tc-sfq(8) — Linux manual page


TC(8)                             Linux                            TC(8)

NAME         top

       sfq - Stochastic Fairness Queueing

SYNOPSIS         top

       tc qdisc ...  [ divisor hashtablesize ] [ limit packets ] [
       perturb seconds ] [ quantum bytes ] [ flows number ] [ depth
       number ] [ headdrop ] [ redflowlimit bytes ] [ min bytes ] [ max
       bytes ] [ avpkt bytes ] [ burst packets ] [ probability P ] [ ecn
       ] [ harddrop ]

DESCRIPTION         top

       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         top

       On enqueueing, each packet is assigned to a hash bucket, based on
       the packets hash value.  This hash value is either obtained from
       an external flow classifier (use tc filter to set them), or a
       default internal classifier if no external classifier has been

       When the internal classifier is used, sfq uses

       (i)    Source address

       (ii)   Destination address

       (iii)  Source and Destination 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 source. 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, sfqs internal
       hashing algorithm may be perturbed at configurable intervals so
       that the unfairness lasts only for a short while. Perturbation
       may however cause some inadvertent packet reordering to occur.
       After linux-3.3, there is no packet reordering problem, but
       possible packet drops if rehashing hits one limit (number of
       flows or packets per flow)

       When dequeuing, each hashbucket with data is queried in a round
       robin fashion.

       Before linux-3.3, 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.

       After linux-3.3, maximum length of SFQ is 65535 packets, and
       divisor limit is 65536.  In case of overflow, tail-drop is
       performed on the fullest bucket, unless headdrop was requested.

PARAMETERS         top

              Can be used to set a different hash table size, available
              from kernel 2.6.39 onwards.  The specified divisor must be
              a power of two and cannot be larger than 65536.  Default
              value: 1024.

       limit  Upper limit of the SFQ. Can be used to reduce the default
              length of 127 packets.  After linux-3.3, it can be raised.

       depth  Limit of packets per flow (after linux-3.3). Default to
              127 and can be lowered.

              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 or losses. Advised value: 60 This value
              has no effect when external flow classification is used.
              Its better to increase divisor value to lower risk of hash

              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.

       flows  After linux-3.3, it is possible to change the default
              limit of flows.  Default value is 127

              Default SFQ behavior is to perform tail-drop of packets
              from a flow.  You can ask a headdrop instead, as this is
              known to provide a better feedback for TCP flows.

              Configure the optional RED module on top of each SFQ flow.
              Random Early Detection principle is to perform packet
              marks or drops in a probabilistic way.  (man tc-red for
              details about RED)
              redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.

       min    Average queue size at which marking becomes a possibility.
              Defaults to max /3

       max    At this average queue size, the marking probability is
              maximal. Defaults to redflowlimit /4

              Maximum  probability  for  marking, specified as a
              floating point number from 0.0 to 1.0. Default value is

       avpkt  Specified in bytes. Used with burst to determine the time
              constant for average queue size calculations. Default
              value is 1000

       burst  Used for determining how fast the average queue size is
              influenced by the real queue size.
              Default value is :
              (2 * min + max) / (3 * avpkt)

       ecn    RED can either 'mark' or 'drop'. Explicit Congestion
              Notification allows RED to notify remote hosts that their
              rate exceeds the amount of bandwidth available. Non-ECN
              capable hosts can only be notified by dropping a packet.
              If this parameter is specified, packets which indicate
              that their hosts honor ECN will only be marked and not
              dropped, unless the queue size hits depth packets.

              If average flow queue size is above max bytes, this
              parameter forces a drop instead of ecn marking.

EXAMPLE & USAGE         top

       To attach to device ppp0:

       # tc qdisc add dev ppp0 root sfq

       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

       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.

       It is possible to use external classifiers with sfq, for example
       to hash traffic based only on source/destination ip addresses:

       # tc filter add ... flow hash keys src,dst perturb 30 divisor

       Note that the given divisor should match the one used by sfq. If
       you have changed the sfq default of 1024, use the same value for
       the flow hash filter, too.

       Example of sfq with optional RED mode :

       # tc qdisc add dev eth0 parent 1:1 handle 10: sfq limit 3000
       flows 512 divisor 16384
         redflowlimit 100000 min 8000 max 60000 probability 0.20 ecn

SOURCE         top

       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,

       o      See also: M. Shreedhar and George Varghese "Efficient Fair
              Queuing using Deficit Round Robin", Proc. SIGCOMM 95.

SEE ALSO         top

       tc(8), tc-red(8)

AUTHORS         top

       Alexey N. Kuznetsov, <>, Eric Dumazet

       This manpage maintained by bert hubert <>

COLOPHON         top

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iproute2                     24 January 2012                       TC(8)

Pages that refer to this page: ovs-vswitchd.conf.db(5)tc(8)tc-drr(8)tc-flow(8)tc-sfb(8)