Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs)
draft-ietf-iri-bidi-guidelines-03

Abstract

This specification gives guidelines for selection, use, and presentation of International Resource Identifiers (IRIs) which include characters with inherent right-to-left (rtl) writing direction.

Status of this Memo

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This Internet-Draft will expire on April 24, 2013.

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Table of Contents

1.  Introduction
    1.1.  Overview
    1.2.  Availability
    1.3.  Notation
2.  Logical Storage and Visual Presentation
3.  Bidi IRI Structure
4.  Input of Bidi IRIs
5.  Examples
6.  IANA Considerations
7.  Security Considerations
8.  Acknowledgements
9.  Main Changes Since RFC 3987
10.  References
    10.1.  Normative References
    10.2.  Informative References
Appendix A.  List of ASCII Symbols and their Bidirectional Character Types
§  Authors' Addresses

1.  Introduction

1.1.  Overview

Some UCS characters, such as those used in the Arabic and Hebrew scripts, have an inherent right-to-left (rtl) writing direction as opposed to characters, such as those in the Latin script, that have an inherent left-to-right (ltr) direction. IRIs containing rtl characters (called bidirectional IRIs or Bidi IRIs) require additional attention because of the non-trivial relation between their logical and visual ordering. The logical order represents the order in which characters are stored on computers and read by people. The visual order is the order in which the characters appear (or are expected to appear) on a computer display or printout.

Generally, alphabetic characters in scripts like Arabic and Hebrew are drawn rtl while numbers are drawn ltr. Symbols such as slash ('/') and period ('.') take their visual direction from the surrounding characters. A list of all ASCII symbols with their bidirectional character type and their function in URIs and IRIs is given in Appendix A (List of ASCII Symbols and their Bidirectional Character Types).

Because of this complex interaction between the logical representation, the visual representation, and the syntax of a Bidi IRI, a balance is needed between various requirements. The main requirements are:

1.

user-predictable conversion between visual and logical representation;

2.

the ability to include a wide range of characters in various parts of the IRI; and

3.

minor or no changes or restrictions for implementations.

1.2.  Availability

This document is available in (line-printer ready) plaintext ASCII and in PDF. It is also available in HTML from
http://www.sw.it.aoyama.ac.jp/2012/pub/draft-ietf-iri-bidi-guidelines-03.html, and in UTF-8 plaintext from
http://www.sw.it.aoyama.ac.jp/2012/pub/draft-ietf-iri-bidi-guidelines-03.utf8.txt. While all these versions are identical in their technical content, the HTML, PDF, and UTF-8 plaintext versions show non-Unicode characters directly. This often makes it easier to understand examples, and readers are therefore strongly advised to consult one of these versions in preference to or as a supplement to the ASCII version.

This version of this document contains bidirectional examples. In order to correctly understand the examples, it is important to view this document with a viewer that correctly implements the Unicode Bidirectional Algorithm [UNI9] (Davis, M., “The Unicode Bidirectional Algorithm,” September 2012.). Many text viewers and text editors, and all major browsers, currently implement the Unicode Bidirectional Algorithm. Also, all users who are reading RTL text on a regular basis have viewers that implement this algorithm, because otherwise, they would be unable to read even the simplest texts. In order to check whether a viewer implements the Unicode Bidirectional Algorithm, please observe the following three lines:
FEDCBA ,EDCBA ,DCBA, CBA, BA, A
ب, بت, بتث, بتثج, بتثجح, بتثجحخ
א, אב, אבג, אבגד, אבגדה, אבגדהו
The first line contains upper-case Latin letters, the second line contains Arabic letters, and the third line contains Hebrew letters. Your viewer will be okay if in all three lines, the shortest word (one character) is on the right, and the longest word (six characters) on the left, the words are getting longer and longer from right to left, and the commas are between the words, but on the right of the spaces. Otherwise, please use another viewer. In the second line, the characters in each word should all be connected, and change shape slighly on context. In the first and third line, no characters should be connected.

1.3.  Notation

In this document, "Bidi Notation", abbreviated "BN" is used for the given Bidi IRI examples as follows: Lower case letters a-z stand for characters that are written with a left to right ordering (such as Latin characters), whereas upper case letters A-Z represent characters that are written right to left (such as Arabic or Hebrew characters). Numbers and symbols are the same.

