Internationalized Resource Identifiers M. Duerst
(iri) Aoyama Gakuin University
Internet-Draft L. Masinter
Intended status: BCP Adobe
Expires: September 10, 2012 A. Allawi
Diwan Software Limited
March 9, 2012
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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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 . . . . . . . . . . . . . . . . . . . . . . 5
4. Input of Bidi IRIs . . . . . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
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 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
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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
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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.
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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
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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"
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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
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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.
[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 Unicode 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 Bidi IRIs",
<http://www.w3.org/International/iri-edit/BidiExamples>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
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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/
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
Adil Allawi
Diwan Software Limited
37-39 Peckham Road
London SE5 8UH
United Kingdom
Phone: +44 7718 785850
Fax: +44 20 72525444
Email: adil@diwan.com
URI: http://ironymark.diwan.com/
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