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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                                               dynamicsoft
Expires: August 12, 2003                                     A. Johnston
                                                                WorldCom
                                                       February 11, 2003


          Session Initiation Protocol Call Control - Transfer
                   draft-ietf-sipping-cc-transfer-01

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 12, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document describes providing Call Transfer capabilities in the
   Session Initiation Protocol (SIP). 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.    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.    Using REFER to achieve Call Transfer . . . . . . . . . . . .  4
   5.    Basic Transfer . . . . . . . . . . . . . . . . . . . . . . .  5
   5.1   Successful Transfer  . . . . . . . . . . . . . . . . . . . .  6
   5.2   Failed Transfer  . . . . . . . . . . . . . . . . . . . . . .  9
   5.2.1 Target Busy  . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.2.2 Transfer Target does not answer  . . . . . . . . . . . . . . 10
   6.    Transfer with Consultation Hold  . . . . . . . . . . . . . . 11
   6.1   Exposing transfer target . . . . . . . . . . . . . . . . . . 11
   6.2   Protecting transfer target . . . . . . . . . . . . . . . . . 13
   6.3   Attended Transfer  . . . . . . . . . . . . . . . . . . . . . 16
   6.4   Recovery when one party does not support REFER . . . . . . . 19
   6.5   Attended Transfer when Contact URI is Not Globally
         Routable . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   6.6   Aborting a Consultation Hold . . . . . . . . . . . . . . . . 24
   6.7   Attended Transfer Fallback to Basic Transfer . . . . . . . . 25
   7.    Transfer with Referred-By  . . . . . . . . . . . . . . . . . 26
   8.    Transfer with multiple parties . . . . . . . . . . . . . . . 27
   9.    Changes from draft-sipping-cc-transfer-00  . . . . . . . . . 28
   10.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 28
   11.   Security Considerations  . . . . . . . . . . . . . . . . . . 28
   12.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 29
         Normative References . . . . . . . . . . . . . . . . . . . . 29
         Informative References . . . . . . . . . . . . . . . . . . . 29
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 30
         Intellectual Property and Copyright Statements . . . . . . . 31





















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1. Overview

   This document describes providing Call Transfer capabilities and
   requirements in SIP [1]. This work is part of the Multiparty Call
   Control Framework [5].

   The mechanisms discussed here are most closely related to traditional
   basic and consultation hold transfers. This document does not discuss
   transfer scenarios involving ad-hoc conferences (where all parties
   involved are briefly in a conference until this transferor drops
   out).

   This document details the use of REFER method [2] and Replaces [3]
   header field to achieve call transfer.

   A user agent that fully supports the transfer mechanisms described in
   this document MUST support REFER[2] and Replaces[3] in addition to
   RFC 3261 [1].

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:

        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.

        Screener -    receives a call ultimately intended for the Recipient
                      and transfers the calling party to the Recipient if
                      appropriate.




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        Recipient -   the party the Originator is ultimately connected to.


3. 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 (this is significantly different from the requirements
       of the earlier BYE-Also approach to transfer).

   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.


4. Using REFER to achieve Call Transfer

   A REFER [2] 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 [1] or the conferencing extensions



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   provided by this framework.

5. 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
   REFER and NOTIFY methods in Allow header fields in INVITE, 200 OK to
   INVITE, and OPTIONS.

   The diagrams below show indicate the first line of each message. The
   first column of the figure shows the Call-ID 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".


















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5.1 Successful Transfer


              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |            INVITE  |                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |            ACK     |                    |
         Call-ID:1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |  ACK               |                    |
         Call-ID:1 |------------------->|                    |
                   |  REFER F1          |                    |
         Call-ID:1 |------------------->|                    |
                   |  202 Accepted      |                    |
         Call-ID:1 |<-------------------|                    |
                   | NOTIFY (100 Trying) F2                  |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |                    |  INVITE F3         |
         Call-ID:2 |                    |------------------->|
                   |                    |  200 OK            |
         Call-ID:2 |                    |<-------------------|
                   |                    |  ACK               |
         Call-ID:2 |                    |------------------->|
                   |  NOTIFY (200 OK) F4|                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |  BYE               |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |                    |             BYE    |
         Call-ID:2 |                    |<-------------------|
                   |                    |             200 OK |
         Call-ID:2 |                    |------------------->|

    Figure 1. Basic Transfer Call Flow.


