rfc2045.txt

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   For example, the "charset" parameter is applicable to any subtype of
   "text", while the "boundary" parameter is required for any subtype of
   the "multipart" media type.

   There are NO globally-meaningful parameters that apply to all media
   types.  Truly global mechanisms are best addressed, in the MIME
   model, by the definition of additional Content-* header fields.

   An initial set of seven top-level media types is defined in RFC 2046.
   Five of these are discrete types whose content is essentially opaque
   as far as MIME processing is concerned.  The remaining two are
   composite types whose contents require additional handling by MIME
   processors.

   This set of top-level media types is intended to be substantially
   complete.  It is expected that additions to the larger set of
   supported types can generally be accomplished by the creation of new
   subtypes of these initial types.  In the future, more top-level types
   may be defined only by a standards-track extension to this standard.
   If another top-level 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.





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5.1.  Syntax of the Content-Type Header Field

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

     content := "Content-Type" ":" type "/" subtype
                *(";" parameter)
                ; Matching of media type and subtype
                ; is ALWAYS case-insensitive.

     type := discrete-type / composite-type

     discrete-type := "text" / "image" / "audio" / "video" /
                      "application" / extension-token

     composite-type := "message" / "multipart" / extension-token

     extension-token := ietf-token / x-token

     ietf-token := <An extension token defined by a
                    standards-track RFC and registered
                    with IANA.>

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

     subtype := extension-token / iana-token

     iana-token := <A publicly-defined extension token. Tokens
                    of this form must be registered with IANA
                    as specified in RFC 2048.>

     parameter := attribute "=" value

     attribute := token
                  ; Matching of attributes
                  ; is ALWAYS case-insensitive.

     value := token / quoted-string

     token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
                 or tspecials>

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



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

   Note also that a subtype specification is MANDATORY -- it may not be
   omitted from a Content-Type header field.  As such, there are no
   default subtypes.

   The type, subtype, and parameter names are not case sensitive.  For
   example, TEXT, Text, and TeXt are all equivalent top-level media
   types.  Parameter values are normally case sensitive, but sometimes
   are interpreted in a case-insensitive fashion, depending on the
   intended use.  (For example, multipart boundaries are case-sensitive,
   but the "access-type" parameter for message/External-body is not
   case-sensitive.)

   Note that the value of a quoted string parameter does not include the
   quotes.  That is, the quotation marks in a quoted-string are not a
   part of the value of the parameter, but are merely used to delimit
   that parameter value.  In addition, comments are allowed in
   accordance with RFC 822 rules for structured header fields.  Thus the
   following two forms

     Content-type: text/plain; charset=us-ascii (Plain text)

     Content-type: text/plain; charset="us-ascii"

   are completely equivalent.

   Beyond this syntax, the only syntactic constraint on the definition
   of subtype names is the desire that their uses must not conflict.
   That is, it would be undesirable to have two different communities
   using "Content-Type: application/foobar" to mean two different
   things.  The process of defining new media subtypes, then, is not
   intended to be a mechanism for imposing restrictions, but simply a
   mechanism for publicizing their definition and usage.  There are,
   therefore, two acceptable mechanisms for defining new media subtypes:

    (1)   Private values (starting with "X-") may be defined
          bilaterally between two cooperating agents without
          outside registration or standardization. Such values
          cannot be registered or standardized.

    (2)   New standard values should be registered with IANA as
          described in RFC 2048.

   The second document in this set, RFC 2046, defines the initial set of
   media types for MIME.



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5.2.  Content-Type Defaults

   Default RFC 822 messages without a MIME Content-Type header are taken
   by this protocol to be plain text in the US-ASCII character set,
   which can be explicitly specified as:

     Content-type: text/plain; charset=us-ascii

   This default is assumed if no Content-Type header field is specified.
   It is also recommend that this default be assumed when a
   syntactically invalid Content-Type header field is encountered. In
   the presence of a MIME-Version header field and the absence of any
   Content-Type header field, a receiving User Agent can also assume
   that plain US-ASCII text was the sender's intent.  Plain US-ASCII
   text may still be assumed in the absence of a MIME-Version or the
   presence of an syntactically invalid Content-Type header field, but
   the sender's intent might have been otherwise.

6.  Content-Transfer-Encoding Header Field

   Many media types which could be usefully transported via email are
   represented, in their "natural" format, as 8bit character or binary
   data.  Such data cannot be transmitted over some transfer protocols.
   For example, RFC 821 (SMTP) restricts mail messages to 7bit US-ASCII
   data with lines no longer than 1000 characters including any trailing
   CRLF line separator.

