Network Working Group J. Klensin Internet-Draft January 18, 2007 Expires: July 22, 2007 ASCII Escaping of Unicode Characters draft-klensin-unicode-escapes-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on July 22, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract There are a number of circumstances in which an escape mechanism is needed in conjunction with a protocol to encode characters that cannot by represented or transmitted directly. With ASCII coding the traditional escape has been either the decimal or hexadecimal offset 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 the options now in use and makes a proposal for general use in IETF protocols. Klensin Expires July 22, 2007 [Page 1] Internet-Draft Unicode Escapes January 2007 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Context and Background . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3. Discussion List . . . . . . . . . . . . . . . . . . . . . . 3 2. Proposal for a Standard Form . . . . . . . . . . . . . . . . . 4 3. Rationale and Other Alternatives . . . . . . . . . . . . . . . 4 3.1. Unicode Table Position versus UTF-8 Octets . . . . . . . . 4 3.2. Presentation Variants for Unicode Table Position . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Intellectual Property and Copyright Statements . . . . . . . . . . 8 Klensin Expires July 22, 2007 [Page 2] Internet-Draft Unicode Escapes January 2007 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 by represented or transmitted directly. With ASCII [ASCII] coding the traditional escape has been either the decimal or hexadecimal offset 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 extended ASCII, 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. In particular, we have seen fairly extensive use of both hexadecimal representations of the UTF-8 encoding [RFC3629] of a character and variations on the U+NNNN[N[N]] notation commonly used in conjunction with the Unicode Standard. This document proposes that a specific variation on the latter SHOULD be used in protocols unless other considerations apply and explains that choice. 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. 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]. 1.3. Discussion List Discussion of this document should be addressed to the discuss@apps.ietf.org mailing list. Klensin Expires July 22, 2007 [Page 3] Internet-Draft Unicode Escapes January 2007 2. Proposal for a Standard Form For the reasons discussed in the next section, the forms \UNNNNNNNN (for any Unicode character) and \uNNNN (for Unicode characters in plane 0) are generally preferred for use when an ASCII escape for embedded Unicode characters is needed in protocols. Specifically, in ABNF [RFC4234], EmbeddedUnicodeChar = BMP-form Full-form Hex-quad = 4*4 HexDigit BMP-form = "\u" Hex-quad Full-form = "\U" 2*2 Hex-quad HexDigit = "0" / "1" / "2"/ "3"/ "4"/ "5"/ "6"/ "7"/ "8"/ "9"/ "A"/ "B" / "C"/ "D"/ "E"/ "F" This form SHOULD be used in IETF protocols that require Unicode character escaping unless there are substantial reasons for using something else. For the convenience of the reader, it is generally preferred for documentation in IETF-related running text as well (e.g., in RFCs) although the U+NNNN form MAY be used when Unicode character encoding is clearly expected. 3. Rationale and Other Alternatives There are many different ways to designate, encode, or call out a Unicode character. Given adequate decoding facilities, all of these other than the formal character name are equivalent. However, when information about characters is to be processed by people, information about the Unicode code point is preferable to a further encoding of the encoded form of the character and it is desirable to reduce confusion by designating one form as preferable. These issues are discussed in the following subsections. 3.1. Unicode Table Position versus UTF-8 Octets There are two major families of ways to represent Unicode characters. One uses the code point position in the table in some representation (see the next section), the other encodes the octets of the UTF-8 encoding. 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 point forms be used rather than the UTF-8 ones. 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 Klensin Expires July 22, 2007 [Page 4] Internet-Draft Unicode Escapes January 2007 presentation form should reference the Unicode tables for characters and to do so as simply as possible. o The nature of UTF-8 implies that a decimal or hexadecimal numeral representation of UTF-8 requires conversion to the UTF-8 form, then conversion from the UTF-8 form to a Unicode character position form in order to look the character up 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 character code position form is generally more compact than forms based on coding UTF-8 octets, sometimes much more compact. The same considerations that apply to encoding of UTF-8 octets also apply to more compact ACE encodings such as the "bootstring" encoding [RFC3492] with or without its "Punycode" profile. 3.2. Presentation Variants for Unicode Table Position There are a number of different ways to represent a Unicode code point position. The forms suggested here -- "\U" followed by eight hexadecimal efforts for general use and, optionally, "\u" followed by four hexadecimal digits for references to Unicode Plane 0 (the "BMP") -- were chosen because of their use in several programming languages, notably the "new character" extensions to ISO Standard C [ISO-C-Chars]. Other forms that were considered, and that may sometimes be encountered and justified, include: o Perl uses the form \x(NNN...). 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'NNN...' 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. o Java uses the form \uNNNN, but can represent characters outside Plane 0 (i.e., above U+FFFF) only by the use of surrogate pairs. Decoding (or de-mapping) surrogates raises some of the same issues as the use of UTF-8 octets discussed above. Codings that depend on surrogates SHOULD NOT be used. For characters in Plane 0, the Java form is identical to the recommended Plane 0-only form recommended above. o HTML and XML use the form &#xNNNN;. Like the Perl form, this form has a clear terminator, reducing ambiguity. However, it is generally considered ugly and awkward outside of its native HTML, Klensin Expires July 22, 2007 [Page 5] Internet-Draft Unicode Escapes January 2007 XML, and similar contexts. There is one significant disadvantage of the recommended form. The use of a case variation (between "u" for the four digit form and "U" for the six digit form) may not seem natural in environments in which upper and lower case 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). There appears to be consensus that existing standards and wide current use outweigh that objection. 4. Security Considerations This document proposes a specific mechanism for encoding Unicode characters when other considerations do not apply. Since the encoding is 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 mechanism 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. 5. 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. 6. References 6.1. 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. Klensin Expires July 22, 2007 [Page 6] Internet-Draft Unicode Escapes January 2007 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [Unicode] The Unicode Consortium, "The Unicode Standard, Version 5.0", 2006. (Addison-Wesley, 2006. ISBN 0-321-48091-0). 6.2. 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-Chars] International Organization for Standardization, "Information technology -- Programming languages, their environments and system software inferfaces -- Extensions for the programming language C to support new character data types", ISO/IEC TR 19769:2004, July 2004. [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. Author's Address John C Klensin 1770 Massachusetts Ave, #322 Cambridge, MA 02140 USA Phone: +1 617 245 1457 Email: john-ietf@jck.com Klensin Expires July 22, 2007 [Page 7] Internet-Draft Unicode Escapes January 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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