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However, this does not imply that all interaction with such resources is limited to the default interaction protocol.
For example, information retrieval systems often make use of proxies to interact with a multitude of URI schemes, such as HTTP proxies being used to access "ftp" and "wais" resources. Proxies can also to provide enhanced services, such as annotation proxies that combine normal information retrieval with additional metadata retrieval to provide a seamless, multidimensional view of resources using the same protocols and user agents as the non-annotated Web. Likewise, future protocols may be defined that encompass our current systems, using entirely different interaction mechanisms, without changing the existing identifier schemes.
Dereferencing a URI generally involves a succession of steps as described in multiple specifications and implemented by the agent. In this example, the URI is "http: Precisely which representation s are retrieved depends on a number of factors, including:. Assuming that a representation has been successfully retrieved, the expressive power of the representation's format will affect how precisely the representation provider communicates resource state.
If the representation communicates the state of the resource inaccurately, this inaccuracy or ambiguity may lead to confusion among users about what the resource is. Some communities, such as the ones developing the Semantic Web, seek to provide a framework for accurately communicating the semantics of a resource in a machine readable way. Machine readable semantics may alleviate some of the ambiguity associated with natural language descriptions of resources.
A representation is data that encodes information about resource state. Representations do not necessarily describe the resource, or portray a likeness of the resource, or represent the resource in other senses of the word "represent". Representations of a resource may be sent or received using interaction protocols. These protocols in turn determine the form in which representations are conveyed on the Web. Just as it is important to reuse existing URI schemes whenever possible, there are significant benefits to using media typed octet streams for representations even in the unusual case where a new URI scheme and associated protocol is to be defined.
The Internet media type mechanism does have some limitations. Emma's Web browser starts up an SVG implementation to view the image. It passes it the original URI including the fragment, "http: The semantics of a fragment identifier are defined by the set of representations that might result from a retrieval action on the primary resource.
The fragment's format and resolution are therefore dependent on the type of a potentially retrieved representation, even though such a retrieval is only performed if the URI is dereferenced. If no such representation exists, then the semantics of the fragment are considered unknown and, effectively, unconstrained. Fragment identifier semantics are orthogonal to URI schemes and thus cannot be redefined by URI scheme specifications.
Interpretation of the fragment identifier is performed solely by the agent that dereferences a URI; the fragment identifier is not passed to other systems during the process of retrieval. This means that some intermediaries in Web architecture such as proxies have no interaction with fragment identifiers and that redirection in HTTP [ RFC ], for example does not account for fragments.
Content negotiation refers to the practice of making available multiple representations via the same URI.
Negotiation between the requesting agent and the server determines which representation is served usually with the goal of serving the "best" representation a receiving agent can process. HTTP is an example of a protocol that enables representation providers to use content negotiation. Individual data formats may define their own rules for use of the fragment identifier syntax for specifying different types of subsets, views, or external references that are identifiable as secondary resources by that media type.
Therefore, representation providers must manage content negotiation carefully when used with a URI that contains a fragment identifier.
Consider an example where the owner of the URI "http: Three situations can arise:. The second case is a server management error: The third case is not a server management error. It is a means by which the Web can grow. Because the Web is a distributed system in which formats and agents are deployed in a non-uniform manner, Web architecture does not constrain authors to only use "lowest common denominator" formats.
Content authors may take advantage of new data formats while still ensuring reasonable backward-compatibility for agents that do not yet implement them. In case three, behavior by the receiving agent should vary depending on whether the negotiated format defines fragment identifier semantics. When a received data format does not define fragment identifier semantics, the agent should not perform silent error recovery unless the user has given consent; see [ CUAP ] for additional suggested agent behavior in this case.
Successful communication between two parties depends on a reasonably shared understanding of the semantics of exchanged messages, both data and metadata. At times, there may be inconsistencies between a message sender's data and metadata.
