31.1.1.0.2 Round-Trip Time Estimation and RTO Timeout Selection

Four variables are used to estimate the round-trip time and set the retransmission timer: rtt_, srtt_, rttvar_, tcpTick_, and backoff_. TCP initializes rttvar to and backoff to 1. When any future retransmission timer is set, it's timeout is set to the current time plus seconds, where is the current backoff value, is the value of tcpTick, is the value of srtt, and is the value of rttvar.

Round-trip time samples arrive with new ACKs. The RTT sample is computed as the difference between the current time and a ``time echo'' field in the ACK packet. When the first sample is taken, its value is used as the initial value for srtt_. Half the first sample is used as the initial value for rttvar_. For subsequent samples, the values are updated as follows: