Uniform Resource Identifier (URI): Generic Syntax
draft-fielding-uri-rfc2396bis-07

Network Working Group T. Berners-Lee
Internet-Draft W3C/MIT
Updates: 1738 (if approved) R. Fielding
Obsoletes: 2732, 2396, 1808 (if approved) Day Software
 L. Masinter
Expires: March 26, 2005 Adobe
 September 25, 2004
 Uniform Resource Identifier (URI): Generic Syntax
 draft-fielding-uri-rfc2396bis-07
Status of this Memo
 This document is an Internet-Draft and is subject to all provisions
 of section 3 of RFC 3667. By submitting this Internet-Draft, each
 author represents that any applicable patent or other IPR claims of
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 which he or she become aware will be disclosed, in accordance with
 RFC 3668.
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Copyright Notice
 Copyright (C) The Internet Society (2004).
Abstract
 A Uniform Resource Identifier (URI) is a compact sequence of
 characters for identifying an abstract or physical resource. This
 specification defines the generic URI syntax and a process for
 resolving URI references that might be in relative form, along with
 guidelines and security considerations for the use of URIs on the
 Internet. The URI syntax defines a grammar that is a superset of all
 valid URIs, such that an implementation can parse the common
 components of a URI reference without knowing the scheme-specific
 requirements of every possible identifier. This specification does
 not define a generative grammar for URIs; that task is performed by
 the individual specifications of each URI scheme.
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Editorial Note
 Discussion of this draft and comments to the editors should be sent
 to the uri@w3.org mailing list. An issues list and version history
 is available at <http://gbiv.com/protocols/uri/rev-2002/issues.html>.
Table of Contents
 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
 1.1 Overview of URIs . . . . . . . . . . . . . . . . . . . . . 4
 1.1.1 Generic Syntax . . . . . . . . . . . . . . . . . . . . 6
 1.1.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 7
 1.1.3 URI, URL, and URN . . . . . . . . . . . . . . . . . . 7
 1.2 Design Considerations . . . . . . . . . . . . . . . . . . 7
 1.2.1 Transcription . . . . . . . . . . . . . . . . . . . . 7
 1.2.2 Separating Identification from Interaction . . . . . . 9
 1.2.3 Hierarchical Identifiers . . . . . . . . . . . . . . . 10
 1.3 Syntax Notation . . . . . . . . . . . . . . . . . . . . . 11
 2. Characters . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 2.1 Percent-Encoding . . . . . . . . . . . . . . . . . . . . . 12
 2.2 Reserved Characters . . . . . . . . . . . . . . . . . . . 12
 2.3 Unreserved Characters . . . . . . . . . . . . . . . . . . 13
 2.4 When to Encode or Decode . . . . . . . . . . . . . . . . . 13
 2.5 Identifying Data . . . . . . . . . . . . . . . . . . . . . 14
 3. Syntax Components . . . . . . . . . . . . . . . . . . . . . . 16
 3.1 Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . 16
 3.2 Authority . . . . . . . . . . . . . . . . . . . . . . . . 17
 3.2.1 User Information . . . . . . . . . . . . . . . . . . . 18
 3.2.2 Host . . . . . . . . . . . . . . . . . . . . . . . . . 18
 3.2.3 Port . . . . . . . . . . . . . . . . . . . . . . . . . 21
 3.3 Path . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
 3.4 Query . . . . . . . . . . . . . . . . . . . . . . . . . . 23
 3.5 Fragment . . . . . . . . . . . . . . . . . . . . . . . . . 24
 4. Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
 4.1 URI Reference . . . . . . . . . . . . . . . . . . . . . . 25
 4.2 Relative Reference . . . . . . . . . . . . . . . . . . . . 26
 4.3 Absolute URI . . . . . . . . . . . . . . . . . . . . . . . 26
 4.4 Same-document Reference . . . . . . . . . . . . . . . . . 27
 4.5 Suffix Reference . . . . . . . . . . . . . . . . . . . . . 27
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 5. Reference Resolution . . . . . . . . . . . . . . . . . . . . . 28
 5.1 Establishing a Base URI . . . . . . . . . . . . . . . . . 28
 5.1.1 Base URI Embedded in Content . . . . . . . . . . . . . 29
 5.1.2 Base URI from the Encapsulating Entity . . . . . . . . 29
 5.1.3 Base URI from the Retrieval URI . . . . . . . . . . . 30
 5.1.4 Default Base URI . . . . . . . . . . . . . . . . . . . 30
 5.2 Relative Resolution . . . . . . . . . . . . . . . . . . . 30
 5.2.1 Pre-parse the Base URI . . . . . . . . . . . . . . . . 30
 5.2.2 Transform References . . . . . . . . . . . . . . . . . 31
 5.2.3 Merge Paths . . . . . . . . . . . . . . . . . . . . . 32
 5.2.4 Remove Dot Segments . . . . . . . . . . . . . . . . . 32
 5.3 Component Recomposition . . . . . . . . . . . . . . . . . 34
 5.4 Reference Resolution Examples . . . . . . . . . . . . . . 34
 5.4.1 Normal Examples . . . . . . . . . . . . . . . . . . . 35
 5.4.2 Abnormal Examples . . . . . . . . . . . . . . . . . . 35
 6. Normalization and Comparison . . . . . . . . . . . . . . . . . 36
 6.1 Equivalence . . . . . . . . . . . . . . . . . . . . . . . 37
 6.2 Comparison Ladder . . . . . . . . . . . . . . . . . . . . 37
 6.2.1 Simple String Comparison . . . . . . . . . . . . . . . 38
 6.2.2 Syntax-based Normalization . . . . . . . . . . . . . . 39
 6.2.3 Scheme-based Normalization . . . . . . . . . . . . . . 40
 6.2.4 Protocol-based Normalization . . . . . . . . . . . . . 41
 7. Security Considerations . . . . . . . . . . . . . . . . . . . 41
 7.1 Reliability and Consistency . . . . . . . . . . . . . . . 41
 7.2 Malicious Construction . . . . . . . . . . . . . . . . . . 42
 7.3 Back-end Transcoding . . . . . . . . . . . . . . . . . . . 42
 7.4 Rare IP Address Formats . . . . . . . . . . . . . . . . . 43
 7.5 Sensitive Information . . . . . . . . . . . . . . . . . . 44
 7.6 Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 44
 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 45
 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
 10.1 Normative References . . . . . . . . . . . . . . . . . . . . 46
 10.2 Informative References . . . . . . . . . . . . . . . . . . . 46
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 48
 A. Collected ABNF for URI . . . . . . . . . . . . . . . . . . . . 49
 B. Parsing a URI Reference with a Regular Expression . . . . . . 51
 C. Delimiting a URI in Context . . . . . . . . . . . . . . . . . 52
 D. Changes from RFC 2396 . . . . . . . . . . . . . . . . . . . . 53
 D.1 Additions . . . . . . . . . . . . . . . . . . . . . . . . 53
 D.2 Modifications . . . . . . . . . . . . . . . . . . . . . . 54
 E. Instructions to RFC Editor . . . . . . . . . . . . . . . . . . 56
 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
 Intellectual Property and Copyright Statements . . . . . . . . 61
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1. Introduction
 A Uniform Resource Identifier (URI) provides a simple and extensible
 means for identifying a resource. This specification of URI syntax
 and semantics is derived from concepts introduced by the World Wide
 Web global information initiative, whose use of such identifiers
 dates from 1990 and is described in "Universal Resource Identifiers
 in WWW" [RFC1630], and is designed to meet the recommendations laid
 out in "Functional Recommendations for Internet Resource Locators"
 [RFC1736] and "Functional Requirements for Uniform Resource Names"
 [RFC1737].
 This document obsoletes [RFC2396], which merged "Uniform Resource
 Locators" [RFC1738] and "Relative Uniform Resource Locators"
 [RFC1808] in order to define a single, generic syntax for all URIs.
 It contains the updates from, and obsoletes, [RFC2732], which
 introduced syntax for IPv6 addresses. It excludes those portions of
 RFC 1738 that defined the specific syntax of individual URI schemes;
 those portions will be updated as separate documents. The process
 for registration of new URI schemes is defined separately by [BCP35].
 Advice for designers of new URI schemes can be found in [RFC2718].
 All significant changes from RFC 2396 are noted in Appendix D.
 This specification uses the terms "character" and "coded character
 set" in accordance with the definitions provided in [BCP19], and
 "character encoding" in place of what [BCP19] refers to as a
 "charset".
1.1 Overview of URIs
 URIs are characterized as follows:
 Uniform
 Uniformity provides several benefits: it allows different types of
 resource identifiers to be used in the same context, even when the
 mechanisms used to access those resources may differ; it allows
 uniform semantic interpretation of common syntactic conventions
 across different types of resource identifiers; it allows
 introduction of new types of resource identifiers without
 interfering with the way that existing identifiers are used; and,
 it allows the identifiers to be reused in many different contexts,
 thus permitting new applications or protocols to leverage a
 pre-existing, large, and widely-used set of resource identifiers.
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 Resource
 This specification does not limit the scope of what might be a
 resource; rather, the term "resource" is used in a general sense
 for whatever might be identified by a URI. Familiar examples
 include an electronic document, an image, a source of information
 with consistent purpose (e.g., "today's weather report for Los
 Angeles"), a service (e.g., an HTTP to SMS gateway), a collection
 of other resources, and so on. A resource is not necessarily
 accessible via the Internet; e.g., human beings, corporations, and
 bound books in a library can also be resources. Likewise,
 abstract concepts can be resources, such as the operators and
 operands of a mathematical equation, the types of a relationship
 (e.g., "parent" or "employee"), or numeric values (e.g., zero,
 one, and infinity).
 Identifier
 An identifier embodies the information required to distinguish
 what is being identified from all other things within its scope of
 identification. Our use of the terms "identify" and "identifying"
 refer to this purpose of distinguishing one resource from all
 other resources, regardless of how that purpose is accomplished
 (e.g., by name, address, context, etc.). These terms should not
 be mistaken as an assumption that an identifier defines or
 embodies the identity of what is referenced, though that may be
 the case for some identifiers. Nor should it be assumed that a
 system using URIs will access the resource identified: in many
 cases, URIs are used to denote resources without any intention
 that they be accessed. Likewise, the "one" resource identified
 might not be singular in nature (e.g., a resource might be a named
 set or a mapping that varies over time).
 A URI is an identifier, consisting of a sequence of characters
 matching the syntax rule named <URI> in Section 3, that enables
 uniform identification of resources via a separately defined,
 extensible set of naming schemes (Section 3.1). How that
 identification is accomplished, assigned, or enabled is delegated to
 each scheme specification.
 This specification does not place any limits on the nature of a
 resource, the reasons why an application might wish to refer to a
 resource, or the kinds of system that might use URIs for the sake of
 identifying resources. This specification does not require that a
 URI persists in identifying the same resource over all time, though
 that is a common goal of all URI schemes. Nevertheless, nothing in
 this specification prevents an application from limiting itself to
 particular types of resources, or to a subset of URIs that maintains
 characteristics desired by that application.
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 URIs have a global scope and are interpreted consistently regardless
 of context, though the result of that interpretation may be in
 relation to the end-user's context. For example, "http://localhost/"
 has the same interpretation for every user of that reference, even
 though the network interface corresponding to "localhost" may be
 different for each end-user: interpretation is independent of access.
 However, an action made on the basis of that reference will take
 place in relation to the end-user's context, which implies that an
 action intended to refer to a single, globally unique thing must use
 a URI that distinguishes that resource from all other things. URIs
 that identify in relation to the end-user's local context should only
 be used when the context itself is a defining aspect of the resource,
 such as when an on-line help manual refers to a file on the
 end-user's filesystem (e.g., "file:///etc/hosts").
1.1.1 Generic Syntax
 Each URI begins with a scheme name, as defined in Section 3.1, that
 refers to a specification for assigning identifiers within that
 scheme. As such, the URI syntax is a federated and extensible naming
 system wherein each scheme's specification may further restrict the
 syntax and semantics of identifiers using that scheme.
 This specification defines those elements of the URI syntax that are
 required of all URI schemes or are common to many URI schemes. It
 thus defines the syntax and semantics that are needed to implement a
 scheme-independent parsing mechanism for URI references, such that
 the scheme-dependent handling of a URI can be postponed until the
 scheme-dependent semantics are needed. Likewise, protocols and data
 formats that make use of URI references can refer to this
 specification as defining the range of syntax allowed for all URIs,
 including those schemes that have yet to be defined, thus decoupling
 the evolution of identification schemes from the evolution of
 protocols, data formats, and implementations that make use of URIs.
