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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5589

SIPPING WG                                                     R. Sparks
Internet-Draft                                          Estacado Systems
Intended status: BCP                                    A. Johnston, Ed.
Expires: March 7, 2009                                             Avaya
                                                               D. Petrie
                                                               SIPez LLC
                                                       September 3, 2008


          Session Initiation Protocol Call Control - Transfer
                   draft-ietf-sipping-cc-transfer-10

Status of this Memo

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   This Internet-Draft will expire on March 7, 2009.

Abstract

   This document describes providing Call Transfer capabilities in the
   Session Initiation Protocol (SIP).  SIP extensions such as REFER and
   Replaces are used to provide a number of transfer services including
   blind transfer, consultative transfer, and attended transfer.  This
   work is part of the SIP multiparty call control framework.







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Table of Contents

   1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Actors and Roles . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Using REFER to achieve Call Transfer . . . . . . . . . . . . .  5
   6.  Basic Transfer . . . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  Successful Transfer  . . . . . . . . . . . . . . . . . . .  7
     6.2.  Transfer with Dialog Reuse . . . . . . . . . . . . . . . . 11
     6.3.  Failed Transfer  . . . . . . . . . . . . . . . . . . . . . 15
       6.3.1.  Target Busy  . . . . . . . . . . . . . . . . . . . . . 15
       6.3.2.  Transfer Target does not answer  . . . . . . . . . . . 17
   7.  Transfer with Consultation Hold  . . . . . . . . . . . . . . . 18
     7.1.  Exposing transfer target . . . . . . . . . . . . . . . . . 18
     7.2.  Protecting transfer target . . . . . . . . . . . . . . . . 19
     7.3.  Attended Transfer  . . . . . . . . . . . . . . . . . . . . 24
     7.4.  Recovery when one party does not support REFER . . . . . . 27
     7.5.  Attended transfer when Contact URI is not known to
           route to a unique user agent.  . . . . . . . . . . . . . . 28
     7.6.  Semi-Attended Transfer . . . . . . . . . . . . . . . . . . 35
     7.7.  Attended Transfer Fallback to Basic Transfer . . . . . . . 40
   8.  Transfer with Referred-By  . . . . . . . . . . . . . . . . . . 42
   9.  Transfer as an Ad-Hoc Conference . . . . . . . . . . . . . . . 48
   10. Transfer with multiple parties . . . . . . . . . . . . . . . . 51
   11. Gateway Transfer Issues  . . . . . . . . . . . . . . . . . . . 53
     11.1. Coerce Gateway Hairpins to the Same Gateway  . . . . . . . 53
     11.2. Consultative Turned Blind Gateway Glare  . . . . . . . . . 54
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 54
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 54
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 55
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 55
     15.2. Informative References . . . . . . . . . . . . . . . . . . 56
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56
   Intellectual Property and Copyright Statements . . . . . . . . . . 58















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1.  Overview

   This document describes providing Call Transfer capabilities and
   requirements in SIP [RFC3261].  This work is part of the Multiparty
   Call Control Framework [I-D.ietf-sipping-cc-framework].

   The mechanisms discussed here are most closely related to traditional
   basic and consultation hold transfers.

   This document details the use of REFER method [RFC3515] and Replaces
   [RFC3891] header field to achieve call transfer.

   A user agent that fully supports the transfer mechanisms described in
   this document supports REFER[RFC3515] and Replaces[RFC3891] in
   addition to RFC 3261 [RFC3261].  A user agent should use a Contact
   URI which meets the requirements in Section 8.1.1.8 of RFC 3261.  A
   compliant user agent supports the Target-Dialog header field
   [RFC4538].


2.  Actors and Roles

   There are three actors in a given transfer event, each playing one of
   the following roles:

     Transferee -      the party being transferred to the Transfer
                       Target.

     Transferor -      the party initiating the transfer

     Transfer Target - the new party being introduced into a
                       call with the Transferee.

   The following roles are used to describe transfer requirements and
   scenarios:
















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    Originator -  wishes to place a call to the Recipient. This
                  actor is the source of the first INVITE in a
                  session, to either a Facilitator or a Screener.

    Facilitator - receives a call or out-of-band request from the
                  Originator, establishes a call to the Recipient
                  through the Screener, and connects the
                  Originator to the Recipient.  Typically, a Facilitator
                  acts on behalf of the Originator.

    Screener -    receives a call ultimately intended for the
                  Recipient and transfers the calling party to
                  the Recipient if appropriate. Typically, a Screener
                  acts on behalf of the Recipient.


    Recipient -   the party the Originator is ultimately
                  connected to.


3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].


4.  Requirements

   1.  Any party in a SIP session MUST be able to transfer any other
       party in that session at any point in that session.
   2.  The Transferor and the Transferee MUST NOT be removed from a
       session as part of a transfer transaction.

             At first glance, requirement 2 may seem to indicate
             that the user experience in a transfer must be
             significantly different from what a current PBX or
             Centrex user expects. As the call-flows in this
             document show, this is not the case. A client MAY
             preserve the current experience. In fact, without
             this requirement, some forms of the current
             experience (ringback on transfer failure
             for instance) will be lost.

   3.  The Transferor MUST know whether or not the transfer was
       successful.




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   4.  The Transferee MUST be able to replace an existing dialog with a
       new dialog.
   5.  The Transferor and Transferee SHOULD indicate their support for
       the primitives required to achieve transfer.
   6.  The Transferor SHOULD provide the Transfer Target and Transferee
       with information about the nature and progress of the transfer
       operation being attempted.


             To meet this requirement, the transfer operation can
             be modeled as an ad-hoc conference between three
             parties, as discussed in Section 8.


5.  Using REFER to achieve Call Transfer

   A REFER [RFC3515] can be issued by the Transferor to cause the
   Transferee to issue an INVITE to the Transfer-Target.  Note that a
   successful REFER transaction does not terminate the session between
   the Transferor and the Transferee.  If those parties wish to
   terminate their session, they must do so with a subsequent BYE
   request.  The media negotiated between the transferee and the
   transfer target is not affected by the media that had been negotiated
   between the transferor and the transferee.  In particular, the INVITE
   issued by the Transferee will have the same SDP body it would have if
   he Transferee had initiated that INVITE on its own.  Further, the
   disposition of the media streams between the Transferor and the
   Transferee is not altered by the REFER method.

   Agents may alter a session's media through additional signaling.  For
   example, they may make use of the SIP hold re-INVITE [RFC3261] or
   conferencing extensions described in the conferencing framework
   [RFC4353].

   To perform the transfer, the transferor and transferee could reuse an
   existing dialog established by an INVITE to send the REFER.  This
   would result in a single dialog shared by two uses - an invite usage
   and a subscription usage.  The call flows for this are shown in
   detail in Section 6.2.  However, the approach described in this
   document is to avoid dialog reuse.  The issues and difficulties
   associated with dialog reuse are described in
   [I-D.ietf-sipping-dialogusage].

   Motivations for reusing the existing dialog include:
   1.  There was no way to ensure that a REFER on a new dialog would
       reach the particular endpoint involved in a transfer.  Many
       factors, including details of implementations and changes in
       proxy routing between an INVITE and a REFER could cause the REFER



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       to be sent to the wrong place.  Sending the REFER down the
       existing dialog ensured it got to the endpoint we were already
       talking to.
   2.  It was unclear how to associate an existing invite usage with a
       REFER arriving on a new dialog, where it was completely obvious
       what the association was when the REFER came on the invite
       usage's dialog.
   3.  There were concerns with authorizing out-of-dialog REFERs.  The
       authorization policy for REFER in most implementations piggybacks
       on the authorization policy for INVITE (which is, in most cases,
       based simply on "I placed or answered this call").

   GRUUs [I-D.ietf-sip-gruu] can be used to address problem 1.  Problem
   2 can be addressed using the Target-Dialog header field defined in
   [RFC4538].  In the immediate term, this solution to problem 2 allows
   the existing REFER authorization policy to be reused.

   As a result, if the Transferee supports the target-dialog extension
   and the Transferor knows the Contact URI is routable outside the
   dialog, the REFER SHOULD be sent in a new dialog.  If the nature of
   the Contact URI is not known or if support for the target-dialog
   extension is not known, the REFER should be sent inside the existing
   dialog.  A Transferee must be prepared to receive a REFER either
   inside or outside a dialog.  One way that a Transferor could know
   that a Contact URI is routable outside a dialog is by validation
   (e.g. sending an OPTIONS and receiving a response) or if it satisfies
   the properties described in the GRUU specification
   [I-D.ietf-sip-gruu].

   In most of the following examples, the Transferor is in the
   atlanta.example.com domain, the Transferee is in the
   biloxi.example.com, and the Transfer Target is in the
   chicago.example.com domain.


6.  Basic Transfer

   Basic Transfer consists of the Transferor providing the Transfer
   Target's contact to the Transferee.  The Transferee attempts to
   establish a session using that contact and reports the results of
   that attempt to the Transferor.  The signaling relationship between
   the Transferor and Transferee is not terminated, so the call is
   recoverable if the Transfer Target cannot be reached.  Note that the
   Transfer Target's contact information has been exposed to the
   Transferee.  The provided contact can be used to make new calls in
   the future.

   The participants in a basic transfer should indicate support for the



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   REFER and NOTIFY methods in Allow header fields in INVITE, 200 OK to
   INVITE, and OPTIONS messages.  Participants should also indicate
   support for Target-Dialog in the Supported header field.

   The diagrams below show the first line of each message.  The first
   column of the figure shows the dialog used in that particular
   message.  In these diagrams, media is managed through re-INVITE
   holds, but other mechanisms (mixing multiple media streams at the UA
   or using the conferencing extensions for example) are valid.
   Selected message details are shown labeled as message F1, F2, etc.

   Each of the flows below shows the dialog between the Transferor and
   the Transferee remaining connected (on hold) during the REFER
   process.  While this provides the greatest flexibility for recovery
   from failure, it is not necessary.  If the Transferor's agent does
   not wish to participate in the remainder of the REFER process and has
   no intention of assisting with recovery from transfer failure, it
   could emit a BYE to the Transferee as soon as the REFER transaction
   completes.  This flow is sometimes known as "unattended transfer" or
   "blind transfer".

   Figure 1 shows transfer when the Transferee utilizes a GRUU and
   supports the target-dialog extension and indicates this to the
   Transferor.  As a result, the Transferor sends the REFER outside the
   INVITE dialog.  The Transferee is able to match this REFER to the
   existing dialog using the Target-Dialog header field in the refer
   which references the existing dialog.

6.1.  Successful Transfer






















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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |          INVITE F1 |                    |
           dialog1 |<-------------------|                    |
                   |          200 OK F2 |                    |
           dialog1 |------------------->|                    |
                   |            ACK     |                    |
           dialog1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |
                   |  REFER F3 (Target-Dialog:1)             |
           dialog2 |------------------->|                    |
                   |  202 Accepted      |                    |
           dialog2 |<-------------------|                    |
                   | NOTIFY (100 Trying) F4                  |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |                    |  INVITE F5         |
           dialog3 |                    |------------------->|
                   |                    |  200 OK            |
           dialog3 |                    |<-------------------|
                   |                    |  ACK               |
           dialog3 |                    |------------------->|
                   |  NOTIFY (200 OK) F6|                    |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |  BYE               |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |                    |             BYE    |
           dialog3 |                    |<-------------------|
                   |                    |             200 OK |
           dialog3 |                    |------------------->|

   Figure 1.  Basic Transfer Call Flow.


   F1 INVITE Transferee -> Transferor

   INVITE sips:transferor@atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432



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   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transferor -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   To: <sips:transferor@atlanta.example.com>;tag=31kdl4i3k
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Type: application/sdp
   Content-Length: ...


   F3 REFER Transferor -> Transferee

   REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKna9
   Max-Forwards: 70
   To: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Require: tdialog
   Refer-To: <sips:transfertarget@chicago.example.com>
   Target-Dialog: 090459243588173445;local-tag=7553452
    ;remote-tag=31kdl4i3k
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Length: 0


   F4 NOTIFY Transferee -> Transferor




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   NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=1928301774
   From: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
    ;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 73 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, tdialog
   Event: refer
   Subscription-State: active;expires=60
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 100 Trying


   F5 INVITE Transferee -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas41234
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=j3kso3iqhq
   Call-ID: 90422f3sd23m4g56832034
   CSeq: 521 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   Content-Type: application/sdp
   Content-Length: ...


   F6 NOTIFY Transferee -> Transferor

   NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=1928301774
   From: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
    ;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 74 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, tdialog
   Event: refer
   Subscription-State: terminated;reason=noresource



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   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


6.2.  Transfer with Dialog Reuse

   In this scenario, the Transferor does not know the properties of the
   Transferee's Contact URI or does not know that the Transferee
   supports the Target-Dialog header field.  As a result, the REFER is
   sent inside the INVITE dialog.







































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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |         INVITE F1  |                    |
           dialog1 |<-------------------|                    |
                   |         200 OK F2  |                    |
           dialog1 |------------------->|                    |
                   |            ACK     |                    |
           dialog1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |
                   |  REFER F3          |                    |
           dialog1 |------------------->|                    |
                   |  202 Accepted      |                    |
           dialog1 |<-------------------|                    |
                   | NOTIFY (100 Trying) F4                  |
           dialog1 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog1 |------------------->|                    |
                   |                    |  INVITE F5         |
           dialog2 |                    |------------------->|
                   |                    |  200 OK            |
           dialog2 |                    |<-------------------|
                   |                    |  ACK               |
           dialog2 |                    |------------------->|
                   |  NOTIFY (200 OK) F6|                    |
           dialog1 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog1 |------------------->|                    |
                   |  BYE               |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |                    |             BYE    |
           dialog2 |                    |<-------------------|
                   |                    |             200 OK |
           dialog2 |                    |------------------->|

   Figure 2.  Transfer with Dialog Reuse.


   F1 INVITE Transferee -> Transferor

   INVITE sips:transferor@atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432



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   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferee@192.0.2.4>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transferor -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   To: <sips:transferor@atlanta.example.com>;tag=31kdl4i3k
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Type: application/sdp
   Content-Length: ...


   F3 REFER Transferor -> Transferee

   REFER sips:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKna9
   Max-Forwards: 70
   To: <sips:transferee@biloxi.example.com>;tag=7553452
   From: <sips:transferor@atlanta.example.com>;tag=31kdl4i3k
   Call-ID: 090459243588173445
   CSeq: 314159 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Refer-To: <sips:transfertarget@chicago.example.com>
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Length: 0


   F4 NOTIFY Transferee -> Transferor

   NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70



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   To: <sips:transferor@atlanta.example.com>;tag=31kdl4i3k
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29888 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer
   Subscription-State: active;expires=60
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 100 Trying


   F5 INVITE Transferee -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas41234
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=j3kso3iqhq
   Call-ID: 90422f3sd23m4g56832034
   CSeq: 521 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferee@192.0.2.4>
   Content-Type: application/sdp
   Content-Length: ...


   F6 NOTIFY Transferee -> Transferor

   NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=31kdl4i3k
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29889 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK




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6.3.  Failed Transfer

   This section shows examples of failed transfer attempts.  After the
   transfer failure occurs, the Transferor takes the Transferee off hold
   and resumes the session.

6.3.1.  Target Busy












































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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
                   |            INVITE  |                    |
           dialog1 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog1 |------------------->|                    |
                   |            ACK     |                    |
           dialog1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |
                   |  REFER (Target-Dialog:1)                |
           dialog2 |------------------->|                    |
                   |  202 Accepted      |                    |
           dialog2 |<-------------------|                    |
                   | NOTIFY (100 Trying)|                    |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |                    |  INVITE            |
           dialog3 |                    |------------------->|
                   |                    |  486 Busy Here     |
           dialog3 |                    |<-------------------|
                   |                    |  ACK               |
           dialog3 |                    |------------------->|
                   | NOTIFY (486 Busy Here)                  |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |  INVITE (unhold)   |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |
                   |  BYE               |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |

   Figure 3.  Failed Transfer - Target Busy






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6.3.2.  Transfer Target does not answer


              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |            INVITE  |                    |
           dialog1 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog1 |------------------->|                    |
                   |            ACK     |                    |
           dialog1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |
                   |  REFER             |                    |
           dialog2 |------------------->|                    |
                   |  202 Accepted      |                    |
           dialog2 |<-------------------|                    |
                   | NOTIFY (100 Trying)|                    |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |                    |  INVITE            |
           dialog3 |                    |------------------->|
                   |                    |  180 Ringing       |
           dialog3 |                    |<-------------------|
                   |          (Transferee gets tired of waiting)
                   |                    |  CANCEL            |
           dialog3 |                    |------------------->|
                   |                    |  200 OK (CANCEL)   |
           dialog3 |                    |<-------------------|
                   |                 487 Request Cancelled (INVITE)
           dialog3 |                    |<-------------------|
                   |                    |  ACK               |
           dialog3 |                    |------------------->|
                   |    NOTIFY (487 Request Cancelled)       |
           dialog2 |<-------------------|                    |
                   |            200 OK  |                    |
           dialog2 |------------------->|                    |
                   |  INVITE (unhold)   |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |
                   |  ACK               |                    |
           dialog1 |------------------->|                    |



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                   |  BYE               |                    |
           dialog1 |------------------->|                    |
                   |  200 OK            |                    |
           dialog1 |<-------------------|                    |

   Figure 4.  Failed Transfer - Target Does Not Answer.


7.  Transfer with Consultation Hold

   Transfer with Consultation Hold involves a session between the
   transferor and the transfer target before the transfer actually takes
   place.  This is implemented with SIP Hold and Transfer as described
   above.

   A nice feature is for the transferor to let the target know that the
   session relates to an intended transfer.  Since many UAs render the
   display name in the From header field to the user, a consultation
   INVITE could contain a string such as "Incoming consultation from
   Transferor with intent to transfer Transferee", where the display
   names of the transferor and transferee are included in the string.

7.1.  Exposing transfer target

   The transferor places the transferee on hold, establishes a call with
   the transfer target to alert them to the impending transfer,
   terminates the connection with the transfer target, then proceeds
   with transfer as above.  This variation can be used to provide an
   experience similar to that expected by current PBX and Centrex users.

   To (hopefully) improve clarity, non-REFER transactions have been
   collapsed into one indicator with the arrow showing the direction of
   the request.


















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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
           dialog1 | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
           dialog1 | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
           dialog2 | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
           dialog2 | BYE/200 OK         |                    |
                   |---------------------------------------->|
           dialog3 | REFER              |                    |
                   |------------------->|                    |
           dialog3 | 202 Accepted       |                    |
                   |<-------------------|                    |
           dialog3 | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
           dialog3 |            200 OK  |                    |
                   |------------------->|                    |
           dialog4 |                    |  INVITE/200 OK/ACK |
                   |                    |------------------->|
           dialog3 | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
           dialog3 |            200 OK  |                    |
                   |------------------->|                    |
           dialog1 | BYE/200 OK         |                    |
                   |------------------->|                    |
           dialog4 |                    |         BYE/200 OK |
                   |                    |<-------------------|

   Figure 5.  Transfer with Consultation Hold - Exposing Transfer
   Target.

7.2.  Protecting transfer target

   The transferor places the transferee on hold, establishes a call with
   the transfer target and then reverses their roles, transferring the
   original transfer target to the original transferee.  This has the
   advantage of hiding information about the original transfer target
   from the original transferee.  On the other hand, the Transferee's
   experience is different that in current systems.  The Transferee is
   effectively "called back" by the Transfer Target.

   One of the problems with this simplest implementation of a target
   protecting transfer is that the transferee is receiving a new call
   from the transfer-target.  Unless the transferee's agent has a
   reliable way to associate this new call with the call it already has
   with the transferor, it will have to alert the new call on another



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   appearance.  If this, or some other call-waiting-like UI were not
   available, the transferee might be stuck returning a Busy-Here to the
   transfer target, effectively preventing the transfer.  There are many
   ways that that correlation could be provided.  The dialog parameters
   could be provided directly as header parameters in the Refer-To: URI
   for example.  The Replaces mechanism [RFC3891] uses this approach and
   solves this problem nicely.

   For the flow below, dialog1 means dialog identifier 1, and consists
   of the parameters of the Replaces header for dialog 1.  In [RFC3891]
   this is the Call-ID, To-tag and From-tag.

   Note that the transferee's agent emits a BYE to the transferor's
   agent as an immediate consequence of processing the Replaces header.

   The Transferor knows that both the Transferee and the Transfer Target
   support the Replaces header from the Supported: replaces header
   contained in the 200 OK responses from both.

   In this scenario, the Transferee utilizes a GRUU as a Contact URI for
   reasons discussed in Section 6.3.

   Note that the conventions used in the SIP Torture Test Messages
   [RFC4475] document are reused, specifically the <allOneLine> tag.



























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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
         dialog1   | INVITE/200 OK/ACK F1 F2                 |
                   |<-------------------|                    |
         dialog1   | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
         dialog2   | INVITE/200 OK/ACK F3 F4                 |
                   |---------------------------------------->|
         dialog2   | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
         dialog3   | REFER (Target-Dialog:2,                 |
                   |  Refer-To:sips:Transferee?Replaces=1) F5|
                   |---------------------------------------->|
         dialog3   | 202 Accepted       |                    |
                   |<----------------------------------------|
         dialog3   | NOTIFY (100 Trying)|                    |
                   |<----------------------------------------|
         dialog3   |                    |            200 OK  |
                   |---------------------------------------->|
         dialog4   |         INVITE (Replaces:dialog1)/200 OK/ACK F6
                   |                    |<-------------------|
         dialog1   | BYE/200 OK         |                    |
                   |<-------------------|                    |
         dialog3   | NOTIFY (200 OK)    |                    |
                   |<----------------------------------------|
         dialog3   |                    |            200 OK  |
                   |---------------------------------------->|
         dialog2   | BYE/200 OK         |                    |
                   |---------------------------------------->|
                   |              (transferee and target converse)
         dialog4   |                    |  BYE/200 OK        |
                   |                    |------------------->|

   Figure 6.  Transfer Protecting Transfer Target.


   F1 INVITE Transferee -> Transferor

   INVITE sips:transferor@atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu



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   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transferor -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   To: <sips:transferor@atlanta.example.com>;tag=31431
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Type: application/sdp
   Content-Length: ...


   F3 INVITE Transferor -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 592435881734450904
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Require: replaces
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=384i32lw3>
   Content-Type: application/sdp
   Content-Length: ...


   F4 200 OK Transfer Target -> Transferor

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
    ;received=192.0.2.1
   To: <sips:transfertarget@chicago.example.com>;tag=9m2n3wq
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 592435881734450904
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog



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   Contact: <sips:482n4z24kdg@chicago.example.com;gr=8594958>
   Content-Type: application/sdp
   Content-Length: ...


   F5 REFER Transferor -> Transfer Target

   REFER sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sips:482n4z24kdg@chicago.example.com;gr=8594958>
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Require: tdialog
   <allOneLine>
   Refer-To: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha
   ?Replaces=090459243588173445%3Bto-tag%3D7553452%3Bfrom-tag%3D31431>
   </allOneLine>
   Target-Dialog: 592435881734450904;local-tag=9m2n3wq
    ;remote-tag=763231
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Length: 0


   F6 INVITE Transfer Target -> Transferee

   INVITE sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
   Via: SIP/2.0/TLS client.chicago.example.com;branch=z9hG4bKnaslu84
   Max-Forwards: 70
   To: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   From: <sips:transfertarget@chicago.example.com>;tag=341234
   Call-ID: kmzwdle3dl3d08
   CSeq: 41 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Contact: <sips:482n4z24kdg@chicago.example.com;gr=8594958>
   Replaces: 090459243588173445;to-tag=7553452;from-tag=31431
   Content-Type: application/sdp
   Content-Length: ...









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7.3.  Attended Transfer

   The transferor places the transferee on hold, establishes a call with
   the transfer target to alert them to the impending transfer, places
   the target on hold, then proceeds with transfer using an escaped
   Replaces header field in the Refer-To header.  This is another common
   service expected by current PBX and Centrex users.

   The Contact URI of the Transfer Target SHOULD be used by the
   Transferor as the Refer-To URI, unless the URI is suspected or known
   to not be routable outside the dialog.  Otherwise, the Address of
   Record (AOR) of the Transfer Target should be used.  That is, the
   same URI that the Transferor used to establish the session with the
   Transfer Target should be used.  In case the triggered INVITE is
   routed to a different User Agent than the Transfer Target, the
   Require: replaces header field should be used in the triggered
   INVITE.  (This is to prevent an incorrect User Agent which does not
   support Replaces from ignoring the Replaces and answering the INVITE
   without a dialog match.)

   It is possible that proxy/service routing may prevent the triggered
   INVITE from reaching the same User Agent.  If this occurs, the
   triggered invite will fail with a timout, 403, 404, etc error.  The
   Transferee MAY then retry the transfer with the Refer-To URI set to
   the Contact URI.


























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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK F1 F2                 |
                   |<-------------------|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE/200 OK/ACK F3 F4                 |
                   |---------------------------------------->|
          dialog2  | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
          dialog3  | REFER (Target-Dialog:1,                 |
                   |  Refer-To:sips:TransferTarget?Replaces=2) F5
                   |------------------->|                    |
          dialog3  | 202 Accepted       |                    |
                   |<-------------------|                    |
          dialog3  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog4  |        INVITE (Replaces:dialog2)/200 OK/ACK F6
                   |                    |------------------->|
          dialog2  | BYE/200 OK         |                    |
                   |<----------------------------------------|
          dialog3  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog4  |                    |         BYE/200 OK |
                   |                    |<-------------------|

   Figure 7.  Attended Transfer Call Flow.


   F1 INVITE Transferee -> Transferor

   INVITE sips:transferor@atlanta.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>



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


   F2 200 OK Transferor -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   To: <sips:transferor@atlanta.example.com>;tag=31431
   From: <sips:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu, tdialog
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Type: application/sdp
   Content-Length: ...


   F3 INVITE Transferor -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 592435881734450904
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Require: replaces
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=384i32lw3>
   Content-Type: application/sdp
   Content-Length: ...


   F4 200 OK Transfer Target -> Transferor

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
    ;received=192.0.2.1
   To: <sips:transfertarget@chicago.example.com>;tag=9m2n3wq
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 592435881734450904
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces, gruu
   Contact: <sips:482n4z24kdg@chicago.example.com;gr=8594958>



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


   F5 REFER Transferor -> Transferee

   REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Require: tdialog
   <allOneLine>
   Refer-To: <sips:482n4z24kdg@chicago.example.com;gr=8594958?
   Replaces=592435881734450904%3Bto-tag%3D9m2n3wq%3Bfrom-tag3D763231>
   </allOneLine>
   Target-Dialog: 592435881734450904;local-tag=9m2n3wq
    ;remote-tag=763231
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Length: 0


   F6 INVITE Transferee -> Transfer Target

   INVITE sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
   Max-Forwards: 70
   To: <sips:482n4z24kdg@chicago.example.com;gr=8594958>
   From: <sips:transferee@biloxi.example.com>;tag=954
   Call-ID: kmzwdle3dl3d08
   CSeq: 41 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: gruu, replaces, tdialog
   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   Replaces: 592435881734450904;to-tag=9m2n3wq;from-tag=763231
   Content-Type: application/sdp
   Content-Length: ...


7.4.  Recovery when one party does not support REFER

   If protecting or exposing the transfer target is not a concern, it is
   possible to complete a transfer with consultation hold when only the
   transferor and one other party support REFER.  Note that a 405 Method
   Not Allowed might be returned instead of the 501 Not Implemented
   response.



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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
          dialog2  | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
          dialog3  | REFER (Target-Dialog:1,                 |
                   |    Refer-To:sips:TransferTarget?Replaces=2)
                   |------------------->|                    |
          dialog3  | 501 Not Implemented                     |
                   |<-------------------|                    |
          dialog4  | REFER (Refer-To:sips:Transferee?Replaces=dialog1)
                   |---------------------------------------->|
          dialog4  | 202 Accepted       |                    |
                   |<----------------------------------------|
          dialog4  | NOTIFY (100 Trying)|                    |
                   |<----------------------------------------|
          dialog4  |                    |            200 OK  |
                   |---------------------------------------->|
          dialog5  |             INVITE (Replaces:dialog1)/200 OK/ACK
                   |                    |<-------------------|
          dialog4  | NOTIFY (200 OK)    |                    |
                   |<----------------------------------------|
          dialog4  |                    |            200 OK  |
                   |---------------------------------------->|
          dialog1  | BYE/200 OK         |                    |
                   |<-------------------|                    |
          dialog2  | BYE/200 OK         |                    |
                   |---------------------------------------->|
          dialog5  |                    |  BYE/200 OK        |
                   |                    |------------------->|

   Figure 8.  Recovery when one party does not support REFER.

7.5.  Attended transfer when Contact URI is not known to route to a
      unique user agent.

   It is a requirement of RFC3261 that a Contact URI be globally
   routable even outside the dialog.  However, due to RFC2543 User
   Agents and some architectures (NAT/Firewall traversal, screening
   proxies, ALGs, etc.) this will not always be the case.  As a result,
   the method of Attended Transfer shown in Figures 6, 7, and 8 should
   only be used if the Contact URI is known to be routable outside the



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   dialog.

   Figure 9 shows such a scenario where the Transfer Target Contact URI
   is not routable outside the dialog, so the triggered INVITE is sent
   to the AOR of the Transfer Target.

       Transferor           Transferee  Screening       Transfer
           |                  |           Proxy         Target
           |                  |             |             |
   dialog1 | INVITE/200 OK/ACK|             |             |
           |<-----------------|             |             |
   dialog1 | INVITE (hold)/200 OK/ACK       |             |
           |----------------->|             |             |
   dialog2 | INVITE/200 OK/ACK F1 F2        |             |
           |--------------------------------|------------>|
   dialog2 | INVITE (hold)/200 OK/ACK                     |
           |--------------------------------|------------>|
   dialog1 | REFER (Refer-To:sips:TargetAOR               |
           |         ?Replaces=dialog2&Require=replaces) F3
           |----------------->|             |             |
   dialog1 | 202 Accepted     |             |             |
           |<-----------------|             |             |
   dialog1 | NOTIFY (100 Trying)            |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog4 |INVITE (Replaces:dialog2,Require:replaces)/200 OK/ACK F6
           |                  |------------>|------------>|
   dialog2 | BYE/200 OK       |             |             |
           |<-------------------------------|<------------|
   dialog1 | NOTIFY (200 OK) F7             |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog1 | BYE/200 OK       |             |             |
           |----------------->|             |             |
   dialog3 |                  |             |  BYE/200 OK |
           |                  |<------------|-------------|

   Figure 9.  Attended Transfer Call Flow with a Contact URI not known
   to be Globally Routable


   F1 INVITE Transferor -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK76
   Max-Forwards: 70



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   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferor@pc33.atlanta.example.com>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transfer Target -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
    ;received=192.0.2.1
   To: <sips:transfertarget@chicago.example.com>;tag=9m2n3wq
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transfertarget@client.chicago.example.com>
   Content-Type: application/sdp
   Content-Length: ...


   F3 REFER Transferor -> Transferee

   REFER sips:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 REFER
   <allOneLine>
   Refer-To: <sips:transfertarget@chicago.example.com?Replaces=
   090459243588173445%3Bto-tag%3D9m2n3wq%3Bfrom-tag%3D763231
   &Require=replaces>
   <allOneLine>
   Contact: <sips:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F4 INVITE Transferee -> Transfer Target

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



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   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=954
   Call-ID: 20482817324945934422930
   CSeq: 42 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferee@192.0.2.4>
   Replaces: 090459243588173445;to-tag=9m2n3wq;from-tag=763231
   Require: replaces
   Content-Type: application/sdp
   Content-Length: ...


   F5 NOTIFY Transferee -> Transferor

   NOTIFY sips:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=1928301774
   From: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 76 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer;id=98873867
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   Figure 10 shows a failure case in which the AOR URI fails to reach
   the transfer Target.  As a result, the transfer is retried with the
   Contact URI which succeeds.

   Note that there is still no guarantee that the correct endpoint will
   be reached, and the result of this second REFER may also be a
   failure.  In that case, the Transferor could fall back to unattended
   transfer or give up on the transfer entirely.  Since two REFERs are
   sent within the dialog creating two distinct subscriptions, the
   Transferee uses the 'id' parameter in the Event header field to
   distinguish notifications for the two subscriptions.






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       Transferor           Transferee  Screening      Transfer
           |                  |           Proxy         Target
           |                  |             |             |
   dialog1 | INVITE/200 OK/ACK|             |             |
           |<-----------------|             |             |
   dialog1 | INVITE (hold)/200 OK/ACK       |             |
           |----------------->|             |             |
   dialog2 | INVITE/200 OK/ACK F1 F2        |             |
           |--------------------------------|------------>|
   dialog2 | INVITE (hold)/200 OK/ACK                     |
           |--------------------------------|------------>|
   dialog1 | REFER (Refer-To:sips:TargetAOR?              |
           |       Replaces=dialog2&Require=replaces) F3  |
           |----------------->|             |             |
   dialog1 | 202 Accepted     |             |             |
           |<-----------------|             |             |
   dialog1 | NOTIFY (100 Trying)            |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog3 |                  |INVITE (Replaces:dialog2,  |
           |                  | Require:replaces)/403/ACK |
           |                  |------------>|             |
   dialog1 | NOTIFY (403 Forbidden) F4      |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog1 |REFER(Refer-To:sips:TargetContact?Replaces=dialog2) F5
           |----------------->|             |             |
   dialog1 | 202 Accepted     |             |             |
           |<-----------------|             |             |
   dialog1 | NOTIFY (100 Trying)            |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog4 |                INVITE (Replaces:dialog2)/200 OK/ACK F6
           |                  |------------>|------------>|
   dialog2 | BYE/200 OK       |             |             |
           |<-------------------------------|<------------|
   dialog1 | NOTIFY (200 OK) F7             |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog1 | BYE/200 OK       |             |             |
           |----------------->|             |             |
   dialog3 |                  |             |  BYE/200 OK |
           |                  |<------------|-------------|




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   Figure 10.  Attended Transfer Call Flow with non-routable Contact URI
   and AOR Failure


   F1 INVITE Transferor -> Transfer Target

   INVITE sips:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK76
   Max-Forwards: 70
   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferor@pc33.atlanta.example.com>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transfer Target -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
    ;received=192.0.2.1
   To: <sips:transfertarget@chicago.example.com>;tag=9m2n3wq
   From: <sips:transferor@atlanta.example.com>;tag=763231
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transfertarget@client.chicago.example.com>
   Content-Type: application/sdp
   Content-Length: ...


   F3 REFER Transferor -> Transferee

   REFER sips:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   <allOneLine>
   Refer-To: <sips:transfertarget@chicago.example.com?Replaces=
   090459243588173445%3Bto-tag%3D9m2n3wq%3Bfrom-tag%3D763231



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   &Require=replaces>
   </allOneLine>
   Contact: <sips:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F4 NOTIFY Transferee -> Transferor

   NOTIFY sips:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=1928301774
   From: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 74 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer;id=314159
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 403 Forbidden


   F5 REFER Transferor -> Transferee

   REFER sips:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 REFER
   <allOneLine>
   Refer-To: <sips:transfertarget@client.chicago.example.com
   ?Replaces=090459243588173445%3Bto-tag%3D9m2n3wq
   %3Bfrom-tag%3D763231>
   </allOneLine>
   Contact: <sips:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F6 INVITE Transferee -> Transfer Target

   INVITE sips:transfertarget@client.chicago.example.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
   Max-Forwards: 70



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   To: <sips:transfertarget@chicago.example.com>
   From: <sips:transferee@biloxi.example.com>;tag=954
   Call-ID: 20482817324945934422930
   CSeq: 42 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sips:transferee@192.0.2.4>
   Replaces: 090459243588173445;to-tag=9m2n3wq;from-tag=763231
   Content-Type: application/sdp
   Content-Length: ...


   F7 NOTIFY Transferee -> Transferor

   NOTIFY sips:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sips:transferor@atlanta.example.com>;tag=1928301774
   From: <sips:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 76 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Event: refer;id=314160
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   To prevent this scenario from happening, the Transfer Target should
   use a Contact URI which is routable outside the dialog, which will
   result in the call flow of Figure 7.

7.6.  Semi-Attended Transfer

   In any of the consultation hold flows above, the Transferor may
   decide to terminate its attempt to contact the Transfer target before
   that session is established.  Most frequently, that will be the end
   of the scenario, but in some circumstances, the transferor may wish
   to proceed with the transfer action.  For example, the Transferor may
   wish to complete the transfer knowing that the transferee will end up
   eventually talking to the transfer-target's voice-mail service.  Some
   PBX systems support this feature, sometimes called "semi-attended
   transfer", that is effectively a hybrid between a fully attended
   transfer and an unattended transfer.  A call flow is shown in Figure
   11.  In this flow, the Transferor's User Agent continues the transfer



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   as an attended transfer even after the Transferor hangs up.  Note
   that media must be played to the Transfer Target upon answer -
   otherwise, the Target may hang up and the resulting transfer
   operation will fail.

              Transferor           Transferee            Transfer
                   |                    |                 Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK F1 F2                 |
                   |<-------------------|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE             |                    |
                   |---------------------------------------->|
          dialog2  |                    |       180 Ringing  |
                   |<----------------------------------------|
                Transferor hangs up but wants transfer to continue
                   |                    |                    |
                   | User Agent continues transfer operation |
                   |                    |                    |
          dialog2  |                    |           200 OK   |
                   |<----------------------------------------|
          dialog2  | ACK                |                    |
                   |---------------------------------------->|
          dialog2  | Media Played to keep Target from hanging up
                   |========================================>|
          dialog3  | REFER (Target-Dialog:1,                 |
                   |  Refer-To:sips:TransferTarget?Replaces=2)
                   |------------------->|                    |
          dialog3  | 202 Accepted       |                    |
                   |<-------------------|                    |
          dialog3  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog4  |             INVITE (Replaces:dialog2)/200 OK/ACK
                   |                    |------------------->|
          dialog2  | BYE/200 OK         |                    |
                   |<----------------------------------------|
          dialog3  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog4  |                    |         BYE/200 OK |
                   |                    |<-------------------|




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   Figure 11.  Recommended Semi-Attended Transfer Call Flow.

   Two other possible semi-attended transfer call flows are shown in
   Figures 12 and 13.  However, these call flows are NOT RECOMMENDED due
   to a race conditions.  In both of these flows, when the Transferor
   hangs up, the Transferor attempts to revert to unattended transfer by
   sending a CANCEL to the Target.  This can result in two race
   conditions.  One is that the Target answers despite the CANCEL and
   the resulting unattended transfer fails.  This race condition can be
   eliminated by the Transferor waiting to send the REFER until the 487
   response from the Target is returned.  Instead of a 487, a 200 OK may
   return indicating that the Target has answered the consultation call.
   In this, case the call flow in Figure 13 must be followed.  In this
   flow, the Transferor must play some kind of media to the Target to
   prevent the Target from hanging up, or the Transfer will fail.  That
   is, the human at the Transfer Target will hear silence from when they
   answer (message F1) until the transfer completes (F3 and they are
   talking to the Transferee unless some media is played (F2).

   The second race condition occurs in Figure 12 if the Transfer Target
   goes "off hook" after the CANCEL is received and the 487 returned.
   This may result in a 486 Busy Here response to the unattended
   transfer.

   The recommended call flow of Figure 11 does not utilize a CANCEL and
   does not suffer from these race conditions.

























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              Transferor           Transferee            Transfer
                   |                    |                 Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE                                  |
                   |---------------------------------------->|
          dialog2  | 180 Ringing                             |
                   |<----------------------------------------|
                   |                                         |
                   |  Transferor gives up waiting            |
                   |                                         |
          dialog2  | CANCEL                                  |
                   |---------------------------------------->|
          dialog2  | 200 OK                                  |
                   |<----------------------------------------|
          dialog2  | 487 Request Terminated                  |
                   |<----------------------------------------|
          dialog2  | ACK                                     |
                   |---------------------------------------->|
          dialog3  | REFER (Target-Dialog:1) F3              |
                   |------------------->|                    |
          dialog3  | 202 Accepted       |                    |
                   |<-------------------|                    |
          dialog3  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog4  |                INVITE/200 OK/ACK        |
                   |                    |------------------->|
          dialog3  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog4  |                    |         BYE/200 OK |
                   |                    |<-------------------|

   Figure 12.  Semi-Attended Transfer as Blind Transfer Call Flow.  (Not
   Recommended)








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              Transferor           Transferee            Transfer
                   |                    |                 Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE                                  |
                   |---------------------------------------->|
          dialog2  | 180 Ringing                             |
                   |<----------------------------------------|
                   |                                         |
                   |Transferor gives up waiting but Target answers
                   |                                         |
          dialog2  | CANCEL                                  |
                   |---------------------------------------->|
          dialog2  | 200 OK (CANCEL)                         |
                   |<----------------------------------------|
          dialog2  | 200 OK (INVITE) F1                      |
                   |<----------------------------------------|
          dialog2  | ACK                                     |
                   |---------------------------------------->|
          dialog2  | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
                   |  Tones or media played avoid silence F2 |
                   |========================================>|
          dialog1  |REFER (Refer-To:sips:TransferTarget      |
                   |                      ?Replaces=dialog2) |
                   |------------------->|                    |
          dialog1  | 202 Accepted       |                    |
                   |<-------------------|                    |
          dialog1  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog3  |         INVITE (Replaces:dialog2)/200 OK/ACK F3
                   |                    |------------------->|
          dialog2  | BYE/200 OK         |                    |
                   |<----------------------------------------|
          dialog1  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog3  |                    |         BYE/200 OK |
                   |                    |<-------------------|




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   Figure 13.  Semi-Attended Transfer as Attended Transfer Call Flow.
   (Not Recommended)

7.7.  Attended Transfer Fallback to Basic Transfer

   In this flow, an attempted attended transfer fails so the transferor
   falls back to basic transfer.

   The call flow in Figure 14 shows the use of Require: replaces in the
   INVITE sent by the Transferor to the Transfer Target in which the
   Transferor's intention at the time of sending the INVITE to the
   Transfer Target was known to be to complete an attended transfer.
   Since the Target does not support Replaces, the INVITE is rejected
   with a 420 Bad Extension response, and the Transferor switches from
   attended transfer to basic transfer immediately.

              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
          dialog1  |   OPTIONS/200 OK   |                    |
                   |------------------->|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE (Require:replaces)               |
                   |---------------------------------------->|
          dialog2  |                     420 Bad Extension   |
                   |<----------------------------------------|
          dialog2  |    ACK                                  |
                   |---------------------------------------->|
          dialog1  | REFER (Refer-To:sips:TransferTarget)    |
                   |------------------->|                    |
          dialog1  |    202 Accepted    |                    |
                   |<-------------------|                    |
          dialog1  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog3  |                    |  INVITE/200 OK/ACK |
                   |                    |------------------->|
          dialog1  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog3  |                    |         BYE/200 OK |



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

   Figure 14.  Attended Transfer Fallback to Basic Transfer using
   Require:replaces.

   Figure 15 shows the use of OPTIONS when the Transferee and Transfer
   Target do not explicitly indicate support for the REFER method and
   Replaces header fields in Allow and Supported header fields and the
   Transferor did not have the intention of performing an attended
   transfer when the INVITE to the Target was sent.  In dialog1, the
   Transferor determines using OPTIONS that the Transferee does support
   REFER and Replaces.  As a result, the Transferor begins the attended
   transfer by placing the Transferee on hold and calling the Transfer
   Target.  Using an OPTIONS in dialog2, the Transferor determines that
   the Target does not support either REFER or Replaces, making attended
   transfer impossible.  The Transferor then ends dialog2 by sending a
   BYE then sends a REFER to the Transferee using the AOR URI of the
   Transfer Target.

































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              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
          dialog1  | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
          dialog1  |   OPTIONS/200 OK   |                    |
                   |------------------->|                    |
          dialog1  | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
          dialog2  | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
          dialog2  | OPTIONS/200 OK     |                    |
                   |---------------------------------------->|
          dialog2  |    BYE/200 OK      |                    |
                   |---------------------------------------->|
          dialog3  |REFER (Target-Dialog:1,                  |
                   |          Refer-To:sips:TransferTarget)  |
                   |------------------->|                    |
          dialog3  |    202 Accepted    |                    |
                   |<-------------------|                    |
          dialog3  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog4  |                    |  INVITE/200 OK/ACK |
                   |                    |------------------->|
          dialog3  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog3  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog4  |                    |         BYE/200 OK |
                   |                    |<-------------------|

   Figure 15.  Attended Transfer Fallback to Basic Transfer.


8.  Transfer with Referred-By

   In the previous examples, the Transfer Target does not have
   definitive information about what party initiated the transfer, or,
   in some cases, even that transfer is taking place.  The Referred-By
   mechanism [RFC3892] provides a way for the Transferor to provide the
   Transferee with a way to let the Transfer Target know what party
   initiated the transfer.

   The simplest and least secure approach just involves the inclusion of



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   the Referred-By header field in the REFER which is then copied into
   the triggered INVITE.  However, a more secure mechanism involving the
   Referred-By security token which is generated and signed by the
   Transferor and passed in a message body to the Transferee then to the
   Transfer Target.

   The call flow would be identical to Figure 7.  However, the REFER and
   triggered INVITE messages for this flow showing the Referred-By
   mechanism are shown below.

   Note that the conventions used in the SIP Torture Test Messages
   [RFC4475] document are reused, specifically the <hex> and
   <allOneLine> tags.


   F5 REFER Transferor -> Transferee

   REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
   Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK392039842
   Max-Forwards: 70
   To: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   From: <sips:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314160 REFER
   <allOneLine>
   Refer-To: <sips:482n4z24kdg@chicago.example.com;gr=8594958
   ?Replaces=090459243588173445%3Bto-tag%3D9m2n3wq%3Bfrom-tag
   %3D763231&Require=replaces>
   </allOneLine>
   Supported: gruu, replaces, tdialog
   Require: tdialog
   Referred-By: <sips:transferor@atlanta.example.com>
     ;cid="20398823.2UWQFN309shb3@atlanta.example.com"
   Target-Dialog: 592435881734450904;local-tag=9m2n3wq;remote-tag=763231
   Contact: <sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d>
   Content-Type: multipart/mixed; boundary=unique-boundary-1
   Content-Length: 3267

   --unique-boundary-1
   Content-ID: <20398823.2UWQFN309shb3@atlanta.example.com>

   Content-Length: 2961
   Content-Type: multipart/signed;
                 protocol="application/pkcs-7-signature";
                 micalg=sha1;
                 boundary="----590F24D439B31E08745DEF0CD9397189"





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   ------590F24D439B31E08745DEF0CD9397189
   Content-Type: message/sipfrag

   Date: Thu, 18 Sep 2003 13:07:43 GMT
   <allOneLine>
   Refer-To: <sips:482n4z24kdg@chicago.example.com;gr=8594958
   ?Replaces=090459243588173445%3B
   to-tag%3D9m2n3wq%3Bfrom-tag%3D763231&Require=replaces>
   </allOneLine>
   Referred-By: <sips:transferor@atlanta.example.com>
     ;cid="20398823.2UWQFN309shb3@atlanta.example.com"

   ------590F24D439B31E08745DEF0CD9397189
   Content-Type: application/pkcs-7-signature; name="smime.p7s"
   Content-Transfer-Encoding: binary
   Content-Disposition: attachment; filename="smime.p7s"

   <hex>3082088806092A86
   4886F70D010702A082087930820875020101310B300906052B0E03021A050030
   0B06092A864886F70D010701A082067A30820339308202A2A003020102020800
   90008902240001300D06092A864886F70D01010505003070310B300906035504
   0613025553311330110603550408130A43616C69666F726E69613111300F0603
   550407130853616E4A6F7365310E300C060355040A1305736970697431293027
   060355040B135369706974546573744365727469666963617465417574686F72
   697479301E170D3033313032313134343332355A170D31333130313831343433
   32355A3062310B3009060355040613025553311330110603550408130A43616C
   69666F726E69613111300F0603550407130853616E4A6F7365310E300C060355
   040A13057369706974311B30190603550403141273656E646572406578616D70
   6C652E6F726730819F300D06092A864886F70D010101050003818D0030818902
   818100CB8302060F12C8FA2D1786922CA173DCEB80BF1B1B8AF74A310C6975A5
   56A7630FB6E044D9E994DCD49AFF7976C462D7A8E74ECBF98723AEBF2796EDDD
   6263577C6C2B77DC7C300B533DEDB5FB8EB3827FD6FC9B37B9A0DE829F1B1081
   D632A8AD9FB00A860928E88F87E0B979BA65294AC7D6D2D18A78C86B4FA73387
   4E230203010001A381E93081E6301D0603551D1104163014811273656E646572
   406578616D706C652E6F726730090603551D1304023000301D0603551D0E0416
   041440FF1C0C1BB8684CA917839D70E97DF8DD5B60D130819A0603551D230481
   9230818F80146B461714EA94762580546E1354DAA1E35414A1B6A174A4723070
   310B3009060355040613025553311330110603550408130A43616C69666F726E
   69613111300F0603550407130853616E4A6F7365310E300C060355040A130573
   6970697431293027060355040B13536970697454657374436572746966696361
   7465417574686F72697479820100300D06092A864886F70D0101050500038181
   006FFE1A3B5CE807C3DD2CFDF6E9787F491C84DBF7DCD11DB2D6A8887D2FE3F2
   2E9C6894994282E50AA0DFFE1CBD4EC2C20217831FC2AD360FF1C0DE1DE1E870
   102CFA99EE504C7DC0D8752A63294AC748DDDEFADE55C6D051F1CD54CFE7C153
   278962A53CEF61B875C1FD3C74E972242CBA0131B3B8C607BF95B378212CA9A7
   5E30820339308202A2A00302010202080090008902240001300D06092A864886
   F70D01010505003070310B300906035504061302555331133011060355040813
   0A43616C69666F726E69613111300F0603550407130853616E4A6F7365310E30



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   0C060355040A1305736970697431293027060355040B13536970697454657374
   4365727469666963617465417574686F72697479301E170D3033313032313134
   343332355A170D3133313031383134343332355A3062310B3009060355040613
   025553311330110603550408130A43616C69666F726E69613111300F06035504
   07130853616E4A6F7365310E300C060355040A13057369706974311B30190603
   550403141273656E646572406578616D706C652E6F726730819F300D06092A86
   4886F70D010101050003818D0030818902818100CB8302060F12C8FA2D178692
   2CA173DCEB80BF1B1B8AF74A310C6975A556A7630FB6E044D9E994DCD49AFF79
   76C462D7A8E74ECBF98723AEBF2796EDDD6263577C6C2B77DC7C300B533DEDB5
   FB8EB3827FD6FC9B37B9A0DE829F1B1081D632A8AD9FB00A860928E88F87E0B9
   79BA65294AC7D6D2D18A78C86B4FA733874E230203010001A381E93081E6301D
   0603551D1104163014811273656E646572406578616D706C652E6F7267300906
   03551D1304023000301D0603551D0E0416041440FF1C0C1BB8684CA917839D70
   E97DF8DD5B60D130819A0603551D2304819230818F80146B461714EA94762580
   546E1354DAA1E35414A1B6A174A4723070310B30090603550406130255533113
   30110603550408130A43616C69666F726E69613111300F060355040713085361
   6E4A6F7365310E300C060355040A1305736970697431293027060355040B1353
   69706974546573744365727469666963617465417574686F7269747982010030
   0D06092A864886F70D0101050500038181006FFE1A3B5CE807C3DD2CFDF6E978
   7F491C84DBF7DCD11DB2D6A8887D2FE3F22E9C6894994282E50AA0DFFE1CBD4E
   C2C20217831FC2AD360FF1C0DE1DE1E870102CFA99EE504C7DC0D8752A63294A
   C748DDDEFADE55C6D051F1CD54CFE7C153278962A53CEF61B875C1FD3C74E972
   242CBA0131B3B8C607BF95B378212CA9A75E318201D6308201D2020101307C30
   70310B3009060355040613025553311330110603550408130A43616C69666F72
   6E69613111300F0603550407130853616E4A6F7365310E300C060355040A1305
   736970697431293027060355040B135369706974546573744365727469666963
   617465417574686F7269747902080090008902240001300906052B0E03021A05
   00A081B1301806092A864886F70D010903310B06092A864886F70D010701301C
   06092A864886F70D010905310F170D3034303132363139313831345A30230609
   2A864886F70D01090431160414408CCA5772916A968204FD24CC24EDAEAD3943
   95305206092A864886F70D01090F31453043300A06082A864886F70D0307300E
   06082A864886F70D030202020080300D06082A864886F70D0302020140300706
   052B0E030207300D06082A864886F70D0302020128300D06092A864886F70D01
   010105000481807795329BB23B8BB9F72526AB9CC22D93B9A37A2E69A0171D3C
   C417DD394F0A5FD4F8B082733CD9F2E26F6991031F7FF2EAD31640718502FB4C
   822771211E6228C793DA4DBBA2159227C221030FE9088CD659578EB862568087
   8E63D306487A740A197A3970594CF47DD385643B1DC49FF767A3D2B428388966
   79089AAD95767F</hex>

   ------590F24D439B31E08745DEF0CD9397189--

   --unique_boundary-1


   F6 INVITE Transferee -> Transfer Target

   INVITE sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
   Via: SIP/2.0/TLS referee.example;branch=z9hG4bKffe209934aac



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   To: <sips:482n4z24kdg@chicago.example.com;gr=8594958>
   From: <sips:transferee@biloxi.example.com>;tag=2909034023
   Call-ID: fe9023940-a3465@referee.example
   CSeq: 889823409 INVITE
   Max-Forwards: 70
   Contact: <sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha>
   Referred-By: <sips:transferor@atlanta.example.com>
       ;cid="20398823.2UWQFN309shb3@atlanta.example.com"
   Replaces:090459243588173445;to-tag=9m2n3wq;from-
     tag=76323
   Require: replaces
   Supported: gruu, replaces, tdialog
   Content-Type: multipart/mixed; boundary=my-boundary-9
   Content-Length: 3432

   --my-boundary-9
   Content-Type: application/sdp

   Content-Length: 156

   v=0
   o=referee 2890844526 2890844526 IN IP4 referee.example
   s=Session SDP
   c=IN IP4 referee.example
   t=0 0
   m=audio 49172 RTP/AVP 0
   a=rtpmap:0 PCMU/8000

   --my-boundary-9
   Content-Length: 2961
   Content-Type: multipart/signed;
                 protocol="application/pkcs-7-signature";
                 micalg=sha1;
                 boundary="----590F24D439B31E08745DEF0CD9397189"

   ------590F24D439B31E08745DEF0CD9397189
   Content-Type: message/sipfrag

   Date: Thu, 18 Sep 2003 13:07:43 GMT

   <allOneLine>
   Refer-To: <sips:transfertarget@chicago.example.com;
   Replaces=090459243588173445%3B
   to-tag%3D9m2n3wq%3Bfrom-tag%3D763231&Require=replaces>
   </allOneLine>
   Referred-By: <sips:transferor@atlanta.example.com>
     ;cid="20398823.2UWQFN309shb3@atlanta.example.com"




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   ------590F24D439B31E08745DEF0CD9397189
   Content-Type: application/pkcs-7-signature; name="smime.p7s"
   Content-Transfer-Encoding: binary
   Content-Disposition: attachment; filename="smime.p7s"

