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                       Alternate Formats: rfc1521.ps
  ------------------------------------------------------------------------

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.

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
   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

   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

   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
            textual information in a number of character sets and
            formatted text description languages in a standardized
            manner.

       2.b. A "multipart" Content-Type value, which can be used to
            combine several body parts, possibly of differing types of
            data, into a single message.

       2.c. An "application" Content-Type value, which can be used to
            transmit application data or binary data, and hence, among
            other uses, to implement an electronic mail file transfer
            service.

       2.d. A "message" Content-Type value, for encapsulating another
            mail message.

       2.e An "image" Content-Type value, for transmitting still image
            (picture) data.

       2.f. An "audio" Content-Type value, for transmitting audio or
            voice data.

       2.g. A "video" Content-Type value, for transmitting video or
            moving image data, possibly with audio as part of the
            composite video data format.

   3. A Content-Transfer-Encoding header field, which can be used to
       specify an auxiliary encoding that was applied to the data in
       order to allow it to pass through mail transport mechanisms which
       may have data or character set limitations.

   4. Two additional header fields that can be used to further describe
       the data in a message body, the Content-ID and Content-
       Description header fields.

   MIME has been carefully designed as an extensible mechanism, and it
   is expected that the set of content-type/subtype pairs and their
   associated parameters will grow significantly with time.  Several
   other MIME fields, notably including character set names, are likely
   to have new values defined over time.  In order to ensure that the
   set of such values is developed in an orderly, well-specified, and
   public manner, MIME defines a registration process which uses the
   Internet Assigned Numbers Authority (IANA) as a central registry for
   such values.  Appendix E provides details about how IANA registration
   is accomplished.

   Finally, to specify and promote interoperability, Appendix A of this
   document provides a basic applicability statement for a subset of the
   above mechanisms that defines a minimal level of "conformance" with
   this document.

      HISTORICAL NOTE: Several of the mechanisms described in this
      document may seem somewhat strange or even baroque at first
      reading.  It is important to note that compatibility with existing
      standards AND robustness across existing practice were two of the
      highest priorities of the working group that developed this
      document.  In particular, compatibility was always favored over
      elegance.

   MIME was first defined and published as RFCs 1341 and 1342 [RFC-1341]
   [RFC-1342].  This document is a relatively minor updating of RFC
   1341, and is intended to supersede it.  The differences between this
   document and RFC 1341 are summarized in Appendix H.  Please refer to
   the current edition of the "IAB Official Protocol Standards" for the
   standardization state and status of this protocol.  Several other RFC
   documents will be of interest to the MIME implementor, in particular
   [RFC 1343], [RFC-1344], and [RFC-1345].

2.    Notations, Conventions, and Generic BNF Grammar

   This document is being published in two versions, one as plain ASCII
   text and one as PostScript (PostScript is a trademark of Adobe
   Systems Incorporated.).  While the text version is the official
   specification, some will find the PostScript version easier to read.
   The textual contents are identical.  An Andrew-format copy of this
   document is also available from the first author (Borenstein).

   Although the mechanisms specified in this document are all described
   in prose, most are also described formally in the modified BNF
   notation of RFC 822.  Implementors will need to be familiar with this
   notation in order to understand this specification, and are referred
   to RFC 822 for a complete explanation of the modified BNF notation.

   Some of the modified BNF in this document makes reference to
   syntactic entities that are defined in RFC 822 and not in this
   document.  A complete formal grammar, then, is obtained by combining
   the collected grammar appendix of this document with that of RFC 822
   plus the modifications to RFC 822 defined in RFC 1123, which
   specifically changes the syntax for `return', `date' and `mailbox'.

   The term CRLF, in this document, refers to the sequence of the two
   ASCII characters CR (13) and LF (10) which, taken together, in this
   order, denote a line break in RFC 822 mail.

   The term "character set" is used in this document to refer to a
   method used with one or more tables to convert encoded text to a
   series of octets.  This definition is intended to allow various kinds
   of text encodings, from simple single-table mappings such as ASCII to
   complex table switching methods such as those that use ISO 2022's
   techniques.  However, a MIME character set name must fully specify
   the mapping to be performed.

   The term "message", when not further qualified, means either the
   (complete or "top-level") message being transferred on a network, or
   a message encapsulated in a body of type "message".

   The term "body part", in this document, means one of the parts of the
   body of a multipart entity. A body part has a header and a body, so
   it makes sense to speak about the body of a body part.

   The term "entity", in this document, means either a message or a body
   part.  All kinds of entities share the property that they have a
   header and a body.

   The term "body", when not further qualified, means the body of an
   entity, that is the body of either a message or of a body part.

      NOTE: The previous four definitions are clearly circular.  This is
      unavoidable, since the overall structure of a MIME message is
      indeed recursive.

   In this document, all numeric and octet values are given in decimal
   notation.

   It must be noted that Content-Type values, subtypes, and parameter
   names as defined in this document are case-insensitive.  However,
   parameter values are case-sensitive unless otherwise specified for
   the specific parameter.

      FORMATTING NOTE: This document has been carefully formatted for
      ease of reading.  The PostScript version of this document, in
      particular, places notes like this one, which may be skipped by
      the reader, in a smaller, italicized, font, and indents it as
      well.  In the text version, only the indentation is preserved, so
      if you are reading the text version of this you might consider
      using the PostScript version instead. However, all such notes will
      be indented and preceded by "NOTE:" or some similar introduction,
      even in the text version.

      The primary purpose of these non-essential notes is to convey
      information about the rationale of this document, or to place this
      document in the proper historical or evolutionary context.  Such
      information may be skipped by those who are focused entirely on
      building a conformant implementation, but may be of use to those
      who wish to understand why this document is written as it is.

      For ease of recognition, all BNF definitions have been placed in a
      fixed-width font in the PostScript version of this document.

3.    The MIME-Version Header Field

   Since RFC 822 was published in 1982, there has really been only one
   format standard for Internet messages, and there has been little
   perceived need to declare the format standard in use.  This document
   is an independent document that complements RFC 822. Although the
   extensions in this document have been defined in such a way as to be
   compatible with RFC 822, there are still circumstances in which it
   might be desirable for a mail-processing agent to know whether a
   message was composed with the new standard in mind.

   Therefore, this document defines a new header field, "MIME-Version",
   which is to be used to declare the version of the Internet message
   body format standard in use.

   Messages composed in accordance with this document MUST include such

   a header field, with the following verbatim text:

   MIME-Version: 1.0

   The presence of this header field is an assertion that the message
   has been composed in compliance with this document.

   Since it is possible that a future document might extend the message
   format standard again, a formal BNF is given for the content of the
   MIME-Version field:

   version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

   Thus, future format specifiers, which might replace or extend "1.0",
   are constrained to be two integer fields, separated by a period.  If
   a message is received with a MIME-version value other than "1.0", it
   cannot be assumed to conform with this specification.

   Note that the MIME-Version header field is required at the top level
   of a message. It is not required for each body part of a multipart
   entity.  It is required for the embedded headers of a body of type
   "message" if and only if the embedded message is itself claimed to be
   MIME-conformant.

   It is not possible to fully specify how a mail reader that conforms
   with MIME as defined in this document should treat a message that
   might arrive in the future with some value of MIME-Version other than
   "1.0".  However, conformant software is encouraged to check the
   version number and at least warn the user if an unrecognized MIME-
   version is encountered.

   It is also worth noting that version control for specific content-
   types is not accomplished using the MIME-Version mechanism.  In
   particular, some formats (such as application/postscript) have
   version numbering conventions that are internal to the document
   format.  Where such conventions exist, MIME does nothing to supersede
   them.  Where no such conventions exist, a MIME type might use a
   "version" parameter in the content-type field if necessary.

   NOTE TO IMPLEMENTORS: All header fields defined in this document,
   including MIME-Version, Content-type, etc., are subject to the
   general syntactic rules for header fields specified in RFC 822.  In
   particular, all can include comments, which means that the following
   two MIME-Version fields are equivalent:

                    MIME-Version: 1.0
                    MIME-Version: 1.0 (Generated by GBD-killer 3.7)

4.    The Content-Type Header Field

   The purpose of the Content-Type field is to describe the data
   contained in the body fully enough that the receiving user agent can
   pick an appropriate agent or mechanism to present the data to the
   user, or otherwise deal with the data in an appropriate manner.

   HISTORICAL NOTE: The Content-Type header field was first defined in
   RFC 1049.  RFC 1049 Content-types used a simpler and less powerful
   syntax, but one that is largely compatible with the mechanism given
   here.

   The Content-Type header field is used to specify the nature of the
   data in the body of an entity, by giving type and subtype
   identifiers, and by providing auxiliary information that may be
   required for certain types.  After the type and subtype names, the
   remainder of the header field is simply a set of parameters,
   specified in an attribute/value notation.  The set of meaningful
   parameters differs for the different types.  In particular, there are
   NO globally-meaningful parameters that apply to all content-types.
   Global mechanisms are best addressed, in the MIME model, by the
   definition of additional Content-* header fields.  The ordering of
   parameters is not significant.  Among the defined parameters is a
   "charset" parameter by which the character set used in the body may
   be declared. Comments are allowed in accordance with RFC 822 rules
   for structured header fields.

   In general, the top-level Content-Type is used to declare the general
   type of data, while the subtype specifies a specific format for that
   type of data.  Thus, a Content-Type of "image/xyz" is enough to tell
   a user agent that the data is an image, even if the user agent has no
   knowledge of the specific image format "xyz".  Such information can
   be used, for example, to decide whether or not to show a user the raw
   data from an unrecognized subtype -- such an action might be
   reasonable for unrecognized subtypes of text, but not for
   unrecognized subtypes of image or audio.  For this reason, registered
   subtypes of audio, image, text, and video, should not contain
   embedded information that is really of a different type.  Such
   compound types should be represented using the "multipart" or
   "application" types.

   Parameters are modifiers of the content-subtype, and do not
   fundamentally affect the requirements of the host system.  Although
   most parameters make sense only with certain content-types, others
   are "global" in the sense that they might apply to any subtype.  For
   example, the "boundary" parameter makes sense only for the
   "multipart" content-type, but the "charset" parameter might make
   sense with several content-types.

   An initial set of seven Content-Types is defined by this document.
   This set of top-level names is intended to be substantially complete.
   It is expected that additions to the larger set of supported types
   can generally be accomplished by the creation of new subtypes of
   these initial types.  In the future, more top-level types may be
   defined only by an extension to this standard.  If another primary
   type is to be used for any reason, it must be given a name starting
   with "X-" to indicate its non-standard status and to avoid a
   potential conflict with a future official name.

   In the Augmented BNF notation of RFC 822, a Content-Type header field
   value is defined as follows:

     content  :=   "Content-Type"  ":"  type  "/"  subtype  *(";"
     parameter)
               ; case-insensitive matching of type and subtype

     type :=          "application"     / "audio"
               / "image"           / "message"
               / "multipart"  / "text"
               / "video"           / extension-token
               ; All values case-insensitive

     extension-token :=  x-token / iana-token

     iana-token := <a publicly-defined extension token,
               registered with IANA, as specified in
               appendix E>

     x-token := <The two characters "X-" or "x-" followed, with
                 no intervening white space, by any token>

     subtype := token ; case-insensitive

     parameter := attribute "=" value

     attribute := token   ; case-insensitive

     value := token / quoted-string

     token  :=  1*<any (ASCII) CHAR except SPACE, CTLs,
                   or tspecials>

     tspecials :=  "(" / ")" / "<" / ">" / "@"
                /  "," / ";" / ":" / "\" / <">
                /  "/" / "[" / "]" / "?" / "="
               ; Must be in quoted-string,
               ; to use within parameter values

   Note that the definition of "tspecials" is the same as the RFC 822
   definition of "specials" with the addition of the three characters
   "/", "?", and "=", and the removal of ".".

   Note also that a subtype specification is MANDATORY.  There are no
   default subtypes.

   The type, subtype, and parameter names are not case sensitive.  For
   example, TEXT, Text, and TeXt are all equivalent.  Parameter values
   are normally case sensitive, but certain parameters are interpreted
   to be case-insensitive, depending on the intended use.  (For example,
   multipart boundaries are case-sensitive, but the "access-type" for
   message/External-body is not case-sensitive.)

   Beyond this syntax, the only constraint on the definition of subtype
   names is the desire that their uses must not conflict.  That is, it
   would be undesirable to have two different communities using
   "Content-Type: application/foobar" to mean two different things.  The
   process of defining new content-subtypes, then, is not intended to be
   a mechanism for imposing restrictions, but simply a mechanism for
   publicizing the usages. There are, therefore, two acceptable
   mechanisms for defining new Content-Type subtypes:

            1.  Private values (starting with "X-") may be
                defined bilaterally between two cooperating
                agents without outside registration or
                standardization.

            2.  New standard values must be documented,
                registered with, and approved by IANA, as
                described in Appendix E.  Where intended for
                public use, the formats they refer to must
                also be defined by a published specification,
                and possibly offered for standardization.

   The seven standard initial predefined Content-Types are detailed in
   the bulk of this document.  They are:

    text -- textual information.  The primary subtype,
         "plain", indicates plain (unformatted) text.  No
         special software is required to get the full
         meaning of the text, aside from support for the
         indicated character set.  Subtypes are to be used
         for enriched text in forms where application
         software may enhance the appearance of the text,
         but such software must not be required in order to
         get the general idea of the content.  Possible
         subtypes thus include any readable word processor

         format.  A very simple and portable subtype,
         richtext, was defined in RFC 1341, with a future
         revision expected.

    multipart -- data consisting of multiple parts of
         independent data types.  Four initial subtypes
         are defined, including the primary "mixed"
         subtype, "alternative" for representing the same
         data in multiple formats, "parallel" for parts
         intended to be viewed simultaneously, and "digest"
         for multipart entities in which each part is of
         type "message".

    message -- an encapsulated message.  A body of
         Content-Type "message" is itself all or part of a
         fully formatted RFC 822 conformant message which
         may contain its own different Content-Type header
         field.  The primary subtype is "rfc822".  The
         "partial" subtype is defined for partial messages,
         to permit the fragmented transmission of bodies
         that are thought to be too large to be passed
         through mail transport facilities.  Another
         subtype, "External-body", is defined for
         specifying large bodies by reference to an
         external data source.

    image -- image data.  Image requires a display device
         (such as a graphical display, a printer, or a FAX
         machine) to view the information.  Initial
         subtypes are defined for two widely-used image
         formats, jpeg and gif.

    audio -- audio data, with initial subtype "basic".
         Audio requires an audio output device (such as a
         speaker or a telephone) to "display" the contents.

    video -- video data.  Video requires the capability to
         display moving images, typically including
         specialized hardware and software.  The initial
         subtype is "mpeg".

    application -- some other kind of data, typically
         either uninterpreted binary data or information to
         be processed by a mail-based application.  The
         primary subtype, "octet-stream", is to be used in
         the case of uninterpreted binary data, in which
         case the simplest recommended action is to offer
         to write the information into a file for the user.

