|
NAME | SYNOPSIS | DESCRIPTION | ALGORITHM | PARAMETERS | EXAMPLE & USAGE | SEE ALSO | AUTHOR | COLOPHON |
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
⟨git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git⟩
on 2017-07-05. 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-netem(8), tc-prio(8)