supported.pod

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

First we start with which characters to include.  We call this
collection of characters I<character repertoire>.

=item *

Then we have to give each character a unique ID so your computer can
tell the difference between 'a' and 'A'.  This itemized character
repertoire is now a I<character set>.

=item *

If your computer can grow the character set without further
processing, you can go ahead and use it.  This is called a I<coded
character set> (CCS) or I<raw character encoding>.  ASCII is used this
way for most cases.

=item *

But in many cases, especially multi-byte CJK encodings, you have to
tweak a little more.  Your network connection may not accept any data
with the Most Significant Bit set, and your computer may not be able to
tell if a given byte is a whole character or just half of it.  So you
have to I<encode> the character set to use it.

A I<character encoding scheme> (CES) determines how to encode a given
character set, or a set of multiple character sets.  7bit ISO-2022 is
an example of a CES.  You switch between character sets via I<escape
sequences>.

=back

Technically, or mathematically, speaking, a character set encoded in
such a CES that maps character by character may form a CCS.  EUC is such
an example.  The CES of EUC is as follows:

=over 2

=item *

Map ASCII unchanged.

=item *

Map such a character set that consists of 94 or 96 powered by N
members by adding 0x80 to each byte.

=item *

You can also use 0x8e and 0x8f to indicate that the following sequence of
characters belongs to yet another character set.  To each following byte
is added the value 0x80.

=back

By carefully looking at the encoded byte sequence, you can find that the
byte sequence conforms a unique number.  In that sense, EUC is a CCS
generated by a CES above from up to four CCS (complicated?).  UTF-8
falls into this category.  See L<perlUnicode/"UTF-8"> to find out how
UTF-8 maps Unicode to a byte sequence.

You may also have found out by now why 7bit ISO-2022 cannot comprise
a CCS.  If you look at a byte sequence \x21\x21, you can't tell if
it is two !'s or IDEOGRAPHIC SPACE.  EUC maps the latter to \xA1\xA1
so you have no trouble differentiating between "!!". and S<"  ">.

=head1 Encoding Classification (by Anton Tagunov and Dan Kogai)

This section tries to classify the supported encodings by their 
applicability for information exchange over the Internet and to 
choose the most suitable aliases to name them in the context of 
such communication.

=over 2

=item * 

To (en|de)code encodings marked by C<(**)>, you need 
C<Encode::HanExtra>, available from CPAN.

=back

Encoding names

  US-ASCII    UTF-8    ISO-8859-*  KOI8-R
  Shift_JIS   EUC-JP   ISO-2022-JP ISO-2022-JP-1
  EUC-KR      Big5     GB2312

are registered with IANA as preferred MIME names and may
be used over the Internet.

C<Shift_JIS> has been officialized by JIS X 0208:1997.
L<Microsoft-related naming mess> gives details.

C<GB2312> is the IANA name for C<EUC-CN>.
See L<Microsoft-related naming mess> for details.

C<GB_2312-80> I<raw> encoding is available as C<gb2312-raw>
with Encode. See L<Encode::CN> for details.

  EUC-CN
  KOI8-U        [RFC2319]

have not been registered with IANA (as of March 2002) but
seem to be supported by major web browsers. 
The IANA name for C<EUC-CN> is C<GB2312>.

  KS_C_5601-1987

is heavily misused.
See L<Microsoft-related naming mess> for details.

C<KS_C_5601-1987> I<raw> encoding is available as C<kcs5601-raw>
with Encode. See L<Encode::KR> for details.

  UTF-16 UTF-16BE UTF-16LE

are IANA-registered C<charset>s. See [RFC 2781] for details.
Jungshik Shin reports that UTF-16 with a BOM is well accepted
by MS IE 5/6 and NS 4/6. Beware however that

=over 2

=item *

C<UTF-16> support in any software you're going to be
using/interoperating with has probably been less tested
then C<UTF-8> support

=item *

C<UTF-8> coded data seamlessly passes traditional
command piping (C<cat>, C<more>, etc.) while C<UTF-16> coded
data is likely to cause confusion (with its zero bytes,
for example)

=item *

it is beyond the power of words to describe the way HTML browsers
encode non-C<ASCII> form data. To get a general impression, visit
L<http://ppewww.ph.gla.ac.uk/~flavell/charset/form-i18n.html>.
While encoding of form data has stabilized for C<UTF-8> encoded pages
(at least IE 5/6, NS 6, and Opera 6 behave consistently), be sure to
expect fun (and cross-browser discrepancies) with C<UTF-16> encoded
pages!

