Network Working Group R. Sparks
Request for Comments: 5589 Tekelec
BCP: 149 A. Johnston, Ed.
Category: Best Current Practice Avaya
D. Petrie
SIPez LLC
June 2009
Session Initiation Protocol (SIP) Call Control - Transfer
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
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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.
Sparks, et al. Best Current Practice [Page 1]
RFC 5589 SIP CC Transfer June 2009
Table of Contents
1. Overview ........................................................3
2. Actors and Roles ................................................3
3. Terminology .....................................................4
4. Requirements ....................................................4
5. Using REFER to Achieve Call Transfer ............................5
6. Basic Transfer ..................................................6
6.1. Successful Transfer ........................................8
6.2. Transfer with Dialog Reuse ................................11
6.3. Failed Transfer ...........................................15
6.3.1. Target Busy ........................................16
6.3.2. Transfer Target Does Not Answer ....................17
7. Transfer with Consultation Hold ................................18
7.1. Exposing Transfer Target ..................................18
7.2. Protecting Transfer Target ................................19
7.3. Attended Transfer .........................................24
7.4. Recovery When One Party Does Not Support REFER ............28
7.5. Attended Transfer When Contact URI Is Not Known to
Route to a User Agent .....................................29
7.6. Semi-Attended Transfer ....................................37
7.7. Attended Transfer Fallback to Basic Transfer ..............42
8. Transfer with Referred-By ......................................45
9. Transfer as an Ad Hoc Conference ...............................49
10. Transfer with Multiple Parties ................................52
11. Gateway Transfer Issues .......................................54
11.1. Coerce Gateway Hairpins to the Same Gateway ..............54
11.2. Consultative Turned Blind Gateway Glare ..................55
12. Security Considerations .......................................55
13. Acknowledgments ...............................................56
14. References ....................................................56
14.1. Normative References .....................................56
14.2. Informative References ...................................57
Sparks, et al. Best Current Practice [Page 2]
RFC 5589 SIP CC Transfer June 2009
1. Overview
This document describes providing Call Transfer capabilities and
requirements in SIP [RFC3261]. This work is part of the multiparty
call control framework [CC-FRMWRK].
The mechanisms discussed here are most closely related to
traditional, basic, and consultation hold transfers.
This document details the use of the REFER method [RFC3515] and
Replaces [RFC3891] header field to achieve call transfer.
A User Agent (UA) that fully supports the transfer mechanisms
described in this document supports REFER [RFC3515] and Replaces
[RFC3891] in addition to RFC 3261 [RFC3261]. A User Agent should use
a Contact URI that meets the requirements in Section 8.1.1.8 of RFC
3261. A compliant User Agent supports the Target-Dialog header field
[RFC4538].
2. Actors and Roles
There are three actors in a given transfer event, each playing one of
the following roles:
Transferee: the party being transferred to the Transfer
Target.
Transferor: the party initiating the transfer.
Transfer Target: the new party being introduced into a call with
the Transferee.
The following roles are used to describe transfer requirements and
scenarios:
Originator: wishes to place a call to the Recipient. This
actor is the source of the first INVITE in a
session, to either a Facilitator or a Screener.
Facilitator: receives a call or out-of-band request from the
Originator, establishes a call to the Recipient
through the Screener, and connects the Originator
to the Recipient. Typically, a Facilitator acts
on behalf of the Originator.
Sparks, et al. Best Current Practice [Page 3]
RFC 5589 SIP CC Transfer June 2009
Screener: receives a call ultimately intended for the
Recipient and transfers the calling party to the
Recipient if appropriate. Typically, a Screener
acts on behalf of the Recipient.
Recipient: the party to which the Originator is ultimately
connected.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC2119].
4. Requirements
1. Any party in a SIP session must be able to transfer any other
party in that session at any point in that session.
2. The Transferor and the Transferee must not be removed from a
session as part of a transfer transaction.
At first glance, requirement 2 may seem to indicate
that the user experience in a transfer must be
significantly different from what a current Private Branch
Exchange (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.
Sparks, et al. Best Current Practice [Page 4]
RFC 5589 SIP CC Transfer June 2009
To meet this requirement, the transfer operation can
be modeled as an ad hoc conference between three
parties, as discussed in Section 9.
5. Using REFER to Achieve Call Transfer
A REFER [RFC3515] can be issued by the Transferor to cause the
Transferee to issue an INVITE to the Transfer Target. Note that a
successful REFER transaction does not terminate the session between
the Transferor and the Transferee. If those parties wish to
terminate their session, they must do so with a subsequent BYE
request. The media negotiated between the transferee and the
Transfer Target is not affected by the media that had been negotiated
between the Transferor and the Transferee. In particular, the INVITE
issued by the Transferee will have the same Session Description
Protocol (SDP) body it would have if the Transferee had initiated
that INVITE on its own. Further, the disposition of the media
streams between the Transferor and the Transferee is not altered by
the REFER method.
Agents may alter a session's media through additional signaling. For
example, they may make use of the SIP hold re-INVITE [RFC3261] or
conferencing extensions described in the conferencing framework
[RFC4353].
To perform the transfer, the Transferor and Transferee could reuse an
existing dialog established by an INVITE to send the REFER. This
would result in a single dialog shared by two uses -- an invite usage
and a subscription usage. The call flows for this are shown in
detail in Section 6.2. However, the approach described in this
document is to avoid dialog reuse. The issues and difficulties
associated with dialog reuse are described in [RFC5057].
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 to which we were
already talking.
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.
