3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

■

When the message arrives at Device B, Device B inserts its own timestamp 
11:00:01 am (T

2

) into the packet.

■

When the NTP message leaves Device B, Device B inserts its own timestamp 
11:00:02 am (T

3

) into the packet.

■

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:

■

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

).

■

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.

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

Figure 110   Server/client mode

Server

Clock synchronization 

request 

Response 

Network

Client

Works in server mode 
automatically and sends
a response packet

Filters and selects a clock
and synchronizes the local 
clock to that of the preferred 
server