SFB(8) Linux SFB(8)
sfb - Stochastic Fair Blue
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
Stochastic Fair Blue is a classless qdisc to manage congestion based on
packet loss and link utilization history 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.
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 account-
ing. Each flow is mapped into a bin of each level using a per-level
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 sec-
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.
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 section 4.4 of the SFB reference. While one set is
used to manage the queue, a second set is warmed up: Whenever a
flow is then determined to be non-responsive, the marking proba-
bilities in the second set are updated. When the rehashing hap-
pens, these bins will be used to manage the queue and all non-
responsive flows can be rate-limited immediately. This value
determines how much time has to pass before the 2nd set will
start to be warmed up. Defaults to one minute, should be lower
limit Hard limit on the real (not average) total queue size in pack-
ets. Further packets are dropped. Defaults to the transmit
queue length of the device the qdisc is attached to.
max Maximum length of a buckets queue, in packets, before packets
start being dropped. Should be sightly larger than target , but
should not be set to values exceeding 1.5 times that of target .
Defaults to 25.
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.
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
Value used to decrease the marking probability when the queue is
found to be empty. Must be between 0 and 1.0. Defaults to
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
The number of packets a flow is permitted to exceed the penalty
rate before packets start being dropped. Defaults to 20 pack-
This qdisc exposes additional statistics via 'tc -s qdisc' output.
The number of packets dropped before a per-flow queue was full.
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.
The number of packets dropped because a per-flow queue was full.
High bucketdrop may point to a high number of aggressive, short-
The number of packets dropped due to reaching limit. This should
normally be 0.
marked The number of packets marked with ECN.
The length of the current longest per-flow (virtual) queue.
The maximum per-flow drop probability. 1 means that some flows
have been detected as non-reactive.
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 maintains virtual queue states, the inner qdisc must not
drop a packet previously queued. Furthermore, if a buckets queue has a
very high marking rate, this implementation will start dropping packets
instead of marking them, as such a situation points to either bad con-
gestion, or an unresponsive flow.
EXAMPLE & USAGE
To attach to interface $DEV, using default options:
# tc qdisc add dev $DEV handle 1: root sfb
Only use destination ip addresses for assigning packets to bins, per-
turbing hash results every 10 minutes:
# tc filter add dev $DEV parent 1: handle 1 flow hash keys dst perturb
tc(8), tc-red(8), tc-sfq(8)
o W. Feng, D. Kandlur, D. Saha, K. Shin, BLUE: A New Class of
Active Queue Management Algorithms, U. Michigan CSE-TR-387-99,
This SFB implementation was contributed by Juliusz Chroboczek and Eric
iproute2 August 2011 SFB(8)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2021
All Rights Reserved.