dmc

mime-p1-rfc2045.txt (72932B)

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 Network Working Group                                          N. Freed
 Request for Comments: 2045                                     Innosoft
 Obsoletes: 1521, 1522, 1590                               N. Borenstein
 Category: Standards Track                                 First Virtual
                                                           November 1996
 
 
                  Multipurpose Internet Mail Extensions
                             (MIME) Part One:
                    Format of Internet Message Bodies
 
 Status of this Memo
 
    This document specifies an Internet standards track protocol for the
    Internet community, and requests discussion and suggestions for
    improvements.  Please refer to the current edition of the "Internet
    Official Protocol Standards" (STD 1) for the standardization state
    and status of this protocol.  Distribution of this memo is unlimited.
 
 Abstract
 
    STD 11, RFC 822, defines a message representation protocol specifying
    considerable detail about US-ASCII message headers, and leaves the
    message content, or message body, as flat US-ASCII text.  This set of
    documents, collectively called the Multipurpose Internet Mail
    Extensions, or MIME, redefines the format of messages to allow for
 
     (1)   textual message bodies in character sets other than
           US-ASCII,
 
     (2)   an extensible set of different formats for non-textual
           message bodies,
 
     (3)   multi-part message bodies, and
 
     (4)   textual header information in character sets other than
           US-ASCII.
 
    These documents are based on earlier work documented in RFC 934, STD
    11, and RFC 1049, but extends and revises them.  Because RFC 822 said
    so little about message bodies, these documents are largely
    orthogonal to (rather than a revision of) RFC 822.
 
    This initial document specifies the various headers used to describe
    the structure of MIME messages. The second document, RFC 2046,
    defines the general structure of the MIME media typing system and
    defines an initial set of media types. The third document, RFC 2047,
    describes extensions to RFC 822 to allow non-US-ASCII text data in
 
 
 
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    Internet mail header fields. The fourth document, RFC 2048, specifies
    various IANA registration procedures for MIME-related facilities. The
    fifth and final document, RFC 2049, describes MIME conformance
    criteria as well as providing some illustrative examples of MIME
    message formats, acknowledgements, and the bibliography.
 
    These documents are revisions of RFCs 1521, 1522, and 1590, which
    themselves were revisions of RFCs 1341 and 1342.  An appendix in RFC
    2049 describes differences and changes from previous versions.
 
 Table of Contents
 
    1. Introduction .........................................    3
    2. Definitions, Conventions, and Generic BNF Grammar ....    5
    2.1 CRLF ................................................    5
    2.2 Character Set .......................................    6
    2.3 Message .............................................    6
    2.4 Entity ..............................................    6
    2.5 Body Part ...........................................    7
    2.6 Body ................................................    7
    2.7 7bit Data ...........................................    7
    2.8 8bit Data ...........................................    7
    2.9 Binary Data .........................................    7
    2.10 Lines ..............................................    7
    3. MIME Header Fields ...................................    8
    4. MIME-Version Header Field ............................    8
    5. Content-Type Header Field ............................   10
    5.1 Syntax of the Content-Type Header Field .............   12
    5.2 Content-Type Defaults ...............................   14
    6. Content-Transfer-Encoding Header Field ...............   14
    6.1 Content-Transfer-Encoding Syntax ....................   14
    6.2 Content-Transfer-Encodings Semantics ................   15
    6.3 New Content-Transfer-Encodings ......................   16
    6.4 Interpretation and Use ..............................   16
    6.5 Translating Encodings ...............................   18
    6.6 Canonical Encoding Model ............................   19
    6.7 Quoted-Printable Content-Transfer-Encoding ..........   19
    6.8 Base64 Content-Transfer-Encoding ....................   24
    7. Content-ID Header Field ..............................   26
    8. Content-Description Header Field .....................   27
    9. Additional MIME Header Fields ........................   27
    10. Summary .............................................   27
    11. Security Considerations .............................   27
    12. Authors' Addresses ..................................   28
    A. Collected Grammar ....................................   29
 
 
 
 
 
 
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 1.  Introduction
 
    Since its publication in 1982, RFC 822 has defined the standard
    format of textual mail messages on the Internet.  Its success has
    been such that the RFC 822 format has been adopted, wholly or
    partially, well beyond the confines of the Internet and the Internet
    SMTP transport defined by RFC 821.  As the format has seen wider use,
    a number of limitations have proven increasingly restrictive for the
    user community.
 
    RFC 822 was intended to specify a format for text messages.  As such,
    non-text messages, such as multimedia messages that might include
    audio or images, are simply not mentioned.  Even in the case of text,
    however, RFC 822 is inadequate for the needs of mail users whose
    languages require the use of character sets richer than US-ASCII.
    Since RFC 822 does not specify mechanisms for mail containing audio,
    video, Asian language text, or even text in most European languages,
    additional specifications are needed.
 
    One of the notable limitations of RFC 821/822 based mail systems is
    the fact that they limit the contents of electronic mail messages to
    relatively short lines (e.g. 1000 characters or less [RFC-821]) of
    7bit US-ASCII.  This forces users to convert any non-textual data
    that they may wish to send into seven-bit bytes representable as
    printable US-ASCII characters before invoking a local mail UA (User
    Agent, a program with which human users send and receive mail).
    Examples of such encodings currently used in the Internet include
    pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in
    RFC 1421, the Andrew Toolkit Representation [ATK], and many others.
 
    The limitations of RFC 822 mail become even more apparent as gateways
    are designed to allow for the exchange of mail messages between RFC
    822 hosts and X.400 hosts.  X.400 [X400] specifies mechanisms for the
    inclusion of non-textual material within electronic mail messages.
    The current standards for the mapping of X.400 messages to RFC 822
    messages specify either that X.400 non-textual material must be
    converted to (not encoded in) IA5Text format, or that they must be
    discarded, notifying the RFC 822 user that discarding has occurred.
    This is clearly undesirable, as information that a user may wish to
    receive is lost.  Even though a user agent may not have the
    capability of dealing with the non-textual material, the user might
    have some mechanism external to the UA that can extract useful
    information from the material.  Moreover, it does not allow for the
    fact that the message may eventually be gatewayed back into an X.400
    message handling system (i.e., the X.400 message is "tunneled"
    through Internet mail), where the non-textual information would
    definitely become useful again.
 
