CLUE WG                                              M. Duckworth, Ed.
Internet Draft                                                  Polycom
Intended status: Standards Track                           A. Pepperell
Expires: April 19, 2014                                           Acano
                                                              S. Wenger
                                                       October 19, 2013

                Framework for Telepresence Multi-Streams


   This document defines a framework for a protocol to enable devices
   in a telepresence conference to interoperate.  The protocol enables
   communication of information about multiple media streams so a
   sending system and receiving system can make reasonable decisions
   about transmitting, selecting and rendering the media streams.
   This protocol is used in addition to SIP signaling for setting up a
   telepresence session.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in

   This Internet-Draft will expire on April 19, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with
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   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................3
   2. Terminology....................................................3
   3. Definitions....................................................4
   4. Overview & Motivation..........................................6
   5. Overview of the Framework/Model................................9
   6. Spatial Relationships.........................................15
   7. Media Captures and Capture Scenes.............................16
      7.1. Media Captures...........................................16
         7.1.1. Media Capture Attributes............................17
      7.2. Capture Scene............................................22
         7.2.1. Capture Scene attributes............................25
         7.2.2. Capture Scene Entry attributes......................25
      7.3. Simultaneous Transmission Set Constraints................26
   8. Encodings.....................................................28
      8.1. Individual Encodings.....................................28
      8.2. Encoding Group...........................................29
   9. Associating Captures with Encoding Groups.....................30
   10. Consumer's Choice of Streams to Receive from the Provider....31
      10.1. Local preference........................................33
      10.2. Physical simultaneity restrictions......................33
      10.3. Encoding and encoding group limits......................33
   11. Extensibility................................................34
   12. Examples - Using the Framework (Informative).................34
      12.1. Provider Behavior.......................................34
         12.1.1. Three screen Endpoint Provider.....................35
         12.1.2. Encoding Group Example.............................42
         12.1.3. The MCU Case.......................................42
      12.2. Media Consumer Behavior.................................43
         12.2.1. One screen Media Consumer..........................44
         12.2.2. Two screen Media Consumer configuring the example..44

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         12.2.3. Three screen Media Consumer configuring the example45
   13. Acknowledgements.............................................45
   14. IANA Considerations..........................................45
   15. Security Considerations......................................46
   16. Changes Since Last Version...................................46
   17. Authors' Addresses...........................................50

1. Introduction

   Current telepresence systems, though based on open standards such
   as RTP [RFC3550] and SIP [RFC3261], cannot easily interoperate with
   each other.  A major factor limiting the interoperability of
   telepresence systems is the lack of a standardized way to describe
   and negotiate the use of the multiple streams of audio and video
   comprising the media flows.  This document provides a framework for
   protocols to enable interoperability by handling multiple streams
   in a standardized way.  The framework is intended to support the
   use cases described in draft-ietf-clue-telepresence-use-cases and
   to meet the requirements in draft-ietf-clue-telepresence-

   The basic session setup for the use cases is based on SIP [RFC3261]
   and SDP offer/answer [RFC3264]. In addition to basic SIP & SDP
   offer/answer, CLUE specific signaling is required to exchange the
   information describing the multiple media streams. The motivation
   for this framework, an overview of the signaling, and information
   required to be exchanged is described in subsequent sections of
   this document.  The signaling details and data model are provided
   in subsequent documents.

2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC 2119

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3. Definitions

   The terms defined below are used throughout this document and
   companion documents and they are normative.  In order to easily
   identify the use of a defined term, those terms are capitalized.

   Advertisement: a CLUE message a Media Provider sends to a Media
   Consumer describing specific aspects of the content of the media,
   the formatting of the media streams it can send, and any
   restrictions it has in terms of being able to provide certain
   Streams simultaneously.

   Audio Capture: Media Capture for audio.  Denoted as ACn in the
   example cases in this document.

   Camera-Left and Right: For Media Captures, camera-left and camera-
   right are from the point of view of a person observing the rendered
   media.  They are the opposite of Stage-Left and Stage-Right.

   Capture: Same as Media Capture.

   Capture Device: A device that converts audio and video input into
   an electrical signal, in most cases to be fed into a media encoder.

   Capture Encoding: A specific encoding of a Media Capture, to be
   sent by a Media Provider to a Media Consumer via RTP.

   Capture Scene: a structure representing a spatial region containing
   one or more Capture Devices, each capturing media representing a
   portion of the region. The spatial region represented by a Capture
   Scene MAY or may not correspond to a real region in physical space,
   such as a room.  A Capture Scene includes attributes and one or
   more Capture Scene Entries, with each entry including one or more
   Media Captures.

   Capture Scene Entry: a list of Media Captures of the same media
   type that together form one way to represent the entire Capture

   Conference: used as defined in [RFC4353], A Framework for
   Conferencing within the Session Initiation Protocol (SIP).

   Configure Message: A CLUE message a Media Consumer sends to a Media
   Provider specifying which content and media streams it wants to

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   receive, based on the information in a corresponding Advertisement

   Consumer: short for Media Consumer.

   Encoding or Individual Encoding: a set of parameters representing a
   way to encode a Media Capture to become a Capture Encoding.

   Encoding Group: A set of encoding parameters representing a total
   media encoding capability to be sub-divided across potentially
   multiple Individual Encodings.

   Endpoint: The logical point of final termination through receiving,
   decoding and rendering, and/or initiation through capturing,
   encoding, and sending of media streams.  An endpoint consists of
   one or more physical devices which source and sink media streams,
   and exactly one [RFC4353] Participant (which, in turn, includes
   exactly one SIP User Agent).  Endpoints can be anything from
   multiscreen/multicamera rooms to handheld devices.

   Front: the portion of the room closest to the cameras.  In going
   towards back you move away from the cameras.

   MCU: Multipoint Control Unit (MCU) - a device that connects two or
   more endpoints together into one single multimedia conference
   [RFC5117].  An MCU includes an [RFC4353] like Mixer, without the
   [RFC4353] requirement to send media to each participant.

   Media: Any data that, after suitable encoding, can be conveyed over
   RTP, including audio, video or timed text.

   Media Capture: a source of Media, such as from one or more Capture
   Devices or constructed from other Media streams.

   Media Consumer: an Endpoint or middle box that receives Media

   Media Provider: an Endpoint or middle box that sends Media streams

   Model: a set of assumptions a telepresence system of a given vendor
   adheres to and expects the remote telepresence system(s) also to
   adhere to.

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   Plane of Interest: The spatial plane containing the most relevant
   subject matter.

   Provider: Same as Media Provider.

   Render: the process of generating a representation from a media,
   such as displayed motion video or sound emitted from loudspeakers.

   Simultaneous Transmission Set: a set of Media Captures that can be
   transmitted simultaneously from a Media Provider.

   Spatial Relation: The arrangement in space of two objects, in
   contrast to relation in time or other relationships.  See also
   Camera-Left and Right.

   Stage-Left and Right: For Media Captures, Stage-left and Stage-
   right are the opposite of Camera-left and Camera-right.  For the
   case of a person facing (and captured by) a camera, Stage-left and
   Stage-right are from the point of view of that person.

   Stream: a Capture Encoding sent from a Media Provider to a Media
   Consumer via RTP [RFC3550].

   Stream Characteristics: the media stream attributes commonly used
   in non-CLUE SIP/SDP environments (such as: media codec, bit rate,
   resolution, profile/level etc.) as well as CLUE specific
   attributes, such as the Capture ID or a spatial location.

   Video Capture: Media Capture for video.  Denoted as VCn in the
   example cases in this document.

   Video Composite: A single image that is formed, normally by an RTP
   mixer inside an MCU, by combining visual elements from separate

4. Overview & Motivation

   This section provides an overview of the functional elements
   defined in this document to represent a telepresence system.  The
   motivations for the framework described in this document are also

   Two key concepts introduced in this document are the terms "Media
   Provider" and "Media Consumer". A Media Provider represents the
   entity that is sending the media and a Media Consumer represents

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   the entity that is receiving the media. A Media Provider provides
   Media in the form of RTP packets, a Media Consumer consumes those
   RTP packets.  Media Providers and Media Consumers can reside in
   Endpoints or in middleboxes such as Multipoint Control Units
   (MCUs).  A Media Provider in an Endpoint is usually associated
   with the generation of media for Media Captures; these Media
   Captures are typically sourced from cameras, microphones, and the
   like.  Similarly, the Media Consumer in an Endpoint is usually
   associated with renderers, such as screens and loudspeakers.  In
   middleboxes, Media Providers and Consumers can have the form of
   outputs and inputs, respectively, of RTP mixers, RTP translators,
   and similar devices.  Typically, telepresence devices such as
   Endpoints and middleboxes would perform as both Media Providers
   and Media Consumers, the former being concerned with those
   devices' transmitted media and the latter with those devices'
   received media.  In a few circumstances, a CLUE Endpoint middlebox
   includes only Consumer or Provider functionality, such as
   recorder-type Consumers or webcam-type Providers.

   The motivations for the framework outlined in this document
   include the following:

   (1) Endpoints in telepresence systems typically have multiple Media
   Capture and Media Render devices, e.g., multiple cameras and
   screens. While previous system designs were able to set up calls
   that would capture media using all cameras and display media on all
   screens, for example, there is no mechanism that can associate
   these Media Captures with each other in space and time.

   (2) The mere fact that there are multiple capture and rendering
   devices, each of which may be configurable in aspects such as zoom,
   leads to the difficulty that a variable number of such devices can
   be used to capture different aspects of a region.  The Capture
   Scene concept allows for the description of multiple setups for
   those multiple capture devices that could represent sensible
   operation points of the physical capture devices in a room, chosen
   by the operator.  A Consumer can pick and choose from those
   configurations based on its rendering abilities and inform the
   Provider about its choices.  Details are provided in section 7.

