RFCs in HTML Format

RFC 1521

         MIME (Multipurpose Internet Mail Extensions) Part One:
                Mechanisms for Specifying and Describing
                 the Format of Internet Message Bodies

Borenstein & Freed                                              [Page 1]

RFC 1521 MIME September 1993 Table of Contents 1. Introduction....................................... 3 2. Notations, Conventions, and Generic BNF Grammar.... 6 3. The MIME-Version Header Field...................... 7 4. The Content-Type Header Field...................... 9 5. The Content-Transfer-Encoding Header Field......... 13 5.1. Quoted-Printable Content-Transfer-Encoding......... 18 5.2. Base64 Content-Transfer-Encoding................... 21 6. Additional Content-Header Fields................... 23 6.1. Optional Content-ID Header Field................... 23 6.2. Optional Content-Description Header Field.......... 24 7. The Predefined Content-Type Values................. 24 7.1. The Text Content-Type.............................. 24 7.1.1. The charset parameter.............................. 25 7.1.2. The Text/plain subtype............................. 28 7.2. The Multipart Content-Type......................... 28 7.2.1. Multipart: The common syntax...................... 29 7.2.2. The Multipart/mixed (primary) subtype.............. 34 7.2.3. The Multipart/alternative subtype.................. 34 7.2.4. The Multipart/digest subtype....................... 36 7.2.5. The Multipart/parallel subtype..................... 37 7.2.6. Other Multipart subtypes........................... 37 7.3. The Message Content-Type........................... 38 7.3.1. The Message/rfc822 (primary) subtype............... 38 7.3.2. The Message/Partial subtype........................ 39 7.3.3. The Message/External-Body subtype.................. 42 The "ftp" and "tftp" access-types............... 44 The "anon-ftp" access-type...................... 45 The "local-file" and "afs" access-types......... 45 The "mail-server" access-type................... 45 Examples and Further Explanations............... 46 7.4. The Application Content-Type....................... 49 7.4.1. The Application/Octet-Stream (primary) subtype..... 50 7.4.2. The Application/PostScript subtype................. 50 7.4.3. Other Application subtypes......................... 53 7.5. The Image Content-Type............................. 53 7.6. The Audio Content-Type............................. 54 7.7. The Video Content-Type............................. 54 7.8. Experimental Content-Type Values................... 54 8. Summary............................................ 56 9. Security Considerations............................ 56 10. Authors' Addresses................................. 57 11. Acknowledgements................................... 58 Appendix A -- Minimal MIME-Conformance.................... 60 Appendix B -- General Guidelines For Sending Email Data... 63 Appendix C -- A Complex Multipart Example................. 66 Appendix D -- Collected Grammar........................... 68 Borenstein & Freed [Page 2]
RFC 1521 MIME September 1993 Appendix E -- IANA Registration Procedures................ 72 E.1 Registration of New Content-type/subtype Values...... 72 E.2 Registration of New Access-type Values for Message/external-body............................ 73 Appendix F -- Summary of the Seven Content-types.......... 74 Appendix G -- Canonical Encoding Model.................... 76 Appendix H -- Changes from RFC 1341....................... 78 References................................................ 80 1. Introduction Since its publication in 1982, STD 11, RFC 822 [RFC 822] has defined the standard format of textual mail messages on the Internet. Its success has been such that the RFC 822 format has been adopted, wholly or partially, well beyond the confines of the Internet and the Internet SMTP transport defined by STD 10, RFC 821 [RFC 821]. As the format has seen wider use, a number of limitations have proven increasingly restrictive for the user community. RFC 822 was intended to specify a format for text messages. As such, non-text messages, such as multimedia messages that might include audio or images, are simply not mentioned. Even in the case of text, however, RFC 822 is inadequate for the needs of mail users whose languages require the use of character sets richer than US ASCII [US-ASCII]. Since RFC 822 does not specify mechanisms for mail containing audio, video, Asian language text, or even text in most European languages, additional specifications are needed. One of the notable limitations of RFC 821/822 based mail systems is the fact that they limit the contents of electronic mail messages to relatively short lines of seven-bit ASCII. This forces users to convert any non-textual data that they may wish to send into seven- bit bytes representable as printable ASCII characters before invoking a local mail UA (User Agent, a program with which human users send and receive mail). Examples of such encodings currently used in the Internet include pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in RFC 1421, the Andrew Toolkit Representation [ATK], and many others. The limitations of RFC 822 mail become even more apparent as gateways are designed to allow for the exchange of mail messages between RFC 822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the inclusion of non-textual body parts within electronic mail messages. The current standards for the mapping of X.400 messages to RFC 822 messages specify either that X.400 non-textual body parts must be converted to (not encoded in) an ASCII format, or that they must be discarded, notifying the RFC 822 user that discarding has occurred. This is clearly undesirable, as information that a user may wish to Borenstein & Freed [Page 3]
RFC 1521 MIME September 1993 receive is lost. Even though a user's UA may not have the capability of dealing with the non-textual body part, the user might have some mechanism external to the UA that can extract useful information from the body part. Moreover, it does not allow for the fact that the message may eventually be gatewayed back into an X.400 message handling system (i.e., the X.400 message is "tunneled" through Internet mail), where the non-textual information would definitely become useful again. This document describes several mechanisms that combine to solve most of these problems without introducing any serious incompatibilities with the existing world of RFC 822 mail. In particular, it describes: 1. A MIME-Version header field, which uses a version number to declare a message to be conformant with this specification and allows mail processing agents to distinguish between such messages and those generated by older or non-conformant software, which is presumed to lack such a field. 2. A Content-Type header field, generalized from RFC 1049 [RFC 1049], which can be used to specify the type and subtype of data in the body of a message and to fully specify the native representation (encoding) of such data. 2.a. A "text" Content-Type value, which can be used to represent textual information in a number of character sets and formatted text description languages in a standardized manner. 2.b. A "multipart" Content-Type value, which can be used to combine several body parts, possibly of differing types of data, into a single message. 2.c. An "application" Content-Type value, which can be used to transmit application data or binary data, and hence, among other uses, to implement an electronic mail file transfer service. 2.d. A "message" Content-Type value, for encapsulating another mail message. 2.e An "image" Content-Type value, for transmitting still image (picture) data. 2.f. An "audio" Content-Type value, for transmitting audio or voice data. Borenstein & Freed [Page 4]
RFC 1521 MIME September 1993 2.g. A "video" Content-Type value, for transmitting video or moving image data, possibly with audio as part of the composite video data format. 3. A Content-Transfer-Encoding header field, which can be used to specify an auxiliary encoding that was applied to the data in order to allow it to pass through mail transport mechanisms which may have data or character set limitations. 4. Two additional header fields that can be used to further describe the data in a message body, the Content-ID and Content- Description header fields. MIME has been carefully designed as an extensible mechanism, and it is expected that the set of content-type/subtype pairs and their associated parameters will grow significantly with time. Several other MIME fields, notably including character set names, are likely to have new values defined over time. In order to ensure that the set of such values is developed in an orderly, well-specified, and public manner, MIME defines a registration process which uses the Internet Assigned Numbers Authority (IANA) as a central registry for such values. Appendix E provides details about how IANA registration is accomplished. Finally, to specify and promote interoperability, Appendix A of this document provides a basic applicability statement for a subset of the above mechanisms that defines a minimal level of "conformance" with this document. HISTORICAL NOTE: Several of the mechanisms described in this document may seem somewhat strange or even baroque at first reading. It is important to note that compatibility with existing standards AND robustness across existing practice were two of the highest priorities of the working group that developed this document. In particular, compatibility was always favored over elegance. MIME was first defined and published as RFCs 1341 and 1342 [RFC 1341] [RFC 1342]. This document is a relatively minor updating of RFC 1341, and is intended to supersede it. The differences between this document and RFC 1341 are summarized in Appendix H. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Several other RFC documents will be of interest to the MIME implementor, in particular [RFC 1343], [RFC 1344], and [RFC 1345]. Borenstein & Freed [Page 5]
