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hwk2-Su-10 - CNT5106C Computer Networks Summer 2010...

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CNT5106C Computer Networks, Summer 2010 Instructor: Prof. Ahmed Helmy Homework #2 On the Transport Layer, Congestion Control and TCP [Date Assigned: Jun 17 th , 2010. Due Date: Jun 23 rd ] Q1. (8 points: 4 points each) I. Consider a reliable data transfer protocol that uses only negative acknowledgements. Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? II. Now suppose the sender has a lot of data to send and the end-to-end connection experiences few losses. In this second case, would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? Q2. (12 points: 4 points each case) Consider a transport protocol that uses connection- oriented network service. Suppose that the transport protocol uses a credit allocation flow control scheme (such as TCP), and the network protocol uses a sliding-window scheme. What relationship should there exist between the dynamic window of the transport protocol and the fixed window of the network protocol, such that the transport protocol is effective? Consider the following cases: I. There is one-to-one relationship between a transport layer connection and network layer connection. II. A transport layer connection may be split among multiple network layer connections. III. Multiple transport layer connections may be multiplexed in one network layer connection. Q3. (14 points: 6 + 8) I. Describe (with the aid of a graph) the different phases of network load/overload outlining the degrees of congestion with increase of load. Indicate the point of congestion collapse and explain why it occurs. (6 points) II. Where does TCP operate on that graph? Explain for the various phases of TCP; slow start, congestion avoidance (due to timeout), fast retransmit-fast recovery triggered by duplicate ACKs. (8 points) Q4.
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