Sparks, et al. Best Current Practice [Page 5]
RFC 5589 SIP CC Transfer June 2009
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").
Globally Routable UA URIs (GRUUs) [SIP-GRUU] can be used to address
problem 1. Problem 2 can be addressed using the Target-Dialog header
field defined in [RFC4538]. In the immediate term, this solution to
problem 2 allows the existing REFER authorization policy to be
reused.
As a result, if the Transferee supports the target-dialog extension
and the Transferor knows the Contact URI is routable outside the
dialog, the REFER SHOULD be sent in a new dialog. If the nature of
the Contact URI is not known or if support for the target-dialog
extension is not known, the REFER SHOULD be sent inside the existing
dialog. A Transferee MUST be prepared to receive a REFER either
inside or outside a dialog. One way that a Transferor could know
that a Contact URI is routable outside a dialog is by validation
(e.g., sending an OPTIONS and receiving a response) or if it
satisfies the properties described in the GRUU specification
[SIP-GRUU].
This document does not prescribe the flows and examples precisely as
they are shown, but rather the flows illustrate the principles for
best practice for the transfer feature. The call flows represent
well-reviewed examples of SIP usage to implement transfer with REFER,
which are Best Common Practice according to IETF consensus.
In most of the following examples, the Transferor is in the
atlanta.example.com domain, the Transferee is in the
biloxi.example.com, and the Transfer Target is in the
chicago.example.com domain.
6. Basic Transfer
Basic Transfer consists of the Transferor providing the Transfer
Target's contact to the Transferee. The Transferee attempts to
establish a session using that contact and reports the results of
that attempt to the Transferor. The signaling relationship between
the Transferor and Transferee is not terminated, so the call is
recoverable if the Transfer Target cannot be reached. Note that the
Transfer Target's contact information has been exposed to the
Transferee. The provided contact can be used to make new calls in
the future.
Sparks, et al. Best Current Practice [Page 6]
RFC 5589 SIP CC Transfer June 2009
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. Participants SHOULD also indicate
support for Target-Dialog in the Supported header field.
The diagrams below show the first line of each message. The first
column of the figure shows the dialog used in that particular
message. In these diagrams, media is managed through re-INVITE
holds, but other mechanisms (mixing multiple media streams at the UA
or using the conferencing extensions, for example) are valid.
Selected message details are shown labeled as message F1, F2, etc.
Each of the flows below shows the dialog between the Transferor and
the Transferee remaining connected (on hold) during the REFER
process. While this provides the greatest flexibility for recovery
from failure, it is not necessary. If the Transferor's agent does
not wish to participate in the remainder of the REFER process and has
no intention of assisting with recovery from transfer failure, it
could emit a BYE to the Transferee as soon as the REFER transaction
completes. This flow is sometimes known as "unattended transfer" or
"blind transfer".
Figure 1 shows transfer when the Transferee utilizes a GRUU and
supports the target-dialog extension and indicates this to the
Transferor. As a result, the Transferor sends the REFER outside the
INVITE dialog. The Transferee is able to match this REFER to the
existing dialog using the Target-Dialog header field in the refer
which references the existing dialog.
Sparks, et al. Best Current Practice [Page 7]
RFC 5589 SIP CC Transfer June 2009
6.1. Successful Transfer
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
Sparks, et al. Best Current Practice [Page 8]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
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: ;tag=31kdl4i3k
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
Content-Type: application/sdp
Content-Length: ...
F3 REFER Transferor -> Transferee
REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKna9
Max-Forwards: 70
To:
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 REFER
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Require: tdialog
Refer-To:
Target-Dialog: 090459243588173445;local-tag=7553452
;remote-tag=31kdl4i3k
Contact:
Content-Length: 0
Sparks, et al. Best Current Practice [Page 9]
RFC 5589 SIP CC Transfer June 2009
F4 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
Max-Forwards: 70
To: ;tag=1928301774
From:
;tag=a6c85cf
Call-ID: a84b4c76e66710
CSeq: 73 NOTIFY
Contact:
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:
From: ;tag=j3kso3iqhq
Call-ID: 90422f3sd23m4g56832034
CSeq: 521 REFER
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 10]
RFC 5589 SIP CC Transfer June 2009
F6 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
Max-Forwards: 70
To: ;tag=1928301774
From:
;tag=a6c85cf
Call-ID: a84b4c76e66710
CSeq: 74 NOTIFY
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, tdialog
Event: refer
Subscription-State: terminated;reason=noresource
Content-Type: message/sipfrag
Content-Length: ...
SIP/2.0 200 OK
6.2. Transfer with Dialog Reuse
In this scenario, the Transferor does not know the properties of the
Transferee's Contact URI or does not know that the Transferee
supports the Target-Dialog header field. As a result, the REFER is
sent inside the INVITE dialog.
Sparks, et al. Best Current Practice [Page 11]
RFC 5589 SIP CC Transfer June 2009
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
Sparks, et al. Best Current Practice [Page 12]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
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: ;tag=31kdl4i3k
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces
Contact:
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: ;tag=7553452
From: ;tag=31kdl4i3k
Call-ID: 090459243588173445
CSeq: 314159 REFER
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Refer-To:
Contact:
Content-Length: 0
Sparks, et al. Best Current Practice [Page 13]
RFC 5589 SIP CC Transfer June 2009
F4 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
Max-Forwards: 70
To: ;tag=31kdl4i3k
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29888 INVITE
Contact:
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:
From: ;tag=j3kso3iqhq
Call-ID: 90422f3sd23m4g56832034
CSeq: 521 REFER
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 14]
RFC 5589 SIP CC Transfer June 2009
F6 NOTIFY Transferee -> Transferor
NOTIFY sips:4889445d8kjtk3@atlanta.example.com;gr=723jd2d SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnas432
Max-Forwards: 70
To: ;tag=31kdl4i3k
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29889 INVITE
Contact:
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
6.3. Failed Transfer
This section shows examples of failed transfer attempts. After the
transfer failure occurs, the Transferor takes the Transferee off hold
and resumes the session.
