s between an access node and SMF) is exchanged between the SMF and the 5G access node which is transparent to the AMF. The NG application protocol enables message exchange between a 5G access node and the 152 Chapter 1 AMF over N2 interface using SCTP protocol, which guarantees delivery of signaling mes- sages between AMF and 5G access node. 1.2.2.3.2 User-Plane Protocol Stacks The protocol stack for the user-plane transport related to a PDU session is illustrated in Fig. 1.72. The PDU layer corresponds to the PDU that is transported between the UE and the PDN during a PDU session. The PDU session type can be IPv6 or Ethernet for trans- porting IP packets or Ethernet frames. The GPRS tunneling protocol for the user plane (GTP-U) supports multiplexing of the traffic from different PDU sessions by tunneling user data over N3 interface (i.e., between the 5G access node and the UPF) in the core network. GTP encapsulates all end-user PDUs and provides encapsulation per-PDU-session. This layer also transports the marking associated with a QoS flow [3]. The 5G encapsulation layer supports multiplexing the traffic from different PDU sessions over N9 interface (i.e., an interface between different UPFs). It provides encapsulation per PDU session and carries the marking associated with the QoS flows. The 5G access node protocol stack is a set of protocols/layers which are related to the access network as described in the previous sections. The number of UPF entities in the data path is not con- strained by the 3GPP specifications; therefore, there could be none or more than one UPF entities in the data path of a PDU session that may not support PDU session anchor func- tionality for that PDU session. In certain cases, there is an uplink classifier or a branching point in the data path of a PDU session, which does not act as the non-PDU session anchor UPF. In that case, there are multiple N9 interfaces branching out of the uplink classifier/ branching point, each leading to different PDU session anchors. Application PDU lay