To understand the working of the network time protocol, you need to understand about stratum.
It's all about reliability.
The highest level of reliability is the atomic clock.
That's because atomic clocks are highly accurate in terms of stratum. The atomic clock is getting Stratum 1.
Now, in time synchronization, you might have servers that synchronize directly with the atomic clock.
And if you have a server that synchronizes with the atomic clock, well, it gets
the stratum of two, which indicates that they are very close to the time source.
Now, if you have another level of time servers that is
synchronizing with the Stratum 2 server, then these get Stratum 3.
And then there is you, your local server that needs to make a choice how to synchronize.
Now, how is that going to work? Well, typically the server is reaching out to multiple servers.
Let's say 1 stratum 2, 1 stratum 3, 1 stratum 1.
And then the NTP server has a choice.
So that means that you have three different servers. And if everything is going all right, you get the time from three
different, different servers. And if one of these servers drops, then there is no problem. Now, it's very easy to understand.
Your local NTP process will always go for the highest stratum. So if it can reach
the atomic clock, you will get a Stratum 2 on your own server. Which means that
if you are synchronizing as a Stratum 2 with this Stratum 3 server, well, that's interesting.
You will never synchronize with that because yourself you are more reliable. Right now you
can also configure your own server with its local stratum for local time configuration. It's
pretty common as a backup that your server is synchronized as stratum 10.
And that means that you indicate that your stratum is not very reliable. But
if anything goes wrong in the Internet communication and other time clients in your
network needs time, they can always fetch the time from your local server.
Sometimes you will also find that a stratum of 16 is communicate well.
If 16 is communicated, that means that this time server is not reliable.
So that will be excluded from synchronization in the NTP protocol.