Re: [AVT] time alignment capability

[Chuck Harrison <cfharr at erols.com>]

I agree with Joe that synchronizing system clocks is properly
dealt with separately from RTP. The RTP architecture includes
a concept of system time, used in RTCP messages, which we *call*
"NTP time" or "wallclock time" but which will presumably be
maintained by PTP or GPS where those are available.

The availability of a superbly accurate system clock throughout
the network does not directly address the primary issue I'm
discussing because real media sources and sinks are rarely phase
locked to the system clock. Nonetheless the fundamental concept
of RTP -- cross-timestamping the media clocks against the system
clock -- is very powerful and can be used as a tool to manage the
larger media synchronization tasks even in the face of network
delay and jitter.

Peace,
  Chuck

Joe Kelsey wrote:
> 
[...]
> So, essentially this problem is completely outside of RTP.  If
> you require synchronized devices, you have to use something
> outside of RTP.  Adding this to RTP would be a complete waste of time.
> 
> /Joe
> 
> >>> "Tom-PT Taylor" <taylor at nortel.com> 3/2/2006 2:00 PM >>>
> So does it make sense to build this capability into RTCP? Or are you
> suggesting a separate protocol session between nodes?
> 
> Joe Kelsey wrote:
> > Fundamentally, neither the RTP time codes nor NTP is sufficient for precise synchronization.  If you want accurate and fast synchronization between devices, then you have to use something like PTP, since NTP takes much too long to synchronize and does not do it to a sufficient resolution.
> >
> > /Joe
> >
> >
> >>>>Chuck Harrison <cfharr at erols.com> 3/2/2006 1:32 PM >>>
> >
> > I believe that there *are* significant use cases for something
> > like Dave's "payload format independent capability". Long-time
> > participants in this forum will have heard my refrain before:
> >
> > RTP's original design never addressed or intended to address the
> > issue of long-term phase correlation among sender and receiver
> > sample clocks. In neither theory nor practice are RTP media
> > sources or sinks expected to be phase synchronous to NTP time,
> > and drift does occur.
> >
[...]

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