стохастическая организация очередей справедливости (Stochastic Fairness Queueing)
Имя (Name)
sfq - Stochastic Fairness Queueing
Синопсис (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 ]
Описание (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.
Алгоритм (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 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 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.
Параметры (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. 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
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.
Примеры (Examples)
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
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
Источники (Sources)
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.
Смотри также (See also)
tc(8), tc-red(8)
