How to synchronize the ball in a network pong game?

Question

I’m developing a multiplayer network pong game, my first game ever. The current state is, I’ve running the physic engine with the same configurations on the server and the clients. The own paddle movement is predicted and get just confirmed by the authoritative server. Is a difference detected between them, I correct the position at the client by interpolation. The opponent paddle is also interpolated 200ms to 100ms in the past, because the server is broadcasting snapshots every 100ms to each client.

So far it works very well, but now I have to simulate the ball and have a problem to understanding the procedure.

I’ve read Valve’s (and many other) articles about fast-paced multiplayer several times and understood their approach. Maybe I can compare my ball with their bullets, but their advantage is, the bullets are not visible. When I have to display the ball, and see my paddle in the present, the opponent in the past and the server is somewhere between it, how can I synchronize the ball over all instances and ensure, that the it got ever hit by the paddle even if the paddle is fast moving? Currently my ball’s position is simply set by a server update, so it can happen, that the ball bounces back, even if the paddle is some pixel away (because of a delayed server position).

Until now I’ve got no synced clock over all instances. I’m sending a client step index with each update to the server. If the server did his job, he sends the snapshot with the last step index of each client back to the clients. Now I’m looking for the stored position at the returned step index and compare them. Do I need a common clock to sync the ball?

EDIT:
I've tried to sync a common clock for the server and all clients with a timestamp. But I think it's better to use an own stepping instead of a timestamp (so I don't need to calculate with the ping and so on - and the timestamp will never be exact). The physics are running 60 times per second and now I use this for keeping them synchronized. Is that a good way?

When the ball gets calculated by each client, the angle after bouncing can differ because of the different position of the paddles (the opponent is 200ms in the past). When the server is sending his ball position, velocity and angle (because he knows the position of each paddle and is authoritative), the ball could be in a very different position because of the different angles after bouncing (because the clients receive the server data after 100ms). How is it possible to interpolate such a huge difference?

Answer 1

As you are developing your first game, I think you should try the simplest but brute-force method first. Then you will experience the first exciting result, then you will get courage and try the better methods.

Like Source Engine method, process the game play in one side and send every object state to the other for each 1/30 second. This is a brute method but it works in LAN environment.

Now you will find problems that occur WAN environment where latency is more than 1/30 second.

I am not sure it works actually, but I think that:

1. Assume that the movement of ball does not change by only player's hit.
2. Send ball position P, velocity and player A position only when player hits the ball to B.
3. At B, receive it but process it as if time L is already passed (L=latency between A and B * 2) However, rendered ball should be keep its previous movement until it reaches the ball position P.

Values which never are affected can be masqueraded, even if it is time value. :)