| Internet-Draft | Specifying Unicode | September 2023 |
| Bray & Hoffman | Expires 4 March 2024 | [Page] |
This document describes how to specify the use of Unicode characters in a helpful and unambiguous way.¶
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When a protocol or data format has text fields, that text is normally composed of Unicode [UNICODE] characters, to support use by speakers of many languages. IETF policy mandates this [RFC2277]. Unfortunately, the Unicode Standard does not define the term "Unicode character" in a way that is useful for technical specifications.¶
Protocols and data formats SHOULD describe exactly which selection of the available Unicode characters are to be used. The term "character repertoire" is normally applied to an encoding standard; in this document it describes selected subsets of the Unicode characters. Authors should have a way to concisely and exactly reference a stable specification that identifies a protocol or data format's character repertoire¶
This document describes and names several subsets that have been popular choices in specification character repertoires, and suggests one new subset. The goal is to provide a convenient target for cross-reference from other specifications which discuss character repertoires.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
In this document, the numeric values assigned to Unicode characters are provided in hexadecimal. In the text, Unicode’s standard "U+" notation [RFC5137] is used. For example, "A", decimal 65, would be expressed as U+0041, and "😉" (Winking Face), decimal 128,521, would be U+1F609.¶
Certain groups of numeric values described in Section 3 and Section 4 are given in ABNF [RFC5234]. In ABNF, the hexadecimal values for characters are preceded by "%x" rather than "U+".¶
All the numeric ranges in this document are inclusive.¶
The Unicode Standard's definition of "Unicode character" is conceptual. However, each Unicode character is assigned an integer identifier in the range U+0000 through U+10FFFF. These numbers are used to represent the characters in computer memory and storage systems and, in specifications, to specify the allowed repertoires of Unicode characters.¶
The numbers assigned to Unicode characters are called “code points”; there are potentially 1,114,112 of them. As of 2023, fewer than 150,000 characters have had code points assigned. While the inclusion of unassigned code points in text data is undesirable, it is difficult to specify that it should be avoided, because unassigned code points regularly become assigned as new characters are added to Unicode. Fortunately, the occurrence of unassigned code points in texts is generally unlikely to cause software to malfunction.¶
Unicode describes a variety of "transformation formats", ways to encode code points in bytes of computer memory. A survey of transformation formats is beyond the scope of this document. However, it is useful to note that the "UTF-16" transformation format represents each code point with one or two 16-bit chunks, and the “UTF-8” transformation format uses variable-length byte sequences.¶
Use of the UTF-8 transformation format is mandated by the IETF [RFC2277] and widely used for interoperable data formats such as JSON, YAML, and XML.¶
Definition D10a in section 3.4 of [UNICODE] defines seven code point types. Three types of code points are assigned to constructs which are not actually characters or whose value as Unicode characters is questionable: "Control", "Surrogate", and "Noncharacter".¶
A total of 2,048 code points, in the range U+D800-U+DFFF, are divided into two blocks called "high surrogates" and "low surrogates"; collectively the 2,048 code points are referred to as "surrogates". Surrogates may only be used in Unicode texts encoded in UTF-16, where a high-surrogate/low-surrogate pair represents a code point greater than U+FFFF.¶
A surrogate which occurs as a singleton, or in an improperly-composed pair, or in text encoded in any transformation format other than UTF-16, has no meaning and may cause malfunction in software that encounters it. In particular, it is impossible to represent a surrogate in well-formed UTF-8.¶
Section 23.1 of [UNICODE] introduces the "Control Codes" for compatibility with legacy pre-Unicode standards. They comprise 65 code points in the ranges U+0000-U+001F ("C0 Controls") and U+0080-U+009F (“C1 Controls”), plus U+007F, "DEL".¶
The C0 Controls include the newline (U+000A), carriage return (U+000D), and Tab (U+0009); this document refers to these three characters as the "useful controls".¶
Aside from the useful controls, the control codes are mostly obsolete and generally lack interoperable semantics. This document uses the phrase "useless controls" to describe control codes that are not useful controls.¶
Since the code points for C0 Controls include the 32 smallest integers including zero, they are likely to occur in data as a result of programming errors.¶
Certain code points are classified as "noncharacters", and [UNICODE] asserts in multiple chapters that they are not designed or used for open interchange.¶
Code points are organized into 17 "planes", each containing 216 code points. The last two code points in each plane are noncharacters: U+00FFFE, U+00FFFF, U+01FFFE, U+01FFF, U+02FFFE, U+02FFFF, and so on, up to U+10FFFE, U+10FFFF.¶
The code points in the range U+FDD0 to U+FDEF are noncharacters.¶
