Network Working Group M. Shand
Request for Comments: 5715 S. Bryant
Category: Informational Cisco Systems
December 2009
A Framework for Loop-Free Convergence
Abstract
A micro-loop is a packet forwarding loop that may occur transiently
among two or more routers in a hop-by-hop, hop-by-hop packet forwarding paradigm.
This framework provides a summary of the causes and consequences of
micro-loops and enables the reader to form a judgement on whether
micro-looping is an issue that needs to be addressed in specific
networks. It also provides a survey of the currently proposed
mechanisms that may be used to prevent or to suppress the formation
of micro-loops when an IP or MPLS network undergoes topology change
due to failure, repair, or management action. When sufficiently fast
convergence is not available and the topology is susceptible to
micro-loops, use of one or more of these mechanisms may be desirable.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Nature of Micro-Loops . . . . . . . . . . . . . . . . . . 4
3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Micro-Loop Control Strategies . . . . . . . . . . . . . . . . 6
5. Loop Mitigation . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Fast Convergence . . . . . . . . . . . . . . . . . . . . . 8
5.2. PLSN . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Micro-Loop Prevention . . . . . . . . . . . . . . . . . . . . 10
6.1. Incremental Cost Advertisement . . . . . . . . . . . . . . 10
6.2. Nearside Tunneling . . . . . . . . . . . . . . . . . . . . 11
6.3. Farside Tunnels . . . . . . . . . . . . . . . . . . . . . 13
6.4. Distributed Tunnels . . . . . . . . . . . . . . . . . . . 13
6.5. Packet Marking . . . . . . . . . . . . . . . . . . . . . . 14
6.6. MPLS New Labels . . . . . . . . . . . . . . . . . . . . . 14
6.7. Ordered FIB Update . . . . . . . . . . . . . . . . . . . . 16
6.8. Synchronised FIB Update . . . . . . . . . . . . . . . . . 17
7. Using PLSN in Conjunction with Other Methods . . . . . . . . . 18
8. Loop Suppression . . . . . . . . . . . . . . . . . . . . . . . 19
9. Compatibility Issues . . . . . . . . . . . . . . . . . . . . . 19
10. Comparison of Loop-Free Convergence Methods . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
13. Informative References . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
When there is a change to the network topology (due to the failure or
restoration of a link or router, or as a result of management
action), the routers need to converge on a common view of the new
topology and the paths to be used for forwarding traffic to each
destination. During this process, referred to as a routing
transition, packet delivery between certain source/destination pairs
may be disrupted. This occurs due to the time it takes for the
topology change to be propagated around the network together with the
time it takes each individual router to determine and then update the
forwarding information base (FIB) for the affected destinations.
During this transition, packets may be lost due to the continuing
attempts to use the failed component and due to forwarding loops.
Forwarding loops arise due to the inconsistent FIBs that occur as a
result of the difference in time taken by routers to execute the
transition process. This is a problem that may occur in both IP
networks and MPLS networks that use the label distribution protocol
(LDP) [RFC5036] as the label switched path (LSP) signaling protocol.
The service failures caused by routing transitions are largely hidden
by higher-level protocols that retransmit the lost data. However,
new Internet services could emerge that are more sensitive to the
packet disruption that occurs during a transition. To make the
transition transparent to their users, these services would require a
short routing transition. Ideally, routing transitions would be
completed in zero time with no packet loss.
Regardless of how optimally the mechanisms involved have been
designed and implemented, it is inevitable that a routing transition
will take some minimum interval that is greater than zero. This has
led to the development of a traffic engineering (TE) fast-reroute
mechanism for MPLS [RFC4090]. Alternative mechanisms that might be
deployed in an MPLS network or an IP network are current work items
in the IETF [RFC5714]. The repair mechanism may, however, be
disrupted by the formation of micro-loops during the period between
the time when the failure is announced and the time when all FIBs
have been updated to reflect the new topology.
One method of mitigating the effects of micro-loops is to ensure that
the network reconverges in a sufficiently short time that these
effects are inconsequential. Another method is to design the network
topology to minimise or even eliminate the possibility of micro-
loops.
The propensity to form micro-loops is highly topology dependent, and
algorithms are available to identify which links in a network are
subject to micro-looping. In topologies that are critically
susceptible to the formation of micro-loops, there is little point in
introducing new mechanisms to provide fast reroute without also
deploying mechanisms that prevent the disruptive effects of micro-
loops. Unless micro-loop prevention is used in these topologies,
packets may not reach the repair and micro-looping packets may cause
congestion, resulting in further packet loss.
