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RFC 1341 - MIME (Multipurpose Internet Mail Extensions): Mechani


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            Network Working Group               N. Borenstein, Bellcore
            Request for Comments: 1341               N. Freed, Innosoft
                                                              June 1992

                   MIME  (Multipurpose Internet Mail Extensions):

                      Mechanisms for Specifying and Describing
                       the Format of Internet Message Bodies

          Status of this Memo

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

          Abstract

            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  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.   It is
            recognized that such extensions are necessary, and they  are
            the subject of a companion document [RFC -1342].

            A table of contents appears at the end of this document.

            Borenstein & Freed                                  [Page i]

            1    Introduction

            Since its publication in 1982, 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 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 1113,
            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 that either X.400  non-textual  body  parts
            should  be converted to (not encoded in) an ASCII format, or
            that they should be discarded, notifying the  RFC  822  user
            that  discarding has occurred.  This is clearly undesirable,
            as information that a user may  wish  to  receive  is  lost.
            Even  though  a  user's  UA  may  not have the capability of
            dealing with the non-textual body part, the user might  have
            some  mechanism  external  to the UA that can extract useful
            information from the body part.  Moreover, it does not allow
            for  the  fact  that the message may eventually be gatewayed
            back into an X.400 message handling system (i.e., the  X.400
            message  is  "tunneled"  through  Internet  mail), where the
            non-textual  information  would  definitely  become   useful
            again.

            Borenstein & Freed                                  [Page 1]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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
                      a mail message.

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

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

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

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

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

            Borenstein & Freed                                  [Page 2]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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  F 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.

            2    Notations, Conventions, and Generic BNF Grammar

            This document is being published in  two  versions,  one  as
            plain  ASCII  text  and  one  as  PostScript.  The latter is
            recommended, though 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.

            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",  wherever  it  is  used  in  this
            document,  refers  to a coded character set, in the sense of
            ISO character set standardization  work,  and  must  not  be

            Borenstein & Freed                                  [Page 3]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            misinterpreted as meaning "a set of characters."

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

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

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

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

            Note : the previous four definitions are  clearly  circular.
            This  is  unavoidable,  since the overal 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  compliant
            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.

            Borenstein & Freed                                  [Page 4]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            3    The MIME-Version Header Field

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

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

            Messages composed in  accordance  with  this  document  MUST
            include  such  a  header  field, with the following verbatim
            text:

            MIME-Version: 1.0

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

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

            MIME-Version := text

            Thus, future  format  specifiers,  which  might  replace  or
            extend  "1.0", are (minimally) constrained by the definition
            of "text", which appears in RFC 822.

            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-compliant.

            Borenstein & Freed                                  [Page 5]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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.   The  ordering  of  parameters  is  not significant.
            Among the defined parameters is  a  "charset"  parameter  by
            which  the  character  set used in the body may be declared.
            Comments are allowed in accordance with RFC  822  rules  for
            structured header fields.

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

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

            An initial set of seven Content-Types  is  defined  by  this
            document.   This  set  of  top-level names is intended to be
            substantially complete.  It is expected  that  additions  to
            the   larger   set  of  supported  types  can  generally  be

            Borenstein & Freed                                  [Page 6]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

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

            Content-Type := type "/" subtype *[";" parameter]

            type :=          "application"     / "audio"
                      / "image"           / "message"
                      / "multipart"  / "text"
                      / "video"           / x-token

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

            subtype := token

            parameter := attribute "=" value

            attribute := token

            value := token / quoted-string

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

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

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

            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

            Borenstein & Freed                                  [Page 7]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 F.  Where intended  for
                      public  use,  the  formats they refer to must
                      also be defined by a published specification,
                      and possibly offered for standardization.

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

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

            Borenstein & Freed                                  [Page 8]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 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.
                      Two  additional  subtypes, "ODA" and "PostScript",
                      are defined for transporting  ODA  and  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   entails   several  securityconsiderations,
                      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, either by error or by an
            older user agent, 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.

            Borenstein & Freed                                  [Page 9]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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  1000
            character lines.

            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:

            Content-Transfer-Encoding := "BASE64" / "QUOTED-PRINTABLE" /
                                         "8BIT"   / "7BIT" /
                                         "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,

            Borenstein & Freed                                 [Page 10]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            "Content-Transfer-Encoding:  7BIT"   is   assumed   if   the
            Content-Transfer-Encoding header field is not present.

            The values "8bit", "7bit", and "binary" all  imply  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.   "7bit"  means  that the data is all represented as
            short lines of US-ASCII data.  "8bit" means that  the  lines
            are  short,  but  there  may be non-ASCII characters (octets
            with the high-order bit set).  "Binary" means that not  only
            may non-ASCII characters be present, but also that the lines
            are not necessarily short enough for SMTP transport.

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

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

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

            Borenstein & Freed                                 [Page 11]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            e.g.,    "Content-Transfer-Encoding:     x-my-new-encoding".
            However, unlike Content-Types and subtypes, the creation  of
            new   Content-Transfer-Encoding  values  is  explicitly  and
            strongly  discouraged,  as  it  seems   likely   to   hinder
            interoperability  with  little potential benefit.  Their use
            is allowed only  as  the  result  of  an  agreement  between
            cooperating user agents.

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

            It should be noted that email is character-oriented, so that
            the  mechanisms  described  here are mechanisms for encoding
            arbitrary byte 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 should 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

            Borenstein & Freed                                 [Page 12]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            "message" Content-Types.  All encodings that are desired for
            bodies of type multipart or message  must  be  done  at  the
            innermost  level,  by encoding the actual body that needs to
            be encoded.

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

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

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

            Borenstein & Freed                                 [Page 13]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            in base64 data should be  converted  to  a  quoted-printable
            line break, but ONLY when converting text data.

            NOTE  ON  CANONICAL  ENCODING  MODEL:     There   was   some
            confusion,  in  earlier  drafts  of this memo, regarding the
            model for when email data was to be converted  to  canonical
            form  and  encoded, and in particular how this process would
            affect the treatment of CRLFs, given that the representation
            of  newlines  varies greatly from system to system. For this
            reason, a canonical  model  for  encoding  is  presented  as
            Appendix H.

            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 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

            Borenstein & Freed                                 [Page 14]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

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

                 Rule #4 (Line Breaks): A line  break  in  a  text  body
                 part,   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.  If isolated CRs and LFs, or LF CR
                 and CR LF sequences are allowed  to  appear  in  binary
                 data  according  to  the  canonical  form, they must be
                 represented   using  the  "=0D",  "=0A",  "=0A=0D"  and
                 "=0D=0A" notations respectively.

