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rfc:rfc2049

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.
  (2)   Recognize the Content-Transfer-Encoding header field
        and decode all received data encoded by either quoted-
        printable or base64 implementations.  The identity
        transformations 7bit, 8bit, and binary must also be
        recognized.
        Any non-7bit data that is sent without encoding must be
        properly labelled with a content-transfer-encoding of
        8bit or binary, as appropriate.  If the underlying
        transport does not support 8bit or binary (as SMTP
        [RFC-821] does not), the sender is required to both
        encode and label data using an appropriate Content-
        Transfer-Encoding such as quoted-printable or base64.
  (3)   Must treat any unrecognized Content-Transfer-Encoding
        as if it had a Content-Type of "application/octet-
        stream", regardless of whether or not the actual
        Content-Type is recognized.
  (4)   Recognize and interpret the Content-Type header field,
        and avoid showing users raw data with a Content-Type
        field other than text.  Implementations  must be able
        to send at least text/plain messages, with the
        character set specified with the charset parameter if
        it is not US-ASCII.
  (5)   Ignore any content type parameters whose names they do
        not recognize.
  (6)   Explicitly handle the following media type values, to
        at least the following extents:
        Text:
  1. - Recognize and display "text" mail with the

character set "US-ASCII."

  1. - Recognize other character sets at least to the

extent of being able to inform the user about what

          character set the message uses.

Freed & Borenstein Standards Track [Page 3] RFC 2049 MIME Conformance November 1996

  1. - Recognize the "ISO-8859-*" character sets to the

extent of being able to display those characters that

          are common to ISO-8859-* and US-ASCII, namely all
          characters represented by octet values 1-127.
  1. - For unrecognized subtypes in a known character

set, show or offer to show the user the "raw" version

          of the data after conversion of the content from
          canonical form to local form.
  1. - Treat material in an unknown character set as if

it were "application/octet-stream".

        Image, audio, and video:
  1. - At a minumum provide facilities to treat any

unrecognized subtypes as if they were

          "application/octet-stream".
        Application:
  1. - Offer the ability to remove either of the quoted-

printable or base64 encodings defined in this

          document if they were used and put the resulting
          information in a user file.
        Multipart:
  1. - Recognize the mixed subtype. Display all relevant

information on the message level and the body part

          header level and then display or offer to display
          each of the body parts individually.
  1. - Recognize the "alternative" subtype, and avoid

showing the user redundant parts of

          multipart/alternative mail.
  1. - Recognize the "multipart/digest" subtype,

specifically using "message/rfc822" rather than

          "text/plain" as the default media type for body parts
          inside "multipart/digest" entities.
  1. - Treat any unrecognized subtypes as if they were

"mixed".

Freed & Borenstein Standards Track [Page 4] RFC 2049 MIME Conformance November 1996

        Message:
  1. - Recognize and display at least the RFC822 message

encapsulation (message/rfc822) in such a way as to

          preserve any recursive structure, that is, displaying
          or offering to display the encapsulated data in
          accordance with its media type.
  1. - Treat any unrecognized subtypes as if they were

"application/octet-stream".

  (7)   Upon encountering any unrecognized Content-Type field,
        an implementation must treat it as if it had a media
        type of "application/octet-stream" with no parameter
        sub-arguments.  How such data are handled is up to an
        implementation, but likely options for handling such
        unrecognized data include offering the user to write it
        into a file (decoded from its mail transport format) or
        offering the user to name a program to which the
        decoded data should be passed as input.
  (8)   Conformant user agents are required, if they provide
        non-standard support for non-MIME messages employing
        character sets other than US-ASCII, to do so on
        received messages only. Conforming user agents must not
        send non-MIME messages containing anything other than
        US-ASCII text.
        In particular, the use of non-US-ASCII text in mail
        messages without a MIME-Version field is strongly
        discouraged as it impedes interoperability when sending
        messages between regions with different localization
        conventions. Conforming user agents MUST include proper
        MIME labelling when sending anything other than plain
        text in the US-ASCII character set.
        In addition, non-MIME user agents should be upgraded if
        at all possible to include appropriate MIME header
        information in the messages they send even if nothing
        else in MIME is supported.  This upgrade will have
        little, if any, effect on non-MIME recipients and will
        aid MIME in correctly displaying such messages.  It
        also provides a smooth transition path to eventual
        adoption of other MIME capabilities.
  (9)   Conforming user agents must ensure that any string of
        non-white-space printable US-ASCII characters within a
        "*text" or "*ctext" that begins with "=?" and ends with

Freed & Borenstein Standards Track [Page 5] RFC 2049 MIME Conformance November 1996

        "?=" be a valid encoded-word.  ("begins" means: At the
        start of the field-body or immediately following
        linear-white-space; "ends" means: At the end of the
        field-body or immediately preceding linear-white-
        space.) In addition, any "word" within a "phrase" that
        begins with "=?" and ends with "?=" must be a valid
        encoded-word.
  (10)  Conforming user agents must be able to distinguish
        encoded-words from "text", "ctext", or "word"s,
        according to the rules in section 4, anytime they
        appear in appropriate places in message headers.  It
        must support both the "B" and "Q" encodings for any
        character set which it supports.  The program must be
        able to display the unencoded text if the character set
        is "US-ASCII".  For the ISO-8859-* character sets, the
        mail reading program must at least be able to display
        the characters which are also in the US-ASCII set.
 A user agent that meets the above conditions is said to be MIME-
 conformant.  The meaning of this phrase is that it is assumed to be
 "safe" to send virtually any kind of properly-marked data to users of
 such mail systems, because such systems will at least be able to
 treat the data as undifferentiated binary, and will not simply splash
 it onto the screen of unsuspecting users.
 There is another sense in which it is always "safe" to send data in a
 format that is MIME-conformant, which is that such data will not
 break or be broken by any known systems that are conformant with RFC
 821 and RFC 822.  User agents that are MIME-conformant have the
 additional guarantee that the user will not be shown data that were
 never intended to be viewed as text.

3. Guidelines for Sending Email Data

 Internet email is not a perfect, homogeneous system.  Mail may become
 corrupted at several stages in its travel to a final destination.
 Specifically, email sent throughout the Internet may travel across
 many networking technologies. Many networking and mail technologies
 do not support the full functionality possible in the SMTP transport
 environment.  Mail traversing these systems is likely to be modified
 in order that it can be transported.
 There exist many widely-deployed non-conformant MTAs in the Internet.
 These MTAs, speaking the SMTP protocol, alter messages on the fly to
 take advantage of the internal data structure of the hosts they are
 implemented on, or are just plain broken.

Freed & Borenstein Standards Track [Page 6] RFC 2049 MIME Conformance November 1996

 The following guidelines may be useful to anyone devising a data
 format (media type) that is supposed to survive the widest range of
 networking technologies and known broken MTAs unscathed.  Note that
 anything encoded in the base64 encoding will satisfy these rules, but
 that some well-known mechanisms, notably the UNIX uuencode facility,
 will not.  Note also that anything encoded in the Quoted-Printable
 encoding will survive most gateways intact, but possibly not some
 gateways to systems that use the EBCDIC character set.
  (1)   Under some circumstances the encoding used for data may
        change as part of normal gateway or user agent
        operation.  In particular, conversion from base64 to
        quoted-printable and vice versa may be necessary.  This
        may result in the confusion of CRLF sequences with line
        breaks in text bodies.  As such, the persistence of
        CRLF as something other than a line break must not be
        relied on.
  (2)   Many systems may elect to represent and store text data
        using local newline conventions.  Local newline
        conventions may not match the RFC822 CRLF convention --
        systems are known that use plain CR, plain LF, CRLF, or
        counted records.  The result is that isolated CR and LF
        characters are not well tolerated in general; they may
        be lost or converted to delimiters on some systems, and
        hence must not be relied on.
  (3)   The transmission of NULs (US-ASCII value 0) is
        problematic in Internet mail.  (This is largely the
        result of NULs being used as a termination character by
        many of the standard runtime library routines in the C
        programming language.) The practice of using NULs as
        termination characters is so entrenched now that
        messages should not rely on them being preserved.
  (4)   TAB (HT) characters may be misinterpreted or may be
        automatically converted to variable numbers of spaces.
        This is unavoidable in some environments, notably those
        not based on the US-ASCII character set.  Such
        conversion is STRONGLY DISCOURAGED, but it may occur,
        and mail formats must not rely on the persistence of
        TAB (HT) characters.
  (5)   Lines longer than 76 characters may be wrapped or
        truncated in some environments.  Line wrapping or line
        truncation imposed by mail transports is STRONGLY
        DISCOURAGED, but unavoidable in some cases.
        Applications which require long lines must somehow

Freed & Borenstein Standards Track [Page 7] RFC 2049 MIME Conformance November 1996

        differentiate between soft and hard line breaks.  (A
        simple way to do this is to use the quoted-printable
        encoding.)
  (6)   Trailing "white space" characters (SPACE, TAB (HT)) on
        a line may be discarded by some transport agents, while
        other transport agents may pad lines with these
        characters so that all lines in a mail file are of
        equal length.  The persistence of trailing white space,
        therefore, must not be relied on.
  (7)   Many mail domains use variations on the US-ASCII
        character set, or use character sets such as EBCDIC
        which contain most but not all of the US-ASCII
        characters.  The correct translation of characters not
        in the "invariant" set cannot be depended on across
        character converting gateways.  For example, this
        situation is a problem when sending uuencoded
        information across BITNET, an EBCDIC system.  Similar
        problems can occur without crossing a gateway, since
        many Internet hosts use character sets other than US-
        ASCII internally.  The definition of Printable Strings
        in X.400 adds further restrictions in certain special
        cases.  In particular, the only characters that are
        known to be consistent across all gateways are the 73
        characters that correspond to the upper and lower case
        letters A-Z and a-z, the 10 digits 0-9, and the
        following eleven special characters:
          "'"  (US-ASCII decimal value 39)
          "("  (US-ASCII decimal value 40)
          ")"  (US-ASCII decimal value 41)
          "+"  (US-ASCII decimal value 43)
          ","  (US-ASCII decimal value 44)
          "-"  (US-ASCII decimal value 45)
          "."  (US-ASCII decimal value 46)
          "/"  (US-ASCII decimal value 47)
          ":"  (US-ASCII decimal value 58)
          "="  (US-ASCII decimal value 61)
          "?"  (US-ASCII decimal value 63)
        A maximally portable mail representation will confine
        itself to relatively short lines of text in which the
        only meaningful characters are taken from this set of
        73 characters.  The base64 encoding follows this rule.
  (8)   Some mail transport agents will corrupt data that
        includes certain literal strings.  In particular, a

Freed & Borenstein Standards Track [Page 8] RFC 2049 MIME Conformance November 1996

        period (".") alone on a line is known to be corrupted
        by some (incorrect) SMTP implementations, and a line
        that starts with the five characters "From " (the fifth
        character is a SPACE) are commonly corrupted as well.
        A careful composition agent can prevent these
        corruptions by encoding the data (e.g., in the quoted-
        printable encoding using "=46rom " in place of "From "
        at the start of a line, and "=2E" in place of "." alone
        on a line).
 Please note that the above list is NOT a list of recommended
 practices for MTAs.  RFC 821 MTAs are prohibited from altering the
 character of white space or wrapping long lines.  These BAD and
 invalid practices are known to occur on established networks, and
 implementations should be robust in dealing with the bad effects they
 can cause.

4. Canonical Encoding Model

 There was some confusion, in earlier versions of these documents,
 regarding the model for when email data was to be converted to
 canonical form and encoded, and in particular how this process would
 affect the treatment of CRLFs, given that the representation of
 newlines varies greatly from system to system.  For this reason, a
 canonical model for encoding is presented below.
 The process of composing a MIME entity can be modeled as being done
 in a number of steps.  Note that these steps are roughly similar to
 those steps used in PEM [RFC-1421] and are performed for each
 "innermost level" body:
  (1)   Creation of local form.
        The body to be transmitted is created in the system's
        native format.  The native character set is used and,
        where appropriate, local end of line conventions are
        used as well.  The body may be a UNIX-style text file,
        or a Sun raster image, or a VMS indexed file, or audio
        data in a system-dependent format stored only in
        memory, or anything else that corresponds to the local
        model for the representation of some form of
        information.  Fundamentally, the data is created in the
        "native" form that corresponds to the type specified by
        the media type.

Freed & Borenstein Standards Track [Page 9] RFC 2049 MIME Conformance November 1996

  (2)   Conversion to canonical form.
        The entire body, including "out-of-band" information
        such as record lengths and possibly file attribute
        information, is converted to a universal canonical
        form.  The specific media type of the body as well as
        its associated attributes dictate the nature of the
        canonical form that is used.  Conversion to the proper
        canonical form may involve character set conversion,
        transformation of audio data, compression, or various
        other operations specific to the various media types.
        If character set conversion is involved, however, care
        must be taken to understand the semantics of the media
        type, which may have strong implications for any
        character set conversion, e.g. with regard to
        syntactically meaningful characters in a text subtype
        other than "plain".
        For example, in the case of text/plain data, the text
        must be converted to a supported character set and
        lines must be delimited with CRLF delimiters in
        accordance with RFC 822.  Note that the restriction on
        line lengths implied by RFC 822 is eliminated if the
        next step employs either quoted-printable or base64
        encoding.
  (3)   Apply transfer encoding.
        A Content-Transfer-Encoding appropriate for this body
        is applied.  Note that there is no fixed relationship
        between the media type and the transfer encoding.  In
        particular, it may be appropriate to base the choice of
        base64 or quoted-printable on character frequency
        counts which are specific to a given instance of a
        body.
  (4)   Insertion into entity.
        The encoded body is inserted into a MIME entity with
        appropriate headers. The entity is then inserted into
        the body of a higher-level entity (message or
        multipart) as needed.
 Conversion from entity form to local form is accomplished by
 reversing these steps. Note that reversal of these steps may produce
 differing results since there is no guarantee that the original and
 final local forms are the same.

