Unix System Administration Handbook 1997 October/usah_oct97.iso/rfc/2000s/rfc2046.txt

Text File | 1996-11-25 | 106KB | 2,468 lines

Network Working Group                                          N. Freed
Request for Comments: 2046                                     Innosoft
Obsoletes: 1521, 1522, 1590                               N. Borenstein
Category: Standards Track                                 First Virtual
                                                          November 1996


                 Multipurpose Internet Mail Extensions
                            (MIME) Part Two:
                              Media Types

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   STD 11, RFC 822 defines a message representation protocol specifying
   considerable detail about US-ASCII message headers, but which leaves
   the message content, or message body, as flat US-ASCII text.  This
   set of documents, collectively called the Multipurpose Internet Mail
   Extensions, or MIME, redefines the format of messages to allow for

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

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

    (3)   multi-part message bodies, and

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

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

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



Freed & Borenstein          Standards Track                     [Page 1]

RFC 2046                      Media Types                  November 1996


   data in Internet mail header fields. The fourth document, RFC 2048,
   specifies various IANA registration procedures for MIME-related
   facilities.  The fifth and final document, RFC 2049, describes MIME
   conformance criteria as well as providing some illustrative examples
   of MIME message formats, acknowledgements, and the bibliography.

   These documents are revisions of RFCs 1521 and 1522, which themselves
   were revisions of RFCs 1341 and 1342.  An appendix in RFC 2049
   describes differences and changes from previous versions.

Table of Contents

   1. Introduction .........................................    3
   2. Definition of a Top-Level Media Type .................    4
   3. Overview Of The Initial Top-Level Media Types ........    4
   4. Discrete Media Type Values ...........................    6
   4.1 Text Media Type .....................................    6
   4.1.1 Representation of Line Breaks .....................    7
   4.1.2 Charset Parameter .................................    7
   4.1.3 Plain Subtype .....................................   11
   4.1.4 Unrecognized Subtypes .............................   11
   4.2 Image Media Type ....................................   11
   4.3 Audio Media Type ....................................   11
   4.4 Video Media Type ....................................   12
   4.5 Application Media Type ..............................   12
   4.5.1 Octet-Stream Subtype ..............................   13
   4.5.2 PostScript Subtype ................................   14
   4.5.3 Other Application Subtypes ........................   17
   5. Composite Media Type Values ..........................   17
   5.1 Multipart Media Type ................................   17
   5.1.1 Common Syntax .....................................   19
   5.1.2 Handling Nested Messages and Multiparts ...........   24
   5.1.3 Mixed Subtype .....................................   24
   5.1.4 Alternative Subtype ...............................   24
   5.1.5 Digest Subtype ....................................   26
   5.1.6 Parallel Subtype ..................................   27
   5.1.7 Other Multipart Subtypes ..........................   28
   5.2 Message Media Type ..................................   28
   5.2.1 RFC822 Subtype ....................................   28
   5.2.2 Partial Subtype ...................................   29
   5.2.2.1 Message Fragmentation and Reassembly ............   30
   5.2.2.2 Fragmentation and Reassembly Example ............   31
   5.2.3 External-Body Subtype .............................   33
   5.2.4 Other Message Subtypes ............................   40
   6. Experimental Media Type Values .......................   40
   7. Summary ..............................................   41
   8. Security Considerations ..............................   41
   9. Authors' Addresses ...................................   42



Freed & Borenstein          Standards Track                     [Page 2]

RFC 2046                      Media Types                  November 1996


   A. Collected Grammar ....................................   43

1.  Introduction

   The first document in this set, RFC 2045, defines a number of header
   fields, including Content-Type. The Content-Type field is used to
   specify the nature of the data in the body of a MIME entity, by
   giving media type and subtype identifiers, and by providing auxiliary
   information that may be required for certain media 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
   ordering of parameters is not significant.

   In general, the top-level media type is used to declare the general
   type of data, while the subtype specifies a specific format for that
   type of data.  Thus, a media 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 "text", "image", "audio", and "video" should
   not contain embedded information that is really of a different type.
   Such compound formats should be represented using the "multipart" or
   "application" types.

   Parameters are modifiers of the media subtype, and as such do not
   fundamentally affect the nature of the content.  The set of
   meaningful parameters depends on the media type and subtype.  Most
   parameters are associated with a single specific subtype.  However, a
   given top-level media type may define parameters which are applicable
   to any subtype of that type.  Parameters may be required by their
   defining media type or subtype or they may be optional.  MIME
   implementations must also ignore any parameters whose names they do
   not recognize.

   MIME's Content-Type header field and media type mechanism has been
   carefully designed to be extensible, and it is expected that the set
   of media type/subtype pairs and their associated parameters will grow
   significantly over time.  Several other MIME facilities, such as
   transfer encodings and "message/external-body" access types, 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 sets up a registration process which uses the
   Internet Assigned Numbers Authority (IANA) as a central registry for
   MIME's various areas of extensibility.  The registration process for
   these areas is described in a companion document, RFC 2048.



Freed & Borenstein          Standards Track                     [Page 3]

RFC 2046                      Media Types                  November 1996


   The initial seven standard top-level media type are defined and
   described in the remainder of this document.

2.  Definition of a Top-Level Media Type

   The definition of a top-level media type consists of:

    (1)   a name and a description of the type, including
          criteria for whether a particular type would qualify
          under that type,

    (2)   the names and definitions of parameters, if any, which
          are defined for all subtypes of that type (including
          whether such parameters are required or optional),

    (3)   how a user agent and/or gateway should handle unknown
          subtypes of this type,

    (4)   general considerations on gatewaying entities of this
          top-level type, if any, and

    (5)   any restrictions on content-transfer-encodings for
          entities of this top-level type.

3.  Overview Of The Initial Top-Level Media Types

   The five discrete top-level media types are:

    (1)   text -- textual information.  The subtype "plain" in
          particular indicates plain text containing no
          formatting commands or directives of any sort. Plain
          text is intended to be displayed "as-is". No special
          software is required to get the full meaning of the
          text, aside from support for the indicated character
          set. Other 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 of "text" thus include any
          word processor format that can be read without
          resorting to software that understands the format.  In
          particular, formats that employ embeddded binary
          formatting information are not considered directly
          readable. A very simple and portable subtype,
          "richtext", was defined in RFC 1341, with a further
          revision in RFC 1896 under the name "enriched".





Freed & Borenstein          Standards Track                     [Page 4]

RFC 2046                      Media Types                  November 1996


    (2)   image -- image data.  "Image" requires a display device
          (such as a graphical display, a graphics printer, or a
          FAX machine) to view the information. An initial
          subtype is defined for the widely-used image format
          JPEG. .  subtypes are defined for two widely-used image
          formats, jpeg and gif.

    (3)   audio -- audio data.  "Audio" requires an audio output
          device (such as a speaker or a telephone) to "display"
          the contents.  An initial subtype "basic" is defined in
          this document.

    (4)   video -- video data.  "Video" requires the capability
          to display moving images, typically including
          specialized hardware and software.  An initial subtype
          "mpeg" is defined in this document.

    (5)   application -- some other kind of data, typically
          either uninterpreted binary data or information to be
          processed by an application.  The 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.  The "PostScript" subtype is also defined
          for the transport of PostScript material.  Other
          expected uses for "application" include spreadsheets,
          data for mail-based scheduling systems, and languages
          for "active" (computational) messaging, and word
          processing formats that are not directly readable.
          Note that security considerations may exist for some
          types of application data, most notably
          "application/PostScript" and any form of active
          messaging.  These issues are discussed later in this
          document.

   The two composite top-level media types are:

    (1)   multipart -- data consisting of multiple entities of
          independent data types.  Four subtypes are initially
          defined, including the basic "mixed" subtype specifying
          a generic mixed set of parts, "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 has a default type of "message/rfc822".






Freed & Borenstein          Standards Track                     [Page 5]

RFC 2046                      Media Types                  November 1996


    (2)   message -- an encapsulated message.  A body of media
          type "message" is itself all or a portion of some kind
          of message object.  Such objects may or may not in turn
          contain other entities.  The "rfc822" subtype is used
          when the encapsulated content is itself an RFC 822
          message.  The "partial" subtype is defined for partial
          RFC 822 messages, to permit the fragmented transmission
          of bodies that are thought to be too large to be passed
          through transport facilities in one piece.  Another
          subtype, "external-body", is defined for specifying
          large bodies by reference to an external data source.

   It should be noted that the list of media 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.

4.  Discrete Media Type Values

   Five of the seven initial media type values refer to discrete bodies.
   The content of these types must be handled by non-MIME mechanisms;
   they are opaque to MIME processors.

4.1.  Text Media Type

   The "text" media type is intended for sending material which is
   principally textual in form.  A "charset" parameter may be used to
   indicate the character set of the body text for "text" subtypes,
   notably including the subtype "text/plain", which is a generic
   subtype for plain text.  Plain text does not provide for or allow
   formatting commands, font attribute specifications, processing
   instructions, interpretation directives, or content markup.  Plain
   text is seen simply as a linear sequence of characters, possibly
   interrupted by line breaks or page breaks.  Plain text may allow the
   stacking of several characters in the same position in the text.
   Plain text in scripts like Arabic and Hebrew may also include
   facilitites that allow the arbitrary mixing of text segments with
   opposite writing directions.

   Beyond plain text, there are many formats for representing what might
   be known as "rich text".  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".



Freed & Borenstein          Standards Track                     [Page 6]

RFC 2046                      Media Types                  November 1996


4.1.1.  Representation of Line Breaks

   The canonical form of any MIME "text" subtype MUST always represent a
   line break as a CRLF sequence.  Similarly, any occurrence of CRLF in
   MIME "text" MUST represent a line break.  Use of CR and LF outside of
   line break sequences is also forbidden.

   This rule applies regardless of format or character set or sets
   involved.

   NOTE: The proper interpretation of line breaks when a body is
   displayed depends on the media type. In particular, while it is
   appropriate to treat a line break as a transition to a new line when
   displaying a "text/plain" body, this treatment is actually incorrect
   for other subtypes of "text" like "text/enriched" [RFC-1896].
   Similarly, whether or not line breaks should be added during display
   operations is also a function of the media type. It should not be
   necessary to add any line breaks to display "text/plain" correctly,
   whereas proper display of "text/enriched" requires the appropriate
   addition of line breaks.

   NOTE: Some protocols defines a maximum line length.  E.g. SMTP [RFC-
   821] allows a maximum of 998 octets before the next CRLF sequence.
   To be transported by such protocols, data which includes too long
   segments without CRLF sequences must be encoded with a suitable
   content-transfer-encoding.

4.1.2.  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=iso-8859-1

   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.  For other subtypes of "text"
   than "text/plain", the semantics of the "charset" parameter should be
   defined to 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 to the implications of multioctet character sets for their
   subtype definitions.



Freed & Borenstein          Standards Track                     [Page 7]

RFC 2046                      Media Types                  November 1996


   The charset parameter for subtypes of "text" gives a name of a
   character set, as "character set" is defined in RFC 2045.  The rules
   regarding line breaks detailed in the previous section must also be
   observed -- a character set whose definition does not conform to
   these rules cannot be used in a MIME "text" subtype.

   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.

   Other media types than subtypes of "text" might choose to employ the
   charset parameter as defined here, but with the CRLF/line break
   restriction removed.  Therefore, all character sets that conform to
   the general definition of "character set" in RFC 2045 can be
   registered for MIME use.

   Note that if the specified character set includes 8-bit characters
   and such characters are used in the body, 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 [RFC-821].

   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 as a media type parameter in the Content-Type
   header field. "US-ASCII" does not indicate an arbitrary 7-bit
   character set, but specifies that all octets in the body must be
   interpreted as characters according to the US-ASCII character set.
   National and application-oriented versions of ISO 646 [ISO-646] are
   usually NOT identical to US-ASCII, and in that case their use in
   Internet mail is explicitly discouraged.  The omission of the ISO 646
   character set from this document is deliberate in this regard.  The
   character set name of "US-ASCII" explicitly refers to the character
   set defined in ANSI X3.4-1986 [US- ASCII].  The new international
   reference version (IRV) of the 1991 edition of ISO 646 is identical
   to US-ASCII.  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 media 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



Freed & Borenstein          Standards Track                     [Page 8]

RFC 2046                      Media Types                  November 1996


   messages containing characters coded according to other versions of
   ISO 646 than US-ASCII and the 1991 IRV, or using code-switching
   procedures (e.g., those of ISO 2022), as well as 8bit or multiple
   octet character encodings MUST use an appropriate character set
   specification to be consistent with MIME.

   The complete US-ASCII character set is listed in ANSI X3.4- 1986.
   Note that the control characters including DEL (0-31, 127) have no
   defined meaning in apart from the combination CRLF (US-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)."  Aside from these
   conventions, any use of the control characters or DEL in a body must
   either occur

    (1)   because a subtype of text other than "plain"
          specifically assigns some additional meaning, or

    (2)   within the context 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: An enormous proliferation of character sets exist beyond US-
   ASCII.  A large number of partially or totally overlapping character
   sets is NOT a good thing.  A SINGLE character set that can be used
   universally for representing all of the world's languages in Internet
   mail would be preferrable.  Unfortunately, existing practice in
   several communities seems to point to the continued use of multiple
   character sets in the near future.  A small number of standard
   character sets are, therefore, defined for Internet use in this
   document.

   The defined charset values are:

    (1)   US-ASCII -- as defined in ANSI X3.4-1986 [US-ASCII].

    (2)   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 10.




Freed & Borenstein          Standards Track                     [Page 9]

RFC 2046                      Media Types                  November 1996


   Characters in the range 128-159 has no assigned meaning in ISO-8859-
   X.  Characters with values below 128 in ISO-8859-X have the same
   assigned meaning as they do in US-ASCII.

   Part 6 of ISO 8859 (Latin/Arabic alphabet) and part 8 (Latin/Hebrew
   alphabet) includes both characters for which the normal writing
   direction is right to left and characters for which it is left to
   right, but do not define a canonical ordering method for representing
   bi-directional text.  The charset values "ISO-8859-6" and "ISO-8859-
   8", however, specify that the visual method is used [RFC-1556].

   All of these character sets are used as pure 7bit or 8bit sets
   without any shift or escape functions.  The meaning of shift and
   escape sequences in these character sets is not defined.

   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.

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

   No character set name other than those defined above 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 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, composition software should always use the "lowest common
   denominator" character set possible.  For example, if a body contains
   only US-ASCII characters, it SHOULD 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
   should be labelled as being in that subset.  This will increase the
   chances that the recipient will be able to view the resulting entity
   correctly.



Freed & Borenstein          Standards Track                    [Page 10]

RFC 2046                      Media Types                  November 1996


4.1.3.  Plain Subtype

   The simplest and most important subtype of "text" is "plain".  This
   indicates plain text that does not contain any formatting commands or
   directives. Plain text is intended to be displayed "as-is", that is,
   no interpretation of embedded formatting commands, font attribute
   specifications, processing instructions, interpretation directives,
   or content markup should be necessary for proper display.  The
   default media type of "text/plain; charset=us-ascii" for Internet
   mail 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.

4.1.4.  Unrecognized Subtypes

   Unrecognized subtypes of "text" should be treated as subtype "plain"
   as long as the MIME implementation knows how to handle the charset.
   Unrecognized subtypes which also specify an unrecognized charset
   should be treated as "application/octet- stream".

4.2.  Image Media Type

   A media type of "image" indicates that the body contains an image.
   The subtype names the specific image format.  These names are not
   case sensitive. An initial subtype is "jpeg" for the JPEG format
   using JFIF encoding [JPEG].

   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 RFC 2048.

   Unrecognized subtypes of "image" should at a miniumum be treated as
   "application/octet-stream".  Implementations may optionally elect to
   pass subtypes of "image" that they do not specifically recognize to a
   secure and robust general-purpose image viewing application, if such
   an application is available.

   NOTE: Using of a generic-purpose image viewing application this way
   inherits the security problems of the most dangerous type supported
   by the application.

4.3.  Audio Media Type

   A media 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.



Freed & Borenstein          Standards Track                    [Page 11]

RFC 2046                      Media Types                  November 1996


   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 single channel audio
   encoded using 8bit ISDN mu-law [PCM] at a sample rate of 8000 Hz.

   Unrecognized subtypes of "audio" should at a miniumum be treated as
   "application/octet-stream".  Implementations may optionally elect to
   pass subtypes of "audio" that they do not specifically recognize to a
   robust general-purpose audio playing application, if such an
   application is available.

4.4.  Video Media Type

   A media type of "video" indicates that the body contains a time-
   varying-picture image, possibly with color and coordinated sound.
   The term 'video' is used in its most generic sense, 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".

   Unrecognized subtypes of "video" should at a minumum be treated as
   "application/octet-stream".  Implementations may optionally elect to
   pass subtypes of "video" that they do not specifically recognize to a
   robust general-purpose video display application, if such an
   application is available.

4.5.  Application Media Type

   The "application" media type is to be used for discrete data which do
   not fit in any of the other categories, and particularly for data to
   be processed by some type of application program.  This is
   information which must be processed by an application before it is
   viewable or usable by a user.  Expected uses for the "application"
   media type include file transfer, spreadsheets, data for mail-based
   scheduling systems, and languages for "active" (computational)
   material.  (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" media type.)




Freed & Borenstein          Standards Track                    [Page 12]

RFC 2046                      Media Types                  November 1996


   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 additional material based on that
   dialog.  More generally, there have been several "active" messaging
   languages developed in which programs in a suitably specialized
   language are transported to a remote location and automatically run
   in the recipient's environment.

   Such applications may be defined as subtypes of the "application"
   media type. This document defines two subtypes:

   octet-stream, and PostScript.

   The subtype of "application" will often be either the name or include
   part of 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".

4.5.1.  Octet-Stream Subtype

   The "octet-stream" subtype is used to indicate that a body contains
   arbitrary binary data.  The set of currently defined parameters is:

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

    (2)   PADDING -- the number of bits of padding that were
          appended to the bit-stream comprising the actual
          contents to produce the enclosed 8bit 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
          8.

   Both of these parameters are optional.

   An additional parameter, "CONVERSIONS", was defined in RFC 1341 but
   has since 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 an
   "application/octet-stream" entity 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.



Freed & Borenstein          Standards Track                    [Page 13]

RFC 2046                      Media Types                  November 1996


   To reduce the danger of transmitting rogue programs, 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 message body as input.

4.5.2.  PostScript Subtype

   A media 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 media type "application/postscript" implies recognition of
   that trademark and all the rights it entails.

   The PostScript language definition provides facilities for internal
   labelling of the specific language features a given program uses.
   This labelling, called the PostScript document structuring
   conventions, or DSC, 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 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 by typical printer environments,
   implementors should consider all of the following before they add
   interactive display of PostScript bodies to their MIME readers.

   The remainder of this section outlines some, though probably not all,
   of the possible problems with the transport of PostScript entities.

    (1)   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.



Freed & Borenstein          Standards Track                    [Page 14]

RFC 2046                      Media Types                  November 1996


          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.

    (2)   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, since the ability to exit
          will probably be unavailable in secure PostScript
          implementations.  Message receiving and displaying
          software should completely disable the ability to make
          retained changes to the PostScript environment by
          eliminating or disabling the "startjob" and
          "exitserver" operations.  If these operations cannot be
          eliminated or completely disabled the password
          associated with them should at least be set to a hard-
          to-guess value.

    (3)   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



Freed & Borenstein          Standards Track                    [Page 15]

RFC 2046                      Media Types                  November 1996


          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
          disable the ability to change system and device
          parameters.  If these operators cannot be completely
          disabled the password associated with them should at
          least be set to a hard-to-guess value.

    (4)   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.

    (5)   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.

    (6)   It is possible to write PostScript that consumes huge
          amounts of various system resources.  It is also
          possible to write PostScript programs that loop
          indefinitely.  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 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.





Freed & Borenstein          Standards Track                    [Page 16]

RFC 2046                      Media Types                  November 1996


    (7)   It is possible to include raw binary information inside
          PostScript in various forms.  This is not recommended
          for use in Internet mail, both because it is not
          supported by all PostScript interpreters and because it
          significantly complicates the use of a MIME Content-
          Transfer-Encoding.  (Without such binary, PostScript
          may typically be viewed as line-oriented data.  The
          treatment of CRLF sequences becomes extremely
          problematic if binary and line-oriented data are mixed
          in a single Postscript data stream.)

    (8)   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.

4.5.3.  Other Application Subtypes

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

5.  Composite Media Type Values

   The remaining two of the seven initial Content-Type values refer to
   composite entities.  Composite entities are handled using MIME
   mechanisms -- a MIME processor typically handles the body directly.

5.1.  Multipart Media Type

   In the case of multipart entities, in which one or more different
   sets of data are combined in a single body, a "multipart" media type
   field must appear in the entity's header.  The body must then contain
   one or more body parts, each preceded by a boundary delimiter line,
   and the last one followed by a closing boundary delimiter line.
   After its boundary delimiter line, each body part then consists of a
   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 an entity and hence 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 usually indicates that the corresponding body has



Freed & Borenstein          Standards Track                    [Page 17]

RFC 2046                      Media Types                  November 1996


   a content-type of "text/plain; charset=US-ASCII".

   The only header fields that have defined meaning for body parts are
   those the names of which begin with "Content-".  All other header
   fields may be ignored in body parts.  Although they should generally
   be retained if at all possible, 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 a JPEG image.  In order to represent
   the latter, the body part must have "Content-Type: message/rfc822",
   and its body (after the blank line) must be the encapsulated message,
   with its own "Content-Type: image/jpeg" 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 parts
   actually are messages, a "digest" subtype is also defined.)

   As stated previously, each body part is preceded by a boundary
   delimiter line that contains the boundary delimiter.  The boundary
   delimiter MUST NOT appear inside any of the encapsulated parts, on a
   line by itself or as the prefix of any line.  This implies that it is
   crucial that the composing agent be able to choose and specify a
   unique boundary parameter value that does not contain the boundary
   parameter value of an enclosing multipart as a prefix.

   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 those of an
   unrecognized subtype.

   As stated in the definition of the Content-Transfer-Encoding field
   [RFC 2045], no encoding other than "7bit", "8bit", or "binary" is
   permitted for entities of type "multipart".  The "multipart" boundary
   delimiters and header fields are always represented as 7bit US-ASCII
   in any case (though the header fields may encode non-US-ASCII header
   text as per RFC 2047) 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.



Freed & Borenstein          Standards Track                    [Page 18]

RFC 2046                      Media Types                  November 1996


5.1.1.  Common Syntax

   This section defines a common syntax for subtypes of "multipart".
   All subtypes of "multipart" must use this syntax.  A simple example
   of a multipart message also appears in this section.  An example of a
   more complex multipart message is given in RFC 2049.

   The Content-Type field for multipart entities requires one parameter,
   "boundary". The boundary delimiter line is then defined as a line
   consisting entirely of two hyphen characters ("-", decimal value 45)
   followed by the boundary parameter value from the Content-Type header
   field, optional linear whitespace, and a terminating CRLF.

   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 boundary
   parameter values 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 invalid
   Content-type fields.  Thus, a typical "multipart" Content-Type header
   field might look like this:

     Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p

   But the following is not valid:

     Content-Type: multipart/mixed; boundary=gc0pJq0M:08jU534c0p

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

     Content-Type: multipart/mixed; boundary="gc0pJq0M:08jU534c0p"

   This Content-Type value indicates that the content consists of one or
   more parts, each with a structure that is syntactically identical to
   an RFC 822 message, except that the header area is allowed to be
   completely empty, and that the parts are each preceded by the line




Freed & Borenstein          Standards Track                    [Page 19]

RFC 2046                      Media Types                  November 1996


     --gc0pJq0M:08jU534c0p

   The boundary delimiter MUST occur at the beginning of a line, i.e.,
   following a CRLF, and the initial CRLF is considered to be attached
   to the boundary delimiter line rather than part of the preceding
   part.  The boundary may be followed by zero or more characters of
   linear whitespace. It is then terminated by either 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.  If no Content-Type
   field is present it is assumed to be "message/rfc822" in a
   "multipart/digest" and "text/plain" otherwise.

   NOTE:  The CRLF preceding the boundary delimiter 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 boundary delimiter line, the first of which is part of
   the preceding body part, and the second of which is part of the
   encapsulation boundary.

   Boundary delimiters must not appear within the encapsulated material,
   and must be no longer than 70 characters, not counting the two
   leading hyphens.

   The boundary delimiter line following the last body part is a
   distinguished delimiter that indicates that no further body parts
   will follow.  Such a delimiter line is identical to the previous
   delimiter lines, with the addition of two more hyphens after the
   boundary parameter value.

     --gc0pJq0M:08jU534c0p--

   NOTE TO IMPLEMENTORS:  Boundary string comparisons must compare the
   boundary value with the beginning of each candidate line.  An exact
   match of the entire candidate line is not required; it is sufficient
   that the boundary appear in its entirety following the CRLF.

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



Freed & Borenstein          Standards Track                    [Page 20]

RFC 2046                      Media Types                  November 1996


   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 boundary delimiters must not appear in the body parts
   being encapsulated, a user agent must exercise care to choose a
   unique boundary parameter value.  The boundary parameter value in the
   example above could have been the result of an algorithm designed to
   produce boundary delimiters 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" boundary
   delimiters for a recipient with an old user agent, but would require
   more attention to the possibility that the boundary delimiter might
   appear at the beginning of some line in the encapsulated part.  The
   simplest boundary delimiter line possible is something like "---",
   with a closing boundary delimiter line 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 <[e-mail]>
     To: Ned Freed <[e-mail]>
     Date: Sun, 21 Mar 1993 23:56:48 -0800 (PST)
     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 composition agents to include an
     explanatory note to non-MIME conformant readers.

     --simple boundary

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

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

     --simple boundary--

     This is the epilogue.  It is also to be ignored.






Freed & Borenstein          Standards Track                    [Page 21]

RFC 2046                      Media Types                  November 1996


   The use of a media 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 uses a different boundary delimiter.  See RFC 2049 for an
   example of nested "multipart" entities.

   The use of the "multipart" media type with only a single body part
   may be useful in certain contexts, and is explicitly permitted.

   NOTE: Experience has shown that a "multipart" media type with a
   single body part is useful for sending non-text media types.  It has
   the advantage of providing the preamble as a place to include
   decoding instructions.  In addition, a number of SMTP gateways move
   or remove the MIME headers, and a clever MIME decoder can take a good
   guess at multipart boundaries even in the absence of the Content-Type
   header and thereby successfully decode the message.

   The only mandatory global parameter for the "multipart" media type is
   the boundary parameter, which consists of 1 to 70 characters from a
   set of characters known to be very robust through mail gateways, and
   NOT ending with white space. (If a boundary delimiter line 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:

     dash-boundary := "--" boundary
                      ; boundary taken from the value of
                      ; boundary parameter of the
                      ; Content-Type field.

     multipart-body := [preamble CRLF]
                       dash-boundary transport-padding CRLF
                       body-part *encapsulation
                       close-delimiter transport-padding
                       [CRLF epilogue]





Freed & Borenstein          Standards Track                    [Page 22]

RFC 2046                      Media Types                  November 1996


     transport-padding := *LWSP-char
                          ; Composers MUST NOT generate
                          ; non-zero length transport
                          ; padding, but receivers MUST
                          ; be able to handle padding
                          ; added by message transports.

     encapsulation := delimiter transport-padding
                      CRLF body-part

     delimiter := CRLF dash-boundary

     close-delimiter := delimiter "--"

     preamble := discard-text

     epilogue := discard-text

     discard-text := *(*text CRLF) *text
                     ; May be ignored or discarded.

     body-part := MIME-part-headers [CRLF *OCTET]
                  ; Lines in a body-part must not start
                  ; with the specified dash-boundary and
                  ; the delimiter must not appear anywhere
                  ; in the body part.  Note that the
                  ; semantics of a body-part differ from
                  ; the semantics of a message, as
                  ; described in the text.

     OCTET := <any 0-255 octet value>

   IMPORTANT:  The free insertion of linear-white-space and RFC 822
   comments between the elements shown in this BNF is NOT allowed since
   this BNF does not specify a structured header field.

   NOTE:  In certain transport enclaves, RFC 822 restrictions such as
   the one that limits bodies to printable US-ASCII characters may not
   be in force. (That is, the transport domains may exist that resemble
   standard Internet mail transport as specified in RFC 821 and assumed
   by RFC 822, 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
   US-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
   US-ASCII characters.



Freed & Borenstein          Standards Track                    [Page 23]

RFC 2046                      Media Types                  November 1996


   NOTE:  Conspicuously missing from the "multipart" type is a notion of
   structured, related body parts. It is recommended that those wishing
   to provide more structured or integrated multipart messaging
   facilities should define subtypes of multipart that are syntactically
   identical but define relationships between the various parts. For
   example, subtypes of multipart could be defined that include a
   distinguished part which in turn is used to specify the relationships
   between the other parts, probably referring to them by their
   Content-ID field.  Old implementations will not recognize the new
   subtype if this approach is used, but will treat it as
   multipart/mixed and will thus be able to show the user the parts that
   are recognized.

5.1.2.  Handling Nested Messages and Multiparts

   The "message/rfc822" subtype defined in a subsequent section of this
   document has no terminating condition other than running out of data.
   Similarly, an improperly truncated "multipart" entity may not have
   any terminating boundary marker, and can turn up operationally due to
   mail system malfunctions.

   It is essential that such entities be handled correctly when they are
   themselves imbedded inside of another "multipart" structure.  MIME
   implementations are therefore required to recognize outer level
   boundary markers at ANY level of inner nesting.  It is not sufficient
   to only check for the next expected marker or other terminating
   condition.

5.1.3.  Mixed Subtype

   The "mixed" subtype of "multipart" 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".

5.1.4.  Alternative Subtype

   The "multipart/alternative" type is syntactically identical to
   "multipart/mixed", but the semantics are different.  In particular,
   each of the body 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 references, 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



Freed & Borenstein          Standards Track                    [Page 24]

RFC 2046                      Media Types                  November 1996


   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 a message
   in a fancy text format in such a way that it can easily be displayed
   anywhere:

     From: Nathaniel Borenstein <[e-mail]>
     To: Ned Freed <[e-mail]>
     Date: Mon, 22 Mar 1993 09:41:09 -0800 (PST)
     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/enriched

       ... RFC 1896 text/enriched version of same message
           goes here ...

