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RFC 5359

 
 
 

Session Initiation Protocol Service Examples

Part 4 of 6, p. 84 to 112
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2.8.  Call Forwarding - Busy


           Alice            Proxy          User B1             User B2
             |                |              |                   |
             |    INVITE F1   |              |                   |
             |--------------->|   INVITE F2  |                   |
             |                |------------->|                   |
             |(100 Trying) F3 |              |                   |
             |<---------------|  486 Busy F4 |                   |
             |                |<-------------|                   |
             |                |     ACK F5   |                   |
             |                |------------->|                   |
             |(181 Call is Being Forwarded) F6                   |
             |<---------------|              |    INVITE F7      |
             |                |--------------------------------->|
             |                |              | 180 Ringing F8    |
             |                |<---------------------------------|
             | 180 Ringing F9 |              |                   |
             |<---------------|              |      200 OK F10   |
             |                |<---------------------------------|
             |    200 OK F11  |              |                   |
             |<---------------|              |                   |
             |     ACK F12    |              |                   |
             |--------------->|              |     ACK F13       |
             |                |--------------------------------->|
             |               Both way RTP Established            |
             |<=================================================>|
             |    BYE F14     |              |                   |
             |--------------->|              |      BYE F15      |
             |                |--------------------------------->|
             |                |              |    200 OK F16     |
             |  200 OK F17    |<---------------------------------|
             |<---------------|              |                   |
             |                |              |                   |

   Bob wants calls to B1 forwarded to B2 if B1 is busy (this information
   is known to the proxy).  Alice calls B1, B1 is busy, the proxy server
   places call to B2.

   Message Details

      F1 INVITE Alice -> Proxy

      INVITE sips:bob@biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
      Max-Forwards: 70

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      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      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 INVITE Proxy -> B1

      INVITE sips:bob@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      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

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      F3 (100 Trying) Proxy -> Alice

      SIP/2.0 100 Trying
      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>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0


      F4 486 Busy Here B1 -> Proxy

      SIP/2.0 486 Busy Here
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
       ;received=192.0.2.54
      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=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0


      F5 ACK Proxy -> B1

      ACK sips:bob@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0


      F6 (181 Call is Being Forwarded) Proxy -> Alice

      SIP/2.0 181 Call is Being Forwarded
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103

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      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=9214d
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0

      /* The proxy now forwards the call to B2. */


      F7 INVITE Proxy -> B2

      INVITE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      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


      F8 180 Ringing B2 -> Proxy

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567

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      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Content-Length: 0


      F9 180 Ringing Proxy -> Alice

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Content-Length: 0


      F10 200 OK B2 -> Proxy

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=bob 2890844527 2890844527 IN IP4 client2.biloxi.example.com
      s=
      c=IN IP4 client2.biloxi.example.com
      t=0 0
      m=audio 3456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000

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      F11 200 OK Proxy -> Alice

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=bob 2890844527 2890844527 IN IP4 client2.biloxi.example.com
      s=
      c=IN IP4 client2.biloxi.example.com
      t=0 0
      m=audio 3456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F12 ACK Alice -> Proxy

      ACK sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bfX
      Route: <sips:ss1.example.com;lr>
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0


      F13 ACK Proxy -> B2

      ACK sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83731
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bfX
       ;received=192.0.2.103
      Max-Forwards: 69

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      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0

      /* RTP streams are established between Alice and B2. */

      /* Alice eventually hangs up with User B2. */


      F14 BYE Alice -> Proxy

      BYE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bW4
      Route: <sips:ss1.example.com;lr>
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F15 BYE Proxy -> B2

      BYE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK837493
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bW4
       ;received=192.0.2.103
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F16 200 OK B2 -> Proxy

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK837493
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061

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       ;branch=z9hG4bK74bW4
       ;received=192.0.2.103
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F17 200 OK Proxy -> Alice

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bW4
       ;received=192.0.2.103
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=7654321
      Call-ID: 12345600@atlanta.example.com
      CSeq:  2 BYE
      Content-Length: 0

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2.9.  Call Forwarding - No Answer


