WDIFF

draft-ietf-822ext-mime-imb-00.txt --> draft-ietf-822ext-mime-imb-01.txt

Network Working Group N. Nathaniel Borenstein
Internet Draft First Virtual Holdings
Expires in six months N. Freed, Innosoft
May 1994

MIME (Multipurpose Ned Freed
<draft-ietf-822ext-mime-imb-01.txt>

Multipurpose Internet Mail Extensions) Extensions
(MIME) Part One:

Mechanisms for Specifying and Describing
the

Format of Internet Message Bodies

<draft-ietf-822ext-mime-imb-00.txt>

November 21, 1994

Status of this Memo

This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-
Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and
months. Internet-Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is inappropriate not appropriate to use
Internet- Drafts
Internet-Drafts as reference material or to cite them other
than as ``work a "working draft" or "work in progress.'' progress".

To learn the current status of any Internet-Draft, please
check the ``1id-abstracts.txt'' 1id-abstracts.txt listing contained in the
Internet- Drafts
Internet-Drafts Shadow Directories on ds.internic.net (US East
Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast),
or munnari.oz.au (Pacific Rim).

Internet Draft MIME Part One November 1994

1. Abstract

STD 11, RFC 822 defines a message representation protocol
which specifies
specifying considerable detail about message headers, but
which leaves the message content, or message body, as flat ASCII
US-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, STD 11, and RFC 1049, but extends and revises
that work. them.
Because RFC 822 said so little about message bodies, this
document is largely orthogonal to (rather than a revision of)
RFC 822.

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In particular, this document is designed to provide facilities
to include multiple objects parts in a single message, to represent
body text in character sets other than US-
ASCII, 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]. [RFC-MIME-HEADERS].

This document is a revision of RFC 1521, which was a revision
of RFC 1341. Significant differences from RFC 1521 are
summarized in Appendix H.

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1 Introduction

Since its publication in 1982, RFC 822 [RFC-822] has defined
the standard format

2. Table 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 Contents

1 Abstract .............................................. 2
2 Table of the Internet Contents ..................................... 3
3 Introduction .......................................... 5
4 Notations, Conventions, and the Internet SMTP transport
defined by 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 Generic BNF Grammar ....... 9
5 MIME Header Fields .................................... 12
5.1 MIME-Version Header Field ........................... 12
5.2 Content-Type Header Field ........................... 14
5.2.1 Syntax of text, however, RFC 822 is inadequate for the needs Content-Type Header Field ........... 15
5.2.2 Definition of mail users whose languages require the use a Top-Level Content-Type ............ 18
5.2.3 Initial Set of
character sets richer than US ASCII [US-ASCII]. Top-Level Content-Types ............ 18
5.3 Content-Transfer-Encoding Header Field .............. 21
5.3.1 Content-Transfer-Encoding Syntax .................. 21
5.3.2 Content-Transfer-Encoding Semantics ............... 22
5.3.3 Quoted-Printable Content-Transfer-Encoding ........ 26
5.3.4 Base64 Content-Transfer-Encoding .................. 30
5.4 Content-ID Header Field ............................. 32
5.5 Content-Description Header Field .................... 33
5.6 Additional MIME Header Fields ....................... 33
6 Predefined Content-Type Values ........................ 34
6.1 Discrete Content-Type Values ........................ 34
6.1.1 Text Content-Type ................................. 34
6.1.1.1 Representation of Line Breaks ................... 35
6.1.1.2 Charset Parameter ............................... 35
6.1.1.3 Plain Subtype ................................... 38
6.1.1.4 Unrecognized Subtypes ........................... 38
6.1.2 Image Content-Type ................................ 39
6.1.3 Audio Content-Type ................................ 39
6.1.4 Video Content-Type ................................ 40
6.1.5 Application Content-Type .......................... 40
6.1.5.1 Octet-Stream Subtype ............................ 41
6.1.5.2 PostScript Subtype .............................. 42
6.1.5.3 Other Application Subtypes ...................... 45
6.2 Composite Content-Type Values ....................... 46
6.2.1 Multipart Content-Type ............................ 46
6.2.1.1 Common Syntax ................................... 48
6.2.1.2 Handling Nested Messages and Multiparts ......... 53
6.2.1.3 Mixed Subtype ................................... 53
6.2.1.4 Alternative Subtype ............................. 53
6.2.1.5 Digest Subtype .................................. 56
6.2.1.6 Parallel Subtype ................................ 57
6.2.1.7 Other Multipart Subtypes ........................ 57
6.2.2 Message Content-Type .............................. 57

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6.2.2.1 RFC822 Subtype .................................. 58
6.2.2.2 Partial Subtype ................................. 58
6.2.2.2.1 Message Fragmentation and Reassembly .......... 59
6.2.2.2.2 Fragmentation and Reassembly Example .......... 60
6.2.2.3 External-Body Subtype ........................... 62
6.2.2.3.1 General External-Body Parameters .............. 64
6.2.2.3.2 The 'ftp' and 'tftp' Access-Types ............. 65
6.2.2.3.3 The 'anon-ftp' Access-Type .................... 66
6.2.2.3.4 The 'local-file' Access-Type .................. 66
6.2.2.3.5 The 'mail-server' Access-Type ................. 66
6.2.2.3.6 Examples and Further Explanations ............. 67
6.2.2.4 Other Message Subtypes .......................... 70
7 Experimental Content-Type Values ...................... 71
8 Summary ............................................... 72
9 Security Considerations ............................... 73
10 Authors' Addresses ................................... 74
11 Acknowledgements ..................................... 75
A MIME Conformance ...................................... 77
B Guidelines For Sending Email Data ..................... 80
C A Complex Multipart Example ........................... 83
D Collected Grammar ..................................... 85
F Summary of the Seven Content-types .................... 88
G Canonical Encoding Model .............................. 91
H Changes from RFC 1521 ................................. 94
I References ............................................ 97

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3. Introduction

Since its publication in 1982, 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 [RFC-822] has defined
the notable limitations standard format of RFC 821/822 based textual mail
systems is messages on the fact Internet.
Its success has been such that they limit the contents of
electronic mail messages RFC 822 format has been
adopted, wholly or partially, well beyond the confines of the
Internet and the Internet SMTP transport defined by 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. 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. 7-bit US-ASCII.
This forces users to convert any non-
textual non-textual data that they
may wish to send into seven-bit bytes representable as
printable ASCII US-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 IA5Text 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 receive is lost. Even though a user's UA user agent
may not have the capability of dealing with the non-textual

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body part, the user might have some mechanism external to the
UA that can extract useful

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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 are 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.

(3) A "text" Content-Type value, Content-Transfer-Encoding header field, which can be
used to
represent textual information in a number of
character sets and formatted text description
languages specify an auxiliary encoding that was applied
to the data in a standardized manner.

2.b. A "multipart" Content-Type value, 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 combine several body parts, possibly of
differing types of data, into
further describe the data in a single message.

2.c. An "application" Content-Type value, body, the Content-ID and
Content-Description header fields.

All of these header fields defined in this document are
subject to the general syntactic rules for header fields
specified in RFC 822. In particular, all of these header
fields can include RFC 822 comments, which have no semantic
content and should be ignored during MIME processing.

The generalized Content-Type header field values can be used
to transmit application data or binary data, identify both discrete and hence, among other uses, to implement an
electronic mail file transfer service.

2.d. composite bodies. The following
types of discrete bodies are currently defined:

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(1) A "message" "text" Content-Type value, for encapsulating
another mail message.

2.e which can be used to
represent textual information in a number of character
sets and formatted text description languages in a
standardized manner.

(2) An "image" Content-Type value, for transmitting still
image (picture) data.

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2.f.

(3) An "audio" Content-Type value, for transmitting audio
or voice data.

2.g.

(4) 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,

(5) An "application" Content-Type value, which can be used
to specify an auxiliary encoding that was applied
to the transmit application data in order to allow it or binary data, and hence,
among other uses, to pass through implement an electronic mail
transport mechanisms which may have data or character
set limitations.

4. file
transfer service.

Two additional header fields that types of composite bodies are currently defined:

(1) A "multipart" Content-Type value, which can be used to further
describe the data in
combine several body parts, possibly of differing types
of data, into a single message.

(2) A "message" Content-Type value, for encapsulating
another message body, the Content-ID and
Content-Description header fields.

MIME has or part of a message.

MIME's Content-Type mechanism has been carefully designed as an extensible mechanism, to
be extensible, and it is expected that the set of content-type/subtype content-
type/subtype pairs and their associated parameters will grow
significantly with time. Several other MIME fields, entities, most
notably
including the list of the name of character set names, sets registered for
MIME usage, 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 sets up 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 MIME's extension
areas. The registration process is accomplished. described in RFC REG [RFC-
REG].

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

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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 RFC 1341 and 1342 [RFC-1341]
and RFC1342 [RFC-1342], then revised as RFCs in RFC 1521 [RFC-1521]
and RFC 1522

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[RFC-1521] [RFC-1522]. This document is a relatively minor
updating of RFC 1521, and is intended to supersede it. The
companion document RFC MIME-HEADERS [RFC-MIME-HEADERS] in turn
supersedes RFC 1522.

The differences between this document and RFC 1521 are
summarized in Appendix H. G. Please refer to the current edition
of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol.
Several RFC 822 and
RFC 1123 [RFC-1123] also provide essential background for MIME
since no conforming implementation of MIME can violate them.
In addition, several other informational RFC documents will be
of interest to the MIME implementor, in particular [RFC 1343], RFC 1344
[RFC-1344], RFC 1345 [RFC-1345], and
[RFC-1345].

2 RFC 1524 [RFC-1524].

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4. Notations, Conventions, and Generic BNF Grammar

This document is being published in two versions, one as
plain ASCII text and one as PostScript1 . The latter is
recommended, though 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
augmented 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 augmented BNF notation.

Some of the modified augmented 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
Appendix D of this document document, the collected grammar, with that the
BNF 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 US-ASCII characters CR (13) (decimal value 13) and LF (10) (decimal
value 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 sequence of
octets into a sequence of characters. Note that unconditional
conversion in the other direction is not required, in that not
all characters may be available in a given character set and a
character set may provide more than one sequence of octets. octets to
represent a particular character. This definition is intended
to allow various kinds of text character encodings, from simple single-
table
single-table mappings such as ASCII US-ASCII to complex table
switching methods such as those that use ISO 2022's
techniques.
__________
1PostScript is a trademark of Adobe Systems Incorporated.

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character set name must fully specify the mapping to be performed.
performed from octets to characters. In particular, use of
external profiling information to determine the exact mapping
is not permitted.

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 part of type
"message".

The term "body part", in this document, means refers to either the a
single part message or one of the parts of in the body of a

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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.

NOTE: The previous four definitions are clearly circular.
This is unavoidable, since the overall structure of a MIME
message is indeed recursive.

"7bit data" refers to data that is all represented as short
lines of US-ASCII. CR (decimal value 13) and LF (decimal
value 10) characters only occur as part of CRLF line
separation sequences and no NULs (US-ASCII value 0) are
allowed.

(1) "8bit data" refers to data that is all represented as
short lines, but there may be non-US-ASCII characters
(octets with the high-order bit set) present. As with
"7bit data" CR and LF characters only occur as part of
CRLF line separation sequences and no NULs are allowed.

(2) "Binary data" refers to data where any sequence of
octets whatsoever is allowed.

"Lines" are defined as sequences of octets separated by a CRLF
sequences. This is consistent with both RFC 821 and RFC 822.
Lines in MIME bodies must also be terminated with a CRLF, but
the terminating CRLF on the last line of the body may properly
be part of a subsequent boundary marker rather than being part
of the body itself.

In this document, all numeric and octet values are given in
decimal notation.

It must be noted that All 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 Notes, such at this one, provide additional
nonessential information which may be skipped by the
reader, in a smaller, italicized, font, and
indents it as well. In the text version, only the
indentation reader

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without missing anything essential. The primary purpose of
these non-essential notes 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.

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The primary purpose of these non-essential notes
is to convey information about to convey information about the
rationale of this document, or to place this document in the
proper historical or evolutionary context. Such information
may in particular 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 certain design choices were
made.

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5. MIME Header Fields

MIME defines a number of recognition, all BNF definitions have
been placed new RFC 822 header fields that are
used to describe the content of messages. These header fields
occur in two contexts:

(1) As part of a fixed-width font in the
PostScript regular RFC 822 message header.

(2) In a MIME body part header within a multipart
construct.

The formal definition of these header fields is as follows:

MIME-message-headers := fields
version CRLF
[ content CRLF ]
[ encoding CRLF ]
[ id CRLF ]
[ description CRLF ]
*( mime-extension-field CRLF )
; The ordering of the header
; fields implied by this document.

3 BNF
; definition should be ignored

MIME-part-headers := [ content CRLF ]
[ encoding CRLF ]
[ id CRLF ]
[ description CRLF ]
*( mime-extension-field CRLF )
; The ordering of the header
; fields implied by this BNF
; definition should be ignored

The syntax of the various specific MIME header fields will be
described in the following sections.

5.1. 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

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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 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:

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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

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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 When checking MIME-Version values any
RFC 822. 822 comment strings that are present must be ignored. In
particular, all can include
comments, which means that the following two four MIME-Version fields are
equivalent:

MIME-Version: 1.0

MIME-Version: 1.0 (Generated (produced by GBD-killer 3.7)

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4 The MetaSend Vx.x)

MIME-Version: (produced by MetaSend Vx.x) 1.0

MIME-Version: 1.(produced by MetaSend Vx.x)0

5.2. 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 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

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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, image, audio, and video, video should
not contain embedded information that is really of a different
type. Such compound types formats should be represented using the
"multipart" or "application" types.

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Parameters are modifiers of the content-subtype, and as such
do not fundamentally affect the requirements nature of the host system.
Although most content. The set
of meaningful parameters make sense only depends on the content-type and
subtype. Most parameters are associated with certain
content-types, others a single specific
subtype. However, a given top-level content-type may define
parameters which are "global" in the sense that they
might apply applicable to any subtype. subtype of that type.
For example, the "charset" parameter is applicable to any
subtype of "text", while the "boundary" parameter makes sense only is required
for any subtype of the "multipart" content-type,
but the "charset" parameter might make sense with several content-type.

There are NO globally-meaningful parameters that apply to all
content-types.

An initial set of seven Content-Types is defined by this
document. This set Truly global mechanisms are best addressed, in
the MIME model, by the definition of additional Content-*
header fields.

An initial set of seven top-level names Content-Types is defined by
this document. Five of these are discrete types whose content
is essentially opaque as far as MIME processing is concerned.
The remaining two are composite types whose contents require
additional handling by MIME processors.

This set of top-level Content-Types 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 a
standards-track extension to this standard. If another
primary top-
level 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.

5.2.1. Syntax of the Content-Type Header Field

In the Augmented BNF notation of RFC 822, a Content-Type
header field value is defined as follows:

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content := "Content-Type" ":" type "/" subtype
*(";" parameter)
; case-insensitive matching Matching of type and subtype is
; ALWAYS case-insensitive

type := "application" discrete-type / "audio" composite-type

discrete-type := "text" / "image" / "message" "audio" / "multipart" "video" / "text"
"application" / "video" extension-token

composite-type := "message" / "multipart" / extension-token
; All values case-insensitive

extension-token := x-token / iana-token / ietf-token / x-token

iana-token := <a publicly-defined extension token,
registered with IANA, as specified in
appendix E>
RFC REG [RFC-REG]>

ietf-token := <a publicly-defined extension token,
initially registered with IANA and
subsequently standardized by the IETF>

x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token>

subtype := token ; case-insensitive

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parameter := attribute "=" value

attribute := token ; case-insensitive

value := token / quoted-string

token := 1*<any (ASCII) (US-ASCII) CHAR except SPACE, CTLs,
or tspecials>

tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
/
"/" / "[" / "]" / "?" / "="
; Must be in quoted-string,
; to use within parameter values

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 ".".

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Note also that a subtype specification is MANDATORY. There MANDATORY -- it may
not be omitted from a Content-Type header field. As such,
there are no default subtypes.

The type, subtype, and parameter names are not case sensitive.
For example, TEXT, Text, and TeXt are all
equivalent. equivalent top-level
Content Types. Parameter values are normally case sensitive,
but certain parameters sometimes are interpreted to be case-
insensitive, in a case-insensitive fashion,
depending on the intended use. (For example, multipart
boundaries are case-sensitive, but the "access-
type" "access-type" parameter
for message/External-body is not case-sensitive.)

Note that the value of a quoted string parameter does not
include the quotes. That is, the quotation marks in a
quoted-string are not a part of the value of an object, the parameter,
but are merely used to delimit that object. parameter value. In
addition, comments are allowed in accordance with RFC 822
rules for structured header fields. Thus the following two forms:
forms

Content-type: text/plain; charset=us-ascii (Plain text)

Content-type: text/plain; charset="us-ascii"

are completely equivalent.

Beyond this syntax, the only syntactic 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

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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 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 RFC REG.

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5.2.2. Definition of a published specification,
and possibly offered for standardization. Top-Level Content-Type

The seven standard initial predefined Content-Types are definition of a top-level content-type consists of:

(1) a name and a description of the type, including
criteria for whether a particular type would qualify
under that type,

(2) the names and definitions of parameters, if any, which
are defined for all subtypes of that type (including
whether such parameters are required or optional),

(3) how a user agent and/or gateway should handle unknown
subtypes of this type,

(4) general considerations on gatewaying objects of this
top-level type, if any, and

(5) any restrictions on content-transfer-encodings for
objects of this top-level type.

5.2.3. Initial Set of Top-Level Content-Types

The initial seven standard top-level Content-Types are
detailed in the bulk of this document. They The five discrete top-
level Content-Types are:

(1) text -- textual information. The primary subtype,
"plain", subtype "plain" in
particular 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 Other 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 format that can be read without
resorting to software that understands the format. In
particular, formats that employ embeddded binary
formatting information are not considered directly
readable. A very simple and portable subtype, richtext,
was defined in RFC 1341 [RFC-1341], with a further
revision in RFC 1563 [RFC-1563].
multipart [RFC-1563] under the name
"enriched".

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(2) image -- data consisting of multiple parts of
independent data types. Four initial image data. Image requires a display device
(such as a graphical display, a graphics printer, or a
FAX machine) to view the information. Initial subtypes
are defined, including the primary "mixed"
subtype, "alternative" defined for representing the same
data in multiple two widely-used image formats, "parallel" for parts
intended to be viewed simultaneously, jpeg and "digest"
for multipart entities in which each part is of
type "message".
message
gif.

(3) audio -- audio data. Audio requires 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

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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 audio output
device (such as a speaker or a telephone) to "display"
the contents. An initial subtype "basic" is defined in
this document.

(4) video -- video data. Video requires the capability to
display moving images, typically including specialized
hardware and software. The An initial subtype "mpeg" is "mpeg".
defined in this document.

(5) 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", 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.
An additional subtype, "PostScript", The "PostScript" subtype is
also defined for transporting the transport of PostScript documents in bodies. material.
Other expected uses for "application" include
spreadsheets, data for mail-based scheduling systems,
and languages for "active" (computational) email. (Note that active email email, and other application data may entail several
word processing formats that are not directly readable.
Note that security considerations, which considerations may exist for some
types of application data, most notably
application/PostScript and any form of active mail.
These issues are discussed later in this memo, particularly in the context document.

The two composite top-level Content-Types are:

(1) multipart -- data consisting of
application/PostScript.)

Default RFC 822 messages multiple parts of
independent data types. Four subtypes 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 initially
defined, including the presence of a MIME-Version header basic "mixed" subtype specifying
a generic mixed set of parts, "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".

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(2) message -- an encapsulated message. A body of
Content-Type "message" is itself all or part of some
kind of message object. Such objects may in turn
contain other messages and body parts of their own.
The "rfc822" subtype is used when the encpsulated
content is itself an RFC 822 message. The "partial"
subtype is defined for partial RFC 822 messages, to
permit the fragmented transmission of bodies that are
thought to be too large to be passed through mail
transport facilities in one piece. Another subtype,
"external-body", is defined for specifying large bodies
by reference to an external data source.

Default RFC 822 messages without a MIME Content-Type header
are taken by this protocol to be plain text in the US-ASCII
character set, which can be explicitly specified as:

Content-type: text/plain; charset=us-ascii

This default is assumed if no Content-Type is specified. 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 Plain US-ASCII text must still be assumed in
the absence of a MIME-Version specification, plain US-ASCII text must still be assumed,

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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 some set of
nonstandard 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.

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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.

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5 The

5.3. Content-Transfer-Encoding Header Field

Many Content-Types which could usefully be usefully 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 (SMTP)
restricts mail messages to 7-bit US-ASCII data with lines no
longer than 1000 characters.

It is necessary, therefore, to define a standard mechanism for re-encoding
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 Proper
labelling of transformation that has been used in order to
represent the body unencoded material 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 less restrictive formats
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 direct use over less restrictive transports is designed to specify
an invertible mapping between the "native" representation of
a type of data and a representation also
desireable. This document specifies that can be readily
exchanged using 7 bit mail transport protocols, such as
those defined encodings will
be indicated by RFC 821 (SMTP). a new "Content-Transfer-Encoding" header
field. This field has not been defined by any previous
standard.

5.3.1. Content-Transfer-Encoding Syntax

The Content-Transfer-Encoding 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 / "8bit" / "binary" /
"quoted-printable" / "base64" / "8bit"
/ "binary"
ietf-token / x-token

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These values are not case sensitive. That is, sensitive -- 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 7-bit mail-ready representation.
This is the default value -- that is,
"Content-Transfer-Encoding: "Content-Transfer-
Encoding: 7BIT" is assumed if the Content-Transfer-Encoding
header field is not present.

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5.3.2. Content-Transfer-Encoding Semantics

This single token actually provides two pieces of information.
It specifies what sort of encoding transformation the body was
subjected to, and it specifies what the domain of the result
is.

Three transformations are currently defined: identity, the
"quoted-printable" encoding, and the "base64" encoding. The
domains are "binary", "8bit" and "7bit".

The values "8bit", "7bit", "8bit", and "binary" all mean that NO the
identity (i.e. NO) encoding transformation has been performed. However,
As such, they are potentially
useful serve simply as indications indicators of the kind domain of data contained in the
object,
body part data, and therefore of provide useful information about the kind sort
of encoding that might
need to be performed needed for transmission in a given
transport system. In particular:

"7bit" means that the data is The terms "7bit data", "8bit data", and
"binary data" are all represented as short
lines defined in Section 4.

The quoted-printable and base64 encodings transform their
input from an arbitrary domain into material in the "7bit"
domain, thus making it safe to carry over restricted
transports. The specific definition of US-ASCII data.
"8bit" means that the lines transformations are short, but there may
given below.

The proper Content-Transfer-Encoding label must always be
non-ASCII characters (octets with the high-order
bit set).
"Binary" means that not only may non-ASCII
used. Labelling unencoded data containing 8-bit 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 as
"7bit" 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 allowed, nor is labelling unencoded non-line-
oriented data in any
bit-width as anything 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 allowed.

Unlike Content-Type subtypes, a proliferation of Content-
Transfer-Encoding values of "binary", "8bit", etc.
may is both undesirable and unnecessary.
However, establishing only a single transformation into the
"7bit" domain does not seem unimportant, in that all of them really
mean "none" -- that is, there has been no possible. There is a tradeoff
between the desire for a compact and efficient encoding of the
largely-binary data and the desire for transport. However, clear
labeling will be a readable encoding of enormous value to gateways
between future mail transport systems with
differing capabilities in transporting
data that
do is mostly, but not meet the restrictions of RFC 821 transport. entirely, 7-bit. For this
reason, at least two encoding mechanisms are necessary: a
"readable" encoding (quoted-printable) and a "dense" encoding
(base64).

Mail transport for unencoded 8-bit data is defined in RFC-1426 [RFC-1426]. RFC 1652
[RFC-1652]. As of the publication of

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no standardized Internet mail transports for which it is
legitimate to include unencoded binary data in mail bodies.

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Thus there are no circumstances in which the "binary"
Content-Transfer-Encoding is actually
legal valid 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 labelled as such using this mechanism.

NOTE: The five values defined for the Content-
Transfer-Encoding 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 Content-Transfer-
Encoding values, but must use an x-token, which is a name
prefixed by "X-" "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 STRONGLY discouraged, as
it seems likely to hinder interoperability with little
potential benefit. Their Such use is therefore allowed only as the
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.) "8bit" or "binary". Even more severe
restrictions apply to some subtypes of the "message" type.

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 8-bit byte. A bit
stream not ending at an 8-bit boundary must be padded with

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zeroes. This document provides a mechanism for noting the
addition of such padding in the case of the application
application/octet-stream Content-Type, which has a "padding"
parameter.

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The encoding mechanisms defined here explicitly encode all
data in ASCII. US-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
US-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 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 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 EXPRESSLY
FORBIDDEN to use any encodings other than "7bit", "8bit", or
"binary" with any Content-Type composite Content-Type, i.e. one that
recursively includes other Content-Type fields, notably fields. Currently the
only composite Content-Types are "multipart" and
"message" Content-Types. "message".
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.

It should also be noted that, by definition, if a
"multipart" or "message" composite
entity has a transfer-encoding value such as "7bit", but one
of the enclosed parts has a less restrictive value such as
"8bit", then either the outer "7bit" labelling is in error,
because 8 bit 8-bit data are included, or the inner "8bit" labelling
placed an unnecessarily high demand on the transport system
because the actual included data were actually 7bit-safe. 7-bit-safe.

NOTE ON ENCODING RESTRICTIONS: Though the prohibition against
using content-transfer-
encodings content-transfer-encodings on composite body data of type multipart or message may

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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 Aside from the
obvious efficiency problems with such multiple encodings, they

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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 bodies 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: CONTENT-
TRANSFER-ENCODING: It may seem that the
Content-Transfer-Encoding 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 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. encoding at all. 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 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

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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 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 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.

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NOTE ON CANONICAL ENCODING MODEL: There was some confusion,
in earlier drafts of this memo, document, 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 content-transfer-encodings and character sets. For this reason, a A
canonical model for encoding is presented as Appendix G.

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5.1 F for
this reason.

5.3.3. 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 US-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 US-ASCII text, the encoded form of the data remains
largely recognizable by humans. A body which is entirely US-
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:

(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
decimal value 12 (ASCII (US-ASCII form feed) can be
represented by "=0C", and the decimal value 61 (ASCII (US-
ASCII EQUAL SIGN) can be represented by "=3D". Except This
rule must be followed except when the following rules
allow an alternative encoding, this
rule is mandatory.

Rule #2: encoding.

(2) (Literal representation) Octets with decimal values of
33 through 60 inclusive, and 62 through 126, inclusive,

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MAY be represented as the ASCII US-ASCII characters which
correspond to those octets (EXCLAMATION POINT through
LESS THAN, and GREATER THAN through TILDE,
respectively).

Rule #3:

(3) (White Space): Space) Octets with values of 9 and 32 MAY be
represented as ASCII US-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 "=" at the end
of an encoded line,

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rule #5) may follow one or more TAB (HT) or SPACE
characters. It follows that an octet with decimal
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

(4) (Line Breaks): Breaks) A line break in a text body,
independent of what its representation is following represented
as a CRLF sequence in the text canonical representation of the data being encoded, form, must be
represented by a (RFC 822) line break, which is also 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,
breaks as CRLF sequences, 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 quoted-
printable encoding of such types. Of course, occurrences of Sequences like "=0D",
"=0A", "=0A=0D" and "=0D=0A" will eventually be encountered.
In general, however, base64 is preferred over quoted-
printable for binary data. routinely appear in
non-text data represented in quoted-printable, of
course.

Note that many implementations may elect to encode the
local representation of various content types directly,
as described in Appendix G. F. In particular, this may
apply to plain text material on systems that use
newline conventions other than CRLF delimiters. Such

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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

(5) (Soft Line Breaks): 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' "soft" line breaks
must be used. An equal sign as the last character on a
encoded line indicates such a non-significant ('soft') ("soft")
line break in the encoded text.

Thus if the "raw" form

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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 as:

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

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transport through EBCDIC gateways is to also quote the US-
ASCII characters

!"#$@[\]^`{|}~

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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 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 / safe-char

safe-char := <any ASCII US-ASCII character except "=",
SPACE, or TAB>
; characters Characters not listed as "mail-safe" in
; Appendix B
; are also not recommended.

octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; octet Octet must be used for characters > 127, =,
; SPACE, or TAB,
; and is recommended for any
; characters not listed in
; Appendix B as
; "mail-safe".

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5.2

IMPORTANT NOTE: The addition of LWSP between the elements
shown in this BNF is NOT allowed since this BNF does not
specify a structured header field.

5.3.4. 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. 1421 [RFC-1421].

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,
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 most-significant-bit first.
That is, the first bit in the stream will be the high-order
bit in the first 8-bit byte, and the eighth bit will be the
low-order bit in the first 8-bit 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

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in Table 1, below, are selected so as to be universally
representable, and the set excludes characters with

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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 encoded 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 arise:
(1) the final quantum of encoding input is an integral
multiple of 24 bits; here, the final unit of encoded output

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will be an integral multiple of 4

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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
invalid 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 base64-encoded parts of
multipart entities because no hyphen characters are used in
the base64 encoding.

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6 Additional Content- Header Fields

6.1 Optional

5.4. 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 labelled 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.

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The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for cacheing caching data
referenced by the message/external-body mechanism. Although
the Content-ID header is generally optional, its use is mandatory
MANDATORY in 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 caching 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. content-
type. This is explained in the section of this document
dealing with multipart/alternative.

6.2 Optional

5.5. 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 Content-Description
header field. This header field is always optional.

description := "Content-Description" ":" *text

The description is presumed to be given in the US-ASCII
character set, although the mechanism specified in [RFC-
1522] RFC MIME-
HEADERS [RFC-MIME-HEADERS] may be used for non-US-ASCII
Content-Description values.

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5.6. Additional MIME Header Fields

Future documents may elect to define additional MIME header
fields for various purposes. Any new header field that
further describes the content of a message should begin with
the string "Content-" to allow such fields which appear in a
message header to be distinguished from ordinary RFC 822
message header fields.

MIME-extension-field := <Any RFC 822 header field which
begins with the string
"Content-">

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7 The

6. 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 E.

6.1. Discrete Content-Type Values

Five of the seven initial Content-Type values refer to
discrete bodies. The content of such entities is handled by
non-MIME mechanisms; they are opaque to MIME processors.

6.1.1. 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, subtype "text/plain",
which indicates plain (unformatted) text. The default Content-
Type
Content-Type for Internet mail if none is specified 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 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
"text/enriched" [RFC-1563].

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6.1.1.1. Representation of Line Breaks

The charset parameter canonical form of any MIME text type MUST represent a line
break as a CRLF sequence. Similarly, any occurrence of CRLF
in text MUST represent a line break. Use of CR and LF outside
of line break sequences is also forbidden.

This rule applies regardless of format or character set or
sets involved.

6.1.1.2. Charset Parameter

A critical parameter that may be specified in the Content-
Type 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

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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
multioctet 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 the name of a byte
stream to glyphs, a mapping which does not require external
profiling information.

An initial list of predefined character set, as
"character set" as defined in Section 4 of this document. The
rules regarding line breaks detailed in the previous section
must also be observed -- a character set whose definition does
not conform to these rules cannot be used in a MIME text type.

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 IAB review and approval. IANA as described in RFC REG.

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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 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 7-bit character code, but specifies that the body
uses character coding that uses the exact correspondence of codes
octets to characters specified in
ASCII. US-ASCII. National use
variations of ISO 646 [ISO-646] are NOT
ASCII US-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" "US-ASCII" explicitly refers to ANSI
X3.4-1986 [US-ASCII] only. The character set name "ASCII" is
reserved and must not be

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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 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]. ANSI X3.4-
1986. Note that the control characters including DEL (0-31,
127) have no defined meaning apart from the combination CRLF
(ASCII
(US-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

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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 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

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we define names for a small number of character sets for which
a strong constituent base exists.

The defined charset values are:

(1) US-ASCII -- as defined in ANSI X3.4-1986 [US-ASCII].

(2) ISO-8859-X -- where "X" is to be replaced, as
necessary, for the parts of ISO-8859 [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.

All of these character sets are used as pure 7- or 8-bit sets
without any shift or escape functions. The meaning of shift
and escape sequences in these character sets is not defined.

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, US-
ASCII, must always be explicitly specified in the Content-Type
field.

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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 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.

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In general, mail-sending software must should always use the
"lowest common denominator" character set possible. For
example, if a body contains only US-ASCII characters, it
must
should 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 should be labeled labelled as being in that subset. This
will increase the chances that the recipient will be able to
view the mail correctly.

7.1.2 The Text/plain subtype

6.1.1.3. Plain Subtype

The primary simplest and most important subtype of text is "plain".
This indicates plain (unformatted) text. The default
Content-Type for Internet mail, "text/plain; charset=us-ascii", 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

6.1.1.4. Unrecognized Subtypes

Unrecognized subtypes of text is should be treated as follows:

text-type := "text" "/" text-subtype [";" "charset" "="
charset]

text-subtype := subtype
"plain" / extension-token as long as the MIME implementation knows how to handle
the charset. Unrecognized subtypes which also specify an
unrecognized 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

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stream".

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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
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Internet Draft MIME Part One November 1994

6.1.2. Image Content-Type field must appear in

A Content-Type of "image" indicates that the entity's
header. The body must then contain one or more "body parts,"
each preceded by contains an encapsulation boundary, and
image. The subtype names the last one
followed by a closing boundary. Each part starts with an
encapsulation boundary, specific image format. These
names are not case sensitive. Two initial subtypes are "jpeg"
for the JPEG format, JFIF encoding, and then contains a body part
consisting "gif" for GIF format
[GIF].

The list of header area, a blank line, and a body area.
Thus a body part image subtypes given here is similar to an RFC 822 message in syntax,
but different in meaning.

A body part neither exclusive nor
exhaustive, and is NOT expected to be interpreted grow as actually being an
RFC 822 message. To begin with, NO header fields more types are
actually required in body parts. A body part that starts
registered with IANA, as described in RFC REG.

Unrecognized subtypes of image should at a blank line, therefore, is allowed and is a body part
for which all default values are to miniumum be assumed. In such a
case, the absence treated
as "application/octet-stream". Implementations may optionally
elect to pass subtypes of image that they do not specifically
recognize to a robust general-purpose image viewing
application, if such an application is available.

6.1.3. Audio Content-Type header field implies

A Content-Type of "audio" indicates that the corresponding body contains
audio data. Although there is plain US-ASCII text. 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
only header fields initial subtype of "basic" is specified to meet this
requirement by providing an absolutely minimal lowest common
denominator audio format. It is expected that have defined meaning richer formats
for body parts
are those higher quality and/or lower bandwidth audio will be
defined by a later document.

The content of the names "audio/basic" subtype is single channel
audio encoded using 8-bit ISDN mu-law [PCM] at a sample rate
of which begin with "Content-". All
other header fields are generally to be ignored in body
parts. Although they 8000 Hz.

Unrecognized subtypes of audio should generally at a miniumum be retained in mail
processing, they treated
as "application/octet-stream". Implementations may be discarded by gateways if necessary.
Such other fields are permitted optionally
elect to appear in body parts but
must not be depended on. "X-" fields may be created for
experimental or private purposes, with the recognition pass subtypes of audio that
the information they contain may be lost at some gateways.

NOTE: The distinction between an RFC 822 message
and do not specifically
recognize to a body part robust general-purpose audio playing
application, if such an application is subtle, but important. A
gateway between available.

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Internet and X.400 mail, for
example, must be able to tell Draft MIME Part One November 1994

6.1.4. Video Content-Type

A Content-Type of "video" indicates that 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,
time-varying-picture image, possibly with
its own "Content-Type: image" header field. color and
coordinated sound. The
use of similar syntax facilitates the conversion
of messages term "video" is used extremely
generically, rather than with reference to body parts, any particular
technology or format, 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

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also defined.)

As stated previously, each body part is preceded by an
encapsulation boundary. not meant to preclude subtypes
such as animated drawings encoded compactly. The encapsulation boundary MUST NOT
appear inside any of the encapsulated parts. Thus, it is
crucial that the composing agent be able subtype
"mpeg" refers to video coded according to choose and
specify the unique boundary MPEG standard
[MPEG].

Note that will separate although in general this document strongly
discourages the parts.

