Miroslav,
According to your diagram, the algorithm would determine the
intersection interval as interval a. The midpoints of all three
intervals would be considered truechimers, since each of the intervals
a, b and c, contain points in the intersection interval.
Dave
Miroslav Lichvar wrote:
On Wed, Jan 05, 2011 at 10:31:15AM -0500, Brian Utterback wrote:
Let's equalize a bit to make it a bit more fair:
<c------------> <b------------->
<a-------------->
So, now, if you were NTP, which would you choose? You are correct in
your assessment that NTP would accept them all as truechimers. You are
correct also that adding a fourth still does not guarantee that you
will throw out the falseticker, but NTP uses intervals at this stage,
not actual servers, so adding another truechimer will guarantee that
the interval used will contain the real time.
Not necessarily.
|
<---------> A
| <-----> B
<--------> C
<-------> D
|
<==> X
Here, B is the only server off, but the result X doesn't contain the
actual time.
I think clockhopping can happen with any number of servers, there just
needs to be two or more similar sources on top of the list sorted by
synchronization distance.
With more servers on the list, the clustering and combining algorithms
will merge them into a single offset and they will not hop. With two
servers, these algorithms cannot function.
Combining doesn't affect clockhopping, it happens after the system
peer is selected.
By the way, over time Dr. Mills has added features to try to suppress
clock hopping as much as possible without compromising the correctness
proofs. With the latest versions, clock hopping may not be so much of
a problem. Bu tit is still an issue. Even if you prefer one clock, it
might be inaccessible for a while and you will hop anyway.
Yes, the maximum anti-clockhopping threshold is a fixed value (1 ms by
default), so it can't work well in all situations. But it can be tuned
with the tos mindist command.
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