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rfc:rfc4736

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 (C) 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 [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 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 [RFC4105] and [RFC4216]).  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.

Vasseur, et al. Informational [Page 3] 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 [RFC2119].

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 [RFC3209].  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 [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 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

Vasseur, et al. Informational [Page 4] 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
  1. R3, R5, R8, and R9 are ABRs.
  1. 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).

Vasseur, et al. Informational [Page 5] 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
 [RFC3209]) 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
  1. 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
  1. 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.

Vasseur, et al. Informational [Page 6] 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 [RFC3209], 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.

Vasseur, et al. Informational [Page 7] RFC 4736 MPLS-TE Loosely Routed LSP November 2006

6.2. Reoptimization Triggers

 There are several possible reoptimization triggers:
  1. Timer-based: A reoptimization is triggered (process evaluating

whether a more optimal path can be found) when a configurable timer

   expires.
  1. Event-driven: A reoptimization is triggered when a particular

network event occurs (such as a "Link-UP" event).

  1. 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 [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:

Vasseur, et al. Informational [Page 8] RFC 4736 MPLS-TE Loosely Routed LSP November 2006

 A path re-evaluation is triggered, and the newly computed path is
 compared to the existing path:
  1. 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.
  1. 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
   [RFC2961].
   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.

Vasseur, et al. Informational [Page 9] 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:
  1. The link (sub-code=7) or the node (sub-code=8) MUST be locally

registered for further reference (the TE database must be updated).

  1. 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 [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 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

 [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.-L., Vasseur, J.-P., and J. Boyle,
            "Requirements for Inter-Area MPLS Traffic Engineering",
            RFC 4105, June 2005.
 [RFC4216]  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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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
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Vasseur, et al. Informational [Page 14]

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