3Com Switch 8800 Advanced Software V5 Configuration Guide

NTP Overview 1065
■ Device A sends Device B an NTP message, which is timestamped when it leaves
Device A. The time stamp is 10:00:00am (T
1
).
■ When this NTP message arrives at Device B, it is timestamped by Device B. The
timestamp is 11:00:01am (T
2
).
■ When the NTP message leaves Device B, Device B timestamps it. The timestamp
is 11:00:02am (T
3
).
■ When Device A receives the NTP message, the local time of Device A is
10:00:03am (T
4
).
Up to now, Device A has sufficient information to calculate the following two
important parameters:
■ The roundtrip delay of NTP message: Delay = (T
4
-T
1
) - (T
3
-T
2
) = 2 seconds.
■ Time difference between Device A and Device B: Offset = ((T
2
-T
1
) + (T
3
-T
4
))/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.
n
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 313.