
Network Working Group                     Nathaniel Borenstein
Internet Draft                                       Ned Freed
                          <draft-ietf-822ext-mime-conf-01.txt>

            Multipurpose Internet Mail Extensions
                      (MIME) Part Five:

              Conformance Criteria and Examples

                         May 5, 1995



                     Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six
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than as a "working draft" or "work in progress".

To learn the current status of any Internet-Draft, please
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or munnari.oz.au (Pacific Rim).


1.  Abstract

STD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message headers,
and leaves the message content, or message body, as flat US-
ASCII text.  This set of documents, collectively called the
Multipurpose Internet Mail Extensions, or MIME, redefines the
format of messages to allow for














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 (1)   textual message bodies in character sets other than
       US-ASCII,

 (2)   non-textual message bodies,

 (3)   multi-part message bodies, and

 (4)   textual header information in character sets other than
       US-ASCII.

These documents are based on earlier work documented in RFC
934, STD 11, and RFC 1049, but extends and revises them.
Because RFC 822 said so little about message bodies, these
documents are largely orthogonal to (rather than a revision
of) RFC 822.

In particular, these documents are designed to provide
facilities to include multiple parts in a single message, to
represent body and header text in character sets other than
US-ASCII, to represent formatted multi-font text messages, to
represent non-textual material such as images and audio
fragments, and generally to facilitate later extensions
defining new types of Internet mail for use by cooperating
mail agents.

The initial document in this set, RFC MIME-IMB, specifies the
various headers used to describe the structure of MIME
messages. The second document defines the general structure of
the MIME media typing system and defines an initial set of
media types.  The third document, RFC MIME-HEADERS, describes
extensions to RFC 822 to allow non-US-ASCII text data in
Internet mail header fields. The fourth document, RFC MIME-
REG, specifies various IANA registration procedures for MIME-
related entities. This fifth and final document describes MIME
conformance criteria as well as providing some illustrative
examples of MIME message formats, acknowledgements, and the
bibliography.

These documents are revisions of RFCs 1521, 1522, and 1590,
which themselves were revisions of RFCs 1341 and 1342.
Appendix B of this document describes differences and changes
from previous versions.








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2.  Table of Contents

1 Abstract ..............................................    1
2 Table of Contents .....................................    3
3 Introduction ..........................................    3
4 MIME Conformance ......................................    3
5 Guidelines for Sending Email Data .....................    6
6 Canonical Encoding Model ..............................    9
7 Summary ...............................................   11
8 Security Considerations ...............................   12
9 Authors' Addresses ....................................   12
10 Acknowledgements .....................................   13
A A Complex Multipart Example ...........................   15
B Changes from RFC 1521, 1522, and 1590 .................   17
C References ............................................   21


3.  Introduction

The first and second documents in this set defined MIME header
field and the initial set of MIME media types. This document
describes what portions  of MIME must be supported by a
conformant MIME implementation. It also describes various
pitfalls of contemporary messaging systems as well as the
canonical encoding model MIME is based on.


4.  MIME Conformance

The mechanisms described in these documents are open-ended.
It is definitely not expected that all implementations will
support all available media types, nor that they will all
share the same extensions.  In order to promote
interoperability, however, it is useful to define the concept
of "MIME-conformance" to define a certain level of
implementation that allows the useful interworking of messages
with content that differs from US-ASCII text.  In this
section, we specify the requirements for such conformance.

A mail user agent that is MIME-conformant MUST:

 (1)   Always generate a "MIME-Version: 1.0" header field in
       any message it creates.







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 (2)   Recognize the Content-Transfer-Encoding header field
       and decode all received data encoded with either the
       quoted-printable or base64 implementations.  Any non-
       7-bit data that is sent without encoding must be
       properly labelled with a content-transfer-encoding of
       8bit or binary, as appropriate.  If the underlying
       transport does not support 8bit or binary (as SMTP
       [RFC821] does not), the sender is required to both
       encode and label data using an appropriate Content-
       Transfer-Encoding such as quoted-printable or base64.

 (3)   Recognize and interpret the Content-Type header field,
       and avoid showing users raw data with a Content-Type
       field other than text.  Be able to send at least
       text/plain messages, with the character set specified
       as a parameter if it is not US-ASCII.

 (4)   Explicitly handle the following media type values, to
       at least the following extents:

       Text:

         -- Recognize and display "text" mail with the
         character set "US-ASCII."

         -- Recognize other character sets at least to the
         extent of being able to inform the user about what
         character set the message uses.

         -- Recognize the "ISO-8859-*" character sets to the
         extent of being able to display those characters that
         are common to ISO-8859-* and US-ASCII, namely all
         characters represented by octet values 1-127.

         -- For unrecognized subtypes in a known character
         set, show or offer to show the user the "raw" version
         of the data after conversion of the content from
         canonical form to local form.

         -- Treat material in an unknown character set as if
         it were "application/octet-stream".

       Image, audio, and video:

         -- At a minumum provide facilities to treat any





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         unrecognized subtypes as if they were
         "application/octet-stream".

       Application:

         -- Offer the ability to remove either of the quoted-
         printable or base64 encodings defined in this
         document if they were used and put the resulting
         information in a user file.

       Multipart:

         -- Recognize the mixed subtype.  Display all relevant
         information on the message level and the body part
         header level and then display or offer to display
         each of the body parts individually.

         -- Recognize the "alternative" subtype, and avoid
         showing the user redundant parts of
         multipart/alternative mail.

         -- Recognize the "multipart/digest" subtype,
         specifically using "message/rfc822" rather than
         "text/plain" as the default media type for body parts
         inside "multipart/digest" entities.

         -- Treat any unrecognized subtypes as if they were
         "mixed".

       Message:

         -- Recognize and display at least the primary
         (RFC822) encapsulation in such a way as to preserve
         any recursive structure, that is, displaying or
         offering to display the encapsulated data in
         accordance with its media type.

         -- Treat any unrecognized subtypes as if they were
         "application/octet-stream".

 (5)   Upon encountering any unrecognized Content-Type field,
       an implementation must treat it as if it had a media
       type of "application/octet-stream" with no parameter
       sub-arguments.  How such data are handled is up to an
       implementation, but likely options for handling such





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       unrecognized data include offering the user to write it
       into a file (decoded from its mail transport format) or
       offering the user to name a program to which the
       decoded data should be passed as input.

A user agent that meets the above conditions is said to be
MIME-conformant.  The meaning of this phrase is that it is
assumed to be "safe" to send virtually any kind of properly-
marked data to users of such mail systems, because such
systems will at least be able to treat the data as
undifferentiated binary, and will not simply splash it onto
the screen of unsuspecting users.

There is another sense in which it is always "safe" to send
data in a format that is MIME-conformant, which is that such
data will not break or be broken by any known systems that are
conformant with RFC 821 and RFC 822.  User agents that are
MIME-conformant have the additional guarantee that the user
will not be shown data that were never intended to be viewed
as text.


5.  Guidelines for Sending Email Data

Internet email is not a perfect, homogeneous system.  Mail may
become corrupted at several stages in its travel to a final
destination.  Specifically, email sent throughout the Internet
may travel across many networking technologies. Many
networking and mail technologies do not support the full
functionality possible in the SMTP transport environment.
Mail traversing these systems is likely to be modified in
order that it can be transported.

There exist many widely-deployed non-conformant MTAs in the
Internet.  These MTAs, speaking the SMTP protocol, alter
messages on the fly to take advantage of the internal data
structure of the hosts they are implemented on, or are just
plain broken.

The following guidelines may be useful to anyone devising a
data format (media type) that is supposed to survive the
widest range of networking technologies and known broken MTAs
unscathed.  Note that anything encoded in the base64 encoding
will satisfy these rules, but that some well-known mechanisms,
notably the UNIX uuencode facility, will not.  Note also that





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anything encoded in the Quoted-Printable encoding will survive
most gateways intact, but possibly not some gateways to
systems that use the EBCDIC character set.

 (1)   Under some circumstances the encoding used for data may
       change as part of normal gateway or user agent
       operation.  In particular, conversion from base64 to
       quoted-printable and vice versa may be necessary.  This
       may result in the confusion of CRLF sequences with line
       breaks in text bodies.  As such, the persistence of
       CRLF as something other than a line break must not be
       relied on.

 (2)   Many systems may elect to represent and store text data
       using local newline conventions.  Local newline
       conventions may not match the RFC822 CRLF convention --
       systems are known that use plain CR, plain LF, CRLF, or
       counted records.  The result is that isolated CR and LF
       characters are not well tolerated in general; they may
       be lost or converted to delimiters on some systems, and
       hence must not be relied on.

 (3)   The transmission of NULs (US-ASCII value 0) is
       problematic in Internet mail.  (This is largely the
       result of NULs being used as a termination character by
       many of the standard runtime library routines in the C
       programming language.) The practice of using NULs as
       termination characters is so entrenched now that
       messages should not rely on them being preserved.

 (4)   TAB (HT) characters may be misinterpreted or may be
       automatically converted to variable numbers of spaces.
       This is unavoidable in some environments, notably those
       not based on the US-ASCII character set.  Such
       conversion is STRONGLY DISCOURAGED, but it may occur,
       and mail formats must not rely on the persistence of
       TAB (HT) characters.

 (5)   Lines longer than 76 characters may be wrapped or
       truncated in some environments.  Line wrapping and line
       truncation are STRONGLY DISCOURAGED, but unavoidable in
       some cases.  Applications which require long lines must
       somehow differentiate between soft and hard line
       breaks.  (A simple way to do this is to use the
       quoted-printable encoding.)





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 (6)   Trailing "white space" characters (SPACE, TAB (HT)) on
       a line may be discarded by some transport agents, while
       other transport agents may pad lines with these
       characters so that all lines in a mail file are of
       equal length.  The persistence of trailing white space,
       therefore, must not be relied on.

 (7)   Many mail domains use variations on the US-ASCII
       character set, or use character sets such as EBCDIC
       which contain most but not all of the US-ASCII
       characters.  The correct translation of characters not
       in the "invariant" set cannot be depended on across
       character converting gateways.  For example, this
       situation is a problem when sending uuencoded
       information across BITNET, an EBCDIC system.  Similar
       problems can occur without crossing a gateway, since
       many Internet hosts use character sets other than US-
       ASCII internally.  The definition of Printable Strings
       in X.400 adds further restrictions in certain special
       cases.  In particular, the only characters that are
       known to be consistent across all gateways are the 73
       characters that correspond to the upper and lower case
       letters A-Z and a-z, the 10 digits 0-9, and the
       following eleven special characters:

         "'"  (US-ASCII decimal value 39)
         "("  (US-ASCII decimal value 40)
         ")"  (US-ASCII decimal value 41)
         "+"  (US-ASCII decimal value 43)
         ","  (US-ASCII decimal value 44)
         "-"  (US-ASCII decimal value 45)
         "."  (US-ASCII decimal value 46)
         "/"  (US-ASCII decimal value 47)
         ":"  (US-ASCII decimal value 58)
         "="  (US-ASCII decimal value 61)
         "?"  (US-ASCII decimal value 63)

       A maximally portable mail representation will confine
       itself to relatively short lines of text in which the
       only meaningful characters are taken from this set of
       73 characters.  The base64 encoding follows this rule.

 (8)   Some mail transport agents will corrupt data that
       includes certain literal strings.  In particular, a
       period (".") alone on a line is known to be corrupted





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       by some (incorrect) SMTP implementations, and a line
       that starts with the five characters "From " (the fifth
       character is a SPACE) are commonly corrupted as well.
       A careful composition agent can prevent these
       corruptions by encoding the data (e.g., in the quoted-
       printable encoding using "=46rom " in place of "From "
       at the start of a line, and "=2E" in place of "." alone
       on a line).

Please note that the above list is NOT a list of recommended
practices for MTAs.  RFC 821 MTAs are prohibited from altering
the character of white space or wrapping long lines.  These
BAD and invalid practices are known to occur on established
networks, and implementations should be robust in dealing with
the bad effects they can cause.


6.  Canonical Encoding Model

There was some confusion, in earlier drafts of these
documents, regarding the model for when email data was to be
converted to canonical form and encoded, and in particular how
this process would affect the treatment of CRLFs, given that
the representation of newlines varies greatly from system to
system.  For this reason, a canonical model for encoding is
presented below.

The process of composing a MIME entity can be modeled as being
done in a number of steps.  Note that these steps are roughly
similar to those steps used in PEM [RFC1421] and are performed
for each "innermost level" body:

 (1)   Creation of local form.

       The body to be transmitted is created in the system's
       native format.  The native character set is used and,
       where appropriate, local end of line conventions are
       used as well.  The body may be a UNIX-style text file,
       or a Sun raster image, or a VMS indexed file, or audio
       data in a system-dependent format stored only in
       memory, or anything else that corresponds to the local
       model for the representation of some form of
       information.  Fundamentally, the data is created in the
       "native" form that corresponds to the type specified by
       the media type.





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 (2)   Conversion to canonical form.

