[ntp:questions] NTP absolute accuracy?
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.
"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:
>>I guessed so since it wouldn't be possible under the network environments
>>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
>>Offset and RTT? Also, I'm wondering whether there is any way to find out
>>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
>>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
> wide range of time offsets (+/- 20 milliseconds), but the offsets
> were centered
> around 0 milliseconds. Overall, my computer serving pretty good time
> to the
> 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
> 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
> 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
> the public internet, but that's only because my system was offset by the
> of asymetry. For computers within my network, they were offset from the
> "absolute" time by about 8 milliseconds.
> So what you need to do is figure out what your tolerance for time is and
> setup your system accordingly. If your tolerance is low enough, then
> simply using
> publically available NTP servers should be good enough (low millisecond
> 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
> with the receivers cross syncronize to each other.
More information about the questions