Uniform Resource Identifier
Encyclopedia
In computing
, a uniform resource identifier (URI) is a string of character
s used to identify
a name or a resource
on the Internet
. Such identification enables interaction with representations of the resource over a network (typically the World Wide Web
) using specific protocols. Schemes specifying a concrete syntax
and associated protocols define each URI.
(URN) functions like a person's name, while a Uniform Resource Locator
(URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it.
The ISBN
system for uniquely identifying books provides a typical example of the use of URNs. ISBN 0486275574 (urn:isbn:0-486-27557-4 ) cites, unambiguously, a specific edition of Shakespeare's play Romeo and Juliet
. To gain access to this object and read the book, one needs its location: a URL address. A typical URL for this book on a Unix-like
operating system would be a file path
such as file:///home/username/RomeoAndJuliet.pdf, identifying the electronic book saved in a file on a local hard disk. So URNs and URLs have complementary purposes.
, specifies the means of acting upon or obtaining the representation: either through description of the primary access mechanism, or through network "location". For example, the URLhttp://www.wikipedia.org/ identifies a resource Wikipedia's home page and implies that a representation of that resource (such as the home page's current HTML
code, as encoded characters
) is obtainable via HTTP
from a network host named www.wikipedia.org. A uniform resource name
(URN) is a URI that identifies a resource by name, in a particular namespace
. One can use a URN to talk about a resource without implying its location or how to access it. The resource does not need to necessarily be network homed. For example, the URNurn:isbn:0-395-36341-1 is a URI that specifies the identifier system, i.e. international standard book number (ISBN), as well as the unique reference within that system and allows one to talk about a book, but doesn't suggest where and how to obtain an actual copy of it.
Technical publications, especially standards produced by the IETF
and by the W3C
, normally no longer use the term URL, as the need to distinguish between URLs and URIs rarely arises. However, in non-technical contexts and in software for the World Wide Web, the term URL remains widely used. Additionally, the term web address (which has no formal definition) often occurs in non-technical publications as a synonym for a URL that uses the 'http' or 'https' scheme.
outlines the work of a joint W3C/IETF working group set up specifically to normalize the divergent views held within the IETF and W3C over the relationship between the various 'UR*' terms and standards. While not published as a full standard by either organization, it has become the basis for the above common understanding and has informed many standards since then.
name (such as "
", "
", "
" or "
") followed by a colon
character, and then by a scheme-specific part. The specifications that govern the schemes determine the syntax and semantics
of the scheme-specific part. However, URI syntax does require all schemes to adhere to a general syntax that (among other things) reserves certain characters for special purposes (without always identifying those purposes). The URI syntax also enforces restrictions on the scheme-specific part in order to (for example) provide for a degree of consistency when the part has a hierarchical structure.
Percent encoding can add extra information to a URI.
proposals for HyperText
implicitly introduced the idea of a URL as a short string representing a resource that is the target of a hyperlink
. At the time, people referred to it as a 'hypertext name'
or 'document name'.
Over the next 3.5 years, as the World Wide Web's core technologies of HTML
(the HyperText Markup Language
), HTTP, and web browser
s developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and the more contentious Uniform Resource Name came to represent the latter.
During the debate over defining URLs and URNs it became evident that the two concepts embodied by the terms were merely aspects of the fundamental, overarching notion of resource identification. In June 1994, the IETF published Berners-Lee's RFC 1630: the first RFC
that (in its non-normative text) acknowledged the existence of URLs and URNs, and, more importantly, defined a formal syntax for Universal Resource Identifiers — URL-like strings whose precise syntaxes and semantics depended on their schemes. In addition, this RFC attempted to summarize the syntaxes of URL schemes in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifier
s.
The publication of RFC 2396 in August 1998 saw the URI syntax become a separate specification and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded. The new RFC changed the significance of the "U" in "URI": it came to represent "Uniform" rather than "Universal". The sections ofRFC 1738 that summarized existing URL schemes migrated into a separate document. IANA
keeps a registry of those schemes; RFC 2717 first described the procedure to register them.
In December 1999, RFC 2732 provided a minor update toRFC 2396 , allowing URIs to accommodate IPv6
addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development of a number of draft revisions under the title rfc2396bis. This community effort, coordinated byRFC 2396 co-author Roy Fielding
, culminated in the publication of RFC 3986 in January 2005. This RFC, the current version of the URI syntax recommended for use on the Internet, rendersRFC 2396 obsolete. It does not, however, render the details of existing URL schemes obsolete; RFC 1738 continues to govern such schemes except where otherwise superseded – RFC 2616 for example, refines the 'http' scheme. Simultaneously, the IETF published the content of RFC 3986 as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol.
In August 2002, RFC 3305 pointed out that the term 'URL' has, despite its widespread use in the vernacular of the Internet-aware public at large, faded into near obsolescence. It now serves only as a reminder that some URIs act as addresses because they have schemes that imply some kind of network accessibility, regardless of whether systems actually use them for that purpose. As URI-based standards such as Resource Description Framework
make evident, resource identification need not suggest the retrieval of resource representations over the Internet, nor need they imply network-based resources at all.
On November 1, 2006, the W3C Technical Architecture Group published 'On Linking Alternative Representations To Enable Discovery And Publishing', a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content.
The Semantic Web
uses the HTTP URI scheme to identify both documents and concepts in the real world: this has caused confusion as to how to distinguish the two. The Technical Architecture Group of W3C (TAG) published an e-mail in June 2005 on how to solve this problem. The e-mail became known as the httpRange-14 resolution.
To expand on this (rather brief) email, W3C published in March 2008 the Interest Group Note Cool URIs for the Semantic Web. This explains the use of content negotiation
and the 303-redirect
code in more detail.
To derive a URI from a URI reference, software converts the URI reference to 'absolute' form by merging it with an absolute 'base' URI according to a fixed algorithm. The system treats the URI reference as relative to the base URI, although in the case of an absolute reference, the base has no relevance. The base URI typically identifies the document containing the URI reference, although this can be overridden by declarations made within the document or as part of an external data transmission protocol. If the base URI includes a fragment identifier, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process.
Web document markup language
s frequently use URI references to point to other resources, such as external documents or specific portions of the same logical document.
or URI reference by attempting to obtain a representation of the resource that it identifies. The 'resolver' component in document processing software generally provides both services.
One can regard a URI reference as a same document reference: a reference to the document containing the URI reference itself. Document processing software can efficiently use its current representation of the document to satisfy the resolution of a same document reference without fetching a new representation. This is only a recommendation, and document processing software can alternatively use other mechanisms to determine whether to obtain a new representation.
The current URI specification ,RFC 3986 , defines a URI reference as a same document reference if, when resolved to absolute form, it equates exactly to the base URI in effect for the reference. Typically, the base URI is the URI of the document containing the reference. XSLT 1.0, for example, has a RFC 3986 also formally defines URI equivalence, which can be used to determine that a URI reference, while not identical to the base URI, still represents the same resource and thus can be considered to be a same document reference.
RFC 2396 prescribed a different method for determining same document references; RFC 3986 made RFC 2396 obsolete, but RFC 2396 still serves as the basis of many specifications and implementations. This specification defines a URI reference as a same document reference if it is an empty string or consists of only the # character followed by an optional fragment.
has a concept of a namespace
, an abstract domain to which a collection of element and attribute names can be assigned. The namespace name (a character string which must adhere to the generic URI syntax) identifies an XML namespace. However, the namespace name is generally not considered to be a URI because the 'URI-ness' of strings is, according to the URI specification, based on their intended use, not just their lexical components. A namespace name also does not necessarily imply any of the semantics of URI schemes; a namespace name beginning with 'http:', for example, likely has nothing to do with the HTTP protocol. XML professionals have debated this thoroughly on the xml dev electronic mailing list
; some feel that a namespace name could be a URI, since the collection of names comprising a particular namespace could be regarded as a resource that is being identified, and since a version of the 'Namespaces in XML' specification says that the namespace name is a URI reference.
But the consensus seems to suggest that a namespace name is just a string that happens to look like a URI, nothing more.
Initially, the namespace name could match the syntax of any non-empty URI reference, but an erratum
to the 'Namespaces In XML Recommendation' later deprecated the use of relative URI references. A separate specification, issued for namespaces for XML 1.1, allows IRI
references, not just URI references, to serve as the basis for namespace names.
To mitigate confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL
(Resource Directory Description Language) developed, though the specification of RDDL has no official standing and no relevant organization (such as W3C) has considered or approved it. An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. Authors of XML documents were encouraged to put RDDL documents in locations such that if a namespace name in their document somehow becomes de-referenced, then an RDDL document would be obtained, thus satisfying the desire among many developers for a namespace name to point to a network-accessible resource.
Information technology
Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...
, a uniform resource identifier (URI) is a string of character
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
s used to identify
Identifier
An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical [countable] object , or physical [noncountable] substance...
a name or a resource
Resource (Web)
The concept of resource is primitive in the Web architecture, and is used in the definition of its fundamental elements. The term was first introduced to refer to targets of Uniform Resource Locators , but its definition has been further extended to include the referent of any Uniform Resource...
on the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
. Such identification enables interaction with representations of the resource over a network (typically the World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
) using specific protocols. Schemes specifying a concrete syntax
Syntax
In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....
and associated protocols define each URI.
Relationship to URL and URN
URIs can be classified as locators (URLs), as names (URNs), or as both. A uniform resource nameUniform Resource Name
A uniform resource name is a uniform resource identifier that uses the urn scheme and does not imply availability of the identified resource. Both URNs and URLs are URIs, and a particular URI may be a name and a locator at the same time.The functional requirements for uniform resource names are...
(URN) functions like a person's name, while a Uniform Resource Locator
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
(URL) resembles that person's street address. In other words: the URN defines an item's identity, while the URL provides a method for finding it.
The ISBN
International Standard Book Number
The International Standard Book Number is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering code created by Gordon Foster, Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H...
system for uniquely identifying books provides a typical example of the use of URNs. ISBN 0486275574 (
Romeo and Juliet
Romeo and Juliet is a tragedy written early in the career of playwright William Shakespeare about two young star-crossed lovers whose deaths ultimately unite their feuding families. It was among Shakespeare's most popular archetypal stories of young, teenage lovers.Romeo and Juliet belongs to a...
. To gain access to this object and read the book, one needs its location: a URL address. A typical URL for this book on a Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
operating system would be a file path
Path (computing)
A path, the general form of a filename or of a directory name, specifies a unique location in a file system. A path points to a file system location by following the directory tree hierarchy expressed in a string of characters in which path components, separated by a delimiting character, represent...
such as file:///home/username/RomeoAndJuliet.pdf, identifying the electronic book saved in a file on a local hard disk. So URNs and URLs have complementary purposes.
Technical view
A URL is a URI that, in addition to identifying a network-homed resourceResource (Web)
The concept of resource is primitive in the Web architecture, and is used in the definition of its fundamental elements. The term was first introduced to refer to targets of Uniform Resource Locators , but its definition has been further extended to include the referent of any Uniform Resource...
, specifies the means of acting upon or obtaining the representation: either through description of the primary access mechanism, or through network "location". For example, the URL
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
code, as encoded characters
Character encoding
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
) is obtainable via HTTP
Hypertext Transfer Protocol
The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
from a network host named www.wikipedia.org. A uniform resource name
Uniform Resource Name
A uniform resource name is a uniform resource identifier that uses the urn scheme and does not imply availability of the identified resource. Both URNs and URLs are URIs, and a particular URI may be a name and a locator at the same time.The functional requirements for uniform resource names are...
(URN) is a URI that identifies a resource by name, in a particular namespace
Namespace (computer science)
A namespace is an abstract container or environment created to hold a logical grouping of unique identifiers or symbols . An identifier defined in a namespace is associated only with that namespace. The same identifier can be independently defined in multiple namespaces...
. One can use a URN to talk about a resource without implying its location or how to access it. The resource does not need to necessarily be network homed. For example, the URN
Technical publications, especially standards produced by the IETF
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
and by the W3C
World Wide Web Consortium
The World Wide Web Consortium is the main international standards organization for the World Wide Web .Founded and headed by Tim Berners-Lee, the consortium is made up of member organizations which maintain full-time staff for the purpose of working together in the development of standards for the...
