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rfc:rfc5567

Network Working Group T. Melanchuk, Ed. Request for Comments: 5567 Rain Willow Communications Category: Informational June 2009

        An Architectural Framework for Media Server Control

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

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 document authors.  All rights reserved.
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 Provisions Relating to IETF Documents in effect on the date of
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 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
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 Without obtaining an adequate license from the person(s) controlling
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Abstract

 This document describes an architectural framework for Media Server
 control.  The primary focus will be to define logical entities that
 exist within the context of Media Server control, and define the
 appropriate naming conventions and interactions between them.

Melanchuk Informational [Page 1] RFC 5567 Mediactrl Architecture June 2009

Table of Contents

 1. Introduction ....................................................2
 2. Terminology .....................................................3
 3. Architecture Overview ...........................................4
 4. SIP Usage .......................................................7
 5. Media Control for IVR Services .................................10
    5.1. Basic IVR Services ........................................11
    5.2. IVR Services with Mid-Call Controls .......................11
    5.3. Advanced IVR Services .....................................11
 6. Media Control for Conferencing Services ........................12
    6.1. Creating a New Conference .................................14
    6.2. Adding a Participant to a Conference ......................14
    6.3. Media Controls ............................................15
    6.4. Floor Control .............................................16
 7. Security Considerations ........................................21
 8. Acknowledgments ................................................22
 9. Contributors ...................................................22
 10. Informative References ........................................23

1. Introduction

 Application Servers host one or more instances of a communications
 application.  Media Servers provide real-time media processing
 functions.  This document presents the core architectural framework
 to allow Application Servers to control Media Servers.  An overview
 of the architecture describing the core logical entities and their
 interactions is presented in Section 3.  The requirements for Media
 Server control are defined in [RFC5167].
 The Session Initiation Protocol (SIP) [RFC3261] is used as the
 session establishment protocol within this architecture.  Application
 Servers use it both to terminate media streams on Media Servers and
 to create and manage control channels for Media Server control
 between themselves and Media Servers.  The detailed model for Media
 Server control together with a description of SIP usage is presented
 in Section 4.
 Several services are described using the framework defined in this
 document.  Use cases for Interactive Voice Response (IVR) services
 are described in Section 5, and conferencing use cases are described
 in Section 6.

Melanchuk Informational [Page 2] RFC 5567 Mediactrl Architecture June 2009

2. Terminology

 The following terms are defined for use in this document in the
 context of Media Server control:
 Application Server (AS):  A functional entity that hosts one or more
    instances of a communication application.  The application server
    may include the conference policy server, the focus, and the
    conference notification server, as defined in [RFC4353].  Also, it
    may include communication applications that use IVR or
    announcement services.
 Media Functions:  Functions available on a Media Server that are used
    to supply media services to the AS.  Some examples are Dual-Tone
    Multi-Frequency (DTMF) detection, mixing, transcoding, playing
    announcement, recording, etc.
 Media Resource Broker (MRB):  A logical entity that is responsible
    for both the collection of appropriate published Media Server (MS)
    information and supplying of appropriate MS information to
    consuming entities.  The MRB is an optional entity and will be
    discussed in a separate document.
 Media Server (MS):  The media server includes the mixer as defined in
    [RFC4353].  The media server plays announcements, it processes
    media streams for functions like DTMF detection and transcoding.
    The media server may also record media streams for supporting IVR
    functions like announcing conference participants.  In the
    architecture for the 3GPP IP Multimedia Subsystem (IMS) a Media
    Server is referred to as a Media Resource Function (MRF).
 Media Services:  Application service requiring media functions such
    as Interactive Voice Response (IVR) or media conferencing.
 Media Session:  From the Session Description Protocol (SDP)
    specification [RFC4566]: "A multimedia session is a set of
    multimedia senders and receivers and the data streams flowing from
    senders to receivers.  A multimedia conference is an example of a
    multimedia session."
 MS Control Channel:  A reliable transport connection between the AS
    and MS used to exchange MS Control PDUs.  Implementations must
    support the Transport Control Protocol (TCP) [RFC0793] and may
    support the Stream Control Transmission Protocol (SCTP) [RFC4960].
    Implementations must support TLS [RFC5246] as a transport-level
    security mechanism although its use in deployments is optional.

Melanchuk Informational [Page 3] RFC 5567 Mediactrl Architecture June 2009

 MS Control Dialog:  A SIP dialog that is used for establishing a
    control channel between the user agent (UA) and the MS.
 MS Control Protocol:  The protocol used for by an AS to control an
    MS.  The MS Control Protocol assumes a reliable underlying
    transport protocol for the MS Control Channel.
 MS Media Dialog:  A SIP dialog between the AS and MS that is used for
    establishing media sessions between a user device such as a SIP
    phone and the MS.
 The definitions for AS, MS, and MRB above are taken from [RFC5167].

3. Architecture Overview

 A Media Server (MS) is a network device that processes media streams.
 Examples of media processing functionality may include:
 o  Control of the Real-Time Protocol (RTP) [RFC3550] streams using
    the Extended RTP Profile for Real-time Transport Control Protocol
    (RTCP)-Based Feedback (RTP/AVPF) [RFC4585].
 o  Mixing of incoming media streams.
 o  Media stream source (for multimedia announcements).
 o  Media stream processing (e.g., transcoding, DTMF detection).
 o  Media stream sink (for multimedia recordings).
 An MS supplies one or more media processing functionalities, which
 may include others than those illustrated above, to an Application
 Server (AS).  An AS is able to send a particular call to a suitable
 MS, either through discovery of the capabilities that a specific MS
 provides or through the use of a Media Resource Broker.
 The type of processing that a Media Server performs on media streams
 is specified and controlled by an Application Server.  Application
 Servers are logical entities that are capable of running one or more
 instances of a communications application.  Examples of Application
 Servers that may interact with a Media Server are an AS acting as a
 Conference 'Focus' as defined in [RFC4353], or an IVR application
 using a Media Server to play announcements and detect DTMF key
 presses.
 Application servers use SIP to establish control channels between
 themselves and MSs.  An MS Control Channel implements a reliable
 transport protocol that is used to carry the MS Control Protocol.  A

