💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc4463.txt captured on 2022-01-08 at 17:54:11.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group S. Shanmugham
Request for Comments: 4463 Cisco Systems, Inc.
Category: Informational P. Monaco
Nuance Communications
B. Eberman
Speechworks Inc.
April 2006
A Media Resource Control Protocol (MRCP)
Developed by Cisco, Nuance, and Speechworks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
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.
Note that this document uses a MIME type 'application/mrcp' which has
not been registered with the IANA, and is therefore not recognized as
a standard IETF MIME type. The historical value of this document as
an ancestor to ongoing standardization in this space, however, makes
the publication of this document meaningful.
Shanmugham, et al. Informational [Page 1]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Abstract
This document describes a Media Resource Control Protocol (MRCP) that
was developed jointly by Cisco Systems, Inc., Nuance Communications,
and Speechworks, Inc. It is published as an RFC as input for further
IETF development in this area.
MRCP controls media service resources like speech synthesizers,
recognizers, signal generators, signal detectors, fax servers, etc.,
over a network. This protocol is designed to work with streaming
protocols like RTSP (Real Time Streaming Protocol) or SIP (Session
Initiation Protocol), which help establish control connections to
external media streaming devices, and media delivery mechanisms like
RTP (Real Time Protocol).
Table of Contents
1. Introduction ....................................................3
2. Architecture ....................................................4
2.1. Resources and Services .....................................4
2.2. Server and Resource Addressing .............................5
3. MRCP Protocol Basics ............................................5
3.1. Establishing Control Session and Media Streams .............5
3.2. MRCP over RTSP .............................................6
3.3. Media Streams and RTP Ports ................................8
4. Notational Conventions ..........................................8
5. MRCP Specification ..............................................9
5.1. Request ...................................................10
5.2. Response ..................................................10
5.3. Event .....................................................12
5.4. Message Headers ...........................................12
6. Media Server ...................................................19
6.1. Media Server Session ......................................19
7. Speech Synthesizer Resource ....................................21
7.1. Synthesizer State Machine .................................22
7.2. Synthesizer Methods .......................................22
7.3. Synthesizer Events ........................................23
7.4. Synthesizer Header Fields .................................23
7.5. Synthesizer Message Body ..................................29
7.6. SET-PARAMS ................................................32
7.7. GET-PARAMS ................................................32
7.8. SPEAK .....................................................33
7.9. STOP ......................................................34
7.10. BARGE-IN-OCCURRED ........................................35
7.11. PAUSE ....................................................37
7.12. RESUME ...................................................37
7.13. CONTROL ..................................................38
7.14. SPEAK-COMPLETE ...........................................40
Shanmugham, et al. Informational [Page 2]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.15. SPEECH-MARKER ............................................41
8. Speech Recognizer Resource .....................................42
8.1. Recognizer State Machine ..................................42
8.2. Recognizer Methods ........................................42
8.3. Recognizer Events .........................................43
8.4. Recognizer Header Fields ..................................43
8.5. Recognizer Message Body ...................................51
8.6. SET-PARAMS ................................................56
8.7. GET-PARAMS ................................................56
8.8. DEFINE-GRAMMAR ............................................57
8.9. RECOGNIZE .................................................60
8.10. STOP .....................................................63
8.11. GET-RESULT ...............................................64
8.12. START-OF-SPEECH ..........................................64
8.13. RECOGNITION-START-TIMERS .................................65
8.14. RECOGNITON-COMPLETE ......................................65
8.15. DTMF Detection ...........................................67
9. Future Study ...................................................67
10. Security Considerations .......................................67
11. RTSP-Based Examples ...........................................67
12. Informative References ........................................74
Appendix A. ABNF Message Definitions ..............................76
Appendix B. Acknowledgements ......................................84
1. Introduction
The Media Resource Control Protocol (MRCP) is designed to provide a
mechanism for a client device requiring audio/video stream processing
to control processing resources on the network. These media
processing resources may be speech recognizers (a.k.a. Automatic-
Speech-Recognition (ASR) engines), speech synthesizers (a.k.a. Text-
To-Speech (TTS) engines), fax, signal detectors, etc. MRCP allows
implementation of distributed Interactive Voice Response platforms,
for example VoiceXML [6] interpreters. The MRCP protocol defines the
requests, responses, and events needed to control the media
processing resources. The MRCP protocol defines the state machine
for each resource and the required state transitions for each request
and server-generated event.
The MRCP protocol does not address how the control session is
established with the server and relies on the Real Time Streaming
Protocol (RTSP) [2] to establish and maintain the session. The
session control protocol is also responsible for establishing the
media connection from the client to the network server. The MRCP
protocol and its messaging is designed to be carried over RTSP or
another protocol as a MIME-type similar to the Session Description
Protocol (SDP) [5].
Shanmugham, et al. Informational [Page 3]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
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 [8].
2. Architecture
The system consists of a client that requires media streams generated
or needs media streams processed and a server that has the resources
or devices to process or generate the streams. The client
establishes a control session with the server for media processing
using a protocol such as RTSP. This will also set up and establish
the RTP stream between the client and the server or another RTP
endpoint. Each resource needed in processing or generating the
stream is addressed or referred to by a URL. The client can now use
MRCP messages to control the media resources and affect how they
process or generate the media stream.
|--------------------|
||------------------|| |----------------------|
|| Application Layer|| ||--------------------||
||------------------|| || TTS | ASR | Fax ||
|| ASR/TTS API || ||Plugin|Plugin|Plugin||
||------------------|| || on | on | on ||
|| MRCP Core || || MRCP | MRCP | MRCP ||
|| Protocol Stack || ||--------------------||
||------------------|| || RTSP Stack ||
|| RTSP Stack || || ||
||------------------|| ||--------------------||
|| TCP/IP Stack ||========IP=========|| TCP/IP Stack ||
||------------------|| ||--------------------||
|--------------------| |----------------------|
MRCP client Real-time Streaming MRCP
media server
2.1. Resources and Services
The server is set up to offer a certain set of resources and services
to the client. These resources are of 3 types.
Transmission Resources
These are resources that are capable of generating real-time streams,
like signal generators that generate tones and sounds of certain
frequencies and patterns, and speech synthesizers that generate
spoken audio streams, etc.
Shanmugham, et al. Informational [Page 4]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Reception Resources
These are resources that receive and process streaming data like
signal detectors and speech recognizers.
Dual Mode Resources
These are resources that both send and receive data like a fax
resource, capable of sending or receiving fax through a two-way RTP
stream.
2.2. Server and Resource Addressing
The server as a whole is addressed using a container URL, and the
individual resources the server has to offer are reached by
individual resource URLs within the container URL.
RTSP Example:
A media server or container URL like,
rtsp://mediaserver.com/media/
may contain one or more resource URLs of the form,
rtsp://mediaserver.com/media/speechrecognizer/
rtsp://mediaserver.com/media/speechsynthesizer/
rtsp://mediaserver.com/media/fax/
3. MRCP Protocol Basics
The message format for MRCP is text based, with mechanisms to carry
embedded binary data. This allows data like recognition grammars,
recognition results, synthesizer speech markup, etc., to be carried
in the MRCP message between the client and the server resource. The
protocol does not address session control management, media
management, reliable sequencing, and delivery or server or resource
addressing. These are left to a protocol like SIP or RTSP. MRCP
addresses the issue of controlling and communicating with the
resource processing the stream, and defines the requests, responses,
and events needed to do that.
3.1. Establishing Control Session and Media Streams
The control session between the client and the server is established
using a protocol like RTSP. This protocol will also set up the
appropriate RTP streams between the server and the client, allocating
ports and setting up transport parameters as needed. Each control
Shanmugham, et al. Informational [Page 5]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
session is identified by a unique session-id. The format, usage, and
life cycle of the session-id is in accordance with the RTSP protocol.
The resources within the session are addressed by the individual
resource URLs.
The MRCP protocol is designed to work with and tunnel through another
protocol like RTSP, and augment its capabilities. MRCP relies on
RTSP headers for sequencing, reliability, and addressing to make sure
that messages get delivered reliably and in the correct order and to
the right resource. The MRCP messages are carried in the RTSP
message body. The media server delivers the MRCP message to the
appropriate resource or device by looking at the session-level
message headers and URL information. Another protocol, such as SIP
[4], could be used for tunneling MRCP messages.
3.2. MRCP over RTSP
RTSP supports both TCP and UDP mechanisms for the client to talk to
the server and is differentiated by the RTSP URL. All MRCP based
media servers MUST support TCP for transport and MAY support UDP.
In RTSP, the ANNOUNCE method/response MUST be used to carry MRCP
request/responses between the client and the server. MRCP messages
MUST NOT be communicated in the RTSP SETUP or TEARDOWN messages.
Currently all RTSP messages are request/responses and there is no
support for asynchronous events in RTSP. This is because RTSP was
designed to work over TCP or UDP and, hence, could not assume
reliability in the underlying protocol. Hence, when using MRCP over
RTSP, an asynchronous event from the MRCP server is packaged in a
server-initiated ANNOUNCE method/response communication. A future
RTSP extension to send asynchronous events from the server to the
client would provide an alternate vehicle to carry such asynchronous
MRCP events from the server.
An RTSP session is created when an RTSP SETUP message is sent from
the client to a server and is addressed to a server URL or any one of
its resource URLs without specifying a session-id. The server will
establish a session context and will respond with a session-id to the
client. This sequence will also set up the RTP transport parameters
between the client and the server, and then the server will be ready
to receive or send media streams. If the client wants to attach an
additional resource to an existing session, the client should send
that session's ID in the subsequent SETUP message.
Shanmugham, et al. Informational [Page 6]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
When a media server implementing MRCP over RTSP receives a PLAY,
RECORD, or PAUSE RTSP method from an MRCP resource URL, it should
respond with an RTSP 405 "Method not Allowed" response. For these
resources, the only allowed RTSP methods are SETUP, TEARDOWN,
DESCRIBE, and ANNOUNCE.
Example 1:
C->S: ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:4
Session:12345678
Content-Type:application/mrcp
Content-Length:223
SPEAK 543257 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C: RTSP/1.0 200 OK
CSeq: 4
Session:12345678
RTP-Info:url=rtsp://media.server.com/media/synthesizer;
seq=9810092;rtptime=3450012
Content-Type:application/mrcp
Content-Length:52
MRCP/1.0 543257 200 IN-PROGRESS
S->C: ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:6
Session:12345678
Shanmugham, et al. Informational [Page 7]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Content-Type:application/mrcp
Content-Length:123
SPEAK-COMPLETE 543257 COMPLETE MRCP/1.0
C->S: RTSP/1.0 200 OK
CSeq:6
For the sake of brevity, most examples from here on show only the
MRCP messages and do not show the RTSP message and headers in which
they are tunneled. Also, RTSP messages such as response that are not
carrying an MRCP message are also left out.
3.3. Media Streams and RTP Ports
A single set of RTP/RTCP ports is negotiated and shared between the
MRCP client and server when multiple media processing resources, such
as automatic speech recognition (ASR) engines and text to speech
(TTS) engines, are used for a single session. The individual
resource instances allocated on the server under a common session
identifier will feed from/to that single RTP stream.
The client can send multiple media streams towards the server,
differentiated by using different synchronized source (SSRC)
identifier values. Similarly the server can use multiple
Synchronized Source (SSRC) identifier values to differentiate media
streams originating from the individual transmission resource URLs if
more than one exists. The individual resources may, on the other
hand, work together to send just one stream to the client. This is
up to the implementation of the media server.
4. Notational Conventions
Since many of the definitions and syntax are identical to HTTP/1.1,
this specification only points to the section where they are defined
rather than copying it. For brevity, [HX.Y] refers to Section X.Y of
the current HTTP/1.1 specification (RFC 2616 [1]).
All the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur form (ABNF) similar to that used
in [H2.1]. It is described in detail in RFC 4234 [3].
The ABNF provided along with the descriptive text is informative in
nature and may not be complete. The complete message format in ABNF
form is provided in Appendix A and is the normative format
definition.
Shanmugham, et al. Informational [Page 8]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
5. MRCP Specification
The MRCP PDU is textual using an ISO 10646 character set in the UTF-8
encoding (RFC 3629 [12]) to allow many different languages to be
represented. However, to assist in compact representations, MRCP
also allows other character sets such as ISO 8859-1 to be used when
desired. The MRCP protocol headers and field names use only the
US-ASCII subset of UTF-8. Internationalization only applies to
certain fields like grammar, results, speech markup, etc., and not to
MRCP as a whole.
Lines are terminated by CRLF, but receivers SHOULD be prepared to
also interpret CR and LF by themselves as line terminators. Also,
some parameters in the PDU may contain binary data or a record
spanning multiple lines. Such fields have a length value associated
with the parameter, which indicates the number of octets immediately
following the parameter.
The whole MRCP PDU is encoded in the body of the session level
message as a MIME entity of type application/mrcp. The individual
MRCP messages do not have addressing information regarding which
resource the request/response are to/from. Instead, the MRCP message
relies on the header of the session level message carrying it to
deliver the request to the appropriate resource, or to figure out who
the response or event is from.
The MRCP message set consists of requests from the client to the
server, responses from the server to the client and asynchronous
events from the server to the client. All these messages consist of
a start-line, one or more header fields (also known as "headers"), an
empty line (i.e., a line with nothing preceding the CRLF) indicating
the end of the header fields, and an optional message body.
generic-message = start-line
message-header
CRLF
[ message-body ]
message-body = *OCTET
start-line = request-line / status-line / event-line
The message-body contains resource-specific and message-specific data
that needs to be carried between the client and server as a MIME
entity. The information contained here and the actual MIME-types
used to carry the data are specified later when addressing the
specific messages.
Shanmugham, et al. Informational [Page 9]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
If a message contains data in the message body, the header fields
will contain content-headers indicating the MIME-type and encoding of
the data in the message body.
5.1. Request
An MRCP request consists of a Request line followed by zero or more
parameters as part of the message headers and an optional message
body containing data specific to the request message.
The Request message from a client to the server includes, within the
first line, the method to be applied, a method tag for that request,
and the version of protocol in use.
request-line = method-name SP request-id SP
mrcp-version CRLF
The request-id field is a unique identifier created by the client and
sent to the server. The server resource should use this identifier
in its response to this request. If the request does not complete
with the response, future asynchronous events associated with this
request MUST carry the request-id.
request-id = 1*DIGIT
The method-name field identifies the specific request that the client
is making to the server. Each resource supports a certain list of
requests or methods that can be issued to it, and will be addressed
in later sections.
method-name = synthesizer-method
/ recognizer-method
The mrcp-version field is the MRCP protocol version that is being
used by the client.
mrcp-version = "MRCP" "/" 1*DIGIT "." 1*DIGIT
5.2. Response
After receiving and interpreting the request message, the server
resource responds with an MRCP response message. It consists of a
status line optionally followed by a message body.
response-line = mrcp-version SP request-id SP status-code SP
request-state CRLF
Shanmugham, et al. Informational [Page 10]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
The mrcp-version field used here is similar to the one used in the
Request Line and indicates the version of MRCP protocol running on
the server.
