[ntp:questions] Number of Stratum 1 & Stratum 2 Peers

William Unruh unruh at invalid.ca
Thu Dec 4 16:00:11 UTC 2014

On 2014-12-04, Rob <nomail at example.com> wrote:
> David Lord <snews at lordynet.org> wrote:
>> Brian Utterback wrote:
>>> On 12/2/2014 4:00 AM, Rob wrote:
>>>> The whole "have 3 servers to select a majority" thing is absolutely not
>>>> required when your servers are accurately synchronized themselves and
>>>> your requirements are "only" within-a-second.  It is true that when you
>>>> have two servers the clients cannot know which one is right, but it is
>>>> trivial to keep servers within a millisecond of eachother with GPS and
>>>> within 10 milliseconds using only network peering.  To that is two
>>>> orders of magnitude better than you require.
>>> Be careful with this generalization. While it may be "trivial", it isn't 
>>> "automatic". I deal with customers all the time that have configured 
>>> exactly two servers on their clients and then are surprised later when 
>>> all of the clients become unsynchronized and start free drifting. I 
>>> always recommend against it. I still think that it takes four to 
>>> guarantee a majority but I don't have proof of that. Someday I will 
>>> spend some time to either prove or disprove it, but alas, time is 
>>> something I don't generally have extra to spend. But you are better off 
>>> with one than two from an operational standpoint.
>> The ntp html docs on selection state that four are needed to
>> guarantee a majority and give an example of this case.
> In practice this problem does not occur when you use only your own
> servers that you monitor and trust, and it confuses people that
> want to setup NTP on their company network.
> They get sent away with "you need to configure and maintain 4 servers
> or better even more".  When the servers either are synced correctly
> or are down, this is not required.

People love to either read or make rules, no matter how senseless. That
way you do not have to think.

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