AT&T did this for Uverse - it’s unicast for 5 seconds then it switches over to multicast. I assume there’s some key frames and h26x codec configuration that’s necessary.

Unfortunately multicast doesn’t work across the internet. It’s typically filtered or ignored I think.

That's correct. The unicast is there to catch up to the next multicast key frame and reduces the channel hop lag.

Multicast works great across the internet but it fails because everyone uses NAT in their home.

There's issues with wifi from memory.

Not sure why multicast wouldn't work from behind nat, assuming the router supported it.

The BBC ran such a trial around the turn of the millennium with a range of ISPs.

http://www.bbc.co.uk/multicast/

BT in the UK do this. It's awesome, but doesn't fix encoding delays like iPlayer is suffering from. Plus, it's over their Network which is a subset of the UK public internet infrastructure.

I had an old blog page about how it was designed but I took it down. May be worth revisiting it. That said the general public prefer unicast so they can watch it when they want, which renders multicast a moot point.

Yep. And they're not the only ones.

Nagra sell a system that does it to ISPs around the world and have done for at least 10 years. I'd been hoping we'd see more of that in the UK, but there has been virtually zero movement probably largely because of our patchy broadband service. I'm assuming the Sky Q boxes are heading in that direction eventually.

Internet multicast turns out to be hard. Now that streaming video is popular enough maybe it will be deemed a problem worth solving. I'm sure it's solvable. But it's hard. Also the trend for most things is video on demand, simultaneous viewing of a live event is rare.