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RFC 5589 SIP CC Transfer June 2009
6.3.1. Target Busy
Transferor Transferee Transfer
| | Target
| | |
| INVITE | |
dialog1 |<-------------------| |
| 200 OK | |
dialog1 |------------------->| |
| ACK | |
dialog1 |<-------------------| |
| INVITE (hold) | |
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
| ACK | |
dialog1 |------------------->| |
| REFER (Target-Dialog:1) |
dialog2 |------------------->| |
| 202 Accepted | |
dialog2 |<-------------------| |
| NOTIFY (100 Trying)| |
dialog2 |<-------------------| |
| 200 OK | |
dialog2 |------------------->| |
| | INVITE |
dialog3 | |------------------->|
| | 486 Busy Here |
dialog3 | |<-------------------|
| | ACK |
dialog3 | |------------------->|
| NOTIFY (486 Busy Here) |
dialog2 |<-------------------| |
| 200 OK | |
dialog2 |------------------->| |
| INVITE (unhold) | |
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
| ACK | |
dialog1 |------------------->| |
| BYE | |
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
Figure 3: Failed Transfer - Target Busy
Sparks, et al. Best Current Practice [Page 16]
RFC 5589 SIP CC Transfer June 2009
6.3.2. Transfer Target Does Not Answer
Transferor Transferee Transfer
| | Target
| INVITE | |
dialog1 |<-------------------| |
| 200 OK | |
dialog1 |------------------->| |
| ACK | |
dialog1 |<-------------------| |
| INVITE (hold) | |
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
| ACK | |
dialog1 |------------------->| |
| REFER | |
dialog2 |------------------->| |
| 202 Accepted | |
dialog2 |<-------------------| |
| NOTIFY (100 Trying)| |
dialog2 |<-------------------| |
| 200 OK | |
dialog2 |------------------->| |
| | INVITE |
dialog3 | |------------------->|
| | 180 Ringing |
dialog3 | |<-------------------|
| (Transferee gets tired of waiting)
| | CANCEL |
dialog3 | |------------------->|
| | 200 OK (CANCEL) |
dialog3 | |<-------------------|
| 487 Request Cancelled (INVITE)
dialog3 | |<-------------------|
| | ACK |
dialog3 | |------------------->|
| NOTIFY (487 Request Cancelled) |
dialog2 |<-------------------| |
| 200 OK | |
dialog2 |------------------->| |
| INVITE (unhold) | |
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
| ACK | |
dialog1 |------------------->| |
| BYE | |
Sparks, et al. Best Current Practice [Page 17]
RFC 5589 SIP CC Transfer June 2009
dialog1 |------------------->| |
| 200 OK | |
dialog1 |<-------------------| |
Figure 4: Failed Transfer - Target Does Not Answer
7. Transfer with Consultation Hold
Transfer with consultation hold involves a session between the
Transferor and the Transfer Target before the transfer actually takes
place. This is implemented with SIP Hold and Transfer as described
above.
A nice feature is for the Transferor to let the target know that the
session relates to an intended transfer. Since many UAs render the
display name in the From header field to the user, a consultation
INVITE could contain a string such as "Incoming consultation from
Transferor with intent to transfer Transferee", where the display
names of the transferor and transferee are included in the string.
7.1. Exposing Transfer Target
The Transferor places the Transferee on hold, establishes a call with
the Transfer Target to alert them to the impending transfer,
terminates the connection with the Transfer Target, then proceeds
with transfer as above. This variation can be used to provide an
experience similar to that expected by current PBX and Centrex users.
To (hopefully) improve clarity, non-REFER transactions have been
collapsed into one indicator with the arrow showing the direction of
the request.
Sparks, et al. Best Current Practice [Page 18]
RFC 5589 SIP CC Transfer June 2009
Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | BYE/200 OK | |
|---------------------------------------->|
dialog3 | REFER | |
|------------------->| |
dialog3 | 202 Accepted | |
|<-------------------| |
dialog3 | NOTIFY (100 Trying)| |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog4 | | INVITE/200 OK/ACK |
| |------------------->|
dialog3 | NOTIFY (200 OK) | |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog1 | BYE/200 OK | |
|------------------->| |
dialog4 | | BYE/200 OK |
| |<-------------------|
Figure 5: Transfer with Consultation Hold - Exposing Transfer Target
7.2. Protecting Transfer Target
The Transferor places the Transferee on hold, establishes a call with
the Transfer Target and then reverses their roles, transferring the
original Transfer Target to the original Transferee. This has the
advantage of hiding information about the original Transfer Target
from the original Transferee. On the other hand, the Transferee's
experience is different than 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
Sparks, et al. Best Current Practice [Page 19]
RFC 5589 SIP CC Transfer June 2009
available, the Transferee might be stuck returning a Busy-Here to the
Transfer Target, effectively preventing the transfer. There are many
ways that correlation could be provided. The dialog parameters could
be provided directly as header parameters in the Refer-To URI, for
example. The Replaces mechanism [RFC3891] uses this approach and
solves this problem nicely.
For the flow below, dialog1 means dialog identifier 1, and consists
of the parameters of the Replaces header for dialog 1. In [RFC3891],
this is the Call-ID, To-tag, and From-tag.
Note that the Transferee's agent emits a BYE to the Transferor's
agent as an immediate consequence of processing the Replaces header.
The Transferor knows that both the Transferee and the Transfer Target
support the Replaces header from the Supported: replaces header
contained in the 200 OK responses from both.
In this scenario, the Transferee utilizes a GRUU as a Contact URI for
reasons discussed in Section 6.3.
Note that the conventions used in the SIP Torture Test Messages
[RFC4475] document are reused, specifically the tag.
Sparks, et al. Best Current Practice [Page 20]
RFC 5589 SIP CC Transfer June 2009
Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK F1 F2 |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK F3 F4 |
|---------------------------------------->|
dialog2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
dialog3 | REFER (Target-Dialog:2, |
| Refer-To:sips:Transferee?Replaces=1) F5|
|---------------------------------------->|
dialog3 | 202 Accepted | |
|<----------------------------------------|
dialog3 | NOTIFY (100 Trying)| |
|<----------------------------------------|
dialog3 | | 200 OK |
|---------------------------------------->|
dialog4 | INVITE (Replaces:dialog1)/200 OK/ACK F6
| |<-------------------|
dialog1 | BYE/200 OK | |
|<-------------------| |
dialog3 | NOTIFY (200 OK) | |
|<----------------------------------------|
dialog3 | | 200 OK |
|---------------------------------------->|
dialog2 | BYE/200 OK | |
|---------------------------------------->|
| (Transferee and target converse)
dialog4 | | BYE/200 OK |
| |------------------->|
Figure 6: Transfer Protecting Transfer Target
Sparks, et al. Best Current Practice [Page 21]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu
Contact:
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: ;tag=31431
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
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:
From: ;tag=763231
Call-ID: 592435881734450904
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Require: replaces
Contact:
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 22]
RFC 5589 SIP CC Transfer June 2009
F4 200 OK Transfer Target -> Transferor
SIP/2.0 200 OK
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
;received=192.0.2.1
To: ;tag=9m2n3wq
From: ;tag=763231
Call-ID: 592435881734450904
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
Content-Type: application/sdp
Content-Length: ...
F5 REFER Transferor -> Transfer Target
REFER sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
Max-Forwards: 70
To:
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 REFER
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Require: tdialog
Refer-To:
Target-Dialog: 592435881734450904;local-tag=9m2n3wq
;remote-tag=763231
Contact:
Content-Length: 0
Sparks, et al. Best Current Practice [Page 23]
RFC 5589 SIP CC Transfer June 2009
F6 INVITE Transfer Target -> Transferee
INVITE sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS client.chicago.example.com;branch=z9hG4bKnaslu84
Max-Forwards: 70
To:
From: ;tag=341234
Call-ID: kmzwdle3dl3d08
CSeq: 41 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Contact:
Replaces: 090459243588173445;to-tag=7553452;from-tag=31431
Content-Type: application/sdp
Content-Length: ...
7.3. Attended Transfer
The Transferor places the Transferee on hold, establishes a call with
the Transfer Target to alert them to the impending transfer, places
the target on hold, then proceeds with transfer using an escaped
Replaces header field in the Refer-To header. This is another common
service expected by current PBX and Centrex users.
The Contact URI of the Transfer Target SHOULD be used by the
Transferor as the Refer-To URI, unless the URI is suspected or known
to not be routable outside the dialog. Otherwise, the Address of
Record (AOR) of the Transfer Target SHOULD be used. That is, the
same URI that the Transferor used to establish the session with the
Transfer Target should be used. In case the triggered INVITE is
routed to a different User Agent than the Transfer Target, the
Require: replaces header field SHOULD be used in the triggered
INVITE. (This is to prevent an incorrect User Agent that 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 timeout, 403, 404, etc. error. The
Transferee MAY then retry the transfer with the Refer-To URI set to
the Contact URI.
Sparks, et al. Best Current Practice [Page 24]
RFC 5589 SIP CC Transfer June 2009
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
Sparks, et al. Best Current Practice [Page 25]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
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: ;tag=31431
From: ;tag=7553452
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu, tdialog
Contact:
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:
From: ;tag=763231
Call-ID: 592435881734450904
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Require: replaces
Contact:
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 26]
RFC 5589 SIP CC Transfer June 2009
F4 200 OK Transfer Target -> Transferor
SIP/2.0 200 OK
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnas432
;received=192.0.2.1
To: ;tag=9m2n3wq
From: ;tag=763231
Call-ID: 592435881734450904
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces, gruu
Contact:
Content-Type: application/sdp
Content-Length: ...
F5 REFER Transferor -> Transferee
REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds9
Max-Forwards: 70
To:
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 REFER
Require: tdialog
Refer-To:
Target-Dialog: 592435881734450904;local-tag=9m2n3wq
;remote-tag=763231
Contact:
Content-Length: 0
Sparks, et al. Best Current Practice [Page 27]
RFC 5589 SIP CC Transfer June 2009
F6 INVITE Transferee -> Transfer Target
INVITE sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
Max-Forwards: 70
To:
From: ;tag=954
Call-ID: kmzwdle3dl3d08
CSeq: 41 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: gruu, replaces, tdialog
Contact:
Replaces: 592435881734450904;to-tag=9m2n3wq;from-tag=763231
Content-Type: application/sdp
Content-Length: ...
