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authorSergey Poznyakoff <gray@gnu.org.ua>2003-03-19 12:51:31 +0000
committerSergey Poznyakoff <gray@gnu.org.ua>2003-03-19 12:51:31 +0000
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+Network Working Group N. Borenstein
+Request for Comments: 1521 Bellcore
+Obsoletes: 1341 N. Freed
+Category: Standards Track Innosoft
+ September 1993
+
+
+ MIME (Multipurpose Internet Mail Extensions) Part One:
+ Mechanisms for Specifying and Describing
+ the Format of Internet Message Bodies
+
+Status of this Memo
+
+ This RFC 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" 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 which
+ specifies considerable detail about message headers, but which leaves
+ the message content, or message body, as flat ASCII text. This
+ document redefines the format of message bodies to allow multi-part
+ textual and non-textual message bodies to be represented and
+ exchanged without loss of information. This is based on earlier work
+ documented in RFC 934 and STD 11, RFC 1049, but extends and revises
+ that work. Because RFC 822 said so little about message bodies, this
+ document is largely orthogonal to (rather than a revision of) RFC
+ 822.
+
+ In particular, this document is designed to provide facilities to
+ include multiple objects in a single message, to represent body text
+ in character sets other than US-ASCII, to represent formatted multi-
+ font text messages, to represent non-textual material such as images
+ and audio fragments, and generally to facilitate later extensions
+ defining new types of Internet mail for use by cooperating mail
+ agents.
+
+ This document does NOT extend Internet mail header fields to permit
+ anything other than US-ASCII text data. Such extensions are the
+ subject of a companion document [RFC-1522].
+
+ This document is a revision of RFC 1341. Significant differences
+ from RFC 1341 are summarized in Appendix H.
+
+
+
+
+
+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
+ 7.3.3.1. The "ftp" and "tftp" access-types............... 44
+ 7.3.3.2. The "anon-ftp" access-type...................... 45
+ 7.3.3.3. The "local-file" and "afs" access-types......... 45
+ 7.3.3.4. The "mail-server" access-type................... 45
+ 7.3.3.5. 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
+
+
+ names for a small number of character sets for which a strong
+ constituent base exists.
+
+ The defined charset values are:
+
+ US-ASCII -- as defined in [US-ASCII].
+
+ ISO-8859-X -- where "X" is to be replaced, as necessary, for the
+ parts of ISO-8859 [ISO-8859]. Note that the ISO 646
+ character sets have deliberately been omitted in favor of
+ their 8859 replacements, which are the designated character
+ sets for Internet mail. As of the publication of this
+ document, the legitimate values for "X" are the digits 1
+ through 9.
+
+ The character sets specified above are the ones that were relatively
+ uncontroversial during the drafting of MIME. This document does not
+ endorse the use of any particular character set other than US-ASCII,
+ and recognizes that the future evolution of world character sets
+ remains unclear. It is expected that in the future, additional
+ character sets will be registered for use in MIME.
+
+ Note that the character set used, if anything other than US-ASCII,
+ must always be explicitly specified in the Content-Type field.
+
+ No other character set name may be used in Internet mail without the
+ publication of a formal specification and its registration with IANA,
+ or by private agreement, in which case the character set name must
+ begin with "X-".
+
+ Implementors are discouraged from defining new character sets for
+ mail use unless absolutely necessary.
+
+ The "charset" parameter has been defined primarily for the purpose of
+ textual data, and is described in this section for that reason.
+ However, it is conceivable that non-textual data might also wish to
+ specify a charset value for some purpose, in which case the same
+ syntax and values should be used.
+
+ In general, mail-sending software must always use the "lowest common
+ denominator" character set possible. For example, if a body contains
+ only US-ASCII characters, it must be marked as being in the US-ASCII
+ character set, not ISO-8859-1, which, like all the ISO-8859 family of
+ character sets, is a superset of US-ASCII. More generally, if a
+ widely-used character set is a subset of another character set, and a
+ body contains only characters in the widely-used subset, it must be
+ labeled as being in that subset. This will increase the chances that
+ the recipient will be able to view the mail correctly.
+
+
+
+Borenstein & Freed [Page 27]
+
+RFC 1521 MIME September 1993
+
+
+7.1.2. The Text/plain subtype
+
+ The primary subtype of text is "plain". This indicates plain
+ (unformatted) text. The default Content-Type for Internet mail,
+ "text/plain; charset=us-ascii", describes existing Internet practice.
+ That is, it is the type of body defined by RFC 822.
+
+ No other text subtype is defined by this document.
+
+ The formal grammar for the content-type header field for text is as
+ follows:
+
+ text-type := "text" "/" text-subtype [";" "charset" "=" charset]
+
+ text-subtype := "plain" / extension-token
+
+ charset := "us-ascii"/ "iso-8859-1"/ "iso-8859-2"/ "iso-8859-3"
+ / "iso-8859-4"/ "iso-8859-5"/ "iso-8859-6"/ "iso-8859-7"
+ / "iso-8859-8" / "iso-8859-9" / extension-token
+ ; case insensitive
+
+7.2. The Multipart Content-Type
+
+ In the case of multiple part entities, in which one or more different
+ sets of data are combined in a single body, a "multipart" Content-
+ Type field must appear in the entity's header. The body must then
+ contain one or more "body parts," each preceded by an encapsulation
+ boundary, and the last one followed by a closing boundary. Each part
+ starts with an encapsulation boundary, and then contains a body part
+ consisting of header area, a blank line, and a body area. Thus a
+ body part is similar to an RFC 822 message in syntax, but different
+ in meaning.
+
+ A body part is NOT to be interpreted as actually being an RFC 822
+ message. To begin with, NO header fields are actually required in
+ body parts. A body part that starts with a blank line, therefore, is
+ allowed and is a body part for which all default values are to be
+ assumed. In such a case, the absence of a Content-Type header field
+ implies that the corresponding body is plain US-ASCII text. The only
+ header fields that have defined meaning for body parts are those the
+ names of which begin with "Content-". All other header fields are
+ generally to be ignored in body parts. Although they should
+ generally be retained in mail processing, they may be discarded by
+ gateways if necessary. Such other fields are permitted to appear in
+ body parts but must not be depended on. "X-" fields may be created
+ for experimental or private purposes, with the recognition that the
+ information they contain may be lost at some gateways.
+
+
+
+
+Borenstein & Freed [Page 28]
+
+RFC 1521 MIME September 1993
+
+
+ NOTE: The distinction between an RFC 822 message and a body part
+ is subtle, but important. A gateway between Internet and X.400
+ mail, for example, must be able to tell the difference between a
+ body part that contains an image and a body part that contains an
+ encapsulated message, the body of which is an image. In order to
+ represent the latter, the body part must have "Content-Type:
+ message", and its body (after the blank line) must be the
+ encapsulated message, with its own "Content-Type: image" header
+ field. The use of similar syntax facilitates the conversion of
+ messages to body parts, and vice versa, but the distinction
+ between the two must be understood by implementors. (For the
+ special case in which all parts actually are messages, a "digest"
+ subtype is also defined.)
+
+ As stated previously, each body part is preceded by an encapsulation
+ boundary. The encapsulation boundary MUST NOT appear inside any of
+ the encapsulated parts. Thus, it is crucial that the composing agent
+ be able to choose and specify the unique boundary that will separate
+ the parts.
+
+ All present and future subtypes of the "multipart" type must use an
+ identical syntax. Subtypes may differ in their semantics, and may
+ impose additional restrictions on syntax, but must conform to the
+ required syntax for the multipart type. This requirement ensures
+ that all conformant user agents will at least be able to recognize
+ and separate the parts of any multipart entity, even of an
+ unrecognized subtype.
+
+ As stated in the definition of the Content-Transfer-Encoding field,
+ no encoding other than "7bit", "8bit", or "binary" is permitted for
+ entities of type "multipart". The multipart delimiters and header
+ fields are always represented as 7-bit ASCII in any case (though the
+ header fields may encode non-ASCII header text as per [RFC-1522]),
+ and data within the body parts can be encoded on a part-by-part
+ basis, with Content-Transfer-Encoding fields for each appropriate
+ body part.
+
+ Mail gateways, relays, and other mail handling agents are commonly
+ known to alter the top-level header of an RFC 822 message. In
+ particular, they frequently add, remove, or reorder header fields.
+ Such alterations are explicitly forbidden for the body part headers
+ embedded in the bodies of messages of type "multipart."
+
+7.2.1. Multipart: The common syntax
+
+ All subtypes of "multipart" share a common syntax, defined in this
+ section. A simple example of a multipart message also appears in
+ this section. An example of a more complex multipart message is
+
+
+
+Borenstein & Freed [Page 29]
+
+RFC 1521 MIME September 1993
+
+
+ given in Appendix C.
+
+ The Content-Type field for multipart entities requires one parameter,
+ "boundary", which is used to specify the encapsulation boundary. The
+ encapsulation boundary is defined as a line consisting entirely of
+ two hyphen characters ("-", decimal code 45) followed by the boundary
+ parameter value from the Content-Type header field.
+
+ NOTE: The hyphens are for rough compatibility with the earlier RFC
+ 934 method of message encapsulation, and for ease of searching for
+ the boundaries in some implementations. However, it should be
+ noted that multipart messages are NOT completely compatible with
+ RFC 934 encapsulations; in particular, they do not obey RFC 934
+ quoting conventions for embedded lines that begin with hyphens.
+ This mechanism was chosen over the RFC 934 mechanism because the
+ latter causes lines to grow with each level of quoting. The
+ combination of this growth with the fact that SMTP implementations
+ sometimes wrap long lines made the RFC 934 mechanism unsuitable
+ for use in the event that deeply-nested multipart structuring is
+ ever desired.
+
+ WARNING TO IMPLEMENTORS: The grammar for parameters on the Content-
+ type field is such that it is often necessary to enclose the
+ boundaries in quotes on the Content-type line. This is not always
+ necessary, but never hurts. Implementors should be sure to study the
+ grammar carefully in order to avoid producing illegal Content-type
+ fields. Thus, a typical multipart Content-Type header field might
+ look like this:
+
+ Content-Type: multipart/mixed;
+ boundary=gc0p4Jq0M2Yt08jU534c0p
+
+ But the following is illegal:
+
+ Content-Type: multipart/mixed;
+ boundary=gc0p4Jq0M:2Yt08jU534c0p
+
+ (because of the colon) and must instead be represented as
+
+ Content-Type: multipart/mixed;
+ boundary="gc0p4Jq0M:2Yt08jU534c0p"
+
+ This indicates that the entity consists of several parts, each itself
+ with a structure that is syntactically identical to an RFC 822
+ message, except that the header area might be completely empty, and
+ that the parts are each preceded by the line
+
+ --gc0p4Jq0M:2Yt08jU534c0p
+
+
+
+Borenstein & Freed [Page 30]
+
+RFC 1521 MIME September 1993
+
+
+ Note that the encapsulation boundary must occur at the beginning of a
+ line, i.e., following a CRLF, and that the initial CRLF is considered
+ to be attached to the encapsulation boundary rather than part of the
+ preceding part. The boundary must be followed immediately either by
+ another CRLF and the header fields for the next part, or by two
+ CRLFs, in which case there are no header fields for the next part
+ (and it is therefore assumed to be of Content-Type text/plain).
+
+ NOTE: The CRLF preceding the encapsulation line is conceptually
+ attached to the boundary so that it is possible to have a part
+ that does not end with a CRLF (line break). Body parts that must
+ be considered to end with line breaks, therefore, must have two
+ CRLFs preceding the encapsulation line, the first of which is part
+ of the preceding body part, and the second of which is part of the
+ encapsulation boundary.
+
+ Encapsulation boundaries must not appear within the encapsulations,
+ and must be no longer than 70 characters, not counting the two
+ leading hyphens.
+
+ The encapsulation boundary following the last body part is a
+ distinguished delimiter that indicates that no further body parts
+ will follow. Such a delimiter is identical to the previous
+ delimiters, with the addition of two more hyphens at the end of the
+ line:
+
+ --gc0p4Jq0M2Yt08jU534c0p--
+
+ There appears to be room for additional information prior to the
+ first encapsulation boundary and following the final boundary. These
+ areas should generally be left blank, and implementations must ignore
+ anything that appears before the first boundary or after the last
+ one.
+
+ NOTE: These "preamble" and "epilogue" areas are generally not used
+ because of the lack of proper typing of these parts and the lack
+ of clear semantics for handling these areas at gateways,
+ particularly X.400 gateways. However, rather than leaving the
+ preamble area blank, many MIME implementations have found this to
+ be a convenient place to insert an explanatory note for recipients
+ who read the message with pre-MIME software, since such notes will
+ be ignored by MIME-compliant software.
+
+ NOTE: Because encapsulation boundaries must not appear in the body
+ parts being encapsulated, a user agent must exercise care to
+ choose a unique boundary. The boundary in the example above could
+ have been the result of an algorithm designed to produce
+ boundaries with a very low probability of already existing in the
+
+
+
+Borenstein & Freed [Page 31]
+
+RFC 1521 MIME September 1993
+
+
+ data to be encapsulated without having to prescan the data.
+ Alternate algorithms might result in more 'readable' boundaries
+ for a recipient with an old user agent, but would require more
+ attention to the possibility that the boundary might appear in the
+ encapsulated part. The simplest boundary possible is something
+ like "---", with a closing boundary of "-----".
+
+ As a very simple example, the following multipart message has two
+ parts, both of them plain text, one of them explicitly typed and one
+ of them implicitly typed:
+
+ From: Nathaniel Borenstein <nsb@bellcore.com>
+ To: Ned Freed <ned@innosoft.com>
+ Subject: Sample message
+ MIME-Version: 1.0
+ Content-type: multipart/mixed; boundary="simple
+ boundary"
+
+ This is the preamble. It is to be ignored, though it
+ is a handy place for mail composers to include an
+ explanatory note to non-MIME conformant readers.
+ --simple boundary
+
+ This is implicitly typed plain ASCII text.
+ It does NOT end with a linebreak.
+ --simple boundary
+ Content-type: text/plain; charset=us-ascii
+
+ This is explicitly typed plain ASCII text.
+ It DOES end with a linebreak.
+
+ --simple boundary--
+ This is the epilogue. It is also to be ignored.
+
+ The use of a Content-Type of multipart in a body part within another
+ multipart entity is explicitly allowed. In such cases, for obvious
+ reasons, care must be taken to ensure that each nested multipart
+ entity must use a different boundary delimiter. See Appendix C for an
+ example of nested multipart entities.
+
+ The use of the multipart Content-Type with only a single body part
+ may be useful in certain contexts, and is explicitly permitted.
+
+ The only mandatory parameter for the multipart Content-Type is the
+ boundary parameter, which consists of 1 to 70 characters from a set
+ of characters known to be very robust through email gateways, and NOT
+ ending with white space. (If a boundary appears to end with white
+ space, the white space must be presumed to have been added by a
+
+
+
+Borenstein & Freed [Page 32]
+
+RFC 1521 MIME September 1993
+
+
+ gateway, and must be deleted.) It is formally specified by the
+ following BNF:
+
+ boundary := 0*69<bchars> bcharsnospace
+
+ bchars := bcharsnospace / " "
+
+ bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" /"_"
+ / "," / "-" / "." / "/" / ":" / "=" / "?"
+
+ Overall, the body of a multipart entity may be specified as
+ follows:
+
+ multipart-body := preamble 1*encapsulation
+ close-delimiter epilogue
+
+ encapsulation := delimiter body-part CRLF
+
+ delimiter := "--" boundary CRLF ; taken from Content-Type field.
+ ; There must be no space
+ ; between "--" and boundary.
+
+ close-delimiter := "--" boundary "--" CRLF ; Again, no space
+ by "--",
+
+ preamble := discard-text ; to be ignored upon receipt.
+
+ epilogue := discard-text ; to be ignored upon receipt.
+
+ discard-text := *(*text CRLF)
+
+ body-part := <"message" as defined in RFC 822,
+ with all header fields optional, and with the
+ specified delimiter not occurring anywhere in
+ the message body, either on a line by itself
+ or as a substring anywhere. Note that the
+ semantics of a part differ from the semantics
+ of a message, as described in the text.>
+
+ NOTE: In certain transport enclaves, RFC 822 restrictions such as
+ the one that limits bodies to printable ASCII characters may not
+ be in force. (That is, the transport domains may resemble
+ standard Internet mail transport as specified in RFC821 and
+ assumed by RFC822, but without certain restrictions.) The
+ relaxation of these restrictions should be construed as locally
+ extending the definition of bodies, for example to include octets
+ outside of the ASCII range, as long as these extensions are
+ supported by the transport and adequately documented in the
+
+
+
+Borenstein & Freed [Page 33]
+
+RFC 1521 MIME September 1993
+
+
+ Content-Transfer-Encoding header field. However, in no event are
+ headers (either message headers or body-part headers) allowed to
+ contain anything other than ASCII characters.
+
+ NOTE: Conspicuously missing from the multipart type is a notion of
+ structured, related body parts. In general, it seems premature to
+ try to standardize interpart structure yet. It is recommended
+ that those wishing to provide a more structured or integrated
+ multipart messaging facility should define a subtype of multipart
+ that is syntactically identical, but that always expects the
+ inclusion of a distinguished part that can be used to specify the
+ structure and integration of the other parts, probably referring
+ to them by their Content-ID field. If this approach is used,
+ other implementations will not recognize the new subtype, but will
+ treat it as the primary subtype (multipart/mixed) and will thus be
+ able to show the user the parts that are recognized.
+
+7.2.2. The Multipart/mixed (primary) subtype
+
+ The primary subtype for multipart, "mixed", is intended for use when
+ the body parts are independent and need to be bundled in a particular
+ order. Any multipart subtypes that an implementation does not
+ recognize must be treated as being of subtype "mixed".
+
+7.2.3. The Multipart/alternative subtype
+
+ The multipart/alternative type is syntactically identical to
+ multipart/mixed, but the semantics are different. In particular,
+ each of the parts is an "alternative" version of the same
+ information.
+
+ Systems should recognize that the content of the various parts are
+ interchangeable. Systems should choose the "best" type based on the
+ local environment and preferences, in some cases even through user
+ interaction. As with multipart/mixed, the order of body parts is
+ significant. In this case, the alternatives appear in an order of
+ increasing faithfulness to the original content. In general, the best
+ choice is the LAST part of a type supported by the recipient system's
+ local environment.
+
+ Multipart/alternative may be used, for example, to send mail in a
+ fancy text format in such a way that it can easily be displayed
+ anywhere:
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 34]
+
+RFC 1521 MIME September 1993
+
+
+ From: Nathaniel Borenstein <nsb@bellcore.com>
+ To: Ned Freed <ned@innosoft.com>
+ Subject: Formatted text mail
+ MIME-Version: 1.0
+ Content-Type: multipart/alternative; boundary=boundary42
+
+ --boundary42
+
+ Content-Type: text/plain; charset=us-ascii
+
+ ...plain text version of message goes here....
+ --boundary42
+ Content-Type: text/richtext
+
+ .... RFC 1341 richtext version of same message goes here ...
+ --boundary42
+ Content-Type: text/x-whatever
+
+ .... fanciest formatted version of same message goes here
+ ...
+ --boundary42--
+
+ In this example, users whose mail system understood the "text/x-
+ whatever" format would see only the fancy version, while other users
+ would see only the richtext or plain text version, depending on the
+ capabilities of their system.
+
+ In general, user agents that compose multipart/alternative entities
+ must place the body parts in increasing order of preference, that is,
+ with the preferred format last. For fancy text, the sending user
+ agent should put the plainest format first and the richest format
+ last. Receiving user agents should pick and display the last format
+ they are capable of displaying. In the case where one of the
+ alternatives is itself of type "multipart" and contains unrecognized
+ sub-parts, the user agent may choose either to show that alternative,
+ an earlier alternative, or both.
+
+ NOTE: From an implementor's perspective, it might seem more
+ sensible to reverse this ordering, and have the plainest
+ alternative last. However, placing the plainest alternative first
+ is the friendliest possible option when multipart/alternative
+ entities are viewed using a non-MIME-conformant mail reader.
+ While this approach does impose some burden on conformant mail
+ readers, interoperability with older mail readers was deemed to be
+ more important in this case.
+
+ It may be the case that some user agents, if they can recognize more
+ than one of the formats, will prefer to offer the user the choice of
+
+
+
+Borenstein & Freed [Page 35]
+
+RFC 1521 MIME September 1993
+
+
+ which format to view. This makes sense, for example, if mail
+ includes both a nicely-formatted image version and an easily-edited
+ text version. What is most critical, however, is that the user not
+ automatically be shown multiple versions of the same data. Either
+ the user should be shown the last recognized version or should be
+ given the choice.
+
+ NOTE ON THE SEMANTICS OF CONTENT-ID IN MULTIPART/ALTERNATIVE: Each
+ part of a multipart/alternative entity represents the same data, but
+ the mappings between the two are not necessarily without information
+ loss. For example, information is lost when translating ODA to
+ PostScript or plain text. It is recommended that each part should
+ have a different Content-ID value in the case where the information
+ content of the two parts is not identical. However, where the
+ information content is identical -- for example, where several parts
+ of type "application/external- body" specify alternate ways to access
+ the identical data -- the same Content-ID field value should be used,
+ to optimize any cacheing mechanisms that might be present on the
+ recipient's end. However, it is recommended that the Content-ID
+ values used by the parts should not be the same Content-ID value that
+ describes the multipart/alternative as a whole, if there is any such
+ Content-ID field. That is, one Content-ID value will refer to the
+ multipart/alternative entity, while one or more other Content-ID
+ values will refer to the parts inside it.
+
+7.2.4. The Multipart/digest subtype
+
+ This document defines a "digest" subtype of the multipart Content-
+ Type. This type is syntactically identical to multipart/mixed, but
+ the semantics are different. In particular, in a digest, the default
+ Content-Type value for a body part is changed from "text/plain" to
+ "message/rfc822". This is done to allow a more readable digest
+ format that is largely compatible (except for the quoting convention)
+ with RFC 934.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 36]
+
+RFC 1521 MIME September 1993
+
+
+ A digest in this format might, then, look something like this:
+
+ From: Moderator-Address
+ To: Recipient-List
+ MIME-Version: 1.0
+ Subject: Internet Digest, volume 42
+ Content-Type: multipart/digest;
+ boundary="---- next message ----"
+
+ ------ next message ----
+
+ From: someone-else
+ Subject: my opinion
+
+ ...body goes here ...
+
+ ------ next message ----
+
+ From: someone-else-again
+ Subject: my different opinion
+
+ ... another body goes here...
+
+ ------ next message ------
+
+7.2.5. The Multipart/parallel subtype
+
+ This document defines a "parallel" subtype of the multipart Content-
+ Type. This type is syntactically identical to multipart/mixed, but
+ the semantics are different. In particular, in a parallel entity,
+ the order of body parts is not significant.
+
+ A common presentation of this type is to display all of the parts
+ simultaneously on hardware and software that are capable of doing so.
+ However, composing agents should be aware that many mail readers will
+ lack this capability and will show the parts serially in any event.
+
+7.2.6. Other Multipart subtypes
+
+ Other multipart subtypes are expected in the future. MIME
+ implementations must in general treat unrecognized subtypes of
+ multipart as being equivalent to "multipart/mixed".
+
+ The formal grammar for content-type header fields for multipart data
+ is given by:
+
+ multipart-type := "multipart" "/" multipart-subtype
+ ";" "boundary" "=" boundary
+
+
+
+Borenstein & Freed [Page 37]
+
+RFC 1521 MIME September 1993
+
+
+ multipart-subtype := "mixed" / "parallel" / "digest"
+ / "alternative" / extension-token
+
+7.3. The Message Content-Type
+
+ It is frequently desirable, in sending mail, to encapsulate another
+ mail message. For this common operation, a special Content-Type,
+ "message", is defined. The primary subtype, message/rfc822, has no
+ required parameters in the Content-Type field. Additional subtypes,
+ "partial" and "External-body", do have required parameters. These
+ subtypes are explained below.
+
+ NOTE: It has been suggested that subtypes of message might be
+ defined for forwarded or rejected messages. However, forwarded
+ and rejected messages can be handled as multipart messages in
+ which the first part contains any control or descriptive
+ information, and a second part, of type message/rfc822, is the
+ forwarded or rejected message. Composing rejection and forwarding
+ messages in this manner will preserve the type information on the
+ original message and allow it to be correctly presented to the
+ recipient, and hence is strongly encouraged.
+
+ As stated in the definition of the Content-Transfer-Encoding field,
+ no encoding other than "7bit", "8bit", or "binary" is permitted for
+ messages or parts of type "message". Even stronger restrictions
+ apply to the subtypes "message/partial" and "message/external-body",
+ as specified below. The message header fields are always US-ASCII in
+ any case, and data within the body can still be encoded, in which
+ case the Content-Transfer-Encoding header field in the encapsulated
+ message will reflect this. Non-ASCII text in the headers of an
+ encapsulated message can be specified using the mechanisms described
+ in [RFC-1522].
+
+ Mail gateways, relays, and other mail handling agents are commonly
+ known to alter the top-level header of an RFC 822 message. In
+ particular, they frequently add, remove, or reorder header fields.
+ Such alterations are explicitly forbidden for the encapsulated
+ headers embedded in the bodies of messages of type "message."
+
+7.3.1. The Message/rfc822 (primary) subtype
+
+ A Content-Type of "message/rfc822" indicates that the body contains
+ an encapsulated message, with the syntax of an RFC 822 message.
+ However, unlike top-level RFC 822 messages, it is not required that
+ each message/rfc822 body must include a "From", "Subject", and at
+ least one destination header.
+
+ It should be noted that, despite the use of the numbers "822", a
+
+
+
+Borenstein & Freed [Page 38]
+
+RFC 1521 MIME September 1993
+
+
+ message/rfc822 entity can include enhanced information as defined in
+ this document. In other words, a message/rfc822 message may be a
+ MIME message.
+
+7.3.2. The Message/Partial subtype
+
+ A subtype of message, "partial", is defined in order to allow large
+ objects to be delivered as several separate pieces of mail and
+ automatically reassembled by the receiving user agent. (The concept
+ is similar to IP fragmentation/reassembly in the basic Internet
+ Protocols.) This mechanism can be used when intermediate transport
+ agents limit the size of individual messages that can be sent.
+ Content-Type "message/partial" thus indicates that the body contains
+ a fragment of a larger message.
+
+ Three parameters must be specified in the Content-Type field of type
+ message/partial: The first, "id", is a unique identifier, as close to
+ a world-unique identifier as possible, to be used to match the parts
+ together. (In general, the identifier is essentially a message-id;
+ if placed in double quotes, it can be any message-id, in accordance
+ with the BNF for "parameter" given earlier in this specification.)
+ The second, "number", an integer, is the part number, which indicates
+ where this part fits into the sequence of fragments. The third,
+ "total", another integer, is the total number of parts. This third
+ subfield is required on the final part, and is optional (though
+ encouraged) on the earlier parts. Note also that these parameters
+ may be given in any order.
+
+ Thus, part 2 of a 3-part message may have either of the following
+ header fields:
+
+ Content-Type: Message/Partial;
+ number=2; total=3;
+ id="oc=jpbe0M2Yt4s@thumper.bellcore.com"
+
+ Content-Type: Message/Partial;
+ id="oc=jpbe0M2Yt4s@thumper.bellcore.com";
+ number=2
+
+ But part 3 MUST specify the total number of parts:
+
+ Content-Type: Message/Partial;
+ number=3; total=3;
+ id="oc=jpbe0M2Yt4s@thumper.bellcore.com"
+
+ Note that part numbering begins with 1, not 0.
+
+ When the parts of a message broken up in this manner are put
+
+
+
+Borenstein & Freed [Page 39]
+
+RFC 1521 MIME September 1993
+
+
+ together, the result is a complete MIME entity, which may have its
+ own Content-Type header field, and thus may contain any other data
+ type.
+
+ Message fragmentation and reassembly: The semantics of a reassembled
+ partial message must be those of the "inner" message, rather than of
+ a message containing the inner message. This makes it possible, for
+ example, to send a large audio message as several partial messages,
+ and still have it appear to the recipient as a simple audio message
+ rather than as an encapsulated message containing an audio message.
+ That is, the encapsulation of the message is considered to be
+ "transparent".
+
+ When generating and reassembling the parts of a message/partial
+ message, the headers of the encapsulated message must be merged with
+ the headers of the enclosing entities. In this process the following
+ rules must be observed:
+
+ (1) All of the header fields from the initial enclosing entity
+ (part one), except those that start with "Content-" and the
+ specific header fields "Message-ID", "Encrypted", and "MIME-
+ Version", must be copied, in order, to the new message.
+
+ (2) Only those header fields in the enclosed message which start
+ with "Content-" and "Message-ID", "Encrypted", and "MIME-Version"
+ must be appended, in order, to the header fields of the new
+ message. Any header fields in the enclosed message which do not
+ start with "Content-" (except for "Message-ID", "Encrypted", and
+ "MIME-Version") will be ignored.
+
+ (3) All of the header fields from the second and any subsequent
+ messages will be ignored.
+
+ For example, if an audio message is broken into two parts, the first
+ part might look something like this:
+
+ X-Weird-Header-1: Foo
+ From: Bill@host.com
+ To: joe@otherhost.com
+ Subject: Audio mail
+ Message-ID: <id1@host.com>
+ MIME-Version: 1.0
+ Content-type: message/partial;
+ id="ABC@host.com";
+ number=1; total=2
+
+ X-Weird-Header-1: Bar
+ X-Weird-Header-2: Hello
+
+
+
+Borenstein & Freed [Page 40]
+
+RFC 1521 MIME September 1993
+
+
+ Message-ID: <anotherid@foo.com>
+ MIME-Version: 1.0
+ Content-type: audio/basic
+ Content-transfer-encoding: base64
+
+ ... first half of encoded audio data goes here...
+
+ and the second half might look something like this:
+
+ From: Bill@host.com
+ To: joe@otherhost.com
+ Subject: Audio mail
+ MIME-Version: 1.0
+ Message-ID: <id2@host.com>
+ Content-type: message/partial;
+ id="ABC@host.com"; number=2; total=2
+
+ ... second half of encoded audio data goes here...
+
+ Then, when the fragmented message is reassembled, the resulting
+ message to be displayed to the user should look something like this:
+
+ X-Weird-Header-1: Foo
+ From: Bill@host.com
+ To: joe@otherhost.com
+ Subject: Audio mail
+ Message-ID: <anotherid@foo.com>
+ MIME-Version: 1.0
+ Content-type: audio/basic
+ Content-transfer-encoding: base64
+
+ ... first half of encoded audio data goes here...
+ ... second half of encoded audio data goes here...
+
+ Note on encoding of MIME entities encapsulated inside message/partial
+ entities: Because data of type "message" may never be encoded in
+ base64 or quoted-printable, a problem might arise if message/partial
+ entities are constructed in an environment that supports binary or
+ 8-bit transport. The problem is that the binary data would be split
+ into multiple message/partial objects, each of them requiring binary
+ transport. If such objects were encountered at a gateway into a 7-
+ bit transport environment, there would be no way to properly encode
+ them for the 7-bit world, aside from waiting for all of the parts,
+ reassembling the message, and then encoding the reassembled data in
+ base64 or quoted-printable. Since it is possible that different
+ parts might go through different gateways, even this is not an
+ acceptable solution. For this reason, it is specified that MIME
+ entities of type message/partial must always have a content-
+
+
+
+Borenstein & Freed [Page 41]
+
+RFC 1521 MIME September 1993
+
+
+ transfer-encoding of 7-bit (the default). In particular, even in
+ environments that support binary or 8-bit transport, the use of a
+ content-transfer-encoding of "8bit" or "binary" is explicitly
+ prohibited for entities of type message/partial.
