rfc1896.txt

<VC++网络游戏建摸与实现>源代码


RFC 1896            text/enriched MIME Content-type        February 1996


               running margin.

     Nofill
          causes the affected text to be displayed without filling.
          That is, the text is displayed without using the rules
          for replacing CRLF pairs with spaces or removing
          consecutive sequences of CRLF pairs. However, the current
          state of the margins and justification is honored; any
          indentation or justification commands are still applied
          to the text within the scope of the "nofill".

   The "center", "flushleft", "flushright", and "flushboth" commands are
   mutually exclusive, and, when nested, the inner command takes
   precedence.

   The "nofill" command is mutually exclusive with the "in" and "out"
   parameters of the "paraindent" command; when they occur in the same
   scope, their behavior is undefined.

   The parameter data for the "paraindent" command may contain multiple
   occurances of the same parameter (i.e. "left", "right", "in", or
   "out").  Each occurance causes the text to be further indented in the
   manner indicated by that parameter. Nested "paraindent" commands
   cause the affected text to be further indented according to the
   parameters. Note that the "in" and "out" parameters for "paraindent"
   are mutually exclusive; when they appear together or when nested
   "paraindent" commands contain both of them, their behavior is
   undefined.

   For purposes of the "in" and "out" parameters, a paragraph is defined
   as text that is delimited by line breaks after applying the rules for
   replacing CRLF pairs with spaces or removing consecutive sequences of
   CRLF pairs. For example, within the scope of an "out", the line
   following each CRLF is made flush with the running margin, and
   subsequent lines are indented. Within the scope of an "in", the first
   line following each CRLF is indented, and subsequent lines remain
   flush to the running margin.

   Whether or not text is justified by default (that is, whether the
   default environment is "flushleft", "flushright", or "flushboth") is
   unspecified, and depends on the preferences of the user, the
   capabilities of the local software and hardware, and the nature of
   the character set in use. On systems where full justification is
   considered undesirable, the "flushboth" environment may be identical
   to the default environment. Note that full justification should never
   be performed inside of "center", "flushleft", "flushright", or
   "nofill" environments.  Note also that for some non-ASCII character
   sets, full justification may be fundamentally inappropriate.



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RFC 1896            text/enriched MIME Content-type        February 1996


   Note that [RFC-1563] defined two additional indentation commands,
   "Indent" and "IndentRight". These commands did not force a line
   break, and therefore their behavior was unpredictable since they
   depended on the margins and character sizes that a particular
   implementation used.  Therefore, their use is deprecated and they
   should be ignored just as other unrecognized commands.

Markup Commands

   Commands in this section, unlike the other text/enriched commands are
   declarative markup commands. Text/enriched is not intended as a full
   markup language, but instead as a simple way to represent common
   formatting commands. Therefore, markup commands are purposely kept to
   a minimum. It is only because each was deemed so prevalent or
   necessary in an e-mail environment that these particular commands
   have been included at all.

     Excerpt
          causes the affected text to be interpreted as a textual
          excerpt from another source, probably a message being
          responded to. Typically this will be displayed using
          indentation and an alternate font, or by indenting lines
          and preceding them with "> ", but such decisions are up
          to the implementation. Note that as with the
          justification commands, the excerpt command implicitly
          begins and ends with a line break if one is not already
          there. Nested "excerpt" commands are acceptable and
          should be interpreted as meaning that the excerpted text
          was excerpted from yet another source. Again, this can be
          displayed using additional indentation, different colors,
          etc.

          Optionally, the "excerpt" command can take a parameter by
          using the "param" command. The format of the data is
          unspecified, but it is intended to uniquely identify the
          text from which the excerpt is taken. With this
          information, an implementation should be able to uniquely
          identify the source of any particular excerpt, especially
          if two or more excerpts in the message are from the same
          source, and display it in some way that makes this
          apparent to the user.

     Lang
          causes the affected text to be interpreted as belonging
          to a particular language. This is most useful when two
          different languages use the same character set, but may
          require a different font or formatting depending on the
          language. For instance, Chinese and Japanese share



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RFC 1896            text/enriched MIME Content-type        February 1996


          similar character glyphs, and in some character sets like
          UNICODE share common code points, but it is considered
          very important that different fonts be used for the two
          languages, especially if they appear together, so that
          meaning is not lost. Also, language information can be
          used to allow for fancier text handling, like spell
          checking or hyphenation.

