课程名称︰高等计算机网路 课程性质︰系选修 课程教师︰逄爱君 开课学院：电资学院 开课系所︰资讯所 考试日期（年月日）︰2010/11/10 考试时限（分钟）：180 是否需发放奖励金：是 （如未明确表示，则不予发放） 1. Please elaborate on the relationship among SNR, BER and PHY. (5%) 2. Hidden Terminal: (15%) A. What is "hidden terminal" problem? (5%) B. How does 802.11 MAC solve the hidden terminal problem? (10%) 3. Please comment respectively on "Indirect Routing" and "Direct Routing" for handling mobility over the Internet. (10%) 4. Multimedia Networking Application: (25%) A. What are the chalenges for multimedia applications over the Internet? (5%) B. Please describe possible solutions to overcome the challenges. (10%) C. Please describe how "adaptive playout delay" works for Internet phone. (10%) 5. Consider the leaky bucket policer that polices the average rate and burst size of a packet flow. We now want to police the peak rate, p as well. Show how the output of this leaky bucket policer can be fed into a second leaky bucket policer so that the two leaky buckets in series police the average rate, peak rate, and burst size. Be sure to give bucket size and token generation rate for the second policer. (10%) 6. Please compare the public-key and symmetric-key for network security. (5%) 7. Message Condifentiality, Message Integrity and Endpoint Authentication. (20%) A. Why do we need "message integrity" and "endpoint authentication" if we already have "message confidentiality"? (5%) B. What is "DIgital Signature"? Why do we have CA for "Digital Signature"? (10%) C. How to break the encryption fo 802.11 WEP? (5%) 8. Consider our authentication protocol ap4.0, in which Alice authenticates herself to Bob, which we saw works well (i.e., we found no flaws in it). Now suppose that while Alice is authenticating herself to Bob, Bob must authenticate himself to Alice. Given a scenario by which Trudy, pretending to be Alice, can now authenticate herself to Bob as Alice. (Hint: Consider that the sequence of operations of ap4.0, one with Trudy initialing and one with Bob initialing, can be arbitrarily interleaved. Pay particular attention to the fact that both Bob and Alice will use a nonce, and that if case is not taken, the same nonce can be used maliciously.) (10%) --

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