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IETF RFC 4736
Reoptimization of Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) Loosely Routed Label Switched Path (LSP)
Last modified on Friday, November 3rd, 2006
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Network Working Group JP. Vasseur, Ed.
Request for Comments: 4736 Cisco Systems, Inc.
Category: Informational Y. Ikejiri
NTT Communications Corporation
R. Zhang
BT Infonet
November 2006
Reoptimization of Multiprotocol Label Switching (MPLS) Traffic
Engineering (TE) Loosely Routed Label Switched Path (LSP)
 Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright © The IETF Trust (2006).
Abstract
This document defines a mechanism for the reoptimization of loosely
routed MPLS and GMPLS (Generalized Multiprotocol Label Switching)
Traffic Engineering (TE) Label Switched Paths (LSPs) signaled with
Resource Reservation Protocol Traffic Engineering (RSVP-TE). This
document proposes a mechanism that allows a TE LSP head-end Label
Switching Router (LSR) to trigger a new path re-evaluation on every
hop that has 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) or that the TE LSP must be reoptimized
(because of maintenance required on the TE LSP path). The proposed
mechanism applies to the cases of intra- and inter-domain (Interior
Gateway Protocol area (IGP area) or Autonomous System) packet and
non-packet TE LSPs following a loosely routed path.
Vasseur, et al. Informational PAGE 1 
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
2.1. Requirements Language ......................................4
3. Establishment of a Loosely Routed TE LSP ........................4
4. Reoptimization of a Loosely Routed TE LSP Path ..................6
5. Signaling Extensions ............................................7
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 ....................10
6.3.3. ERO Caching ........................................10
7. Applicability and Interoperability .............................11
8. IANA Considerations ............................................11
9. Security Considerations ........................................11
10. Acknowledgements ..............................................12
11. References ....................................................12
11.1. Normative References .....................................12
11.2. Informative References ...................................12
Vasseur, et al. Informational PAGE 2
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
1. 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 [RFC 3209] and
[RFC 3473]). 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 Explicit Route Object (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 [RFC 4105] and [RFC 4216]). Both requirements documents specify
the need for some mechanism providing an option for the head-end LSR
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:
(1) The first mechanism allows a head-end LSR to trigger a new path
re-evaluation on every hop that has a next hop defined as a loose
hop or abstract node and get a notification from the mid-point as
to whether a better path exists.
(2) The second mechanism allows a mid-point LSR to explicitly signal
to the head-end LSR either that a better path exists to reach a
loose/abstract hop (compared to the current path) 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.
2. Terminology
ABR: Area Border Router.
ERO: Explicit Route Object.
LSR: Label Switching Router.
TE LSP: Traffic Engineering Label Switched Path.
TE LSP head-end: head/source of the TE LSP.
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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
TE LSP tail-end: tail/destination of the TE LSP.
Interior Gateway Protocol Area (IGP Area): OSPF Area or IS-IS level.
Intra-area TE LSP: A 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 Autonomous Systems (ASes) or sub-ASes (BGP
confederations).
2.1. 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 [RFC 2119].
3. Establishment of a Loosely Routed TE LSP
The aim of this section is purely to summarize the mechanisms
involved in the establishment of a loosely routed TE LSP, as
specified in [RFC 3209]. The reader should see RFC 3209 for a more
detailed description of these 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 [RFC 3209], 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 be either partial (up to
the next loose hop) or complete (set of strict hops up to the TE LSP
destination).
Note that although the examples in the rest of this document are
provided in the context of MPLS inter-area TE, the proposed mechanism
applies equally to loosely routed paths within a single routing
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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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 follows.
<---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 is a strict hop (L=0) and L is a
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 as 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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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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 the TE LSP lifetime, a more
optimal path might appear between an LSR and its next loose hop (for
the sake of illustration, suppose that in the example above 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 same IGP area where the associated topology change
occurred, the head-end cannot make use of this shorter path (and
reroute the LSP using a make-before-break technique as described in
[RFC 3209]) when appropriate. Thus, a new mechanism 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; and
- 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 LSR that a preferable path exists
(a path with a lower cost, where the cost definition is determined
by some metric).
Once the head-end LSR has been notified of the existence of such a
preferable path, it 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
to fail due to some (link or node) required maintenance.
This mechanism allows the head-end LSR to reoptimize a TE LSP by
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.
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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
5. Signaling 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.
5.1. Path Re-Evaluation Request
The following new flag of the SESSION_ATTRIBUTE object (C-Type 1 and
7) is defined:
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 a 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. However,
strictly speaking, the ERO has not changed.
5.2. New Error Value Sub-Codes
As defined in [RFC 3209], the Error Code 25 in the ERROR SPEC object
corresponds to a Notify Error.
This document adds three new Error Value sub-codes:
6 Preferable path exists
7 Local link maintenance required
8 Local node maintenance required
The details about the local maintenance required modes are 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 LSR reoptimization.
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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
6.2. Reoptimization Triggers
There are several 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 that an implementation supporting the extensions
proposed in this document 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": Having determined that
a preferable path (other than the current path) exists or having
the need to perform a link/node local maintenance, a mid-point LSR
explicitly notifies the head-end LSR, which 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 [RFC 2961].
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 PathErr to the head-end
LSR (Error code 25 (Notify), Error sub-code=6 (better path
exists)). At this point, the LSR MAY decide not to propagate this
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). Therefore, 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 with 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
[RFC 2961].
The network administrator may decide to establish some local policy
specifying to ignore such request or not to consider those requests
more frequently than at a certain rate.
The proposed mechanism does not make any assumption of the path
computation method performed by the ERO expansion process.
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RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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), or the
user explicitly requests it. If a preferable path is found, the LSR
sends an RSVP PathErr to the head-end LSR (Error code 25 (Notify),
Error sub-code=6 ("preferable path exists").
There is another circumstance 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 that has 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.
Upon receiving an RSVP PathErr 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 of each other: both the timer and event-driven
reoptimization triggers can be implemented on the head-end or on any
mid-point LSR with a potentially different timer value 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
Vasseur, et al. Informational PAGE 10
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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 for the caching timer is suggested.
7. Applicability and Interoperability
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 already documented in [RFC 3209] and
[RFC 3473].
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 an RSVP PathErr message.
8. IANA Considerations
IANA assigned three new error sub-code values for the RSVP PathErr
Notify message (Error code=25):
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 the existence of a preferable path or the need to
reroute the TE LSP for maintenance purposes. Hence, in the 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
Vasseur, et al. Informational PAGE 11
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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 to accept 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, and Dimitri Papadimitriou for their useful
comments. A special thanks to Adrian Farrel for his very valuable
inputs.
11. References
11.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
and S. Molendini, "RSVP Refresh Overhead Reduction
Extensions", RFC 2961, April 2001.
[RFC 3209] 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.
[RFC 3473] 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
[RFC 4105] Le Roux, J.-L., Vasseur, J.-P., and J. Boyle,
"Requirements for Inter-Area MPLS Traffic Engineering",
RFC 4105, June 2005.
[RFC 4216] Zhang, R. and J.-P. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
Vasseur, et al. Informational PAGE 12
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
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
Vasseur, et al. Informational PAGE 13
RFC 4736 MPLS-TE Loosely Routed LSP November 2006
Full Copyright Statement
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Vasseur, et al. Informational PAGE 14
Reoptimization of Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) Loosely Routed Label Switched Path (LSP)
RFC TOTAL SIZE: 28850 bytes
PUBLICATION DATE: Friday, November 3rd, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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