          The "lang" command requires a parameter using the "param"
          command. The parameter data can be any of the language
          tags specified in [RFC-1766], "Tags for the
          Identification of Languages". These tags are the two
          letter language codes taken from [ISO-639] or can be
          other language codes that are registered according to the
          instructions in the Langauge Tags RFC. Consult that memo
          for further information.

Balancing and Nesting of Formatting Commands

   Pairs of formatting commands must be properly balanced and nested.
   Thus, a proper way to describe text in bold italics is:

     <bold><italic>the-text</italic></bold>

   or, alternately,

     <italic><bold>the-text</bold></italic>

   but, in particular, the following is illegal text/enriched:

     <bold><italic>the-text</bold></italic>

   The nesting requirement for formatting commands imposes a slightly
   higher burden upon the composers of text/enriched bodies, but
   potentially simplifies text/enriched displayers by allowing them to
   be stack-based. The main goal of text/enriched is to be simple enough
   to make multifont, formatted email widely readable, so that those
   with the capability of sending it will be able to do so with
   confidence. Thus slightly increased complexity in the composing
   software was deemed a reasonable tradeoff for simplified reading
   software. Nonetheless, implementors of text/enriched readers are
   encouraged to follow the general Internet guidelines of being
   conservative in what you send and liberal in what you accept. Those
   implementations that can do so are encouraged to deal reasonably with
   improperly nested text/enriched data.






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Unrecognized formatting commands

   Implementations must regard any unrecognized formatting command as
   "no-op" commands, that is, as commands having no effect, thus
   facilitating future extensions to "text/enriched". Private extensions
   may be defined using formatting commands that begin with "X-", by
   analogy to Internet mail header field names.

   In order to formally define extended commands, a new Internet
   document should be published.

White Space in Text/enriched Data

   No special behavior is required for the SPACE or TAB (HT) character.
   It is recommended, however, that, at least when fixed-width fonts are
   in use, the common semantics of the TAB (HT) character should be
   observed, namely that it moves to the next column position that is a
   multiple of 8. (In other words, if a TAB (HT) occurs in column n,
   where the leftmost column is column 0, then that TAB (HT) should be
   replaced by 8-(n mod 8) SPACE characters.) It should also be noted
   that some mail gateways are notorious for losing (or, less commonly,
   adding) white space at the end of lines, so reliance on SPACE or TAB
   characters at the end of a line is not recommended.

Initial State of a text/enriched interpreter

   Text/enriched is assumed to begin with filled text in a variable-
   width font in a normal typeface and a size that is average for the
   current display and user. The left and right margins are assumed to
   be maximal, that is, at the leftmost and rightmost acceptable
   positions.

Non-ASCII character sets

   One of the great benefits of MIME is the ability to use different
   varieties of non-ASCII text in messages. To use non-ASCII text in a
   message, normally a charset parameter is specified in the Content-
   type line that indicates the character set being used. For purposes
   of this RFC, any legal MIME charset parameter can be used with the
   text/enriched Content-type. However, there are two difficulties that
   arise with regard to the text/enriched Content-type when non-ASCII
   text is desired. The first problem involves difficulties that occur
   when the user wishes to create text which would normally require
   multiple non-ASCII character sets in the same text/enriched message.
   The second problem is an ambiguity that arises because of the
   text/enriched use of the "<" character in formatting commands.





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Using multiple non-ASCII character sets

   Normally, if a user wishes to produce text which contains characters
   from entirely different character sets within the same MIME message
   (for example, using Russian Cyrillic characters from ISO 8859-5 and
   Hebrew characters from ISO 8859-8), a multipart message is used.
   Every time a new character set is desired, a new MIME body part is
   started with different character sets specified in the charset
   parameter of the Content-type line. However, using multiple character
   sets this way in text/enriched messages introduces problems. Since a
   change in the charset parameter requires a new part, text/enriched
   formatting commands used in the first part would not be able to apply
   to text that occurs in subsequent parts. It is not possible for
   text/enriched formatting commands to apply across MIME body part
   boundaries.

   [RFC-1341] attempted to get around this problem in the now obsolete
   text/richtext format by introducing different character set
   formatting commands like "iso-8859-5" and "us-ascii". But this, or
   even a more general solution along the same lines, is still
   undesirable: It is common for a MIME application to decide, for
   example, what character font resources or character lookup tables it
   will require based on the information provided by the charset
   parameter of the Content-type line, before it even begins to
   interpret or display the data in that body part. By allowing the
   text/enriched interpreter to subsequently change the character set,
   perhaps to one completely different from the charset specified in the
   Content-type line (with potentially much different resource
   requirements), too much burden would be placed on the text/enriched
   interpreter itself.

