Subject: RE: [AVT] Re: RTP/RTCP Port Sharing (SSRC)

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Fri, 27 Jul 2001 03:40:39 -0700

> 
> > > I strongly believe that any solution we come up with should 
> > > application
> > > generic and not impose limitations that restrict the way in 
> > > which RTP can be
> > > used.
> > 
> > By definition the problem is really restricted to unicast 
> > here, though.
> 
> True but I don't see any reason why it won't provide some benefits to
> multicast scenarios as well.
> 

Actually after thinking about it some more, I think applicability of "the
problem" (and solution) has nothing to do with transport modes (eg, unicast
or multicast) but rather conference modes. 

The proposal works for point-to-point communications (trivial case) and
conference modes where there is a centralized mixing bridge. For instance,
all conference senders send their RTP to the mixer and then the mixer sends
the mixed signal out to all conference listeners (either through multicast
or unicast). In these models there is only one sender relationship for each
participant even though each participant may have knowledge of all other
senders and receivers (and their SSRC's). 

The model does not work for distributed (meshed) conferencing models. For
instance one receiver might restrict the senders to choosing an SSRC within
a certain subspace (eg, 1234xxxx) and another receiver may require another
non-overlapping SSRC subspace (eg 4567xxxx). Thus it would not be possible
to select a SSRC that satisfies all receivers. 

In my mind the centralized conference model will in most cases have a
greater need/benefit for the properties endowed by this proposal (eg, shared
RTP ports/session separation) than a fully distributed conference model.

Robert.

---

From: Jim Murphy <[email-masked]>
Date: Fri, 27 Jul 2001 11:22:40 -0700

A bit off the thread, but I haven't heard anyone mention
the problems this port sharing is going to present to the
forwarding path in routers doing 5 tuple classification
and policing and voice gateways trying to send RTP packets to DSPs and
RTCP packets to some control level code. Not being able to
rely on the 5 tuple is going to present some problems.

Thanks,

Jim