Network Working Group M. Petit-Huguenin Internet-Draft Unaffiliated Updates: 3550 (if approved) G. Zorn, Ed. Intended status: Standards Track Network Zen Expires: April 25, 2013 October 22, 2012 Support for Multiple Clock Rates in an RTP Session draft-ietf-avtext-multiple-clock-rates-06 Abstract This document clarifies the RTP specification when different clock rates are used in an RTP session. It also provides guidance on how to interoperate with legacy RTP implementations that use multiple clock rates. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 25, 2013. Copyright Notice Copyright (c) 2012 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. Petit-Huguenin & Zorn Expires April 25, 2013 [Page 1] Internet-Draft Multiple Clock Rates October 2012 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Legacy RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Different SSRC . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Same SSRC . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2.1. Monotonic timestamps . . . . . . . . . . . . . . . . . 5 3.2.2. Non-monotonic timestamps . . . . . . . . . . . . . . . 6 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. RTP Sender (with RTCP) . . . . . . . . . . . . . . . . . . 7 4.2. RTP Sender (without RTCP) . . . . . . . . . . . . . . . . 7 4.3. RTP Receiver . . . . . . . . . . . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 9 Appendix A. Using a Fixed Clock Rate . . . . . . . . . . . . . . 9 Appendix B. Behavior of Legacy Implementations . . . . . . . . . 10 B.1. libccrtp 2.0.2 . . . . . . . . . . . . . . . . . . . . . . 10 B.2. libmediastreamer0 2.6.0 . . . . . . . . . . . . . . . . . 10 B.3. libpjmedia 1.0 . . . . . . . . . . . . . . . . . . . . . . 10 B.4. Android RTP stack 4.0.3 . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Petit-Huguenin & Zorn Expires April 25, 2013 [Page 2] Internet-Draft Multiple Clock Rates October 2012 1. Introduction The clock rate is a parameter of the payload format. It is often defined as been the same as the sampling rate but it is not always the case (see e.g. the G722 and MPA audio codecs [RFC3551]). An RTP sender can switch between different payloads during the lifetime of an RTP session and because clock rates are defined by payload types, it is possible that the clock rate also varies during an RTP session. Schulzrinne, et al. [RFC3550] lists using multiple clock rates as one of the reasons to not use different payloads on the same SSRC but unfortunately this advice was not always followed and some RTP implementations change the payload in the same SSRC even if the different payloads use different clock rates. This creates three problems: o The method used to calculate the RTP timestamp field in an RTP packet is underspecified. o When the same SSRC is used for different clock rates, it is difficult to know what clock rate was used for the RTP timestamp field in an RTCP SR packet. o When the same SSRC is used for different clock rates, it is difficult to know what clock rate was used for the interarrival jitter field in an RTCP RR packet. Table 1 contains a non-exhaustive list of fields in RTCP packets that uses a clock rate as unit: +---------------------+------------------+-----------+ | Field name | RTCP packet type | Reference | +---------------------+------------------+-----------+ | RTP timestamp | SR | [RFC3550] | | Interarrival jitter | RR | [RFC3550] | | min_jitter | XR Summary Block | [RFC3611] | | max_jitter | XR Summary Block | [RFC3611] | | mean_jitter | XR Summary Block | [RFC3611] | | dev_jitter | XR Summary Block | [RFC3611] | | Interarrival jitter | IJ | [RFC5450] | | RTP timestamp | SMPTETC | [RFC5484] | | Jitter | RSI Jitter Block | [RFC5760] | | Median jitter | RSI Stats Block | [RFC5760] | +---------------------+------------------+-----------+ Table 1 Petit-Huguenin & Zorn Expires April 25, 2013 [Page 3] Internet-Draft Multiple Clock Rates October 2012 This document first tries to list in Section 3 and subsections all of the algorithms known to be used in existing RTP implementations at the time of writing. These sections are not normative. Section 4 and subsections then recommend a unique algorithm that modifies RFC 3550. These sections are normative. 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]. In addition, this document uses the following terms: Clock rate The multiplier used to convert from a wallclock value in seconds to an equivalent RTP timestamp value (without the fixed random offset). Note that RFC 3550 uses various terms like "clock frequency", "media clock rate", "timestamp unit", "timestamp frequency", and "RTP timestamp clock rate" as synonymous to clock rate. RTP Sender A logical network element that sends RTP packets, sends RTCP SR packets, and receives RTCP RR packets. RTP Receiver A logical network element that receives RTP packets, receives RTCP SR packets, and sends RTCP RR packets. 3. Legacy RTP The following sections describe the various ways legacy RTP implementations behave when multiple clock rates are used. Legacy RTP refers to RFC 3550 without the modifications introduced by this document. 3.1. Different SSRC One way of managing multiple clock rates is to use a different SSRC for each different clock rate, as in this case there is no ambiguity on the clock rate used by fields in the RTCP packets. This method also seems to be the original intent of RTP as can be deduced from points 2 and 3 of section 5.2 of RFC 3550. On the other hand changing the SSRC can be a problem for some implementations designed to work only with unicast IP addresses, where having multiple SSRCs is considered a corner case. Lip Petit-Huguenin & Zorn Expires April 25, 2013 [Page 4] Internet-Draft Multiple Clock Rates October 2012 synchronization can also be a problem in the interval between the beginning of the new stream and the first RTCP SR packet. This is not different than what happen at the beginning of the RTP session but it can be more annoying for the end-user. 3.2. Same SSRC The simplest way of managing multiple clock rates is to use the same SSRC for all the payload types regardless of the clock rates. Unfortunately there is no clear definition on how the RTP timestamp should be calculated in this case. The following subsections present the algorithms used in the field. 3.2.1. Monotonic timestamps This method of calculating the RTP timestamp ensures that the value increases monotonically. The formula used by this method is as follows: timestamp = previous_timestamp + (current_capture_time - previous_capture_time) * current_clock_rate The problem with this method is that the jitter calculation on the receiving side gives an invalid result during the transition between two clock rates, as shown in Table 2. The capture and arrival time are in seconds, starting at the beginning of the capture of the first packet; clock rate is in Hz; the RTP timestamp does not include the random offset; the transit, jitter, and average jitter use the clock rate as unit. +-------+-------+-----------+---------+---------+--------+----------+ | Capt. | Clock | RTP | Arrival | Transit | Jitter | Average | | time | rate | timestamp | time | | | jitter | +-------+-------+-----------+---------+---------+--------+----------+ | 0 | 8000 | 0 | 0.1 | 800 | | | | 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 | | 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 | | 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 | | 0.08 | 16000 | 800 | 0.18 | 2080 | 480 | 30 | | 0.1 | 16000 | 1120 | 0.2 | 2080 | 0 | 28 | | 0.12 | 16000 | 1440 | 0.22 | 2080 | 0 | 26 | | 0.14 | 8000 | 1600 | 0.24 | 320 | 720 | 70 | | 0.16 | 8000 | 1760 | 0.26 | 320 | 0 | 65 | +-------+-------+-----------+---------+---------+--------+----------+ Table 2 Petit-Huguenin & Zorn Expires April 25, 2013 [Page 5] Internet-Draft Multiple Clock Rates October 2012 Calculating the correct transit time on the receiving side can be done by using the following formulas: 1. current_capture_time = (current_timestamp - previous_timestamp) / current_clock_rate + previous_capture_time 2. transit = current_clock_rate * (arrival_time - current_capture_time) 3. previous_capture_time = current_capture_time The main problem with this method, in addition to the fact that the jitter calculation described in RFC 3550 cannot be used, is that is it dependent on the previous RTP packets, packets that can be reordered or lost in the network. 3.2.2. Non-monotonic timestamps An alternate way of generating the RTP timestamps is to use the following formula: timestamp = capture_time * clock_rate With this formula, the jitter calculation is correct but the RTP timestamp values are no longer increasing monotonically as shown in Table 3. RFC 3550 states that "[t]he sampling instant MUST be derived from a clock that increments monotonically[...]" but nowhere says that the RTP timestamp must increment monotonically. +-------+-------+-----------+---------+---------+--------+----------+ | Capt. | Clock | RTP | Arrival | Transit | Jitter | Average | | time | rate | timestamp | time | | | jitter | +-------+-------+-----------+---------+---------+--------+----------+ | 0 | 8000 | 0 | 0.1 | 800 | | | | 0.02 | 8000 | 160 | 0.12 | 800 | 0 | 0 | | 0.04 | 8000 | 320 | 0.14 | 800 | 0 | 0 | | 0.06 | 8000 | 480 | 0.16 | 800 | 0 | 0 | | 0.08 | 16000 | 1280 | 0.18 | 1600 | 0 | 0 | | 0.1 | 16000 | 1600 | 0.2 | 1600 | 0 | 0 | | 0.12 | 16000 | 1920 | 0.22 | 1600 | 0 | 0 | | 0.14 | 8000 | 1120 | 0.24 | 800 | 0 | 0 | | 0.16 | 8000 | 1280 | 0.26 | 800 | 0 | 0 | +-------+-------+-----------+---------+---------+--------+----------+ Table 3 The advantage with this method is that it works with the jitter calculation described in RFC 3550, as long as the correct clock rates Petit-Huguenin & Zorn Expires April 25, 2013 [Page 6] Internet-Draft Multiple Clock Rates October 2012 are used. It seems that this is what most implementations are using. 4. Recommendations The following subsections describe behavioral recommendations for RTP senders (with and without RTCP) and RTP receivers. 4.1. RTP Sender (with RTCP) An RTP Sender with RTCP turned on MUST use a different SSRC for each different clock rate. An RTCP BYE MUST be sent and a new SSRC MUST be used if the clock rate switches back to a value already seen in the RTP stream. To accelerate lip synchronization, the next compound RTCP packet sent by the RTP sender MUST contain multiple SR packets, the first one containing the mapping for the current clock rate and the next SR packets containing the mapping for the other clock rates seen during the last period. The RTP extension defined in Perkins & Schierl [RFC6051] MAY be used to accelerate the synchronization. 4.2. RTP Sender (without RTCP) An RTP Sender with RTCP turned off (i.e. by setting the RS and RR bandwidth modifiers [RFC3556] to 0) SHOULD use a different SSRC for each different clock rate but MAY use different clock rates on the same SSRC as long as the RTP timestamp is calculated as explained below: Each time the clock rate changes, the start_offset and capture_start values are calculated with the following formulas: start_offset += (capture_time - capture_start) * previous_clock_rate capture_start = capture_time For the first RTP packet, the values are initialized with the following values: start_offset = random_initial_offset capture_start = capture_time After eventually updating these values, the RTP timestamp is calculated with the following formula: timestamp = (capture_time - capture_start) * clock_rate Petit-Huguenin & Zorn Expires April 25, 2013 [Page 7] Internet-Draft Multiple Clock Rates October 2012 + start_offset Note that in all the formulas, capture_time is the first instant the new timestamp rate is used. 4.3. RTP Receiver An RTP Receiver MUST calculate the jitter using the following formula: D(i,j) = (arrival_time_j * clock_rate_i - timestamp_j) - (arrival_time_i * clock_rate_i - timestamp_i) An RTP Receiver MUST be able to handle a compound RTCP packet with multiple SR packets. 5. Security Considerations This document is not believed to effect the security of the RTP sessions described here in any way. 6. IANA Considerations This document requires no IANA actions. 7. Acknowledgements Thanks to Colin Perkins, Ali C. Begen, Harald Alvestrand, Qin Wu and Magnus Westerlund for their comments, suggestions and questions that helped to improve this document. Thanks to Robert Sparks and the attendees of SIPit 26 for the survey on multiple clock rates interoperability. This document was written with the xml2rfc tool described in Rose [RFC2629]. 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Petit-Huguenin & Zorn Expires April 25, 2013 [Page 8] Internet-Draft Multiple Clock Rates October 2012 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. 8.2. Informative References [I-D.ietf-avt-variable-rate-audio] Wenger, S. and C. Perkins, "RTP Timestamp Frequency for Variable Rate Audio Codecs", draft-ietf-avt-variable-rate-audio-00 (work in progress), October 2004. [RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, June 1999. [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003. [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003. [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [RFC5450] Singer, D. and H. Desineni, "Transmission Time Offsets in RTP Streams", RFC 5450, March 2009. [RFC5484] Singer, D., "Associating Time-Codes with RTP Streams", RFC 5484, March 2009. [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP Flows", RFC 6051, November 2010. Appendix A. Using a Fixed Clock Rate An alternate way of fixing the multiple clock rates issue was proposed in [I-D.ietf-avt-variable-rate-audio]. This document proposed to define a unified clock rate, but the proposal was rejected at IETF 61. Petit-Huguenin & Zorn Expires April 25, 2013 [Page 9] Internet-Draft Multiple Clock Rates October 2012 Appendix B. Behavior of Legacy Implementations B.1. libccrtp 2.0.2 This library uses the formula described in Section 3.2.2. Note that this library uses gettimeofday(2) which is not guaranteed to increment monotonically, like when the clock is adjusted by NTP. B.2. libmediastreamer0 2.6.0 This library (which uses the oRTP library) uses the formula described in Section 3.2.2. Note that in some environments this library uses gettimeofday(2) which is not guaranteed to increment monotonically. B.3. libpjmedia 1.0 This library uses the formula described in Section 3.2.2. B.4. Android RTP stack 4.0.3 This library changes the SSRC each time the format changes, as described in Section 3.1. Authors' Addresses Marc Petit-Huguenin Unaffiliated Email: petithug@acm.org Glen Zorn (editor) Network Zen 227/358 Thanon Sanphawut Bang Na, Bangkok 10260 Thailand Phone: +66 (0) 909-201060 Email: glenzorn@gmail.com Petit-Huguenin & Zorn Expires April 25, 2013 [Page 10]