tc-sfq manpage

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TC(8)				     Linux				 TC(8)

       sfq - Stochastic Fairness Queueing

       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 ]

       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

       SFQ is work-conserving and therefore always delivers a packet if it has
       one available.

       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

       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.

	      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.

	      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.

	      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

	      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.

	      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
	      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

	      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.

	      If average flow queue size is above max  bytes,  this  parameter
	      forces a drop instead of ecn marking.

       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

       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(8), tc-red(8)

       Alexey	N.    Kuznetsov,    <kuznet@ms2.inr.ac.ru>,    Eric    Dumazet

       This manpage maintained by bert hubert <ahu@ds9a.nl>

iproute2			24 January 2012				 TC(8)