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

William Unruh unruh at invalid.ca
Thu Dec 4 15:58:50 UTC 2014


On 2014-12-04, 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.

Under what circumstances? Nothing can guarentee a majority. 100001
cannot guarentee a majority. Except 1 I guess. As long as it is working
it is the majority. Of course if it stops working all together, then not
either. 3 guarentees a majority if only one stops or stops sending out
correct time. etc. 

>
>
> David
>
>> 
>> Brian Utterback



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