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Network Working Group N. Borenstein, Bellcore
INTERNET DRAFTS April 1993
The text/enriched MIME Content-type
Status of this Memo
This document is an Internet Draft. Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.
Internet Drafts are draft documents valid for a maximum of
six months. Internet Drafts may be updated, replaced, or
obsoleted by other documents at any time. It is not
appropriate to use Internet Drafts as reference material or
to cite them other than as a "working draft" or "work in
progress."
Please check the I-D abstract listing contained in each
Internet Draft directory to learn the current status of this
or any other Internet Draft.
Abstract
MIME [RFC-1341, RFC-MIME] 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.
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"
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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 two optional parameters, "opentoken" and "closetoken",
which define the special characters that delimit a
formatting token. By default, these tokens are "<" and ">".
Thus the following two content-type lines are equivalent:
Content-type: text/enriched
Content-type: text/enriched; opentoken="<";
closetoken=">"
Most of this document is written under the assumption that
the default opentoken and closetoken values are used. It is
STRONGLY RECOMMENDED that no other opentoken or closetoken
values be used without a very good reason. The only known
good reason is discussed in the section on "Non-ASCII
Character Sets".
The syntax of "text/enriched" is very simple. It represents
text in a single character set -- US-ASCII by default,
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although a different character set can be specified by the
use of a "charset" parameter, as with the "text/plain" type.
(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 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.)
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:
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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.
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.
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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 recommened, for example, that
"<smaller>" be used to end the effect of "<bigger>". This
is properly done with "</bigger>".
Justification Commands
Initially, text/enriched text is intended to be displayed
fully-justified with appropriate fill, 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 (prefering 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
command 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.
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FlushRight -- causes the affected text to be right-
justified with a ragged left margin.
The center, flushleft, and flushright commands are mutually
exclusive, and, when nested, the inner command takes
precedence.
Note that for some non-ASCII character sets, full
justification may be inappropriate. In these cases, a user
agent may choose not to justify such data.
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 implementations.
A line break is NOT forced by a change of the margin, to
permit the description of "hanging" text. Thus for example
the following text:
Now <indent> is the time for all good horses to come to the
aid of their stable, assuming that </indent> any stable is
really stable.
would be displayed in a 40-character-wide window as follows:
Now is the time for all good horses to
come to the aid of their stable,
assuming that any stable is
really stable.
Miscellaneous Commands
Excerpt -- causes the affected text to be interpreted
as a textual excerpt from another source, probably
a message being responded to. Typically this will
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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 this is the only truly
declarative markup construct in text/enriched, and
as such doesn't fit very well with the other
facilities, but it describes a type of markup that
is very commonly used in email and has no
procedural analogue.) Note that as with the
justification commands, the excerpt command
implicitly begins and ends with a line break if
one is not already there.
Verbatim -- causes the affected text to be displayed
without filling, justification, any interpretation
of embedded formatting commands, or the usual
special rules for CRLF handling. Note, however,
that the end token </verbatim> must still be
recognized.
Comment -- causes the affected text to be interpreted
as a comment, and hence not shown to the reader.
Extension -- marks the affected text as extended
commands. If the extension set in use (as defined
below, under "Extensions to richtext") is not
recognized by the local interpreter, then
"<extension>" and "</extension>" should be
interpreted as synonyms for "<comment>" and
"</comment>".
Note that while the absence of a quoting mechanism makes it
slightly challenging to include the literal string
"</verbatim>" inside of a verbatim environment, it can be
done by breaking up the verbatim segment into two verbatim
segments as follows:
<verbatim>
...slightly challenging to include the literal string
"</</verbatim><verbatim>verbatim>" inside of a verbatim
environment...
</verbatim>
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:
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<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.
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.
A mechanism for formally defining sets of extension commands
is given later in this document.
"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
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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, fully
justified 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
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 characer 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. It is precisely for such cases that the
"opentoken" and "closetoken" content-type parameters were
defined.
When a multibyte character set is used for text/enriched
data, it may make sense to choose an alternate
representation for delimiting formatting tokens. In
particular, it may be most natural to choose a multibyte
string. If such a string is chosen, it MUST be
representable as US-ASCII. That is, each of the octets must
correspond to a normal ASCII octet that can legally appear
in a Content-type parameter. (It is conjectured that there
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will never be a character set in which it is impossible to
choose a multibyte delimiter string that cannot be viewed as
ASCII. If this conjecture is incorrect, a new version of
text/enriched will have to be defined for that character
set.)
Thus, for example, in a 16-bit character set, one might
choose
Content-type: text/enriched; opentoken="<<"; closetoken=">>"
and one could represent the literal 2-octet sequence "<<" as
"<<<<".
Minimal text/enriched conformance
A minimal text/enriched implementation is one that simply
recognizes the beginning and ending of "verbatim"
environments and, outside of them, converts "<<" to "<",
removes everything between a <comment> command and the next
balancing </comment> command, removes all other formatting
commands (all text enclosed in angle brackets), 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 comments. 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.
