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1 | .TH TC 8 "13 December 2001" "iproute2" "Linux" |
2 | .SH NAME | |
3 | tbf \- Token Bucket Filter | |
4 | .SH SYNOPSIS | |
5 | .B tc qdisc ... tbf rate | |
6 | rate | |
7 | .B burst | |
8 | bytes/cell | |
9 | .B ( latency | |
10 | ms | |
11 | .B | limit | |
12 | bytes | |
13 | .B ) [ mpu | |
14 | bytes | |
15 | .B [ peakrate | |
16 | rate | |
17 | .B mtu | |
18 | bytes/cell | |
19 | .B ] ] | |
20 | .P | |
21 | burst is also known as buffer and maxburst. mtu is also known as minburst. | |
22 | .SH DESCRIPTION | |
23 | ||
7bc7fcaa | 24 | The Token Bucket Filter is a classful queueing discipline available for |
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25 | traffic control with the |
26 | .BR tc (8) | |
27 | command. | |
28 | ||
29 | TBF is a pure shaper and never schedules traffic. It is non-work-conserving and may throttle | |
30 | itself, although packets are available, to ensure that the configured rate is not exceeded. | |
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31 | It is able to shape up to 1mbit/s of normal traffic with ideal minimal burstiness, |
32 | sending out data exactly at the configured rates. | |
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33 | |
34 | Much higher rates are possible but at the cost of losing the minimal burstiness. In that | |
35 | case, data is on average dequeued at the configured rate but may be sent much faster at millisecond | |
36 | timescales. Because of further queues living in network adaptors, this is often not a problem. | |
37 | ||
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38 | .SH ALGORITHM |
39 | As the name implies, traffic is filtered based on the expenditure of | |
40 | .B tokens. | |
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41 | Tokens roughly correspond to bytes, with the additional constraint |
42 | that each packet consumes some tokens, no matter how small it is. This | |
43 | reflects the fact that even a zero-sized packet occupies the link for | |
44 | some time. | |
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45 | |
46 | On creation, the TBF is stocked with tokens which correspond to the amount of traffic that can be burst | |
47 | in one go. Tokens arrive at a steady rate, until the bucket is full. | |
48 | ||
49 | If no tokens are available, packets are queued, up to a configured limit. The TBF now | |
50 | calculates the token deficit, and throttles until the first packet in the queue can be sent. | |
51 | ||
52 | If it is not acceptable to burst out packets at maximum speed, a peakrate can be configured | |
53 | to limit the speed at which the bucket empties. This peakrate is implemented as a second TBF | |
54 | with a very small bucket, so that it doesn't burst. | |
55 | ||
56 | To achieve perfection, the second bucket may contain only a single packet, which leads to | |
57 | the earlier mentioned 1mbit/s limit. | |
58 | ||
59 | This limit is caused by the fact that the kernel can only throttle for at minimum 1 'jiffy', which depends | |
60 | on HZ as 1/HZ. For perfect shaping, only a single packet can get sent per jiffy - for HZ=100, this means 100 | |
61 | packets of on average 1000 bytes each, which roughly corresponds to 1mbit/s. | |
62 | ||
63 | .SH PARAMETERS | |
64 | See | |
65 | .BR tc (8) | |
66 | for how to specify the units of these values. | |
67 | .TP | |
68 | limit or latency | |
69 | Limit is the number of bytes that can be queued waiting for tokens to become | |
70 | available. You can also specify this the other way around by setting the | |
71 | latency parameter, which specifies the maximum amount of time a packet can | |
72 | sit in the TBF. The latter calculation takes into account the size of the | |
73 | bucket, the rate and possibly the peakrate (if set). These two parameters | |
74 | are mutually exclusive. | |
75 | .TP | |
76 | burst | |
77 | Also known as buffer or maxburst. | |
78 | Size of the bucket, in bytes. This is the maximum amount of bytes that tokens can be available for instantaneously. | |
79 | In general, larger shaping rates require a larger buffer. For 10mbit/s on Intel, you need at least 10kbyte buffer | |
80 | if you want to reach your configured rate! | |
81 | ||
82 | If your buffer is too small, packets may be dropped because more tokens arrive per timer tick than fit in your bucket. | |
83 | The minimum buffer size can be calculated by dividing the rate by HZ. | |
84 | ||
85 | Token usage calculations are performed using a table which by default has a resolution of 8 packets. | |
86 | This resolution can be changed by specifying the | |
87 | .B cell | |
88 | size with the burst. For example, to specify a 6000 byte buffer with a 16 | |
89 | byte cell size, set a burst of 6000/16. You will probably never have to set | |
90 | this. Must be an integral power of 2. | |
91 | .TP | |
92 | mpu | |
93 | A zero-sized packet does not use zero bandwidth. For ethernet, no packet uses less than 64 bytes. The Minimum Packet Unit | |
94 | determines the minimal token usage (specified in bytes) for a packet. Defaults to zero. | |
95 | .TP | |
96 | rate | |
97 | The speed knob. See remarks above about limits! See | |
98 | .BR tc (8) | |
99 | for units. | |
100 | .PP | |
101 | Furthermore, if a peakrate is desired, the following parameters are available: | |
102 | ||
103 | .TP | |
104 | peakrate | |
a89d5329 | 105 | Maximum depletion rate of the bucket. The peakrate does not |
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106 | need to be set, it is only necessary if perfect millisecond timescale |
107 | shaping is required. | |
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108 | |
109 | .TP | |
110 | mtu/minburst | |
111 | Specifies the size of the peakrate bucket. For perfect accuracy, should be set to the MTU of the interface. | |
112 | If a peakrate is needed, but some burstiness is acceptable, this size can be raised. A 3000 byte minburst | |
113 | allows around 3mbit/s of peakrate, given 1000 byte packets. | |
114 | ||
115 | Like the regular burstsize you can also specify a | |
116 | .B cell | |
117 | size. | |
118 | .SH EXAMPLE & USAGE | |
119 | ||
120 | To attach a TBF with a sustained maximum rate of 0.5mbit/s, a peakrate of 1.0mbit/s, | |
121 | a 5kilobyte buffer, with a pre-bucket queue size limit calculated so the TBF causes | |
122 | at most 70ms of latency, with perfect peakrate behaviour, issue: | |
123 | .P | |
7bc7fcaa | 124 | # tc qdisc add dev eth0 handle 10: root tbf rate 0.5mbit \\ |
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125 | burst 5kb latency 70ms peakrate 1mbit \\ |
126 | minburst 1540 | |
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127 | .P |
128 | To attach an inner qdisc, for example sfq, issue: | |
129 | .P | |
130 | # tc qdisc add dev eth0 parent 10:1 handle 100: sfq | |
131 | .P | |
132 | Without inner qdisc TBF queue acts as bfifo. If the inner qdisc is changed | |
133 | the limit/latency is not effective anymore. | |
134 | .P | |
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135 | |
136 | .SH SEE ALSO | |
137 | .BR tc (8) | |
138 | ||
139 | .SH AUTHOR | |
140 | Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by | |
141 | bert hubert <ahu@ds9a.nl> | |
142 | ||
143 |