Questions are at the bottom.
When adding a time stamp based on the Pi's system time to a data file, I'd like to include some information about how reliable that time is.
For example, if the Pi has not yet made an internet connection since power up, the system time will be quite wrong by hours, days, or worse. If the last opportunity for synching to ntp servers was say 24 hours ago, it could potentially be off by many seconds, and if it were ten minutes ago, it's "good" as far as I'm concerned.
I'm using the following python to capture all of the lines in the response so I can decide later how to interpret them, the [2:] suppresses the header of column labels
stat, msg = commands.getstatusoutput("sudo ntpq -p")
for line in msg.splitlines()[2:]:
print line:
After disconnecting the WiFi, I captured a few responses. According to the page 22.13. Checking the Status of NTP the column labeled when is "how long since last poll (in seconds)" and it increases with time as expected. But I don't understand why between 155 and 434 seconds that "jitter" changes, or between 434 and 687 seconds "delay" changes.
Right now I'm classifying that as an inconsequential bug and ignoring it. To me, the value of "when" is most important
remote refid st t when poll reach delay offset jitter
==============================================================================
*59.124.29.241 118.163.81.61 3 - 155 64 4 15.729 -0.961 17.536
103.226.213.30 .PPS. 1 - 155 64 4 15.423 -0.126 18.942
61.216.153.104 118.163.81.63 3 - 155 64 4 15.871 0.996 16.849
*59.124.29.241 118.163.81.61 3 - 434 64 100 15.729 -0.961 19.605
103.226.213.30 .PPS. 1 - 434 64 100 15.423 -0.126 18.942
61.216.153.104 118.163.81.63 3 - 434 64 100 15.871 0.996 18.834
59.124.29.241 118.163.81.61 3 - 687 64 0 99.582 -37.818 0.000
103.226.213.30 .PPS. 1 - 687 64 0 99.470 -38.008 0.000
61.216.153.104 118.163.81.63 3 - 687 64 0 100.017 -36.608 0.000
After turning WiFi on again and letting it run for a bit, I see the following, which suggests that it's periodically checking.
+59-124-29-241.H 211.22.103.157 3 u 58 64 377 15.857 1.084 1.112
*30-213-226-103- .PPS. 1 u 62 64 137 16.268 1.319 3.777
+61-216-153-104. 118.163.81.63 3 u 132 64 376 16.529 1.836 3.589
+59-124-29-241.H 211.22.103.157 3 u 7 64 377 15.857 1.084 3.690
*30-213-226-103- .PPS. 1 u 10 64 277 16.484 1.211 3.743
+61-216-153-104. 118.163.81.63 3 u 13 64 375 16.529 1.836 3.678
From an earlier experiment, I also received responses like:
+61.216.153.104 211.22.103.157 3 u 8 64 377 18.411 -1.238 16.884
+61.216.153.106 211.22.103.157 3 u 13 64 377 18.180 -0.263 0.915
-59.124.29.241 211.22.103.157 3 u 11 64 377 22.955 -2.006 16.754
*103.18.128.60 140.112.2.189 2 u 7 64 377 17.831 1.108 26.451
and in a separate test, I obtained the following unusual report:
+lithium.constan 18.26.4.105 2 u 18 64 1 216.854 -3.978 1.724
*30-213-226-103- .PPS. 1 u 18 64 1 19.176 1.233 1.815
+59-124-29-241.H 118.163.81.62 3 u 15 64 3 15.366 -1.471 17.176
edit 1: I've taken a look at Install NTP on CentOS which turned up in a search for help trying to understand what "lithium.constan" meant, and I found the command:
ntpdc -c sysinfo
When I use it, I get the following output
system peer: 30-213-226-103-static.chief.net.tw
system peer mode: client
leap indicator: 11
stratum: 2
precision: -20
root distance: 0.01585 s
root dispersion: 0.01375 s
reference ID: [103.226.213.30]
reference time: de849d47.0cf50a40 Sat, Apr 21 2018 1:09:27.050
system flags: auth monitor ntp kernel stats
jitter: 0.001526 s
stability: 0.000 ppm
broadcastdelay: 0.000000 s
authdelay: 0.000000 s
What caught my eye is the "stability" measurement. I'll let it run for a while to see if it becomes non-zero, hoping it might address Question 2 below.
edit 2: Looking at the question timed out, nothing received on ntpdc> loopinfo? I found the command
ntpdc -c loopinfo
Which gives me
offset: 0.000000 s
frequency: -4.696 ppm
poll adjust: 6
watchdog timer: 372 s
ntpdc>
I've turned the WiFi off about five minutes ago so that explains the watchdog timer being 372 seconds. Is the value for frequency -4.696 ppm what I'm looking for?
Question 1: Is using the lowest value for "when" a reasonable way to gauge roughly how long it's been since the system time has been checked against internet servers?
Question 2: Does the ntp daemon make an estimate of system clock drift rate available in any way? If it's say 10 ppm or 100 ppm, is this reported somehow?
Question 3: What do the characters in the first column such as '+', '*', '-', or absence thereof indicate?
System: using Raspbian GNU/Linux 8 (jessie)
