Network Working Group M.P.H. Petit-Huguenin
Internet-Draft Impedance Mismatch
Updates: 3550 (if approved) G. Zorn, Ed.
Intended status: Standards Track Network Zen
Expires: October 24, 2013 April 22, 2013
Support for Multiple Clock Rates in an RTP Session
draft-ietf-avtext-multiple-clock-rates-09
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.
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Petit-Huguenin & Zorn Expires October 24, 2013 [Page 1]
Internet-Draft Multiple Clock Rates April 2013
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Legacy RTP . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Different SSRC . . . . . . . . . . . . . . . . . . . . . 4
3.2. Same SSRC . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1. Monotonic timestamps . . . . . . . . . . . . . . . . 5
3.2.2. Non-monotonic timestamps . . . . . . . . . . . . . . 5
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. RTP Sender (with RTCP) . . . . . . . . . . . . . . . . . 6
4.2. RTP Sender (without RTCP) . . . . . . . . . . . . . . . . 6
4.3. RTP Receiver . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Example Values . . . . . . . . . . . . . . . . . . . 9
Appendix B. Using a Fixed Clock Rate . . . . . . . . . . . . . . 11
Appendix C. Behavior of Legacy Implementations . . . . . . . . . 11
C.1. libccrtp 2.0.2 . . . . . . . . . . . . . . . . . . . . . 11
C.2. libmediastreamer0 2.6.0 . . . . . . . . . . . . . . . . . 11
C.3. libpjmedia 1.0 . . . . . . . . . . . . . . . . . . . . . 12
C.4. Android RTP stack 4.0.3 . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The clock rate is a parameter of the payload format as identified in
RTP and RTCP by the payload type value. It is often defined as being
the same as the sampling rate but that 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 format, it is possible that the clock rate will also vary
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 has not
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