TCP RTO Calculation

What this TCP retransmission timeout question tests

This question probes your understanding of how TCP estimates network latency and adapts its retransmission behaviour in real time. Rather than using a fixed timeout, modern TCP implementations employ a smoothed round-trip time (RTT) estimator that responds to changing network conditions. The question rewards candidates who can explain both the mathematical framework and the practical reasoning behind it.

To answer well, you'll need to discuss how TCP samples RTT measurements from actual data segments, applies exponential smoothing to filter noise, and derives a safety margin that accounts for variance. This is a core topic in TCP/IP Illustrated: The Protocols, Volume 1 and appears regularly in systems and networking interviews at firms that build infrastructure or care deeply about protocol efficiency.

• Smoothed RTT (SRTT) and RTT variance estimation
• Exponential weighted moving averages
• The relationship between timeout duration and retransmission policy
• Karn's algorithm and Jacobson's RTO refinements