UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING

Transcription

1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING ECE361 Computer Networks Midterm March 06, 2017, 6:15PM DURATION: 80 minutes Calculator Type: 2 (non-programmable calculators) Examiner: Hamid S. Timorabadi FAMILY NAME: SOLUTION GIVEN NAME: STUDENT NUMBER: MARKS Q1 /3 Q2 /13 Q3 /4 Q4 /6 Q5 /4 Total /30 ECE361 Midterm 2017 Page 1 of 10

2 Question 1: [3 Marks] In the following questions clearly Circle the correct answer. 1) Consider the following statements. I. SMTP is a push protocol. II. SMTP is used to read s from the server. a. Both I and II are correct. b. Both I and II are incorrect. c. Only I is correct. d. Only II is correct. 2) A webpage has two small-size pictures. The webpage and all three pictures are on the same server. Consider the following two statements. I. If non-persistent HTTP connection with parallel TCP connections is used, it takes approximately 4RTT time to fetch the webpage and the objects. II. If persistent HTTP connection without pipelining is used, it takes approximately 4 RTT times to fetch the webpage and the objects. a. Both I and II are correct. b. Both I and II are incorrect. c. Only I is correct. d. Only II is correct. 3) Choose the incorrect statement from the following: a. In TCP Reno (new version) a retransmission of a missing segment happens when the sender receives three duplicate ACKs b. UDP does not provide flow control and congestion control c. If a TCP receiver receives a packet with sequence number n, then it sends an ACK with sequence number n+1 to the sender d. At a TCP sender, if an ACK is received for packet n, then any unacknowledged packets sent prior to n may be retransmitted e. None of the above. 4) A host sends a packet with destination IP address of: with netmask of: to a recipient within its network. The destination is: a. Another host within the same network. b. The default router of the same network. c. The sender itself and this is a ping packet. d. None of the above. ECE361 Midterm 2017 Page 2 of 10

3 5) Which one of the followings is not supported by IPv6. a. Multicast. b. Unicast. c. Anycast. d. Broadcast. e. None of the above. 6) Which of the following is private IP address? a b c d e. None of the above. ECE361 Midterm 2017 Page 3 of 10

4 Question 2: [13 Marks]: Short answer questions. Note that full marks are assigned to only brief, concise, and detailed answers. a) What is an end-to-end delay of a packet in a network? (1 mark) The difference between the time at which the packet reaches a receiver node and the time of its inception at the sender node. b) What are the delays that constitute the end-to-end delay? Just name them. (1 mark) Processing, queueing, transmission and propagation delays. c) What is flow control? (1 mark) A mechanism to avoid buffer overflow at the receiver. d) Provide 3 differences between HTTP and SMTP? (2 marks) HTTP is a pull protocol and SMTP is a push protocol. SMTP requires each message to be in 7-bit ASCII format and HTTP does not have this restriction. HTTP encapsulates in each object in its on HTTP response message, while SMTP places all the message s objects into one massage object. ECE361 Midterm 2017 Page 4 of 10

5 e) What is the name of model that allows to implement protocol layering and has become a de-facto industry standard for heterogeneous networking? What are the names of its layers? (1 mark) TCP/IP model or Internet protocol stack. Application, transport, network, link and physical layers. f) Explain what is encapsulation? (1 mark) Encapsulation is the process of adding control information at each layer of protocol stack as the data moves from higher layers to the lower layers. g) A host with IP address /25 decides to unicast the other hosts in its network. What is the maximum number of messages sent out by this host? (1 mark) 1 for network (host= 0) 1 for broadcast (host = all 1s) 1 for the default router (typically host address = 1) 1 for the sender himself => remaining is 124. h) Determine the checksum for the binary number Assume 4-bit word and use a module arithmetic of mod (1 mark) ECE361 Midterm 2017 Page 5 of 10

6 i) What does a globally unique address for applications consists of? (1 marks) <IP address, type of protocol, port number> j) Provide the sequence of the steps that were employed in the File Transfer Lab by writing a number (e.g. 1, 2, 3,) beside each step. (2 marks) Client send ftp to Server Client check whether the file exist Client send the first data packet to Server Server get the address of the Client for the first time Client ask for command and filenames Server check the command Server send back ACK Server send yes back to the Client k) Describe one thing you would like to see changed in the class. (Zero mark if your answer is "nothing"). (1 mark) ECE361 Midterm 2017 Page 6 of 10

7 Question 3: [4 Marks] Two nodes A and B are connected by a single store-and-forward router and two links as shown in the Figure below. Links 1 and 2 are dedicated for the traffic between A and B. However, the router is shared between different links. Due to cross traffic from other links, the average queuing delay at the router is 2 ms, i.e. once a packet is received at the router it is delayed by 2 ms. The transmission rate on both links is 15 Mbps and each link is 100 km long. Assume the speed of light is 2x10 8 m/s in the links. a) What is the end-to-end delay for a packet of size 1500 bytes sent from node A to node B? b) Suppose that there is no cross traffic at the router and the queuing delay is zero. Now, compute the endto-end delay for a file of size bytes that is sent from A to B using packets of size 1500 bytes. Part a) On each link: Tx time of a packet (T) = 1500x8/(15x10 6 ) = 0.8 ms Propagation delay (τ) = 10 5 /(2x10 8 ) = 0.5 ms End-to-end delay of a packet = 2x( ) + 2 = 4.6 ms Part b) For the file of size bytes, 10 packets has to be transmitted. End-to-end delay for the file = 2τ + 11T = 9.8 ms ECE361 Midterm 2017 Page 7 of 10

8 Question 4: [6 Marks] Consider the following network. The client A wishes to see a webpage from host B. The local network of A and the local DNS server is freshly setup. Therefore, the IP address of host B is not cached in neither A nor the local DNS server. AA is the Authoritative DNS server for hosts B and C. The one way propagation delay for a DNS query/response message in each network is shown in the figure. As an example, a DNS query from local DNS server to TLD takes 50 ms to reach TLD. Answer the following questions. a) Write down the DNS query/response sequence if client A uses default query scheme to resolve the IP address of host B. Compute the duration for the address resolution. b) Repeat part a) for recursive query scheme. c) After retrieving the html file from B, client A finds a reference to an image that resides in host C. Now, write down the DNS query/response sequence if client A uses default query scheme to resolve the IP address of host C. Compute the duration for the address resolution. Part a) A Local DNS root Local DNS TLD Local DNS AA Local DNS A Duration for the address resolution = x90 + 2x50 + 2x = 420 ms Part b) A Local DNS root TLD AA TLD root Local DNS A Duration for the address resolution = = 460 ms Part c) In this case the IP address will be cached in the Local DNS server. A Local DNS AA Local DNS A Duration for the address resolution of C = 140 ms ECE361 Midterm 2017 Page 8 of 10

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10 Question 5: [4 Marks] Consider the following Figure for TCP Reno where the y-axis is the TCP window size of the sender. Note that the window size of the TCP sender decreases at several points in this Figure. a) Name the event at B that causes the sender to decrease its window size. Triple duplicate ACK b) The event at F indicates that the network has lost a packet? True False Justification: False. It could be due to delays in asymmetric paths. c) Name the event at D that causes the sender to decrease its window size. Timeout d) The event at D can only be caused by the network loss of a packet. True False Justification: False Congestion in either direction could cause RTT > RTO (retransmission timeout). ECE361 Midterm 2017 Page 10 of 10