FTPEXT Working Group B. Curtin
INTERNET DRAFT Defense Information Systems Agency
Expires 16 December 1997 20 February, 1998
Internationalization of the File Transfer Protocol
<draft-ietf-ftpext-intl-ftp-04.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is not appropriate to use
Internet-Drafts as reference material or to cite them other
than as a "working draft" or "work in progress".
To view the entire list of current Internet-Drafts, please
check the 1id-abstracts.txt listing contained in the
Internet-Drafts Shadow Directories on ftp.is.co.za (Africa),
ftp.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West
Coast).
Distribution of this document is unlimited. Please send
comments to the FTP Extension working group (FTPEXT-WG) of the
Internet Engineering Task Force (IETF) at
<ftp-wg@hops.ag.utk.edu>. Subscription address is
<ftp-wg-request@hops.ag.utk.edu>. Discussions of the group are
archived at <URL:ftp://hops.ag.utk.edu/ftp-wg/archives/>.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in RFC 2119 [RFC 2119].
Expires 20 August 1998 [Page 1]
INTERNET DRAFT FTP Internationalization 20 February, 1998
Abstract
The File Transfer Protocol, as defined in RFC 959 [RFC959] and
RFC 1123 Section 4 [RFC1123], is one of the oldest and widely
used protocols on the Internet. The protocol's primary
character set, 7 bit ASCII, has served the protocol well
through the early growth years of the Internet. However, as the
Internet becomes more global, there is a need to support
character sets beyond 7 bit ASCII.
This document addresses the internationalization (I18n) of FTP,
which includes supporting the multiple character sets found
throughout the Internet community. This is achieved by
extending the FTP specification and giving recommendations for
proper internationalization support.
Table of Contents
1 INTRODUCTION....................................................3
2 INTERNATIONALIZATION............................................3
2.1 International Character Set.................................4
2.2 Transfer Encoding...........................................4
3 CONFORMANCE.....................................................5
3.1 General.....................................................5
3.2 International Servers.......................................7
3.3 International Clients.......................................7
4 SECURITY........................................................8
5 ACKNOWLEDGMENTS.................................................8
6 GLOSSARY........................................................8
7 BIBLIOGRAPHY....................................................9
8 AUTHOR'S ADDRESS...............................................10
APPENDIX A - IMPLEMENTATION CONSIDERATIONS......................A-1
A.1 General Considerations....................................A-1
A.2 Transition Considerations.................................A-2
APPENDIX B - SAMPLE CODE AND EXAMPLES...........................B-1
B.1 Valid UTF-8 check.........................................B-1
B.2 Conversions...............................................B-2
B.2.1 Conversion from local character set to UTF-8............B-2
B.2.2 Conversion from UTF-8 to local character set............B-5
B.2.3 ISO/IEC 8859-8 Example.................................B-6
B.2.4 Vendor Codepage Example.................................B-7
B.3 Pseudo Code for translating servers.......................B-8
Expires 20 August 1998 [Page 2 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
1 Introduction
As the Internet grows throughout the world the requirement to
support character sets outside of the ASCII [ASCII] / Latin-1
[ISO-8859] character set becomes ever more urgent. For FTP,
because of the large installed base, it is paramount that this
be done without breaking existing clients and servers. This
document addresses this need. In doing so it defines a solution
which will still allow the installed base to interoperate with
new international clients and servers.
This document enhances the capabilities of the File Transfer
Protocol by removing the 7-bit restrictions on pathnames used
in client commands and server responses, recommends the use of
a Universal Character Set (UCS) ISO/IEC 10646 [ISO-10646], and
recommends a UCS transformation format (UTF) UTF-8 [UTF-8].
The recommendations made in this document are consistent with
the recommendations expressed by the 29 Feb - 1 Mar 1996 IAB
Character Set Workshop as expressed in RFC 2130 [RFC 2130].