Melanchuk Informational [Page 4] RFC 5567 Mediactrl Architecture June 2009

 SIP dialog used to establish a control channel is referred to as an
 MS Control Dialog.
 Application Servers terminate SIP [RFC3261] signaling from SIP User
 Agents and may terminate other signaling outside the scope of this
 document.  They use SIP Third Party Call Control [RFC3725] (3PCC) to
 establish, maintain, and tear down media streams from those SIP UAs
 to a Media Server.  A SIP dialog used by an AS to establish a media
 session on an MS is referred to as an MS Media Dialog.
 Media streams go directly between SIP User Agents and Media Servers.
 Media Servers support multiple types of media.  Common supported RTP
 media types include audio and video, but others such as text and the
 Binary Floor Control Protocol (BFCP) [RFC4583] are also possible.
 This basic architecture, showing session establishment signaling
 between a single AS and MS is shown in Figure 1 below.
         +-------------+                         +--------------+
         |             | SIP (MS Control Dialog) |              |
         | Application |<----------------------->|     Media    |
         |   Server    |                         |    Server    |
         |             |<----------------------->|              |
         +-------------+ SIP (MS Media Dialog)   +--------------+
                     ^                               ^
                      \                              | RTP/SRTP
                       \                             |  audio/
                        \                            | video/etc)
                         \                           |
                          \                          v
                           \                 +--------------+
                            \     SIP        |              |
                             +-------------->|      SIP     |
                                             |  User Agent  |
                                             |              |
                                             +--------------+
                Figure 1: Basic Signaling Architecture
 The architecture must support a many-to-many relationship between
 Application Servers and Media Servers.  In real world deployments, an
 Application Server may interact with multiple Media Servers and/or a
 Media Server may be controlled by more than one Application Server.
 Application Servers can use the SIP URI as described in [RFC4240] to
 request basic functions from Media Servers.  Basic functions are
 characterized as requiring no mid-call interactions between the AS
 and MS.  Examples of these functions are simple announcement-playing

Melanchuk Informational [Page 5] RFC 5567 Mediactrl Architecture June 2009

 or basic conference-mixing where the AS does not need to explicitly
 control the mixing.
 Most services however have interactions between the AS and MS during
 a call or conference.  The type of interactions can be generalized as
 follows:
 o  commands from an AS to an MS to request the application or
    configuration of a function.  The request may apply to a single
    media stream, multiple media streams associated with multiple SIP
    dialogs, or to properties of a conference mix.
 o  responses from an MS to an AS reporting on the status of
    particular commands.
 o  notifications from an MS to an AS that report results from
    commands or notify changes to subscribed status.
 Commands, responses, and notifications are transported using one or
 more dedicated control channels between the Application Server and
 the Media Server.  Dedicated control channels provide reliable,
 sequenced, peer-to-peer transport for Media Server control
 interactions.  Implementations must support the Transport Control
 Protocol (TCP) [RFC0793] and may support the Stream Control
 Transmission Protocol (SCTP) [RFC4960].  Because MS control requires
 sequenced reliable delivery of messages, unreliable protocols such as
 the User Datagram Protocol (UDP) are not suitable.  Implementations
 must support TLS [RFC5246] as a transport-level security mechanism
 although its use in deployments is optional.  A dedicated control
 channel is shown in Figure 2 below.

Melanchuk Informational [Page 6] RFC 5567 Mediactrl Architecture June 2009

           +-------------+                     +--------------+
           |             |                     |              |
           | Application |   MS ctrl channel   |     Media    |
           |   Server    |<------------------->|    Server    |
           |             |                     |              |
           +-------------+                     +--------------+
                                                       ^ ^ ^
                                              RTP/SRTP | | |
                                              (audio/  | | |
                                            video/etc) | | |
                                                       | | v
                                                   +---|-v-------+
                                                 +-|---v-------+ |
                                               +-|-----------+ | |
                                               |             | | |
                                               |     SIP     | | |
                                               | User Agent  | |-+
                                               |             |-+
                                               +-------------+
              Figure 2: Media Server Control Architecture
 Both Application Servers and Media Servers may interact with other
 servers for specific purposes beyond the scope of this document.  For
 example, Application Servers will often communicate with other
 infrastructure components that are usually based on deployment
 requirements with links to back-office data stores and applications.
 Media Servers will often retrieve announcements from external file
 servers.  Also, many Media Servers support IVR dialog services using
 VoiceXML [W3C.REC-voicexml20-20040316].  In this case, the MS
 interacts with other servers using HTTP during standard VoiceXML
 processing.  VoiceXML Media Servers may also interact with speech
 engines (for example, using the Media Resource Control Protocol
 version 2 (MRCPv2)) for speech recognition and generation purposes.
 Some specific types of interactions between Application and Media
 servers are also out of scope for this document.  MS resource
 reservation is one such interaction.  Also, any interactions between
 Application Servers, or between Media Servers, are also out of scope.

