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Network Working Group D. Burke
Internet-Draft Google
Intended status: Informational M. Scott
Expires: January 10, 2008 Genesys
J. Haynie
Hakano Inc
R. Auburn
Voxeo
S. McGlashan
Hewlett-Packard
July 9, 2007
SIP Interface to VoiceXML Media Services
draft-burke-vxml-03.txt
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This document describes a SIP interface to VoiceXML media services,
which is commonly employed between application servers and media
servers offering VoiceXML processing capabilities.
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Comments
Comments are solicited and should be addressed to the authors.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1. IVR Services with Application Servers . . . . . . . . 5
1.1.2. PSTN IVR Service Node . . . . . . . . . . . . . . . . 6
1.1.3. 3GPP IMS Media Resource Function (MRF) . . . . . . . . 7
1.1.4. CCXML <-> VoiceXML Interaction . . . . . . . . . . . . 8
1.1.5. Other Use Cases . . . . . . . . . . . . . . . . . . . 8
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 8
2. VoiceXML Session Establishment and Termination . . . . . . . . 10
2.1. Service Identification . . . . . . . . . . . . . . . . . . 10
2.2. Initiating a VoiceXML Session . . . . . . . . . . . . . . 12
2.3. Preparing a VoiceXML Session . . . . . . . . . . . . . . . 14
2.4. Session Variable Mappings . . . . . . . . . . . . . . . . 14
2.5. Terminating a VoiceXML Session . . . . . . . . . . . . . . 17
2.6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.6.1. Basic Session Establishment . . . . . . . . . . . . . 18
2.6.2. VoiceXML Session Preparation . . . . . . . . . . . . . 18
2.6.3. MRCP Establishment . . . . . . . . . . . . . . . . . . 19
3. Media Support . . . . . . . . . . . . . . . . . . . . . . . . 22
3.1. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . 22
3.2. Early Media . . . . . . . . . . . . . . . . . . . . . . . 22
3.3. Modifying the Media Session . . . . . . . . . . . . . . . 24
3.4. Audio and Video Codecs . . . . . . . . . . . . . . . . . . 24
3.5. DTMF . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4. Returning Data to the Application Server . . . . . . . . . . . 26
4.1. HTTP Mechanism . . . . . . . . . . . . . . . . . . . . . . 26
4.2. SIP Mechanism . . . . . . . . . . . . . . . . . . . . . . 26
5. Outbound Calling . . . . . . . . . . . . . . . . . . . . . . . 29
5.1. Third Party Call Control Mechanism . . . . . . . . . . . . 29
5.2. REFER Mechanism . . . . . . . . . . . . . . . . . . . . . 29
6. Call Transfer . . . . . . . . . . . . . . . . . . . . . . . . 31
6.1. Blind . . . . . . . . . . . . . . . . . . . . . . . . . . 31
6.2. Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.3. Consultation . . . . . . . . . . . . . . . . . . . . . . . 34
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 37
8. Security Considerations . . . . . . . . . . . . . . . . . . . 38
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
10. Changes since last version: . . . . . . . . . . . . . . . . . 40
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
11.1. Normative References . . . . . . . . . . . . . . . . . . . 41
11.2. Informative References . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . . . 46
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1. Introduction
VoiceXML [VXML20], [VXML21] is a World Wide Web Consortium (W3C)
standard for creating audio and video dialogs that feature
synthesized speech, digitized audio, recognition of spoken and DTMF
key input, recording of audio and video, telephony, and mixed
initiative conversations. VoiceXML allows Web-based development and
content delivery paradigms to be used with interactive video and
voice response applications.
This document describes a SIP [RFC3261] interface to VoiceXML media
services, which is commonly employed between Application Servers and
media servers offering VoiceXML processing capabilities. SIP is
responsible for initiating a media session to the VoiceXML media
server and simultaneously triggering the execution of a specified
VoiceXML application.
The interface described here owes its genesis to the draft [SIPVXML]
and leverages a mechanism for identifying dialog media services
described in [RFC4240]. The interface has been updated and extended
to support the W3C Recommendation for VoiceXML 2.0 [VXML20] and
VoiceXML 2.1 [VXML21]. A set of commonly implemented functions and
extensions have been specified including VoiceXML dialog preparation,
outbound calling, video media support, and transfers. VoiceXML
session variable mappings have been defined for SIP with an
extensible mechanism for passing application-specific values into the
VoiceXML application. Mechanisms for returning data to the
Application Server have also been added.
1.1. Use Cases
The VoiceXML media service user in this document is generically
referred to as an Application Server. In practice, it is intended
that the interface defined by this document is applicable across a
wide range of use cases. Several intended use cases are described
below.
1.1.1. IVR Services with Application Servers
SIP Application Servers provide services to users of the network.
Typically, there may be several Application Servers in the same
network, each specialised in providing a particular service.
Throughout this specification and without loss of generality, we
posit the presence of an Application Server specialised in providing
IVR services. A typical configuration for this use case is
illustrated below.
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+--------------+
| |
| Application |\
| Server | \
| | \ HTTP
SIP +--------------+ \
/ \ \
+-------------+ / SIP \ +--------------+
| |/ \| |
| SIP | | VoiceXML |
| User Agent | RTP/SRTP | Media Server |
| |=====================| |
+-------------+ +--------------+
Assuming the Application Server also supports HTTP, the VoiceXML
application may be hosted on it and served up via HTTP [RFC2616].
Note, however, that the Web model allows the VoiceXML application to
be hosted on a separate (HTTP) Application Server from the (SIP)
Application Server that interacts with the VoiceXML Media Server via
this specification. It is also possible for a static VoiceXML
application to be stored locally on the VoiceXML Media Server,
leveraging the VoiceXML 2.1 [VXML21] <data> mechanism to interact
with a Web/Application Server when dynamic behavior is required. The
viability of static VoiceXML applications is further enhanced by the
mechanisms defined in section 2.4, through which the Application
Server can make session-specific information available within the
VoiceXML session context.
The approach described in this document is sometimes termed the
"delegation model" - the Application Server is essentially delegating
programmatic control of the human-machine interactions to one or more
VoiceXML documents running on the VoiceXML Media Server. During the
human-machine interactions, the Application Server remains in the
signaling path and can respond to results returned from the VoiceXML
Media Server or other external network events.
1.1.2. PSTN IVR Service Node
While this document is intended to enable enhanced use of VoiceXML as
a component of larger systems and services, it is intended that
devices that are completely unaware of this specification but that
support [RFC4240] remain capable of invoking VoiceXML services
offered by a VoiceXML Media Server compliant with this document. A
typical configuration for this use case is as follows:
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+-------------+ SIP +--------------+
| |---------------------| |
| IP/PSTN | | VoiceXML |
| Gateway | RTP/SRTP | Media Server |
| |=====================| |
+-------------+ +--------------+
Note also that beyond the invocation and termination of a VoiceXML
dialog, the semantics defined for call transfers using REFER are
intended to be compatible with standard, existing IP/PSTN gateways.
