Media Server Control Markup Language (MSCML) and Protocol :: RFC5022
Network Working Group J. Van Dyke
Request for Comments: 5022 Cantata Technology, Inc.
Obsoletes: 4722 E. Burger, Ed.
Category: Informational BEA Systems, Inc.
A. Spitzer
Bluesocket Inc.
September 2007
Media Server Control Markup Language (MSCML) and Protocol
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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.
Abstract
Media Server Control Markup Language (MSCML) is a markup language
used in conjunction with SIP to provide advanced conferencing and
interactive voice response (IVR) functions. MSCML presents an
application-level control model, as opposed to device-level control
models. One use of this protocol is for communications between a
conference focus and mixer in the IETF SIP Conferencing Framework.
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Table of Contents
1. Introduction ....................................................4
1.1. Conventions Used in This Document ..........................5
2. MSCML Approach ..................................................5
3. Use of SIP Request Methods ......................................6
4. MSCML Design ....................................................8
4.1. Transaction Model ..........................................8
4.2. XML Usage ..................................................9
4.2.1. MSCML Time Values ...................................9
5. Advanced Conferencing ..........................................10
5.1. Conference Model ..........................................10
5.2. Configure Conference Request .......11
5.3. Configure Leg Request .....................13
5.4. Terminating a Conference ..................................14
5.5. Conference Manipulation ...................................15
5.6. Video Conferencing ........................................16
5.7. Conference Events .........................................17
5.8. Conferencing with Personalized Mixes ......................18
5.8.1. MSCML Elements and Attributes for
Personalized Mixes .................................19
5.8.2. Example Usage of Personalized Mixes ................20
6. Interactive Voice Response (IVR) ...............................23
6.1. Specifying Prompt Content .................................24
6.1.1. Use of the Prompt Element ..........................24
6.2. Multimedia Processing for IVR .............................30
6.3. Playing Announcements ..............................31
6.4. Prompt and Collect ..........................32
6.4.1. Control of Digit Buffering and Barge-In ............33
6.4.2. Mapping DTMF Keys to Special Functions .............33
6.4.3. Collection Timers ..................................35
6.4.4. Logging Caller DTMF Input ..........................36
6.4.5. Specifying DTMF Grammars ...........................36
6.4.6. Playcollect Response ...............................37
6.4.7. Playcollect Example ................................38
6.5. Prompt and Record ............................38
6.5.1. Prompt Phase .......................................38
6.5.2. Record Phase .......................................39
6.5.3. Playrecord Example .................................41
6.6. Stop Request .......................................42
7. Call Leg Events ................................................43
7.1. Keypress Events ...........................................43
7.1.1. Keypress Subscription Examples .....................45
7.1.2. Keypress Notification Examples .....................45
7.2. Signal Events .............................................46
7.2.1. Signal Event Examples ..............................47
8. Managing Content ...............................48
8.1. Managecontent Example .....................................50
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9. Fax Processing .................................................51
9.1. Recording a Fax ...............................51
9.2. Sending a Fax ...................................53
10. MSCML Response Attributes and Elements ........................56
10.1. Mechanism ................................................56
10.2. Base Attributes ...............................56
10.3. Response Attributes and Elements for .....57
10.4. Response Attributes and Elements for ..............57
10.4.1. Reporting Content Retrieval Errors ...............58
10.5. Response Attributes and Elements for .......59
10.6. Response Attributes and Elements for ........60
10.7. Response Attributes and Elements for .....61
10.8. Response Attributes and Elements for
and ..........................................61
11. Formal Syntax .................................................62
11.1. Schema ...................................................62
12. IANA Considerations ...........................................73
12.1. IANA Registration of MIME Media Type application/
mediaservercontrol+xml ...................................73
13. Security Considerations .......................................74
14. References ....................................................75
14.1. Normative References .....................................75
14.2. Informative References ...................................76
Appendix A. Regex Grammar Syntax .................................78
Appendix B. Contributors .........................................79
Appendix C. Acknowledgements .....................................79
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1. Introduction
This document describes the Media Server Control Markup Language
(MSCML) and its usage. It describes payloads that one can send to a
media server using standard SIP INVITE and INFO methods and the
capabilities these payloads implement. RFC 4240 [2] describes media
server SIP URI formats.
Prior to MSCML, there was not a standard way to deliver SIP-based
enhanced conferencing. Basic SIP constructs, such as those described
in RFC 4240 [2], serve simple n-way conferencing well. The SIP URI
provides a natural mechanism for identifying a specific SIP
conference, while INVITE and BYE methods elegantly implement
conference join and leave semantics. However, enhanced conferencing
applications also require features such as sizing and resizing, in-
conference IVR operations (e.g., recording and playing participant
names to the full conference), and conference event reporting. MSCML
payloads within standard SIP methods realize these features.
The structure and approach of MSCML satisfy the requirements set out
in RFC 4353 [10]. In particular, MSCML serves as the interface
between the conference server or focus and a centralized conference
mixer. In this case, a media server has the role of the conference
mixer.
