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RFC 6505

 
 
 

A Mixer Control Package for the Media Control Channel Framework

Part 2 of 5, p. 8 to 27
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4.  Element Definitions

   This section defines the XML elements for this package.  The elements
   are defined in the XML namespace specified in Section 8.2.

   The root element is <mscmixer> (Section 4.1).  All other XML elements
   (requests, responses, and notification elements) are contained within
   it.  Child elements describe mixer management (Section 4.2) and audit
   (Section 4.3) functionality.  Response status codes are defined in
   Section 4.6 and type definitions in Section 4.7.

   Implementation of this Control Package MUST address the security
   considerations described in Section 7.

   Implementation of this Control Package MUST adhere to the syntax and
   semantics of XML elements defined in this section and the schema
   (Section 5).  The XML schema supports extensibility by allowing
   attributes and elements from other namespaces.  Implementations MAY

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   support attributes and elements from other (foreign) namespaces.  If
   an MS implementation receives a <mscmixer> element containing
   attributes or elements from another namespace, which it does not
   support, the MS sends a 428 response (Section 4.6).

   Extensible attributes and elements are not described in this section.
   In all other cases where there is a difference in constraints between
   the XML schema and the textual description of elements in this
   section, the textual definition takes priority.

   Some elements in this Control Package contain attributes whose value
   is descriptive text primarily for diagnostic use.  The implementation
   can indicated the language used in the descriptive text by means of a
   'desclang' attribute [RFC2277].  The 'desclang' attribute can appear
   on the root element as well as selected subordinate elements (see
   Section 4.1).  The 'desclang' attribute value on the root element
   applies to all 'desclang' attributes in subordinate elements unless
   the subordinate element has an explicit 'desclang' attribute that
   overrides it.

   Usage examples are provided in Section 6.

4.1.  <mscmixer>

   The <mscmixer> element has the following attributes (in addition to
   standard XML namespace attributes such as 'xmlns'):

   version:  a string specifying the mscmixer package version.  The
      value is fixed as "1.0" for this version of the package.  The
      attribute is mandatory.

   desclang:  specifies the language used in descriptive text attributes
      of subordinate elements (unless the subordinate element provides a
      'desclang' attribute that overrides the value for its descriptive
      text attributes).  The descriptive text attributes on subordinate
      elements include: the 'reason' attribute on <response>
      (Section 4.2.3), <unjoin-notify> (Section 4.2.4.2),
      <conferenceexit> (Section 4.2.4.3), and <auditresponse>
      (Section 4.3.2).  A valid value is a language identifier
      (Section 4.7.7).  The attribute is optional.  The default value is
      "i-default" (BCP 47 [RFC5646]).

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   The <mscmixer> element has the following defined child elements, only
   one of which can occur:

   1.  mixer management elements defined in Section 4.2:

       <createconference>:  create and configure a new conference mixer.
          See Section 4.2.1.1

       <modifyconference>:  modify the configuration of an existing
          conference mixer.  See Section 4.2.1.2

       <destroyconference>:  destroy an existing conference mixer.  See
          Section 4.2.1.3

       <join>:  create and configure media streams between connections
          and/or conferences (for example, add a participant to a
          conference).  See Section 4.2.2.2

       <modifyjoin>:  modify the configuration of joined media streams.
          See Section 4.2.2.3

       <unjoin>:  delete a media stream (for example, remove a
          participant from a conference).  See Section 4.2.2.4

       <response>:  response to a mixer request.  See Section 4.2.3

       <event>:  mixer or subscription notification.  See Section 4.2.4

   2.  audit elements defined in Section 4.3:

       <audit>:  audit package capabilities and managed mixers.  See
          Section 4.3.1

       <auditresponse>:  response to an audit request.  See
          Section 4.3.2

   For example, a request to the MS to create a conference mixer is as
   follows:

   <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer">
    <createconference/>
   </mscmixer>

   And a response from the MS that the conference was successfully
   created is as follows:

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   <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer"
     desclang="en">
    <response status="200" conferenceid="conference1"
      reason="conference created"/>
   </mscmixer>

4.2.  Mixer Elements

   This section defines the mixer management XML elements for this
   Control Package.  These elements are divided into requests,
   responses, and notifications.

