Candidates should answer TWO questions out of THREE. If all three questions are attempted only the first two answers will be marked.
Each question is worth 50 marks.
G5115
1. (a)
Computer Networks
The following output is produced by running ¡®traceroute www.google.com¡¯ in the command line.
1 139.184.0.14 (139.184.0.14) 0.854 ms
2 146.97.146.181 (146.97.146.181) 2.455 ms
3 ae1-0.aldess-rbr1.ja.net (146.97.68.46) 3.540 ms
4 ae23.londpg-sbr2.ja.net (146.97.37.249) 4.878 ms
5 ae29.londhx-sbr1.ja.net (146.97.33.1) 5.599 ms
6 193.62.157.22 (193.62.157.22) 5.579 ms
7 108.170.246.144 (108.170.246.144) 5.991 ms
8 …
When running the same command for a second time after a short time period:
i. The output would be exactly the same as the first time.
ii. The IP values will most likely be different, but the time values will be exactly the same.
iii. The time values will most likely be different, but the IP values will be the same.
iv. Both IP and time values will most likely be different.
v. None of the above.
Briefly explain your answer. [10 marks]
(b) Let us suppose that a number of users share a 10 Mbps link. Each user transmits continuously at 2 Mbps when transmitting, but each user transmits only 10 percent of the time. If circuit switching is used, how many users can be supported?
i. 50 users ii. 20 users
iii. 5 users
iv. No upper limit exists but the link will be congested.
v. None of the above. Briefly explain your answer.
[10 marks]
(c) A user requests a Web page that consists of the base HTML code, three images, one CSS file and two JavaScript files. For this page:
i. The client will send seven HTTP requests (one for each object) and receive seven HTTP responses.
ii. The client will send one HTTP request (for the whole page) and receive one HTTP response.
iii. The client will send one HTTP request (for the base HTML file) and receive seven HTTP responses.
iv. The client will send four HTTP requests (one for each MIME type) and receive seven HTTP responses.
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Computer Networks G5115
v. The client will send six HTTP requests (one for each object except the base HTML file) and receive seven HTTP responses.
Briefly explain your answer. [10 marks]
(d) Provide an example of an iterative DNS query for ¡®www.sussex.ac.uk¡¯ and briefly explain each step until the IP address of the requested host name is resolved. Assume that you have configured your machine to use Google¡¯s DNS as the local DNS server. [10 marks]
(e) Suppose that the University of Sussex has a single local DNS server for all computers in the university. You are an ordinary user (i.e., not a network/system administrator). Can you determine if an external Web site was likely accessed from a computer in your department a couple of seconds ago? Briefly explain your answer. [10 marks]
3 Turn over/
G5115
2. (a)
Computer Networks
Suppose that a TCP server running at Host C listens to port 6000. Suppose that the server is currently receiving requests from two different Hosts, A and B. Which of the following statements is correct?
i. The requests from A and B pass through different sockets at Host C, both having port number 6000.
ii. The requests from A and B pass through the same socket at Host C with port number 6000.
iii. The requests from A and B pass through different sockets at Host C, with randomly generated port numbers.
iv. The requests from A and B pass through the same socket at Host C with a randomly generated port number.
v. None of the above.
Briefly explain your answer. [10 marks]
Hosts A and B are communicating over a TCP connection, and Host B has already received from A and acknowledged all bytes up to byte 550. Suppose Host A then sends two segments to Host B back-to- back. The first and second segments contain 200 and 400 bytes of data, respectively. In the first segment, the sequence number is 10000, the source port number is 45000, and the destination port number is 80. Host B sends an acknowledgement whenever it receives a segment from Host A. In the second segment sent from Host A to B, what is the sequence number, source port number, and destination port number?
(b)
(c)
For the example above, if the second segment arrives before the
first segment, in the acknowledgement of this segment, what is the acknowledgement number, the source port number, and the destination port number?
(d)
Consider the figure below, which plots the evolution of TCP¡¯s congestion window at the beginning of each time unit (where the unit of time is equal to the RTT; i.e. a transmission round). TCP Reno is used here.
i. Seq. no: 10400, source port: 45000, destination port: 80. ii. Seq. no: 10400, source port: 80, destination port: 45000.
iii. Seq. no: 550, source port: 45000, destination port: 80. iv. Seq. no: 10200, source port: 45000, destination port: 80.
v. Seq. no: 750, source port: 45000, destination port: 80.