       The entire body, including "out-of-band" information
       such as record lengths and possibly file attribute
       information, is converted to a universal canonical
       form.  The specific media type of the body as well as
       its associated attributes dictate the nature of the
       canonical form that is used.  Conversion to the proper
       canonical form may involve character set conversion,
       transformation of audio data, compression, or various
       other operations specific to the various media types.
       If character set conversion is involved, however, care
       must be taken to understand the semantics of the media
       type, which may have strong implications for any
       character set conversion, e.g. with regard to
       syntactically meaningful characters in a text subtype
       other than "plain".

       For example, in the case of text/plain data, the text
       must be converted to a supported character set and
       lines must be delimited with CRLF delimiters in
       accordance with RFC 822.  Note that the restriction on
       line lengths implied by RFC 822 is eliminated if the
       next step employs either quoted-printable or base64
       encoding.

 (3)   Apply transfer encoding.

       A Content-Transfer-Encoding appropriate for this body
       is applied.  Note that there is no fixed relationship
       between the media type and the transfer encoding.  In
       particular, it may be appropriate to base the choice of
       base64 or quoted-printable on character frequency
       counts which are specific to a given instance of a
       body.

 (4)   Insertion into entity.

       The encoded object is inserted into a MIME entity with
       appropriate headers.  The entity is then inserted into
       the body of a higher-level entity (message or
       multipart) if needed.

It is vital to note that these steps are only a model; they
are specifically NOT a blueprint for how an actual system





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would be built.  In particular, the model fails to account for
two common designs:

 (1)   In many cases the conversion to a canonical form prior
       to encoding will be subsumed into the encoder itself,
       which understands local formats directly.  For example,
       the local newline convention for text bodies might be
       carried through to the encoder itself along with
       knowledge of what that format is.

 (2)   The output of the encoders may have to pass through one
       or more additional steps prior to being transmitted as
       a message.  As such, the output of the encoder may not
       be conformant with the formats specified by RFC 822.
       In particular, once again it may be appropriate for the
       converter's output to be expressed using local newline
       conventions rather than using the standard RFC 822 CRLF
       delimiters.

Other implementation variations are conceivable as well.  The
vital aspect of this discussion is that, in spite of any
optimizations, collapsings of required steps, or insertion of
additional processing, the resulting messages must be
consistent with those produced by the model described here.
For example, a message with the following header fields:

  Content-type: text/foo; charset=bar
  Content-Transfer-Encoding: base64

must be first represented in the text/foo form, then (if
necessary) represented in the "bar" character set, and finally
transformed via the base64 algorithm into a mail-safe form.


7.  Summary

This document defines what is meant by MIME Conformance. It
also details various problems known to exist in the Internet
email system and how to use MIME to overcome them. Finally, it
describes MIME's canonical encoding model.










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8.  Security Considerations

Security issues are discussed in the second document in this
set, RFC MIME-IMT.


9.  Authors' Addresses

For more information, the authors of this document are best
contacted via Internet mail:

Nathaniel S. Borenstein
First Virtual Holdings
25 Washington Avenue
Morristown, NJ 07960
USA

Email: nsb@nsb.fv.com
Phone: +1 201 540 8967
Fax:   +1 201 993 3032

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA

Email: ned@innosoft.com
Phone: +1 818 919 3600
Fax:   +1 818 919 3614

MIME is a result of the work of the Internet Engineering Task
Force Working Group on Email Extensions.  The chairman of that
group, Greg Vaudreuil, may be reached at:

Gregory M. Vaudreuil
Tigon Corporation
17060 Dallas Parkway
Dallas Texas, 75248

Email: greg.vaudreuil@ons.octel.com
Phone: +1 214 733 2722








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10.  Acknowledgements

This document is the result of the collective effort of a
large number of people, at several IETF meetings, on the
IETF-SMTP and IETF-822 mailing lists, and elsewhere.  Although
any enumeration seems doomed to suffer from egregious
omissions, the following are among the many contributors to
this effort:

