Architecture of the World Wide Web, Volume One


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09/10/12 Architecture of the World Wide Web, Volume One Architecture of the World Wide Web, Volume One W3C Recommendation 15 December 2004 This version:­webarch­20041215/ Latest version: Previous version:­webarch­20041105/ Editors: Ian Jacobs, W3C Norman Walsh, Sun Microsystems, Inc. Authors: See acknowledgments (§8). Please refer to the errata for this document, which may include some normative correction
  09/10/12Architecture of the World Wide Web, Volume One1/‑webarch‑20041215/#uri‑benefits  Architecture of the World Wide Web, VolumeOne W3C Recommendation 15 December 2004 This version: Latest version: Previous version: Editors: Ian Jacobs, W3CNorman Walsh, Sun Microsystems, Inc. Authors: Seeacknowledgments (§8).Please refer to the errata for this document, which may include some normativecorrections.See also translations . Copyright© 2002-2004W3C  ® (MIT,ERCIM,Keio), All Rights Reserved. W3Cliability,trademark, document useandsoftware licensingrules apply. Your interactions with this site are in accordance with our publicandMember privacy statements.  Abstract The WorldWide Web uses relatively simple technologies with sufficient scalability,ef ficiencyand utility that they have resulted in a remarkable information space of interrelated resources, growing across languages, cultures, and media. In an effortto preserve these properties of the information space as the technologies evolve,this architecture document discusses the core design components of the Web.They are identification of resources, representation of resource state, and theprotocols that support the interaction between agents and resources in the space.We relate core design components, constraints, and good practices to theprinciples and properties they support. Status of this document This section describes the status of this document at the time of its publication.  09/10/12Architecture of the World Wide Web, Volume One2/‑webarch‑20041215/#uri‑benefits Other documents may supersede this document. A list of current W3C publicationsand the latest revision of this technical report can be found in theW3C technical reports index at This is the 15 December 2004 Recommendation of “Architecture of the World WideWeb, Volume One.” This document has been reviewed by W3C Members, bysoftware developers, and by other W3C groups and interested parties, and isendorsed by the Director as a W3C Recommendation. It is a stable document andmay be used as reference material or cited from another document. W3C's role inmaking the Recommendation is to draw attention to the specification and topromote its widespread deployment. This enhances the functionality andinteroperability of the Web.This document was developed by W3C'sTechnical Architecture Group (TAG),which, bycharter maintains alist of architectural issues. The scope of this document is a useful subset of those issues; it is not intended to address all of them. The TAG intends to address the remaining (and future) issues now thatVolume One is published as a W3C Recommendation. A completehistory of changesso this document is available. Please send comments on this document (public archive of public-webarch-comments).TAG technical discussion takes place on (public archive of www-tag).This document was produced under theW3C IPR policy of the July 2001 ProcessDocument. The TAG maintains apublic list of patent disclosuresrelevant to this document; that page also includes instructions for disclosing a patent. An individualwho has actual knowledge of a patent which the individual believes containsEssential Claim(s) with respect to this specification should disclose the informationin accordance withsection 6 of the W3C Patent Policy. Table of Contents 1. Introduction 1.1. About this Document1.1.1. Audience of this Document1.1.2. Scope of this Document1.1.3. Principles, Constraints, and Good Practice Notes 2. Identification 2.1. Benefits of URIs2.2. URI/Resource Relationships2.2.1. URI collision2.2.2. URI allocation2.2.3. Indirect Identification2.3. URI Comparisons2.3.1. URI aliases2.3.2. Representation reuse2.4. URI Schemes2.4.1. URI Scheme Registration2.5. URI Opacity2.6. Fragment Identifiers2.7. Future Directions for Identifiers2.7.1. Internationalized identifiers2.7.2. Assertion that two URIs identify the same resource 3. Interaction  09/10/12Architecture of the World Wide Web, Volume One3/‑webarch‑20041215/#uri‑benefits 3.1. Using a URI to Access a Resource3.1.1. Details of retrieving a representation3.2. Representation Types and Internet Media Types3.2.1. Representation types and fragment identifier semantics3.2.2. Fragment identifiers and content negotiation3.3. Inconsistencies between Representation Data and Metadata3.4. Safe Interactions3.4.1. Unsafe interactions and accountability3.5. Representation Management3.5.1. URI persistence3.5.2. Linking and access control3.5.3. Supporting Navigation3.6. Future Directions for Interaction 4. Data Formats 4.1. Binary and Textual Data Formats4.2. Versioning and Extensibility4.2.1. Versioning4.2.2. Versioning and XML namespace policy4.2.3. Extensibility4.2.4. Composition of data formats4.3. Separation of Content, Presentation, and Interaction4.4. Hypertext4.4.1. URI references4.5. XML-Based Data Formats4.5.1. When to use an XML-based format4.5.2. Links in XML4.5.3. XML namespaces4.5.4. Namespace documents4.5.5. QNames in XML4.5.6. XML ID semantics4.5.7. Media types for XML4.5.8. Fragment identifiers in XML4.6. Future Directions for Data Formats 5. General Architecture Principles 5.1. Orthogonal Specifications5.2. Extensibility5.3. Error Handling5.4. Protocol-based Interoperability 6. Glossary7. References 7.1. Architectural Specifications 8. Acknowledgments List of Principles, Constraints, and Good Practice Notes The following principles, constraints, and good practice notes are discussed in thisdocument and listed here for convenience. There is also afree-standing summary. Identification Global Identifiers(principle, 2)Identify with URIs(practice, 2.1)URIs Identify a Single Resource(constraint, 2.2) Avoiding URI aliases(practice, 2.3.1)  09/10/12Architecture of the World Wide Web, Volume One4/‑webarch‑20041215/#uri‑benefits Consistent URI usage(practice, 2.3.1)Reuse URI schemes(practice, 2.4)URI opacity(practice, 2.5) Interaction Reuse representation formats(practice, 3.2)Data-metadata inconsistency(constraint, 3.3)Metadata association(practice, 3.3)Safe retrieval(principle, 3.4) Available representation(practice, 3.5)Reference does not imply dereference(principle, 3.5)Consistent representation(practice, 3.5.1) Data Formats Version information(practice, 4.2.1)Namespace policy(practice, 4.2.2)Extensibility mechanisms(practice, 4.2.3)Extensibility conformance(practice, 4.2.3)Unknown extensions(practice, 4.2.3)Separation of content, presentation, interaction(practice, 4.3)Link identification(practice, 4.4)Web linking(practice, 4.4)Generic URIs(practice, 4.4)Hypertext links(practice, 4.4)Namespace adoption(practice, 4.5.3)Namespace documents(practice, 4.5.4)QNames Indistinguishable from URIs(constraint, 4.5.5)QName Mapping(practice, 4.5.5)XML and text/* (practice, 4.5.7)XML and character encodings(practice, 4.5.7) General Architecture Principles Orthogonality(principle, 5.1)Error recovery(principle, 5.3) 1. Introduction The World Wide Web ( WWW  , or simply Web ) is an information space in which theitems of interest, referred to as resources, are identified by global identifiers calledUniform Resource Identifiers ( URI  ).Examples such as the following travel scenario are used throughout this documentto illustrate typical behavior of  Web agents —people or software acting on thisinformation space. A user agent  acts on behalf of a user. Software agents includeservers, proxies, spiders, browsers, and multimedia players.   Story 
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