sfb(8) — Linux manual page


SFB(8)                              Linux                             SFB(8)

NAME         top

       sfb - Stochastic Fair Blue

SYNOPSIS         top

       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         top

       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

ALGORITHM         top

       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

LIMITATIONS         top

       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 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,

PARAMETERS         top

       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 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.

              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.

              Value used to decrease the marking probability when the queue
              is found to be empty. Must be between 0 and 1.0.  Defaults to

              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.

              The number of packets a flow is permitted to exceed the
              penalty rate before packets start being dropped.  Defaults to
              20 packets.

STATISTICS         top

       This qdisc exposes additional statistics via 'tc -s qdisc' output.
       These are:

              The number of packets dropped before a per-flow queue was

              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.

              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.

              The number of packets dropped due to reaching limit. This
              should normally be 0.

       marked The number of packets marked with ECN.

              The length of the current longest per-flow (virtual) queue.

              The maximum per-flow drop probability. 1 means that some flows
              have been detected as non-reactive.

NOTES         top

       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         top

       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         top

       tc(8), tc-red(8), tc-sfq(8)

SOURCES         top

       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         top

       This SFB implementation was contributed by Juliusz Chroboczek and
       Eric Dumazet.

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
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iproute2                         August 2011                          SFB(8)