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.
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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 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
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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 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.
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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 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].
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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 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
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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 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)
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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 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.
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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 *(";" parameter) ; case-insensitive matching of type and subtype
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 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
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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 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.
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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:
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 as:
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.
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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.
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.
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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 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
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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 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
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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
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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 encoding.
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 world-unique.
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 multipart/alternative.
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 F.
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- ascii".
Beyond plain text, there are many formats for representing what might be known as "extended text" -- text with embedded formatting and presentation information. An interesting characteristic of many such representations is that they are to some extent readable even without the software that interprets them. It is useful, then, to distinguish them, at the highest level, from such unreadable data as
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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 definitions.
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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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 specification.
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 document.
NOTE: Beyond US-ASCII, an enormous proliferation of character sets is possible. It is the opinion of the IETF working group that a large number of character sets is NOT a good thing. We would prefer to specify a single character set that can be used universally for representing all of the world's languages in electronic mail. Unfortunately, existing practice in several communities seems to point to the continued use of multiple character sets in the near future. For this reason, we define
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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 follows:
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:
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
--gc0p4Jq0M:2Yt08jU534c0p
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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 line:
--gc0p4Jq0M2Yt08jU534c0p--
There appears to be room for additional information prior to the first encapsulation boundary and following the final boundary. These areas should generally be left blank, and implementations must ignore anything that appears before the first boundary or after the last one.
NOTE: These "preamble" and "epilogue" areas are generally not used because of the lack of proper typing of these parts and the lack of clear semantics for handling these areas at gateways, particularly X.400 gateways. However, rather than leaving the preamble area blank, many MIME implementations have found this to be a convenient place to insert an explanatory note for recipients who read the message with pre-MIME software, since such notes will be ignored by MIME-compliant software.
NOTE: Because encapsulation boundaries must not appear in the body parts being encapsulated, a user agent must exercise care to choose a unique boundary. The boundary in the example above could have been the result of an algorithm designed to produce boundaries with a very low probability of already existing in the
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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 <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> Subject: Sample message MIME-Version: 1.0 Content-type: multipart/mixed; boundary="simple boundary"
This is the preamble. It is to be ignored, though it is a handy place for mail composers to include an explanatory note to non-MIME conformant readers. --simple boundary
This is implicitly typed plain 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:
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 information.
Systems should recognize that the content of the various parts are interchangeable. Systems should choose the "best" type based on the local environment and preferences, in some cases even through user interaction. As with multipart/mixed, the order of body parts is significant. In this case, the alternatives appear in an order of increasing faithfulness to the original content. In general, the best choice is the LAST part of a type supported by the recipient system's local environment.
Multipart/alternative may be used, for example, to send mail in a fancy text format in such a way that it can easily be displayed anywhere:
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From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> Subject: Formatted text mail MIME-Version: 1.0 Content-Type: multipart/alternative; boundary=boundary42
--boundary42
Content-Type: text/plain; charset=us-ascii
...plain text version of message goes here.... --boundary42 Content-Type: text/richtext
.... RFC 1341 richtext version of same message goes here ... --boundary42 Content-Type: text/x-whatever
.... fanciest formatted version of same message goes here ... --boundary42--
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.
NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each part of a multipart/alternative entity represents the same data, but the mappings between the two are not necessarily without information loss. For example, information is lost when translating ODA to PostScript or plain text. It is recommended that each part should have a different Content-ID value in the case where the information content of the two parts is not identical. 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:
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:
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 type.
Message fragmentation and reassembly: The semantics of a reassembled partial message must be those of the "inner" message, rather than of a message containing the inner message. This makes it possible, for example, to send a large audio message as several partial messages, and still have it appear to the recipient as a simple audio message rather than as an encapsulated message containing an audio message. That is, the encapsulation of the message is considered to be "transparent".
When generating and reassembling the parts of a message/partial message, the headers of the encapsulated message must be merged with the headers of the enclosing entities. In this process the following rules must be observed:
(1) All of the header fields from the initial enclosing entity (part one), except those that start with "Content-" and the specific header fields "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 From: Bill@host.com To: joe@otherhost.com Subject: Audio mail Message-ID: <id1@host.com> MIME-Version: 1.0 Content-type: message/partial; id="ABC@host.com"; number=1; total=2
and the second half might look something like this:
From: Bill@host.com To: joe@otherhost.com Subject: Audio mail MIME-Version: 1.0 Message-ID: <id2@host.com> Content-type: message/partial; id="ABC@host.com"; number=2; total=2
... second half of encoded audio data goes here...
Then, when the fragmented message is reassembled, the resulting message to be displayed to the user should look something like this:
X-Weird-Header-1: Foo From: Bill@host.com To: joe@otherhost.com Subject: Audio mail Message-ID: <anotherid@foo.com> MIME-Version: 1.0 Content-type: audio/basic Content-transfer-encoding: base64
... first half of encoded audio data goes here... ... second half of encoded audio data goes here...
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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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 permitted.
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 optional.
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 fragments.
