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               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
                                                          Jean-Philippe Vasseur(Editor)
                                                                    Cisco Systems, Inc.
                                                                         Yuichi Ikejiri
                                                         NTT Communications Corporation
               
               IETF Internet Draft
               Expires: July, 2003
                                                                        January, 2003
               
               
               
               
               
                              draft-vasseur-mpls-loose-path-reopt-00.txt
               
               
               
               
                      Reoptimization of an explicit loosely routed MPLS TE paths
               
               
               
               
               Status of this Memo
               
               This document is an Internet-Draft and is in full conformance with all
               provisions of Section 10 of RFC2026. Internet-Drafts are
               Working documents of the Internet Engineering Task Force (IETF), its
               areas, and its working groups.  Note that other groups may also
               distribute working documents as Internet-Drafts.
               
               Internet-Drafts are draft documents valid for a maximum of six months
               and may be updated, replaced, or obsoleted by other documents at any
               time. It is inappropriate to use Internet-Drafts as reference material
               or to cite them other than as "work in progress."
               
               The list of current Internet-Drafts can be accessed at
               http://www.ietf.org/ietf/1id-abstracts.txt.
               The list of Internet-Draft Shadow Directories can be accessed at
               http://www.ietf.org/shadow.html.
               
               
               
               
               
               
               
               
               
               
               
               
                Vasseur and Ikejiri                                                 1
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               Abstract
               
               The aim of this document is to propose a mechanism for the
               reoptimization of loosely routed explicit paths. A loosely routed
               explicit path is as a path specified as a combination of strict and
               loose hop(s) that contains at least one loose hop and zero or more
               strict hop(s). The path calculation (ERO expansion) to reach a loose
               hop is made on the previous hop defined in the TE LSP path. This draft
               proposes a mechanism that allows
                  - the TE LSP Head-end LSR to trigger a reoptimization on every loose
                  hops along the path,
                  - an LSR to signal to the TE LSP head-end that a better path exists
                  to reach a loose than the path in use. A better path is defined as a
                  path with a lower cost, where the cost is defined by the metric used
                  to compute the path.
               This primarily applies to inter-area TE when the path is defined as a
               list of loose hops (generally the loose hops are the ABRs) but the
               following mechanism is also applicable to any loosely routed explicit
               paths within a single routing domain.
               
               1.      Establishment of an explicit loosely routed TE LSP path
               
               A loosely routed explicit path is as 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). Loose hops are specified in the ERO object
               of the Path message with the L flag of the Ipv4 prefix sub-object set,
               as defined in RFC3209. In this case, each LSR along path can perform a
               partial route computation to reach the next loose hop and then performs
               an ERO expand, before forwarding the RSVP Path message downstream.
               
               Note that the examples in the rest of this draft will be provided in
               the context of MPLS inter-area TE but the proposed mechanism also
               applies to loosely routed path within a single routing domain.
               Furthermore, this mechanism could also be used in the context of
               loosely routed paths in the context of TE LSPs spanning several
               autonomous systems.
               Also examples will be provided with OSPF as the IGP but the mechanisms
               similarly apply to IS-IS.
               
               An example of an explicit loosely routed TE LSP signalling (see also
               [MULTI-AREA-TE scenario 1]
               
               <---area 1--><-area 0--><-area 2->
               
                R1---R2----R3---R6    R8-----R10
                 |          |    |   / |\    |
                 |          |    | --  | --\ |
                 |          |    |/    |    \|
                 |---R4----R5---R7----R9-----R11
               
               
               
                Vasseur and Ikejiri                                                 2
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               Assumptions
               - R3, R5, R8 and R9 are ABRs
               - A TE LSP1 from R1 (Head-End LSR) to R11 (Tail-end LSR) is defined
               with the following loosely routed path: R1-R3-R8-R11. R3, R8 and R11
               are defined as loose hops.
               
               Step 1: the TE LSP 1 Head-end (R1) builds the following ERO object:
               R1(S)-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 the TE LSP constraint
               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 expand operation to
               reach the next loose hops R8. The new ERO becomes: R6(S)-R7(S)-R8(S)-
               R11(L).
               Step 4: the same procedure applies at R8.
               ...
               
               2.      Reoptimization of an explicit loosely routed TE LSP
               
               Once the TE LSP is set up, the TE LSP is maintained through normal RSVP
               procedures. Then a more optimal path might appear between an LSR and
               its next loose hop (suppose in the example above that a link between R6
               and R8 is added that provides a shorter path between R3 and R8 (R3-R6-
               R8) than the existing R3-R6-R7-R8 path). Currently if the better path
               is not visible from the Head-end LSR, it cannot make use of this better
               path and perform a make before break when appropriate. This is for
               instance the case in the example above as the better path appears in an
               area, which is not the Head-end area.
               
