INTERNET-DRAFT Martin Duerst draft-ietf-idn-uri-01 W3C/Keio University Expires May 2002 November 20, 2001 Internationalized Domain Names in URIs and IRIs Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Abstract This document proposes to upgrade the definitions of URIs [RFC 2396] and IRIs (Internationalized Resource Identifiers, [IRI]) to work consistently with internationalized domain names. 0. Change Log 0.1 Changes from -00 to -01 - Changed requirement for URI/IRI resolvers from MUST to SHOULD - Changed IRI syntax slightly (ichar -> idchar, based on changes in [IRI]) - Various wording changes 1. Introduction Internet domain names serve to identify hosts and services on the Internet in a convenient way. The IETF IDN working group is currently working on extending the character repertoire usable in domain names beyond a subset of US-ASCII. One of the most important places where domain names appear are Uniform Resource Identifiers (URIs, [RFC 2396], as modified by [RFC2732]). However, in the current definition of the generic URI syntax, the restrictions on domain names are 'hard-coded'. In Section 2, this document relaxes these restrictions by updating the syntax, and defines how internationalized domain names are encoded in URIs. URIs are restricted to a subset of US-ASCII. However, IRIs (Internationalized Resource Identifier [IRI]) in general allow non-ASCII characters. But the syntax of IRIs has the same 'hard-coded' restrictions on domain names as the syntax of URIs. In Section 3, this document relaxes these restrictions by updating the IRI syntax. This is done in a way that is compatible with the new syntax for URIs. This means that encoding an internationalized domain name in an URI and encoding the same domain name in an IRI will produce an URI and an IRI that can be converted into each other using the procedures defined in [IRI] for these conversions. 2. URI syntax changes The syntax of URIs [RFC2326] currently contains the following rules relevant to domain names: hostname = *( domainlabel "." ) toplabel [ "." ] domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum toplabel = alpha | alpha *( alphanum | "-" ) alphanum The later two rules are changed as follows: domainlabel = escalphanum | escalphanum *( escalphanum | "-" ) escalphanum toplabel = escalpha | escalpha *( escalphanum | "-" ) escalphanum and the following rules are added: escalphanum = escaped8 | alphanum escalpha = elcaped8 | alpha escaped8 = "%" hexdig8 HEXDIG hexdig8 = <<HEXDIG greater than 7>> The %HH escaping is used to encode characters outside the repertoire of US-ASCII. This is done by first encoding the characters in UTF-8 [RFC 2279], resulting in a sequence of octets, and then escaping these octets according to the rules defined in [RFC2396]. Using UTF-8 assures that this encoding interoperates with IRIs (see Section 3). It is also aligned with the recommendations in [RFC 2277] and [RFC 2718], and is consistent with the URN syntax [RFC2141] as well as recent URL scheme definitions that define encodings of non-ASCII characters based on UTF-8 (e.g., IMAP URLs [RFC 2192] and POP URLs [RFC 2384]). Please note that the use of UTF-8 for encoding internationalized domain names in URIs is independent of the choice of encoding chosen for these names in the DNS protocol. Depending on the choice of encoding for the DNS protocol, an appropriate conversion is necessary. The above syntax rules do not extend the possible domain names based on US-ASCII characters. This is in accordance with the current direction of the IDN WG [IDNWG]. The above rules also do not allow escaping of US-ASCII characters, although this is allowed in the other parts of an URI (except for the special provisions in case of reserved characters). Allowing such escaping would make the syntax rules quite a bit more complicated, would mean that the restrictions on US-ASCII characters can be circumvented by using escaping, or would lead to much simpler syntax rules that don't express these restrictions anymore. Whether escaping of US-ASCII characters is allowed or not, two things should be noted: 1) It is always better not to escape US-ASCII characters in domain names because of the possibility that a resolver does not unescape them. At least purely US-ASCII domain names would then always be resolved by such a processor. 