Network Working Group                     Nathaniel Borenstein
Internet Draft                                       Ned Freed

            Multipurpose Internet Mail Extensions
                       (MIME) Part One:

              Format of Internet Message Bodies

                          March 1996

                     Status of this Memo

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1.  Abstract

STD 11, RFC 822, defines a message representation protocol
specifying considerable detail about US-ASCII message headers,
and leaves the message content, or message body, as flat US-
ASCII text.  This set of documents, collectively called the
Multipurpose Internet Mail Extensions, or MIME, redefines the
format of messages to allow for

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 (1)   textual message bodies in character sets other than

 (2)   an extensible set of different formats for non-textual
       message bodies,

 (3)   multi-part message bodies, and

 (4)   textual header information in character sets other than

These documents are 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, these
documents are largely orthogonal to (rather than a revision
of) RFC 822.

This initial document specifies the various headers used to
describe the structure of MIME messages. The second document,
RFC MIME-IMT, defines the general structure of the MIME media
typing system and defines an initial set of media types. The
third document, RFC MIME-HEADERS, describes extensions to RFC
822 to allow non-US-ASCII text data in Internet mail header
fields. The fourth document, RFC MIME-REG, specifies various
IANA registration procedures for MIME-related facilities. The
fifth and final document, RFC MIME-CONF, describes MIME
conformance criteria as well as providing some illustrative
examples of MIME message formats, acknowledgements, and the

These documents are revisions of RFCs 1521, 1522, and 1590,
which themselves were revisions of RFCs 1341 and 1342.  An
appendix in RFC MIME-CONF describes differences and changes
from previous versions.

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2.  Table of Contents

1 Abstract ..............................................    1
2 Table of Contents .....................................    3
3 Introduction ..........................................    4
4 Definitions, Conventions, and Generic BNF Grammar .....    6
4.1 CRLF ................................................    7
4.2 Character Set .......................................    7
4.3 Message .............................................    8
4.4 Entity ..............................................    8
4.5 Body Part ...........................................    8
4.6 Body ................................................    8
4.7 7bit Data ...........................................    9
4.8 8bit Data ...........................................    9
4.9 Binary Data .........................................    9
4.10 Lines ..............................................    9
5 MIME Header Fields ....................................    9
6 MIME-Version Header Field .............................   10
7 Content-Type Header Field .............................   12
7.1 Syntax of the Content-Type Header Field .............   14
7.2 Content-Type Defaults ...............................   16
8 Content-Transfer-Encoding Header Field ................   17
8.1 Content-Transfer-Encoding Syntax ....................   17
8.2 Content-Transfer-Encodings Semantics ................   17
8.3 New Content-Transfer-Encodings ......................   19
8.4 Interpretation and Use ..............................   19
8.5 Translating Encodings ...............................   21
8.6 Canonical Encoding Model ............................   22
8.7 Quoted-Printable Content-Transfer-Encoding ..........   22
8.8 Base64 Content-Transfer-Encoding ....................   26
9 Content-ID Header Field ...............................   29
10 Content-Description Header Field .....................   30
11 Additional MIME Header Fields ........................   30
12 Summary ..............................................   30
13 Security Considerations ..............................   31
14 Authors' Addresses ...................................   32
A Collected Grammar .....................................   33

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3.  Introduction

Since its publication in 1982, 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.
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 (e.g. 1000 characters
or less [RFC821]) of 7bit 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

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 material
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 material 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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material, the user might have some mechanism external to the
UA that can extract useful information from the material.
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,
       which can be used to specify the media type and subtype
       of data in the body of a message and to fully specify
       the native representation (canonical form) of such

 (3)   A Content-Transfer-Encoding header field, which can be
       used to specify both the encoding transformation that
       was applied to the body and the domain of the result.
       Encoding transformations other than the identity
       transformation are usually applied to 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 the 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 except for
Content-Disposition can include RFC 822 comments, which have
no semantic content and should be ignored during MIME

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Finally, to specify and promote interoperability, RFC MIME-
CONF provides a basic applicability statement for a subset of
the above mechanisms that defines a minimal level of
"conformance" with this document.

HISTORICAL NOTE:  Several of the mechanisms described in this
set of documents 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 set of documents.  In particular,
compatibility was always favored over elegance.

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 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 1345, and RFC

4.  Definitions, Conventions, and Generic BNF Grammar

Although the mechanisms specified in this set of documents 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 set of documents makes named
references to syntax rules defined in RFC 822.  A complete
formal grammar, then, is obtained by combining the collected
grammar appendices in each document in this set 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').

