Network Working Group N. Borenstein
Request for Comments: 1521 Bellcore
Obsoletes: 1341 N. Freed
Category: Standards Track Innosoft
September 1993
MIME (Multipurpose Internet Mail Extensions) Part One:
Mechanisms for Specifying and Describing
the Format of Internet Message Bodies
Status of this Memo
This RFC 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" 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 which
specifies considerable detail about message headers, but which leaves
the message content, or message body, as flat ASCII text. This
document redefines the format of message bodies to allow multi-part
textual and non-textual message bodies to be represented and
exchanged without loss of information. This is based on earlier work
documented in RFC 934 and STD 11, RFC 1049, but extends and revises
that work. Because RFC 822 said so little about message bodies, this
document is largely orthogonal to (rather than a revision of) RFC
822.
In particular, this document is designed to provide facilities to
include multiple objects in a single message, to represent body text
in character sets other than US-ASCII, to represent formatted multi-
font text messages, to represent non-textual material such as images
and audio fragments, and generally to facilitate later extensions
defining new types of Internet mail for use by cooperating mail
agents.
This document does NOT extend Internet mail header fields to permit
anything other than US-ASCII text data. Such extensions are the
subject of a companion document [RFC-1522].
This document is a revision of RFC 1341. Significant differences
from RFC 1341 are summarized in Appendix H.
Borenstein & Freed [Page 1]RFC 1521 MIME September 1993
Table of Contents
1. Introduction....................................... 3
2. Notations, Conventions, and Generic BNF Grammar.... 6
3. The MIME-Version Header Field...................... 7
4. The Content-Type Header Field...................... 9
5. The Content-Transfer-Encoding Header Field......... 13
5.1. Quoted-Printable Content-Transfer-Encoding......... 18
5.2. Base64 Content-Transfer-Encoding................... 21
6. Additional Content-Header Fields................... 23
6.1. Optional Content-ID Header Field................... 23
6.2. Optional Content-Description Header Field.......... 24
7. The Predefined Content-Type Values................. 24
7.1. The Text Content-Type.............................. 24
7.1.1. The charset parameter.............................. 25
7.1.2. The Text/plain subtype............................. 28
7.2. The Multipart Content-Type......................... 28
7.2.1. Multipart: The common syntax...................... 29
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..................... 37
7.2.6. Other Multipart subtypes........................... 37
7.3. The Message Content-Type........................... 38
7.3.1. The Message/rfc822 (primary) subtype............... 38
7.3.2. The Message/Partial subtype........................ 39
7.3.3. The Message/External-Body subtype.................. 42
7.3.3.1. The "ftp" and "tftp" access-types............... 44
7.3.3.2. The "anon-ftp" access-type...................... 45
7.3.3.3. The "local-file" and "afs" access-types......... 45
7.3.3.4. The "mail-server" access-type................... 45
7.3.3.5. Examples and Further Explanations............... 46
7.4. The Application Content-Type....................... 49
7.4.1. The Application/Octet-Stream (primary) subtype..... 50
7.4.2. The Application/PostScript subtype................. 50
7.4.3. Other Application subtypes......................... 53
7.5. The Image Content-Type............................. 53
7.6. The Audio Content-Type............................. 54
7.7. The Video Content-Type............................. 54
7.8. Experimental Content-Type Values................... 54
8. Summary............................................ 56
9. Security Considerations............................ 56
10. Authors' Addresses................................. 57
11. Acknowledgements................................... 58
Appendix A -- Minimal MIME-Conformance.................... 60
Appendix B -- General Guidelines For Sending Email Data... 63
Appendix C -- A Complex Multipart Example................. 66
Appendix D -- Collected Grammar........................... 68
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RFC 1521 MIME September 1993
Appendix E -- IANA Registration Procedures................ 72
E.1 Registration of New Content-type/subtype Values...... 72
E.2 Registration of New Access-type Values
for Message/external-body............................ 73
Appendix F -- Summary of the Seven Content-types.......... 74
Appendix G -- Canonical Encoding Model.................... 76
Appendix H -- Changes from RFC 1341....................... 78
References................................................ 80
1. Introduction
Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined
the standard format of textual mail messages on the Internet. Its
success has been such that the RFC 822 format has been adopted,
wholly or partially, well beyond the confines of the Internet and the
Internet SMTP transport defined by STD 10, RFC 821 [RFC-821]. As the
format has seen wider use, a number of limitations have proven
increasingly restrictive for the user community.
RFC 822 was intended to specify a format for text messages. As such,
non-text messages, such as multimedia messages that might include
audio or images, are simply not mentioned. Even in the case of text,
however, RFC 822 is inadequate for the needs of mail users whose
languages require the use of character sets richer than US ASCII
[US-ASCII]. Since RFC 822 does not specify mechanisms for mail
containing audio, video, Asian language text, or even text in most
European languages, additional specifications are needed.
One of the notable limitations of RFC 821/822 based mail systems is
the fact that they limit the contents of electronic mail messages to
relatively short lines of seven-bit ASCII. This forces users to
convert any non-textual data that they may wish to send into seven-
bit bytes representable as printable ASCII characters before invoking
a local mail UA (User Agent, a program with which human users send
and receive mail). Examples of such encodings currently used in the
Internet include pure hexadecimal, uuencode, the 3-in-4 base 64
scheme specified in RFC 1421, the Andrew Toolkit Representation
[ATK], and many others.
The limitations of RFC 822 mail become even more apparent as gateways
are designed to allow for the exchange of mail messages between RFC
822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
inclusion of non-textual body parts within electronic mail messages.
The current standards for the mapping of X.400 messages to RFC 822
messages specify either that X.400 non-textual body parts must be
converted to (not encoded in) an ASCII format, or that they must be
discarded, notifying the RFC 822 user that discarding has occurred.
This is clearly undesirable, as information that a user may wish to
Borenstein & Freed [Page 3]RFC 1521 MIME September 1993
receive is lost. Even though a user's UA may not have the capability
of dealing with the non-textual body part, the user might have some
mechanism external to the UA that can extract useful information from
the body part. Moreover, it does not allow for the fact that the
message may eventually be gatewayed back into an X.400 message
handling system (i.e., the X.400 message is "tunneled" through
Internet mail), where the non-textual information would definitely
become useful again.
This document describes several mechanisms that combine to solve most
of these problems without introducing any serious incompatibilities
with the existing world of RFC 822 mail. In particular, it
describes:
1. A MIME-Version header field, which uses a version number to
declare a message to be conformant with this specification and
allows mail processing agents to distinguish between such
messages and those generated by older or non-conformant software,
which is presumed to lack such a field.
2. A Content-Type header field, generalized from RFC 1049 [RFC-1049],
which can be used to specify the type and subtype of data in the
body of a message and to fully specify the native representation
(encoding) of such data.
2.a. A "text" Content-Type value, which can be used to represent
textual information in a number of character sets and
formatted text description languages in a standardized
manner.
2.b. A "multipart" Content-Type value, which can be used to
combine several body parts, possibly of differing types of
data, into a single message.
2.c. An "application" Content-Type value, which can be used to
transmit application data or binary data, and hence, among
other uses, to implement an electronic mail file transfer
service.
2.d. A "message" Content-Type value, for encapsulating another
mail message.
2.e An "image" Content-Type value, for transmitting still image
(picture) data.
2.f. An "audio" Content-Type value, for transmitting audio or
voice data.
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2.g. A "video" Content-Type value, for transmitting video or
moving image data, possibly with audio as part of the
composite video data format.
3. A Content-Transfer-Encoding header field, which can be used to
specify an auxiliary encoding that was applied to the data in
order to allow it to pass through mail transport mechanisms which
may have data or character set limitations.
4. Two additional header fields that can be used to further describe
the data in a message body, the Content-ID and Content-
Description header fields.
MIME has been carefully designed as an extensible mechanism, 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 including character set names, 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. Appendix E provides details about how IANA registration
is accomplished.
Finally, to specify and promote interoperability, Appendix A of this
document provides a basic applicability statement for a subset of the
above mechanisms that defines a minimal level of "conformance" with
this document.
HISTORICAL NOTE: Several of the mechanisms described in this
document may seem somewhat strange or even baroque at first
reading. It is important to note that compatibility with existing
standards AND robustness across existing practice were two of the
highest priorities of the working group that developed this
document. In particular, compatibility was always favored over
elegance.
MIME was first defined and published as RFCs 1341 and 1342 [RFC-1341]
[RFC-1342]. This document is a relatively minor updating of RFC
1341, and is intended to supersede it. The differences between this
document and RFC 1341 are summarized in Appendix H. Please refer to
the current edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Several other RFC
documents will be of interest to the MIME implementor, in particular
[RFC 1343], [RFC-1344], and [RFC-1345].
Borenstein & Freed [Page 5]RFC 1521 MIME September 1993
2. Notations, Conventions, and Generic BNF Grammar
This document is being published in two versions, one as plain ASCII
text and one as PostScript (PostScript is a trademark of Adobe
Systems Incorporated.). While the text version is the official
specification, some will find the PostScript version easier to read.
The textual contents are identical. An Andrew-format copy of this
document is also available from the first author (Borenstein).
Although the mechanisms specified in this document are all described
in prose, most are also described formally in the modified BNF
notation of RFC 822. Implementors will need to be familiar with this
notation in order to understand this specification, and are referred
to RFC 822 for a complete explanation of the modified BNF notation.
Some of the modified BNF in this document makes reference to
syntactic entities that are defined in RFC 822 and not in this
document. A complete formal grammar, then, is obtained by combining
the collected grammar appendix of this document with that of RFC 822
plus the modifications to RFC 822 defined in RFC 1123, which
specifically changes the syntax for `return', `date' and `mailbox'.
The term CRLF, in this document, refers to the sequence of the two
ASCII characters CR (13) and LF (10) which, taken together, in this
order, denote a line break in RFC 822 mail.
The term "character set" is used in this document to refer to a
method used with one or more tables to convert encoded text to a
series of octets. This definition is intended to allow various kinds
of text encodings, from simple single-table mappings such as ASCII to
complex table switching methods such as those that use ISO 2022's
techniques. However, a MIME character set name must fully specify
the mapping to be performed.
The term "message", when not further qualified, means either the
(complete or "top-level") message being transferred on a network, or
a message encapsulated in a body of type "message".
The term "body part", in this document, means one of the parts of the
body of a multipart entity. A body part has a header and a body, so
it makes sense to speak about the body of a body part.
The term "entity", in this document, means either a message or a body
part. All kinds of entities share the property that they have a
header and a body.
The term "body", when not further qualified, means the body of an
entity, that is the body of either a message or of a body part.
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RFC 1521 MIME September 1993
NOTE: The previous four definitions are clearly circular. This is
unavoidable, since the overall structure of a MIME message is
indeed recursive.
In this document, all numeric and octet values are given in decimal
notation.
It must be noted that Content-Type values, subtypes, and parameter
names as defined in this document are case-insensitive. However,
parameter values are case-sensitive unless otherwise specified for
the specific parameter.
FORMATTING NOTE: This document has been carefully formatted for
ease of reading. The PostScript version of this document, in
particular, places notes like this one, which may be skipped by
the reader, in a smaller, italicized, font, and indents it as
well. In the text version, only the indentation is preserved, so
if you are reading the text version of this you might consider
using the PostScript version instead. However, all such notes will
be indented and preceded by "NOTE:" or some similar introduction,
even in the text version.
The primary purpose of these non-essential notes is to convey
information about the rationale of this document, or to place this
document in the proper historical or evolutionary context. Such
information may be skipped by those who are focused entirely on
building a conformant implementation, but may be of use to those
who wish to understand why this document is written as it is.
For ease of recognition, all BNF definitions have been placed in a
fixed-width font in the PostScript version of this document.
