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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>