(As of 17 May 07) By default, in BIC adn CUBIC ssthresh is set to 100, so that when cwnd reaches this value it stops growing exponentially and instead grows in a polynomial fashion (cubically). This is in order to prevent a surge in traffic that could itself induce packet loss. For capacity rich networks this behaviour can be over-ridden by setting the initial ssthresh to another value, either higher or 0 (which means there is no limit to exponential growth).
In Linux this is done by changing the value in the file /sys/module/tcp_bic/parameters/initial_ssthresh
– Main.TobyRodwell - 17 May 2007