phpman > man > tc-sfb(8)

SFB(8)                                          Linux                                         SFB(8)



NAME
       sfb - Stochastic Fair Blue

SYNOPSIS
       tc  qdisc ... blue rehash milliseconds db milliseconds limit packets max packets target pack‐
       ets 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 re‐
       act 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 be‐
       comes empty, the probability is decreased. The Stochastic Fair Blue (SFB)  algorithm  is  de‐
       signed 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 oc‐
       cupancy.  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 probabil‐
       ity 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-re‐
       sponsive 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  de‐
              tecting  unrelated,  responsive  flows as being part of a non-responsive flow for pro‐
              longed periods of time.  Defaults to 10 minutes.

       db     Double buffering warmup wait time, in milliseconds.  To avoid destroying the probabil‐
              ity 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 mark‐
              ing probability is increased by increment.  The default value depends on the max  set‐
              ting, 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 up‐
              link 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  im‐
       plementation  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 sit‐
       uation 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 ev‐
       ery 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)