Network Working Group B. Curtin
Request for Comments: 2640 Defense Information Systems Agency
Updates: 959 July 1999
Category: Proposed Standard
 Internationalization of the File Transfer Protocol
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (1999). All Rights Reserved.
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 and languages found
 throughout the Internet community. This is achieved by extending the
 FTP specification and giving recommendations for proper
 internationalization support.
Table of Contents
 ABSTRACT.......................................................1
 1 INTRODUCTION.................................................2
 1.1 Requirements Terminology..................................2
 2 INTERNATIONALIZATION.........................................3
 2.1 International Character Set...............................3
 2.2 Transfer Encoding Set.....................................4
 3 PATHNAMES....................................................5
 3.1 General compliance........................................5
 3.2 Servers compliance........................................6
 3.3 Clients compliance........................................7
 4 LANGUAGE SUPPORT.............................................7
 4.1 The LANG command..........................................8
 4.2 Syntax of the LANG command................................9
 4.3 Feat response for LANG command...........................11
 4.3.1 Feat examples.........................................11
 5 SECURITY CONSIDERATIONS.....................................12
 6 ACKNOWLEDGMENTS.............................................12
 7 GLOSSARY....................................................13
 8 BIBLIOGRAPHY................................................13
 9 AUTHOR'S ADDRESS............................................15
 ANNEX A - IMPLEMENTATION CONSIDERATIONS.......................16
 A.1 General Considerations...................................16
 A.2 Transition Considerations................................18
 ANNEX B - SAMPLE CODE AND EXAMPLES............................19
 B.1 Valid UTF-8 check........................................19
 B.2 Conversions..............................................20
 B.2.1 Conversion from Local Character Set to UTF-8..........20
 B.2.2 Conversion from UTF-8 to Local Character Set..........23
 B.2.3 ISO/IEC 8859-8 Example................................25
 B.2.4 Vendor Codepage Example...............................25
 B.3 Pseudo Code for Translating Servers......................26
 Full Copyright Statement......................................27
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 is 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 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], RECOMMENDs a UCS
 transformation format (UTF) UTF-8 [UTF-8], and defines a new command
 for language negotiation.
 The recommendations made in this document are consistent with the
 recommendations expressed by the IETF policy related to character
 sets and languages as defined in RFC 2277 [RFC2277].
1.1. Requirements Terminology
 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 BCP 14 [BCP14].
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 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
 2279 [RFC2279].
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:
 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 Pathnames
3.1 General compliance
 - 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 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 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.
3.2 Servers compliance
 - Servers MUST support the UTF-8 feature in response to the FEAT
 command [RFC2389]. 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, but are NOT REQUIRED. The one space indentation of the
 feature lines is mandatory [RFC2389].
 - 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 Clients compliance
 - 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.
 - 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. Language Support
 The Character Set Workshop Report [RFC2130] suggests that clients and
 servers SHOULD negotiate a language for "greetings" and "error
 messages". This specification interprets the use of the term "error
 message", by RFC 2130, to mean any explanatory text string returned
 by server-PI in response to a user-PI command.
 Implementers SHOULD note that FTP commands and numeric responses are
 protocol elements. As such, their use is not affected by any guidance
 expressed by this specification.
 Language support of greetings and command responses shall be the
 default language supported by the server or the language supported by
 the server and selected by the client.
 It may be possible to achieve language support through a virtual host
 as described in [MLST]. However, an FTP server might not support
 virtual servers, or virtual servers might be configured to support an
 environment without regard for language. To allow language
 negotiation this specification defines a new LANG command. Clients
 and servers that comply with this specification MUST support the LANG
 command.
4.1 The LANG command
 A new command "LANG" is added to the FTP command set to allow
 server-FTP process to determine in which language to present server
 greetings and the textual part of command responses. The parameter
 associated with the LANG command SHALL be one of the language tags
 defined in RFC 1766 [RFC1766]. If a LANG command without a parameter
 is issued the server's default language will be used.
 Greetings and responses issued prior to language negotiation SHALL be
 in the server's default language. Paragraph 4.5 of [RFC2277] state
 that this "default language MUST be understandable by an English-
 speaking person". This specification RECOMMENDS that the server
 default language be English encoded using ASCII. This text may be
 augmented by text from other languages. Once negotiated, server-PI
 MUST return server messages and textual part of command responses in
 the negotiated language and encoded in UTF-8. Server-PI MAY wish to
 re-send previously issued server messages in the newly negotiated
 language.
 The LANG command only affects presentation of greeting messages and
 explanatory text associated with command responses. No attempt should
 be made by the server to translate protocol elements (FTP commands
 and numeric responses) or data transmitted over the data connection.
 User-PI MAY issue the LANG command at any time during an FTP session.
 In order to gain the full benefit of this command, it SHOULD be
 presented prior to authentication. In general, it will be issued
 after the HOST command [MLST]. Note that the issuance of a HOST or
 REIN command [RFC959] will negate the affect of the LANG command.
