CS 6250 Quiz 4 Questions and Answers

CS 6250 Quiz 4 | Questions and Answers | 2026 Update | 100% Correct - Georgia Institute Of Technology. 1. Which layer is responsible for end-to-end reliability? Answer: Transport layer Rationale: The transport layer (e.g., TCP) ensures reliability, ordering, and retransmission between hosts. 2. TCP congestion control primarily reacts to: A. Link failures B. Packet loss C. Encryption errors D. Routing loops Answer: B Rationale: TCP interprets packet loss as congestion and reduces its sending rate. 3. True or False: UDP provides flow control. Answer: False Rationale: UDP is connectionless and provides no flow or congestion control. 4. What does the TCP congestion window (cwnd) limit? Answer: Amount of unacknowledged data in flight Rationale: cwnd controls how much data TCP can send before receiving ACKs. 5. Slow start increases cwnd: A. Linearly B. Exponentially Answer: B Rationale: cwnd doubles every RTT during slow start. 6. Which event causes TCP to enter fast retransmit? Answer: Three duplicate ACKs Rationale: Duplicate ACKs indicate a likely lost packet. 7. Control plane is responsible for: Answer: Computing routing decisions Rationale: Control plane determines paths; data plane forwards packets. 8. Data plane operates at: A. Per-packet timescale B. Minutes-long timescale Answer: A Rationale: Data plane handles each packet as it arrives. 9. Which protocol is used for intra-domain routing? Answer: OSPF Rationale: OSPF is an interior gateway protocol. 10. Which protocol is used for inter-domain routing? Answer: BGP Rationale: BGP connects autonomous systems. 11. True or False: BGP is a distance-vector protocol. Answer: False Rationale: BGP is a path-vector protocol. 12. Link-state routing requires routers to know: Answer: Entire network topology Rationale: Routers run Dijkstra on a full topology map. 13. Distance-vector routing suffers from: Answer: Count-to-infinity problem Rationale: Incorrect updates propagate slowly. 14. Which algorithm does OSPF use? Answer: Dijkstra’s algorithm Rationale: OSPF computes shortest paths using link-state info. 15. TCP fairness means: Answer: Flows share bandwidth roughly equally Rationale: AIMD promotes fair sharing. 16. AIMD stands for: Answer: Additive Increase, Multiplicative Decrease Rationale: TCP increases slowly, decreases aggressively. 17. True or False: Packet switching reserves bandwidth. Answer: False Rationale: Circuit switching reserves; packet switching does not. 18. What does RTT stand for? Answer: Round-Trip Time Rationale: Time for packet + ACK to return. 19. Which layer handles multiplexing? Answer: Transport layer Rationale: Ports allow multiple apps to share the network. 20. TCP checksum detects: Answer: Bit errors Rationale: Ensures data integrity. 21. Flow control protects: A. Network B. Receiver Answer: B Rationale: Prevents overwhelming the receiver buffer. 22. Congestion control protects: Answer: The network Rationale: Prevents router buffer overflow. 23. True or False: ACKs are cumulative in TCP. Answer: True Rationale: ACK number indicates next expected byte. 24. What causes TCP timeout? Answer: No ACK received within RTO Rationale: Indicates severe congestion or loss. 25. TCP timeout results in: Answer: cwnd reset to 1 MSS Rationale: TCP re-enters slow start. 26. MSS stands for: Answer: Maximum Segment Size Rationale: Max TCP payload size. 27. Which queue discipline drops packets early? Answer: RED Rationale: Random Early Detection avoids global synchronization. 28. True or False: RED eliminates congestion. Answer: False Rationale: It mitigates congestion, not eliminates it. 29. What is global synchronization? Answer: Multiple TCP flows reducing rate simultaneously Rationale: Caused by tail-drop queues. 30. Which metric is used in routing? Answer: Cost Rationale: Could represent delay, hops, or bandwidth. 31. Forwarding is: Answer: Moving packets from input to output port Rationale: Happens in the data plane. 32. Routing is: Answer: Computing paths Rationale: Happens in the control plane. 33. Which device operates at layer 3? Answer: Router Rationale: Uses IP addresses. 34. Switches primarily use: Answer: MAC addresses Rationale: Operate at layer 2. 35. True or False: IP provides reliability. Answer: False Rationale: IP is best-effort. 36. What is an AS? Answer: Autonomous System Rationale: A network under one administrative domain. 37. BGP decisions are based on: Answer: Policies and path attributes Rationale: Not purely shortest path. 38. Which BGP attribute prevents loops? Answer: AS-PATH Rationale: Lists traversed ASes. 39. TCP sequence numbers are used for: Answer: Ordering data Rationale: Ensures correct byte stream reconstruction. 40. What happens when receiver window is zero? Answer: Sender pauses Rationale: Receiver buffer is full. 41. True or False: UDP guarantees delivery order. Answer: False Rationale: No ordering or reliability. 42. Which layer handles encryption (conceptually)? Answer: Application or transport Rationale: TLS sits above TCP. 43. What does pipelining improve? Answer: Throughput Rationale: Allows multiple packets in flight. 44. TCP is: Answer: Connection-oriented Rationale: Requires handshake. 45. What is the purpose of the 3-way handshake? Answer: Synchronize sequence numbers Rationale: Establish reliable connection. 46. Which packet starts a TCP connection? Answer: SYN Rationale: Initiates handshake. 47. True or False: Routing tables change frequently. Answer: False Rationale: Changes are relatively slow. 48. What is head-of-line blocking? Answer: One packet blocks others Rationale: Common in FIFO queues. 