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RFC 5022

 Errata 
Informational
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Media Server Control Markup Language (MSCML) and Protocol

Part 1 of 4, p. 1 to 22
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Network Working Group                                        J. Van Dyke
Request for Comments: 5022                      Cantata Technology, Inc.
Obsoletes: 4722                                           E. Burger, Ed.
Category: Informational                                BEA Systems, Inc.
                                                              A. Spitzer
                                                         Bluesocket Inc.
                                                          September 2007


       Media Server Control Markup Language (MSCML) and Protocol

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.

IESG Note

   This RFC is not a candidate for any level of Internet Standard.  The
   IETF disclaims any knowledge of the fitness of this RFC for any
   purpose and in particular notes that the decision to publish is not
   based on IETF review for such things as security, congestion control,
   or inappropriate interaction with deployed protocols.  The RFC Editor
   has chosen to publish this document at its discretion.  Readers of
   this document should exercise caution in evaluating its value for
   implementation and deployment.  See RFC 3932 for more information.

Abstract

   Media Server Control Markup Language (MSCML) is a markup language
   used in conjunction with SIP to provide advanced conferencing and
   interactive voice response (IVR) functions.  MSCML presents an
   application-level control model, as opposed to device-level control
   models.  One use of this protocol is for communications between a
   conference focus and mixer in the IETF SIP Conferencing Framework.

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Table of Contents

   1. Introduction ....................................................4
      1.1. Conventions Used in This Document ..........................5
   2. MSCML Approach ..................................................5
   3. Use of SIP Request Methods ......................................6
   4. MSCML Design ....................................................8
      4.1. Transaction Model ..........................................8
      4.2. XML Usage ..................................................9
           4.2.1. MSCML Time Values ...................................9
   5. Advanced Conferencing ..........................................10
      5.1. Conference Model ..........................................10
      5.2. Configure Conference Request <configure_conference> .......11
      5.3. Configure Leg Request <configure_leg> .....................13
      5.4. Terminating a Conference ..................................14
      5.5. Conference Manipulation ...................................15
      5.6. Video Conferencing ........................................16
      5.7. Conference Events .........................................17
      5.8. Conferencing with Personalized Mixes ......................18
           5.8.1. MSCML Elements and Attributes for
                  Personalized Mixes .................................19
           5.8.2. Example Usage of Personalized Mixes ................20
   6. Interactive Voice Response (IVR) ...............................23
      6.1. Specifying Prompt Content .................................24
           6.1.1. Use of the Prompt Element ..........................24
      6.2. Multimedia Processing for IVR .............................30
      6.3. Playing Announcements <play> ..............................31
      6.4. Prompt and Collect <playcollect> ..........................32
           6.4.1. Control of Digit Buffering and Barge-In ............33
           6.4.2. Mapping DTMF Keys to Special Functions .............33
           6.4.3. Collection Timers ..................................35
           6.4.4. Logging Caller DTMF Input ..........................36
           6.4.5. Specifying DTMF Grammars ...........................36
           6.4.6. Playcollect Response ...............................37
           6.4.7. Playcollect Example ................................38
      6.5. Prompt and Record <playrecord> ............................38
           6.5.1. Prompt Phase .......................................38
           6.5.2. Record Phase .......................................39
           6.5.3. Playrecord Example .................................41
      6.6. Stop Request <stop> .......................................42
   7. Call Leg Events ................................................43
      7.1. Keypress Events ...........................................43
           7.1.1. Keypress Subscription Examples .....................45
           7.1.2. Keypress Notification Examples .....................45
      7.2. Signal Events .............................................46
           7.2.1. Signal Event Examples ..............................47
   8. Managing Content <managecontent> ...............................48
      8.1. Managecontent Example .....................................50

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   9. Fax Processing .................................................51
      9.1. Recording a Fax <faxrecord> ...............................51
      9.2. Sending a Fax <faxplay> ...................................53
   10. MSCML Response Attributes and Elements ........................56
      10.1. Mechanism ................................................56
      10.2. Base <response> Attributes ...............................56
      10.3. Response Attributes and Elements for <configure_leg> .....57
      10.4. Response Attributes and Elements for <play> ..............57
            10.4.1. Reporting Content Retrieval Errors ...............58
      10.5. Response Attributes and Elements for <playcollect> .......59
      10.6. Response Attributes and Elements for <playrecord> ........60
      10.7. Response Attributes and Elements for <managecontent> .....61
      10.8. Response Attributes and Elements for <faxplay>
            and <faxrecord> ..........................................61
   11. Formal Syntax .................................................62
      11.1. Schema ...................................................62
   12. IANA Considerations ...........................................73
      12.1. IANA Registration of MIME Media Type application/
            mediaservercontrol+xml ...................................73
   13. Security Considerations .......................................74
   14. References ....................................................75
      14.1. Normative References .....................................75
      14.2. Informative References ...................................76
   Appendix A.  Regex Grammar Syntax .................................78
   Appendix B.  Contributors .........................................79
   Appendix C.  Acknowledgements .....................................79

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1.  Introduction

   This document describes the Media Server Control Markup Language
   (MSCML) and its usage.  It describes payloads that one can send to a
   media server using standard SIP INVITE and INFO methods and the
   capabilities these payloads implement.  RFC 4240 [2] describes media
   server SIP URI formats.

