Manual Reference Pages - NET::IDN::STANDARDS (3)
Net::IDN::Standards -- Internationalized Domain Names for Applications (IDNA)
Historically, domain names and host names were restricted to a
limited repertoire of ASCII characters, i.e. letters, digits and
the hypen (i.e. /[A-Z0-9-]/i). Words and names from langauges
that require additional characters (such as diacritics or special
characters) or other scripts could not be used.
Internationalizied Domain Names (IDNs) extend the character
repertoire for domain names from ASCII to Unicode while
maintaining backwards compatibility with software that only
expects and handles ASCII characters.
In order to do so, Unicode domain names are converted to ASCII
using an ASCII-compatible encoding (ACE) called Punycode. On the
wire, converted domain names start with xn--, followed by the
ASCII encoding of the Unicode string. The Unicode version is
typically only shown in applications presenting the domain to the
user (hence Internationalized Domain Names for Applications,
IDNA). Internationalized Resource Identifiers (IRIs), the
Unicode version of URLs, may also include domain names in their
The IDNA specifications, however, do not only cover the actual
Punycode conversion but also include extensive rules for
preparation (mapping and/or validation) of input strings. They
typically define two functions, ToASCII and ToUnicode, which
prepare and convert a domain name to the ACE version or the
"The nice thing about standards is that you have so many to
-- Andrew S. Tanenbaum
While the actual Punycode conversion is stable, there are different
specifications regarding mapping and/or validation (preparation):
IDNA2003, which is defined in RFC 3490
(<http://tools.ietf.org/html/rfc3490>) and related documents, was
the original specification for the internationalization of domain
However, some issues were subsequently identified with IDNA2003:
The specification was tied to Unicode 3.2 and therefore did not
allow characters added in newer versions of Unicode (without
updating the specifications).
Furthermore, a few characters were mapped to other characters or
deleted although they would carry meaning in some languages (i.e.
ss and X were mapped to ss and X; ZWJ and ZWNJ were always
mapped to nothing, although some scripts like Arabic require them
for correct display).
IDNA2008, which is defined in RFC 5890
(<http://tools.ietf.org/html/rfc5890>) and related documents, resolves the
issues found in IDNA2003.
This was done by allowing some characters that would either be
mapped to other characters, mapped to zero and/or cause the
preparation to fail. The new domain names would not be accessible
by IDNA2003 implementations, of course.
However, IDNA2008 also disallowed a large number of characters
that had been allowed in IDNA2003 (mostly symbols). An
implementation of IDNA2008 would therefore no longer be able to
access domain names such as X.com, which had been registered
Unicode Technical Standard #46 (UTS #46,
<http://unicode.org/reports/tr46/>) solves this problem by
allowing domain names that are valid in either IDNA2003 or
This makes UTS #46 the perfect fit for domain lookup (be liberal
in what you accept) but unsuitable for validating domain names
prior to registration (be conservative in what you send).
Claus Faerber <CFAERBER@cpan.org>
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