               This draft proposes a mechanism that allows:
               - the TE LSP Head-end LSR to trigger on every LSR whose next hop is a
               loose hop the re evaluation of the current path in order to detect a
               potential more optimal path,
               - an LSR whose next hop is a loose-hop to signal (using a new ERROR-
               SPEC sub code carried in a Path Error Notify message) to the TE LSP
               head-end that a better path exists (a path with a lower cost, where the
               cost is defined by the metric used to compute the path -                                                                      - see [SEC-
               METRIC], [METRIC]).
               
               Then once the existence of a better path is notified to the Head-end it
               can perform a make before break.
               
               
               
               
               
               
               
                Vasseur and Ikejiri                                                 3
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               3.      Signalling extensions
               
               3.1.     ERO expansion signaling request
               
               For EXPLICIT ROUTE object C-Type 1 and 7, we define a new flag in the
               SESSION_ATTRIBUTE object:
               
                       ERO expansion request:  0x20
               
               This flag indicates to every LSR having a loose hops specified as their
               next hop in the ERO that a new ERO expansion is requested.
               
               
               3.2.    New Path Error sub-code
               
               The format of a Path Error is the following:
               
               <PathErr message> ::= <Common Header> [ <INTEGRITY> ]
               
                                                     <SESSION> <ERROR_SPEC>
               
                                                     [ <POLICY_DATA> ...]
               
                                                    [ <sender descriptor> ]
               
                          <sender descriptor> ::= (see earlier definition)
               
               IPv4 ERROR_SPEC object: Class = 6, C-Type = 1
               
               
                          +-------------+-------------+-------------+-------------+
                          |            IPv4 Error Node Address (4 bytes)          |
                          +-------------+-------------+-------------+-------------+
                          |    Flags    |  Error Code |        Error Value        |
                          +-------------+-------------+-------------+-------------+
               
               Various Error Codes and Error values have been defined in RFC2205 and
               RFC3209.
               
               The ERROR-CODE 25 corresponds to a Path Error - Notify Error. We
               propose to add two new sub-codes:
                        4       Better path exists
                        5       No better path
               
               
               4.      Mode of operation
               
               
               4.1.    TE LSP reroute
               
               
               
                Vasseur and Ikejiri                                                 4
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               The TE LSP reroute (make before break) is always considered as being
               performed on the Head-end LSR.
               
               4.2.    Reoptimization triggers
               
               There are two possible reoptimization triggers:
               - timer-based: a reoptimization is triggered (look for a more optimal
               path) when a configurable timer expires,
               - event-driven: a reoptimization is triggered when a particular event
               happens (such as a ''Link-UP'' event).
               
               
               4.3.    Head-end reoptimization request versus mid-point reoptimization
                   indication
               
               The need for reoptimization (a better path exists) of an explicit
               loosely routed TE LSP can be either:
                       - requested by the Head-end LSR,
                       - determined by any mid-point LSR whose next hop is a loose hop
                       having detected that a better path (than the existing path)
                       exists.
               
               4.3.1.  Head-end reoptimization request
               
               In this mode, when a timer-based reoptimization is triggered on the
               head-end LSR (reoptimization timer has fired) or the operator manually
               requests a reoptimization, the head-end LSR immediately sends a Path
               message with the ''ERO expansion request'' bit of the SESSION_ATTRIBUTE
               object set.
               
               Upon receiving a Path message with the ''ERO expansion request'' bit of
               the SESSION_ATTRIBUTE object set, every LSR, for which the next
               abstract node contained in the ERO is defined as a loose hop, must
               perform a new ERO expansion (path re evaluation):
                       - if a better path can be found to reach the next loose hop
                       (than the path currently in use), the LSR must immediately send
                       a Path Error to the head-end (Error code 25 (Notify), sub-
                       code=4 (better path exists)),
                       - if no better path can be found, the LSR must send a Path
                       Error to the head-end (Error code 25 (Notify), sub-code=5 (no
                       better path))
               
               By better path, we mean a path having a lower cost. By default, an LSR
               uses the IGP metric in their CSPF to detect the shortest path that
               obeys a set of constraints. Note that the head-end 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 in the ERO to
               use the TE metric to determine the best path.
               
               Let call Ln the list of LSRs defined as loose hops in the ERO sent in
               the Path message by the Head-end LSR: Ln=<l1, l2, ..., ln>. Let's now
               
                Vasseur and Ikejiri                                                 5
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               call Pn=<p1, p2, ..., pn> the list of LSRs pi such that li is a next
               (loose) hop of pi for i=1...n
               
               Example 2:
               
               <---area 1--><-area 0--><-area 2->
               
                R1---R2----R3---R6    R8-----R10
                 |          |    |   / |\    |
                 |          |    | --  | --\ |
                 |          |    |/    |    \|
                 |---R4----R5---R7----R9-----R11
               
               A TE LSP1 from R1 (Head-End LSR) to R11 (Tail-end LSR) is defined with
               the following loosely routed path: R1-R3-R8-R11. R3, R8 and R11 are
               defined as loose hops.
               
