Page 1G22.2262: Data Communications and Network Designs: NetworksFinal ExamA user complains that the “person query” application stopped working. It was working yesterday, but it does not work today. The user is on machine 169.125.14.55, and the service is on 169.125.14.21. The user sends thename “jamesdean” to the server, and the user expects an “n” to arrive. But, the user gets no response at all!So, you sniff the network and ask the user to run the application again. When she does, you see the following packets. The Ethernet preamble and checksum have been stripped off. For your information, the letter “n” in hexadecimal is “6e”, and “jamesdean” in hexadecimal is “6a 61 6d 65 73 64 65 61 6e”.0800 2080 f94c 0800 201f bc72 0800 45000026 dbbe 4000 ff11 30c1 a97d 0e37 a97d0e15 f0c2 140e 0012 0000 6a61 6d65 73646561 6e00 0800 a13f 775e 0800 2080 f94c 0800 4500001d eaf3 4000 ff11 2195 a97d 0e15 a97d0e37 140e f0c2 0009 0000 6e00 Question 1: Why doesn’t the user get the “n” as she expects?Question 2: What happened to cause the problem?Question 3: What will you do to fix the problem?Question 4: Was the UDP checksum involved in the problem?Page 2Question 5: What does the 4000 in the second line of the packets mean?The next day, the user stopped complaining, saying the application seems to be working properly. The user is still on machine 169.125.14.55, but she’s now using the service on 169.125.14.50. Just to make sure things are OK, you ask the user to try the application again. The user sends the name “jamesdean” to the server, and the user gets an “n” back. But, you see something strange on the network! When the user runs the application, you see the following packets. The Ethernet preamble and checksum have been stripped off.0800 1ca9 0205 0800 201f bc72 0800 45000026 dbbf 4000 ff11 30a3 a97d 0e37 a97d0e32 f0c2 140e 0012 0000 6a61 6d65 73646561 6e00 ffff ffff ffff 0800 1ca9 0205 0800 4500001d eaf5 4000 ff11 2176 a97d 0e32 a97d0e37 140e f0c2 0009 0000 6e00 Question 6: The response is sent to the broadcast address! What happened to cause that problem?Question 7: Will other machine be affected by the response from 169.125.14.50?Question 8: What will you do to fix the problem?Question 9: Was the UDP checksum involved in the problem?Page 3Question 10: Was the IP header checksum involved in the problem?Consider the following network topology: 169.125.14.55 169.125.14.56 169.125.14.57 169.125.14.1 198.11.109.8 198.11.109.9 198.11.109.34 198.11.109.36 A B C R D E F08:00:3e:a1:92:00 08:00:3e:a1:92:01 08:00:3e:a1:92:02 08:00:3e:a1:92:03 08:00:3e:a1:92:04 08:00:3e:a1:92:05 08:00:3e:a1:92:06 08:00:3e:a1:92:07 Question 11. Suppose the wire corresponding to 169.125.0.0 had a MTU of 500, and an application on 169.125.14.55 wanted to send data consisting of 1360 x’s followed by a 00 to a service on 198.11.109.34. The packet would look like the following: 4 5 00 056d 60db 00 00 ff 11 6ac2 a9 7d 0e 37 c6 0b 6d 22 f1 bc 14 0e 0559 048f 78 78 78 78 78 78 78 78 78 78 78 78 78 . . . . . . . . . . 78 78 78 78 78 78 00IP HeaderUDP Header1360 x’s and a 00Page 4IP on 169.125.14.55 must fragment this packet. Please show the fragments that 169.125.14.55 sends, including IP and UDP checksums.Question 12Suppose the wire corresponding to 169.125.0.0 had an MTU of 500, but the wire corresponding to 198.11.109.0 had a MTU of 420. IP on 169.125.14.55 must fragment the packet as in the previous question,but the router R must fragment it some more! Please show the IP packets that will be placed on the 198.11.109.0 wire (don’t forget that router R decrements the TTL).Consider the following Class B configuration on old Ethernet. For the following questions, you do not have to show padding or Ethernet checksums. 169.124.21.187 169.124.21.188 169.124.21.189 169.124.21.190 169.124.22.15 169.124.22.16 169.124.22.17 169.124.22.18 A B C R D E F08:00:3e:a1:92:00 08:00:3e:a1:92:01 08:00:3e:a1:92:02 08:00:3e:a1:92:03 08:00:3e:a1:92:04 08:00:3e:a1:92:05 08:00:3e:a1:92:06 08:00:3e:a1:92:07Question 13Suppose machine R supports Proxy ARP, and machine A is not using subnets. Suppose machine A issues the following ARP request:ff ff ff ff ff ff 08 00 3e a1 92 00 08 06 00 01 08 00 06 04 00 01 08 00 3e a1 92 00 a9 7c 15 bb00 00 00 00 00 00 a9 7c 15 bcPage 5Please show the ARP reply.Question 14Suppose machine R supports Proxy ARP, and machine A is not using subnets. Suppose machine A issues the following ARP request:ff ff ff ff ff ff 08 00 3e a1 92 00 08 06 00 01 08 00 06 04 00 01 08 00 3e a1 92 00 a9 7c 15 bb00 00 00 00 00 00 a9 7c 16 11Please show the ARP reply.Question 15Suppose machine B supports Gratuitous ARP. Show the old Ethernet frame that B sends when it generates a Gratutious ARP request.Question 16If machine B was erroneously configured to have address 169.124.22.18 and machine R supports Proxy ARP, show the response that B will receive from its Gratuitous ARP request (or write “none” if no response is received).Question 17If machine B was erroneously configured to have address 169.124.22.18 and machine R did not support Proxy ARP (instead, all machines used a subnet mask of 255.255.255.0), show the response that B will receive from its gratuitous ARP request (or write “none” if no response is received).Page 6Question 18Suppose machine R supports Proxy ARP. Assuming E’s ARP table is empty, please show the ARP table of machine E after an application on machine A issues an ARP request for E’s address (or write “empty” if E’s ARP table remains empty).Miscellaneous questionsQuestion 19Suppose machine with IP address 198.21.169.24 sets it Time-To-Live field to the hexadecimal value of f3 and sends a packet to 199.23.40.15. We put a sniffer on network 199.23.40.0, and we see the following packet arrive. Assuming every router only decrements the TTL by 1, how many routers handled this packet? 08 00 5a 47 43 58 00 00 0c 19 b5 d1 08 00 4 5 00 00 1d e1 b5 00 00 e2 11 18c5 c6 15 a9 18 c7 17 28 0f 14 0e 10 0f 00 09 00 00 6eQuestion 20If a UDP checksum results in 0, the sender sends all 1’s instead. Why does it do that?Ethernet