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Versions: (draft-vasseur-ccamp-loose-path-reopt)
00 01 02 RFC 4736
Networking Working Group JP. Vasseur, Ed.
Internet-Draft Cisco Systems, Inc
Proposed Status: Informational Y. Ikejiri
Expires: August 12, 2006 NTT Communications Corporation
R. Zhang
BT Infonet
February 8, 2006
Reoptimization of Multiprotocol Label Switching (MPLS) Traffic
Engineering (TE) loosely routed Label Switch Path (LSP)
draft-ietf-ccamp-loose-path-reopt-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a mechanism for the reoptimization of loosely
routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)
Traffic Engineering (TE) LSPs signaled with RSVP-TE. This document
proposes a mechanism that allows a TE LSP head-end LSR to trigger a
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new path re-evaluation on every hop having a next hop defined as a
loose or abstract hop and a mid-point LSR to signal to the head-end
LSR that a better path exists (compared to the current path in use)
or that the TE LSP must be reoptimized because of some maintenance
required on the TE LSP path. The proposed mechanism applies to the
cases of intra and inter-domain (IGP area or Autonomous System)
packet and non-packet TE LSPs following a loosely routed path.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Establishment of a loosely routed TE LSP . . . . . . . . . . . 4
4. Reoptimization of a loosely routed TE LSP path . . . . . . . . 6
5. Signalling extensions . . . . . . . . . . . . . . . . . . . . 6
5.1. Path re-evaluation request . . . . . . . . . . . . . . . . 7
5.2. New error value sub-codes . . . . . . . . . . . . . . . . 7
6. Mode of operation . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Head-end reoptimization control . . . . . . . . . . . . . 7
6.2. Reoptimization triggers . . . . . . . . . . . . . . . . . 8
6.3. Head-end request versus mid-point explicit
notification functions . . . . . . . . . . . . . . . . . . 8
6.3.1. Head-end request function . . . . . . . . . . . . . . 8
6.3.2. Mid-point explicit notification . . . . . . . . . . . 9
6.3.3. ERO caching . . . . . . . . . . . . . . . . . . . . . 10
7. Applicability and Interoperability . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Terminology
Terminology used in this document
ABR: Area Border Router.
ERO: Explicit Route Object.
LSR: Label Switch Router.
TE LSP: Traffic Engineering Label Switched Path.
TE LSP head-end: head/source of the TE LSP.
TE LSP tail-end: tail/destination of the TE LSP.
IGP Area: OSPF Area or IS-IS level.
Intra-area TE LSP: TE LSP whose path does not transit across areas.
Inter-area TE LSP: A TE LSP whose path transits across at least two
different IGP areas.
Inter-AS MPLS TE LSP: A TE LSP whose path transits across at least
two different ASs or sub-ASs (BGP confederations).
2. Introduction
This document defines a mechanism for the reoptimization of loosely
routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)
Traffic Engineering LSPs signaled with RSVP-TE (see [RFC3209] and
[RFC3473]). A loosely routed LSP is defined as one that does not
contain a full explicit route identifying each LSR along the path of
the LSP at the time it is signaled by the ingress LSR. Such an LSP
is signaled with no ERO, with an ERO that contains at least one loose
hop, or with an ERO that contains an abstract node that is not a
simple abstract node (that is, an abstract node that identifies more
than one LSR).
The Traffic Engineering Working Group (TE WG) has specified a set of
requirements for inter-area and inter-AS MPLS Traffic Engineering
(see [RFC4105] and [RFC4216]). Both requirements documents specify
the need for some mechanism providing an option for the head-end to
control the reoptimization process should a more optimal path exist
in a downstream domain (IGP area or Autonomous System). This
document defines a solution to meet this requirement and proposes two
mechanisms:
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(1) The first mechanism allows a head-end LSR to trigger a new path
re-evaluation on every hop having a next hop defined as a loose hop
or abstract node and get a notification from the mid-point on whether
a better path exists.
(2) The second mechanism allows a mid-point LSR to explicitly signal
to the head-end LSR that either a better path exists to reach a
loose/abstract hop (compared to the current path in use) or that the
TE LSP must be reoptimized because of some maintenance required along
the TE LSP path. In this case, the notification is sent by the mid-
point LSR without being polled by the head-end LSR.
A better path is defined as a lower cost path, where the cost is
determined by the metric used to compute the path.
3. Establishment of a loosely routed TE LSP
The aim of this section is purely to remind the mechanisms involved
in the establishment of a loosely routed TE LSP (in line with
[RFC3209]) and does not introduce any new protocol extensions or
mechanisms.
