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GeezerVison is showing datatracker.ietf.org — RFC 2046: Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types Network Working Group N. Freed Request for Comments: 2046 Innosoft Obsoletes: 1521, 1522, 1590 N. Borenstein Category: Standards Track First Virtual November 1996 RFC 2046 Media Types November 1996 data in Internet mail header fields. The fourth document, RFC 2048, specifies various IANA registration procedures for MIME-related facilities. The fifth and final document, RFC 2049, describes MIME conformance criteria as well as providing some illustrative examples of MIME message formats, acknowledgements, and the bibliography. These documents are revisions of RFCs 1521 and 1522, which themselves were revisions of RFCs 1341 and 1342. An appendix in RFC 2049 describes differences and changes from previous versions. Table of Contents 1. Introduction ......................................... 3 2. Definition of a Top-Level Media Type ................. 4 3. Overview Of The Initial Top-Level Media Types ........ 4 4. Discrete Media Type Values ........................... 6 4.1 Text Media Type ..................................... 6 4.1.1 Representation of Line Breaks ..................... 7 4.1.2 Charset Parameter ................................. 7 4.1.3 Plain Subtype ..................................... 11 4.1.4 Unrecognized Subtypes ............................. 11 4.2 Image Media Type .................................... 11 4.3 Audio Media Type .................................... 11 4.4 Video Media Type .................................... 12 4.5 Application Media Type .............................. 12 4.5.1 Octet-Stream Subtype .............................. 13 4.5.2 PostScript Subtype ................................ 14 4.5.3 Other Application Subtypes ........................ 17 5. Composite Media Type Values .......................... 17 5.1 Multipart Media Type ................................ 17 5.1.1 Common Syntax ..................................... 19 5.1.2 Handling Nested Messages and Multiparts ........... 24 5.1.3 Mixed Subtype ..................................... 24 5.1.4 Alternative Subtype ............................... 24 5.1.5 Digest Subtype .................................... 26 5.1.6 Parallel Subtype .................................. 27 5.1.7 Other Multipart Subtypes .......................... 28 5.2 Message Media Type .................................. 28 5.2.1 RFC822 Subtype .................................... 28 5.2.2 Partial Subtype ................................... 29 5.2.2.1 Message Fragmentation and Reassembly ............ 30 5.2.2.2 Fragmentation and Reassembly Example ............ 31 5.2.3 External-Body Subtype ............................. 33 5.2.4 Other Message Subtypes ............................ 40 6. Experimental Media Type Values ....................... 40 7. Summary .............................................. 41 8. Security Considerations .............................. 41 9. Authors' Addresses ................................... 42 Freed & Borenstein Standards Track [Page 2] RFC 2046 Media Types November 1996 A. Collected Grammar .................................... 43 RFC 2046 Media Types November 1996 The initial seven standard top-level media type are defined and described in the remainder of this document. RFC 2046 Media Types November 1996 (2) image -- image data. "Image" requires a display device (such as a graphical display, a graphics printer, or a FAX machine) to view the information. An initial subtype is defined for the widely-used image format JPEG. . subtypes are defined for two widely-used image formats, jpeg and gif. (3) audio -- audio data. "Audio" requires an audio output device (such as a speaker or a telephone) to "display" the contents. An initial subtype "basic" is defined in this document. (4) video -- video data. "Video" requires the capability to display moving images, typically including specialized hardware and software. An initial subtype "mpeg" is defined in this document. (5) application -- some other kind of data, typically either uninterpreted binary data or information to be processed by an application. The 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. The "PostScript" subtype is also defined for the transport of PostScript material. Other expected uses for "application" include spreadsheets, data for mail-based scheduling systems, and languages for "active" (computational) messaging, and word processing formats that are not directly readable. Note that security considerations may exist for some types of application data, most notably "application/PostScript" and any form of active messaging. These issues are discussed later in this document. The two composite top-level media types are: (1) multipart -- data consisting of multiple entities of independent data types. Four subtypes are initially defined, including the basic "mixed" subtype specifying a generic mixed set of parts, "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 has a default type of "message/rfc822". Freed & Borenstein Standards Track [Page 5] RFC 2046 Media Types November 1996 (2) message -- an encapsulated message. A body of media type "message" is itself all or a portion of some kind of message object. Such objects may or may not in turn contain other entities. The "rfc822" subtype is used when the encapsulated content is itself an RFC 822 message. The "partial" subtype is defined for partial RFC 822 messages, to permit the fragmented transmission of bodies that are thought to be too large to be passed through transport facilities in one piece. Another subtype, "external-body", is defined for specifying large bodies by reference to an external data source. It should be noted that the list of media 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. RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 The charset parameter for subtypes of "text" gives a name of a character set, as "character set" is defined in RFC 2045. The rules regarding line breaks detailed in the previous section must also be observed -- a character set whose definition does not conform to these rules cannot be used in a MIME "text" subtype. 