, normally no longer use the term URL, as the need to distinguish between URLs and URIs rarely arises. However, in non-technical contexts and in software for the World Wide Web, the term URL remains widely used. Additionally, the term web address (which has no formal definition) often occurs in non-technical publications as a synonym for a URL that uses the 'http' or 'https' scheme.
RFC 3305
Much of this discussion comes from RFC3305, titled 'Report from the Joint W3C/IETF URI Planning Interest Group: Uniform Resource Identifiers (URIs), URLs, and Uniform Resource Names (URNs): Clarifications and Recommendations'. This RFCRequest for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
outlines the work of a joint W3C/IETF working group set up specifically to normalize the divergent views held within the IETF and W3C over the relationship between the various 'UR*' terms and standards. While not published as a full standard by either organization, it has become the basis for the above common understanding and has informed many standards since then.
Syntax
The URI syntax consists of a URI schemeURI scheme
In the field of computer networking, a URI scheme is the top level of the Uniform Resource Identifier naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character , and the remainder of the URI called the scheme-specific part...
name (such as "
httpHypertext Transfer Protocol
The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
", "
ftpFile Transfer Protocol
File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
", "
mailtoE-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
" or "
fileComputer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
") followed by a colon
Colon (punctuation)
The colon is a punctuation mark consisting of two equally sized dots centered on the same vertical line.-Usage:A colon informs the reader that what follows the mark proves, explains, or lists elements of what preceded the mark....
character, and then by a scheme-specific part. The specifications that govern the schemes determine the syntax and semantics
Semantics
Semantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....
of the scheme-specific part. However, URI syntax does require all schemes to adhere to a general syntax that (among other things) reserves certain characters for special purposes (without always identifying those purposes). The URI syntax also enforces restrictions on the scheme-specific part in order to (for example) provide for a degree of consistency when the part has a hierarchical structure.
Percent encoding can add extra information to a URI.
Naming, addressing, and identifying resources
URIs and URLs have a shared history. In 1990, Tim Berners-Lee’sTim Berners-Lee
Sir Timothy John "Tim" Berners-Lee, , also known as "TimBL", is a British computer scientist, MIT professor and the inventor of the World Wide Web...
proposals for HyperText
Hypertext
Hypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...
implicitly introduced the idea of a URL as a short string representing a resource that is the target of a hyperlink
Hyperlink
In computing, a hyperlink is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks...
. At the time, people referred to it as a 'hypertext name'
or 'document name'.
Over the next 3.5 years, as the World Wide Web's core technologies of HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
(the HyperText Markup Language
Markup language
A markup language is a modern system for annotating a text in a way that is syntactically distinguishable from that text. The idea and terminology evolved from the "marking up" of manuscripts, i.e. the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts...
), HTTP, and web browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...
s developed, a need to distinguish a string that provided an address for a resource from a string that merely named a resource emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent the former, and the more contentious Uniform Resource Name came to represent the latter.
During the debate over defining URLs and URNs it became evident that the two concepts embodied by the terms were merely aspects of the fundamental, overarching notion of resource identification. In June 1994, the IETF published Berners-Lee's RFC 1630: the first RFC
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
that (in its non-normative text) acknowledged the existence of URLs and URNs, and, more importantly, defined a formal syntax for Universal Resource Identifiers — URL-like strings whose precise syntaxes and semantics depended on their schemes. In addition, this RFC attempted to summarize the syntaxes of URL schemes in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifier
Fragment identifier
In computer hypertext, a fragment identifier is a short string of characters that refers to a resource that is subordinate to another, primary resource...
s.
Refinement of specifications
In December 1994, RFC 1738 formally defined relative and absolute URLs, refined the general URL syntax, defined how to resolve relative URLs to absolute form, and better enumerated the URL schemes then in use. The agreed definition and syntax of URNs had to wait until the publication of RFC 2141 in May 1997.The publication of RFC 2396 in August 1998 saw the URI syntax become a separate specification and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded. The new RFC changed the significance of the "U" in "URI": it came to represent "Uniform" rather than "Universal". The sections of
Internet Assigned Numbers Authority
The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
keeps a registry of those schemes; RFC 2717 first described the procedure to register them.
