tc-sfb(8) - phpMan

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 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 respon-
sive 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 proba-
bility 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-respon-
sive.

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 proba-
bility 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 deter-
mines 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 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.

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 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.

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