2 Internationalization
The File Transfer Protocol was developed when the predominate
character sets were 7 bit ASCII and 8 bit EBCDIC. Today these
character sets cannot support the wide range of characters
needed by multinational systems. Given that there are a number
of character sets in current use that provide more characters
than 7-bit ASCII, it makes sense to decide on a convenient way
to represent the union of those possibilities. To work globally
either requires support of a number of character sets and to be
able to convert between them, or the use of a single preferred
character set. To assure global interoperability this document
RECOMMENDS the latter approach and defines a single character
set, in addition to NVT ASCII and EBCDIC, which is
understandable by all systems. For FTP this character set SHALL
be ISO/IEC 10646:1993. For support of global compatibility it
is STRONGLY RECOMMENDED that clients and servers use UTF-8
encoding when exchanging pathnames. Clients and servers are,
however, under no obligation to perform any conversion on the
contents of a file for operations such as STOR or RETR.
The character set used to store files SHALL remain a local
decision and MAY depend on the capability of local operating
systems. Prior to the exchange of pathnames they should be
converted into a ISO/IEC 10646 format and UTF-8 encoded. This
approach, while allowing international exchange of pathnames,
will still allow backward compatibility with older systems
because the code set positions for ASCII characters are
identical to the one byte sequence in UTF-8.
Sections 2.1 and 2.2 give a brief description of the
international character set and transfer encoding recommended
Expires 20 August 1998 [Page 3 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
by this document. A more thorough description of UTF-8, ISO/IEC
10646, and UNICODE [UNICODE], beyond that given in this
document, can be found in RFC 2044 [RFC2044].
2.1 International Character Set
The character set defined for international support of FTP
SHALL be the Universal Character Set as defined in ISO
10646:1993 as amended. This standard incorporates the character
sets of many existing international, national, and corporate
standards. ISO/IEC 10646 defines two alternate forms of
encoding, UCS-4 and UCS-2. UCS-4 is a four byte (31 bit)
encoding containing 2**31 code positions divided into 128
groups of 256 planes. Each plane consists of 256 rows of 256
cells. UCS-2 is a 2 byte (16 bit) character set consisting of
plane zero or the Basic Multilingual Plane (BMP). Currently,
no codesets have been defined outside of the 2 byte BMP.
The Unicode standard version 2.0 [UNICODE] is consistent with
the UCS-2 subset of ISO/IEC 10646. The Unicode standard version
2.0 includes the repertoire of IS 10646 characters, amendments
1-7 of IS 10646, and editorial and technical corrigenda.
2.2 Transfer Encoding
UCS Transformation Format 8 (UTF-8), in the past referred to as
UTF-2 or UTF-FSS, SHALL be used as a transfer encoding to
transmit the international character set. UTF-8 is a file safe
encoding which avoids the use of byte values that have special
significance during the parsing of pathname character strings.
UTF-8 is an 8 bit encoding of the characters in the UCS. Some
of UTF-8's benefits are that it is compatible with 7 bit ASCII,
so it doesn't affect programs that give special meanings to
various ASCII characters; it is immune to synchronization
errors; its encoding rules allow for easy identification; and
it has enough space to support a large number of character
sets.
UTF-8 encoding represents each UCS character as a sequence of 1
to 6 bytes in length. For all sequences of one byte the most
significant bit is ZERO. For all sequences of more than one
byte the number of ONE bits in the first byte, starting from
the most significant bit position, indicates the number of
bytes in the UTF-8 sequence followed by a ZERO bit. For
example, the first byte of a 3 byte UTF-8 sequence would have
1110 as its most significant bits. Each additional bytes
(continuing bytes) in the UTF-8 sequence, contain a ONE bit
followed by a ZERO bit as their most significant bits. The
remaining free bit positions in the continuing bytes are used
to identify characters in the UCS. The relationship between UCS
and UTF-8 is demonstrated in the following table:
Expires 20 August 1998 [Page 4 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
UCS-4 range(hex) UTF-8 byte sequence(binary)
00000000 - 0000007F 0xxxxxxx
00000080 - 000007FF 110xxxxx 10xxxxxx
00000800 - 0000FFFF 1110xxxx 10xxxxxx 10xxxxxx
00010000 - 001FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
00200000 - 03FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx
10xxxxxx
04000000 - 7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx
10xxxxxx 10xxxxxx
A beneficial property of UTF-8 is that its single byte sequence
is consistent with the ASCII character set. This feature will
allow a transition where old ASCII-only clients can still
interoperate with new servers that support the UTF-8 encoding.
Another feature is that the encoding rules make it very
unlikely that a character sequence from a different character
set will be mistaken for a UTF-8 encoded character sequence.
