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Rich: If you think FENs are too large, (which to me seems a gross
exaggeration at the current state of technology; note that FENs involve some inherent compression, as stretches of empty squares are indicated by a single number), an alternative would be to only transmit the difference between the previous game state.

Note that the FEN standard for normal Chess, unlike what Fergus wrote,
does contain a full description of e.p. and castling rights. The only thing that is missing is the possibility to recognize game-state repetitions in connection with 3-fold repetition draws. In Chess this does not affect legality of a move; in Xiangqi this might be different. But I think that is not really important for communicating moves between entities that play or display, as they can remember the game history. This is the issu if the protocol should be stateless, or not. UCI is an example of a stateless protocol (it sends the engine the entire game history plus opening position for every move the engine has to make), and I think in general this is a very bad idea. It just makes the protocol awkward and cumbersome. Fergus has good reason for wanting the Game Courier protocol to be stateless, though: you have to be able to resume a game halfway, with a freshly-started browser. ICS protocol is not stateless, but solves this problem through bidirectional communication: although the board position is transmitted with every move, the game history is only available on explicit request. So normally, you don't request it, but if you enter a game halfway (recognized by the fact that the move number of the first move you got sent was not 1), you have to make a one-time request for the game history.

As in most games only a small fraction of the cells change state, (this is even true in games like Reversi and Go), you could just send a list that names the altered cells and their new state. E.g. 'e2=0,e4=P;' for e2-e4, and 'e1=0,h1=0,f1=R,g1=K;' for O-O. This would work over an exremely wide range of games. As in many games moving a piece from one cell to another is the only thing that happens most of the time, it can be very useful to make 'x-y'; a short-hand notation for 'x=0,y=P;', when P was the old state of cell x.