Federated means you’re part of a giant tree with some authority on the top (like DNS has with ICANN).
Decentralized means there is no center
I would add that Distributed to me seems there are no inherent “domains” you need to belong to, like for example email is federated but with scuttlebutt you’re identified by your public key and a hub is just a dumb relay and you can have many.
Non-federated routing alternatives include Kademlia (used in BitTorrent) and simple flooding/gossipping (like in Kazaa) or ad-hoc networks
What's the authority at the top of Mastadon? You can have private instances that don't federate with anyone, and there's no coordinated authority that controls any other instance.
For that matter, what is the authority at the top of email? You and I can set up an email server and start talking to each other right now without anyone's permission. We don't have to be connected to anyone else, and we don't have any authority that can control our implementations or even that keeps track of what servers exist.
I don't get the comparison between Email or Mastadon to ICANN.
If you're using Mastodon or email you need a server with a public IP address, which most people don't have. Distributed solutions like Secure Scuttlebutt (SSB) don't require that, so you can pass messages via any available relay server, LAN, or even a sneaker-net.
It's also worth mentioning that most people have to trust their Mastodon / email provider not to snoop in their private messages. Systems like SSB provide true end-to-end encryption that makes it safe to broadcast your private messages to the world.
The Web and email are both federated and thus theoretically decentralized, but in practice they're highly centralized. Yet another federated protocol (sitting on top of an already centralized federated protocol) is probably not a reliable path to decentralization.
Perhaps you could explain?