Apparatus for use within an IP Multimedia Subsystem, IMS, network to handle Session Initiation Protocol, SIP, messages. The apparatus comprises a receiver for receiving a SIP message from a peer IMS node, and a SIP message inspector for inspecting a P-Charging-Vector, PCV, header within a received SIP message in order to detect the presence within the PCV header of a parameter and associated handling determinator. The apparatus further comprises a SIP message handler for determining, on the basis of a handling determinator and without reference to said associated parameter, an action or actions to be applied to said parameter and associated handling determinator.