Network Working Group A. Johnston
Request for Comments: 4579 Avaya
BCP: 119 O. Levin
Category: Best Current Practice Microsoft Corporation
August 2006
Session Initiation Protocol (SIP)
Call Control - Conferencing for User Agents
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.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines conferencing call control features for the
Session Initiation Protocol (SIP). This document builds on the
Conferencing Requirements and Framework documents to define how a
tightly coupled SIP conference works. The approach is explored from
the perspective of different user agent (UA) types: conference-
unaware, conference-aware, and focus UAs. The use of Uniform
Resource Identifiers (URIs) in conferencing, OPTIONS for capabilities
discovery, and call control using REFER are covered in detail with
example call flow diagrams. The usage of the isfocus feature tag is
defined.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. SIP User Agent Conferencing Capability Types ....................3
3.1. Focus UA ...................................................4
3.2. Conference Factory URI .....................................4
3.3. Conference-Unaware UA ......................................5
3.4. Conference-Aware UA ........................................5
4. Usage of the 'isfocus' Feature Parameter ........................6
4.1. General ....................................................6
4.2. Session Establishment ......................................6
4.3. Discovery ..................................................7
5. SIP Conferencing Primitives .....................................7
5.1. INVITE: Joining a Conference Using the Conference
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RFC 4579 SIP CC Conferencing for UAs August 2006
URI - Dial-In ..............................................7
5.2. INVITE: Adding a Participant by the Focus - Dial-Out ......11
5.3. INVITE: Manually Creating a Conference by Dialing
In to a Conferencing Application ..........................15
5.4. INVITE: Creating a Conference Using Ad-Hoc SIP Methods ....16
5.5. REFER: Requesting a Focus to Add a New Resource to
a Conference (Dial Out to a New Participant) ..............18
5.6. REFER: Requesting a User to Dial in to a Conference
Using a Conference URI ....................................21
5.7. REFER with REFER: Requesting a Focus to Refer a
Participant to Dial in to the Conference ..................23
5.8. Join Header Field: Dialing in to a Conference
Using a (3rd Party) Dialog Identifier .....................26
5.9. Replaces Header Field: Switching User Agents
within a Conference .......................................28
5.10. Replaces Header Field: Transferring a Point-to-Point
Session in to a Conference ...............................29
5.11. REFER with BYE: Requesting That the Focus Remove a
Participant from a Conference ............................31
5.12. Deleting a Conference ....................................33
5.13. Discovery of URI Properties Using OPTIONS ................34
6. Security Considerations ........................................36
7. Contributors ...................................................37
8. References .....................................................38
8.1. Normative References ......................................38
8.2. Informative References ....................................38
Appendix A: Creating a Conference by a Conference-Unaware UA.......40
1. Introduction
This specification uses the concepts and definitions from the high
level requirements [14] and the SIP conferencing framework [8]
documents. This approach is applicable to tightly coupled SIP
conferences. In this architecture, a user agent (UA), known as a
participant, establishes a SIP dialog with another UA, known as a
focus. The focus is the central point of control, authentication,
and authorization. This specification defines the operation of a
focus and participant UAs. Note that only the signalling (SIP) needs
to be centralized in this model; the media can be centrally mixed,
distributed, or even multicast. For a full discussion of this
architecture, see the SIP conferencing framework document [8].
The approach described in this document implements key functions in
the conferencing framework using SIP primitives only. This allows
for conducting simple conferences with defined functionalities using
SIP mechanisms and conventions. Many other advanced functions can be
implemented using additional means, but they are not in the scope of
this document.
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This document presents the basic call control (dial-in and dial-out)
conferencing building blocks from the UA perspective. Possible
applications include ad-hoc conferences and scheduled conferences.
Note that a single conference can bridge participants that have
different capabilities and who potentially have joined the conference
by different means (i.e., dial-in, dial-out, scheduled, or ad-hoc).
The call control and dialog manipulation approach is based on the
multiparty framework document [15]. That document defines the basic
approach of service design adopted for SIP, which includes the
following:
- Definition of primitives, not services
- Signaling model independent
- Invoker oriented
- Primitives make full use of URIs
- Include policies for authentication, authorization, logging, etc.
- Define graceful fallback to baseline SIP
The use of opaque URIs and the ability to communicate call control
context information within a URI (as opposed to using service-related
header fields), as discussed in RFC 3087 [11], is fundamental to this
approach.
Capabilities discovery is an important feature of SIP systems, and
conferencing systems can make use of such features. For a UA acting
as a focus in a conference, this specification defines the usage of
the 'isfocus' feature parameter.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 and
indicate requirement levels for compliant implementations [1].
3. SIP User Agent Conferencing Capability Types
From a conferencing perspective, the framework document outlines a
number of possible different SIP components such as conference-
unaware participant, conference-aware participant, and focus.
This document applies the concepts above to the SIP call control part
of the conferencing components. It defines normative behavior of the
SIP UAs in various conferencing situations (referred to later as
"scenarios").
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3.1. Focus UA
A focus, as defined in the framework, hosts a SIP conference and
maintains a SIP signaling relationship with each participant in the
conference. A focus contains a conference-aware user agent that
supports the conferencing call control conventions as defined in this
document.
A focus SHOULD support the conference package RFC 4575 [9], behave as
a notifier for that package, and indicate its support in the Allow-
Events header fields in requests and responses. A focus MAY include
information about the conference in Session Description Protocol
(SDP) bodies sent as part of normal SIP signaling by populating the
Session Information, URI, Email Address, and Phone Number SDP fields.
In order to support advanced features, where a session established
between two endpoints can migrate to a centralized conference, a
focus SHOULD support the Replaces header field [6].
A user agent with focus capabilities could be implemented in end user
equipment and would be used for the creation of ad-hoc conferences.
A dedicated conferencing server, whose primary task is to
simultaneously host conferences of arbitrary type and size, may
allocate and publish a conference factory URI (as defined in the next
section) for creating an arbitrary number of ad-hoc conferences (and
subsequently their focuses) using SIP call control means.
3.2. Conference Factory URI
According to the framework, there are many ways in which a conference
can be created. A conferencing server implementation is free to
choose from these methods, which include non-automated means (such as
an Interactive Voice Response (IVR) system), SIP, or any conference
control protocol.
In order to automatically create an arbitrary number of ad-hoc
conferences (and subsequently their focuses) using SIP call control
means, a globally routable Conference Factory URI can be allocated
and published.
A successful attempt to establish a call to this URI would result in
the automatic creation of a new conference and its focus. As a
result, note that the Conference Factory URI and the newly created
focus URI MAY resolve to different physical devices.
A scenario showing the use of the conference factory URI is shown in
Section 5.4.
