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INFORMATIONAL
Network Working Group O. Levin
Request for Comments: 4245 Microsoft Corporation
Category: Informational R. Even
Polycom
November 2005
High-Level Requirements for Tightly Coupled SIP Conferencing
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 (2005).
Abstract
This document examines a wide range of conferencing requirements for
tightly coupled SIP conferences. Separate documents will map the
requirements to existing protocol primitives, define new protocol
extensions, and introduce new protocols as needed. Together, these
documents will provide a guide for building interoperable SIP
conferencing applications.
Table of Contents
1. Introduction ....................................................2
2. An Overview .....................................................2
3. High-Level Requirements .........................................3
3.1. Discovery Phase ............................................3
3.2. Conference Creation ........................................4
3.3. Conference Termination .....................................4
3.4. Participants' Manipulations ................................4
3.4.1. Participation of a Conference-Unaware User Agent ......5
3.4.2. Dial-Out Scenarios ....................................5
3.4.3. Dial-In Scenarios .....................................5
3.4.4. Third-Party Invitation to a Conference ................6
3.4.5. Participants' Removal .................................6
3.4.6. Participants' Privacy .................................6
3.5. Conference State Information ...............................7
3.5.1. Description ...........................................7
3.5.2. Dissemination of Changes ..............................7
3.5.3. On-demand Information Dissemination ...................8
3.6. Focus Role Migration .......................................8
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3.7. Side-bar Conferences .......................................8
3.8. Cascading of Conferences ...................................9
3.9. SIMPLE and SIP Conferencing Coordination ...................9
4. Security Considerations ........................................10
5. Contributors ...................................................10
6. References .....................................................10
6.1. Normative References ......................................10
1. Introduction
This document examines a wide range of conferencing requirements for
tightly coupled SIP (RFC 3261 [2]) conferencing.
The requirements are grouped by subjects in various areas allowing
solutions to progress in parallel.
Separate documents will map the requirements to existing protocol
primitives, define new protocol extensions, and introduce new
protocols as needed.
Together, these documents will provide a guide for building
interoperable SIP conferencing applications.
The terms "MAY", "SHOULD", and "MUST" are to be interpreted as
described in RFC 2119 [1].
2. An Overview
A SIP conference is an association of SIP user agents (i.e.,
conference participants) with a central point (i.e., a conference
focus), where the focus has direct peer-wise relationships with the
participants by maintaining a separate SIP dialog with each.
The focus is a SIP user agent that has abilities to host SIP
conferences including their creation, maintenance, and manipulation
using SIP call control means and potentially other non-SIP means.
In this tightly coupled model, the SIP conference graph is always a
star. The conference focus maintains the correlation among
conference's dialogs internally.
The conference focus can be implemented either by a participant or by
a separate application server.
In the first case, a focus is typically capable of hosting a simple
ad hoc conference only. We envision that such basic conference can
be established using SIP call control primitives only.
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A dedicated conference server, in addition to the basic features,
offers richer functionality including simultaneous conferences, large
scalable conferences, reserved conferences, and managed conferences.
A conferencing server can support any subset of the advanced
conferencing functions presented in this document.
The media graph of a SIP conference can be centralized,
decentralized, or any combination of both, and potentially differ per
media type. In the centralized case, the media sessions are
established between the focus and each one of the participants. In
the de-centralized (i.e., distributed) case, the media graph is a
(multicast or multi-unicast) mesh among the participants.
Consequently, the media processing (e.g., mixing) can be performed
either by the focus alone or by the participants.
Conference participants and third parties can have different roles
and privileges in a certain conference. For example, conferencing
policy can state that the rights to disconnect from and to invite to
a conference are limited to the conference chair only.
Throughout the document, by conference policies we mean a set of
parameters and rules (e.g., maximum number of participants, needs
chair-person supervision or not, password protected or not, duration,
or a way of media mixing) that are defined at the onset of a
conference. Typically, conference policies would be specified by a
conference creator and need special privileges to be manipulated.
Throughout the document, by a conference state we mean a set of
information describing the conference in progress. This includes
participants' information (such as dialog identifiers), media
sessions in progress, the current loudest speaker, the current chair,
etc.
3. High-Level Requirements
In addition to the requirements presented in this document,
supplementary requirements for conferencing policy, media mixing and
other manipulations, floor control, privilege control, etc. will be
discussed in separate documents.
3.1. Discovery Phase
Some of the requirements presented in this section can be met either
by configuration means or by using proprietary conventions.
Nevertheless, there is consensus that standard means for implementing
these functions by automata MUST be defined.
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REQ-1: Discovery of a location of an arbitrary SIP conferencing
server(s).
REQ-2: Given a SIP Address-of-Record (AOR) of a certain entity,
resolution whether the SIP entity has focus capabilities.
REQ-3: Given a global identifier of a particular conference, locating
the conference focus.