   Request elements are sent to the MS to request a specific mixer
   operation to be executed.  The following request elements are
   defined:

   <createconference>:  create and configure a new a conference mixer.
      See Section 4.2.1.1

   <modifyconference>:  modify the configuration of an existing
      conference mixer.  See Section 4.2.1.2

   <destroyconference>:  destroy an existing conference mixer.  See
      Section 4.2.1.3

   <join>:  create and configure media streams between connections
      and/or conferences (for example, add a participant to a
      conference).  See Section 4.2.2.2

   <modifyjoin>:  modify the configuration of joined media streams.  See
      Section 4.2.2.3

   <unjoin>:  delete a media stream (for example, remove a participant
      from a conference).  See Section 4.2.2.4

   Responses from the MS describe the status of the requested operation.
   Responses are specified in a <response> element (Section 4.2.3) that
   includes a mandatory attribute describing the status in terms of a
   numeric code.  Response status codes are defined in Section 4.6.  The
   MS MUST respond to a request message with a response message.  If the
   MS is not able to process the request and carry out the mixer
   operation (in whole or in part), then the request has failed: the MS
   MUST ensure that no part of the requested mixer operation is carried
   out, and the MS MUST indicate the class of failure using an
   appropriate 4xx response code.  Unless an error response code is
   specified for a class of error within this section, implementations
   follow Section 4.6 in determining the appropriate status code for the
   response.

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   Notifications are sent from the MS to provide updates on the status
   of a mixer operation or subscription.  Notifications are specified in
   an <event> element (Section 4.2.4).

4.2.1.  Conference Elements

4.2.1.1.  <createconference>

   The <createconference> element is sent to the MS to request creation
   of a new conference (multiparty) mixer.

   The <createconference> element has the following attributes:

   conferenceid:  string indicating a unique name for the new
      conference.  If this attribute is not specified, the MS MUST
      create a unique name for the conference.  The value is used in
      subsequent references to the conference (e.g., as conferenceid in
      a <response>).  The attribute is optional.  There is no default
      value.

   reserved-talkers:  indicates the requested number of guaranteed
      speaker slots to be reserved for the conference.  A valid value is
      a non-negative integer (see Section 4.7.2).  The attribute is
      optional.  The default value is 0.

   reserved-listeners:  indicates the requested number of guaranteed
      listener slots to be reserved for the conference.  A valid value
      is a non-negative integer (see Section 4.7.2).  The attribute is
      optional.  The default value is 0.

   The <createconference> element has the following sequence of child
   elements:

   <codecs>:  an element to configure the codecs supported by the
      conference (see Section 4.4).  If codecs are specified, then they
      impose limitations on media capability when the MS attempts to
      join the conference to other entities (see Sections 4.2.2.2 and
      4.2.2.3).  The element is optional.

   <audio-mixing>:  an element to configure the audio mixing
      characteristics of a conference (see Section 4.2.1.4.1).  The
      element is optional.

   <video-layouts>:  an element to configure the video layouts of a
      conference (see Section 4.2.1.4.2).  The element is optional.

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   <video-switch>:  an element to configure the video switch policy for
      the layout of a conference (see Section 4.2.1.4.3).  The element
      is optional.

   <subscribe>:  an element to request subscription to conference
      events. (see Section 4.2.1.4.4).  The element is optional.

   If the 'conferenceid' attribute specifies a value that is already
   used by an existing conference, the MS reports an error (405) and
   MUST NOT create a new conference and MUST NOT affect the existing
   conference.

   If the MS is unable to configure the conference according to
   specified 'reserved-talkers' or 'reserved-listeners' attributes, the
   MS reports an error (420) and MUST NOT create the conference.

   If the MS is unable to configure the conference according to a
   specified <audio-mixing> element, the MS reports an error (421) and
   MUST NOT create the conference.

   If the MS is unable to configure the conference according to a
   specified <video-layouts> element, the MS reports an error (423) and
   MUST NOT create the conference.

   If the MS is unable to configure the conference according to a
   specified <video-switch> element, the MS reports an error (424) and
   MUST NOT create the conference.

   If the MS is unable to configure the conference according to a
   specified <codecs> element, the MS reports an error (425) and MUST
   NOT create the conference.

