[ntp:questions] Is the use of the leap seconds mechanism mandatory?

Martin Burnicki martin.burnicki at meinberg.de
Thu Feb 26 16:40:22 UTC 2009

Hi Alain,

Bartholome, Alain wrote:
> Hi,
> The primary reason why  I need NTP is the synchronization of the systems,
> not the accuracy of time.
> I need to simplify the maintenance of the systems as much as possible.
> I would like to know if it is mandatory to use the leap seconds mechanism
> and installing the up to date leap file in order to have ntpd working
> smoothly.

Basically the time used by NTP is assumed to be UTC. However, in fact it is
just the time provided by the top level reference time source, so in a
closed network you can distribute any time instead of UTC, with or without
leap seconds, if you control the top level time source.

In order to have NTP handle leap seconds correctly it is important that the
leap second is announced properly. Once the top level NTP server is aware
of the leap second it passes the announcement to its clients. In turn the
clients can also act as NTP servers and cann pass the announcement on to
their clients, after they have receive the announcement from their upstream

You have to take care that the last client in the hierarchy receives the
announcement before the leap second occurs. For examples, if there are 2
levels of clients below the top level server which are polling at long
intervals of 1024 seconds then the top level server has to start to
announce the leap second more then 2048 seconds before it occurs. Otherwise
the last clients will miss it.

If you have a GPS receiver as top level ref time source for the top level
NTP daemon then the GPS receiver is internally aware of the upcoming leap
second. The question is which protocol is used to pass the time string to
the NTP daemon. E.g., AFAIK, the NMEA protocol does not support leap second
announcements, so the server will not become aware of an upcoming leap

In this case the leap second file (of course the current version) can make
sure the top level server is aware of the leap second and starts to
announce it down the client chain early enough.

So if you use the file you're on the safe side, and the the effort to supply
the file to the top level server should not be too hard.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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