In the context of this document, a loosely routed LSP is defined as
one that does not contain a full explicit route identifying each LSR
along the path of the LSP at the time it is signaled by the ingress
LSR. Such an LSP is signaled with no ERO, with an ERO that contains
at least one loose hop, or with an ERO that contains an abstract node
that is not a simple abstract node (that is, an abstract node that
identifies more than one LSR). As specified in [RFC3209], loose hops
are listed in the ERO object of the RSVP Path message with the L flag
of the IPv4 or the IPv6 prefix sub-object set.
Each LSR along the path whose next hop is specified as a loose hop or
a non-specific abstract node triggers a path computation (also
referred to as an ERO expansion), before forwarding the RSVP Path
message downstream. The computed path may either be partial (up to
the next loose hop) or complete (set of strict hops up to the TE LSP
destination).
Note that the examples in the rest of this document are provided in
the context of MPLS inter-area TE but the proposed mechanism equally
applies to loosely routed paths within a single routing domain and
across multiple Autonomous Systems. The examples below are provided
with OSPF as the IGP but the described set of mechanisms similarly
apply to IS-IS.
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An example of an explicit loosely routed TE LSP signaling.
<---area 1--><-area 0--><-area 2->
R1---R2----R3---R6 R8---R10
| | | / | \ |
| | | / | \ |
| | | / | \|
R4---------R5---R7----R9---R11
Assumptions
- R3, R5, R8 and R9 are ABRs
- The path of an inter-area TE LSP T1 from R1 (head-end LSR) to R11
(tail-end LSR) is defined on R1 as the following loosely routed path:
R1-R3(loose)-R8(loose)-R11(loose). R3, R8 and R11 are defined as
loose hops.
Step 1: R1 determines that the next hop (R3) is a loose hop (not
directly connected to R1) and then performs an ERO expansion
operation to reach the next loose hops R3. The new ERO becomes:
R2(S)-R3(S)-R8(L)-R11(L) where: S: Strict hop (L=0) L: Loose hop
(L=1)
The R1-R2-R3 path satisfies T1's set of constraints.
Step 2: the RSVP Path message is then forwarded by R1 following the
path specified in the ERO object and reaches R3 with the following
content: R8(L)-R11(L).
Step 3: R3 determines that the next hop (R8) is a loose hop (not
directly connected to R3) and then performs an ERO expansion
operation to reach the next loose hops R8. The new ERO becomes:
R6(S)-R7(S)-R8(S)-R11(L).
Note: in this example, the assumption is made that the path is
computed on a per loose hop basis, also referred to a partial route
computation. Note that other path computation techniques may result
in complete paths (set of strict hops up to the final destination).
Step 4: the same procedure is repeated by R8 to reach T1's
destination (R11).
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4. Reoptimization of a loosely routed TE LSP path
Once a loosely routed explicit TE LSP is set up, it is maintained
through normal RSVP procedures. During TE LSP life time, a more
optimal path might appear between an LSR and its next loose hop (for
the sake of illustration, suppose in the example above that a link
between R6 and R8 is added or restored that provides a preferable
path between R3 and R8 (R3-R6-R8) than the existing R3-R6-R7-R8
path). Since a preferable (e.g. shorter) path might not be visible
from the head-end LSR by means of the IGP if the head-end LSR does
not belong to the head-end IGP area, the head-end cannot make use of
this shorter path (and reroute the LSP using a make-before-break
technique as described in [RFC3209]) when appropriate. Hence, a new
mechanisms specified in this document is required to detect the
existence of such a preferable path and to notify the head-end LSR
accordingly.
This document defines a mechanism that allows:
- A head-end LSR to trigger on every LSR whose next hop is a loose
hop or an abstract node the re-evaluation of the current path in
order to detect a potentially more optimal path,
- A mid-point LSR whose next hop is a loose-hop or an abstract node
to signal (using a new Error value sub-code carried in a RSVP PERR
message) to the head-end LSR that a more preferable path exists (a
path with a lower cost, where the cost definition is determined by
some metric).
Once the existence of such a preferable path has been notified to the
head-end LSR, the head-end LSR can decide (depending on the TE LSP
characteristics) whether to perform a TE LSP graceful reoptimization
such as the "make-before-break" procedure.
There is another scenario whereby notifying the head-end LSR of the
existence of a better path is desirable: if the current path is about
the fail due to some (link or node) required maintenance.
This mechanism allows the head-end LSR to reoptimize a TE LSP making
use of the non disruptive make-before-break procedure if and only if
a preferable path exists and if such a reoptimization is desired.
5. Signalling extensions
A new flag in the SESSION ATTRIBUTE object and new Error value sub-
codes in the ERROR SPEC object are proposed in this document (to be
assigned by IANA).
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5.1. Path re-evaluation request
The following new flag of the SESSION_ATTRIBUTE object (C-Type 1 and
7) is defined (suggested value to be confirmed by IANA):
Path re-evaluation request: 0x20
This flag indicates that a path re-evaluation (of the current path in
use) is requested. Note that this does not trigger any LSP Reroute
but instead just signals the request to evaluate whether a preferable
path exists.
