tbf(8) — Linux manual page


TC(8)                               Linux                              TC(8)

NAME         top

       tbf - Token Bucket Filter

SYNOPSIS         top

       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

DESCRIPTION         top

       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-
       conserving 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         top

       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         top

       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

              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

       Furthermore, if a peakrate is desired, the following parameters are

              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.

              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         top

       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         top


AUTHOR         top

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

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
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iproute2                      13 December 2001                         TC(8)