School of Computing and Information Systems
COMP30023: Computer Systems
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Tutorial Week 12
Network Layer – routing and control protocols
1. Consider the following network, which represents Brisbane, Sydney, Canberra and Mel-
bourne.. Apply Dijkstra’s algorithm to find the shortest path from (B)risbane to (M)elbourne.
The result should be a table like this
step B S C M
1 0 ∞,− ∞,− ∞,−
Put a box around the values for node being “visited”, and underline the values for closed
2. At peak times, the link from Sydney to Melbourne becomes congested. Choose one new set
of link weights so that
• the shortest path between Sydney and Melbourne is the direct link
• the shortest path between Brisbane and Sydney is the direct link
• the shortest path between Brisbane and Melbourne goes via Canberra.
3. The Open Shortest Path First (OSPF) link-state routing protocl specifies that dijkstra’s
algorithm be used to calculate the shortest paths once all the link states are known.
(a) What could possibly go wrong if a different shortest path algorithm were used?
(b) Why is that unlikely to cause problems in practice?
4. Traceroute is often implemented using ICMP echo request packets as the probes.
(a) Is it possible to use other types packets? Why or why not?
(b) What advantages and disadvantages are there in using other types of packets?
5. A DHCP server knows the MAC address of the host that is requesting configuration. This
means that the server can give the host the same IP address each time the host requests one.
(a) What are the advantages of this static assignment over allocating independent of the
MAC address?
(b) What are the advantages of this over statically configuring the hosts?
(c) What are the disadvantages of this compared to statically configuring the hosts?
6. The ping command is used to detect if the interface associated with an IP address is “alive”:
connected and active. It sends an ICMP “echo request” packet to that host, to which the
host replies with an ICMP “echo reply” packet. The sender also typically records the delay
between sending the request and receiving the reply, giving an indication of the round trip
• Given that most systems use interrupts to process arriving packets, is it possible for a
host to reply even if the host has crashed?
• Some hosts give lower priority to ICMP packets than to TCP and UDP packets. How
could this distort the results of ping?
Weekly tutorial participation activity
To obtain a weekly tutorial mark (1% of your overall mark for the subject, totalling to 10% over
the semester), please answer the following questions in Canvas Quiz (called Week 12 Tutorial
Activity). You can have multiple attempts but need to submit the quiz by 11:59 pm AEST on
the day of your tutorial. Only answering all questions correctly will give you the weeks mark.
During the tutorial, your tutor will provide you with the access code that will unlock the quiz for
the corresponding week. The access code is valid only for students in this tutorial. As a result,
we expect you to attend your tutorial class as otherwise the access code you obtain in another
tutorial will not work for you.
If you have a valid reason for not being able to attend your tutorial, please fill in the form accessible
via Canvas by Friday 8pm AEDT of the corresponding week. You will be given an access code
during the next business day. The code will not be provided otherwise, hence, please do not email
the subject coordinator or other staff members including your tutor asking for the code. We will
monitor such requests and may limit the number of times an access code is given to the same
studentthroughout the semester to encourage participation and attendance of the tutorials.
1. For the figure in Question 1, which of the following is not a possible set of shortest paths
and their costs? You may use Bellman’s principle.
(a) BS = 3; SM = 4; BSM = 7
(b) BS = 3; SM = 4; BCM = 6
(c) BS = 4; SM = 4; BSM = 9
(d) BS = 3; SM = 4; BCM = 2
2. How can the cost associated with a node change in Dijkstra’s algorithm?
(a) It can go up, but not down
(b) It can go down, but not up
(c) It can go either up or down
(d) It cannot change
3. ICMP consists mainly of one-way messages, rather than two-way exchanges like most pro-
tocols use. There is one frequent exception. Which command uses it?
(a) traceroute
4. In Question 1, what is the cheapest path that visits all nodes?
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