Internet Engineering Task Force (IETF) D. Burnett

Internet Engineering Task Force (IETF) D. Burnett
Request for Comments: 6787 Voxeo
Category: Standards Track S. Shanmugham
ISSN: 2070-1721 Cisco Systems, Inc.
November 2012

Media Resource Control Protocol Version 2 (MRCPv2)

Abstract

The Media Resource Control Protocol Version 2 (MRCPv2) allows client
hosts to control media service resources such as speech synthesizers,
recognizers, verifiers, and identifiers residing in servers on the
network. MRCPv2 is not a "stand-alone" protocol -- it relies on
other protocols, such as the Session Initiation Protocol (SIP), to
coordinate MRCPv2 clients and servers and manage sessions between
them, and the Session Description Protocol (SDP) to describe,
discover, and exchange capabilities. It also depends on SIP and SDP
to establish the media sessions and associated parameters between the
media source or sink and the media server. Once this is done, the
MRCPv2 exchange operates over the control session established above,
allowing the client to control the media processing resources on the
speech resource server.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6787.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents

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RFC 6787 MRCPv2 November 2012

carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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This document may contain material from IETF Documents or IETF
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than English.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Document Conventions . . . . . . . . . . . . . . . . . . . . 9
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . 10
2.2. State-Machine Diagrams . . . . . . . . . . . . . . . . . 10
2.3. URI Schemes . . . . . . . . . . . . . . . . . . . . . . 11
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1. MRCPv2 Media Resource Types . . . . . . . . . . . . . . 12
3.2. Server and Resource Addressing . . . . . . . . . . . . . 14
4. MRCPv2 Basics . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1. Connecting to the Server . . . . . . . . . . . . . . . . 14
4.2. Managing Resource Control Channels . . . . . . . . . . . 14
4.3. SIP Session Example . . . . . . . . . . . . . . . . . . 17
4.4. Media Streams and RTP Ports . . . . . . . . . . . . . . 22
4.5. MRCPv2 Message Transport . . . . . . . . . . . . . . . . 24
4.6. MRCPv2 Session Termination . . . . . . . . . . . . . . . 24
5. MRCPv2 Specification . . . . . . . . . . . . . . . . . . . . 24
5.1. Common Protocol Elements . . . . . . . . . . . . . . . . 25
5.2. Request . . . . . . . . . . . . . . . . . . . . . . . . 28
5.3. Response . . . . . . . . . . . . . . . . . . . . . . . . 29
5.4. Status Codes . . . . . . . . . . . . . . . . . . . . . . 30
5.5. Events . . . . . . . . . . . . . . . . . . . . . . . . . 31
6. MRCPv2 Generic Methods, Headers, and Result Structure . . . . 32
6.1. Generic Methods . . . . . . . . . . . . . . . . . . . . 32
6.1.1. SET-PARAMS . . . . . . . . . . . . . . . . . . . . . 32
6.1.2. GET-PARAMS . . . . . . . . . . . . . . . . . . . . . 33
6.2. Generic Message Headers . . . . . . . . . . . . . . . . 34
6.2.1. Channel-Identifier . . . . . . . . . . . . . . . . . 35
6.2.2. Accept . . . . . . . . . . . . . . . . . . . . . . . 36

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6.2.3. Active-Request-Id-List . . . . . . . . . . . . . . . 36
6.2.4. Proxy-Sync-Id . . . . . . . . . . . . . . . . . . . 36
6.2.5. Accept-Charset . . . . . . . . . . . . . . . . . . . 37
6.2.6. Content-Type . . . . . . . . . . . . . . . . . . . . 37
6.2.7. Content-ID . . . . . . . . . . . . . . . . . . . . . 38
6.2.8. Content-Base . . . . . . . . . . . . . . . . . . . . 38
6.2.9. Content-Encoding . . . . . . . . . . . . . . . . . . 38
6.2.10. Content-Location . . . . . . . . . . . . . . . . . . 39
6.2.11. Content-Length . . . . . . . . . . . . . . . . . . . 39
6.2.12. Fetch Timeout . . . . . . . . . . . . . . . . . . . 39
6.2.13. Cache-Control . . . . . . . . . . . . . . . . . . . 40
6.2.14. Logging-Tag . . . . . . . . . . . . . . . . . . . . 41
6.2.15. Set-Cookie . . . . . . . . . . . . . . . . . . . . . 42
6.2.16. Vendor-Specific Parameters . . . . . . . . . . . . . 44
6.3. Generic Result Structure . . . . . . . . . . . . . . . . 44
6.3.1. Natural Language Semantics Markup Language . . . . . 45
7. Resource Discovery . . . . . . . . . . . . . . . . . . . . . 46
8. Speech Synthesizer Resource . . . . . . . . . . . . . . . . . 47
8.1. Synthesizer State Machine . . . . . . . . . . . . . . . 48
8.2. Synthesizer Methods . . . . . . . . . . . . . . . . . . 48
8.3. Synthesizer Events . . . . . . . . . . . . . . . . . . . 49
8.4. Synthesizer Header Fields . . . . . . . . . . . . . . . 49
8.4.1. Jump-Size . . . . . . . . . . . . . . . . . . . . . 49
8.4.2. Kill-On-Barge-In . . . . . . . . . . . . . . . . . . 50
8.4.3. Speaker-Profile . . . . . . . . . . . . . . . . . . 51
8.4.4. Completion-Cause . . . . . . . . . . . . . . . . . . 51
8.4.5. Completion-Reason . . . . . . . . . . . . . . . . . 52
8.4.6. Voice-Parameter . . . . . . . . . . . . . . . . . . 52
8.4.7. Prosody-Parameters . . . . . . . . . . . . . . . . . 53
8.4.8. Speech-Marker . . . . . . . . . . . . . . . . . . . 53
8.4.9. Speech-Language . . . . . . . . . . . . . . . . . . 54
8.4.10. Fetch-Hint . . . . . . . . . . . . . . . . . . . . . 54
8.4.11. Audio-Fetch-Hint . . . . . . . . . . . . . . . . . . 55
8.4.12. Failed-URI . . . . . . . . . . . . . . . . . . . . . 55
8.4.13. Failed-URI-Cause . . . . . . . . . . . . . . . . . . 55
8.4.14. Speak-Restart . . . . . . . . . . . . . . . . . . . 56
8.4.15. Speak-Length . . . . . . . . . . . . . . . . . . . . 56
8.4.16. Load-Lexicon . . . . . . . . . . . . . . . . . . . . 57
8.4.17. Lexicon-Search-Order . . . . . . . . . . . . . . . . 57
8.5. Synthesizer Message Body . . . . . . . . . . . . . . . . 57
8.5.1. Synthesizer Speech Data . . . . . . . . . . . . . . 57
8.5.2. Lexicon Data . . . . . . . . . . . . . . . . . . . . 59
8.6. SPEAK Method . . . . . . . . . . . . . . . . . . . . . . 60
8.7. STOP . . . . . . . . . . . . . . . . . . . . . . . . . . 62
8.8. BARGE-IN-OCCURRED . . . . . . . . . . . . . . . . . . . 63
8.9. PAUSE . . . . . . . . . . . . . . . . . . . . . . . . . 65
8.10. RESUME . . . . . . . . . . . . . . . . . . . . . . . . . 66
8.11. CONTROL . . . . . . . . . . . . . . . . . . . . . . . . 67

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8.12. SPEAK-COMPLETE . . . . . . . . . . . . . . . . . . . . . 69
8.13. SPEECH-MARKER . . . . . . . . . . . . . . . . . . . . . 70
8.14. DEFINE-LEXICON . . . . . . . . . . . . . . . . . . . . . 71
9. Speech Recognizer Resource . . . . . . . . . . . . . . . . . 72
9.1. Recognizer State Machine . . . . . . . . . . . . . . . . 74
9.2. Recognizer Methods . . . . . . . . . . . . . . . . . . . 74
9.3. Recognizer Events . . . . . . . . . . . . . . . . . . . 75
9.4. Recognizer Header Fields . . . . . . . . . . . . . . . . 75
9.4.1. Confidence-Threshold . . . . . . . . . . . . . . . . 77
9.4.2. Sensitivity-Level . . . . . . . . . . . . . . . . . 77
9.4.3. Speed-Vs-Accuracy . . . . . . . . . . . . . . . . . 77
9.4.4. N-Best-List-Length . . . . . . . . . . . . . . . . . 78
9.4.5. Input-Type . . . . . . . . . . . . . . . . . . . . . 78
9.4.6. No-Input-Timeout . . . . . . . . . . . . . . . . . . 78
9.4.7. Recognition-Timeout . . . . . . . . . . . . . . . . 79
9.4.8. Waveform-URI . . . . . . . . . . . . . . . . . . . . 79
9.4.9. Media-Type . . . . . . . . . . . . . . . . . . . . . 80
9.4.10. Input-Waveform-URI . . . . . . . . . . . . . . . . . 80
9.4.11. Completion-Cause . . . . . . . . . . . . . . . . . . 80
9.4.12. Completion-Reason . . . . . . . . . . . . . . . . . 83
9.4.13. Recognizer-Context-Block . . . . . . . . . . . . . . 83
9.4.14. Start-Input-Timers . . . . . . . . . . . . . . . . . 83
9.4.15. Speech-Complete-Timeout . . . . . . . . . . . . . . 84
9.4.16. Speech-Incomplete-Timeout . . . . . . . . . . . . . 84
9.4.17. DTMF-Interdigit-Timeout . . . . . . . . . . . . . . 85
9.4.18. DTMF-Term-Timeout . . . . . . . . . . . . . . . . . 85
9.4.19. DTMF-Term-Char . . . . . . . . . . . . . . . . . . . 85
9.4.20. Failed-URI . . . . . . . . . . . . . . . . . . . . . 86
9.4.21. Failed-URI-Cause . . . . . . . . . . . . . . . . . . 86
9.4.22. Save-Waveform . . . . . . . . . . . . . . . . . . . 86
9.4.23. New-Audio-Channel . . . . . . . . . . . . . . . . . 86
9.4.24. Speech-Language . . . . . . . . . . . . . . . . . . 87
9.4.25. Ver-Buffer-Utterance . . . . . . . . . . . . . . . . 87
9.4.26. Recognition-Mode . . . . . . . . . . . . . . . . . . 87
9.4.27. Cancel-If-Queue . . . . . . . . . . . . . . . . . . 88
9.4.28. Hotword-Max-Duration . . . . . . . . . . . . . . . . 88
9.4.29. Hotword-Min-Duration . . . . . . . . . . . . . . . . 88
9.4.30. Interpret-Text . . . . . . . . . . . . . . . . . . . 89
9.4.31. DTMF-Buffer-Time . . . . . . . . . . . . . . . . . . 89
9.4.32. Clear-DTMF-Buffer . . . . . . . . . . . . . . . . . 89
9.4.33. Early-No-Match . . . . . . . . . . . . . . . . . . . 90
9.4.34. Num-Min-Consistent-Pronunciations . . . . . . . . . 90
9.4.35. Consistency-Threshold . . . . . . . . . . . . . . . 90
9.4.36. Clash-Threshold . . . . . . . . . . . . . . . . . . 90
9.4.37. Personal-Grammar-URI . . . . . . . . . . . . . . . . 91
9.4.38. Enroll-Utterance . . . . . . . . . . . . . . . . . . 91
9.4.39. Phrase-Id . . . . . . . . . . . . . . . . . . . . . 91
9.4.40. Phrase-NL . . . . . . . . . . . . . . . . . . . . . 92

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9.4.41. Weight . . . . . . . . . . . . . . . . . . . . . . . 92
9.4.42. Save-Best-Waveform . . . . . . . . . . . . . . . . . 92
9.4.43. New-Phrase-Id . . . . . . . . . . . . . . . . . . . 93
9.4.44. Confusable-Phrases-URI . . . . . . . . . . . . . . . 93
9.4.45. Abort-Phrase-Enrollment . . . . . . . . . . . . . . 93
9.5. Recognizer Message Body . . . . . . . . . . . . . . . . 93
9.5.1. Recognizer Grammar Data . . . . . . . . . . . . . . 93
9.5.2. Recognizer Result Data . . . . . . . . . . . . . . . 97
9.5.3. Enrollment Result Data . . . . . . . . . . . . . . . 98
9.5.4. Recognizer Context Block . . . . . . . . . . . . . . 98
9.6. Recognizer Results . . . . . . . . . . . . . . . . . . . 99
9.6.1. Markup Functions . . . . . . . . . . . . . . . . . . 99
9.6.2. Overview of Recognizer Result Elements and Their
Relationships . . . . . . . . . . . . . . . . . . . 100
9.6.3. Elements and Attributes . . . . . . . . . . . . . . 101
9.7. Enrollment Results . . . . . . . . . . . . . . . . . . . 106
9.7.1. <num-clashes> Element . . . . . . . . . . . . . . . 106
9.7.2. <num-good-repetitions> Element . . . . . . . . . . . 106
9.7.3. <num-repetitions-still-needed> Element . . . . . . . 107
9.7.4. <consistency-status> Element . . . . . . . . . . . . 107
9.7.5. <clash-phrase-ids> Element . . . . . . . . . . . . . 107
9.7.6. <transcriptions> Element . . . . . . . . . . . . . . 107
9.7.7. <confusable-phrases> Element . . . . . . . . . . . . 107
9.8. DEFINE-GRAMMAR . . . . . . . . . . . . . . . . . . . . . 107
9.9. RECOGNIZE . . . . . . . . . . . . . . . . . . . . . . . 111
9.10. STOP . . . . . . . . . . . . . . . . . . . . . . . . . . 118
9.11. GET-RESULT . . . . . . . . . . . . . . . . . . . . . . . 119
9.12. START-OF-INPUT . . . . . . . . . . . . . . . . . . . . . 120
9.13. START-INPUT-TIMERS . . . . . . . . . . . . . . . . . . . 120
9.14. RECOGNITION-COMPLETE . . . . . . . . . . . . . . . . . . 120
9.15. START-PHRASE-ENROLLMENT . . . . . . . . . . . . . . . . 123
9.16. ENROLLMENT-ROLLBACK . . . . . . . . . . . . . . . . . . 124
9.17. END-PHRASE-ENROLLMENT . . . . . . . . . . . . . . . . . 124
9.18. MODIFY-PHRASE . . . . . . . . . . . . . . . . . . . . . 125
9.19. DELETE-PHRASE . . . . . . . . . . . . . . . . . . . . . 125
9.20. INTERPRET . . . . . . . . . . . . . . . . . . . . . . . 125
9.21. INTERPRETATION-COMPLETE . . . . . . . . . . . . . . . . 127
9.22. DTMF Detection . . . . . . . . . . . . . . . . . . . . . 128
10. Recorder Resource . . . . . . . . . . . . . . . . . . . . . . 129
10.1. Recorder State Machine . . . . . . . . . . . . . . . . . 129
10.2. Recorder Methods . . . . . . . . . . . . . . . . . . . . 130
10.3. Recorder Events . . . . . . . . . . . . . . . . . . . . 130
10.4. Recorder Header Fields . . . . . . . . . . . . . . . . . 130
10.4.1. Sensitivity-Level . . . . . . . . . . . . . . . . . 130
10.4.2. No-Input-Timeout . . . . . . . . . . . . . . . . . . 131
10.4.3. Completion-Cause . . . . . . . . . . . . . . . . . . 131
10.4.4. Completion-Reason . . . . . . . . . . . . . . . . . 132
10.4.5. Failed-URI . . . . . . . . . . . . . . . . . . . . . 132

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10.4.6. Failed-URI-Cause . . . . . . . . . . . . . . . . . . 132
10.4.7. Record-URI . . . . . . . . . . . . . . . . . . . . . 132
10.4.8. Media-Type . . . . . . . . . . . . . . . . . . . . . 133
10.4.9. Max-Time . . . . . . . . . . . . . . . . . . . . . . 133
10.4.10. Trim-Length . . . . . . . . . . . . . . . . . . . . 134
10.4.11. Final-Silence . . . . . . . . . . . . . . . . . . . 134
10.4.12. Capture-On-Speech . . . . . . . . . . . . . . . . . 134
10.4.13. Ver-Buffer-Utterance . . . . . . . . . . . . . . . . 134
10.4.14. Start-Input-Timers . . . . . . . . . . . . . . . . . 135
10.4.15. New-Audio-Channel . . . . . . . . . . . . . . . . . 135
10.5. Recorder Message Body . . . . . . . . . . . . . . . . . 135
10.6. RECORD . . . . . . . . . . . . . . . . . . . . . . . . . 135
10.7. STOP . . . . . . . . . . . . . . . . . . . . . . . . . . 136
10.8. RECORD-COMPLETE . . . . . . . . . . . . . . . . . . . . 137
10.9. START-INPUT-TIMERS . . . . . . . . . . . . . . . . . . . 138
10.10. START-OF-INPUT . . . . . . . . . . . . . . . . . . . . . 138
11. Speaker Verification and Identification . . . . . . . . . . . 139
11.1. Speaker Verification State Machine . . . . . . . . . . . 140
11.2. Speaker Verification Methods . . . . . . . . . . . . . . 142
11.3. Verification Events . . . . . . . . . . . . . . . . . . 144
11.4. Verification Header Fields . . . . . . . . . . . . . . . 144
11.4.1. Repository-URI . . . . . . . . . . . . . . . . . . . 144
11.4.2. Voiceprint-Identifier . . . . . . . . . . . . . . . 145
11.4.3. Verification-Mode . . . . . . . . . . . . . . . . . 145
11.4.4. Adapt-Model . . . . . . . . . . . . . . . . . . . . 146
11.4.5. Abort-Model . . . . . . . . . . . . . . . . . . . . 146
11.4.6. Min-Verification-Score . . . . . . . . . . . . . . . 147
11.4.7. Num-Min-Verification-Phrases . . . . . . . . . . . . 147
11.4.8. Num-Max-Verification-Phrases . . . . . . . . . . . . 147
11.4.9. No-Input-Timeout . . . . . . . . . . . . . . . . . . 148
11.4.10. Save-Waveform . . . . . . . . . . . . . . . . . . . 148
11.4.11. Media-Type . . . . . . . . . . . . . . . . . . . . . 148
11.4.12. Waveform-URI . . . . . . . . . . . . . . . . . . . . 148
11.4.13. Voiceprint-Exists . . . . . . . . . . . . . . . . . 149
11.4.14. Ver-Buffer-Utterance . . . . . . . . . . . . . . . . 149
11.4.15. Input-Waveform-URI . . . . . . . . . . . . . . . . . 149
11.4.16. Completion-Cause . . . . . . . . . . . . . . . . . . 150
11.4.17. Completion-Reason . . . . . . . . . . . . . . . . . 151
11.4.18. Speech-Complete-Timeout . . . . . . . . . . . . . . 151
11.4.19. New-Audio-Channel . . . . . . . . . . . . . . . . . 152
11.4.20. Abort-Verification . . . . . . . . . . . . . . . . . 152
11.4.21. Start-Input-Timers . . . . . . . . . . . . . . . . . 152
11.5. Verification Message Body . . . . . . . . . . . . . . . 152
11.5.1. Verification Result Data . . . . . . . . . . . . . . 152
11.5.2. Verification Result Elements . . . . . . . . . . . . 153
11.6. START-SESSION . . . . . . . . . . . . . . . . . . . . . 157
11.7. END-SESSION . . . . . . . . . . . . . . . . . . . . . . 158
11.8. QUERY-VOICEPRINT . . . . . . . . . . . . . . . . . . . . 159

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11.9. DELETE-VOICEPRINT . . . . . . . . . . . . . . . . . . . 160
11.10. VERIFY . . . . . . . . . . . . . . . . . . . . . . . . . 160
11.11. VERIFY-FROM-BUFFER . . . . . . . . . . . . . . . . . . . 160
11.12. VERIFY-ROLLBACK . . . . . . . . . . . . . . . . . . . . 164
11.13. STOP . . . . . . . . . . . . . . . . . . . . . . . . . . 164
11.14. START-INPUT-TIMERS . . . . . . . . . . . . . . . . . . . 165
11.15. VERIFICATION-COMPLETE . . . . . . . . . . . . . . . . . 165
11.16. START-OF-INPUT . . . . . . . . . . . . . . . . . . . . . 166
11.17. CLEAR-BUFFER . . . . . . . . . . . . . . . . . . . . . . 166
11.18. GET-INTERMEDIATE-RESULT . . . . . . . . . . . . . . . . 167
12. Security Considerations . . . . . . . . . . . . . . . . . . . 168
12.1. Rendezvous and Session Establishment . . . . . . . . . . 168
12.2. Control Channel Protection . . . . . . . . . . . . . . . 168
12.3. Media Session Protection . . . . . . . . . . . . . . . . 169
12.4. Indirect Content Access . . . . . . . . . . . . . . . . 169
12.5. Protection of Stored Media . . . . . . . . . . . . . . . 170
12.6. DTMF and Recognition Buffers . . . . . . . . . . . . . . 171
12.7. Client-Set Server Parameters . . . . . . . . . . . . . . 171
12.8. DELETE-VOICEPRINT and Authorization . . . . . . . . . . 171
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 171
13.1. New Registries . . . . . . . . . . . . . . . . . . . . . 171
13.1.1. MRCPv2 Resource Types . . . . . . . . . . . . . . . 171
13.1.2. MRCPv2 Methods and Events . . . . . . . . . . . . . 172
13.1.3. MRCPv2 Header Fields . . . . . . . . . . . . . . . . 173
13.1.4. MRCPv2 Status Codes . . . . . . . . . . . . . . . . 176
13.1.5. Grammar Reference List Parameters . . . . . . . . . 176
13.1.6. MRCPv2 Vendor-Specific Parameters . . . . . . . . . 176
13.2. NLSML-Related Registrations . . . . . . . . . . . . . . 177
13.2.1. 'application/nlsml+xml' Media Type Registration . . 177
13.3. NLSML XML Schema Registration . . . . . . . . . . . . . 178
13.4. MRCPv2 XML Namespace Registration . . . . . . . . . . . 178
13.5. Text Media Type Registrations . . . . . . . . . . . . . 178
13.5.1. text/grammar-ref-list . . . . . . . . . . . . . . . 178
13.6. 'session' URI Scheme Registration . . . . . . . . . . . 180
13.7. SDP Parameter Registrations . . . . . . . . . . . . . . 181
13.7.1. Sub-Registry "proto" . . . . . . . . . . . . . . . . 181
13.7.2. Sub-Registry "att-field (media-level)" . . . . . . . 182
14. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 183
14.1. Message Flow . . . . . . . . . . . . . . . . . . . . . . 183
14.2. Recognition Result Examples . . . . . . . . . . . . . . 192
14.2.1. Simple ASR Ambiguity . . . . . . . . . . . . . . . . 192
14.2.2. Mixed Initiative . . . . . . . . . . . . . . . . . . 192
14.2.3. DTMF Input . . . . . . . . . . . . . . . . . . . . . 193
14.2.4. Interpreting Meta-Dialog and Meta-Task Utterances . 194
14.2.5. Anaphora and Deixis . . . . . . . . . . . . . . . . 195
14.2.6. Distinguishing Individual Items from Sets with
One Member . . . . . . . . . . . . . . . . . . . . . 195
14.2.7. Extensibility . . . . . . . . . . . . . . . . . . . 196

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15. ABNF Normative Definition . . . . . . . . . . . . . . . . . . 196
16. XML Schemas . . . . . . . . . . . . . . . . . . . . . . . . . 211
16.1. NLSML Schema Definition . . . . . . . . . . . . . . . . 211
16.2. Enrollment Results Schema Definition . . . . . . . . . . 213
16.3. Verification Results Schema Definition . . . . . . . . . 214
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 218
17.1. Normative References . . . . . . . . . . . . . . . . . . 218
17.2. Informative References . . . . . . . . . . . . . . . . . 220
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 223
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 223

1. Introduction

MRCPv2 is designed to allow a client device to control media
processing resources on the network. Some of these media processing
resources include speech recognition engines, speech synthesis
engines, speaker verification, and speaker identification engines.
MRCPv2 enables the implementation of distributed Interactive Voice
Response platforms using VoiceXML [W3C.REC-voicexml20-20040316]
browsers or other client applications while maintaining separate
back-end speech processing capabilities on specialized speech
processing servers. MRCPv2 is based on the earlier Media Resource
Control Protocol (MRCP) [RFC4463] developed jointly by Cisco Systems,
Inc., Nuance Communications, and Speechworks, Inc. Although some of
the method names are similar, the way in which these methods are
communicated is different. There are also more resources and more
methods for each resource. The first version of MRCP was essentially
taken only as input to the development of this protocol. There is no
expectation that an MRCPv2 client will work with an MRCPv1 server or
vice versa. There is no migration plan or gateway definition between
the two protocols.

The protocol requirements of Speech Services Control (SPEECHSC)
[RFC4313] include that the solution be capable of reaching a media
processing server, setting up communication channels to the media
resources, and sending and receiving control messages and media
streams to/from the server. The Session Initiation Protocol (SIP)
[RFC3261] meets these requirements.

The proprietary version of MRCP ran over the Real Time Streaming
Protocol (RTSP) [RFC2326]. At the time work on MRCPv2 was begun, the
consensus was that this use of RTSP would break the RTSP protocol or
cause backward-compatibility problems, something forbidden by Section
3.2 of [RFC4313]. This is the reason why MRCPv2 does not run over
RTSP.

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MRCPv2 leverages these capabilities by building upon SIP and the
Session Description Protocol (SDP) [RFC4566]. MRCPv2 uses SIP to set
up and tear down media and control sessions with the server. In
addition, the client can use a SIP re-INVITE method (an INVITE dialog
sent within an existing SIP session) to change the characteristics of
these media and control session while maintaining the SIP dialog
between the client and server. SDP is used to describe the
parameters of the media sessions associated with that dialog. It is
mandatory to support SIP as the session establishment protocol to
ensure interoperability. Other protocols can be used for session
establishment by prior agreement. This document only describes the
use of SIP and SDP.

MRCPv2 uses SIP and SDP to create the speech client/server dialog and
set up the media channels to the server. It also uses SIP and SDP to
establish MRCPv2 control sessions between the client and the server
for each media processing resource required for that dialog. The
MRCPv2 protocol exchange between the client and the media resource is
carried on that control session. MRCPv2 exchanges do not change the
state of the SIP dialog, the media sessions, or other parameters of
the dialog initiated via SIP. It controls and affects the state of
the media processing resource associated with the MRCPv2 session(s).

MRCPv2 defines the messages to control the different media processing
resources and the state machines required to guide their operation.
It also describes how these messages are carried over a transport-
layer protocol such as the Transmission Control Protocol (TCP)
[RFC0793] or the Transport Layer Security (TLS) Protocol [RFC5246].
(Note: the Stream Control Transmission Protocol (SCTP) [RFC4960] is a
viable transport for MRCPv2 as well, but the mapping onto SCTP is not
described in this specification.)

2. Document Conventions

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 [RFC2119].

Since many of the definitions and syntax are identical to those for
the Hypertext Transfer Protocol -- HTTP/1.1 [RFC2616], this
specification refers to the section where they are defined rather
than copying it. For brevity, [HX.Y] is to be taken to refer to
Section X.Y of RFC 2616.

All the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur form (ABNF [RFC5234]).

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The complete message format in ABNF form is provided in Section 15
and is the normative format definition. Note that productions may be
duplicated within the main body of the document for reading
convenience. If a production in the body of the text conflicts with
one in the normative definition, the latter rules.

2.1. Definitions

Media Resource
An entity on the speech processing server that can be
controlled through MRCPv2.

MRCP Server
Aggregate of one or more "Media Resource" entities on
a server, exposed through MRCPv2. Often, 'server' in
this document refers to an MRCP server.

MRCP Client
An entity controlling one or more Media Resources
through MRCPv2 ("Client" for short).

DTMF
Dual-Tone Multi-Frequency; a method of transmitting
key presses in-band, either as actual tones (Q.23
[Q.23]) or as named tone events (RFC 4733 [RFC4733]).

Endpointing
The process of automatically detecting the beginning
and end of speech in an audio stream. This is
critical both for speech recognition and for automated
recording as one would find in voice mail systems.

Hotword Mode
A mode of speech recognition where a stream of
utterances is evaluated for match against a small set
of command words. This is generally employed either
to trigger some action or to control the subsequent
grammar to be used for further recognition.

2.2. State-Machine Diagrams

The state-machine diagrams in this document do not show every
possible method call. Rather, they reflect the state of the resource
based on the methods that have moved to IN-PROGRESS or COMPLETE
states (see Section 5.3). Note that since PENDING requests
essentially have not affected the resource yet and are in the queue
to be processed, they are not reflected in the state-machine
diagrams.

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2.3. URI Schemes

This document defines many protocol headers that contain URIs
(Uniform Resource Identifiers [RFC3986]) or lists of URIs for
referencing media. The entire document, including the Security
Considerations section (Section 12), assumes that HTTP or HTTP over
TLS (HTTPS) [RFC2818] will be used as the URI addressing scheme
unless otherwise stated. However, implementations MAY support other
schemes (such as 'file'), provided they have addressed any security
considerations described in this document and any others particular
to the specific scheme. For example, implementations where the
client and server both reside on the same physical hardware and the
file system is secured by traditional user-level file access controls
could be reasonable candidates for supporting the 'file' scheme.

3. Architecture

A system using MRCPv2 consists of a client that requires the
generation and/or consumption of media streams and a media resource
server that has the resources or "engines" to process these streams
as input or generate these streams as output. The client uses SIP
and SDP to establish an MRCPv2 control channel with the server to use
its media processing resources. MRCPv2 servers are addressed using
SIP URIs.

SIP uses SDP with the offer/answer model described in RFC 3264
[RFC3264] to set up the MRCPv2 control channels and describe their
characteristics. A separate MRCPv2 session is needed to control each
of the media processing resources associated with the SIP dialog
between the client and server. Within a SIP dialog, the individual
resource control channels for the different resources are added or
removed through SDP offer/answer carried in a SIP re-INVITE
transaction.

The server, through the SDP exchange, provides the client with a
difficult-to-guess, unambiguous channel identifier and a TCP port
number (see Section 4.2). The client MAY then open a new TCP
connection with the server on this port number. Multiple MRCPv2
channels can share a TCP connection between the client and the
server. All MRCPv2 messages exchanged between the client and the
server carry the specified channel identifier that the server MUST
ensure is unambiguous among all MRCPv2 control channels that are
active on that server. The client uses this channel identifier to
indicate the media processing resource associated with that channel.
For information on message framing, see Section 5.

SIP also establishes the media sessions between the client (or other
source/sink of media) and the MRCPv2 server using SDP "m=" lines.

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One or more media processing resources may share a media session
under a SIP session, or each media processing resource may have its
own media session.

The following diagram shows the general architecture of a system that
uses MRCPv2. To simplify the diagram, only a few resources are
shown.

MRCPv2 client MRCPv2 Media Resource Server
|--------------------| |------------------------------------|
||------------------|| ||----------------------------------||
|| Application Layer|| ||Synthesis|Recognition|Verification||
||------------------|| || Engine | Engine | Engine ||
||Media Resource API|| || || | || | || ||
||------------------|| ||Synthesis|Recognizer | Verifier ||
|| SIP | MRCPv2 || ||Resource | Resource | Resource ||
||Stack | || || Media Resource Management ||
|| | || ||----------------------------------||
||------------------|| || SIP | MRCPv2 ||
|| TCP/IP Stack ||---MRCPv2---|| Stack | ||
|| || ||----------------------------------||
||------------------||----SIP-----|| TCP/IP Stack ||
|--------------------| || ||
| ||----------------------------------||
SIP |------------------------------------|
| /
|-------------------| RTP
| | /
| Media Source/Sink |------------/
| |
|-------------------|

Figure 1: Architectural Diagram

3.1. MRCPv2 Media Resource Types

An MRCPv2 server may offer one or more of the following media
processing resources to its clients.

Basic Synthesizer
A speech synthesizer resource that has very limited
capabilities and can generate its media stream
exclusively from concatenated audio clips. The speech
data is described using a limited subset of the Speech
Synthesis Markup Language (SSML)
[W3C.REC-speech-synthesis-20040907] elements. A basic
synthesizer MUST support the SSML tags <speak>,
<audio>, <say-as>, and <mark>.

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Speech Synthesizer
A full-capability speech synthesis resource that can
render speech from text. Such a synthesizer MUST have
full SSML [W3C.REC-speech-synthesis-20040907] support.

Recorder
A resource capable of recording audio and providing a
URI pointer to the recording. A recorder MUST provide
endpointing capabilities for suppressing silence at
the beginning and end of a recording, and MAY also
suppress silence in the middle of a recording. If
such suppression is done, the recorder MUST maintain
timing metadata to indicate the actual timestamps of
the recorded media.

DTMF Recognizer
A recognizer resource capable of extracting and
interpreting Dual-Tone Multi-Frequency (DTMF) [Q.23]
digits in a media stream and matching them against a
supplied digit grammar. It could also do a semantic
interpretation based on semantic tags in the grammar.

Speech Recognizer
A full speech recognition resource that is capable of
receiving a media stream containing audio and
interpreting it to recognition results. It also has a
natural language semantic interpreter to post-process
the recognized data according to the semantic data in
the grammar and provide semantic results along with
the recognized input. The recognizer MAY also support
enrolled grammars, where the client can enroll and
create new personal grammars for use in future
recognition operations.

Speaker Verifier
A resource capable of verifying the authenticity of a
claimed identity by matching a media stream containing
spoken input to a pre-existing voiceprint. This may
also involve matching the caller's voice against more
than one voiceprint, also called multi-verification or
speaker identification.

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3.2. Server and Resource Addressing

The MRCPv2 server is a generic SIP server, and is thus addressed by a
SIP URI (RFC 3261 [RFC3261]).

For example:

sip:[email protected] or
sips:[email protected]

4. MRCPv2 Basics

MRCPv2 requires a connection-oriented transport-layer protocol such
as TCP to guarantee reliable sequencing and delivery of MRCPv2
control messages between the client and the server. In order to meet
the requirements for security enumerated in SPEECHSC requirements
[RFC4313], clients and servers MUST implement TLS as well. One or
more connections between the client and the server can be shared
among different MRCPv2 channels to the server. The individual
messages carry the channel identifier to differentiate messages on
different channels. MRCPv2 encoding is text based with mechanisms to
carry embedded binary data. This allows arbitrary data like
recognition grammars, recognition results, synthesizer speech markup,
etc., to be carried in MRCPv2 messages. For information on message
framing, see Section 5.

4.1. Connecting to the Server

MRCPv2 employs SIP, in conjunction with SDP, as the session
establishment and management protocol. The client reaches an MRCPv2
server using conventional INVITE and other SIP requests for
establishing, maintaining, and terminating SIP dialogs. The SDP
offer/answer exchange model over SIP is used to establish a resource
control channel for each resource. The SDP offer/answer exchange is
also used to establish media sessions between the server and the
source or sink of audio.

4.2. Managing Resource Control Channels

The client needs a separate MRCPv2 resource control channel to
control each media processing resource under the SIP dialog. A
unique channel identifier string identifies these resource control
channels. The channel identifier is a difficult-to-guess,
unambiguous string followed by an "@", then by a string token
specifying the type of resource. The server generates the channel
identifier and MUST make sure it does not clash with the identifier
of any other MRCP channel currently allocated by that server. MRCPv2
defines the following IANA-registered types of media processing

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resources. Additional resource types and their associated methods/
events and state machines may be added as described below in
Section 13.

+---------------+----------------------+--------------+
| Resource Type | Resource Description | Described in |
+---------------+----------------------+--------------+
| speechrecog | Speech Recognizer | Section 9 |
| dtmfrecog | DTMF Recognizer | Section 9 |
| speechsynth | Speech Synthesizer | Section 8 |
| basicsynth | Basic Synthesizer | Section 8 |
| speakverify | Speaker Verification | Section 11 |
| recorder | Speech Recorder | Section 10 |
+---------------+----------------------+--------------+

Table 1: Resource Types

The SIP INVITE or re-INVITE transaction and the SDP offer/answer
exchange it carries contain "m=" lines describing the resource
control channel to be allocated. There MUST be one SDP "m=" line for
each MRCPv2 resource to be used in the session. This "m=" line MUST
have a media type field of "application" and a transport type field
of either "TCP/MRCPv2" or "TCP/TLS/MRCPv2". The port number field of
the "m=" line MUST contain the "discard" port of the transport
protocol (port 9 for TCP) in the SDP offer from the client and MUST
contain the TCP listen port on the server in the SDP answer. The
client may then either set up a TCP or TLS connection to that server
port or share an already established connection to that port. Since
MRCPv2 allows multiple sessions to share the same TCP connection,
multiple "m=" lines in a single SDP document MAY share the same port
field value; MRCPv2 servers MUST NOT assume any relationship between
resources using the same port other than the sharing of the
communication channel.

MRCPv2 resources do not use the port or format field of the "m=" line
to distinguish themselves from other resources using the same
channel. The client MUST specify the resource type identifier in the
resource attribute associated with the control "m=" line of the SDP
offer. The server MUST respond with the full Channel-Identifier
(which includes the resource type identifier and a difficult-to-
guess, unambiguous string) in the "channel" attribute associated with
the control "m=" line of the SDP answer. To remain backwards
compatible with conventional SDP usage, the format field of the "m="
line MUST have the arbitrarily selected value of "1".

When the client wants to add a media processing resource to the
session, it issues a new SDP offer, according to the procedures of
RFC 3264 [RFC3264], in a SIP re-INVITE request. The SDP offer/answer

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RFC 6787 MRCPv2 November 2012

exchange carried by this SIP transaction contains one or more
additional control "m=" lines for the new resources to be allocated
to the session. The server, on seeing the new "m=" line, allocates
the resources (if they are available) and responds with a
corresponding control "m=" line in the SDP answer carried in the SIP
response. If the new resources are not available, the re-INVITE
receives an error message, and existing media processing going on
before the re-INVITE will continue as it was before. It is not
possible to allocate more than one resource of each type. If a
client requests more than one resource of any type, the server MUST
behave as if the resources of that type (beyond the first one) are
not available.

MRCPv2 clients and servers using TCP as a transport protocol MUST use
the procedures specified in RFC 4145 [RFC4145] for setting up the TCP
connection, with the considerations described hereby. Similarly,
MRCPv2 clients and servers using TCP/TLS as a transport protocol MUST
use the procedures specified in RFC 4572 [RFC4572] for setting up the
TLS connection, with the considerations described hereby. The
a=setup attribute, as described in RFC 4145 [RFC4145], MUST be
"active" for the offer from the client and MUST be "passive" for the
answer from the MRCPv2 server. The a=connection attribute MUST have
a value of "new" on the very first control "m=" line offer from the
client to an MRCPv2 server. Subsequent control "m=" line offers from
the client to the MRCP server MAY contain "new" or "existing",
depending on whether the client wants to set up a new connection or
share an existing connection, respectively. If the client specifies
a value of "new", the server MUST respond with a value of "new". If
the client specifies a value of "existing", the server MUST respond.
The legal values in the response are "existing" if the server prefers
to share an existing connection or "new" if not. In the latter case,
the client MUST initiate a new transport connection.

When the client wants to deallocate the resource from this session,
it issues a new SDP offer, according to RFC 3264 [RFC3264], where the
control "m=" line port MUST be set to 0. This SDP offer is sent in a
SIP re-INVITE request. This deallocates the associated MRCPv2
identifier and resource. The server MUST NOT close the TCP or TLS
connection if it is currently being shared among multiple MRCP
channels. When all MRCP channels that may be sharing the connection
are released and/or the associated SIP dialog is terminated, the
client or server terminates the connection.

