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 ] [ head-
drop ] [ redflowlimit bytes ] [ min bytes ] [ max bytes ] [ avpkt bytes
] [ burst packets ] [ probability P ] [ ecn ] [ harddrop ]

DESCRIPTION
Stochastic Fairness Queueing is a classless queueing discipline avail-
able for traffic control with the tc(8) command.

SFQ does not shape traffic but only schedules the transmission of pack-
ets, 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 at-
tempt.

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 exter-
nal 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 algo-
rithm may be perturbed at configurable intervals so that the unfairness
lasts only for a short while. Perturbation may however cause some inad-
vertent 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 avail-
able. In case of overflow, tail-drop is performed 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. 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 pro-
vide a better feedback for TCP flows.

redflowlimit
Configure the optional RED module on top of each SFQ flow. Ran-
dom 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. De-
faults 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 con-
stant for average queue size calculations. Default value is 1000

burst Used for determining how fast the average queue size is influ-
enced by the real queue size.
Default value is :
(2 * min + max) / (3 * avpkt)

ecn RED can either 'mark' or 'drop'. Explicit Congestion Notifica-
tion 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 connections and direct non-switched ethernet links.

Most often, cable modems and DSL devices do not fall into this cate-
gory. 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 Queu-
ing using Deficit Round Robin", Proc. SIGCOMM 95.

tc-sfq

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