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Versions: 00 01 02 03 04 RFC 5567

MediaCtrl                                              T. Melanchuk, Ed.
Internet-Draft                                Rain Willow Communications
Expires: May 22, 2008                                  November 19, 2007

          An Architectural Framework for Media  Server Control

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Architecture Overview  . . . . . . . . . . . . . . . . . . . .  5
   4.  SIP Usage  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Media Control for IVR Services . . . . . . . . . . . . . . . . 12
     5.1.  Basic IVR Services . . . . . . . . . . . . . . . . . . . . 13
     5.2.  IVR Services with Mid-call Controls  . . . . . . . . . . . 13
     5.3.  Advanced IVR Services  . . . . . . . . . . . . . . . . . . 13
   6.  Media Control for Conferencing Services  . . . . . . . . . . . 14
     6.1.  Creating a New Conference  . . . . . . . . . . . . . . . . 16
     6.2.  Adding a Participant To a Conference . . . . . . . . . . . 16
     6.3.  Media Controls . . . . . . . . . . . . . . . . . . . . . . 17
     6.4.  Floor Control  . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 25
   11. Informative References . . . . . . . . . . . . . . . . . . . . 26
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 29

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1.  Introduction

   Application Servers host one or more instances of a communications
   application.  Media servers provide real time media processing
   functions.  This documents presents the core arcritectureal 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.

   SIP 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 TCP 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 IVR services are described in Section 5 and
   conferencing use cases are described in Section 6.

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2.  Terminology

   The following additional 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 communications application.

   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):  Assigns specific Media Server resources
      to incoming calls at the request of service applications (i.e., an
      AS), which happens in real time as calls come into the network;
      may acquire knowledge of media server resources utilization that
      it can use to help decide which MS resources to assign to resource
      requests from applications; and employs methods/algorithms to
      determine MS resource assignment.

   Media Server (MS):  A functional entity whose main task is to supply
      real time media related functions to communication applications.
      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 (RFC 4566 [1]): "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:  The connection between the AS and MS used to
      exchange MS Control PDUs.

   MS Control Dialog:  A SIP dialog that is used for establishing a
      control channel between the UA and the MS.

   MS Control Protocol:  The protocol in the MS control channel.

   MS Media Dialog:  A SIP dialog between the AS and Media Server that
      is used for establishing media sessions between a user device such
      as a SIP phone and the Media Server.

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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) [2] streams such as video
      fast update and flow control using Real-Time Control Protocol
      (RTCP) feedback [3].

   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)

   A 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 RFC 4353 [4] or an IVR application
   using a Media Server to play announcements and detect DTMF key

   Application servers use SIP to establish control channels between
   themselves and MSs.  A MS Control Channel is used to carry the
   protocol used for MS control.  A SIP dialog used to establish a
   control channel is referred to as a MS Control Dialog.

   Application Servers terminate SIP [5] signaling from SIP User Agents
   and may terminate other signaling outside the scope of this document.
   They use SIP Third Party Call Control [6] (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 a MS Media Dialog.

   Media streams go directly between SIP User Agents and Media Servers.
   Media Servers support multiple types of media.  Common supported
   media types include audio and video but others such as text and the
   Binary Floor Control Protocol (BFCP) [7] are also possible.  This

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   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 Signalling 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 RFC 4240 [8]
   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
   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

   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 command.

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   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 using the Transport Control Protocol (TCP).  A dedicated
   control channel is shown in Figure 2 below.

             +-------------+                     +--------------+
             |             |                     |              |
             | Application |   MS ctrl channel   |     Media    |
             |   Server    |<------------------->|    Server    |
             |             |                     |              |
             +-------------+                     +--------------+
                                                         ^ ^ ^
                                                RTP/SRTP | | |
                                                (audio/  | | |
                                              video/etc) | | |
                                                         | | 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 [9].  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 MRCPv2, for
   speech recognition and generation purposes.

   Some specific types of interactions between Application and Media

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   servers are also out of scope this document.  MS resource reservation
   is one such interaction.  Also, any interactions between Application
   Servers, or between Media Servers, are also out of scope.

