Network Working Group                     Nathaniel Borenstein
Internet Draft                                       Ned Freed
                           <draft-ietf-822ext-mime-imb-01.txt>

            Multipurpose Internet Mail Extensions
                       (MIME) Part One:

              Format of Internet Message Bodies

                      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. Internet-Drafts may be updated, replaced, or obsoleted
by other documents at any time.  It is not appropriate to use
Internet-Drafts as reference material or to cite them other
than as a "working draft" or "work in progress".

To learn the current status of any Internet-Draft, please
check the 1id-abstracts.txt listing contained in the
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
specifying considerable detail about message headers, but
which leaves the message content, or message body, as flat
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 them.
Because RFC 822 said so little about message bodies, this
document is largely orthogonal to (rather than a revision of)
RFC 822.

In particular, this document is designed to provide facilities
to include multiple parts in a single message, to represent
body text in character sets other than US-ASCII, to represent
formatted multi-font text messages, to represent non-textual
material such as images and audio fragments, and generally to
facilitate later extensions defining new types of Internet
mail for use by cooperating mail agents.

This document does NOT extend Internet mail header fields to
permit anything other than US-ASCII text data.  Such
extensions are the subject of [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 G.





















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2.  Table of Contents


1 Abstract ..............................................    2
2 Table of Contents .....................................    3
3 Introduction ..........................................    5
4 Notations, Conventions, and 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 the Content-Type Header Field ...........   15
5.2.2 Definition of a Top-Level Content-Type ............   18
5.2.3 Initial Set of 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 [RFC-822] has defined
the standard format of textual mail messages on the Internet.
Its success has been such that the RFC 822 format has been
adopted, wholly or partially, well beyond the confines of the
Internet and the Internet SMTP transport defined by 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 7-bit US-ASCII.
This forces users to convert any non-textual data that they
may wish to send into seven-bit bytes representable as
printable 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) 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 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 information from the body part.
Moreover, it does not allow for the fact that the message may
eventually be gatewayed back into an X.400 message handling
system (i.e., the X.400 message is "tunneled" through Internet
mail), where the non-textual information would definitely
become useful again.

This document describes several mechanisms that combine to
solve most of these problems without introducing any serious
incompatibilities with the existing world of RFC 822 mail.  In
particular, it describes:

 (1)   A MIME-Version header field, which uses a version
       number to declare a message to be conformant with this
       specification and allows mail processing agents to
       distinguish between such messages and those generated
       by older or non-conformant software, which 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.

 (3)   A Content-Transfer-Encoding header field, which can be
       used to specify an auxiliary encoding that was applied
       to the data in order to allow it to pass through mail
       transport mechanisms which may have data or character
       set limitations.

 (4)   Two additional header fields that can be used to
       further describe the data in a 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 identify both discrete and composite bodies.  The following
types of discrete bodies are currently defined:





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 (1)   A "text" Content-Type value, which can be used to
       represent textual information in a number of character
       sets and formatted text description languages in a
       standardized manner.

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

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

 (4)   A "video" Content-Type value, for transmitting video or
       moving image data, possibly with audio as part of the
       composite video data format.

 (5)   An "application" Content-Type value, which can be used
       to transmit application data or binary data, and hence,
       among other uses, to implement an electronic mail file
       transfer service.

Two types of composite bodies are currently defined:

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

 (2)   A "message" Content-Type value, for encapsulating
       another message or part of a message.

MIME's Content-Type mechanism has been carefully designed to
be extensible, and it is expected that the set of content-
type/subtype pairs and their associated parameters will grow
significantly with time.  Several other MIME entities, most
notably the list of the name of character 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 sets up a
registration process which uses the Internet Assigned Numbers
Authority (IANA) as a central registry for MIME's extension
areas. The registration process is 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 RFC 1341 [RFC-1341]
and RFC1342 [RFC-1342], then revised in RFC 1521 [RFC-1521]
and RFC 1522 [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 G.  Please refer to the current edition
of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. 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 1344
[RFC-1344], RFC 1345 [RFC-1345], and RFC 1524 [RFC-1524].
























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4.  Notations, Conventions, and Generic BNF Grammar

Although the mechanisms specified in this document are all
described in prose, most are also described formally in the
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 augmented BNF notation.

Some of the 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
Appendix D of this document, the collected grammar, with 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 US-ASCII characters CR (decimal value 13) and LF (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 a 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 to
represent a particular character.  This definition is intended
to allow various kinds of character encodings, from simple
single-table mappings such as US-ASCII to complex table
switching methods such as those that use ISO 2022's
techniques. However, the definition associated with a MIME
character set name must fully specify the mapping to be
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, refers to either the a
single part message or one of the parts 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.  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:  Notes, such at this one, provide additional
nonessential information which may be skipped by the reader





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without missing anything essential. The primary purpose of
these non-essential notes is to convey information about the
rationale of this document, or to place this document in the
proper historical or evolutionary context.  Such information
may 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 certain design choices were
made.










































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5.  MIME Header Fields

MIME defines a number of 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 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 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:

  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".

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:  When checking MIME-Version values any
RFC 822 comment strings that are present must be ignored. In
particular, the following four MIME-Version fields are
equivalent:

  MIME-Version: 1.0

  MIME-Version: 1.0 (produced by 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 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
ordering of parameters is not significant.

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

Parameters are modifiers of the content-subtype, and as such
do not fundamentally affect the nature of the content. The set
of meaningful parameters depends on the content-type and
subtype. Most parameters are associated with a single specific
subtype.  However, a given top-level content-type may define
parameters which are applicable to any subtype of that type.
For example, the "charset" parameter is applicable to any
subtype of "text", while the "boundary" parameter is required
for any subtype of the "multipart" content-type.

There are NO globally-meaningful parameters that apply to all
content-types.  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 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 a
standards-track extension to this standard.  If another 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)
              ; Matching of type and subtype is
              ; ALWAYS case-insensitive

  type := discrete-type / composite-type

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

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

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

  iana-token := <a publicly-defined extension token,
                 registered with IANA, as specified in
                 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 := extension-token

  parameter := attribute "=" value

  attribute := token

  value := token / quoted-string

  token := 1*<any (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 -- 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 top-level
Content Types.  Parameter values are normally case sensitive,
but sometimes are interpreted in a case-insensitive fashion,
depending on the intended use.  (For example, multipart
boundaries are case-sensitive, but the "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 the parameter,
but are merely used to delimit that parameter value.  In
addition, comments are allowed in accordance with RFC 822
rules for structured header fields.  Thus the following two
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 process of defining new
content-subtypes, then, is not intended to be a mechanism for
imposing restrictions, but simply a mechanism for publicizing
the usages.  There are, therefore, two acceptable mechanisms
for defining new Content-Type subtypes:

 (1)   Private values (starting with "X-") may be defined
       bilaterally between two cooperating agents without
       outside registration or standardization.

 (2)   New standard values MUST be documented, registered
       with, and approved by IANA, as described in RFC REG.








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5.2.2.  Definition of a Top-Level Content-Type

The 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. The five discrete top-
level Content-Types are:

 (1)   text -- textual information.  The 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. 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
       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] under the name
       "enriched".





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 (2)   image -- 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 for two widely-used image formats, jpeg and
       gif.

 (3)   audio -- audio data. Audio requires an 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.  An initial subtype "mpeg" is
       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 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.  The "PostScript" subtype is
       also defined for the transport of PostScript material.
       Other expected uses for "application" include
       spreadsheets, data for mail-based scheduling systems,
       and languages for "active" (computational) email, and
       word processing formats that are not directly readable.
       Note that security 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 document.

The two composite top-level Content-Types are:

 (1)   multipart -- data consisting of multiple parts of
       independent data types.  Four subtypes are initially
       defined, including the 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.  Plain US-ASCII text must still be assumed in
the absence of a MIME-Version specification, but the sender's
intent might have been otherwise.

RATIONALE:  In the absence of any Content-Type header field or
MIME-Version header field, it is impossible to be certain that
a message is actually text in the US-ASCII character set,
since it might well be a message that, using 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.


5.3.  Content-Transfer-Encoding Header Field

Many Content-Types which could 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
encoding such data into a 7-bit short-line format.  Proper
labelling of unencoded material in less restrictive formats
for direct use over less restrictive transports is also
desireable.  This document specifies that such encodings will
be indicated by 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" / "8bit" / "binary" /
               "quoted-printable" / "base64" /
               ietf-token / x-token

These values are not case sensitive -- Base64 and BASE64 and
bAsE64 are all equivalent.  An encoding type of 7BIT requires
that the body is already in a 7-bit mail-ready representation.
This is the default value -- that is, "Content-Transfer-
Encoding: 7BIT" is assumed if the Content-Transfer-Encoding
header field is not present.







