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Keywords: audio codec, speech codec
Internet Engineering Task Force (IETF) JM. Valin
Request for Comments: 6569 Mozilla
Category: Informational S. Borilin
ISSN: 2070-1721 SPIRIT DSP
K. Vos
C. Montgomery
Xiph.Org Foundation
R. Chen
Broadcom Corporation
March 2012
Guidelines for Development of an Audio Codec within the IETF
Abstract
This document provides general guidelines for work on developing and
specifying an interactive audio codec within the IETF. These
guidelines cover the development process, evaluation, requirements
conformance, and intellectual property issues.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc6569.
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
carefully, as they describe your rights and restrictions with respect
Valin, et al. Informational [Page 1]
RFC 6569 Codec Guidelines March 2012
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
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Development Process .............................................2
3. Evaluation, Testing, and Characterization .......................5
4. Specifying the Codec ............................................6
5. Intellectual Property ...........................................8
6. Relationship with Other SDOs ...................................10
7. Security Considerations ........................................12
8. Acknowledgments ................................................12
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................12
1. Introduction
This document describes a process for work in the IETF codec WG on
standardization of an audio codec that is optimized for use in
interactive Internet applications and that can be widely implemented
and easily distributed among application developers, service
operators, and end users.
2. Development Process
The process outlined here is intended to make the work on a codec
within the IETF transparent, predictable, and well organized, in a
way that is consistent with [PROCESS]. Such work might involve
development of a completely new codec, adaptation of an existing
codec to meet the requirements of the working group, or integration
of two or more existing codecs that results in an improved codec
combining the best aspects of each. To enable such procedural
transparency, the contributor of an existing codec must be willing to
cede change control to the IETF and should have sufficient knowledge
of the codec to assist in the work of adapting it or applying some of
its technology to the development or improvement of other codecs.
Furthermore, contributors need to be aware that any codec that
results from work within the IETF is likely to be different from any
existing codec that was contributed to the Internet Standards
Process.
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Work on developing an interactive audio codec is expected to proceed
as follows:
1. IETF participants will identify the requirements to be met by an
Internet codec in the form of an Internet-Draft.
2. Interested parties are encouraged to make contributions proposing
existing or new codecs, or elements thereof, to the codec WG as
long as these contributions are within the scope of the WG.
Ideally, these contributions should be in the form of Internet-
Drafts, although other forms of contributions are also possible,
as discussed in [PROCESS].
3. Given the importance of intellectual property rights (IPR) to the
activities of the working group, any IPR disclosures must be made
in a timely way. Contributors are required, as described in
[IPR], to disclose any known IPR, both first and third party.
Timely disclosures are particularly important, since those
disclosures may be material to the decision process of the
working group.
4. As contributions are received and discussed within the working
group, the group should gain a clearer understanding of what is
achievable within the design space. As a result, the authors of
the requirements document should iteratively clarify and improve
their document to reflect the emerging working group consensus.
This is likely to involve collaboration with IETF working groups
in other areas, such as collaboration with working groups in the
Transport area to identify important aspects of packet
transmission over the Internet and to understand the degree of
rate adaptation desirable and with working groups in the RAI area
to ensure that information about and negotiation of the codec can
be easily represented at the signaling layer. In parallel with
this work, interested parties should evaluate the contributions
at a higher level to see which requirements might be met by each
codec.
5. Once a sufficient number of proposals has been received, the
interested parties will identify the strengths, weaknesses, and
innovative aspects of the contributed codecs. This step will
consider not only the codecs as a whole, but also key features of
the individual algorithms (predictors, quantizers, transforms,
etc.).
6. Interested parties are encouraged to collaborate and combine the
best ideas from the various codec contributions into a
consolidated codec definition, representing the merging of some
of the contributions. Through this iterative process, the number
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of proposals will reduce, and consensus will generally form
around one of them. At that point, the working group should
adopt that document as a working group item, forming a baseline
codec.
7. IETF participants should then attempt to iteratively add to or
improve each component of the baseline codec reference
implementation, where by "component" we mean individual
algorithms such as predictors, transforms, quantizers, and
entropy coders. The participants should proceed by trying new
designs, applying ideas from the contributed codecs, evaluating
"proof of concept" ideas, and using their expertise in codec
development to improve the baseline codec. Any aspect of the
baseline codec might be changed (even the fundamental principles
of the codec), or the participants might start over entirely by
scrapping the baseline codec and designing a completely new one.
