draft-ietf-iri-comparison-00.txt		draft-ietf-iri-comparison-01.txt
	Internationalized Resource Identifiers L. Masinter		Internationalized Resource Identifiers L. Masinter
	(iri) Adobe		(iri) Adobe
	Internet-Draft M. Duerst		Internet-Draft M. Duerst
	Intended status: Standards Track Aoyama Gakuin University		Intended status: Standards Track Aoyama Gakuin University
	Expires: February 15, 2012 August 14, 2011		Expires: September 3, 2012 March 2, 2012
	Equivalence and Canonicalization of Internationalized Resource		Equivalence and Canonicalization of Internationalized Resource
	Identifiers (IRIs)		Identifiers (IRIs)
	draft-ietf-iri-comparison-00		draft-ietf-iri-comparison-01
	Abstract		Abstract
	Internationalized Resource Identifiers (IRIs) are unicode strings		Internationalized Resource Identifiers (IRIs) are unicode strings
	used to identify resources on the Internet. Applications that use		used to identify resources on the Internet. Applications that use
	IRIs often define a means of comparing two IRIs to determine when two		IRIs often define a means of comparing two IRIs to determine when two
	IRIs are equivalent for the purpose of that application. Some		IRIs are equivalent for the purpose of that application. Some
	applications also define a method for 'canonicalizing' or		applications also define a method for 'canonicalizing' or
	'normalizing' an IRI -- translating one IRI into another which is		'normalizing' an IRI -- translating one IRI into another which is
	equivalent under the comparison method used.		equivalent under the comparison method used.
	skipping to change at page 1, line 42		skipping to change at page 1, line 42
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on February 15, 2012.		This Internet-Draft will expire on September 3, 2012.
	Copyright Notice		Copyright Notice
	Copyright (c) 2011 IETF Trust and the persons identified as the		Copyright (c) 2012 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	skipping to change at page 3, line 8		skipping to change at page 3, line 8
	Without obtaining an adequate license from the person(s) controlling		Without obtaining an adequate license from the person(s) controlling
	the copyright in such materials, this document may not be modified		the copyright in such materials, this document may not be modified
	outside the IETF Standards Process, and derivative works of it may		outside the IETF Standards Process, and derivative works of it may
	not be created outside the IETF Standards Process, except to format		not be created outside the IETF Standards Process, except to format
	it for publication as an RFC or to translate it into languages other		it for publication as an RFC or to translate it into languages other
	than English.		than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
	2. Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . 5		2. Equivalence . . . . . . . . . . . . . . . . . . . . . . . . . 4
	3. Preparation for Comparison . . . . . . . . . . . . . . . . . . 6		3. Comparison, Equivalence, Normalization and
	4. Comparison Ladder . . . . . . . . . . . . . . . . . . . . . . 6		Canonicalization . . . . . . . . . . . . . . . . . . . . . . . 5
	4.1. Simple String Comparison . . . . . . . . . . . . . . . . . 7		4. Preparation for Comparison . . . . . . . . . . . . . . . . . . 5
	4.2. Syntax-Based Normalization . . . . . . . . . . . . . . . . 8		5. Comparison Ladder . . . . . . . . . . . . . . . . . . . . . . 6
	4.2.1. Case Normalization . . . . . . . . . . . . . . . . . . 8		5.1. Simple String Comparison . . . . . . . . . . . . . . . . . 6
	4.2.2. Character Normalization . . . . . . . . . . . . . . . 8		5.2. Syntax-Based Equivalence . . . . . . . . . . . . . . . . . 7
	4.2.3. Percent-Encoding Normalization . . . . . . . . . . . . 10		5.2.1. Case Equivalence . . . . . . . . . . . . . . . . . . . 8
	4.2.4. Path Segment Normalization . . . . . . . . . . . . . . 10		5.2.2. Unicode Character Normalization . . . . . . . . . . . 8
	4.3. Scheme-Based Normalization . . . . . . . . . . . . . . . . 10		5.2.3. Percent-Encoding Equivalence . . . . . . . . . . . . . 9
	4.4. Protocol-Based Normalization . . . . . . . . . . . . . . . 12		5.2.4. Path Segment Equivalence . . . . . . . . . . . . . . . 10
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 12		5.3. Scheme-Based Comparison . . . . . . . . . . . . . . . . . 10
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12		5.4. Protocol-Based Comparison . . . . . . . . . . . . . . . . 11
	7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13		6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
	7.1. Normative References . . . . . . . . . . . . . . . . . . . 13		7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
	7.2. Informative References . . . . . . . . . . . . . . . . . . 13		8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
			8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
			8.2. Informative References . . . . . . . . . . . . . . . . . . 13
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
	1. Introduction		1. Introduction
	Internationalized Resource Identifiers (IRIs) are unicode strings		Internationalized Resource Identifiers (IRIs) are unicode strings
	used to identify resources on the Internet. Applications that use		used to identify resources on the Internet. Applications that use
	IRIs often define a means of comparing two IRIs to determine when two		IRIs often define a means of comparing two IRIs to determine when two
	IRIs are equivalent for the purpose of that application. Some		IRIs are equivalent for the purpose of that application. Some
	applications also define a method for 'canonicalizing' or		applications also define a method for 'canonicalizing' or
	'normalizing' an IRI -- translating one IRI into another which is		'normalizing' an IRI -- translating one IRI into another which is
	equivalent under the comparison method used.		equivalent under the comparison method used.
