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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>
-
-
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