mid2_solution[1]

mid2_solution[1] - Introduction to Computer Networks 1....

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Introduction to Computer Networks Midterm-2, Fall 2009 1. (5%) What is the purpose for using timers in reliable data transfer protocols? Explain. To handle losses in the channel. If the ACK for a transmitted packet is not received within the duration of the timer for the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is retransmitted. 2. (5%) What is the purpose for using sequence numbers in reliable data transfer protocols? Explain. Sequence numbers are required for a receiver to find out whether an arriving packet contains new data or is a retransmission. 3. (5%) Suppose the UDP receiver computes the Internet checksum for the received UDP segment and finds that it matches the value carried in the checksum field. Can the receiver be absolutely certain that no bit error has occurred? Explain. No, the receiver cannot be absolutely certain that no bit error has occurred. This is because of the manner in which the checksum for the packet is calculated. If the corresponding bits (that would be added together) of two 16-bit words in the packet were 0 and 1 then even if these get flipped to 1 and 0 respectively, the sum still remains the same. Hence, the 1s complement the receiver calculates will also be the same. This means the checksum will verify even if there was transmission error. 4.
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This note was uploaded on 02/17/2010 for the course ACS ACS-3911 taught by Professor Chen during the Spring '10 term at Windsor.

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mid2_solution[1] - Introduction to Computer Networks 1....

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