  Harald Tveit Alvestrand       Marc Andreessen
  Randall Atkinson              Bob Braden
  Philippe Brandon              Brian Capouch
  Kevin Carosso                 Uhhyung Choi
  Peter Clitherow               Dave Collier-Brown
  Cristian Constantinof         John Coonrod
  Mark Crispin                  Dave Crocker
  Stephen Crocker               Terry Crowley
  Walt Daniels                  Jim Davis
  Frank Dawson                  Axel Deininger
  Hitoshi Doi                   Kevin Donnelly
  Steve Dorner                  Keith Edwards
  Chris Eich                    Dana S. Emery
  Johnny Eriksson               Craig Everhart
  Patrik Faltstrom              Erik E. Fair
  Roger Fajman                  Alain Fontaine
  Martin Forssen                James M. Galvin
  Stephen Gildea                Philip Gladstone
  Thomas Gordon                 Keld Simonsen
  Terry Gray                    Phill Gross
  James Hamilton                David Herron
  Mark Horton                   Bruce Howard
  Bill Janssen                  Olle Jarnefors
  Risto Kankkunen               Phil Karn
  Alan Katz                     Tim Kehres
  Neil Katin                    Steve Kille
  Kyuho Kim                     Anders Klemets
  John Klensin                  Valdis Kletniek
  Jim Knowles                   Stev Knowles
  Bob Kummerfeld                Pekka Kytolaakso
  Stellan Lagerstrom            Vincent Lau
  Timo Lehtinen                 Donald Lindsay
  Warner Losh                   Carlyn Lowery
  Laurence Lundblade            Charles Lynn
  John R. MacMillan             Larry Masinter
  Rick McGowan                  Michael J. McInerny





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  Leo Mclaughlin                Goli Montaser-Kohsari
  Tom Moore                     John Gardiner Myers
  Erik Naggum                   Mark Needleman
  Chris Newman                  John Noerenberg
  Mats Ohrman                   Julian Onions
  Michael Patton                David J. Pepper
  Erik van der Poel             Blake C. Ramsdell
  Christer Romson               Luc Rooijakkers
  Marshall T. Rose              Jonathan Rosenberg
  Guido van Rossum              Jan Rynning
  Harri Salminen                Michael Sanderson
  Yutaka Sato                   Markku Savela
  Richard Alan Schafer          Masahiro Sekiguchi
  Mark Sherman                  Bob Smart
  Peter Speck                   Henry Spencer
  Einar Stefferud               Michael Stein
  Klaus Steinberger             Peter Svanberg
  James Thompson                Steve Uhler
  Stuart Vance                  Peter Vanderbilt
  Greg Vaudreuil                Ed Vielmetti
  Larry W. Virden               Ryan Waldron
  Rhys Weatherly                Jay Weber
  Dave Wecker                   Wally Wedel
  Sven-Ove Westberg             Brian Wideen
  John Wobus                    Glenn Wright
  Rayan Zachariassen            David Zimmerman

The authors apologize for any omissions from this list, which
are certainly unintentional.





















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          Appendix A -- A Complex Multipart Example



What follows is the outline of a complex multipart message.
This message has five parts to be displayed serially:  two
introductory plain text parts, an embedded multipart message,
a text/enriched part, and a closing encapsulated text message
in a non-ASCII character set.  The embedded multipart message
has two parts to be displayed in parallel, a picture and an
audio fragment.

  MIME-Version: 1.0
  From: Nathaniel Borenstein <nsb@bellcore.com>
  To: Ned Freed <ned@innosoft.com>
  Date: Fri, 07 Oct 1994 16:15:05 -0700 (PDT)
  Subject: A multipart example
  Content-Type: multipart/mixed;
                boundary=unique-boundary-1

  This is the preamble area of a multipart message.
  Mail readers that understand multipart format
  should ignore this preamble.

  If you are reading this text, you might want to
  consider changing to a mail reader that understands
  how to properly display multipart messages.

  --unique-boundary-1

    ... Some text appears here ...

  [Note that the blank between the boundary and the start
   of the text in this part means no header fields were
   given and this is text in the US-ASCII character set.
   It could have been done with explicit typing as in the
   next part.]

  --unique-boundary-1
  Content-type: text/plain; charset=US-ASCII

  This could have been part of the previous part, but
  illustrates explicit versus implicit typing of body
  parts.






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  --unique-boundary-1
  Content-Type: multipart/parallel; boundary=unique-boundary-2

  --unique-boundary-2
  Content-Type: audio/basic
  Content-Transfer-Encoding: base64

    ... base64-encoded 8000 Hz single-channel
        mu-law-format audio data goes here ...

  --unique-boundary-2
  Content-Type: image/tiff
  Content-Transfer-Encoding: base64

    ... base64-encoded image data goes here ...