The disruptive effect of micro-loops is not confined to periods when
there is a component failure. Micro-loops can, for example, form
when a component is put back into service following repair. Micro-
loops can also form as a result of a network-maintenance action such
as adding a new network component, removing a network component, or
modifying a link cost.
This framework provides a summary of the causes and consequences of
micro-loops and enables the reader to form a judgement on whether
micro-looping is an issue that needs to be addressed in specific
networks. It also provides a survey of the currently proposed micro-
loop mitigation mechanisms. When sufficiently fast convergence is
not available and the topology is susceptible to micro-loops, use of
one or more of these mechanisms may be desirable.
2. The Nature of Micro-Loops
A micro-loop is a packet forwarding loop that may occur transiently
among two or more routers in a hop-by-hop, packet forwarding
paradigm.
Micro-loops may form during the periods when a network is re-
converging following ANY topology change and are caused by
inconsistent FIBs in the routers. During the transition, micro-loops
may occur over a single link between a pair of routers that
temporarily use each other as the next hop for a prefix. Micro-loops
may also form when each router in a cycle of three or more routers
has the next router in the cycle as a next hop for a given prefix.
Cyclic loops may occur if one or more of the following conditions are
met:
1. Asymmetric link costs.
2. An equal-cost path exists between a pair of routers, each of
which makes a different decision regarding which path to use for
forwarding to a particular destination. Note that even routers
that do not implement equal-cost, multi-path (ECMP) forwarding
must make a choice between the available equal-cost paths, and
unless they make the same choice, the condition for cyclic loops
will be fulfilled.
3. Topology changes affecting multiple links, including single node
and line card failures.
Micro-loops have two undesirable side effects: congestion and repair
starvation.
o A looping packet consumes bandwidth until it either escapes as a
result of the re-synchronization of the FIBs or its time to live
(TTL) expires. This transiently increases the traffic over a link
by as much as 128 times, and may cause the link to become
congested. This congestion reduces the bandwidth available to
other traffic (which is not otherwise affected by the topology
change). As a result, the "innocent" traffic using the link
experiences increased latency and is liable to congestive packet
loss.
o In cases where the link or node failure has been protected by a
fast-reroute repair, an inconsistency in the FIBs may prevent some
traffic from reaching the failure, and hence being repaired. The
repair may thus become starved of traffic and thereby rendered
ineffective.
Although micro-loops are usually considered in the context of a
failure, similar problems of congestive packet loss and starvation
may also occur if the topology change is the result of management
action. For example, consider the case where a link is to be taken
out of service by management action. The link can be retained in
service throughout the transition, thus avoiding the need for any
repair. However, if micro-loops form, they may cause congestion loss
and may also prevent traffic from reaching the link.
Unless otherwise controlled, micro-loops may form in any part of the
network that forwards (or in the case of a new link, will forward)
packets over a path that includes the affected topology change. The
time taken to propagate the topology change through the network, and
the non-uniform time taken by each router to calculate the new
shortest path tree (SPT) and update its FIB, contribute to the
duration of the packet disruption caused by the micro-loops. In some
cases, a packet may be subject to disruption from micro-loops that
occur sequentially at links along the path, thus further extending
the period of disruption beyond that required to resolve a single
loop.
3. Applicability
Loop-free convergence techniques are applicable to any situation in
which micro-loops may form, for example, the convergence of a network
following:
1. Component failure
2. Component repair
3. Management withdrawal of a component
4. Management insertion or a component
5. Management change of link cost (either positive or negative)
6. External cost change, for example, change of external gateway as
a result of a BGP change
7. A Shared Risk Link Group (SRLG) failure
In each case, a component may be a link, a set of links, or an entire
router. Throughout this document, we use the term SRLG when
describing the procedure to be followed when multiple failures have
occurred, whether or not they are members of an explicit SRLG. In
the case of multiple independent failures, the loop-prevention method
described for SRLG may be used, provided it is known that all of
these failures have been repaired.
Loop-free convergence techniques are applicable to both IP networks
and MPLS-enabled networks that use LDP, including LDP networks that
use the single-hop tunnel fast-reroute mechanism.
An assessment of whether loop-free convergence techniques are
required should take into account whether or not the interior gateway
protocol (IGP) convergence is sufficiently fast that any micro-loops
are of such short duration that they are not disruptive, and whether
or not the topology is such that micro-loops are likely to form.
4. Micro-Loop Control Strategies
Micro-loop control strategies fall into four basic classes:
1. Micro-loop mitigation
2. Micro-loop prevention
3. Micro-loop suppression
4. Network design to minimise micro-loops
A micro-loop mitigation scheme works by re-converging the network in
such a way that it reduces, but does not eliminate, the formation of
micro-loops. Such schemes cannot guarantee the productive forwarding
of packets during the transition.
A micro-loop-prevention mechanism controls the re-convergence of the
network in such a way that no micro-loops form. Such a micro-loop-
prevention mechanism allows the continued use of any fast repair
method until the network has converged on its new topology and
prevents the collateral damage that occurs to other traffic for the
duration of each micro-loop.
A micro-loop suppression mechanism attempts to eliminate the
collateral damage caused by micro-loops to other traffic. This may
be achieved by, for example, using a packet-monitoring method that
detects that a packet is looping and drops it. Such schemes make no
attempt to productively forward the packet throughout the network
transition.
Highly meshed topologies are less susceptible to micro-loops, thus
networks may be designed to minimise the occurrence of micro-loops by
appropriate link placement and metric settings. However, this
approach may conflict with other design requirements, such as cost
and traffic planning, and may not accurately track the evolution of
the network or temporary changes due to outages.
Note that all known micro-loop-prevention mechanisms and most micro-
loop mitigation mechanisms extend the duration of the re-convergence
process. When the failed component is protected by a fast-reroute
repair, this implies that the converging network requires the repair
to remain in place for longer than would otherwise be the case. The
extended convergence time means any traffic that is not repaired by
an imperfect repair experiences a significantly longer outage than it
would experience with conventional convergence.
When a component is returned to service, or when a network management
action has taken place, this additional delay does not cause traffic
disruption because there is no repair involved. However, the
extended delay is undesirable because it increases the time that the
network takes to be ready for another failure, and hence leaves it
vulnerable to multiple failures.
5. Loop Mitigation
There are two approaches to loop mitigation.
o Fast convergence
o A purpose-designed, loop-mitigation mechanism
5.1. Fast Convergence
The duration of micro-loops is dependent on the speed of convergence.
Improving the speed of convergence may therefore be seen as a loop-
mitigation technique.
5.2. PLSN
The only known purpose-designed, loop-mitigation approach is the Path
Locking with Safe-Neighbors (PLSN) method described in PLSN
[ANALYSIS]. In this method, a micro-loop-free, micro-loop-free next-hop safety
condition is defined as follows:
In a symmetric-cost network, it is safe for router X to change to the
use of neighbor Y as its next hop for a specific destination if the
path through Y to that destination satisfies both of the following
criteria:
1. X considers Y as its loop-free neighbor based on the topology
before the change, AND
2. X considers Y as its downstream neighbor based on the topology
after the change.
In an asymmetric-cost network, a stricter safety condition is needed,
and the criterion is that:
X considers Y as its downstream neighbor based on the topology
both before and after the change.
Based on these criteria, destinations are classified by each router
into three classes:
o Type A destinations: Destinations unaffected by the change (type
A1) and also destinations whose next hop after the change
satisfies the safety criteria (type A2).
o Type B destinations: Destinations that cannot be sent via the new,
primary next hop because the safety criteria are not satisfied,
but that can be sent via another next hop that does satisfy the
safety criteria.
o Type C destinations: All other destinations.
Following a topology change, type A destinations are immediately
changed to go via the new topology. Type B destinations are
immediately changed to go via the next hop that satisfies the safety
criteria, even though this is not the shortest path. Type B
destinations continue to go via this path until all routers have
changed their type C destinations over to the new next hop. Routers
must not change their type C destinations until all routers have
changed their type A2 and B destinations to the new or intermediate
(safe) next hop.
Simulations indicate that this approach produces a significant
reduction in the number of links that are subject to micro-looping.
However, unlike all of the micro-loop prevention methods, it is only
a partial solution. In particular, micro-loops may form on any link
joining a pair of type C routers.
Because routers delay updating their type C destination FIB entries,
they will continue to route towards the failure during the time when
the routers are changing their type A and B destinations, and hence
will continue to productively forward packets, provided that viable
repair paths exist.
