A Framework for Loop-Free Convergence
RFC 5715

Internet Engineering Task Force (IETF)                          M. Shand
Request for Comments: 5715                                     S. Bryant
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                             January 2010

                 A Framework for Loop-Free Convergence

Abstract

   A micro-loop is a packet forwarding loop that may occur transiently
   among two or more routers in a hop-by-hop packet forwarding paradigm.

   This framework provides a summary of the causes and consequences of
   micro-loops and enables the reader to form a judgement on whether
   micro-looping is an issue that needs to be addressed in specific
   networks.  It also provides a survey of the currently proposed
   mechanisms that may be used to prevent or to suppress the formation
   of micro-loops when an IP or MPLS network undergoes topology change
   due to failure, repair, or management action.  When sufficiently fast
   convergence is not available and the topology is susceptible to
   micro-loops, use of one or more of these mechanisms may be desirable.

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/rfc5715.

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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
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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. The Nature of Micro-Loops .......................................4
   3. Applicability ...................................................5
   4. Micro-Loop Control Strategies ...................................6
   5. Loop Mitigation .................................................8
      5.1. Fast Convergence ...........................................8
      5.2. PLSN .......................................................8
   6. Micro-Loop Prevention ..........................................10
      6.1. Incremental Cost Advertisement ............................10
      6.2. Nearside Tunneling ........................................12
      6.3. Farside Tunnels ...........................................13
      6.4. Distributed Tunnels .......................................14
      6.5. Packet Marking ............................................14
      6.6. MPLS New Labels ...........................................15
      6.7. Ordered FIB Update ........................................16
      6.8. Synchronised FIB Update ...................................18
   7. Using PLSN in Conjunction with Other Methods ...................18
   8. Loop Suppression ...............................................19
   9. Compatibility Issues ...........................................20
   10. Comparison of Loop-Free Convergence Methods ...................20
   11. Security Considerations .......................................21
   12. Acknowledgments ...............................................21
   13. Informative References ........................................21

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1.  Introduction

   When there is a change to the network topology (due to the failure or
   restoration of a link or router, or as a result of management
   action), the routers need to converge on a common view of the new
   topology and the paths to be used for forwarding traffic to each
   destination.  During this process, referred to as a routing
   transition, packet delivery between certain source/destination pairs
   may be disrupted.  This occurs due to the time it takes for the
   topology change to be propagated around the network together with the
   time it takes each individual router to determine and then update the
   forwarding information base (FIB) for the affected destinations.
   During this transition, packets may be lost due to the continuing
   attempts to use the failed component and due to forwarding loops.
   Forwarding loops arise due to the inconsistent FIBs that occur as a
   result of the difference in time taken by routers to execute the
   transition process.  This is a problem that may occur in both IP
   networks and MPLS networks that use the label distribution protocol
   (LDP) [RFC5036] as the label switched path (LSP) signaling protocol.

   The service failures caused by routing transitions are largely hidden
   by higher-level protocols that retransmit the lost data.  However,
   new Internet services could emerge that are more sensitive to the
   packet disruption that occurs during a transition.  To make the
   transition transparent to their users, these services would require a
   short routing transition.  Ideally, routing transitions would be
   completed in zero time with no packet loss.

   Regardless of how optimally the mechanisms involved have been
   designed and implemented, it is inevitable that a routing transition
   will take some minimum interval that is greater than zero.  This has
   led to the development of a traffic engineering (TE) fast-reroute
   mechanism for MPLS [RFC4090].  Alternative mechanisms that might be
   deployed in an MPLS network or an IP network are current work items
   in the IETF [RFC5714].  The repair mechanism may, however, be
   disrupted by the formation of micro-loops during the period between
   the time when the failure is announced and the time when all FIBs
   have been updated to reflect the new topology.

   One method of mitigating the effects of micro-loops is to ensure that
   the network reconverges in a sufficiently short time that these
   effects are inconsequential.  Another method is to design the network
   topology to minimise or even eliminate the possibility of micro-
   loops.

   The propensity to form micro-loops is highly topology dependent, and
   algorithms are available to identify which links in a network are
   subject to micro-looping.  In topologies that are critically

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   susceptible to the formation of micro-loops, there is little point in
   introducing new mechanisms to provide fast reroute without also
   deploying mechanisms that prevent the disruptive effects of micro-
   loops.  Unless micro-loop prevention is used in these topologies,
   packets may not reach the repair and micro-looping packets may cause
   congestion, resulting in further packet loss.

   The disruptive effect of micro-loops is not confined to periods when
   there is a component failure.  Micro-loops can, for example, form
   when a component is put back into service following repair.  Micro-
   loops can also form as a result of a network-maintenance action such
   as adding a new network component, removing a network component, or
   modifying a link cost.

