13.1 About the NTP Daemon

The ntpd daemon can synchronise the system clock with remote NTP servers, with local reference clocks, or with GPS and radio time signals. ntpd provides a complete implementation of NTP version 4 (RFC 5905) and is also compatibility with versions 3 (RFC 1305), 2 (RFC 1119), and 1 (RFC 1059).

You can configure ntpd to run in several different modes, as described at http://doc.ntp.org/4.2.6p5/assoc.html, using both symmetric-key and public-key cryptography, as described at http://doc.ntp.org/4.2.6p5/authopt.html.