rohrpost

rfc1341.txt (211117B)

---

 
 
 
 
 
 
             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
 
                  --gc0p4Jq0M2Yt08jU534c0p
 
             Note that the  encapsulation  boundary  must  occur  at  the
             beginning  of  a line, i.e., following a CRLF, and that that
             initial CRLF is considered to be part of  the  encapsulation
             boundary  rather  than  part  of  the preceding part.    The
             boundary must be followed immediately either by another CRLF
             and the header fields for the next part, or by two CRLFs, in
             which case there are no header fields for the next part (and
             it is therefore assumed to be of Content-Type text/plain).
 
             NOTE:   The  CRLF  preceding  the  encapsulation   line   is
             considered  part  of  the boundary so that it is possible to
             have a part that does not end with  a  CRLF  (line   break).
             Body  parts that must be considered to end with line breaks,
             therefore, should have two CRLFs preceding the encapsulation
             line, the first of which is part of the preceding body part,
             and the  second  of  which  is  part  of  the  encapsulation
             boundary.
 
             The requirement that the encapsulation boundary begins  with
             a  CRLF  implies  that  the  body of a multipart entity must
             itself begin with a CRLF before the first encapsulation line
             --  that  is, if the "preamble" area is not used, the entity
             headers must be followed by TWO CRLFs.  This is  indeed  how
             such  entities  should be composed.  A tolerant mail reading
             program, however, may interpret a  body  of  type  multipart
             that  begins  with  an encapsulation line NOT initiated by a
             CRLF  as  also  being  an  encapsulation  boundary,  but   a
             compliant  mail  sending  program  must  not  generate  such
             entities.
 
             Encapsulation  boundaries  must  not   appear   within   the
             encapsulations,  and  must  be no longer than 70 characters,
             not counting the two leading hyphens.
 
             The encapsulation boundary following the last body part is a
             distinguished  delimiter that indicates that no further body
             parts will follow.  Such a delimiter  is  identical  to  the
             previous  delimiters,  with the addition of two more hyphens
             at the end of the line:
 
                  --gc0p4Jq0M2Yt08jU534c0p--
 
             There appears to be room for additional information prior to
             the  first  encapsulation  boundary  and following the final
 
 
 
             Borenstein & Freed                                 [Page 31]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             boundary.  These areas should generally be left  blank,  and
             implementations  should  ignore anything that appears before
             the first boundary or after the last one.
 
             NOTE:  These "preamble" and "epilogue" areas  are  not  used
             because  of the lack of proper typing of these parts and the
             lack  of  clear  semantics  for  handling  these  areas   at
             gateways, particularly X.400 gateways.
 
             NOTE:  Because encapsulation boundaries must not  appear  in
             the  body  parts  being  encapsulated,  a  user  agent  must
             exercise care to choose a unique boundary.  The boundary  in
             the example above could have been the result of an algorithm
             designed to produce boundaries with a very  low  probability
             of  already  existing in the data to be encapsulated without
             having to prescan  the  data.   Alternate  algorithms  might
             result in more 'readable' boundaries for a recipient with an
             old user agent, but would  require  more  attention  to  the
             possibility   that   the   boundary   might  appear  in  the
             encapsulated  part.   The  simplest  boundary  possible   is
             something like "---", with a closing boundary of "-----".
 
             As a very simple example, the  following  multipart  message
             has  two  parts,  both  of  them  plain  text,  one  of them
             explicitly typed and one of them implicitly typed:
 
                  From: Nathaniel Borenstein <nsb@bellcore.com>
                  To:  Ned Freed <ned@innosoft.com>
                  Subject: Sample message
                  MIME-Version: 1.0
                  Content-type: multipart/mixed; boundary="simple
                  boundary"
 
                  This is the preamble.  It is to be ignored, though it
                  is a handy place for mail composers to include an
                  explanatory note to non-MIME compliant readers.
                  --simple boundary
 
                  This is implicitly typed plain ASCII text.
                  It does NOT end with a linebreak.
                  --simple boundary
                  Content-type: text/plain; charset=us-ascii
 
                  This is explicitly typed plain ASCII text.
                  It DOES end with a linebreak.
 
                  --simple boundary--
                  This is the epilogue.  It is also to be ignored.
 
             The use of a Content-Type of multipart in a body part within
             another  multipart  entity  is explicitly allowed.   In such
             cases, for obvious reasons, care must  be  taken  to  ensure
             that  each  nested  multipart  entity  must  use a different
             boundary delimiter. See Appendix C for an example of  nested
 
 
 
             Borenstein & Freed                                 [Page 32]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             multipart entities.
 
             The use of the multipart Content-Type  with  only  a  single
             body  part  may  be  useful  in  certain  contexts,  and  is
             explicitly permitted.
 
             The only mandatory parameter for the multipart  Content-Type
             is  the  boundary  parameter,  which  consists  of  1  to 70
             characters from a set of characters known to be very  robust
             through  email  gateways,  and  NOT ending with white space.
             (If a boundary appears to end with white  space,  the  white
             space  must be presumed to have been added by a gateway, and
             should  be  deleted.)   It  is  formally  specified  by  the
             following BNF:
 
             boundary := 0*69<bchars> bcharsnospace
 
             bchars := bcharsnospace / " "
 
             bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  /
             "_"
                            / "," / "-" / "." / "/" / ":" / "=" / "?"
 
             Overall, the body of a multipart entity may be specified  as
             follows:
 
             multipart-body := preamble 1*encapsulation
                            close-delimiter epilogue
 
             encapsulation := delimiter CRLF body-part
 
             delimiter := CRLF "--" boundary   ; taken from  Content-Type
             field.
                                            ;   when   content-type    is
             multipart
                                          ; There must be no space
                                          ; between "--" and boundary.
 
             close-delimiter := delimiter "--" ; Again, no  space  before
             "--"
 
             preamble :=  *text                  ;  to  be  ignored  upon
             receipt.
 
             epilogue :=  *text                  ;  to  be  ignored  upon
             receipt.
 
             body-part = <"message" as defined in RFC 822,
                      with all header fields optional, and with the
                      specified delimiter not occurring anywhere in
                      the message body, either on a line by itself
                      or as a substring anywhere.  Note that the
 
 
 
 
 
             Borenstein & Freed                                 [Page 33]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                      semantics of a part differ from the semantics
                      of a message, as described in the text.>
 
             NOTE:  Conspicuously missing from the multipart  type  is  a
             notion  of  structured,  related body parts.  In general, it
             seems premature to try to  standardize  interpart  structure
             yet.  It is recommended that those wishing to provide a more
             structured or integrated multipart messaging facility should
             define   a   subtype  of  multipart  that  is  syntactically
             identical, but  that  always  expects  the  inclusion  of  a
             distinguished part that can be used to specify the structure
             and integration of the other parts,  probably  referring  to
             them  by  their Content-ID field.  If this approach is used,
             other implementations will not recognize  the  new  subtype,
             but  will  treat it as the primary subtype (multipart/mixed)
             and will thus be able to show the user the  parts  that  are
             recognized.
 
             7.2.2     The Multipart/mixed (primary) subtype
 
             The primary subtype for multipart, "mixed", is intended  for
             use  when  the body parts are independent and intended to be
             displayed  serially.   Any  multipart   subtypes   that   an
             implementation does not recognize should be treated as being
             of subtype "mixed".
 
             7.2.3     The Multipart/alternative subtype
 
             The multipart/alternative type is syntactically identical to
             multipart/mixed,   but  the  semantics  are  different.   In
             particular, each of the parts is an "alternative" version of
             the same information.  User agents should recognize that the
             content of the various parts are interchangeable.  The  user
             agent  should  either  choose  the  "best" type based on the
             user's environment and preferences, or offer  the  user  the
             available  alternatives.  In general, choosing the best type
             means displaying only the LAST part that can  be  displayed.
             This  may be used, for example, to send mail in a fancy text
             format in such  a  way  that  it  can  easily  be  displayed
             anywhere:
 
             From:  Nathaniel Borenstein <nsb@bellcore.com>
             To: Ned Freed <ned@innosoft.com>
             Subject: Formatted text mail
             MIME-Version: 1.0
             Content-Type: multipart/alternative; boundary=boundary42
 
 
             --boundary42
             Content-Type: text/plain; charset=us-ascii
 
             ...plain text version of message goes here....
 
 
 
 
 
             Borenstein & Freed                                 [Page 34]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             --boundary42
             Content-Type: text/richtext
 
             .... richtext version of same message goes here ...
             --boundary42
             Content-Type: text/x-whatever
 
             .... fanciest formatted version of same  message  goes  here
             ...
             --boundary42--
 
             In this example, users  whose  mail  system  understood  the
             "text/x-whatever"  format  would see only the fancy version,
             while other users would see only the richtext or plain  text
             version, depending on the capabilities of their system.
 
             In general, user agents that  compose  multipart/alternative
             entities  should place the body parts in increasing order of
             preference, that is, with the  preferred  format  last.  For
             fancy  text,  the sending user agent should put the plainest
             format first and the richest format  last.   Receiving  user
             agents  should  pick  and  display  the last format they are
             capable of  displaying.   In  the  case  where  one  of  the
             alternatives  is  itself  of  type  "multipart" and contains
             unrecognized sub-parts, the user agent may choose either  to
             show that alternative, an earlier alternative, or both.
 
             NOTE:  From an implementor's perspective, it might seem more
             sensible  to  reverse  this  ordering, and have the plainest
             alternative last.  However, placing the plainest alternative
             first    is    the    friendliest   possible   option   when
             mutlipart/alternative entities are viewed using a  non-MIME-
             compliant mail reader.  While this approach does impose some
             burden on  compliant  mail  readers,  interoperability  with
             older  mail  readers was deemed to be more important in this
             case.
 
