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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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: [email protected]


  Nathaniel S. Borenstein
  First Virtual Holdings
  25 Washington Avenue
  Morristown, NJ 07960
  USA

  Phone: +1 201 540 8967
  Fax:   +1 201 993 3032
  EMail: [email protected]


  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: [email protected]













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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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