3Com Switch 4800G 24-Port Configuration Manual page 949

NTP Message Format
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The process of system clock synchronization is as follows:
Switch A sends Switch B an NTP message, which is timestamped when it leaves
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Switch A. The time stamp is 10:00:00 am (T1).
When this NTP message arrives at Switch B, it is timestamped by Switch B. The
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timestamp is 11:00:01 am (T2).
When the NTP message leaves Switch B, Switch B timestamps it. The
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timestamp is 11:00:02 am (T3).
When Switch A receives the NTP message, the local time of Switch A is
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10:00:03 am (T4).
Up to now, Switch A has sufficient information to calculate the following two
important parameters:
The roundtrip delay of NTP message: Delay = (T
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Time difference between Switch A and Switch B: Offset = ((T
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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 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 278.
NTP Overview
-T ) - (T -T ) = 2 seconds.
4 1 3 2
-T ) + (T -T
2 1 3
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))/2 =
4