"Perfect forward secrecy" requires synchronous key exchange. The compromise that signal protocol makes is for forward secrecy to "eventually repair" itself whil

"Perfect forward secrecy" requires synchronous key exchange. The compromise that signal protocol makes is for forward secrecy to "eventually repair" itself while in the meanwhile a limited number of messages are potentially vulnerable. That is one of the novel feature of the protocol and it is what allows for async communication without some central server doing all the key mgmt (central key mgmt doesn't have this problem because it's actually synchronous). I am not a cryptographer.