                 Note that many implementation may elect to  encode  the
                 local representation of various content types directly.
                 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 15]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                      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 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.

            Borenstein & Freed                                 [Page 16]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            5.2  Base64 Content-Transfer-Encoding

            The  Base64   Content-Transfer-Encoding   is   designed   to
            represent  arbitrary  sequences  of octets in a form that is
            not 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
            based on the one used in Privacy Enhanced Mail applications,
            as defined in RFC 1113.   The  base64  encoding  is  adapted
            from  RFC  1113, with one change:  base64 eliminates the "*"
            mechanism for embedded clear text.

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

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

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

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

            Borenstein & Freed                                 [Page 17]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                            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.  Output character positions
            which are not required to represent actual  input  data  are
            set  to  the  character  "=".   Since all base64 input is an
            integral number of octets,  only  the  following  cases  can
            arise:  (1)  the  final  quantum  of  encoding  input  is an
            integral multiple of  24  bits;  here,  the  final  unit  of
            encoded  output will be an integral multiple of 4 characters
            with no "=" padding, (2) the final quantum of encoding input
            is  exactly  8  bits; here, the final unit of encoded output
            will  be  two  characters  followed  by  two   "="   padding
            characters,  or  (3)  the final quantum of encoding input is
            exactly 16 bits; here, the final unit of encoded output will
            be three characters followed by one "=" padding character.

            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  should  be converted into CRLF sequences

            Borenstein & Freed                                 [Page 18]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 Optional 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:

            Content-ID := msg-id

            Like  the  Message-ID  values,  Content-ID  values  must  be
            generated to be as unique as possible.

            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.

            Content-Description := *text

            The description is presumed to  be  given  in  the  US-ASCII
            character  set,  although  the  mechanism specified in [RFC-
            1342]  may  be  used  for  non-US-ASCII  Content-Description
            values.

            Borenstein & Freed                                 [Page 19]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 G.

            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.  The primary subtype of text
            is "plain".  This indicates plain (unformatted)  text.   The
            default  Content-Type  for  Internet  mail  is  "text/plain;
            charset=us-ascii".

            Beyond plain text, there are many formats  for  representing
            what might be known as "extended text" -- text with embedded
            formatting and  presentation  information.   An  interesting
            characteristic of many such representations is that they are
            to some extent  readable  even  without  the  software  that
            interprets  them.   It is useful, then, to distinguish them,
            at the highest level, from such unreadable data  as  images,
            audio,  or  text  represented in an unreadable form.  In the
            absence  of  appropriate  interpretation  software,  it   is
            reasonable to show subtypes of text to the user, while it is
            not reasonable to do so with most nontextual data.

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

            7.1.1     The charset parameter

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

            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  as  described  in  Appendix  F,
            although the standardization of their use requires the usual

            Borenstein & Freed                                 [Page 20]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            IAB  review  and  approval.  Note  that  if  the   specified
            character  set  includes  8-bit  data,  a  Content-Transfer-
            Encoding header field and a corresponding  encoding  on  the
            data  are  required  in  order to transmit the body via some
            mail transfer protocols, such as SMTP.

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

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

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

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            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            NOTE:   Beyond  US-ASCII,  an  enormous   proliferation   of
            character  sets  is  possible. It is the opinion of the IETF
            working group that a large number of character sets is NOT a
            good  thing.   We would prefer to specify a single character
            set that can be used universally for representing all of the
            world's   languages   in  electronic  mail.   Unfortunately,
            existing practice in several communities seems to  point  to
            the  continued  use  of  multiple character sets in the near
            future.  For this reason, we define names for a small number
            of  character  sets  for  which  a  strong  constituent base
            exists.    It is our hope  that  ISO  10646  or  some  other
            effort  will  eventually define a single world character set
            which can then be specified for use in Internet mail, but in
            the  advance of that definition we cannot specify the use of
            ISO  10646,  Unicode,  or  any  other  character  set  whose
            definition is, as of this writing, incomplete.

            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.

            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 as described in  Appendix  F,  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  should  always  use  the
            "lowest  common  denominator"  character  set possible.  For
            example, if a body contains  only  US-ASCII  characters,  it

            Borenstein & Freed                                 [Page 22]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            should be marked as being in the US-ASCII character set, not
            ISO-8859-1, which, like all the ISO-8859 family of character
            sets,  is  a  superset  of  US-ASCII.   More generally, if a
            widely-used character set is a subset of  another  character
            set,  and a body contains only characters in the widely-used
            subset, it should be labeled as being in that  subset.  This
            will increase the chances that the recipient will be able to
            view the mail correctly.

            7.1.2     The Text/plain subtype

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

            7.1.3     The Text/richtext subtype

            In order to promote the  wider  interoperability  of  simple
            formatted  text,  this  document defines an extremely simple
            subtype of "text", the "richtext" subtype.  This subtype was
            designed to meet the following criteria:

                 1.  The syntax must be extremely simple to  parse,
                 so  that  even  teletype-oriented mail systems can
                 easily strip away the formatting  information  and
                 leave only the readable text.

                 2.  The syntax must be extensible to allow for new
                 formatting commands that are deemed essential.

                 3.  The capabilities must be extremely limited, to
                 ensure  that  it  can  represent  no  more than is
                 likely to be representable by the  user's  primary
                 word  processor.   While  this  limits what can be
                 sent, it increases the  likelihood  that  what  is
                 sent can be properly displayed.

                 4.  The syntax must be compatible  with  SGML,  so
                 that,  with  an  appropriate  DTD  (Document  Type
                 Definition, the standard mechanism for defining  a
                 document  type  using SGML), a general SGML parser
                 could be made to parse richtext.  However, despite
                 this  compatibility,  the  syntax  should  be  far
                 simpler than full SGML, so that no SGML  knowledge
                 is required in order to implement it.

            The syntax of "richtext" is very simple.  It is assumed,  at
            the  top-level,  to be in the US-ASCII character set, unless
            of course a different charset parameter was specified in the
            Content-type  field.   All  characters represent themselves,
            with the exception of the "<" character (ASCII 60), which is
            used   to  mark  the  beginning  of  a  formatting  command.

