Unable to send UDP packet of size 9400 bytes on Mac

[richardeakin]

We're not sure why, but we can't send a packet over UDP of 9400 bytes on Mac OS X, though it works fine on Windows. We are getting an error callback with error string "Message too long". AFAIK we should be able to send up to 65507, any idea why we can't?

You can test by modifying line 207 of UdpClientApp.cpp(dev branch) to be:

ci::Buffer buffer( 9400 );
mSession->write( buffer );

Maybe we are missing an option or setting?

[pizthewiz]

Doesn't the packet need to be smaller than the MTU? I'd have to think your config has jumbo frames enabled to even send packets of 9400 bytes, my wireless interface seems to be set to 1500:

$  networksetup -getMTU en0
Active MTU: 1500 (Current Setting: 1500)

[f0ster]

IP fragmentation should allow sending packets larger than the MTU, and up to the theoretical limit ~ 65k.

By default OSX won't allow a larger datagram size than 9216 bytes, however. You can change it as so

sudo sysctl -w net.inet.udp.maxdgram=65535

[richardeakin]

Yep, the above sysctl command solved the issue for us, so I'll close this.

[pizthewiz]

IP fragmentation will totally allow packets larger than the MTU, but UDP does not guarantee any ordering so it can be dangerous!