











Network Working Group                                      N. Borenstein

Request for Comments: 1521                                      Bellcore

Obsoletes: 1341                                                 N. Freed

Category: Standards Track                                       Innosoft

                                                          September 1993





         MIME (Multipurpose Internet Mail Extensions) Part One:

                Mechanisms for Specifying and Describing

                 the Format of Internet Message Bodies



Status of this Memo



   This RFC specifies an Internet standards track protocol for the

   Internet community, and requests discussion and suggestions for

   improvements.  Please refer to the current edition of the "Internet

   Official Protocol Standards" for the standardization state and status

   of this protocol.  Distribution of this memo is unlimited.



Abstract



   STD 11, RFC 822 defines a message representation protocol which

   specifies considerable detail about message headers, but which leaves

   the message content, or message body, as flat ASCII text.  This

   document redefines the format of message bodies to allow multi-part

   textual and non-textual message bodies to be represented and

   exchanged without loss of information.  This is based on earlier work

   documented in RFC 934 and STD 11, RFC 1049, but extends and revises

   that work.  Because RFC 822 said so little about message bodies, this

   document is largely orthogonal to (rather than a revision of) RFC

   822.



   In particular, this document is designed to provide facilities to

   include multiple objects in a single message, to represent body text

   in character sets other than US-ASCII, to represent formatted multi-

   font text messages, to represent non-textual material such as images

   and audio fragments, and generally to facilitate later extensions

   defining new types of Internet mail for use by cooperating mail

   agents.



   This document does NOT extend Internet mail header fields to permit

   anything other than US-ASCII text data.  Such extensions are the

   subject of a companion document [RFC-1522].



   This document is a revision of RFC 1341.  Significant differences

   from RFC 1341 are summarized in Appendix H.











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Table of Contents



   1.     Introduction.......................................  3

   2.     Notations, Conventions, and Generic BNF Grammar....  6

   3.     The MIME-Version Header Field......................  7

   4.     The Content-Type Header Field......................  9

   5.     The Content-Transfer-Encoding Header Field......... 13

   5.1.   Quoted-Printable Content-Transfer-Encoding......... 18

   5.2.   Base64 Content-Transfer-Encoding................... 21

   6.     Additional Content-Header Fields................... 23

   6.1.   Optional Content-ID Header Field................... 23

   6.2.   Optional Content-Description Header Field.......... 24

   7.     The Predefined Content-Type Values................. 24

   7.1.   The Text Content-Type.............................. 24

   7.1.1. The charset parameter.............................. 25

   7.1.2. The Text/plain subtype............................. 28

   7.2.   The Multipart Content-Type......................... 28

   7.2.1. Multipart:  The common syntax...................... 29

   7.2.2. The Multipart/mixed (primary) subtype.............. 34

   7.2.3. The Multipart/alternative subtype.................. 34

   7.2.4. The Multipart/digest subtype....................... 36

   7.2.5. The Multipart/parallel subtype..................... 37

   7.2.6. Other Multipart subtypes........................... 37

   7.3.   The Message Content-Type........................... 38

   7.3.1. The Message/rfc822 (primary) subtype............... 38

   7.3.2. The Message/Partial subtype........................ 39

   7.3.3. The Message/External-Body subtype.................. 42

   7.3.3.1.  The "ftp" and "tftp" access-types............... 44

   7.3.3.2.  The "anon-ftp" access-type...................... 45

   7.3.3.3.  The "local-file" and "afs" access-types......... 45

   7.3.3.4.  The "mail-server" access-type................... 45

   7.3.3.5.  Examples and Further Explanations............... 46

   7.4.   The Application Content-Type....................... 49

   7.4.1. The Application/Octet-Stream (primary) subtype..... 50

   7.4.2. The Application/PostScript subtype................. 50

   7.4.3. Other Application subtypes......................... 53

   7.5.   The Image Content-Type............................. 53

   7.6.   The Audio Content-Type............................. 54

   7.7.   The Video Content-Type............................. 54

   7.8.   Experimental Content-Type Values................... 54

   8.     Summary............................................ 56

   9.     Security Considerations............................ 56

   10.    Authors' Addresses................................. 57

   11.    Acknowledgements................................... 58

   Appendix A -- Minimal MIME-Conformance.................... 60

   Appendix B -- General Guidelines For Sending Email Data... 63

   Appendix C -- A Complex Multipart Example................. 66

   Appendix D -- Collected Grammar........................... 68







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   Appendix E -- IANA Registration Procedures................ 72

   E.1  Registration of New Content-type/subtype Values...... 72

   E.2  Registration of New Access-type Values

        for Message/external-body............................ 73

   Appendix F -- Summary of the Seven Content-types.......... 74

   Appendix G -- Canonical Encoding Model.................... 76

   Appendix H -- Changes from RFC 1341....................... 78

   References................................................ 80



1.    Introduction



   Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined

   the standard format of textual mail messages on the Internet.  Its

   success has been such that the RFC 822 format has been adopted,

   wholly or partially, well beyond the confines of the Internet and the

   Internet SMTP transport defined by STD 10, RFC 821 [RFC-821].  As the

   format has seen wider use, a number of limitations have proven

   increasingly restrictive for the user community.



   RFC 822 was intended to specify a format for text messages.  As such,

   non-text messages, such as multimedia messages that might include

   audio or images, are simply not mentioned.  Even in the case of text,

   however, RFC 822 is inadequate for the needs of mail users whose

   languages require the use of character sets richer than US ASCII

   [US-ASCII]. Since RFC 822 does not specify mechanisms for mail

   containing audio, video, Asian language text, or even text in most

   European languages, additional specifications are needed.



   One of the notable limitations of RFC 821/822 based mail systems is

   the fact that they limit the contents of electronic mail messages to

   relatively short lines of seven-bit ASCII.  This forces users to

   convert any non-textual data that they may wish to send into seven-

   bit bytes representable as printable ASCII characters before invoking

   a local mail UA (User Agent, a program with which human users send

   and receive mail). Examples of such encodings currently used in the

   Internet include pure hexadecimal, uuencode, the 3-in-4 base 64

   scheme specified in RFC 1421, the Andrew Toolkit Representation

   [ATK], and many others.



   The limitations of RFC 822 mail become even more apparent as gateways

   are designed to allow for the exchange of mail messages between RFC

   822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the

   inclusion of non-textual body parts within electronic mail messages.

   The current standards for the mapping of X.400 messages to RFC 822

   messages specify either that X.400 non-textual body parts must be

   converted to (not encoded in) an ASCII format, or that they must be

   discarded, notifying the RFC 822 user that discarding has occurred.

   This is clearly undesirable, as information that a user may wish to







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   receive is lost.  Even though a user's UA may not have the capability

   of dealing with the non-textual body part, the user might have some

   mechanism external to the UA that can extract useful information from

   the body part.  Moreover, it does not allow for the fact that the

   message may eventually be gatewayed back into an X.400 message

   handling system (i.e., the X.400 message is "tunneled" through

   Internet mail), where the non-textual information would definitely

   become useful again.



   This document describes several mechanisms that combine to solve most

   of these problems without introducing any serious incompatibilities

   with the existing world of RFC 822 mail.  In particular, it

   describes:



   1. A MIME-Version header field, which uses a version number to

       declare a message to be conformant with this specification and

       allows mail processing agents to distinguish between such

       messages and those generated by older or non-conformant software,

       which is presumed to lack such a field.



   2. A Content-Type header field, generalized from RFC 1049 [RFC-1049],

       which can be used to specify the type and subtype of data in the

       body of a message and to fully specify the native representation

       (encoding) of such data.



       2.a. A "text" Content-Type value, which can be used to represent

            textual information in a number of character sets and

            formatted text description languages in a standardized

            manner.



       2.b. A "multipart" Content-Type value, which can be used to

            combine several body parts, possibly of differing types of

            data, into a single message.



       2.c. An "application" Content-Type value, which can be used to

            transmit application data or binary data, and hence, among

            other uses, to implement an electronic mail file transfer

            service.



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

            mail message.



       2.e An "image" Content-Type value, for transmitting still image

            (picture) data.



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

            voice data.









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       2.g. A "video" Content-Type value, for transmitting video or

            moving image data, possibly with audio as part of the

            composite video data format.



   3. A Content-Transfer-Encoding header field, which can be used to

       specify an auxiliary encoding that was applied to the data in

       order to allow it to pass through mail transport mechanisms which

       may have data or character set limitations.



   4. Two additional header fields that can be used to further describe

       the data in a message body, the Content-ID and Content-

       Description header fields.



   MIME has been carefully designed as an extensible mechanism, and it

   is expected that the set of content-type/subtype pairs and their

   associated parameters will grow significantly with time.  Several

   other MIME fields, notably including character set names, are likely

   to have new values defined over time.  In order to ensure that the

   set of such values is developed in an orderly, well-specified, and

   public manner, MIME defines a registration process which uses the

   Internet Assigned Numbers Authority (IANA) as a central registry for

   such values.  Appendix E provides details about how IANA registration

   is accomplished.



   Finally, to specify and promote interoperability, Appendix A of this

   document provides a basic applicability statement for a subset of the

   above mechanisms that defines a minimal level of "conformance" with

   this document.



      HISTORICAL NOTE: Several of the mechanisms described in this

      document may seem somewhat strange or even baroque at first

      reading.  It is important to note that compatibility with existing

      standards AND robustness across existing practice were two of the

      highest priorities of the working group that developed this

      document.  In particular, compatibility was always favored over

      elegance.



   MIME was first defined and published as RFCs 1341 and 1342 [RFC-1341]

   [RFC-1342].  This document is a relatively minor updating of RFC

   1341, and is intended to supersede it.  The differences between this

   document and RFC 1341 are summarized in Appendix H.  Please refer to

   the current edition of the "IAB Official Protocol Standards" for the

   standardization state and status of this protocol.  Several other RFC

   documents will be of interest to the MIME implementor, in particular

   [RFC 1343], [RFC-1344], and [RFC-1345].













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2.    Notations, Conventions, and Generic BNF Grammar



   This document is being published in two versions, one as plain ASCII

   text and one as PostScript (PostScript is a trademark of Adobe

   Systems Incorporated.).  While the text version is the official

   specification, some will find the PostScript version easier to read.

   The textual contents are identical.  An Andrew-format copy of this

   document is also available from the first author (Borenstein).



   Although the mechanisms specified in this document are all described

   in prose, most are also described formally in the modified BNF

   notation of RFC 822.  Implementors will need to be familiar with this

   notation in order to understand this specification, and are referred

   to RFC 822 for a complete explanation of the modified BNF notation.



   Some of the modified BNF in this document makes reference to

   syntactic entities that are defined in RFC 822 and not in this

   document.  A complete formal grammar, then, is obtained by combining

   the collected grammar appendix of this document with that of RFC 822

   plus the modifications to RFC 822 defined in RFC 1123, which

   specifically changes the syntax for `return', `date' and `mailbox'.



   The term CRLF, in this document, refers to the sequence of the two

   ASCII characters CR (13) and LF (10) which, taken together, in this

   order, denote a line break in RFC 822 mail.



   The term "character set" is used in this document to refer to a

   method used with one or more tables to convert encoded text to a

   series of octets.  This definition is intended to allow various kinds

   of text encodings, from simple single-table mappings such as ASCII to

   complex table switching methods such as those that use ISO 2022's

   techniques.  However, a MIME character set name must fully specify

   the mapping to be performed.



   The term "message", when not further qualified, means either the

   (complete or "top-level") message being transferred on a network, or

   a message encapsulated in a body of type "message".



   The term "body part", in this document, means one of the parts of the

   body of a multipart entity. A body part has a header and a body, so

   it makes sense to speak about the body of a body part.



   The term "entity", in this document, means either a message or a body

   part.  All kinds of entities share the property that they have a

   header and a body.



   The term "body", when not further qualified, means the body of an

   entity, that is the body of either a message or of a body part.







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      NOTE: The previous four definitions are clearly circular.  This is

      unavoidable, since the overall structure of a MIME message is

      indeed recursive.



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

   notation.



   It must be noted that Content-Type values, subtypes, and parameter

   names as defined in this document are case-insensitive.  However,

   parameter values are case-sensitive unless otherwise specified for

   the specific parameter.



      FORMATTING NOTE: This document has been carefully formatted for

      ease of reading.  The PostScript version of this document, in

      particular, places notes like this one, which may be skipped by

      the reader, in a smaller, italicized, font, and indents it as

      well.  In the text version, only the indentation is preserved, so

      if you are reading the text version of this you might consider

      using the PostScript version instead. However, all such notes will

      be indented and preceded by "NOTE:" or some similar introduction,

      even in the text version.



      The primary purpose of these non-essential notes is to convey

      information about the rationale of this document, or to place this

      document in the proper historical or evolutionary context.  Such

      information may be skipped by those who are focused entirely on

      building a conformant implementation, but may be of use to those

      who wish to understand why this document is written as it is.



      For ease of recognition, all BNF definitions have been placed in a

      fixed-width font in the PostScript version of this document.



3.    The MIME-Version Header Field



   Since RFC 822 was published in 1982, there has really been only one

   format standard for Internet messages, and there has been little

   perceived need to declare the format standard in use.  This document

   is an independent document that complements RFC 822. Although the

   extensions in this document have been defined in such a way as to be

   compatible with RFC 822, there are still circumstances in which it

   might be desirable for a mail-processing agent to know whether a

   message was composed with the new standard in mind.



   Therefore, this document defines a new header field, "MIME-Version",

   which is to be used to declare the version of the Internet message

   body format standard in use.



   Messages composed in accordance with this document MUST include such







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   a header field, with the following verbatim text:



   MIME-Version: 1.0



   The presence of this header field is an assertion that the message

   has been composed in compliance with this document.



   Since it is possible that a future document might extend the message

   format standard again, a formal BNF is given for the content of the

   MIME-Version field:



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



   Thus, future format specifiers, which might replace or extend "1.0",

   are constrained to be two integer fields, separated by a period.  If

   a message is received with a MIME-version value other than "1.0", it

   cannot be assumed to conform with this specification.



   Note that the MIME-Version header field is required at the top level

   of a message. It is not required for each body part of a multipart

   entity.  It is required for the embedded headers of a body of type

   "message" if and only if the embedded message is itself claimed to be

   MIME-conformant.



   It is not possible to fully specify how a mail reader that conforms

   with MIME as defined in this document should treat a message that

   might arrive in the future with some value of MIME-Version other than

   "1.0".  However, conformant software is encouraged to check the

   version number and at least warn the user if an unrecognized MIME-

   version is encountered.



   It is also worth noting that version control for specific content-

   types is not accomplished using the MIME-Version mechanism.  In

   particular, some formats (such as application/postscript) have

   version numbering conventions that are internal to the document

   format.  Where such conventions exist, MIME does nothing to supersede

   them.  Where no such conventions exist, a MIME type might use a

   "version" parameter in the content-type field if necessary.



   NOTE TO IMPLEMENTORS: All header fields defined in this document,

   including MIME-Version, Content-type, etc., are subject to the

   general syntactic rules for header fields specified in RFC 822.  In

   particular, all can include comments, which means that the following

   two MIME-Version fields are equivalent:



                    MIME-Version: 1.0

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









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4.    The Content-Type Header Field



   The purpose of the Content-Type field is to describe the data

   contained in the body fully enough that the receiving user agent can

   pick an appropriate agent or mechanism to present the data to the

   user, or otherwise deal with the data in an appropriate manner.



   HISTORICAL NOTE: The Content-Type header field was first defined in

   RFC 1049.  RFC 1049 Content-types used a simpler and less powerful

   syntax, but one that is largely compatible with the mechanism given

   here.



   The Content-Type header field is used to specify the nature of the

   data in the body of an entity, by giving type and subtype

   identifiers, and by providing auxiliary information that may be

   required for certain types.  After the type and subtype names, the

   remainder of the header field is simply a set of parameters,

   specified in an attribute/value notation.  The set of meaningful

   parameters differs for the different types.  In particular, there are

   NO globally-meaningful parameters that apply to all content-types.

   Global mechanisms are best addressed, in the MIME model, by the

   definition of additional Content-* header fields.  The ordering of

   parameters is not significant.  Among the defined parameters is a

   "charset" parameter by which the character set used in the body may

   be declared. Comments are allowed in accordance with RFC 822 rules

   for structured header fields.



   In general, the top-level Content-Type is used to declare the general

   type of data, while the subtype specifies a specific format for that

   type of data.  Thus, a Content-Type of "image/xyz" is enough to tell

   a user agent that the data is an image, even if the user agent has no

   knowledge of the specific image format "xyz".  Such information can

   be used, for example, to decide whether or not to show a user the raw

   data from an unrecognized subtype -- such an action might be

   reasonable for unrecognized subtypes of text, but not for

   unrecognized subtypes of image or audio.  For this reason, registered

   subtypes of audio, image, text, and video, should not contain

   embedded information that is really of a different type.  Such

   compound types should be represented using the "multipart" or

   "application" types.



