< draft-ietf-822ext-mime-imb-01.txt   draft-ietf-822ext-mime-imb-02.txt >
Network Working Group Nathaniel Borenstein Network Working Group Nathaniel Borenstein
Internet Draft Ned Freed Internet Draft Ned Freed
<draft-ietf-822ext-mime-imb-01.txt> <draft-ietf-822ext-mime-imb-02.txt>
Multipurpose Internet Mail Extensions Multipurpose Internet Mail Extensions
(MIME) Part One: (MIME) Part One:
Format of Internet Message Bodies Format of Internet Message Bodies
November 21, 1994 April 11, 1995
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet- groups may also distribute working documents as Internet-
Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six Internet-Drafts are draft documents valid for a maximum of six
skipping to change at page 2, line ? skipping to change at page 2, line ?
than as a "working draft" or "work in progress". than as a "working draft" or "work in progress".
To learn the current status of any Internet-Draft, please To learn the current status of any Internet-Draft, please
check the 1id-abstracts.txt listing contained in the check the 1id-abstracts.txt listing contained in the
Internet-Drafts Shadow Directories on ds.internic.net (US East Internet-Drafts Shadow Directories on ds.internic.net (US East
Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast),
or munnari.oz.au (Pacific Rim). or munnari.oz.au (Pacific Rim).
1. Abstract 1. Abstract
STD 11, RFC 822 defines a message representation protocol STD 11, RFC 822, defines a message representation protocol
specifying considerable detail about message headers, but specifying considerable detail about US-ASCII message headers,
which leaves the message content, or message body, as flat and leaves the message content, or message body, as flat US-
US-ASCII text. This document redefines the format of message ASCII text. This set of documents, collectively called the
bodies to allow multi-part textual and non-textual message Multipurpose Internet Mail Extensions, or MIME, redefines the
bodies to be represented and exchanged without loss of format of messages to allow for
information. This is based on earlier work documented in RFC (1) textual message bodies in character sets other than
US-ASCII,
(2) non-textual message bodies,
(3) multi-part message bodies, and
(4) textual header information in character sets other than
US-ASCII.
These documents are based on earlier work documented in RFC
934, STD 11, and RFC 1049, but extends and revises them. 934, STD 11, and RFC 1049, but extends and revises them.
Because RFC 822 said so little about message bodies, this Because RFC 822 said so little about message bodies, these
document is largely orthogonal to (rather than a revision of) documents are largely orthogonal to (rather than a revision
RFC 822. of) RFC 822.
In particular, this document is designed to provide facilities In particular, these documents are designed to provide
to include multiple parts in a single message, to represent facilities to include multiple parts in a single message, to
body text in character sets other than US-ASCII, to represent represent body and header text in character sets other than
formatted multi-font text messages, to represent non-textual US-ASCII, to represent formatted multi-font text messages, to
material such as images and audio fragments, and generally to represent non-textual material such as images and audio
facilitate later extensions defining new types of Internet fragments, and generally to facilitate later extensions
mail for use by cooperating mail agents. defining new types of Internet mail for use by cooperating
mail agents.
This document does NOT extend Internet mail header fields to This initial document specifies the various headers used to
permit anything other than US-ASCII text data. Such describe the structure of MIME messages. The second document,
extensions are the subject of [RFC-MIME-HEADERS]. 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 entities. The
fifth and final document, RFC MIME-CONF, describes MIME
conformance conformance criteria as well as providing some
illustrative examples of MIME message formats,
acknowledgements, and the bibliography.
This document is a revision of RFC 1521, which was a revision These documents are revisions of RFCs 1521, 1522, and 1590,
of RFC 1341. Significant differences from RFC 1521 are which themselves were revisions of RFCs 1341 and 1342. An
summarized in Appendix G. appendix in RFC MIME-CONF describes differences and changes
from previous versions.
2. Table of Contents 2. Table of Contents
1 Abstract .............................................. 2 1 Abstract .............................................. 1
2 Table of Contents ..................................... 3 2 Table of Contents ..................................... 3
3 Introduction .......................................... 5 3 Introduction .......................................... 4
4 Notations, Conventions, and Generic BNF Grammar ....... 9 4 Notations, Conventions, and Generic BNF Grammar ....... 6
5 MIME Header Fields .................................... 12 4.1 CRLF ................................................ 7
5.1 MIME-Version Header Field ........................... 12 4.2 Character Set ....................................... 7
5.2 Content-Type Header Field ........................... 14 4.3 Message ............................................. 8
5.2.1 Syntax of the Content-Type Header Field ........... 15 4.4 Body Part ........................................... 8
5.2.2 Definition of a Top-Level Content-Type ............ 18 4.5 Entity .............................................. 8
5.2.3 Initial Set of Top-Level Content-Types ............ 18 4.6 Body ................................................ 8
5.3 Content-Transfer-Encoding Header Field .............. 21 4.7 7bit Data ........................................... 8
5.3.1 Content-Transfer-Encoding Syntax .................. 21 4.8 8bit Data ........................................... 9
5.3.2 Content-Transfer-Encoding Semantics ............... 22 4.9 Binary Data ......................................... 9
5.3.3 Quoted-Printable Content-Transfer-Encoding ........ 26 4.10 Lines .............................................. 9
5.3.4 Base64 Content-Transfer-Encoding .................. 30 5 MIME Header Fields .................................... 9
5.4 Content-ID Header Field ............................. 32 6 MIME-Version Header Field ............................. 10
5.5 Content-Description Header Field .................... 33 7 Content-Type Header Field ............................. 12
5.6 Additional MIME Header Fields ....................... 33 7.1 Syntax of the Content-Type Header Field ............. 14
6 Predefined Content-Type Values ........................ 34 7.2 Content-Type Defaults ............................... 16
6.1 Discrete Content-Type Values ........................ 34 8 Content-Transfer-Encoding Header Field ................ 17
6.1.1 Text Content-Type ................................. 34 8.1 Content-Transfer-Encoding Syntax .................... 17
6.1.1.1 Representation of Line Breaks ................... 35 8.2 Content-Transfer-Encodings Sematics ................. 17
6.1.1.2 Charset Parameter ............................... 35 8.3 New Content-Transfer-Encodings ...................... 19
6.1.1.3 Plain Subtype ................................... 38 8.4 Interpretation and Use .............................. 19
6.1.1.4 Unrecognized Subtypes ........................... 38 8.5 Translating Encodings ............................... 21
6.1.2 Image Content-Type ................................ 39 8.6 Canonical Encoding Model ............................ 22
6.1.3 Audio Content-Type ................................ 39 8.7 Quoted-Printable Content-Transfer-Encoding .......... 22
6.1.4 Video Content-Type ................................ 40 8.8 Base64 Content-Transfer-Encoding .................... 26
6.1.5 Application Content-Type .......................... 40 9 Content-ID Header Field ............................... 29
6.1.5.1 Octet-Stream Subtype ............................ 41 10 Content-Description Header Field ..................... 29
6.1.5.2 PostScript Subtype .............................. 42 11 Additional MIME Header Fields ........................ 30
6.1.5.3 Other Application Subtypes ...................... 45 12 Summary .............................................. 30
6.2 Composite Content-Type Values ....................... 46 13 Security Considerations .............................. 30
6.2.1 Multipart Content-Type ............................ 46 14 Authors' Addresses ................................... 31
6.2.1.1 Common Syntax ................................... 48 A Collected Grammar ..................................... 32
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
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
3. Introduction 3. Introduction
Since its publication in 1982, RFC 822 [RFC-822] has defined Since its publication in 1982, RFC 822 [RFC-822] has defined
the standard format of textual mail messages on the Internet. the standard format of textual mail messages on the Internet.
Its success has been such that the RFC 822 format has been Its success has been such that the RFC 822 format has been
adopted, wholly or partially, well beyond the confines of the adopted, wholly or partially, well beyond the confines of the
Internet and the Internet SMTP transport defined by RFC 821 Internet and the Internet SMTP transport defined by RFC 821
[RFC-821]. As the format has seen wider use, a number of [RFC-821]. As the format has seen wider use, a number of
limitations have proven increasingly restrictive for the user limitations have proven increasingly restrictive for the user
community. community.
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might include audio or images, are simply not mentioned. Even might include audio or images, are simply not mentioned. Even
in the case of text, however, RFC 822 is inadequate for the in the case of text, however, RFC 822 is inadequate for the
needs of mail users whose languages require the use of needs of mail users whose languages require the use of
character sets richer than US-ASCII. Since RFC 822 does not character sets richer than US-ASCII. Since RFC 822 does not
specify mechanisms for mail containing audio, video, Asian specify mechanisms for mail containing audio, video, Asian
language text, or even text in most European languages, language text, or even text in most European languages,
additional specifications are needed. additional specifications are needed.
One of the notable limitations of RFC 821/822 based mail One of the notable limitations of RFC 821/822 based mail
systems is the fact that they limit the contents of electronic systems is the fact that they limit the contents of electronic
mail messages to relatively short lines of 7-bit US-ASCII. mail messages to relatively short lines (e.g. 1000 characters
This forces users to convert any non-textual data that they or less [RFC821]) of 7-bit US-ASCII. This forces users to
may wish to send into seven-bit bytes representable as convert any non-textual data that they may wish to send into
printable US-ASCII characters before invoking a local mail UA seven-bit bytes representable as printable US-ASCII characters
(User Agent, a program with which human users send and receive before invoking a local mail UA (User Agent, a program with
mail). Examples of such encodings currently used in the which human users send and receive mail). Examples of such
Internet include pure hexadecimal, uuencode, the 3-in-4 base encodings currently used in the Internet include pure
64 scheme specified in RFC 1421, the Andrew Toolkit hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in
Representation [ATK], and many others. RFC 1421, the Andrew Toolkit Representation [ATK], and many
others.
