[ntp:questions] Synchronizing using servers in stratum 1 and 2

Richard B. gilbert rgilbert88 at comcast.net
Wed Feb 14 19:18:03 UTC 2007

Timo Ruiter wrote:
> Timo Ruiter schreef:
>> Hi all,
>> When using pool.ntp.org the DNS randomly select time servers.  Since 
>> stratum-1 servers and stratum-2 servers share the same pool you will 
>> get a mix of them when using 0.nl.pool.ntp.org, 1.nl.pool.ntp.org, and 
>> 2.nl.pool.ntp.org for instance.
>> When you are assigned *one* stratum-1 server and *two* stratum-2 
>> servers your clock will obviously synchronize to the stratum-1 server. 
>> But that being the case, what is the use of the two stratum-2 servers, 
>> assuming that the stratum-1 server will not go off-line?
>> Could it be so that if the stratum-1 server is not so accurate after 
>> all it would still be selected over the stratum-2 servers (since no 
>> reference stratum-1 server is available)?
>> Will you not be better served instead when you are assigned *three* 
>> stratum-2 servers, so the selection process of ntpd will compare all 
>> three servers equally and choose the best one, switching to another if 
>> that one looks more stable after a while?
>> Regards,
>> Timo
> What I really wanted to know is what is better in terms of stability: 
> *one* stratum-1 server and *two* stratum-2 servers versus *three* 
> stratum-2 servers.
> When having only one stratum-1 server available you might be relying on 
> that server too much; when having three stratum-2 servers (ideally 
> synchronized to three different stratum-1 servers) time *could* be more 
> stable since there are three stratum-1 servers involved, albeit through 
> the stratum-2 servers.
> This is a pure theoretical issue as far as I'm concerned, since who can 
> tell the difference if your servers are 23 milliseconds off...
> So replace "stratum-1" by "stratum-N" and "stratum-2" by 
> "stratum-(N-1)"; the question will remain the same.
> Regards,
> Timo

Think first about adding another server!  Three servers can degenerate 
too easily to the two server case!  When those two servers disagree. . . .

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