When the client wants to tear down the whole session and all its
resources, it MUST issue a SIP BYE request to close the SIP session.
This will deallocate all the control channels and resources allocated
under the session.

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RFC 6787 MRCPv2 November 2012

All servers MUST support TLS. Servers MAY use TCP without TLS in
controlled environments (e.g., not in the public Internet) where both
nodes are inside a protected perimeter, for example, preventing
access to the MRCP server from remote nodes outside the controlled
perimeter. It is up to the client, through the SDP offer, to choose
which transport it wants to use for an MRCPv2 session. Aside from
the exceptions given above, when using TCP, the "m=" lines MUST
conform to RFC4145 [RFC4145], which describes the usage of SDP for
connection-oriented transport. When using TLS, the SDP "m=" line for
the control stream MUST conform to Connection-Oriented Media
(COMEDIA) over TLS [RFC4572], which specifies the usage of SDP for
establishing a secure connection-oriented transport over TLS.

4.3. SIP Session Example

This first example shows the power of using SIP to route to the
appropriate resource. In the example, note the use of a request to a
domain's speech server service in the INVITE to
[email protected]. The SIP routing machinery in the domain
locates the actual server, [email protected], which gets
returned in the 200 OK. Note that "cmid" is defined in Section 4.4.

This example exchange adds a resource control channel for a
synthesizer. Since a synthesizer also generates an audio stream,
this interaction also creates a receive-only Real-Time Protocol (RTP)
[RFC3550] media session for the server to send audio to. The SIP
dialog with the media source/sink is independent of MRCP and is not
shown.

C->S: INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf1
Max-Forwards:6
To:MediaServer <sip:[email protected]>
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314161 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2890844526 2890844526 IN IP4 192.0.2.12
s=-
c=IN IP4 192.0.2.12
t=0 0
m=application 9 TCP/MRCPv2 1
a=setup:active

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a=connection:new
a=resource:speechsynth
a=cmid:1
m=audio 49170 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=recvonly
a=mid:1

S->C: SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf1;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314161 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 2890842808 2890842808 IN IP4 192.0.2.11
s=-
c=IN IP4 192.0.2.11
t=0 0
m=application 32416 TCP/MRCPv2 1
a=setup:passive
a=connection:new
a=channel:32AECB234338@speechsynth
a=cmid:1
m=audio 48260 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=sendonly
a=mid:1

C->S: ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf2
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314161 ACK
Content-Length:0

Example: Add Synthesizer Control Channel

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RFC 6787 MRCPv2 November 2012

This example exchange continues from the previous figure and
allocates an additional resource control channel for a recognizer.
Since a recognizer would need to receive an audio stream for
recognition, this interaction also updates the audio stream to
sendrecv, making it a two-way RTP media session.

C->S: INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf3
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314162 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2890844526 2890844527 IN IP4 192.0.2.12
s=-
c=IN IP4 192.0.2.12
t=0 0
m=application 9 TCP/MRCPv2 1
a=setup:active
a=connection:existing
a=resource:speechsynth
a=cmid:1
m=audio 49170 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=sendrecv
a=mid:1
m=application 9 TCP/MRCPv2 1
a=setup:active
a=connection:existing
a=resource:speechrecog
a=cmid:1

S->C: SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf3;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314162 INVITE

Burnett & Shanmugham Standards Track [Page 19]

RFC 6787 MRCPv2 November 2012

Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 2890842808 2890842809 IN IP4 192.0.2.11
s=-
c=IN IP4 192.0.2.11
t=0 0
m=application 32416 TCP/MRCPv2 1
a=setup:passive
a=connection:existing
a=channel:32AECB234338@speechsynth
a=cmid:1
m=audio 48260 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=sendrecv
a=mid:1
m=application 32416 TCP/MRCPv2 1
a=setup:passive
a=connection:existing
a=channel:32AECB234338@speechrecog
a=cmid:1

C->S: ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf4
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314162 ACK
Content-Length:0

Example: Add Recognizer

This example exchange continues from the previous figure and
deallocates the recognizer channel. Since a recognizer no longer
needs to receive an audio stream, this interaction also updates the
RTP media session to recvonly.

C->S: INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf5
Max-Forwards:6

Burnett & Shanmugham Standards Track [Page 20]

RFC 6787 MRCPv2 November 2012

To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314163 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2890844526 2890844528 IN IP4 192.0.2.12
s=-
c=IN IP4 192.0.2.12
t=0 0
m=application 9 TCP/MRCPv2 1
a=resource:speechsynth
a=cmid:1
m=audio 49170 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=recvonly
a=mid:1
m=application 0 TCP/MRCPv2 1
a=resource:speechrecog
a=cmid:1

S->C: SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf5;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314163 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 2890842808 2890842810 IN IP4 192.0.2.11
s=-
c=IN IP4 192.0.2.11
t=0 0
m=application 32416 TCP/MRCPv2 1
a=channel:32AECB234338@speechsynth
a=cmid:1
m=audio 48260 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=sendonly
a=mid:1

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RFC 6787 MRCPv2 November 2012

m=application 0 TCP/MRCPv2 1
a=channel:32AECB234338@speechrecog
a=cmid:1

C->S: ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf6
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:314163 ACK
Content-Length:0

Example: Deallocate Recognizer

4.4. Media Streams and RTP Ports

Since MRCPv2 resources either generate or consume media streams, the
client or the server needs to associate media sessions with their
corresponding resource or resources. More than one resource could be
associated with a single media session or each resource could be
assigned a separate media session. Also, note that more than one
media session can be associated with a single resource if need be,
but this scenario is not useful for the current set of resources.
For example, a synthesizer and a recognizer could be associated to
the same media session (m=audio line), if it is opened in "sendrecv"
mode. Alternatively, the recognizer could have its own "sendonly"
audio session, and the synthesizer could have its own "recvonly"
audio session.

The association between control channels and their corresponding
media sessions is established using a new "resource channel media
identifier" media-level attribute ("cmid"). Valid values of this
attribute are the values of the "mid" attribute defined in RFC 5888
[RFC5888]. If there is more than one audio "m=" line, then each
audio "m=" line MUST have a "mid" attribute. Each control "m=" line
MAY have one or more "cmid" attributes that match the resource
control channel to the "mid" attributes of the audio "m=" lines it is
associated with. Note that if a control "m=" line does not have a
"cmid" attribute it will not be associated with any media. The
operations on such a resource will hence be limited. For example, if
it was a recognizer resource, the RECOGNIZE method requires an
associated media to process while the INTERPRET method does not. The
formatting of the "cmid" attribute is described by the following
ABNF:

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RFC 6787 MRCPv2 November 2012

cmid-attribute = "a=cmid:" identification-tag
identification-tag = token

To allow this flexible mapping of media sessions to MRCPv2 control
channels, a single audio "m=" line can be associated with multiple
resources, or each resource can have its own audio "m=" line. For
example, if the client wants to allocate a recognizer and a
synthesizer and associate them with a single two-way audio stream,
the SDP offer would contain two control "m=" lines and a single audio
"m=" line with an attribute of "sendrecv". Each of the control "m="
lines would have a "cmid" attribute whose value matches the "mid" of
the audio "m=" line. If, on the other hand, the client wants to
allocate a recognizer and a synthesizer each with its own separate
audio stream, the SDP offer would carry two control "m=" lines (one
for the recognizer and another for the synthesizer) and two audio
"m=" lines (one with the attribute "sendonly" and another with
attribute "recvonly"). The "cmid" attribute of the recognizer
control "m=" line would match the "mid" value of the "sendonly" audio
"m=" line, and the "cmid" attribute of the synthesizer control "m="
line would match the "mid" attribute of the "recvonly" "m=" line.

When a server receives media (e.g., audio) on a media session that is
associated with more than one media processing resource, it is the
responsibility of the server to receive and fork the media to the
resources that need to consume it. If multiple resources in an
MRCPv2 session are generating audio (or other media) to be sent on a
single associated media session, it is the responsibility of the
server either to multiplex the multiple streams onto the single RTP
session or to contain an embedded RTP mixer (see RFC 3550 [RFC3550])
to combine the multiple streams into one. In the former case, the
media stream will contain RTP packets generated by different sources,
and hence the packets will have different Synchronization Source
Identifiers (SSRCs). In the latter case, the RTP packets will
contain multiple Contributing Source Identifiers (CSRCs)
corresponding to the original streams before being combined by the
mixer. If an MRCPv2 server implementation neither multiplexes nor
mixes, it MUST disallow the client from associating multiple such
resources to a single audio stream by rejecting the SDP offer with a
SIP 488 "Not Acceptable" error. Note that there is a large installed
base that will return a SIP 501 "Not Implemented" error in this case.
To facilitate interoperability with this installed base, new
implementations SHOULD treat a 501 in this context as a 488 when it
is received from an element known to be a legacy implementation.

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RFC 6787 MRCPv2 November 2012

4.5. MRCPv2 Message Transport

The MRCPv2 messages defined in this document are transported over a
TCP or TLS connection between the client and the server. The method
for setting up this transport connection and the resource control
channel is discussed in Sections 4.1 and 4.2. Multiple resource
control channels between a client and a server that belong to
different SIP dialogs can share one or more TLS or TCP connections
between them; the server and client MUST support this mode of
operation. Clients and servers MUST use the MRCPv2 channel
identifier, carried in the Channel-Identifier header field in
individual MRCPv2 messages, to differentiate MRCPv2 messages from
different resource channels (see Section 6.2.1 for details). All
MRCPv2 servers MUST support TLS. Servers MAY use TCP without TLS in
controlled environments (e.g., not in the public Internet) where both
nodes are inside a protected perimeter, for example, preventing
access to the MRCP server from remote nodes outside the controlled
perimeter. It is up to the client to choose which mode of transport
it wants to use for an MRCPv2 session.

Most examples from here on show only the MRCPv2 messages and do not
show the SIP messages that may have been used to establish the MRCPv2
control channel.

4.6. MRCPv2 Session Termination

If an MRCP client notices that the underlying connection has been
closed for one of its MRCP channels, and it has not previously
initiated a re-INVITE to close that channel, it MUST send a BYE to
close down the SIP dialog and all other MRCP channels. If an MRCP
server notices that the underlying connection has been closed for one
of its MRCP channels, and it has not previously received and accepted
a re-INVITE closing that channel, then it MUST send a BYE to close
down the SIP dialog and all other MRCP channels.

5. MRCPv2 Specification

Except as otherwise indicated, MRCPv2 messages are Unicode encoded in
UTF-8 (RFC 3629 [RFC3629]) to allow many different languages to be
represented. DEFINE-GRAMMAR (Section 9.8), for example, is one such
exception, since its body can contain arbitrary XML in arbitrary (but
specified via XML) encodings. MRCPv2 also allows message bodies to
be represented in other character sets (for example, ISO 8859-1
[ISO.8859-1.1987]) because, in some locales, other character sets are
already in widespread use. The MRCPv2 headers (the first line of an
MRCP message) and header field names use only the US-ASCII subset of
UTF-8.

Burnett & Shanmugham Standards Track [Page 24]

RFC 6787 MRCPv2 November 2012

Lines are terminated by CRLF (carriage return, then line feed).
Also, some parameters in the message 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.

5.1. Common Protocol Elements

The MRCPv2 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, 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 / response-line / event-line

message-header = 1*(generic-header / resource-header / generic-field)

resource-header = synthesizer-header
/ recognizer-header
/ recorder-header
/ verifier-header

The message-body contains resource-specific and message-specific
data. The actual media types used to carry the data are specified in
the sections defining the individual messages. Generic header fields
are described in Section 6.2.

If a message contains a message body, the message MUST contain
content-headers indicating the media type and encoding of the data in
the message body.

Request, response and event messages (described in following
sections) include the version of MRCP that the message conforms to.
Version compatibility rules follow [H3.1] regarding version ordering,
compliance requirements, and upgrading of version numbers. The
version information is indicated by "MRCP" (as opposed to "HTTP" in
[H3.1]) or "MRCP/2.0" (as opposed to "HTTP/1.1" in [H3.1]). To be
compliant with this specification, clients and servers sending MRCPv2

Burnett & Shanmugham Standards Track [Page 25]

RFC 6787 MRCPv2 November 2012

messages MUST indicate an mrcp-version of "MRCP/2.0". ABNF
productions using mrcp-version can be found in Sections 5.2, 5.3, and
5.5.

mrcp-version = "MRCP" "/" 1*2DIGIT "." 1*2DIGIT

The message-length field specifies the length of the message in
octets, including the start-line, and MUST be the second token from
the beginning of the message. This is to make the framing and
parsing of the message simpler to do. This field specifies the
length of the message including data that may be encoded into the
body of the message. Note that this value MAY be given as a fixed-
length integer that is zero-padded (with leading zeros) in order to
eliminate or reduce inefficiency in cases where the message-length
value would change as a result of the length of the message-length
token itself. This value, as with all lengths in MRCP, is to be
interpreted as a base-10 number. In particular, leading zeros do not
indicate that the value is to be interpreted as a base-8 number.

message-length = 1*19DIGIT

The following sample MRCP exchange demonstrates proper message-length
values. The values for message-length have been removed from all
other examples in the specification and replaced by '...' to reduce
confusion in the case of minor message-length computation errors in
those examples.

C->S: MRCP/2.0 877 INTERPRET 543266
Channel-Identifier:32AECB23433801@speechrecog
Interpret-Text:may I speak to Andre Roy
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:661

<?xml version="1.0"?>

<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">

<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>

Burnett & Shanmugham Standards Track [Page 26]

RFC 6787 MRCPv2 November 2012


<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/2.0 82 543266 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C: MRCP/2.0 634 INTERPRETATION-COMPLETE 543266 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:441

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

All MRCPv2 messages, responses and events MUST carry the Channel-
Identifier header field so the server or client can differentiate
messages from different control channels that may share the same
transport connection.

In the resource-specific header field descriptions in Sections 8-11,
a header field is disallowed on a method (request, response, or
event) for that resource unless specifically listed as being allowed.
Also, the phrasing "This header field MAY occur on method X"
indicates that the header field is allowed on that method but is not
required to be used in every instance of that method.

Burnett & Shanmugham Standards Track [Page 27]

RFC 6787 MRCPv2 November 2012

5.2. Request

An MRCPv2 request consists of a Request line followed by the message
header section 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 the protocol in use.

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

The mrcp-version field is the MRCP protocol version that is being
used by the client.

The message-length field specifies the length of the message,
including the start-line.

Details about the mrcp-version and message-length fields are given in
Section 5.1.

The method-name field identifies the specific request that the client
is making to the server. Each resource supports a subset of the
MRCPv2 methods. The subset for each resource is defined in the
section of the specification for the corresponding resource.

method-name = generic-method
/ synthesizer-method
/ recognizer-method
/ recorder-method
/ verifier-method

The request-id field is a unique identifier representable as an
unsigned 32-bit integer created by the client and sent to the server.
Clients MUST utilize monotonically increasing request-ids for
consecutive requests within an MRCP session. The request-id space is
linear (i.e., not mod(32)), so the space does not wrap, and validity
can be checked with a simple unsigned comparison operation. The
client may choose any initial value for its first request, but a
small integer is RECOMMENDED to avoid exhausting the space in long
sessions. If the server receives duplicate or out-of-order requests,
the server MUST reject the request with a response code of 410.
Since request-ids are scoped to the MRCP session, they are unique
across all TCP connections and all resource channels in the session.

The server resource MUST use the client-assigned identifier in its
response to the request. If the request does not complete

Burnett & Shanmugham Standards Track [Page 28]

RFC 6787 MRCPv2 November 2012

synchronously, future asynchronous events associated with this
request MUST carry the client-assigned request-id.

request-id = 1*10DIGIT

5.3. Response

After receiving and interpreting the request message for a method,
the server resource responds with an MRCPv2 response message. The
response consists of a response line followed by the message header
section and an optional message body containing data specific to the
method.

response-line = mrcp-version SP message-length SP request-id
SP status-code SP request-state CRLF

The mrcp-version field MUST contain the version of the request if
supported; otherwise, it MUST contain the highest version of MRCP
supported by the server.

The message-length field specifies the length of the message,
including the start-line.

Details about the mrcp-version and message-length fields are given in
Section 5.1.

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.

status-code = 3DIGIT

The request-state field indicates if the action 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 or other messages from that resource to the
client with that request-id. The PENDING status means that the
request has been placed in 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 may be delivered with
that request-id.

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

Burnett & Shanmugham Standards Track [Page 29]

RFC 6787 MRCPv2 November 2012

5.4. Status Codes

The status codes are classified under the Success (2xx), Client
Failure (4xx), and Server Failure (5xx) codes.

+------------+--------------------------------------------------+
| Code | Meaning |
+------------+--------------------------------------------------+
| 200 | Success |
| 201 | Success with some optional header fields ignored |
+------------+--------------------------------------------------+

Success (2xx)

+--------+----------------------------------------------------------+
| Code | Meaning |
+--------+----------------------------------------------------------+
| 401 | Method not allowed |
| 402 | Method not valid in this state |
| 403 | Unsupported header field |
| 404 | Illegal value for header field. This is the error for a |
| | syntax violation. |
| 405 | Resource not allocated for this session or does not |
| | exist |
| 406 | Mandatory Header Field Missing |
| 407 | Method or Operation Failed (e.g., Grammar compilation |
| | failed in the recognizer. Detailed cause codes might be |
| | available through a resource-specific header.) |
| 408 | Unrecognized or unsupported message entity |
| 409 | Unsupported Header Field Value. This is a value that is |
| | syntactically legal but exceeds the implementation's |
| | capabilities or expectations. |
| 410 | Non-Monotonic or Out-of-order sequence number in request.|
| 411-420| Reserved for future assignment |
+--------+----------------------------------------------------------+

Client Failure (4xx)

+------------+--------------------------------+
| Code | Meaning |
+------------+--------------------------------+
| 501 | Server Internal Error |
| 502 | Protocol Version not supported |
| 503 | Reserved for future assignment |
| 504 | Message too large |
+------------+--------------------------------+

Server Failure (5xx)

Burnett & Shanmugham Standards Track [Page 30]

RFC 6787 MRCPv2 November 2012

5.5. Events

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. The 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 message. The
header line has the request-id of the corresponding request and
status value. The request-state value is COMPLETE if the request is
done and this was the last event, else 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 highest version of MRCP
running on the server.

The message-length field specifies the length of the message,
including the start-line.

Details about the mrcp-version and message-length fields are given in
Section 5.1.

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
the document.

event-name = synthesizer-event
/ recognizer-event
/ recorder-event
/ verifier-event

The request-id used in the event MUST match the one sent in the
request that caused this event.

The request-state indicates whether the Request/Command causing this
event is complete or still in progress and whether it is the same as
the one mentioned in Section 5.3. The final event for a request has
a COMPLETE status indicating the completion of the request.

Burnett & Shanmugham Standards Track [Page 31]

RFC 6787 MRCPv2 November 2012

6. MRCPv2 Generic Methods, Headers, and Result Structure

MRCPv2 supports a set of methods and header fields that are common to
all resources. These are discussed here; resource-specific methods
and header fields are discussed in the corresponding resource-
specific section of the document.

6.1. Generic Methods

MRCPv2 supports two generic methods for reading and writing the state
associated with a resource.

generic-method = "SET-PARAMS"
/ "GET-PARAMS"

These are described in the following subsections.

6.1.1. SET-PARAMS

The SET-PARAMS method, from the client to the server, tells the
MRCPv2 resource to define parameters for the session, such as voice
characteristics and prosody on synthesizers, recognition timers on
recognizers, etc. If the server accepts and sets all parameters, it
MUST return a response status-code of 200. If it chooses to ignore
some optional header fields that can be safely ignored without
affecting operation of the server, it MUST return 201.

If one or more of the header fields being sent is incorrect, error
403, 404, or 409 MUST be returned as follows:

o If one or more of the header fields being set has an illegal
value, the server MUST reject the request with a 404 Illegal Value
for Header Field.

o If one or more of the header fields being set is unsupported for
the resource, the server MUST reject the request with a 403
Unsupported Header Field, except as described in the next
paragraph.

o If one or more of the header fields being set has an unsupported
value, the server MUST reject the request with a 409 Unsupported
Header Field Value, except as described in the next paragraph.

If both error 404 and another error have occurred, only error 404
MUST be returned. If both errors 403 and 409 have occurred, but not
error 404, only error 403 MUST be returned.

Burnett & Shanmugham Standards Track [Page 32]

RFC 6787 MRCPv2 November 2012

If error 403, 404, or 409 is returned, the response MUST include the
bad or unsupported header fields and their values exactly as they
were sent from the client. Session parameters modified using
SET-PARAMS do not override parameters explicitly specified on
individual requests or requests that are IN-PROGRESS.

C->S: MRCP/2.0 ... SET-PARAMS 543256
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:female
Voice-variant:3

S->C: MRCP/2.0 ... 543256 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth

6.1.2. GET-PARAMS

The GET-PARAMS method, from the client to the server, asks the MRCPv2
resource for its current session parameters, such as voice
characteristics and prosody on synthesizers, recognition timers on
recognizers, etc. For every header field the client sends in the
request without a value, the server MUST include the header field and
its corresponding value in the response. If no parameter header
fields are specified by the client, then the server MUST return all
the settable parameters and their values in the corresponding header
section of the response, including vendor-specific parameters. Such
wildcard parameter requests can be very processing-intensive, since
the number of settable parameters can be large depending on the
implementation. Hence, it is RECOMMENDED that the client not use the
wildcard GET-PARAMS operation very often. Note that GET-PARAMS
returns header field values that apply to the whole session and not
values that have a request-level scope. For example, Input-Waveform-
URI is a request-level header field and thus would not be returned by
GET-PARAMS.

If all of the header fields requested are supported, the server MUST
return a response status-code of 200. If some of the header fields
being retrieved are unsupported for the resource, the server MUST
reject the request with a 403 Unsupported Header Field. Such a
response MUST include the unsupported header fields exactly as they
were sent from the client, without values.

C->S: MRCP/2.0 ... GET-PARAMS 543256
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:
Voice-variant:
Vendor-Specific-Parameters:com.example.param1;
com.example.param2

Burnett & Shanmugham Standards Track [Page 33]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... 543256 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:female
Voice-variant:3
Vendor-Specific-Parameters:com.example.param1="Company Name";
com.example.param2="[email protected]"

6.2. Generic Message Headers

All MRCPv2 header fields, which include both the generic-headers
defined in the following subsections and the resource-specific header
fields defined later, follow the same generic format as that given in
Section 3.1 of RFC 5322 [RFC5322]. Each header field consists of a
name followed by a colon (":") and the value. Header field names are
case-insensitive. The value MAY be preceded by any amount of LWS
(linear white space), though a single SP (space) is preferred.
Header fields may extend over multiple lines by preceding each extra
line with at least one SP or HT (horizontal tab).

generic-field = field-name ":" [ field-value ]
field-name = token
field-value = *LWS field-content *( CRLF 1*LWS field-content)
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, separators, and quoted-string>

The field-content does not include any leading or trailing LWS (i.e.,
linear white space occurring before the first non-whitespace
character of the field-value or after the last non-whitespace
character of the field-value). Such leading or trailing LWS MAY be
removed without changing the semantics of the field value. Any LWS
that occurs between field-content MAY be replaced with a single SP
before interpreting the field value or forwarding the message
downstream.

MRCPv2 servers and clients MUST NOT depend on header field order. It
is RECOMMENDED to send general-header fields first, followed by
request-header or response-header fields, and ending with the entity-
header fields. However, MRCPv2 servers and clients MUST be prepared
to process the header fields in any order. The only exception to
this rule is when there are multiple header fields with the same name
in a message.

Multiple header fields with the same name MAY be present in a message
if and only if the entire value for that header field is defined as a
comma-separated list [i.e., #(values)].

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Since vendor-specific parameters may be order-dependent, it MUST be
possible to combine multiple header fields of the same name into one
"name:value" pair without changing the semantics of the message, by
appending each subsequent value to the first, each separated by a
comma. The order in which header fields with the same name are
received is therefore significant to the interpretation of the
combined header field value, and thus an intermediary MUST NOT change
the order of these values when a message is forwarded.

generic-header = channel-identifier
/ accept
/ active-request-id-list
/ proxy-sync-id
/ accept-charset
/ content-type
/ content-id
/ content-base
/ content-encoding
/ content-location
/ content-length
/ fetch-timeout
/ cache-control
/ logging-tag
/ set-cookie
/ vendor-specific

6.2.1. Channel-Identifier

All MRCPv2 requests, responses, and events MUST contain the Channel-
Identifier header field. The value is allocated by the server when a
control channel is added to the session and communicated to the
client by the "a=channel" attribute in the SDP answer from the
server. The header field value consists of 2 parts separated by the
'@' symbol. The first part is an unambiguous string identifying the
MRCPv2 session. The second part is a string token that specifies one
of the media processing resource types listed in Section 3.1. The
unambiguous string (first part) MUST be difficult to guess, unique
among the resource instances managed by the server, and common to all
resource channels with that server established through a single SIP
dialog.

channel-identifier = "Channel-Identifier" ":" channel-id CRLF
channel-id = 1*alphanum "@" 1*alphanum

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6.2.2. Accept

The Accept header field follows the syntax defined in [H14.1]. The
semantics are also identical, with the exception that if no Accept
header field is present, the server MUST assume a default value that
is specific to the resource type that is being controlled. This
default value can be changed for a resource on a session by sending
this header field in a SET-PARAMS method. The current default value
of this header field for a resource in a session can be found through
a GET-PARAMS method. This header field MAY occur on any request.

6.2.3. Active-Request-Id-List

In a request, this header field indicates the list of request-ids to
which the request applies. This is useful when there are multiple
requests that are PENDING or IN-PROGRESS and the client wants this
request to apply to one or more of these specifically.

In a response, this header field returns the list of request-ids that
the method modified or affected. There could be one or more requests
in a request-state of PENDING or IN-PROGRESS. When a method
affecting one or more PENDING or IN-PROGRESS requests is sent from
the client to the server, the response MUST contain the list of
request-ids that were affected or modified by this command in its
header section.

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 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. The response to the STOP request
contains in the Active-Request-Id-List value the request-ids of all
the SPEAK requests that were terminated. After sending the STOP
response, the server MUST NOT send any SPEAK-COMPLETE or RECOGNITION-
COMPLETE events for the terminated requests.

active-request-id-list = "Active-Request-Id-List" ":"
request-id *("," request-id) CRLF

6.2.4. Proxy-Sync-Id

When any server resource generates a "barge-in-able" event, it also
generates a unique tag. The tag is sent as this header field's value
in an event to the client. The client then acts as an intermediary
among the server resources and sends a BARGE-IN-OCCURRED method to
the synthesizer server resource with the Proxy-Sync-Id it received

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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, intermediated
by the client, to decide if this event has been processed through a
direct interaction of the resources. This header field MAY occur
only on events and the BARGE-IN-OCCURRED method. The name of this
header field contains the word 'proxy' only for historical reasons
and does not imply that a proxy server is involved.

proxy-sync-id = "Proxy-Sync-Id" ":" 1*VCHAR CRLF

6.2.5. Accept-Charset

See [H14.2]. This specifies the acceptable character sets 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 Semantic Markup Language (NLSML) results of a
RECOGNITION-COMPLETE event. This header field is only used on
requests.

6.2.6. Content-Type

See [H14.17]. MRCPv2 supports a restricted set of registered media
types for content, including speech markup, grammar, and recognition
results. The content types applicable to each MRCPv2 resource-type
are specified in the corresponding section of the document and are
registered in the MIME Media Types registry maintained by IANA. The
multipart content type "multipart/mixed" is supported to communicate
multiple of the above mentioned contents, in which case the body
parts MUST NOT contain any MRCPv2-specific header fields. This
header field MAY occur on all messages.

content-type = "Content-Type" ":" media-type-value CRLF

media-type-value = type "/" subtype *( ";" parameter )

type = token

subtype = token

parameter = attribute "=" value

attribute = token

value = token / quoted-string

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6.2.7. Content-ID

This header field contains an ID or name for the content by which it
can be referenced. This header field operates according to the
specification in RFC 2392 [RFC2392] and is required for content
disambiguation in multipart messages. In MRCPv2, whenever the
associated content is stored by either the client or the server, it
MUST be retrievable using this ID. Such content can be referenced
later in a session by addressing it with the 'session' URI scheme
described in Section 13.6. This header field MAY occur on all
messages.

6.2.8. Content-Base

The Content-Base entity-header MAY be used to specify the base URI
for resolving relative URIs 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 multipart media, which in turn can have multiple entities
within it. This header field MAY occur on all messages.

6.2.9. Content-Encoding

The Content-Encoding entity-header is used as a modifier to the
Content-Type. When present, its value indicates what additional
content encoding 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. Note that the SIP session
can be used to determine accepted encodings (see Section 7). This
header field MAY occur on all messages.

content-encoding = "Content-Encoding" ":"
*WSP content-coding
*(*WSP "," *WSP content-coding *WSP )
CRLF

Content codings are defined in [H3.5]. An example of its use is
Content-Encoding:gzip

If multiple encodings have been applied to an entity, the content
encodings MUST be listed in the order in which they were applied.

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6.2.10. Content-Location

The Content-Location entity-header 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. Refer to [H14.14].

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. This header field is provided for optimization purposes
only. The receiver of this header field MAY assume that the entity
being sent is identical to what would have been retrieved or might
already have been 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 this 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. This header field MAY
occur on all messages.

6.2.11. Content-Length

This header field contains the length of the content of the message
body (i.e., after the double CRLF following the last header field).
Unlike in HTTP, it MUST be included in all messages that carry
content beyond the header section. If it is missing, a default value
of zero is assumed. Otherwise, it is interpreted according to
[H14.13]. When a message having no use for a message body contains
one, i.e., the Content-Length is non-zero, the receiver MUST ignore
the content of the message body. This header field MAY occur on all
messages.

content-length = "Content-Length" ":" 1*19DIGIT CRLF

6.2.12. Fetch Timeout

When the recognizer or synthesizer needs to fetch documents or other
resources, this header field controls the corresponding URI access
properties. This defines the timeout for content that the server may

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need to fetch over the network. The value is interpreted to be in
milliseconds and ranges from 0 to an implementation-specific maximum
value. It is RECOMMENDED that servers be cautious about accepting
long timeout values. The default value for this header field is
implementation specific. This header field MAY occur in DEFINE-
GRAMMAR, RECOGNIZE, SPEAK, SET-PARAMS, or GET-PARAMS.

fetch-timeout = "Fetch-Timeout" ":" 1*19DIGIT CRLF

6.2.13. Cache-Control

If the server implements content caching, it MUST adhere to the cache
correctness rules of HTTP 1.1 [RFC2616] when accessing and caching
stored content. In particular, the "expires" and "cache-control"
header fields of the cached URI or document MUST be honored and 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 server for the session or request. The scope of
the directive is based on the method it is sent on. If the directive
is sent on a SET-PARAMS method, it applies for all requests for
external documents the server makes during that session, unless it is
overridden by a Cache-Control header field on an individual request.
If the directives are sent on any other requests, they apply only to
external document requests the server makes for that request. An
empty Cache-Control header field on the GET-PARAMS method is a
request for the server to return the current Cache-Control directives
setting on the server. This header field MAY occur only on requests.

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*19DIGIT

Here, delta-seconds is a decimal time value specifying the number of
seconds since the instant the message response or data was received
by the server.

The different cache-directive options allow the client to ask the
server to override the default cache expiration mechanisms:

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max-age Indicates that the client can tolerate the server
using content 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 a response based on stale data.

min-fresh Indicates that the client is willing to accept a
server response with cached data whose expiration is
no less than its current age plus the specified time
in seconds. If the server's cache time-to-live
exceeds the client-supplied min-fresh value, the
server MUST NOT utilize cached content.

max-stale Indicates that the client is willing to allow a server
to utilize cached data that has exceeded its
expiration time. If "max-stale" is assigned a value,
then the client is willing to allow the server to use
cached data 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 allow the server to use stale data of any
age.

If the server cache is requested to use stale response/data without
validation, it MAY do so 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 to the corresponding URI).

If both the MRCPv2 Cache-Control directive and the cached entry on
the 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.

6.2.14. Logging-Tag

This header field MAY be sent as part of a SET-PARAMS/GET-PARAMS
method to set or retrieve the logging tag for logs generated by the
server. Once set, the value persists until a new value is set or the
session ends. The MRCPv2 server MAY provide a mechanism to create
subsets of 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.

It is RECOMMENDED that clients include in the logging tag information
to identify the MRCPv2 client User Agent, so that one can determine
which MRCPv2 client request generated a given log message at the
server. It is also RECOMMENDED that MRCPv2 clients not log

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personally identifiable information such as credit card numbers and
national identification numbers.

logging-tag = "Logging-Tag" ":" 1*UTFCHAR CRLF

6.2.15. Set-Cookie

Since the associated HTTP client on an MRCPv2 server fetches
documents for processing on behalf of the MRCPv2 client, the cookie
store in the HTTP client of the MRCPv2 server is treated as an
extension of the cookie store in the HTTP client of the MRCPv2
client. This requires that the MRCPv2 client and server be able to
synchronize their common cookie store as needed. To enable the
MRCPv2 client to push its stored cookies to the MRCPv2 server and get
new cookies from the MRCPv2 server stored back to the MRCPv2 client,
the Set-Cookie entity-header field MAY be included in MRCPv2 requests
to update the cookie store on a server and be returned in final
MRCPv2 responses or events to subsequently update the client's own
cookie store. The stored cookies on the server persist for the
duration of the MRCPv2 session and MUST be destroyed at the end of
the session. To ensure support for cookies, MRCPv2 clients and
servers MUST support the Set-Cookie entity-header field.

Note that it is the MRCPv2 client that determines which, if any,
cookies are sent to the server. There is no requirement that all
cookies be shared. Rather, it is RECOMMENDED that MRCPv2 clients
communicate only cookies needed by the MRCPv2 server to process its
requests.

set-cookie = "Set-Cookie:" cookies CRLF
cookies = cookie *("," *LWS cookie)
cookie = attribute "=" value *(";" cookie-av)
cookie-av = "Comment" "=" value
/ "Domain" "=" value
/ "Max-Age" "=" value
/ "Path" "=" value
/ "Secure"
/ "Version" "=" 1*19DIGIT
/ "Age" "=" delta-seconds

set-cookie = "Set-Cookie:" SP set-cookie-string
set-cookie-string = cookie-pair *( ";" SP cookie-av )
cookie-pair = cookie-name "=" cookie-value
cookie-name = token
cookie-value = *cookie-octet / ( DQUOTE *cookie-octet DQUOTE )
cookie-octet = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E
token = <token, defined in [RFC2616], Section 2.2>

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cookie-av = expires-av / max-age-av / domain-av /
path-av / secure-av / httponly-av /
extension-av / age-av
expires-av = "Expires=" sane-cookie-date
sane-cookie-date = <rfc1123-date, defined in [RFC2616], Section 3.3.1>
max-age-av = "Max-Age=" non-zero-digit *DIGIT
non-zero-digit = %x31-39
domain-av = "Domain=" domain-value
domain-value = <subdomain>
path-av = "Path=" path-value
path-value = <any CHAR except CTLs or ";">
secure-av = "Secure"
httponly-av = "HttpOnly"
extension-av = <any CHAR except CTLs or ";">
age-av = "Age=" delta-seconds

The Set-Cookie header field is specified in RFC 6265 [RFC6265]. The
"Age" attribute is introduced in this specification to indicate the
age of the cookie and is OPTIONAL. An MRCPv2 client or server MUST
calculate the age of the cookie according to the age calculation
rules in the HTTP/1.1 specification [RFC2616] and append the "Age"
attribute accordingly. This attribute is provided because time may
have passed since the client received the cookie from an HTTP server.
Rather than having the client reduce Max-Age by the actual age, it
passes Max-Age verbatim and appends the "Age" attribute, thus
maintaining the cookie as received while still accounting for the
fact that time has passed.

The MRCPv2 client or server MUST supply defaults for the "Domain" and
"Path" attributes, as specified in RFC 6265, if they are omitted by
the HTTP origin server. Note that there is no leading dot present in
the "Domain" attribute value in this case. Although an explicitly
specified "Domain" value received via the HTTP protocol may be
modified to include a leading dot, an MRCPv2 client or server MUST
NOT modify the "Domain" value when received via the MRCPv2 protocol.

An MRCPv2 client or server MAY combine multiple cookie header fields
of the same type into a single "field-name:field-value" pair as
described in Section 6.2.

The Set-Cookie header field MAY be specified in any request that
subsequently results in the server performing an HTTP access. When a
server receives new cookie information from an HTTP origin server,
and assuming the cookie store is modified according to RFC 6265, the
server MUST return the new cookie information in the MRCPv2 COMPLETE
response or event, as appropriate, to allow the client to update its
own cookie store.

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The SET-PARAMS request MAY specify the Set-Cookie header field to
update the cookie store on a server. The GET-PARAMS request MAY be
used to return the entire cookie store of "Set-Cookie" type cookies
to the client.

6.2.16. Vendor-Specific Parameters

This set of header fields allows for the client to set or retrieve
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 "="
value

vendor-av-pair-name = 1*UTFCHAR

Header fields of this form MAY be sent in any method (request) and
are used to manage implementation-specific parameters on the server
side. The vendor-av-pair-name follows the reverse Internet Domain
Name convention (see Section 13.1.6 for syntax and registration
information). The value of the vendor attribute is specified after
the "=" symbol and MAY be quoted. For example:

com.example.companyA.paramxyz=256
com.example.companyA.paramabc=High
com.example.companyB.paramxyz=Low

When used in GET-PARAMS to get the current value of these parameters
from the server, this header field value MAY contain a semicolon-
separated list of implementation-specific attribute names.

6.3. Generic Result Structure

Result data from the server for the Recognizer and Verifier resources
is carried as a typed media entity in the MRCPv2 message body of
various events. The Natural Language Semantics Markup Language
(NLSML), an XML markup based on an early draft from the W3C, is the
default standard for returning results back to the client. Hence,
all servers implementing these resource types MUST support the media
type 'application/nlsml+xml'. The Extensible MultiModal Annotation
(EMMA) [W3C.REC-emma-20090210] format can be used to return results
as well. This can be done by negotiating the format at session
establishment time with SDP (a=resultformat:application/emma+xml) or
with SIP (Allow/Accept). With SIP, for example, if a client wants

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results in EMMA, an MRCPv2 server can route the request to another
server that supports EMMA by inspecting the SIP header fields, rather
than having to inspect the SDP.

MRCPv2 uses this representation to convey content among the clients
and servers that generate and make use of the markup. MRCPv2 uses
NSLML specifically to convey recognition, enrollment, and
verification results between the corresponding resource on the MRCPv2
server and the MRCPv2 client. Details of this result format are
fully described in Section 6.3.1.

Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="http://theYesNoGrammar">
<interpretation>
<instance>
<ex:response>yes</ex:response>
</instance>
<input>OK</input>
</interpretation>
</result>

Result Example

6.3.1. Natural Language Semantics Markup Language

The Natural Language Semantics Markup Language (NLSML) is an XML data
structure with elements and attributes designed to carry result
information from recognizer (including enrollment) and verifier
resources. The normative definition of NLSML is the RelaxNG schema
in Section 16.1. Note that the elements and attributes of this
format are defined in the MRCPv2 namespace. In the result structure,
they must either be prefixed by a namespace prefix declared within
the result or must be children of an element identified as belonging
to the respective namespace. For details on how to use XML
Namespaces, see [W3C.REC-xml-names11-20040204]. Section 2 of
[W3C.REC-xml-names11-20040204] provides details on how to declare
namespaces and namespace prefixes.

The root element of NLSML is <result>. Optional child elements are
<interpretation>, <enrollment-result>, and <verification-result>, at
least one of which must be present. A single <result> MAY contain
any or all of the optional child elements. Details of the <result>
and <interpretation> elements and their subelements and attributes

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can be found in Section 9.6. Details of the <enrollment-result>
element and its subelements can be found in Section 9.7. Details of
the <verification-result> element and its subelements can be found in
Section 11.5.2.

7. Resource Discovery

Server resources may be discovered and their capabilities learned by
clients through standard SIP machinery. The client MAY issue a SIP
OPTIONS transaction to a server, which has the effect of requesting
the capabilities of the server. The server MUST respond to such a
request with an SDP-encoded description of its capabilities according
to RFC 3264 [RFC3264]. The MRCPv2 capabilities are described by a
single "m=" line containing the media type "application" and
transport type "TCP/TLS/MRCPv2" or "TCP/MRCPv2". There MUST be one
"resource" attribute for each media resource that the server
supports, and it has the resource type identifier as its value.

The SDP description MUST also contain "m=" lines describing the audio
capabilities and the coders the server supports.

In this example, the client uses the SIP OPTIONS method to query the
capabilities of the MRCPv2 server.

C->S:
OPTIONS sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf7
Max-Forwards:6
To:<sip:[email protected]>
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:63104 OPTIONS
Contact:<sip:[email protected]>
Accept:application/sdp
Content-Length:0

S->C:
SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bf7;received=192.0.32.10
To:<sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:63104 OPTIONS
Contact:<sip:[email protected]>
Allow:INVITE, ACK, CANCEL, OPTIONS, BYE

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Accept:application/sdp
Accept-Encoding:gzip
Accept-Language:en
Supported:foo
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2890844536 2890842811 IN IP4 192.0.2.12
s=-
i=MRCPv2 server capabilities
c=IN IP4 192.0.2.12/127
t=0 0
m=application 0 TCP/TLS/MRCPv2 1
a=resource:speechsynth
a=resource:speechrecog
a=resource:speakverify
m=audio 0 RTP/AVP 0 3
a=rtpmap:0 PCMU/8000
a=rtpmap:3 GSM/8000

Using SIP OPTIONS for MRCPv2 Server Capability Discovery

8. Speech Synthesizer Resource

This resource processes text markup provided by the client and
generates a stream of synthesized speech in real time. Depending
upon the server implementation and capability of this resource, the
client can also dictate parameters of the synthesized speech such as
voice characteristics, speaker speed, etc.

The synthesizer resource is controlled by MRCPv2 requests from the
client. Similarly, the resource can respond to these requests or
generate asynchronous events to the client to indicate conditions of
interest to the client during the generation of the synthesized
speech stream.

This section applies for the following resource types:

o speechsynth

o basicsynth

The capabilities of these resources are defined in Section 3.1.

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8.1. Synthesizer State Machine

The synthesizer maintains a state machine to process MRCPv2 requests
from the client. The state transitions shown below describe the
states of the synthesizer and reflect the state of the request at the
head of the synthesizer resource queue. A SPEAK request in the
PENDING state can be deleted or stopped by a STOP request without
affecting 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 | RESUME |
| | |<---------| |
|<---------| | |
|<---------------------STOP-------------------------|
|----------| | |
| DEFINE-LEXICON | |
| | | |
|<---------| | |
|<---------------BARGE-IN-OCCURRED------------------|

Synthesizer State Machine

8.2. Synthesizer Methods

The synthesizer supports the following methods.

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synthesizer-method = "SPEAK"
/ "STOP"
/ "PAUSE"
/ "RESUME"
/ "BARGE-IN-OCCURRED"
/ "CONTROL"
/ "DEFINE-LEXICON"

8.3. Synthesizer Events

The synthesizer can generate the following events.

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

8.4. Synthesizer Header Fields

A synthesizer method can contain header fields containing request
options and information to augment the Request, Response, or Event it
is associated with.

synthesizer-header = jump-size
/ kill-on-barge-in
/ speaker-profile
/ completion-cause
/ completion-reason
/ voice-parameter
/ prosody-parameter
/ speech-marker
/ speech-language
/ fetch-hint
/ audio-fetch-hint
/ failed-uri
/ failed-uri-cause
/ speak-restart
/ speak-length
/ load-lexicon
/ lexicon-search-order

8.4.1. Jump-Size

This header field MAY be specified in a CONTROL method and controls
the amount to jump forward or backward in an active SPEAK request. A
'+' or '-' indicates a relative value to what is being currently
played. This header field MAY also be specified in a SPEAK request
as a desired offset into the synthesized speech. In this case, the
synthesizer MUST begin speaking from this amount of time into the
speech markup. Note that an offset that extends beyond the end of

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the produced speech will result in audio of length zero. The
different speech length units supported are dependent on the
synthesizer implementation. If the synthesizer resource does not
support a unit for the operation, the resource MUST respond with a
status-code of 409 "Unsupported Header Field Value".

jump-size = "Jump-Size" ":" speech-length-value CRLF

speech-length-value = numeric-speech-length
/ text-speech-length

text-speech-length = 1*UTFCHAR SP "Tag"

numeric-speech-length = ("+" / "-") positive-speech-length

positive-speech-length = 1*19DIGIT SP numeric-speech-unit

numeric-speech-unit = "Second"
/ "Word"
/ "Sentence"
/ "Paragraph"

8.4.2. Kill-On-Barge-In

This header field 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 CRLF

The client MUST send a BARGE-IN-OCCURRED method to the synthesizer
resource when it receives a barge-in-able event from any source.
This source could be a synthesizer resource or signal detector
resource and MAY be either local or distributed. If this header
field is not specified in a SPEAK request or explicitly set by a
SET-PARAMS, the default value for this header field is "true".

If the recognizer or signal detector resource is on the same server
as the synthesizer and both are part of the same session, the server
MAY work with both to provide internal notification to the
synthesizer so that audio may be stopped without having to wait for
the client's BARGE-IN-OCCURRED event.

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

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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).

8.4.3. Speaker-Profile

This header field MAY be part of the SET-PARAMS/GET-PARAMS or SPEAK
request from the client to the server and specifies a URI that
references the profile of the speaker. Speaker profiles are
collections of voice parameters like gender, accent, etc.

speaker-profile = "Speaker-Profile" ":" uri CRLF

8.4.4. 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 the SPEAK request completed.

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 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. |
| 006 | lexicon-load-failure | Lexicon loading failed. |
| 007 | cancelled | A prior SPEAK request failed |
| | | while this one was still in |
| | | the queue. |
+------------+-----------------------+------------------------------+

Synthesizer Resource Completion Cause Codes

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8.4.5. Completion-Reason

This header field MAY be specified in a SPEAK-COMPLETE event coming
from the synthesizer resource to the client. This contains the
reason text behind the SPEAK request completion. This header field
communicates text describing the reason for the failure, such as an
error in parsing the speech markup text.

completion-reason = "Completion-Reason" ":"
quoted-string CRLF

The completion reason text is provided for client use in logs and for
debugging and instrumentation purposes. Clients MUST NOT interpret
the completion reason text.

8.4.6. Voice-Parameter

This set of header fields defines the voice of the speaker.

voice-parameter = voice-gender
/ voice-age
/ voice-variant
/ voice-name

voice-gender = "Voice-Gender:" voice-gender-value CRLF
voice-gender-value = "male"
/ "female"
/ "neutral"
voice-age = "Voice-Age:" 1*3DIGIT CRLF
voice-variant = "Voice-Variant:" 1*19DIGIT CRLF
voice-name = "Voice-Name:"
1*UTFCHAR *(1*WSP 1*UTFCHAR) CRLF

The "Voice-" parameters are derived from the similarly named
attributes of the voice element specified in W3C's Speech Synthesis
Markup Language Specification (SSML)
[W3C.REC-speech-synthesis-20040907]. Legal values for these
parameters are as defined in that specification.

These header fields MAY be sent in SET-PARAMS or GET-PARAMS requests
to define or get default values for the entire session or MAY be sent
in the SPEAK request to define default values for that SPEAK request.
Note that SSML content can itself set these values internal to the
SSML document, of course.

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Voice parameter header fields MAY 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
MUST reject the request with a status-code of 403 "Unsupported Header
Field".

8.4.7. Prosody-Parameters

This set of header fields defines the prosody of the speech.

prosody-parameter = "Prosody-" prosody-param-name ":"
prosody-param-value CRLF

prosody-param-name = 1*VCHAR

prosody-param-value = 1*VCHAR

prosody-param-name is any one of the attribute names under the
prosody element specified in W3C's Speech Synthesis Markup Language
Specification [W3C.REC-speech-synthesis-20040907]. The prosody-
param-value is any one of the value choices of the corresponding
prosody element attribute from that specification.

These header fields MAY be sent in SET-PARAMS or GET-PARAMS requests
to define or 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 change its behavior
on the fly. If the synthesizer resource does not support this
operation, it MUST respond back to the client with a status-code of
403 "Unsupported Header Field".

8.4.8. Speech-Marker

This header field contains timestamp information in a "timestamp"
field. This is a Network Time Protocol (NTP) [RFC5905] timestamp, a
64-bit number in decimal form. It MUST be synced with the Real-Time
Protocol (RTP) [RFC3550] timestamp of the media stream through the
Real-Time Control Protocol (RTCP) [RFC3550].

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Markers are bookmarks that are defined within the markup. Most
speech markup formats provide mechanisms to embed marker fields
within speech texts. The synthesizer generates SPEECH-MARKER events
when it reaches these marker fields. This header field MUST be part
of the SPEECH-MARKER event and contain the marker tag value after the
timestamp, separated by a semicolon. In these events, the timestamp
marks the time the text corresponding to the marker was emitted as
speech by the synthesizer.

This header field MUST also be returned in responses to STOP,
CONTROL, and BARGE-IN-OCCURRED methods, in the SPEAK-COMPLETE event,
and in an IN-PROGRESS SPEAK response. In these messages, if any
markers have been encountered for the current SPEAK, the marker tag
value MUST be the last embedded marker encountered. If no markers
have yet been encountered for the current SPEAK, only the timestamp
is REQUIRED. Note that in these events, the purpose of this header
field is to provide timestamp information associated with important
events within the lifecycle of a request (start of SPEAK processing,
end of SPEAK processing, receipt of CONTROL/STOP/BARGE-IN-OCCURRED).

timestamp = "timestamp" "=" time-stamp-value

time-stamp-value = 1*20DIGIT

speech-marker = "Speech-Marker" ":"
timestamp
[";" 1*(UTFCHAR / %x20)] CRLF

8.4.9. Speech-Language

This header field specifies the default language of the speech data
if the language is not specified in the markup. The value of this
header field MUST follow RFC 5646 [RFC5646] for its values. The
header field MAY occur in SPEAK, SET-PARAMS, or GET-PARAMS requests.

speech-language = "Speech-Language" ":" 1*VCHAR CRLF

8.4.10. Fetch-Hint

When the synthesizer needs to fetch documents or other resources like
speech markup or audio files, this header field controls the
corresponding URI access properties. This provides client policy on
when the synthesizer should retrieve content from the server. A
value of "prefetch" indicates the content MAY be downloaded when the
request is received, whereas "safe" indicates that content MUST NOT

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be downloaded until actually referenced. The default value is
"prefetch". This header field MAY occur in SPEAK, SET-PARAMS, or
GET-PARAMS requests.

fetch-hint = "Fetch-Hint" ":" ("prefetch" / "safe") CRLF

8.4.11. Audio-Fetch-Hint

When the synthesizer needs to fetch documents or other resources like
speech audio files, this header field controls the corresponding URI
access properties. This provides client policy whether or not the
synthesizer is permitted to attempt to optimize speech by pre-
fetching audio. The value is either "safe" to say that audio is only
fetched when it is referenced, never before; "prefetch" to permit,
but not require the implementation 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" ":"
("prefetch" / "safe" / "stream") CRLF

8.4.12. Failed-URI

When a synthesizer method needs a synthesizer to fetch or access a
URI and the access fails, the server SHOULD provide the failed URI in
this header field in the method response, unless there are multiple
URI failures, in which case the server MUST provide one of the failed
URIs in this header field in the method response.

failed-uri = "Failed-URI" ":" absoluteURI CRLF

8.4.13. Failed-URI-Cause

When a synthesizer method needs a synthesizer to fetch or access a
URI and the access fails, the server MUST provide the URI-specific or
protocol-specific response code for the URI in the Failed-URI header
field in the method response through this header field. The value
encoding is UTF-8 (RFC 3629 [RFC3629]) to accommodate any access
protocol -- some access protocols might have a response string
instead of a numeric response code.

failed-uri-cause = "Failed-URI-Cause" ":" 1*UTFCHAR CRLF

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8.4.14. Speak-Restart

When a client issues a CONTROL request to a currently speaking
synthesizer resource to jump backward, and the target jump point is
before the start of the current SPEAK request, the current SPEAK
request MUST restart from the beginning of its speech data and the
server's response to the CONTROL request MUST contain this header
field with a value of "true" indicating a restart.

speak-restart = "Speak-Restart" ":" BOOLEAN CRLF

8.4.15. Speak-Length

This header field MAY be specified in a CONTROL method to control the
maximum length of speech to speak, relative to the current speaking
point in the currently active SPEAK request. If numeric, the value
MUST be a positive integer. If a header field with a Tag unit is
specified, then the speech output continues until the tag is reached
or the SPEAK request is completed, whichever comes first. This
header field MAY be specified in a SPEAK request to indicate the
length to speak from the speech data and is relative to the point in
speech that the SPEAK request starts. The different speech length
units supported are synthesizer implementation dependent. If a
server does not support the specified unit, the server MUST respond
with a status-code of 409 "Unsupported Header Field Value".

speak-length = "Speak-Length" ":" positive-length-value
CRLF

positive-length-value = positive-speech-length
/ text-speech-length

text-speech-length = 1*UTFCHAR SP "Tag"

positive-speech-length = 1*19DIGIT SP numeric-speech-unit

numeric-speech-unit = "Second"
/ "Word"
/ "Sentence"
/ "Paragraph"

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8.4.16. Load-Lexicon

This header field is used to indicate whether a lexicon has to be
loaded or unloaded. The value "true" means to load the lexicon if
not already loaded, and the value "false" means to unload the lexicon
if it is loaded. The default value for this header field is "true".
This header field MAY be specified in a DEFINE-LEXICON method.

load-lexicon = "Load-Lexicon" ":" BOOLEAN CRLF

8.4.17. Lexicon-Search-Order

This header field is used to specify a list of active pronunciation
lexicon URIs and the search order among the active lexicons.
Lexicons specified within the SSML document take precedence over the
lexicons specified in this header field. This header field MAY be
specified in the SPEAK, SET-PARAMS, and GET-PARAMS methods.

lexicon-search-order = "Lexicon-Search-Order" ":"
"<" absoluteURI ">" *(" " "<" absoluteURI ">") CRLF

8.5. Synthesizer Message Body

A synthesizer message can contain additional information associated
with the Request, Response, or Event in its message body.

8.5.1. Synthesizer Speech Data

Marked-up text for the synthesizer to speak is specified as a typed
media entity in the message body. The speech data to be spoken by
the synthesizer can be specified inline by embedding the data in the
message body or by reference by providing a URI for accessing the
data. In either case, the data and the format used to markup the
speech needs to be of a content type supported by the server.

All MRCPv2 servers containing synthesizer resources MUST support both
plain text speech data and W3C's Speech Synthesis Markup Language
[W3C.REC-speech-synthesis-20040907] and hence MUST support the media
types 'text/plain' and 'application/ssml+xml'. Other formats MAY be
supported.

If the speech data is to be fetched by URI reference, the media type
'text/uri-list' (see RFC 2483 [RFC2483]) is used to indicate one or
more URIs that, when dereferenced, will contain the content to be
spoken. If a list of speech URIs is specified, the resource MUST
speak the speech data provided by each URI in the order in which the
URIs are specified in the content.

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MRCPv2 clients and servers MUST support the 'multipart/mixed' media
type. This is the appropriate media type to use when providing a mix
of URI and inline speech data. Embedded within the multipart content
block, there MAY be content for the 'text/uri-list', 'application/
ssml+xml', and/or 'text/plain' media types. The character set and
encoding used in the speech data is specified according to standard
media type definitions. The multipart content MAY also contain
actual audio data. Clients may have recorded audio clips stored in
memory or on a local device and wish to play it as part of the SPEAK
request. The audio portions MAY be sent by the client as part of the
multipart content block. This audio is referenced in the speech
markup data that is another part in the multipart content block
according to the 'multipart/mixed' media type specification.

Content-Type:text/uri-list
Content-Length:...

http://www.example.com/ASR-Introduction.ssml
http://www.example.com/ASR-Document-Part1.ssml
http://www.example.com/ASR-Document-Part2.ssml
http://www.example.com/ASR-Conclusion.ssml

URI List Example

Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Aldine Turnbet
and arrived at <break/>
<say-as interpret-as="vxml:time">0345p</say-as>.</s>

<s>The subject is <prosody
rate="-20%">ski trip</prosody></s>
</p>
</speak>

SSML Example

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Content-Type:multipart/mixed; boundary="break"

--break
Content-Type:text/uri-list
Content-Length:...

http://www.example.com/ASR-Introduction.ssml
http://www.example.com/ASR-Document-Part1.ssml
http://www.example.com/ASR-Document-Part2.ssml
http://www.example.com/ASR-Conclusion.ssml

--break
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams
and arrived at <break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>

<s>The subject is <prosody
rate="-20%">ski trip</prosody></s>
</p>
</speak>
--break--

Multipart Example

8.5.2. Lexicon Data

Synthesizer lexicon data from the client to the server can be
provided inline or by reference. Either way, they are carried as
typed media in the message body of the MRCPv2 request message (see
Section 8.14).

When a lexicon is specified inline in the message, the client MUST
provide a Content-ID for that lexicon as part of the content header
fields. The server MUST store the lexicon associated with that
Content-ID for the duration of the session. A stored lexicon can be
overwritten by defining a new lexicon with the same Content-ID.

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Lexicons that have been associated with a Content-ID can be
referenced through the 'session' URI scheme (see Section 13.6).

If lexicon data is specified by external URI reference, the media
type 'text/uri-list' (see RFC 2483 [RFC2483] ) is used to list the
one or more URIs that may be dereferenced to obtain the lexicon data.
All MRCPv2 servers MUST support the "http" and "https" URI access
mechanisms, and MAY support other mechanisms.

If the data in the message body consists of a mix of URI and inline
lexicon data, the 'multipart/mixed' media type is used. The
character set and encoding used in the lexicon data may be specified
according to standard media type definitions.

8.6. SPEAK Method

The SPEAK request provides the synthesizer resource with the speech
text and initiates speech synthesis and streaming. The SPEAK method
MAY carry voice and prosody header fields that alter the behavior of
the voice being synthesized, as well as a typed media message body
containing the actual marked-up text to be spoken.

The SPEAK method implementation MUST do a fetch of all external URIs
that are part of that operation. If caching is implemented, this URI
fetching MUST conform to the cache-control hints and parameter header
fields associated with the method in deciding whether it is to be
fetched from cache or from the external server. If these hints/
parameters are not specified in the method, the values set for the
session using SET-PARAMS/GET-PARAMS apply. If it was not set for the
session, their default values apply.

When applying voice parameters, there are three levels of precedence.
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 that apply for that SPEAK request only, followed by
the session default values that can be set using the SET-PARAMS
request and apply for subsequent methods invoked during the session.

If the resource was idle at the time the SPEAK request arrived at the
server and the SPEAK method is being actively processed, the resource
responds immediately with a success status code and a request-state
of IN-PROGRESS.

If the resource is in the speaking or paused state when the SPEAK
method arrives at the server, i.e., it is in the middle of processing
a previous SPEAK request, the status returns success with a request-
state of PENDING. The server places the SPEAK request in the
synthesizer resource request queue. The request queue operates

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strictly FIFO: requests are processed serially in order of receipt.
If the current SPEAK fails, all SPEAK methods in the pending queue
are cancelled and each generates a SPEAK-COMPLETE event with a
Completion-Cause of "cancelled".

For the synthesizer resource, SPEAK is the only method that can
return a request-state of IN-PROGRESS or PENDING. When the text has
been synthesized and played into the media stream, the resource
issues a SPEAK-COMPLETE event with the request-id of the SPEAK
request and a request-state of COMPLETE.

C->S: MRCP/2.0 ... SPEAK 543257
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-Age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams and arrived at
<break/>
<say-as interpret-as="vxml:time">0342p</say-as>.
</s>
<s>The subject is
<prosody rate="-20%">ski trip</prosody>
</s>
</p>
</speak>

S->C: MRCP/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059

S->C: MRCP/2.0 ... SPEAK-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Completion-Cause:000 normal
Speech-Marker:timestamp=857206027059

SPEAK Example

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8.7. STOP

The STOP method from the client to the server tells the synthesizer
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 status-code of 200 "Success". If no
Active-Request-Id-List header field is sent in the STOP request, the
server terminates all outstanding SPEAK requests.

If a STOP request successfully terminated one or more PENDING or
IN-PROGRESS SPEAK requests, then the response MUST contain an Active-
Request-Id-List header field enumerating the SPEAK request-ids that
were terminated. Otherwise, there is no Active-Request-Id-List
header field in the response. No SPEAK-COMPLETE events are sent for
such terminated requests.

If a SPEAK request that was IN-PROGRESS and speaking was stopped, the
next pending SPEAK request, if any, becomes IN-PROGRESS at the
resource and enters the speaking state.

If a SPEAK request that was IN-PROGRESS and paused was stopped, the
next pending SPEAK request, if any, becomes IN-PROGRESS and enters
the paused state.

C->S: MRCP/2.0 ... SPEAK 543258
Channel-Identifier:32AECB23433802@speechsynth
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams and arrived at
<break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>
<s>The subject is
<prosody rate="-20%">ski trip</prosody></s>
</p>
</speak>

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S->C: MRCP/2.0 ... 543258 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059

C->S: MRCP/2.0 ... STOP 543259
Channel-Identifier:32AECB23433802@speechsynth

S->C: MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258
Speech-Marker:timestamp=857206039059

STOP Example

8.8. BARGE-IN-OCCURRED

The BARGE-IN-OCCURRED method, when used with the synthesizer
resource, provides a client that has detected a barge-in-able event a
means to communicate the occurrence of the event to the synthesizer
resource.

This method is useful in two scenarios:

1. The client has detected DTMF digits in the input media or some
other barge-in-able event and wants to communicate that to the
synthesizer resource.

2. The recognizer resource and the synthesizer resource are in
different servers. In this case, the client acts as an
intermediary for the two servers. It receives an event from the
recognition resource and sends a BARGE-IN-OCCURRED request 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 (see
Section 8.4.2), and the BARGE-IN-OCCURRED event is received, the
synthesizer MUST immediately stop streaming out audio. It MUST also
terminate any speech requests queued behind the current active one,
irrespective of whether or not they have barge-in enabled. If a
barge-in-able SPEAK request was playing and it was terminated, the
response MUST contain an Active-Request-Id-List header field listing
the request-ids of all SPEAK requests that were terminated. The
server generates no SPEAK-COMPLETE events for these requests.

Burnett & Shanmugham Standards Track [Page 63]

RFC 6787 MRCPv2 November 2012

If there were no SPEAK requests terminated by the synthesizer
resource as a result of the BARGE-IN-OCCURRED method, the server MUST
respond to the BARGE-IN-OCCURRED with a status-code of 200 "Success",
and the response MUST NOT contain an Active-Request-Id-List header
field.

If the synthesizer and recognizer resources are part of the same
MRCPv2 session, they can be optimized for a quicker kill-on-barge-in
response if the recognizer and synthesizer interact directly. In
these cases, the client MUST still react to a START-OF-INPUT event
from the recognizer by invoking the BARGE-IN-OCCURRED method to the
synthesizer. The client MUST invoke the BARGE-IN-OCCURRED if it has
any outstanding requests to the synthesizer resource in either the
PENDING or IN-PROGRESS state.

C->S: MRCP/2.0 ... SPEAK 543258
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-Age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams and arrived at
<break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>
<s>The subject is
<prosody rate="-20%">ski trip</prosody></s>
</p>
</speak>

S->C: MRCP/2.0 ... 543258 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059

C->S: MRCP/2.0 ... BARGE-IN-OCCURRED 543259
Channel-Identifier:32AECB23433802@speechsynth
Proxy-Sync-Id:987654321

Burnett & Shanmugham Standards Track [Page 64]

RFC 6787 MRCPv2 November 2012

S->C:MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258
Speech-Marker:timestamp=857206039059

BARGE-IN-OCCURRED Example

8.9. PAUSE

The PAUSE method from the client to the server tells the synthesizer
resource to pause speech output if it is speaking something. If a
PAUSE method is issued on a session when a SPEAK is not active, the
server MUST respond with a status-code of 402 "Method not valid in
this state". If a PAUSE method is issued on a session when a SPEAK
is active and paused, the server MUST respond with a status-code of
200 "Success". If a SPEAK request was active, the server MUST return
an Active-Request-Id-List header field whose value contains the
request-id of the SPEAK request that was paused.

C->S: MRCP/2.0 ... SPEAK 543258
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-Age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams and arrived at
<break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>

<s>The subject is
<prosody rate="-20%">ski trip</prosody></s>
</p>
</speak>

S->C: MRCP/2.0 ... 543258 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059

Burnett & Shanmugham Standards Track [Page 65]

RFC 6787 MRCPv2 November 2012

C->S: MRCP/2.0 ... PAUSE 543259
Channel-Identifier:32AECB23433802@speechsynth

S->C: MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258

PAUSE Example

8.10. RESUME

The RESUME method from the client to the server tells a paused
synthesizer resource to resume speaking. If a RESUME request is
issued on a session with no active SPEAK request, the server MUST
respond with a status-code of 402 "Method not valid in this state".
If a RESUME request is issued on a session with an active SPEAK
request that is speaking (i.e., not paused), the server MUST respond
with a status-code of 200 "Success". If a SPEAK request was paused,
the server MUST return an Active-Request-Id-List header field whose
value contains the request-id of the SPEAK request that was resumed.

C->S: MRCP/2.0 ... SPEAK 543258
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams and arrived at
<break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>
<s>The subject is
<prosody rate="-20%">ski trip</prosody></s>
</p>
</speak>

Burnett & Shanmugham Standards Track [Page 66]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... 543258 200 IN-PROGRESS@speechsynth
Channel-Identifier:32AECB23433802
Speech-Marker:timestamp=857206027059

C->S: MRCP/2.0 ... PAUSE 543259
Channel-Identifier:32AECB23433802@speechsynth

S->C: MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258

C->S: MRCP/2.0 ... RESUME 543260
Channel-Identifier:32AECB23433802@speechsynth

S->C: MRCP/2.0 ... 543260 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258

RESUME Example

8.11. 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 request the synthesizer to jump forward or backward
in what it is speaking, change speaker rate, speaker parameters, etc.
It affects only the currently IN-PROGRESS SPEAK request. Depending
on the implementation and capability of the synthesizer resource, it
may or may not support the various modifications indicated by header
fields in the CONTROL request.

When a client invokes a CONTROL method to jump forward and the
operation goes beyond the end of the active SPEAK method's text, the
CONTROL request still succeeds. The active SPEAK request completes
and returns a SPEAK-COMPLETE event following the response to the
CONTROL method. If there are more SPEAK requests in the queue, the
synthesizer resource starts at the beginning of the next SPEAK
request in the queue.

When a client invokes a CONTROL method to jump backward and the
operation jumps to the beginning or beyond the beginning of the
speech data of the active SPEAK method, the CONTROL request still
succeeds. The response to the CONTROL request contains the speak-
restart header field, and the active SPEAK request restarts from the
beginning of its speech data.

Burnett & Shanmugham Standards Track [Page 67]

RFC 6787 MRCPv2 November 2012

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 field
containing the request-id of the SPEAK request that was active.

C->S: MRCP/2.0 ... SPEAK 543258
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams
and arrived at <break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>

<s>The subject is <prosody
rate="-20%">ski trip</prosody></s>
</p>
</speak>

S->C: MRCP/2.0 ... 543258 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857205016059

C->S: MRCP/2.0 ... CONTROL 543259
Channel-Identifier:32AECB23433802@speechsynth
Prosody-rate:fast

S->C: MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258
Speech-Marker:timestamp=857206027059

Burnett & Shanmugham Standards Track [Page 68]

RFC 6787 MRCPv2 November 2012

C->S: MRCP/2.0 ... CONTROL 543260
Channel-Identifier:32AECB23433802@speechsynth
Jump-Size:-15 Words

S->C: MRCP/2.0 ... 543260 200 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Active-Request-Id-List:543258
Speech-Marker:timestamp=857206039059

CONTROL Example

8.12. SPEAK-COMPLETE

This is an Event message from the synthesizer resource to the client
that indicates the corresponding SPEAK request was completed. The
request-id field matches the request-id of the SPEAK request that
initiated the speech that just completed. The request-state field is
set to COMPLETE by the server, indicating that this is the last event
with the corresponding request-id. 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, kill-on-barge-in, etc.

C->S: MRCP/2.0 ... SPEAK 543260
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams
and arrived at <break/>
<say-as interpret-as="vxml:time">0342p</say-as>.</s>
<s>The subject is
<prosody rate="-20%">ski trip</prosody></s>
</p>
</speak>

Burnett & Shanmugham Standards Track [Page 69]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... 543260 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059

S->C: MRCP/2.0 ... SPEAK-COMPLETE 543260 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Completion-Cause:000 normal
Speech-Marker:timestamp=857206039059

SPEAK-COMPLETE Example

8.13. SPEECH-MARKER

This is an event generated by the synthesizer resource to the client
when the synthesizer encounters a marker tag in the speech markup it
is currently processing. The value of the request-id field MUST
match that of the corresponding SPEAK request. The request-state
field MUST have the value "IN-PROGRESS" as the speech is still not
complete. The value of the speech marker tag hit, describing where
the synthesizer is in the speech markup, MUST be returned in the
Speech-Marker header field, along with an NTP timestamp indicating
the instant in the output speech stream that the marker was
encountered. The SPEECH-MARKER event MUST also be generated with a
null marker value and output NTP timestamp when a SPEAK request in
Pending-State (i.e., in the queue) changes state to IN-PROGRESS and
starts speaking. The NTP timestamp MUST be synchronized with the RTP
timestamp used to generate the speech stream through standard RTCP
machinery.

C->S: MRCP/2.0 ... SPEAK 543261
Channel-Identifier:32AECB23433802@speechsynth
Voice-gender:neutral
Voice-age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams
and arrived at <break/>

Burnett & Shanmugham Standards Track [Page 70]

RFC 6787 MRCPv2 November 2012

<say-as interpret-as="vxml:time">0342p</say-as>.</s>
<mark name="here"/>
<s>The subject is
<prosody rate="-20%">ski trip</prosody>
</s>
<mark name="ANSWER"/>
</p>
</speak>

S->C: MRCP/2.0 ... 543261 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857205015059

S->C: MRCP/2.0 ... SPEECH-MARKER 543261 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206027059;here

S->C: MRCP/2.0 ... SPEECH-MARKER 543261 IN-PROGRESS
Channel-Identifier:32AECB23433802@speechsynth
Speech-Marker:timestamp=857206039059;ANSWER

S->C: MRCP/2.0 ... SPEAK-COMPLETE 543261 COMPLETE
Channel-Identifier:32AECB23433802@speechsynth
Completion-Cause:000 normal
Speech-Marker:timestamp=857207689259;ANSWER

SPEECH-MARKER Example

8.14. DEFINE-LEXICON

The DEFINE-LEXICON method, from the client to the server, provides a
lexicon and tells the server to load or unload the lexicon (see
Section 8.4.16). The media type of the lexicon is provided in the
Content-Type header (see Section 8.5.2). One such media type is
"application/pls+xml" for the Pronunciation Lexicon Specification
(PLS) [W3C.REC-pronunciation-lexicon-20081014] [RFC4267].

If the server resource is in the speaking or paused state, the server
MUST respond with a failure status-code of 402 "Method not valid in
this state".

If the resource is in the idle state and is able to successfully
load/unload the lexicon, the status MUST return a 200 "Success"
status-code and the request-state MUST be COMPLETE.

Burnett & Shanmugham Standards Track [Page 71]

RFC 6787 MRCPv2 November 2012

If the synthesizer could not define the lexicon for some reason, for
example, because the download failed or the lexicon was in an
unsupported form, the server MUST respond with a failure status-code
of 407 and a Completion-Cause header field describing the failure
reason.

9. Speech Recognizer Resource

The speech recognizer resource receives an incoming voice stream and
provides the client with an interpretation of what was spoken in
textual form.

The recognizer resource is controlled by MRCPv2 requests from the
client. The recognizer resource can both respond to these requests
and generate asynchronous events to the client to indicate conditions
of interest during the processing of the method.

This section applies to the following resource types.

1. speechrecog

2. dtmfrecog

The difference between the above two resources is in their level of
support for recognition grammars. The "dtmfrecog" resource type is
capable of recognizing only DTMF digits and hence accepts only DTMF
grammars. It only generates barge-in for DTMF inputs and ignores
speech. The "speechrecog" resource type can recognize regular speech
as well as DTMF digits and hence MUST support grammars describing
either speech or DTMF. This resource generates barge-in events for
speech and/or DTMF. By analyzing the grammars that are activated by
the RECOGNIZE method, it determines if a barge-in should occur for
speech and/or DTMF. When the recognizer decides it needs to generate
a barge-in, it also generates a START-OF-INPUT event to the client.
The recognizer resource MAY support recognition in the normal or
hotword modes or both (although note that a single "speechrecog"
resource does not perform normal and hotword mode recognition
simultaneously). For implementations where a single recognizer
resource does not support both modes, or simultaneous normal and
hotword recognition is desired, the two modes can be invoked through
separate resources allocated to the same SIP dialog (with different
MRCP session identifiers) and share the RTP audio feed.

The capabilities of the recognizer resource are enumerated below:

Normal Mode Recognition Normal mode recognition tries to match all
of the speech or DTMF against the grammar and returns a no-match
status if the input fails to match or the method times out.

Burnett & Shanmugham Standards Track [Page 72]

RFC 6787 MRCPv2 November 2012

Hotword Mode Recognition Hotword mode is where the recognizer looks
for a match against specific speech grammar or DTMF sequence and
ignores speech or DTMF that does not match. The recognition
completes only if there is a successful match of grammar, if the
client cancels the request, or if there is a non-input or
recognition timeout.

Voice Enrolled Grammars A recognizer resource MAY optionally support
Voice Enrolled Grammars. With this functionality, enrollment is
performed using a person's voice. For example, a list of contacts
can be created and maintained by recording the person's names
using the caller's voice. This technique is sometimes also called
speaker-dependent recognition.

Interpretation A recognizer resource MAY be employed strictly for
its natural language interpretation capabilities by supplying it
with a text string as input instead of speech. In this mode, the
resource takes text as input and produces an "interpretation" of
the input according to the supplied grammar.

Voice enrollment has the concept of an enrollment session. A session
to add a new phrase to a personal grammar involves the initial
enrollment followed by a repeat of enough utterances before
committing the new phrase to the personal grammar. Each time an
utterance is recorded, it is compared for similarity with the other
samples and a clash test is performed against other entries in the
personal grammar to ensure there are no similar and confusable
entries.

Enrollment is done using a recognizer resource. Controlling which
utterances are to be considered for enrollment of a new phrase is
done by setting a header field (see Section 9.4.39) in the Recognize
request.

Interpretation is accomplished through the INTERPRET method
(Section 9.20) and the Interpret-Text header field (Section 9.4.30).

Burnett & Shanmugham Standards Track [Page 73]

RFC 6787 MRCPv2 November 2012

9.1. Recognizer State Machine

The recognizer resource maintains a state machine to process MRCPv2
requests from the client.

Idle Recognizing Recognized
State State State
| | |
|---------RECOGNIZE---->|---RECOGNITION-COMPLETE-->|
|<------STOP------------|<-----RECOGNIZE-----------|
| | |
| |--------| |-----------|
| START-OF-INPUT | GET-RESULT |
| |------->| |---------->|
|------------| | |
| DEFINE-GRAMMAR |----------| |
|<-----------| | START-INPUT-TIMERS |
| |<---------| |
|------| | |
| INTERPRET | |
|<-----| |------| |
| | RECOGNIZE |
|-------| |<-----| |
| STOP |
|<------| |
|<-------------------STOP--------------------------|
|<-------------------DEFINE-GRAMMAR----------------|

Recognizer State Machine

If a recognizer resource supports voice enrolled grammars, starting
an enrollment session does not change the state of the recognizer
resource. Once an enrollment session is started, then utterances are
enrolled by calling the RECOGNIZE method repeatedly. The state of
the speech recognizer resource goes from IDLE to RECOGNIZING state
each time RECOGNIZE is called.

9.2. Recognizer Methods

The recognizer supports the following methods.

recognizer-method = recog-only-method
/ enrollment-method

Burnett & Shanmugham Standards Track [Page 74]

RFC 6787 MRCPv2 November 2012

recog-only-method = "DEFINE-GRAMMAR"
/ "RECOGNIZE"
/ "INTERPRET"
/ "GET-RESULT"
/ "START-INPUT-TIMERS"
/ "STOP"

It is OPTIONAL for a recognizer resource to support voice enrolled
grammars. If the recognizer resource does support voice enrolled
grammars, it MUST support the following methods.

enrollment-method = "START-PHRASE-ENROLLMENT"
/ "ENROLLMENT-ROLLBACK"
/ "END-PHRASE-ENROLLMENT"
/ "MODIFY-PHRASE"
/ "DELETE-PHRASE"

9.3. Recognizer Events

The recognizer can generate the following events.

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

9.4. Recognizer Header Fields

A recognizer message can contain header fields containing request
options and information to augment the Method, Response, or Event
message it is associated with.

recognizer-header = recog-only-header
/ enrollment-header

recog-only-header = confidence-threshold
/ sensitivity-level
/ speed-vs-accuracy
/ n-best-list-length
/ no-input-timeout
/ input-type
/ recognition-timeout
/ waveform-uri
/ input-waveform-uri
/ completion-cause
/ completion-reason
/ recognizer-context-block
/ start-input-timers
/ speech-complete-timeout

Burnett & Shanmugham Standards Track [Page 75]

RFC 6787 MRCPv2 November 2012

/ speech-incomplete-timeout
/ dtmf-interdigit-timeout
/ dtmf-term-timeout
/ dtmf-term-char
/ failed-uri
/ failed-uri-cause
/ save-waveform
/ media-type
/ new-audio-channel
/ speech-language
/ ver-buffer-utterance
/ recognition-mode
/ cancel-if-queue
/ hotword-max-duration
/ hotword-min-duration
/ interpret-text
/ dtmf-buffer-time
/ clear-dtmf-buffer
/ early-no-match

If a recognizer resource supports voice enrolled grammars, the
following header fields are also used.

enrollment-header = num-min-consistent-pronunciations
/ consistency-threshold
/ clash-threshold
/ personal-grammar-uri
/ enroll-utterance
/ phrase-id
/ phrase-nl
/ weight
/ save-best-waveform
/ new-phrase-id
/ confusable-phrases-uri
/ abort-phrase-enrollment

For enrollment-specific header fields that can appear as part of
SET-PARAMS or GET-PARAMS methods, the following general rule applies:
the START-PHRASE-ENROLLMENT method MUST be invoked before these
header fields may be set through the SET-PARAMS method or retrieved
through the GET-PARAMS method.

Note that the Waveform-URI header field of the Recognizer resource
can also appear in the response to the END-PHRASE-ENROLLMENT method.

