This networking enhancement is new in the Solaris Express 1/06 release.
The new TCP socket option, TCP_INIT_CWND, enables an application to override the settings in the initial TCP congestion window, as described in RFC 3390, “Increasing TCP's Initial Window.” By default, TCP sets the initial congestion window at connection set up time and after an idle period. (An idle period is when no traffic occurs between the two ends of the TCP connection. An application can use the TCP_INIT_CWND socket option to set the initial congestion window to a specified number of TCP segments. So the value of this new socket option is used both at the connection start time and after an idle period to set the initial congestion window. The process must have the PRIV_SYS_NET_CONFIG privilege if a number greater than that calculated by RFC 3390 must be specified.
For further information, see the tcp(7P) man page.