To recap what I was saying on the list, here's my proposal:

- Recursives can attempt to connect to any authoritative by probing
  with DoT/DoQ [0]. In this case, they should cleanly fall back to
  Do53 on connect failure and not validate the credential (whether
  WebPKI or DANE) This allows authoritatives to just turn on TLS
  without risk.

- The SVCB record is used to signal that the authoritative expects to
  do TLS and indicates the type of credential (WebPKI, DANE, etc.)
  that the authoritative intends to present.  The recursive should
  insist on TLS in this case and hard fail if it cannot negotiate. [1]
  If there is an overlap between the credentials the recursive
  supports and the ones the authoritative advertises, then the
  recursive should validate the credential and fail if it cannot.  If
  there is no overlap, it should not validate the credential.

- (Optional) We should invent some way for the server to say in
  SVCB that it doesn't have any valid credential (e.g., a reserved
  credential type). This would be a signal you wanted unauthenticated.

This would allow for:

- Unauthenticated to work and be discovered.
- An easy upgrade to authenticated after initial contact (a la
  HSTS etc.)
- A path to fully authenticated from initial contact if we
  ever figure out the parent signaling, because you just need
  to propagate the contents of SVCB (or some subset).

-Ekr


[0] As PaulH says, this won't work with DoH because of ambiguity
about the path portion.

[1] This signal should only be accepted over an authenticated
connection, a la HSTS.
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