rfc1341.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

                      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.



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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
                      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.



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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
            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,



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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
            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,



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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