           Alice            Proxy         User B1             User B2
             |                |              |                   |
             |    INVITE F1   |              |                   |
             |--------------->|   INVITE F2  |                   |
             |(100 Trying) F3 |------------->|                   |
             |<---------------|180 Ringing F4|                   |
             | 180 Ringing F5 |<-------------|                   |
             |<---------------|              |                   |
             |                 Request Timeout                   |
             |                |              |                   |
             |                |   CANCEL F6  |                   |
             |                |------------->|                   |
             |                |   200 OK F7  |                   |
             |                |<-------------|                   |
             |                |     487 F8   |                   |
             |                |<-------------|                   |
             |                |     ACK F9   |                   |
             |                |------------->|                   |
             |(181 Call is Being Forwarded) F10                  |
             |<---------------|              |    INVITE F11     |
             |                |--------------------------------->|
             |                |              | 180 Ringing F12   |
             | 180 Ringing F13|<---------------------------------|
             |<---------------|              |      200 OK F14   |
             |                |<---------------------------------|
             |   200 OK F15   |              |                   |
             |<---------------|              |                   |
             |     ACK F16    |              |                   |
             |--------------->|              |     ACK F17       |
             |                |--------------------------------->|
             |               Both way RTP Established            |
             |<=================================================>|
             |    BYE F18     |              |                   |
             |--------------->|              |      BYE F19      |
             |                |--------------------------------->|
             |                |              |    200 OK F20     |
             |  200 OK F21    |<---------------------------------|
             |<---------------|              |                   |
             |                |              |                   |

   Bob wants calls to B1 forwarded to B2 if B1 is not answered
   (information is known to the proxy server).  Alice calls B1 and no
   one answers.  The proxy server then places the call to B2.

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   Message Details

      F1 INVITE Alice -> Proxy

      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: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      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 INVITE Proxy -> B1

      INVITE sips:bob@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
      s=

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      c=IN IP4 client.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F3 (100 Trying) Proxy -> Alice

      SIP/2.0 100 Trying
      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>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0


      F4 180 Ringing B1 -> Proxy

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=3145678
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client.biloxi.example.com>
      Content-Length: 0


      F5 180 Ringing Proxy -> Alice

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=3145678
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE

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      Contact: <sips:bob@client.biloxi.example.com>
      Content-Length: 0

      /* B1 rings until a configurable timer expires in the proxy.  The
         proxy sends Cancel and proceeds down the list of routes. */


      F6 CANCEL Proxy -> B1

      CANCEL sips:bob@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 CANCEL
      Content-Length: 0


      F7 200 OK B1 -> Proxy

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
       ;received=192.0.2.54
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=329d823
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 CANCEL
      Content-Length: 0


      F8 487 Request Terminated B1 -> Proxy

      SIP/2.0 487 Request Terminated
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
       ;received=192.0.2.54
      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=3145678
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0

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      F9 ACK Proxy -> B1

      ACK sips:bob@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.1
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=3145678
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0


      F10 (181 Call is Being Forwarded) Proxy -> Alice

      SIP/2.0 181 Call is Being Forwarded
      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=9214d
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Content-Length: 0


      F11 INVITE Proxy -> B2

      INVITE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:alice@client.atlanta.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=alice 2890844526 2890844526 IN IP4 client.atlanta.example.com
      s=

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      c=IN IP4 client.atlanta.example.com
      t=0 0
      m=audio 49170 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F12 180 Ringing B2 -> Proxy

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Content-Length: 0


      F13 180 Proxy -> Alice

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Content-Length: 0


      F14 200 OK B2 -> Proxy

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK83749.2
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103

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      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=bob 2890844527 2890844527 IN IP4 client2.biloxi.example.com
      s=
      c=IN IP4 client2.biloxi.example.com
      t=0 0
      m=audio 3456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F15 200 OK Proxy -> Alice

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf9
       ;received=192.0.2.103
      Record-Route: <sips:ss1.example.com;lr>
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:bob@client2.biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Content-Type: application/sdp
      Content-Length: ...

      v=0
      o=bob 2890844527 2890844527 IN IP4 client2.biloxi.example.com
      s=
      c=IN IP4 client2.biloxi.example.com
      t=0 0
      m=audio 3456 RTP/AVP 0
      a=rtpmap:0 PCMU/8000


      F16 ACK Alice -> Proxy

      ACK sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS client.atlanta.example.com:5061

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       ;branch=z9hG4bK74bf3
      Route: <sips:ss1.example.com;lr>
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0