The overriding goal shall be to design a codec that will meet the
requirements defined in the requirements document [CODEC-REQ].
Given the IETF's open standards process, any interested party
will be able to contribute to this work, whether or not they
submitted an Internet-Draft for one of the contributed codecs.
The codec itself should be normatively specified with code in an
Internet-Draft.
8. In parallel with work on the codec reference implementation,
developers and other interested parties should perform evaluation
of the codec as described under Section 3. IETF participants
should define (within the PAYLOAD working group) the codec's
payload format for use with the Real-time Transport Protocol
[RTP]. Ideally, application developers should test the codec by
implementing it in code and deploying it in actual Internet
applications. Unfortunately, developers will frequently wait to
deploy the codec until it is published as an RFC or until a
stable bitstream is guaranteed. As such, this is a nice-to-have
and not a requirement for this process. Lab implementations are
certainly encouraged.
9. The group will produce a testing results document. The document
will be a living document that captures testing done before the
codec stabilized, after it has stabilized, and after the codec
specification is issued as an RFC. The document serves the
purpose of helping the group determine whether the codec meets
the requirements. Any testing done after the codec RFC is issued
helps implementers understand the final performance of the codec.
The process of testing is described in Section 3.
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3. Evaluation, Testing, and Characterization
Lab evaluation of the codec being developed should happen throughout
the development process because it will help ensure that progress is
being made toward fulfillment of the requirements. There are many
ways in which continuous evaluation can be performed. For minor,
uncontroversial changes to the codec, it should usually be sufficient
to use objective measurements (e.g., Perceptual Evaluation of Speech
Quality (PESQ) [ITU-T-P.862], Perceptual Evaluation of Audio Quality
(PEAQ) [ITU-R-BS.1387-1], and segmental signal-to-noise ratio)
validated by informal subjective evaluation. For more complex
changes (e.g., when psychoacoustic aspects are involved) or for
controversial issues, internal testing should be performed. An
example of internal testing would be to have individual participants
rate the decoded samples using one of the established testing
methodologies, such as MUltiple Stimuli with Hidden Reference and
Anchor (MUSHRA) [ITU-R-BS.1534].
Throughout the process, it will be important to make use of the
Internet community at large for real-world distributed testing. This
will enable many different people with different equipment and use
cases to test the codec and report any problems they experience. In
the same way, third-party developers will be encouraged to integrate
the codec into their software (with a warning about the bitstream not
being final) and provide feedback on its performance in real-world
use cases.
Characterization of the final codec must be based on the reference
implementation only (and not on any "private implementation"). This
can be performed by independent testing labs or, if this is not
possible, by testing labs of the organizations that contribute to the
Internet Standards Process. Packet-loss robustness should be
evaluated using actual loss patterns collected from use over the
Internet, rather than theoretical models. The goals of the
characterization phase are to:
o ensure that the requirements have been fulfilled
o guide the IESG in its evaluation of the resulting work
o assist application developers in understanding whether the codec
is suitable for a particular application
The exact methodology for the characterization phase can be
determined by the working group. Because the IETF does not have
testing resources of its own, it has to rely on the resources of its
participants. For this reason, even if the group agrees that a
particular test is important, if no one volunteers to do it, or if
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volunteers do not complete it in a timely fashion, then that test
should be discarded. This ensures that only important tests be done
-- in particular, the tests that are important to participants.
4. Specifying the Codec
Specifying a codec requires careful consideration regarding what is
required versus what is left to the implementation. The following
text provides guidelines for consideration by the working group:
1. Any audio codec specified by the codec working group must include
source code for a normative software implementation, documented
in an Internet-Draft intended for publication as a Standards
Track RFC. This implementation will be used to verify
conformance of an implementation. Although a text description of
the algorithm should be provided, its use should be limited to
helping the reader in understanding the source code. Should the
description contradict the source code, the latter shall take
precedence. For convenience, the source code may be provided in
compressed form, with base64 [BASE64] encoding.
2. Because of the size and complexity of most codecs, it is possible
that even after publishing the RFC, bugs will be found in the
reference implementation, or differences will be found between
the implementation and the text description. As usual, an errata
list should be maintained for the RFC. Although a public
software repository containing the current reference
implementation is desirable, the normative implementation would
still be the RFC.
