tc-tbf(8) - phpMan

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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 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 adap-
       tors, 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 traf-
       fic 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  imple-
       mented 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 per-
              fect 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 bursti-
              ness 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 AT ms2.ru>. This manpage maintained by bert hubert
       <ahu AT ds9a.nl>





iproute2                       13 December 2001                          TC(8)

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