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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 5359

Internet Engineering Task Force                          Alan Johnston
Internet Draft                                                WorldCom
Document: draft-ietf-sipping-service-examples-02.txt     Robert Sparks
Category: Informational                               Chris Cunningham
June 2002                                                Steve Donovan
Expires: December 2002                                     dynamicsoft
                                                         Kevin Summers
                                                                 Sonus



                          SIP Service Examples


   Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026[1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.
   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."
   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.



   Abstract

   This informational document gives examples of SIP (Session Initiation
   Protocol) services.  This covers most features offered in so-called
   Centrex offerings from local exchange carriers and PBX (Private
   Branch Exchange) features.  Most of the services shown in this
   document are implemented in the SIP User Agents, although some
   require the assistance of a SIP Proxy.  Some require some extensions
   to SIP including the REFER method and Replaces header. These features
   are not intended to be an exhaustive set, but rather show
   implementations of common features likely to be implemented on SIP IP
   telephones in a business environment.








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Table of Contents

   1      Overview...................................................3
   1.1   Legend for Message Flows...................................4
   1.2   Document History...........................................4
   2      IP Telephony Services Features Call Flows..................8
   2.1   Call Hold..................................................8
   2.2   Consultation Hold.........................................19
   2.3   Music On Hold.............................................34
   2.4   Unattended Transfer.......................................41
   2.5   Attended Transfer.........................................47
   2.6   Call Forwarding Unconditional.............................58
   2.7   Call Forwarding - Busy....................................63
   2.8   Call Forwarding - No Answer...............................70
   2.9   3-way Conference û Third Party is Added...................78
   2.10  3-way Conference û Third Party Joins......................88
   2.11  Single Line Extension.....................................98
   2.12  Find-Me..................................................114
   2.13  Call Management (Incoming Call Screening)................124
   2.14  Call Management (Outgoing Call Screening)................128
   2.15  Call Park................................................131
   2.16  Call Pickup..............................................139
   2.17  Automatic Redial.........................................146
   3      Security Considerations..................................151
   4      IANA Considerations......................................152
   5      Acknowledgements.........................................153
   6      References...............................................154
   7      Author's Addresses.......................................156

























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1  Overview


   This informational document provides call flows detailing a SIP
   implementation of the following traditional telephony services:

         Call Hold                     Music on Hold
         Unattended Transfer           Consultation Hold
         Unconditional Call Forwarding Attended Transfer
         No Answer Call Forwarding     Busy Call Forwarding
         Single-Line Extension         3-way Call
         Incoming Call Screening       Find-Me
         Call Pickup                   Call Park
         Outgoing Call Screening       Automatic Redial

   It is the hope of the authors that this document will be useful for
   SIP implementors, users, designers, and protocol researchers alike
   and will help further the goal of a standard SIP implementation for
   IP Telephony.  It is envisioned that as changes to the standard and
   additional RFCs are added that this document will reflect those
   changes and represent the current state of a standard SIP IP
   Telephony implementation.

   These flows use SIP as defined by RFC 2543bis [2] and [8]. Note that
   this document is informational, and nothing stated here should be
   taken as normative.  The SIP specification and the other referenced
   documents are definitive as far as protocol issues are concerned.
   Also, these flows do not represent the only way to implement these
   services - other approaches such as 3pcc (Third Party Call Control)
   or Back-to-Back User Agents (B2BUA) may be more appropriate in some
   circumstances.  The design and principles of these service examples
   are described in the the Multiparty Framework document [12].

   These flows assume the functionality described in the SIP Call Flow
   Examples document [3], which explores basic behavior and PSTN
   internetworking. Some of the scenarios described herein make use of
   the SIP method extension REFER [4] and the SIP header extension
   Replaces [5], the SIP header extension Accept-Contact [6], and some
   of the concepts in the 3pcc (third party call control) document [7].
   The SIP Events document [11] describes the use of SUBSCRIBE and
   NOTIFY while the SIP Call Package document [10] describes the dialog
   event package.

   The call flow for the Single Line Extension requires an INVITE
   containing "join" semantics, which request the recipient to provide
   media mixing for a session.  No SIP Working Group document describes
   such a mechanism to date.

   These flows were prepared assuming a network of proxies, registrars,
   PSTN gateways, and other SIP servers that have a pre-established
   trust relationship with each other, secured through other means than
   SIP. User agents wishing to use the services in this network are

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   required to authenticate themselves with an edge proxy using SIP
   Digest. To improve the clarity of this document, authentication of
   User Agents is not explicitly shown in all flows, except where
   authentication directly relates to the service example.

   Each call flow is presented with a textual description of the
   scenario, a message flow diagram showing the messages exchanged
   between separate network elements, and the detailed contents of each
   message shown in the diagram.

1.1       Legend for Message Flows

   Dashed lines (---) represent control messages that are mandatory to
   the call scenario. These control messages can be SIP or PSTN
   signaling.

   Double dashed lines (===) represent media paths between network
   elements.

   Messages with parenthesis around name represent optional control
   messages.

   Messages are identified in the Figures as F1, F2, etc.  This
   references the message details in the table that follows the Figure.
   Comments in the message details are shown in the following form:

          /* Comments. */

1.2       Document History

   The first version of this document was the Internet-Draft
   "draft-sparks-sip-service-examples.txt" October 1999.

   The next version was combined with the SIP Telephony Call Flows
   document into the "draft-ietf-sip-call-flows-00.txt" April 2000.

   This version is based on Section 7 of that document with many of the
   examples extensively rewritten using the REFER method.


1.3       Changes to 00

   - Moddified all REFER flows to include 202 Accepted and NOTIFY
     result message.

   - Removed "Telephony" from title of document.

   - Added note on 2.9 on use of REFER to exit 3-way conference.

   - Modified Music on Hold/Call Park flow (2.2).

   - Modified order of messages in transfers (2.4 and 2.5) to more

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     closely resemble PSTN experience.

   - Added Call Pickup flow (2.13).

   - Added editorial notes on usefulness of Replaces header.

1.4       Changes to 01

   - Added Replaces header to flows.

   - Changed handling of call hold.

   - Added Contact and Referred-By headers and ref parameters to REFER
   messages.

   - Changed some Contacts, Routes, and Request-URIs to IP addresses.

   - Incremented o= line when SDP is updated with hold.

   - Added From tags throughout.

   - Renamed Music On Hold/Call Park to Call Park and added new Music On
   Hold call flow using 3pcc.


1.5       Changes to 02

   - Changed call hold flows to show sendonly attribute based on [8].

   - Removed ref parameter from the Referred-By headers since it is now
   just a generic paramter in the latest draft [4]. (Note that if the
   ref parameter stays, additional characters such as "<", "@", and ">"
   must be escaped.)

   - Removed tags in CANCEL requests and responses.

1.6 Changes to sipping-00

   - Added Max-Forwards

   - Added branch parm and received param to Vias

   - Changed Record-Route and Route behavior to show loose routing.

   - Added Record-Route and Contact to 18x responses

   - Changed Call Pickup to show Replaces with an early dialog

   - Changed description of Single Line Extension service

   - Changed Call Park and Pickup flow to show a different user picking
     up the call.

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   - Added Allow and Supported headers to REFER and replaces flows.


1.7 Changes to -01

   - Added SIP Events and dialog event package to Call Pickup flow.

   - Added Auto Redial call flows

   - Updated based on new Replaces draft with 481 behavior instead of
     using Accept-Contact in attended transfer.

   - Added Single Line Extension call flow.

   - Added Error-Info in call screening flows.

1.8 Changes to -02

   - Added 3-way call flows showing transition from 2 party call.

































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2  IP Telephony Services Features Call Flows

   These call flows show how a number of standard telephony features can
   be implemented using SIP.  They are not meant to represent a complete
   set.  Some calls make use of SIP extensions.

2.1       Call Hold

        User A           Proxy          User B
          |                |              |
          |    INVITE F1   |              |
          |--------------->|              |
          |                |   INVITE F2  |
          |(100 Trying) F3 |------------->|
          |<---------------|              |
          |                |180 Ringing F4|
          | 180 Ringing F5 |<-------------|
          |<---------------|              |
          |                |  200 OK F6   |
          |    200 OK F7   |<-------------|
          |<---------------|              |
          |     ACK F8     |              |
          |--------------->|    ACK F9    |
          |                |------------->|
          |    Both way RTP Established   |
          |<=============================>|
          |                |INVITE(hold) F10
          |INVITE(hold) F11|<-------------|
          |<---------------|              |
          |    200 OK F12  |              |
          |--------------->|   200 OK F13 |
          |                |------------->|
          |                |     ACK F14  |
          |     ACK F15    |<-------------|
          |<---------------|              |
          |           No RTP Sent!        |
          |                |  INVITE F16  |
          |   INVITE F17   |<-------------|
          |<---------------|              |
          |   200 OK F18   |              |
          |--------------->|  200 OK F19  |
          |                |------------->|
          |                |    ACK F20   |
          |     ACK F21    |<-------------|
          |<---------------|              |
          |    Both way RTP Established   |
          |<=============================>|
          |     BYE F22    |              |
          |--------------->|   BYE F23    |
          |                |------------->|
          |                |   200 OK F24 |
          |    200 OK F25  |<-------------|

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          |<---------------|              |
          |                |              |

   User A calls User B, User B places call on hold.  User B then takes
   call off hold. User A hangs up call.  Hold is unidirectional in
   nature.  However, a UA that places the other party on hold will
   generally also stop sending media, resulting in no media exchange
   between the UAs.


   Message Details


   F1 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy 1 -> B

   INVITE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0

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   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 (100 Trying) Proxy 1 -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F4 180 Ringing B -> Proxy 1

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content Length:0


   F5 180 Ringing Proxy 1 -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content Length: 0


   F6 200 OK B -> Proxy 1


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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 200 OK Proxy 1 -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F8 ACK A -> Proxy 1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70

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   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F9 ACK Proxy 1 -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User B places User A on hold.  Note that the version is
      incremented in the o= field of the SDP  */

   F10 INVITE B -> Proxy 1

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F11 INVITE Proxy 1 -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1

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   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   /* A replies with normal SDP */

   F12 200 OK A -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F13 200 OK Proxy 1 -> B

   SIP/2.0 200 OK

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   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 ACK B -> Proxy 1

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F15 ACK Proxy 1 -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User B takes the call off hold */

   F16 INVITE B -> Proxy 1

   INVITE sip:UserA@100.101.102.103 SIP/2.0

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   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844529 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F17 INVITE Proxy 1 -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844529 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F18 200 OK A -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123

Johnston, et al.            Informational                   [Page 15]

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   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F19 200 OK Proxy 1 -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F20 ACK B -> Proxy 1

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 ACK
   Content-Length: 0

Johnston, et al.            Informational                   [Page 16]

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   F21 ACK Proxy 1 -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 ACK
   Content-Length: 0


   /* RTP Media stream re-established.  User A disconnects. */

   F22 BYE A -> Proxy 1

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F23 BYE Proxy 1 -> B

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F24 200 OK B -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103

Johnston, et al.            Informational                   [Page 17]

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   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F25 200 OK Proxy 1 -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0




































Johnston, et al.            Informational                   [Page 18]

