Network Working Group N. Freed
Request for Comments: 2049 Innosoft
Obsoletes: 1521, 1522, 1590 N. Borenstein
Category: Standards Track First Virtual
November 1996
Multipurpose Internet Mail Extensions
(MIME) Part Five:
Conformance Criteria and Examples
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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
(1) textual message bodies in character sets other than
US-ASCII,
(2) an extensible set of different formats for 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.
The initial document in this set, RFC 2045, 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 2047, describes extensions to RFC 822 to allow non-US-
Freed & Borenstein Standards Track [Page 1]RFC 2049 MIME Conformance November 1996
ASCII text data in Internet mail header fields. The fourth document,
RFC 2048, specifies various IANA registration procedures for MIME-
related facilities. 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.
Table of Contents
1. Introduction .......................................... 2
2. MIME Conformance ...................................... 2
3. Guidelines for Sending Email Data ..................... 6
4. Canonical Encoding Model .............................. 9
5. Summary ............................................... 12
6. Security Considerations ............................... 12
7. Authors' Addresses .................................... 12
8. Acknowledgements ...................................... 13
A. A Complex Multipart Example ........................... 15
B. Changes from RFC 1521, 1522, and 1590 ................. 16
C. References ............................................ 20
1. Introduction
The first and second documents in this set define MIME header fields
and the initial set of MIME media types. The third document
describes extensions to RFC822 formats to allow for character sets
other than US-ASCII. 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.
2. 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.
Freed & Borenstein Standards Track [Page 2]
RFC 2049 MIME Conformance November 1996
A mail user agent that is MIME-conformant MUST:
(1) Always generate a "MIME-Version: 1.0" header field in
any message it creates.
(2) Recognize the Content-Transfer-Encoding header field
and decode all received data encoded by either quoted-
printable or base64 implementations. The identity
transformations 7bit, 8bit, and binary must also be
recognized.
Any non-7bit 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
[RFC-821] 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) Must treat any unrecognized Content-Transfer-Encoding
as if it had a Content-Type of "application/octet-
stream", regardless of whether or not the actual
Content-Type is recognized.
(4) Recognize and interpret the Content-Type header field,
and avoid showing users raw data with a Content-Type
field other than text. Implementations must be able
to send at least text/plain messages, with the
character set specified with the charset parameter if
it is not US-ASCII.
(5) Ignore any content type parameters whose names they do
not recognize.
(6) 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.
Freed & Borenstein Standards Track [Page 3]RFC 2049 MIME Conformance November 1996
-- 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
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".
Freed & Borenstein Standards Track [Page 4]RFC 2049 MIME Conformance November 1996
Message:
-- Recognize and display at least the RFC822 message
encapsulation (message/rfc822) 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".
(7) 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
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.
(8) Conformant user agents are required, if they provide
non-standard support for non-MIME messages employing
character sets other than US-ASCII, to do so on
received messages only. Conforming user agents must not
send non-MIME messages containing anything other than
US-ASCII text.
In particular, the use of non-US-ASCII text in mail
messages without a MIME-Version field is strongly
discouraged as it impedes interoperability when sending
messages between regions with different localization
conventions. Conforming user agents MUST include proper
MIME labelling when sending anything other than plain
text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if
at all possible to include appropriate MIME header
information in the messages they send even if nothing
else in MIME is supported. This upgrade will have
little, if any, effect on non-MIME recipients and will
aid MIME in correctly displaying such messages. It
also provides a smooth transition path to eventual
adoption of other MIME capabilities.
(9) Conforming user agents must ensure that any string of
non-white-space printable US-ASCII characters within a
"*text" or "*ctext" that begins with "=?" and ends with
Freed & Borenstein Standards Track [Page 5]RFC 2049 MIME Conformance November 1996
"?=" be a valid encoded-word. ("begins" means: At the
start of the field-body or immediately following
linear-white-space; "ends" means: At the end of the
field-body or immediately preceding linear-white-
space.) In addition, any "word" within a "phrase" that
begins with "=?" and ends with "?=" must be a valid
encoded-word.
(10) Conforming user agents must be able to distinguish
encoded-words from "text", "ctext", or "word"s,
according to the rules in section 4, anytime they
appear in appropriate places in message headers. It
must support both the "B" and "Q" encodings for any
character set which it supports. The program must be
able to display the unencoded text if the character set
is "US-ASCII". For the ISO-8859-* character sets, the
mail reading program must at least be able to display
the characters which are also in the US-ASCII set.
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.
3. 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.
Freed & Borenstein Standards Track [Page 6]RFC 2049 MIME Conformance November 1996
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 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 or line
truncation imposed by mail transports is STRONGLY
DISCOURAGED, but unavoidable in some cases.
Applications which require long lines must somehow
Freed & Borenstein Standards Track [Page 7]RFC 2049 MIME Conformance November 1996
differentiate between soft and hard line breaks. (A
simple way to do this is to use the 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.
(8) Some mail transport agents will corrupt data that
includes certain literal strings. In particular, a
Freed & Borenstein Standards Track [Page 8]RFC 2049 MIME Conformance November 1996
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.