   Parameters are modifiers of the content-subtype, and do not

   fundamentally affect the requirements of the host system.  Although

   most parameters make sense only with certain content-types, others

   are "global" in the sense that they might apply to any subtype.  For

   example, the "boundary" parameter makes sense only for the

   "multipart" content-type, but the "charset" parameter might make

   sense with several content-types.







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   An initial set of seven Content-Types is defined by this document.

   This set of top-level names is intended to be substantially complete.

   It is expected that additions to the larger set of supported types

   can generally be accomplished by the creation of new subtypes of

   these initial types.  In the future, more top-level types may be

   defined only by an extension to this standard.  If another primary

   type is to be used for any reason, it must be given a name starting

   with "X-" to indicate its non-standard status and to avoid a

   potential conflict with a future official name.



   In the Augmented BNF notation of RFC 822, a Content-Type header field

   value is defined as follows:



     content  :=   "Content-Type"  ":"  type  "/"  subtype  *(";"

     parameter)

               ; case-insensitive matching of type and subtype



     type :=          "application"     / "audio"

               / "image"           / "message"

               / "multipart"  / "text"

               / "video"           / extension-token

               ; All values case-insensitive



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



     iana-token := <a publicly-defined extension token,

               registered with IANA, as specified in

               appendix E>



     x-token := <The two characters "X-" or "x-" followed, with

                 no intervening white space, by any token>



     subtype := token ; case-insensitive



     parameter := attribute "=" value



     attribute := token   ; case-insensitive



     value := token / quoted-string



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

                   or tspecials>



     tspecials :=  "(" / ")" / "<" / ">" / "@"

                /  "," / ";" / ":" / "\" / <">

                /  "/" / "[" / "]" / "?" / "="

               ; Must be in quoted-string,

               ; to use within parameter values







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   Note that the definition of "tspecials" is the same as the RFC 822

   definition of "specials" with the addition of the three characters

   "/", "?", and "=", and the removal of ".".



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

   default subtypes.



   The type, subtype, and parameter names are not case sensitive.  For

   example, TEXT, Text, and TeXt are all equivalent.  Parameter values

   are normally case sensitive, but certain parameters are interpreted

   to be case-insensitive, depending on the intended use.  (For example,

   multipart boundaries are case-sensitive, but the "access-type" for

   message/External-body is not case-sensitive.)



   Beyond this syntax, the only constraint on the definition of subtype

   names is the desire that their uses must not conflict.  That is, it

   would be undesirable to have two different communities using

   "Content-Type: application/foobar" to mean two different things.  The

   process of defining new content-subtypes, then, is not intended to be

   a mechanism for imposing restrictions, but simply a mechanism for

   publicizing the usages. There are, therefore, two acceptable

   mechanisms for defining new Content-Type subtypes:



            1.  Private values (starting with "X-") may be

                defined bilaterally between two cooperating

                agents without outside registration or

                standardization.



            2.  New standard values must be documented,

                registered with, and approved by IANA, as

                described in Appendix E.  Where intended for

                public use, the formats they refer to must

                also be defined by a published specification,

                and possibly offered for standardization.



   The seven standard initial predefined Content-Types are detailed in

   the bulk of this document.  They are:



    text -- textual information.  The primary subtype,

         "plain", indicates plain (unformatted) text.  No

         special software is required to get the full

         meaning of the text, aside from support for the

         indicated character set.  Subtypes are to be used

         for enriched text in forms where application

         software may enhance the appearance of the text,

         but such software must not be required in order to

         get the general idea of the content.  Possible

         subtypes thus include any readable word processor







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         format.  A very simple and portable subtype,

         richtext, was defined in RFC 1341, with a future

         revision expected.



    multipart -- data consisting of multiple parts of

         independent data types.  Four initial subtypes

         are defined, including the primary "mixed"

         subtype, "alternative" for representing the same

         data in multiple formats, "parallel" for parts

         intended to be viewed simultaneously, and "digest"

         for multipart entities in which each part is of

         type "message".



    message -- an encapsulated message.  A body of

         Content-Type "message" is itself all or part of a

         fully formatted RFC 822 conformant message which

         may contain its own different Content-Type header

         field.  The primary subtype is "rfc822".  The

         "partial" subtype is defined for partial messages,

         to permit the fragmented transmission of bodies

         that are thought to be too large to be passed

         through mail transport facilities.  Another

         subtype, "External-body", is defined for

         specifying large bodies by reference to an

         external data source.



    image -- image data.  Image requires a display device

         (such as a graphical display, a printer, or a FAX

         machine) to view the information.  Initial

         subtypes are defined for two widely-used image

         formats, jpeg and gif.



    audio -- audio data, with initial subtype "basic".

         Audio requires an audio output device (such as a

         speaker or a telephone) to "display" the contents.



    video -- video data.  Video requires the capability to

         display moving images, typically including

         specialized hardware and software.  The initial

         subtype is "mpeg".



    application -- some other kind of data, typically

         either uninterpreted binary data or information to

         be processed by a mail-based application.  The

         primary subtype, "octet-stream", is to be used in

         the case of uninterpreted binary data, in which

         case the simplest recommended action is to offer

         to write the information into a file for the user.







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         An additional subtype, "PostScript", is defined

         for transporting PostScript documents in bodies.

         Other expected uses for "application" include

         spreadsheets, data for mail-based scheduling

         systems, and languages for "active"

         (computational) email.  (Note that active email

         and other application data may entail several

         security considerations, which are discussed later

         in this memo, particularly in the context of

         application/PostScript.)



   Default RFC 822 messages are typed by this protocol as plain text in

   the US-ASCII character set, which can be explicitly specified as

   "Content-type: text/plain; charset=us-ascii".  If no Content-Type is

   specified, this default is assumed.  In the presence of a MIME-

   Version header field, a receiving User Agent can also assume that

   plain US-ASCII text was the sender's intent.  In the absence of a

   MIME-Version specification, plain US-ASCII text must still be

   assumed, but the sender's intent might have been otherwise.



      RATIONALE: In the absence of any Content-Type header field or

      MIME-Version header field, it is impossible to be certain that a

      message is actually text in the US-ASCII character set, since it

      might well be a message that, using the conventions that predate

      this document, includes text in another character set or non-

      textual data in a manner that cannot be automatically recognized

      (e.g., a uuencoded compressed UNIX tar file).  Although there is

      no fully acceptable alternative to treating such untyped messages

      as "text/plain; charset=us-ascii", implementors should remain

      aware that if a message lacks both the MIME-Version and the

      Content-Type header fields, it may in practice contain almost

      anything.



   It should be noted that the list of Content-Type values given here

   may be augmented in time, via the mechanisms described above, and

   that the set of subtypes is expected to grow substantially.



   When a mail reader encounters mail with an unknown Content-type

   value, it should generally treat it as equivalent to

   "application/octet-stream", as described later in this document.



5.    The Content-Transfer-Encoding Header Field



   Many Content-Types which could usefully be transported via email are

   represented, in their "natural" format, as 8-bit character or binary

   data.  Such data cannot be transmitted over some transport protocols.

   For example, RFC 821 restricts mail messages to 7-bit US-ASCII data

   with lines no longer than 1000 characters.







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   It is necessary, therefore, to define a standard mechanism for re-

   encoding such data into a 7-bit short-line format.  This document

   specifies that such encodings will be indicated by a new "Content-

   Transfer-Encoding" header field.  The Content-Transfer-Encoding field

   is used to indicate the type of transformation that has been used in

   order to represent the body in an acceptable manner for transport.



   Unlike Content-Types, a proliferation of Content-Transfer-Encoding

   values is undesirable and unnecessary.  However, establishing only a

   single Content-Transfer-Encoding mechanism does not seem possible.

   There is a tradeoff between the desire for a compact and efficient

   encoding of largely-binary data and the desire for a readable

   encoding of data that is mostly, but not entirely, 7-bit data.  For

   this reason, at least two encoding mechanisms are necessary: a

   "readable" encoding and a "dense" encoding.



   The Content-Transfer-Encoding field is designed to specify an

   invertible mapping between the "native" representation of a type of

   data and a representation that can be readily exchanged using 7 bit

   mail transport protocols, such as those defined by RFC 821 (SMTP).

   This field has not been defined by any previous standard. The field's

   value is a single token specifying the type of encoding, as

   enumerated below.  Formally:



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



   mechanism :=     "7bit"  ;  case-insensitive

                  / "quoted-printable"

                  / "base64"

                  / "8bit"

                  / "binary"

                  / x-token



   These values are not case sensitive.  That is, Base64 and BASE64 and

   bAsE64 are all equivalent.  An encoding type of 7BIT requires that

   the body is already in a seven-bit mail-ready representation.  This

   is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is

   assumed if the Content-Transfer-Encoding header field is not present.



   The values "8bit", "7bit", and "binary" all mean that NO encoding has

   been performed. However, they are potentially useful as indications

   of the kind of data contained in the object, and therefore of the

   kind of encoding that might need to be performed for transmission in

   a given transport system.  In particular:



       "7bit" means that the data is all represented as short

            lines of US-ASCII data.









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       "8bit" means that the lines are short, but there may be

            non-ASCII characters (octets with the high-order

            bit set).



       "Binary" means that not only may non-ASCII characters

            be present, but also that the lines are not

            necessarily short enough for SMTP transport.



   The difference between "8bit" (or any other conceivable bit-width

   token) and the "binary" token is that "binary" does not require

   adherence to any limits on line length or to the SMTP CRLF semantics,

   while the bit-width tokens do require such adherence.  If the body

   contains data in any bit-width other than 7-bit, the appropriate

   bit-width Content-Transfer-Encoding token must be used (e.g., "8bit"

   for unencoded 8 bit wide data).  If the body contains binary data,

   the "binary" Content-Transfer-Encoding token must be used.



      NOTE: The distinction between the Content-Transfer-Encoding values

      of "binary", "8bit", etc.  may seem unimportant, in that all of

      them really mean "none" -- that is, there has been no encoding of

      the data for transport.  However, clear labeling will be of

      enormous value to gateways between future mail transport systems

      with differing capabilities in transporting data that do not meet

      the restrictions of RFC 821 transport.



      Mail transport for unencoded 8-bit data is defined in RFC-1426

      [RFC-1426].  As of the publication of this document, there are no

      standardized Internet mail transports for which it is legitimate

      to include unencoded binary data in mail bodies.  Thus there are

      no circumstances in which the "binary" Content-Transfer-Encoding

      is actually legal on the Internet.  However, in the event that

      binary mail transport becomes a reality in Internet mail, or when

      this document is used in conjunction with any other binary-capable

      transport mechanism, binary bodies should be labeled as such using

      this mechanism.



      NOTE: The five values defined for the Content-Transfer-Encoding

      field imply nothing about the Content-Type other than the

      algorithm by which it was encoded or the transport system

      requirements if unencoded.



   Implementors may, if necessary, define new Content-Transfer-Encoding

   values, but must use an x-token, which is a name prefixed by "X-" to

   indicate its non-standard status, e.g., "Content-Transfer-Encoding:

   x-my-new-encoding".  However, unlike Content-Types and subtypes, the

   creation of new Content-Transfer-Encoding values is explicitly and

   strongly discouraged, as it seems likely to hinder interoperability

   with little potential benefit.  Their use is allowed only as the







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   result of an agreement between cooperating user agents.



   If a Content-Transfer-Encoding header field appears as part of a

   message header, it applies to the entire body of that message.  If a

   Content-Transfer-Encoding header field appears as part of a body

   part's headers, it applies only to the body of that body part.  If an

   entity is of type "multipart" or "message", the Content-Transfer-

   Encoding is not permitted to have any value other than a bit width

   (e.g., "7bit", "8bit", etc.) or "binary".



   It should be noted that email is character-oriented, so that the

   mechanisms described here are mechanisms for encoding arbitrary octet

   streams, not bit streams.  If a bit stream is to be encoded via one

   of these mechanisms, it must first be converted to an 8-bit byte

   stream using the network standard bit order ("big-endian"), in which

   the earlier bits in a stream become the higher-order bits in a byte.

   A bit stream not ending at an 8-bit boundary must be padded with

   zeroes.  This document provides a mechanism for noting the addition

   of such padding in the case of the application Content-Type, which

   has a "padding" parameter.



   The encoding mechanisms defined here explicitly encode all data in

   ASCII.  Thus, for example, suppose an entity has header fields such

   as:



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

        Content-transfer-encoding: base64



   This must be interpreted to mean that the body is a base64 ASCII

   encoding of data that was originally in ISO-8859-1, and will be in

   that character set again after decoding.



   The following sections will define the two standard encoding

   mechanisms.  The definition of new content-transfer-encodings is

   explicitly discouraged and should only occur when absolutely

   necessary.  All content-transfer-encoding namespace except that

   beginning with "X-" is explicitly reserved to the IANA for future

   use.  Private agreements about content-transfer-encodings are also

   explicitly discouraged.



   Certain Content-Transfer-Encoding values may only be used on certain

   Content-Types.  In particular, it is expressly forbidden to use any

   encodings other than "7bit", "8bit", or "binary" with any Content-

   Type that recursively includes other Content-Type fields, notably the

   "multipart" and "message" Content-Types.  All encodings that are

   desired for bodies of type multipart or message must be done at the

   innermost level, by encoding the actual body that needs to be

   encoded.







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      NOTE ON ENCODING RESTRICTIONS: Though the prohibition against

      using content-transfer-encodings on data of type multipart or

      message may seem overly restrictive, it is necessary to prevent

      nested encodings, in which data are passed through an encoding

      algorithm multiple times, and must be decoded multiple times in

      order to be properly viewed.  Nested encodings add considerable

      complexity to user agents: aside from the obvious efficiency

      problems with such multiple encodings, they can obscure the basic

      structure of a message.  In particular, they can imply that

      several decoding operations are necessary simply to find out what

      types of objects a message contains.  Banning nested encodings may

      complicate the job of certain mail gateways, but this seems less

      of a problem than the effect of nested encodings on user agents.



      NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-

      TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding

      could be inferred from the characteristics of the Content-Type

      that is to be encoded, or, at the very least, that certain

      Content-Transfer-Encodings could be mandated for use with specific

      Content-Types. There are several reasons why this is not the case.

      First, given the varying types of transports used for mail, some

      encodings may be appropriate for some Content-Type/transport

      combinations and not for others.  (For example, in an 8-bit

      transport, no encoding would be required for text in certain

      character sets, while such encodings are clearly required for 7-

      bit SMTP.)  Second, certain Content-Types may require different

      types of transfer encoding under different circumstances. For

      example, many PostScript bodies might consist entirely of short

      lines of 7-bit data and hence require little or no encoding.

      Other PostScript bodies (especially those using Level 2

      PostScript's binary encoding mechanism) may only be reasonably

      represented using a binary transport encoding. Finally, since

      Content-Type is intended to be an open-ended specification

      mechanism, strict specification of an association between

      Content-Types and encodings effectively couples the specification

      of an application protocol with a specific lower-level transport.

      This is not desirable since the developers of a Content-Type

      should not have to be aware of all the transports in use and what

      their limitations are.



      NOTE ON TRANSLATING ENCODINGS: The quoted-printable and base64

      encodings are designed so that conversion between them is

      possible.  The only issue that arises in such a conversion is the

      handling of line breaks.  When converting from quoted-printable to

      base64 a line break must be converted into a CRLF sequence.

      Similarly, a CRLF sequence in base64 data must be converted to a

      quoted-printable line break, but ONLY when converting text data.









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      NOTE ON CANONICAL ENCODING MODEL: There was some confusion, in

      earlier drafts of this memo, regarding the model for when email

      data was to be converted to canonical form and encoded, and in

      particular how this process would affect the treatment of CRLFs,

      given that the representation of newlines varies greatly from

      system to system, and the relationship between content-transfer-

      encodings and character sets.  For this reason, a canonical model

      for encoding is presented as Appendix G.



5.1.  Quoted-Printable Content-Transfer-Encoding



   The Quoted-Printable encoding is intended to represent data that

   largely consists of octets that correspond to printable characters in

   the ASCII character set.  It encodes the data in such a way that the

   resulting octets are unlikely to be modified by mail transport.  If

   the data being encoded are mostly ASCII text, the encoded form of the

   data remains largely recognizable by humans.  A body which is

   entirely ASCII may also be encoded in Quoted-Printable to ensure the

   integrity of the data should the message pass through a character-

   translating, and/or line-wrapping gateway.