There are two broad classes of MSCML functionality. The first class
includes primitives for advanced conferencing, such as conference
configuration, participant leg manipulation, and conference event
reporting. The second class comprises primitives for interactive
voice response (IVR). These include collecting DTMF digits and
playing and recording multimedia content.
MSCML fills the need for IVR and conference control with requests and
responses over a SIP transport. VoiceXML [11] fills the need for IVR
with requests and responses over a HTTP transport. This enables
developers to use whatever model fits their needs best.
In general, a media server offers services to SIP UACs, such as
Application Servers, Feature Servers, and Media Gateway Controllers.
See the IPCC Reference Architecture [12] for definitions of these
terms. It is unlikely, but not prohibited, for end-user SIP UACs to
have a direct signaling relationship with a media server. The term
"client" is used in this document to refer generically to an entity
that interacts with the media server using SIP and MSCML.
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The media server fulfills the role of the Media Resource Function
(MRF) in the IP Multimedia Subsystem (IMS) [13] as described by 3GPP.
MSCML and RFC 4240 [2], upon which MSCML builds, are specifically
focused on the Media resource (Mr) interface which supports
interactions between application logic and the MRF.
This document describes a working framework and protocol with which
there is considerable implementation experience. Application
developers and service providers have created several MSCML-based
services since the availability of the initial version in 2001. This
experience is highly relevant to the ongoing work of the IETF,
particularly the SIP [26], SIPPING [27], MMUSIC [28], and XCON [29]
work groups, the IMS [30] work in 3GPP, and the CCXML work in the
Voice Browser Work Group of the W3C.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
2. MSCML Approach
It is critically important to emphasize that the goal of MSCML is to
provide an application interface that follows the SIP, HTTP, and XML
development paradigm to foster easier and more rapid application
deployment. This goal is reflected in MSCML in two ways.
First, the programming model is that of peer to peer rather than
master-slave. Importantly, this allows the media server to be used
simultaneously for multiple applications rather than be tied to a
single point of control. It also enables standard SIP mechanisms to
be used for media server location and load balancing.
Second, MSCML defines constructs and primitives that are meaningful
at the application level to ensure that programmers are not
distracted by unnecessary complexity. For example, the mixing
resource operates on constructs such as conferences and call
participants rather than directly on individual media streams.
The MSCML paradigm is important to the developer community, in that
developers and operators conceptually write applications about calls,
conferences, and call legs. For the majority of developers and
applications this approach significantly simplifies and speeds
development.
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3. Use of SIP Request Methods
As mentioned above, MSCML payloads may be carried in either SIP
INVITE or INFO requests. The initial INVITE, which creates an
enhanced conference, MAY include an MSCML payload. A subsequent
INVITE to the same Request-URI joins a participant leg to the
conference. This INVITE MAY include an MSCML payload. The initial
INVITE that establishes an IVR session MUST NOT include an MSCML
payload. The client sends all mid-call MSCML payloads for
conferencing and IVR via SIP INFO requests.
SIP INVITE requests that contain both MSCML and Session Description
Protocol (SDP) body parts are used frequently in conferencing
scenarios. Therefore, the media server MUST support message bodies
with the MIME type "multipart/mixed" in SIP INVITE requests.
The media server transports MSCML responses in the final response to
the SIP INVITE containing the matching MSCML request or in a SIP INFO
message. The only allowable final response to a SIP INFO containing
a message body is a 200 OK, per RFC 2976 [3]. Therefore, if the
client sends the MSCML request via SIP INFO, the media server
responds with the MSCML response in a separate INFO request. In
general, these responses are asynchronous in nature and require a
separate transaction due to timing considerations.
There has been considerable debate on the use of the SIP INFO method
for any purpose. Our experience is that MSCML would not have been
possible without it. At the time the first MSCML specification was
published, the first SIP Event Notification draft had just been
submitted as an individual submission. At that time, there was no
mechanism to link SUBSCRIBE/NOTIFY to an existing dialog. This
prevented its use in MSCML, since all events occurred in an INVITE-
established dialog. And while SUBSCRIBE/NOTIFY was well suited for
reporting conference events, its semantics seemed inappropriate for
modifying a participant leg or conference setting where the only
"event" was the success or failure of the request. Lastly, since SIP
INFO was an established RFC, most SIP stack implementations supported
it at that time. We had few, if any, interoperability issues as a
result.
More recent developments have provided additional reasons why
SUBSCRIBE/NOTIFY is not appropriate for use in MSCML. Use of
SUBSCRIBE presents two problems. The first is semantic. The purpose
of SUBSCRIBE is to register interest in User Agent state. However,
using SUBSCRIBE for MSCML results in the SUBSCRIBE modifying the User
Agent state. The second reason SUBSCRIBE is not appropriate is
because MSCML is inherently call based. The association of a SIP
dialog with a call leg means MSCML can be incredibly straightforward.
