have it appear to the recipient as a simple audio message
rather than as an encapsulated message containing an audio
message. That is, the encapsulation of the message is
considered to be "transparent".
When generating and reassembling the parts of a
message/partial message, the headers of the encapsulated
message must be merged with the headers of the enclosing
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entities. In this process the following rules must be
observed:
(1) All of the headers from the initial enclosing
entity (part one), except those that start with
"Content-" and "Message-ID", must be copied, in
order, to the new message.
(2) Only those headers in the enclosed message
which start with "Content-" and "Message-ID" must
be appended, in order, to the headers of the new
message. Any headers in the enclosed message
which do not start with "Content-" (except for
"Message-ID") will be ignored.
(3) All of the headers from the second and any
subsequent messages will be ignored.
For example, if an audio message is broken into two parts,
the first part might look something like this:
X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
Message-ID: id1@host.com
MIME-Version: 1.0
Content-type: message/partial;
id="ABC@host.com";
number=1; total=2
X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Message-ID: anotherid@foo.com
Content-type: audio/basic
Content-transfer-encoding: base64
... first half of encoded audio data goes here...
and the second half might look something like this:
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
MIME-Version: 1.0
Message-ID: id2@host.com
Content-type: message/partial;
id="ABC@host.com"; number=2; total=2
... second half of encoded audio data goes here...
Then, when the fragmented message is reassembled, the
resulting message to be displayed to the user should look
something like this:
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X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail
Message-ID: anotherid@foo.com
MIME-Version: 1.0
Content-type: audio/basic
Content-transfer-encoding: base64
... first half of encoded audio data goes here...
... second half of encoded audio data goes here...
It should be noted that, because some message transfer
agents may choose to automatically fragment large messages,
and because such agents may use different fragmentation
thresholds, it is possible that the pieces of a partial
message, upon reassembly, may prove themselves to comprise a
partial message. This is explicitly permitted.
It should also be noted that the inclusion of a "References"
field in the headers of the second and subsequent pieces of
a fragmented message that references the Message-Id on the
previous piece may be of benefit to mail readers that
understand and track references. However, the generation of
such "References" fields is entirely optional.
7.3.3 The Message/External-Body subtype
The external-body subtype indicates that the actual body
data are not included, but merely referenced. In this case,
the parameters describe a mechanism for accessing the
external data.
When a message body or body part is of type
"message/external-body", it consists of a header, two
consecutive CRLFs, and the message header for the
encapsulated message. If another pair of consecutive CRLFs
appears, this of course ends the message header for the
encapsulated message. However, since the encapsulated
message's body is itself external, it does NOT appear in the
area that follows. For example, consider the following
message:
Content-type: message/external-body; access-
type=local-file;
name=/u/nsb/Me.gif
Content-type: image/gif
THIS IS NOT REALLY THE BODY!
The area at the end, which might be called the "phantom
body", is ignored for most external-body messages. However,
it may be used to contain auxilliary information for some
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such messages, as indeed it is when the access-type is
"mail-server". Of the access-types defined by this
document, the phantom body is used only when the access-type
is "mail-server". In all other cases, the phantom body is
ignored.
The only always-mandatory parameter for message/external-
body is "access-type"; all of the other parameters may be
mandatory or optional depending on the value of access-type.
ACCESS-TYPE -- One or more case-insensitive words,
comma-separated, indicating supported access
mechanisms by which the file or data may be
obtained. Values include, but are not limited to,
"FTP", "ANON-FTP", "TFTP", "AFS", "LOCAL-FILE",
and "MAIL-SERVER". Future values, except for
experimental values beginning with "X-", must be
registered with IANA, as described in Appendix F .
In addition, the following two parameters are optional for
ALL access-types:
EXPIRATION -- The date (in the RFC822 "date-time"
syntax, as extended by RFC1123 to permit 4 digits
in the date field) after which the existence of
the external data is not guaranteed.
SIZE -- The size (in octets) of the data. The
intent of this parameter is to help the recipient
decide whether or not to expend the necessary
resources to retrieve the external data.
PERMISSION -- A field that indicates whether or
not it is expected that clients might also attempt
to overwrite the data. By default, or if
permission is "read", the assumption is that they
are not, and that if the data is retrieved once,
it is never needed again. If PERMISSION is "read-
write", this assumption is invalid, and any local
copy must be considered no more than a cache.