=back

The rule of thumb is to use C<UTF-8> unless you know what
you're doing and unless you really benefit from using C<UTF-16>.

  ISO-IR-165    [RFC1345]
  VISCII
  GB 12345
  GB 18030 (**)  (see links bellow)
  EUC-TW   (**)

are totally valid encodings but not registered at IANA.
The names under which they are listed here are probably the
most widely-known names for these encodings and are recommended
names.

  BIG5PLUS (**)

is a proprietary name. 

=head2 Microsoft-related naming mess

Microsoft products misuse the following names:

=over 2

=item KS_C_5601-1987

Microsoft extension to C<EUC-KR>.

Proper names: C<CP949>, C<UHC>, C<x-windows-949> (as used by Mozilla).

See L<http://lists.w3.org/Archives/Public/ietf-charsets/2001AprJun/0033.html>
for details.

Encode aliases C<KS_C_5601-1987> to C<cp949> to reflect this common
misusage. I<Raw> C<KS_C_5601-1987> encoding is available as
C<kcs5601-raw>.

See L<Encode::KR> for details.

=item GB2312

Microsoft extension to C<EUC-CN>.

Proper names: C<CP936>, C<GBK>.

C<GB2312> has been registered in the C<EUC-CN> meaning at
IANA. This has partially repaired the situation: Microsoft's 
C<GB2312> has become a superset of the official C<GB2312>.

Encode aliases C<GB2312> to C<euc-cn> in full agreement with
IANA registration. C<cp936> is supported separately.
I<Raw> C<GB_2312-80> encoding is available as C<gb2312-raw>.

See L<Encode::CN> for details.

=item Big5

Microsoft extension to C<Big5>.

Proper name: C<CP950>.

Encode separately supports C<Big5> and C<cp950>.

=item Shift_JIS

Microsoft's understanding of C<Shift_JIS>.

JIS has not endorsed the full Microsoft standard however.
The official C<Shift_JIS> includes only JIS X 0201 and JIS X 0208
character sets, while Microsoft has always used C<Shift_JIS>
to encode a wider character repertoire. See C<IANA> registration for
C<Windows-31J>.

As a historical predecessor, Microsoft's variant
probably has more rights for the name, though it may be objected
that Microsoft shouldn't have used JIS as part of the name
in the first place.

Unambiguous name: C<CP932>. C<IANA> name (also used by Mozilla, and
provided as an alias by Encode): C<Windows-31J>.

Encode separately supports C<Shift_JIS> and C<cp932>.

=back

=head1 Glossary

=over 2

=item character repertoire

A collection of unique characters.  A I<character> set in the strictest
sense. At this stage, characters are not numbered.

=item coded character set (CCS)

A character set that is mapped in a way computers can use directly.
Many character encodings, including EUC, fall in this category.

=item character encoding scheme (CES)

An algorithm to map a character set to a byte sequence.  You don't
have to be able to tell which character set a given byte sequence
belongs.  7-bit ISO-2022 is a CES but it cannot be a CCS.  EUC is an
example of being both a CCS and CES.

=item charset (in MIME context)

has long been used in the meaning of C<encoding>, CES.

While the word combination C<character set> has lost this meaning
in MIME context since [RFC 2130], the C<charset> abbreviation has
retained it. This is how [RFC 2277] and [RFC 2278] bless C<charset>:

 This document uses the term "charset" to mean a set of rules for
 mapping from a sequence of octets to a sequence of characters, such
 as the combination of a coded character set and a character encoding
 scheme; this is also what is used as an identifier in MIME "charset="
 parameters, and registered in the IANA charset registry ...  (Note
 that this is NOT a term used by other standards bodies, such as ISO).
 [RFC 2277]

=item EUC

Extended Unix Character.  See ISO-2022.