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Logical Storage and Visual Presentation

When stored or transmitted in digital representation, Bidi IRIs MUST be in full logical order and MUST conform to the IRI syntax rules (which includes the rules relevant to their scheme). This ensures that Bidi IRIs can be processed in the same way as other IRIs.

Bidi IRIs MUST be visually ordered by the Unicode Bidirectional Algorithm [UNIV6] (The Unicode Consortium, “The Unicode Standard, Version 6.2.0 (Mountain View, CA, The Unicode Consortium, 2012, ISBN 978-1-936213-07-8),” October 2012.), [UNI9] (Davis, M., “The Unicode Bidirectional Algorithm,” September 2012.). Bidi IRIs MUST be rendered in the same way as they would be if they were in a left-to-right embedding.

In conformance with the Unicode Bidirectional Algorithm, embedding MAY be done in one of two ways:

1.

precede the IRI with U+202A, LEFT-TO-RIGHT EMBEDDING (LRE), and follow with U+202C, POP DIRECTIONAL FORMATTING (PDF); or

2.

use a higher-level protocol (e.g., the dir='ltr' attribute in HTML).

Preceding and following the Bidi IRI with U+200E, LEFT-TO-RIGHT MARK (LRM) is NOT RECOMMENDED as, there are cases where this may not be sufficient to match full left to right embedding.

There is no requirement to use embedding if the display is still the same without the embedding. For example, a Bidi IRI in a text with left-to-right base directionality (such as used for English or Cyrillic) that is preceded and followed by whitespace and strong left-to-right characters does not need an embedding. Also, a bidirectional relative IRI reference that only contains strong right-to-left characters and weak characters (such as symbols) and that starts and ends with a strong right-to-left character and appears in a text with right-to-left base directionality (such as used for Arabic or Hebrew) and is preceded and followed by whitespace and strong characters does not need an embedding.

However, implementers are RECOMMENDED to use embedding in all cases where they are not completely sure that the display behavior is unaffected without the embedding.

The Unicode Bidirectional Algorithm ([UNI9] (Davis, M., “The Unicode Bidirectional Algorithm,” September 2012.), section 4.3) permits higher-level protocols to influence bidirectional rendering. Such changes by higher-level protocols MUST NOT be used if they change the rendering of IRIs.

The bidirectional formatting characters that may be used before or after the IRI to ensure correct display are not themselves part of the IRI. IRIs MUST NOT contain bidirectional formatting characters (LRM, RLM, LRE, RLE, LRO, RLO, and PDF). They affect the visual rendering of the IRI but do not appear themselves. It would therefore not be possible to input an IRI with such characters correctly.

3.  Bidi IRI Structure

The Unicode Bidirectional Algorithm is designed for general purpose text. To make sure that it does not affect the rendering of Bidi IRIs outside of the requirements of this document, some restrictions on Bidi IRIs are necessary. These restrictions are given in terms of delimiters (structural characters, mostly punctuation such as "@", ".", ":", and "/") and components (usually consisting mostly of letters and digits).

The following syntax rules from the ABNF of [RFC3987bis] (Dürst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” October 2012.) correspond to components for the purpose of Bidi behavior: iuserinfo, ireg-name, isegment, isegment-nz, isegment-nz-nc, ireg-name, iquery, and ifragment.

Specifications that define the syntax of any of the above components MAY divide them further and define smaller parts to be components according to this document. As an example, the restrictions of [RFC3490] (Fältström, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) on bidirectional domain names correspond to treating each label of a domain name as a component for schemes with ireg-name as a domain name. Even where the components are not defined formally, it may be helpful to think about some syntax in terms of components and to apply the relevant restrictions. For example, for the usual name/value syntax in query parts, it is convenient to treat each name and each value as a component. As another example, the extensions in a resource name can be treated as separate components.

For each component, the following restrictions apply:

1.

A component SHOULD NOT use both right-to-left and left-to-right characters.

2.

A component using right-to-left characters SHOULD start with a right-to-left character, and end with a right-to-left character potentially followed by one or more nonspacing mark (bidi class NSM).

The above restrictions are given as "SHOULD"s, rather than as "MUST"s. For IRIs that are never presented visually, they are not relevant. However, for IRIs in general, they are very important to ensure consistent conversion between visual presentation and logical representation, in both directions.

Note:

In some components, the above restrictions may actually be strictly enforced. For example, [RFC3490] (Fältström, P., Hoffman, P., and A. Costello, “Internationalizing Domain Names in Applications (IDNA),” March 2003.) requires that these restrictions apply to the labels of a host name for those schemes where ireg-name is a host name. In some other components (for example, path components) following these restrictions may not be too difficult. For other components, such as parts of the query part, it may be very difficult to enforce the restrictions because the values of query parameters may be arbitrary character sequences.

If the above restrictions cannot be satisfied otherwise, the affected component can always be mapped to URI notation using the general percent-encoding of IRI components, as described in [RFC3987bis] (Dürst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” October 2012.). Please note that the whole component has to be mapped (see also Example 9 below).

4.  Input of Bidi IRIs

Bidi input methods MUST generate Bidi IRIs in logical order while rendering them according to Section 2 (Logical Storage and Visual Presentation). During input, rendering SHOULD be updated after every new character is input to avoid end-user confusion.

5.  Examples

This section gives examples of Bidi IRIs in Bidi Notation. It shows legal IRIs with the relationship between their logical and visual representation and explains how certain phenomena in this relationship may look strange to somebody not familiar with bidirectional behavior, but familiar to users of Arabic and Hebrew. It also shows what happens if the restrictions given in Section 3 (Bidi IRI Structure) are not followed.

To read the bidi text in the examples, read the visual representation from left to right until you encounter a block of rtl text. Read the rtl block (including slashes and other special characters) from right to left, then continue at the next unread ltr character.

Please note that "BN" stands for "Bidi Notation", see Notation. AR stands for Arabic, HE for Hebrew.

Example 1: A single component with rtl characters is inverted:
Logical representation (BN): "http://ab.CDEFGH.ij/kl/mn/op.html"
Visual representation (BN): "http://ab.HGFEDC.ij/kl/mn/op.html"
Visual representation (AR): "http://ab.تثجحخد.ij/kl/mn/op.html"
Visual representation (HE): "http://ab.גדהוזח.ij/kl/mn/op.html"
Components can be read one by one, and each component can be read in its natural direction.

Example 2: More than one consecutive component with rtl characters is inverted as a whole:
Logical representation (BN): "http://ab.CDE.FGH/ij/kl/mn/op.html"
Visual representation (BN): "http://ab.HGF.EDC/ij/kl/mn/op.html"
Visual representation (AR): "http://ab.تثج.حخد/ij/kl/mn/op.html"
Visual representation (HE): "http://ab.גדה.וזח/ij/kl/mn/op.html"
A sequence of rtl components is read rtl, in the same way as a sequence of rtl words is read rtl in a bidi text.

Example 3: All components of an IRI (except for the scheme) are rtl. All rtl components are inverted overall:
Logical representation (BN): "http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV"
Visual representation (BN): "http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA"
Visual representation (AR): "http://اب.تث.جح/خد/ذر/زس?شص=ضط;ظع=غف#قك"
Visual representation (HE): "http://אב.גד.הו/זח/טי/כל?מן=סע;פץ=קר#שת"
The whole IRI (except the scheme) is read rtl. Delimiters between rtl components stay between the respective components; delimiters between ltr and rtl components don't move.

Example 4: Each of several sequences of rtl components is inverted on its own:
Logical representation (BN): "http://AB.CD.ef/gh/IJ/KL.html"
Visual representation (BN): "http://DC.BA.ef/gh/LK/JI.html"
Visual representation (AR): "http://اب.تث.ef/gh/ذر/زس.html"
Visual representation (HE): "http://אב.גד.ef/gh/טי/כל.html"
Each sequence of rtl components is read rtl, in the same way as each sequence of rtl words in an ltr text is read rtl.

