I want to serialize the state mid-game, send it over the network to an identical computer (same CPU, same OS, same binary), load it there, and have the two games run in tandem doing the exact same simulation, without one of them drifting off and going haywire.

In short: I want pop-in, pop-out networking support on my HIGHLY physics-intensive game, where sending object coordinates every few seconds is impossible, due to having thousands of objects, and many clients.

I tried this with Box2D, and saving an object's location/velocity/etc wasn't enough... there's internal state that's not accessible through any public methods.

My current workaround is to force EVERY client to save its entire worldstate and reload it from scratch, whenever a new player connects... but this is obviously bad practice, because it hangs the game for everyone whenever someone new connects. However, it works, with zero desynchronization.

So, anyone know of any other techniques that can help me? Or should I just kiss my project goodbye?