Network Working Group R. Even
Request for Comments: 4597 Polycom
Category: Informational N. Ismail
Cisco Systems, Inc.
July 2006
Conferencing Scenarios
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes multimedia conferencing scenarios. It
describes both basic and advanced conferencing scenarios involving
voice, video, text, and interactive text sessions. These scenarios
will help with the definition and evaluation of the protocols being
developed in the centralized conferencing XCON working group.
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Table of Contents
1. Introduction ....................................................3
2. Basic Conferencing Scenarios ....................................3
2.1. Ad Hoc Conferences .........................................4
2.2. Extension of a Point-to-Point Call to a Multipoint Call ....4
2.3. Reserved Conferences .......................................4
3. Advanced Conferencing Scenarios .................................5
3.1. Extending a Point-to-Point Call to a Multipoint Call .......5
3.2. Lecture Mode Conferences ...................................5
3.3. Conference with Conference-Aware and Unaware Participants ..6
3.4. A Reserved or Ad Hoc Conference with
Conference-Aware Participants ..............................6
3.5. Advanced Conference Features ...............................6
4. Scenarios for Media Policy Control ..............................9
4.1. Video Mixing Scenarios ....................................10
4.2. Typical Video Conferencing Scenario .......................11
4.3. Conference Sidebar Scenario ...............................11
4.4. Coaching Scenario .........................................12
4.5. Presentation and Q & A Session ............................12
4.6. Presence-Enabled Ad Hoc Conference ........................13
4.7. Group Chat Text Conferencing ..............................13
4.8. Interactive Text ..........................................13
4.9. Moderated Group Chat ......................................14
4.10. Text Sidebars ............................................14
4.11. Conference Announcements .................................14
5. Security Considerations ........................................14
6. Acknowledgements ...............................................15
7. Informative References .........................................15
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1. Introduction
This document describes multimedia conferencing scenarios. The
development of these scenarios is intended to help with the
definition and evaluation of the requirements for the centralized
conferencing (XCON) working group. Although this document uses some
definitions and conventions described in the SIP Conferencing
Framework document [1], these scenarios are not specific to SIP. The
document describes basic and advanced conferencing scenarios. The
advanced scenarios assume that the user agents support the set of
XCON protocols, identified in the Framework and Data Model for
Centralized Conferencing [3], in order to take advantage of the
conference functionality. However, note that many of these features
can be implemented today by using an interactive voice response (IVR)
or web interface to control the conferencing application.
The entities comprising the Conferencing System are the conference
that is the center point for signaling and the participants. The
participant who initiated the conference is called the initiating
participant.
The scenarios described here demonstrate different conferencing
services. These services can be offered in a multimedia environment
that benefits from having some support in the user agents that enable
more robust and easier-to-use conferencing services. It is up to the
conferencing system manufacturers and the conferencing service
provider to decide what services can be built and which services are
offered to the end users.
The scenarios describe multimedia examples, but they are applicable
to audio only as well as to audio and video conferences.
Multimedia conferences may include any combination of different media
types such as audio, video, text, interactive text, or presentation
graphics. The conference scenarios are similar, but the media
handling may be dependent on the media type.
2. Basic Conferencing Scenarios
These scenarios enable a conference-unaware participant to create,
join, and participate in a conference. The participant may use out-
of-band signaling to participate in a conference, but this is not
mandatory. The Conferencing System has all the functionality it
needs in order to supply the service offered to the participants.
Typical minimum requirements are that the participant support dual-
tone multi-frequency (DTMF) tones/signal or provide voice responses
to an IVR system.
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2.1. Ad Hoc Conferences
A participant has a service provisioned to him that enables him to
start an ad hoc conference when he calls the Conferencing System.
When the participant wants to start a conference, he calls the
conference service. The participant may be identified by different
means, including request destination, authenticated identity, or an
IVR system using DTMF. The conference is created automatically with
the predefined functionality. The participant who has such a service
notifies the other participants how to call the conference via
external means such as instant message or email.
The participant may have Conferencing System functionality and thus
can create an ad hoc conference using his own user agent. An example
of such a conference is an audio conference initiated by a
participant who has a conference service that enables him to start a
conference when he calls a specific URI. The conference may be
created by the first person calling this URI, or it may be created
only after the owner is authenticated using an IVR system. In the
latter case, the other participants may get an announcement and are
placed on hold if they call the conference before the owner.
2.2. Extension of a Point-to-Point Call to a Multipoint Call
This is a basic case. The initiating participant (PA) is in a
point-to-point call with another participant (PB). PA wants to add a
third participant (PC) to the call. PA cannot provide the
Conferencing System functionality on his user agent nor can the other
participant PB. PA and PB do not support call transfer. PA has a
conferencing service that uses the methods described in 2.1. PA
conveys the conference information to PB in the point-to-point call.
Both participants disconnect and call the Conferencing System. The
Conferencing System may support dial-out (for example, via DTMF),
allowing the initiating participant PA to call the third party PC
through the Conferencing System.
2.3. Reserved Conferences
The reservation for this type of conference is typically done by an
out-of-band mechanism in advance of the actual conference time. The
conference identification, which may be a URI or a phone number with
a pin number, is allocated by the reservation system. It is sent to
all participants through email, IM, etc. The participants join by
using the conference identification. The conference identification
must be routable, enabling the allocation of a conference with free
resources at the time when the conference actually runs. The
Conferencing System can also dial out to the conference participants.
The participants may not be informed that they are in a conference,
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since their User Agent is not conference aware. The participants may
know, via announcement from the Conferencing System, that they are in
a conference and who the other participants are.
3. Advanced Conferencing Scenarios
These scenarios assume user agents that support at least call
transfer service and a way to communicate information on events from
the Conferencing System to the user agent. The Conferencing System
may have the ability to discover the capabilities of the
participants, for example, whether they support call transfer. This
section specifies the dependencies in each scenario. An advanced
conference can be initiated only by a user agent that has advanced
features, but some user agents in the conference may have less
functionality.
3.1. Extending a Point-to-Point Call to a Multipoint Call
The initiating participant PA is in a point-to-point call and wants
to add a third participant. PA can start a multipoint call on a
conferencing bridge known to him. The extension can be without
consultation, which means that PA moves the point-to-point call to
the Conferencing System and then adds the third party (this can be
done in various ways). Alternatively the extension can be done with
consultation, which means that PA puts his current party on hold,
calls the third party, asks him to join the conference, and then
transfers all the participants to the Conferencing System.
3.2. Lecture Mode Conferences
This conference scenario enables a conference with a lecturer who
presents a topic and can allow questions. The lecturer needs to know
who the participants are and needs to be able to give them the right
to speak. The right to speak can be based on floor control or an
out-of-band mechanism.
In general, the lecturer is seen/heard by the conference participants
and often shares a presentation or application with the other
participants.
A participant joining this type of conference can get the identity of
the lecturer and often the identities of the audience participants.
This type of conference may have multiple media streams. For
example, if simultaneous language translation is available, a
participant has the option of selecting the appropriate language
audio stream. Multiple video streams could include the speaker's
face and a whiteboard/demonstration stream.
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3.3. Conference with Conference-Aware and Unaware Participants
A conference can include a mix of participants that are conference-
aware and unaware. Conference-unaware participants may be using a
proxy function that proxies the advanced functionality between the
different protocols and the Conferencing System. For example, an IVR
system or a web page interface can be used to provide additional
functionality.
3.4. A Reserved or Ad Hoc Conference with Conference-Aware Participants
In order to start the conference, the initiating participant calls
the Conferencing System using, for example, a unique identifier. The
Conferencing System may use some authenticating method to qualify the
participant. The other participants may call the Conferencing System
and join the conference. The Conferencing System is able to find the
capabilities of the participants. In case of a reserved conference,
the Conferencing System starts the conference at the scheduled time.
The participants may join by calling the conference URI, or the
Conferencing System may call them. The conference may have privilege
levels associated with a specific conference or participant. The
privileges are for the initiating participant and for a regular
participant; the initiating participant may delegate privileges to
the other participants. The privileges allow functionality as
defined in the next section.
3.5. Advanced Conference Features
The following features can be used in all the advanced conferencing
scenarios. In the examples given in this section, when referring to
a participant that has a functionality, it means a participant with
the right privileges. These scenarios may be available in the
advanced conferencing scenarios and are common in many conferencing
applications. This is not a requirement list, rather some examples
of how specific functions may be used in a conference.
o Add Participants - A participant may add a new participant to the
conference. This can be done, for example, by instructing the
Conferencing System to call the participant or by the first
participant calling the new participant and pointing him to the
conference.
o Delete Participant - A participant may delete participants from
the conference if he can identify them.
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o Changing User Agent/Modes - During the course of a conference, a
participant may switch between user agents with different
capabilities while still remaining part of the conference. For
example, a participant may initially join using a mobile phone and
then switch to a desktop phone. Or a participant may join with a
phone, discover that the conference has video streams available,
and switch to a video phone.
o Changing Media - During the conference, a participant may be able
to select different media streams than the one he had when he
joined the conference. An example is a participant that initially
joined the conference as an audio participant. The participant is
unable to understand the conversation properly, and he learns that
there is also an interactive text available. He will ask to
receive the text stream also.
o Authenticate participants - A participant can authenticate other
participants who want to join the conference. This can be done,
for example, in a video conferencing session by creating a sidebar
between the two participants, allowing the authenticating
participant to talk with the new participant and verify his
identity.
o Authorize participants - A participant can authorize other
participants in order to allow them to join the conference. This
can be done implicitly by assigning a password to the conference
or to each participant and letting the Conferencing System decide
if the new participant is allowed to join. The authorization can
be done explicitly by directing the entered password to the
initiating participant who will authorize each participant. The
conferencing system may use an authentication mechanism to
authenticate the participants.
o Controlling the presentation of media - During the conference, the
participant may be able to manage whose media is being sent to
each participant. For example, the participant may be able to
decide that he wants to be the speaker and all the rest to be
listeners; he may also specify whose media he wants to receive.
The participant may be able to mute a media stream during the
conference.
o Giving privileges - During the conference, the participant may
want to give a privilege to another participant. The assigning of
privileges may be implicit when requested or explicit by asking
the participant to grant a privilege.
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o Side conferences or sidebars - The participant may want to create
a side conference that include some of the main conference
participants. When the side conference is finished, the
participants return to the main conference. A sidebar may have
the same functionality as the main conference. There can be
several sidebar scenarios:
1. A basic sidebar requires that two participants have the
capability to have two calls at the same time, with a point-
to-point call in parallel to the main conference. It is user
agent implementation-specific whether both calls' streams are
mixed automatically or the participants are allowed to
manually switch between them.
2. A conferencing-system-based sidebar uses the Conferencing
System to create the sidebar and compose the relevant sidebar
stream mixes. These mixes can include the main conference as
an incoming stream to the mix. Mechanisms to signal the
creation of the sidebar, invite participants, and control the
mixes should be available.
For example, participants in an audio sidebar may not be heard
by the rest of the conference. However, the main conference
audio may be mixed in the sidebar, but at a lower volume, or
in a different channel. As another example, a sidebar can
have a different media type from the main conference: a video
call can have an audio sidebar where the other participants
can see the sidebar participants talking but cannot hear them;
or an audio or video conference may have a text sidebar.
o Conference information - When a participant joins the conference,
he is announced to the participants. An announcement may be
available when he leaves the conference. The participants may
query the conferencing system for the current participants of a
specific conference. This conference information may include
other information, for example, the media streams available in the
conference.
o Extending of a conference - Reserved conferences and ad hoc
conferences may have a time limit. The Conferencing System
informs the participants when the limit is approaching and may
allow the extension of the conference.
o Adding and removing a media type to the conference - A participant
may want to start a data presentation during a conference. He may
want to distribute this new media to all the participants. The
participant asks the Conferencing System to start the new media
channel and to allow him to send data in the new channel.
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o Audio-only participants - In a multimedia conference, some of the
participants who want to join may have no way to send and receive
all the media types. Typically, they can send and receive audio.
Such participants join the conference as audio-only participants.
The general case is that participants may send and receive only
part of the media streams available in the multimedia conference.
o Passive participants - In a conference, some participants may be
listeners to all or part of the media streams, but may be
invisible to all other participants.
o Recorders - A recorder can be added to the conference. A recorder
can record all streams or a subset of the streams. Recorders may
be turned on and off during the conference. Recorders may be used
for a "role call" scenario in order to record a participant's
name. This name can be announced at a later stage automatically
or based on a participant request. A recorder is a case of a
passive participant.
o Whisper/Private Message - A participant can send a one-way message
(text, audio, or even some other media) to another participant
that is immediately rendered. This differs from a sidebar in that
it is immediate and creates no long-lived session.
o Human operator - A participant may ask for assistance from a human
operator during the conference.
4. Scenarios for Media Policy Control
During a conference, media streams may be controlled by authorized
participants using either a media control protocol or a third-party
application. This section describes some typical media control
scenarios. The conference can be of any size. Some of the media
control scenarios are typical of specific conference sizes. As a
general rule, larger conferences scenarios tend to be more centrally
managed or structured.
The mixing of media in a conference may start when the conference
starts or when the initiating participant joins. In the later case,
early participants may be put on hold and get "music on hold".
The scenarios apply to audio conferences as well as to multimedia
conferences. In the sections below, there is some specific
information about the mixed video layout and interactive text.
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4.1. Video Mixing Scenarios
For video, the participant selects one of a set of predefined video
presentations offered by the server. Each video presentation is
identified by a textual description as well as an image specifying
how the presentation appears on the screen. In this scenario, by
choosing a video presentation, the participant chooses how many video
streams (participants) are viewed at once and the layout of these
video streams on the screen.
The contents of each sub-window can be defined by a conference policy
and/or controlled by authorized participants. It may also be
possible to have multiple mixes per conference, possibly as many as
there are participants. (Note that the same flexibility may be
afforded to audio mixes as well.)
The following is a list of typical video presentations. Other
layouts are available today in commercial products.
- Single view: This presentation typically shows the video of the
loudest speaker.
- Dual view: This presentation shows two streams. If the streams are
to be multiplexed in one image (typical of centralized servers),
the multiplexing can be:
1. Side-by-side windows, with no altered aspect ratio. Thus,
blanking of parts of the image might be necessary if the
streams are to be combined as one image.
2. Side-by-side windows, with altered aspect ratios. Thus,
blanking parts of the image is not necessary. The mixer
handles the cropping of the images.
3. One window above the other, with no altered aspect ratio.
4. One window above the other, with altered aspect ratios.
- Quadrate view: This presentation shows 4 streams. If the streams
are multiplexed into one image (centralized server), they are
arranged in a 2x2 style. Note that in this style the aspect ratios
are maintained.
- 9 sub-picture view: This presentation shows 9 streams. If the
streams are to be multiplexed in one image, they are arranged in a
3x3 style. In the multiplexing case, cropping is performed under
the discretion of the mixer.
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- 16 sub-picture view: This presentation shows 16 streams. If the
streams are to be multiplexed into one image, they are arranged in
a 4x4 style. In this style, the aspect ratios are maintained, and
no cropping or blanking is needed.
- 5+1 sub-picture view: This presentation shows 6 streams. If the
streams are to be multiplexed into one image, then the pictures are
laid so that one sub-window occupies 4/9 of the screen while each
of the other five occupies 1/9 of the screen.
4.2. Typical Video Conferencing Scenario
This scenario is known as voice-activated video switch. Every
participant hears the N loudest participants but does not hear
himself. All the participants see the loudest speaker; the loudest
speaker may see the previous loudest speaker. This mode is typical
for a small conference.
A participant with proper authorization can exclude one or more
participants from the audio or video mix. An indication that they
are not being seen/heard might be displayed to the affected
participants.
A participant with proper authorization can manipulate the gain level
associated with one or more audio streams in the mix.
4.3. Conference Sidebar Scenario
An authorized participant creates a sidebar. The participant selects
whether the sidebar should include the media from the main conference
or not and the audio gain level associated with the main conference
audio.
A participant invites participants to the sidebar, and upon
acceptance they start receiving the sidebar media as specified by the
sidebar creator. If the new participant is not a participant of the
conference, but is just a participant of the sidebar, the participant
only receives the sidebar media without the media of the main
conference.
A participant with the right authorization can move another
participant into the sidebar with no indication, in which case the
participant suddenly starts receiving the sidebar media.
Sidebar participants with the right authorization can select to hear
or not to hear the main conference audio mixed with the sidebar
audio.
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A participant can be a participant to more than one sidebar but can
only actively participate in one.
A participant can jump back and forth between the main conference and
one or more sidebars.
4.4. Coaching Scenario
This is a call center or a remote training session where there is a
supervisor who can monitor the conference. The supervised
participants may be the call center operators or the teachers. A
participant in the conference may be a supervised participant or a
"customer".
The supervisor is a hidden participant and is not part of the
participant roster.
The supervised participants might get an announcement/tone indicating
that the supervisor has joined. The other participants do not hear
the announcement.
The supervisor listens to or sees the session but can only be heard
or seen by the supervised participant.
The supervisor can become a normal participant, in which case the
participants see the supervisor as part of the roster and start
hearing and seeing him.
4.5. Presentation and Q & A Session
An example is an earning call scenario in which a group of presenters
delivers material to a group of people. After the presentation is
finished, a Q & A session is opened.
The conference is created as a panel, and the panel participants are
identified. Only their streams are mixed.
After the end of the presentation, the session chair changes the
conference type to normal, and now streams from all participants may
be mixed. Alternatively, a floor control protocol can be used. The
chair can grant the right to speak by adding the participant, whose
turn it is to ask a question, to the conference mix.
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4.6. Presence-Enabled Ad Hoc Conference
A presence-enabled ad hoc conference, sometimes described as "walkie
talkie" service, is a scenario in which a participant sends media to
the other participants of the conference after receiving a
confirmation of the other participants' availability. For example, a
participant presses a talk button, which checks the presence of the
participants to see if they are available for communication. If they
are, a confirmation tone is played, and the participant can then
talk; as a result, the media is sent to the other participants in the
conference. These types of conferences tend to be long lived, hence
the need for presence to ensure that the other participants are still
available. The ad hoc nature of the conference means that the
participant list can be changed at any time. Floor control can be
used to allow other participants to speak, as the conference is
usually half-duplex in nature.
4.7. Group Chat Text Conferencing
Group chat is a common scenario for text messaging in which a
participant joins (or enters) a chat room in which text messages from
participants are rendered in a single window and attributed to the
participant that sent the message. Changes in conference membership
are often announced in the text window itself (e.g., "Alice has just
entered the room. Bob has just departed."). Note that a real-time
transcription/closed captioning service can provide a similar window
in which audio media is converted into interactive text. "Nicknames"
or aliases are often chosen by participants or assigned by the
Conferencing System and used as handles within the room.
4.8. Interactive Text
Interactive text uses RTP to carry text one character at a time,
providing real-time interactivity, as described in RFC 4103 [2]. The
interactive text session may be the main conference itself, or it may
be used in conjunction with other media types. Interactive text may
be used to represent the audio in the conference using some
translation services. There can be more than one such stream where
each text stream is in a different language. These text streams may
be used as subtitles to the audio stream. The translation from to
text to speech and back is done by transcoders. These transcoders
have similar functionality to transcoders between different audio or
video algorithms.
The conference participants should be able to select to receive text
streams with the conference audio or those without it.
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4.9. Moderated Group Chat
A moderated group chat scenario for text messaging is similar to
group chat, but all text messages sent to the group are filtered/
approved by a moderator. Note that the moderator can be a human or
an application. The moderator also often has the ability to remove
participants and provide feedback on their submissions (e.g., provide
warnings before removal).
4.10. Text Sidebars
Interactive text or instant messaging sidebars are perhaps the most
common sidebars in conferences today. Often the text sessions are
separate from the conference. However, there are some advantages to
having text sessions be a sidebar and as a result a part of the main
conference. For example, a conference that is providing anonymity/
aliases to participants can also provide anonymous/alias sidebars. A
text sidebar can also benefit from other security/logging/recording
services provided by the Conferencing System.
Another use of a text sidebar is a text-only conversation/discussion
between two or more conference participants who are following the
main conference at the same time.
4.11. Conference Announcements
The conference moderator may be able to play announcements to all the
conference participants. An announcement may be prerecorded or
composed by the moderator before it is sent. The announcements may
be text, audio, or audio-visual. An example is a conference with
several audio break-out sessions going on. At some point, the
moderator wants to record an audio message like "In 5 minutes,
everyone please come back to the main meeting" and then play that
message to all the breakout sessions.
5. Security Considerations
Conferences generally have authorization rules about who may or may
not join a conference, what type of media may or may not be used,
etc. This information, sometimes called the conference policy or
common conference information, is used by the Conferencing System to
admit or deny participation in a conference. For the conference
policy to be implemented, the Conferencing System needs to be able to
authenticate potential participants. The methods used depend on the
signaling protocols used by the conference. This can include a
challenge/response mechanism, certificates, shared secret, asserted
identity, etc.
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Conferences often require that their content be confidential. In
addition, secure authorization of participants is incomplete if
access to the media can be gained by unauthorized participants.
Functions for securing the media and for key management and
distribution to authorized participants need to be provided by the
Conferencing System. In some cases, the functions used for
participant authorization can be leveraged for this purpose.
Privacy is an important aspect of conferencing. Users may wish to
join a conference without anyone knowing that they have joined, in
order to silently listen in. In other applications, a participant
may wish just to hide their identity from other participants, but
otherwise let them know of their presence. These functions need to
be provided by the Conferencing System.
These conference-specific security requirements are discussed further
in the XCON framework document.
6. Acknowledgements
Thanks to Brian Rosen for contributing conferencing scenarios.
Thanks to Alan Johnston for going over the document and adding some
more scenarios; to Keith Lantz, Mary Barnes, and Dave Morgan for
carefully reading the document.
7. Informative References
[1] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol (SIP)", RFC 4353, February 2006.
[2] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation",
RFC 4103, June 2005.
[3] Barnes, M., "A Framework and Data Model for Centralized
Conferencing", Work in Progress, June 2006.
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Authors' Addresses
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva 49130
Israel
EMail: roni.even@polycom.co.il
Nermeen Ismail
Cisco Systems, Inc.
170 West Tasman Drive
San Jose 95134
CA USA
EMail: nismail@cisco.com
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