Network Working Group B. Lilly
Request for Comments: 4263 January 2006
Category: Informational
Media Subtype Registration for Media Type text/troff
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
A text media subtype for tagging content consisting of juxtaposed
text and formatting directives as used by the troff series of
programs and for conveying information about the intended processing
steps necessary to produce formatted output is described.
Table of Contents
1. Introduction................................................... 2
2. Requirement Levels............................................. 3
3. Scope of Specification......................................... 3
4. Registration Form.............................................. 3
5. Acknowledgements............................................... 8
6. Security Considerations........................................ 8
7. Internationalization Considerations............................ 8
8. IANA Considerations............................................ 9
Appendix A. Examples.............................................. 10
A.1. Data Format............................................... 10
A.2. Simple Diagram............................................ 11
Appendix B. Disclaimers........................................... 12
Normative References.............................................. 13
Informative References............................................ 13
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1. Introduction
It is sometimes desirable to format text in a particular way for
presentation. One approach is to provide formatting directives in
juxtaposition to the text to be formatted. That approach permits
reading the text in unformatted form (by ignoring the formatting
directives), and it permits relatively simple repurposing of the text
for different media by making suitable alterations to the formatting
directives or the environment in which they operate. One particular
series of related programs for formatting text in accordance with
that model is often referred to generically as "troff", although that
is also the name of a particular lineage of programs within that
generic category for formatting text specifically for typesetter and
typesetter-like devices. A related formatting program within the
generic "troff" category, usually used for character-based output
such as (formatted) plain text, is known as "nroff". For the purpose
of the media type defined here, the entire category will be referred
to simply by the generic "troff" name. Troff as a distinct set of
programs first appeared in the early 1970s [N1.CSTR54], based on the
same formatting approach used by some earlier programs ("runoff" and
"roff"). It has been used to produce documents in various formats,
ranging in length from short memoranda to books (including tables,
diagrams, and other non-textual content). It remains in wide use as
of the date of this document; this document itself was prepared using
the troff family of tools per [I1.RFC2223] and [I2.Lilly05].
The basic format (juxtaposed text and formatting directives) is
extensible and has been used for related formatting of text and
graphical document content. Formating is usually controlled by a set
of macros; a macro package is a set of related formatting tools,
written in troff format (although compressed binary representations
have also been used) and using basic formatting directives to extend
and manage formatting capabilities for document authors. There are a
number of preprocessors that transform a textual description of some
content into the juxtaposed text and formatting directives necessary
to produce some desired output. Preprocessors exist for formatting
of tables of text and non-textual material, mathematical equations,
chemical formulae, general line drawings, graphical representation of
data (in plotted coordinate graphs, bar charts, etc.),
representations of data formats, and representations of the abstract
mathematical construct known as a graph (consisting of nodes and
edges). Many such preprocessors use the same general type of input
format as the formatters, and such input is explicitly within the
scope of the media type described in this document.
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2. Requirement Levels
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", and "MAY" in this document are to be interpreted as
described in [N2.BCP14].
3. Scope of Specification
The described media type refers to input that may be processed by
preprocessors and by a page formatter. It is intended to be used
where content has some text that may be comprehensible (either as
text per se or as a readable description of non-text content) without
machine processing of the content. Where there is little or no
comprehensible text content, this media type SHOULD NOT be used. For
example, while output of the "pic" preprocessor certainly consists of
troff-compatible sequences of formatting directives, the sheer number
of individual directives interspersed with any text that might be
present makes comprehension difficult, whereas the preprocessor input
language (as described in the "Published Specification" section of
the registration below) may provide a concise and comprehensible
description of graphical content. Preprocessor output that includes
a large proportion of formatting directives would best be labeled as
a subtype of the application media type. If particular preprocessor
input content describes only graphical content with little or no
text, and which is not readily comprehensible from a textual
description of the graphical elements, a subtype of the image media
type would be appropriate. The purpose of labeling media content is
to provide information about that content to facilitate use of the
content. Use of a particular label requires some common sense and
judgment, and SHOULD NOT be mechanically applied to content in the
absence of such judgment.
