rfc1563.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                      N. Borenstein
Request for Comments: 1563                                      Bellcore
Obsoletes: 1523                                             January 1994
Category: Informational


                  The text/enriched MIME Content-type

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   MIME [RFC-1341, RFC-1521] defines a format and general framework for
   the representation of a wide variety of data types in Internet mail.
   This document defines one particular type of MIME data, the
   text/enriched type, a refinement of the "text/richtext" type defined
   in RFC 1341.  The text/enriched MIME type is intended to facilitate
   the wider interoperation of simple enriched text across a wide
   variety of hardware and software platforms.

Table of Contents

   The Text/enriched MIME type..............................  2
   Formatting Commands......................................  4
         Font-Alteration Commands...........................  4
         Fill/Justification Commands........................  5
         Indentation Commands...............................  6
         Miscellaneous Commands.............................  6
         Balancing and Nesting of Formatting Commands.......  7
         Unrecognized formatting commands...................  8
   White Space in Text/enriched Data........................  8
   Initial State of a text/enriched interpreter.............  8
   Non-ASCII character sets.................................  8
   Minimal text/enriched conformance........................  9
   Notes for Implementors...................................  9
   Extensions to text/enriched.............................. 10
   An Example............................................... 11
   Security Considerations.................................. 12
   Author's Address......................................... 12
   Acknowledgements......................................... 12
   References............................................... 12
   Appendix A -- A Simple enriched-to-plain Translator in C. 13
   Appendix B -- Differences from RFC 1341 text/richtext.... 15




Borenstein                                                      [Page 1]

RFC 1563             A text/enriched type for MIME          January 1994


The Text/enriched MIME type

   In order to promote the wider interoperability of simple formatted
   text, this document defines an extremely simple subtype of the MIME
   content-type "text", the "text/enriched" subtype.  This subtype was
   designed to meet the following criteria:

      1.  The syntax must be extremely simple to parse,
          so that even teletype-oriented mail systems can
          easily strip away the formatting information and
          leave only the readable text.

      2.  The syntax must be extensible to allow for new
          formatting commands that are deemed essential for
          some application.

      3.  If the character set in use is ASCII or an 8-
          bit ASCII superset, then the raw form of the data
          must be readable enough to be largely
          unobjectionable in the event that it is displayed
          on the screen of the user of a non-MIME-conformant
          mail reader.

      4.  The capabilities must be extremely limited, to
          ensure that it can represent no more than is
          likely to be representable by the user's primary
          word processor.  While this limits what can be
          sent, it increases the likelihood that what is
          sent can be properly displayed.

   This document defines a new MIME content-type, "text/enriched".  The
   content-type line for this type may have one optional parameter, the
   "charset" parameter, with the same values permitted for the
   "text/plain" MIME content-type.

   The syntax of "text/enriched" is very simple.  It represents text in
   a single character set -- US-ASCII by default, although a different
   character set can be specified by the use of the "charset" parameter.
   (The semantics of text/enriched in non-ASCII character sets are
   discussed later in this document.)  All characters represent
   themselves, with the exception of the "<" character (ASCII 60), which
   is used to mark the beginning of a formatting command.  Formatting
   instructions consist of formatting commands surrounded by angle
   brackets ("<>", ASCII 60 and 62).  Each formatting command may be no
   more than 60 characters in length, all in US-ASCII, restricted to the
   alphanumeric and hyphen ("-") characters.  Formatting commands may be
   preceded by a solidus ("/", ASCII 47), making them negations, and
   such negations must always exist to balance the initial opening



Borenstein                                                      [Page 2]

RFC 1563             A text/enriched type for MIME          January 1994


   commands.  Thus, if the formatting command "<bold>" appears at some
   point, there must later be a "</bold>" to balance it.  (NOTE: The 60
   character limit on formatting commands does NOT include the "<", ">",
   or "/" characters that might be attached to such commands.)

   Formatting commands are always case-insensitive.  That is, "bold" and
   "BoLd" are equivalent in effect, if not in good taste.

