Network Working Group H. Alvestrand, Ed. Internet-Draft Google Intended status: Standards Track C. Karp Expires: June 3, 2009 Swedish Museum of Natural History November 30, 2008 An updated IDNA criterion for right-to-left scripts draft-ietf-idnabis-bidi-03 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 June 3, 2009. Abstract The use of right-to-left scripts in internationalized domain names has presented several challenges. This memo discusses some problems with these scripts, and some shortcomings in the 2003 IDNA BIDI criterion. Based on this discussion, it proposes a new BIDI rule for IDNA labels. Alvestrand & Karp Expires June 3, 2009 [Page 1] Internet-Draft IDNA RTL fix November 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Purpose and applicability . . . . . . . . . . . . . . . . 3 1.2. Background and history . . . . . . . . . . . . . . . . . . 3 1.3. Layout of the rest of this document . . . . . . . . . . . 4 1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 2. A replacement for the RFC 3454 BIDI rule . . . . . . . . . . . 6 3. A requirement set for the BIDI rule . . . . . . . . . . . . . 7 4. Examples of issues found . . . . . . . . . . . . . . . . . . . 10 4.1. Dhivehi . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2. Yiddish . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.3. Strings with numbers . . . . . . . . . . . . . . . . . . . 12 5. Troublesome situations and guidelines . . . . . . . . . . . . 12 6. Other issues in need of resolution . . . . . . . . . . . . . . 13 7. Compatibility considerations . . . . . . . . . . . . . . . . . 14 7.1. Backwards compatibility considerations . . . . . . . . . . 14 7.2. Forward compatibility considerations . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 11.1. Normative references . . . . . . . . . . . . . . . . . . . 16 11.2. Informative references . . . . . . . . . . . . . . . . . . 16 Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 17 A.1. Changes from draft-alvestrand-00 to -01 . . . . . . . . . 17 A.2. Changes from alvestrand-01 to -02 . . . . . . . . . . . . 17 A.3. Changes from alvestrand-02 to -03 . . . . . . . . . . . . 17 A.4. Changes from alvestrand-03 to -04 . . . . . . . . . . . . 17 A.5. Changes from draft-alvestrand-04 to draft-ietf -00 . . . . 18 A.6. Changes from idnabis -00 to -01 . . . . . . . . . . . . . 18 A.7. Changes from idnabis -01 to -02 . . . . . . . . . . . . . 18 A.8. Changes from idnabis -02 to -03 . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . . . . 20 Alvestrand & Karp Expires June 3, 2009 [Page 2] Internet-Draft IDNA RTL fix November 2008 1. Introduction 1.1. Purpose and applicability This document's purpose is to establish a rule that can be applied to Internationalized Domain Name (IDN) labels in Unicode form (U-labels) containing right-to-left characters. When labels satisfy the rule, and when certain other conditions are satisfied, they can be used with a minimal chance of these labels being displayed in a confusing way by a bidirectional display algorithm. This specification is not intended to place any requirements on domain names that do not contain right-to-left characters. 1.2. Background and history The IDNA specification "Stringprep" [RFC3454] makes the following statement in its section 6 on the BIDI algorithm: 3) If a string contains any RandALCat character, a RandALCat character MUST be the first character of the string, and a RandALCat character MUST be the last character of the string. (A RandALCat character is a character with unambiguously right-to- left directionality.) The reasoning behind this prohibition was to ensure that every component of a displayed domain name has an unambiguously preferred direction. However, this makes certain words in languages written with right-to-left scripts invalid as IDN labels, and in at least one case means that all the words of an entire language are forbidden as IDN labels. This is illustrated below with examples taken from the Dhivehi and Yiddish languages, as written with the Thaana and Hebrew scripts, respectively. In investigating this problem, it was realized that the RFC 3454 specification did not explicitly state the requirement to be fulfilled, and therefore, it was impossible to tell whether a simple relaxation of the rule would continue to fulfil the requirement. A further investigation led to the conclusion that for one reasonable set of requirements, IDNA2003's BIDI restriction did not fulfil the requirements. This document therefore proposes replacing the RFC 3454 BIDI requirement in its entirety. Alvestrand & Karp Expires June 3, 2009 [Page 3] Internet-Draft IDNA RTL fix November 2008 While the document proposes completely new text, most reasonable labels that were allowed under the old criterion will also be allowed under the new criterion, so the operational impact of the rule change is limited. 