   Prior to MSCML, there was not a standard way to deliver SIP-based
   enhanced conferencing.  Basic SIP constructs, such as those described
   in RFC 4240 [2], serve simple n-way conferencing well.  The SIP URI
   provides a natural mechanism for identifying a specific SIP
   conference, while INVITE and BYE methods elegantly implement
   conference join and leave semantics.  However, enhanced conferencing
   applications also require features such as sizing and resizing, in-
   conference IVR operations (e.g., recording and playing participant
   names to the full conference), and conference event reporting.  MSCML
   payloads within standard SIP methods realize these features.

   The structure and approach of MSCML satisfy the requirements set out
   in RFC 4353 [10].  In particular, MSCML serves as the interface
   between the conference server or focus and a centralized conference
   mixer.  In this case, a media server has the role of the conference
   mixer.

   There are two broad classes of MSCML functionality.  The first class
   includes primitives for advanced conferencing, such as conference
   configuration, participant leg manipulation, and conference event
   reporting.  The second class comprises primitives for interactive
   voice response (IVR).  These include collecting DTMF digits and
   playing and recording multimedia content.

   MSCML fills the need for IVR and conference control with requests and
   responses over a SIP transport.  VoiceXML [11] fills the need for IVR
   with requests and responses over a HTTP transport.  This enables
   developers to use whatever model fits their needs best.

   In general, a media server offers services to SIP UACs, such as
   Application Servers, Feature Servers, and Media Gateway Controllers.
   See the IPCC Reference Architecture [12] for definitions of these
   terms.  It is unlikely, but not prohibited, for end-user SIP UACs to
   have a direct signaling relationship with a media server.  The term
   "client" is used in this document to refer generically to an entity
   that interacts with the media server using SIP and MSCML.

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   The media server fulfills the role of the Media Resource Function
   (MRF) in the IP Multimedia Subsystem (IMS) [13] as described by 3GPP.
   MSCML and RFC 4240 [2], upon which MSCML builds, are specifically
   focused on the Media resource (Mr) interface which supports
   interactions between application logic and the MRF.

   This document describes a working framework and protocol with which
   there is considerable implementation experience.  Application
   developers and service providers have created several MSCML-based
   services since the availability of the initial version in 2001.  This
   experience is highly relevant to the ongoing work of the IETF,
   particularly the SIP [26], SIPPING [27], MMUSIC [28], and XCON [29]
   work groups, the IMS [30] work in 3GPP, and the CCXML work in the
   Voice Browser Work Group of the W3C.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2.  MSCML Approach

   It is critically important to emphasize that the goal of MSCML is to
   provide an application interface that follows the SIP, HTTP, and XML
   development paradigm to foster easier and more rapid application
   deployment.  This goal is reflected in MSCML in two ways.

   First, the programming model is that of peer to peer rather than
   master-slave.  Importantly, this allows the media server to be used
   simultaneously for multiple applications rather than be tied to a
   single point of control.  It also enables standard SIP mechanisms to
   be used for media server location and load balancing.

   Second, MSCML defines constructs and primitives that are meaningful
   at the application level to ensure that programmers are not
   distracted by unnecessary complexity.  For example, the mixing
   resource operates on constructs such as conferences and call
   participants rather than directly on individual media streams.

   The MSCML paradigm is important to the developer community, in that
   developers and operators conceptually write applications about calls,
   conferences, and call legs.  For the majority of developers and
   applications this approach significantly simplifies and speeds
   development.

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3.  Use of SIP Request Methods

   As mentioned above, MSCML payloads may be carried in either SIP
   INVITE or INFO requests.  The initial INVITE, which creates an
   enhanced conference, MAY include an MSCML payload.  A subsequent
   INVITE to the same Request-URI joins a participant leg to the
   conference.  This INVITE MAY include an MSCML payload.  The initial
   INVITE that establishes an IVR session MUST NOT include an MSCML
   payload.  The client sends all mid-call MSCML payloads for
   conferencing and IVR via SIP INFO requests.

   SIP INVITE requests that contain both MSCML and Session Description
   Protocol (SDP) body parts are used frequently in conferencing
   scenarios.  Therefore, the media server MUST support message bodies
   with the MIME type "multipart/mixed" in SIP INVITE requests.

