This section describes the concept of the URI and does not form part of the specification.
Many protocols and systems for document search and retrieval are currently in use, and many more protocols or refinements of existing protocols are to be expected in a field whose expansion is explosive.
These systems are aiming to achieve global search and readership of documents across differing computing platforms, and despite a plethora of protocols and data formats. As protocols evolve, gateways can allow global access to remain possible. As data formats evolve, format conversion programs can preserve global access. There is one area, however, in which it is impractical to make conversions, and that is in the names and addresses used to identify objects. This is because names and addresses of objects are passed on in so many ways, from the backs of envelopes to hypertext objects, and may have a long life.
A common feature of almost all the data models of past and proposed systems is something which can be mapped onto a concept of "object" and some kind of name, address, or identifier for that object. One can therefore define a set of name spaces in which these objects can be said to exist.
Practical systems need to access and mix objects which are part of different existing and proposed systems. Therefore, the concept of the universal set of all objects, and hence the universal set of names and addresses, in all name spaces, becomes important. This allows names in different spaces to be treated in a common way, even though names in different spaces have differing characteristics, as do the objects to which they refer.
This document defines a way to encapsulate a name in any registered name space, and label it with the the name space, producing a member of the universal set. Such an encoded and labelled member of this set is known as a Universal Resource Identifier, or URI.
The universal syntax allows access of objects available using existing protocols, and may be extended with technology.
The specification of the URI syntax does not imply anything about the properties of names and addresses in the various name spaces which are mapped onto the set of URI strings. The properties follow from the specifications of the protocols and the associated usage conventions for each scheme.
For existing Internet access protocols, it is necessary in most cases to define the encoding of the access algorithm into something concise enough to be termed address. URIs which refer to objects accessed with existing protocols are known as "Uniform Resource Locators" (URLs) and are listed here as used in WWW, but to be formally defined in a separate document.
There is currently a drive to define a space of more persistent names than any URLs. These "Uniform Resource Names" are the subject of an IETF working group's discussions. (See Sollins and Masinter, Functional Specifications for URNs, circulated informally.)
The URI syntax and URL forms have been in widespread use by World-Wide Web software since 1990.