MRCP Event Messages

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. Intermediate results and events of the request are indicated to the client through event messages from the server.

An event message consists of an event header line followed by the message header section and an optional message body containing data specific to the event. The header line includes the request-id of the corresponding request and a status value. The request-state value is COMPLETE if the request is done and this was the last event; otherwise it is IN-PROGRESS.

event-line         = mrcp-version SP message-length SP event-name
                                  SP request-id SP request-state CRLF

The mrcp-version used here is identical to the one used in the Request/Response Line and indicates the version of the MRCP protocol running on the server.

mrcp-version       = "MRCP" "/" 1*DIGIT "." 1*DIGIT

The message-length field describes the length of the message (in bytes), including the start-line:

message-length     = 1*19DIGIT

The request-id field is a unique identifier associated with each request. Clients should assign incremental numbers to each unique request, and the Media Server will use this request-id when responding to requests. In event messages, this value indicates which specific request the event relates to.

request-id         = 1*10DIGIT

The event-name identifies the nature of the event generated by the media resource. The set of valid event names depends on the resource generating it. See the corresponding resource-specific section of this article below.

event-name         =  synthesizer-event
                   /  recognizer-event

The request-state field indicates whether the request causing this event is complete or still in progress. The final event for a request has a COMPLETE status indicating the completion of the request.

request-state      =  "COMPLETE"
                   /  "IN-PROGRESS"
                   /  "PENDING"

Recognizer Events

The recognizer can generate the following events.

recognizer-event    =  "START-OF-INPUT"
                    /  "RECOGNITION-COMPLETE"
                    /  "INTERPRETATION-COMPLETE"

The START-OF-INPUT event is issued whenever the recognizer detects the beginning of either speech or a DTMF signal within the RTP audio stream associated with the session. The input-type header field of the START-OF-INPUT event indicates whether the source was speech or DTMF. Note that in the MRCPv1 specification, "START-OF-INPUT" is reported as "START-OF-SPEECH", so when using MRCPv1, the event will be "START-OF-SPEECH", not "START-OF-INPUT".

The RECOGNITION-COMPLETE event is issued when the Media Server finishes a recognition operation. The recognition result is sent in the body of the message. If the server is configured to return a URI to the audio waveform for this request, it will be indicated in the Waveform-URI header field. The client can use this URI to retrieve or play back the audio. The request-state for this event will be COMPLETE to indicate that no further messages will be associated with the request-id.

The INTERPRETATION-COMPLETE event from the recognizer resource to the client indicates that an INTERPRET operation is complete. The interpretation result is sent in the body of the message. The request-state for this event will be COMPLETE to indicate that no further messages will be associated with the request-id. The Completion-Cause header field will be included in this event and will be set to an appropriate value from the table below.

Recognizer Completion-Cause Codes

The Completion-Cause header field is part of a RECOGNITION-COMPLETE event coming from the recognizer resource to the client. It indicates the reason behind the RECOGNIZE method completion. This header field is also sent in DEFINE-GRAMMAR and RECOGNIZE responses if they return with a failure status and a COMPLETE state. In the ABNF below, the 'cause-code' contains a numerical value selected from the Cause-Code column of the following table. The 'cause-name' contains the corresponding token selected from the Cause-Name column.

completion-cause    =  "Completion-Cause" ":" cause-code SP
                        cause-name CRLF
cause-code          =  3DIGIT
cause-name          =  *VCHAR

Cause-Code	Cause-Name	Description
000	success	RECOGNIZE completed with a match or DEFINE-GRAMMAR succeeded in downloading and compiling the grammar.
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	hotword-maxtime	RECOGNIZE in hotword mode completed without a match due to a recognition-timeout.
004	grammar-load-failure	RECOGNIZE failed due to a grammar load failure.
005	grammar-compilation-failure	RECOGNIZE failed due to a grammar compilation failure.
006	recognizer-error	RECOGNIZE request terminated prematurely due to a recognizer error.
007	speech-too-early	RECOGNIZE request terminated because speech was too early. This occurs when the audio stream is already "in-speech" when the RECOGNIZE request is received.
008	success-maxtime	RECOGNIZE request terminated because speech exceeded the maximum time, but what was spoken up to that point was a full match.
009	uri-failure	Failure accessing a URI.
010	language-unsupported	Language not supported.
011	cancelled	A new RECOGNIZE cancelled this one, or a prior RECOGNIZE failed while this one was still in the queue.
012	semantics-failure	Recognition succeeded but semantic interpretation of the recognized input failed. The RECOGNITION-COMPLETE event will contain the recognition result with only input text and no interpretation.
013	partial-match	Speech incomplete timeout expired before there was a full match, but what was spoken up to that point was a partial match to one or more grammars.
014	partial-match-maxtime	The Recognition-Timer expired before a full match was achieved, but what was spoken up to that point was a partial match to one or more grammars.
015	no-match-maxtime	The Recognition-Timer expired. What was spoken up to that point did not match any of the grammars. This cause may also be returned if the recognizer does not support detecting partial grammar matches.
016	grammar-definition-failure	Any DEFINE-GRAMMAR error other than grammar-load-failure and grammar-compilation-failure.

Synthesizer Events

The synthesizer can generate the following events.

synthesizer-event  =  "SPEECH-MARKER"
                   /  "SPEAK-COMPLETE"

The SPEECH-MARKER event is issued whenever the synthesizer reaches a marker tag in the speech markup it is currently processing. The value of the request-id field will match that of the corresponding SPEAK request. The request-state field will have the value "IN-PROGRESS" as the speech is still not complete. The value of the speech marker, describing where the synthesizer is in the speech markup, will be returned in the Speech-Marker header field, along with a timestamp indicating the instant in the output speech stream that the marker was encountered.

The SPEAK-COMPLETE event is issued when the Media Server finishes a SPEAK operation. The request-state for this event will be COMPLETE to indicate that no further messages will be associated with the request-id. The Completion-Cause header field specifies the cause code pertaining to the status and reason of the request completion, such as SPEAK completed normally, barge-in, or an error. The Completion-Cause header field will be included in this event and will be set to an appropriate value from the table below.

Synthesizer Completion-Cause Codes

The Completion-Cause header field is part of a SPEAK-COMPLETE event coming from the synthesizer resource to the client. It indicates the reason behind the SPEAK method completion. This header field is sent in SPEAK responses if they return with a failure status and a COMPLETE state.

completion-cause   =   "Completion-Cause" ":" 3DIGIT SP
                        1*VCHAR 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 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 a synthesizer error.
005	language-unsupported	Language not supported.
006	lexicon-load-failure	Lexicon loading failed.
007	cancelled	A prior SPEAK request failed while this one was still in the queue.

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