	This document gives guidelines and best practices for defining and		This document gives guidelines and best practices for defining and
	using IRI comparison, equivalence, normalization and canonicalization		using IRI comparison, equivalence, normalization and canonicalization
	methods.		methods.
	Things to do:
	o Introductory section on comparison, equivalence, normalization and
	canonicalization.
	o Verify acknowledgements for this component.
	o Verify cross-references from other documents.
	o Consider making 4395bis reference this document and recommend
	scheme definitions describe equivalence specifically.
	o Consider making this document 'update' 3986 in order to resolve
	which one is normative if there are conflicts.
	o alternatively? Consider making this document BCP rather than
	standards track, since it basically gives guidance for protocols
	and applications needing equivalence, and doesn't directly have a
	scope of application?
	o Distingish between IRIs as sequence-of-unicode characters and
	presentations of IRIs.
	o Should we insist that percent-hex encoding equivalence of non-
	reserved characters MUST be always used if there is any
	equivalence at all?
	o Update security considerations to describe security concerns
	specific to comparison.
	o Consider making sections talk about 'equivalent' rather than
	'normalization' where appropriate.
	One of the most common operations on IRIs is simple comparison:		One of the most common operations on IRIs is simple comparison:
	Determining whether two IRIs are equivalent, without using the IRIs		Determining whether two IRIs are equivalent, without using the IRIs
	to access their respective resource(s). A comparison is performed		to access their respective resource(s). A comparison is performed
	whenever a response cache is accessed, a browser checks its history		whenever a response cache is accessed, a browser checks its history
	to color a link, or an XML parser processes tags within a namespace.		to color a link, or an XML parser processes tags within a namespace.
	Extensive normalization prior to comparison of IRIs may be used by		Extensive normalization prior to comparison of IRIs may be used by
	spiders and indexing engines to prune a search space or reduce		spiders and indexing engines to prune a search space or reduce
	duplication of request actions and response storage.		duplication of request actions and response storage.
	IRI comparison is performed for some particular purpose. Protocols		IRI comparison is performed for some particular purpose. Protocols
	or implementations that compare IRIs for different purposes will		or implementations that compare IRIs for different purposes will
	skipping to change at page 5, line 29		skipping to change at page 4, line 44
	use them.		use them.
	2. Equivalence		2. Equivalence
	Because IRIs exist to identify resources, presumably they should be		Because IRIs exist to identify resources, presumably they should be
	considered equivalent when they identify the same resource. However,		considered equivalent when they identify the same resource. However,
	this definition of equivalence is not of much practical use, as there		this definition of equivalence is not of much practical use, as there
	is no way for an implementation to compare two resources to determine		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		if they are "the same" unless it has full knowledge or control of
	them. For this reason, determination of equivalence or difference of		them. For this reason, determination of equivalence or difference of
	IRIs is based on string comparison, perhaps augmented by reference to		IRIs is based on string comparison, augmented by reference to
	additional rules provided by URI scheme definitions. We use the		additional rules provided by scheme definition. We use the terms
	terms "different" and "equivalent" to describe the possible outcomes		"different" and "equivalent" to describe the possible outcomes of
	of such comparisons, but there are many application-dependent		such comparisons, but there are many application-dependent versions
	versions of equivalence.		of equivalence.
