Technical solution to eliminate desync in single-player sessions

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Giving the player the RNG seed at the beginning of the instance means a malicious client knows in advance what the next random outcome will be.

With this alone, you already have lots of hack possibilities. For example, you could have a hack which tells players whether the next monster hit will be blocked or not, when their next attack will be a critical, and so on.

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A malicious client is in full control of the order of events, allowing it to assign specific numbers to specific events, from the pool of random numbers which its allowed.

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Let me just save you some trouble on this point; you can mitigate this, but it means uploading essentially a full replay file of the instance to the server upon instance change for verification. Over a large instance with lots of combat, this would likely be dozens of megabytes in size, severely increasing instance transition speed; keep in mind that most internet connections have significantly better download speed than upload speed, and even GGG's download bandwidth is limited.

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A malicious client is still in full control of whether events occurred at all.

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How so? Boss rolls a crit - whoops nope, that was my crit, the boss gets this other non-crit roll that came afterwards instead. Computers are really good at crunching numbers, and you only need to look at and reorder relatively small subsets of the data, not the whole sequence.

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Seriously? Do you really believe packet loss doesn't occur naturally?

A significant percentage of user packets never make it to their own router, assuming a wireless connection.

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Tuoweit wrote:

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syrioforel wrote:

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Tuoweit wrote:

But the client would have full authority to reorder the actions any way it pleases in order to optimize the outcome for the player, as long as it followed the PRNG sequence.

Computing even a single alternate sequence of actions that would collide with what you actually did would be very difficult.

How so? Boss rolls a crit - whoops nope, that was my crit, the boss gets this other non-crit roll that came afterwards instead. Computers are really good at crunching numbers, and you only need to look at and reorder relatively small subsets of the data, not the whole sequence.

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qwave wrote:

There is not a 'pool of random numbers', its a deterministic seed. You are misunderstanding the nature of how this works.

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qwave wrote:

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How so? Boss rolls a crit - whoops nope, that was my crit, the boss gets this other non-crit roll that came afterwards instead. Computers are really good at crunching numbers, and you only need to look at and reorder relatively small subsets of the data, not the whole sequence.

You are misunderstanding how the deterministic seed works. Your client can't make the boss 'reroll' his attack/crit without corrupting the entire snapshot.

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qwave wrote:

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Couldn't you (the player) simply introduce an artifical delay between actions on the client and sending the info to the server? At the very least this could be exploited to prevent deaths in hardcore.

Let's assume someone had the ability to do this dependably. So what you're suggesting is that I am playing hardcore, and a mob kills me. However, since my client is authoritative, I can simply 'roll myself back' before this death and not actually notify the server that I died.

There are two considerations I would make in this situation:


1. You could only roll yourself back as far as the last snapshot data you streamed. The client will be continually streaming your snapshot in realtime.

2. The server must mandate that it receives a continuous stream of data, otherwise it will forcefully sync you back to the last known state. You might consider this the same thing as the 'desyncs' that we already have. However, this particular 'desync' has nothing to do with being out of 'sync' as much as it has to do with an actual 'loss of connection' (The actual data stream connection has been severed). This would not occur nearly as frequently as the current desync.

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Bullshit. It is a pool of numbers.

Deterministic seed #XXXXXXX = RandomNumber1, RandomNumber2, RandomNumber3, RandomNumber4, ...

When you have a seed, a deterministic random number generator is no longer truly random, it's deterministic. It's a specific set of numbers, and when the client can order events, it's a pool of numbers which can be ordered as it wishes so long as it knows the seed in advance.

Just in case you didn't know this already, GGG already sends the client a random seed for the map layout of an instance. So guess what? We have fully functioning maphacks in the game right now. That is just what happens when you give seeds to the client.