Port :123Network Time Protocol

Network Time Protocol (NTP) is a widely implemented protocol designed to synchronize the clocks of computers over packet-switched, variable-latency data networks. Operating primarily over UDP port 123, NTP works by exchanging time-stamped packets between a client and one or more NTP servers. The client calculates round-trip delay times as well as local clock offsets, then adjusts its time accordingly for high accuracy.

NTP uses a hierarchical system of strata, where stratum 0 devices (high-precision reference clocks like atomic clocks or GPS receivers) feed time information to stratum 1 servers directly connected to them. Lower strata servers acquire the time indirectly from higher levels. This distributed approach improves resilience and accuracy, providing fault tolerance in case some servers become unreachable or unreliable.

While the core protocol is defined in RFC 5905, NTP implementations commonly support various modes, including client-server, symmetric (peer-to-peer), and broadcast/multicast operation. The protocol periodically updates, correcting for clock drift and variations in network latency, enabling devices to maintain synchronization within milliseconds, even over the public internet.