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Network Working Group N. Freed

Request for Comments: 2045 Innosoft

Obsoletes: 1521, 1522, 1590 N. Borenstein

Category: Standards Track First Virtual

November 1996

Multipurpose Internet Mail Extensions

(MIME) Part One:

Format of Internet Message Bodies

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

STD 11, RFC 822, defines a message representation protocol specifying

considerable detail about US-ASCII message headers, and leaves the

message content, or message body, as flat US-ASCII text. This set of

documents, collectively called the Multipurpose Internet Mail

Extensions, or MIME, redefines the format of messages to allow for

(1) textual message bodies in character sets other than

US-ASCII,

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

message bodies,

(3) multi-part message bodies, and

(4) textual header information in character sets other than

US-ASCII.

These documents are based on earlier work documented in RFC 934, STD

11, and RFC 1049, but extends and revises them. Because RFC 822 said

so little about message bodies, these documents are largely

orthogonal to (rather than a revision of) RFC 822.

This initial document specifies the various headers used to describe

the structure of MIME messages. The second document, RFC 2046,

defines the general structure of the MIME media typing system and

defines an initial set of media types. The third document, RFC 2047,

describes extensions to RFC 822 to allow non-US-ASCII text data in

Freed & Borenstein Standards Track [Page 1]

RFC 2045 Internet Message Bodies November 1996

Internet mail header fields. The fourth document, RFC 2048, specifies

various IANA registration procedures for MIME-related facilities. The

fifth and final document, RFC 2049, describes MIME conformance

criteria as well as providing some illustrative examples of MIME

message formats, acknowledgements, and the bibliography.

These documents are revisions of RFCs 1521, 1522, and 1590, which

themselves were revisions of RFCs 1341 and 1342. An appendix in RFC

2049 describes differences and changes from previous versions.

Table of Contents

1. Introduction ......................................... 3

2. Definitions, Conventions, and Generic BNF Grammar .... 5

2.1 CRLF ................................................ 5

2.2 Character Set ....................................... 6

2.3 Message ............................................. 6

2.4 Entity .............................................. 6

2.5 Body Part ........................................... 7

2.6 Body ................................................ 7

2.7 7bit Data ........................................... 7

2.8 8bit Data ........................................... 7

2.9 Binary Data ......................................... 7

2.10 Lines .............................................. 7

3. MIME Header Fields ................................... 8

4. MIME-Version Header Field ............................ 8

5. Content-Type Header Field ............................ 10

5.1 Syntax of the Content-Type Header Field ............. 12

5.2 Content-Type Defaults ............................... 14

6. Content-Transfer-Encoding Header Field ............... 14

6.1 Content-Transfer-Encoding Syntax .................... 14

6.2 Content-Transfer-Encodings Semantics ................ 15

6.3 New Content-Transfer-Encodings ...................... 16

6.4 Interpretation and Use .............................. 16

6.5 Translating Encodings ............................... 18

6.6 Canonical Encoding Model ............................ 19

6.7 Quoted-Printable Content-Transfer-Encoding .......... 19

6.8 Base64 Content-Transfer-Encoding .................... 24

7. Content-ID Header Field .............................. 26

8. Content-Description Header Field ..................... 27

9. Additional MIME Header Fields ........................ 27

10. Summary ............................................. 27

11. Security Considerations ............................. 27

12. Authors’ Addresses .................................. 28

A. Collected Grammar .................................... 29

Freed & Borenstein Standards Track [Page 2]

RFC 2045 Internet Message Bodies November 1996

1. Introduction

Since its publication in 1982, RFC 822 has defined the standard

format of textual mail messages on the Internet. Its success has

been such that the RFC 822 format has been adopted, wholly or

partially, well beyond the confines of the Internet and the Internet

SMTP transport defined by RFC 821. As the format has seen wider use,

a number of limitations have proven increasingly restrictive for the

user community.

RFC 822 was intended to specify a format for text messages. As such,

non-text messages, such as multimedia messages that might include

audio or images, are simply not mentioned. Even in the case of text,

however, RFC 822 is inadequate for the needs of mail users whose

languages require the use of character sets richer than US-ASCII.

Since RFC 822 does not specify mechanisms for mail containing audio,

video, Asian language text, or even text in most European languages,

additional specifications are needed.

One of the notable limitations of RFC 821/822 based mail systems is

the fact that they limit the contents of electronic mail messages to

relatively short lines (e.g. 1000 characters or less [RFC-821]) of

7bit US-ASCII. This forces users to convert any non-textual data

that they may wish to send into seven-bit bytes representable as

printable US-ASCII characters before invoking a local mail UA (User

Agent, a program with which human users send and receive mail).

