Network Working Group J. Klensin Internet-Draft February 15, 2004 Expires: August 15, 2004 Registration of Internationalized Domain Names: Overview and Method draft-klensin-reg-guidelines-02.txt 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. This Internet-Draft will expire on August 15, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract IETF has introduced standards-track mechanisms to enable the use of "internationalized", i.e., non-ASCII, names in the DNS and applications that use it. This has led, in turn, to concerns that characters with similar meanings or appearance could cause user confusion and opportunities for deliberate deception and fraud. Part of this problem can be addressed by limiting, on a per-zone (or per-registry) basis, the specific characters that can be used to be a subset of the list allowed by the standard and by creating "reservations" of labels that might create confusion with those that are permitted. The model for doing this for languages that use characters that originated with Chinese has been extensively developed in another document. This document discusses some of the issues in that design and relates them to considerations and Klensin Expires August 15, 2004 [Page 1] Internet-Draft IDN Registration February 2004 mechanisms that might be appropriate for other languages and scripts, especially those involving alphabetic characters. In particular, it describes some suggested practices for registering internationalized domain names (IDNs) in a zone. Before accepting such registrations of domain names into a zone, the zone's registry should decide which codepoints in the Unicode character set the zone will accept. The registry should also decide whether particular characters in a registered domain name should cause registration of multiple equivalent domain names; these domain names might be added to the zone or blocked from registration. This document also describes how to handle character variants in registering IDNs, and how to publish tables that list the character variants. This document is intended to supply a basis for adapting methods developed for Chinese, Japanese, and Korean to other languages and scripts. If these adaptations are made carefully and with due consideration for local issues, the likelihood of problematic DNS registrations with be significantly reduced. A specific method is introduced that should be applicable (directly, or with minor modifications), to many scripts. Klensin Expires August 15, 2004 [Page 2] Internet-Draft IDN Registration February 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2.1 Characters, variants, registrations, and other issues . . . 5 1.2.2 Confusion, fraud, and cybersquatting . . . . . . . . . . . . 6 1.3 A Review of the JET Guidelines . . . . . . . . . . . . . . . 6 1.3.1 JET model . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2 Reserved Names and Label Packages . . . . . . . . . . . . . 8 1.4 Languages, Scripts, and Variants . . . . . . . . . . . . . . 8 1.4.1 Languages and Scripts . . . . . . . . . . . . . . . . . . . 8 1.4.2 Variant Selection . . . . . . . . . . . . . . . . . . . . . 10 1.5 Reservations and Exclusions . . . . . . . . . . . . . . . . 11 1.5.1 Sequence Exclusions for Valid Characters . . . . . . . . . . 11 1.5.2 Character Pairing Issues . . . . . . . . . . . . . . . . . . 11 1.6 The Registration Bundle . . . . . . . . . . . . . . . . . . 11 1.6.1 Definitions and Structure . . . . . . . . . . . . . . . . . 11 1.6.2 Application of the Registration Bundle . . . . . . . . . . . 12 2. Some Implications of this Approach . . . . . . . . . . . . . 13 3. Required Modifications to JET Model Needed Under Some of the Models Above . . . . . . . . . . . . . . . . . . . . . . 14 4. Conclusions and Recommendations about the General Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5. A Model Table format . . . . . . . . . . . . . . . . . . . . 15 6. A Model Registration Procedure --"CreateBundle" . . . . . . 16 6.1 Description of CreateBundle . . . . . . . . . . . . . . . . 16 7. Security Considerations . . . . . . . . . . . . . . . . . . 17 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 References . . . . . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . 20 Klensin Expires August 15, 2004 [Page 3] Internet-Draft IDN Registration February 2004 1. Introduction 1.1 Background Once work on the basic model for encoding non-ASCII strings in the DNS with IDNA ([1], [2], [3]) was nearing completion, it became clear that it would be desirable for registries to impose additional restrictions on the names that could actually be registered (e.g., see [6]) as a means of reducing potential confusion among characters that were similar in some way. These restrictions were, in many respects, part of a long tradition. For example, while the original DNS specifications [4] permitted any string of octets to be used in a DNS label, they also recommended the use of a much more restricted subset, one that was derived from the much older "hostname" rules [7] and defined by the "LDH" (for "letter digit hyphen", the three permitted types of characters) convention. Enforcement of those restricted rules in registrations was the responsibility of the registry or domain administrator. They were not embedded in the DNS protocol itself, although some applications protocols, notably those concerned with electronic mail, imposed and enforced similar rules. If there are no constraints on registration in a