INTERNET DRAFT  A Framework for Loop-free Convergence        Oct 2006

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

                                                               Oct 2006

                A Framework for Loop-free Convergence

   Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This draft describes 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

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   this document are to be interpreted as described in RFC 2119

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Table of Contents
1. Introduction........................................................4

2. The Nature of Micro-loops...........................................5

3. Applicability.......................................................6

4. Micro-loop Control Strategies.......................................6

5. Loop mitigation.....................................................7

6. Micro-loop Prevention...............................................9
 6.1. Incremental Cost Advertisement...................................9
 6.2. Nearside Tunneling..............................................11
 6.3. Farside Tunnels.................................................12
 6.4. Distributed Tunnels.............................................13
 6.5. Packet Marking..................................................13
 6.6. MPLS New Labels.................................................13
 6.7. Ordered FIB Update..............................................15
 6.8. Synchronised FIB Update.........................................16
7. Using PLSN In Conjunction With Other Methods.......................17

8. Loop Suppression...................................................18

9. Compatibility Issues...............................................19

10. Comparison of Loop-free Convergence Methods.......................19

11. IANA considerations...............................................20

12. Security Considerations...........................................20

13. Intellectual Property Statement...................................20

14. Disclaimer of Validity............................................21

15. copyright Statement...............................................21

16. Normative References..............................................21

17. Informative References............................................21

18. Authors' Addresses................................................22

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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
   occurs in both IP networks and MPLS networks that use LDP [RFC3036]
   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 are emerging which are more sensitive
   to the packet disruption that occurs during a transition. To make
   the transition transparent to their users, these services 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 TE fast-reroute
   mechanism for MPLS [MPLS-TE]. Alternative mechanisms that might be
   deployed in an MPLS network and mechanisms that may be used in an
   IP network are work in progress in the IETF [IPFRR]. Any 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

   There is, however, little point in introducing new mechanisms into
   an IP network to provide fast re-route, without also deploying
   mechanisms that prevent the disruptive effects of micro-loops which

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   may starve the repair or cause congestion loss as a result of
   looping packets.

   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 mechanisms that have been
   proposed to address the micro-loop issue.

2.   The Nature of Micro-loops

   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 a cycle of routers have the next router in
   the cycle as a next hop for a prefix. Cyclic micro-loops always
   include at least one link with an asymmetric cost, and/or at least
   two symmetric cost link cost changes within the convergence time.

   Micro-loops have two undesirable side-effects; congestion and
   repair starvation. A looping packet consumes bandwidth until it
   either escapes as a result of the re-synchronization of the FIBs,
   or its TTL expires. This transiently increases the traffic over a
   link by as much as 128 times, and may cause the link to congest.
   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.

   In cases where the link or node failure has been protected by a
   fast re-route repair, the inconsistency in the FIBs prevents some
   traffic from reaching the failure and hence being repaired. The
   repair may thus become starved of traffic and hence become
   ineffective. Thus in addition to the congestive damage, the repair
   is rendered ineffective by the micro-loop. Similarly, if the
   topology change is the result of management action the link could
   have been retained in service throughout the transition (i.e. the
   link acts as its own repair path), however, if micro-loops form,
   they prevent productive forwarding during the transition.

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   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 may
   significantly extend the duration of the packet disruption caused
   by the micro-loops. In some cases a packet may be subject to
   disruption from micro-loops which 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 [APPL] to any
   situation in which micro-loops may form. For example the
   convergence of a network following:

   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 failure.

   In each case, a component may be a link or a router.
   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.

4.   Micro-loop Control Strategies.

   Micro-loop control strategies fall into three basic classes:

     1. Micro-loop mitigation

     2. Micro-loop prevention

     3. Micro-loop suppression

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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
   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 done by micro-loops to other traffic. This may be
   achieved by, for example, using a packet monitoring method, which
   detects that a packet is looping and drops it. Such schemes make no
   attempt to productively forward the packet throughout the network

   Note that all known micro-loop mitigation and micro-loop prevention
   mechanisms extend the duration of the re-convergence process. When
   the failed component is protected by a fast re-route 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 which 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.