"Read" and "Read-write" are the only defined
values of permission.
The precise semantics of the access-types defined here are
described in the sections that follow.
7.3.3.1 The "ftp" and "tftp" access-types
An access-type of FTP or TFTP indicates that the message
body is accessible as a file using the FTP [RFC-959] or TFTP
[RFC-783] protocols, respectively. For these access-types,
the following additional parameters are mandatory:
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NAME -- The name of the file that contains the
actual body data.
SITE -- A machine from which the file may be
obtained, using the given protocol
Before the data is retrieved, using these protocols, the
user will generally need to be asked to provide a login id
and a password for the machine named by the site parameter.
In addition, the following optional parameters may also
appear when the access-type is FTP or ANON-FTP:
DIRECTORY -- A directory from which the data named
by NAME should be retrieved.
MODE -- A transfer mode for retrieving the
information, e.g. "image".
7.3.3.2 The "anon-ftp" access-type
The "anon-ftp" access-type is identical to the "ftp" access
type, except that the user need not be asked to provide a
name and password for the specified site. Instead, the ftp
protocol will be used with login "anonymous" and a password
that corresponds to the user's email address.
7.3.3.3 The "local-file" and "afs" access-types
An access-type of "local-file" indicates that the actual
body is accessible as a file on the local machine. An
access-type of "afs" indicates that the file is accessible
via the global AFS file system. In both cases, only a
single parameter is required:
NAME -- The name of the file that contains the
actual body data.
The following optional parameter may be used to describe the
locality of reference for the data, that is, the site or
sites at which the file is expected to be visible:
SITE -- A domain specifier for a machine or set of
machines that are known to have access to the data
file. Asterisks may be used for wildcard matching
to a part of a domain name, such as
"*.bellcore.com", to indicate a set of machines on
which the data should be directly visible, while a
single asterisk may be used to indicate a file
that is expected to be universally available,
e.g., via a global file system.
7.3.3.4 The "mail-server" access-type
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The "mail-server" access-type indicates that the actual body
is available from a mail server. The mandatory parameter
for this access-type is:
SERVER -- The email address of the mail server
from which the actual body data can be obtained.
Because mail servers accept a variety of syntax, some of
which is multiline, the full command to be sent to a mail
server is not included as a parameter on the content-type
line. Instead, it may be provided as the "phantom body"
when the content-type is message/external-body and the
access-type is mail-server.
Note that MIME does not define a mail server syntax.
Rather, it allows the inclusion of arbitrary mail server
commands in the phantom body. Implementations should
include the phantom body in the body of the message it sends
to the mail server address to retrieve the relevant data.
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7.3.3.5 Examples and Further Explanations
With the emerging possibility of very wide-area file
systems, it becomes very hard to know in advance the set of
machines where a file will and will not be accessible
directly from the file system. Therefore it may make sense
to provide both a file name, to be tried directly, and the
name of one or more sites from which the file is known to be
accessible. An implementation can try to retrieve remote
files using FTP or any other protocol, using anonymous file
retrieval or prompting the user for the necessary name and
password. If an external body is accessible via multiple
mechanisms, the sender may include multiple parts of type
message/external-body within an entity of type
multipart/alternative.
However, the external-body mechanism is not intended to be
limited to file retrieval, as shown by the mail-server
access-type. Beyond this, one can imagine, for example,
using a video server for external references to video clips.
If an entity is of type "message/external-body", then the
body of the entity will contain the header fields of the
encapsulated message. The body itself is to be found in the
external location. This means that if the body of the
"message/external-body" message contains two consecutive
CRLFs, everything after those pairs is NOT part of the
message itself. For most message/external-body messages,
this trailing area must simply be ignored. However, it is a
convenient place for additional data that cannot be included
in the content-type header field. In particular, if the
"access-type" value is "mail-server", then the trailing area
must contain commands to be sent to the mail server at the
address given by NAME@SITE, where NAME and SITE are the
values of the NAME and SITE parameters, respectively.
The embedded message header fields which appear in the body
of the message/external-body data can be used to declare the
Content-type of the external body. Thus a complete
message/external-body message, referring to a document in
PostScript format, might look like this:
From: Whomever
Subject: whatever
MIME-Version: 1.0
Message-ID: id1@host.com
Content-Type: multipart/alternative; boundary=42
--42
Content-Type: message/external-body;
name="BodyFormats.ps";
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site="thumper.bellcore.com";
access-type=ANON-FTP;
directory="pub";
mode="image";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42
Content-Type: message/external-body;
name="/u/nsb/writing/rfcs/RFC-XXXX.ps";
site="thumper.bellcore.com";
access-type=AFS
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
--42
Content-Type: message/external-body;
access-type=mail-server
server="listserv@bogus.bitnet";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
get rfc-xxxx doc
--42--
Like the message/partial type, the message/external-body
type is intended to be transparent, that is, to convey the
data type in the external body rather than to convey a
message with a body of that type. Thus the headers on the
outer and inner parts must be merged using the same rules as
for message/partial. In particular, this means that the
Content-type header is overridden, but the From and Subject
headers are preserved.
Note that since the external bodies are not transported as
mail, they need not conform to the 7-bit and line length
requirements, but might in fact be binary files. Thus a
Content-Transfer-Encoding is not generally necessary, though
it is permitted.
Note that the body of a message of type "message/external-
body" is governed by the basic syntax for an RFC822
message. In particular, anything before the first
consecutive pair of CRLFs is header information, while
anything after it is body information, which is ignored for
most access-types.
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7.4 The Application Content-Type
The "application" Content-Type is to be used for data which
do not fit in any of the other categories, and particularly
for data to be processed by mail-based uses of application
programs. This is information which must be processed by an
application before it is viewable or usable to a user.
Expected uses for Content-Type application include mail-
based file transfer, spreadsheets, data for mail-based
scheduling systems, and languages for "active"
(computational) email. (The latter, in particular, can pose
security problems which should be understood by
implementors, and are considered in detail in the discussion
of the application/PostScript content-type.)
For example, a meeting scheduler might define a standard
representation for information about proposed meeting dates.
An intelligent user agent would use this information to
conduct a dialog with the user, and might then send further
mail based on that dialog. More generally, there have been
several "active" messaging languages developed in which
programs in a suitably specialized language are sent through
the mail and automatically run in the recipient's
environment.
Such applications may be defined as subtypes of the
"application" Content-Type. This document defines three
subtypes: octet-stream, ODA, and PostScript.
In general, the subtype of application will often be the
name of the application for which the data are intended.
This does not mean, however, that any application program
name may be used freely as a subtype of application. Such
usages must be registered with IANA, as described in
Appendix F.
7.4.1 The Application/Octet-Stream (primary) subtype
The primary subtype of application, "octet-stream", may be
used to indicate that a body contains binary data. The set
of possible parameters includes, but is not limited to:
NAME -- a suggested name for the binary data if
stored as a file.
TYPE -- the general type or category of binary
data. This is intended as information for the
human recipient rather than for any automatic
processing.
CONVERSIONS -- the set of operations that have
been performed on the data before putting it in
the mail (and before any Content-Transfer-Encoding
that might have been applied). If multiple
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conversions have occurred, they must be separated
by commas and specified in the order they were
applied -- that is, the leftmost conversion must
have occurred first, and conversions are undone
from right to left. Note that NO conversion
values are defined by this document. Any
conversion values that that do not begin with "X-"
must be preceded by a published specification and
by registration with IANA, as described in
Appendix F.
PADDING -- the number of bits of padding that were
appended to the bitstream comprising the actual
contents to produce the enclosed byte-oriented
data. This is useful for enclosing a bitstream in
a body when the total number of bits is not a
multiple of the byte size.
The values for these attributes are left undefined at
present, but may require specification in the future. An
example of a common (though UNIX-specific) usage might be:
Content-Type: application/octet-stream;
name=foo.tar.Z; type=tar;
conversions="x-encrypt,x-compress"
However, it should be noted that the use of such conversions
is explicitly discouraged due to a lack of portability and
standardization. The use of uuencode is particularly
discouraged, in favor of the Content-Transfer-Encoding
mechanism, which is both more standardized and more portable
across mail boundaries.
