PDU Session Modification / Release¶

Learning objectives¶

After reading this page, you will be able to:

• Explain the two procedures — PDU Session Modification (changing QoS or adding/removing QoS Flows on an established session) and PDU Session Release (release) — along with the difference in their purposes.
• Explain that each procedure can occur from both the UE-requested (UE-originated) and Network-requested (NW-originated) sides.
• Explain that Modification is realized as the addition, change, or deletion of per-QoS-Flow parameters (QFI/5QI/GBR, etc.).
• Explain the flow of separating control from enforcement — control (PCF/SMF) → enforcement (UPF over N4/PFCP, gNB over N2/NGAP, UE over N1/5GSM NAS).
• Explain that a PCF policy change is a primary trigger for Network-requested Modification.
• Explain the division of roles in which the AMF mediates (the N1/N2 point of contact) while the actual SM processing is driven by the SMF.
• Compare 4G's Bearer Modification / Bearer Deactivation (per EPS Bearer) with 5G's per-PDU-Session / per-QoS-Flow change and release.

Prerequisites¶

• PDU Session Establishment — an essential prerequisite. An established session is the starting point of this chapter.
• QoS — the detailed model of QoS Flow / QFI / 5QI / GBR.
• Service Request — the procedure that brings a CM-IDLE UE back onto the user plane (related to NW-originated Command delivery).
• N1 / N2 / N4 / N7 — the individual interfaces.
• SMF / UPF / PCF / AMF — the NFs involved.

What you'll learn in this chapter¶

• Scope: Modification and Release of an established PDU Session. Each from both the UE-requested / Network-requested origins. The flow from control to enforcement.
• Out of scope:
• The establishment procedure itself → PDU Session Establishment.
• The details of the QoS model (5QI characteristics, AMBR, DRB mapping, etc.) → QoS.
• The internal processing of the radio-side DRB and the details of RRC reconfiguration.

Why — why Modification / Release are needed¶

A PDU session is not "establish it and you're done." During communication the application switches, a video call begins and demands high quality assurance (GBR), or conversely that is no longer needed and reverts to Non-GBR, or the session itself becomes unnecessary and its resources should be returned — such dynamic state changes occur constantly.

• The procedure for changing QoS mid-communication (adding, changing, or deleting QoS Flows) is Modification.
• The procedure for returning resources when a session is no longer needed (releasing the UPF session, TEID, IP, and radio resources) is Release.

Without these, quality stays frozen out of step with reality, or unused resources linger and squeeze network capacity.

Analogy: In terms of a rental contract, Modification is like "rewriting the terms of the contract (adding a room, changing options) while continuing to live in the same place," and Release is like "moving out and returning the keys and the room." Both are operations that arise after move-in (after establishment).

Overview¶

flowchart TD
    A["Established PDU Session (ACTIVE)"] --> B{"Trigger"}
    B -->|"Want to change QoS"| M["Modification (add/change/delete QoS Flow)"]
    B -->|"No longer needed"| R["Release (release resources)"]
    M --> A
    R --> Z["Deleted (equivalent to INACTIVE)"]

Basic Concept — a beginner-friendly explanation¶

1. What Modification is (changing QoS Flows)¶

Modification is the procedure of rewriting the QoS Flows inside a session while keeping the established session intact. A QoS Flow is the finest granularity of 5G QoS, identified by a QFI, and carries parameters such as 5QI (quality characteristics) and GBR (bandwidth guarantee). Modification adds, changes, or deletes QoS Flows. For the detailed QoS model, see QoS.

Enforcement targets three directions. On the UPF, PDR/FAR/QER are updated via the N4(PFCP) Session Modification; on the gNB, the DRB is reconfigured via the N2(NGAP) PDU Session Resource Modify; and to the UE, the N1(5GSM NAS) Modification Command is delivered.

2. What Release is (release)¶

Release is the procedure of deleting the session itself and returning the associated resources. The UPF releases the session and TEID via N4 Session Deletion, the gNB releases the DRB/N3 via N2 PDU Session Resource Release, and the N1 Release Command is delivered to the UE. The SMF releases the Nsmf context, and also stops charging and returns the IP address.

3. UE-requested and Network-requested¶

Even the same Modification / Release comes in two variants depending on who initiates it. UE-requested begins when the UE sends a Request, whereas Network-requested begins for network-side reasons such as a PCF policy change, the SMF's decision, a fault, or charging. In the Network-requested case, if the UE is CM-IDLE, a Service Request (returning to the user plane via paging) may precede in order to deliver the Command (to be confirmed: the conditions depend on the release/implementation).

