Gentoo seems to limit multicast speeds - why?

Malvineous

Hi everyone,

I'm trying to solve a problem with the dvbstream program. It tunes a digital TV broadcast and then multicasts it over the network so that other PCs can watch that TV channel. To cut a long story short, if I unicast the data to a single address it works fine (network bandwidth is 3MB/sec, same as the broadcast) but if I multicast the data to 224.0.1.2 then the network can only transmit at 1.7MB/sec, and the TV broadcast is too corrupted to watch (too many dropped packets.)

I tried the exact same release of dvbstream on a Slackware box with the same network card plugged into the same switch (but with an older kernel) and it worked fine (and has done for the past three years) however I have two Gentoo boxes (2006.0 and 2007.0) and neither of them have ever worked when using multicast.

Either the kernel or Gentoo seem to have enabled some sort of multicast rate limiting, but I've got no idea where to look for this. Does anyone know whether this is true, or have any ideas where I could start looking? "sysctl -a" only has a couple of rate limiting options and they're the same on the broken Gentoo and working Slackware machines, and since one of the Gentoo machines is new there are no iptables rules yet.

Any ideas would be much appreciated!

Jaglover · Posted: Mon Feb 25, 2008 3:35 pm Post subject:

You do have multicast enabled in kernel, do you?

Malvineous · Posted: Mon Feb 25, 2008 9:59 pm Post subject:

Yep, multicast is enabled, and so is multicast routing, although I'm not using that at the moment.

If I ping 224.0.0.1 from anywhere on the network then both the Slackware and Gentoo machines respond, so it seems to be working - I just have no idea why it would be limiting the multicast bandwidth.