5G N2切换端到端信令全链拆解：从Handover Required到路径切换完成

flowchart TD
    A[UE发送Measurement Report] --> B{源gNB评估测量结果}
    B -->|信号质量下降| C{Xn接口是否可用?}
    C -->|是| D[选择Xn切换路径]
    C -->|否| E[选择N2切换路径]
    B -->|负载均衡触发| F{策略是否允许Xn切换?}
    F -->|是| D
    F -->|否| E
    D --> G[Xn Handover Preparation]
    E --> H[N2 Handover Required → AMF]
    H --> I{目标gNB是否在同AMF服务区?}
    I -->|是| J[Intra-AMF N2 Handover]
    I -->|否| K[Inter-AMF N2 Handover + AMF Relocation]

sequenceDiagram
    participant UE
    participant S_gNB as 源gNB
    participant AMF
    participant SMF
    participant UPF
    participant T_gNB as 目标gNB

    Note over UE,T_gNB: === N2切换准备阶段 (Preparation Phase) ===

    UE->>S_gNB: Measurement Report
    Note right of UE: 包含邻区RSRP/RSRQ测量结果

    S_gNB->>AMF: NGAP Handover Required
    Note right of S_gNB: IE: AMF UE NGAP ID, RAN UE NGAP ID,<br/>Handover Type, Cause, Target ID,<br/>PDU Session Resource List,<br/>Source-to-Target Transparent Container,<br/>Direct Forwarding Path Availability

    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of AMF: 每个PDU Session一条请求<br/>包含: Target ID, N2 SM Info

    SMF->>UPF: N4 Session Modification Request
    Note right of SMF: 请求分配UL CN Tunnel Info<br/>用于目标gNB的上行隧道

    UPF-->>SMF: N4 Session Modification Response
    Note left of UPF: 返回: UL CN Tunnel Info,<br/>Data Forwarding Info

    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response
    Note left of SMF: 包含N2 SM Information:<br/>QoS Flow列表、UL CN Tunnel Info、<br/>Data Forwarding可用性指示

    Note over AMF: AMF等待所有SMF响应<br/>或超时后继续

    AMF->>T_gNB: NGAP Handover Request
    Note right of AMF: IE: AMF UE NGAP ID (新分配),<br/>Source-to-Target Transparent Container,<br/>N2 SM Information List,<br/>N2 MM Information (Security Context),<br/>Mobility Restriction List

    Note over T_gNB: 目标gNB执行准入控制<br/>检查无线资源、QoS保障能力

    T_gNB-->>AMF: NGAP Handover Request Acknowledge
    Note left of T_gNB: IE: Target-to-Source Transparent Container,<br/>Admitted PDU Session List + N2 SM Info,<br/>Failed PDU Session List,<br/>Data Forwarding Tunnel Info

    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of AMF: 传递目标gNB的N3 AN Tunnel Info<br/>及Data Forwarding信息

    SMF->>UPF: N4 Session Modification
    Note right of SMF: 配置DL Data Forwarding Tunnel<br/>(直传或经UPF间传)

    UPF-->>SMF: N4 Session Modification Response

    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response
    Note left of SMF: 返回DL Data Forwarding Tunnel Info

    AMF->>S_gNB: NGAP Handover Command
    Note right of AMF: IE: Target-to-Source Transparent Container,<br/>Handover Type, PDU Session Resource List,<br/>DL Forwarding Tunnel Info

HandoverRequired ::= SEQUENCE {
    protocolIEs ProtocolIE-Container { {HandoverRequired-IEs} }
}

HandoverRequired-IEs NGAP-PROTOCOL-IES ::= {
    { AMF UE NGAP ID }           |  -- 必选：AMF侧UE标识
    { RAN UE NGAP ID }           |  -- 必选：RAN侧UE标识
    { Handover Type }            |  -- 必选：切换类型
    { Cause }                    |  -- 必选：触发原因
    { Target ID }                |  -- 必选：目标标识
    { Direct Forwarding Path Availability } |  -- 可选：直传路径可用性
    { PDU Session Resource List } |  -- 必选：PDU会话资源列表
    { Source to Target Transparent Container }  -- 必选：源到目标透传容器
}

flowchart TD
    A[AMF评估数据转发需求] --> B{源gNB与目标gNB间<br/>是否有IP连通性?}
    B -->|是| C{Direct Forwarding Path<br/>Availability = available?}
    B -->|否| F[选择间接转发 Indirect]
    C -->|是| D[选择直接转发 Direct]
    C -->|否| F
    D --> G[源gNB → 目标gNB<br/>直接转发DL数据]
    F --> H{是否需要新UPF?}
    H -->|是| I[SMF选择Intermediate UPF<br/>建立N4会话]
    H -->|否| J[使用现有UPF<br/>配置转发隧道]
    I --> K[源UPF → 中间UPF → 目标gNB]
    J --> K

sequenceDiagram
    participant UE
    participant S_gNB as 源gNB
    participant AMF
    participant SMF
    participant UPF as PSA UPF
    participant T_gNB as 目标gNB

    Note over UE,T_gNB: === N2切换执行阶段 (Execution Phase) ===

    S_gNB->>UE: RRC Reconfiguration (HO Command)
    Note right of S_gNB: 包含目标gNB生成的RRC配置<br/>Target Cell ID, C-RNTI, RACH资源

