日常通信中主要是两个终端（UA）之间的通话，但经常会遇到需要第三者加入共同讨论（或认证）；这就是三方通话；在IMS/SIP系统中三方通话一般是先建立两方通话，再呼叫第三方加入；

1. 三方通话流程

       如下图所示，Alice先呼通Bob,再邀请Carol加入一起通话；

2.三方通话消息内容

F1 INVITE Alice -> Bob 

      INVITE sips:bob@biloxi.example.com SIP/2.0

      Via: SIP/2.0/TLS client.atlanta.example.com:5061

       ;branch=z9hG4bK74bf9

      Max-Forwards: 70

      From: Alice <sips:alice@atlanta.example.com>;tag=1234567

      To: Bob <sips:bob@biloxi.example.com>

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1 INVITE

      Contact: <sips:alice@client.atlanta.example.com>

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces

      Content-Type: application/sdp

      Content-Length: ...

      v=0

      o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com

      s=

      c=IN IP4 client.atlanta.example.com

      t=0 0

      m=audio 49170 RTP/AVP 0

      a=rtpmap:0 PCMU/8000

F2 180 Ringing Bob -> Alice 

      SIP/2.0 180 Ringing

      Via: SIP/2.0/TLS client.atlanta.example.com:5061

       ;branch=z9hG4bK74bf9

       ;received=192.0.2.103

      From: Alice <sips:alice@atlanta.example.com>;tag=1234567

      To: Bob <sips:bob@biloxi.example.com>;tag=23431

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1 INVITE

      Contact: <sips:b54gh42f5@biloxi.example.com>

      Content-Length: 0

F3 200 OK Bob -> Alice

      SIP/2.0 200 OK

      Via: SIP/2.0/TLS client.atlanta.example.com:5061

       ;branch=z9hG4bK74bf9

       ;received=192.0.2.103

      From: Alice <sips:alice@atlanta.example.com>;tag=1234567

      To: Bob <sips:bob@biloxi.example.com>;tag=23431

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1 INVITE

      Contact: <sips:b54gh42f5@biloxi.example.com>

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces, gruu

      Content-Type: application/sdp

      Content-Length: ...

      v=0

      o=bob 2890844527 2890844527 IN IP4 client.biloxi.example.com

      s=

      c=IN IP4 client.biloxi.example.com

      t=0 0

      m=audio 3456 RTP/AVP 0

      a=rtpmap:0 PCMU/8000

F4 ACK Alice -> Bob 

      ACK sips:b54gh42f5@biloxi.example.com SIP/2.0

      Via: SIP/2.0/TLS client.atlanta.example.com:5061

       ;branch=z9hG4bK74bfL

      Max-Forwards: 70

      From: Alice <sips:alice@atlanta.example.com>;tag=1234567

      To: Bob <sips:bob@biloxi.example.com>;tag=23431

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1 ACK

      Content-Length: 0

      /* Alice and Bob have established a session.

         Bob re-INVITEs, changing Contact URIs. */ 

F5 INVITE Bob -> Alice 

      INVITE sips:alice@client.atlanta.example.com SIP/2.0

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnashds

      Max-Forwards: 70

      From: Bob <sips:bob@biloxi.example.com>;tag=23431

      To: Alice <sips:alice@atlanta.example.com>;tag=1234567

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1024 INVITE

      Contact: <sips:bob-[email]Mixer@client.biloxi.example.com[/email]>;isfocus

      Content-Type: application/sdp

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces, gruu

      Content-Length: ...

      v=0

      o=bob 2890844527 2890844528 IN IP4 client.biloxi.example.com

      s=

      c=IN IP4 client.biloxi.example.com

      t=0 0

      m=audio 49172 RTP/AVP 0

      a=rtpmap:0 PCMU/8000 

F6 200 OK Alice -> Bob 

      SIP/2.0 200 OK 

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnashds7

       ;received=192.0.2.113

      From: Bob <sips:bob@biloxi.example.com>;tag=23431

      To: Alice <sips:alice@atlanta.example.com>;tag=1234567

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1024 INVITE

      Contact: <sips:alice@client.atlanta.example.com>

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces

      Content-Type: application/sdp

      Content-Length: ...