RFC 1521 MIME September 1993 2. Notations, Conventions, and Generic BNF Grammar This document is being published in two versions, one as plain ASCII text and one as PostScript (PostScript is a trademark of Adobe Systems Incorporated.). While the text version is the official specification, some will find the PostScript version easier to read. The textual contents are identical. An Andrew-format copy of this document is also available from the first author (Borenstein). Although the mechanisms specified in this document are all described in prose, most are also described formally in the modified BNF notation of RFC 822. Implementors will need to be familiar with this notation in order to understand this specification, and are referred to RFC 822 for a complete explanation of the modified BNF notation. Some of the modified BNF in this document makes reference to syntactic entities that are defined in RFC 822 and not in this document. A complete formal grammar, then, is obtained by combining the collected grammar appendix of this document with that of RFC 822 plus the modifications to RFC 822 defined in RFC 1123, which specifically changes the syntax for `return', `date' and `mailbox'. The term CRLF, in this document, refers to the sequence of the two ASCII characters CR (13) and LF (10) which, taken together, in this order, denote a line break in RFC 822 mail. The term "character set" is used in this document to refer to a method used with one or more tables to convert encoded text to a series of octets. This definition is intended to allow various kinds of text encodings, from simple single-table mappings such as ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, a MIME character set name must fully specify the mapping to be performed. The term "message", when not further qualified, means either the (complete or "top-level") message being transferred on a network, or a message encapsulated in a body of type "message". The term "body part", in this document, means one of the parts of the body of a multipart entity. A body part has a header and a body, so it makes sense to speak about the body of a body part. The term "entity", in this document, means either a message or a body part. All kinds of entities share the property that they have a header and a body. The term "body", when not further qualified, means the body of an entity, that is the body of either a message or of a body part. Borenstein & Freed [Page 6]
RFC 1521 MIME September 1993 NOTE: The previous four definitions are clearly circular. This is unavoidable, since the overall structure of a MIME message is indeed recursive. In this document, all numeric and octet values are given in decimal notation. It must be noted that Content-Type values, subtypes, and parameter names as defined in this document are case-insensitive. However, parameter values are case-sensitive unless otherwise specified for the specific parameter. FORMATTING NOTE: This document has been carefully formatted for ease of reading. The PostScript version of this document, in particular, places notes like this one, which may be skipped by the reader, in a smaller, italicized, font, and indents it as well. In the text version, only the indentation is preserved, so if you are reading the text version of this you might consider using the PostScript version instead. However, all such notes will be indented and preceded by "NOTE:" or some similar introduction, even in the text version. The primary purpose of these non-essential notes is to convey information about the rationale of this document, or to place this document in the proper historical or evolutionary context. Such information may be skipped by those who are focused entirely on building a conformant implementation, but may be of use to those who wish to understand why this document is written as it is. For ease of recognition, all BNF definitions have been placed in a fixed-width font in the PostScript version of this document. 3. The MIME-Version Header Field Since RFC 822 was published in 1982, there has really been only one format standard for Internet messages, and there has been little perceived need to declare the format standard in use. This document is an independent document that complements RFC 822. Although the extensions in this document have been defined in such a way as to be compatible with RFC 822, there are still circumstances in which it might be desirable for a mail-processing agent to know whether a message was composed with the new standard in mind. Therefore, this document defines a new header field, "MIME-Version", which is to be used to declare the version of the Internet message body format standard in use. Messages composed in accordance with this document MUST include such Borenstein & Freed [Page 7]
RFC 1521 MIME September 1993 a header field, with the following verbatim text: MIME-Version: 1.0 The presence of this header field is an assertion that the message has been composed in compliance with this document. Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field: version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT Thus, future format specifiers, which might replace or extend "1.0", are constrained to be two integer fields, separated by a period. If a message is received with a MIME-version value other than "1.0", it cannot be assumed to conform with this specification. Note that the MIME-Version header field is required at the top level of a message. It is not required for each body part of a multipart entity. It is required for the embedded headers of a body of type "message" if and only if the embedded message is itself claimed to be MIME-conformant. It is not possible to fully specify how a mail reader that conforms with MIME as defined in this document should treat a message that might arrive in the future with some value of MIME-Version other than "1.0". However, conformant software is encouraged to check the version number and at least warn the user if an unrecognized MIME- version is encountered. It is also worth noting that version control for specific content- types is not accomplished using the MIME-Version mechanism. In particular, some formats (such as application/postscript) have version numbering conventions that are internal to the document format. Where such conventions exist, MIME does nothing to supersede them. Where no such conventions exist, a MIME type might use a "version" parameter in the content-type field if necessary. NOTE TO IMPLEMENTORS: All header fields defined in this document, including MIME-Version, Content-type, etc., are subject to the general syntactic rules for header fields specified in RFC 822. In particular, all can include comments, which means that the following two MIME-Version fields are equivalent: MIME-Version: 1.0 MIME-Version: 1.0 (Generated by GBD-killer 3.7) Borenstein & Freed [Page 8]
