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

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CNT5106C Computer Networks, Summer 2010 Instructor: Prof. Ahmed Helmy Homework #2 Solution On the Transport Layer, Congestion Control and TCP 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? Ans. 1. I. In a NAK only protocol, the loss of packet x is only detected by the receiver when packet x+1 is received. That is, the receivers receives x-1 and then x+1, only when x+1 is received does the receiver realize that x was missed. If there is a long delay between the transmission of x and the transmission of x+1, then it will be a long time until x can be recovered, under a NAK only protocol. II. On the other hand, if data is being sent often, then recovery under a NAK-only scheme could happen quickly. Moreover, if errors are infrequent, then NAKs are only occasionally sent (when needed), and ACK are never sent – a significant reduction in feedback in the NAK-only case over the ACK-only case. 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. Ans 2. I. If there is one-to-one relationship between network connections and transport connections, then it will do no good to grant credit at the transport level in excess of the window size at the network level. So, in order to be effective, the window size of the transport protocol should be less than that for the network protocol. II. If one transport connection is split among multiple network connections, then the credit/window of the transport protocol should be less than the sum of the network window sizes. III. If multiple transport connections are multiplexed on one network connection their aggregate credit/window should not exceed the network protocol window.
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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
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hwk2-Su-10-sol - CNT5106C Computer Networks, Summer 2010...

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