Re: [DNSOP] A conversational description of sentinel.

[Patrick Mevzek <[e-mail]>]

On 2018-02-05 14:18 -0500, Geoff Huston wrote:
> I thought this was due to some concern over the wording in RFC <mumble>(some IDN
> RFC whose number I can’t recall right now!) over a comment that the UC label 
> should not contain the starting sequence "<letter> <letter> - -”

RFC5890 point 2.3.1

   To facilitate clear description, two new subsets of LDH labels are
   created by the introduction of IDNA.  These are called Reserved LDH
   labels (R-LDH labels) and Non-Reserved LDH labels (NR-LDH labels).
   Reserved LDH labels, known as "tagged domain names" in some other
   contexts, have the property that they contain "--" in the third and
   fourth characters but which otherwise conform to LDH label rules.
   Only a subset of the R-LDH labels can be used in IDNA-aware
   applications.  That subset consists of the class of labels that begin
   with the prefix "xn--" (case independent), but otherwise conform to
   the rules for LDH labels.

And also:

Labels within the class of R-LDH labels that are not prefixed with
   "xn--" are also not valid IDNA labels.  To allow for future use of
   mechanisms similar to IDNA, those labels MUST NOT be processed as
   ordinary LDH labels by IDNA-conforming programs and SHOULD NOT be
   mixed with IDNA labels in the same zone.

   These distinctions among possible LDH labels are only of significance
   for software that is IDNA-aware or for future extensions that use
   extensions based on the same "prefix and encoding" model.  For
   IDNA-aware systems, the valid label types are: A-labels, U-labels,
   and NR-LDH labels.


> Is there a broader concern over the use of double hypens in labels in hostname
> contexts in the DNS?

Double hyphens anywhere that is ok, but at position 3 and 4 it will be a
problem for many registries.

ICANN contract with registries, Specification 6 ("Registry
Interoperability and Continuity Specifications") point 1.1 says:
DNS labels may only include hyphens in the
third and fourth position if they represent valid IDNs (as specified
above) in their ASCII encoding (e.g., “xn--ndk061n”).
</quote>

Many ccTLDs have the same restriction.

-- 
Patrick Mevzek