   This framework provides a summary of the causes and consequences of
   micro-loops and enables the reader to form a judgement on whether
   micro-looping is an issue that needs to be addressed in specific
   networks.  It also provides a survey of the currently proposed micro-
   loop mitigation mechanisms.  When sufficiently fast convergence is
   not available and the topology is susceptible to micro-loops, use of
   one or more of these mechanisms may be desirable.

2.  The Nature of Micro-Loops

   A micro-loop is a packet forwarding loop that may occur transiently
   among two or more routers in a hop-by-hop, packet forwarding
   paradigm.

   Micro-loops may form during the periods when a network is re-
   converging following ANY topology change and are caused by
   inconsistent FIBs in the routers.  During the transition, micro-loops
   may occur over a single link between a pair of routers that
   temporarily use each other as the next hop for a prefix.  Micro-loops
   may also form when each router in a cycle of three or more routers
   has the next router in the cycle as a next hop for a given prefix.

   Cyclic loops may occur if one or more of the following conditions are
   met:

   1.  Asymmetric link costs.

   2.  An equal-cost path exists between a pair of routers, each of
       which makes a different decision regarding which path to use for
       forwarding to a particular destination.  Note that even routers
       that do not implement equal-cost, multi-path (ECMP) forwarding
       must make a choice between the available equal-cost paths, and
       unless they make the same choice, the condition for cyclic loops
       will be fulfilled.

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   3.  Topology changes affecting multiple links, including single node
       and line card failures.

   Micro-loops have two undesirable side effects: congestion and repair
   starvation.

   o  A looping packet consumes bandwidth until it either escapes as a
      result of the re-synchronization of the FIBs or its time to live
      (TTL) expires.  This transiently increases the traffic over a link
      by as much as 128 times, and may cause the link to become
      congested.  This congestion reduces the bandwidth available to
      other traffic (which is not otherwise affected by the topology
      change).  As a result, the "innocent" traffic using the link
      experiences increased latency and is liable to congestive packet
      loss.

   o  In cases where the link or node failure has been protected by a
      fast-reroute repair, an inconsistency in the FIBs may prevent some
      traffic from reaching the failure, and hence being repaired.  The
      repair may thus become starved of traffic and thereby rendered
      ineffective.

   Although micro-loops are usually considered in the context of a
   failure, similar problems of congestive packet loss and starvation
   may also occur if the topology change is the result of management
   action.  For example, consider the case where a link is to be taken
   out of service by management action.  The link can be retained in
   service throughout the transition, thus avoiding the need for any
   repair.  However, if micro-loops form, they may cause congestion loss
   and may also prevent traffic from reaching the link.

   Unless otherwise controlled, micro-loops may form in any part of the
   network that forwards (or in the case of a new link, will forward)
   packets over a path that includes the affected topology change.  The
   time taken to propagate the topology change through the network, and
   the non-uniform time taken by each router to calculate the new
   shortest path tree (SPT) and update its FIB, contribute to the
   duration of the packet disruption caused by the micro-loops.  In some
   cases, a packet may be subject to disruption from micro-loops that
   occur sequentially at links along the path, thus further extending
   the period of disruption beyond that required to resolve a single
   loop.

3.  Applicability

   Loop-free convergence techniques are applicable to any situation in
   which micro-loops may form, for example, the convergence of a network
   following:

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   1.  Component failure

   2.  Component repair

   3.  Management withdrawal of a component

   4.  Management insertion or a component

   5.  Management change of link cost (either positive or negative)

   6.  External cost change, for example, change of external gateway as
       a result of a BGP change

   7.  A Shared Risk Link Group (SRLG) failure

   In each case, a component may be a link, a set of links, or an entire
   router.  Throughout this document, we use the term SRLG when
   describing the procedure to be followed when multiple failures have
   occurred, whether or not they are members of an explicit SRLG.  In
   the case of multiple independent failures, the loop-prevention method
   described for SRLG may be used, provided it is known that all of
   these failures have been repaired.

   Loop-free convergence techniques are applicable to both IP networks
   and MPLS-enabled networks that use LDP, including LDP networks that
   use the single-hop tunnel fast-reroute mechanism.

   An assessment of whether loop-free convergence techniques are
   required should take into account whether or not the interior gateway
   protocol (IGP) convergence is sufficiently fast that any micro-loops
   are of such short duration that they are not disruptive, and whether
   or not the topology is such that micro-loops are likely to form.

4.  Micro-Loop Control Strategies

   Micro-loop control strategies fall into four basic classes:

   1.  Micro-loop mitigation

   2.  Micro-loop prevention

   3.  Micro-loop suppression

   4.  Network design to minimise micro-loops

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   A micro-loop-mitigation scheme works by re-converging the network in
   such a way that it reduces, but does not eliminate, the formation of
   micro-loops.  Such schemes cannot guarantee the productive forwarding
   of packets during the transition.

