it comprose a
   Label Switching Router (LSR) are coresident and related in a
   one-to-one manner. This is necessary to discern a convenience of documentaiton only.

   It should be noted that GMPLS LSRs may be decomposed such that the
   resources available to each link or node
   control plane components are not physically collocated. Further, one
   presence in the network.
   For control plane may control more than one LSR in the collection of such resource information, routing
   protocols, such as OSPF and IS-IS , can
   data plane. These points have several consequences with respect to
   this document:

   o  The nodes, links and resources that are reported as in error, are
      data plane entities.

   o  The nodes, areas and ASs that report that they have attempted
      re-routing, are control plane entities.

   o  Where a single control plane entity is responsible for more than
      one data plane LSR, crankback signaling may be implicit in just
      the same way as LSP establishment signaling may be.

   The above points may be considered self-evident, but are stated
   here for absolute clarity.

   The stylistic convenience of refering to both the control plane
   element responsible for a single LSR and the data plane component of
   that LSR simply as "the LSR", should not be taken to mean that this
   document is applicable only to a colocated one-to-one relationship.
   Further, in the majority case the control plane and data plane
   components are related in a 1:1 ratio and are usually collocated.

2. Introduction and Framework

2.1. Background

   RSVP-TE (RSVP Extensions for LSP Tunnels) [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 an 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.

   Generalized MPLS [RFC3471] and [RFC3473] extends MPLS into networks
   that manage Layer 2, TDM and lambda resources as well as packet
   resources. Thus, crankback routing is also useful in GMPLS networks.

   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. This makes crankback
   routing all the more important in certain GMPLS networks.

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 it 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 the 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 include fast
   restoration. These schemes rely on the existence of a backup LSP to
   protect the primary, but if both the primary and backup paths fail it
   is necessary to re-establish 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 be used to notify the 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 Layer 2, TDM and

   The requirement for end-to-end allocation of lambda resources. In a
   network without wavelength converters, setup requests are
   likely to be blocked more often than resources in a conventional
   MPLS environment because the same
   GMPLS networks without wavelength must be
   allocated at each Optical Cross-Connect on an end-to-end
   explicit path. Furthermore, converters means that end-to-end
   restoration is the only way to recover LSP failures. This implies that makes
   crankback routing would also be rerouting particularly useful in a GMPLS network, in
   particular in dynamic LSP re-routing cases
   (no backup LSP pre-establishment). dynamic LSP re-routing cases (no backup LSP
   pre-establishment).

2.3. Interaction with TE Flooding Mechanisms

   GMPLS uses IGPs (OSPF and IS-IS) to flood traffic engineering (TE)
   information that is used to construct a traffic engineering database
   (TED) which acts as a data source for path computation.

   Crankback signaling is not intended to supplement or replace the
   normal operation of the TE flooding mechanism, since these mechanisms
   are independent of each other. That is, information gathered from
   crankback signaling may be applied to compute an alternate path for
   the LSP for which the information was signaled, but the information
   is not intended to be used to influence the computation of the paths
   of other LSPs.

   Any requirement to rapidly flood updates about resource availability
   so that they may be applied as deltas to the TED and utilized in
   future path computations are out of scope of this document.

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.  For example, experience of using
   release messages in TDM-based networks for analogous implicit and
   explicit re-routing indications purposes provides some guidance. This
   background information is given in Appendix A." A.

   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. path computation point.

   [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, and it is best to know the following information
   separately:

   - where blockage/congestion occurred
   - whether alternate routing "should" be attempted.

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.

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 sections 4.2.1 and 7.

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 network. 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 the 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, each 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.

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 by downstream LSRs
   or until the repair point LSR abandons re-routing attempts. Since
   crankback routing signaling information may happen be returned to the same repair
   point LSR more than once while establishing a specific LSP, the
   repair point LSR SHOULD maintain 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 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.

   Note that the purpose of this history table is to correlate
   information when repeated retry attempts are made by the same LSR.
   For example, suppose that an attempt is made to route from A through
   B, and B returns a failure with crankback information, an attempt may
   be made to route from A through C, and this may also fail with the
   return of crankback information - the next attempt SHOULD NOT be to
   route from A through B, and this may be achieved by use of the
   history table.

