[rtcweb] IP handling: Using mDNS names for host candidates

[Justin Uberti <[e-mail]>]

The Safari team has come up with a clever approach to avoid disclosing
private IP addresses for host candidates. As discussed in this WebKit bug
<https://bugs.webkit.org/show_bug.cgi?id=174500>, the technique works as
follows:

   1. Register a random UUID-based mDNS name when ICE gathering starts
   2. Replace the private IP address by a "{UUID}.local" string in each
   host candidate (and set raddr to 0.0.0.0 for other candidates)
   3. The other party will do mDNS resolution on any candidate having a
   .local suffix, similar to how hostnames in candidates are handled in RFC
   5245, Section 15.1.

This technique is relevant to the IP handling document, as it addresses one
of the lesser problems (private IP disclosure) from the overall problem
statement. While I don't think this will have a large impact on the
document, including the default mode selection, incorporating this
technique would result in some moderate changes:

   - Section 5.1 would mention concealing private IPs in the default case
   as an explicit goal.
   - In Section 6, Mode 2 would change from handling out private IPs to
   handing out mDNS names.
   - This document would have to describe the technique or point to another
   document that describes the technique. mmusic-ice-sip-sdp, Section 4.1
   <https://tools.ietf.org/html/draft-ietf-mmusic-ice-sip-sdp-20#section-4.1>
seems
   like a good option, as it already covers how to handle DNS names in ICE
   candidates.

This is a significant improvement and I think we will want to incorporate
this suggestion. Is this something we could do as part of this WGLC, or
should we look for another option?