#81 Synchronising multiple SSRCs
taken from a thread in mmusic
We have a problem in RTSP when a media stream
needs to contain multiple
RTP SSRCs. This can't be expressed today clearly.
When one is only interested in controlling the media and
not each
individual source the following scenarios should be
supported:
A SINGLE SETUP request can result in one of the
following:
1. A media stream with a single SSRC.
2. A media stream with retransmission with SSRC
multiplexing. This
requires a SSRC for the original and one for the
retransmission packets.
It is an advantage if both can be explicitly signalled, but
is no
requirement.
3. Live session with any number of participants and
unlimited use of
dynamic chosen SSRCs.
Due to clients capability the different scenarios need to
be explicitly
signaled. Scenario 2 will be that from an SDP
description and support of
retransmission will be signaled by requesting AVPF in
the transport
header. However today there exist no way to signal if it
is scenario 1
or 3. So this needs to be resolved in a nice backwards
compatible way. I
think that support for 3 can require support for any extra
signaling
while scenario 1 must work without any extra parts.
By doing all these scenarios with a single SETUP per
media stream we
keep the signaling to a minimum and avoid further
requirements on SDP.
It might be advantageous in the future to create a SDP
attribute
declaring the streams type.
If one desires to control the individual source one either
declare
individual media streams for each source. This will
require use of
ANNOUNCE in dynamic settings. The alternative would
be to create a new
media resource control protocol which is suited for this.
However I
don't see real need to solve this problem in standard
RTSP better than
by using ANNOUNCE.
Discussion
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This was discussed in the telecon the 28th April 2003.
The resolution was to clarify in RTSP that only a single
SSRC one can be given in RTSP. Any furhter SSRC may be
present in the stream, however they must be synchronised
using RTCP.
This has been clarified in the specification.
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No comments, closing bug.