1.3. Layout of the rest of this document Section 2 defines a test, the "BIDI test", that can be used on a domain name label to check how safe it is to use in a domain name of possibly mixed directionality.- one user of that test is the IDNA2008 protocol[I-D.ietf-idnabis-protocol]. Section 3 sets out the requirements for defining a BIDI rule. Section 4 gives detailed examples that serve as justification for the change proposed here. Section 5 to Section 9 describe various situations that can occur when dealing with domain names with characters of different directionality. Only Section 1.4 and Section 2 are normative. 1.4. Terminology In this memo, we use "network order" to describe the sequence of characters as transmitted on the wire or stored in a file; the terms "first", "next", "previous", "before" and "after" are used to refer to the relationship of characters and labels in network order. We use "display order" to talk about the sequence of characters as imaged on a display medium; the terms "left" and "right" are used to refer to the relationship of characters and labels in display order. Most of the time, the examples use the abbreviations for the Unicode BIDI classes to denote the directionality of the characters; in some examples, the convention that uppercase characters are of class R or AL, and lowercase characters are of class L is used - thus, the example string ABC.abc would consist of 3 right-to-left characters and 3 left-to-right characters. The directionality of such examples is determined by context - for instance, in the sentence "ABC.abc is displayed as CBA.abc", the first example string is in network order, the second example string is in display order. The term "paragraph" is used in the sense of the Unicode BIDI specification [UAX9] - it means "a block of text that has an overall Alvestrand & Karp Expires June 3, 2009 [Page 4] Internet-Draft IDNA RTL fix November 2008 direction, either left-to-right or right-to-left", approximately; see UAX 9 for the details. "LTR" and "RTL" are abbreviations for "right to left" and "left to right", respectively. The terminology used for the BIDI properties of Unicode characters is taken from the Unicode Standard[Unicode] For reference, here are the values that the Unicode BIDI property can have: o L - Left-to-right - most letters in LTR scripts o R - Right-to-left - most letters in non-Arabic RTL scripts o AL - Arabic letters - most letters in the Arabic script o EN - European Number (0-9, and Extended Arabic-Indic numbers) o ES - European Number Separator (+ and -) o ET - European Number Terminator (currency symbols, the hash sign, the percent sign and so on) o AN - Arabic Number o CS - Common Number Separator (. , / : et al) o NSM - Non spacing Mark - most combining accents o BN - Boundary Neutral - control characters o B - Paragraph Separator o S - Segment Separator o WS - Whitespace, including the SPACE character o ON - Other Neutrals, including @, &, parentheses, MIDDLE DOT o LRE, LRO, RLE, RLO, PDF - these are "directional control characters", and are not used in IDNA labels. The other terminology used to describe IDNA concepts is defined in [I-D.ietf-idnabis-defs] Alvestrand & Karp Expires June 3, 2009 [Page 5] Internet-Draft IDNA RTL fix November 2008 2. A replacement for the RFC 3454 BIDI rule The following test has been developed for BIDI domain name labels. The requirements that this test satisifes are described in Section 3. A label containing a character of type R, AL or AN MUST satisfy all 7 of the rules below. The main bullets give the rule, subordinate bullets (if any) give justifications or examples of things that break if this rule is not present. 1. Only characters with the BIDI properties L, R, AL, AN, EN, ES, BN, ON and NSM are allowed. * B, S and WS are excluded because they are separators or spaces. * LRE, LRO, RLE, RLO, PDF are excluded because they are BIDI controls. * ET is excluded because the string L ET does not satisfy the Character Grouping requirement. * CS is excluded because the string L CS does not satisfy the Character Grouping requirement. 2. ES and ON are not allowed in the first position * ES R and ON R do not satisfy the Character Grouping requirement. 3. ES and ON, followed by zero or more NSM, is not allowed in the last position * L ON and L ES both fail the Character Grouping requirement. 4. If an R, AL or AN is present, no L may be present. 5. If an EN is present, no AN may be present, and vice versa. 6. The first character may not be an NSM 7. The first character may not be an EN (European Number) or an AN (Arabic Number). * If the character on both sides of a CS is an EN or an AN, the labels fail the Character Grouping requirement. Alvestrand & Karp Expires June 3, 2009 [Page 6] Internet-Draft IDNA RTL fix November 2008 * Some domain names where some of the labels use leading EN and AN may be problem-free, but there's no way of verifying this while looking at a single label in isolation.. * We could achieve stability by barring numbers at the end of labels, but this may be more disruptive in practice. The following guarantees can be made based on the above: o In a domain name consisting of only labels that pass the test, the requirements of Section 3 are satisfied. o In a domain name consisting of only LDH-labels and labels that pass the test, the requirements of Section 3 are satisfied as long as a label that starts with an ASCII digit does not come after a right-to-left label that ends in a digit. No guarantee is given for other combinations. 