Network Working Group J. Klensin

Network Working Group J. Klensin
Request for Comments: 5137 February 2008
BCP: 137
Category: Best Current Practice

ASCII Escaping of Unicode Characters

Status of This Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Abstract

There are a number of circumstances in which an escape mechanism is
needed in conjunction with a protocol to encode characters that
cannot be represented or transmitted directly. With ASCII coding,
the traditional escape has been either the decimal or hexadecimal
numeric value of the character, written in a variety of different
ways. The move to Unicode, where characters occupy two or more
octets and may be coded in several different forms, has further
complicated the question of escapes. This document discusses some
options now in use and discusses considerations for selecting one for
use in new IETF protocols, and protocols that are now being
internationalized.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Context and Background . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Discussion List . . . . . . . . . . . . . . . . . . . . . 4
2. Encodings that Represent Unicode Code Points: Code
Position versus UTF-8 or UTF-16 Octets . . . . . . . . . . . . 4
3. Referring to Unicode Characters . . . . . . . . . . . . . . . 5
4. Syntax for Code Point Escapes . . . . . . . . . . . . . . . . 6
5. Recommended Presentation Variants for Unicode Code Point
Escapes . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Backslash-U with Delimiters . . . . . . . . . . . . . . . 7
5.2. XML and HTML . . . . . . . . . . . . . . . . . . . . . . . 7
6. Forms that Are Normally Not Recommended . . . . . . . . . . . 8
6.1. The C Programming Language: Backslash-U . . . . . . . . . 8
6.2. Perl: A Hexadecimal String . . . . . . . . . . . . . . . . 8
6.3. Java: Escaped UTF-16 . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Appendix A. Formal Syntax for Forms Not Recommended . . . . . . . 12
A.1. The C Programming Language Form . . . . . . . . . . . . . 12
A.2. Perl Form . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Java Form . . . . . . . . . . . . . . . . . . . . . . . . 12

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1. Introduction

1.1. Context and Background

There are a number of circumstances in which an escape mechanism is
needed in conjunction with a protocol to encode characters that
cannot be represented or transmitted directly. With ASCII [ASCII]
coding, the traditional escape has been either the decimal or
hexadecimal numeric value of the character, written in a variety of
different ways. For example, in different contexts, we have seen
%dNN or %NN for the decimal form, %NN, %xNN, X'nn', and %X'NN' for
the hexadecimal form. "%NN" has become popular in recent years to
represent a hexadecimal value without further qualification, perhaps
as a consequence of its use in URLs and their prevalence. There are
even some applications around in which octal forms are used and,
while they do not generalize well, the MIME Quoted-Printable and
Encoded-word forms can be thought of as yet another set of escapes.
So, even for the fairly simple cases of ASCII and standard built by
extending ASCII, such as the ISO 8859 family, we have been living
with several different escaping forms, each the result of some
history.

When one moves to Unicode [Unicode] [ISO10646], where characters
occupy two or more octets and may be coded in several different
forms, the question of escapes becomes even more complicated.
Unicode represents characters as code points: numeric values from 0
to hex 10FFFF. When referencing code points in flowing text, they
are represented using the so-called "U+" notation, as values from
U+0000 to U+10FFFF. When serialized into octets, these code points
can be represented in different forms:

o in UTF-8 with one to four octets [RFC3629]

o in UTF-16 with two or four octets (or one or two seizets -- 16-bit
units)

o in UTF-32 with exactly four octets (or one 32-bit unit)

When escaping characters, we have seen fairly extensive use of
hexadecimal representations of both the serialized forms and
variations on the U+ notation, known as code point escapes.

In accordance with existing best-practices recommendations [RFC2277],
new protocols that are required to carry textual content for human
use SHOULD be designed in such a way that the full repertoire of
Unicode characters may be represented in that text.

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This document proposes that existing protocols being
internationalized, and those that need an escape mechanism, SHOULD
use some contextually appropriate variation on references to code
points as described in Section 2 unless other considerations outweigh
those described here.

This recommendation is not applicable to protocols that already
accept native UTF-8 or some other encoding of Unicode. In general,
when protocols are internationalized, it is preferable to accept
those forms rather than using escapes. This recommendation applies
to cases, including transition arrangements, in which that is not
practical.

In addition to the protocol contexts addressed in this specification,
escapes to represent Unicode characters also appear in presentations
to users, i.e., in user interfaces (UI). The formats specified in,
and the reasoning of, this document may be applicable in UI contexts
as well, but this is not a proposal to standardize UI or presentation
forms.

