🚀 go-pugleaf

Archive-Name: mail/mime-faq/part3
Version: $Id: mime3,v 3.9 1994/10/09 21:35:40 jsweet Rel $
Posting-Frequency: monthly


--
==========================================================
comp.mail.mime frequently asked questions list (FAQ) (3/3)
==========================================================
Part 3: Advanced Topics
~~~~~~
--

Overview
--------
This is part 3 of a Frequently Asked Questions document about MIME,
the multipurpose and multi-media standard for Internet mail.

Part 1 covers frequently asked questions.

Part 2 is a listing of MIME products.

Part 3 covers advanced topics.
--

10) Information
---------------

10.1) MIME-relevant RFCs and other standards

The RFCs mentioned here are mainly relevant to persons building MIME
software.  As an end user, if your mail system is nice to you, you
won't really have to know very much about these things.

RFC and Internet-Drafts are available by anonymous FTP from any decent
archive site.  If you're really stuck, try these URLs:

ftp://ds.internic.net/rfc/
ftp://ds.internic.net/internet-drafts/

MIME is defined in RFC 1521 (MIME Mechanisms for Specifying and
Describing the Format of Internet Message Bodies) and RFC 1522
(Representation of Non-ASCII Text in Internet Message Headers).

These are Internet standards-track protocols.  For the full
implications of this, see RFC 1540 (IAB Official Protocol Standards).
Here is their current status.

    RFC 1521: Draft Elective Standard
    RFC 1522: Draft Elective Standard

These two RFCs do not fully define MIME.  For one thing, they are
based on RFC 822 (Standard for the format of ARPA Internet text
messages), as revised by RFC 1123 (Requirements for Internet hosts -
application and support) and must be read in conjunction with these.

For another, they are extensible.  See 10.2 for a complete list of
registered subtypes.

There are a whole lot of other RFCs that deal with e-mail, including
these.

IAB standards-track RFCs

    RFC 1653  SMTP Service Extension for Message Size Declaration.
    RFC 1652  SMTP Service Extension for 8bit-MIMEtransport.
    RFC 1651  SMTP Service Extensions.
    RFC 1502  X.400 Use of Extended Character Sets
    RFC 1496  Rules for Downgrading Messages from X.400(88) to X.400(84)
              when MIME Content-Types are Present in the Messages
    RFC 1495  Mapping between X.400 and RFC-822 Message Bodies
    RFC 1494  Equivalences between 1988 X.400 and RFC-922 Message Bodies
    RFC 1424  Privacy Enhancement for Internet Electronic Mail: Part IV.
    RFC 1423  Privacy Enhancement for Internet Electronic Mail: Part III.
    RFC 1422  Privacy Enhancement for Internet Electronic Mail: Part II.
    RFC 1421  Privacy Enhancement for Internet Electronic Mail: Part I.
    RFC 1327  Mapping between X.400(1988)/ISO 10021 and RFC 822.
    RFC 1314  File format for the exchange of images in the Internet.

Other RFCs (Informational, Experimental, or Historical)

    RFC 1641  Using Unicode with MIME.
    RFC 1563  The text/enriched MIME Content-type.
    RFC 1556  Handling of Bi-directional Texts in MIME.
    RFC 1489  Registration of a Cyrillic Character Set.
    RFC 1468  Japanese Character Encoding for Internet Messages.
    RFC 1456  Conventions for Encoding the Vietnamese Language.
    RFC 1428  Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIME.
    RFC 1357  Format for emailing bibliographic records.
    RFC 1345  Character Mnemonics & Character Sets.
    RFC 1344  Implications of MIME for Internet mail gateways.
    RFC 1343  User agent configuration mechanism for multimedia mail format
              information.
    RFC 1339  Remote mail checking protocol.
    RFC 1321  MD5 Message-Digest algorithm.
    RFC 1225  Post Office Protocol: Version 3.
    RFC 1211  Problems with the maintenance of large mailing lists.
    RFC 1176  Interactive Mail Access Protocol: Version 2.
    RFC 1197  Using ODA for translating multimedia information.
    RFC 1154  Encoding header field for internet messages.
    RFC 1153  Digest message format.
    RFC 1049  Content-type header field for Internet messages.
    RFC 1036  Standard for interchange of USENET messages.
    RFC 934   Proposed standard for message encapsulation.
    RFC 807   Multimedia mail meeting notes.

--------------------------------

10.2) MIME types

There are registered and unregistered MIME types.  Unregistered MIME
types begin with an "x-" and their meanings generally depend on
private agreements between senders and receivers.  This section lists
registered types and some known unregistered types.

--------------------------------

10.2.1) List of registered MIME types

The latest list of registered MIME types is available from this file:

ftp://isi.edu/in-notes/media-types/media-types

A list of URLs follows for documents relevant to various media types.
The media types are taken from the January, 1994 version of the
aforementioned media-types file, but the URLs below aren't necessarily
representative of the latest list of registered types.  In general,
each <type> has a directory whose name has this form:

media-types/<type>/<subtype>

The <type> directory contains the definitions of the subtypes of the
given <type>/<subtype>.


Application subtypes:

ftp://isi.edu/in-notes/media-types/application/activemessage
ftp://isi.edu/in-notes/media-types/application/andrew
ftp://isi.edu/in-notes/media-types/application/applefile
ftp://isi.edu/in-notes/media-types/application/atomicmail
ftp://isi.edu/in-notes/media-types/application/dec
ftp://isi.edu/in-notes/media-types/application/dca
ftp://isi.edu/in-notes/media-types/application/mac-binhex40
ftp://isi.edu/in-notes/media-types/application/macwriteii
ftp://isi.edu/in-notes/media-types/application/msword
ftp://isi.edu/in-notes/media-types/application/news-message-id
ftp://isi.edu/in-notes/media-types/application/news-transmission 
ftp://isi.edu/in-notes/media-types/application/octet
ftp://isi.edu/in-notes/media-types/application/oda
ftp://isi.edu/in-notes/media-types/application/pdf
ftp://isi.edu/in-notes/media-types/application/postscript
ftp://isi.edu/in-notes/media-types/application/remote-printing
ftp://isi.edu/in-notes/media-types/application/rtf
ftp://isi.edu/in-notes/media-types/application/slate
ftp://isi.edu/in-notes/media-types/application/wita
ftp://isi.edu/in-notes/media-types/application/wordperfect5.1
ftp://isi.edu/in-notes/media-types/application/zip

Audio subtypes:

ftp://isi.edu/in-notes/media-types/audio/basic

Image subtypes:

ftp://isi.edu/in-notes/media-types/image/jpeg
ftp://isi.edu/in-notes/media-types/image/gif
ftp://isi.edu/in-notes/media-types/image/ief
ftp://isi.edu/in-notes/media-types/image/tiff

Message subtypes:

ftp://isi.edu/in-notes/media-types/message/external
ftp://isi.edu/in-notes/media-types/message/partial
ftp://isi.edu/in-notes/media-types/message/rfc822
ftp://isi.edu/in-notes/media-types/message/news

Multipart subtypes:

ftp://isi.edu/in-notes/media-types/multipart/alternative
ftp://isi.edu/in-notes/media-types/multipart/appledouble
ftp://isi.edu/in-notes/media-types/multipart/digest
ftp://isi.edu/in-notes/media-types/multipart/header
ftp://isi.edu/in-notes/media-types/multipart/mixed
ftp://isi.edu/in-notes/media-types/multipart/parallel

Text subtypes:

ftp://isi.edu/in-notes/media-types/text/plain
ftp://isi.edu/in-notes/media-types/text/richtext
ftp://isi.edu/in-notes/media-types/text/tab-separated-values

Video subtypes:

ftp://isi.edu/in-notes/media-types/video/mpeg
ftp://isi.edu/in-notes/media-types/video/quicktime

--------------------------------

10.2.2) List of known unregistered MIME types

Here is a list of some known x-types, x-subtypes, and x-parameters.

The enumeration of these x-types here does not imply any kind of
standardization or open specification.  The meanings of x-types depend
on private agreements between senders and receivers.  Some x-types may
eventually become registered types; see sections 10.2.1 and 11.1.

Just because an x-type is generated by a proprietary mail user agent
doesn't necessarily mean that only that MUA can handle the x-type.
