> Dominique Fober writes: > > [... algorithm in Appendix B of NOSSDAV paper, does it have clock skew > issues?]
Note the use of RTCP in this algorithm -- when new RTCP packets arrive, we check their timestamp to see if they are on time, and if so we update the ACO model parameters. This on-going parameter re-estimation (about once per 5 seconds for sessions with small numbers of receivers) handles the garden-variety "sender and receiver clocks unsynced" issue pretty well -- in practice on CalREN 2, we only see the "late window exhaustion" happen for rarer network and host events that mess up local timing. > Are RTP timestamped packets enough ? I'm not sure ... Another feature of RTCP can come in handy here too -- each RTCP packet supplies both the RTP timestamp and its corresponding wall-clock time, using a 64-bit NTP encoding of time. RTP timestamps are supposed to be "nominal", not "actual" -- the way RTP systems are supposed to get a handle on skew, sync, and other issues is to use these NTP/RTP pairs in RTCP to maintain sync. We didn't use this in our simple "Appendix B" algorithm, but a more sophisticated algorithm could ... > However, fixing the limits (the skew tolerance and the corresponding > time limit) would be useful for the protocol implementation). This is one possible way to proceed -- another is to take a look at whether normative "late/ontime" semantics need to be in the MWPP I-D at all. At Salt Lake, this was another issue that came up -- people were skeptical how needed this normative text was, given that the method of deciding late/ontime was totally implementation dependent. It might be better to simply state that sound playout is implementation dependent, and offer non-normative references to the NOSSDAV paper, the RTESP-Wedel.pdf, and perhaps others as examples for implementors to consider ... ------------------------------------------------------------------------- John Lazzaro -- Research Specialist -- CS Division -- EECS -- UC Berkeley lazzaro [at] cs [dot] berkeley [dot] edu www.cs.berkeley.edu/~lazzaro -------------------------------------------------------------------------
