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rfc:rfc1341
          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.
          Borenstein & Freed                                 [Page 21]
          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
  1. -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:
  1. -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.
  1. -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
  1. -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
  1. -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 ----"
  1. —– next message —-
          From: someone-else
          Subject: my opinion
          ...body goes here ...
  1. —– next message —-
          From: someone-else-again
          Subject: my different opinion
          ... another body goes here...
  1. —– 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
  1. -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
  1. -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
  1. -42

Content-Type: message/external-body;

                    access-type=mail-server
                    server="listserv@bogus.bitnet";
                    expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
               Content-type: application/postscript
               get rfc-xxxx doc
  1. -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.]
  1. -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.
  1. -unique-boundary-1

Content-Type: multipart/parallel;

                    boundary=unique-boundary-2
  1. -unique-boundary-2

Content-Type: audio/basic

               Content-Transfer-Encoding: base64
               ... base64-encoded 8000 Hz single-channel
                   u-law-format audio data goes here....
  1. -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....
  1. -unique-boundary-2–
  1. -unique-boundary-1

Content-type: text/richtext

               This is <bold><italic>richtext.</italic></bold>
               <nl><nl>Isn't it
               <bigger><bigger>cool?</bigger></bigger>
  1. -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 ...
  1. -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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          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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