IMS re-INVITE会话修改与SDP Offer/Answer博弈

sequenceDiagram
    participant UE_A as UE-A (发起方)
    participant P_CSCF_A as P-CSCF-A
    participant PCF_A as PCF/PCRF-A
    participant S_CSCF_A as S-CSCF-A
    participant AS as MMTel AS
    participant S_CSCF_B as S-CSCF-B
    participant P_CSCF_B as P-CSCF-B
    participant PCF_B as PCF/PCRF-B
    participant UE_B as UE-B (接收方)

    Note over UE_A,UE_B: === 阶段1: 呼叫保持 (Hold) ===

    UE_A->>P_CSCF_A: re-INVITE (SDP Offer: a=sendonly)
    Note right of UE_A: Call-ID不变<br/>CSeq递增<br/>SDP: m=audio 5004 RTP/AVP 96<br/>a=sendonly

    P_CSCF_A->>PCF_A: AAR (媒体变更授权)
    PCF_A-->>P_CSCF_A: AAA (QoS策略更新)
    Note over P_CSCF_A,PCF_A: Rx接口：因方向变更<br/>需重新授权QoS

    P_CSCF_A->>S_CSCF_A: re-INVITE (转发)
    S_CSCF_A->>AS: re-INVITE (ISC触发)
    Note over S_CSCF_A,AS: iFC匹配: HOLD业务<br/>AS执行业务逻辑

    AS-->>S_CSCF_A: re-INVITE (AS透传/修改)
    S_CSCF_A->>S_CSCF_B: re-INVITE (转发)
    S_CSCF_B->>P_CSCF_B: re-INVITE (转发)

    P_CSCF_B->>PCF_B: AAR (媒体变更授权)
    PCF_B-->>P_CSCF_B: AAA (QoS策略更新)

    P_CSCF_B->>UE_B: re-INVITE (SDP Offer: a=sendonly)

    UE_B-->>P_CSCF_B: 200 OK (SDP Answer: a=recvonly)
    Note left of UE_B: SDP应答方向：<br/>对端sendonly→本端recvonly

    P_CSCF_B-->>S_CSCF_B: 200 OK
    S_CSCF_B-->>S_CSCF_A: 200 OK
    S_CSCF_A-->>AS: 200 OK
    AS-->>S_CSCF_A: 200 OK (透传)
    S_CSCF_A-->>P_CSCF_A: 200 OK
    P_CSCF_A-->>UE_A: 200 OK (SDP Answer: a=recvonly)

    UE_A->>P_CSCF_A: ACK
    P_CSCF_A->>S_CSCF_A: ACK
    S_CSCF_A->>AS: ACK
    AS-->>S_CSCF_A: ACK
    S_CSCF_A->>S_CSCF_B: ACK
    S_CSCF_B->>P_CSCF_B: ACK
    P_CSCF_B->>UE_B: ACK

    Note over UE_A,UE_B: === 阶段2: 呼叫恢复 (Resume) ===

    UE_A->>P_CSCF_A: re-INVITE (SDP Offer: a=sendrecv)
    Note right of UE_A: 恢复双向媒体<br/>SDP version递增

    P_CSCF_A->>PCF_A: AAR (恢复双向QoS)
    PCF_A-->>P_CSCF_A: AAA

    P_CSCF_A->>S_CSCF_A: re-INVITE
    S_CSCF_A->>AS: re-INVITE
    AS-->>S_CSCF_A: re-INVITE
    S_CSCF_A->>S_CSCF_B: re-INVITE
    S_CSCF_B->>P_CSCF_B: re-INVITE

    P_CSCF_B->>PCF_B: AAR (恢复双向QoS)
    PCF_B-->>P_CSCF_B: AAA

    P_CSCF_B->>UE_B: re-INVITE (SDP Offer: a=sendrecv)
    UE_B-->>P_CSCF_B: 200 OK (SDP Answer: a=sendrecv)

    P_CSCF_B-->>UE_A: 200 OK (经全链路返回)
    UE_A->>UE_B: ACK (经全链路)

// 初始会话
o=- 123456 1 IN IP4 10.1.1.1

// 第一次re-INVITE (Hold)
o=- 123456 2 IN IP4 10.1.1.1
//                   ^ 递增

// 第二次re-INVITE (Resume)
o=- 123456 3 IN IP4 10.1.1.1
//                   ^ 再次递增

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP [2001:db8::1]:5060;branch=z9hG4bK-hold123
Call-ID: a84b4c76de667010@2001:db8::1
From: <sip:[email protected]>;tag=1928301774
To: <sip:[email protected]>;tag=a6c85cf
CSeq: 314162 INVITE        ← CSeq必须递增
Contact: <sip:ue_a@[2001:db8::1]:5060>
Content-Type: application/sdp
Content-Length: ...

v=0
o=- 123456 2 IN IP4 10.1.1.1    ← version从1→2
s=-
c=IN IP4 0.0.0.0                 ← HOLD方案一：地址置0
t=0 0
m=audio 5004 RTP/AVP 96 97
a=rtpmap:96 EVS/16000/1
a=rtpmap:97 AMR-WB/16000/1
a=fmtp:96 evs-mode-switch=1; bw=16000
a=sendonly                       ← 核心变更：方向改为sendonly
a=ptime:20
a=maxptime:240

sequenceDiagram
    participant UE_A as UE-A
    participant P_CSCF as P-CSCF/S-CSCF
    participant UE_B as UE-B

    Note over UE_A,UE_B: Glare场景：双方同时发re-INVITE

    UE_A->>P_CSCF: re-INVITE (SDP Offer-A1)
    UE_B->>P_CSCF: re-INVITE (SDP Offer-B1)

    P_CSCF->>UE_B: 转发 re-INVITE (Offer-A1)
    P_CSCF->>UE_A: 转发 re-INVITE (Offer-B1)

    Note over UE_A: UE-A收到对端re-INVITE<br/>但自己还有未完成的re-INVITE

    UE_A-->>P_CSCF: 491 Request Pending
    Note right of UE_A: Retry-After: 3<br/>(随机值2.1~4s)

    UE_B-->>P_CSCF: 491 Request Pending
    Note left of UE_B: Retry-After: 1.5<br/>(随机值0~2s)

    Note over UE_A,UE_B: 双方各自等待Retry-After后重试

    Note over UE_B: UE_B先到时（1.5s < 3s）
    UE_B->>P_CSCF: re-INVITE (SDP Offer-B1 重试)
    P_CSCF->>UE_A: re-INVITE (Offer-B1)

    UE_A-->>P_CSCF: 200 OK (SDP Answer-A1)
    P_CSCF-->>UE_B: 200 OK (Answer-A1)
    UE_B->>UE_A: ACK

flowchart TD
    A[需要修改会话参数] --> B{Dialog是否已Established?}
    B -->|否, 早期Dialog| C[使用UPDATE]
    B -->|是| D{是否需要同时刷新Dialog?}
    D -->|是| E[使用re-INVITE]
    D -->|否| F{对端是否支持UPDATE?}
    F -->|支持| G[优先使用UPDATE<br/>更轻量无需ACK]
    F -->|不确定| H[使用re-INVITE<br/>兼容性更好]
    G --> I{是否涉及媒体方向变更<br/>如Hold/Resume?}
    I -->|是| J[多数实现用re-INVITE]
    I -->|否| K[使用UPDATE]

sequenceDiagram
    participant UE as UE
    participant P_CSCF as P-CSCF
    participant PCF as PCF/PCRF
    participant SMF as SMF
    participant UPF as UPF

    UE->>P_CSCF: re-INVITE (SDP变更)

    Note over P_CSCF: 1. 解析SDP变更<br/>2. 提取新的带宽/方向参数

    P_CSCF->>PCF: AAR (Diameter)
    Note right of P_CSCF: Media-Component-Description<br/>含新的带宽/方向

    PCF->>PCF: 策略决策<br/>评估新QoS需求

    PCF-->>P_CSCF: AAA (授权结果)
    Note left of PCF: 含授权的QoS参数

    PCF->>SMF: SM策略更新(N7)
    SMF->>UPF: N4 Session Modification
    Note over SMF,UPF: 更新GTP-U过滤器<br/>调整QoS Flow参数

    P_CSCF->>UE: 转发re-INVITE到下一跳

v=0
o=- 123456 4 IN IP4 10.1.1.1        ← version递增到4
s=-
c=IN IP4 10.1.1.1
t=0 0
m=audio 5004 RTP/AVP 96 97          ← 音频流保持
a=rtpmap:96 EVS/16000/1
a=rtpmap:97 AMR-WB/16000/1
a=sendrecv
m=video 5006 RTP/AVPF 98 99         ← 新增视频流
a=rtpmap:98 H265/90000
a=rtpmap:99 H264/90000
a=fmtp:98 profile-id=1;tier-flag=0
a=fmtp:99 profile-level-id=42C01F
a=sendrecv
a=rtcp-fb:98 nack pli               ← PLI关键帧请求
a=rtcp-fb:98 ccm fir                ← FIR全帧内请求

m=video 0 RTP/AVPF 98 99    ← 端口=0表示拒绝该媒体流

stateDiagram-v2
    [*] --> Stable: Dialog Established
    Stable --> InProgress: 发送re-INVITE<br/>(CSeq递增)
    InProgress --> Received491: 收到491<br/>(Glare)
    Received491 --> WaitingRetry: 读取Retry-After
    WaitingRetry --> InProgress: 定时器到期<br/>重发re-INVITE
    InProgress --> Stable: 收到200 OK + 发送ACK<br/>(修改成功)
    InProgress --> Stable: 收到4xx/5xx/6xx<br/>(修改失败,回退原参数)
    InProgress --> ReceivedGlare: 收到对端re-INVITE<br/>(同时自己有待完成re-INVITE)
    ReceivedGlare --> Sent491: 回复491 Request Pending
    Sent491 --> Stable: 收到对端200 OK/失败
    Stable --> [*]: BYE终止会话

# 过滤所有re-INVITE
sip.Method == "INVITE" && sip.CSeq > 1

# 过滤Hold场景的re-INVITE
sip.Method == "INVITE" && sdp.media_attr.contains("sendonly")

# 过滤491响应
sip.Status-Code == 491

# 过滤视频添加re-INVITE
sip.Method == "INVITE" && sdp.media_name.contains("video")

# 过滤特定Call-ID的所有re-INVITE
sip.Call-ID == "a84b4c76de667010" && sip.Method == "INVITE"

P-CSCF →(Diameter/Rx)→ PCF →(N7)→ SMF →(N4)→ UPF