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223 lines
7.0 KiB
Groff
223 lines
7.0 KiB
Groff
.TH TC 8 "24 January 2012" "iproute2" "Linux"
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.SH NAME
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sfq \- Stochastic Fairness Queueing
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.SH SYNOPSIS
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.B tc qdisc ...
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.B [ divisor
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hashtablesize
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.B ] [ limit
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packets
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.B ] [ perturb
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seconds
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.B ] [ quantum
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bytes
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.B ] [ flows
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number
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.B ] [ depth
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number
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.B ] [ headdrop
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.B ] [ redflowlimit
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bytes
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.B ] [ min
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bytes
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.B ] [ max
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bytes
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.B ] [ avpkt
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bytes
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.B ] [ burst
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packets
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.B ] [ probability
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P
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.B ] [ ecn
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.B ] [ harddrop ]
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.SH DESCRIPTION
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Stochastic Fairness Queueing is a classless queueing discipline available for
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traffic control with the
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.BR tc (8)
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command.
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SFQ does not shape traffic but only schedules the transmission of packets, based on 'flows'.
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The goal is to ensure fairness so that each flow is able to send data in turn, thus preventing
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any single flow from drowning out the rest.
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This may in fact have some effect in mitigating a Denial of Service attempt.
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SFQ is work-conserving and therefore always delivers a packet if it has one available.
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.SH ALGORITHM
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On enqueueing, each packet is assigned to a hash bucket, based on the packets hash value.
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This hash value is either obtained from an external flow classifier (use
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.B
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tc filter
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to set them), or a default internal classifier if no external classifier has been configured.
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When the internal classifier is used, sfq uses
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.TP
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(i)
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Source address
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.TP
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(ii)
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Destination address
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.TP
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(iii)
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Source and Destination port
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.P
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If these are available. SFQ knows about ipv4 and ipv6 and also UDP, TCP and ESP.
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Packets with other protocols are hashed based on the 32bits representation of their
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destination and source. A flow corresponds mostly to a TCP/IP connection.
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Each of these buckets should represent a unique flow. Because multiple flows may
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get hashed to the same bucket, sfqs internal hashing algorithm may be perturbed at configurable
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intervals so that the unfairness lasts only for a short while. Perturbation may
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however cause some inadvertent packet reordering to occur. After linux-3.3, there is
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no packet reordering problem, but possible packet drops if rehashing hits one limit
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(number of flows or packets per flow)
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When dequeuing, each hashbucket with data is queried in a round robin fashion.
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Before linux-3.3, the compile time maximum length of the SFQ is 128 packets, which can be spread over
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at most 128 buckets of 1024 available. In case of overflow, tail-drop is performed
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on the fullest bucket, thus maintaining fairness.
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After linux-3.3, maximum length of SFQ is 65535 packets, and divisor limit is 65536.
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In case of overflow, tail-drop is performed on the fullest bucket, unless headdrop was requested.
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.SH PARAMETERS
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.TP
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divisor
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Can be used to set a different hash table size, available from kernel 2.6.39 onwards.
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The specified divisor must be a power of two and cannot be larger than 65536.
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Default value: 1024.
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.TP
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limit
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Upper limit of the SFQ. Can be used to reduce the default length of 127 packets.
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After linux-3.3, it can be raised.
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.TP
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depth
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Limit of packets per flow (after linux-3.3). Default to 127 and can be lowered.
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.TP
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perturb
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Interval in seconds for queue algorithm perturbation. Defaults to 0, which means that
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no perturbation occurs. Do not set too low for each perturbation may cause some packet
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reordering or losses. Advised value: 60
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This value has no effect when external flow classification is used.
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Its better to increase divisor value to lower risk of hash collisions.
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.TP
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quantum
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Amount of bytes a flow is allowed to dequeue during a round of the round robin process.
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Defaults to the MTU of the interface which is also the advised value and the minimum value.
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.TP
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flows
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After linux-3.3, it is possible to change the default limit of flows.
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Default value is 127
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.TP
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headdrop
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Default SFQ behavior is to perform tail-drop of packets from a flow.
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You can ask a headdrop instead, as this is known to provide a better feedback for TCP flows.
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.TP
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redflowlimit
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Configure the optional RED module on top of each SFQ flow.
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Random Early Detection principle is to perform packet marks or drops in a probabilistic way.
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(man tc-red for details about RED)
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.nf
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redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.
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.fi
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.TP
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min
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Average queue size at which marking becomes a possibility. Defaults to
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.B max
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/3
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.TP
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max
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At this average queue size, the marking probability is maximal. Defaults to
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.B redflowlimit
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/4
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.TP
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probability
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Maximum probability for marking, specified as a floating point number from 0.0 to 1.0. Default value is 0.02
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.TP
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avpkt
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Specified in bytes. Used with burst to determine the time constant for average queue size calculations. Default value is 1000
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.TP
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burst
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Used for determining how fast the average queue size is influenced by the real queue size.
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.nf
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Default value is :
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.B (2 * min + max) / (3 * avpkt)
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.fi
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.TP
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ecn
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RED can either 'mark' or 'drop'. Explicit Congestion
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Notification allows RED to notify remote hosts that their rate exceeds the
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amount of bandwidth available. Non-ECN capable hosts can only be notified by
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dropping a packet. If this parameter is specified, packets which indicate
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that their hosts honor ECN will only be marked and not dropped, unless the
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queue size hits
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.B depth
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packets.
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.TP
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harddrop
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If average flow queue size is above
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.B max
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bytes, this parameter forces a drop instead of ecn marking.
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.SH EXAMPLE & USAGE
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To attach to device ppp0:
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.P
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# tc qdisc add dev ppp0 root sfq
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.P
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Please note that SFQ, like all non-shaping (work-conserving) qdiscs, is only useful
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if it owns the queue.
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This is the case when the link speed equals the actually available bandwidth. This holds
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for regular phone modems, ISDN connections and direct non-switched ethernet links.
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.P
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Most often, cable modems and DSL devices do not fall into this category. The same holds
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for when connected to a switch and trying to send data to a congested segment also
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connected to the switch.
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.P
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In this case, the effective queue does not reside within Linux and is therefore not
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available for scheduling.
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.P
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Embed SFQ in a classful qdisc to make sure it owns the queue.
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It is possible to use external classifiers with sfq, for example to hash traffic based only
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on source/destination ip addresses:
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.P
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# tc filter add ... flow hash keys src,dst perturb 30 divisor 1024
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.P
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Note that the given divisor should match the one used by sfq. If you have
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changed the sfq default of 1024, use the same value for the flow hash filter, too.
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.P
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Example of sfq with optional RED mode :
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.P
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# tc qdisc add dev eth0 parent 1:1 handle 10: sfq limit 3000 flows 512 divisor 16384
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redflowlimit 100000 min 8000 max 60000 probability 0.20 ecn headdrop
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.SH SOURCE
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.TP
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o
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Paul E. McKenney "Stochastic Fairness Queuing",
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IEEE INFOCOMM'90 Proceedings, San Francisco, 1990.
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.TP
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o
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Paul E. McKenney "Stochastic Fairness Queuing",
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"Interworking: Research and Experience", v.2, 1991, p.113-131.
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.TP
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o
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See also:
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M. Shreedhar and George Varghese "Efficient Fair
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Queuing using Deficit Round Robin", Proc. SIGCOMM 95.
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.SH SEE ALSO
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.BR tc (8),
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.BR tc-red (8)
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.SH AUTHORS
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Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,
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Eric Dumazet <eric.dumazet@gmail.com>.
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.P
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This manpage maintained by bert hubert <ahu@ds9a.nl>
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