Use it as reference for URL syntax. Take a print out.-Javed



Network Working Group N. Borenstein

Request for Comments: 1521 Bellcore

Obsoletes: 1341 N. Freed

Category: Standards Track Innosoft

September 1993



MIME (Multipurpose Internet Mail Extensions) Part One:

Mechanisms for Specifying and Describing

the Format of Internet Message Bodies


Status of this Memo


This RFC 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" for the standardization state and status

of this protocol. Distribution of this memo is unlimited.




STD 11, RFC 822 defines a message representation protocol which

specifies considerable detail about message headers, but which leaves

the message content, or message body, as flat ASCII text. This

document redefines the format of message bodies to allow multi-part

textual and non-textual message bodies to be represented and

exchanged without loss of information. This is based on earlier work

documented in RFC 934 and STD 11, RFC 1049, but extends and revises

that work. Because RFC 822 said so little about message bodies, this

document is largely orthogonal to (rather than a revision of) RFC



In particular, this document is designed to provide facilities to

include multiple objects in a single message, to represent body text

in character sets other than US-ASCII, to represent formatted multi-

font text messages, to represent non-textual material such as images

and audio fragments, and generally to facilitate later extensions

defining new types of Internet mail for use by cooperating mail



This document does NOT extend Internet mail header fields to permit

anything other than US-ASCII text data. Such extensions are the

subject of a companion document [RFC-1522].


This document is a revision of RFC 1341. Significant differences

from RFC 1341 are summarized in Appendix H.






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Table of Contents


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

2. Notations, Conventions, and Generic BNF Grammar.... 6

3. The MIME-Version Header Field...................... 7

4. The Content-Type Header Field...................... 9

5. The Content-Transfer-Encoding Header Field......... 13

5.1. Quoted-Printable Content-Transfer-Encoding......... 18

5.2. Base64 Content-Transfer-Encoding................... 21

6. Additional Content-Header Fields................... 23

6.1. Optional Content-ID Header Field................... 23

6.2. Optional Content-Description Header Field.......... 24

7. The Predefined Content-Type Values................. 24

7.1. The Text Content-Type.............................. 24

7.1.1. The charset parameter.............................. 25

7.1.2. The Text/plain subtype............................. 28

7.2. The Multipart Content-Type......................... 28

7.2.1. Multipart: The common syntax...................... 29

7.2.2. The Multipart/mixed (primary) subtype.............. 34

7.2.3. The Multipart/alternative subtype.................. 34

7.2.4. The Multipart/digest subtype....................... 36

7.2.5. The Multipart/parallel subtype..................... 37

7.2.6. Other Multipart subtypes........................... 37

7.3. The Message Content-Type........................... 38

7.3.1. The Message/rfc822 (primary) subtype............... 38

7.3.2. The Message/Partial subtype........................ 39

7.3.3. The Message/External-Body subtype.................. 42 The "ftp" and "tftp" access-types............... 44 The "anon-ftp" access-type...................... 45 The "local-file" and "afs" access-types......... 45 The "mail-server" access-type................... 45 Examples and Further Explanations............... 46

7.4. The Application Content-Type....................... 49

7.4.1. The Application/Octet-Stream (primary) subtype..... 50

7.4.2. The Application/PostScript subtype................. 50

7.4.3. Other Application subtypes......................... 53

7.5. The Image Content-Type............................. 53

7.6. The Audio Content-Type............................. 54

7.7. The Video Content-Type............................. 54

7.8. Experimental Content-Type Values................... 54

8. Summary............................................ 56

9. Security Considerations............................ 56

10. Authors' Addresses................................. 57

11. Acknowledgements................................... 58

Appendix A -- Minimal MIME-Conformance.................... 60

Appendix B -- General Guidelines For Sending Email Data... 63

Appendix C -- A Complex Multipart Example................. 66

Appendix D -- Collected Grammar........................... 68




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Appendix E -- IANA Registration Procedures................ 72

E.1 Registration of New Content-type/subtype Values...... 72

E.2 Registration of New Access-type Values

for Message/external-body............................ 73

Appendix F -- Summary of the Seven Content-types.......... 74

Appendix G -- Canonical Encoding Model.................... 76

Appendix H -- Changes from RFC 1341....................... 78

References................................................ 80


1. Introduction


Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined

the standard format of textual mail messages on the Internet. Its

success has been such that the RFC 822 format has been adopted,

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

Internet SMTP transport defined by STD 10, RFC 821 [RFC-821]. As the

format has seen wider use, a number of limitations have proven

increasingly restrictive for the user community.


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

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

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

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

languages require the use of character sets richer than US ASCII

[US-ASCII]. Since RFC 822 does not specify mechanisms for mail

containing audio, video, Asian language text, or even text in most

European languages, additional specifications are needed.


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

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

relatively short lines of seven-bit ASCII. This forces users to

convert any non-textual data that they may wish to send into seven-

bit bytes representable as printable ASCII characters before invoking

a local mail UA (User Agent, a program with which human users send

and receive mail). Examples of such encodings currently used in the

Internet include pure hexadecimal, uuencode, the 3-in-4 base 64

scheme specified in RFC 1421, the Andrew Toolkit Representation

[ATK], and many others.


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

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

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

inclusion of non-textual body parts within electronic mail messages.

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

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

converted to (not encoded in) an ASCII 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




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receive is lost. Even though a user's UA may not have the capability

of dealing with the non-textual body part, the user might have some

mechanism external to the UA that can extract useful information from

the body part. Moreover, it does not allow for the fact that the

message may eventually be gatewayed back into an X.400 message

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

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

become useful again.


This document describes several mechanisms that combine to solve most

of these problems without introducing any serious incompatibilities

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



1. A MIME-Version header field, which uses a version number to

declare a message to be conformant with this specification and

allows mail processing agents to distinguish between such

messages and those generated by older or non-conformant software,

which is presumed to lack such a field.


2. A Content-Type header field, generalized from RFC 1049 [RFC-1049],

which can be used to specify the type and subtype of data in the

body of a message and to fully specify the native representation

(encoding) of such data.


2.a. 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



2.b. 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.c. 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



2.d. A "message" Content-Type value, for encapsulating another

mail message.


2.e An "image" Content-Type value, for transmitting still image

(picture) data.


2.f. An "audio" Content-Type value, for transmitting audio or

voice data.





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2.g. A "video" Content-Type value, for transmitting video or

moving image data, possibly with audio as part of the

composite video data format.


3. A Content-Transfer-Encoding header field, which can be used to

specify an auxiliary encoding that was applied to the data in

order to allow it to pass through mail transport mechanisms which

may have data or character set limitations.


4. Two additional header fields that can be used to further describe

the data in a message body, the Content-ID and Content-

Description header fields.


MIME has been carefully designed as an extensible mechanism, and it

is expected that the set of content-type/subtype pairs and their

associated parameters will grow significantly with time. Several

other MIME fields, notably including character set names, are likely

to have new values defined over time. In order to ensure that the

set of such values is developed in an orderly, well-specified, and

public manner, MIME defines a registration process which uses the

Internet Assigned Numbers Authority (IANA) as a central registry for

such values. Appendix E provides details about how IANA registration

is accomplished.


Finally, to specify and promote interoperability, Appendix A of this

document provides a basic applicability statement for a subset of the

above mechanisms that defines a minimal level of "conformance" with

this document.


HISTORICAL NOTE: Several of the mechanisms described in this

document may seem somewhat strange or even baroque at first

reading. It is important to note that compatibility with existing

standards AND robustness across existing practice were two of the

highest priorities of the working group that developed this

document. In particular, compatibility was always favored over



MIME was first defined and published as RFCs 1341 and 1342 [RFC-1341]

[RFC-1342]. This document is a relatively minor updating of RFC

1341, and is intended to supersede it. The differences between this

document and RFC 1341 are summarized in Appendix H. Please refer to

the current edition of the "IAB Official Protocol Standards" for the

standardization state and status of this protocol. Several other RFC

documents will be of interest to the MIME implementor, in particular

[RFC 1343], [RFC-1344], and [RFC-1345].







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2. Notations, Conventions, and Generic BNF Grammar


This document is being published in two versions, one as plain ASCII

text and one as PostScript (PostScript is a trademark of Adobe

Systems Incorporated.). While the text version is the official

specification, some will find the PostScript version easier to read.

The textual contents are identical. An Andrew-format copy of this

document is also available from the first author (Borenstein).


Although the mechanisms specified in this document are all described

in prose, most are also described formally in the modified BNF

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

notation in order to understand this specification, and are referred

to RFC 822 for a complete explanation of the modified BNF notation.


Some of the modified BNF in this document makes reference to

syntactic entities that are defined in RFC 822 and not in this

document. A complete formal grammar, then, is obtained by combining

the collected grammar appendix of this document with that of RFC 822

plus the modifications to RFC 822 defined in RFC 1123, which

specifically changes the syntax for `return', `date' and `mailbox'.


The term CRLF, in this document, refers to the sequence of the two

ASCII characters CR (13) and LF (10) which, taken together, in this

order, denote a line break in RFC 822 mail.


The term "character set" is used in this document to refer to a

method used with one or more tables to convert encoded text to a

series of octets. This definition is intended to allow various kinds

of text encodings, from simple single-table mappings such as ASCII to

complex table switching methods such as those that use ISO 2022's

techniques. However, a MIME character set name must fully specify

the mapping to be performed.


The term "message", when not further qualified, means either the

(complete or "top-level") message being transferred on a network, or

a message encapsulated in a body of type "message".


The term "body part", in this document, means one of the parts of the

body of a multipart entity. A body part has a header and a body, so

it makes sense to speak about the body of a body part.


The term "entity", in this document, means either a message or a body

part. All kinds of entities share the property that they have a

header and a body.


The term "body", when not further qualified, means the body of an

entity, that is the body of either a message or of a body part.




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NOTE: The previous four definitions are clearly circular. This is

unavoidable, since the overall structure of a MIME message is

indeed recursive.


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



It must be noted that Content-Type values, subtypes, and parameter

names as defined in this document are case-insensitive. However,

parameter values are case-sensitive unless otherwise specified for

the specific parameter.


