CLUE WG M. Duckworth, Ed.
Internet Draft Polycom
Intended status: Standards Track A. Pepperell
Expires: April 19, 2014 Acano
S. Wenger
Vidyo
October 19, 2013
Framework for Telepresence Multi-Streams
draft-ietf-clue-framework-12.txt
Abstract
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
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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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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-
requirements.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[RFC2119].
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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
Scene.
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
message.
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
streams
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
sources.
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
provided.
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
streams.
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
attributes.
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
Advertisement.
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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) |
|=================================>|
| ... |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
| |
| 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
SDP.
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
solicited.
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
cases.
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
flexible:
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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
stream'
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
video)
. A Media Capture is associated with exactly one Capture Scene
. A Media Capture is associated with one or more Capture Scene
Entries
. A Media Capture has exactly one set of spatial information
. A Media Capture can be the source of one or more Capture
Encodings
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
camera).
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
above).
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
Advertisement.
Composed:
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.
Switched:
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
Encoding.
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.
Description:
Human-readable description of the Capture, which could be in
multiple languages.
Presentation:
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
etc.
Note: It is expected that a number of keywords will be defined that
provide more detail on the type of presentation.
View:
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
Language:
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.
Role:
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.
Priority:
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
receive.
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
chosen.
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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
include
. 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
Entry.
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
conference.
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
as:
. 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
format.
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
Provider.
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
considerations).
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
instance).
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
video.
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
shown.
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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
table
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
follows.
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,
switched=false
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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
PiPs".
Audio Captures:
o AC0 (camera-left), encoding group=EG3, content=main, channel
format=mono
o AC1 (camera-right), encoding group=EG3, content=main, channel
format=mono
o AC2 (center) encoding group=EG3, content=main, channel
format=mono
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
sets.)
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
None.
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15. Security Considerations
TBD
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 &
Motivation.
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
Entry.
2. Remove contradiction and change the note about open issue
regarding always responding to Advertisement with a Configure
message.
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
attribute.
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
attribute.
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.,
Johnston,
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
5117,
January 2008.
17. Authors' Addresses
Mark Duckworth (editor)
Polycom
Andover, MA 01810
USA
Email: mark.duckworth@polycom.com
Andrew Pepperell
Acano
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Uxbridge, England
UK
Email: apeppere@gmail.com
Stephan Wenger
Vidyo, Inc.
433 Hackensack Ave.
Hackensack, N.J. 07601
USA
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