Introduction
You’re a programmer. Have you ever wondered how multiplayer games work?
From the outside it seems magical: two or more players sharing a consistent experience across the network like they actually exist together in the same virtual world. But as programmers we know the truth of what is actually going on underneath is quite different from what you see. It turns out that it’s all an illusion. A massive sleight-of-hand. What you perceive as a shared reality is only an approximation unique to your own point of view and place in time.
Read on to discover the various tricks that game programmers use for various genres and the history of their development.
Peer-to-Peer Lockstep
In the beginning games were networked peer-to-peer, with each each computer exchanging information with each other in a star topology. You can still see this model alive today in RTS games, and interestingly for some reason, perhaps because it was the first way – it’s still how most people think that game networking works.
The basic idea is to abstract the game into a series of turns and a set of command messages when processed at the beginning of each turn direct the evolution of the game state. For example: move unit, attack unit, construct building. All that is needed to network this is to run exactly the same set of commands and turns on each player’s machine starting from a common initial state.
Of course this is an overly simplistic explanation and glosses over many subtle points, but it gets across the basic idea of how networking for RTS games work. You can read more about this networking model here: 1500 Archers on a 28.8: Network Programming in Age of Empires and Beyond.
It seems so simple and elegant, but unfortunately there are several limitations:
First, it’s exceptionally difficult to ensure that a game is completely deterministic; that each turn plays out identically on each
machine. For example, one unit could take slightly a different path on two machines, arriving sooner to a battle and saving the day on one machine, while arriving later on the other and erm. not saving the day. Like a butterfly flapping it’s wings and causing a hurricane on the other side of the world, one tiny difference results in complete desynchronization over time.
The next limitation is that in order to ensure that the game plays out identically on all machines it is necessary to wait until all player’s commands for that turn are received before simulating that turn. This means that each player in the game has latency equal to the most lagged player. RTS games typically hide this by providing audio feedback immediately and/or playing cosmetic animation, but ultimately any truly game affecting action may occur only after this delay has passed.
The final limitation occurs because of the way the game synchronizes by sending just the command messages which change the state. In order for this to work it is necessary for all players to start from the same initial state. Typically this means that each player must join up in a lobby before commencing play, although it is technically possible to support late join, this is not common due to the difficulty of capturing and transmitting a completely deterministic starting point in the middle of a live game.
Despite these limitations this model naturally suits RTS games and it still lives on today in games like “Command and Conquer”, “Age of Empires” and “Starcraft”.
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