BEng. (Hons) Telecommunications. Examinations for / Semester 2

Transcription

1 BEng. (Hons) Telecommunications Cohort: BTEL/16B/FT Examinations for / Semester 2 Resit Examinations for BTEL/15B/FT MODULE: NETWORKS MODULE CODE: CAN 1102C Duration: 2 ½ hours Instructions to Candidates: 1. Answer ALL 4 (four) questions. 2. Questions may be answered in any order but your answers must show the question number clearly. 3. Always start a new question on a fresh page. 4. All questions carry equal marks. 5. Total marks 100. This question paper contains 4 questions and 9 pages. Page 1 of 9

2 ANSWER ALL 4 (FOUR) QUESTIONS QUESTION 1: (25 MARKS) Consider the setting of Figure 1.1, and assume that a person in the home network wants to access a webpage on the institution network webserver and that the TCP/IP protocol stack is used for the connection. Fig 1.1: Typical network infrastructure a) Briefly describe the five layers of the TCP/IP protocol stack, name the packets that are used at of the top four layers and list the protocols that the person will use on the top three layers. ( marks) Page 2 of 9

3 b) Assume that the webserver has the address Write down a full DOS command the person can use to check whether the webserver is reachable? (1 mark) c) Write the full command the person should use to get the list of routers that the connection will need to go through to reach that website? (1 mark) d) Assume that there are five routers between the person s laptop and the webserver, draw the protocol stack of the laptop, the five routers and the webserver and illustrate how a packet will flow from the application layer of the laptop to the application layer on the webserver. (3 marks) e) When packets flow on networks, there can be delays. State and briefly describe two types of delays that can happen at any of the routers during the communication with the webserver. In addition, packets can also get lost. Briefly explain why. (4 + 2 marks) f) Instead of being at home, assume that the person is connected to a PC at the institutional network. In Fig 1.1, the PCs are shown to connect directly to the router. That is not the case in practice. i) State the device to which the PC will normally connect. ii) State the protocol that is used by that device identified in i). iii) What is the advantage when connecting to the webserver from the institutional network? (½ + ½ + 1 marks) Page 3 of 9

4 QUESTION 2: (25 MARKS) a) Two types of architectures that are used to obtain resources are client-server and peer-to-peer. i) Why does the server need an always-on, fixed IP address whereas the clients do not? ii) One advantage of the peer-to-peer network is self-scalability. Briefly explain. (2 + 2 marks) b) Consider the diagram below where a web client is connected to a webserver using HTTP: Web client Web server Fig 2.1: HTTP connection between client and server using sockets i) We say that HTTP needs the service of TCP for reliable transport. Briefly explain the meaning of reliable transport. (3 marks) ii) What exactly does a TCP socket consist of? Briefly explain the importance of sockets? (2 + 3 marks) Page 4 of 9

5 iii) Assuming that the person operating the web client has some knowledge of networking concepts and decide to telnet the webserver. If the person types the following code: telnet 80 in a DOS prompt, what will happen? iv) What will the person see if he next type: GET / HTTP/1.1 (presses enter once) host: (presses enter twice) into the DOS prompt? v) If instead of typing GET, the person uses the command HEAD, what difference will that make? c) HTTP uses persistent connections by default. Contrast the meaning of HTTP persistent connections with non-persistent connections. (3 marks) d) TCP provides flow control and congestion control. Very briefly explain these two concepts? (2 + 2 marks) QUESTION 3: (25 MARKS) a) If UDP does not provide reliability, what advantage does it provide that makes it useful? Provide an example of an application that is best suited for UDP. Page 5 of 9

6 (2 + 1 marks) b) We say that a web cache i) reduce response time for client request and ii) reduce traffic on an institution s access link. Briefly explain how these advantages are achieved. (2 + 2 marks) c) Web caches use Conditional GET when connecting to the origin server. Briefly explain the use of conditional GET? d) Assume that Alice wants to send Bob an . Fig 3.1 shows how this operates. Fig 3.1: Process used for Alice to send an to Bob i) Which application layer and transport layer protocols will Alice use? ii) Briefly explain the procedure including the layer 4 protocols that may be involved for the of Alice to reach Bob s mail server? (4 marks) iii) Bob can use IMAP or POP3 to retrieve the mail from Alice. Briefly describe the difference between the two protocols. (2 + 2 marks) e) State and briefly describe the three types of DNS servers in use on the internet. (6 marks) Page 6 of 9

7 QUESTION 4: (25 MARKS) a) Consider a UDP packet as shown in Fig 4.1: Fig 4.1: UDP c i) What is the use of the length field of the UDP header? (1 mark) ii) What is the maximum theoretical size of a UDP packet? Explain your reasoning. (1 + 2 marks) iii) What is the function of the checksum? iv) Where does the payload of the UDP packet originate? Very briefly explain your answer. (1 + 1 marks) b) Consider the graph of Fig 4.2 which describes the congestion mechanism used by TCP. Page 7 of 9

8 Fig 4.2: TCP switching from slow start to congestion avoidance mode Explain the strategies of the two TCP implementations, namely TCPTahoe and TCP Reno with respect to Fig 4.2. (6 marks) c) Two important techniques used by routers are routing and forwarding. Very briefly differentiate between the two. d) One problem faced by the IP layer is fragmentation. Consider Fig 4.3 which depicts such a fragmentation. Page 8 of 9

9 (a) (b) Fig 4.3: a) IP packet with size 4000 bytes that needs to be carried over a link with MTU 1500 bytes, b) the first fragmented packet i) Briefly explain why IP fragmentation occurs. ii) Specify the ID of the first fragmented IP packet (1 mark) iii) Specify the length, ID, fragflag and offset values of the remaining fragmented packets. (6 marks) ***END OF QUESTION PAPER*** Page 9 of 9