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XCON Working Group C. Boulton
Internet-Draft Ubiquity Software Corporation
Expires: January 2, 2006 M. Barnes
Nortel
July 1, 2005
Centralized Conferencing (XCON) Using the Message Session Relay
Protocol (MSRP)
draft-boulton-xcon-msrp-conferencing-01
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Copyright (C) The Internet Society (2005).
Abstract
A Centralized Conference as defined by the XCON working group is both
signaling and protocol agnostic. The primary focus of the XCON work
has been centered on the Session Initiation Protocol for signaling
and Audio/Video for the media types. This document defines the
mechanisms, in the context of the XCON framework, required when using
the Message Session Relay Protocol (MSRP) in a Centralized Conference
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(XCON).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Protocol and Framework Overview . . . . . . . . . . . . . . . 3
3.1 Overview of Operation . . . . . . . . . . . . . . . . . . 5
4. Text Sidebar . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Private Message . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1 Normative References . . . . . . . . . . . . . . . . . . . 10
8.2 Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. Introduction
A Centralized Conference as defined by the XCON working group is both
signaling and protocol agnostic. The primary focus of the XCON work
has been centered on the Session Initiation Protocol for signaling
and Audio/Video for the media types. The requirements to support
conferences of session-based instant messages, private messaging, and
sidebars are introduced in [6]. This document defines the mechanisms
and associated framework elements involved when using the Message
Session Relay Protocol(MSRP) in a Centralized Conference(XCON) in
support of those requirements.
[Editors Note: This document is still in early stages of development
and is intended to invoke discussion. It is not intended to provide
exact solutions at his stage, but rather explores a potential
approach to a solution.]
This document has been constructed in full compliance with both the
XCON Framework[2] document and the SIPPING Conference Framework [3]
document. The XCON Framework provides the data model and interfaces
to be used while the SIPPING Framework provides details of the SIP
signaling protocol operations. For the purpose of this document,
MSRP will be discussed in the context of SIP being the carrying
protocol, as defined in the core MSRP [4] specification.
2. Conventions and Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
3. Protocol and Framework Overview
MSRP is defined as a peer-to-peer protocol that enables a direct
connection between two compliant endpoints, unless an MSRP relay is
inserted in the MSRP signaling path. The MSRP Relay specification
[5] details the associated functionality with a relay in the
signaling path. Centralized conferencing using MSRP can be achieved
by the Conference Server appearing as an MSRP endpoint for Conference
Participants, with the Conference server distributing the messages by
relaying them to each of the conference participants.
Figure 1 provides an illustration of MSRP clients having a direct,
1:1 connection to the Conference server. The MSRP Conference Server
can be roughly categorized as a hybrid MSRP entity that combines both
MSRP client and MSRP relay functionality. An MSRP SEND sent to the
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conference will arrive at the conference server and then be
replicated to all appropriate MSRP sessions.
+--------+
| MSRP |
| Client |
| |
+--------+
|
|
|
|
|
|
v
+-----------+
+--------+ | | +--------+
| MSRP | | MSRP | | MSRP |
| Client |-------------->|Conference |<--------------| Client |
| | | Server | | |
+--------+ | | +--------+
+-----------+
^
|
|
|
|
|
|
+--------+
| XCON |
| Client |
| |
+--------+
Figure 1: Client Connection
The approach in this document is to minimize the impact no the MSRP
protocol, while taking full advantage of the functionality provided
by the XCON and SIPPING Conferencing frameworks. The solution
proposal in this document, as described in Section 3.1, meets many
of the requirements identified in the requirements document for
Multiparty MSRP [6]. Some of the requirements introduce additional
concepts that are not yet fully addressed within the context of the
XCON framework or associated protocol documents, however, they are
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general enough that they should be addressed. For example, the
notion of privacy and anonymity of participants is introduced in the
framework in the context of security, but no discussion of mechanism
is provided. [Editor's Note: Further elaboration of how this
solution proposal meets those requirements is likely required,
although a simple checklist in the appendix, or inline references to
those requirements (e.g. ...solution text...(REQ-GEN-10) might
suffice. ]
An alternative approach, also meeting the Multiparty MSRP
requirements, is documented in [7]. The solution proposed in that
document requires extensions to the base MSRP protocol. It uses the
concept of an "MSRP switch" as the centralized component, whose role
is very similar to the MSRP Conferencing Server in this document.
While that solution doesn't explicitly take advantage of the XCON FW
data model, as it primarily intends to make use of the basic SIP
conferencing framework, it could certainly do so without a large
impact on the basic solution proposal (extending MSRP). One of the
advantages of doing so would be a reuse of the XCON FW model for
sidebars and private conferences and manipulation of the conference
data. [Editor's Note: Ideally, discussion of this document can be
used to further the model for sidebars and private conferences within
the XCON FW document, which is currently very sketchy].
3.1 Overview of Operation
As mentioned in the overview, an MSRP client connecting to a
Conference server has a 1:1 relationship with the MSRP signaling
entity, each having a unique MSRP session ID (session ID's are
contained in MSRP URLs). When referring to MSRP session ID's the
document is making reference to the locally (at Conference Server)
generated Session ID that is inserted into the local Path SDP
attribute and used for MSRP session signaling identification. An
important concept in this proposal is the creation and management of
MSRP sessions. It is important that each MSRP session created, as
identified by the unique session ID, is explicitly tied to an
associated Conference, represented by the Conference Identifier.
This provides the relevant association between MSRP and a Conference.
An example is illustrated in Figure 2.
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-------------------------------------------
| XCON Conference Identifier |
-------------------------------------------
| MSRP Session ID=8asjdhk |
| MSRP Session ID=38iuhds |
| MSRP Session ID=djiowid |
| MSRP Session ID=389hewu |
-------------------------------------------
Figure 2: Session Association
On issuing an MSRP request to any of the MSRP sessions that are
members of a Conference, the message will be replicated to all other
MSRP Sessions that are participants of the Conference.
An MSRP capable client wishing to join a conference uses standardized
XCON mechanisms for creating and joining a Conference. SIP signaling
mechanisms for creating and joining a conference are defined in
SIPPING Conference Framework [3] and non signaling specific
mechanisms are defined in the XCON Framework [2].
Each conference created in accordance with the XCON data model (as
defined in the XCON framework) has a unique internal representation
in the form of a Conference Data object. This representation
provides a unique external key for use in various conference
operations (e.g. Identify a specific conference for use by the Data
Manipulation Protocol [ref]). This document will use the unique
Conference Object identifier, in association with a unique user
identifier, as a means to identify an MSRP session in association
with a conference represented by the Conference Identifier.
[Editors Note: The following is based on current discussion in the
XCON framework and would align appropriately]
This can be achieved as follows:
o Various mechanisms are used to create a conference within an XCON
compliant Conference server. These include using the Data
Manipulation Protocol and SIP signaling to generate a unique
conference identifier from the Conference Factory. The XCON
Conference Identifier might look something like:- xcon:conf=1234.
o It is proposed that the Conference system and MSRP signaling
interface use the Conference Identifier when creating MSRP
sessions for conference participants. As part of the SIP offer/
answer exchange between the Focus and an MSRP client when adding
an MSRP clients to a conference, the focus must create appropriate
SDP that complies with section 8.1 of [4].
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o As part of the SDP construction for the offer/answer the
Conference Focus has to provide an 'msrp:' URL, as defined in
section 6 of [4]. It is proposed that the contents of this URL is
the primary identifier for the associated conference instance.
The unique identifier for an MSRP session is currently located
within the SDP 'a=path' attribute, as shown by the string '2s93i9'
in Figure 3. This session Identifier is created by an MSRP client
for identification purposes and so adding semantics only impacts
the creating entity.
a=path:msrp://alice.example.com:7394/2s93i9;tcp
Figure 3: Example MSRP URL
o A conference server will use the semantics defined for the session
identifier in an 'msrp' URL to identify both a specific conference
and a participating user. The Focus MUST ensure that the
uniqueness of the whole MSRP URL is maintained for compliance to
the core MSRP specification . An example of how this could be
achieved is defined in Figure 4.
a=path:msrp://alice.example.com:7394/Conf=123456~userID=654321~;tcp
Figure 4: Example MSRP URL with Conference Identifier
o In this example the session identifier contains the following
string 'Conf=123456~userID=654321~'. This string can be broken
down to several into pieces of conference specific information.
The first part of the string contains the Conference Identifier
for which this message is intended. An MSRP conference entity
receiving an MSRP SEND request would search for the string 'Conf='
and extract the identifier from the '=' sign to the '~' character.
The MSRP conference entity can then compare with existing
conference instances to see if there is a match. Using semantics
in the session identifier allows the reuse of connections to the
conference system.
o After extracting the conference identifier from the MSRP URL, the
MSRP entity can then extract the unique user identifier. After
the first '~' sign, a unique user identifier can be found of the
form userID=654321~ from Figure 4. This allows the controlling
MSRP entity to internally identify the user issuing the SEND
request and then send the request to all other participants in the
conference instance.
o Using this mechanism does not make any additional requirements on
MSRP clients. The information contained in the MSRP URL is just
viewed as a random session identifier.
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4. Text Sidebar
The Session Based Messaging Conferencing requirements document [6]
identifies the requirement (REQ-GEN-12) to set up a sidebar
conference with one or more participants of the conference.
The concept of a 'sidebar' in an XCON compliant conference system is
fully described in section 7.3 of the XCON Framework document [2].
The creation, manipulation and deletion of sidebars for MSRP based
sessions has the same principles.
