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Zorn, Ed. 5 Intended status: Standards Track Network Zen 6 Expires: October 24, 2013 April 22, 2013 8 Support for Multiple Clock Rates in an RTP Session 9 draft-ietf-avtext-multiple-clock-rates-09 11 Abstract 13 This document clarifies the RTP specification when different clock 14 rates are used in an RTP session. It also provides guidance on how 15 to interoperate with legacy RTP implementations that use multiple 16 clock rates. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at http://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on October 24, 2013. 35 Copyright Notice 37 Copyright (c) 2013 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 54 3. Legacy RTP . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 3.1. Different SSRC . . . . . . . . . . . . . . . . . . . . . 4 56 3.2. Same SSRC . . . . . . . . . . . . . . . . . . . . . . . . 4 57 3.2.1. Monotonic timestamps . . . . . . . . . . . . . . . . 5 58 3.2.2. Non-monotonic timestamps . . . . . . . . . . . . . . 5 59 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6 60 4.1. RTP Sender (with RTCP) . . . . . . . . . . . . . . . . . 6 61 4.2. RTP Sender (without RTCP) . . . . . . . . . . . . . . . . 6 62 4.3. RTP Receiver . . . . . . . . . . . . . . . . . . . . . . 7 63 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 64 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 65 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 66 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 67 8.1. Normative References . . . . . . . . . . . . . . . . . . 8 68 8.2. Informative References . . . . . . . . . . . . . . . . . 8 69 Appendix A. Example Values . . . . . . . . . . . . . . . . . . . 9 70 Appendix B. Using a Fixed Clock Rate . . . . . . . . . . . . . . 11 71 Appendix C. Behavior of Legacy Implementations . . . . . . . . . 11 72 C.1. libccrtp 2.0.2 . . . . . . . . . . . . . . . . . . . . . 11 73 C.2. libmediastreamer0 2.6.0 . . . . . . . . . . . . . . . . . 11 74 C.3. libpjmedia 1.0 . . . . . . . . . . . . . . . . . . . . . 12 75 C.4. Android RTP stack 4.0.3 . . . . . . . . . . . . . . . . . 12 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 78 1. Introduction 80 The clock rate is a parameter of the payload format as identified in 81 RTP and RTCP by the payload type value. It is often defined as being 82 the same as the sampling rate but that is not always the case (see 83 e.g. the G722 and MPA audio codecs [RFC3551]). 85 An RTP sender can switch between different payloads during the 86 lifetime of an RTP session and because clock rates are defined by 87 payload format, it is possible that the clock rate will also vary 88 during an RTP session. Schulzrinne, et al. [RFC3550] lists using 89 multiple clock rates as one of the reasons to not use different 90 payloads on the same SSRC but unfortunately this advice has not 91 always been followed and some RTP implementations change the payload 92 in the same SSRC even if the different payloads use different clock 93 rates. 95 This creates three problems: 97 o The method used to calculate the RTP timestamp field in an RTP 98 packet is underspecified. 100 o When the same SSRC is used for different clock rates, it is 101 difficult to know what clock rate was used for the RTP timestamp 102 field in an RTCP SR packet. 104 o When the same SSRC is used for different clock rates, it is 105 difficult to know what clock rate was used for the interarrival 106 jitter field in an RTCP RR packet. 108 Table 1 contains a non-exhaustive list of fields in RTCP packets that 109 uses a clock rate as unit: 111 +---------------------+------------------+------------+ 112 | Field name | RTCP packet type | Reference | 113 +---------------------+------------------+------------+ 114 | RTP timestamp | SR | [RFC3550] | 115 | | | | 116 | Interarrival jitter | RR | [RFC3550] | 117 | | | | 118 | min_jitter | XR Summary Block | [RFC3611] | 119 | | | | 120 | max_jitter | XR Summary Block | [RFC3611] | 121 | | | | 122 | mean_jitter | XR Summary Block | [RFC3611] | 123 | | | | 124 | dev_jitter | XR Summary Block | [RFC3611] | 125 | | | | 126 | Interarrival jitter | IJ | [RFC5450] | 127 | | | | 128 | RTP timestamp | SMPTETC | [RFC5484] | 129 | | | | 130 | Jitter | RSI Jitter Block | [RFC5760] | 131 | | | | 132 | Median jitter | RSI Stats Block | [RFC5760] | 133 +---------------------+------------------+------------+ 135 Table 1 137 This document first tries to list in Section 3 and subsections all of 138 the algorithms known to be used in existing RTP implementations at 139 the time of writing. These sections are not normative. 