            Borenstein & Freed                                 [Page 23]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Formatting  instructions  consist  of  formatting   commands
            surrounded  by angle brackets ("<>", ASCII 60 and 62).  Each
            formatting command may be no  more  than  40  characters  in
            length,  all in US-ASCII, restricted to the alphanumeric and
            hyphen ("-") characters. Formatting commands may be preceded
            by  a  forward slash or solidus ("/", ASCII 47), making them
            negations, and such negations must always exist  to  balance
            the  initial opening commands, except as noted below.  Thus,
            if the formatting command "<bold>" appears  at  some  point,
            there  must  later  be a "</bold>" to balance it.  There are
            only three exceptions to this "balancing" rule:  First,  the
            command "<lt>" is used to represent a literal "<" character.
            Second, the command "<nl>" is used to represent  a  required
            line  break.   (Otherwise,  CRLFs in the data are treated as
            equivalent to  a  single  SPACE  character.)   Finally,  the
            command  "<np>"  is  used to represent a page break.  (NOTE:
            The 40 character  limit  on  formatting  commands  does  not
            include  the  "<",  ">",  or  "/"  characters  that might be
            attached to such commands.)

            Initially defined formatting commands, not all of which will
            be implemented by all richtext implementations, include:

                 Bold -- causes the subsequent text  to  be  in  a  bold
                      font.
                 Italic -- causes the subsequent text to be in an italic
                      font.
                 Fixed -- causes the subsequent text to be  in  a  fixed
                      width font.
                 Smaller -- causes  the  subsequent  text  to  be  in  a
                      smaller font.
                 Bigger -- causes the subsequent text to be in a  bigger
                      font.
                 Underline  --  causes  the  subsequent   text   to   be
                      underlined.
                 Center -- causes the subsequent text to be centered.
                 FlushLeft -- causes the  subsequent  text  to  be  left
                      justified.
                 FlushRight -- causes the subsequent text  to  be  right
                      justified.
                 Indent -- causes the subsequent text to be indented  at
                      the left margin.
                 IndentRight  --  causes  the  subsequent  text  to   be
                      indented at the right margin.
                 Outdent -- causes the subsequent text to  be  outdented
                      at the left margin.
                 OutdentRight  --  causes  the  subsequent  text  to  be
                      outdented at the right margin.
                 SamePage -- causes the subsequent text to  be  grouped,
                      if possible, on one page.
                 Subscript  --  causes  the  subsequent   text   to   be
                      interpreted as a subscript.

            Borenstein & Freed                                 [Page 24]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 Superscript  --  causes  the  subsequent  text  to   be
                      interpreted as a superscript.
                 Heading -- causes the subsequent text to be interpreted
                      as a page heading.
                 Footing -- causes the subsequent text to be interpreted
                      as a page footing.
                 ISO-8859-X  (for any value of X  that  is  legal  as  a
                      "charset" parameter) -- causes the subsequent text
                      to be  interpreted  as  text  in  the  appropriate
                      character set.
                 US-ASCII  --  causes  the   subsequent   text   to   be
                      interpreted as text in the US-ASCII character set.
                 Excerpt -- causes the subsequent text to be interpreted
                      as   a   textual   excerpt  from  another  source.
                      Typically this will be displayed using indentation
                      and  an  alternate font, but such decisions are up
                      to the viewer.
                 Paragraph  --  causes  the  subsequent   text   to   be
                      interpreted    as   a   single   paragraph,   with
                      appropriate  paragraph  breaks  (typically   blank
                      space) before and after.
                 Signature  --  causes  the  subsequent   text   to   be
                      interpreted  as  a  "signature".  Some systems may
                      wish to display signatures in a  smaller  font  or
                      otherwise set them apart from the main text of the
                      message.
                 Comment -- causes the subsequent text to be interpreted
                      as a comment, and hence not shown to the reader.
                 No-op -- has no effect on the subsequent text.
                 lt -- <lt> is replaced by a literal "<" character.   No
                      balancing </lt> is allowed.
                 nl -- <nl> causes a line break.  No balancing </nl>  is
                      allowed.
                 np -- <np> causes a page break.  No balancing </np>  is
                      allowed.

            Each positive formatting command affects all subsequent text
            until  the matching negative formatting command.  Such pairs
            of formatting commands must be properly balanced and nested.
            Thus, a proper way to describe text in bold italics is:

                      <bold><italic>the-text</italic></bold>

                 or, alternately,

                      <italic><bold>the-text</bold></italic>

                 but,  in  particular,  the  following  is  illegal
                 richtext:

                      <bold><italic>the-text</bold></italic>

            NOTE:   The  nesting  requirement  for  formatting  commands
            imposes  a  slightly  higher  burden  upon  the composers of

            Borenstein & Freed                                 [Page 25]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            richtext  bodies,  but   potentially   simplifies   richtext
            displayers  by  allowing  them  to be stack-based.  The main
            goal of richtext is to be simple enough to  make  multifont,
            formatted  email  widely  readable,  so  that those with the
            capability of  sending  it  will  be  able  to  do  so  with
            confidence.   Thus  slightly  increased  complexity  in  the
            composing software was  deemed  a  reasonable  tradeoff  for
            simplified  reading  software.  Nonetheless, implementors of
            richtext  readers  are  encouraged  to  follow  the  general
            Internet  guidelines  of being conservative in what you send
            and liberal in what you accept.  Those implementations  that
            can  do so are encouraged to deal reasonably with improperly
            nested richtext.

            Implementations  must  regard  any  unrecognized  formatting
            command  as  equivalent to "No-op", thus facilitating future
            extensions to "richtext".  Private extensions may be defined
            using  formatting  commands that begin with "X-", by analogy
            to Internet mail header field names.

            It is worth noting that no special behavior is required  for
            the TAB (HT) character. It is recommended, however, that, at
            least  when  fixed-width  fonts  are  in  use,  the   common
            semantics  of  the  TAB  (HT)  character should be observed,
            namely that it moves to the next column position that  is  a
            multiple  of  8.   (In  other words, if a TAB (HT) occurs in
            column n, where the leftmost column is column 0,  then  that
            TAB   (HT)   should   be  replaced  by  8-(n  mod  8)  SPACE
            characters.)

            Richtext also differentiates between "hard" and "soft"  line
            breaks.   A line break (CRLF) in the richtext data stream is
            interpreted as a "soft" line break,  one  that  is  included
            only for purposes of mail transport, and is to be treated as
            white space by richtext interpreters.  To include  a  "hard"
            line  break (one that must be displayed as such), the "<nl>"
            or "<paragraph> formatting constructs  should  be  used.  In
            general, a soft line break should be treated as white space,
            but when soft line breaks immediately follow  a  <nl>  or  a
            </paragraph>  tag they should be ignored rather than treated
            as white space.

