About the exact time

Everyone is already used to the NTP time synchronization service - it is turned on by default or is easily turned on for most popular operating systems. However, what accuracy is achieved? What are the exact time servers, and which can be used by mere mortals? What are the pitfalls, and how to choose the "right" time server?

Server types

Servers are Stratum 1 and 2, 3 (rarely above).

Stratum 1 - receive the exact time directly from the source of the exact time: atomic clock (for example time-a.nist.gov, accuracy - trillionths of a second) or GPS receiver (ntpx.imvp.ru accuracy - billionths of a second). There are servers that receive accurate time via the CDMA cellular network (millionths of a second). When working with ntpd, you can find out the type of server using the ntpq -np command: “PPS” means GPS, “ACTS” means direct connection to the atomic clock (there are other valuable statistics - ping, “ping” (jitter), server rank (1 , 2 ...)).

It is strictly forbidden for “mere” mortals to access the Stratum-1 servers, as the load on them is already very large (and on many Stratum-1 servers there is no public access at all). It is believed that you can connect to Stratum-1 servers or request access to them if you have an NTP server serving at least 100 clients. A complete list of servers can be seen here: support.ntp.org/bin/view/Servers/StratumOneTimeServers . Naturally, it is advisable to connect to servers in your own country.

Stratum 2 - Get accurate time from Stratum-1 servers. When properly configured and selected, the source servers of the exact time have an error of less than 1ms. Usually everyone can connect, but many servers regularly die from the load (for example, time.windows.com). www.pool.ntp.orgsupports round-robin lists of public Stratum-2 NTP servers. This ensures load balancing and is almost always available. You can connect to these servers at the addresses 0.ru.pool.ntp.org 1.ru.pool.ntp.org 2.ru.pool.ntp.org and 3.ru.pool.ntp.org (this is for Russia, they are selected randomly from the list of ~ 50 servers).

Stratum 3 - receive time from Stratum-2 servers, etc.

Practical accuracy

Stratum-1
This is a graph of the difference in system time with the time received from Stratum-1 servers (canonical.com - Stratum-2 server, for comparison).
* .nist.gov - located in the USA, terrible "outbursts" sometimes occur due to the "big" ping. Error from Russian Stratum-1 servers usually fit in + - 1 ms.


Stratum-2
This is the difference with Stratum-2 servers: 4 from the Russian pool, 1 from the European one, the default ubuntu time server (europium.canonical.com), and the same * .nist.gov: The

servers that have a constant error up to 20 ms.

Summary

• Time on servers and workstations should not be allowed to drift, otherwise the time deviation can reach minutes.
• If the accuracy of + -0.05 seconds suits you, you can not bother with the choice of servers, and synchronize with the default server
• If accuracy up to + -0.01 seconds is needed, you need to find the Stratum-2 NTP in your country, build deviation plots from them (Munin + ntp_peers plugin), and discard the “curves”. Under windows, this of course will be difficult to do. Then install ntpd daemon in linux, and give it 3-6 servers for synchronization (he will choose the most "high-quality" ones).
• If you need accuracy up to + -0.001 seconds. And you will provide the exact time service in your organization - connect to Stratum-1 servers in your country. It is highly advisable to make your service public.
• If you need more accurate 0.001 seconds, you only need to configure your Stratum-1 server from GPS or CDMA. In both cases, you need to either lead the antenna from the street, or for the signal to "finish off" (in the case of GPS, it is unrealistic). Atomic clocks with a volume of less than a cubic meter do not yet, so this option disappears

PS. By the way, by default in Ubuntu, time synchronization occurs once at system startup. If uptime is under half a year, time can go a long way. Install ntpd - it adjusts the time constantly and “smoothly” (without sharp jerks, “smearing” time dilation / acceleration). In Windows, synchronization occurs once a day a week with one “jump”, which can cause difficulties when processing logs, if a big difference runs up.

Comrades!

Give a lot of public NTP servers in Russia! Especially Stratum-1 (with them, we generally have a strain, 2-3 pieces for the entire exUSSR). By the way, the exact time can be taken with Glonass, who will undertake to launch the first Glonass-powered server?

PPS I can’t understand which blog to transfer to, tell me :-)