Independent Submission A. Saleem
Request for Comments: 5707 Y. Xin
Category: Informational RadiSys
ISSN: 2070-1721 G. Sharratt
Consultant
February 2010
Media Server Markup Language (MSML)
Abstract
The Media Server Markup Language (MSML) is used to control and invoke
many different types of services on IP media servers. The MSML
control interface was initially driven by RadiSys with subsequent
significant contributions from Intel, Dialogic, and others in the
industry. Clients can use it to define how multimedia sessions
interact on a media server and to apply services to individuals or
groups of users. MSML can be used, for example, to control media
server conferencing features such as video layout and audio mixing,
create sidebar conferences or personal mixes, and set the properties
of media streams. As well, clients can use MSML to define media
processing dialogs, which may be used as parts of application
interactions with users or conferences. Transformation of media
streams to and from users or conferences as well as interactive voice
response (IVR) dialogs are examples of such interactions, which are
specified using MSML. MSML clients may also invoke dialogs with
individual users or with groups of conference participants using
VoiceXML.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not 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/rfc5707.
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IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. The RFC Editor
has chosen to publish this document at its discretion. Readers of
this document should exercise caution in evaluating its value for
implementation and deployment. See RFC 3932 for more information.
Copyright Notice
Copyright (c) 2010 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.
Table of Contents
1. Introduction ....................................................4
2. Glossary ........................................................5
3. MSML SIP Usage ..................................................6
3.1. SIP INFO ...................................................7
3.2. SIP Control Framework ......................................8
4. Language Structure .............................................15
4.1. Package Scheme ............................................15
4.2. Profile Scheme ............................................18
5. Execution Flow .................................................19
6. Media Server Object Model ......................................21
6.1. Objects ...................................................21
6.2. Identifiers ...............................................23
7. MSML Core Package ..............................................26
7.1. ....................................................26
7.2. ....................................................26
7.3. ..................................................27
7.4. ...................................................27
8. MSML Conference Core Package ...................................28
8.1. Conferences ...............................................28
8.2. Media Streams .............................................29
8.3. ........................................31
8.4. ........................................33
8.5. .......................................34
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8.6. ................................................35
8.7. .............................................36
8.8. ....................................................43
8.9. ............................................45
8.10. .................................................46
8.11. ................................................47
8.12. .................................................47
9. MSML Dialog Packages ...........................................51
9.1. Overview ..................................................51
9.2. Primitives ................................................53
9.3. Events ....................................................55
9.4. MSML Dialog Usage with SIP ................................56
9.5. MSML Dialog Structure and Modularity ......................57
9.6. MSML Dialog Core Package ..................................58
9.7. MSML Dialog Base Package ..................................63
9.8. MSML Dialog Group Package .................................81
9.9. MSML Dialog Transform Package .............................85
9.10. MSML Dialog Speech Package ...............................88
9.11. MSML Dialog Fax Detection Package ........................92
9.12. MSML Dialog Fax Send/Receive Package .....................93
10. MSML Audit Package ...........................................100
10.1. MSML Audit Core Package .................................100
10.2. MSML Audit Conference Package ...........................102
10.3. MSML Audit Connection Package ...........................106
10.4. MSML Audit Dialog Package ...............................108
10.5. MSML Audit Stream Package ...............................110
11. Response Codes ...............................................111
12. MSML Conference Examples .....................................113
12.1. Establishing a Dial-In Conference .......................113
12.2. Example of a Sidebar Audio Conference ...................117
12.3. Example of Removing a Conference ........................118
12.4. Example of Modifying Video Layout .......................118
13. MSML Dialog Examples .........................................120
13.1. Announcement ............................................120
13.2. Voice Mail Retrieval ....................................120
13.3. Play and Record .........................................122
13.4. Speech Recognition ......................................125
13.5. Play and Collect ........................................125
13.6. User Controlled Gain ....................................128
14. MSML Audit Examples ..........................................128
14.1. Audit All Conferences ...................................128
14.2. Audit Conference Dialogs ................................129
14.3. Audit Conference Streams ................................130
14.4. Audit All Connections ...................................131
14.5. Audit Connection Dialogs ................................131
14.6. Audit Connection Streams ................................132
14.7. Audit Connection with Selective States ..................133
15. Future Work ..................................................134
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16. XML Schema ...................................................134
16.1. MSML Core ...............................................136
16.2. MSML Conference Core Package ............................140
16.3. MSML Dialog Packages ....................................148
16.4. MSML Audit Packages .....................................170
17. Security Considerations ......................................176
18. IANA Considerations ..........................................176
18.1. IANA Registrations for 'application' MIME Media Type ....176
18.2. IANA Registrations for 'text' MIME Media Type ...........178
18.3. URN Sub-Namespace Registration ..........................179
18.4. XML Schema Registration .................................180
19. References ...................................................181
19.1. Normative References ....................................181
19.2. Informative References ..................................182
Acknowledgments ..................................................183
1. Introduction
Media servers contain dynamic pools of media resources. Control
agents and other users of media servers (called media server clients)
can define and create many different services based on how they
configure and use those resources. Often, that configuration and the
ways in which those resources interact will be changed dynamically
over the course of a call, to reflect changes in the way that an
application interacts with a user.
For example, a call may undergo an initial IVR dialog before being
placed into a conference. Calls may be moved from a main conference
to a sidebar conference and then back again. Individual calls may be
directly bridged to create small n-way calls or simple sidebars.
None of these change the SIP [n1] dialog or RTP [i3] session. Yet
these do affect the media flow and processing internal to the media
server.
The Media Server Markup Language (MSML) is an XML [n2] language used
to control the flow of media streams and services applied to media
streams within a media server. It is used to invoke many different
types of services on individual sessions, groups of sessions, and
conferences. MSML allows the creation of conferences, bridging
different sessions together, and bridging sessions into conferences.
MSML may also be used to create user interaction dialogs and allows
the application of media transforms to media streams. Media
interaction dialogs created using MSML allow construction of IVR
dialog sessions to individual users as well as to groups of users
participating in a conference. Dialogs may also be specified using
other languages, VoiceXML [n5], which support complete single-party
application logic to be executed on the media server.
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MSML is a transport independent language, such that it does not rely
on underlying transport mechanisms and language semantics are
independent of transport. However, SIP is a typical and commonly
used transport mechanism for MSML, invoked using the SIP URI scheme.
This specification defines using MSML dialogs using SIP as the
transport mechanism.
A network connection may be established with the media server using
SIP. Media received and transmitted on that connection will flow
through different media resources on the media server depending on
the requested service. Basic Network Media Services with SIP [n7]
defines conventions for associating a basic service with a SIP
Request-URI. MSML allows services to be dynamically applied and
changed by a control agent during the lifetime of the SIP dialog.
MSML has been designed to address the control and manipulation of
media processing operations (e.g., announcement, IVR, play and
record, automatic speech recognition (ASR), text to speech (TTS),
fax, video), as well as control and relationships of media streams
(e.g., simple and advanced conferencing). It provides a general-
purpose media server control architecture. MSML can additionally be
used to invoke other more complex IVR languages such as VoiceXML.
The MSML control interface has been widely deployed in the industry,
with numerous client-side and server-side implementations, since
2003. The in-service commercial deployments cover a wide variety of
applications including, but not limited to, IP multimedia
conferencing, network voice services, IVR, IVVR (interactive voice
and video response), and voice/video mail.
2. Glossary
Media Server: a general-purpose platform for executing real-time
media processing tasks. This is a logical function that maps either
to a single physical device or to a portion of a physical device.
Media Server Client: an application that originates MSML requests to
a media server and also referred to as a control agent in this
specification.
Network Connection: a participant that represents the termination on
a media server of one or more RTP [i3] sessions (for example, audio
and video) associated with a call. Network connections are
established and removed using a session establishment protocol such
as SIP. An instance of a network connection is independent of MSML
processing instructions applied to it.
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Dialog: an automated IVR participant. Examples of dialogs may be
announcement players, IVR interfaces, or voice recorders. Dialogs
may be defined in MSML or using VoiceXML [n5].
Conference: an intermediary function that provides multimedia mixing
and other advanced conferencing services. This specification
currently considers conferences with audio and/or video media types,
but is extensible to other media types.
Identifier: a name that is used to refer to a specific instance of an
object on the media server, such as a conference or a dialog.
Identifiers are composed of one or more terms where each term
identifies an object class and instance.
Object: the generic term for a media server entity that terminates,
originates, or processes media. This specification defines four
classes of objects and specifies mechanisms to create them, join them
together, and destroy them.
Participant Object: an object in a media server that sources original
media in a call and/or receives and terminates media in a call.
Intermediary Object: an object in a media server that acts on media
within a call for the benefit of the participants.
Independent Object: an object that can exist on a media server
independent of other objects.
Operator: an intermediary transformer that modifies or transforms a
media stream. Examples of operators may be audio gain controls,
video scaling, or voice masking. MSML defines operators as media
transform objects, which transform media using operations such as
gain control, when applied to media streams.
Media Stream: a single media flow between two objects. A media
stream has a media type and may be unidirectional or bidirectional.
3. MSML SIP Usage
SIP is used to create and modify media sessions with a media server
according to the procedures defined in RFC 3261 [n1]. Often, SIP
third party call control [i4] will be used to create sessions to a
media server on behalf of end users. MSML is used to define and
change the service that a user connected to a media server will
receive. MSML clients are application servers, soft-switches, or
other forms of control agents, and SHOULD have an authorized security
relationship with the media server. MSML itself does not define
authorization mechanisms.
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MSML transactions are originated based upon events that occur in the
application domain. These events may be independent from any media
or user interaction. For example, an application may wish to play an
announcement to a conference warning that its scheduled completion
time is approaching. Applications themselves are structured in many
different ways. Their structure and requirements contribute to their
selection of protocols and languages. To accommodate differing
application needs, MSML has been designed to be neutral to other
languages and independent of the transport used to carry it.
MSML is purposely designed to be transport independent. In this
release of the specification, SIP INFO [i5] and SIP Control Framework
[i11] have been chosen for transport mechanisms for MSML, as
described in the following sections.
3.1. SIP INFO
SIP INVITE and INFO [i5] requests and responses MAY be used to carry
MSML. INFO requests allow asynchronous mid-call messages within SIP
with few additional semantics. In addition, there are existing
widely deployed implementations of that method, it aids in initial
developments that are closely coupled with SIP session establishment,
and it allows MSML to be directly associated with user dialogs when
third party call control is used.
Although INFO is sometimes considered not to be a suitable general-
purpose transport mechanism for messages within SIP, there have been
proposals to make it more acceptable. MSML may evolve to include
other SIP usage and/or to work with other protocols or as a stand-
alone protocol established through SIP, in future releases of this
document.
MSML supports several models for client interaction. When clients
use 3PCC to establish media sessions on behalf of end users, clients
will have a SIP dialog for each media session. MSML MAY be sent on
these dialogs. However the targets of MSML actions are not inferred
from the session associated with the SIP dialog. The targets of MSML
actions are always explicitly specified using identifiers as
previously defined.
An application, after interacting with a user, may want to affect
multiple objects within a media server. For example, tones or
messages are often played to a conference when connections are added
or removed. A separate message may also be played to a participant
as they are joined, or to moderators. Explicit identifiers, that is,
not inferred from a transport mechanism, allow these multiple actions
to be easily grouped into a single transaction sent on any SIP
dialog.
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MSML also supports a model of dedicated control associations. This
supports decoupled application architectures where a client can
control media server services without also establishing all of the
media sessions itself. Control associations are created using SIP,
but they do not have any associated media session. Although
initially INFO messages will be sent on this SIP dialog, just as with
dialogs associated with media sessions, it is possible that in the
future, the SIP dialog will be used to establish a separate control
session (defined in SDP [n9]) that does not use SIP as the transport
for MSML messages.
A media server using MSML also sends asynchronous events to a client
using MSML scripts in SIP INFO. Events are sent based on previous
MSML requests and are sent within the SIP dialog on which the MSML
request that caused the event to be generated was received. If this
dialog no longer exists when the event is generated, the event is
discarded.
Events may be generated during the execution of a dialog created by a
element. For example, dialogs can send events based on
user input. VoiceXML dialogs, on the other hand, generally interact
with other servers outside of MSML using HTTP.
An event is also generated when the execution of a dialog terminates,
because of either completion or failure. The exact information
returned is dependent on the dialog language, the capabilities of the
dialog execution environment, and what was requested by the dialog.
Both MSML and VoiceXML [n5] allow information to be returned when
they exit. These events may be sent in a SIP INFO or a SIP BYE. SIP
BYE is used when the dialog itself specifies that the connection
should be disconnected, for example, through the use of the
element.
Conferences may also generate events based upon their configuration.
An example of this is the notification of the set of active speakers.
3.2. SIP Control Framework
The SIP Control Framework [i11] MAY be used as a transport mechanism
for MSML.
The Control Framework provides a generic approach for establishment
and reporting capabilities of remotely initiated commands. The
framework utilizes many functions provided by the Session Initiation
Protocol (SIP) [n1] for the rendezvous and establishment of a
reliable channel for control interactions. Compared to SIP INFO, the
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SIP Control Framework is a more general-purpose transport mechanism
and one that is not constrained by limitations of the SIP INFO
mechanism.
The Control Framework also introduces the concept of a Control
Package, which is an explicit usage of the Control Framework for a
particular interaction set. This specification has already specified
a list of packages for MSML to control the media server in many
aspects, including basic dialog, advanced conferencing, advanced
dialog, and audit service. Each of these packages has a unique
Control Package name assigned in order for MSML to be used with the
Control Framework.
This section fulfills the mandatory requirement for information that
MUST be specified during the definition of a Control Framework
Package, as detailed in SIP Control Framework [i11].
3.2.1. Control Framework Package Names
The Control Framework [i11] requires a Control Package definition to
specify and register a unique name.
MSML specification defines Control Package names using a hierarchical
scheme to indicate the inherited relationship across packages. For
example, package "msml-x" is derived from package "msml", and package
"msml-x-y" is derived from package "msml-x".
The following is a list of Control Package names reserved by the MSML
specification.
"msml": this Control Package supports MSML Core Package as specified
in section 7.
"msml-conf": this Control Package supports MSML Conference Core
Package as specified in section 8.
"msml-dialog": this Control Package supports MSML Dialog Core Package
as specified in section 9.6.
"msml-dialog-base": this Control Package supports MSML Dialog Base
Package as specified in section 9.7.
"msml-dialog-group": this Control Package supports MSML Dialog Group
Package as specified in section 9.8.
"msml-dialog-transform": this Control Package supports MSML Dialog
Transform Package as specified in section 9.9.
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"msml-dialog-speech": this Control Package supports MSML Dialog
Speech Package as specified in section 9.10.
"msml-dialog-fax-detect": this Control Package supports MSML Dialog
Fax Detection Package as specified in section 9.11.
"msml-dialog-fax-sendrecv": this Control Package supports MSML Dialog
Fax Send/Receive Package as specified in section 9.12.
"msml-audit": this Control Package supports MSML Audit Core Package
as specified in section 10.1.
"msml-audit-conf": this Control Package supports MSML Audit
Conference Package as specified in section 10.2.
"msml-audit-conn": this Control Package supports MSML Audit
Connection Package as specified in section 10.3.
"msml-audit-dialog": this Control Package supports MSML Audit Dialog
Package as specified in section 10.4.
"msml-audit-stream": this Control Package supports MSML Audit Stream
Package as specified in section 10.5.
An application server using the Control Framework as transport for
MSML MUST use one or multiple package names, depending on the service
required from the media server. The package name(s) are identified
in the "Control-Packages" SIP header that is present in the SIP
INVITE dialog request that creates the control channel, as specified
in [i11]. The "Control-Packages" value MAY be re-negotiated via the
SIP re-INVITE mechanism.
3.2.2. Control Framework Messages
The usage of CONTROL, response, and REPORT messages, as defined in
[i11], by each Control Package defined in MSML is different and
described separately in the following sections.
MSML Core Package "msml"
The application server may send a CONTROL message with a body
of MSML request using the following elements to the MS:
: the root element that may contain a list of child
elements that request a specific operation. The child elements
are defined in extended packages (e.g., "msml-conf" and "msml-
dialog"). This element is also the root element that contains
an MSML result and event.
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: sends an event to the specified recipient within the
media server. Specific event types are defined within the
extended packages.
The media server replies with a response message containing a
MSML result using the following elements:
: reports the results of an MSML transaction.
The media server MAY send the MSML event to the application
server, in a REPORT or CONTROL message, using the element
. The actual content of the and which Control
Framework message to use are defined within the extended
packages.
MSML Conference Core Package "msml-conf"
This package extends the MSML Core Package to define a
framework for creation, manipulation, and deletion of a
conference.
The AS can send a CONTROL message with a body of the MSML
request that contains one or multiple conference-related
commands to the MS. The MS then replies with a response
message with a body of the MSML result to indicate whether or
not the request has been fulfilled.
During the lifetime of a conference, whenever an event occurs,
the media server MAY send CONTROL messages containing MSML
events to notify the application server. The application
server SHOULD reply with a response message with no MSML body
to acknowledge the event has been received.
This package does NOT use the REPORT message.
Dialog Core Package "msml-dialog"
This package extends the MSML Core Package to define the
structural framework and abstractions for MSML dialogs.
The application server MAY send CONTROL messages containing a
MSML request using the following elements:
: instantiate an MSML media dialog on a connection
or a conference.
: terminates an MSML dialog.
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: sends an event and an optional namelist to the dialog,
dialog group, or dialog primitive.
: used by the dialog description language to cause the
execution of the MSML dialog to terminate.
For the command, the response message MUST
contain an MSML result that indicates that the dialog has been
started successfully. The MSML result MAY contain
to return the dialog identifier, if the identifier was assigned
by the media server. Subsequently, zero or more MSML events
MAY be initiated by the media server in (update) REPORT
messages to report information gathered during the dialog.
Finally, an MSML event "msml.dialog.exit" SHOULD be generated
in a (terminate) REPORT message when the dialog terminates
(e.g., MSML execution of ).
For the and commands, the response message
contains the final MSML result that indicates that the request
has either been fulfilled or rejected.
Dialog Base Package "msml-dialog-base"
This package extends the MSML Dialog Core Package to define a
set of base functionality for MSML dialogs. The extension
defines individual media primitives, including ,
, , , and , to be
used as child element of . This package does not
change the framework message usage as defined by the MSML
Dialog Core Package.
Dialog Transform Package "msml-dialog-transform"
This package extends the MSML Dialog Core Package to define a
set of transform primitives that works as filter on half-duplex
media streams. The extension defines transform primitives,
including , , , , and ,
that MAY be used as child elements of . This
package does not change the framework message usage as defined
by the MSML Dialog Core Package.
Dialog Group Package "msml-dialog-group"
This package extends the MSML Dialog Core, Base, and Transform
Packages to define a single control flow construct that
specifies concurrent execution of multiple media primitives.
The extension defines the element that MAY be used as a
child element of to enclose multiple media
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primitives, such that they can be executed concurrently. This
package does not change the framework message usage as defined
by the MSML Dialog Core Package.
Dialog Speech Package "msml-dialog-speech"
This package extends the MSML Dialog Core and MSML Base Package
to define functionality that MAY be used for automatic speech
recognition and text to speech. The extension extends the
and the elements.
For , it defines a new child element to
activate grammars or user input rules associated with speech
recognition. For , it defines a new child element
to initiate the text-to-speech service.
This package does not change the framework message usage as
defined by the MSML Dialog Core Package.
Dialog Fax Detection Package "msml-dialog-fax-detect"
This package extends the MSML Dialog Core Package to define
primitives provide fax detection service. The extension
defines a primitive to be used as a child element
of . This package does not change the framework
message usage as defined by the MSML Dialog Core Package.
Dialog Fax Send/Receive Package "msml-dialog-fax-sendrecv"
This package extends the MSML Dialog Core Package to define
primitives that allow a media server to provide fax send or
receive service. The extension defines new primitives
and , to be used as a child element of
. This package does not change the framework
message usage as defined by the MSML Dialog Core Package.
Dialog Audit Core Package "msml-audit"
This package extends the MSML Core Package to define a
framework for auditing media resource(s) allocated on the media
server.
This package follows a simple request/response transaction,
allowing the application server to send CONTROL messages
containing MSML requests. The media server MUST reply
with a response message containing the result. The result is
contained within the element, returning the
queried state information.