   (3) In some cases, physical limitations or other reasons disallow
   the concurrent use of a device in more than one setup.  For
   example, the center camera in a typical three-camera conference
   room can set its zoom objective either to capture only the middle
   few seats, or all seats of a room, but not both concurrently.  The

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   Simultaneous Transmission Set concept allows a Provider to signal
   such limitations.  Simultaneous Transmission Sets are part of the
   Capture Scene description, and discussed in section 7.3.

   (4) Often, the devices in a room do not have the computational
   complexity or connectivity to deal with multiple encoding options
   simultaneously, even if each of these options is sensible in
   certain scenarios, and even if the simultaneous transmission is
   also sensible (i.e. in case of multicast media distribution to
   multiple endpoints).   Such constraints can be expressed by the
   Provider using the Encoding Group concept, described in section 8.

   (5) Due to the potentially large number of RTP flows required for a
   Multimedia Conference involving potentially many Endpoints, each of
   which can have many Media Captures and media renderers, it has
   become common to multiplex multiple RTP media flows onto the same
   transport address, so to avoid using the port number as a
   multiplexing point and the associated shortcomings such as
   NAT/firewall traversal.  While the actual mapping of those RTP
   flows to the header fields of the RTP packets is not subject of
   this specification, the large number of possible permutations of
   sensible options a Media Provider can make available to a Media
   Consumer makes a mechanism desirable that allows to narrow down the
   number of possible options that a SIP offer-answer exchange has to
   consider.  Such information is made available using protocol
   mechanisms specified in this document and companion documents,
   although it should be stressed that its use in an implementation is
   OPTIONAL.  Also, there are aspects of the control of both Endpoints
   and middleboxes/MCUs that dynamically change during the progress of
   a call, such as audio-level based screen switching, layout changes,
   and so on, which need to be conveyed.  Note that these control
   aspects are complementary to those specified in traditional SIP
   based conference management such as BFCP.  An exemplary call flow
   can be found in section 4.

   Finally, all this information needs to be conveyed, and the notion
   of support for it needs to be established.  This is done by the
   negotiation of a "CLUE channel", a data channel negotiated early
   during the initiation of a call.  An Endpoint or MCU that rejects
   the establishment of this data channel, by definition, is not
   supporting CLUE based mechanisms, whereas an Endpoint or MCU that
   accepts it is REQUIRED to use it to the extent specified in this
   document and its companion documents.

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5. Overview of the Framework/Model

   The CLUE framework specifies how multiple media streams are to be
   handled in a telepresence conference.

   A Media Provider (transmitting Endpoint or MCU) describes specific
   aspects of the content of the media and the formatting of the media
   streams it can send in an Advertisement; and the Media Consumer
   responds to the Media Provider by specifying which content and
   media streams it wants to receive in a Configure message.  The
   Provider then transmits the asked-for content in the specified

   This Advertisement and Configure MUST occur during call initiation
   but MAY also happen at any time throughout the call, whenever there
   is a change in what the Consumer wants to receive or (perhaps less
   common) the Provider can send.

   An Endpoint or MCU typically act as both Provider and Consumer at
   the same time, sending Advertisements and sending Configurations in
   response to receiving Advertisements.  (It is possible to be just
   one or the other.)

   The data model is based around two main concepts: a Capture and an
   Encoding.  A Media Capture (MC), such as audio or video, describes
   the content a Provider can send.  Media Captures are described in
   terms of CLUE-defined attributes, such as spatial relationships and
   purpose of the capture.  Providers tell Consumers which Media
   Captures they can provide, described in terms of the Media Capture

   A Provider organizes its Media Captures into one or more Capture
   Scenes, each representing a spatial region, such as a room.  A
   Consumer chooses which Media Captures it wants to receive from each
   Capture Scene.

   In addition, the Provider can send the Consumer a description of
   the Individual Encodings it can send in terms of the media
   attributes of the Encodings, in particular, audio and video
   parameters such as bandwidth, frame rate, macroblocks per second.
   Note that this is OPTIONAL, and intended to minimize the number of
   options a later SDP offer-answer would have to include in the SDP
   in case of complex setups, as should become clearer shortly when
   discussing an outline of the call flow.

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   The Provider can also specify constraints on its ability to provide
   Media, and a sensible design choice for a Consumer is to take these
   into account when choosing the content and Capture Encodings it
   requests in the later offer-answer exchange.  Some constraints are
   due to the physical limitations of devices--for example, a camera
   may not be able to provide zoom and non-zoom views simultaneously.
   Other constraints are system based, such as maximum bandwidth and
   maximum video coding performance measured in macroblocks/second.

   The following diagram illustrates the information contained in an

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   .  Provider Advertisement                                         .
   .                                                                 .
   .        +------------------------+   +--------------------+      .
   .        |       Capture Scene N  |   | Simultaneous       |      .
   .      +-+----------------------+ |   +--------------------+      .
   .      |       Capture Scene 2  | |                               .
   .    +-+----------------------+ | |      +----------------------+ .
   .    |  Capture Scene 1       | | |      |  Encoding Group N    | .
   .    |    +---------------+   | | |    +-+--------------------+ | .
   .    |    | Attributes    |   | | |    |   Encoding Group 2   | | .
   .    |    +---------------+   | | |  +-+--------------------+ | | .
   .    |                        | | |  |   Encoding Group 1   | | | .
   .    |    +----------------+  | | |  |     parameters       | | | .
   .    |    | E n t r i e s  |  | | |  |                      | | | .
   .    |    |  +---------+   |  | | |  | +-------------------+| | | .
   .    |    |  |Attribute|   |  | | |  | | V i d e o         || | | .
   .    |    |  +---------+   |  | | |  | | E n c o d i n g s || | | .
   .    |    |                |  | | |  | | Encoding 1        || | | .
   .    |    | Entry 1        |  | | |  | | (parameters)      || | | .
   .    |    |  (list of MCs) |  | |-+  | +-------------------+| | | .
   .    |    +----|-|--|------+  |-+    |                      | | | .
   .    +---------|-|--|---------+      | +-------------------+| | | .
   .              | |  |                | | A u d i o         || | | .
   .              | |  |                | | E n c o d i n g s || | | .
   .              v |  |                | | Encoding 1        || | | .
   .      +---------|--|--------+       | | (ID,maxBandwidth) || | | .
   .      | Media Capture N     |------>| +-------------------+| | | .
   .    +-+---------v--|------+ |       |                      | | | .
   .    | Media Capture 2     | |       |                      | |-+ .
   .  +-+--------------v----+ |-------->|                      | |   .
   .  | Media Capture  1    | | |       |                      |-+   .
   .  |  +----------------+ |---------->|                      |     .
   .  |  | Attributes     | | |_+       +----------------------+     .
   .  |  +----------------+ |_+                                      .
   .  +---------------------+                                        .
   .                                                                 .

   A very brief outline of the call flow used by a simple system (two
   Endpoints) in compliance with this document can be described as
   follows, and as shown in the following figure.

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         +-----------+                     +-----------+
         | Endpoint1 |                     | Endpoint2 |
         +----+------+                     +-----+-----+
              | INVITE (BASIC SDP+CLUECHANNEL)   |
              |    200 0K (BASIC SDP+CLUECHANNEL)|
              | ACK                              |
              |                                  |
              |     BASIC SDP MEDIA SESSION      |
              |                                  |
              |    CONNECT (CLUE CTRL CHANNEL)   |
              |            ...                   |
              |                                  |
              | ADVERTISEMENT 1                  |
              |                  ADVERTISEMENT 2 |
              |                                  |
              |                      CONFIGURE 1 |
              | CONFIGURE 2                      |
              |                                  |
              | REINVITE (UPDATED SDP)           |
              |              200 0K (UPDATED SDP)|
              | ACK                              |
              |                                  |
              |   UPDATED SDP MEDIA SESSION      |
              |                                  |
              v                                  v

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   An initial offer/answer exchange establishes a basic media session,
   for example audio-only, and a CLUE channel between two Endpoints.
   With the establishment of that channel, the endpoints have
   consented to use the CLUE protocol mechanisms and, therefore, MUST
   adhere to the CLUE protocol suite as outlined herein.

   Over this CLUE channel, the Provider in each Endpoint conveys its
   characteristics and capabilities by sending an Advertisement as
   specified herein.  The Advertisement is typically not sufficient to
   set up all media.  The Consumer in the Endpoint receives the
   information provided by the Provider, and can use it for two
   purposes.  First, it MUST construct and send a CLUE Configure
   message to tell the Provider what the Consumer wishes to receive.
   Second, it MAY, but is not necessarily REQUIRED to, use the
   information provided to tailor the SDP it is going to send during
   the following SIP offer/answer exchange, and its reaction to SDP it
   receives in that step.  It is often a sensible implementation
   choice to do so, as the representation of the media information
   conveyed over the CLUE channel can dramatically cut down on the
   size of SDP messages used in the O/A exchange that follows.
   Spatial relationships associated with the Media can be included in
   the Advertisement, and it is often sensible for the Media Consumer
   to take those spatial relationships into account when tailoring the

   This CLUE exchange MUST be followed by an SDP offer answer exchange
   that not only establishes those aspects of the media that have not
   been "negotiated" over CLUE, but has also the side effect of
   setting up the media transmission itself, involving potentially
   security exchanges, ICE, and whatnot.  This step is plain vanilla
   SIP, with the exception that the SDP used herein, in most (but not
   necessarily all) cases can be considerably smaller than the SDP a
   system would typically need to exchange if there were no pre-
   established knowledge about the Provider and Consumer
   characteristics.  (The need for cutting down SDP size is not quite
   obvious for a point-to-point call involving simple endpoints;
   however, when considering a large multipoint conference involving
   many multi-screen/multi-camera endpoints, each of which can operate
   using multiple codecs for each camera and microphone, it becomes
   perhaps somewhat more intuitive.)

