CS 444N ExamStanford UniversityComputer Science DepartmentCS 444N M. BakerExam Spring 2001CS 444N ExamName:_______________________________________________________________This is a closed-book examination (i.e. no assistance from textbooks, notes, other people,etc.) You have 1 hour and 15 minutes to answer as many questions as possible. Write all ofyour answers directly on this paper. Make your answers as concise as possible. (You needn’tcover every available nano-acre with writing.)Stanford University Honor Code:In accordance with both the letter and spirit of the Honor Code, I didn’t cheat on this exam.Signature:____________________________________________________________Problem Points Score1 152 153 154 155 (extra credit) 5Total 60CS 444N Exam Page 1 of 7Spring 2001Problem 1: (15 points)Without a “primary server,” Bayou provides only a partial ordering over updates in the system. A primary server is needed for total ordering of updates to a database across all hosts in the system.a) Why can’t a total ordering be provided without a primary server?b) What is the purpose of providing this total ordering rather than just a partial ordering?c) What is the disadvantage for a system like Bayou of requiring the existence of a primary server?CS 444N Exam Page 2 of 7Spring 2001Problem 2: (15 points)Two big issues for TCP in a wireless environment are 1) fluctuations and shrinkage of thewindow size, and 2) coarse-grain timeouts.a) What are the causes of each of these problems?b) Specifically, how does each problem affect the connection’s performance?c) Explicit loss notification and selective acknowledgements have been proposed to tackle these problems. What do these techniques do, and which problem or problems does each technique address?CS 444N Exam Page 3 of 7Spring 2001Problem 3: (15 points)Mobile IP was intended to work with legacy correspondent hosts and legacy applications.However, some advantages may be gained if correspondent hosts are aware of mobility incertain ways.a) If a correspondent host is able to do IP-in-IP decapsulation, it may be able to communicate more efficiently with the mobile host despite the presence of ingressfiltering. Why?b) If a correspondent host is able to do IP-in-IP encapsulation, it may be able to communicate more efficiently with the mobile host than is possible with standard Mobile IP. Why?c) If a correspondent host is able, someplace beneath the application level, to maintain a mapping between a mobile host’s local IP address and its home IP address, and receive updates to that mapping from the mobile host, it may be able to communicate more efficiently with the mobile host than is possible with standard Mobile IP. Why?CS 444N Exam Page 4 of 7Spring 2001Problem 4: (15 points)Transformational proxies have emerged as one popular way to access Internet content from "small glass" or "thin" clients. The proxy approach is motivated by the following assumptions (among others):(i) Clients are underpowered (CPU, memory) compared to their desktop counterparts. For example, in Top Gun Wingman (Fox et al.), the client-side "browser" is a much simpler program that doesn't understand HTML at all and doesn't do its own page layout -it relies on the proxy for everything. (ii) The network connecting the client to the proxy is much slower than the network connecting the proxy to the rest of the Internet. For example, one of the earliest uses of image distillation was to compensate for slow 'last mile' links.a) Suppose assumption (i) is relaxed. That is, in five years it will be realistic to havethin clients whose CPU speed and memory are comparable to those of today's desktop PC's, but wireless access is still slow. How would this affect the design of a proxy-based system for Internet access, or would it render such a system obsolete?CS 444N Exam Page 5 of 7Spring 2001b) Suppose assumption (ii) is relaxed. That is, in five years, wide-area wireless bandwidth will be comparable to cable modem today (several hundred Kbits/sec) and will be available essentially everywhere, but thin clients are still relatively underpowered compared to desktop PC's. How would this affect the design of a proxy-based system for Internet access, or would it render such a system obsolete?c) Suppose both assumptions are relaxed. Is there anything compelling left for a proxy to do, or have they become obsolete?CS 444N Exam Page 6 of 7Spring 2001Problem 5: (5 points extra credit)When a mobile host switches to a new host network, its IP address changes. At what level or levels of the system do you think such a name change should be handled (link layer, network layer, transport layer, middleware layer, application layer, user layer)?In general, how transparent or not transparent do you think name changes and other mobility issues (adaptation to bandwidth changes, etc.) should be to the application? To the user?CS 444N Exam Page 7 of 7Spring
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