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Network Working Group                                         M. CrispinRequest for Comments: 4042                             Panda ProgrammingCategory: Informational                                     1 April 2005UTF-9 and UTF-18Efficient Transformation Formats of UnicodeStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   ISO-10646 defines a large character set called the Universal   Character Set (UCS), which encompasses most of the world's writing   systems.  The same set of codepoints is defined by Unicode, which   further defines additional character properties and other   implementation details.  By policy of the relevant standardization   committees, changes to Unicode and amendments and additions to   ISO/IEC 646 track each other, so that the character repertoires and   code point assignments remain in synchronization.   The current representation formats for Unicode (UTF-7, UTF-8, UTF-16)   are not storage and computation efficient on platforms that utilize   the 9 bit nonet as a natural storage unit instead of the 8 bit octet.   This document describes a transformation format of Unicode that takes   advantage of the nonet so that the format will be storage and   computation efficient.1.  Introduction   A number of Internet sites utilize platforms that are not based upon   the traditional 8-bit byte or octet.  One such platform is the PDP-   10, which is based upon a 36-bit word.  On these platforms, it is   wasteful to represent data in octets, since 4 bits are left unused in   each word.  The 9-bit nonet is a much more sensible representation.   Although these platforms support IETF standards, many of these   platforms still utilize a text representation based upon the septet,Crispin                      Informational                      [Page 1]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   which is only suitable for [US-ASCII] (although it has been used for   various ISO 10646 national variants).   To maximize international and multi-lingual interoperability, the IAB   has recommended ([IAB-CHARACTER]) that [ISO-10646] be the default   coded character set.   Although other transformation formats of [UNICODE] exist, and   conceivably can be used on nonet-oriented machines (most notably   [UTF-8]), they suffer significant disadvantages:      [UTF-8]         requires one to three octets to represent codepoints in the         Basic Multilingual Plane (BMP), four octets to represent         [UNICODE] codepoints outside the BMP, and six octets to         represent non-[UNICODE] codepoints.  When stored in nonets,         this results in as many as four wasted bits per [UNICODE]         character.      [UTF-16]         requires a hexadecet to represent codepoints in the BMP, and         two hexadecets to represent [UNICODE] codepoints outside the         BMP.  When stored in nonet pairs, this results in as many as         four wasted bits per [UNICODE] character.  This transformation         format requires complex surrogates to represent codepoints         outside the BMP, and can not represent non-[UNICODE] codepoints         at all.      [UTF-7]         requires one to five septets to represent codepoints in the         BMP, and as many as eight septets to represent codepoints         outside the BMP.  When stored in nonets, this results in as         many as sixteen wasted bits per character.  This transformation         format requires very complex and computationally expensive         shifting and "modified BASE64" processing, and can not         represent non-[UNICODE] codepoints at all.   By comparison, UTF-9 uses one to two nonets to represent codepoints   in the BMP, three nonets to represent [UNICODE] codepoints outside   the BMP, and three or four nonets to represent non-[UNICODE]   codepoints.  There are no wasted bits, and as the examples in this   document demonstrate, the computational processing is minimal.   Transformation between [UTF-8] and UTF-9 is straightforward, with   most of the complexity in the handling of [UTF-8].  It is hoped that   future extensions to protocols such as SMTP will permit the use of   UTF-9 in these protocols between nonet platforms without the use of   [UTF-8] as an "on the wire" format.Crispin                      Informational                      [Page 2]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   Similarly, transformation between [UNICODE] codepoints and UTF-18 is   also quite simple.  Although (like UCS-2) UTF-18 only represents a   subset of the available [UNICODE] codepoints, it encompasses the   non-private codepoints that are currently assigned in [UNICODE].1.1.  Conventions Used in This Document   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 inBCP 14,RFC 2119   [KEYWORDS].2.  Overview   UTF-9 encodes [UNICODE] codepoints in the low order 8 bits of a   nonet, using the high order bit to indicate continuation.  Surrogates   are not used.   [UNICODE] codepoints in the range U+0000 - U+00FF ([US-ASCII] and   Latin 1) are represented by a single nonet; codepoints in the range   U+0100 - U+FFFF (the remainder of the BMP) are represented by two   nonets; and codepoints in the range U+1000 - U+10FFFF (remainder of   [UNICODE]) are represented by three nonets.   Non-[UNICODE] codepoints in [ISO-10646] (that is, codepoints in the   range 0x110000 - 0x7fffffff) can also be represented in UTF-9 by   obvious extension, but this is not discussed further as these   codepoints have been removed from [ISO-10646] by ISO.   