TC(8)				     Linux				 TC(8)



NAME
       tbf - Token Bucket Filter

SYNOPSIS
       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  min-
       burst.

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

       TBF is a pure shaper and never schedules traffic. It  is	 non-work-con-
       serving	and  may  throttle  itself, although packets are available, to
       ensure that the configured rate is not exceeded.	 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.


ALGORITHM
       As  the	name  implies, traffic is filtered based on the expenditure of
       tokens.	Tokens roughly correspond to bytes, with the  additional  con-
       straint	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 calculates 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 configured to limit the speed at which the bucket  emp-
       ties.  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.


PARAMETERS
       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 available. You can also specify this the	 other
	      way around by setting the latency parameter, 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 maximum 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 mini-
	      mum buffer size can be calculated by dividing the rate by HZ.

	      Token  usage  calculations  are performed using a table which by
	      default has a resolution 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 (specified 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:


       peakrate
	      Maximum depletion rate of the bucket. The peakrate does not need
	      to be set, it is only necessary if perfect millisecond timescale
	      shaping is required.


       mtu/minburst
	      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  acceptable, 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.

EXAMPLE & USAGE
       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 handle 10: root tbf rate 0.5mbit \
	 burst 5kb latency 70ms peakrate 1mbit	     \
	 minburst 1540

       To attach an inner qdisc, for example sfq, issue:

       # tc qdisc add dev eth0 parent 10:1 handle 100: sfq

       Without inner qdisc TBF queue acts as bfifo.  If	 the  inner  qdisc  is
       changed the limit/latency is not effective anymore.

SEE ALSO
       tc(8)


AUTHOR
       Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
       bert hubert <ahu@ds9a.nl>



iproute2		       13 December 2001				 TC(8)