tc-sfq(8) - phpMan

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TC(8)                                Linux                               TC(8)

       sfq - Stochastic Fairness Queueing

       tc qdisc ... perturb seconds quantum bytes

       Stochastic Fairness Queueing is a classless queueing discipline available for traf-
       fic 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  avail-

       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

       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.

              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

              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.

       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 seg-
       ment 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.

       o      Paul  E.  McKenney  "Stochastic Fairness Queuing", IEEE INFOCOMM'90 Proceed-
              ings, 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.


       Alexey N. Kuznetsov, <kuznet AT>. This manpage maintained by bert hubert
       <ahu AT>

iproute2                        8 December 2001                          TC(8)

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