SIP multiple 2** responses

Question

i am receiving multiple 2** responses for a call. The problem is that once the call is connected and the server receives the "ACK" packet and the call starts successsfully, server again sends the "OK" response packet back to the callee and recieves the "ACK" packet again for it. This happens multiple times before the call gets terminated automatically. Can somebody explain to me why this is happening that my server is sending the "OK" response even after the call is connected successfully and what can be the possible solution for the problem? Thanks in advance for any help.

Answer 1

A SIP UAS will retransmit the 2xx response multiple times until it receives the ACK request from the UAC. See the SIP RFC chapter "13.3 UAS Processing" for the gory details.

Most often this sort of problem happens when the ACK sent by the UAC is invalid and thus prevents the UAS from matching it to the INVITE transaction. ACK requests have special rules about their construction see 13.2.2.4 2xx Responses. To make things even more difficult the construction of the ACK request differs for 2xx and non-2xx final responses.