   The media server transports MSCML responses in the final response to
   the SIP INVITE containing the matching MSCML request or in a SIP INFO
   message.  The only allowable final response to a SIP INFO containing
   a message body is a 200 OK, per RFC 2976 [3].  Therefore, if the
   client sends the MSCML request via SIP INFO, the media server
   responds with the MSCML response in a separate INFO request.  In
   general, these responses are asynchronous in nature and require a
   separate transaction due to timing considerations.

   There has been considerable debate on the use of the SIP INFO method
   for any purpose.  Our experience is that MSCML would not have been
   possible without it.  At the time the first MSCML specification was
   published, the first SIP Event Notification draft had just been
   submitted as an individual submission.  At that time, there was no
   mechanism to link SUBSCRIBE/NOTIFY to an existing dialog.  This
   prevented its use in MSCML, since all events occurred in an INVITE-
   established dialog.  And while SUBSCRIBE/NOTIFY was well suited for
   reporting conference events, its semantics seemed inappropriate for
   modifying a participant leg or conference setting where the only
   "event" was the success or failure of the request.  Lastly, since SIP
   INFO was an established RFC, most SIP stack implementations supported
   it at that time.  We had few, if any, interoperability issues as a
   result.

   More recent developments have provided additional reasons why
   SUBSCRIBE/NOTIFY is not appropriate for use in MSCML.  Use of
   SUBSCRIBE presents two problems.  The first is semantic.  The purpose
   of SUBSCRIBE is to register interest in User Agent state.  However,
   using SUBSCRIBE for MSCML results in the SUBSCRIBE modifying the User
   Agent state.  The second reason SUBSCRIBE is not appropriate is
   because MSCML is inherently call based.  The association of a SIP
   dialog with a call leg means MSCML can be incredibly straightforward.

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   For example, if one used SUBSCRIBE or other SIP method to send
   commands about some context, one must identify that context somehow.
   Relating commands to the SIP dialog they arrive on defines the
   context for free.  Moreover, it is conceptually easy for the
   developer.  Using NOTIFY to transport MSCML responses is also not
   appropriate, as the NOTIFY would be in response to an implicit
   subscription.  The SIP and SIPPING lists have discussed the dangers
   of implicit subscription.

   In order to guarantee interoperability with this specification, as
   well as with SIP User Agents that are unaware of MSCML, SIP UACs that
   wish to use MSCML services MUST specify a service indicator that
   supports MSCML in the initial INVITE.  RFC 4240 [2] defines the
   service indicator "conf", which MUST be used for MSCML conferencing
   applications.  The service indicator "ivr" MUST be used for MSCML
   interactive voice response applications.  In this specification, only
   "conf" and "ivr" are described.

   The media server MUST support moving the call between services
   through sending the media server a BYE on the existing dialog and
   establishing a new dialog with an INVITE to the desired service.
   Media servers SHOULD support moving between services without
   requiring modification of the previously established SDP parameters.
   This is achieved by sending a re-INVITE on the existing dialog in
   which the Request-URI is modified to specify the new service desired
   by the client.  This eliminates the need for the client to send an
   INVITE to the caller or gateway to establish new SDP parameters.

   The media server, as a SIP UAS, MUST respond appropriately to an
   INVITE that contains an MSCML body.  If MSCML is not supported, the
   media server MUST generate a 415 final response and include a list of
   the supported content types in the response per RFC 3261 [4].  The
   media server MUST also advertise its support of MSCML in responses to
   OPTIONS requests, by including "application/mediaservercontrol+xml"
   as a supported content type in an Accept header.  This alleviates the
   major issues with using INFO for the transport of application data;
   namely, the User Agent's proper interpretation of what is, by design,
   an opaque message request.

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4.  MSCML Design

4.1.  Transaction Model

   To avoid undue complexity, MSCML establishes two rules regarding its
   usage.  The first is that only one MSCML body may be present in a SIP
   request.  The second is that each MSCML body may contain only one
   request or response.  This greatly simplifies transaction management.
   MSCML syntax does provide for the unique identification of multiple
   requests in a single body part.  However, this is not supported in
   this specification.

   Per the guidelines of RFC 3470 [14], MSCML bodies MUST be well formed
   and valid.

   MSCML is a direct request-response protocol.  There are no
   provisional responses, only final responses.  A request may, however,
   result in multiple notifications.  For example, a request for active
   talker reports will result in a notification for each speaker set.
   This maps to the three major element trees for MSCML: <request>,
   <response>, and <notification>.

   Figure 1 shows a request body.  Depending on the command, one can
   send the request in an INVITE or an INFO.  Figure 2 shows a response
   body.  The SIP INFO method transports response bodies.  Figure 3
   shows a notification body.  The SIP INFO method transports
   notifications.

