PDU sessions

Overview

Registration gets a UE onto the network; a PDU session gives it a usable data connection. Establishment is UE-initiated: after registering, the UE asks for a session to a named DNN (Data Network Name, internet in the defaults) on a chosen slice. The core validates the request against the subscription, allocates an IP address, and programs the UPF so packets can flow.

In dflux TinyCore the AMF and SMF both live inside the DMF and talk in-process — the AMF hands the session-management NAS to the SMF directly rather than over an SBI hop. The SMF fetches subscription and policy data from the DPF and drives the UPF over PFCP (the N4 interface).

Establishment step by step

1. Subscription check. The SMF fetches the subscriber's session-management data from the DPF (UDM) and verifies the requested (S-NSSAI, DNN) is provisioned. An unknown slice or DNN is rejected toward the UE rather than silently defaulted.
2. Policy. The SMF asks the DPF (PCF) to create an SM policy for the session.
3. IP allocation. The SMF allocates an IPv4 address for the UE out of the DNN's pool (the default UE pool is 10.45.0.0/16).
4. UPF selection. The SMF picks an associated UPF and allocates an F-SEID for the new PFCP session.
5. PFCP session establishment. The SMF sends a PFCP Session Establishment Request that installs the packet-detection and forwarding rules described below.

PDRs, FARs and QERs

The UPF forwards traffic using rules the SMF installs per session:

• PDR (Packet Detection Rule) — matches packets on an interface. The session gets an uplink PDR (matching on the N3 access side) and a downlink PDR (matching on the N6 core side, by UE IP).
• FAR (Forwarding Action Rule) — says what to do with matched packets. The uplink FAR forwards to the core (N6); the downlink FAR forwards toward the access network (N3).
• QER (QoS Enforcement Rule) — applies the QoS for the flow.

The default QoS flow is non-GBR with QFI 9 (5QI 9), the standard best-effort flow for general internet traffic. The UPF allocates its own N3 uplink F-TEID and answers with the created PDR; the SMF records the assigned TEID on the session context.

User-plane activation

At establishment time the downlink path is not yet usable: the UPF doesn't know the gNB's access-network tunnel endpoint yet, so the downlink FAR is installed as DROP. Only when the gNB answers the PDU Session Resource Setup (supplying its downlink TEID) does the SMF flip the downlink FAR to FORWARD and program the GTP-U outer header toward the gNB. The same FAR-flip path is reused when an idle session's user plane is later re-activated.

Establishment call flow

What comes next

Once the session is up and activated, packets flow through the UPF datapath. See User plane for how the UPF forwards traffic, and Subscribers & identities for the subscription data that gates establishment.