H3c-technologies H3C WX5500E Series Access Controllers Manuel d'utilisateur télécharger pdf (Page 17)

• 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, so Device A synchronizes to Device B.

• It takes 1 second for an NTP message to travel from one device to the other.

Figure 5 Basic work flow of NTP

The synchronization process is as follows:

• Device A sends Device B an NTP message, which is timestamped when it leaves Device A. The

timestamp 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 can calculate the following parameters based on the timestamps:

• 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 a rough description of how NTP works. For more information, see RFC 1305.

NTP message format

NTP uses two types of messages: clock synchronization and NTP control messages. All NTP messages

mentioned in this document refer to NTP clock synchronization messages. NTP control messages are

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H3c-technologies H3C WX5500E Series Access Controllers Manuel d'utilisateur Page 17