   The history table can be discarded by the signaling controller for A
   if the LSP is successfully established through A. The history table
   MAY be retained after the signaling controller for A sends an error
   upstream, however it is questionable what value this provides since a
   future retry as a result of crankback rerouting should not attempt to
   route through A (such is the nature of crankback). If the history
   information is retained for a longer period it SHOULD be discarded
   after a local timeout has expired, and that timer MUST be shorter
   than the timer used by the ingress to re-attempt a failed service
   (note that re-attempting a failed service is received by not the same as making a repair point (while
   it
   re-route attempt after failure).

   It is performing crankback re-routing) it should update not intended that the information in the history table that lists all experienced blockages, and use be used
   to supplement the
   entire gathered information when making a further re-routing attempt. TED for the computation of paths of other LSPs.

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.

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 This document allows an LSR to limit
   the retries per LSP, and assumes that such a limit may will be applied
   either as a configuration item per
   LSP, per node, per area node configuration for those LSRs that are capable
   of rerouting, or per AS. as a network-wide configuration value.

   When the number of retries at a particular node, area or
   AS LSR is exceeded, the LSR handling the current failure
   reports the failure upstream to the next node, area or AS node 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 node 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) LSP 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) LSP 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 during LSP establishment,
      as the Resv is processed. 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 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/AS networks, the ResvErr might be
      intercepted and acted on at an area/AS 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, 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.

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          The ingress node MAY attempt re-routing after
                          failure. 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  The ingress node MAY attempt re-routing after
                          failure. 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
                          error message upstream to the ingress
                          LSR or try to select another egress boundary
                          LSR. Other intermediate nodes SHOULD NOT
                          attempt re-routing. Nodes detecting failures
                          MUST report an error and SHOULD supply
                          crankback information.

   Segment-based Re-routing
                          All nodes
                          Any node MAY attempt re-routing rerouting after
                          failure. it
                          receives an error report and before it passes
                          the error report further upstream. 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 MUST return a protocol
   error indication and SHOULD include full crankback information.

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 bit numbers below are suggested
   and actual values are TBD by IETF consensus.

   Bit     Name and Usage
   Number

      1    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.

      2    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.

      3    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.

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]. [RFC3209]. Additionally,
   the IF_ID ERROR_SPEC Object has scope for carrying TLVs that identify
   the link associated with the error.

   The TLVs for use with this object are defined in [RFC3471], and are
   listed below. 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 further

   Note that TLVs 4 and 5 are defined in [TE-BUNDLE] for use in the IF_ID
   PHOP Object obsoleted by [BUNDLE] and in the IF_ID ERROR_SPEC object SHOULD NOT be
   used to identify component
   links of unnumbered interfaces. Note that the Type values shown here
   are only suggested values interfaces in [TE-BUNDLE] - final values are TBD and
   to be determined by IETF consensus.

   Type Length Format     Description
   --------------------------------------------------------------------
    6     16   Compound   UNUM_COMPONENT_IF_DOWN  (Component interface)
    7     16   Compound   UNUM_COMPONENT_IF_UP    (Component interface) IF_ID ERROR_SPEC objects.

   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.

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

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

   For types 6 and 7 the format of the Value field is already
   defined in [TE-BUNDLE].

   For types 16 14 and 18, 16, they format of the Value field is the same as
   for type 1.

   For types 17 15 and 19, 17, the format of the Value field is the same as for
   type 2.

   For types 20, 21 and 22, type 18 the formats format of the Value fields
   are the same as for types 3, 4 and 5 respectively.

   For types 23 and 24 the Value field is the same as for
   types 6 and 7 respectively. type 3.

   For types 8, 9, 25 6, 7, 19 and 26 20 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 8 and 27 21 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 ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Router Id: ID: 32 bits

          The TE Router Id ID (TLV type 8) or the Router ID (TLV type 21)
          used to identify the node within the IGP.

   For types 11 9 and 28 22 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 for the node. In the case of
          ABRs, this identifies the area where the failure has occurred.

   For types 12 10 and 29 23 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 IS-IS Area Identifier in
          octets. Valid values are from 2 to 11 inclusive.

       ISIS Area Identifier

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

   For types 13 11 and 30 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  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 12, 13 and 31 25 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.