   During the lifetime of a call, further exchanges MAY occur over the
   CLUE channel.  In some cases, those further exchanges lead to a
   modified system behavior of Provider or Consumer (or both) without
   any other protocol activity such as further offer/answer exchanges.

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   For example, voice-activated screen switching, signaled over the
   CLUE channel, ought not to lead to heavy-handed mechanisms like SIP
   re-invites.  However, in other cases, after the CLUE negotiation an
   additional offer/answer exchange becomes necessary.  For example,
   if both sides decide to upgrade the call from a single screen to a
   multi-screen call and more bandwidth is required for the additional
   video channels compared to what was previously negotiated using
   offer/answer, a new O/A exchange is REQUIRED.

   Numerous optimizations are possible, and are the implementer's
   choice.  For example, it can be sensible to establish one or more
   initial media channels during the initial offer/answer exchange,
   which would allow, for example, for a fast startup of audio.
   Depending on the system design, it can be possible to re-use this
   established channel for more advanced media negotiated only by CLUE
   mechanisms, thereby avoiding further offer/answer exchanges.

   Edt. note: The editors are not sure whether the mentioned
   overloading of established RTP channels using only CLUE messages is
   possible, or desired by the WG.  If it were, certainly there is
   need for specification work.  One possible issue: a Provider which
   thinks that it can switch, say, a audio codec algorithm by CLUE
   only, talks to a Consumer which thinks that it has to faithfully
   answer the Providers Advertisement through a Configure, but does
   not dare setting up its internal resource until such time it has
   received its authoritative O/A exchange.  Working group input is

   One aspect of the protocol outlined herein and specified in more
   detail in companion documents is that it makes available
   information regarding the Provider's capabilities to deliver Media,
   and attributes related to that Media such as their spatial
   relationship, to the Consumer.  The operation of the renderer
   inside the Consumer is unspecified in that it can choose to ignore
   some information provided by the Provider, and/or not render media
   streams available from the Provider (although it MUST follow the
   CLUE protocol and, therefore, MUST gracefully receive and respond
   (through a Configure) to the Provider's information).  All CLUE
   protocol mechanisms are OPTIONAL in the Consumer in the sense that,
   while the Consumer MUST be able to receive (and, potentially,
   gracefully acknowledge) CLUE messages, it is free to ignore the

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   information provided therein.  Obviously, this is not a
   particularly sensible design choice in almost all conceivable

   A CLUE-implementing device interoperates with a device that does
   not support CLUE, because the non-CLUE device does, by definition,
   not understand the offer of a CLUE channel in the initial
   offer/answer exchange and, therefore, will reject it. This
   rejection MUST be used as the indication to the CLUE-implementing
   device that the other side of the communication is not compliant
   with CLUE, and to fall back to behavior that does not require CLUE.

   As for the media, Provider and Consumer have an end-to-end
   communication relationship with respect to (RTP transported) media;
   and the mechanisms described herein and in companion documents do
   not change the aspects of setting up those RTP flows and sessions.
   In other words, the RTP media sessions conform to the negotiated
   SDP whether or not CLUE is used.

   Edt. note (StW): what's written below is likely correct, but is not
   the result of the introduction of CLUE, but rather the result of a
   generational overhaul of RTP usage that would have happened with or
   without CLUE.  Suggest to delete the sentences below until begin of
   section 6.  Is having a CLUE RTP Mapping document still the plan?
   If yes, we should have a real draft and a real reference.

   However, some form of RTP multiplexing is likely to be used by CLUE
   devices.  More information about relating RTP flows to CLUE
   entities is in the CLUE RTP Mapping document.

6. Spatial Relationships

   In order for a Consumer to perform a proper rendering, it is often
   necessary or at least helpful for the Consumer to have received
   spatial information about the streams it is receiving.  CLUE
   defines a coordinate system that allows Media Providers to describe
   the spatial relationships of their Media Captures to enable proper
   scaling and spatially sensible rendering of their streams.  The
   coordinate system is based on a few principles:

   o  Simple systems which do not have multiple Media Captures to
      associate spatially need not use the coordinate model.

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   o  Coordinates can either be in real, physical units (millimeters),
      have an unknown scale or have no physical scale.  Systems which
      know their physical dimensions (for example professionally
      installed Telepresence room systems) MUST always provide those
      real-world measurements.  Systems which don't know specific
      physical dimensions but still know relative distances MUST use
      'unknown scale'.  'No scale' is intended to be used where Media
      Captures from different devices (with potentially different
      scales) will be forwarded alongside one another (e.g. in the
      case of a middle box).

      *  "millimeters" means the scale is in millimeters

      *  "Unknown" means the scale is not necessarily millimeters, but
         the scale is the same for every Capture in the Capture Scene.

      *  "No Scale" means the scale could be different for each
         capture- an MCU provider that advertises two adjacent
         captures and picks sources (which can change quickly) from
         different endpoints might use this value; the scale could be
         different and changing for each capture.  But the areas of
         capture still represent a spatial relation between captures.

   o  The coordinate system is Cartesian X, Y, Z with the origin at a
      spatial location of the provider's choosing.  The Provider MUST
      use the same coordinate system with same scale and origin for
      all coordinates within the same Capture Scene.

   The direction of increasing coordinate values is:
   X increases from Camera-Left to Camera-Right
   Y increases from Front to back
   Z increases from low to high (i.e. floor to ceiling)

7. Media Captures and Capture Scenes

   This section describes how Providers can describe the content of
   media to Consumers.

7.1. Media Captures

   Media Captures are the fundamental representations of streams that
   a device can transmit.  What a Media Capture actually represents is

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   o  It can represent the immediate output of a physical source (e.g.
      camera, microphone) or 'synthetic' source (e.g. laptop computer,
      DVD player).

   o  It can represent the output of an audio mixer or video composer

   o  It can represent a concept such as 'the loudest speaker'

   o  It can represent a conceptual position such as 'the leftmost

   To identify and distinguish between multiple instances, video and
   audio captures are labeled.  For instance: VC1, VC2 and AC1, AC2,
   where  VC1 and VC2 refer to two different video captures and AC1
   and AC2 refer to two different audio captures.

   Some key points about Media Captures:

     . A Media Capture is of a single media type (e.g. audio or
     . A Media Capture is associated with exactly one Capture Scene
     . A Media Capture is associated with one or more Capture Scene
     . A Media Capture has exactly one set of spatial information
     . A Media Capture can be the source of one or more Capture

   Each Media Capture can be associated with attributes to describe
   what it represents.

7.1.1. Media Capture Attributes

   Media Capture Attributes describe information about the Captures.
   A Provider can use the Media Capture Attributes to describe the
   Captures for the benefit of the Consumer in the Advertisement
   message.  Media Capture Attributes include:

     . spatial information, such as point of capture, point on line
        of capture, and area of capture, all of which, in combination
        define the capture field of, for example, a camera;
     . Capture multiplexing information (composed/switched video,
        mono/stereo audio, maximum number of simultaneous encodings
        per Capture and so on); and

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     . Other descriptive information to help the Consumer choose
        between captures (description, presentation, view, priority,
        language, role).
     . Control information for use inside the CLUE protocol suite.

   Point of Capture:

   A field with a single Cartesian (X, Y, Z) point value which
   describes the spatial location of the capturing device (such as

   Point on Line of Capture:

   A field with a single Cartesian (X, Y, Z) point value which
   describes a position in space of a second point on the axis of the
   capturing device; the first point being the Point of Capture (see

   Together, the Point of Capture and Point on Line of Capture define
   an axis of the capturing device, for example the optical axis of a
   camera.  The Media Consumer can use this information to adjust how
   it renders the received media if it so chooses.

   Area of Capture:

   A field with a set of four (X, Y, Z) points as a value which
   describe the spatial location of what is being "captured".  By
   comparing the Area of Capture for different Media Captures within
   the same Capture Scene a consumer can determine the spatial
   relationships between them and render them correctly.

   The four points MUST be co-planar, forming a quadrilateral, which
   defines the Plane of Interest for the particular media capture.

   If the Area of Capture is not specified, it means the Media Capture
   is not spatially related to any other Media Capture.

   For a switched capture that switches between different sections
   within a larger area, the area of capture MUST use coordinates for
   the larger potential area.

   Mobility of Capture:

   This attribute indicates whether or not the point of capture, line
   on point of capture, and area of capture values stay the same over

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   time, or are expected to change (potentially frequently).  Possible
   values are static, dynamic, and highly dynamic.

   An example for "dynamic" is a camera mounted on a stand which is
   occasionally hand-carried and placed at different positions in
   order to provide the best angle to capture a work task.  A camera
   worn by a participant who moves around the room is an example for
   "highly dynamic". In either case, the effect is that the capture
   point, capture axis and area of capture change with time.

   The capture point of a static capture MUST NOT move for the life of
   the conference. The capture point of dynamic captures is
   categorized by a change in position followed by a reasonable period
   of stability--in the order of magnitude of minutes. High dynamic
   captures are categorized by a capture point that is constantly
   moving.  If the "area of capture", "capture point" and "line of
   capture" attributes are included with dynamic or highly dynamic
   captures they indicate spatial information at the time of the


   A boolean field which indicates whether or not the Media Capture is
   a mix (audio) or composition (video) of streams.

   This attribute is useful for a media consumer to avoid nesting a
   composed video capture into another composed capture or rendering.
   This attribute is not intended to describe the layout a media
   provider uses when composing video streams.


   A boolean field which indicates whether or not the Media Capture
   represents the (dynamic) most appropriate subset of a 'whole'.
   What is 'most appropriate' is up to the provider and could be the
   active speaker, a lecturer or a VIP.

   Audio Channel Format:

   A field with enumerated values which describes the method of
   encoding used for audio. A value of 'mono' means the Audio Capture
   has one channel.  'stereo' means the Audio Capture has two audio
   channels, left and right.

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   This attribute applies only to Audio Captures.  A single stereo
   capture is different from two mono captures that have a left-right
   spatial relationship.  A stereo capture maps to a single Capture
   Encoding, while each mono audio capture maps to a separate Capture

   Max Capture Encodings:

   An optional attribute indicating the maximum number of Capture
   Encodings that can be simultaneously active for the Media Capture.
   The number of simultaneous Capture Encodings is also limited by the
   restrictions of the Encoding Group for the Media Capture.


   Human-readable description of the Capture, which could be in
   multiple languages.


   This attribute indicates that the capture originates from a
   presentation device, that is one that provides supplementary
   information to a conference through slides, video, still images,
   data etc.  Where more information is known about the capture it MAY
   be expanded hierarchically to indicate the different types of
   presentation media, e.g. presentation.slides, presentation.image

   Note: It is expected that a number of keywords will be defined that
   provide more detail on the type of presentation.


   A field with enumerated values, indicating what type of view the
   capture relates to.  The Consumer can use this information to help
   choose which Media Captures it wishes to receive.  The value MUST
   be one of:

   Room - Captures the entire scene

   Table - Captures the conference table with seated participants

   Individual - Captures an individual participant

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   Lectern - Captures the region of the lectern including the
   presenter, for example in a classroom style conference room

   Audience - Captures a region showing the audience in a classroom
   style conference room


   This attribute indicates one or more languages used in the content
   of the media capture.  Captures MAY be offered in different
   languages in case of multilingual and/or accessible conferences.  A
   Consumer can use this attribute to differentiate between them and
   pick the appropriate one.

   Note that the Language atttribute is defined and meaningful both
   for audio and video captures.  In case of audio captures, the
   meaning is obvious.  For a video capture, "Language" could, for
   example, be sign interpretation or text.


   Edt. Note -- this is a placeholder for a role attribute, as
   discussed in draft-groves-clue-capture-attr.  We expect to continue
   discussing the role attribute in the context of that draft, and
   follow-on drafts, before adding it to this framework document.


   This attribute indicates a relative priority between different
   Media Captures.  The Provider sets this priority, and the Consumer
   MAY use the priority to help decide which captures it wishes to

   The "priority" attribute is an integer which indicates a relative
   priority between captures. For example it is possible to assign a
   priority between two presentation captures that would allow a
   remote endpoint to determine which presentation is more important.
   Priority is assigned at the individual capture level. It represents
   the Provider's view of the relative priority between captures with
   a priority. The same priority number MAY be used across multiple
   captures. It indicates they are equally important. If no priority
   is assigned no assumptions regarding relative important of the
   capture can be assumed.

   Embedded Text:

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   This attribute indicates that a capture provides embedded textual
   information. For example the video capture MAY contain speech to
   text information composed with the video image. This attribute is
   only applicable to video captures and presentation streams with
   visual information.

   Related To:

   This attribute indicates the capture contains additional
   complementary information related to another capture.  The value
   indicates the other capture to which this capture is providing
   additional information.

   For example, a conferences can utilize translators or facilitators
   that provide an additional audio stream (i.e. a translation or
   description or commentary of the conference).  Where multiple
   captures are available, it may be advantageous for a Consumer to
   select a complementary capture instead of or in addition to a
   capture it relates to.

7.2. Capture Scene

   In order for a Provider's individual Captures to be used
   effectively by a Consumer, the provider organizes the Captures into
   one or more Capture Scenes, with the structure and contents of
   these Capture Scenes being sent from the Provider to the Consumer
   in the Advertisement.

   A Capture Scene is a structure representing a spatial region
   containing one or more Capture Devices, each capturing media
   representing a portion of the region.  A Capture Scene includes one
   or more Capture Scene entries, with each entry including one or
   more Media Captures.  A Capture Scene represents, for example, the
   video image of a group of people seated next to each other, along
   with the sound of their voices, which could be represented by some
   number of VCs and ACs in the Capture Scene Entries.  A middle box
   can also describe in Capture Scenes what it constructs from media
   Streams it receives.

   A Provider MAY advertise one or more Capture Scenes .  What
   constitutes an entire Capture Scene is up to the Provider.  A
   simple Provider might typically use one Capture Scene for
   participant media (live video from the room cameras) and another
   Capture Scene for a computer generated presentation.  In more
   complex systems, the use of additional Capture Scenes is also

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   sensible.  For example, a classroom may advertise two Capture
   Scenes involving live video, one including only the camera
   capturing the instructor (and associated audio), the other
   including camera(s) capturing students (and associated audio).

   A Capture Scene MAY (and typically will) include more than one type
   of media.  For example, a Capture Scene can include several Capture
   Scene Entries for Video Captures, and several Capture Scene Entries
   for Audio Captures.  A particular Capture MAY be included in more
   than one Capture Scene Entry.

   A provider MAY express spatial relationships between Captures that
   are included in the same Capture Scene.  However, there is not
   necessarily the same spatial relationship between Media Captures
   that are in different Capture Scenes.  In other words, Capture
   Scenes can use their own spatial measurement system as outlined
   above in section 6.

   A Provider arranges Captures in a Capture Scene to help the
   Consumer choose which captures it wants to render.  The Capture
   Scene Entries in a Capture Scene are different alternatives the
   Provider is suggesting for representing the Capture Scene.  The
   order of Capture Scene Entries within a Capture Scene has no
   significance.  The Media Consumer can choose to receive all Media
   Captures from one Capture Scene Entry for each media type (e.g.
   audio and video), or it can pick and choose Media Captures
   regardless of how the Provider arranges them in Capture Scene
   Entries.  Different Capture Scene Entries of the same media type
   are not necessarily mutually exclusive alternatives.  Also note
   that the presence of multiple Capture Scene Entries (with
   potentially multiple encoding options in each entry) in a given
   Capture Scene does not necessarily imply that a Provider is able to
   serve all the associated media simultaneously (although the
   construction of such an over-rich Capture Scene is probably not
   sensible in many cases).  What a Provider can send simultaneously
   is determined through the Simultaneous Transmission Set mechanism,
   described in section 7.3.

   Captures within the same Capture Scene entry MUST be of the same
   media type - it is not possible to mix audio and video captures in
   the same Capture Scene Entry, for instance.  The Provider MUST be
   capable of encoding and sending all Captures in a single Capture
   Scene Entry simultaneously.  The order of Captures within a Capture
   Scene Entry has no significance.  A Consumer can decide to receive
   all the Captures in a single Capture Scene Entry, but a Consumer

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   could also decide to receive just a subset of those captures.  A
   Consumer can also decide to receive Captures from different Capture
   Scene Entries, all subject to the constraints set by Simultaneous
   Transmission Sets, as discussed in section 7.3.

   When a Provider advertises a Capture Scene with multiple entries,
   it is essentially signaling that there are multiple representations
   of the same Capture Scene available.  In some cases, these multiple
   representations would typically be used simultaneously (for
   instance a "video entry" and an "audio entry").  In some cases the
   entries would conceptually be alternatives (for instance an entry
   consisting of three Video Captures covering the whole room versus
   an entry consisting of just a single Video Capture covering only
   the center if a room).  In this latter example, one sensible choice
   for a Consumer would be to indicate (through its Configure and
   possibly through an additional offer/answer exchange) the Captures
   of that Capture Scene Entry that most closely matched the
   Consumer's number of display devices or screen layout.

   The following is an example of 4 potential Capture Scene Entries
   for an endpoint-style Provider:

   1.  (VC0, VC1, VC2) - left, center and right camera Video Captures

   2.  (VC3) - Video Capture associated with loudest room segment

   3.  (VC4) - Video Capture zoomed out view of all people in the room

   4.  (AC0) - main audio

   The first entry in this Capture Scene example is a list of Video
   Captures which have a spatial relationship to each other.
   Determination of the order of these captures (VC0, VC1 and VC2) for
   rendering purposes is accomplished through use of their Area of
   Capture attributes.  The second entry (VC3) and the third entry
   (VC4) are alternative representations of the same room's video,
   which might be better suited to some Consumers' rendering
   capabilities.  The inclusion of the Audio Capture in the same
   Capture Scene indicates that AC0 is associated with all of those
   Video Captures, meaning it comes from the same spatial region.
   Therefore, if audio were to be rendered at all, this audio would be
   the correct choice irrespective of which Video Captures were

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7.2.1. Capture Scene attributes

   Capture Scene Attributes can be applied to Capture Scenes as well
   as to individual media captures.  Attributes specified at this
   level apply to all constituent Captures.  Capture Scene attributes

     . Human-readable description of the Capture Scene, which could
        be in multiple languages;
     . Scale information (millimeters, unknown, no scale), as
        described in Section 5.