This document does not make general recommendations for processing
Unicode strings or for their contents. It assumes that the strings
that one might want to escape are valid and reasonable and that the
definition of "valid and reasonable" is the province of other
documents. Recommendations about general treatment of Unicode
strings may be found in many places, including the Unicode Standard
itself and the W3C Character Model [W3C-CharMod], as well as specific
rules in individual protocols.

1.2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

Additional Unicode-specific terminology appears in [UnicodeGlossary],
but is not necessary for understanding this specification.

1.3. Discussion List

Discussion of this document should be addressed to the
[email protected] mailing list.

2. Encodings that Represent Unicode Code Points: Code Position versus
UTF-8 or UTF-16 Octets

There are two major families of ways to escape Unicode characters.
One uses the code point in some representation (see the next

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section), the other encodes the octets of the UTF-8 encoding or some
other encoding in some representation. Some other options are
possible, but they have been rare in practice. This specification
recommends that, in the absence of compelling reasons to do
otherwise, the Unicode code points SHOULD be used rather than a
representation of UTF-8 (or UTF-16) octets. There are several
reasons for this, including:

o One reason for the success of many IETF protocols is that they use
human-interpretable text forms to communicate, rather than
encodings that generally require computer programs (or hand
simulation of algorithms) to decode. This suggests that the
presentation form should reference the Unicode tables for
characters and to do so as simply as possible.

o Because of the nature of UTF-8, for a human to interpret a decimal
or hexadecimal numeral representation of UTF-8 octets requires one
or more decoding steps to determine a Unicode code point that can
used to look up the character in a table. That may be appropriate
in some cases where the goal is really to represent the UTF-8 form
but, in general, it just obscures desired information and makes
errors more likely and debugging harder.

o Except for characters in the ASCII subset of Unicode (U+0000
through U+007F), the code point form is generally more compact
than forms based on coding UTF-8 octets, sometimes much more
compact.

The same considerations that apply to representation of the octets of
UTF-8 encoding also apply to more compact ACE encodings such as the
"bootstring" encoding [RFC3492] with or without its "Punycode"
profile.

Similar considerations apply to UTF-16 encoding, such as the \uNNNN
form used in Java (See Section 6.3). While those forms are
equivalent to code point references for the Basic Multilingual Plane
(BMP, Plane 0), a two-stage decoding process is needed to handle
surrogates to access higher planes.

3. Referring to Unicode Characters

Regardless of what decisions are made about escapes for Unicode
characters in protocol or similar contexts, text referring to a
Unicode code point SHOULD use the U+NNNN[N[N]] syntax, as specified
in the Unicode Standard, where the NNNN... string consists of
hexadecimal numbers. Text actually containing a Unicode character
SHOULD use a syntax more suitable for automated processing.

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4. Syntax for Code Point Escapes

There are many options for code point escapes, some of which are
summarized below. All are equivalent in content and semantics -- the
differences lie in syntax. The best choice of syntax for a
particular protocol or other application depends on that application:
one form may simply "fit" better in a given context than others. It
is clear, however, that hexadecimal values are preferable to other
alternatives: Systems based on decimal or octal offsets SHOULD NOT be
used.

Since this specification does not recommend one specific syntax,
protocol specifications that use escapes MUST define the syntax they
are using, including any necessary escapes to permit the escape
sequence to be used literally.

The application designer selecting a format should consider at least
the following factors:

o If similar or related protocols already use one form, it may be
best to select that form for consistency and predictability.

o A Unicode code point can fall in the range from U+0000 to
U+10FFFF. Different escape systems may use four, five, six, or
eight hexadecimal digits. To avoid clever syntax tricks and the
consequent risk of confusion and errors, forms that use explicit
string delimiters are generally preferred over other alternatives.
In many contexts, symmetric paired delimiters are easier to
recognize and understand than visually unrelated ones.

o Syntax forms starting in "\u", without explicit delimiters, have
been used in several different escape systems, including the four
or eight digit syntax of C [ISO-C] (see Section 6.1), the UTF-16
encoding of Java [Java] (see Section 6.3), and some arrangements
that may follow the "\u" with four, five, or six digits. The
possible confusion about which option is actually being used may
argue against use of any of these forms.

o Forms that require decoding surrogate pairs share most of the
problems that appear with encoding of UTF-8 octets. Internet
protocols SHOULD NOT use surrogate pairs.

