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.