On 8/2/06, Lars Eggert <lars.eggert@xxxxxxxxxxxxx> wrote:
Hi, we finally have the results of our voice quality experiments over DCCP written up: http://larseggert.de/tmp/2006-dccp-voip-quality.pdf We'd appreciate any feedback you may have on this! Thanks, Lars -- Lars Eggert NEC Network Laboratories
Lars, Finally read through this in detail and have some feedback. In the first paragraph of the introduction you use "looses" where it should be "loses". I noticed in section IV.A you say no experimental results are available where 0ms delay. I have had problems on Linux also with no delay (well it was 0.26 ms) in that throughput fluctuates significantly. Is this also what you were experiencing? I am wondering whether CCID3 has real problems when delay is very low. In other tests when I have 1.2 msec delay due to added hop the problem goes away. In section IV.B you note that the packet size is significantly smaller than the maximum sized packet size. In section 5.3 of RFC4342 (CCID3) it says that the packet size s can be set, or the MSS can be used (which is what it appears yours does) or the implementation can work out the average packet size. In Linux we allow socket options to set s for the TFRC calculation which helps with packet sizes being substantially smaller than the MSS. However it won't help solve the problem that occurs due to idle periods as you describe later on - but only help for continuous streams. I too agree strongly with the parts about TFRC being passed upon TCP-Reno which is not used in modern stacks in it's original form. When I use iperf on modern TCP variants I see significantly higher throughput than the TCP throughput equation predicts. This does mean that TFRC is using less than it's fair share as you say. Is the source code freely available for your modified ttcp and also for your calculation of r? If it is I would like to be able to look at the code to see if it can be of assistance in my research. This is a great paper overall. Regards, 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