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5. Recommended Presentation Variants for Unicode Code Point Escapes

There are a number of different ways to represent a Unicode code
point position. No one of them appears to be "best" for all
contexts. In addition, when an escape is needed for the escape
mechanism itself, the optimal one of those might differ from one
context to another.

Some forms that are in popular use and that might reasonably be
considered for use in a given protocol are described below and
identified with a current-use context when feasible. The two in this
section are recommended for use in Internet Protocols. Other popular
ones appear in Section 6 with some discussion of their disadvantages.

5.1. Backslash-U with Delimiters

One of the recommended forms is a variation of the many forms that
start in "\u" (See, e.g., Section 6.1, below>), but uses explicit
delimiters for the reasons discussed elsewhere.

Specifically, in ABNF [RFC5234],

EmbeddedUnicodeChar = %x5C.75.27 4*6HEXDIG %x27
; starting with lowercase "\u" and "'" and ending with "'".
; Note that the encodings are considered to be abstractions
; for the relevant characters, not designations of specific
; octets.

HEXDIG = "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" /
"A" / "B" / "C" / "D" / "E" / "F"
; effectively identical with definition in RFC 5234.

Protocol designers of applications using this form should specify a
way to escape the introducing backslash ("\"), if needed. "\\" is one
obvious possibility, but not the only one.

5.2. XML and HTML

The other recommended form is the one used in XML. It uses the form
"&#xNNNN;". Like the Perl form (Section 6.2), this form has a clear
ending delimiter, reducing ambiguity. HTML uses a similar form, but
the semicolon may be omitted in some cases. If that is done, the
advantages of the delimiter disappear so that the HTML form without
the semicolon SHOULD NOT be used. However, this format is often
considered ugly and awkward outside of its native HTML, XML, and
similar contexts.

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In ABNF:

EmbeddedUnicodeChar = %x26.23.78 2*6HEXDIG %x3B
; starts with "&#x" and ends with ";"

Note that a literal "&" can be expressed by "&" when using this
style.

6. Forms that Are Normally Not Recommended

6.1. The C Programming Language: Backslash-U

The forms

\UNNNNNNNN (for any Unicode character) and

\uNNNN (for Unicode characters in plane 0)

are utilized in the C Programming Language [ISO-C] when an ASCII
escape for embedded Unicode characters is needed.

There are disadvantages of this form that may be significant. First,
the use of a case variation (between "u" for the four-digit form and
"U" for the eight-digit form) may not seem natural in environments
where uppercase and lowercase characters are generally considered
equivalent and might be confusing to people who are not very familiar
with Latin-based alphabets (although those people might have even
more trouble reading relevant English text and explanations).
Second, as discussed in Section 4, the very fact that there are
several different conventions that start in \u or \U may become a
source of confusion as people make incorrect assumptions about what
they are looking at.

6.2. Perl: A Hexadecimal String

Perl uses the form \x{NNNN...}. The advantage of this form is that
there are explicit delimiters, resolving the issue of having
variable-length strings or using the case-change mechanism of the
proposed form to distinguish between Plane 0 and more general forms.
Some other programming languages would tend to favor X'NNNN...' forms
for hexadecimal strings and perhaps U'NNNN...' for Unicode-specific
strings, but those forms do not seem to be in use around the IETF.

Note that there is a possible ambiguity in how two-character or low-
numbered sequences in this notation are understood, i.e., that octets
in the range \x(00) through \x(FF) may be construed as being in the
local character set, not as Unicode code points. Because of this
apparent ambiguity, and because IETF documents do not contain

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provision for pragmas (see [PERLUniIntro] for more information about
the "encoding" pragma in Perl and other details), the Perl forms
should be used with extreme caution, if at all.

6.3. Java: Escaped UTF-16

Java [Java] uses the form \uNNNN, but as a reference to UTF-16
values, not to Unicode code points. While it uses a syntax similar
to that described in Section 6.1, this relationship to UTF-16 makes
it, in many respects, more similar to the encodings of UTF-8
discussed above than to an escape that designates Unicode code
points. Note that the UTF-16 form, and hence, the Java escape
notation, can represent characters outside Plane 0 (i.e., above
U+FFFF) only by the use of surrogate pairs, raising some of the same
issues as the use of UTF-8 octets discussed above. For characters in
Plane 0, the Java form is indistinguishable from the Plane 0-only
form described in Section 6.1. If only for that reason, it SHOULD
NOT be used as an escape except in those Java contexts in which it is
natural.