   It is necessary, therefore, to define a standard mechanism for
   encoding such data into a 7bit short line format.  Proper labelling
   of unencoded material in less restrictive formats for direct use over
   less restrictive transports is also desireable.  This document
   specifies that such encodings will be indicated by a new "Content-
   Transfer-Encoding" header field.  This field has not been defined by
   any previous standard.

6.1.  Content-Transfer-Encoding Syntax

   The Content-Transfer-Encoding field's value is a single token
   specifying the type of encoding, as enumerated below.  Formally:

     encoding := "Content-Transfer-Encoding" ":" mechanism

     mechanism := "7bit" / "8bit" / "binary" /
                  "quoted-printable" / "base64" /
                  ietf-token / x-token

   These values are not case sensitive -- Base64 and BASE64 and bAsE64
   are all equivalent.  An encoding type of 7BIT requires that the body



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   is already in a 7bit mail-ready representation.  This is the default
   value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the
   Content-Transfer-Encoding header field is not present.

6.2.  Content-Transfer-Encodings Semantics

   This single Content-Transfer-Encoding token actually provides two
   pieces of information.  It specifies what sort of encoding
   transformation the body was subjected to and hence what decoding
   operation must be used to restore it to its original form, and it
   specifies what the domain of the result is.

   The transformation part of any Content-Transfer-Encodings specifies,
   either explicitly or implicitly, a single, well-defined decoding
   algorithm, which for any sequence of encoded octets either transforms
   it to the original sequence of octets which was encoded, or shows
   that it is illegal as an encoded sequence.  Content-Transfer-
   Encodings transformations never depend on any additional external
   profile information for proper operation. Note that while decoders
   must produce a single, well-defined output for a valid encoding no
   such restrictions exist for encoders: Encoding a given sequence of
   octets to different, equivalent encoded sequences is perfectly legal.

   Three transformations are currently defined: identity, the "quoted-
   printable" encoding, and the "base64" encoding.  The domains are
   "binary", "8bit" and "7bit".

   The Content-Transfer-Encoding values "7bit", "8bit", and "binary" all
   mean that the identity (i.e. NO) encoding transformation has been
   performed.  As such, they serve simply as indicators of the domain of
   the body data, and provide useful information about the sort of
   encoding that might be needed for transmission in a given transport
   system.  The terms "7bit data", "8bit data", and "binary data" are
   all defined in Section 2.

   The quoted-printable and base64 encodings transform their input from
   an arbitrary domain into material in the "7bit" range, thus making it
   safe to carry over restricted transports.  The specific definition of
   the transformations are given below.

   The proper Content-Transfer-Encoding label must always be used.
   Labelling unencoded data containing 8bit characters as "7bit" is not
   allowed, nor is labelling unencoded non-line-oriented data as
   anything other than "binary" allowed.

   Unlike media subtypes, a proliferation of Content-Transfer-Encoding
   values is both undesirable and unnecessary.  However, establishing
   only a single transformation into the "7bit" domain does not seem



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   possible.  There is a tradeoff between the desire for a compact and
   efficient encoding of largely- binary data and the desire for a
   somewhat readable encoding of data that is mostly, but not entirely,
   7bit.  For this reason, at least two encoding mechanisms are
   necessary: a more or less readable encoding (quoted-printable) and a
   "dense" or "uniform" encoding (base64).

   Mail transport for unencoded 8bit data is defined in RFC 1652.  As of
   the initial publication of this document, there are no standardized
   Internet mail transports for which it is legitimate to include
   unencoded binary data in mail bodies.  Thus there are no
   circumstances in which the "binary" Content-Transfer-Encoding is
   actually valid in Internet mail.  However, in the event that binary
   mail transport becomes a reality in Internet mail, or when MIME is
   used in conjunction with any other binary-capable mail transport
   mechanism, binary bodies must be labelled as such using this
   mechanism.

   NOTE: The five values defined for the Content-Transfer-Encoding field
   imply nothing about the media type other than the algorithm by which
   it was encoded or the transport system requirements if unencoded.

6.3.  New Content-Transfer-Encodings

   Implementors may, if necessary, define private Content-Transfer-
   Encoding values, but must use an x-token, which is a name prefixed by
   "X-", to indicate its non-standard status, e.g., "Content-Transfer-
   Encoding: x-my-new-encoding".  Additional standardized Content-
   Transfer-Encoding values must be specified by a standards-track RFC.
   The requirements such specifications must meet are given in RFC 2048.
   As such, all content-transfer-encoding namespace except that
   beginning with "X-" is explicitly reserved to the IETF for future
   use.

   Unlike media types and subtypes, the creation of new Content-
   Transfer-Encoding values is STRONGLY discouraged, as it seems likely
   to hinder interoperability with little potential benefit

6.4.  Interpretation and Use