Richard Fleeson said: > Can anyone recommend a low cost or freeware way to measure packet delay and > packet jitter using NTP associations or IEEE-1588? How accurate an answer do you want? Do you want one-way timing, or round trip? If you turn on logging, ntpd will make log files. One has it's opinion of the local clock accuracy. Another has 4 time stamps from each request-response pair: the time the request left the client the time the request arrived at the server the time the response left the server the time the response arrived at the client NTP assumes the transit times are equal and works out the difference between the client clock and the server clock. If you assume the clocks are equal, you can get the one way transit times. If you want good one-way timing, you need good time at both ends. What do you already have in the way of NTP servers? Assuming you have good-enough servers, either local or nearby, then it's easy to collect data. Just setup ntpd with logging, then add all the servers you want to watch with "noselect". NTP doesn't need a lot of cycles. An old PC or corner of a server will probably be fine. It's easy to try. The crystal on a system that does the timekeeping changes frequency with temperature. On modern systems, the heat varies with load. Things may work better on a box that isn't doing anything else. If your local NTP servers aren't good enough, then you need some way to get better time. The usual approach is GPS. GPS is unlikely to work in a machine room. Too much EMI. It might work in an office window. (Some windows have metalic coatings to reflect sun.) It's easy to try. If not, you will need an antenna on the roof -- or an alternate plan. You can get 1U NTP/GPS boxes. I'm not familiar with what's currently available. The low cost approach is to plug a small GPS unit into a PC. Some assembly (soldering) probably required. Again, I don't have specific suggestions. Sorry. You need PPS (pulse per second) for good timing. The timing on the serial stream is generally crap. With PPS, you need a real serial port or GPIO pin. The polling on USB adds 1ms of noise. There are several GPS-HATs available for Raspberry Pis. If you use a Pi 4, that avoids 125 microseconds (1/8th ms) of noise/jitter on the USB ethernet. Raspberry Pis are hard to find these days. Poke me off list if you want help with any of that. -- These are my opinions. I hate spam.

NTP probes may be rate limited(somewhat common) or intentionally delayed (not common) by middleboxes within the IP transport. One-way delays may vary with the NTP client's UDP source port. Actual application traffic can be sampled using GPS-sync'd packet captures.

Putting a GPS disciplined NTP where you can get signal or being the PPS to where it needs to be renders interference moot. If necessary, fiber to copper media converters are a possibility, but depending on how they’re implemented they may add some skew of their own. It might not hurt to look at Ethernet switching products that support time stamping and synchronous Ethernet to see what’s possible there as well. Get Outlook for iOS<https://aka.ms/o0ukef> ________________________________ From: Steven Sommars via time-nuts <time-nuts@lists.febo.com> Sent: Thursday, March 23, 2023 10:20:17 PM To: Discussion of precise time and frequency measurement <time-nuts@lists.febo.com> Cc: Fleeson, Richard <RFLEESON@c-span.org>; Hal Murray <halmurray@sonic.net>; Steven Sommars <stevesommarsntp@gmail.com> Subject: [time-nuts] Re: Using NTP Server Time To Measure WAN Performance NTP probes may be rate limited(somewhat common) or intentionally delayed (not common) by middleboxes within the IP transport. One-way delays may vary with the NTP client's UDP source port. Actual application traffic can be sampled using GPS-sync'd packet captures. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-leave@lists.febo.com