4. SIP Usage

 The Session Initiation Protocol (SIP) [RFC3261] was developed by the
 IETF for the purposes of initiating, managing, and terminating
 multimedia sessions.  The popularity of SIP has grown dramatically
 since its inception and is now the primary Voice over IP (VoIP)
 protocol.  This includes being selected as the basis for
 architectures such as the IP Multimedia Subsystem (IMS) in 3GPP and

Melanchuk Informational [Page 7] RFC 5567 Mediactrl Architecture June 2009

 included in many of the early live deployments of VoIP-related
 systems.  Media servers are not a new concept in IP telephony
 networks and there have been numerous signaling protocols and
 techniques proposed for their control.  The most popular techniques
 to date have used a combination of SIP and various markup languages
 to convey media service requests and responses.
 As discussed in Section 3 and illustrated in Figure 1, the logical
 architecture described by this document involves interactions between
 an Application Server (AS) and a Media Server (MS).  The SIP
 interactions can be broken into "MS media dialogs" that are used
 between an AS and an MS to establish media sessions between an
 endpoint and a Media Server, and "MS control dialogs" that are used
 to establish and maintain MS control channels.
 SIP is the primary signaling protocol for session signaling and is
 used for all media sessions directed towards a Media Server as
 described in this document.  Media Servers may support other
 signaling protocols but this type of interaction is not considered
 here.  Application Servers may terminate non-SIP signaling protocols
 but must gateway those requests to SIP when interacting with a Media
 Server.
 SIP will also be used for the creation, management, and termination
 of the dedicated MS control channel(s).  Control channel(s) provide
 reliable sequenced delivery of MS Control Protocol messages.  The
 Application and Media Servers use the SDP attributes defined in
 [RFC4145] to allow SIP negotiation of the control channel.  A control
 channel is closed when SIP terminates the corresponding MS control
 dialog.  Further details and example flows are provided in the SIP
 Control Framework [SIP-CTRL-FW].  The SIP Control Framework also
 includes basic control message semantics corresponding to the types
 of interactions identified in Section 3.  It uses the concept of
 "packages" to allow domain-specific protocols to be defined using the
 Extensible Markup Language (XML) [W3C.REC-xml-20060816] format.  The
 MS Control Protocol is made up of one or more packages for the SIP
 Control Framework.
 Using SIP for both media and control dialogs provides a number of
 inherent benefits over other potential techniques.  These include:
 1.  The use of SIP location and rendezvous capabilities, as defined
     in [RFC3263].  This provides core mechanisms for routing a SIP
     request based on techniques such as DNS SRV and NAPTR records.
     The SIP infrastructure makes heavy use of such techniques.
 2.  The security and identity properties of SIP; for example, using
     TLS for reliably and securely connecting to another SIP-based

Melanchuk Informational [Page 8] RFC 5567 Mediactrl Architecture June 2009

     entity.  The SIP protocol has a number of identity mechanisms
     that can be used.  [RFC3261] provides an intra-domain digest-
     based mechanism and [RFC4474] defines a certificate-based inter-
     domain identity mechanism.  SIP with S/MIME provides the ability
     to secure payloads using encrypted and signed certificate
     techniques.
 3.  SIP has extremely powerful and dynamic media-negotiation
     properties as defined in [RFC3261] and [RFC3264].
 4.  The ability to select an appropriate SIP entity based on
     capability sets as discussed in [RFC3840].  This provides a
     powerful function that allows Media Servers to convey a specific
     capability set.  An AS is then free to select an appropriate MS
     based on its requirements.
 5.  Using SIP also provides consistency with IETF protocols and
     usages.  SIP was intended to be used for the creation and
     management of media sessions, and this provides a correct usage
     of the protocol.
 As mentioned previously in this section, media services using SIP are
 fairly well understood.  Some previous proposals suggested using the
 SIP INFO [RFC2976] method as the transport vehicle between the AS and
 MS.  Using SIP INFO in this way is not advised for a number of
 reasons, which include:
 o  INFO is an opaque request with no specific semantics.  A SIP
    endpoint that receives an INFO request does not know what to do
    with it based on SIP signaling.
 o  SIP INFO was not created to carry generic session control
    information along the signaling path, and it should only really be
    used for optional application information, e.g., carrying mid-call
    Public Switched Telephone Network (PSTN) signaling messages
    between PSTN gateways.
 o  SIP INFO traverses the signaling path, which is an inefficient use
    for control messages that can be routed directly between the AS
    and MS.
 o  [RFC3261] contains rules when using an unreliable protocol such as
    UDP.  When a packet reaches a size close to the Maximum
    Transmission Unit (MTU), the protocol should be changed to TCP.
    This type of operation is not ideal when constantly dealing with
    large payloads such as XML-formatted MS control messages.

Melanchuk Informational [Page 9] RFC 5567 Mediactrl Architecture June 2009

5. Media Control for IVR Services

 One of the functions of a Media Server is to assist an Application
 Server that is implementing IVR services by performing media
 processing functions on media streams.  Although "IVR" is somewhat
 generic terminology, the scope of media functions provided by an MS
 addresses the needs for user interaction dialogs.  These functions
 include media transcoding, basic announcements, user input detection
 (via DTMF or speech), and media recording.
 A particular IVR or user dialog application typically requires the
 use of several specific media functions, as described above.  The
 range and complexity of IVR dialogs can vary significantly, from a
 simple single announcement play-back to complex voice mail
 applications.
 As previously discussed, an AS uses SIP [RFC3261] and SDP [RFC4566]
 to establish and configure media sessions to a Media Server.  An AS
 uses the MS control channel, established using SIP, to invoke IVR
 requests and to receive responses and notifications.  This topology
 is shown in Figure 3 below.
    +-------------+             SIP              +-------------+
    | Application |<---------------------------->|   Media     |
    |    Server   | (media & MS Control dialogs) |   Server    |
    |             |                              |             |
    |             |  MS Control Protocol (IVR)   |             |
    |             |<---------------------------->| (IVR media  |
    | (App logic) |       (CtrlChannel)          | functions)  |
    +-------------+                              +-------------+
           ^                                            ^^
            \                                           ||  R
             \                                          ||  T
              \                                         ||  P
               \                                        ||  /
                \                                       ||  S
                 \                                      ||  R
                  \                                     ||  T
                   \                                    ||  P
                    \                                   vv
                     \    call signaling           +-----------+
                      ---------------------------->|   User    |
                            (e.g., SIP)            | Equipment |
                                                   +-----------+
                        Figure 3: IVR Topology

Melanchuk Informational [Page 10] RFC 5567 Mediactrl Architecture June 2009

 The variety in complexity of Application Server IVR services requires
 support for different levels of media functions from the Media Server
 as described in the following sub-sections.