Burnett & Shanmugham Standards Track [Page 76]

RFC 6787 MRCPv2 November 2012

9.4.1. Confidence-Threshold

When a recognizer resource recognizes or matches a spoken phrase with
some portion of the grammar, it associates a confidence level with
that match. The Confidence-Threshold header field tells the
recognizer resource what confidence level the client considers a
successful match. This is a float value between 0.0-1.0 indicating
the recognizer's confidence in the recognition. If the recognizer
determines that there is no candidate match with a confidence that is
greater 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 header field
is implementation specific, as is the interpretation of any specific
value for this header field. Although values for servers from
different vendors are not comparable, it is expected that clients
will tune this value over time for a given server.

confidence-threshold = "Confidence-Threshold" ":" FLOAT CRLF

9.4.2. Sensitivity-Level

To filter out background noise and not mistake it for speech, the
recognizer resource supports a variable level of sound sensitivity.
The Sensitivity-Level header field is a float value between 0.0 and
1.0 and allows the client to set the sensitivity level for the
recognizer. This header field MAY occur in RECOGNIZE, SET-PARAMS, or
GET-PARAMS. A higher value for this header field means higher
sensitivity. The default value for this header field is
implementation specific, as is the interpretation of any specific
value for this header field. Although values for servers from
different vendors are not comparable, it is expected that clients
will tune this value over time for a given server.

sensitivity-level = "Sensitivity-Level" ":" FLOAT CRLF

9.4.3. 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
fewer active sessions per server and vice versa. The value is a
float between 0.0 and 1.0. A value of 0.0 means fastest recognition.
A value of 1.0 means best accuracy. This header field MAY occur in
RECOGNIZE, SET-PARAMS, or GET-PARAMS. The default value for this

Burnett & Shanmugham Standards Track [Page 77]

RFC 6787 MRCPv2 November 2012

header field is implementation specific. Although values for servers
from different vendors are not comparable, it is expected that
clients will tune this value over time for a given server.

speed-vs-accuracy = "Speed-Vs-Accuracy" ":" FLOAT CRLF

9.4.4. 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 recognizer resource
returns only the best match above the confidence threshold. The
client, by setting this header field, can ask the recognition
resource to send it more than one alternative. All alternatives must
still be above the Confidence-Threshold. A value greater than one
does not guarantee that the recognizer will provide the requested
number of alternatives. This header field MAY occur in RECOGNIZE,
SET-PARAMS, or GET-PARAMS. The minimum value for this header field
is 1. The default value for this header field is 1.

n-best-list-length = "N-Best-List-Length" ":" 1*19DIGIT CRLF

9.4.5. Input-Type

When the recognizer detects barge-in-able input and generates a
START-OF-INPUT event, that event MUST carry this header field to
specify whether the input that caused the barge-in was DTMF or
speech.

input-type = "Input-Type" ":" inputs CRLF
inputs = "speech" / "dtmf"

9.4.6. 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 with a Completion-Cause of "no-input-
timeout" and terminate the recognition operation. The client can use
the No-Input-Timeout header field to set this timeout. The value is
in milliseconds and can range from 0 to an implementation-specific
maximum value. This header field MAY occur in RECOGNIZE, SET-PARAMS,
or GET-PARAMS. The default value is implementation specific.

no-input-timeout = "No-Input-Timeout" ":" 1*19DIGIT CRLF

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9.4.7. 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 header field allows the client to set this
timeout value. The value is in milliseconds. The value for this
header field ranges from 0 to an implementation-specific maximum
value. The default value is 10 seconds. This header field MAY occur
in RECOGNIZE, SET-PARAMS, or GET-PARAMS.

recognition-timeout = "Recognition-Timeout" ":" 1*19DIGIT CRLF

9.4.8. Waveform-URI

If the Save-Waveform header field is set to "true", the recognizer
MUST record the incoming audio stream of the recognition into a
stored form and provide a URI for the client to access it. This
header field MUST be present in the RECOGNITION-COMPLETE event if the
Save-Waveform header field was set to "true". The value of the
header field MUST be empty if there was some error condition
preventing the server from recording. Otherwise, the URI generated
by the server MUST be unambiguous across the server and all its
recognition sessions. The content associated with the URI MUST be
available to the client until the MRCPv2 session terminates.

Similarly, if the Save-Best-Waveform header field is set to "true",
the recognizer MUST save the audio stream for the best repetition of
the phrase that was used during the enrollment session. The
recognizer MUST then record the recognized audio and make it
available to the client by returning a URI in the Waveform-URI header
field in the response to the END-PHRASE-ENROLLMENT method. The value
of the header field MUST be empty if there was some error condition
preventing the server from recording. Otherwise, the URI generated
by the server MUST be unambiguous across the server and all its
recognition sessions. The content associated with the URI MUST be
available to the client until the MRCPv2 session terminates. See the
discussion on the sensitivity of saved waveforms in Section 12.

The server MUST also return the size in octets and the duration in
milliseconds of the recorded audio waveform as parameters associated
with the header field.

waveform-uri = "Waveform-URI" ":" ["<" uri ">"
";" "size" "=" 1*19DIGIT
";" "duration" "=" 1*19DIGIT] CRLF

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9.4.9. Media-Type

This header field MAY be specified in the SET-PARAMS, GET-PARAMS, or
the RECOGNIZE methods and tells the server resource the media type in
which to store captured audio or video, such as the one captured and
returned by the Waveform-URI header field.

media-type = "Media-Type" ":" media-type-value
CRLF

9.4.10. Input-Waveform-URI

This optional header field specifies a URI pointing to audio content
to be processed by the RECOGNIZE operation. This enables the client
to request recognition from a specified buffer or audio file.

input-waveform-uri = "Input-Waveform-URI" ":" uri CRLF

9.4.11. Completion-Cause

This header field MUST be 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 MUST be
sent in the 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

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+------------+-----------------------+------------------------------+
| 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 |
| | | grammar load failure. |
| | | |
| 005 | grammar-compilation- | RECOGNIZE failed due to |
| | failure | 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 happens when the |
| | | audio stream is already |
| | | "in-speech" when the |
| | | RECOGNIZE request was |
| | | received. |
| | | |
| 008 | success-maxtime | RECOGNIZE request terminated |
| | | because speech was too long |
| | | but whatever was spoken till |
| | | that point was a full match. |
| | | |
| 009 | uri-failure | Failure accessing a URI. |
| | | |
| 010 | language-unsupported | Language not supported. |
| | | |

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| 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 MUST 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 whatever was |
| | | spoken till that point was a |
| | | partial match to one or more |
| | | grammars. |
| | | |
| 014 | partial-match-maxtime | The Recognition-Timeout |
| | | expired before full match |
| | | was achieved. But whatever |
| | | was spoken till that point |
| | | was a partial match to one |
| | | or more grammars. |
| | | |
| 015 | no-match-maxtime | The Recognition-Timeout |
| | | expired. Whatever was spoken |
| | | till that point did not |
| | | match any of the grammars. |
| | | This cause could also be |
| | | returned if the recognizer |
| | | does not support detecting |
| | | partial grammar matches. |
| | | |
| 016 | grammar-definition- | Any DEFINE-GRAMMAR error |
| | failure | other than |
| | | grammar-load-failure and |
| | | grammar-compilation-failure. |
+------------+-----------------------+------------------------------+

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9.4.12. Completion-Reason

This header field MAY be specified in a RECOGNITION-COMPLETE event
coming from the recognizer resource to the client. This contains the
reason text behind the RECOGNIZE request completion. The server uses
this header field to communicate text describing the reason for the
failure, such as the specific error encountered in parsing a grammar
markup.

The completion reason text is provided for client use in logs and for
debugging and instrumentation purposes. Clients MUST NOT interpret
the completion reason text.

completion-reason = "Completion-Reason" ":"
quoted-string CRLF

9.4.13. Recognizer-Context-Block

This header field 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 typed media message body. If the
server returns a recognizer context block, the response MUST contain
this header field and its value MUST match the Content-ID of the
corresponding media block.

If the SET-PARAMS method contains this header field, it MUST also
contain a message body containing the recognizer context data and a
Content-ID matching this header field value. This Content-ID MUST
match the Content-ID that came with the context data during the
GET-PARAMS operation.

An implementation choosing to use this mechanism to hand off
recognizer context data between servers MUST distinguish its
implementation-specific block of data by using an IANA-registered
content type in the IANA Media Type vendor tree.

recognizer-context-block = "Recognizer-Context-Block" ":"
[1*VCHAR] CRLF

9.4.14. Start-Input-Timers

This header field 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 MUST NOT start the
timers until the client sends a START-INPUT-TIMERS request to the
recognizer. This is useful in the scenario when the recognizer and

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synthesizer engines are not part of the same session. In such
configurations, when a kill-on-barge-in prompt is being played (see
Section 8.4.2), the client wants the RECOGNIZE request to be
simultaneously active so that it can detect and implement kill-on-
barge-in. However, the recognizer SHOULD NOT start the no-input
timers until the prompt is finished. The default value is "true".

start-input-timers = "Start-Input-Timers" ":" BOOLEAN CRLF

9.4.15. 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 generating a no-match result). The Speech-Complete-
Timeout value applies when the recognizer currently has a complete
match against an active grammar, and specifies how long the
recognizer MUST wait for more input before declaring a match. By
contrast, the Speech-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*19DIGIT CRLF

A long Speech-Complete-Timeout value delays the result to the client
and therefore makes the application's response to a user slow. A
short Speech-Complete-Timeout may lead to an utterance being broken
up inappropriately. Reasonable speech complete timeout values are
typically in the range of 0.3 seconds to 1.0 seconds. The value for
this header field ranges from 0 to an implementation-specific maximum
value. The default value for this header field is implementation
specific. This header field MAY occur in RECOGNIZE, SET-PARAMS, or
GET-PARAMS.

9.4.16. 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
Completion-Cause of "partial-match"). The value is in milliseconds.
The value for this header field ranges from 0 to an implementation-
specific maximum value. The default value for this header field is
implementation specific.

speech-incomplete-timeout = "Speech-Incomplete-Timeout" ":" 1*19DIGIT
CRLF

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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 Speech-Complete-Timeout is used when the speech is a complete
match to an active grammar and no further spoken words can continue
to represent a match.

A long Speech-Incomplete-Timeout value delays the result to the
client and therefore makes the application's response to a user 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.

9.4.17. 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 header field ranges from 0 to an implementation-specific maximum
value. The default value is 5 seconds. This header field MAY occur
in RECOGNIZE, SET-PARAMS, or GET-PARAMS.

dtmf-interdigit-timeout = "DTMF-Interdigit-Timeout" ":" 1*19DIGIT CRLF

9.4.18. DTMF-Term-Timeout

This header field specifies the terminating timeout to use when
recognizing DTMF input. The DTMF-Term-Timeout applies only when no
additional input is allowed by the grammar; otherwise, the
DTMF-Interdigit-Timeout applies. The value is in milliseconds. The
value for this header field ranges from 0 to an implementation-
specific maximum value. 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*19DIGIT CRLF

9.4.19. DTMF-Term-Char

This header field specifies the terminating DTMF character for DTMF
input recognition. The default value is NULL, which is indicated by
an empty header field value. This header field MAY occur in
RECOGNIZE, SET-PARAMS, or GET-PARAMS.

dtmf-term-char = "DTMF-Term-Char" ":" VCHAR CRLF

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9.4.20. Failed-URI

When a recognizer needs to fetch or access a URI and the access
fails, the server SHOULD provide the failed URI in this header field
in the method response, unless there are multiple URI failures, in
which case one of the failed URIs MUST be provided in this header
field in the method response.

failed-uri = "Failed-URI" ":" absoluteURI CRLF

9.4.21. Failed-URI-Cause

When a recognizer method needs a recognizer to fetch or access a URI
and the access fails, the server MUST provide the URI-specific or
protocol-specific response code for the URI in the Failed-URI header
field through this header field in the method response. The value
encoding is UTF-8 (RFC 3629 [RFC3629]) to accommodate any access
protocol, some of which might have a response string instead of a
numeric response code.

failed-uri-cause = "Failed-URI-Cause" ":" 1*UTFCHAR CRLF

9.4.22. Save-Waveform

This header field allows the client to request the recognizer
resource to save the audio input to the recognizer. The recognizer
resource MUST then attempt to record the recognized audio, without
endpointing, 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
content is otherwise not available, the recognizer MUST return an
empty Waveform-URI header field. The default value for this field is
"false". This header field MAY occur in RECOGNIZE, SET-PARAMS, or
GET-PARAMS. See the discussion on the sensitivity of saved waveforms
in Section 12.

save-waveform = "Save-Waveform" ":" BOOLEAN CRLF

9.4.23. New-Audio-Channel

This header field MAY be specified in a RECOGNIZE request and allows
the client to tell the server that, from this point on, further input
audio comes from a different audio source, channel, or speaker. If
the recognizer resource had collected any input statistics or
adaptation state, the recognizer resource MUST do what is appropriate
for the specific recognition technology, which includes but is not
limited to discarding any collected input statistics or adaptation
state before starting the RECOGNIZE request. Note that if there are

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multiple resources that are sharing a media stream and are collecting
or using this data, and the client issues this header field to one of
the resources, the reset operation applies to all resources that use
the shared media stream. This helps in a number of use cases,
including where the client wishes to reuse an open recognition
session with an existing media session for multiple telephone calls.

new-audio-channel = "New-Audio-Channel" ":" BOOLEAN
CRLF

9.4.24. 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 MUST follow RFC 5646 [RFC5646] for its
values. This MAY occur in DEFINE-GRAMMAR, RECOGNIZE, SET-PARAMS, or
GET-PARAMS requests.

speech-language = "Speech-Language" ":" 1*VCHAR CRLF

9.4.25. Ver-Buffer-Utterance

This header field lets the client request the server to buffer the
utterance associated with this recognition request into a buffer
available to a co-resident verifier resource. The buffer is shared
across resources within a session and is allocated when a verifier
resource is added to this session. The client MUST NOT send this
header field unless a verifier resource is instantiated for the
session. The buffer is released when the verifier resource is
released from the session.

9.4.26. Recognition-Mode

This header field specifies what mode the RECOGNIZE method will
operate in. The value choices are "normal" or "hotword". If the
value is "normal", the RECOGNIZE starts matching speech and DTMF to
the grammars specified in the RECOGNIZE request. If any portion of
the speech does not match the grammar, the RECOGNIZE command
completes with a no-match status. Timers may be active to detect
speech in the audio (see Section 9.4.14), so the RECOGNIZE method may
complete because of a timeout waiting for speech. If the value of
this header field is "hotword", the RECOGNIZE method operates in
hotword mode, where it only looks for the particular keywords or DTMF

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sequences specified in the grammar and ignores silence or other
speech in the audio stream. The default value for this header field
is "normal". This header field MAY occur on the RECOGNIZE method.

recognition-mode = "Recognition-Mode" ":"
"normal" / "hotword" CRLF

9.4.27. Cancel-If-Queue

This header field specifies what will happen if the client attempts
to invoke another RECOGNIZE method when this RECOGNIZE request is
already in progress for the resource. The value for this header
field is a Boolean. A value of "true" means the server MUST
terminate this RECOGNIZE request, with a Completion-Cause of
"cancelled", if the client issues another RECOGNIZE request for the
same resource. A value of "false" for this header field indicates to
the server that this RECOGNIZE request will continue to completion,
and if the client issues more RECOGNIZE requests to the same
resource, they are queued. When the currently active RECOGNIZE
request is stopped or completes with a successful match, the first
RECOGNIZE method in the queue becomes active. If the current
RECOGNIZE fails, all RECOGNIZE methods in the pending queue are
cancelled, and each generates a RECOGNITION-COMPLETE event with a
Completion-Cause of "cancelled". This header field MUST be present
in every RECOGNIZE request. There is no default value.

cancel-if-queue = "Cancel-If-Queue" ":" BOOLEAN CRLF

9.4.28. Hotword-Max-Duration

This header field MAY be sent in a hotword mode RECOGNIZE request.
It specifies the maximum length of an utterance (in seconds) that
will be considered for hotword recognition. This header field, along
with Hotword-Min-Duration, can be used to tune performance by
preventing the recognizer from evaluating utterances that are too
short or too long to be one of the hotwords in the grammar(s). The
value is in milliseconds. The default is implementation dependent.
If present in a RECOGNIZE request specifying a mode other than
"hotword", the header field is ignored.

hotword-max-duration = "Hotword-Max-Duration" ":" 1*19DIGIT
CRLF

9.4.29. Hotword-Min-Duration

This header field MAY be sent in a hotword mode RECOGNIZE request.
It specifies the minimum length of an utterance (in seconds) that
will be considered for hotword recognition. This header field, along

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with Hotword-Max-Duration, can be used to tune performance by
preventing the recognizer from evaluating utterances that are too
short or too long to be one of the hotwords in the grammar(s). The
value is in milliseconds. The default value is implementation
dependent. If present in a RECOGNIZE request specifying a mode other
than "hotword", the header field is ignored.

hotword-min-duration = "Hotword-Min-Duration" ":" 1*19DIGIT CRLF

9.4.30. Interpret-Text

The value of this header field is used to provide a pointer to the
text for which a natural language interpretation is desired. The
value is either a URI or text. If the value is a URI, it MUST be a
Content-ID that refers to an entity of type 'text/plain' in the body
of the message. Otherwise, the server MUST treat the value as the
text to be interpreted. This header field MUST be used when invoking
the INTERPRET method.

interpret-text = "Interpret-Text" ":" 1*VCHAR CRLF

9.4.31. DTMF-Buffer-Time

This header field MAY be specified in a GET-PARAMS or SET-PARAMS
method and is used to specify the amount of time, in milliseconds, of
the type-ahead buffer for the recognizer. This is the buffer that
collects DTMF digits as they are pressed even when there is no
RECOGNIZE command active. When a subsequent RECOGNIZE method is
received, it MUST look to this buffer to match the RECOGNIZE request.
If the digits in the buffer are not sufficient, then it can continue
to listen to more digits to match the grammar. The default size of
this DTMF buffer is platform specific.

dtmf-buffer-time = "DTMF-Buffer-Time" ":" 1*19DIGIT CRLF

9.4.32. Clear-DTMF-Buffer

This header field MAY be specified in a RECOGNIZE method and is used
to tell the recognizer to clear the DTMF type-ahead buffer before
starting the RECOGNIZE. The default value of this header field is
"false", which does not clear the type-ahead buffer before starting
the RECOGNIZE method. If this header field is specified to be
"true", then the RECOGNIZE will clear the DTMF buffer before starting
recognition. This means digits pressed by the caller before the
RECOGNIZE command was issued are discarded.

clear-dtmf-buffer = "Clear-DTMF-Buffer" ":" BOOLEAN CRLF

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9.4.33. Early-No-Match

This header field MAY be specified in a RECOGNIZE method and is used
to tell the recognizer that it MUST NOT wait for the end of speech
before processing the collected speech to match active grammars. A
value of "true" indicates the recognizer MUST do early matching. The
default value for this header field if not specified is "false". If
the recognizer does not support the processing of the collected audio
before the end of speech, this header field can be safely ignored.

early-no-match = "Early-No-Match" ":" BOOLEAN CRLF

9.4.34. Num-Min-Consistent-Pronunciations

This header field MAY be specified in a START-PHRASE-ENROLLMENT,
SET-PARAMS, or GET-PARAMS method and is used to specify the minimum
number of consistent pronunciations that must be obtained to voice
enroll a new phrase. The minimum value is 1. The default value is
implementation specific and MAY be greater than 1.

num-min-consistent-pronunciations =
"Num-Min-Consistent-Pronunciations" ":" 1*19DIGIT CRLF

9.4.35. Consistency-Threshold

This header field MAY be sent as part of the START-PHRASE-ENROLLMENT,
SET-PARAMS, or GET-PARAMS method. Used during voice enrollment, this
header field specifies how similar to a previously enrolled
pronunciation of the same phrase an utterance needs to be in order to
be considered "consistent". The higher the threshold, the closer the
match between an utterance and previous pronunciations must be for
the pronunciation to be considered consistent. The range for this
threshold is a float value between 0.0 and 1.0. The default value
for this header field is implementation specific.

consistency-threshold = "Consistency-Threshold" ":" FLOAT CRLF

9.4.36. Clash-Threshold

This header field MAY be sent as part of the START-PHRASE-ENROLLMENT,
SET-PARAMS, or GET-PARAMS method. Used during voice enrollment, this
header field specifies how similar the pronunciations of two
different phrases can be before they are considered to be clashing.
For example, pronunciations of phrases such as "John Smith" and "Jon
Smits" may be so similar that they are difficult to distinguish
correctly. A smaller threshold reduces the number of clashes
detected. The range for this threshold is a float value between 0.0

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and 1.0. The default value for this header field is implementation
specific. Clash testing can be turned off completely by setting the
Clash-Threshold header field value to 0.

clash-threshold = "Clash-Threshold" ":" FLOAT CRLF

9.4.37. Personal-Grammar-URI

This header field specifies the speaker-trained grammar to be used or
referenced during enrollment operations. Phrases are added to this
grammar during enrollment. For example, a contact list for user
"Jeff" could be stored at the Personal-Grammar-URI
"http://myserver.example.com/myenrollmentdb/jeff-list". The
generated grammar syntax MAY be implementation specific. There is no
default value for this header field. This header field MAY be sent
as part of the START-PHRASE-ENROLLMENT, SET-PARAMS, or GET-PARAMS
method.

personal-grammar-uri = "Personal-Grammar-URI" ":" uri CRLF

9.4.38. Enroll-Utterance

This header field MAY be specified in the RECOGNIZE method. If this
header field is set to "true" and an Enrollment is active, the
RECOGNIZE command MUST add the collected utterance to the personal
grammar that is being enrolled. The way in which this occurs is
engine specific and may be an area of future standardization. The
default value for this header field is "false".

enroll-utterance = "Enroll-Utterance" ":" BOOLEAN CRLF

9.4.39. Phrase-Id

This header field in a request identifies a phrase in an existing
personal grammar for which enrollment is desired. It is also
returned to the client in the RECOGNIZE complete event. This header
field MAY occur in START-PHRASE-ENROLLMENT, MODIFY-PHRASE, or DELETE-
PHRASE requests. There is no default value for this header field.

phrase-id = "Phrase-ID" ":" 1*VCHAR CRLF

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9.4.40. Phrase-NL

This string specifies the interpreted text to be returned when the
phrase is recognized. This header field MAY occur in START-PHRASE-
ENROLLMENT and MODIFY-PHRASE requests. There is no default value for
this header field.

phrase-nl = "Phrase-NL" ":" 1*UTFCHAR CRLF

9.4.41. Weight

The value of this header field represents the occurrence likelihood
of a phrase in an enrolled grammar. When using grammar enrollment,
the system is essentially constructing a grammar segment consisting
of a list of possible match phrases. This can be thought of to be
similar to the dynamic construction of a <one-of> tag in the W3C
grammar specification. Each enrolled-phrase becomes an item in the
list that can be matched against spoken input similar to the <item>
within a <one-of> list. This header field allows you to assign a
weight to the phrase (i.e., <item> entry) in the <one-of> list that
is enrolled. Grammar weights are normalized to a sum of one at
grammar compilation time, so a weight value of 1 for each phrase in
an enrolled grammar list indicates all items in that list have the
same weight. This header field MAY occur in START-PHRASE-ENROLLMENT
and MODIFY-PHRASE requests. The default value for this header field
is implementation specific.

weight = "Weight" ":" FLOAT CRLF

9.4.42. Save-Best-Waveform

This header field allows the client to request the recognizer
resource to save the audio stream for the best repetition of the
phrase that was used during the enrollment session. The recognizer
MUST attempt to 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 response to the END-PHRASE-ENROLLMENT method. If there
was an error in recording the stream or the audio data is otherwise
not available, the recognizer MUST return an empty Waveform-URI
header field. This header field MAY occur in the START-PHRASE-
ENROLLMENT, SET-PARAMS, and GET-PARAMS methods.

save-best-waveform = "Save-Best-Waveform" ":" BOOLEAN CRLF

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9.4.43. New-Phrase-Id

This header field replaces the ID used to identify the phrase in a
personal grammar. The recognizer returns the new ID when using an
enrollment grammar. This header field MAY occur in MODIFY-PHRASE
requests.

new-phrase-id = "New-Phrase-ID" ":" 1*VCHAR CRLF

9.4.44. Confusable-Phrases-URI

This header field specifies a grammar that defines invalid phrases
for enrollment. For example, typical applications do not allow an
enrolled phrase that is also a command word. This header field MAY
occur in RECOGNIZE requests that are part of an enrollment session.

confusable-phrases-uri = "Confusable-Phrases-URI" ":" uri CRLF

9.4.45. Abort-Phrase-Enrollment

This header field MAY be specified in the END-PHRASE-ENROLLMENT
method to abort the phrase enrollment, rather than committing the
phrase to the personal grammar.

abort-phrase-enrollment = "Abort-Phrase-Enrollment" ":"
BOOLEAN CRLF

9.5. Recognizer Message Body

A recognizer message can carry additional data associated with the
request, response, or event. The client MAY provide the grammar to
be recognized in DEFINE-GRAMMAR or RECOGNIZE requests. When one or
more grammars are specified using the DEFINE-GRAMMAR method, the
server MUST attempt to fetch, compile, and optimize the grammar
before returning a response to the DEFINE-GRAMMAR method. A
RECOGNIZE request MUST completely specify the grammars to be active
during the recognition operation, except when the RECOGNIZE method is
being used to enroll a grammar. During grammar enrollment, such
grammars are OPTIONAL. The server resource sends the recognition
results in the RECOGNITION-COMPLETE event and the GET-RESULT
response. Grammars and recognition results are carried in the
message body of the corresponding MRCPv2 messages.

9.5.1. Recognizer Grammar Data

Recognizer grammar data from the client to the server can be provided
inline or by reference. Either way, grammar data is carried as typed
media entities in the message body of the RECOGNIZE or DEFINE-GRAMMAR

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request. All MRCPv2 servers MUST accept grammars in the XML form
(media type 'application/srgs+xml') of the W3C's XML-based Speech
Grammar Markup Format (SRGS) [W3C.REC-speech-grammar-20040316] and
MAY accept grammars in other formats. Examples include but are not
limited to:

o the ABNF form (media type 'application/srgs') of SRGS

o Sun's Java Speech Grammar Format (JSGF)
[refs.javaSpeechGrammarFormat]

Additionally, MRCPv2 servers MAY support the Semantic Interpretation
for Speech Recognition (SISR)
[W3C.REC-semantic-interpretation-20070405] specification.

When a grammar is specified inline in the request, the client MUST
provide a Content-ID for that grammar as part of the content header
fields. If there is no space on the server to store the inline
grammar, the request MUST return with a Completion-Cause code of 016
"grammar-definition-failure". Otherwise, the server MUST associate
the inline grammar block with that Content-ID and MUST store it on
the server for the duration of the session. However, if the
Content-ID is redefined later in the session through a subsequent
DEFINE-GRAMMAR, the inline grammar previously associated with the
Content-ID MUST be freed. If the Content-ID is redefined through a
subsequent DEFINE-GRAMMAR with an empty message body (i.e., no
grammar definition), then in addition to freeing any grammar
previously associated with the Content-ID, the server MUST clear all
bindings and associations to the Content-ID. Unless and until
subsequently redefined, this URI MUST be interpreted by the server as
one that has never been set.

Grammars that have been associated with a Content-ID can be
referenced through the 'session' URI scheme (see Section 13.6). For
example:
session:[email protected]

Grammar data MAY be specified using external URI references. To do
so, the client uses a body of media type 'text/uri-list' (see RFC
2483 [RFC2483] ) to list the one or more URIs that point to the
grammar data. The client can use a body of media type 'text/
grammar-ref-list' (see Section 13.5.1) if it wants to assign weights
to the list of grammar URI. All MRCPv2 servers MUST support grammar
access using the 'http' and 'https' URI schemes.

If the grammar data the client wishes to be used on a request
consists of a mix of URI and inline grammar data, the client uses the
'multipart/mixed' media type to enclose the 'text/uri-list',

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'application/srgs', or 'application/srgs+xml' content entities. The
character set and encoding used in the grammar data are specified
using to standard media type definitions.

When more than one grammar URI or inline grammar block is specified
in a message body of the RECOGNIZE request, the server interprets
this as a list of grammar alternatives to match against.

Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>


<rule id="people1">
<token lexicon="en-US,fr-CA"> Robert </token>
</rule>

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<rule id="people2">
<one-of>
<item xml:lang="en-US">Robert</item>
<item xml:lang="fr-CA">Robert</item>
</one-of>
</rule>

</grammar>

SRGS Grammar Example

Content-Type:text/uri-list
Content-Length:...

session:[email protected]
http://www.example.com/Directory-Name-List.grxml
http://www.example.com/Department-List.grxml
http://www.example.com/TAC-Contact-List.grxml
session:[email protected]

Grammar Reference Example

Content-Type:multipart/mixed; boundary="break"

--break
Content-Type:text/uri-list
Content-Length:...

http://www.example.com/Directory-Name-List.grxml
http://www.example.com/Department-List.grxml
http://www.example.com/TAC-Contact-List.grxml

--break
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

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<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<rule id="request">
may I speak to
<one-of xml:lang="fr-CA">
<item>Michel Tremblay</item>
<item>Andre Roy</item>
</one-of>
</rule>


<rule id="people1">
<token lexicon="en-US,fr-CA"> Robert </token>
</rule>


<rule id="people2">
<one-of>
<item xml:lang="en-US">Robert</item>
<item xml:lang="fr-CA">Robert</item>
</one-of>
</rule>

</grammar>
--break--

Mixed Grammar Reference Example

9.5.2. Recognizer Result Data

Recognition results are returned to the client in the message body of
the RECOGNITION-COMPLETE event or the GET-RESULT response message as
described in Section 6.3. Element and attribute descriptions for the
recognition portion of the NLSML format are provided in Section 9.6
with a normative definition of the schema in Section 16.1.

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Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="http://www.example.com/theYesNoGrammar">
<interpretation>
<instance>
<ex:response>yes</ex:response>
</instance>
<input>OK</input>
</interpretation>
</result>

Result Example

9.5.3. Enrollment Result Data

Enrollment results are returned to the client in the message body of
the RECOGNITION-COMPLETE event as described in Section 6.3. Element
and attribute descriptions for the enrollment portion of the NLSML
format are provided in Section 9.7 with a normative definition of the
schema in Section 16.2.

9.5.4. Recognizer Context Block

When a client changes servers while operating on the behalf of the
same incoming communication session, this header field allows the
client to collect a block of opaque data from one server and provide
it to another server. This capability is desirable if the client
needs different language support or because the server issued a
redirect. Here, the first recognizer resource may have collected
acoustic and other data during its execution of recognition methods.
After a server switch, communicating this data may allow the
recognizer resource on the new server to provide better recognition.
This block of data is implementation specific and MUST be carried as
media 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, if any, in the
message body as an entity of media type 'application/octets' with a
specific Content-ID. The Content-ID value MUST also be specified in
the Recognizer-Context-Block header field in the GET-PARAMS response.
The SET-PARAMS request wishing to provide this vendor-specific data
MUST send it in the message body as a typed entity with the same

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Content-ID that it received from the GET-PARAMS. The Content-ID MUST
also be sent in the Recognizer-Context-Block header field of the
SET-PARAMS message.

Each speech recognition implementation choosing to use this mechanism
to hand off recognizer context data among servers MUST distinguish
its implementation-specific block of data from other implementations
by choosing a Content-ID that is recognizable among the participating
servers and unlikely to collide with values chosen by another
implementation.

9.6. Recognizer Results

The recognizer portion of NLSML (see Section 6.3.1) represents
information automatically extracted from a user's utterances by a
semantic interpretation component, where "utterance" is to be taken
in the general sense of a meaningful user input in any modality
supported by the MRCPv2 implementation.

9.6.1. Markup Functions

MRCPv2 recognizer resources employ the Natural Language Semantics
Markup Language (NLSML) to interpret natural language speech input
and to format the interpretation for consumption by an MRCPv2 client.

The elements of the markup fall into the following general functional
categories: interpretation, side information, and multi-modal
integration.

9.6.1.1. Interpretation

Elements and attributes represent the semantics of a user's
utterance, including the <result>, <interpretation>, and <instance>
elements. The <result> element contains the full result of
processing one utterance. It MAY contain multiple <interpretation>
elements if the interpretation of the utterance results in multiple
alternative meanings due to uncertainty in speech recognition or
natural language understanding. There are at least two reasons for
providing multiple interpretations:

1. The client application might have additional information, for
example, information from a database, that would allow it to
select a preferred interpretation from among the possible
interpretations returned from the semantic interpreter.

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2. A client-based dialog manager (e.g., VoiceXML
[W3C.REC-voicexml20-20040316]) that was unable to select between
several competing interpretations could use this information to
go back to the user and find out what was intended. For example,
it could issue a SPEAK request to a synthesizer resource to emit
"Did you say 'Boston' or 'Austin'?"

9.6.1.2. Side Information

These are elements and attributes representing additional information
about the interpretation, over and above the interpretation itself.
Side information includes:

1. Whether an interpretation was achieved (the <nomatch> element)
and the system's confidence in an interpretation (the
"confidence" attribute of <interpretation>).

2. Alternative interpretations (<interpretation>)

3. Input formats and Automatic Speech Recognition (ASR) information:
the <input> element, representing the input to the semantic
interpreter.

9.6.1.3. Multi-Modal Integration

When more than one modality is available for input, the
interpretation of the inputs needs to be coordinated. The "mode"
attribute of <input> supports this by indicating whether the
utterance was input by speech, DTMF, pointing, etc. The "timestamp-
start" and "timestamp-end" attributes of <input> also provide for
temporal coordination by indicating when inputs occurred.

9.6.2. Overview of Recognizer Result Elements and Their Relationships

The recognizer elements in NLSML fall into two categories:

1. description of the input that was processed, and

2. description of the meaning which was extracted from the input.

Next to each element are its attributes. In addition, some elements
can contain multiple instances of other elements. For example, a
<result> can contain multiple <interpretation> elements, each of
which is taken to be an alternative. Similarly, <input> can contain
multiple child <input> elements, which are taken to be cumulative.
To illustrate the basic usage of these elements, as a simple example,

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consider the utterance "OK" (interpreted as "yes"). The example
illustrates how that utterance and its interpretation would be
represented in the NLSML markup.

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="http://www.example.com/theYesNoGrammar">
<interpretation>
<instance>
<ex:response>yes</ex:response>
</instance>
<input>OK</input>
</interpretation>
</result>

This example includes only the minimum required information. There
is an overall <result> element, which includes one interpretation and
an input element. The interpretation contains the application-
specific element "<response>", which is the semantically interpreted
result.

9.6.3. Elements and Attributes

9.6.3.1. <result> Root Element

The root element of the markup is <result>. The <result> element
includes one or more <interpretation> elements. Multiple
interpretations can result from ambiguities in the input or in the
semantic interpretation. If the "grammar" attribute does not apply
to all of the interpretations in the result, it can be overridden for
individual interpretations at the <interpretation> level.

Attributes:

1. grammar: The grammar or recognition rule matched by this result.
The format of the grammar attribute will match the rule reference
semantics defined in the grammar specification. Specifically,
the rule reference is in the external XML form for grammar rule
references. The markup interpreter needs to know the grammar
rule that is matched by the utterance because multiple rules may
be simultaneously active. The value is the grammar URI used by
the markup interpreter to specify the grammar. The grammar can
be overridden by a grammar attribute in the <interpretation>
element if the input was ambiguous as to which grammar it
matched. If all interpretation elements within the result
element contain their own grammar attributes, the attribute can
be dropped from the result element.

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<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="http://www.example.com/grammar">
<interpretation>
....
</interpretation>
</result>

9.6.3.2. <interpretation> Element

An <interpretation> element contains a single semantic
interpretation.

Attributes:

1. confidence: A float value from 0.0-1.0 indicating the semantic
analyzer's confidence in this interpretation. A value of 1.0
indicates maximum confidence. The values are implementation
dependent but are intended to align with the value interpretation
for the confidence MRCPv2 header field defined in Section 9.4.1.
This attribute is OPTIONAL.

2. grammar: The grammar or recognition rule matched by this
interpretation (if needed to override the grammar specification
at the <interpretation> level.) This attribute is only needed
under <interpretation> if it is necessary to override a grammar
that was defined at the <result> level. Note that the grammar
attribute for the interpretation element is optional if and only
if the grammar attribute is specified in the <result> element.

Interpretations MUST be sorted best-first by some measure of
"goodness". The goodness measure is "confidence" if present;
otherwise, it is some implementation-specific indication of quality.

The grammar is expected to be specified most frequently at the
<result> level. However, it can be overridden at the
<interpretation> level because it is possible that different
interpretations may match different grammar rules.

The <interpretation> element includes an optional <input> element
containing the input being analyzed, and at least one <instance>
element containing the interpretation of the utterance.

<interpretation confidence="0.75"
grammar="http://www.example.com/grammar">
...
</interpretation>

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9.6.3.3. <instance> Element

The <instance> element contains the interpretation of the utterance.
When the Semantic Interpretation for Speech Recognition format is
used, the <instance> element contains the XML serialization of the
result using the approach defined in that specification. When there
is semantic markup in the grammar that does not create semantic
objects, but instead only does a semantic translation of a portion of
the input, such as translating "coke" to "coca-cola", the instance
contains the whole input but with the translation applied. The NLSML
looks like the markup in Figure 2 below. If there are no semantic
objects created, nor any semantic translation, the instance value is
the same as the input value.

Attributes:

1. confidence: Each element of the instance MAY have a confidence
attribute, defined in the NLSML namespace. The confidence
attribute contains a float value in the range from 0.0-1.0
reflecting the system's confidence in the analysis of that slot.
A value of 1.0 indicates maximum confidence. The values are
implementation dependent, but are intended to align with the
value interpretation for the MRCPv2 header field Confidence-
Threshold defined in Section 9.4.1. This attribute is OPTIONAL.

<instance>
<nameAddress>
<street confidence="0.75">123 Maple Street</street>
<city>Mill Valley</city>
<state>CA</state>
<zip>90952</zip>
</nameAddress>
</instance>
<input>
My address is 123 Maple Street,
Mill Valley, California, 90952
</input>

<instance>
I would like to buy a coca-cola
</instance>
<input>
I would like to buy a coke
</input>

Figure 2: NSLML Example

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9.6.3.4. <input> Element

The <input> element is the text representation of a user's input. It
includes an optional "confidence" attribute, which indicates the
recognizer's confidence in the recognition result (as opposed to the
confidence in the interpretation, which is indicated by the
"confidence" attribute of <interpretation>). Optional "timestamp-
start" and "timestamp-end" attributes indicate the start and end
times of a spoken utterance, in ISO 8601 format [ISO.8601.1988].

Attributes:

1. timestamp-start: The time at which the input began. (optional)

2. timestamp-end: The time at which the input ended. (optional)

3. mode: The modality of the input, for example, speech, DTMF, etc.
(optional)

4. confidence: The confidence of the recognizer in the correctness
of the input in the range 0.0 to 1.0. (optional)

Note that it may not make sense for temporally overlapping inputs to
have the same mode; however, this constraint is not expected to be
enforced by implementations.

When there is no time zone designator, ISO 8601 time representations
default to local time.

There are three possible formats for the <input> element.

1. The <input> element can contain simple text:

<input>onions</input>

A future possibility is for <input> to contain not only text but
additional markup that represents prosodic information that was
contained in the original utterance and extracted by the speech
recognizer. This depends on the availability of ASRs that are
capable of producing prosodic information. MRCPv2 clients MUST
be prepared to receive such markup and MAY make use of it.

