Network Working Group                                           M. Shand
Internet-Draft                                                 S. Bryant
Intended status: Informational                             Cisco Systems
Expires: May 3, 2009                                    October 30, 2008

                       IP Fast Reroute Framework

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   This document provides a framework for the development of IP fast-
   reroute 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

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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Problem Analysis . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Mechanisms for IP Fast-reroute . . . . . . . . . . . . . . . .  7
     4.1.  Mechanisms for fast failure detection  . . . . . . . . . .  7
     4.2.  Mechanisms for repair paths  . . . . . . . . . . . . . . .  8
       4.2.1.  Scope of repair paths  . . . . . . . . . . . . . . . .  9
       4.2.2.  Analysis of repair coverage  . . . . . . . . . . . . .  9
       4.2.3.  Link or node repair  . . . . . . . . . . . . . . . . . 10
       4.2.4.  Maintenance of Repair paths  . . . . . . . . . . . . . 11
       4.2.5.  Multiple failures and Shared Risk Link Groups  . . . . 11
     4.3.  Local Area Networks  . . . . . . . . . . . . . . . . . . . 12
     4.4.  Mechanisms for micro-loop prevention . . . . . . . . . . . 12
   5.  Management Considerations  . . . . . . . . . . . . . . . . . . 12
   6.  Scope and applicability  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   10. Informative References . . . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Terminology

   This section defines words and acronyms used in this draft and other
   drafts discussing IP Fast-reroute.

   D                   Used to denote the destination router under

   Distance_opt(A,B)   The distance of the shortest path from A to B.

   Downstream Path     This is a subset of the loop-free alternates
                       where the neighbor N meet the following

                       Distance_opt(N, D) < Distance_opt(S,D)

   E                   Used to denote the router which is the primary
                       next-hop neighbor to get from S to the
                       destination D. Where there is an ECMP set for the
                       shortest path from S to D, these are referred to
                       as E_1, E_2, etc.

   ECMP                Equal cost multi-path: Where, for a particular
                       destination D, multiple primary next-hops are
                       used to forward traffic because there exist
                       multiple shortest paths from S via different
                       output layer-3 interfaces.

   FIB                 Forwarding Information Base.  The database used
                       by the packet forwarder to determine what actions
                       to perform on a packet.

   IPFRR               IP fast-reroute.

   Link(A->B)          A link connecting router A to router B.

   LFA                 Loop Free Alternate.  This is a neighbor N, that
                       is not a primary next-hop neighbor E, whose
                       shortest path to the destination D does not go
                       back through the router S. The neighbor N must
                       meet the following condition:-

                       Distance_opt(N, D) < Distance_opt(N, S) +
                       Distance_opt(S, D)

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   Loop Free Neighbor  A neighbor N_i, which is not the particular
                       primary neighbor E_k under discussion, and whose
                       shortest path to D does not traverse S. For
                       example, if there are two primary neighbors E_1
                       and E_2, E_1 is a loop-free neighbor with regard
                       to E_2 and vice versa.

   Loop Free Link Protecting Alternate
                       This is a path via a Loop-Free Neighbor N_i which
                       does not go through the particular link of S
                       which is being protected to reach the destination

   Loop Free Node-protecting Alternate
                       This is a path via a Loop-Free Neighbor N_i which
                       does not go through the particular primary
                       neighbor of S which is being protected to reach
                       the destination D.

   N_i                 The ith neighbor of S.

   Primary Neighbor    A neighbor N_i of S which is one of the next hops
                       for destination D in S's FIB prior to any

   R_i_j               The jth neighbor of N_i.

   Routing Transition  The process whereby routers converge on a new
                       topology.  In conventional networks this process
                       frequently causes some disruption to packet

   RPF                 Reverse Path Forwarding.  I.e. checking that a
                       packet is received over the interface which would
                       be used to send packets addressed to the source
                       address of the packet.

   S                   Used to denote a router that is the source of a
                       repair that is computed in anticipation of the
                       failure of a neighboring router denoted as E, or
                       of the link between S and E. It is the viewpoint
                       from which IP Fast-Reroute is described.

   S_i                 The set of neighbors of E, in addition to S,
                       which will independently take the role of S for
                       the traffic they carry.

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   SPF                 Shortest Path First, e.g.  Dijkstra's algorithm.

   SPT                 Shortest path tree

   Upstream Forwarding Loop
                       This is a forwarding loop which involves a set of
                       routers, none of which are directly connected to
                       the link which has caused the topology change
                       that triggered a new SPF in any of the routers.

2.  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

   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

   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 [RFC4090], but the mechanisms employed
   for the backup routes in pure IP networks are necessarily very

   This document provides a framework for the development of this

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3.  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.

   4.  The time taken to re-compute the forwarding tables.  This is
       typically a few mS for a link state protocol using Dijkstra's

   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

   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 4.1.

   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

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   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

   Similarly, micro-loop avoidance can be used in isolation to prevent
   loops arising from pre-planned management action, because the link or
   node being shut down can remain in service for a short time after its
   removal has been announced into the network, and hence it can
   function as its own "repair path".

   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.

4.  Mechanisms for IP Fast-reroute

   The set of mechanisms required for an effective solution to the
   problem can be broken down into the following sub-problems.

4.1.  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.

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   2.  Routing protocol independent protocol detection; for example, The
       Bidirectional Failure Detection protocol [I-D.ietf-bfd-base].

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

4.2.  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 multi-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.  Loop free alternate paths.  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.  Multi-hop repair paths.  When there is no feasible loop free
       alternate 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 loop free alternate paths (as described in [RFC5286]) offer
   the simplest repair paths and would normally be used when they are
   available.  It is anticipated that around 80% of failures (see
   Section 4.2.2) can be repaired using these basic methods alone.

   Multi-hop repair paths are 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 per packet signaling such
       as detecting a "U-turn" [I-D.atlas-ip-local-protect-uturn] ,
       [FIFR] or by marking the packet [SIMULA].

   2.  Mechanisms functionally equivalent to a loose source route which
       is invoked using the normal FIB.  These include tunnels
       [I-D.bryant-ipfrr-tunnels], alternative shortest paths
       [I-D.tian-frr-alt-shortest-path] and label based mechanisms.

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   3.  Mechanisms employing special addresses or labels which are
       installed in the FIBs of all routers with routes pre-computed to
       avoid certain components of the network.  For example

   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 4.2.2) can be repaired by this
   method.  However, to provide complete repair coverage some use of
   longer multi-hop repair paths is generally necessary.

4.2.1.  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 loop free alternate 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

   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.

   It will frequently be the case that the majority of destinations may
   be repaired using only the "basic" repair mechanism, leaving a
   smaller subset of the destinations to be repaired using one of the
   more complex multi-hop methods.  Such a hybrid approach may go some
   way to resolving the conflict between completeness and complexity.

   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.

4.2.2.  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

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   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 four 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-

   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.

   4.  The percentage of packets normally passing though the network
       that will continue to reach their destination.  This requires a
       traffic matrix for the network as part of the analysis.

   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.

4.2.3.  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[I-D.ietf-bfd-base].  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.

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4.2.4.  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

   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 4.2.5).

   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.

4.2.5.  Multiple failures and Shared Risk Link 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 Link Groups are an example of multiple related failures,
   and the more complex aspects of 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.

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4.3.  Local Area Networks

   Protection against partial or complete failure of LANs is more
   complex than the point to point case.  In general there is a tradeoff
   between the simplicity of the repair and the ability to provide
   complete and optimal repair coverage.

4.4.  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 and are summarized in [I-D.ietf-rtgwg-lf-conv-frmwk].
   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

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

   5.  Applicability to scheduled events.

   6.  Applicability to link/node reinstatement.

5.  Management Considerations

   While many of the management requirements will be specific to
   particular IPFRR solutions, the following general aspects need to be

   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.

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       D.  Configuration of failure detection mechanisms.

       E.  Configuration of loop avoidance strategies

   2.  Monitoring

       A.  Notification of links/nodes/destinations which cannot be

       B.  Notification of pre-computed repair paths, and anticipated
           traffic patterns.

       C.  Counts of failure detections, protection invocations and
           packets forwarded over repair paths.

6.  Scope and applicability

   The initial scope of this work is in the context of link state IGPs.
   Link state protocols provide ubiquitous topology information, which
   facilitates the computation of repairs paths.

   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

7.  IANA Considerations

   There are no IANA considerations that arise from this framework

8.  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.

9.  Acknowledgements

   The authors would like to acknowledge contributions made by Alia
   Atlas, Clarence Filsfils, Pierre Francois, Joel Halpern, Stefano
   Previdi and Alex Zinin.

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10.  Informative References

   [FIFR]     Nelakuditi, S., Lee, S., Lu, Y., Zhang, Z., and C. Chuah,
              "Fast local rerouting for handling transient link
              failures."", Tech. Rep. TR-2004-004, 2004.

              Atlas, A., "U-turn Alternates for IP/LDP Fast-Reroute",
              draft-atlas-ip-local-protect-uturn-03 (work in progress),
              March 2006.

              Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
              Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
              (work in progress), November 2007.

              Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-08 (work in progress),
              March 2008.

              Shand, M., Bryant, S., and S. Previdi, "IP Fast Reroute
              Using Not-via Addresses",
              draft-ietf-rtgwg-ipfrr-notvia-addresses-02 (work in
              progress), February 2008.

              Shand, M. and S. Bryant, "A Framework for Loop-free
              Convergence", draft-ietf-rtgwg-lf-conv-frmwk-02 (work in
              progress), February 2008.

              Tian, A., "Fast Reroute using Alternative Shortest Paths",
              draft-tian-frr-alt-shortest-path-01 (work in progress),
              July 2004.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              May 2005.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [SIMULA]   Lysne, O., Kvalbein, A., Cicic, T., Gjessing, S., and A.
              Hansen, "Fast IP Network Recovery using Multiple Routing
              Configurations."", Infocom 10.1109/INFOCOM.2006.227, 2006,

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Authors' Addresses

   Mike Shand
   Cisco Systems
   250, Longwater Avenue.
   Reading, Berks  RG2 6GB


   Stewart Bryant
   Cisco Systems
   250, Longwater Avenue.
   Reading, Berks  RG2 6GB


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Full Copyright Statement

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