H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 488)

54-3
The process of system clock synchronization is as follows:
z
Switch A sends Switch B an NTP message, which is timestamped when it leaves Switch A. The
time stamp is 10:00:00 am (T1).
z
When this NTP message arrives at Switch B, it is timestamped by Switch B. The timestamp is
11:00:01 am (T2).
z
When the NTP message leaves Switch B, Switch B timestamps it. The timestamp is 11:00:02 am
(T3).
z
When Switch A receives the NTP message, the local time of Switch A is 10:00:03 am (T4).
Up to now, Switch A has sufficient information to calculate the following two important parameters:
z
The roundtrip delay of NTP message: Delay = (T
4
–T
1
) – (T
3
-T
2
) = 2 seconds.
z
Time difference between Switch A and Switch B: Offset = ((T
2
-T
1
) + (T
3
-T
4
))/2 = 1 hour.
Based on these parameters, Switch A can synchronize its own clock to the clock of Switch 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
54-2 .