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2.2       Consultation Hold

       User A           Proxy          User B              User C
          |                |              |                   |
          |    INVITE F1   |              |                   |
          |--------------->|   INVITE F2  |                   |
          |                |------------->|                   |
          |(100 Trying) F3 |              |                   |
          |<---------------|180 Ringing F4|                   |
          |                |<-------------|                   |
          | 180 Ringing F5 |              |                   |
          |<---------------|  200 OK F6   |                   |
          |                |<-------------|                   |
          |    200 OK F7   |              |                   |
          |<---------------|              |                   |
          |     ACK F8     |              |                   |
          |--------------->|     ACK F9   |                   |
          |                |------------->|                   |
          |     Both way RTP Established  |                   |
          |<=============================>|                   |
          |                |INVITE(hold) F10                  |
          |INVITE(hold) F11|<-------------|                   |
          |<---------------|              |                   |
          |    200 OK F12  |              |                   |
          |--------------->|   200 OK F13 |                   |
          |                |------------->|                   |
          |                |     ACK F14  |                   |
          |                |<-------------|                   |
          |    ACK F15     |              |                   |
          |<---------------|              |                   |
          |         No RTP Sent!          |                   |
          |                |   INVITE F16 |                   |
          |                |<-------------|                   |
          |                |              |    INVITE F17     |
          |                |--------------------------------->|
          |                |(100 Trying) F18                  |
          |                |------------->|                   |
          |                |              | 180 Ringing F19   |
          |                |<---------------------------------|
          |                | 180 Ringing F20                  |
          |                |------------->|                   |
          |                |              |    200 OK F21     |
          |                |<---------------------------------|
          |                |   200 OK F22 |                   |
          |                |------------->|                   |
          |                |    ACK F23   |                   |
          |                |<-------------|                   |
          |                |              |      ACK F24      |
          |                |--------------------------------->|
          |                |         Both way RTP Established |

Johnston, et al.            Informational                   [Page 19]

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          |                |              |<=================>|
          |                |   BYE F25    |                   |
          |                |<-------------|                   |
          |                |              |       BYE F26     |
          |                |--------------------------------->|
          |                |              |    200 OK F27     |
          |                |<---------------------------------|
          |                |   200 OK F28 |                   |
          |                |------------->|                   |
          |                |   INVITE F29 |                   |
          |    INVITE F30  |<-------------|                   |
          |<---------------|              |                   |
          |    200 OK F31  |              |                   |
          |--------------->|   200 OK F32 |                   |
          |                |------------->|                   |
          |                |     ACK F33  |                   |
          |                |<-------------|                   |
          |    ACK F34     |              |                   |
          |<---------------|              |                   |
          |     Both way RTP Established  |                   |
          |<=============================>|                   |
          |    BYE F35     |              |                   |
          |--------------->|    BYE F36   |                   |
          |                |------------->|                   |
          |                |   200 OK F37 |                   |
          |                |<-------------|                   |
          |  200 OK F38    |              |                   |
          |<---------------|              |                   |
          |                |              |                   |

   User A calls user B.  User B places call on hold.  User B calls User
   C.  User B then disconnects with C, then takes the call with User A
   off hold.  The call ends with B hangs up.

   Message Details


   F1 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com

Johnston, et al.            Informational                   [Page 20]

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   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy 1 -> B

   INVITE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3(100 Trying) Proxy 1 -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F4 180 Ringing B -> Proxy 1

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>

Johnston, et al.            Informational                   [Page 21]

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   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content Length:0


   F5 180 Ringing Proxy 1 -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content Length: 0


   F6 200 OK B -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 200 OK Proxy 1 -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103

Johnston, et al.            Informational                   [Page 22]

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   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F8 ACK A -> Proxy 1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F9 ACK Proxy 1 -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User B places User A on hold. */

   F10 INVITE B -> Proxy 1

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>

Johnston, et al.            Informational                   [Page 23]

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   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F11 INVITE Proxy 1 -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F12 200 OK A -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123

Johnston, et al.            Informational                   [Page 24]

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   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F13 200 OK Proxy 1 -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 ACK B -> Proxy 1

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0

Johnston, et al.            Informational                   [Page 25]

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   F15 ACK Proxy 1 -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F16 INVITE B -> Proxy 1

   INVITE sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 50170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F17 INVITE Proxy 1 -> C

   INVITE sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp

Johnston, et al.            Informational                   [Page 26]

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   Content-Length: ...

   v=0
   o=UserB 2890844834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 50170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F18 (100 Trying) Proxy 1 -> B

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Content-Length: 0


   F19 180 Ringing C -> Proxy 1

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content Length:0


   F20 180 Ringing Proxy 1 -> B

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content Length: 0


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   F21 200 OK C -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F22 200 OK Proxy 1 -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F23 ACK B -> Proxy 1

   ACK sip:UserC@120.121.122.123 SIP/2.0

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   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 1 ACK
   Content-Length: 0


   F24 ACK Proxy 1 -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 1 ACK
   Content-Length: 0


   F25 BYE B -> Proxy 1

   BYE sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 2 BYE
   Content-Length: 0


   F26 BYE Proxy 1 -> C

   BYE sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 2 BYE
   Content-Length: 0


   F27 200 OK C -> Proxy 1

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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 2 BYE
   Content-Length: 0


   F28 200 OK Proxy 1 -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=456654
   Call-ID: 9876543210@there.com
   CSeq: 2 BYE
   Content-Length: 0


   /* User B takes the call off hold */

   F29 INVITE B -> Proxy 1

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844529 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F30 INVITE Proxy 1 -> A


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   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844529 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F31 200 OK A -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F32 200 OK Proxy 1 -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7

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    ;received=120.121.122.123
   Record-Route: <sip:ss1.wcom.com;lr>
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F33 ACK B -> Proxy 1

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 ACK
   Content-Length: 0


   F34 ACK Proxy 1 -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 2 ACK
   Content-Length: 0


   F35 BYE A -> Proxy 1

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70

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   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F36 BYE Proxy 1 -> B

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F37 200 OK B -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F38 200 OK Proxy 1 -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0








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2.3       Music On Hold

         User A          User B     Music Server
          |                |              |
          |    INVITE F1   |              |
          |--------------->|              |
          | 180 Ringing F2 |              |
          |<---------------|              |
          |    200 OK F3   |              |
          |<---------------|              |
          |     ACK F4     |              |
          |--------------->|              |
          |       RTP      |              |
          |<==============>|              |
          |                |              |
          | B places A on hold with music |
          |                |              |
          |                | INVITE no sdp F5
          |                |------------->|
          |                |200 OK sdpM F6|
          |                |<-------------|
          |INVITE (hold) F7|              |
          |<---------------|              |
          | 200 OK sdpA F8 |              |
          |--------------->|              |
          |     ACK F9     |              |
          |<---------------|              |
          |                | ACK sdpA F10 |
          |                |------------->|
          |               RTP             |
          |<==============================|
          |                |              |
          |    B Picks up the Call        |
          |                |    BYE F11   |
          |                |------------->|
          |                | 200 OK F12   |
          |                |<-------------|
          |    INVITE F13  |              |
          |<---------------|              |
          |    200 OK F14  |              |
          |--------------->|              |
          |     ACK F15    |              |
          |<---------------|              |
          |       RTP      |              |
          |<==============>|              |


   In this flow,B places A on hold with music. This is performed by B
   acting as a B2BUA (Back-to-Back User Agent) using 3pcc (third party
   call control) [7] to INVITE a Music Server into the session, which


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   sends RTP music to A.  B picks the call up from hold by sending a BYE
   to the music server and taking A off of hold.

   Note that if A responds to the INVITE with hold SDP with hold SDP in
   the 200 OK, this call flow will not work properly.

   Note also that A will receive RTP packets from two different IP
   addresses during this call flow.


   Message Details


   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 180 Ringing B -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F3 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9

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    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 ACK A -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User B places A on hold with music by acting as a B2BUA and
   sending an INVITE to the Music Server  */

   F5 INVITE B -> Music Server

   INVITE sip:music@server.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: Music <sip:music@server.com>
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F6 200 OK C -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=8675309

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   To: Music <sip:music@server.com>;tag=90210
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 1 INVITE
   Contact: <sip:music@server.com>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=MusicServer 2890844576 2890844576 IN IP4 music.server.com
   s=Session SDP
   c=IN IP4 50.60.70.80
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly

   /* B re-INVITEs A on hold */

   F7 INVITE B -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 music.server.com
   s=Session SDP
   c=IN IP4 50.60.70.80
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   /* A responds with normal SDP */

   F8 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE

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   Contact: <sip:UserB@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F9 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* B passes SDP of A to the Music Server to allow music to be sent
   from B to A */

   F10 ACK B -> Music Server

   ACK sip:music@server.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: Music <sip:music@server.com>;tag=90210
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 1 ACK
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* RTP music is now sent from the Music Server to A */

   /* B picks up the call from the Music Server */

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   F11 BYE B -> Music Server

   BYE sip:music@server.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: Music <sip:music@server.com>;tag=90210
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 2 BYE
   Content-Length: 0


   F12 200 OK Music Server -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: Music <sip:music@server.com>;tag=90210
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 2 BYE
   Content-Length:0


   /* B re-INVITEs taking A off hold */

   F13 INVITE B -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844529 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 200 OK A -> B

   SIP/2.0 200 OK

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   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserB@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F15 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 2 ACK
   Content-Length: 0


   /* Normal media session between A and B is resumed */





















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2.4 Unattended Transfer


       User A              User B               User C
         |      INVITE F1     |                    |
         |<-------------------|                    |
         |   180 Ringing F2   |                    |
         |------------------->|                    |
         |      200 OK F3     |                    |
         |------------------->|                    |
         |        ACK F4      |                    |
         |<-------------------|                    |
         |        RTP         |                    |
         |<==================>|                    |
         |                    |                    |
         |  A performs unattended transfer         |
         |                    |                    |
         | REFER Refer-To:C F5|                    |
         |------------------->|                    |
         |  202 Accepted F6   |                    |
         |<-------------------|                    |
         |       BYE F7       |                    |
         |------------------->|                    |
         |     200 OK F8      |                    |
         |<-------------------|                    |
         |   No RTP Session   | INVITE Referred-By: A F9
         |                    |------------------->|
         |                    |   180 Ringing F10  |
         |                    |<-------------------|
         |                    |     200 OK F11     |
         |                    |<-------------------|
         |                    |       ACK F12      |
         |                    |------------------->|
         |                    |        RTP         |
         |                    |<==================>|
         |      NOTIFY F13    |                    |
         |<-------------------|                    |
         |      200 OK F14    |                    |
         |------------------->|                    |
         |                    |                    |


   User B call User A.  User A then transfers User B to User C, then
   User A disconnects with User B.  User B establishes the session to C
   then reports the success  back to A in the NOTIFY.  If the transfer
   fails, User B can send a new INVITE back to A to re-establish the
   session.

   Note that the BYE could be sent by A after the NOTIFY.  Also, if A
   has flushed all state information, the NOTIFY may receive a 481 Call
   Leg Does Not Exist.

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   Message Details

   F1 INVITE B -> A

   INVITE sip:UserA@here.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 180 Ringing A -> B

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Length: 0


   F3 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY

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   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* Session is established between A and B. */

   /* A peforms unattended transfer of B to C */

   F5 REFER A -> B

   REFER sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 REFER
   Refer-To: <sip:UserC@anywhere.com>
   Referred-By: <UserA@here.com>
   Contact: <sip:UserA@100.101.102.103>
   Content-Length: 0


   F6 202 Accepted B -> A

   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com

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   CSeq: 2 REFER
   Content-Length: 0


   /* A now disconnects with B */

   F7 BYE A -> B

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 3 BYE
   Content-Length: 0


   F8 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 3 BYE
   Content-Length: 0


   /* B attempts the transfer to C */

   F9 INVITE B -> C

   INVITE sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: 7436222@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Referred-By: <UserA@here.com>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844539 2890844539 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113

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   t=3034423821 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F10 180 Ringing C -> B

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=928287
   Call-ID: 7436222@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Length: 0


   F11 200 OK C -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=928287
   Call-ID: 7436222@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844527 2890844527 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F12 ACK B -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=928287
   Call-ID: 7436222@here.com
   CSeq: 1 ACK
   Content-Length: 0

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   /* B and C now have established a session.  B reports success to A
   which A probably ignores. */

   F13 NOTIFY B -> A

   NOTIFY sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 NOTIFY
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=928287
   Call-ID: 7436222@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Type: application/sdp
   Content-Length: ...

