1. EEC-484/584 Computer Networks Lecture 7 Wenbing Zhao wenbing@ieee.org (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book, and on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall)

2. 2 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Outline DNS Resource Records Optional homework problems

3. 3 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Type A RR Type A DNS records map a host name to an IPv4 address

4. 4 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Type NS RR Type NS records within the zone file are authoritative records for the zone's name servers

5. 5 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Type CNAME RR A Type CNAME record maps an alias or nickname to the real or Canonical name

6. 6 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Type MX RR Type MX records Specifies the name and relative preference of mail servers for the zone

7. 7 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Layered Protocols Q1. If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate packets or do packets encapsulate frames? Explain your answer.

8. 8 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Layered Protocols Q2. A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers?

9. 9 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Reliable Communication Q3. When a file is transferred between two computers, two acknowledgement strategies are possible. In the first one, the file is chopped up into packets, which are individually acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the second one, the packets are not acknowledged individually, but the entire file is acknowledged when it arrives. Discuss these two approaches.

10. 10 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao URL Q4. Imagine that someone in the CS Department at Stanford has just written a new program that he wants to distribute by FTP. He puts the program in the FTP directory ftp/pub/freebies/newprog.c. What is the URL for this program likely to be?

11. 11 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Web and Cookies Q5. In Fig. 7-25, www.aportal.com keeps track of user preferences in a cookie. A disadvantage of this scheme is that cookies are limited to 4 KB, so if the preferences are extensive, for example, many stocks, sports teams, types of news stories, weather for multiple cities, specials in numerous product categories, and more, the 4-KB limit may be reached. Design an alternative way to keep track of preferences that does not have this problem.

12. 12 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Web and Cookies Q6. Sloth Bank wants to make on-line banking easy for its lazy customers, so after a customer signs up and is authenticated by a password, the bank returns a cookie containing a customer ID number. In this way, the customer does not have to identify himself or type a password on future visits to the on-line bank. What do you think of this idea? Will it work? Is it a good idea?

13. 13 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao HTTP and Caching Q7. The If-Modified-Since header can be used to check whether a cached page is still valid. Requests can be made for pages containing images, sound, video, and so on, as well as HTML. Do you think the effectiveness of this technique is better or worse for JPEG images as compared to HTML?

14. 14 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Web and Caching Q8. Consider an institutional network connected to the internet. Suppose that the average object size is 900,000 bits and that the average request rate from the institution’s browsers to the origin servers is 1.5 requests per second. The bandwidth of the access link is 1.5 Mbps. Also suppose that the amount of time it takes from when the router on the Internet side of the access link forwards an HTTP request until it receives the response in two seconds on average. Model the total average response time as the sum of the average access delay (that is, the delay from Internet router to institution router) and the average Internet delay. (continued on next slide)

15. 15 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Web and Caching Q8 (cont’d) For the average access delay, use  /(1-  ), where  is the average time required to send an object over the access link and  is the arrival rate of objects to the access link. –Find the total average response time. –Now suppose a cache is installed in the institutional LAN. Suppose the high rate is 0.4. Find the total response time.

16. 16 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao CDN Q9. Under what conditions is using a CDN a bad idea?

17. 17 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao DNS Q10. DNS typically uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved?

18. 18 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao DNS Q11. Although it was not mentioned in the text, an alternative form for a URL is to use the IP address instead of its DNS name. An example of using an IP address is http://192.31.231.66/index.html. How does the browser know whether the name following the scheme is a DNS name or an IP address.

19. 19 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao DNS, Web, HTTP Q12. 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 look-up 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 0 transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?

20. 20 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Physical Layer Q13. Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

21. 21 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Physical Layer Q14. A noiseless 4-kHz channel is sampled every 1 msec. What is the maximum data rate? (Assuming each sample is 16 bits)

22. 22 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Physical Layer Q15. What signal-to-noise ratio is needed to put a T1 carrier (1.544 Mbps) on a 50-kHz line?

23. 23 Spring Semester 2006EEC-484/584: Computer NetworksWenbing Zhao Physical Layer Q16. If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio is 20 dB, what is the maximum achievable data rate?