Computer Networks 4th Ed Andrew S. Tanenbaum [Electronic resources] نسخه متنی

7.5 Summary

Naming in the Internet uses a hierarchical scheme called the domain name system (DNS). At the top level are the well-known generic domains, including com and edu as well as about 200 country domains. DNS is implemented as a distributed database system with servers all over the world. DNS holds records with IP addresses, mail exchanges, and other information. By querying a DNS server, a process can map an Internet domain name onto the IP address used to communicate with that domain.

E-mail is one of the two killer apps for the Internet. Everyone from small children to grandparents now use it. Most e-mail systems in the world use the mail system now defined in RFCs 2821 and 2822. Messages sent in this system use system ASCII headers to define message properties. Many kinds of content can be sent using MIME. Messages are sent using SMTP, which works by making a TCP connection from the source host to the destination host and directly delivering the e-mail over the TCP connection.

The other killer app for the Internet is the World Wide Web. The Web is a system for linking hypertext documents. Originally, each document was a page written in HTML with hyperlinks to other documents. Nowadays, XML is gradually starting to take over from HTML. Also, a large amount of content is dynamically generated, using server-side scripts (PHP, JSP, and ASP), as well as clientside scripts (notably JavaScript). A browser can display a document by establishing a TCP connection to its server, asking for the document, and then closing the connection. These request messages contain a variety of headers for providing additional information. Caching, replication, and content delivery networks are widely used to enhance Web performance.

The wireless Web is just getting started. The first systems are WAP and i-mode, each with small screens and limited bandwidth, but the next generation will be more powerful.

Multimedia is also a rising star in the networking firmament. It allows audio and video to be digitized and transported electronically for display. Audio requires less bandwidth, so it is further along. Streaming audio, Internet radio, and voice over IP are a reality now, with new applications coming along all the time. Video on demand is an up-and-coming area in which there is great interest. Finally, the MBone is an experimental, worldwide digital live television service sent over the Internet.

Problems

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

According to the information given in Fig. 7-3, is little-sister.cs.vu.nl on a class A, B, or C network?

In Fig. 7-3, there is no period after rowboat? Why not?

Make a guess about what the smiley :-X (sometimes written as :-#) might mean.

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?

In addition to being subject to loss, UDP packets have a maximum length, potentially as low as 576 bytes. What happens when a DNS name to be looked up exceeds this length? Can it be sent in two packets?

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

Can a computer have two DNS names that fall in different top-level domains? If so, give a plausible example. If not, explain why not.

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.

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

Electronic mail systems need directories so people's e-mail 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.

A person's e-mail address is his or her login name @ the name of a DNS domain with an MX record. Login names can be first names, last names, initials, and all kinds of other names. Suppose that a large company decided too much e-mail was getting lost because people did not know the login name of the recipient. Is there a way for them to fix this problem without changing DNS? If so, give a proposal and explain how it works. If not, explain why it is impossible.

A binary file is 3072 bytes long. How long will it be if encoded using base64 encoding, with a CR+LF pair inserted after every 80 bytes sent and at the end?

Consider the quoted-printable MIME encoding scheme. Mention a problem not discussed in the text and propose a solution.

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

Suppose that you want to send an MP3 file to a friend, but your friend's ISP limits the amount of incoming mail to 1 MB and the MP3 file is 4 MB. Is there a way to handle this situation by using RFC 822 and MIME?

Suppose that someone sets up a vacation daemon and then sends a message just before logging out. Unfortunately, the recipient has been on vacation for a week and also has a vacation daemon in place. What happens next? Will canned replies go back and forth until somebody returns?

In any standard, such as RFC 822, 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.

Is the vacation daemon 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.

POP3 allows users to fetch and download e-mail from a remote mailbox. Does this mean that the internal format of mailboxes has to be standardized so any POP3 program on the client side can read the mailbox on any mail server? Discuss your answer.

From an ISP's point of view, POP3 and IMAP differ in an important way. POP3 users generally empty their mailboxes every day. IMAP users keep their mail on the server indefinitely. Imagine that you were called in to advise an ISP on which protocol it should support. What considerations would you bring up?

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?

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

When are external viewers needed? How does a browser know which one to use?

Is it possible that when a user clicks on a link with Netscape, 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.

A multithreaded Web server is organized as shown in Fig. 7-21. It takes 500 µsec to accept a request and check the cache. Half the time the file is found in the cache and returned immediately. The other half of the time the module has to block for 9 msec while its disk request is queued and processed. How many modules should the server have to keep the CPU busy all the time (assuming the disk is not a bottleneck)?

The standard http URL assumes that the Web server is listening on port 80. However, it is possible for a Web server to listen to some other port. Devise a reasonable syntax for a URL accessing a file on a nonstandard port.

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/indexl. How does the browser know whether the name following the scheme is a DNS name or an IP address?

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?

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.

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?

In Fig. 7-26, the ALT parameter is set in the <img> tag. Under what conditions does the browser use it, and how?

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

Show the <a> tag that is needed to make the string ''ACM'' be a hyperlink to http://www.acm.org.

Design a form for a new company, Interburger, that allows hamburgers to be ordered via the Internet. The form should include the customer's name, address, and city, as well as a choice of size (either gigantic or immense) and a cheese option. The burgers are to be paid for in cash upon delivery, so no credit card information is needed.

Design a form that requests the user to type in two numbers. When the user clicks on the submit button, the server returns their sum. Write the server side as a PHP script.

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

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).

An HTML page is as follows:

<html> <body> 
<a href="> 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.

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.

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 Florida election in 2000? Why or why not?

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?

Under what conditions is using a CDN a bad idea?

Wireless Web terminals have low bandwidth, which makes efficient coding important. Devise a scheme for transmitting English text efficiently over a wireless link to a WAP device. You may assume that the terminal has several megabytes of ROM and a moderately powerful CPU. Hint: think about how you transmit Japanese, in which each symbol is a word.

A compact disc holds 650 MB of data. Is compression used for audio CDs? Explain your reasoning.

In Fig. 7-57(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?

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?

An audio streaming server has a one-way distance of 50 msec with a media player. It outputs at 1 Mbps. If the media player has a 1-MB buffer, what can you say about the position of the low-water mark and the high-water mark?

The interleaving algorithm of Fig. 7-60 has the advantage of being able to survive an occasional lost packet without introducing a gap in the playback. However, when used for Internet telephony, it also has a small disadvantage. What is it?

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

What is the bit rate for transmitting uncompressed 800 x 600 pixel color frames with 8 bits/pixel at 40 frames/sec?

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

Consider a 100,000-customer video server, where each customer watches two movies per month. Half the movies are served at 8 P.M. How many movies does the server have to transmit at once during this time period? If each movie requires 4 Mbps, how many OC-12 connections does the server need to the network?

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

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 domains.

Numerous people have registered DNS names that consist of a www.word.com where word is a common word. For each of the following categories, list five Web sites and briefly summarize what it is (e.g., www.stomach.com is 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.

Design some emoji of your own using a 12 x 12 bit map. Include boyfriend, girlfriend, professor, and politician.

Write a POP3 server that accepts the following commands: USER, PASS, LIST, RETR, DELE, and QUIT.

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.