=item ISO-2022

A CES that was carefully designed to coexist with ASCII.  There are a 7
bit version and an 8 bit version.  

The 7 bit version switches character set via escape sequence so it
cannot form a CCS.  Since this is more difficult to handle in programs
than the 8 bit version, the 7 bit version is not very popular except for
iso-2022-jp, the I<de facto> standard CES for e-mails.

The 8 bit version can form a CCS.  EUC and ISO-8859 are two examples
thereof.  Pre-5.6 perl could use them as string literals.

=item UCS

Short for I<Universal Character Set>.  When you say just UCS, it means
I<Unicode>.

=item UCS-2

ISO/IEC 10646 encoding form: Universal Character Set coded in two
octets.

=item Unicode

A character set that aims to include all character repertoires of the
world.  Many character sets in various national as well as industrial
standards have become, in a way, just subsets of Unicode.

=item UTF

Short for I<Unicode Transformation Format>.  Determines how to map a
Unicode character into a byte sequence.

=item UTF-16

A UTF in 16-bit encoding.  Can either be in big endian or little
endian.  The big endian version is called UTF-16BE (equal to UCS-2 + 
surrogate support) and the little endian version is called UTF-16LE.

=back

=head1 See Also

L<Encode>, 
L<Encode::Byte>, 
L<Encode::CN>, L<Encode::JP>, L<Encode::KR>, L<Encode::TW>,
L<Encode::EBCDIC>, L<Encode::Symbol>
L<Encode::MIME::Header>, L<Encode::Guess>

=head1 References

=over 2

=item ECMA

European Computer Manufacturers Association
L<http://www.ecma.ch>

=over 2

=item ECMA-035 (eq C<ISO-2022>)

L<http://www.ecma.ch/ecma1/STAND/ECMA-035.HTM> 

The specification of ISO-2022 is available from the link above.

=back

=item IANA

Internet Assigned Numbers Authority
L<http://www.iana.org/>

=over 2

=item Assigned Charset Names by IANA

L<http://www.iana.org/assignments/character-sets>

Most of the C<canonical names> in Encode derive from this list
so you can directly apply the string you have extracted from MIME
header of mails and web pages.

=back

=item ISO

International Organization for Standardization
L<http://www.iso.ch/>

=item RFC

Request For Comments -- need I say more?
L<http://www.rfc-editor.org/>, L<http://www.rfc.net/>,
L<http://www.faqs.org/rfcs/>

=item UC

Unicode Consortium
L<http://www.unicode.org/>

=over 2

=item Unicode Glossary

L<http://www.unicode.org/glossary/>

The glossary of this document is based upon this site.

=back

=back

=head2 Other Notable Sites

=over 2

=item czyborra.com

L<http://czyborra.com/>

Contains a lot of useful information, especially gory details of ISO
vs. vendor mappings.

=item CJK.inf

L<http://www.oreilly.com/people/authors/lunde/cjk_inf.html>

Somewhat obsolete (last update in 1996), but still useful.  Also try

L<ftp://ftp.oreilly.com/pub/examples/nutshell/cjkv/pdf/GB18030_Summary.pdf>

You will find brief info on C<EUC-CN>, C<GBK> and mostly on C<GB 18030>.

=item Jungshik Shin's Hangul FAQ

L<http://jshin.net/faq>

And especially its subject 8.

L<http://jshin.net/faq/qa8.html>

A comprehensive overview of the Korean (C<KS *>) standards.

=item debian.org: "Introduction to i18n"

A brief description for most of the mentioned CJK encodings is
contained in
L<http://www.debian.org/doc/manuals/intro-i18n/ch-codes.en.html>

=back

=head2 Offline sources

=over 2

=item C<CJKV Information Processing> by Ken Lunde

CJKV Information Processing
1999 O'Reilly & Associates, ISBN : 1-56592-224-7

The modern successor of C<CJK.inf>.

Features a comprehensive coverage of CJKV character sets and
encodings along with many other issues faced by anyone trying
to better support CJKV languages/scripts in all the areas of
information processing.

To purchase this book, visit
L<http://www.oreilly.com/catalog/cjkvinfo/>
or your favourite bookstore.

=back

=cut