Examples, observed in practice, of inconsistencies between representation data and metadata include:. Receiving agents should detect protocol inconsistencies and perform proper error recovery. Thus, for example, if the parties responsible for "weather. Nadia's browser can notify Nadia of the problem or notify Nadia and take corrective action. Furthermore, representation providers can help reduce the risk of inconsistencies through careful assignment of representation metadata especially that which applies across representations.
The accuracy of metadata relies on the server administrators, the authors of representations, and the software that they use. Practically, the capabilities of the tools and the social relationships may be the limiting factors.
The accuracy of these and other metadata fields is just as important for dynamic Web resources, where a little bit of thought and programming can often ensure correct metadata for a huge number of resources. Often there is a separation of control between the users who create representations of resources and the server managers who maintain the Web site software. Given that it is generally the Web site software that provides the metadata associated with a resource, it follows that coordination between the server managers and content creators is required.
In particular, content creators need to be able to control the content type for extensibility and the character encoding for proper internationalization. Nadia's retrieval of weather information an example of a read-only query or lookup qualifies as a "safe" interaction; a safe interaction is one where the agent does not incur any obligation beyond the interaction. An agent may incur an obligation through other means such as by signing a contract.
If an agent does not have an obligation before a safe interaction, it does not have that obligation afterwards. Other Web interactions resemble orders more than queries. These unsafe interactions may cause a change to the state of a resource and the user may be held responsible for the consequences of these interactions. Unsafe interactions include subscribing to a newsletter, posting to a list, or modifying a database.
In this context, the word "unsafe" does not necessarily mean "dangerous"; the term "safe" is used in section 9. Nadia decides to book a vacation to Oaxaca at "booking. She provides this information in another form. When she presses the "Purchase" button, her browser opens another network connection to the server at "booking. This is an unsafe interaction ; Nadia wishes to change the state of the system by exchanging money for airline tickets.
The server reads the POST request, and after performing the booking transaction returns a message to Nadia's browser that contains a representation of the results of Nadia's request. Note that neither the data transmitted with the POST nor the data received in the response necessarily correspond to any resource identified by a URI.
Safe interactions are important because these are interactions where users can browse with confidence and where agents including search engines and browsers that pre-cache data for the user can follow hypertext links safely.
Users or agents acting on their behalf do not commit themselves to anything by querying a resource or following a hypertext link. Agents do not incur obligations by retrieving a representation. For instance, it is incorrect to publish a URI that, when followed as part of a hypertext link, subscribes a user to a mailing list.
Remember that search engines may follow such hypertext links. At times, there may be good reasons such as confidentiality requirements or practical limits on URI length to conduct an otherwise safe operation using a mechanism generally reserved for unsafe operations e. Nadia pays for her airline tickets online through a POST interaction as described above. She receives a Web page with confirmation information and wishes to bookmark it so that she can refer to it when she calculates her expenses.
Although Nadia can print out the results, or save them to a file, she would also like to bookmark them. However, in practice, Nadia cannot bookmark her commitment to pay expressed via the POST request or the airline company's acknowledgment and commitment to provide her with a flight expressed via the response to the POST.
There are ways to provide persistent URIs for transaction requests and their results. For transaction requests, user agents can provide an interface for managing transactions where the user agent has incurred an obligation on behalf of the user. Since Nadia finds the Oaxaca weather site useful, she emails a review to her friend Dirk recommending that he check out 'http: Dirk clicks on the resulting hypertext link in the email he receives and is frustrated by a not found.
Dirk tries again the next day and receives a representation with "news" that is two-weeks old. He tries one more time the next day only to receive a representation that claims that the weather in Oaxaca is sunny, even though his friends in Oaxaca tell him by phone that in fact it is raining. Dirk and Nadia conclude that the URI owners are unreliable or unpredictable. Although the URI owner has chosen the Web as a communication medium, the owner has lost two customers due to ineffective representation management.
