Internationalized Resource Identifiers L. Masinter
(iri) Adobe
Internet-Draft M. Duerst
Intended status: Standards Track Aoyama Gakuin University
Expires: September 3, 2012 March 2, 2012
Equivalence and Canonicalization of Internationalized Resource
Identifiers (IRIs)
draft-ietf-iri-comparison-01
Abstract
Internationalized Resource Identifiers (IRIs) are unicode strings
used to identify resources on the Internet. Applications that use
IRIs often define a means of comparing two IRIs to determine when two
IRIs are equivalent for the purpose of that application. Some
applications also define a method for 'canonicalizing' or
'normalizing' an IRI -- translating one IRI into another which is
equivalent under the comparison method used.
This document gives guidelines and best practices for defining and
using IRI comparison, equivalence, normalization and canonicalization
methods.
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
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on September 3, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Comparison, Equivalence, Normalization and
Canonicalization . . . . . . . . . . . . . . . . . . . . . . . 5
4. Preparation for Comparison . . . . . . . . . . . . . . . . . . 5
5. Comparison Ladder . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Simple String Comparison . . . . . . . . . . . . . . . . . 6
5.2. Syntax-Based Equivalence . . . . . . . . . . . . . . . . . 7
5.2.1. Case Equivalence . . . . . . . . . . . . . . . . . . . 8
5.2.2. Unicode Character Normalization . . . . . . . . . . . 8
5.2.3. Percent-Encoding Equivalence . . . . . . . . . . . . . 9
5.2.4. Path Segment Equivalence . . . . . . . . . . . . . . . 10
5.3. Scheme-Based Comparison . . . . . . . . . . . . . . . . . 10
5.4. Protocol-Based Comparison . . . . . . . . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Internationalized Resource Identifiers (IRIs) are unicode strings
used to identify resources on the Internet. Applications that use
IRIs often define a means of comparing two IRIs to determine when two
IRIs are equivalent for the purpose of that application. Some
applications also define a method for 'canonicalizing' or
'normalizing' an IRI -- translating one IRI into another which is
equivalent under the comparison method used.
This document gives guidelines and best practices for defining and
using IRI comparison, equivalence, normalization and canonicalization
methods.
One of the most common operations on IRIs is simple comparison:
Determining whether two IRIs are equivalent, without using the IRIs
to access their respective resource(s). A comparison is performed
whenever a response cache is accessed, a browser checks its history
to color a link, or an XML parser processes tags within a namespace.
Extensive normalization prior to comparison of IRIs may be used by
spiders and indexing engines to prune a search space or reduce
duplication of request actions and response storage.
IRI comparison is performed for some particular purpose. Protocols
or implementations that compare IRIs for different purposes will
often be subject to differing design trade-offs in regards to how
much effort should be spent in reducing aliased identifiers. This
document describes various methods that may be used to compare IRIs,
the trade-offs between them, and the types of applications that might
use them.
2. Equivalence
Because IRIs exist to identify resources, presumably they should be
considered equivalent when they identify the same resource. However,
this definition of equivalence is not of much practical use, as there
is no way for an implementation to compare two resources to determine
if they are "the same" unless it has full knowledge or control of
them. For this reason, determination of equivalence or difference of
IRIs is based on string comparison, augmented by reference to
additional rules provided by scheme definition. We use the terms
"different" and "equivalent" to describe the possible outcomes of
such comparisons, but there are many application-dependent versions
of equivalence.
Even when it is possible to determine that two IRIs are equivalent,
IRI comparison is not sufficient to determine whether two IRIs
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identify different resources. For example, an owner of two different
domain names could decide to serve the same resource from both,
resulting in two different IRIs. Therefore, comparison methods are
designed to minimize false negatives while strictly avoiding false
positives.
