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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.


 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
 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
 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  The "ftp" and "tftp" access-types............... 44  The "anon-ftp" access-type...................... 45  The "local-file" and "afs" access-types......... 45  The "mail-server" access-type................... 45  Examples and Further Explanations............... 46
 7.4.   The Application Content-Type....................... 49
 7.4.1. The Application/Octet-Stream (primary) subtype..... 50
 7.4.2. The Application/PostScript subtype................. 50
 7.4.3. Other Application subtypes......................... 53
 7.5.   The Image Content-Type............................. 53
 7.6.   The Audio Content-Type............................. 54
 7.7.   The Video Content-Type............................. 54
 7.8.   Experimental Content-Type Values................... 54
 8.     Summary............................................ 56
 9.     Security Considerations............................ 56
 10.    Authors' Addresses................................. 57
 11.    Acknowledgements................................... 58
 Appendix A -- Minimal MIME-Conformance.................... 60
 Appendix B -- General Guidelines For Sending Email Data... 63
 Appendix C -- A Complex Multipart Example................. 66
 Appendix D -- Collected Grammar........................... 68

Borenstein & Freed [Page 2] RFC 1521 MIME September 1993

 Appendix E -- IANA Registration Procedures................ 72
 E.1  Registration of New Content-type/subtype Values...... 72
 E.2  Registration of New Access-type Values
      for Message/external-body............................ 73
 Appendix F -- Summary of the Seven Content-types.......... 74
 Appendix G -- Canonical Encoding Model.................... 76
 Appendix H -- Changes from RFC 1341....................... 78
 References................................................ 80

1. Introduction

 Since its publication in 1982, STD 11, RFC 822 [RFC-822] has defined
 the standard format of textual mail messages on the Internet.  Its
 success has been such that the RFC 822 format has been adopted,
 wholly or partially, well beyond the confines of the Internet and the
 Internet SMTP transport defined by STD 10, RFC 821 [RFC-821].  As the
 format has seen wider use, a number of limitations have proven
 increasingly restrictive for the user community.
 RFC 822 was intended to specify a format for text messages.  As such,
 non-text messages, such as multimedia messages that might include
 audio or images, are simply not mentioned.  Even in the case of text,
 however, RFC 822 is inadequate for the needs of mail users whose
 languages require the use of character sets richer than US ASCII
 [US-ASCII]. Since RFC 822 does not specify mechanisms for mail
 containing audio, video, Asian language text, or even text in most
 European languages, additional specifications are needed.
 One of the notable limitations of RFC 821/822 based mail systems is
 the fact that they limit the contents of electronic mail messages to
 relatively short lines of seven-bit ASCII.  This forces users to
 convert any non-textual data that they may wish to send into seven-
 bit bytes representable as printable ASCII characters before invoking
 a local mail UA (User Agent, a program with which human users send
 and receive mail). Examples of such encodings currently used in the
 Internet include pure hexadecimal, uuencode, the 3-in-4 base 64
 scheme specified in RFC 1421, the Andrew Toolkit Representation
 [ATK], and many others.
 The limitations of RFC 822 mail become even more apparent as gateways
 are designed to allow for the exchange of mail messages between RFC
 822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the
 inclusion of non-textual body parts within electronic mail messages.
 The current standards for the mapping of X.400 messages to RFC 822
 messages specify either that X.400 non-textual body parts must be
 converted to (not encoded in) an ASCII format, or that they must be
 discarded, notifying the RFC 822 user that discarding has occurred.
 This is clearly undesirable, as information that a user may wish to

Borenstein & Freed [Page 3] RFC 1521 MIME September 1993

 receive is lost.  Even though a user's UA may not have the capability
 of dealing with the non-textual body part, the user might have some
 mechanism external to the UA that can extract useful information from
 the body part.  Moreover, it does not allow for the fact that the
 message may eventually be gatewayed back into an X.400 message
 handling system (i.e., the X.400 message is "tunneled" through
 Internet mail), where the non-textual information would definitely
 become useful again.
 This document describes several mechanisms that combine to solve most
 of these problems without introducing any serious incompatibilities
 with the existing world of RFC 822 mail.  In particular, it
 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
     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
     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
 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
 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
 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
 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  *(";"
             ; 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
          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
 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
 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
      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

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.
    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
    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
    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
    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
    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

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
 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

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

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-
 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
 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
 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
    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
 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;
 But the following is illegal:
               Content-Type: multipart/mixed;
 (because of the colon) and must instead be represented as
               Content-Type: multipart/mixed;
 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
  1. -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
  1. -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
    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 <>
    To:  Ned Freed <>
    Subject: Sample message
    MIME-Version: 1.0
    Content-type: multipart/mixed; boundary="simple
    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.
  1. -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
 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
 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

Borenstein & Freed [Page 34] RFC 1521 MIME September 1993

 From:  Nathaniel Borenstein <>
 To: Ned Freed <>
 Subject: Formatted text mail
 MIME-Version: 1.0
 Content-Type: multipart/alternative; boundary=boundary42
  1. -boundary42
 Content-Type: text/plain; charset=us-ascii
    ...plain text version of message goes here....
 Content-Type: text/richtext
    .... RFC 1341 richtext version of same message goes here ...
 Content-Type: text/x-whatever
    .... fanciest formatted version of same  message  goes  here
 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.
 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 ----"
  1. —– next message —-
 From: someone-else
 Subject: my opinion
    ...body goes here ...
  1. —– next message —-
 From: someone-else-again
 Subject: my different opinion
    ... another body goes here...
  1. —– 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;
              Content-Type: Message/Partial;
 But part 3 MUST specify the total number of parts:
              Content-Type: Message/Partial;
                   number=3; total=3;
 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
 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
 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
    Subject: Audio mail
    Message-ID: <>
    MIME-Version: 1.0
    Content-type: message/partial;
         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: <>
    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:
    Subject: Audio mail
    MIME-Version: 1.0
    Message-ID: <>
    Content-type: message/partial;
         id=""; 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
    Subject: Audio mail
    Message-ID: <>
    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
 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
 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

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-
    Content-type:  image/gif
    Content-ID: <>
    Content-Transfer-Encoding: binary

Borenstein & Freed [Page 42] RFC 1521 MIME September 1993

 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
    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. 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
 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. 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. 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
    "*", 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. 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. 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
 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: <>
    Content-Type: multipart/alternative; boundary=42
    Content-ID: <>
  1. -42

Content-Type: message/external-body;

         expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
    Content-type: application/postscript
    Content-ID: <>
  1. -42

Content-Type: message/external-body;

         expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
    Content-type: application/postscript
    Content-ID: <>
  1. -42

Content-Type: message/external-body;


Borenstein & Freed [Page 47] RFC 1521 MIME September 1993

         expiration="Fri, 14 Jun 1991 19:13:14 -0400 (EDT)"
    Content-type: application/postscript
    Content-ID: <>
    get RFC-MIME.DOC
  1. -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-
 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
 Such applications may be defined as subtypes of the "application"
 Content-Type.  This document defines two subtypes: octet-stream, and
 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
 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-
 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
 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
 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

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
 Ned Freed
 Innosoft International, Inc.
 250 West First Street
 Suite 240
 Claremont, CA 91711
 Phone:  +1 909 624 7907
 Fax: +1 909 621 5319
 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

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:
  1. - Recognize and display "text" mail

with the character set "US-ASCII."

  1. - Recognize other character sets at

least to the extent of being able

               to inform the user about what
               character set the message uses.
  1. - 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

  1. - 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.
  1. - Recognize and display at least the

primary (822) encapsulation.

  1. - 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.
  1. - Recognize the "alternative" subtype,

and avoid showing the user

               redundant parts of
               multipart/alternative mail.
  1. - Treat any unrecognized subtypes as if

they were "mixed".

  1. - 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
    (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 <>
    To: Ned Freed <>
    Subject: A multipart example
    Content-Type: multipart/mixed;
    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.
       ...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.]
  1. -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.
  1. -unique-boundary-1

Content-Type: multipart/parallel;

  1. -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

  1. -unique-boundary-2

Content-Type: image/gif

    Content-Transfer-Encoding: base64
       ... base64-encoded image data goes here....
  1. -unique-boundary-2–
  1. -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
  1. -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 ...
  1. -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
 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
      ; 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
 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.
    Subject:  Registration of new MIME
    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
    Person & email address to contact for further information:
 E.2  Registration of New Access-type Values
         for Message/external-body
    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
    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
 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
 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
 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
 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
 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


 [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
 [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
 [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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