   F14 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=314159
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345601@here.com
   CSeq: 2 NOTIFY
   Content-Length: 0














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2.5       Attended Transfer

         User A          User B        User C
          |                |              |
          |    INVITE F1   |              |
          |--------------->|              |
          | 180 Ringing F2 |              |
          |<---------------|              |
          |    200 OK F3   |              |
          |<---------------|              |
          |     ACK F4     |              |
          |--------------->|              |
          |       RTP      |              |
          |<==============>|              |
          |INVITE (hold) F5|              |
          |<---------------|              |
          |    200 OK F6   |              |
          |--------------->|              |
          |     ACK F7     |              |
          |<---------------|              |
          |     No RTP     |              |
          |                |  INVITE F8   |
          |                |------------->|
          |                | 180 Ringing F9
          |                |<-------------|
          |                |  200 OK F10  |
          |                |<-------------|
          |                |    ACK F11   |
          |                |------------->|
          |                |     RTP      |
          |                |<============>|
          |                |INVITE (hold) F12
          |                |------------->|
          |                | 200 OK F13   |
          |                |<-------------|
          |                |    ACK F14   |
          |                |------------->|
          |                |     No RTP   |
          | REFER Refer-To: C F15         |
          |<---------------|              |
          |202 Accepted F16|              |
          |--------------->|              |
          |     INVITE Replaces: B F17    |
          |------------------------------>|
          |            200 OK F18         |
          |<------------------------------|
          |             ACK F19           |
          |------------------------------>|
          |               RTP             |
          |<=============================>|
          |                |    BYE F20   |

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          |                |<-------------|
          |                |  200 OK F21  |
          |                |------------->|
          |   NOTIFY F22   |              |
          |--------------->|              |
          |   200 OK F23   |              |
          |<---------------|              |
          |    BYE F24     |              |
          |<---------------|              |
          |   200 OK F25   |              |
          |--------------->|              |

   User A calls User B.  User B puts User A on hold then calls User C to
   announce transfer, then places C on hold.  User B transfers User A to
   User C which replaces the session between B and C.  C then
   disconnects session with B. A reports success of transfer to B, who
   then disconnects with A.  In this example, the Replaces header [5] is
   inserted into the Refer-To URL by B.  Note the presence of the
   Require: Replaces header field in the REFER F15.  This ensures that
   only the correct instance of User C is reached.

   For further details on types of call transfers, refer to [9].

   Message Details


   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 180 Ringing B -> A


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   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F3 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 ACK A -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User A and User B have established a session.  User B puts User A
   on Hold */

   F5 INVITE B -> A


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   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F6 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70

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   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 ACK
   Content-Length: 0


   /* User B calls User C  */

   F8 INVITE B -> C

   INVITE sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 28908445834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423645 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F9 180 Ringing C -> B

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Length: 0


   F10 200 OK C -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309

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   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F11 ACK B -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 ACK
   Content-Length: 0


   /* User B puts User C on hold */

   F12 INVITE B -> C

   INVITE sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 43 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 289084834 2890844835 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423645 0

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   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F13 200 OK C -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 43 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 ACK B -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 43 ACK
   Content-Length: 0

   /* User B Transfers User A to User C.  */

   F15 REFER B -> A

   REFER sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Refer-To: <sip:UserC@anywhere.com?Replaces=

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    sdjfdjfskdf@there.com%3Bto-tag%3D5f35a3%3Bfrom-tag%3D8675309>
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F16 202 Accepted A -> B

   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=1234567
   To: BigGuy <sip:UserA@here.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Content-Length: 0


   /* User A establishes session with User C which replaces the session
   between User B and User C */

   F17 INVITE A -> C

   INVITE sip:UserA@here.com SIP/2.0
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKadfe4ko
   To: OtherGuy <sip:UserC@anywhere.com>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Referred-By: <sip:UserB@there.com>
   Accept-Contact: <sip:UserC@120.121.122.123>;only=true
   Replaces: 12345600@here.com;to-tag=23431;from-tag=8675309
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Require: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844989 2890844989 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423643 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F18 200 OK C -> A

   SIP/2.0 200 OK

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   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKadfe4ko
    ;received=120.121.122.123
   To: OtherGuy <sip:UserC@anywhere.com>;tag=ff3a
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844221 2890844221 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423452 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F19 ACK A -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKadfe4ko
   To: OtherGuy <sip:UserC@anywhere.com>;tag=ff3a
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   /* User C then disconnects from User B */

   F20 BYE C -> B

   BYE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Max-Forwards: 70
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 1 BYE
   Content-Length: 0


   F21 200 OK B -> C

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103

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   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 1 BYE
   Content-Length: 0


   /* User A tells User B that the call has been successfully
   transferred */

   F22 NOTIFY A -> B

   NOTIFY sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKadfe4ko
    ;received=120.121.122.123
   From: OtherGuy <sip:UserC@anywhere.com>;tag=ff3a
   To: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@anywhere.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>


   F23 200 OK B -> C

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: LittleGuy <sip:UserB@there.com>;tag=1234567
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY
   Content-Length: 0


   /* User B disconnects with User A */

   F24 BYE B -> A

   BYE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70

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   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0


   F25 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0




































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2.6       Call Forwarding Unconditional


         User A           Proxy          Gateway
          |                |              |
          |    INVITE F1   |              |
          |--------------->|   INVITE F2  |
          |(100 Trying) F3 |------------->|
          |<---------------|180 Ringing F4|
          | 180 Ringing F5 |<-------------|
          |<---------------|  200 OK F6   |
          |    200 OK F7   |<-------------|
          |<---------------|              |
          |     ACK F8     |              |
          |--------------->|    ACK F9    |
          |                |------------->|
          |    Both way RTP Established   |
          |<=============================>|
          |     BYE F10    |              |
          |--------------->|   BYE F11    |
          |                |------------->|
          |                |   200 OK F12 |
          |    200 OK F13  |<-------------|
          |<---------------|              |
          |                |              |

   User B wants all calls forwarded to the PSTN.  User A calls User B.
   The Proxy server rewrites the request URI, and forwards the INVITE to
   a Gateway.  Details of messaging behind the Gateway are not shown.


   Message Details


   F1 INVITE A -> Proxy

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103

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   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* Proxy forwards call by rewriting Request-URI */

   F2 INVITE Proxy -> Gateway

   INVITE sip:+19727293660@gw1.wcom.com;user=phone SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 (100 Trying) Proxy -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F4 180 Ringing Gateway -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>

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   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:+19727293660@gw1.wcom.com;user=phone>
   Content Length:0


   F5 180 Ringing Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:+19727293660@gw1.wcom.com;user=phone>
   Content Length: 0


   F6 200 OK Gateway -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:+19727293660@gw1.wcom.com;user=phone>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=GATEWAY1 2890844527 2890844527 IN IP4 gatewayone.wcom.com
   s=Session SDP
   c=IN IP4 gatewayone.wcom.com
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103

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   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:+19727293660@gw1.wcom.com;user=phone>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=GATEWAY1 2890844527 2890844527 IN IP4 gatewayone.wcom.com
   s=Session SDP
   c=IN IP4 gatewayone.wcom.com
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F8 ACK A -> Proxy

   ACK sip:+19727293660@gw1.wcom.com;user=phone SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F9 ACK Proxy -> Gateway

   ACK sip:+19727293660@gw1.wcom.com;user=phone SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F10 BYE A -> Proxy 1

   BYE sip:+19727293660@gw1.wcom.com;user=phone SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:+19727293660@gw1.wcom.com;user=phone>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567

Johnston, et al.            Informational                   [Page 61]

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   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F11 BYE Proxy 1 -> Gateway

   BYE sip:+19727293660@gw1.wcom.com;user=phone SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F12 200 OK Gateway -> Proxy 1

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F13 200 OK Proxy 1 -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345600@here.com
   CSeq:  2 BYE
   Content-Length: 0









Johnston, et al.            Informational                   [Page 62]

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2.7       Call Forwarding - Busy

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

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

   Message Details


   F1 INVITE A -> Proxy

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>

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   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy -> B1

   INVITE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 (100 Trying) Proxy -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F4 486 Busy Here B1 -> Proxy

   SIP/2.0 486 Busy Here

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   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F5 ACK Proxy -> B1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


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

   F6 INVITE Proxy -> B2

   INVITE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 180 Ringing B2 -> Proxy


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   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Length: 0


   F8 180 Ringing Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Length: 0


   F9 200 OK B2 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


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   F10 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F11 ACK A -> Proxy

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F12 ACK Proxy -> B2

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* RTP streams are established between A and B2 */

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

   F13 BYE A -> Proxy

   BYE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F14 BYE Proxy -> B2

   BYE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F15 200 OK B2 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F16 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com

Johnston, et al.            Informational                   [Page 68]

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   CSeq:  2 BYE
   Content-Length: 0



















































Johnston, et al.            Informational                   [Page 69]

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2.8       Call Forwarding - No Answer

         User A           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   |                   |
          |                |------------->|                   |
          |                |              |    INVITE F10     |
          |                |--------------------------------->|
          |                |              | 180 Ringing F11   |
          | 180 Ringing F12|<---------------------------------|
          |<---------------|              |      200 OK F13   |
          |                |<---------------------------------|
          |   200 OK F14   |              |                   |
          |<---------------|              |                   |
          |     ACK F15    |              |                   |
          |--------------->|              |     ACK F16       |
          |                |--------------------------------->|
          |               Both way RTP Established            |
          |<=================================================>|
          |    BYE F17     |              |                   |
          |--------------->|              |      BYE F18      |
          |                |--------------------------------->|
          |                |              |    200 OK F19     |
          |  200 OK F20    |<---------------------------------|
          |<---------------|              |                   |
          |                |              |                   |

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


   Message Details

   F1 INVITE A -> Proxy

   INVITE sip:UserB@there.com SIP/2.0

Johnston, et al.            Informational                   [Page 70]

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   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy -> B1

   INVITE sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 (100 Trying) Proxy -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com

Johnston, et al.            Informational                   [Page 71]

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   CSeq: 1 INVITE
   Content-Length: 0


   F4 180 Ringing B1 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB1@110.111.112>
   Content-Length: 0


   F5 180 Ringing Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB1@110.111.112>
   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 sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F7 200 OK B1 -> Proxy

   SIP/2.0 200 OK

Johnston, et al.            Informational                   [Page 72]

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   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F8 487 Request Terminated B1 -> Proxy

   SIP/2.0 487 Request Terminated
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F9 ACK Proxy -> B1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F10 INVITE Proxy -> B2

   INVITE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0

Johnston, et al.            Informational                   [Page 73]

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   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F11 180 Ringing B2 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Length: 0


   F12 180 Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Length: 0


   F13 200 OK B2 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp

Johnston, et al.            Informational                   [Page 74]

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   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F15 ACK A -> Proxy

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F16 ACK Proxy -> B2

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9

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    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* RTP streams are established between A and B2.  User A Hangs Up
   with User B2. */

   F17 BYE A -> Proxy

   BYE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F18 BYE Proxy -> B2

   BYE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F19 200 OK B2 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 2 BYE
   Content-Length: 0


Johnston, et al.            Informational                   [Page 76]

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   F20 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq:  2 BYE
   Content-Length: 0










































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2.9  3-way Conference û Third Party is Added

