Huawei AR150 Series Configuration Manual page 97

Huawei AR150&200&1200&2200&3200 Series Enterprise
Routers
Configuration Guide - Device Management
Figure 5-1 Diagram of NTP implementation
NTP packet
Step1:
Step2:
Step3:
NTP Packet received at 10:00:03
Step4:
The process of synchronizing the system clock is as follows:
1.
2.
3.
4.
5.
NOTE
Issue 05 (2014-01-16)
10:00:00am
RouterA
NTP packet
RouterA
NTP packet 10:00:00am
RouterA
RouterA
RouterA sends an NTP message to RouterB. The message carries an initial timestamp,
10:00:00 a.m. (T1), indicating the time when it leaves RouterA.
When the NTP message reaches RouterB, RouterB adds the timestamp 11: 00:01 a.m. (T2)
to the NTP message, indicting the time when RouterB receives the message.
When the NTP message leaves RouterB, RouterB adds the transmit timestamp 11:00:02
a.m. (T3) to the NTP message, indicating the time when the message leaves RouterB.
When RouterA receives this response message, it adds a new receive timestamp, 10:00:03
a.m. (T4).
RouterA uses the information in the received message to calculate the following two
important parameters:
l Roundtrip delay of the NTP message: Delay = (T4 - T1) - (T3 - T2)
l Clock offset of RouterA by taking RouterB as a reference: Offset = ((T2 - T1) + (T3 -
T4))/2
After the calculation, RouterA knows that the roundtrip delay is 2 seconds and the clock
offset of RouterA is 1 hour. RouterA sets its own clock based on these two parameters to
synchronize its clock with that of RouterB.
The preceding example is only a brief description of the operating principle of NTP. In fact, NTP uses the
standard algorithms in RFC 1305 to ensure the precision of clock synchronization.
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5 NTP Configuration
RouterB
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