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <request>
       ... request body ...
     </request>
   </MediaServerControl>

   Figure 1: MSCML Request Format

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <response>
       ... response body ...
     </response>
   </MediaServerControl>

   Figure 2: MSCML Response Format

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   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <notification>
       ... notification body ...
     </notification>
   </MediaServerControl>

   Figure 3: MSCML Notification Format

   MSCML requests MAY include a client-defined ID attribute for the
   purposes of matching requests and responses.  The values used for
   these IDs need only be unique within the scope of the dialog in which
   the requests are issued.

4.2.  XML Usage

   In the philosophy of XML as a text-based description language, and
   not as a programming language, MSCML makes the choice of many
   attribute values for readability by a human.  Thus, many attributes
   that would often be "boolean" instead take "yes" or "no" values.  For
   example, what does 'report="false"' or 'report="1"' mean?  However,
   'report="yes"' is clearer: I want a report.  Some programmers prefer
   the precision of a boolean.  To satisfy both styles, MSCML defines an
   XML type, "yesnoType", that takes on the values "yes" and "no" as
   well as "true", "false", "1", and "0".

   Many attributes in the MSCML schema have default values.  In order to
   limit demands on the XML parser, MSCML applies these values at the
   protocol, not XML, level.  The MSCML schema documents these defaults
   as XML annotations to the appropriate attribute.

4.2.1.  MSCML Time Values

   For clarity, time values in MSCML are based on the time designations
   described in the Cascading Style Sheets level 2 (CSS2) Specification
   [15].  Their format consists of a number immediately followed by an
   optional time unit identifier of the following form:

      ms: milliseconds (default)
      s: seconds

   If no time unit identifier is present, the value MUST be interpreted
   as being in milliseconds.  As extensions to [15] MSCML allows the
   string values "immediate" and "infinite", which have special meaning
   for certain timers.

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5.  Advanced Conferencing

5.1.  Conference Model

   The advanced conferencing model is a star controller model, with both
   signaling and media directed to a central location.  Figure 4 depicts
   a typical signaling relationship between end users' UACs, a
   conference application server, and a media server.

   RFC 4353 [10] describes this model.  The application server is an
   instantiation of the conference focus.  The media server is an
   instantiation of the media mixer.  Note that user-level constructs,
   such as event notifications, are in the purview of the application
   server.  This is why, for example, the media server sends active
   talker reports using MSCML notifications, while the application
   server would instead use the conference package [16] for individual
   notifications to SIP user agents.  Note that we do not recommend the
   use of the conference package for media server to application server
   notifications because none of the filtering and membership
   information is available at the media server.

      +-------+
      | UAC 1 |---\   Public URI  +-------------+
      +-------+    \ _____________| Application |
                    /    /        |   Server    |     Not shown:
      +-------+    /    /         +-------------+     RTP flows directly
      | UAC 2 |---/    /                 | Private    between UACs and
      +-------+       /                  |   URI      media server
          .          /            +--------------+
          :         /             |              |
      +-------+    /              | Media Server |
      | UAC n |---/               |              |
      +-------+                   +--------------+

                          Figure 4: Conference Model

   Each UAC sends an INVITE to a Public Conference URI.  Presumably, the
   client publishes this URI, or it is an ad hoc URI.  In any event, the
   client generates a Private URI, following the rules specified by RFC
   4240 [2].  That is, the URI is of the following form:

   sip:conf=UniqueID@ms.example.net

   where UniqueID is a unique conference identifier and ms.example.net
   is the host name or IP address of the media server.  There is nothing
   to prevent the UACs from contacting the media server directly.
   However, one would expect the owner of the media server to restrict
   who can use its resources.

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   As for basic conferencing, described by RFC 4240 [2], the first
   INVITE to the media server with a UniqueID creates a conference.
   However, in advanced conferencing, the first INVITE MAY include a
   MSCML <configure_conference> payload rather than the SDP of a
   conference participant.  The <configure_conference> payload conveys
   extended session parameters (e.g., number of participants) that SDP
   does not readily express, but the media server must know to allocate
   the appropriate resources.

   When the conference is created by sending an INVITE containing a
   MSCML <configure_conference> payload, the resulting SIP dialog is
   termed the "Conference Control Leg."  This leg has several useful
   properties.  The lifetime of the conference is the same as that of
   its control leg.  This ensures that the conference remains in
   existence even if all participant legs leave or have not yet arrived.
   In addition, when the client terminates the Conference Control Leg,
   the media server automatically terminates all participant legs.  The
   Conference Control Leg is also used for play or record operations
   to/from the entire conference and for active talker notifications.
   Full conference media operations and active talker report
   subscriptions MUST be executed on the Conference Control Leg.