Clients and servers can use a simple routine to determine if
the character set being exchanged is valid UTF-8. Section B.1
shows a code example of this check.
3 Conformance
3.1 General
- The 7-bit restriction for pathnames exchanged is dropped.
- Many operating system allow the use of spaces <SP>, carriage
return <CR>, and line feed <LF> characters as part of the
pathname. The exchange of pathnames with these special command
characters will cause the pathnames to be parsed improperly.
This is because ftp commands associated with pathnames have
the form:
COMMAND <SP> <pathname> <CRLF>.
To allow the exchange of pathnames containing these
characters, the definition of pathname is changed from
<pathname> ::= <string> ; in BNF format
to
pathname = 1*(%x01..%xFF) ; in ABNF format [ABNF]
To avoid mistaking these characters within pathnames as
special command characters the following rules will apply:
There MUST be only one <SP> between a ftp command and the
pathname. Implementations MUST assume <SP> characters
following the initial <SP> as part of the pathname. For
Expires 20 August 1998 [Page 5 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
example the pathname in STOR <SP><SP><SP>foo.bar<CRLF> is
<SP><SP>foo.bar .
Current implementations, which may allow multiple <SP>
characters as separators between the command and pathname,
MUST assure that they comply with this single <SP>
convention. Note: Implementations which treat 3 character
commands (e.g. CWD, MKD, etc.) as a fixed 4 character
command by padding the command with a trailing <SP> are in
non-compliance to this specification.
When a <CR> character is encountered as part of a pathname it
MUST be padded with a <NUL> character prior to sending the
command. On receipt of a pathname containing a <CR><NUL>
sequence the <NUL> character MUST be stripped away. This
approach is described in the Telnet protocol [RFC854] on pages
11 and 12. For example, to store a pathname foo<CR><LF>boo.bar
the pathname would become foo<CR><NUL><LF>boo.bar prior to
sending the command STOR <SP>foo<CR><NUL><LF>boo.bar<CRLF> .
Upon receipt of the altered pathname the <NUL> character
following the <CR> would be stripped away to form the original
pathname.
- Conforming internationalized clients and servers MUST support
UTF-8 for the transfer and receipt of pathnames. Clients and
servers MAY in addition give users a choice of specifying
interpretation of pathnames in another encoding. Note that
configuring clients and servers to use character sets /
encoding other than UTF-8 is outside of the scope of this
document. While it is recognized that in certain operational
scenarios this may be desirable, this is left as a quality of
implementation and operational issue.
- Pathnames are sequences of bytes. The encoding of names that
are valid UTF-8 sequences is assumed to be UTF-8. The
character set of other names is undefined. Clients and
servers, unless otherwise configured to support a specific
native character set, MUST check for a valid UTF-8 byte
sequence to determine if the pathname being presented is
UTF-8.
- To avoid data loss, clients and servers SHOULD use the UTF-8
encoded pathnames when unable to convert them to a usable code
set.
- There may be cases when the code set / encoding presented to
the server or client cannot be determined. In such cases the
raw bytes SHOULD be used.
Expires 20 August 1998 [Page 6 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
3.2 International Servers
- Servers MUST support the UTF-8 feature in response to the
FEAT command [FEAT]. The UTF-8 feature is a line containing
the exact string "UTF8". This string is not case sensitive,
but SHOULD be transmitted in upper case. The response to a
FEAT command SHOULD be:
C> feat
S> 211- <any descriptive text>
S> ...
S> UTF8
S> ...
S> 211 end
The ellipses indicate placeholders where other features may be
included, and are not required. The one space indentation of
the feature lines is mandatory [FEAT].
- Mirror servers may want to exactly reflect the site that they
are mirroring. In such cases servers MAY store and present the
exact pathname bytes that it received from the main server.
3.3 International Clients
- Clients which do not require display of pathnames are under
no obligation to do so. Non-display clients do not need to
conform to requirements associated with display.
- Clients, which are presented UTF-8 pathnames by the server,
SHOULD parse UTF-8 correctly and attempt to display the
pathname within the limitation of the resources available.
- Clients MUST support the FEAT command and recognize the
"UTF8" feature (defined in 3.2 above) to determine if a server
supports UTF-8 encoding.