All numeric and octet values are given in decimal notation in
this set of documents. All media type values, subtype values,
and parameter names as defined are case-insensitive.  However,
parameter values are case-sensitive unless otherwise specified
for the specific parameter.

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FORMATTING NOTE:  Notes, such at this one, provide additional
nonessential information which may be skipped by the reader
without missing anything essential.  The primary purpose of
these non-essential notes is to convey information about the
rationale of this set of documents, or to place these
documents 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.

4.1.  CRLF

The term CRLF, in this set of documents, refers to the
sequence of octets corresponding to 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.

4.2.  Character Set

The term "character set" is used in MIME to refer to a method
of converting a sequence of octets into a sequence of
characters.  Note that unconditional and unambiguous
conversion in the other direction is not required, in that not
all characters may be representable by a given character set
and a character set may provide more than one sequence of
octets to represent a particular sequence of characters.

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, to be used as character sets.
However, the definition associated with a MIME character set
name must fully specify the mapping to be performed.  In
particular, use of external profiling information to determine
the exact mapping is not permitted.

NOTE: The term "character set" was originally used in MIME
with specifications such as US-ASCII and other 7bit and 8bit
schemes which have a simple mapping from single octets to
single characters. Multi-octet coded character sets and
switching techniques make the situation more complex. For

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example, some communities use the term "character encoding"
for what MIME calls a "character set", while using the phrase
"coded character set" to denote an abstract mapping from
integers (not octets) to characters.

4.3.  Message

The term "message", when not further qualified, means either a
(complete or "top-level") RFC 822 message being transferred on
a network, or a message encapsulated in a body of type
"message/rfc822" or "message/partial".

4.4.  Entity

The term "entity", refers specifically to the MIME-defined
header fields and contents of either a message or one of the
parts in the body of a multipart entity.  The specification of
such entities is the essence of MIME.  Since the contents of
an entity are often called the "body", it makes sense to speak
about the body of an entity.  Any sort of field may be present
in the header of an entity, but only those fields whose names
begin with "content-" actually have any MIME-related meaning.
Note that this does NOT imply thay they have no meaning at all
-- an entity that is also a message has non-MIME header fields
whose meanings are defined by RFC 822.

4.5.  Body Part

The term "body part" refers to an entity inside of a multipart

4.6.  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

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

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4.7.  7bit Data

"7bit data" refers to data that is all represented as
relatively short lines with 998 octets or less between CRLF
line separation sequences [RFC821].  No octets with decimal
values greater than 127 are allowed and neither are NULs
(octets with decimal value 0).  CR (decimal value 13) and LF
(decimal value 10) octets only occur as part of CRLF line
separation sequences.

4.8.  8bit Data

"8bit data" refers to data that is all represented as
relatively short lines with 998 octets or less between CRLF
line separation sequences [RFC821]), but octets with decimal
values greater than 127 may be used.  As with "7bit data" CR
and LF octets only occur as part of CRLF line separation
sequences and no NULs are allowed.

4.9.  Binary Data

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

4.10.  Lines

"Lines" are defined as sequences of octets separated by a CRLF
sequences.  This is consistent with both RFC 821 and RFC 822.
"Lines" only refers to a unit of data in a message, which may
or may not correspond to something that is actually displayed
by a user agent.

5.  MIME Header Fields

MIME defines a number of new RFC 822 header fields that are
used to describe the content of a MIME entity.  These header
fields occur in at least two contexts:

 (1)   As part of a regular RFC 822 message header.

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

The formal definition of these header fields is as follows:

  entity-headers := [ content CRLF ]
                    [ encoding CRLF ]
                    [ id CRLF ]
                    [ description CRLF ]
                    *( MIME-extension-field CRLF )

  MIME-message-headers := entity-headers
                          version CRLF
                          ; The ordering of the header
                          ; fields implied by this BNF
                          ; definition should be ignored.

  MIME-part-headers := entity-headers
                       [ fields ]
                       ; Any field not beginning with
                       ; "content-" can have no defined
                       ; meaning and may be ignored.
                       ; 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.

6.  MIME-Version Header Field

Since RFC 822 was published in 1982, there has really been
only one format standard for Internet messages, and there has
been little perceived need to declare the format standard in
use.  This document is an independent document that
complements RFC 822.  Although the extensions in this document
have been defined in such a way as to be compatible with RFC
822, there are still circumstances in which it might be
desirable for a mail-processing agent to know whether a
message was composed with the new standard in mind.

