optimizing rtt (was: Re: root-server trouble?)

[Barry Margolin]

In article <Pine.LNX.4.10.9908211541540.29772-100000 at lilly.devconsult.de>,
Ingo Luetkebohle  <ingo at devconsult.de> wrote:
>On Fri, 20 Aug 1999, Barry Margolin wrote:
>> BIND already does this automatically.  Whenever it uses any server it
>> records the response time.  When it has a choice of servers for a zone
>> (including the root) it chooses the one with the best past response time.
>
>Hmm. Well, if thats really the case, it doesn't do a very good job. A while
>ago, I set up an ipfw rule to log DNS queries and most of them did go to
>the US servers, which are definetely not the best. Maybe thats related to
>the fact that our named is reloaded every hour.

Reloaded or restarted?  When it first starts up, it doesn't have any data,
and presumably starts from the first one in the list.  Also, it uses a
fuzzy comparison, by grouping together all servers whose response times are
within some delta of each other, to take into account the variability of
individual RTTs.  And finally, every time it uses a server it "penalizes"
it, so that eventually it will try other servers (in case their response
times have improved).

In the long run, it should settle on the ones with good response time.  But
if you restart the server every hour, that's not "the long run" (although
I'd expect that it would learn the best root servers pretty quickly).

-- 
Barry Margolin, barmar at bbnplanet.com
GTE Internetworking, Powered by BBN, Burlington, MA
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