Tech-invite3GPPspaceIETF RFCsSIP
in Index   Prev   Next

RFC 1521

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

Pages: 81
Obsoletes:  1341
Obsoleted by:  20452046204720482049
Updated by:  1590
Part 1 of 3 – Pages 1 to 23
None   None   Next

ToP   noToC   RFC1521 - Page 1
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.
ToP   noToC   RFC1521 - Page 2
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
ToP   noToC   RFC1521 - Page 3
   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
ToP   noToC   RFC1521 - Page 4
   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.
ToP   noToC   RFC1521 - Page 5
       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].
ToP   noToC   RFC1521 - Page 6
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.
ToP   noToC   RFC1521 - Page 7
      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
ToP   noToC   RFC1521 - Page 8
   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)
ToP   noToC   RFC1521 - Page 9
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.
ToP   noToC   RFC1521 - Page 10
   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
ToP   noToC   RFC1521 - Page 11
   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
ToP   noToC   RFC1521 - Page 12
         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.
ToP   noToC   RFC1521 - Page 13
         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.
ToP   noToC   RFC1521 - Page 14
   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.
ToP   noToC   RFC1521 - Page 15
       "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
ToP   noToC   RFC1521 - Page 16
   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
ToP   noToC   RFC1521 - Page 17
      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.
ToP   noToC   RFC1521 - Page 18
      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
ToP   noToC   RFC1521 - Page 19
      "=" 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
ToP   noToC   RFC1521 - Page 20
          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
ToP   noToC   RFC1521 - Page 21
   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.
ToP   noToC   RFC1521 - Page 22
   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
ToP   noToC   RFC1521 - Page 23
   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

(page 23 continued on part 2)

Next Section