I liked the discussion of transmitting game state for network play. Quick summary: Doom sent differences of state, which had to be received and therefore acknowledged. Quake sent the whole game state each time, but compressed, so it wouldn't matter if a transmission was lost.

Neither approach seems optimal from an information theory point of view. The Doom approach needs feedback, but communicating at the optimal rate of noisy channel doesn't need feedback. The Quake approach sends the game state redundantly, but across time simply repeats information, and repetition codes aren't optimal.

It could turn out that in this application the Quake approach is best, because for latency reasons it might not be possible to send long enough blocks for Shannon's theory to apply. The Quake approach is also nice and simple. However, here's the approach I have in mind: send the stream of deltas protected by a code that can cope with some of them being erased, such as a Digital Fountain Code [1]. Each message would contain deltas stored slightly redundantly and XORed with previous deltas. If we have all previous deltas then we are set, otherwise we'll have to wait for another packet or two before we can infer the deltas, but we don't need to bother telling the receiver that we lost a packet.

[1] http://en.wikipedia.org/wiki/Fountain_code — but a much better resource is chapter 50 of http://www.inference.phy.cam.ac.uk/mackay/itila/book.html

EDIT: In response to replies by VMG and JoachimSchipper: I didn't mean to suggest they should have done anything differently. It worked well enough, and they got it out the door; agreed! I just think it's an interesting puzzle to think about.