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Linux 2.6 - man page for tc-tbf (linux section 8)

TC(8)					      Linux					    TC(8)

       tbf - Token Bucket Filter

       tc  qdisc  ...  tbf  rate rate burst bytes/cell ( latency ms | limit bytes ) [ mpu bytes [
       peakrate rate mtu bytes/cell ] ]

       burst is also known as buffer and maxburst. mtu is also known as minburst.

       The Token Bucket Filter is a classless queueing discipline available for  traffic  control
       with the tc(8) command.

       TBF is a pure shaper and never schedules traffic. It is non-work-conserving and may throt-
       tle itself, although packets are available, to ensure that  the	configured  rate  is  not
       exceeded.   On all platforms except for Alpha, it is able to shape up to 1mbit/s of normal
       traffic with ideal minimal burstiness, sending out  data exactly at the configured rates.

       Much higher rates are possible but at the cost of losing the minimal burstiness.  In  that
       case,  data  is	on average dequeued at the configured rate but may be sent much faster at
       millisecond timescales. Because of further queues living  in  network  adaptors,  this  is
       often not a problem.

       Kernels	with a higher 'HZ' can achieve higher rates with perfect burstiness. On Alpha, HZ
       is ten times higher, leading to a 10mbit/s limit to perfection.	These  calculations  hold
       for packets of on average 1000 bytes.

       As  the	name  implies,	traffic  is  filtered based on the expenditure of tokens.  Tokens
       roughly correspond to bytes, with the additional constraint that each packet consumes some
       tokens,	no  matter  how small it is. This reflects the fact that even a zero-sized packet
       occupies the link for some time.

       On creation, the TBF is stocked with tokens which correspond to the amount of traffic that
       can be burst in one go. Tokens arrive at a steady rate, until the bucket is full.

       If no tokens are available, packets are queued, up to a configured limit. The TBF now cal-
       culates the token deficit, and throttles until the first packet in the queue can be sent.

       If it is not acceptable to burst out packets at maximum speed, a peakrate can  be  config-
       ured  to  limit	the  speed at which the bucket empties. This peakrate is implemented as a
       second TBF with a very small bucket, so that it doesn't burst.

       To achieve perfection, the second bucket may contain only a single packet, which leads  to
       the earlier mentioned 1mbit/s limit.

       This  limit  is	caused	by  the  fact  that the kernel can only throttle for at minimum 1
       'jiffy', which depends on HZ as 1/HZ. For perfect shaping, only a single  packet  can  get
       sent  per  jiffy - for HZ=100, this means 100 packets of on average 1000 bytes each, which
       roughly corresponds to 1mbit/s.

       See tc(8) for how to specify the units of these values.

       limit or latency
	      Limit is the number of bytes that can be queued waiting for tokens to become avail-
	      able. You can also specify this the other way around by setting the latency parame-
	      ter, which specifies the maximum amount of time a packet can sit in  the	TBF.  The
	      latter calculation takes into account the size of the bucket, the rate and possibly
	      the peakrate (if set). These two parameters are mutually exclusive.

       burst  Also known as buffer or maxburst.  Size of the bucket, in bytes. This is the  maxi-
	      mum  amount of bytes that tokens can be available for instantaneously.  In general,
	      larger shaping rates require a larger buffer. For 10mbit/s on Intel,  you  need  at
	      least 10kbyte buffer if you want to reach your configured rate!

	      If  your buffer is too small, packets may be dropped because more tokens arrive per
	      timer tick than fit in your bucket.  The minimum buffer size can be  calculated  by
	      dividing the rate by HZ.

	      Token usage calculations are performed using a table which by default has a resolu-
	      tion of 8 packets.  This resolution can be changed by specifying the cell size with
	      the burst. For example, to specify a 6000 byte buffer with a 16 byte cell size, set
	      a burst of 6000/16. You will probably never have to set this. Must be  an  integral
	      power of 2.

       mpu    A  zero-sized packet does not use zero bandwidth. For ethernet, no packet uses less
	      than 64 bytes. The Minimum Packet Unit determines the minimal token  usage  (speci-
	      fied in bytes) for a packet. Defaults to zero.

       rate   The speed knob. See remarks above about limits! See tc(8) for units.

       Furthermore, if a peakrate is desired, the following parameters are available:

	      Maximum  depletion  rate	of  the  bucket. Limited to 1mbit/s on Intel, 10mbit/s on
	      Alpha. The peakrate does not need to be set, it is only necessary if  perfect  mil-
	      lisecond timescale shaping is required.

	      Specifies  the  size of the peakrate bucket. For perfect accuracy, should be set to
	      the MTU of the interface.  If a peakrate is needed, but some burstiness is  accept-
	      able,  this  size  can  be  raised.  A  3000 byte minburst allows around 3mbit/s of
	      peakrate, given 1000 byte packets.

	      Like the regular burstsize you can also specify a cell size.

       To attach a TBF with a sustained maximum rate of 0.5mbit/s, a  peakrate	of  1.0mbit/s,	a
       5kilobyte  buffer, with a pre-bucket queue size limit calculated so the TBF causes at most
       70ms of latency, with perfect peakrate behaviour, issue:

       # tc qdisc add dev eth0 root tbf rate 0.5mbit \
	 burst 5kb latency 70ms peakrate 1mbit	     \
	 minburst 1540


       Alexey N. Kuznetsov,  <kuznet@ms2.inr.ac.ru>.  This  manpage  maintained  by  bert  hubert

iproute2				 13 December 2001				    TC(8)

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