[ntp:questions] NTP no internet connection

Unruh unruh-spam at physics.ubc.ca
Sat Feb 9 23:11:38 UTC 2008


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

>Unruh wrote:
>> "Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>> 
>> 
>>>flyersix wrote:
>>>
>>>>I have a network that I would like to cordinate the time in without
>>>>exposing it to the internet for NTP servers.
>>>>
><snip>
>>>The problem with this is that your clock is not being disciplined.  This 
>>>means that, in addition to not necessarily having the correct time, it 
>>>may not be ticking at exactly 1 second per second.  Even worse, if the 
>>>temperature is not controlled, the clock frequency and, therefore, the 
>>>tick rate, can change from hour to hour.
>> 
>> 
>> So what? The other clocks will simply follow that one, whatever its time.
>> If its clock drift rate is 300ppm there might be trouble, but in general
>> that is not what happens.
>> 
>> 
>> 
>>>Now, imagine other machines trying to synchronize with this server.  I 
>>>think of it as one drunk driver trying to follow another.  Nobody is 
>>>going to crash into anything but the various machines trying to 
>>>synchronize to the server will probably be all over the map.
>> 
>> 
>> What does "all over the map" mean. they will be withing a few tens of
>> microseconds of that server. 

>I mean that a system with a GPS based hardware reference clock 
>synchronizes a mob of other machines more closely than a system using a 
>bunch of internet servers can.  The later case is more of a "moving 

No, it does not synchronize it any better. Both are equally synchronized.
Now that synchronized time will be much closer to what the rest of the
world thinks of as the time, but does not mean that all of the machines
will not report almost exactly ( to 10s of usec) same (wrong) time. The
only advantage of a GPS reference is that the common time will be the same
as the rest of the worlds. 

>target".  Those internet servers MAY know the correct time to within a 
>few microseconds but, by the time the internet is done with the query 
>and response packets, the time is a "Scientific Wild Ass Guess" with an 
>uncertainty of many milliseconds.  As ntpd changes its opinion as to who 
>has the best time, and switches from server to server the time tends to 
>bounce around a bit.  As the internet changes its behavior from 
>night-time not very busy to day-time traffic jam the quality of the time 
>deteriorates.  That "rock solid" GPS reference makes a big difference.


??? He is on a private network with absolutely no access to the network.
What the network time is is irrelevant. 
I do not dispute that having a GPS receiver will make sure that that server
keeps very close ( a few usec) to "true time". But it will make no
difference to how well his various machines are synched to each other.





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