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3.3. Conference-Unaware UA
The simplest user agent can participate in a conference ignoring all
SIP conferencing-related information. The simplest user agent is
able to dial in to a conference and to be invited to a conference.
Any conferencing information is optionally conveyed to/from it using
non-SIP means. Such a user agent would not usually host a conference
(at least, not using SIP explicitly). A conference-unaware UA need
only support RFC 3261 [2]. Call flows for conference-unaware UAs are
not shown in general in this document as they would be identical to
those in the SIP call flows document [13].
Note that the presence of an 'isfocus' feature tag in a Contact
header field will not cause interoperability issues between a focus
and a conference-unaware UA since it will be treated as an unknown
header parameter and ignored, as per standard SIP behavior.
3.4. Conference-Aware UA
A conference-aware user agent supports SIP conferencing call control
conventions defined in this document as a conference participant, in
addition to support of RFC 3261 [2]. A conference-aware UA should be
able to process SIP redirections such as described in Section 8.1.3.4
of RFC 3261.
A conference-aware UA MUST recognize the 'isfocus' feature parameter.
A conference-aware UA SHOULD support REFER [4], SIP events [3], and
the conferencing package [9].
A conference-aware UA SHOULD subscribe to the conference package if
the 'isfocus' parameter is in the remote target URI of a dialog and
if the conference package is listed by a focus in an Allow-Events
header field. The SUBSCRIBE to the conference package SHOULD be sent
outside any INVITE-initiated dialog. A termination of the INVITE
dialog with a BYE does not necessarily terminate the SUBSCRIBE
dialog.
A conference-aware UA MAY render to the user any information about
the conference obtained from the SIP header fields and SDP fields
from the focus.
A conference-aware UA SHOULD render to the user any information about
the conference obtained from the SIP conference package.
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RFC 4579 SIP CC Conferencing for UAs August 2006
4. Usage of the 'isfocus' Feature Parameter
4.1. General
The main design guidelines for the development of SIP extensions and
conventions for conferencing are to define the minimum number of
extensions and to have seamless backward compatibility with
conference-unaware SIP UAs. The minimal requirement for SIP is being
able to express that a dialog is a part of a certain conference
referenced to by a URI. As a result of these extensions, it is
possible to do the following using SIP:
- Create a conference
- Join a conference
- Invite a user to a conference
- Expel a user by third party
- Discover if a URI is a conference URI
- Delete a conference
The approach taken is to use the feature parameter 'isfocus' to
express that a SIP dialog belongs to a conference. The use of
feature parameters in Contact header fields to describe the
characteristics and capabilities of a UA is described in the User
Agent Capabilities document [5], which includes the definition of the
'isfocus' feature parameter.
4.2. Session Establishment
In session establishment, a focus MUST include the 'isfocus' feature
parameter in the Contact header field unless the focus wishes to hide
the fact that it is a focus. To a participant, the feature parameter
will be associated with the remote target URI of the dialog. It is
an indication to a conference-aware UA that the resulting dialog
belongs to a conference, identified by the URI in the Contact header
field, and that the call control conventions defined in this document
can be applied.
By their nature, the conferences supported by this specification are
centralized. Therefore, typically a conferencing system needs to
allocate a SIP conference URI such that SIP requests to this URI are
not forked and are routed to a dedicated conference focus. For
example, a globally accessible SIP conference could be well
constructed with a conference URI using a Globally Routable User
Agent URI (GRUU) (defined in [16]), because of its ability to support
the non-forking and global routability requirements.
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4.3. Discovery
Using the mechanism described in this section, it is possible, given
an opaque URI, to determine if it belongs to a certain conference
(i.e., meaning that it is a conference URI) or not. This discovery
function can be implemented in SIP using an OPTIONS request, and can
be done either inside an active dialog or outside a dialog. A focus
MUST include the 'isfocus' feature parameter in a 200 OK response to
an OPTIONS unless the focus wishes to hide the fact that it is a
focus.
5. SIP Conferencing Primitives
The SIP conferencing call control flows presented in this section are
the call control building blocks for various SIP conferencing
applications as described in the conferencing requirements [14] and
framework [8] documents. The major design goal is that the same SIP
conferencing primitives would be used by user agents having different
conferencing capabilities and implementing different applications.
5.1. INVITE: Joining a Conference Using the Conference URI - Dial-In
In this section, a user knows the conference URI and "dials in" to
join this conference. The focus will authenticate the participant
and apply authorization policy before allowing the participant to
join the conference.
If the UA is the first participant of the conference to dial-in, it
is likely that this INVITE will activate the focus and hence the
conference. However, the conference URI must have been reserved
prior to its use.
If the conference is up and running already, the dialing-in
participant is joined to the conference by its focus.
To join an existing specific conference, a UA will send an INVITE
with the Request-URI set to the conference URI. The focus MUST
include the 'isfocus' feature parameter in the Contact header field
of the 200 OK response to the INVITE.
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An example call flow for joining a conference is shown in Figure 1.
Alice Focus Bob Carol
| | |
| | Carol joins the conference |
| | |
| | INVITE sip:Conf-ID F1 |
| |<----------------------------------------|
| | 180 Ringing F2 |
| |---------------------------------------->|
| | 200 OK Contact:Conf-ID;isfocus F3 |
| |---------------------------------------->|
| | ACK F4 |
| |<----------------------------------------|
| | RTP |
| |<=======================================>|
| | SUBSCRIBE sip:Conf-ID F5 |
| |<----------------------------------------|
| | 200 OK F6 |
| |---------------------------------------->|
| | NOTIFY F7 |
| |---------------------------------------->|
| | 200 OK F8 |
| |<----------------------------------------|
Figure 1. A Participant Joins a Conference Using the Conference URI.
F1 INVITE sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To:
From: Carol ;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 45 INVITE
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Supported: replaces
Content-Type: application/sdp
Content-Length: ...