1.1.3. 3GPP IMS Media Resource Function (MRF)
The 3GPP IP Multimedia Subsystem (IMS) [TS23002] defines a Media
Resource Function (MRF) used to offer media processing services such
as conferencing, transcoding, and prompt/collect. The capabilities
offered by VoiceXML are ideal for offering richer media processing
services in the context of the MRF. In this architecture, the
interface defined here corresponds to the "Mr" interface to the MRFC;
the implementation of this interface might use separated MRFC and
MRFP elements (as per the IMS architecture), or might be an
integrated MRF (as is common practice).
+----------+
| App |
| Server |
+----------+
|
| SIP (ISC)
|
+----------+ SIP (Mr) +--------------+
| S-CSCF |---------------| VoiceXML |
| | | MRF |
+----------+ +--------------+
||
|| RTP/SRTP (Mb)
||
The above diagram is highly simplified and shows a subset of nodes
typically involved in MRF interactions. It should be noted that
while the MRF will primarily be used by the Application Server via
the S-CSCF, it is also possible for calls to be routed directly to
the MRF without the involvement of an Application Server.
Although the above is described in terms of the 3GPP IMS
architecture, it is intended that it is also applicable to 3GPP2,
NGN, and PacketCable architectures that are converging with 3GPP IMS
standards.
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1.1.4. CCXML <-> VoiceXML Interaction
CCXML 1.0 [CCXML10] applications provide services mainly through
controlling the interaction between Connections, Conferences, and
Dialogs. Although CCXML is capable of supporting arbitrary dialog
environments, VoiceXML is commonly used as a dialog environment in
conjunction with CCXML applications; CCXML is specifically designed
to effectively support the use of VoiceXML. CCXML 1.0 defines
language elements that allow for Dialogs to be prepared, started, and
terminated; it further allows for data to be returned by the dialog
environment, for call transfers to be requested (by the dialog) and
responded to by the CCXML application, and for arbitrary eventing
between the CCXML application and running dialog application.
The interface described in this document can be used by CCXML 1.0
implementations to control VoiceXML Media Servers. Note, however,
that some CCXML language features require eventing facilities between
CCXML and VoiceXML sessions that go beyond what is defined in this
specification. For example, VoiceXML-controlled call transfers and
mid-dialog application-defined events cannot be fully realized using
this specification alone. A SIP event package [RFC3265] MAY be used
in addition to this specification to provide extended eventing.
1.1.5. Other Use Cases
In addition to the use cases described in some detail above, there
are a number of other intended use cases that are not described in
detail, such as:
1. Use of a VoiceXML Media Server as an adjunct to an IP-based PBX/
ACD, possibly to provide voicemail/messaging, automated
attendant, or other capabilities.
2. Invocation and control of a VoiceXML session that provides the
voice modality component in a multimodal system.
1.2. Terminology
Application Server: A SIP Application Server hosts and executes
services, in particular by terminating SIP sessions on a media
server. The Application Server MAY also act as an HTTP server
[RFC2616] in interactions with media servers.
VoiceXML Media Server: A VoiceXML interpreter including a SIP-based
interpreter context and the requisite media processing
capabilities to support VoiceXML functionality.
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VoiceXML Session: A VoiceXML Session is a multimedia session
comprising of at least a SIP user agent, a VoiceXML Media Server,
the data streams between them, and an executing VoiceXML
application.
VoiceXML Dialog: Equivalent to VoiceXML Session.
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 [RFC2119].
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2. VoiceXML Session Establishment and Termination
This section describes how to establish a VoiceXML Session, with or
without preparation, and how to terminate a session. This section
also addresses how session information is made available to VoiceXML
applications.
2.1. Service Identification
The SIP Request-URI is used to identify the VoiceXML media service as
defined in [RFC4240]. The user part of the SIP Request-URI is fixed
to "dialog". The initial VoiceXML document is specified with the
"voicexml" parameter. In addition, parameters are defined that
control how the VoiceXML Media Server fetches the specified VoiceXML
document. The list of parameters defined by this specification is as
follows:
voicexml: URI of the initial VoiceXML document to fetch. This will
typically contain an HTTP URI, but may use other URI schemes, for
example to refer to local, static VoiceXML documents. If the
"voicexml" parameter is omitted, the VoiceXML Media Server may
select the initial VoiceXML document by other means, such as by
applying a default, or may reject the request.
maxage: Used to set the max-age value of the Cache-Control header in
conjunction with VoiceXML documents fetched using HTTP, as per
[RFC2616]. If omitted, the VoiceXML Media Server will use a
default value.
maxstale: Used to set the max-stale value of the Cache-Control
header in conjunction with VoiceXML documents fetched using HTTP,
as per [RFC2616]. If omitted, the VoiceXML Media Server will use
a default value.
method: Used to set the HTTP method applied in the fetch of the
initial VoiceXML document. Allowed values are "get" or "post"
(case-insensitive). Default is "get".
postbody: Used to set the application/x-www-form-urlencoded encoded
[HTML4] HTTP body for "post" requests (or is otherwise ignored).
The postbody value is the prepared application/
x-www-form-urlencoded content, subsequently URL-encoded (see note
below).
Other application-specific parameters may be added to the Request-URI
and are exposed in VoiceXML session variables (see section 2.4).
The BNF for the Request-URI is given below:
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DIALOG-URL = sip-ind dialog-ind "@" hostport
dialog-parameters
sip-ind = "sip:" / "sips:"
dialog-ind = "dialog"
dialog-parameters = [ init-parameters ]
[ vxml-parameters ]
[ uri-parameters ]
init-parameters = init-param [ init-parameters ]
init-param = ";" (dialog-param /
maxage-param /
maxstale-param /
method-param /
postbody-param)
dialog-param = "voicexml=" vxml-url ; vxml-url follows the URI
; syntax defined in RFC3986
maxage-param = "maxage=" 1*DIGIT
maxstale-param = "maxstale=" 1*DIGIT
method-param = "method=" ("get" / "post")
postbody-param = "postbody=" token
vxml-parameters = vxml-param [ vxml-parameters ]
vxml-param = ";" vxml-keyword "=" vxml-value
vxml-keyword = token
vxml-value = false /
null /
true /
object /
array /
number /
string ; see RFC4627
Parameters of the Request-URI in subsequent re-INVITEs are ignored.
One consequence of this is that the VoiceXML Media Server cannot be
instructed by the Application Server to change the executing VoiceXML
Application after a VoiceXML Session has been started.
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Incorrectly formed requests MUST be rejected with the appropriate 4xx
class response. If one of the init-parameters is repeated, then the
request MUST be rejected with a 400 Bad Request response.
Note: Special characters in Request-URI parameter values need to be
URL-encoded as required by the SIP URI syntax, for example '?' (%3f),
'=' (%3d), and ';' (%3b). The VoiceXML Media Server MUST therefore
unescape Request-URI parameter values before making use of them or
exposing them to running VoiceXML applications. It is important that
the VoiceXML Media Server only unescape the parameter values once
since the desired VoiceXML URI value could itself be URL encoded, for
example. When a postbody is included, its entire content including
any line breaks (represented by a CR LF pair) is encoded as a single
parameter value following the above rules (such that the line breaks
would be replaced by '%0D%0A', for example).