Architecture¶

flowchart LR
    UE["UE (5GSM)"]
    gNB["gNB / NG-RAN"]
    AMF["AMF (mediator)"]
    SMF["SMF (SM driver)"]
    UPF["UPF (enforcement)"]
    PCF["PCF (policy trigger)"]
    DN["DN"]

    PCF -->|"N7 (policy change)"| SMF
    SMF -->|"N4 (PFCP) enforcement"| UPF
    SMF -->|"SM NAS + N2 SM info"| AMF
    AMF -->|"N2 (NGAP) enforcement"| gNB
    AMF -->|"N1 (NAS) relay"| UE
    gNB -.->|"DRB reconfigure/release"| UE
    UPF --- DN

    classDef core fill:#c8e6c9,stroke:#2e7d32,color:#000;
    class AMF,SMF,UPF,PCF core;

The control path (PCF → SMF) decides "what to change," and the enforcement path (SMF → UPF over N4, AMF → gNB over N2, AMF → UE over N1) actually applies it. This separation of control from enforcement is the design philosophy of the 5GC.

Network Function¶

Interface / Protocol¶

Procedure — Call Flow¶

(A) PDU Session Modification (centered on Network-requested)¶

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant SMF
    participant UPF
    participant PCF

    Note over PCF,SMF: Trigger: PCF policy change
(for the UE-originated case, see the diff note)
    PCF->>SMF: Npcf_SMPolicyControl_UpdateNotify
(PCC rule change — to be confirmed)
    Note over SMF: SMF decides the change
(add/change/delete QoS Flow)
    SMF->>UPF: N4 Session Modification (PFCP)
update PDR/FAR/QER
    UPF-->>SMF: N4 Modification Response
    SMF->>AMF: Namf_Communication_N1N2MessageTransfer
(N1: Modification Command + N2 SM info)
    AMF->>gNB: N2 PDU Session Resource Modify (NGAP)
    Note over gNB,UE: gNB reconfigures the DRB
    gNB-->>AMF: N2 Modify Response
    AMF->>UE: PDU Session Modification Command (5GSM/N1)
    UE->>AMF: PDU Session Modification Complete (5GSM/N1)
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext
    Note over SMF,UPF: Additional N4 updates as needed (count is implementation-dependent — to be confirmed)

Step-by-step (Modification)

1. Trigger: The PCF notifies the SMF of a PCC rule change (Npcf_SMPolicyControl_UpdateNotify — operation name to be confirmed). In the UE-originated case, the UE sends a PDU Session Modification Request(5GSM) (the diff is below).
2. SMF decision: The SMF decides the addition/change/deletion of QoS Flows.
3. N4 enforcement: SMF → UPF updates PDR/FAR/QER via N4 Session Modification(PFCP).
4. N1N2 delivery: SMF → AMF via Namf_Communication_N1N2MessageTransfer (N1 = Modification Command, with N2 SM information enclosed).
5. N2 enforcement: AMF → gNB via N2 PDU Session Resource Modify(NGAP) → the gNB reconfigures the DRB.
6. NAS completion: Modification Command / Complete are exchanged among UE ↔ AMF ↔ SMF.
7. The UE state stays ACTIVE (only the QoS is updated).

Diff from the UE-originated case: The UE first sends a PDU Session Modification Request(5GSM) to the SMF via the AMF, the SMF decides whether it can be accepted (checking with the PCF if necessary), and thereafter it proceeds the same as from step 3 above. If the SMF cannot authorize the request, it returns a Modification Reject (the 5GSM cause value for the reason is to be confirmed).

(B) PDU Session Release¶

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    participant SMF
    participant UPF
    participant PCF

    Note over UE,PCF: Trigger: UE Release Request
or NW-originated (SMF/PCF/AMF)
    SMF->>UPF: N4 Session Deletion (PFCP)
release session/TEID
    UPF-->>SMF: N4 Deletion Response
    SMF->>AMF: Namf_Communication_N1N2MessageTransfer
(N1: Release Command + N2 SM info)
    AMF->>gNB: N2 PDU Session Resource Release (NGAP)
    Note over gNB,UE: gNB releases DRB/N3
    gNB-->>AMF: N2 Release Response
    AMF->>UE: PDU Session Release Command (5GSM/N1)
    UE->>AMF: PDU Session Release Complete (5GSM/N1)
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext
    SMF->>PCF: Npcf_SMPolicyControl_Delete (to be confirmed)
    Note over SMF: stop charging・return IP address
release Nsmf context

Step-by-step (Release)

1. Trigger: The UE sends a PDU Session Release Request(5GSM), or it is NW-originated (a decision by the SMF/PCF/AMF, triggered by policy, charging, faults, deregistration, etc.).
2. N4 deletion: SMF → UPF via N4 Session Deletion(PFCP) → the UPF releases the session, TEID, and forwarding rules.
3. N1N2 delivery: SMF → AMF via N1N2MessageTransfer (N1 = Release Command, with N2 SM information enclosed).
4. N2 release: AMF → gNB via N2 PDU Session Resource Release(NGAP) → the gNB releases the DRB/N3.
5. NAS completion: Release Command / Complete are exchanged among UE ↔ AMF ↔ SMF.
6. Post-processing: The SMF notifies the PCF/charging of the release (Npcf_SMPolicyControl_Delete — to be confirmed), returns the allocated IP, and releases the Nsmf context.
7. The session state goes from ACTIVE → deleted (equivalent to INACTIVE).