	Even when it is possible to determine that two IRIs are equivalent,		Even when it is possible to determine that two IRIs are equivalent,
	IRI comparison is not sufficient to determine whether two IRIs		IRI comparison is not sufficient to determine whether two IRIs
	identify different resources. For example, an owner of two different		identify different resources. For example, an owner of two different
	domain names could decide to serve the same resource from both,		domain names could decide to serve the same resource from both,
	resulting in two different IRIs. Therefore, comparison methods are		resulting in two different IRIs. Therefore, comparison methods are
	designed to minimize false negatives while strictly avoiding false		designed to minimize false negatives while strictly avoiding false
	positives.		positives.
	In testing for equivalence, applications should not directly compare		In testing for equivalence, applications should not directly compare
	relative references; the references should be converted to their		relative references; the references should be converted to their
	respective target IRIs before comparison. When IRIs are compared to		respective target IRIs before comparison. When IRIs are compared to
	select (or avoid) a network action, such as retrieval of a		select (or avoid) a network action, such as retrieval of a
	representation, fragment components (if any) MUST be excluded from		representation, fragment components (if any) MUST be excluded from
	the comparison.		the comparison.
	Applications using IRIs as identity tokens with no relationship to a		Applications using IRIs as identity tokens with no relationship to a
	protocol MUST use the Simple String Comparison (see Section 4.1).		protocol MUST use the Simple String Comparison (see Section 5.1).
	All other applications MUST select one of the comparison practices		All other applications MUST select one of the comparison practices
	from the Comparison Ladder (see Section 4.		from the Comparison Ladder (see Section 5.
	3. Preparation for Comparison		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		Any kind of IRI comparison REQUIRES that any additional contextual
	processing is first performed, including undoing higher-level		processing is first performed, including undoing higher-level
	escapings or encodings in the protocol or format that carries an IRI.		escapings or encodings in the protocol or format that carries an IRI.
	This preprocessing is usually done when the protocol or format is		This preprocessing is usually done when the protocol or format is
	parsed.		parsed.
	Examples of such escapings or encodings are entities and numeric		Examples of such escapings or encodings are entities and numeric
	character references in [HTML4] and [XML1]. As an example,		character references in [HTML4] and [XML1]. As an example,
	"http://example.org/rosé" (in HTML),		"http://example.org/rosé" (in HTML),
	skipping to change at page 6, line 31		skipping to change at page 6, line 23
	what is denoted in this document (see 'Notation' section of		what is denoted in this document (see 'Notation' section of
	[RFC3987bis]) as "http://example.org/rosé" (the "é" here		[RFC3987bis]) as "http://example.org/rosé" (the "é" here
	standing for the actual e-acute character, to compensate for the fact		standing for the actual e-acute character, to compensate for the fact
	that this document cannot contain non-ASCII characters).		that this document cannot contain non-ASCII characters).
	Similar considerations apply to encodings such as Transfer Codings in		Similar considerations apply to encodings such as Transfer Codings in
	HTTP (see [RFC2616]) and Content Transfer Encodings in MIME		HTTP (see [RFC2616]) and Content Transfer Encodings in MIME
	([RFC2045]), although in these cases, the encoding is based not on		([RFC2045]), although in these cases, the encoding is based not on
	characters but on octets, and additional care is required to make		characters but on octets, and additional care is required to make
	sure that characters, and not just arbitrary octets, are compared		sure that characters, and not just arbitrary octets, are compared
	(see Section 4.1).		(see Section 5.1).
	4. Comparison Ladder		5. Comparison Ladder
	In practice, a variety of methods are used to test IRI equivalence.		In practice, a variety of methods are used to test IRI equivalence.
	These methods fall into a range distinguished by the amount of		These methods fall into a range distinguished by the amount of
	processing required and the degree to which the probability of false		processing required and the degree to which the probability of false
	negatives is reduced. As noted above, false negatives cannot be		negatives is reduced. As noted above, false negatives cannot be
	eliminated. In practice, their probability can be reduced, but this		eliminated. In practice, their probability can be reduced, but this
	reduction requires more processing and is not cost-effective for all		reduction requires more processing and is not cost-effective for all
	applications.		applications.