   When a MS has finished processing a <createconference> request, it
   MUST reply with an appropriate <response> element (Section 4.2.3).

   For example, a request to create an audio video conference mixer with
   specified codecs, video layout, video switch, and subscription is as
   follows:

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   <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer">
    <createconference conferenceid="conference1"
          reserved-talkers="1" reserved-listeners="10">
      <codecs>
       <codec name="video">
        <subtype>H264</subtype>
       </codec>
       <codec name="audio">
        <subtype>PCMA</subtype>
       </codec>
      </codecs>
      <audio-mixing type="nbest"/>
      <video-layouts>
       <video-layout min-participants="1"><single-view/></video-layout>
       <video-layout min-participants="2"><dual-view/></video-layout>
       <video-layout min-participants="3"><quad-view/></video-layout>
      </video-layouts>
      <video-switch interval="5"><vas/></video-switch>
      <subscribe>
       <active-talkers-sub interval="4"/>
      </subscribe>
    </createconference>
   </mscmixer>

   A response from the MS if the conference was successfully created is
   as follows:

   <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer">
    <response status="200" conferenceid="conference1"/>
   </mscmixer>

   Alternatively, a response if the MS could not create the conference
   due to a lack of support for the H264 codec is as follows:

   <mscmixer version="1.0" xmlns="urn:ietf:params:xml:ns:msc-mixer">
    <response status="425" conferenceid="conference1"
              reason="H264 codec not supported"/>
   </mscmixer>

4.2.1.2.  <modifyconference>

   The <modifyconference> element is sent to the MS to request
   modification of an existing conference.

   The <modifyconference> element has the following attribute:

   conferenceid:  string indicating the name of the conference to
      modify.  This attribute is mandatory.

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   The <modifyconference> element has the following sequence of child
   elements (one or more):

   <codecs>:  an element to configure the codecs supported by the
      conference (see Section 4.4).  If codecs are specified, then they
      impose limitations in media capability when the MS attempts to
      join the conference to other entities (see Sections 4.2.2.2 and
      4.2.2.3).  Existing conference participants are unaffected by any
      policy change.  The element is optional.

   <audio-mixing>:  an element to configure the audio mixing
      characteristics of a conference (see Section 4.2.1.4.1).  The
      element is optional.

   <video-layouts>:  an element to configure the video layouts of a
      conference (see Section 4.2.1.4.2).  The element is optional.

   <video-switch>:  an element to configure the video switch policy for
      the layout of a conference (see Section 4.2.1.4.3).  The element
      is optional.

   <subscribe>:  an element to request subscription to conference
      events. (see Section 4.2.1.4.4).  The element is optional.

   If the 'conferenceid' attribute specifies the name of a conference
   that does not exist, the MS reports an error (406).

   If the MS is unable to configure the conference according to a
   specified <audio-mixing> element, the MS reports an error (421) and
   MUST NOT modify the conference in any way.

   If the MS is unable to configure the conference according to a
   specified <video-layouts> element, the MS reports an error (423) and
   MUST NOT modify the conference in any way.

   If the MS is unable to configure the conference according to a
   specified <video-switch> element, the MS reports an error (424) and
   MUST NOT modify the conference in any way.

   If the MS is unable to configure the conference according to a
   specified <codecs> element, the MS reports an error (425) and MUST
   NOT modify the conference.

   When a MS has finished processing a <modifyconference> request, it
   MUST reply with an appropriate <response> element (Section 4.2.3).

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4.2.1.3.  <destroyconference>

   The <destroyconference> element is sent to the MS to request
   destruction of an existing conference.

   The <destroyconference> element has the following attribute:

   conferenceid:  string indicating the name of the conference to
      destroy.  This attribute is mandatory.

   The <destroyconference> element does not specify any child elements.

   If the 'conferenceid' attribute specifies the name of a conference
   that does not exist, the MS reports an error (406).

   When a MS has finished processing a <destroyconference> request, it
   MUST reply with an appropriate <response> element (Section 4.2.3).

