Host A                                 Host B
   ------                                 ------
   ADD_ADDR                  ->
   [Echo-flag=0,
    IP#-A2,
    IP#-A2's Address ID,
    HMAC of IP#-A2]

                             <-          ADD_ADDR
                                         [Echo-flag=1,
                                          IP#-A2,
                                          IP#-A2's Address ID,
                                          HMAC of IP#-A2]