   Creation of a Conference Control Leg is RECOMMENDED because full
   advanced conferencing capabilities are not available without it.
   Clients MUST establish the Conference Control Leg in the initial
   INVITE that creates the conference; it cannot be created later.

   Once the client has created the conference with or without the
   Conference Control Leg, participants can be joined to the conference.
   This is achieved by the client's directing an INVITE to the Private
   Conference URI for each participant.  Using the example conference
   URI given above, this would be sip:conf=UniqueID@ms.example.net.

5.2.  Configure Conference Request <configure_conference>

   The <configure_conference> request has two attributes that control
   the resources the media server sets aside for the conference.  These
   are described in the list below.

   Attributes of <configure_conference>:

   o  reservedtalkers - optional (see note), no default value: The
      maximum number of talker legs allocated for the conference.  Note:
      required when establishing the Conference Control Leg but optional
      in subsequent <configure_conference> requests.

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   o  reserveconfmedia - optional, default value "yes": Controls
      allocation of resources to enable playing or recording to or from
      the entire conference.

   When the reservedtalkers+1st INVITE arrives at the media server, the
   media server SHOULD generate a 486 Busy Here response.  Failure to
   send a 486 response to this condition can cause the media server to
   oversubscribe its resources.

      NOTE: It would be symmetric to have a reservedlisteners parameter.
      However, the practical limitation on the media server is the
      number of talkers for a mixer to monitor.  In either case, the
      client regulates who gets into the conference by either proxying
      the INVITEs from the user agent clients or metering to whom it
      gives the conference URI.

   For example, to create a conference with up to 120 active talkers and
   the ability to play audio into the conference or record portions or
   all of the conference full mix, the client specifies both attributes,
   as shown in Figure 6.

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <request>
       <configure_conference reservedtalkers="120"
         reserveconfmedia="yes"/>
     </request>
   </MediaServerControl>

   Figure 6: 120 Speaker MSCML Example

   In addition to these attributes, a <configure_conference> request MAY
   contain a child <subscribe> element.  The <subscribe> element is used
   to request notifications for conference-wide active talker events.
   Detailed information regarding active talker events is contained in
   Section 5.7.

   The client MUST include a <configure_conference> request in the
   initial INVITE which establishes the conference when creating the
   Conference Control Leg. The client server MUST issue asynchronous
   commands, such as <play>, separately (i.e., in INFO messages) to
   avoid ambiguous responses.

   Media operations on the Conference Control leg are performed
   internally, no external RTP streams are involved.  Accordingly, the
   media server does not expect RTP on the Conference Control Leg.
   Therefore, the client MUST send either no SDP or hold SDP in the
   INVITE request containing a <configure_conference> payload.  The

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   media server MUST treat SDP with all media lines set to "inactive" or
   with connection addresses set to 0.0.0.0 (for backwards
   compatibility) as hold SDP.

   The media server sends a response when it has finished processing the
   <configure_conference> request.  The format of the
   <configure_conference> response is detailed in Section 10.2.

5.3.  Configure Leg Request <configure_leg>

   Conference legs have a number of properties the client can modify.
   These are set using the <configure_leg> request.  This request has
   the attributes described in the list below.

   Attributes of <configure_leg>:

   o  type - optional, default value "talker": Consider this leg's audio
      for inclusion in the output mix.  Alternative is "listener".

   o  dtmfclamp - optional, default value "yes": Remove detected DTMF
      digits from the input audio.

   o  toneclamp - optional, default value "yes": Remove tones from the
      input audio.  Tones include call progress tones and the like.

   o  mixmode - optional, default value "full": Be a candidate for the
      full mix.  Alternatives are "mute", to disallow media in the mix,
      "parked", to disconnect the leg's media streams from the
      conference for IVR operations, "preferred", to give this stream
      preferential selection in the mix (i.e., even if not loudest
      talker, include media, if present, from this leg in the mix), and
      "private", which enables personalized mixes.

   In addition to these attributes, there are four child elements
   defined for <configure_leg>.  These are <inputgain>, <outputgain>,
   <configure_team>, and <subscribe>.

   The first two, <inputgain> and <outputgain>, modify the gain applied
   to the input and output audio streams, respectively.  These may
   contain <auto>, to use automatic gain control (AGC) or <fixed>.  The
   <auto> element has the attributes "startlevel", "targetlevel", and
   "silencethreshold".  All the parameters are in dB.  The <fixed>
   element has the attribute "level", which is in dB.  The default for
   both <inputgain> and <outputgain> is <fixed>.  The media server MAY

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   silently cap <inputgain> or <outputgain> requests that exceed the
   gain limits imposed by the platform.