RFC 1521 MIME September 1993 4. The Content-Type Header Field The purpose of the Content-Type field is to describe the data contained in the body fully enough that the receiving user agent can pick an appropriate agent or mechanism to present the data to the user, or otherwise deal with the data in an appropriate manner. HISTORICAL NOTE: The Content-Type header field was first defined in RFC 1049. RFC 1049 Content-types used a simpler and less powerful syntax, but one that is largely compatible with the mechanism given here. The Content-Type header field is used to specify the nature of the data in the body of an entity, by giving type and subtype identifiers, and by providing auxiliary information that may be required for certain types. After the type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The set of meaningful parameters differs for the different types. In particular, there are NO globally-meaningful parameters that apply to all content-types. Global mechanisms are best addressed, in the MIME model, by the definition of additional Content-* header fields. The ordering of parameters is not significant. Among the defined parameters is a "charset" parameter by which the character set used in the body may be declared. Comments are allowed in accordance with RFC 822 rules for structured header fields. In general, the top-level Content-Type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a Content-Type of "image/xyz" is enough to tell a user agent that the data is an image, even if the user agent has no knowledge of the specific image format "xyz". Such information can be used, for example, to decide whether or not to show a user the raw data from an unrecognized subtype -- such an action might be reasonable for unrecognized subtypes of text, but not for unrecognized subtypes of image or audio. For this reason, registered subtypes of audio, image, text, and video, should not contain embedded information that is really of a different type. Such compound types should be represented using the "multipart" or "application" types. Parameters are modifiers of the content-subtype, and do not fundamentally affect the requirements of the host system. Although most parameters make sense only with certain content-types, others are "global" in the sense that they might apply to any subtype. For example, the "boundary" parameter makes sense only for the "multipart" content-type, but the "charset" parameter might make sense with several content-types. Borenstein & Freed [Page 9]
RFC 1521 MIME September 1993 An initial set of seven Content-Types is defined by this document. This set of top-level names is intended to be substantially complete. It is expected that additions to the larger set of supported types can generally be accomplished by the creation of new subtypes of these initial types. In the future, more top-level types may be defined only by an extension to this standard. If another primary type is to be used for any reason, it must be given a name starting with "X-" to indicate its non-standard status and to avoid a potential conflict with a future official name. In the Augmented BNF notation of RFC 822, a Content-Type header field value is defined as follows: content := "Content-Type" ":" type "/" subtype *(";" parameter) ; case-insensitive matching of type and subtype type := "application" / "audio" / "image" / "message" / "multipart" / "text" / "video" / extension-token ; All values case-insensitive extension-token := x-token / iana-token iana-token := <a publicly-defined extension token, registered with IANA, as specified in appendix E> x-token := <The two characters "X-" or "x-" followed, with no intervening white space, by any token> subtype := token ; case-insensitive parameter := attribute "=" value attribute := token ; case-insensitive value := token / quoted-string token := 1*<any (ASCII) CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "=" ; Must be in quoted-string, ; to use within parameter values Borenstein & Freed [Page 10]
RFC 1521 MIME September 1993 Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=", and the removal of ".". Note also that a subtype specification is MANDATORY. There are no default subtypes. The type, subtype, and parameter names are not case sensitive. For example, TEXT, Text, and TeXt are all equivalent. Parameter values are normally case sensitive, but certain parameters are interpreted to be case-insensitive, depending on the intended use. (For example, multipart boundaries are case-sensitive, but the "access-type" for message/External-body is not case-sensitive.) Beyond this syntax, the only constraint on the definition of subtype names is the desire that their uses must not conflict. That is, it would be undesirable to have two different communities using "Content-Type: application/foobar" to mean two different things. The process of defining new content-subtypes, then, is not intended to be a mechanism for imposing restrictions, but simply a mechanism for publicizing the usages. There are, therefore, two acceptable mechanisms for defining new Content-Type subtypes: 1. Private values (starting with "X-") may be defined bilaterally between two cooperating agents without outside registration or standardization. 2. New standard values must be documented, registered with, and approved by IANA, as described in Appendix E. Where intended for public use, the formats they refer to must also be defined by a published specification, and possibly offered for standardization. The seven standard initial predefined Content-Types are detailed in the bulk of this document. They are: text -- textual information. The primary subtype, "plain", indicates plain (unformatted) text. No special software is required to get the full meaning of the text, aside from support for the indicated character set. Subtypes are to be used for enriched text in forms where application software may enhance the appearance of the text, but such software must not be required in order to get the general idea of the content. Possible subtypes thus include any readable word processor Borenstein & Freed [Page 11]
RFC 1521 MIME September 1993 format. A very simple and portable subtype, richtext, was defined in RFC 1341, with a future revision expected. multipart -- data consisting of multiple parts of independent data types. Four initial subtypes are defined, including the primary "mixed" subtype, "alternative" for representing the same data in multiple formats, "parallel" for parts intended to be viewed simultaneously, and "digest" for multipart entities in which each part is of type "message". message -- an encapsulated message. A body of Content-Type "message" is itself all or part of a fully formatted RFC 822 conformant message which may contain its own different Content-Type header field. The primary subtype is "rfc822". The "partial" subtype is defined for partial messages, to permit the fragmented transmission of bodies that are thought to be too large to be passed through mail transport facilities. Another subtype, "External-body", is defined for specifying large bodies by reference to an external data source. image -- image data. Image requires a display device (such as a graphical display, a printer, or a FAX machine) to view the information. Initial subtypes are defined for two widely-used image formats, jpeg and gif. audio -- audio data, with initial subtype "basic". Audio requires an audio output device (such as a speaker or a telephone) to "display" the contents. video -- video data. Video requires the capability to display moving images, typically including specialized hardware and software. The initial subtype is "mpeg". application -- some other kind of data, typically either uninterpreted binary data or information to be processed by a mail-based application. The primary subtype, "octet-stream", is to be used in the case of uninterpreted binary data, in which case the simplest recommended action is to offer to write the information into a file for the user. Borenstein & Freed [Page 12]
RFC 1521 MIME September 1993 An additional subtype, "PostScript", is defined for transporting PostScript documents in bodies. Other expected uses for "application" include spreadsheets, data for mail-based scheduling systems, and languages for "active" (computational) email. (Note that active email and other application data may entail several security considerations, which are discussed later in this memo, particularly in the context of application/PostScript.) Default RFC 822 messages are typed by this protocol as plain text in the US-ASCII character set, which can be explicitly specified as "Content-type: text/plain; charset=us-ascii". If no Content-Type is specified, this default is assumed. In the presence of a MIME- Version header field, a receiving User Agent can also assume that plain US-ASCII text was the sender's intent. In the absence of a MIME-Version specification, plain US-ASCII text must still be assumed, but the sender's intent might have been otherwise. RATIONALE: In the absence of any Content-Type header field or MIME-Version header field, it is impossible to be certain that a message is actually text in the US-ASCII character set, since it might well be a message that, using the conventions that predate this document, includes text in another character set or non- textual data in a manner that cannot be automatically recognized (e.g., a uuencoded compressed UNIX tar file). Although there is no fully acceptable alternative to treating such untyped messages as "text/plain; charset=us-ascii", implementors should remain aware that if a message lacks both the MIME-Version and the Content-Type header fields, it may in practice contain almost anything. It should be noted that the list of Content-Type values given here may be augmented in time, via the mechanisms described above, and that the set of subtypes is expected to grow substantially. When a mail reader encounters mail with an unknown Content-type value, it should generally treat it as equivalent to "application/octet-stream", as described later in this document. 5. The Content-Transfer-Encoding Header Field Many Content-Types which could usefully be transported via email are represented, in their "natural" format, as 8-bit character or binary data. Such data cannot be transmitted over some transport protocols. For example, RFC 821 restricts mail messages to 7-bit US-ASCII data with lines no longer than 1000 characters. Borenstein & Freed [Page 13]