There is a benefit to the community in providing representations. Just because representations are available does not mean that it is always desirable to retrieve them. In fact, in some cases the opposite is true.
Reference does not imply dereference. Dereferencing a URI has a potentially significant cost in computing and bandwidth resources, may have security implications, and may impose significant latency on the dereferencing application. Dereferencing URIs should be avoided except when necessary. As is the case with many human interactions, confidence in interactions via the Web depends on stability and predictability.
For an information resource, persistence depends on the consistency of representations. The representation provider decides when representations are sufficiently consistent although that determination generally takes user expectations into account.
Although persistence in this case is observable as a result of representation retrieval, the term URI persistence is used to describe the desirable property that, once associated with a resource, a URI should continue indefinitely to refer to that resource. The choice of a particular URI scheme provides no guarantee that those URIs will be persistent or that they will not be persistent. For example, HTTP redirection using the 3xx response codes permits servers to tell an agent that further action needs to be taken by the agent in order to fulfill the request for example, a new URI is associated with the resource.
In addition, content negotiation also promotes consistency, as a site manager is not required to define new URIs when adding support for a new format specification. Protocols that do not support content negotiation such as FTP require a new identifier when a new data format is introduced. Improper use of content negotiation can lead to inconsistent representations.
For more discussion about URI persistence, see [ Cool ]. It is reasonable to limit access to a resource for commercial or security reasons, for example , but merely identifying the resource is like referring to a book by title. In exceptional circumstances, people may have agreed to keep titles or URIs confidential for example, a book author and a publisher may agree to keep the URI of page containing additional material secret until after the book is published , otherwise they are free to exchange them.
The owners of a building might have a policy that the public may only enter the building via the main front door, and only during business hours. People who work in the building and who make deliveries to it might use other doors as appropriate. Such a policy would be enforced by a combination of security personnel and mechanical devices such as locks and pass-cards.
One would not enforce this policy by hiding some of the building entrances, nor by requesting legislation requiring the use of the front door and forbidding anyone to reveal the fact that there are other doors to the building. Nadia sends to Dirk the URI of the current article she is reading. With his browser, Dirk follows the hypertext link and is asked to enter his subscriber username and password.
Since Dirk is also a subscriber to services provided by "weather. Thus, the authority for "weather. The Web provides several mechanisms to control access to resources; these mechanisms do not rely on hiding or suppressing URIs for those resources. It is a strength of Web Architecture that links can be made and shared; a user who has found an interesting part of the Web can share this experience just by republishing a URI.
Nadia goes to "http: Dirk goes to "http: Dirk reads Nadia's email and is able to follow the link to the map. Nadia has to start again from "http: For resources that are generated on demand, machine generation of URIs is common.
For resources that might usefully be bookmarked for later perusal, or shared with others, server managers should avoid needlessly restricting the reusability of such URIs.
There remain open questions regarding Web interactions. Since then, data formats have grown in number. Web architecture does not constrain which data formats content providers can use. This flexibility is important because there is constant evolution in applications, resulting in new data formats and refinements of existing formats.
Although Web architecture allows for the deployment of new data formats, the creation and deployment of new formats and agents able to handle them is expensive. Thus, before inventing a new data format or "meta" format such as XML , designers should carefully consider re-using one that is already available.
For a data format to be usefully interoperable between two parties, the parties must agree to a reasonable extent about its syntax and semantics. Shared understanding of a data format promotes interoperability but does not imply constraints on usage; for instance, a sender of data cannot count on being able to constrain the behavior of a data receiver.
Below we describe some characteristics of a data format that facilitate integration into Web architecture. This document does not address generally beneficial characteristics of a specification such as readability, simplicity, attention to programmer goals, attention to user needs, accessibility, nor internationalization.
Binary data formats are those in which portions of the data are encoded for direct use by computer processors, for example 32 bit little-endian two's-complement and 64 bit IEEE double-precision floating-point.