The recommended action for an implementation that receives
application/octet-stream mail is to simply offer to put the
data in a file, with any Content-Transfer-Encoding undone,
or perhaps to use it as input to a user-specified process.
To reduce the danger of transmitting rogue programs through
the mail, it is strongly recommended that implementations
NOT implement a path-search mechanism whereby an arbitrary
program named in the Content-Type parameter (e.g., an
"interpreter=" parameter) is found and executed using the
mail body as input.
7.4.2 The Application/PostScript subtype
A Content-Type of "application/postscript" indicates a
PostScript program. The language is defined in
[POSTSCRIPT]. It is recommended that Postscript as sent
through email should use Postscript document structuring
conventions if at all possible, and correctly.
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The execution of general-purpose PostScript interpreters
entails serious security risks, and implementors are
discouraged from simply sending PostScript email bodies to
"off-the-shelf" interpreters. While it is usually safe to
send PostScript to a printer, where the potential for harm
is greatly constrained, implementors should consider all of
the following before they add interactive display of
PostScript bodies to their mail readers.
The remainder of this section outlines some, though probably
not all, of the possible problems with sending PostScript
through the mail.
Dangerous operations in the PostScript language include, but
may not be limited to, the PostScript operators deletefile,
renamefile, filenameforall, and file. File is only
dangerous when applied to something other than standard
input or output. Implementations may also define additional
nonstandard file operators; these may also pose a threat to
security. Filenameforall, the wildcard file search
operator, may appear at first glance to be harmless. Note,
however, that this operator has the potential to reveal
information about what files the recipient has access to,
and this information may itself be sensitive. Message
senders should avoid the use of potentially dangerous file
operators, since these operators are quite likely to be
unavailable in secure PostScript implementations. Message-
receiving and -displaying software should either completely
disable all potentially dangerous file operators or take
special care not to delegate any special authority to their
operation. These operators should be viewed as being done by
an outside agency when interpreting PostScript documents.
Such disabling and/or checking should be done completely
outside of the reach of the PostScript language itself; care
should be taken to insure that no method exists for
reenabling full-function versions of these operators.
The PostScript language provides facilities for exiting the
normal interpreter, or server, loop. Changes made in this
"outer" environment are customarily retained across
documents, and may in some cases be retained semipermanently
in nonvolatile memory. The operators associated with exiting
the interpreter loop have the potential to interfere with
subsequent document processing. As such, their unrestrained
use constitutes a threat of service denial. PostScript
operators that exit the interpreter loop include, but may
not be limited to, the exitserver and startjob operators.
Message-sending software should not generate PostScript that
depends on exiting the interpreter loop to operate. The
ability to exit will probably be unavailable in secure
PostScript implementations. Message-receiving and
-displaying software should, if possible, disable the
ability to make retained changes to the PostScript
environment. Eliminate the startjob and exitserver commands.
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If these commands cannot be eliminated, at least set the
password associated with them to a hard-to-guess value.
PostScript provides operators for setting system-wide and
device-specific parameters. These parameter settings may be
retained across jobs and may potentially pose a threat to
the correct operation of the interpreter. The PostScript
operators that set system and device parameters include, but
may not be limited to, the setsystemparams and setdevparams
operators. Message-sending software should not generate
PostScript that depends on the setting of system or device
parameters to operate correctly. The ability to set these
parameters will probably be unavailable in secure PostScript
implementations. Message-receiving and -displaying software
should, if possible, disable the ability to change system
and device parameters. If these operators cannot be
disabled, at least set the password associated with them to
a hard-to-guess value.
Some PostScript implementations provide nonstandard
facilities for the direct loading and execution of machine
code. Such facilities are quite obviously open to
substantial abuse. Message-sending software should not
make use of such features. Besides being totally hardware-
specific, they are also likely to be unavailable in secure
implementations of PostScript. Message-receiving and
-displaying software should not allow such operators to be
used if they exist.
PostScript is an extensible language, and many, if not most,
implementations of it provide a number of their own
extensions. This document does not deal with such extensions
explicitly since they constitute an unknown factor.
Message-sending software should not make use of nonstandard
extensions; they are likely to be missing from some
implementations. Message-receiving and -displaying software
should make sure that any nonstandard PostScript operators
are secure and don't present any kind of threat.
