Re: Minimum TTL question

At 11:59 AM 7/1/2003 -0500, Pete Ehlke wrote:
>On Tue, Jul 01, 2003 at 04:28:02PM +0000, Barry Margolin wrote:
> >
> > Conceivably there may be other DNS servers that raise short TTLs up to some
> > minimum, but I haven't heard of them and it would be a violation of the
> > protocol (conversely, there's nothing too wrong with a ceiling on the TTL,
> > since cached data can be discarded any time for a variety of reasons,
> > although it would be quite antisocial to set the ceiling too low).
> >
>AOL are widely rumoured to ignore 'low' TTLs and raise them up to some
>(equally rumoured) minimum, though I've never heard a definitive answer
>from AOL as to whether they actually do this, and if so, what the value
>of 'low' is.
>
>I do note that if it's true, it is a practice that, if widely deployed,
>would render AOL's MX load balancing, for which they no doubt pay an
>exhorbitant ammount of money, useless.

Well, here is what started the question in my mind. We do use Foundry
hardware in our two data centers, and they do a reasonable job of load
balancing using DNS as the mechanism. Because I "thought" there was a
practical minimum of 15 to 30 minutes, that's what I set it to.

But, when I observe what traffic does when we do a "DNS roll" (this is
where we want to take one of the data centers off-line for maintenance, so
we change the DNS to point to a specific data center), 4 hours after the
change, I'll still be seeing one access a second at the "down" center. A
majority of traffic moves, yes, but there is still this "refuses to move"
traffic that bothers me.

That leads me to believe that it isn't just "AOL" doing this :( Thus the
question of a practical minimum TTL.

I guess, too, while it use to be bad form to have a tiny TTL, it is
becoming less so, with the advent of hardware like the 3DNS, and people's
DNS hardware is just getting better at handling the "update" load?

Thanks for the feedback, btw. It helps A LOT.


===
Bob Gahl Bicycle (Ryan Vanguard) Mobile || @
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