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...

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 cu
ently 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
emote server at the same time using one public IP. Assume PC-1 under switch-1 is using TCP port XXXXXXXXXXfor 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 co
ect 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 e
or in one of the aforementioned points
o 3 marks: Average explanation, but there are e
ors two points as mentioned above
o 2 marks: Mostly inco
ect and have e
ors in three points as mentioned above
o 1 marks: You have tried to answer but there are many e
ors, one point is mostly co
ect
and rest of the points are inco
ect (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. Co
ectly written router entry for PC-1 with all components in the entry (private, public and ports) 
Human Resource
XXXXXXXXXX/24
Server-2
XXXXXXXXXX/24
Accounting Serve
XXXXXXXXXX/24
Fa0/0
XXXXXXXXXX/24
Fa0/1
XXXXXXXXXX/24
S0/0
XXXXXXXXXX/27
Server-1
XXXXXXXXXX/24
PC-1
XXXXXXXXXX/24
PC-2
XXXXXXXXXX/24
Internet
Remote Serve
Switch-1 Switch-2
o 6 Mark. You have written the entry in the table with one e
or.
o 4 Mark. You have written the entry in the table with two e
ors.
o 2 Mark. You have written the entry in the table with three e
ors.
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. Co
ectly written two addresses
o 1 Mark. Co
ectly 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
iefly 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
emote, 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
iefly 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 inco
ect.
o 1 mark: Your answer is very poor and/or mostly inco
ect, missing components in both ICMP
and IP packet, however there is some demonstration of understanding / some co
ect aspects
to your answer.
Fa0/0
XXXXXXXXXX/28
Fa0/1
XXXXXXXXXX/27
PC-A
XXXXXXXXXX/28
PC-B
XXXXXXXXXX/27
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 co
ectly shown calculation of network
addresses (with result in decimal format).
o 3 marks: Your explanation is co
ect 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 inco
ect.
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 inco
ect.
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 co
ectly the steps of frame delivery to next device’s IP protocol, decapsulation,
outing 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 co
ect but one steps has inco
ect explanation/missing one step
mentioned in the above.
o 4 marks: You explained well however you have e
ors in any two steps mentioned in the above.
o 3 marks: Your explanation is average and have e
ors in any three steps mentioned in the above.
o 2 marks: Your explanation is largely inco
ect and have four inco
ect steps mentioned in the above.
o 1 marks: Your explanation is mostly inco
ect, have some co
ect 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
outing 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 co
ectly and mentioned the names.
o 1 marks: You have identified the protocols co
ectly and mentioned the names, but one was
wrong
o 0 marks: You have failed to answer co
ectly/or did not attempt.
2. Co
ect 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 co
ect components, but some parts are missing and/or
inco
ect.
o 2 mark: Some aspect of your answer is co
ect and at the same time this has e
or in half of
the part.
o 1 mark: Your answer is very poor and/or mostly inco
ect, however there is some
demonstration of understanding.
o 0 marks: You have failed to answer co
ectly/or did not attempt.

4. Network addresses (2 marks):
o 1 marks for each co
ect answer for network address
o 0 Marks: Not attempted or inco
ect.

5. Convergence (3 marks):
o 3 marks: You have clearly explained and compared the convergence of protocols
o 2 marks: Your explanation has co
ect components, but some parts are missing and/or
inco
ect.
o 1 mark: Your answer is very poor and/or mostly inco
ect, however there is some
demonstration of understanding.
o 0 marks: You have failed to answer co
ectly/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 e
or control for this communication.
An example flow diagram of data and acknowledgement exchange between the nodes has been presented
in the following.

Fa0/0
XXXXXXXXXX/24
XXXXXXXXXX/24
Fa0/0
XXXXXXXXXX/24
S0/0
XXXXXXXXXX/24
S0/0
XXXXXXXXXX/24
XXXXXXXXXX/24


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 a
ived 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.
) Show all timer activities (starts and so on) at the sender side with clear indication of sending and
eceiving 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.