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. Other media types than subtypes of "text" might choose to employ the charset parameter as defined here, but with the CRLF/line break restriction removed. Therefore, all character sets that conform to the general definition of "character set" in RFC 2045 can be registered for MIME use. Note that if the specified character set includes 8-bit characters and such characters are used in the body, 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 [RFC-821]. 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 as a media type parameter in the Content-Type header field. "US-ASCII" does not indicate an arbitrary 7-bit character set, but specifies that all octets in the body must be interpreted as characters according to the US-ASCII character set. National and application-oriented versions of ISO 646 [ISO-646] are usually NOT identical to US-ASCII, and in that case their use in Internet mail is explicitly discouraged. The omission of the ISO 646 character set from this document is deliberate in this regard. The character set name of "US-ASCII" explicitly refers to the character set defined in ANSI X3.4-1986 [US- ASCII]. The new international reference version (IRV) of the 1991 edition of ISO 646 is identical to US-ASCII. 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 media type and character set is to permit the receiver to unambiguously determine how the sender intended the coded message to be interpreted, assuming anything other than "strict ASCII" as the default would risk unintentional and incompatible changes to the semantics of messages now being transmitted. This also implies that Freed & Borenstein Standards Track [Page 8] RFC 2046 Media Types November 1996 messages containing characters coded according to other versions of ISO 646 than US-ASCII and the 1991 IRV, or using code-switching procedures (e.g., those of ISO 2022), as well as 8bit or multiple octet character encodings MUST use an appropriate character set specification to be consistent with MIME. The complete US-ASCII character set is listed in ANSI X3.4- 1986. Note that the control characters including DEL (0-31, 127) have no defined meaning in apart from the combination CRLF (US-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)." Aside from these conventions, any use of the control characters or DEL in a body must either occur (1) because a subtype of text other than "plain" specifically assigns some additional meaning, or (2) within the context 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: An enormous proliferation of character sets exist beyond US- ASCII. A large number of partially or totally overlapping character sets is NOT a good thing. A SINGLE character set that can be used universally for representing all of the world's languages in Internet mail would be preferrable. Unfortunately, existing practice in several communities seems to point to the continued use of multiple character sets in the near future. A small number of standard character sets are, therefore, defined for Internet use in this document. The defined charset values are: (1) US-ASCII -- as defined in ANSI X3.4-1986 [US-ASCII]. (2) 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 10. Freed & Borenstein Standards Track [Page 9] RFC 2046 Media Types November 1996 Characters in the range 128-159 has no assigned meaning in ISO-8859- X. Characters with values below 128 in ISO-8859-X have the same assigned meaning as they do in US-ASCII. Part 6 of ISO 8859 (Latin/Arabic alphabet) and part 8 (Latin/Hebrew alphabet) includes both characters for which the normal writing direction is right to left and characters for which it is left to right, but do not define a canonical ordering method for representing bi-directional text. The charset values "ISO-8859-6" and "ISO-8859- 8", however, specify that the visual method is used [RFC-1556]. All of these character sets are used as pure 7bit or 8bit sets without any shift or escape functions. The meaning of shift and escape sequences in these character sets is not defined. The character sets specified above are the ones that were relatively uncontroversial during the drafting of MIME. This document does not endorse the use of any particular character set other than US-ASCII, and recognizes that the future evolution of world character sets remains unclear. Note that the character set used, if anything other than US- ASCII, must always be explicitly specified in the Content-Type field. No character set name other than those defined above may be used in Internet mail without the publication of a formal specification and its registration with IANA, or by private agreement, in which case the character set name must begin with "X-". Implementors are discouraged from defining new character sets 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, composition software should always use the "lowest common denominator" character set possible. For example, if a body contains only US-ASCII characters, it SHOULD 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 should be labelled as being in that subset. This will increase the chances that the recipient will be able to view the resulting entity correctly. Freed & Borenstein Standards Track [Page 10] RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 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 single channel audio encoded