1. Definitions and Notation

The following sections define terms used in this document.

1.1. Requirements Notation

This document occasionally uses terms that appear in capital letters. When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate particular requirements of this specification. A discussion of the meanings of these terms appears in [RFC-2119].

1.2. Character

A member of a set of elements used for the organization, control, or representation of data.

1.3. Charset

The term "charset" (referred to as a "character set" in previous versions of this document) is used here to refer to a method of converting a sequence of octets into a sequence of characters. This conversion may also optionally produce additional control information such as directionality indicators.

Note that unconditional and unambiguous conversion in the other direction is not required, in that not all characters may be representable by a given charset and a charset may provide more than one sequence of octets to represent a particular sequence of characters.

This definition is intended to allow charsets to be defined in a variety of different ways, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, the definition associated with a charset name must fully specify the mapping to be performed. In particular, use of external profiling information to determine the exact mapping is not permitted.

HISTORICAL NOTE: The term "character set" was originally used in MIME to describe such straightforward schemes as US-ASCII and ISO-8859-1 which consist of a small set of characters and a simple one-to-one mapping from single octets to single characters. Multi-octet character encoding schemes and switching techniques make the situation much more complex. As such, the definition of this term was revised to emphasize both the conversion aspect of the process, and the term itself has been changed to "charset" to emphasize that it is not, after all, just a set of characters. A discussion of these issues as well as specification of standard terminology for use in the IETF appears in RFC 2130.

1.4. Coded Character Set

A Coded Character Set (CCS) is a one-to-one mapping from a set of abstract characters to a set of integers. Examples of coded character sets are ISO 10646 [ISO-10646], US-ASCII [US-ASCII], and the ISO-8859 series [ISO-8859].

1.5. Character Encoding Scheme

A Character Encoding Scheme (CES) is a mapping from a Coded Character Set or several coded character sets to a set of octet sequences. A given CES is sometimes associated with a single CCS; for example, UTF-8 applies only to ISO 10646.