1CBQ(8) Linux CBQ(8)
2
6 CBQ - Class Based Queueing
7
9 tc qdisc ... dev dev ( parent classid | root) [ handle major: ] cbq
10 avpkt bytes bandwidth rate [ cell bytes ] [ ewma log ] [ mpu bytes ]
11 tc class ... dev dev parent major:[minor] [ classid major:minor ] cbq
13 allot bytes [ bandwidth rate ] [ rate rate ] prio priority [ weight
14 weight ] [ minburst packets ] [ maxburst packets ] [ ewma log ] [ cell
15 bytes ] avpkt bytes [ mpu bytes ] [ bounded isolated ] [ split handle &
16 defmap defmap ] [ estimator interval timeconstant ]
17
20 Class Based Queueing is a classful qdisc that implements a rich
21 linksharing hierarchy of classes. It contains shaping elements as well
22 as prioritizing capabilities. Shaping is performed using link idle time
23 calculations based on the timing of dequeue events and underlying link
24 bandwidth.
25
28 Shaping is done using link idle time calculations, and actions taken if
29 these calculations deviate from set limits.
30 When shaping a 10mbit/s connection to 1mbit/s, the link will be idle
32 90% of the time. If it isn't, it needs to be throttled so that it IS
33 idle 90% of the time.
34 From the kernel's perspective, this is hard to measure, so CBQ instead
36 derives the idle time from the number of microseconds (in fact,
37 jiffies) that elapse between requests from the device driver for more
38 data. Combined with the knowledge of packet sizes, this is used to
39 approximate how full or empty the link is.
40 This is rather circumspect and doesn't always arrive at proper results.
42 For example, what is the actual link speed of an interface that is not
43 really able to transmit the full 100mbit/s of data, perhaps because of
44 a badly implemented driver? A PCMCIA network card will also never
45 achieve 100mbit/s because of the way the bus is designed - again, how
46 do we calculate the idle time?
47 The physical link bandwidth may be ill defined in case of not-quite-
49 real network devices like PPP over Ethernet or PPTP over TCP/IP. The
50 effective bandwidth in that case is probably determined by the effi‐
51 ciency of pipes to userspace - which not defined.
52 During operations, the effective idletime is measured using an exponen‐
54 tial weighted moving average (EWMA), which considers recent packets to
55 be exponentially more important than past ones. The Unix loadaverage is
56 calculated in the same way.
57 The calculated idle time is subtracted from the EWMA measured one, the
59 resulting number is called 'avgidle'. A perfectly loaded link has an
60 avgidle of zero: packets arrive exactly at the calculated interval.
61 An overloaded link has a negative avgidle and if it gets too negative,
63 CBQ throttles and is then 'overlimit'.
64 Conversely, an idle link might amass a huge avgidle, which would then
66 allow infinite bandwidths after a few hours of silence. To prevent
67 this, avgidle is capped at maxidle.
68 If overlimit, in theory, the CBQ could throttle itself for exactly the
70 amount of time that was calculated to pass between packets, and then
71 pass one packet, and throttle again. Due to timer resolution con‐
72 straints, this may not be feasible, see the minburst parameter below.
73
76 Within the one CBQ instance many classes may exist. Each of these
77 classes contains another qdisc, by default tc-pfifo(8).
78 When enqueueing a packet, CBQ starts at the root and uses various meth‐
80 ods to determine which class should receive the data. If a verdict is
81 reached, this process is repeated for the recipient class which might
82 have further means of classifying traffic to its children, if any.
83 CBQ has the following methods available to classify a packet to any
85 child classes.
86 (i) skb->priority class encoding. Can be set from userspace by an
88 application with the SO_PRIORITY setsockopt. The skb->priority
89 class encoding only applies if the skb->priority holds a
90 major:minor handle of an existing class within this qdisc.
91 (ii) tc filters attached to the class.
93 (iii) The defmap of a class, as set with the split & defmap parame‐
95 ters. The defmap may contain instructions for each possible
96 Linux packet priority.
97 Each class also has a level. Leaf nodes, attached to the bottom of the
100 class hierarchy, have a level of 0.
101
103 Classification is a loop, which terminates when a leaf class is found.
104 At any point the loop may jump to the fallback algorithm.
105 The loop consists of the following steps:
107 (i) If the packet is generated locally and has a valid classid
109 encoded within its skb->priority, choose it and terminate.
110 (ii) Consult the tc filters, if any, attached to this child. If these
113 return a class which is not a leaf class, restart loop from the
114 class returned. If it is a leaf, choose it and terminate.
