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