The request-id used in the response MUST match the one sent in the
corresponding request message.
The status-code field is a 3-digit code representing the success or
failure or other status of the request.
The request-state field indicates if the job initiated by the Request
is PENDING, IN-PROGRESS, or COMPLETE. The COMPLETE status means that
the Request was processed to completion and that there will be no
more events from that resource to the client with that request-id.
The PENDING status means that the job has been placed on a queue and
will be processed in first-in-first-out order. The IN-PROGRESS
status means that the request is being processed and is not yet
complete. A PENDING or IN-PROGRESS status indicates that further
Event messages will be delivered with that request-id.
request-state = "COMPLETE"
/ "IN-PROGRESS"
/ "PENDING"
5.2.1. Status Codes
The status codes are classified under the Success(2XX) codes and the
Failure(4XX) codes.
5.2.1.1. Success 2xx
200 Success
201 Success with some optional parameters ignored.
5.2.1.2. Failure 4xx
401 Method not allowed
402 Method not valid in this state
403 Unsupported Parameter
404 Illegal Value for Parameter
405 Not found (e.g., Resource URI not initialized
or doesn't exist)
406 Mandatory Parameter Missing
407 Method or Operation Failed (e.g., Grammar compilation
failed in the recognizer. Detailed cause codes MAY BE
available through a resource specific header field.)
408 Unrecognized or unsupported message entity
Shanmugham, et al. Informational [Page 11]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
409 Unsupported Parameter Value
421-499 Resource specific Failure codes
5.3. Event
The server resource may need to communicate a change in state or the
occurrence of a certain event to the client. These messages are used
when a request does not complete immediately and the response returns
a status of PENDING or IN-PROGRESS. The intermediate results and
events of the request are indicated to the client through the event
message from the server. Events have the request-id of the request
that is in progress and is generating these events and status value.
The status value is COMPLETE if the request is done and this was the
last event, else it is IN-PROGRESS.
event-line = event-name SP request-id SP request-state SP
mrcp-version CRLF
The mrcp-version used here is identical to the one used in the
Request/Response Line and indicates the version of MRCP protocol
running on the server.
The request-id used in the event should match the one sent in the
request that caused this event.
The request-state indicates if the Request/Command causing this event
is complete or still in progress, and is the same as the one
mentioned in Section 5.2. The final event will contain a COMPLETE
status indicating the completion of the request.
The event-name identifies the nature of the event generated by the
media resource. The set of valid event names are dependent on the
resource generating it, and will be addressed in later sections.
event-name = synthesizer-event
/ recognizer-event
5.4. Message Headers
MRCP header fields, which include general-header (Section 5.4) and
resource-specific-header (Sections 7.4 and 8.4), follow the same
generic format as that given in Section 2.1 of RFC 2822 [7]. Each
header field consists of a name followed by a colon (":") and the
field value. Field names are case-insensitive. The field value MAY
be preceded by any amount of linear whitespace (LWS), though a single
SP is preferred. Header fields can be extended over multiple lines
by preceding each extra line with at least one SP or HT.
Shanmugham, et al. Informational [Page 12]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
message-header = 1*(generic-header / resource-header)
The order in which header fields with differing field names are
received is not significant. However, it is "good practice" to send
general-header fields first, followed by request-header or response-
header fields, and ending with the entity-header fields.
Multiple message-header fields with the same field-name MAY be
present in a message if and only if the entire field value for that
header field is defined as a comma-separated list (i.e., #(values)).
It MUST be possible to combine the multiple header fields into one
"field-name:field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each
separated by a comma. Therefore, the order in which header fields
with the same field-name are received is significant to the
interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded.
Generic Headers
generic-header = active-request-id-list
/ proxy-sync-id
/ content-id
/ content-type
/ content-length
/ content-base
/ content-location
/ content-encoding
/ cache-control
/ logging-tag
All headers in MRCP will be case insensitive, consistent with HTTP
and RTSP protocol header definitions.
5.4.1. Active-Request-Id-List
In a request, this field indicates the list of request-ids to which
it should apply. This is useful when there are multiple Requests
that are PENDING or IN-PROGRESS and you want this request to apply to
one or more of these specifically.
In a response, this field returns the list of request-ids that the
operation modified or were in progress or just completed. There
could be one or more requests that returned a request-state of
PENDING or IN-PROGRESS. When a method affecting one or more PENDING
Shanmugham, et al. Informational [Page 13]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
or IN-PROGRESS requests is sent from the client to the server, the
response MUST contain the list of request-ids that were affected in
this header field.
The active-request-id-list is only used in requests and responses,
not in events.
For example, if a STOP request with no active-request-id-list is sent
to a synthesizer resource (a wildcard STOP) that has one or more
SPEAK requests in the PENDING or IN-PROGRESS state, all SPEAK
requests MUST be cancelled, including the one IN-PROGRESS. In
addition, the response to the STOP request would contain the
request-id of all the SPEAK requests that were terminated in the
active-request-id-list. In this case, no SPEAK-COMPLETE or
RECOGNITION-COMPLETE events will be sent for these terminated
requests.
active-request-id-list = "Active-Request-Id-List" ":" request-id
*("," request-id) CRLF
5.4.2. Proxy-Sync-Id
When any server resource generates a barge-in-able event, it will
generate a unique Tag and send it as a header field in an event to
the client. The client then acts as a proxy to the server resource
and sends a BARGE-IN-OCCURRED method (Section 7.10) to the
synthesizer server resource with the Proxy-Sync-Id it received from
the server resource. When the recognizer and synthesizer resources
are part of the same session, they may choose to work together to
achieve quicker interaction and response. Here, the proxy-sync-id
helps the resource receiving the event, proxied by the client, to
decide if this event has been processed through a direct interaction
of the resources.
proxy-sync-id = "Proxy-Sync-Id" ":" 1*ALPHA CRLF
5.4.3. Accept-Charset
See [H14.2]. This specifies the acceptable character set for
entities returned in the response or events associated with this
request. This is useful in specifying the character set to use in
the Natural Language Semantics Markup Language (NLSML) results of a
RECOGNITON-COMPLETE event.
Shanmugham, et al. Informational [Page 14]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
5.4.4. Content-Type
See [H14.17]. Note that the content types suitable for MRCP are
restricted to speech markup, grammar, recognition results, etc., and
are specified later in this document. The multi-part content type
"multi-part/mixed" is supported to communicate multiple of the above
mentioned contents, in which case the body parts cannot contain any
MRCP specific headers.
5.4.5. Content-Id
This field contains an ID or name for the content, by which it can be
referred to. The definition of this field conforms to RFC 2392 [14],
RFC 2822 [7], RFC 2046 [13] and is needed in multi-part messages. In
MRCP whenever the content needs to be stored, by either the client or
the server, it is stored associated with this ID. Such content can
be referenced during the session in URI form using the session:URI
scheme described in a later section.
5.4.6. Content-Base
The content-base entity-header field may be used to specify the base
URI for resolving relative URLs within the entity.
content-base = "Content-Base" ":" absoluteURI CRLF
Note, however, that the base URI of the contents within the entity-
body may be redefined within that entity-body. An example of this
would be a multi-part MIME entity, which in turn can have multiple
entities within it.
5.4.7. Content-Encoding
The content-encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional
content coding has been applied to the entity-body, and thus what
decoding mechanisms must be applied in order to obtain the media-type
referenced by the content-type header field. Content-encoding is
primarily used to allow a document to be compressed without losing
the identity of its underlying media type.
content-encoding = "Content-Encoding" ":"
*WSP content-coding
*(*WSP "," *WSP content-coding *WSP )
CRLF
content-coding = token
Shanmugham, et al. Informational [Page 15]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
token = 1*(alphanum / "-" / "." / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
Content coding is defined in [H3.5]. An example of its use is
Content-Encoding:gzip
If multiple encodings have been applied to an entity, the content
codings MUST be listed in the order in which they were applied.
5.4.8. Content-Location
The content-location entity-header field MAY BE used to supply the
resource location for the entity enclosed in the message when that
entity is accessible from a location separate from the requested
resource's URI.
content-location = "Content-Location" ":" ( absoluteURI /
relativeURI ) CRLF
The content-location value is a statement of the location of the
resource corresponding to this particular entity at the time of the
request. The media server MAY use this header field to optimize
certain operations. When providing this header field, the entity
being sent should not have been modified from what was retrieved from
the content-location URI.
For example, if the client provided a grammar markup inline, and it
had previously retrieved it from a certain URI, that URI can be
provided as part of the entity, using the content-location header
field. This allows a resource like the recognizer to look into its
cache to see if this grammar was previously retrieved, compiled, and
cached. In which case, it might optimize by using the previously
compiled grammar object.
If the content-location is a relative URI, the relative URI is
interpreted relative to the content-base URI.
5.4.9. Content-Length
This field contains the length of the content of the message body
(i.e., after the double CRLF following the last header field).
Unlike HTTP, it MUST be included in all messages that carry content
beyond the header portion of the message. If it is missing, a
default value of zero is assumed. It is interpreted according to
[H14.13].
Shanmugham, et al. Informational [Page 16]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
5.4.10. Cache-Control
If the media server plans on implementing caching, it MUST adhere to
the cache correctness rules of HTTP 1.1 (RFC2616), when accessing and
caching HTTP URI. In particular, the expires and cache-control
headers of the cached URI or document must be honored and will always
take precedence over the Cache-Control defaults set by this header
field. The cache-control directives are used to define the default
caching algorithms on the media server for the session or request.
The scope of the directive is based on the method it is sent on. If
the directives are sent on a SET-PARAMS method, it SHOULD apply for
all requests for documents the media server may make in that session.
If the directives are sent on any other messages, they MUST only
apply to document requests the media server needs to make for that
method. An empty cache-control header on the GET-PARAMS method is a
request for the media server to return the current cache-control
directives setting on the server.
cache-control = "Cache-Control" ":" *WSP cache-directive
*( *WSP "," *WSP cache-directive *WSP )
CRLF
cache-directive = "max-age" "=" delta-seconds
/ "max-stale" "=" delta-seconds
/ "min-fresh" "=" delta-seconds
delta-seconds = 1*DIGIT
Here, delta-seconds is a time value to be specified as an integer
number of seconds, represented in decimal, after the time that the
message response or data was received by the media server.
These directives allow the media server to override the basic
expiration mechanism.
max-age
Indicates that the client is OK with the media server using a
response whose age is no greater than the specified time in
seconds. Unless a max-stale directive is also included, the
client is not willing to accept the media server using a stale
response.
min-fresh
Indicates that the client is willing to accept the media server
using a response whose freshness lifetime is no less than its
current age plus the specified time in seconds. That is, the
Shanmugham, et al. Informational [Page 17]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
client wants the media server to use a response that will still be
fresh for at least the specified number of seconds.
max-stale
Indicates that the client is willing to accept the media server
using a response that has exceeded its expiration time. If max-
stale is assigned a value, then the client is willing to accept
the media server using a response that has exceeded its expiration
time by no more than the specified number of seconds. If no value
is assigned to max-stale, then the client is willing to accept the
media server using a stale response of any age.
The media server cache MAY BE requested to use stale response/data
without validation, but only if this does not conflict with any
"MUST"-level requirements concerning cache validation (e.g., a
"must-revalidate" cache-control directive) in the HTTP 1.1
specification pertaining the URI.
If both the MRCP cache-control directive and the cached entry on the
media server include "max-age" directives, then the lesser of the two
values is used for determining the freshness of the cached entry for
that request.
5.4.11. Logging-Tag
This header field MAY BE sent as part of a SET-PARAMS/GET-PARAMS
method to set the logging tag for logs generated by the media server.
Once set, the value persists until a new value is set or the session
is ended. The MRCP server should provide a mechanism to subset its
output logs so that system administrators can examine or extract only
the log file portion during which the logging tag was set to a
certain value.
MRCP clients using this feature should take care to ensure that no
two clients specify the same logging tag. In the event that two
clients specify the same logging tag, the effect on the MRCP server's
output logs in undefined.
logging-tag = "Logging-Tag" ":" 1*ALPHA CRLF
Shanmugham, et al. Informational [Page 18]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
6. Media Server
The capability of media server resources can be found using the RTSP
DESCRIBE mechanism. When a client issues an RTSP DESCRIBE method for
a media resource URI, the media server response MUST contain an SDP
description in its body describing the capabilities of the media
server resource. The SDP description MUST contain at a minimum the
media header (m-line) describing the codec and other media related
features it supports. It MAY contain another SDP header as well, but
support for it is optional.
The usage of SDP messages in the RTSP message body and its
application follows the SIP RFC 2543 [4], but is limited to media-
related negotiation and description.
6.1. Media Server Session
As discussed in Section 3.2, a client/server should share one RTSP
session-id for the different resources it may use under the same
session. The client MUST allocate a set of client RTP/RTCP ports for
a new session and MUST NOT send a Session-ID in the SETUP message for
the first resource. The server then creates a Session-ID and
allocates a set of server RTP/RTCP ports and responds to the SETUP
message.
If the client wants to open more resources with the same server under
the same session, it will send the session-id (that it got in the
earlier SETUP response) in the SETUP for the new resource. A SETUP
message with an existing session-id tells the server that this new
resource will feed from/into the same RTP/RTCP stream of that
existing session.
If the client wants to open a resource from a media server that is
not where the first resource came from, it will send separate SETUP
requests with no session-id header field in them. Each server will
allocate its own session-id and return it in the response. Each of
them will also come back with their own set of RTP/RTCP ports. This
would be the case when the synthesizer engine and the recognition
engine are on different servers.
The RTSP SETUP method SHOULD contain an SDP description of the media
stream being set up. The RTSP SETUP response MUST contain an SDP
description of the media stream that it expects to receive and send
on that session.
The SDP description in the SETUP method from the client SHOULD
describe the required media parameters like codec, Named Signaling
Event (NSE) payload types, etc. This could have multiple media
Shanmugham, et al. Informational [Page 19]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
headers (i.e., m-lines) to allow the client to provide the media
server with more than one option to choose from.
The SDP description in the SETUP response should reflect the media
parameters that the media server will be using for the stream. It
should be within the choices that were specified in the SDP of the
SETUP method, if one was provided.
