** note: this is, unfortunately, already out of date

arx_socket
	encapsulates the UDP socket.
	points to the associated arx_mux.

arx_peer
	identifies some remote host that is talking to us.
	for now it will probably just be an IP address / port number
	pair, but could be something more clever later on if need be.

arx_mux
	has an associated arx_socket, and a table of arx_peer's and
	arx_conn's.

	receives packets from arx_socket, parses them enough to figure
	out which connection should get them, and hands them to the
	appropriate arx_conn (creating one if appropriate.)

arx_conn
	one for each rx connection.
	created and referenced by arx_mux in the conntab.
	can create and reference up to 4 arx_call's, which are the
	latest calls for each of the 4 rx channels at that time.

arx_call
	must be associated with an arx_conn, but can later be detached.
	(for example, the call is done, and the arx_conn wants to use
	the call slot for something else, but client still hasn't read
	the data or somesuch.  so we must be able to exist without an
	arx_conn.)