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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 prevent‐
ing 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 inter‐
vals 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 prob‐
lem, 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 per‐
formed 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. Af‐
ter 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 red‐‐
flowlimit /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 capa‐
ble 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 band‐
width. 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 de‐
fault 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 Experi‐
ence", v.2, 1991, p.113-131.

o See also: M. Shreedhar and George Varghese "Efficient Fair Queuing using Deficit Round
Robin", Proc. SIGCOMM 95.