            Putting all this  together,  the  following  "text/richtext"
            body fragment:

                      <bold>Now</bold> is the time for
                      <italic>all</italic> good men
                       <smaller>(and <lt>women>)</smaller> to
                      <ignoreme></ignoreme> come

                      to the aid of their
                      <nl>

            Borenstein & Freed                                 [Page 26]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                      beloved <nl><nl>country. <comment> Stupid
                      quote! </comment> -- the end

            represents the following  formatted  text  (which  will,  no
            doubt,  look  cryptic  in  the  text-only  version  of  this
            document):

                 Now is the time for all good men (and <women>)  to
                 come to the aid of their
                 beloved

                 country. -- the end

            Richtext conformance:  A minimal richtext implementation  is
            one  that  simply  converts "<lt>" to "<", converts CRLFs to
            SPACE, converts <nl> to a newline according to local newline
            convention,  removes  everything between a <comment> command
            and the next balancing </comment> command, and  removes  all
            other  formatting  commands  (all  text  enclosed  in  angle
            brackets).

            NOTE ON THE RELATIONSHIP OF RICHTEXT TO SGML:   Richtext  is
            decidedly  not  SGML,  and  must  not  be  used to transport
            arbitrary SGML  documents.   Those  who  wish  to  use  SGML
            document  types as a mail transport format must define a new
            text or application subtype, e.g.,  "text/sgml-dtd-whatever"
            or   "application/sgml-dtd-whatever",   depending   on   the
            perceived readability  of  the  DTD  in  use.   Richtext  is
            designed  to  be  compatible  with SGML, and specifically so
            that it will be possible to define a richtext DTD if one  is
            needed.   However,  this  does not imply that arbitrary SGML
            can be called richtext, nor that richtext implementors  have
            any  need  to  understand  SGML;  the  description  in  this
            document is a complete definition of richtext, which is  far
            simpler than complete SGML.

            NOTE ON THE INTENDED USE OF RICHTEXT:  It is recognized that
            implementors  of  future  mail  systems  will want rich text
            functionality  far  beyond  that   currently   defined   for
            richtext.   The  intent  of  richtext is to provide a common
            format for expressing that functionality in a form in  which
            much  of  it, at least, will be understood by interoperating
            software.  Thus,  in  particular,  software  with  a  richer
            notion  of  formatted  text  than  richtext  can  still  use
            richtext as its basic representation, but can extend it with
            new  formatting  commands and by hiding information specific
            to that software  system  in  richtext  comments.   As  such
            systems  evolve,  it  is  expected  that  the  definition of
            richtext  will  be  further  refined  by  future   published
            specifications,  but  richtext  as  defined  here provides a
            platform on which evolutionary refinements can be based.

            IMPLEMENTATION NOTE:  In  some  environments,  it  might  be
            impossible  to combine certain richtext formatting commands,

            Borenstein & Freed                                 [Page 27]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            whereas in  others  they  might  be  combined  easily.   For
            example,  the  combination  of  <bold>  and  <italic>  might
            produce bold italics on systems that support such fonts, but
            there  exist  systems that can make text bold or italicized,
            but not both.  In  such  cases,  the  most  recently  issued
            recognized formatting command should be preferred.

            One of the major goals in the design of richtext was to make
            it  so  simple  that  even  text-only mailers will implement
            richtext-to-plain-text  translators,  thus  increasing   the
            likelihood  that  multifont  text  will become "safe" to use
            very widely.  To demonstrate this simplicity,  an  extremely
            simple  35-line  C program that converts richtext input into
            plain text output is included in Appendix D.

            Borenstein & Freed                                 [Page 28]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            7.2  The Multipart Content-Type

            In the case of multiple part messages, 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 encapsulation 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
            should  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.

            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,

            Borenstein & Freed                                 [Page 29]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 7-bit ASCII in any
            case, and data within the body parts can  be  encoded  on  a
            part-by-part  basis,  with  Content-Transfer-Encoding fields
            for each appropriate body part.

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

            7.2.1     Multipart:  The common syntax

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

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

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

            Thus, a typical multipart Content-Type  header  field  might
            look like this:

                 Content-Type: multipart/mixed;

            Borenstein & Freed                                 [Page 30]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                      boundary=gc0p4Jq0M2Yt08jU534c0p

            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

                 --gc0p4Jq0M2Yt08jU534c0p

            Note that the  encapsulation  boundary  must  occur  at  the
            beginning  of  a line, i.e., following a CRLF, and that that
            initial CRLF is considered to be part of  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
            considered  part  of  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, should 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.

            The requirement that the encapsulation boundary begins  with
            a  CRLF  implies  that  the  body of a multipart entity must
            itself begin with a CRLF before the first encapsulation line
            --  that  is, if the "preamble" area is not used, the entity
            headers must be followed by TWO CRLFs.  This is  indeed  how
            such  entities  should be composed.  A tolerant mail reading
            program, however, may interpret a  body  of  type  multipart
            that  begins  with  an encapsulation line NOT initiated by a
            CRLF  as  also  being  an  encapsulation  boundary,  but   a
            compliant  mail  sending  program  must  not  generate  such
            entities.

            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

            Borenstein & Freed                                 [Page 31]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            boundary.  These areas should generally be left  blank,  and
            implementations  should  ignore anything that appears before
            the first boundary or after the last one.

            NOTE:  These "preamble" and "epilogue" areas  are  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.

            NOTE:  Because encapsulation boundaries must not  appear  in
            the  body  parts  being  encapsulated,  a  user  agent  must
            exercise care to choose a unique boundary.  The boundary  in
            the example above could have been the result of an algorithm
            designed to produce boundaries with a very  low  probability
            of  already  existing in the data to be encapsulated without
            having to prescan  the  data.   Alternate  algorithms  might
            result in more 'readable' boundaries for a recipient with an
            old user agent, but would  require  more  attention  to  the
            possibility   that   the   boundary   might  appear  in  the
            encapsulated  part.   The  simplest  boundary  possible   is
            something like "---", with a closing boundary of "-----".

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

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

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

            Borenstein & Freed                                 [Page 32]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            multipart entities.

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

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

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

            close-delimiter := delimiter "--" ; Again, no  space  before
            "--"

            preamble :=  *text                  ;  to  be  ignored  upon
            receipt.

            epilogue :=  *text                  ;  to  be  ignored  upon
            receipt.

