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課程名稱︰計算機網路
課程性質︰必修
課程教師：逄愛君
開課學院：電資學院
開課系所︰資工系
考試日期（年月日）︰2014.11.12
考試時限（分鐘）：
試題 :
1. (5pts) Suppose we want to send a file of 210,000 bits from Host A to Host B
over a circuit-switched network. Suppose that all links in the network use
TDM with 8 slots and have a bit rate of 1.2 Mbps. Also suppose that it
takes 600 msec to establish an end-to-end circuit before Host A can begin
to transmit the file. How long does it take to send the file?
2. (10pts) Consider a packet of length L which begins at end system A and
travels over three links to a destination end system. These three links are
connected by two packet switches. Let d_i, s_i, and R_i denote the length,
propagation speed, and the transmission rate of link i, for i = 1, 2, 3.
The packet switch delays each packet by d_proc. Assuming no queuing delays,
in terms of d_i, s_i, R_i, (i = 1, 2, 3), and L, what is the total
end-to-end delay for the packet? Suppose now the packet is 2,000 bytes,
the propagation speed on all three links is 2*10^8 m/s, the transmission
rates of all three links are 4 Mbps, the packet switch processing delay is
3 msec, the length of the first link is 7,000 km, the length of the second
link is 5,000 km, and the length of the last link is 8,000 km. For these
values, what is the end-to-end delay?
3. (10pts) True or false?
a. Processes on two different end system communicate with each other by
exchanging messages across the computer network.
b. A client server architecture achieves perfect security.
c. Socket is a hardware interface through which a process sends messages
into, and receives messages from the network.
d. No data loss is tolerated in multimedia applications such as
conversational audio/video.
e. Developing a new network application for the Internet often requires one
to decide whether to choose UDP or TCP.
4. (10pts) Suppose within your Web browser you click on a link to obtain a
Web page. The IP address for the associated URL is not cached in your local
host, so a DNS lookup is necessary to obtain the IP address. Suppose that
n DNS servers are visited before your host receives the IP address from
DNS; visiting k of them incurs an RTT of D_1 per DNS and visiting each of
the remaining incurs an RTT of D_2. Further suppose that the Web page
associated with the link contains m very small objects. Suppose the HTTP
running is non-persistent and let RTT_0 denote the RTT between the local
host and the server for each object. Assuming zero transmission time of
each object, how much time elapses from when the client clicks on the link
until the client receives all the objects?
5. (10pts) Suppose Bob joins a BitTorrent torrent, but he does not want to
upload any data to any other peers (so called free-riding).
a. Bob claims that he can receive a complete copy of the file that is
shared by the swarm. Is Bobs claim possible? Why or why not?
b. Bob further claims that he can further make his "free-riding" more
efficient by using a collection of multiple computers (with distinct
IP addresses) in the computer lab in his department. How can he do that?
6. (8pts) What is the trade-off in designing a DHT with respect to the number
of neighbors?
7. (5pts) If you would like to access the Internet at home, what factors do
you consider for your access network and physical media?
8. (10pts) Protocol layers
a. Please describe the advantages and disadvantages of protocol layering.
(5pts)
b. Please elaborate on the main functions of each protocol layer of the
Internet. (5pts)
9. (5pts) Describe how a botnet can be created, and how it can be used for a
DDoS attack.
10. (10pts) By using the following example, please describe how Web caching
can significantly reduce the delay in receiving a requested object with
low-cost installation.
http://i.imgur.com/epQrARR.png
11. (7pts) What is traffic intensity? Please elaborate on the relationship
between the traffic intensity and average queuing delay?
12. (10pts) Please describe how demultiplexing works in Transport layer, and
compare connectionless and connection-oriented demultiplexing.
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