   Clients most commonly manipulate only the input gain for a conference
   leg and rely on the mixer to set an optimum output gain based on the
   inputs currently in the mix.  However, as described above, MSCML does
   allow for manipulation of the output gain as well.  Some of the IVR
   commands, such as <play>, enable control of the output gain for
   content playback operations.  The interaction of conference output
   gain and IVR playback gain controls is described in Section 6.1.1.
   Note that <inputgain> and <outputgain> settings apply only to
   conference legs and do not apply to IVR sessions.

   The <configure_team> element is used to create and manipulate groups
   for personalized mixes.  Details of personalized mixes are discussed
   in Section 5.8.

   The <subscribe> element is used to request notifications for call leg
   related events, such as asynchronous DTMF digit reports.  Detailed
   information regarding call leg events is discussed in Section 7.

   If the default parameters are acceptable for the leg the client
   wishes to enter into the conference, then a normal SIP INVITE, with
   no MSCML body, is sufficient.  However, if the client wishes to
   modify one or more of the parameters, the client can include a MSCML
   body in addition to the SDP body.

   The client can modify the conference leg parameters during the
   conference by issuing a SIP INFO on the dialog representing the
   conference leg.  Of course, the client cannot modify SDP in an INFO
   message.

   The media server sends a response when it has finished processing the
   <configure_leg> request.  The format of the <configure_leg> response
   is detailed in Section 10.3.

5.4.  Terminating a Conference

   To remove a leg from the conference, the client issues a SIP BYE
   request on the selected dialog representing the conference leg.

   The client can terminate all legs in a conference by issuing a SIP
   BYE request on the Conference Control Leg.  If one or more
   participants are still in the conference when the media server
   receives a SIP BYE request on the Conference Control Leg, the media
   server issues SIP BYE requests on all remaining conference legs to
   ensure cleanup of the legs.

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   The media server returns a 200 OK to the SIP BYE request as it sends
   BYE requests to the other legs.  This is because we cannot issue a
   provisional response to a non-INVITE request, yet the teardown of the
   other legs may exceed the retransmission timer limits of the original
   request.  While the conference is being cleaned up, the media server
   MUST reject any new INVITEs to the terminated conference with a 486
   Busy Here response.  This response indicates that the specified
   conference cannot accept any new members, pending deletion.

5.5.  Conference Manipulation

   Once the conference has begun, the client can manipulate the
   conference as a whole or a particular participant leg by issuing
   commands on the associated SIP dialog.  For example, by sending MSCML
   requests on the Conference Control Leg the client can request that
   the media server record the conference, play a prompt to the
   conference, or request reports on active talker events.  Similarly,
   the client may mute a participant leg, configure a personalized mix
   or request reports for call leg events, such as DTMF keypresses.

   Figure 7 shows an example of an MSCML command that plays a prompt to
   all conference participants.

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <request>
       <play>
         <prompt>
           <audio url="http://prompts.example.net/en_US/welcome.au"/>
         </prompt>
       </play>
     </request>
   </MediaServerControl>

   Figure 7: Full Conference Audio Command - Play

   A client can modify a leg by issuing an INFO on the dialog associated
   with the participant leg.  For example, Figure 8 mutes a conference
   leg.

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <request>
       <configure_leg mixmode="mute"/>
     </request>
   </MediaServerControl>

   Figure 8: Sample Change Leg Command

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   In Figure 7, we saw a request to play a prompt to the entire
   conference.  The client can also request to play a prompt to an
   individual call leg.  In that case, the MSCML request is issued
   within the SIP dialog of the desired conference participant.

   Section 6 describes the interactive voice response (IVR) services
   offered by MSCML.  If an IVR command arrives on the control channel,
   it takes effect on the whole conference.  This is a mechanism for
   playing prompts to the entire conference (e.g., announcing new
   participants).  If an IVR command arrives on an individual leg, it
   only affects that leg.  This is a mechanism for interacting with
   users, such as the creation of "waiting rooms", allowing a user to
   mute themselves using key presses, allowing a moderator to out-dial,
   etc.

   A participant leg MUST be configured with mixmode="parked" prior to
   the issuance of any IVR commands with prompt content ('prompturl'
   attribute or <prompt> element).  Parking the leg isolates the
   participant's input and output media from the conference and allows
   use of those streams for playing and recording purposes.  However,
   the mixmode has no effect if just digit collection or recording is
   desired. <playcollect> and <playrecord> requests without prompt
   content MAY be sent on participant legs without setting
   mixmode="parked".

5.6.  Video Conferencing

   MSCML-controlled advanced conferences, as well as RFC 4240 [2]
   controlled basic conferences, implicitly support video conferencing
   in the form of video switching.  In video switching, the video stream
   of the loudest talker (with some hysteresis) is sent to all
   participants other than that talker.  The loudest talker receives the
   video stream from the immediately prior loudest talker.