        User A           User B        Bridge         User C
          |       RTP      |              |              |
          |<==============>|              |              |
          |INVITE (hold) F1|              |              |
          |<---------------|              |              |
          |    200 OK F2   |              |              |
          |--------------->|              |              |
          |     ACK F3     |              |              |
          |<---------------|  INVITE F4   |              |
          |     No RTP     |------------->|              |
          |                |  200 OK F5   |              |
          |                |<-------------|              |
          |                |    ACK F6    |              |
          |                |------------->|              |
          |                |     RTP      |              |
          |                |<============>|              |
          | REFER Refer-To: Bridge F7     |              |
          |<---------------|              |              |
          |202 Accepted F8 |              |              |
          |--------------->|              |              |
          |           INVITE F9           |              |
          |------------------------------>|              |
          |            200 OK F10         |              |
          |<------------------------------|              |
          |             ACK F11           |              |
          |------------------------------>|              |
          |               RTP             |              |
          |<=============================>|              |
          |   NOTIFY F12   |              |              |
          |--------------->|              |              |
          |   200 OK F13   |              |              |
          |<---------------|              |              |
          |    BYE F14     |              |              |
          |<---------------|              |              |
          |   200 OK F15   |              |              |
          |--------------->| REFER Refer-To: C F16       |
          |                |------------->|              |
          |                |202 Accepted F17             |
          |                |<-------------|  INVITE F18  |
          |                |              |------------->|
          |                |              |   180 F19    |
          |                |              |<-------------|
          |                |              |  200 OK F20  |
          |                |              |<-------------|
          |                |              |    ACK F21   |
          |                |              |------------->|
          |                |              |     RTP      |
          |                |  NOTIFY F22  |<============>|
          |                |<-------------|              |

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          |                |  200 OK F23  |              |
          |                |------------->|              |

   In this scenario, A and B are in a 2-party call (session) when B
   wishes to add C into the conversation.  B is not capable of media
   mixing, but does have call control capabilities and a conference
   bridge which is capable of mixing.  B first places A on hold
   (optional step) then establishes a session with the bridge. B then
   transfers A to the bridge using a special conference-URI which
   resolves to the bridge but identifies B as the owner of the
   conference.  A sends an INVITE to that URI and establishes a session
   with the bridge, which mixes the media of A and B.  Once B has
   confirmation that A has joined the bridge, B then sends a refer to
   the bridge to invite C into the conference which causes the bridge to
   send an INVITE to C.  If C accepts the INVITE, C is added to the
   session and the 3 party conference is complete.  Additional parties
   could be added to the conference in the same way (B sending a REFER
   to the conference bridge) or by them sending an INVITE to the bridge
   with BÆs conference URI.

   In this example, User BÆs URI is sip:UserB@there.com while User BÆs
   conference Bridge is sip:UserB-Bridge@there.com.  As an alternative
   to this conference URI which would require registration, the
   conference URI could be the contact sip:UserB-Bridge@110.111.112.114
   which would not require registration.

   Message Details


   /* User A and User B have established a session.  User B puts User A
   on Hold */

   F1 INVITE B -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0

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   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F2 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 ACK
   Content-Length: 0


   /* User B calls Conference Bridge  */

   F4 INVITE B -> Bridge

   INVITE sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE

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   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 28908445834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423645 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F5 200 OK Bridge -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.114>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB-Bridge 2890844922 2890844922 IN IP4 bridge.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F6 ACK B -> Bridge

   ACK sip:UserB-Bridge@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 ACK
   Content-Length: 0


   /* User B Transfers User A to the Bridge.  */

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   F7 REFER B -> A

   REFER sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Refer-To: <sip:UserB-Bridge@there.com>
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F8 202 Accepted A -> B

   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=1234567
   To: BigGuy <sip:UserA@here.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Content-Length: 0


   /* User A establishes session with the Bridge */

   F9 INVITE A -> Bridge

   INVITE sip:UserB-Bridge@here.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
   To: OtherGuy <sip:UserC@anywhere.com>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844221 2890844221 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423452 0
   m=audio 49172 RTP/AVP 0

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   a=rtpmap:0 PCMU/8000



   F10 200 OK Bridge -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
    ;received=100.1101.102.103
   To: OtherGuy <sip:UserB-Bridge@there.com>;tag=ff3a
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.14>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB-Bridge 2890844922 2890844922 IN IP4 bridge.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423619 0
   m=audio 3468 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F11 ACK A -> Bridge

   ACK sip:UserB-Bridge@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
   Max-Forwards: 70
   To: OtherGuy <sip:UserB-Bridge@there.com>;tag=ff3a
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   /* User A tells User B that the call has been successfully
      transferred */

   F12 NOTIFY A -> B

   NOTIFY sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY

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   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   F13 200 OK B -> C

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: LittleGuy <sip:UserB@there.com>;tag=1234567
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY
   Content-Length: 0


   /* User B disconnects with User A since they are now on the bridge
      together */

   F14 BYE B -> A

   BYE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0


   F15 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0


   /* User B asks Bridge to add C to the conference. It is implicit that
      any REFER sent by B to the bridge involves mixing the resulting
      media into the existing conference. */

   F16 REFER B -> Bridge


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   REFER sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 43 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Refer-To: <sip:UserC@anywhere.com>
   Referred-By: <sip:UserB@there.com>
   Content-Length: 0


   F17 202 Accepted Bridge -> B

   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 43 REFER
   Content-Length: 0


   /* Bridge calls User C  */

   F18 INVITE Bridge -> C

   INVITE sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Bridge <sip:UserB-Bridge@there.com>;tag=8493jw423U
   To: OtherGuy <sip:UserC@anywhere.com>
   Call-ID: 32j2241adf12@bridge.there.com
   CSeq: 42 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.114>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Referred-By: <sip:UserB@there.com>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 28908445834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423645 0
   m=audio 3470 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


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   F19 180 Ringing C -> Bridge

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.114
   From: Bridge <sip:UserB-Bridge@there.com>;tag=8493jw423U
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: 32j2241adf12@bridge.there.com
   CSeq: 42 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Content-Length: 0


   F20 200 OK C -> Bridge

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.114
   From: Bridge <sip:UserB-Bridge@there.com>;tag=8493jw423U
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: 32j2241adf12@bridge.there.com
   CSeq: 42 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F21 ACK Bridge -> C

   ACK sip:UserC@120.121.122.123 SIP/2.0
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: OtherGuy <sip:UserC@anywhere.com>;tag=5f35a3
   Call-ID: 32j2241adf12@bridge.there.com
   CSeq: 42 ACK
   Content-Length: 0


   /* Bridge confirms that C has been added to the conference. */


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   F22 NOTIFY Bridge -> B

   NOTIFY sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 5453 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   F23 200 OK B -> Bridge

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.114
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 5453 NOTIFY
   Content-Length: 0
























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2.10   3-way Conference û Third Party Joins

        User A           User B        Bridge         User C
          |                |              |              |
          |       RTP      |              |  INVITE F1   |
          |<==============>|              |<-------------|
          |                |              |    180 F2    |
          |                |   NOTIFY F3  |------------->|
          |                |<-------------|              |
          |                |  200 OK F4   |              |
          |INVITE (hold) F5|------------->|              |
          |<---------------|              |              |
          |    200 OK F6   |              |              |
          |--------------->|              |              |
          |     ACK F7     |              |              |
          |<---------------|  INVITE F8   |              |
          |     No RTP     |------------->|              |
          |                |  200 OK F9   |              |
          |                |<-------------|              |
          |                |    ACK F10   |              |
          |                |------------->|              |
          |                |     RTP      |              |
          |                |<============>|  200 OK F11  |
          |                |              |------------->|
          |                |              |    ACK F12   |
          |                |              |<-------------|
          |                |              |     RTP      |
          |                |  NOTIFY F13  |<============>|
          |                |<-------------|              |
          |                |  200 OK F14  |              |
          |                |------------->|              |
          | REFER Refer-To: Bridge F15    |              |
          |<---------------|              |              |
          |202 Accepted F16|              |              |
          |--------------->|              |              |
          |           INVITE F17          |              |
          |------------------------------>|              |
          |            200 OK F18         |              |
          |<------------------------------|              |
          |             ACK F19           |              |
          |------------------------------>|              |
          |               RTP             |              |
          |<=============================>|              |
          |   NOTIFY F20   |              |              |
          |--------------->|              |              |
          |   200 OK F21   |              |              |
          |<---------------|              |              |
          |    BYE F22     |              |              |
          |<---------------|              |              |
          |   200 OK F23   |              |              |
          |--------------->|              |              |

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   In this scenario, A and B are in a 2 party call and C wishes to join
   resulting in a three party call.  C could have learned BÆs conference
   URI using some non-SIP means, or possibly from a REFER sent by B.  C
   sends an INVITE to BÆs conference-URI which reaches the bridge.  It
   is assumed that B has subscribed to the dialog state of his
   conference bridge, so B receives a NOTIFY from the bridge indicating
   that C wishes to join the conference.  If B allows C to join, B puts
   A on hold and establishes a session with the bridge.  Once B is on
   the bridge, the bridge then establishes the session with C and mixes
   the media of B and C.  B then transfers A to the bridge by
   transfering A to BÆs conference-URI.  When A has successfully joined
   the bridge, B receives notification and terminates the session with
   A.  Additional parties can join the conference using the conference-
   URI.

   Message Details


   /* User A and User B have established a session.  User C joins by
   calling BÆs conference URI */

   F1 INVITE C -> Bridge

   INVITE sip:UserB-Bridge@there.com SIP/2.0
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg
   To: Bridge <sip:UserB-Bridge@there.com>
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com
   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 180 Ringing Bridge -> C

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg

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   To: Bridge <sip:UserB-Bridge@there.com>;tag=dkfkk32o410
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com
   CSeq: 1 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.114>
   Content-Length: 0


   /* Bridge notifies B that C wishes to join the call. */

   F3 NOTIFY Bridge -> B

   NOTIFY sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 5453 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 180 Ringing
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg
   To: Bridge <sip:UserB-Bridge@there.com>;tag=dkfkk32o410
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com


   F4 200 OK B -> Bridge

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.114
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 5453 NOTIFY
   Content-Length: 0


   /* B prepares to add C into the call by putting A on hold */

   F5 INVITE B -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567

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   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844528 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=sendonly


   F6 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F7 ACK B -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1024 ACK
   Content-Length: 0

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   /* User B calls Conference Bridge  */

   F8 INVITE B -> Bridge

   INVITE sip:UserB-Bridge@there.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 28908445834 2890844834 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423645 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F9 200 OK Bridge -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.114>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB-Bridge 2890844922 2890844922 IN IP4 bridge.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



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   F10 ACK B -> Bridge

   ACK sip:UserB-Bridge@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=8675309
   To: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 42 ACK
   Content-Length: 0


   /* Once B is on the bridge, the bridge answers C and adds C into the
      conference */

   F11 200 OK Bridge -> C

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg
   To: Bridge <sip:UserB-Bridge@there.com>;tag=dkfkk32o410
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com
   CSeq: 1 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 28908445834 2890844834 IN IP4 bridge.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423645 0
   m=audio 3468 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F12 ACK C -> Bridge

   ACK sip:UserB-Bridge@there.com SIP/2.0
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg
   To: Bridge <sip:UserB-Bridge@there.com>;tag=dkfkk32o410
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com
   CSeq: 1 INVITE
   Content-Length: 0


   /* Bridge notifies B that C has joined the call. */

   F13 NOTIFY Bridge -> B

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   NOTIFY sip:UserC@anywhere.com SIP/2.0
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 5454 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK
   From: OtherGuy <sip:UserC@anywhere.com>;tag=3241kd34fg
   To: Bridge <sip:UserB-Bridge@there.com>;tag=dkfkk32o410
   Call-ID: 25i345k435j25FUJjudjss@anywhere.com