As an example, the following SIP Request-URI identifies the use of
VoiceXML media services, with
'http://appserver.example.com/promptcollect.vxml' as the initial
VoiceXML document, to be fetched with max-age/max-stale values of
3600s/0s respectively:
sip:dialog@mediaserver.example.com; \
voicexml=http://appserver.example.com/promptcollect.vxml; \
maxage=3600;maxstale=0
2.2. Initiating a VoiceXML Session
A VoiceXML Session is initiated via the Application Server using a
SIP INVITE or REFER (see section 5.2). Typically, the Application
Server will be specialized in providing VoiceXML services. At a
minimum, the Application Server may behave as a simple proxy by
rewriting the Request-URI received from the User Agent to a Request-
URI suitable for consumption by the VoiceXML Media Server (as
specified in section 2.1). For example, a User Agent might present a
dialed number:
tel:+1-201-555-0123
which the Application Server maps to a directory assistance
application on the VoiceXML Media Server with a Request-URI of:
sip:dialog@ms1.example.com; \
voicexml=http://as1.example.com/da.vxml
The Application Server SHOULD insert its own URI in the Record-Route
header so that it remains in the signaling path for subsequent
signaling related to the session. This is of particular importance
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for call transfers so that upstream Application Servers or proxy
servers see signaling originating from the Application Server and not
the VoiceXML Media Server itself. Certain header values in the
INVITE message to the VoiceXML Media Server are mapped into VoiceXML
session variables and are specified in section 2.4.
On receipt of the INVITE, the VoiceXML Media Server issues a
provisional response, 100 Trying, and commences the fetch of the
initial VoiceXML document. The 200 OK response indicates that the
VoiceXML document has been fetched and parsed correctly and is ready
for execution. Application execution commences on receipt of the ACK
(except if the dialog is being prepared as specified in section 2.3).
Note that the 100 Trying response will usually be sent on receipt of
the INVITE in accordance with [RFC3261], since the VoiceXML Media
Server cannot in general guarantee that the initial fetch will
complete in less than 200 ms. However, certain implementations may
be able to guarantee response times to the initial INVITE, and thus
may not need to send a 100 Trying response.
As an optimization, prior to sending the 200 OK response, the
VoiceXML Media Server MAY execute the application up to the point of
the first VoiceXML waiting state or prompt flush.
A VoiceXML Media Server, like any SIP User Agent, may be unable to
accept the INVITE request for a variety of reasons. For instance, an
SDP offer contained in the INVITE might require the use of codecs
that are not supported by the Media Server. In such cases, the Media
Server should respond as defined by [RFC3261]. However, there are
error conditions specific to VoiceXML, as follows:
1. If the Request-URI does not conform to this specification, a 400
Bad Request MUST be returned (unless it is used to select other
services not defined by this specification).
2. If the Request-URI does not include a "voicexml" parameter, and
the VoiceXML Media Server does not elect to use a default page,
the VoiceXML Media Server MUST return a final response of 400 Bad
Request, and SHOULD include a Warning header with a 3-digit code
of 399 and a human readable error message.
3. If the VoiceXML document cannot be fetched or parsed, the
VoiceXML Media Server MUST return a final response of 500 Server
Internal Error and SHOULD include a Warning header with a 3-digit
code of 399 and a human readable error message.
Informational note: Certain applications may pass a significant
amount of data to the VoiceXML dialog in the form of Request-URI
parameters. This may cause the total size of the INVITE request to
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exceed the MTU of the underlying network. In such cases,
applications/implementations must take care either to use a transport
appropriate to these larger messages (such as TCP), or to use
alternative means of passing the required information to the VoiceXML
dialog (such as supplying a unique session identifier in the initial
VoiceXML URI and later using that identifier as a key to retrieve
data from the HTTP server). This note also applies if the dialog is
started using a REFER request as described in section 5.2.
2.3. Preparing a VoiceXML Session
In certain scenarios, it is beneficial to prepare a VoiceXML Session
for execution prior to running it. A previously prepared VoiceXML
Session is expected to execute with minimal delay when instructed to
do so.
If a media-less SIP dialog is established with the initial INVITE to
the VoiceXML Media Server, the VoiceXML Application will not execute
after receipt of the ACK. To run the VoiceXML Application, the AS
must issue a re-INVITE to establish a media session.
A media-less SIP dialog can be established by sending SDP containing
no media lines in the initial INVITE. Alternatively, if no SDP is
sent in the initial INVITE, the VoiceXML Media Server will include an
offer in the 200 OK message, which can be responded to with an answer
in the ACK with the media port(s) set to 0.
Once a VoiceXML Application is running, a re-INVITE which disables
the media streams (i.e. sets the ports to 0) will not otherwise
affect the executing application (except that recognition actions
initiated while the media streams are disabled will result in noinput
timeouts).
2.4. Session Variable Mappings
The standard VoiceXML session variables are assigned values according
to:
session.connection.local.uri: Evaluates to the SIP URI specified in
the To: header of the initial INVITE (or REFER).
session.connection.remote.uri: Evaluates to the SIP URI specified in
the From: header of the initial INVITE (or REFER).
session.connection.redirect: This array is populated by information
contained in the History-Info [RFC4244] header in the initial
INVITE or is otherwise undefined. Each entry (hi-entry) in the
History-Info header is mapped, in reverse order, into an element
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of the session.connection.redirect array. Properties of each
element of the array are determined as follows:
* uri - Set to the hi-targeted-to-uri value of the History-Info
entry
* pi - Set to 'true' if hi-targeted-to-uri contains a
'Privacy=history' parameter, or if the INVITE Privacy header
includes 'history'; 'false' otherwise
* si - Set to the value of the 'si' parameter if it exists,
undefined otherwise
* reason - Set verbatim to the value of the 'Reason' parameter of
hi-targeted-to-uri
session.connection.protocol.name: Evaluates to "sip". Note that
this is intended to reflect the use of SIP in general, and does
not distinguish between whether the media server was accessed via
SIP or SIPS procedures.
session.connection.protocol.version: Evaluates to "2.0".
session.connection.protocol.sip.headers: This is an associative
array where each key in the array is the non-compact name of a SIP
header in the initial INVITE converted to lower-case (note the
case conversion does not apply to the header value). If multiple
header fields of the same field name are present, the values are
combined into a single comma-separated value. Implementations
MUST at a minimum include the Call-ID header and MAY include other
headers. For example,
session.connection.protocol.sip.headers["call-id"] evaluates to
the Call-ID of the SIP dialog.
session.connection.protocol.sip.requesturi: This is an associative
array where the array keys and values are formed from the URI
parameters on the SIP Request-URI of the initial INVITE (or
REFER). The array key is the URI parameter name. The
corresponding array value is derived from the URI parameter value
according to the following rules:
* If the URI parameter name is an init-param or dialog-param, the
corresponding array value is obtained by evaluating the URI
parameter value as a string.
* If the URI parameter name is a vxml-param, the corresponding
array value is obtained by evaluating the URI parameter value
as a "JSON value" [RFC4627].
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* If the URI parameter name is present but its value is omitted,
the value is an empty string.