5.1. Basic IVR Services

 For simple basic announcement requests, the MS control channel, as
 depicted in Figure 3 above, is not required.  Simple announcement
 requests may be invoked on the Media Server using the SIP URI
 mechanism defined in [RFC4240].  This interface allows no digit
 detection or collection of user input and no mid-call dialog control.
 However, many applications only require basic media services, and the
 processing burden on the Media Server to support more complex
 interactions with the AS would not be needed in that case.

5.2. IVR Services with Mid-Call Controls

 For more complex IVR dialogs, which require mid-call interaction and
 control between the Application Server and the Media Server, the MS
 control channel (as shown in Figure 3 above) is used to invoke
 specific media functions on the Media Server.  These functions
 include, but are not limited to, complex announcements with barge-in
 facility, user-input detection and reporting (e.g., DTMF) to an
 Application Server, DTMF and voice-activity controlled recordings,
 etc.  Composite services, such as play-collect and play-record, are
 also addressed by this model.
 Mid-call control also allows Application Servers to subscribe to IVR-
 related events and for the Media Server to notify the AS when these
 events occur.  Examples of such events are announcement completion
 events, record completion events, and reporting of collected DTMF
 digits.

5.3. Advanced IVR Services

 Although IVR services with mid-call control, as described above,
 provide a comprehensive set of media functions expected from a Media
 Server, the advanced IVR services model allows a higher level of
 abstraction describing application logic, as provided by VoiceXML, to
 be executed on the Media Server.  Invocation of VoiceXML IVR dialogs
 may be via the "Prompt and Collect" mechanism of [RFC4240].
 Additionally, the IVR control protocol can be extended to allow
 VoiceXML requests to also be invoked over the MS control channel.
 VoiceXML IVR services invoked on the Media Server may require an HTTP
 interface (not shown in Figure 3) between the Media Server and one or
 more back-end servers that host or generate VoiceXML documents.  The
 back-end server(s) may or may not be physically separate from the
 Application Server.

Melanchuk Informational [Page 11] RFC 5567 Mediactrl Architecture June 2009

6. Media Control for Conferencing Services

 [RFC4353] describes the overall architecture and protocol components
 needed for multipoint conferencing using SIP.  The framework for
 centralized conferencing [RFC5239] extends the framework to include a
 protocol between the user and the conferencing server.  [RFC4353]
 describes the conferencing server decomposition but leaves the
 specifics open.
 This section describes the decomposition and discusses the
 functionality of the decomposed functional units.  The conferencing
 factory and the conference focus are part of the Application Server
 described in this document.
 An Application Server uses SIP Third Party Call Control [RFC3725] to
 establish media sessions from SIP user agents to a Media Server.  The
 same mechanism is used by the Application Server as described in this
 section to add/remove participants to/from a conference, as well as
 to handle the involved media streams set up on a per-user basis.
 Since the XCON framework has been conceived as protocol-agnostic when
 talking about the Call Signaling Protocol used by users to join a
 conference, an XCON-compliant Application Server will have to take
 care of gatewaying non-SIP signaling negotiations.  This is in order
 to set up and make available valid SIP media sessions between itself
 and the Media Server, while still keeping the non-SIP interaction
 with the user in a transparent way.

Melanchuk Informational [Page 12] RFC 5567 Mediactrl Architecture June 2009

              +------------+             +------------+
              |            | SIP (2m+1c) |            |
              | Application|-------------|   Media    |
              |   Server   |             |   Server   |
              |  (Focus)   |-------------|  (Mixer)   |
              |            | CtrlChannel |            |
              +------------+             +------------+
                  |      \                    .. .
                  |       \\            RTP...   .
                  |         \\           ..      .
                  |     H.323  \\      ...       .
              SIP |             \\ ...           .RTP
                  |              ..\             .
                  |           ...   \\           .
                  |        ...        \\         .
                  |      ..             \\       .
                  |   ...                 \\     .
                  | ..                      \    .
             +-----------+              +-----------+
             |Participant|              |Participant|
             +-----------+              +-----------+
                     Figure 4: Conference Topology
 To complement the functionality provided by 3PCC and by the XCON
 control protocol, the Application Server makes use of a dedicated
 Media Server control channel in order to set up and manage media
 conferences on the Media Server.  Figure 4 shows the signaling and
 media paths for a two-participant conference.  The three SIP dialogs
 between the AS and MS establish one control session (1c) and two
 media sessions (2m) from the participants (one originally signaled
 using H.323 and then gatewayed into SIP and one signaled directly in
 SIP).
 As a conference focus, the Application Server is responsible for
 setting up and managing a media conference on the Media Servers, in
 order to make sure that all the media streams provided in a
 conference are available to its participants.  This is achieved by
 using the services of one or more mixer entities (as described in RFC
 4353), whose role as part of the Media Server is described in this
 section.  Services required by the Application Server include, but
 are not limited to, means to set up, handle, and destroy a new media
 conference, adding and removing participants from a conference,
 managing media streams in a conference, controlling the layout and
 the mixing configuration for each involved media, allowing per-user
 custom media profiles, and so on.