    S_gNB->>AMF: NGAP Uplink RAN Status Transfer
    Note right of S_gNB: PDCP SN状态、HFN<br/>保障数据连续性

    Note over S_gNB,T_gNB: DL数据转发开始<br/>(Direct或Indirect)

    Note over UE: UE断开源gNB<br/>同步到目标gNB

    UE->>T_gNB: RACH Preamble (如需)
    T_gNB-->>UE: RACH Response
    UE->>T_gNB: RRC Reconfiguration Complete

    T_gNB->>AMF: NGAP Handover Notify
    Note right of T_gNB: 通知AMF: UE已成功到达目标侧

    Note over AMF: AMF触发路径切换

    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of AMF: 指示路径切换到目标gNB

    SMF->>UPF: N4 Session Modification
    Note right of SMF: 切换DL隧道到目标gNB<br/>释放源gNB隧道

    UPF->>S_gNB: End Marker Packets
    Note left of UPF: 在旧N3隧道上发送End Marker<br/>标记数据流边界

    UPF->>T_gNB: DL Data (新路径)
    Note left of UPF: 开始通过新N3隧道发送下行数据

    UPF-->>SMF: N4 Session Modification Response

    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response

    AMF->>S_gNB: NGAP UE Context Release Command
    Note right of AMF: 释放源gNB上的UE上下文

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

    Note over UE,T_gNB: === N2切换完成 ===

sequenceDiagram
    participant S_gNB as 源gNB
    participant S_AMF as 源AMF
    participant T_AMF as 目标AMF
    participant SMF
    participant T_gNB as 目标gNB

    S_gNB->>S_AMF: Handover Required
    Note right of S_AMF: 源AMF发现Target ID<br/>不在自己服务区

    S_AMF->>T_AMF: Namf_Communication_CreateUEContext Request
    Note right of S_AMF: N14接口：传递完整UE上下文<br/>SUPI, PDU Session列表,<br/>安全上下文, 移动性限制

    T_AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request
    Note right of T_AMF: 通知SMF AMF已变更<br/>传递T-AMF ID

    SMF-->>T_AMF: Nsmf_PDUSession_UpdateSMContext Response
    Note left of SMF: 返回N2 SM Information

    T_AMF->>T_gNB: Handover Request
    T_gNB-->>T_AMF: Handover Request Acknowledge

    T_AMF->>S_AMF: Namf_Communication_CreateUEContext Response
    Note right of T_AMF: N14接口：返回Target-to-Source<br/>Transparent Container

    S_AMF->>S_gNB: Handover Command

    Note over S_gNB,T_gNB: UE执行切换...

    T_gNB->>T_AMF: Handover Notify
    T_AMF->>S_AMF: N1N2MessageTransfer (Notification)
    S_AMF->>S_gNB: UE Context Release Command

stateDiagram-v2
    state "UE状态" as UE {
        [*] --> RRC_CONNECTED: 已建立连接
        RRC_CONNECTED --> HO_Executing: 收到RRC Reconfig(HO)
        HO_Executing --> RRC_CONNECTED: RRC Reconfig Complete
        HO_Executing --> RRC_Reestablishment: 切换失败/RLF
    }

    state "源gNB状态" as S_gNB {
        [*] --> Serving: 正常服务UE
        Serving --> HO_Preparing: 发送Handover Required
        HO_Preparing --> HO_Executing: 收到Handover Command
        HO_Executing --> Data_Forwarding: 开始数据转发
        Data_Forwarding --> Resource_Releasing: 收到UE Context Release
        Resource_Releasing --> [*]
    }

    state "AMF状态" as AMF_State {
        [*] --> Idle: 无HO活动
        Idle --> Preparation: 收到Handover Required
        Preparation --> Waiting_SMF: 调用SMF UpdateSMContext
        Waiting_SMF --> Sending_HORequest: 所有SMF响应/超时
        Sending_HORequest --> Waiting_HOAck: 等待目标gNB
        Waiting_HOAck --> Forwarding_Setup: 收到HO Request ACK
        Forwarding_Setup --> HO_Command_Sent: 转发隧道就绪
        HO_Command_Sent --> Waiting_HONotify: 等待Handover Notify
        Waiting_HONotify --> Path_Switching: 收到Handover Notify
        Path_Switching --> Cleanup: 路径切换完成
        Cleanup --> Idle
    }

# NGAP Handover Required
tshark -r capture.pcap -Y "ngap.procedureCode == 9 && ngap.initiatingMessage"

# NGAP Handover Request (AMF → 目标gNB)
tshark -r capture.pcap -Y "ngap.procedureCode == 14 && ngap.handoverType"

# NGAP Handover Request Acknowledge
tshark -r capture.pcap -Y "ngap.procedureCode == 14 && ngap.successfulOutcome"

# NGAP Handover Command
tshark -r capture.pcap -Y "ngap.procedureCode == 9 && ngap.successfulOutcome"

# NGAP Handover Notify
tshark -r capture.pcap -Y "ngap.procedureCode == 15"

# NGAP Uplink/Downlink RAN Status Transfer
tshark -r capture.pcap -Y "ngap.procedureCode == 22 || ngap.procedureCode == 23"

# 所有NGAP切换相关消息
tshark -r capture.pcap -Y "ngap.procedureCode >= 9 && ngap.procedureCode <= 23"