[Editors note: see current sidebar proposal in the XCON FW relating
to cloning a conference object with no associated time reference.
This provides a current sub-conference.]
Creating a sidebar representation for MSRP would have internal
ramifications in a Conference System. It would involve the creation
of a cloned conference object (see editor's note above) that
associates the appropriate Conference users. This would then involve
a new session being established to convey the Sidebar information.
The session would be created in the same manner as described in
Section 3.1 with a new 'conf=' parameter in the 'msrp' session ID but
the consistent 'userID'. This would again create unique session
identifiers that can be associated with the sidebar instance. The
Conference system would also manage the mapping between the sidebar
and the main conference.
[Editors note: Lots more detail to insert.]
5. Private Message
The Session Based Messaging Conferencing Requirements document [6]
identifies a set of requirements to support the ability to send a
private message to another participant in a conference. A private
message would not be seen by any other conference participants.
The mechanism for achieving this follows on from the semantics
contained in the construction of the identifying 'a=path' attribute,
as described in Section 3.1. This example used the 'conf=' and
'userID=' identifiers in the session identifier of the 'msrp' URL.
It is proposed that if a user wishes to send a private message to
another user in a conference it extends the use of such identifiers.
This can be achieved using an optional 'priv=' identifier. If this
identifier exists within a conference, then the SEND message is only
distributed to the one location contained in the 'priv=' identifier.
If the identifier does not exist within a Conference, the SEND
message SHOULD NOT be distributed to the Conference members and
should be rejected with an appropriate error response.
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An example of an 'msrp' URL containing the 'priv=' identifier is
contained in Figure 5
a=path:msrp://alice.example.com:7394/Conf=123456~userID=654321~priv=abcdef~;tcp
Figure 5: Example MSRP Private Message URL
[Editors note: Need to explain how a conference client obtains
individual participant identifiers. The initial inclination is to
propose the conference package rather than define a new Event
package, but this is also a more general question relating to the
XCON FW, as well (i.e. we need to complete and agree the work on
identifiers there to better understand the general approach).]
This section describes a mechanism to send a private message to one
other participant in a conference. If there is a desire to involve
more than one other participant in the private messaging, then a
sidebar would be used.
6. Security Considerations
As discussed in the XCON Framework, there are a wide variety of
potential attacks related to conferencing, due to the natural
involvement of multiple endpoints and the many, often user-invoked,
capabilities provided by the conferencing system. Examples of
attacks in the context of MSRP conferencing would include the
following: an endpoint attempting to receive the messages for
conferences in which it is not authorized to participate, an
endpoint attempting to disconnect other users, and theft of service,
by an endpoint, in attempting to create conferences it is not allowed
to create.
Since this solution makes use of existing protocols (e.g. MSRP,
Conference Control Protocol, SIP), it also re-uses the security
solutions for those protocols and the associated authorization
mechanisms. There are security issues associated with the
authorization to specifically perform the MSRP conferencing
capability. Since, this solution makes use of the XCON framework, it
makes use of the policy associated with the Conference Object to
ensure that only authorized entities are able to manipulate the data
to access the capabilities. This solution also makes use of the
privacy and security of the identity of a user in the conference, as
discussed in the XCON Framework.
[Editor's Note: Are there any security issues unique to MSRP
conferencing that aren't covered by based MSRP, MSRP relays or the
conferencing framework? ]
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7. Acknowledgements
The authors would like to thank Miguel Garcia-Martin for his comments
on this document and discussion of the solution options.
8. References
8.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
8.2 Informative References
[2] Barnes, M., "A Framework and Data Model for Centralized
Conferencing", draft-ietf-xcon-framework-00 (work in progress),
May 2005.
[3] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol",
draft-ietf-sipping-conferencing-framework-05 (work in progress),
May 2005.
[4] Campbell, B., "The Message Session Relay Protocol",
draft-ietf-simple-message-sessions-10 (work in progress),
February 2005.
[5] Jennings, C. and R. Mahy, "Relay Extensions for the Message
Sessions Relay Protocol (MSRP)",
draft-ietf-simple-msrp-relays-04 (work in progress), June 2005.
[6] Niemi, A. and M. Garcia-Martin, "Requirements for Private
Messaging in Centralized Conference Environments",
draft-garcia-xcon-private-messaging-reqs-01 (work in progress),
June 2005.
[7] Niemi, A. and M. Garcia-Martin, "Multi-party Message Sessions
using the Message Session Relay Protocol (MSRP)",
draft-niemi-simple-chat-02 (work in progress), February 2005.
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Authors' Addresses
Chris Boulton
Ubiquity Software Corporation
Building 3
Wern Fawr Lane
St Mellons
Cardiff, South Wales CF3 5EA
Email: cboulton@ubiquitysoftware.com
Mary Barnes
Nortel
2201 Lakeside Blvd
Richardson, TX
Email: mary.barnes@nortel.com
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