In December 1999, RFC 2732 provided a minor update to
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development of a number of draft revisions under the title rfc2396bis. This community effort, coordinated by
Roy Fielding
Roy Thomas Fielding is an American computer scientist, one of the principal authors of the HTTP specification, an authority on computer network architecture and co-founder of the Apache HTTP Server project....
, culminated in the publication of RFC 3986 in January 2005. This RFC, the current version of the URI syntax recommended for use on the Internet, renders
In August 2002, RFC 3305 pointed out that the term 'URL' has, despite its widespread use in the vernacular of the Internet-aware public at large, faded into near obsolescence. It now serves only as a reminder that some URIs act as addresses because they have schemes that imply some kind of network accessibility, regardless of whether systems actually use them for that purpose. As URI-based standards such as Resource Description Framework
Resource Description Framework
The Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
make evident, resource identification need not suggest the retrieval of resource representations over the Internet, nor need they imply network-based resources at all.
On November 1, 2006, the W3C Technical Architecture Group published 'On Linking Alternative Representations To Enable Discovery And Publishing', a guide to best practices and canonical URIs for publishing multiple versions of a given resource. For example, content might differ by language or by size to adjust for capacity or settings of the device used to access that content.
The Semantic Web
Semantic Web
The Semantic Web is a collaborative movement led by the World Wide Web Consortium that promotes common formats for data on the World Wide Web. By encouraging the inclusion of semantic content in web pages, the Semantic Web aims at converting the current web of unstructured documents into a "web of...
uses the HTTP URI scheme to identify both documents and concepts in the real world: this has caused confusion as to how to distinguish the two. The Technical Architecture Group of W3C (TAG) published an e-mail in June 2005 on how to solve this problem. The e-mail became known as the httpRange-14 resolution.
To expand on this (rather brief) email, W3C published in March 2008 the Interest Group Note Cool URIs for the Semantic Web. This explains the use of content negotiation
Content negotiation
Content negotiation is a mechanism defined in the HTTP specification that makes it possible to serve different versions of a document at the same URI, so that user agents can specify which version fit their capabilities the best...
and the 303-redirect
HTTP 303
The HTTP response status code 303 See Other is the correct manner in which to redirect web applications to a new URI, particularly after an HTTP POST has been performed....
code in more detail.
URI reference
A URI reference may take the form of a full URI, or just the scheme-specific portion of one, or even some trailing component thereof – even the empty string. An optional fragment identifier, preceded by #, may be present at the end of a URI reference. The part of the reference before the # indirectly identifies a resource, and the fragment identifier identifies some portion of that resource.To derive a URI from a URI reference, software converts the URI reference to 'absolute' form by merging it with an absolute 'base' URI according to a fixed algorithm. The system treats the URI reference as relative to the base URI, although in the case of an absolute reference, the base has no relevance. The base URI typically identifies the document containing the URI reference, although this can be overridden by declarations made within the document or as part of an external data transmission protocol. If the base URI includes a fragment identifier, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process.
Web document markup language
Markup language
A markup language is a modern system for annotating a text in a way that is syntactically distinguishable from that text. The idea and terminology evolved from the "marking up" of manuscripts, i.e. the revision instructions by editors, traditionally written with a blue pencil on authors' manuscripts...
s frequently use URI references to point to other resources, such as external documents or specific portions of the same logical document.
Uses of URI references in markup languages
- In HTMLHTMLHyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
, the value of thesrcattribute of theimgelement provides a URI reference, as does the value of thehrefattribute of theaorlinkelement. - In XMLXMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
, the system identifierSystem identifierA system identifier is a document processing construct introduced in the HyTime markup language as a supplement to SGML. It was subsequently incorporated into the HTML and XML markup languages....
appearing after theSYSTEMkeyword in a DTDDocument Type DefinitionDocument Type Definition is a set of markup declarations that define a document type for SGML-family markup languages...
is a fragmentless URI reference. - In XSLTXSLTXSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
, the value of thehrefattribute of thexsl:importelement/instruction is a URI reference; likewise the first argument to thedocumentfunction.