   A micro-loop-prevention mechanism controls the re-convergence of the
   network in such a way that no micro-loops form.  Such a micro-loop-
   prevention mechanism allows the continued use of any fast repair
   method until the network has converged on its new topology and
   prevents the collateral damage that occurs to other traffic for the
   duration of each micro-loop.

   A micro-loop-suppression mechanism attempts to eliminate the
   collateral damage caused by micro-loops to other traffic.  This may
   be achieved by, for example, using a packet-monitoring method that
   detects that a packet is looping and drops it.  Such schemes make no
   attempt to productively forward the packet throughout the network
   transition.

   Highly meshed topologies are less susceptible to micro-loops, thus
   networks may be designed to minimise the occurrence of micro-loops by
   appropriate link placement and metric settings.  However, this
   approach may conflict with other design requirements, such as cost
   and traffic planning, and may not accurately track the evolution of
   the network or temporary changes due to outages.

   Note that all known micro-loop-prevention mechanisms and most micro-
   loop-mitigation mechanisms extend the duration of the re-convergence
   process.  When the failed component is protected by a fast-reroute
   repair, this implies that the converging network requires the repair
   to remain in place for longer than would otherwise be the case.  The
   extended convergence time means any traffic that is not repaired by
   an imperfect repair experiences a significantly longer outage than it
   would experience with conventional convergence.

   When a component is returned to service, or when a network management
   action has taken place, this additional delay does not cause traffic
   disruption because there is no repair involved.  However, the
   extended delay is undesirable because it increases the time that the
   network takes to be ready for another failure, and hence leaves it
   vulnerable to multiple failures.

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5.  Loop Mitigation

   There are two approaches to loop mitigation.

   o  Fast convergence

   o  A purpose-designed, loop-mitigation mechanism

5.1.  Fast Convergence

   The duration of micro-loops is dependent on the speed of convergence.
   Improving the speed of convergence may therefore be seen as a loop-
   mitigation technique.

5.2.  PLSN

   The only known purpose-designed, loop-mitigation approach is the Path
   Locking with Safe-Neighbors (PLSN) method described in PLSN
   [ANALYSIS].  In this method, a micro-loop-free next-hop safety
   condition is defined as follows:

   In a symmetric-cost network, it is safe for router X to change to the
   use of neighbor Y as its next hop for a specific destination if the
   path through Y to that destination satisfies both of the following
   criteria:

   1.  X considers Y as its loop-free neighbor based on the topology
       before the change, AND

   2.  X considers Y as its downstream neighbor based on the topology
       after the change.

   In an asymmetric-cost network, a stricter safety condition is needed,
   and the criterion is that:

      X considers Y as its downstream neighbor based on the topology
      both before and after the change.

   Based on these criteria, destinations are classified by each router
   into three classes:

   o  Type A destinations: Destinations unaffected by the change (type
      A1) and also destinations whose next hop after the change
      satisfies the safety criteria (type A2).

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   o  Type B destinations: Destinations that cannot be sent via the new,
      primary next hop because the safety criteria are not satisfied,
      but that can be sent via another next hop that does satisfy the
      safety criteria.

   o  Type C destinations: All other destinations.

   Following a topology change, type A destinations are immediately
   changed to go via the new topology.  Type B destinations are
   immediately changed to go via the next hop that satisfies the safety
   criteria, even though this is not the shortest path.  Type B
   destinations continue to go via this path until all routers have
   changed their type C destinations over to the new next hop.  Routers
   must not change their type C destinations until all routers have
   changed their type A2 and B destinations to the new or intermediate
   (safe) next hop.

   Simulations indicate that this approach produces a significant
   reduction in the number of links that are subject to micro-looping.
   However, unlike all of the micro-loop-prevention methods, it is only
   a partial solution.  In particular, micro-loops may form on any link
   joining a pair of type C routers.

   Because routers delay updating their type C destination FIB entries,
   they will continue to route towards the failure during the time when
   the routers are changing their type A and B destinations, and hence
   will continue to productively forward packets, provided that viable
   repair paths exist.

   A backwards-compatibility issue arises with PLSN.  If a router is not
   capable of micro-loop control, it will not correctly delay its FIB
   update.  If all such routers had only type A destinations, this loop-
   mitigation mechanism would work as it was designed.  Alternatively,
   if all such incapable routers had only type C destinations, the
   "loop-prevention" announcement mechanism used to trigger the tunnel-
   based schemes (see Sections 6.2 to 6.4) could be used to cause the
   type A and B destinations to be changed, with the incapable routers
   and routers having type C destinations delaying until they received
   the "real" announcement.  Unfortunately, these two approaches are
   mutually incompatible.

