MidtTermSolution - CNT4704 Computer Networking Mid-Term...

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CNT4704: Computer Networking Mid-Term Exam Solution Prof. Cliff Zou Oct. 14, 2010 Question 1: Knowledge questions (20points) Answer each of the following questions briefly, i.e., in a few sentences. a). What is the major differences between TCP and UDP? Why DNS uses UDP instead of TCP for its service? (1). TCP is connection oriented, which needs to set up a connection first; UDP is connection-less and does not need to set up connection. (2). TCP is reliable data transfer. UDP does not guarantee data to reach the destination reliably. (3). TCP has flow control and congestion control; UDP does not. DNS service is very short-lived: one-round of DNS service only needs the client to send one packet to query and the server to send back one packet for response (typically)----it takes only about one RTT time for the service. Using TCP will add another RTT time to set up TCP connection, which is very inefficient. UDP does not have this issue. b). According to the course’s textbook, how many layers are included in the Internet (give their names too)? Give one example of application, or protocol, or device, that corresponding to each layer. From top to down: Application layer (HTTP), transport layer (TCP), network layer (router), link layer (Ethernet), physical layer (optic fiber). c). What does “TTL” mean? What is the usage of TTL in DNS resource record? What is the usage of TTL in packet IP header? TTL is “time-to-live”. In DNS resource record, “TTL” specifies the amount of time (in seconds) DNS servers and applications are allowed to cache the record. In a packet IP header, TTL specifies the maximum routers the packet could pass through. Each router will decrease this value by 1 before sending it out, and drop a packet if it has a TTL=0. Traceroute program uses this property to measure network path and delay.
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