paragraphs, sentences, and markers.
o The ability to increase and decrease playout speed.
o The ability to fast-forward and fast-rewind the audio, where
snippets of audio are played as the server moves forwards or
backwards in time.
o The ability to pause and resume playout.
o The ability to increase and decrease playout volume.
These controls SHOULD be made easily available to users through the
client user interface and through per-user customization capabilities
of the client. This is particularly important for hearing-impaired
users, who will likely desire settings and control regimes different
from those that would be acceptable for non-impaired users.
4.6. Session Parameters
The SPEECHSC framework MUST support the specification of session
parameters, such as language, prosody, and voicing.
4.7. Speech Markers
The SPEECHSC framework MUST accommodate speech markers, with
capability at least as flexible as that provided in SSML [1]. The
framework MUST further provide an efficient mechanism for reporting
that a marker has been reached during playout.
5. ASR Requirements
5.1. Requesting Automatic Speech Recognition
The SPEECHSC framework MUST allow a Media Processing Entity or
Application Server to request the ASR Server to perform automatic
speech recognition on an RTP stream, returning the results over
SPEECHSC.
5.2. XML
The SPEECHSC framework assumes that all ASR servers support the
VoiceXML speech recognition grammar specification (SRGS) for speech
recognition [2].
5.3. Grammar Requirements
5.3.1. Grammar Specification
The SPEECHSC framework assumes all ASR servers are capable of
accepting grammar specifications either "by value" (embedded in the
protocol) or "by reference" (e.g., by de-referencing a URI embedded
in the protocol). The latter MUST allow the indication of a grammar
already known to, or otherwise "built in" to, the server. The
framework and protocol further SHOULD exploit the ability to store
and later retrieve by reference large grammars that were originally
supplied by the client.
5.3.2. Explicit Indication of Grammar Format
The SPEECHSC framework protocol MUST be able to explicitly convey the
grammar format in which the grammar is encoded and MUST be extensible
to allow for conveying new grammar formats as they are defined.
5.3.3. Grammar Sharing
The SPEECHSC framework SHOULD exploit sharing grammars across
sessions for servers that are capable of doing so. This supports
applications with large grammars for which it is unrealistic to
dynamically load. An example is a city-country grammar for a weather
service.
5.4. Session Parameters
The SPEECHSC framework MUST accommodate at a minimum all of the
protocol parameters currently defined in Media Resource Control
Protocol (MRCP) [10] In addition, there SHOULD be a capability to
reset parameters within a session.
5.5. Input Capture
The SPEECHSC framework MUST support a method directing the ASR Server
to capture the input media stream for later analysis and tuning of
the ASR engine.
6. Speaker Identification and Verification Requirements
6.1. Requesting SI/SV
The SPEECHSC framework MUST allow a Media Processing Entity to
request the SI/SV Server to perform speaker identification or
verification on an RTP stream, returning the results over SPEECHSC.
6.2. Identifiers for SI/SV
The SPEECHSC framework MUST accommodate an identifier for each
verification resource and permit control of that resource by ID,
because voiceprint format and contents are vendor specific.
6.3. State for Multiple Utterances
The SPEECHSC framework MUST work with SI/SV servers that maintain
state to handle multi-utterance verification.
6.4. Input Capture
The SPEECHSC framework MUST support a method for capturing the input
media stream for later analysis and tuning of the SI/SV engine. The
framework may assume all servers are capable of doing so. In
addition, the framework assumes that the captured stream contains
enough timestamp context (e.g., the NTP time range from the RTP
Control Protocol (RTCP) packets, which corresponds to the RTP
timestamps of the captured input) to ascertain after the fact exactly
when the verification was requested.
6.5. SI/SV Functional Extensibility
The SPEECHSC framework SHOULD be extensible to additional functions
associated with SI/SV, such as prompting, utterance verification, and
retraining.
7. Duplexing and Parallel Operation Requirements
One very important requirement for an interactive speech-driven
system is that user perception of the quality of the interaction
depends strongly on the ability of the user to interrupt a prompt or
rendered TTS with speech. Interrupting, or barging, the speech
output requires more than energy detection from the user’s direction.
Many advanced systems halt the media towards the user by employing
the ASR engine to decide if an utterance is likely to be real speech,
as opposed to a cough, for example.
7.1. Full Duplex Operation
To achieve low latency between utterance detection and halting of
playback, many implementations combine the speaking and ASR
functions. The SPEECHSC framework MUST support such full-duplex
implementations.
