CLUE WG A. Romanow
Internet-Draft Cisco Systems
Intended status: Informational S. Botzko
Expires: February 20, 2012 Polycom
August 19, 2011
Requirements for Telepresence Multi-Streams
draft-ietf-clue-telepresence-requirements-00.txt
Abstract
This memo discusses the requirements for a specification that enables
telepresence interoperability, by describing the relationship between
multiple RTP streams. In addition, the problem statement and
definitions are also covered herein.
Status of this Memo
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This Internet-Draft will expire on February 20, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Telepresence systems greatly improve collaboration. In a
telepresence conference (as used herein), the goal is to create an
environment that gives the users a feeling of (co-located) presence -
the feeling that a local user is in the same room with other local
users and the remote parties. Currently, systems from different
vendors often do not interoperate because they do the same tasks
differently, as discussed in the Problem Statement section below.
The approach taken in this memo is to set requirements for a future
specification that, when fullfilled by an implementation of the
specification, provide for interoperability between IETF protocol
based telepresence systems. It is anticipated that a solution for
the requirements set out in this memo likely involves the exchange of
adequate information about participating sites; information that is
currently not standardized by the IETF.
The purpose of this document is to describe the requirements for a
specification that enables interworking between different SIP-based
[RFC3261] telepresence systems, by exchanging and negotiating
appropriate information. Non IETF protocol based systems, such as
those based on ITU-T Rec. H.323, are out of scope. These
requirements are for the specification, they are not requirements on
the telepresence systems implementing the solution/protocol that will
be specified.
Telepresence systems of different vendors, today, can follow
radically different architectural approaches while offering a similar
user experience. It is not the intention of CLUE to dictate
telepresence architectural and implementation choices. CLUE enables
interoperability between telepresence systems by exchanging
information about the systems' characteristics. Systems can use this
information to control their behavior to allow for interoperability
between those systems.
In a telepresence session, required are at least one sending and one
receiving endpoint. Most telepresence endpoints are full-duplex in
that they are both sending and receiving. Some, especially
multiparty telepresence sessions include more than two endpoints, and
centralized infrastructure such as Multipoint Control Units (MCUs) or
equivalent. CLUE specifies the syntax, semantics, and control flow
of information to enable the best possible user experience at those
endpoints.
Sending endpoints, or MCUs, are not mandated to use any of the CLUE
specifications that describe their capabilities, attributes, or
behavior. Similarly, it is not envisioned that endpoints or MCUs
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must ever take into account information received. However, by making
available as much information as possible, and by taking into account
as much information as has been received or exchanged, MCUs and
endpoints are expected to select operation modes that enable the best
possible user experience under their constraints.
The document structure is as follows: Definitions are set out,
followed by a description of the problem of telepresence
interoperability that led to this work. Then the requirements to a
specification addressing the current shortcomings are enumerated and
discussed.
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 definitions are from draft-wenger-clue-definitions-00-01.txt.
The editor's notes are not included here.
Audio Mixing: refers to the accumulation of scaled audio signals
to produce a single audio stream. See RTP Topologies, [RFC5117].
Conference: used as defined in [RFC4353], A Framework for
Conferencing within the Session Initiation Protocol (SIP).
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). In contrast to an
endpoint, an MCU may also send and receive media streams, but it
is not the initiator nor the final terminator in the sense that
Media is Captured or Rendered. Endpoints can be anything from
multiscreen/multicamera rooms to handheld devices.
Endpoint Characteristics: include placement of Capture and
Rendering Devices, capture/render angle, resolution of cameras and
screens, spatial location and mixing parameters of microphones.
Endpoint characteristics are not specific to individual media
streams sent by the endpoint.
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Layout: How rendered media streams are spatially arranged with
respect to each other on a single screen/mono audio telepresence
endpoint, and how rendered media streams are arranged with respect
to each other on a multiple screen/speaker telepresence endpoint.
Note that audio as well as video is encompassed by the term
layout--in other words, included is the placement of audio streams
on speakers as well as video streams on video screens.
Left: to be interpreted as a stage direction, see also
[StageDirection(Wikipedia)]
Local: Sender and/or receiver physically co-located ("local") in
the context of the discussion.
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] Mixer.
Media: Any data that, after suitable encoding, can be conveyed
over RTP, including audio, video or timed text.
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.
Remote: Sender and/or receiver on the other side of the
communication channel (depending on context); not Local. A remote
can be an Endpoint or an MCU.
Render: the process of generating a representation from a media,
such as displayed motion video or sound emitted from loudspeakers.
Right: to be interpreted as stage direction, see also
[StageDirection(Wikipedia)]
Telepresence: an environment that gives non co-located users or
user groups a feeling of (co-located) presence - the feeling that
a Local user is in the same room with other Local users and the
Remote parties. The inclusion of Remote parties is achieved
through multimedia communication including at least audio and
video signals of high fidelity.
4. Problem Statement
In order to create the "being there" or telepresence experience,
media inputs need to be transported, received, and coordinated.
Different telepresence systems take diverse approaches in crafting a
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solution. Or, implement similar solutions quite differently.
They use disparate techniques, and they describe, control and
negotiate media in dissimilar fashions. Such diversity creates an
interoperability problem. The same issues are solved in different
ways by different systems, so that they are not directly
interoperable. This makes interworking difficult at best and
sometimes impossible.
Worse, many telepresence use proprietry protocol extensions to solve
telepresence-related problems, even if those extensions are based on
common standards such as SIP.
Some degree of interworking between systems from different vendors is
possible through transcoding and translation. This requires
additional devices, which are expensive, often not entirely
automatic, and sometimes introduce unwelcome side effects such as
additional delay or degrading performance. Specialized knowledge is
currently required to operate a telepresence conference with
endpoints from different vendors, for example to configure
transcoding and translating devices. Often such conferences do not
commence as planned, or are interrupted by difficulties that arise.
The general problem that needs to be solved can be described as
follows. Today, the transmitting side sends audio and video captures
based upon an implicitely assumed model for rendering a realistic
depiction from this information. If the receiving side belongs to
the same vendor, it works with the same model and renders the
information according to the model implicitely assumed by the vendor.
However, if the receiver and the sender are from different vendors,
the models they each have for rendering presence can and usually do
differ. The result can be that the telepresence systems actually
connect, but the user experience suffers, for example because one
system assumes that the first video stream stems from the right
camera, whereas the other assumes the first video stream stems from
the left camera.
It is as if Alice and Bob are at different sites. Alice needs to
tell Bob information about what her camera and sound equipment see at
her site so that Bob's receiver can create a display that will
capture the important characteristics of her site. Alice and Bob
need to agree on what the salient characteristics are as well as how
to represent and communicate them. Characteristics include number,
placement, capture/render angle, resolution of cameras and screens,
spatial location and audio mixing parameters of microphones.
The telepresence multi-stream work seeks to describe the sender
situation in a way that allows the receiver to render it
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realistically, though it may have a different rendering model than
the sender; and for the receiver to provide information to the sender
in order to help the sender create adequate content for interworking.
5. Requirements
Although some aspects of these requirements can be met by existing
technology, such as SDP, or H.264, nonetheless we state them here to
have a complete record of what the requirements for CLUE are, whether
new work is needed or they can be met by existing technology.
Figuring this out will be part of the solution development, rather
than part of the requirements.
REQMT-1: The solution MUST support a description of the spatial
arrangement of source video images sent in video streams
which enables a satisfactory reproduction at the receiver
of the original scene. This applies to each site in a
point to point or a multipoint meeting and refers to the
spatial ordering within a site, not to the ordering of
images between sites.
REQMT-1a: The solution MUST support a means of allowing
the preservation of the order of images in the
captured scene. For example, if John is to
Susan's right in the image capture, John is
also to Susan's right in the rendered image.
REQMT-1b: The solution MUST support a means of allowing
the preservation of order of images in the
scene in two dimensions- horizontal and
vertical.
REQMT-2: The solution MUST support a description of the spatial
arrangement of captured source audio sent in audio streams
which enables a satisfactory reproduction at the receiver
in a spatially correct manner. This applies to each site
in a point to point or a multipoint meeting and refers to
the spatial ordering within a site, not the ordering of
channels between sites.
REQMT-2a: The solution MUST support a means of preserving
the spatial order of audio in the captured
scene. For example, if John sounds as if he is
at Susan's right in the captured audio, John
voice is also placed at Susan's right in the
rendered image.
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REQMT-2b: The solution MUST support a means to identify
monaural, stereophonic (2.0), and 3.0 (left,
center, right) audio channels.
REQMT-2c: The solution MUST NOT preclude the use of
binaural audio. Edt. [The discussion of
binaural is unresolved and current text is
problematic. It will be changed once the issue
has been resolved.]
REQMT-3: The solution MUST support a mechanism to enable a
satisfactory spatial matching between audio and video
streams coming from the same endpoints.
REQMT-3a: The solution MUST enable individual audio
streams to be associated with one or more video
image captures, and individual video image
captures to be associated with one or more
audio captures, for the purpose of rendering
proper position.
REQMT-3b: The solution MUST enable individual audio
streams to be rendered in any desired spatial
position.
Edt: Use case is off-camera positioning of
audio-only streams. Deferred, wait till
discussion of rendering is resolved.
REQMT-4: The solution MUST enable interoperability between
endpoints that have a different number of similar devices.
For example, one endpoint may have 1 screen, 1 speaker, 1
camera, 1 mic, and another endpoint may have 3 screens, 2
speakers, 3 cameras and 2 mics. Or, in a multi-point
conference, one endpoint may have one screen, another may
have 2 screens and a third may have 3 screens. This
includes endpoints where the number of devices of a given
type is zero.
REQMT-5: The solution MUST support means of enabling
interoperability between telepresence endpoints where
cameras are of different picture aspect ratios.
REQMT-6: The solution MUST provide scaling information which
enables rendering of a video image at the actual size of
the captured scene.
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REQMT-7: The solution MUST support means of enabling
interoperability between telepresence endpoints where
displays are of different resolutions.
REQMT-8: The solution MUST support methods for handling different
bit rates in the same conference.
REQMT-9: The solution MUST support means of enabling
interoperability between endpoints that send and receive
different numbers of media streams.
REQMT-10: The solution MUST make it possible for endpoints without
support for telepresence extensions to participate in a
telepresence session with those that do.
REQMT-11: The solution MUST support a mechanism for determining
whether or not an endpoint or MCU is capable of
telepresence extensions.
REQMT-12: The solution MUST support a means to enable more than two
sites to participate in a teleconference.
REQMT-13: The solution MUST support both transcoding and switching
approaches to providing multipoint conferences.
REQMT-14: The solution MUST support mechanisms to make possible for
either or both site switching or segment switching. [Edt:
this needs rewording. Deferred until layout discussion is
resolved.]
REQMT-15: The solution MUST support mechanisms for presentations in
such a way that:
* Presentations can have different sources
* Presentations can be seen by all
* There can be variation in placement, number and size of
presentations
REQMT-16: The solution MUST include extensibility mechanisms.
6. Acknowledgements
This draft has benefitted from all the comments on the mailing list
and a number of discussions. So many people contributed that it is
not possible to list them all.
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7. IANA Considerations
TBD
8. Security Considerations
TBD
9. 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.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[StageDirection(Wikipedia)]
Wikipedia, "Blocking (stage), available from http://
en.wikipedia.org/wiki/Stage_direction#Stage_directions",
May 2011, <http://en.wikipedia.org/wiki/
Stage_direction#Stage_directions>.
Authors' Addresses
Allyn Romanow
Cisco Systems
San Jose, CA 95134
USA
Email: allyn@cisco.com
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Stephen Botzko
Polycom
Andover, MA 01810
US
Email: stephen.botzko@polycom.com
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