Reference no: EM131858489

TEXTBOOK - COMPUTER NETWORKS, FIFTH EDITION BY ANDREW S. TANENBAUM AND DAVID J. WETHERALL

Chapter 7 - THE APPLICATION LAYER

PROBLEMS

1. Many business computers have three distinct and worldwide unique identifiers. What are they?

2. In Fig. 7-4, there is no period after laserjet. Why not?

3. Consider a situation in which a cyberterrorist makes all the DNS servers in the world crash simultaneously. How does this change one's ability to use the Internet?

4. DNS 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?

5. John wants to have an original domain name and uses a randomized program to generate a secondary domain name for him. He wants to register this domain name in the com generic domain. The domain name that was generated is 253 characters long. Will the com registrar allow this domain name to be registered?

6. Can a machine with a single DNS name have multiple IP addresses? How could this occur?

7. The number of companies with a Web site has grown explosively in recent years. As a result, thousands of companies are registered in the com domain, causing a heavy load on the top-level server for this domain. Suggest a way to alleviate this problem without changing the naming scheme (i.e., without introducing new top-level domain names). It is permitted that your solution requires changes to the client code.

8. Some email systems support a Content Return: header field. It specifies whether the body of a message is to be returned in the event of non-delivery. Does this field belong to the envelope or to the header?

9. Electronic mail systems need directories so people's email addresses can be looked up. To build such directories, names should be broken up into standard components (e.g., first name, last name) to make searching possible. Discuss some problems that must be solved for a worldwide standard to be acceptable.

10. A large law firm, which has many employees, provides a single email address for each employee. However, the firm did not explicitly define the format of the login. Thus, some employees use their first names as their login names, some use their last names, some use their initials, etc.

11. A binary file is 4560 bytes long. How long will it be if encoded using base64 en-coding, with a CR+LF pair inserted after every 110 bytes sent and at the end?

12. Name five MIME types not listed in this book. You can check your browser or the Internet for information.

13. Suppose that you want to send an MP3 file to a friend, but your friend's ISP limits the size of each incoming message to 1 MB and the MP3 file is 4 MB. Is there a way to handle this situation by using RFC 5322 and MIME?

14. Suppose that John just set up an auto-forwarding mechanism on his work email address, which receives all of his business-related emails, to forward them to his person-al email address, which he shares with his wife. John's wife was unaware of this, and activated a vacation agent on their personal account. Because John forwarded his email, he did not set up a vacation daemon on his work machine. What happens when an email is received at John's work email address?

15. In any standard, such as RFC 5322, a precise grammar of what is allowed is needed so that different implementations can interwork. Even simple items have to be defined carefully. The SMTP headers allow white space between the tokens. Give two plausible alternative definitions of white space between tokens.

16. Is the vacation agent part of the user agent or the message transfer agent? Of course, it is set up using the user agent, but does the user agent actually send the replies? Explain your answer.

17. In a simple version of the Chord algorithm for peer-to-peer lookup, searches do not use the finger table. Instead, they are linear around the circle, in either direction. Can a node accurately predict which direction it should search in? Discuss your answer.

18. IMAP allows users to fetch and download email from a remote mailbox. Does this mean that the internal format of mailboxes has to be standardized so any IMAP program on the client side can read the mailbox on any mail server? Discuss your answer.

19. Consider the Chord circle of Fig. 7-71. Suppose that node 18 suddenly goes online. Which of the finger tables shown in the figure are affected? How?

20. Does Webmail use POP3, IMAP, or neither? If one of these, why was that one chosen? If neither, which one is it closer to in spirit?

21. When Web pages are sent out, they are prefixed by MIME headers. Why?

22. Is it possible that when a user clicks on a link with Firefox, a particular helper is started, but clicking on the same link in Internet Explorer causes a completely different helper to be started, even though the MIME type returned in both cases is identical? Explain your answer.

23. 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. Use this information to explain why a DNS name cannot end with a digit.

24. Imagine that someone in the math department at Stanford has just written a new document including a proof that he wants to distribute by FTP for his colleagues to review. He puts the program in the FTP directory ftp/pub/forReview/newProof.pdf. What is the URL for this program likely to be?

