http:/ /www.someSchool.edu/someDepartment/picture.Gif
has www.Someschool.edu for a hostname and /somedepartment/picture.gif for a path name. Because Web browsers (such as Internet Explorer and Firefox) implement the client side of HTI'P, in the context of the Web, we will use the words browser and client interchangeably. Web servers, which implement the server side of HTTP, house Web objects, each addressable by a URL. Popular Web servers include Apache and Microsoft Internet Information Server.
HTTP defines how Web clients request Web pages from Web servers and how servers transfer Web pages to clients. We discuss the interaction between client and server in detail later, but the general idea is illustrated. When a user requests a Web page (for example, clicks on a hyperlink), the browser sends HTTP request messages for the objects in the page to the server. The server receives the requests and responds with HTTP response messages that contain the objects.
HTTP uses TCP as its underlying transport protocol (rather than running on top of UDP). The HTTP client first initiates a TCP connection with the server. Once the connection is established, the browser and the server processes access TCP through their socket interfaces on the client side the socket interface is the door between the client process and the TCP connection; on the server side it is the door between the server process and the TCP connection. The client sends HTTP request messages into its socket interface and receives HTTP response
messages from its socket interface. Similarly, the HTTP server receives request messages from its socket interface and sends response messages into its socket interface. Once the client sends a message into its socket interface, the message is out of the client's hands and is "in the hands" of TCP. TCP provides a reliable data transfer service to HTTP. This implies that each HTTP request message sent by a client process eventually arrives intact at the server; similarly, each HT1'P response message sent by the server process eventually arrives intact at the client. Here we see one of the great advantages of a layered architecture HTTP need not worry about lost data or the details of how TCP recovers from loss or reordering of data within the network: That is the job of TCP and the protocols in the lower layers of the protocol stack. It is important to note, that the server sends requested files to clients without storing any state information about the client. If a particular client asks for the same object twice in a period of a few seconds, the server does not respond by saying that it just served the object to the client; instead, the server resends the object, as it has completely forgotten what it did earlier. Because an FTTP server maintains no information about the clients, HTTP is said to be a stateless protocol. We also remark that the Web uses the client-server application architecture. A Web server is always on, with a fixed address and it services requests from potentially millions of different browsers.
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