Is caching necessary?

[Brad Knowles]

At 11:45 PM -0400 2005-04-29, Kevin Darcy wrote:

>  The only legitimate reason for forwarding to a central cache, when one
>  has the option available to query Internet nameservers directly, is if
>  the constellation of local network topology, query patterns, TTL values,
>  etc. happen to all align so as to make your average and/or worst-case
>  query latency better that way.

	No, there's another legitimate reason.  When you have a business 
need to guarantee that all internal servers see the same answers for 
a given question (modulo TTLs), but you can't point all those servers 
directly at a single caching/recursive server.

	For example, at AOL I had a problem where mail would come in from 
a given sender to a given recipient via one particular server, and 
everything would work fine.  However, mail from the same sender to 
the same recipient via a different server would find different 
information in the DNS (perhaps the delegation was broken, or there 
was a lame server, or somesuch), and we would end up rejecting the 
message.

	Try doing that with ten million users, handling tens of millions 
of messages per day, and even if this kind of thing only has a 
one-in-a-thousand chance of happening, that means you have a very 
large number of extremely irate customers who are all looking to use 
explosive devices in new and creative ways.


	Performance is one valid reason where forwarding may be used, but 
there are others.

-- 
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.