tc qdisc ...  divisor hashtablesize limit packets perturb seconds quan-
       tum bytes

       Stochastic Fairness Queueing is a classless queueing discipline  avail-
       able for traffic control with the tc(8) command.

       SFQ does not shape traffic but only schedules the transmission of pack-
       ets, based on 'flows'.  The goal is to ensure  fairness  so  that  each
       flow is able to send data in turn, thus preventing any single flow from
       drowning out the rest.

       This may in fact have some effect in mitigating  a  Denial  of  Service

       SFQ is work-conserving and therefore always delivers a packet if it has
       one available.

       On enqueueing, each packet is assigned to a hash bucket, based  on  the
       packets  hash value.  This hash value is either obtained from an exter-
       nal flow classifier (use tc filter to set them), or a default  internal
       classifier if no external classifier has been configured.

       When the internal classifier is used, sfq uses

       (i)    Source address

       (ii)   Destination address

       (iii)  Source port

       If these are available. SFQ knows about ipv4 and ipv6 and also UDP, TCP
       and ESP.  Packets with other protocols are hashed based on  the  32bits
       representation  of  their  destination and the socket they belong to. A
       flow corresponds mostly to a TCP/IP connection.

       Each of these buckets should represent a unique flow. Because  multiple
       flows  may  get  hashed to the same bucket, sfqs internal hashing algo-
       rithm may be perturbed at configurable intervals so that the unfairness
       lasts only for a short while. Perturbation may however cause some inad-
       vertent packet reordering to occur.

       When dequeuing, each hashbucket with data is queried in a  round  robin

       The compile time maximum length of the SFQ is 128 packets, which can be
       spread over at most 128 buckets of 1024 available. In case of overflow,
       tail-drop  is  performed  on the fullest bucket, thus maintaining fair-

              to 0, which means that no perturbation occurs. Do  not  set  too
              low  for  each  perturbation  may  cause some packet reordering.
              Advised value: 10 This value has no effect  when  external  flow
              classification is used.

              Amount  of  bytes a flow is allowed to dequeue during a round of
              the round robin process.  Defaults to the MTU of  the  interface
              which is also the advised value and the minimum value.

       To attach to device ppp0:

       # tc qdisc add dev ppp0 root sfq perturb 10

       Please note that SFQ, like all non-shaping (work-conserving) qdiscs, is
       only useful if it owns the queue.  This is the case when the link speed
       equals  the  actually available bandwidth. This holds for regular phone
       modems, ISDN connections and direct non-switched ethernet links.

       Most often, cable modems and DSL devices do not fall  into  this  cate-
       gory. The same holds for when connected to a switch  and trying to send
       data to a congested segment also connected to the switch.

       In this case, the effective queue does not reside within Linux  and  is
       therefore not available for scheduling.

       Embed SFQ in a classful qdisc to make sure it owns the queue.

       It  is  possible  to  use external classifiers with sfq, for example to
       hash traffic based only on source/destination ip addresses:

       # tc filter add ... flow hash keys src,dst perturb 30 divisor 1024 Note
       that  the  given  divisor should match the one used by sfq. If you have
       changed the sfq default of 1024, use the same value for the  flow  hash
       filter, too.

       o      Paul E. McKenney "Stochastic Fairness Queuing", IEEE INFOCOMM'90
              Proceedings, San Francisco, 1990.

       o      Paul E. McKenney "Stochastic Fairness  Queuing",  "Interworking:
              Research and Experience", v.2, 1991, p.113-131.

       o      See also: M. Shreedhar and George Varghese "Efficient Fair Queu-
              ing using Deficit Round Robin", Proc. SIGCOMM 95.

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