## Unequal data path times brain teaser - need answer

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### Unequal data path times brain teaser - need answer

I read this brain teaser years ago, and I never figured it out. It bubbles up in my noggin every one in a while and frustrates me. I might have the details wrong, so straighten me out if you know it. Here it is:

You have two computers connected on a network. They are the only two. They have perfect internal clocks but they are not synced together. The network has a consistent but unequal delay, so when A sends a message to B say it takes 300 milliseconds, but when B sends a message to B it takes 700 milliseconds. Or it could be 200 / 800 milliseconds, or 400 / 600. We don't know which path takes what amount of time, but we know that whatever it is, the individual path times do not change. How do we sync the clocks perfectly?
Cervisiae Amatorem

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Joined: Mon Aug 29, 2011 5:47 pm UTC

### Re: Unequal data path times brain teaser - need answer

I'm pretty sure this is impossible to solve without some additional factor. No matter how you communicate, you will never be able to tell the difference between the real situation and one where the other computer's clock is 100 ms off and both delays are different by 100 ms. You can easily find the total combined delay, but you cannot break it down further than that because every possible round trip communication will involve both delays.

If a third computer were added to the network with its own set of delays, then it might be possible to isolate individual delays by clever routing and comparing of message traffic times, but with just two there's only the one path so no comparisons are possible.
douglasm

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### Re: Unequal data path times brain teaser - need answer

This has come up before, and the outcome was that what douglasm is saying is right.

viewtopic.php?f=3&t=6531
ameretrifle wrote:Magic space feudalism is therefore a viable idea.

jestingrabbit

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Location: Sydney

### Re: Unequal data path times brain teaser - need answer

Thank you jestingrabbit! That was the original post I read years ago and I couldn't remember the specifics well enough to find it searching. It's bugged me ever since.
Cervisiae Amatorem

Posts: 21
Joined: Mon Aug 29, 2011 5:47 pm UTC

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