Exercise 4 (9.-14.10.2001)

1. Stop and wait -protocol.
   1. Why is it enough to have one bit sequence number (only using numbers 0 and 1) in stop and wait protocol? Simulate the protocol and based on that reach conglusion that no more numbers are needed to distinquish the messages sent.
   2. Is it also necessary to add numbers to ACK and NAK acknowledgements? Simulate the protocol again to find out situations where unnumbered ACK or NAK can cause problems.

2. UDP and TCP use one's complement in their checksums.
   1. Calculate the UDP checksum for the following three 8 bit words: 0101 0101, 0111 0000, 1100 1100.
   2. How does the receiver detect that an error has occured in the transfer?
   3. Is it possible that the receiver does not detect one bit error (ie, the change of one bit from on to zero or vice verse)? What about two or more bit errors?

3. The operation of sliding window protocols
   Suppose an error burst destroys frame D(n+1), the acknowledgement of the previous frame and the following acknowlegdement. Simulate, in the situation, the operation of
   1. the Go-Back -N protocol,
   2. the selective repeat protocol.
   3. Would, in this situation, the negative acknowledgement NAK be useful for either of these protocols? Could there be any disadvantages when using NAK?
   4. Is it necessary to have a timer in both these protocols?

4. 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?

5. Slow start
   1. Why is slow start used? What are the advantages? Can it have disadvantages? If yes, then in what situations?
   2. Consider the effect of using slow start on a line with 100-msec round-trip time and no congestion, so the acknowledgements arrive in time. The receive window is 18 KB and the maximum segment size is 2 KB. The threshold value at first is 30 KB. How long does it take before the first full window can be sent?
   3. 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?

6. An email application is sending a rather large email message containing pictures into the email application of the receiver using TCP protocol. Suppose that there are no errors or other problems in the transmission. Explain what happens in the TCP layer, what TCP segments are sent, in which order and when between the mail systems of the receiver and the sender.