               Ln=<R3,R8,R11>
               Pn=<R1,R3,R8>
               
               As soon as a positive response is received from an LSR pi (sub-code=4,
               ''Better path exists''), the Head-end LSR must:
                               - clear the ''ERO expansion request'' bit,
                               - perform a make before break
               
               If no positive response is received then the Head-end LSR must keep
               refreshing the current TE LSP.
               
               The Head-end LSR must not clear the ''ERO expansion request'' bit until:
               - all LSRs <p1, ..pn> have responded to the request,
               or
               - at least 4 consecutive Path messages have been sent with this bit
               set.
               
               Note that if RSVP Path messages are sent in reliable mode (see
               RFC2961), then this number of 4 can be reduced to 1.
               
               
               4.3.2.  Mid-point reoptimization indication
               
               In this mode, an LSR whose next abstract node is a loose hop can
               locally trigger an ERO expansion (when a configurable timer expires or
               on event-driven basis (link-up event for example)). If a better path is
               found compared to the existing one, the LSR sends a Path Error to the
               head-end (Error code 25 (Notify), sub-code=4 (better path exists)). The
               Head-end LSR must then immediately perform a make before break.
               
               Note that those modes are not exclusive: both the timer and even-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.
               
                Vasseur and Ikejiri                                                 6
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               
               
               4.3.3.  ERO caching
               
               Once a mid-point LSR has determined that a better path exists (after a
               reoptimization request has been received by the Head-end LSR or the
               retopimization timer on the mid-point has fired), the more optimal path
               should be cached on the mid-point for a limited amount of time to avoid
               having to recompute a route once the Head-LSR performs a make before
               break.
               
               
               5.      Interoperability
               
               An LSR non supporting the ''ERO expansion request'' bit of the SESSION-
               ATTRIBUTE object should just ignore it.
               
               Any Head-end LSR non supporting this draft receiving a Path Error
               Notify message with sub-code = 4 or 5 should just ignore the Path
               message.
               
               
               6.      Security Considerations
               
               The practice described in this draft does not raise specific security
               issues beyond those of existing TE.
               
               
               7.      Acknowledgment
               
               The authors would like to thank Carol Iturralde and Miya Khono for
               their useful and valuable comments.
               
               
               8.      Intellectual Property
               
               The contributor represents that he has disclosed the existence of any
               proprietary or intellectual property rights in the contribution that
               are reasonably and personally known to the contributor.  The
               contributor does not represent that he personally knows of all
               potentially pertinent proprietary and intellectual property rights
               owned or claimed by the organization he represents (if any) or third
               parties.
               
               
               References
               
               [TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS,
               RFC2702, September 1999.
               
               
               
                Vasseur and Ikejiri                                                 7
               
               
               
               draft-vasseur-mpls-loose-path-reopt-00.txt                January 2003
               
               
               [OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
               katz-yeung-ospf-traffic-05.txt, June 2001.
               
               [ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
               ietf-isis-traffic-03.txt, June 2001.
               
               [RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels",
               RFC3209, December 2001.
               
               [METRICS] Fedyk et al, ''Multiple Metrics for Traffic Engineering with
               IS-IS and OSPF'', draft-fedyk-isis-ospf-te-metrics-01.txt, November
               2000.
               
               [DS-TE] Le Faucheur et al, ''Requirements for support of Diff-Serv-aware
               MPLS Traffic Engineering'', draft-ietf-tewg-diff-te-reqts-01.txt, June
               2001.
               
               [MULTI-AREA-TE] Kompella at all, ''Multi-area MPLS Traffic Engineering'',
               draft-kompella-mpls-multiarea-te-03.txt, June 2002.
               
               [PATH-COMP] Vasseur et al, ''RSVP Path computation request and reply
               messages'',  draft-vasseur-mpls-computation-rsvp-03.txt, November 2001.
               
               [SEC-METRIC] Le Faucheur et all,'' Use of Interior Gateway Protocol
               (IGP) Metric as a second MPLS Traffic Engineering Metric'', draft-ietf-
               tewg-te-metric-igp-02.txt, September, 2002.
               
               
               Authors' addresses:
               
               Jean Philippe Vasseur
               Cisco Systems, Inc.
               300 Apollo Drive
               Chelmsford, MA 01824
               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
               
               
               
               
               
               
               
               
               
                Vasseur and Ikejiri                                                 8
               

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