25. In Fig. 7-22, 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.

26. Sloth Bank wants to make online 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 online bank. What do you think of this idea? Will it work? Is it a good idea?

27. (a) Consider the following HTML tag:

<h1 title=''this is the header''> HEADER 1 </h1>

Under what conditions does the browser use the TITLE attribute, and how?

(b) How does the TITLE attribute differ from the ALT attribute?

28. How do you make an image clickable in HTML? Give an example.

29. Write an HTML page that includes a link to the email address [email protected]. What happens when a user clicks this link?

30. Write an XML page for a university registrar listing multiple students, each having a name, an address, and a GPA.

31. For each of the following applications, tell whether it would be (1) possible and (2) better to use a PHP script or JavaScript, and why:

(a) Displaying a calendar for any requested month since September 1752.

(b) Displaying the schedule of flights from Amsterdam to New York.

(c) Graphing a polynomial from user-supplied coefficients.

32. Write a program in JavaScript that accepts an integer greater than 2 and tells whether it is a prime number. Note that JavaScript has if and while statements with the same syntax as C and Java. The modulo operator is %. If you need the square root of x, use Math.sqrt (x).

33. An HTML page is as follows:

<html> <body>

<a href="www.info-source.com/welcome.html"> Click here for info </a>

</body> </html>

If the user clicks on the hyperlink, a TCP connection is opened and a series of lines is sent to the server. List all the lines sent.

34. 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? Think carefully about what ''effectiveness'' means and explain your answer.

35. On the day of a major sporting event, such as the championship game in some popular sport, many people go to the official Web site. Is this a flash crowd in the same sense as the 2000 Florida presidential election? Why or why not?

36. Does it make sense for a single ISP to function as a CDN? If so, how would that work? If not, what is wrong with the idea?

37. Assume that compression is not used for audio CDs. How many MB of data must the compact disc contain in order to be able to play two hours of music?

38. In Fig. 7-42(c), quantization noise occurs due to the use of 4-bit samples to represent nine signal values. The first sample, at 0, is exact, but the next few are not. What is the percent error for the samples at 1/32, 2/32, and 3/32 of the period?

39. Could a psychoacoustic model be used to reduce the bandwidth needed for Internet telephony? If so, what conditions, if any, would have to be met to make it work? If not, why not?

40. An audio streaming server has a one-way ''distance'' of 100 msec to a media player. It outputs at 1 Mbps. If the media player has a 2-MB buffer, what can you say about the position of the low-water mark and the high-water mark?

41. Does voice over IP have the same problems with firewalls that streaming audio does? Discuss your answer.

42. What is the bit rate for transmitting uncompressed 1200 × 800 pixel color frames with 16 bits/pixel at 50 frames/sec?

43. Can a 1-bit error in an MPEG frame affect more than the frame in which the error occurs? Explain your answer.

44. Consider a 50,000-customer video server, where each customer watches three movies per month. Two-thirds of the movies are served at 9 P.M. How many movies does the server have to transmit at once during this time period? If each movie requires 6 Mbps, how many OC-12 connections does the server need to the network?

45. Suppose that Zipf's law holds for accesses to a 10,000-movie video server. If the server holds the most popular 1000 movies in memory and the remaining 9000 on disk, give an expression for the fraction of all references that will be to memory. Write a little program to evaluate this expression numerically.

46. Some cybersquatters have registered domain names that are misspellings of common corporate sites, for example, www.microsfot.com. Make a list of at least five such do-mains.

47. Numerous people have registered DNS names that consist of www.word.com, where word is a common word. For each of the following categories, list five such Web sites and briefly summarize what it is (e.g., www.stomach.com belongs to a gastroenterologist on Long Island). Here is the list of categories: animals, foods, household objects, and body parts. For the last category, please stick to body parts above the waist.

48. Rewrite the server of Fig. 6-6 as a true Web server using the GET command for HTTP 1.1. It should also accept the Host message. The server should maintain a cache of files recently fetched from the disk and serve requests from the cache when possible.