   Note that simulations indicate that in most topologies treating type
   B destinations as type C results in only a small degradation in loop
   prevention.  Also note that simulation results indicate that in
   production networks where some, but not all, links have asymmetric
   costs, using the stricter asymmetric-cost criterion actually reduces
   the number of loop-free destinations because fewer destinations can
   be classified as type A or B.

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   This mechanism operates identically for:

   o  events that degrade the topology (e.g., link failure),

   o  events that improve the topology (e.g., link restoration), and

   o  shared risk link group (SRLG) failure.

6.  Micro-Loop Prevention

   Eight micro-loop-prevention methods have been proposed:

   1.  Incremental cost advertisement

   2.  Nearside tunneling

   3.  Farside tunneling

   4.  Distributed tunnels

   5.  Packet marking

   6.  New MPLS labels

   7.  Ordered FIB update

   8.  Synchronized FIB update

6.1.  Incremental Cost Advertisement

   When a link fails, the cost of the link is normally changed from its
   assigned metric to "infinity" in one step.  However, it can be proved
   [OPT] that no micro-loops will form if the link cost is increased in
   suitable increments, and the network is allowed to stabilize before
   the next cost increment is advertised.  Once the link cost has been
   increased to a value greater than that of the lowest alternative cost
   around the link, the link may be disabled without causing a micro-
   loop.

   The criterion for a link cost change to be safe is that any link that
   is subjected to a cost change of x can only cause loops in a part of
   the network that has a cyclic cost less than or equal to x.  Because
   there may exist links that have a cost of one in each direction,
   resulting in a cyclic cost of two, this can result in the link cost
   having to be raised in increments of one.  However, the increment can
   be larger where the minimum cost permits.  Recent work [OPT] has

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   shown that there are a number of optimizations that can be applied to
   the problem in order to determine the exact set of cost values
   required, and hence minimise the number of increments.

   It will be appreciated that when a link is returned to service, its
   cost is reduced in small steps from "infinity" to its final cost,
   thereby providing similar micro-loop prevention during a "good-news"
   event.  Note that the link cost may be decreased from "infinity" to
   any value greater than that of the lowest alternative cost around the
   link in one step without causing a micro-loop.

   When the failure is an SRLG, the link cost increments must be
   coordinated across all failing members of the SRLG.  This may be
   achieved by completing the transition of one link before starting the
   next or by interleaving the changes.

   The incremental cost change approach has the advantage over all other
   currently known loop-prevention schemes in that it requires no change
   to the routing protocol.  It will work in any network because it does
   not require any cooperation from the other routers in the network.

   Where the micro-loop-prevention mechanism is being used to support a
   planned reconfiguration of the network, the extended total
   reconvergence time resulting from the multiple increments is of
   limited consequence, particularly where the number of increments have
   been optimized.  This, together with the ability to implement this
   technique in isolation, makes this method a good candidate for use
   with such management-initiated changes.

   Where the micro-loop-prevention mechanism is being used to support
   failure recovery, the number of increments required, and hence the
   time taken to fully converge, is significant even for small numbers
   of increments.  This is because, for the duration of the transition,
   some parts of the network continue to use the old forwarding path,
   and hence use any repair mechanism for an extended period.  In the
   case of a failure that cannot be fully repaired, some destinations
   may therefore become unreachable for an extended period.  In
   addition, the network may be vulnerable to a second failure for the
   duration of the controlled re-convergence.

   Where large metrics are used and no optimization (such as that
   described above) is performed, the incremental cost method can be
   extremely slow.  However, in cases where the per-link metric is
   small, either because small values have been assigned by the network
   designers or because of restrictions implicit in the routing protocol
   (e.g., RIP restricts the metric, and BGP using the autonomous system

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   (AS) path length frequently uses an effective metric of one or a very
   small integer for each inter AS hop), the number of required
   increments can be acceptably small even without optimizations.

6.2.  Nearside Tunneling

   This mechanism works by creating an overlay network using tunnels
   whose path is not affected by the topology change and then carrying
   the traffic affected by the change in that new network.  When all the
   traffic is in the new, tunnel-based network, the real network is
   allowed to converge on the new topology.  Because all the traffic
   that would be affected by the change is carried in the overlay
   network, no micro-loops form.

   When a failure is detected (or a link is withdrawn from service), the
   router adjacent to the failure issues a new "loop-prevention" routing
   message announcing the topology change.  This message is propagated
   through the network by all routers but is only understood by routers
   capable of using one of the tunnel-based, micro-loop-prevention
   mechanisms.

   Each of the micro-loop-preventing routers builds a tunnel to the
   closest router adjacent to the failure.  They then determine which of
   their traffic would transit the failure and place that traffic in the
   tunnel.  When all of these tunnels are in place (determined, for
   example, by waiting a suitable interval), the failure is announced as
   normal.  Because these tunnels will be unaffected by the transition
   and because the routers protecting the link will continue the repair
   (or forward across the link being withdrawn), no traffic will be
   disrupted by the failure.  When the network has converged, these
   tunnels are withdrawn, allowing traffic to be forwarded along its
   new, "natural" path.  The order of tunnel insertion and withdrawal is
   not important, provided that the tunnels are all in place before the
   normal announcement is issued and that the repair remains in place
   until normal convergence has completed.

   This method completes in bounded time and is generally much faster
   than the incremental cost method.  Depending on the exact design, it
   completes in two or three flood-SPF-FIB update cycles.

   At the time at which the failure is announced as normal, micro-loops
   may form within isolated islands of non-micro-loop-preventing
   routers.  However, only traffic entering the network via such routers
   can micro-loop.  All traffic entering the network via a micro-loop-
   preventing router will be tunneled correctly to the nearest repairing
   router -- including, if necessary, being tunneled via a non-micro-
   loop-preventing router -- and will not micro-loop.

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   Where there is no requirement to prevent the formation of micro-loops
   involving non-micro-loop-preventing routers, a single, "normal"
   announcement may be made and a local timer used to determine the time
   at which transition from tunneled forwarding to normal forwarding
   over the new topology may commence.

   This technique has the disadvantage that it requires traffic to be
   tunneled during the transition.  This is an issue in IP networks
   because not all router designs are capable of high-performance IP
   tunneling.  It is also an issue in MPLS networks because the
   encapsulating router has to know the label set that the decapsulating
   router is distributing.

   A further disadvantage of this method is that it requires cooperation
   from all the routers within the routing domain to fully protect the
   network against micro-loops.

   When a new link is added, the mechanism is run in "reverse".  When
   the loop-prevention announcement is heard, routers determine which
   traffic they will send over the new link and tunnel that traffic to
   the router on the near side of that link.  This path will not be
   affected by the presence of the new link.  When the "normal"
   announcement is heard, they then update their FIB to send the traffic
   normally, according to the new topology.  Any traffic encountering a
   router that has not yet updated its FIB will be tunneled to the near
   side of the link, and will therefore not loop.

   When a management change to the topology is required, again exactly
   the same mechanism protects against micro-looping of packets by the
   micro-loop-preventing routers.

   When the failure is an SRLG, the required strategy is to classify
   traffic according the furthest failing member of the SRLG that it
   will traverse on its way to the destination, and to tunnel that
   traffic to the repairing router for that SRLG member.  This will
   require multiple tunnel destinations -- in the limiting case, one per
   SRLG member.

6.3.  Farside Tunnels

   Farside tunneling loop prevention requires the loop-preventing
   routers to place all of the traffic that would traverse the failure
   in one or more tunnels terminating at the router (or, in the case of
   node failure, routers) at the far side of the failure.  The
   properties of this method are a more uniform distribution of repair
   traffic than is achieved using the nearside tunnel method and, in the
   case of node failure, a reduction in the decapsulation load on any
   single router.

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   Unlike the nearside tunnel method (which uses normal routing to the
   repairing router), this method requires the use of a repair path to
   the farside router.  This may be provided by the not-via [NOT-VIA]
   mechanism, in which case no further computation is needed.

   The mode of operation is otherwise identical to the nearside
   tunneling loop-prevention method (Section 6.2).

6.4.  Distributed Tunnels

   In the distributed tunnels loop-prevention method, each router
   calculates its own repair and forwards traffic affected by the
   failure using that repair.  Unlike the fast reroute (FRR) case, the
   actual failure is known at the time of the calculation.  The
   objective of the loop-preventing routers is to get the packets that
   would have gone via the failure into Q-space [FRR-TUNN] using routers
   that are in P-space.  Because packets are decapsulated on entry to
   Q-space, rather than being forced to go to the farside of the
   failure, more optimum routing may be achieved.  This method is
   subject to the same reachability constraints described in [FRR-TUNN].

   The mode of operation is otherwise identical to the nearside
   tunneling loop-prevention method (Section 6.2).

   An alternative distributed tunnel mechanism is for all routers to
   tunnel to the not-via address [NOT-VIA] associated with the failure.

6.5.  Packet Marking

   If packets could be marked in some way, this information could be
   used to assign them to one of:

   o  the new topology,

   o  the old topology, or

   o  a transition topology.

   They would then be correctly forwarded during the transition.  This
   mechanism works identically for both "bad-news" and "good-news"
   events.  It also works identically for SRLG failure.  There are three
   problems with this solution:

   o  A packet-marking bit may not be available, for example, a network
      supporting both the differentiated services architecture [RFC2475]
      and explicit congestion notification [RFC3168] uses all eight bits
      of the IPv4 Type of Service field.

