An important issue is how many readers and writers can exchange information at a port.
Different approaches impose various restrictions on the access to ports.
At the other extreme, the free port allows any number of readers and writers. Unfortunately, implementing free ports can be quite costly. The queue associated with the port is kept at a site which, in general, is remote to both a sender and a receiver. Thus both sends and receives result in messages being sent to this site. The former put messages in this queue and the latter request messages from it.