Network Working Group R. Sparks
Internet-Draft dynamicsoft
Expires: January 16, 2002 July 18, 2001
SIP Call Control - Transfer
draft-ietf-sip-cc-transfer-05
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes providing Call Transfer capabilites in SIP.
Transfer capabilities. This work is part of the Call Control
Framework.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Changes from draft-sparks-sip-cc-transfer-04 . . . . . . . . 3
3. Actors and Roles . . . . . . . . . . . . . . . . . . . . . . 3
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Using REFER to achieve Call Transfer . . . . . . . . . . . . 4
6. Basic Transfer . . . . . . . . . . . . . . . . . . . . . . . 5
6.1 Successful Transfer . . . . . . . . . . . . . . . . . . . . 6
6.2 Failed Transfer . . . . . . . . . . . . . . . . . . . . . . 7
6.2.1 Target Busy . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2.2 Transfer Target does not answer . . . . . . . . . . . . . . 8
7. Transfer with Consultation Hold . . . . . . . . . . . . . . 9
7.1 Exposing transfer target . . . . . . . . . . . . . . . . . . 9
7.2 Protecting transfer target . . . . . . . . . . . . . . . . . 10
7.3 Recovery when one party does not support REFER . . . . . . . 10
7.4 Consultation Hold in the presence of forking proxies . . . . 11
7.5 Using the Replaces header to improve the Consultation Hold
experience . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.5.1 Consultation Hold protecting transfer target . . . . . . . . 12
7.5.2 Recovering from one party not supporting the Replaces header 13
7.6 Aborting a Consultation Hold . . . . . . . . . . . . . . . . 14
8. Transfer with multiple parties . . . . . . . . . . . . . . . 14
9. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
References . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . 16
Full Copyright Statement . . . . . . . . . . . . . . . . . . 17
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1. Overview
This document describes providing Call Transfer capabilites in SIP.
Transfer capabilities. This work is part of the Call Control
Framework.
The mechanisms discussed here are most closely related to traditional
basic and consultation hold transfers. This document does not
discuss transfer scenarios involving ad-hoc conferences (where all
parties involved are briefly in a conference until this transferor
drops out).
Editor's note: Per working group consensus, draft-ietf-sip-cc-
transfer-04 was split into two drafts. This document details the use
of REFER to achieve call transfer. The definition of REFER itself
was removed to draft-ietf-sip-refer-00
2. Changes from draft-sparks-sip-cc-transfer-04
o Split the draft
o Removed the contested distinction between attended and unattended
transfer (involving an ad-hoc conference).
o Added new failure and recovery flows
o Added flow demonstrating the use of the Replaces header to affect
user experience
3. 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.
4. Requirements
1. Any party in a SIP session MUST be able to transfer any other
party in that session at any point in that session.
2. The Transferor and the Transferee MUST NOT be removed from a
session as part of a transfer transaction.
At first glance, requirement 2 may seem to indicate
that the user experience in a transfer must be
significantly different from what a current PBX or
Centrex user expects. As the call-flows in this
document show, this is not the case. A client MAY
preserve the current experience. In fact, without
this requirement, some forms of the current
experience (ringback on transfer failure
for instance) will be lost.
3. The Transferor MUST know whether or not the transfer was
successful (this is significantly different from the requirements
of the earlier BYE-Also approach to transfer).
5. Using REFER to achieve Call Transfer
A REFER [3] 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
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Transferee had initiated that INVITE on its own. Further, the
disposition of the media streams between the Transferor and the
Transferee is not altered by the REFER method. Agents may alter a
session's media through additional signaling. For example, they may
make use of the SIP hold re-INVITE [1] or the conferencing extensions
provided by this framework.
6. Basic Transfer
Basic Transfer consists of the Transferor providing the Transfer
Target's contact to the Transferee. The Transferee attempts to
establish a session using that contact and reports the results of
that attempt to the Transferor. The signaling relationship between
the Transferor and Transferee is not terminated, so the call is
recoverable if the Transfer Target cannot be reached. Note that the
Transfer Target's contact information has been exposed to the
Transferee. The provided contact can be used to make new calls in
the future.