- Character semantics of other names shall remain undefined. If
a client detects that a server is non UTF-8, it SHOULD change
its display appropriately. How a client implementation handles
non UTF-8 is a quality of implementation issue. It MAY try to
assume some other encoding, give the user a chance to try to
assume something, or save encoding assumptions for a server
from one FTP session to another.
- Glyph rendering is outside the scope of this document. How a
client presents characters it cannot display is a quality of
implementation issue. This document RECOMMENDS that octets
corresponding to non-displayable characters SHOULD be
presented in URL %HH format defined in RFC 1738 [RFC1738].
They MAY, however, display them as question marks, with their
UCS hexadecimal value, or in any other suitable fashion.
Expires 20 August 1998 [Page 7 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
- Many existing clients interpret 8-bit pathnames as being in
the local character set. They MAY continue to do so for
pathnames that are not valid UTF-8.
4 Security
This document addresses the support of character sets beyond 1
byte. Conformance to this document should not induce a security
threat.
5 Acknowledgments
The following people have contributed to this document:
D. J. Bernstein
Martin J. Duerst
Mark Harris
Paul Hethmon
Alun Jones
James Matthews
Keith Moore
Sandra O'Donnell
Benjamin Riefenstahl
Stephen Tihor
(and others from the FTPEXT working group)
6 Glossary
BIDI - abbreviation for Bi-directional, a reference to mixed
right-to-left and left-to-right text.
Character Set - a collection of characters used to represent
textual information in which each character has a numeric value
Code Set - (see character set).
Glyph - a character image represented on a display device.
I18N - "I eighteen N", the first and last letters of the word
"internationalization" and the eighteen letters in between.
UCS-2 - the ISO/IEC 10646 two octet Universal Character Set
form.
UCS-4 - the ISO/IEC 10646 four octet Universal Character Set
form.
UTF-8 - the UCS Transformation Format represented in 8 bits.
Expires 20 August 1998 [Page 8 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
UTF-16 - A 16-bit format including the BMP (directly encoded)
and surrogate pairs to represent characters in planes 01-16;
equivalent to Unicode.
7 Bibliography
[ABNF]
D. Crocker, P. Overell, Augmented BNF for Syntax
Specifications: ABNF, RFC 2234, November 1997.
[ASCII]
ANSI X3.4:1986 Coded Character Sets - 7 Bit American National
Standard Code for Information Interchange (7-bit ASCII)
[FEAT]
Hethmon, P., Elz, R., "Feature Negotiation Mechanism for the
File Transfer Protocol", Work in Progress, <draft-ietf-ftpext-
feat-02.txt> November 1997.
[ISO-8859]
ISO 8859. International standard -- Information processing --
8-bit single-byte coded graphic character sets -- Part 1:
Latin alphabet No. 1 (1987) -- Part 2: Latin alphabet No. 2
(1987) -- Part 3: Latin alphabet No. 3 (1988) -- Part 4: Latin
alphabet No. 4 (1988) -- Part 5: Latin/Cyrillic alphabet
(1988) -- Part 6: Latin/Arabic alphabet (1987) -- Part :
Latin/Greek alphabet (1987) -- Part 8: Latin/Hebrew alphabet
(1988) -- Part 9: Latin alphabet No. 5 (1989) -- Part10: Latin
alphabet No. 6 (1992)
[ISO-10646]
ISO/IEC 10646-1:1993. International standard -- Information
technology -- Universal multiple-octet coded character set
(UCS) -- Part 1: Architecture and basic multilingual plane.
[RFC854]
J. Postel, J Reynolds, "Telnet Protocol Specification", RFC
854, May 1983.
[RFC959]
J. Postel, J Reynolds, "File Transfer Protocol (FTP)", RFC
959, October 1985.
Expires 20 August 1998 [Page 9 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
[RFC1123]
R. Braden, "Requirements for Internet Hosts -- Application and
Support", RFC 1123, October 1989.
[RFC1738]
T. Berners-Lee, L. Masinter, M.McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[RFC2044]
F. Yergeau, "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
[RFC 2119]
S. Bradner, " Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2130]
C. Weider, C. Preston, K.Simonsen, H. Alvestrand, " The Report
of the IAB Character Set Workshop held 29 February - 1 March,
1996", RFC 2130, April, 1997.
