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Network Working Group N. Borenstein, Bellcore Internet Draft: MIME N. Freed, Innosoft February 1993 MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any other Internet Draft. This document is a PROPOSED revision to RFC 1341. Significant differences from RFC 1341 are summarized in Appendix H. In addition, in the PostScript version of this memo, ALL differences from RFC 1341 are indicated by a strikethrough bar across the text. (Change bars were not available.) Distribution of this memo is unlimited. Please send comments to ietf-822@dimacs.rutgers.edu. Abstract 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page i] information. This is based on earlier work documented in RFC 934 and 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. It is recognized that such extensions are necessary, and they are the subject of a companion document [RFC -1342]. Borenstein & Freed DRAFT - expires 8/1/93 [Page ii] THIS PAGE INTENTIONALLY LEFT BLANK. The table of contents should be inserted after this page. Borenstein & Freed DRAFT - expires 8/1/93 [Page iii] 1 Introduction Since its publication in 1982, 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 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 1113, 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 that either X.400 non-textual body parts must be converted to (not encoded in) an ASCII format, or that they must be discarded, notifying the RFC 822 user that discarding has occurred. This is clearly undesirable, as information that a user may wish to receive is lost. Even though a user's UA may not have the capability of dealing with the non-textual body part, the user might have some Borenstein & Freed DRAFT - expires 8/1/93 [Page 1] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 a mail message. 2.e An "image" Content-Type value, for transmitting still image (picture) data. Borenstein & Freed DRAFT - expires 8/1/93 [Page 2] MIME Multipurpose Internet Mail ExtensionFebruary 1993 2.f. An "audio" Content-Type value, for transmitting audio or voice data. 2.g. A "video" Content-Type value, for transmitting video or moving image data, possibly with audio as part of the composite video data format. 3. A Content-Transfer-Encoding header field, which can be used to specify an auxiliary encoding that was applied to the data in order to allow it to pass through mail transport mechanisms which may have data or character set limitations. 4. Three 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 RFC 1341 [RFC-1341]. This document is a relatively minor updating of that Borenstein & Freed DRAFT - expires 8/1/93 [Page 3] MIME Multipurpose Internet Mail ExtensionFebruary 1993 document, 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, particular [RFC-1342], [RFC 1343], [RFC-1344], and [RFC-1345]. 2 Notations, Conventions, and Generic BNF Grammar This document is being published in two versions, one as plain ASCII text and one as PostScript1 . The latter is recommended, though 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. 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", wherever it is used in this document, refers to a unique mapping of a byte stream to glyphs, a mapping which does not require external profiling information. For example, bare "ISO 10646" can't be the charset parameter, because it requires several language information for the unique mapping to glyphs. However, this term can refer to multibyte character sets and to extension techniques such as those used in ISO 2022. __________ 1PostScript is a trademark of Adobe Systems Incorporated. Borenstein & Freed DRAFT - expires 8/1/93 [Page 4] MIME Multipurpose Internet Mail ExtensionFebruary 1993 The term "message", when not further qualified, means either the (complete or "top-level") message being transferred on a network, or a message encapsulated in a body of type "message". The term "body part", in this document, means one of the parts of the body of a multipart entity. A body part has a header and a body, so it makes sense to speak about the body of a body part. The term "entity", in this document, means either a message or a body part. All kinds of entities share the property that they have a header and a body. The term "body", when not further qualified, means the body of an entity, that is the body of either a message or of a body part. Note : the previous four definitions are clearly circular. This is unavoidable, since the overal 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 5] MIME Multipurpose Internet Mail ExtensionFebruary 1993 proper historical or evolutionary context. Such information may be skipped by those who are focused entirely on building a conformant implementation, but may be of use to those who wish to understand why this document is written as it is. For ease of recognition, all BNF definitions have been placed in a fixed-width font in the PostScript version of this document. 3 The MIME-Version Header Field Since RFC 822 was published in 1982, there has really been only one format standard for Internet messages, and there has been little perceived need to declare the format standard in use. This document is an independent document that complements RFC 822. Although the extensions in this document have been defined in such a way as to be compatible with RFC 822, there are still circumstances in which it might be desirable for a mail-processing agent to know whether a message was composed with the new standard in mind. Therefore, this document defines a new header field, "MIME- Version", which is to be used to declare the version of the Internet message body format standard in use. Messages composed in accordance with this document MUST include such a header field, with the following verbatim text: MIME-Version: 1.0 The presence of this header field is an assertion that the message has been composed in compliance with this document. Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field: MIME-Version := 1*DIGIT "." 1*DIGIT Borenstein & Freed DRAFT - expires 8/1/93 [Page 6] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Thus, future format specifiers, which might replace or extend "1.0", are (minimally) constrained by the definition of "text", which appears in RFC 822. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 7] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 8] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. UNIVERSAL "FILENAME" PARAMETER: For any given content-type, the set of valid parameters is defined as part of the definition of the content-type itself. However, a single exception to this rule is the "filename" parameter, which may be included as part of any content-type field, and is to be interpreted as a suggested file name in the event that the user wishes to write out the data (in its unencoded form) to a file. 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 Extended BNF notation of RFC 822, a Content-Type header field value is defined as follows: Borenstein & Freed DRAFT - expires 8/1/93 [Page 9] MIME Multipurpose Internet Mail ExtensionFebruary 1993 content := "Content-Type" ":" type "/" subtype *(";" parms) ; case-insensitive matching of type and subtype type := "application" / "audio" / "image" / "message" / "multipart" / "text" / "video" / extension-token ; All values case-insensitive extension-token := x-token / iana-token iana-token := <a publicly-defined extension token, registered with IANA, as specified in appendix E> x-token := <The two characters "X-" or "x-" followed, with no intervening white space, by any token> subtype := token parms := filename / parameter filename := "filename" "=" "value" ; suggested name to use if body-part is stored locally parameter := attribute "=" value attribute := token value := token / quoted-string token := 1*<any (ASCII) CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "=" ; Must be in quoted-string, ; to use withing parameter values Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=", and the removal of ".". Borenstein & Freed DRAFT - expires 8/1/93 [Page 10] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 11] MIME Multipurpose Internet Mail ExtensionFebruary 1993 The seven standard initial predefined Content-Types are detailed in the bulk of this document. They are: text -- textual information. The primary subtype, "plain", indicates plain (unformatted) text. No special software is required to get the full meaning of the text, aside from support for the indicated character set. Subtypes are to be used for enriched text in forms where application software may enhance the appearance of the text, but such software must not be required in order to get the general idea of the content. Possible subtypes thus include any readable word processor format. A very simple and portable subtype, richtext, was defined in RFC 1341, with a future revision expected. multipart -- data consisting of multiple parts of independent data types. Four initial subtypes are defined, including the primary "mixed" subtype, "alternative" for representing the same data in multiple formats, "parallel" for parts intended to be viewed simultaneously, and "digest" for multipart entities in which each part is of type "message". message -- an encapsulated message. A body of Content-Type "message" is itself 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 12] MIME Multipurpose Internet Mail ExtensionFebruary 1993 subtype is "mpeg". application -- some other kind of data, typically either uninterpreted binary data or information to be processed by a mail-based application. The primary subtype, "octet-stream", is to be used in the case of uninterpreted binary data, in which case the simplest recommended action is to offer to write the information into a file for the user. An additional subtype, "PostScript", is defined for transporting PostScript documents in bodies. Other expected uses for "application" include spreadsheets, data for mail-based scheduling systems, and languages for "active" (computational) email. (Note that active email entails 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, either by error or by an older user agent, 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 13] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 1000 character lines. 