Internet Engineering Task Force Curtis Villamizar
INTERNET-DRAFT ANS
draft-ietf-idr-route-damp-00 Ravi Chandra
Cisco
Ramesh Govindan
ISI
October 30, 1997
BGP Route Flap Damping
Status of this Memo
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Abstract
A usage of the BGP routing protocol is described which is capable of
reducing the routing traffic passed on to routing peers and therefore
the load on these peers without adversely affecting route convergence
time for relatively stable routes. This technique has been
implemented in commercial products supporting BGP. The technique is
also applicable to IDRP.
The overall goals are:
o to provide a mechanism capable of reducing router processing load
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caused by instability
o in doing so prevent sustained routing oscillations
o to do so without sacrificing route convergence time for generally
well behaved routes.
This must be accomplished keeping other goals of BGP in mind:
o pack changes into a small number of updates
o preserve consistent routing
o minimal addition space and computational overhead
Excessive updates to reachability state has been widespread in the
Internet. This observation was made in the early 1990s by many people
involved in Internet operations and remains to case to date. These
excessive updates are not necessarily periodic so route oscillation
would be a misleading term. The informal term used to describe this
effect is ``route flap''. The techniques described here are now
widely deployed and are commonly referred to as ``route flap
damping''.
1 Overview
It is necessary to reduce the amount of routing traffic (the number of
update message) generated by BGP in order to limit processing
requirements. The primary contributors of processing load resulting
from BGP updates are the BGP decision process and adding and removing
forwarding entries.
Consider the following example. A widely deployed BGP implementation
may tend to fail due to high routing update volume. For example, it
may be unable to maintain it's BGP or IGP sessions if sufficiently
loaded. The failure of one router can further contribute to the load
on other routers. This additional load may cause failures in other
instances of the same implementation or other implementations with a
similar weakness. In the worst case, a stable oscillation could
result. Such worse cases have already been observed in practice.
A BGP implementation must be prepared for a large volume of routing
traffic. A BGP implementation cannot rely upon the sender to
sufficiently shield it from route instabilities. The guidelines here
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are designed to prevent sustained oscillations, but do not eliminate
the need for robust and efficient implementations. The mechanisms
described here allow routing instability to be contained at an AS
border router bordering the instability.
2 Methods of Limiting Route Advertisement
Two methods of controlling the frequency of route advertisement are
described here. The first involves fixed timers. The fixed timer
technique has no space overhead per route but has the disadvantage of
slowing route convergence for the normal case where a route does not
have a history of instability. The second method overcomes this
limitation at the expense of maintaining some additional space
overhead. The additional overhead includes a small amount of state
per route and a very small processing overhead.
It is possible and desirable to combine both techniques. In practice,
fixed timers have been set to very short time intervals and have
proven useful to pack routes (NLRI) into a smaller number of updates
when routes arrive in separate updates.
Seldom are fixed timers set to the tens of minutes to hours that would
be necessary to actually damp route flap. To do so would produce the
undesirable effect of severely limiting routing convergence.
2.1 Existing Fixed Timer Recommendations
BGP-3 does not make specific recommendations in this area [1]. The
short section entitled ``Frequency of Route Selection'' simply
recommends that something be done and makes broad statements regarding
certain properties that are desirable or undesirable.
BGP4 retains the ``Frequency of Route Advertisement'' section and adds
a ``Frequency of Route Origination'' section. BGP-4 describes a
method of limiting route advertisement involving a fixed
(configurable) MinRouteAdvertisementInterval timer and fixed
MinASOriginationInterval timer [5]. The recommended timer values of
MinRouteAdvertisementInterval is 30 seconds and
MinASOriginationInterval is 15 seconds.
2.2 Desirable Properties of Damping Algorithms
Before describing damping algorithms the objectives need to be clearly
defined. Some key properties are examined to clarify the design
rationale.
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The overall objective is to reduce the route update load without
limiting convergence time for well behaved routes. To accomplish
this, criteria must be defined for well behaved and poorly behaved
routes. An algorithm must be defined which allows poorly behaved
routes to be identified. Ideally, this measure would be a prediction
of the future stability of a route.
Any delay in propagation of well behaved routes should be minimal.
Some delay is tolerable to support better packing of updates. Delay
of poorly behave routes should, if possible, be proportional to a
measure of the expected future instability of the route. Delay in
propagating an unstable route should cause the unstable route to be
suppressed until there is some degree of confidence that the route has
stabilized.
If a large number of route changes are received in separate updates
over some very short period of time and these updates have the
potential to be combined into a single update then these should be
packed as efficiently as possible before propagating further. Some
small delay in propagating well behaved routes is tolerable and is
necessary to allow better packing of updates.
Where routes are unstable, use and announcement of the routes should
be suppressed rather than suppressing their removal. Where one route
to a destination is stable, and another route to the same destination
is somewhat unstable, if possible, the unstable route should be
suppressed more aggressively than if there were no alternate path.
Routing consistency within an AS is very important. Only very minimal
delay of internal BGP (IBGP) should be done. Routing consistency
across AS boundaries is also very important. It is highly undesirable
to advertise a route that is different from the route that is being
used, except for a very minimal time. It is more desirable to
suppress the acceptance of a route (and therefore the use of that
route in the IGP) rather than suppress only the redistribution.
