CLUE WG M. Duckworth, Ed.
Internet Draft Polycom
Intended status: Informational A. Pepperell
Expires: November 16, 2013 Acano
S. Wenger
Vidyo
May 16, 2013
Framework for Telepresence Multi-Streams
draft-ietf-clue-framework-10.txt
Abstract
This document offers a framework for a protocol that enables
devices in a telepresence conference to interoperate by specifying
the relationships between multiple media streams.
Status of this Memo
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This Internet-Draft will expire on November 16, 2013.
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Table of Contents
1. Introduction...................................................3
2. Terminology....................................................5
3. Definitions....................................................5
4. Overview of the Framework/Model................................8
5. Spatial Relationships.........................................13
6. Media Captures and Capture Scenes.............................14
6.1. Media Captures...........................................14
6.1.1. Media Capture Attributes............................15
6.2. Capture Scene............................................19
6.2.1. Capture Scene attributes............................22
6.2.2. Capture Scene Entry attributes......................22
6.3. Simultaneous Transmission Set Constraints................23
7. Encodings.....................................................25
7.1. Individual Encodings.....................................25
7.2. Encoding Group...........................................27
8. Associating Captures with Encoding Groups.....................28
9. Consumer's Choice of Streams to Receive from the Provider.....29
9.1. Local preference.........................................31
9.2. Physical simultaneity restrictions.......................31
9.3. Encoding and encoding group limits.......................32
10. Extensibility................................................32
11. Examples - Using the Framework...............................32
11.1. Provider Behavior.......................................33
11.1.1. Three screen Endpoint Provider.....................33
11.1.2. Encoding Group Example.............................40
11.1.3. The MCU Case.......................................41
11.2. Media Consumer Behavior.................................42
11.2.1. One screen Media Consumer..........................42
11.2.2. Two screen Media Consumer configuring the example..43
11.2.3. Three screen Media Consumer configuring the example43
12. Acknowledgements.............................................44
13. IANA Considerations..........................................44
14. Security Considerations......................................44
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15. Changes Since Last Version...................................44
16. Authors' Addresses...........................................48
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 draft provides a framework for a
protocol to enable interoperability by handling multiple streams in
a standardized way. It 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.
Conceptually distinguished are Media Providers and Media Consumers.
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 may include only Consumer or Provider functionality, such
as recorder-type Consumers or webcam-type Providers.
Motivations for this document (and, in fact, for the existence of
the CLUE protocol) include:
(1) Endpoints according to this document can, and usually do, have
multiple Media Captures and Media Renderers, that is, for example,
multiple cameras and screens. While previous system designs were
able to set up calls that would light up all screens and cameras
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(or equivalent), what was missing was a mechanism that can
associate the 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 6.
(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
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 6.3.
(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 may be sensible in
certain environments, and even if the simultaneous transmission may
also be 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 7.
(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, a sensible
system design is 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 may 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,
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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.
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].
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.
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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
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.
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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.
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].
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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 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 occurs as a minimum 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
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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 require to include in the
SDP in case of complex setups, as should become clearer shortly
when discussing an outline of the call flow.
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 constraints, such as maximum
bandwidth and maximum macroblocks/second.
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.
+-----------+ +-----------+
| Endpoint1 | | Endpoint2 |
+----+------+ +-----+-----+
| INVITE (BASIC SDP+CLUECHANNEL) |
|--------------------------------->|
| 200 0K (BASIC SDP+CLUECHANNEL)|
|<---------------------------------|
| ACK |
|--------------------------------->|
| |
|<################################>|
| BASIC SDP MEDIA SESSION |
|<################################>|
| |
| CONNECT (CLUE CTRL CHANNEL) |
|=================================>|
| ... |
|<================================>|
| CLUE CTRL CHANNEL ESTABLISHED |
|<================================>|
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| |
| ADVERTISEMENT 1 |
|*********************************>|
| ADVERTISEMENT 2 |
|<*********************************|
| |
| CONFIGURE 1 |
|<*********************************|
| CONFIGURE 2 |
|*********************************>|
| |
| REINVITE (UPDATED SDP) |
|--------------------------------->|
| 200 0K (UPDATED SDP)|
|<---------------------------------|
| ACK |
|--------------------------------->|
| |
|<################################>|
| UPDATED SDP MEDIA SESSION |
|<################################>|
| |
v v
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 have to adhere to
them.
