Internet Engineering Task Force (IETF) E. Ivov, Ed.

Internet Engineering Task Force (IETF) E. Ivov, Ed.
Request for Comments: 6465 Jitsi
Category: Standards Track E. Marocco, Ed.
ISSN: 2070-1721 Telecom Italia
J. Lennox
Vidyo
December 2011

A Real-time Transport Protocol (RTP) Header Extension for
Mixer-to-Client Audio Level Indication

Abstract

This document describes a mechanism for RTP-level mixers in audio
conferences to deliver information about the audio level of
individual participants. Such audio level indicators are transported
in the same RTP packets as the audio data they pertain to.

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/rfc6465.

Copyright Notice

Copyright (c) 2011 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
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.

Ivov, et al. Standards Track [Page 1]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................4
3. Protocol Operation ..............................................4
4. Audio Levels ....................................................5
5. Signaling Information ...........................................7
6. Security Considerations .........................................9
7. IANA Considerations ............................................10
8. Acknowledgments ................................................10
9. References .....................................................10
9.1. Normative References ......................................10
9.2. Informative References ....................................11
Appendix A. Reference Implementation ..............................12
A.1. AudioLevelCalculator.java .................................12

1. Introduction

"A Framework for Conferencing with the Session Initiation Protocol
(SIP)" [RFC4353] presents an overall architecture for multi-party
conferencing. Among others, the framework borrows from RTP [RFC3550]
and extends the concept of a mixer entity "responsible for combining
the media streams that make up a conference, and generating one or
more output streams that are delivered to recipients". Every
participant would hence receive, in a flat single stream, media
originating from all the others.

Using such centralized mixer-based architectures simplifies support
for conference calls on the client side, since they would hardly
differ from one-to-one conversations. However, the method also
introduces a few limitations. The flat nature of the streams that a
mixer would output and send to participants makes it difficult for
users to identify the original source of what they are hearing.

Mechanisms that allow the mixer to send to participants cues on
current speakers (e.g., the contributing source (CSRC) fields in RTP
[RFC3550]) only work for speaking/silent binary indications. There
are, however, a number of use cases where one would require more
detailed information. Possible examples include the presence of
background chat/noise/music/typing, someone breathing noisily in
their microphone, or other cases where identifying the source of the
disturbance would make it easy to remove it (e.g., by sending a
private IM to the concerned party asking them to mute their
microphone). A more advanced scenario could involve an intense
discussion between multiple participants that the user does not
personally know. Audio level information would help better recognize
the speakers by associating with them complex (but still human
readable) characteristics like loudness and speed, for example.

Ivov, et al. Standards Track [Page 2]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

One way of presenting such information in a user-friendly manner
would be for a conferencing client to attach audio level indicators
to the corresponding participant-related components in the user
interface. One possible example is displayed in Figure 1, where
levels can help users determine that Alice is currently the active
speaker, Carol is mute, and Bob and Dave are sending some background
noise.

________________________
| |
| 00:42 | Weekly Call |
|________________________|
| |
| |
| Alice |====== | (S) |
| |
| Bob |= | |
| |
| Carol | | (M) |
| |
| Dave |=== | |
| |
|________________________|

Figure 1: Displaying Detailed Speaker Information to the User by
Including Audio Level for Every Participant

Implementing a user interface like the above requires analysis of the
media sent from other participants. In a conventional audio
conference, this is only possible for the mixer, since all other
conference participants are generally receiving a single, flat audio
stream and therefore have no immediate way of determining individual
audio levels.

This document specifies an RTP extension header that allows such
mixers to deliver audio level information to conference participants
by including it directly in the RTP packets transporting the
corresponding audio data.

The header extension in this document is different than, but
complementary to, the one defined in [RFC6464], which defines a
mechanism by which clients can indicate to audio mixers the levels of
the audio in the packets they send.

Ivov, et al. Standards Track [Page 3]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

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].

3. Protocol Operation

According to RFC 3550 [RFC3550], a mixer is expected to include in
outgoing RTP packets a list of identifiers (CSRC IDs) indicating the
sources that contributed to the resulting stream. The presence of
such CSRC IDs allows RTP clients to determine, in a binary way, the
active speaker(s) in any given moment. The RTP Control Protocol
(RTCP) also provides a basic mechanism to map the CSRC IDs to user
identities through the CNAME field. More advanced mechanisms can
exist, depending on the signaling protocol used to establish and
control a conference. In the case of the Session Initiation Protocol
[RFC3261], for example, "A Session Initiation Protocol (SIP) Event
Package for Conference State" [RFC4575] defines a <src-id> tag that
binds CSRC IDs to media streams and SIP URIs.

