28.1.1.0.2 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 b is the current backoff value, t is the value of tcpTick, a is the value of srtt, and v 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: