CCAMP WG
Internet Draft Jean-Philippe Vasseur (Ed)
Proposed status: Standard Cisco Systems
Yuichi Ikejiri
NTT Communications
Corporation
Raymond Zhang
Infonet Service Corporation
Document: draft-vasseur-ccamp-loose-path-
reopt-02.txt
Expires: January 2005 July 2004
Reoptimization of MPLS Traffic Engineering loosely routed LSP
draft-vasseur-ccamp-loose-path-reopt-02.txt
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Abstract
This document defines a mechanism for the reoptimization of loosely
routed MPLS Traffic Engineering LSPs. A loosely routed LSP follows a
path specified as a combination of strict and loose hop(s) that
contains at least one loose hop and zero or more strict hop(s). The
path calculation (which implies an ERO expansion) to reach a loose
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hop is performed by the previous hop defined in the TE LSP path. This
document proposes a mechanism that allows:
- The TE LSP head-end LSR to trigger a new path re-evaluation on
every hop having a next hop defined as a loose hop,
- A mid-point LSR to signal to the head-end LSR that either a better
path exists to reach a loose hop (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. A better path is defined as
a lower cost path, where the cost is determined by the metric used to
compute the path.
The proposed mechanism applies to intra-domain and inter-domain (IGP
area or Autonomous System) packet and non-packet TE LSPs when the
path is defined as a list of loose hops or when a strict hop is a
non-specific abstract node (e.g. IGP area, Autonomous Systems).
Conventions used in this document
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 [ii].
Table of contents
1. Introduction...................................................3
2. Establishment of a loosely routed TE LSP.......................3
3. Reoptimization of a loosely routed TE LSP path.................4
4. Signalling extensions..........................................5
4.1 Path re-evaluation request.................................5
4.2 New error value sub-code...................................5
5. Mode of operation..............................................6
5.1 Head-end reoptimization control............................6
5.2 Reoptimization triggers....................................6
5.3 Head-end request versus mid-point explicit notification modes
...............................................................6
5.3.1 Head-end request mode.......................................7
5.3.2 Mid-point explicit notification mode........................8
5.3.3 ERO caching.................................................9
6. Interoperability...............................................9
7. Security considerations........................................9
8. Acknowledgments................................................9
9. Intellectual property considerations...........................9
9.1 IPR Disclosure Acknowledgement............................10
10. References...................................................10
Normative references.............................................10
Informative references...........................................10
11. Author's Addresses...........................................11
Full Copyright Statement.........................................11
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1. Introduction
The Traffic Engineering Work Group has specified a set of
requirements for inter-area [INTER-AREA-TE-REQ] and inter-AS [INTER-
AS-TE-REQ] MPLS Traffic Engineering. 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, in
addition to a mechanism to notify a Head-end LSR of the existence of
such a more optimal path or the need to reoptimize due to some
maintenance required in a downstream domain.
2. Establishment of a loosely routed TE LSP
A loosely routed explicit path is a path specified as a combination
of strict and loose hop(s) that contains at least one loose hop and a
set of zero or more strict hop(s). 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, as defined in [RSVP-TE]. In this case,
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 path computation may either be performed by
means of CSPF or any Path Computation Element (PCE) and can be
partial (up to the next loose hop) or complete (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.
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
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- The path 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 either by means of CSPF or
any other PCE-based path computation method. 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 obeys T1Æs set of constraints.
Step 2: the RSVP Path message is then forwarded by R1 following the
ERO path 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 either by means of CSPF or
any other PCE-based path computation method. 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 PCE-based mechanisms may also allow for full
route computation (up to the final destination).
Step 4: the same procedure applies at R8 to reach T1Æs destination
(R11).
3. 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 it 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) when
appropriate. Hence, some mechanism is required to detect the
existence of such a preferable path and to notify the head-end
accordingly.
This document defines a mechanism that allows:
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- 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 potential 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
Path Error message) to the head-end that a more preferable path
exists (a path with a lower cost, where the cost definition is
determined by some metric).
Then once the existence of such a preferable path is 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.
There is another scenario whereby notifying the head-end 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 (see also
[GR-SHUT]).
This allows the head-end 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.
4. Signalling extensions
New ERO flags and Error value sub-codes are proposed in this document
(to be assigned by IANA).
4.1 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 signal 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.
4.2 New error value sub-code
As defined in [RSVP-TE], the ERROR-CODE 25 in ERROR SPEC object
corresponds to a Notify Error.
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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 5.3.2
5. Mode of operation
5.1 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.
5.2 Reoptimization triggers
There are three possible reoptimization triggers:
- 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 ææLink-UPÆÆ 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.
5.3 Head-end request versus mid-point explicit notification modes
This document defines two modes:
1) ææHead-end requesting modeÆÆ: the request for a new path
evaluation of a loosely routed TE LSP is requested by the head-
end LSR.