In testing for equivalence, applications should not directly compare
relative references; the references should be converted to their
respective target IRIs before comparison. When IRIs are compared to
select (or avoid) a network action, such as retrieval of a
representation, fragment components (if any) MUST be excluded from
the comparison.
Applications using IRIs as identity tokens with no relationship to a
protocol MUST use the Simple String Comparison (see Section 5.1).
All other applications MUST select one of the comparison practices
from the Comparison Ladder (see Section 5.
3. Comparison, Equivalence, Normalization and Canonicalization
In general, when considering a set of items or strings, there are
several interrelated concepts. A comparison method determines,
between two items in the set, their relationship. In particular, a
comparison method for determining equivalence might result in a
determination that two (different) items are equivalent, known to be
different, or that equivalence isn't determined.
One way to define a comparison for equivalence is to define a a
normalization or canonicalization algorithm. For each item in a set
of equivalent items, one of them could be designated the "normal" or
"canonical" form.
These general concepts are used with IRIs in this document, and in
other circumstances, where a mapping from one sequence of Unicode
characters to another one could be described as a "normalization"
algorithm.
In general, this document tries to stay with the "equivalence" or
"comparison" methods, become some times the mathematical notion of
"normalization" results in forms that ordinary users might not
consider "normal" in an ordinary sense.
4. Preparation for Comparison
Any kind of IRI comparison REQUIRES that any additional contextual
processing is first performed, including undoing higher-level
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escapings or encodings in the protocol or format that carries an IRI.
This preprocessing is usually done when the protocol or format is
parsed.
Examples of such escapings or encodings are entities and numeric
character references in [HTML4] and [XML1]. As an example,
"http://example.org/rosé" (in HTML),
"http://example.org/rosé" (in HTML or XML), and
"http://example.org/rosé" (in HTML or XML) are all resolved into
what is denoted in this document (see 'Notation' section of
[RFC3987bis]) as "http://example.org/rosé" (the "é" here
standing for the actual e-acute character, to compensate for the fact
that this document cannot contain non-ASCII characters).
Similar considerations apply to encodings such as Transfer Codings in
HTTP (see [RFC2616]) and Content Transfer Encodings in MIME
([RFC2045]), although in these cases, the encoding is based not on
characters but on octets, and additional care is required to make
sure that characters, and not just arbitrary octets, are compared
(see Section 5.1).
5. Comparison Ladder
In practice, a variety of methods are used to test IRI equivalence.
These methods fall into a range distinguished by the amount of
processing required and the degree to which the probability of false
negatives is reduced. As noted above, false negatives cannot be
eliminated. In practice, their probability can be reduced, but this
reduction requires more processing and is not cost-effective for all
applications.
If this range of comparison practices is considered as a ladder, the
following discussion will climb the ladder, starting with practices
that are cheap but have a relatively higher chance of producing false
negatives, and proceeding to those that have higher computational
cost and lower risk of false negatives.
5.1. Simple String Comparison
If two IRIs, when considered as character strings, are identical,
then it is safe to conclude that they are equivalent. This type of
equivalence test has very low computational cost and is in wide use
in a variety of applications, particularly in the domain of parsing.
It is also used when a definitive answer to the question of IRI
equivalence is needed that is independent of the scheme used and that
can be calculated quickly and without accessing a network. An
example of such a case is XML Namespaces ([XMLNamespace]).
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Testing strings for equivalence requires some basic precautions.
This procedure is often referred to as "bit-for-bit" or "byte-for-
byte" comparison, which is potentially misleading. Testing strings
for equality is normally based on pair comparison of the characters
that make up the strings, starting from the first and proceeding
until both strings are exhausted and all characters are found to be
equal, until a pair of characters compares unequal, or until one of
the strings is exhausted before the other.
This character comparison requires that each pair of characters be
put in comparable encoding form. For example, should one IRI be
stored in a byte array in UTF-8 encoding form and the second in a
UTF-16 encoding form, bit-for-bit comparisons applied naively will
produce errors. It is better to speak of equality on a character-
for-character rather than on a byte-for-byte or bit-for-bit basis.
In practical terms, character-by-character comparisons should be done
codepoint by codepoint after conversion to a common character
encoding form. When comparing character by character, the comparison
function MUST NOT map IRIs to URIs, because such a mapping would
create additional spurious equivalences. It follows that an IRI
SHOULD NOT be modified when being transported if there is any chance
that this IRI might be used in a context that uses Simple String
Comparison.
False negatives are caused by the production and use of IRI aliases.
Unnecessary aliases can be reduced, regardless of the comparison
method, by consistently providing IRI references in an already
normalized form (i.e., a form identical to what would be produced
after normalization is applied, as described below). Protocols and
data formats often limit some IRI comparisons to simple string
comparison, based on the theory that people and implementations will,
in their own best interest, be consistent in providing IRI
references, or at least be consistent enough to negate any efficiency
that might be obtained from further normalization.
5.2. Syntax-Based Equivalence
Implementations may use logic based on the definitions provided by
this specification to reduce the probability of false negatives.
This processing is moderately higher in cost than character-for-
character string comparison. For example, an application using this
approach could reasonably consider the following two IRIs equivalent:
example://a/b/c/%7Bfoo%7D/rosé
eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
Web user agents, such as browsers, typically apply this type of IRI
normalization when determining whether a cached response is
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available. Syntax-based normalization includes such techniques as
case normalization, character normalization, percent-encoding
normalization, and removal of dot-segments.
5.2.1. Case Equivalence
For all IRIs, the hexadecimal digits within a percent-encoding
triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
should be considered equivalent to forms which use uppercase letters
for the digits A-F.
When an IRI uses components of the generic syntax, the component
syntax equivalence rules always apply; namely, that the scheme and
US-ASCII only host are case insensitive and therefore should be
normalized to lowercase. For example, the URI
"HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
Case equivalence for non-ASCII characters in IRI components that are
IDNs are discussed in Section 5.3. The other generic syntax
components are assumed to be case sensitive unless specifically
defined otherwise by the scheme.
Creating schemes that allow case-insensitive syntax components
containing non-ASCII characters should be avoided. Case
normalization of non-ASCII characters can be culturally dependent and
is always a complex operation. The only exception concerns non-ASCII
host names for which the character normalization includes a mapping
step derived from case folding.
5.2.2. Unicode Character Normalization
The Unicode Standard [UNIV6] defines various equivalences between
sequences of characters for various purposes. Unicode Standard Annex
#15 [UTR15] defines various Normalization Forms for these
equivalences, in particular Normalization Form C (NFC, Canonical
Decomposition, followed by Canonical Composition) and Normalization
Form KC (NFKC, Compatibility Decomposition, followed by Canonical
Composition).
IRIs already in Unicode MUST NOT be normalized before parsing or
interpreting. In many non-Unicode character encodings, some text
cannot be represented directly. For example, the word "Vietnam" is
natively written "Việt Nam" (containing a LATIN SMALL LETTER E
WITH CIRCUMFLEX AND DOT BELOW) in NFC, but a direct transcoding from
the windows-1258 character encoding leads to "Việt Nam"
(containing a LATIN SMALL LETTER E WITH CIRCUMFLEX followed by a
COMBINING DOT BELOW). Direct transcoding of other 8-bit encodings of
Vietnamese may lead to other representations.
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Equivalence of IRIs MUST rely on the assumption that IRIs are
appropriately pre-character-normalized rather than apply character
normalization when comparing two IRIs. The exceptions are conversion
from a non-digital form, and conversion from a non-UCS-based
character encoding to a UCS-based character encoding. In these
cases, NFC or a normalizing transcoder using NFC MUST be used for
interoperability. To avoid false negatives and problems with
transcoding, IRIs SHOULD be created by using NFC. Using NFKC may
avoid even more problems; for example, by choosing half-width Latin
letters instead of full-width ones, and full-width instead of half-
width Katakana.
