Network Time Protocol
Atomic clocks and other time keeping devices all around the planet supply a precise signal.
Network Time Protocol allow clients to synchronize their time by calculating round trip delays and time against signaled time. A kernel interface adjtime() moves the clock.
Abuse of the service will result in a packet with Kiss Of Death (KOD) set which should be respected.
- David Mill's wonderful book Network Time Protocol.
- The newsgroup comp.protocol.ntp