UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING

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1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING ECE361 Computer Networks Midterm March 09, 2016, 6:15PM DURATION: 75 minutes Calculator Type: 2 (non-programmable calculators) Examiner: Hamid S. Timorabadi FAMILY NAME: GIVEN NAME: STUDENT NUMBER: MARKS Q1 /12 Q2 /12 Q3 /10 Q4 /21 Q5 /7 Q6 /15 Total /77 ECE361 Midterm 2016 Page 1 of 14

2 Question 1: [12 Marks] Answer the following questions by circling the correct answer. (1 mark each) 1- In the Internet, all packets from source A to destination B follow the same path through the networks. False True 2- IP fragments are reassembled at the destination. False True 3- Sliding window achieves higher throughput than Stop-and-Go. False True 4- Computer networks that provide only a connection service at the network layer are called virtual Circuit. False True 5- In a datagram network, routers keep no state about the end-to-end connections. False True 6- CIDR is the addressing scheme currently used in the Internet. False True 7- The number of addresses represented by the IP prefix /24 is? (1 points) a) 254 b) 510 c) 256 d) The HTTP protocol. a) Is stateless b) Uses persistent connections c) Can use non-persistent connections d) All of the responses above are correct. 9- Round-trip time (RTT) is estimated based on. a) A weighted average RTT that is fixed after several samples b) A weighted average RTT that is continuously updated c) Sampling of routes to determine minimum cost paths d) None of these responses is correct. ECE361 Midterm 2016 Page 2 of 14

3 10- Which is true about DHCP? a) DHCP is used for dynamic routing protocol configuration b) DHCP is used to reliably and dynamically transfer file to the network c) DHCP is used to ease the burden of manual allocation of IP address parameters d) DHCP is used to convert dotted decimal IP addresses to a human-readable format 11- In the Transmission Control Protocol (TCP), acknowledgements (ACKs) are used for: a) Error control b) Flow control c) Congestion control d) Error control and flow control e) Error control, flow control, and congestion control 12- Which is the highest layer implemented in routers? a) Network layer b) Link Layer c) Transport layer d) Application layer e) Physical layer ECE361 Midterm 2016 Page 3 of 14

4 Question 2: [12 Marks]: Short answer questions: 1- (3 Marks) Name three kinds of switching found inside a router. i. Switching via memory ii. Bus iii. Crossbar (or interconnected networks). 2- (5 Marks) Consider the following fields in the IP packet header, with the additional answer of none of the above. a) 16-bit Identifier field b) 16-bit length c) 8-bit TTL d) 8-bit upper layer e) 16-bit checksum f) 4-bit header length g) None of the above i. Which one(s) is used for fragmentation? a) 16-bit Identifier Field, ii. Which one(s) can be used to indicate the presence of options? b) and f) 16-bit Length, 4-bit header length iii. Which one(s) is used to prevent packets from looping forever? c) 8-bit TTL iv. Which one(s) is used to detect corruption of the packet payload? e) 16-bit Checksum v. Which one(s) is used to instruct the network stack how to handle incoming packets? d) 8-bit upper layer 3- (4 Marks) What are the DNS record types for the: i. IP address of a given hostname: A ii. Hostname of given domain s mail server: MX iii. Hostname with a given IP address: PTR iv. Hostname of authoritative DNS server for a given domain: NS ECE361 Midterm 2016 Page 4 of 14

5 Question 3: [10 Marks] Routing Tables Part A: (5 Marks) A router has the following routing table entries: Network Address Interface /19 eth /21 eth /21 eth /25 eth3 Default eth4 For each of the following IP addresses, what does the router do if the router receives packets with the following destination addresses? Circle the correct answer. i a) Sends through interface eth0 b) Sends through interface eth1 c) Sends through interface eth2 d) Sends through interface eth3 e) Sends through interface eth4 f) Drops the packet. g) None of the above. ii a) Sends through interface eth0 b) Sends through interface eth1 c) Sends through interface eth2 d) Sends through interface eth3 e) Sends through interface eth4 f) Drops the packet. g) None of the above. iii a) Sends through interface eth0 b) Sends through interface eth1 c) Sends through interface eth2 d) Sends through interface eth3 e) Sends through interface eth4 f) Drops the packet. g) None of the above. iv a) Sends through interface eth0 b) Sends through interface eth1 c) Sends through interface eth2 d) Sends through interface eth3 e) Sends through interface eth4 f) Drops the packet. g) None of the above. ECE361 Midterm 2016 Page 5 of 14

6 v a) Sends through interface eth0 b) Sends through interface eth1 c) Sends through interface eth2 d) Sends through interface eth3 e) Sends through interface eth4 f) Drops the packet. g) None of the above. Part B: (5 Marks) Aggregate the following routing table to the highest degree possible and write your answer in the table provided. Network Address Interface /20 eth /21 eth /21 eth /25 eth /25 eth /24 eth /23 eth1 Network Address Interface /19 eth /22 Eth1 Details for part B Note: Bold face indicates network prefix /20: /21: /21: Can be aggregated to /19 ECE361 Midterm 2016 Page 6 of 14

7 For the next four entries, write third and fourth byte of the IP address: /25: /25: /24: /23: first four entries can be summarized to: /23 First five entries can be summarized to: /22 ECE361 Midterm 2016 Page 7 of 14

