Resolver behaviour with rrset

[Sebastian Castro Avila]

Hi all:

AFAIK, when a resolver queries for a RR it will receive a "referal answer"  
(I don't know anything, but better go here) or an "authoritative answer"  
(the record you asked for has this answers).

The server will return the RR set in some order (not important for the  
question).

Taking an answer from Barry Margolin about RR sets and Round Robin, that  
says:

=============================================
The resolver remembers the response times of the servers, and prefers
the one that has the best record.  However, the matching is fuzzy, so
close times are considered equivalent, and it load balances among them
(I think it's random selection rather than true round robin, but in the
long run they should both produce equal load).
=============================================

What resolvers implements this behaviour?
How does it work if a server became unreachable?
How often the resolver updates the response time for the whole list of  
known servers?
Where can I get more information about this "feature"?

Thanks in advance

-- 
Sebastian E. Castro Avila             sebastian at nic.cl
Administrador de DNS, NIC Chile
Fono: (2) 9407705                  Fax  : (2) 9407701