[xiph-rtp] Difficulties with several RTP streams for a Vorbis stream.

[Ramón García]

[sorry if sent twice; I think that this message did not reach the mailing list]

You said that if a host that is listenting to an multicast address
only receives packets of the ports it is listenting to. Unfortunately,
this is not correct. IP multicasting works at the IP level. Ports are
a feature of TCP and UDP. IP multicast transports the payload of UDP,
including the port number, totaly blindly. Thus when a host suscribes
an IP multicast address, it is suscribing everything.  See, for
instance, RFC 1112, Appendix I (the format of the IGMP packet used for
subscription does not include port, only group address). This is like
when one sends a packet to a host to a port it is not listening to.
The packet arrives in the host anyway, it is up to the operating
system to decide what to do with it (and return an error if no
application is listening).

It is not imposible to implement Vorbis over RTP by sending through
several channels, but it would be very involved. Consider that every
RTP session must be handled by instances of the same RTP processor,
and that one RTP processor must intercept the output of another.
Multimedia frameworks are designed with the idea that any data stream
produced or consumed by a processor is time-based data. RTP sessions
should be processed uniformly.