         An additional subtype, "PostScript", is defined
         for transporting PostScript documents in bodies.
         Other expected uses for "application" include
         spreadsheets, data for mail-based scheduling
         systems, and languages for "active"
         (computational) email.  (Note that active email
         and other application data may entail several
         security considerations, which are discussed later
         in this memo, particularly in the context of
         application/PostScript.)

   Default RFC 822 messages are typed by this protocol as plain text in
   the US-ASCII character set, which can be explicitly specified as
   "Content-type: text/plain; charset=us-ascii".  If no Content-Type is
   specified, this default is assumed.  In the presence of a MIME-
   Version header field, a receiving User Agent can also assume that
   plain US-ASCII text was the sender's intent.  In the absence of a
   MIME-Version specification, plain US-ASCII text must still be
   assumed, but the sender's intent might have been otherwise.

      RATIONALE: In the absence of any Content-Type header field or
      MIME-Version header field, it is impossible to be certain that a
      message is actually text in the US-ASCII character set, since it
      might well be a message that, using the conventions that predate
      this document, includes text in another character set or non-
      textual data in a manner that cannot be automatically recognized
      (e.g., a uuencoded compressed UNIX tar file).  Although there is
      no fully acceptable alternative to treating such untyped messages
      as "text/plain; charset=us-ascii", implementors should remain
      aware that if a message lacks both the MIME-Version and the
      Content-Type header fields, it may in practice contain almost
      anything.

   It should be noted that the list of Content-Type values given here
   may be augmented in time, via the mechanisms described above, and
   that the set of subtypes is expected to grow substantially.

   When a mail reader encounters mail with an unknown Content-type
   value, it should generally treat it as equivalent to
   "application/octet-stream", as described later in this document.

5.    The Content-Transfer-Encoding Header Field

   Many Content-Types which could usefully be transported via email are
   represented, in their "natural" format, as 8-bit character or binary
   data.  Such data cannot be transmitted over some transport protocols.
   For example, RFC 821 restricts mail messages to 7-bit US-ASCII data
   with lines no longer than 1000 characters.

   It is necessary, therefore, to define a standard mechanism for re-
   encoding such data into a 7-bit short-line format.  This document
   specifies that such encodings will be indicated by a new "Content-
   Transfer-Encoding" header field.  The Content-Transfer-Encoding field
   is used to indicate the type of transformation that has been used in
   order to represent the body in an acceptable manner for transport.

   Unlike Content-Types, a proliferation of Content-Transfer-Encoding
   values is undesirable and unnecessary.  However, establishing only a
   single Content-Transfer-Encoding mechanism does not seem possible.
   There is a tradeoff between the desire for a compact and efficient
   encoding of largely-binary data and the desire for a readable
   encoding of data that is mostly, but not entirely, 7-bit data.  For
   this reason, at least two encoding mechanisms are necessary: a
   "readable" encoding and a "dense" encoding.

   The Content-Transfer-Encoding field is designed to specify an
   invertible mapping between the "native" representation of a type of
   data and a representation that can be readily exchanged using 7 bit
   mail transport protocols, such as those defined by RFC 821 (SMTP).
   This field has not been defined by any previous standard. The field's
   value is a single token specifying the type of encoding, as
   enumerated below.  Formally:

   encoding := "Content-Transfer-Encoding" ":" mechanism

   mechanism :=     "7bit"  ;  case-insensitive
                  / "quoted-printable"
                  / "base64"
                  / "8bit"
                  / "binary"
                  / x-token

   These values are not case sensitive.  That is, Base64 and BASE64 and
   bAsE64 are all equivalent.  An encoding type of 7BIT requires that
   the body is already in a seven-bit mail-ready representation.  This
   is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is
   assumed if the Content-Transfer-Encoding header field is not present.

   The values "8bit", "7bit", and "binary" all mean that NO encoding has
   been performed. However, they are potentially useful as indications
   of the kind of data contained in the object, and therefore of the
   kind of encoding that might need to be performed for transmission in
   a given transport system.  In particular:

       "7bit" means that the data is all represented as short
            lines of US-ASCII data.

       "8bit" means that the lines are short, but there may be
            non-ASCII characters (octets with the high-order
            bit set).

       "Binary" means that not only may non-ASCII characters
            be present, but also that the lines are not
            necessarily short enough for SMTP transport.

   The difference between "8bit" (or any other conceivable bit-width
   token) and the "binary" token is that "binary" does not require
   adherence to any limits on line length or to the SMTP CRLF semantics,
   while the bit-width tokens do require such adherence.  If the body
   contains data in any bit-width other than 7-bit, the appropriate
   bit-width Content-Transfer-Encoding token must be used (e.g., "8bit"
   for unencoded 8 bit wide data).  If the body contains binary data,
   the "binary" Content-Transfer-Encoding token must be used.

      NOTE: The distinction between the Content-Transfer-Encoding values
      of "binary", "8bit", etc.  may seem unimportant, in that all of
      them really mean "none" -- that is, there has been no encoding of
      the data for transport.  However, clear labeling will be of
      enormous value to gateways between future mail transport systems
      with differing capabilities in transporting data that do not meet
      the restrictions of RFC 821 transport.

      Mail transport for unencoded 8-bit data is defined in RFC-1426
      [RFC-1426].  As of the publication of this document, there are no
      standardized Internet mail transports for which it is legitimate
      to include unencoded binary data in mail bodies.  Thus there are
      no circumstances in which the "binary" Content-Transfer-Encoding
      is actually legal on the Internet.  However, in the event that
      binary mail transport becomes a reality in Internet mail, or when
      this document is used in conjunction with any other binary-capable
      transport mechanism, binary bodies should be labeled as such using
      this mechanism.

      NOTE: The five values defined for the Content-Transfer-Encoding
      field imply nothing about the Content-Type other than the
      algorithm by which it was encoded or the transport system
      requirements if unencoded.

   Implementors may, if necessary, define new Content-Transfer-Encoding
   values, but must use an x-token, which is a name prefixed by "X-" to
   indicate its non-standard status, e.g., "Content-Transfer-Encoding:
   x-my-new-encoding".  However, unlike Content-Types and subtypes, the
   creation of new Content-Transfer-Encoding values is explicitly and
   strongly discouraged, as it seems likely to hinder interoperability
   with little potential benefit.  Their use is allowed only as the

   result of an agreement between cooperating user agents.

   If a Content-Transfer-Encoding header field appears as part of a
   message header, it applies to the entire body of that message.  If a
   Content-Transfer-Encoding header field appears as part of a body
   part's headers, it applies only to the body of that body part.  If an
   entity is of type "multipart" or "message", the Content-Transfer-
   Encoding is not permitted to have any value other than a bit width
   (e.g., "7bit", "8bit", etc.) or "binary".

   It should be noted that email is character-oriented, so that the
   mechanisms described here are mechanisms for encoding arbitrary octet
   streams, not bit streams.  If a bit stream is to be encoded via one
   of these mechanisms, it must first be converted to an 8-bit byte
   stream using the network standard bit order ("big-endian"), in which
   the earlier bits in a stream become the higher-order bits in a byte.
   A bit stream not ending at an 8-bit boundary must be padded with
   zeroes.  This document provides a mechanism for noting the addition
   of such padding in the case of the application Content-Type, which
   has a "padding" parameter.

   The encoding mechanisms defined here explicitly encode all data in
   ASCII.  Thus, for example, suppose an entity has header fields such
   as:

        Content-Type: text/plain; charset=ISO-8859-1
        Content-transfer-encoding: base64

   This must be interpreted to mean that the body is a base64 ASCII
   encoding of data that was originally in ISO-8859-1, and will be in
   that character set again after decoding.

   The following sections will define the two standard encoding
   mechanisms.  The definition of new content-transfer-encodings is
   explicitly discouraged and should only occur when absolutely
   necessary.  All content-transfer-encoding namespace except that
   beginning with "X-" is explicitly reserved to the IANA for future
   use.  Private agreements about content-transfer-encodings are also
   explicitly discouraged.

   Certain Content-Transfer-Encoding values may only be used on certain
   Content-Types.  In particular, it is expressly forbidden to use any
   encodings other than "7bit", "8bit", or "binary" with any Content-
   Type that recursively includes other Content-Type fields, notably the
   "multipart" and "message" Content-Types.  All encodings that are
   desired for bodies of type multipart or message must be done at the
   innermost level, by encoding the actual body that needs to be
   encoded.

      NOTE ON ENCODING RESTRICTIONS: Though the prohibition against
      using content-transfer-encodings on data of type multipart or
      message may seem overly restrictive, it is necessary to prevent
      nested encodings, in which data are passed through an encoding
      algorithm multiple times, and must be decoded multiple times in
      order to be properly viewed.  Nested encodings add considerable
      complexity to user agents: aside from the obvious efficiency
      problems with such multiple encodings, they can obscure the basic
      structure of a message.  In particular, they can imply that
      several decoding operations are necessary simply to find out what
      types of objects a message contains.  Banning nested encodings may
      complicate the job of certain mail gateways, but this seems less
      of a problem than the effect of nested encodings on user agents.

      NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-
      TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding
      could be inferred from the characteristics of the Content-Type
      that is to be encoded, or, at the very least, that certain
      Content-Transfer-Encodings could be mandated for use with specific
      Content-Types. There are several reasons why this is not the case.
      First, given the varying types of transports used for mail, some
      encodings may be appropriate for some Content-Type/transport
      combinations and not for others.  (For example, in an 8-bit
      transport, no encoding would be required for text in certain
      character sets, while such encodings are clearly required for 7-
      bit SMTP.)  Second, certain Content-Types may require different
      types of transfer encoding under different circumstances. For
      example, many PostScript bodies might consist entirely of short
      lines of 7-bit data and hence require little or no encoding.
      Other PostScript bodies (especially those using Level 2
      PostScript's binary encoding mechanism) may only be reasonably
      represented using a binary transport encoding. Finally, since
      Content-Type is intended to be an open-ended specification
      mechanism, strict specification of an association between
      Content-Types and encodings effectively couples the specification
      of an application protocol with a specific lower-level transport.
      This is not desirable since the developers of a Content-Type
      should not have to be aware of all the transports in use and what
      their limitations are.

      NOTE ON TRANSLATING ENCODINGS: The quoted-printable and base64
      encodings are designed so that conversion between them is
      possible.  The only issue that arises in such a conversion is the
      handling of line breaks.  When converting from quoted-printable to
      base64 a line break must be converted into a CRLF sequence.
      Similarly, a CRLF sequence in base64 data must be converted to a
      quoted-printable line break, but ONLY when converting text data.

      NOTE ON CANONICAL ENCODING MODEL: There was some confusion, in
      earlier drafts of this memo, 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, and the relationship between content-transfer-
      encodings and character sets.  For this reason, a canonical model
      for encoding is presented as Appendix G.

5.1.  Quoted-Printable Content-Transfer-Encoding

   The Quoted-Printable encoding is intended to represent data that
   largely consists of octets that correspond to printable characters in
   the ASCII character set.  It encodes the data in such a way that the
   resulting octets are unlikely to be modified by mail transport.  If
   the data being encoded are mostly ASCII text, the encoded form of the
   data remains largely recognizable by humans.  A body which is
   entirely ASCII may also be encoded in Quoted-Printable to ensure the
   integrity of the data should the message pass through a character-
   translating, and/or line-wrapping gateway.

   In this encoding, octets are to be represented as determined by the
   following rules:

      Rule #1: (General 8-bit representation) Any octet, except those
      indicating a line break according to the newline convention of the
      canonical (standard) form of the data being encoded, may be
      represented by an "=" followed by a two digit hexadecimal
      representation of the octet's value.  The digits of the
      hexadecimal alphabet, for this purpose, are "0123456789ABCDEF".
      Uppercase letters must be used when sending hexadecimal data,
      though a robust implementation may choose to recognize lowercase
      letters on receipt.  Thus, for example, the value 12 (ASCII form
      feed) can be represented by "=0C", and the value 61 (ASCII EQUAL
      SIGN) can be represented by "=3D".  Except when the following
      rules allow an alternative encoding, this rule is mandatory.

      Rule #2: (Literal representation) Octets with decimal values of 33
      through 60 inclusive, and 62 through 126, inclusive, MAY be
      represented as the ASCII characters which correspond to those
      octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN
      through TILDE, respectively).

      Rule #3: (White Space): Octets with values of 9 and 32 MAY be
      represented as ASCII TAB (HT) and SPACE characters, respectively,
      but MUST NOT be so represented at the end of an encoded line. Any
      TAB (HT) or SPACE characters on an encoded line MUST thus be
      followed on that line by a printable character.  In particular, an

      "=" at the end of an encoded line, indicating a soft line break
      (see rule #5) may follow one or more TAB (HT) or SPACE characters.
      It follows that an octet with value 9 or 32 appearing at the end
      of an encoded line must be represented according to Rule #1.  This
      rule is necessary because some MTAs (Message Transport Agents,
      programs which transport messages from one user to another, or
      perform a part of such transfers) are known to pad lines of text
      with SPACEs, and others are known to remove "white space"
      characters from the end of a line.  Therefore, when decoding a
      Quoted-Printable body, any trailing white space on a line must be
      deleted, as it will necessarily have been added by intermediate
      transport agents.

      Rule #4 (Line Breaks): A line break in a text body, independent of
      what its representation is following the canonical representation
      of the data being encoded, must be represented by a (RFC 822) line
      break, which is a CRLF sequence, in the Quoted-Printable encoding.
      Since the canonical representation of types other than text do not
      generally include the representation of line breaks, no hard line
      breaks (i.e.  line breaks that are intended to be meaningful and
      to be displayed to the user) should occur in the quoted-printable
      encoding of such types.  Of course, occurrences of "=0D", "=0A",
      "0A=0D" and "=0D=0A" will eventually be encountered.  In general,
      however, base64 is preferred over quoted-printable for binary
      data.

      Note that many implementations may elect to encode the local
      representation of various content types directly, as described in
      Appendix G.  In particular, this may apply to plain text material
      on systems that use newline conventions other than CRLF
      delimiters. Such an implementation is permissible, but the
      generation of line breaks must be generalized to account for the
      case where alternate representations of newline sequences are
      used.

      Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES
      that encoded lines be no more than 76 characters long. If longer
      lines are to be encoded with the Quoted-Printable encoding, 'soft'
      line breaks must be used. An equal sign as the last character on a
      encoded line indicates such a non-significant ('soft') line break
      in the encoded text. Thus if the "raw" form of the line is a
      single unencoded line that says:

          Now's the time for all folk to come to the aid of
          their country.

      This can be represented, in the Quoted-Printable encoding, as

          Now's the time =
          for all folk to come=
           to the aid of their country.

      This provides a mechanism with which long lines are encoded in
      such a way as to be restored by the user agent.  The 76 character
      limit does not count the trailing CRLF, but counts all other
      characters, including any equal signs.