             It may be the case  that  some  user  agents,  if  they  can
             recognize more than one of the formats, will prefer to offer
             the user the choice of which format  to  view.   This  makes
             sense, for example, if mail includes both a nicely-formatted
             image version and an easily-edited text  version.   What  is
             most  critical,  however, is that the user not automatically
             be shown multiple versions of the  same  data.   Either  the
             user  should  be shown the last recognized version or should
             explicitly be given the choice.
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 35]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             7.2.4     The Multipart/digest subtype
 
             This document defines a "digest" subtype  of  the  multipart
             Content-Type.   This  type  is  syntactically  identical  to
             multipart/mixed,  but  the  semantics  are  different.    In
             particular,  in a digest, the default Content-Type value for
             a   body   part   is   changed    from    "text/plain"    to
             "message/rfc822".   This  is  done  to allow a more readable
             digest format that is largely  compatible  (except  for  the
             quoting convention) with RFC 934.
 
             A digest in this format might,  then,  look  something  like
             this:
 
             From: Moderator-Address
             MIME-Version: 1.0
             Subject:  Internet Digest, volume 42
             Content-Type: multipart/digest;
                  boundary="---- next message ----"
 
 
             ------ next message ----
 
             From: someone-else
             Subject: my opinion
 
             ...body goes here ...
 
             ------ next message ----
 
             From: someone-else-again
             Subject: my different opinion
 
             ... another body goes here...
 
             ------ next message ------
 
             7.2.5     The Multipart/parallel subtype
 
             This document defines a "parallel" subtype of the  multipart
             Content-Type.   This  type  is  syntactically  identical  to
             multipart/mixed,  but  the  semantics  are  different.    In
             particular,  in  a  parallel  entity,  all  of the parts are
             intended to be presented in parallel, i.e.,  simultaneously,
             on  hardware  and  software  that  are  capable of doing so.
             Composing agents should be aware that many mail readers will
             lack this capability and will show the parts serially in any
             event.
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 36]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             7.3  The Message Content-Type
 
             It is frequently desirable, in sending mail, to  encapsulate
             another  mail  message. For this common operation, a special
             Content-Type, "message", is defined.  The  primary  subtype,
             message/rfc822,  has  no required parameters in the Content-
             Type field.  Additional subtypes, "partial"  and  "External-
             body",  do  have  required  parameters.   These subtypes are
             explained below.
 
             NOTE:  It has been suggested that subtypes of message  might
             be  defined  for  forwarded  or rejected messages.  However,
             forwarded and rejected messages can be handled as  multipart
             messages  in  which  the  first part contains any control or
             descriptive  information,  and  a  second  part,   of   type
             message/rfc822,   is  the  forwarded  or  rejected  message.
             Composing rejection and forwarding messages in  this  manner
             will  preserve  the type information on the original message
             and allow it to be correctly presented to the recipient, and
             hence is strongly encouraged.
 
             As stated in the definition of the Content-Transfer-Encoding
             field, no encoding other than "7bit", "8bit", or "binary" is
             permitted for messages  or  parts  of  type  "message".  The
             message  header  fields are always US-ASCII in any case, and
             data within the body can still be encoded, in which case the
             Content-Transfer-Encoding  header  field in the encapsulated
             message will reflect this.  Non-ASCII text in the headers of
             an   encapsulated   message   can  be  specified  using  the
             mechanisms described in [RFC-1342].
 
             Mail gateways, relays, and other mail  handling  agents  are
             commonly  known  to alter the top-level header of an RFC 822
             message.   In particular, they frequently  add,  remove,  or
             reorder  header  fields.   Such  alterations  are explicitly
             forbidden for  the  encapsulated  headers  embedded  in  the
             bodies of messages of type "message."
 
             7.3.1     The Message/rfc822 (primary) subtype
 
             A Content-Type of "message/rfc822" indicates that  the  body
             contains  an encapsulated message, with the syntax of an RFC
             822 message.
 
             7.3.2     The Message/Partial subtype
 
             A subtype of message, "partial",  is  defined  in  order  to
             allow  large  objects  to  be  delivered as several separate
             pieces  of  mail  and  automatically  reassembled   by   the
             receiving  user  agent.   (The  concept  is  similar  to  IP
             fragmentation/reassembly in the basic  Internet  Protocols.)
             This  mechanism  can  be  used  when  intermediate transport
             agents limit the size of individual  messages  that  can  be
             sent.   Content-Type  "message/partial"  thus indicates that
 
 
 
             Borenstein & Freed                                 [Page 37]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             the body contains a fragment of a larger message.
 
             Three parameters must be specified in the Content-Type field
             of  type  message/partial:  The  first,  "id",  is  a unique
             identifier,  as  close  to  a  world-unique  identifier   as
             possible,  to  be  used  to  match  the parts together.  (In
             general, the identifier  is  essentially  a  message-id;  if
             placed  in  double  quotes,  it  can  be  any message-id, in
             accordance with the BNF for  "parameter"  given  earlier  in
             this  specification.)   The second, "number", an integer, is
             the part number, which indicates where this part  fits  into
             the  sequence  of  fragments.   The  third, "total", another
             integer, is the total number of parts. This  third  subfield
             is  required  on  the  final  part,  and  is optional on the
             earlier parts. Note also that these parameters may be  given
             in any order.
 
             Thus, part 2 of a 3-part message  may  have  either  of  the
             following header fields:
 
                  Content-Type: Message/Partial;
                       number=2; total=3;
                       id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
 
                  Content-Type: Message/Partial;
                       id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
                       number=2
 
             But part 3 MUST specify the total number of parts:
 
                  Content-Type: Message/Partial;
                       number=3; total=3;
                       id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
 
             Note that part numbering begins with 1, not 0.
 
             When the parts of a message broken up in this manner are put
             together,  the  result is a complete RFC 822 format message,
             which may have its own Content-Type header field,  and  thus
             may contain any other data type.
 
             Message fragmentation and reassembly:  The  semantics  of  a
             reassembled  partial  message  must  be those of the "inner"
             message, rather than  of  a  message  containing  the  inner
             message.   This  makes  it  possible, for example, to send a
             large audio message as several partial messages,  and  still
             have  it  appear  to the recipient as a simple audio message
             rather than as an encapsulated message containing  an  audio
             message.   That  is,  the  encapsulation  of  the message is
             considered to be "transparent".
 
             When  generating   and   reassembling   the   parts   of   a
             message/partial  message,  the  headers  of the encapsulated
             message must be merged with the  headers  of  the  enclosing
 
 
 
             Borenstein & Freed                                 [Page 38]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             entities.  In  this  process  the  following  rules  must be
             observed:
 
                  (1) All of the headers from the initial  enclosing
                  entity  (part  one),  except those that start with
                  "Content-" and "Message-ID", must  be  copied,  in
                  order, to the new message.
 
                  (2) Only those headers  in  the  enclosed  message
                  which  start with "Content-" and "Message-ID" must
                  be appended, in order, to the headers of  the  new
                  message.   Any  headers  in  the  enclosed message
                  which do not start  with  "Content-"  (except  for
                  "Message-ID") will be ignored.
 
                  (3) All of the headers from  the  second  and  any
                  subsequent messages will be ignored.
 
             For example, if an audio message is broken into  two  parts,
             the first part might look something like this:
 
                  X-Weird-Header-1: Foo
                  From: Bill@host.com
                  To: joe@otherhost.com
                  Subject: Audio mail
                  Message-ID: id1@host.com
                  MIME-Version: 1.0
                  Content-type: message/partial;
                       id="ABC@host.com";
                       number=1; total=2
 
                  X-Weird-Header-1: Bar
                  X-Weird-Header-2: Hello
                  Message-ID: anotherid@foo.com
                  Content-type: audio/basic
                  Content-transfer-encoding: base64
 
                  ... first half of encoded audio data goes here...
 
             and the second half might look something like this:
 
                  From: Bill@host.com
                  To: joe@otherhost.com
                  Subject: Audio mail
                  MIME-Version: 1.0
                  Message-ID: id2@host.com
                  Content-type: message/partial;
                       id="ABC@host.com"; number=2; total=2
 
                  ... second half of encoded audio data goes here...
 
             Then,  when  the  fragmented  message  is  reassembled,  the
             resulting  message  to  be displayed to the user should look
             something like this:
 
 
 
             Borenstein & Freed                                 [Page 39]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  X-Weird-Header-1: Foo
                  From: Bill@host.com
                  To: joe@otherhost.com
                  Subject: Audio mail
                  Message-ID: anotherid@foo.com
                  MIME-Version: 1.0
                  Content-type: audio/basic
                  Content-transfer-encoding: base64
 
                  ... first half of encoded audio data goes here...
                  ... second half of encoded audio data goes here...
 
             It should be  noted  that,  because  some  message  transfer
             agents  may choose to automatically fragment large messages,
             and because such  agents  may  use  different  fragmentation
             thresholds,  it  is  possible  that  the pieces of a partial
             message, upon reassembly, may prove themselves to comprise a
             partial message.  This is explicitly permitted.
 
             It should also be noted that the inclusion of a "References"
             field  in the headers of the second and subsequent pieces of
             a fragmented message that references the Message-Id  on  the
             previous  piece  may  be  of  benefit  to  mail readers that
             understand and track references. However, the generation  of
             such "References" fields is entirely optional.
 
             7.3.3     The Message/External-Body subtype
 
             The external-body subtype indicates  that  the  actual  body
             data are not included, but merely referenced.  In this case,
             the  parameters  describe  a  mechanism  for  accessing  the
             external data.
 
             When  a   message   body   or   body   part   is   of   type
             "message/external-body",   it  consists  of  a  header,  two
             consecutive  CRLFs,  and  the   message   header   for   the
             encapsulated  message.  If another pair of consecutive CRLFs
             appears, this of course ends  the  message  header  for  the
             encapsulated   message.   However,  since  the  encapsulated
             message's body is itself external, it does NOT appear in the
             area  that  follows.   For  example,  consider the following
             message:
 
                  Content-type: message/external-body; access-
                  type=local-file;
                       name=/u/nsb/Me.gif
 
                  Content-type:  image/gif
 
                  THIS IS NOT REALLY THE BODY!
 