FORMATTING NOTE: This document has been carefully formatted for

ease of reading. The PostScript version of this document, in

particular, places notes like this one, which may be skipped by

the reader, in a smaller, italicized, font, and indents it as

well. In the text version, only the indentation is preserved, so

if you are reading the text version of this you might consider

using the PostScript version instead. However, all such notes will

be indented and preceded by "NOTE:" or some similar introduction,

even in the text version.


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 be skipped by those who are focused entirely on

building a conformant implementation, but may be of use to those

who wish to understand why this document is written as it is.


For ease of recognition, all BNF definitions have been placed in a

fixed-width font in the PostScript version of this document.


3. The MIME-Version Header Field


Since RFC 822 was published in 1982, there has really been only one

format standard for Internet messages, and there has been little

perceived need to declare the format standard in use. This document

is an independent document that complements RFC 822. Although the

extensions in this document have been defined in such a way as to be

compatible with RFC 822, there are still circumstances in which it

might be desirable for a mail-processing agent to know whether a

message was composed with the new standard in mind.


Therefore, this document defines a new header field, "MIME-Version",

which is to be used to declare the version of the Internet message

body format standard in use.


Messages composed in accordance with this document MUST include such




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a header field, with the following verbatim text:


MIME-Version: 1.0


The presence of this header field is an assertion that the message

has been composed in compliance with this document.


Since it is possible that a future document might extend the message

format standard again, a formal BNF is given for the content of the

MIME-Version field:


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


Thus, future format specifiers, which might replace or extend "1.0",

are constrained to be two integer fields, separated by a period. If

a message is received with a MIME-version value other than "1.0", it

cannot be assumed to conform with this specification.


Note that the MIME-Version header field is required at the top level

of a message. It is not required for each body part of a multipart

entity. It is required for the embedded headers of a body of type

"message" if and only if the embedded message is itself claimed to be



It is not possible to fully specify how a mail reader that conforms

with MIME as defined in this document should treat a message that

might arrive in the future with some value of MIME-Version other than

"1.0". However, conformant software is encouraged to check the

version number and at least warn the user if an unrecognized MIME-

version is encountered.


It is also worth noting that version control for specific content-

types is not accomplished using the MIME-Version mechanism. In

particular, some formats (such as application/postscript) have

version numbering conventions that are internal to the document

format. Where such conventions exist, MIME does nothing to supersede

them. Where no such conventions exist, a MIME type might use a

"version" parameter in the content-type field if necessary.


NOTE TO IMPLEMENTORS: All header fields defined in this document,

including MIME-Version, Content-type, etc., are subject to the

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

particular, all can include comments, which means that the following

two MIME-Version fields are equivalent:


MIME-Version: 1.0

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





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4. The Content-Type Header Field


The purpose of the Content-Type field is to describe the data

contained in the body fully enough that the receiving user agent can

pick an appropriate agent or mechanism to present the data to the

user, or otherwise deal with the data in an appropriate manner.


HISTORICAL NOTE: The Content-Type header field was first defined in

RFC 1049. RFC 1049 Content-types used a simpler and less powerful

syntax, but one that is largely compatible with the mechanism given



The Content-Type header field is used to specify the nature of the

data in the body of an entity, by giving type and subtype

identifiers, and by providing auxiliary information that may be

required for certain types. After the type and subtype names, the

remainder of the header field is simply a set of parameters,

specified in an attribute/value notation. The set of meaningful

parameters differs for the different types. In particular, there are

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

Global mechanisms are best addressed, in the MIME model, by the

definition of additional Content-* header fields. The ordering of

parameters is not significant. Among the defined parameters is a

"charset" parameter by which the character set used in the body may

be declared. Comments are allowed in accordance with RFC 822 rules

for structured header fields.


In general, the top-level Content-Type is used to declare the general

type of data, while the subtype specifies a specific format for that

type of data. Thus, a Content-Type of "image/xyz" is enough to tell

a user agent that the data is an image, even if the user agent has no

knowledge of the specific image format "xyz". Such information can

be used, for example, to decide whether or not to show a user the raw

data from an unrecognized subtype -- such an action might be

reasonable for unrecognized subtypes of text, but not for

unrecognized subtypes of image or audio. For this reason, registered

subtypes of audio, image, text, and video, should not contain

embedded information that is really of a different type. Such

compound types should be represented using the "multipart" or

"application" types.


Parameters are modifiers of the content-subtype, and do not

fundamentally affect the requirements of the host system. Although

most parameters make sense only with certain content-types, others

are "global" in the sense that they might apply to any subtype. For

example, the "boundary" parameter makes sense only for the

"multipart" content-type, but the "charset" parameter might make

sense with several content-types.




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An initial set of seven Content-Types is defined by this document.

This set of top-level names is intended to be substantially complete.

It is expected that additions to the larger set of supported types

can generally be accomplished by the creation of new subtypes of

these initial types. In the future, more top-level types may be

defined only by an extension to this standard. If another primary

type is to be used for any reason, it must be given a name starting

with "X-" to indicate its non-standard status and to avoid a

potential conflict with a future official name.


In the Augmented BNF notation of RFC 822, a Content-Type header field

value is defined as follows:


content := "Content-Type" ":" type "/" subtype *(";"


; case-insensitive matching of type and subtype


type := "application" / "audio"

/ "image" / "message"

/ "multipart" / "text"

/ "video" / extension-token

; All values case-insensitive


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


iana-token := <a publicly-defined extension token,

registered with IANA, as specified in

appendix E>


x-token := <The two characters "X-" or "x-" followed, with

no intervening white space, by any token>


subtype := token ; case-insensitive


parameter := attribute "=" value


attribute := token ; case-insensitive


value := token / quoted-string


token := 1*<any (ASCII) CHAR except SPACE, CTLs,

or tspecials>


tspecials := "(" / ")" / "<" / ">" / "@"

/ "," / ";" / ":" / "\" / <">

/ "/" / "[" / "]" / "?" / "="

; Must be in quoted-string,

; to use within parameter values




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Note that the definition of "tspecials" is the same as the RFC 822

definition of "specials" with the addition of the three characters

"/", "?", and "=", and the removal of ".".


Note also that a subtype specification is MANDATORY. There are no

default subtypes.


The type, subtype, and parameter names are not case sensitive. For

example, TEXT, Text, and TeXt are all equivalent. Parameter values

are normally case sensitive, but certain parameters are interpreted

to be case-insensitive, depending on the intended use. (For example,

multipart boundaries are case-sensitive, but the "access-type" for

message/External-body is not case-sensitive.)


Beyond this syntax, the only constraint on the definition of subtype

names is the desire that their uses must not conflict. That is, it

would be undesirable to have two different communities using

"Content-Type: application/foobar" to mean two different things. The

process of defining new content-subtypes, then, is not intended to be

a mechanism for imposing restrictions, but simply a mechanism for

publicizing the usages. There are, therefore, two acceptable

mechanisms for defining new Content-Type subtypes:


1. Private values (starting with "X-") may be

defined bilaterally between two cooperating

agents without outside registration or



2. New standard values must be documented,

registered with, and approved by IANA, as

described in Appendix E. Where intended for

public use, the formats they refer to must

also be defined by a published specification,

and possibly offered for standardization.


The seven standard initial predefined Content-Types are detailed in

the bulk of this document. They are:


text -- textual information. The primary subtype,

"plain", indicates plain (unformatted) text. No

special software is required to get the full

meaning of the text, aside from support for the

indicated character set. Subtypes are to be used

for enriched text in forms where application

software may enhance the appearance of the text,

but such software must not be required in order to

get the general idea of the content. Possible

subtypes thus include any readable word processor




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format. A very simple and portable subtype,

richtext, was defined in RFC 1341, with a future

revision expected.


multipart -- data consisting of multiple parts of

independent data types. Four initial subtypes

are defined, including the primary "mixed"

subtype, "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".


message -- an encapsulated message. A body of

Content-Type "message" is itself all or part of a

fully formatted RFC 822 conformant message which

may contain its own different Content-Type header

field. The primary subtype is "rfc822". The

"partial" subtype is defined for partial messages,

to permit the fragmented transmission of bodies

that are thought to be too large to be passed

through mail transport facilities. Another

subtype, "External-body", is defined for

specifying large bodies by reference to an

external data source.


image -- image data. Image requires a display device

(such as a graphical display, a printer, or a FAX

machine) to view the information. Initial

subtypes are defined for two widely-used image

formats, jpeg and gif.


audio -- audio data, with initial subtype "basic".

Audio requires an audio output device (such as a

speaker or a telephone) to "display" the contents.


video -- video data. Video requires the capability to

display moving images, typically including

specialized hardware and software. The initial

subtype is "mpeg".


application -- some other kind of data, typically

either uninterpreted binary data or information to

be processed by a mail-based application. The

primary subtype, "octet-stream", 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.




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An additional subtype, "PostScript", is defined

for transporting PostScript documents in bodies.

Other expected uses for "application" include

spreadsheets, data for mail-based scheduling

systems, and languages for "active"

(computational) email. (Note that active email

and other application data may entail several

security considerations, which are discussed later

in this memo, particularly in the context of



Default RFC 822 messages are typed by this protocol as plain text in

the US-ASCII character set, which can be explicitly specified as

"Content-type: text/plain; charset=us-ascii". If no Content-Type is

specified, this default is assumed. In the presence of a MIME-

Version header field, a receiving User Agent can also assume that

plain US-ASCII text was the sender's intent. In the absence of a

MIME-Version specification, plain US-ASCII text must still be

assumed, but the sender's intent might have been otherwise.


RATIONALE: In the absence of any Content-Type header field or

MIME-Version header field, it is impossible to be certain that a

message is actually text in the US-ASCII character set, since it

might well be a message that, using the 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



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. The Content-Transfer-Encoding Header Field


Many Content-Types which could usefully be transported via email are

represented, in their "natural" format, as 8-bit character or binary

data. Such data cannot be transmitted over some transport protocols.

For example, RFC 821 restricts mail messages to 7-bit US-ASCII data

with lines no longer than 1000 characters.




