The Hypertext Transfer Protocol , the Web's ( ) protocol , is at the heart of the Web. HTTP is implemented in two programs : a ( ) program and a server program . The client program and server program executing on different end systems , talk to each other by ( ) HTTP messages . HTTP defines how Web clients request Web pages from servers and how servers transfer web pages to clients . when a user (请作答此空) a web page , the browser sends http request messages for the objects in the page to the server . The server ( ) the requests and responds with http response messages that contain the objects .

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