   Successfully destroying the conference (status code 200) will result
   in all connection or conference participants being removed from the
   conference mixer, <unjoin-notify> notification events
   (Section 4.2.4.2) being sent for each conference participant, and a
   <conferenceexit> notification event (Section 4.2.4.3) indicating that
   conference has exited.  A <response> with any other status code
   indicates that the conference mixer still exists and participants are
   still joined to the mixer.

4.2.1.4.  Conference Configuration

   The elements in this section are used to establish and modify the
   configuration of conferences.

4.2.1.4.1.  <audio-mixing>

   The <audio-mixing> element defines the configuration of the
   conference audio mix.

   The <audio-mixing> element has the following attributes:

   type:  is a string indicating the audio stream mixing policy.
      Defined values are: "nbest" (where the N best (loudest)
      participant signals are mixed) and "controller" (where the
      contributing participant(s) is/are selected by the controlling AS
      via an external floor control protocol).  The attribute is
      optional.  The default value is "nbest".

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   n: indicates the number of eligible participants included in the
      conference audio mix.  An eligible participant is a participant
      who contributes audio to the conference.  Inclusion is based on
      having the greatest audio energy.  A valid value is a non-negative
      integer (see Section 4.7.2).  A value of 0 indicates that all
      participants contributing audio to the conference are included in
      the audio mix.  The default value is 0.  The element is optional.

   If the 'type' attribute does not have the value "nbest", the MS
   ignores the 'n' attribute.

   The <audio-mixing> element has no child elements.

   For example, a fragment where the audio-mixing policy is set to
   "nbest" with 3 participants to be included is as follows:

      <audio-mixing type="nbest" n="3"/>

   If the conference had 200 participants of whom 30 contributed audio,
   then there would be 30 eligible participants for the audio mix.  Of
   these, the 3 loudest participants would have their audio included in
   the conference.

4.2.1.4.2.  <video-layouts>

   The <video-layouts> element describes the video presentation layout
   configuration for participants providing a video input stream to the
   conference.  This element allows multiple video layouts to be
   specified so that the MS automatically changes layout depending on
   the number of video-enabled participants.

   The <video-layouts> element has no attributes.

   The <video-layouts> element has the following sequence of child
   elements (one or more):

   <video-layout>:  element describing a video layout
      (Section 4.2.1.4.2.1).

   If the MS does not support video conferencing at all, or does not
   support multiple video layouts, or does not support a specific video
   layout, the MS reports an 423 error in the response to the request
   element containing the <video-layouts> element.

   An MS MAY support more than one <video-layout> element, although only
   one layout can be active at a time.  A <video-layout> is active if
   the number of participants in the conference is equal to or greater
   than the value of its 'min-participants' attribute, but less than the

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   value of the 'min-participants' attribute for any other <video-
   layout> element.  An MS reports an error (400) if more than one
   <video-layout> has the same value for the 'min-participants'
   attribute.  When the number of regions within the active layout is
   greater than the number of participants in the conference, the
   display of unassigned regions is implementation-specific.

   The assignment of participant video streams to regions within the
   layout is according to the video switch policy specified by the
   <video-switch> element (Section 4.2.1.4.3).

   For example, a fragment describing a single layout is as follows:

   <video-layouts>
     <video-layout><single-view/></video-layout>
   </video-layouts>

   A fragment describing a sequence of layouts is as follows:

   <video-layouts>
     <video-layout min-participants="1"><single-view/></video-layout>
     <video-layout min-participants="2"><dual-view/></video-layout>
     <video-layout min-participants="3"><quad-view/></video-layout>
     <video-layout min-participants="5"><multiple-3x3/></video-layout>
   </video-layouts>

   When the conference has one participant providing a video input
   stream to the conference, then the single-view format is used.  When
   the conference has two such participants, the dual-view layout is
   used.  When the conference has three or four participants, the quad-
   view layout is used.  When the conference has five or more
   participants, the multiple-3x3 layout is used.

4.2.1.4.2.1.  <video-layout>

   The <video-layout> element describes a video layout containing one or
   more regions in which participant video input streams are displayed.

   The <video-layout> element has the following attribute:

   min-participants:  the minimum number of conference participants
      needed to allow this layout to be active.  A valid value is a
      positive integer (see Section 4.7.3).  The attribute is optional.
      The default value is 1.