7.4. Recovery When One Party Does Not Support REFER
If protecting or exposing the Transfer Target is not a concern, it is
possible to complete a transfer with consultation hold when only the
transferor and one other party support REFER. Note that a 405 Method
Not Allowed might be returned instead of the 501 Not Implemented
response.
Sparks, et al. Best Current Practice [Page 28]
RFC 5589 SIP CC Transfer June 2009
Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
dialog3 | REFER (Target-Dialog:1, |
| Refer-To:sips:TransferTarget?Replaces=2)
|------------------->| |
dialog3 | 501 Not Implemented |
|<-------------------| |
dialog4 | REFER (Refer-To:sips:Transferee?Replaces=dialog1)
|---------------------------------------->|
dialog4 | 202 Accepted | |
|<----------------------------------------|
dialog4 | NOTIFY (100 Trying)| |
|<----------------------------------------|
dialog4 | | 200 OK |
|---------------------------------------->|
dialog5 | INVITE (Replaces:dialog1)/200 OK/ACK
| |<-------------------|
dialog4 | NOTIFY (200 OK) | |
|<----------------------------------------|
dialog4 | | 200 OK |
|---------------------------------------->|
dialog1 | BYE/200 OK | |
|<-------------------| |
dialog2 | BYE/200 OK | |
|---------------------------------------->|
dialog5 | | BYE/200 OK |
| |------------------->|
Figure 8: Recovery When One Party Does Not Support REFER
7.5. Attended Transfer When Contact URI Is Not Known to Route to a
Unique User Agent
It is a requirement of RFC 3261 that a Contact URI be globally
routable even outside the dialog. However, due to RFC 2543 User
Agents and some architectures (NAT/Firewall traversal, screening
proxies, Application Layer Gateways (ALGs), etc.) this will not
Sparks, et al. Best Current Practice [Page 29]
RFC 5589 SIP CC Transfer June 2009
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 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
Sparks, et al. Best Current Practice [Page 30]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=763231
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
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: ;tag=9m2n3wq
From: ;tag=763231
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
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: ;tag=a6c85cf
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314160 REFER
Refer-To:
Contact:
Content-Length: 0
Sparks, et al. Best Current Practice [Page 31]
RFC 5589 SIP CC Transfer June 2009
F4 INVITE Transferee -> Transfer Target
INVITE sips:transfertarget@chicago.example.com SIP/2.0
Via: SIP/2.0/TLS 192.0.2.4;branch=z9hG4bKnaslu82
Max-Forwards: 70
To:
From: ;tag=954
Call-ID: 20482817324945934422930
CSeq: 42 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
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: ;tag=1928301774
From: ;tag=a6c85cf
Call-ID: a84b4c76e66710
CSeq: 76 NOTIFY
Contact:
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, at which point it 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.
Sparks, et al. Best Current Practice [Page 32]
RFC 5589 SIP CC Transfer June 2009
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 |
| |<------------|-------------|
Sparks, et al. Best Current Practice [Page 33]
RFC 5589 SIP CC Transfer June 2009
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:
From: ;tag=763231
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
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: ;tag=9m2n3wq
From: ;tag=763231
Call-ID: 090459243588173445
CSeq: 29887 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 34]
RFC 5589 SIP CC Transfer June 2009
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: ;tag=a6c85cf
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 REFER
Refer-To:
Contact:
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: ;tag=1928301774
From: ;tag=a6c85cf
Call-ID: a84b4c76e66710
CSeq: 74 NOTIFY
Contact:
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
Sparks, et al. Best Current Practice [Page 35]
RFC 5589 SIP CC Transfer June 2009
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: ;tag=a6c85cf
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314160 REFER
Refer-To:
Contact:
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
To:
From: ;tag=954
Call-ID: 20482817324945934422930
CSeq: 42 INVITE
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, NOTIFY
Supported: replaces
Contact:
Replaces: 090459243588173445;to-tag=9m2n3wq;from-tag=763231
Content-Type: application/sdp
Content-Length: ...
Sparks, et al. Best Current Practice [Page 36]
RFC 5589 SIP CC Transfer June 2009
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: ;tag=1928301774
From: ;tag=a6c85cf
Call-ID: a84b4c76e66710
CSeq: 76 NOTIFY
Contact:
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 that is routable outside the dialog, which will
result in the call flow of Figure 7.
7.6. Semi-Attended Transfer
In any of the consultation hold flows above, the Transferor may
decide to terminate its attempt to contact the Transfer Target before
that session is established. Most frequently, that will be the end
of the scenario, but in some circumstances, the Transferor may wish
to proceed with the transfer action. For example, the Transferor may
wish to complete the transfer knowing that the Transferee will end up
eventually talking to the Transfer Target's voicemail 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
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.
Sparks, et al. Best Current Practice [Page 37]
RFC 5589 SIP CC Transfer June 2009
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 |
| |<-------------------|
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 race conditions. In both of these flows, when the Transferor
Sparks, et al. Best Current Practice [Page 38]
RFC 5589 SIP CC Transfer June 2009
hangs up, the Transferor attempts to revert to unattended transfer by
sending a CANCEL to the target. This can result in two race
conditions. One is that the target answers despite the CANCEL and
the resulting unattended transfer fails. This race condition can be
eliminated by the Transferor waiting to send the REFER until the 487
response from the target is returned. Instead of a 487, a 200 OK may
be returned 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.