5.   Loop mitigation

   The only known loop mitigation approach is the Path Locking with
   safe-neighbors (PLSN) method described in [ZININ]. 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

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     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:

   Type A destinations: Destinations unaffected by the change and also
   destinations whose next hop after the change satisfies the safety

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

   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 Type 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

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   loop mitigation mechanism would work as it was designed.
   Alternatively, if all such incapable routers had only type C
   destinations, the "covert" announcement mechanism used to trigger
   the tunnel based schemes could be used to cause the Type A and Type
   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.

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

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   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
   which 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 which 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. Determining
   the minimum link cost in the network is trivial, but unfortunately,
   calculating the optimum increment at each step is thought to be a
   costly calculation.

   This approach has the advantage that it requires no change to the
   routing protocol. It will work in any network that uses a link-
   state IGP because it does not require any co-operation from the
   other routers in the network. However the method can be extremely
   slow, particularly if large metrics are used. 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 become unreachable for an extended period.

   Where the micro-loop prevention mechanism was being used to support
   a fast re-route repair the network may be vulnerable to a second
   failure for the duration of the controlled re-convergence.

   Where the micro-loop prevention mechanism was being used to support
   a reconfiguration of the network the extended time is less of an
   issue. In this case, because the real forwarding path is available
   throughout the whole transition, there is no conflict between
   concurrent change actions throughout the network.

   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 members of the SRLG. This may be achieved by
   completing the transition of one link before starting the next, or
   by interleaving the changes. This can be achieved without the need
   for any protocol extensions, by for example, using existing
   identifiers to establish the ordering and the arrival of LSP/LSAs
   to trigger the generation of the next increment.

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6.2.     Nearside Tunneling

   This mechanism works by creating an overlay network using tunnels
   whose path is not effected by the topology change and 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 ("covert") 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, the failure
   is then 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.

   This method completes in bounded time, and is 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.

   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.

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   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 labels set that the
   decapsulating router is distributing.

   A further disadvantage of this method is that it requires co-
   operation 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 "covert" 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 first member of the SRLG that it will
   traverse on its way to the destination, and to tunnel that traffic
   to the router that is closest to 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 a achieved using the nearside tunnel method, and in
   the case of node failure, a reduction in the decapsulation load on
   any single router.

   Unlike the nearside tunnel method (which uses normal routing to the
   repairing router), this method requires the use of a repair path to

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   the farside router. This may be provided by the 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 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 G-space [TUNNEL] using routers that are in F-
   space. Because packets are decapsulated on entry to G-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 [TUNNEL].

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

6.5.     Packet Marking

   If packets could be marked in some way, this information could be
   used to assign them to one of: the new topology, the old topology
   or a transition topology. They would then be correctly forwarded
   during the transition. This could, for example, be achieved by
   allocating a Type of Service bit to the task [RFC791]. 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:

     1) The packet marking bit may not available.

     2) The mechanism would introduce a non-standard forwarding

     3) Packet marking using either the old or the new topology would
       double the size of the FIB, however some optimizations may be

6.6.     MPLS New Labels

   In an MPLS network that is using LDP [LDP] 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.

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   As described in Section 6.2, the repairing routers issue a covert
   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 which reaches the destination without
   traversing the failure (i.e. the next hop is in F-space with
   respect to the failure [TUNNEL]). 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 loop-free convergence with tunnels
   method (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 covert announcement, an overt announcement
   of the failure is issued. This announcement MUST NOT be issued
   until such time as all routers have carried out all of their covert
   announcement activities. On receipt of the overt 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 covert 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.

        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 time later, 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.

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   As with other method methods this new labels may be modified to
   provide loop prevention for "good news". There are also a number of
   optimizations of this method. Further details will be provided in a
   forthcoming draft.

