Now the asynchronous communication with portal seems to be correct.
On the transmiter side, we spin on mppa_aio_error() after asynchronous write to verify when the transmission finishes.
Then, we call mppa_aio_return() to be able perform a new asynchronous write.
On the receiver side, we just use mppa_aio_rearm() to block until receiving the message.
The function mppa_aio_rearm() does a mppa_aio_wait() followed by mppa_aio_read().