   Since the hyphen character ("-") is represented as itself in the
   Quoted-Printable encoding, care must be taken, when encapsulating a
   quoted-printable encoded body in a multipart entity, to ensure that
   the encapsulation boundary does not appear anywhere in the encoded
   body.  (A good strategy is to choose a boundary that includes a
   character sequence such as "=_" which can never appear in a quoted-
   printable body.  See the definition of multipart messages later in
   this document.)

      NOTE: The quoted-printable encoding represents something of a
      compromise between readability and reliability in transport.
      Bodies encoded with the quoted-printable encoding will work
      reliably over most mail gateways, but may not work perfectly over
      a few gateways, notably those involving translation into EBCDIC.
      (In theory, an EBCDIC gateway could decode a quoted-printable body
      and re-encode it using base64, but such gateways do not yet
      exist.)  A higher level of confidence is offered by the base64
      Content-Transfer-Encoding.  A way to get reasonably reliable
      transport through EBCDIC gateways is to also quote the ASCII
      characters

             !"#$@[\]^`{|}~

      according to rule #1.  See Appendix B for more information.

   Because quoted-printable data is generally assumed to be line-
   oriented, it is to be expected that the representation of the breaks
   between the lines of quoted printable data may be altered in
   transport, in the same manner that plain text mail has always been
   altered in Internet mail when passing between systems with differing
   newline conventions.  If such alterations are likely to constitute a
   corruption of the data, it is probably more sensible to use the
   base64 encoding rather than the quoted-printable encoding.

   WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-
   printable, care must be taken to encode CR and LF characters as "=0D"
   and "=0A", respectively.  In particular, a CRLF sequence in binary
   data should be encoded as "=0D=0A".  Otherwise, if CRLF were
   represented as a hard line break, it might be incorrectly decoded on

   platforms with different line break conventions.

   For formalists, the syntax of quoted-printable data is described by
   the following grammar:

   quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
        ; Maximum line length of 76 characters excluding CRLF

   ptext := octet /<any ASCII character except "=", SPACE, or TAB>
        ; characters not listed as "mail-safe" in Appendix B
        ; are also not recommended.

   octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
        ; octet must be used for characters > 127, =, SPACE, or TAB,
        ; and is recommended for any characters not listed in
        ; Appendix B as "mail-safe".

5.2.  Base64 Content-Transfer-Encoding

   The Base64 Content-Transfer-Encoding is designed to represent
   arbitrary sequences of octets in a form that need not be humanly
   readable.  The encoding and decoding algorithms are simple, but the
   encoded data are consistently only about 33 percent larger than the
   unencoded data.  This encoding is virtually identical to the one used
   in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.
   The base64 encoding is adapted from RFC 1421, with one change: base64
   eliminates the "*" mechanism for embedded clear text.

   A 65-character subset of US-ASCII is used, enabling 6 bits to be
   represented per printable character. (The extra 65th character, "=",
   is used to signify a special processing function.)

      NOTE: This subset has the important property that it is
      represented identically in all versions of ISO 646, including US
      ASCII, and all characters in the subset are also represented
      identically in all versions of EBCDIC.  Other popular encodings,
      such as the encoding used by the uuencode utility and the base85
      encoding specified as part of Level 2 PostScript, do not share
      these properties, and thus do not fulfill the portability
      requirements a binary transport encoding for mail must meet.

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters. Proceeding from left to right, a
   24-bit input group is formed by concatenating 3 8-bit input groups.
   These 24 bits are then treated as 4 concatenated 6-bit groups, each
   of which is translated into a single digit in the base64 alphabet.
   When encoding a bit stream via the base64 encoding, the bit stream
   must be presumed to be ordered with the most-significant-bit first.

   That is, the first bit in the stream will be the high-order bit in
   the first byte, and the eighth bit will be the low-order bit in the
   first byte, and so on.

   Each 6-bit group is used as an index into an array of 64 printable
   characters. The character referenced by the index is placed in the
   output string. These characters, identified in Table 1, below, are
   selected so as to be universally representable, and the set excludes
   characters with particular significance to SMTP (e.g., ".", CR, LF)
   and to the encapsulation boundaries defined in this document (e.g.,
   "-").

                            Table 1: The Base64 Alphabet

      Value Encoding  Value Encoding  Value Encoding  Value Encoding
           0 A            17 R            34 i            51 z
           1 B            18 S            35 j            52 0
           2 C            19 T            36 k            53 1
           3 D            20 U            37 l            54 2
           4 E            21 V            38 m            55 3
           5 F            22 W            39 n            56 4
           6 G            23 X            40 o            57 5
           7 H            24 Y            41 p            58 6
           8 I            25 Z            42 q            59 7
           9 J            26 a            43 r            60 8
          10 K            27 b            44 s            61 9
          11 L            28 c            45 t            62 +
          12 M            29 d            46 u            63 /
          13 N            30 e            47 v
          14 O            31 f            48 w         (pad) =
          15 P            32 g            49 x
          16 Q            33 h            50 y

   The output stream (encoded bytes) must be represented in lines of no
   more than 76 characters each.  All line breaks or other characters
   not found in Table 1 must be ignored by decoding software.  In base64
   data, characters other than those in Table 1, line breaks, and other
   white space probably indicate a transmission error, about which a
   warning message or even a message rejection might be appropriate
   under some circumstances.

   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  A full encoding quantum is
   always completed at the end of a body.  When fewer than 24 input bits
   are available in an input group, zero bits are added (on the right)
   to form an integral number of 6-bit groups.  Padding at the end of
   the data is performed using the '=' character.  Since all base64
   input is an integral number of octets, only the following cases can

   arise: (1) the final quantum of encoding input is an integral
   multiple of 24 bits; here, the final unit of encoded output will be
   an integral multiple of 4 characters with no "=" padding, (2) the
   final quantum of encoding input is exactly 8 bits; here, the final
   unit of encoded output will be two characters followed by two "="
   padding characters, or (3) the final quantum of encoding input is
   exactly 16 bits; here, the final unit of encoded output will be three
   characters followed by one "=" padding character.

   Because it is used only for padding at the end of the data, the
   occurrence of any '=' characters may be taken as evidence that the
   end of the data has been reached (without truncation in transit).  No
   such assurance is possible, however, when the number of octets
   transmitted was a multiple of three.

   Any characters outside of the base64 alphabet are to be ignored in
   base64-encoded data.  The same applies to any illegal sequence of
   characters in the base64 encoding, such as "====="

   Care must be taken to use the proper octets for line breaks if base64
   encoding is applied directly to text material that has not been
   converted to canonical form.  In particular, text line breaks must be
   converted into CRLF sequences prior to base64 encoding. The important
   thing to note is that this may be done directly by the encoder rather
   than in a prior canonicalization step in some implementations.

      NOTE: There is no need to worry about quoting apparent
      encapsulation boundaries within base64-encoded parts of multipart
      entities because no hyphen characters are used in the base64
      encoding.

6.    Additional Content-Header Fields

6.1.  Optional Content-ID Header Field

   In constructing a high-level user agent, it may be desirable to allow
   one body to make reference to another.  Accordingly, bodies may be
   labeled using the "Content-ID" header field, which is syntactically
   identical to the "Message-ID" header field:

   id :=  "Content-ID" ":" msg-id
   Like the Message-ID values, Content-ID values must be generated to be
   world-unique.

   The Content-ID value may be used for uniquely identifying MIME
   entities in several contexts, particularly for cacheing data
   referenced by the message/external-body mechanism.  Although the
   Content-ID header is generally optional, its use is mandatory in

   implementations which generate data of the optional MIME Content-type
   "message/external-body".  That is, each message/external-body entity
   must have a Content-ID field to permit cacheing of such data.

   It is also worth noting that the Content-ID value has special
   semantics in the case of the multipart/alternative content-type.
   This is explained in the section of this document dealing with
   multipart/alternative.

6.2.  Optional Content-Description Header Field

   The ability to associate some descriptive information with a given
   body is often desirable. For example, it may be useful to mark an
   "image" body as "a picture of the Space Shuttle Endeavor."  Such text
   may be placed in the Content-Description header field.

   description := "Content-Description" ":" *text

   The description is presumed to be given in the US-ASCII character
   set, although the mechanism specified in [RFC-1522] may be used for
   non-US-ASCII Content-Description values.

7.    The Predefined Content-Type Values

   This document defines seven initial Content-Type values and an
   extension mechanism for private or experimental types.  Further
   standard types must be defined by new published specifications.  It
   is expected that most innovation in new types of mail will take place
   as subtypes of the seven types defined here.  The most essential
   characteristics of the seven content-types are summarized in Appendix
   F.

7.1  The Text Content-Type

   The text Content-Type is intended for sending material which is
   principally textual in form.  It is the default Content-Type.  A
   "charset" parameter may be used to indicate the character set of the
   body text for some text subtypes, notably including the primary
   subtype, "text/plain", which indicates plain (unformatted) text.  The
   default Content-Type for Internet mail is "text/plain; charset=us-
   ascii".

   Beyond plain text, there are many formats for representing what might
   be known as "extended text" -- text with embedded formatting and
   presentation information.  An interesting characteristic of many such
   representations is that they are to some extent readable even without
   the software that interprets them.  It is useful, then, to
   distinguish them, at the highest level, from such unreadable data as

   images, audio, or text represented in an unreadable form.  In the
   absence of appropriate interpretation software, it is reasonable to
   show subtypes of text to the user, while it is not reasonable to do
   so with most nontextual data.

   Such formatted textual data should be represented using subtypes of
   text.  Plausible subtypes of text are typically given by the common
   name of the representation format, e.g., "text/richtext" [RFC-1341].

7.1.1.     The charset parameter

   A critical parameter that may be specified in the Content-Type field
   for text/plain data is the character set.  This is specified with a
   "charset" parameter, as in:

        Content-type: text/plain; charset=us-ascii

   Unlike some other parameter values, the values of the charset
   parameter are NOT case sensitive.  The default character set, which
   must be assumed in the absence of a charset parameter, is US-ASCII.

   The specification for any future subtypes of "text" must specify
   whether or not they will also utilize a "charset" parameter, and may
   possibly restrict its values as well.  When used with a particular
   body, the semantics of the "charset" parameter should be identical to
   those specified here for "text/plain", i.e., the body consists
   entirely of characters in the given charset.  In particular, definers
   of future text subtypes should pay close attention the the
   implications of multibyte character sets for their subtype
   definitions.

   This RFC specifies the definition of the charset parameter for the
   purposes of MIME to be a unique mapping of a byte stream to glyphs, a
   mapping which does not require external profiling information.

   An initial list of predefined character set names can be found at the
   end of this section.  Additional character sets may be registered
   with IANA, although the standardization of their use requires the
   usual IESG [RFC-1340] review and approval.  Note that if the
   specified character set includes 8-bit data, a Content-Transfer-
   Encoding header field and a corresponding encoding on the data are
   required in order to transmit the body via some mail transfer
   protocols, such as SMTP.

   The default character set, US-ASCII, has been the subject of some
   confusion and ambiguity in the past.  Not only were there some
   ambiguities in the definition, there have been wide variations in
   practice.  In order to eliminate such ambiguity and variations in the

   future, it is strongly recommended that new user agents explicitly
   specify a character set via the Content-Type header field.  "US-
   ASCII" does not indicate an arbitrary seven-bit character code, but
   specifies that the body uses character coding that uses the exact
   correspondence of codes to characters specified in ASCII.  National
   use variations of ISO 646 [ISO-646] are NOT ASCII and their use in
   Internet mail is explicitly discouraged. The omission of the ISO 646
   character set is deliberate in this regard.  The character set name
   of "US-ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only.
   The character set name "ASCII" is reserved and must not be used for
   any purpose.

      NOTE: RFC 821 explicitly specifies "ASCII", and references an
      earlier version of the American Standard.  Insofar as one of the
      purposes of specifying a Content-Type and character set is to
      permit the receiver to unambiguously determine how the sender
      intended the coded message to be interpreted, assuming anything
      other than "strict ASCII" as the default would risk unintentional
      and incompatible changes to the semantics of messages now being
      transmitted.  This also implies that messages containing
      characters coded according to national variations on ISO 646, or
      using code-switching procedures (e.g., those of ISO 2022), as well
      as 8-bit or multiple octet character encodings MUST use an
      appropriate character set specification to be consistent with this
      specification.

   The complete US-ASCII character set is listed in [US-ASCII].  Note
   that the control characters including DEL (0-31, 127) have no defined
   meaning apart from the combination CRLF (ASCII values 13 and 10)
   indicating a new line.  Two of the characters have de facto meanings
   in wide use: FF (12) often means "start subsequent text on the
   beginning of a new page"; and TAB or HT (9) often (though not always)
   means "move the cursor to the next available column after the current
   position where the column number is a multiple of 8 (counting the
   first column as column 0)." Apart from this, any use of the control
   characters or DEL in a body must be part of a private agreement
   between the sender and recipient.  Such private agreements are
   discouraged and should be replaced by the other capabilities of this
   document.

      NOTE: Beyond US-ASCII, an enormous proliferation of character sets
      is possible. It is the opinion of the IETF working group that a
      large number of character sets is NOT a good thing.  We would
      prefer to specify a single character set that can be used
      universally for representing all of the world's languages in
      electronic mail.  Unfortunately, existing practice in several
      communities seems to point to the continued use of multiple
      character sets in the near future.  For this reason, we define

      names for a small number of character sets for which a strong
      constituent base exists.

   The defined charset values are:

   US-ASCII -- as defined in [US-ASCII].

        ISO-8859-X -- where "X" is to be replaced, as necessary, for the
             parts of ISO-8859 [ISO-8859].  Note that the ISO 646
             character sets have deliberately been omitted in favor of
             their 8859 replacements, which are the designated character
             sets for Internet mail.  As of the publication of this
             document, the legitimate values for "X" are the digits 1
             through 9.

   The character sets specified above are the ones that were relatively
   uncontroversial during the drafting of MIME.  This document does not
   endorse the use of any particular character set other than US-ASCII,
   and recognizes that the future evolution of world character sets
   remains unclear.  It is expected that in the future, additional
   character sets will be registered for use in MIME.

   Note that the character set used, if anything other than US-ASCII,
   must always be explicitly specified in the Content-Type field.

   No other character set name may be used in Internet mail without the
   publication of a formal specification and its registration with IANA,
   or by private agreement, in which case the character set name must
   begin with "X-".

   Implementors are discouraged from defining new character sets for
   mail use unless absolutely necessary.

   The "charset" parameter has been defined primarily for the purpose of
   textual data, and is described in this section for that reason.
   However, it is conceivable that non-textual data might also wish to
   specify a charset value for some purpose, in which case the same
   syntax and values should be used.

