A study on ACK message processing mechanism in SIP transaction layer

According to the RFC3161, ACKs corresponding to 2xx responses to INVITE is handled in the transaction user as separate messages without any transactions. And the transaction user receives all retransmitted ACKs without any filtering. In this paper, ACK transactions and the finite state machines for those are defined. And the rules for matching messages to transactions are also redefined so that the retransmission of ACKs and the filtering of the retransmitted ACKs are treated in the transaction layer.