The work of the web-network, as well as the work of other Internet services, is implemented using the "client-server" technology. As clients in a web-based network, the programs are web-browsers (browser, web-browser), and as servers - web-servers (HTTP-servers) , which accept HTTP requests from browsers, perform their processing, and send HTTP responses. The client, which is usually a web browser, sends requests to the web server for the HTTP protocol (HTTP request) to receive resources specified by URLs. Resources can be HTML-pages, graphic files (images), media streams or other data that the client needs. In response, the web server will give the client an HTTP response that includes the requested data.
Web browsers is client software that allows users to execute requests to web servers over HTTP, receive responses from them, usually containing HTML documents, analyze them and display them in a convenient way. users form. The browser allows you to perform the following basic operations:
• support for interaction using different protocols (HTTP, FTP);
• Display content on the screen of different types of resources (HTML documents, images, multimedia documents, etc.),
• Cross-referencing, which are contained in web pages, by generating HTTP requests to web servers;
• Enter user data using Forms and transfer them to the web using GET or POST methods.
Almost all popular browsers are distributed free of charge. According to Netmarketshare, in January 2013 the most popular browsers were as follows: Microsoft Internet Explorer - 55.14%; Firefox - 19,24%; Chrome - 17.48%; Safari - 5.24%; Opera - 1.75% (percentage shows the share of all computers in the world on which this type of browser is installed).
The main functions of browsers are:
• generation and transmission of requests to web servers on behalf of users as a result of the transition to hyperlinks; explicit input of the URL; sending form data (pressing the submit key); analysis of HTML pages that require additional resources (for example, images, audio files, etc.);
• getting answers from web-based systems and their interpretation to create a visual representation for the user. In the simplest case, it requires checking some response headers, such as Content-Type, to determine what actions are required and what kind of rendering (rendering) is required;
• visualization of the received data in the browser window or with the help of programs, depending on the type of content in the HTTP response.
In addition to these functions, the browser also performs many other functions, depending on the values of the status code and the response headers:
• Caching . Each browser performs a temporary storage of a copy of the resources received from the web servers (supports the local cache).
• Authentication and authorization. A Web server may require authorization to retrieve a resource if the appropriate security settings have been set for it. In this case, the browser will request authentication data (name and password) from the user or use the previously entered data and send it to the web server.
Support states . To write and maintain the state between requests and responses, the web server can send cookies to the browser in the response headers. The browser must store the information contained in the cookie and return it to the server in the headers of subsequent requests.
• Requesting Supporting Data Elements. A typical web page contains links to images, Java applets, multimedia files and other supporting resources. In this case, for the correct display of such a page, the browser must first obtain them from the server (i.e., make relevant requests). This is done without any user interaction.
• Perform actions in response to other headers and status codes. HTTP headers and status codes may provide additional processing instructions. Such instructions can indicate a problem with access to the resource, or they can tell the browser to redirect the request to another address. They can also inform the browser about the support of the current connection (it must remain open) so that subsequent requests are sent using the same connection.
• Visualization of complex objects. Most browsers support content types like text/html, text/plain, image/gif and image/jpeg. This means that the browser includes functionality for displaying such content in its window, without the need to install additional software. To display or play other (more complex) objects (such as audio, video and multimedia), the browser must provide support, i.e., it should be possible to call external auxiliary applications or plug-ins that are required for display and playback of such objects.
• Error handling. Browsers must have facilities for handling connection errors, incorrect server responses, and other similar situations.
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