   The <video-layout> element has one child element specifying the video
   layout.  An MS MAY support the predefined video layouts defined in
   the conference information data model for centralized conferencing

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   (XCON) [RFC6501]: <single-view>, <dual-view>, <dual-view-crop>,
   <dual-view-2x1>, <dual-view-2x1-crop>, <quad-view>, <multiple-3x3>,
   <multiple-4x4>, and <multiple-5x1>.

   The MS MAY support other video layouts.  Non-XCON layouts MUST be
   specified using an element from a namespace other than the one used
   in this specification, for example:

   <video-layout>
    <mylayout xmlns='http://example.com/foo'>my-single-view</mylayout>
   </video-layout>

   If the MS does not support the specified video layout configuration,
   then the MS reports a 423 error (Section 4.6) in the response to the
   request element containing the <video-layout> element.

   Each video layout has associated with it one or more regions.  The
   XCON layouts are associated with the following named regions:

   <single-view/>:  layout with one stream in a single region as shown
      in Figure 1.

                               +-----------+
                               |           |
                               |           |
                               |     1     |
                               |           |
                               |           |
                               +-----------+

                      Figure 1: single-view video layout

   <dual-view/>:  layout presenting two streams side-by-side in two
      regions as shown in Figure 2.  The MS MUST NOT alter the aspect
      ratio of each stream to fit the region, and hence the MS might
      need to blank out part of each region.

                         +-----------+-----------+
                         |           |           |
                         |           |           |
                         |     1     |     2     |
                         |           |           |
                         |           |           |
                         +-----------+-----------+

                       Figure 2: dual-view video layout

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   <dual-view-crop/>:  layout presenting two streams side-by-side in two
      regions as shown in Figure 3.  The MS MUST alter the aspect ratio
      of each stream to fit its region so that no blanking is required.

                         +-----------+-----------+
                         |           |           |
                         |           |           |
                         |     1     |     2     |
                         |           |           |
                         |           |           |
                         +-----------+-----------+

                    Figure 3: dual-view-crop video layout

   <dual-view-2x1/>:  layout presenting two streams, one above the
      other, in two regions as shown in Figure 4.  The MS MUST NOT alter
      the aspect ratio of each stream to fit its region, and hence the
      MS might need to blank out part of each region.

                               +-----------+
                               |           |
                               |           |
                               |     1     |
                               |           |
                               |           |
                               +-----------+
                               |           |
                               |           |
                               |     2     |
                               |           |
                               |           |
                               +-----------+

                     Figure 4: dual-view-2x1 video layout

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   <dual-view-2x1-crop/>:  layout presenting two streams one above the
      other in two regions as shown in Figure 5.  The MS MUST alter the
      aspect ratio of each stream to fit its region so that no blanking
      is required.

                               +-----------+
                               |           |
                               |           |
                               |     1     |
                               |           |
                               |           |
                               +-----------+
                               |           |
                               |           |
                               |     2     |
                               |           |
                               |           |
                               +-----------+

                  Figure 5: dual-view-2x1-crop video layout

   <quad-view/>:  layout presenting four equal-sized regions in a 2x2
      layout as shown in Figure 6.  Typically, the aspect ratio of the
      streams is preserved, so blanking is required.

                         +-----------+-----------+
                         |           |           |
                         |           |           |
                         |     1     |     2     |
                         |           |           |
                         |           |           |
                         +-----------+-----------+
                         |           |           |
                         |           |           |
                         |     3     |     4     |
                         |           |           |
                         |           |           |
                         +-----------+-----------+

                       Figure 6: quad-view video layout

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   <multiple-3x3/>:  layout presenting nine equal-sized regions in a 3x3
      layout as shown in Figure 7.  Typically, the aspect ratio of the
      streams is preserved, so blanking is required.

                   +-----------+-----------+-----------+
                   |           |           |           |
                   |           |           |           |
                   |     1     |     2     |     3     |
                   |           |           |           |
                   |           |           |           |
                   +-----------+-----------+-----------+
                   |           |           |           |
                   |           |           |           |
                   |     4     |     5     |     6     |
                   |           |           |           |
                   |           |           |           |
                   +-----------+-----------+-----------+
                   |           |           |           |
                   |           |           |           |
                   |    7      |     8     |     9     |
                   |           |           |           |
                   |           |           |           |
                   +-----------+-----------+-----------+

                     Figure 7: multiple-3x3 video layout

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   <multiple-4x4/>:  layout presenting 16 equal-sized regions in a 4x4
      layout as shown in Figure 8.  Typically, the aspect ratio of the
      streams is preserved, so blanking is required.

