(i) Consult filters attached to the class. If sent to a leafnode, we are done. Otherwise, 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).
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. 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 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 smoothes out measurements into a moving average. The EWMA LOG determines how much smoothing 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 underlying link. Actual shaping is done in classes.
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 number 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 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.
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, numbered 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' 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 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 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.
