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I know that synchronization is a huge topic, so I have minimized the problem to this example case.

Let's say, Alice and Bob are playing a P2P game, fighting against each other. If Alice hits Bob, how should I do the network component to make Bob's HP decrease?

I can think of two approaches:

  1. Alice perform a Bob.HP--, then send Bob's reduced HP to Bob.
  2. Alice send a "I just hit Bob" signal to Bob. Bob checks it, and reduce its own HP, then send his new HP to everyone including Alice.

I think the second approach is better because I don't think a player in a P2P game should be able to modify other players' private fields. Otherwise cheating would be too easy, right?

My philosophy is that in a P2P game especially, a player's attributes and all attributes of its belonging objects should only be updated by the player himself. However, I can't prove that this is right. Could someone give me some evidence? Thanks :)

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Unfortunately, without a (trusted, independent) 3rd party, stopping the other party from cheating is impossible. The design of your game may limit the extent of cheating; however, in any situation where either party can be (and is equally likely to be) hostile, you need somebody they both will trust. – Clockwork-Muse Sep 6 '12 at 20:46

It's not good practice to directly alter other players' properties, so I would recommend your second option.

Though I would also recommend adding checks to make sure that peers don't spam "I hit you" messages without actually having hit the player.

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To limit cheating as much as possible without a server, the only messages you should be sending should be player input. Then both clients simulate the gameplay locally, and come to the same solution. So if you're making a fighting game, player A says, "I'm starting my kick" and 6 frames later player B thinks, "welp I got hit by the kick now" instead of player A potentially sending "I hit you for 1 billion damage" 20 times every frame. – TheMaster42 Sep 6 '12 at 23:15

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