NTP Server

Mon Oct 25 00:43:50 UTC 2010

Regarding leap seconds:

A modern OS kernel using the NTP daemon to control time will always experience monotonic time. Negative leap seconds should result in the local clock slowing slightly until the local time matches the NTP-derived time.

This is in strong contrast to what can happen when ntpdate or similar queries are used to adjust system time, as in the following example from a popular CPE log where the local CPU clock tended to run fast:

Oct 24 04:27:59 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 05:28:00 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 06:28:00 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 07:28:01 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 08:28:01 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 09:28:02 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 10:28:03 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 11:28:04 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 12:28:04 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 13:28:05 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 14:28:05 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 15:28:05 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 16:28:06 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 17:28:07 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 18:28:08 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).
Oct 24 19:28:08 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted +0 seconds).
Oct 24 20:28:09 192.168.1.1 HomeN ntp: Clock synchronized to network time server time.apple.com (adjusted -1 seconds).

Regarding local time servers:

This is the situation in which local clients, at least those with UNIX or UNIX-like OS can take advantage of ntpd and local time servers to have consistent and monotonic time across your network with a measure of insulation from external vagaries. Yes, I have run ntpd on Windows systems, but have no quotable experience with the current Windows version (Windows 7).

James R. Cutler
james.cutler at consultant.com