课程名称︰计算机网路 课程性质︰系必修 课程教师︰逄爱君 开课学院：电资学院 开课系所︰资工系 考试日期（年月日）︰2008.4.16 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） 试题 : 九十六学年度第二学期 (April 16, 2008) 计算机网路 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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