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This package does NOT use the REPORT message.
Dialog Audit Conference Package "msml-audit-conf"
This package extends the MSML Audit Core Package to define
conference specific states that MAY be queried via the
command and the corresponding response MUST be returned by the
element. This package does not change the
framework message usage as defined by the MSML Audit Core
Package.
Dialog Audit Connection Package "msml-audit-conn"
This package extends the MSML Audit Core Package to define
connection specific states that MAY be queried via the
command and the corresponding response MUST be returned by the
element. This package does not change the
framework message usage as defined by the MSML Audit Core
Package.
Dialog Audit Dialog Package "msml-audit-dialog"
This package extends the MSML Audit Core Package to define
dialog specific states that MAY be queried via the
command and the corresponding response MUST be returned by the
element. This package does not change the
framework message usage as defined by the MSML Audit Core
Package.
Dialog Audit Stream Package "msml-audit-stream"
This package extends the MSML Audit Core Package to define
stream specific states that MAY be queried via the
command and the corresponding response MUST returned by the
element. This package does not change the
framework message usage as defined by the MSML Audit Core
Package.
3.2.3. Common XML Support
The XML schema described in [i11] MUST be supported by all Control
Packages defined by MSML. However, the "connection-id" value MUST be
constructed as defined by MSML (i.e., the identifier MUST contain a
local dialog tag only, while the SIP Control Framework [i11] requires
that the "connection-id" contain both local and remote dialog tags).
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3.2.4. Control Message Body
A valid CONTROL body message MUST conform to the MSML schema, as
included in this specification, for the MSML package(s) used.
3.2.5. REPORT Message Body
A valid REPORT body message MUST conform to the MSML schema, as
included in this specification, for the MSML package(s) used.
4. Language Structure
4.1. Package Scheme
The primary mechanism for extending MSML is the "package". A package
is an integrated set of one or more XML schemas that define
additional features and functions via new or extended use of elements
and attributes. Each package, except for those defined in the
current document, is defined in a separate standards document, e.g.,
an Internet Draft or an RFC. All packages that extend the base MSML
functionality MUST include references to the MSML base set of schemas
provided in the Internet Drafts. A schema in a package MUST only
extend MSML; that is, it must not alter the existing specification.
A particular MSML script will include references to all the schemas
defining the packages whose elements and attributes it makes use of.
A particular script MUST reference MSML base and optionally extension
package(s). See the IANA Considerations section.
Each package MUST define its own namespace so that elements or
attributes with the same name in different packages do not conflict.
A script using a particular element or attribute MUST prefix the
namespace name on that element or attribute's name if it is defined
in a package (as opposed to being defined in the base).
MSML consists of a core package that provides structure without
support for any specific feature set. Additional packages, relying
on the core package, provide functional features. Any combination of
additional packages may be used along with the core package. The
following describes the set of MSML packages defined in this
document.
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+--------------------------------------------------------+
| MSML Core |
+--------------------------------------------------------+
/ \ \
+--------+ +--------+ +-------+
| Dialog | | Conf | | Audit |
| Core | | Core | | Core |
+--------+ +--------+ +-------+
________ \_______________________________________ |
------------------------------------------------ |
/ \ \ \ \ \ |
+------+ +---------+ +------+ +------+ +------+ +-------+ |
|Dialog| |Dialog | |Dialog| |Dialog| |Dialog| |Dialog | |
|Base | |Transform| |Group | |Speech| |Fax | |Fax | |
+------+ +---------+ +------+ +------+ |Detect| |Send/ | |
+------+ |Receive| |
+-------+ |
________________________|
-------------------------
/ \ \ \
+-----+ +-----+ +------+ +------+
|Audit| |Audit| |Audit | |Audit |
|Conf | |Conn | |Dialog| |Stream|
+-----+ +-----+ +------+ +------+
o MSML Core Package (Mandatory)
Describes the minimum base framework that MUST be implemented to
support additional core packages.
o MSML Conference Core Package (Conditionally Mandatory, for
Conferencing)
Describes the audio and multimedia basic and advanced conferencing
package that MAY be implemented.
o MSML Dialog Core Package (Conditionally Mandatory, for Dialogs)
Describes the dialog core package that MUST be implemented for any
dialog services. However, systems supporting conferencing only,
MAY omit support for MSML dialogs. The MSML Dialog Core Package
specifies the framework within which additional dialog packages
are supported. The MSML Dialog Base Package MUST be supported,
while all other dialog packages MAY be supported.
o MSML Dialog Base Package (Conditionally Mandatory, for Dialogs)
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o MSML Dialog Group Package (Optional)
o MSML Dialog Transform Package (Optional)
o MSML Dialog Fax Detection Package (Optional)
o MSML Dialog Fax Send/Receive Package (Optional)
o MSML Dialog Speech Package (Optional)
o MSML Audit Core Package (Conditionally Mandatory, for Auditing)
Describes the audit core package that MUST be implemented to
support auditing services. The MSML audit core package specifies
the framework within which additional audit packages are
supported.
o MSML Audit Conference Package (Conditionally Mandatory, for
Auditing Conference, Conference Dialog, and Conference Stream)
o MSML Audit Connection Package (Conditionally Mandatory, for
Auditing Connection, Connection Dialog, and Connection Stream)
o MSML Audit Dialog Package (Conditionally Mandatory, for Auditing
Dialog, and MUST be used with either MSML Audit Conference
Package or MSML Audit Connection Package)
o MSML Audit Stream Package (Conditionally Mandatory, for Auditing
Stream, and MUST be used with either MSML Audit Conference
Package or MSML Audit Connection Package)
The formal process for defining extensions to MSML dialogs is to
define a new package. The new package MUST provide a text
description of what extensions are included and how they work. It
MUST also define an XML schema file (if applicable) that defines the
new package (which may be through extension, restriction of an
existing package, or a specific profile of an existing package).
Dependencies upon other packages MUST be stated. For example, a
package that extends or restricts has a dependency on the original
package specification. Finally, the new package MUST be assigned a
unique name and version.
The types of things that can be defined in new packages are:
o new primitives
o extensions to existing primitives (events, shadow variables,
attributes, content)
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o new recognition grammars for existing primitives
o new markup languages for speech generation
o languages for specifying a topology schema
o new predefined topology schemas
o new variables / segment types (sets & languages)
o new control flow elements
MSML packages are assembled together to form a specific MSML profile
that is shared between different implementations. The base MSML
dialog profiles that are defined in this document consist of the MSML
Core Package, MSML Dialog Core Package, MSML Dialog Base Package,
MSML Dialog Group Package, MSML Transform Package, MSML Fax Packages,
and the MSML Speech Package.
MSML extension packages, which define primitives, MUST define the
following for each primitive within the package:
o the function that the primitive performs
o the attributes that may be used to tailor its behavior
o the events that it is capable of understanding
o the shadow variables that provide access to information
determined as a result of the primitive's operation
The mechanism used to ensure that a media server and its client share
a compatible set of packages is not defined. Currently, it is
expected that provisioning will be used, possibly coupled with a
future auditing capability. Additionally, when used in SIP networks,
packages could be defined using feature tags and the procedures
defined for Indicating User Agent Capabilities in SIP [i1] used to
allow a media server to describe its capabilities to other user
agents.
4.2. Profile Scheme
Not all devices and applications using MSML will need to support the
entire MSML schema. For example, a media processing device might
support only audio announcements, only audio simple conferencing, or
only multimedia IVR. It is highly desirable to have a system for
describing what portion of MSML a particular media processing device
or control agent supports.
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The package scheme described earlier allows MSML functionality to be
functionally grouped, relying on the MSML core package. This scheme
allows a portion of the complete MSML specification to be
implemented, on a per-package basis, and also creates a framework for
future extension packages. However, within a given package, in some
cases, only a subset of the package functionality may be required.
In order to support subsets of packages, with greater degree of
granularity than at the package level, a profile scheme is required.
MSML package profiles would identify a subset of a given MSML package
with specific definitions of elements and attributes. Each MSML
package profile MUST be accompanied by one or more corresponding
schemas. To use the examples above, there could be an audio
announcements profile of the MSML Dialog Base Package, an audio
simple conferencing profile of the MSML Conference Core Package, and
a multimedia IVR profile of the MSML Dialog Base Package.
MSML package profiles MUST be published separately from the MSML
specification, in one or more standards documents (e.g., Internet
Drafts or RFCs) dedicated to MSML package profiles. Profiles would
not be registered with IANA and any organization would additionally
be free to create its own profile(s) if required.
5. Execution Flow
MSML assumes a model where there is a single control context within a
media server for MSML processing. That context may have one or many
SIP [n1] dialogs associated with it. It is assumed that any SIP
dialogs associated with the MSML control context have been
authorized, as appropriate, by mechanisms outside the scope of MSML.
A media server control context maintains information about the state
of all media objects and media streams within a media server. It
receives and processes all MSML requests from authorized SIP dialogs
and receives all events generated internally by media objects and
sends them on the appropriate SIP dialog. An MSML request is able to
create new media objects and streams, and to modify or destroy any
existing media objects and streams.
An MSML request may simply specify a single action for a media server
to undertake. In this case, the document is very similar to a simple
command request. Often, though, it may be more natural for a client
to request multiple actions at one time, or the client would like
several actions to be closely coordinated by the media server.
Multiple MSML elements received in a single request MUST be processed
sequentially in document order.
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An example of the first scenario would be to create a conference and
join it with an initial participant. An example of the second case
would be to unjoin one or more participants from a main conference
and join them to a sidebar conference. In the first scenario,
network latencies may not be an issue, but it is simpler for the
client to combine the requests. In the second case, the added
network latency between separate requests could mean perceptible
audio loss to the participant.
Each MSML request is processed as a single transaction. A media
server MUST ensure that it has the necessary resources available to
carry out the complete transaction before executing any elements of
the request. If it does not have sufficient resources, it MUST
return a 520 response and MUST NOT execute the transaction.
The MSML request MUST be checked for well-formedness and validated
against the schema prior to executing any elements. This allows XML
[n2] errors to reported immediately and minimizes failures within a
transaction and the corresponding execution of only part of the
transaction.
Each element is expected to execute immediately. Elements such as
, which take an unpredictable amount of time, are
"forked" and executed in a separate thread (see MSML Dialog
Packages). Once successfully forked, execution continues with the
element following the . As such, MSML does not provide
mechanisms to sequence or coordinate other operations with dialog
elements.