A backwards-compatibility issue arises with PLSN. If a router is not
capable of micro-loop control, it will not correctly delay its FIB
update. If all such routers had only type A destinations, this loop-
mitigation mechanism would work as it was designed. Alternatively,
if all such incapable routers had only type C destinations, the
"loop-prevention" announcement mechanism used to trigger the tunnel-
based schemes (see Sections 6.2 to 6.4) could be used to cause the
type A and B destinations to be changed, with the incapable routers
and routers having type C destinations delaying until they received
the "real" announcement. Unfortunately, these two approaches are
mutually incompatible.
Note that simulations indicate that in most topologies treating type
B destinations as type C results in only a small degradation in loop
prevention. Also note that simulation results indicate that in
production networks where some, but not all, links have asymmetric
costs, using the stricter asymmetric-cost criterion actually reduces
the number of loop-free destinations because fewer destinations can
be classified as type A or B.
This mechanism operates identically for:
o events that degrade the topology (e.g., link failure),
o events that improve the topology (e.g., link restoration), and
o shared risk link group (SRLG) failure.
6. Micro-Loop Prevention
Eight micro-loop prevention methods have been proposed:
1. Incremental cost advertisement
2. Nearside tunneling
3. Farside tunneling
4. Distributed tunnels
5. Packet marking
6. New MPLS labels
7. Ordered FIB update
8. Synchronized FIB update
6.1. Incremental Cost Advertisement
When a link fails, the cost of the link is normally changed from its
assigned metric to "infinity" in one step. However, it can be proved
[OPT] that no micro-loops will form if the link cost is increased in
suitable increments, and the network is allowed to stabilize before
the next cost increment is advertised. Once the link cost has been
increased to a value greater than that of the lowest alternative cost
around the link, the link may be disabled without causing a micro-
loop.
The criterion for a link cost change to be safe is that any link that
is subjected to a cost change of x can only cause loops in a part of
the network that has a cyclic cost less than or equal to x. Because
there may exist links that have a cost of one in each direction,
resulting in a cyclic cost of two, this can result in the link cost
having to be raised in increments of one. However, the increment can
be larger where the minimum cost permits. Recent work [OPT] has
shown that there are a number of optimizations that can be applied to
the problem in order to determine the exact set of cost values
required, and hence minimise the number of increments.
It will be appreciated that when a link is returned to service, its
cost is reduced in small steps from "infinity" to its final cost,
thereby providing similar micro-loop prevention during a "good-news"
event. Note that the link cost may be decreased from "infinity" to
any value greater than that of the lowest alternative cost around the
link in one step without causing a micro-loop.
When the failure is an SRLG, the link cost increments must be
coordinated across all failing members of the SRLG. This may be
achieved by completing the transition of one link before starting the
next or by interleaving the changes.
The incremental cost change approach has the advantage over all other
currently known loop-prevention schemes in that it requires no change
to the routing protocol. It will work in any network because it does
not require any cooperation from the other routers in the network.
Where the micro-loop-prevention mechanism is being used to support a
planned reconfiguration of the network, the extended total
reconvergence time resulting from the multiple increments is of
limited consequence, particularly where the number of increments have
been optimized. This, together with the ability to implement this
technique in isolation, makes this method a good candidate for use
with such management-initiated changes.
Where the micro-loop-prevention mechanism is being used to support
failure recovery, the number of increments required, and hence the
time taken to fully converge, is significant even for small numbers
of increments. This is because, for the duration of the transition,
some parts of the network continue to use the old forwarding path,
and hence use any repair mechanism for an extended period. In the
case of a failure that cannot be fully repaired, some destinations
may therefore become unreachable for an extended period. In
addition, the network may be vulnerable to a second failure for the
duration of the controlled re-convergence.
Where large metrics are used and no optimization (such as that
described above) is performed, the incremental cost method can be
extremely slow. However, in cases where the per-link metric is
small, either because small values have been assigned by the network
designers or because of restrictions implicit in the routing protocol
(e.g., RIP restricts the metric, and BGP using the autonomous system
(AS) path length frequently uses an effective metric of one or a very
small integer for each inter AS hop), the number of required
increments can be acceptably small even without optimizations.
6.2. Nearside Tunneling
This mechanism works by creating an overlay network using tunnels
whose path is not affected by the topology change and then carrying
the traffic affected by the change in that new network. When all the
traffic is in the new, tunnel-based network, the real network is
allowed to converge on the new topology. Because all the traffic
that would be affected by the change is carried in the overlay
network, no micro-loops form.
When a failure is detected (or a link is withdrawn from service), the
router adjacent to the failure issues a new "loop-prevention" routing
message announcing the topology change. This message is propagated
through the network by all routers but is only understood by routers
capable of using one of the tunnel-based, micro-loop-prevention
mechanisms.