      v=0

      o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com

      s=

      c=IN IP4 client.atlanta.example.com

      t=0 0

      m=audio 49170 RTP/AVP 0

      a=rtpmap:0 PCMU/8000

F7 ACK Bob -> Alice 

      ACK sips:alice@client.atlanta.example.com SIP/2.0

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnash3G

      Max-Forwards: 70

      From: Bob <sips:bob@biloxi.example.com>;tag=23431

      To: Alice <sips:alice@atlanta.example.com>;tag=1234567

      Call-ID: [email]12345600@atlanta.example.com[/email]

      CSeq: 1024 ACK

      Content-Length: 0

      /* Bob calls Carol. */ 

F8 INVITE Bob -> Carol 

      INVITE sips:carol@chicago.example.com SIP/2.0

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnashJfd

      Max-Forwards: 70

      From: Bob <sips:bob@biloxi.example.com>;tag=8675309

      To: Carol <sips:carol@chicago.example.com>

      Call-ID: [email]sdjfdjfskdf@biloxi.example.com[/email]

      CSeq: 42 INVITE

      Contact: <sips:bob-[email]Mixer@client.biloxi.example.com[/email]>;isfocus

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces, gruu

      Content-Type: application/sdp

      Content-Length: ...

      v=0

      o=bob 28908445834 2890844834 IN IP4 client.biloxi.example.com

      s=

      c=IN IP4 client.biloxi.example.com

      t=0 0

      m=audio 48174 RTP/AVP 0

      a=rtpmap:0 PCMU/8000

F9 180 Ringing Carol -> Bob 

      SIP/2.0 200 OK

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnashJfd

       ;received=192.0.2.113

      From: Bob <sips:bob@biloxi.example.com>;tag=8675309

      To: Carol <sips:carol@chicago.example.com>;tag=341313

      Call-ID: [email]sdjfdjfskdf@biloxi.example.com[/email]

      CSeq: 42 INVITE

      Contact: <sips:carol@client.chicago.example.com>

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces

      Content-Length: 0

F10 200 OK Carol -> Bob 

      SIP/2.0 200 OK

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnashJfd

       ;received=192.0.2.113

      From: Bob <sips:bob@biloxi.example.com>;tag=8675309

      To: Carol <sips:carol@chicago.example.com>;tag=341313

      Call-ID: [email]sdjfdjfskdf@biloxi.example.com[/email]

      CSeq: 42 INVITE

      Contact: <sips:carol@client.chicago.example.com>

      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

      Supported: replaces

      Content-Type: application/sdp

      Content-Length: ...

      v=0

      o=carol 2890844922 2890844922 IN IP4 client.chicago.example.com

      s=

      c=IN IP4 client.chicago.example.com

      t=0 0

      m=audio 3456 RTP/AVP 0

      a=rtpmap:0 PCMU/8000

F11 ACK Bob -> Carol 

      ACK sips:carol@client.chicago.example.com SIP/2.0

      Via: SIP/2.0/TLS client.biloxi.example.com:5061

       ;branch=z9hG4bKnash431

      Max-Forwards: 70

      From: Bob <sips:bob@biloxi.example.com>;tag=8675309

      To: Carol <sips:carol@chicago.example.com>;tag=341313

      Call-ID: [email]sdjfdjfskdf@biloxi.example.com[/email]

      CSeq: 42 ACK

      Content-Length: 0

      /* Bob's mixer now mixes media from both Alice and Carol

         to create the 3-way conference. */

*消息中空行为字段间隔（space)

原文来自：RFC5359

整理：kangguoying20201029

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