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rfc:bcp:bcp87

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].

Le Faucheur, et al. Best Current Practice [Page 1] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

 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

Le Faucheur, et al. Best Current Practice [Page 2] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

 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

Le Faucheur, et al. Best Current Practice [Page 3] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

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.

Le Faucheur, et al. Best Current Practice [Page 4] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

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].

Le Faucheur, et al. Best Current Practice [Page 5] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

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.

Le Faucheur, et al. Best Current Practice [Page 6] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

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

Le Faucheur, et al. Best Current Practice [Page 7] RFC 3785 IGP Metric as a second MPLS TE Metric May 2004

8. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  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
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Le Faucheur, et al. Best Current Practice [Page 8]

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