In addition, the array's toString() function returns the full SIP
Request-URI. For example, assuming a Request-URI of sip:dialog@
example.com;voicexml=http://ajax.com;obj={"x":1,"y":true} then
session.connection.protocol.sip.requesturi["voicexml"] evaluates
to "http://ajax.com",
session.connection.protocol.sip.requesturi["obj"].x evaluates to 1
(type Number), session.connection.protocol.sip.requesturi["obj"].y
evaluates to true (type Boolean), and
session.connection.protocol.sip.requesturi evaluates to the
complete Request-URI.
session.connection.aai: Evaluates to
session.connection.protocol.sip.requesturi["aai"]
session.connection.ccxml: Evaluates to
session.connection.protocol.sip.requesturi["ccxml"]
session.connection.protocol.sip.media: This is an array where each
array element is an object with the following properties:
* type: - This required property indicates the type of the media
associated with the stream. The value is a string. It is
strongly recommended that the following values are used for
common types of media: "audio" for audio media, and "video" for
video media.
* direction: - This required property indicates the
directionality of the media relative to
session.connection.originator. Defined values are sendrecv,
sendonly, recvonly, and inactive.
* format: - This property is optional. If defined, the value of
the property is an array. Each array element is an object
which specifies information about one format of the media
(there is an array element for each payload type on the
m-line). The object contains at least one property called name
whose value is the MIME subtype of the media format (MIME
subtypes are registered in [RFC4855]). Other properties may be
defined with string values; these correspond to required and,
if defined, optional parameters of the format.
As a consequence of this definition, there is an array entry in
session.connection.protocol.sip.media for each non-disabled m-line
for the negotiated media session. Note that this session variable
is updated if the media session characteristics for the VoiceXML
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Session change (i.e. due to a re-INVITE). For an example,
consider a connection with bi-directional G.711 mu-law audio
sampled at 8kHz. In this case,
session.connection.protocol.sip.media[0].type evaluates to
"audio", session.connection.protocol.sip.media[0].direction to
"sendrecv", and
session.connection.protocol.sip.media[0].format[0].name evaluates
to "audio/PCMU" and
session.connection.protocol.sip.media[0].format[0].rate evaluates
to "8000".
Note that when accessing SIP headers and Request-URI parameters via
the session.connection.protocol.sip.headers and
session.connection.protocol.sip.requesturi associative arrays defined
above, applications can choose between two semantically equivalent
ways of referring to the array. For example, either of the following
can be used to access a Request-URI parameter named 'foo':
session.connection.protocol.sip.requesturi["foo"]
session.connection.protocol.sip.requesturi.foo
However, it is important to note that not all SIP header names or
Request-URI parameter names are valid ECMAScript identifiers, and as
such, can only be accessed using the first form (array notation).
For example, the Call-ID header can only be accessed as
session.connection.protocol.sip.headers["call-id"]; attempting to
access the same value as
session.connection.protocol.sip.headers.call-id would result in an
error.
2.5. Terminating a VoiceXML Session
The Application Server can terminate a VoiceXML Session by issuing a
BYE to the VoiceXML Media Server. Upon receipt of a BYE in the
context of an existing VoiceXML Session, the VoiceXML Media Server
MUST send a 200 OK response, and MUST throw a
'connection.disconnect.hangup' event to the VoiceXML application. If
the Reason header [RFC3326] is present on the BYE Request, then the
value of the Reason header is provided verbatim via the '_message'
variable within the catch element's anonymous variable scope.
The VoiceXML Media Server may also initiate termination of the
session by issuing a BYE request. This will typically occur as a
result of encoutering a <disconnect> or <exit> in the VoiceXML
application, due to the VoiceXML application running to completion,
or due to unhandled errors within the VoiceXML application.
See Section 4 for mechanisms to return data to the Application
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Server.
2.6. Examples
2.6.1. Basic Session Establishment
This example illustrates an Application Server setting up a VoiceXML
Session on behalf of a User Agent.
SIP VoiceXML HTTP
User Application Media Application
Agent Server Server Server
| | | |
|(1) INVITE [offer] | | |
|------------------->|(2) INVITE [offer] | |
|(3) 100 Trying |------------------->| |
|<-------------------|(4) 100 Trying | |
| |<-------------------| |
| | | |
| | |(5) GET |
| | |------------------->|
| | |(6) 200 OK [VXML] |
| | |<-------------------|
| | | |
| |(7) 200 OK [answer] | |
|(8) 200 OK [answer] |<-------------------| |
|<-------------------| | |
|(9) ACK | | |
|------------------->|(10) ACK | |
| |------------------->| (execute |
|(11) RTP/SRTP | | VoiceXML |
|.........................................| application) |
| | | |
2.6.2. VoiceXML Session Preparation
This example demonstrates the preparation of a VoiceXML Session. In
this example, the VoiceXML session is prepared prior to placing an
outbound call to a User Agent, and is started as soon as the User
Agent answers.
The [answer1:0] notation is used to indicate an SDP answer with the
media ports set to 0.
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SIP VoiceXML HTTP
User Application Media Application
Agent Server Server Server
| | | |
| |(1) INVITE | |
| |-------------------->| |
| |(2) 100 Trying | |
| |<--------------------| |
| | | |
| | |(3) GET |
| | |------------------->|
| | |(4) 200 OK [VXML] |
| | |<-------------------|
| | | |
| |(5) 200 OK [offer1] | |
| |<--------------------| |
| |(6) ACK [answer1:0] | |
|(7) INVITE |-------------------->| |
|<-------------------| | |
|(8) 200 OK [offer2] | | |
|------------------->|(9) INVITE [offer2] | |
| |-------------------->| |
| |(10) 100 Trying | |
| |<--------------------| |
| |(11) 200 OK [answer2]| |
|(12) ACK [answer2] |<--------------------| |
|<-------------------|(13) ACK | |
| |-------------------->| (execute |
|(14) RTP/SRTP | VoiceXML |
|..........................................| application) |
| | | |
2.6.3. MRCP Establishment
MRCP [MRCPv2] is a protocol that enables clients such as a VoiceXML
Media Server to control media service resources such as speech
synthesizers, recognizers, verifiers and identifiers residing in
servers on the network.
The example below illustrates how a VoiceXML Media Server may
establish an MRCP session in response to an initial INVITE.
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VoiceXML HTTP
User Media MRCPv2 Application
Agent Server Server Server
| | | |
|(1) INVITE [offer1] | | |
|------------------->| | |
|(2) 100 Trying | | |
|<-------------------|(3) GET | |
| |---------------------------------------->|
| | | |
| |(4) 200 OK [VXML] | |
| |<----------------------------------------|
| | | |
| |(5) INVITE [offer2] | |
| |--------------------->| |
| | | |
| |(6) 200 OK [answer2] | |
| |<---------------------| |
| | | |
| |(7) ACK | |
| |--------------------->| |
| | | |
| |(8) MRCP connection | |
| |<-------------------->| |
|(9) 200 OK [answer1]| | |
|<-------------------| | |
| | | |
|(10) ACK | | |
|------------------->| | |
| | | |
|(11) RTP/SRTP | | |
.............................................| |
| | | |
In this example, the VoiceXML Media Server is responsible for
establishing a session with the MRCPv2 Media Resource Server prior to
sending the 200 OK response to the initial INVITE. The VoiceXML
Media Server will perform the appropriate offer/answer with the
MRCPv2 Media Resource Server based on the SDP capabilities of the
Application Server and the MRCPv2 Media Resource Server. The
VoiceXML Media Server will change the offer received from step 1 to
establish a MRCPv2 session in step (5) and will re-write the SDP to
include an m-line for each MRCPv2 resource to be used and other
required SDP modifications as specified by MRCPv2. Once the VoiceXML
Media Server performs the offer/answer with the MRCPv2 Media Resource
Server, it will establish a MRCPv2 control channel in step (8). The
MRCPv2 resource is deallocated when the VoiceXML Media Server
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receives or sends a BYE (not shown).