3. The MIME-Version Header Field
Since RFC 822 was published in 1982, there has really been only one
format standard for Internet messages, and there has been little
perceived need to declare the format standard in use. This document
is an independent document that complements RFC 822. Although the
extensions in this document have been defined in such a way as to be
compatible with RFC 822, there are still circumstances in which it
might be desirable for a mail-processing agent to know whether a
message was composed with the new standard in mind.
Therefore, this document defines a new header field, "MIME-Version",
which is to be used to declare the version of the Internet message
body format standard in use.
Messages composed in accordance with this document MUST include such
Borenstein & Freed [Page 7]RFC 1521 MIME September 1993
a header field, with the following verbatim text:
MIME-Version: 1.0
The presence of this header field is an assertion that the message
has been composed in compliance with this document.
Since it is possible that a future document might extend the message
format standard again, a formal BNF is given for the content of the
MIME-Version field:
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
Thus, future format specifiers, which might replace or extend "1.0",
are constrained to be two integer fields, separated by a period. If
a message is received with a MIME-version value other than "1.0", it
cannot be assumed to conform with this specification.
Note that the MIME-Version header field is required at the top level
of a message. It is not required for each body part of a multipart
entity. It is required for the embedded headers of a body of type
"message" if and only if the embedded message is itself claimed to be
MIME-conformant.
It is not possible to fully specify how a mail reader that conforms
with MIME as defined in this document should treat a message that
might arrive in the future with some value of MIME-Version other than
"1.0". However, conformant software is encouraged to check the
version number and at least warn the user if an unrecognized MIME-
version is encountered.
It is also worth noting that version control for specific content-
types is not accomplished using the MIME-Version mechanism. In
particular, some formats (such as application/postscript) have
version numbering conventions that are internal to the document
format. Where such conventions exist, MIME does nothing to supersede
them. Where no such conventions exist, a MIME type might use a
"version" parameter in the content-type field if necessary.
NOTE TO IMPLEMENTORS: All header fields defined in this document,
including MIME-Version, Content-type, etc., are subject to the
general syntactic rules for header fields specified in RFC 822. In
particular, all can include comments, which means that the following
two MIME-Version fields are equivalent:
MIME-Version: 1.0
MIME-Version: 1.0 (Generated by GBD-killer 3.7)
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4. The Content-Type Header Field
The purpose of the Content-Type field is to describe the data
contained in the body fully enough that the receiving user agent can
pick an appropriate agent or mechanism to present the data to the
user, or otherwise deal with the data in an appropriate manner.
HISTORICAL NOTE: The Content-Type header field was first defined in
RFC 1049. RFC 1049 Content-types used a simpler and less powerful
syntax, but one that is largely compatible with the mechanism given
here.
The Content-Type header field is used to specify the nature of the
data in the body of an entity, by giving type and subtype
identifiers, and by providing auxiliary information that may be
required for certain types. After the type and subtype names, the
remainder of the header field is simply a set of parameters,
specified in an attribute/value notation. The set of meaningful
parameters differs for the different types. In particular, there are
NO globally-meaningful parameters that apply to all content-types.
Global mechanisms are best addressed, in the MIME model, by the
definition of additional Content-* header fields. The ordering of
parameters is not significant. Among the defined parameters is a
"charset" parameter by which the character set used in the body may
be declared. Comments are allowed in accordance with RFC 822 rules
for structured header fields.
In general, the top-level Content-Type is used to declare the general
type of data, while the subtype specifies a specific format for that
type of data. Thus, a Content-Type of "image/xyz" is enough to tell
a user agent that the data is an image, even if the user agent has no
knowledge of the specific image format "xyz". Such information can
be used, for example, to decide whether or not to show a user the raw
data from an unrecognized subtype -- such an action might be
reasonable for unrecognized subtypes of text, but not for
unrecognized subtypes of image or audio. For this reason, registered
subtypes of audio, image, text, and video, should not contain
embedded information that is really of a different type. Such
compound types should be represented using the "multipart" or
"application" types.
Parameters are modifiers of the content-subtype, and do not
fundamentally affect the requirements of the host system. Although
most parameters make sense only with certain content-types, others
are "global" in the sense that they might apply to any subtype. For
example, the "boundary" parameter makes sense only for the
"multipart" content-type, but the "charset" parameter might make
sense with several content-types.
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RFC 1521 MIME September 1993
An initial set of seven Content-Types is defined by this document.
This set of top-level names is intended to be substantially complete.
It is expected that additions to the larger set of supported types
can generally be accomplished by the creation of new subtypes of
these initial types. In the future, more top-level types may be
defined only by an extension to this standard. If another primary
type is to be used for any reason, it must be given a name starting
with "X-" to indicate its non-standard status and to avoid a
potential conflict with a future official name.
In the Augmented BNF notation of RFC 822, a Content-Type header field
value is defined as follows:
content := "Content-Type" ":" type "/" subtype *(";"
parameter)
; case-insensitive matching of type and subtype
type := "application" / "audio"
/ "image" / "message"
/ "multipart" / "text"
/ "video" / extension-token
; All values case-insensitive
extension-token := x-token / iana-token
iana-token := <a publicly-defined extension token,
registered with IANA, as specified in
appendix E>
x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>
subtype := token ; case-insensitive
parameter := attribute "=" value
attribute := token ; case-insensitive
value := token / quoted-string
token := 1*<any (ASCII) CHAR except SPACE, CTLs,
or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@"
/ "," / ";" / ":" / "\" / <">
/ "/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values
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Note that the definition of "tspecials" is the same as the RFC 822
definition of "specials" with the addition of the three characters
"/", "?", and "=", and the removal of ".".
Note also that a subtype specification is MANDATORY. There are no
default subtypes.
The type, subtype, and parameter names are not case sensitive. For
example, TEXT, Text, and TeXt are all equivalent. Parameter values
are normally case sensitive, but certain parameters are interpreted
to be case-insensitive, depending on the intended use. (For example,
multipart boundaries are case-sensitive, but the "access-type" for
message/External-body is not case-sensitive.)
Beyond this syntax, the only constraint on the definition of subtype
names is the desire that their uses must not conflict. That is, it
would be undesirable to have two different communities using
"Content-Type: application/foobar" to mean two different things. The
process of defining new content-subtypes, then, is not intended to be
a mechanism for imposing restrictions, but simply a mechanism for
publicizing the usages. There are, therefore, two acceptable
mechanisms for defining new Content-Type subtypes:
1. Private values (starting with "X-") may be
defined bilaterally between two cooperating
agents without outside registration or
standardization.
2. New standard values must be documented,
registered with, and approved by IANA, as
described in Appendix E. Where intended for
public use, the formats they refer to must
also be defined by a published specification,
and possibly offered for standardization.
The seven standard initial predefined Content-Types are detailed in
the bulk of this document. They are:
text -- textual information. The primary subtype,
"plain", indicates plain (unformatted) text. No
special software is required to get the full
meaning of the text, aside from support for the
indicated character set. Subtypes are to be used
for enriched text in forms where application
software may enhance the appearance of the text,
but such software must not be required in order to
get the general idea of the content. Possible
subtypes thus include any readable word processor
Borenstein & Freed [Page 11]RFC 1521 MIME September 1993
format. A very simple and portable subtype,
richtext, was defined in RFC 1341, with a future
revision expected.
multipart -- data consisting of multiple parts of
independent data types. Four initial subtypes
are defined, including the primary "mixed"
subtype, "alternative" for representing the same
data in multiple formats, "parallel" for parts
intended to be viewed simultaneously, and "digest"
for multipart entities in which each part is of
type "message".
message -- an encapsulated message. A body of
Content-Type "message" is itself all or part of a
fully formatted RFC 822 conformant message which
may contain its own different Content-Type header
field. The primary subtype is "rfc822". The
"partial" subtype is defined for partial messages,
to permit the fragmented transmission of bodies
that are thought to be too large to be passed
through mail transport facilities. Another
subtype, "External-body", is defined for
specifying large bodies by reference to an
external data source.
image -- image data. Image requires a display device
(such as a graphical display, a printer, or a FAX
machine) to view the information. Initial
subtypes are defined for two widely-used image
formats, jpeg and gif.
audio -- audio data, with initial subtype "basic".
Audio requires an audio output device (such as a
speaker or a telephone) to "display" the contents.
video -- video data. Video requires the capability to
display moving images, typically including
specialized hardware and software. The initial
subtype is "mpeg".
application -- some other kind of data, typically
either uninterpreted binary data or information to
be processed by a mail-based application. The
primary subtype, "octet-stream", is to be used in
the case of uninterpreted binary data, in which
case the simplest recommended action is to offer
to write the information into a file for the user.
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An additional subtype, "PostScript", is defined
for transporting PostScript documents in bodies.
Other expected uses for "application" include
spreadsheets, data for mail-based scheduling
systems, and languages for "active"
(computational) email. (Note that active email
and other application data may entail several
security considerations, which are discussed later
in this memo, particularly in the context of
application/PostScript.)
Default RFC 822 messages are typed by this protocol as plain text in
the US-ASCII character set, which can be explicitly specified as
"Content-type: text/plain; charset=us-ascii". If no Content-Type is
specified, this default is assumed. In the presence of a MIME-
Version header field, a receiving User Agent can also assume that
plain US-ASCII text was the sender's intent. In the absence of a
MIME-Version specification, plain US-ASCII text must still be
assumed, but the sender's intent might have been otherwise.
RATIONALE: In the absence of any Content-Type header field or
MIME-Version header field, it is impossible to be certain that a
message is actually text in the US-ASCII character set, since it
might well be a message that, using the conventions that predate
this document, includes text in another character set or non-
textual data in a manner that cannot be automatically recognized
(e.g., a uuencoded compressed UNIX tar file). Although there is
no fully acceptable alternative to treating such untyped messages
as "text/plain; charset=us-ascii", implementors should remain
aware that if a message lacks both the MIME-Version and the
Content-Type header fields, it may in practice contain almost
anything.
It should be noted that the list of Content-Type values given here
may be augmented in time, via the mechanisms described above, and
that the set of subtypes is expected to grow substantially.
When a mail reader encounters mail with an unknown Content-type
value, it should generally treat it as equivalent to
"application/octet-stream", as described later in this document.
5. The Content-Transfer-Encoding Header Field
Many Content-Types which could usefully be transported via email are
represented, in their "natural" format, as 8-bit character or binary
data. Such data cannot be transmitted over some transport protocols.
For example, RFC 821 restricts mail messages to 7-bit US-ASCII data
with lines no longer than 1000 characters.
Borenstein & Freed [Page 13]RFC 1521 MIME September 1993
It is necessary, therefore, to define a standard mechanism for re-
encoding such data into a 7-bit short-line format. This document
specifies that such encodings will be indicated by a new "Content-
Transfer-Encoding" header field. The Content-Transfer-Encoding field
is used to indicate the type of transformation that has been used in
order to represent the body in an acceptable manner for transport.
Unlike Content-Types, a proliferation of Content-Transfer-Encoding
values is undesirable and unnecessary. However, establishing only a
single Content-Transfer-Encoding mechanism does not seem possible.
There is a tradeoff between the desire for a compact and efficient
encoding of largely-binary data and the desire for a readable
encoding of data that is mostly, but not entirely, 7-bit data. For
this reason, at least two encoding mechanisms are necessary: a
"readable" encoding and a "dense" encoding.
The Content-Transfer-Encoding field is designed to specify an
invertible mapping between the "native" representation of a type of
data and a representation that can be readily exchanged using 7 bit
mail transport protocols, such as those defined by RFC 821 (SMTP).