2. An <input> tag can also contain additional <input> tags. Having
additional input elements allows the representation to support
future multi-modal inputs as well as finer-grained speech
information, such as timestamps for individual words and word-
level confidences.

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<input>
<input mode="speech" confidence="0.5"
timestamp-start="2000-04-03T0:00:00"
timestamp-end="2000-04-03T0:00:00.2">fried</input>
<input mode="speech" confidence="1.0"
timestamp-start="2000-04-03T0:00:00.25"
timestamp-end="2000-04-03T0:00:00.6">onions</input>
</input>

3. Finally, the <input> element can contain <nomatch> and <noinput>
elements, which describe situations in which the speech
recognizer received input that it was unable to process or did
not receive any input at all, respectively.

9.6.3.5. <nomatch> Element

The <nomatch> element under <input> is used to indicate that the
semantic interpreter was unable to successfully match any input with
confidence above the threshold. It can optionally contain the text
of the best of the (rejected) matches.

<interpretation>
<instance/>
<input confidence="0.1">
<nomatch/>
</input>
</interpretation>
<interpretation>
<instance/>
<input mode="speech" confidence="0.1">
<nomatch>I want to go to New York</nomatch>
</input>
</interpretation>

9.6.3.6. <noinput> Element

<noinput> indicates that there was no input -- a timeout occurred in
the speech recognizer due to silence.
<interpretation>
<instance/>
<input>
<noinput/>
</input>
</interpretation>

If there are multiple levels of inputs, the most natural place for
<nomatch> and <noinput> elements to appear is under the highest level
of <input> for <noinput>, and under the appropriate level of

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<interpretation> for <nomatch>. So, <noinput> means "no input at
all" and <nomatch> means "no match in speech modality" or "no match
in DTMF modality". For example, to represent garbled speech combined
with DTMF "1 2 3 4", the markup would be:
<input>
<input mode="speech"><nomatch/></input>
<input mode="dtmf">1 2 3 4</input>
</input>

Note: while <noinput> could be represented as an attribute of input,
<nomatch> cannot, since it could potentially include PCDATA content
with the best match. For parallelism, <noinput> is also an element.

9.7. Enrollment Results

All enrollment elements are contained within a single
<enrollment-result> element under <result>. The elements are
described below and have the schema defined in Section 16.2. The
following elements are defined:

1. num-clashes

2. num-good-repetitions

3. num-repetitions-still-needed

4. consistency-status

5. clash-phrase-ids

6. transcriptions

7. confusable-phrases

9.7.1. <num-clashes> Element

The <num-clashes> element contains the number of clashes that this
pronunciation has with other pronunciations in an active enrollment
session. The associated Clash-Threshold header field determines the
sensitivity of the clash measurement. Note that clash testing can be
turned off completely by setting the Clash-Threshold header field
value to 0.

9.7.2. <num-good-repetitions> Element

The <num-good-repetitions> element contains the number of consistent
pronunciations obtained so far in an active enrollment session.

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9.7.3. <num-repetitions-still-needed> Element

The <num-repetitions-still-needed> element contains the number of
consistent pronunciations that must still be obtained before the new
phrase can be added to the enrollment grammar. The number of
consistent pronunciations required is specified by the client in the
request header field Num-Min-Consistent-Pronunciations. The returned
value must be 0 before the client can successfully commit a phrase to
the grammar by ending the enrollment session.

9.7.4. <consistency-status> Element

The <consistency-status> element is used to indicate how consistent
the repetitions are when learning a new phrase. It can have the
values of consistent, inconsistent, and undecided.

9.7.5. <clash-phrase-ids> Element

The <clash-phrase-ids> element contains the phrase IDs of clashing
pronunciation(s), if any. This element is absent if there are no
clashes.

9.7.6. <transcriptions> Element

The <transcriptions> element contains the transcriptions returned in
the last repetition of the phrase being enrolled.

9.7.7. <confusable-phrases> Element

The <confusable-phrases> element contains a list of phrases from a
command grammar that are confusable with the phrase being added to
the personal grammar. This element MAY be absent if there are no
confusable phrases.

9.8. DEFINE-GRAMMAR

The DEFINE-GRAMMAR method, from the client to the server, provides
one or more grammars and requests the server to access, fetch, and
compile the grammars as needed. The DEFINE-GRAMMAR method
implementation MUST do a fetch of all external URIs that are part of
that operation. If caching is implemented, this URI fetching MUST
conform to the cache control hints and parameter header fields
associated with the method in deciding whether the URIs should be
fetched from cache or from the external server. If these hints/
parameters are not specified in the method, the values set for the
session using SET-PARAMS/GET-PARAMS apply. If it was not set for the
session, their default values apply.

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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
process the supplied grammars, the server MUST return a success code
status and the request-state MUST be COMPLETE.

If the recognizer resource could not define the grammar for some
reason (for example, if the download failed, the grammar failed to
compile, or the grammar was in an unsupported form), the MRCPv2
response for the DEFINE-GRAMMAR method MUST contain a failure status-
code of 407 and contain a Completion-Cause header field describing
the failure reason.

C->S:MRCP/2.0 ... DEFINE-GRAMMAR 543257
Channel-Identifier:32AECB23433801@speechrecog
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<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/2.0 ... 543257 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success

Burnett & Shanmugham Standards Track [Page 108]

RFC 6787 MRCPv2 November 2012

C->S:MRCP/2.0 ... DEFINE-GRAMMAR 543258
Channel-Identifier:32AECB23433801@speechrecog
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0">

<rule id="request">
I need help
</rule>

S->C:MRCP/2.0 ... 543258 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success

C->S:MRCP/2.0 ... DEFINE-GRAMMAR 543259
Channel-Identifier:32AECB23433801@speechrecog
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
"http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xmlns="http://www.w3.org/2001/06/grammar" xml:lang="en"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/06/grammar
http://www.w3.org/TR/speech-grammar/grammar.xsd"
version="1.0" mode="voice" root="basicCmd">

<meta name="author" content="Stephanie Williams"/>

<rule id="basicCmd" scope="public">
<example> please move the window </example>
<example> open a file </example>

<ruleref
uri="http://grammar.example.com/politeness.grxml#startPolite"/>

Burnett & Shanmugham Standards Track [Page 109]

RFC 6787 MRCPv2 November 2012

<ruleref uri="#command"/>
<ruleref
uri="http://grammar.example.com/politeness.grxml#endPolite"/>
</rule>

<rule id="command">
<ruleref uri="#action"/> <ruleref uri="#object"/>
</rule>

<rule id="action">
<one-of>
<item weight="10"> open <tag>open</tag> </item>
<item weight="2"> close <tag>close</tag> </item>
<item weight="1"> delete <tag>delete</tag> </item>
<item weight="1"> move <tag>move</tag> </item>
</one-of>
</rule>

<rule id="object">
<item repeat="0-1">
<one-of>
<item> the </item>
<item> a </item>
</one-of>
</item>

<one-of>
<item> window </item>
<item> file </item>
<item> menu </item>
</one-of>
</rule>

</grammar>

S->C:MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success

C->S:MRCP/2.0 ... RECOGNIZE 543260
Channel-Identifier:32AECB23433801@speechrecog
N-Best-List-Length:2
Content-Type:text/uri-list
Content-Length:...

Burnett & Shanmugham Standards Track [Page 110]

RFC 6787 MRCPv2 November 2012

session:[email protected]
session:[email protected]
session:[email protected]

S->C:MRCP/2.0 ... 543260 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C:MRCP/2.0 ... START-OF-INPUT 543260 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C:MRCP/2.0 ... RECOGNITION-COMPLETE 543260 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Waveform-URI:<http://web.media.com/session123/audio.wav>;
size=124535;duration=2340
Content-Type:application/x-nlsml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

Define Grammar Example

9.9. RECOGNIZE

The RECOGNIZE method from the client to the server requests the
recognizer to start recognition and provides it with one or more
grammar references for grammars to match against the input media.
The RECOGNIZE method can carry header fields to control the
sensitivity, confidence level, and the level of detail in results
provided by the recognizer. These header field values override the
current values set by a previous SET-PARAMS method.

The RECOGNIZE method can request the recognizer resource to operate
in normal or hotword mode as specified by the Recognition-Mode header
field. The default value is "normal". If the resource could not
start a recognition, the server MUST respond with a failure status-

Burnett & Shanmugham Standards Track [Page 111]

RFC 6787 MRCPv2 November 2012

code of 407 and a Completion-Cause header field in the response
describing the cause of failure.

The RECOGNIZE request uses the message body to specify the grammars
applicable to the request. The active grammar(s) for the request can
be specified in one of three ways. If the client needs to explicitly
control grammar weights for the recognition operation, it MUST employ
method 3 below. The order of these grammars specifies the precedence
of the grammars that is used when more than one grammar in the list
matches the speech; in this case, the grammar with the higher
precedence is returned as a match. This precedence capability is
useful in applications like VoiceXML browsers to order grammars
specified at the dialog, document, and root level of a VoiceXML
application.

1. The grammar MAY be placed directly in the message body as typed
content. If more than one grammar is included in the body, the
order of inclusion controls the corresponding precedence for the
grammars during recognition, with earlier grammars in the body
having a higher precedence than later ones.

2. The body MAY contain a list of grammar URIs specified in content
of media type 'text/uri-list' [RFC2483]. The order of the URIs
determines the corresponding precedence for the grammars during
recognition, with highest precedence first and decreasing for
each URI thereafter.

3. The body MAY contain a list of grammar URIs specified in content
of media type 'text/grammar-ref-list'. This type defines a list
of grammar URIs and allows each grammar URI to be assigned a
weight in the list. This weight has the same meaning as the
weights described in Section 2.4.1 of the Speech Grammar Markup
Format (SRGS) [W3C.REC-speech-grammar-20040316].

In addition to performing recognition on the input, the recognizer
MUST also enroll the collected utterance in a personal grammar if the
Enroll-Utterance header field is set to true and an Enrollment is
active (via an earlier execution of the START-PHRASE-ENROLLMENT
method). If so, and if the RECOGNIZE request contains a Content-ID
header field, then the resulting grammar (which includes the personal
grammar as a sub-grammar) can be referenced through the 'session' URI
scheme (see Section 13.6).

If the resource was able to successfully start the recognition, the
server MUST return a success status-code and a request-state of
IN-PROGRESS. This means that the recognizer is active and that the
client MUST be prepared to receive further events with this
request-id.

Burnett & Shanmugham Standards Track [Page 112]

RFC 6787 MRCPv2 November 2012

If the resource was able to queue the request, the server MUST return
a success code and request-state of PENDING. This means that the
recognizer is currently active with another request and that this
request has been queued for processing.

If the resource could not start a recognition, the server MUST
respond with a failure status-code of 407 and a Completion-Cause
header field in the response describing the cause of failure.

For the recognizer resource, RECOGNIZE and INTERPRET are the only
requests that return a 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 timeout without a
match or for some other reason, the recognizer resource MUST send the
client a RECOGNITION-COMPLETE event (or INTERPRETATION-COMPLETE, if
INTERPRET was the request) with the result of the recognition and a
request-state of COMPLETE.

Large grammars can take a long time for the server to compile. For
grammars that are used repeatedly, the client can improve server
performance by issuing 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' URI scheme
(see Section 13.6). This also applies in general if the client wants
to repeat recognition with a previous inline grammar.

The RECOGNIZE method implementation MUST do a fetch of all external
URIs that are part of that operation. If caching is implemented,
this URI fetching MUST conform to the cache control hints and
parameter header fields associated with the method in deciding
whether it should be fetched from cache or from the external server.
If these hints/parameters are not specified in the method, the values
set for the session using SET-PARAMS/GET-PARAMS apply. If it was not
set for the session, their default values apply.

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 an audio flow is started by the client at the same time
as the RECOGNIZE method is sent, either the audio or the RECOGNIZE
can arrive at the recognizer first. As another example, the client
may choose to continuously send audio to the server and signal the
server to recognize using the RECOGNIZE method. Mechanisms to
resolve this condition are outside the scope of this specification.
The recognizer can expect the media to start flowing when it receives
the RECOGNIZE request, but it MUST NOT buffer anything it receives
beforehand in order to preserve the semantics that application
authors expect with respect to the input timers.

Burnett & Shanmugham Standards Track [Page 113]

RFC 6787 MRCPv2 November 2012

When a RECOGNIZE method has been received, the recognition is
initiated on the stream. The No-Input-Timer MUST be started at this
time if the Start-Input-Timers header field is specified as "true".
If this header field is set to "false", the No-Input-Timer MUST be
started when it receives the START-INPUT-TIMERS method from the
client. The Recognition-Timeout MUST be started when the recognition
resource detects speech or a DTMF digit in the media stream.

For recognition when not in hotword mode:

When the recognizer resource detects speech or a DTMF digit in the
media stream, it MUST send the START-OF-INPUT event. When enough
speech has been collected for the server to process, the recognizer
can try to match the collected speech with the active grammars. If
the speech collected at this point fully matches with any of the
active grammars, the Speech-Complete-Timer is started. If it matches
partially with one or more of the active grammars, with more speech
needed before a full match is achieved, then the Speech-Incomplete-
Timer is started.

1. When the No-Input-Timer expires, the recognizer MUST complete
with a Completion-Cause code of "no-input-timeout".

2. The recognizer MUST support detecting a no-match condition upon
detecting end of speech. The recognizer MAY support detecting a
no-match condition before waiting for end-of-speech. If this is
supported, this capability is enabled by setting the Early-No-
Match header field to "true". Upon detecting a no-match
condition, the RECOGNIZE MUST return with "no-match".

3. When the Speech-Incomplete-Timer expires, the recognizer SHOULD
complete with a Completion-Cause code of "partial-match", unless
the recognizer cannot differentiate a partial-match, in which
case it MUST return a Completion-Cause code of "no-match". The
recognizer MAY return results for the partially matched grammar.

4. When the Speech-Complete-Timer expires, the recognizer MUST
complete with a Completion-Cause code of "success".

Burnett & Shanmugham Standards Track [Page 114]

RFC 6787 MRCPv2 November 2012

5. When the Recognition-Timeout expires, one of the following MUST
happen:

5.1. If there was a partial-match, the recognizer SHOULD
complete with a Completion-Cause code of "partial-match-
maxtime", unless the recognizer cannot differentiate a
partial-match, in which case it MUST complete with a
Completion-Cause code of "no-match-maxtime". The
recognizer MAY return results for the partially matched
grammar.

5.2. If there was a full-match, the recognizer MUST complete
with a Completion-Cause code of "success-maxtime".

5.3. If there was a no match, the recognizer MUST complete with
a Completion-Cause code of "no-match-maxtime".

For recognition in hotword mode:

Note that for recognition in hotword mode the START-OF-INPUT event is
not generated when speech or a DTMF digit is detected.

1. When the No-Input-Timer expires, the recognizer MUST complete
with a Completion-Cause code of "no-input-timeout".

2. If at any point a match occurs, the RECOGNIZE MUST complete with
a Completion-Cause code of "success".

3. When the Recognition-Timeout expires and there is not a match,
the RECOGNIZE MUST complete with a Completion-Cause code of
"hotword-maxtime".

4. When the Recognition-Timeout expires and there is a match, the
RECOGNIZE MUST complete with a Completion-Cause code of "success-
maxtime".

5. When the Recognition-Timeout is running but the detected speech/
DTMF has not resulted in a match, the Recognition-Timeout MUST be
stopped and reset. It MUST then be restarted when speech/DTMF is
again detected.

Below is a complete example of using RECOGNIZE. It shows the call to
RECOGNIZE, the IN-PROGRESS and START-OF-INPUT status messages, and
the final RECOGNITION-COMPLETE message containing the result.

Burnett & Shanmugham Standards Track [Page 115]

RFC 6787 MRCPv2 November 2012

C->S:MRCP/2.0 ... RECOGNIZE 543257
Channel-Identifier:32AECB23433801@speechrecog
Confidence-Threshold:0.9
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<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/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C:MRCP/2.0 ... START-OF-INPUT 543257 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C:MRCP/2.0 ... RECOGNITION-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Waveform-URI:<http://web.media.com/session123/audio.wav>;
size=424252;duration=2543
Content-Type:application/nlsml+xml
Content-Length:...

Burnett & Shanmugham Standards Track [Page 116]

RFC 6787 MRCPv2 November 2012

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

Below is an example of calling RECOGNIZE with a different grammar.
No status or completion messages are shown in this example, although
they would of course occur in normal usage.

C->S: MRCP/2.0 ... RECOGNIZE 543257
Channel-Identifier:32AECB23433801@speechrecog
Confidence-Threshold:0.9
Fetch-Timeout:20
Content-Type:application/srgs+xml
Content-Length:...

<?xml version="1.0"? Version="1.0" mode="voice"
root="Basic md">
<rule id="rule_list" scope="public">
<one-of>
<item weight=10>
<ruleref uri=
"http://grammar.example.com/world-cities.grxml#canada"/>
</item>
<item weight=1.5>
<ruleref uri=
"http://grammar.example.com/world-cities.grxml#america"/>
</item>
<item weight=0.5>
<ruleref uri=
"http://grammar.example.com/world-cities.grxml#india"/>
</item>
</one-of>
</rule>

Burnett & Shanmugham Standards Track [Page 117]

RFC 6787 MRCPv2 November 2012

9.10. STOP

The STOP method from the client to the server tells the resource to
stop recognition if a request is active. If a RECOGNIZE request is
active and the STOP request successfully terminated it, then the
response header section 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 is 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, the response MUST contain a status-code of 200 "Success".

C->S: MRCP/2.0 ... RECOGNIZE 543257
Channel-Identifier:32AECB23433801@speechrecog
Confidence-Threshold:0.9
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<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/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

C->S: MRCP/2.0 ... STOP 543258 200
Channel-Identifier:32AECB23433801@speechrecog

Burnett & Shanmugham Standards Track [Page 118]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... 543258 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Active-Request-Id-List:543257

9.11. GET-RESULT

The GET-RESULT method from the client to the server MAY be issued
when the recognizer resource 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 server receives this
request, it re-computes and returns the results according to the
recognition constraints provided in the GET-RESULT request.

The GET-RESULT request can specify constraints such as a different
confidence-threshold or n-best-list-length. This capability is
OPTIONAL for MRCPv2 servers and the automatic speech recognition
engine in the server MUST return a status of unsupported feature if
not supported.

C->S: MRCP/2.0 ... GET-RESULT 543257
Channel-Identifier:32AECB23433801@speechrecog
Confidence-Threshold:0.9

S->C: MRCP/2.0 ... 543257 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

Burnett & Shanmugham Standards Track [Page 119]

RFC 6787 MRCPv2 November 2012

9.12. START-OF-INPUT

This is an event from the server to the client indicating that the
recognizer resource has detected speech or a DTMF digit in the media
stream. This event is useful in implementing kill-on-barge-in
scenarios when a synthesizer resource is in a different session from
the recognizer resource and hence is not aware of an incoming audio
source (see Section 8.4.2). In these cases, it is up to the client
to act as an intermediary and respond to this event by issuing a
BARGE-IN-OCCURRED event to the synthesizer resource. The recognizer
resource also MUST send a Proxy-Sync-Id header field with a unique
value for this event.

This event MUST be generated by the server, irrespective of whether
or not the synthesizer and recognizer are on the same server.

9.13. START-INPUT-TIMERS

This request is sent from the client to the recognizer resource when
it knows that a kill-on-barge-in prompt has finished playing (see
Section 8.4.2). This is useful in the scenario when the recognition
and synthesizer engines are not in the same session. When a kill-on-
barge-in prompt is being played, the client may want a RECOGNIZE
request to be simultaneously active so that it can detect and
implement kill-on-barge-in. But at the same time the client doesn't
want the recognizer to start the no-input timers until the prompt is
finished. The Start-Input-Timers header field in the RECOGNIZE
request allows the client to say whether or not the timers should be
started immediately. If not, the recognizer resource MUST NOT start
the timers until the client sends a START-INPUT-TIMERS method to the
recognizer.

9.14. RECOGNITION-COMPLETE

This is an event from the recognizer resource to the client
indicating that the recognition completed. The recognition result is
sent in the body of the MRCPv2 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 server MUST maintain the recognizer context containing the
results and the audio waveform input of that recognition until the
next RECOGNIZE request is issued for that resource or the session
terminates. If the server returns a URI to the audio waveform, it
MUST do so in a Waveform-URI header field in the RECOGNITION-COMPLETE
event. The client can use this URI to retrieve or playback the
audio.

Burnett & Shanmugham Standards Track [Page 120]

RFC 6787 MRCPv2 November 2012

Note, if an enrollment session was active, the RECOGNITION-COMPLETE
event can contain either recognition or enrollment results depending
on what was spoken. The following example shows a complete exchange
with a recognition result.

C->S: MRCP/2.0 ... RECOGNIZE 543257
Channel-Identifier:32AECB23433801@speechrecog
Confidence-Threshold:0.9
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>


<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">


<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<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/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C: MRCP/2.0 ... START-OF-INPUT 543257 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

Burnett & Shanmugham Standards Track [Page 121]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... RECOGNITION-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Waveform-URI:<http://web.media.com/session123/audio.wav>;
size=342456;duration=25435
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

If the result were instead an enrollment result, the final message
from the server above could have been:

S->C: MRCP/2.0 ... RECOGNITION-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version= "1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="Personal-Grammar-URI">
<enrollment-result>
<num-clashes> 2 </num-clashes>
<num-good-repetitions> 1 </num-good-repetitions>
<num-repetitions-still-needed>
1
</num-repetitions-still-needed>
<consistency-status> consistent </consistency-status>
<clash-phrase-ids>
<item> Jeff </item> <item> Andre </item>
</clash-phrase-ids>
<transcriptions>
<item> m ay b r ow k er </item>
<item> m ax r aa k ah </item>
</transcriptions>

Burnett & Shanmugham Standards Track [Page 122]

RFC 6787 MRCPv2 November 2012

<confusable-phrases>
<item>
<phrase> call </phrase>
<confusion-level> 10 </confusion-level>
</item>
</confusable-phrases>
</enrollment-result>
</result>

9.15. START-PHRASE-ENROLLMENT

The START-PHRASE-ENROLLMENT method from the client to the server
starts a new phrase enrollment session during which the client can
call RECOGNIZE multiple times to enroll a new utterance in a grammar.
An enrollment session consists of a set of calls to RECOGNIZE in
which the caller speaks a phrase several times so the system can
"learn" it. The phrase is then added to a personal grammar (speaker-
trained grammar), so that the system can recognize it later.

Only one phrase enrollment session can be active at a time for a
resource. The Personal-Grammar-URI identifies the grammar that is
used during enrollment to store the personal list of phrases. Once
RECOGNIZE is called, the result is returned in a RECOGNITION-COMPLETE
event and will contain either an enrollment result OR a recognition
result for a regular recognition.

Calling END-PHRASE-ENROLLMENT ends the ongoing phrase enrollment
session, which is typically done after a sequence of successful calls
to RECOGNIZE. This method can be called to commit the new phrase to
the personal grammar or to abort the phrase enrollment session.

The grammar to contain the new enrolled phrase, specified by
Personal-Grammar-URI, is created if it does not exist. Also, the
personal grammar MUST ONLY contain phrases added via a phrase
enrollment session.

The Phrase-ID passed to this method is used to identify this phrase
in the grammar and will be returned as the speech input when doing a
RECOGNIZE on the grammar. The Phrase-NL similarly is returned in a
RECOGNITION-COMPLETE event in the same manner as other Natural
Language (NL) in a grammar. The tag-format of this NL is
implementation specific.

If the client has specified Save-Best-Waveform as true, then the
response after ending the phrase enrollment session MUST contain the
location/URI of a recording of the best repetition of the learned
phrase.

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RFC 6787 MRCPv2 November 2012

C->S: MRCP/2.0 ... START-PHRASE-ENROLLMENT 543258
Channel-Identifier:32AECB23433801@speechrecog
Num-Min-Consistent-Pronunciations:2
Consistency-Threshold:30
Clash-Threshold:12
Personal-Grammar-URI:<personal grammar uri>
Phrase-Id:<phrase id>
Phrase-NL:<NL phrase>
Weight:1
Save-Best-Waveform:true

S->C: MRCP/2.0 ... 543258 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog

9.16. ENROLLMENT-ROLLBACK

The ENROLLMENT-ROLLBACK method discards the last live utterance from
the RECOGNIZE operation. The client can invoke this method when the
caller provides undesirable input such as non-speech noises, side-
speech, commands, utterance from the RECOGNIZE grammar, etc. Note
that this method does not provide a stack of rollback states.
Executing ENROLLMENT-ROLLBACK twice in succession without an
intervening recognition operation has no effect the second time.

C->S: MRCP/2.0 ... ENROLLMENT-ROLLBACK 543261
Channel-Identifier:32AECB23433801@speechrecog

S->C: MRCP/2.0 ... 543261 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog

9.17. END-PHRASE-ENROLLMENT

The client MAY call the END-PHRASE-ENROLLMENT method ONLY during an
active phrase enrollment session. It MUST NOT be called during an
ongoing RECOGNIZE operation. To commit the new phrase in the
grammar, the client MAY call this method once successive calls to
RECOGNIZE have succeeded and Num-Repetitions-Still-Needed has been
returned as 0 in the RECOGNITION-COMPLETE event. Alternatively, the
client MAY abort the phrase enrollment session by calling this method
with the Abort-Phrase-Enrollment header field.

If the client has specified Save-Best-Waveform as "true" in the
START-PHRASE-ENROLLMENT request, then the response MUST contain a
Waveform-URI header whose value is the location/URI of a recording of
the best repetition of the learned phrase.

C->S: MRCP/2.0 ... END-PHRASE-ENROLLMENT 543262
Channel-Identifier:32AECB23433801@speechrecog

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S->C: MRCP/2.0 ... 543262 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Waveform-URI:<http://mediaserver.com/recordings/file1324.wav>;
size=242453;duration=25432

9.18. MODIFY-PHRASE

The MODIFY-PHRASE method sent from the client to the server is used
to change the phrase ID, NL phrase, and/or weight for a given phrase
in a personal grammar.

If no fields are supplied, then calling this method has no effect.

C->S: MRCP/2.0 ... MODIFY-PHRASE 543265
Channel-Identifier:32AECB23433801@speechrecog
Personal-Grammar-URI:<personal grammar uri>
Phrase-Id:<phrase id>
New-Phrase-Id:<new phrase id>
Phrase-NL:<NL phrase>
Weight:1

S->C: MRCP/2.0 ... 543265 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog

9.19. DELETE-PHRASE

The DELETE-PHRASE method sent from the client to the server is used
to delete a phase that is in a personal grammar and was added through
voice enrollment or text enrollment. If the specified phrase does
not exist, this method has no effect.

C->S: MRCP/2.0 ... DELETE-PHRASE 543266
Channel-Identifier:32AECB23433801@speechrecog
Personal-Grammar-URI:<personal grammar uri>
Phrase-Id:<phrase id>

S->C: MRCP/2.0 ... 543266 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog

9.20. INTERPRET

The INTERPRET method from the client to the server takes as input an
Interpret-Text header field containing the text for which the
semantic interpretation is desired, and returns, via the
INTERPRETATION-COMPLETE event, an interpretation result that is very
similar to the one returned from a RECOGNIZE method invocation. Only

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portions of the result relevant to acoustic matching are excluded
from the result. The Interpret-Text header field MUST be included in
the INTERPRET request.

Recognizer grammar data is treated in the same way as it is when
issuing a RECOGNIZE method call.

If a RECOGNIZE, RECORD, or another INTERPRET operation is already in
progress for the resource, the server MUST reject the request with a
response having a status-code of 402 "Method not valid in this
state", and a COMPLETE request state.

C->S: MRCP/2.0 ... INTERPRET 543266
Channel-Identifier:32AECB23433801@speechrecog
Interpret-Text:may I speak to Andre Roy
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>

<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">

<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>


<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/2.0 ... 543266 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

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S->C: MRCP/2.0 ... INTERPRETATION-COMPLETE 543266 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

9.21. INTERPRETATION-COMPLETE

This event from the recognizer resource to the client indicates that
the INTERPRET operation is complete. The interpretation result is
sent in the body of the MRCP message. The request state MUST be set
to COMPLETE.

The Completion-Cause header field MUST be included in this event and
MUST be set to an appropriate value from the list of cause codes.

C->S: MRCP/2.0 ... INTERPRET 543266
Channel-Identifier:32AECB23433801@speechrecog
Interpret-Text:may I speak to Andre Roy
Content-Type:application/srgs+xml
Content-ID:<[email protected]>
Content-Length:...

<?xml version="1.0"?>

<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">

<rule id="yes">
<one-of>
<item xml:lang="fr-CA">oui</item>
<item xml:lang="en-US">yes</item>
</one-of>
</rule>

Burnett & Shanmugham Standards Track [Page 127]

RFC 6787 MRCPv2 November 2012


<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/2.0 ... 543266 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

S->C: MRCP/2.0 ... INTERPRETATION-COMPLETE 543266 200 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

9.22. DTMF Detection

Digits received as DTMF tones are delivered to the recognition
resource in the MRCPv2 server in the RTP stream according to RFC 4733
[RFC4733]. The Automatic Speech Recognizer (ASR) MUST support RFC
4733 to recognize digits, and it MAY support recognizing DTMF tones
[Q.23] in the audio.

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RFC 6787 MRCPv2 November 2012

10. Recorder Resource

This resource captures received audio and video and stores it as
content pointed to by a URI. The main usages of recorders are

1. to capture speech audio that may be submitted for recognition at
a later time, and

2. recording voice or video mails.

Both these applications require functionality above and beyond those
specified by protocols such as RTSP [RFC2326]. This includes audio
endpointing (i.e., detecting speech or silence). The support for
video is OPTIONAL and is mainly capturing video mails that may
require the speech or audio processing mentioned above.

A recorder MUST provide endpointing capabilities for suppressing
silence at the beginning and end of a recording, and it MAY also
suppress silence in the middle of a recording. If such suppression
is done, the recorder MUST maintain timing metadata to indicate the
actual time stamps of the recorded media.

See the discussion on the sensitivity of saved waveforms in
Section 12.

10.1. Recorder State Machine

Idle Recording
State State
| |
|---------RECORD------->|
| |
|<------STOP------------|
| |
|<--RECORD-COMPLETE-----|
| |
| |--------|
| START-OF-INPUT |
| |------->|
| |
| |--------|
| START-INPUT-TIMERS |
| |------->|
| |

Recorder State Machine

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RFC 6787 MRCPv2 November 2012

10.2. Recorder Methods

The recorder resource supports the following methods.

recorder-method = "RECORD"
/ "STOP"
/ "START-INPUT-TIMERS"

10.3. Recorder Events

The recorder resource can generate the following events.

recorder-event = "START-OF-INPUT"
/ "RECORD-COMPLETE"

10.4. Recorder Header Fields

Method invocations for the recorder resource can contain resource-
specific header fields containing request options and information to
augment the Method, Response, or Event message it is associated with.

recorder-header = sensitivity-level
/ no-input-timeout
/ completion-cause
/ completion-reason
/ failed-uri
/ failed-uri-cause
/ record-uri
/ media-type
/ max-time
/ trim-length
/ final-silence
/ capture-on-speech
/ ver-buffer-utterance
/ start-input-timers
/ new-audio-channel

10.4.1. Sensitivity-Level

To filter out background noise and not mistake it for speech, the
recorder can support a variable level of sound sensitivity. The
Sensitivity-Level header field is a float value between 0.0 and 1.0
and allows the client to set the sensitivity level for the recorder.
This header field MAY occur in RECORD, SET-PARAMS, or GET-PARAMS. A
higher value for this header field means higher sensitivity. The
default value for this header field is implementation specific.

sensitivity-level = "Sensitivity-Level" ":" FLOAT CRLF

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10.4.2. No-Input-Timeout

When recording is started and there is no speech detected for a
certain period of time, the recorder can send a RECORD-COMPLETE event
to the client and terminate the record operation. The No-Input-
Timeout header field can set this timeout value. The value is in
milliseconds. This header field MAY occur in RECORD, SET-PARAMS, or
GET-PARAMS. The value for this header field ranges from 0 to an
implementation-specific maximum value. The default value for this
header field is implementation specific.

no-input-timeout = "No-Input-Timeout" ":" 1*19DIGIT CRLF

10.4.3. Completion-Cause

This header field MUST be part of a RECORD-COMPLETE event from the
recorder resource to the client. This indicates the reason behind
the RECORD method completion. This header field MUST be sent in the
RECORD 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-silence | RECORD completed with a |
| | | silence at the end. |
| 001 | success-maxtime | RECORD completed after |
| | | reaching maximum recording |
| | | time specified in record |
| | | method. |
| 002 | no-input-timeout | RECORD failed due to no |
| | | input. |
| 003 | uri-failure | Failure accessing the record |
| | | URI. |
| 004 | error | RECORD request terminated |
| | | prematurely due to a |
| | | recorder error. |
+------------+-----------------------+------------------------------+

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10.4.4. Completion-Reason

This header field MAY be present in a RECORD-COMPLETE event coming
from the recorder resource to the client. It contains the reason
text behind the RECORD request completion. This header field
communicates text describing the reason for the failure.

The completion reason text is provided for client use in logs and for
debugging and instrumentation purposes. Clients MUST NOT interpret
the completion reason text.

completion-reason = "Completion-Reason" ":"
quoted-string CRLF

10.4.5. Failed-URI

When a recorder method needs to post the audio to a URI and access to
the URI fails, the server MUST provide the failed URI in this header
field in the method response.

failed-uri = "Failed-URI" ":" absoluteURI CRLF

10.4.6. Failed-URI-Cause

When a recorder method needs to post the audio to a URI and access to
the URI fails, the server MAY provide the URI-specific or protocol-
specific response code through this header field in the method
response. The value encoding is UTF-8 (RFC 3629 [RFC3629]) to
accommodate any access protocol -- some access protocols might have a
response string instead of a numeric response code.

failed-uri-cause = "Failed-URI-Cause" ":" 1*UTFCHAR
CRLF

10.4.7. Record-URI

When a recorder method contains this header field, the server MUST
capture the audio and store it. If the header field is present but
specified with no value, the server MUST store the content locally
and generate a URI that points to it. This URI is then returned in
either the STOP response or the RECORD-COMPLETE event. If the header
field in the RECORD method specifies a URI, the server MUST attempt
to capture and store the audio at that location. If this header
field is not specified in the RECORD request, the server MUST capture
the audio, MUST encode it, and MUST send it in the STOP response or
the RECORD-COMPLETE event as a message body. In this case, the

Burnett & Shanmugham Standards Track [Page 132]

RFC 6787 MRCPv2 November 2012

response carrying the audio content MUST include a Content ID (cid)
[RFC2392] value in this header pointing to the Content-ID in the
message body.

The server MUST also return the size in octets and the duration in
milliseconds of the recorded audio waveform as parameters associated
with the header field.

Implementations MUST support 'http' [RFC2616], 'https' [RFC2818],
'file' [RFC3986], and 'cid' [RFC2392] schemes in the URI. Note that
implementations already exist that support other schemes.

record-uri = "Record-URI" ":" ["<" uri ">"
";" "size" "=" 1*19DIGIT
";" "duration" "=" 1*19DIGIT] CRLF

10.4.8. Media-Type

A RECORD method MUST contain this header field, which specifies to
the server the media type of the captured audio or video.

media-type = "Media-Type" ":" media-type-value
CRLF

10.4.9. Max-Time

When recording is started, this specifies the maximum length of the
recording in milliseconds, calculated from the time the actual
capture and store begins and is not necessarily the time the RECORD
method is received. It specifies the duration before silence
suppression, if any, has been applied by the recorder resource.
After this time, the recording stops and the server MUST return a
RECORD-COMPLETE event to the client having a request-state of
COMPLETE. This header field MAY occur in RECORD, SET-PARAMS, or GET-
PARAMS. The value for this header field ranges from 0 to an
implementation-specific maximum value. A value of 0 means infinity,
and hence the recording continues until one or more of the other stop
conditions are met. The default value for this header field is 0.

max-time = "Max-Time" ":" 1*19DIGIT CRLF

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10.4.10. Trim-Length

This header field MAY be sent on a STOP method and specifies the
length of audio to be trimmed from the end of the recording after the
stop. The length is interpreted to be in milliseconds. The default
value for this header field is 0.

trim-length = "Trim-Length" ":" 1*19DIGIT CRLF

10.4.11. Final-Silence

When the recorder is started and the actual capture begins, this
header field specifies the length of silence in the audio that is to
be interpreted as the end of the recording. This header field MAY
occur in RECORD, SET-PARAMS, or GET-PARAMS. The value for this
header field ranges from 0 to an implementation-specific maximum
value and is interpreted to be in milliseconds. A value of 0 means
infinity, and hence the recording will continue until one of the
other stop conditions are met. The default value for this header
field is implementation specific.

final-silence = "Final-Silence" ":" 1*19DIGIT CRLF

10.4.12. Capture-On-Speech

If "false", the recorder MUST start capturing immediately when
started. If "true", the recorder MUST wait for the endpointing
functionality to detect speech before it starts capturing. This
header field MAY occur in the RECORD, SET-PARAMS, or GET-PARAMS. The
value for this header field is a Boolean. The default value for this
header field is "false".

capture-on-speech = "Capture-On-Speech " ":" BOOLEAN CRLF

10.4.13. Ver-Buffer-Utterance

This header field is the same as the one described for the verifier
resource (see Section 11.4.14). This tells the server to buffer the
utterance associated with this recording request into the
verification buffer. Sending this header field is permitted only if
the verification buffer is for the session. This buffer is shared
across resources within a session. It gets instantiated when a
verifier resource is added to this session and is released when the
verifier resource is released from the session.

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RFC 6787 MRCPv2 November 2012

10.4.14. Start-Input-Timers

This header field MAY be sent as part of the RECORD request. A value
of "false" tells the recorder resource to start the operation, but
not to start the no-input timer until the client sends a START-INPUT-
TIMERS request to the recorder resource. This is useful in the
scenario when the recorder and synthesizer resources are not part of
the same session. When a kill-on-barge-in prompt is being played,
the client may want the RECORD request to be simultaneously active so
that it can detect and implement kill-on-barge-in (see
Section 8.4.2). But at the same time, the client doesn't want the
recorder resource to start the no-input timers until the prompt is
finished. The default value is "true".

start-input-timers = "Start-Input-Timers" ":"
BOOLEAN CRLF

10.4.15. New-Audio-Channel

This header field is the same as the one described for the recognizer
resource (see Section 9.4.23).

10.5. Recorder Message Body

If the RECORD request did not have a Record-URI header field, the
STOP response or the RECORD-COMPLETE event MUST contain a message
body carrying the captured audio. In this case, the message carrying
the audio content has a Record-URI header field with a Content ID
value pointing to the message body entity that contains the recorded
audio. See Section 10.4.7 for details.

10.6. RECORD

The RECORD request places the recorder resource in the recording
state. Depending on the header fields specified in the RECORD
method, the resource may start recording the audio immediately or
wait for the endpointing functionality to detect speech in the audio.
The audio is then made available to the client either in the message
body or as specified by Record-URI.

The server MUST support the 'https' URI scheme and MAY support other
schemes. Note that, due to the sensitive nature of voice recordings,
any protocols used for dereferencing SHOULD employ integrity and
confidentiality, unless other means, such as use of a controlled
environment (see Section 4.2), are employed.

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RFC 6787 MRCPv2 November 2012

If a RECORD operation is already in progress, invoking this method
causes the server to issue a response having a status-code of 402
"Method not valid in this state" and a request-state of COMPLETE.

If the Record-URI is not valid, a status-code of 404 "Illegal Value
for Header Field" is returned in the response. If it is impossible
for the server to create the requested stored content, a status-code
of 407 "Method or Operation Failed" is returned.