It is possible to write PostScript that consumes huge
amounts of various system resources. It is also possible to
write PostScript programs that loop infinitely. Both types
of programs have the potential to cause damage if sent to
unsuspecting recipients. Message-sending software should
avoid the construction and dissemination of such programs,
which is antisocial. Message-receiving and -displaying
software should provide appropriate mechanisms to abort
processing of a document after a reasonable amount of time
has elapsed. In addition, PostScript interpreters should be
limited to the consumption of only a reasonable amount of
any given system resource.
Finally, bugs may exist in some PostScript interpreters
which could possibly be exploited to gain unauthorized
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access to a recipient's system. Apart from noting this
possibility, there is no specific action to take to prevent
this, apart from the timely correction of such bugs if any
are found.
7.4.3 The Application/ODA subtype
The "ODA" subtype of application is used to indicate that a
body contains information encoded according to the Office
Document Architecture [ODA] standards, using the ODIF
representation format. For application/oda, the Content-
Type line should also specify an attribute/value pair that
indicates the document application profile (DAP), using the
key word "profile". Thus an appropriate header field might
look like this:
Content-Type: application/oda; profile=Q112
Consult the ODA standard [ODA] for further information.
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7.5 The Image Content-Type
A Content-Type of "image" indicates that the bodycontains an
image. The subtype names the specific image format. These
names are case insensitive. Two initial subtypes are "jpeg"
for the JPEG format, JFIF encoding, and "gif" for GIF format
[GIF].
The list of image subtypes given here is neither exclusive
nor exhaustive, and is expected to grow as more types are
registered with IANA, as described in Appendix F.
7.6 The Audio Content-Type
A Content-Type of "audio" indicates that the body contains
audio data. Although there is not yet a consensus on an
"ideal" audio format for use with computers, there is a
pressing need for a format capable of providing
interoperable behavior.
The initial subtype of "basic" is specified to meet this
requirement by providing an absolutely minimal lowest common
denominator audio format. It is expected that richer
formats for higher quality and/or lower bandwidth audio will
be defined by a later document.
The content of the "audio/basic" subtype is audio encoded
using 8-bit ISDN u-law [PCM]. When this subtype is present,
a sample rate of 8000 Hz and a single channel is assumed.
7.7 The Video Content-Type
A Content-Type of "video" indicates that the body contains a
time-varying-picture image, possibly with color and
coordinated sound. The term "video" is used extremely
generically, rather than with reference to any particular
technology or format, and is not meant to preclude subtypes
such as animated drawings encoded compactly. The subtype
"mpeg" refers to video coded according to the MPEG standard
[MPEG].
Note that although in general this document strongly
discourages the mixing of multiple media in a single body,
it is recognized that many so-called "video" formats include
a representation for synchronized audio, and this is
explicitly permitted for subtypes of "video".
7.8 Experimental Content-Type Values
A Content-Type value beginning with the characters "X-" is a
private value, to be used by consenting mail systems by
mutual agreement. Any format without a rigorous and public
definition must be named with an "X-" prefix, and publicly
specified values shall never begin with "X-". (Older
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versions of the widely-used Andrew system use the "X-BE2"
name, so new systems should probably choose a different
name.)
In general, the use of "X-" top-level types is strongly
discouraged. Implementors should invent subtypes of the
existing types whenever possible. The invention of new
types is intended to be restricted primarily to the
development of new media types for email, such as digital
odors or holography, and not for new data formats in
general. In many cases, a subtype of application will be
more appropriate than a new top-level type.
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Summary
Using the MIME-Version, Content-Type, and Content-Transfer-
Encoding header fields, it is possible to include, in a
standardized way, arbitrary types of data objects with RFC
822 conformant mail messages. No restrictions imposed by
either RFC821 or RFC822 are violated, and care has been
taken to avoid problems caused by additional restrictions
imposed by the characteristics of some Internet mail
transport mechanisms (see Appendix B). The "multipart" and
"message" Content-Types allow mixing and hierarchical
structuring of objects of different types in a single
message. Further Content-Types provide a standardized
mechanism for tagging messages or body parts as audio,
image, or several other kinds of data. A distinguished
parameter syntax allows further specification of data format
details, particularly the specification of alternate
character sets. Additional optional header fields provide
mechanisms for certain extensions deemed desirable by many
implementors. Finally, a number of useful Content-Types are
defined for general use by consenting user agents, notably
text/richtext, message/partial, and message/external-body.
