CMPSCI 453 Instructor V. Arun Fall Midterm 11/07/2007

Transcription

1 CMPSCI 453 Instructor V. Arun Fall Midterm 11/07/2007 Read before starting: 1. Put down you name on the first page NOW. 2. Put down you initials on the top right corner of each page 3. This answer book has 8 pages. 4. In all answers, clearly state the units. Use standard units such as bps, Kbps, or Mbps, not KB/msec and such. 5. Use variable names before plugging in actual numbers. This way, you are likely to get more partial credit if your final answer is incorrect. 6. Be precise for explain -type questions. Giving long-winded vague answers is unlikely to get you much partial credit. You will also lose time. 7. Be a smart exam-taker. Answer questions you are confident of first. The points roughly reflect the most number of minutes you should spend on the question. 8. The space provided roughly reflects how long your answer should be. Feel free to ask the TA for rough sheets. Attach any rough sheets used. 9. Different parts of a question are mostly independent. So, if you can not do one part, don t let it bother you. Just move on to the next part. 10. If you feel a question does not provide some information necessary to solve a problem, make the most reasonable assumption. Or ask the TA. 11. There is no negative scoring in any question. 12. You have 80 minutes to score 75 points. If you need more time, request the TA. If the TA receives 10 or more requests, he will automatically extend the time by 10 minutes. Good luck! Name: Problem 1 out of 16 Problem 2 out of 24 Problem 3 out of 20 Problem 4 out of 15 Total out of 75 1

2 Problem 1 (Quickies, 16 points): For non-objective type questions below, there may be more than one correct answer. You should try to provide the best one. A. (True or false) TCP checksums always detect one flipped bit, and may sometimes but not always detect two or more flipped bits. (1) B. (True or false) A virtual circuit number globally identifies a connection in a circuit-switched network. (1) C. Google hires you as the p2p expert to reduce its YouTube bandwidth costs. Give two reasons why you might recommend BitTorrent over Kazaa. (2) D. Give a reason why you might recommend Napster over Kazaa. (1) E. UMass s OIT hires you as the p2p expert to reduce its rising bandwidth costs due the rising use of BitTorrent. How would you address their problem. (1) F. What is the relation between the maximum window size k and the size of the sequence number scheme N in (i) Go-Back-N, and (ii) Selective repeat? (2) 2

3 G. If a router s forwarding table has the following entries: [10, 0], [100, 1], [111, 2], [other, 3]. The first entry is a prefix and the second is an interface number. What is the most number of addresses that get forwarded via interface 0? (2) H. If a Web server has open 10 TCP connections, it uses a total of socket(s) and ports(s). (2) I. Upon receipt of each ack that acknowledges new data, a TCP with window W segments increases its window by in slow start and by in congestion avoidance. (2) J. A TCP flow is in slow start with a current window of 17 segments. The value of ssthresh is 64. The window in the next round is 22. How would you explain this? [Hint: No loss or timeout occurs.] (2) End of Problem 1. 3

4 Problem 2 (Earth to moon TCP, 24 points): NASA engineers have developed a new wireless technology to communicate with a space station on the moon. The wireless channel supports a bandwidth of C = 10Gbps, but has an unusually high packet corruption rate of p = 16%, i.e., 16% of packets received have irrecoverable errors. The average distance of the moon from the earth is D = 384,000Km. NASA decides to use TCP for reliable transport with an MSS of 1500B. [Note: If you don t remember the TCP throughput formula, ask the TA and he will note a 3 point penalty on the top right corner of this page.] A. What the TCP throughput T? Why might it fluctuate during different seasons? (3+2) B. Disappointed with the throughput, NASA hires you to address the problem. You tell the NASA engineers that the packet loss rate must be less than p* for TCP to achieve full utilization. What is p*? (5) C. What is the window size W required to saturate the capacity? (4) D. The engineers go back to the drawing board and fix some bugs and tell you they brought the wireless channel s packet loss rate down to 0.1%. They also tell you they can do no better. You observe that TCP still can not saturate the capacity. State briefly how might you fix TCP s utilization problem. (4) 4

5 E. Does TCP use NACKs? Does TCP use cumulative acks? Does TCP use SACKs? For each of these three mechanisms, describe in one sentence when they are useful. (3+3) End of Problem 2. 5

6 Problem 3 (NAT and Skype, 20 points): Alice wishes to make a Skype call to Bob. Both Alice and Bob are at home behind their NAT routers, A and B respectively. Refer the picture below. Both Alice and Bob maintain keep alive connections with the skype server S. When Alice makes the call, the following steps occur: 1) Alice sends S a message requesting a call to Bob, 2) S sends Carol s IP address and a port number to Alice, 3) S sends Carol s IP address and the same port number to Bob, 4) Alice sends a call setup message to Carol, 5) Bob sends a call setup message to Carol and the call has been set up at this point, 6) Bob says Hello and the corresponding voice frame is sent to Carol, 7) Carol forwards the voice frame to Alice. All communication uses UDP. Alice s IP address is and A s IP address is Bob s IP address is and B s IP address is Carol s IP address is She sends and receives all messages on port S s IP address is S sends and receives all messages on port Alice s next unused local port number at step 4 is Bob s next unused local port number at step 5 is A s next unused port number in its NAT table at step 4 is B s next unused port number in its NAT table at step 5 is In the following, a 4-tuple refers to a <source IP address, destination IP address, source port number, destination port number>. [Note: This question is easier to answer than read.] A. In step 1, what are the source and destination IP addresses in the (i) packet from Alice to A, (ii) packet from A to S? (1+1) 6

7 B. In step 4, what is the (i) 4-tuple in the packet from Alice to A, (ii) from A to Carol? (3+3) C. Just after step 4, what is the NAT table entry created at A? Assume the NAT table has two columns: the first is a 4-tuple, the second has the format <IP address, port number>. (4) D. In step 7, what is the (i) 4-tuple in the packet from Carol to A, (ii) from A to Alice? (3+3) End of Problem 3. 7

8 Problem 4 (Reliable transport FSM, 15 points): Suppose we have two network entities, A and B. B has a supply of data messages that will be sent to A according to the following conventions. When A gets a request from the layer above to get the next data (D) message from B, A must send a request (R) message to B on the A-to-B channel. Only when B receives an R message can it send a data (D) message back to A on the B- to-a channel. A should deliver exactly one copy of each D message to the layer above. R messages can be lost (but not corrupted) in the A-to-B channel; D messages, once sent, are always delivered correctly. The delay along both channels is unknown and variable. Assume no reordering occurs. Design (give an FSM description of) a protocol that incorporates the appropriate mechanisms to compensate for the loss-prone A-to-B channel and implements message passing to the layer above at entity A, as discussed above. Use only those mechanisms that are absolutely necessary. Use a pencil or a rough sheet before finalizing your answer. 8