Signal Flow¶

State Machine¶

stateDiagram-v2
    [*] --> INACTIVE
    INACTIVE --> ACTIVE: PDU Session Establishment (separate chapter)
    ACTIVE --> ACTIVE: Modification (QoS change, state maintained)
    ACTIVE --> INACTIVE: PDU Session Release
    INACTIVE --> [*]

• Modification updates only the QoS Flow parameters while keeping the session ACTIVE (a self-transition).
• Release transitions from ACTIVE → INACTIVE (deleted).

Packet Analysis (Wireshark)¶

Main Display Filters¶

What to look for in the decode¶

• Modification: the QFI / 5QI being changed, the QoS Flow being added/deleted, and the PDR/FAR/QER being updated (PFCP).
• Release: the TEID being released (PFCP/GTP-U), the PFCP session being deleted, and the PDU Session Resource being released over N2.

The nesting structure of SM NAS¶

5GSM (SM NAS) is carried nested inside 5GMM (MM NAS) (5GMM > 5GSM). The AMF handles this container transparently, and the substance of the 5GSM is interpreted by the SMF. In Wireshark, nas-5gs.sm appears nested inside nas-5gs.mm.

Configuration¶

Only structural-level aspects are shown (no specific values are given).

• Dynamic QoS changes are chiefly triggered by the PCF's PCC rules. In a static configuration, the default QoS tied to the DNN/slice is used (key names are implementation-dependent).
• On Release, the address is returned to the IP pool as the UPF deletes the session, and the TEID is released.
• Whether Modification/Release is permitted depends on subscriber data, policy, and authorization.

Trouble Shooting¶

Comparison with EPC¶

Release differences¶

• Rel-15: The basic procedures for PDU Session Modification / Release are specified.
• Rel-16 / Rel-17 and later: Additional features and extensions (e.g., procedural differences accompanying specific features) vary by release.

3GPP Specification¶

FAQ¶

Summary¶

• An established PDU session has operational procedures for changing QoS mid-communication (Modification) and for releasing it (Release).
• Both occur from both the UE-requested / Network-requested origins, and Network-requested Modification is chiefly triggered by a PCF policy change.
• Modification is the per-QoS-Flow addition/change/deletion, and the state stays ACTIVE. Release is the deletion of the session, transitioning from ACTIVE to equivalent to INACTIVE.
• The flow of control (PCF/SMF) → enforcement (UPF over N4/PFCP, gNB over N2/NGAP, UE over N1/5GSM NAS) is common to both.
• The AMF mediates; the SMF is the SM driver. In contrast to 4G's Bearer Modification/Deactivation (per EPS Bearer), 5G is per PDU Session/QoS Flow.
• Basis specs (subsection numbers to be confirmed): TS 23.502 §4.3.3/§4.3.4, TS 24.501 §6.4 series, TS 23.501 §5.6/§5.7, TS 29.244, TS 29.502, TS 29.512, TS 38.413.

Practice — comprehension check and exercises¶

Comprehension check¶

Exercise: Reorder the main steps of PDU Session Release¶

Reorder the following steps (NW-originated PDU Session Release) into the correct order.

a. AMF → gNB: N2 PDU Session Resource Release
b. SMF → UPF: N4 Session Deletion (release session/TEID)
c. UE ↔ AMF ↔ SMF: Release Command / Complete
d. SMF → AMF: N1N2MessageTransfer (N1: Release Command + N2 SM info)
e. SMF notifies PCF/charging of the release, returns the IP, and releases the Nsmf context

Next Step¶

• PDU Session Establishment — prerequisite. The establishment procedure.
• QoS — the detailed model of QoS Flow / 5QI / GBR.
• Network Slicing — the relationship between slices and PDU sessions.
• Service Request — returning a CM-IDLE UE to the user plane.
• Related NFs: SMF / UPF / PCF / AMF
• Related IFs: N1 / N2 / N4 / N7
• Dictionaries: Message Dictionary / Timer Dictionary / Cause Dictionary / Protocol Dictionary / NF Dictionary / Interface Dictionary