Francois Le Faucheur
                                                           Ramesh Uppili
                                                     Cisco Systems, Inc.

                                                          Alain Vedrenne
                                                           Pierre Merckx
                                                                  Equant

                                                          Thomas Telkamp
                                                         Global Crossing

IETF Internet Draft
Expires: January, 2002
Document: draft-lefaucheur-te-metric-igp-00.txt         July, 2001



                Use of IGP Metric as a second TE Metric



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.


Abstract

   This draft describes a common practice on how the existing IGP
   Metric can be used as an alternative metric to the TE Metric for
   Constraint Based Routing of MPLS TE Tunnels. This effectively
   results in the ability to perform Constraint Based Routing with
   optimization of one metric (e.g. link bandwidth) for some TE Tunnels
   (e.g. Data Trunks) while optimizing another metric (e.g. propagation
   delay) for some other TE Tunnels with different requirements (e.g.
   Voice Trunks).

   No protocol extensions or modifications are required. This text
   documents current router implementations and deployment practices.

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                    IGP Metric as second TE Metric           July 2001


1.      I-D Summary Information

   This section contains the information submitted on the
   "idsummary@subip.ietf.org" list of the Sub-IP area.

   RELATED DOCUMENTS:
   Complements:
   http://www.ietf.org/internet-drafts/draft-fedyk-isis-ospf-te-
   metrics-01.txt
   Compete: None.

   WHERE DOES IT FIT IN THE PICTURE OF THE SUB-IP WORK:
   This fits into TE-WG.

   WHY IS IT TARGETED AT THIS WG
   This directly addresses specific items of the TEWG charter:
   "- BCP documents on ISP uses, requirements, desires (TEBCPs)
   - TE interoperability & implementation informational notes (TEIMP) "

   JUSTIFICATION
   The draft directly addresses a items of the charter and relates to
   existing implementations and planned deployments.


2.      Introduction

   IGP routing protocols (OSPF and IS-IS) as well as MPLS Signaling
   protocols (RSVP-TE and CR-LDP) have been extended (as specified in
   [ISIS-TE], [OSPF-TE], [RSVP-TE] and [CR-LDP]) in order to support
   the traffic engineering functionality as defined in [TE-REQ].

   These IGP routing protocol extensions currently include
   advertisement of a single additional TE Metric to be used for
   Constraint Based Routing of TE Tunnels.

   However, the objective of traffic engineering is to optimize the use
   and the performance of the network. So it seems relevant that TE
   tunnel placement may be optimized according to different
   optimization criteria. For example, some Service Providers want to
   perform traffic engineering of different classes of service
   separately so that each class of Service is transported on a
   different TE Tunnel. One example motivation for doing so is to apply
   different fast restoration policies to the different Classes of
   Service. Another example motivation is to take advantage of separate
   Constraint Based Routing in order to meet the different QoS
   objectives of each Class of Service. To achieve different QoS
   objectives may require enforcement by Constraint Based Routing of
   different bandwidth constraints for the different Classes of Service
   as defined in [DS-TE]. In some Service Provider environments, it
   also requires optimizing on a different metric during Constraint
   Based Routing.

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                    IGP Metric as second TE Metric           July 2001


   The most common scenario for a different metric calls for
   optimization of a metric reflecting delay (mainly propagation delay)
   when Constraint Based Routing TE LSPs that will be transporting
   voice, while optimizing a more usual metric (e.g. reflecting link
   bandwidth) when Constraint Based Routing TE LSPs that will be
   transporting data.

   [METRICS] proposes extensions so that multiple TE Metrics can be
   advertised in the IGP. If/once those are fully specified and
   implemented, they will address the above scenario. However this
   draft describes how the above scenario is currently addressed in the
   meantime by existing implementations and deployments, without any
   additional IGP extensions beyond [ISIS-TE] and [OSPF-TE], by
   effectively using the IGP Metric as a "second" TE Metric.


3.      Common Practice

   In current MPLS TE deployments, the metric used by network
   administrators for Constraint Based Routing of TE LSPs carrying data
   traffic is often the same as the metric used for Shortest Path
   Routing. Where this is the case, the Constraint Based Routing
   algorithm running on the Head-End LSR can use the IGP Metric
   advertised in the IGP to compute paths for data TE LSPs instead of
   the advertised TE Metric. The TE Metric can then be used to convey
   another metric (e.g. a delay-based metric) which can be used by the
   Constraint Based Routing algorithm on the Head-End LSR to compute
   path for the TE LSPs with different requirements (e.g. Voice TE
   LSP).

   Other services providers use a delay-based metric as the IGP metric.
   In that case, the Constraint Based Routing algorithm running on the
   Head-End LSR can use the IGP Metric advertised in the IGP to compute
   paths for delay-sensitive TE LSPs (e.g. Voice TE LSPs) instead of
   the advertised TE Metric. The TE Metric can then be used to convey
   another metric (e.g. bandwidth based metric) which can be used by
   the Constraint Based Routing algorithm to compute paths for the data
   TE LSPs.

   More generally, the TE Metric can be used to carry any arbitrary
   metric that may be useful for Constraint Based Routing of the set of
   LSPs which need optimization on another metric than the IGP metric.

3.1.    Head-End LSR Implementation Practice

   A Head-End LSR implements the current practice by:
   - Allowing configuration, for each TE LSP to be routed, of whether
     the IGP Metric or the TE Metric is to be used by the Constraint
     Based Routing algorithm.