3. It is the intention of the group to allow the greatest possible
choice of freedom in implementing the specification.
Accordingly, the number of binding RFC 2119 [KEYWORDS] keywords
will be the minimum that still allows interoperable
implementations. In practice, this generally means that only the
decoder needs to be normative, so that the encoder can improve
over time. This also enables different trade-offs between
quality and complexity.
4. To reduce the risk of bias towards certain CPU/DSP (central
processing unit / digital signal processor) architectures,
ideally the decoder specification should not require "bit-exact"
conformance with the reference implementation. In that case, the
output of a decoder implementation should only be "close enough"
to the output of the reference decoder, and a comparison tool
should be provided along with the codec to verify objectively
that the output of a decoder is likely to be perceptually
indistinguishable from that of the reference decoder. An
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implementation may still wish to produce an output that is bit-
exact with the reference implementation to simplify the testing
procedure.
5. To ensure freedom of implementation, decoder-side-only error
concealment does not need to be specified, although the reference
implementation should include the same packet-loss concealment
(PLC) algorithm as used in the testing phase. Is it up to the
working group to decide whether minimum requirements on PLC
quality will be required for compliance with the specification.
Obviously, any information signaled in the bitstream intended to
aid PLC needs to be specified.
6. An encoder implementation should not be required to make use of
all the "features" (tools) in the bitstream definition. However,
the codec specification may require that an encoder
implementation be able to generate any possible bitrate. Unless
a particular "profile" is defined in the specification, the
decoder must be able to decode all features of the bitstream.
The decoder must also be able to handle any combination of bits,
even combinations that cannot be generated by the reference
encoder. It is recommended that the decoder specification shall
define how the decoder should react to "impossible" packets
(e.g., reject or consider as valid). However, an encoder must
never generate packets that do not conform to the bitstream
definition.
7. Compressed test vectors should be provided as a means to verify
conformance with the decoder specification. These test vectors
should be designed to exercise as much of the decoder code as
possible.
8. While the exact encoder will not be specified, it is recommended
to specify objective measurement targets for an encoder, below
which use of a particular encoder implementation is not
recommended. For example, one such specification could be: "the
use of an encoder whose PESQ mean opinion score (MOS) is better
than 0.1 below the reference encoder in the following conditions
is not recommended".
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5. Intellectual Property
Producing an unencumbered codec is desirable for the following
reasons:
o It is the experience of a wide variety of application developers
and service providers that encumbrances such as licensing and
royalties make it difficult to implement, deploy, and distribute
multimedia applications for use by the Internet community.
o It is beneficial to have low-cost options whenever possible,
because innovation -- the hallmark of the Internet -- is hampered
when small development teams cannot deploy an application because
of usage-based licensing fees and royalties.
o Many market segments are moving away from selling hard-coded
hardware devices and toward freely distributing end-user software;
this is true of numerous large application providers and even
telcos themselves.
o Compatibility with the licensing of typical open source
applications implies the need to avoid encumbrances, including
even the requirement to obtain a license for implementation,
deployment, or use (even if the license does not require the
payment of a fee).
Therefore, a codec that can be widely implemented and easily
distributed among application developers, service operators, and end
users is preferred. Many existing codecs that might fulfill some or
most of the technical attributes listed above are encumbered in
various ways. For example, patent holders might require that those
wishing to implement the codec in software, deploy the codec in a
service, or distribute the codec in software or hardware need to
request a license, enter into a business agreement, pay licensing
fees or royalties, or adhere to other special conditions or
restrictions. Because such encumbrances have made it difficult to
widely implement and easily distribute high-quality codecs across the
entire Internet community, the working group prefers unencumbered
technologies in a way that is consistent with BCP 78 [IPR] and BCP 79
[TRUST]. In particular, the working group shall heed the preference
stated in BCP 79: "In general, IETF working groups prefer
technologies with no known IPR claims or, for technologies with
claims against them, an offer of royalty-free licensing." Although
this preference cannot guarantee that the working group will produce
an unencumbered codec, the working group shall follow and adhere to
the spirit of BCP 79. The working group cannot explicitly rule out
the possibility of adopting encumbered technologies; however, the
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working group will try to avoid encumbered technologies that require
royalties or other encumbrances that would prevent such technologies
from being easy to redistribute and use.
