[ntp:questions] NTP tunning for OWD measurements

pret3nder at gmail.com pret3nder at gmail.com
Sat Oct 27 17:28:53 UTC 2012


The accuracy requirement is not written on stone,
but <1ms would be the goal we're aiming for.

I think you made some confusions on the units 
there (usec and sec, when I think those are in ms),
but I got your point. The way NTP works, it estimates 
the one-way delays as RTT/2 and uses that to correct 
the computer clock to UTC. So, without having an external
source on both ends that can correct the computer
clock with a delay <1ms, I can't get that accuracy, right?


On Saturday, October 27, 2012 6:15:30 PM UTC+1, unruh wrote:
> On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
> 
> > OWAMP can refer to One-Way Active Measurement Protocol,
> 
> > see http://tools.ietf.org/html/rfc4656, or the tool that follows
> 
> > that protocol to measure one-way delay, 
> 
> > see http://www.internet2.edu/performance/owamp/
> 
> >
> 
> > NREN, as stated before, means National Research and Education Network.
> 
> >
> 
> > @unruh: yes, I was talking in the range of microseconds,
> 
> > not single microsecond accuracy!
> 
> 
> 
> Then why do you not say WHAT your accuracy requirement is? You still
> 
> have not done so. 
> 
> 
> 
> 
> 
> > This is the current output of ntpq -p on X1 server:
> 
> >
> 
> > [alias at x1 ntp]# ntpq -p
> 
> >      remote           refid      st t when poll reach   delay   offset  jitter
> 
> >=============================================
> 
> > *time01.nren.pt  .DCFa.           1 u    7   16  377    5.661    0.021   0.015
> 
> >  time02.nren.pt  .DCFa.           1 u    6   16  377    1.240    0.078   0.026
> 
> >  time04.nren.pt  .DCFa.           1 u    2   16  377    2.324    0.102   0.009
> 
> >  time03.nren.pt  .DCFa.           1 u    -   16  377    8.757    0.109   0.011
> 
> >
> 
> > If the delay, offset and jitter values are shown in milliseconds,
> 
> > I am led to believe X1 currently has an offset to time01 stratum
> 
> > 1 server of 21 microseconds, right? If S1 has the same offset,
> 
> > I can expect a maximum error of 42 microseconds on the owd 
> 
> > measurements, ignoring the errors caused by the delay asymmetry.
> 
> > Is this correct?
> 
> 
> 
> No. It means that the current offset -- the difference between the
> 
> measurement of the mean time between sent and received-- for time01 is
> 
> .021 us. That does NOT mean that your system is withing .021 sec on the
> 
> true time. Note that all four of the time01-4 are presumably equivalent
> 
> clocks and the scatter for the offset is more like .1ms. The delays are
> 
> 5ms, so the best you can say is that the real time lies between +- 2.6ms
> 
> since yo uhave no idea what the difference between outgoing and incoming
> 
> delay times is. THAT IS WHY YOU NEED GPS ON BOTH ENDS.
> 
> 
> 
> 
> 
> >
> 
> > Pedro
> 
> >
> 
> > 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:
> 
> >> 
> 
> >> > Hi all, and thank you for your answers. I'm afraid I might not have
> 
> >> 
> 
> >> > been clear about my objectives, so I'll try to explain clearer.
> 
> >> 
> 
> >> > I'll also try and keep the lines smaller, and please, excuse me if
> 
> >> 
> 
> >> > I make any mistakes, as english is not my native language.
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > This project involves a NREN, which interconnects several institutions.
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> I give up?  WHAT IS "NREN"?????
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> <snip>



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