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NAME

       sfq - Stochastic Fairness Queueing

SYNOPSIS

       tc qdisc ...  divisor hashtablesize limit packets perturb seconds quantum bytes

DESCRIPTION

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

       SFQ does not shape traffic but only  schedules  the  transmission  of  packets,  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 attempt.

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

ALGORITHM

       On enqueueing, each packet is assigned to a hash bucket, based on the packets hash  value.
       This  hash value is either obtained from an external 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  algorithm  may  be  perturbed  at
       configurable intervals so that the unfairness lasts only for a short  while.  Perturbation
       may however cause some inadvertent packet reordering to occur.

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

       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 fairness.

PARAMETERS

       divisor
              Can  be  used  to  set  a  different  hash table size, available from kernel 2.6.39
              onwards.  The specified divisor must be a power of two and cannot  be  larger  than
              65536.  Default value: 1024.

       limit  Upper limit of the SFQ. Can be used to reduce the default length of 128 packets.

       perturb
              Interval  in  seconds  for queue algorithm perturbation. Defaults 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.

       quantum
              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.

EXAMPLE & USAGE

       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 category. 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.

SOURCE

       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 Queuing using Deficit
              Round Robin", Proc. SIGCOMM 95.

SEE ALSO

       tc(8)

AUTHOR

       Alexey N. Kuznetsov,  <kuznet@ms2.inr.ac.ru>.  This  manpage  maintained  by  bert  hubert
       <ahu@ds9a.nl>