How would the execution differ in an asyncRFC vs sRFC??

As per my knowledge the RFC connection are made by basis people.

Abapers role is to create the function module with RFC enabled .

Let's say in a report prog we call an RFC ,it's maintained as sRFC .so after the FM call the system has to reply with ACKD.

What happens when it does not reply , maybe the system is down.

So does it throw an error or goes for a dump