[ntp:questions] Asymmetric Network and Delay Questions.
Brad Knowles
brad at stop.mail-abuse.org
Thu Jul 14 14:00:02 UTC 2005
At 8:32 AM -0500 2005-07-14, Matt Kinard wrote:
> What are the implications of running ntpdate out of the crontab in
> an asymmetric case?
There's another factor to consider in military applications -- clock drift.
During Desert Shield/Desert Storm while I was working in the J4
LRC in the NMCC in the Pentagon, we kept getting reports of the
Patriot missile batteries being used for point defense that kept
crashing every 24 hours. The problem turned out to be that they set
their clock once on bootup and after 24 hours had gone by the clocks
had drifted enough that the computers were not able to perform the
necessary ballistic calculations, and they would crash.
As a result, they weren't very effective for the Israelis in
trying to defend themselves from Saddam's Scud missiles.
Clock drift could also cause missiles or smart bombs to
mis-calculate their position relative to GPS and wind up hitting the
Chinese embassy, instead of a house known to be used by enemy
combatants. A surprisingly small error in time can result in a
surprisingly large error in space.
Clock drift is a serious problem in most applications that are
time sensitive, but military and aerospace applications are the ones
I know of that are most sensitive to even the smallest errors.
Of course, I'm sure that you've already looked at these kinds of
failure modes twelve ways from Sunday, given where you appear to be
working and the kind of work you're presumably doing.
But I can tell you that I definitely would not want to be on any
aircraft where critical systems were not reasonably well
synchronized. I would not want people anywhere remotely close to me
using highly destructive weapons that were not adequately controlled
and synchronized. If I found out that a particular company had cut
corners in this area, then I would be obliged to warn as many people
away as I could.
I came close to blowing the whistle on some of the stuff I saw
going on during Desert Shield/Desert Storm, and I would not hesitate
to do that in the future if I felt the need.
--
Brad Knowles, <brad at stop.mail-abuse.org>
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
-- Benjamin Franklin (1706-1790), reply of the Pennsylvania
Assembly to the Governor, November 11, 1755
SAGE member since 1995. See <http://www.sage.org/> for more info.
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