using 8bit ISDN mu-law [PCM] at a sample rate of 8000 Hz. Unrecognized subtypes of "audio" should at a miniumum be treated as "application/octet-stream". Implementations may optionally elect to pass subtypes of "audio" that they do not specifically recognize to a robust general-purpose audio playing application, if such an application is available. RFC 2046 Media Types November 1996 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 additional material based on that dialog. More generally, there have been several "active" messaging languages developed in which programs in a suitably specialized language are transported to a remote location and automatically run in the recipient's environment. Such applications may be defined as subtypes of the "application" media type. This document defines two subtypes: octet-stream, and PostScript. The subtype of "application" will often be either the name or include part of 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". RFC 2046 Media Types November 1996 To reduce the danger of transmitting rogue programs, 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 message body as input. RFC 2046 Media Types November 1996 Note, however, that this operator has the potential to reveal information about what files the recipient has access to, and this information may itself be sensitive. Message senders should avoid the use of potentially dangerous file operators, since these operators are quite likely to be unavailable in secure PostScript implementations. Message receiving and displaying software should either completely disable all potentially dangerous file operators or take special care not to delegate any special authority to their operation. These operators should be viewed as being done by an outside agency when interpreting PostScript documents. Such disabling and/or checking should be done completely outside of the reach of the PostScript language itself; care should be taken to insure that no method exists for re-enabling full- function versions of these operators. (2) The PostScript language provides facilities for exiting the normal interpreter, or server, loop. Changes made in this "outer" environment are customarily retained across documents, and may in some cases be retained semipermanently in nonvolatile memory. The 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, since the ability to exit will probably be unavailable in secure PostScript implementations. Message receiving and displaying software should completely disable the ability to make retained changes to the PostScript environment by eliminating or disabling the "startjob" and "exitserver" operations. If these operations cannot be eliminated or completely disabled the password associated with them should at least be set to a hard- to-guess value. (3) 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 Freed & Borenstein Standards Track [Page 15] RFC 2046 Media Types November 1996 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 disable the ability to change system and device parameters. If these operators cannot be completely disabled the password associated with them should at least be set to a hard-to-guess value. (4) 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. (5) PostScript is an extensible language, and many, if not most, implementations of it provide a number of their own extensions. This document does not deal with such extensions explicitly since they constitute an unknown factor. Message sending software should not make use of nonstandard extensions; they are likely to be missing from some implementations. Message receiving and displaying software should make sure that any nonstandard PostScript operators are secure and don't present any kind of threat. (6) It is possible to write PostScript that consumes huge amounts of various system resources. It is also possible to write PostScript programs that loop indefinitely. 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 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. Freed & Borenstein Standards Track [Page 16] RFC 2046 Media Types November 1996 (7) It is possible to include raw binary information inside PostScript in various forms. This is not recommended for use in Internet mail, both because it is not supported by all PostScript interpreters and because it significantly complicates the use of a MIME Content- Transfer-Encoding. (Without such binary, PostScript may typically be viewed as line-oriented data. The treatment of CRLF sequences becomes extremely problematic if binary and line-oriented data are mixed in a single Postscript data stream.) (8) 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. RFC 2046 Media Types November 1996 a content-type of "text/plain; charset=US-ASCII". The only header fields that have defined meaning for body parts are those the names of which begin with "Content-". All other header fields may be ignored in body parts. Although they should generally be retained if at all possible, 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. NOTE: The distinction between an RFC 822 message and a body part is subtle, but important. A gateway between Internet and X.400 mail, for example, must be able to tell the difference between a body part that contains an image and a body part that contains an encapsulated message, the body of which is a JPEG image. In order to represent the latter, the body part must have "Content-Type: message/rfc822", and its body (after the blank line) must be the encapsulated message, with its own "Content-Type: image/jpeg" 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 parts actually are messages, a "digest" subtype is also defined.) As stated previously, each body part is preceded by a boundary delimiter line that contains the boundary delimiter. The boundary delimiter MUST NOT appear inside any of the encapsulated parts, on a line by itself or as the prefix of any line. This implies that it is crucial that the composing agent be able to choose and specify a unique boundary parameter value that does not contain the boundary parameter value of an enclosing multipart as a prefix. 