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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 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
       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 per-
       turbed 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 fair-
       ness.


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  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.


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 Queu-
              ing using Deficit Round Robin", Proc. SIGCOMM 95.


SEE ALSO
       tc(8)


AUTHOR
       Alexey N. Kuznetsov, <kuznetATms2.ru>. This manpage maintained by
       bert hubert <ahuATds9a.nl>





iproute2                        8 December 2001                          TC(8)