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

   following rules:



      Rule #1: (General 8-bit representation) Any octet, except those

      indicating a line break according to the newline convention of the

      canonical (standard) form of the data being encoded, may be

      represented by an "=" followed by a two digit hexadecimal

      representation of the octet's value.  The digits of the

      hexadecimal alphabet, for this purpose, are "0123456789ABCDEF".

      Uppercase letters must be used when sending hexadecimal data,

      though a robust implementation may choose to recognize lowercase

      letters on receipt.  Thus, for example, the value 12 (ASCII form

      feed) can be represented by "=0C", and the value 61 (ASCII EQUAL

      SIGN) can be represented by "=3D".  Except when the following

      rules allow an alternative encoding, this rule is mandatory.



      Rule #2: (Literal representation) Octets with decimal values of 33

      through 60 inclusive, and 62 through 126, inclusive, MAY be

      represented as the ASCII characters which correspond to those

      octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN

      through TILDE, respectively).



      Rule #3: (White Space): Octets with values of 9 and 32 MAY be

      represented as ASCII TAB (HT) and SPACE characters, respectively,

      but MUST NOT be so represented at the end of an encoded line. Any

      TAB (HT) or SPACE characters on an encoded line MUST thus be

      followed on that line by a printable character.  In particular, an







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      "=" at the end of an encoded line, indicating a soft line break

      (see rule #5) may follow one or more TAB (HT) or SPACE characters.

      It follows that an octet with value 9 or 32 appearing at the end

      of an encoded line must be represented according to Rule #1.  This

      rule is necessary because some MTAs (Message Transport Agents,

      programs which transport messages from one user to another, or

      perform a part of such transfers) are known to pad lines of text

      with SPACEs, and others are known to remove "white space"

      characters from the end of a line.  Therefore, when decoding a

      Quoted-Printable body, any trailing white space on a line must be

      deleted, as it will necessarily have been added by intermediate

      transport agents.



      Rule #4 (Line Breaks): A line break in a text body, independent of

      what its representation is following the canonical representation

      of the data being encoded, must be represented by a (RFC 822) line

      break, which is a CRLF sequence, in the Quoted-Printable encoding.

      Since the canonical representation of types other than text do not

      generally include the representation of line breaks, no hard line

      breaks (i.e.  line breaks that are intended to be meaningful and

      to be displayed to the user) should occur in the quoted-printable

      encoding of such types.  Of course, occurrences of "=0D", "=0A",

      "0A=0D" and "=0D=0A" will eventually be encountered.  In general,

      however, base64 is preferred over quoted-printable for binary

      data.



      Note that many implementations may elect to encode the local

      representation of various content types directly, as described in

      Appendix G.  In particular, this may apply to plain text material

      on systems that use newline conventions other than CRLF

      delimiters. Such an implementation is permissible, but the

      generation of line breaks must be generalized to account for the

      case where alternate representations of newline sequences are

      used.



      Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES

      that encoded lines be no more than 76 characters long. If longer

      lines are to be encoded with the Quoted-Printable encoding, 'soft'

      line breaks must be used. An equal sign as the last character on a

      encoded line indicates such a non-significant ('soft') line break

      in the encoded text. Thus if the "raw" form of the line is a

      single unencoded line that says:



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

          their country.



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









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          Now's the time =

          for all folk to come=

           to the aid of their country.



      This provides a mechanism with which long lines are encoded in

      such a way as to be restored by the user agent.  The 76 character

      limit does not count the trailing CRLF, but counts all other

      characters, including any equal signs.



   Since the hyphen character ("-") is represented as itself in the

   Quoted-Printable encoding, care must be taken, when encapsulating a

   quoted-printable encoded body in a multipart entity, to ensure that

   the encapsulation boundary does not appear anywhere in the encoded

   body.  (A good strategy is to choose a boundary that includes a

   character sequence such as "=_" which can never appear in a quoted-

   printable body.  See the definition of multipart messages later in

   this document.)



      NOTE: The quoted-printable encoding represents something of a

      compromise between readability and reliability in transport.

      Bodies encoded with the quoted-printable encoding will work

      reliably over most mail gateways, but may not work perfectly over

      a few gateways, notably those involving translation into EBCDIC.

      (In theory, an EBCDIC gateway could decode a quoted-printable body

      and re-encode it using base64, but such gateways do not yet

      exist.)  A higher level of confidence is offered by the base64

      Content-Transfer-Encoding.  A way to get reasonably reliable

      transport through EBCDIC gateways is to also quote the ASCII

      characters



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



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



   Because quoted-printable data is generally assumed to be line-

   oriented, it is to be expected that the representation of the breaks

   between the lines of quoted printable data may be altered in

   transport, in the same manner that plain text mail has always been

   altered in Internet mail when passing between systems with differing

   newline conventions.  If such alterations are likely to constitute a

   corruption of the data, it is probably more sensible to use the

   base64 encoding rather than the quoted-printable encoding.



   WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-

   printable, care must be taken to encode CR and LF characters as "=0D"

   and "=0A", respectively.  In particular, a CRLF sequence in binary

   data should be encoded as "=0D=0A".  Otherwise, if CRLF were

   represented as a hard line break, it might be incorrectly decoded on







Borenstein & Freed                                             [Page 20]



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   platforms with different line break conventions.



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

   the following grammar:



   quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)

        ; Maximum line length of 76 characters excluding CRLF



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

        ; characters not listed as "mail-safe" in Appendix B

        ; are also not recommended.



   octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")

        ; octet must be used for characters > 127, =, SPACE, or TAB,

        ; and is recommended for any characters not listed in

        ; Appendix B as "mail-safe".



5.2.  Base64 Content-Transfer-Encoding



   The Base64 Content-Transfer-Encoding is designed to represent

   arbitrary sequences of octets in a form that need not be humanly

   readable.  The encoding and decoding algorithms are simple, but the

   encoded data are consistently only about 33 percent larger than the

   unencoded data.  This encoding is virtually identical to the one used

   in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.

   The base64 encoding is adapted from RFC 1421, with one change: base64

   eliminates the "*" mechanism for embedded clear text.



   A 65-character subset of US-ASCII is used, enabling 6 bits to be

   represented per printable character. (The extra 65th character, "=",

   is used to signify a special processing function.)



      NOTE: This subset has the important property that it is

      represented identically in all versions of ISO 646, including US

      ASCII, and all characters in the subset are also represented

      identically in all versions of EBCDIC.  Other popular encodings,

      such as the encoding used by the uuencode utility and the base85

      encoding specified as part of Level 2 PostScript, do not share

      these properties, and thus do not fulfill the portability

      requirements a binary transport encoding for mail must meet.



   The encoding process represents 24-bit groups of input bits as output

   strings of 4 encoded characters. Proceeding from left to right, a

   24-bit input group is formed by concatenating 3 8-bit input groups.

   These 24 bits are then treated as 4 concatenated 6-bit groups, each

   of which is translated into a single digit in the base64 alphabet.

   When encoding a bit stream via the base64 encoding, the bit stream

   must be presumed to be ordered with the most-significant-bit first.







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   That is, the first bit in the stream will be the high-order bit in

   the first byte, and the eighth bit will be the low-order bit in the

   first byte, and so on.



   Each 6-bit group is used as an index into an array of 64 printable

   characters. The character referenced by the index is placed in the

   output string. These characters, identified in Table 1, below, are

   selected so as to be universally representable, and the set excludes

   characters with particular significance to SMTP (e.g., ".", CR, LF)

   and to the encapsulation boundaries defined in this document (e.g.,

   "-").



                            Table 1: The Base64 Alphabet



      Value Encoding  Value Encoding  Value Encoding  Value Encoding

           0 A            17 R            34 i            51 z

           1 B            18 S            35 j            52 0

           2 C            19 T            36 k            53 1

           3 D            20 U            37 l            54 2

           4 E            21 V            38 m            55 3

           5 F            22 W            39 n            56 4

           6 G            23 X            40 o            57 5

           7 H            24 Y            41 p            58 6

           8 I            25 Z            42 q            59 7

           9 J            26 a            43 r            60 8

          10 K            27 b            44 s            61 9

          11 L            28 c            45 t            62 +

          12 M            29 d            46 u            63 /

          13 N            30 e            47 v

          14 O            31 f            48 w         (pad) =

          15 P            32 g            49 x

          16 Q            33 h            50 y



   The output stream (encoded bytes) must be represented in lines of no

   more than 76 characters each.  All line breaks or other characters

   not found in Table 1 must be ignored by decoding software.  In base64

   data, characters other than those in Table 1, line breaks, and other

   white space probably indicate a transmission error, about which a

   warning message or even a message rejection might be appropriate

   under some circumstances.



   Special processing is performed if fewer than 24 bits are available

   at the end of the data being encoded.  A full encoding quantum is

   always completed at the end of a body.  When fewer than 24 input bits

   are available in an input group, zero bits are added (on the right)

   to form an integral number of 6-bit groups.  Padding at the end of

   the data is performed using the '=' character.  Since all base64

   input is an integral number of octets, only the following cases can







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   arise: (1) the final quantum of encoding input is an integral

   multiple of 24 bits; here, the final unit of encoded output will be

   an integral multiple of 4 characters with no "=" padding, (2) the

   final quantum of encoding input is exactly 8 bits; here, the final

   unit of encoded output will be two characters followed by two "="

   padding characters, or (3) the final quantum of encoding input is

   exactly 16 bits; here, the final unit of encoded output will be three

   characters followed by one "=" padding character.



   Because it is used only for padding at the end of the data, the

   occurrence of any '=' characters may be taken as evidence that the

   end of the data has been reached (without truncation in transit).  No

   such assurance is possible, however, when the number of octets

   transmitted was a multiple of three.



   Any characters outside of the base64 alphabet are to be ignored in

   base64-encoded data.  The same applies to any illegal sequence of

   characters in the base64 encoding, such as "====="



   Care must be taken to use the proper octets for line breaks if base64

   encoding is applied directly to text material that has not been

   converted to canonical form.  In particular, text line breaks must be

   converted into CRLF sequences prior to base64 encoding. The important

   thing to note is that this may be done directly by the encoder rather

   than in a prior canonicalization step in some implementations.



      NOTE: There is no need to worry about quoting apparent

      encapsulation boundaries within base64-encoded parts of multipart

      entities because no hyphen characters are used in the base64

      encoding.



6.    Additional Content-Header Fields



6.1.  Optional Content-ID Header Field



   In constructing a high-level user agent, it may be desirable to allow

   one body to make reference to another.  Accordingly, bodies may be

   labeled using the "Content-ID" header field, which is syntactically

   identical to the "Message-ID" header field:



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

   Like the Message-ID values, Content-ID values must be generated to be

   world-unique.



   The Content-ID value may be used for uniquely identifying MIME

   entities in several contexts, particularly for cacheing data

   referenced by the message/external-body mechanism.  Although the

   Content-ID header is generally optional, its use is mandatory in







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   implementations which generate data of the optional MIME Content-type

   "message/external-body".  That is, each message/external-body entity

   must have a Content-ID field to permit cacheing of such data.



   It is also worth noting that the Content-ID value has special

   semantics in the case of the multipart/alternative content-type.

   This is explained in the section of this document dealing with

   multipart/alternative.



6.2.  Optional Content-Description Header Field



   The ability to associate some descriptive information with a given

   body is often desirable. For example, it may be useful to mark an

   "image" body as "a picture of the Space Shuttle Endeavor."  Such text

   may be placed in the Content-Description header field.



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



   The description is presumed to be given in the US-ASCII character

   set, although the mechanism specified in [RFC-1522] may be used for

   non-US-ASCII Content-Description values.



7.    The Predefined Content-Type Values



   This document defines seven initial Content-Type values and an

   extension mechanism for private or experimental types.  Further

   standard types must be defined by new published specifications.  It

   is expected that most innovation in new types of mail will take place

   as subtypes of the seven types defined here.  The most essential

   characteristics of the seven content-types are summarized in Appendix

   F.



7.1  The Text Content-Type



   The text Content-Type is intended for sending material which is

   principally textual in form.  It is the default Content-Type.  A

   "charset" parameter may be used to indicate the character set of the

   body text for some text subtypes, notably including the primary

   subtype, "text/plain", which indicates plain (unformatted) text.  The

   default Content-Type for Internet mail is "text/plain; charset=us-

   ascii".



   Beyond plain text, there are many formats for representing what might

   be known as "extended text" -- text with embedded formatting and

   presentation information.  An interesting characteristic of many such

   representations is that they are to some extent readable even without

   the software that interprets them.  It is useful, then, to

   distinguish them, at the highest level, from such unreadable data as







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   images, audio, or text represented in an unreadable form.  In the

   absence of appropriate interpretation software, it is reasonable to

   show subtypes of text to the user, while it is not reasonable to do

   so with most nontextual data.



   Such formatted textual data should be represented using subtypes of

   text.  Plausible subtypes of text are typically given by the common

   name of the representation format, e.g., "text/richtext" [RFC-1341].



7.1.1.     The charset parameter



   A critical parameter that may be specified in the Content-Type field

   for text/plain data is the character set.  This is specified with a

   "charset" parameter, as in:



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



   Unlike some other parameter values, the values of the charset

   parameter are NOT case sensitive.  The default character set, which

   must be assumed in the absence of a charset parameter, is US-ASCII.



   The specification for any future subtypes of "text" must specify

   whether or not they will also utilize a "charset" parameter, and may

   possibly restrict its values as well.  When used with a particular

   body, the semantics of the "charset" parameter should be identical to

   those specified here for "text/plain", i.e., the body consists

   entirely of characters in the given charset.  In particular, definers

   of future text subtypes should pay close attention the the

   implications of multibyte character sets for their subtype

   definitions.



   This RFC specifies the definition of the charset parameter for the

   purposes of MIME to be a unique mapping of a byte stream to glyphs, a

   mapping which does not require external profiling information.



   An initial list of predefined character set names can be found at the

   end of this section.  Additional character sets may be registered

   with IANA, although the standardization of their use requires the

   usual IESG [RFC-1340] review and approval.  Note that if the

   specified character set includes 8-bit data, a Content-Transfer-

   Encoding header field and a corresponding encoding on the data are

   required in order to transmit the body via some mail transfer

   protocols, such as SMTP.



   The default character set, US-ASCII, has been the subject of some

   confusion and ambiguity in the past.  Not only were there some

   ambiguities in the definition, there have been wide variations in

   practice.  In order to eliminate such ambiguity and variations in the







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   future, it is strongly recommended that new user agents explicitly

   specify a character set via the Content-Type header field.  "US-

   ASCII" does not indicate an arbitrary seven-bit character code, but

   specifies that the body uses character coding that uses the exact

   correspondence of codes to characters specified in ASCII.  National

   use variations of ISO 646 [ISO-646] are NOT ASCII and their use in

   Internet mail is explicitly discouraged. The omission of the ISO 646

   character set is deliberate in this regard.  The character set name

   of "US-ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only.

   The character set name "ASCII" is reserved and must not be used for

   any purpose.



      NOTE: RFC 821 explicitly specifies "ASCII", and references an

      earlier version of the American Standard.  Insofar as one of the

      purposes of specifying a Content-Type and character set is to

      permit the receiver to unambiguously determine how the sender

      intended the coded message to be interpreted, assuming anything

      other than "strict ASCII" as the default would risk unintentional

      and incompatible changes to the semantics of messages now being

      transmitted.  This also implies that messages containing

      characters coded according to national variations on ISO 646, or

      using code-switching procedures (e.g., those of ISO 2022), as well

      as 8-bit or multiple octet character encodings MUST use an

      appropriate character set specification to be consistent with this

      specification.



   The complete US-ASCII character set is listed in [US-ASCII].  Note

   that the control characters including DEL (0-31, 127) have no defined

   meaning apart from the combination CRLF (ASCII values 13 and 10)

   indicating a new line.  Two of the characters have de facto meanings

   in wide use: FF (12) often means "start subsequent text on the

   beginning of a new page"; and TAB or HT (9) often (though not always)

   means "move the cursor to the next available column after the current

   position where the column number is a multiple of 8 (counting the

   first column as column 0)." Apart from this, any use of the control

   characters or DEL in a body must be part of a private agreement

   between the sender and recipient.  Such private agreements are

   discouraged and should be replaced by the other capabilities of this

   document.



      NOTE: Beyond US-ASCII, an enormous proliferation of character sets

      is possible. It is the opinion of the IETF working group that a

      large number of character sets is NOT a good thing.  We would

      prefer to specify a single character set that can be used

      universally for representing all of the world's languages in

      electronic mail.  Unfortunately, existing practice in several

      communities seems to point to the continued use of multiple

      character sets in the near future.  For this reason, we define







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      names for a small number of character sets for which a strong

      constituent base exists.



