partly

Network Working Group N. Freed Request for Comments: 2045 Innosoft Obsoletes: 1521, 1522, 1590 N. Borenstein Category: Standards Track First Virtual November 1996 Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Abstract STD 11, RFC 822, defines a message representation protocol specifying considerable detail about US-ASCII message headers, and leaves the message content, or message body, as flat US-ASCII text. This set of documents, collectively called the Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages to allow for (1) textual message bodies in character sets other than US-ASCII, (2) an extensible set of different formats for non-textual message bodies, (3) multi-part message bodies, and (4) textual header information in character sets other than US-ASCII. These documents are based on earlier work documented in RFC 934, STD 11, and RFC 1049, but extends and revises them. Because RFC 822 said so little about message bodies, these documents are largely orthogonal to (rather than a revision of) RFC 822. This initial document specifies the various headers used to describe the structure of MIME messages. The second document, RFC 2046, defines the general structure of the MIME media typing system and defines an initial set of media types. The third document, RFC 2047, describes extensions to RFC 822 to allow non-US-ASCII text data in Freed & Borenstein Standards Track [Page 1] RFC 2045 Internet Message Bodies November 1996 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, 1522, and 1590, 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. Definitions, Conventions, and Generic BNF Grammar .... 5 2.1 CRLF ................................................ 5 2.2 Character Set ....................................... 6 2.3 Message ............................................. 6 2.4 Entity .............................................. 6 2.5 Body Part ........................................... 7 2.6 Body ................................................ 7 2.7 7bit Data ........................................... 7 2.8 8bit Data ........................................... 7 2.9 Binary Data ......................................... 7 2.10 Lines .............................................. 7 3. MIME Header Fields ................................... 8 4. MIME-Version Header Field ............................ 8 5. Content-Type Header Field ............................ 10 5.1 Syntax of the Content-Type Header Field ............. 12 5.2 Content-Type Defaults ............................... 14 6. Content-Transfer-Encoding Header Field ............... 14 6.1 Content-Transfer-Encoding Syntax .................... 14 6.2 Content-Transfer-Encodings Semantics ................ 15 6.3 New Content-Transfer-Encodings ...................... 16 6.4 Interpretation and Use .............................. 16 6.5 Translating Encodings ............................... 18 6.6 Canonical Encoding Model ............................ 19 6.7 Quoted-Printable Content-Transfer-Encoding .......... 19 6.8 Base64 Content-Transfer-Encoding .................... 24 7. Content-ID Header Field .............................. 26 8. Content-Description Header Field ..................... 27 9. Additional MIME Header Fields ........................ 27 10. Summary ............................................. 27 11. Security Considerations ............................. 27 12. Authors' Addresses .................................. 28 A. Collected Grammar .................................... 29 Freed & Borenstein Standards Track [Page 2] RFC 2045 Internet Message Bodies November 1996 1. Introduction Since its publication in 1982, RFC 822 has defined the standard format of textual mail messages on the Internet. Its success has been such that the RFC 822 format has been adopted, wholly or partially, well beyond the confines of the Internet and the Internet SMTP transport defined by RFC 821. 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. 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 (e.g. 1000 characters or less [RFC-821]) of 7bit US-ASCII. This forces users to convert any non-textual data that they may wish to send into seven-bit bytes representable as printable US-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 material 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 material must be converted to (not encoded in) IA5Text format, or that they must be discarded, notifying the RFC 822 user that discarding has occurred. This is clearly undesirable, as information that a user may wish to receive is lost. Even though a user agent may not have the capability of dealing with the non-textual material, the user might have some mechanism external to the UA that can extract useful information from the material. 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. Freed & Borenstein Standards Track [Page 3] RFC 2045 Internet Message Bodies November 1996 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 MIME and allows mail processing agents to distinguish between such messages and those generated by older or non-conformant software, which are presumed to lack such a field. (2) A Content-Type header field, generalized from RFC 1049, which can be used to specify the media type and subtype of data in the body of a message and to fully specify the native representation (canonical form) of such data. (3) A Content-Transfer-Encoding header field, which can be used to specify both the encoding transformation that was applied to the body and the domain of the result. Encoding transformations other than the identity transformation are usually applied to 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 body, the Content-ID and Content-Description header fields. All of the header fields defined in this document are subject to the general