Duke University CompSci 356 Midterm Spring 2016

Transcription

1 Duke University CompSci 356 Midterm Spring 2016 Name (Print):, (Family name) (Given name) Student ID Number: Date of Exam: Feb 25, 2016 Time Period: 11:45am-1pm Number of Exam Pages: 15 (including this cover page) Exam Type: close book/notes. You may bring one sheet of your own notes. Marking Scheme: Question Score 1 /22 2 /15 3 /12 4 /8 5 /25 6 /14 Survey /4 Total /100

2 For your convenience, this page includes figures that describe packet formats for Ethernet, ARP, IP, and ICMP. (a) ARP request or reply format. Destination address Source address Type Payload 6 bytes 6 bytes 2 bytes bytes (b) Ethernet packet format. (c) IP packet format. (d) ICMP header format. Page 2 of 15

3 1. (22 pts) Questions and short answers. (a) (4 pts) Alice opens a web browser from her laptop and downloads a web page from cnn. com. Describe the sequence of protocol layers her web request goes through before it leaves her laptop. (b) (4 pts) Which error detection algorithm(s) we learned in class cannot detect all two-bit errors? Page 3 of 15

4 (c) (4 pts) How does a network adapter detect the start and the end of a data frame? Describe at least one feasible solution and how it works. (d) (6pts)ExplainwhytheEthernethasbothamaximumframelengthlimitandaminimum frame length limit. Page 4 of 15

5 (e) (4 pts) What are the advantages of the sliding window protocol over the stop-and-wait protocol? Page 5 of 15

6 A 20ms 2Mbps B Figure 1: A link connecting two nodes. 2. (15 pts) Nodes A and B are connected by a 2Mbps link with a one-way propagation delay 20ms. Please answer the following questions. Node A needs to send a 1MB file to node B. Suppose the packet size is 1KB. (You can assume 1KB=1000B, and 1M B=1000KB.) (a) (2 pts) What is the transmission latency of a single packet? (b) (3 pts) Suppose A and B run a sliding window protocol to ensure reliable delivery. An ACK packet is 40 bytes. You can ignore the transmission latency of an ACK and the packet processing latency at both nodes A and B. What s the round trip latency for A to send a packet and receive an ACK from B. Page 6 of 15

7 (c) (5 pts) What s the minimum sending sliding window size to keep the link fully utilized? (d) (5 pts) How long does it take for A to transmit a 1MB file to B? Page 7 of 15

8 A 10ms 1Gbps B Figure 2: A link connecting two nodes. 3. (12 pts) Suppose node A and B are connected by a single link. They run a sliding window protocol between them. Node A has a sending window size of five packets, and node B has a receiving window size of one packet. Node A sets a timeout value 3s for each outstanding (i.e. unacked) packet. The link speed between A and B is 1Gbps, and the one-way propagation delay is 10ms. Answer the following questions. (a) (2 pts) Using only the sender side sliding window size constraint, how many outstanding packets can node A send to node B without receiving an ACK? (b) (2 pts) Suppose before the upper level application reads the first packet from B s sliding window buffer, the second packet arrives at B. What will node B do when it receives the second packet? (c) (2 pts) Suppose right after the application reads the first packet, the third packet from A arrives at B. What will node B do when it receives the third packet? Page 8 of 15

9 (d) (4 pts) How long is it likely to take for A to receive an acknowledgement for the third data packet? You can assume the transmission latency of each packet or ack is much less than the propagation delay and ignore it in your calculation. Please explain your answer and state your assumptions. (e) (2 pts) Assuming the receiver side window size cannot be increased, what can you suggest to shorten the time it takes to reliably send data packets from A to B? Page 9 of 15

10 host C Ethernet 1 1 Ethernet 0 Bridge 0 host A host B Figure 3: Learning bridge. 4. (8 pts) An Ethernet bridge Bridge connects two Ethernets as shown in Figure 3. Suppose Bridge starts with an empty MAC forwarding table. Answering the following questions. (a) (2 pts) When host A sends a Ping packet to host B, how will Bridge forward the packet? What s content of bridge B s forwarding table after it receives the Ping packet? You can use MAC x to denote a host x s MAC address. MAC address Port Number (b) (2 pts) When host B sends a Ping reply to host A, how will Bridge forward the packet? What s the content of Bridge s forwarding table after it receives the Ping reply packet? MAC address Port Number Page 10 of 15