Example 5: Example 2, applied to components of different kinds:
Logical representation (BN): "http://ab.cd.EF/GH/ij/kl.html"
Visual representation (BN): "http://ab.cd.HG/FE/ij/kl.html"
Visual representation (AR): "http://ab.cd.جح/خد/ij/kl.html"
Visual representation (HE): "http://ab.cd.הו/זח/ij/kl.html"
The inversion of the domain name label and the path component may be unexpected, but it is consistent with other bidi behavior. For reassurance that the domain component really is "ab.cd.EF", it may be helpful to read aloud the visual representation following the Unicode Bidirectional Algorithm. After "http://ab.cd." one reads the RTL block "E-F-slash-G-H", which corresponds to the logical representation.

Example 6: Same as Example 5, with more rtl components:
Logical representation (BN): "http://ab.CD.EF/GH/IJ/kl.html"
Visual representation (BN): "http://ab.JI/HG/FE.DC/kl.html"
Visual representation (AR): "http://ab.تث.جح/خد/ذر/kl.html"
Visual representation (HE): "http://ab.גד.הו/זח/טי/kl.html"
The inversion of the domain name labels and the path components may be easier to identify because the delimiters also move.

Example 7: A single rtl component includes digits:
Logical representation (BN): "http://ab.CDE123FGH.ij/kl/mn/op.html"
Visual representation (BN): "http://ab.HGF123EDC.ij/kl/mn/op.html"
Visual representation (AR): "http://ab.تثج123حخد.ij/kl/mn/op.html"
Visual representation (HE): "http://ab.גדה123וזח.ij/kl/mn/op.html"
Numbers are written ltr in all cases but are treated as an additional embedding inside a run of rtl characters. This is completely consistent with usual bidirectional text.

Example 8 (not allowed): Numbers are at the start or end of an rtl component:
Logical representation (BN): "http://ab.cd.ef/GH1/2IJ/KL.html"
Visual representation (BN): "http://ab.cd.ef/LK/JI1/2HG.html"
Visual representation (AR): "http://ab.cd.ef/خد1/2ذر/زس.html"
Visual representation (HE): "http://ab.cd.ef/זח1/2טי/כל.html"
The sequence "1/2" is interpreted by the Bidirectional Algorithm as a fraction, fragmenting the components and leading to confusion. There are other characters that are interpreted in a special way close to numbers; in particular, "+", "-", "#", "$", "%", ",", ".", and ":".

Example 9 (not allowed): The numbers in the previous example are percent-encoded:
Logical representation (BN): "http://ab.cd.ef/GH%31/%32IJ/KL.html"
Visual representation (BN): "http://ab.cd.ef/LK/JI%32/%31HG.html"
Visual representation (AR): "http://ab.cd.ef/خد%31/%32ذر/زس.html"
Visual representation (HE): "http://ab.cd.ef/זח%31/%32טי/כל.html"

Example 10 (allowed but not recommended):
Logical representation (BN): "http://ab.CDEFGH.123/kl/mn/op.html"
Visual representation (BN): "http://ab.123.HGFEDC/kl/mn/op.html"
Visual representation (AR): "http://ab.تثجحخد.123/kl/mn/op.html"
Visual representation (HE): "http://ab.גדהוזח.123/kl/mn/op.html"
Components consisting of only numbers are allowed (it would be rather difficult to prohibit them), but these may interact with adjacent RTL components in ways that are not easy to predict.

Example 11 (allowed but not recommended):
Logical representation (BN): "http://ab.CDEFGH.123ij/kl/mn/op.html"
Visual representation (BN): "http://ab.123.HGFEDCij/kl/mn/op.html"
Visual representation (AR): "http://ab.تثجحخد.123ij/kl/mn/op.html"
Visual representation (HE): "http://ab.גדהוזח.123ij/kl/mn/op.html"
Components consisting of numbers and left-to-right characters are allowed, but these may interact with adjacent RTL components in ways that are not easy to predict.

6.  IANA Considerations

This document makes no changes to IANA registries.

7.  Security Considerations

Confusion can occur with bidirectional IRIs, if the restrictions in Section 3 (Bidi IRI Structure) are not followed. The same visual representation may be interpreted as different logical representations, and vice versa. It is also very important that a correct Unicode bidirectional implementation be used.

