Network Time Protocol (NTP)
NTP enables the synchronization of computer clocks distributed across the network by ensuring accurate local timekeeping with reference to some particular time on the Internet. NTP communicates between clients and servers using the User Datagram Protocol on port No.123. The NTP software package includes a background program known as a daemon or service, which synchronizes the computer’s clock to a particular reference time such as radio clock or a certain device connected to a network.
NTP uses a systematic, hierarchical level of clock sources for its reference. Each level is called a stratum and has a layer number that usually begins with zero. The stratum level serves as an indicator of the distance from the reference clock in order to avoid cyclic dependence in the hierarchy. However, the stratum does not represent the quality or reliability of time.