+
+ It should be noted that, because some message transfer agents may
+ choose to automatically fragment large messages, and because such
+ agents may use different fragmentation thresholds, it is possible
+ that the pieces of a partial message, upon reassembly, may prove
+ themselves to comprise a partial message. This is explicitly
+ permitted.
+
+ It should also be noted that the inclusion of a "References" field in
+ the headers of the second and subsequent pieces of a fragmented
+ message that references the Message-Id on the previous piece may be
+ of benefit to mail readers that understand and track references.
+ However, the generation of such "References" fields is entirely
+ optional.
+
+ Finally, it should be noted that the "Encrypted" header field has
+ been made obsolete by Privacy Enhanced Messaging (PEM), but the rules
+ above are believed to describe the correct way to treat it if it is
+ encountered in the context of conversion to and from message/partial
+ fragments.
+
+7.3.3. The Message/External-Body subtype
+
+ The external-body subtype indicates that the actual body data are not
+ included, but merely referenced. In this case, the parameters
+ describe a mechanism for accessing the external data.
+
+ When an entity is of type "message/external-body", it consists of a
+ header, two consecutive CRLFs, and the message header for the
+ encapsulated message. If another pair of consecutive CRLFs appears,
+ this of course ends the message header for the encapsulated message.
+ However, since the encapsulated message's body is itself external, it
+ does NOT appear in the area that follows. For example, consider the
+ following message:
+
+ Content-type: message/external-body; access-
+ type=local-file;
+
+ name="/u/nsb/Me.gif"
+
+ Content-type: image/gif
+ Content-ID: <id42@guppylake.bellcore.com>
+ Content-Transfer-Encoding: binary
+
+
+
+
+Borenstein & Freed [Page 42]
+
+RFC 1521 MIME September 1993
+
+
+ THIS IS NOT REALLY THE BODY!
+
+ The area at the end, which might be called the "phantom body", is
+ ignored for most external-body messages. However, it may be used to
+ contain auxiliary information for some such messages, as indeed it is
+ when the access-type is "mail-server". Of the access-types defined
+ by this document, the phantom body is used only when the access-type
+ is "mail-server". In all other cases, the phantom body is ignored.
+
+ The only always-mandatory parameter for message/external-body is
+ "access-type"; all of the other parameters may be mandatory or
+ optional depending on the value of access-type.
+
+ ACCESS-TYPE -- A case-insensitive word, indicating the supported
+ access mechanism by which the file or data may be obtained.
+ Values include, but are not limited to, "FTP", "ANON-FTP", "TFTP",
+ "AFS", "LOCAL-FILE", and "MAIL-SERVER". Future values, except for
+ experimental values beginning with "X-" must be registered with
+ IANA, as described in Appendix E .
+
+ In addition, the following three parameters are optional for ALL
+ access-types:
+
+ EXPIRATION -- The date (in the RFC 822 "date-time" syntax, as
+ extended by RFC 1123 to permit 4 digits in the year field) after
+ which the existence of the external data is not guaranteed.
+
+ SIZE -- The size (in octets) of the data. The intent of this
+ parameter is to help the recipient decide whether or not to expend
+ the necessary resources to retrieve the external data. Note that
+ this describes the size of the data in its canonical form, that
+ is, before any Content- Transfer-Encoding has been applied or
+ after the data have been decoded.
+
+ PERMISSION -- A case-insensitive field that indicates whether or
+ not it is expected that clients might also attempt to overwrite
+ the data. By default, or if permission is "read", the assumption
+ is that they are not, and that if the data is retrieved once, it
+ is never needed again. If PERMISSION is "read-write", this
+ assumption is invalid, and any local copy must be considered no
+ more than a cache. "Read" and "Read-write" are the only defined
+ values of permission.
+
+ The precise semantics of the access-types defined here are described
+ in the sections that follow.
+
+ The encapsulated headers in ALL message/external-body entities MUST
+ include a Content-ID header field to give a unique identifier by
+
+
+
+Borenstein & Freed [Page 43]
+
+RFC 1521 MIME September 1993
+
+
+ which to reference the data. This identifier may be used for
+ cacheing mechanisms, and for recognizing the receipt of the data when
+ the access-type is "mail-server".
+
+ Note that, as specified here, the tokens that describe external-body
+ data, such as file names and mail server commands, are required to be
+ in the US-ASCII character set. If this proves problematic in
+ practice, a new mechanism may be required as a future extension to
+ MIME, either as newly defined access-types for message/external-body
+ or by some other mechanism.
+
+ As with message/partial, it is specified that MIME entities of type
+ message/external-body must always have a content-transfer-encoding of
+ 7-bit (the default). In particular, even in environments that
+ support binary or 8-bit transport, the use of a content-transfer-
+ encoding of "8bit" or "binary" is explicitly prohibited for entities
+ of type message/external-body.
+
+7.3.3.1. The "ftp" and "tftp" access-types
+
+ An access-type of FTP or TFTP indicates that the message body is
+ accessible as a file using the FTP [RFC-959] or TFTP [RFC-783]
+ protocols, respectively. For these access-types, the following
+ additional parameters are mandatory:
+
+ NAME -- The name of the file that contains the actual body data.
+
+ SITE -- A machine from which the file may be obtained, using the
+ given protocol. This must be a fully qualified domain name, not a
+ nickname.
+
+ Before any data are retrieved, using FTP, the user will generally
+ need to be asked to provide a login id and a password for the machine
+ named by the site parameter. For security reasons, such an id and
+ password are not specified as content-type parameters, but must be
+ obtained from the user.
+
+ In addition, the following parameters are optional:
+
+ DIRECTORY -- A directory from which the data named by NAME should
+ be retrieved.
+
+ MODE -- A case-insensitive string indicating the mode to be used
+ when retrieving the information. The legal values for access-type
+ "TFTP" are "NETASCII", "OCTET", and "MAIL", as specified by the
+ TFTP protocol [RFC-783]. The legal values for access-type "FTP"
+ are "ASCII", "EBCDIC", "IMAGE", and "LOCALn" where "n" is a
+ decimal integer, typically 8. These correspond to the
+
+
+
+Borenstein & Freed [Page 44]
+
+RFC 1521 MIME September 1993
+
+
+ representation types "A" "E" "I" and "L n" as specified by the FTP
+ protocol [RFC-959]. Note that "BINARY" and "TENEX" are not valid
+ values for MODE, but that "OCTET" or "IMAGE" or "LOCAL8" should be
+ used instead. IF MODE is not specified, the default value is
+ "NETASCII" for TFTP and "ASCII" otherwise.
+
+7.3.3.2. The "anon-ftp" access-type
+
+ The "anon-ftp" access-type is identical to the "ftp" access type,
+ except that the user need not be asked to provide a name and password
+ for the specified site. Instead, the ftp protocol will be used with
+ login "anonymous" and a password that corresponds to the user's email
+ address.
+
+7.3.3.3. The "local-file" and "afs" access-types
+
+ An access-type of "local-file" indicates that the actual body is
+ accessible as a file on the local machine. An access-type of "afs"
+ indicates that the file is accessible via the global AFS file system.
+ In both cases, only a single parameter is required:
+
+ NAME -- The name of the file that contains the actual body data.
+
+ The following optional parameter may be used to describe the locality
+ of reference for the data, that is, the site or sites at which the
+ file is expected to be visible:
+
+ SITE -- A domain specifier for a machine or set of machines that
+ are known to have access to the data file. Asterisks may be used
+ for wildcard matching to a part of a domain name, such as
+ "*.bellcore.com", to indicate a set of machines on which the data
+ should be directly visible, while a single asterisk may be used to
+ indicate a file that is expected to be universally available,
+ e.g., via a global file system.
+
+7.3.3.4. The "mail-server" access-type
+
+ The "mail-server" access-type indicates that the actual body is
+ available from a mail server. The mandatory parameter for this
+ access-type is:
+
+ SERVER -- The email address of the mail server from which the
+ actual body data can be obtained.
+
+ Because mail servers accept a variety of syntaxes, some of which is
+ multiline, the full command to be sent to a mail server is not
+ included as a parameter on the content-type line. Instead, it is
+ provided as the "phantom body" when the content-type is
+
+
+
+Borenstein & Freed [Page 45]
+
+RFC 1521 MIME September 1993
+
+
+ message/external-body and the access- type is mail-server.
+
+ An optional parameter for this access-type is:
+
+ SUBJECT -- The subject that is to be used in the mail that is sent
+ to obtain the data. Note that keying mail servers on Subject lines
+ is NOT recommended, but such mail servers are known to exist.
+
+ Note that MIME does not define a mail server syntax. Rather, it
+ allows the inclusion of arbitrary mail server commands in the phantom
+ body. Implementations must include the phantom body in the body of
+ the message it sends to the mail server address to retrieve the
+ relevant data.
+
+ It is worth noting that, unlike other access-types, mail-server
+ access is asynchronous and will happen at an unpredictable time in
+ the future. For this reason, it is important that there be a
+ mechanism by which the returned data can be matched up with the
+ original message/external-body entity. MIME mailservers must use the
+ same Content-ID field on the returned message that was used in the
+ original message/external-body entity, to facilitate such matching.
+
+7.3.3.5. Examples and Further Explanations
+
+ With the emerging possibility of very wide-area file systems, it
+ becomes very hard to know in advance the set of machines where a file
+ will and will not be accessible directly from the file system.
+ Therefore it may make sense to provide both a file name, to be tried
+ directly, and the name of one or more sites from which the file is
+ known to be accessible. An implementation can try to retrieve remote
+ files using FTP or any other protocol, using anonymous file retrieval
+ or prompting the user for the necessary name and password. If an
+ external body is accessible via multiple mechanisms, the sender may
+ include multiple parts of type message/external-body within an entity
+ of type multipart/alternative.
+
+ However, the external-body mechanism is not intended to be limited to
+ file retrieval, as shown by the mail-server access-type. Beyond
+ this, one can imagine, for example, using a video server for external
+ references to video clips.
+
+ If an entity is of type "message/external-body", then the body of the
+ entity will contain the header fields of the encapsulated message.
+ The body itself is to be found in the external location. This means
+ that if the body of the "message/external-body" message contains two
+ consecutive CRLFs, everything after those pairs is NOT part of the
+ message itself. For most message/external-body messages, this
+ trailing area must simply be ignored. However, it is a convenient
+
+
+
+Borenstein & Freed [Page 46]
+
+RFC 1521 MIME September 1993
+
+
+ place for additional data that cannot be included in the content-type
+ header field. In particular, if the "access-type" value is "mail-
+ server", then the trailing area must contain commands to be sent to
+ the mail server at the address given by the value of the SERVER
+ parameter.
+
+ The embedded message header fields which appear in the body of the
+ message/external-body data must be used to declare the Content-type
+ of the external body if it is anything other than plain ASCII text,
+ since the external body does not have a header section to declare its
+ type. Similarly, any Content-transfer-encoding other than "7bit"
+ must also be declared here. Thus a complete message/external-body
+ message, referring to a document in PostScript format, might look
+ like this:
+
+ From: Whomever
+ To: Someone
+ Subject: whatever
+ MIME-Version: 1.0
+ Message-ID: <id1@host.com>
+ Content-Type: multipart/alternative; boundary=42
+ Content-ID: <id001@guppylake.bellcore.com>
+
+ --42
+ Content-Type: message/external-body;
+ name="BodyFormats.ps";
+ site="thumper.bellcore.com";
+ access-type=ANON-FTP;
+ directory="pub";
+ mode="image";
+ expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
+
+ Content-type: application/postscript
+ Content-ID: <id42@guppylake.bellcore.com>
+
+ --42
+ Content-Type: message/external-body;
+ name="/u/nsb/writing/rfcs/RFC-MIME.ps";
+ site="thumper.bellcore.com";
+ access-type=AFS
+ expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
+
+ Content-type: application/postscript
+ Content-ID: <id42@guppylake.bellcore.com>
+
+ --42
+ Content-Type: message/external-body;
+ access-type=mail-server
+
+
+
+Borenstein & Freed [Page 47]
+
+RFC 1521 MIME September 1993
+
+
+ server="listserv@bogus.bitnet";
+ expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
+
+ Content-type: application/postscript
+ Content-ID: <id42@guppylake.bellcore.com>
+
+ get RFC-MIME.DOC
+
+ --42--
+
+ Note that in the above examples, the default Content-transfer-
+ encoding of "7bit" is assumed for the external postscript data.
+
+ Like the message/partial type, the message/external-body type is
+ intended to be transparent, that is, to convey the data type in the
+ external body rather than to convey a message with a body of that
+ type. Thus the headers on the outer and inner parts must be merged
+ using the same rules as for message/partial. In particular, this
+ means that the Content-type header is overridden, but the From and
+ Subject headers are preserved.
+
+ Note that since the external bodies are not transported as mail, they
+ need not conform to the 7-bit and line length requirements, but might
+ in fact be binary files. Thus a Content-Transfer-Encoding is not
+ generally necessary, though it is permitted.
+
+ Note that the body of a message of type "message/external-body" is
+ governed by the basic syntax for an RFC 822 message. In particular,
+ anything before the first consecutive pair of CRLFs is header
+ information, while anything after it is body information, which is
+ ignored for most access-types.
+
+ The formal grammar for content-type header fields for data of type
+ message is given by:
+
+ message-type := "message" "/" message-subtype
+
+ message-subtype := "rfc822"
+ / "partial" 2#3partial-param
+ / "external-body" 1*external-param
+ / extension-token
+
+ partial-param := (";" "id" "=" value)
+ / (";" "number" "=" 1*DIGIT)
+ / (";" "total" "=" 1*DIGIT)
+ ; id & number required; total required for last part
+
+ external-param := (";" "access-type" "=" atype)
+
+
+
+Borenstein & Freed [Page 48]
+
+RFC 1521 MIME September 1993
+
+
+ / (";" "expiration" "=" date-time)
+ ; Note that date-time is quoted
+ / (";" "size" "=" 1*DIGIT)
+ / (";" "permission" "=" ("read" / "read-write"))
+ ; Permission is case-insensitive
+ / (";" "name" "=" value)
+ / (";" "site" "=" value)
+ / (";" "dir" "=" value)
+ / (";" "mode" "=" value)
+ / (";" "server" "=" value)
+ / (";" "subject" "=" value)
+ ; access-type required;others required based on access-type
+
+ atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
+ / "afs" / "mail-server" / extension-token
+ ; Case-insensitive
+
+7.4. The Application Content-Type
+
+ The "application" Content-Type is to be used for data which do not
+ fit in any of the other categories, and particularly for data to be
+ processed by mail-based uses of application programs. This is
+ information which must be processed by an application before it is
+ viewable or usable to a user. Expected uses for Content-Type
+ application include mail-based file transfer, spreadsheets, data for
+ mail-based scheduling systems, and languages for "active"
+ (computational) email. (The latter, in particular, can pose security
+ problems which must be understood by implementors, and are considered
+ in detail in the discussion of the application/PostScript content-
+ type.)
+
+ For example, a meeting scheduler might define a standard
+ representation for information about proposed meeting dates. An
+ intelligent user agent would use this information to conduct a dialog
+ with the user, and might then send further mail based on that dialog.
+ More generally, there have been several "active" messaging languages
+ developed in which programs in a suitably specialized language are
+ sent through the mail and automatically run in the recipient's
+ environment.
+
+ Such applications may be defined as subtypes of the "application"
+ Content-Type. This document defines two subtypes: octet-stream, and
+ PostScript.
+
+ In general, the subtype of application will often be the name of the
+ application for which the data are intended. This does not mean,
+ however, that any application program name may be used freely as a
+ subtype of application. Such usages (other than subtypes beginning
+
+
+
+Borenstein & Freed [Page 49]
+
+RFC 1521 MIME September 1993
+
+
+ with "x-") must be registered with IANA, as described in Appendix E.
+
+7.4.1. The Application/Octet-Stream (primary) subtype
+
+ The primary subtype of application, "octet-stream", may be used to
+ indicate that a body contains binary data. The set of possible
+ parameters includes, but is not limited to:
+
+ TYPE -- the general type or category of binary data. This is
+ intended as information for the human recipient rather than for
+ any automatic processing.
+
+ PADDING -- the number of bits of padding that were appended to the
+ bit-stream comprising the actual contents to produce the enclosed
+ byte-oriented data. This is useful for enclosing a bit-stream in
+ a body when the total number of bits is not a multiple of the byte
+ size.
+
+ An additional parameter, "conversions", was defined in [RFC-1341] but
+ has been removed.
+
+ RFC 1341 also defined the use of a "NAME" parameter which gave a
+ suggested file name to be used if the data were to be written to a
+ file. This has been deprecated in anticipation of a separate
+ Content-Disposition header field, to be defined in a subsequent RFC.
+
+ The recommended action for an implementation that receives
+ application/octet-stream mail is to simply offer to put the data in a
+ file, with any Content-Transfer-Encoding undone, or perhaps to use it
+ as input to a user-specified process.
+
+ To reduce the danger of transmitting rogue programs through the mail,
+ it is strongly recommended that implementations NOT implement a
+ path-search mechanism whereby an arbitrary program named in the
+ Content-Type parameter (e.g., an "interpreter=" parameter) is found
+ and executed using the mail body as input.
+
+7.4.2. The Application/PostScript subtype
+
+ A Content-Type of "application/postscript" indicates a PostScript
+ program. Currently two variants of the PostScript language are
+ allowed; the original level 1 variant is described in [POSTSCRIPT]
+ and the more recent level 2 variant is described in [POSTSCRIPT2].
+
+ PostScript is a registered trademark of Adobe Systems, Inc. Use of
+ the MIME content-type "application/postscript" implies recognition of
+ that trademark and all the rights it entails.
+
+
+
+
+Borenstein & Freed [Page 50]
+
+RFC 1521 MIME September 1993
+
+
+ The PostScript language definition provides facilities for internal
+ labeling of the specific language features a given program uses. This
+ labeling, called the PostScript document structuring conventions, is
+ very general and provides substantially more information than just
+ the language level.
+
+ The use of document structuring conventions, while not required, is
+ strongly recommended as an aid to interoperability. Documents which
+ lack proper structuring conventions cannot be tested to see whether
+ or not they will work in a given environment. As such, some systems
+ may assume the worst and refuse to process unstructured documents.
+
+ The execution of general-purpose PostScript interpreters entails
+ serious security risks, and implementors are discouraged from simply
+ sending PostScript email bodies to "off-the-shelf" interpreters.
+ While it is usually safe to send PostScript to a printer, where the
+ potential for harm is greatly constrained, implementors should
+ consider all of the following before they add interactive display of
+ PostScript bodies to their mail readers.
+
+ The remainder of this section outlines some, though probably not all,
+ of the possible problems with sending PostScript through the mail.
+
+ Dangerous operations in the PostScript language include, but may not
+ be limited to, the PostScript operators deletefile, renamefile,
+ filenameforall, and file. File is only dangerous when applied to
+ something other than standard input or output. Implementations may
+ also define additional nonstandard file operators; these may also
+ pose a threat to security. Filenameforall, the wildcard file search
+ operator, may appear at first glance to be harmless. Note, however,
+ that this operator has the potential to reveal information about what
+ files the recipient has access to, and this information may itself be
+ sensitive. Message senders should avoid the use of potentially
+ dangerous file operators, since these operators are quite likely to
+ be unavailable in secure PostScript implementations. Message-
+ receiving and -displaying software should either completely disable
+ all potentially dangerous file operators or take special care not to
+ delegate any special authority to their operation. These operators
+ should be viewed as being done by an outside agency when interpreting
+ PostScript documents. Such disabling and/or checking should be done
+ completely outside of the reach of the PostScript language itself;
+ care should be taken to insure that no method exists for re-enabling
+ full-function versions of these operators.
+
+ The PostScript language provides facilities for exiting the normal
+ interpreter, or server, loop. Changes made in this "outer"
+ environment are customarily retained across documents, and may in
+ some cases be retained semipermanently in nonvolatile memory. The
+
+
+
+Borenstein & Freed [Page 51]
+
+RFC 1521 MIME September 1993
+
+
+ operators associated with exiting the interpreter loop have the
+ potential to interfere with subsequent document processing. As such,
+ their unrestrained use constitutes a threat of service denial.
+ PostScript operators that exit the interpreter loop include, but may
+ not be limited to, the exitserver and startjob operators. Message-
+ sending software should not generate PostScript that depends on
+ exiting the interpreter loop to operate. The ability to exit will
+ probably be unavailable in secure PostScript implementations.
+ Message-receiving and -displaying software should, if possible,
+ disable the ability to make retained changes to the PostScript
+ environment, and eliminate the startjob and exitserver commands. If
+ these commands cannot be eliminated, the password associated with
+ them should at least be set to a hard-to-guess value.
+
+ PostScript provides operators for setting system-wide and device-
+ specific parameters. These parameter settings may be retained across
+ jobs and may potentially pose a threat to the correct operation of
+ the interpreter. The PostScript operators that set system and device
+ parameters include, but may not be limited to, the setsystemparams
+ and setdevparams operators. Message-sending software should not
+ generate PostScript that depends on the setting of system or device
+ parameters to operate correctly. The ability to set these parameters
+ will probably be unavailable in secure PostScript implementations.
+ Message-receiving and -displaying software should, if possible,
+ disable the ability to change system and device parameters. If these
+ operators cannot be disabled, the password associated with them
+ should at least be set to a hard-to-guess value.
+
+ Some PostScript implementations provide nonstandard facilities for
+ the direct loading and execution of machine code. Such facilities
+ are quite obviously open to substantial abuse. Message-sending
+ software should not make use of such features. Besides being totally
+ hardware- specific, they are also likely to be unavailable in secure
+ implementations of PostScript. Message-receiving and -displaying
+ software should not allow such operators to be used if they exist.
+
+ PostScript is an extensible language, and many, if not most,
+ implementations of it provide a number of their own extensions. This
+ document does not deal with such extensions explicitly since they
+ constitute an unknown factor. Message-sending software should not
+ make use of nonstandard extensions; they are likely to be missing
+ from some implementations. Message-receiving and -displaying software
+ should make sure that any nonstandard PostScript operators are secure
+ and don't present any kind of threat.
+
+ It is possible to write PostScript that consumes huge amounts of
+ various system resources. It is also possible to write PostScript
+ programs that loop infinitely. Both types of programs have the
+
+
+
+Borenstein & Freed [Page 52]
+
+RFC 1521 MIME September 1993
+
+
+ potential to cause damage if sent to unsuspecting recipients.
+ Message-sending software should avoid the construction and
+ dissemination of such programs, which is antisocial. Message-
+ receiving and -displaying software should provide appropriate
+ mechanisms to abort processing of a document after a reasonable
+ amount of time has elapsed. In addition, PostScript interpreters
+ should be limited to the consumption of only a reasonable amount of
+ any given system resource.
+
+ Finally, bugs may exist in some PostScript interpreters which could
+ possibly be exploited to gain unauthorized access to a recipient's
+ system. Apart from noting this possibility, there is no specific
+ action to take to prevent this, apart from the timely correction of
+ such bugs if any are found.
+
+7.4.3. Other Application subtypes
+
+ It is expected that many other subtypes of application will be
+ defined in the future. MIME implementations must generally treat any
+ unrecognized subtypes as being equivalent to application/octet-
+ stream.
+
+ The formal grammar for content-type header fields for application
+ data is given by:
+
+ application-type := "application" "/" application-subtype
+
+ application-subtype := ("octet-stream" *stream-param)
+ / "postscript" / extension-token
+
+ stream-param := (";" "type" "=" value)
+ / (";" "padding" "=" padding)
+
+ padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"
+
+7.5. The Image Content-Type
+
+ A Content-Type of "image" indicates that the body contains an image.
+ The subtype names the specific image format. These names are case
+ insensitive. Two initial subtypes are "jpeg" for the JPEG format,
+ JFIF encoding, and "gif" for GIF format [GIF].
+
+ The list of image subtypes given here is neither exclusive nor
+ exhaustive, and is expected to grow as more types are registered with
+ IANA, as described in Appendix E.
+
+ The formal grammar for the content-type header field for data of type
+ image is given by:
+
+
+
+Borenstein & Freed [Page 53]
+
+RFC 1521 MIME September 1993
+
+
+ image-type := "image" "/" ("gif" / "jpeg" / extension-token)
+
+7.6. The Audio Content-Type
+
+ A Content-Type of "audio" indicates that the body contains audio
+ data. Although there is not yet a consensus on an "ideal" audio
+ format for use with computers, there is a pressing need for a format
+ capable of providing interoperable behavior.
+
+ The initial subtype of "basic" is specified to meet this requirement
+ by providing an absolutely minimal lowest common denominator audio
+ format. It is expected that richer formats for higher quality and/or
+ lower bandwidth audio will be defined by a later document.
+
+ The content of the "audio/basic" subtype is audio encoded using 8-bit
+ ISDN mu-law [PCM]. When this subtype is present, a sample rate of
+ 8000 Hz and a single channel is assumed.
+
+ The formal grammar for the content-type header field for data of type
+ audio is given by:
+
+ audio-type := "audio" "/" ("basic" / extension-token)
+
+7.7. The Video Content-Type
+
+ A Content-Type of "video" indicates that the body contains a time-
+ varying-picture image, possibly with color and coordinated sound.
+ The term "video" is used extremely generically, rather than with
+ reference to any particular technology or format, and is not meant to
+ preclude subtypes such as animated drawings encoded compactly. The
+ subtype "mpeg" refers to video coded according to the MPEG standard
+ [MPEG].
+
+ Note that although in general this document strongly discourages the
+ mixing of multiple media in a single body, it is recognized that many
+ so-called "video" formats include a representation for synchronized
+ audio, and this is explicitly permitted for subtypes of "video".
+
+ The formal grammar for the content-type header field for data of type
+ video is given by:
+
+ video-type := "video" "/" ("mpeg" / extension-token)
+
+7.8. Experimental Content-Type Values
+
+ A Content-Type value beginning with the characters "X-" is a private
+ value, to be used by consenting mail systems by mutual agreement.
+ Any format without a rigorous and public definition must be named
+
+
+
+Borenstein & Freed [Page 54]
+
+RFC 1521 MIME September 1993
+
+
+ with an "X-" prefix, and publicly specified values shall never begin
+ with "X-". (Older versions of the widely-used Andrew system use the
+ "X-BE2" name, so new systems should probably choose a different
+ name.)
+
+ In general, the use of "X-" top-level types is strongly discouraged.
+ Implementors should invent subtypes of the existing types whenever
+ possible. The invention of new types is intended to be restricted
+ primarily to the development of new media types for email, such as
+ digital odors or holography, and not for new data formats in general.
+ In many cases, a subtype of application will be more appropriate than
+ a new top-level type.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 55]
+
+RFC 1521 MIME September 1993
+
+
+8. Summary
+
+ Using the MIME-Version, Content-Type, and Content-Transfer-Encoding
+ header fields, it is possible to include, in a standardized way,
+ arbitrary types of data objects with RFC 822 conformant mail
+ messages. No restrictions imposed by either RFC 821 or RFC 822 are
+ violated, and care has been taken to avoid problems caused by
+ additional restrictions imposed by the characteristics of some
+ Internet mail transport mechanisms (see Appendix B). The "multipart"
+ and "message" Content-Types allow mixing and hierarchical structuring
+ of objects of different types in a single message. Further Content-
+ Types provide a standardized mechanism for tagging messages or body
+ parts as audio, image, or several other kinds of data. A
+ distinguished parameter syntax allows further specification of data
+ format details, particularly the specification of alternate character
+ sets. Additional optional header fields provide mechanisms for
+ certain extensions deemed desirable by many implementors. Finally, a
+ number of useful Content-Types are defined for general use by
+ consenting user agents, notably message/partial, and
+ message/external-body.
+
+9. Security Considerations
+
+ Security issues are discussed in Section 7.4.2 and in Appendix F.
+ Implementors should pay special attention to the security
+ implications of any mail content-types that can cause the remote
+ execution of any actions in the recipient's environment. In such
+ cases, the discussion of the application/postscript content-type in
+ Section 7.4.2 may serve as a model for considering other content-
+ types with remote execution capabilities.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 56]
+
+RFC 1521 MIME September 1993
+
+
+10. Authors' Addresses
+
+ For more information, the authors of this document may be contacted
+ via Internet mail:
+
+ Nathaniel S. Borenstein
+ MRE 2D-296, Bellcore
+ 445 South St.
+ Morristown, NJ 07962-1910
+
+ Phone: +1 201 829 4270
+ Fax: +1 201 829 7019
+ Email: nsb@bellcore.com
+
+
+ Ned Freed
+ Innosoft International, Inc.
+ 250 West First Street
+ Suite 240
+ Claremont, CA 91711
+
+ Phone: +1 909 624 7907
+ Fax: +1 909 621 5319
+ Email: ned@innosoft.com
+
+ MIME is a result of the work of the Internet Engineering Task Force
+ Working Group on Email Extensions. The chairman of that group, Greg
+ Vaudreuil, may be reached at:
+
+ Gregory M. Vaudreuil
+ Tigon Corporation
+ 17060 Dallas Parkway
+ Dallas Texas, 75248
+
+ Phone: +1 214-733-2722
+ EMail: gvaudre@cnri.reston.va.us
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 57]
+
+RFC 1521 MIME September 1993
+
+
+11. Acknowledgements
+
+ This document is the result of the collective effort of a large
+ number of people, at several IETF meetings, on the IETF-SMTP and
+ IETF-822 mailing lists, and elsewhere. Although any enumeration
+ seems doomed to suffer from egregious omissions, the following are
+ among the many contributors to this effort:
+
+ Harald Tveit Alvestrand Timo Lehtinen
+ Randall Atkinson John R. MacMillan
+ Philippe Brandon Rick McGowan
+ Kevin Carosso Leo Mclaughlin
+ Uhhyung Choi Goli Montaser-Kohsari
+ Cristian Constantinof Keith Moore
+ Mark Crispin Tom Moore
+ Dave Crocker Erik Naggum
+ Terry Crowley Mark Needleman
+ Walt Daniels John Noerenberg
+ Frank Dawson Mats Ohrman
+ Hitoshi Doi Julian Onions
+ Kevin Donnelly Michael Patton
+ Keith Edwards David J. Pepper
+ Chris Eich Blake C. Ramsdell
+ Johnny Eriksson Luc Rooijakkers
+ Craig Everhart Marshall T. Rose
+ Patrik Faeltstroem Jonathan Rosenberg
+ Erik E. Fair Jan Rynning
+ Roger Fajman Harri Salminen
+ Alain Fontaine Michael Sanderson
+ James M. Galvin Masahiro Sekiguchi
+ Philip Gladstone Mark Sherman
+ Thomas Gordon Keld Simonsen
+ Phill Gross Bob Smart
+ James Hamilton Peter Speck
+ Steve Hardcastle-Kille Henry Spencer
+ David Herron Einar Stefferud
+ Bruce Howard Michael Stein
+ Bill Janssen Klaus Steinberger
+ Olle Jaernefors Peter Svanberg
+ Risto Kankkunen James Thompson
+ Phil Karn Steve Uhler
+ Alan Katz Stuart Vance
+ Tim Kehres Erik van der Poel
+ Neil Katin Guido van Rossum
+ Kyuho Kim Peter Vanderbilt
+ Anders Klemets Greg Vaudreuil
+ John Klensin Ed Vielmetti
+ Valdis Kletniek Ryan Waldron
+
+
+
+Borenstein & Freed [Page 58]
+
+RFC 1521 MIME September 1993
+
+
+ Jim Knowles Wally Wedel
+ Stev Knowles Sven-Ove Westberg
+ Bob Kummerfeld Brian Wideen
+ Pekka Kytolaakso John Wobus
+ Stellan Lagerstrom Glenn Wright
+ Vincent Lau Rayan Zachariassen
+ Donald Lindsay David Zimmerman
+ Marc Andreessen Bob Braden
+ Brian Capouch Peter Clitherow
+ Dave Collier-Brown John Coonrod
+ Stephen Crocker Jim Davis
+ Axel Deininger Dana S Emery
+ Martin Forssen Stephen Gildea
+ Terry Gray Mark Horton
+ Warner Losh Carlyn Lowery
+ Laurence Lundblade Charles Lynn
+ Larry Masinter Michael J. McInerny
+ Jon Postel Christer Romson
+ Yutaka Sato Markku Savela
+ Richard Alan Schafer Larry W. Virden
+ Rhys Weatherly Jay Weber
+ Dave Wecker
+
+The authors apologize for any omissions from this list, which are
+certainly unintentional.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 59]
+
+RFC 1521 MIME September 1993
+
+
+Appendix A -- Minimal MIME-Conformance
+
+ The mechanisms described in this document are open-ended. It is
+ definitely not expected that all implementations will support all of
+ the Content-Types described, nor that they will all share the same
+ extensions. In order to promote interoperability, however, it is
+ useful to define the concept of "MIME-conformance" to define a
+ certain level of implementation that allows the useful interworking
+ of messages with content that differs from US ASCII text. In this
+ section, we specify the requirements for such conformance.
+
+ A mail user agent that is MIME-conformant MUST:
+
+ 1. Always generate a "MIME-Version: 1.0" header field.
+
+ 2. Recognize the Content-Transfer-Encoding header field, and
+ decode all received data encoded with either the quoted-printable
+ or base64 implementations. Encode any data sent that is not in
+ seven-bit mail-ready representation using one of these
+ transformations and include the appropriate Content-Transfer-
+ Encoding header field, unless the underlying transport mechanism
+ supports non-seven-bit data, as SMTP does not.
+
+ 3. Recognize and interpret the Content-Type header field, and
+ avoid showing users raw data with a Content-Type field other than
+ text. Be able to send at least text/plain messages, with the
+ character set specified as a parameter if it is not US-ASCII.
+
+ 4. Explicitly handle the following Content-Type values, to at
+ least the following extents:
+
+ Text:
+
+ -- Recognize and display "text" mail
+ with the character set "US-ASCII."
+
+ -- Recognize other character sets at
+ least to the extent of being able
+ to inform the user about what
+ character set the message uses.
+
+ -- Recognize the "ISO-8859-*" character
+ sets to the extent of being able to
+ display those characters that are
+ common to ISO-8859-* and US-ASCII,
+ namely all characters represented
+ by octet values 0-127.
+
+
+
+
+Borenstein & Freed [Page 60]
+
+RFC 1521 MIME September 1993
+
+
+ -- For unrecognized subtypes, show or
+ offer to show the user the "raw"
+ version of the data after
+ conversion of the content from
+ canonical form to local form.
+
+ Message:
+
+ -- Recognize and display at least the
+ primary (822) encapsulation.
+
+ Multipart:
+
+ -- Recognize the primary (mixed)
+ subtype. Display all relevant
+ information on the message level
+ and the body part header level and
+ then display or offer to display
+ each of the body parts individually.
+
+ -- Recognize the "alternative" subtype,
+ and avoid showing the user
+ redundant parts of
+ multipart/alternative mail.
+
+ -- Treat any unrecognized subtypes as if
+ they were "mixed".
+
+ Application:
+
+ -- Offer the ability to remove either of
+ the two types of Content-Transfer-
+ Encoding defined in this document
+ and put the resulting information
+ in a user file.
+
+ 5. Upon encountering any unrecognized Content- Type, an
+ implementation must treat it as if it had a Content-Type of
+ "application/octet-stream" with no parameter sub-arguments. How
+ such data are handled is up to an implementation, but likely
+ options for handling such unrecognized data include offering the
+ user to write it into a file (decoded from its mail transport
+ format) or offering the user to name a program to which the
+ decoded data should be passed as input. Unrecognized predefined
+ types, which in a MIME-conformant mailer might still include
+ audio, image, or video, should also be treated in this way.
+
+ A user agent that meets the above conditions is said to be MIME-
+
+
+
+Borenstein & Freed [Page 61]
+
+RFC 1521 MIME September 1993
+
+
+ conformant. The meaning of this phrase is that it is assumed to be
+ "safe" to send virtually any kind of properly-marked data to users of
+ such mail systems, because such systems will at least be able to
+ treat the data as undifferentiated binary, and will not simply splash
+ it onto the screen of unsuspecting users. There is another sense in
+ which it is always "safe" to send data in a format that is MIME-
+ conformant, which is that such data will not break or be broken by
+ any known systems that are conformant with RFC 821 and RFC 822. User
+ agents that are MIME-conformant have the additional guarantee that
+ the user will not be shown data that were never intended to be viewed
+ as text.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 62]
+
+RFC 1521 MIME September 1993
+
+
+Appendix B -- General Guidelines For Sending Email Data
+
+ Internet email is not a perfect, homogeneous system. Mail may become
+ corrupted at several stages in its travel to a final destination.
+ Specifically, email sent throughout the Internet may travel across
+ many networking technologies. Many networking and mail technologies
+ do not support the full functionality possible in the SMTP transport
+ environment. Mail traversing these systems is likely to be modified
+ in such a way that it can be transported.
+
+ There exist many widely-deployed non-conformant MTAs in the Internet.
+ These MTAs, speaking the SMTP protocol, alter messages on the fly to
+ take advantage of the internal data structure of the hosts they are
+ implemented on, or are just plain broken.
+
+ The following guidelines may be useful to anyone devising a data
+ format (Content-Type) that will survive the widest range of
+ networking technologies and known broken MTAs unscathed. Note that
+ anything encoded in the base64 encoding will satisfy these rules, but
+ that some well-known mechanisms, notably the UNIX uuencode facility,
+ will not. Note also that anything encoded in the Quoted-Printable
+ encoding will survive most gateways intact, but possibly not some
+ gateways to systems that use the EBCDIC character set.
+
+ (1) Under some circumstances the encoding used for data may change
+ as part of normal gateway or user agent operation. In particular,
+ conversion from base64 to quoted-printable and vice versa may be
+ necessary. This may result in the confusion of CRLF sequences with
+ line breaks in text bodies. As such, the persistence of CRLF as
+ something other than a line break must not be relied on.
+
+ (2) Many systems may elect to represent and store text data using
+ local newline conventions. Local newline conventions may not match
+ the RFC822 CRLF convention -- systems are known that use plain CR,
+ plain LF, CRLF, or counted records. The result is that isolated
+ CR and LF characters are not well tolerated in general; they may
+ be lost or converted to delimiters on some systems, and hence must
+ not be relied on.
+
+ (3) TAB (HT) characters may be misinterpreted or may be
+ automatically converted to variable numbers of spaces. This is
+ unavoidable in some environments, notably those not based on the
+ ASCII character set. Such conversion is STRONGLY DISCOURAGED, but
+ it may occur, and mail formats must not rely on the persistence of
+ TAB (HT) characters.
+
+ (4) Lines longer than 76 characters may be wrapped or truncated in
+ some environments. Line wrapping and line truncation are STRONGLY
+
+
+
+Borenstein & Freed [Page 63]
+
+RFC 1521 MIME September 1993
+
+
+ DISCOURAGED, but unavoidable in some cases. Applications which
+ require long lines must somehow differentiate between soft and
+ hard line breaks. (A simple way to do this is to use the quoted-
+ printable encoding.)
+
+ (5) Trailing "white space" characters (SPACE, TAB (HT)) on a line
+ may be discarded by some transport agents, while other transport
+ agents may pad lines with these characters so that all lines in a
+ mail file are of equal length. The persistence of trailing white
+ space, therefore, must not be relied on.
+
+ (6) Many mail domains use variations on the ASCII character set,
+ or use character sets such as EBCDIC which contain most but not
+ all of the US-ASCII characters. The correct translation of
+ characters not in the "invariant" set cannot be depended on across
+ character converting gateways. For example, this situation is a
+ problem when sending uuencoded information across BITNET, an
+ EBCDIC system. Similar problems can occur without crossing a
+ gateway, since many Internet hosts use character sets other than
+ ASCII internally. The definition of Printable Strings in X.400
+ adds further restrictions in certain special cases. In
+ particular, the only characters that are known to be consistent
+ across all gateways are the 73 characters that correspond to the
+ upper and lower case letters A-Z and a-z, the 10 digits 0-9, and
+ the following eleven special characters:
+
+ "'" (ASCII code 39)
+ "(" (ASCII code 40)
+ ")" (ASCII code 41)
+ "+" (ASCII code 43)
+ "," (ASCII code 44)
+ "-" (ASCII code 45)
+ "." (ASCII code 46)
+ "/" (ASCII code 47)
+ ":" (ASCII code 58)
+ "=" (ASCII code 61)
+ "?" (ASCII code 63)
+
+ A maximally portable mail representation, such as the base64
+ encoding, will confine itself to relatively short lines of text in
+ which the only meaningful characters are taken from this set of 73
+ characters.
+
+ (7) Some mail transport agents will corrupt data that includes
+ certain literal strings. In particular, a period (".") alone on a
+ line is known to be corrupted by some (incorrect) SMTP
+ implementations, and a line that starts with the five characters
+ "From " (the fifth character is a SPACE) are commonly corrupted as
+
+
+
+Borenstein & Freed [Page 64]
+
+RFC 1521 MIME September 1993
+
+
+ well. A careful composition agent can prevent these corruptions
+ by encoding the data (e.g., in the quoted-printable encoding,
+ "=46rom " in place of "From " at the start of a line, and "=2E" in
+ place of "." alone on a line.
+
+ Please note that the above list is NOT a list of recommended
+ practices for MTAs. RFC 821 MTAs are prohibited from altering the
+ character of white space or wrapping long lines. These BAD and
+ illegal practices are known to occur on established networks, and
+ implementations should be robust in dealing with the bad effects they
+ can cause.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 65]
+
+RFC 1521 MIME September 1993
+
+
+Appendix C -- A Complex Multipart Example
+
+ What follows is the outline of a complex multipart message. This
+ message has five parts to be displayed serially: two introductory
+ plain text parts, an embedded multipart message, a richtext part, and
+ a closing encapsulated text message in a non-ASCII character set.
+ The embedded multipart message has two parts to be displayed in
+ parallel, a picture and an audio fragment.
+
+ MIME-Version: 1.0
+ From: Nathaniel Borenstein <nsb@bellcore.com>
+ To: Ned Freed <ned@innosoft.com>
+ Subject: A multipart example
+ Content-Type: multipart/mixed;
+ boundary=unique-boundary-1
+
+ This is the preamble area of a multipart message.
+ Mail readers that understand multipart format
+ should ignore this preamble.
+ If you are reading this text, you might want to
+ consider changing to a mail reader that understands
+ how to properly display multipart messages.
+ --unique-boundary-1
+
+ ...Some text appears here...
+ [Note that the preceding blank line means
+ no header fields were given and this is text,
+ with charset US ASCII. It could have been
+ done with explicit typing as in the next part.]
+
+ --unique-boundary-1
+ Content-type: text/plain; charset=US-ASCII
+
+ This could have been part of the previous part,
+ but illustrates explicit versus implicit
+ typing of body parts.
+
+ --unique-boundary-1
+ Content-Type: multipart/parallel;
+ boundary=unique-boundary-2
+
+
+ --unique-boundary-2
+ Content-Type: audio/basic
+ Content-Transfer-Encoding: base64
+
+ ... base64-encoded 8000 Hz single-channel
+ mu-law-format audio data goes here....
+
+
+
+Borenstein & Freed [Page 66]
+
+RFC 1521 MIME September 1993
+
+
+ --unique-boundary-2
+ Content-Type: image/gif
+ Content-Transfer-Encoding: base64
+
+ ... base64-encoded image data goes here....
+
+ --unique-boundary-2--
+
+ --unique-boundary-1
+ Content-type: text/richtext
+
+ This is <bold><italic>richtext.</italic></bold>
+ <smaller>as defined in RFC 1341</smaller>
+ <nl><nl>Isn't it
+ <bigger><bigger>cool?</bigger></bigger>
+
+ --unique-boundary-1
+ Content-Type: message/rfc822
+
+ From: (mailbox in US-ASCII)
+ To: (address in US-ASCII)
+ Subject: (subject in US-ASCII)
+ Content-Type: Text/plain; charset=ISO-8859-1
+ Content-Transfer-Encoding: Quoted-printable
+
+ ... Additional text in ISO-8859-1 goes here ...
+
+ --unique-boundary-1--
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 67]
+
+RFC 1521 MIME September 1993
+
+
+Appendix D -- Collected Grammar
+
+ This appendix contains the complete BNF grammar for all the syntax
+ specified by this document.
+
+ By itself, however, this grammar is incomplete. It refers to several
+ entities that are defined by RFC 822. Rather than reproduce those
+ definitions here, and risk unintentional differences between the two,
+ this document simply refers the reader to RFC 822 for the remaining
+ definitions. Wherever a term is undefined, it refers to the RFC 822
+ definition.
+
+ application-subtype := ("octet-stream" *stream-param)
+ / "postscript" / extension-token
+
+ application-type := "application" "/" application-subtype
+
+ attribute := token ; case-insensitive
+
+ atype := "ftp" / "anon-ftp" / "tftp" / "local-file"
+ / "afs" / "mail-server" / extension-token
+ ; Case-insensitive
+
+ audio-type := "audio" "/" ("basic" / extension-token)
+
+ body-part := <"message" as defined in RFC 822,
+ with all header fields optional, and with the
+ specified delimiter not occurring anywhere in
+ the message body, either on a line by itself
+ or as a substring anywhere.>
+
+ NOTE: In certain transport enclaves, RFC 822 restrictions such as
+ the one that limits bodies to printable ASCII characters may not
+ be in force. (That is, the transport domains may resemble
+ standard Internet mail transport as specified in RFC821 and
+ assumed by RFC822, but without certain restrictions.) The
+ relaxation of these restrictions should be construed as locally
+ extending the definition of bodies, for example to include octets
+ outside of the ASCII range, as long as these extensions are
+ supported by the transport and adequately documented in the
+ Content-Transfer-Encoding header field. However, in no event are
+ headers (either message headers or body-part headers) allowed to
+ contain anything other than ASCII characters.
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 68]
+
+RFC 1521 MIME September 1993
+
+
+ boundary := 0*69<bchars> bcharsnospace
+
+ bchars := bcharsnospace / " "
+
+ bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_"
+ / "," / "-" / "." / "/" / ":" / "=" / "?"
+
+ charset := "us-ascii" / "iso-8859-1" / "iso-8859-2"/ "iso-8859-3"
+ / "iso-8859-4" / "iso-8859-5" / "iso-8859-6" / "iso-8859-7"
+ / "iso-8859-8" / "iso-8859-9" / extension-token
+ ; case insensitive
+
+ close-delimiter := "--" boundary "--" CRLF;Again,no space by "--",
+
+ content := "Content-Type" ":" type "/" subtype *(";" parameter)
+ ; case-insensitive matching of type and subtype
+
+ delimiter := "--" boundary CRLF ;taken from Content-Type field.
+ ; There must be no space
+ ; between "--" and boundary.
+
+ description := "Content-Description" ":" *text
+
+ discard-text := *(*text CRLF)
+
+ encapsulation := delimiter body-part CRLF
+
+ encoding := "Content-Transfer-Encoding" ":" mechanism
+
+ epilogue := discard-text ; to be ignored upon receipt.
+
+ extension-token := x-token / iana-token
+
+ external-param := (";" "access-type" "=" atype)
+ / (";" "expiration" "=" date-time)
+
+ ; Note that date-time is quoted
+ / (";" "size" "=" 1*DIGIT)
+ / (";" "permission" "=" ("read" / "read-write"))
+ ; Permission is case-insensitive
+ / (";" "name" "=" value)
+ / (";" "site" "=" value)
+ / (";" "dir" "=" value)
+ / (";" "mode" "=" value)
+ / (";" "server" "=" value)
+ / (";" "subject" "=" value)
+ ;access-type required; others required based on access-type
+
+
+
+
+Borenstein & Freed [Page 69]
+
+RFC 1521 MIME September 1993
+
+
+ iana-token := <a publicly-defined extension token,
+ registered with IANA, as specified in
+ appendix E>
+
+ id := "Content-ID" ":" msg-id
+
+ image-type := "image" "/" ("gif" / "jpeg" / extension-token)
+
+ mechanism := "7bit" ; case-insensitive
+ / "quoted-printable"
+ / "base64"
+ / "8bit"
+ / "binary"
+ / x-token
+
+ message-subtype := "rfc822"
+ / "partial" 2#3partial-param
+ / "external-body" 1*external-param
+ / extension-token
+
+ message-type := "message" "/" message-subtype
+
+ multipart-body :=preamble 1*encapsulation close-delimiter epilogue
+
+ multipart-subtype := "mixed" / "parallel" / "digest"
+ / "alternative" / extension-token
+
+ multipart-type := "multipart" "/" multipart-subtype
+ ";" "boundary" "=" boundary
+
+ 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".
+
+ padding := "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7"
+
+ parameter := attribute "=" value
+
+ partial-param := (";" "id" "=" value)
+ / (";" "number" "=" 1*DIGIT)
+ / (";" "total" "=" 1*DIGIT)
+ ; id & number required;total required for last part
+
+ preamble := discard-text ; to be ignored upon receipt.
+
+ ptext := octet / <any ASCII character except "=", SPACE, or TAB>
+
+
+
+Borenstein & Freed [Page 70]
+
+RFC 1521 MIME September 1993
+
+
+ ; characters not listed as "mail-safe" in Appendix B
+ ; are also not recommended.
+
+ quoted-printable := ([*(ptext / SPACE / TAB) ptext] ["="] CRLF)
+ ; Maximum line length of 76 characters excluding CRLF
+
+ stream-param := (";" "type" "=" value)
+ / (";" "padding" "=" padding)
+
+ subtype := token ; case-insensitive
+
+ text-subtype := "plain" / extension-token
+
+ text-type := "text" "/" text-subtype [";" "charset" "=" charset]
+
+ token := 1*<any (ASCII) CHAR except SPACE, CTLs, or tspecials>
+
+ tspecials := "(" / ")" / "<" / ">" / "@"
+ / "," / ";" / ":" / "\" / <">
+ / "/" / "[" / "]" / "?" / "="
+ ; Must be in quoted-string,
+ ; to use within parameter values
+
+
+ type := "application" / "audio" ; case-insensitive
+ / "image" / "message"
+ / "multipart" / "text"
+ / "video" / extension-token
+ ; All values case-insensitive
+
+ value := token / quoted-string
+
+ version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
+
+ video-type := "video" "/" ("mpeg" / extension-token)
+
+ x-token := <The two characters "X-" or "x-" followed, with no
+ intervening white space, by any token>
+
+
+
+
+
+
+
+
+
+
+
+
+
+Borenstein & Freed [Page 71]
+
+RFC 1521 MIME September 1993
+
+
+Appendix E -- IANA Registration Procedures
+
+ MIME has been carefully designed to have extensible mechanisms, and
+ it is expected that the set of content-type/subtype pairs and their
+ associated parameters will grow significantly with time. Several
+ other MIME fields, notably character set names, access-type
+ parameters for the message/external-body type, and possibly even
+ Content-Transfer-Encoding values, are likely to have new values
+ defined over time. In order to ensure that the set of such values is
+ developed in an orderly, well-specified, and public manner, MIME
+ defines a registration process which uses the Internet Assigned
+ Numbers Authority (IANA) as a central registry for such values.
+
+ In general, parameters in the content-type header field are used to
+ convey supplemental information for various content types, and their
+ use is defined when the content-type and subtype are defined. New
+ parameters should not be defined as a way to introduce new
+ functionality.
+
+ In order to simplify and standardize the registration process, this
+ appendix gives templates for the registration of new values with
+ IANA. Each of these is given in the form of an email message
+ template, to be filled in by the registering party.
+
+ E.1 Registration of New Content-type/subtype Values
+
+ Note that MIME is generally expected to be extended by subtypes. If
+ a new fundamental top-level type is needed, its specification must be
+ published as an RFC or submitted in a form suitable to become an RFC,
+ and be subject to the Internet standards process.
+
+ To: IANA@isi.edu
+ Subject: Registration of new MIME
+ content-type/subtype
+
+ MIME type name:
+
+ (If the above is not an existing top-level MIME type,
+ please explain why an existing type cannot be used.)
+
+ MIME subtype name:
+
+ Required parameters:
+
+ Optional parameters:
+
+ Encoding considerations:
+
+
+
+
+Borenstein & Freed [Page 72]
+
+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.
+
+ [RFC-1340] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
+ 1340, USC/Information Sciences Institute, July 1992.
+
+
+
+
+Borenstein & Freed [Page 81]
+

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