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A Uniform Resource Identifier (URI), is a compact string of characters used to identify or name a resource. The main purpose of this identification is to enable interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols. URIs are defined in schemes defining a specific syntax and associated protocols. Relationship to URL and URN A URI can be classified as a locator or a name or both. A Uniform Resource Locator (URL) is a URI that, in addition to identifying a resource, provides means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL The contemporary point of view among the working group that oversees URIs is that the terms URL and URN are context-dependent aspects of URIs, and rarely need to be distinguished. In technical publications, especially standards produced by the IETF and the W3C, the term URL has long been deprecated, as it is rarely necessary to distinguish between URLs and URIs. However, in nontechnical contexts and in software for the World Wide Web, the term URL remains ubiquitous. Additionally, the term web address, which has no formal definition, is often used in nontechnical publications as a synonym for URL or URI, although it generally refers only to 'http' and 'https' URIs. Syntax The URI syntax is essentially a URI scheme name like "http", "ftp", "mailto", "urn", "mms", "rtsp", etc., followed by a colon character, and then a scheme-specific part. The syntax and semantics of the scheme-specific part are determined by the specifications that govern the schemes, although the URI syntax does force all schemes to adhere to a certain generic syntax that, among other things, reserves certain characters for special purposes, without always saying what those purposes are. 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 is an often misunderstood aspect of URI syntax. See also URI generic syntax Naming, addressing, and identifying resources URIs and URLs have a shared history. The idea of a URL — a short string representing a resource that is the target of a hyperlink — was implicitly introduced in late 1990 in Tim Berners-Lee's proposals for HyperText *. At the time, it was called a hypertext name or document name* 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 browsers were developed, a need to distinguish between strings that provide an address for resources and those that merely name resources emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent strings used for the former purpose, and the more contentious Uniform Resource Name came to represent strings used for the latter purpose. During the debate over how to best define 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. So, 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 syntax and semantics were dependent upon their scheme. In addition, this RFC attempted to summarize the syntax of URL schemes that were in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifiers. Refinement of specifications In December 1994, RFC 1738 was published in order to formally define relative and absolute URLs, refine the general URL syntax, define how relative URLs were to be resolved to absolute form, and better enumerate the URL schemes that were in use at the time. The definition and syntax of URNs was not settled upon until the publication of RFC 2141 in May 1997. With the publication of RFC 2396 in 1998, the URI syntax became a separate specification, and most parts of RFCs 1630 and 1738 became obsolete. In the new RFC, the "U" in "URI" was changed to represent "Uniform" rather than "Universal", and all parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded. Only those portions of In December 1999, RFC 2732 provided a minor update to In August 2002, RFC 3305 pointed out that the term URL has, despite its ubiquity 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 they are actually being used for that purpose. As URI-based standards such as Resource Description Framework make evident, resource identification need not be coupled with the retrieval of resource representations over the Internet, nor does it need to be associated with network-bound resources at all. URI reference A URI reference is another type of string that represents a URI, and, in turn, the resource identified by that URI. The distinction between a URI and a URI reference is not often maintained in informal usage, but protocol documents should not allow for ambiguity. 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 " In order to derive a URI from a URI reference, the URI reference is converted to "absolute" form by merging it with an absolute "base" URI, according to a fixed algorithm. The URI reference is considered to be relative to the base URI, although if the reference itself is absolute, then the base is irrelevant. The base URI is typically the URI that 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 a fragment identifier is present in the base URI, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process. In web document markup languages, URI references are frequently used in places where there is a need to point to other resources, such as external documents or specific portions of the same logical document. Examples of absolute URIs URI resolution To "resolve" a URI means either to convert a relative URI reference to absolute form, or to 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. A URI reference may be considered to be a same-document reference: a reference to the document containing the URI reference itself. Document processing software is encouraged to use its current representation of the document to satisfy the resolution of a same-document reference; a new representation should not be fetched. This is only a recommendation, and document processing software is free to use other mechanisms to determine whether obtaining a new representation is warranted. According to the current URI specification, document() function that, in effect, implements this functionality. Same-document references were determined differently according to Relation to XML namespaces XML has a concept of a namespace, an abstract domain to which a collection of element and attribute names can be assigned. An XML namespace is identified by a character string, the namespace name, which must adhere to the generic URI syntax. However, the namespace name is not considered to be a URI because the "URI-ness" of strings is, according to the URI specification, based on how they are intended to be used, 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. There has been much debate about this among XML professionals 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 considered to be a resource that is being identified, and since the Namespaces in XML specification says that the namespace name is a URI reference. The consensus seems to be, though, that a namespace name is just a string that happens to look like a URI, nothing more. Initially, the namespace name was allowed to match the syntax of any non-empty URI reference, but the use of relative URI references was later deprecated by an erratum to the Namespaces In XML Recommendation. A separate specification was issued for namespaces for XML 1.1, and allows IRI references, not just URI references, to be used as the basis for namespace names. In order to mitigate the confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL was developed. An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. XML document authors were encouraged to put RDDL documents in locations such that if a namespace name in their document was somehow dereferenced, 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 | |||||||
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