1TC(8) Linux TC(8)
2
6 tc - show / manipulate traffic control settings
7
9 tc qdisc [ add | change | replace | link ] dev DEV [ parent qdisc-id |
10 root ] [ handle qdisc-id ] qdisc [ qdisc specific parameters ]
11 tc class [ add | change | replace ] dev DEV parent qdisc-id [ classid
13 class-id ] qdisc [ qdisc specific parameters ]
14 tc filter [ add | change | replace ] dev DEV [ parent qdisc-id | root ]
16 protocol protocol prio priority filtertype [ filtertype specific param‐
17 eters ] flowid flow-id
18 tc [-s | -d ] qdisc show [ dev DEV ]
20 tc [-s | -d ] class show dev DEV
22 tc filter show dev DEV
24
27 Tc is used to configure Traffic Control in the Linux kernel. Traffic
28 Control consists of the following:
29 SHAPING
32 When traffic is shaped, its rate of transmission is under con‐
33 trol. Shaping may be more than lowering the available bandwidth
34 - it is also used to smooth out bursts in traffic for better
35 network behaviour. Shaping occurs on egress.
36 SCHEDULING
39 By scheduling the transmission of packets it is possible to
40 improve interactivity for traffic that needs it while still
41 guaranteeing bandwidth to bulk transfers. Reordering is also
42 called prioritizing, and happens only on egress.
43 POLICING
46 Where shaping deals with transmission of traffic, policing per‐
47 tains to traffic arriving. Policing thus occurs on ingress.
48 DROPPING
51 Traffic exceeding a set bandwidth may also be dropped forthwith,
52 both on ingress and on egress.
53 Processing of traffic is controlled by three kinds of objects: qdiscs,
56 classes and filters.
57
60 qdisc is short for 'queueing discipline' and it is elementary to under‐
61 standing traffic control. Whenever the kernel needs to send a packet to
62 an interface, it is enqueued to the qdisc configured for that inter‐
63 face. Immediately afterwards, the kernel tries to get as many packets
64 as possible from the qdisc, for giving them to the network adaptor
65 driver.
66 A simple QDISC is the 'pfifo' one, which does no processing at all and
68 is a pure First In, First Out queue. It does however store traffic when
69 the network interface can't handle it momentarily.
70
73 Some qdiscs can contain classes, which contain further qdiscs - traffic
74 may then be enqueued in any of the inner qdiscs, which are within the
75 classes. When the kernel tries to dequeue a packet from such a class‐
76 ful qdisc it can come from any of the classes. A qdisc may for example
77 prioritize certain kinds of traffic by trying to dequeue from certain
78 classes before others.
79
82 A filter is used by a classful qdisc to determine in which class a
83 packet will be enqueued. Whenever traffic arrives at a class with sub‐
84 classes, it needs to be classified. Various methods may be employed to
85 do so, one of these are the filters. All filters attached to the class
86 are called, until one of them returns with a verdict. If no verdict was
87 made, other criteria may be available. This differs per qdisc.
88 It is important to notice that filters reside within qdiscs - they are
90 not masters of what happens.
91
94 The classless qdiscs are:
95 [p|b]fifo
97 Simplest usable qdisc, pure First In, First Out behaviour. Lim‐
98 ited in packets or in bytes.
99 pfifo_fast
101 Standard qdisc for 'Advanced Router' enabled kernels. Consists
102 of a three-band queue which honors Type of Service flags, as
103 well as the priority that may be assigned to a packet.
104 red Random Early Detection simulates physical congestion by randomly
106 dropping packets when nearing configured bandwidth allocation.
107 Well suited to very large bandwidth applications.
108 sfq Stochastic Fairness Queueing reorders queued traffic so each
110 'session' gets to send a packet in turn.
111 tbf The Token Bucket Filter is suited for slowing traffic down to a
113 precisely configured rate. Scales well to large bandwidths.
114
116 In the absence of classful qdiscs, classless qdiscs can only be
117 attached at the root of a device. Full syntax:
118 tc qdisc add dev DEV root QDISC QDISC-PARAMETERS
120 To remove, issue
122 tc qdisc del dev DEV root
124 The pfifo_fast qdisc is the automatic default in the absence of a con‐
126 figured qdisc.
127
130 The classful qdiscs are:
131 CBQ Class Based Queueing implements a rich linksharing hierarchy of
133 classes. It contains shaping elements as well as prioritizing
134 capabilities. Shaping is performed using link idle time calcula‐
135 tions based on average packet size and underlying link band‐
136 width. The latter may be ill-defined for some interfaces.
