CBQ(8) Linux CBQ(8)
NAME
CBQ - Class Based Queueing
SYNOPSIS
tc qdisc ... dev dev ( parent classid | root) [ handle major: ] cbq [ allot bytes ] avpkt bytes bandwidth rate
[ cell bytes ] [ ewma log ] [ mpu bytes ]
tc class ... dev dev parent major:[minor] [ classid major:minor ] cbq allot bytes [ bandwidth 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 timecon‐
stant ]
DESCRIPTION
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 ALGORITHM
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.
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 cal‐
culated 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 bandwidths 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 below.
CLASSIFICATION
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.
In the absence of uncommon configuration options, the process is rather easy. At each node we look for an
instruction, and then go to the class the instruction refers us to. If the class found is a barren leaf-node
(without children), we enqueue the packet there. If it is not yet a leaf node, we do the whole thing over
again starting from that node.
The following actions are performed, in order at each node we visit, until one sends us to another node, or
terminates the process.
(i) Consult filters attached to the class. If sent to a leafnode, we are done. Otherwise, restart.
For more details, see tc-cbq-details(8).
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 priority 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 priorities.
Classes by default borrow bandwidth from their siblings. A class can be prevented from doing so by declaring
it 'bounded'. A class can also indicate its unwillingness to lend out bandwidth by being 'isolated'.
QDISC
The root of a CBQ qdisc 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.
handle major:
Like all other qdiscs, the CBQ can be assigned a handle. Should consist only of a major number, fol‐
lowed by a colon. Optional, but very useful if classes will be generated within this qdisc.
allot bytes
This allotment is the 'chunkiness' of link sharing and is used for determining packet transmission time
tables. The qdisc allot differs slightly from the class allot discussed below. Optional. Defaults to a
reasonable value, related to avpkt.
avpkt bytes
The average size of a packet is needed for calculating maxidle, and is also used for making sure
'allot' has a safe value. Mandatory.
bandwidth rate
To determine the idle time, CBQ must know the bandwidth of your underlying physical interface, or par‐
ent qdisc. This is a vital parameter, more about it later. Mandatory.
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 transmis‐
sion time calculations - packets smaller than this value are still deemed to have this size. Defaults
to zero.
ewma log
When CBQ needs to measure the average idle time, it does so using an Exponentially Weighted Moving
Average which smooths out measurements into a moving average. The EWMA LOG determines how much smooth‐
ing occurs. Lower values imply greater sensitivity. Must be between 0 and 31. Defaults to 5.
A CBQ qdisc does not shape out of its own accord. It only needs to know certain parameters about the underly‐
ing link. Actual shaping is done in classes.
weight weight
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 mat‐
ter. Defaults to the configured rate, unless the priority of this class is maximal, in which case it is
set to 1.
allot bytes
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 above. Silently capped at a minimum of 3/2
avpkt. Mandatory.
prio priority
In the round-robin process, classes with the lowest priority field are tried for packets first. Manda‐
tory.
avpkt See the QDISC section.
rate rate
Maximum rate this class and all its children combined can send at. Mandatory.
bandwidth rate
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.
maxburst
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 this parameter.
minburst
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. Optional.
minidle
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 at -10us. Optional.
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' configures 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 mea‐
surement or, conversely, to the sensitivity of the average to short bursts. Higher values mean less
sensitivity.
BUGS
The actual bandwidth of the underlying link may not be known, for example in the case of PPoE or PPTP connec‐
tions which in fact may send over a pipe, instead of over a physical device. CBQ is quite resilient to major
errors in the configured bandwidth, probably a the cost of coarser shaping.
Default kernels rely on coarse timing information for making decisions. These may make shaping precise in the
long term, but inaccurate on second long scales.
See tc-cbq-details(8) for hints on how to improve this.
SOURCES
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 Guaranteed 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.
SEE ALSO
tc(8)
AUTHOR
Alexey N. Kuznetsov, <[email protected]>. This manpage maintained by bert hubert <[email protected]>
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