Exercise 11  (26. -30.4.)  (Tanenbaum pp. 502-573)
 

1. Answer shortly the following questions ("review type" problem: answers are usually  found  in
     lecture notes or in Tanenbaum's book).
     a) What happens when two users try to connect to each other at the same time?
         What happens  if both  sides  disconnect at the same time?
     b) Is it reasonable to use in transport layer the same kind of buffering as in the data link layer?
         If not, what kind of buffering should be used?
     c) How does  transport layer recover from disconnection  caused   by  the crash of  the machine
         supporting one communicating  transport entity?
     d) What is the silly window syndrome? How it can be prevented?
     e) Where is  the Nagles Algorithm needed and how does  the algorithm   work?
      f) How does  the TCP-protocol handle congestion control?
      g) What does indirect TCP mean and what problem it tries to solve?
      h) Why does UDP exist? Would it  not be enough to just  let user processes send  raw IP packets?
      i) What protocols exist in the AAL Layer? How well do these protocols carry out their tasks.

2. Discuss the advantages and disadvantages of credit versus sliding window protocols.  (Credit protocol = the receiver informs the sender how many messages  the sender is allowed to send.)

3. a) 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?
     b) 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?

4. a)  What is the payload size of the maximum length message that fits in a single AAL 3/4.
     b) When a 1024 byte message is sent with AAL 3/4, what is the efficiency obtained? In other
          words, what fraction of the bits transmitted are useful data bits? Repeat the problem for AAL 5.

5.  Explain what happens when a message is transmitted  from an application process A to another
     application process B. The application processes are situated in hosts of  different LANs  in  the
     same Internet area. Go through all the layers of the TCP/IP protocol from the application program
     to the LAN.  Essential concerns : who does what,  from where it gets  all necessary information
     (OSPF protocol can be skipped), what information unit is dealt with in each layer.

6.  a)  The lecturer thinks that the  course could  be further improved. What is your opinion of the
          following issues:
         -  what part of the material  was most unnecessary
          - what subjects  should  have been treated more from the point of view of
                  -  an application programmer (assumed background Rinnakkaisohjelmistot
                     (Concurrent Systems)  course)
                  -  a user of data communication (mobile, ATM, wireless...) ?
      b)  Fill in  (now or later) the class feedback form for Data Communication course.
 

This is the last problem set for the Data Communication course. The  2. exam is on Friday 7.5.1999, Porthania I.