CBQ(8) Linux CBQ(8)
CBQ - Class Based Queueing
tc qdisc ... dev dev ( parent classid | root) [ handle major: ] cbq avpkt bytes bandwidth
rate [ cell bytes ] [ ewma log ] [ mpu bytes ]
tc class ... dev dev parent major:[minor] [ classid major:minor ] cbq allot bytes [ band-
width rate ] [ rate rate ] prio priority [ weight weight ] [ minburst packets ] [ maxburst
packets ] [ ewma log ] [ cell bytes ] avpkt bytes [ mpu bytes ] [ bounded isolated ] [
split handle & defmap defmap ] [ estimator interval timeconstant ]
Class Based Queueing is a classful qdisc that implements a rich linksharing hierarchy of
classes. It contains shaping elements as well as prioritizing capabilities. Shaping is
performed using link idle time calculations based on the timing of dequeue events and
underlying link bandwidth.
Shaping is done using link idle time calculations, and actions taken if these calculations
deviate from set limits.
When shaping a 10mbit/s connection to 1mbit/s, the link will be idle 90% of the time. If
it isn't, it needs to be throttled so that it IS idle 90% of the time.
From the kernel's perspective, this is hard to measure, so CBQ instead derives the idle
time from the number of microseconds (in fact, jiffies) that elapse between requests from
the device driver for more data. Combined with the knowledge of packet sizes, this is
used to approximate how full or empty the link is.
This is rather circumspect and doesn't always arrive at proper results. For example, what
is the actual link speed of an interface that is not really able to transmit the full
100mbit/s of data, perhaps because of a badly implemented driver? A PCMCIA network card
will also never achieve 100mbit/s because of the way the bus is designed - again, how do
we calculate the idle time?
The physical link bandwidth may be ill defined in case of not-quite-real network devices
like PPP over Ethernet or PPTP over TCP/IP. The effective bandwidth in that case is proba-
bly determined by the efficiency of pipes to userspace - which not defined.
During operations, the effective idletime is measured using an exponential weighted moving
average (EWMA), which considers recent packets to be exponentially more important than
past ones. The Unix loadaverage is calculated in the same way.
The calculated idle time is subtracted from the EWMA measured one, the resulting number is
called 'avgidle'. A perfectly loaded link has an avgidle of zero: packets arrive exactly
at the calculated interval.
An overloaded link has a negative avgidle and if it gets too negative, CBQ throttles and
is then 'overlimit'.
Conversely, an idle link might amass a huge avgidle, which would then allow infinite band-
widths after a few hours of silence. To prevent this, avgidle is capped at maxidle.
If overlimit, in theory, the CBQ could throttle itself for exactly the amount of time that
was calculated to pass between packets, and then pass one packet, and throttle again. Due
to timer resolution constraints, this may not be feasible, see the minburst parameter
Within the one CBQ instance many classes may exist. Each of these classes contains another
qdisc, by default tc-pfifo(8).
When enqueueing a packet, CBQ starts at the root and uses various methods to determine
which class should receive the data. If a verdict is reached, this process is repeated for
the recipient class which might have further means of classifying traffic to its children,
CBQ has the following methods available to classify a packet to any child classes.
(i) skb->priority class encoding. Can be set from userspace by an application with the
SO_PRIORITY setsockopt. The skb->priority class encoding only applies if the
skb->priority holds a major:minor handle of an existing class within this qdisc.
(ii) tc filters attached to the class.
(iii) The defmap of a class, as set with the split & defmap parameters. The defmap may
contain instructions for each possible Linux packet priority.
Each class also has a level. Leaf nodes, attached to the bottom of the class hierarchy,
have a level of 0.
Classification is a loop, which terminates when a leaf class is found. At any point the
loop may jump to the fallback algorithm.
The loop consists of the following steps:
(i) If the packet is generated locally and has a valid classid encoded within its
skb->priority, choose it and terminate.
