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The SMF (Session Management Function) is responsible for creating, updating and removing PDU (Protocol Data Unit) sessions and managing session context with the UPF (User Plane Function). The SMF injects routing rules to the selected UPFs.

A routing rule corresponds to an entry in the context table of the UPF. This context table contains four fields:

1 – a correspondence field (PDR (Packet Detection Rule));

2 – a routing field NH (next hop: IP address, tunnel number TEID (Tunnel End Identifier) or SR (Segment Routing)) to find the next node;

3 – the quality of service to be applied to the flow (QER (QoS Enhancement Rules));

4 – the measurement reports to be applied to the flow (URR (Usage Reporting Rules)).

The SMF is responsible for the session management for each DNN and by network slice (S-NSSAI), based on the user profile stored at the UDR.

When requesting a session to be established, the SMF selects a UPF or queries the NRF (Network Repository Function) to obtain the address of the UPF.

The SMF grants an IPv4 or IPv6 address to the mobile. An IP address is provided for each PDU session, based on the address range of the PSA (PDU Session Anchor) selected to join the IP data network. The address range is obtained by either directly querying the selected UPF or by querying the NRF. If the assigned IPv4 address is a private address, the UPF entity performs NAPT (Network Address and Port Translation) in order to translate the IP address and TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) port numbers.

At the end of the IP session, when the mobile enters the standby state, the SMF releases the session by removing the context at the UPF.

In the event of incoming packets, if the mobile is in the idle state, the SMF sends a notification to the AMF (Downlink Data Notification).