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3. Media Support
This section describes the mandatory and optional media support
required by this interface.
3.1. Offer/Answer
The VoiceXML Media Server MUST support the standard offer/answer
mechanism of [RFC3264]. In particular, if an SDP offer is not
present in the INVITE, the VoiceXML Media Server will make an offer
in the 200 OK response listing its supported codecs.
3.2. Early Media
The VoiceXML Media Server MAY support early establishment of media
streams by sending a 183 Session Progress provisional response to the
initial INVITE. This allows the Application Server to establish
media streams between a user agent and the VoiceXML Media Server
while the initial VoiceXML document is being processed. This is
useful primarily for minimizing the delay in starting a VoiceXML
Session, since media stream establishment and initial VoiceXML
document processing can occur in parallel. This can be particularly
important in cases where the session with the user agent has already
been established, since the user agent is already "connected". The
following flow demonstrates the use of early media:
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SIP VoiceXML HTTP
User Application Media Application
Agent Server Server Server
| | | |
|..(existing session)..| | |
| |(1) INVITE | |
| |------------------>| |
| |(2) 183 [offer] | |
|(3) re-INVITE [offer] |<------------------| |
|<---------------------| | |
|(4) 200 OK [answer] | | |
|--------------------->| | |
|(5) ACK | | |
|<---------------------| | |
| | (6) PRACK [answer]| |
| |------------------>| |
| | (7) PRACK 200 OK | |
| |<------------------| |
|(8) RTP/SRTP | | |
|..........................................| |
| | |(9) HTTP GET |
| | |------------------>|
| | |(10) 200 OK [VXML] |
| | |<------------------|
| | | |
| |(11) 200 OK | |
| |<------------------| |
| |(12) ACK | |
| |------------------>| (execute |
| | | VoiceXML |
| | | application) |
| | | |
In the figure shown above, although step 9 (HTTP GET) is shown
occuring after the early media offer/answer exchange (starting in
step 2), the intent is that the fetching of the VoiceXML document
happens concurrently with the negotiation of early media.
Note that the offer of early media by a VoiceXML Media Server does
not imply that the referenced VoiceXML application can always be
fetched and executed successfully. For instance, if the HTTP
Application Server were to return a 4xx response in step 10 above, or
if the provided VoiceXML content was not valid, the VoiceML Media
Server would still return a 500 response (as per section 2.2). At
this point, it would be the responsibility of the application server
to tear down any media streams established with the media server.
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The use of early media is substantially complicated if the SDP
supplied in the 183 Session Progress differs from that supplied in
the 200 OK. Therefore, if a VoiceXML Media Server generates a 183
Session Progress provisional response containing SDP, it MUST return
identical SDP when generating the 200 OK final response (i.e. the
"gateway model" in [RFC3960]).
Early media is not optimal in all circumstances; for instance, when
handling an incoming call, a 183 Session Progress propagated by the
Application Server to the user agent will typically stop the
"ringback tone" a user would otherwise hear. Furthermore, a 183
Session Progress provisional response does not guarantee that the
VoiceXML application will be executed successfully - the subsequent
fetching of the VoiceXML document could fail.
Finally, the example above assumed the User Agent supported re-
INVITE. If it didn't (i.e. returned a 488 Not Acceptable Here), the
Application Server would have issued a CANCEL to the VoiceXML Media
Server.
3.3. Modifying the Media Session
The VoiceXML Media Server MUST allow the media session to be modified
via a re-INVITE and SHOULD support the UPDATE method [RFC3311] for
the same purpose. In particular, it MUST be possible to change
streams between sendrecv, sendonly, and recvonly as specified in
[RFC3264].
Unidirectional streams are useful for announcement- or listening-only
(hotword). The preferred mechanism for putting the media session on
hold is specified in [RFC3264], i.e. the UA modifies the stream to be
sendonly and mutes its own stream. Modification of the media session
does not affect VoiceXML application execution (except that
recognition actions initiated while on hold will result in noinput
timeouts).
3.4. Audio and Video Codecs
For the purposes of achieving a basic level of interoperability, this
section specifies a minimal subset of codecs and RTP [RFC3550]
payload formats that MUST be supported by the VoiceXML Media Server.
For audio-only applications, G.711 mu-law and A-law MUST be supported
using the RTP payload type 0 and 8 [RFC3551]. Other codecs and
payload formats MAY be supported.
Video telephony applications, which employ a video stream in addition
to the audio stream, are possible in VoiceXML 2.0/2.1 through the use
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of multimedia file container formats such as the .3gp [TS26244] and
.mp4 formats [IEC14496-14]. Video support is optional for this
specification. If video is supported then:
1. H.263 Baseline [RFC4629] MUST be supported. For legacy reasons,
the 1996 version of H.263 MAY be supported using the RTP payload
format defined in [RFC2190] (payload type 34 [RFC3551]).
2. AMR-NB audio [RFC4867] SHOULD be supported.
3. MPEG-4 video [RFC3016] SHOULD be supported.
4. MPEG-4 AAC audio [RFC3016] SHOULD be supported.
5. Other codecs and payload formats MAY be supported.
Video record operations carried out by the VoiceXML Media Server
typically require receipt of an intra-frame before the recording can
commence. The VoiceXML Media Server SHOULD use the mechanism
described in [RFC4585] to request that a new intra-frame be sent.
3.5. DTMF
DTMF events [RFC4733] MUST be supported. When the user agent does
not indicate support for [RFC4733] the VoiceXML Media Server MAY
perform DTMF detection using other means such as detecting DTMF tones
in the audio stream. Implementation note: the reason why only
[RFC4733] telephone-events must be used when the user agent indicates
support of it is to avoid the risk of double detection of DTMF if
detection on the audio stream was simultaneously applied.
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4. Returning Data to the Application Server
This section discusses the mechanisms for returning data (e.g.
collected utterance or digit information) from the VoiceXML Media
Server to the Application Server.
4.1. HTTP Mechanism
At any time during the execution of the VoiceXML application, data
can be returned to the Application Server via a HTTP POST using
standard VoiceXML elements such as <submit> or <subdialog>. Notably,
the <data> element in VoiceXML 2.1 [VXML21] allows data to be sent to
the Application Server efficiently without requiring a VoiceXML page
transition and is ideal for short VoiceXML applications such as
"prompt and collect".
For most applications, it is necessary to correlate the information
being passed over HTTP with a particular VoiceXML Session. One way
this can be achieved is to include the SIP Call-ID (accessible in
VoiceXML via the session.connection.protocol.sip.headers array)
within the HTTP POST fields. Alternatively, a unique "POST-back URI"
can be specified as an application-specific URI parameter in the
Request-URI of the initial INVITE (accessible in VoiceXML via the
session.connection.protocol.sip.requesturi array).