8.  Acknowledgements

This document was derived from [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) and [RFC3987bis] (Dürst, M., Masinter, L., and M. Suignard, “Internationalized Resource Identifiers (IRIs),” October 2012.) and the acknowledgments of those documents apply. Shunsuke Oshima (大嶋 俊介) provided the data for Appendix A (List of ASCII Symbols and their Bidirectional Character Types).

9.  Main Changes Since RFC 3987

This section describes the main changes since [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.).

• Separated out the section on bidi in [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) to this document.
• Added examples in Arabic and Hebrew, which can be seen in html/pdf/utf8.txt versions.
• Allowed NSMs at the end of components, for Dhivehi, Yiddish,...
• TODO: check for major changes between RFC3987 and draft -02.

Note to RFC Editor: Please remove this paragraph before publication. Detailled change logs are available in the IETF tools subversion repository at http://trac.tools.ietf.org/wg/iri/trac/log/draft-ietf-iri-3987bis/draft-ietf-iri-bidi-guidelines.xml.

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  List of ASCII Symbols and their Bidirectional Character Types

To help understand the influence of various symbols on IRI display, this appendix lists all of them, giving the character itself, the Unicode codepoint, the character name, the bidirectional character type (BCT) and the rule and relevance in the IRI syntax.

The most important ones in practice are ":", delimining schem and port (CS, Common Number Separator), "/" to indicate generic (hierarchical) schemes and as a path separator (CS, Common Number Separator), "?" to introduce a query part (ON, Other Neutral), "#" to introduce a fragment identifier (ET, European Number Terminator), "." to separate labels in a domain name (CS, Common Number Separator), "&" to separate form parameters (ON, Other Neutral), and "@" to separate user information (ON, Other Neutral).

Char Codepoint  Character Name       BCT  IRI syntax
-------------------------------------------------------------
"#"  U+0023     NUMBER SIGN          ET   gen-delims, fragments
"/"  U+002F     SOLIDUS              CS   gen-delims, paths
":"  U+003A     COLON                CS   gen-delims, scheme, port
"?"  U+003F     QUESTION MARK        ON   gen-delims, query part
"@"  U+0040     COMMERCIAL AT        ON   gen-delims, user
"["  U+005B     LEFT SQUARE BRACKET  ON   gen-delims
"]"  U+005D     RIGHT SQUARE BRACKET ON   gen-delims
"%"  U+0025     PERCENT SIGN         ET   pcd-encoded
"!"  U+0021     EXCLAMATION MARK     ON   sub-delims
","  U+002C     COMMA                CS   sub-delims
"+"  U+002B     PLUS SIGN            ES   sub-delims
"$"  U+0024     DOLLAR SIGN          ET   sub-delims
"("  U+0028     LEFT PARENTHESIS     ON   sub-delims
"'"  U+0027     APOSTROPHE           ON   sub-delims
")"  U+0029     RIGHT PARENTHESIS    ON   sub-delims
"*"  U+002A     ASTERISK             ON   sub-delims
";"  U+003B     SEMICOLON            ON   sub-delims
"="  U+003D     EQUALS SIGN          ON   sub-delims, forms
"&"  U+0026     AMPERSAND            ON   sub-delims, forms
"."  U+002E     FULL STOP            CS   unreserved, domain names
"-"  U+002D     HYPHEN-MINUS         ES   unreserved
"_"  U+005F     LOW LINE             ON   unreserved
"~"  U+007E     TILDE                ON   unreserved
" "  U+0020     SPACE                WS   excluded, delim
'"'  U+0022     QUOTATION MARK       ON   excluded, delim
"\"  U+005C     REVERSE SOLIDUS      ON   excluded, unwise
"^"  U+005E     CIRCUMFLEX ACCENT    ON   excluded, unwise
"<"  U+003C     LESS-THAN SIGN       ON   excluded, delim
">"  U+003E     GREATER-THAN SIGN    ON   excluded, delim
"`"  U+0060     GRAVE ACCENT         ON   excluded, unwise
"|"  U+007C     VERTICAL LINE        ON   excluded, unwise
"{"  U+007B     LEFT CURLY BRACKET   ON   excluded, delim
"}"  U+007D     RIGHT CURLY BRACKET  ON   excluded, delim

Authors' Addresses