3. A requirement set for the BIDI rule One issue with RFC 3454 was that it did not give an explicit justification for the BIDI rule, thus it was hard to tell if a modified rule would continue to fulfil the purpose for which the RFC 3454 rule was written. This document proposes an explicit justification, by stating a set of requirements for which it is possible to test whether or not the modified rule fulfils the requirement. All the text in this document assumes that text containing the labels under consideration will be displayed using the Unicode bidirectional algorithm [UAX9]. The requirements proposed are these: o Label Uniqueness: No two labels, when presented in display order in the same paragraph, should have the same sequence of characters without also having the same sequence of characters in network order, both when the paragraph has LTR direction and when the paragraph has RTL direction. (This is the criterion that is explicit in RFC 3454). (Note that a label displayed in an RTL paragraph may display the same as a different label displayed in a LTR paragraph, and still satisfy this criterion.) o Character Grouping: When displaying a string of labels, using the Unicode BIDI algorithm to reorder the characters for display, the Alvestrand & Karp Expires June 3, 2009 [Page 7] Internet-Draft IDNA RTL fix November 2008 characters of each label should remain grouped between the characters delimiting the labels, both when the string is embedded in a paragraph with LTR direction and when it's embedded in a paragraph with RTL direction. Several stronger statements were considered and rejected, because they seem to be impossible to fulfil within the constraints of the Unicode bidirectional algorithm. These include: o The appearance of a label should be unaffected by its embedding context. This proved impossible even for ASCII labels; the label "123-456" will have a different display order in an RTL context than in a LTR context. o The sequence of labels should be consistent with network order. This proved impossible - a domain name consisting of the labels (in network order) L1.R1.R2.L2 will be displayed as L1.R2.R1.L2 in an LTR context. (In a RTL context, it will be displayed as L2.R2.R1.L1). o The Label Uniqueness property should hold true between LTR paragraphs and RTL paragraphs. This was shown to be unsound. o No two domain names should be displayed the same, even under differing directionality. This was shown to be unsound, since the domain name (network) ABC.abc will have display order CBA.abc in an LTR context and abc.CBA in an RTL context, while the domain name (network) abc.ABC will have display order abc.CBA in an LTR context and CBA.abc in an RTL context. One specific requirement was thought to be problematic, but turned out to be satisfied by a string that obeys the proposed rules: o The Character Grouping requirement should be satisfied when directional controls (LRE, RLE, RLO, LRO, PDF) are used in the same paragraph (outside of the labels). Because these controls affect presentation order in non-obvious ways, by affecting the "sor" and "eor" properties of the Unicode BIDI algorithm, the conditions above require extra testing in order to figure out whether or not they influence the display of the domain name. Testing found that for the strings allowed under the rule presented in this document, directional controls do not influence the display of the domain name. In the following descriptions, first-level bullets are used to indicate rules or normative statements; second-level bullets are commentary. Alvestrand & Karp Expires June 3, 2009 [Page 8] Internet-Draft IDNA RTL fix November 2008 The Character Grouping requirement can be more formally stated as: o Let "Delimiterchars" be a set of characters with the Unicode BIDI properties CS, WS, ON. (These are commonly used to delimit labels - both the FULL STOP and the space are included.) * ET, though it commonly occurs next to domain names in practice, is problematic: the context R CS L EN ET (for instance A.a1%) makes the label L EN not satisfy the character grouping requirement. * ES commonly occurs in labels as HYPHEN-MINUS, but could also be used as a delimiter (for instance, the plus sign). It is left out here. o Let "unproblematic label" be a label that either satisfies the requirements, or does not contain any character with the BIDI properties R, AL or AN, and does not begin