TCP Timeout Calculation and Handling

EstimatedRTT = (1 – α) * EstimatedRTT + α * SampleRTT

DevRTT = (1 – β) * DevRTT + β * | SampleRTT – EstimatedRTT |

TimeoutInterval = EstimatedRTT + 4 * DevRTT

An initial TimeoutInterval value of 1 second is recommended [RFC 6298]. Also, when a timeout occurs, the value of TimeoutInterval is doubled to avoid a premature timeout occurring for a subsequent segment that will soon be acknowledged. However, as soon as a segment is received and EstimatedRTT is updated, the TimeoutInterval is again computed using the formula above.

TCP never computes a SampleRTT for a segment that has been retransmitted; it only measures SampleRTT for segments that have been transmitted once.

each time TCP retransmits, it sets the next timeout interval to twice the previous value, rather than deriving it from the last EstimatedRTT and DevRTT