            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

            Borenstein & Freed                                 [Page 33]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                     semantics of a part differ from the semantics
                     of a message, as described in the text.>

            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 intended to be
            displayed  serially.   Any  multipart   subtypes   that   an
            implementation does not recognize should 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.  User agents should recognize that the
            content of the various parts are interchangeable.  The  user
            agent  should  either  choose  the  "best" type based on the
            user's environment and preferences, or offer  the  user  the
            available  alternatives.  In general, choosing the best type
            means displaying only the LAST part that can  be  displayed.
            This  may be used, for example, to send mail in a fancy text
            format in such  a  way  that  it  can  easily  be  displayed
            anywhere:

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

            --boundary42
            Content-Type: text/plain; charset=us-ascii

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

            Borenstein & Freed                                 [Page 34]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            --boundary42
            Content-Type: text/richtext

            .... 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  should 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
            mutlipart/alternative entities are viewed using a  non-MIME-
            compliant mail reader.  While this approach does impose some
            burden on  compliant  mail  readers,  interoperability  with
            older  mail  readers was deemed to be more important in this
            case.

            It may be the case  that  some  user  agents,  if  they  can
            recognize more than one of the formats, will prefer to offer
            the user the choice of which format  to  view.   This  makes
            sense, for example, if mail includes both a nicely-formatted
            image version and an easily-edited text  version.   What  is
            most  critical,  however, is that the user not automatically
            be shown multiple versions of the  same  data.   Either  the
            user  should  be shown the last recognized version or should
            explicitly be given the choice.

            Borenstein & Freed                                 [Page 35]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            7.2.4     The Multipart/digest subtype

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

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

            From: Moderator-Address
            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,  all  of the parts are
            intended to be presented in parallel, i.e.,  simultaneously,
            on  hardware  and  software  that  are  capable of doing so.
            Composing agents should be aware that many mail readers will
            lack this capability and will show the parts serially in any
            event.

            Borenstein & Freed                                 [Page 36]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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".  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-1342].

            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.

            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

            Borenstein & Freed                                 [Page 37]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 on the
            earlier parts. Note also that these parameters may be  given
            in any order.

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

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

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

            But part 3 MUST specify the total number of parts:

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

            Note that part numbering begins with 1, not 0.

            When the parts of a message broken up in this manner are put
            together,  the  result is a complete RFC 822 format message,
            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

            Borenstein & Freed                                 [Page 38]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            entities.  In  this  process  the  following  rules  must be
            observed:

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

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

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

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

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

                 X-Weird-Header-1: Bar
                 X-Weird-Header-2: Hello
                 Message-ID: anotherid@foo.com
                 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:

            Borenstein & Freed                                 [Page 39]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 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...

            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.

            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  a   message   body   or   body   part   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

                 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 auxilliary  information  for  some

            Borenstein & Freed                                 [Page 40]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 -- One or more case-insensitive words,
                 comma-separated,   indicating   supported   access
                 mechanisms 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 F .

            In addition, the following two 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 date 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.

                 PERMISSION -- A 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.

            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:

            Borenstein & Freed                                 [Page 41]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 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

            Before the data is retrieved,  using  these  protocols,  the
            user  will  generally need to be asked to provide a login id
            and a password for the machine named by the site parameter.

            In addition, the  following  optional  parameters  may  also
            appear when the access-type is FTP or ANON-FTP:

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

                 MODE  --  A  transfer  mode  for  retrieving   the
                 information, e.g. "image".

            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

            Borenstein & Freed                                 [Page 42]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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  syntax,  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  may  be provided as the "phantom body"
            when  the  content-type  is  message/external-body  and  the
            access-type is mail-server.

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

            Borenstein & Freed                                 [Page 43]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            7.3.3.5  Examples and Further Explanations

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

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

            If an entity is of type  "message/external-body",  then  the
            body  of  the  entity  will contain the header fields of the
            encapsulated message.  The body itself is to be found in the
            external  location.   This  means  that  if  the body of the
            "message/external-body"  message  contains  two  consecutive
            CRLFs,  everything  after  those  pairs  is  NOT part of the
            message itself.  For  most  message/external-body  messages,
            this trailing area must simply be ignored.  However, it is a
            convenient place for additional data that cannot be included
            in  the  content-type  header field.   In particular, if the
            "access-type" value is "mail-server", then the trailing area
            must  contain  commands to be sent to the mail server at the
            address given by NAME@SITE, where  NAME  and  SITE  are  the
            values of the NAME and SITE parameters, respectively.

            The embedded message header fields which appear in the  body
            of the message/external-body data can be used to declare the
            Content-type  of  the  external  body.   Thus   a   complete
            message/external-body  message,  referring  to a document in
            PostScript format, might look like this:

                 From: Whomever
                 Subject: whatever
                 MIME-Version: 1.0
                 Message-ID: id1@host.com
                 Content-Type: multipart/alternative; boundary=42

                 --42
                 Content-Type: message/external-body;
                      name="BodyFormats.ps";

            Borenstein & Freed                                 [Page 44]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                      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

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

                 Content-type: application/postscript

                 --42
                 Content-Type: message/external-body;
                      access-type=mail-server
                      server="listserv@bogus.bitnet";
                      expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

                 Content-type: application/postscript

                 get rfc-xxxx doc

                 --42--

            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.

            Borenstein & Freed                                 [Page 45]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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   should   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 three
            subtypes: octet-stream, ODA, 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  must  be  registered  with  IANA,  as  described  in
            Appendix F.

            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:

                 NAME -- a suggested name for the  binary  data  if
                 stored as a file.

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

                 CONVERSIONS -- the set  of  operations  that  have
                 been  performed  on  the data before putting it in
                 the mail (and before any Content-Transfer-Encoding
                 that   might   have  been  applied).  If  multiple

            Borenstein & Freed                                 [Page 46]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 conversions have occurred, they must be  separated
                 by  commas  and  specified  in the order they were
                 applied -- that is, the leftmost conversion   must
                 have  occurred  first,  and conversions are undone
                 from right  to  left.   Note  that  NO  conversion
                 values   are   defined   by  this  document.   Any
                 conversion values that that do not begin with "X-"
                 must  be preceded by a published specification and
                 by  registration  with  IANA,  as   described   in
                 Appendix F.

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

            The values  for  these  attributes  are  left  undefined  at
            present,  but  may  require specification in the future.  An
            example of a common (though UNIX-specific) usage might be:

                 Content-Type:  application/octet-stream;
                      name=foo.tar.Z; type=tar;
                      conversions="x-encrypt,x-compress"

            However, it should be noted that the use of such conversions
            is  explicitly  discouraged due to a lack of portability and
            standardization.   The  use  of  uuencode  is   particularly
            discouraged,   in  favor  of  the  Content-Transfer-Encoding
            mechanism, which is both more standardized and more portable
            across mail boundaries.

            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.    The   language   is   defined   in
            [POSTSCRIPT].  It is recommended  that  Postscript  as  sent
            through  email  should  use  Postscript document structuring
            conventions if at all possible, and correctly.

