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As from the title, I would like to know what sources “pool.ntp.org” and other big NTP service providers use in order to ensure they are in sync and serving clients the exact date and time.

Marco
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2 Answers2

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Big providers will use an atomic clock - normally not direct but via radio. Europe and America have at least one clock sending time via Radio.

Then you have navigation entworks - GPS and the russian and european alternatives. By definition all of them use an atomic clock in every satellite (that is how you measure distance to them).

TomTom
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  • But would they be in "pool.ntp.org" ? When orgs get that big they often keep such NTP servers to themselves (unless they are related to public service etc)... – Anon Feb 08 '19 at 10:11
  • Ah -no? OBVIOUSLY not. And obviously systems in pools define their stratum. Unless they lie... – TomTom Feb 08 '19 at 10:31
  • @TomTom is entwork actually a typo? I’ve read it twice on both your A and your comment to he other A. Also could you clarify your last “unless they lie” statement in this comment? – Marco Feb 09 '19 at 06:55
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This will usually depend on which starta server you happen to pick and what they use as their source(s). Those sources can be:

  • Another NTP server (usually from a lower strata)
  • Signals based on time from an atomic clock:
    • Via a satellite signal (e.g. GPS)
    • Via a radio signal (e.g. the UK has a radio broadcast known as the The Time from NPL)

Generally it is only people who are re-serving time tend to need access to very low strata refclocks and NTP servers (see the Rules of Engagement NTP page) but it is situation and courtesy enforcing this rule.

Regarding pool.ntp.org: because it is a volunteer effort about distribution load, the source used by a server within the pool could be nearly anything. The following is from the How do I join pool.ntp.org? page:

Note that it is not required that your server is a stratum 1 or 2 server - as this project is about load distribution mostly, there is no reason why a stratum 3 or even stratum 4 server shouldn't join.

Sometimes it is possible to see the source chosen by an NTP server you are connected to (this will depend on your NTP client, the permissions of the NTP server in question etc). For example, this is what I see when connected to Apple's euro timeserver:

$ ntpq -pn
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 17.253.108.125  .GPSs.           1 u    2  512    1   25.700    1.495   0.467

References:

Anon
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  • "a nearby atomic clock" - make that "an atomic clock". At lease in europe germany sends a time signal that can be picked up via Radio all over europe and that qualifies you as Stratum 1. I think another radio source is in the UK and one in the USA - not sure about Africa and China. – TomTom Feb 08 '19 at 08:13
  • Also in addition to GPS you have also the russian naivgation entwork. All those depend on the navigation signals acutally being backed by atomic clocks which are terribly in sync - it is how they work. So, those are direct similar worldwide availabl signals. – TomTom Feb 08 '19 at 08:14