4.2. SIP Mechanism
Data can be returned to the Application Server via the expr or
namelist attribute on <exit> or the namelist attribute on
<disconnect>. A VoiceXML Media Server MUST support encoding of the
expr / namelist data in the message body of a BYE request sent from
the VoiceXML Media Server as a result of encountering the <exit> or
<disconnect> element. A VoiceXML Media Server MAY support inclusion
of the expr / namelist data in the message body of the 200 OK message
in response to a received BYE request (i.e. when the VoiceXML
Application responds to the connection.disconnect.hangup event and
subsequently executes an <exit> element with the expr or namelist
attribute specified).
Note that sending expr/namelist data in the 200 OK response requires
that the VoiceXML Media Server delay the final response to the
received BYE request until the VoiceXML Application's post-disconnect
final processing state terminates. This mechanism is subject to the
constraint that the VoiceXML Media Server must respond before the
UAC's timer F expires (defaults to 32 seconds). Moreover, for
unreliable transports, the UAC will retransmit the BYE request
according to the rules of [RFC3261]. The VoiceXML Media Server
SHOULD implement the recommendations of [RFC4320] regarding when to
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send the 100 Trying provisional response to the BYE request.
If a VoiceXML Application executes a <disconnect> [VXML21] and then
subsequently executes an <exit> with namelist information, the
namelist information from the <exit> element is discarded.
Namelist variables are first converted to to their JSON value
equivalent [RFC4627] and encoded in the message body using the
application/x-www-form-urlencoded format content type [HTML4]. The
behavior resulting from specifying a recording variable in the
namelist or an ECMAScript object with circular references is not
defined. If the expr attribute is specified on the <exit> element
instead of the namelist attribute, the reserved name __exit is used.
To allow the application server to differentiate between a BYE
resulting from a <disconnect> from one resulting from an <exit>, the
reserved name __reason is used, with a value of "disconnect" (without
brackets) to reflect the use of VoiceXML's <disconnect> element, and
a value of "exit" (without brackets) to an explicit <exit> in the
VoiceXML document. If the session terminates for other reasons (such
as the media server encountering an error), this parameter may be
omitted, or may take on platform-specific values prefixed with an
underscore.
This specification extends the application/x-www-form-urlencoded by
replacing non-ASCII characters with one or more octets of the UTF-8
representation of the character, with each octet in turn replaced by
%HH, where HH represents the uppercase hexadecimal notation for the
octet value and % is a literal character. As a consequence, the
Content-Type header field in a BYE message containing expr/namelist
data MUST be set to application/x-www-form-urlencoded;charset=utf-8.
The following table provides some examples of <exit> usage and the
corresponding result content.
+----------------------------------------------------------------+
|<exit> Usage | Result Content |
|------------------------------|---------------------------------|
|<exit/> | __reason=exit |
|<exit expr="5"/> | __exit=5&__reason=exit |
|<exit expr="'done'"/> | __exit="done"&__reason=exit |
|<exit expr="userAuthorized"/> | __exit=true&__reason=exit |
|<exit namelist="pin errors"/> | pin=1234&errors=0&__reason=exit |
+----------------------------------------------------------------+
assuming the following VoiceXML variables and values:
userAuthorized = true
pin = 1234
errors = 0
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For example, consider the VoiceXML snippet:
...
<exit namelist="id pin"/>
...
If id equals 1234 and pin equals 9999, say, the BYE message would
look similar to:
BYE sip:user@pc33.example.com SIP/2.0
Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnashds10
Max-Forwards: 70
From: sip:dialog@example.com;tag=a6c85cf
To: sip:user@example.com;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 231 BYE
Content-Type: application/x-www-form-urlencoded;charset=utf-8
Content-Length: 30
id=1234&pin=9999&__reason=exit
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5. Outbound Calling
Outbound calls can be triggered via the Application Server using
either third party call control [RFC3725] or with the SIP REFER
mechanism [RFC3515].
5.1. Third Party Call Control Mechanism
Flow IV from [RFC3725] is recommended in conjunction with the
VoiceXML Session preparation mechanism. This flow has several
advantages over others, namely:
1. Selection of a VoiceXML Media Server and preparation of the
VoiceXML Application can occur before the call is placed to avoid
the callee experiencing delays.
2. Avoids timing difficulties that could occur with other flows due
to the time taken to fetch and parse the initial VoiceXML
document.
3. The flow is IPv6 compatible.
An example flow for an Application Server initiated outbound call is
provided in section 2.6.2.
5.2. REFER Mechanism
The Application Server can place a REFER request to the VoiceXML
Media Server outside of a SIP dialog to initiate an outbound call.
The Request-URI in the REFER is constructed identical to that of an
INVITE to the VoiceXML Media Server and carries the same semantics.
The Refer-To header contains the URI for the VoiceXML Media Server to
place the call to.
On receipt of the REFER request, the VoiceXML Media Server MUST issue
a provisional response, 100 Trying. The 202 Accepted response
indicates that the VoiceXML document has been fetched and parsed
correctly. The VoiceXML Media Server proceeds to place the outbound
INVITE and will execute the application after the ACK is sent.
If the VoiceXML Session cannot be started, then the VoiceXML Media
Server MUST respond to the REFER request using the procedure defined
in section 2.2 above.
An example is of the REFER initiated outbound call is given below.
The NOTIFY messages, which contain message/sipfrag bodies [RFC3515],
allow the Application Server to monitor the progress of the outbound
call attempt.
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Note: An in-dialog REFER will result in a 403 Forbidden response.
HTTP VoiceXML SIP
Application Media Application User
Server Server Server Agent
| | | |
| |(1) REFER | |
| |<-------------------| |
| |(2) 100 Trying | |
| |------------------->| |
| |(3) NOTIFY | |
| |------------------->| |
| |(4) 200 OK | |
| |<-------------------| |
|(5) GET | | |
|<-------------------| | |
|(6) 200 OK [VXML] | | |
|------------------->| | |
| |(7) 202 Accepted | |
| |------------------->| |
| |(8) INVITE [offer] |
| |------------------------------------>|
| |(9) 200 OK [answer] |
| |<------------------------------------|
| |(10) NOTIFY | |
| |------------------->| |
| |(11) 200 OK | |
| |<-------------------| |
| |(12) ACK |
| |------------------------------------>|
| |(13) RTP/SRTP |
| |.....................................|
| | |
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6. Call Transfer
While VoiceXML is at its core a dialog language, it also provides
optional call transfer capability. VoiceXML's transfer capability is
particularly suited to the PSTN IVR Service Node use-case described
in section 1.1.2. It is NOT RECOMMENDED to use VoiceXML's call
transfer capability in networks involving Application Servers.
Rather, the Application Server itself can provide call routing
functionality by taking signaling actions based on the data returned
to it from the VoiceXML Media Server via HTTP or in the SIP BYE
message.
