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START Network Working Group Audio-Video Transport Working Group Request for Comments: 1889 H. Schulzrinne Category: Standards Track GMD Fokus S. Casner Precept Software, Inc. R. Frederick Xerox Palo Alto Research Center V. Jacobson Lawrence Berkeley National Laboratory January 1996 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. 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. Table of Contents 1. Introduction ........................................ 3 2. RTP Use Scenarios ................................... 5 2.1 Simple Multicast Audio Conference ................... 5 2.2 Audio and Video Conference .......................... 6 2.3 Mixers and Translators .............................. 6 3. Definitions ......................................... 7 4. Byte Order, Alignment, and Time Format .............. 9 5. RTP Data Transfer Protocol .......................... 10 5.1 RTP Fixed Header Fields ............................. 10 5.2 Multiplexing RTP Sessions ........................... 13 Schulzrinne, et al Standards Track [Page 1]  RFC 1889 RTP January 1996 5.3 Profile-Specific Modifications to the RTP Header..... 14 5.3.1 RTP Header Extension ................................ 14 6. RTP Control Protocol -- RTCP ........................ 15 6.1 RTCP Packet Format .................................. 17 6.2 RTCP Transmission Interval .......................... 19 6.2.1 Maintaining the number of session members ........... 21 6.2.2 Allocation of source description bandwidth .......... 21 6.3 Sender and Receiver Reports ......................... 22 6.3.1 SR: Sender report RTCP packet ....................... 23 6.3.2 RR: Receiver report RTCP packet ..................... 28 6.3.3 Extending the sender and receiver reports ........... 29 6.3.4 Analyzing sender and receiver reports ............... 29 6.4 SDES: Source description RTCP packet ................ 31 6.4.1 CNAME: Canonical end-point identifier SDES item ..... 32 6.4.2 NAME: User name SDES item ........................... 34 6.4.3 EMAIL: Electronic mail address SDES item ............ 34 6.4.4 PHONE: Phone number SDES item ....................... 34 6.4.5 LOC: Geographic user location SDES item ............. 35 6.4.6 TOOL: Application or tool name SDES item ............ 35 6.4.7 NOTE: Notice/status SDES item ....................... 35 6.4.8 PRIV: Private extensions SDES item .................. 36 6.5 BYE: Goodbye RTCP packet ............................ 37 6.6 APP: Application-defined RTCP packet ................ 38 7. RTP Translators and Mixers .......................... 39 7.1 General Description ................................. 39 7.2 RTCP Processing in Translators ...................... 41 7.3 RTCP Processing in Mixers ........................... 43 7.4 Cascaded Mixers ..................................... 44 8. SSRC Identifier Allocation and Use .................. 44 8.1 Probability of Collision ............................ 44 8.2 Collision Resolution and Loop Detection ............. 45 9. Security ............................................ 49 9.1 Confidentiality ..................................... 49 9.2 Authentication and Message Integrity ................ 50 10. RTP over Network and Transport Protocols ............ 51 11. Summary of Protocol Constants ....................... 51 11.1 RTCP packet types ................................... 52 11.2 SDES types .......................................... 52 12. RTP Profiles and Payload Format Specifications ...... 53 A. Algorithms .......................................... 56 A.1 RTP Data Header Validity Checks ..................... 59 A.2 RTCP Header Validity Checks ......................... 63 A.3 Determining the Number of RTP Packets Expected and Lost ................................................ 63 A.4 Generating SDES RTCP Packets ........................ 64 A.5 Parsing RTCP SDES Packets ........................... 65 A.6 Generating a Random 32-bit Identifier ............... 66 A.7 Computing the RTCP Transmission Interval ............ 68 Schulzrinne, et al Standards Track [Page 2]  RFC 1889 RTP January 1996 A.8 Estimating the Interarrival Jitter .................. 71 B. Security Considerations ............................. 72 C. Addresses of Authors ................................ 72 D. Bibliography ........................................ 73 1. Introduction This memorandum specifies the real-time transport protocol (RTP), which provides end-to-end delivery services for data with real-time characteristics, such as interactive audio and video. Those services include payload type identification, sequence numbering, timestamping and delivery monitoring. Applications typically run RTP on top of UDP to make use of its multiplexing and checksum services; both protocols contribute parts of the transport protocol functionality. However, RTP may be used with other suitable underlying network or transport protocols (see Section 10). RTP supports data transfer to multiple destinations using multicast distribution if provided by the underlying network. Note that RTP itself does not provide any mechanism to ensure timely delivery or provide other quality-of-service guarantees, but relies on lower-layer services to do so. It does not guarantee delivery or prevent out-of-order delivery, nor does it assume that the underlying network is reliable and delivers packets in sequence. The sequence numbers included in RTP allow the receiver to reconstruct the sender's packet