7.2.2. Capture Scene Entry attributes

   A Capture Scene can include one or more Capture Scene Entries in
   addition to the Capture Scene wide attributes described above.
   Capture Scene Entry attributes apply to the Capture Scene Entry as
   a whole, i.e. to all Captures that are part of the Capture Scene

   Capture Scene Entry attributes include:

     . Human-readable description of the Capture Scene Entry, which
        could be in multiple languages;
     . Scene-switch-policy: {site-switch, segment-switch}

   A media provider uses this scene-switch-policy attribute to
   indicate its support for different switching policies.  If a
   provider supports both policies, it MAY advertise separate Capture
   Scene Entries containing separate Captures, each entry with a
   separate scene-switch-policy value.  If the provider does not
   support any of these policies, it MUST omit this attribute.

   The "site-switch" policy means all captures are switched at the
   same time to keep captures from the same endpoint site together.
   Let's say the speaker is at site A and everyone else is at a
   "remote" site.

   When the room at site A shown, all the camera images from site A
   are forwarded to the remote sites.  Therefore at each receiving
   remote site, all the screens display camera images from site A.

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   This can be used to preserve full size image display, and also
   provide full visual context of the displayed far end, site A. In
   site switching, there is a fixed relation between the cameras in
   each room and the displays in remote rooms.  The room or
   participants being shown can be switched from time to time based
   on, for example, who is speaking or by manual control.

   The "segment-switch" policy means different captures can switch at
   different times, and can be coming from different endpoints.  Still
   using site A as where the speaker is, and "remote" to refer to all
   the other sites, in segment switching, rather than sending all the
   images from site A, only the image containing the speaker at site A
   is shown.  The camera images of the current speaker and previous
   speakers (if any) are forwarded to the other sites in the

   Therefore the screens in each site are usually displaying images
   from different remote sites - the current speaker at site A and the
   previous ones.  This strategy can be used to preserve full size
   image display, and also capture the non-verbal communication
   between the speakers.  In segment switching, the display depends on
   the activity in the remote rooms - generally, but not necessarily
   based on audio / speech detection.

7.3. Simultaneous Transmission Set Constraints

   In many practical cases, a Provider has constraints or limitations
   on its ability to send Captures simultaneously.  One type of
   limitation is caused by the physical limitations of capture
   mechanisms; these constraints are represented by a simultaneous
   transmission set.  The second type of limitation reflects the
   encoding resources available, such as bandwidth or video encoding
   throughput (macroblocks/second).  This type of constraint is
   captured by encoding groups, discussed below.

   Some Endpoints or MCUs can send multiple Captures simultaneously,
   however sometimes there are constraints that limit which Captures
   can be sent simultaneously with other Captures.  A device may not
   be able to be used in different ways at the same time.  Provider
   Advertisements are made so that the Consumer can choose one of
   several possible mutually exclusive usages of the device.  This
   type of constraint is expressed in a Simultaneous Transmission Set,
   which lists all the Captures of a particular media type (e.g.
   audio, video, text) that can be sent at the same time.  There are

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   different Simultaneous Transmission Sets for each media type in the
   Advertisement.  This is easier to show in an example.

   Consider the example of a room system where there are three cameras
   each of which can send a separate capture covering two persons
   each- VC0, VC1, VC2.  The middle camera can also zoom out (using an
   optical zoom lens) and show all six persons, VC3.  But the middle
   camera cannot be used in both modes at the same time - it has to
   either show the space where two participants sit or the whole six
   seats, but not both at the same time.  As a result, VC1 and VC3
   cannot be sent simultaneously.

   Simultaneous transmission sets are expressed as sets of the Media
   Captures that the Provider could transmit at the same time (though,
   in some cases, it is not intuitive to do so).  In this example the
   two simultaneous sets are shown in Table 1.  If a Provider
   advertises one or more mutually exclusive Simultaneous Transmission
   Sets, then for each media type the Consumer MUST ensure that it
   chooses Media Captures that lie wholly within one of those
   Simultaneous Transmission Sets.

                           | Simultaneous Sets |
                           | {VC0, VC1, VC2}   |
                           | {VC0, VC3, VC2}   |

                Table 1: Two Simultaneous Transmission Sets

   A Provider OPTIONALLY can include the simultaneous sets in its
   provider Advertisement.  These simultaneous set constraints apply
   across all the Capture Scenes in the Advertisement.  It is a syntax
   conformance requirement that the simultaneous transmission sets
   MUST allow all the media captures in any particular Capture Scene
   Entry to be used simultaneously.

   For shorthand convenience, a Provider MAY describe a Simultaneous
   Transmission Set in terms of Capture Scene Entries and Capture
   Scenes.  If a Capture Scene Entry is included in a Simultaneous
   Transmission Set, then all Media Captures in the Capture Scene
   Entry are included in the Simultaneous Transmission Set.  If a
   Capture Scene is included in a Simultaneous Transmission Set, then
   all its Capture Scene Entries (of the corresponding media type) are

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   included in the Simultaneous Transmission Set.  The end result
   reduces to a set of Media Captures in either case.

   If an Advertisement does not include Simultaneous Transmission
   Sets, then the Provider MUST be able to provide all Capture Scenes
   simultaneously.  If multiple capture Scene Entries are in a Capture
   Scene then the Consumer chooses at most one Capture Scene Entry per
   Capture Scene for each media type.

   If an Advertisement includes multiple Capture Scene Entries in a
   Capture Scene then the Consumer MAY choose one Capture Scene Entry
   for each media type, or MAY choose individual Captures based on the
   Simultaneous Transmission Sets.

8. Encodings

   Individual encodings and encoding groups are CLUE's mechanisms
   allowing a Provider to signal its limitations for sending Captures,
   or combinations of Captures, to a Consumer.  Consumers can map the
   Captures they want to receive onto the Encodings, with encoding
   parameters they want.  As for the relationship between the CLUE-
   specified mechanisms based on Encodings and the SIP Offer-Answer
   exchange, please refer to section 4.

8.1. Individual Encodings

   An Individual Encoding represents a way to encode a Media Capture
   to become a Capture Encoding, to be sent as an encoded media stream
   from the Provider to the Consumer.  An Individual Encoding has a
   set of parameters characterizing how the media is encoded.

   Different media types have different parameters, and different
   encoding algorithms may have different parameters.  An Individual
   Encoding can be assigned to at most one Capture Encoding at any
   given time.

   The parameters of an Individual Encoding represent the maximum
   values for certain aspects of the encoding.  A particular
   instantiation into a Capture Encoding MAY use lower values than
   these maximums if that is applicable for the media in question.
   For example, most video codec specifications require a conformant
   decoder to decode resolutions and frame rates smaller than what has
   been negotiated as a maximum, so downgrading the CLUE maximum
   values for macroblocks/second is appropriate.  On the other hand,
   downgrading the sample rate of G.711 audio below 8kHz is not

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   specified in G.711 and therefore not applicable in the sense
   described here.

   Individual Encoding parameters are represented in SDP [RFC4566],
   not in CLUE messages.  For example, for a video encoding using
   H.26x compression technologies, this can include parameters such

     . Maximum bandwidth;
     . Maximum picture size in pixels;
     . Maxmimum number of pixels to be processed per second;

   The bandwidth parameter is the only one that specifically relates
   to a CLUE Advertisement, as it can be further constrained by the
   maximum group bandwidth in an Encoding Group.

8.2. Encoding Group

   An Encoding Group includes a set of one or more Individual
   Encodings, and parameters that apply to the group as a whole.  By
   grouping multiple individual Encodings together, an Encoding Group
   describes additional constraints on bandwidth for the group.

   The Encoding Group data structure contains:

     . Maximum bitrate for all encodings in the group combined;
     . A list of identifiers for audio and video encodings,
        respectively, belonging to the group.

   When the Individual Encodings in a group are instantiated into
   Capture Encodings, each Capture Encoding has a bitrate that MUST be
   less than or equal to the max bitrate for the particular individual
   encoding.  The "maximum bitrate for all encodings in the group"
   parameter gives the additional restriction that the sum of all the
   individual capture encoding bitrates MUST be less than or equal to
   the this group value.

   The following diagram illustrates one example of the structure of a
   media provider's Encoding Groups and their contents.

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   |             Media Provider                      |
   |                                                 |
   |  ,--------------------------------------.       |
   |  | ,--------------------------------------.     |
   |  | | ,--------------------------------------.   |
   |  | | |          Encoding Group              |   |
   |  | | | ,-----------.                        |   |
   |  | | | |           | ,---------.            |   |
   |  | | | |           | |         | ,---------.|   |
   |  | | | | Encoding1 | |Encoding2| |Encoding3||   |
   |  `.| | |           | |         | `---------'|   |
   |    `.| `-----------' `---------'            |   |
   |      `--------------------------------------'   |

                    Figure 1: Encoding Group Structure

   A Provider advertises one or more Encoding Groups.  Each Encoding
   Group includes one or more Individual Encodings.  Each Individual
   Encoding can represent a different way of encoding media.  For
   example one Individual Encoding may be 1080p60 video, another could
   be 720p30, with a third being CIF, all in, for example, H.264

   While a typical three codec/display system might have one Encoding
   Group per "codec box" (physical codec, connected to one camera and
   one screen), there are many possibilities for the number of
   Encoding Groups a Provider may be able to offer and for the
   encoding values in each Encoding Group.

   There is no requirement for all Encodings within an Encoding Group
   to be instantiated at the same time.

9. Associating Captures with Encoding Groups

   Every Capture MUST be associated with at least one Encoding Group,
   which is used to instantiate that Capture into one or more Capture
   Encodings.  More than one Capture MAY use the same Encoding Group.

   The maximum number of streams that can result from a particular
   Encoding Group constraint is equal to the number of individual
   Encodings in the group.  The actual number of Capture Encodings
   used at any time MAY be less than this maximum.  Any of the

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   Captures that use a particular Encoding Group can be encoded
   according to any of the Individual Encodings in the group.  If
   there are multiple Individual Encodings in the group, then the
   Consumer can configure the Provider, via a Configure message, to
   encode a single Media Capture into multiple different Capture
   Encodings at the same time, subject to the Max Capture Encodings
   constraint, with each capture encoding following the constraints of
   a different Individual Encoding.

   It is a protocol conformance requirement that the Encoding Groups
   MUST allow all the Captures in a particular Capture Scene Entry to
   be used simultaneously.

10. Consumer's Choice of Streams to Receive from the Provider

   After receiving the Provider's Advertisement message (that includes
   media captures and associated constraints), the Consumer composes
   its reply to the Provider in the form of a Configure message.  The
   Consumer is free to use the information in the Advertisement as it
   chooses, but there are a few obviously sensible design choices,
   which are outlined below.

   If multiple Providers connect to the same Consumer (i.e. in a n
   MCU-less multiparty call), it is the responsibility of the Consumer
   to compose Configures for each Provider that both fulfill each
   Provider's constraints as expressed in the Advertisement, as well
   as its own capabilities.

   In an MCU-based multiparty call, the MCU can logically terminate
   the Advertisement/Configure negotiation in that it can hide the
   characteristics of the receiving endpoint and rely on its own
   capabilities (transcoding/transrating/...) to create Media Streams
   that can be decoded at the Endpoint Consumers.  The timing of an
   MCU's sending of Advertisements (for its outgoing ports) and
   Configures (for its incoming ports, in response to Advertisements
   received there) is up to the MCU and implementation dependent.

   As a general outline, A Consumer can choose, based on the
   Advertisement it has received, which Captures it wishes to receive,
   and which Individual Encodings it wants the Provider to use to
   encode the Captures.  Each Capture has an Encoding Group ID
   attribute which specifies which Individual Encodings are available
   to be used for that Capture.

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   A Configure Message includes a list of Capture Encodings.  These
   are the Capture Encodings the Consumer wishes to receive from the
   Provider.  Each Capture Encoding refers to one Media Capture, one
   Individual Encoding, and includes the encoding parameter values.  A
   Configure Message does not include references to Capture Scenes or
   Capture Scene Entries.

   For each Capture the Consumer wants to receive, it configures one
   or more of the encodings in that capture's encoding group.  The
   Consumer does this by telling the Provider, in its Configure
   Message, parameters such as the resolution, frame rate, bandwidth,
   etc. for each Capture Encodings for its chosen Captures.  Upon
   receipt of this Configure from the Consumer, common knowledge is
   established between Provider and Consumer regarding sensible
   choices for the media streams and their parameters.  The setup of
   the actual media channels, at least in the simplest case, is left
   to a following offer-answer exchange.  Optimized implementations
   MAY speed up the reaction to the offer-answer exchange by reserving
   the resources at the time of finalization of the CLUE handshake.

   Edt. Note (StW): is the sentence below still correct?

   Even more advanced devices MAY choose to establish media streams
   without an offer-answer exchange, for example by overloading
   existing 5 tuple connections with the negotiated media.

   In order to meaningfully create and send an initial Configure, the
   Consumer needs to have received at least one Advertisement from the

   In addition, the Consumer can send a Configure at any time during
   the call.  The Configure MUST be valid according to the most
   recently received Advertisement.  The Consumer can send a Configure
   either in response to a new Advertisement from the Provider or on
   its own, for example because of a local change in conditions
   (people leaving the room, connectivity changes, multipoint related

   When choosing which Media Streams to receive from the Provider, and
   the encoding characteristics of those Media Streams, the Consumer
   advantageously takes several things into account: its local
   preference, simultaneity restrictions, and encoding limits.

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10.1. Local preference

   A variety of local factors influence the Consumer's choice of
   Media Streams to be received from the Provider:

   o  if the Consumer is an Endpoint, it is likely that it would
      choose, where possible, to receive video and audio Captures that
      match the number of display devices and audio system it has

   o  if the Consumer is a middle box such as an MCU, it MAY choose to
      receive loudest speaker streams (in order to perform its own
      media composition) and avoid pre-composed video Captures

   o  user choice (for instance, selection of a new layout) MAY result
      in a different set of Captures, or different encoding
      characteristics, being required by the Consumer

10.2. Physical simultaneity restrictions

   Often there are physical simultaneity constraints of the Provider
   that affect the Provider's ability to simultaneously send all of
   the captures the Consumer would wish to receive.  For instance, a
   middle box such as an MCU, when connected to a multi-camera room
   system, might prefer to receive both individual video streams of
   the people present in the room and an overall view of the room
   from a single camera.  Some Endpoint systems might be able to
   provide both of these sets of streams simultaneously, whereas
   others might not (if the overall room view were produced by
   changing the optical zoom level on the center camera, for

10.3. Encoding and encoding group limits

   Each of the Provider's encoding groups has limits on bandwidth and
   computational complexity, and the constituent potential encodings
   have limits on the bandwidth, computational complexity, video
   frame rate, and resolution that can be provided.  When choosing
   the Captures to be received from a Provider, a Consumer device
   MUST ensure that the encoding characteristics requested for each
   individual Capture fits within the capability of the encoding it
   is being configured to use, as well as ensuring that the combined
   encoding characteristics for Captures fit within the capabilities
   of their associated encoding groups.  In some cases, this could
   cause an otherwise "preferred" choice of capture encodings to be
   passed over in favor of different Capture Encodings--for instance,

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   if a set of three Captures could only be provided at a low
   resolution then a three screen device could switch to favoring a
   single, higher quality, Capture Encoding.

11. Extensibility

   One important characteristics of the Framework is its
   extensibility.  Telepresence is a relatively new industry and
   while we can foresee certain directions, we also do not know
   everything about how it will develop.  The standard for
   interoperability and handling multiple streams must be future-
   proof. The framework itself is inherently extensible through
   expanding the data model types.  For example:

   o  Adding more types of media, such as telemetry, can done by
      defining additional types of Captures in addition to audio and

   o  Adding new functionalities , such as 3-D, say, may require
      additional attributes describing the Captures.

   o  Adding a new codecs, such as H.265, can be accomplished by
      defining new encoding variables.

   The infrastructure is designed to be extended rather than
   requiring new infrastructure elements.  Extension comes through
   adding to defined types.

12. Examples - Using the Framework (Informative)

   This section gives some examples, first from the point of view of
   the Provider, then the Consumer.

12.1. Provider Behavior

   This section shows some examples in more detail of how a Provider
   can use the framework to represent a typical case for telepresence
   rooms.  First an endpoint is illustrated, then an MCU case is

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12.1.1. Three screen Endpoint Provider

   Consider an Endpoint with the following description:

   3 cameras, 3 displays, a 6 person table

   o  Each camera can provide one Capture for each 1/3 section of the

   o  A single Capture representing the active speaker can be provided
      (voice activity based camera selection to a given encoder input
      port implemented locally in the Endpoint)

   o  A single Capture representing the active speaker with the other
      2 Captures shown picture in picture within the stream can be
      provided (again, implemented inside the endpoint)

   o  A Capture showing a zoomed out view of all 6 seats in the room
      can be provided

   The audio and video Captures for this Endpoint can be described as

   Video Captures:

   o  VC0- (the camera-left camera stream), encoding group=EG0,
      switched=false, view=table

   o  VC1- (the center camera stream), encoding group=EG1,
      switched=false, view=table

   o  VC2- (the camera-right camera stream), encoding group=EG2,
      switched=false, view=table

   o  VC3- (the loudest panel stream), encoding group=EG1,
      switched=true, view=table

   o  VC4- (the loudest panel stream with PiPs), encoding group=EG1,
      composed=true, switched=true, view=room

   o  VC5- (the zoomed out view of all people in the room), encoding
      group=EG1, composed=false, switched=false, view=room

   o  VC6- (presentation stream), encoding group=EG1, presentation,

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   The following diagram is a top view of the room with 3 cameras, 3
   displays, and 6 seats.  Each camera is capturing 2 people.  The
   six seats are not all in a straight line.

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      ,-. d
     (   )`--.__        +---+
      `-' /     `--.__  |   |
    ,-.  |            `-.._ |_-+Camera 2 (VC2)
   (   ).'        ___..-+-''`+-+
    `-' |_...---''      |   |
    ,-.c+-..__          +---+
   (   )|     ``--..__  |   |
    `-' |             ``+-..|_-+Camera 1 (VC1)
    ,-. |            __..--'|+-+
   (   )|     __..--'   |   |
    `-'b|..--'          +---+
    ,-. |``---..___     |   |
   (   )\          ```--..._|_-+Camera 0 (VC0)
    `-'  \             _..-''`-+
     ,-. \      __.--'' |   |
    (   ) |..-''        +---+
     `-' a

   The two points labeled b and c are intended to be at the midpoint
   between the seating positions, and where the fields of view of the
   cameras intersect.

   The plane of interest for VC0 is a vertical plane that intersects
   points 'a' and 'b'.

   The plane of interest for VC1 intersects points 'b' and 'c'. The
   plane of interest for VC2 intersects points 'c' and 'd'.

   This example uses an area scale of millimeters.

   Areas of capture:

       bottom left    bottom right  top left         top right
   VC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
   VC1 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
   VC2 (  673,3000,0) (2011,2850,0) (  673,3000,757) (2011,3000,757)
   VC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
   VC4 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
   VC5 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
   VC6 none

   Points of capture:
   VC0 (-1678,0,800)

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   VC1 (0,0,800)
   VC2 (1678,0,800)
   VC3 none
   VC4 none
   VC5 (0,0,800)
   VC6 none

   In this example, the right edge of the VC0 area lines up with the
   left edge of the VC1 area.  It doesn't have to be this way.  There
   could be a gap or an overlap.  One additional thing to note for
   this example is the distance from a to b is equal to the distance
   from b to c and the distance from c to d.  All these distances are
   1346 mm. This is the planar width of each area of capture for VC0,
   VC1, and VC2.

   Note the text in parentheses (e.g. "the camera-left camera
   stream") is not explicitly part of the model, it is just
   explanatory text for this example, and is not included in the
   model with the media captures and attributes.  Also, the
   "composed" boolean attribute doesn't say anything about how a
   capture is composed, so the media consumer can't tell based on
   this attribute that VC4 is composed of a "loudest panel with

   Audio Captures:

   o  AC0 (camera-left), encoding group=EG3, content=main, channel

   o  AC1 (camera-right), encoding group=EG3, content=main, channel

   o  AC2 (center) encoding group=EG3, content=main, channel

   o  AC3 being a simple pre-mixed audio stream from the room (mono),
      encoding group=EG3, content=main, channel format=mono

   o  AC4 audio stream associated with the presentation video (mono)
      encoding group=EG3, content=slides, channel format=mono

   Areas of capture:

       bottom left    bottom right  top left         top right

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   AC0 (-2011,2850,0) (-673,3000,0) (-2011,2850,757) (-673,3000,757)
   AC1 (  673,3000,0) (2011,2850,0) (  673,3000,757) (2011,3000,757)
   AC2 ( -673,3000,0) ( 673,3000,0) ( -673,3000,757) ( 673,3000,757)
   AC3 (-2011,2850,0) (2011,2850,0) (-2011,2850,757) (2011,3000,757)
   AC4 none

   The physical simultaneity information is:

      Simultaneous transmission set #1 {VC0, VC1, VC2, VC3, VC4, VC6}

      Simultaneous transmission set #2 {VC0, VC2, VC5, VC6}

   This constraint indicates it is not possible to use all the VCs at
   the same time.  VC5 can not be used at the same time as VC1 or VC3
   or VC4.  Also, using every member in the set simultaneously may
   not make sense - for example VC3(loudest) and VC4 (loudest with
   PIP).  (In addition, there are encoding constraints that make
   choosing all of the VCs in a set impossible.  VC1, VC3, VC4, VC5,
   VC6 all use EG1 and EG1 has only 3 ENCs.  This constraint shows up
   in the encoding groups, not in the simultaneous transmission

   In this example there are no restrictions on which audio captures
   can be sent simultaneously.

   Encoding Groups:

   This example has three encoding groups associated with the video
   captures.  Each group can have 3 encodings, but with each
   potential encoding having a progressively lower specification.  In
   this example, 1080p60 transmission is possible (as ENC0 has a
   maxPps value compatible with that).  Significantly, as up to 3
   encodings are available per group, it is possible to transmit some
   video captures simultaneously that are not in the same entry in
   the capture scene.  For example VC1 and VC3 at the same time.

   It is also possible to transmit multiple capture encodings of a
   single video capture.  For example VC0 can be encoded using ENC0
   and ENC1 at the same time, as long as the encoding parameters
   satisfy the constraints of ENC0, ENC1, and EG0, such as one at
   4000000 bps and one at 2000000 bps.

   encodeGroupID=EG0, maxGroupBandwidth=6000000
       encodeID=ENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
                      maxPps=124416000, maxBandwidth=4000000

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       encodeID=ENC1, maxWidth=1280, maxHeight=720, maxFrameRate=30,
                      maxPps=27648000, maxBandwidth=4000000
       encodeID=ENC2, maxWidth=960, maxHeight=544, maxFrameRate=30,
                      maxPps=15552000, maxBandwidth=4000000
   encodeGroupID=EG1  maxGroupBandwidth=6000000
       encodeID=ENC3, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
                      maxPps=124416000, maxBandwidth=4000000
       encodeID=ENC4, maxWidth=1280, maxHeight=720, maxFrameRate=30,
                      maxPps=27648000, maxBandwidth=4000000
       encodeID=ENC5, maxWidth=960, maxHeight=544, maxFrameRate=30,
                      maxPps=15552000, maxBandwidth=4000000
   encodeGroupID=EG2  maxGroupBandwidth=6000000
       encodeID=ENC6, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
                      maxPps=124416000, maxBandwidth=4000000
       encodeID=ENC7, maxWidth=1280, maxHeight=720, maxFrameRate=30,
                      maxPps=27648000, maxBandwidth=4000000
       encodeID=ENC8, maxWidth=960, maxHeight=544, maxFrameRate=30,
                      maxPps=15552000, maxBandwidth=4000000

                Figure 2: Example Encoding Groups for Video

   For audio, there are five potential encodings available, so all
   five audio captures can be encoded at the same time.

   encodeGroupID=EG3, maxGroupBandwidth=320000
       encodeID=ENC9, maxBandwidth=64000
       encodeID=ENC10, maxBandwidth=64000
       encodeID=ENC11, maxBandwidth=64000
       encodeID=ENC12, maxBandwidth=64000
       encodeID=ENC13, maxBandwidth=64000

                Figure 3: Example Encoding Group for Audio

   Capture Scenes:

   The following table represents the capture scenes for this
   provider. Recall that a capture scene is composed of alternative
   capture scene entries covering the same spatial region.  Capture
   Scene #1 is for the main people captures, and Capture Scene #2 is
   for presentation.

   Each row in the table is a separate Capture Scene Entry

                           | Capture Scene #1 |

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                           | VC0, VC1, VC2    |
                           | VC3              |
                           | VC4              |
                           | VC5              |
                           | AC0, AC1, AC2    |
                           | AC3              |

                           | Capture Scene #2 |
                           | VC6              |
                           | AC4              |

   Different capture scenes are unique to each other, non-
   overlapping. A consumer can choose an entry from each capture
   scene.  In this case the three captures VC0, VC1, and VC2 are one
   way of representing the video from the endpoint.  These three
   captures should appear adjacent next to each other.
   Alternatively, another way of representing the Capture Scene is
   with the capture VC3, which automatically shows the person who is
   talking.  Similarly for the VC4 and VC5 alternatives.

   As in the video case, the different entries of audio in Capture
   Scene #1 represent the "same thing", in that one way to receive
   the audio is with the 3 audio captures (AC0, AC1, AC2), and
   another way is with the mixed AC3.  The Media Consumer can choose
   an audio capture entry it is capable of receiving.

   The spatial ordering is understood by the media capture attributes
   Area of Capture and Point of Capture.

   A Media Consumer would likely want to choose a capture scene entry
   to receive based in part on how many streams it can simultaneously
   receive.  A consumer that can receive three people streams would
   probably prefer to receive the first entry of Capture Scene #1
   (VC0, VC1, VC2) and not receive the other entries.  A consumer
   that can receive only one people stream would probably choose one
   of the other entries.

   If the consumer can receive a presentation stream too, it would
   also choose to receive the only entry from Capture Scene #2 (VC6).

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12.1.2. Encoding Group Example

   This is an example of an encoding group to illustrate how it can
   express dependencies between encodings.

   encodeGroupID=EG0 maxGroupBandwidth=6000000
       encodeID=VIDENC0, maxWidth=1920, maxHeight=1088,
         maxFrameRate=60, maxPps=62208000, maxBandwidth=4000000
       encodeID=VIDENC1, maxWidth=1920, maxHeight=1088,
         maxFrameRate=60, maxPps=62208000, maxBandwidth=4000000
       encodeID=AUDENC0, maxBandwidth=96000
       encodeID=AUDENC1, maxBandwidth=96000
       encodeID=AUDENC2, maxBandwidth=96000

   Here, the encoding group is EG0.  Although the encoding group is
   capable of transmitting up to 6Mbit/s, no individual video
   encoding can exceed 4Mbit/s.

   This encoding group also allows up to 3 audio encodings, AUDENC<0-
   2>. It is not required that audio and video encodings reside
   within the same encoding group, but if so then the group's overall
   maxBandwidth value is a limit on the sum of all audio and video
   encodings configured by the consumer.  A system that does not wish
   or need to combine bandwidth limitations in this way should
   instead use separate encoding groups for audio and video in order
   for the bandwidth limitations on audio and video to not interact.

   Audio and video can be expressed in separate encoding groups, as
   in this illustration.

   encodeGroupID=EG0 maxGroupBandwidth=6000000
       encodeID=VIDENC0, maxWidth=1920, maxHeight=1088,
         maxFrameRate=60, maxPps=62208000, maxBandwidth=4000000
       encodeID=VIDENC1, maxWidth=1920, maxHeight=1088,
         maxFrameRate=60, maxPps=62208000, maxBandwidth=4000000
   encodeGroupID=EG1 maxGroupBandwidth=500000
       encodeID=AUDENC0, maxBandwidth=96000
       encodeID=AUDENC1, maxBandwidth=96000
       encodeID=AUDENC2, maxBandwidth=96000

12.1.3. The MCU Case

   This section shows how an MCU might express its Capture Scenes,
   intending to offer different choices for consumers that can handle
   different numbers of streams.  A single audio capture stream is

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   provided for all single and multi-screen configurations that can
   be associated (e.g. lip-synced) with any combination of video
   captures at the consumer.

   | Capture Scene #1   | note
   | VC0                | video capture for single screen consumer
   | VC1, VC2           | video capture for 2 screen consumer
   | VC3, VC4, VC5      | video capture for 3 screen consumer
   | VC6, VC7, VC8, VC9 | video capture for 4 screen consumer
   | AC0                | audio capture representing all participants

   If / when a presentation stream becomes active within the
   conference the MCU might re-advertise the available media as:

        | Capture Scene #2 | note                                 |
        | VC10             | video capture for presentation       |
        | AC1              | presentation audio to accompany VC10 |

12.2. Media Consumer Behavior

   This section gives an example of how a Media Consumer might behave
   when deciding how to request streams from the three screen
   endpoint described in the previous section.

   The receive side of a call needs to balance its requirements,
   based on number of screens and speakers, its decoding capabilities
   and available bandwidth, and the provider's capabilities in order
   to optimally configure the provider's streams.  Typically it would
   want to receive and decode media from each Capture Scene
   advertised by the Provider.

   A sane, basic, algorithm might be for the consumer to go through
   each Capture Scene in turn and find the collection of Video

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   Captures that best matches the number of screens it has (this
   might include consideration of screens dedicated to presentation
   video display rather than "people" video) and then decide between
   alternative entries in the video Capture Scenes based either on
   hard-coded preferences or user choice.  Once this choice has been
   made, the consumer would then decide how to configure the
   provider's encoding groups in order to make best use of the
   available network bandwidth and its own decoding capabilities.

12.2.1. One screen Media Consumer

   VC3, VC4 and VC5 are all different entries by themselves, not
   grouped together in a single entry, so the receiving device should
   choose between one of those.  The choice would come down to
   whether to see the greatest number of participants simultaneously
   at roughly equal precedence (VC5), a switched view of just the
   loudest region (VC3) or a switched view with PiPs (VC4).  An
   endpoint device with a small amount of knowledge of these
   differences could offer a dynamic choice of these options, in-
   call, to the user.

12.2.2. Two screen Media Consumer configuring the example

   Mixing systems with an even number of screens, "2n", and those
   with "2n+1" cameras (and vice versa) is always likely to be the
   problematic case.  In this instance, the behavior is likely to be
   determined by whether a "2 screen" system is really a "2 decoder"
   system, i.e., whether only one received stream can be displayed
   per screen or whether more than 2 streams can be received and
   spread across the available screen area.  To enumerate 3 possible
   behaviors here for the 2 screen system when it learns that the far
   end is "ideally" expressed via 3 capture streams:

   1. Fall back to receiving just a single stream (VC3, VC4 or VC5 as
      per the 1 screen consumer case above) and either leave one
      screen blank or use it for presentation if / when a
      presentation becomes active.

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   2. Receive 3 streams (VC0, VC1 and VC2) and display across 2
      screens (either with each capture being scaled to 2/3 of a
      screen and the center capture being split across 2 screens) or,
      as would be necessary if there were large bezels on the
      screens, with each stream being scaled to 1/2 the screen width
      and height and there being a 4th "blank" panel.  This 4th panel
      could potentially be used for any presentation that became
      active during the call.

   3. Receive 3 streams, decode all 3, and use control information
      indicating which was the most active to switch between showing
      the left and center streams (one per screen) and the center and
      right streams.

   For an endpoint capable of all 3 methods of working described
   above, again it might be appropriate to offer the user the choice
   of display mode.

12.2.3. Three screen Media Consumer configuring the example

   This is the most straightforward case - the Media Consumer would
   look to identify a set of streams to receive that best matched its
   available screens and so the VC0 plus VC1 plus VC2 should match
   optimally.  The spatial ordering would give sufficient information
   for the correct video capture to be shown on the correct screen,
   and the consumer would either need to divide a single encoding
   group's capability by 3 to determine what resolution and frame
   rate to configure the provider with or to configure the individual
   video captures' encoding groups with what makes most sense (taking
   into account the receive side decode capabilities, overall call
   bandwidth, the resolution of the screens plus any user preferences
   such as motion vs sharpness).

13. Acknowledgements

   Allyn Romanow and Brian Baldino were authors of early versions.
   Mark Gorzyinski contributed much to the approach.  We want to
   thank Stephen Botzko for helpful discussions on audio.

14. IANA Considerations


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15. Security Considerations


16. Changes Since Last Version

   NOTE TO THE RFC-Editor: Please remove this section prior to
   publication as an RFC.

   Changes from 11 to 12:

     1. Ticket #44. Remove note questioning about requiring a
        Consumer to send a Configure after receiving Advertisement.

     2. Ticket #43. Remove ability for consumer to choose value of
        attribute for scene-switch-policy.

     3. Ticket #36. Remove computational complexity parameter,
        MaxGroupPps, from Encoding Groups.

     4. Reword the Abstract and parts of sections 1 and 4 (now 5)
        based on Mary's suggestions as discussed on the list.  Move
        part of the Introduction into a new section Overview &

     5. Add diagram of an Advertisement, in the Overview of the
        Framework/Model section.

     6. Change Intended Status to Standards Track.

     7. Clean up RFC2119 keyword language.

   Changes from 10 to 11:

     1. Add description attribute to Media Capture and Capture Scene

     2. Remove contradiction and change the note about open issue
        regarding always responding to Advertisement with a Configure

     3. Update example section, to cleanup formatting and make the
        media capture attributes and encoding parameters consistent
        with the rest of the document.

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   Changes from 09 to 10:

     1. Several minor clarifications such as about SDP usage, Media
        Captures, Configure message.

     2. Simultaneous Set can be expressed in terms of Capture Scene
        and Capture Scene Entry.

     3. Removed Area of Scene attribute.

     4. Add attributes from draft-groves-clue-capture-attr-01.

     5. Move some of the Media Capture attribute descriptions back
        into this document, but try to leave detailed syntax to the
        data model.  Remove the OUTSOURCE sections, which are already
        incorporated into the data model document.

   Changes from 08 to 09:

     1. Use "document" instead of "memo".

     2. Add basic call flow sequence diagram to introduction.

     3. Add definitions for Advertisement and Configure messages.

     4. Add definitions for Capture and Provider.

     5. Update definition of Capture Scene.

     6. Update definition of Individual Encoding.

     7. Shorten definition of Media Capture and add key points in the
        Media Captures section.

     8. Reword a bit about capture scenes in overview.

     9. Reword about labeling Media Captures.

     10. Remove the Consumer Capability message.

     11. New example section heading for media provider behavior

     12. Clarifications in the Capture Scene section.

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     13. Clarifications in the Simultaneous Transmission Set section.

     14. Capitalize defined terms.

     15. Move call flow example from introduction to overview section

     16. General editorial cleanup

     17. Add some editors' notes requesting input on issues

     18. Summarize some sections, and propose details be outsourced
        to other documents.

   Changes from 06 to 07:

     1. Ticket #9.  Rename Axis of Capture Point attribute to Point
        on Line of Capture.  Clarify the description of this

     2. Ticket #17.  Add "capture encoding" definition.  Use this new
        term throughout document as appropriate, replacing some usage
        of the terms "stream" and "encoding".

     3. Ticket #18.  Add Max Capture Encodings media capture

     4. Add clarification that different capture scene entries are
        not necessarily mutually exclusive.

   Changes from 05 to 06:

   1. Capture scene description attribute is a list of text strings,
      each in a different language, rather than just a single string.

   2. Add new Axis of Capture Point attribute.

   3. Remove appendices A.1 through A.6.

   4. Clarify that the provider must use the same coordinate system
      with same scale and origin for all coordinates within the same
      capture scene.

   Changes from 04 to 05:

   1. Clarify limitations of "composed" attribute.

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   2. Add new section "capture scene entry attributes" and add the
      attribute "scene-switch-policy".

   3. Add capture scene description attribute and description
      language attribute.

   4. Editorial changes to examples section for consistency with the
      rest of the document.

   Changes from 03 to 04:

   1. Remove sentence from overview - "This constitutes a significant
      change ..."

   2. Clarify a consumer can choose a subset of captures from a
      capture scene entry or a simultaneous set (in section "capture
      scene" and "consumer's choice...").

   3. Reword first paragraph of Media Capture Attributes section.

   4. Clarify a stereo audio capture is different from two mono audio
      captures (description of audio channel format attribute).

   5. Clarify what it means when coordinate information is not
      specified for area of capture, point of capture, area of scene.

   6. Change the term "producer" to "provider" to be consistent (it
      was just in two places).

   7. Change name of "purpose" attribute to "content" and refer to
      RFC4796 for values.

   8. Clarify simultaneous sets are part of a provider advertisement,
      and apply across all capture scenes in the advertisement.

   9. Remove sentence about lip-sync between all media captures in a
      capture scene.

   10.   Combine the concepts of "capture scene" and "capture set"
      into a single concept, using the term "capture scene" to
      replace the previous term "capture set", and eliminating the
      original separate capture scene concept.

   Informative References

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   Edt. Note: Decide which of these really are Normative References.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G.,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J., Schulzrinne, H., "An Offer/Answer Model
              with the Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              February 2006.

   [RFC4579]  Johnston, A., Levin, O., "SIP Call Control -
              Conferencing for User Agents", RFC 4579, August 2006

   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC
              January 2008.

17. Authors' Addresses

   Mark Duckworth (editor)
   Andover, MA  01810


   Andrew Pepperell

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   Uxbridge, England


   Stephan Wenger
   Vidyo, Inc.
   433 Hackensack Ave.
   Hackensack, N.J. 07601


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