Network Working Group M. Shand Internet Draft Expiration Date: Dec 2004 Cisco Systems June 2004 IP Fast Reroute Framework
draft-ietf-rtgwg-ipfrr-framework-00.txtdraft-ietf-rtgwg-ipfrr-framework-01.txt Status of this Memo This document is an Internet-DraftBy submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and isany of which I become aware will be disclosed, in full conformanceaccordance with all provisions of Section 10 ofRFC 2026.3668. 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 obsolete 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 document provides a framework for the development of IP fast re- route mechanisms which provide protection against link or router failure by invoking locally determined repair paths. Unlike MPLS Fast-reroute, the mechanisms are applicable to a network employing conventional IP routing and forwarding. An essential part of such mechanisms is the prevention of packet loss caused by the loops which normally occur during the re-convergence of the network following a failure. Terminology This section defines words, acronyms, and actions used in this draft.1. Introduction When a link or node failure occurs in a routed network, there is inevitably a period of disruption to the delivery of traffic until the network re-converges on the new topology. Packets for destinations which were previously reached by traversing the failed component may be dropped or may suffer looping. Traditionally such disruptions have lasted for periods of at least several seconds, and most applications have been constructed to tolerate such a quality of service. Recent advances in routers have reduced this interval to under a second for carefully configured networks using link state IGPs. However, new Internet services are emerging which may be sensitive to periods of traffic loss which are orders of magnitude shorter than this. Addressing these issues is difficult because the distributed nature of the network imposes an intrinsic limit on the minimum convergence time which can be achieved. However, there is an alternative approach, which is to compute backup routes that allow the failure to be repaired locally by the router(s) detecting the failure without the immediate need to inform other routers of the failure. In this case, the disruption time can be limited to the small time taken to detect the adjacent failure and invoke the backup routes. This is analogous to the technique employed by MPLS Fast Reroute [MPLSFRR], but the mechanisms employed for the backup routes in pure IP networks are necessarily very different. This document provides a framework for the development of this approach. 2. Problem Analysis The duration of the packet delivery disruption caused by a conventional routing transition is determined by a number of factors: 1. The time taken to detect the failure. This may be of the order of a few mS when it can be detected at the physical layer, up to several tens of seconds when a routing protocol hello is employed. During this period packets will be unavoidably lost. 2. The time taken for the local router to react to the failure. This will typically involve generating and flooding new routing updates, perhaps after some hold-down delay, and re-computing the router's FIB. 3. The time taken to pass the information about the failure to other routers in the network. In the absence of routing protocol packet loss, this is typically between 10mS and 100mS per hop in a well designed router.hop. 4. The time taken to re-compute the forwarding tables. This is typically a few mS for a link state protocol using Dijkstra's algorithm. 5. The time taken to load the revised forwarding tables into the forwarding hardware. This time is very implementation dependant and also depends on the number of prefixes affected by the failure, but may be several hundred mS. The disruption will last until the routers adjacent to the failure have completed steps 1 and 2, and then all the routers in the network whose paths are affected by the failure have completed the remaining steps. The initial packet loss is caused by the router(s) adjacent to the failure continuing to attempt to transmit packets across the failure until it is detected. This loss is unavoidable, but the detection time can be reduced to a few tens of mS as described in section 3.1. Subsequent packet loss is caused by the "micro-loops" which form because of temporary inconsistencies between routers' forwarding tables. These occur as a result of the different times at which routers update their forwarding tables to reflect the failure. These variable delays are caused by steps 3, 4 and 5 above and in many routers it is step 5 which is both the largest factor and which has the greatest variance between routers. The large variance arises from implementation differences and from the differing impact that a failure has on each individual router. For example, the number of prefixes affected by the failure may vary dramatically from one router to another. In order to achieve packet disruption times which are commensurate with the failure detection times it is necessary to perform two distinct tasks: 1. Provide a mechanism for the router(s) adjacent to the failure to rapidly invoke a repair path, which is unaffected by any subsequent re-convergence. 2. Provide a mechanism to prevent the effects of micro loops during subsequent re-convergence. Performing the first task without the second will result in the repair path being starved of traffic and hence being redundant. Performing the second without the first will result in traffic being discarded by the router(s) adjacent to the failure. Both tasks are necessary for an effective solution to the problem. However, repair paths can be used in isolation where the failure is short-lived. The repair paths can be kept in place until the failure is repaired and there is no need to advertise the failure to other routers. Similarly, micro loop avoidance can be used in isolation to prevent loops arising from pre-planned management action. Note that micro-loops can also occur when a link or node is restored to service and thus a micro-loop avoidance mechanism is required for both link up and link down cases. 