How ntp works – H3C Technologies H3C MSR 50 User Manual
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How NTP works
shows how NTP synchronizes the system time between two devices, in this example, Device A
and Device B. Assume that:
•
Prior to the time synchronization, the time of Device A is set to 10:00:00 am and that of Device B
is set to 11:00:00 am.
•
Device B is used as the NTP server. Device A is to be synchronized to Device B.
•
It takes 1 second for an NTP message to travel from Device A to Device B, and from Device B to
Device A.
Figure 10 Basic work flow of NTP
The synchronization process is as follows:
•
Device A sends Device B an NTP message, which is timestamped when it leaves Device A. The
timestamp is 10:00:00 am (T1).
•
When this NTP message arrives at Device B, it is timestamped 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).
Now, Device A can calculate the following parameters based on the timestamps:
•
The roundtrip delay of an NTP message: Delay = (T4–T1) – (T3-T2) = 2 seconds.
•
The 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 a rough description of how NTP works. For more information, see RFC 1305.
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