normuri.txt		normiri.txt
	6. Normalization and Comparison
	One of the most common operations on URIs is simple comparison:		5. Normalization and Comparison
	determining if two URIs are equivalent without using the URIs to
	access their respective resource(s). A comparison is performed every
	time 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 URIs is often used by
	spiders and indexing engines to prune a search space or reduce
	duplication of request actions and response storage.
	URI comparison is performed in respect to some particular purpose,		Note: The structure and much of the material for this section is
			taken from section 6 of [RFCYYYY]; the differences are due to the
			specifics of IRIs.
			One of the most common operations on IRIs is simple comparison:
			determining if two IRIs are equivalent without using the IRIs or the
			mapped URIs to access their respective resource(s). A comparison is
			performed every time 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 in respect to some particular purpose,
	and implementations with differing purposes will often be subject to		and implementations with differing purposes will often be subject to
	differing design trade-offs in regards to how much effort should be		differing design trade-offs in regards to how much effort should be
	spent in reducing aliased identifiers. This section describes a		spent in reducing aliased identifiers. This section describes a
	variety of methods that may be used to compare URIs, the trade-offs		variety of methods that may be used to compare IRIs, the trade-offs
	between them, and the types of applications that might use them.		between them, and the types of applications that might use them.
	6.1 Equivalence		5.1 Equivalence
	Since URIs exist to identify resources, presumably they should be		Since 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,
	such a definition of equivalence is not of much practical use, since		such a definition of equivalence is not of much practical use, since
	there is no way for an implementation to compare two resources that		there is no way for an implementation to compare two resources that
	are not under its own control. For this reason, determination of		are not under its own control. For this reason, determination of
	equivalence or difference of URIs is based on string comparison,		equivalence or difference of IRIs is based on string comparison,
	perhaps augmented by reference to additional rules provided by URI		perhaps augmented by reference to additional rules provided by URI
	scheme definitions. We use the terms "different" and "equivalent" to		scheme definitions. We use the terms "different" and "equivalent" to
	describe the possible outcomes of such comparisons, but there are		describe the possible outcomes of such comparisons, but there are
	many application-dependent versions of equivalence.		many applicationdependent versions of equivalence.
	Even though it is possible to determine that two URIs are equivalent,		Even though it is possible to determine that two IRIs are equivalent,
	URI comparison is not sufficient to determine if two URIs identify		IRI comparison is not sufficient to determine if two IRIs identify
	different resources. For example, an owner of two different domain		different resources. For example, an owner of two different domain
	names could decide to serve the same resource from both, resulting in		names could decide to serve the same resource from both, resulting in
	two different URIs. Therefore, comparison methods are designed to		two different IRIs. Therefore, comparison methods are designed to
	minimize false negatives while strictly avoiding false positives.		minimize false negatives while strictly avoiding false 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 URIs before comparison. When URIs are being		respective target IRIs before comparison. When IRIs are being
	compared for the purpose of selecting (or avoiding) a network action,		compared for the purpose of selecting (or avoiding) a network action,
	such as retrieval of a representation, fragment components (if any)		such as retrieval of a representation, fragment components (if any)
	should be excluded from the comparison.		should be excluded from the comparison.
	6.2 Comparison Ladder		Applications using IRIs as identity tokens with no relationship to a
			protocol MUST use the Simple String Comparison (see Section 5.3.1).
			All other applications MUST select one of the comparison practices
			from the Comparison Ladder (see Section 5.3, or, after IRI-to-URI
			conversion, select one of the comparison practices from the URI
			comparison ladder [RFCYYYY], Section 6.2.
	A variety of methods are used in practice to test URI equivalence.		5.2 Preparation for Comparison
			Any kind of IRI comparison REQUIRES that all escapings or encodings
			in the protocol or format that carries an IRI are resolved. This is
			usually done when parsing the protocol or format. 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) all get resolved into
			what is denoted in this document (see Section 1.4) 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 not based on characters, but
			on octets, and additional care is required to make sure that
			characters, and not just arbitrary octets, are compared (see Section
			5.3.1).
			5.3 Comparison Ladder
			A variety of methods are used in practice 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 those		following discussion will climb the ladder, starting with those
	practices that are cheap but have a relatively higher chance of		practices that are cheap but have a relatively higher chance of
	producing false negatives, and proceeding to those that have higher		producing false negatives, and proceeding to those that have higher
	computational cost and lower risk of false negatives.		computational cost and lower risk of false negatives.
	6.2.1 Simple String Comparison		5.3.1 Simple String Comparison
	If two URIs, considered as character strings, are identical, then it		If two IRIs, considered as character strings, are identical, then it
	is safe to conclude that they are equivalent. This type of		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
			and when a definitive answer to the question of IRI equivalence is
			needed that is independent of the scheme used and can be calculated
			quickly and without accessing a network. An example of such a case
			is XML Namespaces ([XMLNamespace]).
	Testing strings for equivalence requires some basic precautions.		Testing strings for equivalence requires some basic precautions.
	This procedure is often referred to as "bit-for-bit" or		This procedure is often referred to as "bit-for-bit" or
	"byte-for-byte" comparison, which is potentially misleading. Testing		"byte-for-byte" comparison, which is potentially misleading. Testing
	of strings for equality is normally based on pairwise comparison of		of strings for equality is normally based on pairwise comparison of
	the characters that make up the strings, starting from the first and		the characters that make up the strings, starting from the first and
	proceeding until both strings are exhausted and all characters found		proceeding until both strings are exhausted and all characters found
	to be equal, a pair of characters compares unequal, or one of the		to be equal, a pair of characters compares unequal, or one of the
	strings is exhausted before the other.		strings is exhausted before the other.
	Such character comparisons require that each pair of characters be		Such character comparisons require that each pair of characters be
	put in comparable form. For example, should one URI be stored in a		put in comparable encoding form. For example, should one IRI be
	byte array in EBCDIC encoding, and the second be in a Java String		stored in a byte array in UTF-8 encoding form, and the second be in a
	object (UTF-16), bit-for-bit comparisons applied naively will produce		UTF-16 encoding form, bit-for-bit comparisons applied naively will
	errors. It is better to speak of equality on a		produce errors. It is better to speak of equality on a
	character-for-character rather than byte-for-byte or bit-for-bit		character-for-character rather than byte-for-byte or bit-for-bit
	basis. In practical terms, character-by-character comparisons should		basis. In practical terms, character-by-character comparisons should
	be done codepoint-by-codepoint after conversion to a common character		be done codepoint-by-codepoint after conversion to a common character
	encoding.		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 IRIs SHOULD
			NOT be modified when being transported if there is any chance that
			this IRI might be used as an identifier.
	False negatives are caused by the production and use of URI aliases.		False negatives are caused by the production and use of IRI aliases.
	Unnecessary aliases can be reduced, regardless of the comparison		Unnecessary aliases can be reduced, regardless of the comparison
	method, by consistently providing URI references in an		method, by consistently providing IRI references in an
	already-normalized form (i.e., a form identical to what would be		already-normalized form (i.e., a form identical to what would be
	produced after normalization is applied, as described below).		produced after normalization is applied, as described below).
	Protocols and data formats often choose to limit some URI comparisons		Protocols and data formats often choose to limit some IRI comparisons
	to simple string comparison, based on the theory that people and		to simple string comparison, based on the theory that people and
	implementations will, in their own best interest, be consistent in		implementations will, in their own best interest, be consistent in
	providing URI references, or at least consistent enough to negate any		providing IRI references, or at least consistent enough to negate any
	efficiency that might be obtained from further normalization.		efficiency that might be obtained from further normalization.
	6.2.2 Syntax-based Normalization		5.3.2 Syntax-based Normalization
	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.
	Such processing is moderately higher in cost than		Such processing is moderately higher in cost than
	character-for-character string comparison. For example, an		character-for-character string comparison. For example, an
	application using this approach could reasonably consider the		application using this approach could reasonably consider the
	following two URIs equivalent:		following two IRIs equivalent:
	example://a/b/c/%7Bfoo%7D		example://a/b/c/%7Bfoo%7D/rosé
	eXAMPLE://a/./b/../b/%63/%7bfoo%7d		eXAMPLE://a/./b/../b/%63/%7bfoo%7d/ros%C3%A9
	Web user agents, such as browsers, typically apply this type of URI		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, percent-encoding normalization, and removal of		case normalization, character normalization, percent-encoding
	dot-segments.		normalization, and removal of dot-segments.
	6.2.2.1 Case Normalization		5.3.2.1 Case Normalization
	For all URIs, 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 normalized to use uppercase letters for the digits A-F.
