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Computer Networks Assignment 1

General instructions

This tutor-marked assignment (TMA) should be done after you have completed your study of Unit 1 and Unit 2. Your assignment must be written in MS-WORD or HTML format and, in either case, it must begin with a cover page containing the course number and title, assignment number, your name and your student id, as well as the time you spent on the assignment. The file containing the cover page must be named TMA1.doc or TMA1.htm. If you have more than one file for the assignment, those files and all other files should be accessible through the file containing the cover page, either directly or indirectly through hyperlinks.

Organization of your assignment files
You need to create a root directory or folder named comp347 somewhere on the file system of your computer, and keep all your work related to the course under that directory; for this assignment you need to create a subdirectory or subfolder named tma1 and put all the necessary files for the assignment under that dedicated subdirectory. When you submit your assignment, you must zip the entire subdirectory of tma1 in a zip file named tma1.zip, and send the compressed file for marking through the appropriate drop box on the course home page.

Part one: (30%) short answer questions

1. Run Traceroute (or Tracert on Windows, or whatever similar utility on your computer) between a source and destination in Canada (or in the country where you live) at three different time of the day, summarize your findings at each of the times in terms of average and standard deviation of the round-trip delays, number of routers in the path, and explain your findings. Please read the article at https://support.microsoft.com/en-us/kb/314868 if you are not familiar with the tool.

2. What are the five layers in the Internet protocol stack? Draw a diagram showing what each layer does and how they are related to each other.
What are packet switches, circuit switches, and message switches respectively? Give two examples of packet-switched networks and explain how they work and what features they have.

3. What are processing delay, queuing delay, transmission delay, and propagation delay respectively? Explain in detail why the traffic intensity should be no greater than one (1) when designing a network system?

4. What is Web-caching? Why is Web-caching important especially for big corporations? What problem does Conditional Get in HTTP want to solve? Download Apache from httpd.apache.org, and install and run it as a proxy server on your computer, then write a brief report on what you have done to make it work and how you use it from your Web browser. You are encouraged to search on the internet for Apache as proxy server, select the most relevant articles to read, and include the titles and links in the report.

5. Suppose you have a web-based email account, such as Gmail, and you have just sent a message to a friend, Alice, who accesses her mail from her mail server using IMAP. Further assume that both you and Alice are using smartphone to access emails. Discuss in detail how the message went from your smartphone to Alices smartphone, by listing the series of application-layer protocols that were used to move the message between the two smartphones, as well as all events that occurred at each application layer protocol.

Part two: (70%) solve the following network problems and show your work in detail

Q2.1. (10%) one important factor in the total delay of a store-and-forward packet-switching network is how long it takes to store and forward a packet through a switch. If the switching time of each switching device is 1 microsecond, is this likely to be a major factor in the response of a client-server system where the client is a computer you use to access the internet, and the server is somewhere in Europe? Explain your answer. You can assume the average propagation speed in all media involved is 2/3 the speed of light in a vacuum; you may calculate or estimate the distance between the server and your computer; you need to find out the number of routers/switches in between using Traceroute or Tracert.

Q2.2. (10%) Carefully study Overlay Network in the textbook and other sources. Consider an overlay network with M active peers, with each pair of peers having an active TCP connection. Additionally, suppose that the TCP connections pass through a total of N routers. How many nodes and edges are there in the corresponding overlay network? Draw a graph to show the network for M = 7 and N = 3.

Q2.3. (15%) Consider sending a file of F bytes over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays at any link. The propagation delay on each link is Tp seconds on average.

a) Suppose the network is a packet-switched virtual-circuit network. Assume the VC setup time is Ts seconds. Suppose the sending layers add a total of h bits of header to the file, and the size of each packet is P. How many packets are needed to send the file? How long does it take to send all the packets from source to destination?

b) Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose the size of each packet is P including 2h bits of header. How many packets are needed to send the file? How long does it take to send the file to its destination?

c) Finally, suppose that the network is a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is R bps. Assuming Ts setup time and h bits of header appended to the file, how long does it take to send the file to its destination?

Q2.4. (20%) In a computer network, two hosts, A and B, are separated by 1,000 kilometres and are connected by a direct link of R = 5 Mbps. Suppose the propagation speed over the link is 2*10^8 meters/sec.

a) What is propagation delay Tprop from A to B through the direct link?

b) What is the bandwidth-delay product R*Tprop?

c) Consider sending a file of 512KB 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?

d) Based on the results from b and c, what does the bandwidth-delay product imply?

e) What is the width (in meters) of a bit in the link?

f) 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.

Q2.5. (15%) Suppose within your Web browser you click on a link to obtain a Web page. The content of the web page is not cached in the local caching server, and the IP address for the associated URL is not cached in your local host either, so that a DNS look-up is necessary to obtain the IP address. Suppose that n DNS servers need to be visited before your host receives the IP address from DNS, and each of successive visits incur round trip delay of RTT1, RTT2 … RTTn respectively.

Further suppose that three images, sized 100KB, 120KB and 80KB respectively, are embedded in the Web page associated with the link, which are stored on the same web server as the web page itself, while the size of the Web page is 4KB. Let RTT0 denote the round trip delay between your computer and the server containing the web page, and let R denote the average end-to-end transmission rate between your computer and the Web server. How much time elapses from when you click on the link until you see the entire content of the web page associated with the link? Your calculation should include all the RTTs and transmission delays and show them in a properly simplified mathematical formula.

 

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