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 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 throt-
tle 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.
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 cal-
culates 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 config-
ured 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 avail-
able. You can also specify this the other way around by setting the latency parame-
ter, 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 maxi-
mum 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 resolu-
tion 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 (speci-
fied 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:
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 mil-
lisecond 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 accept-
able, 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 \
Alexey N. Kuznetsov, <email@example.com>. This manpage maintained by bert hubert
iproute2 13 December 2001 TC(8)