RFC 1521 MIME September 1993 It is necessary, therefore, to define a standard mechanism for re- encoding such data into a 7-bit short-line format. This document specifies that such encodings will be indicated by a new "Content- Transfer-Encoding" header field. The Content-Transfer-Encoding field is used to indicate the type of transformation that has been used in order to represent the body in an acceptable manner for transport. Unlike Content-Types, a proliferation of Content-Transfer-Encoding values is undesirable and unnecessary. However, establishing only a single Content-Transfer-Encoding mechanism does not seem possible. There is a tradeoff between the desire for a compact and efficient encoding of largely-binary data and the desire for a readable encoding of data that is mostly, but not entirely, 7-bit data. For this reason, at least two encoding mechanisms are necessary: a "readable" encoding and a "dense" encoding. The Content-Transfer-Encoding field is designed to specify an invertible mapping between the "native" representation of a type of data and a representation that can be readily exchanged using 7 bit mail transport protocols, such as those defined by RFC 821 (SMTP). This field has not been defined by any previous standard. The field's value is a single token specifying the type of encoding, as enumerated below. Formally: encoding := "Content-Transfer-Encoding" ":" mechanism mechanism := "7bit" ; case-insensitive / "quoted-printable" / "base64" / "8bit" / "binary" / x-token These values are not case sensitive. That is, Base64 and BASE64 and bAsE64 are all equivalent. An encoding type of 7BIT requires that the body is already in a seven-bit mail-ready representation. This is the default value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the Content-Transfer-Encoding header field is not present. The values "8bit", "7bit", and "binary" all mean that NO encoding has been performed. However, they are potentially useful as indications of the kind of data contained in the object, and therefore of the kind of encoding that might need to be performed for transmission in a given transport system. In particular: "7bit" means that the data is all represented as short lines of US-ASCII data. Borenstein & Freed [Page 14]
RFC 1521 MIME September 1993 "8bit" means that the lines are short, but there may be non-ASCII characters (octets with the high-order bit set). "Binary" means that not only may non-ASCII characters be present, but also that the lines are not necessarily short enough for SMTP transport. The difference between "8bit" (or any other conceivable bit-width token) and the "binary" token is that "binary" does not require adherence to any limits on line length or to the SMTP CRLF semantics, while the bit-width tokens do require such adherence. If the body contains data in any bit-width other than 7-bit, the appropriate bit-width Content-Transfer-Encoding token must be used (e.g., "8bit" for unencoded 8 bit wide data). If the body contains binary data, the "binary" Content-Transfer-Encoding token must be used. NOTE: The distinction between the Content-Transfer-Encoding values of "binary", "8bit", etc. may seem unimportant, in that all of them really mean "none" -- that is, there has been no encoding of the data for transport. However, clear labeling will be of enormous value to gateways between future mail transport systems with differing capabilities in transporting data that do not meet the restrictions of RFC 821 transport. Mail transport for unencoded 8-bit data is defined in RFC 1426 [RFC 1426]. As of the publication of this document, there are no standardized Internet mail transports for which it is legitimate to include unencoded binary data in mail bodies. Thus there are no circumstances in which the "binary" Content-Transfer-Encoding is actually legal on the Internet. However, in the event that binary mail transport becomes a reality in Internet mail, or when this document is used in conjunction with any other binary-capable transport mechanism, binary bodies should be labeled as such using this mechanism. NOTE: The five values defined for the Content-Transfer-Encoding field imply nothing about the Content-Type other than the algorithm by which it was encoded or the transport system requirements if unencoded. Implementors may, if necessary, define new Content-Transfer-Encoding values, but must use an x-token, which is a name prefixed by "X-" to indicate its non-standard status, e.g., "Content-Transfer-Encoding: x-my-new-encoding". However, unlike Content-Types and subtypes, the creation of new Content-Transfer-Encoding values is explicitly and strongly discouraged, as it seems likely to hinder interoperability with little potential benefit. Their use is allowed only as the Borenstein & Freed [Page 15]
RFC 1521 MIME September 1993 result of an agreement between cooperating user agents. If a Content-Transfer-Encoding header field appears as part of a message header, it applies to the entire body of that message. If a Content-Transfer-Encoding header field appears as part of a body part's headers, it applies only to the body of that body part. If an entity is of type "multipart" or "message", the Content-Transfer- Encoding is not permitted to have any value other than a bit width (e.g., "7bit", "8bit", etc.) or "binary". It should be noted that email is character-oriented, so that the mechanisms described here are mechanisms for encoding arbitrary octet streams, not bit streams. If a bit stream is to be encoded via one of these mechanisms, it must first be converted to an 8-bit byte stream using the network standard bit order ("big-endian"), in which the earlier bits in a stream become the higher-order bits in a byte. A bit stream not ending at an 8-bit boundary must be padded with zeroes. This document provides a mechanism for noting the addition of such padding in the case of the application Content-Type, which has a "padding" parameter. The encoding mechanisms defined here explicitly encode all data in ASCII. Thus, for example, suppose an entity has header fields such as: Content-Type: text/plain; charset=ISO-8859-1 Content-transfer-encoding: base64 This must be interpreted to mean that the body is a base64 ASCII encoding of data that was originally in ISO-8859-1, and will be in that character set again after decoding. The following sections will define the two standard encoding mechanisms. The definition of new content-transfer-encodings is explicitly discouraged and should only occur when absolutely necessary. All content-transfer-encoding namespace except that beginning with "X-" is explicitly reserved to the IANA for future use. Private agreements about content-transfer-encodings are also explicitly discouraged. Certain Content-Transfer-Encoding values may only be used on certain Content-Types. In particular, it is expressly forbidden to use any encodings other than "7bit", "8bit", or "binary" with any Content- Type that recursively includes other Content-Type fields, notably the "multipart" and "message" Content-Types. All encodings that are desired for bodies of type multipart or message must be done at the innermost level, by encoding the actual body that needs to be encoded. Borenstein & Freed [Page 16]
