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" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > In article <[EMAIL PROTECTED]>, > Juyong Do <[EMAIL PROTECTED]> 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. > _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
