THE UNIVERSITY OF SUSSEX
BSc and MComp SECOND YEAR EXAMINATION May/June 2018 (A2)
Computer Networks
Assessment Period: May/June 2018 (A2)
DO NOT TURN OVER UNTIL INSTRUCTED TO BY THE LEAD INVIGILATOR
Candidates should answer TWO questions out of THREE. If all three questions are attempted only the first two answers will be marked.
The time allowed is TWO hours.
Each question is worth 50 marks.
At the end of the examination the question paper and any answer books/answer sheets, used or unused, will be collected from you before you leave the examination room.
Candidate Number
G5115
G5115
1. (a)
Computer Networks
A network switch receives a packet p and determines the outbound link to which it should be forwarded. When p arrives, one other packet is halfway done being transmitted on this outbound link and four other packets are waiting to be transmitted. Packets are transmitted in order of arrival. Suppose all packets are 1500 bytes and the link rate is 2 Mbps. What is the queuing delay for packet p? [10 marks]
(b) Which layers in the Internet protocol stack does a router process? What is the purpose of each one of these? [10 marks]
(c) The HTTP protocol is stateless. Explain what that means. What are the implications of HTTP¡¯s statelessness? What mechanism is used to provide state with HTTP, and how does this mechanism work?
[20 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]
2
Computer Networks G5115
2. (a)BrieflydescribetheconnectionterminationprocessinTCP.Whichofthe communicationendpointscaninitiatetheprocessandwhy? [20marks]
(b) Explain why and how the size of the receiver¡¯s window (rwnd) changes throughout the duration of a TCP connection. [10 marks]
(c) At time t, the congestion window of a TCP sender is 2000 bytes long. The maximum segment size (MSS) used by the connection is 1000 bytes. What is the size of the congestion window after it sends out 2 packets (of MSS each) and receives acknowledgements for all of them if the connection is in slow-start mode and why? Suppose the receiver sends one acknowledgement per packet and that the sender has always data available to send. [10 marks]
(d) Hosts A and B are communicating over a TCP connection. Host B has already received from A and acknowledged all bytes up to byte 20000. Suppose Host A then sends two segments to Host B back-to- back. The first and second segments contain 300 and 100 bytes of data, respectively. In the first segment, the sequence number is 300000, the source port number is 4000, and the destination port number is 1000. Host B sends an acknowledgement whenever it receives a segment from Host A.
i. In the second segment sent from Host A to B, what is the sequence number, source port number, and destination port number and why? [5 marks]
ii. 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 and why? [5 marks]
3 Turn over/
G5115
3. (a)
Computer Networks
Consider a router that interconnects four subnets: Subnet 1, Subnet 2, Subnet 3 and Subnet 4. Suppose all of the interfaces in each of these four subnets are required to have the prefix 195.200.0.0/23 (i.e., a /23 CIDR portion of the IP space is provided). Also suppose that Subnet 1 is required to support 100 interfaces, Subnet 2 is required to support 110 interfaces, Subnet 3 is required to support 60 interfaces and Subnet 4 is required to support 80 interfaces. What would be the network and broadcast addresses (of the form a.b.c.d/x) of these four subnets, assuming the most economical allocation of IP addresses to the given subnets? [20 marks]
Given the IP routing table below, which interfaces are used for packets with the following destination IP addresses?
(b)
IP Subnet
0.0.0.0 (default gateway) 145.0.0.0/8 145.40.0.0/16 145.40.50.0/24
i. 145.40.50.3 ii. 145.40.50.40 iii. 145.40.40.50 iv. 145.40.40.40 v. 145.50.40.40
Outgoing interface if1
if2
if3
if4
(c)
(d)
Suppose nodes A, B, and C each attach to the same broadcast Ethernet LAN (through their adapters). If A sends an IP datagram to B with the encapsulating Ethernet frame addressed to the MAC address of B, will C¡¯s adapter process this frame? If so, will C¡¯s adapter pass the IP datagram in this frame to the network layer C? How could A learn B¡¯s MAC address? [10 marks]
Which problem of symmetric key cryptography does public key cryptography solve and how? [10 marks]
4 End of Paper
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