PCU <---> SGSN protocol trace

Dear list, I just appear to have a strange UE behavior where the UE tries to Attach (both CS and PS), the Osmo-SGSN sends an Attach accept, but the UE never responds with an Attach complete, instead after 10 seconds it tries to re-attach. I had an issue quite similar like this earlies where the UE requests for some IEs or the SGSN sends soething the UE don't like. The point is, I would like to do protocol trace between the SGSN and the PCU, and I managed to see the traffic with Wireshark, but it was not able to dissect/recognise the protocols over the UDP layer. Can someone please help me how to configure Wireshark to be able to dissect the involved protocols? Thanks! Csaba