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: Borenstein & Freed DRAFT - expires 8/1/93 [Page 14] MIME Multipurpose Internet Mail ExtensionFebruary 1993 encoding := "Content-Transfer-Encoding ":" mechanism mechanism := "7bit" / "quoted-printable" / "quoted-base64" / "8bit" / "binary" / x-token These values are not case sensitive. That is, Base64 and BASE64 and bAsE64 are all equivalent. An encoding type of 7BIT requires that the body is already in a seven-bit mail- ready representation. This is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the Content-Transfer-Encoding header field is not present. The values "8bit", "7bit", and "binary" all mean that NO encoding has been performed. However, they are potentially useful as indications of the kind of data contained in the object, and therefore of the kind of encoding that might need to be performed for transmission in a given transport system. In particular: "7bit" means that the data is all represented as short lines of US-ASCII data. "8bit" means that the lines are short, but there may be non-ASCII characters (octets with the high-order bit set). "Binary" means that not only may non-ASCII characters be present, but also that the lines are not necessarily short enough for SMTP transport. The difference between "8bit" (or any other conceivable bit-width token) and the "binary" token is that "binary" does not require adherence to any limits on line length or to the SMTP CRLF semantics, while the bit-width tokens do require such adherence. If the body contains data in any bit-width other than 7-bit, the appropriate bit-width Content-Transfer-Encoding token must be used (e.g., "8bit" for unencoded 8 bit wide data). If the body contains binary data, the "binary" Content-Transfer-Encoding token must be used. NOTE: The distinction between the Content- Transfer-Encoding values of "binary," "8bit," etc. may seem unimportant, in that all of them really Borenstein & Freed DRAFT - expires 8/1/93 [Page 15] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. As of the publication of this document, there are no standardized Internet transports for which it is legitimate to include unencoded 8-bit or binary data in mail bodies. Thus there are no circumstances in which the "8bit" or "binary" Content-Transfer-Encoding is actually legal on the Internet. However, in the event that 8-bit or binary mail transport becomes a reality in Internet mail, or when this document is used in conjunction with any other 8-bit or binary-capable transport mechanism, 8-bit or binary bodies should be labeled as such using this mechanism. NOTE: The five values defined for the Content- Transfer-Encoding field imply nothing about the Content-Type other than the algorithm by which it was encoded or the transport system requirements if unencoded. Implementors may, if necessary, define new Content- Transfer-Encoding values, but must use an x-token, which is a name prefixed by "X-" to indicate its non-standard status, e.g., "Content-Transfer-Encoding: x-my-new-encoding". However, unlike Content-Types and subtypes, the creation of new Content-Transfer-Encoding values is explicitly and strongly discouraged, as it seems likely to hinder interoperability with little potential benefit. Their use is allowed only as the result of an agreement between cooperating user agents. If a Content-Transfer-Encoding header field appears as part of a message header, it applies to the entire body of that message. If a Content-Transfer-Encoding header field appears as part of a body part's headers, it applies only to the body of that body part. If an entity is of type "multipart" or "message", the Content-Transfer-Encoding is not permitted to have any value other than a bit width (e.g., "7bit", "8bit", etc.) or "binary". Borenstein & Freed DRAFT - expires 8/1/93 [Page 16] MIME Multipurpose Internet Mail ExtensionFebruary 1993 It should be noted that email is character-oriented, so that the mechanisms described here are mechanisms for encoding arbitrary byte streams, not bit streams. If a bit stream is to be encoded via one of these mechanisms, it must first be converted to an 8-bit byte stream using the network standard bit order ("big-endian"), in which the earlier bits in a stream become the higher-order bits in a byte. A bit stream not ending at an 8-bit boundary must be padded with zeroes. This document provides a mechanism for noting the addition of such padding in the case of the application Content-Type, which has a "padding" parameter. The encoding mechanisms defined here explicitly encode all data in ASCII. Thus, for example, suppose an entity has header fields such as: Content-Type: text/plain; charset=ISO-8859-1 Content-transfer-encoding: base64 This must be interpreted to mean that the body is a base64 ASCII encoding of data that was originally in ISO-8859-1, and will be in that character set again after decoding. The following sections will define the two standard encoding mechanisms. The definition of new content-transfer- encodings is explicitly discouraged and should only occur when absolutely necessary. All content-transfer-encoding namespace except that beginning with "X-" is explicitly reserved to the IANA for future use. Private agreements about content-transfer-encodings are also explicitly discouraged. Certain Content-Transfer-Encoding values may only be used on certain Content-Types. In particular, it is expressly forbidden to use any encodings other than "7bit", "8bit", or "binary" with any Content-Type that recursively includes other Content-Type fields, notably the "multipart" and "message" Content-Types. All encodings that are desired for bodies of type multipart or message must be done at the innermost level, by encoding the actual body that needs to be encoded. NOTE ON ENCODING RESTRICTIONS: Though the prohibition against using content-transfer- encodings on data of type multipart or message may seem overly restrictive, it is necessary to Borenstein & Freed DRAFT - expires 8/1/93 [Page 17] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 18] MIME Multipurpose Internet Mail ExtensionFebruary 1993 limitations are. NOTE ON TRANSLATING ENCODINGS: The quoted- printable and base64 encodings are designed so that conversion between them is possible. The only issue that arises in such a conversion is the handling of line breaks. When converting from quoted-printable to base64 a line break must be converted into a CRLF sequence. Similarly, a CRLF sequence in base64 data must be converted to a quoted-printable line break, but ONLY when converting text data. NOTE ON CANONICAL ENCODING MODEL: There was some confusion, in earlier drafts of this memo, regarding the model for when email data was to be converted to canonical form and encoded, and in particular how this process would affect the treatment of CRLFs, given that the representation of newlines varies greatly from system to system. 