Some code in ntpd applies a dither using ntp_random. However,
ntp_random (which is also used elsewhere, e.g. in getting an
initial association ID) generates only positive integers,
resulting in a one-sided offset bias. The following patch
compensates for this bias by offsetting the return value
from ntp_random to produce approximately zero mean dither
in both positive and negative offset directions in the
functions which use ntp_random for dither.
*** libntp/systime.c.orig Wed Dec 9 02:36:35 2009
--- libntp/systime.c Sat Nov 19 09:10:31 2011
***************
*** 62,67 ****
--- 62,68 ----
)
{
double dtemp;
+ long correction = (1L << 30) + 1L; /* compensate for ntp_random bias */
#if defined(HAVE_CLOCK_GETTIME) || defined(HAVE_GETCLOCK)
struct timespec ts; /* seconds and nanoseconds */
***************
*** 78,86 ****
now->l_i = (int32)ts.tv_sec + JAN_1970;
dtemp = 0;
if (sys_tick > FUZZ)
! dtemp = ntp_random() * 2. / FRAC * sys_tick * 1e9;
else if (sys_tick > 0)
! dtemp = ntp_random() * 2. / FRAC;
dtemp = (ts.tv_nsec + dtemp) * 1e-9 + sys_residual;
if (dtemp >= 1.) {
dtemp -= 1.;
--- 79,87 ----
now->l_i = (int32)ts.tv_sec + JAN_1970;
dtemp = 0;
if (sys_tick > FUZZ)
! dtemp = (ntp_random() - correction) * 2. / FRAC * sys_tick *
1e9;
else if (sys_tick > 0)
! dtemp = (ntp_random() - correction) * 2. / FRAC;
dtemp = (ts.tv_nsec + dtemp) * 1e-9 + sys_residual;
if (dtemp >= 1.) {
dtemp -= 1.;
***************
*** 102,110 ****
now->l_i = tv.tv_sec + JAN_1970;
dtemp = 0;
if (sys_tick > FUZZ)
! dtemp = ntp_random() * 2. / FRAC * sys_tick * 1e6;
else if (sys_tick > 0)
! dtemp = ntp_random() * 2. / FRAC;
dtemp = (tv.tv_usec + dtemp) * 1e-6 + sys_residual;
if (dtemp >= 1.) {
dtemp -= 1.;
--- 103,111 ----
now->l_i = tv.tv_sec + JAN_1970;
dtemp = 0;
if (sys_tick > FUZZ)
! dtemp = (ntp_random() - correction) * 2. / FRAC * sys_tick *
1e6;
else if (sys_tick > 0)
! dtemp = (ntp_random() - correction) * 2. / FRAC;
dtemp = (tv.tv_usec + dtemp) * 1e-6 + sys_residual;
if (dtemp >= 1.) {
dtemp -= 1.;
*** ntpd/ntp_io.c.orig Sat Dec 25 04:40:36 2010
--- ntpd/ntp_io.c Sat Oct 29 08:12:06 2011
***************
*** 3344,3350 ****
tvp->tv_sec, tvp->tv_usec));
nts.l_i = tvp->tv_sec + JAN_1970;
dtemp = (tvp->tv_usec
! + (ntp_random() * 2. / FRAC)) / 1e6;
nts.l_uf = (u_int32)(dtemp * FRAC);
#ifdef DEBUG_TIMING
{
--- 3344,3350 ----
tvp->tv_sec, tvp->tv_usec));
nts.l_i = tvp->tv_sec + JAN_1970;
dtemp = (tvp->tv_usec
! + ((ntp_random() - (1L << 30) - 1) * 2. /
FRAC)) / 1e6;
nts.l_uf = (u_int32)(dtemp * FRAC);
#ifdef DEBUG_TIMING
{
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