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rfc:bcp:bcp119

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

Johnston & Levin Best Current Practice [Page 1] 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.

Johnston & Levin Best Current Practice [Page 2] RFC 4579 SIP CC Conferencing for UAs August 2006

 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:
  1. Definition of primitives, not services
  2. Signaling model independent
  3. Invoker oriented
  4. Primitives make full use of URIs
  5. Include policies for authentication, authorization, logging, etc.
  6. 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").

Johnston & Levin Best Current Practice [Page 3] RFC 4579 SIP CC Conferencing for UAs August 2006

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.

Johnston & Levin Best Current Practice [Page 4] RFC 4579 SIP CC Conferencing for UAs August 2006

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.

Johnston & Levin Best Current Practice [Page 5] 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:
  1. Create a conference
  2. Join a conference
  3. Invite a user to a conference
  4. Expel a user by third party
  5. Discover if a URI is a conference URI
  6. 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.

Johnston & Levin Best Current Practice [Page 6] RFC 4579 SIP CC Conferencing for UAs August 2006

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.

Johnston & Levin Best Current Practice [Page 7] RFC 4579 SIP CC Conferencing for UAs August 2006

 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: <sip:3402934234@conf.example.com>
       From: Carol <sip:carol@chicago.example.com>;tag=32331
       Call-ID: d432fa84b4c76e66710
       CSeq: 45 INVITE
       Contact: <sip:carol@client.chicago.example.com>
       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)

Johnston & Levin Best Current Practice [Page 8] 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: <sip:3402934234@conf.example.com>;tag=733413
       From: Carol <sip:carol@chicago.example.com>;tag=32331
       Call-ID: d432fa84b4c76e66710
       CSeq: 45 INVITE
       Contact: <sip:3402934234@conf.example.com>;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: <sip:3402934234@conf.example.com>
       From: Carol <sip:carol@chicago.example.com>;tag=43524545
       Call-ID: k3l43id034ksereree
       CSeq: 22 SUBSCRIBE
       Contact: <sip:carol@client.chicago.example.com>
       Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
        SUBSCRIBE, NOTIFY
       Event: conference
       Accept: application/conference-info+xml
       Supported: replaces
       Content-Length: 0

Johnston & Levin Best Current Practice [Page 9] 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 <sip:carol@chicago.example.com>;tag=43524545
       From: <sip:3402934234@conf.example.com>;tag=a3343df32
       Call-ID: k3l43id034ksereree
       CSeq: 34321 NOTIFY
       Contact: <sip:3402934234@conf.example.com>;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: ...
       <conference-info version="0" state="full"
        entity="sip:3402934234@conf.example.com">
        <conference-description>
         <conf-uris>
          <entry>
           <uri>tel:+18882934234</uri>
          </entry>
         </conf-uris>
        </conference-description>
        <users>
         <user entity="sip:carol@chicago.example.com" state="full">
          <display-text>Carol</display-text>
          <endpoint entity="sip:carol@client.chicago.example.com">
           <status>connected</status>
           <joining-method>dialed-in</joining-method>
           <media id="1">
            <display-text>Main Audio</display-text>
            <type>audio</type>
            <src-id>583398</src-id>
            <status>sendrecv</status>
           </media>
           <media id="2">
            <type>video</type>
            <src-id>345212</src-id>
            <status>sendrecv</status>
           </media>
          </endpoint>
         </user>
        </users>
       </conference-info>

Johnston & Levin Best Current Practice [Page 10] 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 <sip:carol@chicago.example.com>;tag=43524545
       From: <sip:3402934234@conf.example.com>;tag=a3343df32
       Call-ID: k3l43id034ksereree
       CSeq: 34321 NOTIFY
       Contact: <sip:3402934234@conf.example.com>;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: ...

Johnston & Levin Best Current Practice [Page 12] RFC 4579 SIP CC Conferencing for UAs August 2006

       <conference-info version="0" state="full"
        entity="sip:3402934234@conf.example.com">
        <conference-description>
         <conf-uris>
          <entry>
           <uri>tel:+18882934234</uri>
          </entry>
         </conf-uris>
        </conference-description>
        <users>
         <user entity="sip:alice@atlanta.example.com" state="full">
          <display-text>Alice</display-text>
          <endpoint entity="sip:alice@client.atlanta.example.com">
           <status>connected</status>
           <joining-method>dialed-in</joining-method>
           <media id="3">
            <display-text>Main Audio</display-text>
            <type>audio</type>
            <src-id>647231</src-id>
            <status>sendrecv</status>
           </media>
           <media id="4">
            <type>video</type>
            <src-id>21345</src-id>
            <status>sendrecv</status>
           </media>
          </endpoint>
         </user>
         <user entity="sip:carol@chicago.example.com" state="full">
          <display-text>Carol</display-text>
          <endpoint entity="sip:carol@client.chicago.example.com">
           <status>connected</status>
           <joining-method>dialed-out</joining-method>
           <media id="1">
            <display-text>Main Audio</display-text>
            <type>audio</type>
            <src-id>583398</src-id>
            <status>sendrecv</status>
           </media>
           <media id="2">
            <type>video</type>
            <src-id>345212</src-id>
            <status>sendrecv</status>
           </media>
          </endpoint>
         </user>
        </users>
       </conference-info>