syntactic rules for header fields specified in RFC 822. In particular, all of these header fields except for Content-Disposition can include RFC 822 comments, which have no semantic content and should be ignored during MIME processing. Finally, to specify and promote interoperability, RFC 2049 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 set of documents 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 set of documents. In particular, compatibility was always favored over elegance. Freed & Borenstein Standards Track [Page 4] RFC 2045 Internet Message Bodies November 1996 Please refer to the current edition of the "Internet Official Protocol Standards" for the standardization state and status of this protocol. RFC 822 and STD 3, RFC 1123 also provide essential background for MIME since no conforming implementation of MIME can violate them. In addition, several other informational RFC documents will be of interest to the MIME implementor, in particular RFC 1344, RFC 1345, and RFC 1524. 2. Definitions, Conventions, and Generic BNF Grammar Although the mechanisms specified in this set of documents are all described in prose, most are also described formally in the augmented BNF notation of RFC 822. Implementors will need to be familiar with this notation in order to understand this set of documents, and are referred to RFC 822 for a complete explanation of the augmented BNF notation. Some of the augmented BNF in this set of documents makes named references to syntax rules defined in RFC 822. A complete formal grammar, then, is obtained by combining the collected grammar appendices in each document in this set with the BNF of RFC 822 plus the modifications to RFC 822 defined in RFC 1123 (which specifically changes the syntax for `return', `date' and `mailbox'). All numeric and octet values are given in decimal notation in this set of documents. All media type values, subtype values, and parameter names as defined are case-insensitive. However, parameter values are case-sensitive unless otherwise specified for the specific parameter. FORMATTING NOTE: Notes, such at this one, provide additional nonessential information which may be skipped by the reader without missing anything essential. The primary purpose of these non- essential notes is to convey information about the rationale of this set of documents, or to place these documents in the proper historical or evolutionary context. Such information may in particular 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 certain design choices were made. 2.1. CRLF The term CRLF, in this set of documents, refers to the sequence of octets corresponding to the two US-ASCII characters CR (decimal value 13) and LF (decimal value 10) which, taken together, in this order, denote a line break in RFC 822 mail. Freed & Borenstein Standards Track [Page 5] RFC 2045 Internet Message Bodies November 1996 2.2. Character Set The term "character set" is used in MIME to refer to a method of converting a sequence of octets into a sequence of characters. Note that unconditional and unambiguous conversion in the other direction is not required, in that not all characters may be representable by a given character set and a character set may provide more than one sequence of octets to represent a particular sequence of characters. This definition is intended to allow various kinds of character encodings, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO 2022's techniques, to be used as character sets. However, the definition associated with a MIME character set name must fully specify the mapping to be performed. In particular, use of external profiling information to determine the exact mapping is not permitted. NOTE: The term "character set" was originally to describe such straightforward schemes as US-ASCII and ISO-8859-1 which have a simple one-to-one mapping from single octets to single characters. Multi-octet coded character sets and switching techniques make the situation more complex. For example, some communities use the term "character encoding" for what MIME calls a "character set", while using the phrase "coded character set" to denote an abstract mapping from integers (not octets) to characters. 2.3. Message The term "message", when not further qualified, means either a (complete or "top-level") RFC 822 message being transferred on a network, or a message encapsulated in a body of type "message/rfc822" or "message/partial". 2.4. Entity The term "entity", refers specifically to the MIME-defined header fields and contents of either a message or one of the parts in the body of a multipart entity. The specification of such entities is the essence of MIME. Since the contents of an entity are often called the "body", it makes sense to speak about the body of an entity. Any sort of field may be present in the header of an entity, but only those fields whose names begin with "content-" actually have any MIME-related meaning. Note that this does NOT imply thay they have no meaning at all -- an entity that is also a message has non- MIME header fields whose meanings are defined by RFC 822. Freed & Borenstein Standards Track [Page 6] RFC 2045 Internet Message Bodies November 1996 2.5. Body Part The term "body part" refers to an entity inside of a multipart entity. 2.6. Body The term "body", when not further qualified, means the body of an entity, that is, the body of either a message or of a body part. NOTE: The previous four definitions are clearly circular. This is unavoidable, since the overall structure of a MIME message is indeed recursive. 2.7. 7bit Data "7bit data" refers to data that is all represented as relatively short lines with 998 octets or less between CRLF line separation sequences [RFC-821]. No octets with decimal values greater than 127 are allowed and neither are NULs (octets with decimal value 0). CR (decimal value 13) and LF (decimal value 10) octets only occur as part of CRLF line separation sequences. 2.8. 8bit Data "8bit data" refers to data that is all represented as relatively short lines with 998 octets or less between CRLF line separation sequences [RFC-821]), but octets with decimal values greater than 127 may be used. As with "7bit data" CR and LF octets only occur as part of CRLF line separation sequences and no NULs are allowed. 2.9. Binary Data "Binary data" refers to data where any sequence of octets whatsoever is allowed. 2.10. Lines "Lines" are defined as sequences of octets separated by a CRLF sequences. This is consistent with both RFC 821 and RFC 822. "Lines" only refers to a unit of data in a message, which may or may not correspond to something that is actually displayed by a user agent. Freed & Borenstein Standards Track [Page 7] RFC 2045 Internet Message Bodies November 1996 3. MIME Header Fields MIME defines a number of new RFC 822 header fields that are used to describe the content of a MIME entity. These header fields occur in at least two contexts: (1) As part of a regular RFC 822 message header. (2) In a MIME body part header within a multipart construct. The formal definition of these header fields is as follows: entity-headers := [ content CRLF ] [ encoding CRLF ] [ id CRLF ] [ description CRLF ] *( MIME-extension-field CRLF ) MIME-message-headers := entity-headers fields version CRLF ; The ordering of the header ; fields implied by this BNF ; definition should be ignored. MIME-part-headers := entity-headers [ fields ] ; Any field not beginning with ; "content-" can have no defined ; meaning and may be ignored. ; The ordering of the header ; fields implied by this BNF ; definition should be ignored. The syntax of the various specific MIME header fields will be described in the following sections. 4. 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 specification 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. Freed & Borenstein Standards Track [Page 8] RFC 2045 Internet Message Bodies November 1996 Therefore, this document defines a new header field, "MIME-Version", which is to be used to declare the version of the Internet message body format standard in use. Messages composed in accordance with this document MUST include such a header field, with the following verbatim text: MIME-Version: 1.0 The presence of this header field is an assertion that the message has been composed in compliance with this document. Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field: 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 document. 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/rfc822" or "message/partial" 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". It is also worth noting that version control for specific media 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 media format. Where such conventions exist, MIME does nothing to supersede them. Where no such conventions exist, a MIME media type might use a "version" parameter in the content-type field if necessary. Freed & Borenstein Standards Track [Page 9] RFC 2045 Internet Message Bodies November 1996 NOTE TO IMPLEMENTORS: When checking MIME-Version values any RFC 822 comment strings that are present must be ignored. In particular, the following four MIME-Version fields are equivalent: MIME-Version: 1.0 MIME-Version: 1.0 (produced by MetaSend Vx.x) MIME-Version: (produced by MetaSend Vx.x) 1.0 MIME-Version: 1.(produced by MetaSend Vx.x)0 In the absence of a MIME-Version field, a receiving mail user agent (whether conforming to MIME requirements or not) may optionally choose to interpret the body of the message according to local conventions. Many such conventions are currently in use and it should be noted that in practice non-MIME messages can contain just about anything. It is impossible to be certain that a non-MIME mail message is actually plain text in the US-ASCII character set since it might well be a message that, using some set of nonstandard local conventions that predate MIME, includes text in another character set or non- textual data presented in a manner that cannot be automatically recognized (e.g., a uuencoded compressed UNIX tar file). 5. 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. The value in this field is called a media type. HISTORICAL NOTE: The Content-Type header field was first defined in RFC 1049. RFC 1049 used a simpler and less powerful syntax, but one that is largely compatible with the mechanism given here. The Content-Type header field specifies the nature of the data in the body of an entity by giving media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. After the media type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute=value notation. The ordering of parameters is not significant. Freed & Borenstein Standards Track [Page 10] RFC 2045 Internet Message Bodies November 1996 In general, the top-level media type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a media 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 text, image, audio, and video should not contain embedded information that is really of a different type. Such compound formats should be represented using the "multipart" or "application" types. Parameters are modifiers of the media subtype, and as such do not fundamentally affect the nature of the content. The set of meaningful parameters depends on the media type and subtype. Most parameters are associated with a single specific subtype. However, a given top-level media type may define parameters which are applicable to any subtype of that type. Parameters may be required by their defining content type or subtype or they may be optional. MIME implementations must ignore any parameters whose names they do not recognize. For example, the "charset" parameter is applicable to any subtype of "text", while the "boundary" parameter is required for any subtype of the "multipart" media type. There are NO globally-meaningful parameters that apply to all media types. Truly global mechanisms are best addressed, in the MIME model, by the definition of additional Content-* header fields. An initial set of seven top-level media types is defined in RFC 2046. Five of these are discrete types whose content is essentially opaque as far as MIME processing is concerned. The remaining two are composite types whose contents require additional handling by MIME processors. This set of top-level media types 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 a standards-track extension to this standard. If another top-level 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. Freed & Borenstein Standards Track [Page 11] RFC 2045 Internet Message Bodies November 1996 5.1. Syntax of the Content-Type Header Field In the Augmented BNF notation of RFC 822, a Content-Type header field value is defined as follows: content := "Content-Type" ":" type "/" subtype *(";" parameter) ; Matching of media type and subtype ; is ALWAYS case-insensitive. type := discrete-type / composite-type discrete-type := "text" / "image" / "audio" / "video" / "application" / extension-token composite-type := "message" / "multipart" / extension-token extension-token := ietf-token / x-token ietf-token := <An extension token defined by a standards-track RFC and registered with IANA.> x-token := <The two characters "X-" or "x-" followed, with no intervening white space, by any token> subtype := extension-token / iana-token iana-token := <A publicly-defined extension token. Tokens of this form must be registered with IANA as specified in RFC 2048.> parameter := attribute "=" value attribute := token ; Matching of attributes ; is ALWAYS case-insensitive. value := token / quoted-string token := 1*<any (US-ASCII) CHAR except SPACE, CTLs, or tspecials> tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> "/" / "[" / "]" / "?" / "=" ; Must be in quoted-string, ; to use within parameter values Freed & Borenstein Standards Track [Page 12] RFC 2045 Internet Message Bodies November 1996 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 -- it may not be omitted from a Content-Type header field. As such, there are no default subtypes. The type, subtype, and parameter names are not case sensitive. For example, TEXT, Text, and TeXt are all equivalent top-level media types. Parameter values are normally case sensitive, but sometimes are interpreted in a case-insensitive fashion, depending on the intended use. (For example, multipart boundaries are case-sensitive, but the "access-type" parameter for message/External-body is not case-sensitive.) Note that the value of a quoted string parameter does not include the quotes. That is, the quotation marks in a quoted-string are not a part of the value of the parameter, but are merely used to delimit that parameter value. In addition, comments are allowed in accordance with RFC 822 rules for structured header fields. Thus the following two forms Content-type: text/plain; charset=us-ascii (Plain text) Content-type: text/plain; charset="us-ascii" are completely equivalent. Beyond this syntax, the only syntactic 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 media subtypes, then, is not intended to be a mechanism for imposing restrictions, but simply a mechanism for publicizing their definition and usage. There are, therefore, two acceptable mechanisms for defining new media subtypes: (1) Private values (starting with "X-") may be defined bilaterally between two cooperating agents without outside registration or standardization. Such values cannot be registered or standardized. (2) New standard values should be registered with IANA as described in RFC 2048. The second document in this set, RFC 2046, defines the initial set of media types for MIME. Freed & Borenstein Standards Track [Page 13] RFC 2045 Internet Message Bodies November 1996 5.2. Content-Type Defaults Default RFC 822 messages without a MIME Content-Type header are taken by this protocol to be plain text in the US-ASCII character set, which can be explicitly specified as: Content-type: text/plain; charset=us-ascii This default is assumed if no Content-Type header field is specified. It is also recommend that this default be assumed when a syntactically invalid Content-Type header field is encountered. In the presence of a MIME-Version header field and the absence of any Content-Type header field, a receiving User Agent can also assume that plain US-ASCII text was the sender's intent. Plain US-ASCII text may still be assumed in the absence of a MIME-Version or the presence of an syntactically invalid Content-Type header field, but the sender's intent might have been otherwise. 6. Content-Transfer-Encoding Header Field Many media types which could be usefully transported via email are represented, in their "natural" format, as 8bit character or binary data. Such data cannot be transmitted over some transfer protocols. For example, RFC 821 (SMTP) restricts mail messages to 7bit US-ASCII data with lines no longer than 1000 characters including any trailing CRLF line separator. It is necessary, therefore, to define a standard mechanism for encoding such data into a 7bit short line format. Proper labelling of unencoded material in less restrictive formats for direct use over less restrictive transports is also desireable. This document specifies that such encodings will be indicated by a new "Content- Transfer-Encoding" header field. This field has not been defined by any previous standard. 6.1. Content-Transfer-Encoding Syntax The Content-Transfer-Encoding field's value is a single token specifying the type of encoding, as enumerated below. Formally: encoding := "Content-Transfer-Encoding" ":" mechanism mechanism := "7bit" / "8bit" / "binary" / "quoted-printable" / "base64" / ietf-token / x-token These values are not case sensitive -- Base64 and BASE64 and bAsE64 are all equivalent. An encoding type of 7BIT requires that the body Freed & Borenstein Standards Track [Page 14] RFC 2045 Internet Message Bodies November 1996 is already in a 7bit 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. 6.2. Content-Transfer-Encodings Semantics This single Content-Transfer-Encoding token actually provides two pieces of information. It specifies what sort of encoding transformation the body was subjected to and hence what decoding operation must be used to restore it to its original form, and it specifies what the domain of the result is. The transformation part of any Content-Transfer-Encodings specifies, either explicitly or implicitly, a single, well-defined decoding algorithm, which for any sequence of encoded octets either transforms it to the original sequence of octets which was encoded, or shows that it is illegal as an encoded sequence. Content-Transfer- Encodings transformations never depend on any additional external profile information for proper operation. Note that while decoders must produce a single, well-defined output for a valid encoding no such restrictions exist for encoders: Encoding a given sequence of octets to different, equivalent encoded sequences is perfectly legal. Three transformations are currently defined: identity, the "quoted- printable" encoding, and the "base64" encoding. The domains are "binary", "8bit" and "7bit". The Content-Transfer-Encoding values "7bit", "8bit", and "binary" all mean that the identity (i.e. NO) encoding transformation has been performed. As such, they serve simply as indicators of the domain of the body data, and provide useful information about the sort of encoding that might be needed for transmission in a given transport system. The terms "7bit data", "8bit data", and "binary data" are all defined in Section 2. The quoted-printable and base64 encodings transform their input from an arbitrary domain into material in the "7bit" range, thus making it safe to carry over restricted transports. The specific definition of the transformations are given below. The proper Content-Transfer-Encoding label must always be used. Labelling unencoded data containing 8bit characters as "7bit" is not allowed, nor is labelling unencoded non-line-oriented data as anything other than "binary" allowed. Unlike media subtypes, a proliferation of Content-Transfer-Encoding values is both undesirable and unnecessary. However, establishing only a single transformation into the "7bit" domain does not seem Freed & Borenstein Standards Track [Page 15] RFC 2045 Internet Message Bodies November 1996 possible. There is a tradeoff between the desire for a compact and efficient encoding of largely- binary data and the desire for a somewhat readable encoding of data that is mostly, but not entirely, 7bit. For this reason, at least two encoding mechanisms are necessary: a more or less readable encoding (quoted-printable) and a "dense" or "uniform" encoding (base64). Mail transport for unencoded 8bit data is defined in RFC 1652. As of the initial 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 valid in Internet mail. However, in the event that binary mail transport becomes a reality in Internet mail, or when MIME is used in conjunction with any other binary-capable mail transport mechanism, binary bodies must be labelled as such using this mechanism. NOTE: The five values defined for the Content-Transfer-Encoding field imply nothing about the media type other than the algorithm by which it was encoded or the transport system requirements if unencoded. 6.3. New Content-Transfer-Encodings Implementors may, if necessary, define private 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". Additional standardized Content- Transfer-Encoding values must be specified by a standards-track RFC. The requirements such specifications must meet are given in RFC 2048. As such, all content-transfer-encoding namespace except that beginning with "X-" is explicitly reserved to the IETF for future use. Unlike media types and subtypes, the creation of new Content- Transfer-Encoding values is STRONGLY discouraged, as it seems likely to hinder interoperability with little potential benefit 6.4. Interpretation and Use 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 an entity's headers, it applies only to the body of that entity. If an entity is of type "multipart" the Content-Transfer-Encoding is not permitted to have any value other than "7bit", "8bit" or "binary". Even more severe restrictions apply to some subtypes of the "message" type. Freed & Borenstein Standards Track [Page 16] RFC 2045 Internet Message Bodies November 1996 It should be noted that most media types are defined in terms of octets rather than bits, 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 8bit 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 8bit byte. A bit stream not ending at an 8bit boundary must be padded with zeroes. RFC 2046 provides a mechanism for noting the addition of such padding in the case of the application/octet-stream media type, which has a "padding" parameter. The encoding mechanisms defined here explicitly encode all data in US-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 US-ASCII encoding of data that was originally in ISO-8859-1, and will be in that character set again after decoding. Certain Content-Transfer-Encoding values may only be used on certain media types. In particular, it is EXPRESSLY FORBIDDEN to use any encodings other than "7bit", "8bit", or "binary" with any composite media type, i.e. one that recursively includes other Content-Type fields. Currently the only composite media types are "multipart" and "message". 