   The defined charset values are:



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



        ISO-8859-X -- where "X" is to be replaced, as necessary, for the

             parts of ISO-8859 [ISO-8859].  Note that the ISO 646

             character sets have deliberately been omitted in favor of

             their 8859 replacements, which are the designated character

             sets for Internet mail.  As of the publication of this

             document, the legitimate values for "X" are the digits 1

             through 9.



   The character sets specified above are the ones that were relatively

   uncontroversial during the drafting of MIME.  This document does not

   endorse the use of any particular character set other than US-ASCII,

   and recognizes that the future evolution of world character sets

   remains unclear.  It is expected that in the future, additional

   character sets will be registered for use in MIME.



   Note that the character set used, if anything other than US-ASCII,

   must always be explicitly specified in the Content-Type field.



   No other character set name may be used in Internet mail without the

   publication of a formal specification and its registration with IANA,

   or by private agreement, in which case the character set name must

   begin with "X-".



   Implementors are discouraged from defining new character sets for

   mail use unless absolutely necessary.



   The "charset" parameter has been defined primarily for the purpose of

   textual data, and is described in this section for that reason.

   However, it is conceivable that non-textual data might also wish to

   specify a charset value for some purpose, in which case the same

   syntax and values should be used.



   In general, mail-sending software must always use the "lowest common

   denominator" character set possible.  For example, if a body contains

   only US-ASCII characters, it must be marked as being in the US-ASCII

   character set, not ISO-8859-1, which, like all the ISO-8859 family of

   character sets, is a superset of US-ASCII.  More generally, if a

   widely-used character set is a subset of another character set, and a

   body contains only characters in the widely-used subset, it must be

   labeled as being in that subset.  This will increase the chances that

   the recipient will be able to view the mail correctly.







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7.1.2.     The Text/plain subtype



   The primary subtype of text is "plain".  This indicates plain

   (unformatted) text.  The default Content-Type for Internet mail,

   "text/plain; charset=us-ascii", describes existing Internet practice.

   That is, it is the type of body defined by RFC 822.



   No other text subtype is defined by this document.



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

   follows:



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



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



   charset := "us-ascii"/ "iso-8859-1"/ "iso-8859-2"/ "iso-8859-3"

          / "iso-8859-4"/ "iso-8859-5"/ "iso-8859-6"/ "iso-8859-7"

          / "iso-8859-8" / "iso-8859-9" / extension-token

                    ; case insensitive



7.2.  The Multipart Content-Type



   In the case of multiple part entities, in which one or more different

   sets of data are combined in a single body, a "multipart" Content-

   Type field must appear in the entity's header. The body must then

   contain one or more "body parts," each preceded by an encapsulation

   boundary, and the last one followed by a closing boundary.  Each part

   starts with an encapsulation boundary, and then contains a body part

   consisting of header area, a blank line, and a body area.  Thus a

   body part is similar to an RFC 822 message in syntax, but different

   in meaning.



   A body part is NOT to be interpreted as actually being an RFC 822

   message.  To begin with, NO header fields are actually required in

   body parts.  A body part that starts with a blank line, therefore, is

   allowed and is a body part for which all default values are to be

   assumed.  In such a case, the absence of a Content-Type header field

   implies that the corresponding body is plain US-ASCII text.  The only

   header fields that have defined meaning for body parts are those the

   names of which begin with "Content-".  All other header fields are

   generally to be ignored in body parts.  Although they should

   generally be retained in mail processing, they may be discarded by

   gateways if necessary.  Such other fields are permitted to appear in

   body parts but must not be depended on.  "X-" fields may be created

   for experimental or private purposes, with the recognition that the

   information they contain may be lost at some gateways.









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      NOTE: The distinction between an RFC 822 message and a body part

      is subtle, but important. A gateway between Internet and X.400

      mail, for example, must be able to tell the difference between a

      body part that contains an image and a body part that contains an

      encapsulated message, the body of which is an image.  In order to

      represent the latter, the body part must have "Content-Type:

      message", and its body (after the blank line) must be the

      encapsulated message, with its own "Content-Type: image" header

      field.  The use of similar syntax facilitates the conversion of

      messages to body parts, and vice versa, but the distinction

      between the two must be understood by implementors.  (For the

      special case in which all parts actually are messages, a "digest"

      subtype is also defined.)



   As stated previously, each body part is preceded by an encapsulation

   boundary.  The encapsulation boundary MUST NOT appear inside any of

   the encapsulated parts.  Thus, it is crucial that the composing agent

   be able to choose and specify the unique boundary that will separate

   the parts.



   All present and future subtypes of the "multipart" type must use an

   identical syntax.  Subtypes may differ in their semantics, and may

   impose additional restrictions on syntax, but must conform to the

   required syntax for the multipart type.  This requirement ensures

   that all conformant user agents will at least be able to recognize

   and separate the parts of any multipart entity, even of an

   unrecognized subtype.



   As stated in the definition of the Content-Transfer-Encoding field,

   no encoding other than "7bit", "8bit", or "binary" is permitted for

   entities of type "multipart".  The multipart delimiters and header

   fields are always represented as 7-bit ASCII in any case (though the

   header fields may encode non-ASCII header text as per [RFC-1522]),

   and data within the body parts can be encoded on a part-by-part

   basis, with Content-Transfer-Encoding fields for each appropriate

   body part.



   Mail gateways, relays, and other mail handling agents are commonly

   known to alter the top-level header of an RFC 822 message.  In

   particular, they frequently add, remove, or reorder header fields.

   Such alterations are explicitly forbidden for the body part headers

   embedded in the bodies of messages of type "multipart."



7.2.1.     Multipart:  The common syntax



   All subtypes of "multipart" share a common syntax, defined in this

   section.  A simple example of a multipart message also appears in

   this section.  An example of a more complex multipart message is







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   given in Appendix C.



   The Content-Type field for multipart entities requires one parameter,

   "boundary", which is used to specify the encapsulation boundary.  The

   encapsulation boundary is defined as a line consisting entirely of

   two hyphen characters ("-", decimal code 45) followed by the boundary

   parameter value from the Content-Type header field.



      NOTE: The hyphens are for rough compatibility with the earlier RFC

      934 method of message encapsulation, and for ease of searching for

      the boundaries in some implementations. However, it should be

      noted that multipart messages are NOT completely compatible with

      RFC 934 encapsulations; in particular, they do not obey RFC 934

      quoting conventions for embedded lines that begin with hyphens.

      This mechanism was chosen over the RFC 934 mechanism because the

      latter causes lines to grow with each level of quoting.  The

      combination of this growth with the fact that SMTP implementations

      sometimes wrap long lines made the RFC 934 mechanism unsuitable

      for use in the event that deeply-nested multipart structuring is

      ever desired.



   WARNING TO IMPLEMENTORS: The grammar for parameters on the Content-

   type field is such that it is often necessary to enclose the

   boundaries in quotes on the Content-type line.  This is not always

   necessary, but never hurts.  Implementors should be sure to study the

   grammar carefully in order to avoid producing illegal Content-type

   fields. Thus, a typical multipart Content-Type header field might

   look like this:



                 Content-Type: multipart/mixed;

                      boundary=gc0p4Jq0M2Yt08jU534c0p



   But the following is illegal:



                 Content-Type: multipart/mixed;

                      boundary=gc0p4Jq0M:2Yt08jU534c0p



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



                 Content-Type: multipart/mixed;

                      boundary="gc0p4Jq0M:2Yt08jU534c0p"



   This indicates that the entity consists of several parts, each itself

   with a structure that is syntactically identical to an RFC 822

   message, except that the header area might be completely empty, and

   that the parts are each preceded by the line



                 --gc0p4Jq0M:2Yt08jU534c0p







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   Note that the encapsulation boundary must occur at the beginning of a

   line, i.e., following a CRLF, and that the initial CRLF is considered

   to be attached to the encapsulation boundary rather than part of the

   preceding part.  The boundary must be followed immediately either by

   another CRLF and the header fields for the next part, or by two

   CRLFs, in which case there are no header fields for the next part

   (and it is therefore assumed to be of Content-Type text/plain).



      NOTE: The CRLF preceding the encapsulation line is conceptually

      attached to the boundary so that it is possible to have a part

      that does not end with a CRLF (line break). Body parts that must

      be considered to end with line breaks, therefore, must have two

      CRLFs preceding the encapsulation line, the first of which is part

      of the preceding body part, and the second of which is part of the

      encapsulation boundary.



   Encapsulation boundaries must not appear within the encapsulations,

   and must be no longer than 70 characters, not counting the two

   leading hyphens.



   The encapsulation boundary following the last body part is a

   distinguished delimiter that indicates that no further body parts

   will follow.  Such a delimiter is identical to the previous

   delimiters, with the addition of two more hyphens at the end of the

   line:



                 --gc0p4Jq0M2Yt08jU534c0p--



   There appears to be room for additional information prior to the

   first encapsulation boundary and following the final boundary.  These

   areas should generally be left blank, and implementations must ignore

   anything that appears before the first boundary or after the last

   one.



      NOTE: These "preamble" and "epilogue" areas are generally not used

      because of the lack of proper typing of these parts and the lack

      of clear semantics for handling these areas at gateways,

      particularly X.400 gateways.  However, rather than leaving the

      preamble area blank, many MIME implementations have found this to

      be a convenient place to insert an explanatory note for recipients

      who read the message with pre-MIME software, since such notes will

      be ignored by MIME-compliant software.



      NOTE: Because encapsulation boundaries must not appear in the body

      parts being encapsulated, a user agent must exercise care to

      choose a unique boundary.  The boundary in the example above could

      have been the result of an algorithm designed to produce

      boundaries with a very low probability of already existing in the







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      data to be encapsulated without having to prescan the data.

      Alternate algorithms might result in more 'readable' boundaries

      for a recipient with an old user agent, but would require more

      attention to the possibility that the boundary might appear in the

      encapsulated part.  The simplest boundary possible is something

      like "---", with a closing boundary of "-----".



   As a very simple example, the following multipart message has two

   parts, both of them plain text, one of them explicitly typed and one

   of them implicitly typed:



      From: Nathaniel Borenstein <nsb@bellcore.com>

      To:  Ned Freed <ned@innosoft.com>

      Subject: Sample message

      MIME-Version: 1.0

      Content-type: multipart/mixed; boundary="simple

      boundary"



      This is the preamble.  It is to be ignored, though it

      is a handy place for mail composers to include an

      explanatory note to non-MIME conformant readers.

      --simple boundary



      This is implicitly typed plain ASCII text.

      It does NOT end with a linebreak.

      --simple boundary

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



      This is explicitly typed plain ASCII text.

      It DOES end with a linebreak.



      --simple boundary--

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



   The use of a Content-Type of multipart in a body part within another

   multipart entity is explicitly allowed.  In such cases, for obvious

   reasons, care must be taken to ensure that each nested multipart

   entity must use a different boundary delimiter. See Appendix C for an

   example of nested multipart entities.



   The use of the multipart Content-Type with only a single body part

   may be useful in certain contexts, and is explicitly permitted.



   The only mandatory parameter for the multipart Content-Type is the

   boundary parameter, which consists of 1 to 70 characters from a set

   of characters known to be very robust through email gateways, and NOT

   ending with white space.  (If a boundary appears to end with white

   space, the white space must be presumed to have been added by a







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

   following BNF:



   boundary := 0*69<bchars> bcharsnospace



   bchars := bcharsnospace / " "



   bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+" /"_"

                 / "," / "-" / "." / "/" / ":" / "=" / "?"



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

   follows:



   multipart-body := preamble 1*encapsulation

                  close-delimiter epilogue



   encapsulation := delimiter body-part CRLF



   delimiter := "--" boundary CRLF ; taken from Content-Type field.

                                   ; There must be no space

                                   ; between "--" and boundary.



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

   by "--",



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



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



   discard-text := *(*text CRLF)



   body-part := <"message" as defined in RFC 822,

             with all header fields optional, and with the

             specified delimiter not occurring anywhere in

             the message body, either on a line by itself

             or as a substring anywhere.  Note that the

             semantics of a part differ from the semantics

             of a message, as described in the text.>



      NOTE: In certain transport enclaves, RFC 822 restrictions such as

      the one that limits bodies to printable ASCII characters may not

      be in force.  (That is, the transport domains may resemble

      standard Internet mail transport as specified in RFC821 and

      assumed by RFC822, but without certain restrictions.)  The

      relaxation of these restrictions should be construed as locally

      extending the definition of bodies, for example to include octets

      outside of the ASCII range, as long as these extensions are

      supported by the transport and adequately documented in the







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      Content-Transfer-Encoding header field. However, in no event are

      headers (either message headers or body-part headers) allowed to

      contain anything other than ASCII characters.



      NOTE: Conspicuously missing from the multipart type is a notion of

      structured, related body parts.  In general, it seems premature to

      try to standardize interpart structure yet.  It is recommended

      that those wishing to provide a more structured or integrated

      multipart messaging facility should define a subtype of multipart

      that is syntactically identical, but that always expects the

      inclusion of a distinguished part that can be used to specify the

      structure and integration of the other parts, probably referring

      to them by their Content-ID field.  If this approach is used,

      other implementations will not recognize the new subtype, but will

      treat it as the primary subtype (multipart/mixed) and will thus be

      able to show the user the parts that are recognized.



7.2.2.     The Multipart/mixed (primary) subtype



   The primary subtype for multipart, "mixed", is intended for use when

   the body parts are independent and need to be bundled in a particular

   order.  Any multipart subtypes that an implementation does not

   recognize must be treated as being of subtype "mixed".



7.2.3.     The Multipart/alternative subtype



   The multipart/alternative type is syntactically identical to

   multipart/mixed, but the semantics are different.  In particular,

   each of the parts is an "alternative" version of the same

   information.



   Systems should recognize that the content of the various parts are

   interchangeable.  Systems should choose the "best" type based on the

   local environment and preferences, in some cases even through user

   interaction.  As with multipart/mixed, the order of body parts is

   significant.  In this case, the alternatives appear in an order of

   increasing faithfulness to the original content. In general, the best

   choice is the LAST part of a type supported by the recipient system's

   local environment.



   Multipart/alternative may be used, for example, to send mail in a

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

   anywhere:

















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   From:  Nathaniel Borenstein <nsb@bellcore.com>

   To: Ned Freed <ned@innosoft.com>

   Subject: Formatted text mail

   MIME-Version: 1.0

   Content-Type: multipart/alternative; boundary=boundary42



   --boundary42



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



      ...plain text version of message goes here....

   --boundary42

   Content-Type: text/richtext



      .... RFC 1341 richtext version of same message goes here ...

   --boundary42

   Content-Type: text/x-whatever



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

      ...

   --boundary42--



   In this example, users whose mail system understood the "text/x-

   whatever" format would see only the fancy version, while other users

   would see only the richtext or plain text version, depending on the

   capabilities of their system.



   In general, user agents that compose multipart/alternative entities

   must place the body parts in increasing order of preference, that is,

   with the preferred format last.  For fancy text, the sending user

   agent should put the plainest format first and the richest format

   last.  Receiving user agents should pick and display the last format

   they are capable of displaying.  In the case where one of the

   alternatives is itself of type "multipart" and contains unrecognized

   sub-parts, the user agent may choose either to show that alternative,

   an earlier alternative, or both.



      NOTE: From an implementor's perspective, it might seem more

      sensible to reverse this ordering, and have the plainest

      alternative last.  However, placing the plainest alternative first

      is the friendliest possible option when multipart/alternative

      entities are viewed using a non-MIME-conformant mail reader.

      While this approach does impose some burden on conformant mail

      readers, interoperability with older mail readers was deemed to be

      more important in this case.



   It may be the case that some user agents, if they can recognize more

   than one of the formats, will prefer to offer the user the choice of







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   which format to view.  This makes sense, for example, if mail

   includes both a nicely-formatted image version and an easily-edited

   text version.  What is most critical, however, is that the user not

   automatically be shown multiple versions of the same data.  Either

   the user should be shown the last recognized version or should be

   given the choice.



   NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each

   part of a multipart/alternative entity represents the same data, but

   the mappings between the two are not necessarily without information

   loss.  For example, information is lost when translating ODA to

   PostScript or plain text.  It is recommended that each part should

   have a different Content-ID value in the case where the information

   content of the two parts is not identical.  However, where the

   information content is identical -- for example, where several parts

   of type "application/external- body" specify alternate ways to access

   the identical data -- the same Content-ID field value should be used,

   to optimize any cacheing mechanisms that might be present on the

   recipient's end.  However, it is recommended that the Content-ID

   values used by the parts should not be the same Content-ID value that

   describes the multipart/alternative as a whole, if there is any such

   Content-ID field.  That is, one Content-ID value will refer to the

   multipart/alternative entity, while one or more other Content-ID

   values will refer to the parts inside it.