This document describes an RTP header extension that allows mixers to
indicate the audio level of every contributing conference participant
(CSRC) in addition to simply indicating their on/off status. This
new header extension uses the general mechanism for RTP header
extensions as described in [RFC5285].

Each instance of this header contains a list of one-octet audio
levels expressed in -dBov, with values from 0 to 127 representing 0
to -127 dBov (see Figures 2 and 3). Appendix A provides a reference
implementation indicating one way of obtaining such values from raw
audio samples.

Every audio level value pertains to the CSRC identifier located at
the corresponding position in the CSRC list. In other words, the
first value would indicate the audio level of the conference
participant represented by the first CSRC identifier in that packet,
and so forth. The number and order of these values MUST therefore
match the number and order of the CSRC IDs present in the same
packet.

When encoding audio level information, a mixer SHOULD include in a
packet information that corresponds to the audio data being
transported in that same packet. It is important that these values
follow the actual stream as closely as possible. Therefore, a mixer
SHOULD also calculate the values after the original contributing
stream has undergone possible processing such as level normalization,
and noise reduction, for example.

Ivov, et al. Standards Track [Page 4]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

It can sometimes happen that a conference involves more than a single
mixer. In such cases, each of the mixers MAY choose to relay the
CSRC list and audio level information they receive from peer mixers
(as long as the total CSRC count remains below 16). Given that the
maximum audio level is not precisely defined by this specification,
it is likely that in such situations average audio levels would be
perceptibly different for the participants located behind the
different mixers.

4. Audio Levels

The audio level header extension carries the level of the audio in
the RTP payload of the packet with which it is associated. This
information is carried in an RTP header extension element as defined
by "A General Mechanism for RTP Header Extensions" [RFC5285].

The payload of the audio level header extension element can be
encoded using either the one-byte or two-byte header defined in
[RFC5285]. Figures 2 and 3 show sample audio level encodings with
each of these header formats.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=2 |0| level 1 |0| level 2 |0| level 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Sample Audio Level Encoding Using the
One-Byte Header Format

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=3 |0| level 1 |0| level 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| level 3 | 0 (pad) | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Sample Audio Level Encoding Using the
Two-Byte Header Format

In the case of the one-byte header format, the 4-bit len field is the
number minus one of data bytes (i.e., audio level values) transported
in this header extension element following the one-byte header.
Therefore, the value zero in this field indicates that one byte of
data follows. In the case of the two-byte header format, the 8-bit
len field contains the exact number of audio levels carried in the

Ivov, et al. Standards Track [Page 5]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

extension. RFC 3550 [RFC3550] only allows RTP packets to carry a
maximum of 15 CSRC IDs. Given that audio levels directly refer to
CSRC IDs, implementations MUST NOT include more than 15 audio level
values. The maximum value allowed in the len field is therefore 14
for the one-byte header format and 15 for the two-byte header format.

Note: Audio levels in this document are defined in the same manner
as is audio noise level in the RTP Payload Comfort Noise
specification [RFC3389]. In [RFC3389], the overall magnitude of
the noise level in comfort noise 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 itself is packed into the seven
least significant bits of the single byte of the header extension,
shown in Figures 2 and 3. 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 unused and always set to 0.

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 highest-intensity
signal encodable by the payload format. (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], 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 [ITU.G711].)

The audio level for digital silence -- for a muted audio source, for
example -- MUST be represented as 127 (-127 dBov), regardless of the
dynamic range of the encoded audio format.

The audio level header extension only carries the level of the audio
in the RTP payload of the packet with which it is associated, with no
long-term averaging or smoothing applied. That level is measured as
a root mean square of all the samples in the measured range.

To simplify implementation of the encoding procedures described here,
this specification provides a sample Java implementation (see
Appendix A) of an audio level calculator that helps obtain such
values from raw linear Pulse Code Modulation (PCM) audio samples.

Ivov, et al. Standards Track [Page 6]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

5. Signaling Information

The URI for declaring the audio level header extension in a Session
Description Protocol (SDP) extmap attribute and mapping it to a local
extension header identifier is
"urn:ietf:params:rtp-hdrext:csrc-audio-level". There is no
additional setup information needed for this extension (i.e., no
extension attributes).

An example attribute line in the SDP for a conference might be:

a=extmap:7 urn:ietf:params:rtp-hdrext:csrc-audio-level

The above mapping will most often be provided per media stream (in
the media-level section(s) of SDP, i.e., after an "m=" line) or
globally if there is more than one stream containing audio level
indicators in a session.