Processing within a transaction MUST stop if any errors occur.
Elements that were executed prior to the error are not rolled back.
It is the responsibility of the client to determine appropriate
actions based upon the results indicated in the response. Most
elements MAY contain an optional "mark" attribute. The value of that
attribute from the last successfully executed element MUST be
returned in an error response. Note that errors that occur during
the execution of a dialog occur outside the context of an MSML
transaction. These errors will be indicated in an asynchronous
event.
Transaction results are returned as part of the SIP request response.
The transaction results indicate the success or failure of the
transaction. The result MUST also include identifiers for any
objects created by a media server for which the client did not
provide an instance name. Additionally, if the transaction fails,
the reason for the failure MUST be returned, as well as an indication
of how much of the transaction was executed before the failure
occurred SHOULD be returned.
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6. Media Server Object Model
Media servers are general-purpose platforms for executing real-time
media processing tasks. These tasks range in complexity from simple
ones such as serving announcements, to complex ones, such as speech
interfaces, centralized multimedia conferencing, and sophisticated
gaming applications.
Calls are established to a media server using SIP. Clients will
often use SIP third party call control (3PCC) [i4] to establish calls
to a media server on behalf of end users. However MSML does not
require that 3PCC be used, only that the client and the media server
share a common identifier for the call and its associated RTP [i3]
sessions.
Objects represent entities that source, sink, or modify media
streams. A media streams is a bidirectional or unidirectional media
flow between objects on a media server. The following subsections
define the classes of objects that exist on a media server and the
way these are identified in MSML.
6.1. Objects
A media object is an endpoint of one or more media streams. It may
be a connection that terminates RTP sessions from the network or a
resource that transforms or manipulates media. MSML defines four
classes of media objects. Each class defines the basic properties of
how object instances are used within a media server. However, most
classes require that the function of specific instances be defined by
the client, using MSML or other languages such as VoiceXML.
The following classes of media processing objects are defined. The
class names are given in parentheses:
o network connection (conn)
o conference (conf)
o dialog (dialog)
Network connection is an abstraction for the media processing
resources involved in terminating the RTP session(s) of a call. For
audio services, a connection instance presents a full-duplex audio
stream interface within a media server. Multimedia connections have
multiple media streams of different media types, each corresponding
to an RTP session. Network connections get instantiated through SIP
[n1].
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A conference represents the media resources and state information
required for a single logical mix of each media type in the
conference (e.g., audio and video). MSML models multiple mixes/views
of the same media type as separate conferences. Each conference has
multiple inputs. Inputs may be divided into classes that allow an
application to request different media treatment for different
participants. For example, the video streams for some participants
may be assigned to fixed regions of the screen while those for other
participants may only be shown when they are speaking.
A conference has a single logical output per media type. For each
participant, it consists of the audio conference mix, less any
contributed audio of the participant, and the video mix shared by all
conference participants. Video conferences using voice activated
switching have an optional ability to show the previous speaker to
the current speaker.
Conferences are instantiated using the element.
The content of the element specifies the
parameters of the audio and/or video mixes.
Dialogs are a class of objects that represent automated participants.
They are similar to network connections from a media flow perspective
and may have one or more media streams as the abstraction for their
interface within a media server. Unlike connections, however,
dialogs are created and destroyed through MSML, and the media server
itself implements the dialog participant. Dialogs are instantiated
through the element. Contents of the
element define the desired or expected dialog behavior. Dialogs may
also be invoked by referencing VoiceXML as the dialog description
language.
Operators are functions that are used to filter or transform a media
stream. The function that an instance of an operator fulfills is
defined as a property of the media stream. Operators may be
unidirectional or bidirectional and have a media type.
Unidirectional operators reflect simple atomic functions such as
automatic gain control, filtering tones from conferences, or applying
specific gain values to a stream. Unidirectional operators have a
single media input, which is connected to the media stream from one
object, and a single media output, which is connected to the media
stream of a different object.
Bidirectional operators have two media inputs and two media outputs.
One media input and output is associated with the stream to one
object, and the other input and output is associated with a stream to
a different object. Bidirectional objects may treat the media
differently in each direction. For example, an operator could be
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defined that changed the media sent to a connection based upon
recognized speech or dual-tone multi-frequency (DTMF) received from
the connection. Operators are implicitly instantiated when streams
are created or modified using the elements and ,
respectively.
The relationships between the different object classes (conf, conn,
and dialog) are shown in the figure below.
+--------------------------------------+
| Media Server |
| |
|------+ ,---. |
| | +------+ / \ |
<== RTP ==>| conn |<---->| oper |<---->( conf ) |
| | +------+ \ / |
|------+ `---' |
| ^ ^ |
| | | |
| | +------+ +------+ | |
| | | | | | | |
| +-->|dialog| |dialog|<---+ |
| | | | | |
| +------+ +------+ |
+--------------------------------------+
A single, full-duplex instance of each object class is shown together
with common relationships between them. An operator (such as gain)
is shown between a connection and a conference and dialogs are shown
participating both with an individual connection and with a
conference. The figure is not meant to imply only one-to-one
relationships. Conferences will often have hundreds of participants,
and either connections or conferences may be interacting with more
than one dialog. For example, one dialog may be recording a
conference while other dialogs announce participants joining or
leaving the conference.
6.2. Identifiers
Objects are referenced using identifiers that are composed of one or
more terms. Each term specifies an object class and names a specific
instance within that class. The object class and instance are
separated by a colon ":" in an identifier term.
Identifiers are assigned to objects when they are first created. In
general, either the MSML client or a media server may specify the
instance name for an object. Objects for which a client does not
assign an instance name will be assigned one by a media server.
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Media server assigned instance names are returned to the client as a
complete object identifier in the response to the request that
created the object.
It is meaningful for some classes of objects to exist independently
on a media server. Network connections may be created through SIP at
any time. MSML can then be used to associate their media with other
objects as required to create services. Conferences may be created
and have specific resources reserved waiting for participant
connections.
Objects from these two classes, connections and conferences, are
considered independent objects since they can exist on a standalone
basis. Identifiers for independent objects consist of a single term
as defined above. For example, identifiers for a conference and
connection could be "conf:abc" or "conn:1234" respectively. Clients
that choose to assign instance names to independent objects must use
globally unique instance names. One way to create globally unique
names is to include the domain name of the client as part of the
name.
Dialogs are created to provide a service to independent objects.
Dialogs may act as a participant in a conference or interact with a
connection similar to a two-participant call. Dialogs depend upon
the existence of independent objects, and this is reflected in the
composition of their identifiers. Operators modify the media flow
between other objects, such as application of gain between a
connection and a conference. As operators are merely media transform
primitives defined as properties of the media stream, they are not
represented by identifiers and created implicitly.
Identifiers for dialogs are composed of a structured list of slash
('/') separated terms. The left-most term of the identifier must
specify a conference or connection. This serves as the root for the
identifier. An example of an identifier for a dialog acting as a
conference participant could be:
conf:abc/dialog:recorder
All objects except connections are created using MSML. Connections
are created when media sessions get established through SIP. There
are several options clients and media servers can use to establish a
shared instance name for a connection and its media streams.
When media servers support multiple media types, the instance name
SHOULD be a call identifier that can be used to identify the
collection of RTP sessions associated with a call. When MSML is used
in conjunction with SIP and third party call control, the call
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identifier MUST be the same as the local tag assigned by the media
server to identify the SIP dialog. This will be the tag the media
server adds to the "To" header in its response to an initial invite
transaction. RFC 3261 requires the tag values to be globally unique.
An example of a connection identifier is: conn:74jgd63956ts.
With third party call control, the MSML client acts as a back-to-back
user agent (B2BUA) to establish the media sessions. SIP dialogs are
established between the client and the media server allowing the use
of the media server local tag as a connection identifier. If third
party call control is not used, a SIP event package MAY be used to
allow a media server to notify new sessions to a client that has
subscribed to this information.
Identifiers as described above allow every object in a media server
to be uniquely addressed. They can also be used to refer to multiple
objects. There are two ways in which this can currently be done:
wildcards
common instance names
An identifier can reference multiple objects when a wildcard is used
as an instance name. MSML reserves the instance name composed of a
single asterisk ('*') to mean all objects that have the same
identifier root and class. Instance names containing an asterisk
cannot be created. Wildcards MUST only be used as the right-most
term of an identifier and MUST NOT be used as part of the root for
dialog identifiers. Wildcards are only allowed where explicitly
indicated below.
The following are examples of valid wildcards:
conf:abc/dialog:*
conn:*
An example of illegal wildcard usage is:
conf:*/dialog:73849
Although identifiers share a common syntax, MSML elements restrict
the class of objects that are valid in a given context. As an
example, although it is valid to join two connections together, it is
not valid to join two IVR dialogs.
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7. MSML Core Package
This section describes the core MSML package that MUST be supported
in order to use any other MSML packages. The core MSML package
defines a framework, without explicit functionality, over which
functional packages are used.
7.1. is the root element. When received by a media server, it
defines the set of operations that form a single MSML request.
Operations are requested by the contents of the element. Each
operation MAY appear zero or more times as children of .
Specific operations are defined within the conference package and in
the set of dialog packages.
The results of a request or the contents of events sent by a media
server are also enclosed within the element. The results of
the transaction are included as a body in the response to the SIP
request that contained the transaction. This response will contain
any identifiers that the media server assigned to newly created
objects. All messages that a media server generates are correlated
to an object identifier. Objects and identifiers are discussed in
section 6 (Media Server Object Model).
Attributes:
version: "1.1" Mandatory
7.2.
Events are used to affect the behavior of different objects within a
media server. The element is used to send an event to the
specified recipient within the media server.
Attributes:
event: the name of an event. Mandatory.
target: an object identifier. When the identifier is for a
dialog, it may optionally be appended with a slash "/" followed by
the target to be included in an MSML dialog . Mandatory.
valuelist: a list of zero or more parameters that are included
with the event.
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mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document should be unique.
7.3.
The element is used to report the results of an MSML
transaction. It is included as a body in the final response to the
SIP request that initiated the transaction. An optional child
element may include text that expands on the meaning of
error responses. Response codes are defined in section 11 (Response
Codes).