            Borenstein & Freed                                 [Page 47]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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
            reenabling full-function versions of these operators.

            The PostScript language provides facilities for exiting  the
            normal  interpreter,  or  server, loop. Changes made in this
            "outer"  environment   are   customarily   retained   across
            documents, and may in some cases be retained semipermanently
            in nonvolatile memory. The operators associated with exiting
            the  interpreter  loop  have the potential to interfere with
            subsequent document processing. As such, their  unrestrained
            use  constitutes  a  threat  of  service denial.  PostScript
            operators that exit the interpreter loop  include,  but  may
            not  be  limited  to, the exitserver and startjob operators.
            Message-sending software should not generate PostScript that
            depends  on  exiting  the  interpreter  loop to operate. The
            ability to exit  will  probably  be  unavailable  in  secure
            PostScript     implementations.     Message-receiving    and
            -displaying  software  should,  if  possible,  disable   the
            ability   to   make   retained  changes  to  the  PostScript
            environment. Eliminate the startjob and exitserver commands.

            Borenstein & Freed                                 [Page 48]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            If  these  commands  cannot  be eliminated, at least set the
            password associated with them 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,  at least set the password associated with them to
            a hard-to-guess value.

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

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

            It is  possible  to  write  PostScript  that  consumes  huge
            amounts  of various system resources. It is also possible to
            write PostScript programs that loop infinitely.  Both  types
            of  programs  have  the potential to cause damage if sent to
            unsuspecting recipients.   Message-sending  software  should
            avoid  the  construction and dissemination of such programs,
            which  is  antisocial.   Message-receiving  and  -displaying
            software  should  provide  appropriate  mechanisms  to abort
            processing of a document after a reasonable amount  of  time
            has  elapsed. In addition, PostScript interpreters should be
            limited to the consumption of only a  reasonable  amount  of
            any given system resource.

            Finally, bugs may  exist  in  some  PostScript  interpreters
            which  could  possibly  be  exploited  to  gain unauthorized

            Borenstein & Freed                                 [Page 49]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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     The Application/ODA subtype

            The "ODA" subtype of application is used to indicate that  a
            body  contains  information  encoded according to the Office
            Document  Architecture  [ODA]   standards,  using  the  ODIF
            representation  format.   For  application/oda, the Content-
            Type line should also specify an attribute/value  pair  that
            indicates  the document application profile (DAP), using the
            key word "profile".  Thus an appropriate header field  might
            look like this:

            Content-Type:  application/oda; profile=Q112

            Consult the ODA standard [ODA] for further information.

            Borenstein & Freed                                 [Page 50]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            7.5  The Image Content-Type

            A Content-Type of "image" indicates that the bodycontains 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 F.

            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 u-law [PCM]. When this subtype is present,
            a sample rate of 8000 Hz and a single channel is assumed.

            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".

            7.8  Experimental Content-Type Values

            A Content-Type value beginning with the characters "X-" is a
            private  value,  to  be  used  by consenting mail systems by
            mutual agreement.  Any format without a rigorous and  public
            definition  must  be named with an "X-" prefix, and publicly
            specified  values  shall  never  begin  with  "X-".   (Older

            Borenstein & Freed                                 [Page 51]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 52]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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
            text/richtext, message/partial, and message/external-body.

            Borenstein & Freed                                 [Page 53]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Acknowledgements

            This document is the result of the collective  effort  of  a
            large  number  of  people,  at several IETF meetings, on the
            IETF-SMTP  and  IETF-822  mailing  lists,   and   elsewhere.
            Although   any  enumeration  seems  doomed  to  suffer  from
            egregious  omissions,  the  following  are  among  the  many
            contributors to this effort:

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

            Borenstein & Freed                                 [Page 54]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Pekka Kytolaakso              John Wobus
            Stellan Lagerstr.m            Glenn Wright
            Vincent Lau                   Rayan Zachariassen
            Donald Lindsay                David Zimmerman
            The authors apologize for  any  omissions  from  this  list,
            which are certainly unintentional.

            Borenstein & Freed                                 [Page 55]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix A -- Minimal MIME-Conformance

            The mechanisms described in this  document  are  open-ended.
            It  is definitely not expected that all implementations will
            support all of the Content-Types described,  nor  that  they
            will  all  share  the  same extensions.  In order to promote
            interoperability,  however,  it  is  useful  to  define  the
            concept  of  "MIME-conformance" to define a certain level of
            implementation  that  allows  the  useful  interworking   of
            messages  with  content that differs from US ASCII text.  In
            this  section,  we  specify  the   requirements   for   such
            conformance.

            A mail user agent that is MIME-conformant MUST:

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

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

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

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

                 Text:
                      -- Recognize  and  display  "text"  mail
                           with the character set "US-ASCII."
                      -- Recognize  other  character  sets  at
                           least  to  the extent of being able
                           to  inform  the  user  about   what
                           character set the message uses.
                      -- Recognize the "ISO-8859-*"  character
                           sets to the extent of being able to
                           display those characters  that  are
                           common  to ISO-8859-* and US-ASCII,
                           namely all  characters  represented
                           by octet values 0-127.
                      -- For unrecognized  subtypes,  show  or
                           offer  to  show  the user the "raw"
                           version of the data.  An ability at

            Borenstein & Freed                                 [Page 56]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                           least to convert "text/richtext" to
                           plain text, as shown in Appendix D,
                           is encouraged, but not required for
                           conformance.
                 Message:
                      --Recognize and  display  at  least  the
                           primary (822) encapsulation.
                 Multipart:
                      --   Recognize   the   primary   (mixed)
                           subtype.    Display   all  relevant
                           information on  the  message  level
                           and  the body part header level and
                           then display or  offer  to  display
                           each     of    the    body    parts
                           individually.
                      -- Recognize the "alternative"  subtype,
                           and    avoid   showing   the   user
                           redundant         parts          of
                           multipart/alternative mail.
                      -- Treat any unrecognized subtypes as if
                           they were "mixed".
                 Application:
                      -- Offer the ability to remove either of
                           the  two types of Content-Transfer-
                           Encoding defined in  this  document
                           and  put  the resulting information
                           in a user file.

                 5.  Upon encountering  any  unrecognized  Content-
                 Type, an implementation must treat it as if it had
                 a Content-Type of "application/octet-stream"  with
                 no  parameter  sub-arguments.  How  such  data are
                 handled is up to  an  implementation,  but  likely
                 options   for   handling  such  unrecognized  data
                 include offering the user to write it into a  file
                 (decoded   from  its  mail  transport  format)  or
                 offering the user to name a program to  which  the
                 decoded   data   should   be   passed   as  input.
                 Unrecognized predefined types, which  in  a  MIME-
                 conformant   mailer  might  still  include  audio,
                 image, or video, should also be  treated  in  this
                 way.