4. Registration Form
The registration procedure and form are specified in [I3.RFC4288].
Type name: text
Subtype name: troff
Required parameters: none
Optional parameters:
charset: Must be a charset registered for use with MIME text types
[N3.RFC2046], except where transport protocols are explicitly
exempted from that restriction. Specifies the charset of the
media content. With traditional source content, this will be
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the default "US-ASCII" charset. Some recent versions of troff
processing software can handle Unicode input charsets; however,
there may be interoperability issues if the input uses such a
charset (see "Interoperability considerations" below).
process: Human-readable additional information for formatting,
including environment variables, preprocessor arguments and
order, formatter arguments, and postprocessors. The parameter
value may need to be quoted or encoded as provided for by
[N4.RFC2045] as amended by [N5.RFC2231] and [N6.Errata].
Generating implementations must not encode executable content
and other implementations must not attempt any execution or
other interpretation of the parameter value, as the parameter
value may be prose text. Implementations SHOULD present the
parameter (after reassembly of continuation parameters, etc.)
as information related to the media type, particularly if the
media content is not immediately available (e.g., as with
message/external-body composite media [N3.RFC2046]).
resources: Lists any additional files or programs that are
required for formatting (e.g., via .cf, .nx, .pi, .so, and/or
.sy directives).
versions: Human-readable indication of any known specific versions
of preprocessors, formatter, macro packages, postprocessors,
etc., required to process the content.
Encoding considerations:
7bit is adequate for traditional troff provided line endings are
canonicalized per [N3.RFC2046]. Transfer of this media type
content via some transport mechanisms may require or benefit
from encoding into a 7bit range via a suitable encoding method
such as the ones described in [N4.RFC2045]. In particular,
some lines in this media type might begin or end with
whitespace, and that leading and/or trailing whitespace might
be discarded or otherwise mangled if the media type is not
encoded for transport.
8bit may be used with Unicode characters represented as a series
of octets using the utf-8 charset [I4.RFC3629], where transport
methods permit 8bit content and where content line length is
suitable. Transport encoding considerations for robustness may
also apply, and if a suitable 8bit encoding mechanism is
standardized, it might be applicable for protection of media
during transport.
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binary may be necessary when raw Unicode is used or where line
lengths exceed the allowable maximum for 7bit and 8bit content
[N4.RFC2045], and may be used in environments (e.g., HTTP
[I5.RFC2616]) where Unicode characters may be transferred via a
non-MIME charset such as UTF-16 [I6.RFC2781].
framed encoding MAY be used, but is not required and is not
generally useful with this media type.
Restrictions on usage: none
Security considerations: Some troff directives (.sy and .pi) can
cause arbitrary external programs to be run. Several troff
directives (.so, .nx, and .cf) may read external files (and/or
devices on systems that support device input via file system
semantics) during processing. Several preprocessors have similar
features. Some implementations have a "safe" mode that disables
some of these features. Formatters and preprocessors are
programmable, and it is possible to provide input which specifies
an infinite loop, which could result in denial of service, even in
implementations that restrict use of directives that access
external resources. Users of this media type SHOULD be vigilant
of the potential for damage that may be caused by careless
processing of media obtained from untrusted sources.
Processing of this media type other than by facilities that strip
or ignore potentially dangerous directives, and processing by
preprocessors and/or postprocessors, SHOULD NOT be invoked
automatically (i.e., without user confirmation). In some cases,
as this is a text media type (i.e., it contains text that is
comprehensible without processing), it may be sufficient to
present the media type with no processing at all. However, like
any other text media, this media type may contain control
characters, and implementers SHOULD take precautions against
untoward consequences of sending raw control characters to display
devices.
Users of this media type SHOULD carefully scrutinize suggested
command lines associated with the "process" parameter, contained
in comments within the media, or conveyed via external mechanisms,
both for attempts at social engineering and for the effects of
ill-considered values of the parameter. While some
implementations may have "safe" modes, those using this media type
MUST NOT presume that they are available or active.
Comments may be included in troff source; comments are not
formatted for output. However, they are of course readable in the
troff document source. Authors should be careful about
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information placed in comments, as such information may result in
a leak of information, or may have other undesirable consequences.
