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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 5285 (Obsoleted by RFC 8285) == Outdated reference: A later version (-05) exists of draft-ietf-avtcore-srtp-encrypted-header-ext-00 == Outdated reference: A later version (-06) exists of draft-ietf-avtext-mixer-to-client-audio-level-05 Summary: 1 error (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT J. Lennox, Ed. 3 Internet-Draft Vidyo 4 Intended status: Standards Track E. Ivov 5 Expires: March 23, 2012 Jitsi 6 E. Marocco 7 Telecom Italia 8 September 20, 2011 10 A Real-Time Transport Protocol (RTP) Header Extension for Client-to- 11 Mixer Audio Level Indication 12 draft-ietf-avtext-client-to-mixer-audio-level-05 14 Abstract 16 This document defines a mechanism by which packets of Real-Time 17 Transport Protocol (RTP) audio streams can indicate, in an RTP header 18 extension, the audio level of the audio sample carried in the RTP 19 packet. In large conferences, this can reduce the load on an audio 20 mixer or other middlebox which wants to forward only a few of the 21 loudest audio streams, without requiring it to decode and measure 22 every stream that is received. 24 Status of this Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at http://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on March 23, 2012. 41 Copyright Notice 43 Copyright (c) 2011 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 3. Audio Levels . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 4. Signaling (Setup) Information . . . . . . . . . . . . . . . . 5 62 5. Considerations on Use . . . . . . . . . . . . . . . . . . . . 6 63 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 64 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 65 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 66 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 67 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 68 Appendix A. Changes From Earlier Versions . . . . . . . . . . . . 9 69 A.1. Changes From Draft -03 . . . . . . . . . . . . . . . . . . 9 70 A.2. Changes From Draft -02 . . . . . . . . . . . . . . . . . . 9 71 A.3. Changes From Draft -01 . . . . . . . . . . . . . . . . . . 9 72 A.4. Changes From Individual Submission Draft -01 . . . . . . . 10 73 A.5. Changes From Individual Submission Draft -00 . . . . . . . 10 74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 76 1. Introduction 78 In a centralized Real-Time Transport Protocol (RTP) [RFC3550] audio 79 conference, an audio mixer or forwarder receives audio streams from 80 many or all of the conference participants. It then selectively 81 forwards some of them to other participants in the conference. In 82 large conferences, it is possible that such a server might be 83 receiving a large number of streams, of which only a few are intended 84 to be forwarded to the other conference participants. 86 In such a scenario, in order to pick the audio streams to forward, a 87 centralized server needs to decode, measure audio levels, and 88 possibly perform voice activity detection on audio data from a large 89 number of streams. The need for such processing limits the size or 90 number of conferences such a server can support. 92 As an alternative, this document defines an RTP header extension 93 [RFC5285] through which senders of audio packets can indicate the 94 audio level of the packets' payload, reducing the processing load for 95 a server. 97 The header extension in this draft is different than, but 98 complementary with, the one defined in 99 [I-D.ietf-avtext-mixer-to-client-audio-level], which defines a 100 mechanism by which audio mixers can indicate to clients the levels of 101 the contributing sources that made up the mixed audio. 103 2. Terminology 105 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 106 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 107 document are to be interpreted as described in RFC 2119 [RFC2119] and 108 indicate requirement levels for compliant implementations. 110 3. Audio Levels 112 The audio level header extension carries the level of the audio in 113 the RTP [RFC3550] payload of the packet it is associated with. This 114 information is carried in an RTP header extension element as defined 115 by the "General Mechanism for RTP Header Extensions" [RFC5285]. 