(SDP not shown)
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RFC 4579 SIP CC Conferencing for UAs August 2006
F3 SIP/2.0 200 OK
Via: SIP/2.0/UDP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83;received=192.0.2.4
To: ;tag=733413
From: Carol ;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 45 INVITE
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Allow-Events: dialog, conference
Accept: application/sdp, application/conference-info+xml,
message/sipfrag
Supported: replaces, join, gruu
Content-Type: application/sdp
Content-Length: ...
v=0
o=focus431 2890844526 2890842807 IN IP4 ms5.conf.example.com
s=-
i=Example Conference Hosted by Example.com
u=http://conf.example.com/3402934234
e=3402934234@conf-help.example.com
p=+1-888-2934234
c=IN IP4 ms5.conf.example.com
t=0 0
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
F5 SUBSCRIBE sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.chicago.example.com
;branch=z9hG4bKdf334
Max-Forwards: 70
To:
From: Carol ;tag=43524545
Call-ID: k3l43id034ksereree
CSeq: 22 SUBSCRIBE
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Event: conference
Accept: application/conference-info+xml
Supported: replaces
Content-Length: 0
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RFC 4579 SIP CC Conferencing for UAs August 2006
F7 NOTIFY sip:carol@chicago.example.com SIP/2.0
Via: SIP/2.0/UDP ms5.conf.example.com;branch=z9hG4bK3343d1
Max-Forwards: 70
To: Carol ;tag=43524545
From: ;tag=a3343df32
Call-ID: k3l43id034ksereree
CSeq: 34321 NOTIFY
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Event: conference
Accept: application/sdp, message/sipfrag
Subscription-State: active;expires=3600
Supported: replaces, join, gruu
Content-Type: application/conference-info+xml
Content-Length: ...
tel:+18882934234
Carol
connected
dialed-in
Main Audio
audio
583398
sendrecv
video
345212
sendrecv
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RFC 4579 SIP CC Conferencing for UAs August 2006
5.2. INVITE: Adding a Participant by the Focus - Dial-Out
To directly add a participant to a conference, a focus SHOULD send an
INVITE to the participant containing a Contact header field with the
conference URI and the 'isfocus' feature parameter.
Note that a conference-unaware UA would simply ignore the
conferencing information and treat the session (from a SIP
perspective) as a point-to-point session. This is because standard
RFC 3261 [2] behavior is to ignore unknown header parameters such as
'isfocus'.
An example call flow is shown in Figure 2. It is assumed that Alice
is already a participant of the conference. The focus invites Carol
to the conference by sending an INVITE. After the session is
established, Carol subscribes to the conference URI. It is important
to note that there is no dependency on Carol's SUBSCRIBE (F5) and the
NOTIFY to Alice (F9) -- they occur asynchronously and independently.
Johnston & Levin Best Current Practice [Page 11]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| | |
| | |
| Focus "dials out" to add Carol to the conference |
| | |
| | INVITE Contact:Conf-ID;isfocus F1 |
| |---------------------------------------->|
| | 180 Ringing F2 |
| |<----------------------------------------|
| | 200 OK F3 |
| |<----------------------------------------|
| | ACK F4 |
| |---------------------------------------->|
| | RTP |
| |<=======================================>|
| | SUBSCRIBE sip:Conf-ID F5 |
| |<----------------------------------------|
| | 200 OK F6 |
| |---------------------------------------->|
| | NOTIFY F7 |
| |---------------------------------------->|
| | 200 OK F8 |
| |<----------------------------------------|
| NOTIFY F9 | |
|<-------------------| |
| 200 OK F10 | |
|------------------->| |
Figure 2. A Focus "Dials Out" to Add a Participant to the Conference.
F7 NOTIFY sip:carol@chicago.example.com SIP/2.0
Via: SIP/2.0/UDP ms5.conf.example.com;branch=z9hG4bK3343d1
Max-Forwards: 70
To: Carol ;tag=43524545
From: ;tag=a3343df32
Call-ID: k3l43id034ksereree
CSeq: 34321 NOTIFY
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Event: conference
Accept: application/sdp, message/sipfrag
Subscription-State: active;expires=3600
Supported: replaces, gruu
Content-Type: application/conference-info+xml
Content-Length: ...
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RFC 4579 SIP CC Conferencing for UAs August 2006
tel:+18882934234
Alice
connected
dialed-in
Main Audio
audio
647231
sendrecv
video
21345
sendrecv
Carol
connected
dialed-out
Main Audio
audio
583398
sendrecv
video
345212
sendrecv
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RFC 4579 SIP CC Conferencing for UAs August 2006
F9 NOTIFY sip:alice@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP ms5.conf.example.com;branch=z9hG4bK3432
Max-Forwards: 70
To: Alice ;tag=43524545
From: ;tag=a3343df32
Call-ID: 8820450524545
CSeq: 998 NOTIFY
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Event: conference
Accept: application/sdp, message/sipfrag
Subscription-State: active;expires=2450
Supported: replaces, gruu
Content-Type: application/conference-info+xml
Content-Length: ...
Carol
connected
dialed-out
Main Audio
audio
583398
sendrecv
video
345212
sendrecv
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5.3. INVITE: Manually Creating a Conference by Dialing in to a
Conferencing Application
In this section, a user sends an INVITE to a conference server
application. The application (such as an IVR system or a web page)
is implemented because the system requires additional input from the
user before it is able to create a conference. After a normal dialog
is established, additional information is received and the conference
together with its focus are created. Since the UA is now in a dialog
with a focus, the focus will re-INVITE the user with the conference
URI in Contact with the 'isfocus' feature parameter.
Alternatively, the additional information can be provided by the user
during an early dialog (see RFC 3261 [2] for a discussion of early
dialogs in SIP). This could be accomplished by a 183 Session
Progress response sent by the conferencing application. After the
conference is created, the conference URI would then be returned in a
Contact in the 200 OK.
Note that since this flow is all about human interaction with a
conferencing application, any errors and failures will be returned to
the human (recorded announcements, error tones, etc.).
As discussed in the conferencing framework, the conference URI must
be unique across all distinct conferences within the same domain. In
general, the user part of a conference URI will contain a pseudo
random string.
An example call flow is shown in Figure 3. In this example, Alice
uses a conference application that is triggered when Alice sends an
INVITE to the conference application. In this example, Conf-App is
used to represent the conference application URI. Alice's
conference-aware UA learns of the existence of the conference from
the 'isfocus' feature parameter and subscribes to the conference
package to receive notifications of the conference state.
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RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
| Alice establishes session with conference application. |
| | | |
| INVITE sip:Conf-App F1 | |
|------------------->| | |
| 180 Ringing F2 | | |
|<-------------------| | |
| 200 OK F3 | | |
|<-------------------| | |
| ACK F4 | | |
|------------------->| | |
| RTP | | |
|<==================>| | |
| | | |
| Alice uses the application to create the conference. |
| | | |
| INVITE Contact:Conf-ID;isfocus F5 | |
|<-------------------| | |
| 200 OK F6 | | |
|------------------->| | |
| ACK F7 | | |
|<-------------------| | |
| RTP | | |
|<==================>| | |
| | | |
| SUBSCRIBE sip:Conf-ID F8 | |
|------------------->| | |
| 200 OK F9 | | |
|<-------------------| | |
| NOTIFY F10 | | |
|<-------------------| | |
| 200 OK F11 | | |
|------------------->| | |
Figure 3. A Participant Creates a Conference Using an Application.
