wdiff rfc9839.original rfc9839.txt

Network Working Group

Internet Engineering Task Force (IETF) T. Bray
Internet-Draft
Request for Comments: 9839 Textuality Services
Intended status:
Category: Standards Track P. Hoffman
Expires: 28 November 2025
ISSN: 2070-1721 ICANN
27 May
August 2025

Unicode Character Repertoire Subsets
draft-bray-unichars-15

Abstract

This document discusses subsets of the Unicode character repertoire
for use in protocols and data formats, formats and specifies three subsets
recommended for use in IETF specifications.

Status of This Memo

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This Internet-Draft will expire on 28 November 2025.
https://www.rfc-editor.org/info/rfc9839.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Characters and Code Points . . . . . . . . . . . . . . . . . 3
2.1. Encoding forms . . . . . . . . . . . . . . . . . . . . . 4 Forms
2.2. Problematic Code Points . . . . . . . . . . . . . . . . . 4
2.2.1. Surrogates . . . . . . . . . . . . . . . . . . . . . 5
2.2.2. Control Codes . . . . . . . . . . . . . . . . . . . . 5
2.2.3. Noncharacters . . . . . . . . . . . . . . . . . . . . 5
3. Dealing With with Problematic Code Points . . . . . . . . . . . . 6
4. Subsets . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Unicode Scalars . . . . . . . . . . . . . . . . . . . . . 7
4.2. XML Characters . . . . . . . . . . . . . . . . . . . . . 7
4.3. Unicode Assignables . . . . . . . . . . . . . . . . . . . 8
5. Using Subsets . . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References
8.1. Normative References . . . . . . . . . . . . . . . . . . . . 9
9.
8.2. Informative References . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

Protocols and data formats frequently contain or are made up of
textual data. Such text is normally composed of Unicode [UNICODE]
characters, to support use by speakers of many languages. Unicode
characters are represented by numeric code points, and the "set of
all Unicode code points" is generally not a good choice for use in
text fields. Unicode recognizes different types of code points, not
all of which are appropriate in protocols, protocols or even associated with
characters. Therefore, even if the desire is to support "all Unicode
characters"
characters", a subset of the Unicode code point repertoire should be
specified. Subsets such as those discussed in this document are
appropriate choices when more-specific limitations do not apply.

In this document, "subset" means a subset of the Unicode character
repertoire. This document specifies subsets that exclude some or all
of the code points that are "problematic" as defined in Section 2.2.
Authors should have a way to concisely and exactly reference a stable
specification that identifies which subset a protocol or data format
accepts.

This document discusses issues that apply in choosing subsets, names
two subsets that have been popular in practice, and suggests one new
subset. The intended use is to serve as a convenient target for
cross-reference from other specifications whose authors wish to
exclude problematic code points from the data format or protocol
being specified.

Note that this document only provides guidance on avoiding the use of
code points which that cannot be used for interoperable interchange of
Unicode textual data. Dealing with strings, particularly in the
context of user interfaces, requires addressing language, text
rendering direction, alternate representations of the same abstract
character, and so on. These issues, among many others, led to many
efforts by the Unicode Consortium, IETF efforts like by the IETF such as [IDN]
and [PRECIS], and W3C internationalization efforts by W3C such as
[W3C-CHAR]. The results of these efforts should be consulted by
anyone engaging in such work.

1.1. Notation

In this document, the numeric values assigned to Unicode characters
are provided in hexadecimal. This document uses Unicode's standard
notation of "U+" followed by four or more hexadecimal digits. For
example, "A", decimal 65, is expressed as U+0041, and "🖤" (Black
Heart), decimal 128,420, is U+1F5A4.

Groups of numeric values described in Section 4 are given in ABNF
[RFC5234]. In ABNF, hexadecimal values are preceded by "%x" rather
than "U+".

All the numeric ranges in this document are inclusive.

The subsets are described in ABNF.

2. Characters and Code Points

Definition D9 in section Section 3.4 of [UNICODE] defines "Unicode codespace"
as "a range of integers from 0 to 10FFFF_16". Definition D10 defines
"code point" as "Any value in the Unicode codespace".

