Obviously, each world needs to report on the position of the paddle (left or right) that its player controls, but what about the ball? Should I just let the worlds handle that with on-tick, or should I check that the ball is in the same spot on both worlds every so often, or should I have the universe server control the ball and tell the world everything except their own paddle position each time something changes?
In terms of generic network programming, you should only send non-deterministic events (movement of the paddle) and let the clients figure out the movement of the ball.
I would send all paddle movements between clients and an extra message for when the ball hits a paddle that contains the position of the paddle and ball, to ensure that the other client starts from the right state when the ball bounces.
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