Problems for Exercise 2 (4.2.2004)

1. Answer shortly the following questions. The answers are usually almost directly found from the course book.
   1. What protocols of the Internet protocol stack must be running in a host machine, what protocols in a router?
   2. What is meant by error control, flow control and congestion control?
   3. What is File Transfer Protocol (FTP) and how does it differ from HTTP?
   4. What do SMTP and HTTP protocols have in common? How do they differ?

2. Find out what are the different ways you can reach Internet in Finland. What kind of different access networks or ways to connect there exist? What are their transmission rates? What special equipments are needed? What are the usage costs?

3. X bits of the user's data are sent as consecutive packages to a destination k hops away. The size of a network package is p databits and h header bits (and x>>p+h). The speed of data transmission is b bps and the propagation delay is insignificant and can be ignored. What value for p minimizes the total transfer time?

4. Suppose that machines A and B are communicating using a geostatinary satellite that is situated about 36 000 kms from the earth. The transmission rate of the satellite link is 1 Gbps. The propagation speed for the bits is 200 000 km/s (about 2/3 of the speed of light).
   1. How long is the propagation delay when the machine A is sending to the machine B.
   2. How long does it take to transmit a short 1500 byte message from the machine A to the satellite?
   3. How many bits can the satellite link carry at the same time when the bits are travelling serially one after another? How long then is one bit?

5. Surfing the Web.
   1. An eager internet user, student Iitu Iivari sits in the computer science department surfing the Web and opens the URL link http://www.encyclopedia.com/articles/12910.html from the document he is reading by clicking the mouse. What happens in the application layer? Who are communicating and how? What messages are sent?
   2. Try out the HTTP protocol and see its messages with your own eyes. Make a telnet connection to eg. the WWW server telnet cis.poly.edu 80 and ask for the home page of professor Ross: GET /~ross/index.html HTTP/1.0. What happens when you ask for the page /~ross/banana.html? What happens if you use HEAD instead of GET? Can you make a telnet connection to the Computer Science Department WWW-server www.cs.helsinki.fi and GET the homepage of the Data Communications I course (GET /u/marttine/tili/tIk04/kotisivu.html HTTP/1.0)?

6. What are benefits from using cookies? Who benefit from them? What drawbacks they have? Can cookies be security risks? Is it possible to disable cookies? What results can follow from disabling? Do you know what cookies you have in your files?