   F1 REFER Transferor -> Transferee



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   REFER sip:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKna9
   Max-Forwards: 70
   To: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sip:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Refer-To: <sip:transfertarget@chicago.example.com>
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F2 NOTIFY Transferee -> Transferor

   NOTIFY sip:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sip:transferor@atlanta.example.com>;tag=1928301774
   From: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 73 NOTIFY
   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


   F3 INVITE Transferee -> Transfer Target

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




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   F4 NOTIFY Transferee -> Transferor

   NOTIFY sip:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sip:transferor@atlanta.example.com>;tag=1928301774
   From: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 74 NOTIFY
   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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5.2 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.

5.2.1 Target Busy


              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |                    |                    |
                   |            INVITE  |                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |            ACK     |                    |
         Call-ID:1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |  ACK               |                    |
         Call-ID:1 |------------------->|                    |
                   |  REFER             |                    |
         Call-ID:1 |------------------->|                    |
                   |  202 Accepted      |                    |
         Call-ID:1 |<-------------------|                    |
                   | NOTIFY (100 Trying)|                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |                    |  INVITE            |
         Call-ID:2 |                    |------------------->|
                   |                    |  486 Busy Here     |
         Call-ID:2 |                    |<-------------------|
                   |                    |  ACK               |
         Call-ID:2 |                    |------------------->|
                   |    NOTIFY (503 Service Unavailable)     |
                   | or NOTIFY (486 Busy Here)               |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |  INVITE (unhold)   |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |  ACK               |                    |



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         Call-ID:1 |------------------->|                    |
                   |  BYE               |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |

    Figure 2. Failed Transfer - Target Busy

5.2.2 Transfer Target does not answer


              Transferor           Transferee             Transfer
                   |                    |                  Target
                   |            INVITE  |                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |            ACK     |                    |
         Call-ID:1 |<-------------------|                    |
                   |  INVITE (hold)     |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |  ACK               |                    |
         Call-ID:1 |------------------->|                    |
                   |  REFER             |                    |
         Call-ID:1 |------------------->|                    |
                   |  202 Accepted      |                    |
         Call-ID:1 |<-------------------|                    |
                   | NOTIFY (100 Trying)|                    |
         Call-ID:1 |<-------------------|                    |
                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |                    |  INVITE            |
         Call-ID:2 |                    |------------------->|
                   |                    |  180 Ringing       |
         Call-ID:2 |                    |<-------------------|
                   |                    |   (Transferee gets tired of waiting)
                   |                    |  CANCEL            |
         Call-ID:2 |                    |------------------->|
                   |                    |  200 OK (CANCEL)   |
         Call-ID:2 |                    |<-------------------|
                   |                    |  487 Request Cancelled (INVITE)
         Call-ID:2 |                    |<-------------------|
                   |                    |  ACK               |
         Call-ID:2 |                    |------------------->|
                   |    NOTIFY (487 Request Cancelled)       |
         Call-ID:1 |<-------------------|                    |



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                   |            200 OK  |                    |
         Call-ID:1 |------------------->|                    |
                   |  INVITE (unhold)   |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |
                   |  ACK               |                    |
         Call-ID:1 |------------------->|                    |
                   |  BYE               |                    |
         Call-ID:1 |------------------->|                    |
                   |  200 OK            |                    |
         Call-ID:1 |<-------------------|                    |

    Figure 3. Failed Transfer - Target Does Not Answer.

6. 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.

