What is the difference between direct message passing and indirect message passing?

În direct message passing communication, the endpoints (client and server) communicate via direct addressing. This means that the sender knows the exact address (ip for ex) of the receiver.

In indirect communication, the link between the sender and the receiver is done via a port instead of direct addresses. The port acts as a mailbox for the messages and the sender doesn't need to know the exact ip of the receiver. The sender simply sends the message via a port and the receiver collects the message from that port. This way of communicating has several advantages over the direct way (for example it allows multipoint and multipath communication)