11 (c) (2 pts) Suppose host C sends a Ping packet to host B. How will Bridge forward the packet? What s the content of Bridge s forwarding table after it receives the Ping packet? MAC address Port Number (d) (2 pts)hostb replies hostc spingpacket. How will Bridge forwardthepacket? What s the content of Bridge s forwarding table after it receives the ping reply packet? MAC address Port Number Page 11 of 15

12 G 2 B4 0 1 A B! 0 0 B2 1 0 B C D E 1 0 B B6 F Figure 4: Finding the spanning tree. 5. (25 pts) Fun finding the spanning tree. Figure 4 shows an extended Ethernet topology. Suppose all bridges run the spanning tree algorithm to prevent forwarding loops. For simplicity, assume the bridges send out protocol messages synchronously and at t = 0, all bridges start sending out the first round of the spanning tree configuration messages, and send out the following rounds of messages every second. Answering the following questions. (a) (5 pts) At what time will the spanning tree algorithm converge? Please explain your answer. Page 12 of 15

13 (b) (15 pts) When the spanning tree protocol converges, what s the status of each bridge s ports? You can use the letter R for root ports, D for designated ports, and B for blocking ports. B 1 Port 0: Port 1: B 2 Port 0: Port 1: Port 2: B 3 Port 0: Port 1: Port 2: B 4 Port 0: Port 1: Port 2: B 5 Port 0: Port 1: B 6 Port 0: Port 1: (c) (5 pts) Suppose a packet is sent from the Ethernet F. All bridges forwarding tables are empty. How will the packet reach the Ethernet G? Please list the sequence of bridges and Ethernets the packet traverses. Page 13 of 15

14 6. (14 pts) A host A has the following entries in its IP forwarding table. Answer the following questions. Destination Network mask Gateway Interface eth eth1 (a) (2pts)SupposehostAissendinganIPpacketptothedestinationIPaddress To which interface will it send the packet p? (b) (2 pts) Host A s ARP cache is empty. As we learned in class, the host needs to send an ARP request before it can send out the packet. What s the target IP protocol address in the host s ARP request? (You can find the ARP and Ethernet packet format on the second page of the exam.) (c) (2 pts) What s the destination Ethernet address in this ARP request s Ethernet header? (d) (2 pts) Let MAC(IP) denote the Ethernet address of an IP address. For instance, M AC( ) is the Ethernet address corresponding to the interface with the IP address Suppose A receives a valid ARP reply. Now host A can send out the packet p. In the outgoing Ethernet frame that encapsulates this packet, what s the destination Ethernet address, and the destination IP address of the Ethernet frame? (e) (2 pts) Now host A is sending another packet p to the IP address To which interface will A send the packet? (f) (2 pts) Similarly, A needs to send an ARP request. What s the target protocol address in A s ARP request? (g) (2 pts) SupposeAagain receives a valid reply and is ready to send out packet p. What s the destination Ethernet address and the destination IP address in the outgoing Ethernet frame that encapsulates pacekt p? Page 14 of 15

15 Midterm Survey (4 pts) We, the course staff of CompSci 356, would like to hear what you think of the course. Please provide your feedback for the following questions. Please tear off this page from your exam papers so that your opinions will remain anonymous. We will write down your name when you turn in the survey and credit you for your feedback. Thank you! 1. (1 pt) Has the course met your expectations of an undergraduate networking class so far? If not, please explain the areas where the course has not met your expectations. 2. (1 pt) Do you have any suggestion on how the course materials might be improved? 3. (1 pt) Do you have any suggestion on how the instructor might improve her teaching quality? 4. (1 pt) Do you have any suggestion on how the TA might improve his teaching quality? Page 15 of 15