At a party this past week, the question came up about how accurate NTP could be. The need was accuracy under 1ms. Interesting question and I've never looked for that accuracy. I thought that somewhere I had read that it was possible to get to around 10us, but didn't have any concrete data. So off to Google's search engine.
There are several articles on NTP accuracy. The best of them seems to be one by
Dave Mills (the primary investigator on NTP). NTP v4 with kernel mods to
support it, is capable of much better than 1ms accuracy, possibly as good as
1ns. According to his article, NTP v3 is accurate to 1-2ms in a LAN and 10s of
ms in WAN nets.
http://www.cis.udel.edu/~mills/ntp.html
Other articles suggest that with an accurate time source, such as a GPS time
source, NTP is accurate to 50us, but the links on the Linux kernel support say
that accuracy of a few ms are possible.
http://www.atomic-clock.galleon.eu.com/support/ntp-time-server-accuracy.html
Another article says that it is dependent on the predictability of network delays (i.e. a low jitter network).
http://www.postel.org/pipermail/end2end-interest/2003-April/002925.html
So, the typical network answer applies: "It Depends". ;-) ;-)
-Terry