Exercise 5 (17.-20. September 2000)

1. Agreement on a lunch together
   1. Alice sends Bob e-mail about going to lunch together. Bob answers and agrees to have lunch with Alice. Outside the weather is as terrible as usual and neither Bob nor Alice is willing to out, if she /he is not absolutely sure that also the other is coming. How do they continue to exchange messages?
   2. Alice and Bob can agree on the lunch also by phone. Why is it easier? Or is it easier?
   3. A variation: a bank application makes a money transfer from an account a in the bank M to an account b in the O. How can this be made to succeed? How does this differ from Alice and Bob going to have lunch? Or is there any difference at all?

2. The effeciency of TCP
   1. A TCP machine is sending windows of 65 535 bytes over a 1-Gbps channel that has a 10-msec one-way delay. What is the maximum throughput achievable? What is the line efficiency?
   2. In a network that has a maximum TPDU size of 128 bytes, a maximum TPDU lifetime of 30 sec, and an 8 bit sequence number, what is the maximum data rate per connection?

3. Slow start
   1. Consider the effect of using slow start on a line with 10-msec round-trip time and no congestion. The receive window is 24 KB and the maximum segment size is 2 KB. How long does it take before the first full window can be sent?
   2. After having sent the full reveiver window of segments, the sender does not receive the acknowledgement in time, and the retransmission timer times out. How does the sender now continue to send after that?
   3. If, instead of the missing acknowledgement, the sender would have received several duplicate acknowledgements. How would it have continued the sending in this case?

4. An eager Internet-user, student Ida Iwanhoe is surfing the net at the University of Helsinki and opens, with a mouse click, from the document she is reading an URL http://www.encyclopedia.com/articles/12910.html. What all things happen after that?
   1. What do the different parts of the URL mean and what they are used for?
   2. What happens at the application layer? Who are communicating and how? What messages are sent? What transport layes services are used to send the messages?
   3. How is the network layer able to carry the segments of the transport layer to their receiver trough many different networks? What different routing functions are needed?

5. A student T. Terävä from the university of Helsinki is sending email to his friend M. Smart at the University of Berkeley, California. He starts his mail application, writes a short message "Hello! How are you?" addressed to M.Smart@cs.berkeley.edu and sends it. What happens to the message after this?
   1. What does the mail application on the sender side do to the message? How does it know where the message is going to and how and in what form does it give it to the transport layer?
   2. What does the transport entity on the sender side do to the message? What does it do before it gives the message to the network layer? It what form does it give the message to the network layer?
   3. What does the network entity on the sender side do to the message? What and in what form does it give to the Ethernet MAC layer?
   4. How does the Ethernet MAC layer of the sender side transmit the message into the LAN
   5. How does the message proceed in the Internet and finally reach the LAN of the reveiver and the mail application of the receiver?

6. Evaluation of teaching and other feedback from the course
   1. This Data Communication course was now lectured, for the first time, as a 2 cu compulsatory course for all cum laude students in Computer Science. 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. The Data Communication II course that will be lectured next spring will cover some areas of data communication more deeply. What things, already in this course or other, should be, in your opinion, included into that course? What subjects or things are of interest to you?
   3. Fill in (now or later but remember that teaching evalution for this course will be closed 30.11.2000) the class feedback form for Data Communication I course (http://www.cs.helsinki.fi/kurssit/kyselyt/syksy_2000/).