7.2. Multiple Services in Parallel
Good spoken user interfaces typically depend upon the ease with which
the user can accomplish his or her task. When making use of speaker
identification or verification technologies, user interface
improvements often come from the combination of the different
technologies: simultaneous identity claim and verification (on the
same utterance), simultaneous knowledge and voice verification (using
ASR and verification simultaneously). Using ASR and verification on
the same utterance is in fact the only way to support rolling or
dynamically-generated challenge phrases (e.g., "say 51723"). The
SPEECHSC framework MUST support such parallel service
implementations.
7.3. Combination of Services
It is optionally of interest that the SPEECHSC framework support more
complex remote combination and controls of speech engines:
o Combination in series of engines that may then act on the input or
output of ASR, TTS, or Speaker recognition engines. The control
MAY then extend beyond such engines to include other audio input
and output processing and natural language processing.
o Intermediate exchanges and coordination between engines.
o Remote specification of flows between engines.
These capabilities MAY benefit from service discovery mechanisms
(e.g., engines, properties, and states discovery).
8. Additional Considerations (Non-Normative)
The framework assumes that Session Description Protocol (SDP) will be
used to describe media sessions and streams. The framework further
assumes RTP carriage of media. However, since SDP can be used to
describe other media transport schemes (e.g., ATM) these could be
used if they provide the necessary elements (e.g., explicit
timestamps).
The working group will not be defining distributed speech recognition
(DSR) methods, as exemplified by the European Telecommunications
Standards Institute (ETSI) Aurora project. The working group will
not be recreating functionality available in other protocols, such as
SIP or SDP.
TTS looks very much like playing back a file. Extending RTSP looks
promising for when one requires VCR controls or markers in the text
to be spoken. When one does not require VCR controls, SIP in a
framework such as Network Announcements [12] works directly without
modification.
ASR has an entirely different set of characteristics. For barge-in
support, ASR requires real-time return of intermediate results.
Barring the discovery of a good reuse model for an existing protocol,
this will most likely become the focus of SPEECHSC.
9. Security Considerations
Protocols relating to speech processing must take security and
privacy into account. Many applications of speech technology deal
with sensitive information, such as the use of Text-to-Speech to read
financial information. Likewise, popular uses for automatic speech
recognition include executing financial transactions and shopping.
There are at least three aspects of speech processing security that
intersect with the SPEECHSC requirements -- securing the SPEECHSC
protocol itself, implementing and deploying the servers that run the
protocol, and ensuring that utilization of the technology for
providing security functions is appropriate. Each of these aspects
in discussed in the following subsections. While some of these
considerations are, strictly speaking, out of scope of the protocol
itself, they will be carefully considered and accommodated during
protocol design, and will be called out as part of the applicability
statement accompanying the protocol specification(s). Privacy
considerations are discussed as well.
9.1. SPEECHSC Protocol Security
The SPEECHSC protocol MUST in all cases support authentication,
authorization, and integrity, and SHOULD support confidentiality.
For privacy-sensitive applications, the protocol MUST support
confidentiality. We envision that rather than providing
protocol-specific security mechanisms in SPEECHSC itself, the
resulting protocol will employ security machinery of either a
containing protocol or the transport on which it runs. For example,
we will consider solutions such as using Transport Layer Security
(TLS) for securing the control channel, and Secure Realtime Transport
Protocol (SRTP) for securing the media channel. Third-party
dependencies necessitating transitive trust will be minimized or
explicitly dealt with through the authentication and authorization
aspects of the protocol design.
9.2. Client and Server Implementation and Deployment
Given the possibly sensitive nature of the information carried,
SPEECHSC clients and servers need to take steps to ensure
confidentiality and integrity of the data and its transformations to
and from spoken form. In addition to these general considerations,
certain SPEECHSC functions, such as speaker verification and
identification, employ voiceprints whose privacy, confidentiality,
and integrity must be maintained. Similarly, the requirement to
support input capture for analysis and tuning can represent a privacy
vulnerability because user utterances are recorded and could be
either revealed or replayed inappropriately. Implementers must take
care to prevent the exploitation of any centralized voiceprint
database and the recorded material from which such voiceprints may be
derived. Specific actions that are recommended to minimize these
threats include:
o End-to-end authentication, confidentiality, and integrity
protection (like TLS) of access to the database to minimize the
exposure to external attack.
o Database protection measures such as read/write access control and
local login authentication to minimize the exposure to insider
threats.
o Copies of the database, especially ones that are maintained at
off-site locations, need the same protection as the operational
database.