All present and future subtypes mixing of the "multipart" type must
use an identical syntax. Subtypes may differ multiple media in their
semantics, a single body, it
is recognized that many so-called "video" formats include a
representation for synchronized audio, and may impose additional restrictions on syntax,
but must conform to the required syntax this is explicitly
permitted for the multipart
type. This requirement ensures that all conformant user
agents will subtypes of "video".

Unrecognized subtypes of video should at least a minumum be able treated
as "application/octet-stream". Implementations may optionally
elect to recognize and separate the
parts of any multipart entity, even pass subtypes of video that they do not specifically
recognize to a robust general-purpose video display
application, if such an unrecognized
subtype.

As stated in the definition of the Content-Transfer-Encoding
field, no encoding other than "7bit", "8bit", or "binary" application is
permitted for entities of type "multipart". available.

6.1.5. Application Content-Type

The multipart
delimiters and header fields are always represented as 7-bit
ASCII "application" Content-Type is to be used for discrete data
which do not fit in any case (though of the header fields may encode non-
ASCII header text as per [RFC-1522]), other categories, and
particularly for data within the
body parts can to be encoded on 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 part-by-part basis, with
Content-Transfer-Encoding fields user. Expected uses for each appropriate body
part.

Mail gateways, relays, Content-Type application include
mail-based file transfer, spreadsheets, data for mail-based
scheduling systems, and other mail handling agents are
commonly known to alter the top-level header of an RFC 822
message. In languages for "active" (computational)
email. (The latter, in particular, they frequently add, remove, or
reorder header fields. Such alterations can pose security problems
which must be understood by implementors, and are explicitly
forbidden for the body part headers embedded considered
in detail in the bodies
of messages of type "multipart."

7.2.1 Multipart: The common syntax

All subtypes discussion of "multipart" share the application/PostScript
content-type.)

For example, a common syntax, defined
in this section. A simple example of meeting scheduler might define a multipart message
also appears in this section. standard
representation for information about proposed meeting dates.
An example of intelligent user agent would use this information to
conduct a more complex
multipart message is given dialog with the user, and might then send further
mail based on that dialog. More generally, there have been

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several "active" messaging languages developed in Appendix C.

The Content-Type field for multipart entities requires one
parameter, "boundary", which is used to specify
programs in a suitably specialized language are sent through
the
encapsulation boundary. The encapsulation boundary is mail and automatically run in the recipient's environment.

Such applications may be defined as a line consisting entirely subtypes of the
"application" Content-Type. This document defines two hyphen
characters ("-", decimal code 45) followed by
subtypes: octet-stream, and PostScript.

The subtype of application will often be the boundary

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parameter value from name of the Content-Type header field.

NOTE: The hyphens are
application for rough compatibility
with which the earlier RFC 934 method data are intended. This does not
mean, however, that any application program name may be used
freely as a subtype of message
encapsulation, and for ease application. Usage of searching for the
boundaries in some implementations. However, it
should any subtype
(other than subtypes beginning with "x-") must be noted that multipart messages are NOT
completely compatible registered
with RFC 934 encapsulations; IANA, as described in particular, they do not obey RFC 934 quoting
conventions for embedded lines REG.

6.1.5.1. Octet-Stream Subtype

The "octet-stream" subtype is used to indicate that begin with
hyphens. This mechanism was chosen over a body
contains arbitrary binary data. The set of currently defined
parameters is:

(1) TYPE -- the RFC
934 mechanism because the latter causes lines to
grow with each level of quoting. The combination general type or category of this growth with the fact that SMTP
implementations sometimes wrap long lines made the
RFC 934 mechanism unsuitable binary data.
This is intended as information for use in the event
that deeply-nested multipart structuring is ever
desired.

WARNING TO IMPLEMENTORS: The grammar human recipient
rather than for parameters on any automatic processing.

(2) PADDING -- the
Content-type field is such number of bits of padding that it is often necessary were
appended to
enclose the boundaries in quotes on bit-stream comprising the Content-type line.
This is not always necessary, but never hurts. Implementors
should be sure actual
contents to study produce the grammar carefully enclosed 8-bit byte-oriented
data. This is useful for enclosing a bit-stream 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
body when the following total number of bits is illegal:

Content-Type: multipart/mixed;
boundary=gc0p4Jq0M:2Yt08jU534c0p

(because not a multiple of
8.

Both of these parameters are optional.

An additional parameter, "CONVERSIONS", was defined in RFC
1341 but has since been removed. RFC 1341 also defined the colon) and must instead
use of a "NAME" parameter which gave a suggested file name to
be represented as

Content-Type: multipart/mixed;
boundary="gc0p4Jq0M:2Yt08jU534c0p"

This indicates that used if the entity consists data were to be written to a file. This has
been deprecated in anticipation of several parts,
each itself with a structure that is syntactically identical separate Content-
Disposition header field, to be defined in a subsequent RFC.

The recommended action for an RFC 822 message, except implementation that receives
application/octet-stream mail is to simply offer to put the header area might be
completely empty, and that the parts are each preceded by
the line

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data in a file, with any Content-Transfer-Encoding undone, or

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Internet Draft MIME Part One November 1994

--gc0p4Jq0M:2Yt08jU534c0p

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

perhaps to be attached use it as input to a user-specified process.

To reduce the
encapsulation boundary rather than part danger of transmitting rogue programs through
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 mail, it is therefore assumed to be of Content-Type
text/plain).

NOTE: The CRLF preceding strongly recommended that implementations NOT
implement a path-search mechanism whereby an arbitrary program
named in the encapsulation line Content-Type parameter (e.g., an "interpreter="
parameter) is conceptually attached to found and executed using the boundary so that
it is possible to have a part that does not end
with mail body as input.

6.1.5.2. PostScript Subtype

A Content-Type of "application/postscript" indicates a CRLF (line break). Body parts that must
be considered to end with line breaks, therefore,
must have
PostScript program. Currently two CRLFs preceding variants of the encapsulation
line, PostScript
language are allowed; the first of which original level 1 variant is part of the preceding
body part,
described in [POSTSCRIPT] and the second of which more recent level 2 variant
is part described in [POSTSCRIPT2].

PostScript is a registered trademark of Adobe Systems, Inc.
Use of the
encapsulation boundary.

Encapsulation boundaries must not appear within the
encapsulations, MIME content-type "application/postscript" implies
recognition of that trademark and must be no longer than 70 characters,
not counting all the two leading hyphens. rights it entails.

The encapsulation boundary following PostScript language definition provides facilities for
internal labelling of the last body part is a
distinguished delimiter that indicates that no further body
parts will follow. Such specific language features a delimiter is identical to the
previous delimiters, with given
program uses. This labelling, called the addition of two PostScript document
structuring conventions, or DSC, is very general and provides
substantially more hyphens
at information than just the end language level.
The use of the line:

--gc0p4Jq0M2Yt08jU534c0p--

There appears document structuring conventions, while not
required, is strongly recommended as an aid to
interoperability. Documents which lack proper structuring
conventions cannot be room for additional information prior tested 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 see whether or after the last one.

NOTE: These "preamble" and "epilogue" areas are
generally not used because of they will
work in a given environment. As such, some systems may assume
the lack of proper
typing worst and refuse to process unstructured documents.

The execution of these parts 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 lack of clear
semantics potential for handling these areas at gateways,
particularly X.400 gateways. However, rather than
leaving the preamble area blank, many MIME

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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 harm is
greatly constrained by MIME-compliant
software.

NOTE: Because encapsulation boundaries must not
appear in typical printer environments,
implementors should consider all of the body parts being encapsulated, a
user agent must exercise care following before they
add interactive display of PostScript bodies to choose a unique
boundary. their mail
readers.

The boundary in the example above could
have been the result remainder of an algorithm designed to
produce boundaries with a very low probability this section outlines some, though probably
not all, of
already existing in the data to be encapsulated
without having to prescan the data. Alternate
algorithms might result in more 'readable'
boundaries for a recipient possible problems with an old user agent,
but would require more attention to the
possibility that sending PostScript

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through the boundary might appear mail.

(1) Dangerous operations in the
encapsulated part. The simplest boundary possible 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 like "---", with a closing boundary
of "-----".

As other than standard input or output.
Implementations may also define additional nonstandard
file operators; these may also pose a very simple example, threat to
security. "Filenameforall", the following multipart message wildcard file search
operator, may appear at first glance to be harmless.
Note, however, that this operator 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 potential to
reveal information about what files the recipient has
access to, and this information may itself be ignored, though it
is a handy place for mail composers
sensitive. Message senders should avoid the use of
potentially dangerous file operators, since these
operators are quite likely to include an
explanatory note 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 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.

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It DOES end with a linebreak.

--simple boundary--
This is the epilogue. It is also delegate any special authority to
their operation. These operators should be ignored.

The use viewed as
being done by an outside agency when interpreting
PostScript documents. Such disabling and/or checking
should be done completely outside of a Content-Type the reach of multipart in a body part within
another multipart entity is explicitly allowed. In such
cases, for obvious reasons, the
PostScript language itself; care must should be taken to ensure
insure that each nested multipart entity must use a different
boundary delimiter. See Appendix C no method exists for an example re-enabling full-
function versions of nested
multipart entities. these operators.

(2) The use of the multipart Content-Type with only a single
body part 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 useful retained
semipermanently in certain contexts, and is
explicitly permitted. nonvolatile memory. The only mandatory parameter for operators
associated with exiting the multipart Content-Type
is interpreter loop have the boundary parameter, which consists of 1
potential to 70
characters from interfere with subsequent document
processing. As such, their unrestrained use constitutes
a set threat of characters known to service denial. PostScript operators that
exit the interpreter loop include, but may not be very robust
through email gateways,
limited to, the exitserver and NOT ending with white space.
(If a boundary appears startjob operators.
Message sending software should not generate PostScript
that depends on exiting the interpreter loop to end with white space,
operate, since the white
space must ability to exit will probably be presumed
unavailable in secure PostScript implementations.
Message receiving and displaying software should

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completely disable the ability to have been added make retained changes
to the PostScript environment by a gateway, eliminating or
disabling the "startjob" and
must "exitserver" operations.
If these operations cannot be deleted.) It is formally specified by the following
BNF:

boundary := 0*69<bchars> bcharsnospace

bchars := bcharsnospace / " "

bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" /
"_"
/ "," / "-" / "." / "/" / ":" / "=" / "?"

Overall, eliminated or completely
disabled the body of a multipart entity may password associated with them should at
least 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

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; between "--" and boundary.

close-delimiter := "--" boundary "--" CRLF
; Again, no space by "--",

preamble := discard-text ; to be ignored
upon receipt.

epilogue := discard-text ; set to a hard-to-guess value.

(3) PostScript provides operators for setting system-wide
and device-specific parameters. These parameter
settings may be ignored
upon receipt.

discard-text := *(*text CRLF)

body-part := <"message" as defined in RFC 822,
with all header fields optional, retained across jobs 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 may
potentially pose a part differ from threat to the semantics correct operation of a message, as described in the text.>

NOTE: In certain transport enclaves, RFC 822
restrictions such as
the one interpreter. The PostScript operators that limits bodies to
printable ASCII characters set
system and device parameters include, but may not be in force. (That
is,
limited to, the transport domains may resemble standard
Internet mail transport as specified in RFC821 "setsystemparams" and
assumed by RFC822, but without certain restrictions.)
The relaxation of these restrictions "setdevparams"
operators. Message sending software should be
construed as locally extending not
generate PostScript that depends on the definition setting of
bodies, for example
system or device parameters to include octets outside of the
ASCII range, as long as operate correctly. The
ability to set these extensions are supported
by the transport and adequately documented in the
Content-Transfer-Encoding header field. However, parameters will probably be
unavailable 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 secure PostScript implementations.
Message receiving and displaying software should define a subtype of multipart that
is syntactically identical, but that always
expects
disable the inclusion of a distinguished part that

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can be used ability to specify the structure change system and
integration of the other parts, probably referring
to them by their Content-ID field. device
parameters. If this
approach is used, other implementations will not
recognize the new subtype, but will treat it as these operators cannot be completely
disabled the primary subtype (multipart/mixed) and will
thus password associated with them should at
least be able set to show the user a hard-to-guess value.

(4) Some PostScript implementations provide nonstandard
facilities for the parts that direct loading and execution of
machine code. Such facilities are
recognized.

7.2.2 The Multipart/mixed (primary) subtype

The primary subtype for multipart, "mixed", is intended for quite obviously open
to substantial abuse. Message sending software should
not make use when the body parts of such features. Besides being totally
hardware-specific, they are independent and need also likely to be
bundled
unavailable in a particular order. Any multipart subtypes that
an implementation does not recognize must be treated as
being secure implementations of subtype "mixed".

7.2.3 The Multipart/alternative subtype

The multipart/alternative type is syntactically identical PostScript.
Message receiving and displaying software should not
allow such operators to
multipart/mixed, but the semantics are different. In
particular, each of the parts be used if they exist.

(5) PostScript is an "alternative" version extensible language, and many, if not
most, implementations of
the same information.

Systems 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 recognize that the content not make use
of the various
parts nonstandard extensions; they are interchangeable. Systems likely to be
missing from some implementations. Message receiving
and displaying software should choose the
"best" type based on the local environment make sure that any
nonstandard PostScript operators are secure and preferences,
in some cases even through user interaction. As with
multipart/mixed, the order don't
present any kind of body parts threat.

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(6) It is significant. In
this case, the alternatives appear in an order possible to write PostScript that consumes huge
amounts of increasing
faithfulness various system resources. It is also
possible to write PostScript programs that loop
indefinitely. Both types of programs have the original content. In general,
potential to cause damage if sent to unsuspecting
recipients. Message-sending software should avoid the best
choice
construction and dissemination of such programs, which
is the LAST part antisocial. Message receiving and displaying
software should provide appropriate mechanisms to abort
processing of a type supported by the recipient
system's local environment.

Multipart/alternative may be used, for example, to send mail
in document after a fancy text format in such a way that it can easily be
displayed anywhere:

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

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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 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 reasonable amount of their system.
time has elapsed. 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 addition, PostScript interpreters
should pick and display be limited to the last format they are
capable consumption of displaying. In the case where one only a
reasonable amount of the
alternatives any given system resource.

(7) It is itself of type "multipart" and contains
unrecognized sub-parts, the user agent may choose either possible to
show that alternative, an earlier alternative, or both.

NOTE: From an implementor's perspective, include raw binary information inside
PostScript in various forms. This is not recommended
for use in email, both because it might
seem more sensible to reverse this ordering, is not supported by
all PostScript interpreters and
have the plainest alternative last. However,
placing because it
significantly complicates the plainest alternative first is the
friendliest possible option when
multipart/alternative entities are viewed using use of 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 MIME Content-
Transfer-Encoding. (Without such binary, PostScript
may typically be more important viewed as line-oriented data. The
treatment of CRLF sequences becomes extremely
problematic if binary and line-oriented data are mixed
in this
case.

It a single Postscript data stream.)

(8) 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 case that some user agents, timely correction of such
bugs if they can
recognize more than one any are found.

6.1.5.3. Other Application Subtypes

It is expected that many other subtypes of the formats, application will prefer be
defined in the future. MIME implementations must at a minimum
treat any unrecognized subtypes as being equivalent to offer

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Internet Draft MIME Part One November 1994

the user the choice

6.2. Composite Content-Type Values

The remaining two of which format the seven initial Content-Type values
refer to view. This makes
sense, for example, if mail includes both composite entities. Composite entities are handled
using MIME mechanisms -- a nicely-formatted
image version and an easily-edited text version. What is
most critical, however, is that MIME processor typically handles
the user not automatically
be shown body directly.

6.2.1. Multipart Content-Type

In the case of multiple versions 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 same data. Either the
user should be shown the last recognized version entity's
header. The body must then contain one or should
be given more "body parts,"
each preceded by an encapsulation boundary, and the choice.

NOTE ON THE SEMANTICS OF CONTENT-ID IN
MULTIPART/ALTERNATIVE: last one
followed by a closing boundary. Each part starts with an
encapsulation boundary, and then contains a body part
consisting of a multipart/alternative
entity represents the same data, but the mappings between
the two are not necessarily without information loss. For
example, information header area, a blank line, and a body area.
Thus a body part is lost when translating ODA similar to
PostScript or plain text. It is recommended that each part
should have a an RFC 822 message in syntax,
but different Content-ID value in the case where
the information content of the two parts is not identical.
However, where the information content meaning.

A body part is identical -- for
example, where several parts of type "message/external-body"
specify alternate ways NOT to access the identical data -- the
same Content-ID field value should be used, to optimize any
cacheing mechanisms interpreted as actually being an RFC
822 message. To begin with, NO header fields are actually
required in body parts. A body part that might be present on the recipient's
end. However, it starts with a blank
line, therefore, is recommended that the Content-ID allowed and is a body part for which all
default values
used by the parts should not are to be assumed. In such a case, the same Content-ID value
absence of a Content-Type header indicates that describes the multipart/alternative as
corresponding body has a whole, if
there is any such Content-ID field. That is, one Content-ID
value will refer to content-type of "text/plain;
charset=US-ASCII"".

The only header fields that have defined meaning for body
parts are those the multipart/alternative entity, while
one or more names of which begin with "Content-". All
other Content-ID values will refer header fields are generally to the parts
inside it.

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7.2.4 The Multipart/digest subtype

This document defines a "digest" subtype of the multipart
Content-Type. This type is syntactically identical 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
multipart/mixed, appear in body parts but must not be
depended on. "X-" fields may be created for experimental or
private purposes, with the semantics are different. In
particular, in a digest, recognition that the default Content-Type value for information
they contain may be lost at some gateways.

NOTE: The distinction between an RFC 822 message and a body
part is changed from "text/plain" to
"message/rfc822". This is done subtle, but important. A gateway between Internet and
X.400 mail, for example, must be able to allow tell the difference
between a more readable
digest format body part that is largely compatible (except for contains an image and a body part
that contains an encapsulated message, the
quoting convention) with RFC 934.

A digest in this format might, then, look something like
this:

From: Moderator-Address
To: Recipient-List
MIME-Version: 1.0
Subject: body of which is a

Expires May 1995 [Page 46]

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 Draft MIME Part One November 1994

GIF image. In order to represent the latter, the body goes here...

