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RFC 5714 - IP Fast Reroute Framework


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Internet Engineering Task Force (IETF)                          M. Shand
Request for Comments: 5714                                     S. Bryant
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                             January 2010

                       IP Fast Reroute Framework

Abstract

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

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5714.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Scope and Applicability  . . . . . . . . . . . . . . . . . . .  5
   4.  Problem Analysis . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Mechanisms for IP Fast-Reroute . . . . . . . . . . . . . . . .  7
     5.1.  Mechanisms for Fast Failure Detection  . . . . . . . . . .  7
     5.2.  Mechanisms for Repair Paths  . . . . . . . . . . . . . . .  8
       5.2.1.  Scope of Repair Paths  . . . . . . . . . . . . . . . .  9
       5.2.2.  Analysis of Repair Coverage  . . . . . . . . . . . . .  9
       5.2.3.  Link or Node Repair  . . . . . . . . . . . . . . . . . 10
       5.2.4.  Maintenance of Repair Paths  . . . . . . . . . . . . . 10
       5.2.5.  Local Area Networks  . . . . . . . . . . . . . . . . . 11
       5.2.6.  Multiple Failures and Shared Risk Link Groups  . . . . 11
     5.3.  Mechanisms for Micro-Loop Prevention . . . . . . . . . . . 12
   6.  Management Considerations  . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 14

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 that 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 that may be sensitive to
   periods of traffic loss that 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 that 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
   different.

   This document provides a framework for the development of this
   approach.

   Note that in order to further minimize the impact on user
   applications, it may be necessary to design the network such that
   backup paths with suitable characteristics (for example, capacity
   and/or delay) are available for the algorithms to select.  Such
   considerations are outside the scope of this document.

2.  Terminology

   This section defines words and acronyms used in this document and
   other documents discussing IP fast-reroute.

   D                   Used to denote the destination router under
                       discussion.

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

   Downstream Path     This is a subset of the loop-free alternates
                       where the neighbor N meets the following
                       condition:
                       Distance_opt(N, D) < Distance_opt(S,D)

   E                   Used to denote the router that is the primary
                       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.  A neighbor N, that is not a
                       primary 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)

   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
                       A path via a Loop-Free Neighbor N_i that reaches
                       destination D without going through the
                       particular link of S that is being protected.  In
                       some cases, the path to D may go through the
                       primary neighbor E.

   Loop-Free Node-Protecting Alternate
                       A path via a Loop-Free Neighbor N_i that reaches
                       destination D without going through the
                       particular primary neighbor (E) of S that is
                       being protected.

   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
                       failure.

   R_i_j               The jth neighbor of N_i.

   Repair Path         The path used by a repairing node to send traffic
                       that it is unable to send via the normal path
                       owing to a failure.

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

   RPF                 Reverse Path Forwarding, i.e., checking that a
                       packet is received over the interface that 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.

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

   SPT                 Shortest path tree

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

3.  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.

   Complete protection against multiple unrelated failures is out of
   scope of this work.

4.  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 milliseconds 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 10 milliseconds and 100
       milliseconds per hop.

   4.  The time taken to re-compute the forwarding tables.  This is
       typically a few milliseconds 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 dependent
       and also depends on the number of prefixes affected by the
       failure, but may be several hundred milliseconds.

   The disruption will last until the routers adjacent to the failure
   have completed steps 1 and 2, and until 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 milliseconds as described in
   Section 5.1.

   In some topologies, subsequent packet loss may be caused by the
   "micro-loops" which may form as a result of temporary inconsistencies
   between routers' forwarding tables [RFC5715].  These inconsistencies
   are caused by steps 3, 4, and 5 above, and in many routers it is step
   5 that is both the largest factor and that 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 reduce packet disruption times to a duration commensurate
   with the failure detection times, two mechanisms may be required:

   a.  A mechanism for the router(s) adjacent to the failure to rapidly
       invoke a repair path, which is unaffected by any subsequent re-
       convergence.

   b.  In topologies that are susceptible to micro-loops, a micro-loop
       control mechanism may be required [RFC5715].

   Performing the first task without the second may 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.

   Repair paths may always be used in isolation where the failure is
   short-lived.  In this case, the repair paths can be kept in place
   until the failure is repaired, therefore there is no need to
   advertise the failure to other routers.

   Similarly, micro-loop avoidance may be used in isolation to prevent
   loops arising from pre-planned management action.  In which case 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 may also occur when a link or node is restored
   to service, and thus a micro-loop avoidance mechanism may be required
   for both link up and link down cases.

5.  Mechanisms for IP Fast-Reroute

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

5.1.  Mechanisms for Fast Failure Detection

   It is critical that the failure detection time is minimized.  A
   number of well-documented approaches are possible, such as:

   1.  Physical detection; for example, loss of light.

   2.  Protocol detection that is routing protocol independent; for
       example, the Bidirectional Failure Detection protocol [BFD].