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   o  The mechanism would introduce a non-standard forwarding procedure.

   o  Packet marking using either the old or the new topology would
      double the size of the FIB; however, some optimizations may be
      possible.

6.6.  MPLS New Labels

   In an MPLS network that is using [RFC5036] for label distribution,
   loop-free convergence can be achieved through the use of new labels
   when the path that a prefix will take through the network changes.

   As described in Section 6.2, the repairing routers issue a loop-
   prevention announcement to start the loop-free convergence process.
   All loop-preventing routers calculate the new topology and determine
   whether their FIB needs to be changed.  If there is no change in the
   FIB, they take no part in the following process.

   The routers that need to make a change to their FIB consider each
   change and check the new next hop to determine whether it will use a
   path in the OLD topology that reaches the destination without
   traversing the failure (i.e., the next hop is in P-space with respect
   to the failure [FRR-TUNN]).  If so, the FIB entry can be immediately
   updated.  For all of the remaining FIB entries, the router issues a
   new label to each of its neighbors.  This new label is used to lock
   the path during the transition in a similar manner to the previously
   described method for loop-free convergence with tunnels
   (Section 6.2).  Routers receiving a new label install it in their FIB
   for MPLS label translation, but do not yet remove the old label and
   do not yet use this new label to forward IP packets, i.e., they
   prepare to forward using the new label on the new path but do not use
   it yet.  Any packets received continue to be forwarded the old way,
   using the old labels, towards the repair.

   At some time after the loop-prevention announcement, a normal routing
   announcement of the failure is issued.  This announcement must not be
   issued until such time as all routers have carried out all of their
   activities that were triggered by the loop-prevention announcement.
   On receipt of the normal announcement, all routers that were delaying
   convergence move to their new path for both the new and the old
   labels.  This involves changing the IP address entries to use the new
   labels AND changing the old labels to forward using the new labels.

   Because the new label path was installed during the loop-prevention
   phase, packets reach their destinations as follows:

   o  If they do not go via any router using a new label, they go via
      the repairing router and the repair.

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   o  If they meet any router that is using the new labels, they get
      marked with the new labels and reach their destination using the
      new path, back-tracking if necessary.

   When all routers have changed to the new path, the network is
   converged.  At some later time, when it can be assumed that all
   routers have moved to using the new path, the FIB can be cleaned up
   to remove the, now redundant, old labels.

   As with other methods, the new labels may be modified to provide loop
   prevention for "good news".  There are also a number of optimizations
   of this method.

6.7.  Ordered FIB Update

   The ordered FIB loop prevention method is described in "Loop-free
   convergence using oFIB" [oFIB].  Micro-loops occur following a
   failure or a cost increase, when a router closer to the failed
   component revises its routes to take account of the failure before a
   router that is further away.  By analyzing the reverse shortest path
   tree (rSPT) over which traffic is directed to the failed component in
   the old topology, it is possible to determine a strict ordering that
   ensures that nodes closer to the root always process the failure
   after any nodes further away, and hence micro-loops are prevented.

   When the failure has been announced, each router waits a multiple of
   the convergence timer [LF-TIMERS].  The multiple is determined by the
   node's position in the rSPT, and the delay value is chosen to
   guarantee that a node can complete its processing within this time.
   The convergence time may be reduced by employing a signaling
   mechanism to notify the parent when all the children have completed
   their processing, and hence when it is safe for the parent to
   instantiate its new routes.

   The property of this approach is therefore that it imposes a delay
   that is bounded by the network diameter, although in many cases it
   will be much less.

   When a link is returned to service, the convergence process above is
   reversed.  A router first determines its distance (in hops) from the
   new link in the NEW topology.  Before updating its FIB, it then waits
   a time equal to the value of that distance multiplied by the
   convergence timer.

   It will be seen that network-management actions can similarly be
   undertaken by treating a cost increase in a manner similar to a
   failure and a cost decrease similar to a restoration.

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   The ordered FIB mechanism requires all nodes in the domain to operate
   according to these procedures, and the presence of non-cooperating
   nodes can give rise to loops for any traffic that traverses them (not
   just traffic that is originated through them).  Without additional
   mechanisms, these loops could remain in place for a significant time.

   It should be noted that this method requires per-router ordering but
   not per-prefix ordering.  A router must wait its turn to update its
   FIB, but it should then update its entire FIB.

   When an SRLG failure occurs, a router must classify traffic into the
   classes that pass over each member of the SRLG.  Each router is then
   independently assigned a ranking with respect to each SRLG member for
   which they have a traffic class.  These rankings may be different for
   each traffic class.  The prefixes of each class are then changed in
   the FIB according to the ordering of their specific ranking.  Again,
   as for the single failure case, signaling may be used to speed up the
   convergence process.

   Note that the special SRLG case of a full or partial node failure can
   be dealt with without using per-prefix ordering by running a single
   reverse-SPF computation rooted at the failed node (or common point of
   the subset of failing links in the partial case).

   There are two classes of signaling optimization that can be applied
   to the ordered FIB loop-prevention method:

   o  When the router makes NO change, it can signal immediately.  This
      significantly reduces the time taken by the network to process
      long chains of routers that have no change to make to their FIB.

   o  When a router HAS changed, it can signal that it has completed.
      This is more problematic since this may be difficult to determine,
      particularly in a distributed architecture, and the optimization
      obtained is the difference between the actual time taken to make
      the FIB change and the worst-case timer value.  This saving could
      be of the order of one second per hop.

   There is another method of executing ordered FIB that is based on
   pure signaling [SIG].  Methods that use signaling as an optimization
   are safe because eventually they fall back on the established IGP
   mechanisms that ensure that networks converge under conditions of
   packet loss.  However, a mechanism that relies on signaling in order
   to converge requires a reliable signaling mechanism that must be
   proven to recover from any failure circumstance.

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6.8.  Synchronised FIB Update

   Micro-loops form because of the asynchronous nature of the FIB update
   process during a network transition.  In many router architectures,
   it is the time taken to update the FIB itself that is the dominant
   term.  One approach would be to have two FIBs and, in a synchronized
   action throughout the network, to switch from the old to the new.
   One way to achieve this synchronized change would be to signal or
   otherwise determine the wall clock time of the change and then
   execute the change at that time, using NTP [RFC1305] to synchronize
   the wall clocks in the routers.

   This approach has a number of major issues.  Firstly, two complete
   FIBs are needed, which may create a scaling issue; secondly, a
   suitable network-wide synchronization method is needed.  However,
   neither of these are insurmountable problems.

   Since the FIB change synchronization will not be perfect, there may
   be some interval during which micro-loops form.  Whether this scheme
   is classified as a micro-loop-prevention mechanism or a micro-loop-
   mitigation mechanism within this taxonomy is therefore dependent on
   the degree of synchronization achieved.

   This mechanism works identically for both "bad-news" and "good-news"
   events.  It also works identically for SRLG failure.  Further
   consideration needs to be given to interoperating with routers that
   do not support this mechanism.  Without a suitable interoperating
   mechanism, loops may form for the duration of the synchronization
   delay.

7.  Using PLSN in Conjunction with Other Methods

   All of the tunnel methods and packet marking can be combined with
   PLSN (see Section 5.2 of this document and [ANALYSIS]) to reduce the
   traffic that needs to be protected by the advanced method.
   Specifically, all traffic could use PLSN except traffic between a
   pair of routers, both of which consider the destination to be type C.
   The type-C-to-type-C traffic would be protected from micro-looping
   through the use of a loop-prevention method.

   However, determining whether the new next-hop router considers a
   destination to be type C may be computationally intensive.  An
   alternative approach would be to use a loop-prevention method for all
   local type C destinations.  This would not require any additional
   computation, but would require the additional loop-prevention method
   to be used in cases that would not have generated loops (i.e., when
   the new next-hop router considered this to be a type A or B
   destination).

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   The amount of traffic that would use PLSN is highly dependent on the
   network topology and the specific change, but would be expected to be
   in the range of 70% to 90% in typical networks.

   However, PLSN cannot be combined safely with ordered FIB.  Consider
   the network fragment shown below:

                      R
                     /|\
                    / | \
                  1/ 2|  \3
                  /   |   \    cost S->T = 10
           Y-----X----S----T   cost T->S = 1
           |  1     2      |
           |1              |
           D---------------+
                  20

   On failure of link XY, according to PLSN, S will regard R as a safe
   neighbor for traffic to D.  However, the ordered FIB rank of both R
   and T will be zero, and hence these can change their FIBs during the
   same time interval.  If R changes before T, then a loop will form
   around R, T, and S.  This can be prevented by using a stronger safety
   condition than PLSN currently specifies, at the cost of introducing
   more type C routers, and hence reducing the PLSN coverage.

8.  Loop Suppression

   A micro-loop-suppression mechanism recognizes that a packet is
   looping and drops it.  One such approach would be for a router to
   recognize, by some means, that it had seen the same packet before.
   It is difficult to see how sufficiently reliable discrimination could
   be achieved without some form of per-router signature, such as route
   recording.  A packet-recognizing approach therefore seems infeasible.

   An alternative approach would be to recognize that a packet was
   looping by recognizing that it was being sent back to the place from
   which it had just come.  This would work for the types of loop that
   form in symmetric-cost networks, but would not suppress the cyclic
   loops that form in asymmetric networks or as a result of multiple
   failures.

   This mechanism operates identically for both "bad-news" events,
   "good-news" events, and SRLG failure.

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9.  Compatibility Issues

   Deployment of any micro-loop-control mechanism is a major change to a
   network.  Full consideration must be given to interoperation between
   routers that are capable of micro-loop control and those that are
   not.  Additionally, there may be a desire to limit the complexity of
   micro-loop control by choosing a method based purely on its
   simplicity.  Any such decision must take into account that if a more
   capable scheme is needed in the future, its deployment might be
   complicated by interaction with the scheme previously deployed.

10.  Comparison of Loop-Free Convergence Methods

   PLSN [ANALYSIS] is an efficient mechanism to prevent the formation of
   micro-loops but is only a partial solution.  It is a useful adjunct
   to some of the complete solutions but may need modification.

   Incremental cost advertisement in its simplest form is impractical as
   a general solution because it takes too long to complete.  Optimized
   incremental cost advertisement, however, completes in much less time
   and requires no assistance from other routers in the network.  It is
   therefore useful for network-reconfiguration operations.

   Packet marking is probably impractical because of the need to find
   the marking bit and to change the forwarding behavior.

   Of the remaining methods, distributed tunnels is significantly more
   complex than nearside or farside tunnels and should only be
   considered if there is a requirement to distribute the tunnel
   decapsulation load.

   Synchronised FIBs is a fast method but has the issue that a suitable
   synchronization mechanism needs to be defined.  One method would be
   to use NTP [RFC1305]; however, the coupling of routing convergence to
   a protocol that uses the network may be a problem.  During the
   transition, there will be some micro-looping for a short interval
   because it is not possible to achieve complete synchronization of the
   FIB changeover.

   The ordered FIB mechanism has the major advantage that it is a
   control-plane-only solution.  However, SRLGs require a per-
   destination calculation and the convergence delay may be high,
   bounded by the network diameter.  The use of signaling as an
   accelerator may reduce the number of destinations that experience the
   full delay, and hence reduce the total re-convergence time to an
   acceptable period.

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   The nearside and farside tunnel methods deal relatively easily with
   SRLGs and uncorrelated changes.  The convergence delay would be
   small.  However, these methods require the use of tunneled
   forwarding, which is not supported on all router hardware, and raises
   issues of forwarding performance.  When used with PLSN, the amount of
   traffic that was tunneled would be significantly reduced, thus
   reducing the forwarding performance concerns.  If the selected repair
   mechanism requires the use of tunnels, then a tunnel-based loop
   prevention scheme may be acceptable.

11.  Security Considerations

   This document analyzes the problem of micro-loops and summarizes a
   number of potential solutions that have been proposed.  These
   solutions require only minor modifications to existing routing
   protocols and therefore do not add additional security risks.
   However, a full security analysis would need to be provided within
   the specification of a particular solution proposed for deployment.

12.  Acknowledgments

   The authors would like to acknowledge contributions to this document
   made by Clarence Filsfils.

13.  Informative References

   [ANALYSIS]   Zinin, A., "Analysis and Minimization of Microloops in
                Link-state Routing Protocols", Work in Progress,
                October 2005.

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

   [LF-TIMERS]  Atlas, A., Bryant, S., and M. Shand, "Synchronisation of
                Loop Free Timer Values", Work in Progress,
                February 2008.

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

   [OPT]        Francois, P., Shand, M., and O. Bonaventure, "Disruption
                free topology reconfiguration in OSPF networks", IEEE
                INFOCOM May 2007, Anchorage.

   [RFC1305]    Mills, D., "Network Time Protocol (Version 3)
                Specification, Implementation", RFC 1305, March 1992.

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   [RFC2475]    Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
                and W. Weiss, "An Architecture for Differentiated
                Services", RFC 2475, December 1998.

   [RFC3168]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
                of Explicit Congestion Notification (ECN) to IP",
                RFC 3168, September 2001.

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

   [RFC5036]    Andersson, L., Minei, I., and B. Thomas, "LDP
                Specification", RFC 5036, October 2007.

   [RFC5714]    Shand, M. and S. Bryant, "IP Fast Reroute Framework",
                RFC 5714, January 2010.

   [SIG]        Francois, P. and O. Bonaventure, "Avoiding transient
                loops during IGP convergence", IEEE INFOCOM March 2005,
                Miami.

   [oFIB]       Francois, P., "Loop-free convergence using oFIB", Work
                in Progress, February 2008.

Authors' Addresses

   Mike Shand
   Cisco Systems
   250, Longwater Ave,
   Green Park, Reading,  RG2 6GB
   United Kingdom

   EMail: mshand@cisco.com

   Stewart Bryant
   Cisco Systems
   250, Longwater Ave,
   Green Park, Reading,  RG2 6GB
   United Kingdom

   EMail: stbryant@cisco.com

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