happen in the TCP stack, this interface calls the specified callback to
handle the situation according to the currently selected congestion
control algorithm.
A new sysctl node was created: net.inet.tcp.congctl.{available,selected}
with obvious meanings.
The old net.inet.tcp.newreno MIB was removed.
The API is discussed in tcp_congctl(9).
In the near future, it will be possible to selected a congestion control
algorithm on a per-socket basis.
Discussed on tech-net and reviewed by <yamt>.
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