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It is necessary, therefore, to define a standard mechanism for re-

encoding such data into a 7-bit short-line format. This document

specifies that such encodings will be indicated by a new "Content-

Transfer-Encoding" header field. The Content-Transfer-Encoding field

is used to indicate the type of transformation that has been used in

order to represent the body in an acceptable manner for transport.


Unlike Content-Types, a proliferation of Content-Transfer-Encoding

values is undesirable and unnecessary. However, establishing only a

single Content-Transfer-Encoding mechanism does not seem possible.

There is a tradeoff between the desire for a compact and efficient

encoding of largely-binary data and the desire for a readable

encoding of data that is mostly, but not entirely, 7-bit data. For

this reason, at least two encoding mechanisms are necessary: a

"readable" encoding and a "dense" encoding.


The Content-Transfer-Encoding field is designed to specify an

invertible mapping between the "native" representation of a type of

data and a representation that can be readily exchanged using 7 bit

mail transport protocols, such as those defined by RFC 821 (SMTP).

This field has not been defined by any previous standard. The field's

value is a single token specifying the type of encoding, as

enumerated below. Formally:


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


mechanism := "7bit" ; case-insensitive

/ "quoted-printable"

/ "base64"

/ "8bit"

/ "binary"

/ x-token


These values are not case sensitive. That is, Base64 and BASE64 and

bAsE64 are all equivalent. An encoding type of 7BIT requires that

the body is already in a seven-bit mail-ready representation. This

is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is

assumed if the Content-Transfer-Encoding header field is not present.


The values "8bit", "7bit", and "binary" all mean that NO encoding has

been performed. However, they are potentially useful as indications

of the kind of data contained in the object, and therefore of the

kind of encoding that might need to be performed for transmission in

a given transport system. In particular:


"7bit" means that the data is all represented as short

lines of US-ASCII data.





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"8bit" means that the lines are short, but there may be

non-ASCII characters (octets with the high-order

bit set).


"Binary" means that not only may non-ASCII characters

be present, but also that the lines are not

necessarily short enough for SMTP transport.


The difference between "8bit" (or any other conceivable bit-width

token) and the "binary" token is that "binary" does not require

adherence to any limits on line length or to the SMTP CRLF semantics,

while the bit-width tokens do require such adherence. If the body

contains data in any bit-width other than 7-bit, the appropriate

bit-width Content-Transfer-Encoding token must be used (e.g., "8bit"

for unencoded 8 bit wide data). If the body contains binary data,

the "binary" Content-Transfer-Encoding token must be used.


NOTE: The distinction between the Content-Transfer-Encoding values

of "binary", "8bit", etc. may seem unimportant, in that all of

them really mean "none" -- that is, there has been no encoding of

the data for transport. However, clear labeling will be of

enormous value to gateways between future mail transport systems

with differing capabilities in transporting data that do not meet

the restrictions of RFC 821 transport.


Mail transport for unencoded 8-bit data is defined in RFC-1426

[RFC-1426]. As of the publication of this document, there are no

standardized Internet mail transports for which it is legitimate

to include unencoded binary data in mail bodies. Thus there are

no circumstances in which the "binary" Content-Transfer-Encoding

is actually legal on the Internet. However, in the event that

binary mail transport becomes a reality in Internet mail, or when

this document is used in conjunction with any other binary-capable

transport mechanism, binary bodies should be labeled as such using

this mechanism.


NOTE: The five values defined for the Content-Transfer-Encoding

field imply nothing about the Content-Type other than the

algorithm by which it was encoded or the transport system

requirements if unencoded.


Implementors may, if necessary, define new Content-Transfer-Encoding

values, but must use an x-token, which is a name prefixed by "X-" to

indicate its non-standard status, e.g., "Content-Transfer-Encoding:

x-my-new-encoding". However, unlike Content-Types and subtypes, the

creation of new Content-Transfer-Encoding values is explicitly and

strongly discouraged, as it seems likely to hinder interoperability

with little potential benefit. Their use is allowed only as the




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result of an agreement between cooperating user agents.


If a Content-Transfer-Encoding header field appears as part of a

message header, it applies to the entire body of that message. If a

Content-Transfer-Encoding header field appears as part of a body

part's headers, it applies only to the body of that body part. If an

entity is of type "multipart" or "message", the Content-Transfer-

Encoding is not permitted to have any value other than a bit width

(e.g., "7bit", "8bit", etc.) or "binary".


It should be noted that email is character-oriented, so that the

mechanisms described here are mechanisms for encoding arbitrary octet

streams, not bit streams. If a bit stream is to be encoded via one

of these mechanisms, it must first be converted to an 8-bit byte

stream using the network standard bit order ("big-endian"), in which

the earlier bits in a stream become the higher-order bits in a byte.

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

zeroes. This document provides a mechanism for noting the addition

of such padding in the case of the application Content-Type, which

has a "padding" parameter.


The encoding mechanisms defined here explicitly encode all data in

ASCII. Thus, for example, suppose an entity has header fields such



Content-Type: text/plain; charset=ISO-8859-1

Content-transfer-encoding: base64


This must be interpreted to mean that the body is a base64 ASCII

encoding of data that was originally in ISO-8859-1, and will be in

that character set again after decoding.


The following sections will define the two standard encoding

mechanisms. The definition of new content-transfer-encodings is

explicitly discouraged and should only occur when absolutely

necessary. All content-transfer-encoding namespace except that

beginning with "X-" is explicitly reserved to the IANA for future

use. Private agreements about content-transfer-encodings are also

explicitly discouraged.


Certain Content-Transfer-Encoding values may only be used on certain

Content-Types. In particular, it is expressly forbidden to use any

encodings other than "7bit", "8bit", or "binary" with any Content-

Type that recursively includes other Content-Type fields, notably the

"multipart" and "message" Content-Types. All encodings that are

desired for bodies of type multipart or message must be done at the

innermost level, by encoding the actual body that needs to be





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NOTE ON ENCODING RESTRICTIONS: Though the prohibition against

using content-transfer-encodings on data of type multipart or

message may seem overly restrictive, it is necessary to prevent

nested encodings, in which data are passed through an encoding

algorithm multiple times, and must be decoded multiple times in

order to be properly viewed. Nested encodings add considerable

complexity to user agents: aside from the obvious efficiency

problems with such multiple encodings, they can obscure the basic

structure of a message. In particular, they can imply that

several decoding operations are necessary simply to find out what

types of objects a message contains. Banning nested encodings may

complicate the job of certain mail gateways, but this seems less

of a problem than the effect of nested encodings on user agents.



TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding

could be inferred from the characteristics of the Content-Type

that is to be encoded, or, at the very least, that certain

Content-Transfer-Encodings could be mandated for use with specific

Content-Types. There are several reasons why this is not the case.

First, given the varying types of transports used for mail, some

encodings may be appropriate for some Content-Type/transport

combinations and not for others. (For example, in an 8-bit

transport, no encoding would be required for text in certain

character sets, while such encodings are clearly required for 7-

bit SMTP.) Second, certain Content-Types may require different

types of transfer encoding under different circumstances. For

example, many PostScript bodies might consist entirely of short

lines of 7-bit data and hence require little or no encoding.

Other PostScript bodies (especially those using Level 2

PostScript's binary encoding mechanism) may only be reasonably

represented using a binary transport encoding. Finally, since

Content-Type is intended to be an open-ended specification

mechanism, strict specification of an association between

Content-Types and encodings effectively couples the specification

of an application protocol with a specific lower-level transport.

This is not desirable since the developers of a Content-Type

should not have to be aware of all the transports in use and what

their limitations are.


NOTE ON TRANSLATING ENCODINGS: The quoted-printable and base64

encodings are designed so that conversion between them is

possible. The only issue that arises in such a conversion is the

handling of line breaks. When converting from quoted-printable to

base64 a line break must be converted into a CRLF sequence.

Similarly, a CRLF sequence in base64 data must be converted to a

quoted-printable line break, but ONLY when converting text data.





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NOTE ON CANONICAL ENCODING MODEL: There was some confusion, in

earlier drafts of this 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, and the relationship between content-transfer-

encodings and character sets. For this reason, a canonical model

for encoding is presented as Appendix G.


5.1. Quoted-Printable Content-Transfer-Encoding


The Quoted-Printable encoding is intended to represent data that

largely consists of octets that correspond to printable characters in

the ASCII character set. It encodes the data in such a way that the

resulting octets are unlikely to be modified by mail transport. If

the data being encoded are mostly ASCII text, the encoded form of the

data remains largely recognizable by humans. A body which is

entirely ASCII may also be encoded in Quoted-Printable to ensure the

integrity of the data should the message pass through a character-

translating, and/or line-wrapping gateway.


In this encoding, octets are to be represented as determined by the

following rules:


Rule #1: (General 8-bit representation) Any octet, except those

indicating a line break according to the newline convention of the

canonical (standard) form of the data being encoded, may be

represented by an "=" followed by a two digit hexadecimal

representation of the octet's value. The digits of the

hexadecimal alphabet, for this purpose, are "0123456789ABCDEF".

Uppercase letters must be used when sending hexadecimal data,

though a robust implementation may choose to recognize lowercase

letters on receipt. Thus, for example, the value 12 (ASCII form

feed) can be represented by "=0C", and the value 61 (ASCII EQUAL

SIGN) can be represented by "=3D". Except when the following

rules allow an alternative encoding, this rule is mandatory.


Rule #2: (Literal representation) Octets with decimal values of 33

through 60 inclusive, and 62 through 126, inclusive, MAY be

represented as the ASCII characters which correspond to those


through TILDE, respectively).


Rule #3: (White Space): Octets with values of 9 and 32 MAY be

represented as ASCII TAB (HT) and SPACE characters, respectively,

but MUST NOT be so represented at the end of an encoded line. Any

TAB (HT) or SPACE characters on an encoded line MUST thus be

followed on that line by a printable character. In particular, an




Borenstein & Freed [Page 18]



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"=" at the end of an encoded line, indicating a soft line break

(see rule #5) may follow one or more TAB (HT) or SPACE characters.