7.2.4.     The Multipart/digest subtype



   This document defines a "digest" subtype of the multipart Content-

   Type.  This type is syntactically identical to multipart/mixed, but

   the semantics are different.  In particular, in a digest, the default

   Content-Type value for a body part is changed from "text/plain" to

   "message/rfc822".  This is done to allow a more readable digest

   format that is largely compatible (except for the quoting convention)

   with RFC 934.



































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



   From: Moderator-Address

   To: Recipient-List

   MIME-Version: 1.0

   Subject:  Internet Digest, volume 42

   Content-Type: multipart/digest;

        boundary="---- next message ----"



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



   From: someone-else

   Subject: my opinion



      ...body goes here ...



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



   From: someone-else-again

   Subject: my different opinion



      ... another body goes here...



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



7.2.5.     The Multipart/parallel subtype



   This document defines a "parallel" subtype of the multipart Content-

   Type.  This type is syntactically identical to multipart/mixed, but

   the semantics are different.  In particular, in a parallel entity,

   the order of body parts is not significant.



   A common presentation of this type is to display all of the parts

   simultaneously on hardware and software that are capable of doing so.

   However, composing agents should be aware that many mail readers will

   lack this capability and will show the parts serially in any event.



7.2.6.     Other Multipart subtypes



   Other multipart subtypes are expected in the future.  MIME

   implementations must in general treat unrecognized subtypes of

   multipart as being equivalent to "multipart/mixed".



   The formal grammar for content-type header fields for multipart data

   is given by:



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

                  ";" "boundary" "=" boundary







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   multipart-subtype := "mixed" / "parallel" / "digest"

                  / "alternative" / extension-token



7.3.  The Message Content-Type



   It is frequently desirable, in sending mail, to encapsulate another

   mail message. For this common operation, a special Content-Type,

   "message", is defined.  The primary subtype, message/rfc822, has no

   required parameters in the Content-Type field.  Additional subtypes,

   "partial" and "External-body", do have required parameters.  These

   subtypes are explained below.



      NOTE: It has been suggested that subtypes of message might be

      defined for forwarded or rejected messages.  However, forwarded

      and rejected messages can be handled as multipart messages in

      which the first part contains any control or descriptive

      information, and a second part, of type message/rfc822, is the

      forwarded or rejected message.  Composing rejection and forwarding

      messages in this manner will preserve the type information on the

      original message and allow it to be correctly presented to the

      recipient, and hence is strongly encouraged.



   As stated in the definition of the Content-Transfer-Encoding field,

   no encoding other than "7bit", "8bit", or "binary" is permitted for

   messages or parts of type "message".  Even stronger restrictions

   apply to the subtypes "message/partial" and "message/external-body",

   as specified below.  The message header fields are always US-ASCII in

   any case, and data within the body can still be encoded, in which

   case the Content-Transfer-Encoding header field in the encapsulated

   message will reflect this.  Non-ASCII text in the headers of an

   encapsulated message can be specified using the mechanisms described

   in [RFC-1522].



   Mail gateways, relays, and other mail handling agents are commonly

   known to alter the top-level header of an RFC 822 message.  In

   particular, they frequently add, remove, or reorder header fields.

   Such alterations are explicitly forbidden for the encapsulated

   headers embedded in the bodies of messages of type "message."



7.3.1.     The Message/rfc822 (primary) subtype



   A Content-Type of "message/rfc822" indicates that the body contains

   an encapsulated message, with the syntax of an RFC 822 message.

   However, unlike top-level RFC 822 messages, it is not required that

   each message/rfc822 body must include a "From", "Subject", and at

   least one destination header.



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







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   message/rfc822 entity can include enhanced information as defined in

   this document.  In other words, a message/rfc822 message may be a

   MIME message.



7.3.2.     The Message/Partial subtype



   A subtype of message, "partial", is defined in order to allow large

   objects to be delivered as several separate pieces of mail and

   automatically reassembled by the receiving user agent.  (The concept

   is similar to IP fragmentation/reassembly in the basic Internet

   Protocols.)  This mechanism can be used when intermediate transport

   agents limit the size of individual messages that can be sent.

   Content-Type "message/partial" thus indicates that the body contains

   a fragment of a larger message.



   Three parameters must be specified in the Content-Type field of type

   message/partial: The first, "id", is a unique identifier, as close to

   a world-unique identifier as possible, to be used to match the parts

   together.  (In general, the identifier is essentially a message-id;

   if placed in double quotes, it can be any message-id, in accordance

   with the BNF for "parameter" given earlier in this specification.)

   The second, "number", an integer, is the part number, which indicates

   where this part fits into the sequence of fragments.  The third,

   "total", another integer, is the total number of parts. This third

   subfield is required on the final part, and is optional (though

   encouraged) on the earlier parts.  Note also that these parameters

   may be given in any order.



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

   header fields:



                Content-Type: Message/Partial;

                     number=2; total=3;

                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com"



                Content-Type: Message/Partial;

                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com";

                     number=2



   But part 3 MUST specify the total number of parts:



                Content-Type: Message/Partial;

                     number=3; total=3;

                     id="oc=jpbe0M2Yt4s@thumper.bellcore.com"



   Note that part numbering begins with 1, not 0.



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







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   together, the result is a complete MIME entity, which may have its

   own Content-Type header field, and thus may contain any other data

   type.



   Message fragmentation and reassembly: The semantics of a reassembled

   partial message must be those of the "inner" message, rather than of

   a message containing the inner message.  This makes it possible, for

   example, to send a large audio message as several partial messages,

   and still have it appear to the recipient as a simple audio message

   rather than as an encapsulated message containing an audio message.

   That is, the encapsulation of the message is considered to be

   "transparent".



   When generating and reassembling the parts of a message/partial

   message, the headers of the encapsulated message must be merged with

   the headers of the enclosing entities.  In this process the following

   rules must be observed:



      (1) All of the header fields from the initial enclosing entity

      (part one), except those that start with "Content-" and the

      specific header fields "Message-ID", "Encrypted", and "MIME-

      Version", must be copied, in order, to the new message.



      (2) Only those header fields in the enclosed message which start

      with "Content-" and "Message-ID", "Encrypted", and "MIME-Version"

      must be appended, in order, to the header fields of the new

      message.  Any header fields in the enclosed message which do not

      start with "Content-" (except for "Message-ID", "Encrypted", and

      "MIME-Version") will be ignored.



      (3) All of the header fields from the second and any subsequent

      messages will be ignored.



   For example, if an audio message is broken into two parts, the first

   part might look something like this:



      X-Weird-Header-1: Foo

      From: Bill@host.com

      To: joe@otherhost.com

      Subject: Audio mail

      Message-ID: <id1@host.com>

      MIME-Version: 1.0

      Content-type: message/partial;

           id="ABC@host.com";

           number=1; total=2



      X-Weird-Header-1: Bar

      X-Weird-Header-2: Hello







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      Message-ID: <anotherid@foo.com>

      MIME-Version: 1.0

      Content-type: audio/basic

      Content-transfer-encoding: base64



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



   and the second half might look something like this:



      From: Bill@host.com

      To: joe@otherhost.com

      Subject: Audio mail

      MIME-Version: 1.0

      Message-ID: <id2@host.com>

      Content-type: message/partial;

           id="ABC@host.com"; number=2; total=2



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



   Then, when the fragmented message is reassembled, the resulting

   message to be displayed to the user should look something like this:



      X-Weird-Header-1: Foo

      From: Bill@host.com

      To: joe@otherhost.com

      Subject: Audio mail

      Message-ID: <anotherid@foo.com>

      MIME-Version: 1.0

      Content-type: audio/basic

      Content-transfer-encoding: base64



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

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



   Note on encoding of MIME entities encapsulated inside message/partial

   entities: Because data of type "message" may never be encoded in

   base64 or quoted-printable, a problem might arise if message/partial

   entities are constructed in an environment that supports binary or

   8-bit transport.  The problem is that the binary data would be split

   into multiple message/partial objects, each of them requiring binary

   transport.  If such objects were encountered at a gateway into a 7-

   bit transport environment, there would be no way to properly encode

   them for the 7-bit world, aside from waiting for all of the parts,

   reassembling the message, and then encoding the reassembled data in

   base64 or quoted-printable.  Since it is possible that different

   parts might go through different gateways, even this is not an

   acceptable solution.  For this reason, it is specified that MIME

   entities of type message/partial must always have a content-







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   transfer-encoding of 7-bit (the default).  In particular, even in

   environments that support binary or 8-bit transport, the use of a

   content-transfer-encoding of "8bit" or "binary" is explicitly

   prohibited for entities of type message/partial.



   It should be noted that, because some message transfer agents may

   choose to automatically fragment large messages, and because such

   agents may use different fragmentation thresholds, it is possible

   that the pieces of a partial message, upon reassembly, may prove

   themselves to comprise a partial message.  This is explicitly

   permitted.



   It should also be noted that the inclusion of a "References" field in

   the headers of the second and subsequent pieces of a fragmented

   message that references the Message-Id on the previous piece may be

   of benefit to mail readers that understand and track references.

   However, the generation of such "References" fields is entirely

   optional.



   Finally, it should be noted that the "Encrypted" header field has

   been made obsolete by Privacy Enhanced Messaging (PEM), but the rules

   above are believed to describe the correct way to treat it if it is

   encountered in the context of conversion to and from message/partial

   fragments.



7.3.3.     The Message/External-Body subtype



   The external-body subtype indicates that the actual body data are not

   included, but merely referenced.  In this case, the parameters

   describe a mechanism for accessing the external data.



   When an entity is of type "message/external-body", it consists of a

   header, two consecutive CRLFs, and the message header for the

   encapsulated message.  If another pair of consecutive CRLFs appears,

   this of course ends the message header for the encapsulated message.

   However, since the encapsulated message's body is itself external, it

   does NOT appear in the area that follows.  For example, consider the

   following message:



      Content-type: message/external-body; access-

      type=local-file;



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



      Content-type:  image/gif

      Content-ID: <id42@guppylake.bellcore.com>

      Content-Transfer-Encoding: binary









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      THIS IS NOT REALLY THE BODY!



   The area at the end, which might be called the "phantom body", is

   ignored for most external-body messages.  However, it may be used to

   contain auxiliary information for some such messages, as indeed it is

   when the access-type is "mail-server".  Of the access-types defined

   by this document, the phantom body is used only when the access-type

   is "mail-server".  In all other cases, the phantom body is ignored.



   The only always-mandatory parameter for message/external-body is

   "access-type"; all of the other parameters may be mandatory or

   optional depending on the value of access-type.



      ACCESS-TYPE -- A case-insensitive word, indicating the supported

      access mechanism by which the file or data may be obtained.

      Values include, but are not limited to, "FTP", "ANON-FTP", "TFTP",

      "AFS", "LOCAL-FILE", and "MAIL-SERVER".  Future values, except for

      experimental values beginning with "X-" must be registered with

      IANA, as described in Appendix E .



   In addition, the following three parameters are optional for ALL

   access-types:



      EXPIRATION -- The date (in the RFC 822 "date-time" syntax, as

      extended by RFC 1123 to permit 4 digits in the year field) after

      which the existence of the external data is not guaranteed.



      SIZE -- The size (in octets) of the data.  The intent of this

      parameter is to help the recipient decide whether or not to expend

      the necessary resources to retrieve the external data.  Note that

      this describes the size of the data in its canonical form, that

      is, before any Content- Transfer-Encoding has been applied or

      after the data have been decoded.



      PERMISSION -- A case-insensitive field that indicates whether or

      not it is expected that clients might also attempt to overwrite

      the data.  By default, or if permission is "read", the assumption

      is that they are not, and that if the data is retrieved once, it

      is never needed again.  If PERMISSION is "read-write", this

      assumption is invalid, and any local copy must be considered no

      more than a cache.  "Read" and "Read-write" are the only defined

      values of permission.



   The precise semantics of the access-types defined here are described

   in the sections that follow.



   The encapsulated headers in ALL message/external-body entities MUST

   include a Content-ID header field to give a unique identifier by







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   which to reference the data.  This identifier may be used for

   cacheing mechanisms, and for recognizing the receipt of the data when

   the access-type is "mail-server".



   Note that, as specified here, the tokens that describe external-body

   data, such as file names and mail server commands, are required to be

   in the US-ASCII character set.  If this proves problematic in

   practice, a new mechanism may be required as a future extension to

   MIME, either as newly defined access-types for message/external-body

   or by some other mechanism.



   As with message/partial, it is specified that MIME entities of type

   message/external-body must always have a content-transfer-encoding of

   7-bit (the default).  In particular, even in environments that

   support binary or 8-bit transport, the use of a content-transfer-

   encoding of "8bit" or "binary" is explicitly prohibited for entities

   of type message/external-body.



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



   An access-type of FTP or TFTP indicates that the message body is

   accessible as a file using the FTP [RFC-959] or TFTP [RFC-783]

   protocols, respectively.  For these access-types, the following

   additional parameters are mandatory:



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



      SITE -- A machine from which the file may be obtained, using the

      given protocol. This must be a fully qualified domain name, not a

      nickname.



   Before any data are retrieved, using FTP, the user will generally

   need to be asked to provide a login id and a password for the machine

   named by the site parameter.  For security reasons, such an id and

   password are not specified as content-type parameters, but must be

   obtained from the user.



   In addition, the following parameters are optional:



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

      be retrieved.



      MODE -- A case-insensitive string indicating the mode to be used

      when retrieving the information.  The legal values for access-type

      "TFTP" are "NETASCII", "OCTET", and "MAIL", as specified by the

      TFTP protocol [RFC-783].  The legal values for access-type "FTP"

      are "ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a

      decimal integer, typically 8.  These correspond to the







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      representation types "A" "E" "I" and "L n" as specified by the FTP

      protocol [RFC-959].  Note that "BINARY" and "TENEX" are not valid

      values for MODE, but that "OCTET" or "IMAGE" or "LOCAL8" should be

      used instead.  IF MODE is not specified, the default value is

      "NETASCII" for TFTP and "ASCII" otherwise.



7.3.3.2.  The "anon-ftp" access-type



   The "anon-ftp" access-type is identical to the "ftp" access type,

   except that the user need not be asked to provide a name and password

   for the specified site.  Instead, the ftp protocol will be used with

   login "anonymous" and a password that corresponds to the user's email

   address.



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



   An access-type of "local-file" indicates that the actual body is

   accessible as a file on the local machine.  An access-type of "afs"

   indicates that the file is accessible via the global AFS file system.

   In both cases, only a single parameter is required:



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



   The following optional parameter may be used to describe the locality

   of reference for the data, that is, the site or sites at which the

   file is expected to be visible:



      SITE -- A domain specifier for a machine or set of machines that

      are known to have access to the data file.  Asterisks may be used

      for wildcard matching to a part of a domain name, such as

      "*.bellcore.com", to indicate a set of machines on which the data

      should be directly visible, while a single asterisk may be used to

      indicate a file that is expected to be universally available,

      e.g., via a global file system.



7.3.3.4.  The "mail-server" access-type



   The "mail-server" access-type indicates that the actual body is

   available from a mail server.  The mandatory parameter for this

   access-type is:



      SERVER -- The email address of the mail server from which the

      actual body data can be obtained.



   Because mail servers accept a variety of syntaxes, some of which is

   multiline, the full command to be sent to a mail server is not

   included as a parameter on the content-type line.  Instead, it is

   provided as the "phantom body" when the content-type is







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



   An optional parameter for this access-type is:



      SUBJECT -- The subject that is to be used in the mail that is sent

      to obtain the data. Note that keying mail servers on Subject lines

      is NOT recommended, but such mail servers are known to exist.



   Note that MIME does not define a mail server syntax.  Rather, it

   allows the inclusion of arbitrary mail server commands in the phantom

   body.  Implementations must include the phantom body in the body of

   the message it sends to the mail server address to retrieve the

   relevant data.



   It is worth noting that, unlike other access-types, mail-server

   access is asynchronous and will happen at an unpredictable time in

   the future.  For this reason, it is important that there be a

   mechanism by which the returned data can be matched up with the

   original message/external-body entity.  MIME mailservers must use the

   same Content-ID field on the returned message that was used in the

   original message/external-body entity, to facilitate such matching.



7.3.3.5.  Examples and Further Explanations



   With the emerging possibility of very wide-area file systems, it

   becomes very hard to know in advance the set of machines where a file

   will and will not be accessible directly from the file system.

   Therefore it may make sense to provide both a file name, to be tried

   directly, and the name of one or more sites from which the file is

   known to be accessible.  An implementation can try to retrieve remote

   files using FTP or any other protocol, using anonymous file retrieval

   or prompting the user for the necessary name and password.  If an

   external body is accessible via multiple mechanisms, the sender may

   include multiple parts of type message/external-body within an entity

   of type multipart/alternative.



   However, the external-body mechanism is not intended to be limited to

   file retrieval, as shown by the mail-server access-type.  Beyond

   this, one can imagine, for example, using a video server for external

   references to video clips.



   If an entity is of type "message/external-body", then the body of the

   entity will contain the header fields of the encapsulated message.

   The body itself is to be found in the external location.  This means

   that if the body of the "message/external-body" message contains two

   consecutive CRLFs, everything after those pairs is NOT part of the

   message itself.  For most message/external-body messages, this

   trailing area must simply be ignored.  However, it is a convenient







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   place for additional data that cannot be included in the content-type

   header field.  In particular, if the "access-type" value is "mail-

   server", then the trailing area must contain commands to be sent to

   the mail server at the address given by the value of the SERVER

   parameter.



   The embedded message header fields which appear in the body of the

   message/external-body data must be used to declare the Content-type

   of the external body if it is anything other than plain ASCII text,

   since the external body does not have a header section to declare its

   type.  Similarly, any Content-transfer-encoding other than "7bit"

   must also be declared here.  Thus a complete message/external-body

   message, referring to a document in PostScript format, might look

   like this:



      From: Whomever

      To: Someone

      Subject: whatever

      MIME-Version: 1.0

      Message-ID: <id1@host.com>

      Content-Type: multipart/alternative; boundary=42

      Content-ID: <id001@guppylake.bellcore.com>



      --42

      Content-Type: message/external-body;

           name="BodyFormats.ps";

           site="thumper.bellcore.com";

           access-type=ANON-FTP;

           directory="pub";

           mode="image";

           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"



      Content-type: application/postscript

      Content-ID: <id42@guppylake.bellcore.com>



      --42

      Content-Type: message/external-body;

           name="/u/nsb/writing/rfcs/RFC-MIME.ps";

           site="thumper.bellcore.com";

           access-type=AFS

           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"



      Content-type: application/postscript

      Content-ID: <id42@guppylake.bellcore.com>



      --42

      Content-Type: message/external-body;

           access-type=mail-server







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           server="listserv@bogus.bitnet";

           expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"



      Content-type: application/postscript

      Content-ID: <id42@guppylake.bellcore.com>



      get RFC-MIME.DOC



      --42--



   Note that in the above examples, the default Content-transfer-

   encoding of "7bit" is assumed for the external postscript data.



   Like the message/partial type, the message/external-body type is

   intended to be transparent, that is, to convey the data type in the

   external body rather than to convey a message with a body of that

   type.  Thus the headers on the outer and inner parts must be merged

   using the same rules as for message/partial.  In particular, this

   means that the Content-type header is overridden, but the From and

   Subject headers are preserved.



   Note that since the external bodies are not transported as mail, they

   need not conform to the 7-bit and line length requirements, but might

   in fact be binary files.  Thus a Content-Transfer-Encoding is not

   generally necessary, though it is permitted.



   Note that the body of a message of type "message/external-body" is

   governed by the basic syntax for an RFC 822 message.  In particular,

   anything before the first consecutive pair of CRLFs is header

   information, while anything after it is body information, which is

   ignored for most access-types.



   The formal grammar for content-type header fields for data of type

   message is given by:



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



   message-subtype := "rfc822"

                   / "partial" 2#3partial-param

                   / "external-body" 1*external-param

                   / extension-token



   partial-param :=     (";" "id" "=" value)

              /  (";" "number" "=" 1*DIGIT)

              /  (";" "total" "=" 1*DIGIT)

         ; id & number required; total  required  for  last part



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







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              / (";" "expiration" "=" date-time)

                   ; Note that date-time is quoted

              / (";" "size" "=" 1*DIGIT)

              / (";"  "permission"  "="  ("read"  /  "read-write"))

                   ; Permission is case-insensitive

              / (";" "name" "="  value)

              / (";" "site" "=" value)

              / (";" "dir" "=" value)

              / (";" "mode" "=" value)

              / (";" "server" "=" value)

              / (";" "subject" "=" value)

          ; access-type required;others required based on access-type



   atype := "ftp" / "anon-ftp" / "tftp" / "local-file"

                  / "afs" / "mail-server" / extension-token

                  ; Case-insensitive



7.4.  The Application Content-Type



   The "application" Content-Type is to be used for data which do not

   fit in any of the other categories, and particularly for data to be

   processed by mail-based uses of application programs.  This is

   information which must be processed by an application before it is

   viewable or usable to a user.  Expected uses for Content-Type

   application include mail-based file transfer, spreadsheets, data for

   mail-based scheduling systems, and languages for "active"

   (computational) email.  (The latter, in particular, can pose security

   problems which must be understood by implementors, and are considered

   in detail in the discussion of the application/PostScript content-

   type.)



   For example, a meeting scheduler might define a standard

   representation for information about proposed meeting dates.  An

   intelligent user agent would use this information to conduct a dialog

   with the user, and might then send further mail based on that dialog.

   More generally, there have been several "active" messaging languages

   developed in which programs in a suitably specialized language are

   sent through the mail and automatically run in the recipient's

   environment.



   Such applications may be defined as subtypes of the "application"

   Content-Type.  This document defines two subtypes: octet-stream, and

   PostScript.



   In general, the subtype of application will often be the name of the

   application for which the data are intended.  This does not mean,

   however, that any application program name may be used freely as a

   subtype of application.  Such usages (other than subtypes beginning







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



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



   The primary subtype of application, "octet-stream", may be used to

   indicate that a body contains binary data.  The set of possible

   parameters includes, but is not limited to:



      TYPE -- the general type or category of binary data.  This is

      intended as information for the human recipient rather than for

      any automatic processing.



      PADDING -- the number of bits of padding that were appended to the

      bit-stream comprising the actual contents to produce the enclosed

      byte-oriented data.  This is useful for enclosing a bit-stream in

      a body when the total number of bits is not a multiple of the byte

      size.



   An additional parameter, "conversions", was defined in [RFC-1341] but

   has been removed.



   RFC 1341 also defined the use of a "NAME" parameter which gave a

   suggested file name to be used if the data were to be written to a

   file.  This has been deprecated in anticipation of a separate

   Content-Disposition header field, to be defined in a subsequent RFC.



   The recommended action for an implementation that receives

   application/octet-stream mail is to simply offer to put the data in a

   file, with any Content-Transfer-Encoding undone, or perhaps to use it

   as input to a user-specified process.



   To reduce the danger of transmitting rogue programs through the mail,

   it is strongly recommended that implementations NOT implement a

   path-search mechanism whereby an arbitrary program named in the

   Content-Type parameter (e.g., an "interpreter=" parameter) is found

   and executed using the mail body as input.



7.4.2.     The Application/PostScript subtype



   A Content-Type of "application/postscript" indicates a PostScript

   program.  Currently two variants of the PostScript language are

   allowed; the original level 1 variant is described in [POSTSCRIPT]

   and the more recent level 2 variant is described in [POSTSCRIPT2].



   PostScript is a registered trademark of Adobe Systems, Inc.  Use of

   the MIME content-type "application/postscript" implies recognition of

   that trademark and all the rights it entails.









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   The PostScript language definition provides facilities for internal

   labeling of the specific language features a given program uses. This

   labeling, called the PostScript document structuring conventions, is

   very general and provides substantially more information than just

   the language level.



   The use of document structuring conventions, while not required, is

   strongly recommended as an aid to interoperability.  Documents which

   lack proper structuring conventions cannot be tested to see whether

   or not they will work in a given environment.  As such, some systems

   may assume the worst and refuse to process unstructured documents.



   The execution of general-purpose PostScript interpreters entails

   serious security risks, and implementors are discouraged from simply

   sending PostScript email bodies to "off-the-shelf" interpreters.

   While it is usually safe to send PostScript to a printer, where the

   potential for harm is greatly constrained, implementors should

   consider all of the following before they add interactive display of

   PostScript bodies to their mail readers.



   The remainder of this section outlines some, though probably not all,

   of the possible problems with sending PostScript through the mail.



   Dangerous operations in the PostScript language include, but may not

   be limited to, the PostScript operators deletefile, renamefile,

   filenameforall, and file.  File is only dangerous when applied to

   something other than standard input or output. Implementations may

   also define additional nonstandard file operators; these may also

   pose a threat to security.  Filenameforall, the wildcard file search

   operator, may appear at first glance to be harmless. Note, however,

   that this operator has the potential to reveal information about what

   files the recipient has access to, and this information may itself be

   sensitive.  Message senders should avoid the use of potentially

   dangerous file operators, since these operators are quite likely to

   be unavailable in secure PostScript implementations.  Message-

   receiving and -displaying software should either completely disable

   all potentially dangerous file operators or take special care not to

   delegate any special authority to their operation. These operators

   should be viewed as being done by an outside agency when interpreting

   PostScript documents.  Such disabling and/or checking should be done

   completely outside of the reach of the PostScript language itself;

   care should be taken to insure that no method exists for re-enabling

   full-function versions of these operators.



   The PostScript language provides facilities for exiting the normal

   interpreter, or server, loop. Changes made in this "outer"

   environment are customarily retained across documents, and may in

   some cases be retained semipermanently in nonvolatile memory. The







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   operators associated with exiting the interpreter loop have the

   potential to interfere with subsequent document processing. As such,

   their unrestrained use constitutes a threat of service denial.

   PostScript operators that exit the interpreter loop include, but may

   not be limited to, the exitserver and startjob operators.  Message-

   sending software should not generate PostScript that depends on

   exiting the interpreter loop to operate. The ability to exit will

   probably be unavailable in secure PostScript implementations.

   Message-receiving and -displaying software should, if possible,

   disable the ability to make retained changes to the PostScript

   environment, and eliminate the startjob and exitserver commands.  If

   these commands cannot be eliminated, the password associated with

   them should at least be set to a hard-to-guess value.



   PostScript provides operators for setting system-wide and device-

   specific parameters. These parameter settings may be retained across

   jobs and may potentially pose a threat to the correct operation of

   the interpreter.  The PostScript operators that set system and device

   parameters include, but may not be limited to, the setsystemparams

   and setdevparams operators.  Message-sending software should not

   generate PostScript that depends on the setting of system or device

   parameters to operate correctly. The ability to set these parameters

   will probably be unavailable in secure PostScript implementations.

   Message-receiving and -displaying software should, if possible,

   disable the ability to change system and device parameters.  If these

   operators cannot be disabled, the password associated with them

   should at least be set to a hard-to-guess value.



   Some PostScript implementations provide nonstandard facilities for

   the direct loading and execution of machine code.  Such facilities

   are quite obviously open to substantial abuse.  Message-sending

   software should not make use of such features. Besides being totally

   hardware- specific, they are also likely to be unavailable in secure

   implementations of PostScript.  Message-receiving and -displaying

   software should not allow such operators to be used if they exist.



   PostScript is an extensible language, and many, if not most,

   implementations of it provide a number of their own extensions. This

   document does not deal with such extensions explicitly since they

   constitute an unknown factor.  Message-sending software should not

   make use of nonstandard extensions; they are likely to be missing

   from some implementations. Message-receiving and -displaying software

   should make sure that any nonstandard PostScript operators are secure

   and don't present any kind of threat.



   It is possible to write PostScript that consumes huge amounts of

   various system resources. It is also possible to write PostScript

   programs that loop infinitely.  Both types of programs have the







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   potential to cause damage if sent to unsuspecting recipients.

   Message-sending software should avoid the construction and

   dissemination of such programs, which is antisocial.  Message-

   receiving and -displaying software should provide appropriate

   mechanisms to abort processing of a document after a reasonable

   amount of time has elapsed. In addition, PostScript interpreters

   should be limited to the consumption of only a reasonable amount of

   any given system resource.



   Finally, bugs may exist in some PostScript interpreters which could

   possibly be exploited to gain unauthorized access to a recipient's

   system.  Apart from noting this possibility, there is no specific

   action to take to prevent this, apart from the timely correction of

   such bugs if any are found.



7.4.3.     Other Application subtypes



   It is expected that many other subtypes of application will be

   defined in the future.  MIME implementations must generally treat any

   unrecognized subtypes as being equivalent to application/octet-

   stream.



   The formal grammar for content-type header fields for application

   data is given by:



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



   application-subtype := ("octet-stream" *stream-param)

                       / "postscript" / extension-token



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

                       / (";" "padding" "=" padding)



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



7.5.  The Image Content-Type



   A Content-Type of "image" indicates that the body contains an image.

   The subtype names the specific image format.  These names are case

   insensitive.  Two initial subtypes are "jpeg" for the JPEG format,

   JFIF encoding, and "gif" for GIF format [GIF].



   The list of image subtypes given here is neither exclusive nor

   exhaustive, and is expected to grow as more types are registered with

   IANA, as described in Appendix E.



   The formal grammar for the content-type header field for data of type

   image is given by:







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



7.6.  The Audio Content-Type



   A Content-Type of "audio" indicates that the body contains audio

   data.  Although there is not yet a consensus on an "ideal" audio

   format for use with computers, there is a pressing need for a format

   capable of providing interoperable behavior.



   The initial subtype of "basic" is specified to meet this requirement

   by providing an absolutely minimal lowest common denominator audio

   format.  It is expected that richer formats for higher quality and/or

   lower bandwidth audio will be defined by a later document.



   The content of the "audio/basic" subtype is audio encoded using 8-bit

   ISDN mu-law [PCM].  When this subtype is present, a sample rate of

   8000 Hz and a single channel is assumed.



   The formal grammar for the content-type header field for data of type

   audio is given by:



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



7.7.  The Video Content-Type



   A Content-Type of "video" indicates that the body contains a time-

   varying-picture image, possibly with color and coordinated sound.

   The term "video" is used extremely generically, rather than with

   reference to any particular technology or format, and is not meant to

   preclude subtypes such as animated drawings encoded compactly.  The

   subtype "mpeg" refers to video coded according to the MPEG standard

   [MPEG].



   Note that although in general this document strongly discourages the

   mixing of multiple media in a single body, it is recognized that many

   so-called "video" formats include a representation for synchronized

   audio, and this is explicitly permitted for subtypes of "video".



   The formal grammar for the content-type header field for data of type

   video is given by:



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



7.8.  Experimental Content-Type Values



   A Content-Type value beginning with the characters "X-" is a private

   value, to be used by consenting mail systems by mutual agreement.

   Any format without a rigorous and public definition must be named







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   with an "X-" prefix, and publicly specified values shall never begin

   with "X-".  (Older versions of the widely-used Andrew system use the

   "X-BE2" name, so new systems should probably choose a different

   name.)



   In general, the use of "X-" top-level types is strongly discouraged.

   Implementors should invent subtypes of the existing types whenever

   possible.  The invention of new types is intended to be restricted

   primarily to the development of new media types for email, such as

   digital odors or holography, and not for new data formats in general.

   In many cases, a subtype of application will be more appropriate than

   a new top-level type.















































































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8. Summary



   Using the MIME-Version, Content-Type, and Content-Transfer-Encoding

   header fields, it is possible to include, in a standardized way,

   arbitrary types of data objects with RFC 822 conformant mail

   messages.  No restrictions imposed by either RFC 821 or RFC 822 are

   violated, and care has been taken to avoid problems caused by

   additional restrictions imposed by the characteristics of some

   Internet mail transport mechanisms (see Appendix B). The "multipart"

   and "message" Content-Types allow mixing and hierarchical structuring

   of objects of different types in a single message.  Further Content-

   Types provide a standardized mechanism for tagging messages or body

   parts as audio, image, or several other kinds of data.  A

   distinguished parameter syntax allows further specification of data

   format details, particularly the specification of alternate character

   sets.  Additional optional header fields provide mechanisms for

   certain extensions deemed desirable by many implementors.  Finally, a

   number of useful Content-Types are defined for general use by

   consenting user agents, notably message/partial, and

   message/external-body.



9. Security Considerations



   Security issues are discussed in Section 7.4.2 and in Appendix F.

   Implementors should pay special attention to the security

   implications of any mail content-types that can cause the remote

   execution of any actions in the recipient's environment.  In such

   cases, the discussion of the application/postscript content-type in

   Section 7.4.2 may serve as a model for considering other content-

   types with remote execution capabilities.











































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10. Authors' Addresses



   For more information, the authors of this document may be contacted

   via Internet mail:



   Nathaniel S. Borenstein

   MRE 2D-296, Bellcore

   445 South St.

   Morristown, NJ 07962-1910



   Phone: +1 201 829 4270

   Fax:  +1 201 829 7019

   Email: nsb@bellcore.com





   Ned Freed

   Innosoft International, Inc.