   Media servers MUST ensure that participants receive video media
   compatible with their session.  For example, a participant who has
   established an H.263 video stream will not receive video from another
   participant employing H.264 media.  Media servers SHOULD implement
   video transcoding to minimize media incompatibilities between
   participants.

   The media server MUST switch video streams only when it receives a
   refresh video frame.  A refresh frame contains all the video
   information required to decode that frame (i.e., there is no
   dependency on data from previous video frames).

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   Refresh frames are large and generally sent infrequently to conserve
   network bandwidth.  The media server MUST implement standard
   mechanisms to request that the new loudest talker's video encoder
   transmits a refresh frame to ensure that video can be switched
   quickly.

5.7.  Conference Events

   A client can subscribe for periodic active talker event reports that
   indicate which participants are included in the conference mix.  As
   these are conference-level events, the subscription and notifications
   are sent on the Conference Control Leg.

   Media servers MAY impose limits on the minimum interval for active
   talker reports for performance reasons.  If the client request is
   below the imposed minimum, the media server SHOULD set the interval
   to the minimum value supported.  To limit unnecessary notification
   traffic, the media server SHOULD NOT send a report if the active
   talker information for the conference has not changed during the
   reporting interval.

   A request for an active talker report is in Figure 9.  The active
   talker report enumerates the current call legs in the mix.

   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <request>
       <configure_conference>
         <subscribe>
           <events>
             <activetalkers report="yes" interval="60s"/>
           </events>
         </subscribe>
       </configure_conference>
     </request>
   </MediaServerControl>

   Figure 9: Active Talker Request

   Event notifications are sent in SIP INFO messages.  Figure 10 shows
   an example of a report.

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   <?xml version="1.0" encoding="utf-8"?>
   <MediaServerControl version="1.0">
     <notification>
       <conference uniqueid="ab34h76z" numtalkers="47">
         <activetalkers>
           <talker callid="myhost4sn123"/>
           <talker callid="myhost2sn456"/>
           <talker callid="myhost12sn78"/>
         </activetalkers>
       </conference>
     </notification>
   </MediaServerControl>

   Figure 10: Active Talker Event Example

   The value of the "callid" attribute in the <talker> element
   corresponds to the value of the SIP Call-ID header of the associated
   dialog.  This enables the client to associate the active talker with
   a specific participant leg.

5.8.  Conferencing with Personalized Mixes

   MSCML enables clients to create personalized mixes through the
   <configure_team> element for scenarios where the standard mixmode
   settings do not provide sufficient control.  The <configure_team>
   element is a child of <configure_leg>.

   To create personalized mixes, the client has to identify the
   relationships among the participants.  This is accomplished by
   manipulating two MSCML objects.  These objects are:

   1.  The list of team members (<teammate> elements), set using
       <configure_team>

   2.  The mixmode attribute set through <configure_leg>

   The media server uses the values of these objects to determine which
   audio inputs to combine for output to the participant.  In a normal
   conference, each participant hears the conference mix minus their own
   input if they are part of the mixed output.  The team list enables
   the client to specify other participants that the leg can hear in
   addition to the normal mixed output.  Note that personalized mix
   settings apply only to audio media and do not affect video switching.

   Team relationships are implicitly symmetric.  If the client sets
   participant A as a team member of participant B, then the media
   server automatically sets participant B as a team member for A.

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   The id attribute set through <configure_leg> is used to identify the
   various participants.  A unique ID MUST be assigned to each
   participant included in a personalized mix.  The IDs used MUST be
   unique within the scope of the conference in which they appear.

   By itself, the team list only defines those participants that the leg
   can hear.  The mixmode attribute of each team member determines
   whether to include their audio input in the personalized mix.  If the
   client sets the teammate's mixmode to private, then it is part of the
   mix.  If the mixmode is set to any other value, it is not.

5.8.1.  MSCML Elements and Attributes for Personalized Mixes

   Control of personalized mixes rely on two major MSCML elements:

   1.  <configure_leg>, using the mixmode attribute setting
       mixmode="private"

   2.  <configure_team>

   The <configure_team> element allows the user to make the participants
   members of a team within a specific conference.  It is a child of the
   <configure_leg> parent element.

   The client sends the <configure_team> element in a <configure_leg>
   request in either a SIP INVITE or SIP INFO.

   o  In an INVITE, to join a participant whose properties differ from
      the properties established for the conference as a whole.

   o  In an INFO, to change the properties for an existing leg.

   The two attributes of the configure_team element are "id" and
   "action".  The id attribute MUST contain the unique ID of the leg
   being modified, as set in the original <configure_leg> request.  The
   action attribute can take on the values "add", "delete", "query", and
   "set".  The default value is "query".  This attribute allows the user
   to modify the team list.  Table 1 describes the actions that can be
   performed on the team list.

