Department of Computing
Macquarie University
COMP3250 Computer Networks
Assignment Two 2020

Worth: 60 Marks
Weight: 15%
Lecturer In-Charge: Nardin Hanna
Due Date: Week 12, Thursday 29th Oct 2020 at 5 pm.
Topics Covered:
• TCP
• Security
• IPv6

1. TCP (15 Marks)
Host A and B are communicating over a TCP connection, and Host B has already received
from A all bytes up to byte 126. Suppose Host A then sends two segments to Host B back-
to-back. The first and second segments contain 80 and 40 bytes of data, respectively. In
the first segment, the sequence number is 127, the source port number is 3024, and the
destination port number is 80. Host B sends an acknowledgement whenever it receives a
segment from Host A.
a. (3 marks) In the second segment sent from Host A to B, what are the sequence number,
source port number, and destination port number?
b. (3 marks) If the first segment arrives before the second segment, in the acknowledgement of
the first arriving segment, what is the acknowledgement number, the source port number, and
the destination port number?
c. (4 marks) If the second segment arrives before the first segment, in the acknowledgement of
the first arriving segment, what is the acknowledgement number?
d. (5 marks) Suppose the two segments sent by A arrive in order at B. B sends
acknowledgements for both segments, but the first acknowledgement is lost and the second
acknowledgement arrives after the first timeout interval. Draw a timing diagram showing these
segments and all other segments and acknowledgements sent (you can assume no other
segment loss). For each segment in your diagram, provide the sequence number and the number
of bytes of data; for each acknowledgement that you add, show the acknowledgement number.

2. Security (25 marks)
Question 1 (15 marks)
Outline a security architecture for the distributed environment shown below, where users
wish to access enterprise services from various remote locations such as home, airport
and other branch offices.
a. (5 marks) Analyse the security threats which can arise in such an environment. State any
assumptions that you are making. (300-400 words)
b. (5 marks) Specify the types of security services that would be needed to counteract these
security threats and what type of security mechanisms could support these services. (300-
400 words)
c. (5 marks) Identify the types of security components that can be used to provide these
security services and mechanisms and where they would be placed. (300-400 words)

Question 2 (10 marks)
a. (5 marks) IPSec can be used to prevent replay attacks on IP datagrams. How does
IPSec accomplish this goal? Explain your answer in detail. (200-250 words)

b. (5 marks) In IPSec, the ESP header provides both authentication as well as
confidentiality services whereas the AH header simply provides authentication
service. Why would you still want to apply both AH and ESP on a packet? (150-200
words)

3. IPV6 (20 marks)
Enumerate and evaluate at least five differences between IPv4 and IPv6. A starting point for
this would be the differences in the IP headers. The most basic example is that the address
space has expanded from 32 bits to 128 bits. This addresses the enumeration part which is
syntax oriented (the syntax). But you should evaluate it as well. That means finding out why
they might have made such a change (the reason). You should also investigate the effect this
has on the protocol and how it works (the semantics). Is a change made to tidy up a detail, for
performance (to make routing easier or faster), etc.
Your approach to this should be to gain some insight into the IP protocol and indeed into the
design and evolution of protocols in general, what mistakes can be made and how to avoid
them.
Each difference you evaluate should take between 150 and 250 words. You should use
tables and graphics and structure your answer well so that a reader can quickly get an
overview and then read the detail.

Assignment and Submission Guidelines
Assessment
For all questions in this assignment not only content but also presentation will affect your
mark. You will lose marks (and not necessarily only a small portion) if there are problems with
the presentation, particularly with clarity.
This means that your answers to each question should be a coherent statement and that the
spelling and grammar of your submission will be considered in assessing its presentation.
Note
• Answers must be within the specified word limit. This is an absolute word limit and no excess
will be allowed.
• Assumptions (if any) must be stated clearly in your answers.
Remember, there may not be one right answer for some of the questions. Rather, your
explanations do need to present your case clearly. The explanations you provide do not have
to be long, concise is preferred to meandering.
For full marks, your answers should all be clear, coherent, and correct.
The standards of marking described in the unit outline will be applied to this assignment as
relevant to the assignment topics.
In addition, the following standards will be applied in marking this assignment:
• Spelling and grammar:
o Assignment submissions with more than 4 spelling or grammatical errors will not achieve a
grade higher than distinction; submissions with more than 8 such errors will not achieve a grade
higher than credit.
• Clarity:
o Ambiguous or poorly worded answers will receive a grade no more than a pass for the
individual question.
o Minor issues of clarity will receive a grade no more than credit for the individual question.
• Correctness of approach taken, and answer obtained:
o Incorrect answers with the correct logic or approach will receive no more than a pass for the
individual question.
o Correct answers with incorrect logic or approach will receive no more than pass for the
individual question.
o Incorrect answers with no explanation of the approach taken or with the incorrect approach
will receive a fail grade for the individual question.
The questions will be marked individually, the marks totalled, and a final grade assigned that
is no more than indicated by the total marks, and no more than allowed by the standards
specified above and in the unit outline.
Bibliography Guidelines
In your bibliography be careful to identify the source of all material that you use and to clearly
identify any quotations that you make.
Any quotations that are not properly identified will be considered as plagiarism and will attract
a penalty. For more information on how to write bibliographies, you can find resources on the
Internet such as How to Write a Bibliography (an introduction) or Bibliography Guides. We
strictly recommend ACM and IEEE bibliography style.
Font Guidelines
It is strongly recommended to use Times New Roman with a font size 12.
Submission
• Online submission via Turnitin.
Assignments will normally be marked and returned online. There are no hardcopy submissions
for written assignments.
Ensure you submit the correct file. The submission process shows you a complete preview of
your entire assignment after you have uploaded it but before you have submitted it. Carefully
check through every single page to ensure everything is there and the correct version has been
uploaded, and only then press CONFIRM.
Multiple submissions may be possible via Turnitin prior to the final due date and time of an
assessment task and originality reports will be made available to the students.
Teaching staff will use the report to judge whether plagiarism has occurred and whether
penalties should apply for breaches of the Academic Honesty Policy. Any similar text
identified by Turnitin will be considered carefully to see if it is indeed a breach of the Academic
Honesty Policy