5.4. INVITE: Creating a Conference Using Ad-Hoc SIP Methods
This section addresses creating a conference by using ad-hoc SIP
means. The conference factory URI (as defined in Section 3.2) is
used to automatically create the conference in this example. This is
different from the previous scenario in that no human intervention is
required -- an automaton can create the conference and add
participants. Since the conference does not need to be scheduled or
reserved, but is created "on the fly", it is an "ad-hoc" conference
creation.
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The benefit of this approach is that the conference URI need not be
known to the user; instead it is created by a focus and used by the
participants' UAs. The main difference between this scenario and
Section 5.3 is that no user intervention (IVR, web page form, etc.)
is required to create the conference.
The SIP URI of the conference factory can be provisioned in the UA
(as in a "create new conference" button on a SIP phone) or can be
discovered using other means.
A SIP entity (such as conferencing server) can distinguish this
INVITE request as a request to create a new ad-hoc conference from a
request to join an existing conference by the Request-URI. That is,
although both requests may route to the same application, the
differing services requested can be identified by the differing URIs
in the request itself.
Assuming that all security and policy requirements have been met, a
new conference will be created with the Contact URI returned in the
200 OK being the conference URI. The Contact header field MUST
contain the 'isfocus' feature parameter to indicate that this URI is
for a conference.
An example call flow is shown in Figure 4. Note that Conf-Factory is
shorthand for the conference factory URI and Conf-ID Is short for the
conference URI. In this flow, Alice has a conference-aware UA and
creates a conference by sending an INVITE to the conference factory
URI. The conference factory application creates the conference and
redirects Alice to the focus using a 302 Moved Temporarily response.
Note that with proxy recursion as part of normal RFC 3261 [2]
behavior, Alice may never see the redirect but may just receive the
responses from the focus starting with message F5. Once the media
session is established, Alice subscribes to the conference URI
obtained through the Contact in the 200 OK response from the focus.
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RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Conf-Factory App Focus Bob
| | | |
| Alice creates the conference. | |
| | | |
| INVITE sip:Conf-Factory F1 | |
|------------------->| | |
| 302 Moved Contact:Conf-ID;isfocus F2 | |
|<-------------------| | |
| ACK F3 | | |
|------------------->| | |
| INVITE sip:Conf-ID F4 | |
|---------------------------------------->| |
| 180 Ringing F5 | |
|<----------------------------------------| |
| 200 OK Contact:Conf-ID;isfocus F6 | |
|<----------------------------------------| |
| ACK F7 | |
|---------------------------------------->| |
| RTP | |
|<=======================================>| |
| | |
| Alice subscribes to the conference URI. | |
| | |
| SUBSCRIBE sip:Conf-ID F8 | |
|---------------------------------------->| |
| 200 OK F9 | |
|<----------------------------------------| |
| NOTIFY F10 | |
|<----------------------------------------| |
| 200 OK F11 | |
|---------------------------------------->| |
Figure 4. Creation of a Conference Using SIP Ad-Hoc Methods.
5.5. REFER: Requesting a Focus to Add a New Resource to a Conference
(Dial Out to a New Participant)
A SIP conference URI can be used to inject different kinds of
information into the conference. Examples include new participants,
new real-time media sources, new IM messages, and pointers to passive
information references (such as HTTP URIs).
To request that the focus add a new information resource to the
specified conference, any SIP UA can send a REFER to the conference
URI with a Refer-To containing the URI of the new resource. Since
this REFER is sent to the conference URI and not the conference
factory URI, the semantics to the focus are to bring the resource
into the conference and make it visible to the conference
Johnston & Levin Best Current Practice [Page 18]
RFC 4579 SIP CC Conferencing for UAs August 2006
participants. The resultant focus procedures are dependent both on
the nature of the new resource (as expressed by its URI) and the
policy of the focus regarding IM, central vs. distributed real-time
media processing, and so on.
The scenario for adding a new UA participant is important to support
because it works even if the new participant does not support REFER
and transfer call control -- only the requesting participant and the
focus need to support the REFER and transfer call control.
Upon receipt of the REFER containing a Refer-To header with a SIP
URI, the focus SHOULD send an INVITE to the new participant
identified by the Refer-To SIP URI containing a Contact header field
with the conference URI and the 'isfocus' feature parameter.
A conference-unaware UA would simply ignore the conferencing
information and treat the session (from a SIP perspective) as a
point-to-point session.
An example call flow is shown in Figure 5. While this flow shows the
use of REFER to add a new participant to the conference, the
mechanism can generally add a resource as identified by a URI to the
conference. It is assumed that Alice is already a participant of the
conference. Alice sends a REFER to the conference URI. The focus
invites Carol to the conference by sending an INVITE. After the
session is established, Carol subscribes to the conference URI. It
is important to note that there is no dependency on Carol's SUBSCRIBE
(F11) and the NOTIFY to Alice (F15) -- they occur asynchronously and
independently.
Johnston & Levin Best Current Practice [Page 19]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| | |
| REFER sip:Conf-ID Refer-To:Carol F1 | |
|------------------->| |
| 202 Accepted F2 | |
|<-------------------| |
| NOTIFY (Trying) F3 |
|<-------------------| |
| 200 OK F4 | |
|------------------->| |
| | |
| Focus "dials out" to join Carol to the conference |
| | |
| | INVITE Contact:Conf-ID;isfocus F5 |
| |---------------------------------------->|
| | 180 Ringing F6 |
| |<----------------------------------------|
| | 200 OK F7 |
| |<----------------------------------------|
| | ACK F8 |
| |---------------------------------------->|
| | RTP |
| |<=======================================>|
| NOTIFY (OK) F9 | |
|<-------------------| |
| 200 OK F10 | |
|------------------->| |
| | SUBSCRIBE sip:Conf-ID F11 |
| |<----------------------------------------|
| | 200 OK F12 |
| |---------------------------------------->|
| | NOTIFY F13 |
| |---------------------------------------->|
| | 200 OK F14 |
| |<----------------------------------------|
| NOTIFY F15 | |
|<-------------------| |
| 200 OK F16 | |
|------------------->| |
Figure 5. Participant Requests That the Focus Add a Participant to
the Conference.
Johnston & Levin Best Current Practice [Page 20]
RFC 4579 SIP CC Conferencing for UAs August 2006
F1 REFER sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com;branch=z9hG4bKg4534
Max-Forwards: 70
To:
From: Alice ;tag=5534562
Call-ID: 849392fklgl43
CSeq: 476 REFER
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Accept: application/sdp, message/sipfrag
Refer-To:
Supported: replaces
Content-Length: 0
5.6. REFER: Requesting a User to Dial in to a Conference Using a
Conference URI
A participant wishing to add a new participant will request this
participant to send an INVITE to the conference URI. This can be
done using a non-SIP means (such as passing or publishing the
conference URI in an email, IM, or web page). If a non-SIP means is
used, then the flow and requirements are identical to Section 5.1.
