[LARTC] tbf and sfq interaction???

[Jon Zeeff <jzeeff@internet2.edu>]

How do sfq and tbf interact when packets need to be dropped?


Example:

two flows are entering the linux router, flow A is 1 mbit and flow B is 3 
mbits (total = 4).  We use tbf to limit outgoing bandwidth to 2 mbits 
(further downstream there is a bottleneck).

If we just used tbf, it would be dropping 50% of all packets - ie, flow A 
would see about 500K
of it's traffic getting through and flow B would see about 1.5mbits getting 
through.

This isn't "fair" - flow A should not have any of its packets dropped and 
flow B should be
limited to 1 mbit.    So we add sfq.

What's not clear to me is if this will accomplish what I want.  If the tbf 
algorithm occurs before
sfq, tbf will drop 50% of all packets.  After that, sfq will have no 
effect.  If the sfq algorithm
is performed first, then all 4 mbits of the resulting traffic is sent to 
tbf, then tbf will drop
packets from both flows.

It would seem that there needs to be some interaction between the 
algorithms - tbf needs to
signal to sqf that more packets can be sent.  But the packet dropping 
should occur in sqf (where fairness is understood) not in tbf.

So question:  will it work as I want or is a new qdisk needed?

note: I actually have hundreds of flows, so I don't want to classify them 
manually.  I also don't
want hard allocations of bandwidth.  If there is only flow B, 2 mbits of it 
should be allowed to pass.
Thanks.




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