AVT J. Lennox
Internet-Draft Vidyo
Intended status: Standards Track October 20, 2009
Expires: April 23, 2010
A Real-Time Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication
draft-lennox-avt-rtp-audio-level-exthdr-01
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Abstract
This document defines a mechanism by which packets of Real-Time
Transport Protocol (RTP) audio streams can indicate, in an RTP header
extension, the audio level of the audio sample carried in the RTP
packet. In large conferences, this can reduce the load on an audio
mixer or other middlebox which wants to forward only a few of the
loudest audio streams, without requiring it to decode and measure
every stream that is received.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Audio Levels . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Signaling (Setup) Information . . . . . . . . . . . . . . . . . 5
5. Considerations on Use . . . . . . . . . . . . . . . . . . . . . 5
6. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Changes From Earlier Versions . . . . . . . . . . . . 8
A.1. Changes From Individual Submission Draft -00 . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
In a centralized Real-Time Transport Protocol (RTP) [RFC3550] audio
conference, an audio mixer or forwarder receives audio streams from
many or all of the conference participants. It then selectively
forwards some of them to other participants in the conference. In
large conferences, it is possible that such a server might be
receiving a large number of streams, of which only a few should be
forwarded to the other conference participants.
In such a scenario, in order to pick the audio streams to forward, a
centralized server needs to decode, measure audio levels, and
possibly perform voice activity detection on audio data from a large
number of streams. The need for such processing limits the size or
number of conferences such a server can support.
As an alternative, this document defines an RTP header extension
[RFC5285] through which senders of audio packets can indicate the
audio level of the packets' payload, reducing the processing load for
a server.
The header extension in this draft is different to, but complementary
with, the one defined in [I-D.ivov-avt-slic], which defines a
mechanism by which audio mixers can indicate to clients the levels of
the contributing sources that made up the mixed audio.
2. Terminology
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] and
indicate requirement levels for compliant implementations.
3. Audio Levels
The audio level header extension carries both the level of the audio
carried in the RTP payload of the packet it is associated with, as
well as an indication as to whether voice activity has been detected
in the packet.
The form of the audio level extension block is as follows:
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=0 |V| level |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1
The length field takes the value 0 to indicate that 1 byte follows.
The audio level is defined in the same manner as is audio noise level
in the RTP Comfort Noise [RFC3389] specification. In that
specification, the overall magnitude of the noise level is encoded
into the first byte of the payload, with spectral information about
the noise in subsequent bytes. This specification's audio level
parameter is defined so as to be identical to the comfort noise
payload's noise-level byte.
The magnitude of the audio level is packed into the seven least
significant bits of the single byte of the header extension, shown in
Figure 1. The least significant bit of the audio level magnitude is
packed into the least significant bit of the byte. The most
significant bit of the byte is used as a separate flag bit "V",
defined below.
The audio level is expressed in -dBov, with values from 0 to 127
representing 0 to -127 dBov. dBov is the level, in decibels, relative
to the overload point of the system, i.e. the maximum-amplitude
signal that can be handled by the system without clipping. (Note:
Representation relative to the overload point of a system is
particularly useful for digital implementations, since one does not
need to know the relative calibration of the analog circuitry.) For
example, in the case of u-law (audio/pcmu) audio [ITU.G711.1988], the
0 dBov reference would be a square wave with values +/- 8031. (This
translates to 6.18 dBm0, relative to u-law's dBm0 definition in Table
6 of G.711.)
In addition, a flag bit (labeled V) indicates whether the encoder
believes the audio packet contains voice activity (1) or does not
(0). The voice activity detection algorithm is unspecified and left
implementation-specific.
The audio level for digital silence (e.g. all-0 pcmu audio), for
example for a muted audio source, MAY be represented as 127 (-127
dBov), regardless of the dynamic range of the encoded audio format.
When this header extension is used with RTP data sent using the RTP
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Payload for Redundant Audio Data [RFC2198], the header's data
describes the contents of the primary encoding.
4. Signaling (Setup) Information
The URI for declaring this header extension in an extmap attribute is
"urn:ietf:params:rtp-hdrext:audio-level". There is no additional
setup information needed for this extension (no extensionattributes).
5. Considerations on Use
Mixers and forwarders generally should not base audio forwarding
decisions directly on packet-by-packet audio level information, but
rather should apply some analysis of the audio levels and trends.
This general rule applies whether audio levels are provided by
endpoints (as defined in this document), or are calculated at a
server, as would be done in the absence of this information. This
section discusses several issues that mixers and forwarders may wish
to take into account. (Note that this section provides design
guidance only, and is not normative.)
