CS/ECE 438: Communication Networks for Computers Final Exam Spring 2007 Study Guide Short Answers The final examination will be held 8:00 to 11:00 p.m. on Tuesday, May 8th. The exam will begin at 8:00 p.m. sharp, so please arrive before 8:00 to settle in before the test begins. No extensions will be granted to those who are late, nor will any non-emergency excuse for absence be accepted after May 1st. You may not consult any materials during the exam: no textbooks, no crib sheets, no calculator, etc. The final exam will contain parameterized problems and short-answer questions. The parameterized problems are close to those found here and in the homework. The short-answer questions require you to explain or comment on a topic relevant to the course in twenty words or less. As on the midterm, please keep your short answers under 25 words. This study guide contains one hundred and twenty-five short answer questions. Each short-answer question on the exam will differ from one of the short-answer questions in this study guide in the wording used. On the exam, you must show all work and reasoning, writing both work and solution legibly, and should box all answers. If the course staff cannot read a solution, no credit will be given. Short-Answer Questions 1. Explain the exposed terminal problem and how it is solved. 2. Name the OSI layer or layers in which medium access control (MAC) is addressed and state whether MAC is typically handled in hardware, in software, or in both in the Internet architecture. 3. Explain the effect of layering on end-to-end bandwidth. 4. Name the OSI layer or layers in which framing is addressed and state whether framing is typically handled in hardware, in software, or in both in the Internet architecture. 5. Show that the final parity check in a horizontal and vertical parity check code, if taken as the modulo 2 sum of all data bits, is equal to the modulo 2 sum of the horizontal parity checks and also equal to the modulo 2 sum of the vertical parity checks. 6. For a small data packet, which is more relevant, bandwidth or latency? Explain. 7. Describe the benefits of error correction over error detection. 8. Give two arguments against IP reassembly in routers. 9. List three components that contribute to end-to-end latency. 10. The sequence number field in the TCP header is 32 bits long, which is big enough to cover over 4 billion bytes of data. Even if this many bytes were never transferred over a single connection, why might the sequence number still wrap around from 232 — 1 to 0? 11. Explain the circumstances that give rise to the count-to-infinity problem. 12. Assuming SWS=3, RWS=1, and independent timeouts per packet, construct a minimal timeline such that timeouts for packets in the send window are neither monotonically increasing nor monotonically decreasing. 13. Name the OSI layer or layers in which error detection is addressed and state whether error detection is typically handled in hardware, in software, or in both in the Internet architecture. 14. For TCP, why does the maximum packet lifetime, T, have to be large enough to ensure that not only the packet, but also its acknowledgements, have vanished? 15. Caching is an important mechanism whereby frequently used information is replicated in order to provide fast access at different physical locations. Name three instances of caching discussed in the course that arise in the context of standard Internet operation. 16. At what OSI layer do Internet routers typically operate? 17. What is the difference between a shared tree and a source-based tree in the context of multicast routing? 18. Why are differentiated services approaches to supporting QoS considered more scalable than integrated services approaches? 19. Explain how a receiver detects the end of a frame with length-based framing. 20. How can one address variability of bandwidth bottlenecks to receivers of a multicast group (i.e. source can send 10Mbps, one member can receive entire 10Mbps, but other member can only receive 10Kbps)? 21. What Hamming distance is necessary for n-bit error detection? n-bit error correction?22. If SWS=RWS=5 in a sliding window protocol, if packet numbers do not wrap around, if packets do not arrive out of order, and if the next frame expected (NFE) is currently 17, why can't the receiver next receive a packet with sequence number 10? 23. Explain how a receiver detects the end of a frame with sentinel-based framing. 24. Why will UDP require a checksum with IPv6? 25. Why might a host using an IEEE 802.11 access point (AP) continue scanning for other access points? 26. Explain the meaning of the error, "address already in use," sometimes returned from calls to bind. 27. Name the four components that uniquely specify a TCP connection and the length of each component in bits. 28. What does TCP use in addition to an estimate of RTT to calculate timeouts for adaptive retransmission? 29. Suppose a dynamic routing algorithm is employed to try to make routing tables correspond to least cost paths. What types of routing metrics are prone to producing load oscillations? 30. Explain in words (no equations) what the memoryless property of a random, exponentially distributed lifetime is. 31. What does UDP provide in addition to those services contained in IP? 32. What is the difference between congestion avoidance and congestion control? 33. How does TCP guarantee that new connections do not receive segments from previous incarnations of the connection? 34. Describe the responsibilities of hosts and routers using DECbit to avoid congestion. 35. Give an argument why the leaky bucket algorithm should allow just one packet per tick, independent of how large the packet is. 36. What is the purpose of the protocol field in the IPv4 header? 37. List five services demanded by many applications but not provided by IP (nor typically provided by user-level code). 38. Explain the fundamental conflict between tolerating burstiness and controlling network congestion. 39. Why does TCP begin by multiplicatively increasing its congestion window? What is "slow" about this approach? 40. Having ARP table entries time out after 10 -15 minutes is an attempt at a reasonable compromise. Describe the problems that can occur if the timeout value is too small or too large. 41. Give an example of scheduling discipline
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