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:
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 access-types:
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.
7.3.3.1. 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 nickname.
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.
7.3.3.2. 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.
7.3.3.3. 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 "*.bellcore.com", to indicate a set of machines on which the data should be directly visible, while a single asterisk may be used to indicate a file that is expected to be universally available, e.g., via a global file system.
7.3.3.4. 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.
7.3.3.5. 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 parameter.
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:
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:
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- type.)
For example, a meeting scheduler might define a standard representation for information about proposed meeting dates. An intelligent user agent would use this information to conduct a dialog with the user, and might then send further mail based on that dialog. More generally, there have been several "active" messaging languages developed in which programs in a suitably specialized language are sent through the mail and automatically run in the recipient's environment.
Such applications may be defined as subtypes of the "application" Content-Type. This document defines two subtypes: octet-stream, and PostScript.
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 size.
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- stream.
The formal grammar for content-type header fields for application data is given by:
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:
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:
A Content-Type of "video" indicates that the body contains a time- varying-picture image, possibly with color and coordinated sound. The term "video" is used extremely generically, rather than with reference to any particular technology or format, and is not meant to preclude subtypes such as animated drawings encoded compactly. The subtype "mpeg" refers to video coded according to the MPEG standard [MPEG].
Note that although in general this document strongly discourages the mixing of multiple media in a single body, it is recognized that many so-called "video" formats include a representation for synchronized audio, and this is explicitly permitted for subtypes of "video".
The formal grammar for the content-type header field for data of type video is given by:
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 name.)
In general, the use of "X-" top-level types is strongly discouraged. Implementors should invent subtypes of the existing types whenever possible. The invention of new types is intended to be restricted primarily to the development of new media types for email, such as digital odors or holography, and not for new data formats in general. In many cases, a subtype of application will be more appropriate than a new top-level type.
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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 message/external-body.
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
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:
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:
Text:
-- Recognize and display "text" mail with the character set "US-ASCII."
-- Recognize other character sets at least to the extent of being able to inform the user about what character set the message uses.
-- Recognize the "ISO-8859-*" character sets to the extent of being able to display those characters that are common to ISO-8859-* and US-ASCII, namely all characters represented by octet values 0-127.
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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.
Message:
-- Recognize and display at least the primary (822) encapsulation.
Multipart:
-- 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".
Application:
-- 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:
A maximally portable mail representation, such as the base64 encoding, will confine itself to relatively short lines of text in which the only meaningful characters are taken from this set of 73 characters.
(7) Some mail transport agents will corrupt data that includes certain literal strings. In particular, a period (".") alone on a line is known to be corrupted by some (incorrect) SMTP implementations, and a line that starts with the five characters "From " (the fifth character is a SPACE) are commonly corrupted as
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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 <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> Subject: A multipart example Content-Type: multipart/mixed; boundary=unique-boundary-1
This is the preamble area of a multipart message. Mail readers that understand multipart format should ignore this preamble. If you are reading this text, you might want to consider changing to a mail reader that understands how to properly display multipart messages. --unique-boundary-1
...Some text appears here... [Note that the 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.]
This is <bold><italic>richtext.</italic></bold> <smaller>as defined in RFC 1341</smaller> <nl><nl>Isn't it <bigger><bigger>cool?</bigger></bigger>
--unique-boundary-1 Content-Type: message/rfc822
From: (mailbox in US-ASCII) To: (address in US-ASCII) Subject: (subject in US-ASCII) Content-Type: Text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: Quoted-printable
... Additional text in ISO-8859-1 goes here ...
--unique-boundary-1--
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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 definition.
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.
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".
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 functionality.
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.
To: IANA@isi.edu Subject: Registration of new MIME content-type/subtype
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 case.)
Person & email address to contact for further information:
E.2 Registration of New Access-type Values for Message/external-body
To: IANA@isi.edu 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 RFC-to-be.)
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.
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 problems. ________________________________________________________________ Content-type: image
Subtypes defined by this document: jpeg, gif
Important Parameters: none
Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: audio
Subtypes defined by this document: basic
Important Parameters: none
Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: video
Subtypes defined by this document: mpeg
Important Parameters: none
Encoding notes: base64 generally preferred
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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 delimiters.
Other implementation variations are conceivable as well. The vital aspect of this discussion is that, in spite of any optimizations, collapsings of required steps, or insertion of additional processing, the resulting messages must be consistent with those produced by the model described here. For example, a message with the following header fields:
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 parts.
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 cases.
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 removed.
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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References
[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.
[ISO-2022] International Standard--Information Processing--ISO 7-bit and 8-bit coded character sets--Code extension techniques, ISO 2022:1986.
[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.
[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.
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[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 1992.
[RFC-1344] Borenstein, N., "Implications of MIME for Internet Mail Gateways", RFC 1344, Bellcore, June 1992.
[RFC-1345] Simonsen, K., "Character Mnemonics & Character Sets", RFC 1345, Rationel Almen Planlaegning, June 1992.
[RFC-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.