             The area at the end, which  might  be  called  the  "phantom
             body", is ignored for most external-body messages.  However,
             it may be used to contain auxilliary  information  for  some
 
 
 
             Borenstein & Freed                                 [Page 40]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             such  messages,  as  indeed  it  is  when the access-type is
             "mail-server".   Of  the  access-types   defined   by   this
             document, the phantom body is used only when the access-type
             is "mail-server".  In all other cases, the phantom  body  is
             ignored.
 
             The only always-mandatory  parameter  for  message/external-
             body  is  "access-type";  all of the other parameters may be
             mandatory or optional depending on the value of access-type.
 
                  ACCESS-TYPE -- One or more case-insensitive words,
                  comma-separated,   indicating   supported   access
                  mechanisms by  which  the  file  or  data  may  be
                  obtained.  Values include, but are not limited to,
                  "FTP", "ANON-FTP",  "TFTP",  "AFS",  "LOCAL-FILE",
                  and   "MAIL-SERVER".  Future  values,  except  for
                  experimental values beginning with "X-",  must  be
                  registered with IANA, as described in Appendix F .
 
             In addition, the following two parameters are  optional  for
             ALL access-types:
 
                  EXPIRATION -- The date (in the RFC 822 "date-time"
                  syntax, as extended by RFC 1123 to permit 4 digits
                  in the date field) after which  the  existence  of
                  the external data is not guaranteed.
 
                  SIZE -- The size (in octets)  of  the  data.   The
                  intent  of this parameter is to help the recipient
                  decide whether or  not  to  expend  the  necessary
                  resources to retrieve the external data.
 
                  PERMISSION -- A field that  indicates  whether  or
                  not it is expected that clients might also attempt
                  to  overwrite  the  data.   By  default,   or   if
                  permission  is "read", the assumption is that they
                  are not, and that if the data is  retrieved  once,
                  it  is never needed again. If PERMISSION is "read-
                  write", this assumption is invalid, and any  local
                  copy  must  be  considered  no  more than a cache.
                  "Read"  and  "Read-write"  are  the  only  defined
                  values of permission.
 
             The precise semantics of the access-types defined  here  are
             described in the sections that follow.
 
             7.3.3.1  The "ftp" and "tftp" access-types
 
             An access-type of FTP or TFTP  indicates  that  the  message
             body is accessible as a file using the FTP [RFC-959] or TFTP
             [RFC-783] protocols, respectively.  For these  access-types,
             the following additional parameters are mandatory:
 
 
 
 
 
             Borenstein & Freed                                 [Page 41]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  NAME -- The name of the  file  that  contains  the
                  actual body data.
 
                  SITE -- A machine  from  which  the  file  may  be
                  obtained, using the given protocol
 
             Before the data is retrieved,  using  these  protocols,  the
             user  will  generally need to be asked to provide a login id
             and a password for the machine named by the site parameter.
 
             In addition, the  following  optional  parameters  may  also
             appear when the access-type is FTP or ANON-FTP:
 
                  DIRECTORY -- A directory from which the data named
                  by NAME should be retrieved.
 
                  MODE  --  A  transfer  mode  for  retrieving   the
                  information, e.g. "image".
 
             7.3.3.2  The "anon-ftp" access-type
 
             The "anon-ftp" access-type is identical to the "ftp"  access
             type,  except  that  the user need not be asked to provide a
             name and password for the specified site.  Instead, the  ftp
             protocol  will be used with login "anonymous" and a password
             that corresponds to the user's email address.
 
             7.3.3.3  The "local-file" and "afs" access-types
 
             An access-type of "local-file"  indicates  that  the  actual
             body  is  accessible  as  a  file  on the local machine.  An
             access-type of "afs" indicates that the file  is  accessible
             via  the  global  AFS  file  system.   In both cases, only a
             single parameter is required:
 
                  NAME -- The name of the  file  that  contains  the
                  actual body data.
 
             The following optional parameter may be used to describe the
             locality  of  reference  for  the data, that is, the site or
             sites at which the file is expected to be visible:
 
                  SITE -- A domain specifier for a machine or set of
                  machines that are known to have access to the data
                  file.  Asterisks may be used for wildcard matching
                  to   a   part   of   a   domain   name,   such  as
                  "*.bellcore.com", to indicate a set of machines on
                  which the data should be directly visible, while a
                  single asterisk may be used  to  indicate  a  file
                  that  is  expected  to  be  universally available,
                  e.g., via a global file system.
 
             7.3.3.4  The "mail-server" access-type
 
 
 
 
             Borenstein & Freed                                 [Page 42]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             The "mail-server" access-type indicates that the actual body
             is  available  from  a mail server.  The mandatory parameter
             for this access-type is:
 
                  SERVER -- The email address  of  the  mail  server
                  from which the actual body data can be obtained.
 
             Because mail servers accept a variety  of  syntax,  some  of
             which  is  multiline,  the full command to be sent to a mail
             server is not included as a parameter  on  the  content-type
             line.   Instead,  it  may  be provided as the "phantom body"
             when  the  content-type  is  message/external-body  and  the
             access-type is mail-server.
 
             Note that  MIME  does  not  define  a  mail  server  syntax.
             Rather,  it  allows  the  inclusion of arbitrary mail server
             commands  in  the  phantom  body.   Implementations   should
             include the phantom body in the body of the message it sends
             to the mail server address to retrieve the relevant data.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 43]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             7.3.3.5  Examples and Further Explanations
 
             With  the  emerging  possibility  of  very  wide-area   file
             systems,  it becomes very hard to know in advance the set of
             machines where a  file  will  and  will  not  be  accessible
             directly  from the file system.  Therefore it may make sense
             to provide both a file name, to be tried directly,  and  the
             name of one or more sites from which the file is known to be
             accessible.  An implementation can try  to  retrieve  remote
             files  using FTP or any other protocol, using anonymous file
             retrieval or prompting the user for the necessary  name  and
             password.   If  an  external body is accessible via multiple
             mechanisms, the sender may include multiple  parts  of  type
             message/external-body    within    an    entity    of   type
             multipart/alternative.
 
             However, the external-body mechanism is not intended  to  be
             limited  to  file  retrieval,  as  shown  by the mail-server
             access-type.  Beyond this, one  can  imagine,  for  example,
             using a video server for external references to video clips.
 
             If an entity is of type  "message/external-body",  then  the
             body  of  the  entity  will contain the header fields of the
             encapsulated message.  The body itself is to be found in the
             external  location.   This  means  that  if  the body of the
             "message/external-body"  message  contains  two  consecutive
             CRLFs,  everything  after  those  pairs  is  NOT part of the
             message itself.  For  most  message/external-body  messages,
             this trailing area must simply be ignored.  However, it is a
             convenient place for additional data that cannot be included
             in  the  content-type  header field.   In particular, if the
             "access-type" value is "mail-server", then the trailing area
             must  contain  commands to be sent to the mail server at the
             address given by NAME@SITE, where  NAME  and  SITE  are  the
             values of the NAME and SITE parameters, respectively.
 
             The embedded message header fields which appear in the  body
             of the message/external-body data can be used to declare the
             Content-type  of  the  external  body.   Thus   a   complete
             message/external-body  message,  referring  to a document in
             PostScript format, might look like this:
 
                  From: Whomever
                  Subject: whatever
                  MIME-Version: 1.0
                  Message-ID: id1@host.com
                  Content-Type: multipart/alternative; boundary=42
 
 
                  --42
                  Content-Type: message/external-body;
                       name="BodyFormats.ps";
 
 
 
 
 
             Borenstein & Freed                                 [Page 44]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                       site="thumper.bellcore.com";
                       access-type=ANON-FTP;
                       directory="pub";
                       mode="image";
                       expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
 
                  Content-type: application/postscript
 
                  --42
                  Content-Type: message/external-body;
                       name="/u/nsb/writing/rfcs/RFC-XXXX.ps";
                       site="thumper.bellcore.com";
                       access-type=AFS
                       expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
 
                  Content-type: application/postscript
 
                  --42
                  Content-Type: message/external-body;
                       access-type=mail-server
                       server="listserv@bogus.bitnet";
                       expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
 
                  Content-type: application/postscript
 
                  get rfc-xxxx doc
 
                  --42--
 
             Like the  message/partial  type,  the  message/external-body
             type  is  intended to be transparent, that is, to convey the
             data type in the external  body  rather  than  to  convey  a
             message  with  a body of that type.  Thus the headers on the
             outer and inner parts must be merged using the same rules as
             for  message/partial.   In  particular,  this means that the
             Content-type header is overridden, but the From and  Subject
             headers are preserved.
 
             Note that since the external bodies are not  transported  as
             mail,  they  need  not  conform to the 7-bit and line length
             requirements, but might in fact be  binary  files.   Thus  a
             Content-Transfer-Encoding is not generally necessary, though
             it is permitted.
 
             Note that the body of a message of  type  "message/external-
             body"  is  governed  by  the  basic  syntax  for  an RFC 822
             message.   In  particular,   anything   before   the   first
             consecutive  pair  of  CRLFs  is  header  information, while
             anything after it is body information, which is ignored  for
             most access-types.
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 45]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             7.4  The Application Content-Type
 
             The "application" Content-Type is to be used for data  which
             do  not fit in any of the other categories, and particularly
             for data to be processed by mail-based uses  of  application
             programs.  This is information which must be processed by an
             application before it is  viewable  or  usable  to  a  user.
             Expected  uses  for  Content-Type  application include mail-
             based  file  transfer,  spreadsheets,  data  for  mail-based
             scheduling    systems,    and    languages    for   "active"
             (computational) email.  (The latter, in particular, can pose
             security    problems   which   should   be   understood   by
             implementors, and are considered in detail in the discussion
             of the application/PostScript content-type.)
 
