BE Computer Engineering (Pokhara University) Computer Networks (PU, CMP 344) Question Paper 2079

(a) Compare and contrast the OSI reference model and the TCP/IP protocol suite, clearly identifying the correspondence between their layers. (b) For a message originating at the application layer of a sender and travelling to the application layer of a receiver, describe the process of encapsulation and decapsulation, naming the protocol data unit (PDU) at each layer. (c) Why does the practical Internet follow the TCP/IP model rather than the OSI model? Justify with at least two reasons.

An organization has been allocated the class C network address 192.168.20.0/24 and needs to create subnets for five departments having 50, 25, 12, 10 and 6 hosts respectively. (a) Using Variable Length Subnet Masking (VLSM), design an efficient subnetting scheme. For each subnet give the subnet address, subnet mask (in both dotted-decimal and prefix notation), the usable host range and the broadcast address. (b) State how much address space (if any) remains unused after the allocation. (c) Explain why VLSM is more efficient than fixed-length subnetting for this scenario.

(a) Differentiate between distance-vector and link-state routing protocols with respect to the information exchanged, convergence behaviour and scalability. (b) Apply Dijkstra's shortest-path algorithm to compute the shortest paths from node A to all other nodes in a network with the following bidirectional links and costs: A-B = 2, A-C = 5, B-C = 1, B-D = 7, C-D = 3, D-E = 1, C-E = 6. Show the iterative steps and the resulting routing table at node A. (c) Explain the count-to-infinity problem in distance-vector routing and describe one technique used to mitigate it.

(a) Describe the TCP three-way handshake used for connection establishment, including the flags and sequence/acknowledgement numbers exchanged. (b) Explain how TCP provides reliable, in-order delivery using sequence numbers, acknowledgements and retransmission timers. (c) With a suitable example, explain TCP flow control using the sliding window mechanism and contrast it with TCP congestion control. (d) State two application scenarios where UDP is preferred over TCP and justify your choice.

Explain the working of the Domain Name System (DNS). Describe, with a diagram, the steps involved in resolving the hostname www.pu.edu.np to its IP address using iterative and recursive queries.

(a) Differentiate between non-persistent and persistent HTTP connections. (b) What is the role of cookies in HTTP, given that HTTP is a stateless protocol? Explain with an example of a typical e-commerce session.

Compare a hub, a switch and a router in terms of the OSI layer at which they operate, their handling of collision and broadcast domains, and their forwarding decision criteria. Where would each device be appropriately deployed in a campus network?

(a) Explain the hidden terminal problem in wireless LANs. (b) Describe how the CSMA/CA mechanism along with RTS/CTS handshaking is used in IEEE 802.11 to address this problem.

Describe the architecture of the Simple Network Management Protocol (SNMP). In your answer, explain the roles of the manager, the agent, the Management Information Base (MIB) and the SNMP messages (Get, Set, Trap).

(a) What is a Virtual LAN (VLAN)? Explain how VLANs segment a switched network and improve security and broadcast control. (b) Briefly explain the purpose of VLAN trunking and the IEEE 802.1Q tagging mechanism.

Draw and label the format of a UDP datagram header and a TCP segment header. Highlight three header fields present in TCP but absent in UDP, and explain the function of each.

(a) Differentiate between IPv4 and IPv6 addressing in terms of address length, header complexity and address configuration. (b) Briefly explain any two transition mechanisms (such as dual stack, tunnelling or NAT64) used to migrate from IPv4 to IPv6.