 
 
 
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    This document describes several mechanisms that combine to solve most
    of these problems without introducing any serious incompatibilities
    with the existing world of RFC 822 mail.  In particular, it
    describes:
 
     (1)   A MIME-Version header field, which uses a version
           number to declare a message to be conformant with MIME
           and allows mail processing agents to distinguish
           between such messages and those generated by older or
           non-conformant software, which are presumed to lack
           such a field.
 
     (2)   A Content-Type header field, generalized from RFC 1049,
           which can be used to specify the media type and subtype
           of data in the body of a message and to fully specify
           the native representation (canonical form) of such
           data.
 
     (3)   A Content-Transfer-Encoding header field, which can be
           used to specify both the encoding transformation that
           was applied to the body and the domain of the result.
           Encoding transformations other than the identity
           transformation are usually applied to data in order to
           allow it to pass through mail transport mechanisms
           which may have data or character set limitations.
 
     (4)   Two additional header fields that can be used to
           further describe the data in a body, the Content-ID and
           Content-Description header fields.
 
    All of the header fields defined in this document are subject to the
    general syntactic rules for header fields specified in RFC 822.  In
    particular, all of these header fields except for Content-Disposition
    can include RFC 822 comments, which have no semantic content and
    should be ignored during MIME processing.
 
    Finally, to specify and promote interoperability, RFC 2049 provides a
    basic applicability statement for a subset of the above mechanisms
    that defines a minimal level of "conformance" with this document.
 
    HISTORICAL NOTE:  Several of the mechanisms described in this set of
    documents may seem somewhat strange or even baroque at first reading.
    It is important to note that compatibility with existing standards
    AND robustness across existing practice were two of the highest
    priorities of the working group that developed this set of documents.
    In particular, compatibility was always favored over elegance.
 
 
 
 
 
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    Please refer to the current edition of the "Internet Official
    Protocol Standards" for the standardization state and status of this
    protocol.  RFC 822 and STD 3, RFC 1123 also provide essential
    background for MIME since no conforming implementation of MIME can
    violate them.  In addition, several other informational RFC documents
    will be of interest to the MIME implementor, in particular RFC 1344,
    RFC 1345, and RFC 1524.
 
 2.  Definitions, Conventions, and Generic BNF Grammar
 
    Although the mechanisms specified in this set of documents are all
    described in prose, most are also described formally in the augmented
    BNF notation of RFC 822. Implementors will need to be familiar with
    this notation in order to understand this set of documents, and are
    referred to RFC 822 for a complete explanation of the augmented BNF
    notation.
 
    Some of the augmented BNF in this set of documents makes named
    references to syntax rules defined in RFC 822.  A complete formal
    grammar, then, is obtained by combining the collected grammar
    appendices in each document in this set with the BNF of RFC 822 plus
    the modifications to RFC 822 defined in RFC 1123 (which specifically
    changes the syntax for `return', `date' and `mailbox').
 
    All numeric and octet values are given in decimal notation in this
    set of documents. All media type values, subtype values, and
    parameter names as defined are case-insensitive.  However, parameter
    values are case-sensitive unless otherwise specified for the specific
    parameter.
 
    FORMATTING NOTE:  Notes, such at this one, provide additional
    nonessential information which may be skipped by the reader without
    missing anything essential.  The primary purpose of these non-
    essential notes is to convey information about the rationale of this
    set of documents, or to place these documents in the proper
    historical or evolutionary context.  Such information may in
    particular be skipped by those who are focused entirely on building a
    conformant implementation, but may be of use to those who wish to
    understand why certain design choices were made.
 
 2.1.  CRLF
 
    The term CRLF, in this set of documents, refers to the sequence of
    octets corresponding to the two US-ASCII characters CR (decimal value
    13) and LF (decimal value 10) which, taken together, in this order,
    denote a line break in RFC 822 mail.
 
 
 
 
 
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 2.2.  Character Set
 
    The term "character set" is used in MIME to refer to a method of
    converting a sequence of octets into a sequence of characters.  Note
    that unconditional and unambiguous conversion in the other direction
    is not required, in that not all characters may be representable by a
    given character set and a character set may provide more than one
    sequence of octets to represent a particular sequence of characters.
 
    This definition is intended to allow various kinds of character
    encodings, from simple single-table mappings such as US-ASCII to
    complex table switching methods such as those that use ISO 2022's
    techniques, to be used as character sets.  However, the definition
    associated with a MIME character set name must fully specify the
    mapping to be performed.  In particular, use of external profiling
    information to determine the exact mapping is not permitted.
 
    NOTE: The term "character set" was originally to describe such
    straightforward schemes as US-ASCII and ISO-8859-1 which have a
    simple one-to-one mapping from single octets to single characters.
    Multi-octet coded character sets and switching techniques make the
    situation more complex. For example, some communities use the term
    "character encoding" for what MIME calls a "character set", while
    using the phrase "coded character set" to denote an abstract mapping
    from integers (not octets) to characters.
 
 2.3.  Message
 
    The term "message", when not further qualified, means either a
    (complete or "top-level") RFC 822 message being transferred on a
    network, or a message encapsulated in a body of type "message/rfc822"
    or "message/partial".
 
 2.4.  Entity
 
    The term "entity", refers specifically to the MIME-defined header
    fields and contents of either a message or one of the parts in the
    body of a multipart entity.  The specification of such entities is
    the essence of MIME.  Since the contents of an entity are often
    called the "body", it makes sense to speak about the body of an
    entity.  Any sort of field may be present in the header of an entity,
    but only those fields whose names begin with "content-" actually have
    any MIME-related meaning.  Note that this does NOT imply thay they
    have no meaning at all -- an entity that is also a message has non-
    MIME header fields whose meanings are defined by RFC 822.
 
 
 
 
 
 
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 2.5.  Body Part
 
    The term "body part" refers to an entity inside of a multipart
    entity.
 
 2.6.  Body
 
    The term "body", when not further qualified, means the body of an
    entity, that is, the body of either a message or of a body part.
 
    NOTE:  The previous four definitions are clearly circular.  This is
    unavoidable, since the overall structure of a MIME message is indeed
    recursive.
 