Example:
C->S:
SETUP rtsp://media.server.com/recognizer/ RTSP/1.0
CSeq:1
Transport:RTP/AVP;unicast;client_port=46456-46457
Content-Type:application/sdp
Content-Length:190
v=0
o=- 123 456 IN IP4 10.0.0.1
s=Media Server
p=+1-888-555-1212
c=IN IP4 0.0.0.0
t=0 0
m=audio 46456 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
S->C:
RTSP/1.0 200 OK
CSeq:1
Session:0a030258_00003815_3bc4873a_0001_0000
Transport:RTP/AVP;unicast;client_port=46456-46457;
server_port=46460-46461
Content-Length:190
Content-Type:application/sdp
v=0
o=- 3211724219 3211724219 IN IP4 10.3.2.88
s=Media Server
c=IN IP4 0.0.0.0
t=0 0
m=audio 46460 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
Shanmugham, et al. Informational [Page 20]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
If an SDP description was not provided in the RTSP SETUP method, then
the media server may decide on parameters of the stream but MUST
specify what it chooses in the SETUP response. An SDP announcement
is only returned in a response to a SETUP message that does not
specify a Session. That is, the server will not return an SDP
announcement for the synthesizer SETUP of a session already
established with a recognizer.
C->S:
SETUP rtsp://media.server.com/recognizer/ RTSP/1.0
CSeq:1
Transport:RTP/AVP;unicast;client_port=46498
S->C:
RTSP/1.0 200 OK
CSeq:1
Session:0a030258_000039dc_3bc48a13_0001_0000
Transport:RTP/AVP;unicast; client_port=46498;
server_port=46502-46503
Content-Length:193
Content-Type:application/sdp
v=0
o=- 3211724947 3211724947 IN IP4 10.3.2.88
s=Media Server
c=IN IP4 0.0.0.0
t=0 0
m=audio 46502 RTP/AVP 0 101
a=rtpmap:0 pcmu/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15
7. Speech Synthesizer Resource
This resource is capable of converting text provided by the client
and generating a speech stream in real-time. Depending on the
implementation and capability of this resource, the client can
control parameters like voice characteristics, speaker speed, etc.
The synthesizer resource is controlled by MRCP requests from the
client. Similarly, the resource can respond to these requests or
generate asynchronous events to the server to indicate certain
conditions during the processing of the stream.
Shanmugham, et al. Informational [Page 21]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.1. Synthesizer State Machine
The synthesizer maintains states because it needs to correlate MRCP
requests from the client. The state transitions shown below describe
the states of the synthesizer and reflect the request at the head of
the queue. A SPEAK request in the PENDING state can be deleted or
stopped by a STOP request and does not affect the state of the
resource.
Idle Speaking Paused
State State State
| | |
|----------SPEAK------->| |--------|
|<------STOP------------| CONTROL |
|<----SPEAK-COMPLETE----| |------->|
|<----BARGE-IN-OCCURRED-| |
| |--------| |
| CONTROL |-----------PAUSE--------->|
| |------->|<----------RESUME---------|
| | |----------|
| | PAUSE |
| | |--------->|
| |--------|----------| |
| BARGE-IN-OCCURRED | SPEECH-MARKER |
| |------->|<---------| |
|----------| | |------------|
| STOP | SPEAK |
| | | |----------->|
|<---------| |
|<-------------------STOP--------------------------|
7.2. Synthesizer Methods
The synthesizer supports the following methods.
synthesizer-method = "SET-PARAMS"
/ "GET-PARAMS"
/ "SPEAK"
/ "STOP"
/ "PAUSE"
/ "RESUME"
/ "BARGE-IN-OCCURRED"
/ "CONTROL"
Shanmugham, et al. Informational [Page 22]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.3. Synthesizer Events
The synthesizer may generate the following events.
synthesizer-event = "SPEECH-MARKER"
/ "SPEAK-COMPLETE"
7.4. Synthesizer Header Fields
A synthesizer message may contain header fields containing request
options and information to augment the Request, Response, or Event of
the message with which it is associated.
synthesizer-header = jump-target ; Section 7.4.1
/ kill-on-barge-in ; Section 7.4.2
/ speaker-profile ; Section 7.4.3
/ completion-cause ; Section 7.4.4
/ voice-parameter ; Section 7.4.5
/ prosody-parameter ; Section 7.4.6
/ vendor-specific ; Section 7.4.7
/ speech-marker ; Section 7.4.8
/ speech-language ; Section 7.4.9
/ fetch-hint ; Section 7.4.10
/ audio-fetch-hint ; Section 7.4.11
/ fetch-timeout ; Section 7.4.12
/ failed-uri ; Section 7.4.13
/ failed-uri-cause ; Section 7.4.14
/ speak-restart ; Section 7.4.15
/ speak-length ; Section 7.4.16
Parameter Support Methods/Events/Response
jump-target MANDATORY SPEAK, CONTROL
logging-tag MANDATORY SET-PARAMS, GET-PARAMS
kill-on-barge-in MANDATORY SPEAK
speaker-profile OPTIONAL SET-PARAMS, GET-PARAMS,
SPEAK, CONTROL
completion-cause MANDATORY SPEAK-COMPLETE
voice-parameter MANDATORY SET-PARAMS, GET-PARAMS,
SPEAK, CONTROL
prosody-parameter MANDATORY SET-PARAMS, GET-PARAMS,
SPEAK, CONTROL
vendor-specific MANDATORY SET-PARAMS, GET-PARAMS
speech-marker MANDATORY SPEECH-MARKER
speech-language MANDATORY SET-PARAMS, GET-PARAMS, SPEAK
fetch-hint MANDATORY SET-PARAMS, GET-PARAMS, SPEAK
audio-fetch-hint MANDATORY SET-PARAMS, GET-PARAMS, SPEAK
fetch-timeout MANDATORY SET-PARAMS, GET-PARAMS, SPEAK
Shanmugham, et al. Informational [Page 23]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
failed-uri MANDATORY Any
failed-uri-cause MANDATORY Any
speak-restart MANDATORY CONTROL
speak-length MANDATORY SPEAK, CONTROL
7.4.1. Jump-Target
This parameter MAY BE specified in a CONTROL method and controls the
jump size to move forward or rewind backward on an active SPEAK
request. A + or - indicates a relative value to what is being
currently played. This MAY BE specified in a SPEAK request to
indicate an offset into the speech markup that the SPEAK request
should start speaking from. The different speech length units
supported are dependent on the synthesizer implementation. If it
does not support a unit or the operation, the resource SHOULD respond
with a status code of 404 "Illegal or Unsupported value for
parameter".
jump-target = "Jump-Size" ":" speech-length-value CRLF
speech-length-value = numeric-speech-length
/ text-speech-length
text-speech-length = 1*ALPHA SP "Tag"
numeric-speech-length= ("+" / "-") 1*DIGIT SP
numeric-speech-unit
numeric-speech-unit = "Second"
/ "Word"
/ "Sentence"
/ "Paragraph"
7.4.2. Kill-On-Barge-In
This parameter MAY BE sent as part of the SPEAK method to enable
kill-on-barge-in support. If enabled, the SPEAK method is
interrupted by DTMF input detected by a signal detector resource or
by the start of speech sensed or recognized by the speech recognizer
resource.
kill-on-barge-in = "Kill-On-Barge-In" ":" boolean-value CRLF
boolean-value = "true" / "false"
If the recognizer or signal detector resource is on, the same server
as the synthesizer, the server should be intelligent enough to
recognize their interactions by their common RTSP session-id and work
with each other to provide kill-on-barge-in support. The client
needs to send a BARGE-IN-OCCURRED method to the synthesizer resource
when it receives a barge-in-able event from the synthesizer resource
Shanmugham, et al. Informational [Page 24]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
or signal detector resource. These resources MAY BE local or
distributed. If this field is not specified, the value defaults to
"true".
7.4.3. Speaker Profile
This parameter MAY BE part of the SET-PARAMS/GET-PARAMS or SPEAK
request from the client to the server and specifies the profile of
the speaker by a URI, which may be a set of voice parameters like
gender, accent, etc.
speaker-profile = "Speaker-Profile" ":" uri CRLF
7.4.4. Completion Cause
This header field MUST be specified in a SPEAK-COMPLETE event coming
from the synthesizer resource to the client. This indicates the
reason behind the SPEAK request completion.
completion-cause = "Completion-Cause" ":" 1*DIGIT SP 1*ALPHA
CRLF
Cause-Code Cause-Name Description
000 normal SPEAK completed normally.
001 barge-in SPEAK request was terminated because
of barge-in.
002 parse-failure SPEAK request terminated because of a
failure to parse the speech markup text.
003 uri-failure SPEAK request terminated because, access
to one of the URIs failed.
004 error SPEAK request terminated prematurely due
to synthesizer error.
005 language-unsupported
Language not supported.
7.4.5. Voice-Parameters
This set of parameters defines the voice of the speaker.
voice-parameter = "Voice-" voice-param-name ":"
voice-param-value CRLF
voice-param-name is any one of the attribute names under the voice
element specified in W3C's Speech Synthesis Markup Language
Specification [9]. The voice-param-value is any one of the value
choices of the corresponding voice element attribute specified in the
above section.
Shanmugham, et al. Informational [Page 25]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
These header fields MAY BE sent in SET-PARAMS/GET-PARAMS request to
define/get default values for the entire session or MAY BE sent in
the SPEAK request to define default values for that speak request.
Furthermore, these attributes can be part of the speech text marked
up in Speech Synthesis Markup Language (SSML).
These voice parameter header fields can also be sent in a CONTROL
method to affect a SPEAK request in progress and change its behavior
on the fly. If the synthesizer resource does not support this
operation, it should respond back to the client with a status of
unsupported.
7.4.6. Prosody-Parameters
This set of parameters defines the prosody of the speech.
prosody-parameter = "Prosody-" prosody-param-name ":"
prosody-param-value CRLF
prosody-param-name is any one of the attribute names under the
prosody element specified in W3C's Speech Synthesis Markup Language
Specification [9]. The prosody-param-value is any one of the value
choices of the corresponding prosody element attribute specified in
the above section.
These header fields MAY BE sent in SET-PARAMS/GET-PARAMS request to
define/get default values for the entire session or MAY BE sent in
the SPEAK request to define default values for that speak request.
Furthermore, these attributes can be part of the speech text marked
up in SSML.
The prosody parameter header fields in the SET-PARAMS or SPEAK
request only apply if the speech data is of type text/plain and does
not use a speech markup format.
These prosody parameter header fields MAY also be sent in a CONTROL
method to affect a SPEAK request in progress and to change its
behavior on the fly. If the synthesizer resource does not support
this operation, it should respond back to the client with a status of
unsupported.
Shanmugham, et al. Informational [Page 26]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.4.7. Vendor-Specific Parameters
This set of headers allows for the client to set vendor-specific
parameters.
vendor-specific = "Vendor-Specific-Parameters" ":"
vendor-specific-av-pair
*[";" vendor-specific-av-pair] CRLF
vendor-specific-av-pair = vendor-av-pair-name "="
vendor-av-pair-value
This header MAY BE sent in the SET-PARAMS/GET-PARAMS method and is
used to set vendor-specific parameters on the server side. The
vendor-av-pair-name can be any vendor-specific field name and
conforms to the XML vendor-specific attribute naming convention. The
vendor-av-pair-value is the value to set the attribute to and needs
to be quoted.
When asking the server to get the current value of these parameters,
this header can be sent in the GET-PARAMS method with the list of
vendor-specific attribute names to get separated by a semicolon.
7.4.8. Speech Marker
This header field contains a marker tag that may be embedded in the
speech data. Most speech markup formats provide mechanisms to embed
marker fields between speech texts. The synthesizer will generate
SPEECH-MARKER events when it reaches these marker fields. This field
SHOULD be part of the SPEECH-MARKER event and will contain the marker
tag values.
speech-marker = "Speech-Marker" ":" 1*ALPHA CRLF
7.4.9. Speech Language
This header field specifies the default language of the speech data
if it is not specified in the speech data. The value of this header
field should follow RFC 3066 [16] for its values. This MAY occur in
SPEAK, SET-PARAMS, or GET-PARAMS request.
speech-language = "Speech-Language" ":" 1*ALPHA CRLF
Shanmugham, et al. Informational [Page 27]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.4.10. Fetch Hint
When the synthesizer needs to fetch documents or other resources like
speech markup or audio files, etc., this header field controls URI
access properties. This defines when the synthesizer should retrieve
content from the server. A value of "prefetch" indicates a file may
be downloaded when the request is received, whereas "safe" indicates
a file that should only be downloaded when actually needed. The
default value is "prefetch". This header field MAY occur in SPEAK,
SET-PARAMS, or GET-PARAMS requests.
fetch-hint = "Fetch-Hint" ":" 1*ALPHA CRLF
7.4.11. Audio Fetch Hint
When the synthesizer needs to fetch documents or other resources like
speech audio files, etc., this header field controls URI access
properties. This defines whether or not the synthesizer can attempt
to optimize speech by pre-fetching audio. The value is either "safe"
to say that audio is only fetched when it is needed, never before;
"prefetch" to permit, but not require the platform to pre-fetch the
audio; or "stream" to allow it to stream the audio fetches. The
default value is "prefetch". This header field MAY occur in SPEAK,
SET-PARAMS, or GET-PARAMS requests.
audio-fetch-hint = "Audio-Fetch-Hint" ":" 1*ALPHA CRLF
7.4.12. Fetch Timeout
When the synthesizer needs to fetch documents or other resources like
speech audio files, etc., this header field controls URI access
properties. This defines the synthesizer timeout for resources the
media server may need to fetch from the network. This is specified
in milliseconds. The default value is platform-dependent. This
header field MAY occur in SPEAK, SET-PARAMS, or GET-PARAMS.
fetch-timeout = "Fetch-Timeout" ":" 1*DIGIT CRLF
7.4.13. Failed URI
When a synthesizer method needs a synthesizer to fetch or access a
URI, and the access fails, the media server SHOULD provide the failed
URI in this header field in the method response.
failed-uri = "Failed-URI" ":" Url CRLF
Shanmugham, et al. Informational [Page 28]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.4.14. Failed URI Cause
When a synthesizer method needs a synthesizer to fetch or access a
URI, and the access fails, the media server SHOULD provide the URI
specific or protocol-specific response code through this header field
in the method response. This field has been defined as alphanumeric
to accommodate all protocols, some of which might have a response
string instead of a numeric response code.
failed-uri-cause = "Failed-URI-Cause" ":" 1*ALPHA CRLF
7.4.15. Speak Restart
When a CONTROL jump backward request is issued to a currently
speaking synthesizer resource and the jumps beyond the start of the
speech, the current SPEAK request re-starts from the beginning of its
speech data and the response to the CONTROL request would contain
this header indicating a restart. This header MAY occur in the
CONTROL response.
speak-restart = "Speak-Restart" ":" boolean-value CRLF
7.4.16. Speak Length
This parameter MAY BE specified in a CONTROL method to control the
length of speech to speak, relative to the current speaking point in
the currently active SPEAK request. A "-" value is illegal in this
field. If a field with a Tag unit is specified, then the media must
speak until the tag is reached or the SPEAK request complete,
whichever comes first. This MAY BE specified in a SPEAK request to
indicate the length to speak in the speech data and is relative to
the point in speech where the SPEAK request starts. The different
speech length units supported are dependent on the synthesizer
implementation. If it does not support a unit or the operation, the
resource SHOULD respond with a status code of 404 "Illegal or
Unsupported value for parameter".
speak-length = "Speak-Length" ":" speech-length-value
CRLF
7.5. Synthesizer Message Body
A synthesizer message may contain additional information associated
with the Method, Response, or Event in its message body.
