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What this shows

This tool walks through the TCP Retransmission Timeout (RTO) algorithm (RFC 6298 - Jacobson/Karels). The RTO determines how long TCP waits for an acknowledgment before assuming a segment is dropped and retransmitting it.

How to use it

  • Enter a round-trip time (RTT) in milliseconds and click Calculate (or press Enter) to add a round trip. Alternatively, Add Random adds a random RTT measurement within 10% of the previous measurement.
  • Each round trip shows its full calculation, line by line. Round Trip 0 is the starting RTO before any measurement, Round Trip 1 sets up SRTT and VAR from the first measurement, and every round trip after that applies the full formula.
  • Click a round trip's header to expand or collapse it, or use the button at the top right of the Add RTT Measurement box to expand or collapse them all.
  • Only the most recent round trip can be deleted, using the trash icon at the top right of its header.
  • The Glossary explains each variable and the color coding.

Rounding

Each calculation is worked out exactly, then rounded to a whole millisecond on its final line; a note shows the exact value whenever rounding changed it. Those whole-millisecond values are what carry into the next round trip. RFC 6298 doesn't call for rounding; this tool rounds only to keep the numbers readable.

A note on the RFC 6298 bounds

RFC 6298 sets a minimum RTO of 1 second, and allows a maximum as long as it's at least 60 seconds. Many operating systems use their own minimum and maximum values. To keep the arithmetic clear, this tool does not set any artificial minimum or maximum and instead shows you the direct result of the TCP Retransmission Timeout algorithm.

Feedback?

Feedback? Bug reports? Questions? Mention it on my Discord server: pracnet.net/discord.

RTT Round-Trip Time. The time between sending a data segment and receiving its corresponding acknowledgment.
SRTT Smoothed RTT. A weighted average of RTT measurements that softens the impact of brief spikes and dips.
VAR RTT Variation. Tracks how much SRTT differs from measured RTTs over time to determine how much safety margin to add to the RTO.
GAP Error signal. The absolute difference between the previous SRTT↑ and the latest RTT measurement. Used in the calculation for VAR.
RTO Retransmission Timeout. How long TCP waits for an acknowledgment before retransmitting the oldest unacknowledged segment.
currentprevious↑ Color shading. Full-color pills show values calculated in the current step. Faded pills marked ↑ show values carried over from the previous step.
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