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RFC 2781 UTF-16, an encoding of ISO 10646 February 2000 Big-endian text labelled with UTF-16, with a BOM: FE FF D8 08 DF 45 00 3D 00 52 00 61 Little-endian text labelled with UTF-16, with a BOM: FF FE 08 D8 45 DF 3D 00 52 00 61 006. Versions of the standards ISO/IEC 10646 is updated from time to time by published amendments; similarly, different versions of the Unicode standard exist: 1.0, 1.1, 2.0, 2.1, and 3.0 as of this writing. Each new version replaces the previous one, but implementations, and more significantly data, are not updated instantly. In general, the changes amount to adding new characters, which does not pose particular problems with old data. Amendment 5 to ISO/IEC 10646, however, has moved and expanded the Korean Hangul block, thereby making any previous data containing Hangul characters invalid under the new version. Unicode 2.0 has the same difference from Unicode 1.1. The official justification for allowing such an incompatible change was that no significant implementations and data containing Hangul existed, a statement that is likely to be true but remains unprovable. The incident has been dubbed the "Korean mess", and the relevant committees have pledged to never, ever again make such an incompatible change. New versions, and in particular any incompatible changes, have consequences regarding MIME character encoding labels, to be discussed in Appendix A.7. IANA Considerations IANA is to register the character sets found in Appendixes A.1, A.2, and A.3 according to RFC 2278, using registration templates found in those appendixes.8. Security Considerations UTF-16 is based on the ISO 10646 character set, which is frequently being added to, as described in Section 6 and Appendix A of this document. Processors must be able to handle characters that are not defined at the time that the processor was created in such a way as to not allow an attacker to harm a recipient by including unknown characters. Processors that handle any type of text, including text encoded as UTF-16, must be vigilant in checking for control characters that might reprogram a display terminal or keyboard. Similarly, processorsHoffman & Yergeau Informational [Page 8] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000 that interpret text entities (such as looking for embedded programming code), must be careful not to execute the code without first alerting the recipient. Text in UTF-16 may contain special characters, such as the OBJECT REPLACEMENT CHARACTER (0xFFFC), that might cause external processing, depending on the interpretation of the processing program and the availability of an external data stream that would be executed. This external processing may have side-effects that allow the sender of a message to attack the receiving system. Implementors of UTF-16 need to consider the security aspects of how they handle illegal UTF-16 sequences (that is, sequences involving surrogate pairs that have illegal values or unpaired surrogates). It is conceivable that in some circumstances an attacker would be able to exploit an incautious UTF-16 parser by sending it an octet sequence that is not permitted by the UTF-16 syntax, causing it to behave in some anomalous fashion.9. References [CHARPOLICY] Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998. [CHARSET-REG] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2278, January 1998. [HTTP-1.1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [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. 22 amendments and two technical corrigenda have been published up to now. UTF-16 is described in Annex Q, published as Amendment 1. Many other amendments are currently at various stages of standardization. A second edition is in preparation, probably to be published in 2000; in this new edition, UTF-16 will probably be described in Annex C. [MUSTSHOULD] 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 3.0", ISBN 0-201-61633-5. Described atHoffman & Yergeau Informational [Page 9] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000 <http://www.unicode.org/unicode/standard/versions/Unicode3.0.html>. [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [WORKSHOP] Weider, C., Preston, C., Simonsen, K., Alvestrand, H., Atkinson, R., Crispin., M. and P. Svanberg, "Report of the IAB Character Set Workshop", RFC 2130, April 1997.10. Acknowledgments Deborah Goldsmith wrote a great deal of the initial wording for this specification. Martin Duerst proposed numerous significant changes. Other significant contributors include: Mati Allouche Walt Daniels Mark Davis Ned Freed Asmus Freytag Lloyd Honomichl Dan Kegel Murata Makoto Larry Masinter Markus Scherer Keld Simonsen Ken Whistler Some of the text in this specification was copied from [UTF-8], and that document was worked on by many people. Please see the acknowledgments section in that document for more