man/man8/tc-sfq.8 - platform/external/iproute2 - Gitiles

 .TH TC 8 "24 January 2012" "iproute2" "Linux"
 .SH NAME
 sfq \- Stochastic Fairness Queueing
 .SH SYNOPSIS
 .B tc qdisc ...
 .B [ divisor
 hashtablesize
 .B ] [ limit
 packets
 .B ] [ perturb
 seconds
 .B ] [ quantum
 bytes
 .B ] [ flows
 number
 .B ] [ depth
 number
 .B ] [ headdrop
 .B ] [ redflowlimit
 bytes
 .B ] [ min
 bytes
 .B ] [ max
 bytes
 .B ] [ avpkt
 bytes
 .B ] [ burst
 packets
 .B ] [ probability
 P
 .B ] [ ecn
 .B ] [ harddrop ]
 .SH DESCRIPTION

 Stochastic Fairness Queueing is a classless queueing discipline available for
 traffic control with the
 .BR 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.
 .SH 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
 .B
 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
 .TP
 (i)
 Source address
 .TP
 (ii)
 Destination address
 .TP
 (iii)
 Source and Destination port
 .P
 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.

 .SH PARAMETERS
 .TP
 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.
 .TP
 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.
 .TP
 depth
 Limit of packets per flow (after linux-3.3). Default to 127 and can be lowered.
 .TP
 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.
 .TP
 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.
 .TP
 flows
 After linux-3.3, it is possible to change the default limit of flows.
 Default value is 127
 .TP
 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.
 .TP
 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)
 .nf
 redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.
 .fi
 .TP
 min
 Average queue size at which marking becomes a possibility. Defaults to
 .B max
 /3
 .TP
 max
 At this average queue size, the marking probability is maximal. Defaults to
 .B redflowlimit
 /4
 .TP
 probability
 Maximum  probability  for  marking, specified as a floating point number from 0.0 to 1.0. Default value is 0.02
 .TP
 avpkt
 Specified in bytes. Used with burst to determine the time constant for average queue size calculations. Default value is 1000
 .TP
 burst
 Used for determining how fast the average queue size is influenced by the real queue size.
 .nf
 Default value is :
 .B (2 * min + max) / (3 * avpkt)
 .fi
 .TP
 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
 .B depth
 packets.
 .TP
 harddrop
 If average flow queue size is above
 .B max
 bytes, this parameter forces a drop instead of ecn marking.
 .SH EXAMPLE & USAGE

 To attach to device ppp0:
 .P
 # tc qdisc add dev ppp0 root sfq
 .P
 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.
 .P
 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.
 .P
 In this case, the effective queue does not reside within Linux and is therefore not
 available for scheduling.
 .P
 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:
 .P
 # tc filter add ... flow hash keys src,dst perturb 30 divisor 1024
 .P
 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.

 .P
 Example of sfq with optional RED mode :
 .P
 # 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

 .SH SOURCE
 .TP
 o
 Paul E. McKenney "Stochastic Fairness Queuing",
 IEEE INFOCOMM'90 Proceedings, San Francisco, 1990.

 .TP
 o
 Paul E. McKenney "Stochastic Fairness Queuing",
 "Interworking: Research and Experience", v.2, 1991, p.113-131.

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

 .SH SEE ALSO
 .BR tc (8),
 .BR tc-red (8)

 .SH AUTHORS
 Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,
 Eric Dumazet <eric.dumazet@gmail.com>.
 .P
 This manpage maintained by bert hubert <ahu@ds9a.nl>
	.TH TC 8 "24 January 2012" "iproute2" "Linux"
	.SH NAME
	sfq \- Stochastic Fairness Queueing
	.SH SYNOPSIS
	.B tc qdisc ...
	.B [ divisor
	hashtablesize
	.B ] [ limit
	packets
	.B ] [ perturb
	seconds
	.B ] [ quantum
	bytes
	.B ] [ flows
	number
	.B ] [ depth
	number
	.B ] [ headdrop
	.B ] [ redflowlimit
	bytes
	.B ] [ min
	bytes
	.B ] [ max
	bytes
	.B ] [ avpkt
	bytes
	.B ] [ burst
	packets
	.B ] [ probability
	P
	.B ] [ ecn
	.B ] [ harddrop ]
	.SH DESCRIPTION

	Stochastic Fairness Queueing is a classless queueing discipline available for
	traffic control with the
	.BR 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.
	.SH 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
	.B
	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
	.TP
	(i)
	Source address
	.TP
	(ii)
	Destination address
	.TP
	(iii)
	Source and Destination port
	.P
	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.

	.SH PARAMETERS
	.TP
	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.
	.TP
	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.
	.TP
	depth
	Limit of packets per flow (after linux-3.3). Default to 127 and can be lowered.
	.TP
	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.
	.TP
	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.
	.TP
	flows
	After linux-3.3, it is possible to change the default limit of flows.
	Default value is 127
	.TP
	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.
	.TP
	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)
	.nf
	redflowlimit configures the hard limit on the real (not average) queue size per SFQ flow in bytes.
	.fi
	.TP
	min
	Average queue size at which marking becomes a possibility. Defaults to
	.B max
	/3
	.TP
	max
	At this average queue size, the marking probability is maximal. Defaults to
	.B redflowlimit
	/4
	.TP
	probability
	Maximum probability for marking, specified as a floating point number from 0.0 to 1.0. Default value is 0.02
	.TP
	avpkt
	Specified in bytes. Used with burst to determine the time constant for average queue size calculations. Default value is 1000
	.TP
	burst
	Used for determining how fast the average queue size is influenced by the real queue size.
	.nf
	Default value is :
	.B (2 * min + max) / (3 * avpkt)
	.fi
	.TP
	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
	.B depth
	packets.
	.TP
	harddrop
	If average flow queue size is above
	.B max
	bytes, this parameter forces a drop instead of ecn marking.
	.SH EXAMPLE & USAGE

	To attach to device ppp0:
	.P
	# tc qdisc add dev ppp0 root sfq
	.P
	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.
	.P
	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.
	.P
	In this case, the effective queue does not reside within Linux and is therefore not
	available for scheduling.
	.P
	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:
	.P
	# tc filter add ... flow hash keys src,dst perturb 30 divisor 1024
	.P
	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.

	.P
	Example of sfq with optional RED mode :
	.P
	# 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

	.SH SOURCE
	.TP
	o
	Paul E. McKenney "Stochastic Fairness Queuing",
	IEEE INFOCOMM'90 Proceedings, San Francisco, 1990.

	.TP
	o
	Paul E. McKenney "Stochastic Fairness Queuing",
	"Interworking: Research and Experience", v.2, 1991, p.113-131.

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

	.SH SEE ALSO
	.BR tc (8),
	.BR tc-red (8)

	.SH AUTHORS
	Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,
	Eric Dumazet <eric.dumazet@gmail.com>.
	.P
	This manpage maintained by bert hubert <ahu@ds9a.nl>