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rfc:rfc2046

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

  1. -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.
  1. -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 <nsb@bellcore.com>
   To: Ned Freed <ned@innosoft.com>
   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.
  1. -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.
  1. -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 <nsb@bellcore.com>
   To: Ned Freed <ned@innosoft.com>
   Date: Mon, 22 Mar 1993 09:41:09 -0800 (PST)
   Subject: Formatted text mail
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary=boundary42
  1. -boundary42

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

     ... plain text version of message goes here ...
  1. -boundary42

Content-Type: text/enriched

     ... RFC 1896 text/enriched version of same message
         goes here ...
  1. -boundary42

Content-Type: application/x-whatever

     ... fanciest version of same message goes here ...
  1. -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 ----"
  1. —– main boundary —-
     ...Introductory text or table of contents...
  1. —– main boundary —-

Content-Type: multipart/digest;

                 boundary="---- next message ----"
  1. —– next message —-
   From: someone-else
   Date: Fri, 26 Mar 1993 11:13:32 +0200
   Subject: my opinion
     ...body goes here ...
  1. —– next message —-
   From: someone-else-again
   Date: Fri, 26 Mar 1993 10:07:13 -0500
   Subject: my different opinion
     ... another body goes here ...
  1. —– next message ——
  1. —– 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=jpbe0M2Yt4s@thumper.bellcore.com"
   Content-Type: Message/Partial;
                 id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
                 number=2
 But the third piece MUST specify the total number of fragments:
   Content-Type: Message/Partial; number=3; total=3;
                 id="oc=jpbe0M2Yt4s@thumper.bellcore.com"
 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: Bill@host.com
   To: joe@otherhost.com
   Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
   Subject: Audio mail (part 1 of 2)
   Message-ID: <id1@host.com>
   MIME-Version: 1.0
   Content-type: message/partial; id="ABC@host.com";

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: <anotherid@foo.com>
   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: Bill@host.com
   To: joe@otherhost.com
   Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
   Subject: Audio mail (part 2 of 2)
   MIME-Version: 1.0
   Message-ID: <id2@host.com>
   Content-type: message/partial;
                 id="ABC@host.com"; number=2; total=2
     ... second half of encoded audio data goes here ...
 Then, when the fragmented message is reassembled, the resulting
 message to be displayed to the user should look something like this:
   X-Weird-Header-1: Foo
   From: Bill@host.com
   To: joe@otherhost.com
   Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
   Subject: Audio mail
   Message-ID: <anotherid@foo.com>
   MIME-Version: 1.0
   Content-type: audio/basic
   Content-transfer-encoding: base64
     ... first half of encoded audio data goes here ...
     ... second half of encoded audio data goes here ...
 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: <id42@guppylake.bellcore.com>
   Content-Transfer-Encoding: binary
   THIS IS NOT REALLY THE BODY!
 The area at the end, which might be called the "phantom body", is
 ignored for most external-body messages.  However, it may be used to
 contain auxiliary information for some such messages, as indeed it is
 when the access-type is "mail- server".  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: <id1@host.com>
   Content-Type: multipart/alternative; boundary=42
   Content-ID: <id001@guppylake.bellcore.com>
  1. -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: <id42@guppylake.bellcore.com>
  1. -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: <id42@guppylake.bellcore.com>
  1. -42

Content-Type: message/external-body;

                 access-type=mail-server
                 server="listserv@bogus.bitnet";
                 expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
   Content-type: application/postscript
   Content-ID: <id42@guppylake.bellcore.com>
   get RFC-MIME.DOC
  1. -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: +1 818 919 3600
 Fax:   +1 818 919 3614
 EMail: ned@innosoft.com
 Nathaniel S. Borenstein
 First Virtual Holdings
 25 Washington Avenue
 Morristown, NJ 07960
 USA
 Phone: +1 201 540 8967
 Fax:   +1 201 993 3032
 EMail: nsb@nsb.fv.com
 MIME is a result of the work of the Internet Engineering Task Force
 Working Group on RFC 822 Extensions.  The chairman of that group,
 Greg Vaudreuil, may be reached at:
 Gregory M. Vaudreuil
 Octel Network Services
 17080 Dallas Parkway
 Dallas, TX 75248-1905
 USA
 EMail: Greg.Vaudreuil@Octel.Com

Freed & Borenstein Standards Track [Page 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]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2046.txt · Last modified: 1996/11/26 21:26 by 127.0.0.1

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