2) Because of the principle of syntax uniformity for URIs, it is always more prudent to take into account the possibility that US-ASCII characters are escaped. Only the restrictions on US-ASCII characters are expressed in the rules above. However, all the other restrictions on internationalized domain names that are defined by the IDN WG [IDNWG] MUST be respected. The work of the IDN WG currently includes some procedures for name preparation. Before encoding an internationalized domain name in an URI, this preparation step SHOULD be applied. However, the URI resolver SHOULD also apply name preparation. 3. IRI syntax changes The syntax of IRIs [IRI] currently contains the following rules relevant to domain names: hostname = *( domainlabel "." ) toplabel [ "." ] domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum toplabel = alpha | alpha *( alphanum | "-" ) alphanum The later two rules are changed as follows: domainlabel = intalphanum | intalphanum *( intalphanum | "-" ) intalphanum toplabel = intalpha | intalpha *( intalphanum | "-" ) intalphanum and the following rules are added: intalphanum = idchar | alphanum | escaped8 intalpha = idchar | alpha | escaped8 escaped8 = "%" hexdig8 HEXDIG hexdig8 = <<HEXDIG greater than 7>> idchar = << any character of the UCS [ISO10646] of U+00A0 and beyond, subject to limitations in Section 3.1. of [IRI] >> With respect to the allowed domain names based on US-ASCII characters, the same considerations as in Section 2 apply. As in Section 2, all the other restrictions on internationalized domain names that will be defined by the IDN WG MUST be respected. Also, before encoding an internationalized domain name in an IRI, name preparation SHOULD be applied. However, the IRI resolver SHOULD also apply name preparation. It is expected that the rules in Section 3.1 of [IRI] will be less restrictive than the rules for internationalized domain names, so that no escaping is necessary. Nevertheless, escaping is allowed for cases where not all characters can be directly represented. 4. Security Considerations The security considerations of [RFC 2396] and [IRI] and those applying to internationalized domain names apply. There may be an increased potential to smuggle escaped US-ASCII-based domain names across firewalls, although because of the uniform syntax principle for URIs, such a potential is already existing. Acknowledgements Looking forward for comments. Will acknowledge them here! Copyright Copyright (C) The Internet Society, 1997. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Author's address Martin J. Duerst W3C/Keio University 5322 Endo, Fujisawa 252-8520 Japan email@example.com http://www.w3.org/People/D%C3%BCrst/ Tel/Fax: +81 466 49 1170 Note: Please write "Duerst" with u-umlaut wherever possible, e.g. as "Dürst" in XML and HTML. References [IDNWG] IETF Internationalized Domain Name (idn) Working Group. Information at http://www.ietf.org/html.charters/idn-charter.html. [IRI] L. Masinter, M. Duerst, "Internationalized Resource Identifiers (IRI)", Internet Draft, November 2001, <http://www.ietf.org/internet-drafts/draft-masinter-url-i18n-08.txt>, work in progress. [ISO10646] ISO/IEC, Information Technology - Universal Multiple-Octet Coded Character Set (UCS) - Part 1: Architecture and Basic Multilingual Plane, Oct. 2000, with amendments. [RFC 2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", March 1997. [RFC 2141] R. Moats, "URN Syntax", May 1997. [RFC 2192] C. Newman, "IMAP URL Scheme", September 1997. [RFC 2277] H. Alvestrad, "IETF Policy on Character Sets and Languages". [RFC 2279] F. Yergeau. "UTF-8, a transformation format of ISO 10646.", January 1998. [RFC 2384] R. Gellens, "POP URL Scheme", August 1998. [RFC 2396] T.Berners-Lee, R.Fielding, L.Masinter. "Uniform Resource Identifiers (URI): Generic Syntax." August 1998. [RFC 2640] B. Curtis, "Internationalization of the File Transfer Protocol", July 1999. [RFC 2718] L. Masinter, H. Alvestrand, D. Zigmond, R. Petke, "Guidelines for new URL Schemes", November 1999. [RFC 2732] R. Hinden, B. Carpenter, L. Masinter, "Format for Literal IPv6 Addresses in URL's", December 1999.