Melanchuk Informational [Page 13] RFC 5567 Mediactrl Architecture June 2009

 As a mixer entity, in such a multimedia conferencing scenario, the
 Media Server receives a set of media streams of the same type (after
 transcoding if needed) and then takes care of combining the received
 media in a type-specific manner, redistributing the result to each
 authorized participant.  The way each media stream is combined, as
 well as the media-related policies, is properly configured and
 handled by the Application Server by means of a dedicated MS control
 channel.
 To summarize, the AS needs to be able to manage Media Servers at a
 conference and participant level.

6.1. Creating a New Conference

 When a new conference is created, as a result of a previous
 conference scheduling or of the first participant dialing in to a
 specified URI, the Application Server must take care of appropriately
 creating a media conference on the Media Server.  It does so by
 sending an explicit request to the Media Server.  This can be by
 means of an MS control channel message.  This request may contain
 detailed information upon the desired settings and policies for the
 conference (e.g., the media to involve, the mixing configuration for
 them, the relevant identifiers, etc.).  The Media Server validates
 such a request and takes care of allocating the needed resources to
 set up the media conference.
 Application Servers may use mechanisms other than sending requests
 over the control channel to establish conferences on a Media Server,
 and then subsequently use the control channel to control the
 conference.  Examples of other mechanisms to create a conference
 include using the Request-URI mechanism of [RFC4240] or the
 procedures defined in [RFC4579].
 Once done, the MS informs the Application Server about the result of
 the request.  Each conference will be referred to by a specific
 identifier, which both the Application Server and the Media Server
 will include in subsequent transactions related to the same
 conference (e.g., to modify the settings of an extant conference).

6.2. Adding a Participant to a Conference

 As stated before, an Application Server uses SIP 3PCC to establish
 media sessions from SIP user agents to a Media Server.  The URI that
 the AS uses in the INVITE to the MS may be one associated with the
 conference on the MS.  More likely however, the media sessions are
 first established to the Media Server using a URI for the Media
 Server and then subsequently joined to the conference using the MS

Melanchuk Informational [Page 14] RFC 5567 Mediactrl Architecture June 2009

 Control Protocol.  This allows IVR dialogs to be performed prior to
 joining the conference.
 The AS as a 3PCC correlates the media session negotiation between the
 UA and the MS, in order to appropriately establish all the needed
 media streams based on the conference policies.

6.3. Media Controls

 The XCON Common Data Model [XCON-DM] currently defines some basic
 media-related controls, which conference-aware participants can take
 advantage of in several ways, e.g., by means of an XCON conference
 control protocol or IVR dialogs.  These controls include the
 possibility to modify the participants' own volume for audio in the
 conference, configure the desired layout for incoming video streams,
 mute/unmute oneself, and pause/unpause one's own video stream.  Such
 controls are exploited by conference-aware participants through the
 use of dedicated conference control protocol requests to the
 Application Server.  The Application Server takes care of validating
 such requests and translates them into the Media Server Control
 Protocol, before forwarding them over the MS Control Channel to the
 MS.  According to the directives provided by the Application Server,
 the Media Server manipulates the involved media streams accordingly.
                +------------+                  +------------+
                |            | 'Include audio   |            |
                | Application|  sent by user X  |   Media    |
                |   Server   |  in conf Y mix'  |   Server   |
                |  (Focus)   |----------------->|  (Mixer)   |
                |            |   (MS CtrlChn)   |            |
                +------^-----+                  +------------+
                       |                          ..
                       |                       ...
                       | 'Unmute me'        ... RTP
                       |   (XCON)        ...
                       |              ...
                       |           ...
                +-----------+   ...
                |Participant|...
                +-----------+
        Figure 5: Conferencing Example: Unmuting A Participant
 The Media Server may need to inform the AS of events like in-band
 DTMF tones during the conference.

Melanchuk Informational [Page 15] RFC 5567 Mediactrl Architecture June 2009

6.4. Floor Control

 The XCON framework introduces "floor control" functionality as an
 enhancement upon [RFC4575].  Floor control is a means to manage joint
 or exclusive access to shared resources in a (multiparty)
 conferencing environment.  Floor control is not a mandatory mechanism
 for a conferencing system implementation, but it provides advanced
 media input control features for conference-aware participants.  Such
 a mechanism allows for coordinated and moderated access to any set of
 resources provided by the conferencing system.  To do so, a so-called
 floor is associated to a set of resources, thus representing for
 participants the right to access and manipulate the related resources
 themselves.  In order to take advantage of the floor control
 functionality, a specific protocol, the Binary Floor Control
 Protocol, has been specified [RFC4582].  [RFC4583] provides a way for
 SIP UAs to set up a BFCP connection towards the Floor Control Server
 and exploit floor control by means of a Connection-Oriented Media
 (COMEDIA) [RFC4145] negotiation.
 In the context of the AS-MS interaction, floor control constitutes a
 further means to control participants' media streams.  A typical
 example is a floor associated with the right to access the shared
 audio channel in a conference.  A participant who is granted such a
 floor is granted by the conferencing system the right to talk, which
 means that its audio frames are included by the MS in the overall
 audio conference mix.  Similarly, when the floor is revoked, the
 participant is muted in the conference, and its audio is excluded
 from the final mix.
 The BFCP defines a Floor Control Server (FCS) and the floor chair.
 It is clear that the floor chair making decisions about floor
 requests is part of the application logic.  This implies that when
 the role of floor chair in a conference is automated, it will
 normally be part of the AS.
 The example makes it clear that there can be a direct or indirect
 interaction between the Floor Control Server and the Media Server, in
 order to correctly bind each floor to its related set of media
 resources.  Besides, a similar interaction is needed between the
 Floor Control Server and the Application Server as well, since the
 latter must be aware of all the associations between floors and
 resources, in order to opportunely orchestrate the related bindings
 with the element responsible for such resources (e.g., the Media
 Server when talking about audio and/or video streams) and the
 operations upon them (e.g., mute/unmute a participant in a
 conference).  For this reason, the Floor Control Server can be co-

