[Freeswitch-users] FreeSWITCH, NTP daemon and clock drift

[Steven Ayre]

Anthony,

There's previously been a discussion captured on
http://wiki.freeswitch.org/wiki/Clock regarding the internal clock time FS
keeps.

Currently FreeSWITCH uses the monotonic clock and ignores system time. On
machines where the time is very unreliable, NTP isn't in use and the
sysadmin is changing the time manually or by ntpdate the system time can
make large jumps - and I can indeed see the reasoning that having FS use
its own clock on these systems is more reliable as it prevents billing time
being lost because of a large clock jump.

However, on systems correctly running ntpd that have clock drift the system
time will constantly be being corrected and won't experience large jumps
and in these cases the system time will be more accurate than FreeSWITCH's
internal time.

On such systems although FreeSWITCH might not be losing billing seconds
from clock jumps (which shouldn't normally happen) the clock drift will
mean that there is also that clock drift present in FreeSWITCH's CDRs - and
these could conceivably cause billing disputes where a customer's CDRs do
not have the same times because their times are more accurate than FS's due
to that clock drift. This is unlikely to be large enough to affect billed
minutes much, but could put a large number of calls on the wrong billing
rate (eg if FS's internal time in the CDRs identifies calls as on a peak
rate when the customer believes it is on an offpeak rate). On a badly
drifting system over a month on a large volume of calls that could become a
noticeable discrepancy.

Obviously there is the sync_clock option, but it seems you shouldn't need
to run that frequently (and would that cause any sideeffects?) Running it
infrequently would cause the same time-jumping behaviour FS's internal time
was designed to avoid, and sync_clock_when_idle isn't possible on busier
systems.

On systems running ntpd perhaps it would be worth adding an opt-in option
that lets FreeSWITCH use the system time and rely on ntpd (but keeping the
present behaviour as the default)?

Your thoughts please...

Warm regards,
-Steve
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