The portions of data so represented include numeric values, pointers, and compressed data of all sorts. A textual data format is one in which the data is specified in a defined encoding as a sequence of characters.
Although XML-based formats are textual, many XML-based formats do not consist primarily of phrases in natural language. In principle, all data can be represented using textual formats. In practice, some types of content e. The trade-offs between binary and textual data formats are complex and application-dependent.
Binary formats can be substantially more compact, particularly for complex pointer-rich data structures. Also, they can be consumed more rapidly by agents in those cases where they can be loaded into memory and used with little or no conversion.
Note, however, that such cases are relatively uncommon as such direct use may open the door to security issues that can only practically be addressed by examining every aspect of the data structure in detail. Textual formats are usually more portable and interoperable. Textual formats also have the considerable advantage that they can be directly read by human beings and understood, given sufficient documentation.
This can simplify the tasks of creating and maintaining software, and allow the direct intervention of humans in the processing chain without recourse to tools more complex than the ubiquitous text editor. Finally, it simplifies the necessary human task of learning about new data formats; this is called the "view source" effect. It is important to emphasize that intuition as to such matters as data size and processing speed is not a reliable guide in data format design; quantitative studies are essential to a correct understanding of the trade-offs.
Therefore, designers of a data format specification should make a considered choice between binary and textual format design. In a perfect world, language designers would invent languages that perfectly met the requirements presented to them, the requirements would be a perfect model of the world, they would never change over time, and all implementations would be perfectly interoperable because the specifications would have no variability.
In the real world, language designers imperfectly address the requirements as they interpret them, the requirements inaccurately model the world, conflicting requirements are presented, and they change over time. As a result, designers negotiate with users, make compromises, and often introduce extensibility mechanisms so that it is possible to work around problems in the short term.
In the long term, they produce multiple versions of their languages, as the problem, and their understanding of it, evolve. The resulting variability in specifications, languages, and implementations introduces interoperability costs. Extensibility and versioning are strategies to help manage the natural evolution of information on the Web and technologies used to represent that information.
For more information about how these strategies introduce variability and how that variability impacts interoperability, see Variability in Specifications. See also "Web Architecture: There is typically a long transition period during which multiple versions of a format, protocol, or agent are simultaneously in use. Nadia and Dirk are designing an XML data format to encode data about the film industry.
They provide for extensibility by using XML namespaces and creating a schema that allows the inclusion, in certain places, of elements from any namespace. When they revise their format, Nadia proposes a new optional lang attribute on the film element. Dirk feels that such a change requires them to assign a new namespace name, which might require changes to deployed software. Nadia explains to Dirk that their choice of extensibility strategy in conjunction with their namespace policy allows certain changes that do not affect conformance of existing content and software, and thus no change to the namespace identifier is required.
They chose this policy to help them meet their goals of reducing the cost of change. Dirk and Nadia have chosen a particular namespace change policy that allows them to avoid changing the namespace name whenever they make changes that do not affect conformance of deployed content and software. They might have chosen a different policy, for example that any new element or attribute has to belong to a namespace other than the original one.
Whatever the chosen policy, it should set clear expectations for users of the format. In general, changing the namespace name of an element completely changes the element name. If "a" and "b" are bound to two different URIs, a: Practically speaking, this means that deployed applications will have to be upgraded in order to recognize the new language; the cost of this upgrade may be very high.
It follows that there are significant tradeoffs to be considered when deciding on a namespace change policy. If a vocabulary has no extensibility points that is, if it does not allow elements or attributes from foreign namespaces or have a mechanism for dealing with unrecognized names from the same namespace , it may be absolutely necessary to change the namespace name. Languages that allow some form of extensibility without requiring a change to the namespace name are more likely to evolve gracefully.
As an example of a change policy designed to reflect the variable stability of a namespace, consider the W3C namespace policy for documents on the W3C Recommendation track. The policy sets expectations that the Working Group responsible for the namespace may modify it in any way until a certain point in the process "Candidate Recommendation" at which point W3C constrains the set of possible changes to the namespace in order to promote stable implementations.