Melanchuk Informational [Page 16] RFC 5567 Mediactrl Architecture June 2009

 located with either the Media Server or the Application Server, as
 long as both elements are allowed to interact with the Floor Control
 Server by means of some kind of protocol.
 In the following text, both the approaches will be described in order
 to better explain the interactions between the involved components in
 both the topologies.
 When the AS and the FCS are co-located, the scenario is quite
 straightforward.  In fact, it can be considered as a variation of the
 case depicted in Figure 5.  The only relevant difference is that in
 this case the action the AS commands on the control channel is
 triggered by a change in the floor control status instead of a
 specific control requested by a participant himself.  The sequence
 diagram in Figure 6 describes the interaction between the involved
 parties in a typical scenario.  It assumes that a BFCP connection
 between the UA and the FCS (which we assume is co-located with the
 AS) has already been negotiated and established, and that the UA has
 been made aware of all the relevant identifiers and floors-resources-
 associations (e.g., by means of [RFC4583]).  It also assumes that the
 AS has previously configured the media mixing on the MS using the MS
 control channel.  Every frame the UA might be sending on the related
 media stream is currently being dropped by the MS, since the UA still
 isn't authorized to use the resource.  For a SIP UA, this state could
 be consequent to a 'sendonly' field associated to the media stream in
 a re-INVITE originated by the MS.  It is worth pointing out that the
 AS has to make sure that no user media control mechanisms, such as
 mentioned in the previous sub-section, can override the floor
 control.

Melanchuk Informational [Page 17] RFC 5567 Mediactrl Architecture June 2009

   UA                                   AS                         MS
   (Floor Participant)                 (FCS)
   |                                     |                          |
   |<===================== One-way RTP stream ======================|
   |                                     |                          |
   | FloorRequest(BFCP)                  |                          |
   |------------------------------------>|                          |
   |                                     |                          |
   |   FloorRequestStatus[PENDING](BFCP) |                          |
   |<------------------------------------|                          |
   |                                     |--+ apply                 |
   |                                     |  | policies              |
   |                                     |<-+ to request            |
   |                                     |                          |
   |  FloorRequestStatus[ACCEPTED](BFCP) |                          |
   |<------------------------------------|                          |
   |                                     |                          |
   .                                     .                          .
   .                                     .                          .
   |                                     |                          |
   |   FloorRequestStatus[GRANTED](BFCP) |                          |
   |<------------------------------------|                          |
   |                                     | 'Unmute UA' (CtrlChn)    |
   |                                     |------------------------->|
   |                                     |                          |
   |<==================== Bidirectional RTP stream ================>|
   |                                     |                          |
   .                                     .                          .
   .                                     .                          .
        Figure 6: Conferencing Example: Floor Control Call Flow
 A UA, which also acts as a floor participant, sends a "FloorRequest"
 to the floor control server (FCS, which is co-located with the AS),
 stating his will to be granted the floor associated with the audio
 stream in the conference.  The AS answers the UA with a
 "FloorRequestStatus" message with a PENDING status, meaning that a
 decision on the request has not been made yet.  The AS, according to
 the BFCP policies for this conference, makes a decision on the
 request, i.e., accepting it.  Note that this decision might be
 relayed to another participant in case he has previously been
 assigned as chair of the floor.  Assuming the request has been
 accepted, the AS notifies the UA about the decision with a new
 "FloorRequestStatus", this time with an ACCEPTED status in it.  The
 ACCEPTED status of course only means that the request has been
 accepted, which doesn't mean the floor has been granted yet.  Once
 the queue management in the FCS, according to the specified
 algorithms for scheduling, states that the floor request previously

Melanchuk Informational [Page 18] RFC 5567 Mediactrl Architecture June 2009

 made by the UA can be granted, the AS sends a new
 "FloorRequestStatus" to the UA with a GRANTED status, and takes care
 of unmuting the participant in the conference by sending a directive
 to the MS through the control channel.  Once the UA receives the
 notification stating his request has been granted, he can start
 sending its media, aware of the fact that now his media stream won't
 be dropped by the MS.  In case the session has been previously
 updated with a 'sendonly' associated to the media stream, the MS must
 originate a further re-INVITE stating that the media stream flow is
 now bidirectional ('sendrecv').
 As mentioned before, this scenario envisages an automated floor chair
 role, where it's the AS, according to some policies, which makes
 decisions on floor requests.  The case of a chair role performed by a
 real person is exactly the same, with the difference that the
 incoming request is not directly handled by the AS according to its
 policies, but it is instead forwarded to the floor control
 participant that the chair UA is exploiting.  The decision on the
 request is then communicated by the chair UA to the AS-FCS by means
 of a 'ChairAction' message.
 The rest of this section will instead explore the other scenario,
 which assumes that the interaction between AS-FCS happens through the
 MS control channel.  This scenario is compliant with the H.248.19
 document related to conferencing in 3GPP.  The following sequence
 diagram describes the interaction between the involved parties in the
 same use-case scenario that has been explored for the previous
 topology: consequently, the diagram makes exactly the same
 assumptions that have been made for the previously described
 scenario.  This means that the scenario again assumes that a BFCP
 connection between the UA and the FCS has already been negotiated and
 established, and that the UA has been made aware of all the relevant
 identifiers and floors-resources-associations.  It also assumes that
 the AS has previously configured the media mixing on the MS using the
 MS control channel.  This time it includes identifying the BFCP-
 moderated resources, establishing basic policies and instructions
 about chair identifiers for each resource, and subscribing to events
 of interest, because the FCS is not co-located with the AS anymore.
 Additionally, a BFCP session has been established between the AS
 (which in this scenario acts as a floor chair) and the FCS (MS).
 Every frame the UA might be sending on the related media stream is
 currently being dropped by the MS, since the UA still isn't
 authorized to use the resource.  For a SIP UA, this state could be
 consequent to a 'sendonly' field associated to the media stream in a
 re-INVITE originated by the MS.  Again, it is worth pointing out that
 the AS has to make sure that no user media control mechanisms, such
 as mentioned in the previous sub-section, can override the floor
 control.

