Gary E. Miller <g...@rellim.com>: > Yo Eric! > > On Thu, 19 Jan 2017 14:30:35 -0500 > "Eric S. Raymond" <e...@thyrsus.com> wrote: > > > Gary E. Miller <g...@rellim.com>: > > > > - to fuzz the low-order bits of the clock. > > > > > > Hmm, can you expand on this a bit? Which clock? How much fuzz? > > > Does this degrade anything? > > > > Whenever ntpd polls the system clock, it fuzzes the lowest-order > > digits of the result. The amount of fuzz to apply is bounded by half > > the measured interval between system clock ticks. > > Hmm, and how much would that typically be?
I...don't actually know. Probably less than a microsecond. It should be
available as the value of the system variable "fuzz" (internally sys_fuzz)
but I see
ntpq> rv 0 fuzz
fuzz=0.001
which makes me suspect a units problem somewhere.
The way it's measured is by doing two unfuzzed calls to get time
immediately adjacent to each other: see measure_tick_fuzz() in
ntp_proto.c.
I haven't looked at that code closely. It could be buggy.
> > That shouldn't degrade anything. I presume it's a measure to foil
> > timing attacks of some sort. Daniel might be able to say more.
>
> At first glance this seems reasonable, but my experience with the
> GR-601W suggests otherwise. My experience with the GR-601W shows that
> ntpd can find a timing edge and hold onto it very well.
>
> Whe ends up is there will be an offset of up to 1/1024 sec from the
> correct time, but the jitter is much less.
>
> Can you point me to this code? I want to rip it out and do a test.
libntp/systime.c
Have fun. You'll probably do a better job of ayditing that code than I would.
--
<a href="http://www.catb.org/~esr/";>Eric S. Raymond</a>
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