RFCs in HTML Format

RFC 1341

            Network Working Group               N. Borenstein, Bellcore
            Request for Comments: 1341               N. Freed, Innosoft
                                                              June 1992

                   MIME  (Multipurpose Internet Mail Extensions):

                      Mechanisms for Specifying and Describing
                       the Format of Internet Message Bodies

            Borenstein & Freed                                  [Page i]

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 should be converted to (not encoded in) an ASCII format, or that they should be discarded, notifying the RFC 822 user that discarding has occurred. This is clearly undesirable, as information that a user may wish to receive is lost. Even though a user's UA may not have the capability of dealing with the non-textual body part, the user might have some mechanism external to the UA that can extract useful information from the body part. Moreover, it does not allow for the fact that the message may eventually be gatewayed back into an X.400 message handling system (i.e., the X.400 message is "tunneled" through Internet mail), where the non-textual information would definitely become useful again. Borenstein & Freed [Page 1]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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. 2.f. An "audio" Content-Type value, for transmitting audio or voice data. 2.g. A "video" Content-Type value, for transmitting video or moving image data, possibly with audio as part of the composite video data format. 3. A Content-Transfer-Encoding header field, which can be used to specify an auxiliary encoding that was applied to the data in order to allow it to pass through mail transport mechanisms which may have data or character set limitations. 4. Two optional header fields that can be used to further describe the data in a message body, the Content-ID and Content-Description header fields. Borenstein & Freed [Page 2]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 F 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. 2 Notations, Conventions, and Generic BNF Grammar This document is being published in two versions, one as plain ASCII text and one as PostScript. 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 coded character set, in the sense of ISO character set standardization work, and must not be Borenstein & Freed [Page 3]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 misinterpreted as meaning "a set of characters." 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 proper historical or evolutionary context. Such information may be skipped by those who are focused entirely on building a compliant 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. Borenstein & Freed [Page 4]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 := text 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-compliant. Borenstein & Freed [Page 5]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 might apply to any subtype. For example, the "boundary" parameter makes sense only for the "multipart" content-type, but the "charset" parameter might make sense with several content-types. An initial set of seven Content-Types is defined by this document. This set of top-level names is intended to be substantially complete. It is expected that additions to the larger set of supported types can generally be Borenstein & Freed [Page 6]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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: Content-Type := type "/" subtype *[";" parameter] type := "application" / "audio" / "image" / "message" / "multipart" / "text" / "video" / x-token x-token := <The two characters "X-" followed, with no intervening white space, by any token> subtype := token parameter := attribute "=" value attribute := token value := token / quoted-string token := 1*<any CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in / "," / ";" / ":" / "\" / <"> ; quoted-string, / "/" / "[" / "]" / "?" / "." ; to use within / "=" ; parameter values Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=". 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 Borenstein & Freed [Page 7]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 F. Where intended for public use, the formats they refer to must also be defined by a published specification, and possibly offered for standardization. The seven standard initial predefined Content-Types are detailed in the bulk of this document. They are: text -- textual information. The primary subtype, "plain", indicates plain (unformatted) text. No special software is required to get the full meaning of the text, aside from support for the indicated character set. Subtypes are to be used for enriched text in forms where application software may enhance the appearance of the text, but such software must not be required in order to get the general idea of the content. Possible subtypes thus include any readable word processor format. A very simple and portable subtype, richtext, is defined in this document. 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. Borenstein & Freed [Page 8]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 image -- image data. Image requires a display device (such as a graphical display, a printer, or a FAX machine) to view the information. Initial subtypes are defined for two widely-used image formats, jpeg and gif. audio -- audio data, with initial subtype "basic". Audio requires an audio output device (such as a speaker or a telephone) to "display" the contents. video -- video data. Video requires the capability to display moving images, typically including specialized hardware and software. The initial subtype is "mpeg". application -- some other kind of data, typically either uninterpreted binary data or information to be processed by a mail-based application. The primary subtype, "octet-stream", is to be used in the case of uninterpreted binary data, in which case the simplest recommended action is to offer to write the information into a file for the user. Two additional subtypes, "ODA" and "PostScript", are defined for transporting ODA and 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 securityconsiderations, 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 [Page 9]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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: Content-Transfer-Encoding := "BASE64" / "QUOTED-PRINTABLE" / "8BIT" / "7BIT" / "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, Borenstein & Freed [Page 10]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 "Content-Transfer-Encoding: 7BIT" is assumed if the Content-Transfer-Encoding header field is not present. The values "8bit", "7bit", and "binary" all