Network Working Group M. Crispin
Request for Comments: 4042 Panda Programming
Category: Informational 1 April 2005
UTF-9 and UTF-18
Efficient Transformation Formats of Unicode
Status 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 in BCP 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.
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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 33
4. 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 600101
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5. Sample Routines
5.1. [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].
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; 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 in ftp://panda.com/tops-20/utools.mac.txt or
from lingling.panda.com via the file <UTF9>UTOOLS.MAC via ANONYMOUS
[FTP].
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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. References
7.1. 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.
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[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.COM
Crispin Informational [Page 8]RFC 4042 UTF-9 and UTF-18 1 April 2005
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Crispin Informational [Page 9]