             For example, a meeting scheduler  might  define  a  standard
             representation for information about proposed meeting dates.
             An intelligent user agent  would  use  this  information  to
             conduct  a dialog with the user, and might then send further
             mail based on that dialog. More generally, there  have  been
             several  "active"  messaging  languages  developed  in which
             programs in a suitably specialized language are sent through
             the   mail   and   automatically   run  in  the  recipient's
             environment.
 
             Such  applications  may  be  defined  as  subtypes  of   the
             "application"  Content-Type.   This  document  defines three
             subtypes: octet-stream, ODA, and PostScript.
 
             In general, the subtype of application  will  often  be  the
             name  of  the  application  for which the data are intended.
             This does not mean, however, that  any  application  program
             name  may  be used freely as a subtype of application.  Such
             usages  must  be  registered  with  IANA,  as  described  in
             Appendix F.
 
             7.4.1     The Application/Octet-Stream (primary) subtype
 
             The primary subtype of application, "octet-stream",  may  be
             used  to indicate that a body contains binary data.  The set
             of possible parameters includes, but is not limited to:
 
                  NAME -- a suggested name for the  binary  data  if
                  stored as a file.
 
                  TYPE -- the general type  or  category  of  binary
                  data.   This  is  intended  as information for the
                  human recipient  rather  than  for  any  automatic
                  processing.
 
                  CONVERSIONS -- the set  of  operations  that  have
                  been  performed  on  the data before putting it in
                  the mail (and before any Content-Transfer-Encoding
                  that   might   have  been  applied).  If  multiple
 
 
 
             Borenstein & Freed                                 [Page 46]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  conversions have occurred, they must be  separated
                  by  commas  and  specified  in the order they were
                  applied -- that is, the leftmost conversion   must
                  have  occurred  first,  and conversions are undone
                  from right  to  left.   Note  that  NO  conversion
                  values   are   defined   by  this  document.   Any
                  conversion values that that do not begin with "X-"
                  must  be preceded by a published specification and
                  by  registration  with  IANA,  as   described   in
                  Appendix F.
 
                  PADDING -- the number of bits of padding that were
                  appended  to  the  bitstream comprising the actual
                  contents to  produce  the  enclosed  byte-oriented
                  data.  This is useful for enclosing a bitstream in
                  a body when the total number  of  bits  is  not  a
                  multiple of the byte size.
 
             The values  for  these  attributes  are  left  undefined  at
             present,  but  may  require specification in the future.  An
             example of a common (though UNIX-specific) usage might be:
 
                  Content-Type:  application/octet-stream;
                       name=foo.tar.Z; type=tar;
                       conversions="x-encrypt,x-compress"
 
             However, it should be noted that the use of such conversions
             is  explicitly  discouraged due to a lack of portability and
             standardization.   The  use  of  uuencode  is   particularly
             discouraged,   in  favor  of  the  Content-Transfer-Encoding
             mechanism, which is both more standardized and more portable
             across mail boundaries.
 
             The recommended action for an implementation  that  receives
             application/octet-stream  mail is to simply offer to put the
             data in a file, with any  Content-Transfer-Encoding  undone,
             or perhaps to use it as input to a user-specified process.
 
             To reduce the danger of transmitting rogue programs  through
             the  mail,  it  is strongly recommended that implementations
             NOT implement a path-search mechanism whereby  an  arbitrary
             program  named  in  the  Content-Type  parameter  (e.g.,  an
             "interpreter=" parameter) is found and  executed  using  the
             mail body as input.
 
             7.4.2     The Application/PostScript subtype
 
             A  Content-Type  of  "application/postscript"  indicates   a
             PostScript    program.    The   language   is   defined   in
             [POSTSCRIPT].  It is recommended  that  Postscript  as  sent
             through  email  should  use  Postscript document structuring
             conventions if at all possible, and correctly.
 
 
 
 
 
             Borenstein & Freed                                 [Page 47]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             The execution  of  general-purpose  PostScript  interpreters
             entails   serious   security  risks,  and  implementors  are
             discouraged from simply sending PostScript email  bodies  to
             "off-the-shelf"  interpreters.   While it is usually safe to
             send PostScript to a printer, where the potential  for  harm
             is  greatly constrained, implementors should consider all of
             the  following  before  they  add  interactive  display   of
             PostScript bodies to their mail readers.
 
             The remainder of this section outlines some, though probably
             not  all,  of  the possible problems with sending PostScript
             through the mail.
 
             Dangerous operations in the PostScript language include, but
             may  not be limited to, the PostScript operators deletefile,
             renamefile,  filenameforall,  and  file.    File   is   only
             dangerous  when  applied  to  something  other than standard
             input or output. Implementations may also define  additional
             nonstandard  file operators; these may also pose a threat to
             security.     Filenameforall,  the  wildcard   file   search
             operator,  may  appear at first glance to be harmless. Note,
             however, that this operator  has  the  potential  to  reveal
             information  about  what  files the recipient has access to,
             and this  information  may  itself  be  sensitive.   Message
             senders  should  avoid the use of potentially dangerous file
             operators, since these operators  are  quite  likely  to  be
             unavailable  in secure PostScript implementations.  Message-
             receiving and -displaying software should either  completely
             disable  all  potentially  dangerous  file operators or take
             special care not to delegate any special authority to  their
             operation. These operators should be viewed as being done by
             an outside agency when  interpreting  PostScript  documents.
             Such  disabling  and/or  checking  should be done completely
             outside of the reach of the PostScript language itself; care
             should  be  taken  to  insure  that  no  method  exists  for
             reenabling full-function versions of these operators.
 
             The PostScript language provides facilities for exiting  the
             normal  interpreter,  or  server, loop. Changes made in this
             "outer"  environment   are   customarily   retained   across
             documents, and may in some cases be retained semipermanently
             in nonvolatile memory. The operators associated with exiting
             the  interpreter  loop  have the potential to interfere with
             subsequent document processing. As such, their  unrestrained
             use  constitutes  a  threat  of  service denial.  PostScript
             operators that exit the interpreter loop  include,  but  may
             not  be  limited  to, the exitserver and startjob operators.
             Message-sending software should not generate PostScript that
             depends  on  exiting  the  interpreter  loop to operate. The
             ability to exit  will  probably  be  unavailable  in  secure
             PostScript     implementations.     Message-receiving    and
             -displaying  software  should,  if  possible,  disable   the
             ability   to   make   retained  changes  to  the  PostScript
             environment. Eliminate the startjob and exitserver commands.
 
 
 
             Borenstein & Freed                                 [Page 48]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             If  these  commands  cannot  be eliminated, at least set the
             password associated with them to a hard-to-guess value.
 
             PostScript provides operators for  setting  system-wide  and
             device-specific  parameters. These parameter settings may be
             retained across jobs and may potentially pose  a  threat  to
             the  correct  operation  of the interpreter.  The PostScript
             operators that set system and device parameters include, but
             may  not be limited to, the setsystemparams and setdevparams
             operators.  Message-sending  software  should  not  generate
             PostScript  that  depends on the setting of system or device
             parameters to operate correctly. The ability  to  set  these
             parameters will probably be unavailable in secure PostScript
             implementations. Message-receiving and -displaying  software
             should,  if  possible,  disable the ability to change system
             and  device  parameters.  If  these  operators   cannot   be
             disabled,  at least set the password associated with them to
             a hard-to-guess value.
 
             Some   PostScript   implementations   provide    nonstandard
             facilities  for  the direct loading and execution of machine
             code.  Such  facilities  are  quite    obviously   open   to
             substantial  abuse.    Message-sending  software  should not
             make use of such features. Besides being  totally  hardware-
             specific,  they  are also likely to be unavailable in secure
             implementations  of  PostScript.     Message-receiving   and
             -displaying  software  should not allow such operators to be
             used if they exist.
 
             PostScript is an extensible language, and many, if not most,
             implementations   of  it  provide  a  number  of  their  own
             extensions. This document does not deal with such extensions
             explicitly   since   they   constitute  an  unknown  factor.
             Message-sending software should not make use of  nonstandard
             extensions;   they  are  likely  to  be  missing  from  some
             implementations. Message-receiving and -displaying  software
             should  make  sure that any nonstandard PostScript operators
             are secure and don't present any kind of threat.
 
             It is  possible  to  write  PostScript  that  consumes  huge
             amounts  of various system resources. It is also possible to
             write PostScript programs that loop infinitely.  Both  types
             of  programs  have  the potential to cause damage if sent to
             unsuspecting recipients.   Message-sending  software  should
             avoid  the  construction and dissemination of such programs,
             which  is  antisocial.   Message-receiving  and  -displaying
             software  should  provide  appropriate  mechanisms  to abort
             processing of a document after a reasonable amount  of  time
             has  elapsed. In addition, PostScript interpreters should be
             limited to the consumption of only a  reasonable  amount  of
             any given system resource.
 
             Finally, bugs may  exist  in  some  PostScript  interpreters
             which  could  possibly  be  exploited  to  gain unauthorized
 
 
 
             Borenstein & Freed                                 [Page 49]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             access to a  recipient's  system.  Apart  from  noting  this
             possibility,  there is no specific action to take to prevent
             this, apart from the timely correction of such bugs  if  any
             are found.
 
             7.4.3     The Application/ODA subtype
 
             The "ODA" subtype of application is used to indicate that  a
             body  contains  information  encoded according to the Office
             Document  Architecture  [ODA]   standards,  using  the  ODIF
             representation  format.   For  application/oda, the Content-
             Type line should also specify an attribute/value  pair  that
             indicates  the document application profile (DAP), using the
             key word "profile".  Thus an appropriate header field  might
             look like this:
 
             Content-Type:  application/oda; profile=Q112
 
             Consult the ODA standard [ODA] for further information.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 50]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             7.5  The Image Content-Type
 
             A Content-Type of "image" indicates that the bodycontains an
             image.   The subtype names the specific image format.  These
             names are case insensitive.  Two initial subtypes are "jpeg"
             for the JPEG format, JFIF encoding, and "gif" for GIF format
             [GIF].
 