It is clearly not possible to accurately predict the future stability
of a route. The recent history of stability is generally regarded as
a good basis for estimating the likelihood of future stability. The
criteria that is used to distinguish well behaved from poorly behaved
routes is therefore based on the recent history of stability of the
route. There is no simple direct quantitative expression of recent
stability so a figure of merit must be defined. Some desirable
characteristics of this figure of merit would be that the farther in
the past that instability occurred, the less it's affect on the figure
of merit and that the instability measure would be cumulative rather
than reflecting only the most recent event.
The algorithms should behave such that for routes which have a history
of stability but make a few transitions, those transitions should be
made quickly. If transitions continue, advertisement of the route
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should be suppressed. There should be some memory of prior instabil-
ity. The degree to which prior instability is considered should be
gradually reduced as long as the route remains announced and stable.
2.3 Design Choices
After routes have been accepted their readvertisement will be briefly
suppressed to improve packing of updates. There may be a lengthy
suppression of the acceptance of an external route. How long a route
will be suppressed is based on a figure of merit that is expected to
be loosely correlated to the probability of future instability of a
route. Routes with high figure of merit values will be suppressed.
An exponential decay algorithm was chosen as the basis for reducing
the figure of merit over time. These choices should be viewed as
suggestions for implementation.
An exponential decay function has the property that previous
instability can be remembered for a fairly long time. The rate at
which the instability figure of merit decays slows as time goes on.
Exponential decay is a transitive function.
f(f(figure-of-merit, t1), t2) = f(figure-of-merit, t1+t2)
This transitive property allows the decay for a long period to be
computed in a single operation regardless of the current value
(figure-of-merit). As a performance optimization, the decay can be
applied in fixed time increments. Given a desired decay half life,
the decay for a single time increment can be computed ahead of time.
The decay for multiple time increments is expressed below.
f(figure-of-merit, n * t0) = f(figure-of-merit, t0) ** n = K
** n
The values of K ** n can be precomputed for a reasonable number of
``n'' and stored in an array. The value of ``K'' is always less than
one. The array size can be bounded since the value quickly approaches
zero. This makes the decay easy to compute using an array bound
check, an array lookup and a single multiply regardless as to how much
time has elapsed.
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3 Limiting Route Advertisements using Fixed Timers
This method of limiting route advertisements involves the use of fixed
timers applied to the process of sending routes. It's primary purpose
is to improve the packing of routes in BGP update messages. The delay
in advertising a stable route should be bounded and minimal. The
delay in advertising an unreachable need not be zero, but should also
be bounded and should probably have a separate bound set less than or
equal to the bound for a reachable advertisement.
Routes that need to be readvertised can be marked in the RIB or an
external set of structures maintained, which references the RIB.
Periodically, a subset of the marked routes can be flushed. This is
fairly straightforward and accomplishes the objectives. Computation
for too simple an implementation may be order N squared. To avoid N
squared performance, some form of data structure is needed to group
routes with common attributes.
Any implementation should packs updates efficiently, provide a minimum
readvertisement delay, provide a bounds on the maximum readvertisement
delay that would be experienced solely as a result of the algorithm
used to provide a minimum delay, and must be computationally efficient
in the presence of a very large number of candidates for
readvertisement.
4 Stability Sensitive Suppression of Route Advertisement
This method of limiting route advertisements uses a measure of route
stability applied on a per route basis. This technique is applied
when receiving updates from external peers only (EBGP). Applying this
technique to IBGP learned routes or to advertisement to IBGP or EBGP
peers after making a route selection can result in routing loops.
A figure of merit based on a measure of instability is maintained on a
per route basis. This figure of merit is used in the decision to
suppress the use of the route. Routes with high figure of merit are
suppressed. Each time a route is withdrawn, the figure of merit is
incremented. While the route is not changing the figure of merit
value is decayed exponentially with separate decay rates depending on
whether the route is stable and reachable or has been stable and
unreachable. The decay rate may be slower when the route is unreach-
able, or the stability figure of merit could remain fixed (not decay
at all) while the route remains unreachable. Whether to decay un-
reachable routes at the same rate, a slower rate, or not at all is an im-
plementation choice. Decaying at a slower rate is recommended.
An very efficient implementation is suggested in the following
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sections. The implementation only requires computation for the routes
contained in an update, when an update is received or withdrawn (as
opposed to the simplistic approach of periodically decaying each
route). The suggested implementation involves only a small number of
simple operations, and can be implemented using scaled integers.
The behavior of unstable routes is believed to be fairly predictable.
Severely flapping routes will often be advertised and withdrawn at
regular time intervals corresponding to the timers of a particular
protocol (the IGP or exterior protocol in use where the problem
exists). Marginal circuits or mild congestion can result in a long
term pattern of occasional brief route withdrawal or occasional brief
connectivity.
4.1 Single vs. Multiple Configuration Parameter Sets
The behavior of the algorithm is modified by a number of configurable
parameters. It is possible to configure separate sets of parameters
designed to handle short term severe route flap and chronic milder
route flap (a pattern of occasional drops over a long time period).
The former would require a fast decay and low threshold (allowing a
small number of consecutive flaps to cause a route to be suppressed,
but allowing it to be reused after a relatively short period of
stability). The latter would require a very slow decay and a higher
threshold and might be appropriate for routes for which there was an
alternate path of similar bandwidth.