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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 "7bit", "8bit", and "binary" all mean that the
identity (i.e. NO) encoding transformation has been performed.
As such, they serve simply as indicators of the domain of the
body part data, and provide useful information about the sort
of encoding that might be needed for transmission in a given
transport system. The terms "7bit data", "8bit data", and
"binary data" are all 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 the transformations are
given below.

The proper Content-Transfer-Encoding label must always be
used. Labelling unencoded data containing 8-bit characters as
"7bit" is not allowed, nor is labelling unencoded non-line-
oriented data as anything other than "binary" allowed.

Unlike Content-Type subtypes, a proliferation of Content-
Transfer-Encoding values is both undesirable and unnecessary.
However, establishing only a single transformation into the
"7bit" domain does not seem possible.  There is a tradeoff
between the desire for a compact and efficient encoding of
largely-binary data and the desire for a readable encoding of
data that is mostly, but not entirely, 7-bit.  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 1652
[RFC-1652].  As of the publication of this document, there are
no standardized Internet mail transports for which it is
legitimate to include unencoded binary data in mail bodies.





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

NOTE:  The five values defined for the Content-Transfer-
Encoding field imply nothing about the Content-Type other than
the algorithm by which it was encoded or the transport system
requirements if unencoded.

Implementors may, if necessary, define new Content-Transfer-
Encoding values, but must use an x-token, which is a name
prefixed by "X-", to indicate its non-standard status, e.g.,
"Content-Transfer-Encoding:  x-my-new-encoding".  However,
unlike Content-Types and subtypes, the creation of new
Content-Transfer-Encoding values is STRONGLY discouraged, as
it seems likely to hinder interoperability with little
potential benefit.  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" the
Content-Transfer-Encoding is not permitted to have any value
other than "7bit", "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
zeroes.  This document provides a mechanism for noting the
addition of such padding in the case of the
application/octet-stream Content-Type, which has a "padding"
parameter.







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The encoding mechanisms defined here explicitly encode all
data in 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
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
is explicitly discouraged and should only occur when
absolutely necessary.  All content-transfer-encoding namespace
except that beginning with "X-" is explicitly reserved to the
IANA for future use.  Private agreements about content-
transfer-encodings are also explicitly discouraged.

Certain Content-Transfer-Encoding values may only be used on
certain Content-Types.  In particular, it is EXPRESSLY
FORBIDDEN to use any encodings other than "7bit", "8bit", or
"binary" with any composite Content-Type, i.e.  one that
recursively includes other Content-Type fields.  Currently the
only composite Content-Types are "multipart" and "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 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 data are included, or the inner "8bit" labelling
placed an unnecessarily high demand on the transport system
because the actual included data were actually 7-bit-safe.

NOTE ON ENCODING RESTRICTIONS:  Though the prohibition against
using content-transfer-encodings on composite body data may
seem overly restrictive, it is necessary to prevent nested
encodings, in which data are passed through an encoding
algorithm multiple times, and must be decoded multiple times
in order to be properly viewed.  Nested encodings add
considerable complexity to user agents:  Aside from the
obvious efficiency problems with such multiple encodings, they





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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 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: It may seem that the Content-Transfer-
Encoding could be inferred from the characteristics of the
Content-Type that is to be encoded, or, at the very least,
that certain Content-Transfer-Encodings could be mandated for
use with specific Content-Types.  There are several reasons
why this is not the case.  First, given the varying types of
transports used for mail, some encodings may be appropriate
for some Content-Type/transport combinations and not for
others.  (For example, in an 8-bit transport, no encoding
would be required for text in certain character sets, while
such encodings are clearly required for 7-bit SMTP.)

Second, certain Content-Types may require different types of
transfer encoding under different circumstances.  For example,
many PostScript bodies might consist entirely of short lines
of 7-bit data and hence require no 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 is intended to be an open-ended specification mechanism,
strict specification of an association between Content-Types
and encodings effectively couples the specification of an
application protocol with a specific lower-level transport.
This is not desirable since the developers of a Content-Type
should not have to be aware of all the transports in use and
what their limitations are.

NOTE ON TRANSLATING ENCODINGS:  The quoted-printable and
base64 encodings are designed so that conversion between them
is possible.  The only issue that arises in such a conversion
is the handling of line breaks.  When converting from quoted-
printable to base64 a line break must be converted into a CRLF
sequence.  Similarly, a CRLF sequence in base64 data must be
converted to a quoted-printable line break, but ONLY when
converting text data.







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NOTE ON CANONICAL ENCODING MODEL:  There was some confusion,
in earlier drafts of this 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 and character sets.  A
canonical model for encoding is presented as Appendix 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 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 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:

 (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 (US-ASCII form feed) can be
       represented by "=0C", and the decimal value 61 (US-
       ASCII EQUAL SIGN) can be represented by "=3D". This
       rule must be followed except when the following rules
       allow an alternative 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 US-ASCII characters which
       correspond to those octets (EXCLAMATION POINT through
       LESS THAN, and GREATER THAN through TILDE,
       respectively).

 (3)   (White Space) Octets with values of 9 and 32 MAY be
       represented as 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, indicating a soft line break (see
       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.

 (4)   (Line Breaks) A line break in a text body, represented
       as a CRLF sequence in the text canonical 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 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 encoding of such types. Sequences like "=0D",
       "=0A", "=0A=0D" and "=0D=0A" will 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 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.

 (5)   (Soft Line Breaks) The Quoted-Printable encoding
       REQUIRES that encoded lines be no more than 76
       characters long.  If longer lines are to be encoded
       with the Quoted-Printable encoding, "soft" line breaks
       must be used.  An equal sign as the last character on a
       encoded line indicates such a non-significant ("soft")
       line break in the encoded text.

Thus if the "raw" form of the line is a single unencoded line
that says:

  Now's the time for all folk to come to the aid of their country.

This can be represented, in the Quoted-Printable encoding, as:

  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.  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

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

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

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





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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 [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, and all characters in the subset are also
represented identically in all versions of EBCDIC.  Other
popular encodings, such as the encoding used by the uuencode
utility and the base85 encoding specified as part of Level 2
PostScript, do not share these properties, and thus do not
fulfill the portability requirements a binary transport
encoding for mail must meet.

The encoding process represents 24-bit groups of input bits as
output strings of 4 encoded characters.  Proceeding from left
to right, a 24-bit input group is formed by concatenating 3
8-bit input groups.  These 24 bits are then treated as 4
concatenated 6-bit groups, each of which is translated into a
single digit in the base64 alphabet.  When encoding a bit
stream via the base64 encoding, the bit stream must be
presumed to be ordered with the most-significant-bit first.
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 particular
significance to SMTP (e.g., ".", CR, LF) and to the
encapsulation boundaries defined in this document (e.g., "-").

                 Table 1: The Base64 Alphabet

  Value Encoding  Value Encoding  Value Encoding  Value Encoding
      0 A            17 R            34 i            51 z
      1 B            18 S            35 j            52 0
      2 C            19 T            36 k            53 1
      3 D            20 U            37 l            54 2
      4 E            21 V            38 m            55 3
      5 F            22 W            39 n            56 4
      6 G            23 X            40 o            57 5
      7 H            24 Y            41 p            58 6
      8 I            25 Z            42 q            59 7
      9 J            26 a            43 r            60 8
     10 K            27 b            44 s            61 9
     11 L            28 c            45 t            62 +
     12 M            29 d            46 u            63 /
     13 N            30 e            47 v
     14 O            31 f            48 w         (pad) =
     15 P            32 g            49 x
     16 Q            33 h            50 y

The encoded output stream 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 characters with no "="
padding, (2) the final quantum of encoding input is exactly 8
bits; here, the final unit of encoded output will be two
characters followed by two "=" padding characters, or (3) the
final quantum of encoding input is exactly 16 bits; here, the
final unit of encoded output will be three characters followed
by one "=" padding character.

Because it is used only for padding at the end of the data,
the occurrence of any "=" characters may be taken as evidence
that the end of the data has been reached (without truncation
in transit).  No such assurance is possible, however, when the
number of octets transmitted was a multiple of three.

Any characters outside of the base64 alphabet are to be
ignored in base64-encoded data.  The same applies to any
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 parts of
multipart entities because no hyphen characters are used in
the base64 encoding.


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 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 caching data
referenced by the message/external-body mechanism.  Although
the Content-ID header is generally optional, its use is
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 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.  This is explained in the section of this document
dealing with multipart/alternative.


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
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 MIME-
HEADERS [RFC-MIME-HEADERS] may be used for non-US-ASCII
Content-Description values.