First of all, audio levels should generally be measured over longer
intervals than that of a single audio packet. In order to avoid
false-positives for short bursts of sound (such as a cough or a
dropped microphone), it is often useful to require that a
participant's audio level be maintained for some period of time
before considering it to be "real", i.e. some type of low-pass filter
should be applied to the audio levels. Note, though, that such
filtering must be balanced with the need to avoid clipping of the
beginning of a speaker's speech.
Additionally, different participants may have their audio input set
differently. It may be useful to apply some sort of automatic gain
control to the audio levels. There are a number of possible
approaches to acheiving this, e.g. by measuring peak audio levels, by
average audio levels during speech, or by measuring background audio
levels (average audio level levels during non-speech).
6. Limitations
The audio levels carried by the extension header defined by this
document are defined as dBov, decibels below system overload.
In principle, it could be more useful to have, instead, dB SPL,
decibels of sound pressure level. In traditional telephony systems,
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telephone handsets were calibrated such that a particular (e.g.)
u-law audio level, or analog voltage, corresponded to a particular
sound pressure level at the handset's mouthpiece.
However, in many environments, this information is not available.
Notably, PC soundcard hardware can only determine the levels of mic-
or line-in at the hardware input, and operating systems usually allow
further adjustments of audio input levels without providing
information about these transformations to applications.
Furthermore, in many circumstances, such as speech synthesis or mixed
audio, an "audio" signal may in fact never have actually existed as
sound pressure at all.
Thus, while information about the correspondance between dB SPL and
dBov, or encoded audio, could be useful, this document does not
attempt to define it. If there are circumstances in which this
information would be useful, a separate header extension would be
straightforward to define. (The information carried by such a header
extension could indeed be useful independently from the information
in the header extension defined by this document.)
7. Security Considerations
A malicious endpoint could choose to set the values in this header
extension falsely, so as to falsely claim that audio or voice is or
is not present. It is not clear what could be gained by falsely
claiming that audio is not present, but an endpoint falsely claiming
that audio is present could perform a denial-of-service attack on an
audio conference, so as to send silence to suppress other conference
members' audio. Thus, a device relying on audio level data from
untrusted endpoints SHOULD periodically audit the level information
transmitted, taking appropriate corrective action if endpoints appear
to be sending incorrect data. (Note that endpoints MAY choose to
measure audio levels prior to encoding, so some degree of discrepancy
SHOULD be tolerated.)
In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP
header extensions are authenticated but not encrypted. When this
header extension is used, audio levels are therefore visible on a
packet-by-packet basis to an attacker passively observing the audio
stream. As discussed in [I-D.perkins-avt-srtp-vbr-audio], such an
attacker might be able to infer information about the conversation,
possibly with phoneme-level resolution. In scenarios where this is a
concern, additional mechanisms SHOULD be used to protect the
confidentiality of the header extension. One solution would be
header extension encryption
[I-D.lennox-avt-srtp-encrypted-extension-headers].
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8. IANA Considerations
This document defines a new extension URI to the RTP Compact Header
Extensions subregistry of the Real-Time Transport Protocol (RTP)
Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:audio-level
Description: Audio Level
Contact: jonathan@vidyo.com
Reference: RFC XXXX
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
September 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
9.2. Informative References
[I-D.ivov-avt-slic]
Ivov, E. and E. Marocco, "A Real-Time Transport Protocol
(RTP) Extension Header for Mixer-to- client Audio Level
Indication", draft-ivov-avt-slic-01 (work in progress),
October 2009.
[]
Lennox, J., "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)",
draft-lennox-avt-srtp-encrypted-extension-headers-00 (work
in progress), October 2009.
[I-D.perkins-avt-srtp-vbr-audio]
Perkins, C., "Guidelines for the use of Variable Bit Rate
Audio with Secure RTP",
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draft-perkins-avt-srtp-vbr-audio-01 (work in progress),
July 2009.
[ITU.G711.1988]
International Telecommunications Union, "Pulse code
modulation (PCM) of voice frequencies", ITU-
T Recommendation G.711, November 1988.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
Appendix A. Changes From Earlier Versions
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
A.1. Changes From Individual Submission Draft -00
o The draft name has been changed to clarify that this document
defines Client-To-Mixer Audio Levels, to more clearly distinguish
it from [I-D.ivov-avt-slic].
o The header extension format has been changed from a two-byte to a
one-byte payload, eliminating the 7 reserved bits and the one
must-be-zero bit.
o The sections Considerations on Use (Section 5) and Limitations
(Section 6) have been added.
o It has been noted that senders MAY indicate -127 dBov for digital
silence, and that level measurement MAY be done prior to encoding
audio.
o A reference to [I-D.lennox-avt-srtp-encrypted-extension-headers]
has been added to the security considerations.
o The term "header extension" is now used consistentenly throughout
the document (as opposed to "extension header").
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Author's Address
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Sixth Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
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