Each of the micro-loop-preventing routers builds a tunnel to the
closest router adjacent to the failure. They then determine which of
their traffic would transit the failure and place that traffic in the
tunnel. When all of these tunnels are in place (determined, for
example, by waiting a suitable interval), the failure is announced as
normal. Because these tunnels will be unaffected by the transition
and because the routers protecting the link will continue the repair
(or forward across the link being withdrawn), no traffic will be
disrupted by the failure. When the network has converged, these
tunnels are withdrawn, allowing traffic to be forwarded along its
new, "natural" path. The order of tunnel insertion and withdrawal is
not important, provided that the tunnels are all in place before the
normal announcement is issued and that the repair remains in place
until normal convergence has completed.
This method completes in bounded time and is generally much faster
than the incremental cost method. Depending on the exact design, it
completes in two or three flood-SPF-FIB update cycles.
At the time at which the failure is announced as normal, micro-loops
may form within isolated islands of non-micro-loop-preventing
routers. However, only traffic entering the network via such routers
can micro-loop. All traffic entering the network via a micro-loop-
preventing router will be tunneled correctly to the nearest repairing
router -- including, if necessary, being tunneled via a non-micro-
loop-preventing router -- and will not micro-loop.
Where there is no requirement to prevent the formation of micro-loops
involving non-micro-loop-preventing routers, a single, "normal"
announcement may be made and a local timer used to determine the time
at which transition from tunneled forwarding to normal forwarding
over the new topology may commence.
This technique has the disadvantage that it requires traffic to be
tunneled during the transition. This is an issue in IP networks
because not all router designs are capable of high-performance IP
tunneling. It is also an issue in MPLS networks because the
encapsulating router has to know the label set that the decapsulating
router is distributing.
A further disadvantage of this method is that it requires cooperation
from all the routers within the routing domain to fully protect the
network against micro-loops.
When a new link is added, the mechanism is run in "reverse". When
the loop-prevention announcement is heard, routers determine which
traffic they will send over the new link and tunnel that traffic to
the router on the near side of that link. This path will not be
affected by the presence of the new link. When the "normal"
announcement is heard, they then update their FIB to send the traffic
normally, according to the new topology. Any traffic encountering a
router that has not yet updated its FIB will be tunneled to the near
side of the link, and will therefore not loop.
When a management change to the topology is required, again exactly
the same mechanism protects against micro-looping of packets by the
micro-loop-preventing routers.
When the failure is an SRLG, the required strategy is to classify
traffic according the furthest failing member of the SRLG that it
will traverse on its way to the destination, and to tunnel that
traffic to the repairing router for that SRLG member. This will
require multiple tunnel destinations -- in the limiting case, one per
SRLG member.
6.3. Farside Tunnels
Farside tunneling loop prevention requires the loop-preventing
routers to place all of the traffic that would traverse the failure
in one or more tunnels terminating at the router (or, in the case of
node failure, routers) at the far side of the failure. The
properties of this method are a more uniform distribution of repair
traffic than is achieved using the nearside tunnel method and, in the
case of node failure, a reduction in the decapsulation load on any
single router.
Unlike the nearside tunnel method (which uses normal routing to the
repairing router), this method requires the use of a repair path to
the farside router. This may be provided by the not-via [NOT-VIA]
mechanism, in which case no further computation is needed.
The mode of operation is otherwise identical to the nearside
tunneling loop-prevention method (Section 6.2).
6.4. Distributed Tunnels
In the distributed tunnels loop-prevention method, each router
calculates its own repair and forwards traffic affected by the
failure using that repair. Unlike the fast reroute (FRR) case, the
actual failure is known at the time of the calculation. The
objective of the loop-preventing routers is to get the packets that
would have gone via the failure into Q-space [FRR-TUNN] using routers
that are in P-space. Because packets are decapsulated on entry to
Q-space, rather than being forced to go to the farside of the
failure, more optimum routing may be achieved. This method is
subject to the same reachability constraints described in [FRR-TUNN].
The mode of operation is otherwise identical to the nearside
tunneling loop-prevention method (Section 6.2).
An alternative distributed tunnel mechanism is for all routers to
tunnel to the not-via address [NOT-VIA] associated with the failure.
6.5. Packet Marking
If packets could be marked in some way, this information could be
used to assign them to one of:
o the new topology,
o the old topology, or
o a transition topology.
