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Annotated edit history of tc-sfq(8) version 2, including all changes. View license author blame.
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1 perry 1 TC
2 !!!TC
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 ALGORITHM
7 PARAMETERS
8 EXAMPLE
9 SOURCE
10 SEE ALSO
11 AUTHOR
12 ----
13 !!NAME
14
15
16 sfq - Stochastic Fairness Queueing
17 !!SYNOPSIS
18
19
20 __tc qdisc ... perturb__ seconds __quantum__
21 bytes
22 !!DESCRIPTION
23
24
25 Stochastic Fairness Queueing is a classless queueing
26 discipline available for traffic control with the
27 tc(8) command.
28
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30 SFQ does not shape traffic but only schedules the
31 transmission of packets, based on 'flows'. The goal is to
32 ensure fairness so that each flow is able to send data in
33 turn, thus preventing any single flow from drowning out the
34 rest.
35
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37 This may in fact have some effect in mitigating a Denial of
38 Service attempt.
39
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41 SFQ is work-conserving and therefore always delivers a
42 packet if it has one available.
43 !!ALGORITHM
44
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46 On enqueueing, each packet is assigned to a hash bucket,
47 based on
48
49
50 (i)
51
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53 Source address
54
55
56 (ii)
57
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59 Destination address
60
61
62 (iii)
63
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65 Source port
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68 If these are available. SFQ knows about ipv4 and ipv6 and
69 also UDP, TCP and ESP. Packets with other protocols are
70 hashed based on the 32bits representation of their
71 destination and the socket they belong to. A flow
72 corresponds mostly to a TCP/IP connection.
73
74
75 Each of these buckets should represent a unique flow.
76 Because multiple flows may get hashed to the same bucket,
77 the hashing algorithm is perturbed at configurable intervals
78 so that the unfairness lasts only for a short while.
79 Perturbation may however cause some inadvertent packet
80 reordering to occur.
81
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83 When dequeuing, each hashbucket with data is queried in a
84 round robin fashion.
85
86
87 The compile time maximum length of the SFQ is 128 packets,
88 which can be spread over at most 128 buckets of 1024
89 available. In case of overflow, tail-drop is performed on
90 the fullest bucket, thus maintaining fairness.
91 !!PARAMETERS
92
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94 perturb
95
96
97 Interval in seconds for queue algorithm perturbation.
98 Defaults to 0, which means that no perturbation occurs. Do
99 not set too low for each perturbation may cause some packet
100 reordering. Advised value: 10
101
102
103 quantum
104
105
106 Amount of bytes a flow is allowed to dequeue during a round
107 of the round robin process. Defaults to the MTU of the
108 interface which is also the advised value and the minimum
109 value.
110 !!EXAMPLE !!
111
112
113 To attach to device ppp0:
114
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116 # tc qdisc add dev ppp0 root sfq perturb 10
117
118
119 Please note that SFQ, like all non-shaping (work-conserving)
120 qdiscs, is only useful if it owns the queue. This is the
121 case when the link speed equals the actually available
122 bandwidth. This holds for regular phone modems, ISDN
123 connections and direct non-switched ethernet
124 links.
125
126
127 Most often, cable modems and DSL devices do not fall into
128 this category. The same holds for when connected to a switch
129 and trying to send data to a congested segment also
130 connected to the switch.
131
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133 In this case, the effective queue does not reside within
134 Linux and is therefore not available for
135 scheduling.
136
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138 Embed SFQ in a classful qdisc to make sure it owns the
139 queue.
140 !!SOURCE
141
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143 o
144
145
2 perry 146 Paul E. !McKenney
1 perry 147
148
149 o
150
151
2 perry 152 Paul E. !McKenney
1 perry 153
154
155 o
156
157
158 See also: M. Shreedhar and George Varghese
159 !!SEE ALSO
160
161
162 tc(8)
163 !!AUTHOR
164
165
166 Alexey N. Kuznetsov,
167 ----
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