Network Working Group Kireeti Kompella Internet Draft Juniper Networks Expiration Date: September 2001 Yakov Rekhter Juniper Networks Multi-area MPLS Traffic Engineering draft-kompella-mpls-multiarea-te-01.txt 1. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 except that the right to produce derivative works is not granted. 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. 2. Abstract An ISIS/OSPF routing domain may consists of multiple areas. This document postulates a set of mechanisms, and then outlines how these mechanisms could be used to establish/maintain Traffic Engineering LSPs that span multiple areas. draft-kompella-mpls-multiarea-te-01.txt [Page 1] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 3. Set of mechanisms In this section we postulate a set of mechanisms that could be used to construct LSPs that span multiple ISIS/OSPF areas. The actual application of these mechanisms to construct such LSPs is covered in the next section. We assume that the mechanisms listed below are either already available, or could be realized via extensions to the existing signaling (RSVP/CR-LDP) and/or routing (ISIS/OSPF) protocols used for MPLS Traffic Engineering. In this document we use the OSPF term "backbone area". In the context of ISIS this term means the set of L2 routers. 3.1. Passing constraints An LSR that acts as a head-end of an LSP should be able to pass the constraints associated with this LSP to some other node. The other node may, or may not be along the path taken by the LSP. Note that the existing signalling (RSVP/CR-LDP) already provides a way to pass some of the constraints, like resource affinity. A more general way to provide this functionality is described in [Kompella]. 3.2. Loose hops An LSR should be able to specify loose ERO hops, and let some intermediate LSR(s) along the path to expand it to strict hops. Note that the ability to specify loose hops is already available. 3.3. Path computation server An LSR that originates an LSP should be able to "ask" some other node to compute the path for the LSP. The node that computes the path may, or may not be along the path taken by the LSP. The node may compute either the whole path, or a segment of the path. In the latter case the computed path would include loose hops. This mechanism requires the ability of the LSR that originates the LSP to pass the constraints associated with that LSP to the node that is going to compute the path for the LSP. It also requires the ability of the LSR that originates the LSP to pass the address of LSR at the tail-end of the LSP to the node that is going to compute the path for the LSP, and for the node that is going to compute the path for the LSP the ability to pass back to the node that originates the LSP the ERO that contains the results of the path computation. draft-kompella-mpls-multiarea-te-01.txt [Page 2] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 Once the path computation server computes the path, if the server is not along the path, the server is no longer responsible for maintaining the feasibility of the path (for one thing, the server may not even know whether the LSR established the path in the first place). 3.4. Acquiring topology/resources information for multiple areas A node should be able to obtain the topology and TE information of not just its own area, but other areas as well. This information may be subject to filtering (e.g., the node could obtain only the information about FAs in other areas). The node should be able to perform Constrained SPF (CSPF) based on this information. An (existing) example of a node that has the topology and TE information for several areas is an OSPF Area Border Router (ABR) - an ABR has the topology and TE information for the backbone area, plus all the other areas connected to that ABR. An extreme case is when a node obtains the topology and TE information of the whole OSPF/ISIS domain. 4. Constructing multi-area TE LSPs Consider the situation where the head-end of a TE LSP is in a different ISIS/OSPF area than the tail-end of a TE LSP. We'll refer to the area that the head-end is in as the head-end area. We'll refer to the area that the tail-end is in as the tail-end area. Given the set of the mechanisms outlined in the previous section, the following sub-sections outline some of the possible scenarios that use these mechanisms in order to construct TE LSPs that span multiple OSPF/ISIS areas. In the context of this document we define an optimal route as a route that would be computed in the absence of areas. In other words, an optimal route in a given network that is composed of a single OSPF/ISIS routing domain is a route that is computed if the network isn't partitioned into areas. It is easy to construct examples that show that partitioning a network into areas, and the resuling loss of routing information may lead to sub-optimal routing, in a sense that the computed routes would be different than optimal routes. That is true irrespective of a particular scheme to aggregate/abstract/summarize routing information. One corollary of this is that it is impossible to draft-kompella-mpls-multiarea-te-01.txt [Page 3] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 guarantee optimal routing in the presence of routing information aggregation/abstraction/summarization. 