It may also be desirable to configure different thresholds for routes
with roughly equivalent alternate paths than for routes where the
alternate paths have a lower bandwidth or tend to be congested. This
can be solved by associating a different set of parameters with
different ranges of preference values. Parameter selection could be
based on BGP LOCAL_PREF.
Parameter selection could also be based on whether an alternate route
was known. A route would be considered if, for any applicable
parameter set, an alternate route with the specified preference value
existed and the figure of merit associated with the parameter set did
not indicate a need to suppress the route. A less aggressive
suppression would be applied to the case where no alternate route at
all existed. In the simplest case, a more aggressive suppression
would be applied if any alternate route existed. Only the highest
preference (most preferred) value needs to be specified, since the
ranges may overlap.
It might also be desirable to configure a different set of thresholds
for routes which rely on switched services and may disconnect at times
to reduce connect charges. Such routes might be expected to change
state somewhat more often, but should be suppressed if continuous
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state changes indicate instability.
While not essential, it might be desirable to be able to configure
multiple sets of configuration parameters per route. It may also be
desirable to be able to configure sets of parameters that only
correspond to a set of routes (identified by AS path, peer router,
specific destinations or other means). Experience may dictate how
much flexibility is needed and how to best to set the parameters.
Whether to allow different damping parameter sets for different
routes, and whether to allow multiple figures of merit per route is an
implementation choice.
Parameter selection can also be based on prefix length. The rationale
is that longer prefixes tend to reach less end systems and are less
important and these less important prefixes can be damped more
aggressively. This technique is in fairly widespread use. Small
sites or those with dense address allocation who are multihomed are
often reachable by long prefixes which are not easily aggregated.
These sites tend to dispute the choice of prefix length for parameter
selection. Advocates of the technique point out that it encourages
better aggregation.
4.2 Configuration Parameters
At configuration time, a number of parameters may be specified by the
user. The configuration parameters are expressed in units meaningful
to the user. These differ from the parameters used at run time which
are in unit convenient for computation. The run time parameters are
derived from the configuration parameters. Suggested configuration
parameters are listed below.
cutoff threshold (cut)
This value is expressed as a number of route withdrawals. It is
the value above which a route advertisement will be suppressed.
reuse threshold (reuse)
This value is expressed as a number of route withdrawals. It is
the value below which a suppressed route will now be used again.
maximum hold down time (T-hold)
This value is the maximum time a route can be suppressed no matter
how unstable it has been prior to this period of stability.
decay half life while reachable (decay-ok)
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This value is the time duration in minutes or seconds during which
the accumulated stability figure of merit will be reduced by half
if the route if considered reachable (whether suppressed or not).
decay half life while unreachable (decay-ng)
This value is the time duration in minutes or seconds during which
the accumulated stability figure of merit will be reduced by half
if the route if considered unreachable. If not specified or set to
zero, no decay will occur while a route remains unreachable.
decay memory limit (Tmax-ok or Tmax-ng)
This is the maximum time that any memory of previous instability
will be retained given that the route's state remains unchanged,
whether reachable or unreachable. This parameter is generally used
to determine array sizes.
There may be multiple sets of the parameters above as described in
Section 4.1. The configuration parameters listed below would be
applied system wide. These include the time granularity of all
computations, and the parameters used to control reevaluation of
routes that have previously been suppressed.
time granularity (delta-t)
This is the time granularity in seconds used to perform all decay
computations.
reuse list time granularity (delta-reuse)
This is the time interval between evaluations of the reuse lists.
Each reuse lists corresponds to an additional time increment.
reuse list memory reuse-list-max
This is the time value corresponding to the last reuse list. This
may be the maximum value of T-hold for all parameter sets of may be
configured.
number of reuse lists (reuse-list-size)
This is the number of reuse lists. It may be determined from
reuse-list-max or set explicitly.
A necessary optimization is described in Section 4.8.6 that involves
an array referred to as the ``reuse index array''. A reuse index
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Figure 1: Instability figure of merit for flap at a constant rate
array is needed for each decay rate in use. The reuse index array is
used to estimate which reuse list to place a route when it is
suppressed. Proper placement avoids the need to periodically evaluate
decay to determine if a route can be reused. Using the reuse index
array avoids the need to compute a logarithm to determine placement.
One additional system wide parameter can be introduced.
reuse index array size (reuse-index-array-size)
This is the size of reuse index arrays. This size determines the
accuracy with which suppressed routes can be placed within the set
of reuse lists when suppressed for a long time.
4.3 Guidelines for Setting Parameters
The decay half life should be set to a time considerably longer than
the period of the route flap it is intended to address. If for
example, the decay is set to ten minutes and a route is withdrawn and
readvertised exactly every ten minutes, the route would continue to
flap if the cutoff was set to a value of 2 or above.
The stability figure of merit itself is an accumulated time decayed
total. This must be kept in mind in setting the decay time, cutoff
values and reuse values. For example, if a route flaps at four times
the decay rate, it will reach 3 in 4 cycles, 4 in 6 cycles, 5 in 10
cycles, and will converge at about 6.3. At twice the decay time, it
will reach 3 in 7 cycles, and converge at a value of less than 3.5.