   In general, mail-sending software must always use the "lowest common
   denominator" character set possible.  For example, if a body contains
   only US-ASCII characters, it must be marked as being in the US-ASCII
   character set, not ISO-8859-1, which, like all the ISO-8859 family of
   character sets, is a superset of US-ASCII.  More generally, if a
   widely-used character set is a subset of another character set, and a
   body contains only characters in the widely-used subset, it must be
   labeled as being in that subset.  This will increase the chances that
   the recipient will be able to view the mail correctly.

7.1.2.     The Text/plain subtype

   The primary subtype of text is "plain".  This indicates plain
   (unformatted) text.  The default Content-Type for Internet mail,
   "text/plain; charset=us-ascii", describes existing Internet practice.
   That is, it is the type of body defined by RFC 822.

   No other text subtype is defined by this document.

   The formal grammar for the content-type header field for text is as
   follows:

   text-type := "text" "/" text-subtype [";" "charset" "=" charset]

   text-subtype := "plain" / extension-token

   charset := "us-ascii"/ "iso-8859-1"/ "iso-8859-2"/ "iso-8859-3"
          / "iso-8859-4"/ "iso-8859-5"/ "iso-8859-6"/ "iso-8859-7"
          / "iso-8859-8" / "iso-8859-9" / extension-token
                    ; case insensitive

7.2.  The Multipart Content-Type

   In the case of multiple part entities, in which one or more different
   sets of data are combined in a single body, a "multipart" Content-
   Type field must appear in the entity's header. The body must then
   contain one or more "body parts," each preceded by an encapsulation
   boundary, and the last one followed by a closing boundary.  Each part
   starts with an encapsulation boundary, and then contains a body part
   consisting of header area, a blank line, and a body area.  Thus a
   body part is similar to an RFC 822 message in syntax, but different
   in meaning.

   A body part is NOT to be interpreted as actually being an RFC 822
   message.  To begin with, NO header fields are actually required in
   body parts.  A body part that starts with a blank line, therefore, is
   allowed and is a body part for which all default values are to be
   assumed.  In such a case, the absence of a Content-Type header field
   implies that the corresponding body is plain US-ASCII text.  The only
   header fields that have defined meaning for body parts are those the
   names of which begin with "Content-".  All other header fields are
   generally to be ignored in body parts.  Although they should
   generally be retained in mail processing, they may be discarded by
   gateways if necessary.  Such other fields are permitted to appear in
   body parts but must not be depended on.  "X-" fields may be created
   for experimental or private purposes, with the recognition that the
   information they contain may be lost at some gateways.

      NOTE: The distinction between an RFC 822 message and a body part
      is subtle, but important. A gateway between Internet and X.400
      mail, for example, must be able to tell the difference between a
      body part that contains an image and a body part that contains an
      encapsulated message, the body of which is an image.  In order to
      represent the latter, the body part must have "Content-Type:
      message", and its body (after the blank line) must be the
      encapsulated message, with its own "Content-Type: image" header
      field.  The use of similar syntax facilitates the conversion of
      messages to body parts, and vice versa, but the distinction
      between the two must be understood by implementors.  (For the
      special case in which all parts actually are messages, a "digest"
      subtype is also defined.)

   As stated previously, each body part is preceded by an encapsulation
   boundary.  The encapsulation boundary MUST NOT appear inside any of
   the encapsulated parts.  Thus, it is crucial that the composing agent
   be able to choose and specify the unique boundary that will separate
   the parts.

   All present and future subtypes of the "multipart" type must use an
   identical syntax.  Subtypes may differ in their semantics, and may
   impose additional restrictions on syntax, but must conform to the
   required syntax for the multipart type.  This requirement ensures
   that all conformant user agents will at least be able to recognize
   and separate the parts of any multipart entity, even of an
   unrecognized subtype.

   As stated in the definition of the Content-Transfer-Encoding field,
   no encoding other than "7bit", "8bit", or "binary" is permitted for
   entities of type "multipart".  The multipart delimiters and header
   fields are always represented as 7-bit ASCII in any case (though the
   header fields may encode non-ASCII header text as per [RFC-1522]),
   and data within the body parts can be encoded on a part-by-part
   basis, with Content-Transfer-Encoding fields for each appropriate
   body part.

   Mail gateways, relays, and other mail handling agents are commonly
   known to alter the top-level header of an RFC 822 message.  In
   particular, they frequently add, remove, or reorder header fields.
   Such alterations are explicitly forbidden for the body part headers
   embedded in the bodies of messages of type "multipart."

7.2.1.     Multipart:  The common syntax

   All subtypes of "multipart" share a common syntax, defined in this
   section.  A simple example of a multipart message also appears in
   this section.  An example of a more complex multipart message is

   given in Appendix C.

   The Content-Type field for multipart entities requires one parameter,
   "boundary", which is used to specify the encapsulation boundary.  The
   encapsulation boundary is defined as a line consisting entirely of
   two hyphen characters ("-", decimal code 45) followed by the boundary
   parameter value from the Content-Type header field.

      NOTE: The hyphens are for rough compatibility with the earlier RFC
      934 method of message encapsulation, and for ease of searching for
      the boundaries in some implementations. However, it should be
      noted that multipart messages are NOT completely compatible with
      RFC 934 encapsulations; in particular, they do not obey RFC 934
      quoting conventions for embedded lines that begin with hyphens.
      This mechanism was chosen over the RFC 934 mechanism because the
      latter causes lines to grow with each level of quoting.  The
      combination of this growth with the fact that SMTP implementations
      sometimes wrap long lines made the RFC 934 mechanism unsuitable
      for use in the event that deeply-nested multipart structuring is
      ever desired.

   WARNING TO IMPLEMENTORS: The grammar for parameters on the Content-
   type field is such that it is often necessary to enclose the
   boundaries in quotes on the Content-type line.  This is not always
   necessary, but never hurts.  Implementors should be sure to study the
   grammar carefully in order to avoid producing illegal Content-type
   fields. Thus, a typical multipart Content-Type header field might
   look like this:

                 Content-Type: multipart/mixed;
                      boundary=gc0p4Jq0M2Yt08jU534c0p

   But the following is illegal:

                 Content-Type: multipart/mixed;
                      boundary=gc0p4Jq0M:2Yt08jU534c0p

   (because of the colon) and must instead be represented as

                 Content-Type: multipart/mixed;
                      boundary="gc0p4Jq0M:2Yt08jU534c0p"

   This indicates that the entity consists of several parts, each itself
   with a structure that is syntactically identical to an RFC 822
   message, except that the header area might be completely empty, and
   that the parts are each preceded by the line

                 --gc0p4Jq0M:2Yt08jU534c0p

   Note that the encapsulation boundary must occur at the beginning of a
   line, i.e., following a CRLF, and that the initial CRLF is considered
   to be attached to the encapsulation boundary rather than part of the
   preceding part.  The boundary must be followed immediately either by
   another CRLF and the header fields for the next part, or by two
   CRLFs, in which case there are no header fields for the next part
   (and it is therefore assumed to be of Content-Type text/plain).

      NOTE: The CRLF preceding the encapsulation line is conceptually
      attached to the boundary so that it is possible to have a part
      that does not end with a CRLF (line break). Body parts that must
      be considered to end with line breaks, therefore, must have two
      CRLFs preceding the encapsulation line, the first of which is part
      of the preceding body part, and the second of which is part of the
      encapsulation boundary.

   Encapsulation boundaries must not appear within the encapsulations,
   and must be no longer than 70 characters, not counting the two
   leading hyphens.

   The encapsulation boundary following the last body part is a
   distinguished delimiter that indicates that no further body parts
   will follow.  Such a delimiter is identical to the previous
   delimiters, with the addition of two more hyphens at the end of the
   line:

                 --gc0p4Jq0M2Yt08jU534c0p--

   There appears to be room for additional information prior to the
   first encapsulation boundary and following the final boundary.  These
   areas should generally be left blank, and implementations must ignore
   anything that appears before the first boundary or after the last
   one.

      NOTE: These "preamble" and "epilogue" areas are generally not used
      because of the lack of proper typing of these parts and the lack
      of clear semantics for handling these areas at gateways,
      particularly X.400 gateways.  However, rather than leaving the
      preamble area blank, many MIME implementations have found this to
      be a convenient place to insert an explanatory note for recipients
      who read the message with pre-MIME software, since such notes will
      be ignored by MIME-compliant software.

      NOTE: Because encapsulation boundaries must not appear in the body
      parts being encapsulated, a user agent must exercise care to
      choose a unique boundary.  The boundary in the example above could
      have been the result of an algorithm designed to produce
      boundaries with a very low probability of already existing in the

      data to be encapsulated without having to prescan the data.
      Alternate algorithms might result in more 'readable' boundaries
      for a recipient with an old user agent, but would require more
      attention to the possibility that the boundary might appear in the
      encapsulated part.  The simplest boundary possible is something
      like "---", with a closing boundary of "-----".

   As a very simple example, the following multipart message has two
   parts, both of them plain text, one of them explicitly typed and one
   of them implicitly typed:

      From: Nathaniel Borenstein <nsb@bellcore.com>
      To:  Ned Freed <ned@innosoft.com>
      Subject: Sample message
      MIME-Version: 1.0
      Content-type: multipart/mixed; boundary="simple
      boundary"

      This is the preamble.  It is to be ignored, though it
      is a handy place for mail composers to include an
      explanatory note to non-MIME conformant readers.
      --simple boundary

      This is implicitly typed plain ASCII text.
      It does NOT end with a linebreak.
      --simple boundary
      Content-type: text/plain; charset=us-ascii

      This is explicitly typed plain ASCII text.
      It DOES end with a linebreak.

      --simple boundary--
      This is the epilogue.  It is also to be ignored.

   The use of a Content-Type of multipart in a body part within another
   multipart entity is explicitly allowed.  In such cases, for obvious
   reasons, care must be taken to ensure that each nested multipart
   entity must use a different boundary delimiter. See Appendix C for an
   example of nested multipart entities.

   The use of the multipart Content-Type with only a single body part
   may be useful in certain contexts, and is explicitly permitted.

   The only mandatory parameter for the multipart Content-Type is the
   boundary parameter, which consists of 1 to 70 characters from a set
   of characters known to be very robust through email gateways, and NOT
   ending with white space.  (If a boundary appears to end with white
   space, the white space must be presumed to have been added by a

   gateway, and must be deleted.)  It is formally specified by the
   following BNF:

   boundary := 0*69<bchars> bcharsnospace

   bchars := bcharsnospace / " "

   bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+" /"_"
                 / "," / "-" / "." / "/" / ":" / "=" / "?"

   Overall, the body of a multipart entity may be specified  as
   follows:

   multipart-body := preamble 1*encapsulation
                  close-delimiter epilogue

   encapsulation := delimiter body-part CRLF

   delimiter := "--" boundary CRLF ; taken from Content-Type field.
                                   ; There must be no space
                                   ; between "--" and boundary.

   close-delimiter := "--" boundary "--" CRLF ; Again, no space
   by "--",

   preamble := discard-text   ;  to  be  ignored upon receipt.

   epilogue := discard-text   ;  to  be  ignored upon receipt.

   discard-text := *(*text CRLF)

   body-part := <"message" as defined in RFC 822,
             with all header fields optional, and with the
             specified delimiter not occurring anywhere in
             the message body, either on a line by itself
             or as a substring anywhere.  Note that the
             semantics of a part differ from the semantics
             of a message, as described in the text.>

      NOTE: In certain transport enclaves, RFC 822 restrictions such as
      the one that limits bodies to printable ASCII characters may not
      be in force.  (That is, the transport domains may resemble
      standard Internet mail transport as specified in RFC821 and
      assumed by RFC822, but without certain restrictions.)  The
      relaxation of these restrictions should be construed as locally
      extending the definition of bodies, for example to include octets
      outside of the ASCII range, as long as these extensions are
      supported by the transport and adequately documented in the

      Content-Transfer-Encoding header field. However, in no event are
      headers (either message headers or body-part headers) allowed to
      contain anything other than ASCII characters.

      NOTE: Conspicuously missing from the multipart type is a notion of
      structured, related body parts.  In general, it seems premature to
      try to standardize interpart structure yet.  It is recommended
      that those wishing to provide a more structured or integrated
      multipart messaging facility should define a subtype of multipart
      that is syntactically identical, but that always expects the
      inclusion of a distinguished part that can be used to specify the
      structure and integration of the other parts, probably referring
      to them by their Content-ID field.  If this approach is used,
      other implementations will not recognize the new subtype, but will
      treat it as the primary subtype (multipart/mixed) and will thus be
      able to show the user the parts that are recognized.

7.2.2.     The Multipart/mixed (primary) subtype

   The primary subtype for multipart, "mixed", is intended for use when
   the body parts are independent and need to be bundled in a particular
   order.  Any multipart subtypes that an implementation does not
   recognize must be treated as being of subtype "mixed".

7.2.3.     The Multipart/alternative subtype

   The multipart/alternative type is syntactically identical to
   multipart/mixed, but the semantics are different.  In particular,
   each of the parts is an "alternative" version of the same
   information.

   Systems should recognize that the content of the various parts are
   interchangeable.  Systems should choose the "best" type based on the
   local environment and preferences, in some cases even through user
   interaction.  As with multipart/mixed, the order of body parts is
   significant.  In this case, the alternatives appear in an order of
   increasing faithfulness to the original content. In general, the best
   choice is the LAST part of a type supported by the recipient system's
   local environment.

   Multipart/alternative may be used, for example, to send mail in a
   fancy text format in such a way that it can easily be displayed
   anywhere:

   From:  Nathaniel Borenstein <nsb@bellcore.com>
   To: Ned Freed <ned@innosoft.com>
   Subject: Formatted text mail
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary=boundary42

   --boundary42

   Content-Type: text/plain; charset=us-ascii

      ...plain text version of message goes here....
   --boundary42
   Content-Type: text/richtext

      .... RFC 1341 richtext version of same message goes here ...
   --boundary42
   Content-Type: text/x-whatever

      .... fanciest formatted version of same  message  goes  here
      ...
   --boundary42--

   In this example, users whose mail system understood the "text/x-
   whatever" format would see only the fancy version, while other users
   would see only the richtext or plain text version, depending on the
   capabilities of their system.

   In general, user agents that compose multipart/alternative entities
   must place the body parts in increasing order of preference, that is,
   with the preferred format last.  For fancy text, the sending user
   agent should put the plainest format first and the richest format
   last.  Receiving user agents should pick and display the last format
   they are capable of displaying.  In the case where one of the
   alternatives is itself of type "multipart" and contains unrecognized
   sub-parts, the user agent may choose either to show that alternative,
   an earlier alternative, or both.

      NOTE: From an implementor's perspective, it might seem more
      sensible to reverse this ordering, and have the plainest
      alternative last.  However, placing the plainest alternative first
      is the friendliest possible option when multipart/alternative
      entities are viewed using a non-MIME-conformant mail reader.
      While this approach does impose some burden on conformant mail
      readers, interoperability with older mail readers was deemed to be
      more important in this case.

