(第5空)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 ( ) 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 .