Problem Set 5 (Tuesday 20.2.2001)

1. Something to ponder about.
   1. How is the IPv6 protocol able to find out the the maximum possible packet size in a route?
   2. What reasons there could be to prefer using ECN instead of Source Quench ICMP to inform about congestion in the Internet?

2. The TCP layer gets data to send from an application. Suppose the amount of data is 15 Kbytes and in the net it is possible to send segments of 1500 databytes. Explain alltogether what kind of segments and in which order are transmitted between the sender and the receiver in order to get the data transmission carried out. Explain also the content of the relevant fields in these segments. You can assume that the data transmission is not constrained by the receiver winfdow neither by the congestion window.

3. Show as a diagram how the 'basic' TCP (TCP with slow start, retransmission timer, fast retransmit and fast recovery) handles the following situations.
   1. In the very beginning of the transmit the very first packet arrives corrupted to the receiver. The size of the congestion window is 2 MSS.
   2. The size of the congestion window is 8 MSS. At first the sender succeeds to one segment correctly, butr after that an error burst corrups the following 3 packets.
   3. The size of the congestion window is 8 MSS. The second segment is routed to a congested router and it arrives to the receiver first after the fourth sent segment. Other segments arrive in order.
   In all these cases the round-trip-time RTT is 200 ms and value of the retransmission timer is set to 3 * RTT. The transmission is constrained only by the congestion window.

4. Show with a diagram how the different cases of the former problem will be handled with a TCP using limited transmit. In what situations limited transmit seems to be usefull?

5. Show with a diagram how the different cases of the problem 3 will be handled with a TCP using the SACK option (Selective ACK). In what situations the SACK option seems to be usefull?

6. A TCP connection is using ECN (Explicit Congestion Notification).
   1. What kind of segments the sender and the receiver must exhange when establishing the connection?
   2. The third segment sent by the sender is routed to a congested router. What happens to that segment? How do the sender and the receiver behave in this situation? The size of the congestion window is 4 segments and the receiver acknowledges each segment is receives. The one way delay is about 100 ms and the sender is sending with speed of 20 segments per second.