[dns-operations] More Aggressive prefetch for popular names

[Giovane Moura]

> The flip side of this argument is that prefetching when a new request
> is expected within the next TTL helps the first users after the TTL 
> expiration. For 30-second TTLs, this could easily be of value to
> more than one user, particularly if the authoritative response is
> slow.

Yeah, you have a point here.

> Prefetching without a perceived need is indeed wasteful.
That's what I had in mind in the previous e-mail,  prefetching  all
domains in cache is a bad idea.

>However, if
> the name was served from the cache during the last 25% of the TTL,
> that's a good indication that it will be requested again after the
> TTL has expired. 
I think we could really use some large-scale measurement studies on
caches to understand their actual behavior. Things in the wild tend to
behave sometimes quite differently from what we expect.

While I agree with the general idea, it would be nice to see if that's
what happens in the wild.

Any resolver ops in here that have some data on this?

> Using this non-aggressive pre-fetching "requested
> from the cache during the end of lifetime" rule seems useful to
> resolver users while only increasing the authoritative load in the
> less common cases.
"only increasing the authoritative load in the less common cases."
That's where it can get tricky. In theory, yes, but in practice it's
hard to estimate the aggregate effects from such policy -- it may as
well lead to some unintended collateral damage -- and IMO we need more
studies on this.

/giovane