   F14 200 OK B -> Bridge

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP bridge.there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.114
   Max-Forwards: 70
   From: My Bridge <sip:UserB-Bridge@there.com>;tag=5f35a3
   To: LittleGuy <sip:UserB@there.com>;tag=8675309
   Call-ID: sdjfdjfskdf@there.com
   CSeq: 54534 NOTIFY
   Content-Length: 0


   /* User B Transfers User A to the Bridge.  */

   F15 REFER B -> A

   REFER sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Refer-To: <sip:UserB-Bridge@there.com>
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F16 202 Accepted A -> B


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   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=110.111.112.113
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1025 REFER
   Content-Length: 0


   /* User A establishes session with the Bridge */

   F17 INVITE A -> Bridge

   INVITE sip:UserB-Bridge@here.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
   To: OtherGuy <sip:UserC@anywhere.com>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844221 2890844221 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423452 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



   F18 200 OK Bridge -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
    ;received=100.1101.102.103
   To: OtherGuy <sip:UserB-Bridge@there.com>;tag=ff3a
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB-Bridge@110.111.112.14>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

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   v=0
   o=UserB-Bridge 2890844922 2890844922 IN IP4 bridge.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423619 0
   m=audio 3468 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F19 ACK A -> Bridge

   ACK sip:UserB-Bridge@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bKadfe4ko
   Max-Forwards: 70
   To: OtherGuy <sip:UserB-Bridge@there.com>;tag=ff3a
   From: BigGuy <sip:UserA@here.com>;tag=3461
   Call-ID: 9435674543@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   /* User A tells User B that the call has been successfully
      transferred */

   F20 NOTIFY A -> B

   NOTIFY sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   F21 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: 12345678900@here.com
   CSeq: 2 NOTIFY
   Content-Length: 0


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   /* User B disconnects with User A since they are now on the bridge
      together */

   F22 BYE B -> A

   BYE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0


   F23 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=23431
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345678900@here.com
   CSeq: 1026 BYE
   Content-Length: 0



























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2.11 Single Line Extension


         User A      Forking Proxy       User   User   User
                                          B1     B2     B3
          |                |              |      |      |
          |    INVITE F1   |              |      |      |
          |--------------->|   INVITE F2  |      |      |
          |                |------------->|      |      |
          |                |       INVITE F3     |      |
          |                |-------------------->|      |
          |                |          INVITE F4  |      |
          |                |--------------------------->|
          | 100 Trying F5  |              |      |      |
          |<---------------|180 Ringing F6|      |      |
          |                |<-------------|      |      |
          | 180 Ringing F7 |              |      |      |
          |<---------------|              |      |      |
          |                | 480 Not Logged In F8|      |
          |                |<--------------------|      |
          |                |              ACK F9 |      |
          |                |-------------------->|      |
          |                |         180 Ringing F10    |
          |     180 F11    |<---------------------------|
          |<---------------|            200 OK F12      |
          |   200 OK F13   |<---------------------------|
          |<---------------|              |      |      |
          |                |   CANCEL 14  |      |      |
          |                |------------->|      |      |
          |                |  200 OK F15  |      |      |
          |                |<-------------|      |      |
          |                |    487 F16   |      |      |
          |                |<-------------|      |      |
          |                |    ACK F17   |      |      |
          |                |------------->|      |      |
          |     ACK F18    |              |      |      |
          |--------------->|           ACK F19   |      |
          |                |--------------------------->|
          |               Both way RTP Established      |
          |<===========================================>|
          |                |          NOTIFY F20        |
          |                |<---------------------------|
          |                |   NOTIFY F21 |      |      |
          |                |------------->|      |      |
          |                |       NOTIFY F22    |      |
          |                |-------------------->|      |
          |                |          NOTIFY F23 |      |
          |                |--------------------------->|
          |                |   200 OK F24 |      |      |
          |                |<-------------|      |      |
          |                |          200 OK F25        |

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          |                |--------------------------->|
          |                |   200 OK F26 |      |
          |                |<--------------------|      |
          |                |     482 Loop Detected F27  |
          |                |<---------------------------|
          |                |                            |
          |         User B2 wants to join the call      |
          |                |                            |
          |                |   INVITE "Join" F28 |      |
          |                |<--------------------|      |
          |                |   INVITE "Join" F29 |      |
          |                |------------->|      |      |
          |                |      INVITE "Join" F30     |
          |                |-------------------->|      |
          |                |          INVITE "Join" F31 |
          |                |--------------------------->|
          |                |    100 Trying F32   |      |
          |                |-------------------->|      |
          |                | 481 Dialog Does Not Exist F33
          |                |<-------------|      |      |
          |                |     ACK F34  |      |      |
          |                |------------->|      |      |
          |                | 482 Loop Detected F35      |
          |                |<--------------------|      |
          |                |            ACK F36  |      |
          |                |-------------------->|      |
          |                |               200 OK F37   |
          |                |<---------------------------|
          |                |          200 OK F38 |      |
          |                |-------------------->|      |
          |                |                     |ACK F39
          |                |                     |----->|
          |                |             Both way RTP Established
          |                |                     |<====>|  B3 mixes RTP
          |                |                     |      |

   In Single Line Extension a call will ring several extensions at the
   same time (forking).  The extension to answer the call becomes the
   active set.  If other extensions pick up, the call becomes a
   conference call.

   In this flow, A calls B which results in the INVITE being forked to
   three devices, B1, B2, and B3.  B3 answers the call and is connected
   to A.  B3 sends a NOTIFY containing the dialog information to B,
   which is forked to B1 and B2.  A user at B2 wishes to join the call
   and sends an INVITE with "join" semantics to B to request this.  This
   INVITE is forked and is accepted by B3 who then performs the mixing
   function.  Note that this INVITE with "join" semantics has not yet
   been defined in a WG document yet.




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   Note that if B3 is not capable of mixing the media, B3 would transfer
   both A and B2 to a conferencing bridge which would perform the
   mixing.

   The use of SUBSCRIBE and NOTIFY is defined in [11] while the dialog
   Event package is defined in [10].

   Message Details


   F1 INVITE A -> Proxy

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t= 0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy -> B1

   INVITE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t= 0 0

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   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 INVITE Proxy -> B2

   INVITE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 INVITE Proxy -> B3

   INVITE sip:UserB3@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.3
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



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   F5 100 Trying Proxy -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F6 180 Ringing B1 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112>
   Content-Length: 0


   F7 180 Ringing Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112>
   Content-Length: 0


   F8 480 Not Logged In B2 -> Proxy

   SIP/2.0 480 Not Logged In
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567

Johnston, et al.            Informational                  [Page 102]

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   To: LittleGuy <sip:UserB@there.com>;tag=314756
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F9 ACK Proxy -> B2

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314756
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F10 180 Ringing B3 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=83749.3
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.115>
   Content-Length: 0


   F11 180 Ringing B3 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.115>
   Content-Length: 0


   F12 200 OK B3 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=83749.3
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9

Johnston, et al.            Informational                  [Page 103]

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    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.115>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client3.there.com
   s=Session SDP
   c=IN IP4 110.111.112.115
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F13 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.113.115>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client3.there.com
   s=Session SDP
   c=IN IP4 110.111.112.115
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* Forking Proxy cancels pending branch B1 */

   F14 CANCEL Proxy -> B1

   CANCEL sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0

Johnston, et al.            Informational                  [Page 104]

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   F15 200 OK B1 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F16 487 Request Terminated B1 -> Proxy

   SIP/2.0 487 Request Terminated
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F17 ACK Proxy -> B1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F18 ACK A -> Proxy

   ACK sip:UserB4@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0



Johnston, et al.            Informational                  [Page 105]

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   F19 ACK Proxy -> B3

   ACK sip:UserB4@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.3
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* RTP streams are established between A and B3*/

   /* User B3 sends a NOTIFY to the group with established dialog
   information. */

   F20 NOTIFY B3 -> Proxy

   NOTIFY sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="partial" entity="sip:UserB@there.com">
   <dialog id="2393lds3204" call-id="12345600@here.com"
      local-tag="7137136" remote-tag="1234567" direction="recipient">
   <state event="2xx">confirmed</state>
   </dialog>
   </dialog-info>


   F21 NOTIFY Proxy -> B1

   NOTIFY sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com

Johnston, et al.            Informational                  [Page 106]

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   CSeq: 1 NOTIFY
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="partial" entity="sip:UserB@there.com">
   <dialog id="2393lds3204" call-id="12345600@here.com"
      local-tag="7137136" remote-tag="1234567" direction="recipient">
   <state event="2xx">confirmed</state>
   </dialog>
   </dialog-info>


   F22 Proxy -> B2

   NOTIFY sip:UserB1@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.2
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="partial" entity="sip:UserB@there.com">
   <dialog id="2393lds3204" call-id="12345600@here.com"
      local-tag="7137136" remote-tag="1234567" direction="recipient">
   <state event="2xx">confirmed</state>
   </dialog>
   </dialog-info>


   F23 NOTIFY Proxy-> B3

   NOTIFY sip:UserB1@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Event: dialog

Johnston, et al.            Informational                  [Page 107]

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   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="partial" entity="sip:UserB@there.com">
   <dialog id="2393lds3204" call-id="12345600@here.com"
      local-tag="7137136" remote-tag="1234567" direction="recipient">
   <state event="2xx">confirmed</state>
   </dialog>
   </dialog-info>


   F24 200 OK B1 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Content-Length: 0


   F25 200 OK Proxy -> B3

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Content-Length: 0


   F26 200 OK B2 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Content-Length: 0


Johnston, et al.            Informational                  [Page 108]

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   F27 200 OK B3 -> Proxy

   SIP/2.0 482 Loop Detected
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.115
   From: LittleGuy <sip:UserB@there.com>;tag=w2Hu8sj
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 8458999275@there.com
   CSeq: 1 NOTIFY
   Content-Length: 0


   /* User B2 decided to join the call.  Note that this INVITE with
   "join" semantics has not yet been defined in a WG document yet.
   Presumably it will use dialog indentifiers from the NOTIFY.  */

   F28 INVITE B2 -> Proxy

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F29 INVITE Proxy -> B1

   INVITE sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE

Johnston, et al.            Informational                  [Page 109]

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   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F30 INVITE Proxy -> B2

   INVITE sip:UserB1@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.2
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



   F31 INVITE Proxy -> B1

   INVITE sip:UserB1@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB2@110.111.112.114>
   Content-Type: application/sdp
   Content-Length: ...

