Initiator                         Responder
-----------                       -----------
HDR(IKE_SA_INIT,MID=0),
SAi1, KEi, Ni,
N(INTERMEDIATE_EXCHANGE_SUPPORTED)  -->

                             <--  HDR(IKE_SA_INIT,MID=0),
                                  SAr1, KEr, Nr, [CERTREQ],
                                  N(INTERMEDIATE_EXCHANGE_SUPPORTED)