zone, people can register characters that increase the risk of misunderstandings, cybersquatting, and other forms of confusion. A similar situation existed even before the introduction of IDNA as exemplified by domain names such as example.com and examp1e.com (note that the latter domain contains the digit "1" instead of the letter "l"). For non-ASCII names (so-called "internationalized domain names" or "IDNs"), the problem was more complicated than that which led to the "LDH" (hostname) rules. In the earlier situation, all protocols, hosts, and DNS zones used ASCII exclusively in practice, so the LDH restriction could reasonably be applied uniformly across the Internet. With the introduction of a very large character repertoire, and different locations and languages considering different characters important, the optimal registration restrictions became, not a global matter, but ones that were different in different areas and, hence, in different DNS zones. For some human languages, there are characters and/or strings that have equivalent or near-equivalent usages. If someone is allowed to register a name with such a character or string, the registry might want to automatically associate all the names that have the same meaning with the registered name. The registry can also decide if the names that came from one registration should go into the zone, be blocked from other people registering them, or a combination of these two actions. Klensin Expires August 15, 2004 [Page 4] Internet-Draft IDN Registration February 2004 To date, the best-developed system for handling registration restrictions for IDNs is the JET Guidelines for Chinese, Japanese, and Korean [5], the so-called "CJK" languages. That system is limited to those languages and, in particular, to their common script base. This document explores the principles behind those guidelines and some of the issues that might arise in trying to adapt them to alphabetic languages. This document describes five things: o The general background and considerations for non-ASCII scripts in names. Just as the JET Guidelines contain some suggestions that may not be applicable to alphabetic scripts, some of the suggestions here, especially the more specific ones, may be applicable to some scripts and not others o Suggested practices for describing character variants o A method for using a zone's character variants to determine which names should be associated with a registration o A format for publishing a zone's table of character variants o A model algorithm for name registration given the presence of language tables. 1.2 Terminology 1.2.1 Characters, variants, registrations, and other issues 1. Characters in this document are given as their Unicode codepoints on U+xxxx format or with their official names. 2. The following terms are used in this document. 3. A "string" is an sequence of one or more characters. 4. This document discusses characters that may have equivalent or near-equivalent characters or strings. The "base character" is the character that has zero or more equivalents. In the JET Guidelines, base characters are referred to as "valid characters". 5. The "variant(s)" are the character(s) and/or string(s) that are equivalent to the base character. Note that these might not be true equivalent characters: a base character might have a mapping to a particular variant character, but that variant character does not have to have a mapping to the base character. Usually, characters or strings to be designated as variants are considered either equivalent or sufficiently similar (by some registry-specific definition) that confusion between them and the base character might occur. 6. The "base registration" is the single name that the registrant requested from the registry. 7. A label (or "name") is described as "registered" if it is actually entered into a domain (i.e., a zone file) by the registry, so that it can be accessed and resolved using standard Klensin Expires August 15, 2004 [Page 5] Internet-Draft IDN Registration February 2004 DNS tools. The JET Guidelines describe a "registered" label as "activated". 8. A "registration bundle" is the set of all labels that comes from expanding the base characters for a single name into their variants. The presence of a label in a registration bundle does not imply that it is registered. In the JET Guidelines, a registration bundle is called an "IDN Package". 9. A "reserved label" is a label in a registration bundle that is not actually registered. 10. A "registry" is the administrative authority for a DNS zone. That is, the registry is the body that enforces, and typically makes, policies that are used in a particular zone in the DNS. 11. A coded character set ("CCS"): A term for a list of characters and the code positions assigned to them. ASCII and Unicode are CCSs. 12. A language: Something spoken by humans, independent of how it is written or coded. ISO Standard 639 and IETF BCP 47 (RFC 3066) [8] list and define codes for identifying languages. 13. Script: a collection of characters (glyphs, independent of coding) that are used together, typically to represent one or more languages. Note that the script for one language may heavily overlap the script for another without their having identical scripts. 