5G PDU会话建立N1/N2/N4信令全链拆解

graph TB
    subgraph "PDU会话建立涉及的参考点"
        UE["UE"]
        RAN["(R)AN<br/>gNB"]
        AMF["AMF"]
        SMF["SMF"]
        UPF["UPF"]
        PCF["PCF"]
        UDM["UDM"]

        UE -->|"N1 (NAS 5GSM)"| RAN
        RAN -->|"N2 (NGAP)"| AMF
        AMF -->|"N11 (Nsmf_PDUSession)"| SMF
        SMF -->|"N4 (PFCP)"| UPF
        SMF -->|"N7 (Npcf_SMPolicyControl)"| PCF
        SMF -->|"N10 (Nudm_SDM)"| UDM
        AMF -->|"N8 (Nudm_UECM)"| UDM
        RAN -->|"N3 (GTP-U)"| UPF
    end

sequenceDiagram
    participant UE
    participant RAN as (R)AN
    participant AMF
    participant SMF
    participant UDM
    participant PCF
    participant UPF

    Note over UE,UPF: Step 1: UE发起NAS消息
    UE->>RAN: NAS Message (N1 SM Container)<br/>PDU Session Establishment Request<br/>[PDU Session ID, DNN, S-NSSAI,<br/>PDU Session Type, SSC Mode, 5GSM Capability]
    RAN->>AMF: N2 Uplink NAS Transport<br/>+ User Location Info + Access Type

    Note over AMF: Step 2: AMF选择SMF
    Note over AMF: 根据S-NSSAI/DNN/UDM订阅信息<br/>执行SMF选择（TS 23.501 §6.3.2）

    Note over AMF,SMF: Step 3: 创建SM上下文
    AMF->>SMF: Nsmf_PDUSession_CreateSMContext Request<br/>(SUPI, DNN, S-NSSAI, PDU Session ID,<br/>AMF ID, N1 SM Container, UE Location)

    Note over SMF,UDM: Step 4: 获取订阅数据
    SMF->>UDM: Nudm_SDM_Get (SUPI, SM Subscription Data)
    UDM-->>SMF: Allowed PDU Session Types, Allowed SSC modes,<br/>Default 5QI/ARP, Session-AMBR
    SMF->>UDM: Nudm_SDM_Subscribe (变更通知)

    Note over SMF: Step 5: 创建SM Context响应
    SMF-->>AMF: Nsmf_PDUSession_CreateSMContext Response<br/>(SM Context ID, Cause)

    Note over SMF,PCF: Step 7: 策略关联建立
    SMF->>PCF: Npcf_SMPolicyControl_Create<br/>(PCC规则请求)
    PCF-->>SMF: Default PCC Rules, QoS参数,<br/>Charging规则

    Note over SMF,UPF: Step 8-10: UPF选择与N4会话建立
    Note over SMF: SMF选择SSC模式和UPF<br/>（TS 23.501 §6.3.3）<br/>分配IP地址/前缀
    SMF->>UPF: N4 Session Establishment Request<br/>(PDR, FAR, QER, URR规则)
    UPF-->>SMF: N4 Session Establishment Response<br/>(CN Tunnel Info / F-TEID)

    Note over SMF,AMF: Step 11: N1N2消息传递
    SMF->>AMF: Namf_Communication_N1N2MessageTransfer<br/>N2 SM Info: QFI, QoS Profile, CN Tunnel Info<br/>N1 SM Container: PDU Session Establishment Accept<br/>(IP地址, QoS Rules, SSC Mode, Session-AMBR)

    Note over AMF,RAN: Step 12: N2 PDU Session请求
    AMF->>RAN: N2 PDU Session Resource Setup Request<br/>(N2 SM Info + NAS Message)

    Note over RAN,UE: Step 13: RRC重配 + NAS Accept
    RAN->>UE: RRC Reconfiguration<br/>(DRB建立 + NAS PDU Session Establishment Accept)
    UE-->>RAN: RRC Reconfiguration Complete

    Note over RAN,AMF: Step 14: N2响应
    RAN->>AMF: N2 PDU Session Resource Setup Response<br/>(AN Tunnel Info / AN F-TEID)

    Note over AMF,SMF: Step 15: 更新SM上下文
    AMF->>SMF: Nsmf_PDUSession_UpdateSMContext Request<br/>(AN Tunnel Info)

    Note over SMF,UPF: Step 16: N4会话修改
    SMF->>UPF: N4 Session Modification Request<br/>(提供AN Tunnel Info + 转发规则)
    UPF-->>SMF: N4 Session Modification Response

    Note over SMF: Step 16c: SMF注册到UDM
    SMF->>UDM: Nudm_UECM_Registration<br/>(SUPI, SMF ID, DNN, S-NSSAI, PDU Session ID)

    Note over SMF,AMF: Step 17: 完成
    SMF-->>AMF: Nsmf_PDUSession_UpdateSMContext Response

flowchart TD
    A[AMF收到PDU Session请求] --> B{Request Type?}
    B -->|Initial Request| C[查询NRF发现SMF<br/>基于S-NSSAI + DNN]
    B -->|Existing PDU Session| D[从UDM获取已注册SMF ID]
    B -->|Emergency Request| E[选择紧急服务专用SMF<br/>TS 23.501 §5.16.4]