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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.
In some environments, it might be impossible to combine
certain text/enriched formatting commands, whereas in others
they might be combined easily. For example, the combination
of <bold> and <italic> might produce bold italics on systems
that support such fonts, but there exist systems that can
make text bold or italicized, but not both. In such cases,
the most recently issued (innermost) recognized formatting
command should be preferred.
One of the major goals in the design of text/enriched was to
make it so simple that even text-only mailers will implement
enriched-to-plain-text translators, thus increasing the
likelihood that enriched text will become "safe" to use very
widely. To demonstrate this simplicity, an extremely simple
C program that converts text/enriched input into plain text
output is included in Appendix A.
Extensions to text/enriched
It is expected that various mail system authors will desire
extensions to text/enriched. The simple syntax of
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text/enriched, and the specification that unrecognized
formatting commands should simply be ignored, are intend to
promote such extensions. To facilitate the evolution and
interoperability of such extensions, this document also
defines an "extensions" parameter by which the use of
publicly-defined text/enriched extensions can be declared as
a comma-separated list of extension names. For example, a
text/enriched object that includes extensions from the
Andrew and Slate extension sets might have a content-type
field of
Content-type: text/enriched;
extensions="Andrew,Slate"
Note, however, that the Andrew and Slate extensions are
hypothetical as of the publication of this document.
An extension will typically define a whole set of extension
commands for a particular purpose or application.
As a useful example of the mechanism, one could define an
extension called "color". If the color extension were used,
a new set of formatting commands would be defined, of the
form: "<colorname>" where colorname is a string that names a
color using some standard naming convention. Thus, mail
that included color might look like:
Subject: Blue moon, lady in red
Content-type: text/enriched, extensions="color"
I want to take my <red>lady</red> to the
<blue>moon</blue>.
Note, however, that this extension is NOT formally defined
by this document, primarily for want of a standard color
naming convention. It could easily be defined by a later
document, however.
Extension names beginning with "x-" may be used
experimentally. Standardized extensions should be
registered with IANA using the process defined in [RFC-
MIME]. Extension names are case-insensitive, so "Color",
"color", and "cOlOR" are equivalent in effect, if not in
good taste.
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Implementations should simply ignore unrecognized
extensions. Since text/enriched extensions define
additional commands, implementations should simply ignore
such commands. This raises the obvious question of why the
extension in use needs to be declared at all. The answer is
that by declaring the extension mechanism in use,
cooperating implementations can extend text/enriched in a
manner that allows them to be sure that both share the same
interpretation of an extended command.
An Example
Putting all this together, the following "text/enriched"
body fragment, presuming the eventual definition of a
"colors" extension:
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Content-type: text/enriched; extensions=color
<bold>Now</bold> is the time for
<italic>all</italic> good men
<smaller>(and <<women>)</smaller> to
<ignoreme>come</ignoreme>
to the aid of their
<red>beloved</red> country. <comment> Stupid
quote! </comment>
<verbatim>
By the way, I think that <smaller>
should
REALLY be called
<tinier>
and that <comment> and </comment> are for
weenies.
-- the end
</verbatim>
represents the following formatted text (which will, no
doubt, look somewhat cryptic in the text-only version of
this document):
Now is the time for all good men (and <women>) to
come
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to the aid of their
beloved country.
By the way, I think that <smaller>
should
REALLY be called
<tinier>
and that <comment> and </comment> are for weenies.
-- the end
where the word "beloved" would be in red on a color display.
Security Considerations
Security issues are not discussed in this memo, as the
mechanism raises no security issues.
Author's Address
For more information, the author of this document may be
contacted via Internet mail:
Nathaniel S. Borenstein
MRE 2D-296, Bellcore
445 South St.
Morristown, NJ 07962-1910
Phone: +1 201 829 4270
Fax: +1 201 829 5963
Email: nsb@bellcore.com
Acknowledgements
This document reflects the input of many contributors,
readers, and implementors of the original MIME
specification, RFC 1341. The current draft also reflects
particular contributions and comments from Terry Crowley and
Rhys Weatherley.
References
[RFC-1341] Borenstein, N., and N. Freed, "MIME
(Multipurpose Internet Mail Extensions): Mechanisms for
Specifying and Describing the Format of Internet Message
Bodies", RFC 1341, June, 1992.
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[RFC-MIME] Borenstein, N., and N. Freed, "MIME
(Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing the Format of
Internet Message Bodies", RFC ********, *****, 1993.
Appendix A -- A Simple enriched-to-plain Translator in C
One of the major goals in the design of the text/enriched
subtype of the text Content-Type is to make formatted text
so simple that even text-only mailers will implement
enriched-to-plain-text translators, thus increasing the
likelihood that multifont text will become "safe" to use
very widely. To demonstrate this simplicity, what follows
is a simple C program that converts text/enriched input into
plain text output. Note that the UNIX newline convention
(the single character represented by "\n") is assumed by
this program.