Presence of the above attribute in the SDP description of a media
stream indicates that RTP packets in that stream, which contain the
level extension defined in this document, will be carrying such an
extension with an ID of 7.

Conferencing clients that support audio level indicators and have no
mixing capabilities would not be able to provide content for this
audio level extension and would hence have to always include the
direction parameter in the "extmap" attribute with a value of
"recvonly". Conference focus entities with mixing capabilities can
omit the direction or set it to "sendrecv" in SDP offers. Such
entities would need to set it to "sendonly" in SDP answers to offers
with a "recvonly" parameter and to "sendrecv" when answering other
"sendrecv" offers.

This specification only defines the use of the audio level extensions
in audio streams. They MUST NOT be advertised with other media
types, such as video or text, for example.

Figures 4 and 5 show two example offer/answer exchanges between a
conferencing client and a focus, and between two conference focus
entities.

Ivov, et al. Standards Track [Page 7]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

SDP Offer:

v=0
o=alice 2890844526 2890844526 IN IP6 host.example.com
s=-
c=IN IP6 host.example.com
t=0 0
m=audio 49170 RTP/AVP 0 4
a=rtpmap:0 PCMU/8000
a=rtpmap:4 G723/8000
a=extmap:1/recvonly urn:ietf:params:rtp-hdrext:csrc-audio-level

SDP Answer:

v=0
i=A Seminar on the session description protocol
o=conf-focus 2890844730 2890844730 IN IP6 focus.example.net
s=-
c=IN IP6 focus.example.net
t=0 0
m=audio 52544 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=extmap:1/sendonly urn:ietf:params:rtp-hdrext:csrc-audio-level

Figure 4: A Client-Initiated Example SDP Offer/Answer Exchange
Negotiating an Audio Stream with One-Way Flow of
Audio Level Information

Ivov, et al. Standards Track [Page 8]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

SDP Offer:

v=0
i=Un seminaire sur le protocole de description des sessions
o=fr-focus 2890844730 2890844730 IN IP6 focus.fr.example.net
s=-
c=IN IP6 focus.fr.example.net
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=extmap:1/sendrecv urn:ietf:params:rtp-hdrext:csrc-audio-level

SDP Answer:

v=0
i=A Seminar on the session description protocol
o=us-focus 2890844526 2890844526 IN IP6 focus.us.example.net
s=-
c=IN IP6 focus.us.example.net
t=0 0
m=audio 52544 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=extmap:1/sendrecv urn:ietf:params:rtp-hdrext:csrc-audio-level

Figure 5: An Example SDP Offer/Answer Exchange between Two Conference
Focus Entities with Mixing Capabilities Negotiating an Audio Stream
with Bidirectional Flow of Audio Level Information

6. Security Considerations

1. This document defines a means of attributing audio level to a
particular participant in a conference. An attacker may try to
modify the content of RTP packets in a way that would make audio
activity from one participant appear to be coming from another
participant.

2. Furthermore, the fact that audio level values would not be
protected even in a Secure Real-time Transport Protocol (SRTP)
session [RFC3711] might be of concern in some cases where the
activity of a particular participant in a conference is
confidential. Also, as discussed in [SRTP-VBR-AUDIO], an
attacker might be able to infer information about the
conversation, possibly with phoneme-level resolution.

3. Both of the above are concerns that stem from the design of the
RTP protocol itself, and they would probably also apply when
using CSRC identifiers in the way specified in RFC 3550
[RFC3550]. It is therefore important that, according to the

Ivov, et al. Standards Track [Page 9]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

needs of a particular scenario, implementors and deployers
consider the use of header extension encryption [SRTP-ENCR-HDR]
or a lower-level security and authentication mechanism such as
IPsec [RFC4301], for example.

7. IANA Considerations

This document defines a new extension URI in 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:csrc-audio-level
Description: Mixer-to-client audio level indicators
Contact: [email protected]
Reference: RFC 6465

8. Acknowledgments

Lyubomir Marinov contributed level measurement and rendering code.

Keith Drage, Roni Even, Miguel A. Garcia, John Elwell, Kevin P.
Fleming, Ingemar Johansson, Michael Ramalho, Magnus Westerlund, and
several others provided helpful feedback over the avt and avtext
mailing lists.

Jitsi's participation in this specification is funded by the NLnet
Foundation.

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.

[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.

