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?