When considering license terms for technologies with IPR claims
against them, some members of the working group have expressed their
preference for license terms that:
o are available to all, worldwide, whether or not they are working
group participants
o extend to all essential claims owned or controlled by the licensor
o do not require payment of royalties, fees, or other consideration
o do not require licensees to adhere to restrictions on usage
(though, licenses that apply only to implementation of the
standard are acceptable)
o do not otherwise impede the ability of the codec to be implemented
in open-source software projects
The following guidelines will help to maximize the odds that the
codec will be unencumbered:
1. In accordance with BCP 79 [IPR], contributed codecs should
preferably use technologies with no known IPR claims or
technologies with an offer of royalty-free licensing.
2. As described in BCP 79, the working group should use technologies
that are perceived by the participants to be safer with regard to
IPR issues.
3. Contributors must disclose IPR as specified in BCP 79.
4. In cases where no royalty-free license can be obtained regarding
a patent, BCP 79 suggests that the working group consider
alternative algorithms or methods, even if they result in lower
quality, higher complexity, or otherwise less desirable
characteristics.
5. In accordance with BCP 78 [TRUST], the source code for the
reference implementation must be made available under a BSD-style
license (or whatever license is defined as acceptable by the IETF
Trust when the Internet-Draft defining the reference
implementation is published).
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Many IPR licenses specify that a license is granted only for
technologies that are adopted by the IETF as a standard. While
reasonable, this has the unintended side effect of discouraging
implementation prior to RFC status. Real-world implementation is
beneficial for evaluation of the codec. As such, entities making IPR
license statements are encouraged to use wording that permits early
implementation and deployment.
IETF participants should be aware that, given the way patents work in
most countries, the resulting codec can never be guaranteed to be
free of patent claims because some patents may not be known to the
contributors, some patent applications may not be disclosed at the
time the codec is developed, and only courts of law can determine the
validity and breadth of patent claims. However, these observations
are no different within the Internet Standards Process than they are
for standardization of codecs within other Standards Development
Organizations (SDOs) (or development of codecs outside the context of
any SDO); furthermore, they are no different for codecs than for
other technologies worked on within the IETF. In all these cases,
the best approach is to minimize the risk of unknowingly incurring
encumbrance on existing patents. Despite these precautions,
participants need to understand that, practically speaking, it is
nearly impossible to _guarantee_ that implementers will not incur
encumbrance on existing patents.
6. Relationship with Other SDOs
It is understood that other SDOs are also involved in the codec
development and standardization, including but not necessarily
limited to:
o The Telecommunication Standardization Sector (ITU-T) of the
International Telecommunication Union (ITU), in particular Study
Group 16
o The Moving Picture Experts Group (MPEG) of the International
Organization for Standardization and International
Electrotechnical Commission (ISO/IEC)
o The European Telecommunications Standards Institute (ETSI)
o The 3rd Generation Partnership Project (3GPP)
o The 3rd Generation Partnership Project 2 (3GPP2)
It is important to ensure that such work does not constitute
uncoordinated protocol development of the kind described in [UNCOORD]
in the following principle:
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[T]he IAB considers it an essential principle of the protocol
development process that only one SDO maintains design authority
for a given protocol, with that SDO having ultimate authority over
the allocation of protocol parameter code-points and over defining
the intended semantics, interpretation, and actions associated
with those code-points.
The work envisioned by this guidelines document is not uncoordinated
in the sense described in the foregoing quote, since the intention of
this process is that two possible outcomes might occur:
1. The IETF adopts an existing audio codec and specifies that it is
the "anointed" IETF Internet codec. In such a case, codec
ownership lies entirely with the SDO that produced the codec, and
not with the IETF.
2. The IETF produces a new codec. Even if this codec uses concepts,
algorithms, or even source code from a codec produced by another
SDO, the IETF codec is a specification unto itself and under
complete control of the IETF. Any changes or enhancements made
by the original SDO to the codecs whose components the IETF used
are not applicable to the IETF codec. Such changes would be
incorporated as a consequence of a revision or extension of the
IETF RFC. In no case should the new codec reuse a name or code
point from another SDO.
