VoNR SIP Session Timer保活机制与422协商博弈

graph TD
    A[UAC提议 Session-Expires: 600<br/>Min-SE: 90] --> B{沿途Proxy检查}
    B -->|Session-Expires >= Proxy Min-SE| C[Proxy放行<br/>可上调Min-SE]
    B -->|Session-Expires < Proxy Min-SE| D[Proxy拒绝 422<br/>携带自己的Min-SE]
    C --> E{UAS检查}
    E -->|Session-Expires >= UAS Min-SE| F[UAS接受<br/>2xx携带协商后的Session-Expires<br/>指定refresher角色]
    E -->|Session-Expires < UAS Min-SE| G[UAS拒绝 422<br/>携带自己的Min-SE]
    D --> H[UAC用422中的Min-SE重发INVITE]
    G --> H
    F --> I[会话建立<br/>refresher方周期性re-INVITE/UPDATE]

sequenceDiagram
    participant UE1 as 主叫UE (UAC)
    participant PCSCF1 as P-CSCF (主叫侧)
    participant SCSCF1 as S-CSCF (主叫侧)
    participant SCSCF2 as S-CSCF (被叫侧)
    participant PCSCF2 as P-CSCF (被叫侧)
    participant UE2 as 被叫UE (UAS)

    Note over UE1,UE2: 阶段1: INVITE协商 — 首次尝试被422拒绝

    UE1->>PCSCF1: INVITE (SDP offer)<br/>Session-Expires: 600;refresher=uac<br/>Min-SE: 90<br/>Supported: timer
    Note right of PCSCF1: P-CSCF策略要求<br/>Min-SE >= 1800
    PCSCF1-->>UE1: 422 Session Interval Too Brief<br/>Min-SE: 1800
    Note left of UE1: UAC收到422后<br/>必须使用422中的Min-SE<br/>重新发起INVITE

    Note over UE1,UE2: 阶段2: 重发INVITE — 协商成功

    UE1->>PCSCF1: INVITE (SDP offer)<br/>Session-Expires: 1800<br/>Min-SE: 1800<br/>Supported: timer
    PCSCF1->>SCSCF1: INVITE (转发)<br/>Session-Expires: 1800<br/>Min-SE: 1800
    SCSCF1->>SCSCF2: INVITE (转发)
    SCSCF2->>PCSCF2: INVITE (转发)
    PCSCF2->>UE2: INVITE (转发)

    UE2->>PCSCF2: 180 Ringing
    PCSCF2->>SCSCF2: 180 Ringing
    SCSCF2->>SCSCF1: 180 Ringing
    SCSCF1->>PCSCF1: 180 Ringing
    PCSCF1->>UE1: 180 Ringing

    UE2->>PCSCF2: 200 OK (SDP answer)<br/>Session-Expires: 1800;refresher=uac
    PCSCF2->>SCSCF2: 200 OK (转发)
    SCSCF2->>SCSCF1: 200 OK (转发)
    SCSCF1->>PCSCF1: 200 OK (转发)
    PCSCF1->>UE1: 200 OK (转发)

    UE1->>PCSCF1: ACK
    PCSCF1->>SCSCF1: ACK → ... → UE2

    Note over UE1,UE2: 会话建立完成<br/>Session-Expires = 1800秒<br/>Refresher = UAC (主叫UE)

    Note over UE1,UE2: 阶段3: 周期性刷新 (1800秒后)

    UE1->>PCSCF1: re-INVITE (无SDP 或 带SDP)<br/>Session-Expires: 1800;refresher=uac
    PCSCF1->>SCSCF1: re-INVITE (转发)
    SCSCF1->>SCSCF2: re-INVITE (转发)
    SCSCF2->>PCSCF2: re-INVITE (转发)
    PCSCF2->>UE2: re-INVITE (转发)

    UE2->>PCSCF2: 200 OK<br/>Session-Expires: 1800;refresher=uac
    PCSCF2->>SCSCF2: 200 OK → ... → UE1