      F17 ACK Proxy -> B2

      ACK sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK8374.1
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74bf3
       ;received=192.0.2.103
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0

      /* RTP streams are established between Alice and B2.
         Alice hangs up with User B2. */


      F18 BYE Alice -> Proxy

      BYE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74b3f
      Route: <sips:ss1.example.com;lr>
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F19 BYE Proxy -> B2

      BYE sips:bob@client2.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK837.1

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      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74b3f
       ;received=192.0.2.103
      Max-Forwards: 69
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F20 200 OK B2 -> Proxy

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS ss1.example.com:5061
       ;branch=z9hG4bK837.1
       ;received=192.0.2.54
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74b3f
       ;received=192.0.2.103
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq: 2 BYE
      Content-Length: 0


      F21 200 OK Proxy -> Alice

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS client.atlanta.example.com:5061
       ;branch=z9hG4bK74b3f
       ;received=192.0.2.103
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=765432
      Call-ID: 12345600@atlanta.example.com
      CSeq:  2 BYE
      Content-Length: 0

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2.10.  3-Way Conference - Third Party Is Added


            Alice           Bob            Carol
             |    INVITE F1   |              |
             |--------------->|              |
             | 180 Ringing F2 |              |
             |<---------------|              |
             |    200 OK F3   |              |
             |<---------------|              |
             |     ACK F4     |              |
             |--------------->|              |
             |       RTP      |              |
             |<==============>|              |
             |   INVITE F5    |              |
             |<---------------|              |
             |    200 OK F6   |              |
             |--------------->|              |
             |     ACK F7     |              |
             |<---------------|  INVITE F8   |
             |                |------------->|
             |                |    180 F9    |
             |                |<-------------|
             |                |  200 OK F10  |
             |                |<-------------|
             |                |  ACK F11     |
             |                |------------->|
             |                |       RTP    |
             |                |<============>|

   In this scenario, Alice and Bob are in a 2-party call (session) when
   Bob wishes to add Carol into the conversation.  Bob is capable of
   media mixing in a 3-party call.  Bob first sends a re-INVITE to
   Alice, changing Contact URIs to one that indicates Bob's mixer and
   acts like a focus.  As a result, Bob includes the "isfocus" feature
   tag [RFC3840] as described in [RFC4579].  Bob then INVITEs Carol
   using the same Contact URI.  Note that Bob could wait to re-INVITE
   Alice until after Carol has answered.  Bob could also put Alice on
   hold before calling Carol.

   Message Details

      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

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      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:b54gh42f5@biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

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      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: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      CSeq: 1024 INVITE
      Contact: <sips:bob-Mixer@client.biloxi.example.com>;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=

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      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: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      CSeq: 1024 ACK
      Content-Length: 0

      /* Bob calls Carol. */

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      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: sdjfdjfskdf@biloxi.example.com
      CSeq: 42 INVITE
      Contact: <sips:bob-Mixer@client.biloxi.example.com>;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: sdjfdjfskdf@biloxi.example.com
      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

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      To: Carol <sips:carol@chicago.example.com>;tag=341313
      Call-ID: sdjfdjfskdf@biloxi.example.com
      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: sdjfdjfskdf@biloxi.example.com
      CSeq: 42 ACK
      Content-Length: 0


      /* Bob's mixer now mixes media from both Alice and Carol
         to create the 3-way conference. */

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2.11.  3-Way Conference - Third Party Joins


            Alice           Bob            Carol
             |    INVITE F1   |              |
             |--------------->|              |
             | 180 Ringing F2 |              |
             |<---------------|              |
             |    200 OK F3   |              |
             |<---------------|              |
             |     ACK F4     |              |
             |--------------->|              |
             |       RTP      |              |
             |<==============>|  INVITE Join:A-B F5
             |                |<-------------|
             |                |    180 F6    |
             |                |------------->|
             |   INVITE F7    |              |
             |<---------------|              |
             |    200 OK F8   |              |
             |--------------->|              |
             |     ACK F9     |              |
             |<---------------|              |
             |                |  200 OK F10  |
             |                |------------->|
             |                |  ACK F11     |
             |                |<-------------|
             |                |       RTP    |
             |                |<============>|


   In this scenario, Alice and Bob are in a 2-party call and Carol
   wishes to join, resulting in a 3-party call.  Carol could have
   learned Bob's dialog identifier using some non-SIP means, or possibly
   from a NOTIFY with the dialog package sent by Bob.  Carol sends an
   INVITE to Bob containing a Join header identifying the dialog between
   Alice and Bob.  Bob re-INVITEs Alice to switch to focus mode and
   includes the "isfocus" feature tag [RFC3840] as described in
   [RFC4579].  Bob then accepts the INVITE from Carol, resulting in the
   3-way call.