4. Canonical Encoding Model
There was some confusion, in earlier versions 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 [RFC-1421] 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.
Freed & Borenstein Standards Track [Page 9]RFC 2049 MIME Conformance November 1996
(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 body 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) as needed.
Conversion from entity form to local form is accomplished by
reversing these steps. Note that reversal of these steps may produce
differing results since there is no guarantee that the original and
final local forms are the same.
Freed & Borenstein Standards Track [Page 10]RFC 2049 MIME Conformance November 1996
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.
NOTE: Some confusion has been caused by systems that represent
messages in a format which uses local newline conventions which
differ from the RFC822 CRLF convention. It is important to note that
these formats are not canonical RFC822/MIME. These formats are
instead *encodings* of RFC822, where CRLF sequences in the canonical
representation of the message are encoded as the local newline
convention. Note that formats which encode CRLF sequences as, for
example, LF are not capable of representing MIME messages containing
binary data which contains LF octets not part of CRLF line separation
sequences.
Freed & Borenstein Standards Track [Page 11]RFC 2049 MIME Conformance November 1996
5. 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.
6. Security Considerations
Security issues are discussed in the second document in this set, RFC
2046.
7. Authors' Addresses
For more information, the authors of this document are best contacted
via Internet mail:
Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
Phone: +1 818 919 3600
Fax: +1 818 919 3614
EMail: ned@innosoft.com
Nathaniel S. Borenstein
First Virtual Holdings
25 Washington Avenue
Morristown, NJ 07960
USA
Phone: +1 201 540 8967
Fax: +1 201 993 3032
EMail: nsb@nsb.fv.com
MIME is a result of the work of the Internet Engineering Task Force
Working Group on RFC 822 Extensions. The chairman of that group,
Greg Vaudreuil, may be reached at:
Gregory M. Vaudreuil
Octel Network Services
17080 Dallas Parkway
Dallas, TX 75248-1905
USA
EMail: Greg.Vaudreuil@Octel.Com
Freed & Borenstein Standards Track [Page 12]
RFC 2049 MIME Conformance November 1996
8. 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
Leo Mclaughlin Goli Montaser-Kohsari
Tom Moore John Gardiner Myers
Erik Naggum Mark Needleman
Chris Newman John Noerenberg
Freed & Borenstein Standards Track [Page 13]RFC 2049 MIME Conformance November 1996
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.
Freed & Borenstein Standards Track [Page 14]RFC 2049 MIME Conformance November 1996
Appendix A -- A Complex Multipart Example
What follows is the outline of a complex multipart message. This
message contains five parts that are to be displayed serially: two
introductory plain text objects, an embedded multipart message, a
text/enriched object, and a closing encapsulated text message in a
non-ASCII character set. The embedded multipart message itself
contains two objects to be displayed in parallel, a picture and an
audio fragment.
MIME-Version: 1.0
From: Nathaniel Borenstein <nsb@nsb.fv.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.
--unique-boundary-1
Content-Type: multipart/parallel; boundary=unique-boundary-2
--unique-boundary-2
Content-Type: audio/basic
Freed & Borenstein Standards Track [Page 15]RFC 2049 MIME Conformance November 1996
Content-Transfer-Encoding: base64
... base64-encoded 8000 Hz single-channel
mu-law-format audio data goes here ...
--unique-boundary-2
Content-Type: image/jpeg
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 1896</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--
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.
Freed & Borenstein Standards Track [Page 16]RFC 2049 MIME Conformance November 1996
(2) BNF describing the overall structure of MIME object
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 informal term ASCII in many
parts of these documents.
(5) The informal concept of a primary subtype has been
removed.
(6) The term "object" was being used inconsistently. The
definition of this term has been clarified, along with
the related terms "body", "body part", and "entity",
and usage has been corrected 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.
(8) The prose and BNF describing the multipart media type
have been changed to make it clear that the body parts
within a multipart object 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) "Message/partial" fragmenters are restricted to
splitting MIME objects only at line boundaries.
(11) 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.
Freed & Borenstein Standards Track [Page 17]RFC 2049 MIME Conformance November 1996
(12) 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.
(13) 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."
(14) A typo was fixed that said "application/external-body"
instead of "message/external-body".
(15) The definition of a character set has been reorganized
to make the requirements clearer.
(16) 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.
(17) 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
requires that NUL characters be preserved, which brings
MIME into alignment with real-world implementations.
(18) 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.
(19) The definition of the quoted-printable encoding now
includes a number of suggestions for how quoted-
printable encoders might best handle improperly encoded
material.
(20) Prose was added to clarify the use of the "7bit",
"8bit", and "binary" transfer-encodings on multipart or
message entities encapsulating "8bit" or "binary" data.
Freed & Borenstein Standards Track [Page 18]RFC 2049 MIME Conformance November 1996
(21) 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.
(22) The various restrictions on subtypes of "message" are
now specified entirely on a subtype by subtype basis.
(23) The definition of "message/rfc822" was changed to
indicate that at least one of the "From", "Subject", or
"Date" headers must be present.
