Uniform Resource Identifier |
A Uniform Resource Identifier (URI), is an Internet protocol element consisting of a short Character string (computer science) of character (computing)s that conform to a certain syntax. The string comprises a name or address that can be used to refer to a resource (computer science). It is a fundamental component of the World Wide Web.
= Relationship to URL and URN =
A URI can be classified as a locator, a name, or both. A ), allows one to talk about a book, but doesn t suggest where and how to obtain an actual copy of it.
The contemporary point of view among the working group that oversees URIs is that the terms URL and URN are context-dependent aspects of URI and rarely need to be distinguished. Furthermore, the term URL is increasingly becoming obsolete, as it is rarely necessary to differentiate between URLs and URIs, in general.
=Syntax=
The URI syntax is essentially a URI scheme name like http , ftp , mailto , Uniform Resource Name , etc., followed by a Colon (punctuation) 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 general 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.
=History=
==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[http://infomesh.net/html/history/early/]. At the time, it was called a hypertext name or document name [http://www.w3.org/History/19921103-hypertext/hypertext/WWW/Addressing/Addressing.html]
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 s.
==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 RFC 1738 that summarized existing URL schemes were not rendered obsolete by RFC 2396.
In December 1999, RFC 2732 provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development a number of draft revisions under the title rfc2396bis . This community effort, coordinated by RFC 2396 co-author Roy Fielding, culminated in the publication of RFC 3986 in January 2005. This RFC is the current version of the URI syntax recommended for use on the Internet, and it renders RFC 2396 obsolete. It does not, however, render the details of existing URL schemes obsolete; those are still governed by RFC 1738, except where otherwise superseded — RFC 2616 for example, refines the http scheme. The content of RFC 3986 was simultaneously published by the IETF 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 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 # , 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.
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. Unless it is an absolute URI already, the URI reference is considered to be relative to the base URI. 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.
== Uses of URI references in markup languages ==
==Examples of absolute URIs==
==Examples of URI references==
= 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, RFC 3986, a URI reference is a same-document reference if, when resolved to absolute form, it is identical to the base URI that is 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 document() function that, in effect, implements this functionality. RFC 3986 also formally defines URI equivalence , which can be used in order 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.
Same-document references were determined differently according to RFC 2396, which was made obsolete by RFC 3986 but is still used as the basis of many specifications and implementations. According to this specification, a URI reference is a same-document reference if it is an empty string or consists of only the # character followed by an optional fragment.
= Relation to XML namespaces =
; 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 arguably misguided desire among developers for a namespace name to point to a network-accessible resource.
= See also =
= External links =
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