TCP-CUBIC: What's TCP Mode?

Question

I'm reading about TCP-CUBIC and I found this phrases:

CUBIC runs in three different modes (Ha, 2008):

If cwnd is less than the window size that (standard) TCP would reach at time t after the last loss event, then CUBIC is in the TCP mode.

If cwnd is less than Wmax, then CUBIC is in the concave region.

If cwnd is larger than Wmax, CUBIC is in the convex region.

What exacly means CUBIC is running in TCP Mode? This is the "balance point"? It is the point where the window is exactly in the middle between the concavely and convexly?

Thanks!

Answer 1

Cubic functions can have both convex and concave portions, meaning that they can grow more slowly in some portions(concave) and more quickly in others(convex). In addition to that CUBIC also has a TCP friendly region that operates when window is small to ensure that CUBIC is not penalized relative to regular TCP. More specifically, the window size of standard TCP in terms of time elapsed.