This field has not been defined by any previous standard. The field's
value is a single token specifying the type of encoding, as
enumerated below. Formally:
encoding := "Content-Transfer-Encoding" ":" mechanism
mechanism := "7bit" ; case-insensitive
/ "quoted-printable"
/ "base64"
/ "8bit"
/ "binary"
/ x-token
These values are not case sensitive. That is, Base64 and BASE64 and
bAsE64 are all equivalent. An encoding type of 7BIT requires that
the body is already in a seven-bit mail-ready representation. This
is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is
assumed if the Content-Transfer-Encoding header field is not present.
The values "8bit", "7bit", and "binary" all mean that NO encoding has
been performed. However, they are potentially useful as indications
of the kind of data contained in the object, and therefore of the
kind of encoding that might need to be performed for transmission in
a given transport system. In particular:
"7bit" means that the data is all represented as short
lines of US-ASCII data.
Borenstein & Freed [Page 14]RFC 1521 MIME September 1993
"8bit" means that the lines are short, but there may be
non-ASCII characters (octets with the high-order
bit set).
"Binary" means that not only may non-ASCII characters
be present, but also that the lines are not
necessarily short enough for SMTP transport.
The difference between "8bit" (or any other conceivable bit-width
token) and the "binary" token is that "binary" does not require
adherence to any limits on line length or to the SMTP CRLF semantics,
while the bit-width tokens do require such adherence. If the body
contains data in any bit-width other than 7-bit, the appropriate
bit-width Content-Transfer-Encoding token must be used (e.g., "8bit"
for unencoded 8 bit wide data). If the body contains binary data,
the "binary" Content-Transfer-Encoding token must be used.
NOTE: The distinction between the Content-Transfer-Encoding values
of "binary", "8bit", etc. may seem unimportant, in that all of
them really mean "none" -- that is, there has been no encoding of
the data for transport. However, clear labeling will be of
enormous value to gateways between future mail transport systems
with differing capabilities in transporting data that do not meet
the restrictions of RFC 821 transport.
Mail transport for unencoded 8-bit data is defined in RFC-1426
[RFC-1426]. As of the publication of this document, there are no
standardized Internet mail transports for which it is legitimate
to include unencoded binary data in mail bodies. Thus there are
no circumstances in which the "binary" Content-Transfer-Encoding
is actually legal on the Internet. However, in the event that
binary mail transport becomes a reality in Internet mail, or when
this document is used in conjunction with any other binary-capable
transport mechanism, binary bodies should be labeled as such using
this mechanism.
NOTE: The five values defined for the Content-Transfer-Encoding
field imply nothing about the Content-Type other than the
algorithm by which it was encoded or the transport system
requirements if unencoded.
Implementors may, if necessary, define new Content-Transfer-Encoding
values, but must use an x-token, which is a name prefixed by "X-" to
indicate its non-standard status, e.g., "Content-Transfer-Encoding:
x-my-new-encoding". However, unlike Content-Types and subtypes, the
creation of new Content-Transfer-Encoding values is explicitly and
strongly discouraged, as it seems likely to hinder interoperability
with little potential benefit. Their use is allowed only as the
Borenstein & Freed [Page 15]RFC 1521 MIME September 1993
result of an agreement between cooperating user agents.
If a Content-Transfer-Encoding header field appears as part of a
message header, it applies to the entire body of that message. If a
Content-Transfer-Encoding header field appears as part of a body
part's headers, it applies only to the body of that body part. If an
entity is of type "multipart" or "message", the Content-Transfer-
Encoding is not permitted to have any value other than a bit width
(e.g., "7bit", "8bit", etc.) or "binary".
It should be noted that email is character-oriented, so that the
mechanisms described here are mechanisms for encoding arbitrary octet
streams, not bit streams. If a bit stream is to be encoded via one
of these mechanisms, it must first be converted to an 8-bit byte
stream using the network standard bit order ("big-endian"), in which
the earlier bits in a stream become the higher-order bits in a byte.
A bit stream not ending at an 8-bit boundary must be padded with
zeroes. This document provides a mechanism for noting the addition
of such padding in the case of the application Content-Type, which
has a "padding" parameter.
The encoding mechanisms defined here explicitly encode all data in
ASCII. Thus, for example, suppose an entity has header fields such
as:
Content-Type: text/plain; charset=ISO-8859-1
Content-transfer-encoding: base64
This must be interpreted to mean that the body is a base64 ASCII
encoding of data that was originally in ISO-8859-1, and will be in
that character set again after decoding.
The following sections will define the two standard encoding
mechanisms. The definition of new content-transfer-encodings is
explicitly discouraged and should only occur when absolutely
necessary. All content-transfer-encoding namespace except that
beginning with "X-" is explicitly reserved to the IANA for future
use. Private agreements about content-transfer-encodings are also
explicitly discouraged.
Certain Content-Transfer-Encoding values may only be used on certain
Content-Types. In particular, it is expressly forbidden to use any
encodings other than "7bit", "8bit", or "binary" with any Content-
Type that recursively includes other Content-Type fields, notably the
"multipart" and "message" Content-Types. All encodings that are
desired for bodies of type multipart or message must be done at the
innermost level, by encoding the actual body that needs to be
encoded.
Borenstein & Freed [Page 16]RFC 1521 MIME September 1993
NOTE ON ENCODING RESTRICTIONS: Though the prohibition against
using content-transfer-encodings on data of type multipart or
message may seem overly restrictive, it is necessary to prevent
nested encodings, in which data are passed through an encoding
algorithm multiple times, and must be decoded multiple times in
order to be properly viewed. Nested encodings add considerable
complexity to user agents: aside from the obvious efficiency
problems with such multiple encodings, they can obscure the basic
structure of a message. In particular, they can imply that
several decoding operations are necessary simply to find out what
types of objects a message contains. Banning nested encodings may
complicate the job of certain mail gateways, but this seems less
of a problem than the effect of nested encodings on user agents.
NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-
TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding
could be inferred from the characteristics of the Content-Type
that is to be encoded, or, at the very least, that certain
Content-Transfer-Encodings could be mandated for use with specific
Content-Types. There are several reasons why this is not the case.
First, given the varying types of transports used for mail, some
encodings may be appropriate for some Content-Type/transport
combinations and not for others. (For example, in an 8-bit
transport, no encoding would be required for text in certain
character sets, while such encodings are clearly required for 7-
bit SMTP.) Second, certain Content-Types may require different
types of transfer encoding under different circumstances. For
example, many PostScript bodies might consist entirely of short
lines of 7-bit data and hence require little or no encoding.
Other PostScript bodies (especially those using Level 2
PostScript's binary encoding mechanism) may only be reasonably
represented using a binary transport encoding. Finally, since
Content-Type is intended to be an open-ended specification
mechanism, strict specification of an association between
Content-Types and encodings effectively couples the specification
of an application protocol with a specific lower-level transport.
This is not desirable since the developers of a Content-Type
should not have to be aware of all the transports in use and what
their limitations are.
NOTE ON TRANSLATING ENCODINGS: The quoted-printable and base64
encodings are designed so that conversion between them is
possible. The only issue that arises in such a conversion is the
handling of line breaks. When converting from quoted-printable to
base64 a line break must be converted into a CRLF sequence.
Similarly, a CRLF sequence in base64 data must be converted to a
quoted-printable line break, but ONLY when converting text data.
Borenstein & Freed [Page 17]RFC 1521 MIME September 1993
NOTE ON 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, and the relationship between content-transfer-
encodings and character sets. For this reason, a canonical model
for encoding is presented as Appendix G.
5.1. Quoted-Printable Content-Transfer-Encoding
The Quoted-Printable encoding is intended to represent data that
largely consists of octets that correspond to printable characters in
the ASCII character set. It encodes the data in such a way that the
resulting octets are unlikely to be modified by mail transport. If
the data being encoded are mostly ASCII text, the encoded form of the
data remains largely recognizable by humans. A body which is
entirely ASCII may also be encoded in Quoted-Printable to ensure the
integrity of the data should the message pass through a character-
translating, and/or line-wrapping gateway.
In this encoding, octets are to be represented as determined by the
following rules:
Rule #1: (General 8-bit representation) Any octet, except those
indicating a line break according to the newline convention of the
canonical (standard) form of the data being encoded, may be
represented by an "=" followed by a two digit hexadecimal
representation of the octet's value. The digits of the
hexadecimal alphabet, for this purpose, are "0123456789ABCDEF".
Uppercase letters must be used when sending hexadecimal data,
though a robust implementation may choose to recognize lowercase
letters on receipt. Thus, for example, the value 12 (ASCII form
feed) can be represented by "=0C", and the value 61 (ASCII EQUAL
SIGN) can be represented by "=3D". Except when the following
rules allow an alternative encoding, this rule is mandatory.
Rule #2: (Literal representation) Octets with decimal values of 33
through 60 inclusive, and 62 through 126, inclusive, MAY be
represented as the ASCII characters which correspond to those
octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN
through TILDE, respectively).
Rule #3: (White Space): Octets with values of 9 and 32 MAY be
represented as ASCII TAB (HT) and SPACE characters, respectively,
but MUST NOT be so represented at the end of an encoded line. Any
TAB (HT) or SPACE characters on an encoded line MUST thus be
followed on that line by a printable character. In particular, an
Borenstein & Freed [Page 18]RFC 1521 MIME September 1993
"=" at the end of an encoded line, indicating a soft line break
(see rule #5) may follow one or more TAB (HT) or SPACE characters.
It follows that an octet with value 9 or 32 appearing at the end
of an encoded line must be represented according to Rule #1. This
rule is necessary because some MTAs (Message Transport Agents,
programs which transport messages from one user to another, or
perform a part of such transfers) are known to pad lines of text
with SPACEs, and others are known to remove "white space"
characters from the end of a line. Therefore, when decoding a
Quoted-Printable body, any trailing white space on a line must be
deleted, as it will necessarily have been added by intermediate
transport agents.
Rule #4 (Line Breaks): A line break in a text body, independent of
what its representation is following the canonical representation
of the data being encoded, must be represented by a (RFC 822) line
break, which is a CRLF sequence, in the Quoted-Printable encoding.
Since the canonical representation of types other than text do not
generally include the representation of line breaks, no hard line
breaks (i.e. line breaks that are intended to be meaningful and
to be displayed to the user) should occur in the quoted-printable
encoding of such types. Of course, occurrences of "=0D", "=0A",
"0A=0D" and "=0D=0A" will eventually be encountered. In general,
however, base64 is preferred over quoted-printable for binary
data.
Note that many implementations may elect to encode the local
representation of various content types directly, as described in
Appendix G. In particular, this may apply to plain text material
on systems that use newline conventions other than CRLF
delimiters. Such an implementation is permissible, but the
generation of line breaks must be generalized to account for the
case where alternate representations of newline sequences are
used.
Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES
that encoded lines be no more than 76 characters long. If longer
lines are to be encoded with the Quoted-Printable encoding, 'soft'
line breaks must be used. An equal sign as the last character on a
encoded line indicates such a non-significant ('soft') line break
in the encoded text. Thus if the "raw" form of the line is a
single unencoded line that says:
Now's the time for all folk to come to the aid of
their country.
This can be represented, in the Quoted-Printable encoding, as
Borenstein & Freed [Page 19]RFC 1521 MIME September 1993
Now's the time =
for all folk to come=
to the aid of their country.
This provides a mechanism with which long lines are encoded in
such a way as to be restored by the user agent. The 76 character
limit does not count the trailing CRLF, but counts all other
characters, including any equal signs.
Since the hyphen character ("-") is represented as itself in the
Quoted-Printable encoding, care must be taken, when encapsulating a
quoted-printable encoded body in a multipart entity, to ensure that
the encapsulation boundary does not appear anywhere in the encoded
body. (A good strategy is to choose a boundary that includes a
character sequence such as "=_" which can never appear in a quoted-
printable body. See the definition of multipart messages later in
this document.)
