(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.






 
RFC 2045                Internet Message Bodies            November 1996


   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.






 
RFC 2045                Internet Message Bodies            November 1996


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