49. What does ECN stand for? Answer: Explicit Congestion Notification Rationale: Signals congestion without loss. 50. TCP throughput is limited by: Answer: RTT and loss rate Rationale: Higher RTT and loss reduce throughput. \All right, let’s finish the set. Here are CS 6250 – Quiz 4 style practice questions 51–100, with answers and clear rationales, same difficulty and coverage as before. CS 6250 – Practice Questions (51–100) Questions • Answers • Rationales 51. What does TCP interpret packet loss as? Answer: Network congestion Rationale: TCP assumes loss is due to congestion, not corruption. 52. True or False: TCP congestion control is end-to-end. Answer: True Rationale: Routers do not explicitly control TCP congestion behavior. 53. Which field in TCP helps detect lost segments? Answer: Sequence number Rationale: Missing sequence numbers indicate loss. 54. What happens to cwnd during congestion avoidance? Answer: Increases linearly Rationale: Additive increase phase. 55. Which queue management drops packets only when full? Answer: Tail drop Rationale: Drops arriving packets after buffer fills. 56. Why is tail drop problematic? Answer: Causes global synchronization Rationale: Multiple TCP flows reduce rates simultaneously. 57. True or False: Flow control depends on receiver buffer size. Answer: True Rationale: Receiver window advertises available buffer space. 58. What does rwnd stand for? Answer: Receiver window Rationale: Limits sender based on receiver capacity. 59. TCP effective window size is: Answer: min(cwnd, rwnd) Rationale: Both congestion and flow control apply. 60. What triggers TCP fast recovery? Answer: Three duplicate ACKs Rationale: Indicates moderate congestion. 61. During fast recovery, cwnd is: Answer: Reduced but not reset to 1 Rationale: TCP avoids slow start. 62. Which protocol resolves IP to MAC? Answer: ARP Rationale: Address Resolution Protocol maps IP → MAC. 63. ARP operates at: Answer: Network–Link boundary Rationale: Bridges layer 2 and 3 addressing. 64. True or False: ARP requests are broadcast. Answer: True Rationale: Sent to all hosts on the LAN. 65. What is forwarding table indexed by? Answer: Destination address Rationale: Used to select output interface. 66. Longest prefix match is used in: Answer: IP forwarding Rationale: Chooses most specific route. 67. Which protocol handles error reporting in IP? Answer: ICMP Rationale: Used for diagnostics and errors. 68. Ping uses which protocol? Answer: ICMP Rationale: Echo request/reply messages. 69. True or False: ICMP runs over TCP. Answer: False Rationale: ICMP runs directly over IP. 70. What does TTL prevent? Answer: Routing loops Rationale: Packet is discarded when TTL reaches zero. 71. Which field decrements at each hop? Answer: TTL Rationale: Reduced by one per router. 72. What does NAT primarily do? Answer: Translates private to public IPs Rationale: Conserves IPv4 address space. 73. True or False: NAT breaks end-to-end connectivity. Answer: True Rationale: External hosts cannot directly initiate connections. 74. Which transport protocol supports multicast? Answer: UDP Rationale: TCP is unicast only. 75. What is goodput? Answer: Useful application-level throughput Rationale: Excludes retransmissions and overhead. 76. What causes bufferbloat? Answer: Excessively large router buffers Rationale: Increases latency under load. 77. Which congestion signal avoids packet loss? Answer: ECN Rationale: Marks packets instead of dropping. 78. True or False: ECN requires router and endpoint support. Answer: True Rationale: Both must understand ECN bits. 79. What is fairness in networking? Answer: Equal bandwidth sharing Rationale: Each flow gets comparable throughput. 80. Which TCP variant improves high-speed performance? Answer: TCP Cubic Rationale: Default in many modern OSes. 81. What problem does Reno improve over Tahoe? Answer: Recovery speed Rationale: Reno avoids slow start after dup ACKs. 82. Which protocol is unreliable but fast? Answer: UDP Rationale: No retransmissions or ordering. 83. What is a socket? Answer: IP + port Rationale: Identifies an endpoint. 84. True or False: Multiple sockets can share the same port. Answer: True Rationale: Distinguished by IP and protocol. 85. Which layer handles segmentation? Answer: Transport layer Rationale: Breaks application data into segments. 86. What is segmentation different from fragmentation? Answer: Segmentation is end-host, fragmentation is network Rationale: IP routers may fragment packets. 87. Why is fragmentation undesirable? Answer: Increases loss impact Rationale: Losing one fragment drops entire packet. 88. What avoids fragmentation? Answer: Path MTU discovery Rationale: Learns smallest MTU on path. 89. True or False: TCP adapts its rate based on RTT. Answer: True Rationale: RTT affects ACK timing. 90. What increases TCP throughput? Answer: Larger window size Rationale: More data in flight. 91. What limits TCP throughput on long paths? Answer: Bandwidth-delay product Rationale: Window must match BDP. 92. Which plane installs forwarding rules? Answer: Control plane Rationale: Programs data plane behavior. 93. SDN separates: Answer: Control and data planes Rationale: Centralizes control logic. 94. True or False: SDN simplifies network management. Answer: True Rationale: Centralized control reduces complexity. 95. What protocol is often used in SDN controllers? Answer: OpenFlow Rationale: Programs switch forwarding tables. 96. What is a match-action table? Answer: Rules mapping headers to actions Rationale: Core SDN forwarding concept. 97. Which metric does TCP NOT consider? Answer: Hop count Rationale: TCP is oblivious to routing metrics. 98. Which layer provides best-effort delivery? Answer: Network layer Rationale: IP does not guarantee delivery. 99. True or False: Reliable delivery requires acknowledgments. Answer: True Rationale: ACKs confirm receipt. 100. The Internet architecture favors: Answer: End-host intelligence Rationale: End-to-end principle.