tc-sfq


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

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  the
       packets  hash value.  This hash value is either obtained from an exter-
       nal 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 algo-
       rithm may be perturbed at configurable intervals so that the unfairness
       lasts only for a short while. Perturbation may however cause some inad-
       vertent 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  avail-
              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.

       perturb
              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.

       quantum
              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

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

       redflowlimit
              Configure the optional RED module on top of each SFQ flow.  Ran-
              dom  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

       probability
              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  con-
              stant for average queue size calculations. Default value is 1000

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

EXAMPLE & USAGE
       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  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.

       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


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), tc-red(8)


AUTHORS
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