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Congestion Control
TCP algorithms for network congestion management
Key Algorithms
Slow Start, Congestion Avoidance, Fast Recovery
TCP Variants
Tahoe, Reno, NewReno, CUBIC
Performance
Optimize throughput while preventing collapse
Congestion Control Algorithms
Slow Start Details
Purpose
Gradually probe network capacity to avoid congestion
Mechanism
Double cwnd every RTT until threshold reached
Formula
cwnd = cwnd + 1 for each ACK receivedKey Phases
- • Start with cwnd = 1 MSS
- • Double every RTT
- • Switch to Congestion Avoidance at ssthresh
TCP Variants Comparison
| TCP Variant | Slow Start | Fast Retransmit | Fast Recovery | Key Feature |
|---|---|---|---|---|
| TCP Tahoe | ✓ | ✓ | ✗ | Returns to slow start on any loss |
| TCP Reno | ✓ | ✓ | ✓ | Avoids slow start on fast retransmit |
| TCP NewReno | ✓ | ✓ | ✓ | Improved multiple loss handling |
| TCP CUBIC | ✓ | ✓ | ✓ | Cubic function for window growth |
📝 Exam Preparation Tips
Key Concepts
- • Congestion window (cwnd) vs receiver window (rwnd)
- • Slow start threshold (ssthresh)
- • AIMD principle (Additive Increase Multiplicative Decrease)
- • Difference between Tahoe and Reno
- • Fast retransmit triggers on 3 duplicate ACKs
Common Questions
- • "Trace TCP congestion window over time" (8 marks)
- • "Compare TCP Tahoe vs Reno" (6 marks)
- • "Explain slow start algorithm" (5 marks)
- • "How does fast retransmit work?" (4 marks)
- • "Calculate throughput with given RTT and loss" (6 marks)