Freed & Borenstein Standards Track [Page 10] RFC 2049 MIME Conformance November 1996

 It is vital to note that these steps are only a model; they are
 specifically NOT a blueprint for how an actual system would be built.
 In particular, the model fails to account for two common designs:
  (1)   In many cases the conversion to a canonical form prior
        to encoding will be subsumed into the encoder itself,
        which understands local formats directly.  For example,
        the local newline convention for text bodies might be
        carried through to the encoder itself along with
        knowledge of what that format is.
  (2)   The output of the encoders may have to pass through one
        or more additional steps prior to being transmitted as
        a message.  As such, the output of the encoder may not
        be conformant with the formats specified by RFC 822.
        In particular, once again it may be appropriate for the
        converter's output to be expressed using local newline
        conventions rather than using the standard RFC 822 CRLF
        delimiters.
 Other implementation variations are conceivable as well.  The vital
 aspect of this discussion is that, in spite of any optimizations,
 collapsings of required steps, or insertion of additional processing,
 the resulting messages must be consistent with those produced by the
 model described here.  For example, a message with the following
 header fields:
   Content-type: text/foo; charset=bar
   Content-Transfer-Encoding: base64
 must be first represented in the text/foo form, then (if necessary)
 represented in the "bar" character set, and finally transformed via
 the base64 algorithm into a mail-safe form.
 NOTE: Some confusion has been caused by systems that represent
 messages in a format which uses local newline conventions which
 differ from the RFC822 CRLF convention.  It is important to note that
 these formats are not canonical RFC822/MIME.  These formats are
 instead *encodings* of RFC822, where CRLF sequences in the canonical
 representation of the message are encoded as the local newline
 convention.  Note that formats which encode CRLF sequences as, for
 example, LF are not capable of representing MIME messages containing
 binary data which contains LF octets not part of CRLF line separation
 sequences.

Freed & Borenstein Standards Track [Page 11] RFC 2049 MIME Conformance November 1996

5. Summary

 This document defines what is meant by MIME Conformance. It also
 details various problems known to exist in the Internet email system
 and how to use MIME to overcome them. Finally, it describes MIME's
 canonical encoding model.

6. Security Considerations

 Security issues are discussed in the second document in this set, RFC
 2046.

7. Authors' Addresses

 For more information, the authors of this document are best contacted
 via Internet mail:
 Ned Freed
 Innosoft International, Inc.
 1050 East Garvey Avenue South
 West Covina, CA 91790
 USA
 Phone: +1 818 919 3600
 Fax:   +1 818 919 3614
 EMail: ned@innosoft.com
 Nathaniel S. Borenstein
 First Virtual Holdings
 25 Washington Avenue
 Morristown, NJ 07960
 USA
 Phone: +1 201 540 8967
 Fax:   +1 201 993 3032
 EMail: nsb@nsb.fv.com
 MIME is a result of the work of the Internet Engineering Task Force
 Working Group on RFC 822 Extensions.  The chairman of that group,
 Greg Vaudreuil, may be reached at:
 Gregory M. Vaudreuil
 Octel Network Services
 17080 Dallas Parkway
 Dallas, TX 75248-1905
 USA
 EMail: Greg.Vaudreuil@Octel.Com

Freed & Borenstein Standards Track [Page 12] RFC 2049 MIME Conformance November 1996

8. Acknowledgements

 This document is the result of the collective effort of a large
 number of people, at several IETF meetings, on the IETF-SMTP and
 IETF-822 mailing lists, and elsewhere.  Although any enumeration
 seems doomed to suffer from egregious omissions, the following are
 among the many contributors to this effort:
   Harald Tveit Alvestrand       Marc Andreessen
   Randall Atkinson              Bob Braden
   Philippe Brandon              Brian Capouch
   Kevin Carosso                 Uhhyung Choi
   Peter Clitherow               Dave Collier-Brown
   Cristian Constantinof         John Coonrod
   Mark Crispin                  Dave Crocker
   Stephen Crocker               Terry Crowley
   Walt Daniels                  Jim Davis
   Frank Dawson                  Axel Deininger
   Hitoshi Doi                   Kevin Donnelly
   Steve Dorner                  Keith Edwards
   Chris Eich                    Dana S. Emery
   Johnny Eriksson               Craig Everhart
   Patrik Faltstrom              Erik E. Fair
   Roger Fajman                  Alain Fontaine
   Martin Forssen                James M. Galvin
   Stephen Gildea                Philip Gladstone
   Thomas Gordon                 Keld Simonsen
   Terry Gray                    Phill Gross
   James Hamilton                David Herron
   Mark Horton                   Bruce Howard
   Bill Janssen                  Olle Jarnefors
   Risto Kankkunen               Phil Karn
   Alan Katz                     Tim Kehres
   Neil Katin                    Steve Kille
   Kyuho Kim                     Anders Klemets
   John Klensin                  Valdis Kletniek
   Jim Knowles                   Stev Knowles
   Bob Kummerfeld                Pekka Kytolaakso
   Stellan Lagerstrom            Vincent Lau
   Timo Lehtinen                 Donald Lindsay
   Warner Losh                   Carlyn Lowery
   Laurence Lundblade            Charles Lynn
   John R. MacMillan             Larry Masinter
   Rick McGowan                  Michael J. McInerny
   Leo Mclaughlin                Goli Montaser-Kohsari
   Tom Moore                     John Gardiner Myers
   Erik Naggum                   Mark Needleman
   Chris Newman                  John Noerenberg