	If this range of comparison practices is considered as a ladder, the		If this range of comparison practices is considered as a ladder, the
	following discussion will climb the ladder, starting with practices		following discussion will climb the ladder, starting with practices
	that are cheap but have a relatively higher chance of producing false		that are cheap but have a relatively higher chance of producing false
	negatives, and proceeding to those that have higher computational		negatives, and proceeding to those that have higher computational
	cost and lower risk of false negatives.		cost and lower risk of false negatives.
	4.1. Simple String Comparison		5.1. Simple String Comparison
	If two IRIs, when considered as character strings, are identical,		If two IRIs, when considered as character strings, are identical,
	then it is safe to conclude that they are equivalent. This type of		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		equivalence test has very low computational cost and is in wide use
	in a variety of applications, particularly in the domain of parsing.		in a variety of applications, particularly in the domain of parsing.
	It is also used when a definitive answer to the question of IRI		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		equivalence is needed that is independent of the scheme used and that
	can be calculated quickly and without accessing a network. An		can be calculated quickly and without accessing a network. An
	example of such a case is XML Namespaces ([XMLNamespace]).		example of such a case is XML Namespaces ([XMLNamespace]).
	skipping to change at page 8, line 5		skipping to change at page 7, line 40
	Unnecessary aliases can be reduced, regardless of the comparison		Unnecessary aliases can be reduced, regardless of the comparison
	method, by consistently providing IRI references in an already		method, by consistently providing IRI references in an already
	normalized form (i.e., a form identical to what would be produced		normalized form (i.e., a form identical to what would be produced
	after normalization is applied, as described below). Protocols and		after normalization is applied, as described below). Protocols and
	data formats often limit some IRI comparisons to simple string		data formats often limit some IRI comparisons to simple string
	comparison, based on the theory that people and implementations will,		comparison, based on the theory that people and implementations will,
	in their own best interest, be consistent in providing IRI		in their own best interest, be consistent in providing IRI
	references, or at least be consistent enough to negate any efficiency		references, or at least be consistent enough to negate any efficiency
	that might be obtained from further normalization.		that might be obtained from further normalization.
	4.2. Syntax-Based Normalization		5.2. Syntax-Based Equivalence
	Implementations may use logic based on the definitions provided by		Implementations may use logic based on the definitions provided by
	this specification to reduce the probability of false negatives.		this specification to reduce the probability of false negatives.
	This processing is moderately higher in cost than character-for-		This processing is moderately higher in cost than character-for-
	character string comparison. For example, an application using this		character string comparison. For example, an application using this
	approach could reasonably consider the following two IRIs equivalent:		approach could reasonably consider the following two IRIs equivalent:
	example://a/b/c/%7Bfoo%7D/rosé		example://a/b/c/%7Bfoo%7D/rosé
	eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9		eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
	Web user agents, such as browsers, typically apply this type of IRI		Web user agents, such as browsers, typically apply this type of IRI
	normalization when determining whether a cached response is		normalization when determining whether a cached response is
	available. Syntax-based normalization includes such techniques as		available. Syntax-based normalization includes such techniques as
	case normalization, character normalization, percent-encoding		case normalization, character normalization, percent-encoding
	normalization, and removal of dot-segments.		normalization, and removal of dot-segments.
	4.2.1. Case Normalization		5.2.1. Case Equivalence
	For all IRIs, the hexadecimal digits within a percent-encoding		For all IRIs, the hexadecimal digits within a percent-encoding
	triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore		triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
	should be normalized to use uppercase letters for the digits A-F.		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		When an IRI uses components of the generic syntax, the component
	syntax equivalence rules always apply; namely, that the scheme and		syntax equivalence rules always apply; namely, that the scheme and
	US-ASCII only host are case insensitive and therefore should be		US-ASCII only host are case insensitive and therefore should be
	normalized to lowercase. For example, the URI		normalized to lowercase. For example, the URI
	"HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".		"HTTP://www.EXAMPLE.com/" is equivalent to "http://www.example.com/".
