Essential System Administration, 3rd Edition

Time Synchronization with NTP

Computers often don’t work right when the hosts on a network have differing ideas about what time it is. For example, DNS servers become very upset when the master server’s and slave servers’ ideas of the current time are significantly different and will not accept zone transfers under such conditions. Also, many security protocols, such as Kerberos, have time-out values that depend on accurate clocks

The Network Time Protocol (NTP) was designed to remedy this situation by automating time synchronization across a network.^[21] The NTP home page is http://www.ntp.org. There is also a lot of useful information available at http://www.eecis.udel.edu/~mills/ntp.htm.

You may wonder how computer clocks get out of synchronization in the first place. Computers contain a oscillator along with some hardware to interface it to the CPU. However, instability in the oscillator (for example, due to temperature changes) and latencies in computer hardware and software cause errors in the system clock (known as wander and jitter , respectively). Thus, over time, the clock settings of different computers that were initially set to the same time will diverge since the errors introduced by their respective hardware will be different.

NTP is designed to deal with these realities in a very sophisticated manner. It has been around since 1980 and was designed and written by Professor David L. Mills of the University of Delaware and his students. This protocol provides time synchronization ...