Freed & Borenstein Standards Track [Page 13] RFC 2049 MIME Conformance November 1996

   Mats Ohrman                   Julian Onions
   Michael Patton                David J. Pepper
   Erik van der Poel             Blake C. Ramsdell
   Christer Romson               Luc Rooijakkers
   Marshall T. Rose              Jonathan Rosenberg
   Guido van Rossum              Jan Rynning
   Harri Salminen                Michael Sanderson
   Yutaka Sato                   Markku Savela
   Richard Alan Schafer          Masahiro Sekiguchi
   Mark Sherman                  Bob Smart
   Peter Speck                   Henry Spencer
   Einar Stefferud               Michael Stein
   Klaus Steinberger             Peter Svanberg
   James Thompson                Steve Uhler
   Stuart Vance                  Peter Vanderbilt
   Greg Vaudreuil                Ed Vielmetti
   Larry W. Virden               Ryan Waldron
   Rhys Weatherly                Jay Weber
   Dave Wecker                   Wally Wedel
   Sven-Ove Westberg             Brian Wideen
   John Wobus                    Glenn Wright
   Rayan Zachariassen            David Zimmerman
 The authors apologize for any omissions from this list, which are
 certainly unintentional.

Freed & Borenstein Standards Track [Page 14] RFC 2049 MIME Conformance November 1996

Appendix A – A Complex Multipart Example

 What follows is the outline of a complex multipart message.  This
 message contains five parts that are to be displayed serially:  two
 introductory plain text objects, an embedded multipart message, a
 text/enriched object, and a closing encapsulated text message in a
 non-ASCII character set.  The embedded multipart message itself
 contains two objects to be displayed in parallel, a picture and an
 audio fragment.
   MIME-Version: 1.0
   From: Nathaniel Borenstein <nsb@nsb.fv.com>
   To: Ned Freed <ned@innosoft.com>
   Date: Fri, 07 Oct 1994 16:15:05 -0700 (PDT)
   Subject: A multipart example
   Content-Type: multipart/mixed;
                 boundary=unique-boundary-1
   This is the preamble area of a multipart message.
   Mail readers that understand multipart format
   should ignore this preamble.
   If you are reading this text, you might want to
   consider changing to a mail reader that understands
   how to properly display multipart messages.
  1. -unique-boundary-1
     ... Some text appears here ...
   [Note that the blank between the boundary and the start
    of the text in this part means no header fields were
    given and this is text in the US-ASCII character set.
    It could have been done with explicit typing as in the
    next part.]
  1. -unique-boundary-1

Content-type: text/plain; charset=US-ASCII

   This could have been part of the previous part, but
   illustrates explicit versus implicit typing of body
   parts.
  1. -unique-boundary-1

Content-Type: multipart/parallel; boundary=unique-boundary-2

  1. -unique-boundary-2

Content-Type: audio/basic

Freed & Borenstein Standards Track [Page 15] RFC 2049 MIME Conformance November 1996

   Content-Transfer-Encoding: base64
     ... base64-encoded 8000 Hz single-channel
         mu-law-format audio data goes here ...
  1. -unique-boundary-2

Content-Type: image/jpeg

   Content-Transfer-Encoding: base64
     ... base64-encoded image data goes here ...
  1. -unique-boundary-2–
  1. -unique-boundary-1

Content-type: text/enriched

   This is <bold><italic>enriched.</italic></bold>
   <smaller>as defined in RFC 1896</smaller>
   Isn't it
   <bigger><bigger>cool?</bigger></bigger>
  1. -unique-boundary-1

Content-Type: message/rfc822

   From: (mailbox in US-ASCII)
   To: (address in US-ASCII)
   Subject: (subject in US-ASCII)
   Content-Type: Text/plain; charset=ISO-8859-1
   Content-Transfer-Encoding: Quoted-printable
     ... Additional text in ISO-8859-1 goes here ...
  1. -unique-boundary-1–

Appendix B – Changes from RFC 1521, 1522, and 1590

 These documents are a revision of RFC 1521, 1522, and 1590.  For the
 convenience of those familiar with the earlier documents, the changes
 from those documents are summarized in this appendix.  For further
 history, note that Appendix H in RFC 1521 specified how that document
 differed from its predecessor, RFC 1341.
  (1)   This document has been completely reformatted and split
        into multiple documents.  This was done to improve the
        quality of the plain text version of this document,
        which is required to be the reference copy.

