[ntp:questions] NTP tunning for OWD measurements

unruh unruh at invalid.ca
Sat Oct 27 19:35:39 UTC 2012


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. 


>
>
> 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:
>> 
>> > 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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