On 2014-11-03 02:19 PM, Warner Losh wrote:
On Nov 3, 2014, at 11:11 AM, Brooks Harris <[email protected]> wrote:
CAUTION about the PTP Epoch. Its not "just nitpicking".
...
We've been advised by PTP experts that A) yes, its confusing, and B) most implementations use a integral-second interpretation,
as in Table B.1. I understand the "escape clause" they use to justify this is the "(POSIX) algorithms" phrase
in Note 1 of 7.2.2 Epoch. By "(POSIX) algorithms" they mean "gmtime()" and (strict) POSIX "ticks"
at 1Hz, so, integral seconds. In any event its really the only interpretation that yields a manageable, practical, implementation
that is consistent with TAI and UTC, NTP, and common-use of POSIX.
A few years ago, I had to produce TAI-like data from a measurement system. We defined the value as
"seconds since 1970" but the technical definition was "number of SI seconds since 1 Jan
1972 00:00:00 UTC + 10 + #seconds-in-1970&71" to avoid the ambiguity. Given that our chief
time scientist suggested this, and they were quite involved in PTP...
I assume you mean "number of SI seconds since 1 Jan 1972 00:00:00 UTC +
10 - #seconds-in-1970&71" ? And the "#seconds-in-1970&71" is (2 * 365 *
86400), right? That would be coincident with the PTP Epoch as
interpreted above, that is, "seconds since 1970 (TAI)".
-Brooks
Warner
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