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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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 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.  A "charset" parameter may be
used to indicate the character set of the body text for some
text subtypes, notably including the subtype "text/plain",
which indicates plain (unformatted) text.  The default
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/enriched" [RFC-1563].






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6.1.1.1.  Representation of Line Breaks

The 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
field for text/plain data is the character set.  This is
specified with a "charset" parameter, as in:

  Content-type: text/plain; charset=iso-8859-1

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
multioctet character sets for their subtype definitions.

This RFC specifies the definition of the charset parameter for
the purposes of MIME to be the name of a 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 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 7-bit character code, but specifies that the body
uses character coding that uses the exact correspondence of
octets to characters specified in US-ASCII.  National use
variations of ISO 646 [ISO-646] are NOT 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" explicitly refers to ANSI
X3.4-1986 [US-ASCII] only.  The character set name "ASCII" is
reserved and must not be used for any purpose.

NOTE: RFC 821 explicitly specifies "ASCII", and references an
earlier version of the American Standard.  Insofar as one of
the purposes of specifying a Content-Type and character set is
to permit the receiver to unambiguously determine how the
sender intended the coded message to be interpreted, assuming
anything other than "strict ASCII" as the default would risk
unintentional and incompatible changes to the semantics of
messages now being transmitted.  This also implies that
messages containing characters coded according to national
variations on ISO 646, or using code-switching procedures
(e.g., those of ISO 2022), as well as 8-bit or multiple octet
character encodings MUST use an appropriate character set
specification to be consistent with this specification.

The complete US-ASCII character set is listed in ANSI X3.4-
1986. Note that the control characters including DEL (0-31,
127) have no defined meaning apart from the combination CRLF
(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 character set that can be
used universally for representing all of the world's languages
in electronic mail.  Unfortunately, existing practice in
several communities seems to point to the continued use of
multiple character sets in the near future.  For this reason,
we define 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].  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, 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 data
might also wish to specify a charset value for some purpose,
in which case the same syntax and values should be used.

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


6.1.1.3.  Plain Subtype

The 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", 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.


6.1.1.4.  Unrecognized Subtypes

Unrecognized subtypes of text should be treated as subtype
"plain" as long as the MIME implementation knows how to handle
the charset. Unrecognized subtypes which also specify an
unrecognized charset should be treated as "application/octet-
stream".





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6.1.2.  Image Content-Type

A Content-Type of "image" indicates that the body contains an
image.  The subtype names the specific image format.  These
names are not case sensitive.  Two initial subtypes are "jpeg"
for the JPEG format, JFIF encoding, and "gif" for GIF format
[GIF].

The list of image subtypes given here is neither exclusive nor
exhaustive, and is expected to grow as more types are
registered with IANA, as described in RFC REG.

Unrecognized subtypes of image should at a miniumum be 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

A Content-Type of "audio" indicates that the body contains
audio data.  Although there is not yet a consensus on an
"ideal" audio format for use with computers, there is a
pressing need for a format capable of providing interoperable
behavior.

The initial subtype of "basic" is specified to meet this
requirement by providing an absolutely minimal lowest common
denominator audio format.  It is expected that richer formats
for higher quality and/or lower bandwidth audio will be
defined by a later document.

The content of the "audio/basic" subtype is single channel
audio encoded using 8-bit ISDN mu-law [PCM] at a sample rate
of 8000 Hz.

Unrecognized subtypes of audio should at a miniumum be treated
as "application/octet-stream". Implementations may optionally
elect to pass subtypes of audio that they do not specifically
recognize to a robust general-purpose audio playing
application, if such an application is available.







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6.1.4.  Video Content-Type

A Content-Type of "video" indicates that the body contains a
time-varying-picture image, possibly with color and
coordinated sound.  The term "video" is used extremely
generically, rather than with reference to any particular
technology or format, and is not meant to preclude subtypes
such as animated drawings encoded compactly.  The subtype
"mpeg" refers to video coded according to the MPEG standard
[MPEG].

Note that although in general this document strongly
discourages the mixing of multiple media in a single body, it
is recognized that many so-called "video" formats include a
representation for synchronized audio, and this is explicitly
permitted for subtypes of "video".

Unrecognized subtypes of video should at a minumum be treated
as "application/octet-stream". Implementations may optionally
elect to pass subtypes of video that they do not specifically
recognize to a robust general-purpose video display
application, if such an application is available.



6.1.5.  Application Content-Type

The "application" Content-Type is to be used for discrete data
which do not fit in any of the other categories, and
particularly for data to be processed by mail-based uses of
application programs.  This is information which must be
processed by an application before it is viewable or usable to
a user.  Expected uses for Content-Type application include
mail-based file transfer, spreadsheets, data for mail-based
scheduling systems, and languages for "active" (computational)
email.  (The latter, in particular, can pose security problems
which must be understood by implementors, and are considered
in detail in the discussion of the application/PostScript
content-type.)

For example, a meeting scheduler might define a standard
representation for information about proposed meeting dates.
An intelligent user agent would use this information to
conduct a dialog with the user, and might then send further
mail based on that dialog.  More generally, there have been





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several "active" messaging languages developed in which
programs in a suitably specialized language are sent through
the mail and automatically run in the recipient's environment.

Such applications may be defined as subtypes of the
"application" Content-Type.  This document defines two
subtypes: octet-stream, and PostScript.

The subtype of application will often be the name of the
application for which the data are intended.  This does not
mean, however, that any application program name may be used
freely as a subtype of application.  Usage of any subtype
(other than subtypes beginning with "x-") must be registered
with IANA, as described in RFC REG.


6.1.5.1.  Octet-Stream Subtype

The "octet-stream" subtype is used to indicate that a body
contains arbitrary binary data.  The set of currently defined
parameters is:

 (1)   TYPE -- the general type or category of binary data.
       This is intended as information for the human recipient
       rather than for any automatic processing.

 (2)   PADDING -- the number of bits of padding that were
       appended to the bit-stream comprising the actual
       contents to produce the enclosed 8-bit byte-oriented
       data. This is useful for enclosing a bit-stream in a
       body when the total number of bits is not a multiple of
       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
use of a "NAME" parameter which gave a suggested file name to
be used if the data were to be written to a file.  This has
been deprecated in anticipation of a separate Content-
Disposition header field, to be defined in a subsequent RFC.

The recommended action for an implementation that receives
application/octet-stream mail is to simply offer to put the
data in a file, with any Content-Transfer-Encoding undone, or





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

To reduce the danger of transmitting rogue programs through
the mail, it is strongly recommended that implementations NOT
implement a path-search mechanism whereby an arbitrary program
named in the Content-Type parameter (e.g., an "interpreter="
parameter) is found and executed using the mail body as input.


6.1.5.2.  PostScript Subtype

A Content-Type of "application/postscript" indicates a
PostScript program.  Currently two variants of the PostScript
language are allowed; the original level 1 variant is
described in [POSTSCRIPT] and the more recent level 2 variant
is described in [POSTSCRIPT2].

PostScript is a registered trademark of Adobe Systems, Inc.
Use of the MIME content-type "application/postscript" implies
recognition of that trademark and all the rights it entails.

The PostScript language definition provides facilities for
internal labelling of the specific language features a given
program uses. This labelling, called the PostScript document
structuring conventions, or DSC, is very general and provides
substantially more information than just the language level.
The use of document structuring conventions, while not
required, is strongly recommended as an aid to
interoperability. Documents which lack proper structuring
conventions cannot be tested to see whether or not they will
work in a given environment. As such, some systems may assume
the worst and refuse to process unstructured documents.

The execution of general-purpose PostScript interpreters
entails serious security risks, and implementors are
discouraged from simply sending PostScript email bodies to
"off-the-shelf" interpreters.  While it is usually safe to
send PostScript to a printer, where the potential for harm is
greatly constrained by typical printer environments,
implementors should consider all of the following before they
add interactive display of PostScript bodies to their mail
readers.

The remainder of this section outlines some, though probably
not all, of the possible problems with sending PostScript





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through the mail.

 (1)   Dangerous operations in the PostScript language
       include, but may not be limited to, the PostScript
       operators "deletefile", "renamefile", "filenameforall",
       and "file".  "File" is only dangerous when applied to
       something other than standard input or output.
       Implementations may also define additional nonstandard
       file operators; these may also pose a threat to
       security. "Filenameforall", the wildcard file search
       operator, may appear at first glance to be harmless.
       Note, however, that this operator has the potential to
       reveal information about what files the recipient has
       access to, and this information may itself be
       sensitive.  Message senders should avoid the use of
       potentially dangerous file operators, since these
       operators are quite likely to be unavailable in secure
       PostScript implementations.  Message receiving and
       displaying software should either completely disable
       all potentially dangerous file operators or take
       special care not to delegate any special authority to
       their operation. These operators should be viewed as
       being done by an outside agency when interpreting
       PostScript documents. Such disabling and/or checking
       should be done completely outside of the reach of the
       PostScript language itself; care should be taken to
       insure that no method exists for re-enabling full-
       function versions of these operators.

