tc-tbf


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 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-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.


       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 root tbf rate 0.5mbit \
         burst 5kb latency 70ms peakrate 1mbit       \
         minburst 1540


iproute2                       13 December 2001                          TC(8)
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