	Case equivalence for non-ASCII characters in IRI components that are		Case equivalence for non-ASCII characters in IRI components that are
	IDNs are discussed in Section 4.3. The other generic syntax		IDNs are discussed in Section 5.3. The other generic syntax
	components are assumed to be case sensitive unless specifically		components are assumed to be case sensitive unless specifically
	defined otherwise by the scheme.		defined otherwise by the scheme.
	Creating schemes that allow case-insensitive syntax components		Creating schemes that allow case-insensitive syntax components
	containing non-ASCII characters should be avoided. Case		containing non-ASCII characters should be avoided. Case
	normalization of non-ASCII characters can be culturally dependent and		normalization of non-ASCII characters can be culturally dependent and
	is always a complex operation. The only exception concerns non-ASCII		is always a complex operation. The only exception concerns non-ASCII
	host names for which the character normalization includes a mapping		host names for which the character normalization includes a mapping
	step derived from case folding.		step derived from case folding.
	4.2.2. Character Normalization		5.2.2. Unicode Character Normalization
	The Unicode Standard [UNIV6] defines various equivalences between		The Unicode Standard [UNIV6] defines various equivalences between
	sequences of characters for various purposes. Unicode Standard Annex		sequences of characters for various purposes. Unicode Standard Annex
	#15 [UTR15] defines various Normalization Forms for these		#15 [UTR15] defines various Normalization Forms for these
	equivalences, in particular Normalization Form C (NFC, Canonical		equivalences, in particular Normalization Form C (NFC, Canonical
	Decomposition, followed by Canonical Composition) and Normalization		Decomposition, followed by Canonical Composition) and Normalization
	Form KC (NFKC, Compatibility Decomposition, followed by Canonical		Form KC (NFKC, Compatibility Decomposition, followed by Canonical
	Composition).		Composition).
	IRIs already in Unicode MUST NOT be normalized before parsing or		IRIs already in Unicode MUST NOT be normalized before parsing or
	skipping to change at page 10, line 6		skipping to change at page 9, line 43
	unclear whether they are case sensitive, case insensitive, or		unclear whether they are case sensitive, case insensitive, or
	something in between (e.g., case sensitive, but with a multiple		something in between (e.g., case sensitive, but with a multiple
	choice selection if the wrong case is used, instead of a direct		choice selection if the wrong case is used, instead of a direct
	negative result). The best recipe is that the creator use a		negative result). The best recipe is that the creator use a
	reasonable capitalization and, when transferring the URI,		reasonable capitalization and, when transferring the URI,
	capitalization never be changed.		capitalization never be changed.
	Various IRI schemes may allow the usage of Internationalized Domain		Various IRI schemes may allow the usage of Internationalized Domain
	Names (IDN) [RFC5890] either in the ireg-name part or elsewhere.		Names (IDN) [RFC5890] either in the ireg-name part or elsewhere.
	Character Normalization also applies to IDNs, as discussed in		Character Normalization also applies to IDNs, as discussed in
	Section 4.3.		Section 5.3.
	4.2.3. Percent-Encoding Normalization		5.2.3. Percent-Encoding Equivalence
	The percent-encoding mechanism (Section 2.1 of [RFC3986]) is a		The percent-encoding mechanism (Section 2.1 of [RFC3986]) is a
	frequent source of variance among otherwise identical IRIs. In		frequent source of variance among otherwise identical IRIs. In
	addition to the case normalization issue noted above, some IRI		addition to the case normalization issue noted above, some IRI
	producers percent-encode octets that do not require percent-encoding,		producers percent-encode octets that do not require percent-encoding,
	resulting in IRIs that are equivalent to their nonencoded		resulting in IRIs that are equivalent to their nonencoded
	counterparts. These IRIs should be normalized by decoding any		counterparts. These IRIs should be normalized by decoding any
	percent-encoded octet sequence that corresponds to an unreserved		percent-encoded octet sequence that corresponds to an unreserved
	character, as described in section 2.3 of [RFC3986].		character, as described in section 2.3 of [RFC3986].