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 those of an unrecognized subtype. As stated in the definition of the Content-Transfer-Encoding field [RFC 2045], no encoding other than "7bit", "8bit", or "binary" is permitted for entities of type "multipart". The "multipart" boundary delimiters and header fields are always represented as 7bit US-ASCII in any case (though the header fields may encode non-US-ASCII header text as per RFC 2047) 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. Freed & Borenstein Standards Track [Page 18] RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 --gc0pJq0M:08jU534c0p The boundary delimiter MUST occur at the beginning of a line, i.e., following a CRLF, and the initial CRLF is considered to be attached to the boundary delimiter line rather than part of the preceding part. The boundary may be followed by zero or more characters of linear whitespace. It is then terminated by either 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. If no Content-Type field is present it is assumed to be "message/rfc822" in a "multipart/digest" and "text/plain" otherwise. NOTE: The CRLF preceding the boundary delimiter line is conceptually attached to the boundary so that it is possible to have a part that does not end with a CRLF (line break). Body parts that must be considered to end with line breaks, therefore, must have two CRLFs preceding the boundary delimiter line, the first of which is part of the preceding body part, and the second of which is part of the encapsulation boundary. Boundary delimiters must not appear within the encapsulated material, and must be no longer than 70 characters, not counting the two leading hyphens. The boundary delimiter line following the last body part is a distinguished delimiter that indicates that no further body parts will follow. Such a delimiter line is identical to the previous delimiter lines, with the addition of two more hyphens after the boundary parameter value. --gc0pJq0M:08jU534c0p-- NOTE TO IMPLEMENTORS: Boundary string comparisons must compare the boundary value with the beginning of each candidate line. An exact match of the entire candidate line is not required; it is sufficient that the boundary appear in its entirety following the CRLF. There appears to be room for additional information prior to the first boundary delimiter line and following the final boundary delimiter line. These areas should generally be left blank, and implementations must ignore anything that appears before the first boundary delimiter line 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 Freed & Borenstein Standards Track [Page 20] RFC 2046 Media Types November 1996 place to insert an explanatory note for recipients who read the message with pre-MIME software, since such notes will be ignored by MIME-compliant software. NOTE: Because boundary delimiters must not appear in the body parts being encapsulated, a user agent must exercise care to choose a unique boundary parameter value. The boundary parameter value in the example above could have been the result of an algorithm designed to produce boundary delimiters 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" boundary delimiters for a recipient with an old user agent, but would require more attention to the possibility that the boundary delimiter might appear at the beginning of some line in the encapsulated part. The simplest boundary delimiter line possible is something like "---", with a closing boundary delimiter line 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 <[email protected]> To: Ned Freed <[email protected]> Date: Sun, 21 Mar 1993 23:56:48 -0800 (PST) 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 composition agents to include an explanatory note to non-MIME conformant readers. --simple boundary This is implicitly typed plain US-ASCII text. It does NOT end with a linebreak. --simple boundary Content-type: text/plain; charset=us-ascii This is explicitly typed plain US-ASCII text. It DOES end with a linebreak. --simple boundary-- This is the epilogue. It is also to be ignored. Freed & Borenstein Standards Track [Page 21] RFC 2046 Media Types November 1996 The use of a media 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 uses a different boundary delimiter. See RFC 2049 for an example of nested "multipart" entities. The use of the "multipart" media type with only a single body part may be useful in certain contexts, and is explicitly permitted. NOTE: Experience has shown that a "multipart" media type with a single body part is useful for sending non-text media types. It has the advantage of providing the preamble as a place to include decoding instructions. In addition, a number of SMTP gateways move or remove the MIME headers, and a clever MIME decoder can take a good guess at multipart boundaries even in the absence of the Content-Type header and thereby successfully decode the message. The only mandatory global parameter for the "multipart" media type is the boundary parameter, which consists of 1 to 70 characters from a set of characters known to be very robust through mail gateways, and NOT ending with white space. (If a boundary delimiter line 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: dash-boundary := "--" boundary ; boundary taken from the value of ; boundary parameter of the ; Content-Type field. multipart-body := [preamble CRLF] dash-boundary transport-padding CRLF body-part *encapsulation close-delimiter transport-padding [CRLF