The Unicode Standard's definition of "Unicode character" is
conceptual. However, each Unicode character is assigned a code
point, used to represent the characters in computer memory and
storage systems and, in specifications, and to specify allowed subsets. subsets in specifications.

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
unassigned code points regularly become assigned when new characters
are added to Unicode, it is usually not a good practice to specify
that unassigned code points should be avoided.

2.1. Encoding forms Forms

Unicode describes a variety of encoding forms, ways to marshal code
points into byte sequences. A survey of these is beyond the scope of
this document. However, it is useful to note that "UTF-16"
represents each code point with one or two 16-bit chunks, while "UTF-
8" uses variable-length byte sequences [RFC3629].

The "IETF Policy on Character Sets and Languages", BCP 18 [RFC2277],
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 single encoding form. UTF-8 is widely used for interoperable data
formats such as JSON, YAML, CBOR, and XML.

2.2. Problematic Code Points

This section classifies as "problematic" all the code points which that can never represent
useful text and and, in some cases cases, can lead to software misbehavior. misbehavior as
"problematic". This is a low bar; the PRECIS [RFC8264] framework's
"IdentifierClass" and "FreeformClass" exclude many more code points which
that can cause problems when displayed to humans, in some cases
presenting security risks. Specifications of fields in protocols and
data formats whose contents are designed for display to and
interactions with humans would benefit from careful consideration of
the issues described by PRECIS; its more-restrictive subsets might be
better choices than those specified in this document.

Definition D10a in section Section 3.4 of [UNICODE] defines seven code point
types. Three types of code points are assigned to entities which that are
not actually characters or whose value as Unicode characters in text
fields is questionable: "Surrogate", "Control", and "Noncharacter".
In this document, "problematic" refers to code points whose type is
"Surrogate" or "Noncharacter", "Noncharacter" and to "legacy controls" as defined in
Section 2.2.2.2 below.

Unicode's definition

Definition D49 in [UNICODE] concerns the "private-use" type type, and section
Section 3.5.10 states that they "are considered to be assigned
characters". Section 23.5 further states that these characters' "use
may be determined by private agreement among cooperating users".
Because private-use code points may have uses based on private
agreements, this document does not classify them as "problematic".

2.2.1. Surrogates

A total of 2,048 code points, in the range U+D800-U+DFFF, is are divided
into two blocks called "high surrogates" and "low surrogates";
collectively
collectively, the 2,048 code points are referred to as "surrogates".
[UNICODE] section
Section 23.6 of [UNICODE] specifies how surrogates may 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 that occurs in text encoded in any encoding form other
than UTF-16 has no meaning. In particular, [UNICODE] section Section 3.9.3 of
[UNICODE] forbids representing a surrogate in UTF-8.

2.2.2. Control Codes

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".

2.2.2.1. Useful Controls

The C0 controls include newline (U+000A), carriage return (U+000D),
and tab (U+0009); this document refers to these three characters as
the "useful controls".

2.2.2.2. Legacy Controls

Aside from the useful controls, both the C0 and C1 control codes are
mostly obsolete and generally lack interoperable semantics. This
document uses the phrase "legacy controls" to describe control codes
that are not useful controls.

Because 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.

2.2.3. Noncharacters

Certain code points are classified as "noncharacters", and [UNICODE]
asserts repeatedly that they are not designed or used for open
interchange.

Code points are organized into 17 "planes", each containing 2^16 code
points. The last two code points in each plane are noncharacters:
U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, U+2FFFE, U+2FFFF, and so on, up to
U+10FFFE, U+10FFFF.

The code points in the range U+FDD0-U+FDEF are noncharacters.

3. Dealing With with Problematic Code Points

[RFC9413], "Maintaining Robust Protocols", provides a thorough
discussion of strategies for dealing with issues in input data.

Different types of problematic code points cause different issues.
Noncharacters and legacy controls are unlikely to cause software
failures, but they cannot usefully be displayed to humans, and they
can be used in attacks based on attempting to display text that
includes them.

