3Com 4200G 12-Port Configuration Manual page 483

Figure 1-1 Implementation principle of NTP
NTP message 10:00:00 am
Device A
1.
Device A
2.
NTP message
Device A
3.
NTP message received at 10:00:03 am
Device A
4.
The procedure of synchronizing the system clock is as follows:
Device A sends an NTP message to Device B, with a timestamp 10:00:00 am (T
it is sent.
When the message arrives at Device B, Device B inserts its own timestamp 11:00:01 am (T
the packet.
When the NTP message leaves Device B, Device B inserts its own timestamp 11:00:02 am (T
into the packet.
When Device A receives the NTP message, the local time of Device A is 10:00:03am (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:
Time offset of Device A relative to Device B:
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.
IP network
NTP message 10:00:00 am
IP network
10:00:00 am 11:00:01 am
IP network
IP network
Delay = (T -T
4 1
Offset = ((T -T ) + (T
2 1
1-3
Device B
11:00:01 am
Device B
11:00:02 am
Device B
Device B
)-(T -T ).
3 2
-T ))/2.
3 4
) identifying when
1
) into
2
)
3