On 2012-10-26, pret3n...@gmail.com <pret3n...@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, pret3n...@gmail.com <pret3n...@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@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 questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