------ next message ------

7.2.5 part
must have "Content-Type: message/rfc822", and its body (after
the blank line) must be the encapsulated message, with its own
"Content-Type: image/gif" header field. The Multipart/parallel subtype

This document defines a "parallel" subtype use of similar
syntax facilitates the multipart
Content-Type. This type is syntactically identical conversion of messages to
multipart/mixed, body parts,
and vice versa, but the semantics are different. In
particular, in a parallel entity, distinction between the order of body two must be
understood by implementors. (For the special case in which
all parts actually are messages, a "digest" subtype is not significant.

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A common presentation of this type also
defined.)

As stated previously, each body part is to display all preceded by an
encapsulation boundary. The encapsulation boundary MUST NOT
appear inside any of the
parts simultaneously on hardware and software encapsulated parts. Thus, it is
crucial that are
capable of doing so. However, the composing agents should agent be
aware that many mail readers will lack this capability able to choose and specify
a unique boundary that will show separate the parts serially in any event.

7.2.6 Other Multipart subtypes

Other multipart parts.

All present and future subtypes are expected in of the future. MIME
implementations "multipart" type must
use an identical syntax. Subtypes may differ 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

multipart-subtype := "mixed" / "parallel" / "digest"
/ "alternative" / extension-token

7.3 The Message Content-Type

It is frequently desirable, in sending mail, their
semantics, and may impose additional restrictions on syntax,
but must conform 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 syntax 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 multipart
type. This requirement ensures that all conformant user
agents will preserve the type
information on the original message and allow it
to at least be correctly presented able to the recipient, recognize and
hence is strongly encouraged. 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

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permitted for messages or parts entities of type "message". Even
stronger restrictions apply to the subtypes
"message/partial" and "message/external-body", as specified
below. "multipart". The message multipart
delimiters and header fields are always represented as 7-bit
US-ASCII in any
case, case (though the header fields may encode
non-US-ASCII header text as per RFC MIME-HEADERS, and data
within the body parts can still be encoded, in
which case the encoded on a part-by-part basis,
with 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, fields for each appropriate
body part.

Message transport agents, relays, and other mail handling agents gateways 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 any body
contains an encapsulated message, with the syntax of part which occurs within an RFC
822 message. However, unlike top-level RFC 822 messages,
the restriction that each message/rfc822 enclosing
multipart body must include part.

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6.2.1.1. Common Syntax

This section defines a
"From", "Date", and at least one destination header is
removed and replaced with the requirement that at least one common syntax for subtypes of "From", "Subject", or "Date"
multipart. All subtypes of multipart must be present.

It should be noted that, despite the use this syntax. A
simple example of the numbers
"822", a message/rfc822 entity can include enhanced
information as defined multipart message also appears in this document. In other words,
section. An example of a
message/rfc822 more complex multipart message may be a MIME message.

7.3.2 The Message/Partial subtype

A subtype of message, "partial", is defined
given in order to
allow large objects Appendix C.

The Content-Type field for multipart entities requires one
parameter, "boundary", which is used to be delivered specify the
encapsulation boundary. The encapsulation boundary is defined
as several separate
pieces a line consisting entirely of mail and automatically reassembled two hyphen characters ("-",
decimal value 45) followed by the
receiving user agent. (The concept is similar to IP
fragmentation/reassembly in boundary parameter value
from 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 header field.

NOTE: The hyphens are for rough compatibility with the body contains a fragment
earlier RFC 934 method of a larger message.

Three parameters must be specified in the Content-Type field message encapsulation, and for ease
of type message/partial: The first, "id", is a unique

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identifier, as close to a world-unique identifier as
possible, to be used to match the parts together. (In
general, searching for the identifier is essentially a message-id; if
placed boundaries in double quotes, some implementations.
However, it can should be any message-id, in
accordance noted that multipart messages are NOT
completely compatible with the BNF for "parameter" given earlier RFC 934 encapsulations; in
this specification.) The second, "number", an integer, is
particular, they do not obey RFC 934 quoting conventions for
embedded lines that begin with hyphens. This mechanism was
chosen over the part number, which indicates where this part fits into RFC 934 mechanism because the sequence latter causes
lines to grow with each level of fragments. quoting. The third, "total", another
integer, is the total number combination of parts. This third subfield
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 required ever
desired.

WARNING TO IMPLEMENTORS: The grammar for parameters on the final part, and
Content-type field is optional (though
encouraged) such that it is often necessary to
enclose the boundaries in quotes on the earlier parts. Note also that these
parameters may Content-type line.
This is not always necessary, but never hurts. Implementors
should be given sure to study the grammar carefully in any order. order to
avoid producing invalid Content-type fields. Thus, part 2 of a 3-part message may have either of the
following typical
multipart Content-Type header fields:

Content-Type: Message/Partial;
number=2; total=3;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com" field might look like this:

Content-Type: Message/Partial;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
number=2 multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p

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, following is not 0.

When the parts valid:

Content-Type: multipart/mixed; boundary=gc0pJq0M:08jU534c0p

(because of a message broken up in this manner are put
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 colon) and reassembly: The semantics of a
reassembled partial message must instead be those of represented as

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Internet Draft MIME Part One November 1994

Content-Type: multipart/mixed; boundary="gc0pJq0M:08jU534c0p"

This Content-Type value indicates that the "inner"
message, rather than content consists of
one or more parts, each with a message containing structure that is syntactically
identical to an RFC 822 message, except that the inner
message. This makes it possible, for example, header area
is allowed to send a
large audio message as several partial messages, be completely empty, and still
have it appear to that the recipient as a simple audio message
rather than as an encapsulated message containing an audio
message. That is, parts are each
preceded by the line

--gc0pJq0M:08jU534c0p

The encapsulation boundary MUST occur at the beginning of a
line, i.e., following a CRLF, and the message initial CRLF is
considered to be "transparent".

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When generating and reassembling the parts of a
message/partial message, the headers of the encapsulated
message must be merged with attached to the headers encapsulation boundary rather
than part of the enclosing
entities. In this process the following rules preceding part. The boundary must be
observed:

(1) All of
followed immediately either by another CRLF and the header
fields from the initial
enclosing entity (part one), except those that
start with "Content-" and for the specific next part, or by two CRLFs, in which case there
are no header fields "Subject", "Message-ID", "Encrypted", and
"MIME-Version",must be copied, in order, for the next part (and it is therefore
assumed to be of Content-Type text/plain).

NOTE: The CRLF preceding the
new message.

(2) Only those header fields in encapsulation line is
conceptually attached to the enclosed
message which start boundary so that it is possible
to have a part that does not end with "Content-" and "Subject",
"Message-ID", "Encrypted", and "MIME-Version" a CRLF (line break).
Body parts that must be appended, in order, considered to end with line breaks,
therefore, must have two CRLFs preceding the header fields encapsulation
line, the first of which is part of the
new message. Any header fields in preceding body part,
and the enclosed
message second of which do not start with "Content-" (except
for "Message-ID", "Encrypted", and "MIME-Version")
will be ignored.

(3) All is part of the header fields from encapsulation boundary.

Encapsulation boundaries must not appear within the second
encapsulations, and
any subsequent messages will must be ignored.

For example, if an audio message is broken into no longer than 70 characters, not
counting the two parts, leading hyphens.

The encapsulation boundary following the first last body part might look something like this:

X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail (part 1 of 2)
Message-ID: <id1@host.com>
MIME-Version: 1.0
Content-type: message/partial;
id="ABC@host.com";
number=1; total=2

X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Message-ID: <anotherid@foo.com>
Subject: Audio mail
MIME-Version: 1.0
Content-type: audio/basic

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Content-transfer-encoding: base64

... first half of encoded audio data goes here...

and is a
distinguished delimiter that indicates that no further body
parts will follow. Such a delimiter is identical to the second half might look something like this:

From: Bill@host.com
To: joe@otherhost.com
Subject: Audio mail (part 2 of 2)
MIME-Version: 1.0
Message-ID: <id2@host.com>
Content-type: message/partial;
id="ABC@host.com"; number=2; total=2

... second half
previous delimiters, with the addition of encoded audio data goes here...

Then, when two more hyphens at
the fragmented message is reassembled, end of the
resulting message line:

--gc0pJq0M:08jU534c0p--

There appears to be displayed room for additional information prior to
the user first encapsulation boundary and following the final
boundary. These areas 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

... generally be left blank, and
implementations must ignore anything that appears before the
first half boundary or after the last one.

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Internet Draft MIME Part One November 1994

NOTE: These "preamble" and "epilogue" areas are generally not
used because of encoded audio data goes here...
... second half the lack of encoded audio data goes here...

Note on encoding proper typing of MIME entities encapsulated inside
message/partial entities: Because data these parts and
the lack 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 clear semantics for handling these areas at
gateways, particularly X.400 gateways. However, rather than
leaving the binary data would preamble area blank, many MIME implementations
have found this to 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 convenient place to properly encode them for the 7-bit
world, aside from waiting insert an
explanatory note for all of the parts, reassembling
the message, and then encoding recipients who read the reassembled data message with
pre-MIME software, since such notes will be ignored by MIME-
compliant software.

NOTE: Because encapsulation boundaries must not appear in
base64 or quoted-printable. Since it is possible that
different the
body parts might go through different gateways, even

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this is not an acceptable solution. For this reason, it is
specified that MIME entities of type message/partial being encapsulated, a user agent must
always have exercise care
to choose a content-transfer-encoding of 7-bit (the
default). In particular, even unique boundary. The boundary in environments that support
binary or 8-bit transport, the use example
above could have been the result of an algorithm designed to
produce boundaries with a content-transfer-
encoding of "8bit" or "binary" is explicitly prohibited for
entities very low probability of type message/partial.

It should already
existing in the data to be noted that, because some message transfer
agents may choose encapsulated without having to automatically fragment large messages,
and because such agents may use different fragmentation
thresholds, it is possible that
prescan the pieces of data. Alternate algorithms might result in more
"readable" boundaries for a partial
message, upon reassembly, may prove themselves recipient with an old user agent,
but would require more attention to comprise a
partial message. This is explicitly permitted.

It should also be noted the possibility that the inclusion of a "References"
field
boundary might appear in the headers of the second and subsequent pieces encapsulated part. The simplest
boundary possible is something like "---", with a closing
boundary of "-----".

As a fragmented message that references the Message-Id on very simple example, the
previous piece may be following multipart message has
two parts, both of benefit to mail readers that
understand them plain text, one of them explicitly
typed and track references. However, the generation one of
such "References" fields them implicitly typed:

From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Date: Sun, 21 Mar 1993 23:56:48 -0800 (PST)
Subject: Sample message
MIME-Version: 1.0
Content-type: multipart/mixed; boundary="simple boundary"

This 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 preamble. It is to treat it if be ignored, though 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 handy place for accessing the
external data.

When mail composers to include an entity
explanatory note to non-MIME conformant readers.

--simple boundary

This 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 implicitly typed plain US-ASCII text.
It does NOT
appear in the area that follows. For example, consider the
following message: end with a linebreak.
--simple boundary
Content-type: message/external-body;

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Internet Draft MIME Part One November 1994

access-type=local-file;
name="/u/nsb/Me.gif"

Content-type: image/gif
Content-ID: <id42@guppylake.bellcore.com>
Content-Transfer-Encoding: binary

THIS IS NOT REALLY THE BODY!

The area at the end, which might be called the "phantom
body",

This is ignored for most external-body messages. However,
it may be used to contain auxiliary information for some
such messages, as indeed it explicitly typed plain US-ASCII text.
It DOES end with a linebreak.

--simple boundary--

This is when the access-type epilogue. It is
"mail-server". Of the access-types defined by this
document, the phantom also to be ignored.

The use of a Content-Type of multipart in a body part within
another multipart entity is used only when the access-type
is "mail-server". explicitly allowed. In all other such
cases, for obvious reasons, care must be taken to ensure that
each nested multipart entity uses a different boundary
delimiter. See Appendix C for an example of nested multipart
entities.

The use of the phantom multipart Content-Type with only a single body
part may be useful in certain contexts, and is
ignored. explicitly
permitted.

The only always-mandatory mandatory global parameter for message/external-
body is "access-type"; all of the other parameters may be
mandatory or optional depending on multipart
Content-Type is the value boundary parameter, which consists of access-type.

ACCESS-TYPE -- A case-insensitive word, indicating
the supported access mechanism by which the file
or data may 1 to
70 characters from a set of characters known to be obtained. Values include, but are
not limited to, "FTP", "ANON-FTP", "TFTP", "AFS",
"LOCAL-FILE", very robust
through email gateways, and "MAIL-SERVER". Future values,
except for experimental values beginning NOT ending with white space. (If a
boundary appears to end with "X-
", white space, the white space must
be registered with IANA, as described in
Appendix E .

In addition, presumed to have been added by a gateway, and must be
deleted.) It is formally specified by the following three parameters are optional for
ALL access-types:

EXPIRATION -- The date (in BNF:

boundary := 0*69<bchars> bcharsnospace

bchars := bcharsnospace / " "

bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" /
"+" / "_" / "," / "-" / "." /
"/" / ":" / "=" / "?"

Overall, the RFC 822 "date-time"
syntax, body of a multipart entity may be specified as extended by
follows:

dash-boundary := "--" boundary
; boundary taken from Content-Type
; field.

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Internet Draft MIME Part One November 1994

multipart-body := preamble dash-boundary
[*LWSP-char] CRLF
body-part *encapsulation
close-delimiter [*LWSP-char]
CRLF epilogue

encapsulation := delimiter [*LWSP-char]
CRLF body-part

delimiter := CRLF dash-boundary

close-delimiter := CRLF dash-boundary "--"

preamble := discard-text

epilogue := discard-text

discard-text := *text *(*text CRLF)
; To be ignored upon receipt.

body-part := <"message" as defined in RFC 1123 to permit 4 digits 822, with all
header fields optional, not starting with the
specified dash-boundary, and with the
delimiter not occurring anywhere in the year field) after which
message body. Note that the existence semantics of a
part differ from the external data is not guaranteed.

SIZE -- The size (in octets) semantics of a message,
as described in the data. text.>

IMPORTANT NOTE: The
intent addition of LWSP between the elements
shown in this parameter BNF is to help the recipient
decide whether or NOT allowed since this BNF does not to expend
specify a structured header field.

NOTE: In certain transport enclaves, RFC 822 restrictions
such as the necessary
resources one that limits bodies to retrieve printable US-ASCII
characters may not be in force. (That is, the external data. Note
that this describes transport
domains may resemble standard Internet mail transport as
specified in RFC 821 and assumed by RFC 822, but without
certain restrictions.) The relaxation of these restrictions
should be construed as locally extending the size definition of
bodies, for example to include octets outside of the data US-ASCII
range, as long as these extensions are supported by the
transport and adequately documented in its
canonical form, that is, before any Content-
Transfer-Encoding has been applied or after the

Borenstein & Freed [Page 50] Content-Transfer-
Encoding header field. However, in no event are headers
(either message headers or body-part headers) allowed to
contain anything other than US-ASCII characters.

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Internet Draft MIME Part One November 1994

data have been decoded.

PERMISSION -- A case-insensitive field that
indicates whether or not

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 expected recommended that
clients might also attempt those wishing to overwrite the data.
By default, provide a more
structured or if permission is "read", the
assumption integrated multipart messaging facility should
define a subtype of multipart that is syntactically identical,
but that they are not, and always expects the inclusion of a distinguished part
that if can be used to specify the
data is retrieved once, it is never needed again. structure and integration of
the other parts, probably referring to them by their Content-
ID field. If PERMISSION is "read-write", this assumption approach is
invalid, used, other implementations
will not recognize the new subtype, but will treat it as the
primary subtype (multipart/mixed) and any local copy must will thus be considered no
more than a cache. "Read" and "Read-write" are
the only defined values of permission.

The precise semantics of able to
show the access-types defined here are
described in user the sections parts that follow. are recognized.

6.2.1.2. Handling Nested Messages and Multiparts

The encapsulated headers "message/rfc822" subtype defined in ALL message/external-body
entities MUST include a Content-ID header field to give a
unique identifier by which to reference the subsequent section
of this document has no terminating condition other than
running out of data. This
identifier Similarly, an improperly truncated
multipart object may be used for cacheing mechanisms, not have any terminating boundary marker,
and for
recognizing the receipt of the data when the access-type does arise in practice due to mail system malfunctions.

It is
"mail-server".

Note that, as specified here, the tokens essential that describe
external-body data, such as file names and mail server
commands, objects be handled correctly when
they are themselves imbedded inside of another multipart
structure. MIME implementations are therefore required to be in
recognize outer level boundary markers at ANY level of inner
nesting. It is not sufficient to only check for 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 next
expected marker or by some other mechanism.

As with message/partial, it terminating condition.

6.2.1.3. Mixed Subtype

The "mixed" subtype of multipart is specified intended for use when the
body parts are independent and need to be bundled in a
particular order. Any multipart subtypes that MIME entities
of type message/external-body an
implementation does not recognize must always have a content-
transfer-encoding be treated as being of 7-bit (the default).
subtype "mixed".

6.2.1.4. Alternative Subtype

The multipart/alternative type is syntactically identical to
multipart/mixed, but the semantics are different. 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 each of FTP or TFTP indicates that the message
body parts is accessible as a file using the FTP [RFC-959] or TFTP
[RFC-783] protocols, respectively. For these access-types,
the following additional parameters are mandatory:

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NAME -- The name of

the file same information.

Systems should recognize that contains the
actual body data.

SITE -- A machine from which the file may be
obtained, using content of the given protocol. This must be
a fully qualified domain name, not a nickname.

Before any data various parts
are retrieved, using FTP, interchangeable. Systems should choose the user will
generally need to be asked to provide a login id "best" type
based on the local environment and a
password for preferences, in some cases
even through user interaction. As with multipart/mixed, 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.
order of body parts is significant. 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 this case, the
mode
alternatives appear in an order of increasing faithfulness to be used when retrieving
the information.
The legal values for access-type "TFTP" are
"NETASCII", "OCTET", and "MAIL", as specified by original content. In general, the TFTP protocol [RFC-783]. The legal values for
access-type "FTP" are "ASCII", "EBCDIC", "IMAGE",
and "LOCALn" where "n" best choice is a decimal integer,
typically 8. These correspond to the
representation types "A" "E" "I" and "L n" as
specified LAST
part of a type supported 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 recipient system's local
environment.

Multipart/alternative may be used instead. IF MODE is not specified,
the default value is "NETASCII" used, 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 example, to provide send mail
in a
name and password for the specified site. Instead, the ftp
protocol will be used with login "anonymous" and fancy text format in such a password way that corresponds to the user's email address.

