Using Unicode with MIME
RFC 1641
Document | Type |
RFC - Experimental
(July 1994; No errata)
Was draft-goldsmith-mime-unicode (individual)
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Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text pdf ps htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 1641 (Experimental) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group D. Goldsmith Request for Comments: 1641 M. Davis Category: Experimental Taligent, Inc. July 1994 Using Unicode with MIME Status of this Memo This memo defines an Experimental Protocol for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Abstract The Unicode Standard, version 1.1, and ISO/IEC 10646-1:1993(E) jointly define a 16 bit character set (hereafter referred to as Unicode) which encompasses most of the world's writing systems. However, Internet mail (STD 11, RFC 822) currently supports only 7- bit US ASCII as a character set. MIME (RFC 1521 and RFC 1522) extends Internet mail to support different media types and character sets, and thus could support Unicode in mail messages. MIME neither defines Unicode as a permitted character set nor specifies how it would be encoded, although it does provide for the registration of additional character sets over time. This document specifies the usage of Unicode within MIME. Motivation Since Unicode is starting to see widespread commercial adoption, users will want a way to transmit information in this character set in mail messages and other Internet media. Since MIME was expressly designed to allow such extensions and is on the standards track for the Internet, it is the most appropriate means for encoding Unicode. RFC 1521 and RFC 1522 do not define Unicode as an allowed character set, but allow registration of additional character sets. In addition to allowing use of Unicode within MIME bodies, another goal is to specify a way of using Unicode that allows text which consists largely, but not entirely, of US-ASCII characters to be represented in a way that can be read by mail clients who do not understand Unicode. This is in keeping with the philosophy of MIME. Such an encoding is described in another document, "UTF-7: A Mail Safe Transformation Format of Unicode" [UTF-7]. Goldsmith & Davis [Page 1] RFC 1641 Using Unicode with MIME July 1994 Overview Several ways of using Unicode are possible. This document specifies both guidelines for use of Unicode within MIME, and a specific usage. The usage specified in this document is a straightforward use of Unicode as specified in "The Unicode Standard, Version 1.1". This encoding is intended for situations where sender and recipient do not want to do a lot of processing, when the text does not consist primarily of characters from the US-ASCII character set, or when sender and receiver are known in advance to support Unicode. Another encoding is intended for situations where the text consists primarily of US-ASCII, with occasional characters from other parts of Unicode. This encoding allows the US-ASCII portion to be read by all recipients without having to support Unicode. This encoding is specified in another document, "UTF-7: A Mail Safe Transformation Format of Unicode" [UTF-7]. Finally, in keeping with the principles set forth in RFC 1521, text which can be represented using the US-ASCII or ISO-8859-x character sets should be so represented where possible, for maximum interoperability. Definitions The definition of character set Unicode: The 16 bit character set Unicode is defined by "The Unicode Standard, Version 1.1". This character set is identical with the character repertoire and coding of the international standard ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2; Subset=300; Implementation Level=3. Note. Unicode 1.1 further specifies the use and interaction of these character codes beyond the ISO standard. However, any valid 10646 BMP (Basic Multilingual Plane) sequence is a valid Unicode sequence, and vice versa; Unicode supplies interpretations of sequences on which the ISO standard is silent as to interpretation. This character set is encoded as sequences of octets, two per 16- bit character, with the most significant octet first. Text with an odd number of octets is ill-formed. Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters in the UCS-2 form are serialized as octets, that the most significant octet appear first. This is also in keeping with Goldsmith & Davis [Page 2] RFC 1641 Using Unicode with MIME July 1994 common network practice of choosing a canonical format for transmission. General Specification of Unicode Character Sets Within MIME The Unicode Standard is currently at version 1.1. Although newShow full document text