 (2)   The PostScript language provides facilities for exiting
       the normal interpreter, or server, loop.  Changes made
       in this "outer" environment are customarily retained
       across documents, and may in some cases be retained
       semipermanently in nonvolatile memory.  The operators
       associated with exiting the interpreter loop have the
       potential to interfere with subsequent document
       processing. As such, their unrestrained use constitutes
       a threat of service denial.  PostScript operators that
       exit the interpreter loop include, but may not be
       limited to, the exitserver and startjob operators.
       Message sending software should not generate PostScript
       that depends on exiting the interpreter loop to
       operate, since the ability to exit will probably be
       unavailable in secure PostScript implementations.
       Message receiving and displaying software should





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       completely disable the ability to make retained changes
       to the PostScript environment by eliminating or
       disabling the "startjob" and "exitserver" operations.
       If these operations cannot be eliminated or completely
       disabled the password associated with them should at
       least be set to a hard-to-guess value.

 (3)   PostScript provides operators for setting system-wide
       and device-specific parameters.  These parameter
       settings may be retained across jobs and may
       potentially pose a threat to the correct operation of
       the interpreter.  The PostScript operators that set
       system and device parameters include, but may not be
       limited to, the "setsystemparams" and "setdevparams"
       operators.  Message sending software should not
       generate PostScript that depends on the setting of
       system or device parameters to operate correctly. The
       ability to set these parameters will probably be
       unavailable in secure PostScript implementations.
       Message receiving and displaying software should
       disable the ability to change system and device
       parameters. If these operators cannot be completely
       disabled the password associated with them should at
       least be set to a hard-to-guess value.

 (4)   Some PostScript implementations provide nonstandard
       facilities for the direct loading and execution of
       machine code.  Such facilities are quite obviously open
       to substantial abuse.  Message sending software should
       not make use of such features. Besides being totally
       hardware-specific, they are also likely to be
       unavailable in secure implementations of PostScript.
       Message receiving and displaying software should not
       allow such operators to be used if they exist.

 (5)   PostScript is an extensible language, and many, if not
       most, implementations of it provide a number of their
       own extensions.  This document does not deal with such
       extensions explicitly since they constitute an unknown
       factor.  Message sending software should not make use
       of nonstandard extensions; they are likely to be
       missing from some implementations.  Message receiving
       and displaying software should make sure that any
       nonstandard PostScript operators are secure and don't
       present any kind of threat.





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 (6)   It is possible to write PostScript that consumes huge
       amounts of various system resources.  It is also
       possible to write PostScript programs that loop
       indefinitely.  Both types of programs have the
       potential to cause damage if sent to unsuspecting
       recipients.  Message-sending software should avoid the
       construction and dissemination of such programs, which
       is antisocial. Message receiving and displaying
       software should provide appropriate mechanisms to abort
       processing of a document after a reasonable amount of
       time has elapsed. In addition, PostScript interpreters
       should be limited to the consumption of only a
       reasonable amount of any given system resource.

 (7)   It is possible to include raw binary information inside
       PostScript in various forms.  This is not recommended
       for use in email, both because it is not supported by
       all PostScript interpreters and because it
       significantly complicates the use of a MIME Content-
       Transfer-Encoding.  (Without such binary, PostScript
       may typically be viewed as line-oriented data.  The
       treatment of CRLF sequences becomes extremely
       problematic if binary and line-oriented data are mixed
       in 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 timely correction of such
       bugs if any are found.


6.1.5.3.  Other Application Subtypes

It is expected that many other subtypes of application will be
defined in the future.  MIME implementations must at a minimum
treat any unrecognized subtypes as being equivalent to
"application/octet-stream".











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6.2.  Composite Content-Type Values

The remaining two of the seven initial Content-Type values
refer to composite entities. Composite entities are handled
using MIME mechanisms -- a MIME processor typically handles
the body directly.


6.2.1.  Multipart Content-Type

In the case of multiple part entities, in which one or more
different sets of data are combined in a single body, a
"multipart" Content-Type field must appear in the entity's
header.  The body must then contain one or more "body parts,"
each preceded by an encapsulation boundary, and the last one
followed by a closing boundary.  Each part starts with an
encapsulation boundary, and then contains a body part
consisting of a header area, a blank line, and a body area.
Thus a body part is similar to an RFC 822 message in syntax,
but different in meaning.

A body part is NOT to be interpreted as actually being an RFC
822 message.  To begin with, NO header fields are actually
required in body parts.  A body part that starts with a blank
line, therefore, is allowed and is a body part for which all
default values are to be assumed.  In such a case, the
absence of a Content-Type header indicates that the
corresponding body has a content-type of "text/plain;
charset=US-ASCII"".

The only header fields that have defined meaning for body
parts are those the names of which begin with "Content-".  All
other header fields are generally to be ignored in body parts.
Although they should generally be retained in mail processing,
they may be discarded by gateways if necessary.  Such other
fields are permitted to appear in body parts but must not be
depended on.  "X-" fields may be created for experimental or
private purposes, with the recognition that the information
they contain may be lost at some gateways.

NOTE:  The distinction between an RFC 822 message and a body
part is subtle, but important.  A gateway between Internet and
X.400 mail, for example, must be able to tell the difference
between a body part that contains an image and a body part
that contains an encapsulated message, the body of which is a





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GIF image.  In order to represent the latter, the body 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 use of similar
syntax facilitates the conversion of messages to body parts,
and vice versa, but the distinction between the two must be
understood by implementors.  (For the special case in which
all parts actually are messages, a "digest" subtype is also
defined.)

As stated previously, each body part is preceded by an
encapsulation boundary.  The encapsulation boundary MUST NOT
appear inside any of the encapsulated parts.  Thus, it is
crucial that the composing agent be able to choose and specify
a unique boundary that will separate the parts.

All present and future subtypes of the "multipart" type must
use an identical syntax.  Subtypes may differ in their
semantics, and may impose additional restrictions on syntax,
but must conform to the required syntax for the multipart
type.  This requirement ensures that all conformant user
agents will at least be able to recognize and separate the
parts of any multipart entity, even of an unrecognized
subtype.

As stated in the definition of the Content-Transfer-Encoding
field, no encoding other than "7bit", "8bit", or "binary" is
permitted for entities of type "multipart".  The multipart
delimiters and header fields are always represented as 7-bit
US-ASCII in any case (though the header fields may encode
non-US-ASCII header text as per RFC MIME-HEADERS, and data
within the body parts can be encoded on a part-by-part basis,
with Content-Transfer-Encoding fields for each appropriate
body part.

Message transport agents, relays, and 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
headers of any body part which occurs within an enclosing
multipart body part.









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6.2.1.1.  Common Syntax

This section defines a common syntax for subtypes of
multipart. All subtypes of multipart must use this syntax. A
simple example of a multipart message also appears in this
section.  An example of a more complex multipart message is
given in Appendix C.

The Content-Type field for multipart entities requires one
parameter, "boundary", which is used to specify the
encapsulation boundary.  The encapsulation boundary is defined
as a line consisting entirely of two hyphen characters ("-",
decimal value 45) followed by the boundary parameter value
from the Content-Type header field.

NOTE:  The hyphens are for rough compatibility with the
earlier RFC 934 method of message encapsulation, and for ease
of searching for the boundaries in some implementations.
However, it should be noted that multipart messages are NOT
completely compatible with RFC 934 encapsulations; in
particular, they do not obey RFC 934 quoting conventions for
embedded lines that begin with hyphens.  This mechanism was
chosen over the RFC 934 mechanism because the latter causes
lines to grow with each level of quoting.  The combination of
this growth with the fact that SMTP implementations sometimes
wrap long lines made the RFC 934 mechanism unsuitable for use
in the event that deeply-nested multipart structuring is ever
desired.

WARNING TO IMPLEMENTORS:  The grammar for parameters on the
Content-type field is such that it is often necessary to
enclose the boundaries in quotes on the Content-type line.
This is not always necessary, but never hurts.  Implementors
should be sure to study the grammar carefully in order to
avoid producing invalid Content-type fields.  Thus, a typical
multipart Content-Type header field might look like this:

  Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08j34c0p

But the following is not valid:

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

(because of the colon) and must instead be represented as






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

This Content-Type value indicates that the content consists of
one or more parts, each with a structure that is syntactically
identical to an RFC 822 message, except that the header area
is allowed to be completely empty, and that the 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 initial CRLF is
considered to be attached to the encapsulation boundary rather
than part of the preceding part.  The boundary must be
followed immediately either by another CRLF and the header
fields for the next part, or by two CRLFs, in which case there
are no header fields for the next part (and it is therefore
assumed to be of Content-Type text/plain).