They would then be correctly forwarded during the transition. This
mechanism works identically for both "bad-news" and "good-news"
events. It also works identically for SRLG failure. There are three
problems with this solution:
o A packet-marking bit may not be available, for example, a network
supporting both the differentiated services architecture [RFC2475]
and explicit congestion notification [RFC3168] uses all eight bits
of the IPv4 Type of Service field.
o The mechanism would introduce a non-standard forwarding procedure.
o Packet marking using either the old or the new topology would
double the size of the FIB; however, some optimizations may be
possible.
6.6. MPLS New Labels
In an MPLS network that is using [RFC5036] for label distribution,
loop-free convergence can be achieved through the use of new labels
when the path that a prefix will take through the network changes.
As described in Section 6.2, the repairing routers issue a loop-
prevention announcement to start the loop-free convergence process.
All loop-preventing routers calculate the new topology and determine
whether their FIB needs to be changed. If there is no change in the
FIB, they take no part in the following process.
The routers that need to make a change to their FIB consider each
change and check the new next hop to determine whether it will use a
path in the OLD topology that reaches the destination without
traversing the failure (i.e., the next hop is in P-space with respect
to the failure [FRR-TUNN]). If so, the FIB entry can be immediately
updated. For all of the remaining FIB entries, the router issues a
new label to each of its neighbors. This new label is used to lock
the path during the transition in a similar manner to the previously
described method for loop-free convergence with tunnels
(Section 6.2). Routers receiving a new label install it in their FIB
for MPLS label translation, but do not yet remove the old label and
do not yet use this new label to forward IP packets, i.e., they
prepare to forward using the new label on the new path but do not use
it yet. Any packets received continue to be forwarded the old way,
using the old labels, towards the repair.
At some time after the loop-prevention announcement, a normal routing
announcement of the failure is issued. This announcement must not be
issued until such time as all routers have carried out all of their
activities that were triggered by the loop-prevention announcement.
On receipt of the normal announcement, all routers that were delaying
convergence move to their new path for both the new and the old
labels. This involves changing the IP address entries to use the new
labels AND changing the old labels to forward using the new labels.
Because the new label path was installed during the loop-prevention
phase, packets reach their destinations as follows:
o If they do not go via any router using a new label, they go via
the repairing router and the repair.
o If they meet any router that is using the new labels, they get
marked with the new labels and reach their destination using the
new path, back-tracking if necessary.
When all routers have changed to the new path, the network is
converged. At some later time, when it can be assumed that all
routers have moved to using the new path, the FIB can be cleaned up
to remove the, now redundant, old labels.
As with other methods, the new labels may be modified to provide loop
prevention for "good news". There are also a number of optimizations
of this method.
6.7. Ordered FIB Update
The ordered FIB loop prevention method is described in "Loop-free
convergence using oFIB" [oFIB]. Micro-loops occur following a
failure or a cost increase, when a router closer to the failed
component revises its routes to take account of the failure before a
router that is further away. By analyzing the reverse shortest path
tree (rSPT) over which traffic is directed to the failed component in
the old topology, it is possible to determine a strict ordering that
ensures that nodes closer to the root always process the failure
after any nodes further away, and hence micro-loops are prevented.
When the failure has been announced, each router waits a multiple of
the convergence timer [LF-TIMERS]. The multiple is determined by the
node's position in the rSPT, and the delay value is chosen to
guarantee that a node can complete its processing within this time.
The convergence time may be reduced by employing a signaling
mechanism to notify the parent when all the children have completed
their processing, and hence when it is safe for the parent to
instantiate its new routes.
The property of this approach is therefore that it imposes a delay
that is bounded by the network diameter, although in many cases it
will be much less.
When a link is returned to service, the convergence process above is
reversed. A router first determines its distance (in hops) from the
new link in the NEW topology. Before updating its FIB, it then waits
a time equal to the value of that distance multiplied by the
convergence timer.
It will be seen that network-management actions can similarly be
undertaken by treating a cost increase in a manner similar to a
failure and a cost decrease similar to a restoration.
The ordered FIB mechanism requires all nodes in the domain to operate
according to these procedures, and the presence of non-cooperating
nodes can give rise to loops for any traffic that traverses them (not
just traffic that is originated through them). Without additional
mechanisms, these loops could remain in place for a significant time.
It should be noted that this method requires per-router ordering but
not per-prefix ordering. A router must wait its turn to update its
FIB, but it should then update its entire FIB.