The behavior of software which that encounters surrogates is unpredictable
and differs among programming-language implementations, even between
different API calls in the same language.

Section 3.9 of [UNICODE] makes it clear that a UTF-8 byte sequence
which
that would map to a surrogate is ill-formed. If a specification
requires that input data be encoded with UTF-8, and if all input were
well-formed, implementors would never have to concern themselves with
surrogates.

Unfortunately, industry experience teaches that problematic code
points, including surrogates, can and do occur in program input where
the source of input data is not controlled by the implementor. In
particular, the specification of JSON allows any code point to appear
in object member names and string values [RFC8259].

For example, the following is a conforming JSON text:

{"example": "\u0000\u0089\uDEAD\uD9BF\uDFFF"}

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 encoded per JSON rules as two
escaped UTF-16 surrogate code points as described in [RFC8259]
section 7. Section 7 of
[RFC8259]. It is unlikely to be useful as the value of a text field.
That value cannot be serialized into well-formed UTF-8, but the
behavior of libraries asked to parse the sample is unpredictable;
some will silently parse this and generate an ill-formed UTF-8
string.

Two reasonable options for dealing with problematic input are either
rejecting text containing problematic code points, points or replacing the
problematic code points with placeholders.

Silently deleting an ill-formed part of a string is a known security
risk. Responding to that risk, [UNICODE] section Section 3.2 of [UNICODE] recommends
dealing with ill-formed byte sequences by signaling an error, error or
replacing problematic code points, ideally with "�" (U+FFFD,
REPLACEMENT CHARACTER).

4. Subsets

This section describes three increasingly restrictive subsets that
can be used in specifying acceptable content for text fields in
protocols and data types. Specifications can refer to these subsets
by the names "Unicode Scalars", "XML Characters", and "Unicode
Assignables".

4.1. Unicode Scalars

Definition D76 in section Section 3.9 of [UNICODE] defines the term "Unicode
scalar value" as "Any Unicode code point except high-surrogate and
low-surrogate code points." points".

The "Unicode Scalars" subset can be expressed as an ABNF production:

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

This subset is the default for CBOR [RFC8949], Concise Binary Object Representation
(CBOR) [RFC8949] and has the advantage of excluding surrogates.
However, it includes legacy controls and noncharacters.

4.2. XML Characters

The XML 1.0 Specification [XML], in its grammar production labeled
"Char", specifies a subset of Unicode code points that excludes
surrogates, legacy C0 controls, and the noncharacters U+FFFE and
U+FFFF.

The "XML Characters" subset can be expressed as an ABNF production:

xml-character =
%x9 / %xA / %xD / ; useful controls
%x20-D7FF / ; exclude surrogates
%xE000-FFFD / ; exclude FFFE and FFFF nonchars
%x100000-10FFFF
%x10000-10FFFF

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, 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.

4.3. Unicode Assignables

This document defines the "Unicode Assignables" subset as all the
Unicode code points that are not problematic. This, a proper subset
of each of the others, comprises all code points that are currently
assigned, excluding legacy control codes, or that might in future be
assigned. assigned
in the future.

Unicode Assignables can be expressed as an ABNF production:

unicode-assignable =
%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. Using Subsets

Many IETF specifications rely on well-known data formats such as
JSON, I-JSON, Internet JSON (I-JSON), CBOR, YAML, and XML. These formats
specify default subsets. For example, JSON allows object member
names and string values to include any Unicode code point, including
all the problematic types.

A protocol based on JSON can be made more robust and implementor-
friendly by restricting the contents of object member names and
string values to one of the subsets described in Section 4.
Equivalent restrictions are possible for other packaging formats such
as I-JSON, XML, YAML, and CBOR.

Note that escaping techniques such as those in the JSON example in
Section 3 cannot be used to circumvent this sort of restriction,
which applies to data content, not textual representation in
packaging formats. If a specification restricted a JSON field value
to the Unicode Assignables, the example would remain a conforming
JSON Text text but the data it represents would not constitute Unicode
Assignable code points.

