stop rewarding laggers, limit client-side-hit-registration to 200ms in the past

There is a video from overwatch devs talking about this exact issue in very simple language. Some of what they are talking about is changes they hadn't yet shipped, but this video is from April 2016, so I presume these things made it into the game.

They specifically discuss a 250ms limit to client-side hit-prediction in the past, and a lag-compensation technique they call "extrapolation", where the shooter's client will dead-reckon "lead" their shots in the direction the client is moving to compensate for the shooter's lag, so lagged clients don't have to "lead" shots, but they are not allowed to hit clients in positions older than 250ms in the past. Because the client is automatically "leading" those shots, if the person is running for cover, the cover can properly block the shot when the person is (on the server) behind cover, even though the shooter is shooting at and hitting a 400ms old target not behind cover. 

They also talk about denying client-side hit registrations when the server knows the target has activated a special ability that mitigates damage. For example, in overwatch netcode, if the server knows wraith activated her tactical, and a lagged client tries to register hits on an old ghost from before the tactical, the server would "deny" those hits because it knows wraith is already invulnerable. 

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Here is a direct link to the part where they talk about the 250ms limit to trusting client side hit-registration..

https://youtu.be/vTH2ZPgYujQ?t=762

Here is a rough transcript:

Another technique we are going to use comes from one of our pro-gamers, Pyro, is using extrapolation.

So, if you play with someone who has a very very high ping, they could be shooting you way in the past, and that feels really really crummy, if you think you've run behind a wall, but they're shooting you from sri-lanka and you get wrecked.

So, we're going to put in a cap to how much we're going to rewind you [to register the shooter hit]. We had a cap before, but it was kind of soft, and it was ridiculously long. We're going to be a little more aggressive with that, and we'll tune it based on your feedback, based on the consequences of that cap. 

So what the cap is gonna do, is basically say there is a limit to how far a shooter can rewind a target in the past [to honor a client side hit-registration]. But we still want to honor this value of favoring the shooter, we don't want you to have to lead the target, so your client will try to do the leading for you, using a technique we call extrapolation, where you're client will understand - 'ohh, there's a delay is higher than this max-rewind time', which will be at 250ms in an upcoming patch, and [the client] will extrapolate the difference, so that most likely, when you shoot at that target, you will register the hit, so long as the target dead-reckoned in that same direction. it is extrapolation, it is a guess, it's almost like prediction, it can be wrong, There are cases where you will mis-predict that shot, For high-rate weapons, we'll show the mis-prediction [on the client], for low rate high damage weapons, we will delay showing the impact [on the client] until it is actually registered on the server.

One of the reasons for this again is just to mitigate the feeling of being shot around corners. The longer that we're doing rewinds [trusting client hit predictions in the past], the worse it feels as the player who is getting shot by that non-exapolating condition. So as soon as we start putting in the extrapolation, it'll tighten up the window in which people are getting shot around corners, and hopefully leading to you guys feeling like the game is alot more fair.