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 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 19] MIME Multipurpose Internet Mail ExtensionFebruary 1993 be used when sending hexadecimal data, though a robust implementation may choose to recognize lowercase letters on receipt. Thus, for example, the value 12 (ASCII form feed) can be represented by "=0C", and the value 61 (ASCII EQUAL SIGN) can be represented by "=3D". Except when the following rules allow an alternative encoding, this rule is mandatory. Rule #2: (Literal representation) Octets with decimal values of 33 through 60 inclusive, and 62 through 126, inclusive, MAY be represented as the ASCII characters which correspond to those octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN through TILDE, respectively). Rule #3: (White Space): Octets with values of 9 and 32 MAY be represented as ASCII TAB (HT) and SPACE characters, respectively, but MUST NOT be so represented at the end of an encoded line. Any TAB (HT) or SPACE characters on an encoded line MUST thus be followed on that line by a printable character. In particular, an "=" at the end of an encoded line, indicating a soft line break (see rule #5) may follow one or more TAB (HT) or SPACE characters. It follows that an octet with value 9 or 32 appearing at the end of an encoded line must be represented according to Rule #1. This rule is necessary because some MTAs (Message Transport Agents, programs which transport messages from one user to another, or perform a part of such transfers) are known to pad lines of text with SPACEs, and others are known to remove "white space" characters from the end of a line. Therefore, when decoding a Quoted-Printable body, any trailing white space on a line must be deleted, as it will necessarily have been added by intermediate transport agents. Rule #4 (Line Breaks): A line break in a text body, independent of what its representation is following the canonical representation of the data being encoded, must be represented by a (RFC 822) line break, which is a CRLF sequence, in the Quoted-Printable encoding. Since the canonical representation of types other than text do not generally include the representation of line breaks, no hard line breaks should occur in the quoted-printable encoding of such types. Of course, Borenstein & Freed DRAFT - expires 8/1/93 [Page 20] MIME Multipurpose Internet Mail ExtensionFebruary 1993 occurences of "=0D", "=0A", "=0A=0D" and "=0D=0A" will eventually be encountered. In general, however, base64 is preferred over quoted-printable for binary data. Note that many implementations may elect to encode the local representation of various content types directly, as described in Appendix G. In particular, this may apply to plain text material on systems that use newline conventions other than CRLF delimiters. Such an implementation is permissible, but the generation of line breaks must be generalized to account for the case where alternate representations of newline sequences are used. Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES that encoded lines be no more than 76 characters long. If longer lines are to be encoded with the Quoted-Printable encoding, 'soft' line breaks must be used. An equal sign as the last character on a encoded line indicates such a non-significant ('soft') line break in the encoded text. Thus if the "raw" form of the line is a single unencoded line that says: Now's the time for all folk to come to the aid of their country. This can be represented, in the Quoted-Printable encoding, as Now's the time = for all folk to come= to the aid of their country. This provides a mechanism with which long lines are encoded in such a way as to be restored by the user agent. The 76 character limit does not count the trailing CRLF, but counts all other characters, including any equal signs. Since the hyphen character ("-") is represented as itself in the Quoted-Printable encoding, care must be taken, when encapsulating a quoted-printable encoded body in a multipart entity, to ensure that the encapsulation boundary does not appear anywhere in the encoded body. (A good strategy is to choose a boundary that includes a character sequence such as "=_" which can never appear in a quoted-printable body. See Borenstein & Freed DRAFT - expires 8/1/93 [Page 21] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 breaks between the lines of quoted printable data may be altered in transport, in the same manner that plain text mail has always been altered in Internet mail when passing between systems with differing newline conventions. If such alterations are likely to constitute a corruption of the data, it is probably more sensible to use the base64 encoding rather than the quoted-printable encoding. WARNING TO IMPLEMENTORS: If binary data are encoded in quoted-printable, care must be taken to encode CR and LF characters as "=0D" and "=0A", respectively. In particular, a CRLF sequence in binary data should be encoded as "=0D=0A". Otherwise, if CRLF were represented as a hard line break, it might be incorrectly decoded on platforms with different line break conventions. For formalists, the syntax of quoted-printable data is describe by the following grammar: quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF) Borenstein & Freed DRAFT - expires 8/1/93 [Page 22] MIME Multipurpose Internet Mail ExtensionFebruary 1993 ; Maximum line length of 78 characters ptext := octet / <any ASCII character except "=", SPACE, or TAB> ; characters in "qp-iffy" are also not recommended qp-iffy := "[" / "]" / <"> / "\" / "@" / "!" / "#" / "$" / "^" / "'" / "{" / "|" / "}" / "~" / "`" octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") ; octet must be used for characters > 127, =, SPACE, or TAB, ; and is recommended for the "qp-iffy" characters too. 5.2 Base64 Content-Transfer-Encoding The Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets in a form that is not 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 1113. The base64 encoding is adapted from RFC 1113, 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 23] MIME Multipurpose Internet Mail ExtensionFebruary 1993 concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base64 alphabet. When encoding a bit stream via the base64 encoding, the bit stream must be presumed to be ordered with the most- significant-bit first. That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit in the first byte, and so on. Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", "CR", "LF") and to the encapsulation boundaries defined in this document (e.g., "-"). Table 1: The Base64 Alphabet Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w (pad) = 15 P 32 g 49 x 16 Q 33 h 50 y The output stream (encoded bytes) must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software. In base64 data, characters other than those in Table 1, line breaks, and other white space probably indicate a transmission error, about which a Borenstein & Freed DRAFT - expires 8/1/93 [Page 24] MIME Multipurpose Internet Mail ExtensionFebruary 1993 warning message or even a message rejection might be appropriate under some circumstances. Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a body. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Padding at the end of the data is performed using the '=' character. Since all base64 input is an integral number of octets, only the following cases can arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character. Because it is used only for padding at the end of the data, the occurrence of any '=' characters may be taken as evidence that the end of the data has been reached (without truncation in transit). To offer the same assurance in the event that padding is not necessary, any base64 data that does not require padding may be terminated by a sequence of four '=' characters ("===="), though such termination is optional. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 25] MIME Multipurpose Internet Mail ExtensionFebruary 1993 6 Additional Content- Header Fields 6.1 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: 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 caching data referenced by the message/external-body mechanism. Although the Content-ID header is generally optional, its use is mandatory in 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 caching 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. Content-Description := *text The description is presumed to be given in the US-ASCII character set, although the mechanism specified in [RFC- 1342] may be used for non-US-ASCII Content-Description values. 6.3 Optional Content-MD5 Header Field Borenstein & Freed DRAFT - expires 8/1/93 [Page 26] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Despite all of the mechanisms MIME provides to attempt to protect data from being damaged in the course of email transport, it is still desirable to have a mechanism for verifying that the data, once decoded, are intact. For this reason, MIME defines the use of an optional header field, Content-MD5, whch may be used as a message integrity check (MIC), to verify that the decoded data are the same data that were initially sent. MD5 is an algorithm for computing a 128 bit "fingerprint" of arbitrary-length data, with a high degree of confidence that any alterations in the data will be reflected in alterations in the fingerprint. The algorithm itself is defined in RFC 1321 [RFC-1321]. This document further specifies the application of that algorithm for use as an integrity check for MIME mail. The MD5 algorithm is computed on the canonical form of the data. In particular, this means that the sender computes the MIC on the raw data, before applying any content- transfer-encoding, and that the receiver also computes the MIC on the raw data, after undoing any content-transfer- encodng. For text data, the algorithm must be computed on data in which the canonical form for newlines applies, that is, in which each newline is represented by CRLF. The output of the MD5 algorithm is a 128 bit signature. When viewed in network byte order (big-endian order), this yields a sequence of 16 octets of binary data. These 16 octets must then be encoded according to the base64 algorithm in order to obtain a value that can legally be placed in a message header field. Thus, if the data in a MIME entity has an MD5 MIC that consists of the (unlikely) 16 octets "Check Integrity!", then that MIME entity's header could contain the field Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ== The Content-MD5 field is completely optional, but its use is recommened wherever complete data integrity is essential. If present, a MIME-conformant reader may choose to use it to verify that the contents of a MIME entity have not been modified during transport. Textual data is regularly altered in the normal delivery of mail (See Appendix B). Because the addition of even trailing Borenstein & Freed DRAFT - expires 8/1/93 [Page 27] MIME Multipurpose Internet Mail ExtensionFebruary 1993 white space will result in a failed MIC, use of a content- transfer-encoding on all MIC protected mail is recommended. Borenstein & Freed DRAFT - expires 8/1/93 [Page 28] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. The primary subtype of text is "plain". This 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 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 data is the character set. This is specified with a "charset" parameter, as in: Content-type: text/plain; charset=us-ascii Borenstein & Freed DRAFT - expires 8/1/93 [Page 29] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. 