[ntp:questions] Re: Reading time offset from ntp variables using ntpq

David L. Mills mills at udel.edu
Thu Feb 16 12:49:40 UTC 2006


Daniel,

Have you considered what an engineer means by "almost exact?" There is 
always an uncertainty in any physical measurement. Yours is no exception.

Dave

Daniel Kabs wrote:

> Hello Professor Mills!
> 
> Please let me get back to your comment:
> 
>> 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.
> 
> 
> The calculation is not complicated, that's correct. Getting the precise 
> time is. Please see the example as follows.
> 
> According to "Plan B", NTP is configured with "disable ntp". I ran a 
> cron job to query the peer clock variables using ntpq -c 'rv 
> <assoc_id_of_peer>' at 9:00 and 10:00. The billboards are attached below 
> as reference.
> 
> During the two "measurements", the offset (peer.offset) increased by 
> 975.174 ms.
> 
> Given the cron jobs where started on time (which I doubt :-), the 
> "measurement" interval was 3600 s, thus the daily time offset of my 
> system clock will be:
> 
> 975.174 / 3600 * 86400 / 1000 s     = 23.40 s
> 
> However, the difference in peer.reftime (8:58:38.253 to 9:58:47.253) 
> gives an interval of 3609 s, thus the extrapolation yields:
> 
> 975.174 / 3609 * 86400 / 1000 s     = 23.35 s
> 
> The difference in peer.org is 3615.973 s, so the rule of proportions gives:
> 
> 975.174 / 3615.973 * 86400 / 1000 s = 23.30 s
> 
> And finally peer.rec gives a time difference of 3616.977 s, making a 
> daily time offset of
> 
> 975.174 / 3616.977 * 86400 / 1000 s = 23.29 s
> 
> 
> I'd like to know, which calculation is most accurate. What is the 
> timestamp to record when reading the offset?
> 
> 
> Cheers
> Daniel Kabs
> 
> 
> PS: The billboards look like this
> # Reading at 9:00 AM
> assID=32468 status=9614 reach, conf, sel_sys.peer, 1 event, event_reach,
> srcadr=10.0.0.254, srcport=123, dstadr=10.0.2.100, dstport=123, leap=00,
> stratum=1, precision=-18, rootdelay=0.000, rootdispersion=6.516,
> refid=DCFa, reach=377, unreach=0, hmode=3, pmode=4, hpoll=6, ppoll=6,
> flash=00 ok, keyid=0, ttl=0, offset=-19583.268, delay=0.718,
> dispersion=3.282, jitter=49.301,
> reftime=c79eaf2e.40eef1ba  Thu, Feb 16 2006  8:58:38.253,
> org=c79eaf2e.c3a5c1c6  Thu, Feb 16 2006  8:58:38.764,
> rec=c79eaf42.628d5410  Thu, Feb 16 2006  8:58:58.384,
> xmt=c79eaf42.5a03d577  Thu, Feb 16 2006  8:58:58.351,
> filtdelay= 33.29    2.73    0.72    2.75    2.80    2.54    5.90    2.70,
> filtoffset= -19604. -19601. -19583. -19565. -19547. -19531. -19516. 
> -19497.,
> filtdisp=   0.03    1.01    1.95    2.90    3.86    4.80    5.79    6.75
> 
> # Reading at 10:00 AM
> assID=32468 status=9614 reach, conf, sel_sys.peer, 1 event, event_reach,
> srcadr=10.0.0.254, srcport=123, dstadr=10.0.2.100, dstport=123, leap=00,
> stratum=1, precision=-18, rootdelay=0.000, rootdispersion=6.714,
> refid=DCFa, reach=377, unreach=0, hmode=3, pmode=4, hpoll=6, ppoll=6,
> flash=00 ok, keyid=0, ttl=0, offset=-20558.442, delay=0.679,
> dispersion=3.734, jitter=45.116,
> reftime=c79ebd47.40cf6be3  Thu, Feb 16 2006  9:58:47.253,
> org=c79ebd4e.bcd4562e  Thu, Feb 16 2006  9:58:54.737,
> rec=c79ebd63.5c88509b  Thu, Feb 16 2006  9:59:15.361,
> xmt=c79ebd63.5a01abd1  Thu, Feb 16 2006  9:59:15.351,
> filtdelay=     9.81    0.70    9.66    0.68    9.13    2.84    0.69    
> 1.74,
> filtoffset= -20618. -20595. -20581. -20558. -20538. -20524. -20508. 
> -20490.,
> filtdisp=      0.03    1.02    2.01    2.99    3.93    4.88    5.82    6.80
> 




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