1. A method for performing network selection and traffic routing, the method comprising: receiving, by a user equipment (UE), an Inter-System Routing Policy (ISRP) rule; receiving, by the UE from a base station, radio access network (RAN) assistance information, wherein the received RAN assistance information is used to evaluate the ISRP rule; evaluating, by the UE, whether the ISRP rule is valid or not by using the received RAN assistance information; and selecting, by the UE, one of the received ISRP rule and the received RAN assistance information to be utilized for performing network selection and traffic routing.