The SIP mechanism to do this utilizes the REFER method.
A UA wishing to add a new participant SHOULD send a REFER request to
the participant with a Refer-To header containing the conference URI.
The requirements are then identical to the dial-in case of Section
5.1. The inviting participant MAY receive notification through the
REFER action that the new participant has been added in addition to
the notification received through the conference package.
An example is shown in Figure 6. In this call flow, it is assumed
that Alice is already a participant of the conference. Alice sends
Bob an "out of band" REFER - that is, a REFER outside of an
established dialog. Should Bob reject the REFER, Alice might try
sending an INVITE to Bob to establish a session first, then send a
REFER within the dialog, effectively transferring Bob into the
conference [17].
Johnston & Levin Best Current Practice [Page 21]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| | |
| | | |
| Alice adds Bob into conference | |
| | | |
| REFER Refer-To:Conf-ID F1 | |
|---------------------------------------->| |
| 202 Accepted F2 | | |
|<----------------------------------------| |
| NOTIFY (Trying) F3| | |
|<----------------------------------------| |
| 200 OK F4 | | |
|---------------------------------------->| |
| | INVITE sip:Conf-ID F5 |
| |<-------------------| |
| | 180 Ringing F6 | |
| |------------------->| |
| | 200 OK Contact:Conf-ID;isfocus F7 |
| |------------------->| |
| | ACK F8 | |
| |<-------------------| |
| | RTP | |
| |<==================>| |
| NOTIFY (OK) F9 | | |
|<----------------------------------------| |
| 200 OK F10 | | |
|---------------------------------------->| |
| NOTIFY F11 | | |
|<-------------------| | |
| 200 OK F12 | | |
|------------------->| | |
| | SUBSCRIBE sip:Conf-ID F13 |
| |<-------------------| |
| | 200 OK F14 | |
| |------------------->| |
| | NOTIFY F15 | |
| |------------------->| |
| | 200 OK F16 | |
| |<-------------------| |
Figure 6. Adding a Participant to an Existing Conference.
Johnston & Levin Best Current Practice [Page 22]
RFC 4579 SIP CC Conferencing for UAs August 2006
5.7. REFER with REFER: Requesting a Focus to Refer a Participant to
Dial in to the Conference
A participant may request that the focus refer a participant into the
conference by sending a REFER method. The Refer-To header field will
have the method set to REFER and an escaped Refer-To header field
containing the conference URI.
Note that in Message F1 below, the Refer-To header field is shown as
continuing across two lines -- this would not be the case in an
actual message; the URI would have continued beyond the formatting
limitations of this document.
This scenario is shown in Figure 7.
Johnston & Levin Best Current Practice [Page 23]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| | |
| Alice asks focus to REFER Bob into conference |
| | | |
|REFER sip:Conf-ID Refer-To:Bob;method=REFER?Refer-To=Conf-ID F1
|------------------->| | |
| 202 Accepted F2 | | |
|<-------------------| | |
| NOTIFY (Trying) F3| | |
|<-------------------| | |
| 200 OK F4 | | |
|------------------->| | |
| Focus REFERs Bob to the conference |
| | | |
| | REFER Refer-To:Conf-ID F5 |
| |------------------->| |
| | 202 Accepted F6 | |
| NOTIFY (202) F7 |<-------------------| |
|<-------------------| NOTIFY (Trying) F8 | |
| 200 OK F9 |<-------------------| |
|------------------->| 200 OK F10 | |
| |------------------->| |
| | INVITE sip:Conf-ID F11 |
| |<-------------------| |
| | 180 Ringing F12 | |
| |------------------->| |
| | 200 OK Contact:Conf-ID;isfocus F13 |
| |------------------->| |
| | ACK F14 | |
| NOTIFY F15 |<-------------------| |
|<-------------------| RTP | |
| 200 OK F16 |<==================>| |
|------------------->| NOTIFY (200) F17 | |
| |<-------------------| |
| | 200 OK F18 | |
| |------------------->| |
| | SUBSCRIBE sip:Conf-ID F17 |
| |<-------------------| |
| | 200 OK F19 | |
| |------------------->| |
| | NOTIFY F20 | |
| |------------------->| |
| | 200 OK F21 | |
| |<-------------------| |
Figure 7. Requesting That the Focus Refer a Participant to a
Conference.
Johnston & Levin Best Current Practice [Page 24]
RFC 4579 SIP CC Conferencing for UAs August 2006
F1 REFER sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com;branch=z9hG4bKg4534
Max-Forwards: 70
To:
From: Alice ;tag=5534562
Call-ID: 849392fklgl43
CSeq: 476 REFER
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Accept: application/sdp, message/sipfrag
Refer-To:
Supported: replaces
Content-Length: 0
F5 REFER sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP ms5.conf.example.com;branch=z9hG4bK33445243
Max-Forwards: 70
To:
From: ;tag=345621412
Call-ID: 5494204
CSeq: 4524323 REFER
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Accept: application/sdp, message/sipfrag
Refer-To:
Supported: join, gruu, replaces
Content-Length: 0
F11 INVITE sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.com;branch=z9hG4bKh3887
Max-Forwards: 70
To:
From: Bob ;tag=32411
Call-ID: 5d4324fa84b4c76e66710
CSeq: 764 INVITE
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Supported: replaces, join
Content-Type: application/sdp
Content-Length: ...
Johnston & Levin Best Current Practice [Page 25]
RFC 4579 SIP CC Conferencing for UAs August 2006
(SDP not shown)
5.8. Join Header Field: Dialing in to a Conference Using a (3rd Party)
Dialog Identifier
Under some circumstances, a participant wanting to join a conference
may only know a dialog identifier of one of the legs of the
conference. The information may have been learned using the dialog
package [18] or some non-SIP means to retrieve this information from
another conference participant.
A UA can request to be added to a conference by sending a request to
the focus containing a Join [7] header field containing a dialog ID
of one leg of the conference (a dialog between another participant
and the focus).
There are other scenarios in which a UA can use the Join header for
certain conferencing call control scenarios. See [7] for further
examples and details.
An example is shown in Figure 8. It is assumed that Alice is a
participant of the conference. The dialog identifier between Alice
and the focus is abbreviated as A-F and is known by Bob. Bob
requests to be added to the conference by sending an INVITE message
F1 to the focus containing a Join header that contains the dialog
identifier A-F. Bob is added into the conference by the focus.
