Guidelines for Internationalized Resource Identifiers with Bi-directional Characters (Bidi IRIs)
draft-ietf-iri-bidi-guidelines-02

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 September 08, 2012.

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Table of Contents

• 1. Introduction
• 1.1. 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. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

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 Latin scripts, 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 the characters are read and stored on computers. The visual order represents the order the characters are drawn on a computer display or printout in the way a human expects to read them.

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 chracters.

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.

1.1. Notation

In this document, "Bidi Notation" 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 Arqbic 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].

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], [UNI9]. 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], 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 mainly for plain 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] 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] 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 and end with right-to-left characters.

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] 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]. 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. 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 are not followed. The examples below can be seen at [BidiEx], in Arabic, Hebrew, and Bidi Notation variants.

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.

Example 1: A single component with rtl characters is inverted:
Logical representation: "http://ab.CDEFGH.ij/kl/mn/op.html"
Visual representation: "http://ab.HGFEDC.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: "http://ab.CDE.FGH/ij/kl/mn/op.html"
Visual representation: "http://ab.HGF.EDC/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: "http://AB.CD.EF/GH/IJ/KL?MN=OP;QR=ST#UV"
Visual representation: "http://VU#TS=RQ;PO=NM?LK/JI/HG/FE.DC.BA"
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: "http://AB.CD.ef/gh/IJ/KL.html"
Visual representation: "http://DC.BA.ef/gh/LK/JI.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: "http://ab.cd.EF/GH/ij/kl.html"
Visual representation: "http://ab.cd.HG/FE/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: "http://ab.CD.EF/GH/IJ/kl.html"
Visual representation: "http://ab.JI/HG/FE.DC/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: "http://ab.CDE123FGH.ij/kl/mn/op.html"
Visual representation: "http://ab.HGF123EDC.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: "http://ab.cd.ef/GH1/2IJ/KL.html"
Visual representation: "http://ab.cd.ef/LK/JI1/2HG.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: "http://ab.cd.ef/GH%31/%32IJ/KL.html",
Visual representation: "http://ab.cd.ef/LK/JI%32/%31HG.html"

Example 10 (allowed but not recommended):
Logical representation: "http://ab.CDEFGH.123/kl/mn/op.html"
Visual representation: "http://ab.123.HGFEDC/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: "http://ab.CDEFGH.123ij/kl/mn/op.html"
Visual representation: "http://ab.123.HGFEDCij/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 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] and [RFC3987bis] and the acknowledgments of those documents apply.

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses