Colin, Thomas, all,the draft is describing the proposed header extension and RTCP SR request quite good, however:
Isn't the NTP timestamp generation subject to the same problems as in RTCP? I could easily send an RTCP SR as well for every RTP packet within a sampling instance of one (in terms of RTP time), i.e. there could be an RTP packet and an RTCP SR packet containing the same RTP timestamp if I were able to generate them virtually at the same time (just a few ns away). Clearly, this is not datarate-efficient nor sensible, but I don't see a big difference to the proposed approach.
So what about the source for the NTP time: Should the NTP time be filtered at the sender before inclusion into the HDR_EXT? Because if not, the NTP timestamp would either be only related to the RTP timeline (bad - then we have two different timelines, one via RTCP, the other in the HDR_EXT) or be jittery (bad because not instantly useful, though the same timeline as RTCP).
An additional comment on the draft is that section 4.3 is quite complex and hard to understand, though (I think) correct.
And the last one: What was wrong again with signaling/knowing that two streams come from the same source/encoder and would be perfectly aligned after reception of one NTP-RTP mapping per stream or even directly aligned (same RTP timeline)? I tried to find the answer in the list archive, but maybe you can give me a hint.
Cheers - Stefan Colin Perkins schrieb:
The changes in this version are primarily editorial; we're aware that not all the open issues have been addressed yet. Any further comments are welcomed.Chairs: we'd like this to be considered as an AVT working group draft, as indicated privately.Cheers, Colin Begin forwarded message:From: Internet-Drafts at ietf.org Date: 9 March 2009 17:15:02 GMT To: i-d-announce at ietf.org Subject: I-D Action:draft-perkins-avt-rapid-rtp-sync-03.txt Reply-To: internet-drafts at ietf.orgA 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-03.txt Pages : 17 Date : 2009-03-09 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-03.txtInternet-Drafts are also available by anonymous FTP at: ftp://ftp.ietf.org/internet-drafts/ Below is the data which will enable a MIME compliant mail reader implementation to automatically retrieve the ASCII version of the Internet-Draft. Content-Type: text/plain Content-ID: <2009-03-09100610.I-D at ietf.org> _______________________________________________ I-D-Announce mailing list I-D-Announce at ietf.org https://www.ietf.org/mailman/listinfo/i-d-announce Internet-Draft directories: http://www.ietf.org/shadow.html or ftp://ftp.ietf.org/ietf/1shadow-sites.txt_______________________________________________ Audio/Video Transport Working Group avt at ietf.org https://www.ietf.org/mailman/listinfo/avt
-- Dipl.-Ing. Stefan Döhla | Email: stefan.doehla at iis.fraunhofer.de Multimedia Transport | Phone: +49 (0)9131 776 6042 (!NEW!) Audio Department | Fax: +49 (0)9131 776 6099 (!NEW!) Fraunhofer IIS | Am Wolfmantel 33 | 91058 Erlangen, Germany | Web: http://www.iis.fraunhofer.de/amm