14. Charset: An IETF-invented term to describe, more or less, the combination of a script, a CCS that encodes that script, and rules for serializing the bytes when those are stored on a computer or transmitted over the network. The last four of these definitions are redundant with, but deliberately somewhat less precise than, the definitions in [12], which also provides sources. The two sets of definitions are intended to be consistent. 1.2.2 Confusion, fraud, and cybersquatting The term "confusion" is used very generically in this document to cover the entire range from accidental user misperception of the relationship between characters with some characteristic in common (typically appearance, sound, or meaning) to cybersquatting and [other] deliberate fraudulent attempts to exploit those relationships. 1.3 A Review of the JET Guidelines 1.3.1 JET model In the JET Guidelines model, a prospective registrant approaches the registry for a zone (perhaps through an intermediate registrar) with Klensin Expires August 15, 2004 [Page 6] Internet-Draft IDN Registration February 2004 a candidate base registration --a proposed name to be registered-- and a list of languages in which that name is to be interpreted. The languages are defined according to the fairly high-resolution coding of [8] -- Chinese as used on the mainland of the People's Republic of China ("zh-cn") can, at registry option, be coded differently and represented by a separate table compared to Chinese as used in Taiwan ("zh-tw"). The design of the JET Guidelines took one important constraint as a basis: IDNA was treated as a firm standard. A procedure that modified some portion of the IDNA functions, or was a variant on them, was considered a violation of those standards and should not be encouraged (or, probably, even permitted). Each registry is expected to construct (or obtain) a table for each language it considers relevant and appropriate. These tables list, for the particular zone, the characters permitted for that language. If a character does not appear as a "valid code point" (called a "base character" in the rest of this document) in that table, then a name containing it cannot be registered. If multiple languages are listed for the registration, then the character must appear in the tables for each of those languages. The tables may also contain columns that specify alternate or variant forms of the valid character. If these variants appear, they are used to synthesize labels that are alternatives to the original one. These labels are all reserved and can be registered or "activated" (placed into the DNS) only by the action or request of the original registrant; some (the "preferred variant labels") are typically registered automatically. The zone is expected to establish appropriate policies for situations in which the variant forms of one label conflict with already-reserved or already-registered labels. Most of these concepts were introduced because of concerns about specific issues with CJK characters, beginning from the requirement that the use of Simplified Chinese by some registrants and Traditional Chinese by others not be permitted to create confusion or opportunities for fraud. While they may be applicable to registry tables contructed for alphabetic scripts, the transfer should be done with care, since many analogies are not exact. Some of the important issues are discussed in the sections that follow. The JET model may be considered as a specialized variation on the model and method presented by the rest of this document. Other languages or scripts may require other variations Klensin Expires August 15, 2004 [Page 7] Internet-Draft IDN Registration February 2004 1.3.2 Reserved Names and Label Packages A basic assumption of the JET model is that, if the properties of Unicode [9], [10], IDNA, or the evolution of specific characters, cause two strings to appear similar enough to cause confusion, either or both should be registered by the same party or one of them should become unregisterable. The definition of "appear similar enough" will differ for different cultures and circumstances --and hence DNS zones-- but the principle is fairly general. In the JET model, all of the "variant" strings are identified, some are placed into the DNS automatically, and others are simply reserved and can be activated, if at all, only by the original registrant. Other zones might find other policies appropriate. For example, a zone might conclude that having similar strings registered in the DNS was undesirable. If so, the list of variant labels would be used only to build a list of names that would be reserved and not able to be registered. 1.4 Languages, Scripts, and Variants 1.4.1 Languages and Scripts Conversations about scripts -- collections of characters associated with particular languages -- are common when discussing character sets and codes. But the boundaries between one script and another are not well-defined. The Unicode Standard [9][10], for example, does not define them at all, even though it is structured in terms of usually-related blocks of characters. The issue is complicated by the common origin of most alphabetic scripts (Cf. [11]), with certain character-symbols appearing in the scripts associated with multiple languages, sometimes with very different sounds or meanings. This differs from the CJK situation in which, if a character appears in more than one of the relevant languages, it will almost always have the same interpretation in each one and, at least for the subset of characters that actually are ideographs, pronunciation is expected to vary widely while meaning is preserved. At least in part because of that