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

   When configuring packet-based failure detection mechanisms it is
   important that consideration be given to the likelihood and
   consequences of false indications of failure.  The incidence of false
   indication of failure may be minimized by appropriately prioritizing
   the transmission, reception, and processing of the packets used to
   detect link or node failure.  Note that this is not an issue that is
   specific to IPFRR.

5.2.  Mechanisms for Repair Paths

   Once a failure has been detected by one of the above mechanisms,
   traffic that 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 that 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 5.2.2) 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:

   a.  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" [UTURN], [FIFR] or by
       marking the packet [SIMULA].

   b.  Mechanisms functionally equivalent to a loose source route that
       is invoked using the normal FIB.  These include tunnels
       [TUNNELS], alternative shortest paths [ALT-SP], and label-based
       mechanisms.

   c.  Mechanisms employing special addresses or labels that are
       installed in the FIBs of all routers with routes pre-computed to
       avoid certain components of the network.  For example, see
       [NOTVIA].

   In many cases, a repair path that reaches two hops away from the
   router detecting the failure will suffice, and it is anticipated that
   around 98% of failures (see Section 5.2.2) can be repaired by this
   method.  However, to provide complete repair coverage, some use of
   longer multi-hop repair paths is generally necessary.

5.2.1.  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) that
   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 trade-off 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.

5.2.2.  Analysis of Repair Coverage

   The repair coverage obtained is dependent on the repair strategy and
   highly dependent on the detailed topology and metrics.  Estimates of
   the repair coverage quoted in this document are for illustrative
   purposes only and may not be always be achievable.

   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 four possibilities:

   1.  The percentage of links (or nodes) that can be fully protected
       (i.e., 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 that can be 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) that 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.

5.2.3.  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 [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 optimize an already invoked
   default strategy.

5.2.4.  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 that 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 5.2.6).

   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
   that 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.

5.2.5.  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 trade-
   off between the simplicity of the repair and the ability to provide
   complete and optimal repair coverage.

5.2.6.  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 (SRLGs) are an example of multiple related
   failures, and the more complex aspects of their protection are a
   matter for further study.

   One specific example of an SRLG that 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.

5.3.  Mechanisms for Micro-Loop Prevention

   Ensuring the absence of micro-loops is important not only because
   they can cause packet loss in traffic that is affected by the
   failure, but because by saturating a link with looping packets micro-
   loops can cause congestion.  This congestion can then lead to routers
   discarding traffic that 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 [RFC5715].  The following factors
   are significant in their classification:

   1.  Partial or complete protection against micro-loops.

   2.  Convergence delay.

   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.

   7.  Topological constraints.

6.  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 or per-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 and operational support

       A.  Notification of links/nodes/destinations that 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.

       D.  Testing repairs.

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.

   Where the chosen solution uses tunnels it is necessary to ensure that
   the tunnel is not used as an attack vector.  One method of addressing
   this is to use a set of tunnel endpoint addresses that are excluded
   from use by user traffic.

   There is a compatibility issue between IPFRR and reverse path
   forwarding (RPF) checking.  Many of the solutions described in this
   document result in traffic arriving from a direction inconsistent
   with a standard RPF check.  When a network relies on RPF checking for
   security purposes, an alternative security mechanism will need to be
   deployed in order to permit IPFRR to used.

   Because the repair path will often be of a different length than the
   pre-failure path, security mechanisms that rely on specific Time to
   Live (TTL) values will be adversely affected.

8.  Acknowledgements

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

9.  Informative References

   [ALT-SP]   Tian, A., "Fast Reroute using Alternative Shortest Paths",
              Work in Progress, July 2004.

   [BFD]      Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", Work in Progress, January 2010.

   [FIFR]     Nelakuditi, S., Lee, S., Lu, Y., Zhang, Z., and C. Chuah,
              "Fast Local Rerouting for Handling Transient Link
              Failures", IEEE/ACM Transactions on Networking, Vol. 15,
              No. 2, DOI 10.1109/TNET.2007.892851, available
              from http://www.ieeexplore.ieee.org, April 2007.

   [NOTVIA]   Shand, M., Bryant, S., and S. Previdi, "IP Fast Reroute
              Using Not-via Addresses", Work in Progress, July 2009.

   [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.

   [RFC5715]  Shand, M. and S. Bryant, "A Framework for Loop-Free
              Convergence", RFC 5715, January 2010.

   [SIMULA]   Kvalbein, A., Hansen, A., Cicic, T., Gjessing, S., and O.
              Lysne, "Fast IP Network Recovery using Multiple Routing
              Configurations", Infocom 10.1109/INFOCOM.2006.227,
              available from http://www.ieeexplore.ieee.org, April 2006.

   [TUNNELS]  Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
              Fast Reroute using tunnels", Work in Progress,
              November 2007.

   [UTURN]    Atlas, A., "U-turn Alternates for IP/LDP Fast-Reroute",
              Work in Progress, February 2006.

Authors' Addresses

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

   EMail: mshand@cisco.com

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

   EMail: stbryant@cisco.com

 

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