Suppose I want a single server that can serve requests over both HTTP (port 80) and HTTPS (port 443).
To do this I would need to
~ create a single stream socket bound to both ports and listen on it
~ create a single datagram socket bound to both ports and recv on it
~+ create two stream sockets, one bound to each port, and listen on both of them
~ create two datagram sockets, one bound to each port, and recv on both of the
~~

After creating two stream sockets and listening on both of them as in the previous question,
the code would then have to:
~ alternately call receive on the two sockets
~ alternately call accept on the two sockets
~ simultaneously (somehow) call recv on the two sockets
~+ simultaneously (somehow) call accept on the two sockets
~~

After successfully accepting a connection, the server would:
~ send and receive on the listening socket
~ send and receive on the bound socket
~+ send and receive on the socket returned by accept
~ send and receive on the socket returned by connect
~~

After successfully connecting to the server, the client would:
~ send and receive on the listening socket
~ send and receive on the socket returned by connect
~+ send and receive on the socket returned by socket