with a character with the BIDI property EN. (Informally, "it does not start with a number".) A label L satisfies the Character Grouping requirement when, for any Delimiter Character D1 and D2, and for any label S1 and S2 that is either a label satisfying the requirements or an unproblematic label, the following holds true: If the string formed by concatenating S1, D1, L, D2 and S2 is reordered according to the BIDI algorithm, then all the characters of L in the reordered string are between D1 and D2, and no other characters are between D1 and D2, both if the overall paragraph direction is LTR and if the overall paragraph direction is RTL. Note that the definition is self-referential, since S1 and S2 are constrained to be "legal" by this definition; this makes testing changes to proposed rules a little complex, but does not create problems for testing whether or not a given proposed rule satisfies the criterion. (The "zero-length" case represents the case where a domain name is next to something that isn't a domain name, separated by a delimiter character). The Label Uniqueness requirement can be formally stated as: If two labels L and L', embedded as for the test above, displayed in paragraphs with the same directionality, are reordered by the BIDI algorithm into the same sequence of codepoints, at most one of the labels L and L' is a legal label. Alvestrand & Karp Expires June 3, 2009 [Page 9] Internet-Draft IDNA RTL fix November 2008 4. Examples of issues found 4.1. Dhivehi Dhivehi, the official language of the Maldives, is written with the Thaana script. This displays some of the characteristics of Arabic script, including its directional properties, and the indication of vowels by the diacritical marking of consonantal base characters. This marking is obligatory, and both double vowels and syllable-final consonants are indicated by the marking of special unvoiced characters. Every Dhivehi word therefore ends with a combining mark. The word for "computer", which is romanized as "konpeetaru", is written with the following sequence of Unicode code points: U+0786 THAANA LETTER KAAFU (AL) U+07AE THAANA OBOFILI (NSM) U+0782 THAANA LETTER NOONU (AL) U+07B0 THAANA SUKUN (NSM) U+0795 THAANA LETTER PAVIYANI (AL) U+07A9 THAANA LETTER EEBEEFILI (AL) U+0793 THAANA LETTER TAVIYANI (AL) U+07A6 THAANA ABAFILI (NSM) U+0783 THAANA LETTER RAA (AL) U+07AA THAANA UBIUFILI (NSM) The directionality class of U+07AA in the Unicode database [Unicode] is NSM (non-spacing mark), which is not R or AL; a conformant implementation of the IDNA2003 algorithm will say that "this is not in RandALCat", and refuse to encode the string. 4.2. Yiddish Yiddish is one of several languages written with the Hebrew script (others include Hebrew and Ladino). This is basically a consonantal alphabet (also termed an "abjad") but Yiddish is written using an extended form that is fully vocalic. The vowels are indicated in several ways, of which one is by repurposing letters that are consonants in Hebrew. Other letters are used both as vowels and Alvestrand & Karp Expires June 3, 2009 [Page 10] Internet-Draft IDNA RTL fix November 2008 consonants, with combining marks, called "points", used to differentiate between them. Finally, some base characters can indicate several different vowels, which are also disambiguated by combining marks. Pointed characters can appear in word-final position and may therefore also be needed at the end of labels. This is not an invariable attribute of a Yiddish string and there is thus greater latitude here than there is with Dhivehi. The organization now known as the "YIVO Institute for Jewish Research" developed orthographic rules for modern Standard Yiddish during the 1930s on the basis of work conducted in several venues since earlier in that century. These are given in, "The Standardized Yiddish Orthography: Rules of Yiddish Spelling" [SYO], and are taken as normatively descriptive of modern Standard Yiddish in any context where that notion is deemed relevant. They have been applied exclusively in all Yiddish dictionaries published since their establishment, and are similarly dominant in academic and bibliographic regards. It therefore appears appropriate for this repertoire also to be supported fully by IDNA. This presents no difficulty with characters in initial and medial positions, but pointed characters are regularly used in final position as well. All of the characters in the SYO repertoire appear in both marked and unmarked form with one exception: the HEBREW LETTER PE (U+05E4). The SYO only permits this with a HEBREW POINT DAGESH (U+05BC), providing the Yiddish equivalent to the Latin letter "p", or a HEBREW POINT RAFE (U+05BF), equivalent to the Latin letter "f". There is, however, a separate unpointed allograph, the HEBREW LETTER FINAL PE (U+05E3), for the latter character when it appears in final position. The constraint on the use of the SYO repertoire resulting from