(ii) Consult the tc filters, if any, attached to this child. If these return a class
which is not a leaf class, restart loop from the class returned. If it is a leaf,
choose it and terminate.
(iii) If the tc filters did not return a class, but did return a classid, try to find a
class with that id within this qdisc. Check if the found class is of a lower level
than the current class. If so, and the returned class is not a leaf node, restart
the loop at the found class. If it is a leaf node, terminate. If we found an
upward reference to a higher level, enter the fallback algorithm.
(iv) If the tc filters did not return a class, nor a valid reference to one, consider
the minor number of the reference to be the priority. Retrieve a class from the
defmap of this class for the priority. If this did not contain a class, consult the
defmap of this class for the BEST_EFFORT class. If this is an upward reference, or
no BEST_EFFORT class was defined, enter the fallback algorithm. If a valid class
was found, and it is not a leaf node, restart the loop at this class. If it is a
leaf, choose it and terminate. If neither the priority distilled from the classid,
nor the BEST_EFFORT priority yielded a class, enter the fallback algorithm.
The fallback algorithm resides outside of the loop and is as follows.
(i) Consult the defmap of the class at which the jump to fallback occured. If the
defmap contains a class for the priority of the class (which is related to the TOS
field), choose this class and terminate.
(ii) Consult the map for a class for the BEST_EFFORT priority. If found, choose it, and
(iii) Choose the class at which break out to the fallback algorithm occurred. Terminate.
The packet is enqueued to the class which was chosen when either algorithm terminated. It
is therefore possible for a packet to be enqueued *not* at a leaf node, but in the middle
of the hierarchy.
LINK SHARING ALGORITHM
When dequeuing for sending to the network device, CBQ decides which of its classes will be
allowed to send. It does so with a Weighted Round Robin process in which each class with
packets gets a chance to send in turn. The WRR process starts by asking the highest prior-
ity classes (lowest numerically - highest semantically) for packets, and will continue to
do so until they have no more data to offer, in which case the process repeats for lower
CERTAINTY ENDS HERE, ANK PLEASE HELP
Each class is not allowed to send at length though - they can only dequeue a configurable
amount of data during each round.
If a class is about to go overlimit, and it is not bounded it will try to borrow avgidle
from siblings that are not isolated. This process is repeated from the bottom upwards. If
a class is unable to borrow enough avgidle to send a packet, it is throttled and not asked
for a packet for enough time for the avgidle to increase above zero.
I REALLY NEED HELP FIGURING THIS OUT. REST OF DOCUMENT IS PRETTY CERTAIN AGAIN.
The root qdisc of a CBQ class tree has the following parameters:
parent major:minor | root
This mandatory parameter determines the place of the CBQ instance, either at the
root of an interface or within an existing class.
Like all other qdiscs, the CBQ can be assigned a handle. Should consist only of a
major number, followed by a colon. Optional.
For calculations, the average packet size must be known. It is silently capped at a
minimum of 2/3 of the interface MTU. Mandatory.
To determine the idle time, CBQ must know the bandwidth of your underlying physical
interface, or parent qdisc. This is a vital parameter, more about it later. Manda-
cell The cell size determines he granularity of packet transmission time calculations.
Has a sensible default.
mpu A zero sized packet may still take time to transmit. This value is the lower cap
for packet transmission time calculations - packets smaller than this value are
still deemed to have this size. Defaults to zero.
When CBQ needs to measure the average idle time, it does so using an Exponentially
Weighted Moving Average which smoothes out measurements into a moving average. The
EWMA LOG determines how much smoothing occurs. Defaults to 5. Lower values imply
greater sensitivity. Must be between 0 and 31.
A CBQ qdisc does not shape out of its own accord. It only needs to know certain parameters
about the underlying link. Actual shaping is done in classes.
Classes have a host of parameters to configure their operation.
Place of this class within the hierarchy. If attached directly to a qdisc and not
to another class, minor can be omitted. Mandatory.