NOTE:  The CRLF preceding the encapsulation line is
conceptually attached to the boundary so that it is possible
to have a part that does not end with a CRLF (line  break).
Body parts that must be considered to end with line breaks,
therefore, must have two CRLFs preceding the encapsulation
line, the first of which is part of the preceding body part,
and the second of which is part of the encapsulation boundary.

Encapsulation boundaries must not appear within the
encapsulations, and must be no longer than 70 characters, not
counting the two leading hyphens.

The encapsulation boundary following the last body part is a
distinguished delimiter that indicates that no further body
parts will follow.  Such a delimiter is identical to the
previous delimiters, with the addition of two more hyphens at
the end of the line:

  --gc0pJq0M:08jU534c0p--

There appears to be room for additional information prior to
the first encapsulation boundary and following the final
boundary.  These areas should generally be left blank, and
implementations must ignore anything that appears before the
first boundary or after the last one.






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NOTE:  These "preamble" and "epilogue" areas are generally not
used because of the lack of proper typing of these parts and
the lack of clear semantics for handling these areas at
gateways, particularly X.400 gateways.  However, rather than
leaving the preamble area blank, many MIME implementations
have found this to be a convenient place to insert an
explanatory note for recipients who read the message with
pre-MIME software, since such notes will be ignored by MIME-
compliant software.

NOTE:  Because encapsulation boundaries must not appear in the
body parts being encapsulated, a user agent must exercise care
to choose a unique boundary.  The boundary in the example
above could have been the result of an algorithm designed to
produce boundaries with a very low probability of already
existing in the data to be encapsulated without having to
prescan the data.  Alternate algorithms might result in more
"readable" boundaries for a recipient with an old user agent,
but would require more attention to the possibility that the
boundary might appear in the encapsulated part.  The simplest
boundary possible is something like "---", with a closing
boundary of "-----".

As a very simple example, the following multipart message has
two parts, both of them plain text, one of them explicitly
typed and one of them implicitly typed:

  From: Nathaniel Borenstein <nsb@bellcore.com>
  To: Ned Freed <ned@innosoft.com>
  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 the preamble.  It is to be ignored, though it
  is a handy place for mail composers to include an
  explanatory note to non-MIME conformant readers.

  --simple boundary

  This is implicitly typed plain US-ASCII text.
  It does NOT end with a linebreak.
  --simple boundary
  Content-type: text/plain; charset=us-ascii






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  This is explicitly typed plain US-ASCII text.
  It DOES end with a linebreak.

  --simple boundary--

  This is the epilogue.  It is also to be ignored.

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

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

The only mandatory global parameter for the multipart
Content-Type is the boundary parameter, which consists of 1 to
70 characters from a set of characters known to be very robust
through email gateways, and NOT ending with white space. (If a
boundary appears to end with white space, the white space must
be presumed to have been added by a gateway, and must be
deleted.)  It is formally specified by the following BNF:

  boundary := 0*69<bchars> bcharsnospace

  bchars := bcharsnospace / " "

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

Overall, the body of a multipart entity may be specified as
follows:

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










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  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 822, with all
                header fields optional, not starting with the
                specified dash-boundary, and with the
                delimiter not occurring anywhere in the
                message body.  Note that the semantics of a
                part differ from the semantics of a message,
                as described in the text.>

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.

NOTE:  In certain transport enclaves, RFC 822 restrictions
such as the one that limits bodies to printable US-ASCII
characters may not be in force.  (That is, the 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 definition of
bodies, for example to include octets outside of the US-ASCII
range, as long as these extensions are supported by the
transport and adequately documented in the Content-Transfer-
Encoding header field.  However, in no event are headers
(either message headers or body-part headers) allowed to
contain anything other than US-ASCII characters.





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NOTE:  Conspicuously missing from the multipart type is a
notion of structured, related body parts.  In general, it
seems premature to try to standardize interpart structure yet.
It is recommended that those wishing to provide a more
structured or integrated multipart messaging facility should
define a subtype of multipart that is syntactically identical,
but that always expects the inclusion of a distinguished part
that can be used to specify the structure and integration of
the other parts, probably referring to them by their Content-
ID field.  If this approach is used, other implementations
will not recognize the new subtype, but will treat it as the
primary subtype (multipart/mixed) and will thus be able to
show the user the parts that are recognized.


6.2.1.2.  Handling Nested Messages and Multiparts

The "message/rfc822" subtype defined in a subsequent section
of this document has no terminating condition other than
running out of data.  Similarly, an improperly truncated
multipart object may not have any terminating boundary marker,
and does arise in practice due to mail system malfunctions.

It is essential that such objects be handled correctly when
they are themselves imbedded inside of another multipart
structure.  MIME implementations are therefore required to
recognize outer level boundary markers at ANY level of inner
nesting.  It is not sufficient to only check for the next
expected marker or other terminating condition.


6.2.1.3.  Mixed Subtype

The "mixed" subtype of multipart is intended for use when the
body parts are independent and need to be bundled in a
particular order.  Any multipart subtypes that an
implementation does not recognize must be treated as being of
subtype "mixed".


6.2.1.4.  Alternative Subtype

The multipart/alternative type is syntactically identical to
multipart/mixed, but the semantics are different.  In
particular, each of the parts is an "alternative" version of





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the same information.

Systems should recognize that the content of the various parts
are interchangeable.  Systems should choose the "best" type
based on the local environment and preferences, in some cases
even through user interaction.  As with multipart/mixed, the
order of body parts is significant.  In this case, the
alternatives appear in an order of increasing faithfulness to
the original content. In general, the best choice is the LAST
part of a type supported by the recipient system's local
environment.

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

  From: Nathaniel Borenstein <nsb@bellcore.com>
  To: Ned Freed <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 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
"application/x-whatever" format would see only the fancy
version, while other users would see only the enriched or
plain text version, depending on the capabilities of their





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system.

In general, user agents that compose multipart/alternative
entities must place the body parts in increasing order of
preference, that is, with the preferred format last.  For
fancy text, the sending user agent should put the plainest
format first and the richest format last.  Receiving user
agents should pick and display the last format they are
capable of displaying.  In the case where one of the
alternatives is itself of type "multipart" and contains
unrecognized sub-parts, the user agent may choose either to
show that alternative, an earlier alternative, or both.

NOTE:  From an implementor's perspective, it might seem more
sensible to reverse this ordering, and have the plainest
alternative last.  However, placing the plainest alternative
first is the friendliest possible option when
multipart/alternative entities are viewed using a non-MIME-
conformant mail reader.  While this approach does impose some
burden on conformant mail readers, interoperability with older
mail readers was deemed to be more important in this case.

It may be the case that some user agents, if they can
recognize more than one of the formats, will prefer to offer
the user the choice of which format to view.  This makes
sense, for example, if mail includes both a nicely-formatted
image version and an easily-edited text version.  What is most
critical, however, is that the user not automatically be shown
multiple versions of the same data.  Either the user should be
shown the last recognized version or should be given the
choice.

NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE:
Each part of a multipart/alternative entity represents the
same data, but the mappings between the two are not
necessarily without information loss.  For example,
information is lost when translating ODA to PostScript or
plain text.  It is recommended that each part should have a
different Content-ID value in the case where the information
content of the two parts is not identical. And when the
information content is identical -- for example, where several
parts of type "message/external-body" specify alternate ways
to access the identical data -- the same Content-ID field
value should be used, to optimize any caching mechanisms that
might be present on the recipient's end. However, the





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Content-ID values used by the parts should NOT be the same
Content-ID value that describes the multipart/alternative as a
whole, if there is any such Content-ID field.  That is, one
Content-ID value will refer to the multipart/alternative
entity, while one or more other Content-ID values will refer
to the parts inside it.


6.2.1.5.  Digest Subtype

This document defines a "digest" subtype of the multipart
Content-Type.  This type is syntactically identical to
multipart/mixed, but the semantics are different.  In
particular, in a digest, the default Content-Type value for a
body part is changed from "text/plain" to "message/rfc822".
This is done to allow a more readable digest format that is
largely compatible (except for the quoting convention) with
RFC 934.

A digest in this 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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  ------ next message ------


6.2.1.6.  Parallel Subtype

This document defines a "parallel" subtype of the multipart
Content-Type.  This type is syntactically identical to
multipart/mixed, but the semantics are different.  In
particular, in a parallel entity, the order of body parts is
not significant.

