Uniform Resource Identifier: Difference between revisions
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{{distinguish2|[[uniform resource locator]] (URL) or [[uniform resource name]] (URN)}} |
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<!--{{Technical|date=May 2012}}--> |
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In [[information technology|computing]], a '''uniform resource identifier''' ('''URI''') is a [[character string (computer science)|string]] of [[character (computing)|character]]s used to [[identifier|identify]] a name or a [[web resource]]. Such identification enables interaction with representations of the web resource over a network (typically the [[World Wide Web]]) using specific [[protocol (computing)|protocol]]s. Schemes specifying a concrete [[syntax]] and associated protocols define each URI. |
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==Syntax== |
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{{See also|URI scheme#Generic syntax|l1=URI generic syntax}} |
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The URI syntax consists of a [[URI scheme]] name (such as "[[HyperText Transfer Protocol|<code>http</code>]]", "[[File Transfer Protocol|<code>ftp</code>]]", "[[Mailto|<code>mailto<code>]]", "[[CRID|<code>crid<code>]]" or "[[File URI scheme|<code>file</code>]]") followed by a [[colon (punctuation)|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. |
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''[[Percent encoding]]'' can add extra information to a URI. |
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==History== |
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=== Naming, addressing, and identifying resources === |
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URIs and URLs have a shared history. In 1994, [[Tim Berners-Lee|Tim Berners-Lee’s]] proposals for [[Hypertext|HyperText]]<ref> |
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{{cite web |
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| first = Sean B. |
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| last = Palmer |
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| title = The Early History of HTML |
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| url = http://infomesh.net/html/history/early/ |
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| accessdate = 2009-04-30 |
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}} |
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</ref> |
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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'<ref name="w3_naming"> |
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{{cite web |
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|url= http://www.w3.org/History/19921103-hypertext/hypertext/WWW/Addressing/Addressing.html |
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|title=W3 Naming Schemes|publisher=[[World Wide Web Consortium|W3]] |
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|accessdate=2009-07-24 |
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| quote = The format of a hypertext name consists of the name of the naming sub-scheme to be used, then a name in a format particular to that sub-scheme, then an optional anchor identifier within the document. For example, the format is for all internet-based access methods: |
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<br> |
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scheme : // host.domain:port / path / path # anchor |
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}} |
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</ref> |
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or 'document name'. |
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Over the next three and a half 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. |
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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 [[Request For Comments|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. |
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===Refinement of specifications=== |
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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. |
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The publication of RFC 2396 in August 1998 saw the URI syntax become a separate specification<ref>{{cite web|url=http://www.faqs.org/rfcs/rfc2396.html |title=FAQS.org |publisher=FAQS.org |date= |accessdate=2012-04-03}}</ref> and most of the parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded by the [[IETF]]. The new RFC changed the significance of the "U" in "URI": it came to represent "Uniform" rather than "Universal". The sections of <nowiki>RFC 1738</nowiki> that summarized existing URL schemes migrated into a separate document.<ref>This separate document is not explicitly linked{{By whom|date=April 2009}}, RFC 2717 and RFC 4395 point to the IANA registry as the official URI scheme registry.</ref> [[Internet Assigned Numbers Authority|IANA]] keeps a registry of those schemes;<ref>{{cite web|url=http://www.iana.org/assignments/uri-schemes.html |title=IANA registry of URI schemes |publisher=Iana.org |date=2012-03-26 |accessdate=2012-04-03}}</ref> RFC 2717 first described the procedure to register them. |
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In December 1999, RFC 2732 provided a minor update to <nowiki>RFC 2396</nowiki>, 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 by <nowiki>RFC 2396</nowiki> co-author [[Roy Fielding]], culminated in the publication of RFC 3986 in January 2005. This RFC, {{as of | 2009 | alt = as of 2009}} the current version of the URI syntax recommended for use on the Internet, renders <nowiki>RFC 2396</nowiki> obsolete. It does not, however, render the details of existing URL schemes obsolete; <nowiki>RFC 1738</nowiki> continues to govern such schemes except where otherwise superseded – RFC 2616 for example, refines the 'http' scheme. Simultaneously, the IETF published the content of <nowiki>RFC 3986</nowiki> as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol. |
