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DESCRIPTION

Conceptually, tokens accumulate in a bucket at the average tokenrate, up to the bucketsize. The driver can dequeue packets as long as there are positive amount of tokens, and the length of the dequeued packet is subtracted from the remaining tokens. Tokens can be negative as a deficit, and packets are not dequeued from the interface queue until the tokens become positive again. The tokenrate limits the average rate, and the bucketsize limits the maximum burst size.

Limiting the burst size is essential to packet scheduling, since the scheduler schedules packets backlogged at the network interface. Limiting the burst size is also needed for drivers which dequeues more packets than they can send and end up with discarding excess packets.

When the tokenrate is set to higher than the actual transmission rate, the transmission complete interrupt will trigger the next dequeue. On the other hand, when the tokenrate is set to lower than the actual transmission rate, the transmission complete interrupt would occur before the tokens become positive. In this case, the next dequeue will be triggered by a timer event. Because the kernel timer has a limited granularity, a larger bucketsize is required for a higher tokenrate.

The interface parameter is a string of the form “name unit”, for example, “en0”.

The tokenrate parameter specifies the average rate in bits per second, and “K” or “M” can be appended to tokenrate as a short hand of “Kilo-bps” or “Mega-bps”, respectively. When tokenrate is omitted, tbrconfig displays the current parameter values.

The bucketsize parameter specifies the bucket size in bytes, and “K” can be appended to bucketsize as a short hand of “Kilo-bytes”. When bucketsize is omitted, tbrconfig assumes the regulator is driven by transmission complete interrupts and, using heuristics, assigns a small bucket size according to the tokenrate. When the keyword “auto” is given as bucketsize, tbrconfig assumes the regulator is driven by the kernel timer, and computes the bucket size from tokenrate and the kernel clock frequency.

If the -d flag is passed before an interface name, tbrconfig will remove the token bucket regulator for the specified interface.

Optionally, the -a flag may be used instead of an interface name. This flag instructs tbrconfig to display information about all interfaces in the system.

# tbrconfig en0 10M 8K

# tbrconfig en0 3M auto