RFC 1341
MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies
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
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.
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.
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
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.
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
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.
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 RFC 821 or RFC 822 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.
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
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.
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
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
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.
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)
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
73 characters.
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 illegal practices are known to occur
on established networks, and implementions should be robust
in dealing with the bad effects they can cause.
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
... 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--
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;
}
} /* 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.
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 RFC 822. Rather than reproduce those definitions here, and risk unintentional differences between the two, this document simply refers the reader to RFC 822 for the remaining definitions. Wherever a term is undefined, it refers to the RFC 822 definition. attribute := token body-part = <"message" as defined in RFC 822, with all header fields optional, and with the specified delimiter not occurring anywhere in the message body, either on a line by itself or as a substring anywhere.> boundary := 0*69<bchars> bcharsnospace bchars := bcharsnospace / " " bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_" / "," / "-" / "." / "/" / ":" / "=" / "?" close-delimiter := delimiter "--" Content-Description := *text Content-ID := msg-id Content-Transfer-Encoding := "BASE64" / "QUOTED- PRINTABLE" / "8BIT" / "7BIT" / "BINARY" / x-token Content-Type := type "/" subtype *[";" parameter] delimiter := CRLF "--" boundary ; taken from Content-Type field. ; when content-type is multipart ; There should be no space ; between "--" and boundary. encapsulation := delimiter CRLF body-part epilogue := *text ; to be ignored upon receipt.
MIME-Version := 1*text
multipart-body := preamble 1*encapsulation close-delimiter
epilogue
parameter := attribute "=" value
preamble := *text ; to be ignored upon
receipt.
subtype := token
token := 1*<any CHAR except SPACE, CTLs, or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in
/ "," / ";" / ":" / "\" / <"> ; quoted-string,
/ "/" / "[" / "]" / "?" / "." ; to use within
/ "=" ; parameter values
type := "application" / "audio" ; case-
insensitive
/ "image" / "message"
/ "multipart" / "text"
/ "video" / x-token
value := token / quoted-string
x-token := <The two characters "X-" followed, with no
intervening white space, by any token>
Appendix F -- IANA Registration Procedures MIME has been carefully designed to have extensible mechanisms, and it is expected that the set of content- type/subtype pairs and their associated parameters will grow significantly with time. Several other MIME fields, notably character set names, access-type parameters for the message/external-body type, conversions parameters for the application type, and possibly even Content-Transfer- Encoding values, are likely to have new values defined over time. In order to ensure that the set of such values is developed in an orderly, well-specified, and public manner, MIME defines a registration process which uses the Internet Assigned Numbers Authority (IANA) as a central registry for such values. In general, parameters in the content-type header field are used to convey supplemental information for various content types, and their use is defined when the content-type and subtype are defined. New parameters should not be defined as a way to introduce new functionality. In order to simplify and standardize the registration process, this appendix gives templates for the registration of new values with IANA. Each of these is given in the form of an email message template, to be filled in by the registering party. F.1 Registration of New Content-type/subtype Values Note that MIME is generally expected to be extended by subtypes. If a new fundamental top-level type is needed, its specification should be published as an RFC or submitted in a form suitable to become an RFC, and be subject to the Internet standards process. To: IANA@isi.edu Subject: Registration of new MIME content-type/subtype MIME type name: (If the above is not an existing top-level MIME type, please explain why an existing type cannot be used.) MIME subtype name: Required parameters: Optional parameters: Encoding considerations: Security considerations:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be if a new top-level type is being defined, and
must be a publicly available specification in any
case.)
Person & email address to contact for further
information:
F.2 Registration of New Character Set Values
To: IANA@isi.edu
Subject: Registration of new MIME character set value
MIME character set name:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be or an international standard.)
Person & email address to contact for further
information:
F.3 Registration of New Access-type Values for
Message/external-body
To: IANA@isi.edu
Subject: Registration of new MIME Access-type for
Message/external-body content-type
MIME access-type name:
Required parameters:
Optional parameters:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be.)
Person & email address to contact for further
information:
F.4 Registration of New Conversions Values for Application
To: IANA@isi.edu
Subject: Registration of new MIME Conversions value
for Application content-type
MIME Conversions name:
Published specification:
(The published specification must be an Internet RFC or
RFC-to-be.)