 2.7.  7bit Data
 
    "7bit data" refers to data that is all represented as relatively
    short lines with 998 octets or less between CRLF line separation
    sequences [RFC-821].  No octets with decimal values greater than 127
    are allowed and neither are NULs (octets with decimal value 0).  CR
    (decimal value 13) and LF (decimal value 10) octets only occur as
    part of CRLF line separation sequences.
 
 2.8.  8bit Data
 
    "8bit data" refers to data that is all represented as relatively
    short lines with 998 octets or less between CRLF line separation
    sequences [RFC-821]), but octets with decimal values greater than 127
    may be used.  As with "7bit data" CR and LF octets only occur as part
    of CRLF line separation sequences and no NULs are allowed.
 
 2.9.  Binary Data
 
    "Binary data" refers to data where any sequence of octets whatsoever
    is allowed.
 
 2.10.  Lines
 
    "Lines" are defined as sequences of octets separated by a CRLF
    sequences.  This is consistent with both RFC 821 and RFC 822.
    "Lines" only refers to a unit of data in a message, which may or may
    not correspond to something that is actually displayed by a user
    agent.
 
 
 
 
 
 
 
 
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 3.  MIME Header Fields
 
    MIME defines a number of new RFC 822 header fields that are used to
    describe the content of a MIME entity.  These header fields occur in
    at least two contexts:
 
     (1)   As part of a regular RFC 822 message header.
 
     (2)   In a MIME body part header within a multipart
           construct.
 
    The formal definition of these header fields is as follows:
 
      entity-headers := [ content CRLF ]
                        [ encoding CRLF ]
                        [ id CRLF ]
                        [ description CRLF ]
                        *( MIME-extension-field CRLF )
 
      MIME-message-headers := entity-headers
                              fields
                              version CRLF
                              ; The ordering of the header
                              ; fields implied by this BNF
                              ; definition should be ignored.
 
      MIME-part-headers := entity-headers
                           [ fields ]
                           ; Any field not beginning with
                           ; "content-" can have no defined
                           ; meaning and may be ignored.
                           ; The ordering of the header
                           ; fields implied by this BNF
                           ; definition should be ignored.
 
    The syntax of the various specific MIME header fields will be
    described in the following sections.
 
 4.  MIME-Version Header Field
 
    Since RFC 822 was published in 1982, there has really been only one
    format standard for Internet messages, and there has been little
    perceived need to declare the format standard in use.  This document
    is an independent specification that complements RFC 822.  Although
    the extensions in this document have been defined in such a way as to
    be compatible with RFC 822, there are still circumstances in which it
    might be desirable for a mail-processing agent to know whether a
    message was composed with the new standard in mind.
 
 
 
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    Therefore, this document defines a new header field, "MIME-Version",
    which is to be used to declare the version of the Internet message
    body format standard in use.
 
    Messages composed in accordance with this document MUST include such
    a header field, with the following verbatim text:
 
      MIME-Version: 1.0
 
    The presence of this header field is an assertion that the message
    has been composed in compliance with this document.
 
    Since it is possible that a future document might extend the message
    format standard again, a formal BNF is given for the content of the
    MIME-Version field:
 
      version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 
    Thus, future format specifiers, which might replace or extend "1.0",
    are constrained to be two integer fields, separated by a period.  If
    a message is received with a MIME-version value other than "1.0", it
    cannot be assumed to conform with this document.
 
    Note that the MIME-Version header field is required at the top level
    of a message.  It is not required for each body part of a multipart
    entity.  It is required for the embedded headers of a body of type
    "message/rfc822" or "message/partial" if and only if the embedded
    message is itself claimed to be MIME-conformant.
 
    It is not possible to fully specify how a mail reader that conforms
    with MIME as defined in this document should treat a message that
    might arrive in the future with some value of MIME-Version other than
    "1.0".
 
    It is also worth noting that version control for specific media types
    is not accomplished using the MIME-Version mechanism.  In particular,
    some formats (such as application/postscript) have version numbering
    conventions that are internal to the media format.  Where such
    conventions exist, MIME does nothing to supersede them.  Where no
    such conventions exist, a MIME media type might use a "version"
    parameter in the content-type field if necessary.
 
 
 
 
 
 
 
 
 
 
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    NOTE TO IMPLEMENTORS:  When checking MIME-Version values any RFC 822
    comment strings that are present must be ignored.  In particular, the
    following four MIME-Version fields are equivalent:
 
      MIME-Version: 1.0
 
      MIME-Version: 1.0 (produced by MetaSend Vx.x)
 
      MIME-Version: (produced by MetaSend Vx.x) 1.0
 
      MIME-Version: 1.(produced by MetaSend Vx.x)0
 
    In the absence of a MIME-Version field, a receiving mail user agent
    (whether conforming to MIME requirements or not) may optionally
    choose to interpret the body of the message according to local
    conventions.  Many such conventions are currently in use and it
    should be noted that in practice non-MIME messages can contain just
    about anything.
 
    It is impossible to be certain that a non-MIME mail message is
    actually plain text in the US-ASCII character set since it might well
    be a message that, using some set of nonstandard local conventions
    that predate MIME, includes text in another character set or non-
    textual data presented in a manner that cannot be automatically
    recognized (e.g., a uuencoded compressed UNIX tar file).
 
 5.  Content-Type Header Field
 
    The purpose of the Content-Type field is to describe the data
    contained in the body fully enough that the receiving user agent can
    pick an appropriate agent or mechanism to present the data to the
    user, or otherwise deal with the data in an appropriate manner. The
    value in this field is called a media type.
 
    HISTORICAL NOTE:  The Content-Type header field was first defined in
    RFC 1049.  RFC 1049 used a simpler and less powerful syntax, but one
    that is largely compatible with the mechanism given here.
 
    The Content-Type header field specifies the nature of the data in the
    body of an entity by giving media type and subtype identifiers, and
    by providing auxiliary information that may be required for certain
    media types.  After the media type and subtype names, the remainder
    of the header field is simply a set of parameters, specified in an
    attribute=value notation.  The ordering of parameters is not
    significant.
 
 
 
 
 
 
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    In general, the top-level media type is used to declare the general
    type of data, while the subtype specifies a specific format for that
    type of data.  Thus, a media type of "image/xyz" is enough to tell a
    user agent that the data is an image, even if the user agent has no
    knowledge of the specific image format "xyz".  Such information can
    be used, for example, to decide whether or not to show a user the raw
    data from an unrecognized subtype -- such an action might be
    reasonable for unrecognized subtypes of text, but not for
    unrecognized subtypes of image or audio.  For this reason, registered
    subtypes of text, image, audio, and video should not contain embedded
    information that is really of a different type.  Such compound
    formats should be represented using the "multipart" or "application"
    types.
 