similarity of meaning, it made sense in the JET case to permit a registration to specfy multiple languages, to verify that the characters in the label string were valid for each, and then to generate variant labels using each language in turn. For many alphabetic languages, it may make sense to prohibit the label string submitted for registration from being associated with more than one language. Indeed, "one label, one language" has been suggested as an important barrier against common sources of "look-alike" confusion. For example, the imposition of that rule in a zone would prevent the insertion of a few Greek or Cyrillic characters with shapes identical to the Latin ones into what was otherwise a Latin-based string. For a particular table, the list of valid characters may be thought of as the script associated with the relevant language, with the Klensin Expires August 15, 2004 [Page 8] Internet-Draft IDN Registration February 2004 understanding that the table design does not prevent the same character from appearing in the tables for multiple languages. Indeed, this notion of a locally, and specifically, identified script can be turned around: while the tables are referred to as "language tables", they are associated with languages only insofar as thinking about the character structure and word forms associated with a given language helps to inform the construction of a table. A country like Finland, for example, might select among o One table each for Finnish, Swedish, and English characters and conventions, permitting a string to be registered in one, two, or all three languages (although a three-language registration would presumably prohibit any characters that did not appear in all three languages). o One table each, but with a "one label, one language" rule for the zone. o A combined table based on the observation that all three writing systems were based on Roman characters and that the possibilities for confusion that were of interest to the registry would not be reduced by "language" differentiation. Regardless of what decisions were made about those languages and scripts, if they also decided to permit registrations of labels containing Cyrillic characters, they might have a separate table for them. That table might contain some Roman-derived characters (either as base characters or as variants) just as some CJK tables do. See also Section 2, below. It is also worth stressing, as the JET Guidelines do, that no tables or systems of this type -- even if identified with languages as a means of defining or describing those tables -- can assure linguistic or even syntactic correctness of labels with regard to that language. That level of assurance may not be possible without human intervention or at least dictionary lookups of complete proposed labels. It may even not be desirable to attempt that level of correctness (see Section 2). Of course, if any language-based tests or constraints, including "one label, one language", are to be applied to limit those sources of confusion, each zone must have a table for each language in which it expects to accept registrations; the notion of a single combined table for the zone is, in the general case, simply unworkable. One could use a single table for the zone if the intent were to impose only minimal restrictions, e.g., to force alphabetic and numeric characters only and exclude symbols and punctuation. That type of restriction might be useful in eliminating some problems, such as those of unreadable labels, but would be unlikely to be very helpful Klensin Expires August 15, 2004 [Page 9] Internet-Draft IDN Registration February 2004 with, e.g., confusion caused by similar-looking characters. 1.4.2 Variant Selection The area of character variants is rife with problems. There is no universal agreement about which base characters have variants, or if they do, what those variants are. For example, in some regions of the world and in some languages, LATIN SMALL LETTER O WITH DIAERESIS and LATIN SMALL LETTER O WITH STROKE are variants of each other, while in other regions, most people would think that LATIN SMALL LETTER O WITH DIAERESIS has no variants. In some cases, the list of variants is difficult to enumerate. For example, it required several years for the Chinese language community to create variant tables for use in IDNA, and it remains, at the time of this writing, questionable how widely those tables will be accepted among users of Chinese from areas of the world other than those represented by the group that created them. Thus, the first thing a registry should ask is whether or not any of the characters that they want to use have variants. If not, the registry's work is much simpler. This is not to say that a registry should ignore variants if they exist: adding variants after a registry has started to take registrations is nearly as difficult administratively as removing characters from the list of acceptable characters. That is, if a registry later decides that two characters are variants of each other, and there are actively-used names in the zones that differ only on the new variants, the registry might have to transfer ownership of one of the names to a different owner, using some process that is certain to be controversial. The list of character variants used in a zone should be stable. Although it is possible to add variants for characters later, doing so can cause confusing with registrants. Of course, zone managers should inform all current