Metamail and MH, for example, permit you to set up your own mechanisms
to handle various standard and non-standard content types.  In
particular, it may simply be a matter of invoking some commercial
application to handle data used by that application.  For example,
FrameMaker or FrameViewer might be run to handle a content type of
application/x-framemaker.  (In the case of Frame documents, there are
several ways to handle this---see Frame Technical Note 1359 or consult
the comp.text.frame FAQ.)  The Metamail source distribution comes with
pre-defined mailcap entries for handling some x-types; these may offer
clues about how to configure your own mail user agent.

Not all of the x-types listed here begin with "x-".  Although such
non-standard types may contravene the MIME specification, the fact
remains that someone out there is generating them.  Listing such types
here is not intended to enshrine such types.

{ NOTE: some of the meanings of these x-types are GUESSES by the FAQ
  maintainer.  Please let us know about incorrect guesses, and, if
  possible, supply a URL pointing to information about the x-type.
  
  And please feel free to let us know about whatever wacko or not-so-wacko
  x-types that your UAs may unleash on an unsuspecting world.  If you
  have a URL for a document that describes the format, so much the
  better.  Please at least let us know what applications are generating 
  the x-types in question.  }


Application types:

application/octet-stream; type=tar; x-conversions=compress
                        MH 6.8: viamail; see tar(1) and compress(1)

application/x-aiff      Z-Mail: AIFF audio data
application/x-bcpio     MHonArc: bcpio data
application/x-bitmap    Z-Mail: X11 bitmaps
application/x-cpio      MHonArc: cpio archives
application/x-csh       MHonArc: csh scripts
application/x-dvi       MHonArc: TeX DVI data
application/x-framemaker  Z-Mail: FrameMaker documents
application/x-gtar      MHonArc: GNU tar archives
application/x-hdf       MHonArc: hdf data
application/x-inventor  Z-Mail: for Inventor files
application/x-island-draw   Z-Mail: IslandDraw files
application/x-island-paint  Z-Mail: IslandPaint files
application/x-island-write  Z-Mail: IslandWrite files
application/x-jot       Z-Mail: Jot documents
application/x-latex     MHonArc: LaTeX documents
application/x-metamail-patch  metamail: patches to metamail
application/x-mif       MHonArc: Frame MIF documents
application/x-movie     Z-Mail: MoviePlayer documents
application/x-netcdf    MHonArc: netcdf data
application/x-sgi       Z-Mail: SGI ImageWorks documents
application/x-sh        MHonArc: sh scripts
application/x-shar      MHonArc: shell archives
application/x-showcase  Z-Mail: Showcase documents
application/x-sv4cpio   MHonArc: SVR4 cpio archives
application/x-sv4crc    MHonArc: SVR4 crc data
application/x-tar       MHonArc: tar archives
application/x-tcl       MHonArc: tcl programs
application/x-tex       MHonArc: TeX documents
application/x-texinfo   MHonArc: GNU texinfo documents
application/x-troff     MHonArc: plain troff documents
application/x-troff-man MHonArc: troff -man documents
application/x-troff-me  MHonArc: troff -me documents
application/x-troff-ms  MHonArc: troff -ms documents
application/x-ustar     MHonArc: ustar data
application/x-wais-source  MHonArc: WAIS sources
application/x-wingz     Z-Mail: Wingz documents
application/x-xpm1      Z-Mail: OL pixmap files
application/x-zm-fax    Z-Mail: Z-Fax documents


Audio types:

audio/x-aiff            MHonArc: AIFF audio data
audio/x-wav             MHonArc: WAV audio data
audio/x-macaudio        Iride: NOT sampled Macintosh audio

audio/x-next            MH 6.8: self-describing audio data
  see ftp://ftp.ics.uci.edu/mh/contrib/multimedia/mhn-tutorial.ps
                

Image types:

image/x-cmu-raster      MHonArc: CMU raster data
image/x-pbm             MHonArc: portable bit map data
image/x-pgm             MHonArc: PGM data
image/x-pict            MHonArc: Mactinosh PICT data
image/x-pnm             MHonArc
image/x-portable-anymap   MHonArc 
image/x-portable-bitmap   MHonArc 
image/x-portable-graymap  MHonArc 
image/x-portable-pixmap   MHonArc
image/x-ppm             MHonArc
image/x-rgb             MHonArc
image/x-xbitmap         MHonArc: in-lines into the HTML
image/x-xbm             MHonArc: in-lines into the HTML
image/x-xpixmap         MHonArc
image/x-xpm             MHonArc
image/x-xwd             MHonArc
image/x-xwindowdump     MHonArc: X window dump


Text types:

text/html               MHonArc
text/x-html             MHonArc
text/x-setext           MHonArc
text/x-usenet-FAQ       Ohio State WWW FAQ document format


Video types:

video/x-msvideo         MHonArc: Microsoft video data
video/x-sgi-movie       MHonArc: SGI movie data


Other types:

x-be2                   old Andrew format
x-sun-attachment        Sun MicroSystems mailtool
x-zm-multipart          old Z-Mail format


--------------------------------

10.3) Internet Engineering Task Force (IETF) working groups

The IETF working group on Privacy-Enhanced Mail (PEM) has developed
extensions that permit confidentiality, authentication, and integrity
to be provided in a manner backwards compatible with RFC 821 and 
RFC 822.  Work is underway to align PEM and MIME which will provide 
real security to MIME e-mail.