Shanmugham, et al. Informational [Page 29]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.5.1. Synthesizer Speech Data
Marked-up text for the synthesizer to speak is specified as a MIME
entity in the message body. The message to be spoken by the
synthesizer can be specified inline (by embedding the data in the
message body) or by reference (by providing the URI to the data). In
either case, the data and the format used to markup the speech needs
to be supported by the media server.
All media servers MUST support plain text speech data and W3C's
Speech Synthesis Markup Language [9] at a minimum and, hence, MUST
support the MIME types text/plain and application/synthesis+ssml at a
minimum.
If the speech data needs to be specified by URI reference, the MIME
type text/uri-list is used to specify the one or more URIs that will
list what needs to be spoken. If a list of speech URIs is specified,
speech data provided by each URI must be spoken in the order in which
the URI are specified.
If the data to be spoken consists of a mix of URI and inline speech
data, the multipart/mixed MIME-type is used and embedded with the
MIME-blocks for text/uri-list, application/synthesis+ssml or
text/plain. The character set and encoding used in the speech data
may be specified according to standard MIME-type definitions. The
multi-part MIME-block can contain actual audio data in .wav or Sun
audio format. This is used when the client has audio clips that it
may have recorded, then stored in memory or a local device, and that
it currently needs to play as part of the SPEAK request. The audio
MIME-parts can be sent by the client as part of the multi-part MIME-
block. This audio will be referenced in the speech markup data that
will be another part in the multi-part MIME-block according to the
multipart/mixed MIME-type specification.
Example 1:
Content-Type:text/uri-list
Content-Length:176
http://www.cisco.com/ASR-Introduction.sml
http://www.cisco.com/ASR-Document-Part1.sml
http://www.cisco.com/ASR-Document-Part2.sml
http://www.cisco.com/ASR-Conclusion.sml
Example 2:
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
Shanmugham, et al. Informational [Page 30]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
Example 3:
Content-Type:multipart/mixed; boundary="--break"
--break
Content-Type:text/uri-list
Content-Length:176
http://www.cisco.com/ASR-Introduction.sml
http://www.cisco.com/ASR-Document-Part1.sml
http://www.cisco.com/ASR-Document-Part2.sml
http://www.cisco.com/ASR-Conclusion.sml
--break
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
--break
Shanmugham, et al. Informational [Page 31]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.6. SET-PARAMS
The SET-PARAMS method, from the client to server, tells the
synthesizer resource to define default synthesizer context
parameters, like voice characteristics and prosody, etc. If the
server accepted and set all parameters, it MUST return a Response-
Status of 200. If it chose to ignore some optional parameters, it
MUST return 201.
If some of the parameters being set are unsupported or have illegal
values, the server accepts and sets the remaining parameters and MUST
respond with a Response-Status of 403 or 404, and MUST include in the
response the header fields that could not be set.
Example:
C->S:SET-PARAMS 543256 MRCP/1.0
Voice-gender:female
Voice-category:adult
Voice-variant:3
S->C:MRCP/1.0 543256 200 COMPLETE
7.7. GET-PARAMS
The GET-PARAMS method, from the client to server, asks the
synthesizer resource for its current synthesizer context parameters,
like voice characteristics and prosody, etc. The client SHOULD send
the list of parameters it wants to read from the server by listing a
set of empty parameter header fields. If a specific list is not
specified then the server SHOULD return all the settable parameters
including vendor-specific parameters and their current values. The
wild card use can be very intensive as the number of settable
parameters can be large depending on the vendor. Hence, it is
RECOMMENDED that the client does not use the wildcard GET-PARAMS
operation very often.
Example:
C->S:GET-PARAMS 543256 MRCP/1.0
Voice-gender:
Voice-category:
Voice-variant:
Vendor-Specific-Parameters:com.mycorp.param1;
com.mycorp.param2
S->C:MRCP/1.0 543256 200 COMPLETE
Voice-gender:female
Voice-category:adult
Voice-variant:3
Shanmugham, et al. Informational [Page 32]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Vendor-Specific-Parameters:com.mycorp.param1="Company Name";
com.mycorp.param2="124324234@mycorp.com"
7.8. SPEAK
The SPEAK method from the client to the server provides the
synthesizer resource with the speech text and initiates speech
synthesis and streaming. The SPEAK method can carry voice and
prosody header fields that define the behavior of the voice being
synthesized, as well as the actual marked-up text to be spoken. If
specific voice and prosody parameters are specified as part of the
speech markup text, it will take precedence over the values specified
in the header fields and those set using a previous SET-PARAMS
request.
When applying voice parameters, there are 3 levels of scope. The
highest precedence are those specified within the speech markup text,
followed by those specified in the header fields of the SPEAK request
and, hence, apply for that SPEAK request only, followed by the
session default values that can be set using the SET-PARAMS request
and apply for the whole session moving forward.
If the resource is idle and the SPEAK request is being actively
processed, the resource will respond with a success status code and a
request-state of IN-PROGRESS.
If the resource is in the speaking or paused states (i.e., it is in
the middle of processing a previous SPEAK request), the status
returns success and a request-state of PENDING. This means that this
SPEAK request is in queue and will be processed after the currently
active SPEAK request is completed.
For the synthesizer resource, this is the only request that can
return a request-state of IN-PROGRESS or PENDING. When the text to
be synthesized is complete, the resource will issue a SPEAK-COMPLETE
event with the request-id of the SPEAK message and a request-state of
COMPLETE.
Example:
C->S:SPEAK 543257 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
Shanmugham, et al. Informational [Page 33]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543257 200 IN-PROGRESS
S->C:SPEAK-COMPLETE 543257 COMPLETE MRCP/1.0
Completion-Cause:000 normal
7.9. STOP
The STOP method from the client to the server tells the resource to
stop speaking if it is speaking something.
The STOP request can be sent with an active-request-id-list header
field to stop the zero or more specific SPEAK requests that may be in
queue and return a response code of 200(Success). If no active-
request-id-list header field is sent in the STOP request, it will
terminate all outstanding SPEAK requests.
If a STOP request successfully terminated one or more PENDING or
IN-PROGRESS SPEAK requests, then the response message body contains
an active-request-id-list header field listing the SPEAK request-ids
that were terminated. Otherwise, there will be no active-request-
id-list header field in the response. No SPEAK-COMPLETE events will
be sent for these terminated requests.
If a SPEAK request that was IN-PROGRESS and speaking was stopped, the
next pending SPEAK request, if any, would become IN-PROGRESS and move
to the speaking state.
If a SPEAK request that was IN-PROGRESS and in the paused state was
stopped, the next pending SPEAK request, if any, would become
IN-PROGRESS and move to the paused state.
Shanmugham, et al. Informational [Page 34]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Example:
C->S:SPEAK 543258 MRCP/1.0
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543258 200 IN-PROGRESS
C->S:STOP 543259 200 MRCP/1.0
S->C:MRCP/1.0 543259 200 COMPLETE
Active-Request-Id-List:543258
7.10. BARGE-IN-OCCURRED
The BARGE-IN-OCCURRED method is a mechanism for the client to
communicate a barge-in-able event it detects to the speech resource.
This event is useful in two scenarios,
1. The client has detected some events like DTMF digits or other
barge-in-able events and wants to communicate that to the
synthesizer.
2. The recognizer resource and the synthesizer resource are in
different servers. In which case the client MUST act as a Proxy
and receive event from the recognition resource, and then send a
BARGE-IN-OCCURRED method to the synthesizer. In such cases, the
BARGE-IN-OCCURRED method would also have a proxy-sync-id header
field received from the resource generating the original event.
If a SPEAK request is active with kill-on-barge-in enabled, and the
BARGE-IN-OCCURRED event is received, the synthesizer should stop
streaming out audio. It should also terminate any speech requests
queued behind the current active one, irrespective of whether they
Shanmugham, et al. Informational [Page 35]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
have barge-in enabled or not. If a barge-in-able prompt was playing
and it was terminated, the response MUST contain the request-ids of
all SPEAK requests that were terminated in its active-request-id-
list. There will be no SPEAK-COMPLETE events generated for these
requests.
If the synthesizer and the recognizer are on the same server, they
could be optimized for a quicker kill-on-barge-in response by having
them interact directly based on a common RTSP session-id. In these
cases, the client MUST still proxy the recognition event through a
BARGE-IN-OCCURRED method, but the synthesizer resource may have
already stopped and sent a SPEAK-COMPLETE event with a barge-in
completion cause code. If there were no SPEAK requests terminated as
a result of the BARGE-IN-OCCURRED method, the response would still be
a 200 success, but MUST not contain an active-request-id-list header
field.
C->S:SPEAK 543258 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543258 200 IN-PROGRESS
C->S:BARGE-IN-OCCURRED 543259 200 MRCP/1.0
Proxy-Sync-Id:987654321
S->C:MRCP/1.0 543259 200 COMPLETE
Active-Request-Id-List:543258
Shanmugham, et al. Informational [Page 36]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.11. PAUSE
The PAUSE method from the client to the server tells the resource to
pause speech, if it is speaking something. If a PAUSE method is
issued on a session when a SPEAK is not active, the server SHOULD
respond with a status of 402 or "Method not valid in this state". If
a PAUSE method is issued on a session when a SPEAK is active and
paused, the server SHOULD respond with a status of 200 or "Success".
If a SPEAK request was active, the server MUST return an active-
request-id-list header with the request-id of the SPEAK request that
was paused.
C->S:SPEAK 543258 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543258 200 IN-PROGRESS
C->S:PAUSE 543259 MRCP/1.0
S->C:MRCP/1.0 543259 200 COMPLETE
Active-Request-Id-List:543258
7.12. RESUME
The RESUME method from the client to the server tells a paused
synthesizer resource to continue speaking. If a RESUME method is
issued on a session when a SPEAK is not active, the server SHOULD
respond with a status of 402 or "Method not valid in this state". If
a RESUME method is issued on a session when a SPEAK is active and
speaking (i.e., not paused), the server SHOULD respond with a status
Shanmugham, et al. Informational [Page 37]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
of 200 or "Success". If a SPEAK request was active, the server MUST
return an active-request-id-list header with the request-id of the
SPEAK request that was resumed
Example:
C->S:SPEAK 543258 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543258 200 IN-PROGRESS
C->S:PAUSE 543259 MRCP/1.0
S->C:MRCP/1.0 543259 200 COMPLETE
Active-Request-Id-List:543258
C->S:RESUME 543260 MRCP/1.0
S->C:MRCP/1.0 543260 200 COMPLETE
Active-Request-Id-List:543258
7.13. CONTROL
The CONTROL method from the client to the server tells a synthesizer
that is speaking to modify what it is speaking on the fly. This
method is used to make the synthesizer jump forward or backward in
what it is being spoken, change speaker rate and speaker parameters,
etc. It affects the active or IN-PROGRESS SPEAK request. Depending
on the implementation and capability of the synthesizer resource, it
may allow this operation or one or more of its parameters.
Shanmugham, et al. Informational [Page 38]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
When a CONTROL to jump forward is issued and the operation goes
beyond the end of the active SPEAK method's text, the request
succeeds. A SPEAK-COMPLETE event follows the response to the CONTROL
method. If there are more SPEAK requests in the queue, the
synthesizer resource will continue to process the next SPEAK method.
When a CONTROL to jump backwards is issued and the operation jumps to
the beginning of the speech data of the active SPEAK request, the
response to the CONTROL request contains the speak-restart header.
These two behaviors can be used to rewind or fast-forward across
multiple speech requests, if the client wants to break up a speech
markup text into multiple SPEAK requests.
If a SPEAK request was active when the CONTROL method was received,
the server MUST return an active-request-id-list header with the
Request-id of the SPEAK request that was active.
Example:
C->S:SPEAK 543258 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543258 200 IN-PROGRESS
C->S:CONTROL 543259 MRCP/1.0
Prosody-rate:fast
S->C:MRCP/1.0 543259 200 COMPLETE
Active-Request-Id-List:543258
C->S:CONTROL 543260 MRCP/1.0
Shanmugham, et al. Informational [Page 39]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Jump-Size:-15 Words
S->C:MRCP/1.0 543260 200 COMPLETE
Active-Request-Id-List:543258
7.14. SPEAK-COMPLETE
This is an Event message from the synthesizer resource to the client
indicating that the SPEAK request was completed. The request-id
header field WILL match the request-id of the SPEAK request that
initiated the speech that just completed. The request-state field
should be COMPLETE indicating that this is the last Event with that
request-id, and that the request with that request-id is now
complete. The completion-cause header field specifies the cause code
pertaining to the status and reason of request completion such as the
SPEAK completed normally or because of an error or kill-on-barge-in,
etc.
Example:
C->S:SPEAK 543260 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:MRCP/1.0 543260 200 IN-PROGRESS
S->C:SPEAK-COMPLETE 543260 COMPLETE MRCP/1.0
Completion-Cause:000 normal
Shanmugham, et al. Informational [Page 40]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
7.15. SPEECH-MARKER
This is an event generated by the synthesizer resource to the client
when it hits a marker tag in the speech markup it is currently
processing. The request-id field in the header matches the SPEAK
request request-id that initiated the speech. The request-state
field should be IN-PROGRESS as the speech is still not complete and
there is more to be spoken. The actual speech marker tag hit,
describing where the synthesizer is in the speech markup, is returned
in the speech-marker header field.
Example:
C->S:SPEAK 543261 MRCP/1.0
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<mark name="here"/>
<sentence>The subject is
<prosody rate="-20%">ski trip</prosody>
</sentence>
<mark name="ANSWER"/>
</paragraph>
</speak>
S->C:MRCP/1.0 543261 200 IN-PROGRESS
S->C:SPEECH-MARKER 543261 IN-PROGRESS MRCP/1.0
Speech-Marker:here
S->C:SPEECH-MARKER 543261 IN-PROGRESS MRCP/1.0
Speech-Marker:ANSWER
S->C:SPEAK-COMPLETE 543261 COMPLETE MRCP/1.0
Completion-Cause:000 normal
Shanmugham, et al. Informational [Page 41]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8. Speech Recognizer Resource
The speech recognizer resource is capable of receiving an incoming
voice stream and providing the client with an interpretation of what
was spoken in textual form.
8.1. Recognizer State Machine
The recognizer resource is controlled by MRCP requests from the
client. Similarly, the resource can respond to these requests or
generate asynchronous events to the server to indicate certain
conditions during the processing of the stream. Hence, the
recognizer maintains states to correlate MRCP requests from the
client. The state transitions are described below.
