Re: [LARTC] RED vs. Tail Drop and docs

[bert hubert <ahu@ds9a.nl>]

On Fri, Dec 07, 2001 at 12:08:25PM -0500, Gregory Maxwell wrote:

Greg, good to see that ou are still alive :-) 

> A further problem is that Linux based shaper boxes are often not directly
> connected to the 'slow link', but instead there is a separate router which is
> taking in a much faster interface, performing it's own (often tail drop)
> queuing, and pushing out on the slow interface. This configuration is the
> biggest contributer to traffic control not behaving as expected.

You need to own the queue to shape it, yes, and in order to make this
happen, your linux box must be the slowest device in the chain.

This is why I am thinking of the 'stack' qdisc which would work like this:


     ___ 10:___
    /  /    \  \
   /   |    |   \
10:1 10:2 10:3 10:4
   <-   <-   <-
 
Enqueue()s always happen at the highest class, 10:4 in this case, dequeue()s
happen via 10:1, 10:2, 10:3 and 10:4. This would allow you to 'stack' RED,
SFQ and TBF, for example.

Haven't worked out how this would work in the kernel though. The question is
if we should 'push' or 'pull' - should a dequeue first always try to dequeue
10:4 to 10:3, and then from 10:3 to 10:2 etc etc.

What I really miss by the way is a way to poll devices for their queue
length, so 'downstream' devices would be able to own the queue so to speak.

Regards,

bert

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