Francois Le Faucheur Ramesh Uppili Cisco Systems, Inc. Alain Vedrenne Pierre Merckx Equant Thomas Telkamp Global Crossing IETF Internet Draft Expires: September, 2002 Document: draft-ietf-tewg-te-metric-igp-00.txt March, 2002 Use of IGP Metric as a second TE 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 Working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This draft describes a common practice on how the existing IGP Metric can be used as an alternative metric to the TE Metric for Constraint Based Routing of MPLS TE Tunnels. This effectively results in the ability to perform Constraint Based Routing with optimization of one metric (e.g. link bandwidth) for some TE Tunnels (e.g. Data Trunks) while optimizing another metric (e.g. propagation delay) for some other TE Tunnels with different requirements (e.g. Voice Trunks). No protocol extensions or modifications are required. This text documents current router implementations and deployment practices. Le Faucheur, et. al 1 IGP Metric as second TE Metric March 2002 1. Introduction IGP routing protocols (OSPF and IS-IS) as well as 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 functionality as defined in [TE-REQ]. These IGP routing protocol extensions currently include advertisement of a single additional 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 QoS objectives of each Class of Service. To achieve different QoS objectives may require enforcement by Constraint Based Routing of different bandwidth constraints for the different classes of service as defined in [DS-TE]. In some Service Provider environments, it also requires optimizing on a different metric 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 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. [METRICS] proposes extensions so that multiple TE Metrics can be advertised in the IGP. If/once those are fully specified and implemented, they will address the above scenario. However this draft describes how the above scenario is currently addressed in the meantime 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 Le Faucheur et. al 2 IGP Metric as second TE Metric March 2002 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: Le Faucheur et. al 3 IGP Metric as second TE Metric March 2002 (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). [METRICS] proposes extensions which could be used to relax this constraints when necessary. (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). [METRICS] proposes extensions which could be used to relax this constraint when necessary. (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 a 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 Le Faucheur et. al 4 IGP Metric as second TE Metric March 2002 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 draft does not raise specific security issues beyond those of existing TE. 5. Acknowledgment This document has benefited from discussion with Jean-Philippe Vasseur. References [TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS, RFC2702, September 1999. Le Faucheur et. al 5 IGP Metric as second TE Metric March 2002 [OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-katz-yeung-ospf-traffic-06.txt, October 2001. [ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft- ietf-isis-traffic-03.txt, June 2001. [RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-tunnel-08.txt, February 2001. [CR-LDP] Jamoussi et al., "Constraint-Based LSP Setup using LDP", draft-ietf-mpls-cr-ldp-05.txt, February 2001 [METRICS] Fedyk et al, "Multiple Metrics for Traffic Engineering with IS-IS and OSPF", draft-fedyk-isis-ospf-te-metrics-01.txt, November 2000. [DS-TE] Le Faucheur et al, "Requirements for support of Diff-Serv- aware MPLS Traffic Engineering", draft-ietf-tewg-diff-te-reqts- 02.txt, November 2001. [PATH-COMP] Vasseur et al, "RSVP Path computation request and reply messages", draft-vasseur-mpls-path-computation-rsvp- 01.txt, November 2001. [LSP-HIER] Kompella et al, "LSP Hierarchy with Generalized MPLS TE", draft-ietf-mpls-lsp-hierarchy-04.txt, February 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 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 Le Faucheur et. al 6 IGP Metric as second TE Metric March 2002 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 Olympia 6 1213 NP Hilversum The Netherlands Phone: +31 35 655 651 E-mail: telkamp@gblx.net Le Faucheur et. al 7