 A parser of the generic URI syntax is capable of parsing any URI
 reference into its major components; once the scheme is determined,
 further scheme-specific parsing can be performed on the components.
 In other words, the URI generic syntax is a superset of the syntax of
 all URI schemes.
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1.1.2 Examples
 The following example URIs illustrate several URI schemes and
 variations in their common syntax components:
 ftp://ftp.is.co.za/rfc/rfc1808.txt
 http://www.ietf.org/rfc/rfc2396.txt
 ldap://[2001:db8::7]/c=GB?objectClass?one
 mailto:John.Doe@example.com
 news:comp.infosystems.www.servers.unix
 tel:+1-816-555-1212
 telnet://192.0.2.16:80/
 urn:oasis:names:specification:docbook:dtd:xml:4.1.2
1.1.3 URI, URL, and URN
 A URI can be further classified as a locator, a name, or both. The
 term "Uniform Resource Locator" (URL) refers to the subset of URIs
 that, in addition to identifying a resource, provide a means of
 locating the resource by describing its primary access mechanism
 (e.g., its network "location"). The term "Uniform Resource Name"
 (URN) has been used historically to refer to both URIs under the
 "urn" scheme [RFC2141], which are required to remain globally unique
 and persistent even when the resource ceases to exist or becomes
 unavailable, and to any other URI with the properties of a name.
 An individual scheme does not need to be classified as being just one
 of "name" or "locator". Instances of URIs from any given scheme may
 have the characteristics of names or locators or both, often
 depending on the persistence and care in the assignment of
 identifiers by the naming authority, rather than any quality of the
 scheme. Future specifications and related documentation should use
 the general term "URI", rather than the more restrictive terms URL
 and URN [RFC3305].
1.2 Design Considerations
1.2.1 Transcription
 The URI syntax has been designed with global transcription as one of
 its main considerations. A URI is a sequence of characters from a
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 very limited set: the letters of the basic Latin alphabet, digits,
 and a few special characters. A URI may be represented in a variety
 of ways: e.g., ink on paper, pixels on a screen, or a sequence of
 character encoding octets. The interpretation of a URI depends only
 on the characters used and not how those characters are represented
 in a network protocol.
 The goal of transcription can be described by a simple scenario.
 Imagine two colleagues, Sam and Kim, sitting in a pub at an
 international conference and exchanging research ideas. Sam asks Kim
 for a location to get more information, so Kim writes the URI for the
 research site on a napkin. Upon returning home, Sam takes out the
 napkin and types the URI into a computer, which then retrieves the
 information to which Kim referred.
 There are several design considerations revealed by the scenario:
 o A URI is a sequence of characters that is not always represented
 as a sequence of octets.
 o A URI might be transcribed from a non-network source, and thus
 should consist of characters that are most likely to be able to be
 entered into a computer, within the constraints imposed by
 keyboards (and related input devices) across languages and
 locales.
 o A URI often needs to be remembered by people, and it is easier for
 people to remember a URI when it consists of meaningful or
 familiar components.
 These design considerations are not always in alignment. For
 example, it is often the case that the most meaningful name for a URI
 component would require characters that cannot be typed into some
 systems. The ability to transcribe a resource identifier from one
 medium to another has been considered more important than having a
 URI consist of the most meaningful of components.
 In local or regional contexts and with improving technology, users
 might benefit from being able to use a wider range of characters;
 such use is not defined by this specification. Percent-encoded
 octets (Section 2.1) may be used within a URI to represent characters
 outside the range of the US-ASCII coded character set if such
 representation is allowed by the scheme or by the protocol element in
 which the URI is referenced; such a definition should specify the
 character encoding used to map those characters to octets prior to
 being percent-encoded for the URI.
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1.2.2 Separating Identification from Interaction
 A common misunderstanding of URIs is that they are only used to refer
 to accessible resources. In fact, the URI alone only provides
 identification; access to the resource is neither guaranteed nor
 implied by the presence of a URI. Instead, an operation (if any)
 associated with a URI reference is defined by the protocol element,
 data format attribute, or natural language text in which it appears.
 Given a URI, a system may attempt to perform a variety of operations
 on the resource, as might be characterized by such words as "access",
 "update", "replace", or "find attributes". Such operations are
 defined by the protocols that make use of URIs, not by this
 specification. However, we do use a few general terms for describing
 common operations on URIs. URI "resolution" is the process of
 determining an access mechanism and the appropriate parameters
 necessary to dereference a URI; such resolution may require several
 iterations. To use that access mechanism to perform an action on the
 URI's resource is to "dereference" the URI.
 When URIs are used within information retrieval systems to identify
 sources of information, the most common form of URI dereference is
 "retrieval": making use of a URI in order to retrieve a
 representation of its associated resource. A "representation" is a
 sequence of octets, along with representation metadata describing
 those octets, that constitutes a record of the state of the resource
 at the time that the representation is generated. Retrieval is
 achieved by a process that might include using the URI as a cache key
 to check for a locally cached representation, resolution of the URI
 to determine an appropriate access mechanism (if any), and
 dereference of the URI for the sake of applying a retrieval
 operation. Depending on the protocols used to perform the retrieval,
 additional information might be supplied about the resource (resource
 metadata) and its relation to other resources.
 URI references in information retrieval systems are designed to be
 late-binding: the result of an access is generally determined at the
 time it is accessed and may vary over time or due to other aspects of
 the interaction. Such references are created in order to be used in
 the future: what is being identified is not some specific result that
 was obtained in the past, but rather some characteristic that is
 expected to be true for future results. In such cases, the resource
 referred to by the URI is actually a sameness of characteristics as
 observed over time, perhaps elucidated by additional comments or
 assertions made by the resource provider.
 Although many URI schemes are named after protocols, this does not
 imply that use of such a URI will result in access to the resource
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 via the named protocol. URIs are often used simply for the sake of
 identification. Even when a URI is used to retrieve a representation
 of a resource, that access might be through gateways, proxies,
 caches, and name resolution services that are independent of the
 protocol associated with the scheme name, and the resolution of some
 URIs may require the use of more than one protocol (e.g., both DNS
 and HTTP are typically used to access an "http" URI's origin server
 when a representation isn't found in a local cache).
1.2.3 Hierarchical Identifiers
 The URI syntax is organized hierarchically, with components listed in
 order of decreasing significance from left to right. For some URI
 schemes, the visible hierarchy is limited to the scheme itself:
 everything after the scheme component delimiter (":") is considered
 opaque to URI processing. Other URI schemes make the hierarchy
 explicit and visible to generic parsing algorithms.
 The generic syntax uses the slash ("/"), question mark ("?"), and
 number sign ("#") characters for the purpose of delimiting components
 that are significant to the generic parser's hierarchical
 interpretation of an identifier. In addition to aiding the
 readability of such identifiers through the consistent use of
 familiar syntax, this uniform representation of hierarchy across
 naming schemes allows scheme-independent references to be made
 relative to that hierarchy.
 It is often the case that a group or "tree" of documents has been
 constructed to serve a common purpose, wherein the vast majority of
 URI references in these documents point to resources within the tree
 rather than outside of it. Similarly, documents located at a
 particular site are much more likely to refer to other resources at
 that site than to resources at remote sites. Relative referencing of
 URIs allows document trees to be partially independent of their
 location and access scheme. For instance, it is possible for a
 single set of hypertext documents to be simultaneously accessible and
 traversable via each of the "file", "http", and "ftp" schemes if the
 documents refer to each other using relative references.
 Furthermore, such document trees can be moved, as a whole, without
 changing any of the relative references.
 A relative reference (Section 4.2) refers to a resource by describing
 the difference within a hierarchical name space between the reference
 context and the target URI. The reference resolution algorithm,
 presented in Section 5, defines how such a reference is transformed
 to the target URI. Since relative references can only be used within
 the context of a hierarchical URI, designers of new URI schemes
 should use a syntax consistent with the generic syntax's hierarchical
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 components unless there are compelling reasons to forbid relative
 referencing within that scheme.
 NOTE: Previous specifications used the terms "partial URI" and
 "relative URI" to denote a relative reference to a URI. Since
 some readers misunderstood those terms to mean that relative URIs
 are a subset of URIs, rather than a method of referencing URIs,
 this specification simply refers to them as relative references.
 All URI references are parsed by generic syntax parsers when used.
 However, since hierarchical processing has no effect on an absolute
 URI used in a reference unless it contains one or more dot-segments
 (complete path segments of "." or "..", as described in Section 3.3),
 URI scheme specifications can define opaque identifiers by
 disallowing use of slash characters, question mark characters, and
 the URIs "scheme:." and "scheme:..".
1.3 Syntax Notation
 This specification uses the Augmented Backus-Naur Form (ABNF)
 notation of [RFC2234], including the following core ABNF syntax rules
 defined by that specification: ALPHA (letters), CR (carriage return),
 DIGIT (decimal digits), DQUOTE (double quote), HEXDIG (hexadecimal
 digits), LF (line feed), and SP (space). The complete URI syntax is
 collected in Appendix A.
2. Characters
 The URI syntax provides a method of encoding data, presumably for the
 sake of identifying a resource, as a sequence of characters. The URI
 characters are, in turn, frequently encoded as octets for transport
 or presentation. This specification does not mandate any particular
 character encoding for mapping between URI characters and the octets
 used to store or transmit those characters. When a URI appears in a
 protocol element, the character encoding is defined by that protocol;
 absent such a definition, a URI is assumed to be in the same
 character encoding as the surrounding text.
 The ABNF notation defines its terminal values to be non-negative
 integers (codepoints) based on the US-ASCII coded character set
 [ASCII]. Since a URI is a sequence of characters, we must invert
 that relation in order to understand the URI syntax. Therefore, the
 integer values used by the ABNF must be mapped back to their
 corresponding characters via US-ASCII in order to complete the syntax
 rules.
 A URI is composed from a limited set of characters consisting of
 digits, letters, and a few graphic symbols. A reserved subset of
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 those characters may be used to delimit syntax components within a
 URI, while the remaining characters, including both the unreserved
 set and those reserved characters not acting as delimiters, define
 each component's identifying data.
2.1 Percent-Encoding
 A percent-encoding mechanism is used to represent a data octet in a
 component when that octet's corresponding character is outside the
 allowed set or is being used as a delimiter of, or within, the
 component. A percent-encoded octet is encoded as a character
 triplet, consisting of the percent character "%" followed by the two
 hexadecimal digits representing that octet's numeric value. For
 example, "%20" is the percent-encoding for the binary octet
 "00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
 character (SP). Section 2.4 describes when percent-encoding and
 decoding is applied.
 pct-encoded = "%" HEXDIG HEXDIG
 The uppercase hexadecimal digits 'A' through 'F' are equivalent to
 the lowercase digits 'a' through 'f', respectively. Two URIs that
 differ only in the case of hexadecimal digits used in percent-encoded
 octets are equivalent. For consistency, URI producers and
 normalizers should use uppercase hexadecimal digits for all
 percent-encodings.
2.2 Reserved Characters
 URIs include components and subcomponents that are delimited by
 characters in the "reserved" set. These characters are called
 "reserved" because they may (or may not) be defined as delimiters by
 the generic syntax, by each scheme-specific syntax, or by the
 implementation-specific syntax of a URI's dereferencing algorithm.
 If data for a URI component would conflict with a reserved
 character's purpose as a delimiter, then the conflicting data must be
 percent-encoded before forming the URI.
 reserved = gen-delims / sub-delims
 gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
 sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
 / "*" / "+" / "," / ";" / "="
 The purpose of reserved characters is to provide a set of delimiting
 characters that are distinguishable from other data within a URI.
 URIs that differ in the replacement of a reserved character with its
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 corresponding percent-encoded octet are not equivalent.
 Percent-encoding a reserved character, or decoding a percent-encoded
 octet that corresponds to a reserved character, will change how the
 URI is interpreted by most applications. Thus, characters in the
 reserved set are protected from normalization and are therefore safe
 to be used by scheme-specific and producer-specific algorithms for
 delimiting data subcomponents within a URI.
 A subset of the reserved characters (gen-delims) are used as
 delimiters of the generic URI components described in Section 3. A
 component's ABNF syntax rule will not use the reserved or gen-delims
 rule names directly; instead, each syntax rule lists the characters
 allowed within that component (i.e., not delimiting it) and any of
 those characters that are also in the reserved set are "reserved" for
 use as subcomponent delimiters within the component. Only the most
 common subcomponents are defined by this specification; other
 subcomponents may be defined by a URI scheme's specification, or by
 the implementation-specific syntax of a URI's dereferencing
 algorithm, provided that such subcomponents are delimited by
 characters in the reserved set allowed within that component.
 URI producing applications should percent-encode data octets that
 correspond to characters in the reserved set. However, if a reserved
 character is found in a URI component and no delimiting role is known
 for that character, then it should be interpreted as representing the
 data octet corresponding to that character's encoding in US-ASCII.
2.3 Unreserved Characters
 Characters that are allowed in a URI but do not have a reserved
 purpose are called unreserved. These include uppercase and lowercase
 letters, decimal digits, hyphen, period, underscore, and tilde.
 unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
 URIs that differ in the replacement of an unreserved character with
 its corresponding percent-encoded US-ASCII octet are equivalent: they
 identify the same resource. However, URI comparison implementations
 do not always perform normalization prior to comparison Section 6.
 For consistency, percent-encoded octets in the ranges of ALPHA
 (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
 underscore (%5F), or tilde (%7E) should not be created by URI
 producers and, when found in a URI, should be decoded to their
 corresponding unreserved character by URI normalizers.
2.4 When to Encode or Decode
 Under normal circumstances, the only time that octets within a URI
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 are percent-encoded is during the process of producing the URI from
 its component parts. It is during that process that an
 implementation determines which of the reserved characters are to be
 used as subcomponent delimiters and which can be safely used as data.
 Once produced, a URI is always in its percent-encoded form.
 When a URI is dereferenced, the components and subcomponents
 significant to the scheme-specific dereferencing process (if any)
 must be parsed and separated before the percent-encoded octets within
 those components can be safely decoded, since otherwise the data may
 be mistaken for component delimiters. The only exception is for
 percent-encoded octets corresponding to characters in the unreserved
 set, which can be decoded at any time. For example, the octet
 corresponding to the tilde ("~") character is often encoded as "%7E"
 by older URI processing implementations; the "%7E" can be replaced by
 "~" without changing its interpretation.
 Because the percent ("%") character serves as the indicator for
 percent-encoded octets, it must be percent-encoded as "%25" in order
 for that octet to be used as data within a URI. Implementations must
 not percent-encode or decode the same string more than once, since
 decoding an already decoded string might lead to misinterpreting a
 percent data octet as the beginning of a percent-encoding, or vice
 versa in the case of percent-encoding an already percent-encoded
 string.
2.5 Identifying Data
 URI characters provide identifying data for each of the URI
 components, serving as an external interface for identification
 between systems. Although the presence and nature of the URI
 production interface is hidden from clients that use its URIs, and
 thus beyond the scope of the interoperability requirements defined by
 this specification, it is a frequent source of confusion and errors
 in the interpretation of URI character issues. Implementers need to
 be aware that there are multiple character encodings involved in the
 production and transmission of URIs: local name and data encoding,
 public interface encoding, URI character encoding, data format
 encoding, and protocol encoding.
 The first encoding of identifying data is the one in which the local
 names or data are stored. URI producing applications (a.k.a., origin
 servers) will typically use the local encoding as the basis for
 producing meaningful names. The URI producer will transform the
 local encoding to one that is suitable for a public interface, and
 then transform the public interface encoding into the restricted set
 of URI characters (reserved, unreserved, and percent-encodings).
 Those characters are, in turn, encoded as octets to be used as a
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 reference within a data format (e.g., a document charset), and such
 data formats are often subsequently encoded for transmission over
 Internet protocols.
 For most systems, an unreserved character appearing within a URI
 component is interpreted as representing the data octet corresponding
 to that character's encoding in US-ASCII. Consumers of URIs assume
 that the letter "X" corresponds to the octet "01011000", and there is
 no harm in making that assumption even when it is incorrect. A
 system that internally provides identifiers in the form of a
 different character encoding, such as EBCDIC, will generally perform
 character translation of textual identifiers to UTF-8 [STD63] (or
 some other superset of the US-ASCII character encoding) at an
 internal interface, thereby providing more meaningful identifiers
 than simply percent-encoding the original octets.
 For example, consider an information service that provides data,
 stored locally using an EBCDIC-based filesystem, to clients on the
 Internet through an HTTP server. When an author creates a file on
 that filesystem with the name "Laguna Beach", their expectation is
 that the "http" URI corresponding to that resource would also contain
 the meaningful string "Laguna%20Beach". If, however, that server
 produces URIs using an overly-simplistic raw octet mapping, then the
 result would be a URI containing
 "%D3%81%87%A4%95%81@%C2%85%81%83%88". An internal transcoding
 interface fixes that problem by transcoding the local name to a
 superset of US-ASCII prior to producing the URI. Naturally, proper
 interpretation of an incoming URI on such an interface requires that
 percent-encoded octets be decoded (e.g., "%20" to SP) before the
 reverse transcoding is applied to obtain the local name.
 In some cases, the internal interface between a URI component and the
 identifying data that it has been crafted to represent is much less
 direct than a character encoding translation. For example, portions
 of a URI might reflect a query on non-ASCII data, numeric coordinates
 on a map, etc. Likewise, a URI scheme may define components with
 additional encoding requirements that are applied prior to forming
 the component and producing the URI.
 When a new URI scheme defines a component that represents textual
 data consisting of characters from the Unicode character set [UCS],
 the data should be encoded first as octets according to the UTF-8
 character encoding [STD63], and then only those octets that do not
 correspond to characters in the unreserved set should be
 percent-encoded. For example, the character A would be represented
 as "A", the character LATIN CAPITAL LETTER A WITH GRAVE would be
 represented as "%C3%80", and the character KATAKANA LETTER A would be
 represented as "%E3%82%A2".
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3. Syntax Components
 The generic URI syntax consists of a hierarchical sequence of
 components referred to as the scheme, authority, path, query, and
 fragment.
 URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
 hier-part = "//" authority path-abempty
 / path-absolute
 / path-rootless
 / path-empty
 The scheme and path components are required, though path may be empty
 (no characters). When authority is present, the path must either be
 empty or begin with a slash ("/") character. When authority is not
 present, the path cannot begin with two slash characters ("//").
 These restrictions result in five different ABNF rules for a path
 (Section 3.3), only one of which will match any given URI reference.
 The following are two example URIs and their component parts:
 foo://example.com:8042/over/there?name=ferret#nose
 \_/ \______________/\_________/ \_________/ \__/
 | | | | |
 scheme authority path query fragment
 | _____________________|__
 / \ / \
 urn:example:animal:ferret:nose
3.1 Scheme
 Each URI begins with a scheme name that refers to a specification for
 assigning identifiers within that scheme. As such, the URI syntax is
 a federated and extensible naming system wherein each scheme's
 specification may further restrict the syntax and semantics of
 identifiers using that scheme.
 Scheme names consist of a sequence of characters beginning with a
 letter and followed by any combination of letters, digits, plus
 ("+"), period ("."), or hyphen ("-"). Although scheme is
 case-insensitive, the canonical form is lowercase and documents that
 specify schemes must do so using lowercase letters. An
 implementation should accept uppercase letters as equivalent to
 lowercase in scheme names (e.g., allow "HTTP" as well as "http"), for
 the sake of robustness, but should only produce lowercase scheme
 names, for consistency.
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 scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
 Individual schemes are not specified by this document. The process
 for registration of new URI schemes is defined separately by [BCP35].
 The scheme registry maintains the mapping between scheme names and
 their specifications. Advice for designers of new URI schemes can be
 found in [RFC2718]. URI scheme specifications must define their own
 syntax such that all strings matching their scheme-specific syntax
 will also match the <absolute-URI> grammar, as described in
 Section 4.3.
 When presented with a URI that violates one or more scheme-specific
 restrictions, the scheme-specific resolution process should flag the
 reference as an error rather than ignore the unused parts; doing so
 reduces the number of equivalent URIs and helps detect abuses of the
 generic syntax that might indicate the URI has been constructed to
 mislead the user (Section 7.6).
3.2 Authority
 Many URI schemes include a hierarchical element for a naming
 authority, such that governance of the name space defined by the
 remainder of the URI is delegated to that authority (which may, in
 turn, delegate it further). The generic syntax provides a common
 means for distinguishing an authority based on a registered name or
 server address, along with optional port and user information.
 The authority component is preceded by a double slash ("//") and is
 terminated by the next slash ("/"), question mark ("?"), or number
 sign ("#") character, or by the end of the URI.
 authority = [ userinfo "@" ] host [ ":" port ]
 URI producers and normalizers should omit the ":" delimiter that
 separates host from port if the port component is empty. Some
 schemes do not allow the userinfo and/or port subcomponents.
 If a URI contains an authority component, then the path component
 must either be empty or begin with a slash ("/") character.
 Non-validating parsers (those that merely separate a URI reference
 into its major components) will often ignore the subcomponent
 structure of authority, treating it as an opaque string from the
 double-slash to the first terminating delimiter, until such time as
 the URI is dereferenced.
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3.2.1 User Information
 The userinfo subcomponent may consist of a user name and, optionally,
 scheme-specific information about how to gain authorization to access
 the resource. The user information, if present, is followed by a
 commercial at-sign ("@") that delimits it from the host.
 userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
 Use of the format "user:password" in the userinfo field is
 deprecated. Applications should not render as clear text any data
 after the first colon (":") character found within a userinfo
 subcomponent unless the data after the colon is the empty string
 (indicating no password). Applications may choose to ignore or
 reject such data when received as part of a reference, and should
 reject the storage of such data in unencrypted form. The passing of
 authentication information in clear text has proven to be a security
 risk in almost every case where it has been used.
 Applications that render a URI for the sake of user feedback, such as
 in graphical hypertext browsing, should render userinfo in a way that
 is distinguished from the rest of a URI, when feasible. Such
 rendering will assist the user in cases where the userinfo has been
 misleadingly crafted to look like a trusted domain name
 (Section 7.6).
3.2.2 Host
 The host subcomponent of authority is identified by an IP literal
 encapsulated within square brackets, an IPv4 address in
 dotted-decimal form, or a registered name. The host subcomponent is
 case-insensitive. The presence of a host subcomponent within a URI
 does not imply that the scheme requires access to the given host on
 the Internet. In many cases, the host syntax is used only for the
 sake of reusing the existing registration process created and
 deployed for DNS, thus obtaining a globally unique name without the
 cost of deploying another registry. However, such use comes with its
 own costs: domain name ownership may change over time for reasons not
 anticipated by the URI producer. In other cases, the data within the
 host component identifies a registered name that has nothing to do
 with an Internet host. We use the name "host" for the ABNF rule
 because that is its most common purpose, not its only purpose, and
 thus should not be considered as semantically limiting the data
 within it.
 host = IP-literal / IPv4address / reg-name
 The syntax rule for host is ambiguous because it does not completely
 distinguish between an IPv4address and a reg-name. In order to
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 disambiguate the syntax, we apply the "first-match-wins" algorithm:
 If host matches the rule for IPv4address, then it should be
 considered an IPv4 address literal and not a reg-name. Although host
 is case-insensitive, producers and normalizers should use lowercase
 for registered names and hexadecimal addresses for the sake of
 uniformity, while only using uppercase letters for percent-encodings.
 A host identified by an Internet Protocol literal address, version 6
 [RFC3513] or later, is distinguished by enclosing the IP literal
 within square brackets ("[" and "]"). This is the only place where
 square bracket characters are allowed in the URI syntax. In
 anticipation of future, as-yet-undefined IP literal address formats,
 an optional version flag may be used to indicate such a format
 explicitly rather than relying on heuristic determination.
 IP-literal = "[" ( IPv6address / IPvFuture ) "]"
 IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
 The version flag does not indicate the IP version; rather, it
 indicates future versions of the literal format. As such,
 implementations must not provide the version flag for existing IPv4
 and IPv6 literal addresses. If a URI containing an IP-literal that
 starts with "v" (case-insensitive), indicating that the version flag
 is present, is dereferenced by an application that does not know the
 meaning of that version flag, then the application should return an
 appropriate error for "address mechanism not supported".
