7.7.   The Video Content-Type............................. 54
   7.8.   Experimental Content-Type Values................... 54
   8.     Summary............................................ 56
   9.     Security Considerations............................ 56
   10.    Authors' Addresses................................. 57
   11.    Acknowledgements................................... 58
   Appendix A -- Minimal MIME-Conformance.................... 60
   Appendix B -- General Guidelines For Sending Email Data... 63
   Appendix C -- A Complex Multipart Example................. 66
   Appendix D -- Collected Grammar........................... 68




 
RFC 1521                          MIME                    September 1993


   Appendix E -- IANA Registration Procedures................ 72
   E.1  Registration of New Content-type/subtype Values...... 72
   E.2  Registration of New Access-type Values
        for Message/external-body............................ 73
   Appendix F -- Summary of the Seven Content-types.......... 74
   Appendix G -- Canonical Encoding Model.................... 76
   Appendix H -- Changes from RFC 1341....................... 78
   References................................................ 80

1.    Introduction

   Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined
   the standard format of textual mail messages on the Internet.  Its
   success has been such that the RFC 822 format has been adopted,
   wholly or partially, well beyond the confines of the Internet and the
   Internet SMTP transport defined by STD 10, RFC 821 [RFC-821].  As the
   format has seen wider use, a number of limitations have proven
   increasingly restrictive for the user community.

   RFC 822 was intended to specify a format for text messages.  As such,
   non-text messages, such as multimedia messages that might include
   audio or images, are simply not mentioned.  Even in the case of text,
   however, RFC 822 is inadequate for the needs of mail users whose
   languages require the use of character sets richer than US ASCII
   [US-ASCII]. Since RFC 822 does not specify mechanisms for mail
   containing audio, video, Asian language text, or even text in most
   European languages, additional specifications are needed.

   One of the notable limitations of RFC 821/822 based mail systems is
   the fact that they limit the contents of electronic mail messages to
   relatively short lines of seven-bit ASCII.  This forces users to
   convert any non-textual data that they may wish to send into seven-
   bit bytes representable as printable ASCII characters before invoking
   a local mail UA (User Agent, a program with which human users send
   and receive mail). Examples of such encodings currently used in the
   Internet include pure hexadecimal, uuencode, the 3-in-4 base 64
   scheme specified in RFC 1421, the Andrew Toolkit Representation
   [ATK], and many others.

   The limitations of RFC 822 mail become even more apparent as gateways
   are designed to allow for the exchange of mail messages between RFC
   822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
   inclusion of non-textual body parts within electronic mail messages.
   The current standards for the mapping of X.400 messages to RFC 822
   messages specify either that X.400 non-textual body parts must be
   converted to (not encoded in) an ASCII format, or that they must be
   discarded, notifying the RFC 822 user that discarding has occurred.
   This is clearly undesirable, as information that a user may wish to




 
RFC 1521                          MIME                    September 1993


   receive is lost.  Even though a user's UA may not have the capability
   of dealing with the non-textual body part, the user might have some
   mechanism external to the UA that can extract useful information from
   the body part.  Moreover, it does not allow for the fact that the
   message may eventually be gatewayed back into an X.400 message
   handling system (i.e., the X.400 message is "tunneled" through
   Internet mail), where the non-textual information would definitely
   become useful again.

   This document describes several mechanisms that combine to solve most
   of these problems without introducing any serious incompatibilities
   with the existing world of RFC 822 mail.  In particular, it
   describes:

   1. A MIME-Version header field, which uses a version number to
       declare a message to be conformant with this specification and
       allows mail processing agents to distinguish between such
       messages and those generated by older or non-conformant software,
       which is presumed to lack such a field.

   2. A Content-Type header field, generalized from RFC 1049 [RFC-1049],
       which can be used to specify the type and subtype of data in the
       body of a message and to fully specify the native representation
       (encoding) of such data.

       2.a. A "text" Content-Type value, which can be used to represent