Exercises 6

1. A HTTP Request message carried in a TCP segment carried in a IP packet carried in a link layer frame
   1. What kind of TCP segment carries the HTTP Request message in the TCP connection? What information is included in the header fields of this TCP segment? What TCP segments have already been sent before this TCP segment carrying the HTTP Request message?
   2. In what kind of IP packets the request message is transferred? What information do the header fields of these IP packets contain?
   3. What kind of link layer frame carries the request message in an Ethernet? What information do the header fields of the frame contain?
   It is enough to describe the header field contents pretty generally.

2. LANs
   The router R2 in the picture receives from the router R1 a datagram that is addressed to the host A in its own Ethernet LAN. The datagram is the HTTP reply message to the the HTTP request message host A has sent earlier. The sender is a host B somewhere in the Internet. The LAN of the router R2 contains of several LANs that are connected by switches and hubs.
   1. Draw a picture that shows all the different protocol headers and the data included in the packet received by R2.
   2. How does the packet look when R2 sends it to the switch? Draw a picture that especially shows the addresses used in different layers.
   3. How does the switch connecting several LANs know to which LAN it has to forward the frame?
   4. What does the hub do when it recieves the frame?
   5. Where and why it migth be necessary to use the ARP protocol? How it is used and which devices might use it?

3. CRC
   CRC is used for error checking and the generator polynome is X**3 +x +1.
   1. Data to be sent is 1100 1001. Show the frame with CRC error checking bit added.
   2. A frame 110101001 is received. Is it corrupted? Give reasons.

4. Ethernet
   1. Show how the stations A, B and C using the CSMA/CD protocol, handle a situation where
      - station A is sending,
      - during the sending of A:s frame stations B and C want to start sending.
      Explain their function until all the stations have succeeded to send their frame without collisions.
   2. Suppose that just after A has stopped its sending the station D also wants to send its frame. Is it possible that station D can succeed sending its frame before stations B and C? If yes, then show how.

5. E-mail transmission in the Internet
   Student T. Terävä from the University of Helsinki sends email to her friend M. Smart to the University of Berkeley in California. She starts a mail program in her PC, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it. What happens to the message after that? (Notice that many parts of this problem has been discussed already in the previous exercises. So it is not necessary to go through them very thoroughly, just to get the general picture.)
   1. What does the mail system of the sender do to the message? How does the mail server know where the message is going to and in which form does it deliver the message to the TCP transport layer?
   2. What does the sender side TCP layer do to the message? What does the TCP layer do before it gives the message to IP layer? How and it what form does it give the message to the network layer?
   3. What does the sender side IP layer do to the message? How and what does the network layer give to the MAC layer of the Ethernet LAN?
   4. How does the sender side MAC layer send the message to the Ethernet LAN?
   5. How does the message proceed in the Internet and finally arrive in the LAN of the receiver and to the mail system of the receiver?

6. Evaluation of teaching and other feedback from the course
   1. The aim of this Introduction to Data Communication course is to provide all major students in Computer Science the basic knowledge in data communication in a 4 cr course.
      Do you think that the course and its content fullfills the requirements for that kind of course? What part of the course material, in your opinion, is unnecessary and should be dropped out? What things should be covered more thoroughly?
   2. Did you, in your opinion, get from the course a clear enough and wide enough picture of the possibilities and problems of data communications? What subjects were too difficult and complicated? What subjects were too simple?
   3. Fill in the class feedback form for the course Introduction to Data Communication just now or after the course exam.