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. For example, bare "ISO 10646" can't be the charset parameter, because it requires several language information for the unique mapping to glyphs. An initial list of predefined character set names can be found at the end of this section. Additional character sets may be registered with IANA as described in Appendix E, although the standardization of their use requires the usual IAB 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 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 30] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 names for a small number of character sets for which a strong constituent base exists. It is our hope that ISO 10646 or some other effort will eventually define a single world character set which can then be specified for use in Internet mail, but in the advance of that definition we cannot specify the use of ISO 10646, Unicode, or any other character set whose Borenstein & Freed DRAFT - expires 8/1/93 [Page 31] MIME Multipurpose Internet Mail ExtensionFebruary 1993 definition is, as of this writing, incomplete. 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. 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 as described in Appendix E, 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 32] MIME Multipurpose Internet Mail ExtensionFebruary 1993 view the mail correctly. 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: text-type := "text" "/" text-subtype [";" "charset" "=" charset] text-subtype := "plain" / "richtext" charset := "us-ascii" / "iso-8859-1" / "iso-8859-2" / "iso- 8859-3" / "iso-8859-4" / "iso-8859-5" / "iso-8859-6" / "iso- 8859-7" / "iso-8859-8" / "iso-8859-9" / extension-token ; case insensitive Borenstein & Freed DRAFT - expires 8/1/93 [Page 33] MIME Multipurpose Internet Mail ExtensionFebruary 1993 7.2 The Multipart Content-Type In the case of multiple part entities, in which one or more different sets of data are combined in a single body, a "multipart" Content-Type field must appear in the entity's header. The body must then contain one or more "body parts," each preceded by an encapsulation boundary, and the last one followed by a closing boundary. Each part starts with an encapsulation boundary, and then contains a body part consisting of header area, a blank line, and a body area. Thus a body part is similar to an RFC 822 message in syntax, but different in meaning. A body part is NOT to be interpreted as actually being an RFC 822 message. To begin with, NO header fields are actually required in body parts. A body part that starts with a blank line, therefore, is allowed and is a body part for which all default values are to be assumed. In such a case, the absence of a Content-Type header field implies that the corresponding body is plain US-ASCII text. The only header fields that have defined meaning for body parts are those the names of which begin with "Content-". All other header fields are generally to be ignored in body parts. Although they should generally be retained in mail processing, they may be discarded by gateways if necessary. Such other fields are permitted to appear in body parts but must not be depended on. "X-" fields may be created for experimental or private purposes, with the recognition that the information they contain may be lost at some gateways. 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 34] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 they may encode non-ASCII text as per [RFC-1342]), 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 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 35] MIME Multipurpose Internet Mail ExtensionFebruary 1993 NOTE: The hyphens are for rough compatibility with the earlier RFC 934 method of message encapsulation, and for ease of searching for the boundaries in some implementations. However, it should be noted that multipart messages are NOT completely compatible with RFC 934 encapsulations; in particular, they do not obey RFC 934 quoting conventions for embedded lines that begin with hyphens. This mechanism was chosen over the RFC 934 mechanism because the latter causes lines to grow with each level of quoting. The combination of this growth with the fact that SMTP implementations sometimes wrap long lines made the RFC 934 mechanism unsuitable for use in the event that deeply-nested multipart structuring is ever desired. WARNING TO IMPLEMENTORS: The grammar for parameters on the Content-type field is such that it is often necessary to enclose the boundaries in quotes on the Content-type line. This is not always necessary, but never hurts. Implementors should be sure to study the grammar carefully in order to avoid producing illegal Content-type fields. Thus, a typical multipart Content-Type header field might look like this: Content-Type: multipart/mixed; boundary=gc0p4Jq0M2Yt08jU534c0p But the following is illegal: Content-Type: multipart/mixed; boundary=gc0p4Jq0M:2Yt08jU534c0p (because of the colon) and must instead be represented as Content-Type: multipart/mixed; boundary="gc0p4Jq0M:2Yt08jU534c0p" 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 36] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Note that the encapsulation boundary must occur at the beginning of a line, i.e., following a CRLF, and that that initial CRLF is considered to be part of 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 considered part of 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 37] MIME Multipurpose Internet Mail ExtensionFebruary 1993 who read the message with pre-MIME software, since such notes will be ignored by MIME-compliant software. NOTE: Because encapsulation boundaries must not appear in the body parts being encapsulated, a user agent must exercise care to choose a unique boundary. The boundary in the example above could have been the result of an algorithm designed to produce boundaries with a very low probability of already existing in the data to be encapsulated without having to prescan the data. Alternate algorithms might result in more 'readable' boundaries for a recipient with an old user agent, but would require more attention to the possibility that the boundary might appear in the encapsulated part. The simplest boundary possible is something like "---", with a closing boundary of "-----". As a very simple example, the following multipart message has two parts, both of them plain text, one of them explicitly typed and one of them implicitly typed: From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> 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-- Borenstein & Freed DRAFT - expires 8/1/93 [Page 38] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 gateway, and must be deleted.) It is formally specified by the following BNF: boundary := 0*69<bchars> bcharsnospace bchars := bcharsnospace / " " bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_" / "," / "-" / "." / "/" / ":" / "=" / "?" Overall, the body of a multipart entity may be specified as follows: multipart-body := preamble 1*encapsulation close-delimiter epilogue encapsulation := delimiter body-part CRLF delimiter := "--" boundary CRLF ; taken from Content-Type field. ; There must be no space ; between "--" and boundary. close-delimiter := "--" boundary "--" CRLF ; Again, no space by "--", Borenstein & Freed DRAFT - expires 8/1/93 [Page 39] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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: 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 intended to be displayed serially. 