Connection Timeout Calculator
Derive the RFC 6298 retransmission timeout and the total time TCP spends retrying before it gives up.
Inputs
Total Time Before Giving Up
6.20s
Initial RTO
200ms
Final Retry Interval
3,200ms
Total Timeout
6,200ms
Failure Detection Budget
6.24s
Step by step
Values used
Smoothed round-trip time (SRTT) = 40 ms; RTT variance (RTTVAR) = 10 ms; Retransmission attempts = 5 attempts; Minimum RTO floor = 200 ms
Connection Timeout
RTO = max(floor, SRTT + 4 × RTTVAR), and the backoff sum is RTO × (2^retries − 1).
Total Time Before Giving Up
= 6.20 s
Initial RTO
= 200 ms
Final Retry Interval
= 3,200 ms
Total Timeout
= 6,200 ms
Failure Detection Budget
= 6.24 s
How it works
RFC 6298 sets the retransmission timeout from the smoothed RTT plus four times its variance, then doubles it after every failed attempt. Summing that geometric series gives the wall-clock time before the stack reports the connection dead. This number is your worst-case failure detection time: it decides how long a client hangs on a dead server, so it has to be shorter than the health check and failover budget above it.
Formula
Connection Timeout
RTO = max(floor, SRTT + 4 × RTTVAR), and the backoff sum is RTO × (2^retries − 1).
- SRTT
- Smoothed round-trip time estimate
- RTTVAR
- Round-trip time variation
- floor
- Minimum RTO the stack enforces (1 s in the RFC, 200 ms on Linux)
Frequently Asked Questions
How is Connection Timeout calculated?
RTO = max(floor, SRTT + 4 × RTTVAR), and the backoff sum is RTO × (2^retries − 1). RFC 6298 sets the retransmission timeout from the smoothed RTT plus four times its variance, then doubles it after every failed attempt. Summing that geometric series gives the wall-clock time before the stack reports the connection dead.
Why does Connection Timeout matter?
This number is your worst-case failure detection time: it decides how long a client hangs on a dead server, so it has to be shorter than the health check and failover budget above it.
What values do I need to enter?
This calculator takes 4 inputs: Smoothed round-trip time (SRTT), RTT variance (RTTVAR), Retransmission attempts, Minimum RTO floor. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why is the RTO floor so much larger than the RTT?
The floor guards against a spuriously low RTT estimate causing needless retransmissions. In datacentre networks with sub-millisecond RTTs the floor dominates completely, which is why incast recovery is often measured in hundreds of milliseconds.