tc-sfq(8) - phpMan

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TC(8)                                         Linux                                         TC(8)

       sfq - Stochastic Fairness Queueing

       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 ]

       Stochastic Fairness Queueing is a classless queueing discipline available for traffic con-
       trol 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.

       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 config-
       urable 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  per-
       formed 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

              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 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  red-
              flowlimit /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 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.

       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  band-
       width.  This holds for regular phone modems, ISDN connections and direct non-switched eth-
       ernet 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 con-
       nected to the switch.

       In this case, the effective queue does not reside within Linux and is therefore not avail-
       able 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 headdrop

       o      Paul  E.  McKenney "Stochastic Fairness Queuing", IEEE INFOCOMM'90 Proceedings, San
              Francisco, 1990.

       o      Paul E. McKenney "Stochastic Fairness Queuing", "Interworking: Research and Experi-
              ence", v.2, 1991, p.113-131.

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

       tc(8), tc-red(8)

       Alexey N. Kuznetsov, <kuznet AT>, Eric Dumazet <eric.dumazet AT>.

       This manpage maintained by bert hubert <ahu AT>

iproute2                                 24 January 2012                                    TC(8)

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