1. 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?
2. Consider the following string of ASCII characters that were captured by Wireshark when the browser sent an HTTP GET message (i. e., this is the actual content of an HTTP GET message). The characters < lf> are carriage return and line- feed characters (that is, the italized character string in the text below represents the single carriage-return character that was contained at that point in the HTTP header). Answer the following questions, indicating where in the HTTP GET message below you find the answer.
GET / cs453/ index. html HTTP/ 1.1< cr>< lf> Host: gai a. cs. umass. edu< cr>< lf> a.cs.umass.edu User- Agent: Mozilla/ 5.0 ( Windows; U; Windows NT 5.1; en- US; rv: 1.7.2) Gec ko/ 20040804 Netscape/ 7.2 ( ax) < cr>< lf> Accept: ex t/ xml, application/ xml, application/ xhtml+ xml, text / html; q= 0.9, text/ plain; q= 0.8, image/ png,*/*; text/plain;q=0.8,image/png,*/*; q= 0.5 < cr>< lf> Accept- Language: en- us, en; q= 0.5< cr>< en-us,en;q=0.5< lf> Accept- Encoding: zip, deflate< cr>< lf> Accept- Charset: zip,deflateAccept-Charset: ISO - 8859- 1, utf- 8; q= -8859-1,utf-8;q= 0.7,*; q= 0.7< cr>< lf> -8859-1,utf-8;q=0.7,*;q=0.7 Keep- Alive: 300< cr> < lf> Connection: keep- alive< cr>< Connection:keep-alive< lf>< cr>< lf>
a. What is the URL of the document requested by the browser?
b. What version of HTTP is the browser running?
c. Does the browser request a non-persistent or a persistent connection?
d. What is the IP address of the host on which the browser is running?
e. What type of browser initiates this message? Why is the browser type needed in an HTTP request message?
3. Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of RTT_1, ... ... RTT_n . Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT_0 denote the RTT between the local host and the server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?
4. How does SMTP mark the end of a message body? How about HTTP? Can HTTP use the same method as SMTP to mark the end of a message body? Explain.
5. 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.
6. Consider distributing a file of F = 15 Gbits to N peers. The server has an upload rate of u s = 30 Mbps, and each peer has a download rate of d i = 2 Mbps and an upload rate of u. For N = 10, 100, and 1,000 and u = 300 Kbps, 700 Kbps, and 2 Mbps, prepare a chart giving the minimum distribution time for each of the combinations of N and u for both client- server distribution and P2P distribution.
7. Suppose Bob joins a BitTorrent torrent, but he does not want to upload any data to any other peers (so called free-riding).
a. Bob claims that he can receive a complete copy of the file that is shared by the swarm. Is Bob's claim possible? Why or why not?
b. Bob further claims that he can further make his "free-riding" more efficient by using a collection of multiple computers (with distinct IP addresses) in the computer lab in his department. How can he do that?
8. In the circular DHT example in Section 2.6.2, suppose that peer 3 learns that peer 5 has left. How does peer 3 update its successor state information? Which peer is now its first successor? Its second successor?