registrants when the registration policy for the zone changes. This includes when IDN characters are allowed in the zone the first time, when characters are added later, and when character variant tables change. In many languages there are two variants for a character, but one variant is strongly preferred. A registry might only allow the base registration in the preferred form, or it might allow any form for the base registration. If the variant tables are created carefully, the resulting bundles will be the same, but some registries will give special status to the base registration such as its appearance in whois databases. Klensin Expires August 15, 2004 [Page 10] Internet-Draft IDN Registration February 2004 1.5 Reservations and Exclusions 1.5.1 Sequence Exclusions for Valid Characters The JET Guidelines are based on processing only single characters. Any processing of pairs or longer sequences of characters are left to what that document describes as "additional processing" -- procedures specifically permitted by the Guildlines but defined by a registry in addition to the variant table processing specified in the Guidelines themselves. A different zone, with different needs, could use a modified version of the table structure, or different types of additional processing, to prohibit, as well as accept, particular sequences of characters by marking them as invalid. Other modifications or extensions might be designed to prevent certain letters from appearing at the beginning or end of labels. The use of regular expressions in the "valid characters" column might be one way to implement these types of restrictions. In particular, in some scripts derived from Roman characters, sequences that have historically been typographically represented by single "ligature" or "digraph" characters may also be represented by the separate characters (e.g., "ae" (U+00E6) or "ij" (U+0133)). If it is desired to either prohibit these, or to treat them as variants, some extensions to the single-character JET model may be needed (as may be some careful thinking about IDNA (especially nameprep), since some of these combinations are excluded there). 1.5.2 Character Pairing Issues Some character pairings -- the use of a character form (glyph) in one language and a different form with the same properties in a related one -- closely approximate the issues with mapping between Traditional and Simplified Chinese although the history is different. For example, it might be useful to have "o" with a stroke (U+00F8) as a variant for "o" with diaeresis above it (U+00F6) (and the equivalent upper-case pair) in a Swedish table, and vice versa in a Norwegian one, or to prohibit one of these characters entirely in each table. In a German table, U+00F8 would presumably be prohibited, while U+00F6 might have "oe" as a variant. Obviously, if the relevant language of registration is unknown, this type of variant matching cannot be applied in any sensible way. 1.6 The Registration Bundle 1.6.1 Definitions and Structure As one of its critical innovations, the JET model defines an "IDN package", known in this document as a "registration bundle", which Klensin Expires August 15, 2004 [Page 11] Internet-Draft IDN Registration February 2004 consists of the primary registered string (which is used as the name of the bundle), the information about the language table(s) used, the variant labels for that string, and indications of which of those labels are registered in the relevant zone file ("activated" in the JET terminology). Registration bundles are also atomic -- one can not add or remove variant labels from one without unregistering the entire package. A label exists in only one registration bundle at a time; if a new label is registered that would generate a variant that matches one that appears in an existing package, that variant simply is not included in the second package. A subsequent deregistration of the first package does not cause the variant to be added to the second. While it might be possible to change this in other models, the JET conclusion was that other options would be far too complex to implement and operate and would cause many new types of name conflicts. 1.6.2 Application of the Registration Bundle A registry has three options for how to handle the case where the registration bundle has more than one label. The policy options are: 1. Resolve all labels in the zone, making the zone information identical to that of the registered label. This option will cause end users to be able to find names with variants more easily, but will result in larger zone files. For some language tables, the zone file could become so large that it could negatively affect the ability of the registry to perform name resolution. If the base registration contains several characters that have equivalents, the owner could end up having to take care of large number of zones. For instance, if DIGIT ONE is a variant of LATIN SMALL LETTER L, the owner of the domain name all-lollypops.example.com will have to manage 32 zones. 2. Block all labels other than the registered label so they cannot be registered in the future. This option does not increase the size of the zone file and provides maximum safety against false positives, but it may cause end users to not be able to find names with variants that they would expect. If the base registration contains characters that have equivalents, Internet users who don't know what the base characters used in the registration will not know what character to type in to get a DNS response. For instance, if DIGIT ONE is a variant of LATIN SMALL LETTER L, and LATIN SMALL LETTER L is a variant of DIGIT ONE, the user who sees "pale.example.com" will no know whether to type a "1" or a "l" after the "pa" in the first label. 