A common presentation of this type is to display all of the
parts simultaneously on hardware and software that are capable
of doing so.  However, composing agents should be aware that
many mail readers will lack this capability and will show the
parts serially in any event.


6.2.1.7.  Other Multipart Subtypes

Other multipart subtypes are expected in the future.  MIME
implementations must in general treat unrecognized subtypes of
multipart as being equivalent to "multipart/mixed".



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 "rfc822"
subtype of "message" is 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 first part contains any control or
descriptive information, and a second part, of type
message/rfc822, is the forwarded or rejected message.
Composing rejection and forwarding messages in this manner
will preserve the type information on the original message and
allow it to be correctly presented to the recipient, and hence
is strongly encouraged.







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Subtypes of message often impose restrictions on what
encodings are allowed.  These restrictions are described in
conjunction with each specific subtype.

Mail gateways, relays, and other mail handling agents are
commonly known to alter the top-level header of an RFC 822
message.  In particular, they frequently add, remove, or
reorder header fields.  Such alterations are explicitly
forbidden for the encapsulated headers embedded in the bodies
of messages of type "message."


6.2.2.1.  RFC822 Subtype

A Content-Type of "message/rfc822" indicates that the body
contains an encapsulated message, with the syntax of an RFC
822 message.  However, unlike top-level RFC 822 messages, the
restriction that each message/rfc822 body must include a
"From", "Date", and at least one destination header is 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
permitted for parts of type "message/rfc822". The message
header fields are always US-ASCII in any case, and data within
the body can still be encoded, in which case the Content-
Transfer-Encoding header field in the encapsulated message
will reflect this.  Non-US-ASCII text in the headers of an
encapsulated message can be specified using the mechanisms
described in RFC MIME-HEADERS.

It should be noted that, despite the use of the numbers "822",
a message/rfc822 entity can include enhanced information as
defined in this document.  In other words, a 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 several separate pieces of mail and automatically
reassembled by the receiving user agent.  (The concept is
similar to IP fragmentation and reassembly in the basic
Internet Protocols.)  This mechanism can be used when
intermediate transport agents limit the size of individual





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messages that can be sent.  Content-Type "message/partial"
thus indicates that the body contains a fragment of a larger
message.

Three parameters must be specified in the Content-Type field
of type message/partial:  The first, "id", is a unique
identifier, as close to a world-unique identifier as possible,
to be used to match the parts together.  (In general, the
identifier is essentially a message-id; if placed in double
quotes, it can be ANY message-id, in accordance with the BNF
for "parameter" given earlier in this specification.)  The
second, "number", an integer, is the part number, which
indicates where this part fits into the sequence of fragments.
The third, "total", another integer, is the total number of
parts.  This third subfield is required on the final part, and
is optional (though encouraged) on the earlier parts.  Note
also that these parameters may be given in any order.

Thus, part 2 of a 3-part message may have either of the
following header fields:

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

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

But part 3 MUST specify the total number of parts:

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

Note that part numbering begins with 1, not 0.

When the parts of a message broken up in this manner are put
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.


6.2.2.2.1.  Message Fragmentation and Reassembly

The semantics of a reassembled partial message must be those
of the "inner" message, rather than of a message containing





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the inner message.  This makes it possible, for example, to
send a large audio message as several partial messages, and
still have it appear to the recipient as a simple audio
message rather than as an encapsulated message containing an
audio message.  That is, the encapsulation of the message is
considered to be "transparent".

When generating and reassembling the parts of a
message/partial message, the headers of the encapsulated
message must be merged with the headers of the enclosing
entities.  In this process the following rules must be
observed:

 (1)   All of the header fields from the initial enclosing
       entity (part one), except those that start with
       "Content-" and the specific header fields "Subject",
       "Message-ID", "Encrypted", and "MIME-Version", must be
       copied, in order, to the new message.

 (2)   Only those header fields in the enclosed message which
       start with "Content-" and "Subject", "Message-ID",
       "Encrypted", and "MIME-Version" must be appended, in
       order, to the header fields of the new message.  Any
       header fields in the enclosed message which do not
       start with "Content-" (except for "Message-ID",
       "Encrypted", and "MIME-Version") will be ignored.

 (3)   All of the header fields from the second and any
       subsequent messages will be ignored.


6.2.2.2.2.  Fragmentation and Reassembly Example

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

  X-Weird-Header-1: Foo
  From: Bill@host.com
  To: joe@otherhost.com
  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 the 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 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 fragmented message is reassembled, the
resulting message to be displayed to the user should look
something like this:

  X-Weird-Header-1: Foo
  From: Bill@host.com
  To: joe@otherhost.com
  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 never be encoded in base64
or quoted-printable, a problem might arise if message/partial
entities are constructed in an environment that supports
binary or 8-bit transport.  The problem is that the binary
data would be split into multiple message/partial messages,





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each of them requiring binary transport.  If such messages
were encountered at a gateway into a 7-bit transport
environment, there would be no way to properly encode them for
the 7-bit world, aside from waiting for all of the fragments,
reassembling the inner message, and 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 an acceptable solution.  For this
reason, it is specified that 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 use of a content-
transfer-encoding of "8bit" or "binary" is explicitly
prohibited for entities of type message/partial.

Because some message transfer agents may choose to
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 comprise a partial
message.  This is explicitly permitted.

The inclusion of a "References" field in the headers of the
second and subsequent pieces of a fragmented message that
references the Message-Id on the previous piece may be of
benefit to mail readers that understand and track references.
However, the generation of such "References" fields is
entirely optional.

Finally, it should be noted that the "Encrypted" header field
has been made obsolete by Privacy Enhanced Messaging (PEM)
[RFC1421, RFC1422, RFC1423, and RFC1424], but the rules above
are nevertheless believed to describe the correct way to treat
it if it is encountered in the context of conversion to and
from message/partial fragments.


6.2.2.3.  External-Body Subtype

The external-body subtype indicates that the actual body data
are not included, but merely referenced.  In this case, the
parameters describe a mechanism for accessing the external
data.







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When an entity is of type "message/external-body", it consists
of a header, two consecutive CRLFs, and the message header for
the encapsulated message.  If another pair of consecutive
CRLFs appears, this of course ends the message header for the
encapsulated message.  However, since the encapsulated
message's body is itself external, it does NOT appear in the
area that follows.  For example, consider the following
message:

  Content-type: message/external-body;
                access-type=local-file;
                name="/u/nsb/Me.gif"
  Content-type: image/gif
  Content-ID: <id42@guppylake.bellcore.com>
  Content-Transfer-Encoding: binary

  THIS IS NOT REALLY THE BODY!

The area at the end, which might be called the "phantom body",
is ignored for most external-body messages.  However, it may
be used to contain auxiliary information for some such
messages, as indeed it is when the access-type is "mail-
server".  The only access-type defined in this document that
uses the phantom body is "mail-server", but other access-types
may be defined in the future in other documents that use this
area.

The 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 data.  This identifier
may be used for caching mechanisms, and for recognizing the
receipt of the data when the access-type is "mail-server".

Note that, as specified here, the tokens that describe
external-body data, such as file names and mail server
commands, are required to be in the US-ASCII character set.
If this proves problematic in practice, a new mechanism may be
required as a future extension to MIME, either as newly
defined access-types for message/external-body or by some
other mechanism.

As with message/partial, MIME entities of type
message/external-body MUST have a content-transfer-encoding of
7-bit (the default).  In particular, even in environments that
support binary or 8-bit transport, the use of a content-





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transfer-encoding of "8bit" or "binary" is explicitly
prohibited for entities of type message/external-body.


6.2.2.3.1.  General External-Body Parameters

The parameters that may be used with any message/external-body
are:

 (1)   ACCESS-TYPE -- A word indicating the supported access
       mechanism by which the file or data may be obtained.
       This word is not 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 described in RFC REG. This
       parameter is unconditionally mandatory and MUST be
       present on EVERY message/external-body.

 (2)   EXPIRATION -- The date (in the RFC 822 "date-time"
       syntax, as extended by RFC 1123 to permit 4 digits in
       the year field) after which the existence of the
       external data is not guaranteed.  This parameter may be
       used with ANY access-type and is ALWAYS optional.

 (3)   SIZE -- The size (in octets) of the data.  The intent
       of this parameter is to help the recipient decide
       whether or not to expend the necessary resources to
       retrieve the external data.  Note that this describes
       the size of the data in its canonical form, that is,
       before any Content-Transfer-Encoding has been applied
       or after the data have been decoded.  This parameter
       may be used with ANY access-type and is ALWAYS
       optional.

