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Versions: (draft-vasseur-ccamp-loose-path-reopt) 00 01 02 RFC 4736

draft-ietf-ccamp-loose-path-reopt-00.txt                 August 2004

   CCAMP WG
   Internet Draft                                    Jean-Philippe Vasseur
   Proposed status: Standard                                      (Editor)
                                                           Cisco Systems
                                                            Yuichi Ikejiri
                                                        NTT Communications
                                                               Corporation
                                                             Raymond Zhang
                                               Infonet Service Corporation
   Document: draft-ietf-ccamp-loose-path-
   reopt-00.txt
   Expires: February 2005                                      August 2004


    Reoptimization of MPLS Traffic Engineering loosely routed LSP

                draft-ietf-ccamp-loose-path-reopt-00.txt


Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or 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.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [i].

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Abstract

   This document defines a mechanism for the reoptimization of loosely
   routed MPLS and GMPLS 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).


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   The path calculation (which implies an ERO expansion) to reach a
   loose 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
   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 ôLink-UPö 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) ôHead-end requesting modeö: the request for a new path
        evaluation of a loosely routed TE LSP is requested by the head-
        end LSR.

        2) ôMid-point explicit notificationö: 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 ôPath
   re-evaluation requestö 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 ôPath re-evaluation requestö
   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 ôPath re-evaluation requestö 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 ôPreferable path
        existsö 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 ôPath re-evaluation requestö 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 ôPath re-
        evaluationö 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 ôPath re-evaluation requestö 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 (ôpreferable path existsö).

   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 ôPath re-evaluation requestö 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, 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.

10. References

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", 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. "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, "MPLS Inter-AS Traffic Engineering
   requirements", draft-ietf-tewg-interas-mpls-te-req-06.txt, February
   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-00.txt, August 2004, Work in progress


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   [INTER-DOMAIN-SIG] Ayyangar A. and Vasseur JP., "Inter domain MPLS
   Traffic Engineering - RSVP-TE extensions ", draft-ayyangar-ccamp-
   inter-domain-rsvp-te-00.txtö, July 2004, Work in progress.

   [INTER-DOMAIN-PATH-COMP] Vasseur JP., Ayyangar A., "Inter-domain
   Traffic Engineering LSP path computation methods", draft-vasseur-
   ccamp-inter-domain-path-comp-00.txt, July 2004, Work in progress.

   [GR-SHUT], Z. Ali et al, "Graceful Shutdown in MPLS Traffic
   Engineering Networks", 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

   "Copyright (C) The Internet Society (year).  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
   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


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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."



















































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