The IETF MIME working group is not actively considering significant
changes to the specifications.  However the WG still exists as a forum
for MIME developers, as a home for interpretation questions, and to
handle any problems or ambiguities that might arise in MIME.

--

11) Developers' FAQs
--------------------

11.1) How can I register a new MIME type?

The procedures for registering new content types, character set
values, access types, and conversions parameters with IANA (the
Internet Assigned Numbers Authority) are documented in RFC 1590.

--------------------------------

11.2) What's ESMTP, and how does it affect MIME?

ESMTP (Extended Simple Mail Transfer Protocol) is a mechanism by which
extensions to "traditional" (RFC 821) SMTP can be negotiated by client
and server.  The mechanism (RFC 1651) is open-ended; so far two
extensions have been defined.

Message size declaration (RFC 1653) offers a graceful way for servers
to limit the size of message they are prepared to accept.  (With SMTP,
the only possibility is for the server to discard the message after it
has been sent in its entirety.  There is no way for the client to know
that it was the size of the message that caused the problem.)

When a message is returned to the user as being too large to deliver,
one possible approach might be to fragment the message using the MIME
Message/Partial mechanism, and resubmit it.

Depending on the exact reason for the "too large" rejection, this may
or may not be a good idea.  For example, the limitation may reflect
the recipient's disk quota, in which case the fragmented message will
not be fully deliverable either.

The possibility of fragmentation should, therefore, be left to the
user's discretion (not performed automatically by the SMTP client).

8bit-MIMEtransport (RFC 1652) opens up the possibility of sending 8bit
data in mail messages, without having to use base64, quoted-printable,
or another encoding, and without the breakage that can result from
sending 8bit data to an unsuspecting RFC 821 SMTP server.  RFC 1428
(Transition of Internet Mail from Just-Send-8 to 8bit-SMTP/MIME)
discusses some of the implications of this.

--------------------------------

11.3) Where can I get some sample MIME messages?

ftp://thumper.bellcore.com/pub/nsb/samples/

--------------------------------

11.4) Wouldn't MIME be better if it did <foo>?

This question is asked for various values of <foo>.  Perhaps the most
common is "multilevel encodings": see the next question.  There are
a couple general points that apply to all <foo>.

1. Please remember that MIME is the result of a lot of work by a lot
of persons, over a long time (look at the Acknowledgements section of
RFC 1521).  A great many ideas, probably including yours, were
considered.  In many cases, there were conflicting goals, such as
simplicity and interoperability on the one hand, and power and
flexibility on the other.

2. If you really think you've got an original idea which would improve
MIME, the correct place to pursue it is not this newsgroup, but the
working group mailing list (having first read the archives, to check
that it really is new).  Yes, this is going to be a lot more work than
posting a news article.

--------------------------------

11.5) So what about multilevel encodings?

MIME uses a two-level encoding scheme.  The original object (for
example, a picture, or a text document) is encoded using a well
defined mechanism appropriate to that object (perhaps GIF for the
picture, and text/enriched for the document).  Then a second encoding
is used to ensure that the first encoding can be transmitted intact
(probably base64 for the GIF, and quoted printable for the
text/enriched document).  

