Google video chat works faster on local LAN than across a WAN connection

Question

Recently the internet speed on our college LAN has dropped drastically. The average file download speed is now 13 Kbps. Google talk's video chat remains unbelievably fast when done with someone within the college's LAN. It is practically unusable with anyone who is not in the college network. My college has a proxy server through which all computers inside the college's LAN connect to the internet. I suspect the problem is due to the proxy server.

How does g-talk open a video chat? Is it something in the mechanism that speeds up video chat between two clients within the same network? Since all computers in the college's LAN use the same proxy, their IP would appear to be the same to the Google server.

Answer 1

It looks like Google Chat video will attempt to directly connect first (via the LAN); if that is not possible it will attempt to connect via the Google infrastructure. So there are two reasons why it will be faster when talking to someone on your university's network.

One: UDP vs TCP When connecting to someone on your network directly, you will probably not go through a firewall, rather just routers. There is also a high probability you will not be NATed. What this means is that Google Chat will use UDP to send the video back and forth. UDP is what is known is a "connectionless" protocol which means it will send data without confirmation. There will be no attempt to control traffic based on collision detection. However, when going to someone outside the university's network it will probably be firewalled and NATed, that will generally block UDP (the firewall, that is). Google chat will detect that and use TCP which will not be as efficient for video.

Two: Issues going over a gateway. Since now all the traffic is going through one IP it will be slowed down. There will also be a latency issue because it is further away.

Source: https://developers.google.com/talk/libjingle/important_concepts#connections