Has anyone used a federated protocol?

I am probably speaking almost 100% from ignorance here, but I'll give it a crack.

I imagine almost all federated protocols basically boil down to DNS.

The records for DNS can be stored any way you want. What matters is that a domain name server offers an API where you can ask well formatted questions and get well formatted responses.

The protocol tells you what you can ask for, but is agnostic about how the server satisfies the protocol.

  .
  com.
  ycombinator.com.
  news.ycombinator.com.

Each of these is a step through a federated architecture where "." is a coordinating authority. You trust "." and therefore implicitly trust "com." and therefore implicitly trust "ycombinator.com." and therefore implicitly trust "news.ycombinator.com."

Depends on the protocol, and sometimes the situation, really.

Maybe an example with Matrix?

I have my own Matrix home server, it sits in my bedroom.

I have a friend with his own Matrix home server, and it sits in his living room.

The chatrooms we are both in are synced to both of our home servers.

If all users in the room are on my home server, all of the data is on my home server.

Like ... SMTP?