[ntp:questions] Re: ntpd, boot time, and hot plugging

Brad Knowles brad at stop.mail-abuse.org
Sun Feb 6 19:08:16 UTC 2005

At 8:46 AM -0500 2005-02-06, Richard B. Gilbert wrote:

>  I'm suggesting that ntpd query the usual suspects using iburst and
>  then unconditionally set (not slew) the clock.

	I will allow that perhaps there should be a startup mode where 
this behaviour is used, but I do not believe that this should be the 

	If nothing else, we have the 34 year problem whereby you could 
easily have your clock mis-set to a completely inappropriate value, 
if it's not set closely enough on startup.

>                                                  Assuming that you have
>  a more or less accurate drift file, and use it, why would that not give
>  a fast startup and a time accurate to within, say, twenty milliseconds?
>  The time budget would be something like eight seconds to send four
>  queries, get responses, and make initial time and frequency settings.
>  Compared with 214 seconds to remove 107 milliseconds of a 127
>  millisecond offset, that looks pretty good when you are in a hurry to
>  get you system back on line.

	I don't think that setting a hard eight second limit would be 
wise.  If nothing else, there are potentially serious delays that can 
be caused while waiting for DNS queries to be answered.

	If your primary nameserver is located on the same machine as your 
ntpd server, there could be very nasty name resolution deadlock 
issues which might result, if your startup sequence chooses to start 
named after ntpd, instead of the reverse.  Throw DNSSEC into the mix, 
and you could be in for a very serious world of hurt.

	If all goes well, even without a good local Stratum 1 timeserver, 
and without a good drift file, you can get pretty much full startup 
of ntpd in about eight seconds.

	But then all hell could break loose if things don't go well, and 
tying yourself to a hard eight second time limit would be about the 
worst possible thing you could do under those circumstances.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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