             The list of image subtypes given here is  neither  exclusive
             nor  exhaustive,  and  is expected to grow as more types are
             registered with IANA, as described in Appendix F.
 
             7.6  The Audio Content-Type
 
             A Content-Type of "audio" indicates that the  body  contains
             audio  data.   Although  there  is not yet a consensus on an
             "ideal" audio format for use  with  computers,  there  is  a
             pressing   need   for   a   format   capable   of  providing
             interoperable behavior.
 
             The initial subtype of "basic" is  specified  to  meet  this
             requirement by providing an absolutely minimal lowest common
             denominator  audio  format.   It  is  expected  that  richer
             formats for higher quality and/or lower bandwidth audio will
             be defined by a later document.
 
             The content of the "audio/basic" subtype  is  audio  encoded
             using  8-bit ISDN u-law [PCM]. When this subtype is present,
             a sample rate of 8000 Hz and a single channel is assumed.
 
             7.7  The Video Content-Type
 
             A Content-Type of "video" indicates that the body contains a
             time-varying-picture   image,   possibly   with   color  and
             coordinated sound.   The  term  "video"  is  used  extremely
             generically,  rather  than  with reference to any particular
             technology or format, and is not meant to preclude  subtypes
             such  as animated drawings encoded compactly.    The subtype
             "mpeg" refers to video coded according to the MPEG  standard
             [MPEG].
 
             Note  that  although  in  general  this  document   strongly
             discourages  the  mixing of multiple media in a single body,
             it is recognized that many so-called "video" formats include
             a   representation  for  synchronized  audio,  and  this  is
             explicitly permitted for subtypes of "video".
 
             7.8  Experimental Content-Type Values
 
             A Content-Type value beginning with the characters "X-" is a
             private  value,  to  be  used  by consenting mail systems by
             mutual agreement.  Any format without a rigorous and  public
             definition  must  be named with an "X-" prefix, and publicly
             specified  values  shall  never  begin  with  "X-".   (Older
 
 
 
             Borenstein & Freed                                 [Page 51]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             versions  of  the  widely-used Andrew system use the "X-BE2"
             name, so new systems  should  probably  choose  a  different
             name.)
 
             In general, the use of  "X-"  top-level  types  is  strongly
             discouraged.   Implementors  should  invent  subtypes of the
             existing types whenever  possible.   The  invention  of  new
             types   is  intended  to  be  restricted  primarily  to  the
             development of new media types for email,  such  as  digital
             odors  or  holography,  and  not  for  new  data  formats in
             general. In many cases, a subtype  of  application  will  be
             more appropriate than a new top-level type.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 52]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Summary
 
             Using the MIME-Version, Content-Type, and  Content-Transfer-
             Encoding  header  fields,  it  is  possible to include, in a
             standardized way, arbitrary types of data objects  with  RFC
             822  conformant  mail  messages.  No restrictions imposed by
             either RFC 821 or RFC 822 are violated, and  care  has  been
             taken  to  avoid  problems caused by additional restrictions
             imposed  by  the  characteristics  of  some  Internet   mail
             transport  mechanisms  (see Appendix B). The "multipart" and
             "message"  Content-Types  allow  mixing   and   hierarchical
             structuring  of  objects  of  different  types  in  a single
             message.  Further  Content-Types  provide   a   standardized
             mechanism  for  tagging  messages  or  body  parts as audio,
             image, or several other  kinds  of  data.   A  distinguished
             parameter syntax allows further specification of data format
             details,  particularly  the   specification   of   alternate
             character  sets.  Additional  optional header fields provide
             mechanisms for certain extensions deemed desirable  by  many
             implementors.  Finally, a number of useful Content-Types are
             defined for general use by consenting user  agents,  notably
             text/richtext, message/partial, and message/external-body.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 53]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Acknowledgements
 
             This document is the result of the collective  effort  of  a
             large  number  of  people,  at several IETF meetings, on the
             IETF-SMTP  and  IETF-822  mailing  lists,   and   elsewhere.
             Although   any  enumeration  seems  doomed  to  suffer  from
             egregious  omissions,  the  following  are  among  the  many
             contributors to this effort:
 
             Harald Tveit Alvestrand       Timo Lehtinen
             Randall Atkinson              John R. MacMillan
             Philippe Brandon              Rick McGowan
             Kevin Carosso                 Leo Mclaughlin
             Uhhyung Choi                  Goli Montaser-Kohsari
             Cristian Constantinof         Keith Moore
             Mark Crispin                  Tom Moore
             Dave Crocker                  Erik Naggum
             Terry Crowley                 Mark Needleman
             Walt Daniels                  John Noerenberg
             Frank Dawson                  Mats Ohrman
             Hitoshi Doi                   Julian Onions
             Kevin Donnelly                Michael Patton
             Keith Edwards                 David J. Pepper
             Chris Eich                    Blake C. Ramsdell
             Johnny Eriksson               Luc Rooijakkers
             Craig Everhart                Marshall T. Rose
             Patrik Faeltstroem              Jonathan Rosenberg
             Erik E. Fair                  Jan Rynning
             Roger Fajman                  Harri Salminen
             Alain Fontaine                Michael Sanderson
             James M. Galvin               Masahiro Sekiguchi
             Philip Gladstone              Mark Sherman
             Thomas Gordon                 Keld Simonsen
             Phill Gross                   Bob Smart
             James Hamilton                Peter Speck
             Steve Hardcastle-Kille        Henry Spencer
             David Herron                  Einar Stefferud
             Bruce Howard                  Michael Stein
             Bill Janssen                  Klaus Steinberger
             Olle Jaernefors                Peter Svanberg
             Risto Kankkunen               James Thompson
             Phil Karn                     Steve Uhler
             Alan Katz                     Stuart Vance
             Tim Kehres                    Erik van der Poel
             Neil Katin                    Guido van Rossum
             Kyuho Kim                     Peter Vanderbilt
             Anders Klemets                Greg Vaudreuil
             John Klensin                  Ed Vielmetti
             Valdis Kletniek               Ryan Waldron
             Jim Knowles                   Wally Wedel
             Stev Knowles                  Sven-Ove Westberg
             Bob Kummerfeld                Brian Wideen
 
 
 
 
 
             Borenstein & Freed                                 [Page 54]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Pekka Kytolaakso              John Wobus
             Stellan Lagerstr.m            Glenn Wright
             Vincent Lau                   Rayan Zachariassen
             Donald Lindsay                David Zimmerman
             The authors apologize for  any  omissions  from  this  list,
             which are certainly unintentional.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 55]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix A -- Minimal MIME-Conformance
 
             The mechanisms described in this  document  are  open-ended.
             It  is definitely not expected that all implementations will
             support all of the Content-Types described,  nor  that  they
             will  all  share  the  same extensions.  In order to promote
             interoperability,  however,  it  is  useful  to  define  the
             concept  of  "MIME-conformance" to define a certain level of
             implementation  that  allows  the  useful  interworking   of
             messages  with  content that differs from US ASCII text.  In
             this  section,  we  specify  the   requirements   for   such
             conformance.
 
             A mail user agent that is MIME-conformant MUST:
 
                  1.  Always generate a "MIME-Version:  1.0"  header
                  field.
 
                  2.  Recognize the Content-Transfer-Encoding header
                  field,  and  decode all received data encoded with
                  either    the    quoted-printable    or     base64
                  implementations.    Encode  any  data sent that is
                  not in seven-bit mail-ready  representation  using
                  one  of  these  transformations  and  include  the
                  appropriate    Content-Transfer-Encoding    header
                  field,  unless  the underlying transport mechanism
                  supports non-seven-bit data, as SMTP does not.
 
                  3.   Recognize  and  interpret  the   Content-Type
                  header  field,  and  avoid  showing users raw data
                  with a Content-Type field  other  than  text.   Be
                  able  to  send  at least text/plain messages, with
                  the character set specified as a parameter  if  it
                  is not US-ASCII.
 
                  4.  Explicitly handle the  following  Content-Type
                  values, to at least the following extents:
 
                  Text:
                       -- Recognize  and  display  "text"  mail
                            with the character set "US-ASCII."
                       -- Recognize  other  character  sets  at
                            least  to  the extent of being able
                            to  inform  the  user  about   what
                            character set the message uses.
                       -- Recognize the "ISO-8859-*"  character
                            sets to the extent of being able to
                            display those characters  that  are
                            common  to ISO-8859-* and US-ASCII,
                            namely all  characters  represented
                            by octet values 0-127.
                       -- For unrecognized  subtypes,  show  or
                            offer  to  show  the user the "raw"
                            version of the data.  An ability at
 
 
 
             Borenstein & Freed                                 [Page 56]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                            least to convert "text/richtext" to
                            plain text, as shown in Appendix D,
                            is encouraged, but not required for
                            conformance.
                  Message:
                       --Recognize and  display  at  least  the
                            primary (822) encapsulation.
                  Multipart:
                       --   Recognize   the   primary   (mixed)
                            subtype.    Display   all  relevant
                            information on  the  message  level
                            and  the body part header level and
                            then display or  offer  to  display
                            each     of    the    body    parts
                            individually.
                       -- Recognize the "alternative"  subtype,
                            and    avoid   showing   the   user
                            redundant         parts          of
                            multipart/alternative mail.
                       -- Treat any unrecognized subtypes as if
                            they were "mixed".
                  Application:
                       -- Offer the ability to remove either of
                            the  two types of Content-Transfer-
                            Encoding defined in  this  document
                            and  put  the resulting information
                            in a user file.
 