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4.  SIP Usage

   The Session Initiation Protocol (SIP) [5] 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 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

   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' - used between an
   AS and a MS to establish media sessions between an endpoint and a
   Media Server, and 'MS control dialogs' - which 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

   SIP will also be used for the creation, management and termination of
   the dedicated MS control channel(s).  A control channel uses TCP to
   provide reliable sequenced delivery of MS Control Protocol messages.
   The Application and Media Servers use the SDP attributes defined in
   [10] to allow SIP negotiation of a TCP connection.  Further details
   and example flows are provided in the SIP Control Framework [11].
   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) [12] 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:

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   1.  The use of SIP location and rendezvous capabilities, as defined
       in [13].  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
       entity.  The SIP protocol has a number of Identity mechanisms
       that can be used.  RFC 3261 provides an intra-domain digest-based
       mechanism and [14] defines a certificate based inter-domain
       identity mechanism.  SIP with S/MIME provides the ability to
       secure payloads using encrypted and signed certificate

   3.  SIP has extremely powerful and dynamic media negotiation
       properties as defined in RFC 3261 and RFC 3264 [15].

   4.  The ability to select an appropriate SIP entity based on
       capability sets as discussed in RFC 3840 [16].  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 [17] 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
      PSTN signaling messages between PSTN gateways.

   o  SIP INFO traverses the signaling path which is an inefficient use
      for control messages which can be routed directly between the AS
      and MS.

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   o  RFC 3261 contains rules when using an un-reliable 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.

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5.  Media Control for IVR Services

   One of the functions of a Media Server is to assist an Application
   Server implementing IVR services by performing media processing
   functions on media streams.  Although IVR is somewhat generic
   terminology, the scope of media functions provided by a 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

   As previously discussed, an AS uses SIP [5] and SDP [1] 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           +-----------+
                        ---------------------------->|     UE    |
                             (e.g. SIP)              +-----------+

                          Figure 3: IVR Topology

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   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 RFC 4240 [8].  This interface allows no user
   input digit detection and collection 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 this 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 speech 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 these events when
   they occur.  Examples of such events are announcement completion
   events, record completion events, and reporting of collected DTMF

5.3.  Advanced IVR Services

   Although IVR Services with Mid-call Control, as described above,
   provides 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 RFC 4240.
   Additionally, VoiceXML dialog services may be invoked over the MS
   control channel, as shown in Figure 3 above.  VoiceXML IVR services
   invoked on the Media Server require an HTTP interface between the
   Media Server and one or more back-end servers that host or generate
   VoiceXML documents.  These server(s) may or may not be physically
   separate from the Application Sever.

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6.  Media Control for Conferencing Services

   RFC 4353 [4] describes the overall architecture and protocol
   components needed for multipoint conferencing using SIP.  The
   framework for centralized conferencing [18]
   [draft-ietf-xcon-framework-08] extends the framework to include a
   protocol between the user and the conferencing server.  RFC 4353
   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 [6] 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, in order to set up
   and make available valid SIP media session between itself and the
   Media Server, while still keeping the non-SIP interaction with the
   user in a transparent way.

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                +------------+             +------------+
                |            | 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 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 two media sessions (2m) from
   participants, one originally signaled using H.323 and then gatewayed
   into SIP and one signaled directly in SIP, and one control session

   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 the all 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
   RFC4353, 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, controling the layout and the
   mixing configuration for each involved media, allowing per-user
   custom media profiles and so on.

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   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

   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 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 a 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, relevant identifiers, etc.).  The Media Server validates such a
   request and takes care of allocating the needed resources to set up
   the media conference.

   There is another way using SIP-based mechanisms such as [8] or [19]
   using pre-defined conference profiles and then using the MS control
   channel afterwards to control the conference if needed.

   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 same
   mechanism is used by the Application Server to add participants to a
   conference.  When a SIP UA places an INVITE to a URI associated with
   an existing conference, the Application Server forwards it to the MS
   associated with 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.

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6.3.  Media Controls

   The XCON Common Data Model [20] currently defines some basic media-
   related controls, which conference-aware participants can take
   advantage of in several ways, e.g. by means of a 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)        ...
                         |              ...
                         |           ...
                  +-----------+   ...

          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.

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

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   media input control features for conference-aware users.  Such
   mechanism allows for a 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
   users 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 [21].  RFC4583 [7] 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 COMEDIA [10] negotiation.

   In the context of the AS-MS interaction, floor control constitutes a
   further means to control users' media streams.  A typical example is
   a floor associated with the right to access the shared audio channel
   in a conference.  A user 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 user 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 floor chair role 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 user in a conference).  For
   this reason, the Floor Control Server can be co-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.  Though neither of the two topologies is
   prohibited, the rest of this section will only explore the first
   scenario, assuming the interaction between AS-FCS to happen through
   the MS control channel.  This scenario is compliant with the H.248.19
   document related to conferencing in 3GPP.  The second scenario is
   clear and does not need any specific MS control protocol.

