[ntp:questions] NTP tunning for OWD measurements
pret3nder at gmail.com
pret3nder at gmail.com
Sat Oct 27 19:48:01 UTC 2012
Ok, thanks for clearing that up for me. Makes sense :-)
On Saturday, October 27, 2012 8:35:40 PM UTC+1, unruh wrote:
> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
>
> > Yes, circuit switched networks don't have this problem,
>
> > for sure.
>
> > If, for example, having a GPS on S1, and using it as a
>
> > timeserver for X1, and considering the incoming and outgoing
>
> > delay on that path are equal, when measuring the OWD
>
> > between S1 and X1, I would get a measurement with an accuracy
>
> > equal to the offset reported by NTP, right?
>
>
>
> It it really were equal, then the time on X1 would not have an offset,
>
> since ntpd would drive them to equality. The problem is that they are
>
> not equal. and thus you get fluctuations in the offset and ntpd tried to
>
> drive the clock to eliminate that offset. If the two delays are equal
>
> then you really have nothing to measure. The one way equals half the
>
> delay,always.
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> >
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> >
>
> > On Saturday, October 27, 2012 7:34:46 PM UTC+1, unruh wrote:
>
> >> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
>
> >>
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> >> > The accuracy requirement is not written on stone,
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> >>
>
> >> > but <1ms would be the goal we're aiming for.
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> >>
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> >> >
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> >>
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> >> > I think you made some confusions on the units
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> >>
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> >> > there (usec and sec, when I think those are in ms),
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> >>
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> >>
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> >>
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> >> usec and msec I should have said.
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> >>
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> >>
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> >>
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> >> > but I got your point. The way NTP works, it estimates
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> >>
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> >> > the one-way delays as RTT/2 and uses that to correct
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> >>
>
> >> > the computer clock to UTC. So, without having an external
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> >>
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> >>
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> >>
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> >> Yes.
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> >>
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> >>
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> >>
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> >> > source on both ends that can correct the computer
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> >>
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> >> > clock with a delay <1ms, I can't get that accuracy, right?
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> >>
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> >>
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> >>
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> >> Yes. All you can say is that it lies between all of the delay being on
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> >>
>
> >> the outgoing trip and all the delay on the ingoing. Now, I agree that is
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> >>
>
> >> unlikely, but certainly if say the outgoing delay is .6 of the total
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> >>
>
> >> delay and the ingoing is .4, you could not tell that. For example the
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> >>
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> >> outgoing path could go through different routers, switches etc, than the
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> >>
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> >> ingoing path. Eg, Toronto to Vancouver could go through Dallas while
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> >>
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> >> Vancouver to Toronto could go through chicago instead. (and the
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> >>
>
> >> equivalent for shorter hops). Each packet is independent and is routed
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> >>
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> >> independently of any prior packets passing either way. The internet is
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> >>
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> >> set up to handle each packet independently on purpose to increase fault
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> >>
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> >> tolerance. An internet connection does not establish a permanant two way
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> >>
>
> >> link between machine as old telephone calls did.
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> >>
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> >>
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> >>
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> >>
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> >>
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> >> >
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> >>
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> >> >
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> >>
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> >> > On Saturday, October 27, 2012 6:15:30 PM UTC+1, unruh wrote:
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> >>
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> >> >> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
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> >>
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> >> >>
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> >>
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> >> >> > OWAMP can refer to One-Way Active Measurement Protocol,
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> >>
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> >> >>
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> >>
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> >> >> > see http://tools.ietf.org/html/rfc4656, or the tool that follows
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> >>
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> >> >>
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> >>
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> >> >> > that protocol to measure one-way delay,
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> >>
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> >> >>
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> >>
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> >> >> > see http://www.internet2.edu/performance/owamp/
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
>
> >> >> > NREN, as stated before, means National Research and Education Network.
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
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> >> >> > @unruh: yes, I was talking in the range of microseconds,
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> >>
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> >> >>
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> >>
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> >> >> > not single microsecond accuracy!