             +-----------+-----------+-----------+-----------+
             |           |           |           |           |
             |           |           |           |           |
             |     1     |     2     |     3     |     4     |
             |           |           |           |           |
             |           |           |           |           |
             +-----------+-----------+-----------+-----------+
             |           |           |           |           |
             |           |           |           |           |
             |     5     |     6     |     7     |     8     |
             |           |           |           |           |
             |           |           |           |           |
             +-----------+-----------+-----------+-----------+
             |           |           |           |           |
             |           |           |           |           |
             |     9     |    10     |    11     |    12     |
             |           |           |           |           |
             |           |           |           |           |
             +-----------+-----------+-----------+-----------+
             |           |           |           |           |
             |           |           |           |           |
             |    13     |    14     |    15     |    16     |
             |           |           |           |           |
             |           |           |           |           |
             +-----------+-----------+-----------+-----------+

                     Figure 8: multiple-4x4 video layout

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   <multiple-5x1/>:  layout presents a 5x1 layout as shown in Figure 9
      where one region will occupy 4/9 of the mixed video stream, while
      the others will each occupy 1/9 of the stream.  Typically, the
      aspect ratio of the streams is preserved, so blanking is required.

                   +-----------------------+-----------+
                   |                       |           |
                   |                       |           |
                   |                       |     2     |
                   |                       |           |
                   |                       |           |
                   |           1           +-----------+
                   |                       |           |
                   |                       |           |
                   |                       |     3     |
                   |                       |           |
                   |                       |           |
                   +-----------+-----------+-----------+
                   |           |           |           |
                   |           |           |           |
                   |    4      |     5     |     6     |
                   |           |           |           |
                   |           |           |           |
                   +-----------+-----------+-----------+

                     Figure 9: multiple-5x1 video layout

4.2.1.4.3.  <video-switch>

   The <video-switch> element describes the configuration of the
   conference policy for how participants' input video streams are
   assigned to regions within the active video layout.

   The <video-switch> element has the following child elements defined
   (one child occurrence only) to indicate the video-switching policy of
   the conference:

   <vas/>:  (Voice-Activated Switching) enables automatic display of the
      loudest speaker participant that is contributing both audio and
      video to the conference mix.  Participants who do not provide an
      audio stream are not considered for automatic display.  If a
      participant provides more than one audio stream, then the policy
      for inclusion of such a participant in the VAS is implementation-
      specific; an MS could select one stream, sum audio streams, or
      ignore the participant for VAS consideration.  If there is only
      one region in the layout, then the loudest speaker is displayed
      there.  If more than one region is available, then the loudest
      speaker is displayed in the largest region (if any), and then in

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      the first region from the top-left corner of the layout.  The MS
      assigns the remaining regions based on the priority mechanism
      described in Section 4.2.1.4.3.1.

   <controller/>:  enables manual control over video switching.  The
      controller AS determines how the regions are assigned based on an
      external floor control policy.  The MS receives <join>,
      <modifyjoin>, and <unjoin> commands with a <stream> element
      (Section 4.2.2.5) indicating the region where the stream is
      displayed.  If no explicit region is specified, the MS assigns the
      region based on the priority mechanism described in
      Section 4.2.1.4.3.1.

   An MS MAY support other video-switching policies.  Other policies
   MUST be specified using an element from a namespace other than the
   one used in this specification.  For example:

   <video-switch>
    <mypolicy xmlns='http://example.com/foo'/>
   </video-switch>

   The <video-switch> element has the following attributes:

   interval:  specifies the period between video switches as a number of
      seconds.  In the case of <vas/> policy, a speaker needs to be the
      loudest speaker for the interval before the switch takes place.  A
      valid value is a non-negative integer (see Section 4.7.2).  A
      value of 0 indicates that switching is applied immediately.  The
      attribute is optional.  The default value is 3 (seconds).

