25-2
z NTP can unicast, multicast or broadcast protocol messages.
How NTP Works
Figure 25-1 shows the basic workflow of NTP. Device A and Device B are interconnected over a
network. They have their own independent system clocks, which need to be automatically
synchronized through NTP. For an easy understanding, we assume that:
z Prior to system clock synchronization between Device A and Device B, the clock of Device A is set
to 10:00:00 am while that of Device B is set to 11:00:00 am.
z Device B is used as the NTP time server, namely, Device A synchronizes its clock to that of
Device B.
z It takes 1 second for an NTP message to travel from one device to the other.
Figure 25-1 Basic work flow of NTP
IP network
IP network
IP network
IP network
Device BDevice A
Device B
Device A
Device B
Device A
Device B
Device A
10:00:00 am 11:00:01 am
10:00:00 am
NTP message 10:00:00 am 11:00:01 am 11:00:02 am
NTP message
NTP message
NTP message received at 10:00:03 am
1.
3.
2.
4.
The process of system clock synchronization is as follows:
z Device A sends Device B an NTP message, which is timestamped when it leaves Device A. The
time stamp is 10:00:00 am (T1).
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.
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