6.1 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
                   |                    |                    |
         Call-ID:1 | INVITE/200 OK/ACK  |                    |
                   |<-------------------|                    |
         Call-ID:1 | INVITE (hold)/200 OK/ACK                |
                   |------------------->|                    |
         Call-ID:2 | INVITE/200 OK/ACK  |                    |
                   |---------------------------------------->|
         Call-ID:2 | BYE/200 OK         |                    |
                   |---------------------------------------->|
         Call-ID:1 | REFER              |                    |
                   |------------------->|                    |
         Call-ID:1 | 202 Accepted       |                    |
                   |<-------------------|                    |
         Call-ID:1 | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
         Call-ID:1 |            200 OK  |                    |
                   |------------------->|                    |
         Call-ID:3 |                    |  INVITE/200 OK/ACK |
                   |                    |------------------->|
         Call-ID:1 | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
         Call-ID:1 |            200 OK  |                    |
                   |------------------->|                    |
         Call-ID:1 | BYE/200 OK         |                    |
                   |------------------->|                    |
         Call-ID:3 |                    |         BYE/200 OK |
                   |                    |<-------------------|

    Figure 4. Transfer with Consultation Hold - Exposing Transfer
   Target.



















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6.2 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
   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 [3] 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 [3] 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.


















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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  |                    |
                   |---------------------------------------->|
         dialog2   | INVITE (hold)/200 OK/ACK                |
                   |---------------------------------------->|
         dialog2   | REFER (Refer-To:sip:Transferee?Replaces=dialog1) F3
                   |---------------------------------------->|
         dialog2   | 202 Accepted       |                    |
                   |<----------------------------------------|
         dialog2   | NOTIFY (100 Trying)|                    |
                   |<----------------------------------------|
         dialog2   |                    |            200 OK  |
                   |---------------------------------------->|
         dialog3   |                 INVITE (Replaces:dialog1)/200 OK/ACK F4
                   |                    |<-------------------|
         dialog1   | BYE/200 OK         |                    |
                   |<-------------------|                    |
         dialog2   | NOTIFY (200 OK)    |                    |
                   |<----------------------------------------|
         dialog2   |                    |            200 OK  |
                   |---------------------------------------->|
         dialog2   | BYE/200 OK         |                    |
                   |---------------------------------------->|
                   |                    |  (transferee and target converse)
         dialog3   |                    |  BYE/200 OK        |
                   |                    |------------------->|


    Figure 5. Transfer Protecting Transfer Target.


   F1 INVITE Transferee -> Transferor

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



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   Contact: <sip:transferee@92.0.2.4>
   Content-Type: application/sdp
   Content-Length: ...


   F2 200 OK Transferor -> Transferee

   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnas432
   To: <sip:transferor@atlanta.example.com>;tag=31431
   From: <sip:transferee@biloxi.example.com>;tag=7553452
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Type: application/sdp
   Content-Length: ...


   F3 REFER Transferor -> Transfer Target


   REFER sip:transfertarget@client.chicago.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sip:transfertarget@chicago.example.com>;tag=a6c85cf
   From: <sip:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Refer-To: <sip:transferee@192.0.2.4;Replaces=
    090459243588173445%3Bto-tag%3D31431%3Bfrom-tag%3D7553452>
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F4 INVITE Transfer Target -> Transferee

   INVITE sip:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/UDP client.chicago.com;branch=z9hG4bKnaslu84
   Max-Forwards: 70
   To: <sip:transferee@biloxi.example.com>
   From: <sip:transfertarget@chicago.example.com>;tag=341234
   Call-ID: kmzwdle3dl3d08
   CSeq: 41 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY



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   Supported: replaces
   Contact: <sip:transfertarget@client.chicago.com>
   Replaces: 090459243588173445;to-tag=31431;from-tag=7553452
   Content-Type: application/sdp
   Content-Length: ...







6.3 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.
































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              Transferor           Transferee             Transfer
                   |                    |                  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:sip:TransferTarget?Replaces=dialog2) F3
                   |------------------->|                    |
          dialog1  | 202 Accepted       |                    |
                   |<-------------------|                    |
          dialog1  | NOTIFY (100 Trying)|                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog3  |                INVITE (Replaces:dialog2)/200 OK/ACK F4
                   |                    |------------------->|
          dialog2  | BYE/200 OK         |                    |
                   |<----------------------------------------|
          dialog1  | NOTIFY (200 OK)    |                    |
                   |<-------------------|                    |
          dialog1  |            200 OK  |                    |
                   |------------------->|                    |
          dialog1  | BYE/200 OK         |                    |
                   |------------------->|                    |
          dialog3  |                    |         BYE/200 OK |
                   |                    |<-------------------|

    Figure 6. Attended Transfer Call Flow.