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >> Then why do you not say WHAT your accuracy requirement is? You still
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> >>
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> >> >>
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> >>
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> >> >> have not done so.
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >> > This is the current output of ntpq -p on X1 server:
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
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> >> >> > [alias at x1 ntp]# ntpq -p
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> >>
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> >> >>
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> >>
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> >> >> > remote refid st t when poll reach delay offset jitter
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> >>
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> >> >>
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> >>
>
> >> >> >=============================================
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> >>
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> >> >>
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> >>
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> >> >> > *time01.nren.pt .DCFa. 1 u 7 16 377 5.661 0.021 0.015
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> >>
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> >> >>
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> >>
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> >> >> > time02.nren.pt .DCFa. 1 u 6 16 377 1.240 0.078 0.026
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> >>
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> >> >>
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> >>
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> >> >> > time04.nren.pt .DCFa. 1 u 2 16 377 2.324 0.102 0.009
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> >>
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> >> >>
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> >>
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> >> >> > time03.nren.pt .DCFa. 1 u - 16 377 8.757 0.109 0.011
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
>
> >> >> > If the delay, offset and jitter values are shown in milliseconds,
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> >>
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> >> >>
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> >>
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> >> >> > I am led to believe X1 currently has an offset to time01 stratum
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> >>
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> >> >>
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> >>
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> >> >> > 1 server of 21 microseconds, right? If S1 has the same offset,
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> >>
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> >> >>
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> >>
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> >> >> > I can expect a maximum error of 42 microseconds on the owd
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> >>
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> >> >>
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> >>
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> >> >> > measurements, ignoring the errors caused by the delay asymmetry.
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> >>
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> >> >>
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> >>
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> >> >> > Is this correct?
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >> No. It means that the current offset -- the difference between the
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> >>
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> >> >>
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> >>
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> >> >> measurement of the mean time between sent and received-- for time01 is
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> >>
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> >> >>
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> >>
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> >> >> .021 us. That does NOT mean that your system is withing .021 sec on the
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> >>
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> >> >>
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> >>
>
> >> >> true time. Note that all four of the time01-4 are presumably equivalent
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> >>
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> >> >>
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> >>
>
> >> >> clocks and the scatter for the offset is more like .1ms. The delays are
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> >>
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> >> >>
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> >>
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> >> >> 5ms, so the best you can say is that the real time lies between +- 2.6ms
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> >>
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> >> >>
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> >>
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> >> >> since yo uhave no idea what the difference between outgoing and incoming
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> >>
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> >> >>
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> >>
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> >> >> delay times is. THAT IS WHY YOU NEED GPS ON BOTH ENDS.
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
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> >> >> > Pedro
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> >>
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> >> >>
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> >>
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> >> >> >
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> >>
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> >> >>
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> >>
>
> >> >> > On Saturday, October 27, 2012 5:19:09 PM UTC+1, Richard B. Gilbert wrote:
>
> >>
>
> >> >>
>
> >>
>
> >> >> >> On 10/26/2012 5:56 PM, pret3nder at gmail.com wrote:
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> >>
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> >> >>
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> >>
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> >> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >> > Hi all, and thank you for your answers. I'm afraid I might not have
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> >>
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> >> >>
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> >>
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> >> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >> > been clear about my objectives, so I'll try to explain clearer.
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> >>
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> >> >>
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> >>
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> >> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >> > I'll also try and keep the lines smaller, and please, excuse me if
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> >>
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> >> >>
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> >>
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> >> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >> > I make any mistakes, as english is not my native language.
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> >>
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> >> >>
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> >>
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> >> >> >>
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> >> >>
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> >> >> >> >
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> >>
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> >> >>
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> >> >> >>
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> >>
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> >> >>
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> >>
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> >> >> >> > This project involves a NREN, which interconnects several institutions.
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> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >>
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> >> >> >> I give up? WHAT IS "NREN"?????
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> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >> >> >>
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> >> >>
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> >>
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> >> >> >> <snip>
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> >>
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> >> >
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> >
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