input: [0] 6. [UNIX] Process rendevous is peer to peer. The basic communication mechanisms involve two active threads of control coming together at some queue. The transferred data may be buffered by the kernel, but it fundamentally isn't going anywhere until the recipient asks to receive it. [EROS] 
output: Process rendevous is pass-through. The key IPC rendevous mechanism is that an active caller calls an inactive recipient. This is necessitated by the persistence model but makes multithreading quite difficult to manage in certain situations.