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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 Timo Lehtinen
Randall Atkinson John R. MacMillan
Philippe Brandon Rick McGowan
Kevin Carosso Leo Mclaughlin
Uhhyung Choi Goli Montaser-Kohsari
Cristian Constantinof Keith Moore
Mark Crispin Tom Moore
Dave Crocker Erik Naggum
Terry Crowley Mark Needleman
Walt Daniels John Noerenberg
Frank Dawson Mats Ohrman
Hitoshi Doi Julian Onions
Kevin Donnelly Michael Patton
Keith Edwards David J. Pepper
Chris Eich Blake C. Ramsdell
Johnny Eriksson Luc Rooijakkers
Craig Everhart Marshall T. Rose
Patrik Faeltstroem Jonathan Rosenberg
Erik E. Fair Jan Rynning
Roger Fajman Harri Salminen
Alain Fontaine Michael Sanderson
James M. Galvin Masahiro Sekiguchi
Philip Gladstone Mark Sherman
Thomas Gordon Keld Simonsen
Phill Gross Bob Smart
James Hamilton Peter Speck
Steve Hardcastle-Kille Henry Spencer
David Herron Einar Stefferud
Bruce Howard Michael Stein
Bill Janssen Klaus Steinberger
Olle Jaernefors Peter Svanberg
Risto Kankkunen James Thompson
Phil Karn Steve Uhler
Alan Katz Stuart Vance
Tim Kehres Erik van der Poel
Neil Katin Guido van Rossum
Kyuho Kim Peter Vanderbilt
Anders Klemets Greg Vaudreuil
John Klensin Ed Vielmetti
Valdis Kletniek Ryan Waldron
Jim Knowles Wally Wedel
Stev Knowles Sven-Ove Westberg
Bob Kummerfeld Brian Wideen
Borenstein & Freed [Page 54]
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Pekka Kytolaakso John Wobus
Stellan Lagerstr.m Glenn Wright
Vincent Lau Rayan Zachariassen
Donald Lindsay David Zimmerman
The authors apologize for any omissions from this list,
which are certainly unintentional.
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Appendix A -- Minimal MIME-Conformance
The mechanisms described in this document are open-ended.
It is definitely not expected that all implementations will
support all of the Content-Types described, 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.
2. Recognize the Content-Transfer-Encoding header
field, and decode all received data encoded with
either the quoted-printable or base64
implementations. Encode any data sent that is
not in seven-bit mail-ready representation using
one of these transformations and include the
appropriate Content-Transfer-Encoding header
field, unless the underlying transport mechanism
supports non-seven-bit data, as SMTP does not.
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 Content-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 0-127.
-- For unrecognized subtypes, show or
offer to show the user the "raw"
version of the data. An ability at
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least to convert "text/richtext" to
plain text, as shown in Appendix D,
is encouraged, but not required for
conformance.
Message:
--Recognize and display at least the
primary (822) encapsulation.
Multipart:
-- Recognize the primary (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.
-- Treat any unrecognized subtypes as if
they were "mixed".
Application:
-- Offer the ability to remove either of
the two types of Content-Transfer-
Encoding defined in this document
and put the resulting information
in a user file.
5. Upon encountering any unrecognized Content-
Type, an implementation must treat it as if it had
a Content-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.
Unrecognized predefined types, which in a MIME-
conformant mailer might still include audio,
image, or video, should also be treated in this
way.
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
RFC821 and RFC822. User agents that are MIME-conformant
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have the additional guarantee that the user will not be
shown data that were never intended to be viewed as text.
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RFC1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
Appendix B -- General 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 such a way 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 (Content-Type) that will 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 body
parts. As such, the persistence of CRLF as
something other than a line break should 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
should not be relied on.
(3) 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 ASCII
character set. Such conversion is STRONGLY
DISCOURAGED, but it may occur, and mail formats
should not rely on the persistence of TAB (HT)
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characters.
