What's the best way to synchronize times to mi

Question

From what I understand, the crystals on PC's are notorious for clock skew. If clocks are always skewing, what is the best way to synchronize clocks between machines with millisecond accuracy and precision? From what I've found, NTP and PTP are possible solutions, but I was wondering if anybody had any experience on stackoverflow.com!

I understand NTP is the popular choice, but am wondering if anybody has had any experience with PTP (IEEE1588)

Answer 1

You cannot synchronize machines to the level of milliseconds by exchanging data, because any data exchange itself already takes at least milliseconds to happen and thus spoils your result! Even protocols that try to first measure how long a data transfer takes and then sending out the time info (taking the measured delay into account) are just a bit better than average but they are still not good since not every data transfer takes equal time (just constantly ping a server on the Internet and see how every ping has a different delay).

The only way to really synchronize two computers in the milliseconds range is by having them both obtain the time from the same source via a transfer method that has no unknown or constantly changing delay. E.g. if both receive a satellite signal, that broadcasts the time. The signal will always have a constant delay (from satellite to earth) and they are both receiving it almost within the same nanosecond.

Germany for example has a radio controlled time. Somewhere in the country is an atomic clock (that has correct time to the nanosecond for hundreds of years) and some sender permanently broadcast the current time on a given frequency all over the country. Alarm clocks and even wristwatches exist that can receive this time and permanently synchronize with it (well, not really permanently, most models do that only once every 24 hours to save battery runtime). Such receiver devices also exist for computers and come with software that can permanently synchronize your computer clock with that time signal.

As far I know GPS also sends time information (either that, or the time can be calculated somehow from the GPS information, I'm not too familiar with the GPS protocol). So attaching a GPS receiver to both computers can probaly also get them synchronized to the millisecond. If your synchronization is done via the Internet however, don't expect a better synchronization than one computer being at most 20 milliseconds off.

To update on the commenter,
NTP is not that accurate as people love to claim here:

NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions.

Source: Wikipedia

I would rather keep them all in sync without any network involved and farther keeping them in sync to the official GMT time and here GPS is probably the only way to get really accurate results on all machines (and that not only down to the ms, actually down to microseconds).

Answer 2

I use NTP throughout the whole network at my company and it works rather well. The key is to have one authoritative server on a local network and have every machine on the network synchronize with it. The best is to have a radio clock installed on that server. NTP is great because it does not just correct the clock once in a while, but it actually calculates and correct clock frequency making it more accurate.

Once I had NTP setup on the network I opened like five VNC session to different server and sat there watching the clock. The clocks on all server were in sync withing milliseconds, and this is right after setup. It gets more accurate as it runs.

Answer 3

As you've already suggested, NTP is the industry standard solution to this problem, but it either requires Internet connectivity or a stratum 0 source (an accurate hardware clock, like a GPS receiver with a computer interface).

If you're using Internet connectivity, consider using the NTP Pool.