   For type 32 26 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 8 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 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                       Link Identifiers                        ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Link Identifiers:

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

7.3 Guidance for Use of IF_ID ERROR_SPEC TLVs

7.3.1 General Principles

   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 3 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 4 or 7 5 SHOULD NOT be used as defined described in this
   [BUNDLE] and component links should be identified using the
   principles described in that document.

   A sender MAY include additional TLVs from the range 8 6 through 33 27
   to report crankback information, although this information will at
   most only be used for logging.

   If crankback 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 3 as described in [RFC3473]
   [RFC3473], [BUNDLE], and the previous paragraph. Additional TLVs SHOULD
   also be included to report further information. The following section
   gives advice on which TLVs should be used under different
   circumstances, and which TLVs must be supported by LSRs.

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

   Note that all LSRs MUST be prepared to receive and forward any TLV as
   per [RFC3473]. This includes TLVs of type 4 or 5 as defined in
   [RFC3473] and obsoleted by [BUNDLE]. There is, however, no
   requirement for an LSR to actively process any but the error report
   TLVs. An LSR that proposes to perform crankback re-routing SHOULD
   support receipt and processing of all of the fundamental crankback
   TLVs, and is RECOMMENDED to support the receipt and processing of
   the additional crankback TLVs.

   It should be noted, however, that some assumptions about the TLVs
   that will be used MAY be made based on the deployment scenarios. For
   example, a router that is deployed in a single-area network does not
   need to support the receipt and processing of TLV types 28 22 and 29. 23.
   Those TLVs might be inserted in an IF_ID ERROR_SPEC object, but would
   not need to be processed by the receiver of a PathErr message.

7.3.2 Error Report TLVs

   Error Report TLVs are those in the range 1 through 7. 3. (Note that
   the obsoleted TLVs 4 and 5 may be considered in this category, but
   SHOULD NOT be used.)

   As stated above, when crankback information is reported, the IF_ID
   ERROR_SPEC object MUST be used. When the IF_ID ERROR_SPEC object is
   used, at least one of the TLVs in the range 1 through 7 3 MUST be
   present. The choice of which TLV to use will be dependent on the
   circumstance of the error and device capabilities. For example, a
   device that does not support IPv6 will not need the ability to
   create a TLV of type 2. Note, however, that such a device MUST still
   be prepared to receive and process all error report TLVs.

7.3.3 Fundamental Crankback TLVs

   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. 11, although TLVs 4 and 5 may be considered to be excluded
   from this category by dint of having been obsoleted.

   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 8, 9, 10 and 13, 11 MAY, however, be omitted
   according to local policy and relevance of the information.

7.3.4 Additional Crankback TLVs

   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 12, 13, 14 and 17 15 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.

   Reporting nodes SHOULD also supply TLVs from the range 14 12 through 26 20
   as appropriate for reporting the error. The reporting nodes MAY also
   supply TLVs from the range 27 21 through 33. 27.

   Note that in deciding whether a TLV in the range 14 12 through 26 20 "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). 16).

   Four TLVs (27, 28, 29 (21, 22, 23 and 30) 24) 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 ID
   (TLV 27) 21) 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.

   The last three TLVs (31, 32, (25, 26, and 33) 27) provide additional information
   for recomputation points. The reporting node (or some node forwarding
   the error) may 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.

7.3.5 Grouping TLVs by Failure Location

   Further guidance as to the inclusion of crankback TLVs can be given
   by grouping the TLVs according to the location of the failure and the
   context within which it is reported. For example, a TLV that reports
   an area identifier would only need to be included as the crankback
   error report transits an area boundary.

   Although discussion of aggregation of crankback information is out of
   the scope of this document, it should be noted that this topic is
   closely aligned to the information presented here.

