RFC 4463
A Media Resource Control Protocol (MRCP) Developed by Cisco, Nuance, and Speechworks
Pages: 86
Informational
Informational
Part 4 of 4 – Pages 67 to 86
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
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
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
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>
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
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
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
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.
[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.
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
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 )
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
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
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
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
/ 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
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
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)
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
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