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TC(8)					      Linux					    TC(8)

NAME
       sfq - Stochastic Fairness Queueing

SYNOPSIS
       tc qdisc ... perturb seconds quantum bytes

DESCRIPTION
       Stochastic Fairness Queueing is a classless queueing discipline available for traffic con-
       trol 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
       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

       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  band-
       width.  This holds for regular phone modems, ISDN connections and direct non-switched eth-
       ernet 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 con-
       nected to the switch.

       In this case, the effective queue does not reside within Linux and is therefore not avail-
       able 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 Experi-
	      ence", 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>

iproute2				 8 December 2001				    TC(8)
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