 A host identified by an IPv6 literal address is represented inside
 the square brackets without a preceding version flag. The ABNF
 provided here is a translation of the text definition of an IPv6
 literal address provided in [RFC3513]. A 128-bit IPv6 address is
 divided into eight 16-bit pieces. Each piece is represented
 numerically in case-insensitive hexadecimal, using one to four
 hexadecimal digits (leading zeroes are permitted). The eight encoded
 pieces are given most-significant first, separated by colon
 characters. Optionally, the least-significant two pieces may instead
 be represented in IPv4 address textual format. A sequence of one or
 more consecutive zero-valued 16-bit pieces within the address may be
 elided, omitting all their digits and leaving exactly two consecutive
 colons in their place to mark the elision.
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 IPv6address = 6( h16 ":" ) ls32
 / "::" 5( h16 ":" ) ls32
 / [ h16 ] "::" 4( h16 ":" ) ls32
 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
 / [ *4( h16 ":" ) h16 ] "::" ls32
 / [ *5( h16 ":" ) h16 ] "::" h16
 / [ *6( h16 ":" ) h16 ] "::"
 ls32 = ( h16 ":" h16 ) / IPv4address
 ; least-significant 32 bits of address
 h16 = 1*4HEXDIG
 ; 16 bits of address represented in hexadecimal
 A host identified by an IPv4 literal address is represented in
 dotted-decimal notation (a sequence of four decimal numbers in the
 range 0 to 255, separated by "."), as described in [RFC1123] by
 reference to [RFC0952]. Note that other forms of dotted notation may
 be interpreted on some platforms, as described in Section 7.4, but
 only the dotted-decimal form of four octets is allowed by this
 grammar.
 IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
 dec-octet = DIGIT ; 0-9
 / %x31-39 DIGIT ; 10-99
 / "1" 2DIGIT ; 100-199
 / "2" %x30-34 DIGIT ; 200-249
 / "25" %x30-35 ; 250-255
 A host identified by a registered name is a sequence of characters
 that is usually intended for lookup within a locally-defined host or
 service name registry, though the URI's scheme-specific semantics may
 require that a specific registry (or fixed name table) be used
 instead. The most common name registry mechanism is the Domain Name
 System (DNS). A registered name intended for lookup in the DNS uses
 the syntax defined in Section 3.5 of [RFC1034] and Section 2.1 of
 [RFC1123]. Such a name consists of a sequence of domain labels
 separated by ".", each domain label starting and ending with an
 alphanumeric character and possibly also containing "-" characters.
 The rightmost domain label of a fully qualified domain name in DNS
 may be followed by a single "." and should be followed by one if it
 is necessary to distinguish between the complete domain name and some
 local domain.
 reg-name = *( unreserved / pct-encoded / sub-delims )
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 If the URI scheme defines a default for host, then that default
 applies when the host subcomponent is undefined or when the
 registered name is empty (zero length). For example, the "file" URI
 scheme is defined such that no authority, an empty host, and
 "localhost" all mean the end-user's machine, whereas the "http"
 scheme considers a missing authority or empty host to be invalid.
 This specification does not mandate a particular registered name
 lookup technology and therefore does not restrict the syntax of
 reg-name beyond that necessary for interoperability. Instead, it
 delegates the issue of registered name syntax conformance to the
 operating system of each application performing URI resolution, and
 that operating system decides what it will allow for the purpose of
 host identification. A URI resolution implementation might use DNS,
 host tables, yellow pages, NetInfo, WINS, or any other system for
 lookup of registered names. However, a globally-scoped naming
 system, such as DNS fully-qualified domain names, is necessary for
 URIs that are intended to have global scope. URI producers should
 use names that conform to the DNS syntax, even when use of DNS is not
 immediately apparent, and should limit such names to no more than 255
 characters in length.
 The reg-name syntax allows percent-encoded octets in order to
 represent non-ASCII registered names in a uniform way that is
 independent of the underlying name resolution technology; such
 non-ASCII characters must first be encoded according to UTF-8 [STD63]
 and then each octet of the corresponding UTF-8 sequence must be
 percent-encoded to be represented as URI characters. URI producing
 applications must not use percent-encoding in host unless it is used
 to represent a UTF-8 character sequence. When a non-ASCII registered
 name represents an internationalized domain name intended for
 resolution via the DNS, the name must be transformed to the IDNA
 encoding [RFC3490] prior to name lookup. URI producers should
 provide such registered names in the IDNA encoding, rather than a
 percent-encoding, if they wish to maximize interoperability with
 legacy URI resolvers.
3.2.3 Port
 The port subcomponent of authority is designated by an optional port
 number in decimal following the host and delimited from it by a
 single colon (":") character.
 port = *DIGIT
 A scheme may define a default port. For example, the "http" scheme
 defines a default port of "80", corresponding to its reserved TCP
 port number. The type of port designated by the port number (e.g.,
 TCP, UDP, SCTP, etc.) is defined by the URI scheme. URI producers
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 and normalizers should omit the port component and its ":" delimiter
 if port is empty or its value would be the same as the scheme's
 default.
3.3 Path
 The path component contains data, usually organized in hierarchical
 form, that, along with data in the non-hierarchical query component
 (Section 3.4), serves to identify a resource within the scope of the
 URI's scheme and naming authority (if any). The path is terminated
 by the first question mark ("?") or number sign ("#") character, or
 by the end of the URI.
 If a URI contains an authority component, then the path component
 must either be empty or begin with a slash ("/") character. If a URI
 does not contain an authority component, then the path cannot begin
 with two slash characters ("//"). In addition, a URI reference
 (Section 4.1) may be a relative-path reference, in which case the
 first path segment cannot contain a colon (":") character. The ABNF
 requires five separate rules to disambiguate these cases, only one of
 which will match the path substring within a given URI reference. We
 use the generic term "path component" to describe the URI substring
 matched by the parser to one of these rules.
 path = path-abempty ; begins with "/" or is empty
 / path-absolute ; begins with "/" but not "//"
 / path-noscheme ; begins with a non-colon segment
 / path-rootless ; begins with a segment
 / path-empty ; zero characters
 path-abempty = *( "/" segment )
 path-absolute = "/" [ segment-nz *( "/" segment ) ]
 path-noscheme = segment-nz-nc *( "/" segment )
 path-rootless = segment-nz *( "/" segment )
 path-empty = 0<pchar>
 segment = *pchar
 segment-nz = 1*pchar
 segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
 ; non-zero-length segment without any colon ":"
 pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
 A path consists of a sequence of path segments separated by a slash
 ("/") character. A path is always defined for a URI, though the
 defined path may be empty (zero length). Use of the slash character
 to indicate hierarchy is only required when a URI will be used as the
 context for relative references. For example, the URI
 <mailto:fred@example.com> has a path of "fred@example.com", whereas
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 the URI <foo://info.example.com?fred> has an empty path.
 The path segments "." and "..", also known as dot-segments, are
 defined for relative reference within the path name hierarchy. They
 are intended for use at the beginning of a relative-path reference
 (Section 4.2) for indicating relative position within the
 hierarchical tree of names. This is similar to their role within
 some operating systems' file directory structure to indicate the
 current directory and parent directory, respectively. However,
 unlike a file system, these dot-segments are only interpreted within
 the URI path hierarchy and are removed as part of the resolution
 process (Section 5.2).
 Aside from dot-segments in hierarchical paths, a path segment is
 considered opaque by the generic syntax. URI-producing applications
 often use the reserved characters allowed in a segment for the
 purpose of delimiting scheme-specific or dereference-handler-specific
 subcomponents. For example, the semicolon (";") and equals ("=")
 reserved characters are often used for delimiting parameters and
 parameter values applicable to that segment. The comma (",")
 reserved character is often used for similar purposes. For example,
 one URI producer might use a segment like "name;v=1.1" to indicate a
 reference to version 1.1 of "name", whereas another might use a
 segment like "name,1.1" to indicate the same. Parameter types may be
 defined by scheme-specific semantics, but in most cases the syntax of
 a parameter is specific to the implementation of the URI's
 dereferencing algorithm.
3.4 Query
 The query component contains non-hierarchical data that, along with
 data in the path component (Section 3.3), serves to identify a
 resource within the scope of the URI's scheme and naming authority
 (if any). The query component is indicated by the first question
 mark ("?") character and terminated by a number sign ("#") character
 or by the end of the URI.
 query = *( pchar / "/" / "?" )
 The characters slash ("/") and question mark ("?") may represent data
 within the query component. Beware that some older, erroneous
 implementations may not handle such data correctly when used as the
 base URI for relative references (Section 5.1), apparently because
 they fail to to distinguish query data from path data when looking
 for hierarchical separators. However, since query components are
 often used to carry identifying information in the form of
 "key=value" pairs, and one frequently used value is a reference to
 another URI, it is sometimes better for usability to avoid
 percent-encoding those characters.
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3.5 Fragment
 The fragment identifier component of a URI allows indirect
 identification of a secondary resource by reference to a primary
 resource and additional identifying information. The identified
 secondary resource may be some portion or subset of the primary
 resource, some view on representations of the primary resource, or
 some other resource defined or described by those representations. A
 fragment identifier component is indicated by the presence of a
 number sign ("#") character and terminated by the end of the URI.
 fragment = *( pchar / "/" / "?" )
 The semantics of a fragment identifier are defined by the set of
 representations that might result from a retrieval action on the
 primary resource. The fragment's format and resolution is therefore
 dependent on the media type [RFC2046] of a potentially retrieved
 representation, even though such a retrieval is only performed if the
 URI is dereferenced. If no such representation exists, then the
 semantics of the fragment are considered unknown and, effectively,
 unconstrained. Fragment identifier semantics are independent of the
 URI scheme and thus cannot be redefined by scheme specifications.
 Individual media types may define their own restrictions on, or
 structure within, the fragment identifier syntax for specifying
 different types of subsets, views, or external references that are
 identifiable as secondary resources by that media type. If the
 primary resource has multiple representations, as is often the case
 for resources whose representation is selected based on attributes of
 the retrieval request (a.k.a., content negotiation), then whatever is
 identified by the fragment should be consistent across all of those
 representations: each representation should either define the
 fragment such that it corresponds to the same secondary resource,
 regardless of how it is represented, or the fragment should be left
 undefined by the representation (i.e., not found).
 As with any URI, use of a fragment identifier component does not
 imply that a retrieval action will take place. A URI with a fragment
 identifier may be used to refer to the secondary resource without any
 implication that the primary resource is accessible or will ever be
 accessed.
 Fragment identifiers have a special role in information retrieval
 systems as the primary form of client-side indirect referencing,
 allowing an author to specifically identify those aspects of an
 existing resource that are only indirectly provided by the resource
 owner. As such, the fragment identifier is not used in the
 scheme-specific processing of a URI; instead, the fragment identifier
 is separated from the rest of the URI prior to a dereference, and
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 thus the identifying information within the fragment itself is
 dereferenced solely by the user agent and regardless of the URI
 scheme. Although this separate handling is often perceived to be a
 loss of information, particularly in regards to accurate redirection
 of references as resources move over time, it also serves to prevent
 information providers from denying reference authors the right to
 selectively refer to information within a resource. Indirect
 referencing also provides additional flexibility and extensibility to
 systems that use URIs, since new media types are easier to define and
 deploy than new schemes of identification.
 The characters slash ("/") and question mark ("?") are allowed to
 represent data within the fragment identifier. Beware that some
 older, erroneous implementations may not handle such data correctly
 when used as the base URI for relative references (Section 5.1).
4. Usage
 When applications make reference to a URI, they do not always use the
 full form of reference defined by the "URI" syntax rule. In order to
 save space and take advantage of hierarchical locality, many Internet
 protocol elements and media type formats allow an abbreviation of a
 URI, while others restrict the syntax to a particular form of URI.
 We define the most common forms of reference syntax in this
 specification because they impact and depend upon the design of the
 generic syntax, requiring a uniform parsing algorithm in order to be
 interpreted consistently.
4.1 URI Reference
 URI-reference is used to denote the most common usage of a resource
 identifier.
 URI-reference = URI / relative-ref
 A URI-reference is either a URI or a relative reference. If the
 URI-reference's prefix does not match the syntax of a scheme followed
 by its colon separator, then the URI-reference is a relative
 reference.
 A URI-reference is typically parsed first into the five URI
 components, in order to determine what components are present and
 whether or not the reference is relative, after which each component
 is parsed for its subparts and their validation. The ABNF of
 URI-reference, along with the "first-match-wins" disambiguation rule,
 is sufficient to define a validating parser for the generic syntax.
 Readers familiar with regular expressions should see Appendix B for
 an example of a non-validating URI-reference parser that will take
 any given string and extract the URI components.
