[Captive-portals] Fixing RFC 7710

[Martin Thomson <[e-mail]>]

We've had a number of discussions in the captive portals group about
fixing RFC 7710.

Erik and I would like to propose a plan for that work.  We would keep
this to addressing the issues that we have identified thus far.
Namely:

1. The purpose of the URI is not well defined.  We would reference the
capport architecture and API documents for that.  The group would need
to decide between:
  a. point to the API
  b. point to a login page

2. There isn't a clear way to signal that there is no captive portal
in the network.  It has been suggested that we use a special URL -
e.g., urn:ietf:params:capport:unrestricted. Alternatively, we could
privilege the empty string, but that doesn't have as clear a signal of
intent.

3. RFC 7710 states that the URL SHOULD use an address literal.  This
works at odds with the idea of using HTTPS.

Is there anyone who is willing to take on this work?  We aim to start
and complete this work in <1 meeting cycle, starting in London.

For the authors of RFC 7710, let us know if you have any concerns.