[ntp:questions] The libntp resumee...

Unruh unruh-spam at physics.ubc.ca
Sat Sep 6 00:31:59 UTC 2008


"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>Kay Hayen wrote:
>> Hello Richard,
>> 
>> you wrote:
>> 
>>> The "rules" about how often to query a daemon are not all that
>>> complicated.  The fact that there ARE rules is due to some history;
>>> google for "Netgear Wisconsin" for the sordid details.  For a "second
>>> opinion" google for "DLink PHK".
>> 
>> Fascinating reads indeed, thanks for the pointers. 
>> 
>> What worried me more was how often we can query the local ntpd before it will 
>> have an adverse effect. Meantime I somehow I sought to convince me I should 
>> be able to convince myself that ntpq requests are served at a different 
>> priority (other socket) than ntpd requests are. I didn't find 2 sockets 
>> though.
>> 
>>> Briefly, you use the defaults for MINPOLL and MAXPOLL.  You may use the
>>> "iburst" keyword in a server statement for fast startup.  You may use
>>> the "burst" keyword ONLY with the permission of the the server's owner.
>>> 99.99% of NTP installations will work very well using these rules".  If
>>> yours does not, ask here for help!
>> 
>> Now speaking about our system, not the middleware, with connections as 
>> follows: 
>> 
>> External NTPs <-> 2 entry hosts <-> 8 other hosts.
>> 
>What do you mean by "entry hosts"?

>> And iburst and minpoll=maxpoll=5 to improve the results.

>Use the default values of minpoll and maxpoll!  Ntpd will adjust the 
>polling interval within those limits.  Ntpd is far smarter than you or 

Well, you have too much faith in ntp. It is a whole series of comprimises,
many set up in the days when one second network delays were not unknown. 
And one of ht ekey design criteria in that minpoll/maxpoll is to relieve
congestion on the servers. IF he is using his own servers (not outside
servers) then he can decrease the minpoll/maxpoll pairs ( after all the
refclocks run at minpoll=maxpoll 4) But there is a tradeoff. because of the
design of ntp, if you choose a low maxpoll, you will keep the phase errors
smaller, but at the expense of larger drift errors. (it basically averages
over a time interval a few times longer than the maxpoll interval) A longer
timebase means a longer lever arm for determining the drift. But at the
expense of not having as much data to beat down the statistical errors in
the offset. 

Thus, with ntp if you want an accurate determination of the clock drift,
use a longer poll ( eg if there is a chance of your system loosing
connectivity for a few days) If you want lower phase noise while connected,
use a shorter poll. But remember that servers out there will get extremely
upset if you query them too often. 

Essentially you want to be working the Allan minimum to get rid of both
short and long term. But NTP does not determine where that is. It simply
assumes a value. That assumption is not necessarily very good. 
(Close by clock servers, with heavily used machines-- lots of temp
fluctuations-- and the optimum point is much shorter than the assumption.
Ie, statistical errors are much smaller than clock drift errors. 


>I.  It will normally start by using minpoll and increase the interval 
>after it has initial synchronization.  If network conditions deteriorate 
>it will decrease the poll interval and increase it as conditions 
>improve.  IOW it will use the optimum poll interval for the conditions 
>then obtaining.  If you configured seven servers, you might observe ntpd 
>using seven DIFFERENT poll intervals, one for each server because seven 
>different servers will be reached by at least seven different network paths!
>> 
>> Currently we observe that both entry hosts can both become restricted due to 
>> large offsets on other hosts, so they become restricted and that will make 
>> the software refuse to go on. Ideally that would not happen.
>> 
>> I will try to formulate questions:
>> 
>> When the other hosts synchronize to the entry hosts of our system, don't the 
>> other hosts ntpd know when and how much these entry hosts changed their time 
>> due to input? 
>> 
>> Would NTP would be more robust if we would configure routing on the entry 
>> hosts, so that they can all speak directly with the external NTPs on their 
>> own?
>> 
>> Is the use of ntpdate before starting ntpd recommended and/or does the iburst 
>> option replace it?
>> 
>> Best regards,
>> Kay Hayen




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