	When a URI 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
	host are case-insensitive and therefore should be normalized to		US-ASCII only host are case-insensitive and therefore should be
	lowercase. For example, the URI <HTTP://www.EXAMPLE.com/> is		normalized to lowercase. For example, the URI
	equivalent to <http://www.example.com/>. The other generic syntax		<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 5.3.3. 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 (see Section 6.2.3).		defined otherwise by the scheme.
	6.2.2.2 Percent-Encoding Normalization		Creating schemes that allow case-insensitive syntax components
			containing non US-ASCII characters should be avoided because such a
			case normalization may be cultural dependant 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.
	The percent-encoding mechanism (Section 2.1) is a frequent source of		5.3.2.2 Character Normalization
	variance among otherwise identical URIs. In addition to the case
	normalization issue noted above, some URI producers percent-encode
	octets that do not require percent-encoding, resulting in URIs that
	are equivalent to their non-encoded counterparts. Such URIs should
	be normalized by decoding any percent-encoded octet that corresponds
	to an unreserved character, as described in Section 2.3.
	6.2.2.3 Path Segment Normalization		The Unicode Standard [UNIV4] 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).
			Equivalence of IRIs MUST rely on the assumption that IRIs are
			appropriately pre-character-normalized, rather than applying
			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 an 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 using NFC. Using NFKC may
			avoid even more problems, for example by choosing half-width Latin
			letters instead of full-width, and full-width Katakana instead of
			half-width.
			As an example, http://www.example.org/r&#xE9;sum&#xE9;.html (in XML
			Notation) is in NFC. On the other hand,
			http://www.example.org/re&#x301;sume&#x301;.html is not in NFC. The
			former uses precombined e-acute characters, the latter uses 'e'
			characters followed by combining acute accents. Both usages are
			defined to be canonically equivalent in [UNIV4].
			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 upper-case/lower-case problems in
			character-normalized as possible (i.e. NFC or even NFKC). URIs.
			Some parts of a URI are case-insensitive (domain name). For
			others, it is unclear whether they are case-sensitive or
			case-insensitive, or something in between (e.g. case-sensitive,
			but if the wrong case is used, a multiple choice selection is
			provided instead of a direct negative result). The best recipe is
			that the creator uses a reasonable capitalization, and when
			transferring the URI, that capitalization is never changed.
			Various IRI schemes may allow the usage of Internationalized Domain
			Names (IDN) [RFC3490] either in the ireg-name part or elsewhere.
			Character Normalization also applies to IDNs, as discussed in Section
			5.3.3.
			5.3.2.3 Percent-Encoding Normalization
			The percent-encoding mechanism (Section 2.1 of [RFCYYYY]) 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. Such IRIs should be normalized by decoding any
			percent-encoded octet sequence that corresponds to an unreserved
			character, as described in Section 2.3 of [RFCYYYY].
			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 (see Section 3.1), 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 the purpose of local comparison.
			5.3.2.4 Path Segment Normalization
	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) and are removed as part of		within relative references (Section 4.1 of [RFCYYYY]) and are removed
	the reference resolution process (Section 5.2). However, some		as part of the reference resolution process (Section 5.2 of
	deployed implementations incorrectly assume that reference resolution		[RFCYYYY]). However, some implementations may incorrectly assume
	is not necessary when the reference is already a URI, and thus fail		that reference resolution is not necessary when the reference is
	to remove dot-segments when they occur in non-relative paths. URI		already an IRI, and thus fail to remove dot-segments when they occur
	normalizers should remove dot-segments by applying the		in non-relative paths. IRI normalizers should remove dot-segments by
	remove_dot_segments algorithm to the path, as described in		applying the remove_dot_segments algorithm to the path, as described
	Section 5.2.4.		in Section 5.2.4 of [RFCYYYY].
	6.2.3 Scheme-based Normalization		5.3.3 Scheme-based Normalization
	The syntax and semantics of URIs 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,
	since the "http" scheme makes use of an authority component, has a		since 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 URIs are equivalent:		"/", the following four IRIs are equivalent:
	http://example.com		http://example.com
	http://example.com/		http://example.com/
	http://example.com:/		http://example.com:/
	http://example.com:80/		http://example.com:80/
			In general, an IRI that uses the generic syntax for authority with an
	In general, a URI that uses the generic syntax for authority with an
	empty path should be normalized to a path of "/"; likewise, an		empty path should be normalized to a path of "/"; likewise, an
	explicit ":port", where the port is empty or the default for the		explicit ":port", where the port is empty or the default for the
	scheme, is equivalent to one where the port and its ":" delimiter are		scheme, is equivalent to one where the port and its ":" delimiter are
	elided, and thus should be removed by scheme-based normalization.		elided, and thus should be removed by scheme-based normalization.
