In ntp_proto.c the delay and offset are computed as follows:
t34 = t3 - t4;
t21 - t2 - t1;
p_del = t21 - t34;
offset = (t21 - t3)/2.;
where t1 = client send time
t2 = server receive time
t3 = server send time
t4 = client receive time.
However, t1 and t4 are not really in seconds if the client clock is slewing.
That is, the
difference t4 - t1 will be shorter than seconds if the clock is being slowed
down
and larger if the clock is being sped up. Hence the clock slew may be a
source of
variation that is not presently being accounted for.
One some systems the frequency of the Performance Counter (PC) is constant
because it is driven by the High Performance Event Timer (HPET). And
according to one article on the Internet, the PC can be made to be driven
by the HPET on Win 8 by this edit: " run cmd as admin and paste
'bcdedit /set {current} useplatformclock Yes'".
Would NTPD have less variation in offset and delay if, say, t4 were measured
by the difference in PC readings between the time t1 is measured and the
time T4 is presently measured?
Charles Elliott
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