	For actual resolution, differences in percent-encoding (except for		For actual resolution, differences in percent-encoding (except for
	the percent-encoding of reserved characters) MUST always result in		the percent-encoding of reserved characters) MUST always result in
	the same resource. For example, "http://example.org/~user",		the same resource. For example, "http://example.org/~user",
	"http://example.org/%7euser", and "http://example.org/%7Euser", must		"http://example.org/%7euser", and "http://example.org/%7Euser", must
	resolve to the same resource.		resolve to the same resource.
	If this kind of equivalence is to be tested, the percent-encoding of		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		both IRIs to be compared has to be aligned; for example, by
	converting both IRIs to URIs (see Section 3.1), eliminating escape		converting both IRIs to URIs, eliminating escape differences in the
	differences in the resulting URIs, and making sure that the case of		resulting URIs, and making sure that the case of the hexadecimal
	the hexadecimal characters in the percent-encoding is always the same		characters in the percent-encoding is always the same (preferably
	(preferably upper case). If the IRI is to be passed to another		upper case). If the IRI is to be passed to another application or
	application or used further in some other way, its original form MUST		used further in some other way, its original form MUST be preserved.
	be preserved. The conversion described here should be performed only		The conversion described here should be performed only for local
	for local comparison.		comparison.
	4.2.4. Path Segment Normalization		5.2.4. Path Segment Equivalence
	The complete path segments "." and ".." are intended only for use		The complete path segments "." and ".." are intended only for use
	within relative references (Section 4.1 of [RFC3986]) and are removed		within relative references (Section 4.1 of [RFC3986]) and are removed
	as part of the reference resolution process (Section 5.2 of		as part of the reference resolution process (Section 5.2 of
	[RFC3986]). However, some implementations may incorrectly assume		[RFC3986]). However, some implementations may incorrectly assume
	that reference resolution is not necessary when the reference is		that reference resolution is not necessary when the reference is
	already an IRI, and thus fail to remove dot-segments when they occur		already an IRI, and thus fail to remove dot-segments when they occur
	in non-relative paths. IRI normalizers should remove dot-segments by		in non-relative paths. IRI normalizers should remove dot-segments by
	applying the remove_dot_segments algorithm to the path, as described		applying the remove_dot_segments algorithm to the path, as described
	in Section 5.2.4 of [RFC3986].		in Section 5.2.4 of [RFC3986].
	4.3. Scheme-Based Normalization		5.3. Scheme-Based Comparison
	The syntax and semantics of IRIs vary from scheme to scheme, as		The syntax and semantics of IRIs vary from scheme to scheme, as
	described by the defining specification for each scheme.		described by the defining specification for each scheme.
	Implementations may use scheme-specific rules, at further processing		Implementations may use scheme-specific rules, at further processing
	cost, to reduce the probability of false negatives. For example,		cost, to reduce the probability of false negatives. For example,
	because the "http" scheme makes use of an authority component, has a		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		default port of "80", and defines an empty path to be equivalent to
	"/", the following four IRIs are equivalent:		"/", the following four IRIs are equivalent:
	http://example.com		http://example.com
	skipping to change at page 12, line 9		skipping to change at page 11, line 45
	be resolved. For legibility purposes, they SHOULD NOT be converted		be resolved. For legibility purposes, they SHOULD NOT be converted
	into ASCII Compatible Encoding (ACE).		into ASCII Compatible Encoding (ACE).
	Scheme-based normalization may also consider IDN components and their		Scheme-based normalization may also consider IDN components and their
	conversions to punycode as equivalent. As an example,		conversions to punycode as equivalent. As an example,
	"http://résumé.example.org" may be considered equivalent to		"http://résumé.example.org" may be considered equivalent to
	"http://xn--rsum-bpad.example.org".		"http://xn--rsum-bpad.example.org".
	Other scheme-specific normalizations are possible.		Other scheme-specific normalizations are possible.
