Section: Linux (8)
Updated: 13 December 2001
tc qdisc ... tbf rate rate burst bytes/cell ( latency ms | limit bytes ) [ mpu bytes [ peakrate rate mtu bytes/cell ] ]
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.
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.
Furthermore, if a peakrate is desired, the following parameters are available:
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:
burst 5kb latency 70ms peakrate 1mbit
To attach an inner qdisc, for example sfq, issue:
Without inner qdisc TBF queue acts as bfifo. If the inner qdisc is changed the limit/latency is not effective anymore.
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