RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One

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<h1 align="center">RFC 1521 (RFC1521)</h1>
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<p><strong>Alternate Formats:</strong> <a href="http://www.faqs.org/ftp/rfc/rfc1521.txt">rfc1521.txt</a> | <a href="http://www.faqs.org/ftp/rfc/rfc1521.ps">rfc1521.ps</a> | <a href="http://www.faqs.org/ftp/rfc/rfc1521.pdf">rfc1521.pdf</a></p>
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<h3 align="center">RFC 1521 - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies</h3>
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<pre class="blockcode">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, <a href="http://www.faqs.org/rfcs/rfc822.html">RFC 822</a> 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 <a href="http://www.faqs.org/rfcs/rfc934.html">RFC 934</a> and STD 11, <a href="http://www.faqs.org/rfcs/rfc1049.html">RFC 1049</a>, but extends and revises
that work.  Because <a href="http://www.faqs.org/rfcs/rfc822.html">RFC 822</a> 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 [<a href="http://www.faqs.org/rfcs/rfc1522.html">RFC-1522</a>].

This document is a revision of <a href="http://www.faqs.org/rfcs/rfc1341.html">RFC 1341</a>.  Significant differences
from <a href="http://www.faqs.org/rfcs/rfc1341.html">RFC 1341</a> are summarized in Appendix H.

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