Johnston, et al.            Informational                  [Page 110]

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   v=0
   o=UserB 2890844527 2890844527 IN IP4 client2.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=0 0
   m=audio 3458 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F32 100 Trying Proxy -> B2

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Content-Length: 0


   /* B1 does not have an established dialog, so it returns a 481 */

   F33 481 Dialog Does Not Exist B1 -> Proxy

   SIP/2.0 481 Dialog Does Not Exist
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=68324lk42jd
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Content-Length: 0


   F34 ACK Proxy -> B1

   ACK sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=68324lk42jd
   Call-ID: 524251233@there.com
   CSeq: 1 ACK
   Content-Length: 0


   F35 482 Loop Detected B2 -> Proxy

   SIP/2.0 482 Loop Detected

Johnston, et al.            Informational                  [Page 111]

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   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=134lkwe
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Content-Length: 0


   F36 ACK Proxy -> B2

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.2
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=134lkwe
   Call-ID: 524251233@there.com
   CSeq: 1 ACK
   Content-Length: 0


   /* B3 has an existing dialog and agrees to mix B2 into the session.
   */

   F37 200 OK User B3 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK4558a.3
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=3891908734
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.115>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client3.there.com
   s=Session SDP
   c=IN IP4 110.111.112.115
   t=0 0
   m=audio 3460 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F38 200 OK User B3 -> Proxy

   SIP/2.0 200 OK

Johnston, et al.            Informational                  [Page 112]

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   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
    ;received=110.111.112.114
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=3891908734
   Call-ID: 524251233@there.com
   CSeq: 1 INVITE
   Contact: <sip:UserB3@110.111.112.115>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client3.there.com
   s=Session SDP
   c=IN IP4 110.111.112.115
   t=0 0
   m=audio 3460 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F39 ACK B2 -> B3

   ACK sip:UserB3@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKq0qwli6ewlk
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=4524524312
   To: LittleGuy <sip:UserB@there.com>;tag=3891908734
   Call-ID: 524251233@there.com
   CSeq: 1 ACK
   Content-Length: 0























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2.12 Find-Me

         User A           Proxy           User   User   User   User
                                           B1     B2     B3     B4
          |                |              |      |      |      |
          |    INVITE F1   |              |      |      |      |
          |--------------->|   INVITE F2  |      |      |      |
          |                |------------->|      |      |      |
          |(100 Trying) F3 |              |      |      |      |
          |<---------------|180 Ringing F4|      |      |      |
          |                |<-------------|      |      |      |
          | 180 Ringing F5 |              |      |      |      |
          |<---------------|              |      |      |      |
          |                |    Timeout   |      |      |      |
          |                |              |      |      |      |
          |                |   CANCEL F6  |      |      |      |
          |                |------------->|      |      |      |
          |                |   200 OK F7  |      |      |      |
          |                |<-------------|      |      |      |
          |                |     487 F8   |      |      |      |
          |                |<-------------|      |      |      |
          |                |     ACK F9   |      |      |      |
          |                |------------->|      |      |      |
          |                |     INVITE F10      |      |      |
          |                |-------------------->|      |      |
          |                |480 Not Logged In F11|      |      |
          |                |<--------------------|      |      |
          |                |              ACK F12|      |      |
          |                |-------------------->|      |      |
          |                |          INVITE F13        |      |
          |                |--------------------------->|      |
          |                |      486 Busy Here F14     |      |
          |                |<---------------------------|      |
          |                |            ACK F15         |      |
          |                |--------------------------->|      |
          |                |              INVITE F16           |
          |                |---------------------------------->|
          |                |            180 Ringing F17        |
          |     180 F18    |<----------------------------------|
          |<---------------|               200 OK F19          |
          |   200 OK F20   |<----------------------------------|
          |<---------------|                                   |
          |     ACK F21    |                                   |
          |--------------->|                 ACK F22           |
          |                |---------------------------------->|
          |               Both way RTP Established             |
          |<==================================================>|
          |                |                  BYE F23          |
          |     BYE F24    |<----------------------------------|
          |<---------------|                                   |
          |    200 OK F25  |                                   |

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          |--------------->|                 200 OK F26        |
          |                |---------------------------------->|
          |                |                                   |

   A call to a user will attempt to locate that user by calling
   locations from a list of contacts.  The  location to answer the call
   becomes the active set, no other sets may join the call.

   While this flow shows a sequential search, the search could be
   accomplished using parallel forking, as in the previous example.

   Message Details


   F1 INVITE A -> Proxy

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t= 0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 INVITE Proxy -> B1

   INVITE sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...


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   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t= 0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F3 (100 Trying) Proxy -> A

   SIP/2.0 100 Trying
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F4 180 Ringing B1 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112>
   Content-Length: 0


   F5 180 Ringing Proxy -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112>
   Content-Length: 0


   /* B1 rings for until a configurable timer in the Proxy expires. The

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   Proxy then sends Cancel and proceeds down the list of routes. */

   F6 CANCEL Proxy -> B1

   CANCEL sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F7 200 OK B1 -> Proxy

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   From: BigGuy <sip:UserA@here.com>
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F8 487 Request Terminated B1 -> Proxy

   SIP/2.0 487 Request Terminated
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F9 ACK Proxy -> B1

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=765432
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0



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   F10 INVITE Proxy -> B2

   INVITE sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F11 480 Not Logged In B2 -> Proxy

   SIP/2.0 480 Not Logged In
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
    ;received=1.2.3.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314756
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F12 ACK Proxy -> B2

   ACK sip:UserB2@110.111.112.114 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.2
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314756
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F13 INVITE Proxy -> B3

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   INVITE sip:UserB3@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.3
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F14 486 Busy Here B3 -> Proxy

   SIP/2.0 486 Busy Here
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.3
    ;received=1.2.3.4
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F15 ACK Proxy -> B3

   ACK sip:UserB3@110.111.112.115 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.3
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7654321
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F16 INVITE Proxy -> B4

   INVITE sip:UserB4@110.111.112.116 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=83749.4

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   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F17 180 Ringing B4 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=83749.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB4@110.111.112.116>
   Content-Length: 0


   F18 180 Ringing B4 -> Proxy

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB4@110.111.112.116>
   Content-Length: 0


   F19 200 OK B4 -> Proxy


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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=83749.4
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB4@110.111.112.116>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client4.there.com
   s=Session SDP
   c=IN IP4 110.111.112.116
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F20 200 OK Proxy -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Record-Route: <sip:ss1.wcom.com;lr>
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB4@110.111.112.113.116>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client4.there.com
   s=Session SDP
   c=IN IP4 110.111.112.116
   t=0 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F21 ACK A -> Proxy

   ACK sip:UserB4@110.111.112.116 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567

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   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F22 ACK Proxy -> B4

   ACK sip:UserB4@110.111.112.116 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   Max-Forwards: 69
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7137136
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* RTP streams are established between A and B4*/

   /* User B4 Hangs Up with User A. */

   F23 BYE B4 -> Proxy

   BYE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Route: <sip:ss1.wcom.com;lr>
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=7137136
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 BYE
   Content-Length: 0


   F24 BYE Proxy -> A

   BYE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   Max-Forwards: 69
   From: LittleGuy <sip:UserB@there.com>;tag=7137136
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 BYE
   Content-Length: 0


   F25 200 OK A -> Proxy

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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
    ;received=1.2.3.4
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=7137136
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 BYE
   Content-Length: 0


   F26 200 OK Proxy -> B4

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=7137136
   To: BigGuy <sip:UserA@here.com>;tag=1234567
   Call-ID: 12345600@here.com
   CSeq: 1 BYE
   Content-Length: 0






























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2.13 Call Management (Incoming Call Screening)

         User A           Proxy          User B
          |                |              |
          |    INVITE F1   |              |
          |------------------------------>|
          | 305 Use Proxy F2              |
          |<------------------------------|
          |     ACK F3     |              |
          |------------------------------>|
          |    INVITE F4   |              |
          |--------------->|              |
          | 407 Proxy Authorization F5    |
          |<---------------|              |
          |     ACK F6     |              |
          |--------------->|              |
          |    INVITE F7   |              |
          |--------------->|              |
          | 403 Screening Failure (Terminating) F8
          |<---------------|              |
          |     ACK F9     |              |
          |--------------->|              |
          |                |              |

   User B has an incoming call screening list, User A is included on the
   list of addresses User B will not accept calls from.  User A attempts
   to call user B.  Messages F1, F2, and F3 are included to show that
   User B does not accept INVITEs that have not been screened by the
   proxy.

   Note that call screening can not be done using the From header û
   instead some form of authentication credentials must be used.


   Message Details


   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0

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   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* User B only accepts INVITEs that have been screened by the proxy
   */

   F2 305 Use Proxy B -> A

   SIP/2.0 305 Use Proxy
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=342123
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:ss1.wcom.com>
   Content-Length: 0


   F3 ACK A -> B

   ACK sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=342123
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* A retries the call through the proxy */

   F4 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0

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   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* Proxy 1 challenges User A for authentication */

   F5 407 Proxy Authorization Required Proxy 1 -> A

   SIP/2.0 407 Proxy Authorization Required
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7886765
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Proxy-Authenticate: Digest realm="wcom.com",
    domain="sip:ss1.wcom.com", nonce="ea9c8e88df84f1cec4341ae6cbe5a359",
    opaque="", stale=FALSE, algorithm=MD5
   Content-Length: 0


   F6 ACK A -> Proxy 1

   ACK sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=7886765
   Call-ID: 12345600@here.com
   CSeq: 2 ACK
   Content-Length: 0


   /* User A responds by sending an INVITE with authentication
   credentials in it.  */

   F7 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 3 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Proxy-Authorization: Digest username="UserA",
    realm="wcom.com", nonce="ae9137be1c87d175c2dd63302a0d6e0a",

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    opaque="", uri="sip:UserB@there.com",
    response="bbaec39f943bdcb3620d90afc548a45c"
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F8 403 Screening Failure (Terminating) Proxy 1 -> A

   SIP/2.0 403 Screening Failure (Terminating)
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=ffe254
   Call-ID: 12345600@here.com
   CSeq: 3 INVITE
   Error-Info: <sip:screen-failure-term-ann@annoucement.company.com>
   Content-Length: 0


   F9 ACK A -> Proxy 1

   ACK sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=ffe254
   Call-ID: 12345600@here.com
   CSeq: 3 ACK
   Content-Length: 0
















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2.14 Call Management (Outgoing Call Screening)

         User A           Proxy          User B
          |                |              |
          |    INVITE F1   |              |
          |--------------->|              |
          | 407 Proxy Authorization F2    |
          |<---------------|              |
          |     ACK F3     |              |
          |--------------->|              |
          |    INVITE F4   |              |
          |--------------->|              |
          | 403 Screening Failure (Originating) F5
          |<---------------|              |
          |     ACK F6     |              |
          |--------------->|              |
          |                |              |

   User A has an outgoing call screening list, User B is included on the
   list of addresses User A will not be able to place a call to.  User A
   attempts to call user B.

   Message Details


   F1 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* Proxy 1 challenges User A for authentication */

   F2 407 Proxy Authorization Required Proxy 1 -> A


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   SIP/2.0 407 Proxy Authorization Required
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=90210
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Proxy-Authenticate: Digest realm="wcom.com",
    domain="sip:ss1.wcom.com", nonce="ea9c8e88df84f1cec4341ae6cbe5a359",
    opaque="", stale=FALSE, algorithm=MD5
   Content-Length: 0


   F3 ACK A -> Proxy 1

   ACK sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=90210
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User A responds be sending an INVITE with authentication
   credentials in it.  */

   F4 INVITE A -> Proxy 1

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Proxy-Authorization: Digest username="UserA", realm="wcom.com",
   nonce="cb360afc54bbaec39f943bd820d9a45c", opaque="",
    uri="sip:UserB@there.com",
    response="b9d2e5bcdec9f69ab2a9b44f270285a6"
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

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   F5 403 Screening Failure (Originating) Proxy 1 -> A

   SIP/2.0 403 Screening Failure (Originating)
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=18017
   Call-ID: 12345600@here.com
   CSeq: 2 INVITE
   Error-Info: <sip:screen-failure-orig-ann@annoucement.company.com>
   Content-Length: 0


   F6 ACK A -> Proxy 1

   ACK sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=18017
   Call-ID: 12345600@here.com
   CSeq: 2 ACK
   Content-Length: 0




























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2.15      Call Park

         User A         User B     Park Server       User C
          |              |              |              |
          |   INVITE F1  |              |              |
          |------------->|              |              |
          |180 Ringing F2|              |              |
          |<-------------|              |              |
          |  200 OK F3   |              |              |
          |<-------------|              |              |
          |    ACK F4    |              |              |
          |------------->|              |              |
          |  RTP Media   |              |              |
          |<============>|              |              |
          |      User B Parks Call      |              |
          |              |   REFER Refer-To: A F5      |
          |              |------------->|              |
          |              |    202 F6    |              |
          |              |<-------------|              |
          |  INVITE F7 Replaces: B      |              |
          |<----------------------------|              |
          |          200 OK F8          |              |
          |---------------------------->|              |
          |           ACK F9            |              |
          |<----------------------------|              |
          |           RTP Music         |              |
          |<===========================>|              |
          |     BYE F10  |              |              |
          |------------->|  NOTIFY F12  |              |
          |  200 OK F11  |<-------------|              |
          |<-------------|  200 OK F13  |              |
          |              |------------->|              |
          |              |   User C picks up the call  |
          |              |              |              |
          |           INVITE Replaces: Park Server F14 |
          |<-------------------------------------------|
          |              |              |   200 F15    |
          |------------------------------------------->|
          |              |              |    ACK F16   |
          |<-------------------------------------------|
          |                  RTP Media                 |
          |<==========================================>|
          |           BYE F17           |              |
          |---------------------------->|              |
          |          200 OK F18         |              |
          |<----------------------------|              |
          |       No more RTP Music     |              |

   In this example, User A calls User B.  User B then parks the call at
   the Park Server by sending a REFER to the Park Server.  The server
   sends an INVITE to A which replaces the session between A and B.  The

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   call is accepted by A and causes A to send a BYE to B. User B
   receives notification of the successful park, and also receives the
   Call-ID in the application/sip body of the NOTIFY response.  User C
   wishes to retrieve the call, a new INVITE is sent to A which replaces
   the session with the Park Server.  User A accepts the call and sends
   a BYE to the Park Server. Note that if the Park Server did not return
   the dialog identifiers (Call-ID, To and From tags) in the NOTIFY, C
   could send a SUBSCRIBE to retrieve this information.