	For example, the second URI above is the normal form for the "http"		For example, the second IRI above is the normal form for the "http"
	scheme.		scheme.
	Another case where normalization varies by scheme is in the handling		Another case where normalization varies by scheme is in the handling
	of an empty authority component or empty host subcomponent. For many		of an empty authority component or empty host subcomponent. For many
	scheme specifications, an empty authority or host is considered an		scheme specifications, an empty authority or host is considered an
	error; for others, it is considered equivalent to "localhost" or the		error; for others, it is considered equivalent to "localhost" or the
	end-user's host. When a scheme defines a default for authority and a		end-user's host. When a scheme defines a default for authority and
	URI reference to that default is desired, the reference should be		an IRI reference to that default is desired, the reference should be
	normalized to an empty authority for the sake of uniformity, brevity,		normalized to an empty authority for the sake of uniformity, brevity,
	and internationalization. If, however, either the userinfo or port		and internationalization. If, however, either the userinfo or port
	subcomponent is non-empty, then the host should be given explicitly		subcomponent is non-empty, then the host should be given explicitly
	even if it matches the default.		even if it matches the default.
	Normalization should not remove delimiters when their associated		Normalization should not remove delimiters when their associated
	component is empty unless licensed to do so by the scheme		component is empty unless licensed to do so by the scheme
	specification. For example, the URI "http://example.com/?" cannot be		specification. For example, the IRI "http://example.com/?" cannot be
	assumed to be equivalent to any of the examples above. Likewise, the		assumed to be equivalent to any of the examples above. Likewise, the
	presence or absence of delimiters within a userinfo subcomponent is		presence or absence of delimiters within a userinfo subcomponent is
	usually significant to its interpretation. The fragment component is		usually significant to its interpretation. The fragment component is
	not subject to any scheme-based normalization; thus, two URIs that		not subject to any scheme-based normalization; thus, two IRIs that
	differ only by the suffix "#" are considered different regardless of		differ only by the suffix "#" are considered different regardless of
	the scheme.		the scheme.
	Some schemes define additional subcomponents that consist of		Some IRI schemes may allow the usage of Internationalized Domain
	case-insensitive data, giving an implicit license to normalizers to		Names (IDN) [RFC3490] either in their ireg-name part or elsewhere.
	convert such data to a common case (e.g., all lowercase). For		When in use in IRIs, those names SHOULD be validated using the
	example, URI schemes that define a subcomponent of path to contain an		ToASCII operation defined in [RFC3490], with the flags
	Internet hostname, such as the "mailto" URI scheme, cause that		"UseSTD3ASCIIRules" and "AllowUnassigned". An IRI containing an
	subcomponent to be case-insensitive and thus subject to case		invalid IDN cannot successfully be resolved. Validated IDN
	normalization (e.g., "mailto:Joe@Example.COM" is equivalent to		components of IRIs SHOULD be character normalized using the Nameprep
	"mailto:Joe@example.com" even though the generic syntax considers the		process [RFC3491]; however, for legibility purposes, they SHOULD NOT
	path component to be case-sensitive).		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://r&#xE9;sum&#xE9;.example.org may be considered equivalent to
			http://xn--rsum-bpad.example.org
	Other scheme-specific normalizations are possible.		Other scheme-specific normalizations are possible.
	6.2.4 Protocol-based Normalization		5.3.4 Protocol-based Normalization
	Web spiders, for which substantial effort to reduce the incidence of		Web spiders, for which substantial effort to reduce the incidence of
	false negatives is often cost-effective, are observed to implement		false negatives is often cost-effective, are observed to implement
	even more aggressive techniques in URI comparison. For example, if		even more aggressive techniques in IRI comparison. For example, if
	they observe that a URI such as		they observe that an IRI such as
	http://example.com/data		http://example.com/data
	redirects to a URI 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
	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).
End of changes.
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