the proscription of combining marks at the end of RTL strings thus reduces to nothing more, or less, than the equivalent of saying that a string of Latin characters cannot end with the letter "p". It must also be noted that the HEBREW LETTER PE with HEBREW POINT DAGESH is characteristic of almost all traditional Yiddish orthographies that predate (or remain in use in parallel to) the SYO, being the first pointed character to appear in any of them. A more general instantiation of the basic problem can be seen in the representation of the YIVO acronym. This is written with the Hebrew letters YOD YOD HIRIQ VAV VAV ALEF QAMATS, where HIRIQ and QAMATS are combining points: U+05D9 HEBREW LETTER YOD (R) U+05B4 HEBREW POINT HIRIQ (NSM) Alvestrand & Karp Expires June 3, 2009 [Page 11] Internet-Draft IDNA RTL fix November 2008 U+05D5 HEBREW LETTER VAV (R) U+05D0 HEBREW LETTER ALEF (R) U+05B8 HEBREW POINT QAMATS (NSM) The directionality class of U+05B8 HEBREW POINT QAMATS in the Unicode database is NSM, which again causes the IDNA2003 algorithm to reject the string. It may also be noted that all of the combined characters mentioned above exist in precomposed form at separate positions in the Unicode chart. However, by invoking Stringprep, the IDNA2003 algorithm also rejects those codepoints, for reasons not discussed here. 4.3. Strings with numbers By requiring that the first or last character of a string be category R or AL, RFC 3454 prohibited a string containing right-to-left characters from ending with a number. Consider the strings ALEF 5 (HEBREW LETTER ALEF + DIGIT FIVE) and 5 ALEF. Displayed in a LTR context, the first one will be displayed from left to right as 5 ALEF (with the 5 being considered right-to- left because of the leading ALEF), while 5 ALEF will be displayed in exactly the same order (5 taking the direction from context). Clearly, only one of those should be permitted as a registered label, but barring them both seems to require justification. 5. Troublesome situations and guidelines There are situations in which labels that satisfy the rule above will be displayed in a surprising fashion; the most important of these is the case where a label ending in a character with BIDI property AL, AN or R occurs before a label beginning with a character of BIDI property EN. In that case, the number will appear to move into the label containing the right-to-left character, violating the Character Grouping requirement. If the label that occurs after the right-to-left label itself satisfies the BIDI criterion, the requirements will be satisfied in all cases (this is the reason why the criterion talks about strings containing L in some cases). However, the WG concluded that this could not be required for several reasons: Alvestrand & Karp Expires June 3, 2009 [Page 12] Internet-Draft IDNA RTL fix November 2008 o There is a large current deployment of ASCII domain names starting with digits. These cannot possibly be invalidated. o Domain names are often constructed piecemeal, for instance by combining a string with the content of a search list. This may occur after IDNA processing, and thus in part of the code that is not IDNA-aware, making detection of the undesirable combination impossible. o Even if a label is registered under a "safe" label, there may be a DNAME [RFC2672]with an "unsafe" label that points to the "safe" label, thus creating seemingly-valid names that would not satisfy the criterion. o Wildcards create the odd situation where a label is "valid" (can be looked up successfully) without the zone owner's knowing that this label exists. So an owner of a zone whose name starts with a digit and contains a wildcard has no way of controlling whether or not names with RTL labels in them are looked up in his zone. So rather than trying to suggest rules that disallow all such undesirable situations, this document merely warns about the possibility. 6. Other issues in need of resolution This document concerns itself only with the rules that are needed when dealing with domain names with characters that have differing BIDI properties, and considers characters only in terms of their BIDI properties. All other issues with these scripts have to be considered in other contexts. One such issue is the need to keep numbers separate; several scripts, such as Arabic, have multiple sets of numbers. The algorithm in this document disallows occurrences of AN-class characters ("Arabic-Indic digits", U+0660 to U+0669) together with EN-class characters (which includes "European" digits, U+0030 to U+0039 and "extended Arabic- Indic digits", U+06F0 to U+06F9), but does not help in preventing the mixing of, for instance, Bengali digits (U+09E6 to U+09EF) and Gujarati digits (U+0AE6 to U+0AEF), both of which have BIDI class L. A registry or script community that wishes to create rules for the mixing of digits in a label will be able to specify these restrictions at the registry