   Note that this call is a special case of call transfer.

   Note also that this flow could also be used for Music on Hold.


   Message Details.


   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 180 Ringing B -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


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   F3 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F4 ACK A -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 1 ACK
   Content-Length: 0


   /* User B REFERs Park Server to establish session with A which
   replaces the established session between A and B.  Note that there is
   no session established between B and the Park Server. */

   F5 REFER B -> Park Server

   REFER sip:park@server.com SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=02134
   To: Park Server <sip:park@server.com>
   Call-ID: 4802029847@there.com
   CSeq: 1 REFER
   Refer-To: <sip:UserA@here.com?Replaces=
    12345601@here.com%3Bfrom-tag%3D314159%3Bto-tag%3D1234567&Accept-
    Contact=%3Csip%3AUserA@100.101.102.103%3C%3Bonly%3Dtrue>
   Referred-By: <sip:UserB@there.com>

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   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F6 202 Accepted Park Server -> B

   SIP/2.0 202 Accepted
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: LittleGuy <sip:UserB@there.com>;tag=02134
   To: Park Server <sip:park@server.com>;tag=56323
   Call-ID: 4802029847@there.com
   CSeq: 1 REFER
   Content-Length: 0


   /* Park Server places call to User A to replace session between A and
   B */

   F7 INVITE Park Server -> A

   INVITE sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP server.com:5060
   Max-Forwards: 70
   From: <sip:park@server.com>;tag=0111
   To: <sip:UserA@100.101.102.103>
   Call-ID: a5-75-34-12-76@server.com
   CSeq: 1 INVITE
   Referred-By: <sip:UserB@there.com>
   Contact: <sip:park@server.com>
   Replaces: 12345601@here.com;from-tag=314159;to-tag=1234567
   Accept-Contact: <sip:UserA@100.101.102.103>;only=true
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=ParkServer 2890844576 2890844576 IN IP4 Park.server.com
   s=Session SDP
   c=IN IP4 50.60.70.80
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F8 200 OK A-> Park Server

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server.com:5060
   From: <sip:park@server.com>;tag=0111
   To: BigGuy <sip:UserA@100.101.102.103>;tag=098594
   Call-ID: a5-75-34-12-76@server.com

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   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   a=recvonly


   F9 ACK Park Server -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP server.com:5060
   Max-Forwards: 70
   From: <sip:park@server.com>;tag=0111
   To: BigGuy <sip:UserA@100.101.102.103>;tag=098594
   Call-ID: a5-75-34-12-76@server.com
   CSeq: 1 ACK
   Content-Length: 0


   F10 BYE A -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0


   F11 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP there.com:5060;branch=z9hG4bKnashds7
    ;received=120.121.122.123
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=314159
   Call-ID: 12345601@here.com
   CSeq: 2 BYE
   Content-Length: 0



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   /*  Park Server reports success back to B by returning all the SIP
   headers in 200 OK response  */

   F12 NOTIFY Park Server -> B

   NOTIFY sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   To: LittleGuy <sip:UserB@there.com>;tag=02134
   Max-Forwards: 70
   From: Park Server <sip:park@server.com>;tag=56323
   Call-ID: 4802029847@there.com
   CSeq: 1 NOTIFY
   Event: refer
   Content-Type: application/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server.com:5060
   From: <sip:park@server.com>;tag=0111
   To: BigGuy <sip:UserA@100.101.102.103>;tag=098594
   Call-ID: a5-75-34-12-76@server.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>


   F13 200 OK B -> Park Server

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: LittleGuy <sip:UserB@there.com>;tag=02134
   From: Park Server <sip:park@server.com>;tag=56323
   Call-ID: 4802029847@there.com
   CSeq: 1 NOTIFY
   Content-Length: 0


   /* User A is now parked at the Park Server */

   /* User C picks up the call by sending an INVITE to A which replaces
   the existing session with the Park/Park Server.  C needs to know the
   dialog information to construct the Replaces header. */


   F14 INVITE C -> A

   INVITE sip:UserA@here.com SIP/2.0
   Via: SIP/2.0/UDP anywhere.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5893461
   To: BigGuy <sip:UserA@here.com>
   Call-ID: 6485356@anywhere.com

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   CSeq: 1 INVITE
   Contact: <sip:UserC@120.121.122.123>
   Replaces: a5-75-34-12-76@server.com;from-tag=098594;to-tag=0111
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserC 2890844922 2890844922 IN IP4 client.anywhere.com
   s=Session SDP
   c=IN IP4 120.121.122.123
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F15 200 OK A -> C

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=120.121.122.123
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5893461
   To: BigGuy <sip:UserA@here.com>;tag=222
   Call-ID: 6485356@anywhere.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844527 2890844527 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F16 ACK C -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: OtherGuy <sip:UserC@anywhere.com>;tag=5893461
   To: BigGuy <sip:UserA@here.com>;tag=222
   Call-ID: 6485356@anywhere.com
   CSeq: 1 ACK
   Content-Length: 0


   /* A replaces the session to the Park Server with the new session
   with C and generates a BYE to disconnect the Park Server */

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   F17 BYE A -> Park Server

   BYE sip:park@server.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=098594
   To: <sip:park@server.com>;tag=0111
   Call-ID: a5-75-34-12-76@server.com
   CSeq: 1 BYE
   Content-Length: 0


   F18 200 OK Park Server -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=098594
   To: <sip:park@server.com>;tag=0111
   Call-ID: a5-75-34-12-76@server.com
   CSeq: 1 BYE
   Content-Length: 0






























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2.16 Call Pickup

         User A       User B             User D
          |              |                   |
          |   INVITE F1  |                   |
          |------------->|                   |
          |180 Ringing F2|                   |
          |<-------------|                   |
          |              |   SUBSCRIBE F3    |
          |              |<------------------|
          |              |     200 OK F4     |
          |              |------------------>|
          |              |     NOTIFY F5     |
          |              |------------------>|
          |              |     200 OK F6     |
          |              |<------------------|
          |            INVITE Replaces:B  F7 |
          |<---------------------------------|
          |              |     200 OK F8     |
          |--------------------------------->|
          |   CANCEL F9  |                   |
          |------------->|                   |
          |  200 OK F10  |                   |
          |<-------------|                   |
          |    487 F11   |                   |
          |<-------------|                   |
          |    ACK F12   |                   |
          |------------->|                   |
          |                    ACK F13       |
          |<---------------------------------|
          |              |     NOTIFY F14    |
          |              |------------------>|
          |              |       481 F15     |
          |              |<------------------|
          |                                  |
          |   Both way RTP Established       |
          |<================================>|
          |                     BYE F16      |
          |--------------------------------->|
          |                   200 OK F17     |
          |<---------------------------------|
          |                                  |

   User B and D are part of a work group at there.com that can pick up
   each others calls. User A calls B who does not answer.  User D wishes
   to pick up the call and sends a SUBSCRIBE to B to retrieve the dialog
   information.  D then generates an INVITE with a Replaces to A.  A
   answers the INVITE and sends a CANCEL to stop BÆs phone ringing. Note
   that the order of the CANCEL/ACK sequence in F11 through F20 is not
   significant.


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   Message Details

   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000



   F2 180 Ringing B1 -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@littleguy.there.com>
   Content-Length: 0


   /* User B decides to pick up the call */

   F3 SUBSCRIBE  D -> B

   SUBSCRIBE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP smallguy.there.com:5060;branch=z9hG4bK74bf
   Max-Forwards: 70
   From: SmallGuy <sip:UserD@there.com>;tag=8675309
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 1 INVITE

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   Contact: <sip:UserD@smallguy.there.com>
   Event: dialog
   Accept: application/dialog-info+xml
   Content-Length: 0


   F4 200 OK B -> D

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP smallguy.there.com:5060;branch=z9hG4bK74bf
    ;received=110.111.112.114
   Max-Forwards: 70
   From: SmallGuy <sip:UserD@there.com>;tag=8675309
   To: LittleGuy <sip:UserB@there.com>;tag=31451098
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 1 INVITE
   Content-Length: 0


   F5 NOTIFY  B -> D

   NOTIFY sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP littleguy.there.com:5060;branch=z9hG4bK74bf
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=31451098
   To: SmallGuy <sip:UserD@there.com>;tag=8675309
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 1 NOTIFY
   Contact: <sip:UserD@littleguy.there.com>
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="full" entity="sip:UserB@there.com">
   <dialog id="94992014524" call-id="12345600@here.com"
      local-tag="3145678" remote-tag="1234567" direction="recipient">
   <state>early</state>
   </dialog>
   </dialog-info>

   F6 200 OK  D -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP littleguy.there.com:5060;branch=z9hG4bK74bf
   From: LittleGuy <sip:UserB@there.com>;tag=31451098
   To: SmallGuy <sip:UserD@there.com>;tag=8675309
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 1 NOTIFY
   Contact: <sip:UserD@littleguy.there.com>
   Content-Length: 0

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   F7 INVITE  D -> A

   INVITE sip:UserA@here.com SIP/2.0
   Via: SIP/2.0/UDP smallguy.there.com:5060;branch=z9hG4bK74bf
   Max-Forwards: 70
   From: SmallGuy <sip:UserD@there.com>;tag=8675309
   To: BigGuy <sip:UserA@here.com>
   Call-ID: 563456212@b2.there.com
   CSeq: 1 INVITE
   Accept-Contact: <sip:UserA@100.101.102.103>;only=true
   Replaces: 12345600@here.com;to-tag=314578;from-tag=1234567
   Contact: <sip:UserB2@110.111.112.114>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890843122 2890843122 IN IP4 smallguy.there.com
   s=Session SDP
   c=IN IP4 110.111.112.114
   t=3034423619 0
   m=audio 5342 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   /* A matches the dialog information in the Replaces header and
   accepts the INVITE */

   F8 200 OK  A -> D

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP smallguy.there.com:5060;branch=z9hG4bK74bf
    ;received=110.111.112.114
   From: SmallGuy <sip:UserD@here.com>;tag=8675309
   To: BigGuy <sip:UserA@here.com>;tag=131256
   Call-ID: 563456212@b2.there.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 289084543 289084543 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=0 0
   m=audio 49172 RTP/AVP 0

