UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING

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1 UNIVERSITY OF TORONTO FACULTY OF APPLIED SCIENCE AND ENGINEERING FINAL EXAMINATION: April 25, 2017 DURATION: 2V2 hrs. ECE361 - Computer Networks Exam Type: B Calculator Type: 2 (non-programmable calculators) Examiner: Hamid S. Tirnorabadi LAST NAME: FIRST NAME: STUDENT NUMBER: Note: Full marks are only applied to legible solutions with full details. MARKS Qi /19 Q2 /31 Q3 /8 Q4 /19 Q5 /13 Total /90 ECE361 Final Exam 2017 Page 1 of 12

2 19 Marks] Qi: Multiple choice, only one correct answer unless stated. 1. Circle the correct answer. Given a URL ARP is used to obtain the destination MAC address ARP is a plug-and-play protocol If host A and host B are in two different networks connected by a router, then host A may use ARP to obtain the MAC address of host B. None of the above are correct 2. Circle all the correct answers (more than one). DHCP is an IP layer protocol DHCP uses UDP to send and receive messages DHCP is used for assigning IP addresses to hosts in a network 3. A digital audio stream consists of 20,000 samples per second, where each sample consists of 10 bits. Suppose music is represented in stereo, with one stream for each ear, and a song lasts five minutes. How many bytes are required to store the song? 15, Bytes 1.000,000 Bytes 7,500,000 Bytes 1,875,000 Bytes 4. A server imposter impersonates a legitimate server to gain sensitive information from a client. Circle all the security requirements that are at risk. Privacy. Authentication. Non-repudiation. Availability. 5. A network advertises the CTDR network number /22 (and no other numbers). Which IP addresses could the network own? ECE361 Final Exam 2017 Page 2 of 12

3 6. Which of the following are true about HTTP headers and connection management? All HTTP responses must either close a connection or include a Content- Length: header to signify to end of a HTTP response body. Persistent HTTP connections can have lower latency than non-persistent connections because they can avoid performing a new TCP handshake for each HTTP request. When transferring many small Web objects, the only performance difference between persistent and non-persistent HTTP connections is related to connection establishment. HTTP headers have a fixed size. 7. Why would an Internet Service Provider (ISP) select a route with a longer AS-PATH over a route with a shorter AS-PATH. Circle all the correct answers. To avoid an undesirable AS, e.g. that performs censorship or eaves dropping. To avoid a performance problem, e.g. high latency or low throughput. To ASes that are not part of its peering network. 8. Host A initiates a TCP connection with host B by sending a SYN packet, and B responds with a SYN-ACK packet. Upon receiving the SYN-ACK packet, A responds with an ACK packet. Part A: What is the earliest time that A can start sending data to B? Immediately after sending the SYN packet to B Immediately after receiving the SYN-ACK packet from B Immediately after receiving data from B None of the above. Part B: What is the earliest time that B can start sending data to A? Immediately after receiving the SYN packet from A Immediately after sending the SYN-ACK packet to A Immediately after receiving the ACK packet from A Immediately after receiving data from A ECE36I Final Exam 2017 Page 3of12

4 [31 Marks] Q2: Short answer questions. Traceroute can sometimes return a path that does not even exist in the Internet topology. e.g., hop i may not be connected to hop 1+1. Give one reason why this might happen. (2 points) Part A: Suppose Ethernet was the only existing LAN technology, so every host in the Internet was part of a local Ethernet and thus had a globally-unique Ethernet address. Your friend suggests that IP is not necessarily anymore, and the entire Internet could just be one large, switched Ethernet instead. Give two reasons why using existing Ethernet protocols for this is a bad idea from a networking perspective (i.e. do not consider security or privacy). (3 points) Part B: What about the other way around, why do we not simply assign IP addresses to network adaptors, instead of dealing with both MAC (Ethernet) and IP addresses? Give one reason. (2 points) What is an intra-as routing protocol? Names two widely used intra-as protocols and present any two contrasting features between these protocols. (4 points) ECE361 Final Exam 2017 Page 4of12

5 Suppose an organization owns the block of addresses of the form /17. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. What are the prefixes (in slash notation) for the four subnets? (4 points) Knowing that ARP employs broadcast traffic. List another protocol that generates broadcast traffic, and explain why broadcasting is used. (2 points) What is the stopping condition for traceroute? (1 point) What is a nonce? Explain. (1 point) Name four applications of NAT. (4 points) ECE36I Final Exam 2O17 Page 5of12

6 9. There are multiple types of messages involved in the Text Conferencing Lab such as LOGIN message. The message could be from client to server or from server to client. List 8 types (other than LOGIN) of messages you used in this Lab. If you do not remember the exact wording, then use your own words to specify the type, or provide a one-line description for the message. (8 points) ECE36I Final Exam 2017 Page 6of12

7 [8 Marks] Q3: Part A: Suppose an organization runs a Network Address Translator (NAT). When a packet arrives from the external Internet, what field(s) determine which internal IP address to use as the destination address for the packet? What other fields in the IP and TCP/UDP header does the NAT change? (2 points) Part B: Suppose the NAT then places the IP packet in an Ethernet frame to send to the receiving host. How does the NAT know what destination MAC address to use? (2 points) Part C: Once the Ethernet frame reaches the destination host, how does the host know what network-layer protocol (e.g. IP, IPX, Appletalk) should be used to parse the payload? (1 point) Part D: How does the operating system know if the packet is a UDP packet or a TCP packet? (1 point) Part E: How does the operating system know which process (and which socket) should receive the packet? (2 points) ECE36I Final Exam 2017 Page 7of 12

8 [19 Marks] Q4: In the following questions nodes use Distance-Vector routing to compute the shortest distances. Part a: In a network, node A has two neighbor nodes B and C. The distances from A to B and from A to C are 1 and 2, respectively. Let D be another node in the network. Node B sent an update (D, x) to A, i.e. B is advertising to A that its shortest distance to D is x. Similarly, Node C sent an update (D, 1/x) to A. Write down the Bellman-Ford equation and use it to compute the shortest distance from A to D in terms of x. What is the maximum possible distance from A to D in this network? (3 points) Part b: Consider the following graph. Fill the table below showing the first two iterations of Distance-Vector routing in node A. The entries should be of the form (Next hop, distance). Assume that exchanges of routing information and routing table updates are synchronous (i.e. they happen at the same time at all nodes). (8 points) Iteration B C D E Initialization (-1) (-1,) (-1,) (-1,) ECE36I Final Exam 2017 Page 8of12

9 Part C: For the graph in Part b, fill the following table showing the updates sent from B and C to A at the end of first two iterations of Distance-Vector routing, when split horizon and reverse poisoning is used and when it is not. Please follow the convention used in Part a for writing down the updates, for example, if B sends an update to A that its distance to D is 6, then write (D, 6) under column B in the following table. (8 points) End of iteration Split horizon and reverse poisoning Updates sent by B Updates sent by C Without split horizon or reverse poisoning Updates sent by Updates sent by B C ECE361 Final Exam 2017 Page 9 of 12

10 [13 Marks] Q5 Part A: Subnetting: Using CIDR a forwarding table is created as shown below. What is a table with a minimum number of entries that will still be able to forward packets correctly? Explain the reasoning behind this approach that reduces the table. (7 points) Prefix /9 etho /10 etho /10 etho Outgoing Interface Prefix Outgoing Interface /11 ethi /12 ethi /13 ethl /Il ethl /11 ethi /11 ethi /13 eth /12 eth2 default eth3 default eth3 ECE36I Final Exam 2017 Page 10 of 12

11 Part B: Routed/Bridged network: The Figure provided below shows a network with two bridges and one router. Each interface is labeled with an IP address (e.g. Hi. ip) and a MAC address (e.g. HJ.rnac). Consider that host Hi is sending a packet to host H2. Then answer the following questions. R2h mic SRC R2 R2 CAL p R2c. itc DST H [ iric Ra.p // R2arnu H-1-1p H7 mac BLip B! LLp Hc.niac Blip B I bjnac How many (datalink) networks are shown above? (1 point) Just before the packet reaches bridge BI, what is its layer 2 destination? (1 point) Just before the packet reaches bridge 132, what is its layer 2 source? (1 point) Just after the packet leaves router R2, what is its layer 3 source? (1 point) When HI sends out an ARP query, what is the reply to that query? (1 point) J Does the entry 132amac appear in B I's forwarding table? (1 point) ECE36I Final Exam 2017 Page 11 of 12

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