4.1. Scenario 1 Path computation is done on a per area basis. The head-end LSR computes strict hops within its own area. The head-end LSR then initiates LSP path setup. The setup includes the information about the constraints associated with the LSP. The ABR in the head-end area uses the topology and TE information of the backbone area, as well as the information about the constraints associated with the LSP (the ABR receives this information as part of the LSP setup) to compute strict hops within the backbone area to the ABR in the tail-end area. The ABR in the tail-end area uses the topology and TE information of the tail-end area, as well as the information about the constraints associated with the LSP (the ABR receives this information as part of the LSP setup) to compute strict hops within the tail-end area from itself to the tail-end LSR. Handling link/node failures could be done as a two-phase approach, where in the first phase a failure is handled within an area where the failure occurs, and only if that fails, the head-end LSR is notified of the failure and is expected to handle it. To direct failure notifications to the ABR of the area where the failure occurs (rather than to the head-end LSR), the ABR should use the Notify Request Object. Note that since the choice of ABR in the head-end area is determined by only the information in the head-end area, the inability to find a route across multiple areas doesn't mean that the route doesn't exist - it may mean that another ABR in the head-end area should be chosen, and/or it may mean than another ABR in the tail-end area should be chosen. Thus the failure to find a route may require to try another ABRs. The total number of such attempts could be as large as H*T (where H is the number of ABRs in the head-end area, and T is the number of ABRs in the tail-end area). In this scenario the only information that has to be distributed across area boundaries are the reachability information associated with routers' interface addresses (including loopback addresses, if an LSP is to be terminated on a router loopback address). Both OSPF and ISIS already provide mechanisms to accomplish this. draft-kompella-mpls-multiarea-te-01.txt [Page 4] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 4.2. Scenario 2 The head-end LSR requests an ABR in its (head-end) area to compute the path all the way from the LSR to the ABR in the destination area. This is possible because the ABR in the head-end area maintains the topology and resource information both for the head-end area and for the backbone area. The ABR in the destination area then computes the rest of the path. Note that the ABR in the source area that computes the path all the way to the ABR in the destination area may, or may not be on the path taken by the LSP. In this scenario the only information that has to be distributed across area boundaries are the reachability information associated with routers' interface addresses (including loopback addresses, if an LSP is to be terminated on a router loopback address). Both OSPF and ISIS already provide mechanisms to accomplish this. This scenario is likely to reduce the number of LSP setup failures, relative to the scenario 1. This is because in contrast with the first scenario, there is no need to try multiple ABRs in the head-end area. 4.3. Scenario 3 The head-end LSR obtains the TE information from the backbone area, and uses it to compute the path all the way to the ABR in the tail- end area. The ABR in the tail-end area computes the rest of the path. If the head-end area contains LSRs that don't originate LSPs, then these LSRs need not maintain the TE information from the backbone area. This scenario is similar to scenario 2, except for the entity that computes path through the head-end and the backbone areas, and the amount of information that the originating LSR has to maintain. With respect to the amount of information, the originating LSR maintains the same information as the ABR in the head-end area. draft-kompella-mpls-multiarea-te-01.txt [Page 5] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 4.4. Scenario 4 The head-end LSR requests an entity that has the entire network (all areas) topology to compute the whole path. Except for the entity that has the entire network topology, in this scenario the only information that has to be distributed across area boundaries are the reachability information associated with routers' interface addresses (including loopback addresses, if an LSP is to be terminated on a router loopback address). Both OSPF and ISIS already provide mechanisms to accomplish this. Note that this is the only scenario that results in computing optimal routes. 