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課程名稱︰計算機網路
課程性質︰系必修
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）︰2008.4.16
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
九十六學年度第二學期 (April 16, 2008)
計算機網路 期中考
1.(15%) Circuit Switching vs. Packet Switching
a. What is Circuit Switching? What is Packet Switching?
b. Which switching approach can support more network users?
c. Compare virtual-circuit networks and datagram networks.
2.(10%) Why do we need the layering for the network architecture?Please explain
(a) what the layering is, and
(b) the pros and cons for the layering.
3.(15%) Packet Loss and Delay
a. How do loss and delay occur in the Internet?
Link1 Link2 Link3
Host A-----------Router 1-----------------Router 2-------------Host B
D=500km D=2000km D=1000km
b. There are two routers between the source host A and the destination
host B as shown above. Suppose that the processing delay is negligible.
All packets consist of L = 200 bits. and the transmission rates of
Link 1, Link 2, and Link 3 are R = 10kbps. Also, the propagation speed
along each link is S = 2.5 x 10^8 m/s. Let ρ denote the traffic
intensity. Suppose that the queueing delay is derived as ρL/R(1-ρ).
Let ρ = 0.5 for Router 2. Find the end-to-end packet delay from
Host A to Host N.
4.(15%) Reliable Data Transfer
a. Draw the FSM for the receiver side of protocol rtd 3.0.
b. In protocol rdt 3.0 the ACK packets flowing from the receiver to
the sender do not have sequence numbers. Why is it that our ACK packets
do not require sequence numbers?
5.(10%) Within the web browser, a link is clicked to obtain a web page. Suppose
that a DNS look-up is necessary to obtain the IP address, because IP address
for the associated URL is not cached. Suppose that n DNS servers are visited
before your host receives the IP address from DNS; the successive visits
incur an RTT of RTT1, ..., RTTn. Further suppose that the Web page associated
with the link contains exactly one object, consisting of a small amount of
HTML text. Let RTT0 denote the RTT between the local host and the server
containing the object. Assuming zero transmission time of the object. how
much time elapses from when the client clicks on the link until the client
receives the object?
6.(10%) HTTP and SMTP
a. Briefly explain the differences between the GET and Conditional GET
commands?
b. Why HTTP/1.1 uses persistent TCP connection for web accessing?
c. Compare the data-access models between HTTP and SMTP
7.(10%) Consider an overlay network using query flooding. Suppose every
participating peer maintains TCP connections to at leat four distinct peers
at all times. Suppose Peer X, which has five TCP connections to other peers,
wants to leave.
a. Consider the case of a graceful departure, that is, Peer X
explicitly closes its application, thereby gracefully closing its
five TCP connections. What actions would each of the five formerly
connected peers take?
b. Suppose that Peer X abruptly disconnects from the Internet without
notifying its five neighbors that it is closing the TCP connections.
What would happen?
8.(15%) Suppose Client A initiates a Telnet session with Server S. At about the
same time. Client B also initiates a Telnet session with Server S. Provide
possible source and destination port numbers for
a. The segments sent from A to S.
b. The segments sent from B to S.
c. The segments sent from S to A.
d. The segments sent from S to B.
e. If A and B are different hosts, is it possible that the source port
number in the segments from A to S is the same as that from B to S?
f. How about if they are the same host?
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