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+.TH TC 8 "24 January 2012" "iproute2" "Linux"
+.SH NAME
+sfq \- Stochastic Fairness Queueing
+.SH SYNOPSIS
+.B tc qdisc ...
+.B [ divisor
+hashtablesize
+.B ] [ limit
+packets
+.B ] [ perturb
+seconds
+.B ] [ quantum
+bytes
+.B ] [ flows
+number
+.B ] [ depth
+number
+.B ] [ headdrop
+.B ] [ redflowlimit
+bytes
+.B ] [ min
+bytes
+.B ] [ max
+bytes
+.B ] [ avpkt
+bytes
+.B ] [ burst
+packets
+.B ] [ probability
+P
+.B ] [ ecn
+.B ] [ harddrop ]
+.SH DESCRIPTION
+
+Stochastic Fairness Queueing is a classless queueing discipline available for
+traffic control with the
+.BR 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.
+.SH 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
+.B
+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
+.TP
+(i)
+Source address
+.TP
+(ii)
+Destination address
+.TP
+(iii)
+Source and Destination port
+.P
+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.
+
+.SH PARAMETERS
+.TP
+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.
+.TP
+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.
+.TP
+depth
+Limit of packets per flow (after linux-3.3). Default to 127 and can be lowered.
+.TP
+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.
+.TP
+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.
+.TP
+flows
+After linux-3.3, it is possible to change the default limit of flows.
+Default value is 127
+.TP
+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.
+.TP
+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)
+.nf
+redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.
+.fi
+.TP
+min
+Average queue size at which marking becomes a possibility. Defaults to
+.B max
+/3
+.TP
+max
+At this average queue size, the marking probability is maximal. Defaults to
+.B redflowlimit
+/4
+.TP
+probability
+Maximum  probability  for  marking, specified as a floating point number from 0.0 to 1.0. Default value is 0.02
+.TP
+avpkt
+Specified in bytes. Used with burst to determine the time constant for average queue size calculations. Default value is 1000
+.TP
+burst
+Used for determining how fast the average queue size is influenced by the real queue size.
+.nf
+Default value is :
+.B (2 * min + max) / (3 * avpkt)
+.fi
+.TP
+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
+.B depth
+packets.
+.TP
+harddrop
+If average flow queue size is above
+.B max
+bytes, this parameter forces a drop instead of ecn marking.
+.SH EXAMPLE & USAGE
+
+To attach to device ppp0:
+.P
+# tc qdisc add dev ppp0 root sfq
+.P
+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.
+.P
+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.
+.P
+In this case, the effective queue does not reside within Linux and is therefore not
+available for scheduling.
+.P
+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:
+.P
+# tc filter add ... flow hash keys src,dst perturb 30 divisor 1024
+.P
+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.
+
+.P
+Example of sfq with optional RED mode :
+.P
+# 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
+
+.SH SOURCE
+.TP
+o
+Paul E. McKenney "Stochastic Fairness Queuing",
+IEEE INFOCOMM'90 Proceedings, San Francisco, 1990.
+
+.TP
+o
+Paul E. McKenney "Stochastic Fairness Queuing",
+"Interworking: Research and Experience", v.2, 1991, p.113-131.
+
+.TP
+o
+See also:
+M. Shreedhar and George Varghese "Efficient Fair
+Queuing using Deficit Round Robin", Proc. SIGCOMM 95.
+
+.SH SEE ALSO
+.BR tc (8),
+.BR tc-red (8)
+
+.SH AUTHORS
+Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,
+Eric Dumazet <eric.dumazet@gmail.com>.
+.P
+This manpage maintained by bert hubert <ahu@ds9a.nl>