RFC1523 - The text/enriched MIME Content-type

Network Working Group N. Borenstein

Request for Comments: 1523 Bellcore

Category: Informational September 1993

The text/enriched MIME Content-type

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard. 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 RFC1341. 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" 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

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.

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>".

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 (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 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.

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 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.

Nofill -- causes the affected text to be displayed without

filling or justification, and hence without any special

handling of CRLFs, but with all remaining text/enriched

features continuing to apply.

Param -- Marks the affected text as command parameters, to be

interpreted or ignored by the text/enriched interpreter,

but NOT to be shown to the reader.

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:

...slightly challenging to include the literal string

"</</verbatim><verbatim>verbatim>" inside of a verbatim

environment...

</verbatim>

Note that the above example demonstrates that it is not desirable for

an implementation to break lines between tokens. In particular,

there should not be a line break inserted between the "</verbatim>"

and the "<verbatim>" that follows it.

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:

or, alternately,

but, in particular, the following is illegal

text/enriched:

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.

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, 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 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 multibyte character sets address this

problem internally. For example, the ISO-2022 family of character

sets can 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.

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 <param>

command and the next balancing </param> 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

<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.

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 text/enriched, and

the specification that unrecognized formatting commands should simply

be ignored, are intend to promote such extensions.

Beyond simply defining new formatting commands, however, it may

sometimes be necessary to define formatting commands that can take

arguments. This is the intended use of the <param> construct. In

particular, software that wished to extend text/enriched to include

colored text might define an "x-color" environment which always began

with a color name parameter, to indicate the desired color for the

affected text.

An Example

Putting all this together, the following "text/enriched" body

fragment:

From: Nathaniel Borenstein <nsb@bellcore.com>

To: Ned Freed <ned@innosoft.com>

Content-type: text/enriched

<bold>Now</bold> is the time for

<italic>all</italic> good men

<smaller>(and <<women>)</smaller> to

to the aid of their

<x-color><param>red</param>beloved</x-color>country.

By the way, I think that <smaller>

should

REALLY be called

and that I am always right.

-- 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

to the aid of their

beloved country.

By the way, I think that <smaller>

should

REALLY be called

and that I am always right.

-- the end

where the word "beloved" would be in red on a color display if the

receiving software implemented the "x-color" extension.

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, RFC1341. This memo

also reflects particular contributions and comments from Terry

Crowley and Rhys Weatherley.

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 local newline convention (the single character

represented by "\n") is assumed by this program, but that special

CRLF handling might be necessary on some systems.

#include <stdio.h>

#include <ctype.h>

main() {

int c, i, paramct=0, newlinect=0, verbatim=0,

nofill=0;

char token[62], *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);

if (c == '<') {

if (paramct <= 0) putc(c, stdout);

} else {

ungetc(c, stdin);

for (i=0, p=token; (c=getc(stdin))

!= EOF && c != '>'; i++) {

if (i < sizeof(token)-1) *p++ =

isupper(c) ? tolower(c) : c;

}

*p = '\0';

if (c == EOF) break;

if (strcmp(token, "param") == 0)

paramct++;

else if (strcmp(token, "verbatim")

== 0)

verbatim = 1;

else if (strcmp(token, "nofill") ==

nofill++;

else if (strcmp(token, "/param") ==

paramct--;

else if (strcmp(token, "/nofill")

== 0)

nofill--;

}

} else {

if (paramct > 0)

; /* ignore params */

else if (c == '\n' && verbatim == 0 &&

nofill <= 0)

if (++newlinect > 1) {

putc(c, stdout);

} else {

putc(' ', stdout);

}

else {

newlinect = 0;

putc(c, stdout);

}

/* The following line is only needed with line-

buffering */

putc('\n', stdout);

exit(0);

}

lc2strncmp(s1, s2, len)

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 RFC1341 text/richtext

Text/enriched is a clarification, simplification, and refinement of

the type defined as text/richtext in RFC1341. 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.

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" and "nofill" commands did not exist in

text/richtext.

5. The "param" command did not exist in text/richtext.

6. The following commands from text/richtext have been REMOVED from

text/enriched: <COMMENT>, <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.

References

[RFC-1341] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet

Mail Extensions): Mechanisms for Specifying and Describing the Format

of Internet Message Bodies", RFC1341, Bellcore, Innosoft, June 1992.

[RFC-1521] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet

Mail Extensions) Part One: Mechanisms for Specifying and Describing

the Format of Internet Message Bodies", RFC1521, September 1993.