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

                                                           Atsushi Iwata
                                                         Norihito Fujita
                                                         NEC Corporation

                                                    Gerald R. Ash (AT&T)

                                                    Simon Marshall-Unitt
                                                    Data Connection Ltd.
                                                               July

                                                            October 2004

           Crankback Signaling Extensions for MPLS Signaling
                 <draft-ietf-ccamp-crankback-02.txt>
                 <draft-ietf-ccamp-crankback-03.txt>

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
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   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

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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   information used to compute a path may be out of date.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 (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.

Table of Contents

   Section A : Problem Statement

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

   Section B : Solution

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

Section A : Problem Statement

0. Changes

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

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

   - Update IPR and copyright
   - Update references

0.2 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 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.

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
   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
   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 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 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."

   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
   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 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 or until the LSR abandons re-
   routing 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.

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 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 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 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]. 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 TLVs are defined in [TE-BUNDLE] for use in the IF_ID
   PHOP Object and in the IF_ID ERROR_SPEC object to identify component
   links of unnumbered interfaces. Note that the Type values shown here
   are only suggested values 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)

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

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

   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 for the node. In the case of
          ABRs, this identifies the area where the failure has occurred.

   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.

   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.

7.3 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 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 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 as described in [RFC3473] 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]. 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
   single-area network does not need to support the receipt and
   processing of TLV types 28 and 29. 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.

   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 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.

   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.

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 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.

   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).

   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.

   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.

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

7.3.6 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.

7.4. 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, 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.

7.4.3 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
   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 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.

7.5. 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 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.

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]
   and [RFC 3473].

   A new error value is defined for the error code "Routing
   Problem". "Re-routing limit exceeded" indicates that re-
   routing re-routing
   has failed because the number of crankback re-
   routing 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 are not
   currently tracked by IANA. This might be a good
   opportunity to move them under IANA control. The 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 ERROR SPEC object. Thanks for a
   welcome and very thorough review by Dimitri Papadimitriou.

   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-03.txt, March 2003,
                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-06.txt
                April daft-ietf-ccamp-gmpls-ason-reqts-07.txt
                October 2004, 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, November 2003
                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,
                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>,
                <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,
                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

   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. 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).  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
   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).