RFC 1521 MIME September 1993 NOTE ON ENCODING RESTRICTIONS: Though the prohibition against using content-transfer-encodings on data of type multipart or message may seem overly restrictive, it is necessary to prevent nested encodings, in which data are passed through an encoding algorithm multiple times, and must be decoded multiple times in order to be properly viewed. Nested encodings add considerable complexity to user agents: aside from the obvious efficiency problems with such multiple encodings, they can obscure the basic structure of a message. In particular, they can imply that several decoding operations are necessary simply to find out what types of objects a message contains. Banning nested encodings may complicate the job of certain mail gateways, but this seems less of a problem than the effect of nested encodings on user agents. NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT- TRANSFER-ENCODING: It may seem that the Content-Transfer-Encoding could be inferred from the characteristics of the Content-Type that is to be encoded, or, at the very least, that certain Content-Transfer-Encodings could be mandated for use with specific Content-Types. There are several reasons why this is not the case. First, given the varying types of transports used for mail, some encodings may be appropriate for some Content-Type/transport combinations and not for others. (For example, in an 8-bit transport, no encoding would be required for text in certain character sets, while such encodings are clearly required for 7- bit SMTP.) Second, certain Content-Types may require different types of transfer encoding under different circumstances. For example, many PostScript bodies might consist entirely of short lines of 7-bit data and hence require little or no encoding. Other PostScript bodies (especially those using Level 2 PostScript's binary encoding mechanism) may only be reasonably represented using a binary transport encoding. Finally, since Content-Type is intended to be an open-ended specification mechanism, strict specification of an association between Content-Types and encodings effectively couples the specification of an application protocol with a specific lower-level transport. This is not desirable since the developers of a Content-Type should not have to be aware of all the transports in use and what their limitations are. NOTE ON TRANSLATING ENCODINGS: The quoted-printable and base64 encodings are designed so that conversion between them is possible. The only issue that arises in such a conversion is the handling of line breaks. When converting from quoted-printable to base64 a line break must be converted into a CRLF sequence. Similarly, a CRLF sequence in base64 data must be converted to a quoted-printable line break, but ONLY when converting text data. Borenstein & Freed [Page 17]
RFC 1521 MIME September 1993 NOTE ON CANONICAL ENCODING MODEL: There was some confusion, in earlier drafts of this memo, regarding the model for when email data was to be converted to canonical form and encoded, and in particular how this process would affect the treatment of CRLFs, given that the representation of newlines varies greatly from system to system, and the relationship between content-transfer- encodings and character sets. For this reason, a canonical model for encoding is presented as Appendix G. 5.1. Quoted-Printable Content-Transfer-Encoding The Quoted-Printable encoding is intended to represent data that largely consists of octets that correspond to printable characters in the ASCII character set. It encodes the data in such a way that the resulting octets are unlikely to be modified by mail transport. If the data being encoded are mostly ASCII text, the encoded form of the data remains largely recognizable by humans. A body which is entirely ASCII may also be encoded in Quoted-Printable to ensure the integrity of the data should the message pass through a character- translating, and/or line-wrapping gateway. In this encoding, octets are to be represented as determined by the following rules: Rule #1: (General 8-bit representation) Any octet, except those indicating a line break according to the newline convention of the canonical (standard) form of the data being encoded, may be represented by an "=" followed by a two digit hexadecimal representation of the octet's value. The digits of the hexadecimal alphabet, for this purpose, are "0123456789ABCDEF". Uppercase letters must be used when sending hexadecimal data, though a robust implementation may choose to recognize lowercase letters on receipt. Thus, for example, the value 12 (ASCII form feed) can be represented by "=0C", and the value 61 (ASCII EQUAL SIGN) can be represented by "=3D". Except when the following rules allow an alternative encoding, this rule is mandatory. Rule #2: (Literal representation) Octets with decimal values of 33 through 60 inclusive, and 62 through 126, inclusive, MAY be represented as the ASCII characters which correspond to those octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN through TILDE, respectively). Rule #3: (White Space): Octets with values of 9 and 32 MAY be represented as ASCII TAB (HT) and SPACE characters, respectively, but MUST NOT be so represented at the end of an encoded line. Any TAB (HT) or SPACE characters on an encoded line MUST thus be followed on that line by a printable character. In particular, an Borenstein & Freed [Page 18]
RFC 1521 MIME September 1993 "=" at the end of an encoded line, indicating a soft line break (see rule #5) may follow one or more TAB (HT) or SPACE characters. It follows that an octet with value 9 or 32 appearing at the end of an encoded line must be represented according to Rule #1. This rule is necessary because some MTAs (Message Transport Agents, programs which transport messages from one user to another, or perform a part of such transfers) are known to pad lines of text with SPACEs, and others are known to remove "white space" characters from the end of a line. Therefore, when decoding a Quoted-Printable body, any trailing white space on a line must be deleted, as it will necessarily have been added by intermediate transport agents. Rule #4 (Line Breaks): A line break in a text body, independent of what its representation is following the canonical representation of the data being encoded, must be represented by a (RFC 822) line break, which is a CRLF sequence, in the Quoted-Printable encoding. Since the canonical representation of types other than text do not generally include the representation of line breaks, no hard line breaks (i.e. line breaks that are intended to be meaningful and to be displayed to the user) should occur in the quoted-printable encoding of such types. Of course, occurrences of "=0D", "=0A", "0A=0D" and "=0D=0A" will eventually be encountered. In general, however, base64 is preferred over quoted-printable for binary data. Note that many implementations may elect to encode the local representation of various content types directly, as described in Appendix G. In particular, this may apply to plain text material on systems that use newline conventions other than CRLF delimiters. Such an implementation is permissible, but the generation of line breaks must be generalized to account for the case where alternate representations of newline sequences are used. Rule #5 (Soft Line Breaks): The Quoted-Printable encoding REQUIRES that encoded lines be no more than 76 characters long. If longer lines are to be encoded with the Quoted-Printable encoding, 'soft' line breaks must be used. An equal sign as the last character on a encoded line indicates such a non-significant ('soft') line break in the encoded text. Thus if the "raw" form of the line is a single unencoded line that says: Now's the time for all folk to come to the aid of their country. This can be represented, in the Quoted-Printable encoding, as Borenstein & Freed [Page 19]
RFC 1521 MIME September 1993 Now's the time = for all folk to come= to the aid of their country. This provides a mechanism with which long lines are encoded in such a way as to be restored by the user agent. The 76 character limit does not count the trailing CRLF, but counts all other characters, including any equal signs. Since the hyphen character ("-") is represented as itself in the Quoted-Printable encoding, care must be taken, when encapsulating a quoted-printable encoded body in a multipart entity, to ensure that the encapsulation boundary does not appear anywhere in the encoded body. (A good strategy is to choose a boundary that includes a character sequence such as "=_" which can never appear in a quoted- printable body. See the definition of multipart messages later in this document.) NOTE: The quoted-printable encoding represents something of a compromise between readability and reliability in transport. Bodies encoded with the quoted-printable encoding will work reliably over most mail gateways, but may not work perfectly over a few gateways, notably those involving translation into EBCDIC. (In theory, an EBCDIC gateway could decode a quoted-printable body and re-encode it using base64, but such gateways do not yet exist.) A higher level of confidence is offered by the base64 Content-Transfer-Encoding. A way to get reasonably reliable transport through EBCDIC gateways is to also quote the ASCII characters !"#$@[\]^`{|}~ according to rule #1. See Appendix B for more information. Because quoted-printable data is generally assumed to be line- oriented, it is to be expected that the representation of the breaks between the lines of quoted printable data may be altered in transport, in the same manner that plain text mail has always been altered in Internet mail when passing between systems with differing newline conventions. If such alterations are likely to constitute a corruption of the data, it is probably more sensible to use the base64 encoding rather than the quoted-printable encoding. WARNING TO IMPLEMENTORS: If binary data are encoded in quoted- printable, care must be taken to encode CR and LF characters as "=0D" and "=0A", respectively. In particular, a CRLF sequence in binary data should be encoded as "=0D=0A". Otherwise, if CRLF were represented as a hard line break, it might be incorrectly decoded on Borenstein & Freed [Page 20]