It follows that an octet with value 9 or 32 appearing at the end

of an encoded line must be represented according to Rule #1. This

rule is necessary because some MTAs (Message Transport Agents,

programs which transport messages from one user to another, or

perform a part of such transfers) are known to pad lines of text

with SPACEs, and others are known to remove "white space"

characters from the end of a line. Therefore, when decoding a

Quoted-Printable body, any trailing white space on a line must be

deleted, as it will necessarily have been added by intermediate

transport agents.


Rule #4 (Line Breaks): A line break in a text body, independent of

what its representation is following the canonical representation

of the data being encoded, must be represented by a (RFC 822) line

break, which is a CRLF sequence, in the Quoted-Printable encoding.

Since the canonical representation of types other than text do not

generally include the representation of line breaks, no hard line

breaks (i.e. line breaks that are intended to be meaningful and

to be displayed to the user) should occur in the quoted-printable

encoding of such types. Of course, occurrences of "=0D", "=0A",

"0A=0D" and "=0D=0A" will eventually be encountered. In general,

however, base64 is preferred over quoted-printable for binary



Note that many implementations may elect to encode the local

representation of various content types directly, as described in

Appendix G. In particular, this may apply to plain text material

on systems that use newline conventions other than CRLF

delimiters. Such an implementation is permissible, but the

generation of line breaks must be generalized to account for the

case where alternate representations of newline sequences are



Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES

that encoded lines be no more than 76 characters long. If longer

lines are to be encoded with the Quoted-Printable encoding, 'soft'

line breaks must be used. An equal sign as the last character on a

encoded line indicates such a non-significant ('soft') line break

in the encoded text. Thus if the "raw" form of the line is a

single unencoded line that says:


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

their country.


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





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RFC 1521 MIME September 1993



Now's the time =

for all folk to come=

to the aid of their country.


This provides a mechanism with which long lines are encoded in

such a way as to be restored by the user agent. The 76 character

limit does not count the trailing CRLF, but counts all other

characters, including any equal signs.


Since the hyphen character ("-") is represented as itself in the

Quoted-Printable encoding, care must be taken, when encapsulating a

quoted-printable encoded body in a multipart entity, to ensure that

the encapsulation boundary does not appear anywhere in the encoded

body. (A good strategy is to choose a boundary that includes a

character sequence such as "=_" which can never appear in a quoted-

printable body. See the definition of multipart messages later in

this document.)


NOTE: The quoted-printable encoding represents something of a

compromise between readability and reliability in transport.

Bodies encoded with the quoted-printable encoding will work

reliably over most mail gateways, but may not work perfectly over

a few gateways, notably those involving translation into EBCDIC.

(In theory, an EBCDIC gateway could decode a quoted-printable body

and re-encode it using base64, but such gateways do not yet

exist.) A higher level of confidence is offered by the base64

Content-Transfer-Encoding. A way to get reasonably reliable

transport through EBCDIC gateways is to also quote the ASCII





according to rule #1. See Appendix B for more information.


Because quoted-printable data is generally assumed to be line-

oriented, it is to be expected that the representation of the breaks

between the lines of quoted printable data may be altered in

transport, in the same manner that plain text mail has always been

altered in Internet mail when passing between systems with differing

newline conventions. If such alterations are likely to constitute a

corruption of the data, it is probably more sensible to use the

base64 encoding rather than the quoted-printable encoding.


WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-

printable, care must be taken to encode CR and LF characters as "=0D"

and "=0A", respectively. In particular, a CRLF sequence in binary

data should be encoded as "=0D=0A". Otherwise, if CRLF were

represented as a hard line break, it might be incorrectly decoded on




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platforms with different line break conventions.


For formalists, the syntax of quoted-printable data is described by

the following grammar:


quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)

; Maximum line length of 76 characters excluding CRLF


ptext := octet /<any ASCII character except "=", SPACE, or TAB>

; characters not listed as "mail-safe" in Appendix B

; are also not recommended.


octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")

; octet must be used for characters > 127, =, SPACE, or TAB,

; and is recommended for any characters not listed in

; Appendix B as "mail-safe".


5.2. Base64 Content-Transfer-Encoding


The Base64 Content-Transfer-Encoding is designed to represent

arbitrary sequences of octets in a form that need not be humanly

readable. The encoding and decoding algorithms are simple, but the

encoded data are consistently only about 33 percent larger than the

unencoded data. This encoding is virtually identical to the one used

in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421.

The base64 encoding is adapted from RFC 1421, with one change: base64

eliminates the "*" mechanism for embedded clear text.


A 65-character subset of US-ASCII is used, enabling 6 bits to be

represented per printable character. (The extra 65th character, "=",

is used to signify a special processing function.)


NOTE: This subset has the important property that it is

represented identically in all versions of ISO 646, including US

ASCII, and all characters in the subset are also represented

identically in all versions of EBCDIC. Other popular encodings,

such as the encoding used by the uuencode utility and the base85

encoding specified as part of Level 2 PostScript, do not share

these properties, and thus do not fulfill the portability

requirements a binary transport encoding for mail must meet.


The encoding process represents 24-bit groups of input bits as output

strings of 4 encoded characters. Proceeding from left to right, a

24-bit input group is formed by concatenating 3 8-bit input groups.

These 24 bits are then treated as 4 concatenated 6-bit groups, each

of which is translated into a single digit in the base64 alphabet.

When encoding a bit stream via the base64 encoding, the bit stream

must be presumed to be ordered with the most-significant-bit first.




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That is, the first bit in the stream will be the high-order bit in

the first byte, and the eighth bit will be the low-order bit in the

first byte, and so on.


Each 6-bit group is used as an index into an array of 64 printable

characters. The character referenced by the index is placed in the

output string. These characters, identified in Table 1, below, are

selected so as to be universally representable, and the set excludes

characters with particular significance to SMTP (e.g., ".", CR, LF)

and to the encapsulation boundaries defined in this document (e.g.,



Table 1: The Base64 Alphabet


Value Encoding Value Encoding Value Encoding Value Encoding

0 A 17 R 34 i 51 z

1 B 18 S 35 j 52 0

2 C 19 T 36 k 53 1

3 D 20 U 37 l 54 2

4 E 21 V 38 m 55 3

5 F 22 W 39 n 56 4

6 G 23 X 40 o 57 5

7 H 24 Y 41 p 58 6

8 I 25 Z 42 q 59 7

9 J 26 a 43 r 60 8

10 K 27 b 44 s 61 9

11 L 28 c 45 t 62 +

12 M 29 d 46 u 63 /

13 N 30 e 47 v

14 O 31 f 48 w (pad) =

15 P 32 g 49 x

16 Q 33 h 50 y


The output stream (encoded bytes) must be represented in lines of no

more than 76 characters each. All line breaks or other characters

not found in Table 1 must be ignored by decoding software. In base64

data, characters other than those in Table 1, line breaks, and other

white space probably indicate a transmission error, about which a

warning message or even a message rejection might be appropriate

under some circumstances.


Special processing is performed if fewer than 24 bits are available

at the end of the data being encoded. A full encoding quantum is

always completed at the end of a body. When fewer than 24 input bits

are available in an input group, zero bits are added (on the right)

to form an integral number of 6-bit groups. Padding at the end of

the data is performed using the '=' character. Since all base64

input is an integral number of octets, only the following cases can




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arise: (1) the final quantum of encoding input is an integral

multiple of 24 bits; here, the final unit of encoded output will be

an integral multiple of 4 characters with no "=" padding, (2) the

final quantum of encoding input is exactly 8 bits; here, the final

unit of encoded output will be two characters followed by two "="

padding characters, or (3) the final quantum of encoding input is

exactly 16 bits; here, the final unit of encoded output will be three

characters followed by one "=" padding character.


Because it is used only for padding at the end of the data, the

occurrence of any '=' characters may be taken as evidence that the

end of the data has been reached (without truncation in transit). No

such assurance is possible, however, when the number of octets

transmitted was a multiple of three.


Any characters outside of the base64 alphabet are to be ignored in

base64-encoded data. The same applies to any illegal sequence of

characters in the base64 encoding, such as "====="


Care must be taken to use the proper octets for line breaks if base64

encoding is applied directly to text material that has not been

converted to canonical form. In particular, text line breaks must be

converted into CRLF sequences prior to base64 encoding. The important

thing to note is that this may be done directly by the encoder rather

than in a prior canonicalization step in some implementations.


NOTE: There is no need to worry about quoting apparent

encapsulation boundaries within base64-encoded parts of multipart

entities because no hyphen characters are used in the base64



6. Additional Content-Header Fields


6.1. Optional Content-ID Header Field


In constructing a high-level user agent, it may be desirable to allow

one body to make reference to another. Accordingly, bodies may be

labeled using the "Content-ID" header field, which is syntactically

identical to the "Message-ID" header field:


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

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



The Content-ID value may be used for uniquely identifying MIME

entities in several contexts, particularly for cacheing data

referenced by the message/external-body mechanism. Although the

Content-ID header is generally optional, its use is mandatory in




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implementations which generate data of the optional MIME Content-type

"message/external-body". That is, each message/external-body entity

must have a Content-ID field to permit cacheing of such data.


It is also worth noting that the Content-ID value has special

semantics in the case of the multipart/alternative content-type.

This is explained in the section of this document dealing with



6.2. Optional Content-Description Header Field


The ability to associate some descriptive information with a given

body is often desirable. For example, it may be useful to mark an

"image" body as "a picture of the Space Shuttle Endeavor." Such text

may be placed in the Content-Description header field.


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


The description is presumed to be given in the US-ASCII character

set, although the mechanism specified in [RFC-1522] may be used for

non-US-ASCII Content-Description values.