level; rules can also be specified at the protocol level, but while the example above involves right-to- left characters, this is not inherently a bidi problem. Another set of issues concerns the proper display of IDNs with a Alvestrand & Karp Expires June 3, 2009 [Page 13] Internet-Draft IDNA RTL fix November 2008 mixture of LTR and RTL labels, or only RTL labels. It is unrealistic to expect that applications will display domain names using embedded formatting codes between their labels (for one thing, no reliable algorithms for identifying domain names in running text exist); thus, the display order will be determined by the bidirectional algorithm. Thus, a sequence (in network order) of R1.R2.ltr will be displayed in the order 2R.1R.ltr in an LTR context, which might surprise someone expecting to see labels displayed in hierarchical order. Again, this memo does not attempt to suggest a solution to this problem. 7. Compatibility considerations 7.1. Backwards compatibility considerations As with any change to an existing standard, it is important to consider what happens with existing implementations when the change is introduced. The following troublesome cases have been noted: o Old program used to input the newly-allowed string. If the old program checks the input against RFC 3454, the string will not be allowed, and that domain name will remain inaccessible. o Old program is asked to display the newly-allowed string, and checks it against RFC 3454 before displaying. The program will perform some kind of fallback, most likely displaying the string in A-label form. o Old program tries to display the newly-allowed string. If the old program has code for displaying the last character of a string that is different from the code used to display the characters in the middle of the string, display may be inconsistent and cause confusion. One particular example of the last case is if a program chooses to examine the last character (in network order) of a string in order to determine its directionality, rather than its first; if it finds an NSM character and tries to display the string as if it was a left-to- right string, the resulting display may be interesting, but not useful. The editors believe that these cases will have less harmful impact in practice than continuing to deny the use of words from the languages for which these strings are necessary as IDN labels. This specification does not forbid using leading European numbers in Alvestrand & Karp Expires June 3, 2009 [Page 14] Internet-Draft IDNA RTL fix November 2008 ASCII-only labels, since this would in conflict with a large installed base of such labels, and would increase the scope of the specification from RTL labels to all labels. The harm resulting from this limitation of scope is described in Section 5. Zone managers, both registries and private zone managers, can check for this particular condition before they allow registration of any string with right-to-left characters in it; generally it is best to not allow registration of any right-to-left strings in a zone where the label at the level above begins with a digit. 7.2. Forward compatibility considerations This text is, intentionally, specified strictly in terms of the Unicode BIDI properties. The determination that the condition is sufficient to fulfil the criteria depends on the Unicode BIDI algorithm; it is unlikely that drastic changes will be made to this algorithm. However, the determination of validity for any string depends on the Unicode BIDI property values, which are not declared immutable by the Unicode Consortium. Furthermore, the behaviour of the algorithm for any given character is likely to be linguistically and culturally sensitive, so that it's not unlikely that later versions of the Unicode standard may change the BIDI properties assigned to certain Unicode characters. This memo does not propose a solution for this problem. 8. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 9. Security Considerations This modification will allow some strings to be used in Stringprep contexts that are not allowed today. It is possible that differences in the interpretation of the specification between old and new implementations could pose a security risk, but it is difficult to envision any specific instantiation of this. Any rational attempt to compute, for instance, a hash over an identifier processed by Stringprep would use network order for its computation, and thus be unaffected by the changes proposed here. Alvestrand & Karp Expires June 3, 2009 [Page 15] Internet-Draft IDNA RTL fix November 2008 While it is not believed to pose a problem, if display routines had been written with specific knowledge of the RFC 3454 Stringprep prohibitions, it is possible that the potential problems noted under "backwards compatibility" could cause new kinds of confusion. 