    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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    RFC 1521                          MIME                    September 1993

       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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    RFC 1521                          MIME                    September 1993

             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.

    Borenstein & Freed                                             [Page 13]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 14]

    RFC 1521                          MIME                    September 1993

           "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

    Borenstein & Freed                                             [Page 15]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 16]

    RFC 1521                          MIME                    September 1993

          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.

    Borenstein & Freed                                             [Page 17]

    RFC 1521                          MIME                    September 1993

          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

    Borenstein & Freed                                             [Page 18]

    RFC 1521                          MIME                    September 1993

          "=" 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

    Borenstein & Freed                                             [Page 19]

    RFC 1521                          MIME                    September 1993

              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]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 21]

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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

    Borenstein & Freed                                             [Page 22]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 23]

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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

    Borenstein & Freed                                             [Page 24]

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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

    Borenstein & Freed                                             [Page 25]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 26]

    RFC 1521                          MIME                    September 1993

          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.

    Borenstein & Freed                                             [Page 27]

    RFC 1521                          MIME                    September 1993

    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.

    Borenstein & Freed                                             [Page 28]

    RFC 1521                          MIME                    September 1993

          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

    Borenstein & Freed                                             [Page 29]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 30]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 31]

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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

    Borenstein & Freed                                             [Page 32]

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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

    Borenstein & Freed                                             [Page 33]

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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:

    Borenstein & Freed                                             [Page 34]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 35]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 36]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 37]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 38]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 39]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 40]

    RFC 1521                          MIME                    September 1993

          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-

    Borenstein & Freed                                             [Page 41]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 42]

    RFC 1521                          MIME                    September 1993

          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

    Borenstein & Freed                                             [Page 43]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 44]

    RFC 1521                          MIME                    September 1993

          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

    Borenstein & Freed                                             [Page 45]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 46]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 47]

    RFC 1521                          MIME                    September 1993

               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)

    Borenstein & Freed                                             [Page 48]

    RFC 1521                          MIME                    September 1993

                  / (";" "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

    Borenstein & Freed                                             [Page 49]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 50]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 51]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 52]

    RFC 1521                          MIME                    September 1993

       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:

    Borenstein & Freed                                             [Page 53]

    RFC 1521                          MIME                    September 1993

       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

    Borenstein & Freed                                             [Page 54]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 55]

    RFC 1521                          MIME                    September 1993

    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.

    Borenstein & Freed                                             [Page 56]

    RFC 1521                          MIME                    September 1993

    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

    Borenstein & Freed                                             [Page 57]

    RFC 1521                          MIME                    September 1993

    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

    Borenstein & Freed                                             [Page 58]

    RFC 1521                          MIME                    September 1993

                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.

    Borenstein & Freed                                             [Page 59]

    RFC 1521                          MIME                    September 1993

    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.

    Borenstein & Freed                                             [Page 60]

    RFC 1521                          MIME                    September 1993

                -- 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-

    Borenstein & Freed                                             [Page 61]

    RFC 1521                          MIME                    September 1993

       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.

    Borenstein & Freed                                             [Page 62]

    RFC 1521                          MIME                    September 1993

    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

    Borenstein & Freed                                             [Page 63]

    RFC 1521                          MIME                    September 1993

          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

    Borenstein & Freed                                             [Page 64]

    RFC 1521                          MIME                    September 1993

          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.

    Borenstein & Freed                                             [Page 65]

    RFC 1521                          MIME                    September 1993

    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....

    Borenstein & Freed                                             [Page 66]

    RFC 1521                          MIME                    September 1993

          --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--

    Borenstein & Freed                                             [Page 67]

    RFC 1521                          MIME                    September 1993

    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.

    Borenstein & Freed                                             [Page 68]

    RFC 1521                          MIME                    September 1993

       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>

    Borenstein & Freed                                             [Page 70]

    RFC 1521                          MIME                    September 1993

            ; 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]

    RFC 1521                          MIME                    September 1993

          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]

    RFC 1521                          MIME                    September 1993

       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]

    RFC 1521                          MIME                    September 1993

       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]