   <hex>3082088806092A86

   4886F70D010702A082087930820875020101310B300906052B0E03021A050030
   0B06092A864886F70D010701A082067A30820339308202A2A003020102020800
   90008902240001300D06092A864886F70D01010505003070310B300906035504
   0613025553311330110603550408130A43616C69666F726E69613111300F0603
   550407130853616E4A6F7365310E300C060355040A1305736970697431293027
   060355040B135369706974546573744365727469666963617465417574686F72
   697479301E170D3033313032313134343332355A170D31333130313831343433
   32355A3062310B3009060355040613025553311330110603550408130A43616C
   69666F726E69613111300F0603550407130853616E4A6F7365310E300C060355
   040A13057369706974311B30190603550403141273656E646572406578616D70
   6C652E6F726730819F300D06092A864886F70D010101050003818D0030818902
   818100CB8302060F12C8FA2D1786922CA173DCEB80BF1B1B8AF74A310C6975A5
   56A7630FB6E044D9E994DCD49AFF7976C462D7A8E74ECBF98723AEBF2796EDDD
   6263577C6C2B77DC7C300B533DEDB5FB8EB3827FD6FC9B37B9A0DE829F1B1081
   D632A8AD9FB00A860928E88F87E0B979BA65294AC7D6D2D18A78C86B4FA73387
   4E230203010001A381E93081E6301D0603551D1104163014811273656E646572
   406578616D706C652E6F726730090603551D1304023000301D0603551D0E0416
   041440FF1C0C1BB8684CA917839D70E97DF8DD5B60D130819A0603551D230481
   9230818F80146B461714EA94762580546E1354DAA1E35414A1B6A174A4723070
   310B3009060355040613025553311330110603550408130A43616C69666F726E
   69613111300F0603550407130853616E4A6F7365310E300C060355040A130573
   6970697431293027060355040B13536970697454657374436572746966696361
   7465417574686F72697479820100300D06092A864886F70D0101050500038181
   006FFE1A3B5CE807C3DD2CFDF6E9787F491C84DBF7DCD11DB2D6A8887D2FE3F2
   2E9C6894994282E50AA0DFFE1CBD4EC2C20217831FC2AD360FF1C0DE1DE1E870
   102CFA99EE504C7DC0D8752A63294AC748DDDEFADE55C6D051F1CD54CFE7C153
   278962A53CEF61B875C1FD3C74E972242CBA0131B3B8C607BF95B378212CA9A7
   5E30820339308202A2A00302010202080090008902240001300D06092A864886
   F70D01010505003070310B300906035504061302555331133011060355040813
   0A43616C69666F726E69613111300F0603550407130853616E4A6F7365310E30
   0C060355040A1305736970697431293027060355040B13536970697454657374
   4365727469666963617465417574686F72697479301E170D3033313032313134
   343332355A170D3133313031383134343332355A3062310B3009060355040613
   025553311330110603550408130A43616C69666F726E69613111300F06035504
   07130853616E4A6F7365310E300C060355040A13057369706974311B30190603
   550403141273656E646572406578616D706C652E6F726730819F300D06092A86
   4886F70D010101050003818D0030818902818100CB8302060F12C8FA2D178692
   2CA173DCEB80BF1B1B8AF74A310C6975A556A7630FB6E044D9E994DCD49AFF79
   76C462D7A8E74ECBF98723AEBF2796EDDD6263577C6C2B77DC7C300B533DEDB5
   FB8EB3827FD6FC9B37B9A0DE829F1B1081D632A8AD9FB00A860928E88F87E0B9
   79BA65294AC7D6D2D18A78C86B4FA733874E230203010001A381E93081E6301D



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   0603551D1104163014811273656E646572406578616D706C652E6F7267300906
   03551D1304023000301D0603551D0E0416041440FF1C0C1BB8684CA917839D70
   E97DF8DD5B60D130819A0603551D2304819230818F80146B461714EA94762580
   546E1354DAA1E35414A1B6A174A4723070310B30090603550406130255533113
   30110603550408130A43616C69666F726E69613111300F060355040713085361
   6E4A6F7365310E300C060355040A1305736970697431293027060355040B1353
   69706974546573744365727469666963617465417574686F7269747982010030
   0D06092A864886F70D0101050500038181006FFE1A3B5CE807C3DD2CFDF6E978
   7F491C84DBF7DCD11DB2D6A8887D2FE3F22E9C6894994282E50AA0DFFE1CBD4E
   C2C20217831FC2AD360FF1C0DE1DE1E870102CFA99EE504C7DC0D8752A63294A
   C748DDDEFADE55C6D051F1CD54CFE7C153278962A53CEF61B875C1FD3C74E972
   242CBA0131B3B8C607BF95B378212CA9A75E318201D6308201D2020101307C30
   70310B3009060355040613025553311330110603550408130A43616C69666F72
   6E69613111300F0603550407130853616E4A6F7365310E300C060355040A1305
   736970697431293027060355040B135369706974546573744365727469666963
   617465417574686F7269747902080090008902240001300906052B0E03021A05
   00A081B1301806092A864886F70D010903310B06092A864886F70D010701301C
   06092A864886F70D010905310F170D3034303132363139313831345A30230609
   2A864886F70D01090431160414408CCA5772916A968204FD24CC24EDAEAD3943
   95305206092A864886F70D01090F31453043300A06082A864886F70D0307300E
   06082A864886F70D030202020080300D06082A864886F70D0302020140300706
   052B0E030207300D06082A864886F70D0302020128300D06092A864886F70D01
   010105000481807795329BB23B8BB9F72526AB9CC22D93B9A37A2E69A0171D3C
   C417DD394F0A5FD4F8B082733CD9F2E26F6991031F7FF2EAD31640718502FB4C
   822771211E6228C793DA4DBBA2159227C221030FE9088CD659578EB862568087
   8E63D306487A740A197A3970594CF47DD385643B1DC49FF767A3D2B428388966
   79089AAD95767F</hex>

   ------590F24D439B31E08745DEF0CD9397189--

   --my-boundary-9--



9.  Transfer as an Ad-Hoc Conference

   In this flow, shown in Figure 16, Bob does an attended transfer of
   Alice to Carol.  In order to keep both Alice and Carol fully informed
   of the nature and state of the transfer operation, Bob acts as a
   focus[RFC4579] and hosts an ad-hoc conference involving Alice, Bob,
   and Carol.  Alice and Carol subscribe to the conference
   package[RFC4575] of Bob's focus, which allows them to know the exact
   status of the operation.  After the transfer operation is complete,
   Bob deletes the conference.

   This call flow meets requirement 6 of Section 3.  NOTIFY messages
   related to the refer package are indicated as NOTIFY (refer), while
   NOTIFYs related to the Conference Info package are indicated as



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   NOTIFY (Conf-Info).

   Note that any type of semi-attended transfer in which media mixing or
   relaying could be implemented using this model.  In addition to
   simply mixing, the focus could introduce additional media signals
   such as simulated ring tone or on hold announcements to improve the
   user experience.

    Alice                  Bob                 Carol
       |                    |                    |
       | INVITE             |                    |
       |------------------->|                    |
       |   180 Ringing      |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |<-------------------|                    |
       |        ACK         |                    |
       |------------------->|                    |
       |        RTP         |                    |
       |<==================>|                    |
       |                    |                    |
    Bob places Alice on hold and begins acting like a focus
       |                    |                    |
       | INVITE (hold) Contact:Conf-ID;isfocus   |
       |<-------------------|                    |
       |    200 OK          |                    |
       |------------------->|                    |
       |        ACK         |                    |
       |<-------------------|                    |
       |                    |                    |
       | Alice subscribes to the conference package
       |                    |                    |
       | SUBSCRIBE sip:Conf-ID                   |
       |------------------->|                    |
       |     200 OK         |                    |
       |<-------------------|                    |
       | NOTIFY (Conf-Info) |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |------------------->|                    |
       |                    |                    |
       |       Bob begins consultation operation |
       |                    |                    |
       |INVITE Require:replaces Contact:Conf-ID;isfocus
       |                    |------------------->|
       |                    |   180 Ringing      |
       |                    |<-------------------|
       |                    |     200 OK         |



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       |                    |<-------------------|
       |                    |       ACK          |
       |                    |------------------->|
       |                    |        RTP         |
       |                    |<==================>|
       |                    |                    |
       |Carol subscribes to the conference package
       |                - learns Bob is on hold  |
       |                    |                    |
       |                    |SUBSCRIBE sip:Conf-ID
       |                    |<-------------------|
       |                    |      200 OK        |
       |                    |------------------->|
       |                    | NOTIFY (Conf-Info) |
       |                    |------------------->|
       |                    |      200 OK        |
       |                    |<-------------------|
       |                    |                    |
       | Alice learns that Bob is talking to Carol
       |                    |                    |
       | NOTIFY (Conf-Info) |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |------------------->|                    |
       |                    |  INVITE (hold)     |
       |                    |------------------->|
       |                    |      200 OK        |
       |                    |<-------------------|
       |                    |      ACK           |
       |                    |------------------->|
       |                    |                    |
       | Alice learns that Carol is now on hold  |
       |                    |                    |
       | NOTIFY (Conf-Info) |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |------------------->|                    |
       |                    |                    |
       |           Bob begins transfer operation |
       |                    |                    |
       |     REFER Refer-To: Carol               |
       |<-------------------|                    |
       |     202 Accepted   |                    |
       |------------------->|                    |
       | NOTIFY (Refer)     |                    |
       |------------------->|                    |
       |     200 OK         |                    |
       |<-------------------|                    |



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       |  INVITE Replaces:B-C Contact:Alice      |
       |---------------------------------------->|
       |                 200 OK                  |
       |<----------------------------------------|
       |                   ACK                   |
       |---------------------------------------->|
       |                    RTP                  |
       |<=======================================>|
       |                    |       BYE          |
       |                    |<-------------------|
       |                    |      200 OK        |
       |                    |------------------->|
       | NOTIFY (Refer)     |                    |
       |------------------->|                    |
       |     200 OK         |                    |
       |<-------------------|                    |
       |                    |                    |
       | Bob terminates the ad-hoc conference    |
       |                    |                    |
       |       BYE          |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |------------------->|                    |
       |                    | NOTIFY (Conf-Info) |
       |                    |------------------->|
       |                    |      200 OK        |
       |                    |<-------------------|
       | NOTIFY (Conf-Info) |                    |
       |<-------------------|                    |
       |     200 OK         |                    |
       |------------------->|                    |

   Figure 16. Attended Transfer as an Ad-Hoc Conference.