As an example, "http://www.example.org/résumé.html" (in XML
Notation) is in NFC. On the other hand,
"http://www.example.org/résumé.html" is not in NFC.
The former uses precombined e-acute characters, and the latter uses
"e" characters followed by combining acute accents. Both usages are
defined as canonically equivalent in [UNIV6].
Note: Because it is unknown how a particular sequence of characters
is being treated with respect to character normalization, it would
be inappropriate to allow third parties to normalize an IRI
arbitrarily. This does not contradict the recommendation that
when a resource is created, its IRI should be as character
normalized as possible (i.e., NFC or even NFKC). This is similar
to the uppercase/lowercase problems. Some parts of a URI are case
insensitive (for example, the domain name). For others, it is
unclear whether they are case sensitive, case insensitive, or
something in between (e.g., case sensitive, but with a multiple
choice selection if the wrong case is used, instead of a direct
negative result). The best recipe is that the creator use a
reasonable capitalization and, when transferring the URI,
capitalization never be changed.
Various IRI schemes may allow the usage of Internationalized Domain
Names (IDN) [RFC5890] either in the ireg-name part or elsewhere.
Character Normalization also applies to IDNs, as discussed in
Section 5.3.
5.2.3. Percent-Encoding Equivalence
The percent-encoding mechanism (Section 2.1 of [RFC3986]) is a
frequent source of variance among otherwise identical IRIs. In
addition to the case normalization issue noted above, some IRI
producers percent-encode octets that do not require percent-encoding,
resulting in IRIs that are equivalent to their nonencoded
counterparts. These IRIs should be normalized by decoding any
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percent-encoded octet sequence that corresponds to an unreserved
character, as described in section 2.3 of [RFC3986].
For actual resolution, differences in percent-encoding (except for
the percent-encoding of reserved characters) MUST always result in
the same resource. For example, "http://example.org/~user",
"http://example.org/%7euser", and "http://example.org/%7Euser", must
resolve to the same resource.
If this kind of equivalence is to be tested, the percent-encoding of
both IRIs to be compared has to be aligned; for example, by
converting both IRIs to URIs, eliminating escape differences in the
resulting URIs, and making sure that the case of the hexadecimal
characters in the percent-encoding is always the same (preferably
upper case). If the IRI is to be passed to another application or
used further in some other way, its original form MUST be preserved.
The conversion described here should be performed only for local
comparison.
5.2.4. Path Segment Equivalence
The complete path segments "." and ".." are intended only for use
within relative references (Section 4.1 of [RFC3986]) and are removed
as part of the reference resolution process (Section 5.2 of
[RFC3986]). However, some implementations may incorrectly assume
that reference resolution is not necessary when the reference is
already an IRI, and thus fail to remove dot-segments when they occur
in non-relative paths. IRI normalizers should remove dot-segments by
applying the remove_dot_segments algorithm to the path, as described
in Section 5.2.4 of [RFC3986].
5.3. Scheme-Based Comparison
The syntax and semantics of IRIs vary from scheme to scheme, as
described by the defining specification for each scheme.
Implementations may use scheme-specific rules, at further processing
cost, to reduce the probability of false negatives. For example,
because the "http" scheme makes use of an authority component, has a
default port of "80", and defines an empty path to be equivalent to
"/", the following four IRIs are equivalent:
http://example.com
http://example.com/
http://example.com:/
http://example.com:80/
In general, an IRI that uses the generic syntax for authority with an
empty path should be normalized to a path of "/". Likewise, an
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explicit ":port", for which the port is empty or the default for the
scheme, is equivalent to one where the port and its ":" delimiter are
elided and thus should be removed by scheme-based normalization. For
example, the second IRI above is the normal form for the "http"
scheme.