115 (iii) If the tc filters did not return a class, but did return a clas‐
117 sid, try to find a class with that id within this qdisc. Check
118 if the found class is of a lower level than the current class.
119 If so, and the returned class is not a leaf node, restart the
120 loop at the found class. If it is a leaf node, terminate. If we
121 found an upward reference to a higher level, enter the fallback
122 algorithm.
123 (iv) If the tc filters did not return a class, nor a valid reference
125 to one, consider the minor number of the reference to be the
126 priority. Retrieve a class from the defmap of this class for the
127 priority. If this did not contain a class, consult the defmap of
128 this class for the BEST_EFFORT class. If this is an upward ref‐
129 erence, or no BEST_EFFORT class was defined, enter the fallback
130 algorithm. If a valid class was found, and it is not a leaf
131 node, restart the loop at this class. If it is a leaf, choose it
132 and terminate. If neither the priority distilled from the clas‐
133 sid, nor the BEST_EFFORT priority yielded a class, enter the
134 fallback algorithm.
135 The fallback algorithm resides outside of the loop and is as follows.
137 (i) Consult the defmap of the class at which the jump to fallback
139 occured. If the defmap contains a class for the priority of the
140 class (which is related to the TOS field), choose this class and
141 terminate.
142 (ii) Consult the map for a class for the BEST_EFFORT priority. If
144 found, choose it, and terminate.
145 (iii) Choose the class at which break out to the fallback algorithm
147 occurred. Terminate.
148 The packet is enqueued to the class which was chosen when either algo‐
150 rithm terminated. It is therefore possible for a packet to be enqueued
151 *not* at a leaf node, but in the middle of the hierarchy.
152
155 When dequeuing for sending to the network device, CBQ decides which of
156 its classes will be allowed to send. It does so with a Weighted Round
157 Robin process in which each class with packets gets a chance to send in
158 turn. The WRR process starts by asking the highest priority classes
159 (lowest numerically - highest semantically) for packets, and will con‐
160 tinue to do so until they have no more data to offer, in which case the
161 process repeats for lower priorities.
162 CERTAINTY ENDS HERE, ANK PLEASE HELP
164 Each class is not allowed to send at length though - they can only
166 dequeue a configurable amount of data during each round.
167 If a class is about to go overlimit, and it is not bounded it will try
169 to borrow avgidle from siblings that are not isolated. This process is
170 repeated from the bottom upwards. If a class is unable to borrow enough
171 avgidle to send a packet, it is throttled and not asked for a packet
172 for enough time for the avgidle to increase above zero.
173 I REALLY NEED HELP FIGURING THIS OUT. REST OF DOCUMENT IS PRETTY CER‐
175 TAIN AGAIN.
176
179 The root qdisc of a CBQ class tree has the following parameters:
180 parent major:minor | root
183 This mandatory parameter determines the place of the CBQ
184 instance, either at the root of an interface or within an exist‐
185 ing class.
186 handle major:
188 Like all other qdiscs, the CBQ can be assigned a handle. Should
189 consist only of a major number, followed by a colon. Optional.
190 avpkt bytes
192 For calculations, the average packet size must be known. It is
193 silently capped at a minimum of 2/3 of the interface MTU. Manda‐
194 tory.
195 bandwidth rate
197 To determine the idle time, CBQ must know the bandwidth of your
198 underlying physical interface, or parent qdisc. This is a vital
199 parameter, more about it later. Mandatory.
200 cell The cell size determines he granularity of packet transmission
202 time calculations. Has a sensible default.
203 mpu A zero sized packet may still take time to transmit. This value
205 is the lower cap for packet transmission time calculations -
206 packets smaller than this value are still deemed to have this
207 size. Defaults to zero.
208 ewma log
210 When CBQ needs to measure the average idle time, it does so
211 using an Exponentially Weighted Moving Average which smooths out
212 measurements into a moving average. The EWMA LOG determines how
213 much smoothing occurs. Defaults to 5. Lower values imply greater
214 sensitivity. Must be between 0 and 31.
215 A CBQ qdisc does not shape out of its own accord. It only needs to know
217 certain parameters about the underlying link. Actual shaping is done in
218 classes.
219
222 Classes have a host of parameters to configure their operation.
223 parent major:minor
226 Place of this class within the hierarchy. If attached directly
227 to a qdisc and not to another class, minor can be omitted.
228 Mandatory.
229 classid major:minor
231 Like qdiscs, classes can be named. The major number must be
232 equal to the major number of the qdisc to which it belongs.
233 Optional, but needed if this class is going to have children.
234 weight weight
236 When dequeuing to the interface, classes are tried for traffic
237 in a round-robin fashion. Classes with a higher configured qdisc
238 will generally have more traffic to offer during each round, so
239 it makes sense to allow it to dequeue more traffic. All weights
240 under a class are normalized, so only the ratios matter.
241 Defaults to the configured rate, unless the priority of this
242 class is maximal, in which case it is set to 1.
243 allot bytes
245 Allot specifies how many bytes a qdisc can dequeue during each
246 round of the process. This parameter is weighted using the
247 renormalized class weight described above.
248 priority priority
251 In the round-robin process, classes with the lowest priority
252 field are tried for packets first. Mandatory.
253 rate rate
256 Maximum rate this class and all its children combined can send
257 at. Mandatory.
258 bandwidth rate
261 This is different from the bandwidth specified when creating a
262 CBQ disc. Only used to determine maxidle and offtime, which are
263 only calculated when specifying maxburst or minburst. Mandatory
264 if specifying maxburst or minburst.
265 maxburst
268 This number of packets is used to calculate maxidle so that when
269 avgidle is at maxidle, this number of average packets can be
270 burst before avgidle drops to 0. Set it higher to be more toler‐
271 ant of bursts. You can't set maxidle directly, only via this
272 parameter.
273 minburst
276 As mentioned before, CBQ needs to throttle in case of overlimit.
277 The ideal solution is to do so for exactly the calculated idle
278 time, and pass 1 packet. However, Unix kernels generally have a
279 hard time scheduling events shorter than 10ms, so it is better
280 to throttle for a longer period, and then pass minburst packets
281 in one go, and then sleep minburst times longer.
282 The time to wait is called the offtime. Higher values of min‐
284 burst lead to more accurate shaping in the long term, but to
285 bigger bursts at millisecond timescales.
286 minidle
289 If avgidle is below 0, we are overlimits and need to wait until
290 avgidle will be big enough to send one packet. To prevent a sud‐
291 den burst from shutting down the link for a prolonged period of
292 time, avgidle is reset to minidle if it gets too low.
293 Minidle is specified in negative microseconds, so 10 means that
295 avgidle is capped at -10us.
296 bounded
299 Signifies that this class will not borrow bandwidth from its
300 siblings.
301 isolated
303 Means that this class will not borrow bandwidth to its siblings
304 split major:minor & defmap bitmap[/bitmap]
307 If consulting filters attached to a class did not give a ver‐
308 dict, CBQ can also classify based on the packet's priority.
309 There are 16 priorities available, numbered from 0 to 15.
310 The defmap specifies which priorities this class wants to
312 receive, specified as a bitmap. The Least Significant Bit corre‐
313 sponds to priority zero. The split parameter tells CBQ at which
314 class the decision must be made, which should be a (grand)parent
315 of the class you are adding.
316 As an example, 'tc class add ... classid 10:1 cbq .. split 10:0
318 defmap c0' configures class 10:0 to send packets with priorities
319 6 and 7 to 10:1.
320 The complimentary configuration would then be: 'tc class add ...
322 classid 10:2 cbq ... split 10:0 defmap 3f' Which would send all
323 packets 0, 1, 2, 3, 4 and 5 to 10:1.
324 estimator interval timeconstant
326 CBQ can measure how much bandwidth each class is using, which tc
327 filters can use to classify packets with. In order to determine
328 the bandwidth it uses a very simple estimator that measures once
329 every interval microseconds how much traffic has passed. This
330 again is a EWMA, for which the time constant can be specified,
331 also in microseconds. The time constant corresponds to the slug‐
332 gishness of the measurement or, conversely, to the sensitivity
333 of the average to short bursts. Higher values mean less sensi‐
334 tivity.
335
340 o Sally Floyd and Van Jacobson, "Link-sharing and Resource Manage‐
341 ment Models for Packet Networks", IEEE/ACM Transactions on Net‐
342 working, Vol.3, No.4, 1995
343 o Sally Floyd, "Notes on CBQ and Guarantee Service", 1995
346 o Sally Floyd, "Notes on Class-Based Queueing: Setting Parame‐
349 ters", 1996
350 o Sally Floyd and Michael Speer, "Experimental Results for Class-
353 Based Queueing", 1998, not published.
354
359 tc(8)
360
363 Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>. This manpage maintained by
364 bert hubert <ahu@ds9a.nl>
365iproute2 8 December 2001 CBQ(8)