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 40] MIME Multipurpose Internet Mail ExtensionFebruary 1993 particular, each of the parts is an "alternative" version of the same information. User agents should recognize that the content of the various parts are interchangeable. The user agent should either choose the "best" type based on the user's environment and preferences, or offer the user the available alternatives. In general, choosing the best type means displaying only the LAST part that can be displayed. This may be used, for example, to send mail in a fancy text format in such a way that it can easily be displayed anywhere: From: Nathaniel Borenstein <nsb@bellcore.com> To: Ned Freed <ned@innosoft.com> 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 .... 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 41] MIME Multipurpose Internet Mail ExtensionFebruary 1993 show that alternative, an earlier alternative, or both. NOTE: From an implementor's perspective, it might seem more sensible to reverse this ordering, and have the plainest alternative last. However, placing the plainest alternative first is the friendliest possible option when multipart/alternative entities are viewed using a non-MIME-conformant mail reader. While this approach does impose some burden on conformant mail readers, interoperability with older mail readers was deemed to be more important in this case. It may be the case that some user agents, if they can recognize more than one of the formats, will prefer to offer the user the choice of which format to view. This makes sense, for example, if mail includes both a nicely-formatted image version and an easily-edited text version. What is most critical, however, is that the user not automatically be shown multiple versions of the same data. Either the user should be shown the last recognized version or should explicitly be given the choice. Note on the semantics of Content-ID in multipart/alternative: Because each part of a multipart/alternative entity represents the same data, it is recommended that each part should have the same Content-ID value, if any. However, it is recommended that this 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 another Content- ID value will refer to any of the parts inside it. Borenstein & Freed DRAFT - expires 8/1/93 [Page 42] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. 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, all of the parts are intended to be presented in parallel, i.e., simultaneously, on hardware and software that are capable of doing so. Borenstein & Freed DRAFT - expires 8/1/93 [Page 43] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Composing agents should be aware that many mail readers will lack this capability and will show the parts serially in any event. 7.2.6 Other Multipart subtypes Other multipart subtypes are expected in the future. MIME implementations must in general treat unrecognized subtypes of multipart as being equivalent to "multipart/mixed". The formal grammar for content-type header fields for multipart data is given by: multipart-type := "multipart" "/" multipart-subtype ";" "boundary" "=" boundary multipart-subtype := "mixed" / "parallel" / "digest" / "alternative" / "extension-token" 7.3 The Message Content-Type It is frequently desirable, in sending mail, to encapsulate another mail message. For this common operation, a special Content-Type, "message", is defined. The primary subtype, message/rfc822, has no required parameters in the Content- Type field. Additional subtypes, "partial" and "External- body", do have required parameters. These subtypes are explained below. NOTE: It has been suggested that subtypes of message might be defined for forwarded or rejected messages. However, forwarded and rejected messages can be handled as multipart messages in which the first part contains any control or descriptive information, and a second part, of type message/rfc822, is the forwarded or rejected message. Composing rejection and forwarding messages in this manner will preserve the type information on the original message and allow it to be correctly presented to the recipient, and hence is strongly encouraged. As stated in the definition of the Content-Transfer-Encoding field, no encoding other than "7bit", "8bit", or "binary" is permitted for messages or parts of type "message". Even stronger restrictions apply to the subtype Borenstein & Freed DRAFT - expires 8/1/93 [Page 44] MIME Multipurpose Internet Mail ExtensionFebruary 1993 "message/partial", 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-1342]. 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. 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 45] MIME Multipurpose Internet Mail ExtensionFebruary 1993 is required on the final part, and is optional on the earlier parts. Note also that these parameters may be given in any order. Thus, part 2 of a 3-part message may have either of the following header fields: Content-Type: Message/Partial; number=2; total=3; id="oc=jpbe0M2Yt4s@thumper.bellcore.com" Content-Type: Message/Partial; id="oc=jpbe0M2Yt4s@thumper.bellcore.com"; number=2 But part 3 MUST specify the total number of parts: Content-Type: Message/Partial; number=3; total=3; id="oc=jpbe0M2Yt4s@thumper.bellcore.com" Note that part numbering begins with 1, not 0. When the parts of a message broken up in this manner are put together, the result is a complete RFC 822 format message, 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 46] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 X-Weird-Header-1: Bar X-Weird-Header-2: Hello 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... and the second half might look something like this: From: Bill@host.com To: joe@otherhost.com Subject: Audio mail Borenstein & Freed DRAFT - expires 8/1/93 [Page 47] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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-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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 48] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 a body is of type "message/external-body", it consists of a header, two consecutive CRLFs, and the message header for the encapsulated message. If another pair of consecutive CRLFs appears, this of course ends the message header for the encapsulated message. However, since the encapsulated message's body is itself external, it does NOT appear in the area that follows. For example, consider the following message: Content-type: message/external-body; access- type=local-file; name="/u/nsb/Me.gif" Content-type: image/gif THIS IS NOT REALLY THE BODY! The area at the end, which might be called the "phantom body", is ignored for most external-body messages. However, Borenstein & Freed DRAFT - expires 8/1/93 [Page 49] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 date 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. PERMISSION -- A 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 50] MIME Multipurpose Internet Mail ExtensionFebruary 1993 ALL message/external-body entities MUST include a Content-ID header field to give a unique identifier by which to reference the data. This identifier may be used for caching mechanisms, and for recognizing the receipt of the data when the access-type is "mail-server". 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 optional parameters may also appear when the access-type is FTP or ANON-FTP: DIRECTORY -- A directory from which the data named by NAME should be retrieved. MODE -- A transfer mode for retrieving the information, e.g. "image", with values as specified by the FTP and TFTP protocols [RFC-783] [RFC-959]. If MODE is not specified, a default mode of "ASCII" is to be assumed. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 51] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 52] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 53] MIME Multipurpose Internet Mail ExtensionFebruary 1993 message itself. For most message/external-body messages, this trailing area must simply be ignored. However, it is a convenient 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 NAME@SERVER, where NAME and SERVER are the values of the NAME and SERVER parameters, respectively. The embedded message header fields which appear in the body of the message/external-body data can be used to declare the Content-type of the external body. Thus a complete message/external-body message, referring to a document in PostScript format, might look like this: From: Whomever To: Someone Subject: whatever MIME-Version: 1.0 Message-ID: <id1@host.com> Content-Type: multipart/alternative; boundary=42 --42 Content-Type: message/external-body; name="BodyFormats.ps"; site="thumper.bellcore.com"; access-type=ANON-FTP; directory="pub"; mode="image"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript --42 Content-Type: message/external-body; name="/u/nsb/writing/rfcs/RFC-MIME.ps"; site="thumper.bellcore.com"; access-type=AFS expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript --42 Borenstein & Freed DRAFT - expires 8/1/93 [Page 54] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Content-Type: message/external-body; access-type=mail-server server="listserv@bogus.bitnet"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript get RFC-MIME.DOC --42-- Like the message/partial type, the message/external-body type is intended to be transparent, that