    Parameters are modifiers of the media subtype, and as such do not
    fundamentally affect the nature of the content.  The set of
    meaningful parameters depends on the media type and subtype.  Most
    parameters are associated with a single specific subtype.  However, a
    given top-level media type may define parameters which are applicable
    to any subtype of that type.  Parameters may be required by their
    defining content type or subtype or they may be optional. MIME
    implementations must ignore any parameters whose names they do not
    recognize.
 
    For example, the "charset" parameter is applicable to any subtype of
    "text", while the "boundary" parameter is required for any subtype of
    the "multipart" media type.
 
    There are NO globally-meaningful parameters that apply to all media
    types.  Truly global mechanisms are best addressed, in the MIME
    model, by the definition of additional Content-* header fields.
 
    An initial set of seven top-level media types is defined in RFC 2046.
    Five of these are discrete types whose content is essentially opaque
    as far as MIME processing is concerned.  The remaining two are
    composite types whose contents require additional handling by MIME
    processors.
 
    This set of top-level media types is intended to be substantially
    complete.  It is expected that additions to the larger set of
    supported types can generally be accomplished by the creation of new
    subtypes of these initial types.  In the future, more top-level types
    may be defined only by a standards-track extension to this standard.
    If another top-level type is to be used for any reason, it must be
    given a name starting with "X-" to indicate its non-standard status
    and to avoid a potential conflict with a future official name.
 
 
 
 
 
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 5.1.  Syntax of the Content-Type Header Field
 
    In the Augmented BNF notation of RFC 822, a Content-Type header field
    value is defined as follows:
 
      content := "Content-Type" ":" type "/" subtype
                 *(";" parameter)
                 ; Matching of media type and subtype
                 ; is ALWAYS case-insensitive.
 
      type := discrete-type / composite-type
 
      discrete-type := "text" / "image" / "audio" / "video" /
                       "application" / extension-token
 
      composite-type := "message" / "multipart" / extension-token
 
      extension-token := ietf-token / x-token
 
      ietf-token := <An extension token defined by a
                     standards-track RFC and registered
                     with IANA.>
 
      x-token := <The two characters "X-" or "x-" followed, with
                  no intervening white space, by any token>
 
      subtype := extension-token / iana-token
 
      iana-token := <A publicly-defined extension token. Tokens
                     of this form must be registered with IANA
                     as specified in RFC 2048.>
 
      parameter := attribute "=" value
 
      attribute := token
                   ; Matching of attributes
                   ; is ALWAYS case-insensitive.
 
      value := token / quoted-string
 
      token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
                  or tspecials>
 
      tspecials :=  "(" / ")" / "<" / ">" / "@" /
                    "," / ";" / ":" / "\" / <">
                    "/" / "[" / "]" / "?" / "="
                    ; Must be in quoted-string,
                    ; to use within parameter values
 
 
 
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    Note that the definition of "tspecials" is the same as the RFC 822
    definition of "specials" with the addition of the three characters
    "/", "?", and "=", and the removal of ".".
 
    Note also that a subtype specification is MANDATORY -- it may not be
    omitted from a Content-Type header field.  As such, there are no
    default subtypes.
 
    The type, subtype, and parameter names are not case sensitive.  For
    example, TEXT, Text, and TeXt are all equivalent top-level media
    types.  Parameter values are normally case sensitive, but sometimes
    are interpreted in a case-insensitive fashion, depending on the
    intended use.  (For example, multipart boundaries are case-sensitive,
    but the "access-type" parameter for message/External-body is not
    case-sensitive.)
 
    Note that the value of a quoted string parameter does not include the
    quotes.  That is, the quotation marks in a quoted-string are not a
    part of the value of the parameter, but are merely used to delimit
    that parameter value.  In addition, comments are allowed in
    accordance with RFC 822 rules for structured header fields.  Thus the
    following two forms
 
      Content-type: text/plain; charset=us-ascii (Plain text)
 
      Content-type: text/plain; charset="us-ascii"
 
    are completely equivalent.
 
    Beyond this syntax, the only syntactic constraint on the definition
    of subtype names is the desire that their uses must not conflict.
    That is, it would be undesirable to have two different communities
    using "Content-Type: application/foobar" to mean two different
    things.  The process of defining new media subtypes, then, is not
    intended to be a mechanism for imposing restrictions, but simply a
    mechanism for publicizing their definition and usage.  There are,
    therefore, two acceptable mechanisms for defining new media subtypes:
 
     (1)   Private values (starting with "X-") may be defined
           bilaterally between two cooperating agents without
           outside registration or standardization. Such values
           cannot be registered or standardized.
 
     (2)   New standard values should be registered with IANA as
           described in RFC 2048.
 
    The second document in this set, RFC 2046, defines the initial set of
    media types for MIME.
 
 
 
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 5.2.  Content-Type Defaults
 
    Default RFC 822 messages without a MIME Content-Type header are taken
    by this protocol to be plain text in the US-ASCII character set,
    which can be explicitly specified as:
 
      Content-type: text/plain; charset=us-ascii
 
    This default is assumed if no Content-Type header field is specified.
    It is also recommend that this default be assumed when a
    syntactically invalid Content-Type header field is encountered. In
    the presence of a MIME-Version header field and the absence of any
    Content-Type header field, a receiving User Agent can also assume
    that plain US-ASCII text was the sender's intent.  Plain US-ASCII
    text may still be assumed in the absence of a MIME-Version or the
    presence of an syntactically invalid Content-Type header field, but
    the sender's intent might have been otherwise.
 
 6.  Content-Transfer-Encoding Header Field
 
    Many media types which could be usefully transported via email are
    represented, in their "natural" format, as 8bit character or binary
    data.  Such data cannot be transmitted over some transfer protocols.
    For example, RFC 821 (SMTP) restricts mail messages to 7bit US-ASCII
    data with lines no longer than 1000 characters including any trailing
    CRLF line separator.
 
