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 [ FORMAT ] qdisc show [ dev DEV ]
20 tc [ FORMAT ] class show dev DEV
22 tc filter show dev DEV
24
26
29 Tc is used to configure Traffic Control in the Linux kernel. Traffic
30 Control consists of the following:
31 SHAPING
34 When traffic is shaped, its rate of transmission is under con‐
35 trol. Shaping may be more than lowering the available bandwidth
36 - it is also used to smooth out bursts in traffic for better
37 network behaviour. Shaping occurs on egress.
38 SCHEDULING
41 By scheduling the transmission of packets it is possible to
42 improve interactivity for traffic that needs it while still
43 guaranteeing bandwidth to bulk transfers. Reordering is also
44 called prioritizing, and happens only on egress.
45 POLICING
48 Where shaping deals with transmission of traffic, policing per‐
49 tains to traffic arriving. Policing thus occurs on ingress.
50 DROPPING
53 Traffic exceeding a set bandwidth may also be dropped forthwith,
54 both on ingress and on egress.
55 Processing of traffic is controlled by three kinds of objects: qdiscs,
58 classes and filters.
59
62 qdisc is short for 'queueing discipline' and it is elementary to under‐
63 standing traffic control. Whenever the kernel needs to send a packet to
64 an interface, it is enqueued to the qdisc configured for that inter‐
65 face. Immediately afterwards, the kernel tries to get as many packets
66 as possible from the qdisc, for giving them to the network adaptor
67 driver.
68 A simple QDISC is the 'pfifo' one, which does no processing at all and
70 is a pure First In, First Out queue. It does however store traffic when
71 the network interface can't handle it momentarily.
72
75 Some qdiscs can contain classes, which contain further qdiscs - traffic
76 may then be enqueued in any of the inner qdiscs, which are within the
77 classes. When the kernel tries to dequeue a packet from such a class‐
78 ful qdisc it can come from any of the classes. A qdisc may for example
79 prioritize certain kinds of traffic by trying to dequeue from certain
80 classes before others.
81
84 A filter is used by a classful qdisc to determine in which class a
85 packet will be enqueued. Whenever traffic arrives at a class with sub‐
86 classes, it needs to be classified. Various methods may be employed to
87 do so, one of these are the filters. All filters attached to the class
88 are called, until one of them returns with a verdict. If no verdict was
89 made, other criteria may be available. This differs per qdisc.
90 It is important to notice that filters reside within qdiscs - they are
92 not masters of what happens.
93
96 The classless qdiscs are:
97 [p|b]fifo
99 Simplest usable qdisc, pure First In, First Out behaviour. Lim‐
100 ited in packets or in bytes.
101 pfifo_fast
103 Standard qdisc for 'Advanced Router' enabled kernels. Consists
104 of a three-band queue which honors Type of Service flags, as
105 well as the priority that may be assigned to a packet.
106 red Random Early Detection simulates physical congestion by randomly
108 dropping packets when nearing configured bandwidth allocation.
109 Well suited to very large bandwidth applications.
110 sfq Stochastic Fairness Queueing reorders queued traffic so each
112 'session' gets to send a packet in turn.
113 tbf The Token Bucket Filter is suited for slowing traffic down to a
115 precisely configured rate. Scales well to large bandwidths.
116
118 In the absence of classful qdiscs, classless qdiscs can only be
119 attached at the root of a device. Full syntax:
120 tc qdisc add dev DEV root QDISC QDISC-PARAMETERS
122 To remove, issue
124 tc qdisc del dev DEV root
126 The pfifo_fast qdisc is the automatic default in the absence of a con‐
128 figured qdisc.
129
132 The classful qdiscs are:
133 CBQ Class Based Queueing implements a rich linksharing hierarchy of
135 classes. It contains shaping elements as well as prioritizing
136 capabilities. Shaping is performed using link idle time calcula‐
137 tions based on average packet size and underlying link band‐
138 width. The latter may be ill-defined for some interfaces.
139 HTB The Hierarchy Token Bucket implements a rich linksharing hierar‐
141 chy of classes with an emphasis on conforming to existing prac‐
142 tices. HTB facilitates guaranteeing bandwidth to classes, while
143 also allowing specification of upper limits to inter-class shar‐
144 ing. It contains shaping elements, based on TBF and can priori‐
145 tize classes.
146 PRIO The PRIO qdisc is a non-shaping container for a configurable
148 number of classes which are dequeued in order. This allows for
149 easy prioritization of traffic, where lower classes are only
150 able to send if higher ones have no packets available. To facil‐
151 itate configuration, Type Of Service bits are honored by
152 default.
153
155 Classes form a tree, where each class has a single parent. A class may
156 have multiple children. Some qdiscs allow for runtime addition of
157 classes (CBQ, HTB) while others (PRIO) are created with a static number
158 of children.
159 Qdiscs which allow dynamic addition of classes can have zero or more
161 subclasses to which traffic may be enqueued.
162 Furthermore, each class contains a leaf qdisc which by default has
164 pfifo behaviour though another qdisc can be attached in place. This
165 qdisc may again contain classes, but each class can have only one leaf
166 qdisc.
167 When a packet enters a classful qdisc it can be classified to one of
169 the classes within. Three criteria are available, although not all
170 qdiscs will use all three:
171 tc filters
173 If tc filters are attached to a class, they are consulted first
174 for relevant instructions. Filters can match on all fields of a
175 packet header, as well as on the firewall mark applied by
176 ipchains or iptables.
177 Type of Service
179 Some qdiscs have built in rules for classifying packets based on
180 the TOS field.
181 skb->priority
183 Userspace programs can encode a class-id in the 'skb->priority'
184 field using the SO_PRIORITY option.
185 Each node within the tree can have its own filters but higher level
187 filters may also point directly to lower classes.
188 If classification did not succeed, packets are enqueued to the leaf
190 qdisc attached to that class. Check qdisc specific manpages for
191 details, however.
192
195 All qdiscs, classes and filters have IDs, which can either be specified
196 or be automatically assigned.
197 IDs consist of a major number and a minor number, separated by a colon.
199 QDISCS A qdisc, which potentially can have children, gets assigned a
202 major number, called a 'handle', leaving the minor number names‐
203 pace available for classes. The handle is expressed as '10:'.
204 It is customary to explicitly assign a handle to qdiscs expected
205 to have children.
206 CLASSES
209 Classes residing under a qdisc share their qdisc major number,
210 but each have a separate minor number called a 'classid' that
211 has no relation to their parent classes, only to their parent
212 qdisc. The same naming custom as for qdiscs applies.
213 FILTERS
216 Filters have a three part ID, which is only needed when using a
217 hashed filter hierarchy.
218
220 All parameters accept a floating point number, possibly followed by a
221 unit.
222 Bandwidths or rates can be specified in:
224 kbps Kilobytes per second
226 mbps Megabytes per second
228 kbit Kilobits per second
230 mbit Megabits per second
232 bps or a bare number
234 Bytes per second
235 Amounts of data can be specified in:
237 kb or k
239 Kilobytes
240 mb or m
242 Megabytes
243 mbit Megabits
245 kbit Kilobits
247 b or a bare number
249 Bytes.
250 Lengths of time can be specified in:
252 s, sec or secs
254 Whole seconds
255 ms, msec or msecs
257 Milliseconds
258 us, usec, usecs or a bare number
260 Microseconds.
261
264 The following commands are available for qdiscs, classes and filter:
265 add Add a qdisc, class or filter to a node. For all entities, a par‐
267 ent must be passed, either by passing its ID or by attaching
268 directly to the root of a device. When creating a qdisc or a
269 filter, it can be named with the handle parameter. A class is
270 named with the classid parameter.
271 remove A qdisc can be removed by specifying its handle, which may also
274 be 'root'. All subclasses and their leaf qdiscs are automati‐
275 cally deleted, as well as any filters attached to them.
276 change Some entities can be modified 'in place'. Shares the syntax of
279 'add', with the exception that the handle cannot be changed and
280 neither can the parent. In other words, change cannot move a
281 node.
282 replace
285 Performs a nearly atomic remove/add on an existing node id. If
286 the node does not exist yet it is created.
287 link Only available for qdiscs and performs a replace where the node
290 must exist already.
291
294 The show command has additional formatting options:
295 -s, -stats, -statistics
298 output more statistics about packet usage.
299 -d, -details
302 output more detailed information about rates and cell sizes.
303 -r, -raw
306 output raw hex values for handles.
307 -p, -pretty
310 decode filter offset and mask values to equivalent filter com‐
311 mands based on TCP/IP.
312 -iec print rates in IEC units (ie. 1K = 1024).
315
319 tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
320
322 tc-cbq(8), tc-drr(8), tc-htb(8), tc-sfq(8), tc-red(8), tc-tbf(8), tc-
323 pfifo(8), tc-bfifo(8), tc-pfifo_fast(8),
324 User documentation at http://lartc.org/, but please direct bugreports
325 and patches to: <netdev@vger.kernel.org>
326
329 Manpage maintained by bert hubert (ahu@ds9a.nl)
330iproute2 16 December 2001 TC(8)