[dnsoverhttp] Caching model

[Martin Thomson <[e-mail]>]

See https://github.com/paulehoffman/draft-ietf-doh-dns-over-https/issues/14
and several others.

My request is that we agree on what the model is, then we can talk
about the properties we can extract from that.

My understanding is that the DNS client would consult its local stack
and that stack would use HTTP to talk to a DNS API server.  In that
model, there are at least three caches in play: the local DNS resolver
cache, the HTTP cache and the DNS API server cache.

If those caches are ordered as I describe, and the two DNS caches are
driven based on the TTL, I'm struggling to find a role for the HTTP
cache.  There are things that HTTP can do with caching that would be
nice, but I'm not seeing any way to really access HTTP caching
features in that architecture.

It's a little unfortunate, but in that architecture it would probably
be better to disable HTTP caching entirely.  HTTP caching has a bunch
of features that are more flexible, but if that caching is wedged
between two relatively inflexible caches, it won't have any
opportunity to add value.