Sparks, et al. Best Current Practice [Page 39]
RFC 5589 SIP CC Transfer June 2009
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)
Sparks, et al. Best Current Practice [Page 40]
RFC 5589 SIP CC Transfer June 2009
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 |
| |<-------------------|
Sparks, et al. Best Current Practice [Page 41]
RFC 5589 SIP CC Transfer June 2009
Figure 13: Semi-Attended Transfer as Attended Transfer Call Flow (Not
Recommended)
7.7. Attended Transfer Fallback to Basic Transfer
In this flow, an attempted attended transfer fails so the Transferor
falls back to basic transfer.
The call flow in Figure 14 shows the use of Require: replaces in the
INVITE sent by the Transferor to the Transfer Target in which the
Transferor's intention at the time of sending the INVITE to the
Transfer Target was known to be to complete an attended transfer.
Since the Target does not support Replaces, the INVITE is rejected
with a 420 Bad Extension response, and the Transferor switches from
attended transfer to basic transfer immediately.
Sparks, et al. Best Current Practice [Page 42]
RFC 5589 SIP CC Transfer June 2009
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 |
| |<-------------------|
Figure 14: Attended Transfer Fallback to Basic Transfer Using
Require:replaces
Figure 15 shows the use of OPTIONS when the Transferee and Transfer
Target do not explicitly indicate support for the REFER method and
Replaces header fields in Allow and Supported header fields and the
Transferor did not have the intention of performing an attended
transfer when the INVITE to the Target was sent. In dialog1, the
Transferor determines, using OPTIONS, that the Transferee does
support REFER and Replaces. As a result, the Transferor begins the
attended transfer by placing the Transferee on hold and calling the
Transfer Target. Using an OPTIONS in dialog2, the Transferor
determines that the target does not support either REFER or Replaces,
Sparks, et al. Best Current Practice [Page 43]
RFC 5589 SIP CC Transfer June 2009
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.
Transferor Transferee Transfer
| | Target
| | |
dialog1 | INVITE/200 OK/ACK | |
|<-------------------| |
dialog1 | OPTIONS/200 OK | |
|------------------->| |
dialog1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
dialog2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
dialog2 | OPTIONS/200 OK | |
|---------------------------------------->|
dialog2 | BYE/200 OK | |
|---------------------------------------->|
dialog3 |REFER (Target-Dialog:1, |
| Refer-To:sips:TransferTarget) |
|------------------->| |
dialog3 | 202 Accepted | |
|<-------------------| |
dialog3 | NOTIFY (100 Trying)| |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog4 | | INVITE/200 OK/ACK |
| |------------------->|
dialog3 | NOTIFY (200 OK) | |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog1 | BYE/200 OK | |
|------------------->| |
dialog4 | | BYE/200 OK |
| |<-------------------|
Figure 15: Attended Transfer Fallback to Basic Transfer
Sparks, et al. Best Current Practice [Page 44]
RFC 5589 SIP CC Transfer June 2009
8. Transfer with Referred-By
In the previous examples, the Transfer Target does not have
definitive information about what party initiated the transfer, or,
in some cases, even that transfer is taking place. The Referred-By
mechanism [RFC3892] provides a way for the Transferor to provide the
Transferee with a way to let the Transfer Target know what party
initiated the transfer.
The simplest and least secure approach just involves the inclusion of
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 in Figure 16 shows the Referred-By header field and
body in the REFER F5 and triggered INVITE F6. Note that the Secure/
Multipurpose Internet Mail Extensions (S/MIME) signature is not shown
in the example below. The conventions used in the SIP Torture Test
Messages [RFC4475] document are reused, specifically the and
tags.
Sparks, et al. Best Current Practice [Page 45]
RFC 5589 SIP CC Transfer June 2009
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, Referred-By:Transferor,
| Refer-To:sips:TransferTarget?Replaces=2) F5
|------------------->| |
dialog3 | 202 Accepted | |
|<-------------------| |
dialog3 | NOTIFY (100 Trying)| |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog4 | INVITE (Replaces:dialog2, |
| Referred-By:Transferor )/200 OK/ACK F6
| |------------------->|
dialog2 | BYE/200 OK | |
|<----------------------------------------|
dialog3 | NOTIFY (200 OK) | |
|<-------------------| |
dialog3 | 200 OK | |
|------------------->| |
dialog1 | BYE/200 OK | |
|------------------->| |
dialog4 | | BYE/200 OK |
| |<-------------------|
Figure 16: Attended Transfer Call Flow with Referred-By
Sparks, et al. Best Current Practice [Page 46]
RFC 5589 SIP CC Transfer June 2009
F5 REFER Transferor -> Transferee
REFER sips:3ld812adkjw@biloxi.example.com;gr=3413kj2ha SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bK392039842
Max-Forwards: 70
To:
From: ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314160 REFER
Refer-To:
Supported: gruu, replaces, tdialog
Require: tdialog
Referred-By:
;cid="20398823.2UWQFN309shb3@atlanta.example.com"
Target-Dialog: 592435881734450904;local-tag=9m2n3wq;remote-tag=763231
Contact:
Content-Type: multipart/mixed; boundary=unique-boundary-1
Content-Length: ...