8 Question 4: [21 Marks] IP address allocation Consider the system of subnets shown in the Figure below. The purpose of this problem is to assign address blocks to the subnets. The allocation of addresses is as follows: Subnet A obtains an address block from ARIN; Subnets A1, A2, and A3 obtain their address blocks by acquiring a portion of the address block assigned to Subnet A; Subnets A2.1 and A2.2 obtain their address blocks by acquiring a portion of the address block assigned to Subnet A2. Also, each subnet must support the following number of hosts: Subnet A2.1 must support 200 hosts; Subnet A2.2 must support 100 hosts; Subnets A1 must support 50 hosts; Subnets A2 must support 400 hosts (in addition to those in Subnets A2.1 and A2.2). Subnet A3 must support 100 hosts; Subnets A must support 200 hosts (in addition to those in Subnets A1, A2, and A3). Suppose the regional service provider ARIN has the following address blocks available: Block 1: Block 2: Block 3: Block 4: Block 5: Block 6: ECE361 Midterm 2016 Page 8 of 14

9 Part A: (6 marks) Express each of the address blocks of ARIN as an IP network prefix (IP network address and prefix length) using CIDR notation. Write your final answer clearly in the box provided below. Block # Addresses Your Answer / / / / / /13 Part B: (6 marks) Determine the smallest address block from ARIN that can satisfy the above requirements of the subsystem of subnets. Provide a justification. Subnet A2.1 has 200 hosts address block: 2 8 = 256 /24 subnet Subnet A2.2 has 100 hosts address block: 2 7 = 128 /25 subnet Subnet A1 has 50 hosts address block: 2 6 = 64 /26 subnet Subnet A2 has 400 hosts and must accommodate Subnets A2.1 and A = 784 addresses address block: 2 10 = 1024 /22 subnet Subnet A3 has 100 hosts address block: 2 7 = 128 /25 subnet Subnet A has 200 hosts and must accommodate Subnets A1,A2 and A = 1416 addresses address block: 2 11 = 2048 /21 subnet Block 3 should be allocated. ECE361 Midterm 2016 Page 9 of 14

10 Part C: (6 marks) Use the address block selected in Part B, and create IP network prefixes for each subnet. The prefix of a subnet must have enough addresses to meet the requirements of the hosts as well as the attached subnets. The figure above presents the address block of Block 3, with each row a /24 prefix. The following is one feasible allocation: Subnet Prefix Subnet A: /21 Subnet A2: /22 Subnet A2.1: /24 Subnet A2.2: /25 Subnet A3: /25 Subnet A1: /26 We can solve the allocation as follows: Subnet A gets the entire address block: /21 Subnet A2 gets one half (here first half). Of this half, A2.1 gets one quarter, and A2.2 gets one eighth. Subnet A3 gets 1/16 th of the address block. Subnet A1 gets 1/32th of the address block. Part D: (3 marks) Suppose that the number of hosts in Subnet A (not in any of the other subnets) must be increased. Given your solution to (Part C), what is the maximum number of hosts that can be supported? Total number available: 2048 Assigned to A2: Assigned to A3-128 Assigned to A1-64 Broadcast and network address: -2 Leftover: 830 ECE361 Midterm 2016 Page 10 of 14

11 Question 5: [7 Marks] Consider a node A behind a NAT, sending packets to a node B. We will follow a simple packet exchange of A sending to B, having the packet altered by NAT, and then the response coming back, again altered by NAT, and then arriving at A. You know the following: Packet leaving A: Destination address: Source address: Destination port: 80 Source port: 1067 Packet leaving B: Destination address: Source address: Destination port: 2500 Source port: 80 a) For the packet leaving the NAT towards A, please fill in the blanks: Destination address: Source address: Destination port: 1067 Source port: 80 b) Fill in the following blanks: IP address of the NAT: IP address of node A: IP address of node B: ECE361 Midterm 2016 Page 11 of 14

12 Question 6: [15 Marks]: Calculate the total time required to transfer a 1000-KB file in the following cases, assuming an RTT of 50 ms, a packet size of 1 KB data, and an initial 2 RTT of handshaking before data is sent. Part A: (4 Marks) The bandwidth is 1.5 Mbps, and data packets can be sent continuously. 2 initial RTT s (100ms) KB/1.5Mbps (transmit) + RTT/2 (propagation= 25ms) Mbit/1.5Mbps = sec = 5.458sec. If we pay more careful attention to when a mega is 10^6 versus 2^20 we get 8,192,000 bits/1,500,000bps = sec, for a total delay of sec. Part B: (3 Marks) The bandwidth is 1.5 Mbps, but after we finish sending each data packet we must wait one RTT before sending the next. To the above we add the time for 999 RTTs (the number of RTTs between when packet 1 arrives and packet 1000 arrives), for a total of = Part C: (3 Marks) The bandwidth is infinite, meaning that we take transmit time to be zero, and up to 20 packets can be sent per RTT. This is 49.5 RTTs, plus the initial 2, for seconds. Part D: (5 Marks) The bandwidth is infinite, and during the first RTT we can send one packet (2 (1-1) ), during the second RTT we can send two packets (2 (2-1) ), during the third RTT we can send four (2 (3-1) )), and so on. Right after the handshaking is done we send one packet. One RTT after the handshaking we send two packets. At n th RTTs past the initial handshaking we have sent n = 2 n +1 1 packets. At n = 9 we have thus been able to send all 1,000 packets; the last batch arrives 0.5 RTT later. Total time is RTTs, or sec. ECE361 Midterm 2016 Page 12 of 14

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