     --boundary42
     Content-Type: application/x-whatever

       ... fanciest version of same message goes here ...

     --boundary42--

   In this example, users whose mail systems understood the
   "application/x-whatever" format would see only the fancy version,
   while other users would see only the enriched 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.





Freed & Borenstein          Standards Track                    [Page 25]

RFC 2046                      Media Types                  November 1996


   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 viewer.  While this approach does impose
   some burden on conformant MIME viewers, 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 a message
   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.

   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.  And when the information
   content is identical -- for example, where several parts of type
   "message/external-body" specify alternate ways to access the
   identical data -- the same Content-ID field value should be used, to
   optimize any caching mechanisms that might be present on the
   recipient's end.  However, 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.

5.1.5.  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.

   Note: Though it is possible to specify a Content-Type value for a
   body part in a digest which is other than "message/rfc822", such as a
   "text/plain" part containing a description of the material in the



Freed & Borenstein          Standards Track                    [Page 26]

RFC 2046                      Media Types                  November 1996


   digest, actually doing so is undesireble. The "multipart/digest"
   Content-Type is intended to be used to send collections of messages.
   If a "text/plain" part is needed, it should be included as a seperate
   part of a "multipart/mixed" message.

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

     From: Moderator-Address
     To: Recipient-List
     Date: Mon, 22 Mar 1994 13:34:51 +0000
     Subject: Internet Digest, volume 42
     MIME-Version: 1.0
     Content-Type: multipart/mixed;
                   boundary="---- main boundary ----"

     ------ main boundary ----

       ...Introductory text or table of contents...

     ------ main boundary ----
     Content-Type: multipart/digest;
                   boundary="---- next message ----"

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

     From: someone-else
     Date: Fri, 26 Mar 1993 11:13:32 +0200
     Subject: my opinion

       ...body goes here ...

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

     From: someone-else-again
     Date: Fri, 26 Mar 1993 10:07:13 -0500
     Subject: my different opinion

       ... another body goes here ...

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

     ------ main boundary ------

5.1.6.  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,



Freed & Borenstein          Standards Track                    [Page 27]

RFC 2046                      Media Types                  November 1996


   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.

5.1.7.  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".

5.2.  Message Media Type

   It is frequently desirable, in sending mail, to encapsulate another
   mail message.  A special media type, "message", is defined to
   facilitate this.  In particular, the "rfc822" subtype of "message" is
   used to encapsulate RFC 822 messages.

   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.

   Subtypes of "message" often impose restrictions on what encodings are
   allowed.  These restrictions are described in conjunction with each
   specific subtype.

   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.
   These operations are explicitly forbidden for the encapsulated
   headers embedded in the bodies of messages of type "message."

5.2.1.  RFC822 Subtype

   A media 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, the restriction that each
   "message/rfc822" body must include a "From", "Date", and at least one
   destination header is removed and replaced with the requirement that
   at least one of "From", "Subject", or "Date" must be present.



Freed & Borenstein          Standards Track                    [Page 28]

RFC 2046                      Media Types                  November 1996


   It should be noted that, despite the use of the numbers "822", a
   "message/rfc822" entity isn't restricted to material in strict
   conformance to RFC822, nor are the semantics of "message/rfc822"
   objects restricted to the semantics defined in RFC822. More
   specifically, a "message/rfc822" message could well be a News article
   or a MIME message.

   No encoding other than "7bit", "8bit", or "binary" is permitted for
   the body of a "message/rfc822" entity.  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-US-ASCII text in the
   headers of an encapsulated message can be specified using the
   mechanisms described in RFC 2047.

5.2.2.  Partial Subtype

   The "partial" subtype is defined to allow large entities to be
   delivered as several separate pieces of mail and automatically
   reassembled by a receiving user agent.  (The concept is similar to IP
   fragmentation and 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.  The media type
   "message/partial" thus indicates that the body contains a fragment of
   a larger entity.

   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 8bit
   transport.  The problem is that the binary data would be split into
   multiple "message/partial" messages, each of them requiring binary
   transport.  If such messages were encountered at a gateway into a
   7bit transport environment, there would be no way to properly encode
   them for the 7bit world, aside from waiting for all of the fragments,
   reassembling the inner message, and then encoding the reassembled
   data in base64 or quoted-printable.  Since it is possible that
   different fragments might go through different gateways, even this is
   not an acceptable solution.  For this reason, it is specified that
   entities of type "message/partial" must always have a content-
   transfer-encoding of 7bit (the default).  In particular, even in
   environments that support binary or 8bit transport, the use of a
   content- transfer-encoding of "8bit" or "binary" is explicitly
   prohibited for MIME entities of type "message/partial". This in turn
   implies that the inner message must not use "8bit" or "binary"
   encoding.






Freed & Borenstein          Standards Track                    [Page 29]

RFC 2046                      Media Types                  November 1996


   Because some message transfer agents may choose to automatically
   fragment large messages, and because such agents may use very
   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.

   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
   fragments 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 in RFC 2045.)  The
   second, "number", an integer, is the fragment number, which indicates
   where this fragment fits into the sequence of fragments.  The third,
   "total", another integer, is the total number of fragments.  This
   third subfield is required on the final fragment, and is optional
   (though encouraged) on the earlier fragments.  Note also that these
   parameters may be given in any order.

   Thus, the second piece of a 3-piece message may have either of the
   following header fields:

     Content-Type: Message/Partial; number=2; total=3;
                   id="oc=[e-mail]"

     Content-Type: Message/Partial;
                   id="oc=[e-mail]";
                   number=2

   But the third piece MUST specify the total number of fragments:

     Content-Type: Message/Partial; number=3; total=3;
                   id="oc=[e-mail]"

   Note that fragment numbering begins with 1, not 0.

   When the fragments of an entity broken up in this manner are put
   together, the result is always a complete MIME entity, which may have
   its own Content-Type header field, and thus may contain any other
   data type.

5.2.2.1.  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



Freed & Borenstein          Standards Track                    [Page 30]

RFC 2046                      Media Types                  November 1996


   message containing an audio message.  That is, the encapsulation of
   the message is considered to be "transparent".