When an SRLG failure occurs, a router must classify traffic into the
classes that pass over each member of the SRLG. Each router is then
independently assigned a ranking with respect to each SRLG member for
which they have a traffic class. These rankings may be different for
each traffic class. The prefixes of each class are then changed in
the FIB according to the ordering of their specific ranking. Again,
as for the single failure case, signaling may be used to speed up the
convergence process.
Note that the special SRLG case of a full or partial node failure can
be dealt with without using per-prefix ordering by running a single
reverse-SPF computation rooted at the failed node (or common point of
the subset of failing links in the partial case).
There are two classes of signaling optimization that can be applied
to the ordered FIB loop-prevention method:
o When the router makes NO change, it can signal immediately. This
significantly reduces the time taken by the network to process
long chains of routers that have no change to make to their FIB.
o When a router HAS changed, it can signal that it has completed.
This is more problematic since this may be difficult to determine,
particularly in a distributed architecture, and the optimization
obtained is the difference between the actual time taken to make
the FIB change and the worst-case timer value. This saving could
be of the order of one second per hop.
There is another method of executing ordered FIB that is based on
pure signaling [SIG]. Methods that use signaling as an optimization
are safe because eventually they fall back on the established IGP
mechanisms that ensure that networks converge under conditions of
packet loss. However, a mechanism that relies on signaling in order
to converge requires a reliable signaling mechanism that must be
proven to recover from any failure circumstance.
6.8. Synchronised FIB Update
Micro-loops form because of the asynchronous nature of the FIB update
process during a network transition. In many router architectures,
it is the time taken to update the FIB itself that is the dominant
term. One approach would be to have two FIBs and, in a synchronized
action throughout the network, to switch from the old to the new.
One way to achieve this synchronized change would be to signal or
otherwise determine the wall clock time of the change and then
execute the change at that time, using NTP [RFC1305] to synchronize
the wall clocks in the routers.
This approach has a number of major issues. Firstly, two complete
FIBs are needed, which may create a scaling issue; secondly, a
suitable network-wide synchronization method is needed. However,
neither of these are insurmountable problems.
Since the FIB change synchronization will not be perfect, there may
be some interval during which micro-loops form. Whether this scheme
is classified as a micro-loop-prevention mechanism or a micro-loop-
mitigation mechanism within this taxonomy is therefore dependent on
the degree of synchronization achieved.
This mechanism works identically for both "bad-news" and "good-news"
events. It also works identically for SRLG failure. Further
consideration needs to be given to interoperating with routers that
do not support this mechanism. Without a suitable interoperating
mechanism, loops may form for the duration of the synchronization
delay.
7. Using PLSN in Conjunction with Other Methods
All of the tunnel methods and packet marking can be combined with
PLSN (see Section 5.2 of this document and [ANALYSIS]) to reduce the
traffic that needs to be protected by the advanced method.
Specifically, all traffic could use PLSN except traffic between a
pair of routers, both of which consider the destination to be type C.
The type-C-to-type-C traffic would be protected from micro-looping
through the use of a loop-prevention method.
However, determining whether the new next-hop router considers a
destination to be type C may be computationally intensive. An
alternative approach would be to use a loop-prevention method for all
local type C destinations. This would not require any additional
computation, but would require the additional loop-prevention method
to be used in cases that would not have generated loops (i.e., when
the new next-hop router considered this to be a type A or B
destination).
The amount of traffic that would use PLSN is highly dependent on the
network topology and the specific change, but would be expected to be
in the range of 70% to 90% in typical networks.
However, PLSN cannot be combined safely with ordered FIB. Consider
the network fragment shown below:
R
/|\
/ | \
1/ 2| \3
/ | \ cost S->T = 10
Y-----X----S----T cost T->S = 1
| 1 2 |
|1 |
D---------------+
20
On failure of link XY, according to PLSN, S will regard R as a safe
neighbor for traffic to D. However, the ordered FIB rank of both R
and T will be zero, and hence these can change their FIBs during the
same time interval. If R changes before T, then a loop will form
around R, T, and S. This can be prevented by using a stronger safety
condition than PLSN currently specifies, at the cost of introducing
more type C routers, and hence reducing the PLSN coverage.
8. Loop Suppression
A micro-loop suppression mechanism recognizes that a packet is
looping and drops it. One such approach would be for a router to
recognize, by some means, that it had seen the same packet before.
It is difficult to see how sufficiently reliable discrimination could
be achieved without some form of per-router signature, such as route
recording. A packet-recognizing approach therefore seems infeasible.
