5G N2切换核心网侧NGAP/PFCP三阶段信令与数据转发博弈

sequenceDiagram
    participant UE
    participant S_gNB as Source gNB
    participant AMF
    participant SMF
    participant UPF as UPF(PSA)
    participant T_gNB as Target gNB

    Note over UE,T_gNB: Phase 1: Handover Preparation
    UE->>S_gNB: Measurement Report (Event A3/A5)
    S_gNB->>S_gNB: HO Decision (Xn unavailable)
    S_gNB->>AMF: NGAP Handover Required
    Note right of S_gNB: IE: Target ID, Source-toTarget-TransparentContainer,<br/>PDU Session Resource List,<br/>HO Type (intra5GS)

    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of AMF: IE: HO State=PREPARING,<br/>Target ID, N2 SM Info

    SMF->>UPF: PFCP Session Modification Request
    Note right of SMF: IE: Create FAR(forwarding),<br/>DL PDR with forwarding action

    UPF-->>SMF: PFCP Session Modification Response
    Note right of UPF: IE: F-TEID(forwarding tunnel)

    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response
    Note right of SMF: IE: N2 SM Info,<br/>QoS Flow Profile,<br/>Forwarding Tunnel Info

    AMF->>T_gNB: NGAP Handover Request
    Note right of AMF: IE: UE AggregateMaxBitRate,<br/>PDU Session Resource Setup List,<br/>Security Context,<br/>Source-toTarget-TransparentContainer

    T_gNB->>T_gNB: Allocate Radio Resources
    T_gNB-->>AMF: NGAP Handover Request Acknowledge
    Note right of T_gNB: IE: Target-toSource-TransparentContainer,<br/>PDU Session Resource Admitted List,<br/>UL NG-U UP TNL Info(TEID)

    Note over UE,T_gNB: Phase 2: Handover Execution
    AMF->>S_gNB: NGAP Handover Command
    Note right of AMF: IE: Target-toSource-TransparentContainer

    S_gNB->>UE: RRC Reconfiguration (HO Command)
    Note right of S_gNB: IE: targetCellIdentity,<br/>newUE-Identity,<br/>reconfigurationWithSync

    Note over S_gNB,UPF: Data Forwarding (DL data buffered)
    S_gNB->>T_gNB: DL Forwarding (direct/indirect)

    UE->>T_gNB: RRC Connection Reconfiguration Complete
    Note right of UE: Random Access + RRC Complete

    Note over UE,T_gNB: Phase 3: Handover Completion
    T_gNB->>AMF: NGAP Handover Notify
    Note right of T_gNB: IE: User Location Info(T-gNB)

    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of AMF: IE: HO State=COMPLETED,<br/>UL Tunnel Info(T-gNB N3 TEID)

    SMF->>UPF: PFCP Session Modification Request
    Note right of SMF: IE: Update FAR(DL→T-gNB),<br/>Remove Forwarding FAR

    UPF->>UPF: Send End Marker on old DL path
    UPF-->>SMF: PFCP Session Modification Response

    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response

    AMF->>S_gNB: NGAP UE Context Release Command
    S_gNB-->>AMF: NGAP UE Context Release Complete

graph TB
    subgraph Direct["直连转发 (Direct Forwarding)"]
        S1[Source gNB] -->|Xn-U / 直接IP| T1[Target gNB]
    end

    subgraph Indirect["间接转发 (Indirect Forwarding)"]
        S2[Source gNB] -->|N3 tunnel| UPF1[UPF PSA]
        UPF1 -->|N9/N3 tunnel| T2[Target gNB]
    end

    subgraph NoForward["无转发 (No Forwarding)"]
        S3[Source gNB] -->|丢弃DL数据| X[Discard]
        UPF2[UPF PSA] -->|新N3 tunnel| T3[Target gNB]
    end

    style Direct fill:#e8f5e9
    style Indirect fill:#fff3e0
    style NoForward fill:#ffebee

sequenceDiagram
    participant S_gNB as Source gNB
    participant S_AMF as Source AMF
    participant T_AMF as Target AMF
    participant SMF
    participant UDM

    S_gNB->>S_AMF: NGAP Handover Required
    Note right of S_AMF: S-AMF判断Target ID<br/>不属于自己服务的gNB

    S_AMF->>T_AMF: Namf_Communication_CreateUEContext Request (N14)
    Note right of S_AMF: IE: Target ID,<br/>UE Context(Security,MM State),<br/>PDU Session List,<br/>Source-toTarget-Container

    T_AMF->>UDM: Nudm_UECM_Registration
    T_AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request

    SMF-->>T_AMF: UpdateSMContext Response

    T_AMF->>T_AMF: Forward to Target gNB

    T_AMF-->>S_AMF: Namf_Communication_CreateUEContext Response

    Note over S_AMF,T_AMF: HO Execution...

    T_AMF->>SMF: Path Switch (UpdateSMContext)
    S_AMF->>S_AMF: Release Source UE Context

stateDiagram-v2
    [*] --> HO_None : UE in Connected Mode

    HO_None --> HO_Preparing : Handover Required sent

    HO_Preparing --> HO_Prepared : Handover Request Ack received
    HO_Preparing --> HO_Failed : Handover Preparation Failure<br/>or Handover Cancel

    HO_Prepared --> HO_Executing : Handover Command sent

    HO_Executing --> HO_Completed : Handover Notify received
    HO_Executing --> HO_Failed : Handover Failure (T304 expiry)

    HO_Completed --> HO_None : UE Context Release Complete
    HO_Failed --> HO_None : RRC Reestablishment<br/>or Connection Release

# NGAP Handover Required
ngap.procedureCode == 7 && ngap.handoverRequired

# NGAP Handover Request
ngap.procedureCode == 8 && ngap.handoverRequest

# PFCP Session Modification（HO期间）
pfcp.message_type_value == 53 && pfcp.f_teid

# HTTP/2 Nsmf UpdateSMContext（SBI接口）
http2.header.value contains "sm-contexts" && http2.header.value contains "modify"