How Ntp Works - 3Com 4210G Series Configuration Manual

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:
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.
Device B is used as the NTP time server, namely, Device A synchronizes its clock to that of
Device B.
It takes 1 second for an NTP message to travel from one device to the other.
Figure 25-1 Basic work flow of NTP
NTP message
Device A
1.
Device A
2.
Device A
3.
NTP message received at 10:00:03 am
Device A
4.
The process of system clock synchronization is as follows:
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).
When this NTP message arrives at Device B, it is timestamped by Device B. The timestamp is
11:00:01 am (T2).
When the NTP message leaves Device B, Device B timestamps it. The timestamp is 11:00:02 am
(T3).
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:
The roundtrip delay of NTP message: Delay = (T4–T1) – (T3-T2) = 2 seconds.
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.
10:00:00 am
IP network
NTP message
IP network
NTP message 10:00:00 am
IP network
IP network
Device B
10:00:00 am 11:00:01 am
Device B
11:00:01 am 11:00:02 am
Device B
Device B
25-2