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Network code, simulating the game on the server

Networking and Multiplayer Programming

Started by jimjamjahaa February 10, 2015 01:28 PM

2 comments, last by hplus0603 9 years, 4 months ago

jimjamjahaa

119

Author

February 10, 2015 01:28 PM

I am trying to make a multiplayer platformer with UDP...

Here is what i have so far (from reading various online resources):

Each frame, the client records the current input state and frame sequence number to an "input buffer". The input buffer is sent to the server via UDP each frame.

If/when the server receives this, the server will respond with an acknowledgement message telling the client that it has received inputs up to sequence number X.

If/when the client receives this acknowledgement message, the client will discard the beginning of its input buffer up to the sequence number that the server has acknowledged.

In this way the client streams all its input to the server in a reliable way over UDP, albeit sending some redundant information.

Now to the tricky part:

With lag it is guaranteed that the server receives input that is in the past. Lets say there is 100ms network delay and my game ticks every 16ms. This means about 6 frames will tick in the time it takes to send a message.

Lets say the client and server are synchronised and they are both currently on frame 50. The client sends its buffer containing input information for frames n...50. The server receives this 100ms later, by which time it is on frame 56 or so.

It doesn't make for a very good simulation if we play back this input on the server starting "now" aka frame 56, because the input was supposed to be representative of frames n...50 according to the client.

So i am guessing we need to record on the server the state of the entire world in a "world state buffer". This way when we receive input that was in the past we can rewind the world state to that exact frame, inject the input and play things back out in fast forward to get back to "now".

That doesn't seem too bad... right? I guess that is question number 1: am i on the right track here?

Now the really tricky part:

What happens when you have 2 clients with very different latencies. Lets say "Client A" is on a fast connection (30ms) and "Client B" is on a slow connection (200ms)

Lets say Client A jumps on Client B's head in order to kill him and get some points. Obviously his messages will get there in good time. Lets say Client B dodges this attack with just a few frames to spare. His messages will not get to the server for some time. It is possible that the server will simulate Client B's death, then some miliseconds later receive Client B's input buffer that contains input from a (relatively) long time in the past, rewind the game to a point before Client B had been killed, apply the input and Client B will now survive.

But we already simulated his death and sent that message out that he died.

I guess my second question is how do we avoid situations like this?

Sorry for the somewhat vague nature of this post. It's a really complex issue and it's really melting my brain.

MrGurken

128

February 10, 2015 05:15 PM

Disclaimer; I'm not the best network programmer, so take this with a grain of salt.

First of all, you probably don't want to send packet on every frame. That could easily flood the connection. Try to send packets somewhere around 10-15 times per second, i.e. every 6th frame or so.

Second of all, it seems like you've slightly misunderstood the authoritative server model. The input that the client is sending to the server is what that client is doing at the moment, so the server doesn't rewind and replay, it just applies the input. It is when the server sends the state of that client back to the client, along with the sequence number of the latest received input, that the client rewinds back to that state and applies any input that has been buffered up to that point. That way the state of the game on the server is always correct, but the interpretation of the client may vary.

Example:

Client A starts buffering input on frame 0. On frame 6 he sends 6 frames worth of input to the Server, along with the sequence number 1.

On frame 12 the Server receives the input an applies it to the corresponding character.

On the same frame the Client sends 6 frames of input along with the sequence number 2.

On frame 18 the Server sends the state of the character back to the Client, along with the sequence number 1.

On the same frame the Client sends 6 frames of input along with the sequence number 3.

On frame 24 the Client receives the state of the character along with the sequence number 1.

The client rewinds back to that state and discards any input before sequence number 1.

Since the client has buffered input with sequence number 2 and 3, it applies this to the current state, bringing him back to where he should be.

This is really just a way for the client to prematurely predict the state of the world, so it doesn't have to wait for the roundtrip before acting upon input, thus reducing input latency.

The second question is the same problem as magic bullets. Since you predict the state of the world on the client, you might trick the player into thinking that he is safe, when in fact he isn't. I can't think of any way to mitigate this.

jimjamjahaa

119

Author

February 10, 2015 06:11 PM

Hey thanks so much for your reply.

I think i am getting confused between a few concepts, that's true. I've been reading so much lately! But also i am trying to achieve something slightly more with my design. I am trying to get the server to simulate every clients input for "sequence 1" at the same time. Imagine in your example scenario the server on frame 24 receives input from Client A @ sequence 3 and Client B @ sequence 1. The way you describe it these would be executed at the same time on the server. Tough luck for client B.

I was thinking of buffering input on the server side so that the slowest client has a chance to get its input sent. Then once the buffer is sufficiently full of inputs it can start simulating everyone's "sequence 1" at the same time and continue with a smooth simulation.

But now i am thinking that maybe this unnecessary, adding latency for the faster connections. I just don't know.

hplus0603

11,920

February 11, 2015 04:20 AM

In general, if you structure your messaging as "requests" that need "responses," you're unlikely to get a robust networked simulation.
Each endpoint (clients, server) should send "updates" to the other end. "Updates" should contain all the information needed.
Clients already know what the server needs. Server already knows what the client needs.

Also, what platforms are you targeting?
For PC/Mac/Linux, cheating is rampant, so you really only can send "player input" from the client to the server.
For mobile phones, the platforms are slightly more locked down (at least iOS) and thus you may get away with sending "player actor state" instead.
For consoles, the hardware vendors work very hard on preventing cheating, so many console games get away with each client "owning/simulating" its own actor, and sending updates about it to the others, often echoed through one playerserver.

Now, exactly how you deal with time stamps -- do you display things in the "future" or "past" on each machine (client / server / observer); do you simulate in lockstep or opportunistically; do you guess-and-patch-up or display-known-good? All of these vary based on the exact feel you want from your game. Different choices will give different amounts of local latency, remote latency, perceived latency, rules-bending ability, number of corrections, possibility of catastrophic simulation failure, etc. We can't choose for you!

On these forums, as well as on the web, there's been a few discussions on how Awesomenauts solves multiplayer. That is a high-action multiplayer platformer, so you probably could get some insights into one particular solution by tracking those down.

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