Figure 1 shows the stability figure of merit for route flap at a
constant rate. The time axis is labeled in multiples of the decay
half life. The plots represent route flap with a period of 1/2, 1/3,
1/4, and 1/8 times the decay half life. A ceiling of 4.5 was set,
which can be seen to affect three of the plots, effectively limiting
the time it takes to readvertise the route regardless of the prior
history. With the cutoff and reuse thresholds suggested by the dotted
lines, routes would be suppressed after being declared unreachable 2-3
times and be used again after approximately 2 decay half life periods
of stability.
From either maximum hold time value (Tmax-ok or Tmax-ng), a ratio of
the cutoff to a ceiling can be determined. An integer value for the
ceiling can then be chosen such that overflow will not be a problem
and all other values can be scaled accordingly. If both cutoffs are
specified or if multiple parameter sets are used the highest ceiling
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Figure 2: Separate decay constants when unreachable
will be used.
Figure 2 show the effect of configuring separate decay rates to be
used when the route is reachable or unreachable. The decay rate is
5 times slower when the route is unreachable. In the three case
shown, the period of the route flap is equal to the decay half life
but the route is reachable 1/8 of the time in one, reachable 1/2 the
time in one, and reachable 7/8 of the time in the other. In the last
case the route is not suppressed until after the third unreachable
(when it is above the top threshold after becoming reachable again).
In both Figure 1 and Figure 2, routes would be suppressed. Routes
flapping at the decay half life or less would be withdrawn two or
three times and then remain withdrawn until they had remained stably
announced and stable for on the order of 1 1/2 to 2 1/2 times the
decay half life (given the ceiling in the example).
A larger time granularity will keep table storage down. The time
granularity should be less than a minimal reasonable time between
expected worse case route flaps. It might be reasonable to fix this
parameter at compile time or set a default and strongly recommend that
the user leave it alone. With an exponential decay, array size can be
greatly reduced by setting a period of complete stability after which
the decayed total will be considered zero rather than retaining a tiny
quantity. Alternately, very long decays can be implemented by
multiplying more than once if array bounds are exceeded.
The reuse lists hold suppressed routes grouped according to how long
it will be before the routes are eligible for reuse. Periodically
each list will be advanced by one position and one list removed as de-
scribed in Section 4.8.7. All of the suppressed routes in the removed
list will be reevaluated and either used or placed in another list
according to how much additional time must elapse before the route can
be reused. The last list will always contain all the routes which
will not be advertised for more time than is appropriate for the re-
maining list heads. When the last list advances to the front, some of
the routes will not be ready to be used and will have to be requeued.
The time interval for reconsidering suppressed routes and number of list
heads should be configurable. Reasonable defaults might be 30 seconds and
64 list heads. A route suppressed for a long time would need to be reeval-
uated every 32 minutes.
4.4 Run Time Data Structures
A fixed small amount of per system storage will be required. Where
sets of multiple configuration parameters are used, storage will be
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required per set of parameters. A small amount of per route storage
is required. A set of list heads is needed. These list heads are
used to arrange suppressed routes according to the time remaining
until they can be reused.
If multiple sets of configuration parameters are allowed per route,
there is a need for some means of associating more than one figure of
merit and set of parameters with each route. Building a linked list
of these objects seems like one of a number of reasonable
implementations. Similarly, a means of associating a route to a reuse
list is required. A small overhead will be required for the pointers
needed to implement whatever data structure is chosen for the reuse
lists. The suggested implementation uses a double linked lists and so
requires two pointers per figure of merit.
Each set of configuration parameters can reference decay arrays and
reuse arrays. These arrays should be shared among multiple sets of
parameters since their storage requirement is not negligible. There
will be only one set of reuse list heads for the entire router.
4.4.1 Data Structures for Configuration Parameter Sets
Based on the configuration parameters described in the previous
section, the following values can be computed as scaled integers
directly from the corresponding configuration parameters.
o decay array scale factor (decay-array-scale-factor)
o cutoff value (cut)
o reuse value (reuse)
o figure of merit ceiling (ceiling)
Each configuration parameter set will reference one or two decay
arrays and one or two reuse arrays. Only one array will be needed if
the decay rate is the same while a route is unreachable as while it is
reachable, or if the stability figure of merit does not decay while a
route is unreachable.
4.4.2 Data Structures per Decay Array and Reuse Index Array
The following are also computed from the configuration parameters
though not as directly.
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o decay rate per tick (decay-delta-t)
o decay array size (decay-array-size)
o decay array (decay)
o reuse index array size (reuse-index-array-size)
o reuse index array (reuse-index-array)
For each decay rate specified, an array will be used to store the
value of a computed parameter raised to the power of the index of each
array element. This is to speed computations. The decay rate per
tick is an intermediate value expressed as a real number and used to
compute the values stored in the decay arrays. The array size is
computed from the decay memory limit configuration parameter expressed
as an array size or as a maximum hold time.
The decay array size must be of sufficient size to accommodate the
specified decay memory given the time granularity, or sufficient to
hold the number of array elements until integer rounding produces a
zero result if that value is smaller, or a implementation imposed
reasonable size to prevent configurations which use excessive memory.
Implementations may chose to make the array size shorter and multiply
more than once when decaying a long time interval to reduce storage.
The reuse index arrays serve a similar purpose to the decay arrays.
