A method, and machine readable storage medium is disclosed for a method performed by policy and charging rules function (PCRF) node (PCRN) in a packet switching network, for identifying a Gn/Gp mode of an adjacent network node, in which the adjacent network node which is connected directly to a Serving General Packet Radio Service (GPRS) Support Node (SGSN) node via a Gn/Gp interface, from an adjacent network node connected to a Serving Gateway (S-GW) via a S5/S8 interface, wherein the adjacent network node is a Packet Data Network Gateway (P-GW) node or a Gateway GPRS Support Node (GGSN). The method comprises receiving at the PCRN from the adjacent network node, a Diameter Credit Control Request (CCR) message requesting a IP-CAN session; determining whether the request message includes a SGSN-Address and does not include an AN-GW-Address.