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Internationalized Resource Identifiers M. Duerst
(iri) Aoyama Gakuin University
Internet-Draft L. Masinter
Intended status: BCP Adobe
Expires: February 15, 2012 August 14, 2011
Guidelines for Internationalized Resource Identifiers with Bi-
directional Characters (Bidi IRIs)
draft-ietf-iri-bidi-guidelines-00
Abstract
This specification gives guidelines for selection, use, presentation
of International Resource Identifiers (IRI) which include characters
with in inherent right-to-left (rtl) writing direction.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on February 15, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notation . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Logical Storage and Visual Presentation . . . . . . . . . . . . 3
3. Bidi IRI Structure . . . . . . . . . . . . . . . . . . . . . . 4
4. Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Some UCS characters, such as those used in the Arabic and Hebrew
scripts, have an inherent right-to-left (rtl) writing direction.
IRIs containing these characters (called bidirectional IRIs or Bidi
IRIs) require additional attention because of the non-trivial
relation between logical representation (used for digital
representation and for reading/spelling) and visual representation
(used for display/printing).
Because of the 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 bidirectional examples:
Lower case letters stand for Latin letters or other letters that are
written left to right, whereas upper case letters represent Arabic or
Hebrew letters that are written right to left.
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, bidirectional
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 bidirectional IRIs can be processed in the same way as
other IRIs.
Bidirectional IRIs MUST be rendered by using the Unicode
Bidirectional Algorithm [UNIV6], [UNI9]. Bidirectional IRIs MUST be
rendered in the same way as they would be if they were in a left-to-
right embedding; i.e., as if they were preceded by U+202A, LEFT-TO-
RIGHT EMBEDDING (LRE), and followed by U+202C, POP DIRECTIONAL
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FORMATTING (PDF). Setting the embedding direction can also be done
in a higher-level protocol (e.g., the dir='ltr' attribute in HTML).
There is no requirement to use the above embedding if the display is
still the same without the embedding. For example, a bidirectional
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 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.
In some other cases, using U+200E, LEFT-TO-RIGHT MARK (LRM), may be
sufficient to force the correct display behavior. However, the
details of the Unicode Bidirectional algorithm are not always easy to
understand. Implementers are strongly advised to err on the side of
caution and 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 running
text. To make sure that it does not affect the rendering of
bidirectional IRIs too much, some restrictions on bidirectional 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
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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).
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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 bidirectional IRIs, in Bidi Notation.
It shows legal IRIs with the relationship between 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"
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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 bidi
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 bidi 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.
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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
[ASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[ISO10646]
International Organization for Standardization, "ISO/IEC
10646:2003: Information Technology - Universal Multiple-
Octet Coded Character Set (UCS)", ISO Standard 10646,
December 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
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[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, March 2003.
[RFC3987bis]
Duerst, M., Masinter, L., and M. Suignard,
"Internationalized Resource Identifiers (IRIs)",
August 2011,
<http://tools.ietf.org/id/draft-ietf-iri-3987bis>.
[UNI9] Davis, M., "The Bidirectional Algorithm", Unicode Standard
Annex #9, March 2004,
<http://www.unicode.org/reports/tr9/tr9-13.html>.
[UNIV6] The Unicode Consortium, "The Unicode Standard, Version
6.0.0 (Mountain View, CA, The Unicode Consortium, 2011,
ISBN 978-1-936213-01-6)", October 2010.
9.2. Informative References
[BidiEx] "Examples of bidirectional IRIs",
<http://www.w3.org/International/iri-edit/BidiExamples>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
Authors' Addresses
Martin Duerst
Aoyama Gakuin University
5-10-1 Fuchinobe
Sagamihara, Kanagawa 229-8558
Japan
Phone: +81 42 759 6329
Fax: +81 42 759 6495
Email: duerst@it.aoyama.ac.jp
URI: http://www.sw.it.aoyama.ac.jp/D%C3%BCrst/
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Larry Masinter
Adobe
345 Park Ave
San Jose, CA 95110
U.S.A.
Phone: +1-408-536-3024
Email: masinter@adobe.com
URI: http://larry.masinter.net
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