    It is necessary, therefore, to define a standard mechanism for
    encoding such data into a 7bit short line format.  Proper labelling
    of unencoded material in less restrictive formats for direct use over
    less restrictive transports is also desireable.  This document
    specifies that such encodings will be indicated by a new "Content-
    Transfer-Encoding" header field.  This field has not been defined by
    any previous standard.
 
 6.1.  Content-Transfer-Encoding Syntax
 
    The Content-Transfer-Encoding field's value is a single token
    specifying the type of encoding, as enumerated below.  Formally:
 
      encoding := "Content-Transfer-Encoding" ":" mechanism
 
      mechanism := "7bit" / "8bit" / "binary" /
                   "quoted-printable" / "base64" /
                   ietf-token / x-token
 
    These values are not case sensitive -- Base64 and BASE64 and bAsE64
    are all equivalent.  An encoding type of 7BIT requires that the body
 
 
 
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    is already in a 7bit mail-ready representation.  This is the default
    value -- that is, "Content-Transfer-Encoding: 7BIT" is assumed if the
    Content-Transfer-Encoding header field is not present.
 
 6.2.  Content-Transfer-Encodings Semantics
 
    This single Content-Transfer-Encoding token actually provides two
    pieces of information.  It specifies what sort of encoding
    transformation the body was subjected to and hence what decoding
    operation must be used to restore it to its original form, and it
    specifies what the domain of the result is.
 
    The transformation part of any Content-Transfer-Encodings specifies,
    either explicitly or implicitly, a single, well-defined decoding
    algorithm, which for any sequence of encoded octets either transforms
    it to the original sequence of octets which was encoded, or shows
    that it is illegal as an encoded sequence.  Content-Transfer-
    Encodings transformations never depend on any additional external
    profile information for proper operation. Note that while decoders
    must produce a single, well-defined output for a valid encoding no
    such restrictions exist for encoders: Encoding a given sequence of
    octets to different, equivalent encoded sequences is perfectly legal.
 
    Three transformations are currently defined: identity, the "quoted-
    printable" encoding, and the "base64" encoding.  The domains are
    "binary", "8bit" and "7bit".
 
    The Content-Transfer-Encoding values "7bit", "8bit", and "binary" all
    mean that the identity (i.e. NO) encoding transformation has been
    performed.  As such, they serve simply as indicators of the domain of
    the body data, and provide useful information about the sort of
    encoding that might be needed for transmission in a given transport
    system.  The terms "7bit data", "8bit data", and "binary data" are
    all defined in Section 2.
 
    The quoted-printable and base64 encodings transform their input from
    an arbitrary domain into material in the "7bit" range, thus making it
    safe to carry over restricted transports.  The specific definition of
    the transformations are given below.
 
    The proper Content-Transfer-Encoding label must always be used.
    Labelling unencoded data containing 8bit characters as "7bit" is not
    allowed, nor is labelling unencoded non-line-oriented data as
    anything other than "binary" allowed.
 
    Unlike media subtypes, a proliferation of Content-Transfer-Encoding
    values is both undesirable and unnecessary.  However, establishing
    only a single transformation into the "7bit" domain does not seem
 
 
 
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    possible.  There is a tradeoff between the desire for a compact and
    efficient encoding of largely- binary data and the desire for a
    somewhat readable encoding of data that is mostly, but not entirely,
    7bit.  For this reason, at least two encoding mechanisms are
    necessary: a more or less readable encoding (quoted-printable) and a
    "dense" or "uniform" encoding (base64).
 
    Mail transport for unencoded 8bit data is defined in RFC 1652.  As of
    the initial publication of this document, there are no standardized
    Internet mail transports for which it is legitimate to include
    unencoded binary data in mail bodies.  Thus there are no
    circumstances in which the "binary" Content-Transfer-Encoding is
    actually valid in Internet mail.  However, in the event that binary
    mail transport becomes a reality in Internet mail, or when MIME is
    used in conjunction with any other binary-capable mail transport
    mechanism, binary bodies must be labelled as such using this
    mechanism.
 
    NOTE: The five values defined for the Content-Transfer-Encoding field
    imply nothing about the media type other than the algorithm by which
    it was encoded or the transport system requirements if unencoded.
 
 6.3.  New Content-Transfer-Encodings
 
    Implementors may, if necessary, define private Content-Transfer-
    Encoding values, but must use an x-token, which is a name prefixed by
    "X-", to indicate its non-standard status, e.g., "Content-Transfer-
    Encoding: x-my-new-encoding".  Additional standardized Content-
    Transfer-Encoding values must be specified by a standards-track RFC.
    The requirements such specifications must meet are given in RFC 2048.
    As such, all content-transfer-encoding namespace except that
    beginning with "X-" is explicitly reserved to the IETF for future
    use.
 
    Unlike media types and subtypes, the creation of new Content-
    Transfer-Encoding values is STRONGLY discouraged, as it seems likely
    to hinder interoperability with little potential benefit
 
 6.4.  Interpretation and Use
 
    If a Content-Transfer-Encoding header field appears as part of a
    message header, it applies to the entire body of that message.  If a
    Content-Transfer-Encoding header field appears as part of an entity's
    headers, it applies only to the body of that entity.  If an entity is
    of type "multipart" the Content-Transfer-Encoding is not permitted to
    have any value other than "7bit", "8bit" or "binary".  Even more
    severe restrictions apply to some subtypes of the "message" type.
 
 
 
 
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    It should be noted that most media types are defined in terms of
    octets rather than bits, so that the mechanisms described here are
    mechanisms for encoding arbitrary octet streams, not bit streams.  If
    a bit stream is to be encoded via one of these mechanisms, it must
    first be converted to an 8bit byte stream using the network standard
    bit order ("big-endian"), in which the earlier bits in a stream
    become the higher-order bits in a 8bit byte.  A bit stream not ending
    at an 8bit boundary must be padded with zeroes. RFC 2046 provides a
    mechanism for noting the addition of such padding in the case of the
    application/octet-stream media type, which has a "padding" parameter.
 
    The encoding mechanisms defined here explicitly encode all data in
    US-ASCII.  Thus, for example, suppose an entity has header fields
    such as:
 
      Content-Type: text/plain; charset=ISO-8859-1
      Content-transfer-encoding: base64
 
    This must be interpreted to mean that the body is a base64 US-ASCII
    encoding of data that was originally in ISO-8859-1, and will be in
    that character set again after decoding.
 
    Certain Content-Transfer-Encoding values may only be used on certain
    media types.  In particular, it is EXPRESSLY FORBIDDEN to use any
    encodings other than "7bit", "8bit", or "binary" with any composite
    media type, i.e. one that recursively includes other Content-Type
    fields.  Currently the only composite media types are "multipart" and
    "message".  All encodings that are desired for bodies of type
    multipart or message must be done at the innermost level, by encoding
    the actual body that needs to be encoded.
 
    It should also be noted that, by definition, if a composite entity
    has a transfer-encoding value such as "7bit", but one of the enclosed
    entities has a less restrictive value such as "8bit", then either the
    outer "7bit" labelling is in error, because 8bit data are included,
    or the inner "8bit" labelling placed an unnecessarily high demand on
    the transport system because the actual included data were actually
    7bit-safe.
 
    NOTE ON ENCODING RESTRICTIONS:  Though the prohibition against using
    content-transfer-encodings on composite body data may seem overly
    restrictive, it is necessary to prevent nested encodings, in which
    data are passed through an encoding algorithm multiple times, and
    must be decoded multiple times in order to be properly viewed.
    Nested encodings add considerable complexity to user agents:  Aside
    from the obvious efficiency problems with such multiple encodings,
    they can obscure the basic structure of a message.  In particular,
    they can imply that several decoding operations are necessary simply
 
 
 
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    to find out what types of bodies a message contains.  Banning nested
    encodings may complicate the job of certain mail gateways, but this
    seems less of a problem than the effect of nested encodings on user
    agents.
 
    Any entity with an unrecognized Content-Transfer-Encoding must be
    treated as if it has a Content-Type of "application/octet-stream",
    regardless of what the Content-Type header field actually says.
 
    NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-TRANSFER-
    ENCODING: It may seem that the Content-Transfer-Encoding could be
    inferred from the characteristics of the media that is to be encoded,
    or, at the very least, that certain Content-Transfer-Encodings could
    be mandated for use with specific media types.  There are several
    reasons why this is not the case. First, given the varying types of
    transports used for mail, some encodings may be appropriate for some
    combinations of media types and transports but not for others.  (For
    example, in an 8bit transport, no encoding would be required for text
    in certain character sets, while such encodings are clearly required
    for 7bit SMTP.)
 
    Second, certain media types may require different types of transfer
    encoding under different circumstances.  For example, many PostScript
    bodies might consist entirely of short lines of 7bit data and hence
    require no encoding at all.  Other PostScript bodies (especially
    those using Level 2 PostScript's binary encoding mechanism) may only
    be reasonably represented using a binary transport encoding.
    Finally, since the Content-Type field is intended to be an open-ended
    specification mechanism, strict specification of an association
    between media types and encodings effectively couples the
    specification of an application protocol with a specific lower-level
    transport.  This is not desirable since the developers of a media
    type should not have to be aware of all the transports in use and
    what their limitations are.
 
 6.5.  Translating Encodings
 
    The quoted-printable and base64 encodings are designed so that
    conversion between them is possible.  The only issue that arises in
    such a conversion is the handling of hard line breaks in quoted-
    printable encoding output. When converting from quoted-printable to
    base64 a hard line break in the quoted-printable form represents a
    CRLF sequence in the canonical form of the data. It must therefore be
    converted to a corresponding encoded CRLF in the base64 form of the
    data.  Similarly, a CRLF sequence in the canonical form of the data
    obtained after base64 decoding must be converted to a quoted-
    printable hard line break, but ONLY when converting text data.
 
 
 
 
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 6.6.  Canonical Encoding Model
 
    There was some confusion, in the previous versions of this RFC,
    regarding the model for when email data was to be converted to
    canonical form and encoded, and in particular how this process would
    affect the treatment of CRLFs, given that the representation of
    newlines varies greatly from system to system, and the relationship
    between content-transfer-encodings and character sets.  A canonical
    model for encoding is presented in RFC 2049 for this reason.
 
 6.7.  Quoted-Printable Content-Transfer-Encoding
 
    The Quoted-Printable encoding is intended to represent data that
    largely consists of octets that correspond to printable characters in
    the US-ASCII character set.  It encodes the data in such a way that
    the resulting octets are unlikely to be modified by mail transport.
    If the data being encoded are mostly US-ASCII text, the encoded form
    of the data remains largely recognizable by humans.  A body which is
    entirely US-ASCII may also be encoded in Quoted-Printable to ensure
    the integrity of the data should the message pass through a
    character-translating, and/or line-wrapping gateway.
 
    In this encoding, octets are to be represented as determined by the
    following rules:
 
     (1)   (General 8bit representation) Any octet, except a CR or
           LF that is part of a CRLF line break of the canonical
           (standard) form of the data being encoded, may be
           represented by an "=" followed by a two digit
           hexadecimal representation of the octet's value.  The
           digits of the hexadecimal alphabet, for this purpose,
           are "0123456789ABCDEF".  Uppercase letters must be
           used; lowercase letters are not allowed.  Thus, for
           example, the decimal value 12 (US-ASCII form feed) can
           be represented by "=0C", and the decimal value 61 (US-
           ASCII EQUAL SIGN) can be represented by "=3D".  This
           rule must be followed except when the following rules
           allow an alternative encoding.
 
     (2)   (Literal representation) Octets with decimal values of
           33 through 60 inclusive, and 62 through 126, inclusive,
           MAY be represented as the US-ASCII characters which
           correspond to those octets (EXCLAMATION POINT through
           LESS THAN, and GREATER THAN through TILDE,
           respectively).
 
     (3)   (White Space) Octets with values of 9 and 32 MAY be
           represented as US-ASCII TAB (HT) and SPACE characters,
 
 
 
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           respectively, but MUST NOT be so represented at the end
           of an encoded line.  Any TAB (HT) or SPACE characters
           on an encoded line MUST thus be followed on that line
           by a printable character.  In particular, an "=" at the
           end of an encoded line, indicating a soft line break
           (see rule #5) may follow one or more TAB (HT) or SPACE
           characters.  It follows that an octet with decimal
           value 9 or 32 appearing at the end of an encoded line
           must be represented according to Rule #1.  This rule is
           necessary because some MTAs (Message Transport Agents,
           programs which transport messages from one user to
           another, or perform a portion of such transfers) are
           known to pad lines of text with SPACEs, and others are
           known to remove "white space" characters from the end
           of a line.  Therefore, when decoding a Quoted-Printable
           body, any trailing white space on a line must be
           deleted, as it will necessarily have been added by
           intermediate transport agents.
 
     (4)   (Line Breaks) A line break in a text body, represented
           as a CRLF sequence in the text canonical form, must be
           represented by a (RFC 822) line break, which is also a
           CRLF sequence, in the Quoted-Printable encoding.  Since
           the canonical representation of media types other than
           text do not generally include the representation of
           line breaks as CRLF sequences, no hard line breaks
           (i.e. line breaks that are intended to be meaningful
           and to be displayed to the user) can occur in the
           quoted-printable encoding of such types.  Sequences
           like "=0D", "=0A", "=0A=0D" and "=0D=0A" will routinely
           appear in non-text data represented in quoted-
           printable, of course.
 
           Note that many implementations may elect to encode the
           local representation of various content types directly
           rather than converting to canonical form first,
           encoding, and then converting back to local
           representation.  In particular, this may apply to plain
           text material on systems that use newline conventions
           other than a CRLF terminator sequence.  Such an
           implementation optimization is permissible, but only
           when the combined canonicalization-encoding step is
           equivalent to performing the three steps separately.
 
     (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
 
 
 
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           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:
 
      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 ("-") may be represented as itself in the
    Quoted-Printable encoding, care must be taken, when encapsulating a
    quoted-printable encoded body inside one or more multipart entities,
    to ensure that the boundary delimiter 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 in
    RFC 2046.)
 
    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.  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 US-ASCII characters
 
      !"#$@[\]^`{|}~
 
    according to rule #1.
 
    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
 
 
 
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    corruption of the data, it is probably more sensible to use the
    base64 encoding rather than the quoted-printable encoding.
 
    NOTE: Several kinds of substrings cannot be generated according to
    the encoding rules for the quoted-printable content-transfer-
    encoding, and hence are formally illegal if they appear in the output
    of a quoted-printable encoder. This note enumerates these cases and
    suggests ways to handle such illegal substrings if any are
    encountered in quoted-printable data that is to be decoded.
 
     (1)   An "=" followed by two hexadecimal digits, one or both
           of which are lowercase letters in "abcdef", is formally
           illegal. A robust implementation might choose to
           recognize them as the corresponding uppercase letters.
 
     (2)   An "=" followed by a character that is neither a
           hexadecimal digit (including "abcdef") nor the CR
           character of a CRLF pair is illegal.  This case can be
           the result of US-ASCII text having been included in a
           quoted-printable part of a message without itself
           having been subjected to quoted-printable encoding.  A
           reasonable approach by a robust implementation might be
           to include the "=" character and the following
           character in the decoded data without any
           transformation and, if possible, indicate to the user
           that proper decoding was not possible at this point in
           the data.
 
     (3)   An "=" cannot be the ultimate or penultimate character
           in an encoded object.  This could be handled as in case
           (2) above.
 
     (4)   Control characters other than TAB, or CR and LF as
           parts of CRLF pairs, must not appear. The same is true
           for octets with decimal values greater than 126.  If
           found in incoming quoted-printable data by a decoder, a
           robust implementation might exclude them from the
           decoded data and warn the user that illegal characters
           were discovered.
 
     (5)   Encoded lines must not be longer than 76 characters,
           not counting the trailing CRLF. If longer lines are
           found in incoming, encoded data, a robust
           implementation might nevertheless decode the lines, and
           might report the erroneous encoding to the user.
 
 
 
 
 
 
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    WARNING TO IMPLEMENTORS:  If binary data is 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
    platforms with different line break conventions.
 
    For formalists, the syntax of quoted-printable data is described by
    the following grammar:
 
      quoted-printable := qp-line *(CRLF qp-line)
 
      qp-line := *(qp-segment transport-padding CRLF)
                 qp-part transport-padding
 
      qp-part := qp-section
                 ; Maximum length of 76 characters
 
      qp-segment := qp-section *(SPACE / TAB) "="
                    ; Maximum length of 76 characters
 
      qp-section := [*(ptext / SPACE / TAB) ptext]
 
      ptext := hex-octet / safe-char
 
      safe-char := <any octet with decimal value of 33 through
                   60 inclusive, and 62 through 126>
                   ; Characters not listed as "mail-safe" in
                   ; RFC 2049 are also not recommended.
 
      hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
                   ; Octet must be used for characters > 127, =,
                   ; SPACEs or TABs at the ends of lines, and is
                   ; recommended for any character not listed in
                   ; RFC 2049 as "mail-safe".
 
      transport-padding := *LWSP-char
                           ; Composers MUST NOT generate
                           ; non-zero length transport
                           ; padding, but receivers MUST
                           ; be able to handle padding
                           ; added by message transports.
 
    IMPORTANT:  The addition of LWSP between the elements shown in this
    BNF is NOT allowed since this BNF does not specify a structured
    header field.
 
 
 
 
 
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 6.8.  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.
 
    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, Macintosh binhex 4.0 [RFC-1741], 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 8bit 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.
    That is, the first bit in the stream will be the high-order bit in
    the first 8bit byte, and the eighth bit will be the low-order bit in
    the first 8bit 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 multipart boundary delimiters defined in RFC 2046 (e.g.,
    "-").
 
 
 
 
 
 
 
 
 
 
 
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                     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 encoded output stream 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
    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
 
 
 
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    such assurance is possible, however, when the number of octets
    transmitted was a multiple of three and no "=" characters are
    present.
 
    Any characters outside of the base64 alphabet are to be ignored in
    base64-encoded data.
 
    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 potential boundary
    delimiters within base64-encoded bodies within multipart entities
    because no hyphen characters are used in the base64 encoding.
 
 7.  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
    labelled using the "Content-ID" header field, which is syntactically
    identical to the "Message-ID" header field:
 
      id := "Content-ID" ":" msg-id
 
    Like the Message-ID values, Content-ID values must be generated to be
    world-unique.
 
