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Uniform Resource Identifiers Roy T. Fielding University of California, Irvine

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1 Uniform Resource Identifiers Roy T. Fielding University of California, Irvine

2 2 URI History zWeb Addresses [Berners-Lee]1990 yUniversal Document Identifiers1992 yUniversal Resource IdentifiersRFC 1630 yURL: LocatorsRFC 1736,1738,1808 yURN: NamesRFC 1737,2141 yUniform Resource IdentifiersRFC 2396 zA simple and extensible means of identification y

3 3 Etymology of URI … Identifier zEstablishing identity by reference yname, handle, moniker, location, … yglobal scope zSimple yJust a string of common characters zTranscribable ybar napkins, advertisements, and e-mail ya sequence of characters, not coded character octets zUsable yno additional entry barrier to deployment and use

4 4 Etymology of URI … Resource zWhat we want to identify: Resources ymatch the semantics of a hypermedia reference yephemeral and persistent information ynew and existing information sources zA resource can be anything that has identity ya document or image ya service, e.g., “today’s weather in Irvine” ya collection of other resources ynon-networked objects (e.g., people)

5 5 More precisely, a resource is … zA temporally varying membership function M R (t) that, for each time t, maps to some set of semantically equivalent values zValues may be resource representations or identifiers to other resources zCan map to the empty set, allowing references to be made to a concept before any realization of that concept exists zThe resource is the conceptual mapping, not the entity that corresponds to that mapping at t

6 6 Representations of a Resource zThe Web is designed to manipulate and transfer representations of a resource zA single resource may be associated with multiple representations (content negotiation) zA representation is in the form of a media type yprovides information for this resource yprovides potential hypermedia state transitions zMost representations can be cached zGET URI transfers representation, not resource

7 7 Etymology of URI … Uniform zUniformity allows ydifferent types of resource identification within a single protocol element yuniform semantic interpretation of common syntactic elements; shared implementations yrelative syntactic interpretation independent of scheme yextensibility for new identification schemes ybounds variability along common paths, making it easier to extend the use of URI to new applications

8 8 Uniform Extensibility zURI scheme allows new types to be defined without affecting old uses yfile, news, http, telnet, gopher, wais, ftp, … zNaturally lends itself to table-driven implementations ybrowser uses table to select handler yhandler can be dynamically loaded/downloaded zUnfortunately, ysome people haven’t figured that out yet

9 9 Uniform Hierarchy zScheme defines interpretation and structure : zHierarchical when desired [Engelbart] :// / ? :/ / / : zName components may be meaningful zHierarchy enables relative URI

10 10 Relative URI zAn identifier that obtains global scope when interpreted relative to a base URI zOnly valid when the base URI is well-defined ySaves space yAllows document content to be partially independent of its location and accessibility yDocument trees (groups of inter-related documents) can be moved without changing embedded URI yDocuments can be shared by multiple access schemes and servers

11 11 Design Failures zTranscribable, but not easily spoken yaitch tee tee pee colon slash slash double-you double-you double-you dot eye see es dot you see eye dot ee dee you slash tilde fielding slash zHierarchical path assumes only one root ynot true for FTP resources, leading to ambiguity ygopher path isn’t layered left-to-right zReliance on DNS as only naming authority yvanity hostname explosion yflat namespace under dot com

12 12 Questions? zPlaces to see for more URI design information: zSlides:

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