Midterm I of Computer Communication , Winter 2004 Electrical and Computer Engineering Department, University of Windsor

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1 Name : ID : Midterm I of Computer Communication , Winter 2004 Electrical and Computer Engineering Department, University of Windsor Date : Important Please Read First Write your name, ID number and date on the first page of this booklet before answering the questions, also all loose pages that you may use during the exam. Please read the questions carefully; and understand what is given and what is asked, then give your answer. You will get partial credits for your correct portion of the answer, that s why I encourage you to try to answer all the questions. There are 5 (five) questions in this booklet, answering the first five questions. Sum of the marks of the all questions is 110, but maximum mark you can get from the midterm is 100. The contribution of each question s mark is given next to the question number. If a question has more than one part, then each part s contribution to this question s mark is labeled separately in each part. If you have questions during the exam, please raise your hand, and the instructor will come to you to answer your question. You are allowed to use reminder sheet as I explained in the syllabus. Written portion of the sheet is one side of a letter size paper. If your sheet has more than that written portion, Please inform instructor before you start the exam, if you fail to inform, then you will loose privilege of using this sheet. Calculators are allowed. The exam time is 80 minutes. Good luck. Kemal Tepe February 18,

2 1. (20 points total) Answer the following questions as True or False, and then explain your reasoning very briefly in given space(no more than 2 sentences). (Each part is 2 points.) (a) Flow control protocols, like Stop-and-Wait or Go-Back-N, can only be applied at transport layer. (b) Efficiency of Go-Back-N and Selective Reject depends on their window sizes. (c) An application layer protocol can be designed to have reliable data transfer even it uses UDP as its transport protocol. (d) Two hosts can connect to a single host at the same port number. (e) Video streaming applications use largely UDP as its transport protocol. (f) DNS uses TCP as its transport layer. (g) One of the advantages of network layering is to divide networking tasks into meaningful and manageable subtasks. (h) TCP/IP works only with high speed ADSL or Ethernet. (i) Although UDP is connectionless transport protocol, destination always acknowledges packet receptions to source. (j) TCP port number uniquely identify a connection in the network. 2

3 2. (30 points total) (a) (10 points) Draw protocol stack of TCP/IP protocol suite and briefly explain functions of each layer of TCP/IP? (b) (5 points) What are the differences between 7-layered ISO networking model and TCP /IP networking model? (c) (5 points) What are the differences between Peer-2-Peer application layer architecture and Client-Server application layer architecture? And give one example each? (d) (10 Points) Explain Two-Way and Three-Way Handshake in detail? Explain how Three- Way handshake identifies duplicate acknowledgements? 3

4 3. (20 points total) Suppose each layer has 20 bytes of fixed header size in TCP/IP protocol stack. If the maximum size of physical layer frames is 120 bytes: (a) (8 points) What is the efficiency of the network, which is ratio between actual number of information bits transferred and total number of bits transferred in at the physical layer from node A to node B? (b) (6 points) What is the minimum number of frames used at physical layer to transfer a 1000 bytes file? (c) (6 points) What is the maximum size of the transport protocol data unit (PDU)? 4

5 4. (20 points regular + 5 points bonus, 25 points total) Consider sending a file size of O bits over a path of K links. Suppose each link can transmit at R bit/sec (bps) and has fixed propagation delay of t p, network lightly loaded (no queuing delays), and each link s maximum frame size is M bits (M < O and S = O/M is integer) and header size of the network is h bits: (a) (5 points) How long does it take to send the file in packet switched network over K links? (b) (10 points) How long does the receiver waits for completion of transfer of entire file ( latency) if each link utilize Stop-N-Wait flow control(new packet can not start transmission before receiving an acknowledgement and assume no loss of packets in the links)if there are only 3 links (K = 3)? (Hint: Acknowledgements at link level) (c) (5 points) If we use end-to-end Stop-N-Wait instead of at each link, how the latency changes with same number of links? (d) (Bonus part 5 points) What is the optimum M for Part A if K = 3? 5

6 5. (10 regular points + 5 bonus points, total 15 points) Suppose host A and B are connected with 100 Kilo bit per second link and propagation delay between them is 100 mili seconds, average size of the packets is 1000 bits. If hosts A and B use Go-Back-N flow control: (a) (5 points) Please find a minimum window size N that can achieve 100% link utilization? (suppose application generates enough packets to send for any window size and there are no packet loss.) (b) (5 points) If Go-Back-N is replaced with Selective Reject, does the window size change? (c) (Bonus 5 Points) For the window size you calculated, what is the minimum sequence number? and how many bits do you need in the frame header to carry this sequence numbers? 6