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.\"
.IX Title "Net::IDN::Standards 3"
.TH Net::IDN::Standards 3 "2016-08-24" "perl v5.16.3" "User Contributed Perl Documentation"
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.SH "NAME"
Net::IDN::Standards \-\- Internationalized Domain Names for Applications (IDNA)
.SH "INTRODUCTION"
.IX Header "INTRODUCTION"
Historically, domain names and host names were restricted to a
limited repertoire of \s-1ASCII\s0 characters, i.e. letters, digits and
the hypen (i.e. \f(CW\*(C`/[A\-Z0\-9\-]/i\*(C'\fR). Words and names from langauges
that require additional characters (such as diacritics or special
characters) or other scripts could not be used.
.PP
Internationalizied Domain Names (IDNs) extend the character
repertoire for domain names from \s-1ASCII\s0 to Unicode while
maintaining backwards compatibility with software that only
expects and handles \s-1ASCII\s0 characters.
.PP
In order to do so, Unicode domain names are converted to \s-1ASCII\s0
using an ASCII-compatible encoding (\s-1ACE\s0) called Punycode. On the
wire, converted domain names start with \f(CW\*(C`xn\-\-\*(C'\fR, followed by the
\&\s-1ASCII\s0 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,
\&\s-1IDNA\s0).  Internationalized Resource Identifiers (IRIs), the
Unicode version of URLs, may also include domain names in their
Unicode form.
.PP
The \s-1IDNA\s0 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, \f(CW\*(C`ToASCII\*(C'\fR and \f(CW\*(C`ToUnicode\*(C'\fR, which
prepare and convert a domain name to the \s-1ACE\s0 version or the
Unicode version.
.SH "DIFFERENT STANDARDS"
.IX Header "DIFFERENT STANDARDS"
.Vb 3
\&  "The nice thing about standards is that you have so many to
\&  choose from."
\&                                       \-\- Andrew S. Tanenbaum
.Ve
.PP
While the actual Punycode conversion is stable, there are different
specifications regarding mapping and/or validation (preparation):
.SS "\s-1IDNA2003\s0"
.IX Subsection "IDNA2003"
\&\s-1IDNA2003,\s0 which is defined in \s-1RFC\s0\ 3490
(<http://tools.ietf.org/html/rfc3490>) and related documents, was
the original specification for the internationalization of domain
names.
.PP
However, some issues were subsequently identified with \s-1IDNA2003:\s0
The specification was tied to Unicode\ 3.2 and therefore did not
allow characters added in newer versions of Unicode (without
updating the specifications).
.PP
Furthermore, a few characters were mapped to other characters or
deleted although they would carry meaning in some languages (i.e.
\&'\*8' and 'X' were mapped to 'ss' and 'X'; \s-1ZWJ\s0 and \s-1ZWNJ\s0 were always
mapped to nothing, although some scripts like Arabic require them
for correct display).
.SS "\s-1IDNA2008\s0"
.IX Subsection "IDNA2008"
\&\s-1IDNA2008,\s0 which is defined in \s-1RFC\s0\ 5890
(<http://tools.ietf.org/html/rfc5890>) and related documents, resolves the
issues found in \s-1IDNA2003.\s0
.PP
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 \s-1IDNA2003\s0 implementations, of course.
.PP
However, \s-1IDNA2008\s0 also disallowed a large number of characters
that had been allowed in \s-1IDNA2003 \s0(mostly symbols). An
implementation of \s-1IDNA2008\s0 would therefore no longer be able to
access domain names such as \f(CW\*(C`X.com\*(C'\fR, which had been registered
under \s-1IDNA2003.\s0
.SS "\s-1UTS\s0\ #46"
.IX Subsection "UTS#46"
Unicode Technical Standard #46 (\s-1UTS\s0\ #46,
<http://unicode.org/reports/tr46/>) solves this problem by
allowing domain names that are valid in either \s-1IDNA2003\s0 or
\&\s-1IDNA2008.\s0
.PP
This makes \s-1UTS\s0\ #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).
.SH "AUTHOR"
.IX Header "AUTHOR"
Claus Fa\*:rber <CFAERBER@cpan.org>