UTF-18 encodes [UNICODE] codepoints in the Basic Multilingual Plane   (BMP, plane 0), Supplementary Multilingual Plane (SMP, plane 1),   Supplementary Ideographic Plane (SIP, plane 2), and Supplementary   Special-purpose Plane (SSP, plane 14) in a single 18-bit value.  It   does not encode planes 3 though 13, which are currently unused; nor   planes 15 or 16, which are private spaces.   Normally, UTF-9 and UTF-18 should only be used in the context of 9   bit storage and transport.  Although some protocols, e.g., [FTP],   support transport of nonets, the current IETF protocol suite is quite   deficient in this area.  The IETF is urged to take action to improve   IETF protocol support for nonets.3.  UTF-9 Definition   A UTF-9 stream represents [ISO-10646] codepoints using 9 bit nonets.   The low order 8-bits of a nonet is an octet, and the high order bit   indicates continuation.Crispin                      Informational                      [Page 3]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   UTF-9 does not use surrogates; consequently a UTF-16 value must be   transformed into the UCS-4 equivalent, and U+D800 - U+DBFF are never   transmitted in UTF-9.   Octets of the [UNICODE] codepoint value are then copied into   successive UTF-9 nonets, starting with the most-significant non-zero   octet.  All but the least significant octet have the continuation bit   set in the associated nonet.   Examples:   Character  Name                                UTF-9 (in octal)   ---------  ----                                ----------------    U+0041    LATIN CAPITAL LETTER A              101    U+00C0    LATIN CAPITAL LETTER A WITH GRAVE   300    U+0391    GREEK CAPITAL LETTER ALPHA          403 221    U+611B    <CJK ideograph meaning "love">      541 33    U+10330   GOTHIC LETTER AHSA                  401 403 60    U+E0041   TAG LATIN CAPITAL LETTER A          416 400 101    U+10FFFD  <Plane 16 Private Use, Last>        420 777 375   0x345ecf1b (UCS-4 value not in [UNICODE])      464 536 717 334.  UTF-18 Definition   A UTF-18 stream represents [ISO-10646] codepoints using a pair of 9   bit nonets to form an 18-bit value.   UTF-18 does not use surrogates; consequently a UTF-16 value must be   transformed into the UCS-4 equivalent, and U+D800 - U+DBFF are never   transmitted in UTF-18.   [UNICODE] codepoint values in the range U+0000 - U+2FFFF are copied   as the same value into a UTF-18 value.  [UNICODE] codepoint values in   the range U+E0000 - U+EFFFF are copied as values 0x30000 - 0x3ffff;   that is, these values are shifted by 0x70000.  Other codepoint values   can not be represented in UTF-18.   Examples:   Character  Name                                UTF-18 (in octal)   ---------  ----                                ----------------    U+0041    LATIN CAPITAL LETTER A              000101    U+00C0    LATIN CAPITAL LETTER A WITH GRAVE   000300    U+0391    GREEK CAPITAL LETTER ALPHA          001621    U+611B    <CJK ideograph meaning "love">      060433    U+10330   GOTHIC LETTER AHSA                  201460    U+E0041   TAG LATIN CAPITAL LETTER A          600101Crispin                      Informational                      [Page 4]

RFC 4042                    UTF-9 and UTF-18                1 April 20055.  Sample Routines5.1.  [UNICODE] Codepoint to UTF-9 Conversion   The following routines demonstrate conversion from UCS-4 to UTF-9.   For simplicity, these routines do not do any validity checking.   Routines used in applications SHOULD reject invalid UTF-9 sequences;   that is, the first nonet with a value of 400 octal (0x100), or   sequences that result in an overflow (exceeding 0x10ffff for   [UNICODE]), or codepoints used for UTF-16 surrogates.   ; Return UCS-4 value from UTF-9 string (PDP-10 assembly version)   ; Accepts: P1/ 9-bit byte pointer to UTF-9 string   ; Returns +1: Always, T1/ UCS-4 value, P1/ updated byte pointer   ; Clobbers T2   UT92U4: TDZA T1,T1              ; start with zero   U92U41:  XOR T1,T2              ; insert octet into UCS-4 value           LSH T1,^D8              ; shift UCS-4 value           ILDB T2,P1              ; get next nonet           TRZE T2,400             ; extract octet, any continuation?            JRST U92U41            ; yes, continue           XOR T1,T2               ; insert final octet           POPJ P,   /* Return UCS-4 value from UTF-9 string (C version)    * Accepts: pointer to pointer to UTF-9 string    * Returns: UCS-4 character, nonet pointer updated    */   UINT31 UTF9_to_UCS4 (UINT9 **utf9PP)   {     UINT9 nonet;     UINT31 ucs4;     for (ucs4 = (nonet = *(*utf9PP)++) & 0xff;          nonet & 0x100;          ucs4 |= (nonet = *(*utf9PP)++) & 0xff)       ucs4 <<= 8;     return ucs4;   }5.2.  UTF-9 to UCS-4 Conversion   The following routines demonstrate conversion from UTF-9 to UCS-4.   For simplicity, these routines do not do any validity checking.   