epilogue] Freed & Borenstein Standards Track [Page 22] RFC 2046 Media Types November 1996 transport-padding := *LWSP-char ; Composers MUST NOT generate ; non-zero length transport ; padding, but receivers MUST ; be able to handle padding ; added by message transports. encapsulation := delimiter transport-padding CRLF body-part delimiter := CRLF dash-boundary close-delimiter := delimiter "--" preamble := discard-text epilogue := discard-text discard-text := *(*text CRLF) *text ; May be ignored or discarded. body-part := MIME-part-headers [CRLF *OCTET] ; Lines in a body-part must not start ; with the specified dash-boundary and ; the delimiter must not appear anywhere ; in the body part. Note that the ; semantics of a body-part differ from ; the semantics of a message, as ; described in the text. OCTET := <any 0-255 octet value> IMPORTANT: The free insertion of linear-white-space and RFC 822 comments between the elements shown in this BNF is NOT allowed since this BNF does not specify a structured header field. NOTE: In certain transport enclaves, RFC 822 restrictions such as the one that limits bodies to printable US-ASCII characters may not be in force. (That is, the transport domains may exist that resemble standard Internet mail transport as specified in RFC 821 and assumed by RFC 822, but without certain restrictions.) The relaxation of these restrictions should be construed as locally extending the definition of bodies, for example to include octets outside of the US-ASCII range, as long as these extensions are supported by the transport and adequately documented in the Content- Transfer-Encoding header field. However, in no event are headers (either message headers or body part headers) allowed to contain anything other than US-ASCII characters. Freed & Borenstein Standards Track [Page 23] RFC 2046 Media Types November 1996 NOTE: Conspicuously missing from the "multipart" type is a notion of structured, related body parts. It is recommended that those wishing to provide more structured or integrated multipart messaging facilities should define subtypes of multipart that are syntactically identical but define relationships between the various parts. For example, subtypes of multipart could be defined that include a distinguished part which in turn is used to specify the relationships between the other parts, probably referring to them by their Content-ID field. Old implementations will not recognize the new subtype if this approach is used, but will treat it as multipart/mixed and will thus be able to show the user the parts that are recognized. RFC 2046 Media Types November 1996 best choice is the LAST part of a type supported by the recipient system's local environment. "Multipart/alternative" may be used, for example, to send a message in a fancy text format in such a way that it can easily be displayed anywhere: From: Nathaniel Borenstein <[email protected]> To: Ned Freed <[email protected]> Date: Mon, 22 Mar 1993 09:41:09 -0800 (PST) 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/enriched ... RFC 1896 text/enriched version of same message goes here ... --boundary42 Content-Type: application/x-whatever ... fanciest version of same message goes here ... --boundary42-- In this example, users whose mail systems understood the "application/x-whatever" format would see only the fancy version, while other users would see only the enriched or plain text version, depending on the capabilities of their system. In general, user agents that compose "multipart/alternative" entities must place the body parts in increasing order of preference, that is, with the preferred format last. For fancy text, the sending user agent should put the plainest format first and the richest format last. Receiving user agents should pick and display the last format they are capable of displaying. In the case where one of the alternatives is itself of type "multipart" and contains unrecognized sub-parts, the user agent may choose either to show that alternative, an earlier alternative, or both. Freed & Borenstein Standards Track [Page 25] RFC 2046 Media Types November 1996 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 viewer. While this approach does impose some burden on conformant MIME viewers, 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 a message includes both a nicely- formatted image version and an easily-edited text version. What is most critical, however, is that the user not automatically be shown multiple versions of the same data. Either the user should be shown the last recognized version or should be given the choice. THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each part of a "multipart/alternative" entity represents the same data, but the mappings between the two are not necessarily without information loss. For example, information is lost when translating ODA to PostScript or plain text. It is recommended that each part should have a different Content-ID value in the case where the information content of the two parts is not identical. And when the information content is identical -- for example, where several parts of type "message/external-body" specify alternate ways to access the identical data -- the same Content-ID field value should be used, to optimize any caching mechanisms that might be present on the recipient's end. However, the Content-ID values used by the parts should NOT be the same Content-ID value that describes the "multipart/alternative" as a whole, if there is any such Content-ID field. That is, one Content-ID value will refer to the "multipart/alternative" entity, while one or more other Content-ID values will refer to the parts inside it. RFC 2046 Media Types November 1996 digest, actually doing so is undesireble. The "multipart/digest" Content-Type is intended to be used to send collections