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Network Working Group                                     F. Le FaucheurRequest for Comments: 3785                                     R. UppiliBCP: 87                                              Cisco Systems, Inc.Category: Best Current Practice                              A. Vedrenne                                                               P. Merckx                                                                  Equant                                                              T. Telkamp                                                         Global Crossing                                                                May 2004Use of Interior Gateway Protocol (IGP) Metricas a second MPLS Traffic Engineering (TE) MetricStatus 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 conveyLe 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-LSP2.2.  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         metricLe Faucheur, et al.      Best Current Practice                  [Page 3]

RFC 3785         IGP Metric as a second MPLS TE Metric          May 20042.3.  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 20043.  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 20045.  Acknowledgment   This document has benefited from discussion with Jean-Philippe   Vasseur.6.  References6.1.  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 20047.  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.netLe Faucheur, et al.      Best Current Practice                  [Page 7]

RFC 3785         IGP Metric as a second MPLS TE Metric          May 20048.  Full Copyright Statement   Copyright (C) The Internet Society (2004).  This document is subject   to the rights, licenses and restrictions contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Le Faucheur, et al.      Best Current Practice                  [Page 8]