In UNICAST.down(), when loopback is true, a message is sent back up on the same thread. Because the message is not added to the retransmission table, ordering is undefined, ie. multiple threads sending unicast messages has completely undefined ordering.
Possible solutions:

• We define the semantics of loopback and declare ordering undefined (some apps mightr be OK with this)
• We deprecate and remove loopback as TP already implemements loopback (this was chosen, but needs to be revisited)
• We add the message to the retransmission table and them remove it again and pass it up. essentially what we do in TP, so it is duplicate functionality. Not recommended