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.
AbstractThis 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. 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.
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.
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. 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.
[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. [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.
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