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Computer Networks Homework 1 Chap1

1.14. Suppose two hosts, A and B, are separated by 10,000 kilometers and are connected by a direct link of R =1 Mbps. Suppose the propagation speed over the link is 2.5108 meters/sec. a. Calculate the bandwidth-delay product, Rtprop. b. Consider sending a file of 400,000 bits from Host A to Host B. Suppose the file is sent continuously as one big message. What is the maximum number of bits that will be in the link at any given time? c. Provide an interpretation of the bandwidth-delay product. d. What is the width (in meters) of a bit in the link? Is it longer than a football field? e. Derive a general expression for the width of a bit in terms of the propagation speed s, the transmission rate R, and the length of the link m. 1.17.Refer again to problem 14. a. How long does it take to send the file, assuming it is sent continuously? b. Suppose now the file is broken up into 10 packets with each packet containing 40,000 bits. Suppose that each packet is acknowledged by the receiver and the transmission time of an acknowledgment packet is negligible. Finally, assume that the sender cannot send a packet until the preceding one is acknowledged. How long does it take to send the file? c. Compare the results from (a) and (b).


2.2. Read RFC 959 for FTP. List all of the client commands that are supported by the RFC. 2.3. Visit What are the well-known port numbers for the simple file transfer protocol (SFTP)? For the network news transfer protocol (NNTP)? 2.6. 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 look-up is necessary to obtain the IP address. Suppose that n DNS servers are 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?

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2.9. Consider Figure 2.11, for which there is an institutional network connected to the Internet. Suppose that the average object size is 900,000 bits and that the average request rate from the institution's browsers to the origin servers is 1.5 requests per second. Also suppose that the amount of time it takes from when the router on the Internet side of the access link forwards an HTTP request until it receives the response in two seconds on average (see Section 2.2.6). Model the total average response time as the sum of the average access delay (that is, the delay from Internet router to institution router) and the average Internet delay. For the average access delay, use /(1), where is the average time required to send an object over the access link and is the arrival rate of objects to the access link. a. Find the total average response time. b. Now suppose a cache is installed in the institutional LAN. Suppose the hit rate is 0.4. Find the total response time.

2.14. Suppose you are downloading MP3s using some P2P file-sharing system. The bottleneck in the Internet is your residential access link, which is a 128 kbps full-duplex link. While you are downloading MP3s, all of a sudden 10 other

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users start uploading MP3s from your computer. Assuming that your computer is very powerful, and all of these downloads and uploads are not putting any strain on your computer (CPU, disk I/O, and so on), will the simultaneous uploads--which are also passing through your bottleneck link--show down your downloads? Why or why not? Also answer the same question for when you have 128 kbps upstream and 512 kbps downstream as part of an ADSL connection 2.19. In this problem we explore designing a KaZaA-like system that has ordinary nodes, group leaders, and super-group leaders. a. Suppose each super-group leader is roughly responsible for 200 group leaders, and each group leader is roughly responsible for 200 ordinary peers. How many super-group leaders would be necessary for a network of four million peers? b. What information might each group leader store? What information might each super-group leader store? How might search be performed in such a three-tier design? 2.21. Install and compile the Java programs TCPClient and UDPClient on one host and TCPServer and UDPServer on another host. a. Suppose you run TCPClient before you run TCPServer. What happens? Why? b. Suppose you run UDPClient before you run UDPServer. What happens? Why? c. What happens if you use different port numbers for the client and server sides?

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