NOTE: The quoted-printable encoding represents something of a
compromise between readability and reliability in transport.
Bodies encoded with the quoted-printable encoding will work
reliably over most mail gateways, but may not work perfectly over
a few gateways, notably those involving translation into EBCDIC.
(In theory, an EBCDIC gateway could decode a quoted-printable body
and re-encode it using base64, but such gateways do not yet
exist.) A higher level of confidence is offered by the base64
Content-Transfer-Encoding. A way to get reasonably reliable
transport through EBCDIC gateways is to also quote the ASCII
characters
!"#$@[\]^`{|}~
according to rule #1. See Appendix B for more information.
Because quoted-printable data is generally assumed to be line-
oriented, it is to be expected that the representation of the breaks
between the lines of quoted printable data may be altered in
transport, in the same manner that plain text mail has always been
altered in Internet mail when passing between systems with differing
newline conventions. If such alterations are likely to constitute a
corruption of the data, it is probably more sensible to use the
base64 encoding rather than the quoted-printable encoding.
WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-
printable, care must be taken to encode CR and LF characters as "=0D"
and "=0A", respectively. In particular, a CRLF sequence in binary
data should be encoded as "=0D=0A". Otherwise, if CRLF were
represented as a hard line break, it might be incorrectly decoded on
Borenstein & Freed [Page 20]RFC 1521 MIME September 1993
platforms with different line break conventions.
For formalists, the syntax of quoted-printable data is described by
the following grammar:
quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
; Maximum line length of 76 characters excluding CRLF
ptext := octet /<any ASCII character except "=", SPACE, or TAB>
; characters not listed as "mail-safe" in Appendix B
; are also not recommended.
octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; octet must be used for characters > 127, =, SPACE, or TAB,
; and is recommended for any characters not listed in
; Appendix B as "mail-safe".
5.2. Base64 Content-Transfer-Encoding
The Base64 Content-Transfer-Encoding is designed to represent
arbitrary sequences of octets in a form that need not be humanly
readable. The encoding and decoding algorithms are simple, but the
encoded data are consistently only about 33 percent larger than the
unencoded data. This encoding is virtually identical to the one used
in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.
The base64 encoding is adapted from RFC 1421, with one change: base64
eliminates the "*" mechanism for embedded clear text.
A 65-character subset of US-ASCII is used, enabling 6 bits to be
represented per printable character. (The extra 65th character, "=",
is used to signify a special processing function.)
NOTE: This subset has the important property that it is
represented identically in all versions of ISO 646, including US
ASCII, and all characters in the subset are also represented
identically in all versions of EBCDIC. Other popular encodings,
such as the encoding used by the uuencode utility and the base85
encoding specified as part of Level 2 PostScript, do not share
these properties, and thus do not fulfill the portability
requirements a binary transport encoding for mail must meet.
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating 3 8-bit input groups.
These 24 bits are then treated as 4 concatenated 6-bit groups, each
of which is translated into a single digit in the base64 alphabet.
When encoding a bit stream via the base64 encoding, the bit stream
must be presumed to be ordered with the most-significant-bit first.
Borenstein & Freed [Page 21]RFC 1521 MIME September 1993
That is, the first bit in the stream will be the high-order bit in
the first byte, and the eighth bit will be the low-order bit in the
first byte, and so on.
Each 6-bit group is used as an index into an array of 64 printable
characters. The character referenced by the index is placed in the
output string. These characters, identified in Table 1, below, are
selected so as to be universally representable, and the set excludes
characters with particular significance to SMTP (e.g., ".", CR, LF)
and to the encapsulation boundaries defined in this document (e.g.,
"-").
Table 1: The Base64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The output stream (encoded bytes) must be represented in lines of no
more than 76 characters each. All line breaks or other characters
not found in Table 1 must be ignored by decoding software. In base64
data, characters other than those in Table 1, line breaks, and other
white space probably indicate a transmission error, about which a
warning message or even a message rejection might be appropriate
under some circumstances.
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. A full encoding quantum is
always completed at the end of a body. When fewer than 24 input bits
are available in an input group, zero bits are added (on the right)
to form an integral number of 6-bit groups. Padding at the end of
the data is performed using the '=' character. Since all base64
input is an integral number of octets, only the following cases can
Borenstein & Freed [Page 22]RFC 1521 MIME September 1993
arise: (1) the final quantum of encoding input is an integral
multiple of 24 bits; here, the final unit of encoded output will be
an integral multiple of 4 characters with no "=" padding, (2) the
final quantum of encoding input is exactly 8 bits; here, the final
unit of encoded output will be two characters followed by two "="
padding characters, or (3) the final quantum of encoding input is
exactly 16 bits; here, the final unit of encoded output will be three
characters followed by one "=" padding character.
Because it is used only for padding at the end of the data, the
occurrence of any '=' characters may be taken as evidence that the
end of the data has been reached (without truncation in transit). No
such assurance is possible, however, when the number of octets
transmitted was a multiple of three.
Any characters outside of the base64 alphabet are to be ignored in
base64-encoded data. The same applies to any illegal sequence of
characters in the base64 encoding, such as "====="
Care must be taken to use the proper octets for line breaks if base64
encoding is applied directly to text material that has not been
converted to canonical form. In particular, text line breaks must be
converted into CRLF sequences prior to base64 encoding. The important
thing to note is that this may be done directly by the encoder rather
than in a prior canonicalization step in some implementations.
NOTE: There is no need to worry about quoting apparent
encapsulation boundaries within base64-encoded parts of multipart
entities because no hyphen characters are used in the base64
encoding.
6. Additional Content-Header Fields
6.1. Optional Content-ID Header Field
In constructing a high-level user agent, it may be desirable to allow
one body to make reference to another. Accordingly, bodies may be
labeled using the "Content-ID" header field, which is syntactically
identical to the "Message-ID" header field:
id := "Content-ID" ":" msg-id
Like the Message-ID values, Content-ID values must be generated to be
world-unique.
The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for cacheing data
referenced by the message/external-body mechanism. Although the
Content-ID header is generally optional, its use is mandatory in
Borenstein & Freed [Page 23]RFC 1521 MIME September 1993
implementations which generate data of the optional MIME Content-type
"message/external-body". That is, each message/external-body entity
must have a Content-ID field to permit cacheing of such data.
It is also worth noting that the Content-ID value has special
semantics in the case of the multipart/alternative content-type.
This is explained in the section of this document dealing with
multipart/alternative.
6.2. Optional Content-Description Header Field
The ability to associate some descriptive information with a given
body is often desirable. For example, it may be useful to mark an
"image" body as "a picture of the Space Shuttle Endeavor." Such text
may be placed in the Content-Description header field.
description := "Content-Description" ":" *text
The description is presumed to be given in the US-ASCII character
set, although the mechanism specified in [RFC-1522] may be used for
non-US-ASCII Content-Description values.
7. The Predefined Content-Type Values
This document defines seven initial Content-Type values and an
extension mechanism for private or experimental types. Further
standard types must be defined by new published specifications. It
is expected that most innovation in new types of mail will take place
as subtypes of the seven types defined here. The most essential
characteristics of the seven content-types are summarized in Appendix
F.
7.1 The Text Content-Type
The text Content-Type is intended for sending material which is
principally textual in form. It is the default Content-Type. A
"charset" parameter may be used to indicate the character set of the
body text for some text subtypes, notably including the primary
subtype, "text/plain", which indicates plain (unformatted) text. The
default Content-Type for Internet mail is "text/plain; charset=us-
ascii".
Beyond plain text, there are many formats for representing what might
be known as "extended text" -- text with embedded formatting and
presentation information. An interesting characteristic of many such
representations is that they are to some extent readable even without
the software that interprets them. It is useful, then, to
distinguish them, at the highest level, from such unreadable data as
Borenstein & Freed [Page 24]
RFC 1521 MIME September 1993
images, audio, or text represented in an unreadable form. In the
absence of appropriate interpretation software, it is reasonable to
show subtypes of text to the user, while it is not reasonable to do
so with most nontextual data.
Such formatted textual data should be represented using subtypes of
text. Plausible subtypes of text are typically given by the common
name of the representation format, e.g., "text/richtext" [RFC-1341].
7.1.1. The charset parameter
A critical parameter that may be specified in the Content-Type field
for text/plain data is the character set. This is specified with a
"charset" parameter, as in:
Content-type: text/plain; charset=us-ascii
Unlike some other parameter values, the values of the charset
parameter are NOT case sensitive. The default character set, which
must be assumed in the absence of a charset parameter, is US-ASCII.
The specification for any future subtypes of "text" must specify
whether or not they will also utilize a "charset" parameter, and may
possibly restrict its values as well. When used with a particular
body, the semantics of the "charset" parameter should be identical to
those specified here for "text/plain", i.e., the body consists
entirely of characters in the given charset. In particular, definers
of future text subtypes should pay close attention the the
implications of multibyte character sets for their subtype
definitions.
This RFC specifies the definition of the charset parameter for the
purposes of MIME to be a unique mapping of a byte stream to glyphs, a
mapping which does not require external profiling information.
An initial list of predefined character set names can be found at the
end of this section. Additional character sets may be registered
with IANA, although the standardization of their use requires the
usual IESG [RFC-1340] review and approval. Note that if the
specified character set includes 8-bit data, a Content-Transfer-
Encoding header field and a corresponding encoding on the data are
required in order to transmit the body via some mail transfer
protocols, such as SMTP.
The default character set, US-ASCII, has been the subject of some
confusion and ambiguity in the past. Not only were there some
ambiguities in the definition, there have been wide variations in
practice. In order to eliminate such ambiguity and variations in the
Borenstein & Freed [Page 25]RFC 1521 MIME September 1993
future, it is strongly recommended that new user agents explicitly
specify a character set via the Content-Type header field. "US-
ASCII" does not indicate an arbitrary seven-bit character code, but
specifies that the body uses character coding that uses the exact
correspondence of codes to characters specified in ASCII. National
use variations of ISO 646 [ISO-646] are NOT ASCII and their use in
Internet mail is explicitly discouraged. The omission of the ISO 646
character set is deliberate in this regard. The character set name
of "US-ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only.
The character set name "ASCII" is reserved and must not be used for
any purpose.
NOTE: RFC 821 explicitly specifies "ASCII", and references an
earlier version of the American Standard. Insofar as one of the
purposes of specifying a Content-Type and character set is to
permit the receiver to unambiguously determine how the sender
intended the coded message to be interpreted, assuming anything
other than "strict ASCII" as the default would risk unintentional
and incompatible changes to the semantics of messages now being
transmitted. This also implies that messages containing
characters coded according to national variations on ISO 646, or
using code-switching procedures (e.g., those of ISO 2022), as well
as 8-bit or multiple octet character encodings MUST use an
appropriate character set specification to be consistent with this
specification.
The complete US-ASCII character set is listed in [US-ASCII]. Note
that the control characters including DEL (0-31, 127) have no defined
meaning apart from the combination CRLF (ASCII values 13 and 10)
indicating a new line. Two of the characters have de facto meanings
in wide use: FF (12) often means "start subsequent text on the
beginning of a new page"; and TAB or HT (9) often (though not always)
means "move the cursor to the next available column after the current
position where the column number is a multiple of 8 (counting the
first column as column 0)." Apart from this, any use of the control
characters or DEL in a body must be part of a private agreement
between the sender and recipient. Such private agreements are
discouraged and should be replaced by the other capabilities of this
document.
