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
<draft-bryant-shand-lf-conv-frmwk-03.txt>
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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
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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-02.txt>, October 2006, (work in
progress).
[IPFRR] Shand, M., "IP Fast-reroute Framework",
<draft-ietf-rtgwg-ipfrr-framework-06.txt>,
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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[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",
<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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