Lookup for reserved LDH labels
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.
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