Note that since namespace names are URIs, the owner of a namespace URI has the authority to decide the namespace change policy. Requirements change over time. Successful technologies are adopted and adapted by new users.
Designers can facilitate the transition process by making careful choices about extensibility during the design of a language or protocol specification. In making these choices, the designers must weigh the trade-offs between extensibility, simplicity, and variability. A language without extensibility mechanisms may be simpler and less variable, improving initial interoperability. However, it's likely that changes to that language will be more difficult, possibly more complex and more variable, than if the initial design had provided such mechanisms.
This may decrease interoperability over the long term. Extensibility introduces variability which has an impact on interoperability. However, languages that have no extensibility mechanisms may be extended in ad hoc ways that impact interoperability as well.
One key criterion of the mechanisms provided by language designers is that they allow the extended languages to remain in conformance with the original specification, increasing the likelihood of interoperability. Application needs determine the most appropriate extension strategy for a specification. For example, applications designed to operate in closed environments may allow specification designers to define a versioning strategy that would be impractical at the scale of the Web.
A powerful design approach is for the language to allow either form of extension, but to distinguish explicitly between them in the syntax. Additional strategies include prompting the user for more input and automatically retrieving data from available hypertext links. More complex strategies are also possible, including mixing strategies.
For instance, a language can include mechanisms for overriding standard behavior. Thus, a data format can specify "must ignore" semantics but also allow for extensions that override that semantics in light of application needs for instance, with "must understand" semantics for a particular extension. Extensibility is not free. Providing hooks for extensibility is one of many requirements to be factored into the costs of language design. Experience suggests that the long term benefits of a well-designed extensibility mechanism generally outweigh the costs.
Many modern data format include mechanisms for composition. In principle, these relationships can be mixed and nested arbitrarily. Each application must define how namespaces interact and what effect the namespace of an element has on the element's ancestors, siblings, and descendants. The Web is a heterogeneous environment where a wide variety of agents provide access to content to users with a wide variety of capabilities. It is good practice for authors to create content that can reach the widest possible audience, including users with graphical desktop computers, hand-held devices and mobile phones, users with disabilities who may require speech synthesizers, and devices not yet imagined.
Furthermore, authors cannot predict in some cases how an agent will display or process their content. This separation also facilitates reuse of authored source content across multiple delivery contexts. Sometimes, functional user experiences suited to any delivery context can be generated by using an adaptation process applied to a representation that does not depend on the access mechanism. Separation of content, presentation, interaction.
Note that when content, presentation, and interaction are separated by design, agents need to recombine them. There is a recombination spectrum, with "client does all" at one end and "server does all" at the other. There are advantages to each approach. The server can, for example, enable faster downloads by adjusting links to refer to lower resolution images, smaller video or no video at all.
Similarly, if the content has been authored with multiple branches, the server can remove unused branches before delivery. In addition, by tailoring the content to match the characteristics of a target client, the server can help reduce client side computation. However, specializing content in this manner reduces caching efficiency. On the other hand, designing content that that can be recombined on the client also tends to make that content applicable to a wider range of devices.
This design also improves caching efficiency and offers users more presentation options. Media-dependent style sheets can be used to tailor the content on the client side to particular groups of target devices. For textual content with a regular and repeating structure, the combined size of the text content plus the style sheet is typically less than that of fully recombined content; the savings improve further if the style sheet is reused by other pages.
In practice a combination of both approaches is often used. The design decision about where on this spectrum an application should be placed depends on the power on the client, the power and the load on the server, and the bandwidth of the medium that connects them. If the number of possible clients is unbounded, the application will scale better if more computation is pushed to the client.
Of course, it may not be desirable to reach the widest possible audience. Data formats such as these demonstrate that one can only separate content from presentation or interaction so far; at some point it becomes necessary to talk about presentation.