Idle Recognizing Recognized
State State State
| | |
|---------RECOGNIZE---->|---RECOGNITION-COMPLETE-->|
|<------STOP------------|<-----RECOGNIZE-----------|
| | |
| | |-----------|
| |--------| GET-RESULT |
| START-OF-SPEECH | |---------->|
|------------| |------->| |
| | |----------| |
| DEFINE-GRAMMAR | RECOGNITION-START-TIMERS |
|<-----------| |<---------| |
| | |
| | |
|-------| | |
| STOP | |
|<------| | |
| |
|<-------------------STOP--------------------------|
|<-------------------DEFINE-GRAMMAR----------------|
8.2. Recognizer Methods
The recognizer supports the following methods.
recognizer-method = SET-PARAMS
/ GET-PARAMS
/ DEFINE-GRAMMAR
/ RECOGNIZE
/ GET-RESULT
/ RECOGNITION-START-TIMERS
/ STOP
Shanmugham, et al. Informational [Page 42]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.3. Recognizer Events
The recognizer may generate the following events.
recognizer-event = START-OF-SPEECH
/ RECOGNITION-COMPLETE
8.4. Recognizer Header Fields
A recognizer message may contain header fields containing request
options and information to augment the Method, Response, or Event
message it is associated with.
recognizer-header = confidence-threshold ; Section 8.4.1
/ sensitivity-level ; Section 8.4.2
/ speed-vs-accuracy ; Section 8.4.3
/ n-best-list-length ; Section 8.4.4
/ no-input-timeout ; Section 8.4.5
/ recognition-timeout ; Section 8.4.6
/ waveform-url ; Section 8.4.7
/ completion-cause ; Section 8.4.8
/ recognizer-context-block ; Section 8.4.9
/ recognizer-start-timers ; Section 8.4.10
/ vendor-specific ; Section 8.4.11
/ speech-complete-timeout ; Section 8.4.12
/ speech-incomplete-timeout; Section 8.4.13
/ dtmf-interdigit-timeout ; Section 8.4.14
/ dtmf-term-timeout ; Section 8.4.15
/ dtmf-term-char ; Section 8.4.16
/ fetch-timeout ; Section 8.4.17
/ failed-uri ; Section 8.4.18
/ failed-uri-cause ; Section 8.4.19
/ save-waveform ; Section 8.4.20
/ new-audio-channel ; Section 8.4.21
/ speech-language ; Section 8.4.22
Parameter Support Methods/Events
confidence-threshold MANDATORY SET-PARAMS, RECOGNIZE
GET-RESULT
sensitivity-level Optional SET-PARAMS, GET-PARAMS,
RECOGNIZE
speed-vs-accuracy Optional SET-PARAMS, GET-PARAMS,
RECOGNIZE
n-best-list-length Optional SET-PARAMS, GET-PARAMS,
RECOGNIZE, GET-RESULT
no-input-timeout MANDATORY SET-PARAMS, GET-PARAMS,
RECOGNIZE
Shanmugham, et al. Informational [Page 43]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
recognition-timeout MANDATORY SET-PARAMS, GET-PARAMS,
RECOGNIZE
waveform-url MANDATORY RECOGNITION-COMPLETE
completion-cause MANDATORY DEFINE-GRAMMAR, RECOGNIZE,
RECOGNITON-COMPLETE
recognizer-context-block Optional SET-PARAMS, GET-PARAMS
recognizer-start-timers MANDATORY RECOGNIZE
vendor-specific MANDATORY SET-PARAMS, GET-PARAMS
speech-complete-timeout MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE
speech-incomplete-timeout MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE
dtmf-interdigit-timeout MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE
dtmf-term-timeout MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE
dtmf-term-char MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE
fetch-timeout MANDATORY SET-PARAMS, GET-PARAMS
RECOGNIZE, DEFINE-GRAMMAR
failed-uri MANDATORY DEFINE-GRAMMAR response,
RECOGNITION-COMPLETE
failed-uri-cause MANDATORY DEFINE-GRAMMAR response,
RECOGNITION-COMPLETE
save-waveform MANDATORY SET-PARAMS, GET-PARAMS,
RECOGNIZE
new-audio-channel MANDATORY RECOGNIZE
speech-language MANDATORY SET-PARAMS, GET-PARAMS,
RECOGNIZE, DEFINE-GRAMMAR
8.4.1. Confidence Threshold
When a recognition resource recognizes or matches a spoken phrase
with some portion of the grammar, it associates a confidence level
with that conclusion. The confidence-threshold parameter tells the
recognizer resource what confidence level should be considered a
successful match. This is an integer from 0-100 indicating the
recognizer's confidence in the recognition. If the recognizer
determines that its confidence in all its recognition results is less
than the confidence threshold, then it MUST return no-match as the
recognition result. This header field MAY occur in RECOGNIZE, SET-
PARAMS, or GET-PARAMS. The default value for this field is platform
specific.
confidence-threshold = "Confidence-Threshold" ":" 1*DIGIT CRLF
Shanmugham, et al. Informational [Page 44]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.4.2. Sensitivity Level
To filter out background noise and not mistake it for speech, the
recognizer may support a variable level of sound sensitivity. The
sensitivity-level parameter allows the client to set this value on
the recognizer. This header field MAY occur in RECOGNIZE, SET-
PARAMS, or GET-PARAMS. A higher value for this field means higher
sensitivity. The default value for this field is platform specific.
sensitivity-level = "Sensitivity-Level" ":" 1*DIGIT CRLF
8.4.3. Speed Vs Accuracy
Depending on the implementation and capability of the recognizer
resource, it may be tunable towards Performance or Accuracy. Higher
accuracy may mean more processing and higher CPU utilization, meaning
less calls per media server and vice versa. This parameter on the
resource can be tuned by the speed-vs-accuracy header. This header
field MAY occur in RECOGNIZE, SET-PARAMS, or GET-PARAMS. A higher
value for this field means higher speed. The default value for this
field is platform specific.
speed-vs-accuracy = "Speed-Vs-Accuracy" ":" 1*DIGIT CRLF
8.4.4. N Best List Length
When the recognizer matches an incoming stream with the grammar, it
may come up with more than one alternative match because of
confidence levels in certain words or conversation paths. If this
header field is not specified, by default, the recognition resource
will only return the best match above the confidence threshold. The
client, by setting this parameter, could ask the recognition resource
to send it more than 1 alternative. All alternatives must still be
above the confidence-threshold. A value greater than one does not
guarantee that the recognizer will send the requested number of
alternatives. This header field MAY occur in RECOGNIZE, SET-PARAMS,
or GET-PARAMS. The minimum value for this field is 1. The default
value for this field is 1.
n-best-list-length = "N-Best-List-Length" ":" 1*DIGIT CRLF
8.4.5. No Input Timeout
When recognition is started and there is no speech detected for a
certain period of time, the recognizer can send a RECOGNITION-
COMPLETE event to the client and terminate the recognition operation.
The no-input-timeout header field can set this timeout value. The
value is in milliseconds. This header field MAY occur in RECOGNIZE,
Shanmugham, et al. Informational [Page 45]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
SET-PARAMS, or GET-PARAMS. The value for this field ranges from 0 to
MAXTIMEOUT, where MAXTIMEOUT is platform specific. The default value
for this field is platform specific.
no-input-timeout = "No-Input-Timeout" ":" 1*DIGIT CRLF
8.4.6. Recognition Timeout
When recognition is started and there is no match for a certain
period of time, the recognizer can send a RECOGNITION-COMPLETE event
to the client and terminate the recognition operation. The
recognition-timeout parameter field sets this timeout value. The
value is in milliseconds. The value for this field ranges from 0 to
MAXTIMEOUT, where MAXTIMEOUT is platform specific. The default value
is 10 seconds. This header field MAY occur in RECOGNIZE, SET-PARAMS
or GET-PARAMS.
recognition-timeout = "Recognition-Timeout" ":" 1*DIGIT CRLF
8.4.7. Waveform URL
If the save-waveform header field is set to true, the recognizer MUST
record the incoming audio stream of the recognition into a file and
provide a URI for the client to access it. This header MUST be
present in the RECOGNITION-COMPLETE event if the save-waveform header
field was set to true. The URL value of the header MUST be NULL if
there was some error condition preventing the server from recording.
Otherwise, the URL generated by the server SHOULD be globally unique
across the server and all its recognition sessions. The URL SHOULD
BE available until the session is torn down.
waveform-url = "Waveform-URL" ":" Url CRLF
8.4.8. Completion Cause
This header field MUST be part of a RECOGNITION-COMPLETE event coming
from the recognizer resource to the client. This indicates the
reason behind the RECOGNIZE method completion. This header field
MUST BE sent in the DEFINE-GRAMMAR and RECOGNIZE responses, if they
return with a failure status and a COMPLETE state.
Cause-Code Cause-Name Description
000 success RECOGNIZE completed with a match or
DEFINE-GRAMMAR succeeded in
downloading and compiling the
grammar
Shanmugham, et al. Informational [Page 46]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
001 no-match RECOGNIZE completed, but no match
was found
002 no-input-timeout
RECOGNIZE completed without a match
due to a no-input-timeout
003 recognition-timeout
RECOGNIZE completed without a match
due to a recognition-timeout
004 gram-load-failure
RECOGNIZE failed due grammar load
failure.
005 gram-comp-failure
RECOGNIZE failed due to grammar
compilation failure.
006 error RECOGNIZE request terminated
prematurely due to a recognizer
error.
007 speech-too-early
RECOGNIZE request terminated because
speech was too early.
008 too-much-speech-timeout
RECOGNIZE request terminated because
speech was too long.
009 uri-failure Failure accessing a URI.
010 language-unsupported
Language not supported.
8.4.9. Recognizer Context Block
This parameter MAY BE sent as part of the SET-PARAMS or GET-PARAMS
request. If the GET-PARAMS method contains this header field with no
value, then it is a request to the recognizer to return the
recognizer context block. The response to such a message MAY contain
a recognizer context block as a message entity. If the server
returns a recognizer context block, the response MUST contain this
header field and its value MUST match the content-id of that entity.
If the SET-PARAMS method contains this header field, it MUST contain
a message entity containing the recognizer context data, and a
content-id matching this header field.
This content-id should match the content-id that came with the
context data during the GET-PARAMS operation.
recognizer-context-block = "Recognizer-Context-Block" ":"
1*ALPHA CRLF
Shanmugham, et al. Informational [Page 47]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.4.10. Recognition Start Timers
This parameter MAY BE sent as part of the RECOGNIZE request. A value
of false tells the recognizer to start recognition, but not to start
the no-input timer yet. The recognizer should not start the timers
until the client sends a RECOGNITION-START-TIMERS request to the
recognizer. This is useful in the scenario when the recognizer and
synthesizer engines are not part of the same session. Here, when a
kill-on-barge-in prompt is being played, you want the RECOGNIZE
request to be simultaneously active so that it can detect and
implement kill-on-barge-in. But at the same time, you don't want the
recognizer to start the no-input timers until the prompt is finished.
The default value is "true".
recognizer-start-timers = "Recognizer-Start-Timers" ":"
boolean-value CRLF
8.4.11. Vendor Specific Parameters
This set of headers allows the client to set Vendor Specific
parameters.
This header can be sent in the SET-PARAMS method and is used to set
vendor-specific parameters on the server. The vendor-av-pair-name
can be any vendor-specific field name and conforms to the XML
vendor-specific attribute naming convention. The vendor-av-pair-
value is the value to set the attribute to, and needs to be quoted.
When asking the server to get the current value of these parameters,
this header can be sent in the GET-PARAMS method with the list of
vendor-specific attribute names to get separated by a semicolon.
This header field MAY occur in SET-PARAMS or GET-PARAMS.
8.4.12. Speech Complete Timeout
This header field specifies the length of silence required following
user speech before the speech recognizer finalizes a result (either
accepting it or throwing a nomatch event). The speech-complete-
timeout value is used when the recognizer currently has a complete
match of an active grammar, and specifies how long it should wait for
more input before declaring a match. By contrast, the incomplete
timeout is used when the speech is an incomplete match to an active
grammar. The value is in milliseconds.
speech-complete-timeout = "Speech-Complete-Timeout" ":"
1*DIGIT CRLF
Shanmugham, et al. Informational [Page 48]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
A long speech-complete-timeout value delays the result completion
and, therefore, makes the computer's response slow. A short speech-
complete-timeout may lead to an utterance being broken up
inappropriately. Reasonable complete timeout values are typically in
the range of 0.3 seconds to 1.0 seconds. The value for this field
ranges from 0 to MAXTIMEOUT, where MAXTIMEOUT is platform specific.
The default value for this field is platform specific. This header
field MAY occur in RECOGNIZE, SET-PARAMS, or GET-PARAMS.
8.4.13. Speech Incomplete Timeout
This header field specifies the required length of silence following
user speech, after which a recognizer finalizes a result. The
incomplete timeout applies when the speech prior to the silence is an
incomplete match of all active grammars. In this case, once the
timeout is triggered, the partial result is rejected (with a nomatch
event). The value is in milliseconds. The value for this field
ranges from 0 to MAXTIMEOUT, where MAXTIMEOUT is platform specific.
The default value for this field is platform specific.
speech-incomplete-timeout = "Speech-Incomplete-Timeout" ":"
1*DIGIT CRLF
The speech-incomplete-timeout also applies when the speech prior to
the silence is a complete match of an active grammar, but where it is
possible to speak further and still match the grammar. By contrast,
the complete timeout is used when the speech is a complete match to
an active grammar and no further words can be spoken.
A long speech-incomplete-timeout value delays the result completion
and, therefore, makes the computer's response slow. A short speech-
incomplete-timeout may lead to an utterance being broken up
inappropriately.
The speech-incomplete-timeout is usually longer than the speech-
complete-timeout to allow users to pause mid-utterance (for example,
to breathe). This header field MAY occur in RECOGNIZE, SET-PARAMS,
or GET-PARAMS.
8.4.14. DTMF Interdigit Timeout
This header field specifies the inter-digit timeout value to use when
recognizing DTMF input. The value is in milliseconds. The value for
this field ranges from 0 to MAXTIMEOUT, where MAXTIMEOUT is platform
specific. The default value is 5 seconds. This header field MAY
occur in RECOGNIZE, SET-PARAMS, or GET-PARAMS.
