COMP90007 Internet Technologies Week 8 Workshop
Semester 2, 2021
Suggested solutions
* © University of Melbourne 2021 1
Question 1
A router has just received the following IP addresses: 57.6.96.0/21, 57.6.104.0/21, 57.6.112.0/21 and 57.6.120.0/21. If all of them use the same outgoing line, can they be aggregated? If so, to what? If not, why not?
Answer:
They can be aggregated to 57.6.96.0/19
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Question 2
Why do we need routing algorithms in the Network layer? What are the key categories of routing algorithms?
Answer: Routing algos are needed to help decide on which output line an incoming packet should be transmitted.
Key Categories: 📫Non-Adaptive Algorithms 📫Adaptive Algorithms
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Question 3
Compute the sink tree for Node F in the graph below:
Ans. Refer to Dijkstra’s algorithm on the Slides 53-55 of Network Layer
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Question 4
Distance vector routing is used for the diagram shown below, and the following vectors have just come in to router C: from B: (5, 0, 8,
12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The cost of the links from C to B, D, and E, are 6, 3, and 5, respectively. What is C’s new routing table? Give both the outgoing line to use and the expected delay.
Answer: Using the delays 6, 3, and 5 for B, D, and E, the vectors will be written as:
All Routers
A
B
C
D
E
Via B
11
6
14
18
12
*F
5
8
Via D
19
15
9
3
12
13
Via E
12
11
8
14
5
9
All Routers
A
B
C
D
E
F
Outgoing Line
B
B
–
D
E
B
Expected Delay
11
6
0
3
5
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Question 5
If the Path MTU Discovery is used to send a packet of 1200 bytes from the source to the destination as shown in Figure. The maximum packet size for each network on the path is labelled on the link.
■ How many trials does the source machine need to send this packet?
■ Which routers on the path send ICMP messages to the source?
1200
800 1100 600 800
ABCD
source
destination
Answer: (1) The packet can be delivered after 2 trials
(2) routers A, and C will send ICMP messages “Destination Unreachable” to the source. The initial message is 1200 bytes, and router A will send ICMP message.
After processing, the packet will be 800 bytes, and router C
will send ICMP message, as the MTU of the following network
is 600 bytes. After this trial, the packet can be sent to the
destination
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