[ntp:questions] NTP tunning for OWD measurements

Richard B. Gilbert rgilbert88 at comcast.net
Sun Oct 28 20:42:58 UTC 2012


On 10/27/2012 3:48 PM, pret3nder at gmail.com 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, 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),
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>>>>
>>
>>>>
>>
>>>>
>>
>>>> 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
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>>>>
>>
>>>>> 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.
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>>>>
>>
>>>>
>>
>>>>
>>
>>>>
>>
>>>>
>>
>>>>>
>>
>>>>
>>
>>>>>
>>
>>>>
>>
>>>>> 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,
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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> see http://tools.ietf.org/html/rfc4656, or the tool that follows
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> that protocol to measure one-way delay,
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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> see http://www.internet2.edu/performance/owamp/
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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>>
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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> NREN, as stated before, means National Research and Education Network.
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>>
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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> @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.
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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:
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>>
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> [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.
>>
>>>>
>>
>>>>>>
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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
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>>
>>
>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>> 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.
>>
>>>>
>>
>>>>>>
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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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>>>>
>>
>>>>>>
>>
>>>>
>>
>>>>>>>>> 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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>>>>>>
>>
>>>>
>>
>>>>>>>>> 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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>>>>
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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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>>>>
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>>>>>>>>
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>>>>
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>>>>>>
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>>>>
>>
>>>>>>>> <snip>
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>>>>
>>
>>>>>
>>
>>>
>


Please folks.  All the ">>>>>>"  conveys nothing, at least as far as I
can see.



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