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Manpages TCSection: Linux (8)Updated: 13 December 2001 Index Return to Main Contents NAMEtbf - Token Bucket FilterSYNOPSIStc 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.DESCRIPTIONThe 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. ALGORITHMAs 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. PARAMETERSSee tc(8) for how to specify the units of these values.
Furthermore, if a peakrate is desired, the following parameters are available:
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 \
SEE ALSOtc(8)AUTHORAlexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by bert hubert <ahu@ds9a.nl>
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