   It may be the case that some user agents, if they can recognize more
   than one of the formats, will prefer to offer the user the choice of

   which format to view.  This makes sense, for example, if mail
   includes both a nicely-formatted image version and an easily-edited
   text version.  What is most critical, however, is that the user not
   automatically be shown multiple versions of the same data.  Either
   the user should be shown the last recognized version or should be
   given the choice.

   NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each
   part of a multipart/alternative entity represents the same data, but
   the mappings between the two are not necessarily without information
   loss.  For example, information is lost when translating ODA to
   PostScript or plain text.  It is recommended that each part should
   have a different Content-ID value in the case where the information
   content of the two parts is not identical.  However, where the
   information content is identical -- for example, where several parts
   of type "application/external- body" specify alternate ways to access
   the identical data -- the same Content-ID field value should be used,
   to optimize any cacheing mechanisms that might be present on the
   recipient's end.  However, it is recommended that the Content-ID
   values used by the parts should not be the same Content-ID value that
   describes the multipart/alternative as a whole, if there is any such
   Content-ID field.  That is, one Content-ID value will refer to the
   multipart/alternative entity, while one or more other Content-ID
   values will refer to the parts inside it.

7.2.4.     The Multipart/digest subtype

   This document defines a "digest" subtype of the multipart Content-
   Type.  This type is syntactically identical to multipart/mixed, but
   the semantics are different.  In particular, in a digest, the default
   Content-Type value for a body part is changed from "text/plain" to
   "message/rfc822".  This is done to allow a more readable digest
   format that is largely compatible (except for the quoting convention)
   with RFC 934.

   A digest in this format might, then, look something like this:

   From: Moderator-Address
   To: Recipient-List
   MIME-Version: 1.0
   Subject:  Internet Digest, volume 42
   Content-Type: multipart/digest;
        boundary="---- next message ----"

   ------ next message ----

   From: someone-else
   Subject: my opinion

      ...body goes here ...

   ------ next message ----

   From: someone-else-again
   Subject: my different opinion

      ... another body goes here...

   ------ next message ------

7.2.5.     The Multipart/parallel subtype

   This document defines a "parallel" subtype of the multipart Content-
   Type.  This type is syntactically identical to multipart/mixed, but
   the semantics are different.  In particular, in a parallel entity,
   the order of body parts is not significant.

   A common presentation of this type is to display all of the parts
   simultaneously on hardware and software that are capable of doing so.
   However, composing agents should be aware that many mail readers will
   lack this capability and will show the parts serially in any event.

7.2.6.     Other Multipart subtypes

   Other multipart subtypes are expected in the future.  MIME
   implementations must in general treat unrecognized subtypes of
   multipart as being equivalent to "multipart/mixed".

   The formal grammar for content-type header fields for multipart data
   is given by:

   multipart-type := "multipart" "/" multipart-subtype
                  ";" "boundary" "=" boundary

   multipart-subtype := "mixed" / "parallel" / "digest"
                  / "alternative" / extension-token

7.3.  The Message Content-Type

   It is frequently desirable, in sending mail, to encapsulate another
   mail message. For this common operation, a special Content-Type,
   "message", is defined.  The primary subtype, message/rfc822, has no
   required parameters in the Content-Type field.  Additional subtypes,
   "partial" and "External-body", do have required parameters.  These
   subtypes are explained below.

      NOTE: It has been suggested that subtypes of message might be
      defined for forwarded or rejected messages.  However, forwarded
      and rejected messages can be handled as multipart messages in
      which the first part contains any control or descriptive
      information, and a second part, of type message/rfc822, is the
      forwarded or rejected message.  Composing rejection and forwarding
      messages in this manner will preserve the type information on the
      original message and allow it to be correctly presented to the
      recipient, and hence is strongly encouraged.

   As stated in the definition of the Content-Transfer-Encoding field,
   no encoding other than "7bit", "8bit", or "binary" is permitted for
   messages or parts of type "message".  Even stronger restrictions
   apply to the subtypes "message/partial" and "message/external-body",
   as specified below.  The message header fields are always US-ASCII in
   any case, and data within the body can still be encoded, in which
   case the Content-Transfer-Encoding header field in the encapsulated
   message will reflect this.  Non-ASCII text in the headers of an
   encapsulated message can be specified using the mechanisms described
   in [RFC-1522].

   Mail gateways, relays, and other mail handling agents are commonly
   known to alter the top-level header of an RFC 822 message.  In
   particular, they frequently add, remove, or reorder header fields.
   Such alterations are explicitly forbidden for the encapsulated
   headers embedded in the bodies of messages of type "message."

7.3.1.     The Message/rfc822 (primary) subtype

   A Content-Type of "message/rfc822" indicates that the body contains
   an encapsulated message, with the syntax of an RFC 822 message.
   However, unlike top-level RFC 822 messages, it is not required that
   each message/rfc822 body must include a "From", "Subject", and at
   least one destination header.

   It should be noted that, despite the use of the numbers "822", a

   message/rfc822 entity can include enhanced information as defined in
   this document.  In other words, a message/rfc822 message may be a
   MIME message.

7.3.2.     The Message/Partial subtype

   A subtype of message, "partial", is defined in order to allow large
   objects to be delivered as several separate pieces of mail and
   automatically reassembled by the receiving user agent.  (The concept
   is similar to IP fragmentation/reassembly in the basic Internet
   Protocols.)  This mechanism can be used when intermediate transport
   agents limit the size of individual messages that can be sent.
   Content-Type "message/partial" thus indicates that the body contains
   a fragment of a larger message.

   Three parameters must be specified in the Content-Type field of type
   message/partial: The first, "id", is a unique identifier, as close to
   a world-unique identifier as possible, to be used to match the parts
   together.  (In general, the identifier is essentially a message-id;
   if placed in double quotes, it can be any message-id, in accordance
   with the BNF for "parameter" given earlier in this specification.)
   The second, "number", an integer, is the part number, which indicates
   where this part fits into the sequence of fragments.  The third,
   "total", another integer, is the total number of parts. This third
   subfield is required on the final part, and is optional (though
   encouraged) on the earlier parts.  Note also that these parameters
   may be given in any order.

   Thus, part 2 of a 3-part message may have either of the following
   header fields:

                Content-Type: Message/Partial;
                     number=2; total=3;
                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com"

                Content-Type: Message/Partial;
                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
                     number=2

   But part 3 MUST specify the total number of parts:

                Content-Type: Message/Partial;
                     number=3; total=3;
                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com"

   Note that part numbering begins with 1, not 0.

   When the parts of a message broken up in this manner are put

   together, the result is a complete MIME entity, which may have its
   own Content-Type header field, and thus may contain any other data
   type.

   Message fragmentation and reassembly: The semantics of a reassembled
   partial message must be those of the "inner" message, rather than of
   a message containing the inner message.  This makes it possible, for
   example, to send a large audio message as several partial messages,
   and still have it appear to the recipient as a simple audio message
   rather than as an encapsulated message containing an audio message.
   That is, the encapsulation of the message is considered to be
   "transparent".

   When generating and reassembling the parts of a message/partial
   message, the headers of the encapsulated message must be merged with
   the headers of the enclosing entities.  In this process the following
   rules must be observed:

      (1) All of the header fields from the initial enclosing entity
      (part one), except those that start with "Content-" and the
      specific header fields "Message-ID", "Encrypted", and "MIME-
      Version", must be copied, in order, to the new message.

      (2) Only those header fields in the enclosed message which start
      with "Content-" and "Message-ID", "Encrypted", and "MIME-Version"
      must be appended, in order, to the header fields of the new
      message.  Any header fields in the enclosed message which do not
      start with "Content-" (except for "Message-ID", "Encrypted", and
      "MIME-Version") will be ignored.

      (3) All of the header fields from the second and any subsequent
      messages will be ignored.

   For example, if an audio message is broken into two parts, the first
   part might look something like this:

      X-Weird-Header-1: Foo
      From: Bill@host.com
      To: joe@otherhost.com
      Subject: Audio mail
      Message-ID: <id1@host.com>
      MIME-Version: 1.0
      Content-type: message/partial;
           id="ABC@host.com";
           number=1; total=2

      X-Weird-Header-1: Bar
      X-Weird-Header-2: Hello

      Message-ID: <anotherid@foo.com>
      MIME-Version: 1.0
      Content-type: audio/basic
      Content-transfer-encoding: base64

         ... first half of encoded audio data goes here...

   and the second half might look something like this:

      From: Bill@host.com
      To: joe@otherhost.com
      Subject: Audio mail
      MIME-Version: 1.0
      Message-ID: <id2@host.com>
      Content-type: message/partial;
           id="ABC@host.com"; number=2; total=2

         ... second half of encoded audio data goes here...

   Then, when the fragmented message is reassembled, the resulting
   message to be displayed to the user should look something like this:

      X-Weird-Header-1: Foo
      From: Bill@host.com
      To: joe@otherhost.com
      Subject: Audio mail
      Message-ID: <anotherid@foo.com>
      MIME-Version: 1.0
      Content-type: audio/basic
      Content-transfer-encoding: base64

         ... first half of encoded audio data goes here...
         ... second half of encoded audio data goes here...

   Note on encoding of MIME entities encapsulated inside message/partial
   entities: Because data of type "message" may never be encoded in
   base64 or quoted-printable, a problem might arise if message/partial
   entities are constructed in an environment that supports binary or
   8-bit transport.  The problem is that the binary data would be split
   into multiple message/partial objects, each of them requiring binary
   transport.  If such objects were encountered at a gateway into a 7-
   bit transport environment, there would be no way to properly encode
   them for the 7-bit world, aside from waiting for all of the parts,
   reassembling the message, and then encoding the reassembled data in
   base64 or quoted-printable.  Since it is possible that different
   parts might go through different gateways, even this is not an
   acceptable solution.  For this reason, it is specified that MIME
   entities of type message/partial must always have a content-

   transfer-encoding of 7-bit (the default).  In particular, even in
   environments that support binary or 8-bit transport, the use of a
   content-transfer-encoding of "8bit" or "binary" is explicitly
   prohibited for entities of type message/partial.

   It should be noted that, because some message transfer agents may
   choose to automatically fragment large messages, and because such
   agents may use different fragmentation thresholds, it is possible
   that the pieces of a partial message, upon reassembly, may prove
   themselves to comprise a partial message.  This is explicitly
   permitted.

   It should also be noted that the inclusion of a "References" field in
   the headers of the second and subsequent pieces of a fragmented
   message that references the Message-Id on the previous piece may be
   of benefit to mail readers that understand and track references.
   However, the generation of such "References" fields is entirely
   optional.

   Finally, it should be noted that the "Encrypted" header field has
   been made obsolete by Privacy Enhanced Messaging (PEM), but the rules
   above are believed to describe the correct way to treat it if it is
   encountered in the context of conversion to and from message/partial
   fragments.

7.3.3.     The Message/External-Body subtype

   The external-body subtype indicates that the actual body data are not
   included, but merely referenced.  In this case, the parameters
   describe a mechanism for accessing the external data.

   When an entity is of type "message/external-body", it consists of a
   header, two consecutive CRLFs, and the message header for the
   encapsulated message.  If another pair of consecutive CRLFs appears,
   this of course ends the message header for the encapsulated message.
   However, since the encapsulated message's body is itself external, it
   does NOT appear in the area that follows.  For example, consider the
   following message:

      Content-type: message/external-body; access-
      type=local-file;

           name="/u/nsb/Me.gif"

      Content-type:  image/gif
      Content-ID: <id42@guppylake.bellcore.com>
      Content-Transfer-Encoding: binary

      THIS IS NOT REALLY THE BODY!

   The area at the end, which might be called the "phantom body", is
   ignored for most external-body messages.  However, it may be used to
   contain auxiliary information for some such messages, as indeed it is
   when the access-type is "mail-server".  Of the access-types defined
   by this document, the phantom body is used only when the access-type
   is "mail-server".  In all other cases, the phantom body is ignored.

   The only always-mandatory parameter for message/external-body is
   "access-type"; all of the other parameters may be mandatory or
   optional depending on the value of access-type.

      ACCESS-TYPE -- A case-insensitive word, indicating the supported
      access mechanism by which the file or data may be obtained.
      Values include, but are not limited to, "FTP", "ANON-FTP", "TFTP",
      "AFS", "LOCAL-FILE", and "MAIL-SERVER".  Future values, except for
      experimental values beginning with "X-" must be registered with
      IANA, as described in Appendix E .

   In addition, the following three parameters are optional for ALL
   access-types:

      EXPIRATION -- The date (in the RFC 822 "date-time" syntax, as
      extended by RFC 1123 to permit 4 digits in the year field) after
      which the existence of the external data is not guaranteed.

      SIZE -- The size (in octets) of the data.  The intent of this
      parameter is to help the recipient decide whether or not to expend
      the necessary resources to retrieve the external data.  Note that
      this describes the size of the data in its canonical form, that
      is, before any Content- Transfer-Encoding has been applied or
      after the data have been decoded.

      PERMISSION -- A case-insensitive field that indicates whether or
      not it is expected that clients might also attempt to overwrite
      the data.  By default, or if permission is "read", the assumption
      is that they are not, and that if the data is retrieved once, it
      is never needed again.  If PERMISSION is "read-write", this
      assumption is invalid, and any local copy must be considered no
      more than a cache.  "Read" and "Read-write" are the only defined
      values of permission.

   The precise semantics of the access-types defined here are described
   in the sections that follow.

   The encapsulated headers in ALL message/external-body entities MUST
   include a Content-ID header field to give a unique identifier by

   which to reference the data.  This identifier may be used for
   cacheing mechanisms, and for recognizing the receipt of the data when
   the access-type is "mail-server".

   Note that, as specified here, the tokens that describe external-body
   data, such as file names and mail server commands, are required to be
   in the US-ASCII character set.  If this proves problematic in
   practice, a new mechanism may be required as a future extension to
   MIME, either as newly defined access-types for message/external-body
   or by some other mechanism.

   As with message/partial, it is specified that MIME entities of type
   message/external-body must always have a content-transfer-encoding of
   7-bit (the default).  In particular, even in environments that
   support binary or 8-bit transport, the use of a content-transfer-
   encoding of "8bit" or "binary" is explicitly prohibited for entities
   of type message/external-body.

7.3.3.1.  The "ftp" and "tftp" access-types

   An access-type of FTP or TFTP indicates that the message body is
   accessible as a file using the FTP [RFC-959] or TFTP [RFC-783]
   protocols, respectively.  For these access-types, the following
   additional parameters are mandatory:

      NAME -- The name of the file that contains the actual body data.

      SITE -- A machine from which the file may be obtained, using the
      given protocol. This must be a fully qualified domain name, not a
      nickname.

   Before any data are retrieved, using FTP, the user will generally
   need to be asked to provide a login id and a password for the machine
   named by the site parameter.  For security reasons, such an id and
   password are not specified as content-type parameters, but must be
   obtained from the user.

   In addition, the following parameters are optional:

      DIRECTORY -- A directory from which the data named by NAME should
      be retrieved.

