Host A                                  Host B
   ------                                  ------
   MP_JOIN               ->
   [B's token, A's nonce, 
    A's Address ID, flags]
                         <-                MP_JOIN
                                           [B's HMAC, B's nonce,
                                            B's Address ID, flags]
   ACK + MP_JOIN         ->
   [A's HMAC]

                         <-                ACK