Reliability is added to RTP by having a client acknowledge to the server each of the RTP packets received by the client, and retransmitting from the server to the client any of the packets that remain unacknowledged subsequent to expiration of a predetermined time duration subsequent to the timestamp....http://www.google.de/patents/US7380014?utm_source=gb-gplus-sharePatent US7380014 - Reliable real-time transport protocol