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4.2 Relative Reference
 A relative reference takes advantage of the hierarchical syntax
 (Section 1.2.3) in order to express a URI reference relative to the
 name space of another hierarchical URI.
 relative-ref = relative-part [ "?" query ] [ "#" fragment ]
 relative-part = "//" authority path-abempty
 / path-absolute
 / path-noscheme
 / path-empty
 The URI referred to by a relative reference, also known as the target
 URI, is obtained by applying the reference resolution algorithm of
 Section 5.
 A relative reference that begins with two slash characters is termed
 a network-path reference; such references are rarely used. A
 relative reference that begins with a single slash character is
 termed an absolute-path reference. A relative reference that does
 not begin with a slash character is termed a relative-path reference.
 A path segment that contains a colon character (e.g., "this:that")
 cannot be used as the first segment of a relative-path reference
 because it would be mistaken for a scheme name. Such a segment must
 be preceded by a dot-segment (e.g., "./this:that") to make a
 relative-path reference.
4.3 Absolute URI
 Some protocol elements allow only the absolute form of a URI without
 a fragment identifier. For example, defining a base URI for later
 use by relative references calls for an absolute-URI syntax rule that
 does not allow a fragment.
 absolute-URI = scheme ":" hier-part [ "?" query ]
 URI scheme specifications must define their own syntax such that all
 strings matching their scheme-specific syntax will also match the
 <absolute-URI> grammar. Scheme specifications are not responsible
 for defining fragment identifier syntax or usage, regardless of its
 applicability to resources identifiable via that scheme, since
 fragment identification is orthogonal to scheme definition. However,
 scheme specifications are encouraged to include a wide range of
 examples, including examples that show use of the scheme's URIs with
 fragment identifiers when such usage is appropriate.
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4.4 Same-document Reference
 When a URI reference refers to a URI that is, aside from its fragment
 component (if any), identical to the base URI (Section 5.1), that
 reference is called a "same-document" reference. The most frequent
 examples of same-document references are relative references that are
 empty or include only the number sign ("#") separator followed by a
 fragment identifier.
 When a same-document reference is dereferenced for the purpose of a
 retrieval action, the target of that reference is defined to be
 within the same entity (representation, document, or message) as the
 reference; therefore, a dereference should not result in a new
 retrieval action.
 Normalization of the base and target URIs prior to their comparison,
 as described in Section 6.2.2 and Section 6.2.3, is allowed but
 rarely performed in practice. Normalization may increase the set of
 same-document references, which may be of benefit to some caching
 applications. As such, reference authors should not assume that a
 slightly different, though equivalent, reference URI will (or will
 not) be interpreted as a same-document reference by any given
 application.
4.5 Suffix Reference
 The URI syntax is designed for unambiguous reference to resources and
 extensibility via the URI scheme. However, as URI identification and
 usage have become commonplace, traditional media (television, radio,
 newspapers, billboards, etc.) have increasingly used a suffix of the
 URI as a reference, consisting of only the authority and path
 portions of the URI, such as
 www.w3.org/Addressing/
 or simply a DNS registered name on its own. Such references are
 primarily intended for human interpretation, rather than for
 machines, with the assumption that context-based heuristics are
 sufficient to complete the URI (e.g., most registered names beginning
 with "www" are likely to have a URI prefix of "http://"). Although
 there is no standard set of heuristics for disambiguating a URI
 suffix, many client implementations allow them to be entered by the
 user and heuristically resolved.
 While this practice of using suffix references is common, it should
 be avoided whenever possible and never used in situations where
 long-term references are expected. The heuristics noted above will
 change over time, particularly when a new URI scheme becomes popular,
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 and are often incorrect when used out of context. Furthermore, they
 can lead to security issues along the lines of those described in
 [RFC1535].
 Since a URI suffix has the same syntax as a relative-path reference,
 a suffix reference cannot be used in contexts where a relative
 reference is expected. As a result, suffix references are limited to
 those places where there is no defined base URI, such as dialog boxes
 and off-line advertisements.
5. Reference Resolution
 This section defines the process of resolving a URI reference within
 a context that allows relative references, such that the result is a
 string matching the <URI> syntax rule of Section 3.
5.1 Establishing a Base URI
 The term "relative" implies that there exists a "base URI" against
 which the relative reference is applied. Aside from fragment-only
 references (Section 4.4), relative references are only usable when a
 base URI is known. A base URI must be established by the parser
 prior to parsing URI references that might be relative. A base URI
 must conform to the <absolute-URI> syntax rule (Section 4.3): if the
 base URI is obtained from a URI reference, then that reference must
 be converted to absolute form and stripped of any fragment component
 prior to use as a base URI.
 The base URI of a reference can be established in one of four ways,
 discussed below in order of precedence. The order of precedence can
 be thought of in terms of layers, where the innermost defined base
 URI has the highest precedence. This can be visualized graphically
 as:
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 .----------------------------------------------------------.
 | .----------------------------------------------------. |
 | | .----------------------------------------------. | |
 | | | .----------------------------------------. | | |
 | | | | .----------------------------------. | | | |
 | | | | | <relative-reference> | | | | |
 | | | | `----------------------------------' | | | |
 | | | | (5.1.1) Base URI embedded in content | | | |
 | | | `----------------------------------------' | | |
 | | | (5.1.2) Base URI of the encapsulating entity | | |
 | | | (message, representation, or none) | | |
 | | `----------------------------------------------' | |
 | | (5.1.3) URI used to retrieve the entity | |
 | `----------------------------------------------------' |
 | (5.1.4) Default Base URI (application-dependent) |
 `----------------------------------------------------------'
5.1.1 Base URI Embedded in Content
 Within certain media types, a base URI for relative references can be
 embedded within the content itself such that it can be readily
 obtained by a parser. This can be useful for descriptive documents,
 such as tables of content, which may be transmitted to others through
 protocols other than their usual retrieval context (e.g., E-Mail or
 USENET news).
 It is beyond the scope of this specification to specify how, for each
 media type, a base URI can be embedded. The appropriate syntax, when
 available, is described by the data format specification associated
 with each media type.
5.1.2 Base URI from the Encapsulating Entity
 If no base URI is embedded, the base URI is defined by the
 representation's retrieval context. For a document that is enclosed
 within another entity, such as a message or archive, the retrieval
 context is that entity; thus, the default base URI of a
 representation is the base URI of the entity in which the
 representation is encapsulated.
 A mechanism for embedding a base URI within MIME container types
 (e.g., the message and multipart types) is defined by MHTML
 [RFC2557]. Protocols that do not use the MIME message header syntax,
 but do allow some form of tagged metadata to be included within
 messages, may define their own syntax for defining a base URI as part
 of a message.
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5.1.3 Base URI from the Retrieval URI
 If no base URI is embedded and the representation is not encapsulated
 within some other entity, then, if a URI was used to retrieve the
 representation, that URI shall be considered the base URI. Note that
 if the retrieval was the result of a redirected request, the last URI
 used (i.e., the URI that resulted in the actual retrieval of the
 representation) is the base URI.
5.1.4 Default Base URI
 If none of the conditions described above apply, then the base URI is
 defined by the context of the application. Since this definition is
 necessarily application-dependent, failing to define a base URI using
 one of the other methods may result in the same content being
 interpreted differently by different types of application.
 A sender of a representation containing relative references is
 responsible for ensuring that a base URI for those references can be
 established. Aside from fragment-only references, relative
 references can only be used reliably in situations where the base URI
 is well-defined.
5.2 Relative Resolution
 This section describes an algorithm for converting a URI reference
 that might be relative to a given base URI into the parsed components
 of the reference's target. The components can then be recomposed, as
 described in Section 5.3, to form the target URI. This algorithm
 provides definitive results that can be used to test the output of
 other implementations. Applications may implement relative reference
 resolution using some other algorithm, provided that the results
 match what would be given by this algorithm.
5.2.1 Pre-parse the Base URI
 The base URI (Base) is established according to the procedure of
 Section 5.1 and parsed into the five main components described in
 Section 3. Note that only the scheme component is required to be
 present in a base URI; the other components may be empty or
 undefined. A component is undefined if its associated delimiter does
 not appear in the URI reference; the path component is never
 undefined, though it may be empty.
 Normalization of the base URI, as described in Section 6.2.2 and
 Section 6.2.3, is optional. A URI reference must be transformed to
 its target URI before it can be normalized.
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5.2.2 Transform References
 For each URI reference (R), the following pseudocode describes an
 algorithm for transforming R into its target URI (T):
 -- The URI reference is parsed into the five URI components
 --
 (R.scheme, R.authority, R.path, R.query, R.fragment) = parse(R);
 -- A non-strict parser may ignore a scheme in the reference
 -- if it is identical to the base URI's scheme.
 --
 if ((not strict) and (R.scheme == Base.scheme)) then
 undefine(R.scheme);
 endif;
 if defined(R.scheme) then
 T.scheme = R.scheme;
 T.authority = R.authority;
 T.path = remove_dot_segments(R.path);
 T.query = R.query;
 else
 if defined(R.authority) then
 T.authority = R.authority;
 T.path = remove_dot_segments(R.path);
 T.query = R.query;
 else
 if (R.path == "") then
 T.path = Base.path;
 if defined(R.query) then
 T.query = R.query;
 else
 T.query = Base.query;
 endif;
 else
 if (R.path starts-with "/") then
 T.path = remove_dot_segments(R.path);
 else
 T.path = merge(Base.path, R.path);
 T.path = remove_dot_segments(T.path);
 endif;
 T.query = R.query;
 endif;
 T.authority = Base.authority;
 endif;
 T.scheme = Base.scheme;
 endif;
 T.fragment = R.fragment;
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5.2.3 Merge Paths
 The pseudocode above refers to a "merge" routine for merging a
 relative-path reference with the path of the base URI. This is
 accomplished as follows:
 o If the base URI has a defined authority component and an empty
 path, then return a string consisting of "/" concatenated with the
 reference's path; otherwise,
 o Return a string consisting of the reference's path component
 appended to all but the last segment of the base URI's path (i.e.,
 excluding any characters after the right-most "/" in the base URI
 path, or excluding the entire base URI path if it does not contain
 any "/" characters).
5.2.4 Remove Dot Segments
 The pseudocode also refers to a "remove_dot_segments" routine for
 interpreting and removing the special "." and ".." complete path
 segments from a referenced path. This is done after the path is
 extracted from a reference, whether or not the path was relative, in
 order to remove any invalid or extraneous dot-segments prior to
 forming the target URI. Although there are many ways to accomplish
 this removal process, we describe a simple method using two string
 buffers.
 1. The input buffer is initialized with the now-appended path
 components and the output buffer is initialized to the empty
 string.
 2. While the input buffer is not empty, loop:
 A. If the input buffer begins with a prefix of "../" or "./",
 then remove that prefix from the input buffer; otherwise,
 B. If the input buffer begins with a prefix of "/./" or "/.",
 where "." is a complete path segment, then replace that
 prefix with "/" in the input buffer; otherwise,
 C. If the input buffer begins with a prefix of "/../" or "/..",
 where ".." is a complete path segment, then replace that
 prefix with "/" in the input buffer and remove the last
 segment and its preceding "/" (if any) from the output
 buffer; otherwise,
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 D. If the input buffer consists only of "." or "..", then remove
 that from the input buffer; otherwise,
 E. Move the first path segment in the input buffer to the end of
 the output buffer, including the initial "/" character (if
 any) and any subsequent characters up to, but not including,
 the next "/" character or the end of the input buffer.
 3. Finally, the output buffer is returned as the result of
 remove_dot_segments.
 Note that dot-segments are intended for use in URI references to
 express an identifier relative to the hierarchy of names in the base
 URI. The remove_dot_segments algorithm respects that hierarchy by
 removing extra dot-segments rather than treating them as an error or
 leaving them to be misinterpreted by dereference implementations.
 The following illustrates how the above steps are applied for two
 example merged paths, showing the state of the two buffers after each
 step.
 STEP OUTPUT BUFFER INPUT BUFFER
 1 : /a/b/c/./../../g
 2E: /a /b/c/./../../g
 2E: /a/b /c/./../../g
 2E: /a/b/c /./../../g
 2B: /a/b/c /../../g
 2C: /a/b /../g
 2C: /a /g
 2E: /a/g
 STEP OUTPUT BUFFER INPUT BUFFER
 1 : mid/content=5/../6
 2E: mid /content=5/../6
 2E: mid/content=5 /../6
 2C: mid /6
 2E: mid/6
 Some applications may find it more efficient to implement the
 remove_dot_segments algorithm using two segment stacks rather than
 strings.