 User-PI SHOULD be capable of supporting UTF-8 encoding for the
 language negotiated. Guidance on interpretation and rendering of
 UTF-8, defined in section 3, SHALL apply.
 Although NOT REQUIRED by this specification, a user-PI SHOULD issue a
 FEAT command [RFC2389] prior to a LANG command. This will allow the
 user-PI to determine if the server supports the LANG command and
 which language options.
 In order to aid the server in identifying whether a connection has
 been established with a client which conforms to this specification
 or an older client, user-PI MUST send a HOST [MLST] and/or LANG
 command prior to issuing any other command (other than FEAT
 [RFC2389]). If user-PI issues a HOST command, and the server's
 default language is acceptable, it need not issue a LANG command.
 However, if the implementation does not support the HOST command, a
 LANG command MUST be issued. Until server-PI is presented with either
 a HOST or LANG command it SHOULD assume that the user-PI does not
 comply with this specification.
4.2 Syntax of the LANG command
 The LANG command is defined as follows:
 lang-command = "Lang" [(SP lang-tag)] CRLF
 lang-tag = Primary-tag *( "-" Sub-tag)
 Primary-tag = 1*8ALPHA
 Sub-tag = 1*8ALPHA
 lang-response = lang-ok / error-response
 lang-ok = "200" [SP *(%x00..%xFF) ] CRLF
 error-response = command-unrecognized / bad-argument /
 not-implemented / unsupported-parameter
 command-unrecognized = "500" [SP *(%x01..%xFF) ] CRLF
 bad-argument = "501" [SP *(%x01..%xFF) ] CRLF
 not-implemented = "502" [SP *(%x01..%xFF) ] CRLF
 unsupported-parameter = "504" [SP *(%x01..%xFF) ] CRLF
 The "lang" command word is case independent and may be specified in
 any character case desired. Therefore "LANG", "lang", "Lang", and
 "lAnG" are equivalent commands.
 The OPTIONAL "Lang-tag" given as a parameter specifies the primary
 language tags and zero or more sub-tags as defined in [RFC1766]. As
 described in [RFC1766] language tags are treated as case insensitive.
 If omitted server-PI MUST use the server's default language.
 Server-FTP responds to the "Lang" command with either "lang-ok" or
 "error-response". "lang-ok" MUST be sent if Server-FTP supports the
 "Lang" command and can support some form of the "lang-tag". Support
 SHOULD be as follows:
 - If server-FTP receives "Lang" with no parameters it SHOULD return
 messages and command responses in the server default language.
 - If server-FTP receives "Lang" with only a primary tag argument
 (e.g. en, fr, de, ja, zh, etc.), which it can support, it SHOULD
 return messages and command responses in the language associated
 with that primary tag. It is possible that server-FTP will only
 support the primary tag when combined with a sub-tag (e.g. en-US,
 en-UK, etc.). In such cases, server-FTP MAY determine the
 appropriate variant to use during the session. How server-FTP makes
 that determination is outside the scope of this specification. If
 server-FTP cannot determine if a sub-tag variant is appropriate it
 SHOULD return an "unsupported-parameter" (504) response.
 - If server-FTP receives "Lang" with a primary tag and sub-tag(s)
 argument, which is implemented, it SHOULD return messages and
 command responses in support of the language argument. It is
 possible that server-FTP can support the primary tag of the "Lang"
 argument but not the sub-tag(s). In such cases server-FTP MAY
 return messages and command responses in the most appropriate
 variant of the primary tag that has been implemented. How server-
 FTP makes that determination is outside the scope of this
 specification. If server-FTP cannot determine if a sub-tag variant
 is appropriate it SHOULD return an "unsupported-parameter" (504)
 response.
 For example if client-FTP sends a "LANG en-AU" command and server-FTP
 has implemented language tags en-US and en-UK it may decide that the
 most appropriate language tag is en-UK and return "200 en-AU not
 supported. Language set to en-UK". The numeric response is a protocol
 element and can not be changed. The associated string is for
 illustrative purposes only.
 Clients and servers that conform to this specification MUST support
 the LANG command. Clients SHOULD, however, anticipate receiving a 500
 or 502 command response, in cases where older or non-compliant
 servers do not recognize or have not implemented the "Lang". A 501
 response SHOULD be sent if the argument to the "Lang" command is not
 syntactically correct. A 504 response SHOULD be sent if the "Lang"
 argument, while syntactically correct, is not implemented. As noted
 above, an argument may be considered a lexicon match even though it
 is not an exact syntax match.