If VoiceXML transfer is supported, the mechanism described in this
section MUST be employed. The transfer flows specified here are
selected on the basis that they provide the best interworking across
a wide range of SIP devices. CCXML<->VoiceXML implementations, which
require tight-coupling in the form of bi-directional eventing to
support all transfer types defined in VoiceXML, may benefit from
other approaches, such as the use of SIP event packages [RFC3265].
In what follows, the provisional responses have been omitted for
clarity.
6.1. Blind
The blind transfer sequence is initiated by the VoiceXML Media Server
via a REFER message [RFC3515] on the original SIP dialog. The
Refer-To header contains the URI for the called party, as specified
via the 'dest' or 'destexpr' attributes on the VoiceXML <transfer>
tag.
If the REFER request is accepted, in which case the VoiceXML Media
Server will receive a 2xx response, the VoiceXML Media Server throws
the connection.disconnect.transfer event and will terminate the
VoiceXML Session with a BYE message. For blind transfers,
implementations MAY use [RFC4488] to suppress the implicit
subscription associated with the REFER message.
If the REFER request results in a non-2xx response, the <transfer>'s
form item variable (or event raised) depends on the SIP response and
is specified in the following table. Note that this indicates that
the transfer request was rejected.
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+-------------------------+-----------------------------------+
| SIP Response | <transfer> variable / event |
+-------------------------+-----------------------------------+
| 404 Not Found | error.connection.baddestination |
| 405 Method Not Allowed | error.unsupported.transfer.blind |
| 503 Service Unavailable | error.connection.noresource |
| (No response) | network_busy |
| (Other 3xx/4xx/5xx/6xx) | unknown |
+-------------------------+-----------------------------------+
An example is illustrated below (provisional responses and NOTIFY
messages corresponding to provisional responses have been omitted for
clarity).
User Agent 1 VoiceXML User Agent 2
(Caller) Media Server (Callee)
| | |
|(0) RTP/SRTP | |
|.................| |
| | |
|(1) REFER | <transfer> |
|<----------------| |
|(2) 202 Accepted | |
|---------------->| |
|(3) BYE | |
|<----------------| |
|(4) 200 OK | |
|---------------->| |
| | Stop RTP (0) |
|(5) INVITE |
|---------------------------------->|
|(6) 200 OK |
|<----------------------------------|
|(7) NOTIFY | |
|---------------->| |
|(8) 200 OK | |
|<--------------- | |
|(9) ACK |
|---------------------------------->|
|(10) RTP/SRTP |
|...................................|
| | |
If the "aai" or "aaiexpr" attribute is present on <transfer>, it is
appended to the Refer-To URI as a parameter named "aai" in the REFER
method. Reserved characters are URL-encoded as required for SIP/SIPS
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URIs [RFC3261]. The mapping of values outside of the ASCII range is
platform specific.
6.2. Bridge
The bridge transfer function results in the creation of a small
multi-party session involving the Caller, the VoiceXML Media Server,
and the Callee. The VoiceXML Media Server invites the Callee to the
session and will eject the Callee if the transfer is terminated.
If the "aai" or "aaiexpr" attribute is present on <transfer>, it is
appended to the Request-URI in the INVITE as a URI parameter named
"aai". Reserved characters are URL-encoded as required for SIP/SIPS
URIs [RFC3261]. The mapping of values outside of the ASCII range is
platform specific.
During the transfer attempt, audio specified in the transferaudio
attribute of <transfer> is streamed to User Agent 1. A VoiceXML
Media Server MAY play early media received from the Callee to the
Caller if the transferaudio attribute is omitted.
The bridge transfer sequence is illustrated below. The VoiceXML
Media Server (acting as a UAC) makes a call to User Agent 2 with the
same codecs used by User Agent 1. When the call setup is complete,
RTP flows between User Agent 2 and the VoiceXML Media Server. This
stream is mixed with User Agent 1's.
User Agent 1 VoiceXML User Agent 2
(Caller) Media Server (Callee)
| | |
|(0)RTP/SRTP | |
|...................| |
| | |
| <transfer>|(1)INVITE [offer] |
| |------------------>|
| |(2) 200 OK [answer]|
| |<------------------|
| |(3) ACK |
| |------------------>|
| |(4) RTP/SRTP |
| mix |...................|
| (0)+(4)| |
If a final response is not received from User Agent 2 from the INVITE
and the connecttimeout expires (specified as an attribute of
<transfer>), the VoiceXML Media Server will issue a CANCEL to
terminate the transaction and the <transfer>'s form item variable is
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set to noanswer.
If INVITE results in a non-2xx response, the <transfer>'s form item
variable (or event raised) depends on the SIP response and is
specified in the following table.
+-------------------------+-----------------------------------+
| SIP Response | <transfer> variable / event |
+-------------------------+-----------------------------------+
| 404 Not Found | error.connection.baddestination |
| 405 Method Not Allowed | error.unsupported.transfer.bridge |
| 408 Request Timeout | noanswer |
| 486 Busy Here | busy |
| 503 Service Unavailable | error.connection.noresource |
| (No response) | network_busy |
| (Other 3xx/4xx/5xx/6xx) | unknown |
+-------------------------+-----------------------------------+
The 405 Method Not Allowed response can be used by the AS to
gracefully decline bridge transfers
Once the transfer is established, the VoiceXML Media Server can
"listen" to the media stream from User Agent 1 to perform speech or
DTMF hotword, which when matched, results in a near-end disconnect,
i.e. the VoiceXML Media Server issues a BYE to User Agent 2 and the
VoiceXML Application continues with User Agent 1. A BYE will also be
issued to User Agent 2 if the call duration exceeds the maximum
duration specified in the maxtime attribute on <transfer>.
If User Agent 2 issues a BYE during the transfer, the transfer
terminates and the VoiceXML <transfer>'s form item variable receives
the value far_end_disconnect. If User Agent 1 issues a BYE during
the transfer, the transfer terminates and the VoiceXML event
connection.disconnect.transfer is thrown.
6.3. Consultation
The consultation transfer (also called attended transfer [SIPEX]) is
similar to a blind transfer except that the outcome of the transfer
call setup is known and the Caller is not dropped as a result of an
unsuccessful transfer attempt.
Consultation transfer commences with the same flow as for bridge
transfer except that the RTP streams are not mixed at step (4) and
error.unsupported.transfer.consultation supplants
error.unsupported.transfer.bridge. Assuming a new SIP dialog with
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User Agent 2 is created, the remainder of the sequence follows as
illustrated below (provisional responses and NOTIFY messages
corresponding to provisional responses have been omitted for
clarity). Consultation transfer makes use of the Replaces: header
[RFC3891] such that User Agent 1 calls User Agent 2 and replaces the
latter's SIP dialog with the VoiceXML Media Server with a new SIP
dialog between the Caller and Callee.
