Initiator                         Responder
---------                         ---------

                                  listen_negotiate() \ Await request
                         
request_negotiate()
          M_REQ_NEG      ->       negotiate_step()   \ Open session,
                         <-      M_NEGOTIATE         / start negotiation
negotiate_step()
        M_NEGOTIATE      ->       negotiate_step()   \ Continue
                         <-      M_NEGOTIATE         / negotiation
                         ...
negotiate_wait()                                     \ Insert
        M_WAIT           ->                          / delay
negotiate_step()
        M_NEGOTIATE      ->       negotiate_step()   \ Continue
                         <-      M_NEGOTIATE         / negotiation
negotiate_step()
        M_NEGOTIATE      ->       end_negotiate()    \ End
                         <-      M_END               / negotiation

                                                     \ Process results