Figure 1-1Implementation principle of NTP

NTP message 10:00:00 am

IP network

1. Device A

Device B

NTP message 10:00:00 am 11:00:01 am

IP network

2. Device A

Device B

NTP message

10:00:00 am 11:00:01 am 11:00:02 am

IP network

3. Device A

Device B

 

NTP message received at 10:00:03 am

 

4. Device A

IP network

Device B

The procedure of synchronizing the system clock is as follows:

zDevice A sends an NTP message to Device B, with a timestamp 10:00:00 am (T1) identifying when it is sent.

zWhen the message arrives at Device B, Device B inserts its own timestamp 11:00:01 am (T2) into the packet.

zWhen the NTP message leaves Device B, Device B inserts its own timestamp 11:00:02 am (T3) into the packet.

zWhen receiving a response packet, Device A inserts a new timestamp 10:00:03 am (T4) into it.

At this time, Device A has enough information to calculate the following two parameters:

zDelay for an NTP message to make a round trip between Device A and Device B: Delay = (T4 -T1)-(T3-T2).

zTime offset of Device A relative to Device B:

Offset = ((T2 -T1) + (T3 -T4))/2.

Device A can then set its own clock according to the above information to synchronize its clock to that of Device B.

For detailed information, refer to RFC 1305.

NTP Implementation Modes

According to the network structure and the position of the local device in the network, the local Ethernet device can work in multiple NTP modes to synchronize the clock.

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3Com WX3000 operation manual NTP Implementation Modes, 1Implementation principle of NTP