3. Resolve some labels and block some other labels. This option is likely to cause the most confusion with users because including some variants will cause a name to be found, but using other variants will cause the name to be not found. For example, even Klensin Expires August 15, 2004 [Page 12] Internet-Draft IDN Registration February 2004 if people understood that DIGIT ONE and LATIN SMALL LETTER L were variants, a typical DNS user wouldn't know which character to type because they wouldn't know whether this pair were allocating variants or blocking variants. However, this option can be used to balance the desires of the name owner (that every possible attempt to enter their name will work) with the desires of the zone administrator (to make the zone more manageable and possibly to be compensated for greater amounts of work needed for a single registration). For many circumstances, it may be the most attractive option. In all cases, at least the registered label should appear in the zone. It would be almost impossible to describe to name owners why the name that they asked for is not in the zone, but some other name that they now control is. By implication, if the requested label is already registered, the entire registration request must be rejected. 2. Some Implications of this Approach Historically, DNS labels were considered to be arbitrary identifier strings, without any inherent meaning. Even in ASCII, there was no requirement that labels form words. Labels that could not possibly represent words in any Romance or Germanic language have actually been quite common. In general, in those languages, words contain at least one vowel and do not have embedded numbers. The more one moves toward "language"-based registry restrictions, the less it is going to be possible to construct labels out of fanciful strings. Such strings may make very good identifiers, while being terrible candidates for "words". To take a trivial example using only ASCII characters, "rtr32w", "rtr32x", and "rtr32z" might be very good DNS labels for a particular zone and application, but, given the embedded digits and lack of vowels, would fail even the most superficial of tests for valid Engish word forms. Interestingly, if one is trying to develop an "only words" system, a rather different --but very restrictive-- model could be developed using lookups in a dictionary for the relevant language and a listing of valid business names for the relevant area. If a string did not appear in either, it would not be permitted to be registered. Models effectively equivalent to this one have historically been used to restrict registrations in some country-code top level domains. On the other hand, if look-alike characters are a concern, even that type of rule (or restriction) would still not avoid the need for variants. Consequently, registries applying the principles outlined in this document should be careful not to apply more severe restrictions than are reasonable and appropriate while, at the same time, being aware of how difficult it usually is to add restrictions at a later time. Klensin Expires August 15, 2004 [Page 13] Internet-Draft IDN Registration February 2004 3. Required Modifications to JET Model Needed Under Some of the Models Above The JET model was designed for CJK characters. The discussion above implies that some extensions to it may be needed to handle the characteristics of various alphabetic scripts and the decisions that might be made about them in different zones. Those extensions might include facilities to process: o Two-character (or more) sequences, such as ligatures and typographic spelling conventions, as variants. o Regular expressions or some other mechanism for dealing with string positions of characters (e.g., characters that must, or must not, appear at the beginning or end of strings). o Delimiter breaks to permit multiple languages to be used, separately, within the same label. E.g., is it possible to define a label as consisting of two or more sublabels, each in a different language, with some particular delimiter used to define the boundaries of the sublabels. 4. Conclusions and Recommendations about the General Approach Thinking about the implications of the use in DNS labels of the full range of characters permitted by IDNA has led multiple groups to the conclusion that some restrictions, on a per-registry or per-zone basis, are needed to prevent many forms of user confusion about the actual structure of a name or the word, phrase, or term that it appears to spell out. It appears that the best way to approach such restrictions involves drawing from the language and culture of the community of registrants and users in the relevant zone: if particular characters are likely to be unintelligible to both of those groups, it is probably wise to not permit them to be used in registrations. Registration restrictions can be carried much further than restricting permitted characters to a selected Unicode subset. The idea of a reserved "bundle" of related labels permits probably-confusing combinations or sets of characters to be bound together, under the control of a single registrant. While that registrant