                  5.  Upon encountering  any  unrecognized  Content-
                  Type, an implementation must treat it as if it had
                  a Content-Type of "application/octet-stream"  with
                  no  parameter  sub-arguments.  How  such  data are
                  handled is up to  an  implementation,  but  likely
                  options   for   handling  such  unrecognized  data
                  include offering the user to write it into a  file
                  (decoded   from  its  mail  transport  format)  or
                  offering the user to name a program to  which  the
                  decoded   data   should   be   passed   as  input.
                  Unrecognized predefined types, which  in  a  MIME-
                  conformant   mailer  might  still  include  audio,
                  image, or video, should also be  treated  in  this
                  way.
 
             A user agent that meets the above conditions is said  to  be
             MIME-conformant.   The  meaning of this phrase is that it is
             assumed  to  be  "safe"  to  send  virtually  any  kind   of
             properly-marked  data to users of such mail systems, because
             such systems will at least be able  to  treat  the  data  as
             undifferentiated  binary, and will not simply splash it onto
             the screen of unsuspecting users.   There is  another  sense
             in  which  it is always "safe" to send data in a format that
             is MIME-conformant, which is that such data will  not  break
             or  be  broken by any known systems that are conformant with
             RFC 821 and RFC 822.  User agents that  are  MIME-conformant
 
 
 
             Borenstein & Freed                                 [Page 57]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             have  the  additional  guarantee  that  the user will not be
             shown data that were never intended to be viewed as text.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 58]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix B -- General Guidelines For Sending Email Data
 
             Internet email is not a perfect, homogeneous  system.   Mail
             may  become  corrupted  at several stages in its travel to a
             final destination. Specifically, email sent  throughout  the
             Internet  may  travel  across  many networking technologies.
             Many networking and mail technologies  do  not  support  the
             full   functionality   possible   in   the   SMTP  transport
             environment. Mail traversing these systems is likely  to  be
             modified in such a way that it can be transported.
 
             There exist many widely-deployed non-conformant MTAs in  the
             Internet.  These  MTAs,  speaking  the  SMTP protocol, alter
             messages on the fly to take advantage of the  internal  data
             structure  of the hosts they are implemented on, or are just
             plain broken.
 
             The following guidelines may be useful to anyone devising  a
             data  format  (Content-Type)  that  will  survive the widest
             range of  networking  technologies  and  known  broken  MTAs
             unscathed.    Note  that  anything  encoded  in  the  base64
             encoding will satisfy these rules, but that some  well-known
             mechanisms,  notably  the  UNIX uuencode facility, will not.
             Note also that  anything  encoded  in  the  Quoted-Printable
             encoding will survive most gateways intact, but possibly not
             some gateways to systems that use the EBCDIC character set.
 
                  (1) Under some circumstances the encoding used for
                  data  may change as part of normal gateway or user
                  agent operation. In  particular,  conversion  from
                  base64  to  quoted-printable and vice versa may be
                  necessary. This may result  in  the  confusion  of
                  CRLF  sequences  with  line  breaks  in  text body
                  parts.  As  such,  the  persistence  of  CRLF   as
                  something  other  than  a line break should not be
                  relied on.
 
                  (2) Many systems may elect to represent and  store
                  text  data  using local newline conventions. Local
                  newline conventions may not match the RFC822  CRLF
                  convention -- systems are known that use plain CR,
                  plain LF, CRLF, or counted records.  The result is
                  that isolated CR and LF characters  are  not  well
                  tolerated  in    general;  they  may  be  lost  or
                  converted to delimiters on some systems, and hence
                  should not be relied on.
 
                  (3) TAB (HT) characters may be  misinterpreted  or
                  may be automatically converted to variable numbers
                  of  spaces.    This   is   unavoidable   in   some
                  environments, notably those not based on the ASCII
                  character  set.  Such   conversion   is   STRONGLY
                  DISCOURAGED,  but  it  may occur, and mail formats
                  should not rely on the  persistence  of  TAB  (HT)
 
 
 
             Borenstein & Freed                                 [Page 59]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  characters.
 
                  (4) Lines longer than 76 characters may be wrapped
                  or  truncated  in some environments. Line wrapping
                  and line truncation are STRONGLY DISCOURAGED,  but
                  unavoidable  in  some  cases.  Applications  which
                  require long lines  should  somehow  differentiate
                  between  soft and hard line breaks.  (A simple way
                  to  do  this  is  to  use   the   quoted-printable
                  encoding.)
 
                  (5)  Trailing "white space" characters (SPACE, TAB
                  (HT)) on a line may be discarded by some transport
                  agents, while other transport agents may pad lines
                  with  these characters so that all lines in a mail
                  file are of equal  length.    The  persistence  of
                  trailing  white  space,  therefore,  should not be
                  relied on.
 
                  (6)  Many mail domains use variations on the ASCII
                  character  set,  or  use  character  sets  such as
                  EBCDIC which contain most but not all of  the  US-
                  ASCII  characters.   The  correct  translation  of
                  characters not in the "invariant"  set  cannot  be
                  depended  on across character converting gateways.
                  For example, this  situation  is  a  problem  when
                  sending  uuencoded  information  across BITNET, an
                  EBCDIC system.  Similar problems can occur without
                  crossing  a gateway, since many Internet hosts use
                  character sets other than ASCII  internally.   The
                  definition  of  Printable  Strings  in  X.400 adds
                  further restrictions in certain special cases.  In
                  particular,  the only characters that are known to
                  be consistent  across  all  gateways  are  the  73
                  characters  that correspond to the upper and lower
                  case letters A-Z and a-z, the 10 digits  0-9,  and
                  the following eleven special characters:
 
                                 "'"  (ASCII code 39)
                                 "("  (ASCII code 40)
                                 ")"  (ASCII code 41)
                                 "+"  (ASCII code 43)
                                 ","  (ASCII code 44)
                                 "-"  (ASCII code 45)
                                 "."  (ASCII code 46)
                                 "/"  (ASCII code 47)
                                 ":"  (ASCII code 58)
                                 "="  (ASCII code 61)
                                 "?"  (ASCII code 63)
 
                  A maximally portable mail representation, such  as
                  the   base64  encoding,  will  confine  itself  to
                  relatively short lines of text in which  the  only
                  meaningful  characters  are taken from this set of
 
 
 
             Borenstein & Freed                                 [Page 60]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  73 characters.
 
             Please note that the above list is NOT a list of recommended
             practices  for  MTAs.  RFC  821  MTAs  are  prohibited  from
             altering the character  of  white  space  or  wrapping  long
             lines.   These  BAD and illegal practices are known to occur
             on established networks, and implementions should be  robust
             in dealing with the bad effects they can cause.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 61]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix C -- A Complex Multipart Example
 
             What follows is the outline of a complex multipart  message.
             This  message  has five parts to be displayed serially:  two
             introductory  plain  text  parts,  an   embedded   multipart
             message,  a  richtext  part, and a closing encapsulated text
             message  in  a  non-ASCII  character  set.    The   embedded
             multipart message has two parts to be displayed in parallel,
             a picture and an audio fragment.
 
                  MIME-Version: 1.0
                  From: Nathaniel Borenstein <nsb@bellcore.com>
                  Subject: A multipart example
                  Content-Type: multipart/mixed;
                       boundary=unique-boundary-1
 
                  This is the preamble area of a multipart message.
                  Mail readers that understand multipart format
                  should ignore this preamble.
                  If you are reading this text, you might want to
                  consider changing to a mail reader that understands
                  how to properly display multipart messages.
                  --unique-boundary-1
 
                  ...Some text appears here...
                  [Note that the preceding blank line means
                  no header fields were given and this is text,
                  with charset US ASCII.  It could have been
                  done with explicit typing as in the next part.]
 
                  --unique-boundary-1
                  Content-type: text/plain; charset=US-ASCII
 
                  This could have been part of the previous part,
                  but illustrates explicit versus implicit
                  typing of body parts.
 
                  --unique-boundary-1
                  Content-Type: multipart/parallel;
                       boundary=unique-boundary-2
 
 
                  --unique-boundary-2
                  Content-Type: audio/basic
                  Content-Transfer-Encoding: base64
 
                  ... base64-encoded 8000 Hz single-channel
                      u-law-format audio data goes here....
 
                  --unique-boundary-2
                  Content-Type: image/gif
                  Content-Transfer-Encoding: Base64
 
 
 
 
 
             Borenstein & Freed                                 [Page 62]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  ... base64-encoded image data goes here....
 
                  --unique-boundary-2--
 
                  --unique-boundary-1
                  Content-type: text/richtext
 
                  This is <bold><italic>richtext.</italic></bold>
                  <nl><nl>Isn't it
                  <bigger><bigger>cool?</bigger></bigger>
 
                  --unique-boundary-1
                  Content-Type: message/rfc822
 
                  From: (name in US-ASCII)
                  Subject: (subject in US-ASCII)
                  Content-Type: Text/plain; charset=ISO-8859-1
                  Content-Transfer-Encoding: Quoted-printable
 
                  ... Additional text in ISO-8859-1 goes here ...
 
