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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 minburst.

DESCRIPTION

       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
       throttle 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.

ALGORITHM

       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
       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 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.

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 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
              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. Limited to  1mbit/s  on  Intel,  10mbit/s  on
              Alpha.  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 root tbf rate 0.5mbit \
         burst 5kb latency 70ms peakrate 1mbit       \
         minburst 1540

SEE ALSO

       tc(8)

AUTHOR

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