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Rapid Synchronisation of RTP Flows (RFC6051) Disclosure Number: IPCOM000201075D
Original Publication Date: 2010-Nov-01
Included in the Prior Art Database: 2010-Nov-08
Document File: 44 page(s) / 53K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

C. Perkins: AUTHOR [+2]


When using RTP to deliver multimedia content it's often necessary to synchronise playout of audio and video components of a presentation. This is achieved using information contained in RTP Control Protocol (RTCP) sender report (SR) packets [RFC3550]. These are sent periodically, and the components of a multimedia session cannot be synchronised until sufficient RTCP SR packets have been received for each RTP flow to allow the receiver to establish mappings between the media clock used for each RTP flow, and the common (NTP-format) reference clock used to establish synchronisation.

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Internet Engineering Task Force (IETF)                        C. Perkins Request for Comments: 6051                         University of Glasgow Updates: 3550                                                 T. Schierl Category: Standards Track                                 Fraunhofer HHI ISSN: 2070-1721                                            November 2010

                    Rapid Synchronisation of RTP Flows


   This memo outlines how RTP sessions are synchronised, and discusses    how rapidly such synchronisation can occur.  We show that most RTP    sessions can be synchronised immediately, but that the use of video    switching multipoint conference units (MCUs) or large source-specific    multicast (SSM) groups can greatly increase the synchronisation    delay.  This increase in delay can be unacceptable to some    applications that use layered and/or multi-description codecs.

   This memo introduces three mechanisms to reduce the synchronisation    delay for such sessions.  First, it updates the RTP Control Protocol    (RTCP) timing rules to reduce the initial synchronisation delay for    SSM sessions.  Second, a new feedback packet is defined for use with    the extended RTP profile for RTCP-based feedback (RTP/AVPF), allowing    video switching MCUs to rapidly request resynchronisation.  Finally,    new RTP header extensions are defined to allow rapid synchronisation    of late joiners, and guarantee correct timestamp-based decoding order    recovery for layered codecs in the presence of clock skew.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force    (IETF).  It represents the consensus of the IETF community.  It has    received public review and has been approved for publication by the    Internet Engineering Steering Group (IESG).  Further information on    Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,    and how to provide feedback on it may be obtained at

Perkins & Schierl            Standards Track                    [Page 1]
 RFC 6051                   RTP Synchronisation             November 2010

 Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the    document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal    Provisions Relating to IETF Documents    ( in effect on the date of    publication of this document.  Please review these documents    car...