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TBRCONFIG(8) NetBSD System Manager's Manual TBRCONFIG(8)
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tbrconfig -- configure a token bucket regulator for an output queue
tbrconfig interface [tokenrate [bucketsize]]
tbrconfig -d interface
tbrconfig configures a token bucket regulator for the output network
interface queue. A token bucket regulator limits both the average amount
and instantaneous amount of packets that the underlying driver can
dequeue from the network interface within the kernel.
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 sub-
tracted 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. Limit-
ing 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 trans-
mission 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 exam-
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
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
To configure a token bucket regulator for the interface en0 with 10Mbps
token rate and 8KB bucket size,
# tbrconfig en0 10M 8K
To rate-limit the interface en0 up to 3Mbps,
# tbrconfig en0 3M auto
The tbrconfig command first appeared in WIDE/KAME IPv6 protocol stack kit
as part of ALTQ tools.
NetBSD 5.0 July 25, 2000 NetBSD 5.0