2) ææMid-point explicit notificationÆÆ: 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 which will in
turn decide whether to perform a reoptimization.
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5.3.1 Head-end request mode
In this mode, 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 ææPath
re-evaluation requestÆÆ bit of the SESSION-ATTRIBUTE object set. This
bit is then cleared in subsequent RSVP path messages sent downstream.
Upon receiving a Path message with the ææPath re-evaluation requestÆÆ
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:
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 MUST immediately
send a Path Error to the head-end LSR (Error code 25 (Notify),
Error sub-code=6 (better path exists)). At this point, the LSR
MAY decide to clear the ææPath re-evaluation requestÆÆ bit of the
SESSION-ATTRIBUTE object in subsequent RSVP Path messages sent
downstream: this mode is the RECOMMENDED mode for the reasons
described below.
The sending of a Path Error Notify message ææPreferable path
existsÆÆ 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 ææPath re-evaluation requestÆÆ 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 ææPath re-
evaluationÆÆ bit of the SESSION-ATTRIBUTE object in RSVP path
message sent downstream).
By preferable path, we mean a path having a lower cost. By default,
an LSR uses the TE metric to compute the shortest path that obeys a
set of constraints. Note that the head-end LSR might use the METRIC-
TYPE object (defined in [PATH-COMP]) in its path message to request
the LSR having a next hop defined as a loose hop or an abstract node
in the ERO to use another metric to determine a preferable path.
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If the RSVP Path message with the ææPath re-evaluation requestÆÆ 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 [REFRESH-
REDUCTION].
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: it can
either be local to each LSR in charge of computing the path to the
next loose hop/abstract node or PCE based.
5.3.2 Mid-point explicit notification mode
In this mode, 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 compared to the existing one, the LSR sends
a Path Error to the head-end LSR (Error code 25 (Notify), Error sub-
code=6 (ææpreferable path existsÆÆ).
There are other circumstances whereby a mid-point LSR MAY send an
RSVP PathError 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 mid-point LSR where the
local maintenance must be performed is responsible for sending an
RSVP PathError 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 Path Error 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 PathError message and
insert its own address for confidentiality reason.
Upon receiving a PathError message with Error code 25 and Error sub-
code 7 or 8, the Head-end LSR MUST perform a TE LSP reoptimization.
Note that those modes are not exclusive: both the timer and event-
driven reoptimization triggers can be implemented on the head-end
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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.
5.3.3 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 fired), 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.
6. Interoperability
An LSR not supporting the ææPath re-evaluation requestÆÆ bit of the
SESSION-ATTRIBUTE object SHALL forward it unmodified.
Any head-end LSR not supporting a PathError Error code 25 message
with Error sub-code = 6, 7 or 8 MUST just silently ignore such Path
Error messages.
7. 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 PathError message (Code 25, Error sub-
code=6 or 7) so as to preserve confidentiality across domains.
8. Acknowledgments
The authors would like to thank Carol Iturralde, Miya Kohno, Francois
Le Faucheur, Philip Matthews, Jim Gibson, Raymond Zhang, Jean-Louis
Le Roux, Kenji Kumaki, Anca Zafir for their useful comments. A
special thank to Adrian Farrel for his very valuable inputs.
9. 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
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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.
9.1 IPR Disclosure Acknowledgement
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
10. References
Normative references
[RFC] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, December 2001.
[REFRESH-REDUCTION] Berger et al, ææRSVP Refresh Overhead Reduction
ExtensionsÆÆ, April 2001
Informative references
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over
MPLS, RFC2702, September 1999.
[INTER-AREA-TE-REQ], Le Roux, Vasseur, Boyle et al. ½ Requirements
for Inter-area MPLS Traffic Engineering ©, draft-ietf-tewg-interarea-
mpls-te-req-01, April 2004 (Work in progress).
[INTER-AS-TE-REQ] Zhang et al, ææMPLS Inter-AS Traffic Engineering
requirementsÆÆ, draft-ietf-tewg-interas-mpls-te-req-06.txt, February
2004, Work in progress.
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[INTER-AREA-AS] Vasseur and Ayyangar, ææInter-area and Inter-AS
Traffic EngineeringÆÆ, draft-vasseur-inter-area-AS-TE-00.txt, February
2004, work in progress.
[GR-SHUT], Z. Ali et al, ææGraceful Shutdown in MPLS Traffic
Engineering NetworksÆÆ, draft-ali-ccamp-mpls-graceful-shutdown-00.txt,
June 2004.
11. Author's Addresses
Jean-Philippe Vasseur
CISCO Systems, Inc.
300 Beaver Brook
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
Infonet Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@infonet.com
Full Copyright Statement
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to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
"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."
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