Note: Problem numbers below refer to the 3rd edition of the text.. In all written assignmens, please show as muchof your work as you can. Even if you get a wrong answer, you can get partial credit if you show your work. If you make a mistake, it will also help the grader show you where you made a mistake.

Problem 1. (15 points) Consider the FTP control connection shown in Figure 2.14, in which the FTP client connects to server port 21 using FTP. Consider now the TCP segment flowing from the client to the server (which carries a client command from client to server) and the TCP ACK segment flowing from the server to the client. (i) What are the source and destination port numbers on each of these two segments? Explain your answer.

Suppose now that a second FTP client (running on the same computer as the original client) starts up an FTP connection with the same server while the first client is running. Consider again the TCP segment flowing from the second client to the server and the TCP ACK segment flowing from the server to the second client. (ii) Which port numbers in these two segments must be different from their counterparts flowing to the first client, and which port number must be identical. Explain your answer.

Problem 2. (15 points) Suppose the following three bytes of data: 10010010, 01010101, 10000001. (i) What is the checksum for these three bytes (note that although the Internet checksum is computed using 16-bit summands, you are being asked to compute this using 8-bit summands)? (ii) Show (by introducing bit errors into the three bytes given earlier) two single bit errors in the summands can be detected by the checksum. (iii) Show another example where two single bit errors in the summands can not be detected by the checksum

Problem 3.  (15 points) Consider our simple stop-and-wait protocol, rdt 3.0 and its operation in figure 3.16 in the text. Suppose that the channel can reorder packets that propagating in the channel, i.e., that if the sender sends packet x followed by packet y, that packet y can arrive before packet x. Show (using a timeline as in Figure 3.16) that rdt 3.0 operating under this assumption can result in (i) data being delivered to the recevier-side application layer twice, and (ii) a piece of data never being delivered to the recevier-side application.

Problem 4. (25 points) Problem 14, page 289.

Problem 5. (10 points) Two Quickies.

• Suppose that a web servers has 500 ongoing connections with clients. How many server-side sockets are used? How many server-side port numbers are used for incoming HTTP traffic?
• Consider a sliding window, selective repeat protocol with a window of size N packets. Suppose that the base of the window is at sequence number x (where the sequence number counts packets). (i) Show a time diagram (e.g., as in Figure 3.26) that is it possible for the sender to receive an ACK for a packet with a sequence number smaller than x. (Note: Assume that N is much smaller than the sequence number space, and don't worry about wrap-around).

Problem 6. (20 points) Page 291, problem 27.