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For example, if one used SUBSCRIBE or other SIP method to send
commands about some context, one must identify that context somehow.
Relating commands to the SIP dialog they arrive on defines the
context for free. Moreover, it is conceptually easy for the
developer. Using NOTIFY to transport MSCML responses is also not
appropriate, as the NOTIFY would be in response to an implicit
subscription. The SIP and SIPPING lists have discussed the dangers
of implicit subscription.
In order to guarantee interoperability with this specification, as
well as with SIP User Agents that are unaware of MSCML, SIP UACs that
wish to use MSCML services MUST specify a service indicator that
supports MSCML in the initial INVITE. RFC 4240 [2] defines the
service indicator "conf", which MUST be used for MSCML conferencing
applications. The service indicator "ivr" MUST be used for MSCML
interactive voice response applications. In this specification, only
"conf" and "ivr" are described.
The media server MUST support moving the call between services
through sending the media server a BYE on the existing dialog and
establishing a new dialog with an INVITE to the desired service.
Media servers SHOULD support moving between services without
requiring modification of the previously established SDP parameters.
This is achieved by sending a re-INVITE on the existing dialog in
which the Request-URI is modified to specify the new service desired
by the client. This eliminates the need for the client to send an
INVITE to the caller or gateway to establish new SDP parameters.
The media server, as a SIP UAS, MUST respond appropriately to an
INVITE that contains an MSCML body. If MSCML is not supported, the
media server MUST generate a 415 final response and include a list of
the supported content types in the response per RFC 3261 [4]. The
media server MUST also advertise its support of MSCML in responses to
OPTIONS requests, by including "application/mediaservercontrol+xml"
as a supported content type in an Accept header. This alleviates the
major issues with using INFO for the transport of application data;
namely, the User Agent's proper interpretation of what is, by design,
an opaque message request.
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4. MSCML Design
4.1. Transaction Model
To avoid undue complexity, MSCML establishes two rules regarding its
usage. The first is that only one MSCML body may be present in a SIP
request. The second is that each MSCML body may contain only one
request or response. This greatly simplifies transaction management.
MSCML syntax does provide for the unique identification of multiple
requests in a single body part. However, this is not supported in
this specification.
Per the guidelines of RFC 3470 [14], MSCML bodies MUST be well formed
and valid.
MSCML is a direct request-response protocol. There are no
provisional responses, only final responses. A request may, however,
result in multiple notifications. For example, a request for active
talker reports will result in a notification for each speaker set.
This maps to the three major element trees for MSCML: ,
, and .
Figure 1 shows a request body. Depending on the command, one can
send the request in an INVITE or an INFO. Figure 2 shows a response
body. The SIP INFO method transports response bodies. Figure 3
shows a notification body. The SIP INFO method transports
notifications.
... request body ...
Figure 1: MSCML Request Format
... response body ...
Figure 2: MSCML Response Format
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... notification body ...
Figure 3: MSCML Notification Format
MSCML requests MAY include a client-defined ID attribute for the
purposes of matching requests and responses. The values used for
these IDs need only be unique within the scope of the dialog in which
the requests are issued.
4.2. XML Usage
In the philosophy of XML as a text-based description language, and
not as a programming language, MSCML makes the choice of many
attribute values for readability by a human. Thus, many attributes
that would often be "boolean" instead take "yes" or "no" values. For
example, what does 'report="false"' or 'report="1"' mean? However,
'report="yes"' is clearer: I want a report. Some programmers prefer
the precision of a boolean. To satisfy both styles, MSCML defines an
XML type, "yesnoType", that takes on the values "yes" and "no" as
well as "true", "false", "1", and "0".
Many attributes in the MSCML schema have default values. In order to
limit demands on the XML parser, MSCML applies these values at the
protocol, not XML, level. The MSCML schema documents these defaults
as XML annotations to the appropriate attribute.
4.2.1. MSCML Time Values
For clarity, time values in MSCML are based on the time designations
described in the Cascading Style Sheets level 2 (CSS2) Specification
[15]. Their format consists of a number immediately followed by an
optional time unit identifier of the following form:
ms: milliseconds (default)
s: seconds
If no time unit identifier is present, the value MUST be interpreted
as being in milliseconds. As extensions to [15] MSCML allows the
string values "immediate" and "infinite", which have special meaning
for certain timers.
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5. Advanced Conferencing
5.1. Conference Model
The advanced conferencing model is a star controller model, with both
signaling and media directed to a central location. Figure 4 depicts
a typical signaling relationship between end users' UACs, a
conference application server, and a media server.
RFC 4353 [10] describes this model. The application server is an
instantiation of the conference focus. The media server is an
instantiation of the media mixer. Note that user-level constructs,
such as event notifications, are in the purview of the application
server. This is why, for example, the media server sends active
talker reports using MSCML notifications, while the application
server would instead use the conference package [16] for individual
notifications to SIP user agents. Note that we do not recommend the
use of the conference package for media server to application server
notifications because none of the filtering and membership
information is available at the media server.