The amount of time until a route can be reused can be determined using
a array lookup. The array can be built given the decay rate. The
array is indexed using a scaled integer proportional to the ratio
between a current stability figure of merit value and the value needed
for the route to be reused.
4.4.3 Data Structures per Route
The following information must be maintained per route. A route here
is considered to be a tuple containing at least NLRI prefix, next hop,
and AS path. The tuple may also contain other BGP attributes such as
MULTI_EXIT_DISCRIMINATOR (MED).
stability figure of merit (figure-of-merit)
Each route must have a stability figure of merit per applicable
parameter set.
last time updated (time-update)
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The exact last time updated must be maintained to allow exponential
decay of the accumulated figure of merit to be deferred until the
route might reasonable be considered eligible for a change in
status (having gone from unreachable to reachable or advancing
within the reuse lists).
config block pointer
Any implementation that supports multiple parameter sets must
provide a means of quickly identifying which set of parameters
corresponds to the route currently being considered. For
implementations supporting only parameter sets where all routes
must be treated the same, this pointer is not required.
reuse list traversal pointers
If doubly linked lists are used to implement reuse lists, then two
pointers will be needed, previous and next. Generally there is a
double linked list which is unused when a route is suppressed from
use that can be used for reuse list traversal eliminating the need
for additional pointer storage.
4.5 Processing Configuration Parameters
From the configuration parameters, it is possible to precompute a
number of values that will be used repeatedly and retain these to
speed later computations that will be required frequently.
The methods of scaled integer arithmetic are not described in detail
here. The methods of determining the real values are given.
Translation into scaled integer values and the details of scaled
integer arithmetic are left up to the individual implementations.
figure of merit scale factor ( scale-figure-of-merit )
The ceiling value can be set to be the largest integer that can fit
in half the bits available for an unsigned integer. This will
allow the scaled integers to be multiplied by the scaled decay
value and then shifted down. Implementations may prefer to use
real numbers or may use any integer scaling deemed appropriate for
their architecture.
penalty value and thresholds (as proportional scaled integers)
The figure of merit penalty for one route withdrawal and the cutoff
values must be scaled according to the above scaling factor.
decay rate per tick (decay[1])
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The decay value per increment of time as defined by the time
granularity must be determined (at least initially as a floating
point number). The per tick decay is a number slightly less than
one. It is the Nth root of the one half where N is the half life
divided by the time granularity.
decay[1] = exp ((1 / (decay-rate/delta-t)) * log (1/2))
decay array size (decay-array-size)
The decay array size is the decay memory divided by the time
granularity. If integer truncation brings the value of an array
element to zero, the array can be made smaller. An implementation
should also impose a maximum reasonable array size or allow more
than one multiplication.
decay-array-size = (Tmax/delta-t)
decay array (decay[])
Each i-th element of the decay array is the per tick delay raised
to the i-th power. This might be best done by successive floating
point multiplies followed by scaling and integer rounding or
truncation. The array itself need only be computed at startup.
decay[i] = decay[1] ** i
4.6 Building the Reuse Index Arrays
The reuse lists may be accessed quite frequently if a lot of routes
are flapping sufficiently to be suppressed. A method of speeding the
determination of which reuse list to use for a given route is
suggested. This method is introduced in Section 4.2, its
configuration described in Section 4.4.2 and the algorithms described
in Section 4.8.6 and Section 4.8.7. This section describes building
the reuse list index arrays.
A ratio of the figure of merit of the route under consideration to the
cutoff value is used as the basis for an array lookup. The ratio is
scaled and truncated to an integer and used to index the array. The
array entry is an integer used to determine which reuse list to use.
reuse array maximum ratio (max-ratio)
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This is the maximum ratio between the current value of the
stability figure of merit and the target reuse value that can be
indexed by the reuse array. It may be limited by the ceiling
imposed by the maximum hold time or by the amount of time that the
reuse lists cover.
max-ratio = min(ceiling/reuse, exp((1 /
(half-life/reuse-array-time)) * log(1/2)))
reuse array scale factor ( scale-factor )
Since the reuse array is an estimator, the reuse array scale factor
has to be computed such that the full size of the reuse array is
used.
scale-factor = (max-ratio - 1) / reuse-array-size
reuse index array (reuse)
Each reuse index array entry should contain an index into the reuse
list array pointing to one of the list heads. This index should
corresponding to the reuse list that will be evaluated just after a
route would be eligible for reuse given the ratio of current value
of the stability figure of merit to target reuse value
corresponding the the reuse array entry.
reuse-array[j] = integer(log(1 / (1 + ((j+1) *
(max-ratio-1)))) / reuse-time-granularity)
To determine which reuse queue to place a route which is being
suppressed, the following procedure is used. Divide the current
figure of merit by the cutoff. Subtract one. Multiply by the scale
factor. This is the array index. If it is off the end of the array
use the last queue otherwise look in the array and pick the number of
the queue from the array at that index. This is quite fast and well
worth the setup and storage required.
4.7 A Sample Configuration
A simple example is presented here in which the space overhead is
estimated for a set of configuration parameters. The design here
assumes:
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1. there is a single parameter set used for all routes,
2. decay time for unreachable routes is slower than for reachable
routes
3. the arrays must be full size, rather than allow more than one
multiply per decay operation to reduce the array size.
