[ntp:questions] Re: simple time server

David Woolley david at djwhome.demon.co.uk
Fri Aug 4 06:50:55 UTC 2006


In article <3458.bryanh at giraffe-data.com>,
bryanh at giraffe-data.com (Bryan Henderson) wrote:

> I don't mean "local time" as in 12:00 means solar noon.  I mean the time

Most people, asking here, who use the term, mean wall clock time, although
I did find your wording ambiguous when I first read it.

>   1) Philosophically, the Linux kernel has no business messing with the
>      hardware clock.  That's something that makes more sense done by a

In engineering terms, it is the most sensible place to do it, because
the kernel is in the position to do this at the most accurate time
without excessive overheads.  Note that, on Linux systems, the kernel
can also be made to do the final filtering of NTP updates, so its
time is more accurate than that seen by the NTP user space code.

> The system clock on a Unix system doesn't operate in a time zone at
> all.  It keeps absolute time.  The OS doesn't convert to local time;

Laying aside that relativity implies that there is no such thing as
absolute time, all Posix systems, like Linux and typical Unix systems,
keep UTC time, and not for example, TAI, in terms of how it is interpreted
by their time conversion libraries.  In particular, Posix time cannot
represent negative leap seconds (23:59:60 UTC) and differences in Posix
time give the wrong duration, in seconds, across leap seconds (there is
a small lobby to change it to do so, but I would argue that this would
confuse most business users, especially when dealing with future times).
As such it actually further from any concept of "absolute" time than
is UTC.

There is a somewhat arbitrary epoch, but that is defined in UTC, and is
a quite round time in UTC.

I don't know what you mean by absolute time, but I think it would have
to be TAI, not UTC.

Note that UTC stands for Universal Coordinated Time and it is not the
name of any country's timezone.  (TAI is International Atomic Time.)

What NTP tries to do is to communicate UTC, not the time of a higher
order NTP node, where UTC is ultimately derived from a national
standards atomic clock, which in turn are mutually synchronised
with other such clocks.

> it generates it -- from two pieces of information: 1) absolute time;
> 2) time zone.

That's what you were being told. Again a reasonable thing to tell
someone who is talking about "local time" given the number of people
who expect NTP to communicate wall clock time directly.

One other point, anything that uses the NTP wire formats, but doesn't
use the NTP algorithms from the reference implementation, isn't NTP,
it is only SNTP.




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