UDP doesn't need to be reliable, and indeed the reliable UDP option is quite useless as it tries to fix old messages which nobody wants anymore. I have a much more optimized UDP error correction, which just takes the newest message, as the game can live without getting any messages for a good time also. In the MMO I'm planning, there's no big difference if you are online or offline. When a player goes offline, his character will keep moving and acting using mimic AI, which does similar things as the player would. For example jumping when walking And in fights, the AI would cast the spells which the player usually casts too. When in a group, the player would follow the group leader and assist on the main assist's target.