Melanchuk Informational [Page 19] RFC 5567 Mediactrl Architecture June 2009

   UA                          AS                                  MS
   (Floor Participant)   (Floor Chair)                          (FCS)
   |                           |                                    |
   |<===================== One-way RTP stream ======================|
   |                           |                                    |
   | FloorRequest(BFCP)        |                                    |
   |--------------------------------------------------------------->|
   |                           |                                    |
   |                           |  FloorRequestStatus[PENDING](BFCP) |
   |<---------------------------------------------------------------|
   |                           |  FloorRequestStatus[PENDING](BFCP) |
   |                           |<-----------------------------------|
   |                           |                                    |
   |                           | ChairAction[ACCEPTED] (BFCP)       |
   |                           |----------------------------------->|
   |                           |       ChairActionAck (BFCP)        |
   |                           |<-----------------------------------|
   |                           |                                    |
   |                           | FloorRequestStatus[ACCEPTED](BFCP) |
   |<---------------------------------------------------------------|
   |                           |                                    |
   .                           .                                    .
   .                           .                                    .
   |                           |                                    |
   |                           |  FloorRequestStatus[GRANTED](BFCP) |
   |<---------------------------------------------------------------|
   |                           | 'Floor has been granted' (CtrlChn) |
   |                           |<-----------------------------------|
   |                           |                                    |
   |<==================== Bidirectional RTP stream ================>|
   |                           |                                    |
   .                           .                                    .
   .                           .                                    .
        Figure 7: Conferencing Example: Floor Control Call Flow
 A UA, which also acts as a floor participant, sends a "FloorRequest"
 to the floor control server (FCS, which is co-located with the MS),
 stating his will to be granted the floor associated with the audio
 stream in the conference.  The MS answers the UA with a
 "FloorRequestStatus" message with a PENDING status, meaning that a
 decision on the request has not been made yet.  It then notifies the
 AS, which in this example handles the floor chair role, about the new
 request by forwarding there the received request.  The AS, according
 to the BFCP policies for this conference, makes a decision on the
 request, i.e., accepting it.  It informs the MS about its decision
 through a BFCP "ChairAction" message.  The MS then acknowledges the
 'ChairAction' message and then notifies the UA about the decision

Melanchuk Informational [Page 20] RFC 5567 Mediactrl Architecture June 2009

 with a new "FloorRequestStatus", this time with an ACCEPTED status in
 it.  The ACCEPTED status of course only means that the request has
 been accepted, which doesn't mean the floor has been granted yet.
 Once the queue management in the MS, according to the specified
 algorithms for scheduling, states that the floor request previously
 made by the UA can be granted, the MS sends a new
 "FloorRequestStatus" to the UA with a GRANTED status, and takes care
 of unmuting the participant in the conference.  Once the UA receives
 the notification stating his request has been granted, he can start
 sending its media, aware of the fact that now his media stream won't
 be dropped by the MS.  In case the session has been previously
 updated with a 'sendonly' associated to the media stream, the MS must
 originate a further re-INVITE stating that the media stream flow is
 now bidirectional ('sendrecv').
 This scenario envisages an automated floor chair role, where it's the
 AS, according to some policies, which makes decisions on floor
 requests.  Again, the case of a chair role performed by a real person
 is exactly the same, with the difference that the incoming request is
 not forwarded to the AS but to the floor control participant that the
 chair UA is exploiting.  The decision on the request is communicated
 by means of a 'ChairAction' message in the same way.
 Another typical scenario is a BFCP-moderated conference with no chair
 to manage floor requests.  In such a scenario, the MS has to take
 care of incoming requests according to some predefined policies,
 e.g., always accepting new requests.  In this case, no decisions are
 required by external entities, since all are instantly decided by
 means of policies in the MS.
 As stated before, the case of the FCS co-located with the AS is much
 simpler to understand and exploit.  When the AS has full control upon
 the FCS, including its queue management, the AS directly instructs
 the MS according to the floor status changes, e.g., by instructing
 the MS through the control channel to unmute a participant who has
 been granted the floor associated to the audio media stream.

