Specifying Unicode Character Repertoires in RFCs

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Table of Contents

1. Introduction

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]. Because of the way the Unicode Standard defines the term "Unicode character", the "set of all Unicode characters" is not always useful for technical specifications. Instead, subsets such as those defined in this document are typically used.¶

Protocols and data formats usually need to describe exactly which selection of the available Unicode characters are to be used. The term "character repertoire" is a well-understood concept when 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.¶

2. Character Concepts

The Unicode Standard's definition of "Unicode character" is conceptual. However, each Unicode character is assigned an integer identifier in the range U+0000-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.¶

3. Subsets Defined in the Unicode Standard

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".¶

unicode-code-points =
   %x0-10FFFF

unicode-scalar-values =
   %x0-D7FF / %xE000-10FFFF  ; exclude surrogates

4. Other Definitions

This section lists other ways to specify subsets of the code points beyond those provided by the Unicode Standard itself. Specifications can refer to these repertoires by the names "XML Characters" and "Useful Assignables".¶

xml-chars =
   %x9 / %xA / %xD /   ; useful controls
   %x20-D7FF /         ; exclude surrogates
   %xE000-FFFD/        ; exclude FFFE and FFFF nonchars
   %x100000-10FFFF

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
   %x10000-1FFFD / %x20000-2FFFD / ; (repeat per plane)
   %x30000-3FFFD / %x40000-4FFFD /
   %x50000-5FFFD / %x60000-6FFFD /
   %x70000-7FFFD / %x80000-8FFFD /
   %x90000-9FFFD / %xA0000-AFFFD /
   %xB0000-BFFFD / %xC0000-CFFFD /
   %xD0000-DFFFD / %xE0000-EFFFD /
   %xF0000-FFFFD / %x100000-10FFFD

5. Refining Character Repertoires

Messages interchanged in Internet protocols of the type that IETF specifies are typically packaged into well-known data formats such as JSON, I-JSON, CBOR, YAML, and XML. These packaging formats typically have a default character repertoire. For example, JSON allows member names and string values to include any Unicode code points, including all the problematic types; the following is a legal JSON document:¶

{"example": "\u0000\U0089\uDEAD\u7FFFF"}

The value of the "example" field contains the C0 Control NUL, the C1 Control "CHARACTER TABULATION WITH JUSTIFICATION", an unpaired surrogate, and the noncharacter U+7FFFF. It is unlikely to be useful as the value of a text field. 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.¶

It is unlikely that anyone specifying a new data format would choose to allow this character repertoire.¶

A protocol based on JSON could be made more robust and implementor-friendly by requiring that the contents of member names and string values contain only Useful Assignables (see Section 4.2). An equivalent requirement is possible for other packaging formats such as I-JSON, XML, YAML, and CBOR.¶

6. IANA Considerations

This document makes no requests of IANA.¶

7. Security Considerations

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.¶

Acknowledgements

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.¶

Others who have contributed significantly to this include Rob Sayre and James Manger.¶

Authors' Addresses