The limitations of RFC 822 mail become even more apparent as The limitations of RFC 822 mail become even more apparent as
gateways are designed to allow for the exchange of mail gateways are designed to allow for the exchange of mail
messages between RFC 822 hosts and X.400 hosts. X.400 [X400] messages between RFC 822 hosts and X.400 hosts. X.400 [X400]
specifies mechanisms for the inclusion of non-textual body specifies mechanisms for the inclusion of non-textual body
parts within electronic mail messages. The current standards parts within electronic mail messages. The current standards
for the mapping of X.400 messages to RFC 822 messages specify for the mapping of X.400 messages to RFC 822 messages specify
either that X.400 non-textual body parts must be converted to either that X.400 non-textual body parts must be converted to
(not encoded in) IA5Text format, or that they must be (not encoded in) IA5Text format, or that they must be
discarded, notifying the RFC 822 user that discarding has discarded, notifying the RFC 822 user that discarding has
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particular, it describes: particular, it describes:
(1) A MIME-Version header field, which uses a version (1) A MIME-Version header field, which uses a version
number to declare a message to be conformant with this number to declare a message to be conformant with this
specification and allows mail processing agents to specification and allows mail processing agents to
distinguish between such messages and those generated distinguish between such messages and those generated
by older or non-conformant software, which are presumed by older or non-conformant software, which are presumed
to lack such a field. to lack such a field.
(2) A Content-Type header field, generalized from RFC 1049 (2) A Content-Type header field, generalized from RFC 1049
[RFC-1049], which can be used to specify the type and [RFC-1049], which can be used to specify the media type
subtype of data in the body of a message and to fully and subtype of data in the body of a message and to
specify the native representation (encoding) of such fully specify the native representation (canonical
data. form) of such data.
(3) A Content-Transfer-Encoding header field, which can be (3) A Content-Transfer-Encoding header field, which can be
used to specify an auxiliary encoding that was applied used to specify an auxiliary encoding that was applied
to the data in order to allow it to pass through mail to the data in order to allow it to pass through mail
transport mechanisms which may have data or character transport mechanisms which may have data or character
set limitations. set limitations.
(4) Two additional header fields that can be used to (4) Two additional header fields that can be used to
further describe the data in a body, the Content-ID and further describe the data in a body, the Content-ID and
Content-Description header fields. Content-Description header fields.
All of these header fields defined in this document are All of the header fields defined in this document are subject
subject to the general syntactic rules for header fields to the general syntactic rules for header fields specified in
specified in RFC 822. In particular, all of these header RFC 822. In particular, all of these header fields can
fields can include RFC 822 comments, which have no semantic include RFC 822 comments, which have no semantic content and
content and should be ignored during MIME processing. 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:
(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 Finally, to specify and promote interoperability, RFC MIME-
of this document provides a basic applicability statement for CONF provides a basic applicability statement for a subset of
a subset of the above mechanisms that defines a minimal level the above mechanisms that defines a minimal level of
of "conformance" with this document. "conformance" with this document.
HISTORICAL NOTE: Several of the mechanisms described in this HISTORICAL NOTE: Several of the mechanisms described in this
document may seem somewhat strange or even baroque at first document may seem somewhat strange or even baroque at first
reading. It is important to note that compatibility with reading. It is important to note that compatibility with
existing standards AND robustness across existing practice existing standards AND robustness across existing practice
were two of the highest priorities of the working group that were two of the highest priorities of the working group that
developed this document. In particular, compatibility was developed this document. In particular, compatibility was
always favored over elegance. always favored over elegance.
MIME was first defined and published as RFC 1341 [RFC-1341] Please refer to the current edition of the "IAB Official
and RFC1342 [RFC-1342], then revised in RFC 1521 [RFC-1521] Protocol Standards" for the standardization state and status
and RFC 1522 [RFC-1522]. This document is a relatively minor of this protocol. RFC 822 and RFC 1123 [RFC-1123] also
updating of RFC 1521, and is intended to supersede it. The provide essential background for MIME since no conforming
companion document RFC MIME-HEADERS [RFC-MIME-HEADERS] in turn implementation of MIME can violate them. In addition, several
supersedes RFC 1522. other informational RFC documents will be of interest to the
MIME implementor, in particular RFC 1344 [RFC-1344], RFC 1345
The differences between this document and RFC 1521 are [RFC-1345], and RFC 1524 [RFC-1524].
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].
4. Notations, Conventions, and Generic BNF Grammar 4. Notations, Conventions, and Generic BNF Grammar
Although the mechanisms specified in this document are all Although the mechanisms specified in this document are all
described in prose, most are also described formally in the described in prose, most are also described formally in the
augmented BNF notation of RFC 822. Implementors will need to augmented BNF notation of RFC 822. Implementors will need to
be familiar with this notation in order to understand this be familiar with this notation in order to understand this
specification, and are referred to RFC 822 for a complete specification, and are referred to RFC 822 for a complete
explanation of the augmented BNF notation. explanation of the augmented BNF notation.
Some of the augmented BNF in this document makes reference to Some of the augmented BNF in this document makes reference to
syntactic entities that are defined in RFC 822 and not in this syntactic entities that are defined in RFC 822 and not in this
document. A complete formal grammar, then, is obtained by document. A complete formal grammar, then, is obtained by
Appendix D of this document, the collected grammar, with the combining Appendix A of this document, the collected grammar,
BNF of RFC 822 plus the modifications to RFC 822 defined in with the BNF of RFC 822 plus the modifications to RFC 822
RFC 1123, which specifically changes the syntax for `return', defined in RFC 1123 (which specifically changes the syntax for
`date' and `mailbox'. `return', `date' and `mailbox').
The term CRLF, in this document, refers to the sequence of the In this document, all numeric and octet values are given in
two US-ASCII characters CR (decimal value 13) and LF (decimal decimal notation. All media type values, subtype values, and
value 10) which, taken together, in this order, denote a line parameter names as defined in this document are case-
break in RFC 822 mail. 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
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.
4.1. CRLF
The term CRLF, in this document, 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 this document to refer to 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 a table-based method of converting a sequence of octets into a
octets into a sequence of characters. Note that unconditional sequence of characters. Note that unconditional and
conversion in the other direction is not required, in that not unambiguous conversion in the other direction is not required,
all characters may be available in a given character set and a in that not all characters may be available in a given
character set may provide more than one sequence of octets to character set and a character set may provide more than one
represent a particular character. This definition is intended sequence of octets to represent a particular character. This
to allow various kinds of character encodings, from simple definition is intended to allow various kinds of character
single-table mappings such as US-ASCII to complex table encodings, from simple single-table mappings such as US-ASCII
switching methods such as those that use ISO 2022's to complex table switching methods such as those that use ISO
techniques. However, the definition associated with a MIME 2022's techniques. However, the definition associated with a
character set name must fully specify the mapping to be MIME character set name must fully specify the mapping to be
performed from octets to characters. In particular, use of performed from octets to characters. In particular, use of
external profiling information to determine the exact mapping external profiling information to determine the exact mapping
is not permitted. is not permitted.
HISTORICAL NOTE: The term "character set" originated in the
definition of US-ASCII and similar 7-bit and 8-bit
specifications. These define true sets. However, the advent
of multi-octet character encodings and switching techniques
have transformed character sets into entities that properly
speaking are no longer strictly sets. Some other communities
have adopted the term "character encoding" for what MIME calls
a "character set" as a result.
4.3. Message
The term "message", when not further qualified, means either The term "message", when not further qualified, means either
the (complete or "top-level") message being transferred on a the (complete or "top-level") message being transferred on a
network, or a message encapsulated in a body part of type network, or a message encapsulated in a body part of type
"message". "message".
The term "body part", in this document, refers to either the a 4.4. Body Part
The term "body part", in this document, refers to either a
single part message or one of the parts in the body of a single part message or one of the parts in the body of a
multipart entity. A body part has a header and a body, so it multipart entity. A body part has a header and a body, so it
makes sense to speak about the body of a body part. makes sense to speak about the body of a body part.
4.5. Entity
The term "entity", in this document, means either a message or The term "entity", in this document, means either a message or
a body part. All kinds of entities share the property that a body part. All kinds of entities share the property that
they have a header and a body. they have a header and a body.
4.6. Body
The term "body", when not further qualified, means the body of 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 an entity, that is the body of either a message or of a body
part. part.
NOTE: The previous four definitions are clearly circular. NOTE: The previous four definitions are clearly circular.
This is unavoidable, since the overall structure of a MIME This is unavoidable, since the overall structure of a MIME
message is indeed recursive. message is indeed recursive.
"7bit data" refers to data that is all represented as short 4.7. 7bit Data
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 "7bit data" refers to data that is all represented as
short lines, but there may be non-US-ASCII characters relatively short lines (e.g. 1000 octets or less between CRLF
(octets with the high-order bit set) present. As with line separation sequences [RFC821]). No octets with decimal
"7bit data" CR and LF characters only occur as part of values greater than 127 are allowed and neither are NULs
CRLF line separation sequences and no NULs are allowed. (octets with decimal value 0). CR (decimal value 13) and LF
(decimal value 10) octets only occur as part of CRLF line
separation sequences.
(2) "Binary data" refers to data where any sequence of 4.8. 8bit Data
octets whatsoever is allowed.
"Lines" are defined as sequences of octets separated by a CRLF "8bit data" refers to data that is all represented as
sequences. This is consistent with both RFC 821 and RFC 822. relatively short lines (e.g. 1000 octets or less between CRLF
Lines in MIME bodies must also be terminated with a CRLF, but line separation sequences [RFC821]), but characters with
the terminating CRLF on the last line of the body may properly decimal values greater than 127 may be used. As with "7bit
be part of a subsequent boundary marker rather than being part data" CR and LF octets only occur as part of CRLF line
of the body itself. separation sequences and no NULs are allowed.