	4.4. Protocol-Based Normalization		5.4. Protocol-Based Comparison
	Substantial effort to reduce the incidence of false negatives is		Substantial effort to reduce the incidence of false negatives is
	often cost-effective for web spiders. Consequently, they implement		often cost-effective for web spiders. Consequently, they implement
	even more aggressive techniques in IRI comparison. For example, if		even more aggressive techniques in IRI comparison. For example, if
	they observe that an IRI such as		they observe that an IRI such as
	http://example.com/data		http://example.com/data
	redirects to an IRI differing only in the trailing slash		redirects to an IRI differing only in the trailing slash
	http://example.com/data/		http://example.com/data/
	they will likely regard the two as equivalent in the future. This		they will likely regard the two as equivalent in the future. This
	kind of technique is only appropriate when equivalence is clearly		kind of technique is only appropriate when equivalence is clearly
	indicated by both the result of accessing the resources and the		indicated by both the result of accessing the resources and the
	common conventions of their scheme's dereference algorithm (in this		common conventions of their scheme's dereference algorithm (in this
	skipping to change at page 12, line 29		skipping to change at page 12, line 17
	http://example.com/data/		http://example.com/data/
	they will likely regard the two as equivalent in the future. This		they will likely regard the two as equivalent in the future. This
	kind of technique is only appropriate when equivalence is clearly		kind of technique is only appropriate when equivalence is clearly
	indicated by both the result of accessing the resources and the		indicated by both the result of accessing the resources and the
	common conventions of their scheme's dereference algorithm (in this		common conventions of their scheme's dereference algorithm (in this
	case, use of redirection by HTTP origin servers to avoid problems		case, use of redirection by HTTP origin servers to avoid problems
	with relative references).		with relative references).
	5. Security Considerations		6. Security Considerations
	The primary security difficulty comes from applications choosing the		The primary security difficulty comes from applications choosing the
	wrong equivalence relationship, or two different parties disagreeing		wrong equivalence relationship, or two different parties disagreeing
	on equivalence. This is especially a problem when IRIs are used in		on equivalence. This is especially a problem when IRIs are used in
	security protocols.		security protocols.
	Besides the large character repertoire of Unicode, reasons for		Besides the large character repertoire of Unicode, reasons for
	confusion include different forms of normalization and different		confusion include different forms of normalization and different
	normalization expectations, use of percent-encoding with various		normalization expectations, use of percent-encoding with various
	legacy encodings, and bidirectionality issues. See also [UTR36].		legacy encodings, and bidirectionality issues. See also [UTR36].
	6. Acknowledgements		7. Acknowledgements
	This document was originally derived from [RFC3986] and [RFC3987],		This document was originally derived from [RFC3986] and [RFC3987],
	based on text contributed by Tim Bray.		based on text contributed by Tim Bray.
	7. References		8. References
	7.1. Normative References
			8.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,		[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
	"Internationalizing Domain Names in Applications (IDNA)",		"Internationalizing Domain Names in Applications (IDNA)",
	RFC 3490, March 2003.		RFC 3490, March 2003.
	[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep		[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
	Profile for Internationalized Domain Names (IDN)",		Profile for Internationalized Domain Names (IDN)",
	skipping to change at page 13, line 42		skipping to change at page 13, line 30
	[UNIV6] The Unicode Consortium, "The Unicode Standard, Version		[UNIV6] The Unicode Consortium, "The Unicode Standard, Version
	6.0.0 (Mountain View, CA, The Unicode Consortium, 2011,		6.0.0 (Mountain View, CA, The Unicode Consortium, 2011,
	ISBN 978-1-936213-01-6)", October 2010.		ISBN 978-1-936213-01-6)", October 2010.
	[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",		[UTR15] Davis, M. and M. Duerst, "Unicode Normalization Forms",
	Unicode Standard Annex #15, March 2008,		Unicode Standard Annex #15, March 2008,
	<http://www.unicode.org/unicode/reports/tr15/		<http://www.unicode.org/unicode/reports/tr15/
	tr15-23.html>.		tr15-23.html>.
	7.2. Informative References		8.2. Informative References
	[HTML4] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01		[HTML4] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
	Specification", World Wide Web Consortium Recommendation,		Specification", World Wide Web Consortium Recommendation,
	December 1999,		December 1999,
	<http://www.w3.org/TR/html401/appendix/notes.html#h-B.2>.		<http://www.w3.org/TR/html401/appendix/notes.html#h-B.2>.
	[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail		[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
	Extensions (MIME) Part One: Format of Internet Message		Extensions (MIME) Part One: Format of Internet Message
	Bodies", RFC 2045, November 1996.		Bodies", RFC 2045, November 1996.
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