TCP Congestion Control MCQs

1. Which mechanism does TCP use to control congestion?

a) Leaky Bucket b) Token Bucket c) AIMD (Additive Increase Multiplicative Decrease) d) Static windowing

Correct Answer: c) AIMD (Additive Increase Multiplicative Decrease)

Explanation:
TCP uses AIMD for congestion control - it increases the window size additively during normal operation and decreases it multiplicatively when congestion is detected.

2. In TCP congestion control, what happens when a packet loss is detected?

a) Window size is doubled b) Window size is halved c) Window size remains the same d) Connection is terminated

Correct Answer: b) Window size is halved

Explanation:
When congestion is detected (via packet loss), TCP reduces its congestion window size by half (multiplicative decrease) to reduce transmission rate.

3. What is the purpose of the "Slow Start" phase in TCP congestion control?

a) To gradually increase the window size exponentially b) To reduce the window size to 1 MSS c) To avoid congestion from the beginning d) Both (a) and (c)

Correct Answer: d) Both (a) and (c)

Explanation:
Slow Start begins with a small window size (1 MSS) and grows exponentially until a threshold is reached, allowing TCP to probe available bandwidth while avoiding sudden congestion.

4. Fast Retransmit is triggered when:

a) A single packet is lost b) Three duplicate ACKs are received c) The timeout occurs d) The window size becomes zero

Correct Answer: b) Three duplicate ACKs are received

Explanation:
Fast Retransmit triggers when the sender receives 3 duplicate ACKs (indicating a packet was lost but later packets arrived), allowing quicker recovery than waiting for a timeout.

5. What is the congestion avoidance phase in TCP?

a) Window size increases linearly b) Window size increases exponentially c) Window size decreases d) Window size remains constant

Correct Answer: a) Window size increases linearly

Explanation:
After Slow Start reaches the threshold, TCP enters Congestion Avoidance where the window grows linearly (by 1 MSS per RTT) to carefully probe for additional bandwidth.

6. Which algorithm is used in modern TCP variants like TCP Reno and TCP Cubic?

a) Leaky Bucket b) Token Bucket c) AIMD with Fast Recovery d) Static routing

Correct Answer: c) AIMD with Fast Recovery

Explanation:
Modern TCP variants enhance basic AIMD with Fast Recovery (after Fast Retransmit) to maintain throughput during moderate congestion.

7. What is the key improvement of TCP Vegas over TCP Reno?

a) It uses packet loss as the only congestion signal b) It measures RTT to predict congestion before losses occur c) It ignores congestion signals d) It does not use slow start

Correct Answer: b) It measures RTT to predict congestion before losses occur

Explanation:
TCP Vegas monitors RTT changes to detect growing queueing delays and reduces its window preemptively, avoiding packet loss.

8. Random Early Detection (RED) is used to:

a) Drop packets randomly to avoid congestion b) Increase bandwidth c) Encrypt packets d) Reduce latency

Correct Answer: a) Drop packets randomly to avoid congestion

Explanation:
RED routers drop/mark packets probabilistically as queue lengths increase, providing early congestion notification before buffers overflow.

9. In RED, what is the purpose of the "minimum threshold"?

a) Packets are always dropped below this threshold b) No packets are dropped below this threshold c) All packets are dropped above this threshold d) It has no effect

Correct Answer: b) No packets are dropped below this threshold

Explanation:
The minimum threshold defines the queue level below which no packets are dropped, allowing normal operation during light loads.

10. Which congestion control algorithm is used in data center networks (e.g., DCTCP)?

a) AIMD b) Token Bucket c) Explicit Congestion Notification (ECN) d) Leaky Bucket

Correct Answer: c) Explicit Congestion Notification (ECN)

Explanation:
DCTCP uses ECN marks (rather than packet drops) for precise congestion signaling in low-latency data center environments.

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