Like qdiscs, classes can be named. The major number must be equal to the major num-
ber of the qdisc to which it belongs. Optional, but needed if this class is going
to have children.
When dequeuing to the interface, classes are tried for traffic in a round-robin
fashion. Classes with a higher configured qdisc will generally have more traffic to
offer during each round, so it makes sense to allow it to dequeue more traffic. All
weights under a class are normalized, so only the ratios matter. Defaults to the
configured rate, unless the priority of this class is maximal, in which case it is
set to 1.
Allot specifies how many bytes a qdisc can dequeue during each round of the
process. This parameter is weighted using the renormalized class weight described
In the round-robin process, classes with the lowest priority field are tried for
packets first. Mandatory.
Maximum rate this class and all its children combined can send at. Mandatory.
This is different from the bandwidth specified when creating a CBQ disc. Only used
to determine maxidle and offtime, which are only calculated when specifying
maxburst or minburst. Mandatory if specifying maxburst or minburst.
This number of packets is used to calculate maxidle so that when avgidle is at
maxidle, this number of average packets can be burst before avgidle drops to 0. Set
it higher to be more tolerant of bursts. You can't set maxidle directly, only via
As mentioned before, CBQ needs to throttle in case of overlimit. The ideal solution
is to do so for exactly the calculated idle time, and pass 1 packet. However, Unix
kernels generally have a hard time scheduling events shorter than 10ms, so it is
better to throttle for a longer period, and then pass minburst packets in one go,
and then sleep minburst times longer.
The time to wait is called the offtime. Higher values of minburst lead to more
accurate shaping in the long term, but to bigger bursts at millisecond timescales.
If avgidle is below 0, we are overlimits and need to wait until avgidle will be big
enough to send one packet. To prevent a sudden burst from shutting down the link
for a prolonged period of time, avgidle is reset to minidle if it gets too low.
Minidle is specified in negative microseconds, so 10 means that avgidle is capped
Signifies that this class will not borrow bandwidth from its siblings.
Means that this class will not borrow bandwidth to its siblings
split major:minor & defmap bitmap[/bitmap]
If consulting filters attached to a class did not give a verdict, CBQ can also
classify based on the packet's priority. There are 16 priorities available, num-
bered from 0 to 15.
The defmap specifies which priorities this class wants to receive, specified as a
bitmap. The Least Significant Bit corresponds to priority zero. The split parameter
tells CBQ at which class the decision must be made, which should be a (grand)parent
of the class you are adding.
As an example, 'tc class add ... classid 10:1 cbq .. split 10:0 defmap c0' config-
ures class 10:0 to send packets with priorities 6 and 7 to 10:1.
The complimentary configuration would then be: 'tc class add ... classid 10:2 cbq
... split 10:0 defmap 3f' Which would send all packets 0, 1, 2, 3, 4 and 5 to 10:1.
estimator interval timeconstant
CBQ can measure how much bandwidth each class is using, which tc filters can use to
classify packets with. In order to determine the bandwidth it uses a very simple
estimator that measures once every interval microseconds how much traffic has
passed. This again is a EWMA, for which the time constant can be specified, also in
microseconds. The time constant corresponds to the sluggishness of the measurement
or, conversely, to the sensitivity of the average to short bursts. Higher values
mean less sensitivity.
o Sally Floyd and Van Jacobson, "Link-sharing and Resource Management Models for
Packet Networks", IEEE/ACM Transactions on Networking, Vol.3, No.4, 1995
o Sally Floyd, "Notes on CBQ and Guarantee Service", 1995
o Sally Floyd, "Notes on Class-Based Queueing: Setting Parameters", 1996
o Sally Floyd and Michael Speer, "Experimental Results for Class-Based Queueing",
1998, not published.
Alexey N. Kuznetsov, <firstname.lastname@example.org>. This manpage maintained by bert hubert
iproute2 8 December 2001 CBQ(8)