RTP: A Transport Protocol for Real-Time Applications
RFC 3550

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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