imply 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. "7bit" means that the data is all represented as short lines of US-ASCII data. "8bit" means that the lines are short, but there may be non-ASCII characters (octets with the high-order bit set). "Binary" means that not only may non-ASCII characters be present, but also that the lines are not necessarily short enough for SMTP transport. The difference between "8bit" (or any other conceivable bit-width token) and the "binary" token is that "binary" does not require adherence to any limits on line length or to the SMTP CRLF semantics, while the bit-width tokens do require such adherence. If the body contains data in any bit-width other than 7-bit, the appropriate bit-width Content-Transfer-Encoding token must be used (e.g., "8bit" for unencoded 8 bit wide data). If the body contains binary data, the "binary" Content-Transfer-Encoding token must be used. NOTE: The distinction between the Content-Transfer-Encoding values of "binary," "8bit," etc. may seem unimportant, in that all of them really mean "none" -- that is, there has been no encoding of the data for transport. However, clear labeling will be of enormous value to gateways between future mail transport systems with differing capabilities in transporting data that do not meet the restrictions of RFC 821 transport. 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, Borenstein & Freed [Page 11]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 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. Composing agents should be aware that many mail readers will lack this capability and will show the parts serially in any event. Borenstein & Freed [Page 36]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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". 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. 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 Borenstein & Freed [Page 37]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 the body contains a fragment of a larger message. Three parameters must be specified in the Content-Type field of type message/partial: The first, "id", is a unique identifier, as close to a world-unique identifier as possible, to be used to match the parts together. (In general, the identifier is essentially a message-id; if placed in double quotes, it can be any message-id, in accordance with the BNF for "parameter" given earlier in this specification.) The second, "number", an integer, is the part number, which indicates where this part fits into the sequence of fragments. The third, "total", another integer, is the total number of parts. This third subfield is required on the final part, and is optional 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 Borenstein & Freed [Page 38]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 entities. In this process the following rules must be observed: (1) All of the headers from the initial enclosing entity (part one), except those that start with "Content-" and "Message-ID", must be copied, in order, to the new message. (2) Only those headers in the enclosed message which start with "Content-" and "Message-ID" must be appended, in order, to the headers of the new message. Any headers in the enclosed message which do not start with "Content-" (except for "Message-ID") will be ignored. (3) All of the headers 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 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 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: Borenstein & Freed [Page 39]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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... 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. 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 message body or body part 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, it may be used to contain auxilliary information for some Borenstein & Freed [Page 40]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 -- One or more case-insensitive words, comma-separated, indicating supported access mechanisms 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 F . In addition, the following two 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. 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: Borenstein & Freed [Page 41]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 MIME-Version := 1*text multipart-body := preamble 1*encapsulation close-delimiter epilogue parameter := attribute "=" value preamble := *text ; to be ignored upon receipt. subtype := token token := 1*<any CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in / "," / ";" / ":" / "\" / <"> ; quoted-string, / "/" / "[" / "]" / "?" / "." ; to use within / "=" ; parameter values type := "application" / "audio" ; case- insensitive / "image" / "message" / "multipart" / "text" / "video" / x-token value := token / quoted-string x-token := <The two characters "X-" followed, with no intervening white space, by any token> Borenstein & Freed [Page 67]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Appendix F -- 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, conversions parameters for the application 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. F.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 should be published as an RFC or submitted in a form suitable to become an RFC, and be subject to the Internet standards process. To: IANA@isi.edu Subject: Registration of new MIME content-type/subtype MIME type name: (If the above is not an existing top-level MIME type, please explain why an existing type cannot be used.) MIME subtype name: Required parameters: Optional parameters: Encoding considerations: Security considerations: Borenstein & Freed [Page 68]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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: F.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: F.