   When generating and reassembling the pieces 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)   Fragmentation agents must split messages at line
          boundaries only. This restriction is imposed because
          splits at points other than the ends of lines in turn
          depends on message transports being able to preserve
          the semantics of messages that don't end with a CRLF
          sequence. Many transports are incapable of preserving
          such semantics.

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

    (3)   The header fields in the enclosed message which start
          with "Content-", plus the "Subject", "Message-ID",
          "Encrypted", and "MIME-Version" fields, 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 the
          "Subject", "Message-ID", "Encrypted", and "MIME-
          Version" fields) will be ignored and dropped.

    (4)   All of the header fields from the second and any
          subsequent enclosing messages are discarded by the
          reassembly process.

5.2.2.2.  Fragmentation and Reassembly Example

   If an audio message is broken into two pieces, the first piece might
   look something like this:

     X-Weird-Header-1: Foo
     From: [e-mail]
     To: [e-mail]
     Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
     Subject: Audio mail (part 1 of 2)
     Message-ID: <[e-mail]>
     MIME-Version: 1.0
     Content-type: message/partial; id="[e-mail]";



Freed & Borenstein          Standards Track                    [Page 31]

RFC 2046                      Media Types                  November 1996


                   number=1; total=2

     X-Weird-Header-1: Bar
     X-Weird-Header-2: Hello
     Message-ID: <[e-mail]>
     Subject: Audio mail
     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: [e-mail]
     To: [e-mail]
     Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
     Subject: Audio mail (part 2 of 2)
     MIME-Version: 1.0
     Message-ID: <[e-mail]>
     Content-type: message/partial;
                   id="[e-mail]"; 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: [e-mail]
     To: [e-mail]
     Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
     Subject: Audio mail
     Message-ID: <[e-mail]>
     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 ...

   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.





Freed & Borenstein          Standards Track                    [Page 32]

RFC 2046                      Media Types                  November 1996


   Finally, it should be noted that the "Encrypted" header field has
   been made obsolete by Privacy Enhanced Messaging (PEM) [RFC-1421,
   RFC-1422, RFC-1423, RFC-1424], but the rules above are nevertheless
   believed to describe the correct way to treat it if it is encountered
   in the context of conversion to and from "message/partial" fragments.

5.2.3.  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 a MIME 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.jpeg"

     Content-type: image/jpeg
     Content-ID: <[e-mail]>
     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".  The only access-type defined
   in this document that uses the phantom body is "mail-server", but
   other access-types may be defined in the future in other
   specifications that use this area.

   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 caching
   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.




Freed & Borenstein          Standards Track                    [Page 33]

RFC 2046                      Media Types                  November 1996


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

5.2.3.1.  General External-Body Parameters

   The parameters that may be used with any "message/external- body"
   are:

    (1)   ACCESS-TYPE -- A word indicating the supported access
          mechanism by which the file or data may be obtained.
          This word is not case sensitive.  Values include, but
          are not limited to, "FTP", "ANON-FTP", "TFTP", "LOCAL-
          FILE", and "MAIL-SERVER".  Future values, except for
          experimental values beginning with "X-", must be
          registered with IANA, as described in RFC 2048.
          This parameter is unconditionally mandatory and MUST be
          present on EVERY "message/external-body".

    (2)   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.  This parameter may be
          used with ANY access-type and is ALWAYS optional.

    (3)   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.  This parameter
          may be used with ANY access-type and is ALWAYS
          optional.

    (4)   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",



Freed & Borenstein          Standards Track                    [Page 34]

RFC 2046                      Media Types                  November 1996


          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.  This
          parameter may be used with ANY access-type and is
          ALWAYS optional.

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

5.2.3.2.  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:

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

    (2)   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.

    (3)   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:

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

    (2)   MODE -- A case-insensitive string indicating the mode
          to be used when retrieving the information.  The valid
          values for access-type "TFTP" are "NETASCII", "OCTET",
          and "MAIL", as specified by the TFTP protocol [RFC-
          783].  The valid 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 and 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.



Freed & Borenstein          Standards Track                    [Page 35]

RFC 2046                      Media Types                  November 1996


5.2.3.3.  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 mail
   address.

5.2.3.4.  The 'local-file' Access-Type

   An access-type of "local-file" indicates that the actual body is
   accessible as a file on the local machine.  Two additional parameters
   are defined for this access type:

    (1)   NAME -- The name of the file that contains the actual
          body data.  This parameter is mandatory for the
          "local-file" access-type.

    (2)   SITE -- A domain specifier for a machine or set of
          machines that are known to have access to the data
          file.  This optional parameter is 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.
          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.

5.2.3.5.  The 'mail-server' Access-Type

   The "mail-server" access-type indicates that the actual body is
   available from a mail server.  Two additional parameters are defined
   for this access-type:

    (1)   SERVER -- The addr-spec of the mail server from which
          the actual body data can be obtained.  This parameter
          is mandatory for the "mail-server" access-type.

    (2)   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.  This is an optional
          parameter.






Freed & Borenstein          Standards Track                    [Page 36]

RFC 2046                      Media Types                  November 1996


   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 in the content-type header field.  Instead,
   it is provided as the "phantom body" when the media type is
   "message/external-body" and the access-type is mail-server.

   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.

   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 mail servers must use the same Content-ID field on the returned
   message that was used in the original "message/external-body"
   entities, to facilitate such matching.

5.2.3.6.  External-Body Security Issues

   "Message/external-body" entities give rise to two important security
   issues:

    (1)   Accessing data via a "message/external-body" reference
          effectively results in the message recipient performing
          an operation that was specified by the message
          originator.  It is therefore possible for the message
          originator to trick a recipient into doing something
          they would not have done otherwise.  For example, an
          originator could specify a action that attempts
          retrieval of material that the recipient is not
          authorized to obtain, causing the recipient to
          unwittingly violate some security policy.  For this
          reason, user agents capable of resolving external
          references must always take steps to describe the
          action they are to take to the recipient and ask for
          explicit permisssion prior to performing it.

          The 'mail-server' access-type is particularly
          vulnerable, in that it causes the recipient to send a
          new message whose contents are specified by the
          original message's originator.  Given the potential for
          abuse, any such request messages that are constructed
          should contain a clear indication that they were
          generated automatically (e.g. in a Comments: header
          field) in an attempt to resolve a MIME



Freed & Borenstein          Standards Track                    [Page 37]

RFC 2046                      Media Types                  November 1996


          "message/external-body" reference.