   Beyond tokens delimited by "<" and ">", there are two other special
   processing rules.  First, a literal less-than sign ("<") can be
   represented by a sequence of two such characters, "<<".  Second, line
   breaks (CRLF pairs in standard network representation) are handled
   specially.  In particular, isolated CRLF pairs are translated into a
   single SPACE character.  Sequences of N consecutive CRLF pairs,
   however, are translated into N-1 actual line breaks.  This permits
   long lines of data to be represented in a natural- looking manner
   despite the frequency of line-wrapping in Internet mailers.  When
   preparing the data for mail transport, isolated line breaks should be
   inserted wherever necessary to keep each line shorter than 80
   characters.  When preparing such data for presentation to the user,
   isolated line breaks should be replaced by a single SPACE character,
   and N consecutive CRLF pairs should be presented to the user as N-1
   line breaks.

   Thus text/enriched data that looks like this:

                 This is
                 a single
                 line

                 This is the
                 next line.


                 This is the
                 next paragraph.

   should be displayed by a text/enriched interpreter as follows:


                 This is a single line
                 This is the next line.

                 This is the next paragraph.

   The formatting commands, not all of which will be implemented by all
   implementations, are described in the following sections.




Borenstein                                                      [Page 3]

RFC 1563             A text/enriched type for MIME          January 1994


Formatting Commands

   The text/enriched formatting commands all begin with <commandname>
   and end with </commandname>, affecting the formatting of the text
   between those two tokens.  The commands are described here, grouped
   according to type.

Font-Alteration Commands

   The following formatting commands are intended to alter the font in
   which text is displayed, but not to alter the indentation or
   justification state of the text:

         Bold -- causes the affected text to be in a bold font.  Nested
              bold commands have the same effect as a single bold
              command.

         Italic -- causes the affected text to be in an italic font.
              Nested italic commands have the same effect as a single
              italic command.

         Fixed -- causes the affected text to be in a fixed width font.
              Nested fixed commands have the same effect as a single
              fixed command.

         Smaller -- causes the affected text to be in a smaller font.
              It is recommended that the font size be changed by two
              points, but other amounts may be more appropriate in some
              environments.  Nested smaller commands produce ever-
              smaller fonts, to the limits of the implementation's
              capacity to reasonably display them, after which further
              smaller commands have no incremental effect.

         Bigger -- causes the affected text to be in a bigger font.  It
              is recommended that the font size be changed by two
              points, but other amounts may be more appropriate in some
              environments.  Nested bigger commands produce ever-bigger
              fonts, to the limits of the implementation's capacity to
              reasonably display them, after which further bigger
              commands have no incremental effect.

         Underline -- causes the affected text to be underlined.  Nested
              underline commands have the same effect as a single
              underline command.

   While the "bigger" and "smaller" operators are effectively inverses,
   it is not recommended, for example, that "<smaller>" be used to end
   the effect of "<bigger>".  This is properly done with "</bigger>".



Borenstein                                                      [Page 4]

RFC 1563             A text/enriched type for MIME          January 1994


Fill/Justification Commands

   Initially, text/enriched text is intended to be displayed fully
   filled with appropriate kerning and letter-tracking as suits the
   capabilities of the receiving user agent software.  Actual line width
   is left to the discretion of the receiver, which is expected to fold
   lines intelligently (preferring soft line breaks) to the best of its
   ability.

   The following commands alter that state.  Each of these commands
   force a line break before and after the formatting environment if
   there is not otherwise a line break.  For example, if one of these
   commands occurs anywhere other than the beginning of a line of text
   as presented, a new line is begun.

      Center -- causes the affected text to be centered.

      FlushLeft -- causes the affected text to be left-justified with a
           ragged right margin.

      FlushRight -- causes the affected text to be right-justified with a
           ragged left margin.

      FlushBoth -- causes the affected text to be filled and padded so
           as to create smooth left and right margins, i.e., to be
           fully justified.

      Nofill -- causes the affected text to be displayed without filling
           or justification.

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

   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 justification is
   considered undesirable, the flushboth environment may be identical to
   the default environment.  Note that 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.







Borenstein                                                      [Page 5]

RFC 1563             A text/enriched type for MIME          January 1994


Indentation Commands

   Initially, text/enriched text is displayed using the maximum
   available margins.  Two formatting commands may be used to affect the
   margins.

         Indent -- causes the running left margin to be moved to the
              right.  The recommended indentation change is the width of
              four characters, but this may differ among
              implementations.

         IndentRight -- causes the running right margin to be moved to
              the left.  The recommended indentation change is the width
              of four characters, but this may differ among