Routines used in applications SHOULD reject invalid UCS-4 codepoints;   that is, codepoints used for UTF-16 surrogates or codepoints with   values exceeding 0x10ffff for [UNICODE].Crispin                      Informational                      [Page 5]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   ; Write UCS-4 character to UTF-9 string (PDP-10 assembly version)   ; Accepts: P1/ 9-bit byte pointer to UTF-9 string   ;          T1/ UCS-4 character to write   ; Returns +1: Always, P1/ updated byte pointer   ; Clobbers T1, T2; (T1, T2) must be an accumulator pair   U42UT9: SETO T2,            ; we'll need some of these 1-bits later           ASHC T1,-^D8        ; low octet becomes nonet with high 0-bit   U32U91: JUMPE T1,U42U9X     ; done if no more octets           LSHC T1,-^D8        ; shift next octet into T2           ROT T2,-1           ; turn it into nonet with high 1 bit           PUSHJ P,U42U91      ; recurse for remainder   U42U9X: LSHC T1,^D9         ; get next nonet back from T2           IDPB T1,P1          ; write nonet           POPJ P,   /* Write UCS-4 character to UTF-9 string (C version)    * Accepts: pointer to nonet string    *          UCS-4 character to write    * Returns: updated pointer    */   UINT9 *UCS4_to_UTF9 (UINT9 *utf9P,UINT31 ucs4)   {     if (ucs4 > 0x100) {       if (ucs4 > 0x10000) {         if (ucs4 > 0x1000000)           *utf9P++ = 0x100 | ((ucs4 >> 24) & 0xff);         *utf9P++ = 0x100 | ((ucs4 >> 16) & 0xff);       }       *utf9P++ = 0x100 | ((ucs4 >> 8) & 0xff);     }     *utf9P++ = ucs4 & 0xff;     return utf9P;   }6.  Implementation Experience   As the sample routines demonstrate, it is quite simple to implement   UTF-9 and UTF-18 on a nonet-based architecture.  More sophisticated   routines can be found inftp://panda.com/tops-20/utools.mac.txt or   from lingling.panda.com via the file <UTF9>UTOOLS.MAC via ANONYMOUS   [FTP].Crispin                      Informational                      [Page 6]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   We are now in the process of implementing support for nonet-based   text files and automated transformation between septet, octet, and   nonet textual data.7.  References7.1.  Normative References   [FTP]           Postel, J. and J. Reynolds, "File Transfer Protocol",                   STD 9,RFC 959, October 1985.   [IAB-CHARACTER] Weider, C., Preston, C., Simonsen, K., Alvestrand,                   H., Atkinson, R., Crispin, M., and P. Svanberg, "The                   Report of the IAB Character Set Workshop held 29                   February - 1 March, 1996",RFC 2130, April 1997.   [ISO-10646]     International Organization for Standardization,                   "Information Technology - Universal Multiple-octet                   coded Character Set (UCS)", ISO/IEC Standard 10646,                   comprised of ISO/IEC 10646-1:2000, "Information                   technology - Universal Multiple-Octet Coded Character                   Set (UCS) - Part 1: Architecture and Basic                   Multilingual Plane", ISO/IEC 10646-2:2001,                   "Information technology - Universal Multiple-Octet                   Coded Character Set (UCS) - Part 2:  Supplementary                   Planes" and ISO/IEC 10646-1:2000/Amd 1:2002,                   "Mathematical symbols and other characters".   [KEYWORDS]      Bradner, S., "Key words for use in RFCs to Indicate                   Requirement Levels",BCP 14,RFC 2119, March 1997.   [UNICODE]       The Unicode Consortium, "The Unicode Standard -                   Version 3.2", defined by The Unicode Standard,                   Version 3.0 (Reading, MA, Addison-Wesley, 2000.  ISBN                   0-201-61633-5), as amended by the Unicode Standard                   Annex #27: Unicode 3.1 and by the Unicode Standard                   Annex #28: Unicode 3.2, March 2002.7.2.  Informative References   [US-ASCII]      American National Standards Institute, "Coded                   Character Set - 7-bit American Standard Code for                   Information Interchange", ANSI X3.4, 1986.   [UTF-16]        Hoffman, P. and F. Yergeau, "UTF-16, an encoding of                   ISO 10646",RFC 2781, February 2000.Crispin                      Informational                      [Page 7]

RFC 4042                    UTF-9 and UTF-18                1 April 2005   [UTF-7]         Goldsmith, D. and M. Davis, "UTF-7 A Mail-Safe                   Transformation Format of Unicode",RFC 2152, May                   1997.   [UTF-8]         Sollins, K., "Architectural Principles of Uniform                   Resource Name Resolution",RFC 2276, January 1998.8.  Security Considerations   As with UTF-8, UTF-9 can represent codepoints that are not in   [UNICODE].  Applications should validate UTF-9 strings to ensure that   all codepoints do not exceed the [UNICODE] maximum of U+10FFFF.   The sample routines in this document are for example purposes, and   make no attempt to validate their arguments, e.g., test for overflow   ([UNICODE] values great than 0x10ffff) or codepoints used for   surrogates.  Besides resulting in invalid data, this can also create   covert channels.9.  IANA Considerations   The IANA shall reserve the charset names "UTF-9" and "UTF-18" for   future assignment.Author's Address   Mark R. Crispin   Panda Programming   6158 NE Lariat Loop   Bainbridge Island, WA 98110-2098   Phone: (206) 842-2385   EMail: UTF9@Lingling.Panda.COMCrispin                      Informational                      [Page 8]

RFC 4042                    UTF-9 and UTF-18                1 April 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78 and at www.rfc-editor.org/copyright.html, and   except as set forth therein, the authors retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Crispin                      Informational                      [Page 9]