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Network Working Group H. Schulzrinne Request for Comments: 3550 Columbia University Obsoletes: 1889 S. Casner Category: Standards Track Packet Design R. Frederick Blue Coat Systems Inc. V. Jacobson Packet Design July 2003 RTP: A Transport Protocol for Real-Time Applications Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This memorandum describes RTP, the real-time transport protocol. RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of-service for real-time services. The data transport is augmented by a control protocol (RTCP) to allow monitoring of the data delivery in a manner scalable to large multicast networks, and to provide minimal control and identification functionality. RTP and RTCP are designed to be independent of the underlying transport and network layers. The protocol supports the use of RTP-level translators and mixers. Most of the text in this memorandum is identical to RFC 1889 which it obsoletes. There are no changes in the packet formats on the wire, only changes to the rules and algorithms governing how the protocol is used. The biggest change is an enhancement to the scalable timer algorithm for calculating when to send RTCP packets in order to minimize transmission in excess of the intended rate when many participants join a session simultaneously. Schulzrinne, et al. Standards Track [Page 1]  RFC 3550 RTP July 2003 Table of Contents 1. Introduction ................................................ 4 1.1 Terminology ............................................ 5 2. RTP Use Scenarios ........................................... 5 2.1 Simple Multicast Audio Conference ...................... 6 2.2 Audio and Video Conference ............................. 7 2.3 Mixers and Translators ................................. 7 2.4 Layered Encodings ...................................... 8 3. Definitions ................................................. 8 4. Byte Order, Alignment, and Time Format ...................... 12 5. RTP Data Transfer Protocol .................................. 13 5.1 RTP Fixed Header Fields ................................ 13 5.2 Multiplexing RTP Sessions .............................. 16 5.3 Profile-Specific Modifications to the RTP Header ....... 18 5.3.1 RTP Header Extension ............................ 18 6. RTP Control Protocol -- RTCP ................................ 19 6.1 RTCP Packet Format ..................................... 21 6.2 RTCP Transmission Interval ............................. 24 6.2.1 Maintaining the Number of Session Members ....... 28 6.3 RTCP Packet Send and Receive Rules ..................... 28 6.3.1 Computing the RTCP Transmission Interval ........ 29 6.3.2 Initialization .................................. 30 6.3.3 Receiving an RTP or Non-BYE RTCP Packet ......... 31 6.3.4 Receiving an RTCP BYE Packet .................... 31 6.3.5 Timing Out an SSRC .............................. 32 6.3.6 Expiration of Transmission Timer ................ 32 6.3.7 Transmitting a BYE Packet ....................... 33 6.3.8 Updating we_sent ................................ 34 6.3.9 Allocation of Source Description Bandwidth ...... 34 6.4 Sender and Receiver Reports ............................ 35 6.4.1 SR: Sender Report RTCP Packet ................... 36 6.4.2 RR: Receiver Report RTCP Packet ................. 42 6.4.3 Extending the Sender and Receiver Reports ....... 42 6.4.4 Analyzing Sender and Receiver Reports ........... 43 6.5 SDES: Source Description RTCP Packet ................... 45 6.5.1 CNAME: Canonical End-Point Identifier SDES Item . 46 6.5.2 NAME: User Name SDES Item ....................... 48 6.5.3 EMAIL: Electronic Mail Address SDES Item ........ 48 6.5.4 PHONE: Phone Number SDES Item ................... 49 6.5.5 LOC: Geographic User Location SDES Item ......... 49 6.5.6 TOOL: Application or Tool Name SDES Item ........ 49 6.5.7 NOTE: Notice/Status SDES Item ................... 50 6.5.8 PRIV: Private Extensions SDES Item .............. 50 6.6 BYE: Goodbye RTCP Packet ............................... 51 6.7 APP: Application-Defined RTCP Packet ................... 52 7. RTP Translators and Mixers .................................. 53 7.1 General Description .................................... 53 Schulzrinne, et al. Standards Track [Page 2]  RFC 3550 RTP July 2003 7.2 RTCP Processing in Translators ......................... 55 7.3 RTCP Processing in Mixers .............................. 57 7.4 Cascaded Mixers ........................................ 58 8. SSRC Identifier Allocation and Use .......................... 59 8.1 Probability of Collision ............................... 59 8.2 Collision Resolution and Loop Detection ................ 60 8.3 Use with Layered Encodings ............................. 64 9. Security .................................................... 65 9.1 Confidentiality ........................................ 65 9.2 Authentication and Message Integrity ................... 67 10. Congestion Control .......................................... 67 11. RTP over Network and Transport Protocols .................... 68 12. Summary of Protocol Constants ............................... 69 12.1 RTCP Packet Types ...................................... 70 12.2 SDES Types ............................................. 70 13. RTP Profiles and Payload Format Specifications .............. 71 14. Security Considerations ..................................... 73 15. IANA Considerations ......................................... 73 16. Intellectual Property Rights Statement ...................... 74 17. Acknowledgments ............................................. 74 Appendix A. Algorithms ........................................ 75 Appendix A.1 RTP Data Header Validity Checks ................... 78 Appendix A.2 RTCP Header Validity Checks ....................... 82 Appendix A.3 Determining Number of Packets Expected and Lost ... 83 Appendix A.4 Generating RTCP SDES Packets ...................... 84 Appendix A.5 Parsing RTCP SDES Packet