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-.TH TC 8 "13 December 2001" "iproute2" "Linux"
-.SH NAME
-tbf \- Token Bucket Filter
-.SH SYNOPSIS
-.B tc qdisc ... tbf rate
-rate
-.B burst
-bytes/cell
-.B ( latency
-ms
-.B | limit
-bytes
-.B ) [ mpu
-bytes
-.B [ peakrate
-rate
-.B mtu
-bytes/cell
-.B ] ]
-.P
-burst is also known as buffer and maxburst. mtu is also known as minburst.
-.SH DESCRIPTION
-
-The Token Bucket Filter is a classless queueing discipline available for
-traffic control with the
-.BR 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.
-
-.SH ALGORITHM
-As the name implies, traffic is filtered based on the expenditure of
-.B 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.
-
-.SH PARAMETERS
-See
-.BR tc (8)
-for how to specify the units of these values.
-.TP
-limit or latency
-Limit is the number of bytes that can be queued waiting for tokens to become
-available. You can also specify this the other way around by setting the
-latency parameter, 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.
-.TP
-burst
-Also known as buffer or maxburst.
-Size of the bucket, in bytes. This is the maximum 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 resolution of 8 packets.
-This resolution can be changed by specifying the
-.B 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.
-.TP
-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 (specified in bytes) for a packet. Defaults to zero.
-.TP
-rate
-The speed knob. See remarks above about limits! See
-.BR tc (8)
-for units.
-.PP
-Furthermore, if a peakrate is desired, the following parameters are available:
-
-.TP
-peakrate
-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 millisecond timescale shaping is required.
-
-.TP
-mtu/minburst
-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 acceptable, 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
-.B cell
-size.
-.SH 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:
-.P
-# tc qdisc add dev eth0 root tbf rate 0.5mbit \\
- burst 5kb latency 70ms peakrate 1mbit \\
- minburst 1540
-
-.SH SEE ALSO
-.BR tc (8)
-
-.SH AUTHOR
-Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
-bert hubert <ahu@ds9a.nl>
-
-
git://git.onelab.eu / iproute2.git / blobdiff