Therefore, this document defines a new header field, "MIME-
Version", which is to be used to declare the version of the

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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

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/rfc822" or "message/partial" if and
only if the embedded message is itself claimed to be MIME-

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
media 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 media format.  Where such conventions
exist, MIME does nothing to supersede them.  Where no such
conventions exist, a MIME media 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

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particular, the following four MIME-Version fields are

  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

In the absence of a MIME-Version field, a receiving mail user
agent (whether conforming to MIME requirements or not) may
optionally choose to interpret the body of the message
according to local conventions.  Many such conventions are
currently in use and it should be noted that in practice non-
MIME messages can contain just about anything.

It is impossible to be certain that a non-MIME mail message is
actually plain text in the US-ASCII character set since it
might well be a message that, using some set of nonstandard
local conventions that predate this document, includes text in
another character set or non-textual data presented in a
manner that cannot be automatically recognized (e.g., a
uuencoded compressed UNIX tar file).

7.  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. The value in this field is called a media

HISTORICAL NOTE:  The Content-Type header field was first
defined in RFC 1049.  RFC 1049 used a simpler and less
powerful syntax, but one that is largely compatible with the
mechanism given here.

The Content-Type header field specifies the nature of the data
in the body of an entity by giving media type and subtype
identifiers, and by providing auxiliary information that may
be required for certain media types.  After the media type and

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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 media type is used to declare the
general type of data, while the subtype specifies a specific
format for that type of data.  Thus, a media type of
"image/xyz" is enough to tell a user agent that the data is an
image, even if the user agent has no knowledge of the specific
image format "xyz".  Such information can be used, for
example, to decide whether or not to show a user the raw data
from an unrecognized subtype -- such an action might be
reasonable for unrecognized subtypes of text, but not for
unrecognized subtypes of image or audio.  For this reason,
registered subtypes of 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 media subtype, and as such do
not fundamentally affect the nature of the content.  The set
of meaningful parameters depends on the media type and
subtype.  Most parameters are associated with a single
specific subtype.  However, a given top-level media type may
define parameters which are applicable to any subtype of that
type.  Parameters may be required by their defining content
type or subtype or they may be optional. MIME implementations
must ignore any parameters whose names they do not recognize.

For example, the "charset" parameter is applicable to any
subtype of "text", while the "boundary" parameter is required
for any subtype of the "multipart" media type.

There are NO globally-meaningful parameters that apply to all
media 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 media types is defined in
MIME-IMT.  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.

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This set of top-level media 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.

7.1.  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:

  content := "Content-Type" ":" type "/" subtype
             *(";" parameter)
             ; Matching of media 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 := ietf-token / x-token

  ietf-token := <An extension token defined by a
                 standards-track RFC and registered
                 with IANA.>

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

  subtype := extension-token / iana-token

  iana-token := <A publicly-defined extension token. Tokens
                 of this form must be registered with IANA
                 as specified in RFC MIME-REG.>

  parameter := attribute "=" value

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  attribute := token
               ; Matching of attributes
               ; is ALWAYS case-insensitive.

  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 ".".

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
media 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

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

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

are completely equivalent.

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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
media subtypes, then, is not intended to be a mechanism for
imposing restrictions, but simply a mechanism for publicizing
their definition and usage.  There are, therefore, two
acceptable mechanisms for defining new media subtypes:

 (1)   Private values (starting with "X-") may be defined
       bilaterally between two cooperating agents without
       outside registration or standardization. Such values
       cannot be registered or standardized.

 (2)   New standard values should be registered with IANA as
       described in RFC MIME-REG.

The second document in this set, RFC MIME-IMT, defines the
initial set of media types for MIME.

7.2.  Content-Type Defaults

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 header field is
specified.  It is also recommend that this default be assumed
when a syntactically invalid Content-Type header field is
encountered. In the presence of a MIME-Version header field
and the absence of any Content-Type header field, a receiving
User Agent can also assume that plain US-ASCII text was the
sender's intent.  Plain US-ASCII text may still be assumed in
the absence of a MIME-Version or the presence of an
syntactically invalid Content-Type header field, but the
sender's intent might have been otherwise.

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8.  Content-Transfer-Encoding Header Field

Many media types which could be usefully transported via email
are represented, in their "natural" format, as 8bit character
or binary data.  Such data cannot be transmitted over some
transfer protocols.  For example, RFC 821 (SMTP) restricts
mail messages to 7bit US-ASCII data with lines no longer than
1000 characters including any trailing CRLF line separator.

It is necessary, therefore, to define a standard mechanism for
encoding such data into a 7bit 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

8.1.  Content-Transfer-Encoding Syntax

The Content-Transfer-Encoding field's value is a single token
specifying the type of encoding, as enumerated below.

  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 7bit 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.

8.2.  Content-Transfer-Encodings Semantics

This single Content-Transfer-Encoding 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.