In this document, all numeric and octet values are given in 4.9. Binary Data
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 "Binary data" refers to data where any sequence of octets
nonessential information which may be skipped by the reader whatsoever is allowed.
without missing anything essential. The primary purpose of
these non-essential notes is to convey information about the 4.10. Lines
rationale of this document, or to place this document in the
proper historical or evolutionary context. Such information "Lines" are defined as sequences of octets separated by a CRLF
may in particular be skipped by those who are focused entirely sequences. This is consistent with both RFC 821 and RFC 822.
on building a conformant implementation, but may be of use to
those who wish to understand why certain design choices were
made.
5. MIME Header Fields 5. MIME Header Fields
MIME defines a number of new RFC 822 header fields that are MIME defines a number of new RFC 822 header fields that are
used to describe the content of messages. These header fields used to describe the content of messages. These header fields
occur in two contexts: occur in two contexts:
(1) As part of a regular RFC 822 message header. (1) As part of a regular RFC 822 message header.
(2) In a MIME body part header within a multipart (2) In a MIME body part header within a multipart
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[ id CRLF ] [ id CRLF ]
[ description CRLF ] [ description CRLF ]
*( mime-extension-field CRLF ) *( mime-extension-field CRLF )
; The ordering of the header ; The ordering of the header
; fields implied by this BNF ; fields implied by this BNF
; definition should be ignored ; definition should be ignored
The syntax of the various specific MIME header fields will be The syntax of the various specific MIME header fields will be
described in the following sections. described in the following sections.
5.1. MIME-Version Header Field 6. MIME-Version Header Field
Since RFC 822 was published in 1982, there has really been Since RFC 822 was published in 1982, there has really been
only one format standard for Internet messages, and there has only one format standard for Internet messages, and there has
been little perceived need to declare the format standard in been little perceived need to declare the format standard in
use. This document is an independent document that use. This document is an independent document that
complements RFC 822. Although the extensions in this document complements RFC 822. Although the extensions in this document
have been defined in such a way as to be compatible with RFC have been defined in such a way as to be compatible with RFC
822, there are still circumstances in which it might be 822, there are still circumstances in which it might be
desirable for a mail-processing agent to know whether a desirable for a mail-processing agent to know whether a
message was composed with the new standard in mind. message was composed with the new standard in mind.
skipping to change at page 13, line 44 skipping to change at page 11, line 32
a multipart entity. It is required for the embedded headers a multipart entity. It is required for the embedded headers
of a body of type "message" if and only if the embedded of a body of type "message" if and only if the embedded
message is itself claimed to be MIME-conformant. message is itself claimed to be MIME-conformant.
It is not possible to fully specify how a mail reader that It is not possible to fully specify how a mail reader that
conforms with MIME as defined in this document should treat a conforms with MIME as defined in this document should treat a
message that might arrive in the future with some value of message that might arrive in the future with some value of
MIME-Version other than "1.0". MIME-Version other than "1.0".
It is also worth noting that version control for specific It is also worth noting that version control for specific
content-types is not accomplished using the MIME-Version media types is not accomplished using the MIME-Version
mechanism. In particular, some formats (such as mechanism. In particular, some formats (such as
application/postscript) have version numbering conventions application/postscript) have version numbering conventions
that are internal to the document format. Where such that are internal to the document format. Where such
conventions exist, MIME does nothing to supersede them. Where conventions exist, MIME does nothing to supersede them. Where
no such conventions exist, a MIME type might use a "version" no such conventions exist, a MIME media type might use a
parameter in the content-type field if necessary. "version" parameter in the content-type field if necessary.
NOTE TO IMPLEMENTORS: When checking MIME-Version values any NOTE TO IMPLEMENTORS: When checking MIME-Version values any
RFC 822 comment strings that are present must be ignored. In RFC 822 comment strings that are present must be ignored. In
particular, the following four MIME-Version fields are particular, the following four MIME-Version fields are
equivalent: equivalent:
MIME-Version: 1.0 MIME-Version: 1.0
MIME-Version: 1.0 (produced by MetaSend Vx.x) MIME-Version: 1.0 (produced by MetaSend Vx.x)
MIME-Version: (produced by MetaSend Vx.x) 1.0 MIME-Version: (produced by MetaSend Vx.x) 1.0
MIME-Version: 1.(produced by MetaSend Vx.x)0 MIME-Version: 1.(produced by MetaSend Vx.x)0
5.2. Content-Type Header Field In the absence of a MIME-Version field, a receiving user agent
(whether MIME compliant 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 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).
MIME-compliant user agents are required, if they support any
such nonstandard conventions at all, to do so on received
messages only -- they must not send non-MIME messages
containing anything other than US-ASCII text.
In particular, the use of non-US-ASCII text in messages
without a MIME-Version field is strongly discouraged as it
impedes interoperability when sending messages between regions
with different localization conventions. MIME-compliant user
agents MUST include proper MIME labelling when sending
anything other than plain text in the US-ASCII character set.
In addition, non-MIME user agents should be upgraded if at all
possible to include appropriate MIME header information in the
messages they send even if nothing else in MIME is supported.
This upgrade will have little, if any, effect on non-MIME
recipients and will aid MIME in correctly displaying such
messages. It also provides a smooth transition path to
eventual adoption of other MIME capabilities.
7. Content-Type Header Field
The purpose of the Content-Type field is to describe the data The purpose of the Content-Type field is to describe the data
contained in the body fully enough that the receiving user contained in the body fully enough that the receiving user
agent can pick an appropriate agent or mechanism to present agent can pick an appropriate agent or mechanism to present
the data to the user, or otherwise deal with the data in an the data to the user, or otherwise deal with the data in an
appropriate manner. appropriate manner. The value in this field is called a media
type.
HISTORICAL NOTE: The Content-Type header field was first HISTORICAL NOTE: The Content-Type header field was first
defined in RFC 1049. RFC 1049 Content-types used a simpler defined in RFC 1049. RFC 1049 used a simpler and less
and less powerful syntax, but one that is largely compatible powerful syntax, but one that is largely compatible with the
with the mechanism given here. mechanism given here.
The Content-Type header field is used to specify the nature of The Content-Type header field specifies the nature of the data
the data in the body of an entity, by giving type and subtype in the body of an entity by giving media type and subtype
identifiers, and by providing auxiliary information that may identifiers, and by providing auxiliary information that may
be required for certain types. After the type and subtype be required for certain media types. After the media type and
names, the remainder of the header field is simply a set of subtype names, the remainder of the header field is simply a
parameters, specified in an attribute/value notation. The set of parameters, specified in an attribute=value notation.
ordering of parameters is not significant. The ordering of parameters is not significant.
In general, the top-level Content-Type is used to declare the In general, the top-level media type is used to declare the
general type of data, while the subtype specifies a specific general type of data, while the subtype specifies a specific
format for that type of data. Thus, a Content-Type of 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/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, even if the user agent has no knowledge of the specific
image format "xyz". Such information can be used, for image format "xyz". Such information can be used, for
example, to decide whether or not to show a user the raw data example, to decide whether or not to show a user the raw data
from an unrecognized subtype -- such an action might be from an unrecognized subtype -- such an action might be
reasonable for unrecognized subtypes of text, but not for reasonable for unrecognized subtypes of text, but not for
unrecognized subtypes of image or audio. For this reason, unrecognized subtypes of image or audio. For this reason,
registered subtypes of text, image, audio, and video should registered subtypes of text, image, audio, and video should
not contain embedded information that is really of a different not contain embedded information that is really of a different
type. Such compound formats should be represented using the type. Such compound formats should be represented using the
"multipart" or "application" types. "multipart" or "application" types.
Parameters are modifiers of the content-subtype, and as such Parameters are modifiers of the media subtype, and as such do
do not fundamentally affect the nature of the content. The set not fundamentally affect the nature of the content. The set
of meaningful parameters depends on the content-type and of meaningful parameters depends on the media type and
subtype. Most parameters are associated with a single specific subtype. Most parameters are associated with a single
subtype. However, a given top-level content-type may define specific subtype. However, a given top-level media type may
parameters which are applicable to any subtype of that type. 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 For example, the "charset" parameter is applicable to any
subtype of "text", while the "boundary" parameter is required subtype of "text", while the "boundary" parameter is required
for any subtype of the "multipart" content-type. for any subtype of the "multipart" media type.
There are NO globally-meaningful parameters that apply to all There are NO globally-meaningful parameters that apply to all
content-types. Truly global mechanisms are best addressed, in media types. Truly global mechanisms are best addressed, in
the MIME model, by the definition of additional Content-* the MIME model, by the definition of additional Content-*
header fields. header fields.
An initial set of seven top-level Content-Types is defined by An initial set of seven top-level media types is defined by
this document. Five of these are discrete types whose content this document. Five of these are discrete types whose content
is essentially opaque as far as MIME processing is concerned. is essentially opaque as far as MIME processing is concerned.
The remaining two are composite types whose contents require The remaining two are composite types whose contents require
additional handling by MIME processors. additional handling by MIME processors.
This set of top-level Content-Types is intended to be This set of top-level media types is intended to be
substantially complete. It is expected that additions to the substantially complete. It is expected that additions to the
larger set of supported types can generally be accomplished by larger set of supported types can generally be accomplished by
the creation of new subtypes of these initial types. In the the creation of new subtypes of these initial types. In the
future, more top-level types may be defined only by a future, more top-level types may be defined only by a
standards-track extension to this standard. If another top- standards-track extension to this standard. If another top-
level type is to be used for any reason, it must be given a level type is to be used for any reason, it must be given a
name starting with "X-" to indicate its non-standard status name starting with "X-" to indicate its non-standard status
and to avoid a potential conflict with a future official name. and to avoid a potential conflict with a future official name.
