topological closeness....

If I could _enforce_ caching by traffic interception then
I could construct vaguely logical cache structures as an ISP.

One way to do that is to spoof TCP sessions in the routers :slight_smile:
That is a horrible kludge, though...

But relying on users to configure their end systems with the
identity of the closest cache server using some ill defined
"network metric" is always going to be an erratic affair at best.

How about allocating fixed IP address for cacheing servers?
Routing will take care of locating the "closest" one, and
browser vendors would be able to hardwire the default address.

Although one can tell that they have little clue and probably won't
do anything about it, given such impressive items on their track
record as HTTP 1.0 or Netscape HTML extensions.