Network Working Group Nathaniel Borenstein
Internet Draft Ned Freed
<draft-ietf-822ext-mime-conf-00.txt>
Multipurpose Internet Mail Extensions
(MIME) Part Five:
Conformance Criteria and Examples
April 11, 1995
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
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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
Internet Draft MIME Conformance April 1995
(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:
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-- At a minumum provide facilities to treat any
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
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implementation, but likely options for handling such
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,
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notably the UNIX uuencode facility, will not. Note also that
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
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quoted-printable encoding.)
(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.
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(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
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
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information. Fundamentally, the data is created in the
"native" form that corresponds to the type specified by
the media type.
(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
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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
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.
[]
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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