rfc2045.txt

穿越防火墙技术代码

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.