--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"
------590F24D439B31E08745DEF0CD9397189
Content-Type: message/sipfrag
Date: Thu, 18 Sep 2003 13:07:43 GMT
Refer-To:
Referred-By:
;cid="20398823.2UWQFN309shb3@atlanta.example.com"
------590F24D439B31E08745DEF0CD9397189
Content-Type: application/pkcs-7-signature; name="smime.p7s"
Sparks, et al. Best Current Practice [Page 47]
RFC 5589 SIP CC Transfer June 2009
Content-Transfer-Encoding: binary
Content-Disposition: attachment; filename="smime.p7s"
3082088806092A86
4886F70D010702A082087930820875020101310B300906052B0E03021A050030
. . . (Signature not shown)
8E63D306487A740A197A3970594CF47DD385643B1DC49FF767A3D2B428388966
79089AAD95767F
------590F24D439B31E08745DEF0CD9397189--
--unique_boundary-1
F6 INVITE Transferee -> Transfer Target
INVITE sips:482n4z24kdg@chicago.example.com;gr=8594958 SIP/2.0
Via: SIP/2.0/TLS referee.example;branch=z9hG4bKffe209934aac
To:
From: ;tag=2909034023
Call-ID: fe9023940-a3465@referee.example
CSeq: 889823409 INVITE
Max-Forwards: 70
Contact:
Referred-By:
;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: ...
--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
Sparks, et al. Best Current Practice [Page 48]
RFC 5589 SIP CC Transfer June 2009
--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
Refer-To:
Referred-By:
;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"
3082088806092A86
4886F70D010702A082087930820875020101310B300906052B0E03021A050030
. . . (Signature not shown)
8E63D306487A740A197A3970594CF47DD385643B1DC49FF767A3D2B428388966
79089AAD95767F
------590F24D439B31E08745DEF0CD9397189--
--my-boundary-9--
9. Transfer as an Ad Hoc Conference
In this flow, shown in Figure 17, Bob does an attended transfer of
Alice to Carol. In order to keep both Alice and Carol fully informed
of the nature and state of the transfer operation, Bob acts as a
focus [RFC4579] and hosts an ad hoc conference involving Alice, Bob,
and Carol. Alice and Carol subscribe to the conference package
[RFC4575] of Bob's focus, which allows them to know the exact status
of the operation. After the transfer operation is complete, Bob
deletes the conference.
Sparks, et al. Best Current Practice [Page 49]
RFC 5589 SIP CC Transfer June 2009
This call flow meets requirement 6 of Section 4. 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).
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
| |------------------->|
Sparks, et al. Best Current Practice [Page 50]
RFC 5589 SIP CC Transfer June 2009
| | 180 Ringing |
| |<-------------------|
| | 200 OK |
| |<-------------------|
| | ACK |
| |------------------->|
| | RTP |
| |<==================>|
| | |
|Carol subscribes to the conference package
| - learns Bob is on hold |
| | |
| |SUBSCRIBE sip:Conf-ID
| |<-------------------|
| | 200 OK |
| |------------------->|
| | NOTIFY (Conf-Info) |
| |------------------->|
| | 200 OK |
| |<-------------------|
| | |
| Alice learns that Bob is talking to Carol
| | |
| NOTIFY (Conf-Info) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| | INVITE (hold) |
| |------------------->|
| | 200 OK |
| |<-------------------|
| | ACK |
| |------------------->|
| | |
| Alice learns that Carol is now on hold |
| | |
| NOTIFY (Conf-Info) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| | |
| Bob begins transfer operation |
| | |
| REFER Refer-To: Carol |
|<-------------------| |
| 202 Accepted | |
|------------------->| |
| NOTIFY (Refer) | |
Sparks, et al. Best Current Practice [Page 51]
RFC 5589 SIP CC Transfer June 2009
|------------------->| |
| 200 OK | |
|<-------------------| |
| INVITE Replaces:B-C Contact:Alice |
|---------------------------------------->|
| 200 OK |
|<----------------------------------------|
| ACK |
|---------------------------------------->|
| RTP |
|<=======================================>|
| | BYE |
| |<-------------------|
| | 200 OK |
| |------------------->|
| NOTIFY (Refer) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| | |
| Bob terminates the ad-hoc conference |
| | |
| BYE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| | NOTIFY (Conf-Info) |
| |------------------->|
| | 200 OK |
| |<-------------------|
| NOTIFY (Conf-Info) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
Figure 17: Attended Transfer as an Ad Hoc Conference
10. Transfer with Multiple Parties
In this example, shown in Figure 18, the Originator places a 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 it should not be used as the
design of an implementation.
Sparks, et al. Best Current Practice [Page 52]
RFC 5589 SIP CC Transfer June 2009
Originator Facilitator Screener Recipient
| | | |
1 |INVITE/200 OK/ACK | |"Get Fred for me!"
|----------->| | | "Right away!"
2 |INVITE (hold)/200 OK/ACK | |
|<-----------| | |
2 | |INVITE/200 OK/ACK |"I have a call
| |----------->| |from Mary for Fred"
2 | |INVITE (hold)/200 OK/ACK "Hold please"
| |<-----------| |
3 | | |INVITE/200 OK/ACK
| | |--------->|"You have a call
| | | |from Mary"
| | | | "Put her through"
3 | | |INVITE (hold)/200 OK/ACK
| | |--------->|
4 | |REFER | |
| |<-----------| |
4 | |202 Accepted| |
| |----------->| |
4 | |NOTIFY (100 Trying) |
| |----------->| |
4 | |200 OK | |
| |<-----------| |
5 | |INVITE/200 OK/ACK |
| |---------------------->|"This is Fred"
4 | |NOTIFY (200 OK) | "Please hold for
| |----------->| | Mary"
4 | |200 OK | |
| |<-----------| |
2 | |BYE/200 OK | |
| |<-----------| |
3 | | |BYE/200 OK|
| | |--------->|
5 | |INVITE (hold)/200 OK/ACK
| |---------------------->|
6 |REFER | | |
|<-----------| | |
6 |202 Accepted| | |
|----------->| | |
6 |NOTIFY (100 Trying) | |
|----------->| | |
6 |200 OK | | |
|<-----------| | |
7 |INVITE/200 OK/ACK | |
|----------------------------------->| "Hey Fred"
Sparks, et al. Best Current Practice [Page 53]
RFC 5589 SIP CC Transfer June 2009
6 |NOTIFY (200 OK) | | "Hello Mary"
|----------->| | |
6 |200 OK | | |
|<-----------| | |
1 |BYE/200 OK | | |
|<-----------| | |
5 | |BYE/200 OK | |
| |---------------------->|
7 |BYE/200 OK | | |
|<-----------------------------------| "See you later"
Figure 18: Transfer with Multiple Parties Example
11. Gateway Transfer Issues
A gateway in SIP acts as a User Agent. As a result, the entire
preceding discussion and call flows apply equally well to gateways as
native SIP endpoints. However, there are some gateway-specific
issues that are documented in this section. While this discussion
focuses on the common cases involving Public Switched Telephone
Network (PSTN) gateways, similar situations exist for other gateways,
such as H.323/SIP gateways.
