Isn't plain old UDP already an unstoppable DDOS tool? Multicast doesn't make it that much harder to stop. In fact using it as a DDOS tool seems a bit problematic since the victim would need to join the groups to receive the traffic. Yes a piece of malware on the victim's computer could go and attempt to join every single multicast source on the internet, but it's a self correcting problem since they wouldn't be able to maintain their subscriptions with their link totally saturated. Much easier to stop than normal DDOS attacks.
The problem is that we have never figured out a multicast routing solution that would work at Internet scale. Especially one that can be implemented in hardware on routers.
> we have never figured out a multicast routing solution that would work at Internet scale
Sure we did, it's called bittorrent. Ok, it isn't really multicast and you probably have to sacrifice ordered delivery, but for many of the use-cases where multiple-delivery would have been a good idea, bittorrent has proven to be a very successful "minimum viable multicast".
Bittorrent succeeded while decades of "multicast" research/experiments failed because bittorrent realized the multi-delivery problem was really about managing peers, which isn't solvable at layer-3.
edit: by which I mean: previous attempts at multicasting assumed it was a packet routing problem, when peer management is actually a question for the application layer.
Bittorrent is the opposite of multicast. Instead of aggregating the data into a single channel to save bandwidth, we instead split it up across every single recipient in a huge NxN graph.
This also illustrates the other problem with multicast on the Internet: It's mostly saving bandwidth on the backbone and at the server. The backbone has plenty of bandwidth to spare, and servers are often in data centers these days where bandwidth is not a huge concern.
The use case where someone does video production in their basement and broadcasts it out to millions of people across the internet over their home cable modem connection is just not compelling enough for ISPs and the backbone providers to make Multicast happen. Just put it on Youtube and let Google sort it out.
hmm. Multicast is often used for, like, IPTV. That's a very different task from BitTorrent. Torrents are indeed about managing peers. IPTV is centralized, not p2p, the benefit of multicast for IPTV is that the routers in between the source (ISP) and your client only carry one copy of the stream instead of one stream per client.
At internet scale.. well, it would be nice to have this efficiency for Twitch and YouTube Live. Which are also pretty centralized (CDN) so I don't see how this is about managing peers.
Bittorrent has a P2P streaming protocol called Bittorrent Live which was used to operate a TV service for several years but I have no idea how efficient it is compared to IPTV multicasting or central servers+CDN.
How exactly? Sources have to pass RPF check following ucast path and receivers have to follow the path either to RP or source, or the packets don't get there.
The problem is that we have never figured out a multicast routing solution that would work at Internet scale. Especially one that can be implemented in hardware on routers.