rfc1521.txt

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      this document is used in conjunction with any other binary-capable
      transport mechanism, 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, 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



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RFC 1521                          MIME                    September 1993


   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 octet
   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 must 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 "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.



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




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RFC 1521                          MIME                    September 1993


      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, and the relationship between content-transfer-
      encodings and character sets.  For this reason, a canonical model
      for encoding is presented as Appendix G.

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 (standard) 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 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



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RFC 1521                          MIME                    September 1993


      "=" 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, 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.
      Since the canonical representation of types other than text do not
      generally include the representation of line breaks, no hard line
      breaks (i.e.  line breaks that are intended to be meaningful and
      to be displayed to the user) should occur in the quoted-printable
      encoding of such types.  Of course, occurrences of "=0D", "=0A",
      "0A=0D" and "=0D=0A" will eventually be encountered.  In general,
      however, base64 is preferred over quoted-printable for binary
      data.

      Note that many implementations may elect to encode the local
      representation of various content types directly, as described in
      Appendix G.  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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RFC 1521                          MIME                    September 1993


          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