[ntp:questions] NTP tunning for OWD measurements

pret3nder at gmail.com pret3nder at gmail.com
Fri Oct 26 21:56:57 UTC 2012


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.
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.

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.

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.

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.

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.
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?

Hope I've made myself clearer this time.

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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