5.2.1. Syntax of the Content-Type Header Field 7.1. Syntax of the Content-Type Header Field
In the Augmented BNF notation of RFC 822, a Content-Type In the Augmented BNF notation of RFC 822, a Content-Type
header field value is defined as follows: header field value is defined as follows:
content := "Content-Type" ":" type "/" subtype content := "Content-Type" ":" type "/" subtype
*(";" parameter) *(";" parameter)
; Matching of type and subtype is ; Matching of media type and subtype
; ALWAYS case-insensitive ; is ALWAYS case-insensitive
type := discrete-type / composite-type type := discrete-type / composite-type
discrete-type := "text" / "image" / "audio" / "video" / discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token "application" / extension-token
composite-type := "message" / "multipart" / extension-token composite-type := "message" / "multipart" / extension-token
extension-token := iana-token / ietf-token / x-token extension-token := iana-token / ietf-token / x-token
iana-token := <a publicly-defined extension token, iana-token := <a publicly-defined extension token,
registered with IANA, as specified in registered with IANA, as specified in
RFC REG [RFC-REG]> RFC MIME-REG [RFC-MIME-REG]>
ietf-token := <a publicly-defined extension token, ietf-token := <a publicly-defined extension token,
initially registered with IANA and initially registered with IANA and
subsequently standardized by the IETF> subsequently standardized by the IETF>
x-token := <The two characters "X-" or "x-" followed, with x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token> no intervening white space, by any token>
subtype := extension-token subtype := extension-token
skipping to change at page 17, line 6 skipping to change at page 15, line 34
"," / ";" / ":" / "\" / <"> "," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "=" "/" / "[" / "]" / "?" / "="
; Must be in quoted-string, ; Must be in quoted-string,
; to use within parameter values ; to use within parameter values
Note that the definition of "tspecials" is the same as the RFC Note that the definition of "tspecials" is the same as the RFC
822 definition of "specials" with the addition of the three 822 definition of "specials" with the addition of the three
characters "/", "?", and "=", and the removal of ".". characters "/", "?", and "=", and the removal of ".".
Note also that a subtype specification is MANDATORY -- it may Note also that a subtype specification is MANDATORY -- it may
not be omitted from a Content-Type header field. As such, not be omitted from a Content-Type header field. As such,
there are no default subtypes. there are no default subtypes.
The type, subtype, and parameter names are not case sensitive. The type, subtype, and parameter names are not case sensitive.
For example, TEXT, Text, and TeXt are all equivalent top-level For example, TEXT, Text, and TeXt are all equivalent top-level
Content Types. Parameter values are normally case sensitive, media types. Parameter values are normally case sensitive,
but sometimes are interpreted in a case-insensitive fashion, but sometimes are interpreted in a case-insensitive fashion,
depending on the intended use. (For example, multipart depending on the intended use. (For example, multipart
boundaries are case-sensitive, but the "access-type" parameter boundaries are case-sensitive, but the "access-type" parameter
for message/External-body is not case-sensitive.) for message/External-body is not case-sensitive.)
Note that the value of a quoted string parameter does not Note that the value of a quoted string parameter does not
include the quotes. That is, the quotation marks in a include the quotes. That is, the quotation marks in a
quoted-string are not a part of the value of the parameter, quoted-string are not a part of the value of the parameter,
but are merely used to delimit that parameter value. In but are merely used to delimit that parameter value. In
addition, comments are allowed in accordance with RFC 822 addition, comments are allowed in accordance with RFC 822
skipping to change at page 17, line 36 skipping to change at page 16, line 18
Content-type: text/plain; charset="us-ascii" Content-type: text/plain; charset="us-ascii"
are completely equivalent. are completely equivalent.
Beyond this syntax, the only syntactic constraint on the Beyond this syntax, the only syntactic constraint on the
definition of subtype names is the desire that their uses must definition of subtype names is the desire that their uses must
not conflict. That is, it would be undesirable to have two not conflict. That is, it would be undesirable to have two
different communities using "Content-Type: application/foobar" different communities using "Content-Type: application/foobar"
to mean two different things. The process of defining new to mean two different things. The process of defining new
content-subtypes, then, is not intended to be a mechanism for media subtypes, then, is not intended to be a mechanism for
imposing restrictions, but simply a mechanism for publicizing imposing restrictions, but simply a mechanism for publicizing
the usages. There are, therefore, two acceptable mechanisms their definition and usage. There are, therefore, two
for defining new Content-Type subtypes: acceptable mechanisms for defining new media subtypes:
(1) Private values (starting with "X-") may be defined (1) Private values (starting with "X-") may be defined
bilaterally between two cooperating agents without bilaterally between two cooperating agents without
outside registration or standardization. outside registration or standardization.
(2) New standard values MUST be documented, registered (2) New standard values MUST be documented, registered
with, and approved by IANA, as described in RFC REG. with, and approved by IANA, as described in RFC MIME-
REG [RFC-MIME-REG].
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".
(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 The second document in this set, RFC MIME-IMT, defines the
independent data types. Four subtypes are initially initial set of media types for MIME.
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".
(2) message -- an encapsulated message. A body of 7.2. Content-Type Defaults
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 Default RFC 822 messages without a MIME Content-Type header
are taken by this protocol to be plain text in the US-ASCII are taken by this protocol to be plain text in the US-ASCII
character set, which can be explicitly specified as: character set, which can be explicitly specified as:
Content-type: text/plain; charset=us-ascii Content-type: text/plain; charset=us-ascii
This default is assumed if no Content-Type is specified. In This default is assumed if no Content-Type header field is
the presence of a MIME-Version header field, a receiving User specified. In the presence of a MIME-Version header field, a
Agent can also assume that plain US-ASCII text was the receiving User Agent can also assume that plain US-ASCII text
sender's intent. Plain US-ASCII text must still be assumed in was the sender's intent. Plain US-ASCII text may still be
the absence of a MIME-Version specification, but the sender's assumed in the absence of a MIME-Version specification, but
intent might have been otherwise. 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.
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 8. Content-Transfer-Encoding Header Field
Many Content-Types which could be usefully transported via Many media types which could be usefully transported via email
email are represented, in their "natural" format, as 8-bit are represented, in their "natural" format, as 8-bit character
character or binary data. Such data cannot be transmitted over or binary data. Such data cannot be transmitted over some
some transport protocols. For example, RFC 821 (SMTP) transfer protocols. For example, RFC 821 (SMTP) restricts
restricts mail messages to 7-bit US-ASCII data with lines no mail messages to 7-bit US-ASCII data with lines no longer than
longer than 1000 characters. 1000 characters.
It is necessary, therefore, to define a standard mechanism for It is necessary, therefore, to define a standard mechanism for
encoding such data into a 7-bit short-line format. Proper encoding such data into a 7-bit short line format. Proper
labelling of unencoded material in less restrictive formats labelling of unencoded material in less restrictive formats
for direct use over less restrictive transports is also for direct use over less restrictive transports is also
desireable. This document specifies that such encodings will desireable. This document specifies that such encodings will
be indicated by a new "Content-Transfer-Encoding" header be indicated by a new "Content-Transfer-Encoding" header
field. This field has not been defined by any previous field. This field has not been defined by any previous
standard. standard.
5.3.1. Content-Transfer-Encoding Syntax 8.1. Content-Transfer-Encoding Syntax
The Content-Transfer-Encoding field's value is a single token The Content-Transfer-Encoding field's value is a single token
specifying the type of encoding, as enumerated below. specifying the type of encoding, as enumerated below.
Formally: Formally:
encoding := "Content-Transfer-Encoding" ":" mechanism encoding := "Content-Transfer-Encoding" ":" mechanism
mechanism := "7bit" / "8bit" / "binary" / mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" / "quoted-printable" / "base64" /
ietf-token / x-token ietf-token / x-token
These values are not case sensitive -- Base64 and BASE64 and These values are not case sensitive -- Base64 and BASE64 and
bAsE64 are all equivalent. An encoding type of 7BIT requires bAsE64 are all equivalent. An encoding type of 7BIT requires
that the body is already in a 7-bit mail-ready representation. that the body is already in a 7-bit mail-ready representation.
This is the default value -- that is, "Content-Transfer- This is the default value -- that is, "Content-Transfer-
Encoding: 7BIT" is assumed if the Content-Transfer-Encoding Encoding: 7BIT" is assumed if the Content-Transfer-Encoding
header field is not present. header field is not present.
5.3.2. Content-Transfer-Encoding Semantics 8.2. Content-Transfer-Encodings Sematics
This single token actually provides two pieces of information. This single Content-Transfer-Encoding token actually provides
It specifies what sort of encoding transformation the body was two pieces of information. It specifies what sort of encoding
subjected to, and it specifies what the domain of the result transformation the body was subjected to, and it specifies
is. what the domain of the result is.
Three transformations are currently defined: identity, the Three transformations are currently defined: identity, the
"quoted-printable" encoding, and the "base64" encoding. The "quoted-printable" encoding, and the "base64" encoding. The
domains are "binary", "8bit" and "7bit". domains are "binary", "8bit" and "7bit".
The values "7bit", "8bit", and "binary" all mean that the The Content-Transfer-Encoding values "7bit", "8bit", and
identity (i.e. NO) encoding transformation has been performed. "binary" all mean that the identity (i.e. NO) encoding
As such, they serve simply as indicators of the domain of the transformation has been performed. As such, they serve simply
body part data, and provide useful information about the sort as indicators of the domain of the body part data, and provide
of encoding that might be needed for transmission in a given useful information about the sort of encoding that might be
transport system. The terms "7bit data", "8bit data", and needed for transmission in a given transport system. The
"binary data" are all defined in Section 4. terms "7bit data", "8bit data", and "binary data" are all
defined in Section 4.