might use the package in a way that confused his or her own users, the possibility of turning potential confusion into a hostile attack would be considerably reduced. At the same time, excessive restrictions may make DNS identifiers less useful for their original, intended, purpose: identifying particular hosts and similar resources on the network in an orderly way. Registries creating rules and policies about what can be registered in particular zones -- whether those are based on the JET Guidelines or the suggestions in this document-- should balance the need for restrictions against the need for flexibility in Klensin Expires August 15, 2004 [Page 14] Internet-Draft IDN Registration February 2004 constructing identifiers. The discussion above provides many options that could be selected, defined, and applied in different types in different registries (zones). Registrars would almost certainly prefer systems in which they can predict, at least to a first order approximation, the implications of a particular potential registration to ones in which they cannot. Predictability of that sort probably requires more standards, and less flexibility, than the model itself might suggest. 5. A Model Table format The format of the table is meant to be machine-readable but not human-readable. It is fairly trivial to convert the table into one that can be read by people. Each character in the table is given in the "U+" notation for Unicode characters. The lines of the table are terminated with either a carriage return character (ASCII 0x0D), a linefeed character (ASCII 0x0A), or a sequence of carriage return followed by linefeed (ASCII 0x0D 0x0A). The order of the lines in the table may or may not matter, depending on how the table is constructed. Comment lines in the table are preceded with a "#" character (ASCII 0x2C). Each non-comment line in the table starts with the character that is allowed in the registry, which is also called the "base character". If the base character has any variants, it is followed by a vertical bar character ("|", ASCII 0x7C) and the variant string. If the base character has more than one variant, the variants are separated by a colon (":", ASCII 0x3A). Strings are given with a hyphen ("-", ASCII 0x2D) between each character. Comments beging with a "#" (ASCII 0x2C), and may be preceded by spaces (" ", ASCII 0x20). The following is an example of how a table might look. The entries in this table are purposely silly and should not be used by any registry as the basis for choosing variants. For the example, assume that the registry: o allows the FOR ALL character (U+2200) with no variants o allows the COMPLEMENT character (U+2201) which has a single variant of LATIN CAPITAL LETTER C (U+0043) o allows the PROPORTION character (U+2237) which has one variant which is the string COLON (U+003A) COLON (U+003A) o allows the PARTIAL DIFFERENTIAL character (U+2202) which has two variants: LATIN SMALL LETTER D (U+0064) and GREEK SMALL LETTER DELTA (U+03B4) Klensin Expires August 15, 2004 [Page 15] Internet-Draft IDN Registration February 2004 The table would look like: # An example of a table U+2200 U+2201|U+0043 U+2237|U+003A-U+003A # Note that the variant is a string U+2202|U+0064:U+03B4 Implementors of table processors should remember that there are tens of thousands of characters whose codepoints are greater than 0xFFFF. Thus, any program that assumes that each character in the table is represented in exactly six octets ("U", "+", and exactly four octets representing the character value) will fail with tables that use characters whose value is greater than 0xFFFF. 6. A Model Registration Procedure --"CreateBundle" This procedure has three inputs: o the proposed base registration o the language for the proposed base registration o the processing table associated with that language The output of the process is either failure (the base registration cannot be registered at all), or a registration bundle that contains one or more labels ( always including the base registration). As described earlier, the registration bundle should be stored with its date of creation so that issues with overlapping elements between bundles can later be resolved on a first-come, first-served basis. There are two steps to processing the registration: 1. Check whether the proposed base registration exists in any bundle. If it does, stop immediately with a failure. 2. Process the base registration with the CreateBundle process described below. Note that the process must be executed only once. The process must not be run on any output of the process, only on the proposed base registration. 6.1 Description of CreateBundle The CreateBundle process determines if a registration bundle can be created and, if so, fills that bundle only with valid labels. During the processing, an "temporary bundle" contains partial labels, that is, labels that are being built and are not complete labels. The partial labels in the temporary bundle consist of strings. The steps in the CreateBundle process are: 1. Split the base registration into individual characters, called "candidate characters". Compare every candidate character against the base characters in the table. If any candidate character does Klensin Expires August 15, 2004 [Page 16] Internet-Draft IDN Registration February 2004 not exist in the set of base characters, the system must stop and not register any names (that is, it must not register either the base registration or any labels that would have come from character variants). 