[ntp:questions] basic questions about the leapsecond

Unruh unruh-spam at physics.ubc.ca
Tue Dec 16 19:31:16 UTC 2008


Martin Burnicki <martin.burnicki at meinberg.de> writes:

>David,

>David J Taylor wrote:
>> Unruh wrote:
>> []
>>> From reading the ntp docs, it seems that if you download the
>>> leapseconds file, call it /etc/ntp.leapseconds  and restart ntpd,
>>> then ntpd will read that file, and set the leapsecond flag on your
>>> system in the last month or day.
>> []

>Again, the text above refers to the developeer version *only*. 
> 
>> This seems a lot simpler than the method described in the docs:
>> 
>>   http://support.ntp.org/bin/view/Support/ConfiguringNTP#Section_6.13.
>> 
>> which talk about "crypto directory" and "autokey", and seem (to me) a lot
>> more complicated than just copying a renamed file and restarting ntpd!

>You are right. However, this is how NTP up to the current stable version
>works.

>> There is also the remark: "If the top level NTP server is unable to obtain
>> the leap second file automatically...".  Does this mean that NTP will try
>> and fetch this file by itself?

>Maybe the dev version. AFAIK no stable version does so.

>BTW, you can see if the leap file has been evaluated by running 

>ntpq -c rv

>and checking the "tai=" value. The NIST leap second file does not only
>contain information about the nearest leap second but also about all
>earlier leap seconds and thus allows the NTP daemon to compute the
>difference between UTC and TAI.

As in my previous post, my ntp (4.2.4p4) running with a local shm refclock
which gives it the PPS from my Garmin 18LVC, a remote level 1 source, and a
remote level 2 sever, has tai=0 but has the leap flag set ( certainly not
from my Garmin receiver).
Of course I did NOT have the leapseconds file installed when I started
ntpd.




 

>So if the tai value is 0 then the leap file has *not* been evaluated. If the
>leap file has been evaluated the displayed value is currently 33.

>> Some clarification would be appreciated.

>Martin

Thanks for the clarification.

I assume that there is some special sentences other than the NMEA sentences
which the Meinberg gps receiver uses to deliver the leap second warning. 




More information about the questions mailing list