An alternative approach would be to recognize that a packet was
looping by recognizing that it was being sent back to the place from
which it had just come. This would work for the types of loop that
form in symmetric-cost networks, but would not suppress the cyclic
loops that form in asymmetric networks or as a result of multiple
failures.
This mechanism operates identically for both "bad-news" events,
"good-news" events, and SRLG failure.
9. Compatibility Issues
Deployment of any micro-loop control mechanism is a major change to a
network. Full consideration must be given to interoperation between
routers that are capable of micro-loop control and those that are
not. Additionally, there may be a desire to limit the complexity of
micro-loop control by choosing a method based purely on its
simplicity. Any such decision must take into account that if a more
capable scheme is needed in the future, its deployment might be
complicated by interaction with the scheme previously deployed.
10. Comparison of Loop-Free Convergence Methods
PLSN [ANALYSIS] is an efficient mechanism to prevent the formation of
micro-loops but is only a partial solution. It is a useful adjunct
to some of the complete solutions but may need modification.
Incremental cost advertisement in its simplest form is impractical as
a general solution because it takes too long to complete. Optimized
incremental cost advertisement, however, completes in much less time
and requires no assistance from other routers in the network. It is
therefore useful for network-reconfiguration operations.
Packet marking is probably impractical because of the need to find
the marking bit and to change the forwarding behavior.
Of the remaining methods, distributed tunnels is significantly more
complex than nearside or farside tunnels and should only be
considered if there is a requirement to distribute the tunnel
decapsulation load.
Synchronised FIBs is a fast method but has the issue that a suitable
synchronization mechanism needs to be defined. One method would be
to use NTP [RFC1305]; however, the coupling of routing convergence to
a protocol that uses the network may be a problem. During the
transition, there will be some micro-looping for a short interval
because it is not possible to achieve complete synchronization of the
FIB changeover.
The ordered FIB mechanism has the major advantage that it is a
control-plane-only solution. However, SRLGs require a per-
destination calculation and the convergence delay may be high,
bounded by the network diameter. The use of signaling as an
accelerator may reduce the number of destinations that experience the
full delay, and hence reduce the total re-convergence time to an
acceptable period.
The nearside and farside tunnel methods deal relatively easily with
SRLGs and uncorrelated changes. The convergence delay would be
small. However, these methods require the use of tunneled
forwarding, which is not supported on all router hardware, and raises
issues of forwarding performance. When used with PLSN, the amount of
traffic that was tunneled would be significantly reduced, thus
reducing the forwarding performance concerns. If the selected repair
mechanism requires the use of tunnels, then a tunnel-based loop
prevention scheme may be acceptable.
11. Security Considerations
This document analyzes the problem of micro-loops and summarizes a
number of potential solutions that have been proposed. These
solutions require only minor modifications to existing routing
protocols and therefore do not add additional security risks.
However, a full security analysis would need to be provided within
the specification of a particular solution proposed for deployment.
12. Acknowledgments
The authors would like to acknowledge contributions to this document
made by Clarence Filsfils.
13. Informative References
[ANALYSIS] Zinin, A., "Analysis and Minimization of Microloops in
Link-state Routing Protocols", Work in Progress,
October 2005.
[FRR-TUNN] Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", Work in Progress,
November 2007.
[LF-TIMERS] Atlas, A., Bryant, S., and M. Shand, "Synchronisation of
Loop Free Timer Values", Work in Progress,
February 2008.
[NOT-VIA] Shand, M., Bryant, S., and S. Previdi, "IP Fast Reroute
Using Not-via Addresses", Work in Progress, July 2009.
[OPT] Francois, P., Shand, M., and O. Bonaventure, "Disruption
free topology reconfiguration in OSPF networks", IEEE
INFOCOM May 2007, Anchorage.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, October 2009.
[SIG] Francois, P. and O. Bonaventure, "Avoiding transient
loops during IGP convergence", IEEE INFOCOM March 2005,
Miami.
[oFIB] Francois, P., "Loop-free convergence using oFIB", Work
in Progress, February 2008.
Authors' Addresses
Mike Shand
Cisco Systems
250, Longwater Ave,
Green Park, Reading, RG2 6GB
United Kingdom
EMail: mshand@cisco.com
Stewart Bryant
Cisco Systems
250, Longwater Ave,
Green Park, Reading, RG2 6GB
United Kingdom
EMail: stbryant@cisco.com