#include <stdio.h>
#include <ctype.h>
main() {
int c, i, commct=0, newlinect=0, verbatim=0;
char token[42], *p;
while ((c=getc(stdin)) != EOF) {
if (c == '<') {
if (verbatim != 0) {
for (i=0, p=token; (*p++ = getc(stdin))
!= EOF
&& !lc2strncmp(token, "/verbatim>",
i+1) && i<9; i++) {}
if (i==9) {
verbatim = 0;
} else {
*p = '\0';
putc('<', stdout);
fputs(token, stdout);
}
continue;
} else {
newlinect=0;
c = getc(stdin);
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if (c == '<') {
putc(c, stdout);
} else {
ungetc(c, stdin);
for (i=0, p=token; (c=getc(stdin))
!= EOF && c != '>'; i++) {
if (i < 41) *p++ = isupper(c) ?
tolower(c) : c;
}
*p = '\0';
if (c == EOF) break;
if (strcmp(token, "comment") == 0
|| strcmp(token, "extension")
== 0)
commct++;
else if (strcmp(token, "verbatim")
== 0)
verbatim = 1;
else if (strcmp(token, "/comment")
== 0
|| strcmp(token, "/extension")
== 0)
commct--;
}
}
} else {
if (commct > 0)
; /* ignore comments */
else if (c == '\n' && verbatim == 0)
if (++newlinect > 1) {
putc(c, stdout);
} else {
putc(' ', stdout);
}
else {
newlinect = 0;
putc(c, stdout);
}
}
}
putc('\n', stdout);
exit(0);
}
lc2strncmp(s1, s2, len)
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char *s1, *s2;
int len;
{
if (!s1 || !s2) return (-1);
while (*s1 && *s2 && len > 0) {
if (*s1 != *s2 && (tolower(*s1) != *s2)) return(-
1);
++s1; ++s2; --len;
}
if (len <= 0) return(0);
return((*s1 == *s2) ? 0 : -1);
}
It should be noted that one can do considerably better than
this in displaying text/enriched data on a dumb terminal.
In particular, one can replace font information such as
"bold" with textual emphasis (like *this* or _T_H_I_S_).
One can also properly handle the text/enriched formatting
commands regarding indentation, justification, and others.
However, the above program is all that is necessary in order
to present text/enriched on a dumb terminal without showing
the user any formatting artifacts.
Appendix B -- Differences from RFC 1341 text/richtext
Text/enriched is a clarification, simplification, and
refinement of the type defined as text/richtext in RFC 1341.
For the benefit of those who are already familiar with
text/richtext, or for those who want to exploit the
similarities to be able to display text/richtext data with
their text/enriched software, the differences between the
two are summarized here. Note, however, that text/enriched
is intended to make text/richtext obsolete, so it is not
recommended that new software generate text/richtext.
0. The name "richtext" was changed to "enriched", both to
differentiate the two versions and because "richtext"
created widespread confusion with Microsoft's Rich Text
Format (RTF).
1. Clarifications. Many things were ambiguous or
unspecified in the text/richtext definition, particularly
the initial state and the semantics of richtext with
multibyte character sets. However, such differences are
OPERATIONALLY irrelevant, since the clarifications offered
in this document are at least reasonable interpretations of
the text/richtext specification.
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2. Newline semantics have changed. In text/richtext, all
CRLFs were mapped to spaces, and line breaks were indicated
by "<nl>". This has been replaced by the "n-1" rule for
CRLFs.
3. The representation of a literal "<" character was "<lt>"
in text/richtext, but is "<<" in text/enriched.
4. The "verbatim" command did not exist in text/richtext.
5. The extensions parameter did not exist in text/richtext.
6. The following commands from text/richtext have been
REMOVED from text/enriched: <OUTDENT>, <OUTDENTRIGHT>,
<SAMEPAGE>, <SUBSCRIPT>, <SUPERSCRIPT>, <HEADING>,
<FOOTING>, <ISO-8859-[1-9]>, <US-ASCII>, <PARAGRAPH>,
<SIGNATURE>, <NO-OP>, <LT>, <NL>, and <NP>.
7. All claims of SGML compatibility have been dropped.
However, with the possible exceptions of the new semantics
for CRLF and "<<" can be implemented, text/enriched should
be no less SGML-friendly than text/richtext was.
8. In text/richtext, there were three commands (<NL>, <NP>,
and <LT>) that did not use balanced closing delimiters.
Since all of these have been eliminated, there are NO
exceptions to the nesting/balancing rules in text/enriched.
9. The limit on the size of formatting tokens has been
increased from 40 to 60 characters.
10. The opentoken and closetoken parameters were not
present in text/richtext, which always used "<" and ">", to
the distress of implementers of text/richtext in Japanese.
Borenstein - text/eExpires September 1, 1993 Page 18
