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What this shows

This tool walks through the RFC 6298 (Jacobson/Karels) algorithm TCP uses to compute its Retransmission Timeout (RTO) — how long TCP waits before assuming a segment was lost and resending it.

How to use it

  • Enter a round-trip time (RTT) measurement in milliseconds and click Calculate to add a step, or click Add Random to generate one based on the last measurement.
  • Each step shows the full formula for SRTT, VAR, and RTO. Faded pills marked with an up-arrow are values carried forward from the previous step; solid pills are freshly computed this step.
  • Click a step's header to expand or collapse it. The trash icon on the most recent step lets you undo it.
  • The Variable glossary panel explains RTT, SRTT, VAR, GAP, and RTO.
RTT Round-Trip Time. The raw time measured for a packet to leave the sender, reach the receiver, and return. A new value is observed for each acknowledged segment.
SRTT Smoothed RTT. An exponentially weighted moving average of RTT samples. It tracks the underlying trend while filtering out short-term noise and jitter.
VAR Variance. A smoothed measure of how much each new RTT deviates from SRTT. Higher variance means the network is less predictable, which widens the RTO safety margin.
GAP Error signal. The absolute difference between the previous SRTT and the latest RTT sample. Feeds directly into the VAR update; only appears from Step 3 onward.
RTO Retransmission Timeout. How long TCP waits before assuming a segment was lost and resending it. Computed fresh each step as SRTT + 4×VAR — never carried forward as an input.
new prev↑ Color shading. Full-color pills are freshly computed values (outputs of this step). Faded pills marked with ↑ are values carried in from the previous step. Intermediate arithmetic sub-totals have no pill — they're not named variables.
Please enter a whole number ≥ 1.
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