117 The payload of the audio level header extension element can be 118 encoded using the one-byte or the two-byte header defined in 119 [RFC5285]. Figure 1 and Figure 2 show sample audio level encodings 120 with each of them. 122 0 1 123 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 124 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 125 | ID | len=0 |V| level | 126 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 128 Sample audio level encoding using the one-byte header format 130 Figure 1 132 0 1 2 3 133 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 1 134 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 135 | ID | len=1 |V| level | 0 (pad) | 136 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 138 Sample audio level encoding using the two-byte header format 140 Figure 2 142 Note that, as indicated in [RFC5285] length field in the one-byte 143 header format takes the value 0 to indicate that 1 byte follows. In 144 the two-byte header format on the other hand it takes the value of 1. 146 The magnitude of the audio level itself is packed into the seven 147 least significant bits of the single byte of the header extension, 148 shown in Figure 1 and Figure 2. The least significant bit of the 149 audio level magnitude is packed into the least significant bit of the 150 byte. The most significant bit of the byte is used as a separate 151 flag bit "V", defined below. 153 The audio level is expressed in -dBov, with values from 0 to 127 154 representing 0 to -127 dBov. dBov is the level, in decibels, relative 155 to the overload point of the system, i.e. the maximum-amplitude 156 signal that can be handled by the system without clipping. (Note: 157 Representation relative to the overload point of a system is 158 particularly useful for digital implementations, since one does not 159 need to know the relative calibration of the analog circuitry.) For 160 example, in the case of u-law (audio/pcmu) audio [ITU.G711.1988], the 161 0 dBov reference would be a square wave with values +/- 8031. (This 162 translates to 6.18 dBm0, relative to u-law's dBm0 definition in Table 163 6 of G.711.) 165 The audio level for digital silence, for example for a muted audio 166 source, MUST be represented as 127 (-127 dBov), regardless of the 167 dynamic range of the encoded audio format. 169 The audio level header extension only carries the level of the audio 170 in the RTP payload of the packet it is associated with, with no long- 171 term averaging or smoothing applied. That level is measured as a 172 root mean square of all the samples in the measured range. 174 To simplify implementation of the encoding procedures described here, 175 the reference implementation section in 176 [I-D.ietf-avtext-mixer-to-client-audio-level] provides a sample Java 177 implementation of an audio level calculator that helps obtain such 178 values from raw linear PCM audio samples. 180 In addition, a flag bit (labeled V) optionally indicates whether the 181 encoder believes the audio packet contains voice activity. If the V 182 bit is in use, the value 1 indicates that the encoder believes the 183 audio packet contains voice activity, and the value 0 indicates that 184 the encoder believes it does not. (The voice activity detection 185 algorithm is unspecified and left implementation-specific.) If the V 186 bit is not in use, its value is unspecified and MUST be ignored by 187 receivers. The use of the V bit is signaled using the extension 188 attribute "vad", discussed in Section 4. 190 When this header extension is used with RTP data sent using the RTP 191 Payload for Redundant Audio Data [RFC2198], the header's data 192 describes the contents of the primary encoding. 194 Note: This audio level is defined in the same manner as is audio 195 noise level in the RTP Payload Comfort Noise specification [RFC3389]. 196 In the comfort noise specification, the overall magnitude of the 197 noise level in comfort noise is encoded into the first byte of the 198 payload, with spectral information about the noise in subsequent 199 bytes. This specification's audio level parameter is defined so as 200 to be identical to the comfort noise payload's noise-level byte. 202 4. Signaling (Setup) Information 204 The URI for declaring this header extension in an extmap attribute is 205 "urn:ietf:params:rtp-hdrext:ssrc-audio-level". 207 It has a single extension attribute, named "vad". It takes the form 208 "vad=on" or "vad=off". If the header extension element is signaled 209 with "vad=on", the "V" bit described in Section 3 is in use, and MUST 210 be set by senders. If the header extension element is signaled with 211 "vad=off", the "V" bit is not in use, and its value MUST be ignored 212 by receivers. If the "vad" extension attribute is not specified, the 213 default is "vad=on". 215 An example attribute line in the SDP, for a conference might hence 216 be: 218 a=extmap:6 urn:ietf:params:rtp-hdrext:ssrc-audio-level vad=on 220 The "vad" extension attribute only controls the semantics of this 221 header extension attribute, and does not make any statement about 222 whether the sender is using any other voice activity detection 223 features such as discontinuous transmission, comfort noise, or 224 silence suppression. 226 Using the mechanisms of [RFC5285], an endpoint MAY signal multiple 227 instances of the header extension element, with different values of 228 the vad attribute, so long as these instances use different values 229 for the extension identifier. However, again following the rules of 230 [RFC5285], the semantics chosen for a header extension element 231 (including its vad setting) for a particular extension identifier 232 value MUST NOT be changed within an RTP session. 234 5. Considerations on Use 236 Mixers and forwarders generally ought not base audio forwarding 237 decisions directly on packet-by-packet audio level information, but 238 rather ought to apply some analysis of the audio levels and trends. 239 This general rule applies whether audio levels are provided by 240 endpoints (as defined in this document), or are calculated at a 241 server, as would be done in the absence of this information. This 242 section discusses several issues that mixers and forwarders may wish 243 to take into account. (Note that this section provides design 244 guidance only, and is not normative.) 246 First of all, audio levels generally ought to be measured over longer 247 intervals than that of a single audio packet. In order to avoid 248 false-positives for short bursts of sound (such as a cough or a 249 dropped microphone), it is often useful to require that a 250 participant's audio level be maintained for some period of time 251 before considering it to be "real", i.e. some type of low-pass filter 252 ought to be applied to the audio levels. Note, though, that such 253 filtering must be balanced with the need to avoid clipping of the 254 beginning of a speaker's speech. 256 Additionally, different participants may have their audio input set 257 differently. It may be useful to apply some sort of automatic gain 258 control to the audio levels. There are a number of possible 259 approaches to acheiving this, e.g. by measuring peak audio levels, by 260 average audio levels during speech, or by measuring background audio 261 levels (average audio level levels during non-speech). 263 6. Security Considerations 265 A malicious endpoint could choose to set the values in this header 266 extension falsely, so as to falsely claim that audio or voice is or 267 is not present. It is not clear what could be gained by falsely 268 claiming that audio is not present, but an endpoint falsely claiming 269 that audio is present could perform a denial-of-service attack on an 270 audio conference, so as to send silence to suppress other conference 271 members' audio. Thus, if a device relys on audio level data from 272 untrusted endpoints, it SHOULD periodically audit the level 273 information transmitted, taking appropriate corrective action against 274 endpoints that appear to be sending incorrect data. (However, as it 275 is valid for an endpoint to choose to measure audio levels prior to 276 encoding, some degree of discrepancy could be present. This would 277 not indicate that an endpoint is malicous.) 279 In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP 280 header extensions are authenticated but not encrypted. When this 281 header extension is used, audio levels are therefore visible on a 282 packet-by-packet basis to an attacker passively observing the audio 283 stream. As discussed in [I-D.perkins-avt-srtp-vbr-audio], such an 284 attacker might be able to infer information about the conversation, 285 possibly with phoneme-level resolution. In scenarios where this is a 286 concern, additional mechanisms SHOULD be used to protect the 287 confidentiality of the header extension. This mechanism could be 288 header extension encryption 289 [I-D.ietf-avtcore-srtp-encrypted-header-ext], or a lower-level 290 security and authentication mechanism. 292 7. IANA Considerations 294 This document defines a new extension URI to the RTP Compact Header 295 Extensions subregistry of the Real-Time Transport Protocol (RTP) 296 Parameters registry, according to the following data: 298 Extension URI: urn:ietf:params:rtp-hdrext:ssrc-audio-level 299 Description: Audio Level 300 Contact: jonathan@vidyo.com 301 Reference: RFC XXXX 303 Note to RFC Editor: please replace "RFC XXXX" with the number of this 304 RFC. 306 8. References 308 8.1. Normative References 310 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 311 Requirement Levels", BCP 14, RFC 2119, March 1997. 313 [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., 314 Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- 315 Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, 316 September 1997. 