The new RFC 6055 is quite interesting, it discusses issues of IDNA vs. UTF-8 encodings in the DNS and other namespaces.
By definition any valid IDNA A-label, i.e., an ASCII compatible label starting with xn-- and following the IDNA rules specified in
RFC 5890 and
RFC 5891 (among others), has a corresponding Unicode U-label, i.e., a label containing non-ASCII characters. The IDNA encoding mechanism
punycode tries hard to create short A-labels, because there is an upper limit of 63 octets per label, and another upper limit of 255 octets per FQDN DNS query consisting of zero or more labels.
RFC 6055 does not propose to use redundant UTF-8 DNS entries for raw U-labels in addition to the IDNA A-labels, so I guess there are too many cases where this would anyway not work, e.g., when an otherwise valid U-label consists of more than 63 UTF-8 octets.
When I get "a round tuit" I'll look into a now long expired EAI SPF draft, where using raw UTF-8 labels was the only viable solution for an EAI extension of SPF. Even if nobody implements this draft and/or if EAI never really makes it the last "tombstone" version should reference RFC 5321 (SMTP), RFC 5890, RFC 5891, and RFC 6055.
John Klensin is the author or a co-author of all RFCs mentioned here. I should check what else he published between RFC 5321 and RFC 6055, but it is nice to see that for at least one person in the world "RFC" still works as a "Request For Comments". :-)
