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 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 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 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.


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


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)