7.3.3.3 The "local-file" and "afs" access-types it can easily be
displayed anywhere:

From: Nathaniel Borenstein & <nsb@bellcore.com>
To: Ned Freed [Page 52]

Expires 11/20/94 draft-ietf-822-mime-00.txt May 1994

An access-type <ned@innosoft.com>
Date: Mon, 22 Mar 1993 09:41:09 -0800 (PST)
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 "local-file" indicates that message goes here ...

--boundary42
Content-Type: text/enriched

... RFC 1563 text/enriched version of same message
goes here ...

--boundary42
Content-Type: application/x-whatever

... fanciest version of same message goes here ...

--boundary42--

In this example, users whose mail system understood the actual
body is accessible as a file on
"application/x-whatever" format would see only the local machine. An
access-type of "afs" indicates that fancy
version, while other users would see only the file is accessible
via enriched or
plain text version, depending on the global AFS file capabilities of their

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Internet Draft MIME Part One November 1994

system.

In both cases, only a
single parameter is required:

NAME -- The name of the file general, user agents that contains compose multipart/alternative
entities must place the
actual body data.

The following optional parameter may be used to describe the
locality parts in increasing order of reference for the data,
preference, that is, with the site or
sites at which preferred format last. For
fancy text, the file 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 expected to be visible:

SITE -- A domain specifier for a machine or set itself of
machines that are known to have access to type "multipart" and contains
unrecognized sub-parts, the data
file. Asterisks user agent may be used for wildcard matching choose either 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
show that is expected alternative, an earlier alternative, or both.

NOTE: From an implementor's perspective, it might seem more
sensible 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 reverse this ordering, and have the actual body plainest
alternative last. However, placing the plainest alternative
first is available from the friendliest possible option when
multipart/alternative entities are viewed using a non-MIME-
conformant mail server. The mandatory parameter
for reader. While this access-type is:

SERVER -- The email address of the approach does impose some
burden on conformant mail server
from which the actual body data can be obtained.

Because readers, interoperability with older
mail servers accept a variety of syntaxes, 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
which is multiline, the full command formats, will prefer to be sent offer
the user the choice of which format to a view. This makes
sense, for example, if mail
server is not included as includes both a parameter on the content-type
line. Instead, it is provided as the "phantom body" when
the content-type is message/external-body nicely-formatted
image version and the access-
type an easily-edited text version. What is mail-server.

An optional parameter for this access-type is:

SUBJECT -- The subject that most
critical, however, 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.

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Note that MIME does user not define a mail server syntax.
Rather, it allows the inclusion automatically be shown
multiple versions of arbitrary mail server
commands in the phantom body. Implementations must include same data. Either the phantom body in user should be
shown the body 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 message it sends to
same data, but the
mail server address to retrieve mappings between the relevant data.

It two are not
necessarily without information loss. For example,
information is worth noting that, unlike other access-types, mail-
server access lost when translating ODA to PostScript or
plain text. It is asynchronous and will happen at an
unpredictable time recommended that each part should have a
different Content-ID value in the future. For this reason, it case where the information
content of the two parts is
important that there be a mechanism by which not identical. And when the returned
data can be matched up with
information content is identical -- for example, where several
parts of type "message/external-body" specify alternate ways
to access the original message/external-
body entity. MIME mailservers must use identical data -- the same Content-ID field
value should be used, to optimize any caching mechanisms that
might be present on the returned message that was recipient's end. However, the

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Content-ID values used in by the original
message/external-body entity, to facilitate such matching.

7.3.3.5 Examples and Further Explanations

With parts should NOT be the emerging possibility of very wide-area file
systems, it becomes very hard to know in advance same
Content-ID value that describes the set of
machines where multipart/alternative as a file will and
whole, if there is any such Content-ID field. That is, one
Content-ID value will not be accessible
directly from the file system. Therefore it may make sense
to provide both a file name, refer to be tried directly, and the
name of multipart/alternative
entity, while 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, Content-ID values will refer
to the sender may include multiple parts inside it.

6.2.1.5. Digest Subtype

This document defines a "digest" subtype of type
message/external-body within an entity of type
multipart/alternative.

However, the external-body mechanism multipart
Content-Type. This type is not intended to be
limited syntactically identical to file retrieval, as shown by
multipart/mixed, but the mail-server
access-type. Beyond this, one can imagine, for example,
using semantics are different. In
particular, in 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 digest, the
encapsulated message. The default Content-Type value for a
body itself part is changed from "text/plain" to be found in the
external location. "message/rfc822".
This means is done to allow a more readable digest format that if the body of the
"message/external-body" message contains two consecutive
CRLFs, everything after those pairs is NOT part of
largely compatible (except for the
message itself. For most message/external-body messages, quoting convention) with
RFC 934.

A digest in this trailing area must simply be ignored. However, it is a
convenient place for additional data that cannot be included

Borenstein & Freed [Page 54] format might, then, look something like this:

From: Moderator-Address
To: Recipient-List
Date: Mon, 22 Mar 1994 13:34:51 +0000
Subject: Internet Digest, volume 42
MIME-Version: 1.0
Content-Type: multipart/digest;
boundary="---- next message ----"

------ next message ----

From: someone-else
Date: Fri, 26 Mar 1993 11:13:32 +0200
Subject: my opinion

...body goes here ...

------ next message ----

From: someone-else-again
Date: Fri, 26 Mar 1993 10:07:13 -0500
Subject: my different opinion

... another body goes here ...

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in the content-type header field. In particular, if

------ next message ------

6.2.1.6. Parallel Subtype

This document defines a "parallel" subtype of the
"access-type" value multipart
Content-Type. This type is "mail-server", then the trailing area
must contain commands to be sent syntactically identical to
multipart/mixed, but the mail server at the
address given by the value of the SERVER parameter.

The embedded message header fields which appear semantics are different. In
particular, in a parallel entity, the order of body parts is
not significant.

A common presentation of the message/external-body data must be used this type is to declare
the Content-type display all of the external body if it is anything
other than plain ASCII text, since
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 external body does
not have a header section to declare its type. Similarly,
parts serially in any Content-transfer-encoding other than "7bit" event.

6.2.1.7. Other Multipart Subtypes

Other multipart subtypes are expected in the future. MIME
implementations 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>

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--42
Content-Type: message/external-body;
access-type=mail-server
server="listserv@bogus.bitnet";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

Content-type: application/postscript
Content-ID: <id42@guppylake.bellcore.com>

get RFC-MIME.DOC

--42--

Note that general treat unrecognized subtypes of
multipart as being equivalent to "multipart/mixed".

6.2.2. Message Content-Type

It is frequently desirable, in sending mail, to encapsulate
another mail message. A special Content-Type, "message", is
defined to facilitate this. In particular, the above examples, the default Content-
transfer-encoding "rfc822"
subtype of "7bit" "message" is assumed used to encapsulate RFC 822 messages.

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 external
postscript data.

Like first part contains any control or
descriptive information, and a second part, of type
message/rfc822, is the message/partial type, forwarded or rejected message.
Composing rejection and forwarding messages in this manner
will preserve the message/external-body type is intended information on the original message and
allow it to be transparent, that is, correctly presented to convey the
data type in the external body rather than to convey a
message with a body recipient, and hence
is strongly encouraged.

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Subtypes of that type. Thus the headers message often impose restrictions on the
outer what
encodings are allowed. These restrictions are described in
conjunction with each specific subtype.

Mail gateways, relays, and inner parts must be merged using other mail handling agents are
commonly known to alter the same rules as
for message/partial. top-level header of an RFC 822
message. In particular, this means that the
Content-type they frequently add, remove, or
reorder header is overridden, but the From and Subject
headers are preserved.

Note that since the external bodies fields. Such alterations are not transported as
mail, they need not conform to explicitly
forbidden for the 7-bit and line length
requirements, but might encapsulated headers embedded in fact be binary files. Thus a
Content-Transfer-Encoding is not generally necessary, though
it is permitted.

Note that the body bodies
of a message messages of type "message/external-
body" is governed by "message."

6.2.2.1. RFC822 Subtype

A Content-Type of "message/rfc822" indicates that the body
contains an encapsulated message, with the basic syntax for of an RFC
822 message. In particular, anything before However, unlike top-level RFC 822 messages, the first
consecutive pair of CRLFs is
restriction that each message/rfc822 body must include a
"From", "Date", and at least one destination header information, while
anything after it is body information, which removed
and replaced with the requirement that at least one of "From",
"Subject", or "Date" must be present.

No encoding other than "7bit", "8bit", or "binary" is ignored
permitted for
most access-types. parts of type "message/rfc822". The formal grammar for content-type message
header fields for data
of type message is given by:

message-type := "message" "/" message-subtype

message-subtype := "rfc822"

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/ "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)
/ (";" "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

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7.4 The Application Content-Type

The "application" Content-Type is to be used for data which
do not fit are always US-ASCII in any of the other categories, case, and particularly
for data to within
the body can still be processed by mail-based uses of application
programs. This is information encoded, in 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, case the Content-
Transfer-Encoding header field in particular, the encapsulated message
will reflect this. Non-US-ASCII text in the headers of an
encapsulated message can pose
security problems which must be understood by implementors,
and are considered in detail specified using the mechanisms
described in RFC MIME-HEADERS.

It should be noted that, despite the discussion use of the
application/PostScript content-type.)

For example, a meeting scheduler might define numbers "822",
a standard
representation for message/rfc822 entity can include enhanced 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 as
defined in this document. In other words, a suitably specialized language are sent through
the mail and automatically run in the recipient's
environment.

Such applications message/rfc822
message may be a MIME message.

6.2.2.2. Partial Subtype

The "partial" subtype is defined to allow large entities to be
delivered as subtypes several separate pieces of mail and automatically
reassembled by the
"application" Content-Type. This document defines two
subtypes: octet-stream, receiving user agent. (The concept is
similar to IP fragmentation and PostScript.

In general, reassembly in the subtype of application will often basic
Internet Protocols.) This mechanism can be used when
intermediate transport agents limit the
name size of the application for which the data are intended.
This does not mean, however, individual

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messages that any application program
name may can be used freely as sent. Content-Type "message/partial"
thus indicates that the body contains a subtype fragment of application. Such
usages (other than subtypes beginning with "x-") a larger
message.

Three parameters must be
registered with IANA, as described specified in Appendix E.

7.4.1 The Application/Octet-Stream (primary) subtype

The primary subtype the Content-Type field
of application, "octet-stream", may type message/partial: The first, "id", is a unique
identifier, as close to a world-unique identifier as possible,
to be used to indicate that a body contains binary data. The set
of possible parameters includes, but is not limited to:

TYPE -- match the general type or category of binary
data. This is intended as information for parts together. (In general, the
human recipient rather than for any automatic
processing.

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PADDING --
identifier is essentially a message-id; if placed in double
quotes, it can be ANY message-id, in accordance with the number of bits of padding that were
appended to BNF
for "parameter" given earlier in this specification.) The
second, "number", an integer, is the bit-stream comprising part number, which
indicates where this part fits into the actual
contents to produce sequence of fragments.
The third, "total", another integer, is the enclosed byte-oriented
data. total number of
parts. This third subfield is useful for enclosing a bit-stream 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 body when 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 bits is parts:

Content-Type: Message/Partial; number=3; total=3;
id="oc=jpbe0M2Yt4s@thumper.bellcore.com"

Note that part numbering begins with 1, not a
multiple of 0.

When the byte size.

An additional parameter, "conversions", was defined parts of a message broken up in
[RFC-1341] but has been removed.

RFC 1341 also defined this manner are put
together, the use of result is a "NAME" parameter complete MIME entity, which
gave may have
its own Content-Type header field, and thus may contain any
other data type.

6.2.2.2.1. Message Fragmentation and Reassembly

The semantics of a suggested file name to reassembled partial message must be used if those
of the data were to be
written to a file. This has been deprecated in anticipation "inner" message, rather than of a separate Content-Disposition header field, to be
defined in a subsequent RFC.

The recommended action message containing

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the inner message. This makes it possible, for an implementation that receives
application/octet-stream mail is to simply offer example, to put the
data in
send a file, with any Content-Transfer-Encoding undone,
or perhaps to use it large audio message as input several partial messages, and
still have it appear to a user-specified process.

To reduce the danger of transmitting rogue programs through the mail, it is strongly recommended that implementations
NOT implement recipient as a path-search mechanism whereby simple audio
message rather than as an arbitrary
program named in the Content-Type parameter (e.g., encapsulated message containing an
"interpreter=" parameter) is found and executed using
audio message. That is, 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 encapsulation of the
PostScript language are allowed; the original level 1
variant message is described in [POSTSCRIPT]
considered to be "transparent".

When generating and reassembling the more recent
level 2 variant is described in [POSTSCRIPT2].

PostScript is a registered trademark parts of Adobe Systems, Inc.
Use a
message/partial message, the headers of the MIME content-type "application/postscript"
implies recognition encapsulated
message must be merged with the headers of that trademark and all the rights it
entails.

The PostScript language definition provides facilities for
internal labeling enclosing
entities. In this process the following rules must be
observed:

(1) All of the specific language features a given
program uses. This labeling, called header fields from the PostScript document
structuring conventions, is very general initial enclosing
entity (part one), except those that start with
"Content-" and provides
substantially more information than just the language level.

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The use of document structuring conventions, while not
required, is strongly recommended as an aid to
interoperability. Documents which lack proper structuring
conventions cannot specific header fields "Subject",
"Message-ID", "Encrypted", and "MIME-Version", must be tested
copied, in order, to see whether or not they will
work the new message.

(2) Only those header fields in a given environment. As such, some systems may
assume the worst enclosed message which
start with "Content-" and refuse to process unstructured
documents.

The execution of general-purpose PostScript interpreters
entails serious security risks, "Subject", "Message-ID",
"Encrypted", and implementors are
discouraged from simply sending PostScript email bodies to
"off-the-shelf" interpreters. While it is usually safe to
send PostScript "MIME-Version" must be appended, in
order, 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, header fields of the possible problems with sending PostScript
through the mail.

Dangerous operations new message. Any
header fields in the PostScript language include, but
may enclosed message which do not
start with "Content-" (except for "Message-ID",
"Encrypted", and "MIME-Version") will be limited to, ignored.

(3) All of the PostScript operators deletefile,
renamefile, filenameforall, header fields from the second and file. File any
subsequent messages will be ignored.

6.2.2.2.2. Fragmentation and Reassembly Example

If an audio message 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, broken into two parts, 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, part
might look something like this:

X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
Subject: Audio mail (part 1 of 2)
Message-ID: <id1@host.com>
MIME-Version: 1.0
Content-type: message/partial; id="ABC@host.com";
number=1; total=2

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X-Weird-Header-1: Bar
X-Weird-Header-2: Hello
Message-ID: <anotherid@foo.com>
Subject: Audio mail
MIME-Version: 1.0
Content-type: audio/basic
Content-transfer-encoding: base64

... first half of encoded audio data goes here ...

and this information may itself be sensitive. Message
senders should avoid the use second half might look something like this:

From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
Subject: Audio mail (part 2 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 2)
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 reach of fragmented message is reassembled, the PostScript language itself; care
should
resulting message to be taken displayed to insure that no method exists for re-
enabling full-function versions of these operators.

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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 user should look
something like this:

X-Weird-Header-1: Foo
From: Bill@host.com
To: joe@otherhost.com
Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST)
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 ...

Because data of type "message" may in some cases never be retained semipermanently encoded in nonvolatile memory. The operators associated with
exiting the interpreter loop have the potential to interfere
with subsequent document processing. As such, their
unrestrained use constitutes base64
or quoted-printable, 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 problem might arise if message/partial
entities are constructed in an environment that depends on exiting the interpreter loop to
operate. supports
binary or 8-bit transport. 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 problem is that the startjob and exitserver
commands. If these commands cannot binary
data would be eliminated, the
password associated with split into multiple message/partial messages,

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each of them should requiring binary transport. If such messages
were encountered at least a gateway into a 7-bit transport
environment, there would be set no way to a
hard-to-guess value.

PostScript provides operators properly encode them 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 7-bit world, aside from waiting for all of the interpreter. The PostScript
operators that set system and device parameters include, but
may not be limited to, fragments,
reassembling the setsystemparams inner message, and setdevparams
operators. Message-sending software should then encoding the
reassembled data in base64 or quoted-printable. Since it is
possible that different fragments might go through different
gateways, even this is not generate
PostScript an acceptable solution. For this
reason, it is specified that depends on MIME entities of type
message/partial 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 setting use of system a content-
transfer-encoding of "8bit" or device
parameters "binary" is explicitly
prohibited for entities of type message/partial.

Because some message transfer agents may choose to operate correctly. The ability
automatically fragment large messages, and because such agents
may use very different fragmentation thresholds, it is
possible that the pieces of a partial message, upon
reassembly, may prove themselves to set these
parameters will probably be unavailable comprise a partial
message. This is explicitly permitted.

The inclusion of a "References" field in secure PostScript
implementations. Message-receiving the headers of the
second and -displaying software
should, if possible, disable subsequent pieces of a fragmented message that
references the ability Message-Id on the previous piece may be of
benefit to change system mail readers that understand and device parameters. If these operators cannot be
disabled, track references.
However, the password associated with them generation of such "References" fields is
entirely optional.

Finally, it should at least be set to a hard-to-guess value.

Some PostScript implementations provide nonstandard
facilities for noted that the direct loading "Encrypted" header field
has been made obsolete by Privacy Enhanced Messaging (PEM)
[RFC1421, RFC1422, RFC1423, and execution of machine
code. Such facilities RFC1424], but the rules above
are quite obviously open nevertheless believed to
substantial abuse. Message-sending software should not
make use of such features. Besides being totally hardware-
specific, they are also likely describe the correct way to be unavailable treat
it if it is encountered in secure
implementations the context of PostScript. Message-receiving conversion to and
-displaying software should
from message/partial fragments.

6.2.2.3. External-Body Subtype

The external-body subtype indicates that the actual body data
are not allow such operators to be
used if they exist.

Borenstein & Freed [Page 61] included, but merely referenced. In this case, the
parameters describe a mechanism for accessing the external
data.

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PostScript is

When an extensible language, and many, if not most,
implementations entity is of type "message/external-body", 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 consists
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 a header, two consecutive CRLFs, and don't present any kind of threat.

It is possible to write PostScript that consumes huge
amounts the message header for
the encapsulated message. If another pair of various system resources. It is also possible to
write PostScript programs that loop infinitely. Both types consecutive
CRLFs appears, this of programs have course ends the potential to cause damage if sent to
unsuspecting recipients. Message-sending software should
avoid message header for the construction and dissemination of such programs,
which
encapsulated message. However, since the encapsulated
message's body 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 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

THIS IS NOT REALLY THE BODY!

The area at the end, which might be
limited to called the consumption of only a reasonable amount of
any given system resource.

It "phantom body",
is possible ignored for most external-body messages. However, it may
be used to include raw binary contain auxiliary information inside
PostScript in various forms. This is not recommended for
use in email, both because some such
messages, as indeed it is not supported by all
PostScript interpreters and because it significantly
complicates when the use of a MIME Content-Transfer-Encoding.
(Without such binary, PostScript may typically be viewed as
line-oriented data. access-type is "mail-
server". The treatment of CRLF sequences becomes
extremely problematic if binary and line-oriented data are
mixed in a single Postscript data stream.)

Finally, bugs may exist only access-type defined 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 document that
uses the timely correction of such bugs if any
are found.

7.4.3 Other Application subtypes

It phantom body is expected that many "mail-server", but other subtypes of application will access-types
may be defined in the future. MIME implementations must
generally treat any unrecognized subtypes as being
equivalent to application/octet-stream.

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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 future in other documents that the body contains
an image. use this
area.

The subtype names encapsulated headers in ALL message/external-body entities
MUST include a Content-ID header field to give a unique
identifier by which to reference the specific image format.
These names are case insensitive. Two initial subtypes are
"jpeg" data. This identifier
may be used for the JPEG format, JFIF encoding, caching mechanisms, and "gif" for GIF
format [GIF].

The list recognizing the
receipt of image subtypes given here is neither exclusive
nor exhaustive, and the data when the access-type is expected to grow "mail-server".

Note that, as more types are
registered with IANA, specified here, the tokens that describe
external-body data, such as described file names and mail server
commands, are required to be in Appendix E.

The formal grammar for the content-type header field 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
data message/external-body or by some
other mechanism.

As with message/partial, MIME entities of type image is given by:

image-type := "image" "/" ("gif" / "jpeg" / extension-token)

7.6 The Audio Content-Type

A Content-Type
message/external-body MUST have a content-transfer-encoding of "audio" indicates
7-bit (the default). In particular, even in environments that
support binary or 8-bit transport, 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.

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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

transfer-encoding of 8000 Hz and a single channel "8bit" or "binary" is
assumed.

The formal grammar for the content-type header field explicitly
prohibited for
data entities of type audio is given by:

audio-type := "audio" "/" ("basic" / extension-token)

7.7 message/external-body.

6.2.2.3.1. General External-Body Parameters

The Video Content-Type

A Content-Type of "video" indicates parameters that the body contains a
time-varying-picture image, possibly with color and
coordinated sound. The term "video" is may be used extremely
generically, rather than with reference to any particular
technology message/external-body
are:

(1) ACCESS-TYPE -- A word indicating the supported access
mechanism by which the file or format, and data may be obtained.
This word is not meant to preclude subtypes
such case sensitive. Values include, but
are not limited to, "FTP", "ANON-FTP", "TFTP", "LOCAL-
FILE", and "MAIL-SERVER". Future values, except for
experimental values beginning with "X-", must be
registered with IANA, as animated drawings encoded compactly. described in RFC REG. This
parameter is unconditionally mandatory and MUST be
present on EVERY message/external-body.

(2) EXPIRATION -- The subtype
"mpeg" refers to video coded according to date (in the MPEG standard
[MPEG].

Note that although RFC 822 "date-time"
syntax, as extended by RFC 1123 to permit 4 digits 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 year field) after which the content-type header field for
data existence 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-"
external data is a
private value, to not guaranteed. This parameter may be
used by consenting mail systems by
mutual agreement. Any format without a rigorous and public
definition must be named with an "X-" prefix, ANY access-type 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 ALWAYS optional.

(3) SIZE -- The size (in octets) of the
existing types whenever possible. data. The invention intent
of new

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types this parameter is intended to be restricted primarily to help the
development of new media types for email, such as digital
odors recipient decide
whether or holography, and not for new data formats in
general. In many cases, a subtype of application will be
more appropriate than a new top-level type.

Summary

Using to expend the MIME-Version, Content-Type, and Content-Transfer-
Encoding header fields, it is possible necessary resources to include, in a
standardized way, arbitrary types of data objects
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. This parameter
may be used with RFC
822 conformant mail messages. No restrictions imposed by
either RFC 821 ANY access-type and is ALWAYS
optional.

(4) PERMISSION -- A case-insensitive field that indicates
whether or RFC 822 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 violated,
not, and care has been
taken to avoid problems caused by additional restrictions
imposed by that if the characteristics of some Internet mail
transport mechanisms (see Appendix B). The "multipart" data is retrieved once, it is
never needed again. If PERMISSION is "read-write",
this assumption is invalid, and
"message" Content-Types allow mixing any local copy must be
considered no more than a cache. "Read" and hierarchical
structuring "Read-
write" are the only defined values of objects permission. This
parameter may be used with ANY access-type and is

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ALWAYS optional.

The precise semantics of different types the access-types defined here are
described in a single
message. Further Content-Types provide a standardized
mechanism for tagging messages the sections that follow.

6.2.2.3.2. The 'ftp' and 'tftp' Access-Types

An access-type of FTP or TFTP indicates that the message body parts
is accessible as audio,
image, a file using the FTP [RFC-959] or several other kinds of data. A distinguished
parameter syntax allows further specification of data format
details, particularly TFTP [RFC-
783] protocols, respectively. For these access-types, 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.

Security Considerations

Security issues
following additional parameters 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 mandatory:

(1) NAME -- The name of any actions in the recipient's
environment. In such cases, file that contains the discussion of actual
body data.

(2) SITE -- A machine from which the
application/postscript content-type in Section 7.4.2 file may
serve as be obtained,
using the given protocol. This must be a model for considering other content-types with
remote execution capabilities.

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Authors' Addresses

For more information, fully
qualified domain name, not a nickname.

(3) Before any data are retrieved, using FTP, the authors of this document may user will
generally need to be
contacted via Internet mail:

Nathaniel S. Borenstein
First Virtual Holdings
25 Washington Avenue
Morristown, NJ 07960

Email: nsb@nsb.fv.com
Phone: +1 201 540 8967
Fax: +1 201 993 3032

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 asked to provide a result of login id and a
password for the work of machine named by the Internet Engineering
Task Force Working Group on Email Extensions. The chairman
of that group, Greg Vaudreuil, may site parameter.
For security reasons, such an id and password are not
specified as content-type parameters, but must be reached at:

Gregory M. Vaudreuil
Tigon Corporation
17060 Dallas Parkway
Dallas Texas, 75248
214-733-2722
Email: gvaudre@cnri.reston.va.us

Borenstein & Freed [Page 66]
obtained from the user.

In addition, the following parameters are optional:

(1) DIRECTORY -- A directory from which the data named by
NAME should be retrieved.

(2) MODE -- A case-insensitive string indicating the mode
to be used when retrieving the information. The valid
values for access-type "TFTP" are "NETASCII", "OCTET",
and "MAIL", as specified by the TFTP protocol [RFC-
783]. The valid values for access-type "FTP" are
"ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a
decimal integer, typically 8. These correspond to the
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 and 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.

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Acknowledgements

This document

6.2.2.3.3. The 'anon-ftp' Access-Type

The "anon-ftp" access-type is identical to the result of "ftp" access
type, except that the collective effort of user need not be asked to provide a
large number of people, at several IETF meetings, on the
IETF-SMTP name
and IETF-822 mailing lists, password for the specified site. Instead, the ftp
protocol will be used with login "anonymous" and elsewhere.
Although any enumeration seems doomed a password
that corresponds to suffer from
egregious omissions, the following are among user's email address.

6.2.2.3.4. The 'local-file' Access-Type

An access-type of "local-file" indicates that the many
contributors to this effort:

Harald Tveit Alvestrand Timo Lehtinen
Randall actual body
is accessible as a file on the local machine. Two additional
parameters are defined for this access type:

(1) NAME -- The name of the file that contains the actual
body data. This parameter is mandatory for the "local-
file" access-type.

(2) SITE -- A domain specifier for a machine or set of
machines that are known to have access to the data
file. This optional parameter is 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.
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.

6.2.2.3.5. The 'mail-server' Access-Type

The "mail-server" access-type indicates that the actual body
is available from a mail server. Two additional parameters
are defined for this access-type:

(1) SERVER -- The email address of the mail server from
which the actual body data can be obtained. This
parameter is mandatory for the "mail-server" access-
type.

(2) SUBJECT -- The subject that is to be used in the mail
that is sent to obtain the data. Note that keying mail

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servers on Subject lines is NOT recommended, but such
mail servers are known to exist. This is an optional
parameter.

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 message/external-body and the access-type is
mail-server.

Note that MIME does not define a mail server syntax. Rather,
it allows the inclusion of arbitrary mail server commands in
the phantom body. Implementations must include the phantom
body in the body of the message it sends to the mail server
address to retrieve the relevant data.

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.

6.2.2.3.6. Examples and Further Explanations

When the external-body mechanism is used in conjunction with
the multipart/alternative Content-Type it extends the
functionality of multipart/alternative to include the case
where the same object is provided in the same format but via
different accces mechanisms. When this is done the originator
of the message must order the part first in terms of preferred
formats and then by preferred access mechanisms. The
recipient's viewer should then evaluate the list both in terms
of format and access mechanisms.

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

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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.

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 US-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
Date: Whenever
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"; mode="image";
access-type=ANON-FTP; directory="pub";
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=local-file;
name="/u/nsb/writing/rfcs/RFC-MIME.ps";
site="thumper.bellcore.com";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

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Content-type: application/postscript
Content-ID: <id42@guppylake.bellcore.com>

--42
Content-Type: message/external-body;
access-type=mail-server
server="listserv@bogus.bitnet";
expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"

Content-type: application/postscript
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.

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6.2.2.4. Other Message Subtypes

MIME implementations must in general treat unrecognized
subtypes of message as being equivalent to
"application/octet-stream".

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7. Experimental Content-Type Values

A Content-Type value beginning with the characters "X-" is a
private value, to be used by consenting mail systems by mutual
agreement. Any format without a rigorous and public
definition must be named with an "X-" prefix, and publicly
specified values shall never begin with "X-". (Older versions
of the widely used Andrew system use the "X-BE2" name, so new
systems should probably choose a different name.)

In general, the use of "X-" top-level types is strongly
discouraged. Implementors should invent subtypes of the
existing types whenever possible. The invention of new types
is intended to be restricted primarily to the development of
new media types for email, such as digital odors or
holography, and not for new data formats in general. In many
cases, a subtype of application will be more appropriate than
a new top-level type.

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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.

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9. Security Considerations

Security issues are discussed in the context of the
application/postscript type and in Appendix E. 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 type may serve as
a model for considering other content-types with remote
execution capabilities.

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10. Authors' Addresses

For more information, the authors of this document may be
contacted via Internet mail:

Nathaniel S. Borenstein
First Virtual Holdings
25 Washington Avenue
Morristown, NJ 07960
USA

Email: nsb@nsb.fv.com
Phone: +1 201 540 8967
Fax: +1 201 993 3032

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA

Email: ned@innosoft.com
Phone: +1 818 919 3600
Fax: +1 818919 3614

MIME is a result of the work of the Internet Engineering Task
Force Working Group on Email Extensions. The chairman of that
group, Greg Vaudreuil, may be reached at:

Gregory M. Vaudreuil
Tigon Corporation
17060 Dallas Parkway
Dallas Texas, 75248

Email: greg.vaudreuil@ons.octel.com
Phone: +1 214 733 2722

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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 Marc Andreessen
Randall Atkinson John R. MacMillan Bob Braden
Philippe Brandon Rick McGowan Brian Capouch
Kevin Carosso Leo Mclaughlin Uhhyung Choi Goli Montaser-Kohsari
Peter Clitherow Dave Collier-Brown
Cristian Constantinof Keith Moore John Coonrod
Mark Crispin Tom Moore Dave Crocker Erik Naggum
Stephen Crocker Terry Crowley Mark Needleman
Walt Daniels John Noerenberg Jim Davis
Frank Dawson Mats Ohrman Axel Deininger
Hitoshi Doi Julian Onions Kevin Donnelly Michael Patton
Steve Dorner Keith Edwards David J. Pepper
Chris Eich Blake C. Ramsdell Dana S. Emery
Johnny Eriksson Luc Rooijakkers Craig Everhart Marshall T. Rose
Patrik F.ltstr.m Jonathan Rosenberg Faltstrom Erik E. Fair Jan Rynning
Roger Fajman Harri Salminen Alain Fontaine Michael Sanderson
Martin Forssen James M. Galvin Masahiro Sekiguchi
Stephen Gildea Philip Gladstone Mark Sherman
Thomas Gordon Keld Simonsen
Terry Gray Phill Gross Bob Smart
James Hamilton Peter Speck
Steve Hardcastle-Kille Henry Spencer David Herron Einar Stefferud
Mark Horton Bruce Howard Michael Stein
Bill Janssen Klaus Steinberger Olle J.rnefors Peter Svanberg Jarnefors
Risto Kankkunen James Thompson Phil Karn Steve Uhler
Alan Katz Stuart Vance Tim Kehres Erik van der Poel

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Neil Katin Guido van Rossum Steve Kille
Kyuho Kim Peter Vanderbilt Anders Klemets Greg Vaudreuil
John Klensin Ed Vielmetti Valdis Kletniek Ryan Waldron
Jim Knowles Wally Wedel Stev Knowles Sven-Ove Westberg
Bob Kummerfeld Brian Wideen Pekka Kytolaakso John Wobus
Stellan Lagerstr.m Glenn Wright Lagerstrom Vincent Lau Rayan Zachariassen
Timo Lehtinen 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
John R. MacMillan Larry Masinter
Rick McGowan Michael J. McInerny

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Leo Mclaughlin Goli Montaser-Kohsari
Keith Moore Tom Moore
Erik Naggum Mark Needleman
John Noerenberg Mats Ohrman
Julian Onions Michael Patton
David J. Pepper Erik van der Poel
Jon Postel Blake C. Ramsdell
Christer Romson Luc Rooijakkers
Marshall T. Rose Jonathan Rosenberg
Guido van Rossum Jan Rynning
Harri Salminen Michael Sanderson
Yutaka Sato Markku Savela
Richard Alan Schafer Masahiro Sekiguchi
Mark Sherman Bob Smart
Peter Speck Henry Spencer
Einar Stefferud Michael Stein
Klaus Steinberger Peter Svanberg
James Thompson Steve Uhler
Stuart Vance Peter Vanderbilt
Greg Vaudreuil Ed Vielmetti
Larry W. Virden Ryan Waldron
Rhys Weatherly Jay Weber
Dave Wecker Wally Wedel
Sven-Ove Westberg Brian Wideen
John Wobus Glenn Wright
Rayan Zachariassen David Zimmerman

The authors apologize for any omissions from this list, which are certainly unintentional.

Borenstein & Freed [Page 68]

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Appendix A -- Minimal MIME-Conformance

The mechanisms described in this document are open-ended.
It is definitely not expected that all implementations will
support all of the Content-Types described, nor that they
will all share the same extensions. In order to promote
interoperability, however, it is useful to define the
concept of "MIME-conformance" to define a certain level of
implementation that allows the useful interworking of
messages with content that differs from US ASCII text. In
this section, we specify the requirements for such
conformance.

A mail user agent that is MIME-conformant MUST:

1. Always generate a "MIME-Version: 1.0" header
field.

2. Recognize the Content-Transfer-Encoding header
field, and decode all received data encoded with
either the quoted-printable or base64
implementations. Encode any data sent that is
not in seven-bit mail-ready representation using
one of these transformations and include the
appropriate Content-Transfer-Encoding header
field, unless the underlying transport mechanism
supports non-seven-bit data, as SMTP does not.

3. Recognize and interpret the Content-Type
header field, and avoid showing users raw data
with a Content-Type field other than text. Be
able to send at least text/plain messages, with
the character set specified as a parameter if it
is not US-ASCII.

4. Explicitly handle the following Content-Type
values, to at least the following extents:

Text:
-- Recognize and display "text" mail
with the character set "US-ASCII."
-- Recognize other character sets at
least to the extent of being able
to inform the user about what
character set the message uses.

Borenstein & Freed [Page 69]
are certainly unintentional.

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Appendix A -- Recognize the "ISO-8859-*" character
sets to the extent MIME Conformance

The mechanisms described in this document are open-ended. It
is definitely not expected that all implementations will
support all of being able to
display those characters the Content-Types described, nor that are
common to ISO-8859-* and US-ASCII,
namely they will
all characters represented
by octet values 0-127.
-- For unrecognized subtypes, show or
offer to show share the user same extensions. In order to promote
interoperability, however, it is useful to define the "raw"
version concept
of the data after
conversion "MIME-conformance" to define a certain level of
implementation that allows the useful interworking of messages
with content that differs from
canonical form to local form.
Message:
-- Recognize and display at least US-ASCII text. In this
section, we specify the
primary (822) encapsulation in requirements for such
a way as to preserve any recursive
structure, conformance.

A mail user agent that is, displaying or
offering to display the
encapsulated data in accordance
with its Content-type.
Multipart:
-- is MIME-conformant MUST:

(1) Always generate a "MIME-Version: 1.0" header field.

(2) Recognize the primary (mixed)
subtype. Display all relevant
information on the message level
and the body part Content-Transfer-Encoding header level field
and
then display decode all received data encoded with either the
quoted-printable or offer to display
each base64 implementations. Any non-7-
bit data that is sent without encoding must be properly
labelled with a content-transfer-encoding of 8bit or
binary, as appropriate. If the body parts
individually.
-- underlying transport
does not support 8bit or binary (as SMTP [RFC821] does
not), the snder is required to both encode and label
data using an appropriate Content-Transfer-Encoding
such as quoted-printable or base64.

