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SIT202 Computer Networks Trimester 2, 2018
Problem Solving Report 2 (due 11:59 pm September 7th 2018)
Total Marks =60
This will be converted to 15% of total marks
Question-1: (15 Marks)
1) Your company has recently been expanded and currently 40 users of your company need simultaneous access to the internet. However company has only one public IP address. The network diagram of the company is presented below. Consider an example situations when three computers in switch-1 are communicating with remote server at the same time using one public IP. Assume PC-1 under switch-1 is using TCP port (1020) for this example. The public IP is assigned to the border router’s (NAT router) serial interface. Explain (for PC-1) how packet is sent to and received from the destination remote server by PC-1 through the steps: 1. source and destination addresses in outgoing packets from PC-1; 2. how router keeps track of incoming packet from remote server and forwards to correct sources with private IP inside the company host by using router’s entries.
Marking Guideline:
Overall Explanations (5 marks):? o 5 marks: Explained clearly outgoing packet from PC-1 to router, NAT router’s activities (address change in outgoing packet, router update, address change at incoming packet form destination, how router identify inside source)
o 4 marks: Explained well but there is error in one of the aforementioned points o 3 marks: Average explanation, but there are errors two points as mentioned above o 2 marks: Mostly incorrect and have errors in three points as mentioned above o 1 marks: You have tried to answer but there are many errors, one point is mostly correct and rest of the points are incorrect (as mentioned above)
o 0 marks: You have failed to explain the problem and/or did not attempt.
?
Router entries (8 Marks)? o 8 Mark. Correctly written router entry for PC-1 with all components in the entry (private, public and ports) ?
o 6 Mark. You have written the entry in the table with one error.? o 4 Mark. You have written the entry in the table with two errors.? o 2 Mark. You have written the entry in the table with three errors.? o 0 Mark. You did not try or your answer is wrong.?
?
Source and destination addresses in outgoing packets from PC-1(2 Marks) o 2 Mark. Correctly written two addresses?
o 1 Mark. Correctly written one address, other one is wrong? o 0 Mark. You did not try or your answer is wrong.?

Question-2: (18 Marks)
In the following network diagram, you want to test the connectivity from PC-A to PC-B which are connected through a router in the middle. As part of this you pinged PC-B’s IP address from PC-A. Eventually this causes ICMP at PC-A to create an ICMP echo request packet which is to be delivered to PC-B.
List and briefly describe the detail of IP routing and frame forwarding of the processes that deliver the above ICMP echo request packet to PC-B at the ICMP protocol of PC-B.
Address the following points in your explanation:
1) How IP packet is created from ICMP packet at PC-A, i.e., describe the encapsulation of the ICMP payload into an IP packet (indicating values of relevant IP header fields).
2) How IP at PC-A determines whether PC-B is a local or remote destination including any calculations performed.
3) Once the location is determined in (2), a destination MAC address is required to prepare a frame. If PC-B is in remote, which device’s MAC address will be used for encapsulation of IP packet into a frame; mention the IP of that devices? Explain why this device.
4) Once the frame is prepared and given to the physical layer of PC-A, list and briefly describe the remaining steps to deliver the frame to next device’s IP protocol, including decapsulation, routing decisions, and encapsulation, until the ICMP payload can be processed at PC-B.
Marking Guideline:
1)
Explanation of ICMP packet (3 marks):? o 3 marks: You have clearly explained ICMP packet with required components at PC-A and explained formation of IP packet with minimum required settings.
o 2 marks: Your explanation is mostly complete and clear, however there is one component missing and partly incorrect.
o 1 mark: Your answer is very poor and/or mostly incorrect, missing components in both ICMP and IP packet, however there is some demonstration of understanding / some correct aspects to your answer.
o 0 marks: You have failed to explain the problem and/or did not attempt.
2)
Identification of network addresses (remote or local) (6 marks):? o 6 marks: You have clearly explained the process and correctly shown calculation of network addresses (with result in decimal format).
o 3 marks: Your explanation is correct for one of the two. o 0 marks: You have failed to computation or did not attempt.
3)
IP Address of device (3 marks):? o 3 marks: You have identified the device name and IP of that device and explained this device should be the destination for frame.
o 2 marks: Your explanation shows that you have done mistake in either the device name or IP of that device, and your explanation is partly incorrect. o 1 marks: Your explanation shows that you have done mistake in both the device name and IP of that device, and your explanation is mostly incorrect.
o 0 marks: You have failed to explain the problem and/or did not attempt.
4)
ICMP packet delivery (6 marks):? o 6 marks: You explained correctly the steps of frame delivery to next device’s IP protocol, decapsulation, routing decisions, and encapsulation and rest of the steps of delivery of ICMP packet to PC-B (decapsulation steps, indication of higher layer protocol at IP).
o 5 marks: Your explanation is mostly correct but one steps has incorrect explanation/missing one step mentioned in the above.
o 4 marks: You explained well however you have errors in any two steps mentioned in the above. o 3 marks: Your explanation is average and have errors in any three steps mentioned in the above. o 2 marks: Your explanation is largely incorrect and have four incorrect steps mentioned in the above. o 1 marks: Your explanation is mostly incorrect, have some correct aspect in one of the above steps.
o 0 marks: You have failed to explain and/or did not attempt.