Another case where normalization varies by scheme is in the handling
of an empty authority component or empty host subcomponent. For many
scheme specifications, an empty authority or host is considered an
error; for others, it is considered equivalent to "localhost" or the
end-user's host. When a scheme defines a default for authority and
an IRI reference to that default is desired, the reference should be
normalized to an empty authority for the sake of uniformity, brevity,
and internationalization. If, however, either the userinfo or port
subcomponents are non-empty, then the host should be given explicitly
even if it matches the default.
Normalization should not remove delimiters when their associated
component is empty unless it is licensed to do so by the scheme
specification. For example, the IRI "http://example.com/?" cannot be
assumed to be equivalent to any of the examples above. Likewise, the
presence or absence of delimiters within a userinfo subcomponent is
usually significant to its interpretation. The fragment component is
not subject to any scheme-based normalization; thus, two IRIs that
differ only by the suffix "#" are considered different regardless of
the scheme.
Some IRI schemes allow the usage of Internationalized Domain Names
(IDN) [RFC5890] either in their ireg-name part or elswhere. When in
use in IRIs, those names SHOULD conform to the definition of U-Label
in [RFC5890]. An IRI containing an invalid IDN cannot successfully
be resolved. For legibility purposes, they SHOULD NOT be converted
into ASCII Compatible Encoding (ACE).
Scheme-based normalization may also consider IDN components and their
conversions to punycode as equivalent. As an example,
"http://résumé.example.org" may be considered equivalent to
"http://xn--rsum-bpad.example.org".
Other scheme-specific normalizations are possible.
5.4. Protocol-Based Comparison
Substantial effort to reduce the incidence of false negatives is
often cost-effective for web spiders. Consequently, they implement
even more aggressive techniques in IRI comparison. For example, if
they observe that an IRI such as
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http://example.com/data
redirects to an IRI differing only in the trailing slash
http://example.com/data/
they will likely regard the two as equivalent in the future. This
kind of technique is only appropriate when equivalence is clearly
indicated by both the result of accessing the resources and the
common conventions of their scheme's dereference algorithm (in this
case, use of redirection by HTTP origin servers to avoid problems
with relative references).
6. Security Considerations
The primary security difficulty comes from applications choosing the
wrong equivalence relationship, or two different parties disagreeing
on equivalence. This is especially a problem when IRIs are used in
security protocols.
Besides the large character repertoire of Unicode, reasons for
confusion include different forms of normalization and different
normalization expectations, use of percent-encoding with various
legacy encodings, and bidirectionality issues. See also [UTR36].
7. Acknowledgements
This document was originally derived from [RFC3986] and [RFC3987],
based on text contributed by Tim Bray.
8. References
8.1. Normative References
[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.
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[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987bis]
Duerst, M., Masinter, L., and M. Suignard,
"Internationalized Resource Identifiers (IRIs)", 2011,
<http://tools.ietf.org/id/draft-ietf-iri-3987bis>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[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.
[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
Unicode Standard Annex #15, March 2008,
<http://www.unicode.org/unicode/reports/tr15/
tr15-23.html>.
8.2. Informative References
[HTML4] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium Recommendation,
December 1999,
<http://www.w3.org/TR/html401/appendix/notes.html#h-B.2>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[UTR36] Davis, M. and M. Suignard, "Unicode Security
Considerations", Unicode Technical Report #36,
August 2010, <http://unicode.org/reports/tr36/>.
[XML1] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
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F. Yergeau, "Extensible Markup Language (XML) 1.0 (Forth
Edition)", World Wide Web Consortium Recommendation,
August 2006, <http://www.w3.org/TR/REC-xml>.
[XMLNamespace]
Bray, T., Hollander, D., Layman, A., and R. Tobin,
"Namespaces in XML (Second Edition)", World Wide Web
Consortium Recommendation, August 2006,
<http://www.w3.org/TR/REC-xml-names>.
Authors' Addresses
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
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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