2. Perform the steps in ToASCII for the base registration. If ToASCII fails for the base registration, the system must stop and not register any of the label (that is, it must not register either the base registration or any created labels, even if those labels would have passed ToASCII). If ToASCII succeeds, add the result to the registration bundle. 3. For every candidate character in the base registration, do the following: 1. Create the set of characters that consists of the candidate character and any variants. 2. For each character in the set from the previous step, duplicate the temporary bundle that resulted from the previous candidate character, and add the new character to the end of each partial label. 4. The temporary bundle now contains zero or more labels that consist of Unicode characters. For every label in the temporary bundle, do the following: Process the label with ToASCII to see if ToASCII succeeds. If it does, put the label into the registration bundle. Otherwise, do not process this label from the temporary bundle any further; it will not go into the registration bundle. 5. The result is the registration bundle with the base registration and possibly other labels. Finish. 7. Security Considerations Registration of labels in the DNS that contain essentially unrestricted sequences of arbitrary Unicode characters may introduce several opportunities for either attacks or simple confusion. Some of these risks, such as confusion about which character, of several that look alike), is actually intended, may be associated with the presentation form of DNS names. Others may be linked to databases associated with the DNS, e.g., with the difficulty of finding an entry in a Whois file when it is not clear how to enter, or search for, the characters that make up a name. This document discusses a family of restrictions on the names that can be registered that can be imposed on a DNS zone ("registry") and some possible tools for implementing restrictions of that sort. No plausible set of restrictions will eliminate all problems and sources of confusion: for example, it has often been pointed out that the characters digit-one ("1") and lower case L ("l") can easily be confused in some fonts used to display ASCII. But, to the degree to which security may be aided by sensible risk reduction, these techniques may be Klensin Expires August 15, 2004 [Page 17] Internet-Draft IDN Registration February 2004 helpful. 8. Acknowledgements Discussions in the process of developing the JET Guidelines were vital in developing this document and all of the JET participants are consequently acknowledged. Attempts to explain some of the issues there to, and feedback from, Vint Cerf, Wendy Rickard, and members of the ICANN IDN Committee were also helpful in the thinking leading up to this document. An effort by Paul Hoffman to create a generic specification for registration restrictions of this type helped to inspire this document, which takes a somewhat different, more language-oriented, approach. While the initial version of that document indicated that multiple languages (or multiple language tables) for a single zone were infeasible, more recent versions [13] shifted to inclusion of language-based approaches. The current version of this document incorporates considerable text, and even more ideas, from those drafts, with Paul Hoffman's generous permission. The opinions expressed here are, of course, the sole responsibility of the author. Some of those whose ideas are reflected in this document may disagree with the conclusions the authors have drawn from them. References [1] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [2] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)", RFC 3491, March 2003. [3] Costello, A., "Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)", RFC 3492, March 2003. [4] Mockapetris, P., "Domain names - implementation and specification", RFC 1035, STD 13, November 1987. [5] Seng, J., Ed., Klensin, J., Ed., Rickard, W., Ed., Konishi, K., Huang, K., Qian, H. and Y. Ko, "International Domain Names Registration and Administration Guidelines for Chinese, Japanese, and Korean", draft-jseng-idn-admin-05.txt (work in progress), June 2003. Klensin Expires August 15, 2004 [Page 18] Internet-Draft IDN Registration February 2004 [6] Internet Engineering Steering Group, IETF, "IESG Statement on IDN", IESG Statement IDNstatement.txt, February 2003. [7] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet host table specification", RFC 952, October 1985. [8] Alvestrand, H., "Tags for the Identification of Languages", BCP 47, RFC 3066, January 2001. [9] The Unicode Consortium, "The Unicode Standard--Version 3.0", January 2000. [10] The Unicode Consortium, "Unicode Standard Annex #28", March 2002. [11] Drucker, J., "The Alphabetic Labyrinth: The Letters in History and Imagination", 1995. [12] Hoffman, P., "Terminology Used in Internationalization in the IETF", RFC 3536, May 2003. [13] Hoffman, P., "A Method for Registering Internationalized Domain Names", draft-hoffman-idn-reg-02.txt (work in progress), October 2003. Author's Address John C Klensin 1770 Massachusetts Ave, #322 Cambridge, MA 02140 USA Phone: +1 617 491 5735 EMail: john-ietf@jck.com Klensin Expires August 15, 2004 [Page 19] Internet-Draft IDN Registration February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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