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TC(8) Linux TC(8)

NAME
sfq - Stochastic Fairness Queueing

SYNOPSIS
tc qdisc ... perturb seconds quantum bytes

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 fair-
ness 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

(i) Source address

(ii) Destination address

(iii) Source 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 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 hash-
ing 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 avail-
able. In case of overflow, tail-drop is performed on the fullest bucket, thus maintaining fairness.

PARAMETERS
limit Upper limit of the SFQ. Can be used to reduce the default length of 128 packets.

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

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.

EXAMPLE & USAGE
To attach to device ppp0:

# tc qdisc add dev ppp0 root sfq perturb 10

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.

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.