Freed & Borenstein Standards Track [Page 16] RFC 2049 MIME Conformance November 1996

  (2)   BNF describing the overall structure of MIME object
        headers has been added. This is a documentation change
        only -- the underlying syntax has not changed in any
        way.
  (3)   The specific BNF for the seven media types in MIME has
        been removed.  This BNF was incorrect, incomplete, amd
        inconsistent with the type-indendependent BNF.  And
        since the type-independent BNF already fully specifies
        the syntax of the various MIME headers, the type-
        specific BNF was, in the final analysis, completely
        unnecessary and caused more problems than it solved.
  (4)   The more specific "US-ASCII" character set name has
        replaced the use of the informal term ASCII in many
        parts of these documents.
  (5)   The informal concept of a primary subtype has been
        removed.
  (6)   The term "object" was being used inconsistently.  The
        definition of this term has been clarified, along with
        the related terms "body", "body part", and "entity",
        and usage has been corrected where appropriate.
  (7)   The BNF for the multipart media type has been
        rearranged to make it clear that the CRLF preceeding
        the boundary marker is actually part of the marker
        itself rather than the preceeding body part.
  (8)   The prose and BNF describing the multipart media type
        have been changed to make it clear that the body parts
        within a multipart object MUST NOT contain any lines
        beginning with the boundary parameter string.
  (9)   In the rules on reassembling "message/partial" MIME
        entities, "Subject" is added to the list of headers to
        take from the inner message, and the example is
        modified to clarify this point.
  (10)  "Message/partial" fragmenters are restricted to
        splitting MIME objects only at line boundaries.
  (11)  In the discussion of the application/postscript type,
        an additional paragraph has been added warning about
        possible interoperability problems caused by embedding
        of binary data inside a PostScript MIME entity.

Freed & Borenstein Standards Track [Page 17] RFC 2049 MIME Conformance November 1996

  (12)  Added a clarifying note to the basic syntax rules for
        the Content-Type header field to make it clear that the
        following two forms:
          Content-type: text/plain; charset=us-ascii (comment)
          Content-type: text/plain; charset="us-ascii"
        are completely equivalent.
  (13)  The following sentence has been removed from the
        discussion of the MIME-Version header: "However,
        conformant software is encouraged to check the version
        number and at least warn the user if an unrecognized
        MIME-version is encountered."
  (14)  A typo was fixed that said "application/external-body"
        instead of "message/external-body".
  (15)  The definition of a character set has been reorganized
        to make the requirements clearer.
  (16)  The definition of the "image/gif" media type has been
        moved to a separate document. This change was made
        because of potential conflicts with IETF rules
        governing the standardization of patented technology.
  (17)  The definitions of "7bit" and "8bit" have been
        tightened so that use of bare CR, LF can only be used
        as end-of-line sequences.  The document also no longer
        requires that NUL characters be preserved, which brings
        MIME into alignment with real-world implementations.
  (18)  The definition of canonical text in MIME has been
        tightened so that line breaks must be represented by a
        CRLF sequence.  CR and LF characters are not allowed
        outside of this usage.  The definition of quoted-
        printable encoding has been altered accordingly.
  (19)  The definition of the quoted-printable encoding now
        includes a number of suggestions for how quoted-
        printable encoders might best handle improperly encoded
        material.
  (20)  Prose was added to clarify the use of the "7bit",
        "8bit", and "binary" transfer-encodings on multipart or
        message entities encapsulating "8bit" or "binary" data.

Freed & Borenstein Standards Track [Page 18] RFC 2049 MIME Conformance November 1996

  (21)  In the section on MIME Conformance, "multipart/digest"
        support was added to the list of requirements for
        minimal MIME conformance.  Also, the requirement for
        "message/rfc822" support were strengthened to clarify
        the importance of recognizing recursive structure.
  (22)  The various restrictions on subtypes of "message" are
        now specified entirely on a subtype by subtype basis.
  (23)  The definition of "message/rfc822" was changed to
        indicate that at least one of the "From", "Subject", or
        "Date" headers must be present.
  (24)  The required handling of unrecognized subtypes as
        "application/octet-stream" has been made more explicit
        in both the type definitions sections and the
        conformance guidelines.
  (25)  Examples using text/richtext were changed to
        text/enriched.
  (26)  The BNF definition of subtype has been changed to make
        it clear that either an IANA registered subtype or a
        nonstandard "X-" subtype must be used in a Content-Type
        header field.
  (27)  MIME media types that are simply registered for use and
        those that are standardized by the IETF are now
        distinguished in the MIME BNF.
  (28)  All of the various MIME registration procedures have
        been extensively revised. IANA registration procedures
        for character sets have been moved to a separate
        document that is no included in this set of documents.
  (29)  The use of escape and shift mechanisms in the US-ASCII
        and ISO-8859-X character sets these documents define
        have been clarified: Such mechanisms should never be
        used in conjunction with these character sets and their
        effect if they are used is undefined.
  (30)  The definition of the AFS access-type for
        message/external-body has been removed.
  (31)  The handling of the combination of
        multipart/alternative and message/external-body is now
        specifically addressed.