people who may have contributed indirectly to this document.Hoffman & Yergeau Informational [Page 10] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000A. Charset registrations This memo is meant to serve as the basis for registration of three MIME charsets [CHARSET-REG]. The proposed charsets are "UTF-16BE", "UTF-16LE", and "UTF-16". These strings label objects containing text consisting of characters from the repertoire of ISO/IEC 10646 including all amendments at least up to amendment 5 (Korean block), encoded to a sequence of octets using the encoding and serialization schemes outlined above. Note that "UTF-16BE", "UTF-16LE", and "UTF-16" are NOT suitable for use in media types under the "text" top-level type, because they do not encode line endings in the way required for MIME "text" media types. An exception to this is HTTP, which uses a MIME-like mechanism, but is exempt from the restrictions on the text top-level type (see section 19.4.2 of HTTP 1.1 [HTTP-1.1]). It is noteworthy that the labels described here do not contain a version identification, referring generically to ISO/IEC 10646. This is intentional, the rationale being as follows: A MIME charset is designed to give just the information needed to interpret a sequence of bytes received on the wire into a sequence of characters, nothing more (see RFC 2045, section 2.2, in [MIME]). As long as a character set standard does not change incompatibly, version numbers serve no purpose, because one gains nothing by learning from the tag that newly assigned characters may be received that one doesn't know about. The tag itself doesn't teach anything about the new characters, which are going to be received anyway. Hence, as long as the standards evolve compatibly, the apparent advantage of having labels that identify the versions is only that, apparent. But there is a disadvantage to such version-dependent labels: when an older application receives data accompanied by a newer, unknown label, it may fail to recognize the label and be completely unable to deal with the data, whereas a generic, known label would have triggered mostly correct processing of the data, which may well not contain any new characters. The "Korean mess" (ISO/IEC 10646 amendment 5) is an incompatible change, in principle contradicting the appropriateness of a version independent MIME charset as described above. But the compatibility problem can only appear with data containing Korean Hangul characters encoded according to Unicode 1.1 (or equivalently ISO/IEC 10646 before amendment 5), and there is arguably no such data to worry about, this being the very reason the incompatible change was deemed acceptable.Hoffman & Yergeau Informational [Page 11] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000 In practice, then, a version-independent label is warranted, provided the label is understood to refer to all versions after Amendment 5, and provided no incompatible change actually occurs. Should incompatible changes occur in a later version of ISO/IEC 10646, the MIME charsets defined here will stay aligned with the previous version until and unless the IETF specifically decides otherwise.A.1 Registration for UTF-16BE To: ietf-charsets@iana.org Subject: Registration of new charset Charset name(s): UTF-16BE Published specification(s): This specification Suitable for use in MIME content types under the "text" top-level type: No Person & email address to contact for further information: Paul Hoffman <phoffman@imc.org> Francois Yergeau <fyergeau@alis.com>A.2 Registration for UTF-16LE To: ietf-charsets@iana.org Subject: Registration of new charset Charset name(s): UTF-16LE Published specification(s): This specification Suitable for use in MIME content types under the "text" top-level type: No Person & email address to contact for further information: Paul Hoffman <phoffman@imc.org> Francois Yergeau <fyergeau@alis.com>A.3 Registration for UTF-16 To: ietf-charsets@iana.org Subject: Registration of new charset Charset name(s): UTF-16 Published specification(s): This specificationHoffman & Yergeau Informational [Page 12] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000 Suitable for use in MIME content types under the "text" top-level type: No Person & email address to contact for further information: Paul Hoffman <phoffman@imc.org> Francois Yergeau <fyergeau@alis.com>Authors' Addresses Paul Hoffman Internet Mail Consortium 127 Segre Place Santa Cruz, CA 95060 USA EMail: phoffman@imc.org Francois Yergeau Alis Technologies 100, boul. Alexis-Nihon, Suite 600 Montreal QC H4M 2P2 Canada EMail: fyergeau@alis.comHoffman & Yergeau Informational [Page 13] RFC 2781 UTF-16, an encoding of ISO 10646 February 2000Full Copyright Statement Copyright (C) The Internet Society (2000). 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.Hoffman & Yergeau Informational [Page 14] ⌨️ 快捷键说明
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