141 Section 4 and subsections then recommend a unique algorithm that 142 modifies RFC 3550. These sections are normative. 144 2. Terminology 146 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 147 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 148 document are to be interpreted as described in RFC 2119 [RFC2119]. 149 In addition, this document uses the following terms: 151 Clock rate The multiplier used to convert from a wallclock value 152 in seconds to an equivalent RTP timestamp value 153 (without the fixed random offset). Note that RFC 3550 154 uses various terms like "clock frequency", "media 155 clock rate", "timestamp unit", "timestamp frequency", 156 and "RTP timestamp clock rate" as synonymous to clock 157 rate. 159 RTP Sender A logical network element that sends RTP packets, 160 sends RTCP SR packets, and receives RTCP RR packets. 162 RTP Receiver A logical network element that receives RTP packets, 163 receives RTCP SR packets, and sends RTCP RR packets. 165 3. Legacy RTP 167 The following sections describe the various ways legacy RTP 168 implementations behave when multiple clock rates are used. Legacy 169 RTP refers to RFC 3550 without the modifications introduced by this 170 document. 172 3.1. Different SSRC 174 One way of managing multiple clock rates is to use a different SSRC 175 for each different clock rate, as in this case there is no ambiguity 176 on the clock rate used by fields in the RTCP packets. This method 177 also seems to be the original intent of RTP as can be deduced from 178 points 2 and 3 of section 5.2 of RFC 3550. 180 On the other hand changing the SSRC can be a problem for some 181 implementations designed to work only with unicast IP addresses, 182 where having multiple SSRCs is considered a corner case. Lip 183 synchronization can also be a problem in the interval between the 184 beginning of the new stream and the first RTCP SR packet. This is 185 not different than what happen at the beginning of the RTP session 186 but it can be more annoying for the end-user. 188 3.2. Same SSRC 190 The simplest way of managing multiple clock rates is to use the same 191 SSRC for all the payload types regardless of the clock rates. 193 Unfortunately there is no clear definition on how the RTP timestamp 194 should be calculated in this case. The following subsections present 195 the algorithms used in the field. 197 3.2.1. Monotonic timestamps 199 This method of calculating the RTP timestamp ensures that the value 200 increases monotonically. The formula used by this method is as 201 follows: 203 timestamp = previous_timestamp 204 + (current_capture_time - previous_capture_time) 205 * current_clock_rate 207 The problem with this method is that the jitter calculation on the 208 receiving side gives an invalid result during the transition between 209 two clock rates, as shown in Table 2. The capture and arrival time 210 are in seconds, starting at the beginning of the capture of the first 211 packet; clock rate is in Hz; the RTP timestamp does not include the 212 random offset; the transit, jitter, and average jitter use the clock 213 rate as unit. 215 Calculating the correct transit time on the receiving side can be 216 done by using the following formulas: 218 1. current_capture_time = (current_timestamp - previous_timestamp) / 219 current_clock_rate + previous_capture_time 221 2. transit = current_clock_rate * (arrival_time - 222 current_capture_time) 224 3. previous_capture_time = current_capture_time 226 The main problem with this method, in addition to the fact that the 227 jitter calculation described in RFC 3550 cannot be used, is that is 228 it dependent on the previous RTP packets, packets that can be 229 reordered or lost in the network. 231 3.2.2. Non-monotonic timestamps 233 An alternate way of generating the RTP timestamps is to use the 234 following formula: 236 timestamp = capture_time * clock_rate 237 With this formula, the jitter calculation is correct but the RTP 238 timestamp values are no longer increasing monotonically as shown in 239 Table 3. RFC 3550 states that "[t]he sampling instant MUST be 240 derived from a clock that increments monotonically[...]" but nowhere 241 says that the RTP timestamp must increment monotonically. 243 The advantage with this method is that it works with the jitter 244 calculation described in RFC 3550, as long as the correct clock rates 245 are used. It seems that this is what most implementations are using. 