10. Acknowledgements While the listed editors held the pen, this document represents the joint work and conclusions of an ad hoc design team. In addition to the editors this consisted of, in alphabetic order, Tina Dam, Patrik Faltstrom, and John Klensin. Many further specific contributions and helpful comments were received from the people listed below, and others who have contributed to the development and use of the IDNA protocols. The team wishes in particular to thank Roozbeh Pournader for calling its attention to the issue with the Thaana script, Paul Hoffmann for pointing out the need to be explicit about backwards compatibility considerations, Ken Whistler for suggesting the basis of the formalized "character grouping" requirement, Mark Davis for commentary, Erik van der Poel for careful review, comments and verification of the rulesets, and Marcos Sanz, Andrew Sullivan and Pete Resnick for reviews. 11. References 11.1. Normative references [I-D.ietf-idnabis-defs] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", draft-ietf-idnabis-defs-01 (work in progress), November 2008. [UAX9] Davis, M., "Unicode Standard Annex #9: The Bidirectional Algorithm, revision 15", 03 2005. [Unicode] Unicode, "The Unicode Standard - version 5.1", 2008. 11.2. Informative references [I-D.ietf-idnabis-protocol] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", draft-ietf-idnabis-protocol-06 (work in progress), November 2008. Alvestrand & Karp Expires June 3, 2009 [Page 16] Internet-Draft IDNA RTL fix November 2008 [RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC 2672, August 1999. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [SYO] "The Standardized Yiddish Orthography: Rules of Yiddish Spelling, 6th ed., , New York, ISBN 0-914512-25-0",", 1999. Appendix A. Change log This appendix is intended to be removed by the RFC Editor when this document is published as an RFC. A.1. Changes from draft-alvestrand-00 to -01 Suggested a possible new algorithm. Multiple smaller changes. A.2. Changes from alvestrand-01 to -02 Date of publication updated. Change log added. A.3. Changes from alvestrand-02 to -03 Intro changed to reflect addressing the deeper issues with the BIDI algorithm. Gave formalized criteria for "valid strings", and documented the new set of requirements for strings that satisfy the criteria. Removed most of section 5, "Other problems", and noted that this memo focuses ONLY on issues that can be evaluated by looking at the BIDI properties of characters. A.4. Changes from alvestrand-03 to -04 Added back AN to the list of allowed characters; it had been left out by accident in -03. Removed some rules that were redundant. Alvestrand & Karp Expires June 3, 2009 [Page 17] Internet-Draft IDNA RTL fix November 2008 Added some considerations for backwards compatibility and interaction with ASCII labels that start with a number. Mentioned the issue with DNAME pointing to a zone containing RTL labels in the security considerations section. Wording updates in multiple places, including some spelling errors. Rewrote the introduction section. Split references into "normative" and "informative". A.5. Changes from draft-alvestrand-04 to draft-ietf -00 Changed name of draft. Added a couple of "note in draft" statements to remind the WG of open issues. Noted that BIDI controls in the paragraph are unproblematic with the given ruleset. A.6. Changes from idnabis -00 to -01 Added text to section 5 describing issues with mixture of numbers in labels Addressed some of the issues raised by Mark Davis in March 2008 in regard to document clarity. Changed the formulation of the label uniqueness requirement to be consistent with the text under "Labels with numbers". Spell-checked document. A.7. Changes from idnabis -01 to -02 Changed the domain of applicability to be only labels containing RTL characters, described the conditions under which harm may result from putting RTL labels next to other labels, and how to detect them. A number of clarification and formatting changes in response to reviews. Alvestrand & Karp Expires June 3, 2009 [Page 18] Internet-Draft IDNA RTL fix November 2008 A.8. Changes from idnabis -02 to -03 Rearranged section list so that the normative material is collected at the front. Moved list of BIDI properties into "terminology" Clarified that only terminology and the BIDI test is normative Changed reference to point to -defs for definitions instead of -rationale Minor fixes in response to comments, wording cleanups, removed all tentative language. Authors' Addresses Harald Tveit Alvestrand (editor) Google Beddingen 10 Trondheim, 7014 Norway Email: harald@alvestrand.no Cary Karp Swedish Museum of Natural History Frescativ. 40 Stockholm, 10405 Sweden Phone: +46 8 5195 4055 Fax: Email: ck@nrm.museum URI: Alvestrand & Karp Expires June 3, 2009 [Page 19] Internet-Draft IDNA RTL fix November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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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