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. It should also be noted that, by definition, if a composite entity has a transfer-encoding value such as "7bit", but one of the enclosed entities has a less restrictive value such as "8bit", then either the outer "7bit" labelling is in error, because 8bit data are included, or the inner "8bit" labelling placed an unnecessarily high demand on the transport system because the actual included data were actually 7bit-safe. NOTE ON ENCODING RESTRICTIONS: Though the prohibition against using content-transfer-encodings on composite body data 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 Freed & Borenstein Standards Track [Page 17] RFC 2045 Internet Message Bodies November 1996 to find out what types of bodies 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. Any entity with an unrecognized Content-Transfer-Encoding must be treated as if it has a Content-Type of "application/octet-stream", regardless of what the Content-Type header field actually says. 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 media that is to be encoded, or, at the very least, that certain Content-Transfer-Encodings could be mandated for use with specific media 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 combinations of media types and transports but not for others. (For example, in an 8bit transport, no encoding would be required for text in certain character sets, while such encodings are clearly required for 7bit SMTP.) Second, certain media types may require different types of transfer encoding under different circumstances. For example, many PostScript bodies might consist entirely of short lines of 7bit data and hence require no encoding at all. 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 the Content-Type field is intended to be an open-ended specification mechanism, strict specification of an association between media 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 media type should not have to be aware of all the transports in use and what their limitations are. 6.5. 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 hard line breaks in quoted- printable encoding output. When converting from quoted-printable to base64 a hard line break in the quoted-printable form represents a CRLF sequence in the canonical form of the data. It must therefore be converted to a corresponding encoded CRLF in the base64 form of the data. Similarly, a CRLF sequence in the canonical form of the data obtained after base64 decoding must be converted to a quoted- printable hard line break, but ONLY when converting text data. Freed & Borenstein Standards Track [Page 18] RFC 2045 Internet Message Bodies November 1996 6.6. Canonical Encoding Model There was some confusion, in the previous versions of this RFC, 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. A canonical model for encoding is presented in RFC 2049 for this reason. 6.7. 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 US-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 US-ASCII text, the encoded form of the data remains largely recognizable by humans. A body which is entirely US-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: (1) (General 8bit representation) Any octet, except a CR or LF that is part of a CRLF line break 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; lowercase letters are not allowed. Thus, for example, the decimal value 12 (US-ASCII form feed) can be represented by "=0C", and the decimal value 61 (US- ASCII EQUAL SIGN) can be represented by "=3D". This rule must be followed except when the following rules allow an alternative encoding. (2) (Literal representation) Octets with decimal values of 33 through 60 inclusive, and 62 through 126, inclusive, MAY be represented as the US-ASCII characters which correspond to those octets (EXCLAMATION POINT through LESS THAN, and GREATER THAN through TILDE, respectively). (3) (White Space) Octets with values of 9 and 32 MAY be represented as US-ASCII TAB (HT) and SPACE characters, Freed & Borenstein Standards Track [Page 19] RFC 2045 Internet Message Bodies November 1996 respectively, but MUST NOT be so represented at the end of an encoded line. Any TAB (HT) or SPACE characters on an encoded line MUST thus be followed on that line by a printable character. In particular, an "=" at the end of an encoded line, indicating a soft line break (see rule #5) may follow one or more TAB (HT) or SPACE characters. It follows that an octet with decimal 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 portion 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. (4) (Line Breaks) A line break in a text body, represented as a CRLF sequence in the text canonical form, must be represented by a (RFC 822) line break, which is also a CRLF sequence, in the Quoted-Printable encoding. Since the canonical representation of media types other than text do not generally include the representation of line breaks as CRLF sequences, no hard line breaks (i.e. line breaks that are intended to be meaningful and to be displayed to the user) can occur in the quoted-printable encoding of such types. Sequences like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely appear in non-text data represented in quoted- printable, of course. Note that many implementations may elect to encode the local representation of various content types directly rather than converting to canonical form first, encoding, and then converting back to local representation. In particular, this may apply to plain text material on systems that use newline conventions other than a CRLF terminator sequence. Such