Note: in case of link bundling for instance, although the resulting
ERO might be identical, this might give the opportunity for a mid-
point LSR to locally select another link within a bundle, although
strictly speaking, the ERO has not changed.
5.2. New error value sub-codes
As defined in [RFC3209], the ERROR-CODE 25 in ERROR-SPEC object
corresponds to a Notify Error.
This document adds three new error value sub-codes (suggested values
to be confirmed by IANA):
6 Preferable path exists
7 Local link maintenance required
8 Local node maintenance required
The details about the local maintenance required modes are detailed
in section 6.3.2.
6. Mode of operation
6.1. Head-end reoptimization control
The notification process of a preferable path (shorter path or new
path due to some maintenance required on the current path) is by
nature de-correlated from the reoptimization operation. In other
words, the location where a potentially preferable path is discovered
does not have to be where the TE LSP is actually reoptimized. This
document applies to the context of a head-end reoptimization.
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6.2. Reoptimization triggers
There are several possible reoptimization triggers. For example,
such reoptimization triggers are:
- Timer-based: a reoptimization is triggered (process evaluating
whether a more optimal path can be found) when a configurable timer
expires,
- Event-driven: a reoptimization is triggered when a particular
network event occurs (such as a "Link-UP" event),
- Operator-driven: a reoptimization is manually triggered by the
Operator.
It is RECOMMENDED for an implementation supporting the extensions
proposed in this document to support the aforementioned modes as path
re-evaluation triggers.
6.3. Head-end request versus mid-point explicit notification functions
This document defines two functions:
1) "Head-end requesting function": the request for a new path
evaluation of a loosely routed TE LSP is requested by the head-end
LSR.
2) "Mid-point explicit notification function": a mid-point LSR having
determined that a preferable path (than the current path is use)
exists or having the need to perform a link/node local maintenance
explicitly notifies the head-end LSR that will in turn decide whether
to perform a reoptimization.
6.3.1. Head-end request function
When a timer-based reoptimization is triggered on the head-end LSR or
the operator manually requests a reoptimization, the head-end LSR
immediately sends an RSVP Path message with the "Path re-evaluation
request" bit of the SESSION-ATTRIBUTE object set. This bit is then
cleared in subsequent RSVP path messages sent downstream. In order
to handle the case of a lost Path message, the solution consists of
relying on the reliable messaging mechanism described in [RFC2961].
Upon receiving a Path message with the "Path re-evaluation request"
bit set, every LSR for which the next abstract node contained in the
ERO is defined as a loose hop/abstract node performs the following
set of actions:
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A path re-evaluation is triggered and the newly computed path is
compared to the existing path:
- If a preferable path can be found, the LSR performing the path re-
evaluation MUST immediately send an RSVP PErr to the head-end LSR
(Error code 25 (Notify), Error sub-code=6 (better path exists)). At
this point, the LSR MAY decide to not propagate such bit in
subsequent RSVP Path messages sent downstream for the re-evaluated TE
LSP: this mode is the RECOMMENDED mode for the reasons described
below.
The sending of an RSVP PERR Notify message "Preferable path exists"
to the head-end LSR will notify the head-end LSR of the existence of
a preferable path (e.g in a downstream area/AS or in another location
within a single domain). Hence, triggering additional path re-
evaluations on downstream nodes is unnecessary. The only motivation
to forward subsequent RSVP Path messages with the "Path re-evaluation
request" bit of the SESSION-ATTRIBUTE object set would be to trigger
path re-evaluation on downstream nodes that could in turn cache some
potentially better paths downstream with the objective to reduce the
signaling setup delay, should a reoptimization be performed by the
head-end LSR.
- If no preferable path can be found, the recommended mode is for an
LSR to relay the request (by setting the "Path re-evaluation" bit of
the SESSION-ATTRIBUTE object in RSVP path message sent downstream).
Note that, by preferable path, we mean a path having a lower cost.
If the RSVP Path message with the "Path re-evaluation request" bit
set is lost, then the next request will be sent when the next
reoptimization trigger will occur on the head-end LSR. The solution
to handle RSVP reliable messaging has been defined in [RFC2961].
The network administrator may decide to establish some local policy
specifying to ignore such request or to consider those requests not
more frequently than a certain rate.
The proposed mechanism does not make any assumption of the path
computation method performed by the ERO expansion process.
6.3.2. Mid-point explicit notification
By contrast with the head-end request function, in this case, a mid-
point LSR whose next hop is a loose hop or an abstract node can
locally trigger a path re-evaluation when a configurable timer
expires, some specific events occur (e.g. link-up event for example)
or the user explicitly requests it. If a preferable path is found
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compared to the path in use, the LSR sends an RSVP PERR to the head-
end LSR (Error code 25 (Notify), Error sub-code=6 ("preferable path
exists").