Freed & Borenstein Standards Track [Page 19] RFC 2049 MIME Conformance November 1996

  (32)  Security issues specific to message/external-body are
        now discussed in some detail.

Appendix C – References

 [ATK]
      Borenstein, Nathaniel S., Multimedia Applications
      Development with the Andrew Toolkit, Prentice-Hall, 1990.
 [ISO-2022]
      International Standard -- Information Processing --
      Character Code Structure and Extension Techniques,
      ISO/IEC 2022:1994, 4th ed.
 [ISO-8859]
      International Standard -- Information Processing -- 8-bit
      Single-Byte Coded Graphic Character Sets
      - Part 1: Latin Alphabet No. 1, ISO 8859-1:1987, 1st ed.
      - Part 2: Latin Alphabet No. 2, ISO 8859-2:1987, 1st ed.
      - Part 3: Latin Alphabet No. 3, ISO 8859-3:1988, 1st ed.
      - Part 4: Latin Alphabet No. 4, ISO 8859-4:1988, 1st ed.
      - Part 5: Latin/Cyrillic Alphabet, ISO 8859-5:1988, 1st
      ed.
      - Part 6: Latin/Arabic Alphabet, ISO 8859-6:1987, 1st ed.
      - Part 7: Latin/Greek Alphabet, ISO 8859-7:1987, 1st ed.
      - Part 8: Latin/Hebrew Alphabet, ISO 8859-8:1988, 1st ed.
      - Part 9: Latin Alphabet No. 5, ISO/IEC 8859-9:1989, 1st
      ed.
      International Standard -- Information Technology -- 8-bit
      Single-Byte Coded Graphic Character Sets
      - Part 10: Latin Alphabet No. 6, ISO/IEC 8859-10:1992,
      1st ed.
 [ISO-646]
      International Standard -- Information Technology -- ISO
      7-bit Coded Character Set for Information Interchange,
      ISO 646:1991, 3rd ed..
 [JPEG]
      JPEG Draft Standard ISO 10918-1 CD.
 [MPEG]
      Video Coding Draft Standard ISO 11172 CD, ISO
      IEC/JTC1/SC2/WG11 (Motion Picture Experts Group), May,
      1991.

Freed & Borenstein Standards Track [Page 20] RFC 2049 MIME Conformance November 1996

 [PCM]
      CCITT, Fascicle III.4 - Recommendation G.711, "Pulse Code
      Modulation (PCM) of Voice Frequencies", Geneva, 1972.
 [POSTSCRIPT]
      Adobe Systems, Inc., PostScript Language Reference
      Manual, Addison-Wesley, 1985.
 [POSTSCRIPT2]
      Adobe Systems, Inc., PostScript Language Reference
      Manual, Addison-Wesley, Second Ed., 1990.
 [RFC-783]
      Sollins, K.R., "TFTP Protocol (revision 2)", RFC-783,
      MIT, June 1981.
 [RFC-821]
      Postel, J.B., "Simple Mail Transfer Protocol", STD 10,
      RFC 821, USC/Information Sciences Institute, August 1982.
 [RFC-822]
      Crocker, D., "Standard for the Format of ARPA Internet
      Text Messages", STD 11, RFC 822, UDEL, August 1982.
 [RFC-934]
      Rose, M. and E. Stefferud, "Proposed Standard for Message
      Encapsulation", RFC 934, Delaware and NMA, January 1985.
 [RFC-959]
      Postel, J. and J. Reynolds, "File Transfer Protocol", STD
      9, RFC 959, USC/Information Sciences Institute, October
      1985.
 [RFC-1049]
      Sirbu, M., "Content-Type Header Field for Internet
      Messages", RFC 1049, CMU, March 1988.
 [RFC-1154]
      Robinson, D., and R. Ullmann, "Encoding Header Field for
      Internet Messages", RFC 1154, Prime Computer, Inc., April
      1990.
 [RFC-1341]
      Borenstein, N., and N.  Freed, "MIME (Multipurpose
      Internet Mail Extensions): Mechanisms for Specifying and
      Describing the Format of Internet Message Bodies", RFC
      1341, Bellcore, Innosoft, June 1992.

Freed & Borenstein Standards Track [Page 21] RFC 2049 MIME Conformance November 1996

 [RFC-1342]
      Moore, K., "Representation of Non-Ascii Text in Internet
      Message Headers", RFC 1342, University of Tennessee, June
      1992.
 [RFC-1344]
      Borenstein, N., "Implications of MIME for Internet Mail
      Gateways", RFC 1344, Bellcore, June 1992.
 [RFC-1345]
      Simonsen, K., "Character Mnemonics & Character Sets", RFC
      1345, Rationel Almen Planlaegning, June 1992.
 [RFC-1421]
      Linn, J., "Privacy Enhancement for Internet Electronic
      Mail:  Part I -- Message Encryption and Authentication
      Procedures", RFC 1421, IAB IRTF PSRG, IETF PEM WG,
      February 1993.
 [RFC-1422]
      Kent, S., "Privacy Enhancement for Internet Electronic
      Mail:  Part II -- Certificate-Based Key Management", RFC
      1422, IAB IRTF PSRG, IETF PEM WG, February 1993.
 [RFC-1423]
      Balenson, D., "Privacy Enhancement for Internet
      Electronic Mail:  Part III -- Algorithms, Modes, and
      Identifiers",  IAB IRTF PSRG, IETF PEM WG, February 1993.
 [RFC-1424]
      Kaliski, B., "Privacy Enhancement for Internet Electronic
      Mail:  Part IV -- Key Certification and Related
      Services", IAB IRTF PSRG, IETF PEM WG, February 1993.
 [RFC-1521]
      Borenstein, N., and Freed, N., "MIME (Multipurpose
      Internet Mail Extensions): Mechanisms for Specifying and
      Describing the Format of Internet Message Bodies", RFC
      1521, Bellcore, Innosoft, September, 1993.
 [RFC-1522]
      Moore, K., "Representation of Non-ASCII Text in Internet
      Message Headers", RFC 1522, University of Tennessee,
      September 1993.

Freed & Borenstein Standards Track [Page 22] RFC 2049 MIME Conformance November 1996

 [RFC-1524]
      Borenstein, N., "A User Agent Configuration Mechanism for
      Multimedia Mail Format Information", RFC 1524, Bellcore,
      September 1993.
 [RFC-1543]
      Postel, J., "Instructions to RFC Authors", RFC 1543,
      USC/Information Sciences Institute, October 1993.
 [RFC-1556]
      Nussbacher, H., "Handling of Bi-directional Texts in
      MIME", RFC 1556, Israeli Inter-University Computer
      Center, December 1993.
 [RFC-1590]
      Postel, J., "Media Type Registration Procedure", RFC
      1590, USC/Information Sciences Institute, March 1994.
 [RFC-1602]
      Internet Architecture Board, Internet Engineering
      Steering Group, Huitema, C., Gross, P., "The Internet
      Standards Process -- Revision 2", March 1994.
 [RFC-1652]
      Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,
      Stefferud, E., and Crocker, D., "SMTP Service Extension
      for 8bit-MIME transport", RFC 1652, United Nations
      University, Innosoft, Dover Beach Consulting, Inc.,
      Network Management Associates, Inc., The Branch Office,
      March 1994.
 [RFC-1700]
      Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
      RFC 1700, USC/Information Sciences Institute, October
      1994.
 [RFC-1741]
      Faltstrom, P., Crocker, D., and Fair, E., "MIME Content
      Type for BinHex Encoded Files", December 1994.
 [RFC-1896]
      Resnick, P., and A. Walker, "The text/enriched MIME
      Content-type", RFC 1896, February, 1996.

Freed & Borenstein Standards Track [Page 23] RFC 2049 MIME Conformance November 1996

 [RFC-2045]
      Freed, N., and and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part One: Format of Internet Message
      Bodies", RFC 2045, Innosoft, First Virtual Holdings,
      November 1996.
 [RFC-2046]
      Freed, N., and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Two: Media Types", RFC 2046,
      Innosoft, First Virtual Holdings, November 1996.
 [RFC-2047]
      Moore, K., "Multipurpose Internet Mail Extensions (MIME)
      Part Three: Representation of Non-ASCII Text in Internet
      Message Headers", RFC 2047, University of
      Tennessee, November 1996.
 [RFC-2048]
      Freed, N., Klensin, J., and J. Postel, "Multipurpose
      Internet Mail Extensions (MIME) Part Four: MIME
      Registration Procedures", RFC 2048, Innosoft, MCI,
      ISI, November 1996.
 [RFC-2049]
      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Five: Conformance Criteria and
      Examples", RFC 2049 (this document), Innosoft, First
      Virtual Holdings, November 1996.
 [US-ASCII]
      Coded Character Set -- 7-Bit American Standard Code for
      Information Interchange, ANSI X3.4-1986.
 [X400]
      Schicker, Pietro, "Message Handling Systems, X.400",
      Message Handling Systems and Distributed Applications, E.
      Stefferud, O-j. Jacobsen, and P. Schicker, eds., North-
      Holland, 1989, pp. 3-41.

Freed & Borenstein Standards Track [Page 24]

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