NOTE: Beyond US-ASCII, an enormous proliferation of character sets
is possible. It is the opinion of the IETF working group that a
large number of character sets is NOT a good thing. We would
prefer to specify a single character set that can be used
universally for representing all of the world's languages in
electronic mail. Unfortunately, existing practice in several
communities seems to point to the continued use of multiple
character sets in the near future. For this reason, we define
Borenstein & Freed [Page 26]RFC 1521 MIME September 1993
names for a small number of character sets for which a strong
constituent base exists.
The defined charset values are:
US-ASCII -- as defined in [US-ASCII].
ISO-8859-X -- where "X" is to be replaced, as necessary, for the
parts of ISO-8859 [ISO-8859]. Note that the ISO 646
character sets have deliberately been omitted in favor of
their 8859 replacements, which are the designated character
sets for Internet mail. As of the publication of this
document, the legitimate values for "X" are the digits 1
through 9.
The character sets specified above are the ones that were relatively
uncontroversial during the drafting of MIME. This document does not
endorse the use of any particular character set other than US-ASCII,
and recognizes that the future evolution of world character sets
remains unclear. It is expected that in the future, additional
character sets will be registered for use in MIME.
Note that the character set used, if anything other than US-ASCII,
must always be explicitly specified in the Content-Type field.
No other character set name may be used in Internet mail without the
publication of a formal specification and its registration with IANA,
or by private agreement, in which case the character set name must
begin with "X-".
Implementors are discouraged from defining new character sets for
mail use unless absolutely necessary.
The "charset" parameter has been defined primarily for the purpose of
textual data, and is described in this section for that reason.
However, it is conceivable that non-textual data might also wish to
specify a charset value for some purpose, in which case the same
syntax and values should be used.
In general, mail-sending software must always use the "lowest common
denominator" character set possible. For example, if a body contains
only US-ASCII characters, it must be marked as being in the US-ASCII
character set, not ISO-8859-1, which, like all the ISO-8859 family of
character sets, is a superset of US-ASCII. More generally, if a
widely-used character set is a subset of another character set, and a
body contains only characters in the widely-used subset, it must be
labeled as being in that subset. This will increase the chances that
the recipient will be able to view the mail correctly.
Borenstein & Freed [Page 27]RFC 1521 MIME September 1993
7.1.2. The Text/plain subtype
The primary subtype of text is "plain". This indicates plain
(unformatted) text. The default Content-Type for Internet mail,
"text/plain; charset=us-ascii", describes existing Internet practice.
That is, it is the type of body defined by RFC 822.
No other text subtype is defined by this document.
The formal grammar for the content-type header field for text is as
follows:
text-type := "text" "/" text-subtype [";" "charset" "=" charset]
text-subtype := "plain" / extension-token
charset := "us-ascii"/ "iso-8859-1"/ "iso-8859-2"/ "iso-8859-3"
/ "iso-8859-4"/ "iso-8859-5"/ "iso-8859-6"/ "iso-8859-7"
/ "iso-8859-8" / "iso-8859-9" / extension-token
; case insensitive
7.2. The Multipart Content-Type
In the case of multiple part entities, in which one or more different
sets of data are combined in a single body, a "multipart" Content-
Type field must appear in the entity's header. The body must then
contain one or more "body parts," each preceded by an encapsulation
boundary, and the last one followed by a closing boundary. Each part
starts with an encapsulation boundary, and then contains a body part
consisting of header area, a blank line, and a body area. Thus a
body part is similar to an RFC 822 message in syntax, but different
in meaning.
A body part is NOT to be interpreted as actually being an RFC 822
message. To begin with, NO header fields are actually required in
body parts. A body part that starts with a blank line, therefore, is
allowed and is a body part for which all default values are to be
assumed. In such a case, the absence of a Content-Type header field
implies that the corresponding body is plain US-ASCII text. The only
header fields that have defined meaning for body parts are those the
names of which begin with "Content-". All other header fields are
generally to be ignored in body parts. Although they should
generally be retained in mail processing, they may be discarded by
gateways if necessary. Such other fields are permitted to appear in
body parts but must not be depended on. "X-" fields may be created
for experimental or private purposes, with the recognition that the
information they contain may be lost at some gateways.
Borenstein & Freed [Page 28]RFC 1521 MIME September 1993
NOTE: The distinction between an RFC 822 message and a body part
is subtle, but important. A gateway between Internet and X.400
mail, for example, must be able to tell the difference between a
body part that contains an image and a body part that contains an
encapsulated message, the body of which is an image. In order to
represent the latter, the body part must have "Content-Type:
message", and its body (after the blank line) must be the
encapsulated message, with its own "Content-Type: image" header
field. The use of similar syntax facilitates the conversion of
messages to body parts, and vice versa, but the distinction
between the two must be understood by implementors. (For the
special case in which all parts actually are messages, a "digest"
subtype is also defined.)
As stated previously, each body part is preceded by an encapsulation
boundary. The encapsulation boundary MUST NOT appear inside any of
the encapsulated parts. Thus, it is crucial that the composing agent
be able to choose and specify the unique boundary that will separate
the parts.
All present and future subtypes of the "multipart" type must use an
identical syntax. Subtypes may differ in their semantics, and may
impose additional restrictions on syntax, but must conform to the
required syntax for the multipart type. This requirement ensures
that all conformant user agents will at least be able to recognize
and separate the parts of any multipart entity, even of an
unrecognized subtype.
As stated in the definition of the Content-Transfer-Encoding field,
no encoding other than "7bit", "8bit", or "binary" is permitted for
entities of type "multipart". The multipart delimiters and header
fields are always represented as 7-bit ASCII in any case (though the
header fields may encode non-ASCII header text as per [RFC-1522]),
and data within the body parts can be encoded on a part-by-part
basis, with Content-Transfer-Encoding fields for each appropriate
body part.
Mail gateways, relays, and other mail handling agents are commonly
known to alter the top-level header of an RFC 822 message. In
particular, they frequently add, remove, or reorder header fields.
Such alterations are explicitly forbidden for the body part headers
embedded in the bodies of messages of type "multipart."
7.2.1. Multipart: The common syntax
All subtypes of "multipart" share a common syntax, defined in this
section. A simple example of a multipart message also appears in
this section. An example of a more complex multipart message is
Borenstein & Freed [Page 29]RFC 1521 MIME September 1993
given in Appendix C.
The Content-Type field for multipart entities requires one parameter,
"boundary", which is used to specify the encapsulation boundary. The
encapsulation boundary is defined as a line consisting entirely of
two hyphen characters ("-", decimal code 45) followed by the boundary
parameter value from the Content-Type header field.
NOTE: The hyphens are for rough compatibility with the earlier RFC
934 method of message encapsulation, and for ease of searching for
the boundaries in some implementations. However, it should be
noted that multipart messages are NOT completely compatible with
RFC 934 encapsulations; in particular, they do not obey RFC 934
quoting conventions for embedded lines that begin with hyphens.
This mechanism was chosen over the RFC 934 mechanism because the
latter causes lines to grow with each level of quoting. The
combination of this growth with the fact that SMTP implementations
sometimes wrap long lines made the RFC 934 mechanism unsuitable
for use in the event that deeply-nested multipart structuring is
ever desired.
WARNING TO IMPLEMENTORS: The grammar for parameters on the Content-
type field is such that it is often necessary to enclose the
boundaries in quotes on the Content-type line. This is not always
necessary, but never hurts. Implementors should be sure to study the
grammar carefully in order to avoid producing illegal Content-type
fields. Thus, a typical multipart Content-Type header field might
look like this:
Content-Type: multipart/mixed;
boundary=gc0p4Jq0M2Yt08jU534c0p
But the following is illegal:
Content-Type: multipart/mixed;
boundary=gc0p4Jq0M:2Yt08jU534c0p
(because of the colon) and must instead be represented as
Content-Type: multipart/mixed;
boundary="gc0p4Jq0M:2Yt08jU534c0p"
This indicates that the entity consists of several parts, each itself
with a structure that is syntactically identical to an RFC 822
message, except that the header area might be completely empty, and
that the parts are each preceded by the line
--gc0p4Jq0M:2Yt08jU534c0p
Borenstein & Freed [Page 30]RFC 1521 MIME September 1993
Note that the encapsulation boundary must occur at the beginning of a
line, i.e., following a CRLF, and that the initial CRLF is considered
to be attached to the encapsulation boundary rather than part of the
preceding part. The boundary must be followed immediately either by
another CRLF and the header fields for the next part, or by two
CRLFs, in which case there are no header fields for the next part
(and it is therefore assumed to be of Content-Type text/plain).
NOTE: The CRLF preceding the encapsulation line is conceptually
attached to the boundary so that it is possible to have a part
that does not end with a CRLF (line break). Body parts that must
be considered to end with line breaks, therefore, must have two
CRLFs preceding the encapsulation line, the first of which is part
of the preceding body part, and the second of which is part of the
encapsulation boundary.
Encapsulation boundaries must not appear within the encapsulations,
and must be no longer than 70 characters, not counting the two
leading hyphens.
The encapsulation boundary following the last body part is a
distinguished delimiter that indicates that no further body parts
will follow. Such a delimiter is identical to the previous
delimiters, with the addition of two more hyphens at the end of the
line:
--gc0p4Jq0M2Yt08jU534c0p--
There appears to be room for additional information prior to the
first encapsulation boundary and following the final boundary. These
areas should generally be left blank, and implementations must ignore
anything that appears before the first boundary or after the last
one.
NOTE: These "preamble" and "epilogue" areas are generally not used
because of the lack of proper typing of these parts and the lack
of clear semantics for handling these areas at gateways,
particularly X.400 gateways. However, rather than leaving the
preamble area blank, many MIME implementations have found this to
be a convenient place to insert an explanatory note for recipients
who read the message with pre-MIME software, since such notes will
be ignored by MIME-compliant software.
NOTE: Because encapsulation boundaries must not appear in the body
parts being encapsulated, a user agent must exercise care to
choose a unique boundary. The boundary in the example above could
have been the result of an algorithm designed to produce
boundaries with a very low probability of already existing in the
Borenstein & Freed [Page 31]RFC 1521 MIME September 1993
data to be encapsulated without having to prescan the data.
Alternate algorithms might result in more 'readable' boundaries
for a recipient with an old user agent, but would require more
attention to the possibility that the boundary might appear in the
encapsulated part. The simplest boundary possible is something
like "---", with a closing boundary of "-----".
As a very simple example, the following multipart message has two
parts, both of them plain text, one of them explicitly typed and one
of them implicitly typed:
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Subject: Sample message
MIME-Version: 1.0
Content-type: multipart/mixed; boundary="simple
boundary"
This is the preamble. It is to be ignored, though it
is a handy place for mail composers to include an
explanatory note to non-MIME conformant readers.
--simple boundary
This is implicitly typed plain ASCII text.
It does NOT end with a linebreak.
--simple boundary
Content-type: text/plain; charset=us-ascii
This is explicitly typed plain ASCII text.
It DOES end with a linebreak.
--simple boundary--
This is the epilogue. It is also to be ignored.
The use of a Content-Type of multipart in a body part within another
multipart entity is explicitly allowed. In such cases, for obvious
reasons, care must be taken to ensure that each nested multipart
entity must use a different boundary delimiter. See Appendix C for an
example of nested multipart entities.
The use of the multipart Content-Type with only a single body part
may be useful in certain contexts, and is explicitly permitted.
The only mandatory parameter for the multipart Content-Type is the
boundary parameter, which consists of 1 to 70 characters from a set
of characters known to be very robust through email gateways, and NOT
ending with white space. (If a boundary appears to end with white
space, the white space must be presumed to have been added by a
Borenstein & Freed [Page 32]RFC 1521 MIME September 1993
gateway, and must be deleted.) It is formally specified by the
following BNF:
boundary := 0*69<bchars> bcharsnospace
bchars := bcharsnospace / " "
bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" /"_"
/ "," / "-" / "." / "/" / ":" / "=" / "?"
