Hi everyone, 

I am using Kamailio as an IMS server. The UE first sends its SIP REGISTER over TCP and gets a 401 in response. Then the UE starts sending its packets over ESP. These ESP packets reach P-CSCF but are not delivered to its process by the kernel. We can see ESP packets in P-CSCF using wireshark. The ports used are correct (both client and server) based on their prior negotiations. We have also checked xfrm policies using the command `ip xfrm policy list` and seems right regarding prior negotiations. Our Linux kernel version is 5.4 and the OS is Ubuntu 20.04.  Has anyone any clue of the root cause of this problem? Any ideas are appreciated.

Thanks, 

Aliakbar