For my game networking class last semester I made a networked version of the 1962 game Spacewar. I made this using XNA 4.0 and set the game up on a client-server model where the host is the designated authoritative server that has the final say in health calculations. In the last segment of the video the game has a planet in the center of the screen which attracts all players and will kill them on contact.
In order to reduce the number of packets that I needed to send each packet was split into sections of data that contained a header field and a data field. The header contained a byte to signify the packet type and an unsigned integer to describe the size of the section.
Over the course of the few weeks after I made this I built upon it by adding prediction and smoothing of the player and projectile positions so that the movement of players appeared smooth and accurate to every player when there was a large amount of latency between them and the server.