+-------+
| UAC 1 |---\ Public URI +-------------+
+-------+ \ _____________| Application |
/ / | Server | Not shown:
+-------+ / / +-------------+ RTP flows directly
| UAC 2 |---/ / | Private between UACs and
+-------+ / | URI media server
. / +--------------+
: / | |
+-------+ / | Media Server |
| UAC n |---/ | |
+-------+ +--------------+
Figure 4: Conference Model
Each UAC sends an INVITE to a Public Conference URI. Presumably, the
client publishes this URI, or it is an ad hoc URI. In any event, the
client generates a Private URI, following the rules specified by RFC
4240 [2]. That is, the URI is of the following form:
sip:conf=UniqueID@ms.example.net
where UniqueID is a unique conference identifier and ms.example.net
is the host name or IP address of the media server. There is nothing
to prevent the UACs from contacting the media server directly.
However, one would expect the owner of the media server to restrict
who can use its resources.
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As for basic conferencing, described by RFC 4240 [2], the first
INVITE to the media server with a UniqueID creates a conference.
However, in advanced conferencing, the first INVITE MAY include a
MSCML payload rather than the SDP of a
conference participant. The payload conveys
extended session parameters (e.g., number of participants) that SDP
does not readily express, but the media server must know to allocate
the appropriate resources.
When the conference is created by sending an INVITE containing a
MSCML payload, the resulting SIP dialog is
termed the "Conference Control Leg." This leg has several useful
properties. The lifetime of the conference is the same as that of
its control leg. This ensures that the conference remains in
existence even if all participant legs leave or have not yet arrived.
In addition, when the client terminates the Conference Control Leg,
the media server automatically terminates all participant legs. The
Conference Control Leg is also used for play or record operations
to/from the entire conference and for active talker notifications.
Full conference media operations and active talker report
subscriptions MUST be executed on the Conference Control Leg.
Creation of a Conference Control Leg is RECOMMENDED because full
advanced conferencing capabilities are not available without it.
Clients MUST establish the Conference Control Leg in the initial
INVITE that creates the conference; it cannot be created later.
Once the client has created the conference with or without the
Conference Control Leg, participants can be joined to the conference.
This is achieved by the client's directing an INVITE to the Private
Conference URI for each participant. Using the example conference
URI given above, this would be sip:conf=UniqueID@ms.example.net.
5.2. Configure Conference Request
The request has two attributes that control
the resources the media server sets aside for the conference. These
are described in the list below.
Attributes of :
o reservedtalkers - optional (see note), no default value: The
maximum number of talker legs allocated for the conference. Note:
required when establishing the Conference Control Leg but optional
in subsequent requests.
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o reserveconfmedia - optional, default value "yes": Controls
allocation of resources to enable playing or recording to or from
the entire conference.
When the reservedtalkers+1st INVITE arrives at the media server, the
media server SHOULD generate a 486 Busy Here response. Failure to
send a 486 response to this condition can cause the media server to
oversubscribe its resources.
NOTE: It would be symmetric to have a reservedlisteners parameter.
However, the practical limitation on the media server is the
number of talkers for a mixer to monitor. In either case, the
client regulates who gets into the conference by either proxying
the INVITEs from the user agent clients or metering to whom it
gives the conference URI.
For example, to create a conference with up to 120 active talkers and
the ability to play audio into the conference or record portions or
all of the conference full mix, the client specifies both attributes,
as shown in Figure 6.
Figure 6: 120 Speaker MSCML Example
In addition to these attributes, a request MAY
contain a child element. The element is used
to request notifications for conference-wide active talker events.
Detailed information regarding active talker events is contained in
Section 5.7.
The client MUST include a request in the
initial INVITE which establishes the conference when creating the
Conference Control Leg. The client server MUST issue asynchronous
commands, such as , separately (i.e., in INFO messages) to
avoid ambiguous responses.
Media operations on the Conference Control leg are performed
internally, no external RTP streams are involved. Accordingly, the
media server does not expect RTP on the Conference Control Leg.
Therefore, the client MUST send either no SDP or hold SDP in the
INVITE request containing a payload. The
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media server MUST treat SDP with all media lines set to "inactive" or
with connection addresses set to 0.0.0.0 (for backwards
compatibility) as hold SDP.
The media server sends a response when it has finished processing the
request. The format of the
response is detailed in Section 10.2.
5.3. Configure Leg Request
Conference legs have a number of properties the client can modify.
These are set using the request. This request has
the attributes described in the list below.
Attributes of :
o type - optional, default value "talker": Consider this leg's audio
for inclusion in the output mix. Alternative is "listener".
o dtmfclamp - optional, default value "yes": Remove detected DTMF
digits from the input audio.
o toneclamp - optional, default value "yes": Remove tones from the
input audio. Tones include call progress tones and the like.
o mixmode - optional, default value "full": Be a candidate for the
full mix. Alternatives are "mute", to disallow media in the mix,
"parked", to disconnect the leg's media streams from the
conference for IVR operations, "preferred", to give this stream
preferential selection in the mix (i.e., even if not loudest
talker, include media, if present, from this leg in the mix), and
"private", which enables personalized mixes.
In addition to these attributes, there are four child elements
defined for . These are , ,
, and .
The first two, and , modify the gain applied
to the input and output audio streams, respectively. These may
contain , to use automatic gain control (AGC) or . The
element has the attributes "startlevel", "targetlevel", and
"silencethreshold". All the parameters are in dB. The
element has the attribute "level", which is in dB. The default for
both and is . The media server MAY
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silently cap or requests that exceed the
gain limits imposed by the platform.
Clients most commonly manipulate only the input gain for a conference
leg and rely on the mixer to set an optimum output gain based on the
inputs currently in the mix. However, as described above, MSCML does
allow for manipulation of the output gain as well. Some of the IVR
commands, such as , enable control of the output gain for
content playback operations. The interaction of conference output
gain and IVR playback gain controls is described in Section 6.1.1.
Note that and settings apply only to
conference legs and do not apply to IVR sessions.
The element is used to create and manipulate groups
for personalized mixes. Details of personalized mixes are discussed
in Section 5.8.
The element is used to request notifications for call leg
related events, such as asynchronous DTMF digit reports. Detailed
information regarding call leg events is discussed in Section 7.
If the default parameters are acceptable for the leg the client
wishes to enter into the conference, then a normal SIP INVITE, with
no MSCML body, is sufficient. However, if the client wishes to
modify one or more of the parameters, the client can include a MSCML
body in addition to the SDP body.
The client can modify the conference leg parameters during the
conference by issuing a SIP INFO on the dialog representing the
conference leg. Of course, the client cannot modify SDP in an INFO
message.
The media server sends a response when it has finished processing the
request. The format of the response
is detailed in Section 10.3.
5.4. Terminating a Conference
To remove a leg from the conference, the client issues a SIP BYE
request on the selected dialog representing the conference leg.
The client can terminate all legs in a conference by issuing a SIP
BYE request on the Conference Control Leg. If one or more
participants are still in the conference when the media server
receives a SIP BYE request on the Conference Control Leg, the media
server issues SIP BYE requests on all remaining conference legs to
ensure cleanup of the legs.
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The media server returns a 200 OK to the SIP BYE request as it sends
BYE requests to the other legs. This is because we cannot issue a
provisional response to a non-INVITE request, yet the teardown of the
other legs may exceed the retransmission timer limits of the original
request. While the conference is being cleaned up, the media server
MUST reject any new INVITEs to the terminated conference with a 486
Busy Here response. This response indicates that the specified
conference cannot accept any new members, pending deletion.
5.5. Conference Manipulation
Once the conference has begun, the client can manipulate the
conference as a whole or a particular participant leg by issuing
commands on the associated SIP dialog. For example, by sending MSCML
requests on the Conference Control Leg the client can request that
the media server record the conference, play a prompt to the
conference, or request reports on active talker events. Similarly,
the client may mute a participant leg, configure a personalized mix
or request reports for call leg events, such as DTMF keypresses.
Figure 7 shows an example of an MSCML command that plays a prompt to
all conference participants.
Figure 7: Full Conference Audio Command - Play
A client can modify a leg by issuing an INFO on the dialog associated
with the participant leg. For example, Figure 8 mutes a conference
leg.
Figure 8: Sample Change Leg Command
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In Figure 7, we saw a request to play a prompt to the entire
conference. The client can also request to play a prompt to an
individual call leg. In that case, the MSCML request is issued
within the SIP dialog of the desired conference participant.
Section 6 describes the interactive voice response (IVR) services
offered by MSCML. If an IVR command arrives on the control channel,
it takes effect on the whole conference. This is a mechanism for
playing prompts to the entire conference (e.g., announcing new
participants). If an IVR command arrives on an individual leg, it
only affects that leg. This is a mechanism for interacting with
users, such as the creation of "waiting rooms", allowing a user to
mute themselves using key presses, allowing a moderator to out-dial,
etc.
A participant leg MUST be configured with mixmode="parked" prior to
the issuance of any IVR commands with prompt content ('prompturl'
attribute or element). Parking the leg isolates the
participant's input and output media from the conference and allows
use of those streams for playing and recording purposes. However,
the mixmode has no effect if just digit collection or recording is
desired. and requests without prompt
content MAY be sent on participant legs without setting
mixmode="parked".
5.6. Video Conferencing
MSCML-controlled advanced conferences, as well as RFC 4240 [2]
controlled basic conferences, implicitly support video conferencing
in the form of video switching. In video switching, the video stream
of the loudest talker (with some hysteresis) is sent to all
participants other than that talker. The loudest talker receives the
video stream from the immediately prior loudest talker.
Media servers MUST ensure that participants receive video media
compatible with their session. For example, a participant who has
established an H.263 video stream will not receive video from another
participant employing H.264 media. Media servers SHOULD implement
video transcoding to minimize media incompatibilities between
participants.
The media server MUST switch video streams only when it receives a
refresh video frame. A refresh frame contains all the video
information required to decode that frame (i.e., there is no
dependency on data from previous video frames).
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Refresh frames are large and generally sent infrequently to conserve
network bandwidth. The media server MUST implement standard
mechanisms to request that the new loudest talker's video encoder
transmits a refresh frame to ensure that video can be switched
quickly.
5.7. Conference Events
A client can subscribe for periodic active talker event reports that
indicate which participants are included in the conference mix. As
these are conference-level events, the subscription and notifications
are sent on the Conference Control Leg.
Media servers MAY impose limits on the minimum interval for active
talker reports for performance reasons. If the client request is
below the imposed minimum, the media server SHOULD set the interval
to the minimum value supported. To limit unnecessary notification
traffic, the media server SHOULD NOT send a report if the active
talker information for the conference has not changed during the
reporting interval.
A request for an active talker report is in Figure 9. The active
talker report enumerates the current call legs in the mix.
Figure 9: Active Talker Request
Event notifications are sent in SIP INFO messages. Figure 10 shows
an example of a report.
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Figure 10: Active Talker Event Example
The value of the "callid" attribute in the element
corresponds to the value of the SIP Call-ID header of the associated
dialog. This enables the client to associate the active talker with
a specific participant leg.
5.8. Conferencing with Personalized Mixes
MSCML enables clients to create personalized mixes through the
element for scenarios where the standard mixmode
settings do not provide sufficient control. The
element is a child of .
To create personalized mixes, the client has to identify the
relationships among the participants. This is accomplished by
manipulating two MSCML objects. These objects are:
1. The list of team members ( elements), set using
2. The mixmode attribute set through
The media server uses the values of these objects to determine which
audio inputs to combine for output to the participant. In a normal
conference, each participant hears the conference mix minus their own
input if they are part of the mixed output. The team list enables
the client to specify other participants that the leg can hear in
addition to the normal mixed output. Note that personalized mix
settings apply only to audio media and do not affect video switching.
Team relationships are implicitly symmetric. If the client sets
participant A as a team member of participant B, then the media
server automatically sets participant B as a team member for A.
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The id attribute set through is used to identify the
various participants. A unique ID MUST be assigned to each
participant included in a personalized mix. The IDs used MUST be
unique within the scope of the conference in which they appear.
By itself, the team list only defines those participants that the leg
can hear. The mixmode attribute of each team member determines
whether to include their audio input in the personalized mix. If the
client sets the teammate's mixmode to private, then it is part of the
mix. If the mixmode is set to any other value, it is not.
5.8.1. MSCML Elements and Attributes for Personalized Mixes
Control of personalized mixes rely on two major MSCML elements:
1. , using the mixmode attribute setting
mixmode="private"
2.
The element allows the user to make the participants
members of a team within a specific conference. It is a child of the
parent element.
The client sends the element in a
request in either a SIP INVITE or SIP INFO.
o In an INVITE, to join a participant whose properties differ from
the properties established for the conference as a whole.
o In an INFO, to change the properties for an existing leg.
The two attributes of the configure_team element are "id" and
"action". The id attribute MUST contain the unique ID of the leg
being modified, as set in the original request. The
action attribute can take on the values "add", "delete", "query", and
"set". The default value is "query". This attribute allows the user
to modify the team list. Table 1 describes the actions that can be
performed on the team list.
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+--------+----------------------------------------------------------+
| Action | Description |
+--------+----------------------------------------------------------+
| add | Adds a teammate to the mix. |
| delete | Deletes a teammate from the mix. |
| query | Returns the teammate list to the requestor. This is the |
| | default value. |
| set | Creates a team list when followed by |
| | and also removes all the teammates from the team list |
| | for example, when the creator (originator) of the team |
| | list on that specific conference leg wants to remove all |
| | of the teammates from the team. If the set operation |
| | removes all teammates from a participant, that |
| | participant hears the full conference mix. |
+--------+----------------------------------------------------------+
Table 1: Configure Team Actions
5.8.2. Example Usage of Personalized Mixes
A common use of personalized mixing is to support coaching of one
participant by another. The coaching scenario includes three
participants:
1. The Supervisor, who coaches the agent.
2. The Agent, who interacts with the customer.
3. The Customer, who interacts with the agent.
Table 2 illustrates the details of the coached conference topology.
+-------------+------------+------------+---------+-----------------+
| Participant | ID | Team | Mixmode | Hears |
| | | Members | | |
+-------------+------------+------------+---------+-----------------+
| Supervisor | supervisor | Agent | Private | customer + |
| | | | | agent |
| Agent | agent | Supervisor | Full | customer + |
| | | | | supervisor |
| Customer | customer | none | Full | agent |
+-------------+------------+------------+---------+-----------------+
Table 2: Coached Conference Example
To create this topology, the client performs the following actions:
1. The client joins each leg to the conference, being certain to
include a unique ID in the request. The leg ID
needs to be unique only within the scope of the conference to
which it belongs.
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2. The client configures the teammate list and mixmode of each
participant, as required.
Both actions (steps 1 and 2) may be combined in a single MSCML
request. The following sections detail these actions and their
corresponding MSCML payloads.
5.8.2.1. Create the Conference
Before joining any participants, the client must create the
conference by sending a SIP INVITE that contains an MSCML
request with a unique conference identifier.
5.8.2.2. Joining and Configuring the Coach
Join the coach leg to the conference and configure its desired
properties by sending a SIP INVITE containing a
request. The element sets the leg's unique ID to
supervisor and its mixmode to private.
The corresponding MSCML request is as follows.
Figure 11: Join Coach Request
Note that the client cannot configure the teammate list for the coach
yet, as there are no other participants in the conference. One must
join a participant to the conference before one can add it as a
teammate for another leg.
5.8.2.3. Joining and Configuring the Agent
Join the agent leg to the conference and configure its desired
properties by sending a SIP INVITE containing a
request. The element sets the leg's unique ID to
"agent" and sets the supervisor as a team member of the agent.
Because team member relationships are symmetric, this action also
adds the agent as a team member for the coach.
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The corresponding MSCML request is as follows.
Figure 12: Join Agent Request
Because the desired mixmode for this leg is full, which is the
default value, there is no need to set it explicitly.
5.8.2.4. Joining and Configuring the Client
Join the client leg to the conference and configure its desired
properties by sending a SIP INVITE containing a
request. The element simply sets the leg's unique ID
to "customer". The media server does not need further configuration
because the desired mixmode, full, is the default and the customer
has no team members.
The corresponding MSCML request is as follows.
Figure 13: Join Client Request
Strictly speaking, it is not a requirement that the client give the
customer leg a unique ID because it will not be a team member.
However, when using coached conferencing, we RECOMMEND that one
assign a unique ID to each leg in the initial INVITE request.
Assigning a unique ID eliminates the need to set it later by sending
a SIP INFO if one later desires personalized mixing for the customer
leg.
The conference is now in the desired configuration, shown previously
in Table 2.
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6. Interactive Voice Response (IVR)
In the IVR model, the media server acts as a media-processing proxy
for the UAC. This is particularly useful when the UAC is a media
gateway or other device with limited media processing capability.
The typical use case for MSCML is when there is an application server
that is the MSCML client. The client can use the SIP Service URI
concept (RFC 3087) to initiate a service. The client then uses RFC
4240 [2] to initiate a MSCML session on a media server. These
relationships are shown in Figure 14.
SIP +--------------+
Service URI | Application |
/----------------| Server |
/(e.g., RFC 3087) +--------------+
/ | MSCML
/ SIP | Session
/ +--------------+
+-----+/ RTP | |
| UAC |======================| Media Server |
+-----+ | |
+--------------+
Figure 14: IVR Model
The IVR service supports basic Interactive Voice Response functions,
playing announcements, collecting DTMF digits, and recording, based
on Media Server Control Markup Language (MSCML) directives added to
the message body of a SIP request. The major MSCML IVR requests are
, , and .
Multifunction media servers MUST use the URI conventions described in
RFC 4240 [2]. The service indicator for MSCML IVR MUST be set to
"ivr", as shown in the following example:
sip:ivr@ms.example.net
The VoiceXML IVR service indicator is "dialog". This service
indicator MUST NOT be used for any other interactive voice response
control mechanism.
The media server MUST accept MSCML IVR payloads in INFO requests and
MUST NOT accept MSCML IVR payloads in the initial or subsequent
INVITEs. The INFO method reduces certain timing issues that occur
with INVITEs and requires less processing on both the client and
media server.
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The media server notifies the client that the command has completed
through a message containing final status information and
associated data such as collected DTMF digits.
The media server does not queue IVR requests. If the media server
receives a new IVR request while another is in progress, the media
server stops the first operation and it carries out the new request.
The media server generates a message for the first request
and returns any data collected up to that point. If a client wishes
to stop a request in progress but does not wish to initiate another
operation, it issues a request. This also causes the media
server to generate a message.
The media server treats a SIP re-INVITE that modifies the established
SDP parameters as an implicit request. Examples of such SDP
modifications include receiving hold SDP or removing an audio or
video stream. When this occurs, the media server immediately
terminates the running , , or request
and sends a indicating "reason=stopped".
6.1. Specifying Prompt Content
The MSCML IVR requests support two methods of specifying content to
be delivered to the user. These are the element and the
prompturl attribute. Clients MUST NOT utilize both methods in a
single IVR request. Clients SHOULD use the more flexible
mechanism. Use of the prompturl attribute is deprecated and may not
be supported in future MSCML versions.
