COMP3250
Computer Networks
Assignment 1

Due: Week 8 – Tuesday 29th of September 2020, 6 pm.
Total Marks: 50
Weighting (Value): 15%

Objectives
This assignment will test your knowledge in the following areas:
• IP Header
• Intra domain routing
• CIDR
• Route Aggregation and BGP
• Multicast

1. [IP header] (6 marks)
A router R1 acts as a gateway to a VPN tunnel. At the other end of the
tunnel is another gateway R2. The two routers R1 and R2 are not
directly connected to each other. Suppose that the Router R1 receives
an IP datagram that needs to traverse the VPN tunnel. This implies that
this incoming datagram needs to be encapsulated in another IP
datagram before it can be sent over the VPN tunnel. However, R1’s path
MTU information indicates that this incoming datagram also requires
fragmentation. Now, router R1 has two options to deal with this
situation:
Option 1: Encapsulate the original datagram and then subject the
encapsulated packet to fragmentation.
Option 2: Fragment the original datagram and then encapsulate each
resulting fragment.
Explain which of the two options should the router R1 opt for and why.
(200-250 words)
2. (Intra-Domain Routing) [8 marks Identify and explain some features
that can help make a distance vector protocol loop free. The features
may already be part of the distance vector protocol or they may be
additional features. (350-400 words)
3. (Intra-Domain Routing) [6 marks] Explain how the link state routing
algorithm avoids the usual problems of loops during broadcast
(flooding).
Explain why it is still possible for routing loops to exist in link state
routing, albeit for a short duration. Give a concrete scenario illustrating
creation (and then dissolution) of such a loop. (300-350 words)
4. (Intra-Domain Routing) [6 marks] So far in this lecture series, for intra
domain routing scenarios, we have assumed that the shortest path is
the least cost path where cost may be defined in terms of number of
hops, delay and so on. Most routing protocols simply build the overall
cost of a path by adding individual link costs along the path until all
links for the path are considered. Thereafter, the path costs of all
available paths between source. and destination are compared and the
path with least cost is selected for use. In some situations, this strategy
of selecting the best path using least cost is not applicable. Identify and
explain one scenario where this routing strategy may not work. (300-
350 words)
5. (CIDR-Route Aggregation) [4 marks] Summarize the following range
of IP addresses. Explain clearly how you arrive at the solution showing
all relevant steps.
182.68.1.0
182.68.2.0
182.68.3.0
…
182.68.119.0
182.68.120.0
6. (CIDR-Route Aggregation) [8 marks] Consider the scenario illustrated
in the figure below. Two enterprise routers R1 and R2 are directly
connected to 2 networks 24.2.2.0/24 and 24.2.3.0/24 respectively.
Both R1 and R2 advertise an aggregate route 24.2.2.0/23 to router R3.
Suppose the interface I1 of R1 to the network 24.2.2.0/24 encounters
a failure.
However, R1 continues to advertise the aggregate route 24.2.2.0/23 to
router R3 even after the failure of interface I1. Explain why it is so.
Identify and explain any routing anomalies that are likely to surface in
such a scenario. (250-300 words)



7. (Inter-Domain Routing-BGP) [6 marks] Consider the network shown
below. Here a customer (represented by AS100) is connected to two
providers via two separate routers SYD and MEL. Such a customer is
said to be multihomed, with redundancy and geographical restrictions
being the key reasons to implement multihoming. Both the provider
networks meet at a common Network Access Point (NAP) that acts as
the gateway of last resort (192.210.0.0/16). Customer accepts partial
routes (most interesting routes) to destinations from each provider. It
has the option of deciding which route to use to send and receive
packets for each destination that it accepts. The Customer deploys BGP
to interface with its providers. Explain how the BGP can be configured
to help the customer achieve the following objectives:
• (2 marks) Outbound traffic: Traffic must be forwarded to N1 on the
SYD link and N2 on the MEL link. (100-150 words)
• (2 marks) Default: For all the other routes to destinations beyond
NAP that are not known to the customer, default must be taken in
the primary and backup default manner The primary default must
be the MEL link and the secondary default must be the SYD link.
The SYD link serves as a secondary default link and gets activated
only when the MEL link experiences a failure. (100-150 words)
• (2 marks) Inbound Traffic: For all inbound traffic coming from NAP,
the customer would prefer to accept via MEL (primary) link and not
SYD (Secondary) link. The SYD link gets activated only when the
MEL link experiences a failure. (100-150 words)



8. (Multicast-IGMP) [6 marks] A company has registered to receive 8
different channels of a high bandwidth video stream from a central
facility via 8 different multicast groups. The company’s network has the
capability to support three active streams concurrently. The company
has support for multicasting and uses version 1 of IGMP to manage its
active groups. Explain what problem can stem from the use of IGMP
version 1 in this scenario? (250-300 words)

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.
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.