SIPPING WG R. Sparks
Internet-Draft Estacado Systems
Expires: September 6, 2006 A. Johnston, Ed.
SIPStation
D. Petrie
SIPez LLC
March 5, 2006
Session Initiation Protocol Call Control - Transfer
draft-ietf-sipping-cc-transfer-06
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Copyright Notice
Copyright (C) The Internet Society (2006).
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. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Using REFER to achieve Call Transfer . . . . . . . . . . . . . 5
5. Basic Transfer . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Successful Transfer . . . . . . . . . . . . . . . . . . . 7
5.2. Transfer with Dialog Reuse . . . . . . . . . . . . . . . . 11
5.3. Failed Transfer . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Target Busy . . . . . . . . . . . . . . . . . . . . . 15
5.3.2. Transfer Target does not answer . . . . . . . . . . . 17
6. Transfer with Consultation Hold . . . . . . . . . . . . . . . 18
6.1. Exposing transfer target . . . . . . . . . . . . . . . . . 18
6.2. Protecting transfer target . . . . . . . . . . . . . . . . 19
6.3. Attended Transfer . . . . . . . . . . . . . . . . . . . . 24
6.4. Recovery when one party does not support REFER . . . . . . 27
6.5. Attended transfer when Contact URI is not known to
route to a unique user agent. . . . . . . . . . . . . . . 28
6.6. Semi-Attended Transfer . . . . . . . . . . . . . . . . . . 35
6.7. Attended Transfer Fallback to Basic Transfer . . . . . . . 40
7. Transfer with Referred-By . . . . . . . . . . . . . . . . . . 42
8. Transfer as an Ad-Hoc Conference . . . . . . . . . . . . . . . 48
9. Transfer with multiple parties . . . . . . . . . . . . . . . . 51
10. Gateway Transfer Issues . . . . . . . . . . . . . . . . . . . 53
10.1. Coerce Gateway Hairpins to the Same Gateway . . . . . . . 53
10.2. Consultative Turned Blind Gateway Glare . . . . . . . . . 54
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54
12. Security Considerations . . . . . . . . . . . . . . . . . . . 54
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 55
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 55
14.1. Normative References . . . . . . . . . . . . . . . . . . . 55
14.2. Informative References . . . . . . . . . . . . . . . . . . 55
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 57
Intellectual Property and Copyright Statements . . . . . . . . . . 58
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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 [6].
The mechanisms discussed here are most closely related to traditional
basic and consultation hold transfers.
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]. A user agent should use a Contact URI which meets the
requirements in Section 8.1.1.8 of RFC 3261. A user agent SHOULD
support the Target-Dialog header field [5].
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.
Screener - receives a call ultimately intended for the
Recipient and transfers the calling party to
the Recipient if appropriate.
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.
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.
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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
conferencing extensions described in the conferencing framework [9].
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 5.2. However, the approach described in this
document is to avoid dialog reuse. The issues and difficulties
associated with dialog reuse are described in [12].
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
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 [7] can be used to address problem 1. Problem 2 can be
addressed using the Target-Dialog header field defined in [5]. In
the immediate term, this solution to problem 2 allows the existing
REFER authorization policy to be reused.
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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 [7].
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.
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 messages.
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
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"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.
5.1. 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:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=723jd2d>
Content-Type: application/sdp
Content-Length: ...
F3 REFER Transferor -> Transferee
REFER sips:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKna9
Max-Forwards: 70
To: <sips:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=723jd2d>
Content-Length: 0
F4 NOTIFY Transferee -> Transferor
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NOTIFY sips:4889445d8kjtk3@atlanta.example.com;grid=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:3k3k3ld812adkjw@biloxi.example.com;grid=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:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha>
Content-Type: application/sdp
Content-Length: ...
F6 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;grid=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:3k3k3ld812adkjw@biloxi.example.com;grid=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
5.2. 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;grid=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;grid=723jd2d>
Content-Length: 0
F4 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;grid=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;grid=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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5.3. 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.
5.3.1. 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 (503 Service Unavailable) |
| or 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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5.3.2. 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.
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.
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.
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
| | |
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.
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
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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 [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.
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 [8]
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:1, |
| 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:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=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;grid=384i32lw3>
Content-Type: application/sdp
Content-Length: ...
F4 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: 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;grid=8594958>
Content-Type: application/sdp
Content-Length: ...
F5 REFER Transferor -> Transfer Target
REFER sips:482n4z24kdg@chicago.example.com;grid=8594958 SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
Max-Forwards: 70
To: <sips:482n4z24kdg@chicago.example.com;grid=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:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha
?Replaces=090459243588173445%3Bto-tag%3D31431%3Bfrom-tag%3D7553452>
</allOneLine>
Target-Dialog: 592435881734450904;local-tag=9m2n3wq
;remote-tag=763231
Contact: <sips:4889445d8kjtk3@atlanta.example.com;grid=723jd2d>
Content-Length: 0
F6 INVITE Transfer Target -> Transferee
INVITE sips:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS client.chicago.example.com;branch=z9hG4bKnaslu84
Max-Forwards: 70
To: <sips:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=8594958>
Replaces: 090459243588173445;to-tag=31431;from-tag=7553452
Content-Type: application/sdp
Content-Length: ...