Shanmugham, et al. Informational [Page 49]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
dtmf-interdigit-timeout = "DTMF-Interdigit-Timeout" ":"
1*DIGIT CRLF
8.4.15. DTMF Term Timeout
This header field specifies the terminating timeout to use when
recognizing DTMF input. The value is in milliseconds. The value for
this field ranges from 0 to MAXTIMEOUT, where MAXTIMEOUT is platform
specific. The default value is 10 seconds. This header field MAY
occur in RECOGNIZE, SET-PARAMS, or GET-PARAMS.
dtmf-term-timeout = "DTMF-Term-Timeout" ":" 1*DIGIT CRLF
8.4.16. DTMF-Term-Char
This header field specifies the terminating DTMF character for DTMF
input recognition. The default value is NULL which is specified as
an empty header field. This header field MAY occur in RECOGNIZE,
SET-PARAMS, or GET-PARAMS.
dtmf-term-char = "DTMF-Term-Char" ":" CHAR CRLF
8.4.17. Fetch Timeout
When the recognizer needs to fetch grammar documents, this header
field controls URI access properties. This defines the recognizer
timeout for completing the fetch of the resources the media server
needs from the network. The value is in milliseconds. The value for
this field ranges from 0 to MAXTIMEOUT, where MAXTIMEOUT is platform
specific. The default value for this field is platform specific.
This header field MAY occur in RECOGNIZE, SET-PARAMS, or GET-PARAMS.
8.4.18. Failed URI
When a recognizer method needs a recognizer to fetch or access a URI,
and the access fails, the media server SHOULD provide the failed URI
in this header field in the method response.
8.4.19. Failed URI Cause
When a recognizer method needs a recognizer to fetch or access a URI,
and the access fails, the media server SHOULD provide the URI-
specific or protocol-specific response code through this header field
in the method response. This field has been defined as alphanumeric
to accommodate all protocols, some of which might have a response
string instead of a numeric response code.
Shanmugham, et al. Informational [Page 50]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.4.20. Save Waveform
This header field allows the client to indicate to the recognizer
that it MUST save the audio stream that was recognized. The
recognizer MUST then record the recognized audio and make it
available to the client in the form of a URI returned in the
waveform-uri header field in the RECOGNITION-COMPLETE event. If
there was an error in recording the stream or the audio clip is
otherwise not available, the recognizer MUST return an empty
waveform-uri header field. The default value for this fields is
"false".
save-waveform = "Save-Waveform" ":" boolean-value CRLF
8.4.21. New Audio Channel
This header field MAY BE specified in a RECOGNIZE message and allows
the client to tell the media server that, from that point on, it will
be sending audio data from a new audio source, channel, or speaker.
If the recognition resource had collected any line statistics or
information, it MUST discard it and start fresh for this RECOGNIZE.
This helps in the case where the client MAY want to reuse an open
recognition session with the media server for multiple telephone
calls.
new-audio-channel = "New-Audio-Channel" ":" boolean-value CRLF
8.4.22. Speech Language
This header field specifies the language of recognition grammar data
within a session or request, if it is not specified within the data.
The value of this header field should follow RFC 3066 [16] for its
values. This MAY occur in DEFINE-GRAMMAR, RECOGNIZE, SET-PARAMS, or
GET-PARAMS request.
8.5. Recognizer Message Body
A recognizer message may carry additional data associated with the
method, response, or event. The client may send the grammar to be
recognized in DEFINE-GRAMMAR or RECOGNIZE requests. When the grammar
is sent in the DEFINE-GRAMMAR method, the server should be able to
download compile and optimize the grammar. The RECOGNIZE request
MUST contain a list of grammars that need to be active during the
recognition. The server resource may send the recognition results in
the RECOGNITION-COMPLETE event or the GET-RESULT response. This data
will be carried in the message body of the corresponding MRCP
message.
Shanmugham, et al. Informational [Page 51]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.5.1. Recognizer Grammar Data
Recognizer grammar data from the client to the server can be provided
inline or by reference. Either way, they are carried as MIME
entities in the message body of the MRCP request message. The
grammar specified inline or by reference specifies the grammar used
to match in the recognition process and this data is specified in one
of the standard grammar specification formats like W3C's XML or ABNF
or Sun's Java Speech Grammar Format, etc. All media servers MUST
support W3C's XML based grammar markup format [11] (MIME-type
application/grammar+xml) and SHOULD support the ABNF form (MIME-type
application/grammar).
When a grammar is specified in-line in the message, the client MUST
provide a content-id for that grammar as part of the content headers.
The server MUST store the grammar associated with that content-id for
the duration of the session. A stored grammar can be overwritten by
defining a new grammar with the same content-id. Grammars that have
been associated with a content-id can be referenced through a special
"session:" URI scheme.
Example:
session:help@root-level.store
If grammar data needs to be specified by external URI reference, the
MIME-type text/uri-list is used to list the one or more URI that will
specify the grammar data. All media servers MUST support the HTTP
URI access mechanism.
If the data to be defined consists of a mix of URI and inline grammar
data, the multipart/mixed MIME-type is used and embedded with the
MIME-blocks for text/uri-list, application/grammar or
application/grammar+xml. The character set and encoding used in the
grammar data may be specified according to standard MIME-type
definitions.
When more than one grammar URI or inline grammar block is specified
in a message body of the RECOGNIZE request, it is an active list of
grammar alternatives to listen. The ordering of the list implies the
precedence of the grammars, with the first grammar in the list having
the highest precedence.
Example 1:
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
Shanmugham, et al. Informational [Page 52]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
<!-- multiple language attachment to a token -->
<rule id="people1">
<token lexicon="en-US,fr-CA"> Robert </token>
</rule>
<!-- the equivalent single-language attachment expansion -->
<rule id="people2">
<one-of>
<item xml:lang="en-US">Robert</item>
<item xml:lang="fr-CA">Robert</item>
</one-of>
</rule>
</grammar>
Example 2:
Content-Type:text/uri-list
Content-Length:176
session:help@root-level.store
http://www.cisco.com/Directory-Name-List.grxml
http://www.cisco.com/Department-List.grxml
http://www.cisco.com/TAC-Contact-List.grxml
session:menu1@menu-level.store
Example 3:
Content-Type:multipart/mixed; boundary="--break"
Shanmugham, et al. Informational [Page 53]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
--break
Content-Type:text/uri-list
Content-Length:176
http://www.cisco.com/Directory-Name-List.grxml
http://www.cisco.com/Department-List.grxml
http://www.cisco.com/TAC-Contact-List.grxml
--break
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
<!-- multiple language attachment to a token -->
<rule id="people1">
<token lexicon="en-US,fr-CA"> Robert </token>
</rule>
<!-- the equivalent single-language attachment expansion -->
<rule id="people2">
<one-of>
<item xml:lang="en-US">Robert</item>
<item xml:lang="fr-CA">Robert</item>
</one-of>
</rule>
Shanmugham, et al. Informational [Page 54]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
</grammar>
--break
8.5.2. Recognizer Result Data
Recognition result data from the server is carried in the MRCP
message body of the RECOGNITION-COMPLETE event or the GET-RESULT
response message as MIME entities. All media servers MUST support
W3C's Natural Language Semantics Markup Language (NLSML) [10] as the
default standard for returning recognition results back to the
client, and hence MUST support the MIME-type application/x-nlsml.
Example 1:
Content-Type:application/x-nlsml
Content-Length:104
<?xml version="1.0"?>
<result grammar="http://theYesNoGrammar">
<interpretation>
<instance>
<myApp:yes_no>
<response>yes</response>
</myApp:yes_no>
</instance>
<input>ok</input>
</interpretation>
</result>
8.5.3. Recognizer Context Block
When the client has to change recognition servers within a call, this
is a block of data that the client MAY collect from the first media
server and provide to the second media server. This may be because
the client needs different language support or because the media
server issued an RTSP RE-DIRECT. Here, the first recognizer may have
collected acoustic and other data during its recognition. When we
switch recognition servers, communicating this data may allow the
second recognition server to provide better recognition based on the
acoustic data collected by the previous recognizer. This block of
data is vendor-specific and MUST be carried as MIME-type
application/octets in the body of the message.
This block of data is communicated in the SET-PARAMS and GET-PARAMS
method/response messages. In the GET-PARAMS method, if an empty
recognizer-context-block header field is present, then the recognizer
should return its vendor-specific context block in the message body
as a MIME-entity with a specific content-id. The content-id value
should also be specified in the recognizer-context-block header field
Shanmugham, et al. Informational [Page 55]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
in the GET-PARAMS response. The SET-PARAMS request wishing to
provide this vendor-specific data should send it in the message body
as a MIME-entity with the same content-id that it received from the
GET-PARAMS. The content-id should also be sent in the recognizer-
context-block header field of the SET-PARAMS message.
Each automatic speech recognition (ASR) vendor choosing to use this
mechanism to handoff recognizer context data among its servers should
distinguish its vendor-specific block of data from other vendors by
choosing a unique content-id that they should recognize.
8.6. SET-PARAMS
The SET-PARAMS method, from the client to the server, tells the
recognizer resource to set and modify recognizer context parameters
like recognizer characteristics, result detail level, etc. In the
following sections some standard parameters are discussed. If the
server resource does not recognize an OPTIONAL parameter, it MUST
ignore that field. Many of the parameters in the SET-PARAMS method
can also be used in another method like the RECOGNIZE method. But
the difference is that when you set something like the sensitivity-
level using the SET-PARAMS, it applies for all future requests,
whenever applicable. On the other hand, when you pass sensitivity-
level in a RECOGNIZE request, it applies only to that request.
Example:
C->S:SET-PARAMS 543256 MRCP/1.0
Sensitivity-Level:20
Recognition-Timeout:30
Confidence-Threshold:85
S->C:MRCP/1.0 543256 200 COMPLETE
8.7. GET-PARAMS
The GET-PARAMS method, from the client to the server, asks the
recognizer resource for its current default parameters, like
sensitivity-level, n-best-list-length, etc. The client can request
specific parameters from the server by sending it one or more empty
parameter headers with no values. The server should then return the
settings for those specific parameters only. When the client does
not send a specific list of empty parameter headers, the recognizer
should return the settings for all parameters. The wild card use can
be very intensive as the number of settable parameters can be large
depending on the vendor. Hence, it is RECOMMENDED that the client
does not use the wildcard GET-PARAMS operation very often.
Shanmugham, et al. Informational [Page 56]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Example:
C->S:GET-PARAMS 543256 MRCP/1.0
Sensitivity-Level:
Recognition-Timeout:
Confidence-threshold:
S->C:MRCP/1.0 543256 200 COMPLETE
Sensitivity-Level:20
Recognition-Timeout:30
Confidence-Threshold:85
8.8. DEFINE-GRAMMAR
The DEFINE-GRAMMAR method, from the client to the server, provides a
grammar and tells the server to define, download if needed, and
compile the grammar.
If the server resource is in the recognition state, the DEFINE-
GRAMMAR request MUST respond with a failure status.
If the resource is in the idle state and is able to successfully load
and compile the grammar, the status MUST return a success code and
the request-state MUST be COMPLETE.
If the recognizer could not define the grammar for some reason, say
the download failed or the grammar failed to compile, or the grammar
was in an unsupported form, the MRCP response for the DEFINE-GRAMMAR
method MUST contain a failure status code of 407, and a completion-
cause header field describing the failure reason.
Example:
C->S:DEFINE-GRAMMAR 543257 MRCP/1.0
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
Shanmugham, et al. Informational [Page 57]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
</grammar>
S->C:MRCP/1.0 543257 200 COMPLETE
Completion-Cause:000 success
C->S:DEFINE-GRAMMAR 543258 MRCP/1.0
Content-Type:application/grammar+xml
Content-Id:helpgrammar@root-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<rule id="request">
I need help
</rule>
</grammar>
S->C:MRCP/1.0 543258 200 COMPLETE
Completion-Cause:000 success
C->S:DEFINE-GRAMMAR 543259 MRCP/1.0
Content-Type:application/grammar+xml
Content-Id:request2@field-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<rule id="request">
I need help
</rule>
S->C:MRCP/1.0 543258 200 COMPLETE
Shanmugham, et al. Informational [Page 58]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Completion-Cause:000 success
C->S:DEFINE-GRAMMAR 543259 MRCP/1.0
Content-Type:application/grammar+xml
Content-Id:request2@field-level.store
Content-Length:104
<?xml version="1.0"?>
<grammar xml:lang="en">
<import uri="session:politeness@form-level.store"
name="polite"/>
<rule id="basicCmd" scope="public">
<example> please move the window </example>
<example> open a file </example>
<ruleref import="polite#startPolite"/>
<ruleref uri="#command"/>
<ruleref import="polite#endPolite"/>
</rule>
<rule id="command">
<ruleref uri="#action"/> <ruleref uri="#object"/>
</rule>
<rule id="action">
<choice>
<item weight="10" tag="OPEN"> open </item>
<item weight="2" tag="CLOSE"> close </item>
<item weight="1" tag="DELETE"> delete </item>
<item weight="1" tag="MOVE"> move </item>
</choice>
</rule>
<rule id="object">
<count number="optional">
<choice>
<item> the </item>
<item> a </item>
</choice>
</count>
<choice>
<item> window </item>
<item> file </item>
<item> menu </item>
</choice>
Shanmugham, et al. Informational [Page 59]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
</rule>
</grammar>
S->C:MRCP/1.0 543259 200 COMPLETE
Completion-Cause:000 success
C->S:RECOGNIZE 543260 MRCP/1.0
N-Best-List-Length:2
Content-Type:text/uri-list
Content-Length:176
session:request1@form-level.store
session:request2@field-level.store
session:helpgramar@root-level.store
S->C:MRCP/1.0 543260 200 IN-PROGRESS
S->C:START-OF-SPEECH 543260 IN-PROGRESS MRCP/1.0
S->C:RECOGNITION-COMPLETE 543260 COMPLETE MRCP/1.0
Completion-Cause:000 success
Waveform-URL:http://web.media.com/session123/audio.wav
Content-Type:applicationt/x-nlsml
Content-Length:276
<?xml version="1.0"?>
<result x-model="http://IdentityModel"
xmlns:xf="http://www.w3.org/2000/xforms"
grammar="session:request1@form-level.store">
<interpretation>
<xf:instance name="Person">
<Person>
<Name> Andre Roy </Name>
</Person>
</xf:instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>
8.9. RECOGNIZE
The RECOGNIZE method from the client to the server tells the
recognizer to start recognition and provides it with a grammar to
match for. The RECOGNIZE method can carry parameters to control the
sensitivity, confidence level, and the level of detail in results
provided by the recognizer. These parameters override the current
defaults set by a previous SET-PARAMS method.
Shanmugham, et al. Informational [Page 60]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
If the resource is in the recognition state, the RECOGNIZE request
MUST respond with a failure status.
If the resource is in the Idle state and was able to successfully
start the recognition, the server MUST return a success code and a
request-state of IN-PROGRESS. This means that the recognizer is
active and that the client should expect further events with this
request-id.
If the resource could not start a recognition, it MUST return a
failure status code of 407 and contain a completion-cause header
field describing the cause of failure.
For the recognizer resource, this is the only request that can return
request-state of IN-PROGRESS, meaning that recognition is in
progress. When the recognition completes by matching one of the
grammar alternatives or by a time-out without a match or for some
other reason, the recognizer resource MUST send the client a
RECOGNITON-COMPLETE event with the result of the recognition and a
request-state of COMPLETE.