The diagrams below show indicate the first line of each message. All
messages in a particular diagram share the same Call-ID. In these
diagrams, media is managed through reINVITE holds, but other
mechanisms (mixing multiple media streams at the UA or using the
conferencing extensions for example) are valid.
Each of the flows below shows the call-leg 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 neccessary. 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.
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6.1 Successful Transfer
Transferor Transferee Transfer
| | Target
| INVITE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| ACK | |
|<-------------------| |
| INVITE (hold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
| | INVITE |
| |------------------->|
| | 200 OK |
| |<-------------------|
| | ACK |
| |------------------->|
| NOTIFY (200 OK) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| BYE | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| | BYE |
| |<-------------------|
| | 200 OK |
| |------------------->|
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6.2 Failed Transfer
6.2.1 Target Busy
Transferor Transferee Transfer
| | Target
| | |
| INVITE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| ACK | |
|<-------------------| |
| INVITE (hold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
| | INVITE |
| |------------------->|
| | 486 Busy Here |
| |<-------------------|
| | ACK |
| |------------------->|
| NOTIFY (503 Service Unavailable) |
| or NOTIFY (486 Busy Here) |
|<-------------------| |
| 200 OK | |
|------------------->| |
| INVITE (unhold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| BYE | |
|------------------->| |
| 200 OK | |
|<-------------------| |
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6.2.2 Transfer Target does not answer
Transferor Transferee Transfer
| | Target
| INVITE | |
|<-------------------| |
| 200 OK | |
|------------------->| |
| ACK | |
|<-------------------| |
| INVITE (hold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
| | INVITE |
| |------------------->|
| | 180 Ringing |
| |<-------------------|
| | (Transferee gets tired of waiting)
| | CANCEL |
| |------------------->|
| | 200 OK (CANCEL) |
| |<-------------------|
| | 487 Request Cancelled (INVITE)
| |<-------------------|
| | ACK |
| |------------------->|
| NOTIFY (487 Request Cancelled) |
|<-------------------| |
| 200 OK | |
|------------------->| |
| INVITE (unhold) | |
|------------------->| |
| 200 OK | |
|<-------------------| |
| ACK | |
|------------------->| |
| BYE | |
|------------------->| |
| 200 OK | |
|<-------------------| |
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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.
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.
Transferor Transferee Transfer
| | Target
| | |
Call-ID:1 | INVITE/200 OK/ACK | |
|<-------------------| |
Call-ID:1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
Call-ID:2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
Call-ID:2 | BYE/200 OK | |
|---------------------------------------->|
Call-ID:1 | REFER | |
|------------------->| |
| 202 Accepted | |
|<-------------------| |
Call-ID:1 | | INVITE/200 OK/ACK |
| |------------------->|
Call-ID:1 | NOTIFY (200 OK) | |
|<-------------------| |
| 200 OK | |
|------------------->| |
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:1 | | BYE/200 OK |
| |<-------------------|
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7.2 Protecting transfer target
The transferor places the transferee on hold, establishes a call with
the transfer target and then reverses their roles, transferring the
original transfer target to the original transferee. This has the
advantage of hiding information about the original transfer target
from the original transferee. On the other hand, the Transferee's
experience is different that in current systems. The Transferee is
effectively "called back" by the Transfer Target. If supported, use
of the Replaces header can help improve this experience. Examples of
this usage appear later in this document.
Transferor Transferee Transfer
| | Target
| | |
Call-ID:1 | INVITE/200 OK/ACK | |
|<-------------------| |
Call-ID:1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
Call-ID:2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
Call-ID:2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
Call-ID:2 | REFER | |
|---------------------------------------->|
| 202 Accepted | |
|<----------------------------------------|
Call-ID:2 | | INVITE/200 OK/ACK |
| |<-------------------|
Call-ID:2 | NOTIFY (200 OK) | |
|<----------------------------------------|
| | 200 OK |
|---------------------------------------->|
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:2 | BYE/200 OK | |
|---------------------------------------->|
Call-ID:2 | | BYE/200 OK |
| |------------------->|
7.3 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.