   F1 INVITE Transferor -> Transfer Target

   INVITE sip:transfertarget@chicago.example.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sip:transfertarget@chicago.example.com>
   From: <sip:transferor@atlanta.example.com>;tag=763231
   Call-ID: 090459243588173445
   CSeq: 29887 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Type: application/sdp



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   Content-Length: ...


   F2 200 OK Transfer Target -> Transferee

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


   F3 REFER Transferor -> Transferee

   REFER sip:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sip:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Refer-To: <sip:transfertarget@client.chicago.example.com;Replaces=
    090459243588173445%3Bto-tag%3D9m2n3wq%3Bfrom-tag%3D763231>
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F4 INVITE Transferee -> Transfer Target

   INVITE sip:transfertarget@client.chicago.example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnaslu82
   Max-Forwards: 70
   To: <sip:transfertarget@chicago.example.com>
   From: <sip:transferee@biloxi.example.com>;tag=954
   Call-ID: kmzwdle3dl3d08
   CSeq: 41 INVITE
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
   Supported: replaces
   Contact: <sip:transferee@192.0.2.4>
   Replaces: 090459243588173445;to-tag=9m2n3wq;from-tag=763231
   Content-Type: application/sdp



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   Content-Length: ...



6.4 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.


              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                |
                   |---------------------------------------->|
          dialog1  | REFER (Refer-To:sip:TransferTarget?Replaces=dialog2)
                   |------------------->|                    |
          dialog1  | 501 Not Implemented                     |
                   |<-------------------|                    |
          dialog2  | REFER  (Refer-To:sip:Transferee?Replaces=dialog1)
                   |---------------------------------------->|
          dialog2  | 202 Accepted       |                    |
                   |<----------------------------------------|
          dialog2  | NOTIFY (100 Trying)|                    |
                   |<----------------------------------------|
          dialog2  |                    |            200 OK  |
                   |---------------------------------------->|
          dialog3  |                    |  INVITE (Replaces:dialog1)/200 OK/ACK
                   |                    |<-------------------|
          dialog2  | NOTIFY (200 OK)    |                    |
                   |<----------------------------------------|
                   |                    |            200 OK  |
                   |---------------------------------------->|
          dialog1  | BYE/200 OK         |                    |
                   |<-------------------|                    |
          dialog2  | BYE/200 OK         |                    |
                   |---------------------------------------->|
          dialog3  |                    |  BYE/200 OK        |
                   |                    |------------------->|



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    Figure 7. Recovery when one party does not support REFER.

6.5 Attended Transfer when Contact URI is Not Globally Routable

   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 and 7 may fail
   since they use the Contact URI in the Refer-To header field.  Figure
   8 shows such a scenario involving a Screening Proxy in which the
   transfer initially fails but succeeds on a second try. The failure
   (403 Forbidden, 404 Not Found, or a timeout after no response)
   response is communicated back to the Transferor.  Since this may be
   caused by routing problems with the Contact URI, the Transferor
   retries the REFER this time with Refer-To containing the Address of
   Record (AOR) of the Target (the same URI the Transferor used to reach
   the Target).  However, the use of the AOR URI may result in routing
   features being activated such as forking or sequential searching
   which may result in the triggered INVITE reaching the wrong UA.  To
   prevent an incorrect UA answering the INVITE, a Require: replaces
   header field is included in the Refer-To.  This ensures that only the
   UA which matches the Replaces dialog will answer the INVITE, since
   any incorrect UA which supports Replaces will reply with a 481 and a
   UA which does not support Replaces will reply with a 420.

   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.