 Note: Beware that some older, erroneous implementations will fail
 to separate a reference's query component from its path component
 prior to merging the base and reference paths, resulting in an
 interoperability failure if the query component contains the
 strings "/../" or "/./".
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5.3 Component Recomposition
 Parsed URI components can be recomposed to obtain the corresponding
 URI reference string. Using pseudocode, this would be:
 result = ""
 if defined(scheme) then
 append scheme to result;
 append ":" to result;
 endif;
 if defined(authority) then
 append "//" to result;
 append authority to result;
 endif;
 append path to result;
 if defined(query) then
 append "?" to result;
 append query to result;
 endif;
 if defined(fragment) then
 append "#" to result;
 append fragment to result;
 endif;
 return result;
 Note that we are careful to preserve the distinction between a
 component that is undefined, meaning that its separator was not
 present in the reference, and a component that is empty, meaning that
 the separator was present and was immediately followed by the next
 component separator or the end of the reference.
5.4 Reference Resolution Examples
 Within a representation with a well-defined base URI of
 http://a/b/c/d;p?q
 a relative reference is transformed to its target URI as follows.
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5.4.1 Normal Examples
 "g:h" = "g:h"
 "g" = "http://a/b/c/g"
 "./g" = "http://a/b/c/g"
 "g/" = "http://a/b/c/g/"
 "/g" = "http://a/g"
 "//g" = "http://g"
 "?y" = "http://a/b/c/d;p?y"
 "g?y" = "http://a/b/c/g?y"
 "#s" = "http://a/b/c/d;p?q#s"
 "g#s" = "http://a/b/c/g#s"
 "g?y#s" = "http://a/b/c/g?y#s"
 ";x" = "http://a/b/c/;x"
 "g;x" = "http://a/b/c/g;x"
 "g;x?y#s" = "http://a/b/c/g;x?y#s"
 "" = "http://a/b/c/d;p?q"
 "." = "http://a/b/c/"
 "./" = "http://a/b/c/"
 ".." = "http://a/b/"
 "../" = "http://a/b/"
 "../g" = "http://a/b/g"
 "../.." = "http://a/"
 "../../" = "http://a/"
 "../../g" = "http://a/g"
5.4.2 Abnormal Examples
 Although the following abnormal examples are unlikely to occur in
 normal practice, all URI parsers should be capable of resolving them
 consistently. Each example uses the same base as above.
 Parsers must be careful in handling cases where there are more ".."
 segments in a relative-path reference than there are hierarchical
 levels in the base URI's path. Note that the ".." syntax cannot be
 used to change the authority component of a URI.
 "../../../g" = "http://a/g"
 "../../../../g" = "http://a/g"
 Similarly, parsers must remove the dot-segments "." and ".." when
 they are complete components of a path, but not when they are only
 part of a segment.
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 "/./g" = "http://a/g"
 "/../g" = "http://a/g"
 "g." = "http://a/b/c/g."
 ".g" = "http://a/b/c/.g"
 "g.." = "http://a/b/c/g.."
 "..g" = "http://a/b/c/..g"
 Less likely are cases where the relative reference uses unnecessary
 or nonsensical forms of the "." and ".." complete path segments.
 "./../g" = "http://a/b/g"
 "./g/." = "http://a/b/c/g/"
 "g/./h" = "http://a/b/c/g/h"
 "g/../h" = "http://a/b/c/h"
 "g;x=1/./y" = "http://a/b/c/g;x=1/y"
 "g;x=1/../y" = "http://a/b/c/y"
 Some applications fail to separate the reference's query and/or
 fragment components from the path component before merging it with
 the base path and removing dot-segments. This error is rarely
 noticed, since typical usage of a fragment never includes the
 hierarchy ("/") character, and the query component is not normally
 used within relative references.
 "g?y/./x" = "http://a/b/c/g?y/./x"
 "g?y/../x" = "http://a/b/c/g?y/../x"
 "g#s/./x" = "http://a/b/c/g#s/./x"
 "g#s/../x" = "http://a/b/c/g#s/../x"
 Some parsers allow the scheme name to be present in a relative
 reference if it is the same as the base URI scheme. This is
 considered to be a loophole in prior specifications of partial URI
 [RFC1630]. Its use should be avoided, but is allowed for backward
 compatibility.
 "http:g" = "http:g" ; for strict parsers
 / "http://a/b/c/g" ; for backward compatibility
6. Normalization and Comparison
 One of the most common operations on URIs is simple 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.
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 URI comparison is performed in respect to some particular purpose,
 and implementations with differing purposes will often be subject to
 differing design trade-offs in regards to how much effort should be
 spent in reducing aliased identifiers. This section describes a
 variety of methods that may be used to compare URIs, the trade-offs
 between them, and the types of applications that might use them.
6.1 Equivalence
 Since URIs exist to identify resources, presumably they should be
 considered equivalent when they identify the same resource. However,
 such a definition of equivalence is not of much practical use, since
 there is no way for an implementation to compare two resources that
 are not under its own control. For this reason, determination of
 equivalence or difference of URIs is based on string comparison,
 perhaps augmented by reference to additional rules provided by URI
 scheme definitions. 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 though it is possible to determine that two URIs are equivalent,
 URI comparison is not sufficient to determine if two URIs 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 URIs. 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 URIs before comparison. When URIs are being
 compared for the purpose of selecting (or avoiding) a network action,
 such as retrieval of a representation, fragment components (if any)
 should be excluded from the comparison.
6.2 Comparison Ladder
 A variety of methods are used in practice to test URI 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 those
 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.
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6.2.1 Simple String Comparison
 If two URIs, 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.
 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
 of strings for equality is normally based on pairwise comparison of
 the characters that make up the strings, starting from the first and
 proceeding until both strings are exhausted and all characters found
 to be equal, a pair of characters compares unequal, or one of the
 strings is exhausted before the other.
 Such character comparisons require that each pair of characters be
 put in comparable form. For example, should one URI be stored in a
 byte array in EBCDIC encoding, and the second be in a Java String
 object (UTF-16), bit-for-bit comparisons applied naively will produce
 errors. It is better to speak of equality on a
 character-for-character rather than 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.
 False negatives are caused by the production and use of URI aliases.
 Unnecessary aliases can be reduced, regardless of the comparison
 method, by consistently providing URI 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 choose to limit some URI comparisons
 to simple string comparison, based on the theory that people and
 implementations will, in their own best interest, be consistent in
 providing URI references, or at least consistent enough to negate any
 efficiency that might be obtained from further normalization.
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6.2.2 Syntax-based Normalization
 Implementations may use logic based on the definitions provided by
 this specification to reduce the probability of false negatives.
 Such 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 URIs equivalent:
 example://a/b/c/%7Bfoo%7D
 eXAMPLE://a/./b/../b/%63/%7bfoo%7d
 Web user agents, such as browsers, typically apply this type of URI
 normalization when determining whether a cached response is
 available. Syntax-based normalization includes such techniques as
 case normalization, percent-encoding normalization, and removal of
 dot-segments.
6.2.2.1 Case Normalization
 For all URIs, the hexadecimal digits within a percent-encoding
 triplet (e.g., "%3a" versus "%3A") are case-insensitive and therefore
 should be normalized to use uppercase letters for the digits A-F.
 When a URI uses components of the generic syntax, the component
 syntax equivalence rules always apply; namely, that the scheme and
 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/>. The other generic syntax
 components are assumed to be case-sensitive unless specifically
 defined otherwise by the scheme (see Section 6.2.3).
6.2.2.2 Percent-Encoding Normalization
 The percent-encoding mechanism (Section 2.1) is a frequent source of
 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 complete path segments "." and ".." are intended only for use
 within relative references (Section 4.1) and are removed as part of
 the reference resolution process (Section 5.2). However, some
 deployed implementations incorrectly assume that reference resolution
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 is not necessary when the reference is already a URI, and thus fail
 to remove dot-segments when they occur in non-relative paths. URI
 normalizers should remove dot-segments by applying the
 remove_dot_segments algorithm to the path, as described in
 Section 5.2.4.
6.2.3 Scheme-based Normalization
 The syntax and semantics of URIs 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,
 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
 "/", the following four URIs are equivalent:
 http://example.com
 http://example.com/
 http://example.com:/
 http://example.com:80/
 In general, a URI that uses the generic syntax for authority with an
 empty path should be normalized to a path of "/"; likewise, an
 explicit ":port", where 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 URI 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 a
 URI 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
 subcomponent is 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 licensed to do so by the scheme
 specification. For example, the URI "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 URIs that
 differ only by the suffix "#" are considered different regardless of
 the scheme.
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 Some schemes define additional subcomponents that consist of
 case-insensitive data, giving an implicit license to normalizers to
 convert such data to a common case (e.g., all lowercase). For
 example, URI schemes that define a subcomponent of path to contain an
 Internet hostname, such as the "mailto" URI scheme, cause that
 subcomponent to be case-insensitive and thus subject to case
 normalization (e.g., "mailto:Joe@Example.COM" is equivalent to
 "mailto:Joe@example.com" even though the generic syntax considers the
 path component to be case-sensitive).
 Other scheme-specific normalizations are possible.
6.2.4 Protocol-based Normalization
 Web spiders, for which substantial effort to reduce the incidence of
 false negatives is often cost-effective, are observed to implement
 even more aggressive techniques in URI comparison. For example, if
 they observe that a URI such as
 http://example.com/data
 redirects to a URI 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).
7. Security Considerations
 A URI does not in itself pose a security threat. However, since URIs
 are often used to provide a compact set of instructions for access to
 network resources, care must be taken to properly interpret the data
 within a URI, to prevent that data from causing unintended access,
 and to avoid including data that should not be revealed in plain
 text.
7.1 Reliability and Consistency
 There is no guarantee that, having once used a given URI to retrieve
 some information, the same information will be retrievable by that
 URI in the future. Nor is there any guarantee that the information
 retrievable via that URI in the future will be observably similar to
 that retrieved in the past. The URI syntax does not constrain how a
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 given scheme or authority apportions its name space or maintains it
 over time. Such a guarantee can only be obtained from the person(s)
 controlling that name space and the resource in question. A specific
 URI scheme may define additional semantics, such as name persistence,
 if those semantics are required of all naming authorities for that
 scheme.
7.2 Malicious Construction
 It is sometimes possible to construct a URI such that an attempt to
 perform a seemingly harmless, idempotent operation, such as the
 retrieval of a representation, will in fact cause a possibly damaging
 remote operation to occur. The unsafe URI is typically constructed
 by specifying a port number other than that reserved for the network
 protocol in question. The client unwittingly contacts a site that is
 running a different protocol service and data within the URI contains
 instructions that, when interpreted according to this other protocol,
 cause an unexpected operation. A frequent example of such abuse has
 been the use of a protocol-based scheme with a port component of
 "25", thereby fooling user agent software into sending an unintended
 or impersonating message via an SMTP server.
 Applications should prevent dereference of a URI that specifies a TCP
 port number within the "well-known port" range (0 - 1023) unless the
 protocol being used to dereference that URI is compatible with the
 protocol expected on that well-known port. Although IANA maintains a
 registry of well-known ports, applications should make such
 restrictions user-configurable to avoid preventing the deployment of
 new services.
 When a URI contains percent-encoded octets that match the delimiters
 for a given resolution or dereference protocol (for example, CR and
 LF characters for the TELNET protocol), such percent-encoded octets
 must not be decoded before transmission across that protocol.
 Transfer of the percent-encoding, which might violate the protocol,
 is less harmful than allowing decoded octets to be interpreted as
 additional operations or parameters, perhaps triggering an unexpected
 and possibly harmful remote operation.
7.3 Back-end Transcoding
 When a URI is dereferenced, the data within it is often parsed by
 both the user agent and one or more servers. In HTTP, for example, a
 typical user agent will parse a URI into its five major components,
 access the authority's server, and send it the data within the
 authority, path, and query components. A typical server will take
 that information, parse the path into segments and the query into
 key/value pairs, and then invoke implementation-specific handlers to
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 respond to the request. As a result, a common security concern for
 server implementations that handle a URI, either as a whole or split
 into separate components, is proper interpretation of the octet data
 represented by the characters and percent-encodings within that URI.
 Percent-encoded octets must be decoded at some point during the
 dereference process. Applications must split the URI into its
 components and subcomponents prior to decoding the octets, since
 otherwise the decoded octets might be mistaken for delimiters.