(3) Recognize and interpret the "alternative" subtype, Content-Type header field,
and avoid showing the user
redundant parts of
multipart/alternative mail.
-- Recognize the "multipart/digest"
subtype, specifically using
"message/rfc822" rather users raw data with a Content-Type
field other than
"text/plain" as text. Be able to send at least
text/plain messages, with the default
content-type for encapsulations
inside "multipart/digest" entities.
-- Treat any unrecognized subtypes character set specified
as a parameter if
they were "mixed".
Application:
-- Offer it is not US-ASCII.

(4) Explicitly handle the ability following Content-Type values, to remove either of
at least the two types of Content-Transfer-
Encoding defined in this document following extents:

Text:

-- Recognize and put display "text" mail with the resulting information
in a user file.

Borenstein & Freed [Page 70]
character set "US-ASCII."

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Internet Draft MIME Part One November 1994

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

-- Recognize other character sets at least to an implementation, but likely
options for handling such unrecognized data
include offering the user
extent of being able to write it into a file
(decoded from its mail transport format) or
offering inform the user to name a program to which about what
character set 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 message uses.

-- Recognize the above conditions is said "ISO-8859-*" character sets to be
MIME-conformant. The meaning the
extent of this phrase is being able to display those characters that it is
assumed
are common to be "safe" ISO-8859-* and US-ASCII, namely all
characters represented by octet values 0-127.

-- For unrecognized subtypes in a known character
set, show or offer to send virtually any kind show the user the "raw" version
of
properly-marked the data to users after conversion of such mail systems, because
such systems will at least be able to treat the data content from
canonical form to local form.

-- Treat material in an unknown character set as
undifferentiated binary, and will not simply splash if
it onto were "application/octet-stream".

Image, audio, and video:

-- At a minumum provide facilities to Treat any
unrecognized subtypes as if they were
"application/octet-stream".

Application:

-- Offer the screen ability to remove either of unsuspecting users. There is another sense the quoted-
printable or base64 encodings defined in which it is always "safe" to send data this
document if they were used and put the resulting
information 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 user file.

Multipart:

-- Recognize the mixed subtype. Display all relevant
information on the message level and RFC 822. User agents that are MIME-conformant
have the additional guarantee that 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 will not be
shown data that were never intended to be viewed as text.

Borenstein & Freed [Page 71] redundant parts of
multipart/alternative mail.

-- Recognize the "multipart/digest" subtype,
specifically using "message/rfc822" rather than
"text/plain" as the default content-type for
encapsulations inside "multipart/digest" entities.

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Internet Draft MIME Part One November 1994

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 Treat any unrecognized subtypes as if they were
"mixed".

Message:

-- Recognize and mail technologies do not support the
full functionality possible in display at least the SMTP transport
environment. Mail traversing these systems is likely to be
modified primary
(RFC822) encapsulation in such a way as to preserve
any recursive structure, 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 is, displaying or
offering to take advantage of display the internal encapsulated data
structure of the hosts in
accordance with its Content-type.

-- Treat any unrecognized subtypes as if they are implemented on, or are just
plain broken.

The following guidelines may be useful to anyone devising were
"application/octet-stream".

(5) Upon encountering any unrecognized Content-Type, an
implementation must treat it as if it had a
data format (Content-Type) that will survive the widest
range Content-
Type 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 "application/octet-stream" with no parameter
sub-arguments. How such data are handled is up to systems that use the EBCDIC character set.

(1) Under some circumstances the encoding used an
implementation, but likely options for handling such
unrecognized data
may change as part of normal gateway or include offering the 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, write it
into a file (decoded from its mail transport format) or
offering the persistence of CRLF
as something other than user to name a line break must not be relied
on.

(2) Many systems may elect program to represent and store text
data using local newline conventions. Local newline
conventions may not match which the RFC822 CRLF convention --
systems are known
decoded data should be passed as input.

A user agent that use plain CR, plain LF, CRLF, or
counted records. meets the above conditions is said to be
MIME-conformant. The result meaning of this phrase is that isolated CR and LF
characters are not well tolerated in general; they
may be lost or converted it is
assumed to delimiters on some systems,
and hence must not be relied on.

Borenstein & Freed [Page 72]

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(3) TAB (HT) characters may be misinterpreted or may be
automatically converted "safe" to variable numbers send virtually any kind 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, 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 mail
formats must will not rely on simply splash it onto
the persistence screen of TAB (HT)
characters.

(4) Lines longer than 76 characters may be wrapped or
truncated in some environments. Line wrapping and line
truncation are STRONGLY DISCOURAGED, but unavoidable unsuspecting users.

There is another sense in
some cases. Applications which require long lines must
somehow differentiate between soft and hard line
breaks. (A simple way to do this it is always "safe" to use the
quoted-printable encoding.)

(5) Trailing "white space" characters (SPACE, TAB
(HT)) on send
data in a line may format that is MIME-conformant, which is that such
data will not break or be discarded broken by some transport
agents, while other transport agents may pad lines any known systems that are
conformant with
these characters so RFC 821 and RFC 822. User agents that all lines in a mail file are
of equal length. The persistence of trailing white
space, therefore, must
MIME-conformant have the additional guarantee that the user
will not be relied on.

(6) Many mail domains use variations on the ASCII
character set, or use character sets such shown data that were never intended to be viewed
as EBCDIC
which contain most but text.

Expires May 1995 [Page 79]

Internet Draft MIME Part One November 1994

Appendix B -- Guidelines For Sending Email Data

Internet email is not all of a perfect, homogeneous system. Mail may
become corrupted at several stages in its travel to a final
destination. Specifically, email sent throughout the US-ASCII
characters. The correct translation of characters Internet
may travel across many networking technologies. Many
networking and mail technologies do not support the full
functionality possible in the "invariant" set cannot be depended on across
character converting gateways. For example, this
situation SMTP transport environment.
Mail traversing these systems is likely to be modified in such
a problem when sending uuencoded
information across BITNET, an EBCDIC system. Similar
problems way that it can occur without crossing a gateway, since be transported.

There exist many Internet hosts use character sets other than ASCII
internally. The definition of Printable Strings in
X.400 adds further restrictions widely-deployed non-conformant MTAs in certain special
cases. In particular, the only characters that are
known to be consistent across all gateways are
Internet. These MTAs, speaking the 73
characters that correspond to SMTP protocol, alter
messages on the upper and lower case
letters A-Z and a-z, fly to take advantage of the 10 digits 0-9, and internal data
structure of the hosts they are implemented on, or are just
plain broken.

The following eleven special characters:

"'" (ASCII code 39)
"(" (ASCII code 40)
")" (ASCII code 41)

Borenstein & Freed [Page 73]

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"+" (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 guidelines may be useful to relatively
short lines anyone devising a
data format (Content-Type) that will survive the widest range
of text networking technologies and known broken MTAs unscathed.
Note that anything encoded in which the only meaningful
characters are taken from this set of 73 characters.

(7) Some mail transport agents base64 encoding will satisfy
these rules, but that some well-known mechanisms, notably the
UNIX uuencode facility, will corrupt data not. Note also that
includes certain literal strings. In particular, a
period (".") alone on a line is known to be corrupted
by anything
encoded in the Quoted-Printable encoding will survive most
gateways intact, but possibly not some (incorrect) SMTP implementations, and a line gateways to systems
that starts with use the five characters "From " (the fifth EBCDIC character is a SPACE) are commonly corrupted as well.
A careful composition agent can prevent these
corruptions by encoding set.

(1) Under some circumstances the encoding used for data (e.g., 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 quoted-
printable encoding, "=46rom " in place confusion of "From " at CRLF sequences with line
breaks in text bodies. As such, the start persistence of
CRLF as something other than a line, line break must not be
relied on.

(2) Many systems may elect to represent and "=2E" in place of "." alone on
a line.

Please note that store text data
using local newline conventions. Local newline
conventions may not match the above list is NOT a list of recommended
practices for MTAs. RFC 821 MTAs RFC822 CRLF convention --
systems are prohibited from
altering the character of white space known that use plain CR, plain LF, CRLF, or wrapping long
lines. These BAD
counted records. The result is that isolated CR and illegal practices LF
characters are known not well tolerated in general; they may

Expires May 1995 [Page 80]

Internet Draft MIME Part One November 1994

be lost or converted to occur delimiters on established networks, some systems, and implementations should
hence must not be
robust relied on.

(3) TAB (HT) characters may be misinterpreted or may be
automatically converted to variable numbers of spaces.
This is unavoidable in dealing with some environments, notably those
not based on the bad effects they can cause.

Borenstein & Freed [Page 74]

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Appendix C -- A Complex Multipart Example

What follows US-ASCII character set. Such
conversion is STRONGLY DISCOURAGED, but it may occur,
and mail formats must not rely on the outline persistence of a complex multipart message.
This message has five parts to
TAB (HT) characters.

(4) Lines longer than 76 characters may be displayed serially: two
introductory plain text parts, an embedded multipart
message, a richtext part, and a closing encapsulated text
message wrapped or
truncated in a non-ASCII character set. The embedded
multipart message has two parts to be displayed some environments. Line wrapping and line
truncation are STRONGLY DISCOURAGED, but unavoidable in parallel,
a picture
some cases. Applications which require long lines must
somehow differentiate between soft 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 hard line
breaks. (A simple way to do this is to use the preamble area of
quoted-printable encoding.)

(5) Trailing "white space" characters (SPACE, TAB (HT)) on
a multipart message.
Mail readers line may be discarded by some transport agents, while
other transport agents may pad lines with these
characters so that understand multipart format
should ignore this preamble.
If you are reading this text, you might want to
consider changing to all lines in a mail reader that understands
how to properly display multipart messages.
--unique-boundary-1

...Some text appears here...
[Note that 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 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 US-ASCII
character set, or use character sets such as in the next part.]

--unique-boundary-1
Content-type: text/plain; charset=US-ASCII

This could have been part EBCDIC
which contain most but not all of the previous part,
but illustrates explicit versus implicit
typing US-ASCII
characters. The correct translation of body parts.

--unique-boundary-1
Content-Type: multipart/parallel;
boundary=unique-boundary-2

--unique-boundary-2

Borenstein & Freed [Page 75]

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Content-Type: audio/basic
Content-Transfer-Encoding: base64

... base64-encoded 8000 Hz single-channel
mu-law-format audio data goes here....

--unique-boundary-2
Content-Type: image/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 characters not
in US-ASCII)
Subject: (subject the "invariant" set cannot be depended on across
character converting gateways. For example, this
situation is a problem when sending uuencoded
information across BITNET, an EBCDIC system. Similar
problems can occur without crossing a gateway, since
many Internet hosts use character sets other than US-
ASCII internally. The definition of Printable Strings
in US-ASCII)
Content-Type: Text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: Quoted-printable

... Additional text X.400 adds further restrictions in ISO-8859-1 goes here ...

--unique-boundary-1--

Borenstein & Freed [Page 76]

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Appendix D -- Collected Grammar

This appendix contains certain special
cases. In particular, the complete BNF grammar for only characters that are
known to be consistent across all gateways are the
syntax specified by this document.

By itself, however, this grammar is incomplete. It refers
to several entities 73
characters that are defined by RFC 822. Rather
than reproduce those definitions here, correspond to the upper and risk
unintentional differences between lower case
letters A-Z and a-z, the two, this document
simply refers 10 digits 0-9, and the reader
following eleven special characters:

"'" (US-ASCII decimal value 39)

Expires May 1995 [Page 81]

Internet Draft MIME Part One November 1994

"(" (US-ASCII decimal value 40)
")" (US-ASCII decimal value 41)
"+" (US-ASCII decimal value 43)
"," (US-ASCII decimal value 44)
"-" (US-ASCII decimal value 45)
"." (US-ASCII decimal value 46)
"/" (US-ASCII decimal value 47)
":" (US-ASCII decimal value 58)
"=" (US-ASCII decimal value 61)
"?" (US-ASCII decimal value 63)

A maximally portable mail representation, such as the
base64 encoding, will confine itself to RFC 822 for relatively
short lines of text in which the remaining
definitions. Wherever 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 term
period (".") alone on a line is undefined, it refers known to be corrupted
by some (incorrect) SMTP implementations, and a line
that starts with 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" five characters "From " (the fifth
character is a SPACE) are commonly corrupted as defined well.
A careful composition agent can prevent these
corruptions by encoding the data (e.g., in RFC 822,
with all header fields optional, and with the
specified delimiter not occurring anywhere quoted-
printable encoding, "=46rom " in place of "From " at
the message body, either start of a line, and "=2E" in place of "." alone on
a line by itself
or as line.

Please note that the above list is NOT a substring anywhere.>

NOTE: In certain transport enclaves, list of recommended
practices for MTAs. RFC 822
restrictions such as 821 MTAs are prohibited from altering
the one that limits bodies character of white space or wrapping long lines. These
BAD and invalid practices are known to
printable ASCII characters may not occur on established
networks, and implementations should be robust in force. (That
is, dealing with
the transport domains may resemble standard bad effects they can cause.

Expires May 1995 [Page 82]

Internet mail transport as specified in RFC821 Draft MIME Part One November 1994

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 text/enriched part, and
assumed by RFC822, but without certain restrictions.) a closing encapsulated text message
in a non-ASCII character set. The relaxation of these restrictions should embedded multipart message
has two parts to be
construed as locally extending the definition of
bodies, for displayed in parallel, a picture and an
audio fragment.

MIME-Version: 1.0
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Date: Fri, 07 Oct 1994 16:15:05 -0700 (PDT)
Subject: A multipart example to include octets outside
Content-Type: multipart/mixed;
boundary=unique-boundary-1

This is the preamble area of the
ASCII range, as long as these extensions a multipart message.
Mail readers that understand multipart format
should ignore this preamble.

If you are supported
by 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 transport blank between the boundary and adequately documented in the
Content-Transfer-Encoding header field. However, start
of the text in
no event are headers (either message headers or body- this part headers) allowed to contain anything other than

Borenstein & Freed [Page 77]

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ASCII characters.

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 means no space
; between "--" header fields were
given 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)

Borenstein & Freed [Page 78] this is text in the US-ASCII character set.
It could have been done with explicit typing as in the
next part.]

--unique-boundary-1
Content-type: text/plain; charset=US-ASCII

This could have been part of the previous part, but
illustrates explicit versus implicit typing of body
parts.

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Internet Draft MIME Part One November 1994

; Note that date-time is quoted
/ (";" "size" "=" 1*DIGIT)
/ (";" "permission" "=" ("read" / "read-
write"))
; Permission

--unique-boundary-1
Content-Type: multipart/parallel; boundary=unique-boundary-2

--unique-boundary-2
Content-Type: audio/basic
Content-Transfer-Encoding: base64

... base64-encoded 8000 Hz single-channel
mu-law-format audio data goes here ...

--unique-boundary-2
Content-Type: image/gif
Content-Transfer-Encoding: base64

... base64-encoded image data goes here ...

--unique-boundary-2--

--unique-boundary-1
Content-type: text/enriched

This is case-insensitive
/ (";" "name" "=" value)
/ (";" "site" "=" value)
/ (";" "dir" "=" value)
/ (";" "mode" "=" value)
/ (";" "server" "=" value)
/ (";" "subject" "=" value)
; access-type required; others required based on
access-type

iana-token := <a publicly-defined extension token,
registered with IANA, as specified <bold><italic>enriched.</italic></bold>
<smaller>as defined in RFC 1563</smaller>

Isn't it
<bigger><bigger>cool?</bigger></bigger>

--unique-boundary-1
Content-Type: message/rfc822

From: (mailbox in US-ASCII)
To: (address in US-ASCII)
Subject: (subject in US-ASCII)
Content-Type: Text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: Quoted-printable

... Additional text in ISO-8859-1 goes here ...

--unique-boundary-1--

Expires May 1995 [Page 84]

Internet Draft MIME Part One November 1994

Appendix D -- Collected Grammar

This 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 contains the complete BNF grammar for all the
syntax specified by this document.

By itself, however, this grammar is incomplete. It refers to
several entities that are defined by RFC 822. Rather than
reproduce those definitions here, and risk unintentional
differences between the two, this document simply refers the
reader to RFC 822 for the remaining definitions. Wherever a
term is undefined, it refers to the RFC 822 definition.

attribute := "message" "/" message-subtype

multipart-body token

boundary := preamble 1*encapsulation close-delimiter
epilogue

multipart-subtype 0*69<bchars> bcharsnospace

bchars := "mixed" / "parallel" / "digest"
/ "alternative" bcharsnospace / extension-token

multipart-type := "multipart" "/" multipart-subtype
";" "boundary" "=" boundary

Borenstein & Freed [Page 79]

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octet " "

bcharsnospace := "=" 2(DIGIT / "A" / "B" DIGIT / "C" ALPHA / "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 "?"

body-part := octet / <any ASCII character except "=", SPACE, or
TAB>
; characters not listed <"message" as "mail-safe" defined in Appendix B
; are also RFC 822, with all
header fields optional, not recommended.

quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="]
CRLF)
; Maximum line length starting with the
specified dash-boundary, and with the
delimiter not occurring anywhere in the
message body. Note that the semantics 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 a
part differ from the semantics of a message,
as described in the text.>

close-delimiter := "(" / ")" / "<" / ">" / "@"
/ "," CRLF dash-boundary "--"

composite-type := "message" / ";" "multipart" / extension-token

content := "Content-Type" ":" / "\" / <">
/ type "/" / "[" / "]" / "?" / "="

Borenstein & Freed [Page 80] subtype
*(";" parameter)
; Matching of type and subtype is
; ALWAYS case-insensitive

dash-boundary := "--" boundary
; boundary taken from Content-Type
; field.

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Internet Draft MIME Part One November 1994

delimiter := CRLF dash-boundary

description := "Content-Description" ":" *text

discard-text := *(*text CRLF)
; Must To be in quoted-string,
; to use within parameter values

type ignored upon receipt.

discrete-type := "application" / "audio" ; case-
insensitive "text" / "image" / "message"
/ "multipart" / "text" "audio" / "video" /
"application" / extension-token
; All values case-insensitive

value

encapsulation := token / quoted-string

version delimiter [*LWSP-char]
CRLF body-part

encoding := "MIME-Version" "Content-Transfer-Encoding" ":" 1*DIGIT "." 1*DIGIT

video-type mechanism

epilogue := "video" "/" ("mpeg" discard-text

extension-token := iana-token / ietf-token / extension-token) x-token

iana-token := <The two characters "X-" or "x-" followed, with
no
intervening white space, by any token>

Borenstein & Freed [Page 81]

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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 <a publicly-defined extension token,
registered with IANA, as specified in
RFC REG [REF-REG]>

ietf-token := <a publicly-defined extension token,
initially registered with IANA and
subsequently standardized by the content-type header field are IETF>

id := "Content-ID" ":" msg-id

mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" /
ietf-token / x-token

multipart-body := preamble dash-boundary
[*LWSP-char] CRLF
body-part *encapsulation
close-delimiter [*LWSP-char]
CRLF epilogue

octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used to convey supplemental information for various content
types, characters > 127, =,
; SPACE, or TAB, 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 recommended for the registration
of new values with IANA. Each of these is given in the form
of an email message template, to be filled any
; characters not listed 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 Appendix B as an RFC or submitted
in a form suitable to become an RFC, and be subject to the
; "mail-safe".

