[ntp:questions] NTP tunning for OWD measurements

unruh unruh at invalid.ca
Sat Oct 27 18:34:45 UTC 2012


On 2012-10-27, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
> 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),

usec and msec I should have said.

> 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

Yes.

> source on both ends that can correct the computer
> clock with a delay <1ms, I can't get that accuracy, right?

Yes. All you can say is that it lies between all of the delay being on
the outgoing trip and all the delay on the ingoing. Now, I agree that is
unlikely, but certainly if say the outgoing delay is .6 of the total
delay and the ingoing is .4, you could not tell that. For example the
outgoing path could go through different routers, switches etc, than the
ingoing path. Eg, Toronto to Vancouver could go through Dallas while
Vancouver to Toronto could go through chicago instead.  (and the
equivalent for shorter hops). Each packet is independent and is routed
independently of any prior packets passing either way. The internet is
set up to handle each packet independently on purpose to increase fault
tolerance. An internet connection does not establish a permanant two way
link between machine as old telephone calls did. 


>
>
> 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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