Uniform Resource Identifier

A Uniform Resource Identifier (URI), is an Internet protocol element consisting of a short string of characters that conform to a certain syntax. The string comprises a name or address that can be used to refer to a resource. 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 Uniform Resource Locator (URL) is a URI that, in addition to identifying a resource, provide a means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL http://www.wikipedia.org/ is a URI that identifies a resource and implies that a representation of the resource is obtainable via HTTP from a host named www.wikipedia.org. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to dereference it. For example, the URN urn:ISBN:0-395-36341-1 is a URI that, like an ISBN book number, 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.


The URI syntax is essentially a URI scheme name like "http", "ftp", "mailto", "urn", 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 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.

Creation and registration of URI scheme names

The top level of the URI naming structure is the definition of URI schemes. RFC 2717 describes the procedures for registering new URI schemes. RFC 2718 provides guidelines for the development of new URI schemes. Both of these RFCs are currently undergoing revision.


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[1] (http://infomesh.net/html/history/early/). At the time, it was called a hypertext name or document name[2] (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 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 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

  • In HTML, the value of the src attribute of the img element is a URI reference, as is the value of the href attribute of the a element.
  • In XML, the system identifier appearing after the SYSTEM keyword in a DTD is a fragmentless URI reference;
  • In XSLT, the value of the href attribute of the xsl:import element/instruction is a URI reference, as is the first argument to the document() function.

Examples of absolute URIs

  • http://somehost/absolute/URI/with/absolute/path/to/resource.txt
  • ftp://somehost/resource.txt
  • urn:issn:1535-3613

Examples of URI references

  • http://example/resource.txt#frag01
  • http://somehost/absolute/URI/with/absolute/path/to/resource.txt
  • /relative/URI/with/absolute/path/to/resource.txt
  • relative/path/to/resource.txt
  • ../../../resource.txt
  • resource.txt
  • /resource.txt#frag01
  • #frag01
  • [empty string]

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

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. There has been much debate about this, and some feel that it could be a URI, since the abstract concept that is a particular namespace could be considered to be a resource that is being identified. 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.

In order to mitigate the confusion that began to arise among newcomers to XML from the use of the URI syntax in namespace names, 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

es:URI fr:Uniform Resource Identifier it:Uniform Resource Identifier ko:URI ja:Uniform Resource Identifier nl:Uniform Resource Identifier pl:URI fi:URI zh:统一资源标志符


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