[ntp:questions] NTP absolute accuracy?

Juyong Do jdo at stanford.edu
Tue Jul 3 05:38:04 UTC 2007


Thanks for sharing your experience, John.

That's exactly what I was concerned about, a case w. offset (in your case, 
8ms) but w.o. reported offset (mean close 0 ms although w. some variance) 
since the offset could be critical to my applications.

thanks,
juyong

"John Cochran" <jdc at smof.fiawol.org> wrote in message 
news:f6bh3l$erm$1 at smof.fiawol.org...
> In article <f6bcv5$18p$1 at news.Stanford.EDU>,
> Juyong Do <jdo at stanford.edu> wrote:
>>Thanks Jason.
>>
>>I guessed so since it wouldn't be possible under the network environments 
>>as
>>you pointed out.
>>
>>Then, my question is how to detect a case with absolute time inaccuracy
>>beyond a certain limit? Is the Root Dispersion the best indicator? How 
>>about
>>Offset and RTT? Also, I'm wondering whether there is any way to find out 
>>the
>>status of network connection such as software and hardware delays---are
>>there any parameters about them?
>>
>>Basically, I'm trying to find or combine parameters to detect a certain
>>outage case where absolute time inaccuracy exceeds my limit. Let me know 
>>if
>>anyone has experience on this.
>
> What you're looking for is effectively impossible if your absolute time
> accuracy tolerance is low. As a very simple example, my computer at home 
> runs ntp
> and for a while I was a member of the ntp pool. Here is what I experienced
>   1. When I was syncronizing my computer to a set of public NTP servers, 
> the health
>      checks that the ntp pool server made against my computer showed a 
> relatively
>      wide range of time offsets (+/- 20 milliseconds), but the offsets 
> were centered
>      around 0 milliseconds. Overall, my computer serving pretty good time 
> to the
>      internet.
>
>   2. I later purchased a Garmin GPS 18 LVC receiver and hooked it up to my
>      computer using the PPS interface. After I did that, the health checks 
> made
>      by the NTP pool against my computer were suddenly much more stable 
> with a
>      variance of only 1 to 2 milliseconds. However they had a persistent 
> offset
>      of 8 milliseconds from 0.
>
> Given the above data and given the fact that I'm using an ADSL connection 
> to the
> internet, I can only assume that I have an asymetrical delay to and from 
> the public
> internet. This delay gets canceled out for systems accessing my computer 
> from
> the public internet, but that's only because my system was offset by the 
> amount
> of asymetry. For computers within my network, they were offset from the 
> correct
> "absolute" time by about 8 milliseconds.
>
> So what you need to do is figure out what your tolerance for time is and 
> then
> setup your system accordingly. If your tolerance is low enough, then 
> simply using
> publically available NTP servers should be good enough (low millisecond 
> tolerance
> if you select your servers wisely). If you have tighter tolerances, then 
> you just
> may have to add GPS, and WWV receivers to the mix and have the various 
> computers
> with the receivers cross syncronize to each other.
> 





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