   The following sequence diagram describes the interaction between the
   involved parties in a typical scenario.  It assumes that a BFCP

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   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 includes identifying the BFCP moderated
   resources, establishing basic policies and instructions about chair
   identifiers for each resource, and subscribing to events of interest.
   A BFCP session has been established between the AS (acting 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.  It is worth
   pointing out that the AS has to make sure that no user-provided
   control mechanism, e.g. the CCP mixing controls, can override the
   floor control, when it is exploited.

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     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 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 collocated 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 upon the request has not been taken 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, takes a decision
   upon the request, i.e. accepting it.  It informs the MS about its
   decision through a BFCP 'ChairAction' message.  The MS then

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   acknowledges the 'ChairAction' message and then 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 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 user 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 takes decisions upon floor
   requests.  The case of a chair role impersonated 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 the UA
   is exploiting.  The decision upon 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
   managing 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 is 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 queues 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 user who has been granted
   the floor associated to the audio media stream.

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7.  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.

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8.  IANA Considerations

   This document has no actions for IANA.

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9.  Security Considerations

   This docuent describes the architectural framework to be used for
   media server control and focuses on the interactions between
   Application Servers and Media Servers.  Interactions between end
   users and these servers is outside the scope of this document.

   Because a MS represents a valuable network resource, there should be
   mechanims to authenticate requests from an AS and to insure the
   integrity of those requests.  Using the SIP suite to establish
   control channels and media sessions allows the architecture to
   leverage the security mechanisms of SIP.  Integrity of media server
   control requests and responses should be provided by using a
   transport protocol such as TLS for the control channels.  Finally,
   the mechanisms defined in the SIP Control Framework should be used to
   insure that requests from an AS for a specific resource or media
   session correlate with the AS that owns and is responsible for that
   resource or session.

   Although interactions other than between an AS and an MS are outside
   the scope of this document, an AS uses the mechanisms described here
   to establish media streams to a MS on behalf of end users.  The AS
   should not degrade the privacy of those sessions from that originally
   requested by end users.

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10.  Contributors

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

      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

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11.  Informative References

   [1]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

   [2]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [3]   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.

   [4]   Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol (SIP)", RFC 4353, February 2006.

   [5]   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.

   [6]   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.

   [7]   Camarillo, G., "Session Description Protocol (SDP) Format for
         Binary Floor Control Protocol (BFCP) Streams", RFC 4583,
         November 2006.

   [8]   Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network Media
         Services with SIP", RFC 4240, December 2005.

   [9]   Danielsen, P., Tryphonas, S., Rehor, K., Burnett, D., Hunt, A.,
         Carter, J., Ferrans, J., McGlashan, S., Porter, B., and B.
         Lucas, "Voice Extensible Markup Language (VoiceXML) Version
         2.0", World Wide Web Consortium Recommendation REC-voicexml20-
         20040316, March 2004,

   [10]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [11]  Boulton, C., "A Control Framework for the Session Initiation
         Protocol (SIP)", draft-ietf-mediactrl-sip-control-framework-00
         (work in progress), September 2007.

   [12]  Maler, E., Paoli, J., Bray, T., Yergeau, F., and C. Sperberg-
         McQueen, "Extensible Markup Language (XML) 1.0 (Fourth

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         Edition)", World Wide Web Consortium Recommendation REC-xml-
         20060816, August 2006,

   [13]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
         (SIP): Locating SIP Servers", RFC 3263, June 2002.

   [14]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation Protocol (SIP)",
         RFC 4474, August 2006.

   [15]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [16]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
         User Agent Capabilities in the Session Initiation Protocol
         (SIP)", RFC 3840, August 2004.

   [17]  Donovan, S., "The SIP INFO Method", RFC 2976, October 2000.

   [18]  Barnes, M., Boulton, C., and O. Levin, "A Framework for
         Centralized Conferencing", draft-ietf-xcon-framework-10 (work
         in progress), November 2007.

   [19]  Johnston, A. and O. Levin, "Session Initiation Protocol (SIP)
         Call Control - Conferencing for User Agents", BCP 119,
         RFC 4579, August 2006.

   [20]  Novo, O., Camarillo, G., Morgan, D., and R. Even, "Conference
         Information Data Model for Centralized Conferencing (XCON)",
         draft-ietf-xcon-common-data-model-06 (work in progress),
         October 2007.

   [21]  Camarillo, G., Ott, J., and K. Drage, "The Binary Floor Control
         Protocol (BFCP)", RFC 4582, November 2006.

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Author's Address

   Tim Melanchuk (editor)
   Rain Willow Communications

   Email: tim.melanchuk@gmail.com

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Full Copyright Statement

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