(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.
(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 terminate.
(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.
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.
avpkt bytes For calculations, the average packet size must be known. It is silently capped at a minimum of 2/3 of the interface MTU. 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. 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.
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.
priority priority In the round-robin process, classes with the lowest priority field are tried for packets first. Mandatory.
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.
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.
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 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.