Note that there is a very small number of the second encodings (five,
but three of these are simply indications of what kind of data an
unencoded body part contains), and it is not expected that there will
be many more in the foreseeable future.

The multilevel encodings idea is for a more generalized MIME-like
encoding mechanism that could indicate many arbitrary transformations
of the original object.  For example,

    Content-Type: application/tar; conversions="encrypt,compress,uuencode"

might indicate a UNIX tar file that had been encrypted, then
compressed, then uuencoded.  (This is a fictitious example of how MIME
might have worked; it's not legal MIME.  Don't worry if you've never
heard of some of these transformations.)

This may look like an attractive scheme at first, but it has a number
of problems.

1. If you've been brought up on UNIX and command pipelines, the
implementation of such a scheme seems trivial.  Surely any half-decent
machine can do something similar?  Unfortunately, this turns out to be
true only for a very restricted definition of "half-decent".  In
practice, it would be awfully difficult to implement this on a lot of
systems.  Probably even more systems would not allow new
transformations to be just "slotted in", and would require
recompilation or reshipping whenever a new one came along.

2. Each successive transformation reduces the size of the audience who
can successfully decode the message.  Every MIME mailer must be able
to decode base64 and quoted-printable, so it's guaranteed that you can
at least get back to the raw data.  What if, in the above example, I
have tar, decrypt, uudecode, but no uncompressor?

3. Such a scheme does not increase the scope of the framework defined
by MIME.  If uuencoded, compressed, encrypted tar files are useful
things to sling around, it is entirely possible to define a new MIME
type (presumably a subtype of application) to handle them.

--------------------------------

11.6) Why doesn't MIME include a mechanism for compression?

Compression is a difficult area.  It was considered by the working
group, but no consensus was reached.  There is still work going on in
this area: there may someday be a compressed-64 encoding.

Most compression algorithms have one of more of these undesirable
properties: they are covered by patent, they require the ability to
treat the input as a stream of bits, they use a large data space.  The
chances of finding a truly interoperable compression algorithm are
therefore rather slim.

It is worth noting that most or all of the image and video subtypes
(including GIF, JPEG, TIFF, and MPEG) define their own compression
schemes.

--

12) Acknowledgements
--------------------

Many persons have contributed to this document.

They include: 

Alan Robiette, Alec Henderson, Axel Boldt, Carlyn Lowery, Chris
Pepper, Christophe Wolfhugel, Christopher Davis, Craig Huckabee,
Daniel Fandrich, Daniel Glazman, Dave Curry, Dave Lacey, David Barr,
David Collier-Brown, David Miller, Douglas Boyce, Ed Anselmo, Ed
Greshko, Edward Vielmetti, Erik van der Poel, Gisle Hannemyr, Harald
Alvestrand, Ian Hoyle, James Ford, Jason Beyer, Jay Weber, Jerry Peek,
Jerry Sweet, Joe Ilacqua, Joergen Haegg, John Gardiner Myers, John
Martin, John R MacMillan, John Romine, Joyce Reynolds, Keith Moore,
Larry Salomon Jr, Larry W. Virden, Lars-Gunnar Olsson, Luc
Rooijakkers, Marc VanHeyningen, Mark Crispin, Mark Grand, Marshall
Rose, Martin Wendel, Masanobu Umeda, Michael Parson, Michael Urban,
Nathaniel Borenstein, Ned Freed, Niklas Agren, Olle Jarnefors, Pat
Farrell, Paul Eggert, Piero Serini, Quentin Smart, Ran Atkinson, Ray
Langford, Rich Ragan, Rick Troth, Ron Barak, Sascha Wildner, Steve
Dorner, Steve Hole, Stuart Lynne, Susan Straub, Syd Weinstein, Tim
Goodwin, Tim Kehres, Tommy Wallo, Yehavi Bourvine.

If we've left your name off, please accept our apologies.  Drop us a
note and we'll include it for next time.

Thanks also to the University of California, Irvine, Department of
Information and Computer Science, Einar Stefferud, and Irvine Compiler
Corp., for providing the resources for maintaining this FAQ; and to
Jonathan Kamens, for coordinating the *.answers groups, and for his
post_faq program which brought you this FAQ.

--
End of Part 3
*************
--