Most Web pages require processing on the server. For example, consider a products page used to check the availability of a certain product. When a user selects his product and hits the submit button the page must check on the server to see whether the selected product is available or not. This kind of functionality is achieved by handling server control events. Whenever a user interaction requires processing on the server, the Web page is posted back to the server, processed and is returned back to the browser. This sequence is called round trip.