 (4)   PERMISSION -- A case-insensitive field that indicates
       whether or not it is expected that clients might also
       attempt to overwrite the data.  By default, or if
       permission is "read", the assumption is that they are
       not, and that if the data is retrieved once, it is
       never needed again.  If PERMISSION is "read-write",
       this assumption is invalid, and any local copy must be
       considered no more than a cache.  "Read" and "Read-
       write" are the only defined values of permission.  This
       parameter may be used with ANY access-type and is





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       ALWAYS optional.

The precise semantics of the access-types defined here are
described in 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
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:

 (1)   NAME -- The name of the file that contains the actual
       body data.

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

 (3)   Before any data are retrieved, using FTP, the user will
       generally need to be asked to provide a login id and a
       password for the machine named by the site parameter.
       For security reasons, such an id and password are not
       specified as content-type parameters, but must be
       obtained from the user.

In addition, the following parameters are optional:

 (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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6.2.2.3.3.  The 'anon-ftp' Access-Type

The "anon-ftp" access-type is identical to the "ftp" access
type, except that the user need not be asked to provide a name
and password for the specified site.  Instead, the ftp
protocol will be used with login "anonymous" and a password
that corresponds to the user's email address.


6.2.2.3.4.  The 'local-file' Access-Type

An access-type of "local-file" indicates that the 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              Bob Braden
  Philippe Brandon              Brian Capouch
  Kevin Carosso                 Uhhyung Choi
  Peter Clitherow               Dave Collier-Brown
  Cristian Constantinof         John Coonrod
  Mark Crispin                  Dave Crocker
  Stephen Crocker               Terry Crowley
  Walt Daniels                  Jim Davis
  Frank Dawson                  Axel Deininger
  Hitoshi Doi                   Kevin Donnelly
  Steve Dorner                  Keith Edwards
  Chris Eich                    Dana S. Emery
  Johnny Eriksson               Craig Everhart
  Patrik Faltstrom              Erik E. Fair
  Roger Fajman                  Alain Fontaine
  Martin Forssen                James M. Galvin
  Stephen Gildea                Philip Gladstone
  Thomas Gordon                 Keld Simonsen
  Terry Gray                    Phill Gross
  James Hamilton                David Herron
  Mark Horton                   Bruce Howard
  Bill Janssen                  Olle Jarnefors
  Risto Kankkunen               Phil Karn
  Alan Katz                     Tim Kehres
  Neil Katin                    Steve Kille
  Kyuho Kim                     Anders Klemets
  John Klensin                  Valdis Kletniek
  Jim Knowles                   Stev Knowles
  Bob Kummerfeld                Pekka Kytolaakso
  Stellan Lagerstrom            Vincent Lau
  Timo Lehtinen                 Donald Lindsay
  Warner Losh                   Carlyn Lowery
  Laurence Lundblade            Charles Lynn
  John R. MacMillan             Larry Masinter
  Rick McGowan                  Michael J. McInerny





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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.





















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                Appendix A -- 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. 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 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 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."






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         -- Recognize other character sets at least to the
         extent of being able to inform the user about what
         character set the message uses.

         -- Recognize the "ISO-8859-*" character sets to the
         extent of being able to display those characters that
         are common to ISO-8859-* and US-ASCII, namely all
         characters represented by octet values 0-127.

         -- For unrecognized subtypes in a known character
         set, show or offer to show the user the "raw" version
         of the data after conversion of the content from
         canonical form to local form.

         -- Treat material in an unknown character set as if
         it were "application/octet-stream".

       Image, audio, and video:

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

       Application:

         -- Offer the ability to remove either of the quoted-
         printable or base64 encodings defined in this
         document if they were used and put the resulting
         information in a user file.

       Multipart:

         -- Recognize the mixed subtype.  Display all relevant
         information on the message level and the body part
         header level and then display or offer to display
         each of the body parts individually.

         -- Recognize the "alternative" subtype, and avoid
         showing the user redundant parts of
         multipart/alternative mail.

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





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

       Message:


         -- Recognize and display at least the primary
         (RFC822) encapsulation in such a way as to preserve
         any recursive structure, that is, displaying or
         offering to display the encapsulated data in
         accordance with its Content-type.

         -- Treat any unrecognized subtypes as if they were
         "application/octet-stream".

 (5)   Upon encountering any unrecognized Content-Type, an
       implementation must treat it as if it had a Content-
       Type of "application/octet-stream" with no parameter
       sub-arguments.  How such data are handled is up to an
       implementation, but likely options for handling such
       unrecognized data include offering the user to write it
       into a file (decoded from its mail transport format) or
       offering the user to name a program to which the
       decoded data should be passed as input.

A user agent that meets the above conditions is said to be
MIME-conformant.  The meaning of this phrase is that it is
assumed to be "safe" to send virtually any kind of properly-
marked data to users of such mail systems, because such
systems will at least be able to treat the data as
undifferentiated binary, and will not simply splash it onto
the screen of unsuspecting users.

There is another sense in which it is always "safe" to send
data in a format that is MIME-conformant, which is that such
data will not break or be broken by any known systems that are
conformant with RFC 821 and RFC 822.  User agents that are
MIME-conformant have the additional guarantee that the user
will not be shown data that were never intended to be viewed
as text.










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       Appendix B -- Guidelines For Sending Email Data




Internet email is not a perfect, homogeneous system.  Mail may
become corrupted at several stages in its travel to a final
destination.  Specifically, email sent throughout the Internet
may travel across many networking technologies.  Many
networking and mail technologies do not support the full
functionality possible in the SMTP transport environment.
Mail traversing these systems is likely to be modified in such
a way that it can be transported.

There exist many widely-deployed non-conformant MTAs in the
Internet. These MTAs, speaking the SMTP protocol, alter
messages on the fly to take advantage of the internal data
structure of the hosts they are implemented on, or are just
plain broken.

The following guidelines may be useful to anyone devising a
data format (Content-Type) that will survive the widest range
of networking technologies and known broken MTAs unscathed.
Note that anything encoded in the base64 encoding will satisfy
these rules, but that some well-known mechanisms, notably the
UNIX uuencode facility, will not.  Note also that anything
encoded in the Quoted-Printable encoding will survive most
gateways intact, but possibly not some gateways to systems
that use the EBCDIC character set.

 (1)   Under some circumstances the encoding used for data may
       change as part of normal gateway or user agent
       operation.  In particular, conversion from base64 to
       quoted-printable and vice versa may be necessary.  This
       may result in the confusion of CRLF sequences with line
       breaks in text bodies.  As such, the persistence of
       CRLF as something other than a line break must not be
       relied on.

 (2)   Many systems may elect to represent and store text data
       using local newline conventions.  Local newline
       conventions may not match the RFC822 CRLF convention --
       systems are known that use plain CR, plain LF, CRLF, or
       counted records.  The result is that isolated CR and LF
       characters are not well tolerated in general; they may





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       be lost or converted to delimiters on some systems, and
       hence must not be relied on.

 (3)   TAB (HT) characters may be misinterpreted or may be
       automatically converted to variable numbers of spaces.
       This is unavoidable in some environments, notably those
       not based on the US-ASCII character set.  Such
       conversion is STRONGLY DISCOURAGED, but it may occur,
       and mail formats must not rely on the persistence of
       TAB (HT) characters.

 (4)   Lines longer than 76 characters may be wrapped or
       truncated in some environments.  Line wrapping and line
       truncation are STRONGLY DISCOURAGED, but unavoidable in
       some cases. Applications which require long lines must
       somehow differentiate between soft and hard line
       breaks.  (A simple way to do this is to use the
       quoted-printable encoding.)

 (5)   Trailing "white space" characters (SPACE, TAB (HT)) on
       a line may be discarded by some transport agents, while
       other transport agents may pad lines with these
       characters so that all lines in a mail file are of
       equal length.  The persistence of trailing white space,
       therefore, must not be relied on.

 (6)   Many mail domains use variations on the US-ASCII
       character set, or use character sets such as EBCDIC
       which contain most but not all of the US-ASCII
       characters.  The correct translation of characters not
       in the "invariant" set cannot be depended on across
       character converting gateways.  For example, this
       situation is a problem when sending uuencoded
       information across BITNET, an EBCDIC system.  Similar
       problems can occur without crossing a gateway, since
       many Internet hosts use character sets other than US-
       ASCII internally.  The definition of Printable Strings
       in X.400 adds further restrictions in certain special
       cases.  In particular, the only characters that are
       known to be consistent across all gateways are the 73
       characters that correspond to the upper and lower case
       letters A-Z and a-z, the 10 digits 0-9, and the
       following eleven special characters:

         "'"  (US-ASCII decimal value 39)





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         "("  (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 relatively
       short lines of text in which the only meaningful
       characters are taken from this set of 73 characters.

