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Internet Engineering Task Force (IETF) A. Romanow
Request for Comments: 7262 Cisco Systems
Category: Informational S. Botzko
ISSN: 2070-1721 Polycom
M. Barnes
MLB@Realtime Communications, LLC
June 2014
Requirements for Telepresence Multistreams
Abstract
This memo discusses the requirements for specifications that enable
telepresence interoperability by describing behaviors and protocols
for Controlling Multiple Streams for Telepresence (CLUE). In
addition, the problem statement and related definitions are also
covered herein.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7262.
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RFC 7262 CLUE Telepresence Requirements June 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . 11
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 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 (see Section 4).
The approach taken in this memo is to set requirements for a future
specification(s) that, when fulfilled by an implementation of the
specification(s), 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; this 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. In the context of the requirements in this
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document and related solution documents, this includes both point-to-
point SIP sessions as well as SIP-based conferences as described in
the SIP conferencing framework [RFC4353] and the SIP-based conference
control [RFC4579] specifications. Non-IETF protocol-based systems,
such as those based on ITU-T Rec. H.323 [ITU.H323], 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.
Today, telepresence systems of different vendors can follow radically
different architectural approaches while offering a similar user
experience. CLUE will not dictate telepresence architectural and
implementation choices; however, it will describe a protocol
architecture for CLUE and how it relates to other protocols. 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.
A telepresence session requires at least one sending and one
receiving endpoint. Multiparty telepresence sessions include more
than 2 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
will ever have to take information received into account. 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 for 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].
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3. Definitions
The following terms are used throughout this document and serve as a
reference for other documents.
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 "A Framework for Conferencing
within the Session Initiation Protocol (SIP)" [RFC4353].
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 that source and sink
media streams, and exactly one participant [RFC4353] (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 or 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.
Layout: How rendered media streams are spatially arranged with
respect to each other on a telepresence endpoint with a single
screen and a single loudspeaker, and how rendered media streams
are arranged with respect to each other on a telepresence endpoint
with multiple screens or loudspeakers. Note that audio as well as
video are encompassed by the term layout -- in other words,
included is the placement of audio streams on loudspeakers as well
as video streams on video screens.
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 may include a mixer [RFC4353].
Media: Any data that, after suitable encoding, can be conveyed
over RTP, including audio, video, or timed text.
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Model: a set of assumptions a telepresence system of a given
vendor adheres to and expects the remote telepresence system(s) to
also adhere to.
Remote: Sender and/or receiver on the other side of the
communication channel (depending on context); i.e., 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.
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 a "being there" experience characteristic of
telepresence, media inputs need to be transported, received, and
coordinated between participating systems. Different telepresence
systems take diverse approaches in crafting a solution, or they
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, even if those extensions are based on common standards such as
SIP, many telepresence systems use proprietary protocol extensions to
solve telepresence-related problems.
Some degree of interworking between systems from different vendors is
possible through transcoding and translation. This requires
additional devices, which are expensive, are often not entirely
automatic, and sometimes introduce unwelcome side effects, such as
additional delay or degraded 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
start as planned or are interrupted by difficulties that arise.
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The general problem that needs to be solved can be described as
follows. Today, each endpoint renders the audio and video captures
it receives according to an implicitly assumed model that stipulates
how to produce a realistic depiction of the remote location. If all
endpoints are manufactured by the same vendor, they all share the
same implicit model and render the received captures correctly.
However, if the devices are from different vendors, the models used
for rendering presence can and usually do differ. The result can be
that the telepresence systems actually connect, but the user
experience will suffer, for example one system assumes that the first
video stream is captured from the right camera, whereas the other
assumes the first video stream is captured from the left camera.
If Alice and Bob are at different sites, Alice needs to tell Bob
about the camera and sound equipment arrangement at her site so that
Bob's receiver can create an accurate rendering 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 may
include number, placement, capture/render angle, resolution of
cameras and screens, spatial location, and audio mixing parameters of
microphones.
The telepresence multistream work seeks to describe the sender
situation in a way that allows the receiver to render it
realistically even though it may have a different rendering model
than the sender.
5. Requirements
Although some aspects of these requirements can be met by existing
technology, such as the Session Description Protocol (SDP) [RFC4566],
they are stated here to have a complete record of the requirements
for CLUE. Determining whether a requirement needs new work or not
will be part of the solution development, and is not discussed in
this document. Note that the term "solution" is used in these
requirements to mean the protocol specifications, including
extensions to existing protocols as well as any new protocols,
developed to support the use cases. The solution might introduce
additional functionality that is not mapped directly to these
requirements; e.g., the detailed information carried in the signaling
protocol(s). In cases where the requirements are directly relevant
to specific use cases as described in [RFC7205], a reference to the
use case is provided.
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REQ-1: The solution MUST support a description of the spatial
arrangement of source video images sent in video streams
that 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.
This requirement relates to all the use cases described in
[RFC7205].
REQ-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.
REQ-1b: The solution MUST support a means of allowing the
preservation of order of images in the scene in two
dimensions - horizontal and vertical.
REQ-1c: The solution MUST support a means to identify the
relative location, within a scene, of the point of
capture of individual video captures in three
dimensions.
REQ-1d: The solution MUST support a means to identify the
area of coverage, within a scene, of individual
video captures in three dimensions.
REQ-2: The solution MUST support a description of the spatial
arrangement of captured source audio sent in audio streams
that 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.
This requirement relates to all the use cases described in
[RFC7205], but is particularly important in the
Heterogeneous Systems use case.
REQ-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 on Susan's
right in the captured audio, John voice is also
placed on Susan's right in the rendered image.
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REQ-2b: The solution MUST support a means to identify the
number and spatial arrangement of audio channels
including monaural, stereophonic (2.0), and 3.0
(left, center, right) audio channels.
REQ-2c: The solution MUST support a means to identify the
point of capture of individual audio captures in
three dimensions.
REQ-2d: The solution MUST support a means to identify the
area of coverage of individual audio captures in
three dimensions.
REQ-3: 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.
This requirement relates to all the use cases described in
[RFC7205].
REQ-4: The solution MUST enable interoperability between endpoints
that have a different number of similar devices. For
example, an endpoint may have 1 screen, 1 loudspeaker, 1
camera, 1 mic, and another endpoint may have 3 screens, 2
loudspeakers, 3 cameras and 2 microphones. Or, in a
multipoint conference, an endpoint may have 1 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.
This requirement relates to the Point-to-Point Meeting:
Symmetric and Multipoint Meeting use cases described in
[RFC7205].
REQ-5: The solution MUST support means of enabling interoperability
between telepresence endpoints where cameras are of
different picture aspect ratios.
REQ-6: The solution MUST provide scaling information that enables
rendering of a video image at the actual size of the
captured scene.
REQ-7: The solution MUST support means of enabling interoperability
between telepresence endpoints where displays are of
different resolutions.
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REQ-8: The solution MUST support methods for handling different bit
rates in the same conference.
REQ-9: The solution MUST support means of enabling interoperability
between endpoints that send and receive different numbers of
media streams.
This requirement relates to the Heterogeneous Systems and
Multipoint Meeting use cases.
REQ-10: The solution MUST ensure that endpoints that support
telepresence extensions can establish a session with a SIP
endpoint that does not support the telepresence extensions.
For example, in the case of a SIP endpoint that supports a
single audio and a single video stream, an endpoint that
supports the telepresence extensions would setup a session
with a single audio and single video stream using existing
SIP and SDP mechanisms.
REQ-11: The solution MUST support a mechanism for determining
whether or not an endpoint or MCU is capable of telepresence
extensions.
REQ-12: The solution MUST support a means to enable more than two
endpoints to participate in a teleconference.
This requirement relates to the Multipoint Meeting use case.
REQ-13: The solution MUST support both transcoding and switching
approaches for providing multipoint conferences.
REQ-14: The solution MUST support mechanisms to allow media from one
source endpoint or/and multiple source endpoints to be sent
to a remote endpoint at a particular point in time. Which
media is sent at a point in time may be based on local
policy.
REQ-15: The solution MUST provide mechanisms to support the
following:
* Presentations with different media sources
* Presentations for which the media streams are visible to
all endpoints
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* Multiple, simultaneous presentation media streams,
including presentation media streams that are spatially
related to each other.
The requirement relates to the Presentation use case.
REQ-16: The specification of any new protocols for the solution MUST
provide extensibility mechanisms.
REQ-17: The solution MUST support a mechanism for allowing
information about media captures to change during a
conference.
REQ-18: The solution MUST provide a mechanism for the secure
exchange of information about the media captures.
6. Acknowledgements
This document has benefited from all the comments on the CLUE mailing
list and a number of discussions. So many people contributed that it
is not possible to list them all. However, the comments provided by
Roberta Presta, Christian Groves and Paul Coverdale during WGLC were
particularly helpful in completing the WG document.
7. Security Considerations
REQ-18 identifies the need to securely transport the information
about media captures. It is important to note that session setup for
a telepresence session will use SIP for basic session setup and
either SIP or the Centralized Conferencing Manipulation Protocol
(CCMP) [RFC6503] for a multiparty telepresence session. Information
carried in the SIP signaling can be secured by the SIP security
mechanisms as defined in [RFC3261]. In the case of conference
control using CCMP, the security model and mechanisms as defined in
the Centralized Conferencing (XCON) Framework [RFC5239] and CCMP
[RFC6503] documents would meet the requirement. Any additional
signaling mechanism used to transport the information about media
captures needs to define the mechanisms by which the information is
secure. The details for the mechanisms needs to be defined and
described in the CLUE framework document and related solution
document(s).
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8. 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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents", BCP
119, RFC 4579, August 2006.
[RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
January 2008.
[RFC5239] Barnes, M., Boulton, C., and O. Levin, "A Framework for
Centralized Conferencing", RFC 5239, June 2008.
[RFC6503] Barnes, M., Boulton, C., Romano, S., and H. Schulzrinne,
"Centralized Conferencing Manipulation Protocol", RFC
6503, March 2012.
[RFC7205] Romanow, A., Botzko, S., Duckworth, M., and R. Even, "Use
Cases for Telepresence Multistreams", RFC 7205, April
2014.
[ITU.H323] ITU-T, "Packet-based Multimedia Communications Systems",
ITU-T Recommendation H.323, December 2009.
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Authors' Addresses
Allyn Romanow
Cisco Systems
San Jose, CA 95134
USA
EMail: allyn@cisco.com
Stephen Botzko
Polycom
Andover, MA 01810
USA
EMail: stephen.botzko@polycom.com
Mary Barnes
MLB@Realtime Communications, LLC
EMail: mary.ietf.barnes@gmail.com
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