Examples of such encodings currently used in the Internet include

pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in

RFC 1421, the Andrew Toolkit Representation [ATK], and many others.

The limitations of RFC 822 mail become even more apparent as gateways

are designed to allow for the exchange of mail messages between RFC

822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the

inclusion of non-textual material within electronic mail messages.

The current standards for the mapping of X.400 messages to RFC 822

messages specify either that X.400 non-textual material must be

converted to (not encoded in) IA5Text format, or that they must be

discarded, notifying the RFC 822 user that discarding has occurred.

This is clearly undesirable, as information that a user may wish to

receive is lost. Even though a user agent may not have the

capability of dealing with the non-textual material, the user might

have some mechanism external to the UA that can extract useful

information from the material. Moreover, it does not allow for the

fact that the message may eventually be gatewayed back into an X.400

message handling system (i.e., the X.400 message is "tunneled"

through Internet mail), where the non-textual information would

definitely become useful again.

Freed & Borenstein Standards Track [Page 3]

RFC 2045 Internet Message Bodies November 1996

This document describes several mechanisms that combine to solve most

of these problems without introducing any serious incompatibilities

with the existing world of RFC 822 mail. In particular, it

describes:

(1) A MIME-Version header field, which uses a version

number to declare a message to be conformant with MIME

and allows mail processing agents to distinguish

between such messages and those generated by older or

non-conformant software, which are presumed to lack

such a field.

(2) A Content-Type header field, generalized from RFC 1049,

which can be used to specify the media type and subtype

of data in the body of a message and to fully specify

the native representation (canonical form) of such

data.

(3) A Content-Transfer-Encoding header field, which can be

used to specify both the encoding transformation that

was applied to the body and the domain of the result.

Encoding transformations other than the identity

transformation are usually applied to data in order to

allow it to pass through mail transport mechanisms

which may have data or character set limitations.

(4) Two additional header fields that can be used to

further describe the data in a body, the Content-ID and

Content-Description header fields.

All of the header fields defined in this document are subject to the

general syntactic rules for header fields specified in RFC 822. In

particular, all of these header fields except for Content-Disposition

can include RFC 822 comments, which have no semantic content and

should be ignored during MIME processing.

Finally, to specify and promote interoperability, RFC 2049 provides a

basic applicability statement for a subset of the above mechanisms

that defines a minimal level of "conformance" with this document.

HISTORICAL NOTE: Several of the mechanisms described in this set of

documents may seem somewhat strange or even baroque at first reading.

It is important to note that compatibility with existing standards

AND robustness across existing practice were two of the highest

priorities of the working group that developed this set of documents.

In particular, compatibility was always favored over elegance.

Freed & Borenstein Standards Track [Page 4]

RFC 2045 Internet Message Bodies November 1996

Please refer to the current edition of the "Internet Official

Protocol Standards" for the standardization state and status of this

protocol. RFC 822 and STD 3, RFC 1123 also provide essential

background for MIME since no conforming implementation of MIME can

violate them. In addition, several other informational RFC documents

will be of interest to the MIME implementor, in particular RFC 1344,

RFC 1345, and RFC 1524.

2. Definitions, Conventions, and Generic BNF Grammar

Although the mechanisms specified in this set of documents are all

described in prose, most are also described formally in the augmented

BNF notation of RFC 822. Implementors will need to be familiar with

this notation in order to understand this set of documents, and are

referred to RFC 822 for a complete explanation of the augmented BNF

notation.

Some of the augmented BNF in this set of documents makes named

references to syntax rules defined in RFC 822. A complete formal

grammar, then, is obtained by combining the collected grammar

appendices in each document in this set with the BNF of RFC 822 plus

the modifications to RFC 822 defined in RFC 1123 (which specifically

changes the syntax for ‘return’, ‘date’ and ‘mailbox’).

All numeric and octet values are given in decimal notation in this

set of documents. All media type values, subtype values, and

parameter names as defined are case-insensitive. However, parameter

values are case-sensitive unless otherwise specified for the specific

parameter.

FORMATTING NOTE: Notes, such at this one, provide additional

nonessential information which may be skipped by the reader without

missing anything essential. The primary purpose of these non-

essential notes is to convey information about the rationale of this

set of documents, or to place these documents in the proper

historical or evolutionary context. Such information may in

particular be skipped by those who are focused entirely on building a

conformant implementation, but may be of use to those who wish to

understand why certain design choices were made.

2.1. CRLF

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

octets corresponding to the two US-ASCII characters CR (decimal value

13) and LF (decimal value 10) which, taken together, in this order,

denote a line break in RFC 822 mail.

Freed & Borenstein Standards Track [Page 5]

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