25-2z NTP can unicast, multicast or broadcast protocol messages.How NTP WorksFigure 25-1 shows the basic workflow of NTP. Device A and Device B are interconnected over anetwork. They have their own independent system clocks, which need to be automaticallysynchronized through NTP. For an easy understanding, we assume that:z Prior to system clock synchronization between Device A and Device B, the clock of Device A is setto 10:00:00 am while that of Device B is set to 11:00:00 am.z Device B is used as the NTP time server, namely, Device A synchronizes its clock to that ofDevice B.z It takes 1 second for an NTP message to travel from one device to the other.Figure 25-1 Basic work flow of NTPIP networkIP networkIP networkIP networkDevice BDevice ADevice BDevice ADevice BDevice ADevice BDevice A10:00:00 am 11:00:01 am10:00:00 amNTP message 10:00:00 am 11:00:01 am 11:00:02 amNTP messageNTP messageNTP message received at 10:00:03 am1.3.2.4.The process of system clock synchronization is as follows:z Device A sends Device B an NTP message, which is timestamped when it leaves Device A. Thetime stamp is 10:00:00 am (T1).z When this NTP message arrives at Device B, it is timestamped by Device B. The timestamp is11:00:01 am (T2).z When the NTP message leaves Device B, Device B timestamps it. The timestamp is 11:00:02 am(T3).z When Device A receives the NTP message, the local time of Device A is 10:00:03 am (T4).Up to now, Device A has sufficient information to calculate the following two important parameters:z The roundtrip delay of NTP message: Delay = (T4–T1) – (T3-T2) = 2 seconds.z Time difference between Device A and Device B: Offset = ((T2-T1) + (T3-T4))/2 = 1 hour.Based on these parameters, Device A can synchronize its own clock to the clock of Device B.This is only a rough description of the work mechanism of NTP. For details, refer to RFC 1305.