This section describes popular subsets of the code points that are defined in [UNICODE]. Specifications can refer to these repertoires by the names "Unicode Code Points" and "Unicode Scalar Values".¶
Definition D9 in section 3.4 of [UNICODE] defines the term "Unicode codespace" as "a range of integers from 0 to 10FFFF16". Definition D10 defines the term "Code point" as "Any value in the Unicode codespace".¶
The "Unicode Code Points" subset can be expressed as an ABNF production:¶
unicode-code-points = %x0-10FFFF¶
This subset is notable for including all possible code points. It has been adopted by JSON [RFC8259]. However, it includes all of the code points with problematic types listed above. For example, the sample below is a legal JSON text.¶
{"example": "\u0000\uDEAD\u7FFFF"}¶
The value of the "example" field contains the C0 Control NUL, an unpaired surrogate, and the noncharacter U+7FFFF. It cannot be serialized into legal UTF-8, but many libraries will silently parse this and generate an ill-formed UTF-8 string. Implementors must be prepared to deal with these sorts of problematic code points.¶
Definition D76 in section 3.9 of [UNICODE] defines the term "Unicode scalar value" as "Any Unicode code point except high-surrogate and low-surrogate code points."¶
The "Unicode Scalar Values" subset can be expressed as an ABNF production:¶
unicode-scalar-values = %x0-D7FF / %xE000-10FFFF ; exclude surrogates¶
This subset has the advantage of excluding surrogates, which can never add any value and have the potential to cause problems. This subset has been adopted by I-JSON [RFC7493]. However it includes useless controls and noncharacters. For example, the sample below is a legal I-JSON text.¶
{"example": "\u0000\u7FFFF"}¶
The value of the "example" field can be serialized into legal UTF-8, but is unlikely ever to be useful in practice.¶
This section lists other ways to specify subsets of the code points beyond those provided by the Unicode Standard itself. These subsets may serve as more appropriate character repertoires for some protocols and data formats than those in Section 3, depending on their needs. Specifications can refer to these repertoires by the names "XML Characters" and "Useful Assignables".¶
The XML 1.0 Specification [XML], in its grammar production labeled "Char", specifies a range of Unicode codepoints that excludes surrogates, useless C0 Controls, and the noncharacters U+FFFE and U+FFFF.¶
XML characters exclude surrogates and some but not all useless controls. For example, the sample below is a well-formed XML document.¶
<example>‰翿</example>¶
The "example" element contains the useless DEL control, the useless "CHARACTER TABULATION WITH JUSTIFICATION" control, and the noncharacter U+7FFFF. It is unlikely ever to be useful in practice.¶
The "XML Characters" subset can be expressed as an ABNF production:¶
xml-chars = %x9 / %xA / %xD / ; useful controls %x20-D7FF / ; exclude surrogates %xE000-FFFD/ ; exclude FFFE and FFFF nonchars %x100000-10FFFF¶
While this subset does not exclude all the problematic code points, the C1 Controls are less likely than the C0 Controls to appear erroneously in data, and have not been observed to be a frequent source of problems. Also, the noncharacters greater in value than U+FFFF are rarely encountered.¶
This subset may be especially appropriate for data formats which may be represented in either JSON or XML.¶
For convenience, this document defines the "Useful Assignables" subset as the Unicode code points, excluding the useless controls, surrogates, and noncharacters. This comprises all code points that are currently assigned, or might in future be assigned, to characters that are not legacy control codes, plus the useful controls.¶
Useful Assignables can be expressed as an ABNF production:¶
useful-assignables = %x9 / %xA / %xD / ; useful controls %x20-7E / ; exclude C1 Controls and DEL %xA0-D7FF / ; exclude surrogates %xE000-FDCF ; exclude FDD0 nonchars %xFDF0-FFFD / ; exclude FFFE and FFFF nonchars %x1000-1FFFD / %x2000-2FFFD / ; (repeat per plane) %x3000-3FFFD / %x4000-4FFFD / %x5000-5FFFD / %x6000-6FFFD / %x7000-7FFFD / %x8000-8FFFD / %x9000-9FFFD / %xA000-AFFFD / %xB000-BFFFD / %xC000-CFFFD / %xD000-DFFFD / %xE000-EFFFD / %xF000-FFFFD / %x10000-10FFFD¶
This document makes no requests of IANA.¶
Unicode Security Considerations [TR36] is a wide-ranging survey of the issues implementors should consider while writing software to process Unicode text. Many of the exploits it discusses are aimed at deceiving human readers, but vulnerabilities involving issues such as surrogates and noncharacters are also covered, and in fact can contribute to human-deceiving exploits.¶
Note that the Unicode-character subsets specified in this document include a successively-decreasing number of surrogates and noncharacters, and thus should be less and less susceptible to vulnerabilities. The Section 4.2 subset, "Basic Unicode Characters", excludes all of them.¶
Thanks are due to Guillaume Fortin-Debigaré, who filed an Errata Report against RFC8259, The JavaScript Object Notation, noting frequent references to "Unicode characters", when in fact the RFC formally specifies the use of Unicode code points.¶
Thanks are due to Asmus Freytag for careful review and many constructive suggestions aimed at making the language more consistent with the structure of the Unicode Standard.¶