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NAME | SYNOPSIS | DESCRIPTION | ALGORITHM | LIMITATIONS | PARAMETERS | STATISTICS | NOTES | EXAMPLE & USAGE | SEE ALSO | SOURCES | AUTHORS | COLOPHON |
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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
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.
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 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 probabilities in the
second set are updated. When the rehashing happens, 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 than rehash.
limit Hard limit on the real (not average) total queue size in
packets. 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 slightly 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.
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.
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.
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 congestion, or an unresponsive
flow.
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,
perturbing hash results every 10 minutes:
# tc filter add dev $DEV parent 1: handle 1 flow hash keys dst
perturb 600
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, April 1999.
This SFB implementation was contributed by Juliusz Chroboczek and
Eric Dumazet.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 August 2011 SFB(8)
Pages that refer to this page: tc(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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