See the TAG issues formattingProperties concerning interoperability in the case of formatting properties and names and contentPresentation concerning the separation of semantic and presentational markup. A defining characteristic of the Web is that it allows embedded references to other resources via URIs. When a representation of one resource contains a reference to another resource, expressed with a URI identifying that other resource, this constitutes a link between the two resources.
Additional metadata may also form part of the link see [ XLink10 ], for example. In this document, the term "link" generally means "relationship", not "physical connection". A specification SHOULD provide ways to identify links to other resources, including to secondary resources via fragment identifiers. Formats that allow content authors to use URIs instead of local identifiers promote the network effect: What agents do with a hypertext link is not constrained by Web architecture and may depend on application context.
Users of hypertext links expect to be able to navigate among representations by following links. Data formats that do not allow content authors to create hypertext links lead to the creation of "terminal nodes" on the Web.
Links are commonly expressed using URI references defined in section 4. URI references help in content management by allowing content authors to design a representation locally, i.
This section discusses issues that are specific to such formats. While it is directed at Internet applications with specific reference to protocols, the discussion is generally applicable to Web scenarios as well. XML defines textual data formats that are naturally suited to describing data objects which are hierarchical and processed in a chosen sequence.
It is widely, but not universally, applicable for data formats; an audio or video format, for example, is unlikely to be well suited to expression in XML. Design constraints that would suggest the use of XML include:. Sophisticated linking mechanisms have been invented for XML formats. XPointer allows links to address content that does not have an explicit, named anchor. XLink allows links to have multiple ends and to be expressed either inline or in "link bases" stored external to any or all of the resources identified by the links it contains.
XLink is not the only linking design that has been proposed for XML, nor is it universally accepted as a good design.
See also TAG issue xlinkScope The purpose of an XML namespace defined in [ XMLNS ] is to allow the deployment of XML vocabularies in which element and attribute names are defined in a global environment and to reduce the risk of name collisions in a given document when vocabularies are combined.
Although XML data from different formats such as MathML and SVG can be combined in a single document, in this case there could be ambiguity about which set element was intended.
Anglais seulement Se connecter. Sollicitations pour le paiement de taxes. Pouvoirs Accès aux rapports d'examen préliminaire international en vertu de la règle Brevets et Littérature autre que celle des brevets. Formulaires à l'usage des offices Circulaires du PCT Informations concernant les plans de préparation aux situations d'urgence Services de données destinés aux offices. En pratique Déposer votre demande. Gérer votre demande Le système ePCT à authentification forte offre un accès en ligne au dossier de votre demande internationale et une série de fonctions visant à faciliter le suivi et la gestion pendant la phase internationale.
Contacts Pour des questions liées à: Via ePCT avec ou sans authentification forte Télécopie: Questions d'ordre général ou juridiques sur le PCT Courriel: Dépôt d'une demande PCT auprès du Bureau international agissant en tant qu'office récepteur Courriel: Ces contraintes peuvent être externes ou résulter de phénomènes économiques et sociologiques.
Par exemple, la législation sociale est plus ou moins importante en fonction de la nature des rapports de force sociaux — il en va de même pour la très forte réglementation concernant la sécurité et la qualité des produits et ceci en raison de la pression exercée par les organisations consommateurs.
Certains secteurs demandent des investissements très lourds et peu rentables à court terme. Certains auteurs notamment en Europe continentale ajoutent une sixième force: Porter lui-même évoque dans des écrits plus récents une autre force: Je suis actuellement sur un mini mémoire en marketing: À vrai dire je ne sais pas du tout quoi écrire.
Vous ne parlez pas du diagnostic interne qui parle des ressources et des compétences.
In this type of environment, there is an inevitable tension between interoperability in the short term and the desire for extensibility.
Remember that search engines may follow such hypertext links. Requiring a URI owner to publish a new URI for each change in resource state would lead to a significant number of broken references.