SFB(8) Linux SFB(8)
NAME
sfb - Stochastic Fair Blue
SYNOPSIS
tc qdisc ... blue rehash milliseconds db milliseconds limit packets max packets target packets increment float
decrement float penalty_rate packets per second penalty_burst packets
DESCRIPTION
Stochastic Fair Blue is a classless qdisc to manage congestion based on packet loss and link utilization his‐
tory while trying to prevent non-responsive flows (i.e. flows that do not react to congestion marking or
dropped packets) from impacting performance of responsive flows. Unlike RED, where the marking probability
has to be configured, BLUE tries to determine the ideal marking probability automatically.
ALGORITHM
The BLUE algorithm maintains a probability which is used to mark or drop packets that are to be queued. If the
queue overflows, the mark/drop probability is increased. If the queue becomes empty, the probability is
decreased. The Stochastic Fair Blue (SFB) algorithm is designed to protect TCP flows against non-responsive
flows.
This SFB implementation maintains 8 levels of 16 bins each for accounting. Each flow is mapped into a bin of
each level using a per-level hash value.
Every bin maintains a marking probability, which gets increased or decreased based on bin occupancy. If the
number of packets exceeds the size of that bin, the marking probability is increased. If the number drops to
zero, it is decreased.
The marking probability is based on the minimum value of all bins a flow is mapped into, thus, when a flow
does not respond to marking or gradual packet drops, the marking probability quickly reaches one.
In this case, the flow is rate-limited to penalty_rate packets per second.
LIMITATIONS
Due to SFBs nature, it is possible for responsive flows to share all of its bins with a non-responsive flow,
causing the responsive flow to be misidentified as being non-responsive.
The probability of a responsive flow to be misidentified is dependent on the number of non-responsive flows,
M. It is (1 - (1 - (1 / 16.0)) ** M) **8, so for example with 10 non-responsive flows approximately 0.2% of
responsive flows will be misidentified.
To mitigate this, SFB performs performs periodic re-hashing to avoid misclassification for prolonged periods
of time.
The default hashing method will use source and destination ip addresses and port numbers if possible, and also
supports tunneling protocols. Alternatively, an external classifier can be configured, too.
PARAMETERS
rehash Time interval in milliseconds when queue perturbation occurs to avoid erroneously detecting unrelated,
responsive flows as being part of a non-responsive flow for prolonged periods of time. Defaults to 10
minutes.
db Double buffering warmup wait time, in milliseconds. To avoid destroying the probability history when
rehashing is performed, this implementation maintains a second set of levels/bins as described in sec‐
target The desired average bin length. If the bin queue length reaches this value, the marking probability is
increased by increment. The default value depends on the max setting, with max set to 25 target will
default to 20.
increment
A value used to increase the marking probability when the queue appears to be over-used. Must be
between 0 and 1.0. Defaults to 0.00050.
decrement
Value used to decrease the marking probability when the queue is found to be empty. Must be between 0
and 1.0. Defaults to 0.00005.
penalty_rate
The maximum number of packets belonging to flows identified as being non-responsive that can be
enqueued per second. Once this number has been reached, further packets of such non-responsive flows
are dropped. Set this to a reasonable fraction of your uplink throughput; the default value of 10
packets is probably too small.
penalty_burst
The number of packets a flow is permitted to exceed the penalty rate before packets start being
dropped. Defaults to 20 packets.
STATISTICS
This qdisc exposes additional statistics via 'tc -s qdisc' output. These are:
earlydrop
The number of packets dropped before a per-flow queue was full.
ratedrop
The number of packets dropped because of rate-limiting. If this value is high, there are many non-
reactive flows being sent through sfb. In such cases, it might be better to embed sfb within a classful
qdisc to better control such flows using a different, shaping qdisc.
bucketdrop
The number of packets dropped because a per-flow queue was full. High bucketdrop may point to a high
number of aggressive, short-lived flows.
queuedrop
The number of packets dropped due to reaching limit. This should normally be 0.
marked The number of packets marked with ECN.
maxqlen
The length of the current longest per-flow (virtual) queue.
maxprob
The maximum per-flow drop probability. 1 means that some flows have been detected as non-reactive.
NOTES
SFB automatically enables use of Explicit Congestion Notification (ECN). Also, this SFB implementation does
not queue packets itself. Rather, packets are enqueued to the inner qdisc (defaults to pfifo). Because sfb
# tc filter add dev $DEV parent 1: handle 1 flow hash keys dst perturb 600
SEE ALSO
tc(8), tc-red(8), tc-sfq(8)
SOURCES
o W. Feng, D. Kandlur, D. Saha, K. Shin, BLUE: A New Class of Active Queue Management Algorithms, U.
Michigan CSE-TR-387-99, April 1999.
AUTHORS
This SFB implementation was contributed by Juliusz Chroboczek and Eric Dumazet.
iproute2 August 2011 SFB(8)
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