RFC 1521 MIME September 1993 platforms with different line break conventions. For formalists, the syntax of quoted-printable data is described by the following grammar: quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF) ; Maximum line length of 76 characters excluding CRLF ptext := octet /<any ASCII character except "=", SPACE, or TAB> ; characters not listed as "mail-safe" in Appendix B ; are also not recommended. octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F") ; octet must be used for characters > 127, =, SPACE, or TAB, ; and is recommended for any characters not listed in ; Appendix B as "mail-safe". 5.2. Base64 Content-Transfer-Encoding The Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets in a form that need not be humanly readable. The encoding and decoding algorithms are simple, but the encoded data are consistently only about 33 percent larger than the unencoded data. This encoding is virtually identical to the one used in Privacy Enhanced Mail (PEM) applications, as defined in RFC 1421. The base64 encoding is adapted from RFC 1421, with one change: base64 eliminates the "*" mechanism for embedded clear text. A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.) NOTE: This subset has the important property that it is represented identically in all versions of ISO 646, including US ASCII, and all characters in the subset are also represented identically in all versions of EBCDIC. Other popular encodings, such as the encoding used by the uuencode utility and the base85 encoding specified as part of Level 2 PostScript, do not share these properties, and thus do not fulfill the portability requirements a binary transport encoding for mail must meet. The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base64 alphabet. When encoding a bit stream via the base64 encoding, the bit stream must be presumed to be ordered with the most-significant-bit first. Borenstein & Freed [Page 21]
RFC 1521 MIME September 1993 That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit in the first byte, and so on. Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", CR, LF) and to the encapsulation boundaries defined in this document (e.g., "-"). Table 1: The Base64 Alphabet Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w (pad) = 15 P 32 g 49 x 16 Q 33 h 50 y The output stream (encoded bytes) must be represented in lines of no more than 76 characters each. All line breaks or other characters not found in Table 1 must be ignored by decoding software. In base64 data, characters other than those in Table 1, line breaks, and other white space probably indicate a transmission error, about which a warning message or even a message rejection might be appropriate under some circumstances. Special processing is performed if fewer than 24 bits are available at the end of the data being encoded. A full encoding quantum is always completed at the end of a body. When fewer than 24 input bits are available in an input group, zero bits are added (on the right) to form an integral number of 6-bit groups. Padding at the end of the data is performed using the '=' character. Since all base64 input is an integral number of octets, only the following cases can Borenstein & Freed [Page 22]
RFC 1521 MIME September 1993 arise: (1) the final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output will be an integral multiple of 4 characters with no "=" padding, (2) the final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output will be two characters followed by two "=" padding characters, or (3) the final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output will be three characters followed by one "=" padding character. Because it is used only for padding at the end of the data, the occurrence of any '=' characters may be taken as evidence that the end of the data has been reached (without truncation in transit). No such assurance is possible, however, when the number of octets transmitted was a multiple of three. Any characters outside of the base64 alphabet are to be ignored in base64-encoded data. The same applies to any illegal sequence of characters in the base64 encoding, such as "=====" Care must be taken to use the proper octets for line breaks if base64 encoding is applied directly to text material that has not been converted to canonical form. In particular, text line breaks must be converted into CRLF sequences prior to base64 encoding. The important thing to note is that this may be done directly by the encoder rather than in a prior canonicalization step in some implementations. NOTE: There is no need to worry about quoting apparent encapsulation boundaries within base64-encoded parts of multipart entities because no hyphen characters are used in the base64 encoding. 6. Additional Content-Header Fields 6.1. Optional Content-ID Header Field In constructing a high-level user agent, it may be desirable to allow one body to make reference to another. Accordingly, bodies may be labeled using the "Content-ID" header field, which is syntactically identical to the "Message-ID" header field: id := "Content-ID" ":" msg-id Like the Message-ID values, Content-ID values must be generated to be world-unique. The Content-ID value may be used for uniquely identifying MIME entities in several contexts, particularly for cacheing data referenced by the message/external-body mechanism. Although the Content-ID header is generally optional, its use is mandatory in Borenstein & Freed [Page 23]
RFC 1521 MIME September 1993 implementations which generate data of the optional MIME Content-type "message/external-body". That is, each message/external-body entity must have a Content-ID field to permit cacheing of such data. It is also worth noting that the Content-ID value has special semantics in the case of the multipart/alternative content-type. This is explained in the section of this document dealing with multipart/alternative. 6.2. Optional Content-Description Header Field The ability to associate some descriptive information with a given body is often desirable. For example, it may be useful to mark an "image" body as "a picture of the Space Shuttle Endeavor." Such text may be placed in the Content-Description header field. description := "Content-Description" ":" *text The description is presumed to be given in the US-ASCII character set, although the mechanism specified in [RFC 1522] may be used for non-US-ASCII Content-Description values. 7. The Predefined Content-Type Values This document defines seven initial Content-Type values and an extension mechanism for private or experimental types. Further standard types must be defined by new published specifications. It is expected that most innovation in new types of mail will take place as subtypes of the seven types defined here. The most essential characteristics of the seven content-types are summarized in Appendix F. 7.1 The Text Content-Type The text Content-Type is intended for sending material which is principally textual in form. It is the default Content-Type. A "charset" parameter may be used to indicate the character set of the body text for some text subtypes, notably including the primary subtype, "text/plain", which indicates plain (unformatted) text. The default Content-Type for Internet mail is "text/plain; charset=us- ascii". Beyond plain text, there are many formats for representing what might be known as "extended text" -- text with embedded formatting and presentation information. An interesting characteristic of many such representations is that they are to some extent readable even without the software that interprets them. It is useful, then, to distinguish them, at the highest level, from such unreadable data as Borenstein & Freed [Page 24]