[UNICODE]
The Unicode Consortium, "The Unicode Standard - Version 2.0",
Addison Westley Developers Press, July 1996.
[UTF-8]
ISO/IEC 10646-1:1993 AMENDMENT 2 (1996). UCS Transformation
Format 8 (UTF-8).
8 Author's Address
JIEO
Attn JEBBD (Bill Curtin)
Ft. Monmouth, N.J.
07703-5613
curtinw@ftm.disa.mil
Expires 20 August 1998 [Page 10 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
Annex A - Implementation Considerations
A.1 General Considerations
- Implementers should ensure that their code accounts for
potential problems, such as using a NULL character to
terminate a string or no longer being able to steal the high
order bit for internal use, when supporting the extended
character set.
- Implementers should be aware that there is a chance that
pathnames that are non UTF-8 may be parsed as valid UTF-8. The
probabilities are low for some encoding or statistically zero
to zero for others. A recent non-scientific analysis found
that EUC encoded Japanese words had a 2.7% false reading; SJIS
had a 0.0005% false reading; other encoding such as ASCII or
KOI-8 have a 0% false reading. This probability is highest for
short pathnames and decreases as pathname size increases.
Implementers may want to look for signs that pathnames which
parse as UTF-8 are not valid UTF-8, such as the existence of
multiple local character sets in short pathnames. Hopefully,
as more implementations conform to UTF-8 transfer encoding
there will be a smaller need to guess at the encoding.
- Client developers should be aware that it will be possible
for pathnames to contain mixed characters (e.g.
/Latin1DirectoryName/HebrewFileName). They should be prepared
to handle the Bi-directional (BIDI) display of these character
sets (i.e. right to left display for the directory and left to
right display for the filename). While bi-directional display
is outside the scope of this document and more complicated
than the above example, an algorithm for bi-directional
display can be found in the UNICODE 2.0 [UNICODE] standard.
Also note that pathnames can have different byte ordering yet
be logically and display-wise equivalent due to the insertion
of BIDI control characters at different points during
composition. Also note that mixed character sets may also
present problems with font swapping.
- A server that copies pathnames transparently from a local
filesystem may continue to do so. It is then up to the local
file creators to use UTF-8 pathnames.
- Servers can supports charset labeling of files and/or
directories, such that different pathnames may have different
charsets. The server should attempt to convert all pathnames
to UTF-8, but if it can't then it should leave that name in
its raw form.
Expires 20 August 1998 [Page A-1 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
- Some server's OS do not mandate character sets, but allow
administrators to configure it in the FTP server. These
servers should be configured to use a particular mapping table
(either external or built-in). This will allow the flexibility
of defining different charsets for different directories.
- If the server's OS does not mandate the character set and the
FTP server cannot be configured, the server should simply use
the raw bytes in the file name. They might be ASCII or UTF-8.
- If the server is a mirror, and wants to look just like the
site it is mirroring, it should store the exact file name
bytes that it received from the main server.
A.2 Transition Considerations
-Clients and servers can transition to UTF-8 by either
converting to/from the local encoding, or the users can store
UTF-8 filenames. The former approach is easier on tightly
controlled file systems (e.g. PCs and MACs). The latter
approach is easier on more free form file systems (e.g. Unix).
-For interactive use attention should be focused on user
interface and ease of use. Non-interactive use requires a
consistent and controlled behavior.
-There may be many applications which reference files under
their old raw pathname (e.g. linked URLs). Changing the
pathname to UTF-8 will cause access to the old URL to fail. A
solution may be for the server to act as if there was 2
different pathnames associated with the file. This might be
done internal to the server on controlled file systems or by
using symbolic links on free form systems. While this approach
may work for single file transfer non-interactive use, a non-
interactive transfer of all of the files in a directory will
produce duplicates. Interactive users may be presented with
lists of files which are double the actual number files.
Expires 20 August 1998 [Page A-2 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
Annex B - Sample Code and Examples
B.1 Valid UTF-8 check
The following routine checks if a byte sequence is valid UTF-8.
