[ntp:questions] Re: NTP does not sync when using pool.ntp.org

Dale Worley worley at dragon.ariadne.com
Fri Oct 10 00:20:02 UTC 2003

Adrian 'Dagurashibanipal' von Bidder <grazdan at fortytwo.ch> writes:
> The main reason why this doesn't exist right now is:
> To achieve the desired effect, {1,2,3}.pool.ntp.org would need to be
> disjunct. But since we want load balancing, and not just load every server
> in the pool, we would need to have a few names in each zone still - so we'd
> need >30 servers - for each zone (continental, country zones, too!).
> With currently 90 servers in toto, I don't think this works.

It's not as bad as it looks -- if we list the same 100 servers for all
three lists, and can get the lists to be handed out at random, there
is a 97% chance of the three names resolving differently.

If we use *four* aliases, there is a 94% chance that they all return
different addresses, and a 99.93% chance that they will return at
least three different addresses.

The remaining problem is that if some intermediate cache hands out the
same addresses for all four names.  We can reduce that by putting the
addresses for each alias in different orders.  Then there is only a
problem if the intermediate cache sorts the addresses, which is
possible.  One possible way to avoid that problem would be to leave
one address off the second name, two off the third name, and three off
the fourth name, so that if the server is maintaining four counters to
cycle through the four (sorted) lists, the pointers can't stay
synchronized with each other for more than one cycle through the


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