7. The Predefined Content-Type Values


This document defines seven initial Content-Type values and an

extension mechanism for private or experimental types. Further

standard types must be defined by new published specifications. It

is expected that most innovation in new types of mail will take place

as subtypes of the seven types defined here. The most essential

characteristics of the seven content-types are summarized in Appendix



7.1 The Text Content-Type


The text Content-Type is intended for sending material which is

principally textual in form. It is the default Content-Type. A

"charset" parameter may be used to indicate the character set of the

body text for some text subtypes, notably including the primary

subtype, "text/plain", which indicates plain (unformatted) text. The

default Content-Type for Internet mail is "text/plain; charset=us-



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




Borenstein & Freed [Page 24]



RFC 1521 MIME September 1993



images, audio, or text represented in an unreadable form. In the

absence of appropriate interpretation software, it is reasonable to

show subtypes of text to the user, while it is not reasonable to do

so with most nontextual data.


Such formatted textual data should be represented using subtypes of

text. Plausible subtypes of text are typically given by the common

name of the representation format, e.g., "text/richtext" [RFC-1341].


7.1.1. The 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=us-ascii


Unlike some other parameter values, the values of the charset

parameter are NOT case sensitive. The default character set, which

must be assumed in the absence of a charset parameter, is US-ASCII.


The specification for any future subtypes of "text" must specify

whether or not they will also utilize a "charset" parameter, and may

possibly restrict its values as well. When used with a particular

body, the semantics of the "charset" parameter should be identical to

those specified here for "text/plain", i.e., the body consists

entirely of characters in the given charset. In particular, definers

of future text subtypes should pay close attention the the

implications of multibyte character sets for their subtype



This RFC specifies the definition of the charset parameter for the

purposes of MIME to be a unique mapping of a byte stream to glyphs, a

mapping which does not require external profiling information.


An initial list of predefined character set names can be found at the

end of this section. Additional character sets may be registered

with IANA, although the standardization of their use requires the

usual IESG [RFC-1340] review and approval. 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




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RFC 1521 MIME September 1993



future, it is strongly recommended that new user agents explicitly

specify a character set via the Content-Type header field. "US-

ASCII" does not indicate an arbitrary seven-bit character code, but

specifies that the body uses character coding that uses the exact

correspondence of codes to characters specified in ASCII. National

use variations of ISO 646 [ISO-646] are NOT 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



The complete US-ASCII character set is listed in [US-ASCII]. Note

that the control characters including DEL (0-31, 127) have no defined

meaning apart from the combination CRLF (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



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




Borenstein & Freed [Page 26]



RFC 1521 MIME September 1993



names for a small number of character sets for which a strong

constituent base exists.


The defined charset values are:


US-ASCII -- as defined in [US-ASCII].


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.


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 must always use the "lowest common

denominator" character set possible. For example, if a body contains

only US-ASCII characters, it must be marked as being in the US-ASCII

character set, not ISO-8859-1, which, like all the ISO-8859 family of

character sets, is a superset of US-ASCII. More generally, if a

widely-used character set is a subset of another character set, and a

body contains only characters in the widely-used subset, it must be

labeled as being in that subset. This will increase the chances that

the recipient will be able to view the mail correctly.




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7.1.2. The Text/plain subtype


The primary 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.


The formal grammar for the content-type header field for text is as



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


text-subtype := "plain" / extension-token


charset := "us-ascii"/ "iso-8859-1"/ "iso-8859-2"/ "iso-8859-3"

/ "iso-8859-4"/ "iso-8859-5"/ "iso-8859-6"/ "iso-8859-7"

/ "iso-8859-8" / "iso-8859-9" / extension-token

; case insensitive


7.2. The Multipart Content-Type


In the case of multiple part entities, in which one or more different

sets of data are combined in a single body, a "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 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 field

implies that the corresponding body is plain US-ASCII text. 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.





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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 an image. In order to

represent the latter, the body part must have "Content-Type:

message", and its body (after the blank line) must be the

encapsulated message, with its own "Content-Type: image" 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 the 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 ASCII in any case (though the

header fields may encode non-ASCII header text as per [RFC-1522]),

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.


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 body part headers

embedded in the bodies of messages of type "multipart."


7.2.1. Multipart: The common syntax


All subtypes of "multipart" share a common syntax, defined in this

section. A simple example of a multipart message also appears in

this section. An example of a more complex multipart message is




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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 code 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 illegal Content-type

fields. Thus, a typical multipart Content-Type header field might

look like this:


Content-Type: multipart/mixed;



But the following is illegal:


Content-Type: multipart/mixed;



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


Content-Type: multipart/mixed;



This indicates that the entity consists of several parts, each itself

with a structure that is syntactically identical to an RFC 822

message, except that the header area might be completely empty, and

that the parts are each preceded by the line






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Note that the encapsulation boundary must occur at the beginning of a

line, i.e., following a CRLF, and that the initial CRLF is considered

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





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



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




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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 <>

To: Ned Freed <>

Subject: Sample message

MIME-Version: 1.0

Content-type: multipart/mixed; boundary="simple



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 ASCII text.

It does NOT end with a linebreak.

--simple boundary

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


This is explicitly typed plain ASCII text.

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 must use 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 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




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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



multipart-body := preamble 1*encapsulation

close-delimiter epilogue


encapsulation := delimiter body-part CRLF


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

; There must be no space

; between "--" and boundary.


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

by "--",


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


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


discard-text := *(*text CRLF)


body-part := <"message" as defined in RFC 822,

with all header fields optional, and with the

specified delimiter not occurring anywhere in

the message body, either on a line by itself

or as a substring anywhere. Note that the

semantics of a part differ from the semantics

of a message, as described in the text.>


NOTE: In certain transport enclaves, RFC 822 restrictions such as

the one that limits bodies to printable ASCII characters may not

be in force. (That is, the transport domains may resemble

standard Internet mail transport as specified in RFC821 and

assumed by RFC822, 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 ASCII range, as long as these extensions are

supported by the transport and adequately documented in the




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Content-Transfer-Encoding header field. However, in no event are

headers (either message headers or body-part headers) allowed to

contain anything other than ASCII characters.


NOTE: Conspicuously missing from the multipart type is a notion of

structured, related body parts. In general, it seems premature to

try to standardize interpart structure yet. It is recommended

that those wishing to provide a more structured or integrated

multipart messaging facility 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.


7.2.2. The Multipart/mixed (primary) subtype


The primary subtype for multipart, "mixed", 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".


7.2.3. The Multipart/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



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










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From: Nathaniel Borenstein <>

To: Ned Freed <>

Subject: Formatted text mail

MIME-Version: 1.0

Content-Type: multipart/alternative; boundary=boundary42




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


...plain text version of message goes here....


Content-Type: text/richtext


.... RFC 1341 richtext version of same message goes here ...


Content-Type: text/x-whatever


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




In this example, users whose mail system understood the "text/x-

whatever" format would see only the fancy version, while other users

would see only the richtext or plain text version, depending on the

capabilities 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




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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.



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. However, where the

information content is identical -- for example, where several parts

of type "application/external- body" specify alternate ways to access

the identical data -- the same Content-ID field value should be used,

to optimize any cacheing mechanisms that might be present on the

recipient's end. However, it is recommended that 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.


7.2.4. The Multipart/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.


















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


From: Moderator-Address

To: Recipient-List

MIME-Version: 1.0

Subject: Internet Digest, volume 42

Content-Type: multipart/digest;

boundary="---- next message ----"


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


From: someone-else

Subject: my opinion


...body goes here ...


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


From: someone-else-again

Subject: my different opinion


... another body goes here...


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


7.2.5. The Multipart/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.


7.2.6. 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".


The formal grammar for content-type header fields for multipart data

is given by:


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

";" "boundary" "=" boundary




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multipart-subtype := "mixed" / "parallel" / "digest"

/ "alternative" / extension-token


7.3. The Message Content-Type


It is frequently desirable, in sending mail, to encapsulate another

mail message. For this common operation, a special Content-Type,

"message", is defined. The primary subtype, message/rfc822, has no

required parameters in the Content-Type field. Additional subtypes,

"partial" and "External-body", do have required parameters. These

subtypes are explained below.


NOTE: It has been suggested that subtypes of message might be

defined 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.


As stated in the definition of the Content-Transfer-Encoding field,

no encoding other than "7bit", "8bit", or "binary" is permitted for

messages or parts of type "message". Even stronger restrictions

apply to the subtypes "message/partial" and "message/external-body",

as specified below. 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-ASCII text in the headers of an

encapsulated message can be specified using the mechanisms described

in [RFC-1522].


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."


7.3.1. The Message/rfc822 (primary) 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, it is not required that

each message/rfc822 body must include a "From", "Subject", and at

least one destination header.


It should be noted that, despite the use of the numbers "822", a




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message/rfc822 entity can include enhanced information as defined in

this document. In other words, a message/rfc822 message may be a

MIME message.


7.3.2. The Message/Partial subtype


A subtype of message, "partial", is defined in order to allow large

objects to be delivered as several separate pieces of mail and

automatically reassembled by the receiving user agent. (The concept

is similar to IP fragmentation/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;



Content-Type: Message/Partial;




But part 3 MUST specify the total number of parts:


Content-Type: Message/Partial;

number=3; total=3;



Note that part numbering begins with 1, not 0.


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




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together, the result is a complete MIME entity, which may have its

own Content-Type header field, and thus may contain any other data



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



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 "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 "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.


For example, if an audio message is broken into two parts, the first

part might look something like this:


X-Weird-Header-1: Foo



Subject: Audio mail

Message-ID: <>

MIME-Version: 1.0

Content-type: message/partial;


number=1; total=2


X-Weird-Header-1: Bar

X-Weird-Header-2: Hello




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Message-ID: <>

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:




Subject: Audio mail

MIME-Version: 1.0

Message-ID: <>

Content-type: message/partial;

id=""; 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



Subject: Audio mail

Message-ID: <>

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...


Note on encoding of MIME entities encapsulated inside message/partial

entities: 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 objects, each of them requiring binary

transport. If such objects were encountered at a gateway into a 7-

bit transport environment, there would be no way to properly encode

them for the 7-bit world, aside from waiting for all of the parts,

reassembling the message, and then encoding the reassembled data in

base64 or quoted-printable. Since it is possible that different

parts 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-




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RFC 1521 MIME September 1993



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.


