When a 5G user (UE) browses the Internet and downloads web content, the UP (user) side adds IP headers to the data and then hands it over to the UPF for processing, as described below;
I. UPF Processing
After adding the IP header, the user packets will be routed through the IP network to the UPF, which provides an entry point to the 5G core network. the IP network relies on its lower layers to transmit packets between routers; and the Ethernet operable Layer 2 agreement transmits IP packets between routers;
The UPF is specifically responsible for mapping TCP/IP packets to specific QoS flows belonging to specific PDU sessions by using packet inspection to extract various header fields, which the UPF compares to a set of SDF (Service Data Flow) templates to identify the appropriate PDU sessions and QoS flows. For example, a unique combination of {source IP address 'X'; destination IP address 'Y'; source port number 'J'; destination port number 'K '} in unique combinations to map packets to specific PDU sessions and QoS flows; in addition, the UPF receives a set of SDF templates from the SMF (Session Management Function) during PDU session setup.
II.Data Forwarding
After identifying the appropriate PDU session and QoS flow, the UPF forwards the data to the gNode B using a GTP-U tunnel (the 5G core network architecture may link multiple UPFs - the first UPF must use a GTP-U tunnel to forward the data to another UPF, which then forwards it to the gNode B). Setting up a GTP-U tunnel for each PDU session implies that the TEID (tunnel endpoint identifier) within the GTP-U header identifies the PDU session but not the QoS flow. The “PDU Session Container” is added to the GTP-U header to provide information to identify the QoS flow. Figure 215 shows the structure of the GTP-U header containing the “PDU Session Container” as specified in 3GPP TS 29.281, and the content of the “PDU Session Container” as specified in 3GPP TS 38.415.
III.PDU Session Container
As shown in Figure 216 below, when the value of “PDU Type” is “0”, it means that the PDU is a downlink packet instead of an uplink packet. the PPP (Paging Policy Presence) field indicates whether or not the header contains PPI (Paging Policy Indicator). (Paging Policy Indicator). the UPF may provide PPI to gNode B to provide paging priority that may be triggered by the arrival of a downlink packet - i.e. when the UE is in the RRC Inactive state. the RQI (Reflected QoS Indicator) specifies whether or not Reflected QoS should be applied to this QoS stream.
IV.GTP-U Tunneling
Using the UDP/IP protocol stack, UDP and IP headers are usually added before forwarding packets over the transport network.UDP provides simple connectionless data transfer.The structure of the UDP header is shown in Figure 217 below, where the source and destination ports identify the higher-level application. The higher-level application in this scenario is GTP-U whose registered port number is 2152.
V.GTP-U Headers
Adding IP headers for routing across GTP-U tunnels means that packets now have two IP headers. These are commonly referred to as the internal and external IP headers. Figure 218 shows these two headers; the UPF can use the DSCP field in the external IP header to prioritize packets, and the header associated with the GTP-U tunnel is removed at the far end of the tunnel, that is, at gNode B or, if the core network architecture is using chained UPF, at another UPF.
