Mathematical Formula

Let ${\displaystyle w(t)}$ be the congestion window size indicating the amount of data in flight during time slot ${\displaystyle t}$, ${\displaystyle a}$ (${\displaystyle a>0}$) be the additive increase parameter, and ${\displaystyle b}$ (${\displaystyle 0<b<1}$) be the multiplicative decrease factor.

${\displaystyle w(t+1)={\begin{cases}w(t)+a&{\text{ if congestion is not detected}}\\w(t)\times b&{\text{ if congestion is detected}}\end{cases}}}$

In TCP, after slow start, the additive increase parameter ${\displaystyle a}$ is typically one MSS (maximum segment size) per round-trip time, and the multiplicative decrease factor ${\displaystyle b}$ is typically 1/2.