Set Your Clocks With Pooled NTP Servers

flok writes "Since we all want to have the time correctly set on our servers we all want to synchronize to some ntp-server. Not everyone has such an NTP server available, so that is why www.pool.ntp.org was started. If your server is synced to some discrete timesource like GPS or something like that you can also join the group to help this initiative!"

Comments

NIST?

[AcornWeb]

Uh, what is wrong with time.nist.gov ?

Re:NIST?

[mgarraha]

Unless you are running a stratum 2 server for hundreds of clients, it's polite to stay off the stratum 1 servers. Two or three us.pool.ntp.org servers do almost as well. My ISP's routers are stratum 3 NTP servers, and I use one of those.

Re:time.apple.com

[Twirlip of the Mists]

I remember hearing a few years ago that the folks who ran tick and tock asked that only second-tier time servers sync to them, and that all the "leaf nodes" sync to a second-tier server. That's why I don't use tick or tock any more.

Accuracy vs Precision

[Lord Bitman]

why would anyone want accurate time based on many differing servers accross the world? On a network, the key is to have precise time. That is, the exact same not-neccessarily-correct time on every single computer on the network.
I have never managed to get this suitably set up using NTP. Anyone have success with this and willing to explain?

Re:Accuracy vs Precision

[wizbit]

On a network, the key is to have precise time. That is, the exact same not-neccessarily-correct time on every single computer on the network.

NTP seeks to find coordinated universal time (UTC), that is, the "one true time" as a basis for every local time on the planet. NTP is composed of several stratums of time servers which try to determine UTC using a complex series of algorithms to measure "drift" and deviation from UTC between servers and stratums.

If you need to set your wristwatch, you look at a wall clock for reference, right? If you have a network of wall clocks across 24 time zones, you choose one as a reference and set the rest accordingly. That reference clock might be an atomic clock or a swatch watch; it doesn't particularly matter where the timekeeper is located, only that your relative distance and time differential is measured precisely. In UTC, this is UT1, the 0-median (like Greenwich Mean Time - the time at 0 longitude).

ntpdate and ntpq -pn will give you an idea of the drag between your clock and the timeserver you are connected to - theoretically, when one hour has elapsed on one clock, all clocks should should strike the hour at the same time. Since this is not possible to an infinite degree of precision, the "drag" is the amount of time it takes your clock to "catch up" and strike the hour, whether it's early or late for the period, typically the hour. Extend that out to a year, or more appropriately, several years, and you'll find even clusters of atomic clocks that strike midnight, January 1st in a leap year several nanoseconds ahead of the astronomical date change. Every year, several "leap seconds" are added to account for the differential.

Anyway, the idea is to get your server to acquire, yes, a "not-necessarily-correct time" from a variety of sources and determine the most likely time for your geographic location, usually within a few nanoseconds, and then broadcast that time for every machine on your network to syndicate. The result is not absolute perfection, but a logical use of network resources to acquire a mostly-correct time.

You'd want to set up an NTP server (maybe several) that poll stratum-2 or -3 servers for the time, and ntpd to syndicate the time for the rest of your network. Win XP, Mac OS X, etc. machines can grab the correct time every reboot, or every network logon, or whatever you prefer. The result will invariably result in a slight adjustment every time a client "re-ups" for the right time, but it should be more than sufficient for the accurate synchornization of network-wide tasks.

If you're interested in best-practice scenerios with NTP, you should really check out www.ntp.org.

Re:Use .pool.ntp.org instead...

[MasTRE]

Would have been a good idea if the web page actually listed all the exact host names and not just briefly mentioned their existance as an afterthought.