Network Working Group                                     F. Le Faucheur
Request for Comments: 3785                                     R. Uppili
BCP: 87                                              Cisco Systems, Inc.
Category: Best Current Practice                              A. Vedrenne
                                                               P. Merckx
                                                                  Equant
                                                              T. Telkamp
                                                         Global Crossing
                                                                May 2004


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

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

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).  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

   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

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

   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

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

6.  References

6.1.  Normative References

   [TE-REQ]    Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M. and J.
               McManus, Requirements for Traffic Engineering over MPLS,
               RFC 2702, September 1999.


   [OSPF-TE]   Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering
               (TE) Extensions to OSPF Version 2", RFC 3630, September
               2003.

   [ISIS-TE]   Smit, H. and T. Li, "Intermediate System to Intermediate
               System (IS-IS) Extensions for Traffic Engineering (TE),
               RFC 3784, May 2004.

   [RSVP-TE]   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.

   [CR-LDP]    Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu,
               L., Doolan, P., Worster, T., Feldman, N., Fredette, A.,
               Girish, M., Gray, E., Heinanen, J., Kilty, T. and A.
               Malis, "Constraint-Based LSP Setup using LDP", RFC 3212,
               January 2002.

6.1.  Informative References

   [METRICS]   Fedyk, et al., "Multiple Metrics for Traffic Engineering
               with IS-IS and OSPF", Work in Progress, November 2000.

   [DIFF-TE]   Le Faucheur, F. and W. Lai, "Requirements for Support of
               Differentiated Services-aware MPLS Traffic Engineering",
               RFC 3564, July 2003.

   [PATH-COMP] Vasseur, et al., "RSVP Path computation request and reply
               messages", Work in Progress, June 2002.

   [LSP-HIER]  Kompella, et al., "LSP Hierarchy with Generalized MPLS
               TE", Work in Progress, September 2002.

7.  Authors' Addresses

   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,
   2000 Innovation Drive
   Kanata,
   ONTARIO,
   Canada - K2K 3E8

   Phone: 01-613-254 4578
   Email: ruppili@cisco.com

   Alain Vedrenne
   Equant
   Heraklion, 1041 route des Dolines, BP347
   06906 Sophia Antipolis Cedex
   FRANCE

   Phone: +33 4 92 96 57 22
   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, Ltd.
   Croeselaan 148
   NL-3521CG Utrecht
   The Netherlands

   Phone: +31 30 238 1250
   EMail: telkamp@gblx.net

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