                  --unique-boundary-1--
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 63]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix D -- A Simple Richtext-to-Text Translator in C
 
             One of the major goals in the design of the richtext subtype
             of the text Content-Type is to make formatted text so simple
             that even  text-only  mailers  will  implement  richtext-to-
             plain-text  translators, thus increasing the likelihood that
             multifont text will become "safe" to use  very  widely.   To
             demonstrate  this  simplicity,  what follows is an extremely
             simple 44-line C program that converts richtext  input  into
             plain text output:
 
                  #include <stdio.h>
                  #include <ctype.h>
                  main() {
                      int c, i;
                      char token[50];
 
                      while((c = getc(stdin)) != EOF) {
                          if (c == '<') {
                              for (i=0; (i<49 && (c = getc(stdin)) != '>'
                                        && c != EOF); ++i) {
                                  token[i] = isupper(c) ? tolower(c) : c;
                              }
                              if (c == EOF) break;
                              if (c != '>') while ((c = getc(stdin)) !=
                  '>'
                                        && c != EOF) {;}
                              if (c == EOF) break;
                              token[i] = '\0';
                              if (!strcmp(token, "lt")) {
                                  putc('<', stdout);
                              } else if (!strcmp(token, "nl")) {
                                  putc('\n', stdout);
                              } else if (!strcmp(token, "/paragraph")) {
                                  fputs("\n\n", stdout);
                              } else if (!strcmp(token, "comment")) {
                                  int commct=1;
                                  while (commct > 0) {
                                      while ((c = getc(stdin)) != '<'
                                       && c != EOF) ;
                                      if (c == EOF) break;
                                      for (i=0; (c = getc(stdin)) != '>'
                                         && c != EOF; ++i) {
                                          token[i] = isupper(c) ?
                                           tolower(c) : c;
                                      }
                                      if (c== EOF) break;
                                      token[i] = NULL;
                                      if (!strcmp(token, "/comment")) --
                  commct;
                                      if (!strcmp(token, "comment"))
                  ++commct;
 
 
 
 
 
             Borenstein & Freed                                 [Page 64]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                                  }
                              } /* Ignore all other tokens */
                          } else if (c != '\n') putc(c, stdout);
                      }
                      putc('\n', stdout); /* for good measure */
                  }
             It should be noted that one can do considerably better  than
             this  in  displaying  richtext  data on a dumb terminal.  In
             particular, one can replace font information such as  "bold"
             with textual emphasis (like *this* or   _T_H_I_S_).  One can
             also  properly  handle  the  richtext  formatting   commands
             regarding  indentation, justification, and others.  However,
             the above program is all  that  is  necessary  in  order  to
             present richtext on a dumb terminal.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 65]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix E -- Collected Grammar
 
             This appendix contains the complete BNF grammar for all  the
             syntax specified by this document.
 
             By itself, however, this grammar is incomplete.   It  refers
             to  several  entities  that  are defined by RFC 822.  Rather
             than   reproduce   those   definitions   here,   and    risk
             unintentional  differences  between  the  two, this document
             simply refers the  reader  to  RFC  822  for  the  remaining
             definitions.  Wherever a term is undefined, it refers to the
             RFC 822 definition.
 
             attribute := token
 
             body-part = <"message" as defined in RFC 822,
                      with all header fields optional, and with the
                      specified delimiter not occurring anywhere in
                      the message body, either on a line by itself
                      or as a substring anywhere.>
 
             boundary := 0*69<bchars> bcharsnospace
 
             bchars := bcharsnospace / " "
 
             bcharsnospace :=    DIGIT / ALPHA / "'" / "(" / ")" / "+"  /
             "_"
                            / "," / "-" / "." / "/" / ":" / "=" / "?"
 
             close-delimiter := delimiter "--"
 
             Content-Description := *text
 
             Content-ID := msg-id
 
             Content-Transfer-Encoding  :=      "BASE64"     /   "QUOTED-
             PRINTABLE" /
                                             "8BIT"  / "7BIT" /
                                             "BINARY"     / x-token
 
             Content-Type := type "/" subtype *[";" parameter]
 
             delimiter := CRLF "--" boundary   ; taken from  Content-Type
             field.
                                            ;   when   content-type    is
             multipart
                                          ; There should be no space
                                          ; between "--" and boundary.
 
             encapsulation := delimiter CRLF body-part
 
             epilogue :=  *text                  ;  to  be  ignored  upon
             receipt.
 
 
 
 
             Borenstein & Freed                                 [Page 66]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             MIME-Version := 1*text
 
             multipart-body := preamble  1*encapsulation  close-delimiter
             epilogue
 
             parameter := attribute "=" value
 
             preamble :=  *text                  ;  to  be  ignored  upon
             receipt.
 
             subtype := token
 
             token := 1*<any CHAR except SPACE, CTLs, or tspecials>
 
             tspecials :=  "(" / ")" / "<" / ">" / "@"  ; Must be in
                        /  "," / ";" / ":" / "\" / <">  ; quoted-string,
                        /  "/" / "[" / "]" / "?" / "."  ; to use within
                        /  "="                        ; parameter values
 
 
             type :=            "application"     /  "audio"     ;  case-
             insensitive
                       / "image"           / "message"
                       / "multipart"  / "text"
                       / "video"           / x-token
 
             value := token / quoted-string
 
             x-token := <The two characters "X-" followed, with no
                        intervening white space, by any token>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 67]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix F -- IANA Registration Procedures
 
             MIME  has  been  carefully  designed  to   have   extensible
             mechanisms,  and  it  is  expected  that the set of content-
             type/subtype pairs and their associated parameters will grow
             significantly with time.  Several other MIME fields, notably
             character  set  names,  access-type   parameters   for   the
             message/external-body  type,  conversions parameters for the
             application  type,  and  possibly   even   Content-Transfer-
             Encoding  values, are likely to have new values defined over
             time.  In order to ensure that the set  of  such  values  is
             developed  in an orderly, well-specified, and public manner,
             MIME defines a registration process which uses the  Internet
             Assigned  Numbers Authority (IANA) as a central registry for
             such values.
 
             In general, parameters in the content-type header field  are
             used  to convey supplemental information for various content
             types, and their use is defined when  the  content-type  and
             subtype  are  defined.  New parameters should not be defined
             as a way to introduce new functionality.
 
             In  order  to  simplify  and  standardize  the  registration
             process,  this appendix gives templates for the registration
             of new values with IANA.  Each of these is given in the form
             of  an  email  message  template,  to  be  filled  in by the
             registering party.
 
             F.1  Registration of New Content-type/subtype Values
 
             Note that MIME is  generally  expected  to  be  extended  by
             subtypes.   If  a  new fundamental top-level type is needed,
             its  specification  should  be  published  as  an   RFC   or
             submitted  in  a  form   suitable  to  become an RFC, and be
             subject to the Internet standards process.
 
                  To:  IANA@isi.edu
                  Subject:  Registration of new MIME content-type/subtype
 
                  MIME type name:
 
                  (If the above is not an existing top-level MIME type,
                  please explain why an existing type cannot be used.)
 
                  MIME subtype name:
 
                  Required parameters:
 
                  Optional parameters:
 
                  Encoding considerations:
 
                  Security considerations:
 
 
 
 
             Borenstein & Freed                                 [Page 68]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  Published specification:
 
                  (The published specification must be an Internet RFC or
                  RFC-to-be if a new top-level type is being defined, and
                  must be a publicly available specification in any
                  case.)
 
                  Person & email address to contact for further
                  information:
             F.2  Registration of New Character Set Values
 
                  To:  IANA@isi.edu
                  Subject:  Registration of new MIME character set value
 
                  MIME character set name:
 
                  Published specification:
 
                  (The published specification must be an Internet RFC or
                  RFC-to-be or an international standard.)
 
                  Person & email address to contact for further
                  information:
 
             F.3  Registration of New Access-type Values for
             Message/external-body
 
                  To:  IANA@isi.edu
                  Subject:  Registration of new MIME Access-type for
                       Message/external-body content-type
 
                  MIME access-type name:
 
                  Required parameters:
 
                  Optional parameters:
 
                  Published specification:
 
                  (The published specification must be an Internet RFC or
                  RFC-to-be.)
 
                  Person & email address to contact for further
                  information:
 
 
             F.4  Registration of New Conversions Values for Application
 
                  To:  IANA@isi.edu
                  Subject:  Registration of new MIME Conversions value
                  for Application content-type
 
                  MIME Conversions name:
 
 
 
 
             Borenstein & Freed                                 [Page 69]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
                  Published specification:
 
                  (The published specification must be an Internet RFC or
                  RFC-to-be.)
 
                  Person & email address to contact for further
                  information:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 70]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix G -- Summary of the Seven Content-types
 
             Content-type: text
 
             Subtypes defined by this document:  plain, richtext
 
             Important Parameters: charset
 
             Encoding notes: quoted-printable generally preferred  if  an
                  encoding  is  needed and the character set is mostly an
                  ASCII superset.
 
             Security considerations:  Rich text formats such as TeX  and
                  Troff  often contain mechanisms for executing arbitrary
                  commands or file system operations, and should  not  be
                  used  automatically unless these security problems have
                  been addressed.  Even plain text  may  contain  control
                  characters that can be used to exploit the capabilities
                  of   "intelligent"   terminals   and   cause   security
                  violations.   User  interfaces  designed to run on such
                  terminals should be aware of and try  to  prevent  such
                  problems.
             ________________________________________________________________
 
             Content-type: multipart
 
             Subtypes defined by  this  document:    mixed,  alternative,
                  digest, parallel.
 
             Important Parameters: boundary
 
             Encoding notes: No content-transfer-encoding is permitted.
 
             ________________________________________________________________
 
             Content-type: message
 
             Subtypes  defined  by  this  document:    rfc822,   partial,
                  external-body
 
             Important Parameters: id, number, total
 
             Encoding notes: No content-transfer-encoding is permitted.
 
             ________________________________________________________________
 
             Content-type: application
 
             Subtypes  defined   by   this   document:      octet-stream,
                  postscript, oda
 
             Important Parameters: profile
 
 
 
 
 
             Borenstein & Freed                                 [Page 71]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Encoding notes: base64 generally preferred for  octet-stream
                  or other unreadable subtypes.
 
