Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples
RFC 2049

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Network Working Group                                          N. Freed
Request for Comments: 2049                                     Innosoft
Obsoletes: 1521, 1522, 1590                               N. Borenstein
Category: Standards Track                                 First Virtual
                                                          November 1996

                 Multipurpose Internet Mail Extensions
                           (MIME) Part Five:
                   Conformance Criteria and Examples

Status of this Memo

   This document 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" (STD 1) 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 specifying
   considerable detail about US-ASCII message headers, and leaves the
   message content, or message body, as flat US-ASCII text.  This set of
   documents, collectively called the Multipurpose Internet Mail
   Extensions, or MIME, redefines the format of messages to allow for

    (1)   textual message bodies in character sets other than
          US-ASCII,

    (2)   an extensible set of different formats for non-textual
          message bodies,

    (3)   multi-part message bodies, and

    (4)   textual header information in character sets other than
          US-ASCII.

   These documents are based on earlier work documented in RFC 934, STD
   11, and RFC 1049, but extends and revises them.  Because RFC 822 said
   so little about message bodies, these documents are largely
   orthogonal to (rather than a revision of) RFC 822.

   The initial document in this set, RFC 2045, specifies the various
   headers used to describe the structure of MIME messages. The second
   document defines the general structure of the MIME media typing
   system and defines an initial set of media types.  The third
   document, RFC 2047, describes extensions to RFC 822 to allow non-US-

Freed & Borenstein          Standards Track                     [Page 1]
RFC 2049                    MIME Conformance               November 1996

   ASCII text data in Internet mail header fields. The fourth document,
   RFC 2048, specifies various IANA registration procedures for MIME-
   related facilities. This fifth and final document describes MIME
   conformance criteria as well as providing some illustrative examples
   of MIME message formats, acknowledgements, and the bibliography.

   These documents are revisions of RFCs 1521, 1522, and 1590, which
   themselves were revisions of RFCs 1341 and 1342.  Appendix B of this
   document describes differences and changes from previous versions.

Table of Contents

   1. Introduction ..........................................    2
   2. MIME Conformance ......................................    2
   3. Guidelines for Sending Email Data .....................    6
   4. Canonical Encoding Model ..............................    9
   5. Summary ...............................................   12
   6. Security Considerations ...............................   12
   7. Authors' Addresses ....................................   12
   8. Acknowledgements ......................................   13
   A. A Complex Multipart Example ...........................   15
   B. Changes from RFC 1521, 1522, and 1590 .................   16
   C. References ............................................   20

1.  Introduction

   The first and second documents in this set define MIME header fields
   and the initial set of MIME media types.  The third document
   describes extensions to RFC822 formats to allow for character sets
   other than US-ASCII.  This document describes what portions  of MIME
   must be supported by a conformant MIME implementation. It also
   describes various pitfalls of contemporary messaging systems as well
   as the canonical encoding model MIME is based on.

2.  MIME Conformance

   The mechanisms described in these documents are open-ended.  It is
   definitely not expected that all implementations will support all
   available media types, nor that they will all share the same
   extensions.  In order to promote interoperability, however, it is
   useful to define the concept of "MIME-conformance" to define a
   certain level of implementation that allows the useful interworking
   of messages with content that differs from US-ASCII text.  In this
   section, we specify the requirements for such conformance.

Freed & Borenstein          Standards Track                     [Page 2]
RFC 2049                    MIME Conformance               November 1996

   A mail user agent that is MIME-conformant MUST:

    (1)   Always generate a "MIME-Version: 1.0" header field in
          any message it creates.