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    Command:

    tc-sfq

    
    
    

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

    iproute2 8 December 2001 TC(8)

    
    
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