This is done by checking for the proper tagging of the first
and following bytes to make sure they conform to the UTF-8
format. It then checks to assure that the data part of the UTF-
8 sequence conforms to the proper range allowed by the
encoding. Note: This routine will not detect characters that
have not been assigned and therefore do not exist.
int utf8_valid(const unsigned char *buf, unsigned int len)
{
const unsigned char *endbuf = buf + len;
unsigned char byte2mask=0x00, c;
int trailing = 0; // trailing (continuation)
bytes to follow
while (buf != endbuf)
{
c = *buf++;
if (trailing)
if ((c&0xC0) == 0x80) // Does trailing byte follow UTF-8 format?
{if (byte2mask) // Need to check 2nd byte for proper range?
if (c&byte2mask) // Are appropriate bits set?
byte2mask=0x00;
else
return 0;
trailing--; }
else
return 0;
else
if ((c&0x80) == 0x00) continue; // valid 1 byte UTF-8
else if ((c&0xE0) == 0xC0) // valid 2 byte UTF-8
if (c&0x1E) // Is UTF-8 byte in proper range?
trailing =1;
else
return 0;
else if ((c&0xF0) == 0xE0) // valid 3 byte UTF-8
{if (!(c&0x0F)) // Is UTF-8 byte in proper range?
byte2mask=0x20; // If not set mask to check next byte
trailing = 2;}
else if ((c&0xF8) == 0xF0) // valid 4 byte UTF-8
{if (!(c&0x07)) // Is UTF-8 byte in proper range?
Expires 20 August 1998 [Page B-1 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
byte2mask=0x30; // If not set mask to check next byte
trailing = 3;}
else if ((c&0xFC) == 0xF8) // valid 5 byte UTF-8
{if (!(c&0x03)) // Is UTF-8 byte in proper range?
byte2mask=0x38; // If not set mask to check next byte
trailing = 4;}
else if ((c&0xFE) == 0xFC) // valid 6 byte UTF-8
{if (!(c&0x01)) // Is UTF-8 byte in proper range?
byte2mask=0x3C; // If not set mask to check next byte
trailing = 5;}
else return 0;
}
return trailing == 0;
}
B.2 Conversions
The code examples in this section closely reflect the algorithm
in ISO 10646 and may not present the most efficient solution
for converting to / from UTF-8 encoding. If efficiency is an
issue, implementers should use the appropriate bitwise
operators.
Additional code examples and numerous mapping tables can be
found at the Unicode site, HTTP://www.unicode.org or
FTP://unicode.org.
Note that the conversion examples below assume that the local
character set supported in the operating system is something
other than UCS2/UTF-16. There are some operating systems that
already support UCS2/UTF-16 (notably Plan 9 and Windows NT). In
this case no conversion will be necessary from the local
character set to the UCS.
B.2.1 Conversion from local character set to UTF-8
Conversion from the local filesystem character set to UTF-8
will normally involve a two step process. First convert the
local character set to the UCS; then convert the UCS to UTF-8.
The first step in the process can be performed by maintaining a
mapping table that includes the local character set code and
the corresponding UCS code. For instance the ISO/IEC 8859-8
[ISO-8859] code for the Hebrew letter "VAV" is 0xE4. The
corresponding 4 byte ISO/IEC 10646 code is 0x000005D5.
