NTP Overview 493
■Record for an application when a user logs in to a system, a file is modified, or
Basic Operating Principle of NTP
Figure 131 illustrates the basic operating principle of NTP:
Figure131 Basic Operating Principle of NTP
In Figure 131, Switch A and Switch B are connected using the Ethernet port. They
have independent system clocks. Before implementing automatic clock
synchronization on both Switches, it is assume that:
■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 1 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 (T1) that tells when it left Switch A.
■When the NTP packet arrives at Switch B, Switch B adds a local timestamp
11:00:01am (T2) to it.
■When the NTP packet leaves Switch B, Switch B adds another local timestamp
11:00:02am (T3) to it.
■When Switch A receives the acknowledgement packet, it adds a new timestamp
10:00:03am (T4) to it.
Now Switch A collects enough information to calculate the following two important
parameters:
■The delay for a round trip of an NTP packet travelling between the Switch A and B:
Delay= (T4-T1) - (T3-T2).
■Offset of Switch A clock relative to Switch B clock: offset= ( (T2-T1) + (T4-T3) ) /2.
Network
Network
NTP消息包 10:00:00am
Network
Network
11:00:01am
NTP消息包 10:00:00am 11:00:01am 11:00:02am
NTP消息包 10:00:00am
NTP Packet received at 10:00:03
1.
2.
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4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B