Page "Microkernel" Paragraph 14

In a client-server system, most communication is essentially synchronous, even if using asynchronous primitives, as the typical operation is a client invoking a server and then waiting for a reply.

As it also lends itself to more efficient implementation, modern microkernels generally follow L4's lead and only provide a synchronous IPC primitive.

Asynchronous IPC can be implemented on top by using helper threads.

However, versions of L4 deployed in commercial products have found it necessary to add an asynchronous notification mechanism to better support asynchronous communication.

This signal-like mechanism does not carry data and therefore does not require buffering by the kernel.