Host A (with data to send immediately)  Host B
   ------                                  ------
   MP_CAPABLE                ->
   [flags]
                             <-            MP_CAPABLE
                                           [B's key, flags]
   ACK + MP_CAPABLE + data   ->
   [A's key, B's key, flags, data-level details]


   Host A (with data to send later)        Host B
   ------                                  ------
   MP_CAPABLE                ->
   [flags]
                             <-            MP_CAPABLE
                                           [B's key, flags]
   ACK + MP_CAPABLE          ->
   [A's key, B's key, flags]

   ACK + MP_CAPABLE + data   ->
   [A's key, B's key, flags, data-level details]


   Host A                                  Host B (sending first)
   ------                                  ------
   MP_CAPABLE                ->
   [flags]
                             <-            MP_CAPABLE
                                           [B's key, flags]
   ACK + MP_CAPABLE          ->
   [A's key, B's key, flags]

                             <-            ACK + DSS + data
                                           [data-level details]