Expires May 1995 [Page 86]

Internet standards process.

To: IANA@isi.edu
Subject: Registration of new MIME
content-type/subtype Draft MIME type name:

(If the above is Part One November 1994

parameter := attribute "=" value

preamble := discard-text

ptext := octet / safe-char

quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
; Maximum line length of 76 characters
; excluding CRLF

safe-char := <any US-ASCII character except "=",
SPACE, or TAB>
; Characters not listed as "mail-safe" in
; Appendix B are also not an existing top-level MIME type,
please explain why an existing type cannot be used.)

MIME recommended.

subtype name:

Borenstein & Freed [Page 82]

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Required parameters:

Optional parameters:

Encoding considerations:

Security considerations:

Published specification:

(The published specification must be an Internet RFC := extension-token

token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or
RFC-to-be if a new top-level type is being defined, and
must tspecials>

tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "="
; Must be a publicly available specification in any
case.)

Person & email address quoted-string,
; 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 use within parameter values

type := discrete-type / composite-type

value := token / quoted-string

version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

x-token := <The two characters "X-" or
RFC-to-be.)

Person & email address to contact for further
information:

Borenstein & Freed [Page 83] "x-" followed, with
no intervening white space, by any token>

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Appendix F E -- Summary of the Seven Content-types

Content-type:

Content type: text

Subtypes defined by this document: plain

Important Parameters: parameters: charset

Encoding notes: quoted-printable generally preferred if an
encoding is needed and the character set is mostly an a US-
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

Content type: image

Subtypes defined by this document: mixed, alternative,
digest, parallel. jpeg, gif

Important Parameters: boundary parameters: none

Encoding notes: No content-transfer-encoding is permitted.

________________________________________________________________

Content-type: message base64 generally preferred

Content type: audio

Subtypes defined by this document: rfc822, partial,
external-body basic

Important Parameters: id, number, total, access-type,
expiration, size, permission, name, site, directory,
mode, server, subject parameters: none

Encoding notes: No content-transfer-encoding is permitted.
Specifically, only "7bit" is permitted for

Borenstein & Freed [Page 84] base64 generally preferred

Content type: video

Subtypes defined by this document: mpeg

Important parameters: none

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"message/partial" or "message/external-body", and only
"7bit", "8bit", or "binary" are permitted for other
subtypes of "message".

________________________________________________________________

Content-type:

Encoding notes: base64 generally preferred

Content type: application

Subtypes defined by this document: octet-stream, postscript

Important Parameters: parameters: type, padding

Deprecated Parameters: parameters: name and conversions were defined in
RFC 1341. 1341, and have since been deleted.

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, Severe security problems could result if this
type is used to transmit executable binary programs or programs in
general-purpose interpreted languages, such as LISP programs
or shell scripts, severe
security problems could result. without taking special precautions.
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

Content type: multipart

Subtypes defined by this document: jpeg, gif mixed, alternative,
digest, parallel.

Important Parameters: none parameters: boundary

Encoding notes: base64 generally preferred

________________________________________________________________

Content-type: audio No content-transfer-encoding other than
"7bit", "8bit", or "binary" are permitted.

Content type: message

Subtypes defined by this document: basic

Borenstein & Freed [Page 85] rfc822, partial,
external-body

Important parameters: id, number, total, access-type,
expiration, size, permission, name, site, directory, mode,
server, subject

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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 86] Only "7bit" is permitted for
"message/partial" or "message/external-body", and only
"7bit", "8bit", or "binary" are permitted for other subtypes
of "message".

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Appendix G F -- 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 PEM [RFC1421] and are performed
for each 'innermost level' "innermost level" body:

Step 1.

(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 system-dependent format stored only in
memory, or anything else that corresponds to the local
model for the representation of some form of
information. Fundamentally, the data is created in the
"native" form that corresponds to the type specified by
the type/subtype
information.

Step 2. content type.

(2) Conversion to canonical form.

The entire body, including "out-of-band" information
such as record lengths and possibly file attribute
information, is converted to a universal canonical
form. The specific 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

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any character set conversion, e.g. with regard to
syntactically meaningful characters in a text subtype
other than "plain".

Borenstein & Freed [Page 87]

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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. RFC 822. Note that the restriction on
line lengths implied by RFC822 RFC 822 is eliminated if the
next step employs either quoted-
printable quoted-printable or base64
encoding.

Step 3.

(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.

Borenstein & Freed [Page 88]

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Step 4.

(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. RFC 822.

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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 RFC 822 CRLF
delimiters.

Other implementation variations are conceivable as well. The
vital aspect of this discussion is that, in spite of any
optimizations, collapsings of required steps, or insertion of
additional processing, the resulting messages must be
consistent with those produced by the model described here.
For example, a message with the following header fields:

Content-type: text/foo; charset=bar
Content-Transfer-Encoding: base64

must be first represented in the text/foo form, then (if
necessary) represented in the "bar" character set, and finally
transformed via the base64 algorithm into a mail-
safe mail-safe form.

Borenstein & Freed [Page 89]

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Internet Draft MIME Part One November 1994

Appendix H G -- Changes from RFC 1521

This document is a very minor revision of RFC 1521. For the convenience
of those familiar with RFC 1521, the changes from that
document are summarized in this appendix. For further history,
note that Appendix H in RFC 1521 specified how that document
differed from its predecessor, RFC 1341.

(1) This document has been completely reformatted. This was
done to improve the quality of the plain text version
of this document, which is required to be the reference
copy.

(2) BNF describing the overall structure of MIME message
and part headers has been added. This is a
documentation change only -- the underlying syntax has
not changed in any way.

(3) The specific BNF for the seven content types in MIME
has been removed. This BNF was incorrect, incomplete,
amd inconsistent with the type-indendependent BNF. And
since the type-independent BNF already fully specifies
the syntax of the various MIME headers, the type-
specific BNF was, in the final analysis, completely
unnecessary and caused more problems than it solved.

(4) The more specific "US-ASCII" character set name has
replaced the use of the term ASCII in many parts of
this specification.

(5) The informal concept of a primary subtype has been
removed.

(6) The term "object" was being used inconsistently. This
term has been replaced with the more precise terms
"body", "body part", and "entity" where appropriate.

(7) The BNF for the multipart content-type has been
rearranged to make it clear that the CRLF preceeding
the boundary marker is actually part of the marker
itself rather than the preceeding body part.

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(8) In the rules on reassembling "message/partial" MIME
entities in section 7.3.2,
entities, "Subject" is added to the list of headers to
take from the inner message, and the example is
modified to clarify this point.

(9) In the discussion of the application/postscript type in
section 7.4.2, type,
an additional paragraph has been added warning against the about
possible interoperability problems caused by embedding
of binary data inside a PostScript MIME entity.

(10) Added a clarifying note to the basic syntax rules in
section 4 for
Content-Type to make it clear that the following two forms:

Content-type: text/plain; charset=us-ascii
Content-type: text/plain; charset="us-ascii"

are completely equivalent.

4. In section 7.2.3, a following two
forms:

Content-type: text/plain; charset=us-ascii (comment)

Content-type: text/plain; charset="us-ascii"

are completely equivalent.

(11) The following sentence has been removed from the
discussion of the MIME-Version header: "However,
conformant software is encouraged to check the version
number and at least warn the user if an unrecognized
MIME-version is encountered."

(12) A typo was fixed that said "application/external-body"
instead of "message/external-
body".

5. In section 5, "message/external-body".

(13) The definition of a character set has been reorganized
to make the following paragraph requirements clearer.

(14) The definitions of "7bit" and "8bit" have been
tightened so that use of bare CR, LF, and NUL
characters are no longer allowed.

(15) The definition of canonical text in MIME has been
tightened so that line breaks must be represented by a
CRLF sequence. CR and LF characters are not allowed
outside of this usage. The definition of quoted-
printable encoding has been altered accordingly.

(16) Prose was added to clarify the use of the "7bit" transfer-encoding in "7bit", "8-
bit", and "binary" transfer-encodings on multipart or
message entities encapsulating "8bit" or "binary" data:

It should also be noted that, by definition, if a
"multipart" or "message" entity has a transfer-
encoding value such as "7bit", but one of the
enclosed parts has a less restrictive value such
as "8bit", then either the outer "7bit" labelling
is in error, because 8 bit data are included, or
the inner "8bit" labelling placed an unnecessarily
high demand on the transport system because the
actual included data were actually 7bit-safe.

6. data.

Expires May 1995 [Page 95]

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(17) In Appendix A, "multipart/digest" support was added to
the list of requirements for minimal MIME conformance.

Borenstein & Freed [Page 90]

Expires 11/20/94 draft-ietf-822-mime-00.txt May 1994
Also, the requirement for "message/rfc822" support were
strengthened to clarify the importance of recognizing
recursive structure.

7. In section 7.3.1,

(18) The various restrictions on subtypes of "message" are
now specified entirely on a subtype by subtype basis.

(19) The definition of "message/rfc822" was changed to
indicate that at least one of the "From", "Subject", or
"Date" headers must be present.

(20) The required handling of unrecognized subtypes as
"application/octet-stream" has been made more explicit
in both the type definitions sections and the
conformance guidelines.

(21) Examples using text/richtext were changed to
text/enriched.

(22) The BNF definition of "message/rfc822" was subtype has been changed to indicate make
it clear that at least one of the "From",
"Subject", either an IANA registered subtype or "Date" headers a
nonstandard "X-" subtype must be present.

Borenstein & Freed [Page 91] used in a Content-Type
header field.

(23) The use of escape and shift mechanisms in the US-ASCII
and ISO-8859-X character sets this specification
defines has been clarified: Such mechanisms should
never be used in conjunction with these character sets
and their effect if they are used is undefined.

(24) The definition of the AFS access-type for
message/external-body has been removed.

(25) Entities that are simply registered for use and those
that are standardized by the IETF are now distinguished
in the MIME BNF.

(26) The handling of the combination of
multipart/alternative and message/external-body is now
specifically addressed.

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Appendix H -- 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-- Standard -- Information Processing -- ISO
7-bit and 8-bit coded character sets--Code extension
techniques, Coded Character Sets -- Code Extension
Techniques, ISO 2022:1986.

[ISO-8859]
International Standard -- Information Processing -- 8-bit
Single-Byte Coded Graphic Character Sets -- Part 1: Latin
Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet
No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3,
ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO
8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO
8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6,
1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9:
Latin alphabet No. 5, ISO 8859-9, 1990.

[ISO-646]
International Standard--Information Processing-- Standard -- Information Processing -- ISO
7-bit coded character set for information interchange, 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, "Pulse Code
Modulation (PCM) of Voice Frequencies", Geneva, 1972.

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Internet Draft MIME Part One November 1994

[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.

Borenstein & Freed [Page 92]

Expires 11/20/94 draft-ietf-822-mime-00.txt May 1994

[RFC-783]
Sollins, K.R. K.R., "TFTP Protocol (revision 2)", RFC-783,
MIT, June 1981.

[RFC-821]
Postel, J.B. J.B., "Simple Mail Transfer Protocol", STD 10,
RFC 821, USC/Information Sciences Institute, August 1982.

[RFC-822]
Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC 822, UDEL, August 1982.

[RFC-934]
Rose, M., and E. Stefferud, "Proposed Standard for
Message Encapsulation", RFC 934, Delaware and NMA,
January 1985.

[RFC-959]
Postel, J. and J. Reynolds, "File Transfer Protocol", STD
9, RFC 959, USC/Information Sciences Institute, October
1985.

[RFC-1049]
Sirbu, M., "Content-Type Header Field for Internet
Messages", 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.

Expires May 1995 [Page 98]

Internet Draft MIME Part One November 1994

[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.

Borenstein & Freed [Page 93]

Expires 11/20/94 draft-ietf-822-mime-00.txt May 1994

[RFC-1345]
Simonsen, K., "Character Mnemonics & Character Sets", RFC
1345, Rationel Almen Planlaegning, June 1992.

[RFC-1426] Klensin,

[RFC-1421]
Linn, J., (WG Chair), Freed, N., (Editor),
Rose, M., Stefferud, E., and D. Crocker, "SMTP Service
Extension "Privacy Enhancement for 8bit-MIME transport", Internet Electronic
Mail: Part I -- Message Encryption and Authentication
Procedures", RFC 1426, United Nations
Universit, Innosoft, Dover Beach Consulting, Inc., Network
Management Associates, Inc., The Branch Office, 1421, IAB IRTF PSRG, IETF PEM WG,
February 1993.

[RFC-1522] Moore, K., "Representation of Non-Ascii Text in

[RFC-1422]
Kent, S., "Privacy Enhancement for Internet Message Headers" Electronic
Mail: Part II -- Certificate-Based Key Management", RFC 1522, University of Tennessee,
September
1422, IAB IRTF PSRG, IETF PEM WG, February 1993.

[RFC-1340] Reynolds, J.,

[RFC-1423]
Balenson, D., "Privacy Enhancement for Internet
Electronic Mail: Part III -- Algorithms, Modes, and J. Postel, "Assigned Numbers",
STD 2, RFC 1340, USC/Information Sciences Institute, July
1992.
Identifiers", IAB IRTF PSRG, IETF PEM WG, February 1993.

[RFC-1424]
Kaliski, B., "Privacy Enhancement for Internet Electronic
Mail: Part IV -- Key Certification and Related
Services", IAB IRTF PSRG, IETF PEM WG, February 1993.

[RFC-1521]
Borenstein, N., and N. Freed, "MIME (Multipurpose
Internet Mail Extensions): Mechanisms for Specifying and
Describing the Format of Internet Message Bodies", RFC
1521, Bellcore, Innosoft, September, 1993.

[RFC-1563] Borenstein, N., "The text/enriched MIME Content-
type",

[RFC-1522]
Moore, K., "Representation of Non-ASCII Text in Internet
Message Headers", RFC 1522, University of Tennessee,
September 1993.

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[RFC-1524]
Borenstein, N., "A User Agent Configuration Mechanism for
Multimedia Mail Format Information", RFC 1524, Bellcore,
September 1993.

[RFC-1563]
Borenstein, N., "The text/enriched MIME Content-type",
RFC 1563, Bellcore, January, 1994.

[RFC-1652]
Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,
Stefferud, E., and Crocker, D., "SMTP Service Extension
for 8bit-MIME transport", RFC 1652, United Nations
Universit, Innosoft, Dover Beach Consulting, Inc.,
Network Management Associates, Inc., The Branch Office,
February 1993.

[RFC-1700]
Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
RFC 1563, Bellcore, January, 1700, USC/Information Sciences Institute, October
1994.

Borenstein & Freed [Page 94]

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Borenstein & Freed [Page i]

Table

[RFC-MIME-HEADERS]
Moore, K., "Representation of Contents

1 Introduction....................................... 4
2 Notations, Conventions, and Generic BNF Grammar.... 4
3 The MIME-Version Header Field...................... 6
4 The Content-Type Header Field...................... 8
5 The Content-Transfer-Encoding Header Field......... 14
5.1 Quoted-Printable Content-Transfer-Encoding......... 20
5.2 Base64 Content-Transfer-Encoding................... 24
6 Additional Content- Header Fields.................. 27
6.1 Optional Content-ID Header Field................... 27
6.2 Optional Content-Description Header Field.......... 27
7 The Predefined Content-Type Values................. 28
7.1 The Non-Ascii Text Content-Type.............................. 28
7.1.1 The charset parameter.............................. 28
7.1.2 The Text/plain subtype............................. 32
7.2 The Multipart Content-Type......................... 33
7.2.1 Multipart: The common syntax...................... 34
7.2.2 The Multipart/mixed (primary) subtype.............. 40
7.2.3 The Multipart/alternative subtype.................. 40
7.2.4 The Multipart/digest subtype....................... 43
7.2.5 The Multipart/parallel subtype..................... 43
7.3 The in Internet
Message Content-Type........................... 44
7.3.1 The Message/rfc822 (primary) subtype............... 45
7.3.2 The Message/Partial subtype........................ 45
7.3.3 The Message/External-Body subtype.................. 49
7.4 The Application Content-Type....................... 58
7.4.1 The Application/Octet-Stream (primary) subtype..... 58
7.4.2 The Application/PostScript subtype................. 59
7.4.3 Other Application subtypes......................... 62
7.5 The Image Content-Type............................. 63
7.6 The Audio Content-Type............................. 63
7.7 The Video Content-Type............................. 64
7.8 Experimental Content-Type Values................... 64
Summary............................................ 65
Security Considerations............................ 65
Authors' Addresses................................. 66
Acknowledgements................................... 67
Appendix A -- Minimal MIME-Conformance............. 69
Appendix B -- General Guidelines For Sending Email Data72
Appendix C -- A Complex Multipart Example.......... 75
Appendix D -- Collected Grammar.................... 77
Appendix E -- IANA Registration Procedures......... 82
E.1 Registration Headers", RFC MIME-HEADERS, University of New Content-type/subtype Values..82

Borenstein & Freed [Page ii]

E.2
Tennessee, ?.

[RFC-REG]
Postel, J., "Media Type Registration of New Access-type Values for Message/external-body83
Appendix F -- Summary of the Seven Content-types... 84
Appendix G -- Canonical Encoding Model............. 87
Appendix H -- Changes from Procedure", RFC 1521................ 90
References......................................... 92

Borenstein & Freed REG,
?.

[US-ASCII]
Coded Character Set -- 7-Bit American Standard Code for
Information Interchange, ANSI X3.4-1986.

[X400]
Schicker, Pietro, "Message Handling Systems, X.400",
Message Handling Systems and Distributed Applications, E.
Stefferud, O-j. Jacobsen, and P. Schicker, eds., North-
Holland, 1989, pp. 3-41.

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