6.1.1. Use of the Prompt Element
The element MAY be included in the body of a ,
, or request to specify a prompt sequence
to be delivered to the caller. The prompt sequence consists of one
or more references to physical content files, spoken variables, or
dynamic URLs that return a sub-sequence of files or variables. In
addition, the element has several attributes that control
playback of the included content. These are described in the list
below.
Attributes of :
o baseurl - optional, no default value: For notational convenience,
as well as reducing the MSCML payload size, the "baseurl"
attribute is used to specify a base URL that is prepended to any
other URLs in the sequence that are not fully qualified.
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o delay - optional, default value "0": The "delay" attribute to the
prompt element specifies the time to pause between repetitions of
the sequence. It has no effect on the first iteration of
the sequence. Expressed as a time value (Section 4.2.1) from 0
onwards.
o duration - optional, default value "infinite": The "duration"
attribute to the prompt element controls the maximum amount of
time that may elapse while the media server repeats the sequence.
This allows the client to set an upper bound on the length of
play. Expressed as a time value (Section 4.2.1) from 1ms onwards
or the strings "immediate" and "infinite". "Immediate" directs
the media server to end play immediately, whereas "infinite"
indicates that the media server imposes no limit.
o gain - optional, default value "0": Sets the absolute gain to be
applied to the content contained in . The value of this
attribute is specified in units of dB. The media server MAY
silently cap values that exceed the gain limits imposed by the
platform. The level reverts back to its original value when
playback of the content contained in has been completed.
o gaindelta - optional, default value "0": Sets the relative gain to
be applied to the content contained in . The value of
this attribute is specified in units of dB. The media server MAY
silently cap values which exceed the gain limits imposed by the
platform. The level reverts back to its original value when
playback of the content contained in has been completed.
o rate - optional, default value "0": Specifies the absolute
playback rate of the content relative to normal as either a
positive percentage (faster) or a negative percentage (slower).
Any value that attempts to set the rate above the maximum allowed
or below the minimum allowed silently sets the rate to the maximum
or minimum. The rate reverts back to its original value when
playback of the content contained in has been completed.
o ratedelta - optional, default value "0": Specifies the playback
rate of the content relative to it's current rate as either a
positive percentage (faster) or negative percentage (slower). Any
value that attempts to set the rate above the maximum allowed or
below the minimum allowed silently sets the rate to the maximum or
minimum. The rate reverts back to its original value when
playback of the content contained in has completed.
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o locale - optional, no default value: Specifies the language and
country variant used for resolving spoken variables. The language
is defined as a two-letter code per ISO 639. The country variant
is also defined as a two-letter code per ISO 3166. These codes
are concatenated with a single underscore (%x5F) character.
o offset - optional, default value "0": A time value (Section 4.2.1)
which specifies the time from the beginning of the sequence at
which play is to begin. Offset only applies to the first
repetition; subsequent repetitions begin play at offset 0.
Allowable values are positive time values from 0 onwards. When
the sequence consists of multiple content files, the offset may
select any point in the sequence. If the offset value is greater
than the total time of the sequence, it will "wrap" to the
beginning and continue from there until the media server reaches
the specified offset.
o repeat - optional, default value "1": The "repeat" attribute to
the prompt element controls the number of times the media server
plays the sequence in the element. Allowable values are
integers from 0 on and the string "infinite", which indicates that
repetition should occur indefinitely. For example, "repeat=2"
means that the sequence will be played twice, and "repeat=0",
which is allowed, means that the sequence is not played.
o stoponerror - optional, default value "no": Controls media server
handling and reporting of errors encountered when retrieving
remote content. If set to "yes", content play will end if a fetch
error occurs, and the response will contain details regarding the
failure. If set to "no", the media server will silently move on
to the next URL in the sequence if a fetch failure occurs.
Clients MUST NOT include both 'gain' and 'gaindelta' attributes
within a single element.
When the client explicitly controls the output gain on a conference
leg, as described in Section 5.3, the 'gain' and 'gaindelta'
attributes SHOULD interact with the conference leg output gain
settings in the following manner.
o Conference leg output gain set to : The operation of the
'gain' and 'gaindelta' attributes are unchanged. However, the
baseline gain value before any playback changes are applied is the
value specified for the conference leg.
o Conference leg output gain set to : When playback gain
controls are used, the automatic gain control settings for the leg
are suspended for the duration of the playback operation. The
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operation of the 'gain' and 'gaindelta' attributes are unchanged.
The automatic gain control settings are reinstated when playback
has finished.
Media servers SHOULD support rate controls for content. However,
media servers MAY silently ignore rate change requests if content
limitations do not allow the request to be honored. Clients MUST NOT
include both 'rate' and 'ratedelta' attributes within a single
element.
Figure 16 shows a sample prompt block.
Figure 16: Prompt Block Example
6.1.1.1.