of messages. If a "text/plain" part is needed, it should be included as a seperate part of a "multipart/mixed" message. A digest in this format might, then, look something like this: From: Moderator-Address To: Recipient-List Date: Mon, 22 Mar 1994 13:34:51 +0000 Subject: Internet Digest, volume 42 MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="---- main boundary ----" ------ main boundary ---- ...Introductory text or table of contents... ------ main boundary ---- Content-Type: multipart/digest; boundary="---- next message ----" ------ next message ---- From: someone-else Date: Fri, 26 Mar 1993 11:13:32 +0200 Subject: my opinion ...body goes here ... ------ next message ---- From: someone-else-again Date: Fri, 26 Mar 1993 10:07:13 -0500 Subject: my different opinion ... another body goes here ... ------ next message ------ ------ main boundary ------ RFC 2046 Media Types November 1996 in a parallel entity, the order of body parts is not significant. A common presentation of this type is to display all of the parts simultaneously on hardware and software that are capable of doing so. However, composing agents should be aware that many mail readers will lack this capability and will show the parts serially in any event. RFC 2046 Media Types November 1996 It should be noted that, despite the use of the numbers "822", a "message/rfc822" entity isn't restricted to material in strict conformance to RFC822, nor are the semantics of "message/rfc822" objects restricted to the semantics defined in RFC822. More specifically, a "message/rfc822" message could well be a News article or a MIME message. No encoding other than "7bit", "8bit", or "binary" is permitted for the body of a "message/rfc822" entity. 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-US-ASCII text in the headers of an encapsulated message can be specified using the mechanisms described in RFC 2047. RFC 2046 Media Types November 1996 Because some message transfer agents may choose to automatically fragment large messages, and because such agents may use very 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. 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 fragments 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 in RFC 2045.) The second, "number", an integer, is the fragment number, which indicates where this fragment fits into the sequence of fragments. The third, "total", another integer, is the total number of fragments. This third subfield is required on the final fragment, and is optional (though encouraged) on the earlier fragments. Note also that these parameters may be given in any order. Thus, the second piece of a 3-piece message may have either of the following header fields: Content-Type: Message/Partial; number=2; total=3; id="[email protected]" Content-Type: Message/Partial; id="[email protected]"; number=2 But the third piece MUST specify the total number of fragments: Content-Type: Message/Partial; number=3; total=3; id="[email protected]" Note that fragment numbering begins with 1, not 0. When the fragments of an entity broken up in this manner are put together, the result is always a complete MIME entity, which may have its own Content-Type header field, and thus may contain any other data type. RFC 2046 Media Types November 1996 message containing an audio message. That is, the encapsulation of the message is considered to be "transparent". When generating and reassembling the pieces 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) Fragmentation agents must split messages at line boundaries only. This restriction is imposed because splits at points other than the ends of lines in turn depends on message transports being able to preserve the semantics of messages that don't end with a CRLF sequence. Many transports are incapable of preserving such semantics. (2) All of the header fields from the initial enclosing message, except those that start with "Content-" and the specific header fields "Subject", "Message-ID", "Encrypted", and "MIME-Version", must be copied, in order, to the new message. (3) The header fields in the enclosed message which start with "Content-", plus the "Subject", "Message-ID", "Encrypted", and "MIME-Version" fields, 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 the "Subject", "Message-ID", "Encrypted", and "MIME- Version" fields) will be ignored and dropped. (4) All of the header fields from the second and any subsequent enclosing messages are discarded by the reassembly process. RFC 2046 Media Types November 1996 number=1; total=2 X-Weird-Header-1: Bar X-Weird-Header-2: Hello Message-ID: <[email protected]> Subject: Audio mail 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: [email protected] To: [email protected] Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST) Subject: Audio mail (part 2 of 2) MIME-Version: 1.0 Message-ID: <[email protected]> Content-type: message/partial; id="[email protected]"; 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: [email protected] To: [email protected] Date: Fri, 26 Mar 1993 12:59:38 -0500 (EST) Subject: Audio mail Message-ID: <[email protected]> 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 ... 