If the type specified in the Media-Type header field is not
supported, the server MUST respond with a status-code of 409
"Unsupported Header Field Value" with the Media-Type header field in
its response.

When the recording operation is initiated, the response indicates an
IN-PROGRESS request state. The server MAY generate a subsequent
START-OF-INPUT event when speech is detected. Upon completion of the
recording operation, the server generates a RECORD-COMPLETE event.

C->S: MRCP/2.0 ... RECORD 543257
Channel-Identifier:32AECB23433802@recorder
Record-URI:<file://mediaserver/recordings/myfile.wav>
Media-Type:audio/wav
Capture-On-Speech:true
Final-Silence:300
Max-Time:6000

S->C: MRCP/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

S->C: MRCP/2.0 ... START-OF-INPUT 543257 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

S->C: MRCP/2.0 ... RECORD-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433802@recorder
Completion-Cause:000 success-silence
Record-URI:<file://mediaserver/recordings/myfile.wav>;
size=242552;duration=25645

RECORD Example

10.7. STOP

The STOP method moves the recorder from the recording state back to
the idle state. If a RECORD request is active and the STOP request
successfully terminates it, then the STOP response MUST contain an
Active-Request-Id-List header field containing the RECORD request-id
that was terminated. In this case, no RECORD-COMPLETE event is sent

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RFC 6787 MRCPv2 November 2012

for the terminated request. If there was no recording active, then
the response MUST NOT contain an Active-Request-Id-List header field.
If the recording was a success, the STOP response MUST contain a
Record-URI header field pointing to the recorded audio content or to
a typed entity in the body of the STOP response containing the
recorded audio. The STOP method MAY have a Trim-Length header field,
in which case the specified length of audio is trimmed from the end
of the recording after the stop. In any case, the response MUST
contain a status-code of 200 "Success".

C->S: MRCP/2.0 ... RECORD 543257
Channel-Identifier:32AECB23433802@recorder
Record-URI:<file://mediaserver/recordings/myfile.wav>
Capture-On-Speech:true
Final-Silence:300
Max-Time:6000

S->C: MRCP/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

S->C: MRCP/2.0 ... START-OF-INPUT 543257 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

C->S: MRCP/2.0 ... STOP 543257
Channel-Identifier:32AECB23433802@recorder
Trim-Length:200

S->C: MRCP/2.0 ... 543257 200 COMPLETE
Channel-Identifier:32AECB23433802@recorder
Record-URI:<file://mediaserver/recordings/myfile.wav>;
size=324253;duration=24561
Active-Request-Id-List:543257

STOP Example

10.8. RECORD-COMPLETE

If the recording completes due to no input, silence after speech, or
reaching the max-time, the server MUST generate the RECORD-COMPLETE
event to the client with a request-state of COMPLETE. If the
recording was a success, the RECORD-COMPLETE event contains a Record-
URI header field pointing to the recorded audio file on the server or
to a typed entity in the message body containing the recorded audio.

Burnett & Shanmugham Standards Track [Page 137]

RFC 6787 MRCPv2 November 2012

C->S: MRCP/2.0 ... RECORD 543257
Channel-Identifier:32AECB23433802@recorder
Record-URI:<file://mediaserver/recordings/myfile.wav>
Capture-On-Speech:true
Final-Silence:300
Max-Time:6000

S->C: MRCP/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

S->C: MRCP/2.0 ... START-OF-INPUT 543257 IN-PROGRESS
Channel-Identifier:32AECB23433802@recorder

S->C: MRCP/2.0 ... RECORD-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433802@recorder
Completion-Cause:000 success
Record-URI:<file://mediaserver/recordings/myfile.wav>;
size=325325;duration=24652

RECORD-COMPLETE Example

10.9. START-INPUT-TIMERS

This request is sent from the client to the recorder resource when it
discovers that a kill-on-barge-in prompt has finished playing (see
Section 8.4.2). This is useful in the scenario when the recorder and
synthesizer resources are not in the same MRCPv2 session. When a
kill-on-barge-in prompt is being played, the client wants the RECORD
request to be simultaneously active so that it can detect and
implement kill-on-barge-in. But at the same time, the client doesn't
want the recorder resource to start the no-input timers until the
prompt is finished. The Start-Input-Timers header field in the
RECORD request allows the client to say if the timers should be
started or not. In the above case, the recorder resource does not
start the timers until the client sends a START-INPUT-TIMERS method
to the recorder.

10.10. START-OF-INPUT

The START-OF-INPUT event is returned from the server to the client
once the server has detected speech. This event is always returned
by the recorder resource when speech has been detected. The recorder
resource also MUST send a Proxy-Sync-Id header field with a unique
value for this event.

S->C: MRCP/2.0 ... START-OF-INPUT 543259 IN-PROGRESS
Channel-Identifier:32AECB23433801@recorder
Proxy-Sync-Id:987654321

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RFC 6787 MRCPv2 November 2012

11. Speaker Verification and Identification

This section describes the methods, responses and events employed by
MRCPv2 for doing speaker verification/identification.

Speaker verification is a voice authentication methodology that can
be used to identify the speaker in order to grant the user access to
sensitive information and transactions. Because speech is a
biometric, a number of essential security considerations related to
biometric authentication technologies apply to its implementation and
usage. Implementers should carefully read Section 12 in this
document and the corresponding section of the SPEECHSC requirements
[RFC4313]. Implementers and deployers of this technology are
strongly encouraged to check the state of the art for any new risks
and solutions that might have been developed.

In speaker verification, a recorded utterance is compared to a
previously stored voiceprint, which is in turn associated with a
claimed identity for that user. Verification typically consists of
two phases: a designation phase to establish the claimed identity of
the caller and an execution phase in which a voiceprint is either
created (training) or used to authenticate the claimed identity
(verification).

Speaker identification is the process of associating an unknown
speaker with a member in a population. It does not employ a claim of
identity. When an individual claims to belong to a group (e.g., one
of the owners of a joint bank account) a group authentication is
performed. This is generally implemented as a kind of verification
involving comparison with more than one voice model. It is sometimes
called 'multi-verification'. If the individual speaker can be
identified from the group, this may be useful for applications where
multiple users share the same access privileges to some data or
application. Speaker identification and group authentication are
also done in two phases, a designation phase and an execution phase.
Note that, from a functionality standpoint, identification can be
thought of as a special case of group authentication (if the
individual is identified) where the group is the entire population,
although the implementation of speaker identification may be
different from the way group authentication is performed. To
accommodate single-voiceprint verification, verification against
multiple voiceprints, group authentication, and identification, this
specification provides a single set of methods that can take a list
of identifiers, called "voiceprint identifiers", and return a list of
identifiers, with a score for each that represents how well the input
speech matched each identifier. The input and output lists of
identifiers do not have to match, allowing a vendor-specific group
identifier to be used as input to indicate that identification is to

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be performed. In this specification, the terms "identification" and
"multi-verification" are used to indicate that the input represents a
group (potentially the entire population) and that results for
multiple voiceprints may be returned.

It is possible for a verifier resource to share the same session with
a recognizer resource or to operate independently. In order to share
the same session, the verifier and recognizer resources MUST be
allocated from within the same SIP dialog. Otherwise, an independent
verifier resource, running on the same physical server or a separate
one, will be set up. Note that, in addition to allowing both
resources to be allocated in the same INVITE, it is possible to
allocate one initially and the other later via a re-INVITE.

Some of the speaker verification methods, described below, apply only
to a specific mode of operation.

The verifier resource has a verification buffer associated with it
(see Section 11.4.14). This allows the storage of speech utterances
for the purposes of verification, identification, or training from
the buffered speech. This buffer is owned by the verifier resource,
but other input resources (such as the recognizer resource or
recorder resource) may write to it. This allows the speech received
as part of a recognition or recording operation to be later used for
verification, identification, or training. Access to the buffer is
limited to one operation at time. Hence, when the resource is doing
read, write, or delete operations, such as a RECOGNIZE with
ver-buffer-utterance turned on, another operation involving the
buffer fails with a status-code of 402. The verification buffer can
be cleared by a CLEAR-BUFFER request from the client and is freed
when the verifier resource is deallocated or the session with the
server terminates.

The verification buffer is different from collecting waveforms and
processing them using either the real-time audio stream or stored
audio, because this buffering mechanism does not simply accumulate
speech to a buffer. The verification buffer MAY contain additional
information gathered by the recognizer resource that serves to
improve verification performance.

11.1. Speaker Verification State Machine

Speaker verification may operate in a training or a verification
session. Starting one of these sessions does not change the state of
the verifier resource, i.e., it remains idle. Once a verification or
training session is started, then utterances are trained or verified

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by calling the VERIFY or VERIFY-FROM-BUFFER method. The state of the
verifier resources goes from IDLE to VERIFYING state each time VERIFY
or VERIFY-FROM-BUFFER is called.

Idle Session Opened Verifying/Training
State State State
| | |
|--START-SESSION--->| |
| | |
| |----------| |
| | START-SESSION |
| |<---------| |
| | |
|<--END-SESSION-----| |
| | |
| |---------VERIFY--------->|
| | |
| |---VERIFY-FROM-BUFFER--->|
| | |
| |----------| |
| | VERIFY-ROLLBACK |
| |<---------| |
| | |
| | |--------|
| | GET-INTERMEDIATE-RESULT |
| | |------->|
| | |
| | |--------|
| | START-INPUT-TIMERS |
| | |------->|
| | |
| | |--------|
| | START-OF-INPUT |
| | |------->|
| | |
| |<-VERIFICATION-COMPLETE--|
| | |
| |<--------STOP------------|
| | |
| |----------| |
| | STOP |
| |<---------| |
| | |
|----------| | |
| STOP | |
|<---------| | |

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| |----------| |
| | CLEAR-BUFFER |
| |<---------| |
| | |
|----------| | |
| CLEAR-BUFFER | |
|<---------| | |
| | |
| |----------| |
| | QUERY-VOICEPRINT |
| |<---------| |
| | |
|----------| | |
| QUERY-VOICEPRINT | |
|<---------| | |
| | |
| |----------| |
| | DELETE-VOICEPRINT |
| |<---------| |
| | |
|----------| | |
| DELETE-VOICEPRINT | |
|<---------| | |

Verifier Resource State Machine

11.2. Speaker Verification Methods

The verifier resource supports the following methods.

verifier-method = "START-SESSION"
/ "END-SESSION"
/ "QUERY-VOICEPRINT"
/ "DELETE-VOICEPRINT"
/ "VERIFY"
/ "VERIFY-FROM-BUFFER"
/ "VERIFY-ROLLBACK"
/ "STOP"
/ "CLEAR-BUFFER"
/ "START-INPUT-TIMERS"
/ "GET-INTERMEDIATE-RESULT"

These methods allow the client to control the mode and target of
verification or identification operations within the context of a
session. All the verification input operations that occur within a
session can be used to create, update, or validate against the

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voiceprint specified during the session. At the beginning of each
session, the verifier resource is reset to the state it had prior to
any previous verification session.

Verification/identification operations can be executed against live
or buffered audio. The verifier resource provides methods for
collecting and evaluating live audio data, and methods for
controlling the verifier resource and adjusting its configured
behavior.

There are no dedicated methods for collecting buffered audio data.
This is accomplished by calling VERIFY, RECOGNIZE, or RECORD as
appropriate for the resource, with the header field
Ver-Buffer-Utterance. Then, when the following method is called,
verification is performed using the set of buffered audio.

1. VERIFY-FROM-BUFFER

The following methods are used for verification of live audio
utterances:

1. VERIFY

2. START-INPUT-TIMERS

The following methods are used for configuring the verifier resource
and for establishing resource states:

1. START-SESSION

2. END-SESSION

3. QUERY-VOICEPRINT

4. DELETE-VOICEPRINT

5. VERIFY-ROLLBACK

6. STOP

7. CLEAR-BUFFER

The following method allows the polling of a verification in progress
for intermediate results.

1. GET-INTERMEDIATE-RESULT

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11.3. Verification Events

The verifier resource generates the following events.

verifier-event = "VERIFICATION-COMPLETE"
/ "START-OF-INPUT"

11.4. Verification Header Fields

A verifier resource message can contain header fields containing
request options and information to augment the Request, Response, or
Event message it is associated with.

verification-header = repository-uri
/ voiceprint-identifier
/ verification-mode
/ adapt-model
/ abort-model
/ min-verification-score
/ num-min-verification-phrases
/ num-max-verification-phrases
/ no-input-timeout
/ save-waveform
/ media-type
/ waveform-uri
/ voiceprint-exists
/ ver-buffer-utterance
/ input-waveform-uri
/ completion-cause
/ completion-reason
/ speech-complete-timeout
/ new-audio-channel
/ abort-verification
/ start-input-timers

11.4.1. Repository-URI

This header field specifies the voiceprint repository to be used or
referenced during speaker verification or identification operations.
This header field is required in the START-SESSION, QUERY-VOICEPRINT,
and DELETE-VOICEPRINT methods.

repository-uri = "Repository-URI" ":" uri CRLF

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11.4.2. Voiceprint-Identifier

This header field specifies the claimed identity for verification
applications. The claimed identity MAY be used to specify an
existing voiceprint or to establish a new voiceprint. This header
field MUST be present in the QUERY-VOICEPRINT and DELETE-VOICEPRINT
methods. The Voiceprint-Identifier MUST be present in the START-
SESSION method for verification operations. For identification or
multi-verification operations, this header field MAY contain a list
of voiceprint identifiers separated by semicolons. For
identification operations, the client MAY also specify a voiceprint
group identifier instead of a list of voiceprint identifiers.

voiceprint-identifier = "Voiceprint-Identifier" ":"
vid *[";" vid] CRLF
vid = 1*VCHAR ["." 1*VCHAR]

11.4.3. Verification-Mode

This header field specifies the mode of the verifier resource and is
set by the START-SESSION method. Acceptable values indicate whether
the verification session will train a voiceprint ("train") or verify/
identify using an existing voiceprint ("verify").

Training and verification sessions both require the voiceprint
Repository-URI to be specified in the START-SESSION. In many usage
scenarios, however, the system does not know the speaker's claimed
identity until a recognition operation has, for example, recognized
an account number to which the user desires access. In order to
allow the first few utterances of a dialog to be both recognized and
verified, the verifier resource on the MRCPv2 server retains a
buffer. In this buffer, the MRCPv2 server accumulates recognized
utterances. The client can later execute a verification method and
apply the buffered utterances to the current verification session.

Some voice user interfaces may require additional user input that
should not be subject to verification. For example, the user's input
may have been recognized with low confidence and thus require a
confirmation cycle. In such cases, the client SHOULD NOT execute the
VERIFY or VERIFY-FROM-BUFFER methods to collect and analyze the
caller's input. A separate recognizer resource can analyze the
caller's response without any participation by the verifier resource.

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Once the following conditions have been met:

1. the voiceprint identity has been successfully established through
the Voiceprint-Identifier header fields of the START-SESSION
method, and

2. the verification mode has been set to one of "train" or "verify",

the verifier resource can begin providing verification information
during verification operations. If the verifier resource does not
reach one of the two major states ("train" or "verify") , it MUST
report an error condition in the MRCPv2 status code to indicate why
the verifier resource is not ready for the corresponding usage.

The value of verification-mode is persistent within a verification
session. If the client attempts to change the mode during a
verification session, the verifier resource reports an error and the
mode retains its current value.

verification-mode = "Verification-Mode" ":"
verification-mode-string

verification-mode-string = "train"
/ "verify"

11.4.4. Adapt-Model

This header field indicates the desired behavior of the verifier
resource after a successful verification operation. If the value of
this header field is "true", the server SHOULD use audio collected
during the verification session to update the voiceprint to account
for ongoing changes in a speaker's incoming speech characteristics,
unless local policy prohibits updating the voiceprint. If the value
is "false" (the default), the server MUST NOT update the voiceprint.
This header field MAY occur in the START-SESSION method.

adapt-model = "Adapt-Model" ":" BOOLEAN CRLF

11.4.5. Abort-Model

The Abort-Model header field indicates the desired behavior of the
verifier resource upon session termination. If the value of this
header field is "true", the server MUST discard any pending changes
to a voiceprint due to verification training or verification
adaptation. If the value is "false" (the default), the server MUST
commit any pending changes for a training session or a successful

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verification session to the voiceprint repository. A value of "true"
for Abort-Model overrides a value of "true" for the Adapt-Model
header field. This header field MAY occur in the END-SESSION method.

abort-model = "Abort-Model" ":" BOOLEAN CRLF

11.4.6. Min-Verification-Score

The Min-Verification-Score header field, when used with a verifier
resource through a SET-PARAMS, GET-PARAMS, or START-SESSION method,
determines the minimum verification score for which a verification
decision of "accepted" may be declared by the server. This is a
float value between -1.0 and 1.0. The default value for this header
field is implementation specific.

min-verification-score = "Min-Verification-Score" ":"
[ %x2D ] FLOAT CRLF

11.4.7. Num-Min-Verification-Phrases

The Num-Min-Verification-Phrases header field is used to specify the
minimum number of valid utterances before a positive decision is
given for verification. The value for this header field is an
integer and the default value is 1. The verifier resource MUST NOT
declare a verification 'accepted' unless Num-Min-Verification-Phrases
valid utterances have been received. The minimum value is 1. This
header field MAY occur in START-SESSION, SET-PARAMS, or GET-PARAMS.

num-min-verification-phrases = "Num-Min-Verification-Phrases" ":"
1*19DIGIT CRLF

11.4.8. Num-Max-Verification-Phrases

The Num-Max-Verification-Phrases header field is used to specify the
number of valid utterances required before a decision is forced for
verification. The verifier resource MUST NOT return a decision of
'undecided' once Num-Max-Verification-Phrases have been collected and
used to determine a verification score. The value for this header
field is an integer and the minimum value is 1. The default value is
implementation specific. This header field MAY occur in START-
SESSION, SET-PARAMS, or GET-PARAMS.

num-max-verification-phrases = "Num-Max-Verification-Phrases" ":"
1*19DIGIT CRLF

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11.4.9. No-Input-Timeout

The No-Input-Timeout header field sets the length of time from the
start of the verification timers (see START-INPUT-TIMERS) until the
VERIFICATION-COMPLETE server event message declares that no input has
been received (i.e., has a Completion-Cause of no-input-timeout).
The value is in milliseconds. This header field MAY occur in VERIFY,
SET-PARAMS, or GET-PARAMS. The value for this header field ranges
from 0 to an implementation-specific maximum value. The default
value for this header field is implementation specific.

no-input-timeout = "No-Input-Timeout" ":" 1*19DIGIT CRLF

11.4.10. Save-Waveform

This header field allows the client to request that the verifier
resource save the audio stream that was used for verification/
identification. The verifier resource MUST attempt to record the
audio and make it available to the client in the form of a URI
returned in the Waveform-URI header field in the VERIFICATION-
COMPLETE event. If there was an error in recording the stream, or
the audio content is otherwise not available, the verifier resource
MUST return an empty Waveform-URI header field. The default value
for this header field is "false". This header field MAY appear in
the VERIFY method. Note that this header field does not appear in
the VERIFY-FROM-BUFFER method since it only controls whether or not
to save the waveform for live verification/identification operations.

save-waveform = "Save-Waveform" ":" BOOLEAN CRLF

11.4.11. Media-Type

This header field MAY be specified in the SET-PARAMS, GET-PARAMS, or
the VERIFY methods and tells the server resource the media type of
the captured audio or video such as the one captured and returned by
the Waveform-URI header field.

media-type = "Media-Type" ":" media-type-value
CRLF

11.4.12. Waveform-URI

If the Save-Waveform header field is set to "true", the verifier
resource MUST attempt to record the incoming audio stream of the
verification into a file and provide a URI for the client to access
it. This header field MUST be present in the VERIFICATION-COMPLETE
event if the Save-Waveform header field was set to true by the
client. The value of the header field MUST be empty if there was

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some error condition preventing the server from recording.
Otherwise, the URI generated by the server MUST be globally unique
across the server and all its verification sessions. The content
MUST be available via the URI until the verification session ends.
Since the Save-Waveform header field applies only to live
verification/identification operations, the server can return the
Waveform-URI only in the VERIFICATION-COMPLETE event for live
verification/identification operations.

The server MUST also return the size in octets and the duration in
milliseconds of the recorded audio waveform as parameters associated
with the header field.

waveform-uri = "Waveform-URI" ":" ["<" uri ">"
";" "size" "=" 1*19DIGIT
";" "duration" "=" 1*19DIGIT] CRLF

11.4.13. Voiceprint-Exists

This header field MUST be returned in QUERY-VOICEPRINT and DELETE-
VOICEPRINT responses. This is the status of the voiceprint specified
in the QUERY-VOICEPRINT method. For the DELETE-VOICEPRINT method,
this header field indicates the status of the voiceprint at the
moment the method execution started.

voiceprint-exists = "Voiceprint-Exists" ":" BOOLEAN CRLF

11.4.14. Ver-Buffer-Utterance

This header field is used to indicate that this utterance could be
later considered for speaker verification. This way, a client can
request the server to buffer utterances while doing regular
recognition or verification activities, and speaker verification can
later be requested on the buffered utterances. This header field is
optional in the RECOGNIZE, VERIFY, and RECORD methods. The default
value for this header field is "false".

ver-buffer-utterance = "Ver-Buffer-Utterance" ":" BOOLEAN
CRLF

11.4.15. Input-Waveform-URI

This header field specifies stored audio content that the client
requests the server to fetch and process according to the current
verification mode, either to train the voiceprint or verify a claimed
identity. This header field enables the client to implement the

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buffering use case where the recognizer and verifier resources are in
different sessions and the verification buffer technique cannot be
used. It MAY be specified on the VERIFY request.

input-waveform-uri = "Input-Waveform-URI" ":" uri CRLF

11.4.16. Completion-Cause

This header field MUST be part of a VERIFICATION-COMPLETE event from
the verifier resource to the client. This indicates the cause of
VERIFY or VERIFY-FROM-BUFFER method completion. This header field
MUST be sent in the VERIFY, VERIFY-FROM-BUFFER, and QUERY-VOICEPRINT
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 | VERIFY or |
| | | VERIFY-FROM-BUFFER |
| | | request completed |
| | | successfully. The verify |
| | | decision can be |
| | | "accepted", "rejected", |
| | | or "undecided". |
| 001 | error | VERIFY or |
| | | VERIFY-FROM-BUFFER |
| | | request terminated |
| | | prematurely due to a |
| | | verifier resource or |
| | | system error. |
| 002 | no-input-timeout | VERIFY request completed |
| | | with no result due to a |
| | | no-input-timeout. |
| 003 | too-much-speech-timeout | VERIFY request completed |
| | | with no result due to too |
| | | much speech. |
| 004 | speech-too-early | VERIFY request completed |
| | | with no result due to |
| | | speech too soon. |

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| 005 | buffer-empty | VERIFY-FROM-BUFFER |
| | | request completed with no |
| | | result due to empty |
| | | buffer. |
| 006 | out-of-sequence | Verification operation |
| | | failed due to |
| | | out-of-sequence method |
| | | invocations, for example, |
| | | calling VERIFY before |
| | | QUERY-VOICEPRINT. |
| 007 | repository-uri-failure | Failure accessing |
| | | Repository URI. |
| 008 | repository-uri-missing | Repository-URI is not |
| | | specified. |
| 009 | voiceprint-id-missing | Voiceprint-Identifier is |
| | | not specified. |
| 010 | voiceprint-id-not-exist | Voiceprint-Identifier |
| | | does not exist in the |
| | | voiceprint repository. |
| 011 | speech-not-usable | VERIFY request completed |
| | | with no result because |
| | | the speech was not usable |
| | | (too noisy, too short, |
| | | etc.) |
+------------+--------------------------+---------------------------+

11.4.17. Completion-Reason

This header field MAY be specified in a VERIFICATION-COMPLETE event
coming from the verifier resource to the client. It contains the
reason text behind the VERIFY request completion. This header field
communicates text describing the reason for the failure.

The completion reason text is provided for client use in logs and for
debugging and instrumentation purposes. Clients MUST NOT interpret
the completion reason text.

completion-reason = "Completion-Reason" ":"
quoted-string CRLF

11.4.18. Speech-Complete-Timeout

This header field is the same as the one described for the Recognizer
resource. See Section 9.4.15. This header field MAY occur in
VERIFY, SET-PARAMS, or GET-PARAMS.

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11.4.19. New-Audio-Channel

This header field is the same as the one described for the Recognizer
resource. See Section 9.4.23. This header field MAY be specified in
a VERIFY request.

11.4.20. Abort-Verification

This header field MUST be sent in a STOP request to indicate whether
or not to abort a VERIFY method in progress. A value of "true"
requests the server to discard the results. A value of "false"
requests the server to return in the STOP response the verification
results obtained up to the point it received the STOP request.

abort-verification = "Abort-Verification " ":" BOOLEAN CRLF

11.4.21. Start-Input-Timers

This header field MAY be sent as part of a VERIFY request. A value
of "false" tells the verifier resource to start the VERIFY operation
but not to start the no-input timer yet. The verifier resource MUST
NOT start the timers until the client sends a START-INPUT-TIMERS
request to the resource. This is useful in the scenario when the
verifier and synthesizer resources are not part of the same session.
In this scenario, when a kill-on-barge-in prompt is being played, the
client may want the VERIFY request to be simultaneously active so
that it can detect and implement kill-on-barge-in (see
Section 8.4.2). But at the same time, the client doesn't want the
verifier resource to start the no-input timers until the prompt is
finished. The default value is "true".

start-input-timers = "Start-Input-Timers" ":"
BOOLEAN CRLF

11.5. Verification Message Body

A verification response or event message can carry additional data as
described in the following subsection.

11.5.1. Verification Result Data

Verification results are returned to the client in the message body
of the VERIFICATION-COMPLETE event or the GET-INTERMEDIATE-RESULT
response message as described in Section 6.3. Element and attribute
descriptions for the verification portion of the NLSML format are
provided in Section 11.5.2 with a normative definition of the schema
in Section 16.3.

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11.5.2. Verification Result Elements

All verification elements are contained within a single
<verification-result> element under <result>. The elements are
described below and have the schema defined in Section 16.2. The
following elements are defined:

1. <voiceprint>

2. <incremental>

3. <cumulative>

4. <decision>

5. <utterance-length>

6. <device>

7. <gender>

8. <adapted>

9. <verification-score>

10. <vendor-specific-results>

11.5.2.1. <voiceprint> Element

This element in the verification results provides information on how
the speech data matched a single voiceprint. The result data
returned MAY have more than one such entity in the case of
identification or multi-verification. Each <voiceprint> element and
the XML data within the element describe verification result
information for how well the speech data matched that particular
voiceprint. The list of <voiceprint> element data are ordered
according to their cumulative verification match scores, with the
highest score first.

11.5.2.2. <cumulative> Element

Within each <voiceprint> element there MUST be a <cumulative> element
with the cumulative scores of how well multiple utterances matched
the voiceprint.

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11.5.2.3. <incremental> Element

The first <voiceprint> element MAY contain an <incremental> element
with the incremental scores of how well the last utterance matched
the voiceprint.

11.5.2.4. <Decision> Element

This element is found within the <incremental> or <cumulative>
element within the verification results. Its value indicates the
verification decision. It can have the values of "accepted",
"rejected", or "undecided".

11.5.2.5. <utterance-length> Element

This element MAY occur within either the <incremental> or
<cumulative> elements within the first <voiceprint> element. Its
value indicates the size in milliseconds, respectively, of the last
utterance or the cumulated set of utterances.

11.5.2.6. <device> Element

This element is found within the <incremental> or <cumulative>
element within the verification results. Its value indicates the
apparent type of device used by the caller as determined by the
verifier resource. It can have the values of "cellular-phone",
"electret-phone", "carbon-button-phone", or "unknown".

11.5.2.7. <gender> Element

This element is found within the <incremental> or <cumulative>
element within the verification results. Its value indicates the
apparent gender of the speaker as determined by the verifier
resource. It can have the values of "male", "female", or "unknown".

11.5.2.8. <adapted> Element

This element is found within the first <voiceprint> element within
the verification results. When verification is trying to confirm the
voiceprint, this indicates if the voiceprint has been adapted as a
consequence of analyzing the source utterances. It is not returned
during verification training. The value can be "true" or "false".

11.5.2.9. <verification-score> Element

This element is found within the <incremental> or <cumulative>
element within the verification results. Its value indicates the
score of the last utterance as determined by verification.

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During verification, the higher the score, the more likely it is that
the speaker is the same one as the one who spoke the voiceprint
utterances. During training, the higher the score, the more likely
the speaker is to have spoken all of the analyzed utterances. The
value is a floating point between -1.0 and 1.0. If there are no such
utterances, the score is -1. Note that the verification score is not
a probability value.

11.5.2.10. <vendor-specific-results> Element

MRCPv2 servers MAY send verification results that contain
implementation-specific data that augment the information provided by
the MRCPv2-defined elements. Such data might be useful to clients
who have private knowledge of how to interpret these schema
extensions. Implementation-specific additions to the verification
results schema MUST belong to the vendor's own namespace. In the
result structure, either they MUST be indicated by a namespace prefix
declared within the result, or they MUST be children of an element
identified as belonging to the respective namespace.

The following example shows the results of three voiceprints. Note
that the first one has crossed the verification score threshold, and
the speaker has been accepted. The voiceprint was also adapted with
the most recent utterance.

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="johnsmith">
<adapted> true </adapted>
<incremental>
<utterance-length> 500 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<decision> accepted </decision>
<verification-score> 0.98514 </verification-score>
</incremental>
<cumulative>
<utterance-length> 10000 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<decision> accepted </decision>
<verification-score> 0.96725</verification-score>
</cumulative>
</voiceprint>

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RFC 6787 MRCPv2 November 2012

<voiceprint id="marysmith">
<cumulative>
<verification-score> 0.93410 </verification-score>
</cumulative>
</voiceprint>
<voiceprint uri="juniorsmith">
<cumulative>
<verification-score> 0.74209 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

Verification Results Example 1

In this next example, the verifier has enough information to decide
to reject the speaker.

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:xmpl="http://www.example.org/2003/12/mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="johnsmith">
<incremental>
<utterance-length> 500 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<verification-score> 0.88514 </verification-score>
<xmpl:raspiness> high </xmpl:raspiness>
<xmpl:emotion> sadness </xmpl:emotion>
</incremental>
<cumulative>
<utterance-length> 10000 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<decision> rejected </decision>
<verification-score> 0.9345 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

Verification Results Example 2

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RFC 6787 MRCPv2 November 2012

11.6. START-SESSION

The START-SESSION method starts a speaker verification or speaker
identification session. Execution of this method places the verifier
resource into its initial state. If this method is called during an
ongoing verification session, the previous session is implicitly
aborted. If this method is invoked when VERIFY or VERIFY-FROM-BUFFER
is active, the method fails and the server returns a status-code of
402.

Upon completion of the START-SESSION method, the verifier resource
MUST have terminated any ongoing verification session and cleared any
voiceprint designation.

A verification session is associated with the voiceprint repository
to be used during the session. This is specified through the
Repository-URI header field (see Section 11.4.1).

The START-SESSION method also establishes, through the Voiceprint-
Identifier header field, which voiceprints are to be matched or
trained during the verification session. If this is an
Identification session or if the client wants to do Multi-
Verification, the Voiceprint-Identifier header field contains a list
of semicolon-separated voiceprint identifiers.

The Adapt-Model header field MAY also be present in the START-SESSION
request to indicate whether or not to adapt a voiceprint based on
data collected during the session (if the voiceprint verification
phase succeeds). By default, the voiceprint model MUST NOT be
adapted with data from a verification session.

The START-SESSION also determines whether the session is for a train
or verify of a voiceprint. Hence, the Verification-Mode header field
MUST be sent in every START-SESSION request. The value of the
Verification-Mode header field MUST be one of either "train" or
"verify".

Before a verification/identification session is started, the client
may only request that VERIFY-ROLLBACK and generic SET-PARAMS and
GET-PARAMS operations be performed on the verifier resource. The
server MUST return status-code 402 "Method not valid in this state"
for all other verification operations.

A verifier resource MUST NOT have more than a single session active
at one time.

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RFC 6787 MRCPv2 November 2012

C->S: MRCP/2.0 ... START-SESSION 314161
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/voiceprintdbase/
Voiceprint-Mode:verify
Voiceprint-Identifier:johnsmith.voiceprint
Adapt-Model:true

S->C: MRCP/2.0 ... 314161 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

11.7. END-SESSION

The END-SESSION method terminates an ongoing verification session and
releases the verification voiceprint resources. The session may
terminate in one of three ways:

1. abort - the voiceprint adaptation or creation may be aborted so
that the voiceprint remains unchanged (or is not created).

2. commit - when terminating a voiceprint training session, the new
voiceprint is committed to the repository.

3. adapt - an existing voiceprint is modified using a successful
verification.

The Abort-Model header field MAY be included in the END-SESSION to
control whether or not to abort any pending changes to the
voiceprint. The default behavior is to commit (not abort) any
pending changes to the designated voiceprint.

The END-SESSION method may be safely executed multiple times without
first executing the START-SESSION method. Any additional executions
of this method without an intervening use of the START-SESSION method
have no effect on the verifier resource.

The following example assumes there is either a training session or a
verification session in progress.

C->S: MRCP/2.0 ... END-SESSION 314174
Channel-Identifier:32AECB23433801@speakverify
Abort-Model:true

S->C: MRCP/2.0 ... 314174 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

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RFC 6787 MRCPv2 November 2012

11.8. QUERY-VOICEPRINT

The QUERY-VOICEPRINT method is used to get status information on a
particular voiceprint and can be used by the client to ascertain if a
voiceprint or repository exists and if it contains trained
voiceprints.

The response to the QUERY-VOICEPRINT request contains an indication
of the status of the designated voiceprint in the Voiceprint-Exists
header field, allowing the client to determine whether to use the
current voiceprint for verification, train a new voiceprint, or
choose a different voiceprint.

A voiceprint is completely specified by providing a repository
location and a voiceprint identifier. The particular voiceprint or
identity within the repository is specified by a string identifier
that is unique within the repository. The Voiceprint-Identifier
header field carries this unique voiceprint identifier within a given
repository.

The following example assumes a verification session is in progress
and the voiceprint exists in the voiceprint repository.

C->S: MRCP/2.0 ... QUERY-VOICEPRINT 314168
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/voiceprints/
Voiceprint-Identifier:johnsmith.voiceprint

S->C: MRCP/2.0 ... 314168 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/voiceprints/
Voiceprint-Identifier:johnsmith.voiceprint
Voiceprint-Exists:true

The following example assumes that the URI provided in the
Repository-URI header field is a bad URI.

C->S: MRCP/2.0 ... QUERY-VOICEPRINT 314168
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/bad-uri/
Voiceprint-Identifier:johnsmith.voiceprint

S->C: MRCP/2.0 ... 314168 405 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/bad-uri/
Voiceprint-Identifier:johnsmith.voiceprint
Completion-Cause:007 repository-uri-failure

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RFC 6787 MRCPv2 November 2012

11.9. DELETE-VOICEPRINT

The DELETE-VOICEPRINT method removes a voiceprint from a repository.
This method MUST carry the Repository-URI and Voiceprint-Identifier
header fields.

An MRCPv2 server MUST reject a DELETE-VOICEPRINT request with a 401
status code unless the MRCPv2 client has been authenticated and
authorized. Note that MRCPv2 does not have a standard mechanism for
this. See Section 12.8.

If the corresponding voiceprint does not exist, the DELETE-VOICEPRINT
method MUST return a 200 status code.

The following example demonstrates a DELETE-VOICEPRINT operation to
remove a specific voiceprint.

C->S: MRCP/2.0 ... DELETE-VOICEPRINT 314168
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/bad-uri/
Voiceprint-Identifier:johnsmith.voiceprint

S->C: MRCP/2.0 ... 314168 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

11.10. VERIFY

The VERIFY method is used to request that the verifier resource
either train/adapt the voiceprint or verify/identify a claimed
identity. If the voiceprint is new or was deleted by a previous
DELETE-VOICEPRINT method, the VERIFY method trains the voiceprint.
If the voiceprint already exists, it is adapted and not retrained by
the VERIFY command.

C->S: MRCP/2.0 ... VERIFY 543260
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 543260 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speakverify

When the VERIFY request completes, the MRCPv2 server MUST send a
VERIFICATION-COMPLETE event to the client.

11.11. VERIFY-FROM-BUFFER

The VERIFY-FROM-BUFFER method directs the verifier resource to verify
buffered audio against a voiceprint. Only one VERIFY or VERIFY-FROM-
BUFFER method may be active for a verifier resource at a time.

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RFC 6787 MRCPv2 November 2012

The buffered audio is not consumed by this method and thus VERIFY-
FROM-BUFFER may be invoked multiple times by the client to attempt
verification against different voiceprints.

For the VERIFY-FROM-BUFFER method, the server MAY optionally return
an IN-PROGRESS response before the VERIFICATION-COMPLETE event.

When the VERIFY-FROM-BUFFER method is invoked and the verification
buffer is in use by another resource sharing it, the server MUST
return an IN-PROGRESS response and wait until the buffer is available
to it. The verification buffer is owned by the verifier resource but
is shared with write access from other input resources on the same
session. Hence, it is considered to be in use if there is a read or
write operation such as a RECORD or RECOGNIZE with the
Ver-Buffer-Utterance header field set to "true" on a resource that
shares this buffer. Note that if a RECORD or RECOGNIZE method
returns with a failure cause code, the VERIFY-FROM-BUFFER request
waiting to process that buffer MUST also fail with a Completion-Cause
of 005 (buffer-empty).

The following example illustrates the usage of some buffering
methods. In this scenario, the client first performed a live
verification, but the utterance had been rejected. In the meantime,
the utterance is also saved to the audio buffer. Then, another
voiceprint is used to do verification against the audio buffer and
the utterance is accepted. For the example, we assume both
Num-Min-Verification-Phrases and Num-Max-Verification-Phrases are 1.

C->S: MRCP/2.0 ... START-SESSION 314161
Channel-Identifier:32AECB23433801@speakverify
Verification-Mode:verify
Adapt-Model:true
Repository-URI:http://www.example.com/voiceprints
Voiceprint-Identifier:johnsmith.voiceprint

S->C: MRCP/2.0 ... 314161 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

C->S: MRCP/2.0 ... VERIFY 314162
Channel-Identifier:32AECB23433801@speakverify
Ver-buffer-utterance:true

S->C: MRCP/2.0 ... 314162 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speakverify

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RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... VERIFICATION-COMPLETE 314162 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="johnsmith">
<incremental>
<utterance-length> 500 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> rejected </decision>
<verification-score> 0.05465 </verification-score>
</incremental>
<cumulative>
<utterance-length> 500 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> rejected </decision>
<verification-score> 0.05465 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

C->S: MRCP/2.0 ... QUERY-VOICEPRINT 314163
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/voiceprints/
Voiceprint-Identifier:johnsmith

S->C: MRCP/2.0 ... 314163 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Repository-URI:http://www.example.com/voiceprints/
Voiceprint-Identifier:johnsmith.voiceprint
Voiceprint-Exists:true

C->S: MRCP/2.0 ... START-SESSION 314164
Channel-Identifier:32AECB23433801@speakverify
Verification-Mode:verify
Adapt-Model:true
Repository-URI:http://www.example.com/voiceprints
Voiceprint-Identifier:marysmith.voiceprint

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RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... 314164 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

C->S: MRCP/2.0 ... VERIFY-FROM-BUFFER 314165
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 314165 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... VERIFICATION-COMPLETE 314165 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="marysmith">
<incremental>
<utterance-length> 1000 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> accepted </decision>
<verification-score> 0.98 </verification-score>
</incremental>
<cumulative>
<utterance-length> 1000 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> accepted </decision>
<verification-score> 0.98 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

C->S: MRCP/2.0 ... END-SESSION 314166
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 314166 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

VERIFY-FROM-BUFFER Example

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RFC 6787 MRCPv2 November 2012

11.12. VERIFY-ROLLBACK

The VERIFY-ROLLBACK method discards the last buffered utterance or
discards the last live utterances (when the mode is "train" or
"verify"). The client will likely want to invoke this method when
the user provides undesirable input such as non-speech noises, side-
speech, out-of-grammar utterances, commands, etc. Note that this
method does not provide a stack of rollback states. Executing
VERIFY-ROLLBACK twice in succession without an intervening
recognition operation has no effect on the second attempt.