7. Security Considerations

This document proposes a set of rules for encoding Unicode characters
when other considerations do not apply. Since all of the recommended
encodings are unambiguous and normalization issues are not involved,
it should not introduce any security issues that are not present as a
result of simple use of non-ASCII characters, no matter how they are
encoded. The mechanisms suggested should slightly lower the risks of
confusing users with encoded characters by making the identity of the
characters being used somewhat more obvious than some of the
alternatives.

An escape mechanism such as the one specified in this document can
allow characters to be represented in more than one way. Where
software interprets the escaped form, there is a risk that security
checks, and any necessary checks for, e.g., minimal or normalized
forms, are done at the wrong point.

8. Acknowledgments

This document was produced in response to a series of discussions
within the IETF Applications Area and as part of work on email
internationalization and internationalized domain name updates. It
is a synthesis of a large number of discussions, the comments of the
participants in which are gratefully acknowledged. The help of Mark
Davis in constructing a list of alternative presentations and
selecting among them was especially important.

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Tim Bray, Peter Constable, Stephane Bortzmeyer, Chris Newman, Frank
Ellermann, Clive D.W. Feather, Philip Guenther, Bjoern Hoehrmann,
Simon Josefsson, Bill McQuillan, der Mouse, Phil Pennock, and Julian
Reschke provided careful reading and some corrections and suggestions
on the various working drafts that preceded this document. Taken
together, their suggestions motivated the significant revision of
this document and its recommendations between version -00 and version
-01 and further improvements in the subsequent versions.

9. References

9.1. Normative References

[ISO10646] International Organization for Standardization,
"Information Technology -- Universal Multiple-
Octet Coded Character Set (UCS)", ISO/
IEC 10646:2003, December 2003.

[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of
ISO 10646", STD 63, RFC 3629, November 2003.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.

[Unicode] The Unicode Consortium, "The Unicode Standard,
Version 5.0", 2006.
(Addison-Wesley, 2006. ISBN 0-321-48091-0).

9.2. Informative References

[ASCII] American National Standards Institute (formerly
United States of America Standards Institute),
"USA Code for Information Interchange", ANSI X3.4-
1968, 1968.

ANSI X3.4-1968 has been replaced by newer versions
with slight modifications, but the 1968 version
remains definitive for the Internet.

[ISO-C] International Organization for Standardization,
"Information technology -- Programming languages
-- C", ISO/IEC 9899:1999, 1999.

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[Java] Sun Microsystems, Inc., "Java Language
Specification, Third Edition", 2005, <http://
java.sun.com/docs/books/jls/third_edition/html/
lexical.html#95413p>.

[PERLUniIntro] Hietaniemi, J., "perluniintro", Perl
documentation 5.8.8, 2002,
<http://perldoc.perl.org/perluniintro.html>.

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.

[RFC3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for Internationalized Domain Names in
Applications (IDNA)", RFC 3492, March 2003.

[UnicodeGlossary] The Unicode Consortium, "Glossary of Unicode
Terms", June 2007,
<http://www.unicode.org/glossary>.

[W3C-CharMod] Duerst, M., "Character Model for the World Wide
Web 1.0", W3C Recommendation, February 2005,
<http://www.w3.org/TR/charmod/>.

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Appendix A. Formal Syntax for Forms Not Recommended

While the syntax for the escape forms that are not recommended above
(see Section 6) are not given inline in the hope of discouraging
their use, they are provided in this appendix in the hope that those
who choose to use them will do so consistently. The reader is
cautioned that some of these forms are not defined precisely in the
original specifications and that others have evolved over time in
ways that are not precisely consistent. Consequently, these
definitions are not normative and may not even precisely match
reasonable interpretations of their sources.

The definition of "HEXDIG" for the forms that follow appears in
Section 5.1.

A.1. The C Programming Language Form

Specifically, in ABNF [RFC5234],

EmbeddedUnicodeChar = BMP-form / Full-form

BMP-form = %x5C.75 4HEXDIG ; starting with lowercase "\u"
; The encodings are considered to be abstractions for the
; relevant characters, not designations of specific octets.

Full-form = %x5C.55 8HEXDIG ; starting with uppercase "\U"

A.2. Perl Form

EmbeddedUnicodeChar = %x5C.78 "{" 2*6HEXDIG "}" ; starts with "\x"

A.3. Java Form

EmbeddedUnicodeChar = %x5C.7A 4HEXDIG ; starts with "\u"

Author's Address

John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA

Phone: +1 617 245 1457
EMail: [email protected]

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