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Versions: 00 01 02 03 draft-ietf-rtgwg-lf-conv-frmwk

INTERNET DRAFT  A Framework for Loop-free Convergence        Mar 2006




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

                                                              Mar 2006




                A Framework for Loop-free Convergence
              <draft-bryant-shand-lf-conv-frmwk-02.txt>


  Status of this Memo

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

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF), its areas, and its working groups.  Note that
  other groups may also distribute working documents as
  Internet-Drafts.

  Internet-Drafts are draft documents valid for a maximum of six
  months and may be updated, replaced, or obsoleted by other
  documents at any time.  It is inappropriate to use Internet-Drafts
  as reference material or to cite them other than as "work in
  progress."

  The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/1id-abstracts.html
  The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html

Abstract
  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
  action.

Conventions used in this document

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in

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  this document are to be interpreted as described in RFC 2119
  [RFC2119].



















































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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
  topology.

  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
  transition.

  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
  criteria.

  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
  criteria.

  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
       procedure.

    3) 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 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
  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 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:

                  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
  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
  transition.

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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
  at http://www.ietf.org/ipr.

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





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14.    Disclaimer of Validity

  This document and the information contained herein are provided on
  an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
  REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
  THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
  THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
  ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
  PARTICULAR PURPOSE.


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
  <http://www.ietf.org/internet-drafts/>

  APPL          Bryant, S., Shand, M., "Applicability of Loop-
                 free Convergence", <draft-bryant-shand-lf-
                 applicability-01.txt>, March 2006, (work in
                 progress).

  IPFRR         Shand, M., "IP Fast-reroute Framework",
                 <draft-ietf-rtgwg-ipfrr-framework-05.txt>,
                 March 2006, (work in progress).

  LDP           Andersson, L., Doolan, P., Feldman, N.,
                 Fredette, A. and B. Thomas, "LDP
                 Specification", RFC 3036,
                 January 2001.




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  NTP           RFC1305 Network Time Protocol (Version 3)
                 Specification, Implementation and Analysis. D.
                 Mills. February 2006.

  [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-01.txt>, March 2006 (work in progress).

  RFC791        RFC-791, Internet Protocol Protocol
                 Specification, September 1981

  TIMER         S. Bryant, et. al. , "Synchronisation of Loop
                 Free Timer Values", <draft-atlas-bryant-shand-
                 lf-timers-01.txt>, March 2005
  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",
                 <draft-zinin-microloop-analysis-02.txt>,
                 February 2006 (work in progress).




18.    Authors' Addresses


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



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






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