Manual: tc-sfb

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 decre‐
ment 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 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.

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 Sto‐
chastic 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 de‐
creased.

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 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 sup‐
ports 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, respon‐
sive 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 rehash‐
ing 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 immedi‐
ately. This value determines how much time has to pass before the 2nd set will start to be warmed up. De‐
faults 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 in‐
creased 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. De‐
faults 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 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 situa‐
tion points to either bad congestion, 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, perturbing hash results every 10 minutes:

# tc filter add dev $DEV parent 1: handle 1 flow hash keys dst perturb 600

Manual: tc-sfb

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