I’ve written a diffing algorithm using UDP. You tell it to diff against a previous packet with an id. Every so often you send a full key frame packet so they always stay in sync and have full game state.
Right. That's how video streams work, too. Every once in a while there's a complete frame, but most frames are diffs.
The key here though is that your server keeps the last N ticks of state (probably around 20) and calculates the diff for each player based on the last id they reported seeing. This way missing an update doesn't get you completely out of sync until the next full state sync, it just gets you a larger diff.
Right. That's how video streams work, too. Every once in a while there's a complete frame, but most frames are diffs.