Stratum 0 are the highly exact time sources such as atomic clocks or GPS clocks, these are our reference time devices.
If installed it will run once at boot time to set up your time according to Ubuntu's NTP server.
Later on anytime a new interface comes up it retries to update the time - while doing so it will try to slowly drift time as long as the delta it has to cover isn't too big.
Note: These examples are based on the Cent OS 7 operating system so steps may vary slightly for other Linux distributions.
In this version chronyd is installed by default, however we will still cover the older ntpd for completeness as this is still widely used.
Behind this simple description, there is a lot of complexity - there are tiers of NTP servers, with the tier one NTP servers connected to atomic clocks, and tier two and three servers spreading the load of actually handling requests across the Internet.
Also the client software is a lot more complex than you might think - it has to factor out communication delays, and adjust the time in a way that does not upset all the other processes that run on the server. That shall ensure that no two time syncing services are fighting and also to retain any kind of old behaviour/config that you had through an upgrade.This configuration is defined by ‘rtcsync’ in /etc/The hwclock command can also be used to display the current time of the hardware clock as shown below.NTP servers work based on a layered hierarchy referred to as stratum, starting at stratum 0.But luckily all that complexity is hidden from you! But it also implies that on an upgrade from a former release ntp/ntpdate might still be installed and therefore renders the new systemd based services disabled.timedatectl status Local time: Fri 2016-04-29 UTC Universal time: Fri 2016-04-29 UTC RTC time: Fri 2016-04-29 Time zone: Etc/UTC (UTC, 0000) Network time on: yes NTP synchronized: no RTC in local TZ: no and thereby no more installed by default.Stratum 2 servers then get their time from the stratum 1 servers, while stratum 3 servers synchronize with stratum 2 sources.