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   +--------+----------------------------------------------------------+
   | Action | Description                                              |
   +--------+----------------------------------------------------------+
   | add    | Adds a teammate to the mix.                              |
   | delete | Deletes a teammate from the mix.                         |
   | query  | Returns the teammate list to the requestor.  This is the |
   |        | default value.                                           |
   | set    | Creates a team list when followed by <teammate id="n">   |
   |        | and also removes all the teammates from the team list    |
   |        | for example, when the creator (originator) of the team   |
   |        | list on that specific conference leg wants to remove all |
   |        | of the teammates from the team.  If the set operation    |
   |        | removes all teammates from a participant, that           |
   |        | participant hears the full conference mix.               |
   +--------+----------------------------------------------------------+

                      Table 1: Configure Team Actions

5.8.2.  Example Usage of Personalized Mixes

   A common use of personalized mixing is to support coaching of one
   participant by another.  The coaching scenario includes three
   participants:
   1.  The Supervisor, who coaches the agent.
   2.  The Agent, who interacts with the customer.
   3.  The Customer, who interacts with the agent.

   Table 2 illustrates the details of the coached conference topology.

   +-------------+------------+------------+---------+-----------------+
   | Participant | ID         | Team       | Mixmode | Hears           |
   |             |            | Members    |         |                 |
   +-------------+------------+------------+---------+-----------------+
   | Supervisor  | supervisor | Agent      | Private | customer +      |
   |             |            |            |         | agent           |
   | Agent       | agent      | Supervisor | Full    | customer +      |
   |             |            |            |         | supervisor      |
   | Customer    | customer   | none       | Full    | agent           |
   +-------------+------------+------------+---------+-----------------+

                    Table 2: Coached Conference Example

   To create this topology, the client performs the following actions:

   1.  The client joins each leg to the conference, being certain to
       include a unique ID in the <configure_leg> request.  The leg ID
       needs to be unique only within the scope of the conference to
       which it belongs.

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   2.  The client configures the teammate list and mixmode of each
       participant, as required.

   Both actions (steps 1 and 2) may be combined in a single MSCML
   request.  The following sections detail these actions and their
   corresponding MSCML payloads.

5.8.2.1.  Create the Conference

   Before joining any participants, the client must create the
   conference by sending a SIP INVITE that contains an MSCML
   <configure_conference> request with a unique conference identifier.

5.8.2.2.  Joining and Configuring the Coach

   Join the coach leg to the conference and configure its desired
   properties by sending a SIP INVITE containing a <configure_leg>
   request.  The <configure_leg> element sets the leg's unique ID to
   supervisor and its mixmode to private.

   The corresponding MSCML request is as follows.

   <?xml version="1.0"?>
   <MediaServerControl version="1.0">
     <request>
       <configure_leg id="supervisor" mixmode="private"/>
     </request>
   </MediaServerControl>

   Figure 11: Join Coach Request

   Note that the client cannot configure the teammate list for the coach
   yet, as there are no other participants in the conference.  One must
   join a participant to the conference before one can add it as a
   teammate for another leg.

5.8.2.3.  Joining and Configuring the Agent

   Join the agent leg to the conference and configure its desired
   properties by sending a SIP INVITE containing a <configure_leg>
   request.  The <configure_leg> element sets the leg's unique ID to
   "agent" and sets the supervisor as a team member of the agent.
   Because team member relationships are symmetric, this action also
   adds the agent as a team member for the coach.

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   The corresponding MSCML request is as follows.
   <?xml version="1.0"?>
   <MediaServerControl version="1.0">
    <request>
       <configure_leg id="agent">
         <configure_team action="set">
           <teammate id="supervisor"/>
         </configure_team>
       </configure_leg>
     </request>
   </MediaServerControl>

   Figure 12: Join Agent Request

   Because the desired mixmode for this leg is full, which is the
   default value, there is no need to set it explicitly.

5.8.2.4.  Joining and Configuring the Client

   Join the client leg to the conference and configure its desired
   properties by sending a SIP INVITE containing a <configure_leg>
   request.  The <configure_leg> element simply sets the leg's unique ID
   to "customer".  The media server does not need further configuration
   because the desired mixmode, full, is the default and the customer
   has no team members.

   The corresponding MSCML request is as follows.
   <?xml version="1.0"?>
   <MediaServerControl version="1.0">
     <request>
       <configure_leg id="customer"/>
     </request>
   </MediaServerControl>

   Figure 13: Join Client Request

   Strictly speaking, it is not a requirement that the client give the
   customer leg a unique ID because it will not be a team member.
   However, when using coached conferencing, we RECOMMEND that one
   assign a unique ID to each leg in the initial INVITE request.
   Assigning a unique ID eliminates the need to set it later by sending
   a SIP INFO if one later desires personalized mixing for the customer
   leg.

   The conference is now in the desired configuration, shown previously
   in Table 2.


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