We have ten thousands or more of clients in a local network to be monitored,
thus we need to write every client's ip address into config file, and every time we
add or remove client devices we must edit the monitor's config file manually.
As a contrast, passively logging client's transmit timestamp doesn't need any priori knowledge of
the clients, we could monitor every surviving ntp client and generate an alarm when
one's time/polling interval/root delay or something else is out of threshold automatically.