      MODE -- A case-insensitive string indicating the mode to be used
      when retrieving the information.  The legal values for access-type
      "TFTP" are "NETASCII", "OCTET", and "MAIL", as specified by the
      TFTP protocol [RFC-783].  The legal values for access-type "FTP"
      are "ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a
      decimal integer, typically 8.  These correspond to the

      representation types "A" "E" "I" and "L n" as specified by the FTP
      protocol [RFC-959].  Note that "BINARY" and "TENEX" are not valid
      values for MODE, but that "OCTET" or "IMAGE" or "LOCAL8" should be
      used instead.  IF MODE is not specified, the default value is
      "NETASCII" for TFTP and "ASCII" otherwise.

7.3.3.2.  The "anon-ftp" access-type

   The "anon-ftp" access-type is identical to the "ftp" access type,
   except that the user need not be asked to provide a name and password
   for the specified site.  Instead, the ftp protocol will be used with
   login "anonymous" and a password that corresponds to the user's email
   address.

7.3.3.3.  The "local-file" and "afs" access-types

   An access-type of "local-file" indicates that the actual body is
   accessible as a file on the local machine.  An access-type of "afs"
   indicates that the file is accessible via the global AFS file system.
   In both cases, only a single parameter is required:

      NAME -- The name of the file that contains the actual body data.

   The following optional parameter may be used to describe the locality
   of reference for the data, that is, the site or sites at which the
   file is expected to be visible:

      SITE -- A domain specifier for a machine or set of machines that
      are known to have access to the data file.  Asterisks may be used
      for wildcard matching to a part of a domain name, such as
      "*.bellcore.com", to indicate a set of machines on which the data
      should be directly visible, while a single asterisk may be used to
      indicate a file that is expected to be universally available,
      e.g., via a global file system.

7.3.3.4.  The "mail-server" access-type

   The "mail-server" access-type indicates that the actual body is
   available from a mail server.  The mandatory parameter for this
   access-type is:

      SERVER -- The email address of the mail server from which the
      actual body data can be obtained.

   Because mail servers accept a variety of syntaxes, some of which is
   multiline, the full command to be sent to a mail server is not
   included as a parameter on the content-type line.  Instead, it is
   provided as the "phantom body" when the content-type is

   message/external-body and the access- type is mail-server.

   An optional parameter for this access-type is:

      SUBJECT -- The subject that is to be used in the mail that is sent
      to obtain the data. Note that keying mail servers on Subject lines
      is NOT recommended, but such mail servers are known to exist.

   Note that MIME does not define a mail server syntax.  Rather, it
   allows the inclusion of arbitrary mail server commands in the phantom
   body.  Implementations must include the phantom body in the body of
   the message it sends to the mail server address to retrieve the
   relevant data.

   It is worth noting that, unlike other access-types, mail-server
   access is asynchronous and will happen at an unpredictable time in
   the future.  For this reason, it is important that there be a
   mechanism by which the returned data can be matched up with the
   original message/external-body entity.  MIME mailservers must use the
   same Content-ID field on the returned message that was used in the
   original message/external-body entity, to facilitate such matching.

7.3.3.5.  Examples and Further Explanations

   With the emerging possibility of very wide-area file systems, it
   becomes very hard to know in advance the set of machines where a file
   will and will not be accessible directly from the file system.
   Therefore it may make sense to provide both a file name, to be tried
   directly, and the name of one or more sites from which the file is
   known to be accessible.  An implementation can try to retrieve remote
   files using FTP or any other protocol, using anonymous file retrieval
   or prompting the user for the necessary name and password.  If an
   external body is accessible via multiple mechanisms, the sender may
   include multiple parts of type message/external-body within an entity
   of type multipart/alternative.

   However, the external-body mechanism is not intended to be limited to
   file retrieval, as shown by the mail-server access-type.  Beyond
   this, one can imagine, for example, using a video server for external
   references to video clips.

   If an entity is of type "message/external-body", then the body of the
   entity will contain the header fields of the encapsulated message.
   The body itself is to be found in the external location.  This means
   that if the body of the "message/external-body" message contains two
   consecutive CRLFs, everything after those pairs is NOT part of the
   message itself.  For most message/external-body messages, this
   trailing area must simply be ignored.  However, it is a convenient

   place for additional data that cannot be included in the content-type
   header field.  In particular, if the "access-type" value is "mail-
   server", then the trailing area must contain commands to be sent to
   the mail server at the address given by the value of the SERVER
   parameter.

   The embedded message header fields which appear in the body of the
   message/external-body data must be used to declare the Content-type
   of the external body if it is anything other than plain ASCII text,
   since the external body does not have a header section to declare its
   type.  Similarly, any Content-transfer-encoding other than "7bit"
   must also be declared here.  Thus a complete message/external-body
   message, referring to a document in PostScript format, might look
   like this:

      From: Whomever
      To: Someone
      Subject: whatever
      MIME-Version: 1.0
      Message-ID: <id1@host.com>
      Content-Type: multipart/alternative; boundary=42
      Content-ID: <id001@guppylake.bellcore.com>

      --42
      Content-Type: message/external-body;
           name="BodyFormats.ps";
           site="thumper.bellcore.com";
           access-type=ANON-FTP;
           directory="pub";
           mode="image";
           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

      Content-type: application/postscript
      Content-ID: <id42@guppylake.bellcore.com>

      --42
      Content-Type: message/external-body;
           name="/u/nsb/writing/rfcs/RFC-MIME.ps";
           site="thumper.bellcore.com";
           access-type=AFS
           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

      Content-type: application/postscript
      Content-ID: <id42@guppylake.bellcore.com>

      --42
      Content-Type: message/external-body;
           access-type=mail-server

           server="listserv@bogus.bitnet";
           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

      Content-type: application/postscript
      Content-ID: <id42@guppylake.bellcore.com>

      get RFC-MIME.DOC

      --42--

   Note that in the above examples, the default Content-transfer-
   encoding of "7bit" is assumed for the external postscript data.

   Like the message/partial type, the message/external-body type is
   intended to be transparent, that is, to convey the data type in the
   external body rather than to convey a message with a body of that
   type.  Thus the headers on the outer and inner parts must be merged
   using the same rules as for message/partial.  In particular, this
   means that the Content-type header is overridden, but the From and
   Subject headers are preserved.

   Note that since the external bodies are not transported as mail, they
   need not conform to the 7-bit and line length requirements, but might
   in fact be binary files.  Thus a Content-Transfer-Encoding is not
   generally necessary, though it is permitted.

   Note that the body of a message of type "message/external-body" is
   governed by the basic syntax for an RFC 822 message.  In particular,
   anything before the first consecutive pair of CRLFs is header
   information, while anything after it is body information, which is
   ignored for most access-types.

   The formal grammar for content-type header fields for data of type
   message is given by:

   message-type := "message" "/" message-subtype

   message-subtype := "rfc822"
                   / "partial" 2#3partial-param
                   / "external-body" 1*external-param
                   / extension-token

   partial-param :=     (";" "id" "=" value)
              /  (";" "number" "=" 1*DIGIT)
              /  (";" "total" "=" 1*DIGIT)
         ; id & number required; total  required  for  last part

   external-param :=   (";" "access-type" "=" atype)

              / (";" "expiration" "=" date-time)
                   ; Note that date-time is quoted
              / (";" "size" "=" 1*DIGIT)
              / (";"  "permission"  "="  ("read"  /  "read-write"))
                   ; Permission is case-insensitive
              / (";" "name" "="  value)
              / (";" "site" "=" value)
              / (";" "dir" "=" value)
              / (";" "mode" "=" value)
              / (";" "server" "=" value)
              / (";" "subject" "=" value)
          ; access-type required;others required based on access-type

   atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
                  / "afs" / "mail-server" / extension-token
                  ; Case-insensitive

7.4.  The Application Content-Type

   The "application" Content-Type is to be used for data which do not
   fit in any of the other categories, and particularly for data to be
   processed by mail-based uses of application programs.  This is
   information which must be processed by an application before it is
   viewable or usable to a user.  Expected uses for Content-Type
   application include mail-based file transfer, spreadsheets, data for
   mail-based scheduling systems, and languages for "active"
   (computational) email.  (The latter, in particular, can pose security
   problems which must be understood by implementors, and are considered
   in detail in the discussion of the application/PostScript content-
   type.)

   For example, a meeting scheduler might define a standard
   representation for information about proposed meeting dates.  An
   intelligent user agent would use this information to conduct a dialog
   with the user, and might then send further mail based on that dialog.
   More generally, there have been several "active" messaging languages
   developed in which programs in a suitably specialized language are
   sent through the mail and automatically run in the recipient's
   environment.

   Such applications may be defined as subtypes of the "application"
   Content-Type.  This document defines two subtypes: octet-stream, and
   PostScript.

   In general, the subtype of application will often be the name of the
   application for which the data are intended.  This does not mean,
   however, that any application program name may be used freely as a
   subtype of application.  Such usages (other than subtypes beginning

   with "x-") must be registered with IANA, as described in Appendix E.

7.4.1.     The Application/Octet-Stream (primary) subtype

   The primary subtype of application, "octet-stream", may be used to
   indicate that a body contains binary data.  The set of possible
   parameters includes, but is not limited to:

      TYPE -- the general type or category of binary data.  This is
      intended as information for the human recipient rather than for
      any automatic processing.

      PADDING -- the number of bits of padding that were appended to the
      bit-stream comprising the actual contents to produce the enclosed
      byte-oriented data.  This is useful for enclosing a bit-stream in
      a body when the total number of bits is not a multiple of the byte
      size.

   An additional parameter, "conversions", was defined in [RFC-1341] but
   has been removed.

   RFC 1341 also defined the use of a "NAME" parameter which gave a
   suggested file name to be used if the data were to be written to a
   file.  This has been deprecated in anticipation of a separate
   Content-Disposition header field, to be defined in a subsequent RFC.

   The recommended action for an implementation that receives
   application/octet-stream mail is to simply offer to put the data in a
   file, with any Content-Transfer-Encoding undone, or perhaps to use it
   as input to a user-specified process.

   To reduce the danger of transmitting rogue programs through the mail,
   it is strongly recommended that implementations NOT implement a
   path-search mechanism whereby an arbitrary program named in the
   Content-Type parameter (e.g., an "interpreter=" parameter) is found
   and executed using the mail body as input.

7.4.2.     The Application/PostScript subtype

   A Content-Type of "application/postscript" indicates a PostScript
   program.  Currently two variants of the PostScript language are
   allowed; the original level 1 variant is described in [POSTSCRIPT]
   and the more recent level 2 variant is described in [POSTSCRIPT2].

   PostScript is a registered trademark of Adobe Systems, Inc.  Use of
   the MIME content-type "application/postscript" implies recognition of
   that trademark and all the rights it entails.

   The PostScript language definition provides facilities for internal
   labeling of the specific language features a given program uses. This
   labeling, called the PostScript document structuring conventions, is
   very general and provides substantially more information than just
   the language level.

   The use of document structuring conventions, while not required, is
   strongly recommended as an aid to interoperability.  Documents which
   lack proper structuring conventions cannot be tested to see whether
   or not they will work in a given environment.  As such, some systems
   may assume the worst and refuse to process unstructured documents.

   The execution of general-purpose PostScript interpreters entails
   serious security risks, and implementors are discouraged from simply
   sending PostScript email bodies to "off-the-shelf" interpreters.
   While it is usually safe to send PostScript to a printer, where the
   potential for harm is greatly constrained, implementors should
   consider all of the following before they add interactive display of
   PostScript bodies to their mail readers.

   The remainder of this section outlines some, though probably not all,
   of the possible problems with sending PostScript through the mail.

   Dangerous operations in the PostScript language include, but may not
   be limited to, the PostScript operators deletefile, renamefile,
   filenameforall, and file.  File is only dangerous when applied to
   something other than standard input or output. Implementations may
   also define additional nonstandard file operators; these may also
   pose a threat to security.  Filenameforall, the wildcard file search
   operator, may appear at first glance to be harmless. Note, however,
   that this operator has the potential to reveal information about what
   files the recipient has access to, and this information may itself be
   sensitive.  Message senders should avoid the use of potentially
   dangerous file operators, since these operators are quite likely to
   be unavailable in secure PostScript implementations.  Message-
   receiving and -displaying software should either completely disable
   all potentially dangerous file operators or take special care not to
   delegate any special authority to their operation. These operators
   should be viewed as being done by an outside agency when interpreting
   PostScript documents.  Such disabling and/or checking should be done
   completely outside of the reach of the PostScript language itself;
   care should be taken to insure that no method exists for re-enabling
   full-function versions of these operators.

   The PostScript language provides facilities for exiting the normal
   interpreter, or server, loop. Changes made in this "outer"
   environment are customarily retained across documents, and may in
   some cases be retained semipermanently in nonvolatile memory. The

   operators associated with exiting the interpreter loop have the
   potential to interfere with subsequent document processing. As such,
   their unrestrained use constitutes a threat of service denial.
   PostScript operators that exit the interpreter loop include, but may
   not be limited to, the exitserver and startjob operators.  Message-
   sending software should not generate PostScript that depends on
   exiting the interpreter loop to operate. The ability to exit will
   probably be unavailable in secure PostScript implementations.
   Message-receiving and -displaying software should, if possible,
   disable the ability to make retained changes to the PostScript
   environment, and eliminate the startjob and exitserver commands.  If
   these commands cannot be eliminated, the password associated with
   them should at least be set to a hard-to-guess value.

   PostScript provides operators for setting system-wide and device-
   specific parameters. These parameter settings may be retained across
   jobs and may potentially pose a threat to the correct operation of
   the interpreter.  The PostScript operators that set system and device
   parameters include, but may not be limited to, the setsystemparams
   and setdevparams operators.  Message-sending software should not
   generate PostScript that depends on the setting of system or device
   parameters to operate correctly. The ability to set these parameters
   will probably be unavailable in secure PostScript implementations.
   Message-receiving and -displaying software should, if possible,
   disable the ability to change system and device parameters.  If these
   operators cannot be disabled, the password associated with them
   should at least be set to a hard-to-guess value.

   Some PostScript implementations provide nonstandard facilities for
   the direct loading and execution of machine code.  Such facilities
   are quite obviously open to substantial abuse.  Message-sending
   software should not make use of such features. Besides being totally
   hardware- specific, they are also likely to be unavailable in secure
   implementations of PostScript.  Message-receiving and -displaying
   software should not allow such operators to be used if they exist.

   PostScript is an extensible language, and many, if not most,
   implementations of it provide a number of their own extensions. This
   document does not deal with such extensions explicitly since they
   constitute an unknown factor.  Message-sending software should not
   make use of nonstandard extensions; they are likely to be missing
   from some implementations. Message-receiving and -displaying software
   should make sure that any nonstandard PostScript operators are secure
   and don't present any kind of threat.