(4) 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 should somehow differentiate
between soft and hard line breaks. (A simple way
to do this is to use the quoted-printable
encoding.)
(5) 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, should not be
relied on.
(6) Many mail domains use variations on the 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 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:
"'" (ASCII code 39)
"(" (ASCII code 40)
")" (ASCII code 41)
"+" (ASCII code 43)
"," (ASCII code 44)
"-" (ASCII code 45)
"." (ASCII code 46)
"/" (ASCII code 47)
":" (ASCII code 58)
"=" (ASCII code 61)
"?" (ASCII code 63)
A maximally portable mail representation, such as
the base64 encoding, will confine itself to
relatively short lines of text in which the only
meaningful characters are taken from this set of
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73 characters.
Please note that the above list is NOT a list of recommended
practices for MTAs. RFC821 MTAs are prohibited from
altering the character of white space or wrapping long
lines. These BAD and illegal practices are known to occur
on established networks, and implementions should be robust
in dealing with the bad effects they can cause.
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Appendix C -- 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 richtext 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>
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 preceding blank line means
no header fields were given and this is text,
with charset US ASCII. 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
Content-Transfer-Encoding: base64
... base64-encoded 8000 Hz single-channel
u-law-format audio data goes here....
--unique-boundary-2
Content-Type: image/gif
Content-Transfer-Encoding: Base64
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... base64-encoded image data goes here....
--unique-boundary-2--
--unique-boundary-1
Content-type: text/richtext
This is <bold><italic>richtext.</italic></bold>
<nl><nl>Isn't it
<bigger><bigger>cool?</bigger></bigger>
--unique-boundary-1
Content-Type: message/rfc822
From: (name 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 D -- A Simple Richtext-to-Text Translator in C
One of the major goals in the design of the richtext subtype
of the text Content-Type is to make formatted text so simple
that even text-only mailers will implement richtext-to-
plain-text translators, thus increasing the likelihood that
multifont text will become "safe" to use very widely. To
demonstrate this simplicity, what follows is an extremely
simple 44-line C program that converts richtext input into
plain text output:
#include <stdio.h>
#include <ctype.h>
main() {
int c, i;
char token[50];
while((c = getc(stdin)) != EOF) {
if (c == '<') {
for (i=0; (i<49 && (c = getc(stdin)) != '>'
&& c != EOF); ++i) {
token[i] = isupper(c) ? tolower(c) : c;
}
if (c == EOF) break;
if (c != '>') while ((c = getc(stdin)) !=
'>'
&& c != EOF) {;}
if (c == EOF) break;
token[i] = '\0';
if (!strcmp(token, "lt")) {
putc('<', stdout);
} else if (!strcmp(token, "nl")) {
putc('\n', stdout);
} else if (!strcmp(token, "/paragraph")) {
fputs("\n\n", stdout);
} else if (!strcmp(token, "comment")) {
int commct=1;
while (commct > 0) {
while ((c = getc(stdin)) != '<'
&& c != EOF) ;
if (c == EOF) break;
for (i=0; (c = getc(stdin)) != '>'
&& c != EOF; ++i) {
token[i] = isupper(c) ?
tolower(c) : c;
}
if (c== EOF) break;
token[i] = NULL;
if (!strcmp(token, "/comment")) --
commct;
if (!strcmp(token, "comment"))
++commct;
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}
} /* Ignore all other tokens */
} else if (c != '\n') putc(c, stdout);
}
putc('\n', stdout); /* for good measure */
}
It should be noted that one can do considerably better than
this in displaying richtext data on a dumb terminal. In
particular, one can replace font information such as "bold"
with textual emphasis (like *this* or _T_H_I_S_). One can
also properly handle the richtext formatting commands
regarding indentation, justification, and others. However,
the above program is all that is necessary in order to
present richtext on a dumb terminal.
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Appendix E -- Collected Grammar
This appendix contains the complete BNF grammar for all the
syntax specified by this document.
By itself, however, this grammar is incomplete. It refers
to several entities that are defined by RFC822. Rather
than reproduce those definitions here, and risk
unintentional differences between the two, this document
simply refers the reader to RFC822 for the remaining
definitions. Wherever a term is undefined, it refers to the