RFC 1521 MIME September 1993 images, audio, or text represented in an unreadable form. In the absence of appropriate interpretation software, it is reasonable to show subtypes of text to the user, while it is not reasonable to do so with most nontextual data. Such formatted textual data should be represented using subtypes of text. Plausible subtypes of text are typically given by the common name of the representation format, e.g., "text/richtext" [RFC 1341]. 7.1.1. The charset parameter A critical parameter that may be specified in the Content-Type field for text/plain data is the character set. This is specified with a "charset" parameter, as in: Content-type: text/plain; charset=us-ascii Unlike some other parameter values, the values of the charset parameter are NOT case sensitive. The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII. The specification for any future subtypes of "text" must specify whether or not they will also utilize a "charset" parameter, and may possibly restrict its values as well. When used with a particular body, the semantics of the "charset" parameter should be identical to those specified here for "text/plain", i.e., the body consists entirely of characters in the given charset. In particular, definers of future text subtypes should pay close attention the the implications of multibyte character sets for their subtype definitions. This RFC specifies the definition of the charset parameter for the purposes of MIME to be a unique mapping of a byte stream to glyphs, a mapping which does not require external profiling information. An initial list of predefined character set names can be found at the end of this section. Additional character sets may be registered with IANA, although the standardization of their use requires the usual IESG [RFC 1340] review and approval. Note that if the specified character set includes 8-bit data, a Content-Transfer- Encoding header field and a corresponding encoding on the data are required in order to transmit the body via some mail transfer protocols, such as SMTP. The default character set, US-ASCII, has been the subject of some confusion and ambiguity in the past. Not only were there some ambiguities in the definition, there have been wide variations in practice. In order to eliminate such ambiguity and variations in the Borenstein & Freed [Page 25]
RFC 1521 MIME September 1993 future, it is strongly recommended that new user agents explicitly specify a character set via the Content-Type header field. "US- ASCII" does not indicate an arbitrary seven-bit character code, but specifies that the body uses character coding that uses the exact correspondence of codes to characters specified in ASCII. National use variations of ISO 646 [ISO-646] are NOT ASCII and their use in Internet mail is explicitly discouraged. The omission of the ISO 646 character set is deliberate in this regard. The character set name of "US-ASCII" explicitly refers to ANSI X3.4-1986 [US-ASCII] only. The character set name "ASCII" is reserved and must not be used for any purpose. NOTE: RFC 821 explicitly specifies "ASCII", and references an earlier version of the American Standard. Insofar as one of the purposes of specifying a Content-Type and character set is to permit the receiver to unambiguously determine how the sender intended the coded message to be interpreted, assuming anything other than "strict ASCII" as the default would risk unintentional and incompatible changes to the semantics of messages now being transmitted. This also implies that messages containing characters coded according to national variations on ISO 646, or using code-switching procedures (e.g., those of ISO 2022), as well as 8-bit or multiple octet character encodings MUST use an appropriate character set specification to be consistent with this specification. The complete US-ASCII character set is listed in [US-ASCII]. Note that the control characters including DEL (0-31, 127) have no defined meaning apart from the combination CRLF (ASCII values 13 and 10) indicating a new line. Two of the characters have de facto meanings in wide use: FF (12) often means "start subsequent text on the beginning of a new page"; and TAB or HT (9) often (though not always) means "move the cursor to the next available column after the current position where the column number is a multiple of 8 (counting the first column as column 0)." Apart from this, any use of the control characters or DEL in a body must be part of a private agreement between the sender and recipient. Such private agreements are discouraged and should be replaced by the other capabilities of this document. NOTE: Beyond US-ASCII, an enormous proliferation of character sets is possible. It is the opinion of the IETF working group that a large number of character sets is NOT a good thing. We would prefer to specify a single character set that can be used universally for representing all of the world's languages in electronic mail. Unfortunately, existing practice in several communities seems to point to the continued use of multiple character sets in the near future. For this reason, we define Borenstein & Freed [Page 26]
RFC 1521 MIME September 1993
RFC 1521 MIME September 1993 Security considerations: Published specification: (The published specification must be an Internet RFC or RFC-to-be if a new top-level type is being defined, and must be a publicly available specification in any case.) Person & email address to contact for further information: E.2 Registration of New Access-type Values for Message/external-body To: IANA@isi.edu Subject: Registration of new MIME Access-type for Message/external-body content-type MIME access-type name: Required parameters: Optional parameters: Published specification: (The published specification must be an Internet RFC or RFC-to-be.) Person & email address to contact for further information: Borenstein & Freed [Page 73]
RFC 1521 MIME September 1993 Appendix F -- Summary of the Seven Content-types Content-type: text Subtypes defined by this document: plain Important Parameters: charset Encoding notes: quoted-printable generally preferred if an encoding is needed and the character set is mostly an ASCII superset. Security considerations: Rich text formats such as TeX and Troff often contain mechanisms for executing arbitrary commands or file system operations, and should not be used automatically unless these security problems have been addressed. Even plain text may contain control characters that can be used to exploit the capabilities of "intelligent" terminals and cause security violations. User interfaces designed to run on such terminals should be aware of and try to prevent such problems. ________________________________________________________ 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, access-type, expiration, size, permission, name, site, directory, mode, server, subject Encoding notes: No content-transfer-encoding is permitted. Specifically, only "7bit" is permitted for "message/partial" or "message/external-body", and only "7bit", "8bit", or "binary" are permitted for other subtypes of "message". ______________________________________________________________ Content-type: application Subtypes defined by this document: octet-stream, postscript Important Parameters: type, padding Borenstein & Freed [Page 74]
RFC 1521 MIME September 1993 Deprecated Parameters: name and conversions were defined in RFC 1341. Encoding notes: base64 preferred for unreadable subtypes. Security considerations: This type is intended for the transmission of data to be interpreted by locally-installed programs. If used, for example, to transmit executable binary programs or programs in general-purpose interpreted languages, such as LISP programs or shell scripts, severe security problems could result. Authors of mail-reading agents are cautioned against giving their systems the power to execute mail-based application data without carefully considering the security implications. While it is certainly possible to define safe application formats and even safe interpreters for unsafe formats, each interpreter should be evaluated separately for possible security problems. ________________________________________________________________ Content-type: image Subtypes defined by this document: jpeg, gif Important Parameters: none Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: audio Subtypes defined by this document: basic Important Parameters: none Encoding notes: base64 generally preferred ________________________________________________________________ Content-type: video Subtypes defined by this document: mpeg Important Parameters: none Encoding notes: base64 generally preferred Borenstein & Freed [Page 75]
