I am running squid 3 on Fedora 10. Something is causing an unreasonable number of timeouts when clients access high definition videos on youtube through the proxy.
I am not trying to cache or otherwise manipulate the youtube content. The symptom is that the video always starts transferring OK, but sometimes it hangs partway through. Wireshark initially showed that there was a problem with tcp window scaling. Changing the linux config to reduce the max window size or even disable scaling got rid of most of the timeouts, but the frequency of timeouts when using the proxy still greatly exceeds the frequency of timeouts when the proxy is not used. When I don't use the proxy, I see maybe one or two timeouts in fifty downloads. Using the proxy, I see maybe 25 timeouts in 50 downloads, i.e. half the time it doesn't work. I took a look at http://inodes.org/blog/2006/09/06/tcp-window-scaling-and-kernel-2617/ for discussion of the window scaling issue and http://squidproxy.wordpress.com/2007/06/05/thinsg-to-look-at-if-websites-are-hanging/ for some general hanging discussion. I don't think that ECN or MTU issues are the culprit here, though I will do a bit more investigation. Does anyone have any other bright ideas about why youtube in particular might present this problem? And only with HD videos; the normal definition videos all seem to load fine (15 timeouts in 340 trials). Thanks in advance, - harv