Overall, the body of a multipart entity may be specified as
follows:
multipart-body := preamble 1*encapsulation
close-delimiter epilogue
encapsulation := delimiter body-part CRLF
delimiter := "--" boundary CRLF ; taken from Content-Type field.
; There must be no space
; between "--" and boundary.
close-delimiter := "--" boundary "--" CRLF ; Again, no space
by "--",
preamble := discard-text ; to be ignored upon receipt.
epilogue := discard-text ; to be ignored upon receipt.
discard-text := *(*text CRLF)
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. Note that the
semantics of a part differ from the semantics
of a message, as described in the text.>
NOTE: In certain transport enclaves, RFC 822 restrictions such as
the one that limits bodies to printable ASCII characters may not
be in force. (That is, the transport domains may resemble
standard Internet mail transport as specified in RFC821 and
assumed by RFC822, but without certain restrictions.) The
relaxation of these restrictions should be construed as locally
extending the definition of bodies, for example to include octets
outside of the ASCII range, as long as these extensions are
supported by the transport and adequately documented in the
Borenstein & Freed [Page 33]RFC 1521 MIME September 1993
Content-Transfer-Encoding header field. However, in no event are
headers (either message headers or body-part headers) allowed to
contain anything other than ASCII characters.
NOTE: Conspicuously missing from the multipart type is a notion of
structured, related body parts. In general, it seems premature to
try to standardize interpart structure yet. It is recommended
that those wishing to provide a more structured or integrated
multipart messaging facility should define a subtype of multipart
that is syntactically identical, but that always expects the
inclusion of a distinguished part that can be used to specify the
structure and integration of the other parts, probably referring
to them by their Content-ID field. If this approach is used,
other implementations will not recognize the new subtype, but will
treat it as the primary subtype (multipart/mixed) and will thus be
able to show the user the parts that are recognized.
7.2.2. The Multipart/mixed (primary) subtype
The primary subtype for multipart, "mixed", is intended for use when
the body parts are independent and need to be bundled in a particular
order. Any multipart subtypes that an implementation does not
recognize must be treated as being of subtype "mixed".
7.2.3. The Multipart/alternative subtype
The multipart/alternative type is syntactically identical to
multipart/mixed, but the semantics are different. In particular,
each of the parts is an "alternative" version of the same
information.
Systems should recognize that the content of the various parts are
interchangeable. Systems should choose the "best" type based on the
local environment and preferences, in some cases even through user
interaction. As with multipart/mixed, the order of body parts is
significant. In this case, the alternatives appear in an order of
increasing faithfulness to the original content. In general, the best
choice is the LAST part of a type supported by the recipient system's
local environment.
Multipart/alternative may be used, for example, to send mail in a
fancy text format in such a way that it can easily be displayed
anywhere:
Borenstein & Freed [Page 34]RFC 1521 MIME September 1993
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Subject: Formatted text mail
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary=boundary42
--boundary42
Content-Type: text/plain; charset=us-ascii
...plain text version of message goes here....
--boundary42
Content-Type: text/richtext
.... RFC 1341 richtext version of same message goes here ...
--boundary42
Content-Type: text/x-whatever
.... fanciest formatted version of same message goes here
...
--boundary42--
In this example, users whose mail system understood the "text/x-
whatever" format would see only the fancy version, while other users
would see only the richtext or plain text version, depending on the
capabilities of their system.
In general, user agents that compose multipart/alternative entities
must place the body parts in increasing order of preference, that is,
with the preferred format last. For fancy text, the sending user
agent should put the plainest format first and the richest format
last. Receiving user agents should pick and display the last format
they are capable of displaying. In the case where one of the
alternatives is itself of type "multipart" and contains unrecognized
sub-parts, the user agent may choose either to show that alternative,
an earlier alternative, or both.
NOTE: From an implementor's perspective, it might seem more
sensible to reverse this ordering, and have the plainest
alternative last. However, placing the plainest alternative first
is the friendliest possible option when multipart/alternative
entities are viewed using a non-MIME-conformant mail reader.
While this approach does impose some burden on conformant mail
readers, interoperability with older mail readers was deemed to be
more important in this case.
It may be the case that some user agents, if they can recognize more
than one of the formats, will prefer to offer the user the choice of
Borenstein & Freed [Page 35]RFC 1521 MIME September 1993
which format to view. This makes sense, for example, if mail
includes both a nicely-formatted image version and an easily-edited
text version. What is most critical, however, is that the user not
automatically be shown multiple versions of the same data. Either
the user should be shown the last recognized version or should be
given the choice.
NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each
part of a multipart/alternative entity represents the same data, but
the mappings between the two are not necessarily without information
loss. For example, information is lost when translating ODA to
PostScript or plain text. It is recommended that each part should
have a different Content-ID value in the case where the information
content of the two parts is not identical. However, where the
information content is identical -- for example, where several parts
of type "application/external- body" specify alternate ways to access
the identical data -- the same Content-ID field value should be used,
to optimize any cacheing mechanisms that might be present on the
recipient's end. However, it is recommended that the Content-ID
values used by the parts should not be the same Content-ID value that
describes the multipart/alternative as a whole, if there is any such
Content-ID field. That is, one Content-ID value will refer to the
multipart/alternative entity, while one or more other Content-ID
values will refer to the parts inside it.
7.2.4. The Multipart/digest subtype
This document defines a "digest" subtype of the multipart Content-
Type. This type is syntactically identical to multipart/mixed, but
the semantics are different. In particular, in a digest, the default
Content-Type value for a body part is changed from "text/plain" to
"message/rfc822". This is done to allow a more readable digest
format that is largely compatible (except for the quoting convention)
with RFC 934.
Borenstein & Freed [Page 36]
RFC 1521 MIME September 1993
A digest in this format might, then, look something like this:
From: Moderator-Address
To: Recipient-List
MIME-Version: 1.0
Subject: Internet Digest, volume 42
Content-Type: multipart/digest;
boundary="---- next message ----"
------ next message ----
From: someone-else
Subject: my opinion
...body goes here ...
------ next message ----
From: someone-else-again
Subject: my different opinion
... another body goes here...
------ next message ------
7.2.5. The Multipart/parallel subtype
This document defines a "parallel" subtype of the multipart Content-
Type. This type is syntactically identical to multipart/mixed, but
the semantics are different. In particular, in a parallel entity,
the order of body parts is not significant.
A common presentation of this type is to display all of the parts
simultaneously on hardware and software that are capable of doing so.
However, composing agents should be aware that many mail readers will
lack this capability and will show the parts serially in any event.
7.2.6. Other Multipart subtypes
Other multipart subtypes are expected in the future. MIME
implementations must in general treat unrecognized subtypes of
multipart as being equivalent to "multipart/mixed".
The formal grammar for content-type header fields for multipart data
is given by:
multipart-type := "multipart" "/" multipart-subtype
";" "boundary" "=" boundary
Borenstein & Freed [Page 37]RFC 1521 MIME September 1993
multipart-subtype := "mixed" / "parallel" / "digest"
/ "alternative" / extension-token
7.3. The Message Content-Type
It is frequently desirable, in sending mail, to encapsulate another
mail message. For this common operation, a special Content-Type,
"message", is defined. The primary subtype, message/rfc822, has no
required parameters in the Content-Type field. Additional subtypes,
"partial" and "External-body", do have required parameters. These
subtypes are explained below.
NOTE: It has been suggested that subtypes of message might be
defined for forwarded or rejected messages. However, forwarded
and rejected messages can be handled as multipart messages in
which the first part contains any control or descriptive
information, and a second part, of type message/rfc822, is the
forwarded or rejected message. Composing rejection and forwarding
messages in this manner will preserve the type information on the
original message and allow it to be correctly presented to the
recipient, and hence is strongly encouraged.
As stated in the definition of the Content-Transfer-Encoding field,
no encoding other than "7bit", "8bit", or "binary" is permitted for
messages or parts of type "message". Even stronger restrictions
apply to the subtypes "message/partial" and "message/external-body",
as specified below. The message header fields are always US-ASCII in
any case, and data within the body can still be encoded, in which
case the Content-Transfer-Encoding header field in the encapsulated
message will reflect this. Non-ASCII text in the headers of an
encapsulated message can be specified using the mechanisms described
in [RFC-1522].
Mail gateways, relays, and other mail handling agents are commonly
known to alter the top-level header of an RFC 822 message. In
particular, they frequently add, remove, or reorder header fields.
Such alterations are explicitly forbidden for the encapsulated
headers embedded in the bodies of messages of type "message."
7.3.1. The Message/rfc822 (primary) subtype
A Content-Type of "message/rfc822" indicates that the body contains
an encapsulated message, with the syntax of an RFC 822 message.
However, unlike top-level RFC 822 messages, it is not required that
each message/rfc822 body must include a "From", "Subject", and at
least one destination header.
It should be noted that, despite the use of the numbers "822", a
Borenstein & Freed [Page 38]RFC 1521 MIME September 1993
message/rfc822 entity can include enhanced information as defined in
this document. In other words, a message/rfc822 message may be a
MIME message.
7.3.2. The Message/Partial subtype
A subtype of message, "partial", is defined in order to allow large
objects to be delivered as several separate pieces of mail and
automatically reassembled by the receiving user agent. (The concept
is similar to IP fragmentation/reassembly in the basic Internet
Protocols.) This mechanism can be used when intermediate transport
agents limit the size of individual messages that can be sent.
Content-Type "message/partial" thus indicates that the body contains
a fragment of a larger message.
Three parameters must be specified in the Content-Type field of type
message/partial: The first, "id", is a unique identifier, as close to
a world-unique identifier as possible, to be used to match the parts
together. (In general, the identifier is essentially a message-id;
if placed in double quotes, it can be any message-id, in accordance
with the BNF for "parameter" given earlier in this specification.)
The second, "number", an integer, is the part number, which indicates
where this part fits into the sequence of fragments. The third,
"total", another integer, is the total number of parts. This third
subfield is required on the final part, and is optional (though
encouraged) on the earlier parts. Note also that these parameters
may be given in any order.
Thus, part 2 of a 3-part message may have either of the following
header fields:
Content-Type: Message/Partial;
number=2; total=3;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com"
Content-Type: Message/Partial;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
number=2
But part 3 MUST specify the total number of parts:
Content-Type: Message/Partial;
number=3; total=3;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com"
Note that part numbering begins with 1, not 0.
When the parts of a message broken up in this manner are put
Borenstein & Freed [Page 39]RFC 1521 MIME September 1993
together, the result is a complete MIME entity, which may have its
own Content-Type header field, and thus may contain any other data
type.
Message fragmentation and reassembly: The semantics of a reassembled
partial message must be those of the "inner" message, rather than of
a message containing the inner message. This makes it possible, for
example, to send a large audio message as several partial messages,
and still 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 entities. In this process the following
rules must be observed:
(1) All of the header fields from the initial enclosing entity
(part one), except those that start with "Content-" and the
specific header fields "Message-ID", "Encrypted", and "MIME-
Version", must be copied, in order, to the new message.
(2) Only those header fields in the enclosed message which start
with "Content-" and "Message-ID", "Encrypted", and "MIME-Version"
must be appended, in order, to the header fields of the new
message. Any header fields in the enclosed message which do not
start with "Content-" (except for "Message-ID", "Encrypted", and
"MIME-Version") will be ignored.
(3) All of the header fields 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
Borenstein & Freed [Page 40]RFC 1521 MIME September 1993
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...
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:
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...
Note on encoding of MIME entities encapsulated inside message/partial
entities: Because data of type "message" may never be encoded in
base64 or quoted-printable, a problem might arise if message/partial
entities are constructed in an environment that supports binary or
8-bit transport. The problem is that the binary data would be split
into multiple message/partial objects, each of them requiring binary
transport. If such objects were encountered at a gateway into a 7-
bit transport environment, there would be no way to properly encode
them for the 7-bit world, aside from waiting for all of the parts,
reassembling the message, and then encoding the reassembled data in
base64 or quoted-printable. Since it is possible that different
parts might go through different gateways, even this is not an
acceptable solution. For this reason, it is specified that MIME
entities of type message/partial must always have a content-
Borenstein & Freed [Page 41]RFC 1521 MIME September 1993
transfer-encoding of 7-bit (the default). In particular, even in
environments that support binary or 8-bit transport, the use of a
content-transfer-encoding of "8bit" or "binary" is explicitly
prohibited for entities of type message/partial.
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.
Finally, it should be noted that the "Encrypted" header field has
been made obsolete by Privacy Enhanced Messaging (PEM), but the rules
above are believed to describe the correct way to treat it if it is
encountered in the context of conversion to and from message/partial
fragments.
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 an entity 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
Content-ID: <id42@guppylake.bellcore.com>
Content-Transfer-Encoding: binary
Borenstein & Freed [Page 42]RFC 1521 MIME September 1993
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 auxiliary information for some 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 -- A case-insensitive word, indicating the supported
access mechanism 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 E .
In addition, the following three parameters are optional for ALL
access-types:
EXPIRATION -- The date (in the RFC 822 "date-time" syntax, as
extended by RFC 1123 to permit 4 digits in the year 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. Note that
this describes the size of the data in its canonical form, that
is, before any Content- Transfer-Encoding has been applied or
after the data have been decoded.
PERMISSION -- A case-insensitive 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.
The encapsulated headers in ALL message/external-body entities MUST
include a Content-ID header field to give a unique identifier by
Borenstein & Freed [Page 43]RFC 1521 MIME September 1993
which to reference the data. This identifier may be used for
cacheing mechanisms, and for recognizing the receipt of the data when
the access-type is "mail-server".
Note that, as specified here, the tokens that describe external-body
data, such as file names and mail server commands, are required to be
in the US-ASCII character set. If this proves problematic in
practice, a new mechanism may be required as a future extension to
MIME, either as newly defined access-types for message/external-body
or by some other mechanism.
As with message/partial, it is specified that MIME entities of type
message/external-body must always have a content-transfer-encoding of
7-bit (the default). In particular, even in environments that
support binary or 8-bit transport, the use of a content-transfer-
encoding of "8bit" or "binary" is explicitly prohibited for entities
of type message/external-body.
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:
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. This must be a fully qualified domain name, not a
nickname.
Before any data are retrieved, using FTP, the user will generally
need to be asked to provide a login id and a password for the machine
named by the site parameter. For security reasons, such an id and
password are not specified as content-type parameters, but must be
obtained from the user.
In addition, the following parameters are optional:
DIRECTORY -- A directory from which the data named by NAME should
be retrieved.
MODE -- A case-insensitive string indicating the mode to be used
when retrieving the information. The legal values for access-type
"TFTP" are "NETASCII", "OCTET", and "MAIL", as specified by the
TFTP protocol [RFC-783]. The legal values for access-type "FTP"
are "ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a
decimal integer, typically 8. These correspond to the
Borenstein & Freed [Page 44]RFC 1521 MIME September 1993
representation types "A" "E" "I" and "L n" as specified by the FTP
protocol [RFC-959]. Note that "BINARY" and "TENEX" are not valid
values for MODE, but that "OCTET" or "IMAGE" or "LOCAL8" should be
used instead. IF MODE is not specified, the default value is
"NETASCII" for TFTP and "ASCII" otherwise.
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
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 syntaxes, 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 is
provided as the "phantom body" when the content-type is
Borenstein & Freed [Page 45]RFC 1521 MIME September 1993
message/external-body and the access- type is mail-server.
An optional parameter for this access-type is:
SUBJECT -- The subject that is to be used in the mail that is sent
to obtain the data. Note that keying mail servers on Subject lines
is NOT recommended, but such mail servers are known to exist.
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 must include the phantom body in the body of
the message it sends to the mail server address to retrieve the
relevant data.
It is worth noting that, unlike other access-types, mail-server
access is asynchronous and will happen at an unpredictable time in
the future. For this reason, it is important that there be a
mechanism by which the returned data can be matched up with the
original message/external-body entity. MIME mailservers must use the
same Content-ID field on the returned message that was used in the
original message/external-body entity, to facilitate such matching.
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
Borenstein & Freed [Page 46]RFC 1521 MIME September 1993
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 the value of the SERVER
parameter.
The embedded message header fields which appear in the body of the
message/external-body data must be used to declare the Content-type
of the external body if it is anything other than plain ASCII text,
since the external body does not have a header section to declare its
type. Similarly, any Content-transfer-encoding other than "7bit"
must also be declared here. Thus a complete message/external-body
message, referring to a document in PostScript format, might look
like this:
From: Whomever
To: Someone
Subject: whatever
MIME-Version: 1.0
Message-ID: <id1@host.com>
Content-Type: multipart/alternative; boundary=42
Content-ID: <id001@guppylake.bellcore.com>
--42
Content-Type: message/external-body;
name="BodyFormats.ps";
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
Content-ID: <id42@guppylake.bellcore.com>
--42
Content-Type: message/external-body;
name="/u/nsb/writing/rfcs/RFC-MIME.ps";
site="thumper.bellcore.com";
access-type=AFS
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
Content-ID: <id42@guppylake.bellcore.com>
--42
Content-Type: message/external-body;
access-type=mail-server
Borenstein & Freed [Page 47]RFC 1521 MIME September 1993
server="listserv@bogus.bitnet";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
Content-type: application/postscript
Content-ID: <id42@guppylake.bellcore.com>
get RFC-MIME.DOC
--42--
Note that in the above examples, the default Content-transfer-
encoding of "7bit" is assumed for the external postscript data.
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 RFC 822 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.
The formal grammar for content-type header fields for data of type
message is given by:
message-type := "message" "/" message-subtype
message-subtype := "rfc822"
/ "partial" 2#3partial-param
/ "external-body" 1*external-param
/ extension-token
partial-param := (";" "id" "=" value)
/ (";" "number" "=" 1*DIGIT)
/ (";" "total" "=" 1*DIGIT)
; id & number required; total required for last part
external-param := (";" "access-type" "=" atype)
Borenstein & Freed [Page 48]RFC 1521 MIME September 1993
/ (";" "expiration" "=" date-time)
; Note that date-time is quoted
/ (";" "size" "=" 1*DIGIT)
/ (";" "permission" "=" ("read" / "read-write"))
; Permission is case-insensitive
/ (";" "name" "=" value)
/ (";" "site" "=" value)
/ (";" "dir" "=" value)
/ (";" "mode" "=" value)
/ (";" "server" "=" value)
/ (";" "subject" "=" value)
; access-type required;others required based on access-type
atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
/ "afs" / "mail-server" / extension-token
; Case-insensitive
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 must 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 two subtypes: octet-stream, 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 (other than subtypes beginning
Borenstein & Freed [Page 49]RFC 1521 MIME September 1993
with "x-") must be registered with IANA, as described in Appendix E.
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:
TYPE -- the general type or category of binary data. This is
intended as information for the human recipient rather than for
any automatic processing.
PADDING -- the number of bits of padding that were appended to the
bit-stream comprising the actual contents to produce the enclosed
byte-oriented data. This is useful for enclosing a bit-stream in
a body when the total number of bits is not a multiple of the byte
size.
An additional parameter, "conversions", was defined in [RFC-1341] but
has been removed.
RFC 1341 also defined the use of a "NAME" parameter which gave a
suggested file name to be used if the data were to be written to a
file. This has been deprecated in anticipation of a separate
Content-Disposition header field, to be defined in a subsequent RFC.
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. Currently two variants of the PostScript language are
allowed; the original level 1 variant is described in [POSTSCRIPT]
and the more recent level 2 variant is described in [POSTSCRIPT2].
PostScript is a registered trademark of Adobe Systems, Inc. Use of
the MIME content-type "application/postscript" implies recognition of
that trademark and all the rights it entails.
Borenstein & Freed [Page 50]RFC 1521 MIME September 1993
The PostScript language definition provides facilities for internal
labeling of the specific language features a given program uses. This
labeling, called the PostScript document structuring conventions, is
very general and provides substantially more information than just
the language level.
The use of document structuring conventions, while not required, is
strongly recommended as an aid to interoperability. Documents which
lack proper structuring conventions cannot be tested to see whether
or not they will work in a given environment. As such, some systems
may assume the worst and refuse to process unstructured documents.
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 re-enabling
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
Borenstein & Freed [Page 51]
RFC 1521 MIME September 1993
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, and eliminate the startjob and exitserver commands. If
these commands cannot be eliminated, the password associated with
them should at least be set 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, the password associated with them
should at least be set 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
Borenstein & Freed [Page 52]
RFC 1521 MIME September 1993
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. Other Application subtypes
It is expected that many other subtypes of application will be
defined in the future. MIME implementations must generally treat any
unrecognized subtypes as being equivalent to application/octet-
stream.
The formal grammar for content-type header fields for application
data is given by:
application-type := "application" "/" application-subtype
application-subtype := ("octet-stream" *stream-param)
/ "postscript" / extension-token
stream-param := (";" "type" "=" value)
/ (";" "padding" "=" padding)
padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"
7.5. The Image Content-Type
A Content-Type of "image" indicates that the body contains 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 E.
The formal grammar for the content-type header field for data of type
image is given by:
Borenstein & Freed [Page 53]RFC 1521 MIME September 1993
image-type := "image" "/" ("gif" / "jpeg" / extension-token)
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 mu-law [PCM]. When this subtype is present, a sample rate of
8000 Hz and a single channel is assumed.
The formal grammar for the content-type header field for data of type
audio is given by:
audio-type := "audio" "/" ("basic" / extension-token)
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".
The formal grammar for the content-type header field for data of type
video is given by:
video-type := "video" "/" ("mpeg" / extension-token)
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
Borenstein & Freed [Page 54]RFC 1521 MIME September 1993
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.
Borenstein & Freed [Page 55]
RFC 1521 MIME September 1993
8. 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 message/partial, and
message/external-body.
9. Security Considerations
Security issues are discussed in Section 7.4.2 and in Appendix F.
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 application/postscript content-type in
Section 7.4.2 may serve as a model for considering other content-
types with remote execution capabilities.
Borenstein & Freed [Page 56]
RFC 1521 MIME September 1993
10. 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: +1 909 624 7907
Fax: +1 909 621 5319
Email: ned@innosoft.com
MIME is a result of the work of the Internet Engineering Task Force
Working Group on Email Extensions. The chairman of that group, Greg
Vaudreuil, may be reached at:
Gregory M. Vaudreuil
Tigon Corporation
17060 Dallas Parkway
Dallas Texas, 75248
Phone: +1 214-733-2722
EMail: gvaudre@cnri.reston.va.us
Borenstein & Freed [Page 57]
RFC 1521 MIME September 1993
11. 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
Borenstein & Freed [Page 58]RFC 1521 MIME September 1993
Jim Knowles Wally Wedel
Stev Knowles Sven-Ove Westberg
Bob Kummerfeld Brian Wideen
Pekka Kytolaakso John Wobus
Stellan Lagerstrom Glenn Wright
Vincent Lau Rayan Zachariassen
Donald Lindsay David Zimmerman
Marc Andreessen Bob Braden
Brian Capouch Peter Clitherow
Dave Collier-Brown John Coonrod
Stephen Crocker Jim Davis
Axel Deininger Dana S Emery
Martin Forssen Stephen Gildea
Terry Gray Mark Horton
Warner Losh Carlyn Lowery
Laurence Lundblade Charles Lynn
Larry Masinter Michael J. McInerny
Jon Postel Christer Romson
Yutaka Sato Markku Savela
Richard Alan Schafer Larry W. Virden
Rhys Weatherly Jay Weber
Dave Wecker
The authors apologize for any omissions from this list, which are
certainly unintentional.
Borenstein & Freed [Page 59]RFC 1521 MIME September 1993
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.
Borenstein & Freed [Page 60]RFC 1521 MIME September 1993
-- For unrecognized subtypes, show or
offer to show the user the "raw"
version of the data after
conversion of the content from
canonical form to local form.
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-
Borenstein & Freed [Page 61]RFC 1521 MIME September 1993
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.
Borenstein & Freed [Page 62]
RFC 1521 MIME September 1993
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 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) 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 must 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
Borenstein & Freed [Page 63]RFC 1521 MIME September 1993
DISCOURAGED, but unavoidable in some cases. Applications which
require long lines must somehow differentiate between soft and
hard line breaks. (A simple way to do this is to use the quoted-
printable encoding.)
(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, must 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.
(7) Some mail transport agents will corrupt data that includes
certain literal strings. In particular, a period (".") alone on a
line is known to be corrupted by some (incorrect) SMTP
implementations, and a line that starts with the five characters
"From " (the fifth character is a SPACE) are commonly corrupted as
Borenstein & Freed [Page 64]RFC 1521 MIME September 1993
well. A careful composition agent can prevent these corruptions
by encoding the data (e.g., in the quoted-printable encoding,
"=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
illegal practices are known to occur on established networks, and
implementations should be robust in dealing with the bad effects they
can cause.
Borenstein & Freed [Page 65]RFC 1521 MIME September 1993
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>
To: Ned Freed <ned@innosoft.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
mu-law-format audio data goes here....
Borenstein & Freed [Page 66]RFC 1521 MIME September 1993
--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>
<smaller>as defined in RFC 1341</smaller>
<nl><nl>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--
Borenstein & Freed [Page 67]RFC 1521 MIME September 1993
Appendix D -- 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.
application-subtype := ("octet-stream" *stream-param)
/ "postscript" / extension-token
application-type := "application" "/" application-subtype
attribute := token ; case-insensitive
atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
/ "afs" / "mail-server" / extension-token
; Case-insensitive
audio-type := "audio" "/" ("basic" / extension-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.>
NOTE: In certain transport enclaves, RFC 822 restrictions such as
the one that limits bodies to printable ASCII characters may not
be in force. (That is, the transport domains may resemble
standard Internet mail transport as specified in RFC821 and
assumed by RFC822, but without certain restrictions.) The
relaxation of these restrictions should be construed as locally
extending the definition of bodies, for example to include octets
outside of the ASCII range, as long as these extensions are
supported by the transport and adequately documented in the
Content-Transfer-Encoding header field. However, in no event are
headers (either message headers or body-part headers) allowed to
contain anything other than ASCII characters.
Borenstein & Freed [Page 68]RFC 1521 MIME September 1993
boundary := 0*69<bchars> bcharsnospace
bchars := bcharsnospace / " "
bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_"
/ "," / "-" / "." / "/" / ":" / "=" / "?"
charset := "us-ascii" / "iso-8859-1" / "iso-8859-2"/ "iso-8859-3"
/ "iso-8859-4" / "iso-8859-5" / "iso-8859-6" / "iso-8859-7"
/ "iso-8859-8" / "iso-8859-9" / extension-token
; case insensitive
close-delimiter := "--" boundary "--" CRLF;Again,no space by "--",
content := "Content-Type" ":" type "/" subtype *(";" parameter)
; case-insensitive matching of type and subtype
delimiter := "--" boundary CRLF ;taken from Content-Type field.
; There must be no space
; between "--" and boundary.
description := "Content-Description" ":" *text
discard-text := *(*text CRLF)
encapsulation := delimiter body-part CRLF
encoding := "Content-Transfer-Encoding" ":" mechanism
epilogue := discard-text ; to be ignored upon receipt.
extension-token := x-token / iana-token
external-param := (";" "access-type" "=" atype)
/ (";" "expiration" "=" date-time)
; Note that date-time is quoted
/ (";" "size" "=" 1*DIGIT)
/ (";" "permission" "=" ("read" / "read-write"))
; Permission is case-insensitive
/ (";" "name" "=" value)
/ (";" "site" "=" value)
/ (";" "dir" "=" value)
/ (";" "mode" "=" value)
/ (";" "server" "=" value)
/ (";" "subject" "=" value)
;access-type required; others required based on access-type
Borenstein & Freed [Page 69]RFC 1521 MIME September 1993
iana-token := <a publicly-defined extension token,
registered with IANA, as specified in
appendix E>
id := "Content-ID" ":" msg-id
image-type := "image" "/" ("gif" / "jpeg" / extension-token)
mechanism := "7bit" ; case-insensitive
/ "quoted-printable"
/ "base64"
/ "8bit"
/ "binary"
/ x-token
message-subtype := "rfc822"
/ "partial" 2#3partial-param
/ "external-body" 1*external-param
/ extension-token
message-type := "message" "/" message-subtype
multipart-body :=preamble 1*encapsulation close-delimiter epilogue
multipart-subtype := "mixed" / "parallel" / "digest"
/ "alternative" / extension-token
multipart-type := "multipart" "/" multipart-subtype
";" "boundary" "=" boundary
octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; octet must be used for characters > 127, =, SPACE, or
TAB,
; and is recommended for any characters not listed in
; Appendix B as "mail-safe".
padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"
parameter := attribute "=" value
partial-param := (";" "id" "=" value)
/ (";" "number" "=" 1*DIGIT)
/ (";" "total" "=" 1*DIGIT)
; id & number required;total required for last part
preamble := discard-text ; to be ignored upon receipt.
ptext := octet / <any ASCII character except "=", SPACE, or TAB>
Borenstein & Freed [Page 70]RFC 1521 MIME September 1993
; characters not listed as "mail-safe" in Appendix B
; are also not recommended.
quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
; Maximum line length of 76 characters excluding CRLF
stream-param := (";" "type" "=" value)
/ (";" "padding" "=" padding)
subtype := token ; case-insensitive
text-subtype := "plain" / extension-token
text-type := "text" "/" text-subtype [";" "charset" "=" charset]
token := 1*<any (ASCII) 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" / extension-token
; All values case-insensitive
value := token / quoted-string
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
video-type := "video" "/" ("mpeg" / extension-token)
x-token := <The two characters "X-" or "x-" followed, with no
intervening white space, by any token>
Borenstein & Freed [Page 71]RFC 1521 MIME September 1993
Appendix E -- 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, 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.
E.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 must 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:
Borenstein & Freed [Page 72]RFC 1521 MIME September 1993
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:
E.2 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:
Borenstein & Freed [Page 73]RFC 1521 MIME September 1993
Appendix F -- Summary of the Seven Content-types
Content-type: text
Subtypes defined by this document: plain
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, access-type, expiration,
size, permission, name, site, directory, mode, server, subject
Encoding notes: No content-transfer-encoding is permitted.
Specifically, only "7bit" is permitted for "message/partial" or
"message/external-body", and only "7bit", "8bit", or "binary" are
permitted for other subtypes of "message".
______________________________________________________________
Content-type: application
Subtypes defined by this document: octet-stream, postscript
Important Parameters: type, padding
Borenstein & Freed [Page 74]RFC 1521 MIME September 1993
Deprecated Parameters: name and conversions were
defined in RFC 1341.
Encoding notes: base64 preferred for 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. 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
Borenstein & Freed [Page 75]RFC 1521 MIME September 1993
Appendix G -- 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 entity can be modeled as being done
in a number of steps. Note that these steps are roughly similar to
those steps used in RFC 1421 and are performed for each 'innermost
level' body:
Step 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 specified by the type/subtype information.
Step 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 content 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
content types. If character set conversion is involved, however,
care must be taken to understand the semantics of the content-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 RFC822. Note that the
restriction on line lengths implied by RFC822 is eliminated if the
next step employs either quoted-printable or base64 encoding.
Borenstein & Freed [Page 76]
RFC 1521 MIME September 1993
Step 3. Apply transfer encoding.
A Content-Transfer-Encoding appropriate for this body 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 a body.
Step 4. Insertion into entity.
The encoded object is inserted into a MIME entity with appropriate
headers. The entity is then inserted into the body of a higher-level
entity (message or multipart) if needed.
It is vital to note that these steps are only a model; they are
specifically NOT a blueprint for how an actual system 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 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 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.
Borenstein & Freed [Page 77]RFC 1521 MIME September 1993
Appendix H -- Changes from RFC 1341
This document is a relatively minor revision of RFC 1341. For
the convenience of those familiar with RFC 1341, the technical
changes from that document are summarized in this appendix.
1. The definition of "tspecials" has been changed to no longer
include ".".
2. The Content-ID field is now mandatory for message/external-body
parts.
3. The text/richtext type (including the old Section 7.1.3 and
Appendix D) has been moved to a separate document.
4. The rules on header merging for message/partial data have been
changed to treat the Encrypted and MIME-Version headers as special
cases.
5. The definition of the external-body access-type parameter has
been changed so that it can only indicate a single access method
(which was all that made sense).
6. There is a new "Subject" parameter for message/external-body,
access-type mail-server, to permit MIME-based use of mail servers
that rely on Subject field information.
7. The "conversions" parameter for application/octet-stream has been
removed.
8. Section 7.4.1 now deprecates the use of the "name" parameter for
application/octet-stream, as this will be superseded in the future by
a Content-Disposition header.
9. The formal grammar for multipart bodies has been changed so that
a CRLF is no longer required before the first boundary line.
10. MIME entities of type "message/partial" and "message/external-
body" are now required to use only the "7bit" transfer-encoding.
(Specifically, "binary" and "8bit" are not permitted.)
11. The "application/oda" content-type has been removed.
12. A note has been added to the end of section 7.2.3, explaining
the semantics of Content-ID in a multipart/alternative MIME entity.
13. The formal syntax for the "MIME-Version" field has been
tightened, but in a way that is completely compatible with the only
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RFC 1521 MIME September 1993
version number defined in RFC 1341.
14. In Section 7.3.1, the definition of message/rfc822 has been
relaxed regarding mandatory fields.
All other changes from RFC 1341 were editorial changes and do not
affect the technical content of MIME. Considerable formal grammar
has been added, but this reflects the prose specification that was
already in place.
Borenstein & Freed [Page 79]
RFC 1521 MIME September 1993
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.
[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.
[POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference
Manual, Addison-Wesley, Second Edition, 1990.
[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., "TFTP Protocol (revision 2)", RFC 783, MIT,
June 1981.
[RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, USC/Information Sciences Institute, August 1982.
Borenstein & Freed [Page 80]
RFC 1521 MIME September 1993
[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", STD 11, RFC 1049, CMU, March 1988.
[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-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.
[RFC-1342] Moore, K., "Representation of Non-Ascii Text in Internet
Message Headers", RFC 1342, University of Tennessee, June 1992.
[RFC-1343] Borenstein, N., "A User Agent Configuration Mechanism
for Multimedia Mail Format Information", RFC 1343, Bellcore, 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-1426] Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,
Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIME
transport", RFC 1426, United Nations Universit, Innosoft, Dover Beach
Consulting, Inc., Network Management Associates, Inc., The Branch
Office, February 1993.
[RFC-1522] Moore, K., "Representation of Non-Ascii Text in Internet
Message Headers" RFC 1522, University of Tennessee, September 1993.
[RFC-1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1340, USC/Information Sciences Institute, July 1992.
Borenstein & Freed [Page 81]