       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:sip:TargetContact?Replaces=dialog2) F3
           |----------------->|             |             |
   dialog1 | 202 Accepted     |             |             |



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           |<-----------------|             |             |
   dialog1 | NOTIFY (100 Trying)            |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog3 |                  |  INVITE (Replaces:dialog2)/403/ACK
           |                  |------------>|             |
   dialog1 | NOTIFY (403 Forbidden) F4      |             |
           |<-----------------|             |             |
   dialog1 |          200 OK  |             |             |
           |----------------->|             |             |
   dialog1 |REFER(Refer-To:sip:TargetAOR?Replaces=dialog2&Require=replaces) F5
           |----------------->|             |             |
   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 8. Attended Transfer Call Flow with non-routable Contact URI


   F1 INVITE Transferor -> Transfer Target

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



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   Content-Length: ...


   F2 200 OK Transfer Target -> Transferee

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


   F3 REFER Transferor -> Transferee

   REFER sip:transferee@192.0.2.4 SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKnashds9
   Max-Forwards: 70
   To: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   From: <sip:transferor@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 REFER
   Refer-To: <sip:transfertarget@client.chicago.example.com;Replaces=
    090459243588173445%3Bto-tag%3D9m2n3wq%3Bfrom-tag%3D763231>
   Contact: <sip:transferor@pc33.atlanta.example.com>
   Content-Length: 0


   F4 NOTIFY Transferee -> Transferor

   NOTIFY sip:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sip:transferor@atlanta.example.com>;tag=1928301774
   From: <sip: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=3112
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag



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   Content-Length: ...

   SIP/2.0 403 Forbidden


   F5 REFER Transferor -> Transferee

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


   F6 INVITE Transferee -> Transfer Target

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


   F7 NOTIFY Transferee -> Transferor

   NOTIFY sip:transferor@pc33.atlanta.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnas432
   Max-Forwards: 70
   To: <sip:transferor@atlanta.example.com>;tag=1928301774
   From: <sip:transferee@biloxi.example.com>;tag=a6c85cf
   Call-ID: a84b4c76e66710
   CSeq: 76 NOTIFY
   Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY



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   Supported: replaces
   Event: refer;id=98873867
   Subscription-State: terminated;reason=noresource
   Content-Type: message/sipfrag
   Content-Length: ...

   SIP/2.0 200 OK


   For a UA which requires all request to be routed through a proxy
   (such as for NAT/firewall traversal or screening/feature reasons),
   special care must be taken in constructing a globally routable
   Contact URI.  One approach is to construct a URI which is unique for
   the device which resolves to the proxy.  A registration would then be
   required to bind this URI to a URI which resolves directly to the
   device.

   For example, consider a UA with a username carol and a hostname
   server51.example.com.  A normal Contact would be automatically
   generated in the form:

      Contact: sip:carol@serv51.example.com

   However, if this UA requires that all requests come through a proxy
   server at p1.chicago.com then this Contact will not work as the proxy
   will be bypassed.

   Consider instead a Contact of the form:

      Contact: sip:serv51@example.com

   in which this sip:serv51@example.com URI would be registered by the
   UA against a Contact:

      Contact: sip:carol@serv51.example.com

   which resolves directly to the UA.

   This means that a UA would first register a URI that corresponds to
   the device.  Then, it would register a users URI (AOR) and use the
   device URI that it registered as the Contact URI.

   Other approaches may also be used to generate a globally routable
   Contact URI.

6.6 Aborting a Consultation Hold

   In any of the consultation hold flows above, the Transferor may



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   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, he 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 true implementation of this
   feature requires a short ad-hoc conference between all parties, which
   ensures that no media clipping occurs.  This flow is outside the
   scope of this document.

   For flows that expose the transfer target, this simply becomes a
   basic transfer.

   This scenario is far more complicated for flows that protect the
   transfer target. Since no session is established between the
   transferor and the transfer target, the transfer target's agent would
   have to honor out-of-session REFERs, and somehow indicate what's
   happening via its user interface (this scenario is most likely to
   occur when the transfer-target is away from his agent).

