Jeff Barr's Blog

Things I Like..

Second Life and Windows Networking

In my quest to make sure that I can run the Second Life client where and when I want to, I’ve discovered another interesting network issue that sometimes gets in the way, and I’ve also discovered a way to get around it.

Some hotels and companies use a protocol known as GRE (Generic Route Encapsulation) to safely escort “guest” packets off the premises and onto the internet at large. Due to the fact that this protocol encapsulates lower level packets inside of its own, there’s a length limitation on those lower level packets. Therefore, it is necessary to limit the maximum length of the lower level packet so that it can fit inside the encapsulating packet, which has a maximum length of its own.

The proper term for the length (in octets) of the inner packet is the MTU, or Maximum Transmission Unit. By default this value is apparently 1500 for most modern networks. If the GRE protocol is in effect, it must be changed to a value of 1360 or smaller. Otherwise the packets won’t fit — imagine trying to squeeze a Sumo wrestler into a VW Beetle and you’ll get the picture.

Windows users need to edit the registry to change the MTU. There’s a good article here. My laptop had 4 separate interfaces in the registry, so I went ahead and set the MTU for all of them. I’m sure that the system is now slightly less efficient, but I guess I will live with that.

After the change, and a reboot, I was able to run Second Life in one more place than I was before!