Attributes:
response: a numeric code indicating the overall success or failure
of the transaction, and in the case of failure, an indication of
the reason. Mandatory.
mark: in the case of an error, the value of the mark attribute
from the last successfully executed element that included the mark
attribute.
In the case of failure, a description of the reason SHOULD be
provided using the child element .
Three other child elements allow the response to include identifiers
for objects created by the request but that did not have instance
names specified by the client. Those elements are and
, for objects created through a and
respectively.
7.4.
The element is used to notify an event to a media server
client. Three types of events are defined by the MSML Core Package:
"msml.dialog.exit", "msml.conf.nomedia", and "msml.conf.asn". These
correspond to the termination of an executing dialog, a conference
being automatically deleted when the last participant has left, and
the notification of the current set of active speakers for a
conference, respectively. Events may also be generated by an
executing dialog. In this case, the event type is specified by the
dialog (see MSML Dialog Core Package ).
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Attributes:
name: the type of event. If the event is generated because of the
execution MSML dialog , the value MUST be the value of the
"event" attribute from the element within the MSML Dialog
Core Package. If the event is generated because of the execution
of an , the value MUST be "moml.exit". If the event is
generated because of the execution of a , the value
MUST be "moml.disconnect". If the event is generated because of
an error, the value must be "moml.error". Mandatory.
id: the identifier of the conference or dialog that generated the
event or caused the event to be generated. Mandatory.
has two children, and , which contain the
name and value respectively of each namelist item associated with
the event.
8. MSML Conference Core Package
8.1. Conferences
A conference has a mixer for each type of media that the conference
supports. Each mix has a corresponding description that defines how
the media from participants contributes to that mix. A mixer has
multiple inputs that are combined in a media specific way to create a
single logical output.
The elements that describe the mix for each media type are called
mixer description elements. They are:
defines the parameters for mixing audio media.
defines the composition of a video window.
These elements, defined in sections 8.6 (Audio Mix) and 8.7 (Video
Layout) respectively, are used as content of the
element to establish the initial properties of a conference. The
elements are used within the element to change the
properties of a conference once it has been created, or within the
element to remove individual mixes from the
conference.
Conferences may be terminated by an MSML client using the
element to remove the entire conference or by
removing the last mixer(s) associated with the conference.
Conferences can also be terminated automatically by a media server
based on criteria specified when the conference is created. When the
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conference is deleted, any remaining participants will have their
associated SIP dialogs left unchanged or deleted based on the value
of the "term" attribute specified when the conference was created.
8.2. Media Streams
Objects have at least one media input and output for each type of
media that they support. Each object class defines the number of
input and output objects of that class support. Media streams are
created when objects are joined, either explicitly using or
implicitly when dialogs are created using . Dialog
creation has two stages, allocating and configuring the resources
required for the dialog instance, and implicitly joining those
resources to the dialog target during the dialog execution. Refer to
the MSML Dialog Base Package.
A join operation by default creates a bidirectional audio stream
between two objects. Video and unidirectional streams may also be
created. A media stream is created by connecting the output from one
object to the input of another object and vice versa (assuming a
bidirectional or full-duplex join).
Many objects may only support a single input for each type of media.
Within this specification, only the conference object class supports
an arbitrary number of inputs. When a stream is requested to be
created to an object that already has a stream of the same type
connected to its single input, the result of the request depends upon
the type of the media stream.
Audio mixing is done by summing audio signals. Automatically mixing
audio streams has common and straightforward applications. For
example, the ability to bridge two streams allows for the easy
creation of simple three-way calls or to bridge private announcements
with a (whispered) conference mix for an individual participant. In
the case of general conferences, however, an MSML client SHOULD
create an audio conference and then join participants to the
conference. Conference mixers SHOULD subtract the audio of each
participant from the mix so that they do not hear themselves.
A media server receiving a request that requires joining an audio
stream to the single audio input of an object that already has an
audio stream connected SHOULD automatically bridge the new stream
with the existing stream, creating a mix of the two audio streams.
The maximum number of streams that may be bridged in this manner is
implementation specific. It is RECOMMENDED that a media server
support bridging at least two streams. A media server that cannot
bridge a new stream with any existing streams MUST fail the operation
requesting the join.
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Unlike audio mixing, there are many different ways that two video
streams may be combined and presented. For example, they may be
presented side by side in separate panes, picture in picture, or in a
single pane that displays only a single stream at a time based on a
heuristic such as active speaker. Each of these options creates a
very different presentation and requires significantly different
media resources.
A join operation does not describe how a new stream can be combined
with an existing stream. Therefore, automatic bridging of video is
not supported. A media server MUST fail requests to join a new video
stream to an object that only supports a single video input and
already has a video stream connected to that input. For an object to
have multiple video streams joined to it, the object itself must be
capable in supporting multiple video streams. Conference objects can
support multiple video streams and provide a way to specify the
mixing presentation for the video streams.
A media server MUST NOT establish any streams unless the media server
is able to create all the streams requested by an operation. Streams
are only able to be created if both objects support a media type and
at least one of the following conditions is true:
1. Each object that is to receive media is not already receiving a
stream of that type.
2. Any object that is to receive media and is already receiving a
stream of that type supports receiving an additional stream of
that type. The only class of objects defined in this
specification that directly support receiving multiple streams
of the same type are conferences.
3. The media server is able to automatically bridge media streams
for an object that is to receive media and that is already
receiving a stream of the requested type. The only type of
media defined in this specification that MAY be automatically
bridged is audio.
The directionality of media streams associated with a connection is
modeled independently from what SDP [n9] allows for the corresponding
RTP [i3] sessions. Media servers MUST respect the SDP in what they
actually transmit but MUST NOT allow the SDP to affect the
directionality when joining streams internal to the media server.
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8.3. is used to allocate and configure the media mixing
resources for conferences. A description of the properties for each
type of media mix required for the conference is defined within the
content of the element. Mixer descriptions are
described in Audio Mix and Video Layout sections. When no mixer
descriptions are specified, the default behavior MUST be equivalent
to inclusion of a single .
Clients can request that a media server automatically delete a
conference when a specified condition occurs by using the
"deletewhen" attribute. A value of "nomedia" indicates that the
conference MUST be deleted when no participants remain in the
conference. When this occurs, an "msml.conf.nomedia" event MUST be
notified to the MSML client. A value of "nocontrol" indicates that
the conference MUST be deleted when the SIP [n1] dialog that carries
the element is terminated. When this occurs, a
media server MUST terminate all participant dialogs by sending a BYE
for their associated SIP dialog. A value of "never" MUST leave the
ability to delete a conference under the control of the MSML client.
Attributes:
name: the instance name of the conference. If the attribute is
not present, the media server MUST assign a globally unique name
for the conference. If the attribute is present but the name is
already in use, an error (432) will result and MSML document
execution MUST stop. Events that the conference generates use
this name as the value of their "id" attribute (see section 7.4
()).
deletewhen: defines whether a media server should automatically
delete the conference. Possible values are "nomedia",
"nocontrol", and "never". Default is "nomedia".
term: when true, the media server MUST send a BYE request on all
SIP dialogs still associated with the conference when the
conference is deleted. Setting term equal to false allows clients
to start dialogs on connections once the conference has completed.
Default is "true".
mark: a token that MAY be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document should be unique.
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An example of creating an audio conference is shown below. This
conference allows at most two participants to contend to be heard and
reports the set of active speakers no more frequently than every 10
seconds.
8.3.1.
Conference resources may be reserved by including the
element as a child of . allows the
specification of a set of resources that a media server will reserve
for the conference. Any requests for resources beyond those that
have been reserved should be honored on a best-effort basis by a
media server.
Attributes:
required: boolean that specifies whether should
fail if the requested resources are not available. When set to
false, the conference will be created, with no reserved resources,
if the complete reservation cannot be honored. Default is "true".
8.3.1.1.
The resources to be reserved are defined using . The
contents of these elements describe a resource that is to be
reserved. Descriptions are implementation dependent. Media servers
that support MSML dialogs may use the elements from that package as
the basis for resource descriptions. Each resource element may use
the attribute "n" to define the quantity of the resource to reserve.
For example, the following creates a conference and reserves two
types of resources. One resource element may represent resources
that are shared by all participants of the conference, while the
other may represent resources that are reserved for each of the
expected participants.
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Attributes:
n: number of resources to be reserved. Default is 1.
type: specifies whether the resource is to be reserved by each
individual participant or reserved as a shared conference
resource. Valid values for this attribute are "individual" or
"shared". Default is "individual".
8.4.
All of the properties of an audio mix or the presentation of a video
mix may be changed during the life of a conference using the
element. Changes to an audio mix are requested by
including an element as a child of .
This may also be used to add an audio mixer to the conference if none
was previously allocated. Changes to a video presentation are
requested by including a element as a child of
. Similar to an audio mixer, this may be used to
add a video mixer if none was previously allocated.
Mixers are removed by including a mixer description element within
.
Features and presentation aspects are enabled/added or modified by
including the element(s) that define the feature or presentation
aspect within a mixer description. The complete specification of the
element must be included just as it would be included when the
conference is created. The new definition completely replaces any
previous definition that existed. Only things that are defined by
elements included in the mixer descriptions are affected. Any
existing configuration aspects of a conference, which are not
specified within the element, MUST maintain their
current state in the media server.
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For example, if an MSML client wanted to change the minimum reporting
interval for active speaker notification from that shown in the
Conference Examples section () it would send the
following to the media server:
This would also enable active speaker notification if it had not
previously been enabled. The N-loudest mixing is unaffected.
Multiple elements MAY be included in the mixer descriptions similar
to when conferences are created. For example, in a video conference,
the video mix description () could specify that the
layout of the video being displayed should change such that the
regions currently displaying participants get smaller and new
region(s) are created to support additional participants. A media
server MUST make all of the requested changes or none of the
requested changes.
Additional examples of modifying conferences are presented in the
Conference Examples section.
Attributes:
id: the identifier for a conference. Wildcards MUST NOT be used.
Mandatory.
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all "mark" attributes within an
MSML document SHOULD be unique.
8.5.
Destroy conference is used to delete mixers or to delete the entire
conference and all state and shared resources. When a mixer is
removed, all of the streams joined to that mixer are unjoined. When
a conference is destroyed, SIP dialogs for any remaining participants
MUST be maintained or removed based on the value of the "term"
attribute when the conference was created.