Inappropriate disclosure of this data does not as of the date of this
document represent an exploitable threat, but quite possibly might in
the future. Specific vulnerabilities that might become feasible are
discussed in the next subsection. It is prudent to take measures
such as encrypting the voiceprint database and permitting access only
through programming interfaces enforcing adequate authorization
machinery.
9.3. Use of SPEECHSC for Security Functions
Either speaker identification or verification can be used directly as
an authentication technology. Authorization decisions can be coupled
with speaker verification in a direct fashion through
challenge-response protocols, or indirectly with speaker
identification through the use of access control lists or other
identity-based authorization mechanisms. When so employed, there are
additional security concerns that need to be addressed through the
use of protocol security mechanisms for clients and servers. For
example, the ability to manipulate the media stream of a speaker
verification request could inappropriately permit or deny access
based on impersonation, or simple garbling via noise injection,
making it critical to properly secure both the control and data
channels, as recommended above. The following issues specific to the
use of SI/SV for authentication should be carefully considered:
1. Theft of voiceprints or the recorded samples used to construct
them represents a future threat against the use of speaker
identification/verification as a biometric authentication
technology. A plausible attack vector (not feasible today) is to
use the voiceprint information as parametric input to a
text-to-speech synthesis system that could mimic the user’s voice
accurately enough to match the voiceprint. Since it is not very
difficult to surreptitiously record reasonably large corpuses of
voice samples, the ability to construct voiceprints for input to
this attack would render the security of voice-based biometric
authentication, even using advanced challenge-response
techniques, highly vulnerable. Users of speaker verification for
authentication should monitor technological developments in this
area closely for such future vulnerabilities (much as users of
other authentication technologies should monitor advances in
factoring as a way to break asymmetric keying systems).
2. As with other biometric authentication technologies, a downside
to the use of speech identification is that revocation is not
possible. Once compromised, the biometric information can be
used in identification and authentication to other independent
systems.
3. Enrollment procedures can be vulnerable to impersonation if not
protected both by protocol security mechanisms and some
independent proof of identity. (Proof of identity may not be
needed in systems that only need to verify continuity of identity
since enrollment, as opposed to association with a particular
individual.
Further discussion of the use of SI/SV as an authentication
technology, and some recommendations concerning advantages and
vulnerabilities, can be found in Chapter 5 of [15].
10. Acknowledgements
Eric Burger wrote the original version of these requirements and has
continued to contribute actively throughout their development. He is
a co-author in all but formal authorship, and is instead acknowledged
here as it is preferable that working group co-chairs have
non-conflicting roles with respect to the progression of documents.
11. References
11.1. Normative References
[1] Walker, M., Burnett, D., and A. Hunt, "Speech Synthesis Markup
Language (SSML) Version 1.0", W3C
REC REC-speech-synthesis-20040907, September 2004.
[2] McGlashan, S. and A. Hunt, "Speech Recognition Grammar
Specification Version 1.0", W3C REC REC-speech-grammar-20040316,
March 2004.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[5] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van
Wijk, "User Requirements for the Session Initiation Protocol
(SIP) in Support of Deaf, Hard of Hearing and Speech-impaired
Individuals", RFC 3351, August 2002.
11.2. Informative References
[6] 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.
[7] Andreasen, F. and B. Foster, "Media Gateway Control Protocol
(MGCP) Version 1.0", RFC 3435, January 2003.
[8] Groves, C., Pantaleo, M., Ericsson, LM., Anderson, T., and T.
Taylor, "Gateway Control Protocol Version 1", RFC 3525,
June 2003.
[9] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[10] Shanmugham, S., Monaco, P., and B. Eberman, "MRCP: Media
Resource Control Protocol", Work in Progress.
[11] World Wide Web Consortium, "Voice Extensible Markup Language
(VoiceXML) Version 2.0", W3C Working Draft , April 2002,
<http://www.w3.org/TR/2002/WD-voicexml20-20020424/>.
[12] Burger, E., Ed., Van Dyke, J., and A. Spitzer, "Basic Network
Media Services with SIP", RFC 4240, December 2005.
[13] Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
Location Protocol, Version 2", RFC 2608, June 1999.
[14] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[15] Committee on Authentication Technologies and Their Privacy
Implications, National Research Council, "Who Goes There?:
Authentication Through the Lens of Privacy", Computer Science
and Telecommunications Board (CSTB) , 2003,
<http://www.nap.edu/catalog/10656.html/ >.
Author’s Address
David R. Oran
Cisco Systems, Inc.
7 Ladyslipper Lane
Acton, MA
USA
EMail: oran@cisco.com
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.