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+.TH TC 8 "8 December 2001" "iproute2" "Linux"
+.SH NAME
+sfq \- Stochastic Fairness Queueing
+.SH SYNOPSIS
+.B tc qdisc ... perturb
+seconds
+.B quantum
+bytes
+
+.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
+.TP
+(i)
+Source address
+.TP
+(ii)
+Destination address
+.TP
+(iii)
+Source 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 the socket they belong to. 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, the hashing algorithm is perturbed at configurable 
+intervals so that the unfairness lasts only for a short while. Perturbation may 
+however cause some inadvertent packet reordering to occur.
+
+When dequeuing, each hashbucket with data is queried in a round robin fashion.
+
+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.
+
+.SH PARAMETERS
+.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. Advised value: 10
+.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.
+
+.SH EXAMPLE & USAGE
+
+To attach to device ppp0:
+.P
+# tc qdisc add dev ppp0 root sfq perturb 10
+.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.
+
+.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)
+
+.SH AUTHOR
+Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
+bert hubert <ahu@ds9a.nl>
+
+