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

       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 configured.

       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

       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 collisions.

              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
              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 0.02

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

       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

       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 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.

       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 1024

       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, p.113-131.

       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)

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