Network Time Protocol
Network Time Protocol (NTP) is used for accurate time keeping and can reference atomic clocks that are present on the Internet, for example. NTP is capable of synchronizing clocks within milliseconds and is a useful protocol when reporting error logs (for instance, from Cisco routers).
For NTP, the defined ports are UDP port 123 and TCP 123. NTP can support a connection-orientated server (TCP guarantees delivery) or connectionless (UDP for non-critical applications).
An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary ...