Abstract: A system and method for service differentiation enabling a user to express the level of importance of a session being established and change the level during the session while satisfying QoS profiles. An extension of the 3GPP IMS architecture includes a Session Prioritization Function (SPF) in communication with a S-CSCF and a Context Information Base (CIB) which provides network contextual information. Upon receiving a session initiation or change request, the S-CSCF queries the SPF for a resource allocation/deallocation decision. The SPF consults session prioritization policies and the contextual information to make an informed decision. If the load on the IMS network is normal or light, the SPF utilizes an upper limit policy to limit resources for each service class. Otherwise, the SPF limits the size of multi-party sessions and utilizes soft and hard preemption to transfer network resources between classes.