A system that supports service data flow detection when using a packet modification function, the system comprising: a system infrastructure for interfacing between an application function and an Evolved Packet System, that is configured to implement: a first packet processing function that receives a packet generated by a sending application function, wherein the packet is destined for a receiving application function, wherein the packet includes first identifier information associated with a service data flow and wherein the first packet processing function adds (212, 214) second identifier information to a header of the packet; a packet modification function that receives the packet from the first packet processing function, repositions the second identifier information in the packet and encrypts the packet in a way that prevents use of the first identifier information by a 3 rd Generation Partnership Project, 3GPP, conforming service data flow detection process to map the packet to the service data flow for the sending application function, wherein the repositioning of the second identifier information in the packet enables a packet gateway of an evolved packet system to use the second identifier information instead of the first identifier information in the 3GPP conforming service data flow detection process to map the packet to the service data flow and to a bearer used to send the packet to the receiving application function; and wherein the system infrastructure is configured to convey the encrypted packet, comprising the re-positioned second identifier information, to the Evolved Packet System. a system infrastructure for interfacing between an application function and an Evolved Packet System, that is configured to implement: a first packet processing function that receives a packet generated by a sending application function, wherein the packet is destined for a receiving application function, wherein the packet includes first identifier information associated with a service data flow and wherein the first packet processing function adds (212, 214) second identifier information to a header of the packet; a packet modification function that receives the packet from the first packet processing function, repositions the second identifier information in the packet and encrypts the packet in a way that prevents use of the first identifier information by a 3 rd Generation Partnership Project, 3GPP, conforming service data flow detection process to map the packet to the service data flow for the sending application function, wherein the repositioning of the second identifier information in the packet enables a packet gateway of an evolved packet system to use the second identifier information instead of the first identifier information in the 3GPP conforming service data flow detection process to map the packet to the service data flow and to a bearer used to send the packet to the receiving application function; and wherein the system infrastructure is configured to convey the encrypted packet, comprising the re-positioned second identifier information, to the Evolved Packet System. a first packet processing function that receives a packet generated by a sending application function, wherein the packet is destined for a receiving application function, wherein the packet includes first identifier information associated with a service data flow and wherein the first packet processing function adds (212, 214) second identifier information to a header of the packet; a packet modification function that receives the packet from the first packet processing function, repositions the second identifier information in the packet and encrypts the packet in a way that prevents use of the first identifier information by a 3 rd Generation Partnership Project, 3GPP, conforming service data flow detection process to map the packet to the service data flow for the sending application function, wherein the repositioning of the second identifier information in the packet enables a packet gateway of an evolved packet system to use the second identifier information instead of the first identifier information in the 3GPP conforming service data flow detection process to map the packet to the service data flow and to a bearer used to send the packet to the receiving application function; and wherein the system infrastructure is configured to convey the encrypted packet, comprising the re-positioned second identifier information, to the Evolved Packet System.