I disagree with this reading of the RFC. The RFC explicitly allows the sender to calculate X as if every packet had size 's = MSS'. As usual, the implementation can then implement any simplifications such an assumption would allow. An implementation that assumes 's = MSS' can therefore calculate a sending rate in packets per second. Eddie
OK. Time for some maths with the TCP throughput equation. I am using an online version here backed up with testing with my modified DCCP stack - this matches on runs to within 10% of this equation. The online equation is here - http://wand.net.nz/~perry/max_download.php - in the examples below I have changed rtt to 250 ms and loss to 5 %. In this online calculator the value for s is put into MSS. The reason for this is we are assuming we are sending maximum size packets which may not be applicable for DCCP but is for TCP. So we vary the MSS to reflect s here. Now starting with a s of 1460 bytes we get a throughput (Xcalc) of 15221 bytes/sec. Now t_ipi is s/X_inst and X_inst on average is X_calc (it is just adjusted to prevent oscillation). So t_ipi is 96 msecs. Now change s to 300 bytes we get a Xcalc of 3128 and a t_ipi of 96 msecs. Put in ANY number for s and you will get the same t_ipi of 96 msecs. This illustrates my point that CCID3 is a packet per second based congestion control. Now if we specify the s is 300 to the implementation we get the desired result, if we average it we get the correct result. However we can't switch to a packet based equation because if we do the spec says we should use s=MSS. Lets assume MSS=1460. Now Xcalc is 15221 bytes/sec. t_ipi is 96 msecs. And we can send the correct rate of packets. OK Now working through the example shows that my logic was wrong. I guess thats what defence of ideas is about - proving it and my idea was wrong. I apologise for wasting everybody's time! However this does show an application can't cheat by specifying the wrong packet size which the spec worries about. Maybe my working through this helps resolve this a little... Ian -- Ian McDonald Web: http://wand.net.nz/~iam4 Blog: http://imcdnzl.blogspot.com WAND Network Research Group Department of Computer Science University of Waikato New Zealand - To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