   Resource Failure
            8
            6      DOWNSTREAM_LABEL
            9
            7      UPSTREAM_LABEL
   Interface failures
            1      IPv4
            2      IPv6
            3      IF_INDEX
            4      COMPONENT_IF_DOWNSTREAM (obsoleted)
            5      COMPONENT_IF_UPSTREAM
            6      UNUM_COMPONENT_IF_DOWN
            7      UNUM_COMPONENT_IF_UP
           14   (obsoleted)
           12      ERO_CONTEXT
           15
           13      ERO_NEXT_CONTEXT
           16
           14      PREVIOUS_HOP_IPv4
           17
           15      PREVIOUS_HOP_IPv6
           18
           16      INCOMING_IPv4
           19
           17      INCOMING_IPv6
           20
           18      INCOMING_IF_INDEX
           21      INCOMING_COMP_IF_DOWN
           22      INCOMING_COMP_IF_UP
           23      INCOMING_UNUM_COMP_DOWN
           24      INCOMING_UNUM_COMP_UP
           25
           19      INCOMING_DOWN_LABEL
           26
           20      INCOMING_UP_LABEL
   Node failures
           10
            8      NODE_ID
           27
           21      REPORTING_NODE_ID
   Area failures
           11
            9      OSPF_AREA
           12
           10      ISIS_AREA
           28
           22      REPORTING_OSPF_AREA
           29
           23      REPORTING_ISIS_AREA
           31
           25      PROPOSED_ERO
           32
           26      NODE_EXCLUSIONS
           33
           27      LINK_EXCLUSIONS
   AS failures
           13
           11      AUTONOMOUS_SYSTEM
           30
           24      REPORTING_AS

7.3.6 Alternate Path identification 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.

7.4. Action on Receiving Crankback Information

7.4.1 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.

7.4.2 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 TE 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.

7.4.3 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.

7.4.4 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 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.

7.4.5 Aggregation of Crankback Information

   When a setup blocking error or an error in an established LSP occurs
   and crankback 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 26 and 33 27 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.

7.5. Notification of Errors

7.5.1 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 [RFC2205] 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]
   [RFC2205] to generate more than one PathErr.

7.5.2 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.

7.6. 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] [RFC3209] and [RFC 3473]. [RFC3473].

   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.

7.7. 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 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.

   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 defined in [RFC3471]
   are not currently tracked by IANA. This might be a good
   opportunity to move them under IANA control. is requested to form a
   registry of these values. The new values proposed by this document
   are found in section 7.2.

10.3 LSP_ATTRIBUTES Object

   Three bits are defined for inclusion in the LSP Attributes TLV of
   the LSP_ATTRIBUTES object in section 6.4. Suggested values are
   supplied. IANA is requested to assign those bits.

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 IF_ID ERROR SPEC object. Thanks
   for a welcome and very thorough review by Dimitri Papadimitriou.

   Stephen Shew made useful comments for clarification through the
   ITU-T liaison process.

   Simon Marshall-Unitt made contributions to this draft.

13. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain 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 in BCP 78 and BCP 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 at
   http://www.ietf.org/ipr.

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

14. Normative References

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

   [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.

   [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-ietf-mpls-rsvpte-attributes-04.txt, July 2004,
                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-07.txt
                October 2004, work in progress.

   [BUNDLE]     Kompella, K., Rekhter, Y., and Berger, L., "Link
                Bundling in MPLS Traffic Engineering",
                draft-ietf-mpls-bundle, 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-06.txt, May 2004
                (work in progress).

   [G8080]      ITU-T Recommendation G.808/Y.1304, Architecture for the
                Automatically Switched Optical Network (ASON), November
                2001. For information on the availability of this
                document, please see http://www.itu.int.

   [EXCLUDE]    C-Y. Lee, A. Farrel and S De Cnodder, "Exclude Routes -
                Extension to RSVP-TE",
                draft-ietf-ccamp-rsvp-te-exclude-route-02.txt, July 2004
                (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.

   [TE-BUNDLE]  Z. Ali, A. Farrel, D. Papadimitriou, A. Satyanarayana,
                and A. Zamfir, "Generalized Multi-Protocol Label
                Switching (GMPLS) RSVP-TE signaling using Bundled
                Traffic Engineering (TE) Links",
                draft-dimitri-ccamp-gmpls-rsvp-te-bundled-links-00.txt,
                May 2004, work in progress.

16. Authors' Addresses

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

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

17. Disclaimer of Validity

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

18. Full Copyright Statement

   Copyright (C) The Internet Society (2004). (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

Appendix A. Experience of Crankback in TDM-based Networks

   Experience of using release messages in TDM-based networks for
   analogous repair and re-routing 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.

       *--------------------*  *-----------------*
       |                    |  |                 |
       |  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.

   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).