It should be noted that, because some message transfer agents may

choose to automatically fragment large messages, and because such

agents may use 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



It should also be noted that 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



Finally, it should be noted that the "Encrypted" header field has

been made obsolete by Privacy Enhanced Messaging (PEM), but the rules

above are believed to describe the correct way to treat it if it is

encountered in the context of conversion to and from message/partial



7.3.3. The Message/External-Body subtype


The external-body subtype indicates that the actual body data are not

included, but merely referenced. In this case, the parameters

describe a mechanism 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-





Content-type: image/gif

Content-ID: <>

Content-Transfer-Encoding: binary





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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". Of the access-types defined

by this document, the phantom body is used only when the access-type

is "mail-server". In all other cases, the phantom body is ignored.


The only always-mandatory parameter for message/external-body is

"access-type"; all of the other parameters may be mandatory or

optional depending on the value of access-type.


ACCESS-TYPE -- A case-insensitive word, indicating the supported

access mechanism by which the file or data may be obtained.

Values include, but are not limited to, "FTP", "ANON-FTP", "TFTP",

"AFS", "LOCAL-FILE", and "MAIL-SERVER". Future values, except for

experimental values beginning with "X-" must be registered with

IANA, as described in Appendix E .


In addition, the following three parameters are optional for ALL



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.


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.


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.


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

in the sections that follow.


The encapsulated headers in ALL message/external-body entities MUST

include a Content-ID header field to give a unique identifier by




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which to reference the data. This identifier may be used for

cacheing 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, it is specified that MIME entities of type

message/external-body 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/external-body. 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:


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


SITE -- A machine from which the file may be obtained, using the

given protocol. This must be a fully qualified domain name, not a



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:


DIRECTORY -- A directory from which the data named by NAME should

be retrieved.


MODE -- A case-insensitive string indicating the mode to be used

when retrieving the information. The legal values for access-type

"TFTP" are "NETASCII", "OCTET", and "MAIL", as specified by the

TFTP protocol [RFC-783]. The legal values for access-type "FTP"

are "ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a

decimal integer, typically 8. These correspond to the




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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, but 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. 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. The "local-file" and "afs" access-types


An access-type of "local-file" indicates that the actual body is

accessible as a file on the local machine. An access-type of "afs"

indicates that the file is accessible via the global AFS file system.

In both cases, only a single parameter is required:


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


The following optional parameter may be 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:


SITE -- A domain specifier for a machine or set of machines that

are known to have access to the data file. Asterisks may be used

for wildcard matching to a part of a domain name, such as

"*", 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. The "mail-server" access-type


The "mail-server" access-type indicates that the actual body is

available from a mail server. The mandatory parameter for this

access-type is:


SERVER -- The email address of the mail server from which the

actual body data can be obtained.


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




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


An optional parameter for this access-type is:


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.


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.


It is worth noting that, 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. Examples and Further Explanations


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.


If an entity is of type "message/external-body", then the body of the

entity will contain the header fields of the encapsulated message.

The body itself is to be found in the external location. This means

that if the body of the "message/external-body" message contains two

consecutive CRLFs, everything after those pairs is NOT part of the

message itself. For most message/external-body messages, this

trailing area must simply be ignored. However, it is a convenient




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place for additional data that cannot be included in the content-type

header field. In particular, if the "access-type" value is "mail-

server", then the trailing area must contain commands to be sent to

the mail server at the address given by the value of the SERVER



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 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

Subject: whatever

MIME-Version: 1.0

Message-ID: <>

Content-Type: multipart/alternative; boundary=42

Content-ID: <>



Content-Type: message/external-body;






expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"


Content-type: application/postscript

Content-ID: <>



Content-Type: message/external-body;




expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"


Content-type: application/postscript

Content-ID: <>



Content-Type: message/external-body;





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expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"


Content-type: application/postscript

Content-ID: <>






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.


The formal grammar for content-type header fields for data of type

message is given by:


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


message-subtype := "rfc822"

/ "partial" 2#3partial-param

/ "external-body" 1*external-param

/ extension-token


partial-param := (";" "id" "=" value)

/ (";" "number" "=" 1*DIGIT)

/ (";" "total" "=" 1*DIGIT)

; id & number required; total required for last part


external-param := (";" "access-type" "=" atype)




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/ (";" "expiration" "=" date-time)

; Note that date-time is quoted

/ (";" "size" "=" 1*DIGIT)

/ (";" "permission" "=" ("read" / "read-write"))

; Permission is case-insensitive

/ (";" "name" "=" value)

/ (";" "site" "=" value)

/ (";" "dir" "=" value)

/ (";" "mode" "=" value)

/ (";" "server" "=" value)

/ (";" "subject" "=" value)

; access-type required;others required based on access-type


atype := "ftp" / "anon-ftp" / "tftp" / "local-file"

/ "afs" / "mail-server" / extension-token

; Case-insensitive


7.4. The Application Content-Type


The "application" Content-Type is to be used for 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-



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



Such applications may be defined as subtypes of the "application"

Content-Type. This document defines two subtypes: octet-stream, and



In general, 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. Such usages (other than subtypes beginning




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with "x-") must be registered with IANA, as described in Appendix E.


7.4.1. The Application/Octet-Stream (primary) subtype


The primary subtype of application, "octet-stream", may be used to

indicate that a body contains binary data. The set of possible

parameters includes, but is not limited to:


TYPE -- the general type or category of binary data. This is

intended as information for the human recipient rather than for

any automatic processing.


PADDING -- the number of bits of padding that were appended to the

bit-stream comprising the actual contents to produce the enclosed

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 the byte



An additional parameter, "conversions", was defined in [RFC-1341] but

has 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.


7.4.2. The Application/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.





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The PostScript language definition provides facilities for internal

labeling of the specific language features a given program uses. This

labeling, called the PostScript document structuring conventions, 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, 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.


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.


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




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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. The ability to exit will

probably be unavailable in secure PostScript implementations.

Message-receiving and -displaying software should, if possible,

disable the ability to make retained changes to the PostScript

environment, and eliminate the startjob and exitserver commands. If

these commands cannot be eliminated, the password associated with

them should at least be set to a hard-to-guess value.


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, if possible,

disable the ability to change system and device parameters. If these

operators cannot be disabled, the password associated with them

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


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.


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.


It is possible to write PostScript that consumes huge amounts of

various system resources. It is also possible to write PostScript

programs that loop infinitely. Both types of programs have the




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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.


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.


7.4.3. Other Application subtypes


It is expected that many other subtypes of application will be

defined in the future. MIME implementations must generally treat any

unrecognized subtypes as being equivalent to application/octet-



The formal grammar for content-type header fields for application

data is given by:


application-type := "application" "/" application-subtype


application-subtype := ("octet-stream" *stream-param)

/ "postscript" / extension-token


stream-param := (";" "type" "=" value)

/ (";" "padding" "=" padding)


padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"


7.5. The Image Content-Type


A Content-Type of "image" indicates that the body contains an image.

The subtype names the specific image format. These names are case

insensitive. 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 Appendix E.


The formal grammar for the content-type header field for data of type

image is given by:




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


7.6. The 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 audio encoded using 8-bit

ISDN mu-law [PCM]. When this subtype is present, a sample rate of

8000 Hz and a single channel is assumed.


The formal grammar for the content-type header field for data of type

audio is given by:


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


7.7. The 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



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".


The formal grammar for the content-type header field for data of type

video is given by:


video-type := "video" "/" ("mpeg" / extension-token)


7.8. Experimental Content-Type Values


A Content-Type value beginning with the characters "X-" 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




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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



In general, the use of "X-" top-level types is strongly discouraged.

Implementors should invent subtypes of the existing types whenever

possible. The invention of new types is intended to be restricted

primarily to the development of new media types for email, such as

digital odors or holography, and not for new data formats in general.

In many cases, a subtype of application will be more appropriate than

a new top-level type.








































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8. Summary


Using the MIME-Version, Content-Type, and Content-Transfer-Encoding

header fields, it is possible to include, in a standardized way,

arbitrary types of data objects with RFC 822 conformant mail

messages. No restrictions imposed by either RFC 821 or RFC 822 are

violated, and care has been taken to avoid problems caused by

additional restrictions imposed by the characteristics of some

Internet mail transport mechanisms (see Appendix B). The "multipart"

and "message" Content-Types allow mixing and hierarchical structuring

of objects of different types in a single message. Further Content-

Types provide a standardized mechanism for tagging messages or body

parts as audio, image, or several other kinds of data. A

distinguished parameter syntax allows further specification of data

format details, particularly the specification of alternate character

sets. Additional optional header fields provide mechanisms for

certain extensions deemed desirable by many implementors. Finally, a

number of useful Content-Types are defined for general use by

consenting user agents, notably message/partial, and



9. Security Considerations


Security issues are discussed in Section 7.4.2 and in Appendix F.

Implementors should pay special attention to the security

implications of any mail content-types that can cause the remote

execution of any actions in the recipient's environment. In such

cases, the discussion of the application/postscript content-type in

Section 7.4.2 may serve as a model for considering other content-

types with remote execution capabilities.






















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10. Authors' Addresses


For more information, the authors of this document may be contacted

via Internet mail:


Nathaniel S. Borenstein

MRE 2D-296, Bellcore

445 South St.

Morristown, NJ 07962-1910


Phone: +1 201 829 4270

Fax: +1 201 829 7019




Ned Freed

Innosoft International, Inc.

250 West First Street

Suite 240

Claremont, CA 91711


Phone: +1 909 624 7907

Fax: +1 909 621 5319



MIME is a result of the work of the Internet Engineering Task Force

Working Group on Email Extensions. The chairman of that group, Greg

Vaudreuil, may be reached at:


Gregory M. Vaudreuil

Tigon Corporation

17060 Dallas Parkway

Dallas Texas, 75248


Phone: +1 214-733-2722

















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11. Acknowledgements


This document is the result of the collective effort of a large

number of people, at several IETF meetings, on the IETF-SMTP and

IETF-822 mailing lists, and elsewhere. Although any enumeration

seems doomed to suffer from egregious omissions, the following are

among the many contributors to this effort:


Harald Tveit Alvestrand Timo Lehtinen

Randall Atkinson John R. MacMillan

Philippe Brandon Rick McGowan

Kevin Carosso Leo Mclaughlin

Uhhyung Choi Goli Montaser-Kohsari

Cristian Constantinof Keith Moore

Mark Crispin Tom Moore

Dave Crocker Erik Naggum

Terry Crowley Mark Needleman

Walt Daniels John Noerenberg

Frank Dawson Mats Ohrman

Hitoshi Doi Julian Onions

Kevin Donnelly Michael Patton

Keith Edwards David J. Pepper

Chris Eich Blake C. Ramsdell

Johnny Eriksson Luc Rooijakkers

Craig Everhart Marshall T. Rose

Patrik Faeltstroem Jonathan Rosenberg

Erik E. Fair Jan Rynning

Roger Fajman Harri Salminen

Alain Fontaine Michael Sanderson

James M. Galvin Masahiro Sekiguchi

Philip Gladstone Mark Sherman

Thomas Gordon Keld Simonsen

Phill Gross Bob Smart

James Hamilton Peter Speck

Steve Hardcastle-Kille Henry Spencer

David Herron Einar Stefferud

Bruce Howard Michael Stein

Bill Janssen Klaus Steinberger

Olle Jaernefors Peter Svanberg

Risto Kankkunen James Thompson

Phil Karn Steve Uhler

Alan Katz Stuart Vance

Tim Kehres Erik van der Poel

Neil Katin Guido van Rossum

Kyuho Kim Peter Vanderbilt

Anders Klemets Greg Vaudreuil

John Klensin Ed Vielmetti

Valdis Kletniek Ryan Waldron




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Jim Knowles Wally Wedel

Stev Knowles Sven-Ove Westberg

Bob Kummerfeld Brian Wideen

Pekka Kytolaakso John Wobus

Stellan Lagerstrom Glenn Wright

Vincent Lau Rayan Zachariassen

Donald Lindsay David Zimmerman

Marc Andreessen Bob Braden

Brian Capouch Peter Clitherow

Dave Collier-Brown John Coonrod

Stephen Crocker Jim Davis

Axel Deininger Dana S Emery

Martin Forssen Stephen Gildea

Terry Gray Mark Horton

Warner Losh Carlyn Lowery

Laurence Lundblade Charles Lynn

Larry Masinter Michael J. McInerny

Jon Postel Christer Romson

Yutaka Sato Markku Savela

Richard Alan Schafer Larry W. Virden

Rhys Weatherly Jay Weber

Dave Wecker


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

certainly unintentional.



























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Appendix A -- Minimal MIME-Conformance


The mechanisms described in this document are open-ended. It is

definitely not expected that all implementations will support all of

the Content-Types described, nor that they will all share the same

extensions. In order to promote interoperability, however, it is

useful to define the concept of "MIME-conformance" to define a

certain level of implementation that allows the useful interworking

of messages with content that differs from US ASCII text. In this

section, we specify the requirements for such conformance.


A mail user agent that is MIME-conformant MUST:


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


2. Recognize the Content-Transfer-Encoding header field, and

decode all received data encoded with either the quoted-printable

or base64 implementations. Encode any data sent that is not in

seven-bit mail-ready representation using one of these

transformations and include the appropriate Content-Transfer-

Encoding header field, unless the underlying transport mechanism

supports non-seven-bit data, as SMTP does not.


3. Recognize and interpret the Content-Type header field, and

avoid showing users raw data with a Content-Type field other than

text. Be able to send at least text/plain messages, with the

character set specified as a parameter if it is not US-ASCII.


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

least the following extents:




-- 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.





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-- For unrecognized subtypes, show or

offer to show the user the "raw"

version of the data after

conversion of the content from

canonical form to local form.




-- Recognize and display at least the

primary (822) encapsulation.




-- Recognize the primary (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.


-- Treat any unrecognized subtypes as if

they were "mixed".




-- Offer the ability to remove either of

the two types of Content-Transfer-

Encoding defined in this document

and put the resulting information

in a user file.


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. Unrecognized predefined

types, which in a MIME-conformant mailer might still include

audio, image, or video, should also be treated in this way.


A user agent that meets the above conditions is said to be MIME-




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conformant. The meaning of this phrase is that it is assumed to be

"safe" to send virtually any kind of properly-marked data to users of

such mail systems, because such systems will at least be able to

treat the data as undifferentiated binary, and will not simply splash

it onto the screen of unsuspecting users. There is another sense in

which it is always "safe" to send data in a format that is MIME-

conformant, which is that such data will not break or be broken by

any known systems that are conformant with RFC 821 and RFC 822. User

agents that are MIME-conformant have the additional guarantee that

the user will not be shown data that were never intended to be viewed

as text.









































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Appendix B -- General Guidelines For Sending Email Data


Internet email is not a perfect, homogeneous system. Mail may become

corrupted at several stages in its travel to a final destination.

Specifically, email sent throughout the Internet may travel across

many networking technologies. Many networking 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

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




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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 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

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:


"'" (ASCII code 39)

"(" (ASCII code 40)

")" (ASCII code 41)

"+" (ASCII code 43)

"," (ASCII code 44)

"-" (ASCII code 45)

"." (ASCII code 46)

"/" (ASCII code 47)

":" (ASCII code 58)

"=" (ASCII code 61)

"?" (ASCII code 63)


A maximally portable mail representation, such as the base64

encoding, will confine itself to relatively short lines of text in

which the only meaningful characters are taken from this set of 73



(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




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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

illegal practices are known to occur on established networks, and

implementations should be robust in dealing with the bad effects they

can cause.









































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Appendix C -- A Complex Multipart Example


What follows is the outline of a complex multipart message. This

message has five parts to be displayed serially: two introductory

plain text parts, an embedded multipart message, a richtext 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 <>

To: Ned Freed <>

Subject: A multipart example

Content-Type: multipart/mixed;



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.



...Some text appears here...

[Note that the preceding blank line means

no header fields were given and this is text,

with charset US ASCII. It could have been

done with explicit typing as in the next part.]



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.



Content-Type: multipart/parallel;





Content-Type: audio/basic

Content-Transfer-Encoding: base64


... base64-encoded 8000 Hz single-channel

mu-law-format audio data goes here....




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Content-Type: image/gif

Content-Transfer-Encoding: base64


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





Content-type: text/richtext


This is <bold><italic>richtext.</italic></bold>

<smaller>as defined in RFC 1341</smaller>

<nl><nl>Isn't it




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 ...


























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Appendix D -- Collected Grammar


This appendix contains the complete BNF grammar for all the syntax

specified by this document.


By itself, however, this grammar is incomplete. It refers to several

entities that are defined by RFC 822. Rather than reproduce those

definitions here, and risk unintentional differences between the two,

this document simply refers the reader to RFC 822 for the remaining

definitions. Wherever a term is undefined, it refers to the RFC 822



application-subtype := ("octet-stream" *stream-param)

/ "postscript" / extension-token


application-type := "application" "/" application-subtype


attribute := token ; case-insensitive


atype := "ftp" / "anon-ftp" / "tftp" / "local-file"

/ "afs" / "mail-server" / extension-token

; Case-insensitive


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


body-part := <"message" as defined in RFC 822,

with all header fields optional, and with the

specified delimiter not occurring anywhere in

the message body, either on a line by itself

or as a substring anywhere.>


NOTE: In certain transport enclaves, RFC 822 restrictions such as

the one that limits bodies to printable ASCII characters may not

be in force. (That is, the transport domains may resemble

standard Internet mail transport as specified in RFC821 and

assumed by RFC822, 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 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 ASCII characters.









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boundary := 0*69<bchars> bcharsnospace


bchars := bcharsnospace / " "


bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_"

/ "," / "-" / "." / "/" / ":" / "=" / "?"


charset := "us-ascii" / "iso-8859-1" / "iso-8859-2"/ "iso-8859-3"

/ "iso-8859-4" / "iso-8859-5" / "iso-8859-6" / "iso-8859-7"

/ "iso-8859-8" / "iso-8859-9" / extension-token

; case insensitive


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


content := "Content-Type" ":" type "/" subtype *(";" parameter)

; case-insensitive matching of type and subtype


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

; There must be no space

; between "--" and boundary.


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


discard-text := *(*text CRLF)


encapsulation := delimiter body-part CRLF


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


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


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


external-param := (";" "access-type" "=" atype)

/ (";" "expiration" "=" date-time)


; Note that date-time is quoted

/ (";" "size" "=" 1*DIGIT)

/ (";" "permission" "=" ("read" / "read-write"))

; Permission is case-insensitive

/ (";" "name" "=" value)

/ (";" "site" "=" value)

/ (";" "dir" "=" value)

/ (";" "mode" "=" value)

/ (";" "server" "=" value)

/ (";" "subject" "=" value)

;access-type required; others required based on access-type





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iana-token := <a publicly-defined extension token,

registered with IANA, as specified in

appendix E>


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


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


mechanism := "7bit" ; case-insensitive

/ "quoted-printable"

/ "base64"

/ "8bit"

/ "binary"

/ x-token


message-subtype := "rfc822"

/ "partial" 2#3partial-param

/ "external-body" 1*external-param

/ extension-token


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


multipart-body :=preamble 1*encapsulation close-delimiter epilogue


multipart-subtype := "mixed" / "parallel" / "digest"

/ "alternative" / extension-token


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

";" "boundary" "=" boundary


octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")

; octet must be used for characters > 127, =, SPACE, or


; and is recommended for any characters not listed in

; Appendix B as "mail-safe".


padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"


parameter := attribute "=" value


partial-param := (";" "id" "=" value)

/ (";" "number" "=" 1*DIGIT)

/ (";" "total" "=" 1*DIGIT)

; id & number required;total required for last part


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


ptext := octet / <any ASCII character except "=", SPACE, or TAB>




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; characters not listed as "mail-safe" in Appendix B

; are also not recommended.


quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)

; Maximum line length of 76 characters excluding CRLF


stream-param := (";" "type" "=" value)

/ (";" "padding" "=" padding)


subtype := token ; case-insensitive


text-subtype := "plain" / extension-token


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


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


tspecials := "(" / ")" / "<" / ">" / "@"

/ "," / ";" / ":" / "\" / <">

/ "/" / "[" / "]" / "?" / "="

; Must be in quoted-string,

; to use within parameter values



type := "application" / "audio" ; case-insensitive

/ "image" / "message"

/ "multipart" / "text"

/ "video" / extension-token

; All values case-insensitive


value := token / quoted-string


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


video-type := "video" "/" ("mpeg" / extension-token)


x-token := <The two characters "X-" or "x-" followed, with no

intervening white space, by any token>














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Appendix E -- IANA Registration Procedures


MIME has been carefully designed to have extensible mechanisms, and

it is expected that the set of content-type/subtype pairs and their

associated parameters will grow significantly with time. Several

other MIME fields, notably character set names, access-type

parameters for the message/external-body type, and possibly even

Content-Transfer-Encoding values, are likely to have new values

defined over time. In order to ensure that the set of such values is

developed in an orderly, well-specified, and public manner, MIME

defines a registration process which uses the Internet Assigned

Numbers Authority (IANA) as a central registry for such values.


