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RFC 1341

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MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies

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

                   MIME  (Multipurpose Internet Mail Extensions):

                      Mechanisms for Specifying and Describing
                       the Format of Internet Message Bodies

Status of this Memo

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


            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.

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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

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            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

                 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.

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            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

            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

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            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

            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.

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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

            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

            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.

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4    The Content-Type Header Field

            The purpose of the Content-Type field  is  to  describe  the
            data  contained  in the body fully enough that the receiving
            user agent can pick an appropriate  agent  or  mechanism  to
            present  the  data  to the user, or  otherwise deal with the
            data in an appropriate manner.

            HISTORICAL NOTE:  The Content-Type header  field  was  first
            defined  in RFC 1049.  RFC 1049 Content-types used a simpler
            and less powerful syntax, but one that is largely compatible
            with the mechanism given here.

            The Content-Type  header field is used to specify the nature
            of  the  data  in  the body of an entity, by giving type and
            subtype identifiers, and by providing auxiliary  information
            that may be required for certain types.   After the type and
            subtype names, the remainder of the header field is simply a
            set of parameters, specified in an attribute/value notation.
            The set of meaningful parameters differs for  the  different
            types.   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

            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

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            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

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            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

                 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.

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                 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

            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.

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            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

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,

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            "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

            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,

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            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

            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

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            "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.

            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

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            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
                 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,

                 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

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                 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

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                      Now's the time =
                      for all folk to come=
                       to the aid of their country.

                 This provides a mechanism with  which  long  lines  are
                 encoded  in  such  a  way as to be restored by the user
                 agent.  The 76  character  limit  does  not  count  the
                 trailing   CRLF,   but  counts  all  other  characters,
                 including any equal signs.

            Since the hyphen character ("-") is represented as itself in
            the  Quoted-Printable  encoding,  care  must  be taken, when
            encapsulating a quoted-printable encoded body in a multipart
            entity,  to  ensure that the encapsulation boundary does not
            appear anywhere in the encoded body.  (A good strategy is to
            choose a boundary that includes a character sequence such as
            "=_" which can never appear in a quoted-printable body.  See
            the   definition   of   multipart  messages  later  in  this

            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


            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.

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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

            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., "-").

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                            Table 1: The Base64 Alphabet

               Value Encoding  Value  Encoding   Value  Encoding   Value
                   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

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            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

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