   It is possible to write PostScript that consumes huge amounts of
   various system resources. It is also possible to write PostScript
   programs that loop infinitely.  Both types of programs have the

   potential to cause damage if sent to unsuspecting recipients.
   Message-sending software should avoid the construction and
   dissemination of such programs, which is antisocial.  Message-
   receiving and -displaying software should provide appropriate
   mechanisms to abort processing of a document after a reasonable
   amount of time has elapsed. In addition, PostScript interpreters
   should be limited to the consumption of only a reasonable amount of
   any given system resource.

   Finally, bugs may exist in some PostScript interpreters which could
   possibly be exploited to gain unauthorized access to a recipient's
   system.  Apart from noting this possibility, there is no specific
   action to take to prevent this, apart from the timely correction of
   such bugs if any are found.

7.4.3.     Other Application subtypes

   It is expected that many other subtypes of application will be
   defined in the future.  MIME implementations must generally treat any
   unrecognized subtypes as being equivalent to application/octet-
   stream.

   The formal grammar for content-type header fields for application
   data is given by:

   application-type :=  "application" "/" application-subtype

   application-subtype := ("octet-stream" *stream-param)
                       / "postscript" / extension-token

   stream-param :=  (";" "type" "=" value)
                       / (";" "padding" "=" padding)

   padding := "0" / "1" /  "2" /  "3" / "4" / "5" / "6" / "7"

7.5.  The Image Content-Type

   A Content-Type of "image" indicates that the body contains an image.
   The subtype names the specific image format.  These names are case
   insensitive.  Two initial subtypes are "jpeg" for the JPEG format,
   JFIF encoding, and "gif" for GIF format [GIF].

   The list of image subtypes given here is neither exclusive nor
   exhaustive, and is expected to grow as more types are registered with
   IANA, as described in Appendix E.

   The formal grammar for the content-type header field for data of type
   image is given by:

   image-type := "image" "/" ("gif" / "jpeg" / extension-token)

7.6.  The Audio Content-Type

   A Content-Type of "audio" indicates that the body contains audio
   data.  Although there is not yet a consensus on an "ideal" audio
   format for use with computers, there is a pressing need for a format
   capable of providing interoperable behavior.

   The initial subtype of "basic" is specified to meet this requirement
   by providing an absolutely minimal lowest common denominator audio
   format.  It is expected that richer formats for higher quality and/or
   lower bandwidth audio will be defined by a later document.

   The content of the "audio/basic" subtype is audio encoded using 8-bit
   ISDN mu-law [PCM].  When this subtype is present, a sample rate of
   8000 Hz and a single channel is assumed.

   The formal grammar for the content-type header field for data of type
   audio is given by:

   audio-type := "audio" "/" ("basic" / extension-token)

7.7.  The Video Content-Type

   A Content-Type of "video" indicates that the body contains a time-
   varying-picture image, possibly with color and coordinated sound.
   The term "video" is used extremely generically, rather than with
   reference to any particular technology or format, and is not meant to
   preclude subtypes such as animated drawings encoded compactly.  The
   subtype "mpeg" refers to video coded according to the MPEG standard
   [MPEG].

   Note that although in general this document strongly discourages the
   mixing of multiple media in a single body, it is recognized that many
   so-called "video" formats include a representation for synchronized
   audio, and this is explicitly permitted for subtypes of "video".

   The formal grammar for the content-type header field for data of type
   video is given by:

   video-type := "video" "/" ("mpeg" / extension-token)

7.8.  Experimental Content-Type Values

   A Content-Type value beginning with the characters "X-" is a private
   value, to be used by consenting mail systems by mutual agreement.
   Any format without a rigorous and public definition must be named

   with an "X-" prefix, and publicly specified values shall never begin
   with "X-".  (Older versions of the widely-used Andrew system use the
   "X-BE2" name, so new systems should probably choose a different
   name.)

   In general, the use of "X-" top-level types is strongly discouraged.
   Implementors should invent subtypes of the existing types whenever
   possible.  The invention of new types is intended to be restricted
   primarily to the development of new media types for email, such as
   digital odors or holography, and not for new data formats in general.
   In many cases, a subtype of application will be more appropriate than
   a new top-level type.

8. Summary

   Using the MIME-Version, Content-Type, and Content-Transfer-Encoding
   header fields, it is possible to include, in a standardized way,
   arbitrary types of data objects with RFC 822 conformant mail
   messages.  No restrictions imposed by either RFC 821 or RFC 822 are
   violated, and care has been taken to avoid problems caused by
   additional restrictions imposed by the characteristics of some
   Internet mail transport mechanisms (see Appendix B). The "multipart"
   and "message" Content-Types allow mixing and hierarchical structuring
   of objects of different types in a single message.  Further Content-
   Types provide a standardized mechanism for tagging messages or body
   parts as audio, image, or several other kinds of data.  A
   distinguished parameter syntax allows further specification of data
   format details, particularly the specification of alternate character
   sets.  Additional optional header fields provide mechanisms for
   certain extensions deemed desirable by many implementors.  Finally, a
   number of useful Content-Types are defined for general use by
   consenting user agents, notably message/partial, and
   message/external-body.

9. Security Considerations

   Security issues are discussed in Section 7.4.2 and in Appendix F.
   Implementors should pay special attention to the security
   implications of any mail content-types that can cause the remote
   execution of any actions in the recipient's environment.  In such
   cases, the discussion of the application/postscript content-type in
   Section 7.4.2 may serve as a model for considering other content-
   types with remote execution capabilities.

10. Authors' Addresses

   For more information, the authors of this document may be contacted
   via Internet mail:

   Nathaniel S. Borenstein
   MRE 2D-296, Bellcore
   445 South St.
   Morristown, NJ 07962-1910

   Phone: +1 201 829 4270
   Fax:  +1 201 829 7019
   Email: nsb@bellcore.com

   Ned Freed
   Innosoft International, Inc.
   250 West First Street
   Suite 240
   Claremont, CA 91711

   Phone:  +1 909 624 7907
   Fax: +1 909 621 5319
   Email: ned@innosoft.com

   MIME is a result of the work of the Internet Engineering Task Force
   Working Group on Email Extensions. The chairman of that group, Greg
   Vaudreuil, may be reached at:

   Gregory M. Vaudreuil
   Tigon Corporation
   17060 Dallas Parkway
   Dallas Texas, 75248

   Phone:    +1 214-733-2722
   EMail: gvaudre@cnri.reston.va.us

11. 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       Timo Lehtinen
            Randall Atkinson              John R. MacMillan
            Philippe Brandon              Rick McGowan
            Kevin Carosso                 Leo Mclaughlin
            Uhhyung Choi                  Goli Montaser-Kohsari
            Cristian Constantinof         Keith Moore
            Mark Crispin                  Tom Moore
            Dave Crocker                  Erik Naggum
            Terry Crowley                 Mark Needleman
            Walt Daniels                  John Noerenberg
            Frank Dawson                  Mats Ohrman
            Hitoshi Doi                   Julian Onions
            Kevin Donnelly                Michael Patton
            Keith Edwards                 David J. Pepper
            Chris Eich                    Blake C. Ramsdell
            Johnny Eriksson               Luc Rooijakkers
            Craig Everhart                Marshall T. Rose
            Patrik Faeltstroem            Jonathan Rosenberg
            Erik E. Fair                  Jan Rynning
            Roger Fajman                  Harri Salminen
            Alain Fontaine                Michael Sanderson
            James M. Galvin               Masahiro Sekiguchi
            Philip Gladstone              Mark Sherman
            Thomas Gordon                 Keld Simonsen
            Phill Gross                   Bob Smart
            James Hamilton                Peter Speck
            Steve Hardcastle-Kille        Henry Spencer
            David Herron                  Einar Stefferud
            Bruce Howard                  Michael Stein
            Bill Janssen                  Klaus Steinberger
            Olle Jaernefors               Peter Svanberg
            Risto Kankkunen               James Thompson
            Phil Karn                     Steve Uhler
            Alan Katz                     Stuart Vance
            Tim Kehres                    Erik van der Poel
            Neil Katin                    Guido van Rossum
            Kyuho Kim                     Peter Vanderbilt
            Anders Klemets                Greg Vaudreuil
            John Klensin                  Ed Vielmetti
            Valdis Kletniek               Ryan Waldron

            Jim Knowles                   Wally Wedel
            Stev Knowles                  Sven-Ove Westberg
            Bob Kummerfeld                Brian Wideen
            Pekka Kytolaakso              John Wobus
            Stellan Lagerstrom            Glenn Wright
            Vincent Lau                   Rayan Zachariassen
            Donald Lindsay                David Zimmerman
            Marc Andreessen               Bob Braden
            Brian Capouch                 Peter Clitherow
            Dave Collier-Brown            John Coonrod
            Stephen Crocker               Jim Davis
            Axel Deininger                Dana S Emery
            Martin Forssen                Stephen Gildea
            Terry Gray                    Mark Horton
            Warner Losh                   Carlyn Lowery
            Laurence Lundblade            Charles Lynn
            Larry Masinter                Michael J. McInerny
            Jon Postel                    Christer Romson
            Yutaka Sato                   Markku Savela
            Richard Alan Schafer          Larry W. Virden
            Rhys Weatherly                Jay Weber
            Dave Wecker

The authors apologize for any omissions from this list, which are
certainly unintentional.

Appendix A -- Minimal MIME-Conformance

   The mechanisms described in this document are open-ended.  It is
   definitely not expected that all implementations will support all of
   the Content-Types described, 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.

   A mail user agent that is MIME-conformant MUST:

      1.  Always generate a "MIME-Version: 1.0" header field.

      2.  Recognize the Content-Transfer-Encoding header field, and
      decode all received data encoded with either the quoted-printable
      or base64 implementations.  Encode any data sent that is not in
      seven-bit mail-ready representation using one of these
      transformations and include the appropriate Content-Transfer-
      Encoding header field, unless the underlying transport mechanism
      supports non-seven-bit data, as SMTP does not.

      3.  Recognize and interpret the Content-Type header field, and
      avoid showing users raw data with a Content-Type field other than
      text.  Be able to send at least text/plain messages, with the
      character set specified as a parameter if it is not US-ASCII.

      4.  Explicitly handle the following Content-Type values, to at
      least the following extents:

      Text:

            -- Recognize and display "text" mail
                 with the character set "US-ASCII."

            -- Recognize other character sets at
                 least to the extent of being able
                 to inform the user about what
                 character set the message uses.

            -- 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 0-127.

            -- For unrecognized subtypes, show or
                 offer to show the user the "raw"
                 version of the data after
                 conversion of the content from
                 canonical form to local form.

       Message:

            -- Recognize and display at least the
                 primary (822) encapsulation.

       Multipart:

            -- Recognize the primary (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.

            -- Recognize the "alternative" subtype,
                 and avoid showing the user
                 redundant parts of
                 multipart/alternative mail.

            -- Treat any unrecognized subtypes as if
                 they were "mixed".

       Application:

            -- Offer the ability to remove either of
                 the two types of Content-Transfer-
                 Encoding defined in this document
                 and put the resulting information
                 in a user file.

      5.  Upon encountering any unrecognized Content- Type, an
      implementation must treat it as if it had a Content-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.  Unrecognized predefined
      types, which in a MIME-conformant mailer might still include
      audio, image, or video, should also be treated in this way.

   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.

Appendix B -- General 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 such a way 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.

   The following guidelines may be useful to anyone devising a data
   format (Content-Type) that will 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) 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
      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.

      (4) Lines longer than 76 characters may be wrapped or truncated in
      some environments. Line wrapping and line truncation are STRONGLY

      DISCOURAGED, but unavoidable in some cases. Applications which
      require long lines must somehow differentiate between soft and
      hard line breaks.  (A simple way to do this is to use the quoted-
      printable encoding.)

      (5) 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.

      (6) Many mail domains use variations on the 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
      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:

                        "'"  (ASCII code 39)
                        "("  (ASCII code 40)
                        ")"  (ASCII code 41)
                        "+"  (ASCII code 43)
                        ","  (ASCII code 44)
                        "-"  (ASCII code 45)
                        "."  (ASCII code 46)
                        "/"  (ASCII code 47)
                        ":"  (ASCII code 58)
                        "="  (ASCII code 61)
                        "?"  (ASCII code 63)

      A maximally portable mail representation, such as the base64
      encoding, will confine itself to relatively short lines of text in
      which the only meaningful characters are taken from this set of 73
      characters.

      (7) Some mail transport agents will corrupt data that includes
      certain literal strings.  In particular, a 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,
      "=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
   illegal practices are known to occur on established networks, and
   implementations should be robust in dealing with the bad effects they
   can cause.

Appendix C -- A Complex Multipart Example

   What follows is the outline of a complex multipart message.  This
   message has five parts to be displayed serially: two introductory
   plain text parts, an embedded multipart message, a richtext part, and
   a closing encapsulated text message in a non-ASCII character set.
   The embedded multipart message has two parts to be displayed in
   parallel, a picture and an audio fragment.

      MIME-Version: 1.0
      From: Nathaniel Borenstein <nsb@bellcore.com>
      To: Ned Freed <ned@innosoft.com>
      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.
      --unique-boundary-1

         ...Some text appears here...
      [Note that the preceding blank line means
      no header fields were given and this is text,
      with charset US ASCII.  It could have been
      done with explicit typing as in the next part.]

      --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.

      --unique-boundary-1
      Content-Type: multipart/parallel;
           boundary=unique-boundary-2

      --unique-boundary-2
      Content-Type: audio/basic
      Content-Transfer-Encoding: base64

         ... base64-encoded 8000 Hz single-channel
             mu-law-format audio data goes here....

      --unique-boundary-2
      Content-Type: image/gif
      Content-Transfer-Encoding: base64

         ... base64-encoded image data goes here....

      --unique-boundary-2--

      --unique-boundary-1
      Content-type: text/richtext

      This is <bold><italic>richtext.</italic></bold>
      <smaller>as defined in RFC 1341</smaller>
      <nl><nl>Isn't it
      <bigger><bigger>cool?</bigger></bigger>

      --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 ...

      --unique-boundary-1--

Appendix D -- Collected Grammar

   This appendix contains the complete BNF grammar for all the syntax
   specified by this document.

