topological closeness....


Yes, caching is a good idea (technically).

And who said "AS path length" was a wonderful global metric???
just because the "usual suspects" BGP implementation generally does
route selection based on that attribute doesn't mean that's the
ONLY thing one can do.

Depending on how much additional information
one wished to supply about ASes, their general level of connectivity,
and geographic location, one can conceivably produce hybrid metrics,
probably heuristic, which reflect some sense of "performance".

for example, the server could do round-trip measurements to "sufficiently
interesting" ASes so that it could base its behavior on observations.
the goal wouldn't be to do fine-grained decision-making, but if the
"Bruce Springsteen Ticket Server" caused some reasonably long-lived
congestion, it might be worthwhile redirecting some responses.

The assumption is that this special DNS server might be concentrated
on fielding responses for a special set of servers, like special Web
servers (could be caches), and not general connectivity.

So while few things are perfectly-universal solutions, the prospect
of implementing heuristics that we all use today in getting a sense of how
things are going, and what to try when the first guess is hosed,
seems like a worthwhile attempt.

some will work better than others.

that is neither news, nor a reason to not try it.