The quoted-printable and base64 encodings transform their The quoted-printable and base64 encodings transform their
input from an arbitrary domain into material in the "7bit" input from an arbitrary domain into material in the "7bit"
domain, thus making it safe to carry over restricted range, thus making it safe to carry over restricted
transports. The specific definition of the transformations are transports. The specific definition of the transformations
given below. are given below.
The proper Content-Transfer-Encoding label must always be The proper Content-Transfer-Encoding label must always be
used. Labelling unencoded data containing 8-bit characters as used. Labelling unencoded data containing 8-bit characters as
"7bit" is not allowed, nor is labelling unencoded non-line- "7bit" is not allowed, nor is labelling unencoded non-line-
oriented data as anything other than "binary" allowed. oriented data as anything other than "binary" allowed.
Unlike Content-Type subtypes, a proliferation of Content- Unlike media subtypes, a proliferation of Content-Transfer-
Transfer-Encoding values is both undesirable and unnecessary. Encoding values is both undesirable and unnecessary. However,
However, establishing only a single transformation into the establishing only a single transformation into the "7bit"
"7bit" domain does not seem possible. There is a tradeoff domain does not seem possible. There is a tradeoff between
between the desire for a compact and efficient encoding of the desire for a compact and efficient encoding of largely-
largely-binary data and the desire for a readable encoding of binary data and the desire for a readable encoding of data
data that is mostly, but not entirely, 7-bit. For this that is mostly, but not entirely, 7-bit. For this reason, at
reason, at least two encoding mechanisms are necessary: a least two encoding mechanisms are necessary: a "readable"
"readable" encoding (quoted-printable) and a "dense" encoding encoding (quoted-printable) and a "dense" encoding (base64).
(base64).
Mail transport for unencoded 8-bit data is defined in RFC 1652 Mail transport for unencoded 8-bit data is defined in RFC 1652
[RFC-1652]. As of the publication of this document, there are [RFC-1652]. As of the publication of this document, there are
no standardized Internet mail transports for which it is no standardized Internet mail transports for which it is
legitimate to include unencoded binary data in mail bodies. legitimate to include unencoded binary data in mail bodies.
Thus there are no circumstances in which the "binary" Thus there are no circumstances in which the "binary"
Content-Transfer-Encoding is actually valid on the Internet. Content-Transfer-Encoding is actually valid in Internet mail.
However, in the event that binary mail transport becomes a However, in the event that binary mail transport becomes a
reality in Internet mail, or when this document is used in reality in Internet mail, or when this document is used in
conjunction with any other binary-capable transport mechanism, conjunction with any other binary-capable transport mechanism,
binary bodies should be labelled as such using this mechanism. binary bodies should be labelled as such using this mechanism.
NOTE: The five values defined for the Content-Transfer- NOTE: The five values defined for the Content-Transfer-
Encoding field imply nothing about the Content-Type other than Encoding field imply nothing about the media type other than
the algorithm by which it was encoded or the transport system the algorithm by which it was encoded or the transport system
requirements if unencoded. requirements if unencoded.
Implementors may, if necessary, define new Content-Transfer- 8.3. New Content-Transfer-Encodings
Encoding values, but must use an x-token, which is a name
prefixed by "X-", to indicate its non-standard status, e.g., Implementors may, if necessary, define private Content-
"Content-Transfer-Encoding: x-my-new-encoding". However, Transfer-Encoding values, but must use an x-token, which is a
unlike Content-Types and subtypes, the creation of new name prefixed by "X-", to indicate its non-standard status,
Content-Transfer-Encoding values is STRONGLY discouraged, as e.g., "Content-Transfer-Encoding: x-my-new-encoding".
it seems likely to hinder interoperability with little Additional standardized Content-Transfer-Encoding values must
potential benefit. Such use is therefore allowed only as the be specified by a standards-track RFC. Additional
result of an agreement between cooperating user agents. 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 IANA
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
benefit
8.4. Interpretation and Use
If a Content-Transfer-Encoding header field appears as part of If a Content-Transfer-Encoding header field appears as part of
a message header, it applies to the entire body of that a message header, it applies to the entire body of that
message. If a Content-Transfer-Encoding header field appears message. If a Content-Transfer-Encoding header field appears
as part of a body part's headers, it applies only to the body 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 of that body part. If an entity is of type "multipart" the
Content-Transfer-Encoding is not permitted to have any value Content-Transfer-Encoding is not permitted to have any value
other than "7bit", "8bit" or "binary". Even more severe other than "7bit", "8bit" or "binary". Even more severe
restrictions apply to some subtypes of the "message" type. restrictions apply to some subtypes of the "message" type.
It should be noted that email is character-oriented, so that It should be noted that most media types are defined in terms
the mechanisms described here are mechanisms for encoding of octets rather than bits, so that the mechanisms described
arbitrary octet streams, not bit streams. If a bit stream is here are mechanisms for encoding arbitrary octet streams, not
to be encoded via one of these mechanisms, it must first be bit streams. If a bit stream is to be encoded via one of
converted to an 8-bit byte stream using the network standard these mechanisms, it must first be converted to an 8-bit byte
bit order ("big-endian"), in which the earlier bits in a stream using the network standard bit order ("big-endian"), in
stream become the higher-order bits in a 8-bit byte. A bit which the earlier bits in a stream become the higher-order
stream not ending at an 8-bit boundary must be padded with bits in a 8-bit byte. A bit stream not ending at an 8-bit
zeroes. This document provides a mechanism for noting the boundary must be padded with zeroes. This document provides a
addition of such padding in the case of the mechanism for noting the addition of such padding in the case
application/octet-stream Content-Type, which has a "padding" of the application/octet-stream media type, which has a
parameter. "padding" parameter.
The encoding mechanisms defined here explicitly encode all The encoding mechanisms defined here explicitly encode all
data in US-ASCII. Thus, for example, suppose an entity has data in US-ASCII. Thus, for example, suppose an entity has
header fields such as: header fields such as:
Content-Type: text/plain; charset=ISO-8859-1 Content-Type: text/plain; charset=ISO-8859-1
Content-transfer-encoding: base64 Content-transfer-encoding: base64
This must be interpreted to mean that the body is a 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, US-ASCII encoding of data that was originally in ISO-8859-1,
and will be in that character set again after decoding. 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-Transfer-Encoding values may only be used on
certain Content-Types. In particular, it is EXPRESSLY certain media types. In particular, it is EXPRESSLY FORBIDDEN
FORBIDDEN to use any encodings other than "7bit", "8bit", or to use any encodings other than "7bit", "8bit", or "binary"
"binary" with any composite Content-Type, i.e. one that with any composite media type, i.e. one that recursively
recursively includes other Content-Type fields. Currently the includes other Content-Type fields. Currently the only
only composite Content-Types are "multipart" and "message". composite media types are "multipart" and "message". All
All encodings that are desired for bodies of type multipart or encodings that are desired for bodies of type multipart or
message must be done at the innermost level, by encoding the message must be done at the innermost level, by encoding the
actual body that needs to be encoded. actual body that needs to be encoded.
It should also be noted that, by definition, if a composite It should also be noted that, by definition, if a composite
entity has a transfer-encoding value such as "7bit", but one entity has a transfer-encoding value such as "7bit", but one
of the enclosed parts has a less restrictive value such as of the enclosed parts has a less restrictive value such as
"8bit", then either the outer "7bit" labelling is in error, "8bit", then either the outer "7bit" labelling is in error,
because 8-bit data are included, or the inner "8bit" labelling because 8-bit data are included, or the inner "8bit" labelling
placed an unnecessarily high demand on the transport system placed an unnecessarily high demand on the transport system
because the actual included data were actually 7-bit-safe. because the actual included data were actually 7-bit-safe.
skipping to change at page 25, line 7 skipping to change at page 21, line 4
using content-transfer-encodings on composite body data may using content-transfer-encodings on composite body data may
seem overly restrictive, it is necessary to prevent nested seem overly restrictive, it is necessary to prevent nested
encodings, in which data are passed through an encoding encodings, in which data are passed through an encoding
algorithm multiple times, and must be decoded multiple times algorithm multiple times, and must be decoded multiple times
in order to be properly viewed. Nested encodings add in order to be properly viewed. Nested encodings add
considerable complexity to user agents: Aside from the considerable complexity to user agents: Aside from the
obvious efficiency problems with such multiple encodings, they obvious efficiency problems with such multiple encodings, they
can obscure the basic structure of a message. In particular, can obscure the basic structure of a message. In particular,
they can imply that several decoding operations are necessary they can imply that several decoding operations are necessary
simply to find out what types of bodies a message contains. simply to find out what types of bodies a message contains.
Banning nested encodings may complicate the job of certain Banning nested encodings may complicate the job of certain
mail gateways, but this seems less of a problem than the mail gateways, but this seems less of a problem than the
effect of nested encodings on user agents. effect of nested encodings on user agents.
NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT- NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-
TRANSFER-ENCODING: It may seem that the Content-Transfer- TRANSFER-ENCODING: It may seem that the Content-Transfer-
Encoding could be inferred from the characteristics of the Encoding could be inferred from the characteristics of the
Content-Type that is to be encoded, or, at the very least, media that is to be encoded, or, at the very least, that
that certain Content-Transfer-Encodings could be mandated for certain Content-Transfer-Encodings could be mandated for use
use with specific Content-Types. There are several reasons with specific media types. There are several reasons why this
why this is not the case. First, given the varying types of is not the case. First, given the varying types of transports
transports used for mail, some encodings may be appropriate used for mail, some encodings may be appropriate for some
for some Content-Type/transport combinations and not for combinations of media types and transports but not for others.
others. (For example, in an 8-bit transport, no encoding (For example, in an 8-bit transport, no encoding would be
would be required for text in certain character sets, while required for text in certain character sets, while such
such encodings are clearly required for 7-bit SMTP.) encodings are clearly required for 7-bit SMTP.)