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

The Content-Transfer-Encoding 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 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"
range, 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 8bit characters as
"7bit" is not allowed, nor is labelling unencoded non-line-
oriented data as anything other than "binary" allowed.

Unlike media 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, 7bit.  For this reason, at
least two encoding mechanisms are necessary: a "readable"
encoding (quoted-printable) and a "dense" encoding (base64).

Mail transport for unencoded 8bit data is defined in RFC 1652.
As of the initial publication of this document, there are no
standardized Internet mail transports for which it is
legitimate to include unencoded binary data in mail bodies.
Thus there are no circumstances in which the "binary"
Content-Transfer-Encoding is actually valid in Internet mail.
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.

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

8.3.  New Content-Transfer-Encodings

Implementors may, if necessary, define private 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".
Additional standardized Content-Transfer-Encoding values must
be specified by a standards-track RFC.  Additional
requirements such specifications must meet are given in RFC
REG.  As such, all content-transfer-encoding namespace except
that beginning with "X-" is explicitly reserved to the IETF
for future use.

Unlike media types and subtypes, the creation of new Content-
Transfer-Encoding values is STRONGLY discouraged, as it seems
likely to hinder interoperability with little potential

8.4.  Interpretation and Use

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 an entity's headers, it applies only to the body of
that entity.  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 most media types are defined in terms
of octets rather than bits, 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 8bit 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 8bit byte.  A bit stream not ending at an 8bit

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boundary must be padded with zeroes. RFC MIME-IMT provides a
mechanism for noting the addition of such padding in the case
of the application/octet-stream media type, which has a
"padding" parameter.

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.

Certain Content-Transfer-Encoding values may only be used on
certain media types.  In particular, it is EXPRESSLY FORBIDDEN
to use any encodings other than "7bit", "8bit", or "binary"
with any composite media type, i.e. one that recursively
includes other Content-Type fields.  Currently the only
composite media 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 entities has a less restrictive value such as
"8bit", then either the outer "7bit" labelling is in error,
because 8bit data are included, or the inner "8bit" labelling
placed an unnecessarily high demand on the transport system
because the actual included data were actually 7bit-safe.

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
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.

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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.

Any entity with an unrecognized Content-Transfer-Encoding must
be treated as if it has a Content-Type of "application/octet-
stream", regardless of what the Content-Type header field
actually says.

TRANSFER-ENCODING: It may seem that the Content-Transfer-
Encoding could be inferred from the characteristics of the
media that is to be encoded, or, at the very least, that
certain Content-Transfer-Encodings could be mandated for use
with specific media 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
combinations of media types and transports but not for others.
(For example, in an 8bit transport, no encoding would be
required for text in certain character sets, while such
encodings are clearly required for 7bit SMTP.)

Second, certain media types may require different types of
transfer encoding under different circumstances.  For example,
many PostScript bodies might consist entirely of short lines
of 7bit 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 the
Content-Type field is intended to be an open-ended
specification mechanism, strict specification of an
association between media 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 media type should not have to be aware of
all the transports in use and what their limitations are.

8.5.  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 hard line
breaks in quoted-printable encoding output. When converting
from quoted-printable to base64 a hard line break must be

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converted into a CRLF sequence.  Similarly, a CRLF sequence in
base64 data must be converted to a quoted-printable hard line
break, but ONLY when converting text data.

8.6.  Canonical Encoding Model

There was some confusion, in the previous versions of this
RFC, 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 in RFC MIME-CONF for this reason.

8.7.  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 8bit representation) Any octet, except a CR or
       LF that is part of a CRLF line break 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)

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

 (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 portion 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 media 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.

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       Note that many implementations may elect to encode the
       local representation of various content types directly
       rather than converting to canonical form first,
       encoding, and then converting back to local
       representation.  In particular, this may apply to plain
       text material on systems that use newline conventions
       other than a CRLF terminator sequence.  Such an
       implementation optimization is permissible, but only
       when the combined canonicalization-encoding step is
       equivalent to performing the three steps separately.

 (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 ("-") may be represented as itself
in the Quoted-Printable encoding, care must be taken, when
encapsulating a quoted-printable encoded body inside one or
more multipart entities, to ensure that the boundary delimiter
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 in

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


according to rule #1.

