Rapid Synchronisation of RTP Flows
RFC 6051

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

Abstract

   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
   http://www.rfc-editor.org/info/rfc6051.

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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Synchronisation of RTP Flows ....................................4
      2.1. Initial Synchronisation Delay ..............................5
           2.1.1. Unicast Sessions ....................................5
           2.1.2. Source-Specific Multicast (SSM) Sessions ............6
           2.1.3. Any-Source Multicast (ASM) Sessions .................7
           2.1.4. Discussion ..........................................8
      2.2. Synchronisation for Late Joiners ...........................9
   3. Reducing RTP Synchronisation Delays ............................10
      3.1. Reduced Initial RTCP Interval for SSM Senders .............10
      3.2. Rapid Resynchronisation Request ...........................10
      3.3. In-Band Delivery of Synchronisation Metadata ..............11
   4. Application to Decoding Order Recovery in Layered Codecs .......14
      4.1. In-Band Synchronisation for Decoding Order Recovery .......14
      4.2. Timestamp-Based Decoding Order Recovery ...................15
      4.3. Example ...................................................16
   5. Security Considerations ........................................18
   6. IANA Considerations ............................................19
   7. Acknowledgements ...............................................19
   8. References .....................................................20
      8.1. Normative References ......................................20
      8.2. Informative References ....................................20

Perkins & Schierl            Standards Track                    [Page 2]
RFC 6051                   RTP Synchronisation             November 2010

1.  Introduction