Person & email address to contact for further
information:
Appendix G -- Summary of the Seven Content-types Content-type: text Subtypes defined by this document: plain, richtext Important Parameters: charset Encoding notes: quoted-printable generally preferred if an encoding is needed and the character set is mostly an ASCII superset. Security considerations: Rich text formats such as TeX and Troff often contain mechanisms for executing arbitrary commands or file system operations, and should not be used automatically unless these security problems have been addressed. Even plain text may contain control characters that can be used to exploit the capabilities of "intelligent" terminals and cause security violations. User interfaces designed to run on such terminals should be aware of and try to prevent such problems. ________________________________________________________________ Content-type: multipart Subtypes defined by this document: mixed, alternative, digest, parallel. Important Parameters: boundary Encoding notes: No content-transfer-encoding is permitted. ________________________________________________________________ Content-type: message Subtypes defined by this document: rfc822, partial, external-body Important Parameters: id, number, total Encoding notes: No content-transfer-encoding is permitted. ________________________________________________________________ Content-type: application Subtypes defined by this document: octet-stream, postscript, oda Important Parameters: profile
Encoding notes: base64 generally preferred for octet-stream
or other unreadable subtypes.
Security considerations: This type is intended for the
transmission of data to be interpreted by locally-installed
programs. If used, for example, to transmit executable
binary programs or programs in general-purpose interpreted
languages, such as LISP programs or shell scripts, severe
security problems could result. In general, authors of
mail-reading agents are cautioned against giving their
systems the power to execute mail-based application data
without carefully considering the security implications.
While it is certainly possible to define safe application
formats and even safe interpreters for unsafe formats, each
interpreter should be evaluated separately for possible
security problems.
________________________________________________________________
Content-type: image
Subtypes defined by this document: jpeg, gif
Important Parameters: none
Encoding notes: base64 generally preferred
________________________________________________________________
Content-type: audio
Subtypes defined by this document: basic
Important Parameters: none
Encoding notes: base64 generally preferred
________________________________________________________________
Content-type: video
Subtypes defined by this document: mpeg
Important Parameters: none
Encoding notes: base64 generally preferred
Appendix H -- Canonical Encoding Model There was some confusion, in earlier drafts of this memo, 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 message part can be modelled as being done in a number of steps. Note that these steps are roughly similar to those steps used in RFC1113: Step 1. Creation of local form. The body part 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 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. Step 2. Conversion to canonical form. The entire body part, including "out-of-band" information such as record lengths and possibly file attribute information, is converted to a universal canonical form. The specific content type of the body part 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 content types. 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 RFC822. Note that the restriction on line lengths implied by RFC822 is eliminated if the next step employs either quoted- printable or base64 encoding. Step 3. Apply transfer encoding. A Content-Transfer-Encoding appropriate for this body part is applied. Note that there is no fixed relationship between the content 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 body part.
Step 4. Insertion into message.
The encoded object is inserted into a MIME message with
appropriate body part headers and boundary markers.
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 bodyparts 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 compliant with
the formats specified by RFC822. 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
RFC822 CRLF delimiters.
Other implementation variations are conceivable as well.
The only important aspect of this discussion is that the
resulting messages are consistent with those produced by the
model described here.
References [US-ASCII] Coded Character Set--7-Bit American Standard Code for Information Interchange, ANSI X3.4-1986. [ATK] Borenstein, Nathaniel S., Multimedia Applications Development with the Andrew Toolkit, Prentice-Hall, 1990. [GIF] Graphics Interchange Format (Version 89a), Compuserve, Inc., Columbus, Ohio, 1990. [ISO-2022] International Standard--Information Processing-- ISO 7-bit and 8-bit coded character sets--Code extension techniques, ISO 2022:1986. [ISO-8859] 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/TJC1/SC2/WG11 (Motion Picture Experts Group), May, 1991. [ODA] ISO 8613; Information Processing: Text and Office System; Office Document Architecture (ODA) and Interchange Format (ODIF), Part 1-8, 1989. [PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972, "Pulse Code Modulation (PCM) of Voice Frequencies". [POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference Manual, Addison-Wesley, 1985. [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. [RFC-783] Sollins, K.R. TFTP Protocol (revision 2). June, 1981, MIT, RFC-783. [RFC-821] Postel, J.B. Simple Mail Transfer Protocol. August, 1982, USC/Information Sciences Institute, RFC-821.
[RFC-822] Crocker, D. Standard for the format of ARPA
Internet text messages. August, 1982, UDEL, RFC-822.
