[ntp:questions] Re: NTP Pool rules vs. four-server advice

Richard B. Gilbert rgilbert88 at comcast.net
Mon Dec 19 15:05:39 UTC 2005

Michael Deutschmann wrote:

>I've noticed a conflict between the instructions on the NTP server Pool
>project page and other NTP advice.
>It is commonly advised here to configure ntpd either to slave to 1
>server, or to track 4 or more.  The reason being that, when not slaving
>blindly to one server, NTP's algorithms require 3 good servers, and if you
>know all three will remain good, you may as well blindly track one.
>In contrast, the NTP Pool directions explicitly say to use only 3 pool
>servers when using the pool alone, and only 2 if you have a local
>timeserver at your ISP.  Presumably this also means only 1 pool server if
>you have 2 others, and no pool servers if you have three or more others.
>The intersection of these two rules mean the only possible answer is to
>use one pool server with no backup, or no pool servers at all.
>This means that the [012].*.pool.ntp.org hostnames are of no legitimate
>At present, I have my border NTP server slaved to my ISP's timeserver.
>But that computer went nuts as of last Friday (It's in an endless loop
>cycle of about twenty minutes -- it keeps letting the offsets to it's
>reference servers drift to -2.8s and then resetting itself.), so I'm
>considering adding the pool as a fallback.  But following all the rules,
>this is impossible.
>---- Michael Deutschmann <michael at talamasca.ocis.net>
Any low stratum (1 or 2) NTP server is a fragile resource.   Look at 
what happens to the public open access servers operated by the US 
Government!   The  whole world wants to get time from them and they are 
usually staggering under the load.   While the NIST servers are correct, 
by definition, the time they serve is pretty raggety-ass when it gets to 
you.  This is due to either the load on the servers themselves or 
network congestion or both.

The pool servers, mostly not backed by the government or wealthy 
companies or institutions of higher education, are even more fragile.  
They generally have network connections of rather modest capacities.  I 
suspect that they frequently run, not on the latest and greatest 
hardware, but on the hardware  nobody else wanted!

A lot of people can't place their servers in the pool because:
a.  They don't have a static IP address, or
b.  Their contract with their ISP prohibits them from operating a server

So configure three pool servers and one public access stratum 2 server 
and you comply with both sets of rules.

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