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Transferor Transferee Transfer
| | Target
| | |
Call-ID:1 | INVITE/200 OK/ACK | |
|<-------------------| |
Call-ID:1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
Call-ID:2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
Call-ID:2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
Call-ID:1 | REFER | |
|------------------->| |
Call-ID:1 | 501 Not Implemented
|<-------------------| |
Call-ID:2 | REFER | |
|---------------------------------------->|
| 202 Accepted | |
|<----------------------------------------|
Call-ID:2 | | INVITE/200 OK/ACK |
| |<-------------------|
Call-ID:2 | NOTIFY (200 OK) | |
|<----------------------------------------|
| | 200 OK |
|---------------------------------------->|
Call-ID:1 | BYE/200 OK | |
|------------------->| |
Call-ID:2 | BYE/200 OK | |
|---------------------------------------->|
Call-ID:2 | | BYE/200 OK |
| |------------------->|
7.4 Consultation Hold in the presence of forking proxies
It is worth noting that the examples given above abstract away any
proxies that might be between the three parties. In 4.5.1 for
example, the URL used to reach the Transfer Target may go through a
forking proxy. There is no guarantee that the Transferee's and
Transferor's invitations to the Transfer Target will reach the same
endpoint. If the proxy forked in parallel, both invitations could
cause multiple endpoints to ring. To increase the probability of the
desired behavior of having the referred invite reach and ring only
the same endpoint as the consultation invite, the Transferor SHOULD
issue the REFER request with the Refer-To: header containing the
Contact the Transfer Target provided in its 200 OK to the
Transferor's INVITE. If that REFER fails, the Transferor SHOULD
issue another REFER with the Refer-To: header containing the URL it
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used to reach the Transfer Target, augmented with an Accept-Contact
header containing the Contact the Transfer Target provided.
7.5 Using the Replaces header to improve the Consultation Hold
experience
7.5.1 Consultation Hold protecting transfer target
One of the problems with the simplest implementation of a target
protecting transfer is that the transferee is receiving a new call
from the transfer-target. Unless the transferee's agent has a
reliable way to associate this new call with the call it already has
with the transferor, it will have to alert the new call on another
appearance. If this, or some other call-waiting-like UI were not
available, the transferee might be stuck returning a Busy-Here to the
transfer target, effectively preventing the transfer. There are many
ways that that correlation could be provided. The call leg
parameters could be provided directly as header parameters in the
Refer-To: URL for example. The Replaces mechanism [4] uses this
approach and solves this problem nicely.
For the flow below, clid1 means Call Leg Identifier 1, and consists
of the parameters to the Replaces header for call-leg 1. In [4] 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.
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Transferor Transferee Transfer
| | Target
| | |
clid1 | INVITE/200 OK/ACK | |
|<-------------------| |
clid1 | INVITE (hold)/200 OK/ACK |
|------------------->| |
clid2 | INVITE/200 OK/ACK | |
|---------------------------------------->|
clid2 | INVITE (hold)/200 OK/ACK |
|---------------------------------------->|
clid2 | REFER (Refer-To: sip:transferee?Replaces=clid1)
|---------------------------------------->|
clid2 | 202 Accepted | |
|<----------------------------------------|
clid3 | | INVITE (Replaces=clid1)/200 OK/ACK
| |<-------------------|
clid1 | BYE/200 OK | |
|<-------------------| |
clid2 | NOTIFY (200 OK) | |
|<----------------------------------------|
clid2 | | 200 OK |
|---------------------------------------->|
clid2 | BYE/200 OK | |
|---------------------------------------->|
| | (transferee and target converse)
clid3 | | BYE/200 OK |
| |------------------->|
7.5.2 Recovering from one party not supporting the Replaces header
Similar to the case of recovering from a party not supporting REFER,
the transferor can recover from a party not supporting the Replaces
header, at the potential cost of not protecting the transfer target
and reverting to the non-Replaces user experience.
