How does an FPS like Overwatch have client time run ahead of the server?

In watching the GDC talk about Overwatch netcode, it mentions that the client is always ahead of the server, that the "current tick" on the server is behind that of the client.

From their explanation this makes sense. If the client is ahead of the server, then by the time the client messages arrive at the server, the server will have caught up and can handle them when it wants them.

But how does this get setup to begin with? The client connects to the server, the server starts a game… does the client jump ahead of it’s own accord? (Server says "game is starting, I’m at tick 0", and the client is like "okay, well, based on my ping I’m going to be tick 20 then")

  • If the server is in charge of what’s going on in the game, how does the client even know what’s going on in that future tick? I would assume constant extrapolation of game events from the last server update would be bad. Having the client in the past to allow it to interpolate between server updates would make a lot more sense to me.

  • Or am I wrong in assuming that client and server tick times need to remain in sync, and it is instead more loosely based on server updates and the number of client events that happened since that update was received, rather than the client actually caring about what tick they’re on compared to the server?

It’s the time/tick synchronization, despite one or the other having to be in the past, that I can’t wrap my head around…