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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.
The Contact URI of the Transfer Target SHOULD be used by the
Transferee 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 Transferee 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:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=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;grid=384i32lw3>
Content-Type: application/sdp
Content-Length: ...
F4 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: 592435881734450904
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu
Contact: <sips:482n4z24kdg@chicago.example.com;grid=8594958>
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Content-Type: application/sdp
Content-Length: ...
F5 REFER Transferor -> Transferee
REFER sips:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
Max-Forwards: 70
To: <sips:3k3k3ld812adkjw@biloxi.example.com;grid=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;grid=8594958?
Replaces=592435881734450904%3Bto-tag%3D9m2n3wq%3Bfrom-tag3D763231>
</allOneLine>
Target-Dialog: 592435881734450904;local-tag=9m2n3wq
;remote-tag=763231
Contact: <sips:4889445d8kjtk3@atlanta.example.com;grid=723jd2d>
Content-Length: 0
F6 INVITE Transferee -> Transfer Target
INVITE sips:482n4z24kdg@chicago.example.com;grid=8594958 SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
Max-Forwards: 70
To: <sips:482n4z24kdg@chicago.example.com;grid=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:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha>
Replaces: 592435881734450904;to-tag=9m2n3wq;from-tag=763231
Content-Type: application/sdp
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.
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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.
6.5. 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.
6.6. 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 User Agent 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)
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 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 complet 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 13 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 14. 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
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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 [8]
document are reused, specifically the <hex> and <allOneLine> tags.
F5 REFER Transferor -> Transferee
REFER sips:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK392039842
Max-Forwards: 70
To: <sips:3k3k3ld812adkjw@biloxi.example.com;grid=3413kj2ha>
From: <sips:transferor@atlanta.example.com>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314160 REFER
<allOneLine>
Refer-To: <sips:482n4z24kdg@chicago.example.com;grid=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;grid=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;grid=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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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;grid=8594958 SIP/2.0
Via: SIP/2.0/TLS referee.example;branch=z9hG4bKffe209934aac
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To: <sips:482n4z24kdg@chicago.example.com;grid=8594958>
From: <sips:transferee@biloxi.example.com>;tag=2909034023
Call-ID: fe9023940-a3465@referee.example
CSeq: 889823409 INVITE
Max-Forwards: 70
Contact: <sips:3k3k3ld812adkjw@biloxi.example.com;grid=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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4E230203010001A381E93081E6301D0603551D1104163014811273656E646572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0603551D1104163014811273656E646572406578616D706C652E6F7267300906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</hex>
------590F24D439B31E08745DEF0CD9397189--
--my-boundary-9--
8. Transfer as an Ad-Hoc Conference
In this flow, 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[10] and hosts an
ad-hoc conference involving Alice, Bob, and Carol. Alice and Carol
subscribe to the conference package[11] 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
NOTIFY (Conf-Info).
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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 |
| |<-------------------|
| | ACK |
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| |------------------->|
| | 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 | |
|<-------------------| |
| INVITE Replaces:B-C Contact:Alice |
|---------------------------------------->|
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| 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 15. Attended Transfer as an Ad-Hoc Conference.
9. 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
| | | |
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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"
|----------->| | |
6 |200 OK | | |
|<-----------| | |
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1 |BYE/200 OK | | |
|<-----------| | |
5 | |BYE/200 OK | |
| |---------------------->|
7 |BYE/200 OK | | |
|<-----------------------------------| "See you later"
Figure 16. Transfer with Multiple Parties Example.
10. 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.
10.1. 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
information that relates the target call to the original.
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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.
10.2. 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 user
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.
11. IANA Considerations
None.
12. 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
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
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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.
13. 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.
14. References
14.1. 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 Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[3] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
Mechanism", RFC 3892, September 2004.
[5] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
draft-ietf-sip-target-dialog-03 (work in progress),
December 2005.
14.2. Informative References
[6] Mahy, R., "A Call Control and Multi-party usage framework for
the Session Initiation Protocol (SIP)",
draft-ietf-sipping-cc-framework-05 (work in progress),
October 2005.
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[7] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", draft-ietf-sip-gruu-06 (work in progress),
October 2005.
[8] Sparks, R., "Session Initiation Protocol Torture Test
Messages", draft-ietf-sipping-torture-tests-09 (work in
progress), November 2005.
[9] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-05 (work in
progress), May 2005.
[10] Levin, O., "Session Initiation Protocol Call Control -
Conferencing for User Agents",
draft-ietf-sipping-cc-conferencing-07 (work in progress),
June 2005.
[11] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Conference State",
draft-ietf-sipping-conference-package-12 (work in progress),
July 2005.
[12] Sparks, R., "Multiple Dialog Usages in the Session Initiation
Protocol", draft-sparks-sipping-dialogusage-00 (work in
progress), July 2004.
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Authors' Addresses
Robert J. Sparks
Estacado Systems
Email: RjS@estacado.net
Alan Johnston (editor)
SIPStation
St. Louis, MO 63124
Email: alan@sisptation.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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