3Com 4200G 12-Port - Page 483

1-3
Figure 1-1 Implementation principle of NTP
IP network
IP network
IP network
IP network
Device BDevice 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.
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 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:
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.