While it is possible to overlay text with graphics or otherwise
produce formatting instructions that would visually obscure text
when formatted, such measures do not prevent extracting text from
the document source, and might be ineffective in obscuring text
when formatted electronically, e.g., as PostScript or PDF.
Interoperability considerations: Recent implementations of
formatters, macro packages, and preprocessors may include some
extended capabilities that are not present in earlier
implementations. Use of such extensions obviously limits the
ability to produce consistent formatted output at sites with
implementations that do not support those extensions. Use of any
such extensions in a particular document using this media type
SHOULD be indicated via the "versions" parameter value.
As mentioned in the Introduction, macro packages are troff
documents, and their content may be subject to copyright. That
has led to multiple independent implementations of macro packages,
which may exhibit gross or subtle differences with some content.
Some preprocessors or postprocessors might be unavailable at some
sites. Where some implementation is available, there may be
differences in implementation that affect the output produced.
For example, some versions of the "pic" preprocessor provide the
capability to fill a bounded graphical object; others lack that
capability. Of those that support that feature, there are
differences in whether a solid fill is represented by a value of
0.0 vs. 1.0. Some implementations support only gray-scale output;
others support color.
Preprocessors or postprocessors may depend on additional programs
such as awk, and implementation differences (including bugs) may
lead to different results on different systems (or even on the
same system with a different environment).
There is a wide variation in the capabilities of various
presentation media and the devices used to prepare content for
presentation. Indeed, that is one reason that there are two basic
formatter program types (nroff for output where limited formatting
control is available, and troff where a greater range of control
is possible). Clearly, a document designed to use complex or
sophisticated formatting might not be representable in simpler
media or with devices lacking certain capabilities. Often it is
possible to produce a somewhat inferior approximation; colors
might be represented as gray-scale values, accented characters
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might be produced by overstriking, italics might be represented by
underlining, etc.
Various systems store text with different line ending codings.
For the purpose of transferring this media type between systems or
between applications using MIME methods, line endings MUST use the
canonical CRLF line ending per [N3.RFC2046].
Published specification: [N1.CSTR54]
Applications which use this media type: The following applications in
each sub-category are examples. The lists are not intended to be
exhaustive.
Preprocessors: tbl [I7.CSTR49], grap [I8.CSTR114], pic
[I9.CSTR116], chem [I10.CSTR122], eqn [I11.eqn], dformat
[I12.CSTR142]
Formatters: troff, nroff, Eroff, sqtroff, groff, awf, cawf
Format converters: deroff, troffcvt, unroff, troff2html, mm2html
Macro packages: man [I13.UNIXman1], me [I14.me], mm
[I15.DWBguide], ms [I16.ms], mv [I15.DWBguide], rfc
[I2.Lilly05]
Additional information:
Magic number(s): None; however, the content format is distinctive
(see "Published specification").
File extension(s): Files do not require any specific "extension".
Many are in use as a convenience for mechanized processing of
files, some associated with specific macro packages or
preprocessors; others are ad hoc. File names are orthogonal to
the nature of the content. In particular, while a file name or
a component of a name may be useful in some types of automated
processing of files, the name or component might not be capable
of indicating subtleties such as proportion of textual (as
opposed to image or formatting directive) content. This media
type SHOULD NOT be assigned a relationship with any file
"extension" where content may be untrusted unless there is
provision for human judgment that may be used to override that
relationship for individual files. Where appropriate, a file
name MAY be suggested by a suitable mechanism such as the one
specified in [I17.RFC2183] as amended by [N5.RFC2231] and
[N6.Errata].
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Macintosh File Type Code(s): unknown
Person & email address to contact for further information:
Bruce Lilly
[email protected]
Intended usage: COMMON
Author/Change controller: IESG
Consistency: The media has provision for comments; these are
sometimes used to convey recommended processing commands, to
indicate required resources, etc. To avoid confusing recipients,
senders SHOULD ensure that information specified in optional
parameters is consistent with any related information that may be
contained within the media content.
5. Acknowledgements
The author would like to acknowledge the helpful comments provided by
members of the ietf-types mailing list.