This example is used in later sections. The use of multiple parameter
sets complicates the examples somewhat. Where multiple parameter sets
are allowed for a single route, the decay portion of the algorithm is
repeated for each parameter set. If different routes are allowed to
have different parameter sets, the routes must have pointers to the
parameter sets to keep the time to locate to a minimum, but the
algorithms are otherwise unchanged.
A sample set of configuration parameters and a sample set of
implementation parameters are provided in in the two following list.
1. Configuration Parameters
o cut = 1.25
o reuse = 0.5
o T-hold = 15 mins
o decay-ok = 5 min
o decay-ng = 15 min
o Tmax-ok, Tmax-ng = 15, 30 mins
2. Implementation Parameters
o delta-t = 1 sec
o delta-reuse
o reuse-list-size = 256
o reuse-index-array-size = 1,024
Using these configuration and implementation parameters and the
equations in Section 4.5, the space overhead can be computed. There
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Figure 3: Some fairly long route flap cycles, repeated for 12
minutes, followed by a period of stability.
is a fixed space overhead that is independent of the number of routes.
There is an space requirement associated with a stable route. There
is a larger space requirement associated with an unstable route. The
space requirements for the parameters above are provide in the lists
below.
1. fixed overhead (using parameters from previous example)
o 900 * integer - decay array
o 1,800 * integer - decay array
o 120 * pointer - reuse list-heads
o 2,048 * integer - reuse index arrays
2. overhead per stable route
o pointer - containing null entry
3. overhead per unstable route
o pointer - to a damping structure containing the following
o integer - figure of merit + bit for state
o integer - last time updated
o pointer (optional) to configuration parameter block
o 2 * pointer - reuse list pointers (prev, next)
Figure 3 shows the behavior of the algorithm with the parameters given
above. Four cases are given in this example. In all four, there is a
twelve minute period of route oscillations. Two periods of oscilla-
tion are used, 2 minutes and 4 minutes. Two duty cycles are used, one
in which the route is reachable during 20% of the cycle and the other
where the route is reachable during 80% of the cycle. In all four
cases, the route becomes suppressed after it becomes unreachable the
second time. Once suppressed, it remains suppressed until some period
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after becoming stable. The routes which oscillate over a 4 minute pe-
riod are no longer suppressed within 9-11 minutes after becoming sta-
ble. The routes with a 2 minute period of oscillation are suppressed for
nearly the maximum 15 minute period after becoming stable.
4.8 Processing Routing Protocol Activity
The prior sections concentrate on configuration parameters and their
relationship to the parameters and arrays used at run time and provide
the algorithms for initializing run time storage. This section
provides the steps taken in processing routing events and timer events
when running.
The routing events are:
1. A BGP peer or new route comes up for the first time (or after an
extended down time) (Section 4.8.1)
2. A route becomes unreachable (Section 4.8.2)
3. A route becomes reachable again (Section 4.8.3)
4. A route changes (Section 4.8.4)
5. A peer goes down (Section 4.8.5)
The reuse list is used to provide a means of fast evaluation of route
that had been suppressed, but had been stable long enough to be reused
again. The following two operations are described.
1. Inserting into a reuse list (Section 4.8.6)
2. Reuse list processing every delta-t seconds (Section 4.8.7)
4.8.1 Processing a New Peer or New Routes
When a peer comes up, no action is required if the routes had no
previous history of instability, for example if this is the first time
the peer is coming up and announcing these routes. For each route,
the pointer to the damping structure would be zeroed and route used.
The same action is taken for a new route or a route that has been down
long enough that the figure of merit reached zero and the damping
structure was deleted.
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4.8.2 Processing Unreachable Messages
When a route is withdrawn or changed (Section 4.8.4 describes how a
change is handled), the following procedure is used.
If there is no previous stability history (the damping structure
pointer is zero), then:
1. allocate a damping structure
2. set figure-of-merit = 1
3. withdraw the route
Otherwise, if there is an existing damping structure, then:
1. set t-diff = t-now - t-updated
2. if (t-diff puts you off the end of the array) {
setfigure-of-merit =1
}else {
setfigure-of-merit =figure-of-merit *decay-array-ok [t-diff ]+ 1
if(figure-of-merit >ceiling) {
setfigure-of-merit =ceiling
}
}
3. remove the route from a reuse list if it is on one
4. withdraw the route unless it is already suppressed
In either case then:
1. set t-updated = t-now
2. insert into a reuse list (see Section 4.8.6)
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If there was a stability history, the previous value of the stability
figure of merit is decayed. This is done using the decay array
(decay-array). The index is determined by subtracting the current
time and the last time updated, then dividing by the time granularity.
If the index is zero, the figure of merit is unchanged (no decay). If
it is greater than the array size, it is zeroed. Otherwise use the
index to fetch a decay array element and multiply the figure of merit
by the array element. If using the suggested scaled integer method,
shift down half an integer. Add the scaled penalty for one more un-
reachable (shown above as 1). If the result is above the ceiling re-
place it with the ceiling value. Now update the last time updated field
(preferably taking into account how much time was truncated before doing
the decay calculation).
When a route becomes unreachable, alternate paths must be considered.
This process is complicated slightly if different configuration param-
eters are used in the presence or absence of viable alternate paths.