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. Freed & Borenstein Standards Track [Page 32] RFC 2046 Media Types November 1996 Finally, it should be noted that the "Encrypted" header field has been made obsolete by Privacy Enhanced Messaging (PEM) [RFC-1421, RFC-1422, RFC-1423, RFC-1424], but the rules above are nevertheless believed to describe the correct way to treat it if it is encountered in the context of conversion to and from "message/partial" fragments. RFC 2046 Media Types November 1996 If this proves problematic in practice, a new mechanism may be required as a future extension to MIME, either as newly defined access-types for "message/external-body" or by some other mechanism. As with "message/partial", MIME entities of type "message/external- body" MUST have a content-transfer-encoding of 7bit (the default). In particular, even in environments that support binary or 8bit transport, the use of a content- transfer-encoding of "8bit" or "binary" is explicitly prohibited for entities of type "message/external-body". RFC 2046 Media Types November 1996 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. This parameter may be used with ANY access-type and is ALWAYS optional. The precise semantics of the access-types defined here are described in the sections that follow. RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 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 in the content-type header field. Instead, it is provided as the "phantom body" when the media type is "message/external-body" and the access-type is mail-server. 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. 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 mail servers must use the same Content-ID field on the returned message that was used in the original "message/external-body" entities, to facilitate such matching. RFC 2046 Media Types November 1996 "message/external-body" reference. (2) MIME will sometimes be used in environments that provide some guarantee of message integrity and authenticity. If present, such guarantees may apply only to the actual direct content of messages -- they may or may not apply to data accessed through MIME's "message/external-body" mechanism. In particular, it may be possible to subvert certain access mechanisms even when the messaging system itself is secure. It should be noted that this problem exists either with or without the availabilty of MIME mechanisms. A casual reference to an FTP site containing a document in the text of a secure message brings up similar issues -- the only difference is that MIME provides for automatic retrieval of such material, and users may place unwarranted trust is such automatic retrieval mechanisms. RFC 2046 Media Types November 1996 The embedded message header fields which appear in the body of the "message/external-body" data must be used to declare the media type of the external body if it is anything other than plain US-ASCII text, since the external body does not have a header section to declare its type. Similarly, any Content-transfer-encoding other than "7bit" must also be declared here. Thus a complete "message/external-body" message, referring to an object in PostScript format, might look like this: From: Whomever To: Someone Date: Whenever Subject: whatever MIME-Version: 1.0 Message-ID: <[email protected]> Content-Type: multipart/alternative; boundary=42 Content-ID: <[email protected]> --42 Content-Type: message/external-body; name="BodyFormats.ps"; site="thumper.bellcore.com"; mode="image"; access-type=ANON-FTP; directory="pub"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript Content-ID: <[email protected]> --42 Content-Type: message/external-body; access-type=local-file; name="/u/nsb/writing/rfcs/RFC-MIME.ps"; site="thumper.bellcore.com"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript Content-ID: <[email protected]> --42 Content-Type: message/external-body; access-type=mail-server server="[email protected]"; expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)" Content-type: application/postscript Content-ID: <[email protected]> get RFC-MIME.DOC --42-- Freed & Borenstein Standards Track [Page 39] RFC 2046 Media Types November 1996 Note that in the above examples, the default Content-transfer- encoding of "7bit" is assumed for the external postscript data. Like the "message/partial" type, the "message/external-body" media 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 and Subject fields are overridden, but the From field is preserved. Note that since the external bodies are not transported along with the external body reference, they need not conform to transport limitations that apply to the reference itself. In particular, Internet mail transports may impose 7bit and line length limits, but these do not automatically apply to binary external body references. 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. RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 RFC 2046 Media Types November 1996 Appendix A -- 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 by name to several syntax rules 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. boundary := 0*69<bchars> bcharsnospace bchars := bcharsnospace / " " bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_" / "," / "-" / "." / "/" / ":" / "=" / "?" body-part := <"message" as defined in RFC 822, with all header fields optional, not starting with the specified dash-boundary, and with the delimiter not occurring anywhere in the body part. Note that the semantics of a part differ from the semantics of a message, as described in the text.> close-delimiter := delimiter "--" dash-boundary := "--" boundary ; boundary taken from the value of ; boundary parameter of the ; Content-Type field. delimiter := CRLF dash-boundary discard-text := *(*text CRLF) ; May be ignored or discarded. encapsulation := delimiter transport-padding CRLF body-part epilogue := discard-text multipart-body := [preamble CRLF] dash-boundary transport-padding CRLF body-part *encapsulation Freed & Borenstein Standards Track [Page 43] RFC 2046 Media Types November 1996
close-delimiter transport-padding
[CRLF epilogue]
preamble := discard-text
transport-padding := *LWSP-char
; Composers MUST NOT generate
; non-zero length transport
; padding, but receivers MUST
; be able to handle padding
; added by message transports.
Freed & Borenstein Standards Track [Page 44]
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