137 HTB The Hierarchy Token Bucket implements a rich linksharing hierar‐
139 chy of classes with an emphasis on conforming to existing prac‐
140 tices. HTB facilitates guaranteeing bandwidth to classes, while
141 also allowing specification of upper limits to inter-class shar‐
142 ing. It contains shaping elements, based on TBF and can priori‐
143 tize classes.
144 PRIO The PRIO qdisc is a non-shaping container for a configurable
146 number of classes which are dequeued in order. This allows for
147 easy prioritization of traffic, where lower classes are only
148 able to send if higher ones have no packets available. To facil‐
149 itate configuration, Type Of Service bits are honored by
150 default.
151
153 Classes form a tree, where each class has a single parent. A class may
154 have multiple children. Some qdiscs allow for runtime addition of
155 classes (CBQ, HTB) while others (PRIO) are created with a static number
156 of children.
157 Qdiscs which allow dynamic addition of classes can have zero or more
159 subclasses to which traffic may be enqueued.
160 Furthermore, each class contains a leaf qdisc which by default has
162 pfifo behaviour though another qdisc can be attached in place. This
163 qdisc may again contain classes, but each class can have only one leaf
164 qdisc.
165 When a packet enters a classful qdisc it can be classified to one of
167 the classes within. Three criteria are available, although not all
168 qdiscs will use all three:
169 tc filters
171 If tc filters are attached to a class, they are consulted first
172 for relevant instructions. Filters can match on all fields of a
173 packet header, as well as on the firewall mark applied by
174 ipchains or iptables. See tc-filters(8).
175 Type of Service
177 Some qdiscs have built in rules for classifying packets based on
178 the TOS field.
179 skb->priority
181 Userspace programs can encode a class-id in the 'skb->priority'
182 field using the SO_PRIORITY option.
183 Each node within the tree can have its own filters but higher level
185 filters may also point directly to lower classes.
186 If classification did not succeed, packets are enqueued to the leaf
188 qdisc attached to that class. Check qdisc specific manpages for
189 details, however.
190
193 All qdiscs, classes and filters have IDs, which can either be specified
194 or be automatically assigned.
195 IDs consist of a major number and a minor number, separated by a colon.
197 QDISCS A qdisc, which potentially can have children, gets assigned a
200 major number, called a 'handle', leaving the minor number names‐
201 pace available for classes. The handle is expressed as '10:'.
202 It is customary to explicitly assign a handle to qdiscs expected
203 to have children.
204 CLASSES
207 Classes residing under a qdisc share their qdisc major number,
208 but each have a separate minor number called a 'classid' that
209 has no relation to their parent classes, only to their parent
210 qdisc. The same naming custom as for qdiscs applies.
211 FILTERS
214 Filters have a three part ID, which is only needed when using a
215 hashed filter hierarchy, for which see tc-filters(8).
216
218 All parameters accept a floating point number, possibly followed by a
219 unit.
220 Bandwidths or rates can be specified in:
222 kbps Kilobytes per second
224 mbps Megabytes per second
226 kbit Kilobits per second
228 mbit Megabits per second
230 bps or a bare number
232 Bytes per second
233 Amounts of data can be specified in:
235 kb or k
237 Kilobytes
238 mb or m
240 Megabytes
241 mbit Megabits
243 kbit Kilobits
245 b or a bare number
247 Bytes.
248 Lengths of time can be specified in:
250 s, sec or secs
252 Whole seconds
253 ms, msec or msecs
255 Milliseconds
256 us, usec, usecs or a bare number
258 Microseconds.
259
262 The following commands are available for qdiscs, classes and filter:
263 add Add a qdisc, class or filter to a node. For all entities, a par‐
265 ent must be passed, either by passing its ID or by attaching
266 directly to the root of a device. When creating a qdisc or a
267 filter, it can be named with the handle parameter. A class is
268 named with the classid parameter.
269 remove A qdisc can be removed by specifying its handle, which may also
272 be 'root'. All subclasses and their leaf qdiscs are automati‐
273 cally deleted, as well as any filters attached to them.
274 change Some entities can be modified 'in place'. Shares the syntax of
277 'add', with the exception that the handle cannot be changed and
278 neither can the parent. In other words, change cannot move a
279 node.
280 replace
283 Performs a nearly atomic remove/add on an existing node id. If
284 the node does not exist yet it is created.
285 link Only available for qdiscs and performs a replace where the node
288 must exist already.
289
293 tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
294
296 tc-cbq(8), tc-htb(8), tc-sfq(8), tc-red(8), tc-tbf(8), tc-pfifo(8), tc-
297 bfifo(8), tc-pfifo_fast(8), tc-filters(8)
298
301 Manpage maintained by bert hubert (ahu@ds9a.nl)
302iproute2 16 December 2001 TC(8)