    C --> F[Nsmf_PDUSession_CreateSMContext]
    D --> G[Nsmf_PDUSession_UpdateSMContext]
    E --> F

    F --> H[SMF创建SM Context]
    G --> I[SMF更新已有SM Context]

if UE提供了SSC Mode:
    if 请求的SSC Mode在Allowed SSC Modes中:
        选择UE请求的SSC Mode
    else:
        拒绝并返回允许的SSC Mode列表
else:
    使用Default SSC Mode

特殊规则：静态IP地址/前缀 → 必须SSC Mode 1
紧急服务 → 必须SSC Mode 1

graph LR
    subgraph "N4 PFCP规则体系 (TS 29.244)"
        PDR["PDR<br/>包检测规则<br/>§7.5.2.2"]
        FAR["FAR<br/>转发动作规则<br/>§7.5.2.3"]
        QER["QER<br/>QoS执行规则<br/>§7.5.2.5"]
        URR["URR<br/>使用量报告规则<br/>§7.5.2.4"]
        BAR["BAR<br/>缓存动作规则<br/>§7.5.2.6"]
    end

    PDR -->|"检测到数据包后"| FAR
    PDR -->|"关联QoS"| QER
    PDR -->|"关联计费"| URR
    FAR -->|"需要缓存时"| BAR

    style PDR fill:#e1f5fe
    style FAR fill:#e8f5e9
    style QER fill:#fff3e0
    style URR fill:#fce4ec
    style BAR fill:#f3e5f5

sequenceDiagram
    participant SMF
    participant UPF
    participant RAN

    Note over SMF,RAN: 阶段1: SMF→UPF获取CN Tunnel Info
    SMF->>UPF: N4 Session Establishment Request<br/>(请求分配UL F-TEID)
    UPF-->>SMF: N4 Session Establishment Response<br/>(CN Tunnel Info = UPF DL F-TEID)

    Note over SMF,RAN: 阶段2: SMF→RAN携带CN Tunnel Info
    Note over SMF: CN Tunnel Info通过N2下发给RAN

    Note over SMF,RAN: 阶段3: RAN→SMF返回AN Tunnel Info
    Note over RAN: RAN分配AN UL F-TEID

    Note over SMF,RAN: 阶段4: SMF→UPF更新AN Tunnel Info
    SMF->>UPF: N4 Session Modification Request<br/>(提供AN Tunnel Info)
    UPF-->>SMF: N4 Session Modification Response

    Note over UPF,RAN: N3双向隧道打通！<br/>UL: RAN→UPF (AN F-TEID→UPF F-TEID)<br/>DL: UPF→RAN (UPF F-TEID→AN F-TEID)

stateDiagram-v2
    [*] --> SSC1: 静态IP/紧急服务/默认
    [*] --> SSC2: 运营商策略触发
    [*] --> SSC3: UE请求+订阅允许

    state SSC1 {
        [*] --> PSA_Fixed
        PSA_Fixed: UPF PSA锚点固定不变<br/>IP地址保持不变<br/>移动性由中间UPF/I-SMF处理
    }

    state SSC2 {
        [*] --> Session_Release
        Session_Release: 网络触发释放旧PDU Session<br/>UE立即重建新PDU Session
        Session_Release --> New_PSA: 新UPF PSA
        New_PSA: 新IP地址<br/>先断后建(Break-before-Make)
    }

    state SSC3 {
        [*] --> Make_Before_Break
        Make_Before_Break: 建新PSA+保持旧PSA<br/>IP地址改变但有过渡期
        Make_Before_Break --> Old_Release: 旧PSA超时释放
        Old_Release: 旧IP地址/前缀释放<br/>仅适用于IP类型
    }

1. 网络在现有PDU Session中分配新的IPv6前缀（新PSA）
2. UE同时拥有旧前缀和新前缀
3. 旧前缀通过Router Advertisement的Preferred Lifetime=0标记为deprecated
4. UE平滑迁移到新前缀
5. 旧PSA在计时器超时后释放

1. UE收到SSC Mode 3触发（NAS Session Management消息）
2. UE发起新PDU Session建立（携带Old PDU Session ID）
3. 新PDU Session分配新IP地址
4. 旧PDU Session在NAS指示的超时时间后释放

flowchart TD
    A[UPF收到数据包] --> B{入口方向?}
    B -->|N3 UL| C[匹配UL PDR]
    B -->|N6 DL / N9 DL| D[匹配DL PDR]

    C --> E{PDR匹配成功?}
    D --> E

    E -->|Yes| F[提取QFI、应用QER]
    F --> G{QER Gate Status?}

    G -->|Open| H[执行FAR动作]
    G -->|Closed| I[丢弃数据包]

    H --> J{FAR Action?}
    J -->|FORWARD| K[添加Outer Header<br/>N3/N9 GTP-U封装]
    J -->|DROP| L[静默丢弃]
    J -->|BUFFER| M[缓存 + 触发DDN通知]
    J -->|NOTIFY| N[通知SMF]

    K --> O[更新URR使用量计数器]
    O --> P[转发数据包]