    (2)   MIME will sometimes be used in environments that
          provide some guarantee of message integrity and
          authenticity.  If present, such guarantees may apply
          only to the actual direct content of messages -- they
          may or may not apply to data accessed through MIME's
          "message/external-body" mechanism.  In particular, it
          may be possible to subvert certain access mechanisms
          even when the messaging system itself is secure.

          It should be noted that this problem exists either with
          or without the availabilty of MIME mechanisms.  A
          casual reference to an FTP site containing a document
          in the text of a secure message brings up similar
          issues -- the only difference is that MIME provides for
          automatic retrieval of such material, and users may
          place unwarranted trust is such automatic retrieval
          mechanisms.

5.2.3.7.  Examples and Further Explanations

   When the external-body mechanism is used in conjunction with the
   "multipart/alternative" media type it extends the functionality of
   "multipart/alternative" to include the case where the same entity is
   provided in the same format but via different accces mechanisms.
   When this is done the originator of the message must order the parts
   first in terms of preferred formats and then by preferred access
   mechanisms.  The recipient's viewer should then evaluate the list
   both in terms of format and access mechanisms.

   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 entities of type "message/external-body" within the
   body parts of an enclosing "multipart/alternative" entity.

   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.




Freed & Borenstein          Standards Track                    [Page 38]

RFC 2046                      Media Types                  November 1996


   The embedded message header fields which appear in the body of the
   "message/external-body" data must be used to declare the media type
   of the external body if it is anything other than plain US-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 an object in PostScript
   format, might look like this:

     From: Whomever
     To: Someone
     Date: Whenever
     Subject: whatever
     MIME-Version: 1.0
     Message-ID: <[e-mail]>
     Content-Type: multipart/alternative; boundary=42
     Content-ID: <[e-mail]>

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

     Content-type: application/postscript
     Content-ID: <[e-mail]>

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

     Content-type: application/postscript
     Content-ID: <[e-mail]>

     --42
     Content-Type: message/external-body;
                   access-type=mail-server
                   server="[e-mail]";
                   expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

     Content-type: application/postscript
     Content-ID: <[e-mail]>

     get RFC-MIME.DOC

     --42--



Freed & Borenstein          Standards Track                    [Page 39]

RFC 2046                      Media Types                  November 1996


   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" media
   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 and Subject fields are overridden,
   but the From field is preserved.

   Note that since the external bodies are not transported along with
   the external body reference, they need not conform to transport
   limitations that apply to the reference itself. In particular,
   Internet mail transports may impose 7bit and line length limits, but
   these do not automatically apply to binary external body references.
   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.

5.2.4.  Other Message Subtypes

   MIME implementations must in general treat unrecognized subtypes of
   "message" as being equivalent to "application/octet-stream".

   Future subtypes of "message" intended for use with email should be
   restricted to "7bit" encoding. A type other than "message" should be
   used if restriction to "7bit" is not possible.

6.  Experimental Media Type Values

   A media type value beginning with the characters "X-" is a private
   value, to be used by consenting 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. In many cases, a subtype of "application" will be more
   appropriate than a new top-level type.




Freed & Borenstein          Standards Track                    [Page 40]

RFC 2046                      Media Types                  November 1996


7.  Summary

   The five discrete media types provide provide a standardized
   mechanism for tagging entities as "audio", "image", or several other
   kinds of data. The composite "multipart" and "message" media types
   allow mixing and hierarchical structuring of entities of different
   types in a single message. 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 media
   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 the context of the
   "application/postscript" type, the "message/external-body" type, and
   in RFC 2048.  Implementors should pay special attention to the
   security implications of any media types that can cause the remote
   execution of any actions in the recipient's environment.  In such
   cases, the discussion of the "application/postscript" type may serve
   as a model for considering other media types with remote execution
   capabilities.



























Freed & Borenstein          Standards Track                    [Page 41]

RFC 2046                      Media Types                  November 1996


9.  Authors' Addresses

   For more information, the authors of this document are best contacted
   via Internet mail:

   Ned Freed
   Innosoft International, Inc.
   1050 East Garvey Avenue South
   West Covina, CA 91790
   USA

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


   Nathaniel S. Borenstein
   First Virtual Holdings
   25 Washington Avenue
   Morristown, NJ 07960
   USA

   Phone: [phone]
   Fax:   [phone]
   EMail: [e-mail]


   MIME is a result of the work of the Internet Engineering Task Force
   Working Group on RFC 822 Extensions.  The chairman of that group,
   Greg Vaudreuil, may be reached at:

   Gregory M. Vaudreuil
   Octel Network Services
   17080 Dallas Parkway
   Dallas, TX 75248-1905
   USA

   EMail: [e-mail]













Freed & Borenstein          Standards Track                    [Page 42]

RFC 2046                      Media Types                  November 1996


Appendix A -- 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 by name to
   several syntax rules 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.

     boundary := 0*69<bchars> bcharsnospace

     bchars := bcharsnospace / " "

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

     body-part := <"message" as defined in RFC 822, with all
                   header fields optional, not starting with the
                   specified dash-boundary, and with the
                   delimiter not occurring anywhere in the
                   body part.  Note that the semantics of a
                   part differ from the semantics of a message,
                   as described in the text.>

     close-delimiter := delimiter "--"

     dash-boundary := "--" boundary
                      ; boundary taken from the value of
                      ; boundary parameter of the
                      ; Content-Type field.

     delimiter := CRLF dash-boundary

     discard-text := *(*text CRLF)
                     ; May be ignored or discarded.

     encapsulation := delimiter transport-padding
                      CRLF body-part

     epilogue := discard-text

     multipart-body := [preamble CRLF]
                       dash-boundary transport-padding CRLF
                       body-part *encapsulation



Freed & Borenstein          Standards Track                    [Page 43]

RFC 2046                      Media Types                  November 1996


                       close-delimiter transport-padding
                       [CRLF epilogue]

     preamble := discard-text

     transport-padding := *LWSP-char
                          ; Composers MUST NOT generate
                          ; non-zero length transport
                          ; padding, but receivers MUST
                          ; be able to handle padding
                          ; added by message transports.








































Freed & Borenstein          Standards Track                    [Page 44]