Johnston & Levin Best Current Practice [Page 26]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| | |
| | | |
| Bob requests to be added to the conference. |
| | | |
| | INVITE Join:A-F F1| |
| |<-------------------| |
| | 180 Ringing F2 | |
| |------------------->| |
| | 200 OK Contact:Conf-ID;isfocus F3 |
| |------------------->| |
| | ACK F4 | |
| |<-------------------| |
| | RTP | |
| NOTIFY F5 |<==================>| |
|<-------------------| SUBSCRIBE sip:Conf-ID F6 |
| 200 OK F7 |<-------------------| |
|------------------->| 200 OK F8 | |
| |------------------->| |
| | NOTIFY F9 | |
| |------------------->| |
| | 200 OK F10 | |
| |<-------------------| |
Figure 8. Adding a Participant to an Existing Conference using Join.
F1 INVITE sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.biloxi.com;branch=z9hG4bKh3832
Max-Forwards: 70
To:
From: Bob ;tag=32411
Call-ID: d432fa84b4c76e66710
CSeq: 8 INVITE
Contact:
Join: 3434034-293553453;to-tag=fdj3l34;from-tag=12f331
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Supported: replaces, join
Content-Type: application/sdp
Content-Length: ...
(SDP not shown)
Johnston & Levin Best Current Practice [Page 27]
RFC 4579 SIP CC Conferencing for UAs August 2006
5.9. Replaces Header Field: Switching User Agents within a Conference
Participants in a conference may want to change the user agent (i.e.,
the endpoint or the device) with which they participate in the
conference. This could be done by simply sending a BYE from one user
agent to leave the conference and an INVITE from the other user agent
to rejoin. However, the SIP Replaces [6] primitive is perfectly
suited to this operation.
An example is shown in Figure 9. It is assumed that Alice is a
participant of the conference using user agent #1. The dialog
identifier between Alice's user agent #1 and the focus is abbreviated
as A-F. Alice switches to user agent #2 and sends an INVITE message
F1 to the focus containing a Replaces header that contains the dialog
identifier A-F. Note that this dialog identifier could be learned
through some non-SIP mechanism, or by use of SUBSCRIBE/NOTIFY and the
dialog event package [18]. Alice's user agent #2 is added into the
conference by the focus. The focus sends a BYE to user agent #1.
User agent #1 then automatically terminates the subscription by
sending a SUBSCRIBE with Expires:0 to terminate the subscription.
Note that the participant list (roster) has not necessarily changed
during this scenario, unless detailed information about Alice user
agents (i.e. endpoints) is included in the conference state
notifications. For a full discussion of conference package
notifications, refer to [9].
Johnston & Levin Best Current Practice [Page 28]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice UA#1 Focus Alice UA#2 Carol
| | | |
|<==================>| | |
| | | |
| Alice switches user agents during the conference. |
| | | |
| | INVITE sip:Conf-ID Replaces:A-F F1 |
| |<-------------------| |
| | 200 OK Contact:Conf-ID;isfocus F2 |
| |------------------->| |
| | ACK F3 | |
| |<-------------------| |
| | RTP | |
| |<==================>| |
| BYE F4 | | |
|<-------------------| | |
| 200 OK F5 | | |
|------------------->| | |
| SUBSCRIBE Expires:0 F6 | |
|------------------->| | |
| 200 OK F7 | | |
|<-------------------| | |
| NOTIFY Subscription-State:terminated F8 | |
|<-------------------| | |
| 200 OK F9 | | |
|------------------->| | |
| | SUBSCRIBE sip:Conf-ID F10 |
| |<-------------------| |
| | 200 OK F11 | |
| |------------------->| |
| | NOTIFY F12 | |
| |------------------->| |
| | 200 OK F13 | |
| |<-------------------| |
Figure 9. Switching a User Agent within a Conference.
5.10. Replaces Header Field: Transferring a Point-to-Point Session into
a Conference
This call flow shows how a point-to-point call can be transferred to
a conference call involving an external focus.
Alice and Bob have an established session with a dialog identifier
A-B. Alice joins the conference with the focus by sending an INVITE
to the Conference URI. Alice then sends a REFER request to the focus
to send an INVITE request to the other participant. Alice includes
an escaped Replaces header field in the URI included in the Refer-To
Johnston & Levin Best Current Practice [Page 29]
RFC 4579 SIP CC Conferencing for UAs August 2006
header field. Bob receives the INVITE from the focus and matches the
dialog in the Replaces header field with the dialog with Alice. As a
result, Bob accepts the INVITE, joins the conference, and sends a BYE
to Alice to tear down their point-to-point dialog.
Alice Focus Bob Carol
| | | |
| Alice is in a session with Bob | |
|<=======================================>| |
| | | |
| Alice joins the conference | |
| | | |
| INVITE sip:Conf-ID F1 | |
|------------------->| | |
| 200 OK Contact:sip:Conf-ID;isfocus F2 | |
|<-------------------| | |
| ACK F3 | | |
|------------------->| | |
| SUBSCRIBE F4 | | |
|------------------->| | |
| 200 OK F5 | | |
|<-------------------| | |
| NOTIFY F6 | | |
|<-------------------| | |
| 200 OK F7 | | |
|------------------->| | |
|<==================>| | |
| | | |
| Alice asks focus to REFER Bob into conference |
| | | |
| REFER sip:Conf-ID Refer-To:Bob?Replaces=A-B F8 |
|------------------->| | |
| 202 Accepted F9 | | |
|<-------------------| | |
| NOTIFY (Trying) F10| | |
|<-------------------| | |
| 200 OK F11 | | |
|------------------->| | |
| | | |
| Focus invites Bob to the conference |
| | | |
| | INVITE sip:Conf-ID Replaces:A-B F12 |
| |------------------->| |
| | 200 OK F13 | |
| |<-------------------| |
| | ACK F14 | |
| |------------------->| |
| | RTP | |
Johnston & Levin Best Current Practice [Page 30]
RFC 4579 SIP CC Conferencing for UAs August 2006
| |<==================>| |
| BYE F15 | |
|<----------------------------------------| |
| 200 OK F16 | |
|---------------------------------------->| |
| NOTIFY (200) F17 | | |
|<-------------------| | |
| 200 OK F18 | | |
|------------------->| | |
| NOTIFY F19 | | |
|<-------------------| | |
| 200 OK F20 | | |
|------------------->| | |
| | SUBSCRIBE sip:Conf-ID F21 |
| |<-------------------| |
| | 200 OK F22 | |
| |------------------->| |
| | NOTIFY F23 | |
| |------------------->| |
| | 200 OK F24 | |
| |<-------------------| |
| | | |
Figure 10. Transitioning a Point to Point Session into a Conference.
5.11. REFER with BYE: Requesting That the Focus Remove a Participant
from a Conference
To request that the focus remove a participant from the specified
conference, a properly authorized SIP UA (typically the conference
owner) can send a REFER to the conference URI with a Refer-To
containing the URI of the participant and with the method set to BYE.