6. Security Considerations
Security considerations are discussed in the media registration.
Additional considerations may apply when the media subtype is used in
some contexts (e.g., MIME [I18.RFC2049]).
7. Internationalization Considerations
The optional charset parameter may be used to indicate the charset of
the media type content. In some cases, that content's charset might
be carried through processing for display of text. In other cases,
combinations of octets in particular sequences are used to represent
glyphs that cannot be directly represented in the content charset.
In either of those categories, the language(s) of the text might not
be evident from the character content, and it is RECOMMENDED that a
suitable mechanism (e.g., [I19.RFC3282]) be used to convey text
language where such a mechanism is available [I20.BCP18]. Where
multiple languages are used within a single document, it may be
necessary or desirable to indicate the languages to readers directly
via explicit indication of language in the content. In still other
cases, the media type content (while readable and comprehensible in
text form) represents symbolic or graphical information such as
mathematical equations or chemical formulae, which are largely global
and language independent.
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8. IANA Considerations
IANA shall enter and maintain the registration information in the
media type registry as directed by the IESG.
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Appendix A. Examples
A.1. Data Format
The input:
Content-Type: text/troff ; process="dformat | pic -n | troff -ms"
Here's what an IP packet header looks like:
.begin dformat
style fill off
style bitwid 0.20
style recspread 0
style recht 0.33333
noname
0-3 \0Version
4-7 IHL
8-15 \0Type of Service
16-31 Total Length
noname
0-15 Identification
16-18 \0Flags
19-31 Fragment Offset
noname
0-7 Time to Live
8-15 Protocol
16-31 Header Checksum
noname
0-31 Source Address
noname
0-31 Destination Address
noname
0-23 Options
24-31 Padding
.end
produces as output:
Here's what an IP packet header looks like:
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+-------+-------+---------------+-------------------------------+
|Version| IHL |Type of Service| Total Length |
0------34------78-------------1516----+-----------------------31+
| Identification |Flags| Fragment Offset |
0---------------+-------------1516--1819----------------------31+
| Time to Live | Protocol | Header Checksum |
0--------------78-------------1516----------------------------31+
| Source Address |
0-------------------------------------------------------------31+
| Destination Address |
0-----------------------------------------------+-------------31+
| Options | Padding |
0---------------------------------------------2324------------31+
A.2. Simple Diagram
The input:
Content-Type: text/troff ; process="use pic -n then troff -ms"
The SMTP design can be pictured as:
.DS B
.PS
boxwid = 0.8
# arrow approximation that looks acceptable in troff and nroff
define myarrow X A: [ move right 0.055;\
"<" ljust;line right ($1 - 0.1);">" rjust;\
move right 0.045 ]\
X
User: box ht 0.333333 "User"
FS: box ht 0.666667 "File" "System" with .n at User.s -0, 0.333333
Client: box ht 1.333333 wid 1.1 "Client\-" "SMTP" \
with .sw at FS.se +0.5, 0
"SMTP client" rjust at Client.se -0, 0.166667
move to User.e ; myarrow(0.5)
move to FS.e ; myarrow(0.5)
move to Client.e ; SMTP: myarrow(1.8)
Server: box ht 1.333333 wid 1.1 "Server\-" "SMTP" \
with .sw at Here.x, Client.s.y
box invis ht 0.5 "SMTP" "Commands/Replies" with .s at SMTP.c
box invis ht 0.25 "and Mail" with .n at SMTP.c
"SMTP server" ljust at Server.sw -0, 0.166667
move to Server.e.x, FS.e.y ; myarrow(0.5)
FS2: box ht 0.666667 "File" "System" \
with .sw at Server.se.x +0.5, FS.s.y
.PE
.DE
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produces as output:
The SMTP design can be pictured as:
+-------+ +----------+ +----------+
| User |<-->+ | | |
+-------+ | | SMTP | |
| |Commands/Replies | |
+-------+ | Client- |<--------------->+ Server- | +-------+
| | | SMTP | and Mail | SMTP | | |
| File |<-->+ | | |<-->+ File |
|System | | | | | |System |
+-------+ +----------+ +----------+ +-------+
SMTP client SMTP server
Appendix B. Disclaimers
This document has exactly one (1) author.
