AVT J. Lennox Internet-Draft Vidyo Intended status: Standards Track June 17, 2009 Expires: December 19, 2009 A Real-Time Transport Protocol (RTP) Extension Header for Audio Level Indication draft-lennox-avt-rtp-audio-level-exthdr-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 19, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. Lennox Expires December 19, 2009 [Page 1]
Internet-Draft RTP Extension Header for Audio Level June 2009 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document defines a mechanism by which packets of Real-Time Transport Protocol (RTP) audio streams can indicate, in an RTP extension header, 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. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . . 6 Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Lennox Expires December 19, 2009 [Page 2]
Internet-Draft RTP Extension Header for Audio Level June 2009 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 the relative 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 extension header 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: Lennox Expires December 19, 2009 [Page 3]
Internet-Draft RTP Extension Header for Audio Level June 2009 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID | len=1 |0| level |V| reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1 The length field takes the value 1 to indicate that 2 bytes follow. 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 least significant bits of the first payload byte of the extension header, with the most significant bit unused and set to 0 as shown in Figure 1. The least significant bit of the audio level magnitude is packed into the least significant bit of the byte. 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 byte carries bits providing additional information about the audio payload carried in the media packet. At this time only a single bit is defined. The V bit 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 other bits of the flag byte are reserved. They SHOULD be set to zero by senders and ignored by receivers. When this extension header is used with RTP data sent using the RTP Lennox Expires December 19, 2009 [Page 4]
Internet-Draft RTP Extension Header for Audio Level June 2009 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. Security Considerations A malicious endpoint could choose to set the values in this extension header 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. In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP extension headers are authenticated but not encrypted. When this extension header 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 can infer a great deal of information about the conversation, often with phoneme-level resolution. In scenarios where this is a concern, additional mechanisms SHOULD be used to protect the confidentiality of the extension header. 6. 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 Lennox Expires December 19, 2009 [Page 5]
Internet-Draft RTP Extension Header for Audio Level June 2009 Reference: RFC XXXX 7. References 7.1. 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. 7.2. Informative References [I-D.ivov-avt-slic] Ivov, E. and E. Marocco, "Delivering Conference Participant Sound Level Indicators in RTP Streams", draft-ivov-avt-slic-00 (work in progress), June 2009. [I-D.perkins-avt-srtp-vbr-audio] Perkins, C., "Guidelines for the use of Variable Bit Rate Audio with Secure RTP", draft-perkins-avt-srtp-vbr-audio-00 (work in progress), March 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. Lennox Expires December 19, 2009 [Page 6]
Internet-Draft RTP Extension Header for Audio Level June 2009 Appendix A. Open issues o Should this draft be merged with [I-D.ivov-avt-slic]? o Would it be useful to add a fractional part to the audio level, e.g., to describe the audio level in an 8+8 fixed-point format? Due to the format of RTP extension headers, a third byte for the extension header is essentially "free" if no other RTP extension headers are in use. o Are any other bits useful in the flag byte? o Is there any compelling use case for providing the audio level without voice detection information, and if so, should the two pieces of information be separated? Author's Address Jonathan Lennox Vidyo, Inc. 433 Hackensack Avenue Sixth Floor Hackensack, NJ 07601 US Email: jonathan@vidyo.com Lennox Expires December 19, 2009 [Page 7]