Expires 20 August 1998 [Page B-2 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
The next step is to convert the UCS character code to the UTF-8
encoding. The following routine can be used to determine and
encode the correct number of bytes based on the UCS-4 character
code:
unsigned int ucs4_to_utf8 (unsigned long *ucs4_buf, unsigned int
ucs4_len, unsigned char *utf8_buf)
{
const unsigned long *ucs4_endbuf = ucs4_buf + ucs4_len;
unsigned int utf8_len = 0; // return value for UTF8 size
unsigned char *t_utf8_buf = utf8_buf;// Temporary pointer
// to load UTF8 values
while (ucs4_buf != ucs4_endbuf)
{
if ( *ucs4_buf <= 0x7F) // ASCII chars no conversion needed
{
*t_utf8_buf++ = (unsigned char) *ucs4_buf;
utf8_len++;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x07FF ) // In the 2 byte utf-8 range
{
*t_utf8_buf++= (unsigned char) (0xC0 + (*ucs4_buf/0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=2;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0xFFFF ) /* In the 3 byte utf-8 range. The
values 0x0000FFFE, 0x0000FFFF
and 0x0000D800 - 0x0000DFFF do
not occur in UCS-4 */
{
*t_utf8_buf++= (unsigned char) (0xE0 + (*ucs4_buf/0x1000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=3;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x1FFFFF ) //In the 4 byte utf-8 range
{
*t_utf8_buf++= (unsigned char) (0xF0 +
(*ucs4_buf/0x040000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x10000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
Expires 20 August 1998 B-3 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=4;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x03FFFFFF )//In the 5 byte utf-8 range
{
*t_utf8_buf++= (unsigned char) (0xF8 +
(*ucs4_buf/0x01000000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x040000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x1000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
(*ucs4_buf%0x40));
utf8_len+=5;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x7FFFFFFF )//In the 6 byte utf-8 range
{
*t_utf8_buf++= (unsigned char)
(0xF8 +(*ucs4_buf/0x40000000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x01000000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x040000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x1000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
(*ucs4_buf%0x40));
utf8_len+=6;
ucs4_buf++;
}
}
return (utf8_len);
}
B.2.2 Conversion from UTF-8 to local character set
When moving from UTF-8 encoding to the local character set the
reverse procedure is used. First the UTF-8 encoding is
transformed into the UCS-4 character set. The UCS-4 is then
converted to the local character set from a mapping table (i.e.
Expires 20 August 1998 [Page B-4 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
the opposite of the table used to form the UCS-4 character
code).
To convert from UTF-8 to UCS-4 the free bits (those that do not
define UTF-8 sequence size or signify continuation bytes) in a
UTF-8 sequence are concatenated as a bit string. The bits are
then distributed into a four-byte sequence starting from the
least significant bits. Those bits not assigned a bit in the
four-byte sequence are padded with ZERO bits. The following
routine converts the UTF-8 encoding to UCS-4 character codes:
int utf8_to_ucs4 (unsigned long *ucs4_buf, unsigned int utf8_len,
unsigned char *utf8_buf)
{
const unsigned char *utf8_endbuf = utf8_buf + utf8_len;
unsigned int ucs_len=0;
while (utf8_buf != utf8_endbuf)
{
if ((*utf8_buf & 0x80) == 0x00) /*ASCII chars no conversion
needed */
{
*ucs4_buf++ = (unsigned long) *utf8_buf;
utf8_buf++;
ucs_len++;
}
else
if ((*utf8_buf & 0xE0)== 0xC0) //In the 2 byte utf-8 range
{
*ucs4_buf++ = (unsigned long) (((*utf8_buf - 0xC0) * 0x40)
+ ( *(utf8_buf+1) - 0x80));
utf8_buf += 2;
ucs_len++;
}
else
if ( (*utf8_buf & 0xF0) == 0xE0 ) /*In the 3 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long) (((*utf8_buf - 0xE0) * 0x1000)
+ (( *(utf8_buf+1) - 0x80) * 0x40)
+ ( *(utf8_buf+2) - 0x80));
utf8_buf+=3;
ucs_len++;
}
else
if ((*utf8_buf & 0xF8) == 0xF0) /* In the 4 byte utf-8
range */
{
Expires 20 August 1998 [Page B-5 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xF0) * 0x040000)
+ (( *(utf8_buf+1) - 0x80) * 0x1000)
+ (( *(utf8_buf+2) - 0x80) * 0x40)
+ ( *(utf8_buf+3) - 0x80));
utf8_buf+=4;
ucs_len++;
}
else
if ((*utf8_buf & 0xFC) == 0xF8) /* In the 5 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xF8) * 0x01000000)
+ ((*(utf8_buf+1) - 0x80) * 0x040000)
+ (( *(utf8_buf+2) - 0x80) * 0x1000)
+ (( *(utf8_buf+3) - 0x80) * 0x40)
+ ( *(utf8_buf+4) - 0x80));
utf8_buf+=5;
ucs_len++;
}
else
if ((*utf8_buf & 0xFE) == 0xFC) /* In the 6 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xFC) * 0x40000000)
+ ((*(utf8_buf+1) - 0x80) * 0x010000000)
+ ((*(utf8_buf+2) - 0x80) * 0x040000)
+ (( *(utf8_buf+3) - 0x80) * 0x1000)
+ (( *(utf8_buf+4) - 0x80) * 0x40)
+ ( *(utf8_buf+5) - 0x80));
utf8_buf+=6;
ucs_len++;
}
}
return (ucs_len);
}
B.2.3 ISO/IEC 8859-8 Example
This example demonstrates mapping ISO/IEC 8859-8 character set
to UTF-8 and back to ISO/IEC 8859-8. As noted earlier, the
Hebrew letter "VAV" is convertd from the ISO/IEC 8859-8
character code 0xE4 to the corresponding 4 byte ISO/IEC 10646
code of 0x000005D5 by a simple lookup of a conversion/mapping
file.