 (7)   Some mail transport agents will corrupt data that
       includes certain literal strings.  In particular, a
       period (".") alone on a line is known to be corrupted
       by some (incorrect) SMTP implementations, and a line
       that starts with the five characters "From " (the fifth
       character is a SPACE) are commonly corrupted as well.
       A careful composition agent can prevent these
       corruptions by encoding the data (e.g., in the quoted-
       printable encoding, "=46rom " in place of "From " at
       the start of a line, and "=2E" in place of "." alone on
       a line.

Please note that the above list is NOT a list of recommended
practices for MTAs.  RFC 821 MTAs are prohibited from altering
the character of white space or wrapping long lines.  These
BAD and invalid practices are known to occur on established
networks, and implementations should be robust in dealing with
the bad effects they can cause.
















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          Appendix C -- A Complex Multipart Example




What follows is the outline of a complex multipart message.
This message has five parts to be displayed serially:  two
introductory plain text parts, an embedded multipart message,
a text/enriched part, and a closing encapsulated text message
in a non-ASCII character set.  The embedded multipart message
has two parts to be displayed in parallel, a picture and an
audio fragment.

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

  This is the preamble area of a multipart message.
  Mail readers that understand multipart format
  should ignore this preamble.

  If you are reading this text, you might want to
  consider changing to a mail reader that understands
  how to properly display multipart messages.

  --unique-boundary-1

    ... Some text appears here ...

  [Note that the blank between the boundary and the start
   of the text in this part means no header fields were
   given and this is text in the US-ASCII character set.
   It could have been done with explicit typing as in the
   next part.]

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

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





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  --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 <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--











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               Appendix D -- Collected Grammar




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

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

  attribute := token

  boundary := 0*69<bchars> bcharsnospace

  bchars := bcharsnospace / " "

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

  body-part := <"message" as defined in RFC 822, with all
                header fields optional, not starting with the
                specified dash-boundary, and with the
                delimiter not occurring anywhere in the
                message body.  Note that the semantics of 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 "/" subtype
             *(";" parameter)
             ; Matching of type and subtype is
             ; ALWAYS case-insensitive

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





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  delimiter := CRLF dash-boundary

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

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

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

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

  encoding := "Content-Transfer-Encoding" ":" mechanism

  epilogue := discard-text

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

  iana-token := <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 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 for characters > 127, =,
           ; SPACE, or TAB, and is recommended for any
           ; characters not listed in Appendix B as
           ; "mail-safe".






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  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 recommended.

  subtype := extension-token

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

  tspecials :=  "(" / ")" / "<" / ">" / "@" /
                "," / ";" / ":" / "\" / <">
                "/" / "[" / "]" / "?" / "="
                ; Must be in quoted-string,
                ; to 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 "x-" followed, with
              no  intervening white space, by any token>
















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       Appendix E -- Summary of the Seven Content-types



Content type:  text

  Subtypes defined by this document:  plain

  Important parameters:  charset

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

  Subtypes defined by this document:  jpeg, gif

  Important parameters: none

  Encoding notes: base64 generally preferred

Content type:  audio

  Subtypes defined by this document:  basic

  Important parameters:  none

  Encoding notes: base64 generally preferred

Content type:  video

  Subtypes defined by this document:  mpeg

  Important parameters:  none





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  Encoding notes:  base64 generally preferred

Content type:  application

  Subtypes defined by this document:  octet-stream, postscript

  Important parameters:  type, padding

  Deprecated parameters:  name and conversions were defined in
  RFC 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.  Severe security problems could result if this
  type is used to transmit binary programs or programs in
  general-purpose interpreted languages, such as LISP programs
  or shell scripts, 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:  multipart

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

  Important parameters:  boundary

  Encoding notes: No content-transfer-encoding other than
  "7bit", "8bit", or "binary" are permitted.

Content type:  message

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

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





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  Encoding notes:  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 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 PEM [RFC1421] and are performed
for each "innermost level" body:

 (1)   Creation of local form.

       The body to be transmitted is created in the system's
       native format.  The native character set is used, and
       where appropriate local end of line conventions are
       used as well.  The body may be a UNIX-style text file,
       or a Sun raster image, or a VMS indexed file, or audio
       data in a system-dependent format stored only in
       memory, or anything else that corresponds to the local
       model for the representation of some form of
       information.  Fundamentally, the data is created in the
       "native" form that corresponds to the type specified by
       the 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".

       For example, in the case of text/plain data, the text
       must be converted to a supported character set and
       lines must be delimited with CRLF delimiters in
       accordance with RFC 822.  Note that the restriction on
       line lengths implied by RFC 822 is eliminated if the
       next step employs either quoted-printable or base64
       encoding.

 (3)   Apply transfer encoding.

       A Content-Transfer-Encoding appropriate for this body
       is applied.  Note that there is no fixed relationship
       between the 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.

 (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 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 RFC 822 CRLF
       delimiters.

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

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

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
































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             Appendix G -- Changes from RFC 1521




This document is a 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, "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,
       an additional paragraph has been added warning 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 for
       Content-Type to make it clear that the following two
       forms:

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

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

       are completely equivalent.

 (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".

 (13)  The definition of a character set has been reorganized
       to make the 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", "8-
       bit", and "binary" transfer-encodings on multipart or
       message entities encapsulating "8bit" or "binary" data.





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 (17)  In Appendix A, "multipart/digest" support was added to
       the list of requirements for minimal MIME conformance.
       Also, the requirement for "message/rfc822" support were
       strengthened to clarify the importance of recognizing
       recursive structure.

 (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 subtype has been changed to make
       it clear that either an IANA registered subtype or a
       nonstandard "X-" subtype must be used in a Content-Type
       header field.

 (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




[ATK]
     Borenstein, Nathaniel S., Multimedia Applications
     Development with the Andrew Toolkit, Prentice-Hall, 1990.

[GIF]
     Graphics Interchange Format (Version 89a), Compuserve,
     Inc., Columbus, Ohio, 1990.

[ISO-2022]
     International Standard -- Information Processing -- ISO
     7-bit and 8-bit Coded Character Sets -- Code Extension
     Techniques, ISO 2022:1986.

[ISO-8859]
     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 -- ISO
     7-bit Coded Character Set For Information Interchange,
     ISO 646:1983.

[MPEG]
     Video Coding Draft Standard ISO 11172 CD, ISO
     IEC/TJC1/SC2/WG11 (Motion Picture Experts Group), May,
     1991.

[PCM]
     CCITT, Fascicle III.4 - Recommendation G.711, "Pulse Code
     Modulation (PCM) of Voice Frequencies", Geneva, 1972.







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[POSTSCRIPT]
     Adobe Systems, Inc., PostScript Language Reference
     Manual, Addison-Wesley, 1985.

[POSTSCRIPT2]
     Adobe Systems, Inc., PostScript Language Reference
     Manual, Addison-Wesley, Second Edition, 1990.

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

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

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

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

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

[RFC-1049]
     Sirbu, M., "Content-Type Header Field for Internet
     Messages", STD 11, RFC 1049, CMU, March 1988.

[RFC-1154]
     Robinson, D. and R. Ullmann, "Encoding Header Field for
     Internet Messages", RFC 1154, Prime Computer, Inc., April
     1990.

[RFC-1341]
     Borenstein, N., and N.  Freed, "MIME (Multipurpose
     Internet Mail Extensions): Mechanisms for Specifying and
     Describing the Format of Internet Message Bodies", RFC
     1341, Bellcore, Innosoft, June 1992.






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[RFC-1342]
     Moore, K., "Representation of Non-Ascii Text in Internet
     Message Headers", RFC 1342, University of Tennessee, June
     1992.

[RFC-1344]
     Borenstein, N., "Implications of MIME for Internet Mail
     Gateways", RFC 1344, Bellcore, June 1992.

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

[RFC-1421]
     Linn, J., "Privacy Enhancement for Internet Electronic
     Mail:  Part I -- Message Encryption and Authentication
     Procedures", RFC 1421, IAB IRTF PSRG, IETF PEM WG,
     February 1993.

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

[RFC-1423]
     Balenson, D., "Privacy Enhancement for Internet
     Electronic Mail:  Part III -- Algorithms, Modes, and
     Identifiers",  IAB IRTF PSRG, IETF PEM WG, February 1993.

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

[RFC-1521]
     Borenstein, N., and 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-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 1700, USC/Information Sciences Institute, October
     1994.

[RFC-MIME-HEADERS]
     Moore, K., "Representation of Non-Ascii Text in Internet
     Message Headers", RFC MIME-HEADERS, University of
     Tennessee, ?.

[RFC-REG]
     Postel, J., "Media Type Registration Procedure", RFC 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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