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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. |
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On November 1, 2006, the W3C [[Technical Architecture Group]] published 'On Linking Alternative Representations To Enable Discovery And Publishing',<ref>{{cite web|url=http://www.w3.org/2001/tag/doc/alternatives-discovery.html |title=w3.org |publisher=w3.org |date= |accessdate=2012-04-03}}</ref> 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. |
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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''.<ref>The httpRange-14 resolution consists of three bullet points: see {{cite web |
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|first = Roy T. |
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|last = Fielding |
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|authorlink = Roy Fielding |
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|title = [httpRange-14] Resolved |
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|url = http://lists.w3.org/Archives/Public/www-tag/2005Jun/0039.html |
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|date = 2005-06-18 |
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|accessdate = 2009-07-24 |
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}}, and did not help much to reduce the confusion. |
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</ref> |
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To expand on this (rather brief) email, W3C published in March 2008 the Interest Group Note ''Cool URIs for the Semantic Web''.<ref>{{cite web|url=http://www.w3.org/TR/cooluris/ |title=W3.org |publisher=W3.org |date= |accessdate=2012-04-03}}</ref> This explains the use of [[content negotiation]] and the [[HTTP 303|303-redirect]] code in more detail. |
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==URI reference== |
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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. |
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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. |
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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. |
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===Uses of URI references in markup languages=== |
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* In [[HTML]], the value of the <code>src</code> attribute of the <code>img</code> element provides a URI reference, as does the value of the <code>href</code> attribute of the <code>a</code> or <code>link</code> element. |
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* In [[XML]], the [[system identifier]] appearing after the <code>SYSTEM</code> keyword in a [[Document Type Definition|DTD]] is a fragmentless URI reference. |
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* In [[XSLT]], the value of the <code>href</code> attribute of the <code>xsl:import</code> element/instruction is a URI reference; likewise the first argument to the <code>document()</code> function. |
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===Examples of absolute URIs=== |
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* <tt><nowiki>http://example.org/absolute/URI/with/absolute/path/to/resource.txt</nowiki></tt> |
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* <tt><nowiki>ftp://example.org/resource.txt</nowiki></tt> |
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* <tt><nowiki>urn:</nowiki>[[International Standard Serial Number|issn]]<nowiki>:1535-3613</nowiki></tt> |
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===Examples of URI references=== |
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* <tt><nowiki>http://en.wikipedia.org/wiki/URI#Examples_of_URI_references</nowiki></tt> ("<tt>http</tt>" specifies the 'scheme' name, "<tt>en.wikipedia.org</tt>" is the 'authority', "<tt>/wiki/URI</tt>" the 'path' pointing to this article, and "<tt>#Examples_of_URI_references</tt>" is a 'fragment' pointing to this section.) |
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* <tt><nowiki>http://example.org/absolute/URI/with/absolute/path/to/resource.txt</nowiki></tt> |
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* <tt><nowiki>//example.org/scheme-relative/URI/with/absolute/path/to/resource.txt</nowiki></tt> |
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* <tt>/relative/URI/with/absolute/path/to/resource.txt</tt> |
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* <tt>relative/path/to/resource.txt</tt> |
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* <tt>../../../resource.txt</tt> |
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* <tt>./resource.txt#frag01</tt> |
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* <tt>resource.txt</tt> |
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* <tt>#frag01</tt> |
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* ''(empty string)'' |
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==URI resolution== |
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To ''resolve'' a URI means either to convert a relative URI reference to absolute form, or to [[Dereferenceable Uniform Resource Identifier|dereference a URI]] 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. |
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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. |
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The current URI specification {{as of|2009|lc = on}}, <nowiki>RFC 3986</nowiki>, 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 <code>document()</code> function that, in effect, implements this functionality. <nowiki>RFC 3986</nowiki> 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. |
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<nowiki>RFC 2396</nowiki> prescribed a different method for determining same document references; <nowiki>RFC 3986</nowiki> made <nowiki>RFC 2396</nowiki> obsolete, but <nowiki>RFC 2396</nowiki> 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. |
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==Relation to XML namespaces== |
