[ntp:questions] Reading time offset from ntp variables using ntpq (was: HowTo calibrate system clock frequency using NTP)

Daniel Kabs daniel.kabs at gmx.de
Wed Feb 15 11:32:52 UTC 2006


David L. Mills wrote:
> The peer offset is recorded for each client/server exchange. Let it run 
> for a few hours and record the time of day and the offsets at the 
> beginning and end. Subtract and divide by the interval. It's really not 
> complicated.

"record the time of day" sounds easy, but it is not if I want to measure 
with good precision. Actually that's everything NTP is about and it 
takes a lot to get the time right :-)

I have NTP running using the option "disable ntp". I read the time 
offset using ntpq -c 'rv <assoc_id_of_peer>' at two points in time.

# ntpq -c "rv 32468"
assID=32468 status=9614 reach, conf, sel_sys.peer, 1 event, event_reach,
srcadr=, srcport=123, dstadr=, dstport=123, leap=00,
stratum=1, precision=-18, rootdelay=0.000, rootdispersion=16.220,
refid=DCFa, reach=377, unreach=0, hmode=3, pmode=4, hpoll=6, ppoll=6,
flash=00 ok, keyid=0, ttl=0, offset=-69.648, delay=0.685,
dispersion=2.849, jitter=61.865,
reftime=c79d992c.40e63a5c  Wed, Feb 15 2006 13:12:28.253,
org=c79d9943.421cac08  Wed, Feb 15 2006 13:12:51.258,
rec=c79d9943.58e27e0e  Wed, Feb 15 2006 13:12:51.347,
xmt=c79d9943.57713272  Wed, Feb 15 2006 13:12:51.341,
filtdelay=  5.58    0.68    4.16   5.30    9.84    0.70    0.69  14.76,
filtoffset=-86.17  -69.65  -51.84 -35.91  -20.53    1.51   18.26  29.64,
filtdisp=    0.03    1.02    1.98   2.93    3.89    4.86    5.82    6.80

The offset in this example is offset=-69.648 milliseconds.

I'd like to know: what is the corresponding time of day when the 
client/server exchange took place that lead to this offset measurement.

The above output contains the timestamps "reftime", "org", "rec" and 
"xmt". Which one is the correct one? Maybe it doesn't matter as long as 
I use the same one?

Daniel Kabs

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