6.7 Attended Transfer Fallback to Basic Transfer

   In this flow, an attempted attended transfer fails so the transferor
   falls back to basic transfer.  The use of OPTIONS is shown 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.  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.  (Note
   that the same information could have been determined by including a
   Require: replaces in the initial INVITE in dialog2, which would have
   failed with a 421 response.)  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      |                    |
                   |---------------------------------------->|
          dialog1  | REFER (Refer-To:sip: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 |
                   |                    |<-------------------|

    Figure 9. Attended Transfer Fallback to Basic Transfer.

7. 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 [4] 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
   the Referred-By header field in the REFER which is then copied into
   the triggered INVITE.  However, a more secure mechanism involving the



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   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.  A call flow showing the request is in [4]

8. Transfer with multiple parties

   In this example 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.

            Originator   Facilitator   Screener   Recipient
        Call-ID  |            |            |          |
            1    |INVITE/200 OK/ACK        |          |"Get Fred for me!"
                 |----------->|            |          |     "Right away!"
            1    |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
                 |            |            |--------->|
            2    |            |REFER       |          |
                 |            |<-----------|          |
            2    |            |202 Accepted|          |
                 |            |----------->|          |
            2    |            |NOTIFY (100 Trying)    |
                 |            |----------->|          |
            2    |            |200 OK      |          |
                 |            |<-----------|          |
            2    |            |INVITE/200 OK/ACK      |
                 |            |---------------------->|"This is Fred"
            2    |            |NOTIFY (200 OK)        |  "Please hold for
                 |            |----------->|          |              Mary"
            2    |            |200 OK      |          |
                 |            |<-----------|          |
            2    |            |BYE/200 OK  |          |
                 |            |<-----------|          |
            3    |            |            |BYE/200 OK|
                 |            |            |--------->|
            2    |            |INVITE (hold)/200 OK/ACK



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                 |            |---------------------->|
            1    |REFER       |            |          |
                 |<-----------|            |          |
            1    |202 Accepted|            |          |
                 |----------->|            |          |
            1    |NOTIFY (100 Trying)      |          |
                 |----------->|            |          |
            1    |200 OK      |            |          |
                 |<-----------|            |          |
            1    |INVITE/200 OK/ACK        |          |
                 |----------------------------------->| "Hey Fred"
            1    |NOTIFY (200 OK)          |          |    "Hello Mary"
                 |----------->|            |          |
            1    |200 OK      |            |          |
                 |<-----------|            |          |
            1    |BYE/200 OK  |            |          |
                 |<-----------|            |          |
            2    |            |BYE/200 OK  |          |
                 |            |---------------------->|
            1    |BYE/200 OK  |            |          |
                 |<-----------------------------------| "See you later"

   Figure 10. Transfer with Multiple Parties Example.

9. Changes from draft-sipping-cc-transfer-00

   o  Added section on use of Referred-By header.

   o  Added selected message details.

   o  Added flow for attended transfer with non-globally routable
      Contact URI.

   o  Added flow for attended transfer fallback to unattended transfer.

   o  Added Security Considerations Section.


10. IANA Considerations

   None.

11. 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
   attacks and security approaches are those detailed in the REFER and
   Replaces documents which are briefly summarized in the following



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   paragraphs.  This document addresses the issue of protecting the
   Address of Record URI of a transfer target in Sections 6.1 and 6.2.

   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.

12. 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.

Normative References

   [1]  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.

   [2]  Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-07
        (work in progress), December 2002.

   [3]  Dean, R., Biggs, B. and R. Mahy, "The Session Inititation
        Protocol (SIP) 'Replaces' Header", draft-ietf-sip-replaces-02
        (work in progress), May 2002.

   [4]  Sparks, R., "The Referred-By Mechanism",
        draft-ietf-sip-referredby-00 (work in progress), May 2002.

Informative References

   [5]  Mahy, R., "A Multi-party Application Framework for SIP",
        draft-ietf-sipping-cc-framework-01 (work in progress), July
        2002.








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Authors' Addresses

   Robert J. Sparks
   dynamicsoft
   5100 Tennyson Parkway
   Suite 1200
   Plano, TX  75024

   EMail: rsparks@dynamicsoft.com


   Alan Johnston
   WorldCom
   100 South 4th Street
   St. Louis, MO  63102

   EMail: alan.johnston@wcom.com


































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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