CCAMP Working Group Jean-Philippe Vasseur
IETF Internet Draft (Editor)
Proposed status: Informational Cisco Systems
Yuichi Ikejiri
NTT Communications
Corporation
Raymond Zhang
Infonet Service Corporation
Expires: November 2005 May 2005
Reoptimization of Multiprotocol Label Switching (MPLS) Traffic
Engineering (TE) loosely routed Label Switch Path (LSP)
draft-ietf-ccamp-loose-path-reopt-01.txt
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Copyright (C) The Internet Society (2005). All Rights Reserved.
draft-ietf-ccamp-loose-path-reopt-01.txt January 2005
Abstract
This document defines a mechanism for the reoptimization of loosely
routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)
Traffic Engineering LSPs. 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). This document proposes a
mechanism that allows a TE LSP head-end LSR to trigger a 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.
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.
Table of contents
1. Notice.........................................................3
2. Introduction...................................................3
3. Establishment of a loosely routed TE LSP.......................4
4. Reoptimization of a loosely routed TE LSP path.................5
5. Signalling extensions..........................................6
5.1 Path re-evaluation request.................................6
5.2 New error value sub-codes..................................6
6. Mode of operation..............................................7
6.1 Head-end reoptimization control............................7
6.2 Reoptimization triggers....................................7
6.3 Head-end request versus mid-point explicit notification
modes..........................................................7
5.3.1 Head-end request mode.......................................7
5.3.2 Mid-point explicit notification mode........................9
5.3.3 ERO caching.................................................9
7. Interoperability..............................................10
8. Security considerations.......................................10
9. IANA considerations...........................................10
10. Acknowledgments..............................................10
11. Intellectual property considerations.........................11
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12. References...................................................11
11.1 Normative references........................................11
11.2 Informative references......................................11
13. Authors' Addresses...........................................12
14. Full Copyright Statement.....................................12
1. Notice
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].
2. 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 and
proposes a set of mechanisms that allow:
- 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 or abstract
node,
- A mid-point LSR to 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 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.
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3. Establishment of a loosely routed TE LSP
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 defined in [RFC3209], loose
hops are listed in the Explicit Route Object (ERO) 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.
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)
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L: Loose hop (L=1)
The R1-R2-R3 path obeys T1's 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. 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 some path computation techniques may result in
complete paths (set of strict hops up to the final destination).
Step 4: the same procedure applies at R8 to reach T1's destination
(R11).
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 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:
- 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
RSVP PathErr 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
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characteristics) whether to perform a TE LSP graceful reoptimization
such as the "Make Before Break" procedure defined in [RFC3209].
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.
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.
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).
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 5.3.2
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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.
6.2 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 "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 modes
This document defines two modes:
1) "Head-end requesting mode": 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": 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.
6.3.1 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 "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
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consists of relying on the reliable messaging mechanism described in
[REFRESH-REDUCTION].
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:
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 PathErr 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 PathErr 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 [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.
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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 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
an RSVP PathErr 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 PathErr 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 PathErr 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 PathErr 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 PathErr message and
insert its own address for confidentiality reason.
Upon receiving an RSVP PathErr 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
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 fired), the more
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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
of 5 seconds is suggested.
7. Interoperability
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 PathErr with Error code 25
message and Error sub-code = 6, 7 or 8 MUST just silently ignore such
RSVP PathErr message.
8. 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 PathErr message (Code 25, Error
sub-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.
9. 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
10. Acknowledgments
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.
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11. 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.
12. References
12.1 Normative references
[RFC] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119.
[RFC3209] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC3209, December 2001.
[RFC3473] Berger L. et al.,"Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.
[REFRESH-REDUCTION] Berger et al, "RSVP Refresh Overhead Reduction
Extensions", RFC2961, April 2001.
12.2 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. "Requirements
for Inter-area MPLS Traffic Engineering", draft-ietf-tewg-interarea-
mpls-te-req-03, November 2004.
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[INTER-AS-TE-REQ] Zhang et al, "MPLS Inter-AS Traffic Engineering
requirements", draft-ietf-tewg-interas-mpls-te-req-09.txt, September
2004, Work in progress.
[INTER-DOMAIN-FW] Farrel A., Vasseur JP. and Ayyangar A., "A
Framework for Inter-Domain MPLS Traffic Engineering", draft-ietf-
ccamp-inter-domain-framework-02.txt, May 2005. Work in progress.
[INTER-DOMAIN-SIG] Ayyangar A. and Vasseur JP., "Inter domain GMPLS
Traffic Engineering - RSVP-TE extensions", draft-ietf-ccamp-inter-
domain-rsvp-te-00.txt", February 2005. Work in progress.
[INTER-DOMAIN-PATH-COMP] Vasseur JP., Ayyangar A., "A Per-domain
path computation method for computing Inter-domain Traffic
Engineering (TE) Label Switched Path (LSP)", draft-ietf-ccamp-inter-
domain-pd-path-comp-00.txt, February 2005. Work in progress.
13. Authors' Addresses
Jean-Philippe Vasseur (Editor)
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
14. Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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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.
Vasseur, Ikejiri and Zhang [Page 13]
