rfc9839v1.txt		rfc9839.txt
	skipping to change at line 102 ¶		skipping to change at line 102 ¶
	subset. The intended use is to serve as a convenient target for		subset. The intended use is to serve as a convenient target for
	cross-reference from other specifications whose authors wish to		cross-reference from other specifications whose authors wish to
	exclude problematic code points from the data format or protocol		exclude problematic code points from the data format or protocol
	being specified.		being specified.
	Note that this document only provides guidance on avoiding the use of		Note that this document only provides guidance on avoiding the use of
	code points that cannot be used for interoperable interchange of		code points that cannot be used for interoperable interchange of
	Unicode textual data. Dealing with strings, particularly in the		Unicode textual data. Dealing with strings, particularly in the
	context of user interfaces, requires addressing language, text		context of user interfaces, requires addressing language, text
	rendering direction, alternate representations of the same abstract		rendering direction, alternate representations of the same abstract
	character, and so on. These issues, among many others, led to many		character, and so on. These issues, among many others, led to
	efforts by the Unicode Consortium, efforts by the IETF such as [IDN]		efforts by the Unicode Consortium, efforts by the IETF such as [IDN]
	and [PRECIS], and internationalization efforts by W3C such as		and [PRECIS], and internationalization efforts by W3C such as
	[W3C-CHAR]. The results of these efforts should be consulted by		[W3C-CHAR]. The results of these efforts should be consulted by
	anyone engaging in such work.		anyone engaging in such work.
	1.1. Notation		1.1. Notation
	In this document, the numeric values assigned to Unicode characters		In this document, the numeric values assigned to Unicode characters
	are provided in hexadecimal. This document uses Unicode's standard		are provided in hexadecimal. This document uses Unicode's standard
	notation of "U+" followed by four or more hexadecimal digits. For		notation of "U+" followed by four or more hexadecimal digits. For
	skipping to change at line 143 ¶		skipping to change at line 143 ¶
	storage systems and to specify allowed subsets in specifications.		storage systems and to specify allowed subsets in specifications.
	There are 1,114,112 (17 * 2^16) code points; as of Unicode 16.0		There are 1,114,112 (17 * 2^16) code points; as of Unicode 16.0
	(2024), about 155,000 have been assigned to characters. Since		(2024), about 155,000 have been assigned to characters. Since
	unassigned code points regularly become assigned when new characters		unassigned code points regularly become assigned when new characters
	are added to Unicode, it is usually not a good practice to specify		are added to Unicode, it is usually not a good practice to specify
	that unassigned code points should be avoided.		that unassigned code points should be avoided.
	2.1. Encoding Forms		2.1. Encoding Forms
	Unicode describes a variety of encoding forms, ways to marshal code		Unicode describes a variety of encoding forms that can be used to
	points into byte sequences. A survey of these is beyond the scope of		marshal code points into byte sequences. A survey of these is beyond
	this document. However, it is useful to note that "UTF-16"		the scope of this document. However, it is useful to note that "UTF-
	represents each code point with one or two 16-bit chunks, while "UTF-		16" represents each code point with one or two 16-bit chunks, while
	8" uses variable-length byte sequences [RFC3629].		"UTF-8" uses variable-length byte sequences [RFC3629].
	The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277],		The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277],
	says "Protocols MUST be able to use the UTF-8 charset", which becomes		says "Protocols MUST be able to use the UTF-8 charset", which becomes
	a mandate to use UTF-8 for any protocol or data format that specifies		a mandate to use UTF-8 for any protocol or data format that specifies
	a single encoding form. UTF-8 is widely used for interoperable data		a single encoding form. UTF-8 is widely used for interoperable data
	formats such as JSON, YAML, CBOR, and XML.		formats such as JSON, YAML, CBOR, and XML.
	2.2. Problematic Code Points		2.2. Problematic Code Points
	This section classifies all the code points that can never represent		This section classifies all the code points that can never represent
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