247 4. Recommendations 249 The following subsections describe behavioral recommendations for RTP 250 senders (with and without RTCP) and RTP receivers. 252 4.1. RTP Sender (with RTCP) 254 An RTP Sender with RTCP turned on MUST use a different SSRC for each 255 different clock rate. An RTCP BYE MUST be sent and a new SSRC MUST 256 be used if the clock rate switches back to a value already seen in 257 the RTP stream. 259 To accelerate lip synchronization, the next compound RTCP packet sent 260 by the RTP sender MUST contain multiple SR packets, the first one 261 containing the mapping for the current clock rate and the next SR 262 packets containing the mapping for the other clock rates seen during 263 the last period. 265 The RTP extension defined in Perkins & Schierl [RFC6051] MAY be used 266 to accelerate the synchronization. 268 4.2. RTP Sender (without RTCP) 270 An RTP Sender with RTCP turned off (i.e. by setting the RS and RR 271 bandwidth modifiers [RFC3556] to 0) SHOULD use a different SSRC for 272 each different clock rate but MAY use different clock rates on the 273 same SSRC as long as the RTP timestamp is calculated as explained 274 below: 276 Each time the clock rate changes, the start_offset and capture_start 277 values are calculated with the following formulas: 279 start_offset += (capture_time - capture_start) * previous_clock_rate 280 capture_start = capture_time 282 For the first RTP packet, the values are initialized with the 283 following values: 285 start_offset = random_initial_offset 286 capture_start = capture_time 288 After eventually updating these values, the RTP timestamp is 289 calculated with the following formula: 291 timestamp = (capture_time - capture_start) * clock_rate 292 + start_offset 294 Note that in all the formulas, capture_start is the first instant 295 that the new timestamp rate is used. The output of the above method 296 is exemplified in Table 4. 298 4.3. RTP Receiver 300 An RTP Receiver MUST calculate the jitter using the following 301 formula: 303 D(i,j) = (arrival_time_j * clock_rate_i - timestamp_j) 304 - (arrival_time_i * clock_rate_i - timestamp_i) 306 An RTP Receiver MUST be able to handle a compound RTCP packet with 307 multiple SR packets. 309 5. Security Considerations 311 This document is not believed to effect the security of the RTP 312 sessions described here in any way. 314 6. IANA Considerations 316 This document requires no IANA actions. 318 7. Acknowledgements 320 Thanks to Colin Perkins, Ali C. Begen, Harald Alvestrand, Qin Wu, 321 and Magnus Westerlund for their comments, suggestions and questions 322 that helped to improve this document. 324 Thanks to Bo Burman (who provided the values in Table 4). 326 Thanks to Robert Sparks and the attendees of SIPit 26 for the survey 327 on multiple clock rates interoperability. 329 This document was written with the xml2rfc tool described in Rose 330 [RFC2629]. 332 8. References 334 8.1. Normative References 336 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 337 Requirement Levels", BCP 14, RFC 2119, March 1997. 339 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 340 Jacobson, "RTP: A Transport Protocol for Real-Time 341 Applications", STD 64, RFC 3550, July 2003. 343 8.2. Informative References 345 [I-D.ietf-avt-variable-rate-audio] 346 Wenger, S. and C. Perkins, "RTP Timestamp Frequency for 347 Variable Rate Audio Codecs", draft-ietf-avt-variable-rate- 348 audio-00 (work in progress), October 2004. 350 [RFC2629] Rose, M.T., "Writing I-Ds and RFCs using XML", RFC 2629, 351 June 1999. 353 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 354 Video Conferences with Minimal Control", STD 65, RFC 3551, 355 July 2003. 357 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 358 Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 359 3556, July 2003. 361 [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control 362 Protocol Extended Reports (RTCP XR)", RFC 3611, November 363 2003. 365 [RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in 366 RTP Streams", RFC 5450, March 2009. 368 [RFC5484] Singer, D., "Associating Time-Codes with RTP Streams", RFC 369 5484, March 2009. 371 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control 372 Protocol (RTCP) Extensions for Single-Source Multicast 373 Sessions with Unicast Feedback", RFC 5760, February 2010. 375 [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP 376 Flows", RFC 6051, November 2010. 378 Appendix A. Example Values 380 The following tables illustrate the timestamp and jitter values 381 produced when the various methods discussed in the text are used. 383 The values shown are purely exemplary, illustrative and non- 384 normative. 386 +--------+-------+-----------+---------+---------+--------+---------+ 387 | Capt. | Clock | RTP | Arrival | Transit | Jitter | Average | 388 | time | rate | timestamp | time | | | jitter | 389 +--------+-------+-----------+---------+---------+--------+---------+ 390 | 0 | 8000 | 0 | 0.1 | 800 | | | 391 | | | | | | | | 392 | 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 | 393 | | | | | | | | 394 | 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 | 395 | | | | | | | | 396 | 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 | 397 | | | | | | | | 398 | 0.08 | 16000 | 800 | 0.18 | 2080 | 480 | 30 | 399 | | | | | | | | 400 | 0.1 | 16000 | 1120 | 0.2 | 2080 | 0 | 28 | 401 | | | | | | | | 402 | 0.12 | 16000 | 1440 | 0.22 | 2080 | 0 | 26 | 403 | | | | | | | | 404 | 0.14 | 8000 | 1600 | 0.24 | 320 | 720 | 70 | 405 | | | | | | | | 406 | 0.16 | 8000 | 1760 | 0.26 | 320 | 0 | 65 | 407 +--------+-------+-----------+---------+---------+--------+---------+ 409 Table 2: Monotonic Timestamps 411 +--------+-------+-----------+---------+---------+--------+---------+ 412 | Capt. | Clock | RTP | Arrival | Transit | Jitter | Average | 413 | time | rate | timestamp | time | | | jitter | 414 +--------+-------+-----------+---------+---------+--------+---------+ 415 | 0 | 8000 | 0 | 0.1 | 800 | | | 416 | | | | | | | | 417 | 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 | 418 | | | | | | | | 419 | 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 | 420 | | | | | | | | 421 | 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 | 422 | | | | | | | | 423 | 0.08 | 16000 | 1280 | 0.18 | 1600 | 0 | 0 | 424 | | | | | | | | 425 | 0.1 | 16000 | 1600 | 0.2 | 1600 | 0 | 0 | 426 | | | | | | | | 427 | 0.12 | 16000 | 1920 | 0.22 | 1600 | 0 | 0 | 428 | | | | | | | | 429 | 0.14 | 8000 | 1120 | 0.24 | 800 | 0 | 0 | 430 | | | | | | | | 431 | 0.16 | 8000 | 1280 | 0.26 | 800 | 0 | 0 | 432 +--------+-------+-----------+---------+---------+--------+---------+ 434 Table 3: Non-monotonic Timestamps 436 +--------+-------+-----------+---------+---------+--------+---------+ 437 | Capt. | Clock | RTP | Arrival | Transit | Jitter | Average | 438 | time | rate | timestamp | time | | | jitter | 439 +--------+-------+-----------+---------+---------+--------+---------+ 440 | 0 | 8000 | 0 | 0.1 | 800 | | | 441 | | | | | | | | 442 | 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 | 443 | | | | | | | | 444 | 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 | 445 | | | | | | | | 446 | 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 | 447 | | | | | | | | 448 | 0.08 | 16000 | 640 | 0.18 | 1600 | 0 | 0 | 449 | | | | | | | | 450 | 0.1 | 16000 | 960 | 0.2 | 1600 | 0 | 0 | 451 | | | | | | | | 452 | 0.12 | 16000 | 1280 | 0.22 | 1600 | 0 | 0 | 453 | | | | | | | | 454 | 0.14 | 8000 | 1600 | 0.24 | 320 | 0 | 0 | 455 | | | | | | | | 456 | 0.16 | 8000 | 1760 | 0.26 | 320 | 0 | 0 | 457 +--------+-------+-----------+---------+---------+--------+---------+ 459 Table 4: Recommended Method for RTP Sender (without RTCP) 461 Appendix B. Using a Fixed Clock Rate 463 An alternate way of fixing the multiple clock rates issue was 464 proposed by Wenger & Perkins [I-D.ietf-avt-variable-rate-audio]. 465 This document proposed to define a unified clock rate, but the 466 proposal was rejected at IETF 61. 468 Appendix C. Behavior of Legacy Implementations 470 C.1. libccrtp 2.0.2 472 This library uses the formula described in Section 3.2.2. 474 Note that this library uses gettimeofday(2) which is not guaranteed 475 to increment monotonically, like when the clock is adjusted by NTP. 477 C.2. libmediastreamer0 2.6.0 479 This library (which uses the oRTP library) uses the formula described 480 in Section 3.2.2. 482 Note that in some environments this library uses gettimeofday(2) 483 which is not guaranteed to increment monotonically. 485 C.3. libpjmedia 1.0 487 This library uses the formula described in Section 3.2.2. 489 C.4. Android RTP stack 4.0.3 491 This library changes the SSRC each time the format changes, as 492 described in Section 3.1. 494 Authors' Addresses 496 Marc Petit-Huguenin 497 Impedance Mismatch 499 Email: petithug@acm.org 501 Glen Zorn (editor) 502 Network Zen 503 227/358 Thanon Sanphawut 504 Bang Na, Bangkok 10260 505 Thailand 507 Phone: +66 (0) 8-1000-4155 508 Email: glenzorn@gmail.com