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Unformatted text preview: ) If the CRC is correct, the receiver sends a link-layer ACK to the sender. The ACK has negligible size and reaches the sender instantaneously. The sender and receiver are near each other, so you can ignore the propagation delay. The bit rate is R = 54 Megabits/s, the smallest packet size is 540 bits, and the largest packet size is 5,400 bits. What is the maximum processing time Tp that ensures that the protocol will achieve a throughput of at least 50% of the bit rate of the link in the absence of packet and ACK losses, for any packet size? 3. Suppose the sender in a reliable transport protocol uses an EWMA filter to estimate the smoothed round trip time, srtt, every time it gets an ACK with an RTT sample r. srtt → α · r +(1 − α)· srtt We would like every packet in a window to contribute a weight of at least 1% to the srtt calculation. As the window size increases, should α increase, decrease, or remain the same, to achieve this goal? (You should be able to answer this question without writing any equations.) 4. TCP computes an average round-trip time (RTT) for the connection using an EWMA estimator, as in the previous problem. Suppose that at time 0, the initial estimate, srtt, is equal to the true value, r0 . Suppose that immediately after this time, the RTT for the connection increases to a value R and remains at that value for the remainder of the connection. You may assume that R >> r0 . Suppose that the TCP retransmission timeout value at step n, RTO(n), is set to β · srtt. Calculate the number of RTT samples before we can be sure that there will be no spurious retransmissions. Old TCP implementations used to have β = 2 and α = 1/8. How many samples does this correspond to before spurious retransmissions are avoided, for this problem? (As explained in Section 20.3, TCP now uses the mean linear deviation as its RTO formula. Originally, TCP didn’t incorporate the linear deviation in its RTO formula.) 5. Consider a sliding window protocol between a sender and a receiver. The receiver should deliver packets reliably and in order to its application. The sender correctly maintains the following state variables: unacked pkts – the buffer of unacknowledged packets first unacked – the lowest unacked sequence number (undefined if all packets have been acked) last unacked – the highest unacked sequence number (undefined if all packets have been acked) last sent – the highest sequence number sent so far (whether acknowledged or not) 18 CHAPTER 20. RELIABLE DATA TRANSPORT PROTOCOLS If the receiver gets a packet that is strictly larger than the next one in sequence, it adds the packet to a buffer if not already present. We want to ensure that the size of this buffer of packets awaiting delivery never exceeds a value W ≥ 0. Write down the check(s) that the sender should perform before sending a new packet in terms of the variables mentioned above that ensure the desired property. 6. Alyssa P. Hacker measures that the network path between two computers has a round-trip time (RTT) of 100 milliseconds. The queueing delay is negligible. The speed of the bottleneck link between them is 1 Mbyte/s. Alyssa implements the reliable sliding window protocol studied in 6.02 and runs it between these two computers. The packet size is fixed at 1000 bytes (you can ignore the size of the acknowledgments). There is no other traffic. (a) Alyssa sets the window size to 10 packets. What is the resulting maximum utilization of the bottleneck link? Explain your answer. (b) Alyssa’s implementation of a sliding window protocol uses an 8-bit field for the sequence number in each packet. Assuming that the RTT remains the same, what is the smallest value of the bottleneck link bandwidth (in Mbytes/s) that will cause the protocol to stop working correctly when packet losses occur? Assume that the definition of a window in her protocol is the difference between the last transmitted sequence number and the last in-sequence ACK. (c) Suppose the window size is 10 packets and that the value of the sender’s retransmission timeout is 1 second. A data packet gets lost before it reaches the receiver. The protocol continues and no other packets or acks are lost. The receiver wants to deliver data to the application in order. What is the maximum size, in packets, that the buffer at the receiver can grow to in the sliding window protocol? Answer this question for the two different definitions of a “window” below. i. When the window is the maximum difference between the last transmitted packet and the last in-sequence ACK received at the sender: ii. When the window is the maximum number of unacknowledged packets at the sender: 7. In the reliable transport protocols we studied, the receiver sends an acknowledgment (ACK) saying “I got k” whenever it receives a packet with sequence number k. Ben Bitdiddle invents a different method using cumulative ACKs: whenever the receiver gets a packet, whether in order or no...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

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