H3C Technologies H3C S3100 Series Switches - Manual (Page 465)

Operation Manual – NTP
H3C S3100-52P Ethernet Switch Chapter 1 NTP Configuration
1-3
Network
Network
NTP Packet10:00:00am
Network
Network
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet10:00:00 am
Network
Network
11:00:01 am
10:00:00 am 11:00:01 am 11:00:02 am
10:00:00 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet10:00:00am
Network
Network
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet 10:00:00 am
Network
Network
11:00:01 am
10:00:00 am11:00:01 am11:00:02 am
10:00:00 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet10:00:00am
Network
Network
11:00:01am
10:00:00am 11:00:01am 11:00:02am
10:00:00am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet10:00:00 am
Network
Network
11:00:01 am
10:00:00 am 11:00:01 am 11:00:02 am
10:00:00 am
NTP Packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP Packet10:00:00am
Network
Network
11:00:01am
10:00:00am 11:00:01am 11:00:02am
NTP Packet received at 10:00:03
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP Packet
NTP Packet
Network
Network
NTP packet 10:00:00 am
Network
Network
11:00:01 am
10:00:00 am11:00:01 am11:00:02 am
10:00:00 am
NTP packet received at 10:00:03 am
1.
2.
3.
4.
LS_A
LS_A
LS_A
LS_A
LS_B
LS_B
LS_B
LS_B
NTP packet
NTP packet
Figure 1-1
Implementation principle of NTP
The procedure of synchronizing the system clock is as follows:
z
LS_A sends an NTP packet to LS_B, with a timestamp 10:00:00 am (T
1
)
identifying when it is sent.
z
When the packet arrives at LS_B, LS_B inserts its own timestamp 11:00:01 am (T
2
)
into the packet.
z
When the NTP packet leaves LS_B, LS_B inserts its own timestamp 11:00:02 am
(T
3
) into the packet.
z
When receiving a response packet, LS_A inserts a new timestamp 10:00:03 am
(T
4
) into it.
At this time, LS_A has enough information to calculate the following two parameters:
z
Delay for an NTP packet to make a round trip between LS_A and LS_B:
Delay = (T
4
-T
1
)-(T
3
-T
2
).
z
Time offset of LS_A relative to LS_B:
Offset = ((T
2
-T
1
) + (T
3
-T
4
))/2.
LS_A can then set its own clock according to the above information to synchronize its
clock to that of LS_B.
For detailed information, refer to RFC1305.