INTERNET DRAFT        IP Fast-reroute Framework               June 2004



Network Working Group                                         M. Shand
Internet Draft
Expiration Date: Dec 2004                                Cisco Systems

                                                             June 2004




                       IP Fast Reroute Framework

                draft-ietf-rtgwg-ipfrr-framework-01.txt


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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.






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.  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.


. 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.


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     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.



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   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.


. 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; for example, loss of light.

     2. Routing protocol independent protocol detection; for example,
        The Bidirectional Failure Detection protocol [BFD].

     3. Routing protocol detection; for example, use of "fast 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 can 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


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   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 98% of failures (see section 3.2.2) can be repaired by this
   method. However, to provide 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.





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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.


3.2.4. 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

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   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:


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     1. Partial or complete protection against micro-loops.

     2. Delay imposed upon convergence.

     3. Tolerance of multiple failures (from node failures, and in
        general).

     4. Computational complexity (pre-computed or real time).

     5. Applicability to scheduled events.

     6. Applicability to link/node reinstatement.


. 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.


. 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


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   mechanisms for traffic with a destination outside the IGP domain is
   an important consideration for solutions based on this framework


. IANA considerations

   There are no IANA considerations that arise from this framework
   document.


. 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.


. IPR Disclosure Acknowledgement

   Certain IPR may be applicable to the mechanisms outlined in this
   document. Please check the detailed specifications for possible IPR
   notices.


. Normative References

   Internet-drafts are works in progress available from
   http://www.ietf.org/internet-drafts/


. 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-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.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).




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. Author's Address


   Mike Shand
   Cisco Systems,
   250, Longwater Avenue,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: mshand@cisco.com

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