4.3 Feat response for LANG command
 A server-FTP process that supports the LANG command, and language
 support for messages and command responses, MUST include in the
 response to the FEAT command [RFC2389], a feature line indicating
 that the LANG command is supported and a fact list of the supported
 language tags. A response to a FEAT command SHALL be in the following
 format:
 Lang-feat = SP "LANG" SP lang-fact CRLF
 lang-fact = lang-tag ["*"] *(";" lang-tag ["*"])
 lang-tag = Primary-tag *( "-" Sub-tag)
 Primary-tag= 1*8ALPHA
 Sub-tag = 1*8ALPHA
 The lang-feat response contains the string "LANG" followed by a
 language fact. This string is not case sensitive, but SHOULD be
 transmitted in upper case, as recommended in [RFC2389]. The initial
 space shown in the Lang-feat response is REQUIRED by the FEAT
 command. It MUST be a single space character. More or less space
 characters are not permitted. The lang-fact SHALL include the lang-
 tags which server-FTP can support. At least one lang-tag MUST be
 included with the FEAT response. The lang-tag SHALL be in the form
 described earlier in this document. The OPTIONAL asterisk, when
 present, SHALL indicate the current lang-tag being used by server-FTP
 for messages and responses.
4.3.1 Feat examples
 C> feat
 S> 211- <any descriptive text>
 S> ...
 S> LANG EN*
 S> ...
 S> 211 end
 In this example server-FTP can only support English, which is the
 current language (as shown by the asterisk) being used by the server
 for messages and command responses.
 C> feat
 S> 211- <any descriptive text>
 S> ...
 S> LANG EN*;FR
 S> ...
 S> 211 end
 C> LANG fr
 S> 200 Le response sera changez au francais
 C> feat
 S> 211- <quelconque descriptif texte>
 S> ...
 S> LANG EN;FR*
 S> ...
 S> 211 end
 In this example server-FTP supports both English and French as shown
 by the initial response to the FEAT command. The asterisk indicates
 that English is the current language in use by server-FTP. After a
 LANG command is issued to change the language to French, the FEAT
 response shows French as the current language in use.
 In the above examples ellipses indicate placeholders where other
 features may be included, but are NOT REQUIRED.
5 Security Considerations
 This document addresses the support of character sets beyond 1 byte
 and a new language negotiation command. Conformance to this document
 should not induce a security risk.
6 Acknowledgments
 The following people have contributed to this document:
 D. J. Bernstein
 Martin J. Duerst
 Mark Harris
 Paul Hethmon
 Alun Jones
 Gregory Lundberg
 James Matthews
 Keith Moore
 Sandra O'Donnell
 Benjamin Riefenstahl
 Stephen Tihor
 (and others from the FTPEXT working group)
7 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.
 TF-16 - A 16-bit format including the BMP (directly encoded) and
 surrogate pairs to represent characters in planes 01-16; equivalent
 to Unicode.
8 Bibliography
 [ABNF] Crocker, D. and 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)
 [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)
 [BCP14] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119, March 1997.
 [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.
 [MLST] Elz, R. and P. Hethmon, "Extensions to FTP", Work in
 Progress.
 [RFC854] Postel, J. and J. Reynolds, "Telnet Protocol
 Specification", STD 8, RFC 854, May 1983.
 [RFC959] Postel, J. and J. Reynolds, "File Transfer Protocol
 (FTP)", STD 9, RFC 959, October 1985.
 [RFC1123] Braden, R., "Requirements for Internet Hosts --
 Application and Support", STD 3, RFC 1123, October 1989.
 [RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
 Resource Locators (URL)", RFC 1738, December 1994.
 [RFC1766] Alvestrand, H., "Tags for the Identification of
 Languages", RFC 1766, March 1995.
 [RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H.,
 Atkinson, R., Crispin, M. and P. Svanberg, "Character
 Set Workshop Report", RFC 2130, April 1997.
 [RFC2277] Alvestrand, H., " IETF Policy on Character Sets and
 Languages", RFC 2277, January 1998.
 [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
 10646", RFC 2279, January 1998.
 [RFC2389] Elz, R. and P. Hethmon, "Feature Negotiation Mechanism
 for the File Transfer Protocol", RFC 2389, August 1998.
 [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).
9 Author's Address
 Bill Curtin
 JIEO
 Attn: JEBBD
 Ft. Monmouth, N.J. 07703-5613
 EMail: curtinw@ftm.disa.mil
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.
 - 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
 - Servers which support this specification, when presented a pathname
 from an old client (one which does not support this specification),
 can nearly always tell whether the pathname is in UTF-8 (see B.1)
 or in some other code set. In order to support these older clients,
 servers may wish to default to a non UTF-8 code set. However, how a
 server supports non UTF-8 is outside the scope of this
 specification.
 - Clients which support this specification will be able to determine
 if the server can support UTF-8 (i.e. supports this specification)
 by the ability of the server to support the FEAT command and the
 UTF8 feature (defined in 3.2). If the newer clients determine that
 the server does not support UTF-8 it may wish to default to a
 different code set. Client developers should take into
 consideration that pathnames, associated with older servers, might
 be stored in UTF-8. However, how a client supports non UTF-8 is
 outside the scope of this specification.
 - 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.
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?
 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.
 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 +
 ((*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. 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 */
 {
 *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:
 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.
 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
Full Copyright Statement
 Copyright (C) The Internet Society (1999). All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works. However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.