RTT and timeout

Round Trip Time (RTT) — The time for a segment to arrive at the endpoint, plus the time for the ACK to return to the sender.

Timeout — The time after which if an ACK has not been received, a packet is considered lost.

The timeout parameter in TCP is determined using the RTT and two scalar constants, $\alpha$ and $\beta$ .

$\displaystyle RTT_{est}^{k+1} = (1-\alpha)\cdot RTT_{est}^k + \alpha\cdot RTT_{sample}^k$

(1)

Generally, $\alpha = 0.125$

$\begin{indisplay}\begin{split} timeout^{k+1} &= RTT_{est}^{k+1}+4\cdot RTT_{dev}... ...ev}^k + \beta \vert RTT_{sample}^k - RTT_{est}^k\vert \end{split}\end{indisplay}$

(2)

Generally, $\beta=0.25$ .