sipp tool sends rtp packets to random port when in container

[AndreHeber]

Hello everybody,

I'm not sure if this is the right place for my question. If so, it would be nice to point me to the right direction.

To my issue: I'm using sipp to test the telephony system, which is running in kubernetes. I start sipp in server mode, so that it receives an incoming call and it accepts the call and starts sending prerecorded media, the rtp packets. The packets are sent from udp port 6000 to udp port 6000. When sipp runs natively, everything is fine. When sipp runs on a pod with hostNetwork: true or on docker with --net=host, the rtp streams switches ports in the middle of the stream, e.g. source ports & target ports are suddenly 8173 now. I used strace to verify that sipp is not responsible for the port switch.

I attached the pcap and the strace log:
logs+pcap.tar.gz

Any idea about the reason of this behavior? And how to avoid that?

Thank you very much!