    UE1->>PCSCF1: ACK → ... → UE2
    Note over UE1,UE2: 刷新成功，定时器重置

    Note over UE1,UE2: 阶段4: 刷新失败场景

    UE1-xPCSCF1: re-INVITE (超时无响应)
    Note left of UE1: 定时器到期<br/>UE1发送BYE终止会话
    UE1->>PCSCF1: BYE
    PCSCF1->>SCSCF1: BYE → ... → UE2
    Note over UE1,UE2: 会话终止，资源释放

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:5060;branch=z9hG4bK1234
Max-Forwards: 70
Route: <sip:pcscf.mnc.mcc.3gppnetwork.org:5060;lr>
Route: <sip:scscf.mnc.mcc.3gppnetwork.org:5060;lr>
Supported: timer, 100rel, precondition, update
Session-Expires: 1800;refresher=uac
Min-SE: 1800
Content-Type: application/sdp

SIP/2.0 422 Session Interval Too Brief
Via: SIP/2.0/UDP [5555::aaa:bbb:ccc:ddd]:5060;branch=z9hG4bK1234
From: <sip:[email protected]>;tag=abc123
To: <sip:[email protected]>;tag=def456
Min-SE: 1800

SIP/2.0 200 OK
Via: SIP/2.0/UDP ...
Session-Expires: 1800;refresher=uac

UE: Min-SE = 90
P-CSCF: Min-SE = 1800  
S-CSCF: Min-SE = 2400
AS (TAS/MRFC): Min-SE = 3600

flowchart TD
    A[INVITE携带 Session-Expires<br/>refresher参数?] -->|有, refresher=uac| B[UAS在2xx中确认 refresher=uac<br/>UAC负责刷新]
    A -->|有, refresher=uas| C[UAS在2xx中确认 refresher=uas<br/>UAS负责刷新]
    A -->|无refresher参数| D{UAS在2xx中指定<br/>refresher}
    D -->|UAS选择 uac| E[UAC负责刷新]
    D -->|UAS选择 uas| F[UAS负责刷新]
    A -->|无Session-Expires| G{UAS是否支持timer?}
    G -->|是, UAS主动添加| H[UAS在2xx中添加<br/>Session-Expires;refresher=uas]
    G -->|否| I[无Session Timer<br/>会话无保活]

stateDiagram-v2
    [*] --> Idle: UE开机/无会话

    Idle --> Negotiating: 发送/接收INVITE<br/>携带Session-Expires

    Negotiating --> Rejected422: 收到422<br/>Session Interval Too Brief
    Rejected422 --> Negotiating: 重发INVITE<br/>使用422中的Min-SE

    Negotiating --> Established: 收到/发送2xx<br/>Session-Expires协商完成

    Established --> RefreshPending: Session Timer触发<br/>(到达过期前阈值)

    RefreshPending --> Refreshing: Refresher方发送<br/>re-INVITE/UPDATE

    Refreshing --> Established: 收到2xx<br/>定时器重置

    Refreshing --> RefreshFailed: 超时无响应<br/>或收到错误响应

    RefreshFailed --> Terminating: 定时器彻底到期<br/>发送BYE终止

    Established --> Terminating: 正常挂机<br/>发送/接收BYE

    Terminating --> Idle: 会话资源释放

    note right of Negotiating
        关键IE: Session-Expires
        Min-SE, Supported: timer
    end note

    note right of RefreshPending
        触发时机 = Session-Expires - δ
        δ通常为64*T1 (RFC 3261)
    end note

# 过滤422响应
sip.Status-Code == 422

# 过滤携带Session-Expires的INVITE
sip.Method == "INVITE" && sip.Header.Session-Expires exists

# 过滤re-INVITE（可能是刷新）
sip.Method == "INVITE" && sip.Header.Session-Expires exists && !sip.CSeq.method == "INVITE" 
# 更精确：通过CSeq识别re-INVITE（CSeq > 1）

# 过滤Min-SE header
sip.Header.Min-SE exists