5. Pre-engineered backbone area In certain cases it may be desirable to "pre-engineer" the backbone area by constructing a set of TE LSPs that would be used as FAs by the traffic that has to traverse the backbone area. The scenarios outline above do not preclude this as an option. With a pre-engineered backbone area, a variation of Scenario 3 would be to restrict the backbone information leaked to the non-backbone areas to only the information associated with the FAs in the backbone area. Note that an LSP that starts/ends in a non-backbone area, but traverses the backbone area may use more than one FA. That means, for example, that even if there is no FA between any of the ABRs in the source area and any of the ABRs in the destination area, an LSP could still be established, as long as there are some other FAs that combined together provide a path from the source to the destination area. Stability of the LSPs that are used as FAs in the backbone area is unaffected by the establishment/tear down of the LSPs that start/end in other areas, but traverse the backbone area. Pre-engineering the backbone area enables aggregation of the Label Forwarding State in the backbone area, as a TE LSP that is used as an FA could carry/nest multiple LSPs. One possible application of this feature would be in conjunction with GMPLS controlling OXCs, where OXCs would be placed in the backbone area. draft-kompella-mpls-multiarea-te-01.txt [Page 6] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 6. TE information summary One possible application of summarization of TE information would be to prune off unfeasible ABRs, as follows. An ABR in the destination area would compute two paths: (a) a min hop path to the destination ignoring all other constraints (such as b/w or resource affinity), and (b) a max unreserved b/w path to the destination, again ignoring all other constraints (such as resource affinity or unreserved b/w). The ABR will then advertise the hop count from the first path and the max unreserved b/w from the second path into the backbone area. The ABR in the source area will do exactly the same, and then advertise this information into the source area. An LSR in the source area that needs to originate an LSP could use this information to prune off unfeasible ABRs. For example, if the LSP requires X amount of bandwidth, the LSR should rule out any of the ABRs in its area that advertise less than X amount of unreserved b/w to the LSP's destination. While pruning off unfeasible ABRs, as outlined in the previous paragraph, could reduce the number of LSP setup failures, it is not without drawbacks. For one thing, it increases the amount of control (routing) information that an LSR has to maintain, as well as the volume of the control information that an LSR receives from other LSRs, and has to process. In addition, it adds computational overhead to ABRs, as they need to perform additional computation. Moreover, establishment/tear down of inter-area LSPs increases the frequency with which this computation would need to be performed. Finally, it is important to realize that this approach doesn't eliminate LSP setup failures, as for one thing, the information computed and advertised by ABRs may be "out-of-date". Yet another reason for LSP setup failures is that the unreserved b/w information computed by ABRs doesn't take into account any other constraints, while a particular LSP may be subject not just to unreserved b/w, but to other constraints (e.g., resource affinity) as well. So, even if from the unreserved b/w constraint point of view a particular ABR is feasible for a particular LSP, when taking into account all the constraints associated with that LSP, the ABR may no longer be feasible. And to reiterate the point made earlier, since route computation with this approach uses summarized/aggreated/abstracted information, this approach doesn't guarantee optimal routing. draft-kompella-mpls-multiarea-te-01.txt [Page 7] Internet Draft draft-kompella-mpls-multiarea-te-01.txt March 2001 7. Security Considerations Security issues are not discussed in this document. 8. Acknowledgements We would like to thank Noel Chiappa, Tony Li, Robert Raszuk, and Alex Zinin for helping us with the ideas presented in this document. 9. References [Kompella] Kompella, K., "Carrying Constraints in RSVP" 10. Author Information Kireeti Kompella Juniper Networks, Inc. 1194 N. Mathilda Ave. Sunnyvale, CA 94089 Email: kireeti@juniper.net Yakov Rekhter Juniper Networks, Inc. 1194 N. Mathilda Ave. Sunnyvale, CA 94089 Email: yakov@juniper.net draft-kompella-mpls-multiarea-te-01.txt [Page 8]