3. Mechanisms for IP Fast-route The set of mechanisms required for an effective solution to the problem can be broken down into the following sub-problems. 3.1. Mechanisms for fast failure detection It is critical that the failure detection time is minimized. A number of approaches are possible, such as: 1. Physical detection, such asdetection; for example, loss of light. 2. Routing protocol independent protocol detection; for example, The Bidirectional Failure Detection protocol [BFD][BFD]. 3. Other formsRouting protocol detection; for example, use of "fast hellos"hellos". 3.2. Mechanisms for repair paths Once a failure has been detected by one of the above mechanisms, traffic which previously traversed the failure is transmitted over one or more repair paths. The design of the repair paths should be such that they can be pre-calculated in anticipation of each local failure and made available for invocation with minimal delay. There are three basic categories of repair paths: 1. Equal cost multiple paths (ECMP). Where such paths exist, and one or more of the alternate paths do not traverse the failure, they may trivially be used as repair paths. 2. Downstream paths. (Also known as "loop free feasible alternates".) Such a path exists when a direct neighbor of the router adjacent to the failure has a path to the destination which cannotcan be guaranteed not to traverse the failure. 3. Multihop repair paths. When there is no feasible downstream path it may still be possible to locate a router, which is more than one hop away from the router adjacent to the failure, from which traffic will be forwarded to the destination without traversing the failure. ECMP and downstream paths offer the simplest repair paths and would normally be used when they are available. It is anticipated that around 80% of failures (see section 3.2.2) can be repaired using these alone. Multi-hop repair paths are considerably more complex, both in the computations required to determine their existence, and in the mechanisms required to invoke them. They can be further classified as: 1. Mechanisms where one or more alternate FIBs are pre-computed in all routers and the repaired packet is instructed to be forwarded using a "repair FIB" by some method of signaling such as detecting a "U-turn" [U-TURNS] or marking the packet. 2. Mechanisms functionally equivalent to a loose source route which is invoked using the normal FIB. These include tunnels [TUNNELS] and label based mechanisms. In many cases a repair path which reaches two-hops away from the router detecting the failure will suffice, and it is anticipated that around 95%98% of failures (see section 3.2.2) can be repaired by this method. However, to effectprovide complete repair coverage some use of longer multi-hop repair paths is generally necessary. 3.2.1. Scope of repair paths A particular repair path may be valid for all destinations which require repair or may only be valid for a subset of destinations. If a repair path is valid for a node immediately downstream of the failure, then it will be valid for all destinations previously reachable by traversing the failure. However, in cases where such a repair path is difficult to achieve because it requires a high order multi-hop repair path, it may still be possible to identify lower order repair paths (possibly even downstream paths) which allow the majority of destinations to be repaired. When IPFRR is unable to provide complete repair, it is desirable that the extent of the repair coverage can be determined and reported via network management. There is a tradeoff to be achieved between minimizing the number of repair paths to be computed, and minimizing the overheads incurred in using higher order multi-hop repair paths for destinations for which they are not strictly necessary. However, the computational cost of determining repair paths on an individual destination basis can be very high. The use of repair paths may result in excessive traffic passing over a link, resulting in congestion discard. This reduces the effectiveness of IPFRR. Mechanisms to influence the distribution of repaired traffic to minimize this effect are therefore desirable. 3.2.2. Analysis of repair coverage In some cases the repair strategy will permit the repair of all single link or node failures in the network for all possible destinations. This can be defined as 100% coverage. However, where the coverage is less than 100% it is important for the purposes of comparisons between different proposed repair strategies to define what is meant by such a percentage. There are three possibilities: 1. The percentage of links (or nodes) which can be fully protected for all destinations. This is appropriate where the requirement is to protect all traffic, but some percentage of the possible failures may be identified as being un-protectable. 2. The percentage of destinations which can be fully protected for all link (or node) failures. This is appropriate where the requirement is to protect against all possible failures, but some percentage of destinations may be identified as being un- protectable. 3. For all destinations (d) and for all failures (f), the percentage of the total potential failure cases (d*f) which are protected. This is appropriate where the requirement is an overall "best effort" protection. The coverage obtained is dependent on the repair strategy and highly dependent on the detailed topology and metrics. Any figures quoted in this document are for illustrative purposes only. 3.2.3. Link or node repair A repair path may be computed to protect against failure of an adjacent link, or failure of an adjacent node. In general, link protection is simpler to achieve. A repair which protects against node failure will also protect against link failure for all destinations except those for which the adjacent node is a single point of failure. In some cases it may be necessary to distinguish between a link or node failure in order that the optimal repair strategy is invoked. Methods for link/node failure determination may be based on techniques such as BFD. This determination may be made prior to invoking any repairs, but this will increase the period of packet loss following a failure unless the determination can be performed as part of the failure detection mechanism itself. Alternatively, a subsequent determination can be used to optimise an already invoked default strategy. 