If all of these alternate paths have been suppressed because there had
previously been an alternate route and the new route withdrawal
changes that condition, the suppressed alternate paths must be reeval-
uated. They should be reevaluated in order of normal route prefer-
ence. When one of these alternate routes is encountered that had been
suppressed but is now usable since there is no alternate route, no
further routes need to be reevaluated. This only applies if routes
are given two different reuse thresholds, one for use when there is an al-
ternate path and a higher threshold to use when suppressing the route would
result in making the destination completely unreachable.
4.8.3 Processing Route Advertisements
When a route is readvertised if there is no damping structure, then
the procedure is the same as in Section 4.8.1.
1. don't create a new damping structure
2. use the route
If an damping structure exists, the figure of merit is decayed and the
figure of merit and last time updated fields are updated. A decision
is now made as to whether the route can be used immediately or needs
to be suppressed for some period of time.
1. set t-diff = t-now - t-updated
2. if (t-diff puts you off the end of the array) {
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setfigure-of-merit =0
}else {
set figure-of-merit= figure-of-merit* decay-array-ng[t-diff]
}
3. if ( not suppressed and figure-of-merit < cut ) {
usethe route
}else if( suppressedand figure-of-merit< reuse) {
setstate tonot suppressed
removethe routefrom areuse listif itis on one
usethe route
}else {
setstate tosuppressed
don'tuse theroute
insertinto areuse list(see Section4.8.6)
}
4. if ( figure-of-merit > 0 ) {
set t-updated= t-now
}else {
recovermemory fordamping struct
zeropointer todamping struct
}
If the route is deemed usable, a search for the current best route
must be made. The newly reachable route is then evaluated according
to the BGP protocol rules for route selection.
If the new route is usable, the previous best route is examined.
Prior to coute comparisons, the current best route may have to be
reevaluated if separate parameter sets are used depending on the
presence or absence of an alternate route. If there had been no
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alternate the previous best route may be suppressed.
If the new route is to be suppressed it is placed on a reuse list only
if it would have been preferred to the current best route had the new
route been accepted as stable. There is no reason to queue a route on
a reuse list if after the route becomes usable it would not be used
anyway due to the existence of a more preferred route. Such a route
would not have to be reevaluated unless the preferred route became
unreachable. As specified here, the less preferred route would be
reevaluated and potentially used or potentially added to a reuse list
when processing the withdrawal of a more preferred best route.
4.8.4 Processing Route Changes
If a route is replaced by a peer router by supplying a new path, the
route that is being replaced should be treated as if an unreachable
were received (see Section 4.8.2). This will occur when a peer
somewhere back in the AS path is continuously switching between two AS
paths and that peer is not damping route flap (or applying less
damping). There is no way to determine if one AS path is stable and
the other is flapping, or if they are both flapping. If the cycle is
sufficiently short compared to convergence times neither route through
that peer will deliver packets very reliably. Since there is no way
to affect the peer such that it choses the stable of the two AS paths,
the only viable option is to penalize both routes by considering each change
as an unreachable followed by a route advertisement.
4.8.5 Processing A Peer Router Loss
When a peer routing session is broken, either all individual routes
advertised by that peer may be marked as unstable, or the peering
session itself may be marked as unstable. Marking the peer will save
considerable memory. Since the individual routes are advertised as
unreachable to routers beyond the immediate problem, per route state
will be incurred beyond the peer immediately adjacent to the BGP
session that went down. If the instability continues, the immediately
adjacent router need only keep track of the peer stability history.
The routers beyond that point will receive no further advertisements
or withdrawal of routes and will dispose of the damping structure over
time.
BGP notification through an optional transitive attribute that damping
will already be applied may be considered in the future to reduce the
number of routers that incur damping structure storage overhead.
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4.8.6 Inserting into the Reuse Timer List
The reuse lists are used to provide a means of fast evaluation of
route that had been suppressed, but had been stable long enough to be
reused again. The data structure consists of a series of list heads.
Each list contains a set of routes that are scheduled for reevaluation
at approximately the same time. The set of reuse list heads are
treated as a circular array.
A simple implementation of the circular array of list heads would be
an array containing the list heads with an offset. The offset would
identify the first list. The Nth list would be at the index
corresponding to N plus the offset modulo the number of list heads.
This design will be assumed in the examples that follow.
A key requirement is to be able to insert an entry in the most
appropriate queue with a minimum of computation. The computation is
given only the current value of figure-of-merit. The array, scale,
and bounds are precomputed to map figure-of-merit to the nearest list
head without requiring a logarithm to be computed (see Section 4.5).
1. scale figure-of-merit for the index array lookup producing index
2. check index against the array bound
3. if (within the array bound) {
setindex =reuse-array [index ]
}else {
setindex =reuse-list-size -1
}
4. insertinto thelist
reuse-list[ moduloreuse-list-size (index +offset )]
Choosing the correct reuse list involves only a multiply and shift to
do the scaling, an integer truncation, then an array lookup. The most
common method of implementing a circular array is to use an array and
apply an offset and modulo operation to pick the correct array entry.
The offset is incremented to rotate the the circular array.
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4.8.7 Handling Reuse Timer Events
The granularity of the reuse timer should be more course that that of
the decay timer. As a result, when the reuse timer fires, suppressed
routes should be decayed by multiple increments of decay time. Some
computation can be avoided by always inserting into the reuse list
corresponding to one time increment past reuse eligibility. In cases
where the reuse lists have a longer ``memory'' than the ``decay
memory'' (described above), all of the routes in the first queue will
be available for immediate reuse.