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[[XML]] has a concept of a [[XML Namespace|namespace]], an abstract domain to which a collection of element and attribute names can be assigned.<!-- who or what can do such assignation? --> 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<ref>[[Elliotte Rusty Harold|Harold, Elliote Rusty]] (2004). ''XML 1.1 Bible'', Third Edition, Wiley Publishing Inc., p. 291. ISBN 0-7645-4986-3.</ref> 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.<ref> |
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{{cite web |
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| author = World Wide Web Consortium |
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| authorlink = World Wide Web Consortium |
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| title = Namespaces in XML |
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| url = http://www.aifb.uni-karlsruhe.de/Lehre/Sommer2004/ISWWW/w3c/xml_namespace_1_on_1.pdf |
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| publisher = W3C |
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| format = PDF |
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| doi = |
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| date = 1999-01-14 |
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| accessdate = 2009-09-14 |
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| quote = [Definition:] The attribute's value, a URI reference, is the namespace name identifying the |
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namespace. |
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}} |
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</ref> |
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But the consensus seems to suggest that a namespace name is just a string that happens to look like a URI, nothing more.<!--This topic was beaten to death on the xml dev mailing list in 2000-2002 and beyond; research the discussion there before changing these carefully qualified statements.--><!-- one author of the namespace spec (me) insisted on the URI because it is controlled name space, not just a string. we did not define what happens when dereferenced, but we did insist it was a controlled space--consensus also seems to suggest santa clause is alive but that doesn't make it true. --> |
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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 [[Internationalized Resource Identifier|IRI]] references, not just URI references, to serve as the basis for namespace names. |
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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<ref>{{cite web|url=http://www.rddl.org/ |title=rddl.org |publisher=rddl.org |date= |accessdate=2012-04-03}}</ref> 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{{By whom|date=March 2009}} 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. |
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==See also== |
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{{Selfref|For help on using external links on Wikipedia, see [[Help:URL]] and [[Wikipedia:External links]]}} |
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{{col-begin}} |
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{{col-2}} |
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* [[.arpa]] – uri.arpa is for dynamic discovery |
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* [[Dan Connolly (computer scientist)|Dan Connolly]] |
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* [[CURIE]] (Compact URI) |
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* [[Dereferenceable Uniform Resource Identifier|Dereferenceable URI]] (an [[Hypertext Transfer Protocol|HTTP]] URI) |
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* [[History of the Internet]] |
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* [[Internationalized Resource Identifier]] (IRI) |
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* [[Namespace (programming)]] |
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* [[Percent-encoding]] |
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{{col-2}} |
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* [[Persistent Uniform Resource Locator]] (PURL) |
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* [[Uniform Naming Convention|Uniform naming convention]] (UNC) |
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* [[Uniform Resource Locator]] (URL) |
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* [[Uniform Resource Name]] (URN) |
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* [[URI scheme]] |
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* [[Website]] |
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* [[XRI]] (Extensible Resource Identifier) |
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{{col-end}} |
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==References== |
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{{Reflist|33em}} |
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==External links== |
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{{External links|date=April 2013}} |
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* RFC 3986 / [http://tools.ietf.org/html/std66 STD 66] (2005) – the {{as of | 2009 | alt = current}} generic URI syntax specification |
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* [http://www.iana.org/assignments/uri-schemes.html URI Schemes] – [[Internet Assigned Numbers Authority|IANA]]-maintained registry of URI Schemes |
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* [http://www.w3.org/TR/webarch/#identification Architecture of the World Wide Web, Volume One, §2: Identification] – by W3C |
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* [http://www.w3.org/TR/uri-clarification/ W3C URI Clarification] |
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{{Semantic Web|state=collapsed}} |
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{{URI scheme}} |
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<!-- Please respect alphabetical order --> |
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[[Category:URI schemes]] |
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