Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric
bcp87-02

Versions: 00 01 02 rfc3785                         Best Current Practice
                                                    Francois Le Faucheur
                                                           Ramesh Uppili
                                                     Cisco Systems, Inc.

                                                          Alain Vedrenne
                                                           Pierre Merckx
                                                                  Equant

                                                          Thomas Telkamp
                                                         Global Crossing

IETF Internet Draft
Expires: March, 2003
Document: draft-ietf-tewg-te-metric-igp-02.txt          September, 2002



       Use of Interior Gateway Protocol (IGP) Metric as a second
                    MPLS Traffic Engineering 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
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Abstract

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


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                    IGP Metric as second TE Metric      September 2002

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


1.      Introduction

   Interior Gateway Protocol (IGP) routing protocols (OSPF and IS-IS)
   as well as MultiProtocol Label Switching (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 (TE) functionality as defined in [TE-REQ].

   These IGP routing protocol extensions currently include
   advertisement of a single additional MPLS 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 Quality of
   Service (QoS) objectives of each Class of Service. Depending on QoS
   objectives one may require either (a) enforcement by Constraint
   Based Routing of different bandwidth constraints for the different
   classes of service as defined in [DS-TE], or (b) optimizing on a
   different metric during Constraint Based Routing or (c) both. This
   document discusses how optimizing on a different metric can be
   achieved during Constraint Based Routing.

   The most common scenario for a different metric calls for
   optimization of a metric reflecting delay (mainly propagation delay)
   when Constraint Based Routing TE Label Switched Paths (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.

   Additional IGP protocol extensions could be defined so that multiple
   TE metrics could be advertised in the IGP (as proposed for example
   in [METRICS]) and would thus be available to Constraint Based
   Routing in order to optimize on a different metric. However this
   document describes how optimizing on a different metric can be
   achieved today 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.


2.      Common Practice

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                    IGP Metric as second TE Metric      September 2002


   In current MPLS TE deployments, network administrators often want
   Constraint Based Routing of TE LSPs carrying data traffic to be
   based on the same metric as the metric used for Shortest Path
   Routing. Where this is the case, this practice allows the Constraint
   Based Routing algorithm running on the Head-End LSR to 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).

   In some networks, network administrators configure the IGP metric to
   a value factoring the link propagation delay. In that case, this
   practice allows the Constraint Based Routing algorithm running on
   the Head-End LSR to 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.

2.1.    Head-End LSR Implementation Practice

   A Head-End LSR implements the current practice by:

   (i)    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.

   (ii)   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

2.2.    Network Deployment Practice

   A Service Provider deploys this practice by:

   (i)    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

   (ii)   Configuring, for every TE LSP, whether this LSP is to be
          constraint based routed according to the TE metric or IGP
          metric


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                    IGP Metric as second TE Metric      September 2002

2.3.    Constraints

   The practice described in this document has the following
   constraints:

   (i)    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). Extensions (for example such as
          those proposed in [METRICS]) could be defined in the future
          if necessary to relax this constraints, but this is outside
          the scope of this document.

   (ii)   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). Extensions (for example such as those proposed
          in [METRICS]) could be defined in the future if necessary to
          relax this constraints, but this is outside the scope of this
          document.

   (iii)  it can only be used on links which support an IGP adjacency
          so that an IGP metric is indeed advertised for the link. For
          example, this practice can not be used on Forwarding
          Adjacencies (see [LSP-HIER]).

   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.

2.4.    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 an example proposal on how to signal
   which metric to optimize, to another component involved in path
   computation when RSVP-TE is used as the protocol to signal path
   computation information.


3.      Migration Considerations


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                    IGP Metric as second TE Metric      September 2002

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

   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 "second
   metric" metric, while the intent may be that only the TE-LSPs
   explicitly configured so should be routed based on the "second
   metric".

   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 the subset of TE-LSPs which
           need to be Constraint Based routed using the "second metric"
           (e.g. delay-based), and/or create new TE-LSPs with such a
           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).


4.      Security Considerations

   The practice described in this document does not raise specific
   security issues beyond those of existing TE. Those are discussed in
   the respective security sections of [TE-REQ], [RSVP-TE] and [CR-
   LDP].


5.      Acknowledgment


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                    IGP Metric as second TE Metric      September 2002

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


6.      Normative References

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

   [OSPF-TE] Katz et al, Traffic Engineering Extensions to OSPF Version
   2, draft-katz-yeung-ospf-traffic-07.txt, August 2002.

   [ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
   ietf-isis-traffic-04.txt, August 2001.

   [RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP
   Tunnels", RFC3209, December 2001.

   [CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP",
   RFC3212, January 2002


7.      Informative References

   [METRICS] Fedyk et al, "Multiple Metrics for Traffic Engineering
   with IS-IS and OSPF", draft-fedyk-isis-ospf-te-metrics-01.txt,
   November 2000.

   [DIFF-TE] Le Faucheur et al, "Requirements for support of Diff-Serv-
   aware MPLS Traffic Engineering", draft-ietf-tewg-diff-te-reqts-
   05.txt, June 2002.

   [PATH-COMP] Vasseur et al, "RSVP Path computation request and reply
   messages",  draft-vasseur-mpls-path-computation-rsvp-03.txt, June
   2002.

   [LSP-HIER] Kompella et al, "LSP Hierarchy with Generalized MPLS TE",
   draft-ietf-mpls-lsp-hierarchy-07.txt, June 2002.


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

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                    IGP Metric as second TE Metric      September 2002

   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
   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
   Oudkerkhof 51
   3512 GJ Utrecht
   The Netherlands
   Phone: +31 30 238 1250
   E-mail: telkamp@gblx.net






















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