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, [email protected] <[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. > > > > 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, [email protected] <[email protected]> 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@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 _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
