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Versions: 00 01 02 03 04 05 06 RFC 4920
Network Working Group Adrian Farrel (editor)
Internet Draft Old Dog Consulting
Category: Standards Track
Expires: April 2005 Arun Satyanarayana
Movaz Networks, Inc.
Atsushi Iwata
Norihito Fujita
NEC Corporation
Gerald R. Ash (AT&T)
October 2004
Crankback Signaling Extensions for MPLS Signaling
<draft-ietf-ccamp-crankback-03.txt>
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
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information used to compute a path may be out of date.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.
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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...........................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.................................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............................................17
7.3.3 Fundamental Crankback TLVs...................................17
7.3.4 Additional Crankback TLVs....................................18
7.3.5 Grouping TLVs by Failure Location............................19
7.3.6 Alternate Path identification................................20
7.4. Action on Receiving Crankback Information.....................20
7.4.1 Re-route Attempts............................................20
7.4.2 Location Identifiers of Blocked Links or Nodes...............20
7.4.3 Locating Errors within Loose or Abstract Nodes...............21
7.4.4 When Re-routing Fails........................................21
7.4.5 Aggregation of Crankback Information.........................21
7.5. Notification of Errors........................................22
7.5.1 ResvErr Processing...........................................22
7.5.2 Notify Message Processing....................................22
7.6. Error Values..................................................23
7.7. Backward Compatibility........................................23
8. Routing Protocol Interactions...................................23
9. LSP Restoration Considerations..................................24
9.1. Upstream of the Fault.........................................24
9.2. Downstream of the Fault.......................................25
10. IANA Considerations............................................25
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10.1. Error Codes..................................................25
10.2. IF_ID_ERROR_SPEC TLVs........................................25
10.3. LSP_ATTRIBUTES Object........................................25
11. Security Considerations........................................26
12. Acknowledgments................................................26
13. Intellectual Property Considerations...........................26
14. Normative References...........................................26
15. Informational References.......................................27
16. Authors' Addresses.............................................28
17. Disclaimer of Validity.........................................29
18. Full Copyright Statement.......................................29
A. Experience of Crankback in TDM-based 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].
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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
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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
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 path in the network through "learning models". Reducing
this flooding reduces overhead and can lead to the ability to
support much larger AS sizes.
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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.
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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
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.
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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
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.
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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
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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.
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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.
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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.
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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)
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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.
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For types 12 and 29 the Value field has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | ISIS Area Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ISIS Area Identifier (continued) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length
Length of the actual (non-padded) ISIS Area Identifier
in octets. Valid values are from 2 to 11 inclusive.
ISIS Area Identifier
The variable-length ISIS area identifier. Padded with
trailing zeroes to a four-octet boundary.
For types 13 and 30 the Value field has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Autonomous System Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Autonomous System Number: 32 bits
The AS Number of the associated Autonomous System. Note
that if 16-bit AS numbers are in use, the low order bits
(16 through 31) should be used and the high order bits
(0 through 15) should be set to zero.
For types 14, 15 and 31 the Value field has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ERO Subobjects ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ERO Subobjects:
A sequence of ERO subobjects. Any ERO subobjects are
allowed whether defined in [RFC3209], [RFC3473] or other
documents. Note that ERO subobjects contain their own
type and length fields.
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For type 32 the Value field has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Node Identifiers ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Node Identifiers:
A sequence of TLVs as defined here of types 1, 2 or 10
that indicates downstream nodes that have already
participated in crankback attempts and have been declared
unusable for the current LSP setup attempt.
For type 33 the Value field has the format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Link Identifiers ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link Identifiers:
A sequence of TLVs as defined here of types 3, 4, 5, 6 or 7
that indicates incoming interfaces at downstream nodes that
have already participated in crankback attempts and have
been declared unusable for the current LSP setup attempt.
7.3 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
A. Farrel et al. Page 16
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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 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.
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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
A. Farrel et al. Page 18
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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
A. Farrel et al. Page 19
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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,
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.
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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 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
A. Farrel et al. Page 21
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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)
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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
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.
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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.
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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.
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11. Security Considerations
It should be noted that while the extensions in this document
introduce no new security holes in the protocols, should a malicious
user gain protocol access to the network, the crankback information
might be used to prevent establishment of valid LSPs.
The implementation of re-routing attempt thresholds are
particularly important in this context.
The crankback routing extensions and procedures for LSP restoration
as applied to RSVP-TE introduce no further new security
considerations. Refer to [RFC2205], [RFC3209] and [RFC3473] for a
description of applicable security considerations.
12. Acknowledgments
We would like to thank Juha Heinanen and Srinivas Makam
for their review and comments, and Zhi-Wei Lin for his
considered opinions. Thanks, too, to John Drake for
encouraging us to resurrect this document and consider
the use of the IF-ID ERROR SPEC object. Thanks for a
welcome and very thorough review by Dimitri Papadimitriou.
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.
A. Farrel et al. Page 26
draft-ietf-ccamp-crankback-03.txt October 2004
[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.
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.
A. Farrel et al. Page 27
draft-ietf-ccamp-crankback-03.txt October 2004
[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
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
A. Farrel et al. Page 28
draft-ietf-ccamp-crankback-03.txt October 2004
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.
A. Farrel et al. Page 29
draft-ietf-ccamp-crankback-03.txt October 2004
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
A. Farrel et al. Page 30
draft-ietf-ccamp-crankback-03.txt October 2004
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).
A. Farrel et al. Page 31
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