Initiator                         Responder
-----------                       -----------
HDR(IKE_AUTH,MID=3),
SK(SK_ei3,SK_ai3) 
{IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2, TSi, TSr}  -->
                             <--  HDR(IKE_AUTH,MID=3),
                                  SK(SK_er3,SK_ar3) 
                                  {IDr, [CERT,] AUTH, SAr2, TSi, TSr}