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Internet Engineering Task Force (IETF)                          M. ShandRequest for Comments: 5715                                     S. BryantCategory: Informational                                    Cisco SystemsISSN: 2070-1721                                             January 2010A Framework for Loop-Free ConvergenceAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5715.Shand & Bryant                Informational                     [Page 1]

RFC 5715          A Framework for Loop-Free Convergence     January 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................32. The Nature of Micro-Loops .......................................43. Applicability ...................................................54. Micro-Loop Control Strategies ...................................65. Loop Mitigation .................................................85.1. Fast Convergence ...........................................85.2. PLSN .......................................................86. Micro-Loop Prevention ..........................................106.1. Incremental Cost Advertisement ............................106.2. Nearside Tunneling ........................................126.3. Farside Tunnels ...........................................136.4. Distributed Tunnels .......................................146.5. Packet Marking ............................................146.6. MPLS New Labels ...........................................156.7. Ordered FIB Update ........................................166.8. Synchronised FIB Update ...................................187. Using PLSN in Conjunction with Other Methods ...................188. Loop Suppression ...............................................199. Compatibility Issues ...........................................2010. Comparison of Loop-Free Convergence Methods ...................2011. Security Considerations .......................................2112. Acknowledgments ...............................................2113. Informative References ........................................21Shand & Bryant                Informational                     [Page 2]

RFC 5715          A Framework for Loop-Free Convergence     January 20101.  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 criticallyShand & Bryant                Informational                     [Page 3]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                     [Page 4]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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:Shand & Bryant                Informational                     [Page 5]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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-loopsShand & Bryant                Informational                     [Page 6]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                     [Page 7]

RFC 5715          A Framework for Loop-Free Convergence     January 20105.  Loop Mitigation   There are two approaches to loop mitigation.   o  Fast convergence   o  A purpose-designed, loop-mitigation mechanism5.1.  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).Shand & Bryant                Informational                     [Page 8]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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 Sections6.2 to6.4) 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.Shand & Bryant                Informational                     [Page 9]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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 update6.1.  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] hasShand & Bryant                Informational                    [Page 10]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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 systemShand & Bryant                Informational                    [Page 11]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   (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.Shand & Bryant                Informational                    [Page 12]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 13]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 14]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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 inSection 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.Shand & Bryant                Informational                    [Page 15]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 16]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 17]

RFC 5715          A Framework for Loop-Free Convergence     January 20106.8.  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 (seeSection 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).Shand & Bryant                Informational                    [Page 18]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 19]

RFC 5715          A Framework for Loop-Free Convergence     January 20109.  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.Shand & Bryant                Informational                    [Page 20]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   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.Shand & Bryant                Informational                    [Page 21]

RFC 5715          A Framework for Loop-Free Convergence     January 2010   [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.comShand & Bryant                Informational                    [Page 22]