External Clock Synchronization
External clock synchronization algorithms are mostly centralized using a central master or decentralized. A few of these are discussed next.
Centralized External Clock Synchronization: Cristian's Algorithm
Cristian's algorithm for external clock synchronization was proposed by Flaviu Cristian in his seminal paper  and is essentially a centralized pull algorithm where clocks are synchronized to a physical clock acting as the master and the other clocks are slaves. Figure 2.5 illustrates Cristian's algorithm.
At regular intervals of (5/2pmax) seconds, each slave sends a request to the time server to send the physical time T. If TR be the time instant at which the request is sent, 0 is the time interval after which the server sends the physical
External clock synchronization: Cristian's algorithm.
time T, and TS be the time instant at which the slave receives the communication from the time server, then
The slave corrects the value of T by adding L to it. Several estimates of L are used to obtain a realistic estimate, since communication latencies are variable.