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When there is no element content, deletes the
entire conference. Individual mixers are removed by including a
mixer description element identifying the mix (or mixes) to be
removed as content to . is used
remove audio mixers and is used remove video mixers.
When one or more mixer descriptions are specified, then media server
MUST only delete the specified mixer and MUST NOT affect any other
existing mixers. When or is identified
for individual removal, other feature aspects of the mix MUST NOT be
included. If specified, the media server MUST ignore any such
elements. When the last mixer is removed from a conference, a media
server MUST remove all conference state, leaving or removing any
remaining SIP dialogs as described above.
Attributes:
id: the identifier for a conference. Mandatory.
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all "mark" attributes within an
MSML document SHOULD be unique.
8.6.
The properties of the overall audio mix are specified using the
element.
Attributes:
id: an optional identifier for the audio mix.
samplerate: Integer value specifies the sample rate (in Hz) for
the audio mixer. Optional, default value of 8000.
An example of the description for an audio mix is:
8.6.1.
The element defines that participants contend to be
included in the conference mix based upon their audio energy. When
the element is not present, all participants are mixed.
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Attributes:
n: the number of participants that will be included in the audio
mix based upon having the greatest audio energy. Mandatory.
8.6.2.
The element enables notification of active speakers. Active
speakers MUST be notified using the element with an event
name of "msml.conf.asn". The namelist of the event consists of the
set of active speakers. The name of each item is the string
"speaker" with a value of the connection identifier for the
connection.
Attributes:
ri: the minimum reporting interval defines the minimum duration of
time that must pass before changes to active speakers will be
reported. A value of zero disables active speaker notification.
asth: specifies the active speaker threshold (in unit of dBm0).
Valid value range is 0 to -96. Optional, default is -96.
An example of an active speaker notification is:
speakerconn:hd93tg5hdfspeakerconn:w8cn59vei7speakerconn:p78fnh6sek47fg
8.7.
A video layout is specified using the element. It is
used as a container to hold elements that describe all of the
properties of a video mix. The parameters of the window that
displays the video mix are defined by the element. When the
video mix in composed of multiple panes, the location and
characteristics of the panes are defined by one or more
elements. A element is not required when only a single
video stream is displayed at one time and none of the visual
attributes of regions are required.
Some regions may be used to display a video stream based on a
selection criteria rather than having a video stream of a single
participant continuously presented in the region. One such an
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example is a distance learning lecture where the instructor sees each
of the students periodically displayed in a region. When a region is
used to display one of a number of streams, it is placed as a child
of a element.
Attributes:
type: specifies the language used to define the layout. Layouts
defined using MSML MUST use the value "text/msml-basic-layout".
This is the same convention as defined for the layout package from
the W3C SMIL 2.0 specification [i6]. The default when omitted is
"text/msml-basic-layout".
id: an optional identifier for the video layout.
8.7.1.
The element describes the root window or virtual screen in
which the conference video mix will be displayed. Simple conferences
can display participant video directly within the root window but
more complex conferences will use regions for this purpose. Areas of
the window which are not used to display video will show the root
window background.
All video presentations require a root window. It MUST be present
when a video mix is created and it cannot be deleted; however, its
attributes MAY be changed using the element.
Attributes:
size: the size of the root window specified as one of the five
standard common intermediate formats (e.g., CIF, QCIF).
backgroundcolor: the color for the root window background defined
using the values for the "background-color" property of the CSS2
specification [n10].
backgroundimage: the URI for an image to be displayed as the root
window background. Transparent portions of the image allow the
background color to show through.
8.7.2. elements define video panes that are used to display
participant video streams. Regions are rendered on top of the root
window.
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The size of a region is specified relative to the size of the root
window using the "relativesize" attribute. Relative sizes are
expressed as fractions (e.g., 1/4, 1/3) that preserve the aspect
ratio of the original video stream while allowing for efficient
scaling implementations.
Regions are located on the root window based on the value of the
position attributes "top" and "left". These attributes define the
position of the top left corner of the region as an offset from the
top left corner of the root window. Their values may be expressed
either as a number of pixels or as a percent of the vertical or
horizontal dimension of the root window. Percent values are appended
with a percent ('%') character. Percent values of "33%" and "67%"
should be interpreted as "1/3" and "2/3" to allow easy alignment of
regions whose size is expressed relative to the size of the root
window.
An example of a video layout with six regions is:
+-------+---+
| | 2 |
| 1 +---+
| | 3 |
+---+---+---+
| 6 | 5 | 4 |
+---+---+---+
The area of the root window covered by a region is a function of the
region's position and its size. When areas of different regions
overlap, they are layered in order of their "priority" attribute.
The region with the highest value for the "priority" attribute is
below all other regions and will be hidden by overlapping regions.
The region with the lowest non-zero value for the "priority"
attribute is on top of all other regions and will not be hidden by
overlapping regions. The priority attribute may be assigned values
between 0 and 1. A value of zero disables the region, freeing any
resources associated with the region, and unjoining any video stream
displayed in the region.
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Regions that do not specify a priority will be assigned a priority by
a media server when a conference is created. The first region within
the element that does not specify a priority will be
assigned a priority of one, the second a priority of two, etc. In
this way, all regions that do not explicitly specify a priority will
be underneath all regions that do specify a priority. As well,
within those regions that do not specify a priority, they will be
layered from top to bottom, in the order they appear within the
element.
For example, if a layout was specified as follows:
Then the regions would be layered, from top to bottom, c,a,b,d.
Portions of regions that extend beyond the root window will be
cropped. For example, a layout specified as:
would appear similar to:
+-----------+
| root |
|background |
| +-----+--
| | |//
| | foo |//
+-----+-----+//
|////////
Visual attributes are used to define aspects of the visual appearance
of individual regions. A border may be defined together with a title
and/or logo. Text and logos are displayed as images on top of the
region's video, below all regions with a lower priority. The visual
attributes are "title", "titletextcolor", "titlebackgroundcolor",
"bordercolor", "borderwidth", and "logo".
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Visual attributes can also be defined for individual streams (Video
Stream Properties). When visual attributes are specified as part of
both a region and a stream, those associated with the stream MUST
take precedence. This allows streams that are chosen for display
automatically (Stream Selection) to have proper text and logos
displayed. The region visual attributes are displayed when no stream
is associated with the region.
Two other attributes associated with a region, "blank" and "freeze",
define the state of the video displayed in the region. When the
blank or freeze attribute is assigned the value "true", then the
media server MUST display the region either as a blank region, or the
video image frozen at the last received frame.
These attributes are specified for a region and not allowed for
streams because that appears to be the common use case. Applying
them to streams would allow only that stream to be affected within a
selector while other streams continue to display normally. Except
for personal mixing scenarios, the same effect can be achieved by
having the participant mute their own transmission to the media
server.
Attributes: associated with each region:
id: a name that can be used to refer to the region.
left: the position of the region from the left side of the root
window.
top: the position of the region from the top of the root window.
relativesize: the size of the region expressed as a fraction of
the root window size.
priority: a number between 0 and 1 that is used to define the
precedence when rendering overlapping regions. A value of zero
disables the region.
title: text to be displayed as the title for the region
titletextcolor: the color of the text
titlebackgroundcolor: the color of the text background
bordercolor: the color of the region border
borderwidth: the width of the region border
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logo: the URI of an image file to be displayed
freeze: a boolean value, with a default of "false", that defines
whether the video image should be frozen at the currently
displayed frame
blank: a boolean value, with a default of "false", that defines
whether the region should display black instead of the associated
video stream
8.7.3.
It is often desired that one of several video streams be
automatically selected to be displayed. The element is
used to define the selection criteria and its associated parameters.
The selection algorithm is specified by the "method" attribute.
Currently defined selection methods allow for voice activated
switching and to iterate sequentially through the set of associated
video streams.
The regions that will display the selected video stream are placed as
child elements of the element. Including regions within a
element does not affect their layout with respect to
regions not subject to the selection. For simple video conferences
that display the video directly in the root window, the
element can be placed as a child of . Region elements MUST
NOT be used in this case.
For example, below is a common video layout that allows the video
stream from the currently active speaker to be displayed in the large
region ("1") at the top left of the layout while the streams from
five other participants are displayed in regions located at the
layout periphery.
+-------+---+
| | 2 |
| 1 +---+
| | 3 |
+---+---+---+
| 6 | 5 | 4 |
+---+---+---+
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All selector methods must be defined so that they work if only a
single region is a child of the selector. Selector methods that
support more than one child region MUST specify how the method works
across multiple regions. Media server implementations MAY support
only a single region for methods that are defined to allow multiple
regions.
The selector or region for a participant's video is defined using the
"display" attribute of during a join operation. Specifying
a selector allows the stream to be displayed according to the
criteria defined by the selector method. Specifying a region
supports continuous presence display of participants. Some streams
may be joined with both a selector and a region. In this case, the
value of attribute defines whether the streams
associated with a continuous presence region should be blanked when
the stream is selected for display in one of the selector regions.
Attributes: common to all selector methods are:
id: a name that can be used to refer to the selector.
method: the name of the method used to select the video stream. A
value of "vas" (see the following section, Voice Activated
Switching) MAY be specified.
status: specifies whether the selector is "active" or "disabled".
blankothers: when "true", video streams that are also displayed in
continuous presence regions will have the continuous presence
regions blanked when the stream is displayed in a selection
region.
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8.7.3.1. Voice Activated Switching ("vas")
Voice activated switching (VAS) is used to display the video stream
that correlates with the participant who is currently speaking. It
is specified using a selector method value of "vas".
If the video stream associated with the active speaker is not
currently displayed in a selection region, then it replaces the video
in the region that is displaying the video of the speaker that was
least recently active. If the video of the active speaker is
currently displayed in a selection region, then there is no change to
any region. When VAS is applied to a single region, this has the
effect that the current speaker is displayed in that region.
Attributes:
si: switching interval is the minimum period of time that must
elapse before allowing the video to switch to the active speaker.
speakersees: defines whether the active speaker sees the "current"
speaker (themselves) or the "previous" speaker.
8.8. is used to create one or more streams between two independent
objects. Streams may be audio or video and may be bidirectional or
unidirectional. A bidirectional stream is implicitly composed of two
unidirectional streams that can be manipulated independently. The
streams to be established are specified by elements (section
) as the content of .
Without any content, by default establishes a bidirectional
audio stream. When only a stream of a single type has previously
been created between two objects, or when only a unidirectional
stream exists, can be used to add a stream of another media
type or make the stream bidirectional by including the necessary
elements. Bidirectional streams are made unidirectional by
using (section ) to remove the unidirectional stream
for the direction that is no longer required.
In addition to defining the media type and direction of streams,
elements are also used to establish the properties of
streams, such as gain, voice masking, or tone clamping of audio
streams, or labels and other visual characteristics of video streams.
Properties are often defined asymmetrically for a single direction of
a stream. Creating a bidirectional stream requires two
elements within the , one for each direction, if one direction
is to have different properties from the other direction.
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If a media server can provide services using both compressed or
uncompressed media, the MSML client may need to distinguish within
requests which format is to be used. When compressed streams are
created, both objects must use the same media format or an error
response (450) is generated.
Attributes:
id1: an identifier of either a connection or conference.
Wildcards MUST NOT be used. Mandatory. Any other object class
results in a 440 error.
id2: an identifier of either a connection or conference.
Wildcards MUST NOT be used. Mandatory. Any other object class
results in a 440 error.
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document SHOULD be unique.
For example, consider a call center coaching scenario where a
supervisor can listen to the conversation between an agent and a
customer and provide hints to the agent, which are not heard by the
customer. One join establishes a stream between the agent and the
customer and another join establishes a stream between the agent and
the supervisor. A third join is used to establish a half-duplex
stream from the customer to the supervisor. The media server
automatically bridges the media streams from the customer and the
supervisor for the agent, and from the customer and the agent for the
supervisor.
Assuming the following connections, each with a single audio stream:
conn:supervisor
conn:agent
conn:customer
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The following would create the media flows previously described:
The following example shows joining a participant to a multimedia
conference. It assumes that the conference has a video
presentation region named "topright". The "display" attribute is
explained in the section Video Stream Properties.
8.9.
Media streams can have different properties such as the gain for an
audio stream or a visual label for a video stream. These properties
are specified as the content of elements (section ).
is used to change the properties of a stream by
including one or more elements that are to have their
properties changed.
Stream properties MUST be set as specified by the element as
a child element of element. Any properties not
included in the element when modifying a stream MUST remain
unchanged. Setting a property for only one direction of a
bidirectional stream MUST NOT affect the other direction. The
directionality of streams can be changed by issuing an
followed by a . Any streams that exist between the two objects
that are not included within MUST NOT be affected.
Attributes:
id1: an identifier of either a conference or a connection. The
instance name MUST NOT contain a wildcard if "id2" contains a
wildcard. Mandatory.
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id2: an identifier of either a conference or a connection. The
instance name MUST NOT contain a wildcard if "id1" contains a
wildcard. Mandatory.
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document is RECOMMENDED to be unique.
8.10.
Unjoin removes one or more media streams between two objects. In the
absence of any content in the element, all media streams
between the objects MUST be removed. Individual streams may be
removed by specifying them using elements, while the
unspecified streams MUST NOT be removed. A bidirectional stream is
changed to a unidirectional stream by unjoining the direction that is
no longer required, using the element. Operator elements
MUST NOT be specified within elements when streams are being
unjoined using the element. Any specified stream operators
MUST be ignored.
and may be used together to move a media stream, such
as from a main conference to a sidebar conference.
Attributes:
id1: an identifier of either a conference or a connection. The
instance name MUST NOT contain a wildcard if "id2" contains a
wildcard. Mandatory.
id2: an identifier of either a conference or a connection. The
instance name MUST NOT contain a wildcard if "id1" contains a
wildcard. Mandatory.
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document SHOULD be unique.
The following removes a participant from a conference and plays a
leave tone for the remaining participants in the conference.
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8.11.
Monitor is a specialized unidirectional join that copies the media
that is destined for a connection object. One example of the use for
may be quality monitoring within a conference. The media
stream may be removed using the element (see the section
).
Attributes:
id1: an identifier of the connection to be monitored. Mandatory.
Any other object class results in a 440 error. Wildcards MUST NOT
be used.
id2: an identifier of the object that is to receive the copy of
the media destined to id1. id2 may be a connection or a
conference. Mandatory. Any other object class results in a 440
error. Wildcards MUST NOT be used.
compressed: "true" or "false". Specifies whether the join should
occur before or after compression. When "true", id2 must be a
connection using the same media format as id1 or an error response
(450) is generated. Default is "false".
mark: a token that can be used to identify execution progress in
the case of errors. The value of the mark attribute from the last
successfully executed MSML element is returned in an error
response. Therefore, the value of all mark attributes within an
MSML document SHOULD be unique.
8.12.
Individual streams are specified using the element. They
MAY be included as a child element in any of the stream manipulation
elements , , or .
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The type of the stream is specified using a "media" attribute that
uses values corresponding to the top-level MIME media types as
defined in RFC 2046 [i7]. This specification only addresses audio
and video media. Other specifications may define procedures for
additional types.
A bidirectional stream is identified when no direction attribute
"dir" is present. A unidirectional stream is identified when a
direction attribute is present. The "dir" attribute MUST have a
value of "from-id1" or "to-id1" depending on the required direction.
These values are relative to the identifier attributes of the parent
element.
The compressed attribute is used to distinguish the compressed nature
of the stream when necessary. It is implementation specific what is
used when the attribute is not present. Joining compressed streams
acts much like an RTP [i3] relay.
The properties of the media streams are specified as the content of
elements when the element is used as a child of or
. Stream elements MUST NOT have any content when they
are used as a child of to identify specific streams to
remove.
Some properties are defined within MSML as additional attributes or
child elements of that are media type specific. Ones for
audio streams and video streams are defined in the following two sub-
sections. Operators, viewed as properties of the media stream, MAY
be specified as child elements of the element.
Attributes:
media: "audio" or video". Mandatory
dir: "from-id1" or "to-id1".
compressed: "true" or "false". Specifies whether the stream uses
compressed media. Default is implementation specific.
8.12.1. Audio Stream Properties
Audio mixes can be specified to only mix the N-loudest participants.
However, there may be some "preferred" participants that are always
able to contribute. When audio streams are joined to a conference
that uses N-loudest audio mixing, preferred streams need to be
identified.
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A preferred audio stream is identified using the "preferred"
attribute. The "preferred" attribute MAY be used for an audio stream
that is input to a conference and MUST NOT be used for other streams.
Additional attributes of the element for audio streams are:
Attributes:
preferred: a boolean value that defines whether the stream does
not contend for N-loudest mixing. A value of "true" means that
the stream MUST always be mixed while a value of "false" means
that the stream MAY contend for mixing into a conference when
N-loudest mixing is enabled. Default is "false".
There are two elements that can be used to change the characteristics
of an audio stream as defined below.
8.12.1.1.
The element may be used to adjust the volume of an audio media
stream. It may be set to a specific gain amount, to automatically
adjust the gain to a desired target level, or to mute the stream.
Attributes:
id: an optional identifier that may be referenced elsewhere for
sending events to the gain primitive.
amt: a specific gain to apply specified in dB or the string "mute"
indicating that the stream should be muted. This attribute MUST
NOT be used if "agc" is present.
agc: boolean indicating whether automatic gain control is to be
used. This attribute MUST NOT be used if "amt" is present.
tgtlvl: the desired target level for AGC specified in dBm0. This
attribute MUST be specified if "agc" is set to "true". This
attribute MUST NOT be specified if "agc" is not present.
maxgain: the maximum gain that AGC may apply. Maxgain is
specified in dB. This attribute MUST be used if "agc" is present
and MUST NOT be used when "agc" is not present.
8.12.1.2.
The element is used to filter tones and/or audio-band dtmf
from a media stream.
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Attributes:
dtmf: boolean indicating whether DTMF tones should be removed.
tone: boolean indicating whether other tones should be removed.
8.12.2. Video Stream Properties
Video mixes define a presentation that may have multiple regions,
such as a quad-split. Each region displays the video from one or
more participants. When video streams are joined to such a
conference, the region that will display the video needs to be
specified as part of the join operation.
The region that will display the video is specified using the
"display" attribute. The "display" attribute MUST be used for a
video stream that is input to a conference and MUST NOT be used for
other streams. The value of the attribute MUST identify a
(see the section ) or a (see the section
) that is defined for the conference. A stream MUST NOT be
directly joined to a region that is defined within a selector.
Changing the value of the "display" attribute can be used to change
where in a video presentation layout a video stream is displayed.
Additional attributes of the element for video streams are:
Attributes:
display: the identifier of a video layout region or selector that
is to be used to display the video stream.
override: specifies whether or not the given video stream is the
override source in the region defined by "display" attribute.
Valid values are "true" or "false". Optional, default value is
"false". Only a video stream that is input to a conference can be
the override source. A particular region can have at most one
override source at a time. The most recently joined video stream
with this attribute set to "true" becomes the override source.
When there's an override source in place, its video is always
displayed in the region, regardless of what video selection
algorithm (either a selector or continuous presence mode) is
configured for that region. Once the override source is cleared,
the conference MUST revert back to original video selection
algorithm.
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8.12.2.1.
Some regions of video conferences may display different streams
automatically, such as when voice activated switching is used.
Connections MAY also be joined directly without the use of video
mixing. In these cases, the element may be used to define
visual display properties for a stream.
The element MAY use any of the visual attributes defined for
regions (see the section ). This allows the visual aspects
of regions within a to be tailored to the selected video
stream, or for streams that are directly joined to display a name or
logo.
9. MSML Dialog Packages
9.1. Overview
MSML Dialog Packages define an XML [n2] language for composing
complex media objects from a vocabulary of simple media resource
objects called primitives. It is primarily a descriptive or
declarative language to describe media processing objects. MSML
dialogs operate on a single or multiple streams that are identified
by the MSML document outside the scope of the MSML Dialog Package.
MSML dialogs are intended to be used in different environments. As
such, the language itself does not define how an MSML dialog is used.
Each environment in which an MSML dialog is used must define how it
is used, the set of services provided, and the mechanism for passing
information between the environment and MSML dialog. The specific
mechanisms used to realize the interface between MSML dialog and its
environment are platform specific.
MSML Dialog Packages provide two models for access to media resources
and service creation building blocks. Both models MAY be used in
conjunction with each other in a complementary manner. The first
model (referred to as "Media Primitives and Composites", part of the
mandatory MSML Dialog Base Package) contains media primitives (such
as digit collection and announcements) and composite functions (such
as play and collect combined as a single operation). The second
model (referred to as "Media Groups", part of the optional MSML
Dialog Group Package) allows the ability to define complex customized
interactions, via event passing mechanisms, between media primitives,
if required.
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MSML Dialog Core Package
Defines core framework over which all MSML Dialog Packages
operate.
MSML Dialog Base Package
Media Primitives
or
DTMF digit collection
Playing of Announcements
Generation of DTMF digits
Tone genration
Media recording
Media Composites
Supports play and collect operation.
Composite function with inclusion of play.
Supports play and record operation.
Composite function with inclusion of play.
MSML Dialog Group Package
Allows grouping of media primitives for parallel
execution, with an event exchange mechanism
between the media primitives to achieve
customized media operations. All the above media
primitive elements are accepted within the
group.
The following operations MUST be supported using elements described
above using either the MSML Dialog Base Package or MSML Dialog Group
Package.
Announcement only
Collection only
or
Recording only
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Play and Collect
Play and Record
Additional MSML Dialog Packages are:
o MSML Dialog Transform Package
o MSML Dialog Speech Package
o MSML Fax Detection Package
o MSML Fax Send/Receive Package
MSML dialogs MAY be used to simply expose primitive media resource
objects but will be used more often to describe dialog operations and
media transformation objects that can be controlled via user
interaction.
MSML dialogs do not contain any computation or flow control
constructs. There are no results automatically generated when media
operations complete. Results MUST be explicitly requested using a
or element within the definition of the MSML dialog.
9.2. Primitives
Primitives perform a single function on a media stream or multiple
streams such as generating audio/video, recognizing speech or DTMF,
or adjusting the gain. They may be composed so that primitives
execute concurrently. Primitives not composed for concurrent
execution MUST simply execute sequentially in the order they occur in
an MSML document. All concurrently executing primitives in the same
MSML object (defined in one MSML document) MAY interact with each
other through events (see MSML Dialog Group Package).
Primitives are categorized into one of the following descriptive
categories.
o Recognizers have a media input but no output. They allow
different things within a media stream to be recognized or
detected and for events to be generated based upon received
media.
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o Transformers have one media input and output and may send and
receive events.
o Sources and sinks generate or consume media. They have either
a media input or a media output but not both. They may receive
and generate events.
o Composites combine underlying primitives to provide higher-
level user interaction, without the need for specific event-
based exchange between the primitives. The composite elements
provide a simpler mechanism for more commonly used services,
such as play and collect or play and record.
Primitives may define different media processing behavior (states)
based upon the events that they receive. Primitives that support
different processing states must define their default starting state
and should support the "initial" attribute to allow that state to be
specified when the primitive is instantiated. All primitives must
support the "terminate" event class.
The following types of primitives are defined within this
specification:
Recognizers Transformers Source/Sink Composites
------------------------------------------------------
dtmf/collect agc play dtmf/collect
faxdetect clamp record record
speech gain dtmfgen
vad gate tonegen
relay faxsend
faxrcv
Primitives have shadow variables, similar to those within VoiceXML
[n5], which are automatically assigned values when the primitives are
used. Upon initialization of an MSML dialog context, all shadow
variables have the string value "undefined". Each primitive has its
own instance of shadow variables that are global in scope to the
entire MSML dialog context.
Names SHOULD be assigned to individual primitives when more than one
primitive of the same type is used within one MSML document. Shadow
variables are overwritten if the primitive has not been named and is
instantiated a second time.
Shadow variables cannot be modified under user control. They may be
returned from the MSML dialog context using the element.
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9.3. Events
Events provide the mechanism for primitives to interact with each
other and for an MSML context to interact with its external
environment. The external environment is defined by the way in which
an MSML context has been invoked. This will often be through MSML,
but other languages and protocols such as SIP may also be used.
Every primitive and group conceptually implements their own event
queue. Events sent to them get placed into their associated queue.
Events are removed from their queues and processed in order.
Primitives within a group conceptually have their own thread of
execution. Due to the asynchronous nature of servicing events from
multiple queues, it cannot be assumed that several events sent in
sequence to different queues will be processed in the order in which
they were sent. For example, if recognition of something led to
sending events to both a and a in that order, it is
possible that the may process its event before the .
Primitives each define the set of events that they support and the
behavior associated with their handling of each event. This allows
many types of behaviors to be defined. For example, VCR type
controls can be constructed by defining primitives that support
events corresponding to each control. Media recognition/detection
can be used to cause those events to be generated.
Alternatively, events can be originated elsewhere, such as from a
control agent, and simply received by the primitive implementing the
control. Examples of the use of events include adjusting volume
(gain) and pause and resume of both announcement playout and record
creation.
Primitives act on events based upon the longest match of an event
name. Event names are a period '.' delimited sequence of tokens.
The first token, or the root of the name, can be considered an event
class. Matching allows a standard meaning to be defined and then
extended based upon what triggers an event's generation. For
example, a record primitive has different behavior depending upon
whether it completed because a user stopped speaking or because it
was cancelled. The recording is retained in the first case but not
the second.
Longest match allows new recognizers to be created and used without
changing how existing primitives are defined. For example, a face
recognition capability could be created that generates a
terminate.frowning event when a user looks puzzled. Although no
primitive directly defines this event, it will still effect a generic
terminate action. Primitives that require specialized behavior based
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upon frowning may be extended to support this. As well, the event
can still be exported from the MSML context without requiring that
primitives receiving the event understand facial expressions.
9.4. MSML Dialog Usage with SIP
MSML dialogs MAY be used directly with SIP for dialog interactions
(e.g., IVR or fax). It can be initially invoked as part of the
"Prompt and Collect" service described in "Basic Network Media
Services with SIP" [n7]. That defines service indicators for a small
number of well-defined services using the user part of the SIP
Request-URI (R-URI).
The prompt and collect service uses "dialog" as the service
indicator. URI parameters further refine the specific IVR request.
This document defines an additional parameter "msml-param" for the
dialog service indicator as follows:
dialog-parameters = ";" ( dialog-param [ vxml-parameters ] )
| moml-param
dialog-param = "voicexml=" dialog-url
moml-param = "moml=" moml-url
There are no additional URI parameters when MSML is used as the
dialog language.
MSML dialogs define discrete IVR dialog commands. These commands MAY
be included directly in the body of the INVITE to the "dialog"
service indicator by using the "cid" [n8] URL scheme. This scheme
identifies a message body part that in this case would contain the
MSML dialog request. Note that a multipart message body, containing
a single part, MUST be present even if the INVITE does not contain an
SDP offer. Subsequent MSML dialog requests are sent in the body of
SIP INFO messages as are all messages from a media server.
An example of SIP URI as described above is:
sip:dialog@mediaserver.example.net;\
moml=cid:14864099865376@appserver.example.net
The body part that contained the MSML dialog referenced by the URL
would have a Content-Id header of:
Content-Id: <14864099865376@appserver.example.net>
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The results of executing an or , or of executing a
that has a "target" attribute value equal to "source", are
notified in SIP INFO messages using the element from MSML
Core package. No messages are sent if execution completes normally
without executing one of these elements.
If there is an error during validation or execution, then a media
server MUST notify the error as described above and must include the
namelist items "moml.error.status" and "moml.error.description". The
values for these items are defined in section 11.
A restricted subset of MSML dialogs can also be used with the
"Announcement" service defined in [n7]. This service uses "annc" as
the service indicator and defines parameters that describe an
announcement. The "play=" parameter identifies the URL of a prompt
or a provisioned announcement sequence. The value of the "play="
parameter can refer to an MSML dialog body part using a "cid" URL as
described above. That body part must only contain the
primitive.
Using MSML dialogs enhances the announcement service by allowing the
client to specify a sequence of audio segments rather than requiring
each sequence to be provisioned as well as support for video.
Moreover, MSML dialogs define a standard set of variables in contrast
to [n7] which defines a parameterization mechanism but does not
formally specify any semantics.
If a media server does not understand the "cid" scheme or does not
understand MSML dialogs, it must respond with the SIP response code
"488 - not acceptable here". If the MSML dialog body contains
elements other than the primitive, or there are errors during
validation, a media server must respond with a SIP response code "400
- bad request". Finally, if there is a discrepancy between
parameters specified in the Request-URI and corresponding attributes
defined in the MSML dialog body, the Request-URI parameters must be
silently ignored.
MSML dialogs MUST NOT change the operation of the announcement
service from that defined in [n7]. When the announcement completes,
a media server issues a SIP BYE request. The INFO method MUST NOT
used with the announcement service.
9.5. MSML Dialog Structure and Modularity
MSML is structured as a set of packages. Only the core and base
packages are required. The Dialog Core Package defines the framework
for MSML requests to a media server, without specific functionality.
It consists of the "primitive" abstraction, an abstract element for
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control flow, the sequential execution model, and the element.
That is, the MSML Dialog Core Package allows for the execution of a
sequence of one or more media processing primitives with the ability
to notify events to the invocation environment.
Primitives are contained within the MSML Dialog Base Package, which
defines the basic , , , , , and
elements. Another package, the MSML Dialog Transform
Package, defines the simple half-duplex filters. More advanced
primitives are defined in the speech and fax packages. The MSML
speech package depends on the MSML Dialog Base Package as it extends
the capability of by adding synthesized speech. Finally, the
group execution model, which is currently the only element that
changes the flow of control, is defined in a separate MSML Dialog
Group Package. All of these packages are optional with the exception
that MSML Dialog Core and MSML Dialog Base Packages MUST be
implemented to provide the minimal functionality.
9.6. MSML Dialog Core Package
The MSML Dialog Core Package defines the structural framework and
abstractions for MSML dialogs (via its schema). It also defines the
basic elements that are not part of the core primitive or control
abstractions. This package is dependent on the MSML Core Package.
Events generated by MSML dialogs, such as prompt completion, digits
collected, or dialog termination, are communicated by the media
server via the MSML Core Package (see MSML Core Package ).
MSML dialogs are executed independently from the MSML core context.
When an MSML dialog is started, MSML allocates the dialog control
resources, and if successful, starts those resources executing. MSML
core execution then continues without waiting for the MSML dialog to
complete. This forking of MSML dialog invocation from the MSML core
context is done via the
