Synchronous AS2-MDN

   {Peer1} ----( connect )----> {Peer2}
   {Peer1} -----( send )------> {Peer2}   HTTP Request {AS2-Message}
   {Peer1} <---( receive )----- {Peer2}   HTTP Response {AS2-MDN}

   Asynchronous AS2-MDN

   {Peer1} ----( connect )----> {Peer2}
   {Peer1} -----( send )------> {Peer2}   HTTP Request {AS2-Message}
   {Peer1} <---( receive )----- {Peer2}   HTTP Response (e.g., "200 OK" or "204 No Content")
   {Peer1}*<---( connect )----- {Peer2}
   {Peer1} <--- ( send )------- {Peer2}   HTTP Request {AS2-MDN}
   {Peer1} ----( receive )----> {Peer2}   HTTP Response