is, to convey the data type in the external body rather than to convey a message with a body of that type. Thus the headers on the outer and inner parts must be merged using the same rules as for message/partial. In particular, this means that the Content-type header is overridden, but the From and Subject headers are preserved. Note that since the external bodies are not transported as mail, they need not conform to the 7-bit and line length requirements, but might in fact be binary files. Thus a Content-Transfer-Encoding is not generally necessary, though it is permitted. Note that the body of a message of type "message/external- body" is governed by the basic syntax for an RFC 822 message. In particular, anything before the first consecutive pair of CRLFs is header information, while anything after it is body information, which is ignored for most access-types. The formal grammar for content-type header fields for data of type message is given by: message-type := "message" "/" message-subtype message-subtype := "rfc822" / "partial" 2#3partial-param / "external-body" 1*external-param / extension-token partial-param := (";" "id" "=" value) Borenstein & Freed DRAFT - expires 8/1/93 [Page 55] MIME Multipurpose Internet Mail ExtensionFebruary 1993 / (";" "number" "=" 1*DIGIT) / (";" "total" "=" 1*DIGIT) ; id & number required; total required for last part external-param := (";" "access-type" "=" atype) / (";" "expiration" "=" date-time) / (";" "size" "=" 1*DIGIT) / (";" "permission" "=" ("read" / "read- write")) / (";" "name" "=" value) / (";" "site" "=" value) / (";" "dir" "=" value) / (";" "mode" "=" value) / (";" "server" "=" value) ; access-type required; others required based on access-type atype := "ftp" / "anon-ftp" / "tftp" / "local-file" / "afs" / "mail-server" / extension-token Borenstein & Freed DRAFT - expires 8/1/93 [Page 56] MIME Multipurpose Internet Mail ExtensionFebruary 1993 7.4 The Application Content-Type The "application" Content-Type is to be used for data which do not fit in any of the other categories, and particularly for data to be processed by mail-based uses of application programs. This is information which must be processed by an application before it is viewable or usable to a user. Expected uses for Content-Type application include mail- based file transfer, spreadsheets, data for mail-based scheduling systems, and languages for "active" (computational) email. (The latter, in particular, can pose security problems which must be understood by implementors, and are considered in detail in the discussion of the application/PostScript content-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 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 57] MIME Multipurpose Internet Mail ExtensionFebruary 1993 PADDING -- the number of bits of padding that were appended to the bitstream comprising the actual contents to produce the enclosed byte-oriented data. This is useful for enclosing a bitstream 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. NOTE: RFC 1341 also defined the use of a "NAME" parameter which was identical with the global "FILENAME" parameter defined in this document. It is recommended that implementations treat "NAME" as a synonym for "FILENAME" if it is encountered. However, implementations must generate "FILENAME" rather than "NAME", and preference must be given to "FILENAME" if both parameters appear on a received message. 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]. The PostScript language definition provides facilities for internal labelling of the specific language features a given program uses. This labelling, called the PostScript document structuring conventions, is very general and provides substantially more information than just the language level. The use of document structuring conventions, while not Borenstein & Freed DRAFT - expires 8/1/93 [Page 58] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 reenabling full-function versions of these operators. The PostScript language provides facilities for exiting the normal interpreter, or server, loop. Changes made in this Borenstein & Freed DRAFT - expires 8/1/93 [Page 59] MIME Multipurpose Internet Mail ExtensionFebruary 1993 "outer" environment are customarily retained across documents, and may in some cases be retained semipermanently in nonvolatile memory. The operators associated with exiting the interpreter loop have the potential to interfere with subsequent document processing. As such, their unrestrained use constitutes a threat of service denial. PostScript operators that exit the interpreter loop include, but may not be limited to, the exitserver and startjob operators. Message-sending software should not generate PostScript that depends on exiting the interpreter loop to operate. 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. Eliminate the startjob and exitserver commands. If these commands cannot be eliminated, at least set the password associated with them 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, at least set the password associated with them 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 60] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 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: application-type := "application" "/" application-subtype application-subtype := ("octet-stream" *stream) / "postscript" / extension-token stream := [";" "name" "=" value] Borenstein & Freed DRAFT - expires 8/1/93 [Page 61] MIME Multipurpose Internet Mail ExtensionFebruary 1993 [";" type "=" value] [";" padding "=" padding] padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" 7.5 The Image Content-Type A Content-Type of "image" indicates that the body contains an image. The subtype names the specific image format. These names are case insensitive. Two initial subtypes are "jpeg" for the JPEG format, JFIF encoding, and "gif" for GIF format [GIF]. The list of image subtypes given here is neither exclusive nor exhaustive, and is expected to grow as more types are registered with IANA, as described in Appendix E. The formal grammar for the content-type header field for data of type image is given by: image-type := "image" "/" ("gif" / "jpeg" / extension-token) 7.6 The Audio Content-Type A Content-Type of "audio" indicates that the body contains audio data. Although there is not yet a consensus on an "ideal" audio format for use with computers, there is a pressing need for a format capable of providing interoperable behavior. The initial subtype of "basic" is specified to meet this requirement by providing an absolutely minimal lowest common denominator audio format. It is expected that richer formats for higher quality and/or lower bandwidth audio will be defined by a later document. The content of the "audio/basic" subtype is audio encoded using 8-bit ISDN mu-law [PCM]. When this subtype is present, a sample rate of 8000 Hz and a single channel is assumed. The formal grammar for the content-type header field for data of type audio is given by: Borenstein & Freed DRAFT - expires 8/1/93 [Page 62] MIME Multipurpose Internet Mail ExtensionFebruary 1993 audio-type := "audio" "/" ("basic" / extension-token) 7.7 The Video Content-Type A Content-Type of "video" indicates that the body contains a time-varying-picture image, possibly with color and coordinated sound. The term "video" is used extremely generically, rather than with reference to any particular technology or format, and is not meant to preclude subtypes such as animated drawings encoded compactly. The subtype "mpeg" refers to video coded according to the MPEG standard [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: video-type := "video" "/" ("mpeg" / extension-token) 7.8 Experimental Content-Type Values A Content-Type value beginning with the characters "X-" is a private value, to be used by consenting mail systems by mutual agreement. Any format without a rigorous and public definition must be named 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 63] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 text/richtext, message/partial, and message/external-body. Borenstein & Freed DRAFT - expires 8/1/93 [Page 64] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Acknowledgements This document is the result of the collective effort of a large number of people, at several IETF meetings, on the IETF-SMTP and IETF-822 mailing lists, and elsewhere. Although any enumeration seems doomed to suffer from egregious omissions, the following are among the many contributors to this effort: Harald Tveit Alvestrand Timo Lehtinen Randall Atkinson John R. MacMillan Philippe Brandon Rick McGowan Kevin Carosso Leo Mclaughlin Uhhyung Choi Goli Montaser-Kohsari Cristian Constantinof Keith Moore Mark Crispin Tom Moore Dave Crocker Erik Naggum Terry Crowley Mark Needleman Walt Daniels John Noerenberg Frank Dawson Mats Ohrman Hitoshi Doi Julian Onions Kevin Donnelly Michael Patton Keith Edwards David J. Pepper Chris Eich Blake C. Ramsdell Johnny Eriksson Luc Rooijakkers Craig Everhart Marshall T. Rose Patrik Faeltstroem Jonathan Rosenberg Erik E. Fair Jan Rynning Roger Fajman Harri Salminen Alain Fontaine Michael Sanderson