            A user agent that meets the above conditions is said  to  be
            MIME-conformant.   The  meaning of this phrase is that it is
            assumed  to  be  "safe"  to  send  virtually  any  kind   of
            properly-marked  data to users of such mail systems, because
            such systems will at least be able  to  treat  the  data  as
            undifferentiated  binary, and will not simply splash it onto
            the screen of unsuspecting users.   There is  another  sense
            in  which  it is always "safe" to send data in a format that
            is MIME-conformant, which is that such data will  not  break
            or  be  broken by any known systems that are conformant with
            RFC 821 and RFC 822.  User agents that  are  MIME-conformant

            Borenstein & Freed                                 [Page 57]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            have  the  additional  guarantee  that  the user will not be
            shown data that were never intended to be viewed as text.

            Borenstein & Freed                                 [Page 58]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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 body
                 parts.  As  such,  the  persistence  of  CRLF   as
                 something  other  than  a line break should 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
                 should 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
                 should not rely on the  persistence  of  TAB  (HT)

            Borenstein & Freed                                 [Page 59]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 characters.

                 (4) Lines longer than 76 characters may be wrapped
                 or  truncated  in some environments. Line wrapping
                 and line truncation are STRONGLY DISCOURAGED,  but
                 unavoidable  in  some  cases.  Applications  which
                 require long lines  should  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,  should 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

            Borenstein & Freed                                 [Page 60]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 73 characters.

            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 implementions should be  robust
            in dealing with the bad effects they can cause.

            Borenstein & Freed                                 [Page 61]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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>
                 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
                     u-law-format audio data goes here....

                 --unique-boundary-2
                 Content-Type: image/gif
                 Content-Transfer-Encoding: Base64

            Borenstein & Freed                                 [Page 62]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

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

                 --unique-boundary-2--

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

                 This is <bold><italic>richtext.</italic></bold>
                 <nl><nl>Isn't it
                 <bigger><bigger>cool?</bigger></bigger>

                 --unique-boundary-1
                 Content-Type: message/rfc822

                 From: (name 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 63]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix D -- A Simple Richtext-to-Text Translator in C

            One of the major goals in the design of the richtext subtype
            of the text Content-Type is to make formatted text so simple
            that even  text-only  mailers  will  implement  richtext-to-
            plain-text  translators, thus increasing the likelihood that
            multifont text will become "safe" to use  very  widely.   To
            demonstrate  this  simplicity,  what follows is an extremely
            simple 44-line C program that converts richtext  input  into
            plain text output:

                 #include <stdio.h>
                 #include <ctype.h>
                 main() {
                     int c, i;
                     char token[50];

                     while((c = getc(stdin)) != EOF) {
                         if (c == '<') {
                             for (i=0; (i<49 && (c = getc(stdin)) != '>'
                                       && c != EOF); ++i) {
                                 token[i] = isupper(c) ? tolower(c) : c;
                             }
                             if (c == EOF) break;
                             if (c != '>') while ((c = getc(stdin)) !=
                 '>'
                                       && c != EOF) {;}
                             if (c == EOF) break;
                             token[i] = '\0';
                             if (!strcmp(token, "lt")) {
                                 putc('<', stdout);
                             } else if (!strcmp(token, "nl")) {
                                 putc('\n', stdout);
                             } else if (!strcmp(token, "/paragraph")) {
                                 fputs("\n\n", stdout);
                             } else if (!strcmp(token, "comment")) {
                                 int commct=1;
                                 while (commct > 0) {
                                     while ((c = getc(stdin)) != '<'
                                      && c != EOF) ;
                                     if (c == EOF) break;
                                     for (i=0; (c = getc(stdin)) != '>'
                                        && c != EOF; ++i) {
                                         token[i] = isupper(c) ?
                                          tolower(c) : c;
                                     }
                                     if (c== EOF) break;
                                     token[i] = NULL;
                                     if (!strcmp(token, "/comment")) --
                 commct;
                                     if (!strcmp(token, "comment"))
                 ++commct;

            Borenstein & Freed                                 [Page 64]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                                 }
                             } /* Ignore all other tokens */
                         } else if (c != '\n') putc(c, stdout);
                     }
                     putc('\n', stdout); /* for good measure */
                 }
            It should be noted that one can do considerably better  than
            this  in  displaying  richtext  data on a dumb terminal.  In
            particular, one can replace font information such as  "bold"
            with textual emphasis (like *this* or   _T_H_I_S_).  One can
            also  properly  handle  the  richtext  formatting   commands
            regarding  indentation, justification, and others.  However,
            the above program is all  that  is  necessary  in  order  to
            present richtext on a dumb terminal.

            Borenstein & Freed                                 [Page 65]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix E -- 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.

            attribute := 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.>

            boundary := 0*69<bchars> bcharsnospace

            bchars := bcharsnospace / " "

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

            close-delimiter := delimiter "--"

            Content-Description := *text

            Content-ID := msg-id

            Content-Transfer-Encoding  :=      "BASE64"     /   "QUOTED-
            PRINTABLE" /
                                            "8BIT"  / "7BIT" /
                                            "BINARY"     / x-token

            Content-Type := type "/" subtype *[";" parameter]

            delimiter := CRLF "--" boundary   ; taken from  Content-Type
            field.
                                           ;   when   content-type    is
            multipart
                                         ; There should be no space
                                         ; between "--" and boundary.

            encapsulation := delimiter CRLF body-part

            epilogue :=  *text                  ;  to  be  ignored  upon
            receipt.

            Borenstein & Freed                                 [Page 66]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            MIME-Version := 1*text

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

            parameter := attribute "=" value

            preamble :=  *text                  ;  to  be  ignored  upon
            receipt.

            subtype := token

            token := 1*<any 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"           / x-token

            value := token / quoted-string

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

            Borenstein & Freed                                 [Page 67]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix F -- 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,  conversions parameters for the
            application  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.

            F.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  should  be  published  as  an   RFC   or
            submitted  in  a  form   suitable  to  become an RFC, and be
            subject to the Internet standards process.