User Agent 1 VoiceXML User Agent 2
(Caller) Media Server (Callee)
| | |
|(0) RTP/SRTP | |
|.................|(4) RTP/SRTP |
| |.................|
|(5) REFER | |
|<----------------| |
|(6) 202 Accepted | |
|---------------->| |
|(7) INVITE Replaces:ms1.example.com|
|---------------------------------->|
|(8) 200 OK |
|<----------------------------------|
|(9) ACK |
|---------------------------------->|
|(10) RTP/SRTP |
|...................................|
| |(11) BYE |
| |<----------------|
| |(12) 200 OK |
| |---------------->| Stop
|(13) NOTIFY | | RTP (4)
|---------------->| |
|(14) 200 OK | |
|<----------------| |
|(15) BYE | |
|<----------------| |
|(16) 200 OK | |
|---------------->| Stop |
| | RTP (0) |
If a response other than 202 Accepted is recevied in response to the
REFER request sent to User Agent 1, the transfer terminates, and an
error.unsupported.transfer.consultation event is raised. In
addition, a BYE is sent to User Agent 2 to terminate the established
outbound leg.
The VoiceXML Media Server uses receipt of a NOTIFY message with a
sipfrag message of 200 OK to determine that the consultation transfer
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has succeeded. When this occurs, the connection.disconnect.transfer
event will be thrown to the VoiceXML application, and a BYE is sent
to User Agent 1 to terminate the session. A NOTIFY message with a
non-2xx final response sipfrag message body will result in the
transfer terminating and the associated VoiceXML input item variable
being set to 'unknown'. Note that as a consequence of this
mechanism, implementations MUST NOT use [RFC4488] to suppress the
implicit subscription associated with the REFER message for
consultation transfers.
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7. Contributors
The editors gratefully acknowledge the following individuals and
their companies who contributed to this specification:
R. J. Auburn (Voxeo)
Hans Bjurstrom (Hewlett-Packard)
Dave Burke (Google)
Emily Candell (Comverse)
Peter Danielsen (Lucent)
Brian Frasca (Tellme)
Jeff Haynie (Hakano)
Scott McGlashan (Hewlett-Packard)
Matt Oshry (Tellme)
Mark Scott (Genesys Telecommunications Laboratories, Inc)
Rao Surapaneni (Tellme)
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8. Security Considerations
Exposing network services with well-known addresses may not be
desirable. The VoiceXML Media Server SHOULD authenticate and
authorize requesting endpoints per local policy. This is
particularly important for REFER-initated outbound calls.
Some applications may choose to transfer confidential information to
or from the VoiceXML Media Server. The VoiceXML Media Server SHOULD
implement the sips: and https: schemes to provide data integrity.
The VoiceXML Media Server SHOULD use authentication and TLS when
establishing MRCP control sessions with a MRCPv2 Media Resource
Server.
To mitigate against the possibility for denial of service attacks,
the VoiceXML Media Server SHOULD have local policies such as time-
limiting VoiceXML application execution.
The VoiceXML Media Server SHOULD support Secure RTP (SRTP) [RFC3711]
to provide confidentiality, authentication, and replay protection for
RTP media streams (including RTCP control traffic).
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9. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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10. Changes since last version:
o JSON used for serialization/deserialization of ECMAScript objects
o Added description of "delegation model"
o Clarified transfer not suitable for use in AS/MS architectures
o Added inactive as a permissible value for
session.connection.protocol.sip.media[x].direction
o Clarified that some header / Request-URI parameters can only be
accessed using the array access mechanism
o Minor typographic corrections
o Updated references
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11. References
11.1. Normative References
[HTML4] Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
Specification", W3C Recommendation, Dec 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2190] Zhu, C., "RTP Payload Format for H.263 Video Streams",
RFC 2190, September 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3016] Kikuchi, Y., Nomura, T., Fukunaga, S., Matsui, Y., and H.
Kimata, "RTP Payload Format for MPEG-4 Audio/Visual
Streams", RFC 3016, November 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
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Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, April 2004.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891,
September 2004.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, December 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4240] Burger, E., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[RFC4244] Barnes, M., "An Extension to the Session Initiation
Protocol (SIP) for Request History Information", RFC 4244,
November 2005.
[RFC4320] Sparks, R., "Actions Addressing Identified Issues with the
Session Initiation Protocol's (SIP) Non-INVITE
Transaction", RFC 4320, January 2006.
[RFC4488] Levin, O., "Suppression of Session Initiation Protocol
(SIP) REFER Method Implicit Subscription", RFC 4488,
May 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC4629] Ott, H., Bormann, C., Sullivan, G., Wenger, S., and R.
Burke, et al. Expires January 10, 2008 [Page 42]
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Even, "RTP Payload Format for ITU-T Rec", RFC 4629,
January 2007.
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, February 2007.
[RFC4867] Sjoberg, J., Westerlund, M., Lakaniemi, A., and Q. Xie,
"RTP Payload Format and File Storage Format for the
Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 4867, April 2007.
[VXML20] McGlashan, S., Burnett, D., Carter, J., Danielsen, P.,
Ferrans, J., Hunt, A., Lucas, B., Porter, B., Rehor, K.,
and S. Tryphonas, "Voice Extensible Markup Language
(VoiceXML) Version 2.0", W3C Recommendation, March 2004.
[VXML21] Oshry, M., Auburn, R J., Baggia, P., Bodell, M., Burke,
D., Burnett, D., Candell, E., Kilic, H., McGlashan, S.,
Lee, A., Porter, B., and K. Rehor, "Voice Extensible
Markup Language (VoiceXML) Version 2.1", W3C Candidate
Recommendation, June 2005.
11.2. Informative References
[CCXML10] Auburn, R J., "Voice Browser Call Control: CCXML Version
1.0", W3C Working Draft (work in progress), June 2005.
[IEC14496-14]
"Information technology. Coding of audio-visual objects.
MP4 file format", ISO/IEC ISO/IEC 14496-14:2003,
October 2003.
[MRCPv2] Shanmugham, S. and D. Burnett, "Media Resource Control
Protocol Version 2", draft-ietf-speechsc-mrcpv2-12 (work
in progress), Mar 2007.
[SIPEX] Johnston, A., Sparks, R., Cunningham, C., Donovan, S., and
K. Summers, "Session Initiation Protocol Examples",
draft-ietf-sipping-service-examples (work in progress),
July 2005.
[SIPVXML] Rosenberg, J., Mataga, P., and D. Ladd, "A SIP Interface
to VoiceXML Dialog Servers", draft-rosenberg-sip-vxml-00
(work in progress), July 2001.
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[TS23002] "3rd Generation Partnership Project: Network architecture
(Release 6)", 3GPP TS 23.002 v6.6.0, December 2004.
[TS26244] "Transparent end-to-end packet switched streaming service
(PSS); 3GPP file format (3GP)", 3GPP TS 26.244 v6.4.0,
December 2004.
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Authors' Addresses
Dave Burke
Google
Belgrave House, 76 Buckingham Palace Road
London SW1W 9TQ
United Kingdom
Email: daveburke@google.com
Mark Scott
Genesys
1120 Finch Avenue West, 8th floor
Toronto, Ontario M3J 3H7
Canada
Email: Mark.Scott@genesyslab.com
Jeff Haynie
Hakano Inc
1840 North Creek Circle
Alpharetta, GA 30004
USA
Email: jhaynie@hakano.com
R.J. Auburn
Voxeo
100 East Pine Street #600
Orlando, FL 32801
USA
Email: rj@voxeo.com
Scott McGlashan
Hewlett-Packard
Gustav III:s boulevard 36
SE-16985 Stockholm
Sweden
Email: Scott.McGlashan@hp.com
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