On 10/27/2012 3:48 PM, [email protected] wrote:
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, [email protected] <[email protected]> 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, [email protected] <[email protected]> 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, [email protected] <[email protected]> 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@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, [email protected] 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>
Please folks. All the ">>>>>>" conveys nothing, at least as far as I
can see.
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