A User Agent Configuration Mechanism for Multimedia Mail Format Information
RFC 1343

Document Type RFC - Informational (June 1992; No errata)
Last updated 2013-03-02
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Network Working Group               N. Borenstein, Bellcore
            Request for Comments: 1343                        June 1992

                        A User Agent Configuration Mechanism

                       For Multimedia Mail Format Information

          Status of This Memo

            This is an informational memo for  the  Internet  community,
            and  requests  discussion  and suggestions for improvements.
            This  memo  does   not   specify   an   Internet   standard.
            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
            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

            Borenstein                                          [Page 1]




            RFC 1343       Multimedia Mail Configuration       June 1992

            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-
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