There are other circumstances whereby any mid-point LSR MAY send an
RSVP PERR message with the objective for the TE LSP to be rerouted by
its head-end LSR: when a link or a node will go down for local
maintenance reasons. In this case, the LSR where a local maintenance
must be performed is responsible for sending an RSVP PERR message
with Error code 25 and Error sub-code=7 or 8 depending on the
affected network element (link or node). Then the first upstream
node having performed the ERO expansion MUST perform the following
set of actions:
- The link (sub-code=7) or the node (sub-code=8) MUST be locally
registered for further reference (the TE database must be updated)
- The RSVP PERR message MUST be immediately forwarded upstream to the
head-end LSR. Note that in the case of TE LSP spanning multiple
administrative domains, it may be desirable for the boundary LSR to
modify the RSVP PERR message and insert its own address for
confidentiality reason.
Upon receiving an RSVP PERR message with Error code 25 and Error sub-
code 7 or 8, the Head-end LSR SHOULD perform a TE LSP reoptimization.
Note that the two functions (head-end and mid-point driven) are not
exclusive from each other: both the timer and event-driven
reoptimization triggers can be implemented on the head-end and/or any
mid-point LSR with potentially different timer values for the timer
driven reoptimization case.
A head-end LSR MAY decide upon receiving an explicit mid-point
notification to delay its next path re-evaluation request.
6.3.3. ERO caching
Once a mid-point LSR has determined that a preferable path exists
(after a reoptimization request has been received by the head-end LSR
or the reoptimization timer on the mid-point has expired), the more
optimal path MAY be cached on the mid-point LSR for a limited amount
of time to avoid having to recompute a path once the head-LSR
performs a make-before-break. This mode is optional. A default
value for the caching timer of 5 seconds is suggested.
7. Applicability and Interoperability
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The procedures described in this document are entirely optional
within an MPLS or GMPLS network. Implementations that do not support
the procedures described in this document will interoperate
seamlessly with those that do. Further, an implementation that does
not support the procedures described in this document will not be
impacted or implicated by a neighboring implementation that does
implement the procedures.
An ingress implementation that chooses not to support the procedures
described in this document may still achieve re-optimization by
periodically issuing a speculative make-before-break replacement of
an LSP without trying to discovery whether a more optimal path is
available in a downstream domain. Such a procedure would not be in
conflict with any mechanisms not already documented in [RFC3209] and
[RFC3473].
An LSR not supporting the "Path re-evaluation request" bit of the
SESSION-ATTRIBUTE object SHALL forward it unmodified.
A head-end LSR not supporting an RSVP PERR with Error code 25 message
and Error sub-code = 6, 7 or 8 MUST just silently ignore such RSVP
PERR message.
8. IANA Considerations
IANA will assign a new flag named "Path re-evaluation request" in the
SESSION-ATTRIBUTE object (C-Type 1 and 7) specified in [RFC3209].
Suggested value is (to be confirmed by IANA) 0x20.
IANA will also assign three new error sub-code values for the RSVP
PERR Notify message (Error code=25). Suggested values are (to be
confirmed by IANA):
6 Preferable path exists
7 Local link maintenance required
8 Local node maintenance required
9. Security Considerations
This document defines a mechanism for a mid-point LSR to notify the
head-end LSR of this existence of a preferable path or the need to
reroute the TE LSP for maintenance purposes. Hence, in case of a TE
LSP spanning multiple administrative domains, it may be desirable for
a boundary LSR to modify the RSVP PERR message (Code 25, Error sub-
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code=6,7 or 8) so as to preserve confidentiality across domains.
Furthermore, a head-end LSR may decide to ignore explicit
notification coming from a mid-point residing in another domain.
Similarly, an LSR may decide to ignore (or accept but up to a pre-
defined rate) path re-evaluation requests originated by a head-end
LSR of another domain.
10. Acknowledgements
The authors would like to thank Carol Iturralde, Miya Kohno, Francois
Le Faucheur, Philip Matthews, Jim Gibson, Jean-Louis Le Roux, Kenji
Kumaki, Anca Zafir, Dimitri Papadimitriou for their useful comments.
A special thank to Adrian Farrel for his very valuable inputs.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.
11.2. Informative References
[RFC4105] Le Roux, J., Vasseur, J., and J. Boyle, "Requirements for
Inter-Area MPLS Traffic Engineering", RFC 4105, June 2005.
[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
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Authors' Addresses
JP Vasseur (editor)
Cisco Systems, Inc
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: jpv@cisco.com
Yuichi Ikejiri
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo, 100-8019
Japan
Email: : y.ikejiri@ntt.com
Raymond Zhang
BT Infonet
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@bt.infonet.com
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