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NAME | SYNOPSIS | DESCRIPTION | ALGORITHM | PARAMETERS | EXAMPLE & USAGE | SEE ALSO | AUTHOR | COLOPHON |
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TC(8) Linux TC(8)
tbf - Token Bucket Filter
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.
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.
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.
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. 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.
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.
tc(8)
Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage
maintained by bert hubert <ahu@ds9a.nl>
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-21.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 13 December 2001 TC(8)
Pages that refer to this page: tc(8), tc-htb(8), tc-prio(8)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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