   Therefore, if multiple types of non-ASCII characters are desired in a
   text/enriched document, one of the following two methods must be
   used:

   1. For cases where the different types of non-ASCII text can be
      limited to their own paragraphs with distinct formatting, a
      multipart message can be used with each part having a
      Content-Type of text/enriched and a different charset parameter.
      The one caveat to using this method is that each new part must
      start in the initial state for a text/enriched document. That
      means that all of the text/enriched commands in the preceding
      part must be properly balanced with ending commands before the
      next text/enriched part begins. Also, each text/enriched part
      must begin a new paragraph.






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   2. If different types of non-ASCII text are to appear in the same
      line or paragraph, or if text/enriched formatting (e.g. margins,
      typeface, justification) is required across several different
      types of non-ASCII text, a single text/enriched body part should
      be used with a character set specified that contains all of the
      required characters. For example, a charset parameter of
      "UNICODE-1-1-UTF-7" as specified in [RFC-1642] could be used for
      such purposes. Not only does UNICODE contain all of the
      characters that can be represented in all of the other registered
      ISO 8859 MIME character sets, but UTF-7 is fully compatible with
      other aspects of the text/enriched standard, including the use of
      the "<" character referred to below. Any other character sets
      that are specified for use in MIME which contain different types
      of non-ASCII text can also be used in these instances.

Use of the "<" character in formatting commands

   If the character set specified by the charset parameter on the
   Content-type line is anything other than "US-ASCII", this means that
   the text being described by text/enriched formatting commands is in a
   non-ASCII character set. However, the commands themselves are still
   the same ASCII commands that are defined in this document. This
   creates an ambiguity only with reference to the "<" character, the
   octet with numeric value 60. In single byte character sets, such as
   the ISO-8859 family, this is not a problem; the octet 60 can be
   quoted by including it twice, just as for ASCII. The problem is more
   complicated, however, in the case of multi-byte character sets, where
   the octet 60 might appear at any point in the byte sequence for any
   of several characters.

   In practice, however, most multi-byte character sets address this
   problem internally. For example, the UNICODE character sets can use
   the UTF-7 encoding which preserves all of the important ASCII
   characters in their single byte form. The ISO-2022 family of
   character sets can use certain character sequences to switch back
   into ASCII at any moment.  Therefore it is specified that, before
   text/enriched formatting commands, the prevailing character set
   should be "switched back" into ASCII, and that only those characters
   which would be interpreted as "<" in plain text should be interpreted
   as token delimiters in text/enriched.

   The question of what to do for hypothetical future character sets
   that do not subsume ASCII is not addressed in this memo.








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Minimal text/enriched conformance

   A minimal text/enriched implementation is one that converts "<<" to
   "<", removes everything between a <param> command and the next
   balancing </param> command, removes all other formatting commands
   (all text enclosed in angle brackets), and, outside of <nofill>
   environments, converts any series of n CRLFs to n-1 CRLFs, and
   converts any lone CRLF pairs to SPACE.

Notes for Implementors

   It is recognized that implementors of future mail systems will want
   rich text functionality far beyond that currently defined for
   text/enriched.  The intent of text/enriched is to provide a common
   format for expressing that functionality in a form in which much of
   it, at least, will be understood by interoperating software. Thus, in
   particular, software with a richer notion of formatted text than
   text/enriched can still use text/enriched as its basic
   representation, but can extend it with new formatting commands and by
   hiding information specific to that software system in text/enriched
   <param> constructs. As such systems evolve, it is expected that the
   definition of text/enriched will be further refined by future
   published specifications, but text/enriched as defined here provides
   a platform on which evolutionary refinements can be based.

   An expected common way that sophisticated mail programs will generate
   text/enriched data is as part of a multipart/alternative construct.
   For example, a mail agent that can generate enriched mail in ODA
   format can generate that mail in a more widely interoperable form by
   generating both text/enriched and ODA versions of the same data,
   e.g.:

     Content-type: multipart/alternative; boundary=foo

     --foo
     Content-type: text/enriched

     [text/enriched version of data]
     --foo Content-type: application/oda

     [ODA version of data]
     --foo--

   If such a message is read using a MIME-conformant mail reader that
   understands ODA, the ODA version will be displayed; otherwise, the
   text/enriched version will be shown.





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RFC 1896            text/enriched MIME Content-type        February 1996