Although there is already sufficient codec expertise available among
IETF participants to complete the envisioned work, additional
contributions are welcome within the framework of the Internet
Standards Process in the following ways:
o Individuals who are technical contributors to codec work within
other SDOs can participate directly in codec work within the IETF.
o Other SDOs can contribute their expertise (e.g., codec
characterization and evaluation techniques) and thus facilitate
the testing of a codec produced by the IETF.
o Any SDO can provide input to IETF work through liaison statements.
However, it is important to note that final responsibility for the
development process and the resulting codec will remain with the IETF
as governed by BCP 9 [PROCESS].
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Finally, there is precedent for the contribution of codecs developed
elsewhere to the ITU-T (e.g., Adaptive Multi-Rate Wideband (AMR-WB)
was standardized originally within 3GPP). This is a model to explore
as the IETF coordinates further with the ITU-T in accordance with the
collaboration guidelines defined in [COLLAB].
7. Security Considerations
The procedural guidelines for codec development do not have security
considerations. However, the resulting codec needs to take
appropriate security considerations into account, as outlined in
[SECGUIDE] and in the security considerations of [CODEC-REQ]. More
specifically, the resulting codec must avoid being subject to denial
of service [DOS] and buffer overflows, and should take into
consideration the impact of variable bitrate (VBR) [SRTP-VBR].
8. Acknowledgments
We would like to thank all the other people who contributed directly
or indirectly to this document, including Jason Fischl, Gregory
Maxwell, Alan Duric, Jonathan Christensen, Julian Spittka, Michael
Knappe, Timothy B. Terriberry, Christian Hoene, Stephan Wenger, and
Henry Sinnreich. We would also like to thank Cullen Jennings and
Gregory Lebovitz for their advice. Special thanks to Peter Saint-
Andre, who originally co-authored this document.
9. References
9.1. Normative References
[IPR] Bradner, S., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3979, March 2005.
[PROCESS] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[TRUST] Bradner, S. and J. Contreras, "Rights Contributors
Provide to the IETF Trust", BCP 78, RFC 5378,
November 2008.
9.2. Informative References
[BASE64] Josefsson, S., "The Base16, Base32, and Base64
Data Encodings", RFC 4648, October 2006.
[CODEC-REQ] Valin, J. and K. Vos, "Requirements for an
Internet Audio Codec", RFC 6366, August 2011.
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RFC 6569 Codec Guidelines March 2012
[COLLAB] Fishman, G. and S. Bradner, "Internet Engineering
Task Force and International Telecommunication
Union - Telecommunications Standardization Sector
Collaboration Guidelines", RFC 3356, August 2002.
[DOS] Handley, M., Rescorla, E., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
December 2006.
[ITU-R-BS.1387-1] "Method for objective measurements of perceived
audio quality", ITU-R Recommendation BS.1387-1,
November 2001.
[ITU-R-BS.1534] "Method for the subjective assessment of
intermediate quality levels of coding systems",
ITU-R Recommendation BS.1534, January 2003.
[ITU-T-P.862] "Perceptual evaluation of speech quality (PESQ):
An objective method for end-to-end speech quality
assessment of narrow-band telephone networks and
speech codecs", ITU-T Recommendation P.862,
October 2007.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RTP] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[SECGUIDE] Rescorla, E. and B. Korver, "Guidelines for
Writing RFC Text on Security Considerations",
BCP 72, RFC 3552, July 2003.
[SRTP-VBR] Perkins, C. and JM. Valin, "Guidelines for the Use
of Variable Bit Rate Audio with Secure RTP",
RFC 6562, March 2012.
[UNCOORD] Bryant, S., Morrow, M., and IAB, "Uncoordinated
Protocol Development Considered Harmful",
RFC 5704, November 2009.
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Authors' Addresses
Jean-Marc Valin
Mozilla
650 Castro Street
Mountain View, CA 94041
USA
EMail: jmvalin@jmvalin.ca
Slava Borilin
SPIRIT DSP
EMail: borilin@spiritdsp.com
Koen Vos
EMail: koen.vos@skype.net
Christopher Montgomery
Xiph.Org Foundation
EMail: xiphmont@xiph.org
Raymond (Juin-Hwey) Chen
Broadcom Corporation
EMail: rchen@broadcom.com
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