[Live-devel] Axis multicast crash

[Ross Finlayson]

>>I don't know what these "typical fixes for Axis" are, because I don't
>>think anyone on this mailing list has ever mentioned them before,
>
>I'm not sure if it's what the OP meant, but Axis has a very strange 
>idea of what "multicast" means.
>
>Instead of just sending packets out blindly for anyone to receive 
>who wants, letting the switching fabric take care of pruning off 
>network segments where no one wants to listen, Axis makes the first 
>client send an RTSP request to start the stream.  Then, instead of 
>some interleaved A/V format, you get audio and video elementaries on 
>different ports.

Audio and video being streamed to different ports is how almost all 
audio+video RTP streams work - unicast or multicast.  (The only 
significant exception is when streaming a MPEG Transport Stream; in 
that case the audio+video remains multiplexed inside the Transport 
Stream.)

For multicast streams, the different port numbers (for audio and 
video) are (necessarily) the same for all clients, so (for multicast 
streams) these port numbers (and the IP multicast address) are 
*usually* specified inside the stream's SDP description, which the 
client gets in response to the RTSP "DESCRIBE" command.

For unicast streams, however, the port numbers can differ for 
differing clients (and there will also be server port numbers - 
needed for RTCP).  In this case, all these port numbers are set up in 
the response to the RTSP "SETUP" command.  (This, by the way, is one 
reason why it's difficult to get unicast streaming to work without 
using RTSP - and it's why we don't recommend trying to do unicast 
streaming without using RTSP.)

Now, I suppose it's possible that some servers (e.g., some Axis or 
other cameras) are using the RTSP "SETUP" command to set up the port 
numbers even for multicast streams.  I have seen some servers that 
specify the IP multicast address in the "SETUP" response, rather than 
in the SDP description (i.e., "DESCRIBE" response).  But I hadn't 
heard of any servers doing this for multicast port numbers also.  But 
if anyone has such a server, then please post an example "rtsp://" 
URL online, so we can check to see whether or not our RTSP client 
software works with it.  (If our software doesn't work - and the 
server's use of RTSP is otherwise standards-compliant - then I'm 
willing to consider updating (the latest version of) our client 
software to work with this server.  But again, I'd need an actual 
example stream to work with.)
-- 

Ross Finlayson
Live Networks, Inc.
http://www.live555.com/