In spite of the fact that the author's given name may also be the
surname of other individuals, and the fact that the author's surname
may also be a given name for some females, the author is, and has
always been, male.
The presence of "/SHE", "their", and "authors" (plural) in the
boilerplate sections of this document is irrelevant. The author of
this document is not responsible for the boilerplate text.
Comments regarding the silliness, lack of accuracy, and lack of
precision of the boilerplate text should be directed to the IESG, not
to the author.
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Normative References
[N1.CSTR54] Ossanna, Joseph F., "NROFF/TROFF User's Manual",
Computing Science Technical Report No.54, Bell
Laboratories, Murray Hill, New Jersey, 1976.
[N2.BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[N3.RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types", RFC
2046, November 1996.
[N4.RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[N5.RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
Encoded Word Extensions: Character Sets, Languages,
and Continuations", RFC 2231, November 1997.
[N6.Errata] RFC-Editor errata page,
http://www.rfc-editor.org/errata.html
Informative References
[I1.RFC2223] Postel, J. and J. Reynolds, "Instructions to RFC
Authors", RFC 2223, October 1997.
[I2.Lilly05] Lilly, B., "Writing Internet-Drafts and Requests For
Comments using troff and nroff", Work in Progress, May
2005.
[I3.RFC4288] Freed, N. and J. Klensin, "Media Type Specifications
and Registration Procedures", BCP 13, RFC 4288,
December 2005.
[I4.RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[I5.RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616,
June 1999.
[I6.RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
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[I7.CSTR49] Lesk, M. E., "TBL - A Program for Setting Tables",
Bell Laboratories Computing Science Technical Report
#49, Murray Hill, New Jersey, 1976.
[I8.CSTR114] Bentley, Jon L. and Kernighan, Brian W., "Grap - A
Language for Typesetting Graphs Tutorial and User
Manual", Computing Science Technical Report No.114,
AT&T Bell Laboratories, Murray Hill, New Jersey, 1991.
[I9.CSTR116] Kernighan, Brian W., "Pic - A Graphics Language for
Typesetting User Manual", Computing Science Technical
Report No.116, AT&T Bell Laboratories, Murray Hill,
New Jersey, 1991.
[I10.CSTR122] Bentley, Jon L., Jelinski, Lynn W., and Kernighan,
Brian W., "Chem - A Program for Typesetting Chemical
Diagrams: User Manual", Computing Science Technical
Report No.122, AT&T Bell Laboratories, Murray Hill,
New Jersey, 1992.
[I11.eqn] Kernighan, Brian W, and Cherry, Lorinda L., "A System
for Typesetting Mathematics", Communications of the
ACM 18, 182-193, 1975.
[I12.CSTR142] Bentley, Jon L. "DFORMAT - A Program for Typesetting
Data Formats", Computing Science Technical Report
No.142, AT&T Bell Laboratories, Murray Hill, New
Jersey, 1988.
[I13.UNIXman1] AT&T Bell Laboratories, "UNIX TIME-SHARING SYSTEM
(VOLUME 1) : UNIX Programmer's Manual", Holt,
Rinehart, & Winston, 1979
[I14.me] Allman, Eric P., "Writing Papers With NROFF Using
-me", USD:19, University of California, Berkeley,
Berkeley, California, 1997.
[I15.DWBguide] AT&T Bell Laboratories, "Unix System V Documenter's
Workbench User's Guide", Prentice Hall, 1989
[I16.ms] Lesk, M. E., "Typing Documents on the UNIX System:
Using the -ms Macros with Troff and Nroff", 1978, in
"UNIX TIME-SHARING SYSTEM (VOLUME 2) : UNIX
Programmer's Manual", Holt, Rinehart, & Winston, 1979
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[I17.RFC2183] Troost, R., Dorner, S., and K. Moore, "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August
1997.
[I18.RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Five: Conformance Criteria
and Examples", RFC 2049, November 1996.
[I19.RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282,
May 2002.
[I20.BCP18] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
Author's Address
Bruce Lilly
EMail:
[email protected]
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
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specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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