The requestor does not need to know the dialog information about the
dialog between the focus and the participant who will be removed --
the focus knows this information and fills it when it generates the
BYE request.
An example call flow is shown in Figure 11. It is assumed that Alice
and Carol are already participants of the conference and that Alice
is authorized to remove members from the conference. Alice sends a
REFER to the conference URI with a Refer-To header containing a URI
of the form sip:carol@chicago.example.com;method=BYE.
Johnston & Levin Best Current Practice [Page 31]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>| |
| REFER sip:Conf-ID Refer-To:Carol;method=BYE F1 |
|------------------->| |
| 202 Accepted F2 | |
|<-------------------| |
| NOTIFY (Trying) F3 |
|<-------------------| |
| 200 OK F4 | |
|------------------->| |
| | |
| Focus removes Carol from the conference |
| | |
| | BYE sip:Carol F5 |
| |---------------------------------------->|
| | 200 OK F6 |
| |<----------------------------------------|
| | NOTIFY Subscription-State:terminated F7 |
| |---------------------------------------->|
| | 200 OK F8 |
| |<----------------------------------------|
| NOTIFY (200) F9 | |
|<-------------------| |
| 200 OK F10 | |
|------------------->| |
| NOTIFY F11 | |
|<-------------------| |
| 200 OK F12 | |
|------------------->| |
Figure 11. Participant Requests That the Focus Remove a Participant
from the Conference.
F1 REFER sip:3402934234@conf.example.com SIP/2.0
Via: SIP/2.0/UDP client.atlanta.example.com;branch=z9hG4bKg4534
Max-Forwards: 70
To:
From: Alice ;tag=5534562
Call-ID: 849392fklgl43
CSeq: 476 REFER
Contact:
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Accept: application/sdp, message/sipfrag
Refer-To:
Supported: replaces
Content-Length: 0
Johnston & Levin Best Current Practice [Page 32]
RFC 4579 SIP CC Conferencing for UAs August 2006
F5 BYE sip:carol@client.chicago.example.com SIP/2.0
Via: SIP/2.0/UDP ms5.conf.example.com;branch=z9hG4bK343gf4
Max-Forwards: 70
From: ;tag=5393k2312
To: Carol ;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 78654 BYE
Content-Length: 0
5.12. Deleting a Conference
The default conference policy for conferences created using the
Conference Factory URI is that the conference is deleted when the
creator departs.
Figure 12 shows this call flow in which the creator Alice departs
causing the conference to be deleted. Note that the order of sending
BYEs and final NOTIFYs is not important.
Johnston & Levin Best Current Practice [Page 33]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
|<==================>|<==================>| |
| BYE F1 |<=======================================>|
|------------------->| | |
| 200 OK F2 | | |
|<-------------------| | |
| | BYE F3 | |
| |------------------->| |
| | 200 OK F4 | |
| |<-------------------| |
| | BYE F5 |
| |---------------------------------------->|
| | 200 OK F6 |
| |<----------------------------------------|
| NOTIFY Subscription-State:terminated F7 |
|<-------------------| | |
| 200 OK F8 | | |
|------------------->| NOTIFY Subscription-State:terminated F9 |
| |------------------->| |
| | 200 OK F10 | |
| |<-------------------| |
| | NOTIFY Subscription-State:terminated F11|
| |---------------------------------------->|
| | 200 OK F12 |
| |<----------------------------------------|
Figure 12. Deleting a Conference.
5.13. Discovery of URI Properties Using OPTIONS
A UA MAY send an OPTIONS request to discover if an opaque URI is a
conference URI (resolves to a focus). In addition, the reply to the
OPTIONS request can also indicate support for various SIP call
control extensions used in this document.
Note that the Allow, Accept, Allow-Events, and Supported header
fields should be present in an INVITE from a focus or a 200 OK answer
from the focus to an INVITE as a part of a normal dialog
establishment process.
An example is shown in Figure 13 where Alice sends an OPTIONS to a
URI that resolves to a focus.
Johnston & Levin Best Current Practice [Page 34]
RFC 4579 SIP CC Conferencing for UAs August 2006
Alice Focus Bob Carol
| | | |
| OPTIONS sip:Conf-ID F1 | |
|------------------->| | |
| 200 OK Contact:Conf-ID;isfocus F2 | |
|<-------------------| | |
Figure 13. Participant Queries Capabilities of URI of a Focus.
Following is an example of message detail of message F2 in Figure 13.
Based on the response, Alice's UA learns that the URI is a conference
URI and that the responding UA is focus that supports a number of SIP
call control extensions.
The response details are as follows:
F2 SIP/2.0 200 OK
Via: SIP/2.0/UDP pc33.atlanta.example.com;branch=z9hG4bKhjsas87
;received=192.0.2.4
To: ;tag=93810874
From: Alice ;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 63104 OPTIONS
Contact: ;isfocus
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
SUBSCRIBE, NOTIFY
Allow-Events: refer, conference
Accept: application/sdp, message/sipfrag
Accept-Language: en
Supported: replaces, join, gruu
Content-Type: application/sdp
Content-Length: ...
v=0
o=focus431 2890844563 2890842835 IN IP4 ms5.conf.example.com
s=-
i=Example Conference Hosted by Example.com
u=http://conf.example.com/3402934234
e=3402934234@conf-help.example.com
p=+18882934234
c=IN IP4 ms5.conf.example.com
t=0 0
m=audio 0 RTP/AVP 0 3 5 7
m=video 0 RTP/AVP 31 32
Useful information from each of these headers is detailed in the next
sections.
Johnston & Levin Best Current Practice [Page 35]
RFC 4579 SIP CC Conferencing for UAs August 2006
Allow. The support of methods such as REFER, SUBSCRIBE, and NOTIFY
indicates that the user agent supports call control and SIP events.
Accept. The support of bodies such as message/sipfrag [12] indicates
support of call control.
Allow-Events. Indicates support of event packages such as refer [4]
and conference [9].
Supported. Indicates support of extensions such as replaces, join,
and gruu.
Contact. The presence of the 'isfocus' feature parameter in the
Contact header indicates that the URI is a conference URI and that
the UA is a focus.
6. Security Considerations
This specification defines the interaction between a focus UA and a
participant UA in a conferencing application. As a result, the
security considerations and mechanisms defined in RFC 3261 [2] apply.
However, there are some aspects unique to conferencing that will be
discussed here.
A conference often involves the use of substantial network bandwidth
and computing resources. As a result, authentication is even more
important than in a simple peer-to-peer session. As discussed in the
conferencing framework [8], conferences often have policy related to
conferencing resources. A focus SHOULD authenticate participants
before joining them to a conference and allowing utilization of
conferencing resources. Different policies can be applied by a focus
to different participants based on the result of authentication.
