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 adaptors, 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 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
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 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
SEE ALSO
tc(8)
AUTHOR
Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
bert hubert <ahu@ds9a.nl>