In general, parameters in the content-type header field are used to

convey supplemental information for various content types, and their

use is defined when the content-type and subtype are defined. New

parameters should not be defined as a way to introduce new



In order to simplify and standardize the registration process, this

appendix gives templates for the registration of new values with

IANA. Each of these is given in the form of an email message

template, to be filled in by the registering party.


E.1 Registration of New Content-type/subtype Values


Note that MIME is generally expected to be extended by subtypes. If

a new fundamental top-level type is needed, its specification must be

published as an RFC or submitted in a form suitable to become an RFC,

and be subject to the Internet standards process.



Subject: Registration of new MIME



MIME type name:


(If the above is not an existing top-level MIME type,

please explain why an existing type cannot be used.)


MIME subtype name:


Required parameters:


Optional parameters:


Encoding considerations:





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Security considerations:


Published specification:


(The published specification must be an Internet RFC or

RFC-to-be if a new top-level type is being defined, and

must be a publicly available specification in any



Person & email address to contact for further information:


E.2 Registration of New Access-type Values

for Message/external-body



Subject: Registration of new MIME Access-type for

Message/external-body content-type


MIME access-type name:


Required parameters:


Optional parameters:


Published specification:


(The published specification must be an Internet RFC or



Person & email address to contact for further information:






















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Appendix F -- 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 an ASCII superset.


Security considerations: Rich text formats such as TeX and Troff

often contain mechanisms for executing arbitrary commands or file

system operations, and should not be used automatically unless

these security problems have been addressed. Even plain text may

contain control characters that can be used to exploit the

capabilities of "intelligent" terminals and cause security

violations. User interfaces designed to run on such terminals

should be aware of and try to prevent such problems.



Content-type: multipart


Subtypes defined by this document: mixed, alternative,

digest, parallel.


Important Parameters: boundary


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

Specifically, only "7bit" is permitted for "message/partial" or

"message/external-body", and only "7bit", "8bit", or "binary" are

permitted for other subtypes of "message".


Content-type: application


Subtypes defined by this document: octet-stream, postscript


Important Parameters: type, padding




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Deprecated Parameters: name and conversions were

defined in RFC 1341.


Encoding notes: base64 preferred for unreadable subtypes.


Security considerations: This type is intended for the

transmission of data to be interpreted by locally-installed

programs. If used, for example, to transmit executable

binary programs or programs in general-purpose interpreted

languages, such as LISP programs or shell scripts, severe

security problems could result. 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



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










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Appendix G -- Canonical Encoding Model


There was some confusion, in earlier drafts of this memo, regarding

the model for when email data was to be converted to canonical form

and encoded, and in particular how this process would affect the

treatment of CRLFs, given that the representation of newlines varies

greatly from system to system. For this reason, a canonical model

for encoding is presented below.


The process of composing a MIME entity can be modeled as being done

in a number of steps. Note that these steps are roughly similar to

those steps used in RFC 1421 and are performed for each 'innermost

level' body:


Step 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 specified by the type/subtype information.


Step 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 RFC822. Note that the

restriction on line lengths implied by RFC822 is eliminated if the

next step employs either quoted-printable or base64 encoding.







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Step 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.


Step 4. Insertion into entity.


The encoded object is inserted into a MIME entity with appropriate

headers. The entity is then inserted into the body of a higher-level

entity (message or multipart) if needed.


It is vital to note that these steps are only a model; they are

specifically NOT a blueprint for how an actual system would be built.

In particular, the model fails to account for two common designs:


1. In many cases the conversion to a canonical form prior to

encoding will be subsumed into the encoder itself, which

understands local formats directly. For example, the local

newline convention for text bodies might be carried through to the

encoder itself along with knowledge of what that format is.


2. The output of the encoders may have to pass through one or

more additional steps prior to being transmitted as a message. As

such, the output of the encoder may not be conformant with the

formats specified by RFC822. In particular, once again it may be

appropriate for the converter's output to be expressed using local

newline conventions rather than using the standard RFC822 CRLF



Other implementation variations are conceivable as well. The vital

aspect of this discussion is that, in spite of any optimizations,

collapsings of required steps, or insertion of additional processing,

the resulting messages must be consistent with those produced by the

model described here. For example, a message with the following

header fields:


Content-type: text/foo; charset=bar

Content-Transfer-Encoding: base64


must be first represented in the text/foo form, then (if necessary)

represented in the "bar" character set, and finally transformed via

the base64 algorithm into a mail-safe form.







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Appendix H -- Changes from RFC 1341


This document is a relatively minor revision of RFC 1341. For

the convenience of those familiar with RFC 1341, the technical

changes from that document are summarized in this appendix.


1. The definition of "tspecials" has been changed to no longer

include ".".


2. The Content-ID field is now mandatory for message/external-body



3. The text/richtext type (including the old Section 7.1.3 and

Appendix D) has been moved to a separate document.


4. The rules on header merging for message/partial data have been

changed to treat the Encrypted and MIME-Version headers as special



5. The definition of the external-body access-type parameter has

been changed so that it can only indicate a single access method

(which was all that made sense).


6. There is a new "Subject" parameter for message/external-body,

access-type mail-server, to permit MIME-based use of mail servers

that rely on Subject field information.


7. The "conversions" parameter for application/octet-stream has been



8. Section 7.4.1 now deprecates the use of the "name" parameter for

application/octet-stream, as this will be superseded in the future by

a Content-Disposition header.


9. The formal grammar for multipart bodies has been changed so that

a CRLF is no longer required before the first boundary line.


10. MIME entities of type "message/partial" and "message/external-

body" are now required to use only the "7bit" transfer-encoding.

(Specifically, "binary" and "8bit" are not permitted.)


11. The "application/oda" content-type has been removed.


12. A note has been added to the end of section 7.2.3, explaining

the semantics of Content-ID in a multipart/alternative MIME entity.


13. The formal syntax for the "MIME-Version" field has been

tightened, but in a way that is completely compatible with the only




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version number defined in RFC 1341.


14. In Section 7.3.1, the definition of message/rfc822 has been

relaxed regarding mandatory fields.


All other changes from RFC 1341 were editorial changes and do not

affect the technical content of MIME. Considerable formal grammar

has been added, but this reflects the prose specification that was

already in place.











































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[US-ASCII] Coded Character Set--7-Bit American Standard Code for

Information Interchange, ANSI X3.4-1986.


[ATK] Borenstein, Nathaniel S., Multimedia Applications Development

with the Andrew Toolkit, Prentice-Hall, 1990.


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

Columbus, Ohio, 1990.


[ISO-2022] International Standard--Information Processing--ISO 7-bit

and 8-bit coded character sets--Code extension techniques, ISO



[ISO-8859] 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, Geneva, 1972,

"Pulse Code Modulation (PCM) of Voice Frequencies".


[POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference

Manual, Addison-Wesley, 1985.


[POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference

Manual, Addison-Wesley, Second Edition, 1990.


[X400] Schicker, Pietro, "Message Handling Systems, X.400", Message

Handling Systems and Distributed Applications, E. Stefferud, O-j.

Jacobsen, and P. Schicker, eds., North-Holland, 1989, pp. 3-41.


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

June 1981.


[RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC

821, USC/Information Sciences Institute, August 1982.




Borenstein & Freed [Page 80]



RFC 1521 MIME September 1993



[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC 822, UDEL, August 1982.


[RFC-934] Rose, M., and E. Stefferud, "Proposed Standard for Message

Encapsulation", RFC 934, Delaware and NMA, January 1985.


[RFC-959] Postel, J. and J. Reynolds, "File Transfer Protocol",

STD 9, RFC 959, USC/Information Sciences Institute, October 1985.


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

Messages", STD 11, RFC 1049, CMU, March 1988.


[RFC-1421] Linn, J., "Privacy Enhancement for Internet Electronic Mail:

Part I - Message Encryption and Authentication Procedures", RFC

1421, IAB IRTF PSRG, IETF PEM WG, February 1993.


[RFC-1154] Robinson, D. and R. Ullmann, "Encoding Header Field for

Internet Messages", RFC 1154, Prime Computer, Inc., April 1990.


[RFC-1341] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet

Mail Extensions): Mechanisms for Specifying and Describing the Format

of Internet Message Bodies", RFC 1341, Bellcore, Innosoft, June 1992.


[RFC-1342] Moore, K., "Representation of Non-Ascii Text in Internet

Message Headers", RFC 1342, University of Tennessee, June 1992.


[RFC-1343] Borenstein, N., "A User Agent Configuration Mechanism

for Multimedia Mail Format Information", RFC 1343, Bellcore, June



[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-1426] Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M.,

Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIME

transport", RFC 1426, United Nations Universit, Innosoft, Dover Beach

Consulting, Inc., Network Management Associates, Inc., The Branch

Office, February 1993.


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

Message Headers" RFC 1522, University of Tennessee, September 1993.


[RFC-1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC

1340, USC/Information Sciences Institute, July 1992.





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