RFC 1521 MIME September 1993 Appendix G -- Canonical Encoding Model There was some confusion, in earlier drafts of this memo, regarding the model for when email data was to be converted to canonical form and encoded, and in particular how this process would affect the treatment of CRLFs, given that the representation of newlines varies greatly from system to system. For this reason, a canonical model for encoding is presented below. The process of composing a MIME entity can be modeled as being done in a number of steps. Note that these steps are roughly similar to those steps used in RFC 1421 and are performed for each 'innermost level' body: Step 1. Creation of local form. The body to be transmitted is created in the system's native format. The native character set is used, and where appropriate local end of line conventions are used as well. The body may be a UNIX-style text file, or a Sun raster image, or a VMS indexed file, or audio data in a system-dependent format stored only in memory, or anything else that corresponds to the local model for the representation of some form of information. Fundamentally, the data is created in the "native" form specified by the type/subtype information. Step 2. Conversion to canonical form. The entire body, including "out-of-band" information such as record lengths and possibly file attribute information, is converted to a universal canonical form. The specific content type of the body as well as its associated attributes dictate the nature of the canonical form that is used. Conversion to the proper canonical form may involve character set conversion, transformation of audio data, compression, or various other operations specific to the various content types. If character set conversion is involved, however, care must be taken to understand the semantics of the content-type, which may have strong implications for any character set conversion, e.g. with regard to syntactically meaningful characters in a text subtype other than "plain". For example, in the case of text/plain data, the text must be converted to a supported character set and lines must be delimited with CRLF delimiters in accordance with RFC822. Note that the restriction on line lengths implied by RFC822 is eliminated if the next step employs either quoted-printable or base64 encoding. Borenstein & Freed [Page 76]
RFC 1521 MIME September 1993 Step 3. Apply transfer encoding. A Content-Transfer-Encoding appropriate for this body is applied. Note that there is no fixed relationship between the content type and the transfer encoding. In particular, it may be appropriate to base the choice of base64 or quoted-printable on character frequency counts which are specific to a given instance of a body. Step 4. Insertion into entity. The encoded object is inserted into a MIME entity with appropriate headers. The entity is then inserted into the body of a higher-level entity (message or multipart) if needed. It is vital to note that these steps are only a model; they are specifically NOT a blueprint for how an actual system would be built. In particular, the model fails to account for two common designs: 1. In many cases the conversion to a canonical form prior to encoding will be subsumed into the encoder itself, which understands local formats directly. For example, the local newline convention for text bodies might be carried through to the encoder itself along with knowledge of what that format is. 2. The output of the encoders may have to pass through one or more additional steps prior to being transmitted as a message. As such, the output of the encoder may not be conformant with the formats specified by RFC822. In particular, once again it may be appropriate for the converter's output to be expressed using local newline conventions rather than using the standard RFC822 CRLF delimiters. Other implementation variations are conceivable as well. The vital aspect of this discussion is that, in spite of any optimizations, collapsings of required steps, or insertion of additional processing, the resulting messages must be consistent with those produced by the model described here. For example, a message with the following header fields: Content-type: text/foo; charset=bar Content-Transfer-Encoding: base64 must be first represented in the text/foo form, then (if necessary) represented in the "bar" character set, and finally transformed via the base64 algorithm into a mail-safe form. Borenstein & Freed [Page 77]
RFC 1521 MIME September 1993 Appendix H -- Changes from RFC 1341 This document is a relatively minor revision of RFC 1341. For the convenience of those familiar with RFC 1341, the technical changes from that document are summarized in this appendix. 1. The definition of "tspecials" has been changed to no longer include ".". 2. The Content-ID field is now mandatory for message/external-body parts. 3. The text/richtext type (including the old Section 7.1.3 and Appendix D) has been moved to a separate document. 4. The rules on header merging for message/partial data have been changed to treat the Encrypted and MIME-Version headers as special cases. 5. The definition of the external-body access-type parameter has been changed so that it can only indicate a single access method (which was all that made sense). 6. There is a new "Subject" parameter for message/external-body, access-type mail-server, to permit MIME-based use of mail servers that rely on Subject field information. 7. The "conversions" parameter for application/octet-stream has been removed. 8. Section 7.4.1 now deprecates the use of the "name" parameter for application/octet-stream, as this will be superseded in the future by a Content-Disposition header. 9. The formal grammar for multipart bodies has been changed so that a CRLF is no longer required before the first boundary line. 10. MIME entities of type "message/partial" and "message/external- body" are now required to use only the "7bit" transfer-encoding. (Specifically, "binary" and "8bit" are not permitted.) 11. The "application/oda" content-type has been removed. 12. A note has been added to the end of section 7.2.3, explaining the semantics of Content-ID in a multipart/alternative MIME entity. 13. The formal syntax for the "MIME-Version" field has been tightened, but in a way that is completely compatible with the only Borenstein & Freed [Page 78]
RFC 1521 MIME September 1993 version number defined in RFC 1341. 14. In Section 7.3.1, the definition of message/rfc822 has been relaxed regarding mandatory fields. All other changes from RFC 1341 were editorial changes and do not affect the technical content of MIME. Considerable formal grammar has been added, but this reflects the prose specification that was already in place. Borenstein & Freed [Page 79]
RFC 1521 MIME September 1993 References [US-ASCII] Coded Character Set--7-Bit American Standard Code for Information Interchange, ANSI X3.4-1986. [ATK] Borenstein, Nathaniel S., Multimedia Applications Development with the Andrew Toolkit, Prentice-Hall, 1990. [GIF] Graphics Interchange Format (Version 89a), Compuserve, Inc., Columbus, Ohio, 1990. [ISO-2022] International Standard--Information Processing--ISO 7-bit and 8-bit coded character sets--Code extension techniques, ISO 2022:1986. [ISO-8859] Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990. [ISO-646] International Standard--Information Processing--ISO 7-bit coded character set for information interchange, ISO 646:1983. [MPEG] Video Coding Draft Standard ISO 11172 CD, ISO IEC/TJC1/SC2/WG11 (Motion Picture Experts Group), May, 1991. [PCM] CCITT, Fascicle III.4 - Recommendation G.711, Geneva, 1972, "Pulse Code Modulation (PCM) of Voice Frequencies". [POSTSCRIPT] Adobe Systems, Inc., PostScript Language Reference Manual, Addison-Wesley, 1985. [POSTSCRIPT2] Adobe Systems, Inc., PostScript Language Reference Manual, Addison-Wesley, Second Edition, 1990. [X400] Schicker, Pietro, "Message Handling Systems, X.400", Message Handling Systems and Distributed Applications, E. Stefferud, O-j. Jacobsen, and P. Schicker, eds., North-Holland, 1989, pp. 3-41. [RFC 783] Sollins, K., "TFTP Protocol (revision 2)", RFC 783, MIT, June 1981. [RFC 821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, USC/Information Sciences Institute, August 1982. Borenstein & Freed [Page 80]
RFC 1521 MIME September 1993 [RFC 822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, August 1982. [RFC 934] Rose, M., and E. Stefferud, "Proposed Standard for Message Encapsulation", RFC 934, Delaware and NMA, January 1985. [RFC 959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC 959, USC/Information Sciences Institute, October 1985. [RFC 1049] Sirbu, M., "Content-Type Header Field for Internet Messages", STD 11, RFC 1049, CMU, March 1988. [RFC 1421] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I - Message Encryption and Authentication Procedures", RFC 1421, IAB IRTF PSRG, IETF PEM WG, February 1993. [RFC 1154] Robinson, D. and R. Ullmann, "Encoding Header Field for Internet Messages", RFC 1154, Prime Computer, Inc., April 1990. [RFC 1341] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC 1341, Bellcore, Innosoft, June 1992. [RFC 1342] Moore, K., "Representation of Non-Ascii Text in Internet Message Headers", RFC 1342, University of Tennessee, June 1992. [RFC 1343] Borenstein, N., "A User Agent Configuration Mechanism for Multimedia Mail Format Information", RFC 1343, Bellcore, June 1992 [RFC 1344] Borenstein, N., "Implications of MIME for Internet Mail Gateways", RFC 1344, Bellcore, June 1992. [RFC 1345] Simonsen, K., "Character Mnemonics & Character Sets", RFC 1345, Rationel Almen Planlaegning, June 1992. [RFC 1426] Klensin, J., (WG Chair), Freed, N., (Editor), Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIME transport", RFC 1426, United Nations Universit, Innosoft, Dover Beach Consulting, Inc., Network Management Associates, Inc., The Branch Office, February 1993. [RFC 1522] Moore, K., "Representation of Non-Ascii Text in Internet Message Headers" RFC 1522, University of Tennessee, September 1993.

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