In the above flow, if all of the following are true:
o The Transferee's agent does not support the Replaces header
o The Transferee's agent does not support multiple appearences or
call-waiting and returns Busy-Here to all new INVITEs when engaged
in a call.
o The Transfer-Target's agent is configured to expose the cause of a
REFERenced action failure in its NOTIFY (see the security issues
associated with this choice in [3]).
o The Transferor is willing to expose the Transfer-Target.
then the Transferor can retry the transfer by sending a new REFER to
the Transferee.
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7.6 Aborting a Consultation Hold
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, he may wish to
complete the transfer knowing that the transferee will end up
evenutally talking to the transfer-target's voice-mail service.
For flows that expose the transfer target, this simply becomes a
basic transfer.
This scenario is far more complicated for flows that protect the
transfer target. Since no session is established between the
transferor and the transfer target, the transfer target's agent would
have to honor out-of-session REFERs, and somehow indicate what's
happening via its user interface (this scenario is most likely to
occur when the transfer-target is away from his agent).
8. Transfer with multiple parties
In this example the Originator places call to the Facilitator who
reaches the Recipient through the Screener. The Recipient's contact
information is exposed to the Facilitator and the Originator. This
example is provided for clarification of the semantics of the REFER
method only and should not be used as the design of an
implementation.
Originator Facilitator Screener Recipient
Call-ID | | | |
1 |INVITE/200 OK/ACK | |"Get Fred for me!"
|----------->| | | "Right away!"
1 |INVITE (hold)/200 OK/ACK | |
|<-----------| | |
2 | |INVITE/200 OK/ACK |"I have a call
| |----------->| |from Mary for Fred"
2 | |INVITE (hold)/200 OK/ACK "Hold please"
| |<-----------| |
3 | | |INVITE/200 OK/ACK
| | |--------->|"You have a call
| | | |from Mary"
| | | | "Put her through"
3 | | |INVITE (hold)/200 OK/ACK
| | |--------->|
2 | |REFER | |
| |<-----------| |
| |202 Accepted| |
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| |----------->| |
2 | |INVITE/200 OK/ACK |
| |---------------------->|"This is Fred"
2 | |NOTIFY (200 OK) | "Please hold for
| |----------->| | Mary"
| |200 OK | |
| |<-----------| |
2 | |BYE/200 OK | |
| |<-----------| |
3 | | |BYE/200 OK|
| | |--------->|
2 | |INVITE (hold)/200 OK/ACK
| |---------------------->|
1 |REFER | | |
|<-----------| | |
|202 Accepted| | |
|----------->| | |
1 |INVITE/200 OK/ACK | |
|----------------------------------->| "Hey Fred"
1 |NOTIFY (200 OK) | | "Hello Mary"
|----------->| | |
|200 OK | | |
|<-----------| | |
1 |BYE/200 OK | | |
|<-----------| | |
2 | |BYE/200 OK | |
| |---------------------->|
1 |BYE/200 OK | | |
|<-----------------------------------| "See you later"
9. Open Issues
10. Acknowledgments
This draft is a collaborative product of the SIP working group.
Thanks to Alan Johnston for providing the starting point for the new
error and recovery flows.
References
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[2] Campbell, B., "Framework for SIP Call Control Extensions",
draft-ietf-sip-cc-framework-00 (work in progress), March 2000.
[3] Sparks, R., "The REFER Method", draft-ietf-sip-refer-00 (work in
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progress), July 2001.
[4] Biggs, B. and R. Dean, "The SIP Replaces Header", draft-biggs-
sip-replaces-00 (work in progress), November 2000.
Author's Address
Robert J. Sparks
dynamicsoft
5100 Tennyson Parkway
Suite 1200
Plano, TX 75024
EMail: rsparks@dynamicsoft.com
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Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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