REQ-4: Given a global identifier of a particular conference,
obtaining the conference properties.
REQ-5: Given a global identifier of a particular conference,
obtaining the conference state information.
3.2. Conference Creation
Given a focus location, a means MUST be defined for an interested
entity (including a user agent) to implement the procedures below:
REQ-1: Creation of an ad-hoc conference identifier and the conference
with specified properties.
REQ-2: Creation of a reserved conference identifier for a conference
with specified properties.
REQ-3: Specifying properties upon conference creation in any of the
following ways: default, profiles, and explicitly.
3.3. Conference Termination
REQ-1: Given a conference identifier, a means MUST be defined for a
user agent to disconnect all participants from the conference
and terminate the conference including the release of the
associated resources.
REQ-2: A means MAY be defined for requesting a focus to revert a
two-party conference to a basic SIP point-to-point session
including the release of the associated conferencing resources.
3.4. Participants' Manipulations
Some of the requirements presented in this section can be met by
human intervention, configuration means, or proprietary
conventions. Nevertheless, there is consensus that standard
means for implementing these functions by automata MUST be
defined.
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3.4.1. Participation of a Conference-Unaware User Agent
REQ-1: Focus MUST be able to invite and disconnect an RFC 3261
compliant only SIP user agent to and from a SIP conference.
REQ-2: An RFC 3261 compliant only SIP user agent MUST be able to
dial-in to a particular SIP conference. In this case, only the
human knows that he/she is connected to the conference.
3.4.2. Dial-Out Scenarios
REQ-1: A means MUST be defined for a focus to invite another user
agent to one of the focus' conferences. This procedure MUST
result in the establishment of a single SIP dialog between the
two.
REQ-2: Given an existing SIP dialog between two user agents, if at
least one user agent has focus capabilities, a means MUST be
defined for the conference focus to invite the other user agent
to one of the focus' conferences without additional SIP dialog
establishment.
REQ-3: An invitation to a user agent to join a conference MUST
include a standard indication that it is a conference and the
conference identifier.
3.4.3. Dial-In Scenarios
REQ-1: A means MUST be defined for a user agent to create an ad hoc
conference with default properties (as per "Conference Creation"
REQ-1 above) and to become a participant using a single SIP
dialog.
REQ-2: Given a reserved conference identifier, a means MUST be
defined for a user agent to activate the conference and to
become a participant using a single SIP dialog.
REQ-3: Given a conference identifier of an active conference, a means
MUST be defined for a user agent to dial-in the conference and
to become a participant using a single SIP dialog between the
two.
REQ-4: Given an identifier of one of the dialogs of a particular
active conference, a means MUST be defined for a user agent to
dial-in the conference and to become a participant.
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3.4.4. Third-Party Invitation to a Conference
REQ-1: Given a conference identifier, a means MUST be defined for a
user agent to invite another user agent to this conference.
REQ-2: Given an identifier of one of the dialogs of a particular
active conference, a means MUST be defined for a user agent to
invite another user agent to this conference.
EQ-3: Given a conference identifier, a means SHOULD be defined for a
user agent to invite a list of user agents to this conference (a
so-called "mass invitation").
3.4.5. Participants' Removal
REQ-1: A means MUST be defined for a conference focus to remove a
conference participant from the conference.
REQ-2: Given a conference identifier, a means MUST be defined for a
user agent to remove a participant from the conference.
REQ-3: Given an identifier of one of the dialogs of a particular
active conference, a means MUST be defined for a user agent to
remove a participant from the conference.
REQ-4: Given a conference identifier, a means MUST be defined for a
user agent to remove all the participants from the conference.
REQ-5: Given a conference identifier and a sub-list of participants,
a means MAY be defined for a user agent to remove the specified
participants from the conference (a so-called "mass ejection").
3.4.6. Participants' Privacy
A conference focus SHOULD support the procedures described in this
section. A conference participant MAY support the procedures
described in this section. The requirements imply that "anonymizing"
operations MUST be performed on all: the call control, the media
control, and the media content when appropriate.
REQ-1: A conference participant joins the conference "anonymously";
that is, his/her presence can be announced but without
disclosing his/her identity.
REQ-2: A conference participant requests a focus for anonymous
participation in the conference.
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REQ-3: A conference participant joins a conference in a "hidden
mode"; that is, his/her presence and identity are not to be
disclosed to other participants.
REQ-4: A conference participant requests a focus for participation in
the conference in a hidden mode.
3.5 Conference State Information
3.5.1. Description
By a conference state, we mean a virtual database describing the
conference in progress. This includes different conference aspects:
participants' information (such as dialog identifiers and state),
media sessions in progress (such as current stream contributing
sources and encoding schemes), the current loudest speaker, the
current chair, etc. Conference state is the latest conference
snapshot triggered by changes in participants' state, conference
policy changes, etc.
REQ-1: A conference state virtual database MUST have a modular
definition that is, it MUST be possible to access different
conference aspects independently.
