[ntp:questions] NTP tunning for OWD measurements

pret3nder at gmail.com pret3nder at gmail.com
Fri Oct 26 23:46:50 UTC 2012


An NREN is a National Research and Education Network. 
We are talking about 26 servers spread all over the country 
and in the islands, so your figures are a bit off, I'm afraid. $50 
don't cover the expenses associated with getting a GPS for every
single server, and like I said before, it's simply undoable.

And I'm just a student, I don't get any payment for this, instead 
I pay my tuition - as I should. But this is getting out of subject :-)

Anyway, thanks for your remarks and suggestions.

Pedro



On Saturday, October 27, 2012 12:31:28 AM UTC+1, unruh wrote:
> On 2012-10-26, pret3nder at gmail.com <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.
> 
> 
> 
> Whatever and NREN is. 
> 
> 
> 
> > The current architecture is as follows: two main servers (let's call it S1 and S2)
> 
> > placed in the NREN infrastructure at two different places in the country,
> 
> > and several servers (X1, X2, Xi) placed at the edge of the institutions before mentioned.
> 
> > The objective is to do one-way delay tests between S1/S2 <-> Xi, so we can
> 
> > measure one-way latency between the main servers and the servers placed at each
> 
> > institution.
> 
> 
> 
> To measure one way latency you MUST have accurate time sources at each
> 
> end of the one way link. 
> 
> 
> 
> >
> 
> > With this being said, I'll try now to answer some questions. 
> 
> > I can't use GPS/PPS attached to the institutions servers because one) we do 
> 
> > not have physical access to the servers in the institutions and two) it would 
> 
> > carry an extra cost that the NREN is not willing to afford.
> 
> 
> 
> ??? An adequate  gps can be had for $50, so if I place one at each
> 
> place, the total cost is of order a few hundred dollars. Ie,  less
> 
> than your salary for a week, and certainly less than your salary while
> 
> doing this research. 
> 
> >
> 
> > Also, I don't want to use NTP to directly measure the one-way delay, I have OWAMP
> 
> > (more specifically perfSonar - http://www.internet2.edu/performance/pS-PS/)
> 
> > to do that for me.
> 
> 
> 
> It does not matter what you use to measure the delay, you need accurate
> 
> times at both ends to do so. 
> 
> 
> 
> 
> 
> >
> 
> > Now, I mentioned I have access to four stratum 1 servers (with GPS/PPS attached)
> 
> > which are placed in different locations around the country. I plan to use 
> 
> > these as time servers for both S1/S2 and X1, X2, Xi servers. Currently,
> 
> > I only use one of them, and it's the same across S1/S2 and Xi servers.
> 
> 
> 
> OK, It is up to you. However, there is a reasonable chance that there is
> 
> a differential timing delay to some of your S1/2 X1,2.... machines. 
> 
> 
> 
> >
> 
> > What I really want to know is how far can I go in tuning the NTP 
> 
> > configuration in the stratum 1 servers and in the clients (S1/S2, Xi) 
> 
> > to obtain the best possible measurements from OWAMP.
> 
> 
> 
> You cannot go any further than the differential delay between them. 
> 
> 
> 
> > Or, what can I do to reduce the offset of the clocks of the hosts involved
> 
> > in the measurements, having these resources (access to four stratum 1 
> 
> > servers and a network with almost no jitter at all), so that it has a minimum 
> 
> > impact in the OWD measurements?
> 
> 
> 
> Nothing, since you have ruled out the one thing you can do.
> 
> 
> 
> The delays are affected by speed of light in copper wires, the speed of
> 
> the switches and routers along the paths the signals take, etc. You can
> 
> get an estimate by, as I said, plotting the delay vs the offset. from
> 
> the ntp servers. Since you want to minimize the offset, not the drift
> 
> rate of the clocks, you want to make frequent queries of the ntp
> 
> servers-- but to do that you had better get permission from those
> 
> servers. 
> 
> 
> 
> 
> 
> >
> 
> > Hope I've made myself clearer this time.
> 
> 
> 
> Hope I have made myself clearer.
> 
> 
> 
> >
> 
> > Kind Regards,
> 
> > Pedro
> 
> >
> 
> >
> 
> > On Friday, October 26, 2012 6:14:33 PM UTC+1, unruh wrote:
> 
> >> On 2012-10-26, pret3nder at gmail.com <pret3nder at gmail.com> wrote:
> 
> >> 
> 
> >> > Hi all,
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > my name is Pedro Queir?s and I'm currently working on my master thesis related to open source probes to measure the QoS of internet links.
> 
> >> 
> 
> >> > One of the tools I'm using is OWAMP (http://www.internet2.edu/performance/owamp/), to measure one-way delay between hosts. This tool requires NTP running and synchronizing clocks so that it can measure correctly (to a certain point) the one-way delay between hosts.
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> If the one way delays between the hosts are consistantly different, then
> 
> >> 
> 
> >> ntp is no good. The best way is to put up a gps clock on each computer,
> 
> >> 
> 
> >> and use that to set the time on each machine. Then the ntp offset
> 
> >> 
> 
> >> between the computers, and the roundtrip delay will tell you exactly
> 
> >> 
> 
> >> what the one way delay is for each trip. 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> You can get an estimate of the scatter of the one way delays be plotting
> 
> >> 
> 
> >> the delay vs the offset for the clock. It will show clearly the
> 
> >> 
> 
> >> differences in one way delays, but unfortunately will not show any
> 
> >> 
> 
> >> consistant offset.
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> > Now, after reading some articles on NTP, PTP, RADclock, I'm wondering what can I do to improve the NTP configuration, so clocks on my hosts are as close together as possible and measurements are as accurate as possible..
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> Yes, put a gps clock on each of them. 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> > Following the suggestion on some of the RADclock articles I've read, I've configured my hosts with the minpoll/maxpoll 4 iburst options and I'm only using one stratum 1 server on ntpd.conf.
> 
> >> 
> 
> >> > I have access to four stratum 1 servers scattered around the country (Portugal), using GPS as the source for true time, but I have not messed with their configuration.
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > What I'm looking for are ways to tune the configuration of both stratum 1 servers and the hosts I'm doing the measurements on (clients of the stratum 1 NTP servers), so I can have results as accurate as possible of the one-way delay between hosts.
> 
> >> 
> 
> >> > Please keep in mind that I don't want to use external time sources on my hosts, but I have them on the stratum 1 servers.
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> Why not? They are not expensive. Why not use the best possible setup for
> 
> >> 
> 
> >> your work, instead of trying to kludge together a second best
> 
> >> 
> 
> >> possibility.
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> 
> 
> >> > My network consists of fiber optic links to connect several institutions together, so we have very low jitter. Here is the output of ntpq -p on one of the hosts:
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > [alias at probe ~]$ ntpq -p
> 
> >> 
> 
> >> >      remote           refid      st t when poll reach   delay   offset  jitter
> 
> >> 
> 
> >> >===============================================================
> 
> >> 
> 
> >> > *xxx01.xxx.pt  .DCFa.           1 u   12   16  377    5.686    0.082   0.384
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > If you need more information about my current setup (configurations, setup, locations, distances, etc), feel free to ask.
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > Thank you for your time!
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> >
> 
> >> 
> 
> >> > Kind Regards,
> 
> >> 
> 
> >> > Pedro



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