Second, certain Content-Types may require different types of Second, certain media types may require different types of
transfer encoding under different circumstances. For example, transfer encoding under different circumstances. For example,
many PostScript bodies might consist entirely of short lines many PostScript bodies might consist entirely of short lines
of 7-bit data and hence require no encoding at all. Other of 7-bit data and hence require no encoding at all. Other
PostScript bodies (especially those using Level 2 PostScript's PostScript bodies (especially those using Level 2 PostScript's
binary encoding mechanism) may only be reasonably represented binary encoding mechanism) may only be reasonably represented
using a binary transport encoding. Finally, since Content- using a binary transport encoding. Finally, since the
Type is intended to be an open-ended specification mechanism, Content-Type field is intended to be an open-ended
strict specification of an association between Content-Types specification mechanism, strict specification of an
and encodings effectively couples the specification of an association between media types and encodings effectively
application protocol with a specific lower-level transport. couples the specification of an application protocol with a
This is not desirable since the developers of a Content-Type specific lower-level transport. This is not desirable since
should not have to be aware of all the transports in use and the developers of a media type should not have to be aware of
what their limitations are. all the transports in use and what their limitations are.
NOTE ON TRANSLATING ENCODINGS: The quoted-printable and 8.5. Translating Encodings
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.
NOTE ON CANONICAL ENCODING MODEL: There was some confusion, The quoted-printable and base64 encodings are designed so that
in earlier drafts of this document, regarding the model for conversion between them is possible. The only issue that
when email data was to be converted to canonical form and arises in such a conversion is the handling of line breaks.
encoded, and in particular how this process would affect the When converting from quoted-printable to base64 a line break
treatment of CRLFs, given that the representation of newlines must be converted into a CRLF sequence. Similarly, a CRLF
varies greatly from system to system, and the relationship sequence in base64 data must be converted to a quoted-
between content-transfer-encodings and character sets. A printable line break, but ONLY when converting text data.
canonical model for encoding is presented as Appendix F for
this reason.
5.3.3. Quoted-Printable Content-Transfer-Encoding 8.6. Canonical Encoding Model
There was some confusion, in the predecessors 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 The Quoted-Printable encoding is intended to represent data
that largely consists of octets that correspond to printable that largely consists of octets that correspond to printable
characters in the US-ASCII character set. It encodes the data characters in the US-ASCII character set. It encodes the data
in such a way that the resulting octets are unlikely to be in such a way that the resulting octets are unlikely to be
modified by mail transport. If the data being encoded are modified by mail transport. If the data being encoded are
mostly US-ASCII text, the encoded form of the data remains mostly US-ASCII text, the encoded form of the data remains
largely recognizable by humans. A body which is entirely US- largely recognizable by humans. A body which is entirely US-
ASCII may also be encoded in Quoted-Printable to ensure the ASCII may also be encoded in Quoted-Printable to ensure the
integrity of the data should the message pass through a integrity of the data should the message pass through a
character-translating, and/or line-wrapping gateway. character-translating, and/or line-wrapping gateway.
In this encoding, octets are to be represented as determined In this encoding, octets are to be represented as determined
by the following rules: by the following rules:
(1) (General 8-bit representation) Any octet, except those (1) (General 8-bit representation) Any octet, except a CR
indicating a line break according to the newline or LF that is part of a CRLF line break of the
convention of the canonical (standard) form of the data canonical (standard) form of the data being encoded,
being encoded, may be represented by an "=" followed by may be represented by an "=" followed by a two digit
a two digit hexadecimal representation of the octet's hexadecimal representation of the octet's value. The
value. The digits of the hexadecimal alphabet, for digits of the hexadecimal alphabet, for this purpose,
this purpose, are "0123456789ABCDEF". Uppercase are "0123456789ABCDEF". Uppercase letters must be used
letters must be used when sending hexadecimal data, when sending hexadecimal data, though a robust
though a robust implementation may choose to recognize implementation may choose to recognize lowercase
lowercase letters on receipt. Thus, for example, the letters on receipt. Thus, for example, the decimal
decimal value 12 (US-ASCII form feed) can be value 12 (US-ASCII form feed) can be represented by
represented by "=0C", and the decimal value 61 (US- "=0C", and the decimal value 61 (US-ASCII EQUAL SIGN)
ASCII EQUAL SIGN) can be represented by "=3D". This can be represented by "=3D". This rule must be
rule must be followed except when the following rules followed except when the following rules allow an
allow an alternative encoding. alternative encoding.
(2) (Literal representation) Octets with decimal values of (2) (Literal representation) Octets with decimal values of
33 through 60 inclusive, and 62 through 126, inclusive, 33 through 60 inclusive, and 62 through 126, inclusive,
MAY be represented as the US-ASCII characters which MAY be represented as the US-ASCII characters which
correspond to those octets (EXCLAMATION POINT through correspond to those octets (EXCLAMATION POINT through
LESS THAN, and GREATER THAN through TILDE, LESS THAN, and GREATER THAN through TILDE,
respectively). respectively).
(3) (White Space) Octets with values of 9 and 32 MAY be (3) (White Space) Octets with values of 9 and 32 MAY be
represented as US-ASCII TAB (HT) and SPACE characters, represented as US-ASCII TAB (HT) and SPACE characters,
respectively, but MUST NOT be so represented at the end respectively, but MUST NOT be so represented at the end
of an encoded line. Any TAB (HT) or SPACE characters on of an encoded line. Any TAB (HT) or SPACE characters
an encoded line MUST thus be followed on that line by a on an encoded line MUST thus be followed on that line
printable character. In particular, an "=" at the end by a printable character. In particular, an "=" at the
of an encoded line, indicating a soft line break (see end of an encoded line, indicating a soft line break
rule #5) may follow one or more TAB (HT) or SPACE (see rule #5) may follow one or more TAB (HT) or SPACE
characters. It follows that an octet with decimal characters. It follows that an octet with decimal
value 9 or 32 appearing at the end of an encoded line value 9 or 32 appearing at the end of an encoded line
must be represented according to Rule #1. This rule is must be represented according to Rule #1. This rule is
necessary because some MTAs (Message Transport Agents, necessary because some MTAs (Message Transport Agents,
programs which transport messages from one user to programs which transport messages from one user to
another, or perform a part of such transfers) are known another, or perform a part of such transfers) are known
to pad lines of text with SPACEs, and others are known to pad lines of text with SPACEs, and others are known
to remove "white space" characters from the end of a to remove "white space" characters from the end of a
line. Therefore, when decoding a Quoted-Printable body, line. Therefore, when decoding a Quoted-Printable
any trailing white space on a line must be deleted, as body, any trailing white space on a line must be
it will necessarily have been added by intermediate deleted, as it will necessarily have been added by
transport agents. intermediate transport agents.
(4) (Line Breaks) A line break in a text body, represented (4) (Line Breaks) A line break in a text body, represented
as a CRLF sequence in the text canonical form, must be as a CRLF sequence in the text canonical form, must be
represented by a (RFC 822) line break, which is also a represented by a (RFC 822) line break, which is also a
CRLF sequence, in the Quoted-Printable encoding. Since CRLF sequence, in the Quoted-Printable encoding. Since
the canonical representation of types other than text the canonical representation of media types other than
do not generally include the representation of line text do not generally include the representation of
breaks as CRLF sequences, no hard line breaks (i.e. line breaks as CRLF sequences, no hard line breaks
line breaks that are intended to be meaningful and to (i.e. line breaks that are intended to be meaningful
be displayed to the user) should occur in the quoted- and to be displayed to the user) should occur in the
printable encoding of such types. Sequences like "=0D", quoted-printable encoding of such types. Sequences
"=0A", "=0A=0D" and "=0D=0A" will routinely appear in like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely
non-text data represented in quoted-printable, of appear in non-text data represented in quoted-
course. printable, of course.
Note that many implementations may elect to encode the Note that many implementations may elect to encode the
local representation of various content types directly, local representation of various content types directly
as described in Appendix F. In particular, this may rather than converting to canonical form first,
apply to plain text material on systems that use encoding, and then converting back to local
newline conventions other than CRLF delimiters. Such representation. In particular, this may apply to plain
an implementation is permissible, but the generation of text material on systems that use newline conventions
line breaks must be generalized to account for the case other than a CRLF terminator sequence. Such an
where alternate representations of newline sequences implementation optimization is permissible, but only
are used. when the combined canonicalization-encoding step is
equivalent to performing the three steps separately.
(5) (Soft Line Breaks) The Quoted-Printable encoding (5) (Soft Line Breaks) The Quoted-Printable encoding
REQUIRES that encoded lines be no more than 76 REQUIRES that encoded lines be no more than 76
characters long. If longer lines are to be encoded characters long. If longer lines are to be encoded
with the Quoted-Printable encoding, "soft" line breaks with the Quoted-Printable encoding, "soft" line breaks
must be used. An equal sign as the last character on a must be used. An equal sign as the last character on a
encoded line indicates such a non-significant ("soft") encoded line indicates such a non-significant ("soft")
line break in the encoded text. line break in the encoded text.
Thus if the "raw" form of the line is a single unencoded line Thus if the "raw" form of the line is a single unencoded line
skipping to change at page 28, line 35 skipping to change at page 24, line 38
for all folk to come= for all folk to come=
to the aid of their country. to the aid of their country.
This provides a mechanism with which long lines are encoded in This provides a mechanism with which long lines are encoded in
such a way as to be restored by the user agent. The 76 such a way as to be restored by the user agent. The 76
character limit does not count the trailing CRLF, but counts character limit does not count the trailing CRLF, but counts
all other characters, including any equal signs. all other characters, including any equal signs.