James M. Galvin Masahiro Sekiguchi Philip Gladstone Mark Sherman Thomas Gordon Keld Simonsen Phill Gross Bob Smart James Hamilton Peter Speck Steve Hardcastle-Kille Henry Spencer David Herron Einar Stefferud Bruce Howard Michael Stein Bill Janssen Klaus Steinberger Olle Jaernefors Peter Svanberg Risto Kankkunen James Thompson Phil Karn Steve Uhler Alan Katz Stuart Vance Tim Kehres Erik van der Poel Borenstein & Freed DRAFT - expires 8/1/93 [Page 65] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Neil Katin Guido van Rossum Kyuho Kim Peter Vanderbilt Anders Klemets Greg Vaudreuil John Klensin Ed Vielmetti Valdis Kletniek Ryan Waldron Jim Knowles Wally Wedel Stev Knowles Sven-Ove Westberg Bob Kummerfeld Brian Wideen Pekka Kytolaakso John Wobus Stellan Lagerstr.m Glenn Wright Vincent Lau Rayan Zachariassen Donald Lindsay David Zimmerman Bob Braden Brian Capouch Peter Clitherow Dave Collier-Brown John Coonrod Jim Davis Axel Deininger Dana S Emery Martin Forssen Stephen Gildea Mark Horton Warner Losh Laurence Lundblade Charles Lynn Larry Masinter Michael J. McInerny Jon Postel Christer Romson Yutaka Sato Richard Alan Schafer Larry W. Virden Rhys Weatherly Dave Wecker The authors apologize for any omissions from this list, which are certainly unintentional. Borenstein & Freed DRAFT - expires 8/1/93 [Page 66] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 67] MIME Multipurpose Internet Mail ExtensionFebruary 1993 -- Recognize the "ISO-8859-*" character sets to the extent of being able to display those characters that are common to ISO-8859-* and US-ASCII, namely all characters represented by octet values 0-127. -- For unrecognized subtypes, show or offer to show the user the "raw" version of the data. 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 68] MIME Multipurpose Internet Mail ExtensionFebruary 1993 way. A user agent that meets the above conditions is said to be MIME-conformant. The meaning of this phrase is that it is assumed to be "safe" to send virtually any kind of properly-marked data to users of such mail systems, because such systems will at least be able to treat the data as undifferentiated binary, and will not simply splash it onto the screen of unsuspecting users. There is another sense in which it is always "safe" to send data in a format that is MIME-conformant, which is that such data will not break or be broken by any known systems that are conformant with RFC 821 and RFC 822. User agents that are MIME-conformant have the additional guarantee that the user will not be shown data that were never intended to be viewed as text. Borenstein & Freed DRAFT - expires 8/1/93 [Page 69] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 70] MIME Multipurpose Internet Mail ExtensionFebruary 1993 (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 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: Borenstein & Freed DRAFT - expires 8/1/93 [Page 71] MIME Multipurpose Internet Mail ExtensionFebruary 1993 "'" (ASCII code 39) "(" (ASCII code 40) ")" (ASCII code 41) "+" (ASCII code 43) "," (ASCII code 44) "-" (ASCII code 45) "." (ASCII code 46) "/" (ASCII code 47) ":" (ASCII code 58) "=" (ASCII code 61) "?" (ASCII code 63) A maximally portable mail representation, such as the base64 encoding, will confine itself to relatively short lines of text in which the only meaningful characters are taken from this set of 73 characters. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 72] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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.] --unique-boundary-1 Content-type: text/plain; charset=US-ASCII This could have been part of the previous part, but illustrates explicit versus implicit typing of body parts. --unique-boundary-1 Content-Type: multipart/parallel; boundary=unique-boundary-2 --unique-boundary-2 Borenstein & Freed DRAFT - expires 8/1/93 [Page 73] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Content-Type: audio/basic Content-Transfer-Encoding: base64 ... base64-encoded 8000 Hz single-channel mu-law-format audio data goes here.... --unique-boundary-2 Content-Type: image/gif Content-Transfer-Encoding: Base64 ... base64-encoded image data goes here.... --unique-boundary-2-- --unique-boundary-1 Content-type: text/richtext This is <bold><italic>richtext.</italic></bold> <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-- Borenstein & Freed DRAFT - expires 8/1/93 [Page 74] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. application-subtype := ("octet-stream" *stream) / "postscript" / extension-token application-type := "application" "/" application-subtype attribute := token atype := "ftp" / "anon-ftp" / "tftp" / "local-file" / "afs" / "mail-server" / extension-token audio-type := "audio" "/" ("basic" / extension-token) body-part = <"message" as defined in RFC 822, with all header fields optional, and with the specified delimiter not occurring anywhere in the message body, either on a line by itself or as a substring anywhere.> boundary := 0*69<bchars> bcharsnospace bchars := bcharsnospace / " " bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_" / "," / "-" / "." / "/" / ":" / "=" / "?" charset := "us-ascii" / "iso-8859-1" / "iso-8859-2" / "iso- 8859-3" / "iso-8859-4" / "iso-8859-5" / "iso-8859-6" / "iso- 8859-7" / "iso-8859-8" / "iso-8859-9" / extension-token Borenstein & Freed DRAFT - expires 8/1/93 [Page 75] MIME Multipurpose Internet Mail ExtensionFebruary 1993 ; case insensitive close-delimiter := "--" boundary "--" CRLF ; Again, no space by "--", content := "Content-Type" ":" type "/" subtype *(";" parameter) ; case-insensitive matching of type and subtype Content-Description := *text Content-ID := msg-id delimiter := "--" boundary CRLF ; taken from Content-Type field. ; There must be no space ; between "--" and boundary. encapsulation := delimiter body-part CRLF encoding := "Content-Transfer-Encoding ":" mechanism epilogue := discard-text ; to be ignored upon receipt. extension-token := x-token / iana-token external-param := (";" "access-type" "=" atype) / (";" "expiration" "=" date-time) / (";" "size" "=" 1*DIGIT) / (";" "permission" "=" ("read" / "read- write")) / (";" "name" "=" value) / (";" "site" "=" value) / (";" "dir" "=" value) / (";" "mode" "=" value) / (";" "server" "=" value) ; access-type required; others required based on access-type filename := "filename" "=" "value" ; suggested name to use if body-part is stored locally iana-token := <a publicly-defined extension token, Borenstein & Freed DRAFT - expires 8/1/93 [Page 76] MIME Multipurpose Internet Mail ExtensionFebruary 1993 registered with IANA, as specified in appendix E> qp-iffy := "[" / "]" / <"> / "\" / "@" / "!" / "#" / "$" / "^" / "'" / "{" / "|" / "}" / "~" / "`" image-type := "image" "/" ("gif" / "jpeg" / extension-token) mechanism := "7bit" / "quoted-printable" / "quoted-base64" / "8bit" / "binary" / x-token message-subtype := "rfc822" / "partial" 2#3partial-param / "external-body" 1*external-param / extension-token message-type := "message" "/" message-subtype MIME-Version := 1*DIGIT "." 1*DIGIT multipart-body := preamble 1*encapsulation close-delimiter epilogue multipart-subtype := "mixed" / "parallel" / "digest" / "alternative" / "extension-token" multipart-type := "multipart" "/" multipart-subtype ";" "boundary" "=" boundary discard-text := *[*text CRLF] octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") ; octet must be used for characters > 127, =, SPACE, or TAB, ; and is recommended for the "qp-iffy" characters too. padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" parameter := attribute "=" value Borenstein & Freed DRAFT - expires 8/1/93 [Page 77] MIME Multipurpose Internet Mail ExtensionFebruary 1993 parms := filename / parameter partial-param := (";" "id" "=" value) / (";" "number" "=" 1*DIGIT) / (";" "total" "=" 1*DIGIT) ; id & number required; total required for last part preamble := discard-text ; to be ignored upon receipt. ptext := octet / <any ASCII character except "=", SPACE, or TAB> ; characters in "qp-iffy" are also not recommended quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF) ; Maximum line length of 78 characters stream := [";" "name" "=" value] [";" type "=" value] [";" padding "=" padding] subtype := token text-subtype := "plain" / "richtext" text-type := "text" "/" text-subtype [";" "charset" "=" charset] token := 1*<any (ASCII) CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "=" ; Must be in quoted-string, ; to use withing parameter values type := "application" / "audio" ; case- insensitive / "image" / "message" / "multipart" / "text" Borenstein & Freed DRAFT - expires 8/1/93 [Page 78] MIME Multipurpose Internet Mail ExtensionFebruary 1993 / "video" / extension-token ; All values case-insensitive value := token / quoted-string video-type := "video" "/" ("mpeg" / extension-token) x-token := <The two characters "X-" or "x-" followed, with no intervening white space, by any token> Borenstein & Freed DRAFT - expires 8/1/93 [Page 79] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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: Borenstein & Freed DRAFT - expires 8/1/93 [Page 80] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Required parameters: Optional parameters: Encoding considerations: 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 Character Set Values To: IANA@isi.edu Subject: Registration of new MIME character set value MIME character set name: Published specification: (The published specification must be an Internet RFC or RFC-to-be or an international standard.) Person & email address to contact for further information: E.3 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: Borenstein & Freed DRAFT - expires 8/1/93 [Page 81] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Published specification: (The published specification must be an Internet RFC or RFC-to-be.) Person & email address to contact for further information: Borenstein & Freed DRAFT - expires 8/1/93 [Page 82] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Appendix F -- Summary of the Seven Content-types [Note that the "filename" parameter is valid for all MIME content-types.] ________________________________________________________________ Content-type: text Subtypes defined by this document: plain, richtext Important Parameters: charset Encoding notes: quoted-printable generally preferred if an encoding is needed and the character set is mostly an ASCII superset. Security considerations: Rich text formats such as TeX and Troff often contain mechanisms for executing arbitrary commands or file system operations, and should not be used automatically unless these security problems have been addressed. Even plain text may contain control characters that can be used to exploit the capabilities of "intelligent" terminals and cause security violations. User interfaces designed to run on such terminals should be aware of and try to prevent such problems. ________________________________________________________________ Content-type: multipart Subtypes defined by this document: mixed, alternative, digest, parallel. Important Parameters: boundary Encoding notes: No content-transfer-encoding is permitted. ________________________________________________________________ Content-type: message Subtypes defined by this document: rfc822, partial, external-body Borenstein & Freed DRAFT - expires 8/1/93 [Page 83] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Important Parameters: id, number, total, access-type, expiration, size, perimssion, name, site, directory, mode, server, subject Encoding notes: No content-transfer-encoding is permitted. Specifically, only "7bit" is permitted for "message/partial", 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: filename, type, padding Deprecated Parameter: name is an obsolete synonym for filename 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 84] MIME Multipurpose Internet Mail ExtensionFebruary 1993 ________________________________________________________________ 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 Borenstein & Freed DRAFT - expires 8/1/93 [Page 85] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 RFC1113 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 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. 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. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 86] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 87] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 only important aspect of this discussion is that the resulting messages are consistent with those produced by the model described here. Borenstein & Freed DRAFT - expires 8/1/93 [Page 88] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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 definition of base64 has been enhanced to permit "====" to mark the termination of data. 3. The Content-ID field is now mandatory for message/external-body parts. 4. The text/richtext type (including the old Section 7.1.3 and Appendix D) has been moved to a separate document. 5. The rules on header merging for message/partial data have been changed to treat the Encrypted and MIME-Version headers as special cases. 6. There is a new header field, "Content-MD5", for the long-awaited message integrity check. 7. 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). 8. 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. 9. The "conversions" parameter for application/octet-stream has been removed. 10. A "filename" parameter is now defined for ALL content- types. Section 7.4.1 now deprecates the use of the "name" parameter for application/octet-stream, as this is now superseded by "filename". 11. The formal grammar for multipart bodies has been changed so that a CRLF is no longer required before the first boundary line. Borenstein & Freed DRAFT - expires 8/1/93 [Page 89] MIME Multipurpose Internet Mail ExtensionFebruary 1993 12. MIME entities of type "message/partial" are now required to use only the "7bit" transfer-encoding. (Specifially, "binary" and "8bit" are not permitted.) 13. The "application/oda" content-type has been removed. 14. 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. 15. The formal syntax for the "MIME-Version" field has been tightened, but in a way that is completely compatible with the only version number defined in RFC 1341. 16. 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 90] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. [GIF] Graphics Interchange Format (Version 89a), Compuserve, Inc., Columbus, Ohio, 1990. [ISO-2022] International Standard--Information Processing-- ISO 7-bit and 8-bit coded character sets--Code extension techniques, ISO 2022:1986. [ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990. [ISO-646] International Standard--Information Processing-- ISO 7-bit coded character set for information interchange, ISO 646:1983. [MPEG] Video Coding Draft Standard ISO 11172 CD, ISO IEC/TJC1/SC2/WG11 (Motion Picture Experts Group), May, 1991. [PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972, "Pulse Code Modulation (PCM) of Voice Frequencies". [POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference Manual, Addison-Wesley, 1985. [POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference Manual, Addison-Wesley, Second Edition, 1990. [X400] Schicker, Pietro, "Message Handling Systems, X.400", Message Handling Systems and Distributed Applications, E. Stefferud, O-j. Jacobsen, and P. Schicker, eds., North- Holland, 1989, pp. 3-41. Borenstein & Freed DRAFT - expires 8/1/93 [Page 91] MIME Multipurpose Internet Mail ExtensionFebruary 1993 [RFC-783] Sollins, K.R. TFTP Protocol (revision 2). June, 1981, MIT, RFC-783. [RFC-821] Postel, J.B. Simple Mail Transfer Protocol. August, 1982, USC/Information Sciences Institute, RFC-821. [RFC-822] Crocker, D. Standard for the format of ARPA Internet text messages. August, 1982, UDEL, RFC-822. [RFC-934] Rose, M.T.; Stefferud, E.A. Proposed standard for message encapsulation. January, 1985, Delaware and NMA, RFC-934. [RFC-959] Postel, J.B.; Reynolds, J.K. File Transfer Protocol. October, 1985, USC/Information Sciences Institute, RFC-959. [RFC-1049] Sirbu, M.A. Content-Type header field for Internet messages. March, 1988, CMU, RFC-1049. [RFC-1113] Linn, J. Privacy enhancement for Internet electronic mail: Part I - message encipherment and authentication procedures. August, 1989, IAB Privacy Task Force, RFC-1113. [RFC-1154] Robinson, D.; Ullmann, R. Encoding header field for Internet messages. April, 1990, Prime Computer, Inc., RFC-1154. [RFC-1321] Rivest, R., The M5 Message-Digest Algorithm. April, 1992, MIT & RSA, RFC-1321. [RFC-1342] Moore, Keith, Representation of Non-Ascii Text in Internet Message Headers. June, 1992, University of Tennessee, RFC-1342. [RFC-1343] Borenstein, Nathaniel, A User Agent Configuration Mechanism for Multimedia Mail Format Information. RFC-1343. [RFC-1344] Borenstein, Nathaniel, Implications of MIME for Internet Mail Gateways. June, 1992, Bellcore, RFC-1344 [RFC-1345] Simonsen, Keld, Character Mnemonics & Character Sets. June, 1992, RFC 1345 Borenstein & Freed DRAFT - expires 8/1/93 [Page 92] MIME Multipurpose Internet Mail ExtensionFebruary 1993 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. Borenstein & Freed DRAFT - expires 8/1/93 [Page 93] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Authors' Addresses For more information, the authors of this document may be contacted via Internet mail: Nathaniel S. Borenstein MRE 2D-296, Bellcore 445 South St. Morristown, NJ 07962-1910 Phone: +1 201 829 4270 Fax: +1 201 829 7019 Email: nsb@bellcore.com Ned Freed Innosoft International, Inc. 250 West First Street Suite 240 Claremont, CA 91711 Phone: +1 714 624 7907 Fax: +1 714 621 5319 Email: ned@innosoft.com MIME is a result of the work of the Internet Engineering Task Force Working Group on Email Extensions. The chairman of that group, Greg Vaudreuil, may be reached at: Gregory M. Vaudreuil Corporation for National Research Initiatives Suite 100 1895 Preston White Drive Reston, VA 22091 Phone: +1 703-620-8990 Fax: +1 703-620-0913 Email: gvaudre@cnri.reston.va.us Borenstein & Freed DRAFT - expires 8/1/93 [Page 94] MIME Multipurpose Internet Mail ExtensionFebruary 1993 Borenstein & Freed DRAFT - expires 8/1/93 [Page i]