When it is time to advance the lists, the first queue on the reuse
list must be processed and the circular queue must be rotated. Using
an array and an offset as a circular array (as described in
Section 4.8.6), the algorithm below is repeated every t-reuse seconds.
1. save a pointer to the current zeroth queue head and zero the list
head entry
2. set offset = modulo reuse-list-size ( offset + 1 ), thereby
rotating the circular queue of list-heads
3. if ( the saved list head pointer is non-empty )
foreach entry {
sett-diff =t-now -t-updated
setfigure-of-merit =figure-of-merit *decay-array-ok [t-diff ]
sett-updated =t-now
if( figure-of-merit< reuse)
reusethe route
else
re-insertinto anotherlist (seeSection 4.8.6)
}
The value of the zeroth list head would be saved and the array entry
itself zeroed. The list heads would then be advanced by incrementing
the offset. Starting with the saved head of the old zeroth list, each
route would be reevaluated and used or requeued if it were not ready
for reuse. If a route is used, it must be treated as if it were a new
route advertisement as described in Section 4.8.3.
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5 Implementation Experience
The first implementations of ``route flap damping'' were the route
server daemon (rsd) coding by Ramesh Govindan (ISI) and the Cisco IOS
implementation by Ravi Chandra. Both implementations first became
available in 1995 and have been used extensively. The rsd
implementation has been in use in route servers at the NSF funded
Network Access Points (NAPs) and at other major Internet
interconnects. The Cisco IOS version has been in use by Internet
Service Providers worldwide. The rsd implementation has been
integrated in releases of gated (see http://www.gated.org) and is
available in commercial routers using gated.
Acknowledgements
This work and this document may not have been completed without the
advise, comments and encouragement of Yakov Rekhter (Cisco). Dennis
Ferguson (MCI) provided a description of the algorithms in the gated
BGP implementation and many valuable comments and insights. David
Bolen (ANS) and Jordan Becker (ANS) provided valuable comments,
particularly regarding early simulations. At the time of this writing
two implementations exists. One was led by Ramesh Govindan (ISI) for
the NSF Routing Arbiter project. The second was led by Ravi Chandra
(Cisco). Sean Doran (Sprintlink) and Serpil Bayraktar (ANS) were
among the early independent testers of the Cisco pre-beta
implementation.
References
[1] P. Gross and Y. Rekhter. Application of the border gateway proto-
col in
the internet. Request for Comments (Draft Standard) RFC 1268, In-
ternet Engineering Task Force, October 1991. (Obsoletes RFC1164);
(Obsoleted by RFC1655). ftp://ds.internic.net/rfc/rfc1268.txt.
[2] ISO/IEC. Iso/iec 10747 - information technology - telecommunica-
tions and information exchange between systems - protocol for
exchange of inter-domain routeing information among intermediate
systems to support forwarding of iso
8473 pdus. Technical report, International Organization for Stan-
dardization, August 1994. ftp://merit.edu/pub/iso/idrp.ps.gz.
[3] K. Lougheed and Y. Rekhter. A border gateway protocol 3 (BGP-3).
Request for Comments (Draft Standard) RFC 1267, In-
ternet Engineering Task Force, October 1991. (Obsoletes RFC1163).
ftp://ds.internic.net/rfc/rfc1267.txt.
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[4] Y. Rekhter and P. Gross. Application of the border gateway proto-
col in the internet. Request for Comments (Draft Standard)
RFC 1772, Internet Engineering Task Force, March 1995. (Obsoletes
RFC1655). ftp://ds.internic.net/rfc/rfc1772.txt.
[5] Y. Rekhter and T. Li. A border
gateway protocol 4 (BGP-4). Request for Comments (Draft Standard)
RFC 1771, Internet Engineering Task Force, March 1995. (Obsoletes
RFC1654). ftp://ds.internic.net/rfc/rfc1771.txt.
[6] Y. Rekhter and C. Topolcic. Exchanging routing information across
provider boundaries in the CIDR environment. Request for Comments
(Informational) RFC 1520, Internet Engineering Task Force,
September 1993. ftp://ds.internic.net/rfc/rfc1520.txt.
[7] P. Traina. BGP-4 protocol analysis. Request for Comments (Infor-
mational) RFC 1774, Internet Engineering Task Force, March 1995.
ftp://ds.internic.net/rfc/rfc1774.txt.
[8] P. Traina. Experience with the BGP-4 protocol. Request for Com-
ments (Informational) RFC 1773,
Internet Engineering Task Force, March 1995. (Obsoletes RFC1656).
ftp://ds.internic.net/rfc/rfc1773.txt.
Security Considerations
The practices outlined in this document do not further weaken the
security of the routing protocols. Denial of service is possible in
an already insecure routing environment but these practices only
contribute to the persistence of such attacks and do not impact the
methods of prevention and the methods of determining the source.
Author's Addresses
Curtis Villamizar
ANS Communications
<curtis@ans.net>
Ravi Chandra
Cisco Systems
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<rchandra@cisco.com>
Ramesh Govindan
ISI
<govindan@isi.edu>
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