   250 West First Street

   Suite 240

   Claremont, CA 91711



   Phone:  +1 909 624 7907

   Fax: +1 909 621 5319

   Email: ned@innosoft.com



   MIME is a result of the work of the Internet Engineering Task Force

   Working Group on Email Extensions. The chairman of that group, Greg

   Vaudreuil, may be reached at:



   Gregory M. Vaudreuil

   Tigon Corporation

   17060 Dallas Parkway

   Dallas Texas, 75248



   Phone:    +1 214-733-2722

   EMail: gvaudre@cnri.reston.va.us































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11. Acknowledgements



   This document is the result of the collective effort of a large

   number of people, at several IETF meetings, on the IETF-SMTP and

   IETF-822 mailing lists, and elsewhere.  Although any enumeration

   seems doomed to suffer from egregious omissions, the following are

   among the many contributors to this effort:



            Harald Tveit Alvestrand       Timo Lehtinen

            Randall Atkinson              John R. MacMillan

            Philippe Brandon              Rick McGowan

            Kevin Carosso                 Leo Mclaughlin

            Uhhyung Choi                  Goli Montaser-Kohsari

            Cristian Constantinof         Keith Moore

            Mark Crispin                  Tom Moore

            Dave Crocker                  Erik Naggum

            Terry Crowley                 Mark Needleman

            Walt Daniels                  John Noerenberg

            Frank Dawson                  Mats Ohrman

            Hitoshi Doi                   Julian Onions

            Kevin Donnelly                Michael Patton

            Keith Edwards                 David J. Pepper

            Chris Eich                    Blake C. Ramsdell

            Johnny Eriksson               Luc Rooijakkers

            Craig Everhart                Marshall T. Rose

            Patrik Faeltstroem            Jonathan Rosenberg

            Erik E. Fair                  Jan Rynning

            Roger Fajman                  Harri Salminen

            Alain Fontaine                Michael Sanderson

            James M. Galvin               Masahiro Sekiguchi

            Philip Gladstone              Mark Sherman

            Thomas Gordon                 Keld Simonsen

            Phill Gross                   Bob Smart

            James Hamilton                Peter Speck

            Steve Hardcastle-Kille        Henry Spencer

            David Herron                  Einar Stefferud

            Bruce Howard                  Michael Stein

            Bill Janssen                  Klaus Steinberger

            Olle Jaernefors               Peter Svanberg

            Risto Kankkunen               James Thompson

            Phil Karn                     Steve Uhler

            Alan Katz                     Stuart Vance

            Tim Kehres                    Erik van der Poel

            Neil Katin                    Guido van Rossum

            Kyuho Kim                     Peter Vanderbilt

            Anders Klemets                Greg Vaudreuil

            John Klensin                  Ed Vielmetti

            Valdis Kletniek               Ryan Waldron







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            Jim Knowles                   Wally Wedel

            Stev Knowles                  Sven-Ove Westberg

            Bob Kummerfeld                Brian Wideen

            Pekka Kytolaakso              John Wobus

            Stellan Lagerstrom            Glenn Wright

            Vincent Lau                   Rayan Zachariassen

            Donald Lindsay                David Zimmerman

            Marc Andreessen               Bob Braden

            Brian Capouch                 Peter Clitherow

            Dave Collier-Brown            John Coonrod

            Stephen Crocker               Jim Davis

            Axel Deininger                Dana S Emery

            Martin Forssen                Stephen Gildea

            Terry Gray                    Mark Horton

            Warner Losh                   Carlyn Lowery

            Laurence Lundblade            Charles Lynn

            Larry Masinter                Michael J. McInerny

            Jon Postel                    Christer Romson

            Yutaka Sato                   Markku Savela

            Richard Alan Schafer          Larry W. Virden

            Rhys Weatherly                Jay Weber

            Dave Wecker



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

certainly unintentional.





















































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Appendix A -- Minimal MIME-Conformance



   The mechanisms described in this document are open-ended.  It is

   definitely not expected that all implementations will support all of

   the Content-Types described, nor that they will all share the same

   extensions.  In order to promote interoperability, however, it is

   useful to define the concept of "MIME-conformance" to define a

   certain level of implementation that allows the useful interworking

   of messages with content that differs from US ASCII text.  In this

   section, we specify the requirements for such conformance.



   A mail user agent that is MIME-conformant MUST:



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



      2.  Recognize the Content-Transfer-Encoding header field, and

      decode all received data encoded with either the quoted-printable

      or base64 implementations.  Encode any data sent that is not in

      seven-bit mail-ready representation using one of these

      transformations and include the appropriate Content-Transfer-

      Encoding header field, unless the underlying transport mechanism

      supports non-seven-bit data, as SMTP does not.



      3.  Recognize and interpret the Content-Type header field, and

      avoid showing users raw data with a Content-Type field other than

      text.  Be able to send at least text/plain messages, with the

      character set specified as a parameter if it is not US-ASCII.



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

      least the following extents:



      Text:



            -- Recognize and display "text" mail

                 with the character set "US-ASCII."



            -- Recognize other character sets at

                 least to the extent of being able

                 to inform the user about what

                 character set the message uses.



            -- Recognize the "ISO-8859-*" character

                 sets to the extent of being able to

                 display those characters that are

                 common to ISO-8859-* and US-ASCII,

                 namely all characters represented

                 by octet values 0-127.









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            -- For unrecognized subtypes, show or

                 offer to show the user the "raw"

                 version of the data after

                 conversion of the content from

                 canonical form to local form.



       Message:



            -- Recognize and display at least the

                 primary (822) encapsulation.



       Multipart:



            -- Recognize the primary (mixed)

                 subtype.  Display all relevant

                 information on the message level

                 and the body part header level and

                 then display or offer to display

                 each of the body parts individually.



            -- Recognize the "alternative" subtype,

                 and avoid showing the user

                 redundant parts of

                 multipart/alternative mail.



            -- Treat any unrecognized subtypes as if

                 they were "mixed".



       Application:



            -- Offer the ability to remove either of

                 the two types of Content-Transfer-

                 Encoding defined in this document

                 and put the resulting information

                 in a user file.



      5.  Upon encountering any unrecognized Content- Type, an

      implementation must treat it as if it had a Content-Type of

      "application/octet-stream" with no parameter sub-arguments.  How

      such data are handled is up to an implementation, but likely

      options for handling such unrecognized data include offering the

      user to write it into a file (decoded from its mail transport

      format) or offering the user to name a program to which the

      decoded data should be passed as input.  Unrecognized predefined

      types, which in a MIME-conformant mailer might still include

      audio, image, or video, should also be treated in this way.



   A user agent that meets the above conditions is said to be MIME-







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   conformant.  The meaning of this phrase is that it is assumed to be

   "safe" to send virtually any kind of properly-marked data to users of

   such mail systems, because such systems will at least be able to

   treat the data as undifferentiated binary, and will not simply splash

   it onto the screen of unsuspecting users.  There is another sense in

   which it is always "safe" to send data in a format that is MIME-

   conformant, which is that such data will not break or be broken by

   any known systems that are conformant with RFC 821 and RFC 822.  User

   agents that are MIME-conformant have the additional guarantee that

   the user will not be shown data that were never intended to be viewed

   as text.

















































































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Appendix B -- General Guidelines For Sending Email Data



   Internet email is not a perfect, homogeneous system.  Mail may become

   corrupted at several stages in its travel to a final destination.

   Specifically, email sent throughout the Internet may travel across

   many networking technologies.  Many networking and mail technologies

   do not support the full functionality possible in the SMTP transport

   environment. Mail traversing these systems is likely to be modified

   in such a way that it can be transported.



   There exist many widely-deployed non-conformant MTAs in the Internet.

   These MTAs, speaking the SMTP protocol, alter messages on the fly to

   take advantage of the internal data structure of the hosts they are

   implemented on, or are just plain broken.



   The following guidelines may be useful to anyone devising a data

   format (Content-Type) that will survive the widest range of

   networking technologies and known broken MTAs unscathed.  Note that

   anything encoded in the base64 encoding will satisfy these rules, but

   that some well-known mechanisms, notably the UNIX uuencode facility,

   will not.  Note also that anything encoded in the Quoted-Printable

   encoding will survive most gateways intact, but possibly not some

   gateways to systems that use the EBCDIC character set.



      (1) Under some circumstances the encoding used for data may change

      as part of normal gateway or user agent operation. In particular,

      conversion from base64 to quoted-printable and vice versa may be

      necessary. This may result in the confusion of CRLF sequences with

      line breaks in text bodies. As such, the persistence of CRLF as

      something other than a line break must not be relied on.



      (2) Many systems may elect to represent and store text data using

      local newline conventions. Local newline conventions may not match

      the RFC822 CRLF convention -- systems are known that use plain CR,

      plain LF, CRLF, or counted records.  The result is that isolated

      CR and LF characters are not well tolerated in general; they may

      be lost or converted to delimiters on some systems, and hence must

      not be relied on.



      (3) TAB (HT) characters may be misinterpreted or may be

      automatically converted to variable numbers of spaces.  This is

      unavoidable in some environments, notably those not based on the

      ASCII character set. Such conversion is STRONGLY DISCOURAGED, but

      it may occur, and mail formats must not rely on the persistence of

      TAB (HT) characters.



      (4) Lines longer than 76 characters may be wrapped or truncated in

      some environments. Line wrapping and line truncation are STRONGLY







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      DISCOURAGED, but unavoidable in some cases. Applications which

      require long lines must somehow differentiate between soft and

      hard line breaks.  (A simple way to do this is to use the quoted-

      printable encoding.)



      (5) Trailing "white space" characters (SPACE, TAB (HT)) on a line

      may be discarded by some transport agents, while other transport

      agents may pad lines with these characters so that all lines in a

      mail file are of equal length.  The persistence of trailing white

      space, therefore, must not be relied on.



      (6) Many mail domains use variations on the ASCII character set,

      or use character sets such as EBCDIC which contain most but not

      all of the US-ASCII characters.  The correct translation of

      characters not in the "invariant" set cannot be depended on across

      character converting gateways.  For example, this situation is a

      problem when sending uuencoded information across BITNET, an

      EBCDIC system.  Similar problems can occur without crossing a

      gateway, since many Internet hosts use character sets other than

      ASCII internally.  The definition of Printable Strings in X.400

      adds further restrictions in certain special cases.  In

      particular, the only characters that are known to be consistent

      across all gateways are the 73 characters that correspond to the

      upper and lower case letters A-Z and a-z, the 10 digits 0-9, and

      the following eleven special characters:



                        "'"  (ASCII code 39)

                        "("  (ASCII code 40)

                        ")"  (ASCII code 41)

                        "+"  (ASCII code 43)

                        ","  (ASCII code 44)

                        "-"  (ASCII code 45)

                        "."  (ASCII code 46)

                        "/"  (ASCII code 47)

                        ":"  (ASCII code 58)

                        "="  (ASCII code 61)

                        "?"  (ASCII code 63)



      A maximally portable mail representation, such as the base64

      encoding, will confine itself to relatively short lines of text in

      which the only meaningful characters are taken from this set of 73

      characters.



      (7) Some mail transport agents will corrupt data that includes

      certain literal strings.  In particular, a period (".") alone on a

      line is known to be corrupted by some (incorrect) SMTP

      implementations, and a line that starts with the five characters

      "From " (the fifth character is a SPACE) are commonly corrupted as







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      well.  A careful composition agent can prevent these corruptions

      by encoding the data (e.g., in the quoted-printable encoding,

      "=46rom " in place of "From " at the start of a line, and "=2E" in

      place of "." alone on a line.



   Please note that the above list is NOT a list of recommended

   practices for MTAs.  RFC 821 MTAs are prohibited from altering the

   character of white space or wrapping long lines.  These BAD and

   illegal practices are known to occur on established networks, and

   implementations should be robust in dealing with the bad effects they

   can cause.

















































































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Appendix C -- A Complex Multipart Example



   What follows is the outline of a complex multipart message.  This

   message has five parts to be displayed serially: two introductory

   plain text parts, an embedded multipart message, a richtext part, and

   a closing encapsulated text message in a non-ASCII character set.

   The embedded multipart message has two parts to be displayed in

   parallel, a picture and an audio fragment.



      MIME-Version: 1.0

      From: Nathaniel Borenstein <nsb@bellcore.com>

      To: Ned Freed <ned@innosoft.com>

      Subject: A multipart example

      Content-Type: multipart/mixed;

           boundary=unique-boundary-1



      This is the preamble area of a multipart message.

      Mail readers that understand multipart format

      should ignore this preamble.

      If you are reading this text, you might want to

      consider changing to a mail reader that understands

      how to properly display multipart messages.

      --unique-boundary-1



         ...Some text appears here...

      [Note that the preceding blank line means

      no header fields were given and this is text,

      with charset US ASCII.  It could have been

      done with explicit typing as in the next part.]



      --unique-boundary-1

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



      This could have been part of the previous part,

      but illustrates explicit versus implicit

      typing of body parts.



      --unique-boundary-1

      Content-Type: multipart/parallel;

           boundary=unique-boundary-2





      --unique-boundary-2

      Content-Type: audio/basic

      Content-Transfer-Encoding: base64



         ... base64-encoded 8000 Hz single-channel

             mu-law-format audio data goes here....







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      --unique-boundary-2

      Content-Type: image/gif

      Content-Transfer-Encoding: base64



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



      --unique-boundary-2--



      --unique-boundary-1

      Content-type: text/richtext



      This is <bold><italic>richtext.</italic></bold>

      <smaller>as defined in RFC 1341</smaller>

      <nl><nl>Isn't it

      <bigger><bigger>cool?</bigger></bigger>



      --unique-boundary-1

      Content-Type: message/rfc822



      From: (mailbox in US-ASCII)

      To: (address in US-ASCII)

      Subject: (subject in US-ASCII)

      Content-Type: Text/plain; charset=ISO-8859-1

      Content-Transfer-Encoding: Quoted-printable



         ... Additional text in ISO-8859-1 goes here ...



      --unique-boundary-1--















































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Appendix D -- Collected Grammar



   This appendix contains the complete BNF grammar for all the syntax

   specified by this document.



   By itself, however, this grammar is incomplete.  It refers to several

   entities that are defined by RFC 822.  Rather than reproduce those

   definitions here, and risk unintentional differences between the two,

   this document simply refers the reader to RFC 822 for the remaining

   definitions.  Wherever a term is undefined, it refers to the RFC 822

   definition.



   application-subtype := ("octet-stream" *stream-param)

                       / "postscript" / extension-token



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



   attribute := token    ; case-insensitive



   atype := "ftp" / "anon-ftp" / "tftp" / "local-file"

                  / "afs" / "mail-server" / extension-token

                  ; Case-insensitive



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



   body-part := <"message" as defined in RFC 822,

            with all header fields optional, and with the

            specified delimiter not occurring anywhere in

            the message body, either on a line by itself

            or as a substring anywhere.>



      NOTE: In certain transport enclaves, RFC 822 restrictions such as

      the one that limits bodies to printable ASCII characters may not

      be in force.  (That is, the transport domains may resemble

      standard Internet mail transport as specified in RFC821 and

      assumed by RFC822, but without certain restrictions.)  The

      relaxation of these restrictions should be construed as locally

      extending the definition of bodies, for example to include octets

      outside of the ASCII range, as long as these extensions are

      supported by the transport and adequately documented in the

      Content-Transfer-Encoding header field. However, in no event are

      headers (either message headers or body-part headers) allowed to

      contain anything other than ASCII characters.

















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   boundary := 0*69<bchars> bcharsnospace



   bchars := bcharsnospace / " "



   bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  / "_"

                  / "," / "-" / "." / "/" / ":" / "=" / "?"



   charset := "us-ascii" / "iso-8859-1" / "iso-8859-2"/ "iso-8859-3"

        / "iso-8859-4" / "iso-8859-5" /  "iso-8859-6" / "iso-8859-7"

        / "iso-8859-8" / "iso-8859-9" / extension-token

        ; case insensitive



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



   content  := "Content-Type"  ":" type "/" subtype  *(";" parameter)

             ; case-insensitive matching of type and subtype



   delimiter := "--" boundary CRLF  ;taken from Content-Type field.

                                ; There must be no space

                                ; between "--" and boundary.