A participant will be interacting with a number of other participants
through the focus. As a result, a participant should authenticate
the focus and be sure that the focus used for the conference is
trusted. Normal SIP authentication mechanisms are suitable for
participant and focus authentication, such as SIP Digest utilizing a
shared secret, or certificates, or a secured SIP identity mechanism.
In addition, a focus SHOULD support Secure SIP connections so that
hop-by-hop mutual authentication and confidentiality provided by TLS
can be achieved.
In the SIP dialog between them, a focus utilizes the 'isfocus'
feature tag to indicate that the UA is acting as a focus. As such,
the SIP header fields such as Contact SHOULD have end to end
integrity. A participant and focus SHOULD support an end-to-end
integrity mechanism such as S/MIME.
Johnston & Levin Best Current Practice [Page 36]
RFC 4579 SIP CC Conferencing for UAs August 2006
Once a participant has learned that the other UA is a focus, SIP call
control operations (such as REFER) can be implemented, or a
subscription to the conference package of the focus might be
attempted. The security considerations described in RFC 3515 [4]
apply to any REFER call control operations. A focus and participant
will apply policy to determine which call control operations are
allowed.
A focus accepting subscriptions to the conference package must follow
the security considerations in RFC 4575 [9]. Since notifications can
carry sensitive information, the subscriptions should be
authenticated and the notifications delivered with confidentiality
and integrity protection. Since a participant is not able to
authenticate other participants directly, a participant must rely on
the focus to perform this authentication.
A focus MUST support a participant's request for privacy, either
through conference policy or as expressed through the signaling. For
example, a participant joining a conference and including a Privacy
header field [10] must not have identity information revealed to
other participants by the focus. If other signaling protocols are
used, privacy signaled through them also must be respected.
7. Contributors
We would like to thank Rohan Mahy, Jonathan Rosenberg, Roni Even,
Petri Koskelainen, Brian Rosen, Paul Kyzivat, Eric Burger, and others
in list discussions.
Thanks to Miguel Garcia for his detailed last-call review and
suggestions.
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RFC 4579 SIP CC Conferencing for UAs August 2006
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
[3] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[4] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[5] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[6] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.
[7] Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
"Join" Header", RFC 3911, October 2004.
[8] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol (SIP)", RFC 4353, February 2006.
[9] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference State",
RFC 4575, August 2006.
[10] Peterson, J., "A Privacy Mechanism for the Session Initiation
Protocol (SIP)", RFC 3323, November 2002.
8.2. Informative References
[11] Campbell, B. and R. Sparks, "Control of Service Context using
SIP Request-URI", RFC 3087, April 2001.
[12] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[13] Johnston, A., Donovan, S., Sparks, R., Cunningham, C., and K.
Summers, "Session Initiation Protocol (SIP) Basic Call Flow
Examples", BCP 75, RFC 3665, December 2003.
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RFC 4579 SIP CC Conferencing for UAs August 2006
[14] Levin, O. and R. Even, "High Level Requirements for Tightly
Coupled SIP Conferencing", RFC 4245, November 2005.
[15] Mahy, R., "A Call Control and Multi-party usage framework for
the Session Initiation Protocol (SIP)", Work in Progress,
February 2005.
[16] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", Work in Progress, February 2005.
[17] Sparks, R., Johnston, A., and D. Petrie, "Session Initiation
Protocol Call Control - Transfer", Work in Progress, April
2005.
[18] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
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RFC 4579 SIP CC Conferencing for UAs August 2006
Appendix A: Creating a Conference by a Conference-Unaware UA
This section discusses how a human user operating a conference-
unaware UA can create and add participants to a conference. This
method is described as an appendix since it is NOT RECOMMENDED. The
scenarios involving creating a conference using ad-hoc or manual
means are recommended over this scenario. This scenario is included,
however, for completeness.
A user (human) would choose a conference URI according to system
rules and insert it into the Request-URI of the INVITE. This same
URI is echoed by a focus adhering to certain addressing conventions
(discussed below) in the Contact header by the focus. Additional
participants could be added by non-SIP means (publication of the
chosen conference URI using web pages, email, IM, etc.).
Alternatively, the conference-unaware UA could then add other
participants to the conference using SIP call control by establishing
a session with them, then transferring [17] them to the conference
URI. Note that in this scenario only the user (human) is aware of
the conferencing application, and the conference-unaware UA only need
support RFC 3261 [2] and optionally call transfer.
Making this work does impose certain addressing conventions on a
system. As a service/implementation choice, a system could allow the
creator of the conference to choose the user portion of the
conference URI. However, this requires the URI format to be agreed
upon between a user and the system.
For example, a service provider might reserve the domain
conf.example.com for all conference URIs. Any URI in the domain of
conf.example.com would resolve to the focus. The focus could be
configured to interpret an unknown user part in the conf.example.com
domain as a request for a conference to be created with the
conference URI as the Request-URI. For example, an INVITE sent with
a Request-URI of sip:k32934208ds72@conf.example.com could be routed
to the focus that would then create the conference. This conference
URI should be registered by the newly created focus to become
routable as a conference URI within the conf.example.com domain. The
returned Contact would look as follows:
Contact: ;isfocus
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Note, however, that this approach relies on conventions adopted
between the user (human) and the focus. Also, the approach is not
robust against collisions in the conference names. If a second user
wishing to create a new conference happened to choose the same user
part as an existing conference, the result would be that the second
user would be added into the existing conference instead of creating
a new one.
As a result, methods of conference creation in which the conference
URI is an opaque URI generated by the focus are preferred.
An example call flow is shown in Figure 14. The participant Alice
creates the conference URI (using some convention agreed to with the
focus domain) and sends an INVITE to that URI which creates the
focus. The focus creates the conference and returns the same
conference URI in the 200 OK answer to the INVITE (which is ignored
by the conference-unaware UA).
Alice Focus Bob Carol
| | | |
| Alice creates the conference and chooses the conference URI. |
| | | |
| INVITE sip:Conf-ID F1 | |
|------------------->| | |
| 180 Ringing F2 | | |
|<-------------------| | |
| 200 OK Contact:Conf-ID;isfocus F3 | |
|<-------------------| | |
| ACK F4 | | |
|------------------->| | |
| RTP | | |
|<==================>| | |
Figure 14. Not Recommended: Conferencing Unaware Participant
Creates a Conference
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Authors' Addresses
Alan Johnston
Avaya
St. Louis, MO 63102
EMail: alan@sipstation.com
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: oritl@microsoft.com
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RFC 4579 SIP CC Conferencing for UAs August 2006
Full Copyright Statement
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Johnston & Levin Best Current Practice [Page 43]
RFC, FYI, BCP