10.  Transfer with multiple parties

   In this example shown in Figure 17, the Originator places call to the
   Facilitator who reaches the Recipient through the Screener.  The
   Recipient's contact information is exposed to the Facilitator and the
   Originator.  This example is provided for clarification of the
   semantics of the REFER method only and should not be used as the
   design of an implementation.




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        Originator   Facilitator   Screener   Recipient
       |            |            |          |
    1  |INVITE/200 OK/ACK        |          |"Get Fred for me!"
       |----------->|            |          |     "Right away!"
    2  |INVITE (hold)/200 OK/ACK |          |
       |<-----------|            |          |
    2  |            |INVITE/200 OK/ACK      |"I have a call
       |            |----------->|          |from Mary for Fred"
    2  |            |INVITE (hold)/200 OK/ACK   "Hold please"
       |            |<-----------|          |
    3  |            |            |INVITE/200 OK/ACK
       |            |            |--------->|"You have a call
       |            |            |          |from Mary"
       |            |            |          |  "Put her through"
    3  |            |            |INVITE (hold)/200 OK/ACK
       |            |            |--------->|
    4  |            |REFER       |          |
       |            |<-----------|          |
    4  |            |202 Accepted|          |
       |            |----------->|          |
    4  |            |NOTIFY (100 Trying)    |
       |            |----------->|          |
    4  |            |200 OK      |          |
       |            |<-----------|          |
    5  |            |INVITE/200 OK/ACK      |
       |            |---------------------->|"This is Fred"
    4  |            |NOTIFY (200 OK)        |  "Please hold for
       |            |----------->|          |              Mary"
    4  |            |200 OK      |          |
       |            |<-----------|          |
    2  |            |BYE/200 OK  |          |
       |            |<-----------|          |
    3  |            |            |BYE/200 OK|
       |            |            |--------->|
    5  |            |INVITE (hold)/200 OK/ACK
       |            |---------------------->|
    6  |REFER       |            |          |
       |<-----------|            |          |
    6  |202 Accepted|            |          |
       |----------->|            |          |
    6  |NOTIFY (100 Trying)      |          |
       |----------->|            |          |
    6  |200 OK      |            |          |
       |<-----------|            |          |
    7  |INVITE/200 OK/ACK        |          |
       |----------------------------------->| "Hey Fred"
    6  |NOTIFY (200 OK)          |          |    "Hello Mary"
       |----------->|            |          |



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    6  |200 OK      |            |          |
       |<-----------|            |          |
    1  |BYE/200 OK  |            |          |
       |<-----------|            |          |
    5  |            |BYE/200 OK  |          |
       |            |---------------------->|
    7  |BYE/200 OK  |            |          |
       |<-----------------------------------| "See you later"

   Figure 17.  Transfer with Multiple Parties Example.


11.  Gateway Transfer Issues

   A gateway in SIP acts as a User Agent.  As a result, the entire
   preceding discussion and call flows apply equally well to gateways as
   native SIP endpoints.  However, there are some gateway specific
   issues that are documented in this section.  While this discussion
   focuses on the common cases involving PSTN gateways, similar
   situations exist for other gateways, such as H.323/SIP gateways.

11.1.  Coerce Gateway Hairpins to the Same Gateway

   To illustrate how a hairpin situation can occur in transfer, consider
   this example.  The original call dialog is setup with the transferee
   residing on the PSTN side of a SIP gateway.  The transferor is a SIP
   phone purely in the IP space.  The transfer target is on the PSTN
   side of a SIP gateway as well.  After completing the transfer,
   (regardless of consultative or blind) the transferee is in a call
   with the transfer target (both on the PSTN side of a gateway).  It is
   often desirable to remove the gateway(s) out of the loop.  This is
   likely to only be possible if both legs of the target call are on the
   same gateway.  With both legs on the same gateway, it may be able to
   invoke the analogous transfer on the PSTN side.  Then the target call
   would not involve the gateway.

   So the problem is how to give the proxy enough information so that it
   knows to route the call to the same gateway.  With a simple single
   call that hairpins, the incoming and outgoing leg have the same
   dialog.  The proxy should have enough information to optimize the
   routing.

   In the consultative transfer scenario, it is desirable to coerce the
   consultative INVITE out the same gateway as the original call to be
   transferred.  However there is no way to relate the consultation with
   the original call.  In the consultative case the target call INVITE
   includes the Replaces header which contains dialog information that
   can be used to relate it to the consultation.  However there is no



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   information that relates the target call to the original.

   In the blind transfer scenario, it is desirable to coerce the target
   call onto the same gateway as the original call.  However the same
   problem exists in that the target dialog cannot be related to the
   original dialog.

   In either transfer scenario, it may be desirable to push the transfer
   operation onto the non-SIP side of the gateway.  Presumably this is
   not possible unless all of the legs go out the same gateway.  If the
   gateway supports more than one truck group, it might also be
   necessary to get all of the legs on the same trunk group in order to
   perform the transfer on the non-SIP side of the gateway.

   Solutions to these gateway specific issues may involve new extensions
   to SIP in the future.

11.2.  Consultative Turned Blind Gateway Glare

   In the consultative transfer case turned blind, there is a glare-like
   problem.  The transferor initiates the consultation INVITE, the
   transferor gets impatient and hangs up, transitioning this to a blind
   transfer.  The transfer target on the gateway (connected through a
   PSTN switch to a single line or dumb analog phone) rings.  The user
   answers the phone just after the CANCEL is received by the transfer
   target.  The REFER and INVITE for the target call are sent.  The
   transferee attempts to setup the call on the PSTN side, but gets
   either a busy or lands in the users voicemail as the user has the
   handset in hand and off hook.

   This is another example of a race condition that this call flow can
   cause.  The recommended behavior is to use the approach described in
   Section 6.6.


12.  IANA Considerations

   None.


13.  Security Considerations

   The call transfer flows shown in this document are implemented using
   the REFER and Replaces call control primitives in SIP.  As such, the
   security considerations detailed in the REFER [RFC3515] and Replaces
   [RFC3891] documents MUST be followed, which are briefly summarized in
   the following paragraphs.  This document addresses the issue of
   protecting the Address of Record URI of a transfer target in Sections



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   7.1 and 7.2.

   Any REFER request must be appropriately authenticated and authorized
   using standard SIP mechanisms or calls may be hijacked.  A user agent
   may use local policy or human intervention in deciding whether or not
   to accept a REFER.  In generating NOTIFY responses based on the
   outcome of the triggered request, care should be taken in
   constructing the message/sipfrag body to ensure that no private
   information is leaked.

   An INVITE containing a Replaces header field should only be accepted
   if it has been properly authenticated and authorized using standard
   SIP mechanisms, and the requestor is authorized to perform dialog
   replacement.


14.  Acknowledgments

   This draft is a collaborative product of the SIP working group.
   Thanks to Rohan Mahy for his input on the use of Replaces in
   transfer.


15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC3891]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
              Protocol (SIP) "Replaces" Header", RFC 3891,
              September 2004.

   [RFC3892]  Sparks, R., "The Session Initiation Protocol (SIP)
              Referred-By Mechanism", RFC 3892, September 2004.

   [RFC4538]  Rosenberg, J., "Request Authorization through Dialog
              Identification in the Session Initiation Protocol (SIP)",
              RFC 4538, June 2006.



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15.2.  Informative References

   [I-D.ietf-sipping-cc-framework]
              Mahy, R., Sparks, R., Rosenberg, J., Petrie, D., and A.
              Johnston, "A Call Control and Multi-party usage framework
              for the Session Initiation  Protocol (SIP)",
              draft-ietf-sipping-cc-framework-10 (work in progress),
              April 2008.

   [I-D.ietf-sip-gruu]
              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-15 (work in progress),
              October 2007.

   [RFC4475]  Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
              and H. Schulzrinne, "Session Initiation Protocol (SIP)
              Torture Test Messages", RFC 4475, May 2006.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              February 2006.

   [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
              (SIP) Call Control - Conferencing for User Agents",
              BCP 119, RFC 4579, August 2006.

   [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
              Initiation Protocol (SIP) Event Package for Conference
              State", RFC 4575, August 2006.

   [I-D.ietf-sipping-dialogusage]
              Sparks, R., "Multiple Dialog Usages in the Session
              Initiation Protocol", draft-ietf-sipping-dialogusage-06
              (work in progress), February 2007.


Authors' Addresses

   Robert J. Sparks
   Estacado Systems

   Email: RjS@estacado.net








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   Alan Johnston (editor)
   Avaya
   St. Louis, MO

   Email: alan@sipstation.com


   Daniel Petrie
   SIPez LLC
   34 Robbins Rd.
   Arlington, MA  02476
   US

   Phone: +1 617 273 4000
   Email: dan.ietf AT SIPez DOT com
   URI:   http://www.SIPez.com/



































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Full Copyright Statement

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