The structure I'm envisioning here is that every object has a public pass(m) method that takes an input message and has no other public methods. Every message is a legitimate input to pass(m), and every object can call the pass method of any other object. It's up to each object to decide for itself what it will do with any given message that it receives. If an object wants a response to its message it can identify itself in the message and then hopefully the object being passed the message will choose to pass a message back.

This seems exactly analogous to how a collection of webservers interact. They can only communicate with each other by tossing a chunk of bytes around hoping that the target knows what it's supposed to be doing with those bytes.