Since the hyphen character ("-") is represented as itself in Since the hyphen character ("-") is represented as itself in
the Quoted-Printable encoding, care must be taken, when the Quoted-Printable encoding, care must be taken, when
encapsulating a quoted-printable encoded body in a multipart encapsulating a quoted-printable encoded body inside one or
entity, to ensure that the encapsulation boundary does not more multipart entities, to ensure that the boundary delimiter
appear anywhere in the encoded body. (A good strategy is to does not appear anywhere in the encoded body. (A good
choose a boundary that includes a character sequence such as strategy is to choose a boundary that includes a character
"=_" which can never appear in a quoted-printable body. See sequence such as "=_" which can never appear in a quoted-
the definition of multipart messages later in this document.) printable body. See the definition of multipart messages
later in this document.)
NOTE: The quoted-printable encoding represents something of a NOTE: The quoted-printable encoding represents something of a
compromise between readability and reliability in transport. compromise between readability and reliability in transport.
Bodies encoded with the quoted-printable encoding will work Bodies encoded with the quoted-printable encoding will work
reliably over most mail gateways, but may not work perfectly reliably over most mail gateways, but may not work perfectly
over a few gateways, notably those involving translation into over a few gateways, notably those involving translation into
EBCDIC. A higher level of confidence is offered by the base64 EBCDIC. A higher level of confidence is offered by the base64
Content-Transfer-Encoding. A way to get reasonably reliable Content-Transfer-Encoding. A way to get reasonably reliable
transport through EBCDIC gateways is to also quote the US- transport through EBCDIC gateways is to also quote the US-
ASCII characters ASCII characters
!"#$@[\]^`{|}~ !"#$@[\]^`{|}~
according to rule #1. See Appendix B for more information. according to rule #1.
Because quoted-printable data is generally assumed to be Because quoted-printable data is generally assumed to be
line-oriented, it is to be expected that the representation of line-oriented, it is to be expected that the representation of
the breaks between the lines of quoted printable data may be the breaks between the lines of quoted printable data may be
altered in transport, in the same manner that plain text mail altered in transport, in the same manner that plain text mail
has always been altered in Internet mail when passing between has always been altered in Internet mail when passing between
systems with differing newline conventions. If such systems with differing newline conventions. If such
alterations are likely to constitute a corruption of the data, alterations are likely to constitute a corruption of the data,
it is probably more sensible to use the base64 encoding rather it is probably more sensible to use the base64 encoding rather
than the quoted-printable encoding. than the quoted-printable encoding.
skipping to change at page 29, line 32 skipping to change at page 25, line 36
quoted-printable, care must be taken to encode CR and LF quoted-printable, care must be taken to encode CR and LF
characters as "=0D" and "=0A", respectively. In particular, a characters as "=0D" and "=0A", respectively. In particular, a
CRLF sequence in binary data should be encoded as "=0D=0A". CRLF sequence in binary data should be encoded as "=0D=0A".
Otherwise, if CRLF were represented as a hard line break, it Otherwise, if CRLF were represented as a hard line break, it
might be incorrectly decoded on platforms with different line might be incorrectly decoded on platforms with different line
break conventions. break conventions.
For formalists, the syntax of quoted-printable data is For formalists, the syntax of quoted-printable data is
described by the following grammar: described by the following grammar:
quoted-printable := ([*(ptext / SPACE / TAB) ptext] quoted-printable := qp-line *(CRLF qp-line)
["="] CRLF)
; Maximum line length of 76 characters qp-line := *(qp-segment transport-padding CRLF)
; excluding 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
qp-section := [*(ptext / SPACE / TAB) ptext]
ptext := octet / safe-char ptext := octet / safe-char
safe-char := <any US-ASCII character except "=", safe-char := <any octet with decimal value of 33 through
SPACE, or TAB> 60 inclusive, and 62 through 126>
; Characters not listed as "mail-safe" in ; Characters not listed as "mail-safe" in
; Appendix B are also not recommended. ; RFC MIME-CONF are also not recommended.
octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used for characters > 127, =, ; Octet must be used for characters > 127, =,
; SPACE, or TAB, and is recommended for any ; SPACEs or TABs at the ends of lines, and is
; characters not listed in Appendix B as ; recommended for any character not listed in
; "mail-safe". ; 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 NOTE: The addition of LWSP between the elements IMPORTANT NOTE: The addition of LWSP between the elements
shown in this BNF is NOT allowed since this BNF does not shown in this BNF is NOT allowed since this BNF does not
specify a structured header field. specify a structured header field.
5.3.4. Base64 Content-Transfer-Encoding 8.8. Base64 Content-Transfer-Encoding
The Base64 Content-Transfer-Encoding is designed to represent The Base64 Content-Transfer-Encoding is designed to represent
arbitrary sequences of octets in a form that need not be arbitrary sequences of octets in a form that need not be
humanly readable. The encoding and decoding algorithms are humanly readable. The encoding and decoding algorithms are
simple, but the encoded data are consistently only about 33 simple, but the encoded data are consistently only about 33
percent larger than the unencoded data. This encoding is percent larger than the unencoded data. This encoding is
virtually identical to the one used in Privacy Enhanced Mail virtually identical to the one used in Privacy Enhanced Mail
(PEM) applications, as defined in RFC 1421 [RFC-1421]. (PEM) applications, as defined in RFC 1421 [RFC-1421].
A 65-character subset of US-ASCII is used, enabling 6 bits to A 65-character subset of US-ASCII is used, enabling 6 bits to
skipping to change at page 31, line 6 skipping to change at page 27, line 26
presumed to be ordered with the most-significant-bit first. presumed to be ordered with the most-significant-bit first.
That is, the first bit in the stream will be the high-order 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 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. 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 Each 6-bit group is used as an index into an array of 64
printable characters. The character referenced by the index printable characters. The character referenced by the index
is placed in the output string. These characters, identified is placed in the output string. These characters, identified
in Table 1, below, are selected so as to be universally in Table 1, below, are selected so as to be universally
representable, and the set excludes characters with particular representable, and the set excludes characters with particular
significance to SMTP (e.g., ".", CR, LF) and to the significance to SMTP (e.g., ".", CR, LF) and to the multipart
encapsulation boundaries defined in this document (e.g., "-"). boundary delimiters defined in this document (e.g., "-").
Table 1: The Base64 Alphabet Table 1: The Base64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z 0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0 1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1 2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2 3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3 4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4 5 F 22 W 39 n 56 4
skipping to change at page 32, line 16 skipping to change at page 28, line 36
bits; here, the final unit of encoded output will be two bits; here, the final unit of encoded output will be two
characters followed by two "=" padding characters, or (3) the characters followed by two "=" padding characters, or (3) the
final quantum of encoding input is exactly 16 bits; here, the final quantum of encoding input is exactly 16 bits; here, the
final unit of encoded output will be three characters followed final unit of encoded output will be three characters followed
by one "=" padding character. by one "=" padding character.
Because it is used only for padding at the end of the data, Because it is used only for padding at the end of the data,
the occurrence of any "=" characters may be taken as evidence the occurrence of any "=" characters may be taken as evidence
that the end of the data has been reached (without truncation that the end of the data has been reached (without truncation
in transit). No such assurance is possible, however, when the in transit). No such assurance is possible, however, when the
number of octets transmitted was a multiple of three. number of octets transmitted was a multiple of three and no
"=" characters are present.
Any characters outside of the base64 alphabet are to be Any characters outside of the base64 alphabet are to be
ignored in base64-encoded data. The same applies to any ignored in base64-encoded data.
invalid sequence of characters in the base64 encoding, such as
"====="
Care must be taken to use the proper octets for line breaks if Care must be taken to use the proper octets for line breaks if
base64 encoding is applied directly to text material that has base64 encoding is applied directly to text material that has
not been converted to canonical form. In particular, text not been converted to canonical form. In particular, text
line breaks must be converted into CRLF sequences prior to line breaks must be converted into CRLF sequences prior to
base64 encoding. The important thing to note is that this may base64 encoding. The important thing to note is that this may
be done directly by the encoder rather than in a prior be done directly by the encoder rather than in a prior
canonicalization step in some implementations. canonicalization step in some implementations.
NOTE: There is no need to worry about quoting apparent NOTE: There is no need to worry about quoting potential
encapsulation boundaries within base64-encoded parts of boundary delimiters within base64-encoded parts of multipart
multipart entities because no hyphen characters are used in entities because no hyphen characters are used in the base64
the base64 encoding. encoding.
5.4. Content-ID Header Field 9. Content-ID Header Field
In constructing a high-level user agent, it may be desirable In constructing a high-level user agent, it may be desirable
to allow one body to make reference to another. Accordingly, to allow one body to make reference to another. Accordingly,
bodies may be labelled using the "Content-ID" header field, bodies may be labelled using the "Content-ID" header field,
which is syntactically identical to the "Message-ID" header which is syntactically identical to the "Message-ID" header
field: field:
id := "Content-ID" ":" msg-id id := "Content-ID" ":" msg-id
Like the Message-ID values, Content-ID values must be Like the Message-ID values, Content-ID values must be
generated to be world-unique. generated to be world-unique.
The Content-ID value may be used for uniquely identifying MIME The Content-ID value may be used for uniquely identifying MIME
entities in several contexts, particularly for caching data entities in several contexts, particularly for caching data
referenced by the message/external-body mechanism. Although referenced by the message/external-body mechanism. Although
the Content-ID header is generally optional, its use is the Content-ID header is generally optional, its use is
MANDATORY in implementations which generate data of the MANDATORY in implementations which generate data of the
optional MIME Content-type "message/external-body". That is, optional MIME media type "message/external-body". That is,
each message/external-body entity must have a Content-ID field each message/external-body entity must have a Content-ID field
to permit caching of such data. to permit caching of such data.
It is also worth noting that the Content-ID value has special It is also worth noting that the Content-ID value has special
semantics in the case of the multipart/alternative content- semantics in the case of the multipart/alternative media type.
type. This is explained in the section of this document This is explained in the section of this document dealing with
dealing with multipart/alternative. multipart/alternative.
5.5. Content-Description Header Field 10. Content-Description Header Field
The ability to associate some descriptive information with a The ability to associate some descriptive information with a
given body is often desirable. For example, it may be useful given body is often desirable. For example, it may be useful
to mark an "image" body as "a picture of the Space Shuttle to mark an "image" body as "a picture of the Space Shuttle
Endeavor." Such text may be placed in the Content-Description Endeavor." Such text may be placed in the Content-Description
header field. This header field is always optional. header field. This header field is always optional.
description := "Content-Description" ":" *text description := "Content-Description" ":" *text
The description is presumed to be given in the US-ASCII The description is presumed to be given in the US-ASCII
character set, although the mechanism specified in RFC MIME- character set, although the mechanism specified in RFC MIME-
HEADERS [RFC-MIME-HEADERS] may be used for non-US-ASCII HEADERS [RFC-MIME-HEADERS] may be used for non-US-ASCII
Content-Description values. Content-Description values.
5.6. Additional MIME Header Fields 11. Additional MIME Header Fields
Future documents may elect to define additional MIME header Future documents may elect to define additional MIME header
fields for various purposes. Any new header field that fields for various purposes. Any new header field that
further describes the content of a message should begin with further describes the content of a message should begin with
the string "Content-" to allow such fields which appear in a the string "Content-" to allow such fields which appear in a
message header to be distinguished from ordinary RFC 822 message header to be distinguished from ordinary RFC 822
message header fields. message header fields.
MIME-extension-field := <Any RFC 822 header field which MIME-extension-field := <Any RFC 822 header field which
begins with the string begins with the string
"Content-"> "Content-">
6. Predefined Content-Type Values 12. Summary
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].
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.
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
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.
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".
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.
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
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
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
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
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.
(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".
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
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.
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
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.
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
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.
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.
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
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
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
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 ...
------ 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.
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
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
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
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,
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.
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-
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
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.
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
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
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)"
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.
6.2.2.4. Other Message Subtypes
MIME implementations must in general treat unrecognized
subtypes of message as being equivalent to
"application/octet-stream".
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.
8. Summary
Using the MIME-Version, Content-Type, and Content-Transfer- Using the MIME-Version, Content-Type, and Content-Transfer-
Encoding header fields, it is possible to include, in a Encoding header fields, it is possible to include, in a
standardized way, arbitrary types of data objects with RFC 822 standardized way, arbitrary types of data objects with RFC 822
conformant mail messages. No restrictions imposed by either conformant mail messages. No restrictions imposed by either
RFC 821 or RFC 822 are violated, and care has been taken to RFC 821 or RFC 822 are violated, and care has been taken to
avoid problems caused by additional restrictions imposed by avoid problems caused by additional restrictions imposed by
the characteristics of some Internet mail transport mechanisms the characteristics of some Internet mail transport mechanisms
(see Appendix B). The "multipart" and "message" Content-Types (see RFC MIME-CONF).
allow mixing and hierarchical structuring of objects of
different types in a single message. Further Content-Types
provide a standardized mechanism for tagging messages or body
parts as audio, image, or several other kinds of data. A
distinguished parameter syntax allows further specification of
data format details, particularly the specification of
alternate character sets. Additional optional header fields
provide mechanisms for certain extensions deemed desirable by
many implementors. Finally, a number of useful Content-Types
are defined for general use by consenting user agents, notably
message/partial, and message/external-body.
9. Security Considerations The next document in this set, RFC MIME-IMT, specifies the
media types that can be labelled and transported using these
headers.
Security issues are discussed in the context of the 13. Security Considerations
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.
10. Authors' Addresses Security issues are discussed in the second document in this
set, RFC MIME-IMT.
For more information, the authors of this document may be 14. Authors' Addresses
For more information, the authors of this document are best
contacted via Internet mail: contacted via Internet mail:
Nathaniel S. Borenstein Nathaniel S. Borenstein
First Virtual Holdings First Virtual Holdings
25 Washington Avenue 25 Washington Avenue
Morristown, NJ 07960 Morristown, NJ 07960
USA USA
Email: nsb@nsb.fv.com Email: nsb@nsb.fv.com
Phone: +1 201 540 8967 Phone: +1 201 540 8967
Fax: +1 201 993 3032 Fax: +1 201 993 3032
Ned Freed Ned Freed
Innosoft International, Inc. Innosoft International, Inc.
1050 East Garvey Avenue South 1050 East Garvey Avenue South
West Covina, CA 91790 West Covina, CA 91790
USA USA
Email: ned@innosoft.com Email: ned@innosoft.com
Phone: +1 818 919 3600 Phone: +1 818 919 3600
Fax: +1 818919 3614 Fax: +1 818 919 3614
MIME is a result of the work of the Internet Engineering Task MIME is a result of the work of the Internet Engineering Task
Force Working Group on Email Extensions. The chairman of that Force Working Group on Email Extensions. The chairman of that
group, Greg Vaudreuil, may be reached at: group, Greg Vaudreuil, may be reached at:
Gregory M. Vaudreuil Gregory M. Vaudreuil
Tigon Corporation Tigon Corporation
17060 Dallas Parkway 17060 Dallas Parkway
Dallas Texas, 75248 Dallas Texas, 75248
Email: greg.vaudreuil@ons.octel.com Email: greg.vaudreuil@ons.octel.com
Phone: +1 214 733 2722 Phone: +1 214 733 2722
11. Acknowledgements Appendix A -- Collected Grammar
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
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.
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."
-- 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.
-- 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.
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
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)
"(" (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.
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.
--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--
Appendix D -- Collected Grammar
This appendix contains the complete BNF grammar for all the This appendix contains the complete BNF grammar for all the
syntax specified by this document. syntax specified by this document.
By itself, however, this grammar is incomplete. It refers to By itself, however, this grammar is incomplete. It refers to
several entities that are defined by RFC 822. Rather than several entities that are defined by RFC 822. Rather than
reproduce those definitions here, and risk unintentional reproduce those definitions here, and risk unintentional
differences between the two, this document simply refers the differences between the two, this document simply refers the
reader to RFC 822 for the remaining definitions. Wherever a reader to RFC 822 for the remaining definitions. Wherever a
term is undefined, it refers to the RFC 822 definition. term is undefined, it refers to the RFC 822 definition.
attribute := token 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 composite-type := "message" / "multipart" / extension-token
content := "Content-Type" ":" type "/" subtype content := "Content-Type" ":" type "/" subtype
*(";" parameter) *(";" parameter)
; Matching of type and subtype is ; Matching of media type and subtype
; ALWAYS case-insensitive ; is ALWAYS case-insensitive
dash-boundary := "--" boundary
; boundary taken from Content-Type
; field.
delimiter := CRLF dash-boundary
description := "Content-Description" ":" *text description := "Content-Description" ":" *text
discard-text := *(*text CRLF)
; To be ignored upon receipt.
discrete-type := "text" / "image" / "audio" / "video" / discrete-type := "text" / "image" / "audio" / "video" /
"application" / extension-token "application" / extension-token
encapsulation := delimiter [*LWSP-char]
CRLF body-part
encoding := "Content-Transfer-Encoding" ":" mechanism encoding := "Content-Transfer-Encoding" ":" mechanism
epilogue := discard-text
extension-token := iana-token / ietf-token / x-token extension-token := iana-token / ietf-token / x-token
iana-token := <a publicly-defined extension token, iana-token := <a publicly-defined extension token,
registered with IANA, as specified in registered with IANA, as specified in
RFC REG [REF-REG]> RFC MIME-REG>
ietf-token := <a publicly-defined extension token, ietf-token := <a publicly-defined extension token,
initially registered with IANA and initially registered with IANA and
subsequently standardized by the IETF> subsequently standardized by the IETF>
id := "Content-ID" ":" msg-id id := "Content-ID" ":" msg-id
mechanism := "7bit" / "8bit" / "binary" / mechanism := "7bit" / "8bit" / "binary" /
"quoted-printable" / "base64" / "quoted-printable" / "base64" /
ietf-token / x-token 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 := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
; Octet must be used for characters > 127, =, ; Octet must be used for characters > 127, =,
; SPACE, or TAB, and is recommended for any ; SPACEs or TABs at the ends of lines, and is
; characters not listed in Appendix B as ; recommended for any character not listed in
; "mail-safe". ; RFC MIME-CONF as "mail-safe".
parameter := attribute "=" value parameter := attribute "=" value
preamble := discard-text
ptext := octet / safe-char ptext := octet / safe-char
quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF) qp-line := *(qp-segment transport-padding CRLF)
; Maximum line length of 76 characters qp-part transport-padding
; excluding CRLF
safe-char := <any US-ASCII character except "=", qp-part := qp-section
SPACE, or TAB> ; 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 ; Characters not listed as "mail-safe" in
; Appendix B are also not recommended. ; RFC MIME-CONF are also not recommended.
subtype := extension-token subtype := extension-token
token := 1*<any (US-ASCII) CHAR except SPACE, CTLs, token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
or tspecials> 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 := "(" / ")" / "<" / ">" / "@" / tspecials := "(" / ")" / "<" / ">" / "@" /
"," / ";" / ":" / "\" / <"> "," / ";" / ":" / "\" / <">
"/" / "[" / "]" / "?" / "=" "/" / "[" / "]" / "?" / "="
; Must be in quoted-string, ; Must be in quoted-string,
; to use within parameter values ; to use within parameter values
type := discrete-type / composite-type type := discrete-type / composite-type
value := token / quoted-string value := token / quoted-string
version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
x-token := <The two characters "X-" or "x-" followed, with x-token := <The two characters "X-" or "x-" followed, with
no intervening white space, by any token> no intervening white space, by any token>
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
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
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".
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
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.
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.
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.
(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.
(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.
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.
[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.
[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.
[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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