11.1. Coerce Gateway Hairpins to the Same Gateway
To illustrate how a hairpin situation can occur in transfer, consider
this example. The original call dialog is setup with the Transferee
residing on the PSTN side of a SIP gateway. The Transferor is a SIP
phone purely in the IP space. The Transfer Target is on the PSTN
side of a SIP gateway as well. After completing the transfer,
(regardless of consultative or blind) the Transferee is in a call
with the Transfer Target (both on the PSTN side of a gateway). It is
often desirable to remove the gateway(s) out of the loop. This is
likely to only be possible if both legs of the target call are on the
same gateway. With both legs on the same gateway, it may be able to
invoke the analogous transfer on the PSTN side. Then the target call
would not involve the gateway.
So the problem is how to give the proxy enough information so that it
knows to route the call to the same gateway. With a simple single
call that hairpins, the incoming and outgoing leg have the same
dialog. The proxy should have enough information to optimize the
routing.
In the consultative transfer scenario, it is desirable to coerce the
consultative INVITE out the same gateway as the original call to be
transferred. However, there is no way to relate the consultation
with the original call. In the consultative case, the target call
Sparks, et al. Best Current Practice [Page 54]
RFC 5589 SIP CC Transfer June 2009
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.
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 trunk group, it might also be
necessary to get all of the legs on the same trunk group in order to
perform the transfer on the non-SIP side of the gateway.
Solutions to these gateway specific issues may involve new extensions
to SIP in the future.
11.2. Consultative Turned Blind Gateway Glare
In the consultative transfer case turned blind, there is a glare-like
problem. The Transferor initiates the consultation INVITE, the
Transferor gets impatient and hangs up, transitioning this to a blind
transfer. The Transfer Target on the gateway (connected through a
PSTN switch to a single line or dumb analog phone) rings. The user
answers the phone just after the CANCEL is received by the Transfer
Target. The REFER and INVITE for the target call are sent. The
Transferee attempts to set up the call on the PSTN side, but gets
either a busy response 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 7.6.
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
security considerations detailed in the REFER [RFC3515] and Replaces
[RFC3891] documents MUST be followed, which are briefly summarized in
the following paragraphs. This document addresses the issue of
protecting the Address of Record URI of a Transfer Target in Sections
7.1 and 7.2.
Sparks, et al. Best Current Practice [Page 55]
RFC 5589 SIP CC Transfer June 2009
Any REFER request MUST be appropriately authenticated and authorized
using standard SIP mechanisms or else 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. Special care is needed if the replaced dialog utilizes
additional media streams compared to the original dialog. In this
case, the user MUST authorize the addition of new media streams in a
dialog replacement. For example, the same mechanism used to
authorize the addition of a media stream in a re-INVITE could be
used.
13. Acknowledgments
This document 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session
Initiation Protocol (SIP) "Replaces" Header", RFC 3891,
September 2004.
[RFC3892] Sparks, R., "The Session Initiation Protocol (SIP)
Referred-By Mechanism", RFC 3892, September 2004.
Sparks, et al. Best Current Practice [Page 56]
RFC 5589 SIP CC Transfer June 2009
[RFC4538] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol
(SIP)", RFC 4538, June 2006.
14.2. Informative References
[CC-FRMWRK] Mahy, R., Sparks, R., Rosenberg, J., Petrie, D., and A.
Johnston, "A Call Control and Multi-party usage
framework for the Session Initiation Protocol (SIP)",
Work in Progress, March 2009.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
[RFC4475] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg,
J., and H. Schulzrinne, "Session Initiation Protocol
(SIP) Torture Test Messages", RFC 4475, May 2006.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference
State", RFC 4575, August 2006.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents",
BCP 119, RFC 4579, August 2006.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[SIP-GRUU] Rosenberg, J., "Obtaining and Using Globally Routable
User Agent (UA) URIs (GRUU) in the Session Initiation
Protocol (SIP)", Work in Progress, October 2007.
Sparks, et al. Best Current Practice [Page 57]
RFC 5589 SIP CC Transfer June 2009
Authors' Addresses
Robert Sparks
Tekelec
17210 Campbell Road
Suite 250
Dallas, Texas 75252
USA
EMail: RjS@nostrum.com
Alan Johnston (editor)
Avaya
St. Louis, MO
EMail: alan@sipstation.com
Daniel Petrie
SIPez LLC
Arlington, MA 02476
US
Phone: +1 617 273 4000
EMail: dan.ietf@SIPez.com
URI: http://www.SIPez.com/
Sparks, et al. Best Current Practice [Page 58]
RFC, FYI, BCP