RFC 1521
MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies
Pages: 81
Obsoletes: 1341
Obsoleted by: 2045 2046 2047 2048 2049
Updated by: 1590
Obsoletes: 1341
Obsoleted by: 2045 2046 2047 2048 2049
Updated by: 1590
Part 1 of 3 – Pages 1 to 23
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. Abstract 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 822. 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 agents. This document does NOT extend Internet mail header fields to permit anything other than US-ASCII text data. Such extensions are the subject of a companion document [RFC-1522]. This document is a revision of RFC 1341. Significant differences from RFC 1341 are summarized in Appendix H.
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 7.3.3.1. The "ftp" and "tftp" access-types............... 44 7.3.3.2. The "anon-ftp" access-type...................... 45 7.3.3.3. The "local-file" and "afs" access-types......... 45 7.3.3.4. The "mail-server" access-type................... 45 7.3.3.5. 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
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
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 describes: 1. A MIME-Version header field, which uses a version number to declare a message to be conformant with this specification and allows mail processing agents to distinguish between such messages and those generated by older or non-conformant software, which 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 manner. 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 service. 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.
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
elegance.
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].
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.
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
notation.
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
a header field, with the following verbatim text: MIME-Version: 1.0 The presence of this header field is an assertion that the message has been composed in compliance with this document. Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field: version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT Thus, future format specifiers, which might replace or extend "1.0", are constrained to be two integer fields, separated by a period. If a message is received with a MIME-version value other than "1.0", it cannot be assumed to conform with this specification. Note that the MIME-Version header field is required at the top level of a message. It is not required for each body part of a multipart entity. It is required for the embedded headers of a body of type "message" if and only if the embedded message is itself claimed to be MIME-conformant. It is not possible to fully specify how a mail reader that conforms with MIME as defined in this document should treat a message that might arrive in the future with some value of MIME-Version other than "1.0". 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)
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 here. The Content-Type header field is used to specify the nature of the data in the body of an entity, by giving type and subtype identifiers, and by providing auxiliary information that may be required for certain types. After the type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The 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.
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 *(";" parameter) ; 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
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 standardization. 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
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.
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
application/PostScript.)
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
anything.
It should be noted that the list of Content-Type values given here
may be augmented in time, via the mechanisms described above, and
that the set of subtypes is expected to grow substantially.
When a mail reader encounters mail with an unknown Content-type
value, it should generally treat it as equivalent to
"application/octet-stream", as described later in this document.
5. 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.
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.
"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
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
as:
Content-Type: text/plain; charset=ISO-8859-1
Content-transfer-encoding: base64
This must be interpreted to mean that the body is a base64 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
encoded.
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.
NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-
TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding
could be inferred from the characteristics of the Content-Type
that is to be encoded, or, at the very least, that certain
Content-Transfer-Encodings could be mandated for use with specific
Content-Types. There are several reasons why this is not the case.
First, given the varying types of transports used for mail, some
encodings may be appropriate for some Content-Type/transport
combinations and not for others. (For example, in an 8-bit
transport, no encoding would be required for text in certain
character sets, while such encodings are clearly required for 7-
bit SMTP.) Second, certain Content-Types may require different
types of transfer encoding under different circumstances. For
example, many PostScript bodies might consist entirely of short
lines of 7-bit data and hence require 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.
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
octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN
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
"=" 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
data.
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
used.
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
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
characters
!"#$@[\]^`{|}~
according to rule #1. See Appendix B for more information.
Because quoted-printable data is generally assumed to be line-
oriented, it is to be expected that the representation of the breaks
between the lines of quoted printable data may be altered in
transport, in the same manner that plain text mail has always been
altered in Internet mail when passing between systems with differing
newline conventions. If such alterations are likely to constitute a
corruption of the data, it is probably more sensible to use the
base64 encoding rather than the quoted-printable encoding.
WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-
printable, care must be taken to encode CR and LF characters as "=0D"
and "=0A", respectively. In particular, a CRLF sequence in binary
data should be encoded as "=0D=0A". Otherwise, if CRLF were
represented as a hard line break, it might be incorrectly decoded on
platforms with different line break conventions.
For formalists, the syntax of quoted-printable data is described by
the following grammar:
quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
; Maximum line length of 76 characters excluding CRLF
ptext := octet /<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.
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
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
encoding.
(page 23 continued on part 2)