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                    IGP Metric as second TE Metric           July 2001

   - Enabling the Constraint Based Routing algorithm to make use of
     either the TE Metric or the IGP Metric, depending on the above
     configuration for the considered TE-LSP

3.2.    Network Deployment Practice

   A Service Provider deploys this practice by:
   - Configuring, on every relevant link, the TE Metric to reflect
     whatever  metric is appropriate (e.g. delay-based metric) for
     Constraint Based Routing of some LSPs as an alternative metric to
     the IGP Metric
   - Configuring, for every TE LSP, whether this LSP is to be
     constraint based routed according to the TE Metric or IGP Metric

3.3.    Constraints

   The practice described in this document has the following
   constraints:
   - it only allows TE Tunnels to be routed on either of two metrics
     (i.e. it cannot allow TE Tunnels to be routed on one of three, or
     more, metrics)
   - it can only be used where the IGP Metric is appropriate as one of
     the two metrics to be used for constraint based routing (i.e. it
     cannot allow TE Tunnels to be routed on either of two metrics
     while allowing IGP SPF to be based on a third metric).

   Note that, as with [METRICS], this practice does not recommend that
   the TE Metric and the IGP metric be used simultaneously during path
   computation for a given LSP. This is known to be an NP-complete
   problem.

3.4.    Interoperability

   Where path computation is entirely performed by the Head-End (e.g.
   intra-area operations with path computation on Head-end), this
   practice does not raise any interoperability issue among LSRs since
   the use of one metric or the other is a matter purely local to the
   Head-End LSR.

   Where path computation involves another component than the Head-End
   (e.g. with inter-area operations where path computation is shared
   between the Head-End and Area Boundary Routers or a Path Computation
   Server), this practice requires that which metric to optimize on be
   signaled along with the other constraints (bandwidth, affinity) for
   the LSP. See [PATH-COMP] for a proposal on how to signal with RSVP-
   TE which metric to optimize on.


4.      Migration Considerations

   Service Providers need to consider how to migrate from the current
   implementation to the new one supporting this practice.

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                    IGP Metric as second TE Metric           July 2001


   Although the head-end routers act independently from each other,
   some migration scenarios may require that all head-end routers be
   upgraded to the new implementation to avoid any disruption on
   existing TE-LSPs before two metrics can effectively be used by TE.
   The reason is that routers with current implementation are expected
   to always use the TE metric for Constraint Based Routing of all
   tunnels; so when the TE metric is reconfigured to reflect the
   "second metric" (say to a delay-based metric) on links in the
   network, then all TE-LSPs would get routed based on the delay-based
   metric, while the intent may be that only the TE-LSPs explicitly
   configured so should be routed based on the "second metric" (e.g.
   delay-based).

   A possible migration scenario would look like this:

        1) upgrade software on all head-end routers in the network to
           support this practice
        2) change the TE-LSPs configuration on the head-end routers to
           use the IGP metric (e.g. bandwidth-based) for Constraint
           Based Routing rather than the TE metric
        3) configure TE metric on the links to reflect the "second
           metric" (e.g. delay-based)
        4) modify the LSP configuration of TE-LSPs that should use the
           "second metric" (e.g. delay-based) to be Constraint Based
           Routed based on the TE Metric, and/or create new TE-LSPs
           with such configuration

   It is desirable that step 2 is non-disruptive (i.e. the routing of a
   LSP will not be affected in any way, and the data transmission will
   not be interrupted) by the change of LSP configuration to use "IGP
   Metric" as long as the actual value of the "IGP Metric" and "TE
   Metric" are equal on every link at the time of LSP reconfiguration
   (as would be the case at step 2 in migration scenario above which
   assumed that TE Metric was initially equal to IGP Metric).


5.      Security Considerations


   The practice described in this draft does not raise specific
   security issues beyond those of existing TE.


6.      Acknowledgment

   This document has benefited from discussion with Jean-Philippe
   Vasseur.



References

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                    IGP Metric as second TE Metric           July 2001


   [TE-REQ] Awduche et al, Requirements for Traffic Engineering over
   MPLS, RFC2702, September 1999.

   [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", draft-ietf-mpls-rsvp-lsp-tunnel-08.txt, February 2001.

   [CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
   draft-ietf-mpls-cr-ldp-05.txt, February 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.

   [PATH-COMP] Vasseur et al, "RSVP Path computation request and reply
   messages",  draft-vasseur-mpls-path-computation-rsvp-te-00.txt, July
   2001.


Authors' Address:

   Francois Le Faucheur
   Cisco Systems, Inc.
   Village d'Entreprise Green Side - Batiment T3
   400, Avenue de Roumanille
   06410 Biot-Sophia Antipolis
   France
   Phone: +33 4 97 23 26 19
   Email: flefauch@cisco.com

   Ramesh Uppili
   Cisco Systems, Inc.
   300 Apollo Drive
   Chelmsford, Massachussets 01824
   USA
   Phone: +1 978 244-4949
   Email: ruppili@cisco.com

   Alain Vedrenne
   EQUANT
   400 Galleria Parkway
   Atlanta, Georgia 30339

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                    IGP Metric as second TE Metric           July 2001

   USA
   Phone: +1 (678)-346-3466
   Email: alain.vedrenne@equant.com

   Pierre Merckx
   EQUANT
   1041 route des Dolines - BP 347
   06906 SOPHIA ANTIPOLIS Cedex
   FRANCE
   Phone: +33 (0)492 96 6454
   Email: pierre.merckx@equant.com

   Thomas Telkamp
   Global Crossing
   Olympia 6
   1213 NP Hilversum
   The Netherlands
   Phone: +31 35 655 651
   E-mail: telkamp@gblx.net

































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