Although the mechanisms specified in this document are all described in prose, most are also described formally in the modified BNF notation of RFC 822. Implementors will need to be familiar with this notation in order to understand this specification, and are referred to RFC 822 for a complete explanation of the modified BNF notation.
Some of the modified BNF in this document makes reference to syntactic entities that are defined in RFC 822 and not in this document. A complete formal grammar, then, is obtained by combining the collected grammar appendix of this document with that of RFC 822.
The term CRLF, in this document, refers to the sequence of the two ASCII characters CR (13) and LF (10) which, taken together, in this order, denote a line break in RFC 822 mail.
The term "character set", wherever it is used in this document, refers to a coded character set, in the sense of ISO character set standardization work, and must not be misinterpreted as meaning "a set of characters."
The term "message", when not further qualified, means either the (complete or "top-level") message being transferred on a network, or a message encapsulated in a body of type "message".
The term "body part", in this document, means one of the parts of the body of a multipart entity. A body part has a header and a body, so it makes sense to speak about the body of a body part.
The term "entity", in this document, means either a message or a body part. All kinds of entities share the property that they have a header and a body.
The term "body", when not further qualified, means the body of an entity, that is the body of either a message or of a body part.
Note : the previous four definitions are clearly circular. This is unavoidable, since the overal structure of a MIME message is indeed recursive.
In this document, all numeric and octet values are given in decimal notation.
It must be noted that Content-Type values, subtypes, and parameter names as defined in this document are case- insensitive. However, parameter values are case-sensitive unless otherwise specified for the specific parameter.
FORMATTING NOTE: This document has been carefully formatted for ease of reading. The PostScript version of this document, in particular, places notes like this one, which may be skipped by the reader, in a smaller, italicized, font, and indents it as well. In the text version, only the indentation is preserved, so if you are reading the text version of this you might consider using the PostScript version instead. However, all such notes will be indented and preceded by "NOTE:" or some similar introduction, even in the text version.
The primary purpose of these non-essential notes is to convey information about the rationale of this document, or to place this document in the proper historical or evolutionary context. Such information may be skipped by those who are focused entirely on building a compliant implementation, but may be of use to those who wish to understand why this document is written as it is.
For ease of recognition, all BNF definitions have been placed in a fixed-width font in the PostScript version of this document.