Johnston & Levin Best Current Practice [Page 13] 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 <sip:alice@atlanta.example.com>;tag=43524545
       From: <sip:3402934234@conf.example.com>;tag=a3343df32
       Call-ID: 8820450524545
       CSeq: 998 NOTIFY
       Contact: <sip:3402934234@conf.example.com>;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: ...
       <conference-info version="1" state="partial"
        entity="sip:3402934234@conf.example.com">
        <users>
         <user entity="sip:carol@chicago.example.com" state="full">
          <display-text>Carol</display-text>
          <endpoint entity="sip:carol@client.chicago.example.com">
           <status>connected</status>
           <joining-method>dialed-out</joining-method>
           <media id="1">
            <display-text>Main Audio</display-text>
            <type>audio</type>
            <src-id>583398</src-id>
            <status>sendrecv</status>
           </media>
           <media id="2">
            <type>video</type>
            <src-id>345212</src-id>
            <status>sendrecv</status>
           </media>
          </endpoint>
         </user>
        </users>
       </conference-info>

Johnston & Levin Best Current Practice [Page 14] RFC 4579 SIP CC Conferencing for UAs August 2006

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.

Johnston & Levin Best Current Practice [Page 15] 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.

Johnston & Levin Best Current Practice [Page 16] RFC 4579 SIP CC Conferencing for UAs August 2006

 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.

Johnston & Levin Best Current Practice [Page 17] 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: <sip:3402934234@conf.example.com>
      From: Alice <sip:alice@atlanta.example.com>;tag=5534562
      Call-ID: 849392fklgl43
      CSeq: 476 REFER
      Contact: <sip:alice@alice.example.com>
      Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
       SUBSCRIBE, NOTIFY
      Accept: application/sdp, message/sipfrag
      Refer-To: <sip:carol@chicago.example.com>
      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: <sip:3402934234@conf.example.com>
       From: Alice <sip:alice@atlanta.example.com>;tag=5534562
       Call-ID: 849392fklgl43
       CSeq: 476 REFER
       Contact: <sip:alice@alice.example.com>
       Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
        SUBSCRIBE, NOTIFY
       Accept: application/sdp, message/sipfrag
       Refer-To: <sip:bob@biloxi.example.com;method=REFER
                            ?Refer-To=sip:3402934234%40example.com>
       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: <sip:bob@biloxi.example.com>
       From: <sip:3402934234@conf.example.com>;tag=345621412
       Call-ID: 5494204
       CSeq: 4524323 REFER
       Contact: <sip:3402934234@conf.example.com>;isfocus
       Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
        SUBSCRIBE, NOTIFY
       Accept: application/sdp, message/sipfrag
       Refer-To: <sip:3402934234@conf.example.com>
       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: <sip:3402934234@conf.example.com>
       From: Bob <sip:bob@biloxi.example.com>;tag=32411
       Call-ID: 5d4324fa84b4c76e66710
       CSeq: 764 INVITE
       Contact: <sip:bob@client.biloxi.example.com>
       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: <sip:3402934234@conf.example.com>
      From: Bob <sip:bob@biloxi.example.com>;tag=32411
      Call-ID: d432fa84b4c76e66710
      CSeq: 8 INVITE
      Contact: <sip:bob@client.biloxi.example.com>
      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: <sip:3402934234@conf.example.com>
       From: Alice <sip:alice@atlanta.example.com>;tag=5534562
       Call-ID: 849392fklgl43
       CSeq: 476 REFER
       Contact: <sip:alice@alice.example.com>
       Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER,
        SUBSCRIBE, NOTIFY
       Accept: application/sdp, message/sipfrag
       Refer-To: <sip:carol@chicago.example.com;method=BYE>
       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: <sip:3402934234@conf.example.com>;tag=5393k2312
       To: Carol <sip:carol@chicago.example.com>;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: <sip:3402934234@conf.example.com>;tag=93810874
      From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 63104 OPTIONS
      Contact: <sip:3402934234@conf.example.com>;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.

Johnston & Levin Best Current Practice [Page 37] 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.

Johnston & Levin Best Current Practice [Page 38] 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.

Johnston & Levin Best Current Practice [Page 39] 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: <sip:k32934208ds72@conf.example.com>;isfocus

Johnston & Levin Best Current Practice [Page 40] RFC 4579 SIP CC Conferencing for UAs August 2006

 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

Johnston & Levin Best Current Practice [Page 41] RFC 4579 SIP CC Conferencing for UAs August 2006

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

Johnston & Levin Best Current Practice [Page 42] RFC 4579 SIP CC Conferencing for UAs August 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
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 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 The IETF invites any interested party to bring to its attention any
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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Johnston & Levin Best Current Practice [Page 43]

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