7. Security Considerations

 This document describes the architectural framework to be used for
 Media Server control.  Its focus is the interactions between
 Application Servers and Media Servers.  User agents interact with
 Application Servers by means of signaling protocols such as SIP.
 These interactions are beyond the scope of this document.
 Application Servers are responsible for utilizing the security
 mechanisms of their signaling protocols, combined with application-
 specific policy, to ensure they grant service only to authorized
 users.  Media interactions between user agents and Media Servers are

Melanchuk Informational [Page 21] RFC 5567 Mediactrl Architecture June 2009

 also outside the scope of this document.  Those interactions are at
 the behest of Application Servers, which must ensure that appropriate
 security mechanisms are used.  For example, if the MS is acting as
 the FCS, then the BFCP connection between the user agent and the MS
 is established to the MS by the AS using SIP and the SDP mechanisms
 described in [RFC4583].  BFCP [RFC4582] strongly imposes the use of
 TLS for BFCP.
 Media Servers are valuable network resources and need to be protected
 against unauthorized access.  Application Servers use SIP and related
 standards both to establish control channels to Media Servers and to
 establish media sessions, including BFCP sessions, between an MS and
 end users.  Media servers use the security mechanisms of SIP to
 authenticate requests from Application servers and to ensure the
 integrity of those requests.  Leveraging the security mechanisms of
 SIP ensures that only authorized Application Servers are allowed to
 establish sessions to an MS and to access MS resources through those
 sessions.
 Control channels between an AS and MS carry the MS control protocol,
 which affects both the service seen by end users and the resources
 used on a Media Server.  TLS [RFC5246] must be implemented as the
 transport-level security mechanism for control channels to guarantee
 the integrity of MS control interactions.
 The resources of an MS can be shared by more than one AS.  Media
 Servers must prevent one AS from accessing and manipulating the
 resources that have been assigned to another AS.  This may be
 achieved by an MS associating ownership of a resource to the AS that
 originally allocates it, and then insuring that future requests
 involving that resource correlate to the AS that owns and is
 responsible for it.

8. Acknowledgments

 The authors would like to thank Spencer Dawkins for detailed reviews
 and comments, Gary Munson for suggestions, and Xiao Wang for review
 and feedback.

9. Contributors

 This document is a product of the Media Control Architecture Design
 Team.  In addition to the editor, the following individuals
 constituted the design team and made substantial textual
 contributions to this document:

Melanchuk Informational [Page 22] RFC 5567 Mediactrl Architecture June 2009

    Chris Boulton: cboulton@ubiquity.net
    Martin Dolly: mdolly@att.com
    Roni Even: roni.even@polycom.co.il
    Lorenzo Miniero: lorenzo.miniero@unina.it
    Adnan Saleem: Adnan.Saleem@radisys.com

10. Informative References

 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, September 1981.
 [RFC2976]  Donovan, S., "The SIP INFO Method", RFC 2976,
            October 2000.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            June 2002.
 [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
            Protocol (SIP): Locating SIP Servers", RFC 3263,
            June 2002.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            June 2002.
 [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
            Jacobson, "RTP: A Transport Protocol for Real-Time
            Applications", STD 64, RFC 3550, July 2003.
 [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
            Camarillo, "Best Current Practices for Third Party Call
            Control (3pcc) in the Session Initiation Protocol (SIP)",
            BCP 85, RFC 3725, April 2004.
 [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
            "Indicating User Agent Capabilities in the Session
            Initiation Protocol (SIP)", RFC 3840, August 2004.
 [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
            the Session Description Protocol (SDP)", RFC 4145,
            September 2005.

Melanchuk Informational [Page 23] RFC 5567 Mediactrl Architecture June 2009

 [RFC4240]  Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
            Media Services with SIP", RFC 4240, December 2005.
 [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
            Session Initiation Protocol (SIP)", RFC 4353,
            February 2006.
 [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
            Authenticated Identity Management in the Session
            Initiation Protocol (SIP)", RFC 4474, August 2006.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.
 [RFC4575]  Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
            Initiation Protocol (SIP) Event Package for Conference
            State", RFC 4575, August 2006.
 [RFC4579]  Johnston, A. and O. Levin, "Session Initiation Protocol
            (SIP) Call Control - Conferencing for User Agents",
            BCP 119, RFC 4579, August 2006.
 [RFC4582]  Camarillo, G., Ott, J., and K. Drage, "The Binary Floor
            Control Protocol (BFCP)", RFC 4582, November 2006.
 [RFC4583]  Camarillo, G., "Session Description Protocol (SDP) Format
            for Binary Floor Control Protocol (BFCP) Streams",
            RFC 4583, November 2006.
 [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
            "Extended RTP Profile for Real-time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
            July 2006.
 [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
            RFC 4960, September 2007.
 [RFC5167]  Dolly, M. and R. Even, "Media Server Control Protocol
            Requirements", RFC 5167, March 2008.
 [RFC5239]  Barnes, M., Boulton, C., and O. Levin, "A Framework for
            Centralized Conferencing", RFC 5239, June 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Melanchuk Informational [Page 24] RFC 5567 Mediactrl Architecture June 2009

 [SIP-CTRL-FW]
            Boulton, C., Melanchuk, T., and S. McGlashan, "Media
            Control Channel Framework", Work in Progress,
            February 2009.
 [W3C.REC-voicexml20-20040316]
            Carter, J., Tryphonas, S., Danielsen, P., Burnett, D.,
            Rehor, K., McGlashan, S., Ferrans, J., Porter, B., Lucas,
            B., and A. Hunt, "Voice Extensible Markup Language
            (VoiceXML) Version 2.0", World Wide Web Consortium
            Recommendation REC-voicexml20-20040316, March 2004,
            <http://www.w3.org/TR/2004/REC-voicexml20-20040316>.
 [W3C.REC-xml-20060816]
            Sperberg-McQueen, C., Paoli, J., Bray, T., Maler, E., and
            F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
            Edition)", World Wide Web Consortium Recommendation REC-
            xml-20060816, August 2006,
            <http://www.w3.org/TR/2006/REC-xml-20060816>.
 [XCON-DM]  Novo, O., Camarillo, G., Morgan, D., and J. Urpalainen,
            "Conference Information Data Model for Centralized
            Conferencing (XCON)", Work in Progress, April 2009.

Author's Address

 Tim Melanchuk (editor)
 Rain Willow Communications
 EMail: tim.melanchuk@gmail.com

Melanchuk Informational [Page 25]

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