The UCS-4 character code is transformed into UTF-8 using the
ucs4_to_utf8 routine described earlier by:
Expires 20 August 1998 [Page B-6 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
1. Because the UCS-4 character is between 0x80 and 0x07FF it
will map to a 2 byte UTF-8 sequence.
2. The first byte is defined by (0xC0 + (0x000005D5 / 0x40)) =
0xD7.
3. The second byte is defined by (0x80 + (0x000005D5 % 0x40))
= 0x95.
The UTF-8 encoding is transferred back to UCS-4 by using the
utf8_to_ucs4 routine described earlier by:
1. Because the first byte of the sequence, when the '&'
operator with a value of 0xE0 is applied, will produce 0xC0
(0xD7 & 0xE0 = 0xC0) the UTF-8 is a 2 byte sequence.
2. The four byte UCS-4 character code is produced by (((0xD7
- 0xC0) * 0x40) + (0x95 -0x80)) = 0x000005D5.
Finally, the UCS-4 character code is converted to ISO/IEC
8859-8 character code (using the mapping table which matches
ISO/IEC 8859-8 to UCS-4 ) to produce the original 0xE4 code for
the Hebrew letter "VAV".
B.2.4 Vendor Codepage Example
This example demonstrates the mapping of a codepage to UTF-8
and back to a vendor codepage. Mapping between vendor codepages
can be done in a very similar manner as described above. For
instance both the PC and Mac codepages reflect the character
set from the Thai standard TIS 620-2533. The character code on
both platforms for the Thai letter "SO SO" is 0xAB. This
character can then be mapped into the UCS-4 by way of a
conversion/mapping file to produce the UCS-4 code of 0x0E0B.
The UCS-4 character code is transformed into UTF-8 using the
ucs4_to_utf8 routine described earlier by:
1. Because the UCS-4 character is between 0x0800 and 0xFFFF it
will map to a 3 byte UTF-8 sequence.
2. The first byte is defined by (0xE0 + (0x00000E0B / 0x1000)
= 0xE0.
3. The second byte is defined by (0x80 + ((0x00000E0B / 0x40)
% 0x40))) = 0xB8.
4. The third byte is defined by (0x80 + (0x00000E0B % 0x40)) =
0x8B.
The UTF-8 encoding is transferred back to UCS-4 by using the
utf8_to_ucs4 routine described earlier by:
1. Because the first byte of the sequence, when the '&'
operator with a value of 0xF0 is applied, will produce 0xE0
(0xE0 & 0xF0 = 0xE0) the UTF-8 is a 3 byte sequence.
2. The four byte UCS-4 character code is produced by (((0xE0
- 0xE0) * 0x1000) + ((0xB8 - 0x80) * 0x40) + (0x8B -0x80) =
0x0000E0B.
Expires 20 August 1998 [Page B-7 ]
INTERNET DRAFT FTP Internationalization 20 February, 1998
Finally, the UCS-4 character code is converted to either the PC
or MAC codepage character code (using the mapping table which
matches codepage to UCS-4 ) to produce the original 0xAB code
for the Thai letter "SO SO".
B.3 Pseudo Code for a high-quality translating server
if utf8_valid(fn)
{
attempt to convert fn to the local charset, producing localfn
if (conversion fails temporarily) return error
if (conversion succeeds)
{
attempt to open localfn
if (open fails temporarily) return error
if (open succeeds) return success
}
}
attempt to open fn
if (open fails temporarily) return error
if (open succeeds) return success
return permanent error
Expires 20 August 1998 [Page B-8 ]
