Hi,

I'm was wondering whether this is a common/known problem when developing a realtime (fast paced) multiplayer game:

I have a ball on the client side and server side. If I press the forward key, the ball on the client side received an impulse (1 time applied). This input is sent to the server which also applies the input to the ball on the server.

I notice that the ball on the client is ahead a little bit (due to the latency from the input that was sent to the server) and I expected this.

I see the ball rolling on the client (still moving) and let's say that this is 50m from the starting point. I see it on the server at 40m.

Here is my problem: if I know press the down key arrow to move back, I see that my client is at 45 meter after a while but my server never reached the 50 meter point, and is now at 30 meter.

I was wondering how to exactly fix this kind of problem. I think that there is something wrong with my implementation of applying impulses, which is implemented like so:

- Once a key is pressed/released, an input struct with the keys that are pressed/released is being sent only once to the server (if each individual key is pressed/released, the struct is changed and 1 signal is sent to the server).

- Locally, I have a loop that runs 10 times a second for input: it checks based on the input struct which keys are pressed and applies the impulse accordingly.

- On the server I have another input loop (also 10 times a second). Once an object has been received, the loop applies the forces again depending on the input structure.

That's the general problem of all networked games. You have to design your gameplay, your physics, your graphics, and your networking to all come together to do the right thing for your particular game.

The basics is to make sure that you fix your simulation step, and count all inputs and simulation in step numbers, rather than time.
Then, you can batch N steps of input into a single network packet, and send to the server (and the same going the other way.)
At this point, the server knows that "the player was pressing forward on step 17 and backward on step 18" and thus can move forwards/backwards the same number of steps as the client.

Once you've done that, you will start running into problems where two players interact -- both want to pick up the same powerup, or both will bump into the same simulated block, or one shoots the other, or whatever.
How you handle those cases is what determines the "feel" of your gameplay, and there are many different solutions.

Thanks again hplus0603!

Oh no hplus0603, sorry for my late reply on this again.. I've just had time to get back on the networking learning.

I have a question regarding your comment. Because I need to keep track of the step as well, so here is the current problem:

1. Player presses UP on step 17 (client side). This is a message that is sent to the server. Let's say the server receives it at step 50 (server loop). It starts processing this input.

2. Player presses BACK on step 40 (client side). This is a message that is sent to the server. The server receives it at step 110. (server loop). It starts processing this input.

The server knows that the actual difference between 2 and 1 should be 33 (the amount that was pressed on the client side). However, it started at step 50, and since the other message came at step 110, the server now only knows it should be 33 steps between those key presses on the client. But the server itself is 60 steps late and pressed UP 60 ticks rather than 33.

Should the server correct itself? Because it sounds quite hard to fix this.

Sorry for this confusion.