A method for processing protocol data unit, PDU, session establishment by an access and mobility management function, AMF, node, the method comprising: receiving a PDU session establishment request message and information related to a multi-access, MA, PDU session from a user equipment, UE, through any one of 3rd Generation Partnership Project, 3GPP, access and non-3GPP access, wherein the PDU session establishment request message includes Access Traffic Steering, Switching and Splitting, ATSSS, information, and request indication, and wherein the request indication indicates that the MA PDU session is to be established based on one procedure for a single MA PDU session establishment procedure; selecting a session management function, SMF, node supporting the MA PDU session, based on the information related to the MA PDU session; transmitting the PDU session establishment request message and information about whether the UE is registered over both the 3GPP access and the non-3GPP access to the SMF node supporting the MA PDU session; receiving PDU session establishment accept message from the SMF node; transmitting the PDU session establishment accept message to the UE; receiving a service request message related to the MA PDU session from the UE over one access of the 3GPP access and the non-3GPP access, after the MA PDU session is established, wherein the service request message includes List Of PDU Sessions To Be Activated related to the MA PDU session; and transmitting a request message for requesting activation of user plane resource of the one access to the SMF node, based on that the service request message is received over the one access, wherein the request message enables the SMF node to only activate the user plane resource of the one access. receiving a PDU session establishment request message and information related to a multi-access, MA, PDU session from a user equipment, UE, through any one of 3rd Generation Partnership Project, 3GPP, access and non-3GPP access, wherein the PDU session establishment request message includes Access Traffic Steering, Switching and Splitting, ATSSS, information, and request indication, and wherein the request indication indicates that the MA PDU session is to be established based on one procedure for a single MA PDU session establishment procedure; selecting a session management function, SMF, node supporting the MA PDU session, based on the information related to the MA PDU session; transmitting the PDU session establishment request message and information about whether the UE is registered over both the 3GPP access and the non-3GPP access to the SMF node supporting the MA PDU session; receiving PDU session establishment accept message from the SMF node; transmitting the PDU session establishment accept message to the UE; receiving a service request message related to the MA PDU session from the UE over one access of the 3GPP access and the non-3GPP access, after the MA PDU session is established, wherein the service request message includes List Of PDU Sessions To Be Activated related to the MA PDU session; and transmitting a request message for requesting activation of user plane resource of the one access to the SMF node, based on that the service request message is received over the one access, wherein the request message enables the SMF node to only activate the user plane resource of the one access.