   activespeakermix:  indicates whether or not the active (loudest)
      speaker participant receives a video stream without themselves
      displayed in the case of the <vas/> switching policy.  If enabled,
      the MS needs to generate two video streams for each conference
      mix: one for the active speaker participant without themselves
      displayed (details of this video layout are implementation-
      specific) and one for other participants (as described in the
      <vas/> switching policy above).  A valid value is a boolean (see
      Section 4.7.1).  A value of "true" indicates that a separate video
      mix is generated for the active speaker without themselves being
      displayed.  A value of "false" indicates that all participants
      receive the same video mix.  The attribute is optional.  The
      default value is "false".  If the 'type' attribute is not set to
      <vas/>, the MS ignores this attribute.

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   If the MS does not support the specified video-switching policy or
   other configuration parameters (including separate active speaker
   video mixes), then the MS reports a 424 error (Section 4.6) in the
   response to the request element containing the <video-switch>
   element.

   If the MS receives a <join> or <modifyjoin> request containing a
   <stream> element (Section 4.2.2.5) that specifies a region and the
   conference video-switching policy is set to <vas/>, then the MS
   ignores the region (i.e., conference-switching policy takes
   precedence).

   If the MS receives a <join> or <modifyjoin> request containing a
   <stream> element (Section 4.2.2.5) specifying a region that is not
   defined for the currently active video layout, the MS MUST NOT report
   an error.  Even though the participant is not currently visible, the
   MS displays the participant if the layout changes to one that defines
   the specified region.

   For example, a fragment specifying a <vas/> video-switching policy
   with an interval of 2s

    <video-switch interval="2"><vas/></video-switch>

   For example, a fragment specifying a <controller/> video-switching
   policy where video switching takes place immediately is as follows:

    <video-switch interval="0"><controller/></video-switch>

4.2.1.4.3.1.  Priority Assignment

   In cases where the video-switching policy does not explicitly
   determine the region to which a participant is assigned, the
   following priority assignment mechanism applies:

   1.  Each participant has a (positive integer) priority value: the
       lower the value, the higher the priority.  The priority value is
       determined by the <priority> child element (Section 4.2.2.5.4) of
       <stream>.  If not explicitly specified, the default priority
       value is 100.

   2.  The MS uses priority values to assign participants to regions in
       the video layout which remain unfilled after application of the
       video-switching policy.  The MS MUST dedicate larger and/or more
       prominent portions of the layout to participants with higher
       priority values first (e.g., first, all participants with
       priority 1, then those with 2, 3, etc.).

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   3.  The policy for displaying participants with the same priority is
       implementation-specific.

   The MS applies this priority policy each time the video layout is
   changed or updated.  It is RECOMMENDED that the MS does not move a
   participant from one region to another unless required by the video-
   switching policy when an active video layout is updated.

   This model allows the MS to apply default video layouts after
   applying the video-switching policy.  For example, region 2 is
   statically assigned to Bob, so the priority mechanism only applies to
   regions 1, 3, 4, etc.

4.2.1.4.4.  <subscribe>

   The <subscribe> element is a container for specifying conference
   notification events to which a controlling entity subscribes.
   Notifications of conference events are delivered using the <event>
   element (see Section 4.2.4).

   The <subscribe> element has no attributes, but has the following
   child element:

   <active-talkers-sub>:  subscription to active talker events
      (Section 4.2.1.4.4.1).  The element is optional.

   The MS MUST support a <active-talkers-sub> subscription.  It MAY
   support other event subscriptions (specified using attributes and
   child elements from a foreign namespace).  If the MS does not support
   a subscription specified in a foreign namespace, it sends a
   <response> with a 428 status code (see Section 4.6).

4.2.1.4.4.1.  <active-talkers-sub>

   The <active-talkers-sub> element has the following attribute:

   interval:  the minimum amount of time (in seconds) that elapses
      before further active talker events can be generated.  A valid
      value is a non-negative integer (see Section 4.7.2).  A value of 0
      suppresses further notifications.  The attribute is optional.  The
      default value is 3 (seconds).

   The <active-talkers-sub> element has no child elements.

   Active talker notifications are delivered in the <active-talkers-
   notify> element (Section 4.2.4.1).


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