Yo Hal! On Fri, 10 Mar 2017 14:07:52 -0800 Hal Murray <hmur...@megapathdsl.net> wrote:
> g...@rellim.com said:
> >> There is nothing to normalize in a l_fp All bit patterns are
> >> valid.
> > Sort of. The header notes imply the integral and fractional part
> > may be signed or unsigned. Separately. I have not confirmed if
> > the code use that.
>
> The whole point of l_fp is so you can do 64 bit arithmetic. There is
> no sign bit in the fractional part. It piggybacks on the sign of the
> high half.
Yes, but the overhead of timespec arithmetic is small, and pretty
soon I don';t think we will do any arithmetic in l_fp.
> > Actually, there may be a better way. NTP defines an Epoch on the
> > wire. I do not know when it is sent. We are currently in NTP Epoch
> > 0. In 2036 it goes to NTP Epoch 1. If we can grab that field off
> > the wire we are good.
>
> It's not on the wire. At least not in the main packet format. I
> don't think it is in mode 6. Yet.
I am also baffled. The RFC defines NTP Date Format, but then never uses
it anywhere....
> >> I think the build date of the code will be a good start.
> > Which breaks regression tests. We tried that on gpsd and the
> > results were, to be kind, loud.
>
> Please say more. I don't remember that scene.
When gpsd ws compiled to assume all dates before compile date, then all
the gpsd regressions, which are captures of earlier GPS output failed.
There is a way out. POSIX zero (1 Jan 1970) is NTP 2,208,988,800. So
any NTP before 2,208,988,800 is actually epoch 1, after 8 Feb 2036.
I don't think anyone possibly uses NTP timestamps for pre 1970.
> > Semms I believed someone that misread the RFC. NTP Time is based on
> > 1Jan1990, but when using the Epoch it can uniquely define any time
> > backwards or forward.
>
> s/1990/1900/
Right.
> It's the same problem as the GPS 1024 week.
That does NOT make me feel better... GPS Week rollover is always
a problem.
RGDS
GARY
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