C->S: MRCP/2.0 ... VERIFY-ROLLBACK 314165
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 314165 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

VERIFY-ROLLBACK Example

11.13. STOP

The STOP method from the client to the server tells the verifier
resource to stop the VERIFY or VERIFY-FROM-BUFFER request if one is
active. If such a request is active and the STOP request
successfully terminated it, then the response header section contains
an Active-Request-Id-List header field containing the request-id of
the VERIFY or VERIFY-FROM-BUFFER request that was terminated. In
this case, no VERIFICATION-COMPLETE event is sent for the terminated
request. If there was no verify request active, then the response
MUST NOT contain an Active-Request-Id-List header field. Either way,
the response MUST contain a status-code of 200 "Success".

The STOP method can carry an Abort-Verification header field, which
specifies if the verification result until that point should be
discarded or returned. If this header field is not present or if the
value is "true", the verification result is discarded and the STOP
response does not contain any result data. If the header field is
present and its value is "false", the STOP response MUST contain a
Completion-Cause header field and carry the Verification result data
in its body.

An aborted VERIFY request does an automatic rollback and hence does
not affect the cumulative score. A VERIFY request that was stopped
with no Abort-Verification header field or with the Abort-
Verification header field set to "false" does affect cumulative
scores and would need to be explicitly rolled back if the client does
not want the verification result considered in the cumulative scores.

Burnett & Shanmugham Standards Track [Page 164]

RFC 6787 MRCPv2 November 2012

The following example assumes a voiceprint identity has already been
established.

C->S: MRCP/2.0 ... VERIFY 314177
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 314177 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speakverify

C->S: MRCP/2.0 ... STOP 314178
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 314178 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Active-Request-Id-List:314177

STOP Verification Example

11.14. START-INPUT-TIMERS

This request is sent from the client to the verifier resource to
start the no-input timer, usually once the client has ascertained
that any audio prompts to the user have played to completion.

C->S: MRCP/2.0 ... START-INPUT-TIMERS 543260
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 543260 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

11.15. VERIFICATION-COMPLETE

The VERIFICATION-COMPLETE event follows a call to VERIFY or VERIFY-
FROM-BUFFER and is used to communicate the verification results to
the client. The event message body contains only verification
results.

S->C: MRCP/2.0 ... VERIFICATION-COMPLETE 543259 COMPLETE
Completion-Cause:000 success
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="johnsmith">

Burnett & Shanmugham Standards Track [Page 165]

RFC 6787 MRCPv2 November 2012

<incremental>
<utterance-length> 500 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<decision> accepted </decision>
<verification-score> 0.85 </verification-score>
</incremental>
<cumulative>
<utterance-length> 1500 </utterance-length>
<device> cellular-phone </device>
<gender> male </gender>
<decision> accepted </decision>
<verification-score> 0.75 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

11.16. START-OF-INPUT

The START-OF-INPUT event is returned from the server to the client
once the server has detected speech. This event is always returned
by the verifier resource when speech has been detected, irrespective
of whether or not the recognizer and verifier resources share the
same session.

S->C: MRCP/2.0 ... START-OF-INPUT 543259 IN-PROGRESS
Channel-Identifier:32AECB23433801@speakverify

11.17. CLEAR-BUFFER

The CLEAR-BUFFER method can be used to clear the verification buffer.
This buffer is used to buffer speech during recognition, record, or
verification operations that may later be used by VERIFY-FROM-BUFFER.
As noted before, the buffer associated with the verifier resource is
shared by other input resources like recognizers and recorders.
Hence, a CLEAR-BUFFER request fails if the verification buffer is in
use. This can happen when any one of the input resources that share
this buffer has an active read or write operation such as RECORD,
RECOGNIZE, or VERIFY with the Ver-Buffer-Utterance header field set
to "true".

C->S: MRCP/2.0 ... CLEAR-BUFFER 543260
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 543260 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify

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RFC 6787 MRCPv2 November 2012

11.18. GET-INTERMEDIATE-RESULT

A client can use the GET-INTERMEDIATE-RESULT method to poll for
intermediate results of a verification request that is in progress.
Invoking this method does not change the state of the resource. The
verifier resource collects the accumulated verification results and
returns the information in the method response. The message body in
the response to a GET-INTERMEDIATE-RESULT REQUEST contains only
verification results. The method response MUST NOT contain a
Completion-Cause header field as the request is not yet complete. If
the resource does not have a verification in progress, the response
has a 402 failure status-code and no result in the body.

C->S: MRCP/2.0 ... GET-INTERMEDIATE-RESULT 543260
Channel-Identifier:32AECB23433801@speakverify

S->C: MRCP/2.0 ... 543260 200 COMPLETE
Channel-Identifier:32AECB23433801@speakverify
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
grammar="What-Grammar-URI">
<verification-result>
<voiceprint id="marysmith">
<incremental>
<utterance-length> 50 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> undecided </decision>
<verification-score> 0.85 </verification-score>
</incremental>
<cumulative>
<utterance-length> 150 </utterance-length>
<device> cellular-phone </device>
<gender> female </gender>
<decision> undecided </decision>
<verification-score> 0.65 </verification-score>
</cumulative>
</voiceprint>
</verification-result>
</result>

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RFC 6787 MRCPv2 November 2012

12. Security Considerations

MRCPv2 is designed to comply with the security-related requirements
documented in the SPEECHSC requirements [RFC4313]. Implementers and
users of MRCPv2 are strongly encouraged to read the Security
Considerations section of [RFC4313], because that document contains
discussion of a number of important security issues associated with
the utilization of speech as biometric authentication technology, and
on the threats against systems which store recorded speech, contain
large corpora of voiceprints, and send and receive sensitive
information based on voice input to a recognizer or speech output
from a synthesizer. Specific security measures employed by MRCPv2
are summarized in the following subsections. See the corresponding
sections of this specification for how the security-related machinery
is invoked by individual protocol operations.

12.1. Rendezvous and Session Establishment

MRCPv2 control sessions are established as media sessions described
by SDP within the context of a SIP dialog. In order to ensure secure
rendezvous between MRCPv2 clients and servers, the following are
required:

1. The SIP implementation in MRCPv2 clients and servers MUST support
SIP digest authentication [RFC3261] and SHOULD employ it.

2. The SIP implementation in MRCPv2 clients and servers MUST support
'sips' URIs and SHOULD employ 'sips' URIs; this includes that
clients and servers SHOULD set up TLS [RFC5246] connections.

3. If media stream cryptographic keying is done through SDP (e.g.
using [RFC4568]), the MRCPv2 clients and servers MUST employ the
'sips' URI.

4. When TLS is used for SIP, the client MUST verify the identity of
the server to which it connects, following the rules and
guidelines defined in [RFC5922].

12.2. Control Channel Protection

Sensitive data is carried over the MRCPv2 control channel. This
includes things like the output of speech recognition operations,
speaker verification results, input to text-to-speech conversion,
personally identifying grammars, etc. For this reason, MRCPv2
servers must be properly authenticated, and the control channel must
permit the use of both confidentiality and integrity for the data.
To ensure control channel protection, MRCPv2 clients and servers MUST
support TLS and SHOULD utilize it by default unless alternative

Burnett & Shanmugham Standards Track [Page 168]

RFC 6787 MRCPv2 November 2012

control channel protection is used. When TLS is used, the client
MUST verify the identity of the server to which it connects,
following the rules and guidelines defined in [RFC4572]. If there
are multiple TLS-protected channels between the client and the
server, the server MUST NOT send a response to the client over a
channel for which the TLS identities of the server or client differ
from the channel over which the server received the corresponding
request. Alternative control-channel protection MAY be used if
desired (e.g., Security Architecture for the Internet Protocol
(IPsec) [RFC4301]).

12.3. Media Session Protection

Sensitive data is also carried on media sessions terminating on
MRCPv2 servers (the other end of a media channel may or may not be on
the MRCPv2 client). This data includes the user's spoken utterances
and the output of text-to-speech operations. MRCPv2 servers MUST
support a security mechanism for protection of audio media sessions.
MRCPv2 clients that originate or consume audio similarly MUST support
a security mechanism for protection of the audio. One such mechanism
is the Secure Real-time Transport Protocol (SRTP) [RFC3711].

12.4. Indirect Content Access

MCRPv2 employs content indirection extensively. Content may be
fetched and/or stored based on URI addressing on systems other than
the MRCPv2 client or server. Not all of the stored content is
necessarily sensitive (e.g., XML schemas), but the majority generally
needs protection, and some indirect content, such as voice recordings
and voiceprints, is extremely sensitive and must always be protected.
MRCPv2 clients and servers MUST implement HTTPS for indirect content
access and SHOULD employ secure access for all sensitive indirect
content. Other secure URI schemes such as Secure FTP (FTPS)
[RFC4217] MAY also be used. See Section 6.2.15 for the header fields
used to transfer cookie information between the MRCPv2 client and
server if needed for authentication.

Access to URIs provided by servers introduces risks that need to be
considered. Although RFC 6454 [RFC6454] discusses and focuses on a
same-origin policy, which MRCPv2 does not restrict URIs to, it still
provides an excellent description of the pitfalls of blindly
following server-provided URIs in Section 3 of the RFC. Servers also
need to be aware that clients could provide URIs to sites designed to
tie up the server in long or otherwise problematic document fetches.
MRCPv2 servers, and the services they access, MUST always be prepared
for the possibility of such a denial-of-service attack.

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MRCPv2 makes no inherent assumptions about the lifetime and access
controls associated with a URI. For example, if neither
authentication nor scheme-specific access controls are used, a leak
of the URI is equivalent to a leak of the content. Moreover, MRCPv2
makes no specific demands on the lifetime of a URI. If a server
offers a URI and the client takes a long, long time to access that
URI, the server may have removed the resource in the interim time
period. MRCPv2 deals with this case by using the URI access scheme's
'resource not found' error, such as 404 for HTTPS. How long a server
should keep a dynamic resource available is highly application and
context dependent. However, the server SHOULD keep the resource
available for a reasonable amount of time to make it likely the
client will have the resource available when the client needs the
resource. Conversely, to mitigate state exhaustion attacks, MRCPv2
servers are not obligated to keep resources and resource state in
perpetuity. The server SHOULD delete dynamically generated resources
associated with an MRCPv2 session when the session ends.

One method to avoid resource leakage is for the server to use
difficult-to-guess, one-time resource URIs. In this instance, there
can be only a single access to the underlying resource using the
given URI. A downside to this approach is if an attacker uses the
URI before the client uses the URI, then the client is denied the
resource. Other methods would be to adopt a mechanism similar to the
URLAUTH IMAP extension [RFC4467], where the server sets cryptographic
checks on URI usage, as well as capabilities for expiration,
revocation, and so on. Specifying such a mechanism is beyond the
scope of this document.

12.5. Protection of Stored Media

MRCPv2 applications often require the use of stored media. Voice
recordings are both stored (e.g., for diagnosis and system tuning),
and fetched (for replaying utterances into multiple MRCPv2
resources). Voiceprints are fundamental to the speaker
identification and verification functions. This data can be
extremely sensitive and can present substantial privacy and
impersonation risks if stolen. Systems employing MRCPv2 SHOULD be
deployed in ways that minimize these risks. The SPEECHSC
requirements RFC [RFC4313] contains a more extensive discussion of
these risks and ways they may be mitigated.

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12.6. DTMF and Recognition Buffers

DTMF buffers and recognition buffers may grow large enough to exceed
the capabilities of a server, and the server MUST be prepared to
gracefully handle resource consumption. A server MAY respond with
the appropriate recognition incomplete if the server is in danger of
running out of resources.

12.7. Client-Set Server Parameters

In MRCPv2, there are some tasks, such as URI resource fetches, that
the server does on behalf of the client. To control this behavior,
MRCPv2 has a number of server parameters that a client can configure.
With one such parameter, Fetch-Timeout (Section 6.2.12), a malicious
client could set a very large value and then request the server to
fetch a non-existent document. It is RECOMMENDED that servers be
cautious about accepting long timeout values or abnormally large
values for other client-set parameters.

12.8. DELETE-VOICEPRINT and Authorization

Since this specification does not mandate a specific mechanism for
authentication and authorization when requesting DELETE-VOICEPRINT
(Section 11.9), there is a risk that an MRCPv2 server may not do such
a check for authentication and authorization. In practice, each
provider of voice biometric solutions does insist on its own
authentication and authorization mechanism, outside of this
specification, so this is not likely to be a major problem. If in
the future voice biometric providers standardize on such a mechanism,
then a future version of MRCP can mandate it.

13. IANA Considerations

13.1. New Registries

This section describes the name spaces (registries) for MRCPv2 that
IANA has created and now maintains. Assignment/registration policies
are described in RFC 5226 [RFC5226].

13.1.1. MRCPv2 Resource Types

IANA has created a new name space of "MRCPv2 Resource Types". All
maintenance within and additions to the contents of this name space
MUST be according to the "Standards Action" registration policy. The
initial contents of the registry, defined in Section 4.2, are given
below:

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Resource type Resource description Reference
------------- -------------------- ---------
speechrecog Speech Recognizer [RFC6787]
dtmfrecog DTMF Recognizer [RFC6787]
speechsynth Speech Synthesizer [RFC6787]
basicsynth Basic Synthesizer [RFC6787]
speakverify Speaker Verifier [RFC6787]
recorder Speech Recorder [RFC6787]

13.1.2. MRCPv2 Methods and Events

IANA has created a new name space of "MRCPv2 Methods and Events".
All maintenance within and additions to the contents of this name
space MUST be according to the "Standards Action" registration
policy. The initial contents of the registry, defined by the
"method-name" and "event-name" BNF in Section 15 and explained in
Sections 5.2 and 5.5, are given below.

Name Resource type Method/Event Reference
---- ------------- ------------ ---------
SET-PARAMS Generic Method [RFC6787]
GET-PARAMS Generic Method [RFC6787]
SPEAK Synthesizer Method [RFC6787]
STOP Synthesizer Method [RFC6787]
PAUSE Synthesizer Method [RFC6787]
RESUME Synthesizer Method [RFC6787]
BARGE-IN-OCCURRED Synthesizer Method [RFC6787]
CONTROL Synthesizer Method [RFC6787]
DEFINE-LEXICON Synthesizer Method [RFC6787]
DEFINE-GRAMMAR Recognizer Method [RFC6787]
RECOGNIZE Recognizer Method [RFC6787]
INTERPRET Recognizer Method [RFC6787]
GET-RESULT Recognizer Method [RFC6787]
START-INPUT-TIMERS Recognizer Method [RFC6787]
STOP Recognizer Method [RFC6787]
START-PHRASE-ENROLLMENT Recognizer Method [RFC6787]
ENROLLMENT-ROLLBACK Recognizer Method [RFC6787]
END-PHRASE-ENROLLMENT Recognizer Method [RFC6787]
MODIFY-PHRASE Recognizer Method [RFC6787]
DELETE-PHRASE Recognizer Method [RFC6787]
RECORD Recorder Method [RFC6787]
STOP Recorder Method [RFC6787]
START-INPUT-TIMERS Recorder Method [RFC6787]
START-SESSION Verifier Method [RFC6787]
END-SESSION Verifier Method [RFC6787]
QUERY-VOICEPRINT Verifier Method [RFC6787]
DELETE-VOICEPRINT Verifier Method [RFC6787]
VERIFY Verifier Method [RFC6787]

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VERIFY-FROM-BUFFER Verifier Method [RFC6787]
VERIFY-ROLLBACK Verifier Method [RFC6787]
STOP Verifier Method [RFC6787]
START-INPUT-TIMERS Verifier Method [RFC6787]
GET-INTERMEDIATE-RESULT Verifier Method [RFC6787]
SPEECH-MARKER Synthesizer Event [RFC6787]
SPEAK-COMPLETE Synthesizer Event [RFC6787]
START-OF-INPUT Recognizer Event [RFC6787]
RECOGNITION-COMPLETE Recognizer Event [RFC6787]
INTERPRETATION-COMPLETE Recognizer Event [RFC6787]
START-OF-INPUT Recorder Event [RFC6787]
RECORD-COMPLETE Recorder Event [RFC6787]
VERIFICATION-COMPLETE Verifier Event [RFC6787]
START-OF-INPUT Verifier Event [RFC6787]

13.1.3. MRCPv2 Header Fields

IANA has created a new name space of "MRCPv2 Header Fields". All
maintenance within and additions to the contents of this name space
MUST be according to the "Standards Action" registration policy. The
initial contents of the registry, defined by the "message-header" BNF
in Section 15 and explained in Section 5.1, are given below. Note
that the values permitted for the "Vendor-Specific-Parameters"
parameter are managed according to a different policy. See
Section 13.1.6.

Name Resource type Reference
---- ------------- ---------
Channel-Identifier Generic [RFC6787]
Accept Generic [RFC2616]
Active-Request-Id-List Generic [RFC6787]
Proxy-Sync-Id Generic [RFC6787]
Accept-Charset Generic [RFC2616]
Content-Type Generic [RFC6787]
Content-ID Generic
[RFC2392], [RFC2046], and [RFC5322]
Content-Base Generic [RFC6787]
Content-Encoding Generic [RFC6787]
Content-Location Generic [RFC6787]
Content-Length Generic [RFC6787]
Fetch-Timeout Generic [RFC6787]
Cache-Control Generic [RFC6787]
Logging-Tag Generic [RFC6787]
Set-Cookie Generic [RFC6787]
Vendor-Specific Generic [RFC6787]
Jump-Size Synthesizer [RFC6787]
Kill-On-Barge-In Synthesizer [RFC6787]
Speaker-Profile Synthesizer [RFC6787]

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Completion-Cause Synthesizer [RFC6787]
Completion-Reason Synthesizer [RFC6787]
Voice-Parameter Synthesizer [RFC6787]
Prosody-Parameter Synthesizer [RFC6787]
Speech-Marker Synthesizer [RFC6787]
Speech-Language Synthesizer [RFC6787]
Fetch-Hint Synthesizer [RFC6787]
Audio-Fetch-Hint Synthesizer [RFC6787]
Failed-URI Synthesizer [RFC6787]
Failed-URI-Cause Synthesizer [RFC6787]
Speak-Restart Synthesizer [RFC6787]
Speak-Length Synthesizer [RFC6787]
Load-Lexicon Synthesizer [RFC6787]
Lexicon-Search-Order Synthesizer [RFC6787]
Confidence-Threshold Recognizer [RFC6787]
Sensitivity-Level Recognizer [RFC6787]
Speed-Vs-Accuracy Recognizer [RFC6787]
N-Best-List-Length Recognizer [RFC6787]
Input-Type Recognizer [RFC6787]
No-Input-Timeout Recognizer [RFC6787]
Recognition-Timeout Recognizer [RFC6787]
Waveform-URI Recognizer [RFC6787]
Input-Waveform-URI Recognizer [RFC6787]
Completion-Cause Recognizer [RFC6787]
Completion-Reason Recognizer [RFC6787]
Recognizer-Context-Block Recognizer [RFC6787]
Start-Input-Timers Recognizer [RFC6787]
Speech-Complete-Timeout Recognizer [RFC6787]
Speech-Incomplete-Timeout Recognizer [RFC6787]
Dtmf-Interdigit-Timeout Recognizer [RFC6787]
Dtmf-Term-Timeout Recognizer [RFC6787]
Dtmf-Term-Char Recognizer [RFC6787]
Failed-URI Recognizer [RFC6787]
Failed-URI-Cause Recognizer [RFC6787]
Save-Waveform Recognizer [RFC6787]
Media-Type Recognizer [RFC6787]
New-Audio-Channel Recognizer [RFC6787]
Speech-Language Recognizer [RFC6787]
Ver-Buffer-Utterance Recognizer [RFC6787]
Recognition-Mode Recognizer [RFC6787]
Cancel-If-Queue Recognizer [RFC6787]
Hotword-Max-Duration Recognizer [RFC6787]
Hotword-Min-Duration Recognizer [RFC6787]
Interpret-Text Recognizer [RFC6787]
Dtmf-Buffer-Time Recognizer [RFC6787]
Clear-Dtmf-Buffer Recognizer [RFC6787]
Early-No-Match Recognizer [RFC6787]
Num-Min-Consistent-Pronunciations Recognizer [RFC6787]

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Consistency-Threshold Recognizer [RFC6787]
Clash-Threshold Recognizer [RFC6787]
Personal-Grammar-URI Recognizer [RFC6787]
Enroll-Utterance Recognizer [RFC6787]
Phrase-ID Recognizer [RFC6787]
Phrase-NL Recognizer [RFC6787]
Weight Recognizer [RFC6787]
Save-Best-Waveform Recognizer [RFC6787]
New-Phrase-ID Recognizer [RFC6787]
Confusable-Phrases-URI Recognizer [RFC6787]
Abort-Phrase-Enrollment Recognizer [RFC6787]
Sensitivity-Level Recorder [RFC6787]
No-Input-Timeout Recorder [RFC6787]
Completion-Cause Recorder [RFC6787]
Completion-Reason Recorder [RFC6787]
Failed-URI Recorder [RFC6787]
Failed-URI-Cause Recorder [RFC6787]
Record-URI Recorder [RFC6787]
Media-Type Recorder [RFC6787]
Max-Time Recorder [RFC6787]
Trim-Length Recorder [RFC6787]
Final-Silence Recorder [RFC6787]
Capture-On-Speech Recorder [RFC6787]
Ver-Buffer-Utterance Recorder [RFC6787]
Start-Input-Timers Recorder [RFC6787]
New-Audio-Channel Recorder [RFC6787]
Repository-URI Verifier [RFC6787]
Voiceprint-Identifier Verifier [RFC6787]
Verification-Mode Verifier [RFC6787]
Adapt-Model Verifier [RFC6787]
Abort-Model Verifier [RFC6787]
Min-Verification-Score Verifier [RFC6787]
Num-Min-Verification-Phrases Verifier [RFC6787]
Num-Max-Verification-Phrases Verifier [RFC6787]
No-Input-Timeout Verifier [RFC6787]
Save-Waveform Verifier [RFC6787]
Media-Type Verifier [RFC6787]
Waveform-URI Verifier [RFC6787]
Voiceprint-Exists Verifier [RFC6787]
Ver-Buffer-Utterance Verifier [RFC6787]
Input-Waveform-URI Verifier [RFC6787]
Completion-Cause Verifier [RFC6787]
Completion-Reason Verifier [RFC6787]
Speech-Complete-Timeout Verifier [RFC6787]
New-Audio-Channel Verifier [RFC6787]
Abort-Verification Verifier [RFC6787]
Start-Input-Timers Verifier [RFC6787]
Input-Type Verifier [RFC6787]

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13.1.4. MRCPv2 Status Codes

IANA has created a new name space of "MRCPv2 Status Codes" with the
initial values that are defined in Section 5.4. All maintenance
within and additions to the contents of this name space MUST be
according to the "Specification Required with Expert Review"
registration policy.

13.1.5. Grammar Reference List Parameters

IANA has created a new name space of "Grammar Reference List
Parameters". All maintenance within and additions to the contents of
this name space MUST be according to the "Specification Required with
Expert Review" registration policy. There is only one initial
parameter as shown below.

Name Reference
---- -------------
weight [RFC6787]

13.1.6. MRCPv2 Vendor-Specific Parameters

IANA has created a new name space of "MRCPv2 Vendor-Specific
Parameters". All maintenance within and additions to the contents of
this name space MUST be according to the "Hierarchical Allocation"
registration policy as follows. Each name (corresponding to the
"vendor-av-pair-name" ABNF production) MUST satisfy the syntax
requirements of Internet Domain Names as described in Section 2.3.1
of RFC 1035 [RFC1035] (and as updated or obsoleted by successive
RFCs), with one exception, the order of the domain names is reversed.
For example, a vendor-specific parameter "foo" by example.com would
have the form "com.example.foo". The first, or top-level domain, is
restricted to exactly the set of Top-Level Internet Domains defined
by IANA and will be updated by IANA when and only when that set
changes. The second-level and all subdomains within the parameter
name MUST be allocated according to the "First Come First Served"
policy. It is RECOMMENDED that assignment requests adhere to the
existing allocations of Internet domain names to organizations,
institutions, corporations, etc.

The registry contains a list of vendor-registered parameters, where
each defined parameter is associated with a contact person and
includes an optional reference to the definition of the parameter,
preferably an RFC. The registry is initially empty.

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13.2. NLSML-Related Registrations

13.2.1. 'application/nlsml+xml' Media Type Registration

IANA has registered the following media type according to the process
defined in RFC 4288 [RFC4288].

To: [email protected]

Subject: Registration of media type application/nlsml+xml

MIME media type name: application

MIME subtype name: nlsml+xml

Required parameters: none

Optional parameters:

charset: All of the considerations described in RFC 3023
[RFC3023] also apply to the application/nlsml+xml media type.

Encoding considerations: All of the considerations described in RFC
3023 also apply to the 'application/nlsml+xml' media type.

Security considerations: As with HTML, NLSML documents contain links
to other data stores (grammars, verifier resources, etc.). Unlike
HTML, however, the data stores are not treated as media to be
rendered. Nevertheless, linked files may themselves have security
considerations, which would be those of the individual registered
types. Additionally, this media type has all of the security
considerations described in RFC 3023.

Interoperability considerations: Although an NLSML document is
itself a complete XML document, for a fuller interpretation of the
content a receiver of an NLSML document may wish to access
resources linked to by the document. The inability of an NLSML
processor to access or process such linked resources could result
in different behavior by the ultimate consumer of the data.

Published specification: RFC 6787

Applications that use this media type: MRCPv2 clients and servers

Additional information: none

Magic number(s): There is no single initial octet sequence that is
always present for NLSML files.

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Person & email address to contact for further information:
Sarvi Shanmugham, [email protected]

Intended usage: This media type is expected to be used only in
conjunction with MRCPv2.

13.3. NLSML XML Schema Registration

IANA has registered and now maintains the following XML Schema.
Information provided follows the template in RFC 3688 [RFC3688].

XML element type: schema

URI: urn:ietf:params:xml:schema:nlsml

Registrant Contact: IESG

XML: See Section 16.1.

13.4. MRCPv2 XML Namespace Registration

IANA has registered and now maintains the following XML Name space.
Information provided follows the template in RFC 3688 [RFC3688].

XML element type: ns

URI: urn:ietf:params:xml:ns:mrcpv2

Registrant Contact: IESG

XML: RFC 6787

13.5. Text Media Type Registrations

IANA has registered the following text media type according to the
process defined in RFC 4288 [RFC4288].

13.5.1. text/grammar-ref-list

To: [email protected]

Subject: Registration of media type text/grammar-ref-list

MIME media type name: text

MIME subtype name: text/grammar-ref-list

Required parameters: none

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Optional parameters: none

Encoding considerations: Depending on the transfer protocol, a
transfer encoding may be necessary to deal with very long lines.

Security considerations: This media type contains URIs that may
represent references to external resources. As these resources
are assumed to be speech recognition grammars, similar
considerations as for the media types 'application/srgs' and
'application/srgs+xml' apply.

Interoperability considerations: '>' must be percent encoded in URIs
according to RFC 3986 [RFC3986].

Published specification: The RECOGNIZE method of the MRCP protocol
performs a recognition operation that matches input against a set
of grammars. When matching against more than one grammar, it is
sometimes necessary to use different weights for the individual
grammars. These weights are not a property of the grammar
resource itself but qualify the reference to that grammar for the
particular recognition operation initiated by the RECOGNIZE
method. The format of the proposed 'text/grammar-ref-list' media
type is as follows:

body = *reference
reference = "<" uri ">" [parameters] CRLF
parameters = ";" parameter *(";" parameter)
parameter = attribute "=" value

This specification currently only defines a 'weight' parameter,
but new parameters MAY be added through the "Grammar Reference
List Parameters" IANA registry established through this
specification. Example:

<http://example.com/grammars/field1.gram>
<http://example.com/grammars/field2.gram>;weight="0.85"
<session:[email protected]>;weight="0.9"
<http://example.com/grammars/universals.gram>;weight="0.75"

Applications that use this media type: MRCPv2 clients and servers

Additional information: none

Magic number(s): none

Person & email address to contact for further information:
Sarvi Shanmugham, [email protected]

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RFC 6787 MRCPv2 November 2012

Intended usage: This media type is expected to be used only in
conjunction with MRCPv2.

13.6. 'session' URI Scheme Registration

IANA has registered the following new URI scheme. The information
below follows the template given in RFC 4395 [RFC4395].

URI scheme name: session

Status: Permanent

URI scheme syntax: The syntax of this scheme is identical to that
defined for the "cid" scheme in Section 2 of RFC 2392 [RFC2392].

URI scheme semantics: The URI is intended to identify a data
resource previously given to the network computing resource. The
purpose of this scheme is to permit access to the specific
resource for the lifetime of the session with the entity storing
the resource. The media type of the resource CAN vary. There is
no explicit mechanism for communication of the media type. This
scheme is currently widely used internally by existing
implementations, and the registration is intended to provide
information in the rare (and unfortunate) case that the scheme is
used elsewhere. The scheme SHOULD NOT be used for open Internet
protocols.

Encoding considerations: There are no other encoding considerations
for the 'session' URIs not described in RFC 3986 [RFC3986]

Applications/protocols that use this URI scheme name: This scheme
name is used by MRCPv2 clients and servers.

Interoperability considerations: Note that none of the resources are
accessible after the MCRPv2 session ends, hence the name of the
scheme. For clients who establish one MRCPv2 session only for the
entire speech application being implemented, this is sufficient,
but clients who create, terminate, and recreate MRCP sessions for
performance or scalability reasons will lose access to resources
established in the earlier session(s).

Security considerations: Generic security considerations for URIs
described in RFC 3986 [RFC3986] apply to this scheme as well. The
URIs defined here provide an identification mechanism only. Given
that the communication channel between client and server is
secure, that the server correctly accesses the resource associated

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RFC 6787 MRCPv2 November 2012

with the URI, and that the server ensures session-only lifetime
and access for each URI, the only additional security issues are
those of the types of media referred to by the URI.

Contact: Sarvi Shanmugham, [email protected]

Author/Change controller: IESG, [email protected]

References: This specification, particularly Sections 6.2.7, 8.5.2,
9.5.1, and 9.9.

13.7. SDP Parameter Registrations

IANA has registered the following SDP parameter values. The
information for each follows the template given in RFC 4566
[RFC4566], Appendix B.

13.7.1. Sub-Registry "proto"

"TCP/MRCPv2" value of the "proto" parameter

Contact name, email address, and telephone number: Sarvi Shanmugham,
[email protected], +1.408.902.3875

Name being registered (as it will appear in SDP): TCP/MRCPv2

Long-form name in English: MCRPv2 over TCP

Type of name: proto

Explanation of name: This name represents the MCRPv2 protocol
carried over TCP.

Reference to specification of name: RFC 6787

"TCP/TLS/MRCPv2" value of the "proto" parameter

Contact name, email address, and telephone number: Sarvi Shanmugham,
[email protected], +1.408.902.3875

Name being registered (as it will appear in SDP): TCP/TLS/MRCPv2

Long-form name in English: MCRPv2 over TLS over TCP

Type of name: proto

Explanation of name: This name represents the MCRPv2 protocol
carried over TLS over TCP.

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RFC 6787 MRCPv2 November 2012

Reference to specification of name: RFC 6787

13.7.2. Sub-Registry "att-field (media-level)"

"resource" value of the "att-field" parameter

Contact name, email address, and telephone number: Sarvi Shanmugham,
[email protected], +1.408.902.3875

Attribute name (as it will appear in SDP): resource

Long-form attribute name in English: MRCPv2 resource type

Type of attribute: media-level

Subject to charset attribute? no

Explanation of attribute: See Section 4.2 of RFC 6787 for
description and examples.

Specification of appropriate attribute values: See section
Section 13.1.1 of RFC 6787.

"channel" value of the "att-field" parameter

Contact name, email address, and telephone number: Sarvi Shanmugham,
[email protected], +1.408.902.3875

Attribute name (as it will appear in SDP): channel

Long-form attribute name in English: MRCPv2 resource channel
identifier

Type of attribute: media-level

Subject to charset attribute? no

Explanation of attribute: See Section 4.2 of RFC 6787 for
description and examples.

Specification of appropriate attribute values: See Section 4.2 and
the "channel-id" ABNF production rules of RFC 6787.

"cmid" value of the "att-field" parameter

Contact name, email address, and telephone number: Sarvi Shanmugham,
[email protected], +1.408.902.3875

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RFC 6787 MRCPv2 November 2012

Attribute name (as it will appear in SDP): cmid

Long-form attribute name in English: MRCPv2 resource channel media
identifier

Type of attribute: media-level

Subject to charset attribute? no

Explanation of attribute: See Section 4.4 of RFC 6787 for
description and examples.

Specification of appropriate attribute values: See Section 4.4 and
the "cmid-attribute" ABNF production rules of RFC 6787.

14. Examples

14.1. Message Flow

The following is an example of a typical MRCPv2 session of speech
synthesis and recognition between a client and a server. Although
the SDP "s=" attribute in these examples has a text description value
to assist in understanding the examples, please keep in mind that RFC
3264 [RFC3264] recommends that messages actually put on the wire use
a space or a dash.

The figure below illustrates opening a session to the MRCPv2 server.
This exchange does not allocate a resource or setup media. It simply
establishes a SIP session with the MRCPv2 server.

C->S:
INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg1
Max-Forwards:6
To:MediaServer <sip:[email protected]>
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323123 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2614933546 2614933546 IN IP4 192.0.2.12
s=Set up MRCPv2 control and audio
i=Initial contact
c=IN IP4 192.0.2.12

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RFC 6787 MRCPv2 November 2012

S->C:
SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg1;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323123 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 3000000001 3000000001 IN IP4 192.0.2.11
s=Set up MRCPv2 control and audio
i=Initial contact
c=IN IP4 192.0.2.11

C->S:
ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg2
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323123 ACK
Content-Length:0

The client requests the server to create a synthesizer resource
control channel to do speech synthesis. This also adds a media
stream to send the generated speech. Note that, in this example, the
client requests a new MRCPv2 TCP stream between the client and the
server. In the following requests, the client will ask to use the
existing connection.

C->S:
INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg3
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323124 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

Burnett & Shanmugham Standards Track [Page 184]

RFC 6787 MRCPv2 November 2012

v=0
o=sarvi 2614933546 2614933547 IN IP4 192.0.2.12
s=Set up MRCPv2 control and audio
i=Add TCP channel, synthesizer and one-way audio
c=IN IP4 192.0.2.12
t=0 0
m=application 9 TCP/MRCPv2 1
a=setup:active
a=connection:new
a=resource:speechsynth
a=cmid:1
m=audio 49170 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=recvonly
a=mid:1

S->C:
SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg3;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323124 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 3000000001 3000000002 IN IP4 192.0.2.11
s=Set up MRCPv2 control and audio
i=Add TCP channel, synthesizer and one-way audio
c=IN IP4 192.0.2.11
t=0 0
m=application 32416 TCP/MRCPv2 1
a=setup:passive
a=connection:new
a=channel:32AECB23433801@speechsynth
a=cmid:1
m=audio 48260 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=sendonly
a=mid:1

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RFC 6787 MRCPv2 November 2012

C->S:
ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg4
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323124 ACK
Content-Length:0

This exchange allocates an additional resource control channel for a
recognizer. Since a recognizer would need to receive an audio stream
for recognition, this interaction also updates the audio stream to
sendrecv, making it a two-way audio stream.

C->S:
INVITE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg5
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323125 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=sarvi 2614933546 2614933548 IN IP4 192.0.2.12
s=Set up MRCPv2 control and audio
i=Add recognizer and duplex the audio
c=IN IP4 192.0.2.12
t=0 0
m=application 9 TCP/MRCPv2 1
a=setup:active
a=connection:existing
a=resource:speechsynth
a=cmid:1
m=audio 49170 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=recvonly
a=mid:1
m=application 9 TCP/MRCPv2 1
a=setup:active

Burnett & Shanmugham Standards Track [Page 186]

RFC 6787 MRCPv2 November 2012

a=connection:existing
a=resource:speechrecog
a=cmid:2
m=audio 49180 RTP/AVP 0 96
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=sendonly
a=mid:2

S->C:
SIP/2.0 200 OK
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg5;received=192.0.32.10
To:MediaServer <sip:[email protected]>;tag=62784
From:sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323125 INVITE
Contact:<sip:[email protected]>
Content-Type:application/sdp
Content-Length:...

v=0
o=- 3000000001 3000000003 IN IP4 192.0.2.11
s=Set up MRCPv2 control and audio
i=Add recognizer and duplex the audio
c=IN IP4 192.0.2.11
t=0 0
m=application 32416 TCP/MRCPv2 1
a=channel:32AECB23433801@speechsynth
a=cmid:1
m=audio 48260 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=sendonly
a=mid:1
m=application 32416 TCP/MRCPv2 1
a=channel:32AECB23433801@speechrecog
a=cmid:2
m=audio 48260 RTP/AVP 0
a=rtpmap:0 pcmu/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-15
a=recvonly
a=mid:2

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RFC 6787 MRCPv2 November 2012

C->S:
ACK sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg6
Max-Forwards:6
To:MediaServer <sip:[email protected]>;tag=62784
From:Sarvi <sip:[email protected]>;tag=1928301774
Call-ID:a84b4c76e66710
CSeq:323125 ACK
Content-Length:0

A MRCPv2 SPEAK request initiates speech.

C->S:
MRCP/2.0 ... SPEAK 543257
Channel-Identifier:32AECB23433801@speechsynth
Kill-On-Barge-In:false
Voice-gender:neutral
Voice-age:25
Prosody-volume:medium
Content-Type:application/ssml+xml
Content-Length:...

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>You have 4 new messages.</s>
<s>The first is from Stephanie Williams
<mark name="Stephanie"/>
and arrived at <break/>
<say-as interpret-as="vxml:time">0345p</say-as>.</s>
<s>The subject is <prosody
rate="-20%">ski trip</prosody></s>
</p>
</speak>

S->C:
MRCP/2.0 ... 543257 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechsynth
Speech-Marker:timestamp=857205015059

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RFC 6787 MRCPv2 November 2012

The synthesizer hits the special marker in the message to be spoken
and faithfully informs the client of the event.

S->C: MRCP/2.0 ... SPEECH-MARKER 543257 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechsynth
Speech-Marker:timestamp=857206027059;Stephanie

The synthesizer finishes with the SPEAK request.

S->C: MRCP/2.0 ... SPEAK-COMPLETE 543257 COMPLETE
Channel-Identifier:32AECB23433801@speechsynth
Speech-Marker:timestamp=857207685213;Stephanie

The recognizer is issued a request to listen for the customer
choices.

C->S: MRCP/2.0 ... RECOGNIZE 543258
Channel-Identifier:32AECB23433801@speechrecog
Content-Type:application/srgs+xml
Content-Length:...

