PDU Sessions,
End to End.
Open the pipe that carries every byte — message by message. PDU session anatomy and SSC modes, the full 21-step Establishment flow, CreateSMContext on the wire, how the SMF programs the UPF over N4/PFCP, modification and release, and multi-anchor edge topologies — narrated and fully animated, explained like an expert.
PDU Sessions End-to-End — opening the pipe, message by message
The player screen is a live animation stage — scenes fire exactly as the narration reaches them, step by step. Karaoke subtitles in English, two subtitle modes, and fullscreen.
Every information element in the module
Each message and structured data type the video opens — its interface, direction, purpose and clause. Click any row to open its full Information-Element breakdown. Filter to find any of them.
| Message | Interface | Dir | What it does | Clause |
|---|
The PDU session, at protocol depth
Everything the video draws, as reference you can scan in a trace: what a session is made of, the four N4 rules that run the user plane, the twenty-one steps grouped into four acts, and the multi-anchor topologies. All spec-cited.
IPv4
- value 1
- one IPv4 address
IPv6
- value 2
- prefix + SLAAC
IPv4v6
- value 3
- both families in one session
Ethernet
- value 5
- raw L2 frames · factories, FWA
Unstructured
- value 4
- no IP · machine data
SSC mode 1 — preserve
- Anchor UPF never changes; IP address preserved for the life of the session.
- For voice, VPN — anything that dies if the address changes.
SSC mode 2 — break before make
- Network releases the old session, then builds a new one.
- New IP address; stateless apps reconnect.
SSC mode 3 — make before break
- New anchor built before the old one is removed.
- Two paths briefly → nothing is lost. The edge-mobility mode.
PDR · eyes
- How to recognize a packet
- PDI: interface, F-TEID, UE IP, SDF filters
- points to FAR / QER / URR
- §7.5.2.2
FAR · hands
- What to do: FORW · DROP · BUFF · DUPL
- Outer Header Creation (GTP-U tunnel)
- §7.5.2.3 · Apply Action §8.2.26
QER · traffic cop
- Enforce bit rate: MBR / GBR
- Gate open/closed · stamp the QFI
- §7.5.2.5
URR · accountant
- Count volume / time / events
- Reporting triggers → SMF → CHF
- §7.5.2.4
| Act | Steps | What happens | Key message |
|---|---|---|---|
| 1 · request | 1–5 | UE asks; AMF selects & calls the SMF; SMF fetches subscription and is born as session owner | Nsmf_PDUSession_CreateSMContext |
| 2 · build | 6–10 | optional secondary auth; PCF policy; UPF + SSC mode selected; IP assigned; UPF programmed over N4 | N4 Session Establishment (PDR/FAR/QER/URR) |
| 3 · radio | 11–16 | one transfer splits into N2 (radio) + N1 (UE); RRC builds DRBs; radio reports back; N4 modified for downlink; UECM register | Namf_Comm_N1N2MessageTransfer · UpdateSMContext |
| 4 · flow | 17–21 | UpdateSMContext ack; status notify; IPv6 Router Advertisement; final policy update; unsubscribe — user data flows | N3 user-plane data |
ULCL — Uplink Classifier
- UPF inserted in the path; sorts uplink traffic by destination.
- Edge traffic → local anchor; internet → central anchor. UE sees one IP.
- §5.6.4.2
Branching Point (IPv6)
- UE has two IPv6 prefixes; BP UPF steers by source prefix.
- Same goal, multi-homing flavour. §5.6.4.3
N9 & local PSA
- N9 tunnels user plane UPF↔UPF (anchor to anchor).
- A local PSA at the edge serves game/cache/factory apps at low latency.
| Cause | Meaning | Where to look |
|---|---|---|
| #26 | insufficient resources | UPF / RAN capacity |
| #27 | missing or unknown DNN | subscription / DNN config |
| #28 | unknown PDU session type | UE request vs support |
| #29 | user authentication or authorization failed | DN-AAA secondary auth |
| #43 | invalid PDU session identity | UE / AMF state mismatch |
| #54 | PDU session does not exist | stale session reference |
The rule to remember: the SMF is the brain, the UPF is the muscle, and N4 is the nerve between them. The SMF never touches a user packet — it only writes the four rules and ships them over PFCP. Everything else in this module is a variation on that one idea.
TS 23.501 §5.6 · TS 23.502 §4.3 · TS 24.501 §8.3 & §9.11.4 · TS 29.502 · TS 29.244PDU session mastery check
Questions and answers reshuffle every load. 70%+ to consider Module 3 done.