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



   discard-text := *(*text CRLF)



   encapsulation := delimiter body-part CRLF



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



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



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



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

                  / (";" "expiration" "=" date-time)



                       ; Note that date-time is quoted

                  / (";" "size" "=" 1*DIGIT)

                  / (";"  "permission"  "="  ("read" / "read-write"))

                       ; Permission is case-insensitive

                  / (";" "name" "="  value)

                  / (";" "site" "=" value)

                  / (";" "dir" "=" value)

                  / (";" "mode" "=" value)

                  / (";" "server" "=" value)

                  / (";" "subject" "=" value)

           ;access-type required; others required based on access-type









Borenstein & Freed                                             [Page 69]



RFC 1521                          MIME                    September 1993





   iana-token := <a publicly-defined extension token,

             registered with IANA, as specified in

             appendix E>



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



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



   mechanism :=     "7bit"    ;  case-insensitive

                  / "quoted-printable"

                  / "base64"

                  / "8bit"

                  / "binary"

                  / x-token



   message-subtype := "rfc822"

                  / "partial" 2#3partial-param

                  / "external-body" 1*external-param

                  / extension-token



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



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



   multipart-subtype := "mixed" / "parallel" / "digest"

                  / "alternative" / extension-token



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

                  ";" "boundary" "=" boundary



   octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")

        ; octet must be used for characters > 127, =, SPACE, or

   TAB,

        ; and is recommended for any characters not listed in

        ; Appendix B as "mail-safe".



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



   parameter := attribute "=" value



   partial-param :=     (";" "id" "=" value)

                  /  (";" "number" "=" 1*DIGIT)

                  /  (";" "total" "=" 1*DIGIT)

             ; id & number required;total required for last part



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



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







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        ; characters not listed as "mail-safe" in Appendix B

        ; are also not recommended.



   quoted-printable := ([*(ptext / SPACE /  TAB)  ptext]  ["="] CRLF)

        ; Maximum line length of 76 characters excluding CRLF



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

                / (";" "padding" "=" padding)



   subtype := token  ; case-insensitive



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



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



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



   tspecials :=  "(" / ")" / "<" / ">" / "@"

              /  "," / ";" / ":" / "\" / <">

              /  "/" / "[" / "]" / "?" / "="

             ; Must be in quoted-string,

             ; to use within parameter values





   type :=     "application"     /  "audio"   ; case-insensitive

             / "image"           / "message"

             / "multipart"  / "text"

             / "video"           / extension-token

             ; All values case-insensitive



   value := token / quoted-string



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



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



   x-token := <The two characters "X-" or "x-" followed, with no

              intervening white space, by any token>



























Borenstein & Freed                                             [Page 71]



RFC 1521                          MIME                    September 1993





Appendix E -- IANA Registration Procedures



   MIME has been carefully designed to have extensible mechanisms, and

   it is expected that the set of content-type/subtype pairs and their

   associated parameters will grow significantly with time.  Several

   other MIME fields, notably character set names, access-type

   parameters for the message/external-body type, and possibly even

   Content-Transfer-Encoding values, are likely to have new values

   defined over time.  In order to ensure that the set of such values is

   developed in an orderly, well-specified, and public manner, MIME

   defines a registration process which uses the Internet Assigned

   Numbers Authority (IANA) as a central registry for such values.



   In general, parameters in the content-type header field are used to

   convey supplemental information for various content types, and their

   use is defined when the content-type and subtype are defined.  New

   parameters should not be defined as a way to introduce new

   functionality.



   In order to simplify and standardize the registration process, this

   appendix gives templates for the registration of new values with

   IANA.  Each of these is given in the form of an email message

   template, to be filled in by the registering party.



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



   Note that MIME is generally expected to be extended by subtypes.  If

   a new fundamental top-level type is needed, its specification must be

   published as an RFC or submitted in a form suitable to become an RFC,

   and be subject to the Internet standards process.



      To:  IANA@isi.edu

      Subject:  Registration of new MIME

           content-type/subtype



      MIME type name:



      (If the above is not an existing top-level MIME type,

      please explain why an existing type cannot be used.)



      MIME subtype name:



      Required parameters:



      Optional parameters:



      Encoding considerations:









Borenstein & Freed                                             [Page 72]



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      Security considerations:



      Published specification:



      (The published specification must be an Internet RFC or

      RFC-to-be if a new top-level type is being defined, and

      must be a publicly available specification in any

      case.)



      Person & email address to contact for further information:



   E.2  Registration of New Access-type Values

           for Message/external-body



      To:  IANA@isi.edu

      Subject:  Registration of new MIME Access-type for

           Message/external-body content-type



      MIME access-type name:



      Required parameters:



      Optional parameters:



      Published specification:



      (The published specification must be an Internet RFC or

      RFC-to-be.)



      Person & email address to contact for further information:











































Borenstein & Freed                                             [Page 73]



RFC 1521                          MIME                    September 1993





Appendix F -- Summary of the Seven Content-types



   Content-type: text



   Subtypes defined by this document:  plain



   Important Parameters: charset



   Encoding notes: quoted-printable generally preferred if an encoding

      is needed and the character set is mostly an ASCII superset.



   Security considerations: Rich text formats such as TeX and Troff

      often contain mechanisms for executing arbitrary commands or file

      system operations, and should not be used automatically unless

      these security problems have been addressed.  Even plain text may

      contain control characters that can be used to exploit the

      capabilities of "intelligent" terminals and cause security

      violations.  User interfaces designed to run on such terminals

      should be aware of and try to prevent such problems.



   ________________________________________________________

   Content-type: multipart



   Subtypes defined by  this  document: mixed, alternative,

        digest, parallel.



   Important Parameters: boundary



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



   ________________________________________________________

   Content-type: message



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



   Important Parameters: id, number, total, access-type, expiration,

      size, permission, name, site, directory, mode, server, subject



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

      Specifically, only "7bit" is permitted for "message/partial" or

      "message/external-body", and only "7bit", "8bit", or "binary" are

      permitted for other subtypes of "message".

   ______________________________________________________________

   Content-type: application



   Subtypes defined by this document:  octet-stream, postscript



   Important Parameters:  type, padding







Borenstein & Freed                                             [Page 74]



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   Deprecated Parameters: name and conversions were

                          defined in RFC 1341.



   Encoding notes: base64 preferred for unreadable subtypes.



   Security considerations:  This  type  is  intended  for  the

   transmission  of data to be interpreted by locally-installed

   programs.  If used,  for  example,  to  transmit  executable

   binary  programs  or programs in general-purpose interpreted

   languages, such as LISP programs or  shell  scripts,  severe

   security  problems  could  result.   Authors of mail-reading

   agents are cautioned against giving their systems the  power

   to  execute  mail-based  application  data without carefully

   considering  the  security  implications.    While   it   is

   certainly  possible  to  define safe application formats and

   even safe interpreters for unsafe formats, each  interpreter

   should   be   evaluated  separately  for  possible  security

   problems.

   ________________________________________________________________

   Content-type: image



   Subtypes defined by this document:  jpeg, gif



   Important Parameters: none



   Encoding notes: base64 generally preferred

   ________________________________________________________________

   Content-type: audio



   Subtypes defined by this document:  basic



   Important Parameters: none



   Encoding notes: base64 generally preferred

   ________________________________________________________________

   Content-type: video



   Subtypes defined by this document:  mpeg



   Important Parameters: none



   Encoding notes: base64 generally preferred



















Borenstein & Freed                                             [Page 75]



RFC 1521                          MIME                    September 1993





Appendix G -- Canonical Encoding Model



   There was some confusion, in earlier drafts of this memo, regarding

   the model for when email data was to be converted to canonical form

   and encoded, and in particular how this process would affect the

   treatment of CRLFs, given that the representation of newlines varies

   greatly from system to system.  For this reason, a canonical model

   for encoding is presented below.



   The process of composing a MIME entity can be modeled as being done

   in a number of steps.  Note that these steps are roughly similar to

   those steps used in RFC 1421 and are performed for each 'innermost

   level' body:



   Step 1.  Creation of local form.



   The body to be transmitted is created in the system's native format.

   The native character set is used, and where appropriate local end of

   line conventions are used as well.  The body may be a UNIX-style text

   file, or a Sun raster image, or a VMS indexed file, or audio data in

   a system-dependent format stored only in memory, or anything else

   that corresponds to the local model for the representation of some

   form of information.  Fundamentally, the data is created in the

   "native" form specified by the type/subtype information.



   Step 2.  Conversion to canonical form.



   The entire body, including "out-of-band" information such as record

   lengths and possibly file attribute information, is converted to a

   universal canonical form.  The specific content type of the body as

   well as its associated attributes dictate the nature of the canonical

   form that is used.  Conversion to the proper canonical form may

   involve character set conversion, transformation of audio data,

   compression, or various other operations specific to the various

   content types.  If character set conversion is involved, however,

   care must be taken to understand the semantics of the content-type,

   which may have strong implications for any character set conversion,

   e.g.  with regard to syntactically meaningful characters in a text

   subtype other than "plain".



   For example, in the case of text/plain data, the text must be

   converted to a supported character set and lines must be delimited

   with CRLF delimiters in accordance with RFC822.  Note that the

   restriction on line lengths implied by RFC822 is eliminated if the

   next step employs either quoted-printable or base64 encoding.













Borenstein & Freed                                             [Page 76]



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   Step 3.  Apply transfer encoding.



   A Content-Transfer-Encoding appropriate for this body is applied.

   Note that there is no fixed relationship between the content type and

   the transfer encoding.  In particular, it may be appropriate to base

   the choice of base64 or quoted-printable on character frequency

   counts which are specific to a given instance of a body.



   Step 4.  Insertion into entity.



   The encoded object is inserted into a MIME entity with appropriate

   headers.  The entity is then inserted into the body of a higher-level

   entity (message or multipart) if needed.



   It is vital to note that these steps are only a model; they are

   specifically NOT a blueprint for how an actual system would be built.

   In particular, the model fails to account for two common designs:



      1.  In many cases the conversion to a canonical form prior to

      encoding will be subsumed into the encoder itself, which

      understands local formats directly.  For example, the local

      newline convention for text bodies might be carried through to the

      encoder itself along with knowledge of what that format is.



      2.  The output of the encoders may have to pass through one or

      more additional steps prior to being transmitted as a message.  As

      such, the output of the encoder may not be conformant with the

      formats specified by RFC822.  In particular, once again it may be

      appropriate for the converter's output to be expressed using local

      newline conventions rather than using the standard RFC822 CRLF

      delimiters.



   Other implementation variations are conceivable as well.  The vital

   aspect of this discussion is that, in spite of any optimizations,

   collapsings of required steps, or insertion of additional processing,

   the resulting messages must be consistent with those produced by the

   model described here.  For example, a message with the following

   header fields:



        Content-type: text/foo; charset=bar

        Content-Transfer-Encoding: base64



   must be first represented in the text/foo form, then (if necessary)

   represented in the "bar" character set, and finally transformed via

   the base64 algorithm into a mail-safe form.













Borenstein & Freed                                             [Page 77]



RFC 1521                          MIME                    September 1993





Appendix H -- Changes from RFC 1341



   This document is a relatively minor revision  of  RFC  1341.  For

   the  convenience  of  those familiar with RFC 1341, the technical

   changes from that document are summarized in  this appendix.



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

   include ".".



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

   parts.



   3.  The text/richtext type (including the old Section 7.1.3 and

   Appendix D) has been moved to a separate document.



   4.  The rules on header merging for message/partial data have been

   changed to treat the Encrypted and MIME-Version headers as special

   cases.



   5.  The definition of the external-body access-type parameter has

   been changed so that it can only indicate a single access method

   (which was all that made sense).



   6.  There is a new "Subject" parameter for message/external-body,

   access-type mail-server, to permit MIME-based use of mail servers

   that rely on Subject field information.



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

   removed.



   8.  Section 7.4.1 now deprecates the use of the "name" parameter for

   application/octet-stream, as this will be superseded in the future by

   a Content-Disposition header.



   9.  The formal grammar for multipart bodies has been changed so that

   a CRLF is no longer required before the first boundary line.



   10.  MIME entities of type "message/partial" and "message/external-

   body" are now required to use only the "7bit" transfer-encoding.

   (Specifically, "binary" and "8bit" are not permitted.)



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



   12.  A note has been added to the end of section 7.2.3, explaining

   the semantics of Content-ID in a multipart/alternative MIME entity.



   13.  The formal syntax for the "MIME-Version" field has been

   tightened, but in a way that is completely compatible with the only







Borenstein & Freed                                             [Page 78]



RFC 1521                          MIME                    September 1993





   version number defined in RFC 1341.



   14.  In Section 7.3.1, the definition of message/rfc822 has been

   relaxed regarding mandatory fields.



   All other changes from RFC 1341 were editorial changes and do not

   affect the technical content of MIME.  Considerable formal grammar

   has been added, but this reflects the prose specification that was

   already in place.





















































































Borenstein & Freed                                             [Page 79]



RFC 1521                          MIME                    September 1993





References



   [US-ASCII] Coded Character Set--7-Bit American Standard Code for

   Information Interchange, ANSI X3.4-1986.



   [ATK] Borenstein, Nathaniel S., Multimedia Applications Development

   with the Andrew Toolkit, Prentice-Hall, 1990.



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

   Columbus, Ohio, 1990.



   [ISO-2022] International Standard--Information Processing--ISO 7-bit

   and 8-bit coded character sets--Code extension techniques, ISO

   2022:1986.



   [ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic

   Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987.  Part

   2: Latin alphabet No.  2, ISO 8859-2, 1987.  Part 3: Latin alphabet

   No. 3, ISO 8859-3, 1988.  Part 4: Latin alphabet No.  4, ISO 8859-4,

   1988.  Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.  Part 6:

   Latin/Arabic alphabet, ISO 8859-6, 1987.  Part 7: Latin/Greek

   alphabet, ISO 8859-7, 1987.  Part 8: Latin/Hebrew alphabet, ISO

   8859-8, 1988.  Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.



   [ISO-646] International Standard--Information Processing--ISO 7-bit

   coded character set for information interchange, ISO 646:1983.



   [MPEG] Video Coding Draft Standard ISO 11172 CD, ISO IEC/TJC1/SC2/WG11

   (Motion Picture Experts Group), May, 1991.



   [PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972,

   "Pulse Code Modulation (PCM) of Voice Frequencies".



   [POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference

   Manual, Addison-Wesley, 1985.



   [POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference

   Manual, Addison-Wesley, Second Edition, 1990.



   [X400] Schicker, Pietro, "Message Handling Systems, X.400", Message

   Handling Systems and Distributed Applications, E.  Stefferud, O-j.

   Jacobsen, and P.  Schicker, eds., North-Holland, 1989, pp. 3-41.



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

   June 1981.



   [RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC

   821, USC/Information Sciences Institute, August 1982.







Borenstein & Freed                                             [Page 80]



RFC 1521                          MIME                    September 1993





   [RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text

   Messages", STD 11, RFC 822, UDEL, August 1982.



   [RFC-934] Rose, M., and E. Stefferud, "Proposed Standard for Message

   Encapsulation", RFC 934, Delaware and NMA, January 1985.



   [RFC-959] Postel, J. and J. Reynolds, "File Transfer Protocol",

   STD 9, RFC 959, USC/Information Sciences Institute, October 1985.



   [RFC-1049] Sirbu, M., "Content-Type Header Field for Internet

   Messages", STD 11, RFC 1049, CMU, March 1988.



   [RFC-1421] Linn, J., "Privacy Enhancement for Internet Electronic Mail:

   Part I - Message Encryption and Authentication Procedures", RFC

   1421, IAB IRTF PSRG, IETF PEM WG, February 1993.



   [RFC-1154] Robinson, D. and R. Ullmann, "Encoding Header Field for

   Internet Messages", RFC 1154, Prime Computer, Inc., April 1990.



   [RFC-1341] Borenstein, N., and N.  Freed, "MIME (Multipurpose Internet

   Mail Extensions): Mechanisms for Specifying and Describing the Format

   of Internet Message Bodies", RFC 1341, Bellcore, Innosoft, June 1992.



   [RFC-1342] Moore, K., "Representation of Non-Ascii Text in Internet

   Message Headers", RFC 1342, University of Tennessee, June 1992.



   [RFC-1343] Borenstein, N., "A User Agent Configuration Mechanism

   for Multimedia Mail Format Information", RFC 1343, Bellcore, June

   1992.



   [RFC-1344] Borenstein, N., "Implications of MIME for Internet

   Mail Gateways", RFC 1344, Bellcore, June 1992.



   [RFC-1345] Simonsen, K., "Character Mnemonics & Character Sets",

   RFC 1345, Rationel Almen Planlaegning, June 1992.



   [RFC-1426] Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,

   Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIME

   transport", RFC 1426, United Nations Universit, Innosoft, Dover Beach

   Consulting, Inc., Network Management Associates, Inc., The Branch

   Office, February 1993.



   [RFC-1522] Moore, K., "Representation of Non-Ascii Text in Internet

   Message Headers" RFC 1522, University of Tennessee, September 1993.



   [RFC-1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC

   1340, USC/Information Sciences Institute, July 1992.









Borenstein & Freed                                             [Page 81]