   Message Details

      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

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      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      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: 12345600@atlanta.example.com
      CSeq: 1 INVITE
      Contact: <sips:b54gh42f5@biloxi.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

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      Supported: replaces, join, 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=z9hG4bK74bf6
      Max-Forwards: 70
      From: Alice <sips:alice@atlanta.example.com>;tag=1234567
      To: Bob <sips:bob@biloxi.example.com>;tag=23431
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1 ACK
      Content-Length: 0

      /* Alice and Bob have established a session.
         Carol requests to join the session. */


      F5 INVITE Carol -> Bob

      INVITE sips:bob@biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS chicago.example.com:5061
       ;branch=z9hG4bKnashds7
      Max-Forwards: 70
      From: Carol <sips:carol@chicago.example.com>;tag=8675309
      To: Bob <sips:bob@biloxi.example.com>
      Call-ID: 452k499sk@chicago.example.com
      CSeq: 99 INVITE
      Contact: <sips:carol@client.chicago.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Supported: replaces, join
      Join: 12345600@atlanta.example.com;from-tag=1234567;to-tag=23431
      Content-Type: application/sdp
      Content-Length: ...

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      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


      F6 180 Ringing Bob -> Carol

      SIP/2.0 180 Ringing
      Via: SIP/2.0/TLS chicago.example.com:5061
       ;branch=z9hG4bKnashds7
       ;received=120.
      From: Carol <sips:carol@chicago.example.com>;tag=8675309
      To: Bob <sips:bob@biloxi.example.com>;tag=0982
      Call-ID: 452k499sk@chicago.example.com
      CSeq: 99 INVITE
      Contact: <sips:bob-Mixer@client.biloxi.example.com>;isfocus
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Supported: replaces
      Content-Length: 0


      F7 INVITE Bob -> Alice

      INVITE sips:alice@client.atlanta.example.com SIP/2.0
      Via: SIP/2.0/TLS client.biloxi.example.com:5061
       ;branch=z9hG4bKnashdyKL
      Max-Forwards: 70
      From: Bob <sips:bob@biloxi.example.com>;tag=23431
      To: Alice <sips:alice@atlanta.example.com>;tag=1234567
      Call-ID: 12345600@atlanta.example.com
      CSeq: 1024 INVITE
      Contact: <sips:bob-Mixer@client.biloxi.example.com>;isfocus
      Content-Type: application/sdp
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Supported: replaces, join, 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

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      F8 200 OK Alice -> Bob
      SIP/2.0 200 OK
      Via: SIP/2.0/TLS client.biloxi.example.com:5061
       ;branch=z9hG4bKnashdyKL
       ;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: 12345600@atlanta.example.com
      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


      F9 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: 12345600@atlanta.example.com
      CSeq: 1024 ACK
      Content-Length: 0


      F10 200 OK Bob -> Carol

      SIP/2.0 200 OK
      Via: SIP/2.0/TLS chicago.example.com:5061
       ;branch=z9hG4bKnashds7
       ;received=120.
      From: Carol <sips:carol@chicago.example.com>;tag=8675309
      To: Bob <sips:bob@biloxi.example.com>;tag=0982
      Call-ID: 452k499sk@chicago.example.com
      CSeq: 99 INVITE
      Contact: <sips:bob-Mixer@client.biloxi.example.com>;isfocus

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      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Supported: replaces, join, 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


      F11 ACK OK Carol -> Bob

      ACK sips:bob-Mixer@client.biloxi.example.com SIP/2.0
      Via: SIP/2.0/TLS chicago.example.com:5061
       ;branch=z9hG4bKnash4Gf
      Max-Forwards: 70
      From: Carol <sips:carol@chicago.example.com>;tag=8675309
      To: Bob <sips:bob@biloxi.example.com>;tag=0982
      Call-ID: 452k499sk@chicago.example.com
      CSeq: 99 ACK
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
      Supported: replaces, join
      Content-Length:0


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