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Network Working Group N. Borenstein, Bellcore
Internet Draft March, 1993
A User Agent Configuration Mechanism
For Multimedia Mail Format Information
Status of This Memo
This RFC specifies an informational protocol for the
Internet community, and requests discussion and suggestions
for improvements. Distribution of this memo is unlimited.
Abstract
This memo suggests a file format to be used to inform
multiple mail reading user agent programs about the
locally-installed facilities for handling mail in various
formats. The mechanism is explicitly designed to work with
mail systems based Internet mail as defined by RFC's 821,
822, 934, 1049, 1113, and the Multipurpose Internet Mail
Extensions, known as MIME. However, with some extensions it
could probably be made to work for X.400-based mail systems
as well. The format and mechanism are proposed in a manner
that is generally operating-system independent. However,
certain implementation details will inevitably reflect
operating system differences, some of which will have to be
handled in a uniform manner for each operating system. This
memo makes such situations explicit, and, in an appendix,
suggests a standard behavior under the UNIX operating
system.
Introduction
The electronic mail world is in the midst of a transition
from single-part text-only mail to multi-part, multi-media
mail. In support of this transition, various extensions to
RFC 821 and RFC 822 have been proposed and/or adopted,
notably including MIME [RFC-1341]. Various parties have
demonstrated extremely high-functionality multimedia mail,
but the problem of mail interchange between different user
agents has been severe. In general, only text messages have
been shared between user agents that were not explicitly
designed to work together. This limitation is not
compatible with a smooth transition to a multi-media mail
Borenstein DRAFT - expires 8/1/93 [Page 1]
MAILCAP Multimedia Mail Configuration March 1993
world.
One approach to this transition is to modify diverse sets of
mail reading user agents so that, when they need to display
mail of an unfamiliar (non-text) type, they consult an
external file for information on how to display that file.
That file might say, for example, that if the content-type
of a message is "foo" it can be displayed to the user via
the "displayfoo" program.
This approach means that, with a one-time modification, a
wide variety of mail reading programs can be given the
ability to display a wide variety of types of message.
Moreover, extending the set of media types supported at a
site becomes a simple matter of installing a binary and
adding a single line to a configuration file. Crucial to
this scheme, however, is that all of the user agents agree
on a common representation and source for the configuration
file. This memo proposes such a common representation.
Location of Configuration Information
Each user agent must clearly obtain the configuration
information from a common location, if the same information
is to be used to configure all user agents. However,
individual users should be able to override or augment a
site's configuration. The configuration information should
therefore be obtained from a designated set of locations.
The overall configuration will be obtained through the
virtual concatenation of several individual configuration
files known as mailcap files. The configuration information
will be obtained from the FIRST matching entry in a mailcap
file, where "matching" depends on both a matching content-
type specification, an entry containing sufficient
information for the purposes of the application doing the
searching, and the success of any test in the "test=" field,
if present.
The precise location of the mailcap files is operating-
system dependent. A standard location for UNIX is specified
in Appendix A.
Overall Format of a Mailcap File
Each mailcap file consists of a set of entries that describe
the proper handling of one media type at the local site.
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MAILCAP Multimedia Mail Configuration March 1993
For example, one line might tell how to display a message in
Group III fax format. A mailcap file consists of a sequence
of such individual entries, separated by newlines (according
to the operating system's newline conventions). Blank lines
and lines that start with the "#" character (ASCII 35) are
considered comments, and are ignored. Long entries may be
continued on multiple lines if each non-terminal line ends
with a backslash character ('\', ASCII 92), in which case
the multiple lines are to be treated as a single mailcap
entry. Note that for such "continued" lines, the backslash
must be the last character on the line to be continued.
Thus the overall format of a mailcap file is given, in the
modified BNF of RFC 822, as:
Mailcap-File = *Mailcap-Line
Mailcap-Line = Comment / Mailcap-Entry
Comment = NEWLINE / "#" *CHAR NEWLINE
NEWLINE = <newline as defined by OS convention>
Note that the above specification implies that comments must
appear on lines all to themselves, with a "#" character as
the first character on each comment line.
Format of a Mailcap Entry
Each mailcap entry consists of a number of fields, separated
by semi-colons. The first two fields are required, and must
occur in the specified order. The remaining fields are
optional, and may appear in any order.
The first field is the content-type, which indicates the
type of data this mailcap entry describes how to handle. It
is to be matched against the type/subtype specification in
the "Content-Type" header field of an Internet mail message.
If the subtype is specified as "*", it is intended to match
all subtypes of the named content-type.
The second field, view-command, is a specification of how
the message or body part can be viewed at the local site.
Although the syntax of this field is fully specified, the
semantics of program execution are necessarily somewhat
operating system dependent. UNIX semantics are given in
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MAILCAP Multimedia Mail Configuration March 1993
Appendix A.
The optional fields, which may be given in any order, are as
follows:
-- The "compose" field may be used to specify a program tha