Subject: SSRC field

From: Fonteny Aurelia-AFONTEN1 <[email-masked]>
Date: Thu, 11 May 2000 11:04:55 -0500

Hi,

I'm rigth now working on RTP header compression, and there is one point that
is very unclear to me and I was wondering if someone could clarify it for
me.
Here is the overall problem : 
In RTP header compression (RFC2508), context-states have to be kept to be
able to compress the RTP header : but do the context have to be indexed on
{IPsource, IPdest, Port source, Port destination, SSRC}, or can they simply
be indexed on {IPsource, IPdest, Port source, Port destination} = does the
SSRC field of RTP add an information to {IPsource, IPdest, Port source, Port
destination}, or does {IPsource, IPdest, Port source, Port destination}
itself uniquely defines the ssrc.
 
In other words : 
RFC 1889 says RTP session = a pair of transport addresses (=network address
+ port for RTP and port for RTCP).
Within a RTP session, we define sources by their SSRC and different streams
within a RTP session have different SSRC.
RFC also says :
In a multimedia session, each medium is carried in a separate RTP session.
So, if we have two different streams coming from the same IP source (one
stream for one camera, another for a second camera), do they have to
communicate over two different source Port or is it possible that they both
go through the same Port source and Port destination in RTP?
What is the exact role of SSRC : distinguish between streams coming from
differents sources =( IP addresses + Port only), or also between different
streams for the same (IP source + Port)?
Thanks for your help.
Aurelie