Manual: tc-sfq

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 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 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 exter‐
nal 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 con‐
nection.

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 fair‐
ness.

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. After 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 redflowlimit /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. De‐
fault 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 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 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 bandwidth. This holds for regular phone modems, ISDN con‐
nections 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 default 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 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.

Manual: tc-sfq

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