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: Published specification: (The published specification must be an Internet RFC or RFC-to-be.) Person & email address to contact for further information: F.4 Registration of New Conversions Values for Application To: IANA@isi.edu Subject: Registration of new MIME Conversions value for Application content-type MIME Conversions name: Borenstein & Freed [Page 69]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Published specification: (The published specification must be an Internet RFC or RFC-to-be.) Person & email address to contact for further information: Borenstein & Freed [Page 70]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Appendix G -- Summary of the Seven 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 Important Parameters: id, number, total Encoding notes: No content-transfer-encoding is permitted. ________________________________________________________________ Content-type: application Subtypes defined by this document: octet-stream, postscript, oda Important Parameters: profile Borenstein & Freed [Page 71]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Encoding notes: base64 generally preferred for octet-stream or other 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. In general, authors of mail-reading agents are cautioned against giving their systems the power to execute mail-based application data without carefully considering the security implications. While it is certainly possible to define safe application formats and even safe interpreters for unsafe formats, each interpreter should be evaluated separately for possible security problems. ________________________________________________________________ Content-type: image Subtypes defined by this document: jpeg, gif Important Parameters: none Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: audio Subtypes defined by this document: basic Important Parameters: none Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: video Subtypes defined by this document: mpeg Important Parameters: none Encoding notes: base64 generally preferred Borenstein & Freed [Page 72]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Appendix H -- 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 message part can be modelled as being done in a number of steps. Note that these steps are roughly similar to those steps used in RFC1113: Step 1. Creation of local form. The body part 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 part, 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 part 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. Step 3. Apply transfer encoding. A Content-Transfer-Encoding appropriate for this body part 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 body part. Borenstein & Freed [Page 73]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 Step 4. Insertion into message. The encoded object is inserted into a MIME message with appropriate body part headers and boundary markers. 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 bodyparts 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 compliant 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 [Page 74]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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. [ODA] ISO 8613; Information Processing: Text and Office System; Office Document Architecture (ODA) and Interchange Format (ODIF), Part 1-8, 1989. [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. [X400] Schicker, Pietro, "Message Handling Systems, X.400", Message Handling Systems and Distributed Applications, E. Stefferud, O-j. Jacobsen, and P. Schicker, eds., North- Holland, 1989, pp. 3-41. [RFC 783] Sollins, K.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. Borenstein & Freed [Page 75]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 [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 1342] Moore, Keith, Representation of Non-Ascii Text in Internet Message Headers. June, 1992, University of Tennessee, RFC 1342. Security Considerations Security issues are discussed in Section 7.4.2 and in Appendix G. 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 applicaton/postscript content-type in Section 7.4.2 may serve as a model for considering other content-types with remote execution capabilities. Borenstein & Freed [Page 76]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 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 Borenstein & Freed [Page 77]
RFC 1341MIME: Multipurpose Internet Mail ExtensionsJune 1992 THIS PAGE INTENTIONALLY LEFT BLANK. Please discard this page and place the following table of contents after the title page. Borenstein & Freed [Page i]
Table of Contents 1 Introduction.......................................
1 2 Notations, Conventions, and Generic BNF Grammar.... 3 3 The MIME-Version Header Field...................... 5 4 The Content-Type Header Field...................... 6 5 The Content-Transfer-Encoding Header Field......... 10 5.1 Quoted-Printable Content-Transfer-Encoding......... 14 5.2 Base64 Content-Transfer-Encoding................... 17 6 Additional Optional Content- Header Fields......... 19 6.1 Optional Content-ID Header Field................... 19 6.2 Optional Content-Description Header Field.......... 19 7 The Predefined Content-Type Values................. 20 7.1 The Text Content-Type.............................. 20 7.1.1 The charset parameter.............................. 20 7.1.2 The Text/plain subtype............................. 23 7.1.3 The Text/richtext subtype.......................... 23 7.2 The Multipart Content-Type......................... 29 7.2.1 Multipart: The common syntax...................... 30 7.2.2 The Multipart/mixed (primary) subtype.............. 34 7.2.3 The Multipart/alternative subtype.................. 34 7.2.4 The Multipart/digest subtype....................... 36 7.2.5 The Multipart/parallel subtype..................... 36 7.3 The Message Content-Type........................... 37 7.3.1 The Message/rfc822 (primary) subtype............... 37 7.3.2 The Message/Partial subtype........................ 37 7.3.3 The Message/External-Body subtype.................. 40 7.4 The Application Content-Type....................... 46 7.4.1 The Application/Octet-Stream (primary) subtype..... 46 7.4.2 The Application/PostScript subtype................. 47 7.4.3 The Application/ODA subtype........................ 50 7.5 The Image Content-Type............................. 51 7.6 The Audio Content-Type............................. 51 7.7 The Video Content-Type............................. 51 7.8 Experimental Content-Type Values................... 51 Summary............................................ 53 Acknowledgements................................... 54 Appendix A -- Minimal MIME-Conformance............. 56 Appendix B -- General Guidelines For Sending Email Data59 Appendix C -- A Complex Multipart Example.......... 62 Appendix D -- A Simple Richtext-to-Text Translator in C64 Appendix E -- Collected Grammar.................... 66 Appendix F -- IANA Registration Procedures......... 68 F.1 Registration of New Content-type/subtype Values..68 F.2 Registration of New Character Set Values...... 69 F.3 Registration of New Access-type Values for Message/external-body69 F.4 Registration of New Conversions Values for Application69 Appendix G -- Summary of the Seven Content-types... 71 Appendix H -- Canonical Encoding Model............. 73 References......................................... 75 Security Considerations............................ 76 Authors' Addresses................................. 77 Borenstein & Freed [Page ii]
Borenstein & Freed [Page iii]

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