Re: [AVT] Fwd: I-D Action:draft-perkins-avt-rapid-rtp-sync-02.txt

Hi,

OK with this draft that improve the synchronization time between multiple sessions. I guess its usage is larger than SVC and multi-description codecs.
Some remarks to consider in this document.
1) If a multicast (or broadcast) session has no feedback channel, the source must send peiodically synchronization messages, with a short period (as ~200ms or less, for example).
2) RTP header extension adds 12 bytes. The source must reserve them in all packets in order to avoid fragmentation at IP level, because (I suppose?) insertion of this extension header may be independant of the RTP payload building. This is very bad when packet losses occur. So could be possible to have RTP packets that contain only header and extension for synchronization?
3) The receiver must have confidence with synchronization information it receives. It is the responsability of the sources to give precize information.
4) About variant A and B, it depends the precision that synchronization needs: a 16 bit information for ntp fraction and 8 bits for ntp seconds could be sufficient, because 16 bit fraction give 15 micro-second precision, and period between synchronization messages never exceed 250 seconds. Could a 32 bit word contain all these information? This may be usefull if extension is in a RTP message with data, according to IP fragmentation.

Regards,

Gerard BABONNEAU

-----Message d'origine-----
De : avt-bounces@ietf.org [mailto:avt-bounces@ietf.org] De la part de Colin Perkins
Envoyé : vendredi 13 février 2009 18:39
À : IETF AVT WG
Objet : [AVT] Fwd: I-D Action:draft-perkins-avt-rapid-rtp-sync-02.txt

This draft is a merger between draft-perkins-avt-rapid-rtp- sync-01.txt and draft-schierl-avt-rtp-ntp-for-layered-codecs-00.txt.

Please send any comments to the list. We'd like this to be considered for adoption as an AVT working group draft.

Colin

Begin forwarded message:
> From: Internet-Drafts@ietf.org
> Date: 13 February 2009 14:45:01 GMT
> To: i-d-announce@ietf.org
> Subject: I-D Action:draft-perkins-avt-rapid-rtp-sync-02.txt
> Reply-To: internet-drafts@ietf.org
>
> A New Internet-Draft is available from the on-line Internet-Drafts 
> directories.
>
> 	Title           : Rapid Synchronisation of RTP Flows
> 	Author(s)       : C. Perkins, T. Schierl
> 	Filename        : draft-perkins-avt-rapid-rtp-sync-02.txt
> 	Pages           : 18
> 	Date            : 2009-02-13
>
> This memo outlines how RTP multimedia 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 initial 
> synchronisation delay.  This increase in delay can be unacceptable to 
> some applications that use layered and/or multi-description codecs.
>
> This memo updates the RTP Control Protocol (RTCP) timing rules to 
> reduce the initial synchronisation delay for SSM sessions.  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.  Two 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.
>
> A URL for this Internet-Draft is:
> http://www.ietf.org/internet-drafts/draft-perkins-avt-rapid-rtp-
> sync-02.txt

-- 
Colin Perkins
http://csperkins.org/

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