I am testing a setup with one sip client (tested with sipp and a softphone) and an IMS server that auto answers (so it is a two way RTP-communication between those 2 network elements).
In RTCP, must both the two entities communicating in such a scenario send RTCP packets? I am asking that because when i use the sipphone, it sends some RTCP packets at first and then the server answers, whereas the sipp scenario doesn't send rtcp packets at all and gets no response from that protocol. Also, all the ports for RTP+RTCP are open both in the router and in the testing machine.
Can an IMS server be configured to send RTCP packets (one direction only) on an RTP flow? Does it say anywhere in the RTCP Protocol, that a client has to send RTCP paskets to receive such packets from the server?
You need to negotiate ports for RTP and RTCP first using SIP and SDP before those messages can be sent, so if the server doesn't respond with any SIP message then something else is wrong. Please check all the IP addresses in the outgoing INVITE from SIPP.