   By itself, however, this grammar is incomplete.  It refers to several
   entities that are defined by RFC 822.  Rather than reproduce those
   definitions here, and risk unintentional differences between the two,
   this document simply refers the reader to RFC 822 for the remaining
   definitions.  Wherever a term is undefined, it refers to the RFC 822
   definition.

   application-subtype := ("octet-stream" *stream-param)
                       / "postscript" / extension-token

   application-type :=  "application" "/" application-subtype

   attribute := token    ; case-insensitive

   atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
                  / "afs" / "mail-server" / extension-token
                  ; Case-insensitive

   audio-type := "audio" "/" ("basic" / extension-token)

   body-part := <"message" as defined in RFC 822,
            with all header fields optional, and with the
            specified delimiter not occurring anywhere in
            the message body, either on a line by itself
            or as a substring anywhere.>

      NOTE: In certain transport enclaves, RFC 822 restrictions such as
      the one that limits bodies to printable ASCII characters may not
      be in force.  (That is, the transport domains may resemble
      standard Internet mail transport as specified in RFC821 and
      assumed by RFC822, but without certain restrictions.)  The
      relaxation of these restrictions should be construed as locally
      extending the definition of bodies, for example to include octets
      outside of the ASCII range, as long as these extensions are
      supported by the transport and adequately documented in the
      Content-Transfer-Encoding header field. However, in no event are
      headers (either message headers or body-part headers) allowed to
      contain anything other than ASCII characters.

   boundary := 0*69<bchars> bcharsnospace

   bchars := bcharsnospace / " "

   bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  / "_"
                  / "," / "-" / "." / "/" / ":" / "=" / "?"

   charset := "us-ascii" / "iso-8859-1" / "iso-8859-2"/ "iso-8859-3"
        / "iso-8859-4" / "iso-8859-5" /  "iso-8859-6" / "iso-8859-7"
        / "iso-8859-8" / "iso-8859-9" / extension-token
        ; case insensitive

   close-delimiter := "--" boundary "--" CRLF;Again,no space by "--",

   content  := "Content-Type"  ":" type "/" subtype  *(";" parameter)
             ; case-insensitive matching of type and subtype

   delimiter := "--" boundary CRLF  ;taken from Content-Type field.
                                ; There must be no space
                                ; between "--" and boundary.

   description := "Content-Description" ":" *text

   discard-text := *(*text CRLF)

   encapsulation := delimiter body-part CRLF

   encoding := "Content-Transfer-Encoding" ":" mechanism

   epilogue := discard-text        ;  to  be  ignored upon receipt.

   extension-token :=  x-token / iana-token

   external-param :=   (";" "access-type" "=" atype)
                  / (";" "expiration" "=" date-time)

                       ; Note that date-time is quoted
                  / (";" "size" "=" 1*DIGIT)
                  / (";"  "permission"  "="  ("read" / "read-write"))
                       ; Permission is case-insensitive
                  / (";" "name" "="  value)
                  / (";" "site" "=" value)
                  / (";" "dir" "=" value)
                  / (";" "mode" "=" value)
                  / (";" "server" "=" value)
                  / (";" "subject" "=" value)
           ;access-type required; others required based on access-type

   iana-token := <a publicly-defined extension token,
             registered with IANA, as specified in
             appendix E>

   id :=  "Content-ID" ":" msg-id

   image-type := "image" "/" ("gif" / "jpeg" / extension-token)

   mechanism :=     "7bit"    ;  case-insensitive
                  / "quoted-printable"
                  / "base64"
                  / "8bit"
                  / "binary"
                  / x-token

   message-subtype := "rfc822"
                  / "partial" 2#3partial-param
                  / "external-body" 1*external-param
                  / extension-token

   message-type := "message" "/" message-subtype

   multipart-body :=preamble 1*encapsulation close-delimiter epilogue

   multipart-subtype := "mixed" / "parallel" / "digest"
                  / "alternative" / extension-token

   multipart-type := "multipart" "/" multipart-subtype
                  ";" "boundary" "=" boundary

   octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
        ; octet must be used for characters > 127, =, SPACE, or
   TAB,
        ; and is recommended for any characters not listed in
        ; Appendix B as "mail-safe".

   padding := "0" / "1" /  "2" /  "3" / "4" / "5" / "6" / "7"

   parameter := attribute "=" value

   partial-param :=     (";" "id" "=" value)
                  /  (";" "number" "=" 1*DIGIT)
                  /  (";" "total" "=" 1*DIGIT)
             ; id & number required;total required for last part

   preamble := discard-text       ;  to  be  ignored upon receipt.

   ptext := octet / <any ASCII character except "=", SPACE,  or TAB>

        ; characters not listed as "mail-safe" in Appendix B
        ; are also not recommended.

   quoted-printable := ([*(ptext / SPACE /  TAB)  ptext]  ["="] CRLF)
        ; Maximum line length of 76 characters excluding CRLF

   stream-param :=  (";" "type" "=" value)
                / (";" "padding" "=" padding)

   subtype := token  ; case-insensitive

   text-subtype := "plain" / extension-token

   text-type := "text" "/" text-subtype [";" "charset" "=" charset]

   token  :=  1*<any  (ASCII) CHAR except SPACE, CTLs, or tspecials>

   tspecials :=  "(" / ")" / "<" / ">" / "@"
              /  "," / ";" / ":" / "\" / <">
              /  "/" / "[" / "]" / "?" / "="
             ; Must be in quoted-string,
             ; to use within parameter values

   type :=     "application"     /  "audio"   ; case-insensitive
             / "image"           / "message"
             / "multipart"  / "text"
             / "video"           / extension-token
             ; All values case-insensitive

   value := token / quoted-string

   version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

   video-type := "video" "/" ("mpeg" / extension-token)

   x-token := <The two characters "X-" or "x-" followed, with no
              intervening white space, by any token>

Appendix E -- IANA Registration Procedures

   MIME has been carefully designed to have extensible mechanisms, and
   it is expected that the set of content-type/subtype pairs and their
   associated parameters will grow significantly with time.  Several
   other MIME fields, notably character set names, access-type
   parameters for the message/external-body type, and possibly even
   Content-Transfer-Encoding values, are likely to have new values
   defined over time.  In order to ensure that the set of such values is
   developed in an orderly, well-specified, and public manner, MIME
   defines a registration process which uses the Internet Assigned
   Numbers Authority (IANA) as a central registry for such values.

   In general, parameters in the content-type header field are used to
   convey supplemental information for various content types, and their
   use is defined when the content-type and subtype are defined.  New
   parameters should not be defined as a way to introduce new
   functionality.

   In order to simplify and standardize the registration process, this
   appendix gives templates for the registration of new values with
   IANA.  Each of these is given in the form of an email message
   template, to be filled in by the registering party.

   E.1  Registration of New Content-type/subtype Values

   Note that MIME is generally expected to be extended by subtypes.  If
   a new fundamental top-level type is needed, its specification must be
   published as an RFC or submitted in a form suitable to become an RFC,
   and be subject to the Internet standards process.

      To:  IANA@isi.edu
      Subject:  Registration of new MIME
           content-type/subtype

      MIME type name:

      (If the above is not an existing top-level MIME type,
      please explain why an existing type cannot be used.)

      MIME subtype name:

      Required parameters:

      Optional parameters:

      Encoding considerations:

      Security considerations:

      Published specification:

      (The published specification must be an Internet RFC or
      RFC-to-be if a new top-level type is being defined, and
      must be a publicly available specification in any
      case.)

      Person & email address to contact for further information:

   E.2  Registration of New Access-type Values
           for Message/external-body

      To:  IANA@isi.edu
      Subject:  Registration of new MIME Access-type for
           Message/external-body content-type

      MIME access-type name:

      Required parameters:

      Optional parameters:

      Published specification:

      (The published specification must be an Internet RFC or
      RFC-to-be.)

      Person & email address to contact for further information:

Appendix F -- Summary of the Seven Content-types

   Content-type: text

   Subtypes defined by this document:  plain

   Important Parameters: charset

   Encoding notes: quoted-printable generally preferred if an encoding
      is needed and the character set is mostly an ASCII superset.

   Security considerations: Rich text formats such as TeX and Troff
      often contain mechanisms for executing arbitrary commands or file
      system operations, and should not be used automatically unless
      these security problems have been addressed.  Even plain text may
      contain control characters that can be used to exploit the
      capabilities of "intelligent" terminals and cause security
      violations.  User interfaces designed to run on such terminals
      should be aware of and try to prevent such problems.

   ________________________________________________________
   Content-type: multipart

   Subtypes defined by  this  document: mixed, alternative,
        digest, parallel.

   Important Parameters: boundary

   Encoding notes: No content-transfer-encoding is permitted.

   ________________________________________________________
   Content-type: message

   Subtypes defined by this document: rfc822, partial, external-body

   Important Parameters: id, number, total, access-type, expiration,
      size, permission, name, site, directory, mode, server, subject

   Encoding notes: No content-transfer-encoding is permitted.
      Specifically, only "7bit" is permitted for "message/partial" or
      "message/external-body", and only "7bit", "8bit", or "binary" are
      permitted for other subtypes of "message".
   ______________________________________________________________
   Content-type: application

   Subtypes defined by this document:  octet-stream, postscript

   Important Parameters:  type, padding

   Deprecated Parameters: name and conversions were
                          defined in RFC 1341.

   Encoding notes: base64 preferred for unreadable subtypes.

   Security considerations:  This  type  is  intended  for  the
   transmission  of data to be interpreted by locally-installed
   programs.  If used,  for  example,  to  transmit  executable
   binary  programs  or programs in general-purpose interpreted
   languages, such as LISP programs or  shell  scripts,  severe
   security  problems  could  result.   Authors of mail-reading
   agents are cautioned against giving their systems the  power
   to  execute  mail-based  application  data without carefully
   considering  the  security  implications.    While   it   is
   certainly  possible  to  define safe application formats and
   even safe interpreters for unsafe formats, each  interpreter
   should   be   evaluated  separately  for  possible  security
   problems.
   ________________________________________________________________
   Content-type: image

   Subtypes defined by this document:  jpeg, gif

   Important Parameters: none

   Encoding notes: base64 generally preferred
   ________________________________________________________________
   Content-type: audio

   Subtypes defined by this document:  basic

   Important Parameters: none

   Encoding notes: base64 generally preferred
   ________________________________________________________________
   Content-type: video

   Subtypes defined by this document:  mpeg

   Important Parameters: none

   Encoding notes: base64 generally preferred

Appendix G -- Canonical Encoding Model

   There was some confusion, in earlier drafts of this memo, 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 RFC 1421 and are performed for each 'innermost
   level' body:

   Step 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 specified by the type/subtype information.

   Step 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 content 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
   content types.  If character set conversion is involved, however,
   care must be taken to understand the semantics of the content-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 RFC822.  Note that the
   restriction on line lengths implied by RFC822 is eliminated if the
   next step employs either quoted-printable or base64 encoding.

   Step 3.  Apply transfer encoding.

   A Content-Transfer-Encoding appropriate for this body is applied.
   Note that there is no fixed relationship between the content 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.

   Step 4.  Insertion into entity.

   The encoded object 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) if needed.

   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 RFC822.  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 RFC822 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.

Appendix H -- Changes from RFC 1341

   This document is a relatively minor revision  of  RFC  1341.  For
   the  convenience  of  those familiar with RFC 1341, the technical
   changes from that document are summarized in  this appendix.

   1.  The definition of "tspecials" has been changed to no longer
   include ".".

   2.  The Content-ID field is now mandatory for message/external-body
   parts.

   3.  The text/richtext type (including the old Section 7.1.3 and
   Appendix D) has been moved to a separate document.

   4.  The rules on header merging for message/partial data have been
   changed to treat the Encrypted and MIME-Version headers as special
   cases.

   5.  The definition of the external-body access-type parameter has
   been changed so that it can only indicate a single access method
   (which was all that made sense).

   6.  There is a new "Subject" parameter for message/external-body,
   access-type mail-server, to permit MIME-based use of mail servers
   that rely on Subject field information.

   7.  The "conversions" parameter for application/octet-stream has been
   removed.

   8.  Section 7.4.1 now deprecates the use of the "name" parameter for
   application/octet-stream, as this will be superseded in the future by
   a Content-Disposition header.

   9.  The formal grammar for multipart bodies has been changed so that
   a CRLF is no longer required before the first boundary line.

   10.  MIME entities of type "message/partial" and "message/external-
   body" are now required to use only the "7bit" transfer-encoding.
   (Specifically, "binary" and "8bit" are not permitted.)

   11.  The "application/oda" content-type has been removed.

   12.  A note has been added to the end of section 7.2.3, explaining
   the semantics of Content-ID in a multipart/alternative MIME entity.

   13.  The formal syntax for the "MIME-Version" field has been
   tightened, but in a way that is completely compatible with the only

   version number defined in RFC 1341.

   14.  In Section 7.3.1, the definition of message/rfc822 has been
   relaxed regarding mandatory fields.

   All other changes from RFC 1341 were editorial changes and do not
   affect the technical content of MIME.  Considerable formal grammar
   has been added, but this reflects the prose specification that was
   already in place.

References

   [US-ASCII] Coded Character Set--7-Bit American Standard Code for
   Information Interchange, ANSI X3.4-1986.

   [ATK] Borenstein, Nathaniel S., Multimedia Applications Development
   with the Andrew Toolkit, Prentice-Hall, 1990.

   [GIF] Graphics Interchange Format (Version 89a), Compuserve, Inc.,
   Columbus, Ohio, 1990.

   [ISO-2022] International Standard--Information Processing--ISO 7-bit
   and 8-bit coded character sets--Code extension techniques, ISO
   2022:1986.

   [ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic
   Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987.  Part
   2: Latin alphabet No.  2, ISO 8859-2, 1987.  Part 3: Latin alphabet
   No. 3, ISO 8859-3, 1988.  Part 4: Latin alphabet No.  4, ISO 8859-4,
   1988.  Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.  Part 6:
   Latin/Arabic alphabet, ISO 8859-6, 1987.  Part 7: Latin/Greek
   alphabet, ISO 8859-7, 1987.  Part 8: Latin/Hebrew alphabet, ISO
   8859-8, 1988.  Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.

   [ISO-646] International Standard--Information Processing--ISO 7-bit
   coded character set for information interchange, ISO 646:1983.

   [MPEG] Video Coding Draft Standard ISO 11172 CD, ISO IEC/TJC1/SC2/WG11
   (Motion Picture Experts Group), May, 1991.

   [PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972,
   "Pulse Code Modulation (PCM) of Voice Frequencies".

   [POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference
   Manual, Addison-Wesley, 1985.

   [POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference
   Manual, Addison-Wesley, Second Edition, 1990.

   [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.

   [RFC-783] Sollins, K., "TFTP Protocol (revision 2)", RFC 783, MIT,
   June 1981.

   [RFC-821] Postel, J., "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", STD 11, RFC 1049, CMU, March 1988.

   [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-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.

   [RFC-1342] Moore, K., "Representation of Non-Ascii Text in Internet
   Message Headers", RFC 1342, University of Tennessee, June 1992.

   [RFC-1343] Borenstein, N., "A User Agent Configuration Mechanism
   for Multimedia Mail Format Information", RFC 1343, Bellcore, 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-1426] Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,
   Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIME
   transport", RFC 1426, United Nations Universit, Innosoft, Dover Beach
   Consulting, Inc., Network Management Associates, Inc., The Branch
   Office, February 1993.

   [RFC-1522] Moore, K., "Representation of Non-Ascii Text in Internet
   Message Headers" RFC 1522, University of Tennessee, September 1993.

   [RFC-1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
   1340, USC/Information Sciences Institute, July 1992.

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