COIT20261 Network Routing & Switching: Written Assessment-2 (2018)

This document provides a detailed solution to the COIT20261 Network Routing and Switching (Term 1, 2018) Assessment-2. It includes answers related to network routing concepts, such as determining next-hop addresses and interfaces based on routing tables. The solution also addresses IP fragmentation, calculating fragment sizes, offsets, and flags. Furthermore, it discusses TCP congestion control mechanisms, comparing current TCP congestion controls with Google’s BBR protocol, and explaining the challenges in adopting new network protocols. The assessment solution provides comprehensive explanations and calculations for each question, demonstrating an understanding of network routing and switching principles.

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
First Name:_________________________ Last Name:____________________________
Student ID: __________________________
Question Number Mark
allocated
Marks
earned
Question 1: (10 marks)
1.
2.
3.
4.
5.
6.
7.
8.
Next hop address is 150.3.0.3/16
Interface is m2
Number of hops is 2
First option, Router discarded the packets. Second option, it forwards the
packet out the interface indicated by the default route entry
Interface is m0
Next hop address is 150.3.0.3/16
Destination network is 150.3.0.0/16
q.8 Routing table of router R2:
Prefix Network address Next-hop address Interface
16 150.3.0.0 Direct -
18 150.32.0.0 150.3.0.1 m0
16 Default Network
(150.3.0.0)
- m0
Show the masks in longest mask order using CIDR format
Network address Longest Mask
150.3.0.0 255.255.255.248 (/29)
Given subnet mask is 16. Here only four IP address need. So that we reduce the
mask.
1-7 1
mark
each, q.8
3 marks

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
Question 2: (5 marks)
a) The maximum size of data field in each fragment is (1500 -20= 1480 where IP
Header size 20 bytes
Number of Fragment required = (5400-20) / 1480 =3.6. So the required
fragment is 4
Fragment Bytes Starting Byte Ending Byte Flag
1st Fragment 1480 0 1479 1
2nd Fragment 1480 1480 2959 1
3rd Fragment 1480 2960 4439 1
4th Fragment (5380 –
1480-1480-
1480) = 940
4440 5379 0
First three fragment of flag is 1. Fourth fragment of flag is 0 (Here 1 is pending
fragment, 0 indicates that this is the last fragment)
2.5
b) Fragment Bytes Offset
1st Fragment 1480 0
2nd Fragment 1480 1480/8 = 185
3rd Fragment 1480 2960/8=370
4th Fragment (5380 –
1480-1480-
1480) = 940
4440=555
1.5
c) First three fragment size is 1480. Last fragment size is 940.
From all four fragment is less than the initial datagram size
1

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
Question 3: (10 marks)
1. Write a brief summary of the congestion controls currently available in TCP as
covered in this Unit
Quote from first link:
“TCP was designed to slow down how fast it sends traffic when it senses
congestion, which it determines by monitoring the number of packets lost in
transport”
Our Explanation:
First, TCP sends the data with a small value of window size. If no loss occurs in
segment, TCP increasing the window size to send the data
If loss occurs in the transmission, TCP connection decreases the window size. If
it finds the unused bandwidth in the network, it automatically increases
window size. This process continues simultaneously
It has few security issues. Hackers may generate the duplicate
acknowledgement (ACK) and send to TCP end point. This may result in
congestion collapse of the network
1
2. Identify and explain two problems with current congestion controls in TCP
that are pointed out in the articles
From the first articles: loss-based congestion control can result in abysmal
throughput because it overreacts, multiplicatively decreasing the sending rate
upon packet loss
Low Bottleneck Bandwidth or Queuing Delay Increase :
If overflow the bandwidth in the path, packet loss occurs. TCP decreases the
sending rate upon the packet loss. Time delay occurs when the low bottleneck
bandwidth in the path of the network
From the second articles: loss-based congestion control causes the
"bufferbloat" problem
Bufferbloat: Undesirable latency that comes from network devices (router or
switch or etc.). Network devices should manage the flow of traffic. Otherwise it
should create interruption of network connection.
2

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
3. Summarize in your own words the difference(s) between the current TCP
congestion controls and Google’s new BBR protocol
Sl. No. Current TCP Congestion
Control Google New BBR Protocol
1 It uses four congestion
algorithms (slow start,
congestion avoidance, fast
retransmit, fast recovery)
It uses a model based congestion
algorithm
2 It controls the transmission
rate by controlling the
transmitted segment which
is yet to be acknowledged
segments
First, it determines the throughput
and roundtrip traffic time in various
network. After that it knows how
long it will carry the data to traverse
the network
3 It continues to increase the
window size until the
segment loss occurs. If end
system may have low
capacity to receive,
bottleneck problem may
occur
Secondly, it sends the data with
calculated traffic size. It avoids
bottleneck starvation
4 If there are more segment
loss, it automatically
reduces the window size. It
does not avoid the queue
delay in each
router/switches (path of the
network)
It minimizes the queue at
bottleneck and all along the path
3

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
4. The Network World article points out that it is difficult to get a new protocol
accepted as a global standard for TCP/IP. Why do you think this is the case?
Give carefully thought out reasons for your answer
Creation of a new protocol is not an easy job and it involves a complicated
process. Synthesis and standardisation would consume quite a lot of time since
it requires patient testing through trial and error and troubleshooting. Till this
time, lot of mere suggestions and predictions such as RENO, VEGAS etc. has
been designed, but which are yet to be researched and tested by using
simulation tools such as NS2, GNS3 and etc.
4
Total marks awarded 25 (max)
Less late penalties if applicable
Less plagiarism penalties if applicable
Total marks earned
Markers comments:

COIT20261 Network Routing and Switching (Term 1, 2018)
Assignment item —Written Assessment-2
References:
Anon, (2018). [online] Available at: https://queue.acm.org/detail.cfm?id=3022184 [Accessed 28
Apr. 2018].
Huston, G. (2018). BBR, the new kid on the TCP block | APNIC Blog. [online] APNIC Blog.
Available at: https://blog.apnic.net/2017/05/09/bbr-new-kid-tcp-block/ [Accessed 28 Apr.
2018].
Congleton, N. and Congleton, N. (2018). Bufferbloat: Fix Your Slow Network - PCMech. [online]
PCMech. Available at: https://www.pcmech.com/article/bufferbloat-fix-slow-network/
[Accessed 1 May 2018].
Tools.ietf.org. (2018). RFC 5681 - TCP Congestion Control. [online] Available at:
https://tools.ietf.org/html/rfc5681 [Accessed 1 May 2018].
Tools.ietf.org. (2018). BBR Congestion Control. [online] Available at:
https://tools.ietf.org/id/draft-cardwell-iccrg-bbr-congestion-control-00.html [Accessed 1 May
2018]002E