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

                 MIME type name:

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

                 MIME subtype name:

                 Required parameters:

                 Optional parameters:

                 Encoding considerations:

                 Security considerations:

            Borenstein & Freed                                 [Page 68]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 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:
            F.2  Registration of New Character Set Values

                 To:  IANA@isi.edu
                 Subject:  Registration of new MIME character set value

                 MIME character set name:

                 Published specification:

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

                 Person & email address to contact for further
                 information:

            F.3  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:

            F.4  Registration of New Conversions Values for Application

                 To:  IANA@isi.edu
                 Subject:  Registration of new MIME Conversions value
                 for Application content-type

                 MIME Conversions name:

            Borenstein & Freed                                 [Page 69]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

                 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 70]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix G -- Summary of the Seven Content-types

            Content-type: text

            Subtypes defined by this document:  plain, richtext

            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

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

            ________________________________________________________________

            Content-type: application

            Subtypes  defined   by   this   document:      octet-stream,
                 postscript, oda

            Important Parameters: profile

            Borenstein & Freed                                 [Page 71]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Encoding notes: base64 generally preferred for  octet-stream
                 or other 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.   In  general, 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 72]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Appendix H -- 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 message part can be modelled
            as  being  done in a number of steps.  Note that these steps
            are roughly similar to those steps used in RFC1113:

            Step 1.  Creation of local form.

            The body part 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 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.

            Step 2.  Conversion to canonical form.

            The entire body part,  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 part 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.

            For example, in the case of text/plain data, the  text  must
            be  converted to a supported character set and lines must be
            delimited with CRLF delimiters in  accordance  with  RFC822.
            Note  that the restriction on line lengths implied by RFC822
            is eliminated  if  the  next  step  employs  either  quoted-
            printable or base64 encoding.

            Step 3.  Apply transfer encoding.

            A Content-Transfer-Encoding appropriate for this  body  part
            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 body part.

            Borenstein & Freed                                 [Page 73]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            Step 4.  Insertion into message.

            The encoded object is inserted  into  a  MIME  message  with
            appropriate body part headers and boundary markers.

            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  bodyparts  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 compliant 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  only  important  aspect  of this discussion is that the
            resulting messages are consistent with those produced by the
            model described here.

            Borenstein & Freed                                 [Page 74]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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.

            [ODA] ISO 8613;  Information  Processing:  Text  and  Office
            System;  Office  Document Architecture (ODA) and Interchange
            Format (ODIF), Part 1-8, 1989.

            [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.

            [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.R.  TFTP Protocol (revision 2).  June,
            1981, MIT, RFC-783.

            [RFC-821]  Postel,  J.B.   Simple  Mail  Transfer  Protocol.
            August, 1982, USC/Information Sciences Institute, RFC-821.

            Borenstein & Freed                                 [Page 75]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            [RFC-822]   Crocker, D.  Standard for  the  format  of  ARPA
            Internet  text  messages. August, 1982, UDEL, RFC-822.

            [RFC-934]   Rose, M.T.; Stefferud, E.A.   Proposed  standard
            for    message     encapsulation.  January,   1985, Delaware
            and NMA, RFC-934.

            [RFC-959]   Postel,  J.B.;  Reynolds,  J.K.   File  Transfer
            Protocol.      October,   1985,   USC/Information   Sciences
            Institute, RFC-959.

            [RFC-1049]   Sirbu,  M.A.   Content-Type  header  field  for
            Internet messages.  March, 1988, CMU,  RFC-1049.

            [RFC-1113]   Linn,  J.   Privacy  enhancement  for  Internet
            electronic    mail:  Part    I  -  message  encipherment and
            authentication procedures.   August,  1989, IAB Privacy Task
            Force, RFC-1113.

            [RFC-1154]  Robinson, D.; Ullmann, R.  Encoding header field
            for   Internet   messages.  April,   1990,   Prime Computer,
            Inc., RFC-1154.

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

            Security Considerations

            Security issues  are  discussed  in  Section  7.4.2  and  in
            Appendix  G.   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
            applicaton/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 76]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

            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:  +1 714 624 7907
                                 Fax: +1 714 621 5319
                                Email: ned@innosoft.com

            Borenstein & Freed                                 [Page 77]

            RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992

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            Please discard this page and place the  following  table  of
            contents after the title page.

            Borenstein & Freed                                  [Page i]

                               Table of Contents

            1     Introduction.......................................  1
            2     Notations, Conventions, and Generic BNF Grammar....  3
            3     The MIME-Version Header Field......................  5
            4     The Content-Type Header Field......................  6
            5     The Content-Transfer-Encoding Header Field......... 10
            5.1   Quoted-Printable Content-Transfer-Encoding......... 14
            5.2   Base64 Content-Transfer-Encoding................... 17
            6     Additional Optional Content- Header Fields......... 19
            6.1   Optional Content-ID Header Field................... 19
            6.2   Optional Content-Description Header Field.......... 19
            7     The Predefined Content-Type Values................. 20
            7.1   The Text Content-Type.............................. 20
            7.1.1 The charset parameter.............................. 20
            7.1.2 The Text/plain subtype............................. 23
            7.1.3 The Text/richtext subtype.......................... 23
            7.2   The Multipart Content-Type......................... 29
            7.2.1 Multipart:  The common syntax...................... 30
            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..................... 36
            7.3   The Message Content-Type........................... 37
            7.3.1 The Message/rfc822 (primary) subtype............... 37
            7.3.2 The Message/Partial subtype........................ 37
            7.3.3 The Message/External-Body subtype.................. 40
            7.4   The Application Content-Type....................... 46
            7.4.1 The Application/Octet-Stream (primary) subtype..... 46
            7.4.2 The Application/PostScript subtype................. 47
            7.4.3 The Application/ODA subtype........................ 50
            7.5   The Image Content-Type............................. 51
            7.6   The Audio Content-Type............................. 51
            7.7   The Video Content-Type............................. 51
            7.8   Experimental Content-Type Values................... 51
                  Summary............................................ 53
                  Acknowledgements................................... 54
                  Appendix A -- Minimal MIME-Conformance............. 56
                  Appendix B -- General Guidelines For Sending Email Data59
                  Appendix C -- A Complex Multipart Example.......... 62
                  Appendix D -- A Simple Richtext-to-Text Translator in C64
                  Appendix E -- Collected Grammar.................... 66
                  Appendix F -- IANA Registration Procedures......... 68
                  F.1  Registration of New Content-type/subtype Values..68
                  F.2  Registration of New Character Set Values...... 69
                  F.3  Registration of New Access-type Values for Message/external-body69
                  F.4  Registration of New Conversions Values for Application69
                  Appendix G -- Summary of the Seven Content-types... 71
                  Appendix H -- Canonical Encoding Model............. 73
                  References......................................... 75
                  Security Considerations............................ 76
                  Authors' Addresses................................. 77

            Borenstein & Freed                                 [Page ii]

            Borenstein & Freed                                [Page iii]

 

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