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Nelson Minar
Blog licensed under a Creative Commons License
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I love NTP, the
magic Internet protocol that keeps computer clocks synchronized. One
of the nice things Apple does is run an NTP service for all their
computers. That's how your Mac always knows the right time. Here are
some notes on what they do.
My American MacOS 10.3.9 box has the following /private/etc/ntp.conf:
server time.apple.com minpoll 12 maxpoll 17
That means my Mac asks time.apple.com for the time somewhere between
once an hour and once every day and a half. That's less often
than usual for NTP, presumably an accuracy / scalability tradeoff.
time.apple.com currently resolves to 4 IP addresses, 17.254.0.26, .27, .28, and .31. All of them seem to be in San Jose, California. .26 is the most accurate clock (stratum 2), the others are just behind at stratum 3. Using ntpdc you can find out where .27 gets its time from. They're well synchronized, talking to several stratum 1 clocks. They poll each of those every 128 seconds, aggressive but maybe appropriate for such a well-used clock. They sync around the world, and also run services in Europe and Asia for their clients. |
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