Thus the message queue associated with a port can be kept with the receiver. Output ports, in contrast, allow any number of readers but only one writer. They are easier to implement than free ports since the message queue can be kept with the sender. However, they are not popular since the one client, many server situation is very unusual. Note that sending to an output port does not broadcast the message to all the receivers - a specific message is received by a single server.
Several applications can use more than one kind of port.
For instance a client can enclose a bound report in a request message to the input report of a server, which can then return results to the client by sending messages to the bound port.