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NAME

       sfq - Stochastic Fairness Queueing

SYNOPSIS

       tc qdisc ... 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

       (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, the hashing algorithm is
       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

       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

       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.

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>