Homework 3 Solution

Reference textbook: Computer Networking: A Top-Down Approach, 7th Edition, by James F.
Kurose and Keith W. Ross, published by Addison-Wesley, 2017, ISBN 0-13-359414-9.

All homework assignments must be completed by each student individually. Any copying of someone else's work, or misrepresentation of other work as your own, will be grounds for failing this assignment or the course.

Penalty for late work is 20 points per day late.

There are 6 questions; make sure you answer all these questions.

    1. In protocol rdt3.0, the ACK packets flowing from the receiver to the sender do not have sequence numbers (although they do have an ACK field that contains the sequence number of the packet they are acknowledging). Why is it that our ACK packets do not require sequence numbers?

    2. Suppose that the seven measured SampleRTT values (see Section 3.5.3) are 85 ms, 130 ms, 108 ms, 72 ms, 142 ms, 64 ms, and 153 ms. Compute the EstimatedRTT after each of these SampleRTT values is obtained, using a value of α = 0.2 and assuming that the value of EstimatedRTT was 110 ms just before the first of these seven samples were obtained. Compute also the DevRTT after each sample is obtained, assuming a value of β = 0.25 and assuming the value of DevRTT was 10 ms just before the first of these seven samples was obtained. Last, compute the TCP TimeoutInterval after each of these samples is obtained.

    3. Host A and B are communicating over a TCP connection, and Host B has already received all bytes up through byte 256. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 90 and 140 bytes of data, respectively. In the first segment, the sequence number is 257, the port number is 3120, and the destination port number is 5470. Host B send an acknowledgement whenever it receives a segment from Host A.

        a. In the second segment sent from Host A to Host B, what are the sequence number, source port number, and destination port number?

        b. If the second segment arrives before the first segment, in the acknowledgement of the first arriving segment, what is the acknowledgement number?

        c. If the first segment arrives before the second segment, in the acknowledgement of the first arriving segment, what are the acknowledgement number, the source port number, and the destination port number?




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        d. Suppose the two segments sent by A arrive in order at B. The first acknowledgement is lost and the second acknowledgement arrives after the first timeout interval. Draw a timing diagram, showing these segments and all other segments and acknowledgements sent. (Assume there is no additional packet loss.) For each segment in your figure, provide the sequence number and the number of bytes of data; for each acknowledgement that you add, provide the acknowledgement number.

    4. Consider the GBN and SR protocols. Suppose the sequence number space is of size X. What is the largest allowable sender window that will avoid the occurrence of problems such as that in Figure 3.27 in the above textbook for each of these protocols?

    5. Consider a TCP connection has an initial Threshold of 32 kB and a Maximum Segment Size (MSS) of 6 kB. The receiver advertised window is 40 kB. Suppose all transmission attempts are successful except for a timeout at transmission number 6 and a triple duplicate ACK received (for the same previously transmitted data) on the number 12 transmission. The first transmission attempt is number 0. Find the size of the sender’s congestion window for the first 18 transmission attempts (number 0-17) assuming the sender’s TCP implementation is using the slow-start congestion control scheme.

    6. Compare GBN, SR and TCP (no delayed ACK). Assume that the timeout values for all three protocols are sufficiently long such that 7 consecutive data segments and the corresponding ACKs can be received (if not lost in the channel) by the receiving host (Host B) and the sending host (Host A) respectively. Suppose Host A sends 7 data segments to Host B, and the third segment (sent from A) is lost. In the end, all 7 data segments have been correctly received by Host B.

            a. How many segments has Host A sent in total and how many ACKs has Host B sent in total? What are their sequence numbers? Answer this question for all three protocols.

            b. If the timeout values for all three protocols are much longer than 7 RTT, then which protocol successfully delivers all 7 data segments in the shortest time interval?























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