22.214.171.124.2.4. Maintenance of Repair paths In order to meet the response time goals, it is expected (though not required) that repair paths, and their associated FIB entries, will be pre-computed and installed ready for invocation when a failure is detected. Following invocation the repair paths remain in effect until they are no longer required. This will normally be when the routing protocol has re-converged on the new topology taking into account the failure, and traffic will no longer be using the repair paths. The repair paths have the property that they are unaffected by any topology changes resulting from the failure which caused their instantiation. Therefore there is no need to re-compute them during the convergence period. They may be affected by an unrelated simultaneous topology change, but such events are out of scope of this work (see section 3.2.5). Once the routing protocol has re-converged it is necessary for all repair paths to take account of the new topology. Various optimizations may permit the efficient identification of repair paths which are unaffected by the change, and hence do not require full re- computation. Since the new repair paths will not be required until the next failure occurs, the re-computation may be performed as a background task and be subject to a hold-down, but excessive delay in completing this operation will increase the risk of a new failure occurring before the repair paths are in place. 3.2.5. Multiple failures and Shared Risk Groups Complete protection against multiple unrelated failures is out of scope of this work. However, it is important that the occurrence of a second failure while one failure is undergoing repair should not result in a level of service which is significantly worse than that which would have been achieved in the absence of any repair strategy. Shared Risk Groups are an example of multiple related failures, and their protection is a matter for further study. One specific example of an SRLG which is clearly within the scope of this work is a node failure. This causes the simultaneous failure of multiple links, but their closely defined topological relationship makes the problem more tractable. 3.3. Mechanisms for micro-loop prevention Control of micro-loops is important not only because they can cause packet loss in traffic which is affected by the failure, but because by saturating a link with looping packets they can also cause congestion loss of traffic flowing over that link which would otherwise be unaffected by the failure. A number of solutions to the problem of micro-loop formation have been proposed. The following factors are significant in their classification: 1. Partial or complete protection against micro-loops. 2. Delay imposed upon convergence. 3. Tolerance of multiple failures (from node failures, and in general)general). 4. Computational complexity (pre-computed or real time)time). 5. Applicability to scheduled eventsevents. 6. Applicability to link/node reinstatement. 4. Management Considerations While many of the management requirements will be specific to particular IPFRR solutions, the following general aspects need to be addressed: 1. Configuration a. Enabling/disabling IPFRR support. b. Enabling/disabling protection on a per link/node basis. c. Expressing preferences regarding the links/nodes used for repair paths. d. Configuration of failure detection mechanisms. e. Configuration of loop avoidance strategies. 2. Monitoring a. Notification of links/nodes/destinations which cannot be protected. b. Notification of pre-computed repair paths, and anticipated traffic patterns. c. Counts of failure detections, protection invocations and packets forwarded over repair paths. 5. Scope and applicability Link state protocols provide ubiquitous topology information, which facilitates the computation of repairs paths. Therefore the initial scope of this work is in the context of link state IGPs. Provision of similar facilities in non-link state IGPs and BGP is a matter for further study, but the correct operation of the repair mechanisms for traffic with a destination outside the IGP domain is an important consideration for solutions based on this framework 5.6. IANA considerations There are no IANA considerations that arise from this description of IPFRR. However there may be changes to the IGPs to support IPFRR in which there will be IANA considerations. 6.framework document. 7. Security Considerations This framework document does not itself introduce any security issues, but attention must be paid to the security implications of any proposed solutions to the problem. Acknowledgments8. IPR Disclosure Acknowledgement Certain IPR may be applicable to the mechanisms outlined in this document. Please check the detailed specifications for possible IPR notices. 9. Normative References Internet-drafts are works in progress available from http://www.ietf.org/internet-drafts/ 10. Informative References Internet-drafts are works in progress available from http://www.ietf.org/internet-drafts/ BFD Katz, D., and Ward, D., "Bidirectional Forwarding Detection", draft-katz-ward-bfd-01.txt, August 2003draft-katz-ward-bfd-02.txt, (work in progress). MPLSFRR Pan, P. et al, "Fast Reroute Extensions to RSVP- TE for LSP Tunnels", draft-ietf-mpls-rsvp-lsp- fastreroute-05.txtdraft-ietf-mpls-rsvp-lsp-fastreroute-05.txt TUNNELS Bryant, S. et al, "IP Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-00.txt, (work in progress). U-TURNS Atlas, A. et al, "IP/LDP Local Protection", draft-atlas-ip-local-protect-00.txt, (work in progress). 11. Author's Address Mike Shand Cisco Systems, 250, Longwater Avenue, Green Park, Reading, RG2 6GB, United Kingdom. Email: firstname.lastname@example.org Full copyright statement Copyright (C) The Internet Society (2004). All Rights Reserved.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.