  --unique-boundary-2--

  --unique-boundary-1
  Content-type: text/enriched

  This is <bold><italic>enriched.</italic></bold>
  <smaller>as defined in RFC 1563</smaller>

  Isn't it
  <bigger><bigger>cool?</bigger></bigger>

  --unique-boundary-1
  Content-Type: message/rfc822

  From: (mailbox in US-ASCII)
  To: (address in US-ASCII)
  Subject: (subject in US-ASCII)
  Content-Type: Text/plain; charset=ISO-8859-1
  Content-Transfer-Encoding: Quoted-printable

    ... Additional text in ISO-8859-1 goes here ...

  --unique-boundary-1--











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     Appendix B -- Changes from RFC 1521, 1522, and 1590



These documents are a revision of RFC 1521, 1522, and 1590.
For the convenience of those familiar with the earlier
documents, the changes from those documents are summarized in
this appendix.  For further history, note that Appendix H in
RFC 1521 specified how that document differed from its
predecessor, RFC 1341.

 (1)   This document has been completely reformatted and split
       into multiple documents.  This was done to improve the
       quality of the plain text version of this document,
       which is required to be the reference copy.

 (2)   BNF describing the overall structure of MIME message
       and part headers has been added.  This is a
       documentation change only -- the underlying syntax has
       not changed in any way.

 (3)   The specific BNF for the seven media types in MIME has
       been removed.  This BNF was incorrect, incomplete, amd
       inconsistent with the type-indendependent BNF.  And
       since the type-independent BNF already fully specifies
       the syntax of the various MIME headers, the type-
       specific BNF was, in the final analysis, completely
       unnecessary and caused more problems than it solved.

 (4)   The more specific "US-ASCII" character set name has
       replaced the use of the term ASCII in many parts of
       this specification.

 (5)   The informal concept of a primary subtype has been
       removed.

 (6)   The term "object" was being used inconsistently.  This
       term has been replaced with the more precise terms
       "body", "body part", and "entity" where appropriate.

 (7)   The BNF for the multipart media type has been
       rearranged to make it clear that the CRLF preceeding
       the boundary marker is actually part of the marker
       itself rather than the preceeding body part.






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 (8)   The prose and BNF describing the multipart media type
       have been changed to make it clear that the body parts
       within a multipart entity MUST NOT contain any lines
       beginning with the boundary parameter string.

 (9)   In the rules on reassembling "message/partial" MIME
       entities, "Subject" is added to the list of headers to
       take from the inner message, and the example is
       modified to clarify this point.

 (10)  In the discussion of the application/postscript type,
       an additional paragraph has been added warning about
       possible interoperability problems caused by embedding
       of binary data inside a PostScript MIME entity.

 (11)  Added a clarifying note to the basic syntax rules for
       the Content-Type header field to make it clear that the
       following two forms:

         Content-type: text/plain; charset=us-ascii (comment)

         Content-type: text/plain; charset="us-ascii"

       are completely equivalent.

 (12)  The following sentence has been removed from the
       discussion of the MIME-Version header: "However,
       conformant software is encouraged to check the version
       number and at least warn the user if an unrecognized
       MIME-version is encountered."

 (13)  A typo was fixed that said "application/external-body"
       instead of "message/external-body".

 (14)  The definition of a character set has been reorganized
       to make the requirements clearer.

 (15)  The definition of the "image/gif" media type has been
       moved to a separate document. This change was made
       because of potential conflicts with IETF rules
       governing the standardization of patented technology.

 (16)  The definitions of "7bit" and "8bit" have been
       tightened so that use of bare CR, LF can only be used
       as end-of-line sequences.  The document also no longer





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       requires that NUL characters be preserved, which brings
       MIME into alignment with real-world implementations.

 (17)  The definition of canonical text in MIME has been
       tightened so that line breaks must be represented by a
       CRLF sequence.  CR and LF characters are not allowed
       outside of this usage.  The definition of quoted-
       printable encoding has been altered accordingly.

 (18)  Prose was added to clarify the use of the "7bit", "8-
       bit", and "binary" transfer-encodings on multipart or
       message entities encapsulating "8bit" or "binary" data.

 (19)  In the section on MIME Conformance, "multipart/digest"
       support was added to the list of requirements for
       minimal MIME conformance.  Also, the requirement for
       "message/rfc822" support were strengthened to clarify
       the importance of recognizing recursive structure.

 (20)  The various restrictions on subtypes of "message" are
       now specified entirely on a subtype by subtype basis.

 (21)  The definition of "message/rfc822" was changed to
       indicate that at least one of the "From", "Subject", or
       "Date" headers must be present.

