Confirming source-routed multicast is dead on the public Internet

[johnl at iecc.com (John Levine)]

In article <nycvar.OFS.7.76.4444.1808021118080.22714 at cnex.qbaryna.pbz> you write:
>Multicast is being used in various private IP networks. It seems to work 
>very well for satellite content distribution because multicast doesn't 
>require ack's. Enterprise networks also use multicast.

I would think it'd work fine on private networks, but since there's no
authentication, on the public Internet how could you tell the
multicast you want from random malicious junk on the same IP address?