TC(8)				     Linux				 TC(8)

NAME
       sfq - Stochastic Fairness Queueing

SYNOPSIS
       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
       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
       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
       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
       divisor
	      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.

       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
	      provide a better feedback for TCP flows.

       redflowlimit
	      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

       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
	      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.

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

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
       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 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
	      Queuing using Deficit Round Robin", Proc. SIGCOMM 95.


SEE ALSO
       tc(8), tc-red(8)


AUTHORS
       Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, Eric Dumazet
       <eric.dumazet@gmail.com>.

       This manpage maintained by bert hubert <ahu@ds9a.nl>

iproute2			24 January 2012				 TC(8)