Over this CLUE channel, the Provider in each Endpoint conveys its
characteristics and capabilities by sending an Advertisement as
specified herein (which will typically not be 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 constructs and sends a CLUE Configure message to tell the
Provider what the Consumer wishes to receive. Second, it can, 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
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often sensible for the Media Consumer to take those spatial
relationships into account when tailoring the SDP.
This CLUE exchange is 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 cases can (but not
necessarily must) 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 may not be 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 can occur over the
CLUE channel. In some cases, those further exchanges can lead to a
modified system behavior of Provider or Consumer (or both) without
any other protocol activity such as further offer/answer exchanges.
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 may become 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, that could require a new O/A
exchange.
Numerous optimizations may be possible, and are the implementer's
choice. For example, it may 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 may 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
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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
normative 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 has to follow the
CLUE protocol and, therefore, has to 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
information provided therein. Obviously, this is not a
particularly sensible design choice.
Legacy devices are defined here in as those Endpoints and MCUs that
do not support the setup and use of the CLUE channel. The notion
of a device being a legacy device is established during the initial
offer/answer exchange, in which the legacy device will not
understand the offer for the CLUE channel and, therefore, reject
it. This is the indication for the CLUE-implementing Endpoint or
MCU that the other side of the communication is not compliant with
CLUE, and to fall back to whatever mechanism was used before the
introduction of 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. However, it should be noted that
forms of RTP multiplexing of multiple RTP flows onto the same
transport address are developed concurrently with the CLUE suite of
specifications, and it is widely expected that most, if not all,
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Endpoints or MCUs supporting CLUE will also support those
mechanisms. Some design choices made in this document reflect this
coincidence in spec development timing.
5. 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.
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) should always provide those
real-world measurements. Systems which don't know specific
physical dimensions but still know relative distances should 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.
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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
6. Media Captures and Capture Scenes
This section describes how Providers can describe the content of
media to Consumers.
6.1. Media Captures
Media Captures are the fundamental representations of streams that
a device can transmit. What a Media Capture actually represents is
flexible:
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
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. A Media Capture may be the source of one or more Capture
Encodings
Each Media Capture can be associated with attributes to describe
what it represents.
6.1.1. 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
. Other descriptive information to help the Consumer choose
between captures (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:
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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 should 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 should 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 will stay the same,
or are expected to change frequently. Possible values are static,
dynamic, and highly dynamic.
For example, a camera may be placed at different positions in order
to provide the best angle to capture a work task, or may include a
camera worn by a participant. This would have an effect of changing
the capture point, capture axis and area of capture. In order that
the Consumer can choose to render the capture appropriately, the
Provider can include this attribute to indicate if the camera
location is dynamic or not.
The capture point of a static capture does not move for the life of
the conference. The capture point of dynamic captures is
categorised by a change in position followed by a reasonable period
of stability. High dynamic captures are categorised 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. No information
regarding future spatial information should be assumed.
Composed:
A boolean field which indicates whether or not the Media Capture is
a mix (audio) or composition (video) of streams.
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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.
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.
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.
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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 can be
one of:
Room - Captures the entire scene
Table - Captures the conference table with seated participants
Individual - Captures an individual participant
Lectern - Captures the region of the lectern including the
presenter in a classroom style conference
Audience - Captures a region showing the audience in a classroom
style conference
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 multilingualand/or accessible conferences, so
a Consumer can use this attribute to differentiate between them.
This indicates which language is associated with the capture. For
example: it may provide a language associated with an audio capture
or a language associated with a video capture when sign
interpretation or text is used.
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.
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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 as important. If no
priority is assigned no assumptions regarding relative important of
the capture can be assumed.
Embedded Text:
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 utilise 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.
6.2. 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
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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
may also express Capture Scenes that it constructs from media
Streams it receives.
A Provider may advertise multiple Capture Scenes or just a single
Capture Scene. What constitutes an entire Capture Scene is up to
the Provider. A 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
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 can 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 5.
A Provider arranges Captures in a Capture Scene to help the
Consumer choose which captures it wants. 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
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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 6.3.
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 may decide to receive
all the Captures in a single Capture Scene Entry, but a Consumer
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 6.3.
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
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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.
6.2.1. 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.
6.2.2. 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:
. Scene-switch-policy: {site-switch, segment-switch}
A media provider uses this scene-switch-policy attribute to
indicate its support for different switching policies. In the
provider's Advertisement, this attribute can have multiple values,
which means the provider supports each of the indicated policies.
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The consumer, when it requests media captures from this Capture
Scene Entry, should also include this attribute but with only the
single value (from among the values indicated by the provider)
indicating the Consumer's choice for which policy it wants the
provider to use. The Consumer must choose the same value for all
the Media Captures in the Capture Scene Entry. If the provider
does not support any of these policies, it should 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.
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 is switched from time to time based on 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.
6.3. Simultaneous Transmission Set Constraints
The Provider may have constraints or limitations on its ability to
send Captures. One type is caused by the physical limitations of
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capture mechanisms; these constraints are represented by a
simultaneous transmission set. The second type of limitation
reflects the encoding resources available - bandwidth and
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
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.
Simultaneous transmission sets are expressed as sets of the Media
Captures that the Provider could transmit at the same time (though
it may not make sense 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
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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
included in the Simultaneous Transmission Set. The end result
reduces to a set of Media Captures in any case.
If an Advertisement does not include Simultaneous Transmission
Sets, then all Capture Scenes can be provided 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 should choose one Capture Scene
Entry for each media type, but may choose individual Captures based
on the Simultaneous Transmission Sets.
7. 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.
7.1. 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.
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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 might use lower values than
these maximums.
In general, the parameters of an Individual Encoding have been
chosen to represent those negotiable parameters of media codecs of
the media type that greatly influence computational complexity,
while abstracting from details of particular media codecs used.
The parameters have been chosen with those media codecs in mind
that have seen wide deployment in the video conferencing and
Telepresence industry.
For video codecs (using H.26x compression technologies), those
parameters include:
. Maximum bitrate;
. Maximum picture size in pixels;
. Maxmimum number of pixels to be processed per second; and
. Clue-protocol internal information.
For audio codecs, so far only one parameter has been identified:
. Maximum bitrate.
Edt. note: the maximum number of pixel per second are currently
expressed as H.264maxmbps.
Edt. note: it would be desirable to make the computational
complexity mechanism codec independent so to allow for expressing
that, say, H.264 codecs are less complex than H.265 codecs, and,
therefore, the same hardware can process higher pixel rates for
H.264 than for H.265. To be discussed in the WG.
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7.2. 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 and other parameters
for the group.
The Encoding Group data structure contains:
. Maximum bitrate for all encodings in the group combined;
. Maximum number of pixels per second for all video encodings of
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.
Likewise, the sum of the pixels per second of each instantiated
encoding in the group must not exceed the 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.
8. Associating Captures with Encoding Groups
Every Capture is associated with an 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.
9. 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 repsonsibility 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.
For each Media Capture in the message, the Consumer may also
specify the value of any attributes for which the Provider has
offered a choice, for example the value for the Scene-switch-policy
attribute. 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.
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.
The Consumer must have received at least one Advertisement from the
Provider to be able to create and send a Configure. Each
Advertisement is acknowledged by a corresponding Configure.
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 as
by its own, for example because of a local change in conditions
(people leaving the room, connectivity changes, multipoint related
considerations).
The Consumer need not send a new Configure message to the Provider
when it receives a new Advertisement from the Provider unless the
contents of the new Advertisement cause the Consumer's current
Configure message to become invalid.
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Edt. Note: The editors solicit input from the working group as to
whether or not a Consumer must respond to every Advertisement with
a new Configure message.
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.
9.1. 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
9.2. Physical simultaneity restrictions
There may be physical simultaneity constraints imposed by 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 may not (if the overall room view were produced by
changing the optical zoom level on the center camera, for
instance).
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9.3. 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 favour of different Capture Encodings - for
instance, 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.
10. Extensibility
One of the most 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.
11. Examples - Using the Framework
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EDT. Note: these examples are currently out of date with respect
to H264Mbps codepoints, which will be fixed in the next release
once an agreement about codec computational complexity has been
found. Other than that, the examples are still valid.
EDT Note: remove syntax-like details in these examples, and focus
on concepts for this document. Syntax examples with XML should be
in the data model doc or dedicated example document.
This section gives some examples, first from the point of view of
the Provider, then the Consumer.
11.1. 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.
11.1.1. 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:
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o VC0- (the camera-left camera stream), encoding group=EG0,
content=main, switched=false
o VC1- (the center camera stream), encoding group=EG1,
content=main, switched=false
o VC2- (the camera-right camera stream), encoding group=EG2,
content=main, switched=false
o VC3- (the loudest panel stream), encoding group=EG1,
content=main, switched=true
o VC4- (the loudest panel stream with PiPs), encoding group=EG1,
content=main, composed=true, switched=true
o VC5- (the zoomed out view of all people in the room), encoding
group=EG1, content=main, composed=false, switched=false
o VC6- (presentation stream), encoding group=EG1, content=slides,
switched=false
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
maxMbps value compatible with that) as long as it is the only
active encoding in the group(as maxMbps for the entire encoding
group is also 489600). 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
1080p30 and one at 720p30.
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encodeGroupID=EG0, maxGroupH264Mbps=489600,
maxGroupBandwidth=6000000
encodeID=ENC0, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC1, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC2, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG1 maxGroupH264Mbps=489600
maxGroupBandwidth=6000000
encodeID=ENC3, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC4, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC5, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, maxBandwidth=4000000
encodeGroupID=EG2 maxGroupH264Mbps=489600
maxGroupBandwidth=6000000
encodeID=ENC6, maxWidth=1920, maxHeight=1088, maxFrameRate=60,
maxH264Mbps=489600, maxBandwidth=4000000
encodeID=ENC7, maxWidth=1280, maxHeight=720, maxFrameRate=30,
maxH264Mbps=108000, maxBandwidth=4000000
encodeID=ENC8, maxWidth=960, maxHeight=544, maxFrameRate=30,
maxH264Mbps=61200, 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, maxGroupH264Mbps=0, 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
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Scene #1 is for the main people captures, and Capture Scene #2 is
for presentation.
Each row in the table is a separate entry in the capture scene
+------------------+
| Capture Scene #1 |
+------------------+
| 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 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
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(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).
11.1.2. Encoding Group Example
This is an example of an encoding group to illustrate how it can
express dependencies between encodings.
encodeGroupID=EG0, maxGroupH264Mbps=489600,
maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088,
maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088,
maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, maxBandwidth=96000
Here, the encoding group is EG0. It can transmit up to two
1080p30 capture encodings (Mbps for 1080p = 244800), but it is
capable of transmitting a maxFrameRate of 60 frames per second
(fps). To achieve the maximum resolution (1920 x 1088) the frame
rate is limited to 30 fps. However 60 fps can be achieved at a
lower resolution if required by the consumer. 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.
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encodeGroupID=EG0, maxGroupH264Mbps=489600,
maxGroupBandwidth=6000000
encodeID=VIDENC0, maxWidth=1920, maxHeight=1088,
maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeID=VIDENC1, maxWidth=1920, maxHeight=1088,
maxFrameRate=60,
maxH264Mbps=244800, maxBandwidth=4000000
encodeGroupID=EG1, maxGroupH264Mbps=0, maxGroupBandwidth=500000
encodeID=AUDENC0, maxBandwidth=96000
encodeID=AUDENC1, maxBandwidth=96000
encodeID=AUDENC2, maxBandwidth=96000
11.1.3. 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
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:
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+------------------+--------------------------------------+
| Capture Scene #2 | note |
+------------------+--------------------------------------+
| VC10 | video capture for presentation |
| AC1 | presentation audio to accompany VC10 |
+------------------+--------------------------------------+
11.2. 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
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.
11.2.1. 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.
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11.2.2. 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.
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.
11.2.3. 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
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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).
12. 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.
13. IANA Considerations
TBD
14. Security Considerations
TBD
15. Changes Since Last Version
NOTE TO THE RFC-Editor: Please remove this section prior to
publication as an RFC.
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:
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6. Use "document" instead of "memo".
7. Add basic call flow sequence diagram to introduction.
8. Add definitions for Advertisement and Configure messages.
9. Add definitions for Capture and Provider.
10. Update definition of Capture Scene.
11. Update definition of Individual Encoding.
12. Shorten definition of Media Capture and add key points in
the Media Captures section.
13. Reword a bit about capture scenes in overview.
14. Reword about labeling Media Captures.
15. Remove the Consumer Capability message.
16. New example section heading for media provider behavior
17. Clarifications in the Capture Scene section.
18. Clarifications in the Simultaneous Transmission Set section.
19. Capitalize defined terms.
20. Move call flow example from introduction to overview section
21. General editorial cleanup
22. Add some editors' notes requesting input on issues
23. 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.
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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.
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...").
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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
[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.
[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.
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[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC
5117,
January 2008.
16. Authors' Addresses
Mark Duckworth (editor)
Polycom
Andover, MA 01810
USA
Email: mark.duckworth@polycom.com
Andrew Pepperell
Acano
Uxbridge, England
UK
Email: apeppere@gmail.com
Stephan Wenger
Vidyo, Inc.
433 Hackensack Ave.
Hackensack, N.J. 07601
USA
Email: stewe@stewe.org
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