How NTP works
shows the basic workflow of NTP. Device A and Device B are connected over a network. They
have their own independent system clocks, which need to be automatically synchronized through NTP.
For an easy understanding, assume the following conditions:
Prior to system clock synchronization between Device A and Device B, the clock of Device A is set
to 10:00:00 am while that of Device B is set to 1 1:00:00 am.
Device B is used as the NTP time server (Device A synchronizes its clock to that of Device B).
It takes 1 second for an NTP message to travel from one device to the other.
Figure 17 Basic work flow of NTP
System clock synchronization includes the following process:
Device A sends Device B an NTP message, which is timestamped when it leaves Device A. The time
stamp is 10:00:00 am (T1).
When this NTP message arrives at Device B, it is timestamped by Device B. The timestamp is
1 1:00:01 am (T2).
When the NTP message leaves Device B, Device B timestamps it. The timestamp is 1 1:00:02 am
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 important parameters:
The roundtrip delay of NTP message: Delay = (T4–T1) – (T3-T2) = 2 seconds.
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 more information, see RFC 1305.