6.7.     Ordered FIB Update

   The Ordered FIB loop prevention method is described in [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 which is further away. By
   analyzing the reverse spanning tree over which traffic is directed
   to the failed component in the old topology, it is possible to
   determine a strict ordering which 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 [TIMER]. The multiple is determined by the
   node's position in the reverse spanning tree, 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 was safe for the
   parent to instantiate its new routes.

   The property of this approach is therefore that it imposes a delay
   which 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.

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

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   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 deal with without using per prefix ordering, by running a
   single reverse SPF 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:

     1. 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.

     2. 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 which is based on
   pure signaling [OB]. Methods that use signaling as an optimization
   are safe because eventually they fall back on the established IGP
   mechanisms which 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 which must be
   proven to recover from any failure circumstance.

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

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   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 [NTP] 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 and 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 [ZININ] 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

   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 which would not have
   generated loops (i.e. when the new next-hop router considered this
   to be a type A or B destination).

   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 region %70 to %90 in typical networks.

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

                 / | \
               1/ 2|  \3
               /   |   \    cost S->T = 10
        Y-----X----S----T   cost T->S = 1
        |  1     2      |
        |1              |

   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

   An alternative approach would be to recognize that a packet was
   looping by recognizing that it was being sent back to the place
   that it had just come from. 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.

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

   The problem with this class of micro-loop control strategies is
   that whilst they prevent collateral damage they do nothing to
   enhance the productive forwarding of packets during the network

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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 will
   be complicated by interaction with the scheme previously deployed.

10.    Comparison of Loop-free Convergence Methods

   PLSN [ZININ] 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 is impractical as a general solution
   because it takes too long to complete. However, it is universally
   available, and hence may find use in certain 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 [NTP], 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 is high, bounded
   by the network diameter. The use of signaling as an accelerator
   will 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.    IANA considerations

   There are no IANA considerations that arise from this draft.

12.    Security Considerations

   All micro-loop control mechanisms raise significant security issues
   which must be addressed in their detailed technical description.

13.    Intellectual Property Statement

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

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

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

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14.    Disclaimer of Validity

   This document and the information contained herein are provided on

15.   copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

16.    Normative References

   There are no normative references.

17.    Informative References

   Internet-drafts are works in progress available from

   [APPL]        Bryant, S., Shand, M., "Applicability of Loop-
                 free Convergence", <draft-bryant-shand-lf-
                 applicability-02.txt>, October 2006, (work in

   [IPFRR]       Shand, M., "IP Fast-reroute Framework",
                 October 2006, (work in progress).

   [LDP]         Andersson, L., Doolan, P., Feldman, N.,
                 Fredette, A. and B. Thomas, "LDP
                 Specification", RFC3036,
                 January 2001.

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INTERNET DRAFT  A Framework for Loop-free Convergence        Oct 2006

   [NTP]         RFC1305 Network Time Protocol (Version 3)
                 Specification, Implementation and Analysis. D.
                 Mills. March 1992.

   [OB]          Avoiding transient loops during IGP convergence
                 P. Francois, O. Bonaventure
                 IEEE INFOCOM 2005, March 2005, Miami, Fl., USA

   [OFIB]        Francois et. al., "Loop-free convergence using
                 ordered FIB updates", <draft-francois-ordered-
                 fib-02.txt>, October 2006 (work in progress).

   [RFC2119]     Bradner, S., "Key words for use in RFCs to
                 Indicate Requirement Levels", BCP 14, RFC2119,
                 March 1997.

   [RFC791]      RFC791, Internet Protocol Protocol
                 Specification, September 1981

   [TIMER]       S. Bryant, et. al. , "Synchronisation of Loop
                 Free Timer Values", <draft-atlas-bryant-shand-
                 lf-timers-02.txt>, October 2006
   [TUNNEL]      Bryant, S., Shand, M., "IP Fast Reroute using
                 tunnels", <draft-bryant-ipfrr-tunnels-02.txt>,
                 Apr 2005 (work in progress).

   [ZININ]       Zinin, A., "Analysis and Minimization of
                 Microloops in Link-state Routing Protocols",
                 February 2006 (work in progress).

18.    Authors' Addresses

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

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

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