(24) 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.
(25) Examples using text/richtext were changed to
text/enriched.
(26) 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.
(27) MIME media types that are simply registered for use and
those that are standardized by the IETF are now
distinguished in the MIME BNF.
(28) All of the various MIME registration procedures have
been extensively revised. IANA registration procedures
for character sets have been moved to a separate
document that is no included in this set of documents.
(29) The use of escape and shift mechanisms in the US-ASCII
and ISO-8859-X character sets these documents define
have been clarified: Such mechanisms should never be
used in conjunction with these character sets and their
effect if they are used is undefined.
(30) The definition of the AFS access-type for
message/external-body has been removed.
(31) The handling of the combination of
multipart/alternative and message/external-body is now
specifically addressed.
Freed & Borenstein Standards Track [Page 19]RFC 2049 MIME Conformance November 1996
(32) Security issues specific to message/external-body are
now discussed in some detail.
Appendix C -- References
[ATK]
Borenstein, Nathaniel S., Multimedia Applications
Development with the Andrew Toolkit, Prentice-Hall, 1990.
[ISO-2022]
International Standard -- Information Processing --
Character Code Structure and Extension Techniques,
ISO/IEC 2022:1994, 4th ed.
[ISO-8859]
International Standard -- Information Processing -- 8-bit
Single-Byte Coded Graphic Character Sets
- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987, 1st ed.
- Part 2: Latin Alphabet No. 2, ISO 8859-2:1987, 1st ed.
- Part 3: Latin Alphabet No. 3, ISO 8859-3:1988, 1st ed.
- Part 4: Latin Alphabet No. 4, ISO 8859-4:1988, 1st ed.
- Part 5: Latin/Cyrillic Alphabet, ISO 8859-5:1988, 1st
ed.
- Part 6: Latin/Arabic Alphabet, ISO 8859-6:1987, 1st ed.
- Part 7: Latin/Greek Alphabet, ISO 8859-7:1987, 1st ed.
- Part 8: Latin/Hebrew Alphabet, ISO 8859-8:1988, 1st ed.
- Part 9: Latin Alphabet No. 5, ISO/IEC 8859-9:1989, 1st
ed.
International Standard -- Information Technology -- 8-bit
Single-Byte Coded Graphic Character Sets
- Part 10: Latin Alphabet No. 6, ISO/IEC 8859-10:1992,
1st ed.
[ISO-646]
International Standard -- Information Technology -- ISO
7-bit Coded Character Set for Information Interchange,
ISO 646:1991, 3rd ed..
[JPEG]
JPEG Draft Standard ISO 10918-1 CD.
[MPEG]
Video Coding Draft Standard ISO 11172 CD, ISO
IEC/JTC1/SC2/WG11 (Motion Picture Experts Group), May,
1991.
Freed & Borenstein Standards Track [Page 20]RFC 2049 MIME Conformance November 1996
[PCM]
CCITT, Fascicle III.4 - Recommendation G.711, "Pulse Code
Modulation (PCM) of Voice Frequencies", Geneva, 1972.
[POSTSCRIPT]
Adobe Systems, Inc., PostScript Language Reference
Manual, Addison-Wesley, 1985.
[POSTSCRIPT2]
Adobe Systems, Inc., PostScript Language Reference
Manual, Addison-Wesley, Second Ed., 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.
[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.
Freed & Borenstein Standards Track [Page 21]RFC 2049 MIME Conformance November 1996
[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.
[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.
Freed & Borenstein Standards Track [Page 22]RFC 2049 MIME Conformance November 1996
[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-1556]
Nussbacher, H., "Handling of Bi-directional Texts in
MIME", RFC 1556, Israeli Inter-University Computer
Center, December 1993.
[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.
[RFC-1700]
Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
RFC 1700, USC/Information Sciences Institute, October
1994.
[RFC-1741]
Faltstrom, P., Crocker, D., and Fair, E., "MIME Content
Type for BinHex Encoded Files", December 1994.
[RFC-1896]
Resnick, P., and A. Walker, "The text/enriched MIME
Content-type", RFC 1896, February, 1996.
Freed & Borenstein Standards Track [Page 23]RFC 2049 MIME Conformance November 1996
[RFC-2045]
Freed, N., and and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, Innosoft, First Virtual Holdings,
November 1996.
[RFC-2046]
Freed, N., and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
Innosoft, First Virtual Holdings, November 1996.
[RFC-2047]
Moore, K., "Multipurpose Internet Mail Extensions (MIME)
Part Three: Representation of Non-ASCII Text in Internet
Message Headers", RFC 2047, University of
Tennessee, November 1996.
[RFC-2048]
Freed, N., Klensin, J., and J. Postel, "Multipurpose
Internet Mail Extensions (MIME) Part Four: MIME
Registration Procedures", RFC 2048, Innosoft, MCI,
ISI, November 1996.
[RFC-2049]
Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049 (this document), Innosoft, First
Virtual Holdings, November 1996.
[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.
Freed & Borenstein Standards Track [Page 24]
