CS F303 T1 _ComputerNetworks PDF

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Consists of Computer Networks questions based on Test 1...

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BITS Pilani Hyderabad Campus Computer Networks (CS F303/IS F303) Test 1 (Regular) Weightage: 20% Type: Closed Book Time: 1 hour Date: 27.02.2016 ---------------------------------------------------------------------------------------------------------------------------------(Note: Question paper consists of 2 pages and 6 questions. Assume suitable data if necessary. Write answers legibly and serially. Manage your time as per the weightage of each question. Calculators are allowed.)

Q.1 a) BITS Hyderabad campus LAN and WAN connectivity (partly) is as shown in Fig.1. Your task is to find out the type of access network that is used to connect to the Internet. Are there any private networks (closed communication networks) that are also being used? Explain the purpose of their usage. [2+1]

1 kbps 2000m A

Fig.1 Part of the Network architecture for BITS, Hyderabad

B

Fig.2 An example network

b) If you are asked to build an Internet of Things (IoT) testbed within your hostel wing to smartly monitor the energy usage of all the computing devices that your batchmates (friends) have in their rooms, what types of devices do you need, what types of networks do you need, and what types of software you need to build the testbed. Be brief in your answer. [3] Q.2 a) Differentiate between a hub, a switch, and a router by giving an example of their usage. What are the OSI layers at which these devices operate? Explain why. [3] b) Assume that host ‘A’ wants to send voice data to host ‘B’ over a packet switched network (say, Internet phone). Host ‘A’ converts analog voice to a digital 64 kbps bit stream on the fly and then groups the bits into a 16 byte packet. Only one link with a transmission rate of 1 kbps is available between A and B for communication. The distance between A and B is 2000 meters as shown in Fig.2. As soon as host A gathers a packet, it sends it to B, and as soon as B receives an entire packet, it converts these into an analog signal. How much time elapses from the time a bit is created at host A until the bit is decoded at host B. You are free to assume any missing data if you feel it is needed. [5] Q.3 a) Calculate the Bandwidth-Delay product for a network with Ethernet links having 100 Mbps transmission capacity. Assume an RTT of 100 msec. Recall that a TCP header has 16 bits reserved for Window size. What are its implications in light of your calculations? Explain. [3] b) The run of nslookup (DNS client) on a windows 7 machine is as shown in Fig.3 from a machine within IPC unit of BITS campus. The figure shows three searches or queries being sent to the system and their responses received. Find out the IP address of the DNS server which replied to all the queries. Why there are some non-authoritative and authoritative answers. What type of querying (iterative or recursive) must have been used in these cases to get the responses? Explain why. [3] (P.T.O)

Fig.4 Request from BITS Hyd to get google page through Airtel ISP Fig.3 Multiple searches using nslookup Q.4 a) Fig.4 shows the WAN for accessing google’s home page from BITS, Hyderabad campus using a Chrome browser. List out the steps that will be carried out at different network protocol stack in a chronological order starting from typing the url on the address bar of the browser. Assume that the authoritative DNS server for BITS Hyderabad domain is provided (or resides at) by the ISP, which is Airtel in this case. [4] b) Assume that one device trying to set up a TCP connection sends a SYN and then receives a SYN from the other side before its’ SYN is acknowledged. What will be the end result? Show the segment exchanges from both the sides and also the state transition diagrams clearly. [4] Q.5 The code fragment shown below (discussed in the class) with small modifications is for implementing a concurrent server (partly). What modifications would you like to make in the following code snippet to ensure that the child server process does not serve the client at all? Also, figure out if there are any extra descriptors that are wasted in any of the processes. listen(listenfd, 10000); for ( ; ; ) { clilen = sizeof(cliaddr); connfd = accept(listenfd, (SA *) &cliaddr, &clilen); if ( (childpid = fork()) == 0) { close(listenfd); str_echo(connfd); exit(0); } } [4] Q.6 Assume that the timeout value for Go-Back-N (GBN) protocol is sufficiently long such that 4 consecutive data segments and their corresponding ACKs can be received (if not lost in the channel) by the receiving host (Host B) and the sending host (Host A) respectively. Assume that host A sends 4 data segments to host B, and the 3rd segment (sent from A) is lost in the network. There are no more segment losses after this. Also, assume that the number of bits to represent the sequence number space is 3. Clearly show the segment exchanges between both the sender and the receiver alongwith the sending and receiving window contents. [8] -------------------------------------...