Your Game Doesn't Need UDP (Yet)

You also need to learn how to use TCP for a game, for several reasons:

1. Not all networks pass UDP. It's better to offer a potentially degraded experience than to cut users off from your game entirely.
2. UDP is appropriate for a few very small classes of data, for a few specific genres of game. Not all MMOs necessarily have an open world that you physically run around in. Position-update and health-update messages only care about the latest state (and therefore can go over UDP), but other updates are less time-sensitive and transmitting the full state as an update for complex game systems would be more expensive than just processing a sequence of changes, which is what TCP lets you do.
3. If you re-invent TCP on top of UDP without a thorough prior understanding of TCP, you will get it wrong.
4. If you re-invent TCP on top of UDP without a thorough prior understanding of TCP, you will get it wrong.