[RFC-934] Rose, M.T.; Stefferud, E.A. Proposed standard
for message encapsulation. January, 1985, Delaware
and NMA, RFC-934.
[RFC-959] Postel, J.B.; Reynolds, J.K. File Transfer
Protocol. October, 1985, USC/Information Sciences
Institute, RFC-959.
[RFC-1049] Sirbu, M.A. Content-Type header field for
Internet messages. March, 1988, CMU, RFC-1049.
[RFC-1113] Linn, J. Privacy enhancement for Internet
electronic mail: Part I - message encipherment and
authentication procedures. August, 1989, IAB Privacy Task
Force, RFC-1113.
[RFC-1154] Robinson, D.; Ullmann, R. Encoding header field
for Internet messages. April, 1990, Prime Computer,
Inc., RFC-1154.
[RFC-1342] Moore, Keith, Representation of Non-Ascii Text in
Internet Message Headers. June, 1992, University of
Tennessee, RFC-1342.
Security Considerations
Security issues are discussed in Section 7.4.2 and in
Appendix G. Implementors should pay special attention to
the security implications of any mail content-types that can
cause the remote execution of any actions in the recipient's
environment. In such cases, the discussion of the
applicaton/postscript content-type in Section 7.4.2 may
serve as a model for considering other content-types with
remote execution capabilities.
Authors' Addresses For more information, the authors of this document may be contacted via Internet mail: Nathaniel S. Borenstein MRE 2D-296, Bellcore 445 South St. Morristown, NJ 07962-1910 Phone: +1 201 829 4270 Fax: +1 201 829 7019 Email: nsb@bellcore.com Ned Freed Innosoft International, Inc. 250 West First Street Suite 240 Claremont, CA 91711 Phone: +1 714 624 7907 Fax: +1 714 621 5319 Email: ned@innosoft.com
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Table of Contents
1 Introduction....................................... 1
2 Notations, Conventions, and Generic BNF Grammar.... 3
3 The MIME-Version Header Field...................... 5
4 The Content-Type Header Field...................... 6
5 The Content-Transfer-Encoding Header Field......... 10
5.1 Quoted-Printable Content-Transfer-Encoding......... 14
5.2 Base64 Content-Transfer-Encoding................... 17
6 Additional Optional Content- Header Fields......... 19
6.1 Optional Content-ID Header Field................... 19
6.2 Optional Content-Description Header Field.......... 19
7 The Predefined Content-Type Values................. 20
7.1 The Text Content-Type.............................. 20
7.1.1 The charset parameter.............................. 20
7.1.2 The Text/plain subtype............................. 23
7.1.3 The Text/richtext subtype.......................... 23
7.2 The Multipart Content-Type......................... 29
7.2.1 Multipart: The common syntax...................... 30
7.2.2 The Multipart/mixed (primary) subtype.............. 34
7.2.3 The Multipart/alternative subtype.................. 34
7.2.4 The Multipart/digest subtype....................... 36
7.2.5 The Multipart/parallel subtype..................... 36
7.3 The Message Content-Type........................... 37
7.3.1 The Message/rfc822 (primary) subtype............... 37
7.3.2 The Message/Partial subtype........................ 37
7.3.3 The Message/External-Body subtype.................. 40
7.4 The Application Content-Type....................... 46
7.4.1 The Application/Octet-Stream (primary) subtype..... 46
7.4.2 The Application/PostScript subtype................. 47
7.4.3 The Application/ODA subtype........................ 50
7.5 The Image Content-Type............................. 51
7.6 The Audio Content-Type............................. 51
7.7 The Video Content-Type............................. 51
7.8 Experimental Content-Type Values................... 51
Summary............................................ 53
Acknowledgements................................... 54
Appendix A -- Minimal MIME-Conformance............. 56
Appendix B -- General Guidelines For Sending Email Data59
Appendix C -- A Complex Multipart Example.......... 62
Appendix D -- A Simple Richtext-to-Text Translator in C64
Appendix E -- Collected Grammar.................... 66
Appendix F -- IANA Registration Procedures......... 68
F.1 Registration of New Content-type/subtype Values..68
F.2 Registration of New Character Set Values...... 69
F.3 Registration of New Access-type Values for Message/external-body69
F.4 Registration of New Conversions Values for Application69
Appendix G -- Summary of the Seven Content-types... 71
Appendix H -- Canonical Encoding Model............. 73
References......................................... 75
Security Considerations............................ 76
Authors' Addresses................................. 77