REQ-2: It MUST be possible to aggregate information relating to
different conference aspects in a single report.
REQ-3: A mechanism for extensible definition and registration of
conference state evolving aspects MUST be present.
REQ-4: A default conference state report MUST be defined. It SHOULD
contain a minimal useful set of information (e.g., a list of
current conference participants).
3.5.2. Dissemination of Changes
REQ-1: A means MUST be defined for reporting the conference state
changes to interested parties (including non-conference
participants) in a timely manner.
REQ-2: A means MUST be defined for a SIP user agent to express its
interest in selected state changes only.
REQ-3: A means MUST be defined for a SIP user agent to express the
minimum interval between receiving state change reports.
REQ-4: It MUST be possible to aggregate recent changes in a single
reporting event.
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REQ-5: Default conference state change reports MUST be defined. They
SHOULD contain minimal useful to the participants information
(e.g., participants' joining and leaving the conference).
3.5.3. On-demand Information Dissemination
REQ-1: A means MUST be defined to disseminate any conference state
information to interested parties (including SIP user agents)
on-demand.
REQ-2: A means MUST be defined for an interested party (including a
SIP user agent) to request conference state information of a
particular conference defined by the conference identifier.
REQ-3: A means MUST be defined for an interested party (including a
SIP user agent) to specify the subset of the conference state
information it wants and is capable of receiving.
3.6. Focus Role Migration
REQ-1: A procedure for delegating a focus role by the current focus
to another participant MUST be defined.
REQ-2: A procedure for requesting a conference focus to transfer its
role to another participant MUST be defined.
REQ-3: A procedure for on-demand unconditional transfer of the focus
role to a different participant MUST be defined.
REQ-4: A detection procedure for a focus failure condition MUST be
defined.
3.7. Side-bar Conferences
A standard means MUST be defined in order to implement the operations
defined in this section below.
REQ-1: A user agent (not a conference participant) joins a side-bar
within the conference by SIP means.
REQ-2: A user agent (not a conference participant) is invited to a
side-bar within the conference by SIP means.
REQ-3: A conference participant creates a side-bar conference with
one or more participants in a conference by SIP means.
REQ-4: A conference participant joins a side-bar within the
conference by SIP means.
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REQ-5: A conference participant is invited to a side-bar within the
conference by SIP means.
REQ-6: A conference-unaware user agent (a participant or not) creates
and participates in side-bar conferences. It MAY be achieved by
non-SIP means.
REQ-7: A conference participant creates side-bar conferences within
the conference without establishing any additional SIP dialogs
with the focus. It MAY be achieved by non-SIP means.
REQ-8: A conference participant joins any number of side-bars within
the conference without establishing any additional SIP dialogs
with the focus. It MAY be achieved by non-SIP means.
REQ-9: A conference participant is invited to any number of side-bars
within the conference without establishing any additional SIP
dialogs with the focus. It MAY be achieved by non-SIP means.
3.8. Cascading of Conferences
"Cascading of Conferences" is a term that has different meanings in
different contexts. Some examples are listed below:
- Peer-to-peer chaining of signaling. (Many ways exist to
build the media graph in this case.)
- Conferences have hierarchal signaling relations. (Many ways
exists to build the media graph in this case.)
- "Cascading" is used to distribute the media "mixing" only.
The distribution of signaling is not required.
As it can be seen from the examples, each will define a different set
of requirements.
3.9. SIMPLE and SIP Conferencing Coordination
REQ-1: SIMPLE-based Presence and Instant Messaging architecture
SHOULD fit into the general SIP Conferencing architecture.
REQ-2: A scenario where a multimedia SIP conference and a multiparty
instant messaging conversation take place among the same group
of participants MUST be addressed.
REQ-3: A scenario where a side-bar and/or a sub-IM-conference is
being held as a part of SIP conference MUST be addressed.
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4. Security Considerations
This document discusses high-level requirements for SIP conferencing.
Conferencing has some specific security requirements, which will be
summarized here at a very high level.
All of the operations and functions described in this document need
to be authorized by a focus or a participant. It is expected that
conferences will be governed by a set of authorization rules defined
as a part of the conference policy. In order for the conference
policy to be implemented, the focus needs to be able to authenticate
potential participants. Normal SIP mechanisms including Digest
authentication and certificates can be used [2]. These conference-
specific security requirements will be discussed in detail in the
protocol documents.
Conferencing also has privacy implications. Some of these are
discussed in this document. Standard SIP mechanisms for a user agent
to request privacy should be utilized by a focus and will be detailed
in the protocol documents.
5. Contributors
This work is based on the discussions among the members of the SIP
Conferencing design team.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] 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.
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Authors' Addresses
Orit Levin
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
EMail: oritl@microsoft.com
Roni Even
Polycom
94 Derech Em Hamoshavot
Petach Tikva, Israel
EMail: roni.even@polycom.co.il
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