 Security checks of the data within a URI should be applied after
 decoding the octets. Note, however, that the "%00" percent-encoding
 (NUL) may require special handling and should be rejected if the
 application is not expecting to receive raw data within a component.
 Special care should be taken when the URI path interpretation process
 involves the use of a back-end filesystem or related system
 functions. Filesystems typically assign an operational meaning to
 special characters, such as the "/", "\", ":", "[", and "]"
 characters, and special device names like ".", "..", "...", "aux",
 "lpt", etc. In some cases, merely testing for the existence of such
 a name will cause the operating system to pause or invoke unrelated
 system calls, leading to significant security concerns regarding
 denial of service and unintended data transfer. It would be
 impossible for this specification to list all such significant
 characters and device names; implementers should research the
 reserved names and characters for the types of storage device that
 may be attached to their application and restrict the use of data
 obtained from URI components accordingly.
7.4 Rare IP Address Formats
 Although the URI syntax for IPv4address only allows the common,
 dotted-decimal form of IPv4 address literal, many implementations
 that process URIs make use of platform-dependent system routines,
 such as gethostbyname() and inet_aton(), to translate the string
 literal to an actual IP address. Unfortunately, such system routines
 often allow and process a much larger set of formats than those
 described in Section 3.2.2.
 For example, many implementations allow dotted forms of three
 numbers, wherein the last part is interpreted as a 16-bit quantity
 and placed in the right-most two bytes of the network address (e.g.,
 a Class B network). Likewise, a dotted form of two numbers means the
 last part is interpreted as a 24-bit quantity and placed in the right
 most three bytes of the network address (Class A), and a single
 number (without dots) is interpreted as a 32-bit quantity and stored
 directly in the network address. Adding further to the confusion,
 some implementations allow each dotted part to be interpreted as
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 decimal, octal, or hexadecimal, as specified in the C language (i.e.,
 a leading 0x or 0X implies hexadecimal; otherwise, a leading 0
 implies octal; otherwise, the number is interpreted as decimal).
 These additional IP address formats are not allowed in the URI syntax
 due to differences between platform implementations. However, they
 can become a security concern if an application attempts to filter
 access to resources based on the IP address in string literal format.
 If such filtering is performed, literals should be converted to
 numeric form and filtered based on the numeric value, rather than a
 prefix or suffix of the string form.
7.5 Sensitive Information
 URI producers should not provide a URI that contains a username or
 password which is intended to be secret: URIs are frequently
 displayed by browsers, stored in clear text bookmarks, and logged by
 user agent history and intermediary applications (proxies). A
 password appearing within the userinfo component is deprecated and
 should be considered an error (or simply ignored) except in those
 rare cases where the 'password' parameter is intended to be public.
7.6 Semantic Attacks
 Because the userinfo subcomponent is rarely used and appears before
 the host in the authority component, it can be used to construct a
 URI that is intended to mislead a human user by appearing to identify
 one (trusted) naming authority while actually identifying a different
 authority hidden behind the noise. For example
 ftp://cnn.example.com&story=breaking_news@10.0.0.1/top_story.htm
 might lead a human user to assume that the host is 'cnn.example.com',
 whereas it is actually '10.0.0.1'. Note that a misleading userinfo
 subcomponent could be much longer than the example above.
 A misleading URI, such as the one above, is an attack on the user's
 preconceived notions about the meaning of a URI, rather than an
 attack on the software itself. User agents may be able to reduce the
 impact of such attacks by distinguishing the various components of
 the URI when rendered, such as by using a different color or tone to
 render userinfo if any is present, though there is no general
 panacea. More information on URI-based semantic attacks can be found
 in [Siedzik].
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8. IANA Considerations
 URI scheme names, as defined by <scheme> in Section 3.1, form a
 registered name space that is managed by IANA according to the
 procedures defined in [BCP35]. No IANA actions are required by this
 document.
9. Acknowledgments
 This specification is derived from RFC 2396 [RFC2396], RFC 1808
 [RFC1808], and RFC 1738 [RFC1738]; the acknowledgments in those
 documents still apply. It also incorporates the update (with
 corrections) for IPv6 literals in the host syntax, as defined by
 Robert M. Hinden, Brian E. Carpenter, and Larry Masinter in
 [RFC2732]. In addition, contributions by Gisle Aas, Reese Anschultz,
 Daniel Barclay, Tim Bray, Mike Brown, Rob Cameron, Jeremy Carroll,
 Dan Connolly, Adam M. Costello, John Cowan, Jason Diamond, Martin
 Duerst, Stefan Eissing, Clive D.W. Feather, Al Gilman, Tony Hammond,
 Elliotte Harold, Pat Hayes, Henry Holtzman, Ian B. Jacobs, Michael
 Kay, John C. Klensin, Graham Klyne, Dan Kohn, Bruce Lilly, Andrew
 Main, Dave McAlpin, Ira McDonald, Michael Mealling, Ray Merkert,
 Stephen Pollei, Julian Reschke, Tomas Rokicki, Miles Sabin, Kai
 Schaetzl, Mark Thomson, Ronald Tschalaer, Norm Walsh, Marc Warne,
 Stuart Williams, and Henry Zongaro are gratefully acknowledged.
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10. References
10.1 Normative References
 [ASCII] American National Standards Institute, "Coded Character
 Set -- 7-bit American Standard Code for Information
 Interchange", ANSI X3.4, 1986.
 [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
 Specifications: ABNF", RFC 2234, November 1997.
 [STD63] Yergeau, F., "UTF-8, a transformation format of ISO
 10646", STD 63, RFC 3629, November 2003.
 [UCS] International Organization for Standardization,
 "Information Technology - Universal Multiple-Octet Coded
 Character Set (UCS)", ISO/IEC 10646:2003, December 2003.
10.2 Informative References
 [BCP19] Freed, N. and J. Postel, "IANA Charset Registration
 Procedures", BCP 19, RFC 2978, October 2000.
 [BCP35] Petke, R. and I. King, "Registration Procedures for URL
 Scheme Names", BCP 35, RFC 2717, November 1999.
 [RFC0952] Harrenstien, K., Stahl, M. and E. Feinler, "DoD Internet
 host table specification", RFC 952, October 1985.
 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
 STD 13, RFC 1034, November 1987.
 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
 and Support", STD 3, RFC 1123, October 1989.
 [RFC1535] Gavron, E., "A Security Problem and Proposed Correction
 With Widely Deployed DNS Software", RFC 1535, October
 1993.
 [RFC1630] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
 Unifying Syntax for the Expression of Names and Addresses
 of Objects on the Network as used in the World-Wide Web",
 RFC 1630, June 1994.
 [RFC1736] Kunze, J., "Functional Recommendations for Internet
 Resource Locators", RFC 1736, February 1995.
 [RFC1737] Masinter, L. and K. Sollins, "Functional Requirements for
 Uniform Resource Names", RFC 1737, December 1994.
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 [RFC1738] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform
 Resource Locators (URL)", RFC 1738, December 1994.
 [RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
 1808, June 1995.
 [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
 Extensions (MIME) Part Two: Media Types", RFC 2046,
 November 1996.
 [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
 [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
 Resource Identifiers (URI): Generic Syntax", RFC 2396,
 August 1998.
 [RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S. and D.
 Jensen, "HTTP Extensions for Distributed Authoring --
 WEBDAV", RFC 2518, February 1999.
 [RFC2557] Palme, F., Hopmann, A., Shelness, N. and E. Stefferud,
 "MIME Encapsulation of Aggregate Documents, such as HTML
 (MHTML)", RFC 2557, March 1999.
 [RFC2718] Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
 "Guidelines for new URL Schemes", RFC 2718, November 1999.
 [RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
 Literal IPv6 Addresses in URL's", RFC 2732, December 1999.
 [RFC3305] Mealling, M. and R. Denenberg, "Report from the Joint W3C/
 IETF URI Planning Interest Group: Uniform Resource
 Identifiers (URIs), URLs, and Uniform Resource Names
 (URNs): Clarifications and Recommendations", RFC 3305,
 August 2002.
 [RFC3490] Faltstrom, P., Hoffman, P. and A. Costello,
 "Internationalizing Domain Names in Applications (IDNA)",
 RFC 3490, March 2003.
 [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
 (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [Siedzik] Siedzik, R., "Semantic Attacks: What's in a URL?",
 April 2001, <http://www.giac.org/practical/gsec/
 Richard_Siedzik_GSEC.pdf>.
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Authors' Addresses
 Tim Berners-Lee
 World Wide Web Consortium
 Massachusetts Institute of Technology
 77 Massachusetts Avenue
 Cambridge, MA 02139
 USA
 Phone: +1-617-253-5702
 Fax: +1-617-258-5999
 EMail: timbl@w3.org
 URI: http://www.w3.org/People/Berners-Lee/
 Roy T. Fielding
 Day Software
 5251 California Ave., Suite 110
 Irvine, CA 92617
 USA
 Phone: +1-949-679-2960
 Fax: +1-949-679-2972
 EMail: fielding@gbiv.com
 URI: http://roy.gbiv.com/
 Larry Masinter
 Adobe Systems Incorporated
 345 Park Ave
 San Jose, CA 95110
 USA
 Phone: +1-408-536-3024
 EMail: LMM@acm.org
 URI: http://larry.masinter.net/
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Appendix A. Collected ABNF for URI
 URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
 hier-part = "//" authority path-abempty
 / path-absolute
 / path-rootless
 / path-empty
 URI-reference = URI / relative-ref
 absolute-URI = scheme ":" hier-part [ "?" query ]
 relative-ref = relative-part [ "?" query ] [ "#" fragment ]
 relative-part = "//" authority path-abempty
 / path-absolute
 / path-noscheme
 / path-empty
 scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
 authority = [ userinfo "@" ] host [ ":" port ]
 userinfo = *( unreserved / pct-encoded / sub-delims / ":" )
 host = IP-literal / IPv4address / reg-name
 port = *DIGIT
 IP-literal = "[" ( IPv6address / IPvFuture ) "]"
 IPvFuture = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )
 IPv6address = 6( h16 ":" ) ls32
 / "::" 5( h16 ":" ) ls32
 / [ h16 ] "::" 4( h16 ":" ) ls32
 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
 / [ *4( h16 ":" ) h16 ] "::" ls32
 / [ *5( h16 ":" ) h16 ] "::" h16
 / [ *6( h16 ":" ) h16 ] "::"
 h16 = 1*4HEXDIG
 ls32 = ( h16 ":" h16 ) / IPv4address
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 IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
 dec-octet = DIGIT ; 0-9
 / %x31-39 DIGIT ; 10-99
 / "1" 2DIGIT ; 100-199
 / "2" %x30-34 DIGIT ; 200-249
 / "25" %x30-35 ; 250-255
 reg-name = *( unreserved / pct-encoded / sub-delims )
 path = path-abempty ; begins with "/" or is empty
 / path-absolute ; begins with "/" but not "//"
 / path-noscheme ; begins with a non-colon segment
 / path-rootless ; begins with a segment
 / path-empty ; zero characters
 path-abempty = *( "/" segment )
 path-absolute = "/" [ segment-nz *( "/" segment ) ]
 path-noscheme = segment-nz-nc *( "/" segment )
 path-rootless = segment-nz *( "/" segment )
 path-empty = 0<pchar>
 segment = *pchar
 segment-nz = 1*pchar
 segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
 ; non-zero-length segment without any colon ":"
 pchar = unreserved / pct-encoded / sub-delims / ":" / "@"
 query = *( pchar / "/" / "?" )
 fragment = *( pchar / "/" / "?" )
 pct-encoded = "%" HEXDIG HEXDIG
 unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
 reserved = gen-delims / sub-delims
 gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "@"
 sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
 / "*" / "+" / "," / ";" / "="
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Appendix B. Parsing a URI Reference with a Regular Expression
 Since the "first-match-wins" algorithm is identical to the "greedy"
 disambiguation method used by POSIX regular expressions, it is
 natural and commonplace to use a regular expression for parsing the
 potential five components of a URI reference.
 The following line is the regular expression for breaking-down a
 well-formed URI reference into its components.
 ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
 12 3 4 5 6 7 8 9
 The numbers in the second line above are only to assist readability;
 they indicate the reference points for each subexpression (i.e., each
 paired parenthesis). We refer to the value matched for subexpression
 <n> as $<n>. For example, matching the above expression to
 http://www.ics.uci.edu/pub/ietf/uri/#Related
 results in the following subexpression matches:
 1ドル = http:
 2ドル = http
 3ドル = //www.ics.uci.edu
 4ドル = www.ics.uci.edu
 5ドル = /pub/ietf/uri/
 6ドル = <undefined>
 7ドル = <undefined>
 8ドル = #Related
 9ドル = Related
 where <undefined> indicates that the component is not present, as is
 the case for the query component in the above example. Therefore, we
 can determine the value of the four components and fragment as
 scheme = 2ドル
 authority = 4ドル
 path = 5ドル
 query = 7ドル
 fragment = 9ドル
 and, going in the opposite direction, we can recreate a URI reference
 from its components using the algorithm of Section 5.3.
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Appendix C. Delimiting a URI in Context
 URIs are often transmitted through formats that do not provide a
 clear context for their interpretation. For example, there are many
 occasions when a URI is included in plain text; examples include text
 sent in electronic mail, USENET news messages, and, most importantly,
 printed on paper. In such cases, it is important to be able to
 delimit the URI from the rest of the text, and in particular from
 punctuation marks that might be mistaken for part of the URI.
 In practice, URIs are delimited in a variety of ways, but usually
 within double-quotes "http://example.com/", angle brackets
 <http://example.com/>, or just using whitespace
 http://example.com/
 These wrappers do not form part of the URI.
 In some cases, extra whitespace (spaces, line-breaks, tabs, etc.) may
 need to be added to break a long URI across lines. The whitespace
 should be ignored when extracting the URI.
 No whitespace should be introduced after a hyphen ("-") character.
 Because some typesetters and printers may (erroneously) introduce a
 hyphen at the end of line when breaking a line, the interpreter of a
 URI containing a line break immediately after a hyphen should ignore
 all whitespace around the line break, and should be aware that the
 hyphen may or may not actually be part of the URI.
 Using <> angle brackets around each URI is especially recommended as
 a delimiting style for a reference that contains embedded whitespace.
 The prefix "URL:" (with or without a trailing space) was formerly
 recommended as a way to help distinguish a URI from other bracketed
 designators, though it is not commonly used in practice and is no
 longer recommended.
 For robustness, software that accepts user-typed URI should attempt
 to recognize and strip both delimiters and embedded whitespace.
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 For example, the text:
 Yes, Jim, I found it under "http://www.w3.org/Addressing/",
 but you can probably pick it up from <ftp://foo.example.
 com/rfc/>. Note the warning in <http://www.ics.uci.edu/pub/
 ietf/uri/historical.html#WARNING>.
 contains the URI references
 http://www.w3.org/Addressing/
 ftp://foo.example.com/rfc/
 http://www.ics.uci.edu/pub/ietf/uri/historical.html#WARNING
Appendix D. Changes from RFC 2396
D.1 Additions
 An ABNF rule for URI has been introduced to correspond to one common
 usage of the term: an absolute URI with optional fragment.
 IPv6 (and later) literals have been added to the list of possible
 identifiers for the host portion of an authority component, as
 described by [RFC2732], with the addition of "[" and "]" to the
 reserved set and a version flag to anticipate future versions of IP
 literals. Square brackets are now specified as reserved within the
 authority component and not allowed outside their use as delimiters
 for an IP literal within host. In order to make this change without
 changing the technical definition of the path, query, and fragment
 components, those rules were redefined to directly specify the
 characters allowed.
 Since [RFC2732] defers to [RFC3513] for definition of an IPv6 literal
 address, which unfortunately lacks an ABNF description of
 IPv6address, we created a new ABNF rule for IPv6address that matches
 the text representations defined by Section 2.2 of [RFC3513].
 Likewise, the definition of IPv4address has been improved in order to
 limit each decimal octet to the range 0-255.
 Section 6 (Section 6) on URI normalization and comparison has been
 completely rewritten and extended using input from Tim Bray and
 discussion within the W3C Technical Architecture Group.
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D.2 Modifications
 The ad-hoc BNF syntax of RFC 2396 has been replaced with the ABNF of
 [RFC2234]. This change required all rule names that formerly
 included underscore characters to be renamed with a dash instead. In
 addition, a number of syntax rules have been eliminated or simplified
 to make the overall grammar more comprehensible. Specifications that
 refer to the obsolete grammar rules may be understood by replacing
 those rules according to the following table:
 +----------------+--------------------------------------------------+
 | obsolete rule | translation |
 +----------------+--------------------------------------------------+
 | absoluteURI | absolute-URI |
 | relativeURI | relative-part [ "?" query ] |
 | hier_part | ( "//" authority path-abempty / |
 | | path-absolute ) [ "?" query ] |
 | | |
 | opaque_part | path-rootless [ "?" query ] |
 | net_path | "//" authority path-abempty |
 | abs_path | path-absolute |
 | rel_path | path-rootless |
 | rel_segment | segment-nz-nc |
 | reg_name | reg-name |
 | server | authority |
 | hostport | host [ ":" port ] |
 | hostname | reg-name |
 | path_segments | path-abempty |
 | param | *<pchar excluding ";"> |
 | | |
 | uric | unreserved / pct-encoded / ";" / "?" / ":" |
 | | / "@" / "&" / "=" / "+" / "$" / "," / "/" |
 | | |
 | uric_no_slash | unreserved / pct-encoded / ";" / "?" / ":" |
 | | / "@" / "&" / "=" / "+" / "$" / "," |
 | | |
 | mark | "-" / "_" / "." / "!" / "~" / "*" / "'" |
 | | / "(" / ")" |
 | | |
 | escaped | pct-encoded |
 | hex | HEXDIG |
 | alphanum | ALPHA / DIGIT |
 +----------------+--------------------------------------------------+
 Use of the above obsolete rules for the definition of scheme-specific
 syntax is deprecated.
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 Section 2 on characters has been rewritten to explain what characters
 are reserved, when they are reserved, and why they are reserved even
 when not used as delimiters by the generic syntax. The mark
 characters that are typically unsafe to decode, including the
 exclamation mark ("!"), asterisk ("*"), single-quote ("'"), and open
 and close parentheses ("(" and ")"), have been moved to the reserved
 set in order to clarify the distinction between reserved and
 unreserved and hopefully answer the most common question of scheme
 designers. Likewise, the section on percent-encoded characters has
 been rewritten, and URI normalizers are now given license to decode
 any percent-encoded octets corresponding to unreserved characters.
 In general, the terms "escaped" and "unescaped" have been replaced
 with "percent-encoded" and "decoded", respectively, to reduce
 confusion with other forms of escape mechanisms.
 The ABNF for URI and URI-reference has been redesigned to make them
 more friendly to LALR parsers and reduce complexity. As a result,
 the layout form of syntax description has been removed, along with
 the uric, uric_no_slash, opaque_part, net_path, abs_path, rel_path,
 path_segments, rel_segment, and mark rules. All references to
 "opaque" URIs have been replaced with a better description of how the
 path component may be opaque to hierarchy. The relativeURI rule has
 been replaced with relative-ref to avoid unnecessary confusion over
 whether or not they are a subset of URI. The ambiguity regarding the
 parsing of URI-reference as a URI or a relative-ref with a colon in
 the first segment has been eliminated through the use of five
 separate path matching rules.
 The fragment identifier has been moved back into the section on
 generic syntax components and within the URI and relative-ref rules,
 though it remains excluded from absolute-URI. The number sign ("#")
 character has been moved back to the reserved set as a result of
 reintegrating the fragment syntax.
 The ABNF has been corrected to allow the path component to be empty.
 This also allows an absolute-URI to consist of nothing after the
 "scheme:", as is present in practice with the "dav:" namespace
 [RFC2518] and the "about:" scheme used internally by many WWW browser
 implementations. The ambiguity regarding the boundary between
 authority and path has been eliminated through the use of five
 separate path matching rules.
 Registry-based naming authorities that use the generic syntax are now
 defined within the host rule. This change allows current
 implementations, where whatever name provided is simply fed to the
 local name resolution mechanism, to be consistent with the
 specification and removes the need to re-specify DNS name formats
 here. It also allows the host component to contain percent-encoded
 octets, which is necessary to enable internationalized domain names
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 to be provided in URIs, processed in their native character encodings
 at the application layers above URI processing, and passed to an IDNA
 library as a registered name in the UTF-8 character encoding. The
 server, hostport, hostname, domainlabel, toplabel, and alphanum rules
 have been removed.
 The resolving relative references algorithm of [RFC2396] has been
 rewritten using pseudocode for this revision to improve clarity and
 fix the following issues:
 o [RFC2396] section 5.2, step 6a, failed to account for a base URI
 with no path.
 o Restored the behavior of [RFC1808] where, if the reference
 contains an empty path and a defined query component, then the
 target URI inherits the base URI's path component.
 o The determination of whether a URI reference is a same-document
 reference has been decoupled from the URI parser, simplifying the
 URI processing interface within applications in a way consistent
 with the internal architecture of deployed URI processing
 implementations. The determination is now based on comparison to
 the base URI after transforming a reference to absolute form,
 rather than on the format of the reference itself. This change
 may result in more references being considered "same-document"
 under this specification than would be under the rules given in
 RFC 2396, especially when normalization is used to reduce aliases.
 However, it does not change the status of existing same-document
 references.
 o Separated the path merge routine into two routines: merge, for
 describing combination of the base URI path with a relative-path
 reference, and remove_dot_segments, for describing how to remove
 the special "." and ".." segments from a composed path. The
 remove_dot_segments algorithm is now applied to all URI reference
 paths in order to match common implementations and improve the
 normalization of URIs in practice. This change only impacts the
 parsing of abnormal references and same-scheme references wherein
 the base URI has a non-hierarchical path.
Appendix E. Instructions to RFC Editor
 Prior to publication as an RFC, please remove this section and the
 "Editorial Note" that appears after the Abstract. If [BCP35] or any
 of the normative references are updated prior to publication, the
 associated reference in this document can be safely updated as well.
 This document has been produced using the xml2rfc tool set; the XML
 version can be obtained via the URI listed in the editorial note.
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Index
A
 ABNF 11
 absolute 26
 absolute-path 26
 absolute-URI 26
 access 9
 authority 16, 17
B
 base URI 28
C
 character encoding 4
 character 4
 characters 11
 coded character set 4
D
 dec-octet 20
 dereference 9
 dot-segments 22
F
 fragment 16, 24
G
 gen-delims 12
 generic syntax 6
H
 h16 19
 hier-part 16
 hierarchical 10
 host 18
I
 identifier 5
 IP-literal 19
 IPv4 20
 IPv4address 20
 IPv6 19
 IPv6address 19, 20
 IPvFuture 19
L
 locator 7
 ls32 19
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M
 merge 32
N
 name 7
 network-path 26
P
 path 16, 22
 path-abempty 22
 path-absolute 22
 path-empty 22
 path-noscheme 22
 path-rootless 22
 path-abempty 16
 path-absolute 16
 path-empty 16
 path-rootless 16
 pchar 22
 pct-encoded 12
 percent-encoding 12
 port 21
Q
 query 16, 23
R
 reg-name 20
 registered name 20
 relative 10, 28
 relative-path 26
 relative-ref 26
 remove_dot_segments 32
 representation 9
 reserved 12
 resolution 9, 28
 resource 5
 retrieval 9
S
 same-document 27
 sameness 9
 scheme 16, 16
 segment 22
 segment-nz 22
 segment-nz-nc 22
 sub-delims 12
 suffix 27
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T
 transcription 7
U
 uniform 4
 unreserved 13
 URI grammar
 absolute-URI 26
 ALPHA 11
 authority 16, 17
 CR 11
 dec-octet 20
 DIGIT 11
 DQUOTE 11
 fragment 16, 24, 26
 gen-delims 12
 h16 19
 HEXDIG 11
 hier-part 16
 host 17, 18
 IP-literal 19
 IPv4address 20
 IPv6address 19, 20
 IPvFuture 19
 LF 11
 ls32 19
 mark 13
 OCTET 11
 path 22
 path-abempty 16, 22
 path-absolute 16, 22
 path-empty 16, 22
 path-noscheme 22
 path-rootless 16, 22
 pchar 22, 23, 24
 pct-encoded 12
 port 17, 21
 query 16, 23, 26, 26
 reg-name 20
 relative-ref 25, 26
 reserved 12
 scheme 16, 16, 26
 segment 22
 segment-nz 22
 segment-nz-nc 22
 SP 11
 sub-delims 12
 unreserved 13
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 URI 16, 25
 URI-reference 25
 userinfo 17, 18
 URI 16
 URI-reference 25
 URL 7
 URN 7
 userinfo 17, 18
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