Operation Manual – NTP
H3C S3100 Series Ethernet Switches
Chapter 1 NTP Configuration
1-3
IP network
IP network
IP network
IP network
Device B
Device 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.
Figure 1-1
Implementation principle of NTP
The procedure of synchronizing the system clock is as follows:
z
Device A sends an NTP message to Device B, with a timestamp 10:00:00 am (T
1
)
identifying when it is sent.
z
When the message arrives at Device B, Device B inserts its own timestamp
11:00:01 am (T
2
) into the packet.
z
When the NTP message leaves Device B, Device B inserts its own timestamp
11:00:02 am (T
3
) into the packet.
z
When receiving a response packet, the local time of Device A is 10:00:03 am (T4).
At this time, Device A has enough information to calculate the following two
parameters:
z
Delay for an NTP message to make a round trip between Device A and Device B:
Delay = (T
4
-T
1
)-(T
3
-T
2
).
z
Time offset of Device A relative to Device B:
Offset = ((T
2
-T
1
) + (T
3
-T
4
))/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.
