MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies
RFC 1521

Document Type RFC - Draft Standard (September 1993; No errata)
Updated by RFC 1590
Obsoletes RFC 1341
Last updated 2013-03-02
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Network Working Group                                      N. Borenstein
Request for Comments: 1521                                      Bellcore
Obsoletes: 1341                                                 N. Freed
Category: Standards Track                                       Innosoft
                                                          September 1993

         MIME (Multipurpose Internet Mail Extensions) Part One:
                Mechanisms for Specifying and Describing
                 the Format of Internet Message Bodies

Status of this Memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

Abstract

   STD 11, RFC 822 defines a message representation protocol which
   specifies considerable detail about message headers, but which leaves
   the message content, or message body, as flat ASCII text.  This
   document redefines the format of message bodies to allow multi-part
   textual and non-textual message bodies to be represented and
   exchanged without loss of information.  This is based on earlier work
   documented in RFC 934 and STD 11, RFC 1049, but extends and revises
   that work.  Because RFC 822 said so little about message bodies, this
   document is largely orthogonal to (rather than a revision of) RFC
   822.

   In particular, this document is designed to provide facilities to
   include multiple objects in a single message, to represent body text
   in character sets other than US-ASCII, to represent formatted multi-
   font text messages, to represent non-textual material such as images
   and audio fragments, and generally to facilitate later extensions
   defining new types of Internet mail for use by cooperating mail
   agents.

   This document does NOT extend Internet mail header fields to permit
   anything other than US-ASCII text data.  Such extensions are the
   subject of a companion document [RFC-1522].

   This document is a revision of RFC 1341.  Significant differences
   from RFC 1341 are summarized in Appendix H.

Borenstein & Freed                                              [Page 1]
RFC 1521                          MIME                    September 1993

Table of Contents

   1.     Introduction.......................................  3
   2.     Notations, Conventions, and Generic BNF Grammar....  6
   3.     The MIME-Version Header Field......................  7
   4.     The Content-Type Header Field......................  9
   5.     The Content-Transfer-Encoding Header Field......... 13
   5.1.   Quoted-Printable Content-Transfer-Encoding......... 18
   5.2.   Base64 Content-Transfer-Encoding................... 21
   6.     Additional Content-Header Fields................... 23
   6.1.   Optional Content-ID Header Field................... 23
   6.2.   Optional Content-Description Header Field.......... 24
   7.     The Predefined Content-Type Values................. 24
   7.1.   The Text Content-Type.............................. 24
   7.1.1. The charset parameter.............................. 25
   7.1.2. The Text/plain subtype............................. 28
   7.2.   The Multipart Content-Type......................... 28
   7.2.1. Multipart:  The common syntax...................... 29
   7.2.2. The Multipart/mixed (primary) subtype.............. 34
   7.2.3. The Multipart/alternative subtype.................. 34
   7.2.4. The Multipart/digest subtype....................... 36
   7.2.5. The Multipart/parallel subtype..................... 37
   7.2.6. Other Multipart subtypes........................... 37
   7.3.   The Message Content-Type........................... 38
   7.3.1. The Message/rfc822 (primary) subtype............... 38
   7.3.2. The Message/Partial subtype........................ 39
   7.3.3. The Message/External-Body subtype.................. 42
   7.3.3.1.  The "ftp" and "tftp" access-types............... 44
   7.3.3.2.  The "anon-ftp" access-type...................... 45
   7.3.3.3.  The "local-file" and "afs" access-types......... 45
   7.3.3.4.  The "mail-server" access-type................... 45
   7.3.3.5.  Examples and Further Explanations............... 46
   7.4.   The Application Content-Type....................... 49
   7.4.1. The Application/Octet-Stream (primary) subtype..... 50
   7.4.2. The Application/PostScript subtype................. 50
   7.4.3. Other Application subtypes......................... 53
   7.5.   The Image Content-Type............................. 53
   7.6.   The Audio Content-Type............................. 54
   7.7.   The Video Content-Type............................. 54
   7.8.   Experimental Content-Type Values................... 54
   8.     Summary............................................ 56
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