 (22)  The required handling of unrecognized subtypes as
       "application/octet-stream" has been made more explicit
       in both the type definitions sections and the
       conformance guidelines.

 (23)  Examples using text/richtext were changed to
       text/enriched.

 (24)  The BNF definition of subtype has been changed to make
       it clear that either an IANA registered subtype or a
       nonstandard "X-" subtype must be used in a Content-Type
       header field.

 (25)  The use of escape and shift mechanisms in the US-ASCII
       and ISO-8859-X character sets this specification
       defines has been clarified: Such mechanisms should
       never be used in conjunction with these character sets
       and their effect if they are used is undefined.






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 (26)  The definition of the AFS access-type for
       message/external-body has been removed.

 (27)  Entities that are simply registered for use and those
       that are standardized by the IETF are now distinguished
       in the MIME BNF.

 (28)  The handling of the combination of
       multipart/alternative and message/external-body is now
       specifically addressed.

 (29)  Security issues specific to message/external-body are
       now discussed in some detail.





































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                      Appendix C -- References



[ATK]
     Borenstein, Nathaniel S., Multimedia Applications
     Development with the Andrew Toolkit, Prentice-Hall, 1990.

[ISO-2022]
     International Standard -- Information Processing -- ISO
     7-bit and 8-bit Coded Character Sets -- Code Extension
     Techniques, ISO 2022:1986.

[ISO-8859]
     International Standard -- Information Processing -- 8-bit
     Single-Byte Coded Graphic Character Sets -- Part 1: Latin
     Alphabet No. 1, ISO 8859-1:1987.  Part 2: Latin alphabet
     No. 2, ISO 8859-2, 1987.  Part 3: Latin alphabet No. 3,
     ISO 8859-3, 1988.  Part 4: Latin alphabet No. 4, ISO
     8859-4, 1988.  Part 5: Latin/Cyrillic alphabet, ISO
     8859-5, 1988.  Part 6: Latin/Arabic alphabet, ISO 8859-6,
     1987.  Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
     Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.  Part 9:
     Latin alphabet No. 5, ISO 8859-9, 1990.

[ISO-646]
     International Standard -- Information Processing -- ISO
     7-bit Coded Character Set For Information Interchange,
     ISO 646:1983.

[MPEG]
     Video Coding Draft Standard ISO 11172 CD, ISO
     IEC/JTC1/SC2/WG11 (Motion Picture Experts Group), May,
     1991.

[PCM]
     CCITT, Fascicle III.4 - Recommendation G.711, "Pulse Code
     Modulation (PCM) of Voice Frequencies", Geneva, 1972.












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[POSTSCRIPT]
     Adobe Systems, Inc., PostScript Language Reference
     Manual, Addison-Wesley, 1985.

[POSTSCRIPT2]
     Adobe Systems, Inc., PostScript Language Reference
     Manual, Addison-Wesley, Second Edition, 1990.

[RFC-783]
     Sollins, K.R., "TFTP Protocol (revision 2)", RFC-783,
     MIT, June 1981.

[RFC-821]
     Postel, J.B., "Simple Mail Transfer Protocol", STD 10,
     RFC 821, USC/Information Sciences Institute, August 1982.

[RFC-822]
     Crocker, D., "Standard for the Format of ARPA Internet
     Text Messages", STD 11, RFC 822, UDEL, August 1982.

[RFC-934]
     Rose, M. and E. Stefferud, "Proposed Standard for Message
     Encapsulation", RFC 934, Delaware and NMA, January 1985.

[RFC-959]
     Postel, J. and J. Reynolds, "File Transfer Protocol", STD
     9, RFC 959, USC/Information Sciences Institute, October
     1985.

[RFC-1049]
     Sirbu, M., "Content-Type Header Field for Internet
     Messages", RFC 1049, CMU, March 1988.

[RFC-1154]
     Robinson, D. and R. Ullmann, "Encoding Header Field for
     Internet Messages", RFC 1154, Prime Computer, Inc., April
     1990.













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[RFC-1341]
     Borenstein, N., and N.  Freed, "MIME (Multipurpose
     Internet Mail Extensions): Mechanisms for Specifying and
     Describing the Format of Internet Message Bodies", RFC
     1341, Bellcore, Innosoft, June 1992.

[RFC-1342]
     Moore, K., "Representation of Non-Ascii Text in Internet
     Message Headers", RFC 1342, University of Tennessee, June
     1992.

[RFC-1344]
     Borenstein, N., "Implications of MIME for Internet Mail
     Gateways", RFC 1344, Bellcore, June 1992.

