6. Consider an HTTP client that wants to retrieve a Web document at a given URL. The IP address of the HTTP server is initially unknown. What transport and application-layer protocols besides HTTP are needed in this scenario? Please explain why each protocol is needed (10pts). 7. Suppose you can access the caches in the local DNS servers of your department. Can you propose a way to roughly determine the Web servers (outside your department) that are most popular among the users in your department? Explain (10pts). 8. Read the POP3 RFC, RFC 1939. What is the purpose of the UIDL POP3 command (10pts)