55
How NTP works
Figure 19 shows the basic workflow of NTP. Device A and Device B are connected over a network. They
have their own independent system clocks, which must 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 11:00:00 am.
• Device B is used as the NTP time server, and 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 19 Basic work flow of NTP
System clock synchronization includes the following process:
• Device A sends Device B an NTP message, which is time stamped when it leaves Device A. The time
stamp is 10:00:00 am (T1).
• When this NTP message arrives at Device B, it is time stamped by Device B. The timestamp is
11:00:01 am (T2).
• When the NTP message leaves Device B, Device B timestamps it. The timestamp is 11:00:02 am (T3).
• 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.
To Next Page
Loading...
