Support for multiple clock rates in an RTP session
draft-petithuguenin-avt-multiple-clock-rates-01

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

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Copyright Notice

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Table of Contents

1.  Introduction
2.  Legacy RTP
    2.1.  Different SSRC
    2.2.  Same SSRC
        2.2.1.  Monotonic timestamps
        2.2.2.  Non-monotonic timestamps
3.  Terminology
4.  RTP Sender
5.  RTP Receiver
6.  Interoperability Analysis
    6.1.  Legacy RTP Sender using different SSRC sending to new RTP Receiver
    6.2.  Legacy RTP Sender using same SSRC with monotonic timestamps sending to new RTP Receiver
    6.3.  Legacy RTP Sender using same SSRC with non-monotonic timestamps sending to new RTP Receiver
    6.4.  New RTP Sender using different SSRC sending to legacy RTP Receiver
    6.5.  New RTP Sender using different SSRC sending to new RTP Receiver
    6.6.  New RTP Sender using same SSRC with non-monotonic timestamps to legacy RTP Receiver
    6.7.  New RTP Sender using same SSRC with non-monotonic timestamps to new RTP Receiver
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
Appendix A.  Using a fixed clock rate
Appendix B.  Release notes
    B.1.  Modifications between -01 and -00
§  Author's Address

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 in [RFC3551] (Schulzrinne, H. and S. Casner, “RTP Profile for Audio and Video Conferences with Minimal Control,” July 2003.)).

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. RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [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:

• The method used to calculate the RTP timestamp field in an RTP packet is underspecified.
• 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.
• 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:

This document changes the RTP specification by recommending to use a different SSRC for each clock rate in most of the cases.

2.  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.

[[Perhaps SIPit would be a good place to collect data on the methods used by real implementations]]

2.1.  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 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.

2.2.  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 two variants.

2.2.1.  Monotonic timestamps

The most common method of calculating the RTP timestamp ensures that the value increases monotonically. The formula used by this method is as follow:

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 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.

Calculating the correct transit time on the receiving side can be done by using the following formulas:

(1)

current_time_capture = current_timestamp - previous_timestamp) / current_clock_rate + previous_time_capture

(2)

transit = current_clock_rate * (time_arrival - current_time_capture)

(3)

previous_time_capture = current_time_capture

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. But it seems that this is what most implementations are using.

2.2.2.  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.

The advantage with this method is that it works with the jitter calculation described in RFC 3550, a long as the correct clock rates are used.

3.  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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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.

4.  RTP Sender

An RTP Sender with RTCP turned off (i.e. by setting the RS and RR bandwidth modifiers defined in [RFC3556] (Casner, S., “Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth,” July 2003.) 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 described in Section 2.2.2 (Non-monotonic timestamps).

[[It is probably easier to implement for VoIP products that do not use RTCP and so do not care about lip synchronization or jitter calculation]]

An RTP Sender with RTCP turned on MUST use a different SSRC for each different clock rate.

[[Can the SSRC be reused when switching back to the old clock rate less than 2T? If not should a BYE be sent?]]

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.

[[Is it authorized by the RTCP syntax to have multiple SR in a compound packet?]]

The RTP extension defined in [RAPID‑SYNC] (Perkins, C. and T. Schierl, “Rapid Synchronisation of RTP Flows,” January 2010.) MAY be used to accelerate the synchronization.

5.  RTP Receiver

An RTP Receiver MUST be able to handle clock rate changes either on the same SSRC (Section 2.1 (Different SSRC)) or on different SSRC (Section 2.2.2 (Non-monotonic timestamps)).

[[What about legacy RTP implementations implementing the method in Section 2.2.1 (Monotonic timestamps)?]]

An RTP Receiver MUST be able to handle a compound RTCP packet with multiple SR packets.

For interoperability with legacy RTP implementations, an RTP receiver MAY use the information in two consecutive SR packets to calculate the clock rate used, i.e. if Ni is the NTP timestamp for the SR packet i, Ri the RTP timestamp for the SR packet i and Nj and Rj the NTP timestamp and RTP timestamp for the previous SR packet j, then the clock rate can be guessed as the closest to (Ri - Rj) / (Ni - Nj).

6.  Interoperability Analysis

The next subsections analyze the various combinations between legacy RTP implementations and RTP implementations that follow this document specifications.

6.1.  Legacy RTP Sender using different SSRC sending to new RTP Receiver

Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet.

6.2.  Legacy RTP Sender using same SSRC with monotonic timestamps sending to new RTP Receiver

The new RTP Receiver will not be able to rebuild the correct RTP timestamp so the jitter will be incorrect. Note that this is not different than if a legacy RTP Receiver is used.

6.3.  Legacy RTP Sender using same SSRC with non-monotonic timestamps sending to new RTP Receiver

TBD

6.4.  New RTP Sender using different SSRC sending to legacy RTP Receiver

Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet. Some legacy RTP implementations may have problems when receiving multiple SR packets.

6.5.  New RTP Sender using different SSRC sending to new RTP Receiver

Because a specific clock rate is associated to a specific SSRC, there is no ambiguity in the RTP timestamp received in the RTP packet or SR packet or in the jitter sent in the RR packet.

6.6.  New RTP Sender using same SSRC with non-monotonic timestamps to legacy RTP Receiver

Because this combination is used only when no RTCP packets are exchanged, there is no problem interpreting the RTCP field units. Some legacy RTP implementations may have problems if the jitter clock rates are not correctly managed.

6.7.  New RTP Sender using same SSRC with non-monotonic timestamps to new RTP Receiver

Because this combination is used only when no RTCP packets are exchanged, there is no problem interpreting the RTCP field units.

7.  Security Considerations

TBD

8.  IANA Considerations

No IANA considerations.

9.  Acknowledgements

Thanks to Colin Perkins and Ali C. Begen for their comments, suggestions and questions that helped to improve this document.

This document was written with the xml2rfc tool described in [RFC2629] (Rose, M., “Writing I-Ds and RFCs using XML,” June 1999.).

10.  References

10.1. Normative References

10.2. Informative References

Appendix A.  Using a fixed clock rate

An alternate way of fixing the multiple clock rates issue was proposed in [uRTR] (Wenger, S. and C. Perkins, “RTP Timestamp Frequency for Variable Rate Audio Codecs,” October 2004.). This document proposed to define a unified clock rate, but the proposal was rejected at IETF 61.

Appendix B.  Release notes

This section must be removed before publication as an RFC.

B.1.  Modifications between -01 and -00

• Complete rewrite as a Standard Track I-D modifying RFC 3550.

Author's Address