    The Content-ID value may be used for uniquely identifying MIME
    entities in several contexts, particularly for caching data
    referenced by the message/external-body mechanism.  Although the
    Content-ID header is generally optional, its use is MANDATORY in
    implementations which generate data of the optional MIME media type
    "message/external-body".  That is, each message/external-body entity
    must have a Content-ID field to permit caching of such data.
 
    It is also worth noting that the Content-ID value has special
    semantics in the case of the multipart/alternative media type.  This
    is explained in the section of RFC 2046 dealing with
    multipart/alternative.
 
 
 
 
 
 
 
 
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 8.  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.  This header
    field is always optional.
 
      description := "Content-Description" ":" *text
 
    The description is presumed to be given in the US-ASCII character
    set, although the mechanism specified in RFC 2047 may be used for
    non-US-ASCII Content-Description values.
 
 9.  Additional MIME Header Fields
 
    Future documents may elect to define additional MIME header fields
    for various purposes.  Any new header field that further describes
    the content of a message should begin with the string "Content-" to
    allow such fields which appear in a message header to be
    distinguished from ordinary RFC 822 message header fields.
 
      MIME-extension-field := <Any RFC 822 header field which
                               begins with the string
                               "Content-">
 
 10.  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 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 RFC 2049).
 
    The next document in this set, RFC 2046, specifies the initial set of
    media types that can be labelled and transported using these headers.
 
 11.  Security Considerations
 
    Security issues are discussed in the second document in this set, RFC
    2046.
 
 
 
 
 
 
 
 
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 RFC 2045                Internet Message Bodies            November 1996
 
 
 12.  Authors' Addresses
 
    For more information, the authors of this document are best contacted
    via Internet mail:
 
    Ned Freed
    Innosoft International, Inc.
    1050 East Garvey Avenue South
    West Covina, CA 91790
    USA
 
    Phone: +1 818 919 3600
    Fax:   +1 818 919 3614
    EMail: ned@innosoft.com
 
 
    Nathaniel S. Borenstein
    First Virtual Holdings
    25 Washington Avenue
    Morristown, NJ 07960
    USA
 
    Phone: +1 201 540 8967
    Fax:   +1 201 993 3032
    EMail: nsb@nsb.fv.com
 
 
    MIME is a result of the work of the Internet Engineering Task Force
    Working Group on RFC 822 Extensions.  The chairman of that group,
    Greg Vaudreuil, may be reached at:
 
    Gregory M. Vaudreuil
    Octel Network Services
    17080 Dallas Parkway
    Dallas, TX 75248-1905
    USA
 
    EMail: Greg.Vaudreuil@Octel.Com
 
 
 
 
 
 
 
 
 
 
 
 
 
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 RFC 2045                Internet Message Bodies            November 1996
 
 
 Appendix A -- 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 by name to
    several syntax rules that are defined by RFC 822.  Rather than
    reproduce those definitions here, and risk unintentional differences
    between the two, this document simply refers the reader to RFC 822
    for the remaining definitions. Wherever a term is undefined, it
    refers to the RFC 822 definition.
 
   attribute := token
                ; Matching of attributes
                ; is ALWAYS case-insensitive.
 
   composite-type := "message" / "multipart" / extension-token
 
   content := "Content-Type" ":" type "/" subtype
              *(";" parameter)
              ; Matching of media type and subtype
              ; is ALWAYS case-insensitive.
 
   description := "Content-Description" ":" *text
 
   discrete-type := "text" / "image" / "audio" / "video" /
                    "application" / extension-token
 
   encoding := "Content-Transfer-Encoding" ":" mechanism
 
   entity-headers := [ content CRLF ]
                     [ encoding CRLF ]
                     [ id CRLF ]
                     [ description CRLF ]
                     *( MIME-extension-field CRLF )
 
   extension-token := ietf-token / x-token
 
   hex-octet := "=" 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
                ; Octet must be used for characters > 127, =,
                ; SPACEs or TABs at the ends of lines, and is
                ; recommended for any character not listed in
                ; RFC 2049 as "mail-safe".
 
   iana-token := <A publicly-defined extension token. Tokens
                  of this form must be registered with IANA
                  as specified in RFC 2048.>
 
 
 
 
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   ietf-token := <An extension token defined by a
                  standards-track RFC and registered
                  with IANA.>
 
   id := "Content-ID" ":" msg-id
 
   mechanism := "7bit" / "8bit" / "binary" /
                "quoted-printable" / "base64" /
                ietf-token / x-token
 
   MIME-extension-field := <Any RFC 822 header field which
                            begins with the string
                            "Content-">
 
   MIME-message-headers := entity-headers
                           fields
                           version CRLF
                           ; The ordering of the header
                           ; fields implied by this BNF
                           ; definition should be ignored.
 
   MIME-part-headers := entity-headers
                        [fields]
                        ; Any field not beginning with
                        ; "content-" can have no defined
                        ; meaning and may be ignored.
                        ; The ordering of the header
                        ; fields implied by this BNF
                        ; definition should be ignored.
 
   parameter := attribute "=" value
 
   ptext := hex-octet / safe-char
 
   qp-line := *(qp-segment transport-padding CRLF)
              qp-part transport-padding
 
   qp-part := qp-section
              ; Maximum length of 76 characters
 
   qp-section := [*(ptext / SPACE / TAB) ptext]
 
   qp-segment := qp-section *(SPACE / TAB) "="
                 ; Maximum length of 76 characters
 
   quoted-printable := qp-line *(CRLF qp-line)
 
 
 
 
 
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   safe-char := <any octet with decimal value of 33 through
                60 inclusive, and 62 through 126>
                ; Characters not listed as "mail-safe" in
                ; RFC 2049 are also not recommended.
 
   subtype := extension-token / iana-token
 
   token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
               or tspecials>
 
   transport-padding := *LWSP-char
                        ; Composers MUST NOT generate
                        ; non-zero length transport
                        ; padding, but receivers MUST
                        ; be able to handle padding
                        ; added by message transports.
 
   tspecials :=  "(" / ")" / "<" / ">" / "@" /
                 "," / ";" / ":" / "\" / <">
                 "/" / "[" / "]" / "?" / "="
                 ; Must be in quoted-string,
                 ; to use within parameter values
 
   type := discrete-type / composite-type
 
   value := token / quoted-string
 
   version := "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 
   x-token := <The two characters "X-" or "x-" followed, with
               no  intervening white space, by any token>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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