Sniffing in my network (for absolutely different reason, also -- I'm the admin) I found that one of popular online RPG games (third person perspective, lots of graphics, 3D) uses TCP protocol, when UDP was the right choice. What's the result? To transfer approx 20 bytes of data it needs more than 200 bytes. It uses PSH flag in nearly all packets, so most acks are without data. Also, when a packet loss occurs, it causes a re-transmission. In simple words -- lags.
The funny thing is that when programming using sockets (most popular interface for network programming) it's really small change. I'm wondering about marks the person who designed the game's network things got from networking subjects. If every took such class...