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 := qp-line *(CRLF qp-line)

  qp-line := *(qp-segment transport-padding CRLF)
             qp-part transport-padding

  qp-part := qp-section
             ; Maximum length of 76 characters

  qp-segment := qp-section *(SPACE / TAB) "="
                ; Maximum length of 76 characters

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  qp-section := [*(ptext / SPACE / TAB) ptext]

  ptext := hex-octet / safe-char

  safe-char := <any octet with decimal value of 33 through
               60 inclusive, and 62 through 126>
               ; Characters not listed as "mail-safe" in
               ; RFC MIME-CONF are also not recommended.

  hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
               ; Octet must be used for characters > 127, =,
               ; SPACEs or TABs at the ends of lines, and is
               ; recommended for any character not listed in
               ; RFC MIME-CONF as "mail-safe".

  transport-padding := *LWSP-char
                       ; Composers MUST NOT generate
                       ; non-zero length transport
                       ; padding, but receivers MUST
                       ; be able to handle padding
                       ; added by message transports.

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

8.8.  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.

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

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

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represented identically in all versions of EBCDIC. Other
popular encodings, such as the encoding used by the uuencode
utility, Macintosh binhex 4.0 [RFC-1741], 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
8bit 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 8bit byte, and the eighth bit will be the
low-order bit in the first 8bit byte, and so on.

Each 6-bit group is used as an index into an array of 64
printable characters.  The character referenced by the index
is placed in the output string.  These characters, identified
in Table 1, below, are selected so as to be universally
representable, and the set excludes characters with particular
significance to SMTP (e.g., ".", CR, LF) and to the multipart
boundary delimiters defined in MIME-IMT (e.g., "-").

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                 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

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
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

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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 and no
"=" characters are present.

Any characters outside of the base64 alphabet are to be
ignored in base64-encoded data.

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 potential
boundary delimiters within base64-encoded bodies within
multipart entities because no hyphen characters are used in
the base64 encoding.

9.  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

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

Like the Message-ID values, Content-ID values must be
generated to be world-unique.

The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for 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

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optional MIME media 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 media type.
This is explained in the section of MIME-IMT dealing with

10.  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 may be used for non-US-ASCII Content-Description

11.  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

12.  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 with RFC 822

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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

The next document in this set, RFC MIME-IMT, specifies the
initial set of media types that can be labelled and
transported using these headers.

13.  Security Considerations

Security issues are discussed in the second document in this

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14.  Authors' Addresses

For more information, the authors of this document are best
contacted via Internet mail:

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

Phone: +1 201 540 8967
Fax:   +1 201 993 3032

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

Phone: +1 818 919 3600
Fax:   +1 818 919 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
Octel Network Services
17080 Dallas Parkway
Dallas, TX 75248-1905

Email: Greg.Vaudreuil@Octel.Com

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               Appendix A -- 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 by
name to several syntax rules 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
               ; Matching of attributes
               ; is ALWAYS case-insensitive.

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

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

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

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

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

  entity-headers := [ content CRLF ]
                    [ encoding CRLF ]
                    [ id CRLF ]
                    [ description CRLF ]
                    *( MIME-extension-field CRLF )

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

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  hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
               ; Octet must be used for characters > 127, =,
               ; SPACEs or TABs at the ends of lines, and is
               ; recommended for any character not listed in
               ; RFC MIME-CONF as "mail-safe".

  iana-token := <A publicly-defined extension token. Tokens
                 of this form must be registered with IANA
                 as specified in RFC MIME-REG.>

  ietf-token := <An extension token defined by a
                 standards-track RFC and registered
                 with IANA.>

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

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

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

  MIME-message-headers := entity-headers
                          version CRLF
                          ; The ordering of the header
                          ; fields implied by this BNF
                          ; definition should be ignored.

  MIME-part-headers := entity-headers
                       ; Any field not beginning with
                       ; "content-" can have no defined
                       ; meaning and may be ignored.
                       ; The ordering of the header
                       ; fields implied by this BNF
                       ; definition should be ignored.

  parameter := attribute "=" value

  ptext := hex-octet / safe-char

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  qp-line := *(qp-segment transport-padding CRLF)
             qp-part transport-padding

  qp-part := qp-section
             ; Maximum length of 76 characters

  qp-section := [*(ptext / SPACE / TAB) ptext]

  qp-segment := qp-section *(SPACE / TAB) "="
                ; Maximum length of 76 characters

  quoted-printable := qp-line *(CRLF qp-line)

  safe-char := <any octet with decimal value of 33 through
               60 inclusive, and 62 through 126>
               ; Characters not listed as "mail-safe" in
               ; RFC MIME-CONF are also not recommended.

  subtype := extension-token / iana-token

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

  transport-padding := *LWSP-char
                       ; Composers MUST NOT generate
                       ; non-zero length transport
                       ; padding, but receivers MUST
                       ; be able to handle padding
                       ; added by message transports.

  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>

                    Expires September 1996           [Page 35]