For large grammars that can take a long time to compile and for
grammars that are used repeatedly, the client could issue a DEFINE-
GRAMMAR request with the grammar ahead of time. In such a case, the
client can issue the RECOGNIZE request and reference the grammar
through the "session:" special URI. This also applies in general if
the client wants to restart recognition with a previous inline
grammar.
Note that since the audio and the messages are carried over separate
communication paths there may be a race condition between the start
of the flow of audio and the receipt of the RECOGNIZE method. For
example, if audio flow is started by the client at the same time as
the RECOGNIZE method is sent, either the audio or the RECOGNIZE will
arrive at the recognizer first. As another example, the client may
chose to continuously send audio to the Media server and signal the
Media server to recognize using the RECOGNIZE method. A number of
mechanisms exist to resolve this condition and the mechanism chosen
is left to the implementers of recognizer Media servers.
Example:
C->S:RECOGNIZE 543257 MRCP/1.0
Confidence-Threshold:90
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
Shanmugham, et al. Informational [Page 61]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
</grammar>
S->C:MRCP/1.0 543257 200 IN-PROGRESS
S->C:START-OF-SPEECH 543257 IN-PROGRESS MRCP/1.0
S->C:RECOGNITION-COMPLETE 543257 COMPLETE MRCP/1.0
Completion-Cause:000 success
Waveform-URL:http://web.media.com/session123/audio.wav
Content-Type:application/x-nlsml
Content-Length:276
<?xml version="1.0"?>
<result x-model="http://IdentityModel"
xmlns:xf="http://www.w3.org/2000/xforms"
grammar="session:request1@form-level.store">
<interpretation>
<xf:instance name="Person">
<Person>
<Name> Andre Roy </Name>
</Person>
</xf:instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>
Shanmugham, et al. Informational [Page 62]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.10. STOP
The STOP method from the client to the server tells the resource to
stop recognition if one is active. If a RECOGNIZE request is active
and the STOP request successfully terminated it, then the response
header contains an active-request-id-list header field containing the
request-id of the RECOGNIZE request that was terminated. In this
case, no RECOGNITION-COMPLETE event will be sent for the terminated
request. If there was no recognition active, then the response MUST
NOT contain an active-request-id-list header field. Either
way,method the response MUST contain a status of 200(Success).
Example:
C->S:RECOGNIZE 543257 MRCP/1.0
Confidence-Threshold:90
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
</grammar>
S->C:MRCP/1.0 543257 200 IN-PROGRESS
C->S:STOP 543258 200 MRCP/1.0
Shanmugham, et al. Informational [Page 63]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
S->C:MRCP/1.0 543258 200 COMPLETE
Active-Request-Id-List:543257
8.11. GET-RESULT
The GET-RESULT method from the client to the server can be issued
when the recognizer is in the recognized state. This request allows
the client to retrieve results for a completed recognition. This is
useful if the client decides it wants more alternatives or more
information. When the media server receives this request, it should
re-compute and return the results according to the recognition
constraints provided in the GET-RESULT request.
The GET-RESULT request could specify constraints like a different
confidence-threshold, or n-best-list-length. This feature is
optional and the automatic speech recognition (ASR) engine may return
a status of unsupported feature.
Example:
C->S:GET-RESULT 543257 MRCP/1.0
Confidence-Threshold:90
S->C:MRCP/1.0 543257 200 COMPLETE
Content-Type:application/x-nlsml
Content-Length:276
<?xml version="1.0"?>
<result x-model="http://IdentityModel"
xmlns:xf="http://www.w3.org/2000/xforms"
grammar="session:request1@form-level.store">
<interpretation>
<xf:instance name="Person">
<Person>
<Name> Andre Roy </Name>
</Person>
</xf:instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>
8.12. START-OF-SPEECH
This is an event from the recognizer to the client indicating that it
has detected speech. This event is useful in implementing kill-on-
barge-in scenarios when the synthesizer resource is in a different
session than the recognizer resource and, hence, is not aware of an
incoming audio source. In these cases, it is up to the client to act
Shanmugham, et al. Informational [Page 64]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
as a proxy and turn around and issue the BARGE-IN-OCCURRED method to
the synthesizer resource. The recognizer resource also sends a
unique proxy-sync-id in the header for this event, which is sent to
the synthesizer in the BARGE-IN-OCCURRED method to the synthesizer.
This event should be generated irrespective of whether the
synthesizer and recognizer are in the same media server or not.
8.13. RECOGNITION-START-TIMERS
This request is sent from the client to the recognition resource when
it knows that a kill-on-barge-in prompt has finished playing. This
is useful in the scenario when the recognition and synthesizer
engines are not in the same session. Here, when a kill-on-barge-in
prompt is being played, you want the RECOGNIZE request to be
simultaneously active so that it can detect and implement kill-on-
barge-in. But at the same time, you don't want the recognizer to
start the no-input timers until the prompt is finished. The
parameter recognizer-start-timers header field in the RECOGNIZE
request will allow the client to say if the timers should be started
or not. The recognizer should not start the timers until the client
sends a RECOGNITION-START-TIMERS method to the recognizer.
8.14. RECOGNITON-COMPLETE
This is an Event from the recognizer resource to the client
indicating that the recognition completed. The recognition result is
sent in the MRCP body of the message. The request-state field MUST
be COMPLETE indicating that this is the last event with that
request-id, and that the request with that request-id is now
complete. The recognizer context still holds the results and the
audio waveform input of that recognition until the next RECOGNIZE
request is issued. A URL to the audio waveform MAY BE returned to
the client in a waveform-url header field in the RECOGNITION-COMPLETE
event. The client can use this URI to retrieve or playback the
audio.
Example:
C->S:RECOGNIZE 543257 MRCP/1.0
Confidence-Threshold:90
Content-Type:application/grammar+xml
Content-Id:request1@form-level.store
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
Shanmugham, et al. Informational [Page 65]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
<!-- single language attachment to tokens -->
<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>
<!-- single language attachment to a rule expansion -->
<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
</grammar>
S->C:MRCP/1.0 543257 200 IN-PROGRESS
S->C:START-OF-SPEECH 543257 IN-PROGRESS MRCP/1.0
S->C:RECOGNITION-COMPLETE 543257 COMPLETE MRCP/1.0
Completion-Cause:000 success
Waveform-URL:http://web.media.com/session123/audio.wav
Content-Type:application/x-nlsml
Content-Length:276
<?xml version="1.0"?>
<result x-model="http://IdentityModel"
xmlns:xf="http://www.w3.org/2000/xforms"
grammar="session:request1@form-level.store">
<interpretation>
<xf:instance name="Person">
<Person>
<Name> Andre Roy </Name>
</Person>
</xf:instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>
Shanmugham, et al. Informational [Page 66]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
8.15. DTMF Detection
Digits received as DTMF tones will be delivered to the automatic
speech recognition (ASR) engine in the RTP stream according to RFC
2833 [15]. The automatic speech recognizer (ASR) needs to support
RFC 2833 [15] to recognize digits. If it does not support RFC 2833
[15], it will have to process the audio stream and extract the audio
tones from it.
9. Future Study
Various sections of the recognizer could be distributed into Digital
Signal Processors (DSPs) on the Voice Browser/Gateway or IP Phones.
For instance, the gateway might perform voice activity detection to
reduce network bandwidth and CPU requirement of the automatic speech
recognition (ASR) server. Such extensions are deferred for further
study and will not be addressed in this document.
10. Security Considerations
The MRCP protocol may carry sensitive information such as account
numbers, passwords, etc. For this reason it is important that the
client have the option of secure communication with the server for
both the control messages as well as the media, though the client is
not required to use it. If all MRCP communications happens in a
trusted domain behind a firewall, this may not be necessary. If the
client or server is deployed in an insecure network, communication
happening across this insecure network needs to be protected. In
such cases, the following additional security functionality MUST be
supported on the MRCP server. MRCP servers MUST implement Transport
Layer Security (TLS) to secure the RTSP communication, i.e., the RTSP
stack SHOULD support the rtsps: URI form. MRCP servers MUST support
Secure Real-Time Transport Protocol (SRTP) as an option to send and
receive media.
11. RTSP-Based Examples
The following is an example of a typical session of speech synthesis
and recognition between a client and the server.
Opening the synthesizer. This is the first resource for this
session. The server and client agree on a single Session ID 12345678
and set of RTP/RTCP ports on both sides.
C->S:SETUP rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:2
Transport:RTP/AVP;unicast;client_port=46456-46457
Content-Type:application/sdp
Shanmugham, et al. Informational [Page 67]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Content-Length:190
v=0
o=- 123 456 IN IP4 10.0.0.1
s=Media Server
p=+1-888-555-1212
c=IN IP4 0.0.0.0
t=0 0
m=audio 0 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
S->C:RTSP/1.0 200 OK
CSeq:2
Transport:RTP/AVP;unicast;client_port=46456-46457;
server_port=46460-46461
Session:12345678
Content-Length:190
Content-Type:application/sdp
v=0
o=- 3211724219 3211724219 IN IP4 10.3.2.88
s=Media Server
c=IN IP4 0.0.0.0
t=0 0
m=audio 46460 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
Opening a recognizer resource. Uses the existing session ID and
ports.
C->S:SETUP rtsp://media.server.com/media/recognizer RTSP/1.0
CSeq:3
Transport:RTP/AVP;unicast;client_port=46456-46457;
mode=record;ttl=127
Session:12345678
S->C:RTSP/1.0 200 OK
CSeq:3
Transport:RTP/AVP;unicast;client_port=46456-46457;
server_port=46460-46461;mode=record;ttl=127
Session:12345678
Shanmugham, et al. Informational [Page 68]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
An ANNOUNCE message with the MRCP SPEAK request initiates speech.
C->S:ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:4
Session:12345678
Content-Type:application/mrcp
Content-Length:456
SPEAK 543257 MRCP/1.0
Kill-On-Barge-In:false
Voice-gender:neutral
Voice-category:teenager
Prosody-volume:medium
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>You have 4 new messages.</sentence>
<sentence>The first is from <say-as
type="name">Stephanie Williams</say-as> <mark
name="Stephanie"/>
and arrived at <break/>
<say-as type="time">3:45pm</say-as>.</sentence>
<sentence>The subject is <prosody
rate="-20%">ski trip</prosody></sentence>
</paragraph>
</speak>
S->C:RTSP/1.0 200 OK
CSeq:4
Session:12345678
RTP-Info:url=rtsp://media.server.com/media/synthesizer;
seq=9810092;rtptime=3450012
Content-Type:application/mrcp
Content-Length:456
MRCP/1.0 543257 200 IN-PROGRESS
The synthesizer hits the special marker in the message to be spoken
and faithfully informs the client of the event.
S->C:ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:5
Session:12345678
Shanmugham, et al. Informational [Page 69]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Content-Type:application/mrcp
Content-Length:123
SPEECH-MARKER 543257 IN-PROGRESS MRCP/1.0
Speech-Marker:Stephanie
C->S:RTSP/1.0 200 OK
CSeq:5
The synthesizer finishes with the SPEAK request.
S->C:ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:6
Session:12345678
Content-Type:application/mrcp
Content-Length:123
SPEAK-COMPLETE 543257 COMPLETE MRCP/1.0
C->S:RTSP/1.0 200 OK
CSeq:6
The recognizer is issued a request to listen for the customer
choices.
C->S:ANNOUNCE rtsp://media.server.com/media/recognizer RTSP/1.0
CSeq:7
Session:12345678
RECOGNIZE 543258 MRCP/1.0
Content-Type:application/grammar+xml
Content-Length:104
<?xml version="1.0"?>
<!-- the default grammar language is US English -->
<grammar xml:lang="en-US" version="1.0">
<!-- single language attachment to a rule expansion -->
<rule id="request">
Can I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>
</grammar>
Shanmugham, et al. Informational [Page 70]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
S->C:RTSP/1.0 200 OK
CSeq:7
Content-Type:application/mrcp
Content-Length:123
MRCP/1.0 543258 200 IN-PROGRESS
The client issues the next MRCP SPEAK method in an ANNOUNCE message,
asking the user the question. It is generally RECOMMENDED when
playing a prompt to the user with kill-on-barge-in and asking for
input, that the client issue the RECOGNIZE request ahead of the SPEAK
request for optimum performance and user experience. This way, it is
guaranteed that the recognizer is online before the prompt starts
playing and the user's speech will not be truncated at the beginning
(especially for power users).
C->S:ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:8 Session:12345678 Content-Type:application/mrcp
Content-Length:733
SPEAK 543259 MRCP/1.0
Kill-On-Barge-In:true
Content-Type:application/synthesis+ssml
Content-Length:104
<?xml version="1.0"?>
<speak>
<paragraph>
<sentence>Welcome to ABC corporation.</sentence>
<sentence>Who would you like Talk to.</sentence>
</paragraph>
</speak>
S->C:RTSP/1.0 200 OK
CSeq:8
Content-Type:application/mrcp
Content-Length:123
MRCP/1.0 543259 200 IN-PROGRESS
Since the last SPEAK request had Kill-On-Barge-In set to "true", the
message synthesizer is interrupted when the user starts speaking, and
the client is notified.
Now, since the recognition and synthesizer resources are in the same
session, they worked with each other to deliver kill-on-barge-in. If
the resources were in different sessions, it would have taken a few
more messages before the client got the SPEAK-COMPLETE event from the
Shanmugham, et al. Informational [Page 71]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
synthesizer resource. Whether the synthesizer and recognizer are in
the same session or not, the recognizer MUST generate the START-OF-
SPEECH event to the client.
The client should have then blindly turned around and issued a
BARGE-IN-OCCURRED method to the synthesizer resource. The
synthesizer, if kill-on-barge-in was enabled on the current SPEAK
request, would have then interrupted it and issued SPEAK-COMPLETE
event to the client. In this example, since the synthesizer and
recognizer are in the same session, the client did not issue the
BARGE-IN-OCCURRED method to the synthesizer and assumed that kill-
on-barge-in was implemented between the two resources in the same
session and worked.
The completion-cause code differentiates if this is normal completion
or a kill-on-barge-in interruption.
S->C:ANNOUNCE rtsp://media.server.com/media/recognizer RTSP/1.0
CSeq:9
Session:12345678
Content-Type:application/mrcp
Content-Length:273
START-OF-SPEECH 543258 IN-PROGRESS MRCP/1.0
C->S:RTSP/1.0 200 OK
CSeq:9
S->C:ANNOUNCE rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:10
Session:12345678
Content-Type:application/mrcp
Content-Length:273
SPEAK-COMPLETE 543259 COMPLETE MRCP/1.0
Completion-Cause:000 normal
C->S:RTSP/1.0 200 OK
CSeq:10
The recognition resource matched the spoken stream to a grammar and
generated results. The result of the recognition is returned by the
server as part of the RECOGNITION-COMPLETE event.