6. IANA Considerations

This document has no actions for IANA. IANA actions.

7. Security Considerations

Section 3 of this document discusses security issues.

Unicode Security Considerations [TR36] is a wide-ranging survey of
the issues implementors should consider while writing software to
process Unicode text. Unicode Source Code Handling [TR55] discusses
use of Unicode in programming languages, with a focus on security
issues. Many of the attacks they discuss are aimed at deceiving
human readers, but vulnerabilities involving issues such as
surrogates and noncharacters are also covered, and covered and, in fact fact, can
contribute to human-deceiving exploits.

The Security Considerations security considerations in Section 12 of [RFC8264] generally
applies
apply to this document as well.

Note that the Unicode-character subsets specified in this document
are increasingly restrictive, omitting more and more problematic code
points, and thus should be less and less susceptible to many of these
exploits. The subset in Section 4.3 subset, 4.3, "Unicode Assignables", excludes
all of these code points.

8. References

8.1. Normative References

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[TR36] The Unicode Consortium, Davis, M., Ed. and M. Suignard, Ed., "Unicode Security
Considerations", <https://www.unicode.org/reports/tr36/>.
Note that this reference is to the latest version of this
document, rather than to a specific release. It is not
expected that future updates will affect the referenced
discussions.

[TR55] The Unicode Consortium, Leroy, R., Ed. and M. Davis, Ed., "Unicode Source Code
Handling", <https://www.unicode.org/reports/tr55/>. Note
that this reference is to the latest version of this
document, rather than to a specific release. It is not
expected that future updates will affect the referenced
discussions.

[UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>. Note that this
reference is to the latest version of Unicode, rather than
to a specific release. It is not expected that future
changes in the Unicode Standard will affect the referenced
definitions.

8.2. Informative References

[IDN] "Internationalized Domain Name Working Group",
<https://datatracker.ietf.org/group/idn/>.

[PRECIS] "PRECIS Working Group",
<https://datatracker.ietf.org/group/precis/>.

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
January 1998, <https://www.rfc-editor.org/info/rfc2277>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.

[RFC8264] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.

[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.

[RFC9413] Thomson, M. and D. Schinazi, "Maintaining Robust
Protocols", RFC 9413, DOI 10.17487/RFC9413, June 2023,
<https://www.rfc-editor.org/info/rfc9413>.

[W3C-CHAR] W3C, "Character encodings: Essential concepts",
<https://www.w3.org/International/articles/definitions-
characters/>.

[XML] Bray, T., Ed., Paoli, J., Ed., McQueen, C.M., Ed., Maler,
E., Ed., and F. Yergeau, Ed., "Extensible Markup Language
(XML) 1.0 (Fifth Edition)", W3C Recommendation, 26
November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126/>. Note that
this reference is to a specific release, based on a
history of previous "Edition" releases having changed this
production.

Acknowledgements

Thanks are due to Guillaume Fortin-Debigaré, who filed an Errata
Report errata
report against RFC 8259, The "The JavaScript Object Notation, Notation (JSON) Data
Interchange Format", noting frequent references to "Unicode
characters", when in fact the RFC formally specifies the use of
Unicode Code Points. code points.

Thanks also to Asmus Freytag for careful review and many constructive
suggestions aimed at making the language more consistent with the
structure of the Unicode Standard.

Thanks also to James Manger for the correctness of the ABNF and JSON
samples.

Thanks also to Addison Phillips and the W3C Internationalization
Working Group for helpful suggestions on language and references.

Thoughtful comments during the many iterations draft versions of this draft, document,
which helped tighten up wording and make difficult points clearer,
were contributed by Harald Alvestrand, Martin J J. Dürst, Donald
E. Eastlake, John Klensin, Barry Leiba, Glyn Normington, Peter Saint-
Andre, and Rob Sayre.

Authors' Addresses

Tim Bray
Textuality Services
Email: tbray@textuality.com

Paul Hoffman
ICANN
Email: paul.hoffman@icann.org