3Com 8807 - Page 754

754 CHAPTER 66: NTP CONFIGURATION
Figure 176 Basic operating principle of NTP
In the figure above, Switch A and Switch B are connected through the Ethernet
port. They have independent system clocks. Before implement automatic clock
synchronization on both switches, we assume that:
■ Before synchronizing the system clocks on Switch A and B, the clock on Switch
A is set to 10:00:00am, and that on B is set to 11:00:00am.
■ Switch B serves as an NTP time server. That is, Switch A synchronizes the local
clock with the clock of B.
■ It takes one second to transmit a data packet from either A or B to the opposite
end.
The system clocks are synchronized as follows:
■ Switch A sends an NTP packet to Switch B. The packet carries the timestamp
10:00:00am (T
1
) that tells when it left Switch A.
■ When the NTP packet arrives at Switch B, Switch B adds a local timestamp
11:00:01am (T
2
) to it.
■ When the NTP packet leaves Switch B, Switch B adds another local timestamp
11:00:02am (T
3
) to it.
■ When Switch A receives the acknowledgement packet, it adds a new
timestamp 10:00:03am (T
4
) to it.
Now, Switch A collects enough information to calculate the following two
important parameters:
NTP packet 10:00:00AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP
packet
10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP packet 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.
NTP 10:00:00AMNTP 10:00:00AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.
NTP packet 10:00:00AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP
packet
10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP packet 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.
NTP 10:00:00AMNTP 10:00:00AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP 10:00:00AM 11:00:01AM 11:00:02AM
NETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NETWORK
LS_A LS_B
NETWORKNETWORK
LS_A LS_B
------------- --------------------- ------------ ------------------ ------------- --------------------- -----
NTP Packet received at 10:00:03
1.
2.
3.
4.