[altq:1844] CBQ interna

[Daniel Breest <eljay@gmx.de>]

Hi,

i have two questions concering cbq interna.
First, i was wondering, why the cbq implementation uses not the existing
fifoq as queuing discipline for a class but a cbq internal fifo implementation.
That way, it is not easy, but nearly impossible to use a different elementary
queueing discipline, for example a token bucket queue to shape the flow that
is assigned to a cbq class. Well, i know that a class, that is not allowed
to borrow bandwidth, effective does the shaping of a flow too. But, i think,
for a guaranteed service flow, the allocated bandwidth can be higher than the
one, originally defined in a simple traffic profile due to the error terms of
a router. So, traffic will not be shaped to that profile but to the higher
rate. Therefore it needs a additional component, which also allows the shaping
against a more complex traffic profile (dual token bucket). Please correct
me, if this is not quite correct.   
What are the drawbacks, if the cbq or the hfsc scheduler works on a well
defined queuing interface, say the already existing functions altq_enqueue and
altq_dequeue, which are mapped to the discipline specific functions. Further,
one needs to provide a dropall function and a function that provides the
number of packets that are stored in the queuing discipline. A different way
would be, to implement enhanced functionality via the more flexible request
function. As far as i can see, one drawback is, that it is a lot of work.

Second, the chapter about cbq in the altq dissertation says, that when a
class becomes backlogged, it is placed in a list of active classes. The pointer
to the first active class for every priority level is stored in the "active"
array in the rm_ifdat structure. As far as i can see, a class will be added
to the active list in the rmc_newclass function and will never be removed from
this list, in case the class is no longer backlogged. Is this analysis
correct, and if so, why?

Any comments are appreciated.
--
Daniel Breest

-- 
contact: Daniel.Breest@breest.com

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