Question-3: (12 Marks)
In the following network diagram two routers are connected through a WAN link. The IP addresses are mentioned for different interfaces of routers and hosts. You have studied different types of dynamic routing in the lecture and practical classes. You are required to configure the routers in the diagram below using dynamic routing protocols.
1. Identify two dynamic routing protocols that will work for the routers below.
2. Also mention a dynamic routing protocol that will not work.
3. Explain your answer for 1 and 2, why (1) will work and (2) will not work.
4. What will be the network addresses when you configure the dynamic routing in the routers?
5. Compare the routing protocols in the above with respect to their convergence properties.
Marking Guideline:
1. Routing protocol (2 marks):? o 2 marks: You have identified the protocols correctly and mentioned the names. o 1 marks: You have identified the protocols correctly and mentioned the names, but one was wrong o 0 marks: You have failed to answer correctly/or did not attempt.
2. Correct name of protocol(1 Marks)
3. Explanation (4 marks):? o 4 marks: You have clearly explained the compatibility of protocols and why they will fit.
o 3 marks: Your explanation has correct components, but some parts are missing and/or incorrect. o 2 mark: Some aspect of your answer is correct and at the same time this has error in half of the part.
o 1 mark: Your answer is very poor and/or mostly incorrect, however there is some demonstration of understanding.
o 0 marks: You have failed to answer correctly/or did not attempt.
4. Network addresses (2 marks):? o 1 marks for each correct answer for network address o 0 Marks: Not attempted or incorrect.
5. Convergence (3 marks):? o 3 marks: You have clearly explained and compared the convergence of protocols o 2 marks: Your explanation has correct components, but some parts are missing and/or incorrect.
o 1 mark: Your answer is very poor and/or mostly incorrect, however there is some demonstration of understanding.
o 0 marks: You have failed to answer correctly/or did not attempt.
Question-4: (15 Marks)
Two processes at the application layers of sender and receiver are communicating each other. Transport layers of both ends are using a stop-and-wait protocol for flow and error control for this communication. An example flow diagram of data and acknowledgement exchange between the nodes has been presented in the following.
Create a flow diagram of data and acknowledgement exchange using stop-and-wait protocol similar to the above diagram where sender sends three packets. The first two packets arrived in time and acknowledgement also received. However third packet was delayed and resent. The duplicate third packet was received after the acknowledgement of original one.
a) Show all the exchange of data and acknowledgement for above three packets in your diagram.
b) Show all timer activities (starts and so on) at the sender side with clear indication of sending and receiving time (receiving side).
c) Show all data acknowledgment, sender and receiver sliding windows and with appropriate sequence numbers in the windows for both sides and next packet indicator.
d) Explain the fact how the duplicate third packet and acknowledgements were exchanged.
Marking Guideline
a. (4 marks):? o 4 marks: You have clearly shown all data and acknowledgement from sender to receiver.
o 3 marks: You have done mistake in one of the cases o 2 mark: You diagram has error in two cases. o 1 marks: You diagram has error in three cases.
o 0 Marks: You have failed to answer correctly/or did not attempt.
b. (4 marks):? o 4 marks: You have clearly shown all timers and times, gaps clearly for sender and receiver.
o 3 marks: You have done mistake in one of the cases such as time gap is not clear or you did not mention all states of a timer for the case or any other error that mislead the timer ‘s states in a wrong way
o 2 mark: You have done mistake in two of the cases such as time gap is not clear or you did not mention all states of a timer for the cases or any other error that mislead the timers’ states in a wrong way
o 1 mark: You have done mistake in three of the cases such as time gap is not clear or you did not mention all states of a timer for the cases or any other error that mislead the timers’ states in a wrong way
o 0 Marks: You have failed to answer correctly/or did not attempt.
c. (4 marks):?
o 4 marks: You have clearly shown all data acknowledgment, sender and receiver sliding windows and with appropriate sequence numbers in the windows for both sides and next packet indicator.
o 3 marks: You have done mistake in one of the cases such as time gap is not clear or you did not mention all states of a timer for the case or any other error that mislead the timer ‘s states in a wrong way
o 2 mark: You have done mistake in two of the cases such as time gap is not clear or you did not mention all states of a timer for the cases or any other error that mislead the timers’ states in a wrong way
o 1 mark: You have done mistake in three of the cases such as time gap is not clear or you did not mention all states of a timer for the cases or any other error that mislead the timers’ states in a wrong way
o 0 Marks: You have failed to answer correctly/or did not attempt.
d. (3 marks):? o 3 marks: You have explained very clearly the fact how the duplicate third packet and acknowledgements were exchanged and timer activities and related sliding windows.
o 2 marks: You have explained but your explanations has error in either timer activities or sliding windows in acknowledgement or data.
o 1 mark: Your explanation is mostly incorrect, a minor parts is correct (e.g. just correct in one of above parts mentioned in 2).
o 0 Marks: You have failed to answer correctly/or did not attempt.

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