[Libav-user] IGMPv3 source-specific multicast: what is the purpose of multiple sources?

[David Henry]

While using the relatively new feature of source-specific multicast (patched with http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=75d339e044f9b87dd9aa4bdaee73b1a8323d4a15)
I tried giving 2 sources, instead of a single source.  e.g. udp://eth1@239.19.5.189:3100?sources=172.31.1.21,172.31.2.21
I assumed (incorrectly) that the linux kernel would fail-over to the 2nd source if there was an interruption or some other error with the first source.
Instead, I'm just getting packets from both sources simultaneously, which is not useful for me.  

So the question is:
1) for streaming video, why would one want this sort of behavior? (packets delivered simultaneously from >1 source)
2) is there any way to dynamically implement fail-over at the receiver with ffmpeg & source-specific multicast?

Thanks, David