I did all of the networking for this project. We were trying to make a sloth soccer game with full online networking. Never got a chance to finish it, but the idea is it uses cubic hermite splines & interpolation to replay the physics state from the server back to clients. The tricky part was that clients had to be able to see instant results so the physics state is cheated on the clients to make it look like it is reacting instantly while it interpolates data from the server. The idea is to combine two splines of movement together.

Created in Unreal 4. Coded in blueprint.

Unfortunately, I never got this to work as intended. Unreal uses a non-deterministic physics engine. I learned a lot but I eventually came to conclusion that this would be much easier with a deterministic physics engine. With enough time, I might be able to figure out a good way to do it without deterministic physics. Mostly the trick is getting the results on the client side to accurately predict enough of the movement that the results from the server can be smoothed out to match the correct state. It's a very interesting but difficult problem to solve if you want clients to feel immediate and accurate physics results. With a dedicated server, all players would see very similar results but it was never quite performance I was intending.