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INTERNET DRAFT        IP Fast-reroute Framework              March 2006



Network Working Group                                         M. Shand
Internet Draft                                               S. Bryant
Expiration Date: September 2006                          Cisco Systems

                                                            March 2006




                       IP Fast Reroute Framework

                draft-ietf-rtgwg-ipfrr-framework-05.txt


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.


   Internet-Drafts are working documents of the Internet Engineering
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   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-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 failure.




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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
                        discussion.

   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
                        condition:-

                        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.

   Loop-Free            This is a neighbor N, that is not a primary
   Alternate            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)

   Loop-Free            A neighbor N_i, which is not the particular
   Neighbor             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.



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

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

   Micro-loop           A temporary forwarding loop which exists
                        during a routing transition as a result of
                        temporary inconsistencies between FIBs.

   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.

   Routing              The process whereby routers converge on a new
   transition           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 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.

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

   SPT                  Shortest path tree

   Upstream             This is a forwarding loop which involves a
   Forwarding Loop      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.


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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.



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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, 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.


3. 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.


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 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.



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   ECMP and loop free alternate paths (as described in [BASE]) 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 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" [U-TURNS, 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
        [TUNNELS], alternative shortest paths [ALT-SP] and label based
        mechanisms.

     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 [NOT-VIA].

   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 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
   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.



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   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.


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 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-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.

     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.


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.

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   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
   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 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

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   multiple links, but their closely defined topological relationship
   makes the problem more tractable.


3.3. 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.


3.4. 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 [MICROLOOP]. 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).

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

     5. Applicability to scheduled events.

     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.

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          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

   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


6. IANA considerations

   There are no IANA considerations that arise from this 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.


8. IPR Disclosure Acknowledgement

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

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.


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   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


9. Acknowledgements

   The authors would like to acknowledge contributions made by Alia
   Atlas and Alex Zinin.


10. Normative References

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


11. Informative References

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

   ALT-SP        Tian, A., Chen, N., "Fast Reroute using
                 Alternative Shortest Paths", draft-tian-frr-
                 alt-shortest-path-01.txt, (work in progress)

   BASE          Atlas, A., Zinin, A., "Basic Specification
                 for IP Fast-Reroute: Loop-free Alternates",
                 draft-ietf-rtgwg-ipfrr-spec-base-04.txt,
                 (work in progress)

   BFD           Katz, D. and Ward, D., "Bidirectional
                 Forwarding Detection",
                 draft-ietf-bfd-base-04.txt, (work in
                 progress).

   FIFR          S. Nelakuditi, S. Lee, Y. Yu, Z.-L. Zhang,
                 and C.-N. Chuah, "Fast local rerouting for
                 handling transient link failures.," Tech.
                 Rep. TR-2004-004, University of South
                 Carolina, 2004.

   MPLSFRR       Pan, P. et al, "Fast Reroute Extensions to
                 RSVP-TE for LSP Tunnels", RFC 4090.


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   MICROLOOP     Bryant, S. and Shand, M., "A Framework for
                 Loop-free Convergence",
                 draft-bryant-shand-lf-conv-frmwk-02.txt,
                 (work in progress).

   NOT-VIA       Bryant, S., Previdi, S., Shand, M., "IP Fast
                 Reroute Using Notvia Addresses",
                 draft-bryant-shand-ipfrr-notvia-addresses-
                 02.txt, (work in progress).

   SIMULA        Lysne, O., et al, "Fast IP Network Recovery
                 using Multiple Routing Configurations",
                 http://folk.uio.no/amundk/infocom06.pdf

   TUNNELS       Bryant, S. et al, "IP Fast Reroute using
                 tunnels", draft-bryant-ipfrr-tunnels-02.txt,
                 (work in progress).

   U-TURNS       Atlas, A. et al, "IP/LDP Local Protection",
                 draft-atlas-ip-local-protect-02.txt, (work in
                 progress).





12. Authors' Addresses


   Stewart Bryant
   Cisco Systems,
   250, Longwater Avenue,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: stbryant@cisco.com

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

Full copyright statement

   Copyright (C) The Internet Society (2006). 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.

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   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,

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   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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