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Re: A question about priorities



On Mon, Aug 07, 2000 at 07:56:03PM +0200, Nikhil Bansal wrote:
> We expected now that connection 1 should receive close to 100% of the 
> data, and connection 2 should not receive any data. 
> But it turns that connection 1 gets only 55% of the data and connection
> still gets 45% of the data (this is the case when the total bandwidth
> is small, just 10Mb/s.)
> This 55-45% reading is repeatedly the case for all the runs of our experiment.
> 
> A further puzzling thing is that when we used UDP instead of TCP,
>  the higher priority connection was able to get 100% of the bandwidth.
> 
> Any explanations as to why are things not working out as they should with TCP.

TCP limits its rate based on incoming acks. So you really need to make sure
that the ACKs coming in from the other end have the same priority. Normally
this should be easy when the back channel is not congested, but e.g. where
you're using half duplex ethernet that would not be true (as the 10Mbit/s
figure suggests) and the ACKs would fight with the traffic, causing 
collisions.

Note that TCP also does not echo TOS.


-Andi