             Security considerations:  This  type  is  intended  for  the
             transmission  of data to be interpreted by locally-installed
             programs.  If used,  for  example,  to  transmit  executable
             binary  programs  or programs in general-purpose interpreted
             languages, such as LISP programs or  shell  scripts,  severe
             security  problems  could  result.   In  general, authors of
             mail-reading  agents  are  cautioned  against  giving  their
             systems  the  power  to  execute mail-based application data
             without carefully  considering  the  security  implications.
             While  it  is  certainly possible to define safe application
             formats and even safe interpreters for unsafe formats,  each
             interpreter  should  be  evaluated  separately  for possible
             security problems.
             ________________________________________________________________
 
             Content-type: image
 
             Subtypes defined by this document:  jpeg, gif
 
             Important Parameters: none
 
             Encoding notes: base64 generally preferred
 
             ________________________________________________________________
 
             Content-type: audio
 
             Subtypes defined by this document:  basic
 
             Important Parameters: none
 
             Encoding notes: base64 generally preferred
 
             ________________________________________________________________
 
             Content-type: video
 
             Subtypes defined by this document:  mpeg
 
             Important Parameters: none
 
             Encoding notes: base64 generally preferred
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 72]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Appendix H -- Canonical Encoding Model
 
 
 
             There was some confusion, in earlier drafts  of  this  memo,
             regarding  the model for when email data was to be converted
             to canonical form and encoded, and in  particular  how  this
             process  would affect the treatment of CRLFs, given that the
             representation of newlines varies  greatly  from  system  to
             system.   For this reason, a canonical model for encoding is
             presented below.
 
             The process of composing a MIME message part can be modelled
             as  being  done in a number of steps.  Note that these steps
             are roughly similar to those steps used in RFC1113:
 
             Step 1.  Creation of local form.
 
             The body part to be transmitted is created in  the  system's
             native format.   The native character set is used, and where
             appropriate local end of line conventions are used as  well.
             The may be a UNIX-style text file, or a Sun raster image, or
             a VMS indexed file, or  audio  data  in  a  system-dependent
             format   stored  only  in  memory,  or  anything  else  that
             corresponds to the local model  for  the  representation  of
             some form of information.
 
             Step 2.  Conversion to canonical form.
 
             The entire body part,  including  "out-of-band"  information
             such   as   record   lengths  and  possibly  file  attribute
             information, is converted to  a  universal  canonical  form.
             The  specific  content  type of the body part as well as its
             associated attributes dictate the nature  of  the  canonical
             form  that is used.  Conversion to the proper canonical form
             may involve  character  set  conversion,  transformation  of
             audio   data,   compression,  or  various  other  operations
             specific to the various content types.
 
             For example, in the case of text/plain data, the  text  must
             be  converted to a supported character set and lines must be
             delimited with CRLF delimiters in  accordance  with  RFC822.
             Note  that the restriction on line lengths implied by RFC822
             is eliminated  if  the  next  step  employs  either  quoted-
             printable or base64 encoding.
 
             Step 3.  Apply transfer encoding.
 
             A Content-Transfer-Encoding appropriate for this  body  part
             is  applied.   Note  that  there  is  no  fixed relationship
             between the content  type  and  the  transfer  encoding.  In
             particular,  it  may  be  appropriate  to base the choice of
             base64 or quoted-printable  on  character  frequency  counts
             which are specific to a given instance of body part.
 
 
 
             Borenstein & Freed                                 [Page 73]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Step 4.  Insertion into message.
 
             The encoded object is inserted  into  a  MIME  message  with
             appropriate body part headers and boundary markers.
 
             It is vital to note that these steps are only a model;  they
             are  specifically  NOT  a blueprint for how an actual system
             would be built.  In particular, the model fails  to  account
             for two common designs:
 
                  1.  In many cases the conversion  to  a  canonical
                  form  prior  to encoding will be subsumed into the
                  encoder itself, which  understands  local  formats
                  directly.    For   example,   the   local  newline
                  convention for text  bodyparts  might  be  carried
                  through to the encoder itself along with knowledge
                  of what that format is.
 
                  2.  The output of the encoders may  have  to  pass
                  through  one  or  more  additional  steps prior to
                  being transmitted as  a  message.   As  such,  the
                  output  of  the  encoder may not be compliant with
                  the formats specified by RFC822.   In  particular,
                  once   again   it   may  be  appropriate  for  the
                  converter's output to  be  expressed  using  local
                  newline conventions rather than using the standard
                  RFC822 CRLF delimiters.
 
             Other implementation variations  are  conceivable  as  well.
             The  only  important  aspect  of this discussion is that the
             resulting messages are consistent with those produced by the
             model described here.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 74]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             References
 
             [US-ASCII] Coded Character Set--7-Bit American Standard Code
             for Information Interchange, ANSI X3.4-1986.
 
             [ATK]  Borenstein,  Nathaniel  S.,  Multimedia  Applications
             Development with the Andrew Toolkit, Prentice-Hall, 1990.
 
             [GIF] Graphics Interchange Format (Version 89a), Compuserve,
             Inc., Columbus, Ohio, 1990.
 
             [ISO-2022] International Standard--Information  Processing--
             ISO  7-bit  and  8-bit  coded character sets--Code extension
             techniques, ISO 2022:1986.
 
             [ISO-8859] Information Processing -- 8-bit Single-Byte Coded
             Graphic  Character Sets -- Part 1: Latin Alphabet No. 1, ISO
             8859-1:1987.  Part 2: Latin  alphabet  No.  2,  ISO  8859-2,
             1987.  Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.  Part
             4:  Latin  alphabet  No.  4,  ISO  8859-4,  1988.   Part  5:
             Latin/Cyrillic   alphabet,  ISO  8859-5,  1988.     Part  6:
             Latin/Arabic  alphabet,  ISO  8859-6,   1987.      Part   7:
             Latin/Greek   alphabet,   ISO   8859-7,   1987.     Part  8:
             Latin/Hebrew alphabet, ISO 8859-8, 1988.     Part  9:  Latin
             alphabet No. 5, ISO 8859-9, 1990.
 
             [ISO-646] International  Standard--Information  Processing--
             ISO  7-bit coded  character set for information interchange,
             ISO 646:1983.
 
             [MPEG]  Video  Coding  Draft  Standard  ISO  11172  CD,  ISO
             IEC/TJC1/SC2/WG11 (Motion Picture Experts Group), May, 1991.
 
             [ODA] ISO 8613;  Information  Processing:  Text  and  Office
             System;  Office  Document Architecture (ODA) and Interchange
             Format (ODIF), Part 1-8, 1989.
 
             [PCM] CCITT, Fascicle III.4 - Recommendation G.711,  Geneva,
             1972, "Pulse Code Modulation (PCM) of Voice Frequencies".
 
             [POSTSCRIPT]  Adobe  Systems,  Inc.,   PostScript   Language
             Reference Manual,  Addison-Wesley, 1985.
 
             [X400]  Schicker, Pietro, "Message Handling Systems, X.400",
             Message  Handling  Systems  and Distributed Applications, E.
             Stefferud, O-j. Jacobsen,  and  P.  Schicker,  eds.,  North-
             Holland, 1989, pp. 3-41.
 
             [RFC-783]  Sollins, K.R.  TFTP Protocol (revision 2).  June,
             1981, MIT, RFC-783.
 
             [RFC-821]  Postel,  J.B.   Simple  Mail  Transfer  Protocol.
             August, 1982, USC/Information Sciences Institute, RFC-821.
 
 
 
 
             Borenstein & Freed                                 [Page 75]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             [RFC-822]   Crocker, D.  Standard for  the  format  of  ARPA
             Internet  text  messages. August, 1982, UDEL, RFC-822.
 
             [RFC-934]   Rose, M.T.; Stefferud, E.A.   Proposed  standard
             for    message     encapsulation.  January,   1985, Delaware
             and NMA, RFC-934.
 
             [RFC-959]   Postel,  J.B.;  Reynolds,  J.K.   File  Transfer
             Protocol.      October,   1985,   USC/Information   Sciences
             Institute, RFC-959.
 
             [RFC-1049]   Sirbu,  M.A.   Content-Type  header  field  for
             Internet messages.  March, 1988, CMU,  RFC-1049.
 
             [RFC-1113]   Linn,  J.   Privacy  enhancement  for  Internet
             electronic    mail:  Part    I  -  message  encipherment and
             authentication procedures.   August,  1989, IAB Privacy Task
             Force, RFC-1113.
 
             [RFC-1154]  Robinson, D.; Ullmann, R.  Encoding header field
             for   Internet   messages.  April,   1990,   Prime Computer,
             Inc., RFC-1154.
 
             [RFC-1342] Moore, Keith, Representation of Non-Ascii Text in
             Internet   Message   Headers.   June,  1992,  University  of
             Tennessee, RFC-1342.
 
             Security Considerations
 
             Security issues  are  discussed  in  Section  7.4.2  and  in
             Appendix  G.   Implementors should pay special attention  to
             the security implications of any mail content-types that can
             cause the remote execution of any actions in the recipient's
             environment.   In  such  cases,  the   discussion   of   the
             applicaton/postscript   content-type  in  Section  7.4.2 may
             serve as a model for considering  other  content-types  with
             remote execution capabilities.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
             Borenstein & Freed                                 [Page 76]
 
 
 
 
             RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
 
 
             Authors' Addresses
 
             For more information, the authors of this  document  may  be
             contacted via Internet mail:
 
                                 Nathaniel S. Borenstein
                                  MRE 2D-296, Bellcore
                                      445 South St.
                                Morristown, NJ 07962-1910
 
                                 Phone: +1 201 829 4270
                                  Fax:  +1 201 829 7019
                                 Email: nsb@bellcore.com
 
 
                                        Ned Freed
                              Innosoft International, Inc.
                                  250 West First Street
                                        Suite 240
                                   Claremont, CA 91711
 
                                 Phone:  +1 714 624 7907
                                  Fax: +1 714 621 5319
                                 Email: ned@innosoft.com
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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             Please discard this page and place the  following  table  of
             contents after the title page.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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                                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
 
 
 
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