S->C:ANNOUNCE rtsp://media.server.com/media/recognizer RTSP/1.0
CSeq:11
Session:12345678
Content-Type:application/mrcp
Shanmugham, et al. Informational [Page 72]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Content-Length:733
RECOGNITION-COMPLETE 543258 COMPLETE MRCP/1.0
Completion-Cause:000 success
Waveform-URL:http://web.media.com/session123/audio.wav
Content-Type:application/x-nlsml
Content-Length:104
<?xml version="1.0"?>
<result x-model="http://IdentityModel"
xmlns:xf="http://www.w3.org/2000/xforms"
grammar="session:request1@form-level.store">
<interpretation>
<xf:instance name="Person">
<Person>
<Name> Andre Roy </Name>
</Person>
</xf:instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>
C->S:RTSP/1.0 200 OK
CSeq:11
C->S:TEARDOWN rtsp://media.server.com/media/synthesizer RTSP/1.0
CSeq:12
Session:12345678
S->C:RTSP/1.0 200 OK
CSeq:12
We are done with the resources and are tearing them down. When the
last of the resources for this session are released, the Session-ID
and the RTP/RTCP ports are also released.
C->S:TEARDOWN rtsp://media.server.com/media/recognizer RTSP/1.0
CSeq:13
Session:12345678
S->C:RTSP/1.0 200 OK
CSeq:13
Shanmugham, et al. Informational [Page 73]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
12. Informative References
[1] 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.
[2] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[4] 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.
[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[6] World Wide Web Consortium, "Voice Extensible Markup Language
(VoiceXML) Version 2.0", W3C Candidate Recommendation, March
2004.
[7] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[9] World Wide Web Consortium, "Speech Synthesis Markup Language
(SSML) Version 1.0", W3C Candidate Recommendation, September
2004.
[10] World Wide Web Consortium, "Natural Language Semantics Markup
Language (NLSML) for the Speech Interface Framework", W3C
Working Draft, 30 May 2001.
[11] World Wide Web Consortium, "Speech Recognition Grammar
Specification Version 1.0", W3C Candidate Recommendation, March
2004.
[12] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
[13] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
Shanmugham, et al. Informational [Page 74]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
[14] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[15] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
Telephony Tones and Telephony Signals", RFC 2833, May 2000.
[16] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
Shanmugham, et al. Informational [Page 75]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Appendix A. ABNF Message Definitions
ALPHA = %x41-5A / %x61-7A ; A-Z / a-z
CHAR = %x01-7F ; any 7-bit US-ASCII character,
; excluding NUL
CR = %x0D ; carriage return
CRLF = CR LF ; Internet standard newline
DIGIT = %x30-39 ; 0-9
DQUOTE = %x22 ; " (Double Quote)
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
HTAB = %x09 ; horizontal tab
LF = %x0A ; linefeed
OCTET = %x00-FF ; 8 bits of data
SP = %x20 ; space
WSP = SP / HTAB ; white space
LWS = [*WSP CRLF] 1*WSP ; linear whitespace
SWS = [LWS] ; sep whitespace
UTF8-NONASCII = %xC0-DF 1UTF8-CONT
/ %xE0-EF 2UTF8-CONT
/ %xF0-F7 3UTF8-CONT
/ %xF8-Fb 4UTF8-CONT
/ %xFC-FD 5UTF8-CONT
UTF8-CONT = %x80-BF
param = *pchar
quoted-string = SWS DQUOTE *(qdtext / quoted-pair )
DQUOTE
qdtext = LWS / %x21 / %x23-5B / %x5D-7E
/ UTF8-NONASCII
Shanmugham, et al. Informational [Page 76]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
quoted-pair = "\" (%x00-09 / %x0B-0C
/ %x0E-7F)
token = 1*(alphanum / "-" / "." / "!" / "%" / "*"
/ "_" / "+" / "`" / "'" / "~" )
reserved = ";" / "/" / "?" / ":" / "@" / "&" / "="
/ "+" / "$" / ","
mark = "-" / "_" / "." / "!" / "~" / "*" / "'"
/ "(" / ")"
unreserved = alphanum / mark
char = unreserved / escaped /
":" / "@" / "&" / "=" / "+" / "$" / ","
alphanum = ALPHA / DIGIT
escaped = "%" HEXDIG HEXDIG
absoluteURI = scheme ":" ( hier-part / opaque-part )
relativeURI = ( net-path / abs-path / rel-path )
[ "?" query ]
hier-part = ( net-path / abs-path ) [ "?" query ]
net-path = "//" authority [ abs-path ]
abs-path = "/" path-segments
rel-path = rel-segment [ abs-path ]
rel-segment = 1*( unreserved / escaped / ";" / "@"
/ "&" / "=" / "+" / "$" / "," )
opaque-part = uric-no-slash *uric
uric = reserved / unreserved / escaped
uric-no-slash = unreserved / escaped / ";" / "?" / ":"
/ "@" / "&" / "=" / "+" / "$" / ","
path-segments = segment *( "/" segment )
segment = *pchar *( ";" param )
Shanmugham, et al. Informational [Page 77]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
authority = srvr / reg-name
srvr = [ [ userinfo "@" ] hostport ]
reg-name = 1*( unreserved / escaped / "$" / ","
/ ";" / ":" / "@" / "&" / "=" / "+" )
query = *uric
userinfo = ( user ) [ ":" password ] "@"
user = 1*( unreserved / escaped
/ user-unreserved )
user-unreserved = "&" / "=" / "+" / "$" / "," / ";"
/ "?" / "/"
password = *( unreserved / escaped /
"&" / "=" / "+" / "$" / "," )
hostport = host [ ":" port ]
host = hostname / IPv4address / IPv6reference
hostname = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum
/ alphanum *( alphanum / "-" ) alphanum
toplabel = ALPHA / ALPHA *( alphanum / "-" )
alphanum
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "."
1*3DIGIT
IPv6reference = "[" IPv6address "]"
IPv6address = hexpart [ ":" IPv4address ]
hexpart = hexseq / hexseq "::" [ hexseq ] / "::"
[ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
Shanmugham, et al. Informational [Page 78]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
port = 1*DIGIT
generic-message = start-line
message-header
CRLF
[ message-body ]
message-body = *OCTET
start-line = request-line / status-line / event-line
request-line = method-name SP request-id SP
mrcp-version CRLF
status-line = mrcp-version SP request-id SP
status-code SP request-state CRLF
event-line = event-name SP request-id SP
request-state SP mrcp-version CRLF
message-header = 1*(generic-header / resource-header)
generic-header = active-request-id-list
/ proxy-sync-id
/ content-id
/ content-type
/ content-length
/ content-base
/ content-location
/ content-encoding
/ cache-control
/ logging-tag
; -- content-id is as defined in RFC 2392 and RFC 2046
mrcp-version = "MRCP" "/" 1*DIGIT "." 1*DIGIT
request-id = 1*DIGIT
status-code = 1*DIGIT
active-request-id-list = "Active-Request-Id-List" ":"
request-id *("," request-id) CRLF
proxy-sync-id = "Proxy-Sync-Id" ":" 1*ALPHA CRLF
content-length = "Content-Length" ":" 1*DIGIT CRLF
content-base = "Content-Base" ":" absoluteURI CRLF
Shanmugham, et al. Informational [Page 79]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
content-type = "Content-Type" ":" media-type
media-type = type "/" subtype *( ";" parameter )
type = token
subtype = token
parameter = attribute "=" value
attribute = token
value = token / quoted-string
content-encoding = "Content-Encoding" ":"
*WSP content-coding
*(*WSP "," *WSP content-coding *WSP )
CRLF
content-coding = token
content-location = "Content-Location" ":"
( absoluteURI / relativeURI ) CRLF
cache-control = "Cache-Control" ":"
*WSP cache-directive
*( *WSP "," *WSP cache-directive *WSP )
CRLF
cache-directive = "max-age" "=" delta-seconds
/ "max-stale" "=" delta-seconds
/ "min-fresh" "=" delta-seconds
logging-tag = "Logging-Tag" ":" 1*ALPHA CRLF
resource-header = recognizer-header
/ synthesizer-header
method-name = synthesizer-method
/ recognizer-method
event-name = synthesizer-event
/ recognizer-event
Shanmugham, et al. Informational [Page 80]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
request-state = "COMPLETE"
/ "IN-PROGRESS"
/ "PENDING"
synthesizer-method = "SET-PARAMS"
/ "GET-PARAMS"
/ "SPEAK"
/ "STOP"
/ "PAUSE"
/ "RESUME"
/ "BARGE-IN-OCCURRED"
/ "CONTROL"
synthesizer-event = "SPEECH-MARKER"
/ "SPEAK-COMPLETE"
synthesizer-header = jump-target
/ kill-on-barge-in
/ speaker-profile
/ completion-cause
/ voice-parameter
/ prosody-parameter
/ vendor-specific
/ speech-marker
/ speech-language
/ fetch-hint
/ audio-fetch-hint
/ fetch-timeout
/ failed-uri
/ failed-uri-cause
/ speak-restart
/ speak-length
recognizer-method = "SET-PARAMS"
/ "GET-PARAMS"
/ "DEFINE-GRAMMAR"
/ "RECOGNIZE"
/ "GET-RESULT"
/ "RECOGNITION-START-TIMERS"
/ "STOP"
recognizer-event = "START-OF-SPEECH"
/ "RECOGNITION-COMPLETE"
recognizer-header = confidence-threshold
/ sensitivity-level
/ speed-vs-accuracy
/ n-best-list-length
Shanmugham, et al. Informational [Page 81]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
/ no-input-timeout
/ recognition-timeout
/ waveform-url
/ completion-cause
/ recognizer-context-block
/ recognizer-start-timers
/ vendor-specific
/ speech-complete-timeout
/ speech-incomplete-timeout
/ dtmf-interdigit-timeout
/ dtmf-term-timeout
/ dtmf-term-char
/ fetch-timeout
/ failed-uri
/ failed-uri-cause
/ save-waveform
/ new-audio-channel
/ speech-language
jump-target = "Jump-Size" ":" speech-length-value CRLF
speech-length-value = numeric-speech-length
/ text-speech-length
text-speech-length = 1*ALPHA SP "Tag"
numeric-speech-length =("+" / "-") 1*DIGIT SP
numeric-speech-unit
numeric-speech-unit = "Second"
/ "Word"
/ "Sentence"
/ "Paragraph"
delta-seconds = 1*DIGIT
kill-on-barge-in = "Kill-On-Barge-In" ":" boolean-value CRLF
boolean-value = "true" / "false"
speaker-profile = "Speaker-Profile" ":" absoluteURI CRLF
completion-cause = "Completion-Cause" ":" 1*DIGIT SP
1*ALPHA CRLF
voice-parameter = "Voice-" voice-param-name ":"
voice-param-value CRLF
Shanmugham, et al. Informational [Page 82]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
voice-param-name = 1*ALPHA
voice-param-value = 1*alphanum
prosody-parameter = "Prosody-" prosody-param-name ":"
prosody-param-value CRLF
prosody-param-name = 1*ALPHA
prosody-param-value = 1*alphanum
vendor-specific = "Vendor-Specific-Parameters" ":"
vendor-specific-av-pair
*[";" vendor-specific-av-pair] CRLF
vendor-specific-av-pair = vendor-av-pair-name "="
vendor-av-pair-value
vendor-av-pair-name = 1*ALPHA
vendor-av-pair-value = 1*alphanum
speech-marker = "Speech-Marker" ":" 1*ALPHA CRLF
speech-language = "Speech-Language" ":" 1*ALPHA CRLF
fetch-hint = "Fetch-Hint" ":" 1*ALPHA CRLF
audio-fetch-hint = "Audio-Fetch-Hint" ":" 1*ALPHA CRLF
fetch-timeout = "Fetch-Timeout" ":" 1*DIGIT CRLF
failed-uri = "Failed-URI" ":" absoluteURI CRLF
failed-uri-cause = "Failed-URI-Cause" ":" 1*ALPHA CRLF
speak-restart = "Speak-Restart" ":" boolean-value CRLF
speak-length = "Speak-Length" ":" speech-length-value
CRLF
confidence-threshold = "Confidence-Threshold" ":"
1*DIGIT CRLF
sensitivity-level = "Sensitivity-Level" ":" 1*DIGIT CRLF
speed-vs-accuracy = "Speed-Vs-Accuracy" ":" 1*DIGIT CRLF
n-best-list-length = "N-Best-List-Length" ":" 1*DIGIT CRLF
Shanmugham, et al. Informational [Page 83]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
no-input-timeout = "No-Input-Timeout" ":" 1*DIGIT CRLF
recognition-timeout = "Recognition-Timeout" ":" 1*DIGIT CRLF
waveform-url = "Waveform-URL" ":" absoluteURI CRLF
recognizer-context-block = "Recognizer-Context-Block" ":"
1*ALPHA CRLF
recognizer-start-timers = "Recognizer-Start-Timers" ":"
boolean-value CRLF
speech-complete-timeout = "Speech-Complete-Timeout" ":"
1*DIGIT CRLF
speech-incomplete-timeout = "Speech-Incomplete-Timeout" ":"
1*DIGIT CRLF
dtmf-interdigit-timeout = "DTMF-Interdigit-Timeout" ":"
1*DIGIT CRLF
dtmf-term-timeout = "DTMF-Term-Timeout" ":" 1*DIGIT CRLF
dtmf-term-char = "DTMF-Term-Char" ":" CHAR CRLF
save-waveform = "Save-Waveform" ":" boolean-value CRLF
new-audio-channel = "New-Audio-Channel" ":"
boolean-value CRLF
Appendix B. Acknowledgements
Andre Gillet (Nuance Communications)
Andrew Hunt (SpeechWorks)
Aaron Kneiss (SpeechWorks)
Kristian Finlator (SpeechWorks)
Martin Dragomirecky (Cisco Systems, Inc.)
Pierre Forgues (Nuance Communications)
Suresh Kaliannan (Cisco Systems, Inc.)
Corey Stohs (Cisco Systems, Inc.)
Dan Burnett (Nuance Communications)
Shanmugham, et al. Informational [Page 84]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Authors' Addresses
Saravanan Shanmugham
Cisco Systems, Inc.
170 W. Tasman Drive
San Jose, CA 95134
EMail: sarvi@cisco.com
Peter Monaco
Nuasis Corporation
303 Bryant St.
Mountain View, CA 94041
EMail: peter.monaco@nuasis.com
Brian Eberman
Speechworks, Inc.
695 Atlantic Avenue
Boston, MA 02111
EMail: brian.eberman@speechworks.com
Shanmugham, et al. Informational [Page 85]
RFC 4463 MRCP by Cisco, Nuance, and Speechworks April 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78 and at www.rfc-editor.org/copyright.html, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Shanmugham, et al. Informational [Page 86]