[AVT] Why RTCP is needed for synch after long time in aggregated A/V RTSP session?

Dear Magnus,
Yes, we need. And thanks to this group, people could learn many things
about RTP timestamp as described in the Appendix B of rfc2326bis. Now I
found most major players have been changed to follow it; it was not in
the past.

As for the subject, I would appreciate if we add more text about "why"
in future; I found one sentence from rfc2326bis-04 but failed to find
why.
(page92)

I felt that Ross knows some empirical knowledge. So, I asked him.
Ross, do you mind if you share your experience with us? 

>From my 2 cents, if the given RTSP session is VOD, although the given
session is enough long, the additional work, checking RTCP would not be
necessary to our client codes.

I think Ross mentioned the live RTSP session, in which we need to take
into account more. I would like to learn :->

And inline:

Magnus wrote:
>Okay, we will log a request to clarify that RTP-Info only provides for 
>initial sync and that RTCP is needed for maintaining sync in the case
of 
>clock drift. But there is much more on timestamp handling in 
>http://tools.ietf.org/html/draft-ietf-mmusic-rfc2326bis-14
>than what is in RFC 2326.
>I could also recommend looking at the How To write RTP payload formats 
>that has some discussion on the RTCP synch mechanism or Colin Perkins 
>book on RTP.
>http://tools.ietf.org/html/draft-ietf-avt-rtp-howto-01
Very nice references. Of course, Colin's is bible.

>There is also need to have RTCP for synch in cases such as multiple 
>sources (SSRCs) and live sessions. In addition RTCP is needed for its 
>transport monitoring purpose. The sender can't do any type of
adaptation 
>or avoid causing long term congestion unless RTCP is used.
Sometimes, non-functional requirement like simplicity and modifiability
is more important than additional features. IMHO, thereby allowing an
applications using just RTP without RTCP would be meaningful. At least,
it looks like optional in rfc2326.

Cheers,
Jaehwan

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