[RFC-1345]
     Simonsen, K., "Character Mnemonics & Character Sets", RFC
     1345, Rationel Almen Planlaegning, June 1992.

[RFC-1421]
     Linn, J., "Privacy Enhancement for Internet Electronic
     Mail:  Part I -- Message Encryption and Authentication
     Procedures", RFC 1421, IAB IRTF PSRG, IETF PEM WG,
     February 1993.

[RFC-1422]
     Kent, S., "Privacy Enhancement for Internet Electronic
     Mail:  Part II -- Certificate-Based Key Management", RFC
     1422, IAB IRTF PSRG, IETF PEM WG, February 1993.

[RFC-1423]
     Balenson, D., "Privacy Enhancement for Internet
     Electronic Mail:  Part III -- Algorithms, Modes, and
     Identifiers",  IAB IRTF PSRG, IETF PEM WG, February 1993.

[RFC-1424]
     Kaliski, B., "Privacy Enhancement for Internet Electronic
     Mail:  Part IV -- Key Certification and Related
     Services", IAB IRTF PSRG, IETF PEM WG, February 1993.











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[RFC-1521]
     Borenstein, N. and Freed, N., "MIME (Multipurpose
     Internet Mail Extensions): Mechanisms for Specifying and
     Describing the Format of Internet Message Bodies", RFC
     1521, Bellcore, Innosoft, September, 1993.

[RFC-1522]
     Moore, K., "Representation of Non-ASCII Text in Internet
     Message Headers", RFC 1522, University of Tennessee,
     September 1993.

[RFC-1524]
     Borenstein, N., "A User Agent Configuration Mechanism for
     Multimedia Mail Format Information", RFC 1524, Bellcore,
     September 1993.

[RFC-1543]
     Postel, J., "Instructions to RFC Authors", RFC 1543,
     USC/Information Sciences Institute, October 1993.

[RFC-1563]
     Borenstein, N., "The text/enriched MIME Content-type",
     RFC 1563, Bellcore, January, 1994.

[RFC-1590]
     Postel, J., "Media Type Registration Procedure", RFC
     1590, USC/Information Sciences Institute, March 1994.

[RFC-1602]
     Internet Architecture Board, Internet Engineering
     Steering Group, Huitema, C., Gross, P., "The Internet
     Standards Process -- Revision 2", March 1994.

[RFC-1652]
     Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,
     Stefferud, E., and Crocker, D., "SMTP Service Extension
     for 8bit-MIME transport", RFC 1652, United Nations
     University, Innosoft, Dover Beach Consulting, Inc.,
     Network Management Associates, Inc., The Branch Office,
     March 1994.










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[RFC-1700]
     Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
     RFC 1700, USC/Information Sciences Institute, October
     1994.

[RFC-MIME-IMB]
     Borenstein, N. and Freed, N., "Multipurpose Internet Mail
     Extensions (MIME) Part One: Format of Internet Message
     Bodies", RFC MIME-IMB, Bellcore, Innosoft, April, 1995.

[RFC-MIME-IMT]
     Borenstein, N. and Freed, N., "Multipurpose Internet Mail
     Extensions (MIME) Part Two: Media Types", RFC MIME-IMT,
     Bellcore, Innosoft, April, 1995.

[RFC-MIME-HEADERS]
     Moore, K., "Multipurpose Internet Mail Extensions (MIME)
     Part Three: Representation of Non-Ascii Text in Internet
     Message Headers", RFC MIME-HEADERS, University of
     Tennessee, ?.

[RFC-MIME-REG]
     Postel, J. and Freed, N., "Multipurpose Internet Mail
     Extensions (MIME) Part Four: Media Type Registration
     Procedure", RFC MIME-REG, ISI, Innosoft, April, 1995.

[RFC-MIME-CONF]
     Borenstein, N. and Freed, N., "Multipurpose Internet Mail
     Extensions (MIME) Part Five: Conformance Criteria and
     Examples", RFC MIME-CONF, Bellcore, Innosoft, April,
     1995.

[US-ASCII]
     Coded Character Set -- 7-Bit American Standard Code for
     Information Interchange, ANSI X3.4-1986.

[X400]
     Schicker, Pietro, "Message Handling Systems, X.400",
     Message Handling Systems and Distributed Applications, E.
     Stefferud, O-j. Jacobsen, and P. Schicker, eds., North-
     Holland, 1989, pp. 3-41.









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