NTP Network Time Protocol

The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. Originally designed by David L. Mills at the University of Delaware in 1985, NTP is one of the oldest internet protocols still in use and is maintained by the IETF NTP Working Group. Defined in RFC 5905 (version 4), NTP synchronizes computer clocks to within a few milliseconds of UTC. Key features include: hierarchical stratum architecture (organizing time servers into strata, where stratum 0 are reference clocks like atomic clocks and GPS receivers, stratum 1 servers connect directly to reference clocks, stratum 2 sync from stratum 1, with up to 16 strata), Marzullo algorithm (using intersection intervals to select the most accurate and reliable time sources), clock discipline algorithm (adjusting the local clock using a phase-locked loop accounting for network delay, clock drift, and jitter), symmetric and broadcast modes (peer-to-peer symmetric, client-server, and broadcast/multicast modes), authentication (cryptographic authentication via symmetric keys and Autokey public key infrastructure), reference clocks (supporting GPS, radio clocks like WWVB and DCF77, atomic clocks, and modem-based services), the reference daemon implementation that maintains synchronization and adjusts the system clock, and a global volunteer time server pool providing free service to millions of systems.