<?xml version="1.0"?>

<grammar xmlns="http://www.w3.org/2001/06/grammar"
xml:lang="en-US" version="1.0" root="request">

<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: MRCP/2.0 ... 543258 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog

The client issues the next MRCPv2 SPEAK method.

C->S: MRCP/2.0 ... SPEAK 543259
Channel-Identifier:32AECB23433801@speechsynth
Kill-On-Barge-In:true
Content-Type:application/ssml+xml
Content-Length:...

Burnett & Shanmugham Standards Track [Page 189]

RFC 6787 MRCPv2 November 2012

<?xml version="1.0"?>
<speak version="1.0"
xmlns="http://www.w3.org/2001/10/synthesis"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.w3.org/2001/10/synthesis
http://www.w3.org/TR/speech-synthesis/synthesis.xsd"
xml:lang="en-US">
<p>
<s>Welcome to ABC corporation.</s>
<s>Who would you like to talk to?</s>
</p>
</speak>

S->C: MRCP/2.0 ... 543259 200 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechsynth
Speech-Marker:timestamp=857207696314

This next section of this ongoing example demonstrates how kill-on-
barge-in support works. Since this last SPEAK request had Kill-On-
Barge-In set to "true", when the recognizer (the server) generated
the START-OF-INPUT event while a SPEAK was active, the client
immediately issued a BARGE-IN-OCCURRED method to the synthesizer
resource. The speech synthesizer then terminated playback and
notified the client. The completion-cause code provided the
indication that this was a kill-on-barge-in interruption rather than
a normal completion.

Note that, since the recognition and synthesizer resources are in the
same session on the same server, to obtain a faster response the
server might have internally relayed the start-of-input condition to
the synthesizer directly, before receiving the expected BARGE-IN-
OCCURRED event. However, any such communication is outside the scope
of MRCPv2.

S->C: MRCP/2.0 ... START-OF-INPUT 543258 IN-PROGRESS
Channel-Identifier:32AECB23433801@speechrecog
Proxy-Sync-Id:987654321

C->S: MRCP/2.0 ... BARGE-IN-OCCURRED 543259
Channel-Identifier:32AECB23433801@speechsynth
Proxy-Sync-Id:987654321

S->C: MRCP/2.0 ... 543259 200 COMPLETE
Channel-Identifier:32AECB23433801@speechsynth
Active-Request-Id-List:543258
Speech-Marker:timestamp=857206096314

Burnett & Shanmugham Standards Track [Page 190]

RFC 6787 MRCPv2 November 2012

S->C: MRCP/2.0 ... SPEAK-COMPLETE 543259 COMPLETE
Channel-Identifier:32AECB23433801@speechsynth
Completion-Cause:001 barge-in
Speech-Marker:timestamp=857207685213

The recognizer 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: MRCP/2.0 ... RECOGNITION-COMPLETE 543258 COMPLETE
Channel-Identifier:32AECB23433801@speechrecog
Completion-Cause:000 success
Waveform-URI:<http://web.media.com/session123/audio.wav>;
size=423523;duration=25432
Content-Type:application/nlsml+xml
Content-Length:...

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="session:[email protected]">
<interpretation>
<instance name="Person">
<ex:Person>
<ex:Name> Andre Roy </ex:Name>
</ex:Person>
</instance>
<input> may I speak to Andre Roy </input>
</interpretation>
</result>

Since the client was now finished with the session, including all
resources, it issued a SIP BYE request to close the SIP session.
This caused all control channels and resources allocated under the
session to be deallocated.

C->S: BYE sip:[email protected] SIP/2.0
Via:SIP/2.0/TCP client.atlanta.example.com:5060;
branch=z9hG4bK74bg7
Max-Forwards:6
From:Sarvi <sip:[email protected]>;tag=1928301774
To:MediaServer <sip:[email protected]>;tag=62784
Call-ID:a84b4c76e66710
CSeq:323126 BYE
Content-Length:0

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RFC 6787 MRCPv2 November 2012

14.2. Recognition Result Examples

14.2.1. Simple ASR Ambiguity

System: To which city will you be traveling?
User: I want to go to Pittsburgh.

<?xml version="1.0"?>
<result xmlns="urn:ietf:params:xml:ns:mrcpv2"
xmlns:ex="http://www.example.com/example"
grammar="http://www.example.com/flight">
<interpretation confidence="0.6">
<instance>
<ex:airline>
<ex:to_city>Pittsburgh</ex:to_city>
<ex:airline>
<instance>
<input mode="speech">
I want to go to Pittsburgh
</input>
</interpretation>
<interpretation confidence="0.4"
<instance>
<ex:airline>
<ex:to_city>Stockholm</ex:to_city>
</ex:airline>
</instance>
<input>I want to go to Stockholm</input>
</interpretation>
</result>

14.2.2. Mixed Initiative

System: What would you like?
User: I would like 2 pizzas, one with pepperoni and cheese,
one with sausage and a bottle of coke, to go.

This example includes an order object which in turn contains objects
named "food_item", "drink_item", and "delivery_method". The
representation assumes there are no ambiguities in the speech or
natural language processing. Note that this representation also
assumes some level of intra-sentential anaphora resolution, i.e., to
resolve the two "one"s as "pizza".

<?xml version="1.0"?>
<nl:result xmlns:nl="urn:ietf:params:xml:ns:mrcpv2"
xmlns="http://www.example.com/example"
grammar="http://www.example.com/foodorder">

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RFC 6787 MRCPv2 November 2012

<nl:interpretation confidence="1.0" >
<nl:instance>
<order>
<food_item confidence="1.0">
<pizza>
<ingredients confidence="1.0">
pepperoni
</ingredients>
<ingredients confidence="1.0">
cheese
</ingredients>
</pizza>
<pizza>
<ingredients>sausage</ingredients>
</pizza>
</food_item>
<drink_item confidence="1.0">
<size>2-liter</size>
</drink_item>
<delivery_method>to go</delivery_method>
</order>
</nl:instance>
<nl:input mode="speech">I would like 2 pizzas,
one with pepperoni and cheese, one with sausage
and a bottle of coke, to go.
</nl:input>
</nl:interpretation>
</nl:result>

14.2.3. DTMF Input

A combination of DTMF input and speech is represented using nested
input elements. For example:
User: My pin is (dtmf 1 2 3 4)

<input>
<input mode="speech" confidence ="1.0"
timestamp-start="2000-04-03T0:00:00"
timestamp-end="2000-04-03T0:00:01.5">My pin is
</input>
<input mode="dtmf" confidence ="1.0"
timestamp-start="2000-04-03T0:00:01.5"
timestamp-end="2000-04-03T0:00:02.0">1 2 3 4
</input>
</input>

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RFC 6787 MRCPv2 November 2012

Note that grammars that recognize mixtures of speech and DTMF are not
currently possible in SRGS; however, this representation might be
needed for other applications of NLSML, and this mixture capability
might be introduced in future versions of SRGS.

14.2.4. Interpreting Meta-Dialog and Meta-Task Utterances

Natural language communication makes use of meta-dialog and meta-task
utterances. This specification is flexible enough so that meta-
utterances can be represented on an application-specific basis
without requiring other standard markup.

Here are two examples of how meta-task and meta-dialog utterances
might be represented.

System: What toppings do you want on your pizza?
User: What toppings do you have?

<interpretation grammar="http://www.example.com/toppings">
<instance>
<question>
<questioned_item>toppings<questioned_item>
<questioned_property>
availability
</questioned_property>
</question>
</instance>
<input mode="speech">
what toppings do you have?
</input>
</interpretation>

User: slow down.

<interpretation grammar="http://www.example.com/generalCommandsGrammar">
<instance>
<command>
<action>reduce speech rate</action>
<doer>system</doer>
</command>
</instance>
<input mode="speech">slow down</input>
</interpretation>

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RFC 6787 MRCPv2 November 2012

14.2.5. Anaphora and Deixis

This specification can be used on an application-specific basis to
represent utterances that contain unresolved anaphoric and deictic
references. Anaphoric references, which include pronouns and
definite noun phrases that refer to something that was mentioned in
the preceding linguistic context, and deictic references, which refer
to something that is present in the non-linguistic context, present
similar problems in that there may not be sufficient unambiguous
linguistic context to determine what their exact role in the
interpretation should be. In order to represent unresolved anaphora
and deixis using this specification, one strategy would be for the
developer to define a more surface-oriented representation that
leaves the specific details of the interpretation of the reference
open. (This assumes that a later component is responsible for
actually resolving the reference).

Example: (ignoring the issue of representing the input from the
pointing gesture.)

System: What do you want to drink?
User: I want this. (clicks on picture of large root beer.)

<?xml version="1.0"?>
<nl:result xmlns:nl="urn:ietf:params:xml:ns:mrcpv2"
xmlns="http://www.example.com/example"
grammar="http://www.example.com/beverages.grxml">
<nl:interpretation>
<nl:instance>
<doer>I</doer>
<action>want</action>
<object>this</object>
</nl:instance>
<nl:input mode="speech">I want this</nl:input>
</nl:interpretation>
</nl:result>

14.2.6. Distinguishing Individual Items from Sets with One Member

For programming convenience, it is useful to be able to distinguish
between individual items and sets containing one item in the XML
representation of semantic results. For example, a pizza order might
consist of exactly one pizza, but a pizza might contain zero or more
toppings. Since there is no standard way of marking this distinction
directly in XML, in the current framework, the developer is free to
adopt any conventions that would convey this information in the XML
markup. One strategy would be for the developer to wrap the set of
items in a grouping element, as in the following example.

Burnett & Shanmugham Standards Track [Page 195]

RFC 6787 MRCPv2 November 2012

<order>
<pizza>
<topping-group>
<topping>mushrooms</topping>
</topping-group>
</pizza>
<drink>coke</drink>
</order>

In this example, the programmer can assume that there is supposed to
be exactly one pizza and one drink in the order, but the fact that
there is only one topping is an accident of this particular pizza
order.

Note that the client controls both the grammar and the semantics to
be returned upon grammar matches, so the user of MRCPv2 is fully
empowered to cause results to be returned in NLSML in such a way that
the interpretation is clear to that user.

14.2.7. Extensibility

Extensibility in NLSML is provided via result content flexibility, as
described in the discussions of meta-utterances and anaphora. NLSML
can easily be used in sophisticated systems to convey application-
specific information that more basic systems would not make use of,
for example, defining speech acts.

15. ABNF Normative Definition

The following productions make use of the core rules defined in
Section B.1 of RFC 5234 [RFC5234].

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
UTFCHAR = %x21-7E
/ UTF8-NONASCII
param = *pchar

Burnett & Shanmugham Standards Track [Page 196]

RFC 6787 MRCPv2 November 2012

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

pchar = unreserved / escaped
/ ":" / "@" / "&" / "=" / "+" / "$" / ","

alphanum = ALPHA / DIGIT

BOOLEAN = "true" / "false"

FLOAT = *DIGIT ["." *DIGIT]

escaped = "%" HEXDIG HEXDIG

fragment = *uric

uri = [ absoluteURI / relativeURI ]
[ "#" fragment ]

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 ]

Burnett & Shanmugham Standards Track [Page 197]

RFC 6787 MRCPv2 November 2012

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

Burnett & Shanmugham Standards Track [Page 198]

RFC 6787 MRCPv2 November 2012

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*19DIGIT

; generic-message is the top-level rule

generic-message = start-line message-header CRLF
[ message-body ]

message-body = *OCTET

start-line = request-line / response-line / event-line

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

response-line = mrcp-version SP message-length SP request-id
SP status-code SP request-state CRLF

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

method-name = generic-method
/ synthesizer-method
/ recognizer-method
/ recorder-method
/ verifier-method

generic-method = "SET-PARAMS"
/ "GET-PARAMS"

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

Burnett & Shanmugham Standards Track [Page 199]

RFC 6787 MRCPv2 November 2012

event-name = synthesizer-event
/ recognizer-event
/ recorder-event
/ verifier-event

message-header = 1*(generic-header / resource-header / generic-field)

generic-field = field-name ":" [ field-value ]
field-name = token
field-value = *LWS field-content *( CRLF 1*LWS field-content)
field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations
of token, separators, and quoted-string>

resource-header = synthesizer-header
/ recognizer-header
/ recorder-header
/ verifier-header

generic-header = channel-identifier
/ accept
/ active-request-id-list
/ proxy-sync-id
/ accept-charset
/ content-type
/ content-id
/ content-base
/ content-encoding
/ content-location
/ content-length
/ fetch-timeout
/ cache-control
/ logging-tag
/ set-cookie
/ vendor-specific

; -- content-id is as defined in RFC 2392, RFC 2046 and RFC 5322
; -- accept and accept-charset are as defined in RFC 2616

mrcp-version = "MRCP" "/" 1*2DIGIT "." 1*2DIGIT

message-length = 1*19DIGIT

request-id = 1*10DIGIT

status-code = 3DIGIT

Burnett & Shanmugham Standards Track [Page 200]

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channel-identifier = "Channel-Identifier" ":"
channel-id CRLF

channel-id = 1*alphanum "@" 1*alphanum

active-request-id-list = "Active-Request-Id-List" ":"
request-id *("," request-id) CRLF

proxy-sync-id = "Proxy-Sync-Id" ":" 1*VCHAR CRLF

content-base = "Content-Base" ":" absoluteURI CRLF

content-length = "Content-Length" ":" 1*19DIGIT CRLF

content-type = "Content-Type" ":" media-type-value CRLF

media-type-value = 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

fetch-timeout = "Fetch-Timeout" ":" 1*19DIGIT CRLF

cache-directive = "max-age" "=" delta-seconds
/ "max-stale" ["=" delta-seconds ]
/ "min-fresh" "=" delta-seconds

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delta-seconds = 1*19DIGIT

logging-tag = "Logging-Tag" ":" 1*UTFCHAR CRLF

vendor-specific = "Vendor-Specific-Parameters" ":"
[vendor-specific-av-pair
*(";" vendor-specific-av-pair)] CRLF

vendor-specific-av-pair = vendor-av-pair-name "="
value

vendor-av-pair-name = 1*UTFCHAR

set-cookie = "Set-Cookie:" SP set-cookie-string
set-cookie-string = cookie-pair *( ";" SP cookie-av )
cookie-pair = cookie-name "=" cookie-value
cookie-name = token
cookie-value = *cookie-octet / ( DQUOTE *cookie-octet DQUOTE )
cookie-octet = %x21 / %x23-2B / %x2D-3A / %x3C-5B / %x5D-7E
token = <token, defined in [RFC2616], Section 2.2>

cookie-av = expires-av / max-age-av / domain-av /
path-av / secure-av / httponly-av /
extension-av / age-av
expires-av = "Expires=" sane-cookie-date
sane-cookie-date = <rfc1123-date, defined in [RFC2616], Section 3.3.1>
max-age-av = "Max-Age=" non-zero-digit *DIGIT
non-zero-digit = %x31-39
domain-av = "Domain=" domain-value
domain-value = <subdomain>
path-av = "Path=" path-value
path-value = <any CHAR except CTLs or ";">
secure-av = "Secure"
httponly-av = "HttpOnly"
extension-av = <any CHAR except CTLs or ";">
age-av = "Age=" delta-seconds

; Synthesizer ABNF

synthesizer-method = "SPEAK"
/ "STOP"
/ "PAUSE"
/ "RESUME"
/ "BARGE-IN-OCCURRED"
/ "CONTROL"
/ "DEFINE-LEXICON"

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

synthesizer-header = jump-size
/ kill-on-barge-in
/ speaker-profile
/ completion-cause
/ completion-reason
/ voice-parameter
/ prosody-parameter
/ speech-marker
/ speech-language
/ fetch-hint
/ audio-fetch-hint
/ failed-uri
/ failed-uri-cause
/ speak-restart
/ speak-length
/ load-lexicon
/ lexicon-search-order

jump-size = "Jump-Size" ":" speech-length-value CRLF

speech-length-value = numeric-speech-length
/ text-speech-length

text-speech-length = 1*UTFCHAR SP "Tag"

numeric-speech-length = ("+" / "-") positive-speech-length

positive-speech-length = 1*19DIGIT SP numeric-speech-unit

numeric-speech-unit = "Second"
/ "Word"
/ "Sentence"
/ "Paragraph"

kill-on-barge-in = "Kill-On-Barge-In" ":" BOOLEAN
CRLF

speaker-profile = "Speaker-Profile" ":" uri CRLF

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

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completion-reason = "Completion-Reason" ":"
quoted-string CRLF

voice-parameter = voice-gender
/ voice-age
/ voice-variant
/ voice-name

voice-gender = "Voice-Gender:" voice-gender-value CRLF

voice-gender-value = "male"
/ "female"
/ "neutral"

voice-age = "Voice-Age:" 1*3DIGIT CRLF

voice-variant = "Voice-Variant:" 1*19DIGIT CRLF

voice-name = "Voice-Name:"
1*UTFCHAR *(1*WSP 1*UTFCHAR) CRLF

prosody-parameter = "Prosody-" prosody-param-name ":"
prosody-param-value CRLF

prosody-param-name = 1*VCHAR

prosody-param-value = 1*VCHAR

timestamp = "timestamp" "=" time-stamp-value

time-stamp-value = 1*20DIGIT

speech-marker = "Speech-Marker" ":"
timestamp
[";" 1*(UTFCHAR / %x20)] CRLF

speech-language = "Speech-Language" ":" 1*VCHAR CRLF

fetch-hint = "Fetch-Hint" ":" ("prefetch" / "safe") CRLF

audio-fetch-hint = "Audio-Fetch-Hint" ":"
("prefetch" / "safe" / "stream") CRLF

failed-uri = "Failed-URI" ":" absoluteURI CRLF

failed-uri-cause = "Failed-URI-Cause" ":" 1*UTFCHAR CRLF

speak-restart = "Speak-Restart" ":" BOOLEAN CRLF

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speak-length = "Speak-Length" ":" positive-length-value
CRLF

positive-length-value = positive-speech-length
/ text-speech-length

load-lexicon = "Load-Lexicon" ":" BOOLEAN CRLF

lexicon-search-order = "Lexicon-Search-Order" ":"
"<" absoluteURI ">" *(" " "<" absoluteURI ">") CRLF

; Recognizer ABNF

recognizer-method = recog-only-method
/ enrollment-method

recog-only-method = "DEFINE-GRAMMAR"
/ "RECOGNIZE"
/ "INTERPRET"
/ "GET-RESULT"
/ "START-INPUT-TIMERS"
/ "STOP"

enrollment-method = "START-PHRASE-ENROLLMENT"
/ "ENROLLMENT-ROLLBACK"
/ "END-PHRASE-ENROLLMENT"
/ "MODIFY-PHRASE"
/ "DELETE-PHRASE"

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

recognizer-header = recog-only-header
/ enrollment-header

recog-only-header = confidence-threshold
/ sensitivity-level
/ speed-vs-accuracy
/ n-best-list-length
/ input-type
/ no-input-timeout
/ recognition-timeout
/ waveform-uri
/ input-waveform-uri
/ completion-cause
/ completion-reason
/ recognizer-context-block

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RFC 6787 MRCPv2 November 2012

/ start-input-timers
/ speech-complete-timeout
/ speech-incomplete-timeout
/ dtmf-interdigit-timeout
/ dtmf-term-timeout
/ dtmf-term-char
/ failed-uri
/ failed-uri-cause
/ save-waveform
/ media-type
/ new-audio-channel
/ speech-language
/ ver-buffer-utterance
/ recognition-mode
/ cancel-if-queue
/ hotword-max-duration
/ hotword-min-duration
/ interpret-text
/ dtmf-buffer-time
/ clear-dtmf-buffer
/ early-no-match

enrollment-header = num-min-consistent-pronunciations
/ consistency-threshold
/ clash-threshold
/ personal-grammar-uri
/ enroll-utterance
/ phrase-id
/ phrase-nl
/ weight
/ save-best-waveform
/ new-phrase-id
/ confusable-phrases-uri
/ abort-phrase-enrollment

confidence-threshold = "Confidence-Threshold" ":"
FLOAT CRLF

sensitivity-level = "Sensitivity-Level" ":" FLOAT
CRLF

speed-vs-accuracy = "Speed-Vs-Accuracy" ":" FLOAT
CRLF

n-best-list-length = "N-Best-List-Length" ":" 1*19DIGIT
CRLF

input-type = "Input-Type" ":" inputs CRLF

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inputs = "speech" / "dtmf"

no-input-timeout = "No-Input-Timeout" ":" 1*19DIGIT
CRLF

recognition-timeout = "Recognition-Timeout" ":" 1*19DIGIT
CRLF

waveform-uri = "Waveform-URI" ":" ["<" uri ">"
";" "size" "=" 1*19DIGIT
";" "duration" "=" 1*19DIGIT] CRLF

recognizer-context-block = "Recognizer-Context-Block" ":"
[1*VCHAR] CRLF

start-input-timers = "Start-Input-Timers" ":"
BOOLEAN CRLF

speech-complete-timeout = "Speech-Complete-Timeout" ":"
1*19DIGIT CRLF

speech-incomplete-timeout = "Speech-Incomplete-Timeout" ":"
1*19DIGIT CRLF

dtmf-interdigit-timeout = "DTMF-Interdigit-Timeout" ":"
1*19DIGIT CRLF

dtmf-term-timeout = "DTMF-Term-Timeout" ":" 1*19DIGIT
CRLF

dtmf-term-char = "DTMF-Term-Char" ":" VCHAR CRLF

save-waveform = "Save-Waveform" ":" BOOLEAN CRLF

new-audio-channel = "New-Audio-Channel" ":"
BOOLEAN CRLF

recognition-mode = "Recognition-Mode" ":"
"normal" / "hotword" CRLF

cancel-if-queue = "Cancel-If-Queue" ":" BOOLEAN CRLF

hotword-max-duration = "Hotword-Max-Duration" ":"
1*19DIGIT CRLF

hotword-min-duration = "Hotword-Min-Duration" ":"
1*19DIGIT CRLF

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interpret-text = "Interpret-Text" ":" 1*VCHAR CRLF

dtmf-buffer-time = "DTMF-Buffer-Time" ":" 1*19DIGIT CRLF

clear-dtmf-buffer = "Clear-DTMF-Buffer" ":" BOOLEAN CRLF

early-no-match = "Early-No-Match" ":" BOOLEAN CRLF

num-min-consistent-pronunciations =
"Num-Min-Consistent-Pronunciations" ":" 1*19DIGIT CRLF

consistency-threshold = "Consistency-Threshold" ":" FLOAT
CRLF

clash-threshold = "Clash-Threshold" ":" FLOAT CRLF

personal-grammar-uri = "Personal-Grammar-URI" ":" uri CRLF

enroll-utterance = "Enroll-Utterance" ":" BOOLEAN CRLF

phrase-id = "Phrase-ID" ":" 1*VCHAR CRLF

phrase-nl = "Phrase-NL" ":" 1*UTFCHAR CRLF

weight = "Weight" ":" FLOAT CRLF

save-best-waveform = "Save-Best-Waveform" ":"
BOOLEAN CRLF

new-phrase-id = "New-Phrase-ID" ":" 1*VCHAR CRLF

confusable-phrases-uri = "Confusable-Phrases-URI" ":"
uri CRLF

abort-phrase-enrollment = "Abort-Phrase-Enrollment" ":"
BOOLEAN CRLF

; Recorder ABNF

recorder-method = "RECORD"
/ "STOP"
/ "START-INPUT-TIMERS"

recorder-event = "START-OF-INPUT"
/ "RECORD-COMPLETE"

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RFC 6787 MRCPv2 November 2012

recorder-header = sensitivity-level
/ no-input-timeout
/ completion-cause
/ completion-reason
/ failed-uri
/ failed-uri-cause
/ record-uri
/ media-type
/ max-time
/ trim-length
/ final-silence
/ capture-on-speech
/ ver-buffer-utterance
/ start-input-timers
/ new-audio-channel

record-uri = "Record-URI" ":" [ "<" uri ">"
";" "size" "=" 1*19DIGIT
";" "duration" "=" 1*19DIGIT] CRLF

media-type = "Media-Type" ":" media-type-value CRLF

max-time = "Max-Time" ":" 1*19DIGIT CRLF

trim-length = "Trim-Length" ":" 1*19DIGIT CRLF

final-silence = "Final-Silence" ":" 1*19DIGIT CRLF

capture-on-speech = "Capture-On-Speech " ":"
BOOLEAN CRLF

; Verifier ABNF

verifier-method = "START-SESSION"
/ "END-SESSION"
/ "QUERY-VOICEPRINT"
/ "DELETE-VOICEPRINT"
/ "VERIFY"
/ "VERIFY-FROM-BUFFER"
/ "VERIFY-ROLLBACK"
/ "STOP"
/ "CLEAR-BUFFER"
/ "START-INPUT-TIMERS"
/ "GET-INTERMEDIATE-RESULT"

verifier-event = "VERIFICATION-COMPLETE"
/ "START-OF-INPUT"

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verifier-header = repository-uri
/ voiceprint-identifier
/ verification-mode
/ adapt-model
/ abort-model
/ min-verification-score
/ num-min-verification-phrases
/ num-max-verification-phrases
/ no-input-timeout
/ save-waveform
/ media-type
/ waveform-uri
/ voiceprint-exists
/ ver-buffer-utterance
/ input-waveform-uri
/ completion-cause
/ completion-reason
/ speech-complete-timeout
/ new-audio-channel
/ abort-verification
/ start-input-timers
/ input-type

repository-uri = "Repository-URI" ":" uri CRLF

voiceprint-identifier = "Voiceprint-Identifier" ":"
vid *[";" vid] CRLF
vid = 1*VCHAR ["." 1*VCHAR]

verification-mode = "Verification-Mode" ":"
verification-mode-string

verification-mode-string = "train" / "verify"

adapt-model = "Adapt-Model" ":" BOOLEAN CRLF

abort-model = "Abort-Model" ":" BOOLEAN CRLF

min-verification-score = "Min-Verification-Score" ":"
[ %x2D ] FLOAT CRLF

num-min-verification-phrases = "Num-Min-Verification-Phrases"
":" 1*19DIGIT CRLF

num-max-verification-phrases = "Num-Max-Verification-Phrases"
":" 1*19DIGIT CRLF

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RFC 6787 MRCPv2 November 2012

voiceprint-exists = "Voiceprint-Exists" ":"
BOOLEAN CRLF

ver-buffer-utterance = "Ver-Buffer-Utterance" ":"
BOOLEAN CRLF

input-waveform-uri = "Input-Waveform-URI" ":" uri CRLF

abort-verification = "Abort-Verification " ":"
BOOLEAN CRLF

The following productions add a new SDP session-level attribute. See
Paragraph 5.

cmid-attribute = "a=cmid:" identification-tag

identification-tag = token

16. XML Schemas

16.1. NLSML Schema Definition

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
targetNamespace="urn:ietf:params:xml:ns:mrcpv2"
xmlns="urn:ietf:params:xml:ns:mrcpv2"
elementFormDefault="qualified"
attributeFormDefault="unqualified" >
<xs:annotation>
<xs:documentation> Natural Language Semantic Markup Schema
</xs:documentation>
</xs:annotation>
<xs:include schemaLocation="enrollment-schema.rng"/>
<xs:include schemaLocation="verification-schema.rng"/>
<xs:element name="result">
<xs:complexType>
<xs:sequence>
<xs:element name="interpretation" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="instance">
<xs:complexType mixed="true">
<xs:sequence minOccurs="0">
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="input" minOccurs="0">

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RFC 6787 MRCPv2 November 2012

<xs:complexType mixed="true">
<xs:choice>
<xs:element name="noinput" minOccurs="0"/>
<xs:element name="nomatch" minOccurs="0"/>
<xs:element name="input" minOccurs="0"/>
</xs:choice>
<xs:attribute name="mode"
type="xs:string"
default="speech"/>
<xs:attribute name="confidence"
type="confidenceinfo"
default="1.0"/>
<xs:attribute name="timestamp-start"
type="xs:string"/>
<xs:attribute name="timestamp-end"
type="xs:string"/>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="confidence" type="confidenceinfo"
default="1.0"/>
<xs:attribute name="grammar" type="xs:anyURI"
use="optional"/>
</xs:complexType>
</xs:element>
<xs:element name="enrollment-result"
type="enrollment-contents"/>
<xs:element name="verification-result"
type="verification-contents"/>
</xs:sequence>
<xs:attribute name="grammar" type="xs:anyURI"
use="optional"/>
</xs:complexType>
</xs:element>

<xs:simpleType name="confidenceinfo">
<xs:restriction base="xs:float">
<xs:minInclusive value="0.0"/>
<xs:maxInclusive value="1.0"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>

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16.2. Enrollment Results Schema Definition

<?xml version="1.0" encoding="UTF-8"?>



<grammar datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes"
ns="urn:ietf:params:xml:ns:mrcpv2"
xmlns="http://relaxng.org/ns/structure/1.0">

<start>
<element name="enrollment-result">
<ref name="enrollment-content"/>
</element>
</start>

<define name="enrollment-content">
<interleave>
<element name="num-clashes">
<data type="nonNegativeInteger"/>
</element>
<element name="num-good-repetitions">
<data type="nonNegativeInteger"/>
</element>
<element name="num-repetitions-still-needed">
<data type="nonNegativeInteger"/>
</element>
<element name="consistency-status">
<choice>
<value>consistent</value>
<value>inconsistent</value>
<value>undecided</value>
</choice>
</element>
<optional>
<element name="clash-phrase-ids">
<oneOrMore>
<element name="item">
<data type="token"/>
</element>
</oneOrMore>
</element>
</optional>
<optional>
<element name="transcriptions">
<oneOrMore>

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RFC 6787 MRCPv2 November 2012

<element name="item">
<text/>
</element>
</oneOrMore>
</element>
</optional>
<optional>
<element name="confusable-phrases">
<oneOrMore>
<element name="item">
<text/>
</element>
</oneOrMore>
</element>
</optional>
</interleave>
</define>

</grammar>

16.3. Verification Results Schema Definition
<?xml version="1.0" encoding="UTF-8"?>



<grammar datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes"
ns="urn:ietf:params:xml:ns:mrcpv2"
xmlns="http://relaxng.org/ns/structure/1.0">

<start>
<element name="verification-result">
<ref name="verification-contents"/>
</element>
</start>

<define name="verification-contents">
<element name="voiceprint">
<ref name="firstVoiceprintContent"/>
</element>
<zeroOrMore>
<element name="voiceprint">
<ref name="restVoiceprintContent"/>
</element>
</zeroOrMore>
</define>

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<define name="firstVoiceprintContent">
<attribute name="id">
<data type="string"/>
</attribute>
<interleave>
<optional>
<element name="adapted">
<data type="boolean"/>
</element>
</optional>
<optional>
<element name="needmoredata">
<ref name="needmoredataContent"/>
</element>
</optional>
<optional>
<element name="incremental">
<ref name="firstCommonContent"/>
</element>
</optional>
<element name="cumulative">
<ref name="firstCommonContent"/>
</element>
</interleave>
</define>

<define name="restVoiceprintContent">
<attribute name="id">
<data type="string"/>
</attribute>
<element name="cumulative">
<ref name="restCommonContent"/>
</element>
</define>

<define name="firstCommonContent">
<interleave>
<element name="decision">
<ref name="decisionContent"/>
</element>
<optional>
<element name="utterance-length">
<ref name="utterance-lengthContent"/>
</element>
</optional>
<optional>
<element name="device">
<ref name="deviceContent"/>

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</element>
</optional>
<optional>
<element name="gender">
<ref name="genderContent"/>
</element>
</optional>
<zeroOrMore>
<element name="verification-score">
<ref name="verification-scoreContent"/>
</element>
</zeroOrMore>
</interleave>
</define>

<define name="restCommonContent">
<interleave>
<optional>
<element name="decision">
<ref name="decisionContent"/>
</element>
</optional>
<optional>
<element name="device">
<ref name="deviceContent"/>
</element>
</optional>
<optional>
<element name="gender">
<ref name="genderContent"/>
</element>
</optional>
<zeroOrMore>
<element name="verification-score">
<ref name="verification-scoreContent"/>
</element>
</zeroOrMore>
</interleave>
</define>

<define name="decisionContent">
<choice>
<value>accepted</value>
<value>rejected</value>
<value>undecided</value>
</choice>
</define>

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RFC 6787 MRCPv2 November 2012

<define name="needmoredataContent">
<data type="boolean"/>
</define>

<define name="utterance-lengthContent">
<data type="nonNegativeInteger"/>
</define>

<define name="deviceContent">
<choice>
<value>cellular-phone</value>
<value>electret-phone</value>
<value>carbon-button-phone</value>
<value>unknown</value>
</choice>
</define>

<define name="genderContent">
<choice>
<value>male</value>
<value>female</value>
<value>unknown</value>
</choice>
</define>

<define name="verification-scoreContent">
<data type="float">
<param name="minInclusive">-1</param>
<param name="maxInclusive">1</param>
</data>
</define>

</grammar>

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RFC 6787 MRCPv2 November 2012

17. References

17.1. Normative References

[ISO.8859-1.1987]
International Organization for Standardization,
"Information technology - 8-bit single byte coded graphic
- character sets - Part 1: Latin alphabet No. 1, JTC1/
SC2", ISO Standard 8859-1, 1987.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.

[RFC2392] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.

[RFC2483] Mealling, M. and R. Daniel, "URI Resolution Services
Necessary for URN Resolution", RFC 2483, January 1999.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

[RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
Types", RFC 3023, January 2001.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.

Burnett & Shanmugham Standards Track [Page 218]

RFC 6787 MRCPv2 November 2012

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.

[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.

[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.

[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.

[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.

[RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.

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RFC 6787 MRCPv2 November 2012

[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.

[RFC5922] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, June 2010.

[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.

[W3C.REC-semantic-interpretation-20070405]
Tichelen, L. and D. Burke, "Semantic Interpretation for
Speech Recognition (SISR) Version 1.0", World Wide Web
Consortium Recommendation REC-semantic-
interpretation-20070405, April 2007,
<http://www.w3.org/TR/2007/
REC-semantic-interpretation-20070405>.

[W3C.REC-speech-grammar-20040316]
McGlashan, S. and A. Hunt, "Speech Recognition Grammar
Specification Version 1.0", World Wide Web Consortium
Recommendation REC-speech-grammar-20040316, March 2004,
<http://www.w3.org/TR/2004/REC-speech-grammar-20040316>.

[W3C.REC-speech-synthesis-20040907]
Walker, M., Burnett, D., and A. Hunt, "Speech Synthesis
Markup Language (SSML) Version 1.0", World Wide Web
Consortium Recommendation REC-speech-synthesis-20040907,
September 2004,
<http://www.w3.org/TR/2004/REC-speech-synthesis-20040907>.

[W3C.REC-xml-names11-20040204]
Layman, A., Bray, T., Hollander, D., and R. Tobin,
"Namespaces in XML 1.1", World Wide Web Consortium First
Edition REC-xml-names11-20040204, February 2004,
<http://www.w3.org/TR/2004/REC-xml-names11-20040204>.

17.2. Informative References

[ISO.8601.1988]
International Organization for Standardization, "Data
elements and interchange formats - Information interchange
- Representation of dates and times", ISO Standard 8601,
June 1988.

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RFC 6787 MRCPv2 November 2012

[Q.23] International Telecommunications Union, "Technical
Features of Push-Button Telephone Sets", ITU-T Q.23, 1993.

[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC4217] Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
October 2005.

[RFC4267] Froumentin, M., "The W3C Speech Interface Framework Media
Types: application/voicexml+xml, application/ssml+xml,
application/srgs, application/srgs+xml, application/
ccxml+xml, and application/pls+xml", RFC 4267,
November 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[RFC4313] Oran, D., "Requirements for Distributed Control of
Automatic Speech Recognition (ASR), Speaker
Identification/Speaker Verification (SI/SV), and Text-to-
Speech (TTS) Resources", RFC 4313, December 2005.

[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35,
RFC 4395, February 2006.

[RFC4463] Shanmugham, S., Monaco, P., and B. Eberman, "A Media
Resource Control Protocol (MRCP) Developed by Cisco,
Nuance, and Speechworks", RFC 4463, April 2006.

[RFC4467] Crispin, M., "Internet Message Access Protocol (IMAP) -
URLAUTH Extension", RFC 4467, May 2006.

[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
Digits, Telephony Tones, and Telephony Signals", RFC 4733,
December 2006.

[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.

[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.

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RFC 6787 MRCPv2 November 2012

[W3C.REC-emma-20090210]
Johnston, M., Baggia, P., Burnett, D., Carter, J., Dahl,
D., McCobb, G., and D. Raggett, "EMMA: Extensible
MultiModal Annotation markup language", World Wide Web
Consortium Recommendation REC-emma-20090210,
February 2009,
<http://www.w3.org/TR/2009/REC-emma-20090210>.

[W3C.REC-pronunciation-lexicon-20081014]
Baggia, P., Bagshaw, P., Burnett, D., Carter, J., and F.
Scahill, "Pronunciation Lexicon Specification (PLS)",
World Wide Web Consortium Recommendation
REC-pronunciation-lexicon-20081014, October 2008,
<http://www.w3.org/TR/2008/
REC-pronunciation-lexicon-20081014>.

[W3C.REC-voicexml20-20040316]
Danielsen, P., Porter, B., Hunt, A., Rehor, K., Lucas, B.,
Burnett, D., Ferrans, J., Tryphonas, S., McGlashan, S.,
and J. Carter, "Voice Extensible Markup Language
(VoiceXML) Version 2.0", World Wide Web Consortium
Recommendation REC-voicexml20-20040316, March 2004,
<http://www.w3.org/TR/2004/REC-voicexml20-20040316>.

[refs.javaSpeechGrammarFormat]
Sun Microsystems, "Java Speech Grammar Format Version
1.0", October 1998.

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Appendix A. Contributors

Pierre Forgues
Nuance Communications Ltd.
1500 University Street
Suite 935
Montreal, Quebec
Canada H3A 3S7

EMail: [email protected]

Charles Galles
Intervoice, Inc.
17811 Waterview Parkway
Dallas, Texas 75252
USA

EMail: [email protected]

Klaus Reifenrath
Scansoft, Inc
Guldensporenpark 32
Building D
9820 Merelbeke
Belgium

EMail: [email protected]

Appendix B. Acknowledgements

Andre Gillet (Nuance Communications)
Andrew Hunt (ScanSoft)
Andrew Wahbe (Genesys)
Aaron Kneiss (ScanSoft)
Brian Eberman (ScanSoft)
Corey Stohs (Cisco Systems, Inc.)
Dave Burke (VoxPilot)
Jeff Kusnitz (IBM Corp)
Ganesh N. Ramaswamy (IBM Corp)
Klaus Reifenrath (ScanSoft)
Kristian Finlator (ScanSoft)
Magnus Westerlund (Ericsson)
Martin Dragomirecky (Cisco Systems, Inc.)
Paolo Baggia (Loquendo)
Peter Monaco (Nuance Communications)
Pierre Forgues (Nuance Communications)

Burnett & Shanmugham Standards Track [Page 223]

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Ran Zilca (IBM Corp)
Suresh Kaliannan (Cisco Systems, Inc.)
Skip Cave (Intervoice, Inc.)
Thomas Gal (LumenVox)

The chairs of the SPEECHSC work group are Eric Burger (Georgetown
University) and Dave Oran (Cisco Systems, Inc.).

Many thanks go in particular to Robert Sparks, Alex Agranovsky, and
Henry Phan, who were there at the end to dot all the i's and cross
all the t's.

Authors' Addresses

Daniel C. Burnett
Voxeo
189 South Orange Avenue #1000
Orlando, FL 32801
USA

EMail: [email protected]

Saravanan Shanmugham
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA

EMail: [email protected]

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