318 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 319 Jacobson, "RTP: A Transport Protocol for Real-Time 320 Applications", STD 64, RFC 3550, July 2003. 322 [RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP 323 Header Extensions", RFC 5285, July 2008. 325 8.2. Informative References 327 [I-D.ietf-avtcore-srtp-encrypted-header-ext] 328 Lennox, J., "Encryption of Header Extensions in the Secure 329 Real-Time Transport Protocol (SRTP)", 330 draft-ietf-avtcore-srtp-encrypted-header-ext-00 (work in 331 progress), June 2011. 333 [I-D.ietf-avtext-mixer-to-client-audio-level] 334 Ivov, E., Marocco, E., and J. Lennox, "A Real-Time 335 Transport Protocol (RTP) Header Extension for Mixer-to- 336 Client Audio Level Indication", 337 draft-ietf-avtext-mixer-to-client-audio-level-05 (work in 338 progress), September 2011. 340 [I-D.perkins-avt-srtp-vbr-audio] 341 Perkins, C. and J. Valin, "Guidelines for the use of 342 Variable Bit Rate Audio with Secure RTP", 343 draft-perkins-avt-srtp-vbr-audio-05 (work in progress), 344 December 2010. 346 [ITU.G711.1988] 347 International Telecommunications Union, "Pulse Code 348 Modulation (PCM) of Voice Frequencies", ITU- 349 T Recommendation G.711, November 1988. 351 [RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for 352 Comfort Noise (CN)", RFC 3389, September 2002. 354 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 355 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 356 RFC 3711, March 2004. 358 Appendix A. Changes From Earlier Versions 360 Note to the RFC-Editor: please remove this section prior to 361 publication as an RFC. 363 A.1. Changes From Draft -03 365 o Added vad extension attribute to negotiate use of the V bit. 366 o Addressed editorial comments made on the mailing list. 368 A.2. Changes From Draft -02 370 o Changed encoding related text so that it would cover both the one- 371 byte and the two-byte header formats. 372 o Clarified use of root mean square for dBov calculation 373 o Added references to the sample level calculator in 374 [I-D.ietf-avtext-mixer-to-client-audio-level]. 375 o Changed affiliation for Emil Ivov. 376 o Other minor editorial changes. 378 A.3. Changes From Draft -01 380 o Changed the URI for declaring this header extension from 381 "urn:ietf:params:rtp-hdrext:audio-level" to 382 "urn:ietf:params:rtp-hdrext:ssrc-audio-level" for consistency with 383 [I-D.ietf-avtext-mixer-to-client-audio-level]. 384 o Removed the "Limitations" section; it was discussing a potential 385 extension that consensus indicated was out of scope of this 386 document. 387 o Closed the P.56 open issue. It was agreed on IETF 80 that P.56 is 388 mostly about speech levels and the levels transported by the 389 extension defined here should also be able to serve as an 390 indication for noise. 391 o Closed the open issue about transmitting noise floor information. 392 Noise floor is (loosely) inferrable by observing the per-packet 393 level information over a period of time, so the additional 394 complexity seemed unnecessary. 395 o Editorial changes for consistency with 396 [I-D.ietf-avtext-mixer-to-client-audio-level]. 397 o Moved several descriptions of normative items that previously had 398 only been described in informative sections of the text. 400 o Other editorial clarifications. 402 A.4. Changes From Individual Submission Draft -01 404 o This version is primarily a document refresh. 405 o Emil Ivov and Enrico Marocco have been added as co-authors. 406 o Additional open issues listed. 408 A.5. Changes From Individual Submission Draft -00 410 o The draft name has been changed to clarify that this document 411 defines Client-To-Mixer Audio Levels, to more clearly distinguish 412 it from [I-D.ietf-avtext-mixer-to-client-audio-level]. 413 o The header extension format has been changed from a two-byte to a 414 one-byte payload, eliminating the 7 reserved bits and the one 415 must-be-zero bit. 416 o The sections Considerations on Use (Section 5) and Limitations 417 have been added. 418 o It has been noted that senders MAY indicate -127 dBov for digital 419 silence, and that level measurement MAY be done prior to encoding 420 audio. 421 o A reference to [I-D.ietf-avtcore-srtp-encrypted-header-ext] has 422 been added to the security considerations. 423 o The term "header extension" is now used consistentenly throughout 424 the document (as opposed to "extension header"). 426 Authors' Addresses 428 Jonathan Lennox (editor) 429 Vidyo, Inc. 430 433 Hackensack Avenue 431 Seventh Floor 432 Hackensack, NJ 07601 433 US 435 Email: jonathan@vidyo.com 437 Emil Ivov 438 Jitsi 439 Strasbourg 67000 440 France 442 Email: emcho@jitsi.org 443 Enrico Marocco 444 Telecom Itialia 445 Via G. Reiss Romoli, 274 446 Turin 10148 447 Italy 449 Email: enrico.marocco@telecomitalia.it