H3C Technologies H3C S7500E Series Switches - Manual (Page 67)

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z When this NTP message arrives at Device B, it is timestamped by Device B. The timestamp
is 11:00:01 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.
NTP Message Format
NTP uses two types of messages, clock synchronization message and NTP control message.
An NTP control message is used in environments where network management is needed. As it
is not a must for clock synchronization, it will not be discussed in this document.
All NTP messages mentioned in this document refer to NTP clock synchronization messages.
A clock synchronization message is encapsulated in a UDP message, in the format shown in
Figure 3-2 .