Ivov, et al. Standards Track [Page 10]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

9.2. Informative References

[ITU.G711] International Telecommunication Union, "Pulse Code
Modulation (PCM) of Voice Frequencies",
ITU-T Recommendation G.711, November 1988.

[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.

[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.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.

[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.

[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, August 2006.

[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6465, December 2011.

[SRTP-ENCR-HDR]
Lennox, J., "Encryption of Header Extensions in the Secure
Real-Time Transport Protocol (SRTP)", Work in Progress,
October 2011.

[SRTP-VBR-AUDIO]
Perkins, C. and JM. Valin, "Guidelines for the use of
Variable Bit Rate Audio with Secure RTP", Work
in Progress, July 2011.

Ivov, et al. Standards Track [Page 11]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

Appendix A. Reference Implementation

This appendix contains Java code for a reference implementation of
the level calculation and rendering methods. The code is not
normative and is by no means the only possible implementation. Its
purpose is to help implementors add audio level support to mixers and
clients.

The Java code contains an AudioLevelCalculator class that calculates
the sound pressure level of a signal with specific samples. It can
be used in mixers to generate values suitable for the level extension
headers.

The implementation is provided in Java but does not rely on any of
the language specifics and can be easily ported to another language.

A.1. AudioLevelCalculator.java

<CODE BEGINS>

/*
Copyright (c) 2011 IETF Trust and the persons identified
as authors of the code. All rights reserved.

Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents (http://trustee.ietf.org/license-info).
*/

/**
* Calculates the audio level of specific samples of a signal
* relative to overload.
*/
public class AudioLevelCalculator
{

/**
* Calculates the audio level of a signal with specific
* <tt>samples</tt>.
*
* @param samples the samples whose audio level we need to
* calculate. The samples are specified as an <tt>int</tt>
* array starting at <tt>offset</tt>, extending <tt>length</tt>
* number of elements, and each <tt>int</tt> element in the
* specified range representing a sample whose audio level we

Ivov, et al. Standards Track [Page 12]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

* need to calculate. Though a sample is provided in the
* form of an <tt>int</tt> value, the sample size in bits
* is determined by the caller via <tt>overload</tt>.
*
* @param offset the offset in <tt>samples</tt> at which the
* samples start.
*
* @param length the length of the signal specified in
* <tt>samples<tt>, starting at <tt>offset</tt>.
*
* @param overload the overload (point) of <tt>signal</tt>.
* For example, <tt>overload</tt> can be {@link Byte#MAX_VALUE}
* for 8-bit signed samples or {@link Short#MAX_VALUE} for
* 16-bit signed samples.
*
* @return the audio level of the specified signal.
*/
public static int calculateAudioLevel(
int[] samples, int offset, int length,
int overload)
{
/*
* Calculate the root mean square (RMS) of the signal.
*/
double rms = 0;

for (; offset < length; offset++)
{
double sample = samples[offset];

sample /= overload;
rms += sample * sample;
}
rms = (length == 0) ? 0 : Math.sqrt(rms / length);

/*
* The audio level is a logarithmic measure of the
* rms level of an audio sample relative to a reference
* value and is measured in decibels.
*/
double db;

/*
* The minimum audio level permitted.
*/
final double MIN_AUDIO_LEVEL = -127;

Ivov, et al. Standards Track [Page 13]

RFC 6465 Mixer-to-Client Audio Level Indication December 2011

/*
* The maximum audio level permitted.
*/
final double MAX_AUDIO_LEVEL = 0;

if (rms > 0)
{
/*
* The "zero" reference level is the overload level,
* which corresponds to 1.0 in this calculation, because
* the samples are normalized in calculating the RMS.
*/
db = 20 * Math.log10(rms);

/*
* Ensure that the calculated level is within the minimum
* and maximum range permitted.
*/
if (db < MIN_AUDIO_LEVEL)
db = MIN_AUDIO_LEVEL;
else if (db > MAX_AUDIO_LEVEL)
db = MAX_AUDIO_LEVEL;
}
else
{
db = MIN_AUDIO_LEVEL;
}

return (int)Math.round(db);
}
}

<CODE ENDS>

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RFC 6465 Mixer-to-Client Audio Level Indication December 2011

Authors' Addresses

Emil Ivov (editor)
Jitsi
Strasbourg 67000
France

EMail: [email protected]

Enrico Marocco (editor)
Telecom Italia
Via G. Reiss Romoli, 274
Turin 10148
Italy

EMail: [email protected]

Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US

EMail: [email protected]

Ivov, et al. Standards Track [Page 15]