Briefly explain your answer.
[5 marks]
i. Ack. no: 750, source port: 80, destination port: 45000 ii. Ack. no: 950, source port: 45000, destination port: 80.
iii. Ack. no: 10400, source port: 80, destination port: 45000. iv. Ack. no: 10200, source port: 80, destination port: 45000. v. Ack. no: 10000, source port: 80, destination port: 45000.
Briefly explain your answer.
[5 marks]
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Computer Networks G5115
20
18
16
14
12
10
8 6 4 2 0
1 2 3 4 5 6 7 8 9 1011121314151617181920212223242526272829 Transmission round
In the abstract model for this problem, TCP sends a ¡°flight¡± of packets of size cwnd (the congestion window) at the beginning of each time unit. The result of sending that flight of packets is that either (i) all packets are ACKed at the end of the time unit, (ii) there is a timeout for the first packet, or (iii) there is a triple duplicate ACK for the first packet.
i. What are the time intervals during which TCP is in the slow start phase? [5 marks]
ii. What are the time intervals during which TCP is in the congestion avoidance phase? [5 marks]
iii. What is the event that occurred at the 5th transmission round that triggered a new TCP phase? [5 marks]
iv. What is the event that occurred at the 9th transmission round that triggered a new TCP phase? [5 marks]
v. What is the ssthresh (slow start threshold) value at the 7th transmission round? [5 marks]
vi. What is the ssthresh (slow start threshold) value at the 21th transmission round? [5 marks]
5 Turn over/
cwnd
G5115
3. (a)
Computer Networks
Are the following statements true or false? Briefly explain your answer.
i. Before any forwarding decision can be made on any single packet, routers need to exchange control messages. [5 marks]
ii. In a datagram network, routers need to keep state for each incoming packet in their forwarding table. [5 marks]
(b) Consider a home network with 3 networked devices (A, B and C), all connected to the Internet through a single router that supports the Network Address Translation (NAT) protocol. The ISP is only providing a single public IP address to this network (82.7.30.73). Which one of the following is a valid IP assignment?
i. A: 192.168.0.1/24, B: 192.168.0.2/24, C: 192.168.0.3/24 ii. A: 192.168.1.1/24, B: 192.168.1.2/24, C: 192.168.1.3/24
iii. A: 10.0.0.1/24, B: 10.0.0.2/24, C: 10.0.0.3/24 iv. All of the above
v. None of the above Briefly explain your answer.
[10 marks]
(c) For the network described above, let us assume that one of the devices with IP address 192.168.0.1 (for some IP assignment) has an open connection from source port 30000 to an external server with IP 195.251.252.168 at port 80. Are the following statements true or false? Briefly explain your answer.
i. The source IP address of incoming packets of this connection at the server will be 192.168.0.1. [2 marks]
ii. The source port of incoming packets of this connection at the server could be 8765. [2 marks]
iii. The source IP address of incoming packets of this connection at the server will be 82.7.30.73. [2 marks]
iv. The destination IP address of packets of this connection as they arrive at the NAT router will be 192.168.0.1. [2 marks]
v. The destination IP address of incoming packets of this connection
at the client will be 192.168.0.1.
(d) You are given the IP routing table below.
[2 marks]
IP Subnet
0.0.0.0 (default gateway) 151.0.0.0/8 151.64.0.0/16 151.64.2.0/24 151.64.2.128/25
Outgoing interface if1
if2
if3
if4 if5
Which interfaces are used for forwarding packets with the following destination IP addresses and why?
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i. 151.64.2.148 ii. 151.64.2.21 iii. 151.64.2.35 iv. 151.65.2.12
v. 151.64.2.107
(e) Suppose nodes A, B, and C are attached to the same Ethernet switch (through their adapters). Are the following statements true or false? Briefly explain your answer.
i. If A sends an IP datagram to B with the encapsulating Ethernet frame addressed to the MAC address of B, C¡¯s adapter will process this frame but its network layer will drop it. [5 marks]
ii. The Ethernet switch will send the frame through all its ports except the one where the frame came from, if it has not previously learned which port C is connected to. [5 marks]
Computer Networks
G5115
[2 marks] [2 marks] [2 marks] [2 marks] [2 marks]
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