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   a=rtpmap:0 PCMU/8000



   /* A stops BÆs phone from ringing by sending a CANCEL */

   F9 CANCEL A -> B

   CANCEL sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F10 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 CANCEL
   Content-Length: 0


   F11 487 Request Terminated B1 -> Proxy

   SIP/2.0 487 Request Terminated
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F12 ACK A -> B

   ACK sip:UserB1@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP ss1.wcom.com:5060;branch=z9hG4bK83749.1
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=3145678
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0

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   Content-Length: 0

   F13 ACK  D -> A

   ACK sip:UserA@100.101.102.103 SIP/2.0
   Via: SIP/2.0/UDP b2.there.com:5060;branch=z9hG4bK74bf
   Max-Forwards: 70
   From: LittleGuy <sip:UserB2@there.com>;tag=8675309
   To: BigGuy <sip:UserA@here.com>;tag=131256
   Call-ID: 563456212@b2.there.com
   CSeq: 1 ACK
   Content-Length: 0


   F14 NOTIFY  B -> D

   NOTIFY sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP littleguy.there.com:5060;branch=z9hG4bK74bf
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=31451098
   To: SmallGuy <sip:UserD@there.com>;tag=8675309
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 2 NOTIFY
   Contact: <sip:UserD@littleguy.there.com>
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="1" state="partial" entity="sip:UserB@there.com">
   <dialog id="94992014524" call-id="12345600@here.com"
      local-tag="3145678" remote-tag="1234567" direction="recipient">
   <state>terminated</state>
   </dialog>
   </dialog-info>


   F15 481 Dialog Does Not Exist  D -> B

   SIP/2.0 481 Dialog Does Not Exist
   Via: SIP/2.0/UDP littleguy.there.com:5060;branch=z9hG4bK74bf
   From: LittleGuy <sip:UserB@there.com>;tag=31451098
   To: SmallGuy <sip:UserD@there.com>;tag=8675309
   Call-ID: rt4353gs2egg@smallguy.there.com
   CSeq: 2 NOTIFY
   Contact: <sip:UserD@littleguy.there.com>
   Content-Length: 0



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   /* RTP streams are established between A and D.    User A Hangs Up
   with User B. */

   F16 BYE A -> Proxy

   BYE sip:UserD@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   To: LittleGuy <sip:UserB2@there.com>;tag=8675309
   From: BigGuy <sip:UserA@here.com>;tag=131256
   Call-ID: 563456212@b2.there.com
   CSeq: 2 BYE
   Content-Length: 0


   F17 200 OK D -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   To: LittleGuy <sip:UserB2@there.com>;tag=8675309
   From: BigGuy <sip:UserA@here.com>;tag=131256
   Call-ID: 563456212@b2.there.com
   CSeq:  2 BYE
   Content-Length: 0



























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2.17 Automatic Redial


         User A           User B
          |                |
          |    INVITE F1   |
          |--------------->|
          |486 Busy Here F2|
          |<---------------|  User B is busy
          |     ACK F3     |
          |--------------->|
          |  SUBSCRIBE F4  |
          |--------------->|
          |   200 OK F5    |
          |<---------------|
          |    NOTIFY F6   |
          |<---------------|
          |   200 OK F7    |
          |--------------->|
          |                |
          |    NOTIFY F8   |  User B is now available
          |<---------------|
          |   200 OK F9    |
          |--------------->|
          |                |
          |   INVITE F10   |
          |--------------->|   Session setup successfully
          | 180 Ringing F11|
          |<---------------|
          |   200 OK F12   |
          |<---------------|
          |     ACK F13    |
          |--------------->|
          |  Media Session |
          |<==============>|
          |                |

   User B is initially busy when User A calls.  User A subscribes to
   User BÆs call state using a SUBSCRIBE F4.  User B sends a NOTIFY F8
   when User B is available.  A is alerted, then User A sends an INVITE
   to B to establish the session.


   Message Details


   F1 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567

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   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F2 486 Busy Here

   SIP/2.0 486 Busy Here
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=982039i4
   Call-ID: 12345600@here.com
   CSeq: 1 INVITE
   Content-Length: 0


   F3 ACK A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=1234567
   To: LittleGuy <sip:UserB@there.com>;tag=982039i4
   Call-ID: 12345600@here.com
   CSeq: 1 ACK
   Content-Length: 0


   F4 SUBSCRIBE A -> B

   SUBSCRIBE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=837348234
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 4524526232@here.com
   CSeq: 1 SUBSCRIBE
   Contact: UserA@100.101.102.103
   Event: dialog
   Accept: application/dialog-info+xml
   Content-Length: 0

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   F5 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   From: BigGuy <sip:UserA@here.com>;tag=837348234
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: 4524526232@here.com
   CSeq: 1 SUBSCRIBE
   Contact: UserB@110.111.112.113
   Content-Length: 0


   F6 NOTIFY B -> A

   NOTIFY sip:UserA@here.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=341123
   To: BigGuy <sip:UserA@here.com>;tag=837348234
   Call-ID: 4524526232@here.com
   CSeq: 1 NOTIFY
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="full" entity="sip:UserB@there.com">
   <dialog id="562623442g3">
   <state>confirmed</state>
   </dialog>
   </dialog-info>


   F7 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   From: LittleGuy <sip:UserB@there.com>;tag=341123
   To: BigGuy <sip:UserA@here.com>;tag=837348234
   Call-ID: 4524526232@here.com
   CSeq: 1 NOTIFY
   Content-Length: 0


   /* User B is now available */

   F8 NOTIFY B -> A

   NOTIFY sip:UserA@here.com SIP/2.0

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   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: LittleGuy <sip:UserB@there.com>;tag=341123
   To: BigGuy <sip:UserA@here.com>;tag=837348234
   Call-ID: 4524526232@here.com
   CSeq: 2 NOTIFY
   Event: dialog
   Content-Type: application/dialog-info+xml
   Content-Length: ...

   <?xml version="1.0"?>
   <dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
      version="0" state="full" entity="sip:UserB@there.com">
   <dialog id="562623442g3">
   <state>terminated</state>
   </dialog>
   </dialog-info>


   F9 200 OK A -> B

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   From: LittleGuy <sip:UserB@there.com>;tag=341123
   To: BigGuy <sip:UserA@here.com>;tag=837348234
   Call-ID: 4524526232@here.com
   CSeq: 2 NOTIFY
   Content-Length: 0


   Message Details


   F10 INVITE A -> B

   INVITE sip:UserB@there.com SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=f23fkg14k
   To: LittleGuy <sip:UserB@there.com>
   Call-ID: aoij4i9okitr@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserA@100.101.102.103>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserA 2890844526 2890844526 IN IP4 client.here.com
   s=Session SDP
   c=IN IP4 100.101.102.103
   t=3034423619 0
   m=audio 49170 RTP/AVP 0

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   a=rtpmap:0 PCMU/8000


   F11 180 Ringing B -> A

   SIP/2.0 180 Ringing
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=f23fkg14k
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: aoij4i9okitr@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Length: 0


   F12 200 OK B -> A

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
    ;received=100.101.102.103
   From: BigGuy <sip:UserA@here.com>;tag=f23fkg14k
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: aoij4i9okitr@here.com
   CSeq: 1 INVITE
   Contact: <sip:UserB@110.111.112.113>
   Content-Type: application/sdp
   Content-Length: ...

   v=0
   o=UserB 2890844527 2890844527 IN IP4 client.there.com
   s=Session SDP
   c=IN IP4 110.111.112.113
   t=3034423619 0
   m=audio 3456 RTP/AVP 0
   a=rtpmap:0 PCMU/8000


   F13 ACK A -> B

   ACK sip:UserB@110.111.112.113 SIP/2.0
   Via: SIP/2.0/UDP here.com:5060;branch=z9hG4bK74bf9
   Max-Forwards: 70
   From: BigGuy <sip:UserA@here.com>;tag=f23fkg14k
   To: LittleGuy <sip:UserB@there.com>;tag=23431
   Call-ID: aoij4i9okitr@here.com
   CSeq: 1 ACK
   Content-Length: 0





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3  Security Considerations

   Since many of the examples in this document involve SIP call control,
   either peer-to-peer or 3pcc, the security considerations in the
   Multiparty Framework document [12] apply.
















































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4  IANA Considerations


   None.

















































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5  Acknowledgements


   The Automatic Call Back call flow is based on a call flow by Adam
   Roach.  The Single Line Extension call flow is based on a call flow
   by Jonathan Rosenberg.  The authors wish to thank the following
   individuals for their assistance and review of this call flows
   document: Rohan Mahey, Jonathan Rosenberg, Hemant Agrawal, Henry
   Sinnreich, Dean Willis, David Devanatham, Joe Pizzimenti,  Matt
   Cannon, John Hearty, the whole MCI WorldCom IPOP Design team, Scott
   Orton, Greg Osterhout, Pat Sollee, Doug Weisenberg, Danny Mistry,
   Steve McKinnon, and Denise Ingram, Denise Caballero, Tom Redman, Ilya
   Slain, Pat Sollee, John Truetken, and others from MCI WorldCom, 3Com,
   Cisco, Lucent and Nortel.







































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6  References

   [1] S. Bradner, "The Internet Standards Process -- Revision 3", BCP
       9, RFC 2026, October 1996.

   [2] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston,
       J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
       Session Initiation Protocol", Internet Engineering Task Force,
       February 2002, Work in Progress.

   [3] A. Johnston, S. Donovan, R. Sparks, C. Cunningham, D. Willis, J.
       Rosenberg, and H. Schulzrinne, "SIP Call Flow Examples", Internet
       Draft, Internet Engineering Task Force, February 2002, Work in
       Progress.

   [4] R. Sparks, "The Refer Method", Internet Draft,
       Internet Engineering Task Force, October 2001,
       Work in Progress.

   [5] B. Biggs, R. Dean, and R. Mahy, "The SIP Replaces header,"
       Internet Draft, Internet Engineering Task Force, January 2002,
       Work in Progress.

   [6] H. Schulzrinne and J. Rosenberg, "SIP Caller Preferences and
       Callee Capabilities," Internet Engineering Task Force, June 2001,
       Work in Progress.

   [7] J. Rosenberg, J. Peterson, H. Schulzrinne, and G. Camarillo,
       "Third Party Call Control in SIP," Internet Engineering Task
       Force, March 2001, Work in Progress.

   [8] J. Rosenberg and H. Schulzrinne, "An Offer/Answer Model with
       SDP," Internet Engineering Task Force, October 2001, Work in
       progress.

   [9] R. Sparks, "SIP Call Control û Transfer," Internet Engineering
       Task Force, July 2001, Work in progress.

   [10] J. Rosenberg and H. Schulzrinne, "A Session Initiation Protocol
       (SIP) Event Package for Dialog State," Internet Engineering Task
        Force, June 2002, Work in progress.

   [11] A. Roach, "SIP-Specific Event Notification," Internet
        Engineering Task Force, February 2002, Work in progress.

   [12] R. Mahy, B. Campbell, A. Johnston, D. Petrie, J. Rosenberg, and
        R. Sparks, "A Multi-party Application Framework for SIP,"
        Internet Engineering Task Force, February 2002, Work in
        progress.




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7  Author's Addresses


   Alan Johnston               Email: alan.johnston@wcom.com
                               sip:alan.johnston@siptest.wcom.com

   Robert Sparks               Email: rsparks@dynamicsoft.com

   Chris Cunningham            Email: ccunningham@dynamicsoft.com

   Steve Donovan               Email: sdonovan@dynamicsoft.com

   Kevin Summers               Email: kevin.summers@sonusnet.com



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