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1. Device A sends Device B an NTP message, which is timestamped when it leaves Device A.
The time stamp is 10:00:00 am (T1).
2. When this NTP message arrives at Device B, Device B adds a timestamp showing the time
when the message arrived at Device B. The timestamp is 11:00:01 am (T2).
3. When the NTP message leaves Device B, Device B adds a timestamp showing the time when
the message left Device B. The timestamp is 11:00:02 am (T3).
4. When Device A receives the NTP message, the local time of Device A is 10:00:03 am (T4).
Up to now, Device A can calculate the following parameters based on the timestamps:
• The roundtrip delay of the NTP message: Delay = (T4 – T1) – (T3 – T2) = 2 seconds.
• Time difference between Device A and Device B: Offset = ((T2 – T1) + (T3 – T4)) /2 = 1 hour.
Based on these parameters, Device A can be synchronized to Device B.
This is only a rough description of the work mechanism of NTP. For more information, see the related
protocols and standards.
NTP architecture
NTP uses stratums 1 to 16 to define clock accuracy, as shown in Figure 6. A lower stratum value
represents higher accuracy. Clocks at stratums 1 through 15 are in synchronized state, and clocks at
stratum 16 are not synchronized.
Figure 6 NTP architecture
Typically, a stratum 1 NTP server gets its time from an authoritative time source, such as an atomic
clock, and provides time for other devices as the primary NTP server. The accuracy of each server is
the stratum, with the topmost level (primary servers) assigned as one and each level downwards in
the hierarchy assigned as one greater than the preceding level. NTP uses a stratum to describe how
many NTP hops away a device is from the primary time server. A stratum 2 time server receives its
time from a stratum 1 time server, and so on.
To ensure time accuracy and availability, you can specify multiple NTP servers for a device. The
device selects an optimal NTP server as the clock source based on parameters such as stratum. The
clock that the device selects is called the reference source. For more information about clock
selection, see the related protocols and standards.
Primary servers
(Stratum 1)
Authoritative
clock
Server
Secondary servers
(Stratum 2)
Tertiary servers
(Stratum 3)
Quaternary servers
(Stratum 4)
Client Symmetric
peer
Symmetric
peer
Broadcast/multicast
server
Broadcast/multicast
client
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