[ntp:questions] NTP tunning for OWD measurements

unruh unruh at invalid.ca
Fri Oct 26 17:14:32 UTC 2012

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

> 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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