Problem Set 6 (Tuesday 27.2.2001)

1. Show as a diagram how TCP using both limited transmit and SACK handles the following situations.
   1. In the very beginning of the transmit the very first packet arrives gets lost and does not arrive to the receiver at all. The other packets arrive uncorrupted. 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, but 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 4 * RTT. The transmission is constrained only by the congestion window.

2. Suppose that the SACK option is not used, but instead the partial acknowlegdements of NewReno. How does the TCP handle the situation where after a succesfull transmit of one segment, three following segments will get lost. After that there are no problems in the transmission. The congestion window is 16 MSS.

3. Assume there are 100 active TCP connections passing through a router. The segmets of these connections are arriving in a randomly mixed even stream to the router. Each connection is sending IP packets in 10 ms intervals. So the router gets 100 packets in 10 ms. Suppose the router is using the normal tail drop algorithm for its buffers ie. in congestion situation it drops packets from the tail of the buffer queue.
   In a very bad congestion situation the router has to drop almost at once packets from all these 100 connections, one packet from each connection. At this moment each connection has a congestion window of 32 MSS. How much the load of the network will decrease because of this loss of packets, when
   1. retransmission is due to the retransmission timeout
   2. fast retransmission and fast recovery is used?
   3. How does the load of the network grow, it the TCP connections are recovering and increasing their sending rate in synchronization?

4. RED buffer
   1. How does a RED buffer work?
   2. How much does the load in the problem above decrease and increase, if RED buffers are used? Assume the RED buffer is dropping (or removing) approximately 2 packets from each 100 packets i.e. 2 packets in each 10 ms.

5. There are proposals for two different ways to add quality of service to Internet. Explain the most important features of these proposals. In your opinion which of these ways seems to be the better way to add quality of service to Internet? Is it really necessary to add any Quality of Service at all to Internet? Wouldn't it be enough just to put enough capacity and bandwidh into the lines?

6. Your opinions about this course.
   1. For the first time this course was lectured in this form and content. What subjects were, in your opinion, the most interesting and important ones? What subjects were unnecessary? How should the course be changed in the future? What missing subjects should be included? What items could be dropped from the course?
   2. Fill the teaching evalution form for this course (http://www.cs.helsinki.fi/kurssit/kyselyt/kevat_2001/).