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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 [ 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 ]
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 un-
derlying 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 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 be-
low.
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. Other-
wise, restart.
(ii) Consult the defmap for the priority assigned to this packet, which depends on the
TOS bits. Check if the referral is leafless, otherwise restart.
(iii) Ask the defmap for instructions for the 'best effort' priority. Check the answer
for leafness, otherwise restart.
(iv) If none of the above returned with an instruction, enqueue at this node.
This algorithm makes sure that a packet always ends up somewhere, even while you are busy
building your configuration.
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 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
priorities.
Classes by default borrow bandwidth from their siblings. A class can be prevented from do-
ing 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, followed 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 parent qdisc. This is a vital parameter, more about it later. Manda-
tory.
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.
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 smoothing occurs. Lower values imply greater sensitiv-
ity. 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 underlying link. Actual shaping is done in classes.
CLASSES
Classes have a host of parameters to configure their operation.
parent major:minor
Place of this class within the hierarchy. If attached directly to a qdisc and not
to another class, minor can be omitted. Mandatory.
classid major:minor
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.
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 matter. 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. Mandatory.
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 ac-
curate 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.
bounded
Signifies that this class will not borrow bandwidth from its siblings.
isolated
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.
BUGS
The actual bandwidth of the underlying link may not be known, for example in the case of
PPoE or PPTP connections 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, <kuznet AT ms2.ru>. This manpage maintained by bert hubert
<ahu AT ds9a.nl>
iproute2 16 December 2001 CBQ(8)
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