TCP-Cubic

CUBIC is RTT-independent.

The window size is a cubic function of the time t, which is passed time since the last congestion occurrence.

• W_CUBIC: cwnd size for cubic congestion control mechanism.

• W_max: cwnd size just before the last window reduction.

• C: predefined constant (scaling factor).

• β: cwnd size decrease factor. Window size reduction at the time of loss event is:

  
• W(t*): cwnd size at the time t* of packet loss, that is, Wmax.

When a packet loss is detected, W(t) is reduced as per the previous equation.

Whenever a packet-dropping event occurs, the cwnd is reduced by a factor β, otherwise increased by the α for a successful ACK. For TCP with CUBIC, β=0.3, whereas for QUIC with CUBIC, β=0.3/2=0.15.