Lookup for reserved LDH labels

Vint Cerf vint at google.com
Thu Nov 8 12:39:31 CET 2012

On Thu, Nov 8, 2012 at 4:24 AM, Simon Josefsson <simon at josefsson.org> wrote:

> Andrew Sullivan <ajs at anvilwalrusden.com> writes:
> What is an "restricted LDH label"?  I can't find the definition of that
> term in RFC 5890/5891.

Sorry that should have read "Reserved LDH" or "R-LDH" label.

see page 7, RFC5890.

 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.  That subset is called "XN-labels" in this
   set of documents.  XN-labels are further divided into those whose
   remaining characters (after the "xn--") are valid output of the
   Punycode algorithm [RFC3492] and those that are not (see below).  The
   XN-labels that are valid Punycode output are known as "A-labels" if
   they also meet the other criteria for IDNA-validity described below.
   Because LDH labels (and, indeed, any DNS label) must not be more than
   63 octets in length, the portion of an XN-label derived from the
   Punycode algorithm is limited to no more than 59 ASCII characters.
   Non-Reserved LDH labels are the set of valid LDH labels that do not

have "--" in the third and fourth positions.

> /Simon
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