[Live-devel] RTCP SR clock sync diff

[Mike Frantzen]

> This is incorrect.  The information in incoming RTCP "SR" packets is used
> to generate presentation times from incoming RTP packets' timestamps.  These
> presentation times - like the RTP timestamps themselves - are (necessarily)
> based on the sender's clock (because that was the only clock available to
> the entity (the sender) that created the presentation times).


Please point me at the section of RFC3550 that says that the RTCP SR NTP
timestamp is to be used for a presentation time.

I see it stating that the sender can use any time source for the NTP
timestamp.  It says that it "may be used for intra- and inter-media
synchronization for sources whose NTP timestamps are synchronized" and "may
be used by media-independent receivers to estimate the nominal RTP clock
frequency".

And on page 14 when talking about the RTP/NTP timestamp pairs: "the purpose
is to allow synchronized presentation of all media sampled at the same
time."

Reading too much into the NTP timestamp violates the fundamental principle
of network protocols "be strict about what you send and permissive about
what you accept."  By immediately adjusting the presentation time based on
the NTP timestamp the presentation times become non-monotonic and the
receiver does not know much much when to display the new stream in relation
to the old stream.

.mike
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