Examples of absolute URIs
-
http://example.org/absolute/URI/with/absolute/path/to/resource.txt -
ftp://example.org/resource.txt -
urn: issnInternational Standard Serial NumberAn International Standard Serial Number is a unique eight-digit number used to identify a print or electronic periodical publication. Periodicals published in both print and electronic form may have two ISSNs, a print ISSN and an electronic ISSN...:1535-3613
Examples of URI references
-
http://en.wikipedia.org/wiki/URI#Examples_of_URI_references ("http" specifies the 'scheme' name, "en.wikipedia.org" is the 'authority', "/wiki/URI" the 'path' pointing to this article, and "#Examples_of_URI_references" is a 'fragment' pointing to this section.) -
http://example.org/absolute/URI/with/absolute/path/to/resource.txt -
//example.org/scheme-relative/URI/with/absolute/path/to/resource.txt - /relative/URI/with/absolute/path/to/resource.txt
- relative/path/to/resource.txt
- ../../../resource.txt
- ./resource.txt#frag01
- resource.txt
- #frag01
- (empty string)
URI resolution
To 'resolve' a URI means either to convert a relative URI reference to absolute form, or to dereference a URIDereferenceable Uniform Resource Identifier
A dereferenceable Uniform Resource Identifier or dereferenceable URI is a resource retrieval mechanism that uses any of the internet protocols A dereferenceable Uniform Resource Identifier or dereferenceable URI is a resource retrieval mechanism that uses any of the internet protocols A...
or URI reference by attempting to obtain a representation of the resource that it identifies. The 'resolver' component in document processing software generally provides both services.
One can regard a URI reference as a same document reference: a reference to the document containing the URI reference itself. Document processing software can efficiently use its current representation of the document to satisfy the resolution of a same document reference without fetching a new representation. This is only a recommendation, and document processing software can alternatively use other mechanisms to determine whether to obtain a new representation.
The current URI specification ,
document function that, in effect, implements this functionality. Relation to XML namespaces
XMLXML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
has a concept of a namespace
XML Namespace
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
, an abstract domain to which a collection of element and attribute names can be assigned. The namespace name (a character string which must adhere to the generic URI syntax) identifies an XML namespace. However, the namespace name is generally not considered to be a URI because the 'URI-ness' of strings is, according to the URI specification, based on their intended use, not just their lexical components. A namespace name also does not necessarily imply any of the semantics of URI schemes; a namespace name beginning with 'http:', for example, likely has nothing to do with the HTTP protocol. XML professionals have debated this thoroughly on the xml dev electronic mailing list
Electronic mailing list
An electronic mailing list is a special usage of email that allows for widespread distribution of information to many Internet users. It is similar to a traditional mailing list — a list of names and addresses — as might be kept by an organization for sending publications to...
; some feel that a namespace name could be a URI, since the collection of names comprising a particular namespace could be regarded as a resource that is being identified, and since a version of the 'Namespaces in XML' specification says that the namespace name is a URI reference.
But the consensus seems to suggest that a namespace name is just a string that happens to look like a URI, nothing more.
Initially, the namespace name could match the syntax of any non-empty URI reference, but an erratum
Erratum
An erratum or corrigendum is a correction of a book. An erratum is most commonly issued shortly after its original text is published. Patches to security issues in a computer program are also sometimes called errata. As a general rule, publishers issue an erratum for a production error An erratum...
to the 'Namespaces In XML Recommendation' later deprecated the use of relative URI references. A separate specification, issued for namespaces for XML 1.1, allows IRI
Internationalized Resource Identifier
On the Internet, the Internationalized Resource Identifier is a generalization of the Uniform Resource Identifier . While URIs are limited to a subset of the ASCII character set, IRIs may contain characters from the Universal Character Set , including Chinese or Japanese kanji, Korean, Cyrillic...
references, not just URI references, to serve as the basis for namespace names.
To mitigate confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL
RDDL
In computing, Resource Directory Description Language is an extension of XHTML Basic 1.0. An RDDL document, called a Resource Directory, provides a package of information about some target. The targets which RDDL was designed to describe are XML namespaces...
(Resource Directory Description Language) developed, though the specification of RDDL has no official standing and no relevant organization (such as W3C) has considered or approved it. An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. Authors of XML documents were encouraged to put RDDL documents in locations such that if a namespace name in their document somehow becomes de-referenced, then an RDDL document would be obtained, thus satisfying the desire among many developers for a namespace name to point to a network-accessible resource.
See also
- .arpa.arpaThe domain name arpa is a top-level domain in the Domain Name System of the Internet. It is used exclusively for technical infrastructure purposes...
– uri.arpa is for dynamic discovery - Dereferenceable URIDereferenceable Uniform Resource IdentifierA dereferenceable Uniform Resource Identifier or dereferenceable URI is a resource retrieval mechanism that uses any of the internet protocols A dereferenceable Uniform Resource Identifier or dereferenceable URI is a resource retrieval mechanism that uses any of the internet protocols A...
(an HTTPHypertext Transfer ProtocolThe Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web....
URI) - History of the InternetHistory of the InternetThe history of the Internet starts in the 1950s and 1960s with the development of computers. This began with point-to-point communication between mainframe computers and terminals, expanded to point-to-point connections between computers and then early research into packet switching...
- Internationalized Resource IdentifierInternationalized Resource IdentifierOn the Internet, the Internationalized Resource Identifier is a generalization of the Uniform Resource Identifier . While URIs are limited to a subset of the ASCII character set, IRIs may contain characters from the Universal Character Set , including Chinese or Japanese kanji, Korean, Cyrillic...
(IRI) - Namespace (programming)
- Percent-encodingPercent-encodingPercent-encoding, also known as URL encoding, is a mechanism for encoding information in a Uniform Resource Identifier under certain circumstances. Although it is known as URL encoding it is, in fact, used more generally within the main Uniform Resource Identifier set, which includes both Uniform...
- Persistent Uniform Resource LocatorPersistent Uniform Resource LocatorA persistent uniform resource locator is a Uniform Resource Locator that is used to redirect to the location of the requested Web resource. PURLs redirect HTTP clients using HTTP status codes...
(PURL)
- Uniform naming convention (UNC)
- Uniform Resource LocatorUniform Resource LocatorIn computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
(URL) - Uniform Resource NameUniform Resource NameA uniform resource name is a uniform resource identifier that uses the urn scheme and does not imply availability of the identified resource. Both URNs and URLs are URIs, and a particular URI may be a name and a locator at the same time.The functional requirements for uniform resource names are...
(URN) - URI schemeURI schemeIn the field of computer networking, a URI scheme is the top level of the Uniform Resource Identifier naming structure. All URIs and absolute URI references are formed with a scheme name, followed by a colon character , and the remainder of the URI called the scheme-specific part...
- WebsiteWebsiteA website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...
- XRI (Extensible Resource Identifier)
External links
- RFC 3986 / STD 66 (2005) – the generic URI syntax specification
- RFC 2396 (1998) and RFC 2732 (1999) – obsolete, but widely implemented, version of the generic URI syntax
- RFC 1808 (1995) – obsolete companion to
RFC 1738 covering relative URL processing - RFC 1738 (1994) – mostly obsolete definition of URLUniform Resource LocatorIn computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
schemes and generic URI syntax - RFC 1630 (1994) – the first generic URI syntax specification; first acknowledgement of URLs in an Internet standard
- URI Schemes – IANAInternet Assigned Numbers AuthorityThe Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
-maintained registry of URI Schemes - URI Working Group – coordination center for development of URI standards
- Architecture of the World Wide Web, Volume One, §2: Identification – by W3C
- Example of discussion about names and addresses
- The URI is the Thing – by Paul Downey (2008)
- W3C materials related to addressing
- W3C URI Clarification
- What's a URI and why does it matter? (2008) – from W3C
- The Self-Describing Web (2008) – from W3C