Exercises 6 (26.4.)

1. Distance vetor routing
   For routing the distance vector algorithm is used. Supposing each node in the network below, first knows only the distances to its neighbors. What is the content of routing table of the node E after the node E has changed routing information with its neighbors?

                 1
        A --------------- B
        |               / |
        |              /  |
        |           5 /   |
        |            /    | 
        |           /     |
        |2         /      |15
        |        E        |
        |      /   \      |
        |     /     \     |
        |    /       \    |
        |   / 2    10 \   |
        |  /           \  |
        | /             \ |
        C --------------- D
                1
    

2. HELO message in TCP segment and IP packet
   In the SMTP connection the first message sent is the HELO message.
   1. What kind of TCP segment carries the HELO 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 HELO message?
   2. How are these TCP segments transferred in IP packets? What information do the header fields of these IP packets contain?

3. CRC
   CRC is used for error checking and the generator polynome is X**3 +1.
   1. Data to be sent is 1010 0111. Show the frame with CRC error checking bit added.
   2. CRC checking is used and the generator polynomial is X**3 +1. A frame 110101011 is received. Is it corrupted?

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? (This has been discussed already in the problem 4 of Exercise 3).
   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 Data Communication I course is to provide all cum laude students in Computer Science the basic knowledge in data communication in a 2 cu 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 (Friday 4.5. 9-12 ).